A Novel Phenoplast-Based Structural Electrolyte with High Ionic Conductivity and Fire Resistance for Advanced Energy Storage Composites

A Novel Phenoplast-Based Structural Electrolyte with High Ionic Conductivity and Fire Resistance for Advanced Energy Storage Composites | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article A Novel Phenoplast-Based Structural Electrolyte with High Ionic Conductivity and Fire Resistance for Advanced Energy Storage Composites Zhao Sha, Ziyan Gao, Yingkun Shen, Cheng Wang, Jiangtao Xu, Shuying Wu, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6027287/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Structural energy storage composites are a promising lightweight solution for many applications, enabling structures to store electricity. However, there is a significant challenge in engendering polymer electrolytes with high flame retardancy, ionic conductivity, and mechanical properties. In this study, we introduce a novel fire-resistant polymer electrolyte that is highly ionic conductive and mechanically strong by designing a co-continuous composite electrolyte consisting of phenolic resin (or phenoplasts), ionic liquid (EMIM TFSI), and lithium salt (LiTFSI). The ionic liquid and lithium salt work synergistically as a hybrid surfactant to facilitate the emulsion polymerization of the phenolic resin, forming unique microstructures where the Li⁺ ions complex with hydroxyl groups in the phenoplasts creates ion-conductive pathways in the solid phase. This design yields significantly enhanced ionic conductivities up to 2.87 mS/cm, comparable to those of ionic liquids. For structural electrolyte applications, certain formulations achieved an ionic conductivity of approximately 0.15 mS/cm, a tensile strength around 19 MPa, and a tensile modulus of about 1.2 GPa. These properties demonstrate a well-balanced performance between electrochemical and mechanical characteristics, making the electrolytes suitable for advanced structural energy storage applications. The resulting phenoplast-based electrolyte not only maintains mechanical strength and structural integrity but also achieves the highest flame retardancy rating of V-0. A composite structural supercapacitor fabricated using this electrolyte demonstrated excellent electrochemical performance and safety features. This development presents a significant advancement in creating safe, efficient, and multifunctional materials for advanced structural energy storage applications. Physical sciences/Materials science/Materials for energy and catalysis Scientific community and society/Energy and society Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Structural energy storage composites have attracted much attention as they are able to achieve further light-weight design of composite structures at the system level 1 – 5 . Such energy-storage composite structures hold enormous potential in multiple fields such as automobiles and aircrafts to achieve lighter and better energy efficient product design 6 . However, the current state of structural supercapacitors and batteries integrated within these composites suffers from significantly lower energy density and power density compared to standalone energy storage devices. This deficiency stems from the absence of high-performance electrodes and electrolytes essential for developing integrated structural energy storage composites. As a critical element of structural energy storage composites, solid polymer electrolytes must simultaneously possess mechanical strength and ionic conductivity 7 – 9 . Achieving this dual functionality presents a significant challenge in advancing structural energy storage composites with high power density. Current methodologies typically involve mixing ionically conductive additives, such as lithium salts and ionic liquids, with polymers to impart ionic conductivity while retaining mechanical load-carrying capability. However, optimizing the trade-off between ionic conductivity and mechanical properties remains a critical issue in achieving the desired performance. The traditional polymer matrix materials used in carbon fibre-reinforced polymers (CFRPs), such as epoxy, inherently lack conductivity. For instance, a polymer structural electrolyte formulated with 40 wt.% ionic liquid and 60 wt.% epoxy resin can only achieve an ionic conductivity of 0.116 mS/cm, merely 2.1% of the ionic conductivity measured for ionic liquid (5.5 mS/cm) 10 . This value falls far below the expected conductivity estimated by the rule of mixture. Such limitations may arise due to complex tortuous paths and the boundary layer effect within small ionic channels, hindering efficient ion transport 11 – 13 . To make matters worse, commonly used polymer matrices in composites are highly flammable and emit toxic fumes during combustion, posing substantial fire hazards during the operation of structural energy storage composites. Moreover, with the wide application of composite materials in extreme work condition, the demand for structural energy storage composites capable of withstanding high temperatures has surged alongside their widespread application in extreme work conditions. Potential scenarios include planetary surface activities (e.g., Venus’s surface temperature reaching 480°C) and the powering of electronics in aerospace vehicles close to the engine, etc. However, exposure to high temperatures can degrade the polymer electrolyte, leading to substantial performance losses in the energy storage device. Thus, developing flame-retardant and thermally stable electrolytes for structural energy storage composites is critically important for industries such as oil and gas, military, aerospace, automotive, and electric vehicles. In this study, we propose a method using phenolic resin to develop flame-retardant polymer electrolytes tailored for structural energy storage composites. By incorporating phenolic resin with ionic liquid and lithium salt, the resulting polymer electrolyte can achieve a high ionic conductivity of 2.87 mS/cm, which is of the same order of magnitude as that of the ionic liquid. By adjusting the content of each component, it is possible to develop an electrolyte with balanced electrochemical and mechanical performance. This approach differs from the conventional method of simply mixing epoxy and ionic liquid to develop a bicontinuous electrolyte 10 , 14 , where the ionic liquid and epoxy have limited interaction with each other. The presence of ionic liquid and lithium can help forming unique microstructures with excellent ionic conductivity. This unique combination of phenolic resin, ionic liquid, and lithium salt addresses the challenges of ionic conductivity, mechanical performance, and fire resistance inherent in conventional methods, making the electrolyte well-suited for structural energy storage composites across various applications, including aerospace, automotive, civil engineering, and wearable electronics. Results Electrochemical, Mechanical, and Thermal Properties of Phenolic Resin based Solid Electrolyte The ionic conductivity of an electrolyte is a critical parameter influencing the overall performance of structural energy storage composites 15 . In this work, a novel flame-retardant polymer electrolyte is prepared using phenolic resin, ionic liquid EMIM TFSI (IL), and lithium salt LiTFSI, further details of the process can be found in Supplementary Information and a filed patent 16 . The content of each component was systematically adjusted through experimentation to explore its influence on the ionic conductivity of the solid electrolyte. To begin with, the electrolyte was prepared by simply mixing phenolic resin with the IL. The mass content of IL in the solid electrolyte was increased from 0–50%, and the calculated ionic conductivities of these electrolytes are shown in Fig. 1 a. It was observed that no ionic conductivity was measured even when the IL mass content increased to 30%, consistent with previous studies on epoxy resin-based solid electrolytes 10 . The solid electrolyte became ionically conductive when the IL mass content exceeded 30%, with the electrolyte containing 40% IL achieving an ionic conductivity of about 4.2 × 10 − 3 mS/cm. However, further increasing the IL mass content did not result in a constant increase in the ionic conductivity of the solid electrolyte. It was observed that the solid electrolyte containing 50% IL had an ionic conductivity of about 4.9 × 10 − 4 mS/cm. This is attributed to the density difference between phenolic resin (1.22 g/cm³) and IL (1.52 g/cm³); a high content of IL is more likely to form a layered structure in the solid electrolyte, as shown in the inset of Fig. 1 a, leading to a significant decrease in the electrolyte’s ionic conductivity. In this work, the ionic conductivity of the solid electrolyte was calculated based on the Nyquist plots obtained through EIS measurements, and the bulk resistance was determined by fitting the data with an equivalent circuit, as shown in Fig. 1 b. The equivalent circuit model used was consistent with our previous study 8 . To avoid the formation of a layered structure in the solid electrolyte, the mass ratio between phenolic resin and IL was controlled to be 3:2. Based on this optimized ratio, further modifications to the solid electrolyte were conducted by introducing different contents of lithium salt LiTFSI. Since the lithium salt was first dissolved in IL before mixing with phenolic resin, the content of lithium salt was controlled in terms of the LiTFSI/IL ratio. This method ensured a homogeneous distribution of the lithium salt within the electrolyte, enhancing the ionic pathways and improving overall conductivity. The LiTFSI/IL ratio was increased from 0 to 0.5, which reaches the solubility limit of LiTFSI in EMIM TFSI 17 . Figure 1 c shows Nyquist plots of solid electrolytes containing different amounts of LiTFSI. It is observed that the change in bulk resistance does not consistently correlate with the change in LiTFSI content. For the electrolyte sample without LiTFSI, the bulk resistance is roughly about 100 kΩ. This resistance increases to over 2000 kΩ when the LiTFSI/IL ratio increases to 0.1. However, further increasing the LiTFSI content leads to a decrease in bulk resistance, with values less than 3000 Ω and less than 200 Ω achieved with LiTFSI/IL ratios of 0.2 and 0.5, respectively, as shown in the inset of Fig. 1 c. Therefore, it was found that adding LiTFSI to the electrolyte initially decreases the ionic conductivity, reaching a minimum value of about 1.89 × 10 − 4 mS/cm at a LiTFSI/IL ratio of 0.1. Further increasing the LiTFSI content leads to an increase in ionic conductivity, with the highest value of 2.87 mS/cm achieved at a LiTFSI/IL ratio of 0.5, as shown in Fig. 1 d. Although there is a dip in ionic conductivity occurring around the LiTFSI/IL ratio of 0.1, which looks more pronounced when the y-axis is in logarithmic scale, the overall trend of ionic conductivity appears to increase with the LiTFSI content, as shown in the inset of Fig. 1 d, where the y-axis is in a linear scale. Although the mass ratio between phenolic resin and IL was controlled to be constant, adding more lithium salt can lead to a decrease in phenolic content in the electrolyte, which may potentially contribute to the increase in the electrolyte’s ionic conductivity. To validate this, another set of samples was prepared by controlling the mass ratio of phenolic resin to the combined IL and LiTFSI to be 3:2. However, by adjusting the content of lithium salt in electrolyte, a similar trend in ionic conductivity was observed, as shown in Fig. 1 e. The ionic conductivity of the solid electrolyte reached its minimum value when the LiTFSI/IL ratio was 0.2, then increased to the highest value at a LiTFSI/IL ratio of 0.5. These results indicate that, besides the decrease in phenolic resin content, there is another mechanism existing when adding LiTFSI that increases the ionic conductivity of the solid electrolyte. Notably, the ionic conductivity of IL decreases with the increase of LiTFSI content, as shown in Fig. 1 f, which is consistent with previous reports 17 , 18 . This decrease in the ionic conductivity of IL could explain the initial dip observed in the ionic conductivity curves in Fig. 1 d and Fig. 1 e when adding LiTFSI to the electrolyte, where the ionic conductivity of the electrolyte is initially dominated by the ionic conductivity of IL. To investigate the mechanical properties of the two sets of solid electrolytes mentioned earlier, tensile tests were conducted according to ASTM D638. One set had a fixed phenolic resin to IL mass ratio of 3:2, while the other had a fixed phenolic resin to IL + LiTFSI ratio of 3:2. The results are presented in Figs. 1 g and 1 h. For the electrolytes with a fixed phenolic resin to IL mass ratio of 3:2, the tensile strength remained relatively stable, ranging from approximately 16 to 19 MPa, as the LiTFSI/IL ratio increased up to 0.2. However, further increases in the LiTFSI/IL ratio led to a decrease in tensile strength, reaching the lowest value of 1.11 ± 0.4 MPa at a phenolic resin: IL: LiTFSI ratio of 3:2:1. The tensile modulus of this set of electrolytes followed a similar trend, initially around 1.9 GPa, and began to decrease when the LiTFSI/IL ratio exceeded 0.2, eventually dropping to a minimum value of 0.067 GPa with the highest LiTFSI content (Ph:IL = 3:2:1). The reduction in strength and modulus for electrolytes with high LiTFSI content is primarily attributed to microstructural changes in the electrolyte, which will be discussed in detail in the following section. In contrast, for the electrolytes with a fixed phenolic resin to IL + LiTFSI ratio of 3:2, the tensile strength remained relatively stable even at the highest LiTFSI content. This stability is mainly due to the constant phenolic resin content (60%) in the electrolyte, which helps maintain the mechanical strength. However, the tensile modulus still showed a reduction at higher LiTFSI contents, particularly when the LiTFSI/IL ratio exceeded 0.3, dropping from around 1.9 GPa to about 1.2 GPa. This decrease is also related to the microstructural changes in the electrolyte. For the application of structural energy storage composites, it is essential that the electrolyte exhibits both high ionic conductivity and favourable mechanical properties. Figure 1 i presents a plot