Deciphering the lithium ion conduction mechanism in solid-electrolyte-interphase

Deciphering the lithium ion conduction mechanism in solid-electrolyte-interphase | 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 Deciphering the lithium ion conduction mechanism in solid-electrolyte-interphase Guanglei Cui, Jinran Sun, Jingtong Yan, Fan Li, Jiedong Li, Jun Ma, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2766405/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The solid electrolyte interphase (SEI) plays a critical role in determining the performance of lithium batteries, making it an area of increased scrutiny in research. Although the formation mechanism and structural model of SEI have been extensively reported, understanding the Li + conduction mechanism within SEI has proven challenging due to its complex structure and the lack of advanced in-situ experimental techniques. In this study, we employed isotopic tracer experiments based on high-resolution 6 Li NMR and TOF-SIMS techniques to illustrate the Li + conduction path in the bulk structure of natural SEI. Importantly, in-situ TEM technology was utilized to visualize the Li + conduction pathway through specific inorganic components of SEI, both in the bulk and on the surface. The critical experimental evidence provided by our work demonstrates that grain boundaries are not the sole pathway for Li + conduction in SEI, which is imperative for optimizing the performance of lithium batteries and designing future SEI. Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Materials science Physical sciences/Nanoscience and technology Physical sciences/Materials science/Techniques and instrumentation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The advent of lithium batteries (LBs) has revolutionized the field of energy storage technology owing to their superior energy densities and cycling stability in comparison to other rechargeable battery technologies. 1 The solid electrolyte interphase (SEI) is an integral component of LB and is widely recognized as a crucial factor in determining the performance of LB. Extensive research has been dedicated to this topic since the SEI concept was first proposed by Peled et al. in 1979. 2 – 8 SEI is a multifunctional byproduct layer formed through parasitic reactions between the anode and electrolyte, which consists of a compact inner layer of inorganic components and a porous outer layer of organic materials. 8 – 11 It serves as an electronic insulator and ionic conductor at the interface, preventing electrolyte decomposition while ensuring dynamic stability for reversible lithiation/de-lithiation processes. 12 , 13 As a necessary route of Li + transport at the anode interface, the cognition of the Li + transport mechanism in SEI remains intricate and controversial, which is still considered the “most important but the least understood point in rechargeable LBs”. 11 , 14 The primary scientific debate centers on whether the bulk SEI or only the grain boundary inside the SEI provides the pathway for Li + transport. Recent studies suggest that Li + can diffuse and migrate through the SEI; however, the precise route of Li + transport remains unclear, and the influence of liquid electrolyte utilized cannot be ignored. 15 In fact, the characterization of Li + diffusion in the ultrathin amorphous SEI at the surface of the anode remains an arduous task. Most of the current research on Li + transport mechanism in SEI relies on single-component analysis, such as LiF and Li 2 O, due to practical limitations in technology. 14 , 16 For Li 2 O, bulk Li 2 O can be applied in TEM techniques as the solid electrolyte. 17 – 19 However, the conductivity measured after pressing Li 2 O bulk into a dense disk is very low, which raises doubts among researchers whether it conducts Li + through the bulk phase in the natural SEI. Given this disparity, it still lacks direct experimental evidence to prove the Li + conduction behavior of Li 2 O within the SEI structure. However, antecedent experimental and theoretical inquiries have indicated that LiF bulk has poor ion conductivity (~ 10 − 10 S cm − 1 ). 1,20,21 As such, from this perspective, numerous scholars harbor the belief that the presence of LiF impedes the diffusion of Li + in SEI. 1 , 22 , 23 Nonetheless, recent first-principles calculations have shown that Li + can diffuse either along the surface of LiF or through the interface, implying the possibility that LiF could bolster ion transport in SEI. 24 – 28 Thus far, there is no empirical substantiation to scientifically verify the surface conduction properties of LiF. Unlike other inorganic species in SEI that have been identified for several decades, LiH has only recently been discovered as a key component with high abundance. 29 – 32 Up to date, the contribution of LiH in the SEI to the electrochemical performance of the battery remains controversial. 29 , 31 Previous study has shown that LiH may induce the deteriorated cyclability of Li metal batteries. 29 Meanwhile, the positive impact of LiH on Li + transport in SEI and stabilizing lithium metal anode have also been proposed. 