Solvent Co-Intercalation Electrolyte Unlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures

Solvent Co-Intercalation Electrolyte Unlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures | 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 Solvent Co-Intercalation Electrolyte Unlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures Naiqing Zhang, Meng Li, Zeping Liu, Yu Zhao, Zhaoyu Chen, Yu Zhang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6911919/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Feb, 2026 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Sodium ion batteries (SIBs) are attracting extensive interests due to their low cost and the abundant sodium resources. However, SIBs still suffer severe performance degradation at low temperatures due to the conflict between ion desolvation and diffusion. Herein, we design a co-intercalation ether electrolyte (CIE) to achieve solvent co-intercalation in hard carbon (HC) anode, thereby bypassing the slow desolvation process while ensuring rapid ion diffusion in electrolyte and HC. The optimized solvation structure also promotes the formation of a thin, inorganic-rich SEI, facilitating interfacial ion transport. As a result, the CIE enables HC to deliver excellent low temperature performance, with an initial Coulombic efficiency of 80.5% at -50°C and a capacity retention of 93% after 200 cycles. Moreover, an Ah-level full cell retains 163 Wh kg − 1 at 25°C and 107 Wh kg − 1 at -50°C, demonstrating the practical feasibility of this strategy for all-climate SIBs. This work overcomes the long-standing trade-off between low-temperature ion desolvation and diffusion, offering a new approach for electrolyte design toward all-climate SIBs. Physical sciences/Chemistry/Electrochemistry/Batteries Physical sciences/Energy science and technology Co-intercalation electrolyte Hard carbon Sodium-ion batteries Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The global energy landscape is shifting toward low-carbon solutions and electrochemical energy storage (EES) technology has become essential for supporting the expanding use of clean energy. Sodium ion batteries (SIBs) show significant potential in EES due to the abundant sodium resources and cost-effectiveness. 1–3 Currently, SIBs are approaching commercialization in terms of energy density and cycle life at room temperature. However, the increasing demand for using batteries in extreme environments calls for SIBs with reliable performance at ultra-low temperatures. And the most promising hard carbon (HC) anode in SIBs shows unsatisfactory low temperature performance, especially below − 30 ℃, due to the slow ion diffusion in electrolyte, across solid electrolyte interface (SEI), and within HC itself. 4–6 Even worse, sluggish desolavtion and thicker SEI formation under low temperature further block the ion transport at anode/electrolyte interface. 7,8 These challenges of HC under low temperature are intrinsically connected to the electrolyte, which demands manipulation on electrolyte to ensure efficient Na + storage under low temperature conditions. The weakly solvated electrolytes (WSEs) are proposed as one attractive strategy in improving the low temperature performance of HC by weakening the Na + -solvent interaction. Specifically, the use of low-polarity cyclic ether solvents can weaken their coordination with Na⁺ and thus accelerate the desolvation process. 9,10 While the WSEs bring the conflict of the decrease ionic conductivity in electrolyte, because the weak Na + -solvent interaction lower the solvent’s ability to dissociate sodium salts. For example, Li et al . 11 experimentally compared the ionic conductivity of various cyclic ether-based electrolytes and found that solvents with weaker coordination ability, while facilitating faster desolvation, resulted in electrolytes with lower ionic conductivity. And the WSE developed by Wang et al . 12 showed improved electrochemical performance at low temperatures, while the rate performance at room temperature was far inferior to that of conventional electrolytes with high ionic conductivity. Besides, these WSEs primarily focus on desolavtion process with little-to-no attention to the slow ion diffusion inside HC anode, and they lack validation in Ah-level cell packs under ultra-low temperatures. Hence, the design of tailor-made electrolyte to enhance the low-temperature adaptability of HC anode remains challenging and is of great importance in practical application for all-climate SIBs. Herein, we design a co-intercalation electrolyte (CIE) toward HC anode, successfully overcoming the trade-off between Na⁺ desolvation and ionic conductivity in electrolyte. Our CIE allows ions to bypass the slow desolvation process at interface and achieve sufficient diffusion in electrolyte and HC simultaneously. The free solvent regulation significantly widens the operating temperature of CIE to -50 ℃ while ensuring sufficient solvent co-intercalation for ion storage. Meanwhile, the precisely regulated solvation structure enables the appropriate participation of anions in the solvation sheath, resulting in the formation of a thinner, inorganic-rich SEI. This interphase facilitates the efficient Na⁺ transport across the SEI. As a result, our CIE greatly improves the low temperature performance of HC anode and is further demonstrated in Ah-level SIBs cell pack. Typically, the HC||Na cell using CIE has an initial Coulombic efficiency (ICE) of 93.7% at 25 ℃, and an ICE of 80.5% even at the extreme low temperature of -50 ℃, achieving impressive performance improvement compared to the existing electrolytes for HC. The CIE empowers the HC to deliver a capacity retention of 93.5% after 200 cycles at -50 ℃. Further assembled Ah-level Na 2/3 Ni 1/3 Fe 1/3 Mn 1/3 O 2 (NFM)||HC pouch cell can achieve energy densities of 163 Wh kg − 1 at 25 ℃ and 107 Wh kg − 1 at -50 ℃. The CIE concept offers new insights for designing electrolyte to broaden the operating temperature range of SIBs, especially under ultra-low temperature conditions. Results and Discussion Electrolyte design and solvation structure The co-embedded behavior of the solvent can significantly accelerate ion transport by directly skipping the slow desolvation process. 13–15 This mechanism was initially discovered in graphite anodes, which enabled graphite anodes to be applied to SIBs, 16,17 but the cycling performance was severely affected by the excessive volume expansion of graphite during charge/discharge. 18 In contrast, HC materials have a larger layer spacing and exhibit highly uniform volume expansion and contraction during charge/discharge, providing enough space for solvent embedding/de-embedding. 19 However, the behavior of solvent co-intercalation in HC has not been thoroughly investigated, especially its enhancement mechanism of ion transport rate at low temperatures has not been thoroughly explored. It has been shown that linear ether solvents such as diethylene glycol dimethyl ether (G2) can form a stable chelating coordination structure with Na + , which not only achieves solvent co-embedding and avoids the slow desolvation process, but also exhibits an excellent ability to dissociate sodium salts, thus endowing the electrolyte with a high ionic conductivity. 20,21 However, when the temperature is lower than − 30 ℃, such linear ether-based electrolyte systems experience a sharp drop in ionic conductivity, salt precipitation and even solidification, which severely impede ion transport. 22 To solve this problem, a novel CIE was designed in this work. The design principle is based on the following findings: there are always some solvents in the electrolyte that do not participate in the coordination and only act as ion transport media, which are called free solvents. For this reason, we replaced this part of free solvent with 2-methyloxolane (MO) which has a lower melting point (-136 ℃, Table S1 ) and is less polar. Through this strategy, not only the co-embeddable solvation structure is retained and the ion transport kinetics is accelerated, but also the working temperature range of electrolyte in ultra-low temperature environment is significantly widened by increasing the solvation entropy. In order to highlight the superiority of our designed electrolyte, we also used common ether electrolyte (CEE) and WSE as a control, and the specific electrolyte formulation is shown in Table S2. According to the optical photographs in Figure S1 , it can be seen that CCE starts to have salt precipitation at -30 ℃, and it is even completely solidified at -50 ℃. On the other hand, CIE and WSE remain liquid at -50 ℃, which effectively guarantees the possibility of electrolyte application at ultra-low temperatures. The ion transport mechanisms of various electrolytes at low temperatures are shown in Fig. 1 , among which CIE has the fastest ion transport rate due to the presence of the co-intercalation behavior. In order to investigate the solvation structure of the different electrolytes, theoretical calculations and experimental characterization were carried out. Firstly, the electrostatic potential maps and binding energies with Na + of the two solvents were calculated using density functional theory (DFT) (Figure S2), which suggests that the oxygen atoms in G2 show a stronger negative electronegativity and form a chelating coordination structure with a much higher binding energy than that of MO (-0.29 eV vs. -0.13 eV). This indicates that Na + will be more preferentially coordinated with G2, while MO mainly acts as a co-solvent. Fourier transform infrared spectroscopy (FTIR) was used to investigate the coordination environments in different electrolytes, as shown in Fig. 2 a. The characteristic peaks at 1103 and 1021 cm − 1 are attributed to C-O-C stretching vibrations in pure G2 and MO solvents, respectively. 23,24 After dissolving sodium salt, a new characteristic peak appears at 1081 cm − 1 in CCE and CIE, which was attributed to the coordination of Na + with G2 solvent, while the characteristic peak of 1016 cm − 1 in WSE corresponded to the coordination of Na + with MO solvent. Interestingly, in CIE, the MO solvent characteristic peak remains at 1021 cm − 1 , which indicates that MO is hardly coordinated with Na + . In addition, we obtain similar results by Raman (Fig. 2 b), where both MO in CIE and pure MO characteristic peaks are at 920 cm − 1 . These results suggest that in CIE, MO hardly participates in the solvation structure, and Na + is mainly coordinated to G2. After knowing that MO has almost no interaction with Na + , the existence of interaction between the solvents is further investigated by 1 H nuclear magnetic resonance (NMR), as shown in Fig. 2 c. It can be found that the chemical shifts of G2 are shifted to the downfield after mixing of the two solvents, which implies that there is a dipole interaction between MO and G2. This makes the density of the electron cloud around the hydrogen nucleus of G2 decrease. 25 Such dipole interactions would weaken the coordination of G2 to some extent, thus providing the possibility for anions to enter the solvation structure. As shown in Figs. 2 d and S3, variable-temperature conductivity tests were conducted for three electrolytes. When the temperature is lower than − 20 ℃, the ionic conductivity of CCE decreased rapidly, which was attributed to the precipitation of NaPF 6 in the electrolyte. While CIE maintains a high ionic conductivity at low temperatures and is always higher than WSE. Therefore, compared to the WSE, the CIE designed by us can effectively solve the decrease in ionic conductivity and promote ion transport at low temperatures. Molecular dynamics (MD) simulations were carried out to further investigate the solvation structure of the electrolyte under different temperature gradients. The radial distribution functions (RDF) of different electrolytes are shown in Figs. 2 e-f and S4- 5 . In CEE, the average coordination number of G2 is 1.60 and that of PF 6 − is 0.87. While in CIE, the coordination number of G2 decreases to 1.41, the coordination number of PF 6 − is enhanced to 1.10, and that of MO is 0.11. This is attributed to the introduction of MO which has a dipole interaction with G2, slightly weakening the coordination of G2 and Na + . In addition, MO hardly coordinates with Na + , which is consistent with the results of Raman and FTIR. In WSE, the average coordination number of MO is 0.93 and PF 6 − is 3.05, constituting a typical aggregate-type (AGG) solvation structure rich in anions. The solvation structure at low temperature (-20, -40 ℃) was further simulated, and the changes of the coordination number of each component are compared (Figs. 2 g, S6). It can be seen that with the decrease of temperature, the coordination of the solvent decreased, and more anions entered the solvation structure. In order to investigate the effect of this change on the solvation sheath, we perform a statistical analysis of the various solvent-anion coordination (Figure S7) to accurately describe the typical solvation environments of different electrolytes at different temperatures. As shown in Fig. S8, for CCE at 25 ℃ and − 20 ℃, solvent-separated ion pairs (SSIPs) dominate. While when the temperature further decreases, the solvation structure transforms to be dominated by