of ionic conductivity versus tensile strength for all solid electrolytes studied. Two electrolytes are identified as potential candidates for application in structural energy storage composites. (1) The electrolyte with a phenolic resin (Ph): ionic liquid (IL): LiTFSI mass ratio of 3:2:0.4, denoted by a black star in the figure. This electrolyte is part of a set where the mass ratio of phenolic resin to IL is fixed. (2) The electrolyte with a phenolic resin to IL + LiTFSI mass ratio of 3:2, and an LiTFSI/IL ratio of 0.5, indicated by a red sphere in the figure. This electrolyte belongs to a set where the phenolic resin to IL + LiTFSI ratio is fixed. Both of these electrolytes exhibit comparable ionic conductivity and mechanical strength, demonstrating a well-balanced performance that makes them suitable candidates for use in structural energy storage composites. In this figure, the red dashed line represents the optimal performance target for structural electrolytes, where the multifunctional efficiency factor 6 equals 1 (which is typically less than 1 in most cases). It can be observed that the electrolyte developed in this study has achieved ionic conductivity very close to this ideal target. However, when considering both structural load-bearing capability and ionic conductivity simultaneously, there remains significant room for improvement to further enhance overall performance. Thermogravimetric analysis (TGA) in nitrogen environment was conducted on above phenolic-based electrolyte candidates, in comparison to an epoxy-based electrolyte (Epoxy: IL = 3:2), as shown in Fig. 2 a. The results demonstrate a significant difference in residual mass between the phenolic and epoxy-based electrolytes at elevated temperatures. At 900°C, the phenolic-based electrolytes retain approximately 30% of their initial mass, while the epoxy-based electrolyte retains only 6.8% under identical testing conditions. This higher residue in the phenolic-based electrolytes indicates a more robust structure that resists decomposition, consistent with the known thermal stability characteristics of phenolic materials. More specifically, electrolyte 1 (Ph:IL:LiTFSI = 3:2:0.4) shows a residual mass of 29.7%, slightly lower than that of electrolyte 2 (Ph:(IL + LiTFSI) = 3:2; IL:LiTFSI = 2:1), which has a residual mass of 31.8%. This variation in residual mass is expected and it can be attributed to the higher phenolic resin content in electrolyte 2, which enhances thermal stability. The DTG analysis, as shown in Fig. 2 b, further highlights the differences in decomposition profiles between the phenolic and epoxy resin-based electrolytes. The phenolic-based electrolytes exhibit a single, dominant decomposition peak around 500°C, suggesting a straightforward decomposition process typical of thermally stable materials. In contrast, the epoxy resin-based electrolyte (blue curve) displays a more complex thermal decomposition profile with two distinct peaks in the DTG curve, indicative of a two-step degradation process. The first peak, at around 300°C, is attributed to the decomposition of the epoxy phase in the electrolyte, while the second peak, around 500°C, corresponds to the decomposition of the ionic liquid (IL) component. These attributions are supported by the TG curves of pure epoxy and IL, as shown in Fig. 2 c. It is notable that, although the second peak occurs at a relatively high temperature of 500°C, the epoxy-based electrolyte will lose its structural integrity at the first peak (~ 300°C), where most of the solid epoxy phase in the electrolyte decomposes, leaving only the liquid IL phase. Further insights can be drawn from Fig. 2 c, where the residual masses of pure phenolic resin and LiTFSI are also presented. Pure phenolic resin retains approximately 53.4% of its initial mass at 900°C, which is significantly higher than the residual masses of pure epoxy (8.1%), IL (6%), and LiTFSI (6.4%). This high residual mass of pure phenolic resin is a key factor contributing to the excellent thermal stability of the phenolic resin-based electrolytes developed in this study, making them suitable for high-temperature applications where structural integrity is critical. The superior thermal resistance of the phenolic resin-based electrolyte, as compared to the epoxy resin-based alternative, demonstrates its potential as a stable material for use in environments requiring sustained thermal endurance. Flame retardancy is a critical performance factor for polymer structural electrolytes, especially for applications requiring both structural integrity and safety under high-temperature conditions. To assess the flame retardancy of the phenolic resin-based electrolyte, a standard UL94 vertical burning test was conducted, with an epoxy-based electrolyte (Epoxy = 3:2) included as a comparison. The flammability test progression is illustrated in Fig. 2 d. The results reveal a stark contrast between the two materials: the epoxy-based electrolyte ignites easily, and the flame spreads rapidly, indicating a complete lack of flame retardancy. Due to its high flammability, the epoxy-based electrolyte was unable to achieve any valid UL94 rating. In contrast, the phenolic resin-based electrolyte developed in this study demonstrates outstanding flame retardancy. It could not be ignited at all during the test, achieving the highest possible flame retardancy rating of V-0. This rating reflects the material’s capability to resist ignition and prevent flame propagation, highlighting its potential as a safe and effective structural electrolyte for high-temperature and flame-resistant applications. The excellent flame retardancy of the phenolic-based electrolyte aligns with the intrinsic fire-resistant properties of phenolic resins, further supporting its suitability for use in environments that demand both thermal stability and fire safety. In addition to demonstrating superior thermal stability and flame retardancy, the phenolic-based polymer electrolyte developed in this study significantly expands the boundaries of mechanical and electrochemical performance compared to electrolytes based on epoxy. Figures 2 e and 2 f present a comparative analysis of the mechanical (tensile strength and modulus) and electrochemical (ionic conductivity) properties between the phenolic-based electrolyte developed here and recently reported epoxy-based electrolytes 8 , 10 , 19 – 21 . For reference, the tensile properties of pure epoxy and phenolic polymers, as well as the ionic conductivity of pure IL, are also indicated in the figures. The data reveal that some phenolic-based electrolytes (marked with red stars) achieve performance levels beyond those of epoxy-based electrolytes, particularly in terms of ionic conductivity, which approaches values similar to pure IL. This enhanced ionic conductivity suggests that the phenolic-based electrolytes developed in this study may provide more efficient ion transport pathways, potentially due to the optimized composition and structure of the phenolic matrix. Furthermore, the improved mechanical properties of the phenolic-based electrolyte, as shown by increased tensile strength and modulus, demonstrate its suitability as a structurally robust electrolyte material. These advancements in both mechanical and electrochemical performance make the phenolic-based polymer electrolyte a promising candidate for applications requiring a balance of mechanical durability, high ionic conductivity, and enhanced thermal and flame resistance. Micro Morphology of Phenolic Resin based Solid Electrolyte Investigating the micro morphology of phenolic resin-based solid electrolytes is crucial for understanding how their internal structure affects ionic conductivity and overall performance. Scanning electron microscopy (SEM) was employed to analyse these microstructural features. Figure 3 . Microstructural Analysis of Phenolic Resin-Based Solid Electrolytes: (a) pure phenolic polymer, (b) phenolic resin mixed with ionic liquid (mass ratio 3:2); (c) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.2); (d) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.4); (e) phenolic resin/ionic liquid/lithium salt mixture (3:2:1), EDS analysis of (f) original and (g) washed electrolyte sample with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1. Figure 3a presents the typical fracture surface morphology of the pure phenolic resin polymer. The SEM image reveals numerous pores within the polymer matrix, which result from the release of water and small gas molecules during the curing process. These pores contribute to the high porosity commonly observed in cured phenolic resin, a characteristic feature of this material 22 , 23 . Upon mixing the phenolic resin with an ionic liquid at a mass ratio of 3:2, slight changes in the microstructure were observed. Figure 3b shows the fracture surface morphology of the cured electrolyte. While the pore size remains similar to that of the pure phenolic resin, the void density is notably reduced, dropping to nearly half of the original value. This decrease in porosity is primarily attributed to the reduced phenolic resin content in the mixture, as the ionic liquid occupies part of the matrix, thereby reducing overall pore formation. The addition of lithium salt to the electrolyte further alters the fracture surface morphology. In Fig. 3c, which represents a phenolic resin/ionic liquid/lithium salt ratio of 3:2:0.2, the pore size decreases while pore density increases, indicating that lithium salt impacts the microstructure, likely through interactions with the resin and ionic liquid that influence pore formation and distribution. Significant changes in micro morphology are observed when the lithium salt content is further increased, as shown in Fig. 3d, where the electrolyte has a phenolic resin/ionic liquid/lithium salt ratio of 3:2:0.4. In this sample, a liquid-phase substrate, likely ionic liquid, is clearly visible on the fracture surface. Unlike the surfaces in Figs. 3b and 3c, where the ionic liquid phase is not easily observed and may be isolated by the non-ionic conductive phenolic resin polymer, the higher lithium salt content seems to enable the release of the ionic liquid from isolation. This creates a continuous ionic conductive phase, significantly enhancing the electrolyte’s ionic conductivity. Additionally, some microsphere structures begin to form beneath the ionic liquid phase. This hypothesis is confirmed with further increases in lithium salt content, as demonstrated in Fig. 3e, showing the fracture surface morphology of an electrolyte with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1. This composition exhibits a markedly different micro morphology, characterized by numerous microsphere structures closely bonded together, with a liquid-phase substrate adhering to the surface. The enlarged view in Fig. 3e reveals that the liquid-phase substrate, likely the ionic liquid, forms a continuous phase that covers the entire surface. The resulting structure appears as a highly porous skeleton made of phenolic microspheres, with an ionic conductive liquid phase closely adhering to each surface, leading to enhanced ionic conductivity throughout the electrolyte. Energy Dispersive Spectroscopy (EDS) analysis was conducted on the electrolyte with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1, as shown in Fig. 3f. Characteristic elements such as sulfur (S), nitrogen (N), and fluorine (F) from TFSI- are clearly identified on the electrolyte’s fracture surface, indicating that the liquid-phase substrate adhering to the microsphere structures is indeed the ionic liquid. Another EDS analysis was performed on the same electrolyte after ultrasonic cleaning with acetone and water for over 4 hours, followed by drying in an oven. The liquid-phase substrate was removed after washing, but the S, N, and F elements remained detectable on the surface, suggesting that some lithium salt (LiTFSI) remains integrated within the microsphere structures even after prolonged ultrasonic cleaning. It is noted that neither EDS analysis in Figs. 3f and 3g could detect the lithium (Li) element, which is consistent with the understanding that the lithium K X-ray from Li to K shells is not an allowed transition 24 , 25 due to the azimuthal quantum number ( l ) selection rule, where both energy levels have the same orbital angular momentum ( l = 0 ). This results in a very low likelihood of X-ray emission. Therefore, the absence of Li in the EDS results does not necessarily indicate its absence in the electrolyte. It is believed that lithium salt resides both in the phenolic resin phase and the ionic liquid phase within the electrolyte. Mechanism Analysis on Phenolic Resin based Solid Electrolyte The curing behaviour of phenolic resin-based electrolyte systems were evaluated using Differential Scanning Calorimetry (DSC), as shown in Fig. 4 a. Four different samples were examined: (1) pure phenolic resin (Ph), (2) phenolic resin with ionic liquid (Ph:IL = 3:2), (3) phenolic resin with LiTFSI (Ph:Li = 3:1), and (4) phenolic resin with both ionic liquid and LiTFSI (Ph:IL:Li = 3:2:1). All these samples are in uncured liquid state, and the DSC curves provide valuable insights into the curing reactions and interactions between the resin and the added ionic liquid and lithium salt. The pure phenolic resin (Ph) shows an exothermic peak between 80°C and 110°C, corresponding to the curing process of the resin 26 . This peak reflects the heat release associated with the cross-linking reaction, and the intensity of the peak suggests that the resin undergoes a thorough curing process. Given that this peak serves as a reference for evaluating modified samples, its well-defined location and intensity indicate that, under these conditions, the phenolic resin reaches a high degree of curing with minimal residual reactive sites. In the sample containing ionic liquid (Ph/IL), the exothermic peak appears in a similar temperature range, starting around 80°C but with a slightly lower intensity compared to the pure phenolic resin. Although the onset temperature of curing does not shift significantly, the lower peak intensity may suggest that the plasticizing effect 27 of ionic liquid results in incomplete curing under the current conditions. This incomplete curing could occur because the enhanced chain mobility might prevent all reactive sites from fully cross-linking, as the