29 , 31 To gain an objective and comprehensive understanding of the role of LiH in the SEI, a thorough understanding of the Li + conduction behavior in LiH is necessary. Nevertheless, the extent to which LiH participates in Li + transport within the SEI complex remains unclear and warrants further exploration. In light of these ongoing debates surrounding Li + transport in SEI, Therefore, all the debates related to the Li + transport in SEI emphasize the urgent need for actual experimental evidence to uncover the mysteries of the ion transport mechanism in the SEI. In this article, the Li + conduction pathway within the SEI has been experimentally identified for the first time. The designed isotope tracing experiment based on high-resolution solid-state nuclear magnetic resonance (ssNMR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) demonstrated the bulk Li + conduction behavior of Li 2 O and LiH within the SEI. Furthermore, the application of nanobattery technology based on in-situ transmission electron microscopy (TEM) facilitated the visualization of Li + conduction behaviors of Li 2 O, LiH, and LiF, thereby providing direct experimental evidence of the bulk Li + conduction within Li 2 O, surface Li + conduction of LiF, and electrochemical decomposition of LiH (as depicted in Fig. 1 ). These experimental observations are in excellent agreement with the results obtained from density functional theory (DFT) simulations. The established fundamental understanding of transport mechanism will aid in the innovative design of new electrolytes and additives specifically for the high-quality SEI construction, which could lead to a leap progress on interface engineering of rechargeable lithium batteries. 2. Results and discussion 2.1 Capturing the trails of Li + transport in SEI The isotopic( 6 Li) tracing experiments based on high-resolution NMR technique was elaborately designed to distinguish the Li + conduction pathways in SEI. As illustrated in Fig. 2 a, firstly, by cycling Li-Cu half cells with 6 Li-labeled lithium foil and 6 Li-labeled electrolyte (1 M 6 LiTFSI- Ethylene carbonate/dimethyl carbonate (EC/DMC)) (Fig. 2 b), the pristine SEI rich in 6 Li was obtained on the surface of Cu. Here, the 6 LiTFSI was synthesized via the reaction between 6 Li foil and bistrifluoromethanesulfonimide in ultrapure water. The battery was cycled at a deposition capacity of 1.3 mAh for 5 cycles with an average coulombic efficiency of approximately 90%, which means that the corresponding capacity converted to SEI (including dead lithium) during charge and discharge processes was approximately 0.65 mAh. The Cu foil with original SEI rich in 6 Li was washed 5 times with DMC and then dried for 12 h in a vacuum chamber attached to the glovebox. The SEI on the Cu foils disassembled from 40 identical coin cells was collected and labeled as sample 1. The total mass of sample 1 was 6.4 mg. The large amount of SEI component maintains the confidence of the ssNMR measurement. Then, an all-solid-state 7 Li-Cu battery (ASSLCB) was assembled using the Cu foil with the 6 Li-labeled SEI. The 7 Li metal and the solid-state electrolyte ( 7 LiSiPSCl) of the ASSLCBs were commercially available. It is important to note that the all-solid-state system, rather than the normal liquid system, is implemented in this step to avoid the frequently physical Li exchange between the liquid electrolyte and the SEI, which would interfere with our analysis based on the isotopic tracing technique. Upon discharge of the all-solid-state lithium-copper batteries (ASSLCBs) (Fig. 2 c), the 6 Li present in the SEI was partially replaced by 7 Li from the solid electrolyte, leaving a trail for Li + transport in the bulk SEI. And then, the SEI on Cu were harvested by carefully scraping the deposits on the Cu foils after disassembling all-solid-state 7 Li-Cu batteries, which was marked as sample 2 (total mass is 12.2 mg). The single-pulse 6 Li MAS NMR spectrum of sample 1 is presented in Fig. 2 d, and the peaks located at -0.8 ppm, 1.3 ppm, and 2.4 ppm are assigned to LiF, LiOH, and Li 2 S, respectively. The sharp peak located near 0.2 ppm is attributed to Li 2 CO 3 /Li 2 O 2 / and/or CH 3 OLi. 33 Moreover, the broad peaks situated at 2.9 ppm and 0.9 ppm are ascribed to Li 2 O and LiH, respectively. 33 Importantly, the NMR signal of LiH in the natural SEI is reported for the first time, and it is well consistent with the standard spectrum of LiH (Fig S1 a). The peaks of various species in the spectrum of sample 2 also correspond well with the standard peaks. It should be noted that sample 2 contains the signal of solid-state electrolyte, which is inevitable during SEI harvesting. Fortunately, the sharp peak of the solid-state electrolyte is located near 1.5 ppm (Fig S1 b), and it does not interfere with our analysis of the critical components. Comparing the spectrum of sample 2 with that of the pristine sample 1, a significant change can be found ( Fig. 2 e ) . It is clearly seen that the peaks of LiOH (1.4 ppm), Li 2 S (2.4 ppm), and Li 2 CO 3 /Li 2 O 2 / and/or CH 3 OLi (0.2 ppm) show no significant change, while the intensity of Li 2 O (2.9 ppm) and