contact ion pairs (CIPs), suggesting that more anions will enter the solvation structure as the temperature decreases. For CIE, it always maintains the solvation sheath of the CIPs, a co-embeddable G2-Na + chelating structure is included, and a moderate amount of anions is also introduced, which facilitates the formation of anion-derived, more inorganic, thinner SEI. 26,27 In addition, the temperature-stable solvated structure in CIE guarantees the same co-embedded behavior at low temperatures as at room temperature. In contrast, WSE always maintains the solvation sheath of AGGs, more anions are introduced into the solvation structure. Too many anions can decompose rapidly at anode/electrolyte interface, leading to the accumulation of inorganic substances on the surface and the formation of thicker and inhomogeneous SEI. 28 Therefore, it is necessary to introduce a moderate amount of anions like CIE to form thin SEI for effective ion transport. Figures 2 h and S9 are snapshots of MD simulations of the solvation environments of the three electrolytes. And it is clear that Na + is surrounded by the G2 solvent in the CIE, ensuring the solvent co-intercalation mechanism. The solvation structures of the CIPs are further clarified by DFT, and as shown in Fig. 2 i. Na + coordinated with two G2 molecules (average distance of 2.38 Å) forms the first solvation sheath, while PF 6 − is mainly distributed in the second solvation sheath. Overall, we precisely regulate the solvation structure to retain the co-embeddable chelating coordination for facilitating ion transport, while also enable an appropriate number of anions to enter the solvation sheath for promoting the reconstruction of a more favorable SEI structure. Electrochemical performance To evaluate the electrochemical performance of the electrolyte, HC||Na cells were assembled. The structural characteristics of HC are shown in Figure S10, and for HC anode, the reversible capacity during the first cycle is critical for less lithium loss. As shown in Fig. 3 a, CIE provides a specific capacity of 300.5 mAh g − 1 and an initial Coulombic efficiency (ICE) of 93.7% during the first cycling at 25 ℃, while CEE and WSE provided specific capacities of 299.6 and 300.1 mAh g − 1 and ICE of 87.7% and 91.4%, respectively. The three electrolytes deliver similar reversible capacities, while CIE has the highest ICE. The severe electrolyte decomposition creates a thick SEI and affects ICE, 29,30 which may explain the lower ICE of CCE. For CIE and WSE, the introduction of anions enables the construction of inorganic-rich SEI, and the appropriate number of anions in CIE consumes less Na + during film formation, thus guaranteeing a high ICE for HC. The rate performance of HC is shown in Figure S11, where CIE still provides a specific capacity of 195.0 mAh g − 1 at 2 A g − 1 , while CEE and WSE maintain a specific capacity of 204.6 and 131.4 mAh g − 1 , respectively. This phenomenon suggests that the ionic conductivity play a crucial role in the rate performance at room temperature. The CIE effectively improves the ionic conductivity while retains the co-intercalation behavior, so the rate performance is close to that of CCE. While WSE has poor rate performance at room temperature due to lower ionic conductivity. Figure 3 b further shows the rate performance of HC at -20 ℃. Considering all three electrolytes deliver similar ionic conductivity at this temperature, the variability in rate performance is primarily originated from the mode of ion diffusion and interface composition. Interestingly, the rate performance of WSE is better than that of CCE at low temperature, which is attributed to the formation of a thin inorganic-rich SEI. The thicker SEI of CCE restricts the rate of Na + transport and even hinders solvent co-embedding, thus resulting in poor rate performance at low temperatures. In contrast, CIE with both solvent co-embedding and thin SEI show superior rate performance at low temperatures. The sodium storage performance of HC at low temperature was further tested (Fig. 3 c ) . The HC anode using CIE is able to provide a specific capacity of 269.6 mAh g − 1 at -20 ℃ with an ICE of 91.4%, and maintains more than 80% of the ICE even at ultra-low temperatures of -50 ℃. Such performance is significantly better than existing reports (Fig. 3 d). In contrast, CCE and WSE show lower capacity and ICE at low temperatures (Figure S12), especially CCE, which can hardly work normally at temperatures below − 30 ℃. CIE also exhibits excellent cycling stability (Fig. 3 e), with a capacity retention of 91.0% after 2000 cycles at 500 mA g − 1 (25 ℃). The capacity retention of CCE and WSE is 83.7% and 90.8%, respectively. Moreover, the cycling performance of the CIE at different current densities was also tested (as shown in Figure S13), all of which shows high stability. Furthermore, the cycle stability of HC with electrolytes under ultra-low temperature environment was tested. The capacity retention rate using CIE is as high as 96.1% after 200 cycles at -40 ℃ with 100 mA g − 1 . And the capacity retention rate is still 93.5% after 200 cycles at -50 ℃, and the average coulombic efficiency was 99.8% (Fig. 3 f). Such excellent low-temperature Na + storage performance is attributed to the enhanced overall ion transport process and anion-derived thin SEI, whereas CCE and WSE exhibits poor cycle performance at low temperatures (Figure S14). To sum up, CIE has achieved high reversible and high-rate sodium storage at room temperature and low temperature with excellent cycling stability, which has a high potential for ultra-low-temperature SIB applications. Interfacial chemistry The electrode/electrolyte interface is crucial for ion transport and cycling stability, and different electrolytes determine the structure and composition of the SEI. Firstly, the composition of SEI was characterized by X-ray photoelectron spectroscopy (XPS) (Figs. 4 a-b, S15-17). Three peaks located at 284.8, 286.3, and 289.2 eV are observed in the C 1s spectra, corresponding to C-C, C-O, and O-C = O, respectively. 31 This mainly corresponds to the organic components in the SEI. The higher intensity C-C peaks in the CCE indicate that the SEI contains more organic compounds. In addition, at low temperatures, the intensity of the C-C peaks of all three electrolytes is higher than that at 25 ℃, which indicates that the electrolyte decomposition is more severe and more decomposition products are found in the low-temperature environment. The same results are verified in both Na 1s and F 1s spectra, with an increase in both Na 2 CO 3 and NaF at low temperatures. Combined with the research on solvation structure mentioned above, more anions enter the solvation sheath as the temperature decreases, and anions catalyze the derivation of SEI from inorganic compounds, resulting in a higher content of inorganic components at low temperatures. WSE derived SEI contains the most inorganic compounds, especially NaF, which originates from the decomposition of PF 6 − in the solvation structure. Although NaF has high mechanical strength and can effectively protect the electrode, 32 excessive NaF will increase the interfacial resistance and hinder the migration of Na + . 33,34 In contrast, CIE derived SEI contains an appropriate amount of NaF, which not only protects the electrode but also does not affect the rapid transport of Na + . The structure of SEI was further tested by high-resolution transmission electron microscopy (HRTEM). As shown in Figures S18-19, the HC anode using CCE shows the thickest (~ 32 nm) and inhomogeneous SEI at room temperature. While the SEI of WSE and CIE is thinner, especially the SEI of CIE, which is only about 16 nm. This is attributed to the participation of anions in the decomposition of CIE and WSE to form thinner SEI. At low temperatures, severe electrolyte decomposition leads to thickening of the SEI. However, even at -40 ℃, the SEI of the HC anode using CIE is only 30 nm (Fig. 4 c), while the SEI of WSE increases to 44 nm thickness at low temperatures (Fig. 4 d). This thick SEI significantly hinders the transport of Na + , resulting in poor performance. Time of flight secondary ion mass spectrometry (TOF-SIMS) is a characterization method capable of obtaining the 3D structure of SEI. 35 Figures S20a and 4e show the variation of the content of the main substances on the HC electrode of the CIE electrolyte with sputtering time after cycling at 25 ℃ and − 40 ℃, which is mainly composed of organic components (C 2 HO − , CH 2 O − , PO 2 − ) and inorganic components (NaF 2 − , NaCO 3 − , NaO − ). The peak intensity of each component in the SEI formed at 25 ℃ is slightly lower than that at -40 ℃, indicating that low temperature will cause more decomposition of the electrolyte, which is consistent with the XPS results. Moreover, the structure of SEI formed by WSE at -40 ℃ was also tested (Fig. 4 f). It can be clearly observed that the peak intensity in CIE shows a sharp decrease around 10 s of sputtering, which indicates that the electrolyte decomposition components in CIE are more inclined to be enriched on the electrode surface. While the peak intensity of WSE decreases more gently. In addition, 3D reconstructed images visualize the deep structure of the SEI (Figs. 4 g, S20b and S21), where it is evident that the WSE-formed SEI has a higher content of inorganic compounds and is evenly distributed in deeper layers of the SEI. In contrast, in CIE, both organic and inorganic compounds in the SEI are highly focused at the surface (especially NaCO 3 − and NaO − ), forming a thinner and denser SEI. In conclusion, although WSE formed an inorganic-rich SEI, the excess anions in the solvation sheath cause the SEI to be too thick, which increases the interfacial resistance. Whereas, CIE is able to form an inorganic-rich, thin, and dense SEI, which effectively promote the transport of ions across SEI (Fig. 4 h). 36 Moreover, the thinner SEI will reduce the consumption of Na + , which is the reason why CIE can enable HC anode to have high ICE at both room and low temperatures. Co-intercalation behavior and ion diffusion As confirmed in the above characterizations, the solvation structure of CIE is invariant at room and low temperatures, retaining the chelating coordination structure of G2 with Na + . To confirm the co-intercalation behavior, the sodium storage process was further investigated by in situ Raman (Figs. 5 a, b). There are two characteristic peaks at 1350 cm − 1 and 1580 cm − 1 corresponding to D and G peaks, respectively. As the discharge proceeds, the G peak redshifts, which is due to the insertion of Na + increasing the layer spacing and electron density, thus weakening the C-C bond making it longer. 37 When the discharge depth was further increased, the characteristic peak of the intercalation compound appeared at 1475 cm − 1 for the HC electrode in both electrolyte systems. Interestingly, when CIE is used, a new characteristic peak appeared near 1083 cm − 1 , which corresponds to the Na + -G2 chelate co-embedded in the carbon layer, 38,39 while this peak did not appear when WSE is used. Therefore, this indicates that the solvent G2 can be co-embedded in the carbon layer with Na + in CIE, whereas the solvent is completely desolvated in WSE and only Na + enters the carbon layer. In order to investigate the behavior of the co-intercalation in a low-temperature environment, we discharged the HC||Na cells using CIE and WSE to different potentials at -40 ℃ and then took out the HC electrodes for testing. To ensure credibility of the results, the electrolyte was repeatedly cleaned using solvents to eliminate residual electrolyte. As shown in Figs. 5 c and S22, the G peaks are shifted compared to the raw electrodes, and the characteristic peaks of the Na-C intercalation compounds appeared, which is the same as at room temperature. Moreover, the characteristic peak of G2 co-embedded in the carbon layer appeared only when CIE is used. Figure 5 d shows the XRD pattern of the HC electrode discharged to 0 V under different conditions. compared with the raw electrode, the electrode discharged to 0 V has some small sharp peaks at 30°-35° corresponding to the characteristic peaks of the quasi-metallic state Na, 40 which indicates that the electrode is in a fully discharged state. More importantly, when discharged to 0 V, the (002) peak of HC is shifted to a lower angle. When WSE is used, only Na + is embedded in the carbon layer, and the peak shift is smaller. When CIE is used, the (002) peak shifts to a larger extent, which indicates that the solvent is co-embedded in the carbon layer with Na + . In addition, when charged to 2.5 V, the (002) peak returns to the same angle as the raw electrode (due to the similarity of the XRD spectra of each group of samples after complete desodiation, only one group of samples is provided here as a comparison), which indicates that the co-intercalation has high reversibility. In summary, we have demonstrated that the co-intercalation behavior of CIE exists at both room and low temperatures and this process is reversible. In order to deeply analyze the diffusion kinetics of different electrolytes in the HC bulk phase, a series of experiments and theoretical calculations were carried out. Firstly, the ion diffusion process in HC materials at low temperature was investigated by GITT (Figure S23). CIE has higher diffusion coefficients in both the discharge and charging processes (Figs. 5 e and S24). Especially after discharged to 0.6 V, the diffusion coefficients of CIE have an obvious gap with WSE, and at this time, the intercalation process starts. Higher diffusion coefficients indicate that co-intercalation favors ion transport in the bulk phase. Then, the diffusion behavior of Na + is analyzed by in situ impedance (Figs. 5 f and S25). We fit the R ct (charge transfer impedance) values to analyze the specific interface process (Figure S26). The two electrolytes show a similar trend: during the initial discharge (OCV-1 V), R ct gradually increases, which corresponds to the polarization process. Subsequently, the increase of Na + adsorbed on the active sites increases the electrode conductivity and R ct gradually decreases (1 V − 0.5 V). 