flexible matrix does not stabilize as completely as in the pure resin. For the sample containing LiTFSI (Ph/Li), the exothermic peak starts much earlier, around 50°C, with the peak that extends up to about 100°C. This early onset reflects the catalytic effect of LiTFSI, which lowers the activation energy required for the curing reaction. The Li + ions can form ion-dipole interactions with the oxygen atoms in the phenolic hydroxyl groups 28 , disrupting the hydrogen bonding network within the resin. This disruption increases the mobility of the polymer chains, allowing the cross-linking to initiate at a lower temperature and proceed more vigorously. However, while the curing is highly accelerated, the decreased peak intensity compared to the pure resin’s peak may imply that the curing process, though faster, is not entirely complete. Rapid initiation of the reaction may lead to premature cross-linking in certain areas, limiting the diffusion of reactants and leaving some reactive sites unreacted. The Ph/IL/Li sample, which represents the structural electrolyte developed in this study, combines both ionic liquid and LiTFSI. It exhibits a broad exothermic peak starting around 50°C and extending to approximately 110°C. The early onset is driven by the catalytic action of LiTFSI, while the ionic liquid enhances chain mobility, creating a more flexible curing environment. However, the lower intensity of the exothermic peak compared to the pure phenolic resin suggests incomplete curing. The broad and lower-intensity peak indicates that, while curing begins early, it proceeds in a different manner, potentially resulting in an incomplete cross-linked network. Integration of the exothermic peaks to calculate the heat released during curing, shown in Fig. 4 b, confirms these observations. The pure phenolic resin sample generated a specific heat of approximately 105 mJ/mg. In comparison, the Ph/IL, Ph/Li, and Ph/IL/Li samples produced lower specific heats of 57.1, 52.3, and 34.2 mJ/mg, respectively. Since the curing process primarily occurs within the phenolic resin, it is more appropriate to normalize the specific heat by the weight fraction of phenolic resin in each sample, as represented by the green bars in Fig. 4 b. According to the normalized results, the Ph/IL sample generated a specific heat of 95.2 mJ/mg during curing, indicating a slightly lower curing degree (~ 91%) compared to the pure phenolic resin. For samples containing LiTFSI, the Ph/Li and Ph/IL/Li samples exhibited normalized specific heats of approximately 69.7 and 68.3 mJ/mg, respectively, representing a curing degree of around 66% relative to the pure phenolic resin sample. These findings suggest that while LiTFSI effectively accelerates curing by lowering the initiation temperature, it also has a more pronounced role in hindering complete cross-linking of the phenolic resin. Solid-state carbon-13 ( 13 C) nuclear magnetic resonance (NMR) spectroscopy was performed on four groups of samples to further investigate the interaction mechanisms among phenolic resin, IL, and LiTFSI. The curing of phenolic resin follows a typical condensation polymerization mechanism. During curing, the hydroxyl group (-OH) attached to the aromatic ring in the phenolic monomer reacts with methylene groups (-CH₂-) from a cross-linking agent, forming methylene bridges between aromatic rings and releasing water as a byproduct 29 . This cross-linking reaction connects the benzene ring structures in the phenolic resin via -CH₂- linkages, as shown in Fig. 4 c. It is noteworthy that the -OH groups directly connected to the benzene rings remain unchanged during this cross-linking reaction. Therefore, the carbon atoms in the benzene rings attached to the -OH groups, marked in green, can be used as internal references to evaluate the curing degree of the phenolic resin. In contrast, the carbon atoms from the -CH₂- groups, marked in light orange in the chemical formula of the phenolic resin, reflect the extent of cross-linking in these samples. From the NMR results, the peaks located around 150 ppm correspond to the carbon atoms in the benzene rings directly connected to the -OH groups. The intensity of this peak in all samples has been normalized to a value of 1.00. This normalization allows for the determination of the relative intensity of the carbon atoms from the -CH₂- groups in each sample. For pure phenolic resin, the intensity of the -CH₂- peak is approximately 1.51, indicating that the number of carbon atoms from -CH₂- groups is roughly 1.5 times that of the reference carbon atoms. Similarly calculated, the intensities of the -CH₂- peaks in the Ph/IL, Ph/Li, and Ph/IL/Li samples are about 1.71, 1.36, and 1.18, respectively. The higher intensity observed in the Ph/IL sample may be attributed to the -CH₂- groups present in the EMIM⁺ cation of the ionic liquid, as indicated in the figure. Excluding the Ph/IL sample, the intensity values calculated for the Ph/Li and Ph/IL/Li samples are lower than that of the pure phenolic resin, suggesting a decreased number of -CH₂- groups. This decrease corresponds to a reduced curing degree of the phenolic resin in these two samples. It appears that the presence of LiTFSI influences the phenolic resin to form shorter-chain polymer structures rather than fully cured long-chain polymers, particularly in the Ph/IL/Li sample. Even with the additional -CH₂- groups from the IL, the total amount of -CH₂- is significantly lower than that of the pure phenolic resin. Assuming that the intensity difference (1.71 − 1.51 = 0.20) between the Ph/IL and pure phenolic resin samples is entirely due to the EMIM⁺ from the IL, the actual intensity of the -CH₂- groups originating from the phenolic resin in the Ph/IL/Li sample would be 0.98 (1.18 − 0.20). Compared to the pure phenolic resin (1.51), this represents about 65% of the -CH₂- content, which aligns well with the curing degree (66%) calculated from the DSC results. Fourier-transform infrared spectroscopy (FTIR) was applied to analyze the four samples. According to the results shown in Fig. 4 d, the characteristic absorption bands of the cured pure phenolic resin were identified in the FTIR spectrum. These include peaks at around 3500 cm⁻¹, representing the presence of hydroxyl groups (-OH), and at 3005 cm⁻¹, corresponding to the aromatic C–H stretch 30 . Peaks at 2918 cm⁻¹ and 2849 cm⁻¹ are attributed to the stretching vibrations of methylene C–H groups 30 , 31 . In addition to these characteristic peaks from the phenolic resin, two more peaks were observed in the FTIR spectra of the Ph/IL and Ph/IL/Li samples, located around 3158–3161 cm⁻¹. These peaks correspond to the C–H stretching vibrations of the imidazolium cation (EMIM⁺) ring of the ionic liquid 32 , 33 . Notably, the characteristic peaks at 2918 cm⁻¹ and 2849 cm⁻¹, representing methylene C–H groups from the phenolic resin, were observed in samples Ph, Ph/IL, and Ph/Li but were absent in the Ph/IL/Li sample. This indicates a significantly decreased amount of -CH₂- groups in the Ph/IL/Li sample, which aligns well with the observations from the NMR results. Based on the above characterizations and the observed micro-morphology of the phenolic-based electrolyte, we can unveil the underlying mechanism by which a phenolic-based electrolyte with high loading of LiTFSI forms a unique microstructure. The microspherical structures of the phenolic resin, observed after mixing with the ionic liquid (IL) EMIM TFSI and a high loading of LiTFSI, align with the characteristics of emulsion polymerization. Although EMIM TFSI and LiTFSI are not conventional surfactants typically used to facilitate emulsion polymerization, their combination—especially with a high concentration of LiTFSI—acts as a hybrid surfactant to promote the emulsion polymerization of phenolic resin, resulting in microsphere formations as shown in Fig. 4 e. Specifically, the ionic liquid EMIM TFSI effectively reduces the interfacial tension 34 and forms a robust ionic layer around the resin droplets. Simultaneously, LiTFSI enhances the ionic strength of the aqueous medium (the phenolic resin used in this study contains about 15% water content, and the curing of phenolic resin also produces water as a byproduct) and interacts synergistically with EMIM TFSI as a co-surfactant, stabilizing the emulsion by promoting electrostatic repulsion between dispersed droplets and preventing their coalescence. The presence of both LiTFSI and EMIM TFSI facilitates the formation of a stable emulsion through their complementary interactions—EMIM TFSI provides steric stabilization via its bulky TFSI⁻ anions, while Li⁺ cations contribute to electrostatic stabilization. Upon initiation with a free initiator, which is the catalyst used in this study, polymerization proceeds within the stabilized droplets, resulting in uniformly dispersed phenolic polymer microspheres, eventually forming the microstructure shown in Fig. 3e. The innovative use of LiTFSI and EMIM TFSI in this emulsion polymerization system underscores their potential as effective non-traditional surfactants, offering enhanced stability and control over polymerization kinetics compared to conventional surfactant-based methods. Moreover, through adjusting the content of each component, the emulsion polymerization process can be fine-tuned to achieve an optimal balance between mechanical and electrochemical properties in the resulting polymer electrolyte. As depicted in Fig. 4 f, the phenolic microspheres formed through emulsion polymerization primarily facilitate ionic conductivity, while the phenolic blocks created via conventional condensation polymerization provide mechanical strength. Regarding the significantly enhanced ionic conductivity of the phenolic-based electrolyte achieved by forming such a microstructure, this improvement can be attributed to the complexation between Li⁺ ions and the hydroxyl groups in the formed phenolic microspheres. In addition to the ionic liquid that fully covers the phenolic microspheres—providing a continuous and highly ionically conductive medium—the Li⁺ ions can complex with the oxygen atoms of the hydroxyl groups 35 , as illustrated in Fig. 4 f. This complexation creates additional ion-conductive pathways, substantially enhancing the ionic conductivity of the electrolyte. Performance of Composite Structural Energy Storage Device A composite structural supercapacitor was fabricated using the phenolic-based electrolyte developed in this study. The device was constructed with two carbon nanotube mats (dimensions: 2 cm × 2 cm) serving as electrodes and a glass fibre veil (thickness: 40 µm) as a separator. These materials were selected to balance mechanical strength, thermal stability, and electrochemical performance, with the phenolic-based electrolyte providing dual functionality as both an energy storage medium and a structural matrix. The capacitance of the structural supercapacitor was evaluated using the cyclic voltammetry (CV) method, with scan rates ranging from 1 mV/s to 200 mV/s. The CV curves, shown in Fig. 5 a, exhibit a quasi-rectangular shape with good symmetry, reflecting efficient charge-discharge behaviour and low resistance at the electrode-electrolyte interface. The absence of significant distortion in the CV profiles across a wide range of scan rates indicates stable electrochemical behaviour and compatibility of the phenolic-based electrolyte with the carbon nanotube electrodes. The calculated capacitances under different scan rates are presented in Fig. 5 b. At the lowest scan rate of 1 mV/s, the device achieved its highest capacitance of approximately 45.5 mF. This high value is remarkable considering that the electrodes are composed solely of pure carbon materials without additional conductive additives or active materials. The high capacitance is attributed to the efficient interaction between the phenolic-based electrolyte and the carbon nanotube electrodes, which maximizes the utilization of the electrode surface area. As the scan rate increased, the capacitance gradually decreased due to the limited time for ion diffusion within the electrode pores and the electrolyte matrix. Nevertheless, even at the highest scan rate of 200 mV/s, the device retained a capacitance of approximately 0.8 mF. This retention demonstrates the strong ion transport capability and relatively low internal resistance of the phenolic-based electrolyte, highlighting its potential for applications requiring rapid charge-discharge cycles. To demonstrate the application of the developed phenolic-based electrolyte, four composite structural supercapacitors connected in series were first charged to 2 V using an electrochemical (EC) workstation. After charging, they were disconnected from the EC workstation and connected to a circuit to light up an LED. The structural supercapacitors successfully powered an LED for over 30 seconds, as shown in Fig. 5 c; a video illustrating this process ( Video 1 ) is available in the Supplementary Information . The performance of the composite structural supercapacitor was further demonstrated through another experiment. The same structural supercapacitor was charged using a 9 V alkaline battery for approximately 20 seconds and was then able to successfully light up six LEDs for over 5 minutes, as depicted in Fig. 5 d. A corresponding video ( Video 2 ) can also be found in the Supplementary Information . Additionally, the structural supercapacitor demonstrated the ability to store electricity for extended periods after charging. In a test, the supercapacitor was charged for about 20 seconds, then allowed to rest for approximately 1 minute before being connected to LEDs. After powering the LEDs for 1 minute, it was disconnected, rested for another 1 minute, and reconnected to the LEDs. The supercapacitors continued to function well under these conditions, indicating good charge retention capabilities. A corresponding video ( Video 3 ) can be found in the Supplementary Information. Moreover, it is worth noting that the second and third charge and discharge demonstrations were performed four months after the first one, and the supercapacitors still functioned effectively. This long-term performance further showcases the excellent stability and efficiency of the phenolic-based electrolyte developed in this study. Finally, the flame retardancy of the composite structural supercapacitor was evaluated by subjecting it