LiH (0.9 ppm) significantly decreases, indicating that both Li 2 O and LiH participate in the Li + conduction process. These results can be mutually verified via the analysis of TOF-SIMS (Fig S2). In this study, the samples were sputtered consecutively for 1240 s. And the Li isotopic distribution in Li 2 O or LiH was analyzed by counting all the secondary ion fragments containing only Li-O or Li-H (Table S1 , Fig. 3 ). As illustrated in Fig. 3 , compared to sample 1, the proportion of 6 Li in Li 2 O or LiH in sample 2 decreased significantly. Correspondingly, the proportion of 7 Li in Li 2 O and LiH increased in sample 2, indicating the participation of bulk Li 2 O and LiH in Li + conduction. Additionally, it is noted that the peak intensity of LiF does not display noticeable change, which precludes the possibility of Li + transport in the bulk LiF. However, the possibility of surface conduction of LiF cannot be ruled out, as the surface conduction may not leave traces that can be identified by isotope tracing experiments. As such, the Li + transport mechanism of SEI was further investigated in detail by in-situ transmission electron microscopy. 2.2 The visualization of the Li + conduction pathway in LiH, LiF and Li 2 O 2.2.1 The self-decomposition behavior of LiH To visualize the Li + conduction behavior in the individual component of SEI (LiH, LiF and Li 2 O), nanobatteries were assembled with the structure of carbon nanotube (CNT) cathode, various inorganic particles (LiH, LiF and Li 2 O) as solid electrolyte (SE) casted on Li anode or aluminum current collector (Al) in the in-situ TEM experiments (as shown in Fig S3). A potential is applied to the CNT and the current collector to drive Li + through the CNT-SE interface. The microstructural evolution of the CNT and SE was monitored in real-time. In addition, it is worth mentioning that trace amounts of oxygen are unavoidable in the TEM column although the nanobattery operated under high vacuum. Upon discharge, active lithium metal will react with O 2 immediately, resulting in the formation of a fine Li 2 O layer. Figure 4 a shows the TEM images acquired during the discharge process of the CNT-LiH-Li nanobattery, along with the corresponding EDP of CNT cathode (Fig. 4 b, c) and LiH (Fig. 4 d, e). During discharge process, the conduction direction of Li + is from SE-Li anode to CNT cathode (the video recording part of the process can be found in the Supporting Information (Video S1). The microstructural evolution of the CNT and SE is monitored in real time. The diameter of the CNT expands significantly from 9.5 nm to 40.5 nm during the lithiation process. Interestingly, the gradually changing morphology and the fading of diffraction contrast of LiH in TEM images indicate the structure transition behavior related to the decomposition of LiH during the discharge process (Fig. 4 a). The corresponding diffraction rings of nanocrystalline Li 2 O indicates that the generated Li not only lithiated CNT but also oxidized to Li 2 O rapidly by the trace oxygen in TEM column at the same time (Fig. 4 c). As previously reported, LiH is an unstable hydrogen storage material which decomposes to H 2 and lithium metal. 34 Moreover, in the nanobattery with the CNT-LiH-Al configuration, CNT was also lithiated with the diameter expanded from 14.7 nm to 32.0 nm during discharge process with the absence of lithium source (Video S2, Fig S4). Thus, it is reasonable to conjecture that Li and H 2 are generated through the decomposition of LiH during discharge process at the interface between CNT and LiH, which is validated by the decreased NMR signal of 6 LiH as described above (Fig. 2 e). Correspondingly, as indicated by the TOF-SIMS results, the intensity proportion of 6 LiH decreases obviously (Fig. 3 d). It is worth emphasizing that the decomposition of LiH is the consequence of electrochemical action, rather than the irradiation decomposition, as evidenced by the fact that LiH remains stable structure even under high temperature (130 ℃) with e-beam shower, as clearly demonstrated in Fig S5. Furthermore, the lower calculated Gibbs free energy (ΔG) of the LiH decomposition reaction rationalizes the decomposition behavior of LiH in that specific electrochemical environment (Fig S6a). It is worth mentioning that, as illustrated in Fig S6, the band gap of LiH (2.97 eV) is much narrower than LiF (8.72 eV) and Li 2 O (4.98 eV). Narrower band gap means better electronic conductivity of LiH, which may promote the electrochemical decomposition of LiH during discharge process. Conduction mechanism of LiH conforms to the electrochemical decomposition behavior regularity. It is worth noting that LiH is found to be abundant in the SEI, as previously reported. 30 The lithium metal generated by the self-decomposition process of LiH will exacerbate interface parasitic reactions and electrolyte consumption. Moreover, the unforeseen morphology of lithium metal and subsequent localized lithiation behavior on various anode materials may accelerate anode failure and trigger thermal runaway of the battery at lower temperatures, necessitating further investigation. 