41 As the discharge proceeds further, Na + diffuses in the bulk phase, the electrochemical behavior gradually stabilizes, and the change of R ct tends to be gentle. The trend of R ct values during charging process is mirror image of the discharging process, which indicates that the sodium storage process in HC is reversible. 42 For CIE, this also proves the reversibility of the co-intercalation behavior. The value of Rct at the end of the charging process is smaller than the value at the beginning of the discharge, which is due to the formation of a stable SEI during the first charging and discharging process, which accelerates the ion diffusion kinetics. 43 Moreover, due to the presence of unavoidable defects in the HC material, some Na + is irreversibly adsorbed on the HC surface, increasing the conductivity, reduces the R ct value. Regardless of this, CIE always has smaller R ct values, indicating a faster charge transfer rate. Especially during the discharge process, the R ct values are significantly smaller than those of WSE, which can be attributed to the higher ionic conductivity across interface. We further investigate the diffusion kinetic mechanism of the co-intercalation behavior by DFT calculations. As shown in Fig. 5 g, a chelate of Na + -G2 is introduced into the two carbon layers, and the diffusion barrier is calculated by optimizing different migration paths (Figure S27). Due to the electron-donating effect of oxygen in the solvent G2, the interaction of Na + with the carbon layers is weakened (Figs. 5 h, i). 44 Meanwhile, the co-embedding of the solvent also widens the layer spacing. The synergistic effect of these two key factors leads to a small diffusion energy barrier (0.163 eV, Fig. 5 j), which is lower than that of Na + in the interlayer (0.224eV, Figure S28). Such a lower diffusion energy barrier explains the merit of solvent co-intercalation mechanism. Electrochemical performance of pouch cell To verify the practical applicability of CIE, we assembled a 1.2 Ah (163 Wh kg − 1 ) pouch cell with O3-type NFM cathode and HC anode (Fig. 6 a). Figure 6 b shows the cycling performance of the pouch cell at different temperatures. At 25 ℃, the pouch cell provides a capacity of 1.18 Ah, as the temperature decreases. We verify the possibility of its operation at ultra-low temperatures, at -50 ℃, the pouch cell still provides a capacity of 0.79 Ah (107 Wh kg − 1 ), demonstrating a significantly better low-temperature adaptability than that of conventional electrolytes. Moreover, even at -50 ℃, the charge/discharge curve maintains a stable voltage plateau (Fig. 6 c). Figure 6 d shows the cycling performance of the pouch cell at -20 ℃ with 94.1% capacity retention after 300 cycles. In addition, in order to visually verify the practical application potential, the LED lights are continuously powered at a low temperature of -50 ℃. The fully-charged state pouch cells keep the LED lights working for more than 10 hours (Fig. 6 e). This demonstration confirms that the CIE has good prospects for practical applications, especially at low temperatures. We further compare with some previously reported low-temperature sodium-ion pouch cells (Fig. 6 f, Table S4), and our work is superior to most advanced work, which fully proves that the CIE developed by us has important application value for SIBs in extreme environments. Conclusion In summary, we have developed a co-intercalation electrolyte based on the solvent co-embedding mechanism for efficient Na + ion storage in HC at low temperatures. The MO solvent is added into linear ether G2 to precisely regulate the solvation structure and expand the electrolyte’s low-temperature range via an entropy effect. Experiments and simulations reveal that the Na + -G2 chelating coordination structure can be well co-embedded in HC anode, avoiding sluggish traditional desolvation process and significantly improving the ion diffusion at low temperatures. Meanwhile, the high sodium salt dissociation ability of G2 ensures excellent ionic conductivity in electrolyte. And the dipole interaction between solvents drives an appropriate number of anions into the solvation sheath, inducing the formation of a thin layer of inorganic rich SEI. This multi-scale synergistic design enables highly reversible and high-rate sodium storage at low temperatures. Consequently, the CIE enables HC to deliver an initial Coulombic efficiency of 80.5% at -50 ℃ and a capacity retention of 93% after 200 cycles. Moreover, an Ah-level full cell retains 163 Wh kg − 1 at 25 ℃ and 107 Wh kg − 1 at -50 ℃. This study demonstrates the significance of solvent co-embedding mechanism in improving low-temperature ion storage and provides a new strategy for designing electrolytes toward extreme cold environments. Declarations Corresponding Author Yu Zhang- Department of Chemistry, Stockholm University, Stockholm 10691, Sweden; E-mail: [email protected] Naiqing Zhang- State Key Laboratory of Urban-rural Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150001, China; E-mail: [email protected] & [email protected] https://orcid.org/0000-0002-9528-9673 Acknowledgment This work was supported by the National Natural Science Foundation of China (22379036), the Natural Science Foundation of Heilongjiang Province (No. JQ2021B001) and State Key Laboratory of Robotics and Systems, Harbin Institute of Technology (SKLRS20230A07). The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for NMR analysis. Thanks to eseshi (www.eceshi.com) for the TOF-SIMS test. References Che, C. et al. Challenges and Breakthroughs in Enhancing Temperature Tolerance of Sodium-Ion Batteries. Advanced Materials 36 , 2402291 (2024). https://doi.org/https://doi.org/10.1002/adma.202402291 Li, M., Wang, Y., Zhang, Y. & Zhang, N. Molecular tailoring of biomass precursor to regulate hard carbon microstructure for sodium ion batteries. Chemical Engineering Journal 506 , 160083 (2025). https://doi.org/https://doi.org/10.1016/j.cej.2025.160083 Zhang, F. et al. Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries. Chemical Reviews 124 , 4778-4821 (2024). https://doi.org/10.1021/acs.chemrev.3c00728 Huang, R. et al. Revealing the low-temperature aging mechanisms of the whole life cycle for lithium-ion batteries (nickel-cobalt-aluminum vs. graphite). Journal of Energy Chemistry 106 , 31-43 (2025). https://doi.org/https://doi.org/10.1016/j.jechem.2025.02.043 Zheng, C. et al. Enhancing Low-Temperature Performance of Sodium-Ion Batteries via Anion-Solvent Interactions. Advanced Functional Materials n/a , 2501303 https://doi.org/https://doi.org/10.1002/adfm.202501303 Feng, Y.-H. et al. Monolithic Interphase Enables Fast Kinetics for High-Performance Sodium-Ion Batteries at Subzero Temperature. Angewandte Chemie International Edition 63 , e202403585 (2024). https://doi.org/https://doi.org/10.1002/anie.202403585 Zhang, Y. et al. Electrolyte Design for Lithium-Ion Batteries for Extreme Temperature Applications. Advanced Materials 36 , 2308484 (2024). https://doi.org/https://doi.org/10.1002/adma.202308484 Qin, M., Zeng, Z., Cheng, S. & Xie, J. Challenges and strategies of formulating low-temperature electrolytes in lithium-ion batteries. Interdisciplinary Materials 2 , 308-336 (2023). https://doi.org/https://doi.org/10.1002/idm2.12077 Tang, Z. et al. Electrode–Electrolyte Interfacial Chemistry Modulation for Ultra-High Rate Sodium-Ion Batteries. Angewandte Chemie International Edition 61 , e202200475 (2022). https://doi.org/https://doi.org/10.1002/anie.202200475 Wang, S., Zhang, X.-G., Gu, Y., Tang, S. & Fu, Y. An Ultrastable Low-Temperature Na Metal Battery Enabled by Synergy between Weakly Solvating Solvents. Journal of the American Chemical Society 146 , 3854-3860 (2024). https://doi.org/10.1021/jacs.3c11134 Fang, H. et al. Regulating Ion-Dipole Interactions in Weakly Solvating Electrolyte towards Ultra-Low Temperature Sodium-Ion Batteries. Angewandte Chemie International Edition 63 , e202400539 (2024). https://doi.org/https://doi.org/10.1002/anie.202400539 Deng, L. et al. Self-optimizing weak solvation effects achieving faster low-temperature charge transfer kinetics for high-voltage Na3V2(PO4)2F3 cathode. Energy Storage Materials 44 , 82-92 (2022). https://doi.org/https://doi.org/10.1016/j.ensm.2021.10.012 Cui, R. et al. Research progress of co-intercalation mechanism electrolytes in sodium-ion batteries. Energy Storage Materials 71 , 103627 (2024). https://doi.org/https://doi.org/10.1016/j.ensm.2024.103627 Ferrero, G. A. et al. Solvent Co-Intercalation Reactions for Batteries and Beyond. Chemical Reviews 125 , 3401-3439 (2025). https://doi.org/10.1021/acs.chemrev.4c00805 Divya, M. L., Lee, Y.-S. & Aravindan, V. Solvent Co-intercalation: An Emerging Mechanism in Li-, Na-, and K-Ion Capacitors. ACS Energy Letters 6 , 4228-4244 (2021). https://doi.org/10.1021/acsenergylett.1c01801 Kim, H. et al. Sodium Storage Behavior in Natural Graphite using Ether-based Electrolyte Systems. Advanced Functional Materials 25 , 534-541 (2015). https://doi.org/https://doi.org/10.1002/adfm.201402984 Xu, Z.-L. et al. Tailoring sodium intercalation in graphite for high energy and power sodium ion batteries. Nature Communications 10 , 2598 (2019). https://doi.org/10.1038/s41467-019-10551-z Ma, S. et al. Recent advances in carbon-based anodes for high-performance sodium-ion batteries: Mechanism, modification and characterizations. Materials Today 75 , 334-358 (2024). https://doi.org/https://doi.org/10.1016/j.mattod.2024.04.007 Sun, Y. et al. A “grafting technique” to tailor the interfacial behavior of hard carbon anodes for stable sodium-ion batteries. Energy & Environmental Science 18 , 1911-1919 (2025). https://doi.org/10.1039/D4EE05305B Xiao, K. et al. Unlocking the Effect of Chain Length and Terminal Group on Ethylene Glycol Ether Family Toward Advanced Aqueous Electrolytes. Small 20 , 2306808 (2024). https://doi.org/https://doi.org/10.1002/smll.202306808 Alvarez Ferrero, G. et al. Co-Intercalation Batteries (CoIBs): Role of TiS2 as Electrode for Storing Solvated Na Ions. Advanced Energy Materials 12 , 2202377 (2022). https://doi.org/https://doi.org/10.1002/aenm.202202377 Yang, C. et al. Entropy-Driven Solvation toward Low-Temperature Sodium-Ion Batteries with Temperature-Adaptive Feature. Advanced Materials 35 , 2301817 (2023). https://doi.org/https://doi.org/10.1002/adma.202301817 Wang, M. et al. Temperature-responsive solvation enabled by dipole-dipole interactions towards wide-temperature sodium-ion batteries. Nature Communications 15 , 8866 (2024). https://doi.org/10.1038/s41467-024-53259-5 Cui, Y. et al. A Temperature-Adapted Ultraweakly Solvating Electrolyte for Cold-Resistant Sodium-Ion Batteries. Advanced Energy Materials n/a , 2405363 https://doi.org/https://doi.org/10.1002/aenm.202405363 Huang, Z. et al. Temperature-Robust Solvation Enabled by Solvent Interactions for Low-Temperature Sodium Metal Batteries. Journal of the American Chemical Society 147 , 5162-5171 (2025). https://doi.org/10.1021/jacs.4c15478 Yu, W. et al. Electrochemical formation of bis(fluorosulfonyl)imide-derived solid-electrolyte interphase at Li-metal potential. Nature Chemistry 17 , 246-255 (2025). https://doi.org/10.1038/s41557-024-01689-5 Rakov, D. A. et al. The impact of electrode conductivity on electrolyte interfacial structuring and its implications on the Na0/+ electrochemical performance. Energy & Environmental Science 16 , 3919-3931 (2023). https://doi.org/10.1039/D3EE00864A Li, X. et al. Fast Interfacial Defluorination Kinetics Enables Stable Cycling of Low-Temperature Lithium Metal Batteries. Journal of the American Chemical Society 146 , 17023-17031 (2024). https://doi.org/10.1021/jacs.3c14667 Lin, Q. et al. Deactivating Defects in Graphenes with Al2O3 Nanoclusters to Produce Long-Life and High-Rate Sodium-Ion Batteries. Advanced Energy Materials 9 , 1803078 (2019). https://doi.org/https://doi.org/10.1002/aenm.201803078 Chen, D. et al. Hard carbon for sodium storage: mechanism and optimization strategies toward commercialization. Energy & Environmental Science 14 , 2244-2262 (2021). https://doi.org/10.1039/D0EE03916K Hou, X. et al. Electrolyte Reconfiguration by Cation/Anion Cross-coordination for Highly Reversible and Facile Sodium Storage. Angewandte Chemie International Edition 64 , e202416939 (2025). https://doi.org/https://doi.org/10.1002/anie.202416939 Sun, M.