to a direct methane flame for 1 min. The device was exposed to the flame to simulate extreme thermal conditions and assess its fire-resistant properties. During the flame exposure, some oxidation and pyrolysis occurred in the components of the composite structural supercapacitor, particularly affecting the surface layers. However, the device maintained a high level of structural integrity after this 1-min flame treatment, as illustrated in Fig. 5 e. There were no signs of melting, dripping, or significant deformation, indicating that the core structure remained largely unaffected. Most importantly, the device did not ignite or sustain combustion during and after the flame exposure. This lack of ignition demonstrates the excellent flame retardancy of the phenolic-based structural electrolyte used in this study. Phenolic resins are well-known for their inherent flame-resistant properties due to their ability to form a stable char layer upon heating, which acts as a barrier to heat and mass transfer 36 . The formation of this protective char layer helps to inhibit further thermal degradation and prevents the spread of flames. The enhanced safety provided by the phenolic-based structural electrolyte is crucial for applications where fire resistance is paramount, such as in aerospace, automotive, and construction industries. The ability of the composite structural supercapacitor to withstand direct flame exposure without igniting or losing structural integrity underscores its potential for safe energy storage solutions in environments prone to high temperatures or fire hazards. Conclusion In this study, a phenolic resin-based structural electrolyte was successfully developed for composite structural energy storage devices, integrating energy storage functionality with mechanical strength and enhanced fire safety features. The electrolyte is composed of phenolic resin, the ionic liquid EMIM TFSI, and lithium salt LiTFSI in appropriate ratios. It was discovered that adding LiTFSI to the phenolic resin accelerates curing by lowering the initiation temperature; however, it also significantly affects the cross-linking of the phenolic resin. Combining the ionic liquid and lithium salt acts as a hybrid surfactant that facilitates the emulsion polymerization of the phenolic resin, forming unique microstructures that exhibit excellent ionic conductivity comparable to those of ionic liquids. Furthermore, Li⁺ ions can complex with hydroxyl groups in the phenolic resin, creating additional ion-conductive pathways and contributing to the enhanced ionic conductivity of the electrolyte. Moreover, the phenolic-based electrolyte exhibits excellent flame retardancy, achieving the highest flame retardancy rating of V-0. This property, along with its mechanical and electrochemical performance, makes it a promising candidate for use in composite structural energy storage devices. Overall, the developed phenolic resin-based structural electrolyte offers a combination of load carrying capability, energy storage capability, and enhanced safety features, paving the way for advanced applications in structural energy storage systems. References Snyder, J. F., Gienger, E. B. & Wetzel, E. D. Performance metrics for structural composites with electrochemical multifunctionality. J Compos Mater 49 , 1835–1848 (2015). Greenhalgh, E. S. et al. A critical review of structural supercapacitors and outlook on future research challenges. Compos Sci Technol 235 , 109968 (2023). Anurangi, J., Herath, M., Galhena, D. T. L. & Epaarachchi, J. The use of fibre reinforced polymer composites for construction of structural supercapacitors: a review. Advanced Composite Materials 1–45 doi:10.1080/09243046.2023.2180792. Ishfaq, A. et al. Multifunctional design, feasibility and requirements for structural power composites in future electric air taxis. J Compos Mater 57 , 817–827 (2023). Ransil, A. & Belcher, A. M. Structural ceramic batteries using an earth-abundant inorganic waterglass binder. Nat Commun 12 , 6494 (2021). Sha, Z. et al. Synergies of vertical graphene and manganese dioxide in enhancing the energy density of carbon fibre-based structural supercapacitors. Compos Sci Technol 201 , 108568 (2021). Shirshova, N. et al. Structural supercapacitor electrolytes based on bicontinuous ionic liquid–epoxy resin systems. J. Mater. Chem. A 1 , 15300–15309 (2013). Huang, F. et al. Creating ionic pathways in solid-state polymer electrolyte by using PVA-coated carbon nanofibers. Compos Sci Technol 207 , 108710 (2021). Demir, B., Chan, K. & Searles, D. J. Structural Electrolytes Based on Epoxy Resins and Ionic Liquids: A Molecular-Level Investigation. Macromolecules 53 , 7635–7649 (2020). Huang, F. et al. Surface Functionalization of Electrodes and Synthesis of Dual-Phase Solid Electrolytes for Structural Supercapacitors. ACS Appl Mater Interfaces 14 , 30857–30871 (2022). Ahmed, Z., Bu, Y. & Yobas, L. Conductance Interplay in Ion Concentration Polarization across 1D Nanochannels: Microchannel Surface Shunt and Nanochannel Conductance. Anal Chem 92 , 1252–1259 (2020). Martins, D., Chu, V., Prazeres, D. M. F. & Conde, J. P. Ionic Conductivity Measurements in a SiO2 Nanochannel with PDMS Interconnects. Procedia Chem 1 , 1095–1098 (2009). Schoch, R. B., van Lintel, H. & Renaud, P. Effect of the surface charge on ion transport through nanoslits. Physics of Fluids 17 , 100604 (2005). Wang, Y., Qiao, X., Zhang, C. & Zhou, X. Development of structural supercapacitors with epoxy based adhesive polymer electrolyte. J Energy Storage 26 , 100968 (2019). Zhu, X. et al. Strategies to Boost Ionic Conductivity and Interface Compatibility of Inorganic - Organic Solid Composite Electrolytes. Energy Storage Mater 36 , 291–308 (2021). SHA, Z. & WANG, C. H. Electrolyte for energy storage composites. AU Prov Pat Appln Ser No. 2025900290. (2025). Asenbauer, J., Ben Hassen, N., McCloskey, B. D. & Prausnitz, J. M. Solubilities and ionic conductivities of ionic liquids containing lithium salts. Electrochim Acta 247 , 1038–1043 (2017). Ara, M., Meng, T., Nazri, G.-A., Salley, S. O. & Ng, K. Y. S. Ternary imidazolium-pyrrolidinium-based ionic liquid electrolytes for rechargeable Li-O2 batteries. J Electrochem Soc 161 , A1969 (2014). Kwon, S. J., Kim, T., Jung, B. M., Lee, S. B. & Choi, U. H. Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State Supercapacitors. ACS Appl Mater Interfaces 10 , 35108–35117 (2018). Lee, K. et al. 3D Printing Nanostructured Solid Polymer Electrolytes with High Modulus and Conductivity. Advanced Materials 34 , 2204816 (2022). Javaid, A. Structural polymer composites for energy storage devices. (Imperial College London, 2012). doi:https://doi.org/10.25560/9464. Sha, Z. et al. Enhancing oxidation resistance of carbon fibre reinforced phenolic composites by ZrO2 nanoparticles through out-of-autoclave vacuum infusion. Compos Part A Appl Sci Manuf 180 , 108071 (2024). Natali, M., Kenny, J. & Torre, L. Phenolic matrix nanocomposites based on commercial grade resols: Synthesis and characterization. Compos Sci Technol 70 , 571–577 (2010). Burgess, S., Sagar, J., Holland, J., Li, X. & Bauer, F. Ultra-Low kV EDS – A New Approach to Improved Spatial Resolution, Surface Sensitivity, and Light Element Compositional Imaging and Analysis in the SEM. Micros Today 25 , 20–29 (2017). Goldstein, J. I. et al. Scanning Electron Microscopy and X-Ray Microanalysis . (springer, 2017). Shukla, S. K., Maithani, A. & Srivastava, D. Studies on the effect of concentration of formaldehyde on the synthesis of resole-type epoxidized phenolic resin from renewable resource material. Des Monomers Polym 17 , 69–77 (2014). Rathika, R. & Suthanthiraraj, S. A. Influence of 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide plasticization on zinc-ion conducting PEO/PVdF blend gel polymer electrolyte. Journal of Materials Science: Materials in Electronics 29 , 19632–19643 (2018). Liu, Z. et al. Ion-conductive properties and lithium battery performance of composite polymer electrolytes filled with lignin derivatives. Polym J 56 , 1165–1175 (2024). Sarika, P. R., Nancarrow, P., Khansaheb, A. & Ibrahim, T. Bio-Based Alternatives to Phenol and Formaldehyde for the Production of Resins. Polymers (Basel) 12 , (2020). Jiang, H. et al. The pyrolysis mechanism of phenol formaldehyde resin. Polym Degrad Stab 97 , 1527–1533 (2012). Ge, T., Hu, X., Tang, K. & Wang, D. The Preparation and Properties of Terephthalyl-Alcohol-Modified Phenolic Foam with High Heat Aging Resistance. Polymers (Basel) 11 , (2019). Li, C. et al. Autocatalyzed interfacial thiol–isocyanate click reactions for microencapsulation of ionic liquids. J Mater Sci 55 , 9119–9128 (2020). Kiefer, J., Fries, J. & Leipertz, A. Experimental Vibrational Study of Imidazolium-Based Ionic Liquids: Raman and Infrared Spectra of 1-Ethyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)imide and 1-Ethyl-3-methylimidazolium Ethylsulfate. Appl Spectrosc 61 , 1306–1311 (2007). Smirnova, N. A. & Safonova, E. A. Ionic liquids as surfactants. Russian Journal of Physical Chemistry A 84 , 1695–1704 (2010). Wang, X., Hua, H., Xie, X., Zhang, P. & Zhao, J. Hydroxyl on the filler surface promotes Li+ conduction in PEO all-solid-state electrolyte. Solid State Ion 372 , 115768 (2021). Eslami, Z., Yazdani, F. & Mirzapour, M. A. Thermal and mechanical properties of phenolic-based composites reinforced by carbon fibres and multiwall carbon nanotubes. Compos Part A Appl Sci Manuf 72 , 22–31 (2015). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation.docx Supplementary Information Video1SupplementaryInformation.mp4 Video_1_Supplementary Information Video2SupplementaryInformation.mp4 Video_2_Supplementary Information Video3SupplementaryInformation.mp4 Video_3_Supplementary Information Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6027287","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":416809063,"identity":"280ef9a6-9f21-48f4-9576-335f2f458d6a","order_by":0,"name":"Zhao Sha","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyUlEQVRIiWNgGAWjYHACNgaGCiiTh3gtZ0jWwthGihb5BvZnD37OO5wnPyOB8cHbNgZ5gwMEtDA28Jgb9m47XGxwI4HZcG4bg+EGQlqYGXjYJHi3HU7cIJ3AJs3bxsBIUAsbA/szyb9zDifOn53A/huoxZ6gFqCHzaR5Gw4nNtxOYGMGakkkqEWCmcdMWuZYeuKG+w+bJeeck0ieSUiLfHv7M8k3NdaJ83sOH/zwpszGto+QFqD/YYCxAWQrIfWjYBSMglEwCogBAIQTO7+bbyaCAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-7264-6378","institution":"UNSW Sydney","correspondingAuthor":true,"prefix":"","firstName":"Zhao","middleName":"","lastName":"Sha","suffix":""},{"id":416809064,"identity":"d40b632c-54d9-4b92-912a-b296f2c92691","order_by":1,"name":"Ziyan Gao","email":"","orcid":"","institution":"UNSW Sydney","correspondingAuthor":false,"prefix":"","firstName":"Ziyan","middleName":"","lastName":"Gao","suffix":""},{"id":416809065,"identity":"354da096-7704-477b-9b2e-7d18c2e7f192","order_by":2,"name":"Yingkun Shen","email":"","orcid":"","institution":"UNSW Sydney","correspondingAuthor":false,"prefix":"","firstName":"Yingkun","middleName":"","lastName":"Shen","suffix":""},{"id":416809066,"identity":"3d952134-f716-4856-b5e9-870631ac3e76","order_by":3,"name":"Cheng Wang","email":"","orcid":"","institution":"UNSW Sydney","correspondingAuthor":false,"prefix":"","firstName":"Cheng","middleName":"","lastName":"Wang","suffix":""},{"id":416809067,"identity":"513696a3-009f-4d70-bd28-c0ff7fb6e2d2","order_by":4,"name":"Jiangtao Xu","email":"","orcid":"https://orcid.org/0000-0002-9020-7018","institution":"UNSW Sydney","correspondingAuthor":false,"prefix":"","firstName":"Jiangtao","middleName":"","lastName":"Xu","suffix":""},{"id":416809068,"identity":"e44ab164-0ae7-4f98-a5c9-1b4e7327cf79","order_by":5,"name":"Shuying Wu","email":"","orcid":"","institution":"The University of Sydney","correspondingAuthor":false,"prefix":"","firstName":"Shuying","middleName":"","lastName":"Wu","suffix":""},{"id":416809069,"identity":"859c83ec-1bd4-4d8b-b27e-10b673e2db66","order_by":6,"name":"Sonya Brown","email":"","orcid":"https://orcid.org/0000-0003-2401-280X","institution":"UNSW Sydney","correspondingAuthor":false,"prefix":"","firstName":"Sonya","middleName":"","lastName":"Brown","suffix":""},{"id":416809070,"identity":"abf15a5d-dce2-4d95-8769-f906e6d796ac","order_by":7,"name":"Shuhua Peng","email":"","orcid":"https://orcid.org/0000-0001-5680-9448","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Shuhua","middleName":"","lastName":"Peng","suffix":""},{"id":416809071,"identity":"ddd9c9a7-e1fa-4afd-8009-782a47dfdc16","order_by":8,"name":"Jin Zhang","email":"","orcid":"https://orcid.org/0000-0002-4257-8148","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"","lastName":"Zhang","suffix":""},{"id":416809072,"identity":"8526520f-44b3-44ad-a301-6e8206f6e37e","order_by":9,"name":"Chun Wang","email":"","orcid":"https://orcid.org/0000-0001-6081-1487","institution":"University of New South Wales, Sydney, Australia","correspondingAuthor":false,"prefix":"","firstName":"Chun","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2025-02-14 04:45:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6027287/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6027287/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":76648367,"identity":"5b50fba6-e3e6-4927-ac13-42ae0c7f0d1a","added_by":"auto","created_at":"2025-02-19 09:23:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":287178,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Ionic conductivity of phenolic resin-based solid electrolytes as a function of ionic liquid (IL) content, with the inset illustrating the formation of a layered structure at higher IL concentrations. (b) Nyquist plot and the corresponding equivalent circuit model used to determine the ionic conductivity of an electrolyte containing 60% phenolic resin and 40% IL. (c) Nyquist plots comparing solid electrolytes with varying LiTFSI content. (d) Ionic conductivity of solid electrolytes with a fixed phenolic resin-to-IL mass ratio of 3:2 and different LiTFSI/IL ratios. (e) Ionic conductivity of solid electrolytes with a fixed phenolic resin-to-(IL+LiTFSI) mass ratio of 3:2 and varying LiTFSI/IL ratios. (f) Ionic conductivity of IL with different LiTFSI contents. (g) Tensile strength and modulus of solid electrolytes with a fixed phenolic resin-to-IL mass ratio of 3:2 and varying LiTFSI/IL ratios. (h) Tensile strength and modulus of solid electrolytes with a fixed phenolic resin-to-(IL+LiTFSI) mass ratio of 3:2 and varying LiTFSI/IL ratios. (i) Plot of ionic conductivity versus tensile strength for all solid electrolytes studied in this work.