2.2.2 The surface conduction behavior of LiF In the CNT-LiF-Li nanobattery, the lithiation of CNT during the discharge process indicates the Li + conduction capacity of LiF (Video S3). Remarkably distinct from the self-decomposition behavior of LiH, LiF maintains its stable structure throughout the lithiation process (Fig. 4 i, j). Specifically, CNT thickens gradually as lithiation progresses in the early stages (Fig. 4 f). Subsequently, lithium metal deposits as the discharge capacity surpasses that of CNT. It is noteworthy that the deposit rapidly spreads along the surface of LiF, which is significantly different from the typical lithium deposition morphology (sphere or dendrite at the interface). Under normal circumstances, the deposit would receive Li + from the shortest pathway once the bulk structure dominates ionic conduction, as shown in Fig. 5 b. However, the spread of the deposit along the surface clearly indicates that the surface is the primary pathway for Li + conduction (Fig. 5 a). Here, the signal of Li 2 O is captured from the spreading layer, which is immediately formed via the reaction between the deposited lithium layer and the inevitable oxygen in the TEM column (Fig S7). All of the phenomena observed in the in-situ TEM experiment solidly corroborate the typical surface conduction behavior of LiF, which is also the reason why the signal of LiF did not change significantly in SSNMR analysis. Furthermore, theoretical simulations employing density functional theory (DFT) and climbing image nudged elastic band (CI-NEB) methods were utilized to elucidate the Li + conduction mechanism of LiF (Fig S8-12, S18-19). It is noteworthy that the activation energy ( E a ) was determined as the sum of carrier formation energy and the diffusion barrier, revealing crucial insights into the diffusion properties of Li + in LiF. 35 Specifically, as presented in Fig. 5 , the lowest E a values for surface and bulk diffusion of LiF were found to be 1.26 eV (LiF (110) , surface) and 4.72 eV (direct interstitial), respectively. This implies that the occurrence of bulk diffusion of Li + in LiF is arduous, while surface conduction is identified as the most favorable kinetic pathway to facilitate Li + transport. Accordingly, most of controversies surrounding LiF can be reasonably resolved. The ion conductivity of LiF relies on surface conduction predominantly than bulk conduction. As a constituent of the solid electrolyte interphase (SEI), LiF generated from the interfacial side reaction exists in nanocrystalline form within the SEI, possessing an ultra-high specific surface area that effectively promotes the surface conduction of Li + . This may explain the improvement in battery electrochemical performance observed when a LiF-rich SEI is obtained via the modulation of electrolytes. 24 – 26 However, artificial compact films or particles of LiF cannot provide sufficient efficient pathways for Li + conduction due to their relatively lower specific surface area and longer transport distance. Therefore, it is impractical to evaluate the actual ion conductivity of LiF using artificial films or particles prepared using conventional methods. 2.2.3 The bulk diffusion conduction behavior of Li 2 O The ssNMR and TOF-SIMS analyses have demonstrated the active participation of Li 2 O bulk phase in Li + conduction. Visualization of Li + conduction behavior in Li 2 O was achieved through the designed CNT-Li 2 O-Li nanobattery based on in-situ transmission electron microscopy. As depicted in Fig. 4 k and Video S4, Li 2 O single crystal maintains stable structure during lithiation (Fig. 4 n, o). Whereas, due to the restricted Li + capacity of CNT, lithium metal continuously deposits at the interface between Li 2 O and CNT during late-stage discharge. The results obtained by ssNMR, TOF-SIMS, and in-situ TEM observations indicate that Li + conduction in Li 2 O follows a bulk conduction mechanism, which is clearly different from the surface conduction mechanism of LiF. Corresponding DFT calculation results (Fig. 5 , Fig S13-17, Fig S20-21) indicate that the E a of bulk vacancy (3.71 eV) of Li 2 O is much lower than the E a of LiF (7.41 eV), implying that bulk conduction of Li 2 O is the kinetically favorable pathway with the ability to conduct ions (Fig. 5 ), which is consistent with the experimental results. Although surface conduction in Li 2 O has not been observed experimentally, the possibility of Li 2 O possessing surface conduction capability cannot be ruled out from a theoretical calculation perspective, and specific factors such as the actual distance of ion transfer (particle surface morphology) may limit this capability. As one of the main components of the SEI, the Li 2 O in nanoscale with high abundance guarantees the rapid transport of Li + . It is believed that the reported ionic conductivity of a dense nano-crystalline Li 2 O film (~ 10 − 7 S cm − 1 ) is reliable and closely approximates the realistic value for a single Li 2 O, which is sufficient to maintain fast conduction within the limited transport distance of the SEI (ranging From nanometers to hundreds of nanometers). 