-Y. et al. Reorganizing Helmholtz Adsorption Plane Enables Sodium Layered-Oxide Cathode beyond High Oxidation Limits. Advanced Materials 36 , 2311432 (2024). https://doi.org/https://doi.org/10.1002/adma.202311432 Liang, H.-J. et al. Electrolyte Chemistry toward Ultrawide-Temperature (−25 to 75 °C) Sodium-Ion Batteries Achieved by Phosphorus/Silicon-Synergistic Interphase Manipulation. Journal of the American Chemical Society 146 , 7295-7304 (2024). https://doi.org/10.1021/jacs.3c11776 Guo, Z. et al. Inorganic-Enriched Solid Electrolyte Interphases: A Key to Enhance Sodium-Ion Battery Cycle Stability? Small 20 , 2407425 (2024). https://doi.org/https://doi.org/10.1002/smll.202407425 Xu, H. et al. Impacts of Dissolved Ni2+ on the Solid Electrolyte Interphase on a Graphite Anode. Angewandte Chemie International Edition 61 , e202202894 (2022). https://doi.org/https://doi.org/10.1002/anie.202202894 Feng, Y.-H. et al. Dual-Anionic Coordination Manipulation Induces Phosphorus and Boron-Rich Gradient Interphase Towards Stable and Safe Sodium Metal Batteries. Angewandte Chemie International Edition 64 , e202415644 (2025). https://doi.org/https://doi.org/10.1002/anie.202415644 Liu, M. et al. Reinventing the High-rate Energy Storage of Hard Carbon: the Order-degree Governs the Trade-off of Desolvation-Solid Electrolyte Interphase at Interfaces. Angewandte Chemie International Edition 64 , e202425507 (2025). https://doi.org/https://doi.org/10.1002/anie.202425507 Zeng, F. et al. Innovative discontinuous-SEI constructed in ether-based electrolyte to maximize the capacity of hard carbon anode. Journal of Energy Chemistry 79 , 459-467 (2023). https://doi.org/https://doi.org/10.1016/j.jechem.2022.12.044 Chen, J. et al. Rechargeable Potassium-Ion Full Cells Operating at −40 °C. Angewandte Chemie International Edition 62 , e202307122 (2023). https://doi.org/https://doi.org/10.1002/anie.202307122 Liu, X. et al. Evidence of Quasi-Na Metallic Clusters in Sodium Ion Batteries through In Situ X-Ray Diffraction. Advanced Materials 37 , 2410673 (2025). https://doi.org/https://doi.org/10.1002/adma.202410673 Lu, X. et al. Low-Temperature Carbonized N/O/S-Tri-Doped Hard Carbon for Fast and Stable K-Ions Storage. Advanced Energy Materials 14 , 2303081 (2024). https://doi.org/https://doi.org/10.1002/aenm.202303081 Guan, K. et al. Hierarchical Co-doped hollow Ti3C2Tx tubes with built-in electron/ion transport network for high-performance lithium sulfur batteries. Chemical Engineering Journal 492 , 151978 (2024). https://doi.org/https://doi.org/10.1016/j.cej.2024.151978 Yang, K. et al. Optimizing Kinetics for Enhanced Potassium-Ion Storage in Carbon-Based Anodes. Advanced Functional Materials 33 , 2306190 (2023). https://doi.org/https://doi.org/10.1002/adfm.202306190 Yang, Y. et al. Rechargeable LiNi0.65Co0.15Mn0.2O2||Graphite Batteries Operating at −60 °C. Angewandte Chemie International Edition 61 , e202209619 (2022). https://doi.org/https://doi.org/10.1002/anie.202209619 Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformation202506.docx Supplementary Information - Updated Cite Share Download PDF Status: Published Journal Publication published 06 Feb, 2026 Read the published version in Nature Communications → 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. 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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-6911919","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":479732171,"identity":"86a049e8-8f39-40eb-bb61-ec6be43280bb","order_by":0,"name":"Naiqing Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAApklEQVRIiWNgGAWjYPACGx5+/gbStKTJSM44QJqWwzYGDQlEqpWf3bx1M2/beR4DhgOMHz7mEKGFcc6xstu8bbd5zJkbmCVnbiNCC7NEjtnt3G23eSwbDrAx8xKjhQ2i5RyPwYEEIrXwQLQcIEGLhERa2e2//5J5JGccbCbOL/IzkrfdnHHGzp6fv/ngh4/EaAECAyjN2ECceiQto2AUjIJRMApwAABzizTmMOgyBgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-9528-9673","institution":"Harbin Institute of Technology","correspondingAuthor":true,"prefix":"","firstName":"Naiqing","middleName":"","lastName":"Zhang","suffix":""},{"id":479732172,"identity":"efc42e10-09c3-4bee-9a26-001a7996fb66","order_by":1,"name":"Meng Li","email":"","orcid":"","institution":"Harbin Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Meng","middleName":"","lastName":"Li","suffix":""},{"id":479732173,"identity":"44cc431b-1b8b-474a-8c62-c4be1e806b12","order_by":2,"name":"Zeping Liu","email":"","orcid":"","institution":"Harbin Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zeping","middleName":"","lastName":"Liu","suffix":""},{"id":479732174,"identity":"c2a4632b-1355-4899-ba90-461af13af2b6","order_by":3,"name":"Yu Zhao","email":"","orcid":"","institution":"Harbin Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Zhao","suffix":""},{"id":479732175,"identity":"14f198ff-e871-4d9a-a62e-e155d708c7b0","order_by":4,"name":"Zhaoyu Chen","email":"","orcid":"https://orcid.org/0000-0001-9076-2890","institution":"Harbin Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhaoyu","middleName":"","lastName":"Chen","suffix":""},{"id":479732176,"identity":"6a8a40d2-5fd0-4928-acc3-1470179535c3","order_by":5,"name":"Yu Zhang","email":"","orcid":"https://orcid.org/0000-0001-8869-9167","institution":"Stockholm University","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2025-06-17 08:17:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6911919/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6911919/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-026-69237-y","type":"published","date":"2026-02-06T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85936312,"identity":"7ae7a47e-bc79-445b-b281-8ad2306757af","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":450119,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of ion transport mechanism in various electrolytes at low temperatures.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/a19b791cbf66acb7c8fd81e1.png"},{"id":85936309,"identity":"e37c5ebb-1e92-40f8-ba96-5a4d4f93be47","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":393044,"visible":true,"origin":"","legend":"\u003cp\u003eSolvation structures of various electrolytes. (a) FTIR spectra of various solvents and electrolyte. (b) Raman spectra of various solvents and electrolyte. (c) \u003csup\u003e1\u003c/sup\u003eH NMR spectra of MO, G2, and the G2/MO mixed solvent. (d) Ionic conductivity of various electrolytes at temperatures ranges from 25℃ to -50℃. The RDF of CIE at (e) 25℃ and (f) -40℃. (g) Coordination number of solvents and anions at different temperatures. (h) MD simulation snapshot of solvent environment of CIE. (i) Typical solvated structure of CIE.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/86e1fdf072f3ebe818098a57.png"},{"id":85936311,"identity":"a609af69-e38f-41f7-be8b-bb4a6d2e0fe4","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":449798,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of various electrolytes. (a) Charge/discharge curves for the first three cycles of various electrolytes at 25 ℃ (20 mA g\u003csup\u003e-1\u003c/sup\u003e). (b) Rate performance of various electrolytes at -20 ℃. (c) Charge/discharge curves of HC||Na cells at 20 mA g\u003csup\u003e-1\u003c/sup\u003e in first cycle using CIE at different temperatures. (d) Comparison of the ICE at different temperatures of this work with the work reported in the literature. (e) Cycling performance of HC||Na cells using various electrolytes at 25 ℃ (500 mA g\u003csup\u003e-1\u003c/sup\u003e). (f) Cycling performance of HC||Na cells using CIE at 25 ℃, -40 ℃ and -50 ℃ (100 mA g\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/2aabe730a96e66891c48ccd3.png"},{"id":85936314,"identity":"587fd9ee-c4a9-44ad-9693-ac58bf4b4632","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":478593,"visible":true,"origin":"","legend":"\u003cp\u003eInterface characterization of the HC anode. XPS spectra of HC anode after 10 cycles using (a) CIE and (b) WSE at -40 ℃. HRTEM images of HC anodes after 10 cycles using (c) CIE and (d) WSE at -40 ℃. Normalized TOF-SIMS depth profiles of C\u003csub\u003e2\u003c/sub\u003eHO\u003csup\u003e-\u003c/sup\u003e, CHO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, NaF\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, NaO\u003csup\u003e-\u003c/sup\u003e and NaCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e ionic fragments in HC anode after 10 cycles using (e) CIE and (f) WSE at -40 ℃. (g) 3D reconstruction images of NaF\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, NaCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, NaO\u003csup\u003e-\u003c/sup\u003e and CHO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e resolved by TOF-SIMS in CIE and WSE. (h) Schematic illustration of the SEI formed in CIE and WSE.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/97d2f36a87b66339dcde3d53.png"},{"id":85936562,"identity":"a03b496a-c25a-4895-b9f0-89889bce034c","added_by":"auto","created_at":"2025-07-03 10:37:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":510303,"visible":true,"origin":"","legend":"\u003cp\u003eCo-intercalation behavior and ion diffusion. \u003cem\u003eIn situ \u003c/em\u003eRaman spectra of HC using (a) CIE and (b) WSE at 25 ℃. (c) Raman spectra of HC electrode using CIE at -40 ℃. (d) XRD patterns of the HC electrode after being fully discharged/charged in CIE and WSE. (e) Diffusion coefficients during the discharge process using different electrolytes at -40 ℃. (f) \u003cem\u003eIn situ\u003c/em\u003e EIS results during the discharge process of different electrolytes at -40 ℃. (g) Modelling of Na\u003csup\u003e+\u003c/sup\u003e-G2 chelates in carbon layers. (h) Side and top views of charge density difference of Na\u003csup\u003e+\u003c/sup\u003e-G2 in carbon layers. (j) The diffusion energy barrier.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/9c9b1a0461ccc4137ae22095.png"},{"id":85936317,"identity":"242566e9-53f7-445d-9c66-135181c4444c","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":374638,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of NFM||HC pouch cell. (a) The schematic diagram of the pouch cell. (b) Cycle performance of pouch cell at different temperatures. (c) Galvanostatic charge/discharge profiles of pouch cell at different temperatures. (d) Cycle performance of pouch cell at -20 ℃. (e) Pouch cell powers LED at -50 °C. (f) Comparison of the capacity of pouch cells at low temperatures between this work and previously reported.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/133eb2f989df44198f9ec2b4.png"},{"id":102284806,"identity":"23b41479-1584-4366-9da7-a70f156362e1","added_by":"auto","created_at":"2026-02-10 08:07:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3057860,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/ac7f1386-f41a-4953-ac39-66190a00893f.pdf"},{"id":85936319,"identity":"7a0a0ee1-a91b-4e0c-ac96-6080bbf59467","added_by":"auto","created_at":"2025-07-03 10:29:07","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":12246743,"visible":true,"origin":"","legend":"Supplementary Information - Updated","description":"","filename":"SupportingInformation202506.docx","url":"https://assets-eu.researchsquare.com/files/rs-6911919/v1/341b7b19f465f36c5bf7d6df.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Solvent Co-Intercalation Electrolyte Unlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe global energy landscape is shifting toward low-carbon solutions and electrochemical energy storage (EES) technology has become essential for supporting the expanding use of clean energy. Sodium ion batteries (SIBs) show significant potential in EES due to the abundant sodium resources and cost-effectiveness.\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e Currently, SIBs are approaching commercialization in terms of energy density and cycle life at room temperature. However, the increasing demand for using batteries in extreme environments calls for SIBs with reliable performance at ultra-low temperatures. And the most promising hard carbon (HC) anode in SIBs shows unsatisfactory low temperature performance, especially below \u0026minus;\u0026thinsp;30 ℃, due to the slow ion diffusion in electrolyte, across solid electrolyte interface (SEI), and within HC itself.\u003csup\u003e4\u0026ndash;6\u003c/sup\u003e Even worse, sluggish desolavtion and thicker SEI formation under low temperature further block the ion transport at anode/electrolyte interface.\u003csup\u003e7,8\u003c/sup\u003e These challenges of HC under low temperature are intrinsically connected to the electrolyte, which demands manipulation on electrolyte to ensure efficient Na\u003csup\u003e+\u003c/sup\u003e storage under low temperature conditions.\u003c/p\u003e \u003cp\u003eThe weakly solvated electrolytes (WSEs) are proposed as one attractive strategy in improving the low temperature performance of HC by weakening the Na\u003csup\u003e+\u003c/sup\u003e-solvent interaction. Specifically, the use of low-polarity cyclic ether solvents can weaken their coordination with Na⁺ and thus accelerate the desolvation process.\u003csup\u003e9,10\u003c/sup\u003e While the WSEs bring the conflict of the decrease ionic conductivity in electrolyte, because the weak Na\u003csup\u003e+\u003c/sup\u003e-solvent interaction lower the solvent\u0026rsquo;s ability to dissociate sodium salts. For example, Li \u003cem\u003eet al\u003c/em\u003e.