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/d97a32537d30d4f297cb0674.png"},{"id":76648106,"identity":"e21b443a-12a1-4cfd-93ad-2e104bf93664","added_by":"auto","created_at":"2025-02-19 09:15:02","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":788811,"visible":true,"origin":"","legend":"\u003cp\u003e(a) TGA and (b) results of phenolic-based electrolytes and epoxy-based electrolyte; (c) TGA results of pure phenolic polymer, epoxy polymer, IL and LiTFSI; (c) UL 94 vertical bunning tests on epoxy-based electrolyte and phenolic-based electrolyte; Comparison of mechanical and electrochemical properties of solid polymer electrolytes: (e) tensile strength /ionic conductivity; (f) tensile young’s modulus / ionic conductivity.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/fa5b47e14c34b5febc5bc2dc.png"},{"id":76648369,"identity":"d1bfe140-c1ee-4587-bf80-2e3a7c62fb41","added_by":"auto","created_at":"2025-02-19 09:23:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":3069245,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructural Analysis of Phenolic Resin-Based Solid Electrolytes: (a) pure phenolic polymer, (b) phenolic resin mixed with ionic liquid (mass ratio 3:2); (c) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.2); (d) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.4); (e) phenolic resin/ionic liquid/lithium salt mixture (3:2:1), EDS analysis of (f) original and (g) washed electrolyte sample with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/43481ab5ba2dbb1639366f5e.png"},{"id":76648108,"identity":"ff094924-ca46-4f46-999e-963d1ee0b7bc","added_by":"auto","created_at":"2025-02-19 09:15:02","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":970012,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterizations on phenolic-based electrolyte systems: (a) DSC curves; (b) Exothermic heat generated during the curing process (c) Solid-state nuclear magnetic resonance results on carbon-13 (\u003csup\u003e13\u003c/sup\u003eC); (d) Fourier-transform Infrared Spectroscopy (FTIR) analysis; Schematic diagram of (e) the emulsion polymerization process, (f) phenolic-based structural electrolyte, and (g) additional lithium-ion conduction in phenolic-based structural electrolyte.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/d1f8f72220c29eaeeee83d05.png"},{"id":76648119,"identity":"c3738d3d-cad8-4dc0-9956-6d1af2c9a883","added_by":"auto","created_at":"2025-02-19 09:15:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1283752,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of the composite energy storage device developed with a phenolic-based electrolyte: (a) CV curves and (b) calculated capacitance of devices under different scan rates; demonstration of the composite structural supercapacitor: (c) lighting up an LED; (d) charged by a battery and then lighting up six LEDs for a long duration; (e) flame retardancy validation.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/198d64d85e5da263a73e66df.png"},{"id":77124361,"identity":"86a1a27f-a653-46cd-b310-30d21935383c","added_by":"auto","created_at":"2025-02-25 10:59:26","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8418758,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/32087efd-57a4-497b-beee-686878d852db.pdf"},{"id":76648104,"identity":"508cd0c6-b41d-4929-bcc8-69a50bf81d1e","added_by":"auto","created_at":"2025-02-19 09:15:02","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":35278,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/040f8473798f062b1ac35b1f.docx"},{"id":76649579,"identity":"f255c121-8f4d-4e0c-987b-9e9e04e6080d","added_by":"auto","created_at":"2025-02-19 09:39:03","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":3690658,"visible":true,"origin":"","legend":"Video_1_Supplementary Information","description":"","filename":"Video1SupplementaryInformation.mp4","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/1596c6631ce62b5531067eee.mp4"},{"id":76648129,"identity":"85f88270-313c-4973-b6ac-c51e7721c10e","added_by":"auto","created_at":"2025-02-19 09:15:03","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":29848761,"visible":true,"origin":"","legend":"\u003cp\u003eVideo_2_Supplementary Information\u003c/p\u003e","description":"","filename":"Video2SupplementaryInformation.mp4","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/3319b192172836ce928c9057.mp4"},{"id":76648130,"identity":"796f7640-a160-4e21-a73c-3bd1408167ee","added_by":"auto","created_at":"2025-02-19 09:15:03","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":23114462,"visible":true,"origin":"","legend":"\u003cp\u003eVideo_3_Supplementary Information\u003c/p\u003e","description":"","filename":"Video3SupplementaryInformation.mp4","url":"https://assets-eu.researchsquare.com/files/rs-6027287/v1/2d8755604835cbbf30fb3f8b.mp4"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"A Novel Phenoplast-Based Structural Electrolyte with High Ionic Conductivity and Fire Resistance for Advanced Energy Storage Composites","fulltext":[{"header":"Introduction","content":"\u003cp\u003eStructural energy storage composites have attracted much attention as they are able to achieve further light-weight design of composite structures at the system level \u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Such energy-storage composite structures hold enormous potential in multiple fields such as automobiles and aircrafts to achieve lighter and better energy efficient product design \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, the current state of structural supercapacitors and batteries integrated within these composites suffers from significantly lower energy density and power density compared to standalone energy storage devices. This deficiency stems from the absence of high-performance electrodes and electrolytes essential for developing integrated structural energy storage composites.\u003c/p\u003e \u003cp\u003eAs a critical element of structural energy storage composites, solid polymer electrolytes must simultaneously possess mechanical strength and ionic conductivity \u003csup\u003e\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Achieving this dual functionality presents a significant challenge in advancing structural energy storage composites with high power density. Current methodologies typically involve mixing ionically conductive additives, such as lithium salts and ionic liquids, with polymers to impart ionic conductivity while retaining mechanical load-carrying capability. However, optimizing the trade-off between ionic conductivity and mechanical properties remains a critical issue in achieving the desired performance. The traditional polymer matrix materials used in carbon fibre-reinforced polymers (CFRPs), such as epoxy, inherently lack conductivity. For instance, a polymer structural electrolyte formulated with 40 wt.% ionic liquid and 60 wt.% epoxy resin can only achieve an ionic conductivity of 0.116 mS/cm, merely 2.1% of the ionic conductivity measured for ionic liquid (5.5 mS/cm) \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. This value falls far below the expected conductivity estimated by the rule of mixture. Such limitations may arise due to complex tortuous paths and the boundary layer effect within small ionic channels, hindering efficient ion transport \u003csup\u003e\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo make matters worse, commonly used polymer matrices in composites are highly flammable and emit toxic fumes during combustion, posing substantial fire hazards during the operation of structural energy storage composites. Moreover, with the wide application of composite materials in extreme work condition, the demand for structural energy storage composites capable of withstanding high temperatures has surged alongside their widespread application in extreme work conditions. Potential scenarios include planetary surface activities (e.g., Venus\u0026rsquo;s surface temperature reaching 480\u0026deg;C) and the powering of electronics in aerospace vehicles close to the engine, etc. However, exposure to high temperatures can degrade the polymer electrolyte, leading to substantial performance losses in the energy storage device. Thus, developing flame-retardant and thermally stable electrolytes for structural energy storage composites is critically important for industries such as oil and gas, military, aerospace, automotive, and electric vehicles.\u003c/p\u003e \u003cp\u003eIn this study, we propose a method using phenolic resin to develop flame-retardant polymer electrolytes tailored for structural energy storage composites. By incorporating phenolic resin with ionic liquid and lithium salt, the resulting polymer electrolyte can achieve a high ionic conductivity of 2.87 mS/cm, which is of the same order of magnitude as that of the ionic liquid. By adjusting the content of each component, it is possible to develop an electrolyte with balanced electrochemical and mechanical performance. This approach differs from the conventional method of simply mixing epoxy and ionic liquid to develop a bicontinuous electrolyte\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, where the ionic liquid and epoxy have limited interaction with each other. The presence of ionic liquid and lithium can help forming unique microstructures with excellent ionic conductivity. This unique combination of phenolic resin, ionic liquid, and lithium salt addresses the challenges of ionic conductivity, mechanical performance, and fire resistance inherent in conventional methods, making the electrolyte well-suited for structural energy storage composites across various applications, including aerospace, automotive, civil engineering, and wearable electronics.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eElectrochemical, Mechanical, and Thermal Properties of Phenolic Resin based Solid Electrolyte\u003c/p\u003e \u003cp\u003eThe ionic conductivity of an electrolyte is a critical parameter influencing the overall performance of structural energy storage composites \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. In this work, a novel flame-retardant polymer electrolyte is prepared using phenolic resin, ionic liquid EMIM TFSI (IL), and lithium salt LiTFSI, further details of the process can be found in Supplementary Information and a filed patent\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The content of each component was systematically adjusted through experimentation to explore its influence on the ionic conductivity of the solid electrolyte.\u003c/p\u003e \u003cp\u003eTo begin with, the electrolyte was prepared by simply mixing phenolic resin with the IL. The mass content of IL in the solid electrolyte was increased from 0\u0026ndash;50%, and the calculated ionic conductivities of these electrolytes are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. It was observed that no ionic conductivity was measured even when the IL mass content increased to 30%, consistent with previous studies on epoxy resin-based solid electrolytes \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The solid electrolyte became ionically conductive when the IL mass content exceeded 30%, with the electrolyte containing 40% IL achieving an ionic conductivity of about 4.2 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mS/cm. However, further increasing the IL mass content did not result in a constant increase in the ionic conductivity of the solid electrolyte. It was observed that the solid electrolyte containing 50% IL had an ionic conductivity of about 4.9 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e mS/cm. This is attributed to the density difference between phenolic resin (1.22 g/cm\u0026sup3;) and IL (1.52 g/cm\u0026sup3;); a high content of IL is more likely to form a layered structure in the solid electrolyte, as shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, leading to a significant decrease in the electrolyte\u0026rsquo;s ionic conductivity. In this work, the ionic conductivity of the solid electrolyte was calculated based on the Nyquist plots obtained through EIS measurements, and the bulk resistance was determined by fitting the data with an equivalent circuit, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. The equivalent circuit model used was consistent with our previous study \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo avoid the formation of a layered structure in the solid electrolyte, the mass ratio between phenolic resin and IL was controlled to be 3:2. Based on this optimized ratio, further modifications to the solid electrolyte were conducted by introducing different contents of lithium salt LiTFSI. Since the lithium salt was first dissolved in IL before mixing with phenolic resin, the content of lithium salt was controlled in terms of the LiTFSI/IL ratio. This method ensured a homogeneous distribution of the lithium salt within the electrolyte, enhancing the ionic pathways and improving overall conductivity. The LiTFSI/IL ratio was increased from 0 to 0.5, which reaches the solubility limit of LiTFSI in EMIM TFSI \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec shows Nyquist plots of solid electrolytes containing different amounts of LiTFSI. It is observed that the change in bulk resistance does not consistently correlate with the change in LiTFSI content. For the electrolyte sample without LiTFSI, the bulk resistance is roughly about 100 kΩ. This resistance increases to over 2000 kΩ when the LiTFSI/IL ratio increases to 0.1. However, further increasing the LiTFSI content leads to a decrease in bulk resistance, with values less than 3000 Ω and less than 200 Ω achieved with LiTFSI/IL ratios of 0.2 and 0.5, respectively, as shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. Therefore, it was found that adding LiTFSI to the electrolyte initially decreases the ionic conductivity, reaching a minimum value of about 1.89 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e mS/cm at a LiTFSI/IL ratio of 0.1. Further increasing the LiTFSI content leads to an increase in ionic conductivity, with the highest value of 2.87 mS/cm achieved at a LiTFSI/IL ratio of 0.5, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed. Although there is a dip in ionic conductivity occurring around the LiTFSI/IL ratio of 0.1, which looks more pronounced when the y-axis is in logarithmic scale, the overall trend of ionic conductivity appears to increase with the LiTFSI content, as shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, where the y-axis is in a linear scale.