36 Conversely, the low conductivity (below 10 − 10 S cm − 1 ) observed in conventional bulk Li 2 O pellets is possibly attributed to the unavoidable presence of cracks and voids among the particles, which severely interfere with the continuous conduction during testing, leading to significantly lower measured conductivity values. Previous calculations suggest that the diffusion barrier for bulk conduction is much higher than that for interface conduction. 14 , 35 Considering the abundance of interfaces and grain boundaries in the SEI complex, 37 – 41 it is proposed that the Li + conductivity of Li 2 O in the intricate SEI is significantly higher than that observed in the single crystal investigated in this experiment or previously reported for Li 2 O films. 2.3 Enlightenment for SEI design SEI is a crucial factor determining the safety, the power capability, the morphology of lithium deposition, the calendar and cycle life of the batteries. Rational design of SEI has emerged as a fundamental obstacle to achieving progress in high energy density lithium-based batteries. As revealed in this study, we suggest that an SEI with high ionic conductivity and good stability should contain a high proportion of LiF and Li 2 O to ensure efficient ion transport, stable interfaces, and minimal parasitic reactions at the anode. Conversely, LiH is an unstable component in SEI that can generate lithium metal through self-decomposition, inducing the structural instability of SEI. Furthermore, the resulting morphology of the lithium metal cannot be controlled, which will trigger various failure mechanisms that depend on the properties of the anode material. Therefore, it is recommended that the ideal SEI should have a high content of LiF and Li 2 O, and as low a LiH content as possible, which has been well confirmed by our previous research. The dual-salt electrolyte (0.5 M lithium difuoro(oxalate)borate and 0.5 M lithium bis (trifluoromethane sulfonyl) imide in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (EC/DEC/EMC)) was used to generate a stable SEI rich in LiF and Li 2 O with a low amount of LiH, which enables better cyclability and Coulombic efficiency compared to the cases using the traditional electrolyte of 1 M LiPF 6 EC/DEC/EMC. 29 Furthermore, the proportion of LiH in SEI of the traditional electrolyte increased dramatically (from 0.74–16.55%) upon cycle (from 1st to 20th ), and that proportion of dual-salt electrolyte also increased obviously (from 2.74–16.94%) from 3th to 120th cycle. The significantly increased LiH identified in SEI of both batteries using various electrolyte upon the process of anode failure, mutually validating our above argument. 3. Conclusions The transport mechanism of Li + within SEI is successfully deciphered in this study. The empirical evidence obtained demonstrates that SEI achieves actual Li + conduction through the synergistic actions of LiH, LiF, and Li 2 O. Isotopic tracing experiments using high-resolution 6 Li solid-state nuclear magnetic resonance (ssNMR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicate the bulk-phase Li + conduction behavior of Li 2 O and LiH. In-situ nanobattery transmission electron microscopy (TEM) reveals that Li 2 O conducts Li + via bulk diffusion, which is consistent with the ssNMR results. Interestingly, it is clearly observed that LiH participates in the Li + conduction through self-decomposition, which may induce different effects on performance in various electrode system. Most importantly, the rapid surface Li + conduction behavior of LiF is revealed, confirming the significant advantage of abundant LiF in SEI for promoting Li + transport and rationalizing the empirical conclusion that fluorinated SEI can improve the electrochemical performance of batteries. We anticipate that the fundamental understanding and in-depth insights into the Li + conduction mechanism in SEI provided by this study will make a significant contribution to the rational design of SEI and the leapfrog improvement of battery performance in future work. Declarations Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments This research was supported by the Finance science and technology project of Hainan province (ZDKJ202014), National Natural Science Foundation of China (22179135), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22010600), Taishan Scholars Program for Young Expert of Shandong Province (tsqn202103145), Shandong Energy Institute (SEI I202108), the Key Scientific and Technological Innovation Project of Shandong (No.2022CXGC020301). Special thanks to Dr. Yang Haijun and Dr. Xie Jin from Tsinghua University. Author contributions J.S., S.D. and G.C. conceived this work and designed the experiment. J.Y., J.L., Y.T. and J.H. performed the in-situ TEM experiment. J.S., F.L. and G.H. performed the ssNMR experiment. J.Y., Y.T., and J.H. performed the theoretical calculation. J.S. and S.D. wrote the draft of the manuscript. J.Y., F.L., G.H., Y.T., S.Z., and J.M. contributed feedback and comments for revising the manuscript. 