\u003csup\u003e11\u003c/sup\u003e experimentally compared the ionic conductivity of various cyclic ether-based electrolytes and found that solvents with weaker coordination ability, while facilitating faster desolvation, resulted in electrolytes with lower ionic conductivity. And the WSE developed by Wang \u003cem\u003eet al\u003c/em\u003e.\u003csup\u003e12\u003c/sup\u003e showed improved electrochemical performance at low temperatures, while the rate performance at room temperature was far inferior to that of conventional electrolytes with high ionic conductivity. Besides, these WSEs primarily focus on desolavtion process with little-to-no attention to the slow ion diffusion inside HC anode, and they lack validation in Ah-level cell packs under ultra-low temperatures. Hence, the design of tailor-made electrolyte to enhance the low-temperature adaptability of HC anode remains challenging and is of great importance in practical application for all-climate SIBs.\u003c/p\u003e \u003cp\u003eHerein, we design a co-intercalation electrolyte (CIE) toward HC anode, successfully overcoming the trade-off between Na⁺ desolvation and ionic conductivity in electrolyte. Our CIE allows ions to bypass the slow desolvation process at interface and achieve sufficient diffusion in electrolyte and HC simultaneously. The free solvent regulation significantly widens the operating temperature of CIE to -50 ℃ while ensuring sufficient solvent co-intercalation for ion storage. Meanwhile, the precisely regulated solvation structure enables the appropriate participation of anions in the solvation sheath, resulting in the formation of a thinner, inorganic-rich SEI. This interphase facilitates the efficient Na⁺ transport across the SEI. As a result, our CIE greatly improves the low temperature performance of HC anode and is further demonstrated in Ah-level SIBs cell pack. Typically, the HC||Na cell using CIE has an initial Coulombic efficiency (ICE) of 93.7% at 25 ℃, and an ICE of 80.5% even at the extreme low temperature of -50 ℃, achieving impressive performance improvement compared to the existing electrolytes for HC. The CIE empowers the HC to deliver a capacity retention of 93.5% after 200 cycles at -50 ℃. Further assembled Ah-level Na\u003csub\u003e2/3\u003c/sub\u003eNi\u003csub\u003e1/3\u003c/sub\u003eFe\u003csub\u003e1/3\u003c/sub\u003eMn\u003csub\u003e1/3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (NFM)||HC pouch cell can achieve energy densities of 163 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 25 ℃ and 107 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at -50 ℃. The CIE concept offers new insights for designing electrolyte to broaden the operating temperature range of SIBs, especially under ultra-low temperature conditions.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eElectrolyte design and solvation structure\u003c/h2\u003e \u003cp\u003eThe co-embedded behavior of the solvent can significantly accelerate ion transport by directly skipping the slow desolvation process.\u003csup\u003e13\u0026ndash;15\u003c/sup\u003e This mechanism was initially discovered in graphite anodes, which enabled graphite anodes to be applied to SIBs,\u003csup\u003e16,17\u003c/sup\u003e but the cycling performance was severely affected by the excessive volume expansion of graphite during charge/discharge.\u003csup\u003e18\u003c/sup\u003e In contrast, HC materials have a larger layer spacing and exhibit highly uniform volume expansion and contraction during charge/discharge, providing enough space for solvent embedding/de-embedding.\u003csup\u003e19\u003c/sup\u003e However, the behavior of solvent co-intercalation in HC has not been thoroughly investigated, especially its enhancement mechanism of ion transport rate at low temperatures has not been thoroughly explored. It has been shown that linear ether solvents such as diethylene glycol dimethyl ether (G2) can form a stable chelating coordination structure with Na\u003csup\u003e+\u003c/sup\u003e, which not only achieves solvent co-embedding and avoids the slow desolvation process, but also exhibits an excellent ability to dissociate sodium salts, thus endowing the electrolyte with a high ionic conductivity.\u003csup\u003e20,21\u003c/sup\u003e However, when the temperature is lower than \u0026minus;\u0026thinsp;30 ℃, such linear ether-based electrolyte systems experience a sharp drop in ionic conductivity, salt precipitation and even solidification, which severely impede ion transport.\u003csup\u003e22\u003c/sup\u003e To solve this problem, a novel CIE was designed in this work. The design principle is based on the following findings: there are always some solvents in the electrolyte that do not participate in the coordination and only act as ion transport media, which are called free solvents. For this reason, we replaced this part of free solvent with 2-methyloxolane (MO) which has a lower melting point (-136 ℃, Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) and is less polar. Through this strategy, not only the co-embeddable solvation structure is retained and the ion transport kinetics is accelerated, but also the working temperature range of electrolyte in ultra-low temperature environment is significantly widened by increasing the solvation entropy.\u003c/p\u003e \u003cp\u003eIn order to highlight the superiority of our designed electrolyte, we also used common ether electrolyte (CEE) and WSE as a control, and the specific electrolyte formulation is shown in Table S2. According to the optical photographs in Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, it can be seen that CCE starts to have salt precipitation at -30 ℃, and it is even completely solidified at -50 ℃. On the other hand, CIE and WSE remain liquid at -50 ℃, which effectively guarantees the possibility of electrolyte application at ultra-low temperatures. The ion transport mechanisms of various electrolytes at low temperatures are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, among which CIE has the fastest ion transport rate due to the presence of the co-intercalation behavior.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to investigate the solvation structure of the different electrolytes, theoretical calculations and experimental characterization were carried out. Firstly, the electrostatic potential maps and binding energies with Na\u003csup\u003e+\u003c/sup\u003e of the two solvents were calculated using density functional theory (DFT) (Figure S2), which suggests that the oxygen atoms in G2 show a stronger negative electronegativity and form a chelating coordination structure with a much higher binding energy than that of MO (-0.29 eV vs. -0.13 eV). This indicates that Na\u003csup\u003e+\u003c/sup\u003e will be more preferentially coordinated with G2, while MO mainly acts as a co-solvent. Fourier transform infrared spectroscopy (FTIR) was used to investigate the coordination environments in different electrolytes, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. The characteristic peaks at 1103 and 1021 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are attributed to C-O-C stretching vibrations in pure G2 and MO solvents, respectively.\u003csup\u003e23,24\u003c/sup\u003e After dissolving sodium salt, a new characteristic peak appears at 1081 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in CCE and CIE, which was attributed to the coordination of Na\u003csup\u003e+\u003c/sup\u003e with G2 solvent, while the characteristic peak of 1016 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in WSE corresponded to the coordination of Na\u003csup\u003e+\u003c/sup\u003e with MO solvent. Interestingly, in CIE, the MO solvent characteristic peak remains at 1021 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which indicates that MO is hardly coordinated with Na\u003csup\u003e+\u003c/sup\u003e. In addition, we obtain similar results by Raman (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), where both MO in CIE and pure MO characteristic peaks are at 920 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These results suggest that in CIE, MO hardly participates in the solvation structure, and Na\u003csup\u003e+\u003c/sup\u003e is mainly coordinated to G2. After knowing that MO has almost no interaction with Na\u003csup\u003e+\u003c/sup\u003e, the existence of interaction between the solvents is further investigated by \u003csup\u003e1\u003c/sup\u003eH nuclear magnetic resonance (NMR), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. It can be found that the chemical shifts of G2 are shifted to the downfield after mixing of the two solvents, which implies that there is a dipole interaction between MO and G2. This makes the density of the electron cloud around the hydrogen nucleus of G2 decrease.\u003csup\u003e25\u003c/sup\u003e Such dipole interactions would weaken the coordination of G2 to some extent, thus providing the possibility for anions to enter the solvation structure. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and S3, variable-temperature conductivity tests were conducted for three electrolytes. When the temperature is lower than \u0026minus;\u0026thinsp;20 ℃, the ionic conductivity of CCE decreased rapidly, which was attributed to the precipitation of NaPF\u003csub\u003e6\u003c/sub\u003e in the electrolyte. While CIE maintains a high ionic conductivity at low temperatures and is always higher than WSE. Therefore, compared to the WSE, the CIE designed by us can effectively solve the decrease in ionic conductivity and promote ion transport at low temperatures.\u003c/p\u003e \u003cp\u003eMolecular dynamics (MD) simulations were carried out to further investigate the solvation structure of the electrolyte under different temperature gradients. The radial distribution functions (RDF) of different electrolytes are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-f and S4-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In CEE, the average coordination number of G2 is 1.60 and that of PF\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is 0.87. While in CIE, the coordination number of G2 decreases to 1.41, the coordination number of PF\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is enhanced to 1.10, and that of MO is 0.11. This is attributed to the introduction of MO which has a dipole interaction with G2, slightly weakening the coordination of G2 and Na\u003csup\u003e+\u003c/sup\u003e. In addition, MO hardly coordinates with Na\u003csup\u003e+\u003c/sup\u003e, which is consistent with the results of Raman and FTIR. In WSE, the average coordination number of MO is 0.93 and PF\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is 3.05, constituting a typical aggregate-type (AGG) solvation structure rich in anions. The solvation structure at low temperature (-20, -40 ℃) was further simulated, and the changes of the coordination number of each component are compared (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, S6). It can be seen that with the decrease of temperature, the coordination of the solvent decreased, and more anions entered the solvation structure. In order to investigate the effect of this change on the solvation sheath, we perform a statistical analysis of the various solvent-anion coordination (Figure S7) to accurately describe the typical solvation environments of different electrolytes at different temperatures. As shown in Fig. S8, for CCE at 25 ℃ and \u0026minus;\u0026thinsp;20 ℃, solvent-separated ion pairs (SSIPs) dominate. While when the temperature further decreases, the solvation structure transforms to be dominated by contact ion pairs (CIPs), suggesting that more anions will enter the solvation structure as the temperature decreases. For CIE, it always maintains the solvation sheath of the CIPs, a co-embeddable G2-Na\u003csup\u003e+\u003c/sup\u003e chelating structure is included, and a moderate amount of anions is also introduced, which facilitates the formation of anion-derived, more inorganic, thinner SEI.\u003csup\u003e26,27\u003c/sup\u003e In addition, the temperature-stable solvated structure in CIE guarantees the same co-embedded behavior at low temperatures as at room temperature. In contrast, WSE always maintains the solvation sheath of AGGs, more anions are introduced into the solvation structure. Too many anions can decompose rapidly at anode/electrolyte interface, leading to the accumulation of inorganic substances on the surface and the formation of thicker and inhomogeneous SEI.\u003csup\u003e28\u003c/sup\u003e Therefore, it is necessary to introduce a moderate amount of anions like CIE to form thin SEI for effective ion transport. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh and S9 are snapshots of MD simulations of the solvation environments of the three electrolytes. And it is clear that Na\u003csup\u003e+\u003c/sup\u003e is surrounded by the G2 solvent in the CIE, ensuring the solvent co-intercalation mechanism. The solvation structures of the CIPs are further clarified by DFT, and as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei. Na\u003csup\u003e+\u003c/sup\u003e coordinated with two G2 molecules (average distance of 2.38 \u0026Aring;) forms the first solvation sheath, while PF\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is mainly distributed in the second solvation sheath. Overall, we precisely regulate the solvation structure to retain the co-embeddable chelating coordination for facilitating ion transport, while also enable an appropriate number of anions to enter the solvation sheath for promoting the reconstruction of a more favorable SEI structure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eElectrochemical performance\u003c/h3\u003e\n\u003cp\u003eTo evaluate the electrochemical performance of the electrolyte, HC||Na cells were assembled. The structural characteristics of HC are shown in Figure S10, and for HC anode, the reversible capacity during the first cycle is critical for less lithium loss. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, CIE provides a specific capacity of 300.5 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and an initial Coulombic efficiency (ICE) of 93.7% during the first cycling at 25 ℃, while CEE and WSE provided specific capacities of 299.6 and 300.1 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and ICE of 87.7% and 91.4%, respectively. The three electrolytes deliver similar reversible capacities, while CIE has the highest ICE. The severe electrolyte decomposition creates a thick SEI and affects ICE,\u003csup\u003e29,30\u003c/sup\u003e which may explain the lower ICE of CCE. For CIE and WSE, the introduction of anions enables the construction of inorganic-rich SEI, and the appropriate number of anions in CIE consumes less Na\u003csup\u003e+\u003c/sup\u003e during film formation, thus guaranteeing a high ICE for HC. The rate performance of HC is shown in Figure S11, where CIE still provides a specific capacity of 195.0 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 2 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, while CEE and WSE maintain a specific capacity of 204.6 and 131.4 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. This phenomenon suggests that the ionic conductivity play a crucial role in the rate performance at room temperature. The CIE effectively improves the ionic conductivity while retains the co-intercalation behavior, so the rate performance is close to that of CCE. While WSE has poor rate performance at room temperature due to lower ionic conductivity. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb further shows the rate performance of HC at -20 ℃. Considering all three electrolytes deliver similar ionic conductivity at this temperature, the variability in rate performance is primarily originated from the mode of ion diffusion and interface composition. Interestingly, the rate performance of WSE is better than that of CCE at low temperature, which is attributed to the formation of a thin inorganic-rich SEI. The thicker SEI of CCE restricts the rate of Na\u003csup\u003e+\u003c/sup\u003e transport and even hinders solvent co-embedding, thus resulting in poor rate performance at low temperatures. In contrast, CIE with both solvent co-embedding and thin SEI show superior rate performance at low temperatures. The sodium storage performance of HC at low temperature was further tested (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec\u003cb\u003e)\u003c/b\u003e. The HC anode using CIE is able to provide a specific capacity of 269.6 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at -20 ℃ with an ICE of 91.4%, and maintains more than 80% of the ICE even at ultra-low temperatures of -50 ℃. Such performance is significantly better than existing reports (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). In contrast, CCE and WSE show lower capacity and ICE at low temperatures (Figure S12), especially CCE, which can hardly work normally at temperatures below \u0026minus;\u0026thinsp;30 ℃. CIE also exhibits excellent cycling stability (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee), with a capacity retention of 91.0% after 2000 cycles at 500 mA g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (25 ℃). The capacity retention of CCE and WSE is 83.7% and 90.8%, respectively. Moreover, the cycling performance of the CIE at different current densities was also tested (as shown in Figure S13), all of which shows high stability. Furthermore, the cycle stability of HC with electrolytes under ultra-low temperature environment was tested. The capacity retention rate using CIE is as high as 96.1% after 200 cycles at -40 ℃ with 100 mA g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. And the capacity retention rate is still 93.5% after 200 cycles at -50 ℃, and the average coulombic efficiency was 99.8% (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). Such excellent low-temperature Na\u003csup\u003e+\u003c/sup\u003e storage performance is attributed to the enhanced overall ion transport process and anion-derived thin SEI, whereas CCE and WSE exhibits poor cycle performance at low temperatures (Figure S14). To sum up, CIE has achieved high reversible and high-rate sodium storage at room temperature and low temperature with excellent cycling stability, which has a high potential for ultra-low-temperature SIB applications.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eInterfacial chemistry\u003c/h3\u003e\n\u003cp\u003eThe electrode/electrolyte interface is crucial for ion transport and cycling stability, and different electrolytes determine the structure and composition of the SEI. Firstly, the composition of SEI was characterized by X-ray photoelectron spectroscopy (XPS) (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-b, S15-17). Three peaks located at 284.8, 286.3, and 289.2 eV are observed in the C 1s spectra, corresponding to C-C, C-O, and O-C\u0026thinsp;=\u0026thinsp;O, respectively.\u003csup\u003e31\u003c/sup\u003e This mainly corresponds to the organic components in the SEI. The higher intensity C-C peaks in the CCE indicate that the SEI contains more organic compounds. In addition, at low temperatures, the intensity of the C-C peaks of all three electrolytes is higher than that at 25 ℃, which indicates that the electrolyte decomposition is more severe and more decomposition products are found in the low-temperature environment. The same results are verified in both Na 1s and F 1s spectra, with an increase in both Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and NaF at low temperatures. Combined with the research on solvation structure mentioned above, more anions enter the solvation sheath as the temperature decreases, and anions catalyze the derivation of SEI from inorganic compounds, resulting in a higher content of inorganic components at low temperatures. WSE derived SEI contains the most inorganic compounds, especially NaF, which originates from the decomposition of PF\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the solvation structure. Although NaF has high mechanical strength and can effectively protect the electrode,\u003csup\u003e32\u003c/sup\u003e excessive NaF will increase the interfacial resistance and hinder the migration of Na\u003csup\u003e+\u003c/sup\u003e.\u003csup\u003e33,34\u003c/sup\u003e In contrast, CIE derived SEI contains an appropriate amount of NaF, which not only protects the electrode but also does not affect the rapid transport of Na\u003csup\u003e+\u003c/sup\u003e. The structure of SEI was further tested by high-resolution transmission electron microscopy (HRTEM). As shown in Figures S18-19, the HC anode using CCE shows the thickest (~\u0026thinsp;32 nm) and inhomogeneous SEI at room temperature. While the SEI of WSE and CIE is thinner, especially the SEI of CIE, which is only about 16 nm. This is attributed to the participation of anions in the decomposition of CIE and WSE to form thinner SEI. At low temperatures, severe electrolyte decomposition leads to thickening of the SEI. However, even at -40 ℃, the SEI of the HC anode using CIE is only 30 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), while the SEI of WSE increases to 44 nm thickness at low temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). This thick SEI significantly hinders the transport of Na\u003csup\u003e+\u003c/sup\u003e, resulting in poor performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTime of flight secondary ion mass spectrometry (TOF-SIMS) is a characterization method capable of obtaining the 3D structure of SEI.\u003csup\u003e35\u003c/sup\u003e Figures S20a and \u003cb\u003e4e\u003c/b\u003e show the variation of the content of the main substances on the HC electrode of the CIE electrolyte with sputtering time after cycling at 25 ℃ and \u0026minus;\u0026thinsp;40 ℃, which is mainly composed of organic components (C\u003csub\u003e2\u003c/sub\u003eHO\u003csup\u003e\u0026minus;\u003c/sup\u003e, CH\u003csub\u003e2\u003c/sub\u003eO\u003csup\u003e\u0026minus;\u003c/sup\u003e, PO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and inorganic components (NaF\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NaCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NaO\u003csup\u003e\u0026minus;\u003c/sup\u003e). The peak intensity of each component in the SEI formed at 25 ℃ is slightly lower than that at -40 ℃, indicating that low temperature will cause more decomposition of the electrolyte, which is consistent with the XPS results. Moreover, the structure of SEI formed by WSE at -40 ℃ was also tested (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). It can be clearly observed that the peak intensity in CIE shows a sharp decrease around 10 s of sputtering, which indicates that the electrolyte decomposition components in CIE are more inclined to be enriched on the electrode surface. While the peak intensity of WSE decreases more gently. In addition, 3D reconstructed images visualize the deep structure of the SEI (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg, S20b and S21), where it is evident that the WSE-formed SEI has a higher content of inorganic compounds and is evenly distributed in deeper layers of the SEI. In contrast, in CIE, both organic and inorganic compounds in the SEI are highly focused at the surface (especially NaCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NaO\u003csup\u003e\u0026minus;\u003c/sup\u003e), forming a thinner and denser SEI. In conclusion, although WSE formed an inorganic-rich SEI, the excess anions in the solvation sheath cause the SEI to be too thick, which increases the interfacial resistance. Whereas, CIE is able to form an inorganic-rich, thin, and dense SEI, which effectively promote the transport of ions across SEI (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh).\u003csup\u003e36\u003c/sup\u003e Moreover, the thinner SEI will reduce the consumption of Na\u003csup\u003e+\u003c/sup\u003e, which is the reason why CIE can enable HC anode to have high ICE at both room and low temperatures.\u003c/p\u003e\n\u003ch3\u003e\u003c/h3\u003e\n\u003cdiv class=\"Heading\"\u003e\u003cb\u003eCo-intercalation behavior and ion diffusion\u003c/b\u003e\u003c/div\u003e \u003cp\u003eAs confirmed in the above characterizations, the solvation structure of CIE is invariant at room and low temperatures, retaining the chelating coordination structure of G2 with Na\u003csup\u003e+\u003c/sup\u003e. To confirm the co-intercalation behavior, the sodium storage process was further investigated by \u003cem\u003ein situ\u003c/em\u003e Raman (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b). There are two characteristic peaks at 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1580 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponding to D and G peaks, respectively. As the discharge proceeds, the G peak redshifts, which is due to the insertion of Na\u003csup\u003e+\u003c/sup\u003e increasing the layer spacing and electron density, thus weakening the C-C bond making it longer.\u003csup\u003e37\u003c/sup\u003e When the discharge depth was further increased, the characteristic peak of the intercalation compound appeared at 1475 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the HC electrode in both electrolyte systems. Interestingly, when CIE is used, a new characteristic peak appeared near 1083 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which corresponds to the Na\u003csup\u003e+\u003c/sup\u003e-G2 chelate co-embedded in the carbon layer,\u003csup\u003e38,39\u003c/sup\u003e while this peak did not appear when WSE is used. Therefore, this indicates that the solvent G2 can be co-embedded in the carbon layer with Na\u003csup\u003e+\u003c/sup\u003e in CIE, whereas the solvent is completely desolvated in WSE and only Na\u003csup\u003e+\u003c/sup\u003e enters the carbon layer. In order to investigate the behavior of the co-intercalation in a low-temperature environment, we discharged the HC||Na cells using CIE and WSE to different potentials at -40 ℃ and then took out the HC electrodes for testing. To ensure credibility of the results, the electrolyte was repeatedly cleaned using solvents to eliminate residual electrolyte. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and S22, the G peaks are shifted compared to the raw electrodes, and the characteristic peaks of the Na-C intercalation compounds appeared, which is the same as at room temperature. Moreover, the characteristic peak of G2 co-embedded in the carbon layer appeared only when CIE is used. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed shows the XRD pattern of the HC electrode discharged to 0 V under different conditions. compared with the raw electrode, the electrode discharged to 0 V has some small sharp peaks at 30\u0026deg;-35\u0026deg; corresponding to the characteristic peaks of the quasi-metallic state Na,\u003csup\u003e40\u003c/sup\u003e which indicates that the electrode is in a fully discharged state. More importantly, when discharged to 0 V, the (002) peak of HC is shifted to a lower angle. When WSE is used, only Na\u003csup\u003e+\u003c/sup\u003e is embedded in the carbon layer, and the peak shift is smaller. When CIE is used, the (002) peak shifts to a larger extent, which indicates that the solvent is co-embedded in the carbon layer with Na\u003csup\u003e+\u003c/sup\u003e. In addition, when charged to 2.5 V, the (002) peak returns to the same angle as the raw electrode (due to the similarity of the XRD spectra of each group of samples after complete desodiation, only one group of samples is provided here as a comparison), which indicates that the co-intercalation has high reversibility. In summary, we have demonstrated that the co-intercalation behavior of CIE exists at both room and low temperatures and this process is reversible.\u003c/p\u003e \u003cp\u003eIn order to deeply analyze the diffusion kinetics of different electrolytes in the HC bulk phase, a series of experiments and theoretical calculations were carried out. Firstly, the ion diffusion process in HC materials at low temperature was investigated by GITT (Figure S23). CIE has higher diffusion coefficients in both the discharge and charging processes (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and S24). Especially after discharged to 0.6 V, the diffusion coefficients of CIE have an obvious gap with WSE, and at this time, the intercalation process starts. Higher diffusion coefficients indicate that co-intercalation favors ion transport in the bulk phase. Then, the diffusion behavior of Na\u003csup\u003e+\u003c/sup\u003e is analyzed by \u003cem\u003ein situ\u003c/em\u003e impedance (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef and S25). We fit the R\u003csub\u003ect\u003c/sub\u003e (charge transfer impedance) values to analyze the specific interface process (Figure S26). The two electrolytes show a similar trend: during the initial discharge (OCV-1 V), R\u003csub\u003ect\u003c/sub\u003e gradually increases, which corresponds to the polarization process. Subsequently, the increase of Na\u003csup\u003e+\u003c/sup\u003e adsorbed on the active sites increases the electrode conductivity and R\u003csub\u003ect\u003c/sub\u003e gradually decreases (1 V \u0026minus;\u0026thinsp;0.5 V).\u003csup\u003e41\u003c/sup\u003e As the discharge proceeds further, Na\u003csup\u003e+\u003c/sup\u003e diffuses in the bulk phase, the electrochemical behavior gradually stabilizes, and the change of R\u003csub\u003ect\u003c/sub\u003e tends to be gentle. The trend of R\u003csub\u003ect\u003c/sub\u003e values during charging process is mirror image of the discharging process, which indicates that the sodium storage process in HC is reversible.\u003csup\u003e42\u003c/sup\u003e For CIE, this also proves the reversibility of the co-intercalation behavior. The value of Rct at the end of the charging process is smaller than the value at the beginning of the discharge, which is due to the formation of a stable SEI during the first charging and discharging process, which accelerates the ion diffusion kinetics.\u003csup\u003e43\u003c/sup\u003e Moreover, due to the presence of unavoidable defects in the HC material, some Na\u003csup\u003e+\u003c/sup\u003e is irreversibly adsorbed on the HC surface, increasing the conductivity, reduces the R\u003csub\u003ect\u003c/sub\u003e value. Regardless of this, CIE always has smaller R\u003csub\u003ect\u003c/sub\u003e values, indicating a faster charge transfer rate. Especially during the discharge process, the R\u003csub\u003ect\u003c/sub\u003e values are significantly smaller than those of WSE, which can be attributed to the higher ionic conductivity across interface. We further investigate the diffusion kinetic mechanism of the co-intercalation behavior by DFT calculations. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg, a chelate of Na\u003csup\u003e+\u003c/sup\u003e-G2 is introduced into the two carbon layers, and the diffusion barrier is calculated by optimizing different migration paths (Figure S27). Due to the electron-donating effect of oxygen in the solvent G2, the interaction of Na\u003csup\u003e+\u003c/sup\u003e with the carbon layers is weakened (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh, i).\u003csup\u003e44\u003c/sup\u003e Meanwhile, the co-embedding of the solvent also widens the layer spacing. The synergistic effect of these two key factors leads to a small diffusion energy barrier (0.163 eV, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ej), which is lower than that of Na\u003csup\u003e+\u003c/sup\u003e in the interlayer (0.224eV, Figure S28). Such a lower diffusion energy barrier explains the merit of solvent co-intercalation mechanism.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eElectrochemical performance of pouch cell\u003c/h3\u003e\n\u003cp\u003eTo verify the practical applicability of CIE, we assembled a 1.2 Ah (163 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) pouch cell with O3-type NFM cathode and HC anode (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb shows the cycling performance of the pouch cell at different temperatures. At 25 ℃, the pouch cell provides a capacity of 1.18 Ah, as the temperature decreases. We verify the possibility of its operation at ultra-low temperatures, at -50 ℃, the pouch cell still provides a capacity of 0.79 Ah (107 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), demonstrating a significantly better low-temperature adaptability than that of conventional electrolytes. Moreover, even at -50 ℃, the charge/discharge curve maintains a stable voltage plateau (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed shows the cycling performance of the pouch cell at -20 ℃ with 94.1% capacity retention after 300 cycles. In addition, in order to visually verify the practical application potential, the LED lights are continuously powered at a low temperature of -50 ℃. The fully-charged state pouch cells keep the LED lights working for more than 10 hours (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). This demonstration confirms that the CIE has good prospects for practical applications, especially at low temperatures. We further compare with some previously reported low-temperature sodium-ion pouch cells (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, Table S4), and our work is superior to most advanced work, which fully proves that the CIE developed by us has important application value for SIBs in extreme environments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we have developed a co-intercalation electrolyte based on the solvent co-embedding mechanism for efficient Na\u003csup\u003e+\u003c/sup\u003e ion storage in HC at low temperatures. The MO solvent is added into linear ether G2 to precisely regulate the solvation structure and expand the electrolyte\u0026rsquo;s low-temperature range \u003cem\u003evia\u003c/em\u003e an entropy effect. Experiments and simulations reveal that the Na\u003csup\u003e+\u003c/sup\u003e-G2 chelating coordination structure can be well co-embedded in HC anode, avoiding sluggish traditional desolvation process and significantly improving the ion diffusion at low temperatures. Meanwhile, the high sodium salt dissociation ability of G2 ensures excellent ionic conductivity in electrolyte. And the dipole interaction between solvents drives an appropriate number of anions into the solvation sheath, inducing the formation of a thin layer of inorganic rich SEI. This multi-scale synergistic design enables highly reversible and high-rate sodium storage at low temperatures. Consequently, the CIE enables HC to deliver an initial Coulombic efficiency of 80.5% at -50 ℃ and a capacity retention of 93% after 200 cycles. Moreover, an Ah-level full cell retains 163 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 25 ℃ and 107 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at -50 ℃. This study demonstrates the significance of solvent co-embedding mechanism in improving low-temperature ion storage and provides a new strategy for designing electrolytes toward extreme cold environments.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCorresponding Author\u003c/h2\u003e\n\u003cp\u003eYu Zhang- Department of Chemistry, Stockholm University, Stockholm 10691, Sweden; E-mail: [email protected]\u003c/p\u003e\n\u003cp\u003eNaiqing Zhang- State Key Laboratory of Urban-rural Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150001, China; E-mail: [email protected] \u0026amp; [email protected]\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;https://orcid.org/0000-0002-9528-9673 \u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eAcknowledgment\u003c/h2\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (22379036), the Natural Science Foundation of Heilongjiang Province (No. JQ2021B001) and State Key Laboratory of Robotics and Systems, Harbin Institute of Technology (SKLRS20230A07). The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for NMR analysis. Thanks to eseshi (www.eceshi.com) for the TOF-SIMS test.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eChe, C.\u003cem\u003e et al.\u003c/em\u003e Challenges and Breakthroughs in Enhancing Temperature Tolerance of Sodium-Ion Batteries. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2402291 (2024). https://doi.org/https://doi.org/10.1002/adma.202402291\u003c/li\u003e\n\u003cli\u003eLi, M., Wang, Y., Zhang, Y. \u0026amp; Zhang, N. Molecular tailoring of biomass precursor to regulate hard carbon microstructure for sodium ion batteries. \u003cem\u003eChemical Engineering Journal\u003c/em\u003e \u003cstrong\u003e506\u003c/strong\u003e, 160083 (2025). https://doi.org/https://doi.org/10.1016/j.cej.2025.160083\u003c/li\u003e\n\u003cli\u003eZhang, F.\u003cem\u003e et al.\u003c/em\u003e Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries. \u003cem\u003eChemical Reviews\u003c/em\u003e \u003cstrong\u003e124\u003c/strong\u003e, 4778-4821 (2024). https://doi.org/10.1021/acs.chemrev.3c00728\u003c/li\u003e\n\u003cli\u003eHuang, R.\u003cem\u003e et al.\u003c/em\u003e Revealing the low-temperature aging mechanisms of the whole life cycle for lithium-ion batteries (nickel-cobalt-aluminum vs. graphite). \u003cem\u003eJournal of Energy Chemistry\u003c/em\u003e \u003cstrong\u003e106\u003c/strong\u003e, 31-43 (2025). https://doi.org/https://doi.org/10.1016/j.jechem.2025.02.043\u003c/li\u003e\n\u003cli\u003eZheng, C.\u003cem\u003e et al.\u003c/em\u003e Enhancing Low-Temperature Performance of Sodium-Ion Batteries via Anion-Solvent Interactions. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003en/a\u003c/strong\u003e, 2501303 https://doi.org/https://doi.org/10.1002/adfm.202501303\u003c/li\u003e\n\u003cli\u003eFeng, Y.-H.\u003cem\u003e et al.\u003c/em\u003e Monolithic Interphase Enables Fast Kinetics for High-Performance Sodium-Ion Batteries at Subzero Temperature. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e63\u003c/strong\u003e, e202403585 (2024). https://doi.org/https://doi.org/10.1002/anie.202403585\u003c/li\u003e\n\u003cli\u003eZhang, Y.\u003cem\u003e et al.\u003c/em\u003e Electrolyte Design for Lithium-Ion Batteries for Extreme Temperature Applications. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2308484 (2024). https://doi.org/https://doi.org/10.1002/adma.202308484\u003c/li\u003e\n\u003cli\u003eQin, M., Zeng, Z., Cheng, S. \u0026amp; Xie, J. Challenges and strategies of formulating low-temperature electrolytes in lithium-ion batteries. \u003cem\u003eInterdisciplinary Materials\u003c/em\u003e \u003cstrong\u003e2\u003c/strong\u003e, 308-336 (2023). https://doi.org/https://doi.org/10.1002/idm2.12077\u003c/li\u003e\n\u003cli\u003eTang, Z.\u003cem\u003e et al.\u003c/em\u003e Electrode\u0026ndash;Electrolyte Interfacial Chemistry Modulation for Ultra-High Rate Sodium-Ion Batteries. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e61\u003c/strong\u003e, e202200475 (2022). https://doi.org/https://doi.org/10.1002/anie.202200475\u003c/li\u003e\n\u003cli\u003eWang, S., Zhang, X.-G., Gu, Y., Tang, S. \u0026amp; Fu, Y. An Ultrastable Low-Temperature Na Metal Battery Enabled by Synergy between Weakly Solvating Solvents. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e146\u003c/strong\u003e, 3854-3860 (2024). https://doi.org/10.1021/jacs.3c11134\u003c/li\u003e\n\u003cli\u003eFang, H.\u003cem\u003e et al.\u003c/em\u003e Regulating Ion-Dipole Interactions in Weakly Solvating Electrolyte towards Ultra-Low Temperature Sodium-Ion Batteries. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e63\u003c/strong\u003e, e202400539 (2024). https://doi.org/https://doi.org/10.1002/anie.202400539\u003c/li\u003e\n\u003cli\u003eDeng, L.\u003cem\u003e et al.\u003c/em\u003e Self-optimizing weak solvation effects achieving faster low-temperature charge transfer kinetics for high-voltage Na3V2(PO4)2F3 cathode. \u003cem\u003eEnergy Storage Materials\u003c/em\u003e \u003cstrong\u003e44\u003c/strong\u003e, 82-92 (2022). https://doi.org/https://doi.org/10.1016/j.ensm.2021.10.012\u003c/li\u003e\n\u003cli\u003eCui, R.