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlthough the mass ratio between phenolic resin and IL was controlled to be constant, adding more lithium salt can lead to a decrease in phenolic content in the electrolyte, which may potentially contribute to the increase in the electrolyte\u0026rsquo;s ionic conductivity. To validate this, another set of samples was prepared by controlling the mass ratio of phenolic resin to the combined IL and LiTFSI to be 3:2. However, by adjusting the content of lithium salt in electrolyte, a similar trend in ionic conductivity was observed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee. The ionic conductivity of the solid electrolyte reached its minimum value when the LiTFSI/IL ratio was 0.2, then increased to the highest value at a LiTFSI/IL ratio of 0.5. These results indicate that, besides the decrease in phenolic resin content, there is another mechanism existing when adding LiTFSI that increases the ionic conductivity of the solid electrolyte. Notably, the ionic conductivity of IL decreases with the increase of LiTFSI content, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef, which is consistent with previous reports \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. This decrease in the ionic conductivity of IL could explain the initial dip observed in the ionic conductivity curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee when adding LiTFSI to the electrolyte, where the ionic conductivity of the electrolyte is initially dominated by the ionic conductivity of IL.\u003c/p\u003e \u003cp\u003eTo investigate the mechanical properties of the two sets of solid electrolytes mentioned earlier, tensile tests were conducted according to ASTM D638. One set had a fixed phenolic resin to IL mass ratio of 3:2, while the other had a fixed phenolic resin to IL\u0026thinsp;+\u0026thinsp;LiTFSI ratio of 3:2. The results are presented in Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh.\u003c/p\u003e \u003cp\u003eFor the electrolytes with a fixed phenolic resin to IL mass ratio of 3:2, the tensile strength remained relatively stable, ranging from approximately 16 to 19 MPa, as the LiTFSI/IL ratio increased up to 0.2. However, further increases in the LiTFSI/IL ratio led to a decrease in tensile strength, reaching the lowest value of 1.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 MPa at a phenolic resin: IL: LiTFSI ratio of 3:2:1. The tensile modulus of this set of electrolytes followed a similar trend, initially around 1.9 GPa, and began to decrease when the LiTFSI/IL ratio exceeded 0.2, eventually dropping to a minimum value of 0.067 GPa with the highest LiTFSI content (Ph:IL\u0026thinsp;=\u0026thinsp;3:2:1). The reduction in strength and modulus for electrolytes with high LiTFSI content is primarily attributed to microstructural changes in the electrolyte, which will be discussed in detail in the following section.\u003c/p\u003e \u003cp\u003eIn contrast, for the electrolytes with a fixed phenolic resin to IL\u0026thinsp;+\u0026thinsp;LiTFSI ratio of 3:2, the tensile strength remained relatively stable even at the highest LiTFSI content. This stability is mainly due to the constant phenolic resin content (60%) in the electrolyte, which helps maintain the mechanical strength. However, the tensile modulus still showed a reduction at higher LiTFSI contents, particularly when the LiTFSI/IL ratio exceeded 0.3, dropping from around 1.9 GPa to about 1.2 GPa. This decrease is also related to the microstructural changes in the electrolyte.\u003c/p\u003e \u003cp\u003eFor the application of structural energy storage composites, it is essential that the electrolyte exhibits both high ionic conductivity and favourable mechanical properties. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei presents a plot of ionic conductivity versus tensile strength for all solid electrolytes studied. Two electrolytes are identified as potential candidates for application in structural energy storage composites. (1) The electrolyte with a phenolic resin (Ph): ionic liquid (IL): LiTFSI mass ratio of 3:2:0.4, denoted by a black star in the figure. This electrolyte is part of a set where the mass ratio of phenolic resin to IL is fixed. (2) The electrolyte with a phenolic resin to IL\u0026thinsp;+\u0026thinsp;LiTFSI mass ratio of 3:2, and an LiTFSI/IL ratio of 0.5, indicated by a red sphere in the figure. This electrolyte belongs to a set where the phenolic resin to IL\u0026thinsp;+\u0026thinsp;LiTFSI ratio is fixed. Both of these electrolytes exhibit comparable ionic conductivity and mechanical strength, demonstrating a well-balanced performance that makes them suitable candidates for use in structural energy storage composites. In this figure, the red dashed line represents the optimal performance target for structural electrolytes, where the multifunctional efficiency factor \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e equals 1 (which is typically less than 1 in most cases). It can be observed that the electrolyte developed in this study has achieved ionic conductivity very close to this ideal target. However, when considering both structural load-bearing capability and ionic conductivity simultaneously, there remains significant room for improvement to further enhance overall performance.\u003c/p\u003e \u003cp\u003eThermogravimetric analysis (TGA) in nitrogen environment was conducted on above phenolic-based electrolyte candidates, in comparison to an epoxy-based electrolyte (Epoxy: IL\u0026thinsp;=\u0026thinsp;3:2), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. The results demonstrate a significant difference in residual mass between the phenolic and epoxy-based electrolytes at elevated temperatures. At 900\u0026deg;C, the phenolic-based electrolytes retain approximately 30% of their initial mass, while the epoxy-based electrolyte retains only 6.8% under identical testing conditions. This higher residue in the phenolic-based electrolytes indicates a more robust structure that resists decomposition, consistent with the known thermal stability characteristics of phenolic materials. More specifically, electrolyte 1 (Ph:IL:LiTFSI\u0026thinsp;=\u0026thinsp;3:2:0.4) shows a residual mass of 29.7%, slightly lower than that of electrolyte 2 (Ph:(IL\u0026thinsp;+\u0026thinsp;LiTFSI)\u0026thinsp;=\u0026thinsp;3:2; IL:LiTFSI\u0026thinsp;=\u0026thinsp;2:1), which has a residual mass of 31.8%. This variation in residual mass is expected and it can be attributed to the higher phenolic resin content in electrolyte 2, which enhances thermal stability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe DTG analysis, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, further highlights the differences in decomposition profiles between the phenolic and epoxy resin-based electrolytes. The phenolic-based electrolytes exhibit a single, dominant decomposition peak around 500\u0026deg;C, suggesting a straightforward decomposition process typical of thermally stable materials. In contrast, the epoxy resin-based electrolyte (blue curve) displays a more complex thermal decomposition profile with two distinct peaks in the DTG curve, indicative of a two-step degradation process. The first peak, at around 300\u0026deg;C, is attributed to the decomposition of the epoxy phase in the electrolyte, while the second peak, around 500\u0026deg;C, corresponds to the decomposition of the ionic liquid (IL) component. These attributions are supported by the TG curves of pure epoxy and IL, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. It is notable that, although the second peak occurs at a relatively high temperature of 500\u0026deg;C, the epoxy-based electrolyte will lose its structural integrity at the first peak (~\u0026thinsp;300\u0026deg;C), where most of the solid epoxy phase in the electrolyte decomposes, leaving only the liquid IL phase.\u003c/p\u003e \u003cp\u003eFurther insights can be drawn from Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, where the residual masses of pure phenolic resin and LiTFSI are also presented. Pure phenolic resin retains approximately 53.4% of its initial mass at 900\u0026deg;C, which is significantly higher than the residual masses of pure epoxy (8.1%), IL (6%), and LiTFSI (6.4%). This high residual mass of pure phenolic resin is a key factor contributing to the excellent thermal stability of the phenolic resin-based electrolytes developed in this study, making them suitable for high-temperature applications where structural integrity is critical. The superior thermal resistance of the phenolic resin-based electrolyte, as compared to the epoxy resin-based alternative, demonstrates its potential as a stable material for use in environments requiring sustained thermal endurance.\u003c/p\u003e \u003cp\u003eFlame retardancy is a critical performance factor for polymer structural electrolytes, especially for applications requiring both structural integrity and safety under high-temperature conditions. To assess the flame retardancy of the phenolic resin-based electrolyte, a standard UL94 vertical burning test was conducted, with an epoxy-based electrolyte (Epoxy\u0026thinsp;=\u0026thinsp;3:2) included as a comparison. The flammability test progression is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed. The results reveal a stark contrast between the two materials: the epoxy-based electrolyte ignites easily, and the flame spreads rapidly, indicating a complete lack of flame retardancy. Due to its high flammability, the epoxy-based electrolyte was unable to achieve any valid UL94 rating. In contrast, the phenolic resin-based electrolyte developed in this study demonstrates outstanding flame retardancy. It could not be ignited at all during the test, achieving the highest possible flame retardancy rating of V-0. This rating reflects the material\u0026rsquo;s capability to resist ignition and prevent flame propagation, highlighting its potential as a safe and effective structural electrolyte for high-temperature and flame-resistant applications. The excellent flame retardancy of the phenolic-based electrolyte aligns with the intrinsic fire-resistant properties of phenolic resins, further supporting its suitability for use in environments that demand both thermal stability and fire safety.\u003c/p\u003e \u003cp\u003eIn addition to demonstrating superior thermal stability and flame retardancy, the phenolic-based polymer electrolyte developed in this study significantly expands the boundaries of mechanical and electrochemical performance compared to electrolytes based on epoxy. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef present a comparative analysis of the mechanical (tensile strength and modulus) and electrochemical (ionic conductivity) properties between the phenolic-based electrolyte developed here and recently reported epoxy-based electrolytes \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. For reference, the tensile properties of pure epoxy and phenolic polymers, as well as the ionic conductivity of pure IL, are also indicated in the figures. The data reveal that some phenolic-based electrolytes (marked with red stars) achieve performance levels beyond those of epoxy-based electrolytes, particularly in terms of ionic conductivity, which approaches values similar to pure IL. This enhanced ionic conductivity suggests that the phenolic-based electrolytes developed in this study may provide more efficient ion transport pathways, potentially due to the optimized composition and structure of the phenolic matrix. Furthermore, the improved mechanical properties of the phenolic-based electrolyte, as shown by increased tensile strength and modulus, demonstrate its suitability as a structurally robust electrolyte material. These advancements in both mechanical and electrochemical performance make the phenolic-based polymer electrolyte a promising candidate for applications requiring a balance of mechanical durability, high ionic conductivity, and enhanced thermal and flame resistance.\u003c/p\u003e \u003cp\u003eMicro Morphology of Phenolic Resin based Solid Electrolyte\u003c/p\u003e \u003cp\u003eInvestigating the micro morphology of phenolic resin-based solid electrolytes is crucial for understanding how their internal structure affects ionic conductivity and overall performance. Scanning electron microscopy (SEM) was employed to analyse these microstructural features.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure 3\u003c/b\u003e. Microstructural Analysis of Phenolic Resin-Based Solid Electrolytes: (a) pure phenolic polymer, (b) phenolic resin mixed with ionic liquid (mass ratio 3:2); (c) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.2); (d) phenolic resin/ionic liquid/lithium salt mixture (3:2:0.4); (e) phenolic resin/ionic liquid/lithium salt mixture (3:2:1), EDS analysis of (f) original and (g) washed electrolyte sample with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;3a presents the typical fracture surface morphology of the pure phenolic resin polymer. The SEM image reveals numerous pores within the polymer matrix, which result from the release of water and small gas molecules during the curing process. These pores contribute to the high porosity commonly observed in cured phenolic resin, a characteristic feature of this material \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eUpon mixing the phenolic resin with an ionic liquid at a mass ratio of 3:2, slight changes in the microstructure were observed. Figure\u0026nbsp;3b shows the fracture surface morphology of the cured electrolyte. While the pore size remains similar to that of the pure phenolic resin, the void density is notably reduced, dropping to nearly half of the original value. This decrease in porosity is primarily attributed to the reduced phenolic resin content in the mixture, as the ionic liquid occupies part of the matrix, thereby reducing overall pore formation.\u003c/p\u003e \u003cp\u003eThe addition of lithium salt to the electrolyte further alters the fracture surface morphology. In Fig.