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Jain, A., Miyaoka, H. & Ichikawa, T. Destabilization of lithium hydride by the substitution of group 14 elements: A review. International Journal of Hydrogen Energy 41 , 5969-5978, doi:10.1016/j.ijhydene.2016.02.069 (2016). Ma, X.-X. et al. The Origin of Fast Lithium-Ion Transport in the Inorganic Solid Electrolyte Interphase on Lithium Metal Anodes. Small Structures 3 , doi:10.1002/sstr.202200071 (2022). Nojabaee, M., Küster, K., Starke, U., Popovic, J. & Maier, J. Solid Electrolyte Interphase Evolution on Lithium Metal in Contact with Glyme-Based Electrolytes. Small 16 , doi:10.1002/smll.202000756 (2020). Sun, S. Y. et al. The Crucial Role of Electrode Potential of a Working Anode in Dictating the Structural Evolution of Solid Electrolyte Interphase. Angewandte Chemie 61 , e202208743, doi:10.1002/anie.202208743 (2022). Han, B. et al. Poor Stability of Li2 CO3 in the Solid Electrolyte Interphase of a Lithium-Metal Anode Revealed by Cryo-Electron Microscopy. Advanced materials 33 , e2100404, doi:10.1002/adma.202100404 (2021). Han, B. et al. Additive stabilization of SEI on graphite observed using cryo-electron microscopy. Energy & Environmental Science 14 , 4882-4889, doi:10.1039/d1ee01678d (2021). Cheng, D. et al. Unveiling the Stable Nature of the Solid Electrolyte Interphase between Lithium Metal and LiPON via Cryogenic Electron Microscopy. Joule 4 , 2484-2500, doi:10.1016/j.joule.2020.08.013 (2020). Lin, R., He, Y. & Wang, C. Characterization of the structure and chemistry of the solid–electrolyte interface by cryo-EM leads to high-performance solid-state Li-metal batteries. Nature nanotechnology 7 , 23685-23693, doi:10.1021/acsami.5b07517 (2022). Methods The preparation of 6 LiTFSI The lithium metal ( 6 Li, purchased from Shanghai Sigma-Aldrich Co., Ltd) was reacted and bistrifluoromethanesulfonimide (purchased from Shanghai Aladdin Bio-chem Technology Co., Ltd) in ultrapure water in argon atmosphere. And then the product was transferred to a vacuum oven for full drying (110 ℃). Electrochemical measurements Electrochemical tests were conducted using a coin-type cell (LIR 2032) built in a glove box filled with argon. The half-cells were assembled with a polyethylene film (Celgard) as the separator, a lithium metal electrode ( 6 Li), and a copper current collector. And the liquid electrolyte is 1 M 6 LiTFSI in 1:1 EC/DMC (by weight). The half-cells were cycled with 0.9 mA cm − 2 . For sample 1, the SEI layers formed in each half cell were harvested by carefully scraping the SEI layer from the Cu electrodes after disassembling the cells that were in a state of full charge (i.e., all Li was stripped from the Cu electrode) in the Ar glovebox without scraping the Cu metal. The scraped off layers were washed using the solvent of electrolyte, i.e., DMC for five times, and then dried in a vacuum chamber attached to the glovebox for 12 h. For sample 2, the Cu foil as well as the attached SEI was washed using the solvent of electrolyte, i.e., DMC for five times after disassembling the coin cells, and dried in a vacuum chamber attached to the glovebox for 12 h. Then, that sample was applied in a new solid-state battery with configuration of 7 Li-solid state electrolyte-Cu (SEI). The solid-state battery was discharged with 0.2 mA cm − 2 . The SEI layers formed in each cell were harvested by carefully scraping the SEI layer from the Cu electrodes after disassembling the all solid-state batteries (i.e., all Li was stripped from the Cu electrode) in the Ar glovebox without scraping the Cu metal. ssNMR ssNMR experiments were performed on a Bruker AVANCE NEO 800 spectrometer with a standard-bore magnet under a magnetic field of 18.8 T in combination of a 3.2 mm HX DVT probe at 6 Li and 7 Li observed frequencies of 117.79 MHz and 311.07 MHz, respectively. For 6 Li,single-pulse NMR experiments were performed with a π/12 pulse of 0.65 μs, 10 s recycle delay and 20 kHz MAS rate. For 7 Li,single-pulse NMR experiments were performed with a π/2 pulse of 4.6 μs, 10 s recycle delay and 20 kHz MAS rate. TOF-SIMs The time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) depth profiles and 3D-compositional analysis were obtained by using a TOF-SIMS 5-100 (ION-TOF GmbH) instrument in a positive mode, which was equipped with Ar-gas cluster sputtering ion source (5 keV) and Bi 3 + ion beam as the analysis beam (30 keV). Synthesis of Arc-Discharged CNTs The CNTs used in this study were prepared by an electrical arc method. The anode was an asphalt-derived carbon rod, and the cathode was a high-purity graphite electrode. To facilitate the collection of CNTs and improve the purity, a wire net was placed on the top of the two electrodes in the chamber and the distance between the wire net and the electrodes was about 5 cm. The buffer gas pressure was set in the range of 0.04–0.05 MPa during the arc discharge experiment; meanwhile, the DC voltage and current for arcing were controlled at 18–20 V and 60–80 A, respectively. By manually advancing the anode that was consumed during the experiment, the distance between the two electrodes was kept at about 1–3 mm. In-Situ Environmental TEM of the Nanobattery Construction and Operation The experiments were conducted in a FEI Titan G2 ETEM. CNT-SEI-Li