\u003cem\u003e et al.\u003c/em\u003e Research progress of co-intercalation mechanism electrolytes in sodium-ion batteries. \u003cem\u003eEnergy Storage Materials\u003c/em\u003e \u003cstrong\u003e71\u003c/strong\u003e, 103627 (2024). https://doi.org/https://doi.org/10.1016/j.ensm.2024.103627\u003c/li\u003e\n\u003cli\u003eFerrero, G. A.\u003cem\u003e et al.\u003c/em\u003e Solvent Co-Intercalation Reactions for Batteries and Beyond. \u003cem\u003eChemical Reviews\u003c/em\u003e \u003cstrong\u003e125\u003c/strong\u003e, 3401-3439 (2025). https://doi.org/10.1021/acs.chemrev.4c00805\u003c/li\u003e\n\u003cli\u003eDivya, M. L., Lee, Y.-S. \u0026amp; Aravindan, V. Solvent Co-intercalation: An Emerging Mechanism in Li-, Na-, and K-Ion Capacitors. \u003cem\u003eACS Energy Letters\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 4228-4244 (2021). https://doi.org/10.1021/acsenergylett.1c01801\u003c/li\u003e\n\u003cli\u003eKim, H.\u003cem\u003e et al.\u003c/em\u003e Sodium Storage Behavior in Natural Graphite using Ether-based Electrolyte Systems. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e25\u003c/strong\u003e, 534-541 (2015). https://doi.org/https://doi.org/10.1002/adfm.201402984\u003c/li\u003e\n\u003cli\u003eXu, Z.-L.\u003cem\u003e et al.\u003c/em\u003e Tailoring sodium intercalation in graphite for high energy and power sodium ion batteries. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, 2598 (2019). https://doi.org/10.1038/s41467-019-10551-z\u003c/li\u003e\n\u003cli\u003eMa, S.\u003cem\u003e et al.\u003c/em\u003e Recent advances in carbon-based anodes for high-performance sodium-ion batteries: Mechanism, modification and characterizations. \u003cem\u003eMaterials Today\u003c/em\u003e \u003cstrong\u003e75\u003c/strong\u003e, 334-358 (2024). https://doi.org/https://doi.org/10.1016/j.mattod.2024.04.007\u003c/li\u003e\n\u003cli\u003eSun, Y.\u003cem\u003e et al.\u003c/em\u003e A \u0026ldquo;grafting technique\u0026rdquo; to tailor the interfacial behavior of hard carbon anodes for stable sodium-ion batteries. \u003cem\u003eEnergy \u0026amp; Environmental Science\u003c/em\u003e \u003cstrong\u003e18\u003c/strong\u003e, 1911-1919 (2025). https://doi.org/10.1039/D4EE05305B\u003c/li\u003e\n\u003cli\u003eXiao, K.\u003cem\u003e et al.\u003c/em\u003e Unlocking the Effect of Chain Length and Terminal Group on Ethylene Glycol Ether Family Toward Advanced Aqueous Electrolytes. \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e20\u003c/strong\u003e, 2306808 (2024). https://doi.org/https://doi.org/10.1002/smll.202306808\u003c/li\u003e\n\u003cli\u003eAlvarez Ferrero, G.\u003cem\u003e et al.\u003c/em\u003e Co-Intercalation Batteries (CoIBs): Role of TiS2 as Electrode for Storing Solvated Na Ions. \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 2202377 (2022). https://doi.org/https://doi.org/10.1002/aenm.202202377\u003c/li\u003e\n\u003cli\u003eYang, C.\u003cem\u003e et al.\u003c/em\u003e Entropy-Driven Solvation toward Low-Temperature Sodium-Ion Batteries with Temperature-Adaptive Feature. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e35\u003c/strong\u003e, 2301817 (2023). https://doi.org/https://doi.org/10.1002/adma.202301817\u003c/li\u003e\n\u003cli\u003eWang, M.\u003cem\u003e et al.\u003c/em\u003e Temperature-responsive solvation enabled by dipole-dipole interactions towards wide-temperature sodium-ion batteries. \u003cem\u003eNature Communications\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 8866 (2024). https://doi.org/10.1038/s41467-024-53259-5\u003c/li\u003e\n\u003cli\u003eCui, Y.\u003cem\u003e et al.\u003c/em\u003e A Temperature-Adapted Ultraweakly Solvating Electrolyte for Cold-Resistant Sodium-Ion Batteries. \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e \u003cstrong\u003en/a\u003c/strong\u003e, 2405363 https://doi.org/https://doi.org/10.1002/aenm.202405363\u003c/li\u003e\n\u003cli\u003eHuang, Z.\u003cem\u003e et al.\u003c/em\u003e Temperature-Robust Solvation Enabled by Solvent Interactions for Low-Temperature Sodium Metal Batteries. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e147\u003c/strong\u003e, 5162-5171 (2025). https://doi.org/10.1021/jacs.4c15478\u003c/li\u003e\n\u003cli\u003eYu, W.\u003cem\u003e et al.\u003c/em\u003e Electrochemical formation of bis(fluorosulfonyl)imide-derived solid-electrolyte interphase at Li-metal potential. \u003cem\u003eNature Chemistry\u003c/em\u003e \u003cstrong\u003e17\u003c/strong\u003e, 246-255 (2025). https://doi.org/10.1038/s41557-024-01689-5\u003c/li\u003e\n\u003cli\u003eRakov, D. A.\u003cem\u003e et al.\u003c/em\u003e The impact of electrode conductivity on electrolyte interfacial structuring and its implications on the Na0/+ electrochemical performance. \u003cem\u003eEnergy \u0026amp; Environmental Science\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 3919-3931 (2023). https://doi.org/10.1039/D3EE00864A\u003c/li\u003e\n\u003cli\u003eLi, X.\u003cem\u003e et al.\u003c/em\u003e Fast Interfacial Defluorination Kinetics Enables Stable Cycling of Low-Temperature Lithium Metal Batteries. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e146\u003c/strong\u003e, 17023-17031 (2024). https://doi.org/10.1021/jacs.3c14667\u003c/li\u003e\n\u003cli\u003eLin, Q.\u003cem\u003e et al.\u003c/em\u003e Deactivating Defects in Graphenes with Al2O3 Nanoclusters to Produce Long-Life and High-Rate Sodium-Ion Batteries. \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, 1803078 (2019). https://doi.org/https://doi.org/10.1002/aenm.201803078\u003c/li\u003e\n\u003cli\u003eChen, D.\u003cem\u003e et al.\u003c/em\u003e Hard carbon for sodium storage: mechanism and optimization strategies toward commercialization. \u003cem\u003eEnergy \u0026amp; Environmental Science\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 2244-2262 (2021). https://doi.org/10.1039/D0EE03916K\u003c/li\u003e\n\u003cli\u003eHou, X.\u003cem\u003e et al.\u003c/em\u003e Electrolyte Reconfiguration by Cation/Anion Cross-coordination for Highly Reversible and Facile Sodium Storage. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e64\u003c/strong\u003e, e202416939 (2025). https://doi.org/https://doi.org/10.1002/anie.202416939\u003c/li\u003e\n\u003cli\u003eSun, M.-Y.\u003cem\u003e et al.\u003c/em\u003e Reorganizing Helmholtz Adsorption Plane Enables Sodium Layered-Oxide Cathode beyond High Oxidation Limits. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2311432 (2024). https://doi.org/https://doi.org/10.1002/adma.202311432\u003c/li\u003e\n\u003cli\u003eLiang, H.-J.\u003cem\u003e et al.\u003c/em\u003e Electrolyte Chemistry toward Ultrawide-Temperature (\u0026minus;25 to 75 \u0026deg;C) Sodium-Ion Batteries Achieved by Phosphorus/Silicon-Synergistic Interphase Manipulation. \u003cem\u003eJournal of the American Chemical Society\u003c/em\u003e \u003cstrong\u003e146\u003c/strong\u003e, 7295-7304 (2024). https://doi.org/10.1021/jacs.3c11776\u003c/li\u003e\n\u003cli\u003eGuo, Z.\u003cem\u003e et al.\u003c/em\u003e Inorganic-Enriched Solid Electrolyte Interphases: A Key to Enhance Sodium-Ion Battery Cycle Stability? \u003cem\u003eSmall\u003c/em\u003e \u003cstrong\u003e20\u003c/strong\u003e, 2407425 (2024). https://doi.org/https://doi.org/10.1002/smll.202407425\u003c/li\u003e\n\u003cli\u003eXu, H.\u003cem\u003e et al.\u003c/em\u003e Impacts of Dissolved Ni2+ on the Solid Electrolyte Interphase on a Graphite Anode. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e61\u003c/strong\u003e, e202202894 (2022). https://doi.org/https://doi.org/10.1002/anie.202202894\u003c/li\u003e\n\u003cli\u003eFeng, Y.-H.\u003cem\u003e et al.\u003c/em\u003e Dual-Anionic Coordination Manipulation Induces Phosphorus and Boron-Rich Gradient Interphase Towards Stable and Safe Sodium Metal Batteries. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e64\u003c/strong\u003e, e202415644 (2025). https://doi.org/https://doi.org/10.1002/anie.202415644\u003c/li\u003e\n\u003cli\u003eLiu, M.\u003cem\u003e et al.\u003c/em\u003e Reinventing the High-rate Energy Storage of Hard Carbon: the Order-degree Governs the Trade-off of Desolvation-Solid Electrolyte Interphase at Interfaces. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e64\u003c/strong\u003e, e202425507 (2025). https://doi.org/https://doi.org/10.1002/anie.202425507\u003c/li\u003e\n\u003cli\u003eZeng, F.\u003cem\u003e et al.\u003c/em\u003e Innovative discontinuous-SEI constructed in ether-based electrolyte to maximize the capacity of hard carbon anode. \u003cem\u003eJournal of Energy Chemistry\u003c/em\u003e \u003cstrong\u003e79\u003c/strong\u003e, 459-467 (2023). https://doi.org/https://doi.org/10.1016/j.jechem.2022.12.044\u003c/li\u003e\n\u003cli\u003eChen, J.\u003cem\u003e et al.\u003c/em\u003e Rechargeable Potassium-Ion Full Cells Operating at \u0026minus;40\u0026thinsp;\u0026deg;C. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202307122 (2023). https://doi.org/https://doi.org/10.1002/anie.202307122\u003c/li\u003e\n\u003cli\u003eLiu, X.\u003cem\u003e et al.\u003c/em\u003e Evidence of Quasi-Na Metallic Clusters in Sodium Ion Batteries through In Situ X-Ray Diffraction. \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e37\u003c/strong\u003e, 2410673 (2025). https://doi.org/https://doi.org/10.1002/adma.202410673\u003c/li\u003e\n\u003cli\u003eLu, X.\u003cem\u003e et al.\u003c/em\u003e Low-Temperature Carbonized N/O/S-Tri-Doped Hard Carbon for Fast and Stable K-Ions Storage. \u003cem\u003eAdvanced Energy Materials\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 2303081 (2024). https://doi.org/https://doi.org/10.1002/aenm.202303081\u003c/li\u003e\n\u003cli\u003eGuan, K.\u003cem\u003e et al.\u003c/em\u003e Hierarchical Co-doped hollow Ti3C2Tx tubes with built-in electron/ion transport network for high-performance lithium sulfur batteries. \u003cem\u003eChemical Engineering Journal\u003c/em\u003e \u003cstrong\u003e492\u003c/strong\u003e, 151978 (2024). https://doi.org/https://doi.org/10.1016/j.cej.2024.151978\u003c/li\u003e\n\u003cli\u003eYang, K.\u003cem\u003e et al.\u003c/em\u003e Optimizing Kinetics for Enhanced Potassium-Ion Storage in Carbon-Based Anodes. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e33\u003c/strong\u003e, 2306190 (2023). https://doi.org/https://doi.org/10.1002/adfm.202306190\u003c/li\u003e\n\u003cli\u003eYang, Y.\u003cem\u003e et al.\u003c/em\u003e Rechargeable LiNi0.65Co0.15Mn0.2O2||Graphite Batteries Operating at \u0026minus;60\u0026thinsp;\u0026deg;C. \u003cem\u003eAngewandte Chemie International Edition\u003c/em\u003e \u003cstrong\u003e61\u003c/strong\u003e, e202209619 (2022). https://doi.org/https://doi.org/10.1002/anie.202209619\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Co-intercalation electrolyte, Hard carbon, Sodium-ion batteries","lastPublishedDoi":"10.21203/rs.3.rs-6911919/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6911919/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSodium ion batteries (SIBs) are attracting extensive interests due to their low cost and the abundant sodium resources. However, SIBs still suffer severe performance degradation at low temperatures due to the conflict between ion desolvation and diffusion. Herein, we design a co-intercalation ether electrolyte (CIE) to achieve solvent co-intercalation in hard carbon (HC) anode, thereby bypassing the slow desolvation process while ensuring rapid ion diffusion in electrolyte and HC. The optimized solvation structure also promotes the formation of a thin, inorganic-rich SEI, facilitating interfacial ion transport. As a result, the CIE enables HC to deliver excellent low temperature performance, with an initial Coulombic efficiency of 80.5% at -50\u0026deg;C and a capacity retention of 93% after 200 cycles. Moreover, an Ah-level full cell retains 163 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 25\u0026deg;C and 107 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at -50\u0026deg;C, demonstrating the practical feasibility of this strategy for all-climate SIBs. This work overcomes the long-standing trade-off between low-temperature ion desolvation and diffusion, offering a new approach for electrolyte design toward all-climate SIBs.\u003c/p\u003e","manuscriptTitle":"Solvent Co-Intercalation Electrolyte Unlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-03 10:29:02","doi":"10.21203/rs.3.rs-6911919/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"45e92b61-e9f4-45a8-80a0-48e11351d40c","owner":[],"postedDate":"July 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":50943680,"name":"Physical sciences/Chemistry/Electrochemistry/Batteries"},{"id":50943681,"name":"Physical sciences/Energy science and technology"}],"tags":[],"updatedAt":"2026-02-10T08:07:48+00:00","versionOfRecord":{"articleIdentity":"rs-6911919","link":"https://doi.org/10.1038/s41467-026-69237-y","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2026-02-06 05:00:00","publishedOnDateReadable":"February 6th, 2026"},"versionCreatedAt":"2025-07-03 10:29:02","video":"","vorDoi":"10.1038/s41467-026-69237-y","vorDoiUrl":"https://doi.org/10.1038/s41467-026-69237-y","workflowStages":[]},"version":"v1","identity":"rs-6911919","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6911919","identity":"rs-6911919","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}