\u0026nbsp;3c, which represents a phenolic resin/ionic liquid/lithium salt ratio of 3:2:0.2, the pore size decreases while pore density increases, indicating that lithium salt impacts the microstructure, likely through interactions with the resin and ionic liquid that influence pore formation and distribution.\u003c/p\u003e \u003cp\u003eSignificant changes in micro morphology are observed when the lithium salt content is further increased, as shown in Fig.\u0026nbsp;3d, where the electrolyte has a phenolic resin/ionic liquid/lithium salt ratio of 3:2:0.4. In this sample, a liquid-phase substrate, likely ionic liquid, is clearly visible on the fracture surface. Unlike the surfaces in Figs.\u0026nbsp;3b and 3c, where the ionic liquid phase is not easily observed and may be isolated by the non-ionic conductive phenolic resin polymer, the higher lithium salt content seems to enable the release of the ionic liquid from isolation. This creates a continuous ionic conductive phase, significantly enhancing the electrolyte\u0026rsquo;s ionic conductivity. Additionally, some microsphere structures begin to form beneath the ionic liquid phase.\u003c/p\u003e \u003cp\u003eThis hypothesis is confirmed with further increases in lithium salt content, as demonstrated in Fig.\u0026nbsp;3e, showing the fracture surface morphology of an electrolyte with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1. This composition exhibits a markedly different micro morphology, characterized by numerous microsphere structures closely bonded together, with a liquid-phase substrate adhering to the surface. The enlarged view in Fig.\u0026nbsp;3e reveals that the liquid-phase substrate, likely the ionic liquid, forms a continuous phase that covers the entire surface. The resulting structure appears as a highly porous skeleton made of phenolic microspheres, with an ionic conductive liquid phase closely adhering to each surface, leading to enhanced ionic conductivity throughout the electrolyte.\u003c/p\u003e \u003cp\u003eEnergy Dispersive Spectroscopy (EDS) analysis was conducted on the electrolyte with a phenolic resin/ionic liquid/lithium salt ratio of 3:2:1, as shown in Fig.\u0026nbsp;3f. Characteristic elements such as sulfur (S), nitrogen (N), and fluorine (F) from TFSI- are clearly identified on the electrolyte\u0026rsquo;s fracture surface, indicating that the liquid-phase substrate adhering to the microsphere structures is indeed the ionic liquid. Another EDS analysis was performed on the same electrolyte after ultrasonic cleaning with acetone and water for over 4 hours, followed by drying in an oven. The liquid-phase substrate was removed after washing, but the S, N, and F elements remained detectable on the surface, suggesting that some lithium salt (LiTFSI) remains integrated within the microsphere structures even after prolonged ultrasonic cleaning.\u003c/p\u003e \u003cp\u003eIt is noted that neither EDS analysis in Figs.\u0026nbsp;3f and 3g could detect the lithium (Li) element, which is consistent with the understanding that the lithium K X-ray from Li to K shells is not an allowed transition\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e due to the azimuthal quantum number (\u003cem\u003el\u003c/em\u003e) selection rule, where both energy levels have the same orbital angular momentum (\u003cem\u003el\u0026thinsp;=\u0026thinsp;0\u003c/em\u003e). This results in a very low likelihood of X-ray emission. Therefore, the absence of Li in the EDS results does not necessarily indicate its absence in the electrolyte. It is believed that lithium salt resides both in the phenolic resin phase and the ionic liquid phase within the electrolyte.\u003c/p\u003e \u003cp\u003eMechanism Analysis on Phenolic Resin based Solid Electrolyte\u003c/p\u003e \u003cp\u003eThe curing behaviour of phenolic resin-based electrolyte systems were evaluated using Differential Scanning Calorimetry (DSC), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. Four different samples were examined: (1) pure phenolic resin (Ph), (2) phenolic resin with ionic liquid (Ph:IL\u0026thinsp;=\u0026thinsp;3:2), (3) phenolic resin with LiTFSI (Ph:Li\u0026thinsp;=\u0026thinsp;3:1), and (4) phenolic resin with both ionic liquid and LiTFSI (Ph:IL:Li\u0026thinsp;=\u0026thinsp;3:2:1). All these samples are in uncured liquid state, and the DSC curves provide valuable insights into the curing reactions and interactions between the resin and the added ionic liquid and lithium salt.\u003c/p\u003e \u003cp\u003eThe pure phenolic resin (Ph) shows an exothermic peak between 80\u0026deg;C and 110\u0026deg;C, corresponding to the curing process of the resin \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. This peak reflects the heat release associated with the cross-linking reaction, and the intensity of the peak suggests that the resin undergoes a thorough curing process. Given that this peak serves as a reference for evaluating modified samples, its well-defined location and intensity indicate that, under these conditions, the phenolic resin reaches a high degree of curing with minimal residual reactive sites.\u003c/p\u003e \u003cp\u003eIn the sample containing ionic liquid (Ph/IL), the exothermic peak appears in a similar temperature range, starting around 80\u0026deg;C but with a slightly lower intensity compared to the pure phenolic resin. Although the onset temperature of curing does not shift significantly, the lower peak intensity may suggest that the plasticizing effect\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e of ionic liquid results in incomplete curing under the current conditions. This incomplete curing could occur because the enhanced chain mobility might prevent all reactive sites from fully cross-linking, as the flexible matrix does not stabilize as completely as in the pure resin.\u003c/p\u003e \u003cp\u003eFor the sample containing LiTFSI (Ph/Li), the exothermic peak starts much earlier, around 50\u0026deg;C, with the peak that extends up to about 100\u0026deg;C. This early onset reflects the catalytic effect of LiTFSI, which lowers the activation energy required for the curing reaction. The Li\u0026thinsp;+\u0026thinsp;ions can form ion-dipole interactions with the oxygen atoms in the phenolic hydroxyl groups \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e, disrupting the hydrogen bonding network within the resin. This disruption increases the mobility of the polymer chains, allowing the cross-linking to initiate at a lower temperature and proceed more vigorously. However, while the curing is highly accelerated, the decreased peak intensity compared to the pure resin\u0026rsquo;s peak may imply that the curing process, though faster, is not entirely complete. Rapid initiation of the reaction may lead to premature cross-linking in certain areas, limiting the diffusion of reactants and leaving some reactive sites unreacted.\u003c/p\u003e \u003cp\u003eThe Ph/IL/Li sample, which represents the structural electrolyte developed in this study, combines both ionic liquid and LiTFSI. It exhibits a broad exothermic peak starting around 50\u0026deg;C and extending to approximately 110\u0026deg;C. The early onset is driven by the catalytic action of LiTFSI, while the ionic liquid enhances chain mobility, creating a more flexible curing environment. However, the lower intensity of the exothermic peak compared to the pure phenolic resin suggests incomplete curing. The broad and lower-intensity peak indicates that, while curing begins early, it proceeds in a different manner, potentially resulting in an incomplete cross-linked network.\u003c/p\u003e \u003cp\u003eIntegration of the exothermic peaks to calculate the heat released during curing, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, confirms these observations. The pure phenolic resin sample generated a specific heat of approximately 105 mJ/mg. In comparison, the Ph/IL, Ph/Li, and Ph/IL/Li samples produced lower specific heats of 57.1, 52.3, and 34.2 mJ/mg, respectively. Since the curing process primarily occurs within the phenolic resin, it is more appropriate to normalize the specific heat by the weight fraction of phenolic resin in each sample, as represented by the green bars in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. According to the normalized results, the Ph/IL sample generated a specific heat of 95.2 mJ/mg during curing, indicating a slightly lower curing degree (~\u0026thinsp;91%) compared to the pure phenolic resin. For samples containing LiTFSI, the Ph/Li and Ph/IL/Li samples exhibited normalized specific heats of approximately 69.7 and 68.3 mJ/mg, respectively, representing a curing degree of around 66% relative to the pure phenolic resin sample. These findings suggest that while LiTFSI effectively accelerates curing by lowering the initiation temperature, it also has a more pronounced role in hindering complete cross-linking of the phenolic resin.\u003c/p\u003e \u003cp\u003eSolid-state carbon-13 (\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC) nuclear magnetic resonance (NMR) spectroscopy was performed on four groups of samples to further investigate the interaction mechanisms among phenolic resin, IL, and LiTFSI. The curing of phenolic resin follows a typical condensation polymerization mechanism. During curing, the hydroxyl group (-OH) attached to the aromatic ring in the phenolic monomer reacts with methylene groups (-CH₂-) from a cross-linking agent, forming methylene bridges between aromatic rings and releasing water as a byproduct \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. This cross-linking reaction connects the benzene ring structures in the phenolic resin via -CH₂- linkages, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is noteworthy that the -OH groups directly connected to the benzene rings remain unchanged during this cross-linking reaction. Therefore, the carbon atoms in the benzene rings attached to the -OH groups, marked in green, can be used as internal references to evaluate the curing degree of the phenolic resin. In contrast, the carbon atoms from the -CH₂- groups, marked in light orange in the chemical formula of the phenolic resin, reflect the extent of cross-linking in these samples.\u003c/p\u003e \u003cp\u003eFrom the NMR results, the peaks located around 150 ppm correspond to the carbon atoms in the benzene rings directly connected to the -OH groups. The intensity of this peak in all samples has been normalized to a value of 1.00. This normalization allows for the determination of the relative intensity of the carbon atoms from the -CH₂- groups in each sample. For pure phenolic resin, the intensity of the -CH₂- peak is approximately 1.51, indicating that the number of carbon atoms from -CH₂- groups is roughly 1.5 times that of the reference carbon atoms. Similarly calculated, the intensities of the -CH₂- peaks in the Ph/IL, Ph/Li, and Ph/IL/Li samples are about 1.71, 1.36, and 1.18, respectively.\u003c/p\u003e \u003cp\u003eThe higher intensity observed in the Ph/IL sample may be attributed to the -CH₂- groups present in the EMIM⁺ cation of the ionic liquid, as indicated in the figure. Excluding the Ph/IL sample, the intensity values calculated for the Ph/Li and Ph/IL/Li samples are lower than that of the pure phenolic resin, suggesting a decreased number of -CH₂- groups. This decrease corresponds to a reduced curing degree of the phenolic resin in these two samples.\u003c/p\u003e \u003cp\u003eIt appears that the presence of LiTFSI influences the phenolic resin to form shorter-chain polymer structures rather than fully cured long-chain polymers, particularly in the Ph/IL/Li sample. Even with the additional -CH₂- groups from the IL, the total amount of -CH₂- is significantly lower than that of the pure phenolic resin. Assuming that the intensity difference (1.71\u0026thinsp;\u0026minus;\u0026thinsp;1.51\u0026thinsp;=\u0026thinsp;0.20) between the Ph/IL and pure phenolic resin samples is entirely due to the EMIM⁺ from the IL, the actual intensity of the -CH₂- groups originating from the phenolic resin in the Ph/IL/Li sample would be 0.98 (1.18\u0026thinsp;\u0026minus;\u0026thinsp;0.20). Compared to the pure phenolic resin (1.51), this represents about 65% of the -CH₂- content, which aligns well with the curing degree (66%) calculated from the DSC results.\u003c/p\u003e \u003cp\u003eFourier-transform infrared spectroscopy (FTIR) was applied to analyze the four samples. According to the results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, the characteristic absorption bands of the cured pure phenolic resin were identified in the FTIR spectrum. These include peaks at around 3500 cm⁻\u0026sup1;, representing the presence of hydroxyl groups (-OH), and at 3005 cm⁻\u0026sup1;, corresponding to the aromatic C\u0026ndash;H stretch \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Peaks at 2918 cm⁻\u0026sup1; and 2849 cm⁻\u0026sup1; are attributed to the stretching vibrations of methylene C\u0026ndash;H groups \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. In addition to these characteristic peaks from the phenolic resin, two more peaks were observed in the FTIR spectra of the Ph/IL and Ph/IL/Li samples, located around 3158\u0026ndash;3161 cm⁻\u0026sup1;. These peaks correspond to the C\u0026ndash;H stretching vibrations of the imidazolium cation (EMIM⁺) ring of the ionic liquid \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Notably, the characteristic peaks at 2918 cm⁻\u0026sup1; and 2849 cm⁻\u0026sup1;, representing methylene C\u0026ndash;H groups from the phenolic resin, were observed in samples Ph, Ph/IL, and Ph/Li but were absent in the Ph/IL/Li sample. This indicates a significantly decreased amount of -CH₂- groups in the Ph/IL/Li sample, which aligns well with the observations from the NMR results.