nanobattery was constructed with the structure of a SEI nanoparticle coated Li anode and a single carbon nanotube (CNT) as the cathode for the discharge reactions. The electrochemical reaction was controlled by external biasing using a potentiostat (PiciFemto V3ST). When a negative bias was applied to the CNT against the Li counter electrode, the discharge reaction took place. For the CNT-SEI nanobattery, SEI nanoparticle was casted on the Al rod as the anode and single (CNT) as the cathode. Computation Methods All the DFT calculations were conducted with the Vienna Ab initio simulation package (VASP) using projector augmented waves (PAW) pseudo potentials. The exchange-correction energy was described by the Perdew–Burke–Ernzerhof (PBE) model under the generalized gradient approximation (GGA). The cutoff energy of 520 eV, and the convergence criteria for the ionic and the electronic relaxation were set to 0.02 eV Å −1 and 10 –5 eV, respectively. Climbing image nudge elastic band calculations (CINEB) was employed to explore the Li + conduction pathway. The initial state (IS) structure and final state (FS) structure were firstly optimized, then images were created and relaxed between the IS structure and FS structure to determine the minimum energy pathway of the Li + conduction. Besides, gamma-centered k-point mesh with reciprocal space resolution of 2π×0.03 Å -1 were chosen. For the density of states (DOS) calculation, gamma-centered k-point mesh with reciprocal space resolution of 2π×0.02 Å -1 were chosen. Additional Declarations There is NO Competing Interest. Supplementary Files VideoS1CNTLiHLi.mp4 Video S1 CNT-LiH-Li VideoS2CNTLiHAl.mp4 Video S2 CNT-LiH-Al VideoS3CNTLiFLi.mp4 Video S3-CNT-LiF-Li VideoS4CNTLi2OLi.mp4 Video S4 CNT-Li2O-Li SupplementaryInformation.docx Supplementary Information Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-2766405","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":191840668,"identity":"f24b9a57-3d42-4c2d-af03-46498399bdec","order_by":0,"name":"Guanglei Cui","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0ElEQVRIiWNgGAWjYFACxgaGBxUMPAYMPKRoSTgD1gJkEQ0S2xgYiNfCPyO5+UXiPGsZc/be4w8YauwY+GcT0ClxI7HNInFbOo9lz7nEBoZjyQwSdw7g12IgkdhmkLjtMI/BjRzDBga2A0CRBGK0zAFquf8GqOUfcVqaHyQ2gGzhMWxgbCNCi8SZh20MCcdAfslLnJHYl8wjcYOAFv729McfPtRY25uznz3w4cM3Ozn+GQS0AAGbBAMDM4QJVExUGmD+ANcyCkbBKBgFowAbAADZ6ELHST5mhwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-5987-7569","institution":"Qingdao Institute of Bioenergy and Bioprocess Technology, College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Shandong Energy Institute","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Guanglei","middleName":"","lastName":"Cui","suffix":""},{"id":191840669,"identity":"d56d1410-016c-4ef0-8155-0a907b3ee9e4","order_by":1,"name":"Jinran Sun","email":"","orcid":"","institution":"Qingdao Institute of Bioenergy and Bioprocess Technology, College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Shandong Energy Institute","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jinran","middleName":"","lastName":"Sun","suffix":""},{"id":191840670,"identity":"46ddcc7d-9082-40ba-b7d2-a6cd4e01729e","order_by":2,"name":"Jingtong Yan","email":"","orcid":"","institution":"Clean Nano Energy Center, State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P. 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Inserts: Schematic diagram of \u003csup\u003e6\u003c/sup\u003eLiTFSI preparation. \u003cstrong\u003ec,\u003c/strong\u003e Voltage profiles of the \u003csup\u003e7\u003c/sup\u003eLi-Cu(\u003csup\u003e6\u003c/sup\u003eLi-SEI) all-solid-state half-cell. \u003cstrong\u003ed,\u003c/strong\u003e The single-pulse \u003csup\u003e6\u003c/sup\u003eLi MAS NMR spectrum of the sample 1. Inserts: Quantification of the phase fractions obtained by corresponding ssNMR data. \u003cstrong\u003ee,\u003c/strong\u003e The single-pulse \u003csup\u003e6\u003c/sup\u003eLi MAS NMR spectrum of the sample 2. Inserts: Quantification of the phase fractions obtained by corresponding ssNMR data.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/98c6dc6178a43f9b9f27ad24.png"},{"id":41023011,"identity":"bafe3c4c-ab57-4b04-bfcc-25b06eb01031","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":59658,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProportions of \u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eLi and \u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e7\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eLi in Li\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO, LiH and the corresponding schematic in sample 1 and sample 2, as measured by the TOF-SIMS technique, respectively. a,\u003c/strong\u003e Li\u003csub\u003e2\u003c/sub\u003eO in sample 1. \u003cstrong\u003eb\u003c/strong\u003e, Li\u003csub\u003e2\u003c/sub\u003eO in sample 2. \u003cstrong\u003ec\u003c/strong\u003e, LiH in sample 1. \u003cstrong\u003ed\u003c/strong\u003e, LiH in sample 2. The proportion of \u003csup\u003e6\u003c/sup\u003eLi is defined as the intensity of \u003csup\u003e6\u003c/sup\u003eLi divided by the total intensity of both \u003csup\u003e6\u003c/sup\u003eLi and \u003csup\u003e7\u003c/sup\u003eLi in the specific sample, while the proportion of \u003csup\u003e7\u003c/sup\u003eLi is defined as the intensity of \u003csup\u003e7\u003c/sup\u003eLi divided by the total intensity of both isotopes in the specific sample.