\u003c/p\u003e \u003cp\u003eBased on the above characterizations and the observed micro-morphology of the phenolic-based electrolyte, we can unveil the underlying mechanism by which a phenolic-based electrolyte with high loading of LiTFSI forms a unique microstructure. The microspherical structures of the phenolic resin, observed after mixing with the ionic liquid (IL) EMIM TFSI and a high loading of LiTFSI, align with the characteristics of emulsion polymerization. Although EMIM TFSI and LiTFSI are not conventional surfactants typically used to facilitate emulsion polymerization, their combination\u0026mdash;especially with a high concentration of LiTFSI\u0026mdash;acts as a hybrid surfactant to promote the emulsion polymerization of phenolic resin, resulting in microsphere formations as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. Specifically, the ionic liquid EMIM TFSI effectively reduces the interfacial tension \u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e and forms a robust ionic layer around the resin droplets. Simultaneously, LiTFSI enhances the ionic strength of the aqueous medium (the phenolic resin used in this study contains about 15% water content, and the curing of phenolic resin also produces water as a byproduct) and interacts synergistically with EMIM TFSI as a co-surfactant, stabilizing the emulsion by promoting electrostatic repulsion between dispersed droplets and preventing their coalescence. The presence of both LiTFSI and EMIM TFSI facilitates the formation of a stable emulsion through their complementary interactions\u0026mdash;EMIM TFSI provides steric stabilization via its bulky TFSI⁻ anions, while Li⁺ cations contribute to electrostatic stabilization. Upon initiation with a free initiator, which is the catalyst used in this study, polymerization proceeds within the stabilized droplets, resulting in uniformly dispersed phenolic polymer microspheres, eventually forming the microstructure shown in Fig.\u0026nbsp;3e. The innovative use of LiTFSI and EMIM TFSI in this emulsion polymerization system underscores their potential as effective non-traditional surfactants, offering enhanced stability and control over polymerization kinetics compared to conventional surfactant-based methods. Moreover, through adjusting the content of each component, the emulsion polymerization process can be fine-tuned to achieve an optimal balance between mechanical and electrochemical properties in the resulting polymer electrolyte. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the phenolic microspheres formed through emulsion polymerization primarily facilitate ionic conductivity, while the phenolic blocks created via conventional condensation polymerization provide mechanical strength. Regarding the significantly enhanced ionic conductivity of the phenolic-based electrolyte achieved by forming such a microstructure, this improvement can be attributed to the complexation between Li⁺ ions and the hydroxyl groups in the formed phenolic microspheres. In addition to the ionic liquid that fully covers the phenolic microspheres\u0026mdash;providing a continuous and highly ionically conductive medium\u0026mdash;the Li⁺ ions can complex with the oxygen atoms of the hydroxyl groups \u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ef. This complexation creates additional ion-conductive pathways, substantially enhancing the ionic conductivity of the electrolyte.\u003c/p\u003e \u003cp\u003ePerformance of Composite Structural Energy Storage Device\u003c/p\u003e \u003cp\u003eA composite structural supercapacitor was fabricated using the phenolic-based electrolyte developed in this study. The device was constructed with two carbon nanotube mats (dimensions: 2 cm \u0026times; 2 cm) serving as electrodes and a glass fibre veil (thickness: 40 \u0026micro;m) as a separator. These materials were selected to balance mechanical strength, thermal stability, and electrochemical performance, with the phenolic-based electrolyte providing dual functionality as both an energy storage medium and a structural matrix. The capacitance of the structural supercapacitor was evaluated using the cyclic voltammetry (CV) method, with scan rates ranging from 1 mV/s to 200 mV/s. The CV curves, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, exhibit a quasi-rectangular shape with good symmetry, reflecting efficient charge-discharge behaviour and low resistance at the electrode-electrolyte interface. The absence of significant distortion in the CV profiles across a wide range of scan rates indicates stable electrochemical behaviour and compatibility of the phenolic-based electrolyte with the carbon nanotube electrodes. The calculated capacitances under different scan rates are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. At the lowest scan rate of 1 mV/s, the device achieved its highest capacitance of approximately 45.5 mF. This high value is remarkable considering that the electrodes are composed solely of pure carbon materials without additional conductive additives or active materials. The high capacitance is attributed to the efficient interaction between the phenolic-based electrolyte and the carbon nanotube electrodes, which maximizes the utilization of the electrode surface area. As the scan rate increased, the capacitance gradually decreased due to the limited time for ion diffusion within the electrode pores and the electrolyte matrix. Nevertheless, even at the highest scan rate of 200 mV/s, the device retained a capacitance of approximately 0.8 mF. This retention demonstrates the strong ion transport capability and relatively low internal resistance of the phenolic-based electrolyte, highlighting its potential for applications requiring rapid charge-discharge cycles.\u003c/p\u003e \u003cp\u003eTo demonstrate the application of the developed phenolic-based electrolyte, four composite structural supercapacitors connected in series were first charged to 2 V using an electrochemical (EC) workstation. After charging, they were disconnected from the EC workstation and connected to a circuit to light up an LED. The structural supercapacitors successfully powered an LED for over 30 seconds, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ec; a video illustrating this process (\u003cb\u003eVideo 1\u003c/b\u003e) is available in the \u003cb\u003eSupplementary Information\u003c/b\u003e. The performance of the composite structural supercapacitor was further demonstrated through another experiment. The same structural supercapacitor was charged using a 9 V alkaline battery for approximately 20 seconds and was then able to successfully light up six LEDs for over 5 minutes, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ed. A corresponding video (\u003cb\u003eVideo 2\u003c/b\u003e) can also be found in the \u003cb\u003eSupplementary Information\u003c/b\u003e. Additionally, the structural supercapacitor demonstrated the ability to store electricity for extended periods after charging. In a test, the supercapacitor was charged for about 20 seconds, then allowed to rest for approximately 1 minute before being connected to LEDs. After powering the LEDs for 1 minute, it was disconnected, rested for another 1 minute, and reconnected to the LEDs. The supercapacitors continued to function well under these conditions, indicating good charge retention capabilities. A corresponding video (\u003cb\u003eVideo 3\u003c/b\u003e) can be found in the \u003cb\u003eSupplementary Information.\u003c/b\u003e Moreover, it is worth noting that the second and third charge and discharge demonstrations were performed four months after the first one, and the supercapacitors still functioned effectively. This long-term performance further showcases the excellent stability and efficiency of the phenolic-based electrolyte developed in this study.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFinally, the flame retardancy of the composite structural supercapacitor was evaluated by subjecting it to a direct methane flame for 1 min. The device was exposed to the flame to simulate extreme thermal conditions and assess its fire-resistant properties. During the flame exposure, some oxidation and pyrolysis occurred in the components of the composite structural supercapacitor, particularly affecting the surface layers. However, the device maintained a high level of structural integrity after this 1-min flame treatment, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ee. There were no signs of melting, dripping, or significant deformation, indicating that the core structure remained largely unaffected. Most importantly, the device did not ignite or sustain combustion during and after the flame exposure. This lack of ignition demonstrates the excellent flame retardancy of the phenolic-based structural electrolyte used in this study. Phenolic resins are well-known for their inherent flame-resistant properties due to their ability to form a stable char layer upon heating, which acts as a barrier to heat and mass transfer\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The formation of this protective char layer helps to inhibit further thermal degradation and prevents the spread of flames. The enhanced safety provided by the phenolic-based structural electrolyte is crucial for applications where fire resistance is paramount, such as in aerospace, automotive, and construction industries. The ability of the composite structural supercapacitor to withstand direct flame exposure without igniting or losing structural integrity underscores its potential for safe energy storage solutions in environments prone to high temperatures or fire hazards.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, a phenolic resin-based structural electrolyte was successfully developed for composite structural energy storage devices, integrating energy storage functionality with mechanical strength and enhanced fire safety features. The electrolyte is composed of phenolic resin, the ionic liquid EMIM TFSI, and lithium salt LiTFSI in appropriate ratios. It was discovered that adding LiTFSI to the phenolic resin accelerates curing by lowering the initiation temperature; however, it also significantly affects the cross-linking of the phenolic resin. Combining the ionic liquid and lithium salt acts as a hybrid surfactant that facilitates the emulsion polymerization of the phenolic resin, forming unique microstructures that exhibit excellent ionic conductivity comparable to those of ionic liquids. Furthermore, Li⁺ ions can complex with hydroxyl groups in the phenolic resin, creating additional ion-conductive pathways and contributing to the enhanced ionic conductivity of the electrolyte. Moreover, the phenolic-based electrolyte exhibits excellent flame retardancy, achieving the highest flame retardancy rating of V-0. This property, along with its mechanical and electrochemical performance, makes it a promising candidate for use in composite structural energy storage devices. Overall, the developed phenolic resin-based structural electrolyte offers a combination of load carrying capability, energy storage capability, and enhanced safety features, paving the way for advanced applications in structural energy storage systems.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSnyder, J. F., Gienger, E. B. \u0026amp; Wetzel, E. D. Performance metrics for structural composites with electrochemical multifunctionality. \u003cem\u003eJ Compos Mater\u003c/em\u003e \u003cstrong\u003e49\u003c/strong\u003e, 1835\u0026ndash;1848 (2015).\u003c/li\u003e\n\u003cli\u003eGreenhalgh, E. S. \u003cem\u003eet al.\u003c/em\u003e A critical review of structural supercapacitors and outlook on future research challenges. \u003cem\u003eCompos Sci Technol\u003c/em\u003e \u003cstrong\u003e235\u003c/strong\u003e, 109968 (2023).\u003c/li\u003e\n\u003cli\u003eAnurangi, J., Herath, M., Galhena, D. T. L. \u0026amp; Epaarachchi, J. 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Thermal and mechanical properties of phenolic-based composites reinforced by carbon fibres and multiwall carbon nanotubes. \u003cem\u003eCompos Part A Appl Sci Manuf\u003c/em\u003e \u003cstrong\u003e72\u003c/strong\u003e, 22\u0026ndash;31 (2015).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6027287/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6027287/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eStructural energy storage composites are a promising lightweight solution for many applications, enabling structures to store electricity. However, there is a significant challenge in engendering polymer electrolytes with high flame retardancy, ionic conductivity, and mechanical properties. In this study, we introduce a novel fire-resistant polymer electrolyte that is highly ionic conductive and mechanically strong by designing a co-continuous composite electrolyte consisting of phenolic resin (or phenoplasts), ionic liquid (EMIM TFSI), and lithium salt (LiTFSI). The ionic liquid and lithium salt work synergistically as a hybrid surfactant to facilitate the emulsion polymerization of the phenolic resin, forming unique microstructures where the Li⁺ ions complex with hydroxyl groups in the phenoplasts creates ion-conductive pathways in the solid phase. This design yields significantly enhanced ionic conductivities up to 2.87 mS/cm, comparable to those of ionic liquids. For structural electrolyte applications, certain formulations achieved an ionic conductivity of approximately 0.15 mS/cm, a tensile strength around 19 MPa, and a tensile modulus of about 1.2 GPa. These properties demonstrate a well-balanced performance between electrochemical and mechanical characteristics, making the electrolytes suitable for advanced structural energy storage applications. The resulting phenoplast-based electrolyte not only maintains mechanical strength and structural integrity but also achieves the highest flame retardancy rating of V-0. A composite structural supercapacitor fabricated using this electrolyte demonstrated excellent electrochemical performance and safety features. This development presents a significant advancement in creating safe, efficient, and multifunctional materials for advanced structural energy storage applications.\u003c/p\u003e","manuscriptTitle":"A Novel Phenoplast-Based Structural Electrolyte with High Ionic Conductivity and Fire Resistance for Advanced Energy Storage Composites","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-19 09:14:57","doi":"10.21203/rs.3.rs-6027287/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fdd27131-3b33-449d-9e2f-f04a464cb80b","owner":[],"postedDate":"February 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":44436254,"name":"Physical sciences/Materials science/Materials for energy and catalysis"},{"id":44436255,"name":"Scientific community and society/Energy and society"}],"tags":[],"updatedAt":"2025-02-25T10:51:15+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-19 09:14:57","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6027287","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6027287","identity":"rs-6027287","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}