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/ce72a23e4866c5956b9689af.png"},{"id":41023012,"identity":"04da65a4-0c63-4a9b-9421-624df991604e","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1424976,"visible":true,"origin":"","legend":"\u003cp\u003eCharacterization of the nanobattery during the discharge process. \u003cstrong\u003ea\u003c/strong\u003e, Typical TEM image CNT-LiH-Li nanobattery and corresponding electron diffraction patterns (EDP) of CNT(\u003cstrong\u003eb,c\u003c/strong\u003e) and LiH(\u003cstrong\u003ed,e\u003c/strong\u003e). \u003cstrong\u003ef\u003c/strong\u003e, Typical TEM image of CNT-LiF-Li nanobattery and corresponding EDP of CNT(\u003cstrong\u003eg,h\u003c/strong\u003e) and LiF(\u003cstrong\u003ei,j\u003c/strong\u003e). \u003cstrong\u003ek\u003c/strong\u003e, Typical TEM image of CNT-Li\u003csub\u003e2\u003c/sub\u003eO-Li nanobattery and corresponding EDP of CNT(\u003cstrong\u003el,m\u003c/strong\u003e) and Li\u003csub\u003e2\u003c/sub\u003eO (\u003cstrong\u003en,o\u003c/strong\u003e).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/d5415e09f3bdef38acb686ba.png"},{"id":41023014,"identity":"dba99163-c752-45e8-9ca0-0fe49a7ca20e","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":205174,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic and calculation of Li\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e+ \u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003econduction behavior in nanobattery. a,\u003c/strong\u003e CNT-LiF-Li.\u003cstrong\u003e b, \u003c/strong\u003eCNT-Li\u003csub\u003e2\u003c/sub\u003eO-Li. c, The E\u003csub\u003ea\u003c/sub\u003e of surface conduction of LiF. \u003cstrong\u003ed,\u003c/strong\u003e The E\u003csub\u003ea\u003c/sub\u003e of surface conduction of Li\u003csub\u003e2\u003c/sub\u003eO. \u003cstrong\u003ee,\u003c/strong\u003e The E\u003csub\u003ea\u003c/sub\u003e of bulk conduction of LiF. \u003cstrong\u003ef,\u003c/strong\u003e The E\u003csub\u003ea\u003c/sub\u003e of bulk conduction of Li\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/861797735449f13e76421375.png"},{"id":41024587,"identity":"53be9792-20a3-4b32-b036-3774dcd7a640","added_by":"auto","created_at":"2023-08-03 15:44:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2525596,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/728fd85b-4abc-44ec-b525-5148c12effaf.pdf"},{"id":41023018,"identity":"ab7662d9-87e3-4d0e-917e-84e47b303703","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":9503843,"visible":true,"origin":"","legend":"Video S1 CNT-LiH-Li","description":"","filename":"VideoS1CNTLiHLi.mp4","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/1948ea66f803eda01b769e3e.mp4"},{"id":41023960,"identity":"8384516a-e58f-4df8-bba9-aac201251b02","added_by":"auto","created_at":"2023-08-03 15:36:41","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":7552285,"visible":true,"origin":"","legend":"\u003cp\u003eVideo S2 CNT-LiH-Al\u003c/p\u003e","description":"","filename":"VideoS2CNTLiHAl.mp4","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/5da49137ada100f3347ca16f.mp4"},{"id":41023015,"identity":"5e119571-f68c-46a8-8a88-13bb4dd9695e","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":7542422,"visible":true,"origin":"","legend":"Video S3-CNT-LiF-Li","description":"","filename":"VideoS3CNTLiFLi.mp4","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/8096121daa3732db559d94f9.mp4"},{"id":41023017,"identity":"36b514b1-7acb-44e3-a93d-a30ca6c8d01a","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"mp4","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":9659200,"visible":true,"origin":"","legend":"Video S4 CNT-Li2O-Li","description":"","filename":"VideoS4CNTLi2OLi.mp4","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/2524f1815c8322ed3bbb9ed0.mp4"},{"id":41023019,"identity":"461e4ca1-53c4-4a17-a92b-2910db7b3658","added_by":"auto","created_at":"2023-08-03 15:28:41","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":35760246,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information\u003c/p\u003e","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-2766405/v1/edc915a00db027c73663dbb1.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Deciphering the lithium ion conduction mechanism in solid-electrolyte-interphase","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe advent of lithium batteries (LBs) has revolutionized the field of energy storage technology owing to their superior energy densities and cycling stability in comparison to other rechargeable battery technologies.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e The solid electrolyte interphase (SEI) is an integral component of LB and is widely recognized as a crucial factor in determining the performance of LB. Extensive research has been dedicated to this topic since the SEI concept was first proposed by Peled et al. in 1979. \u003csup\u003e\u003cspan additionalcitationids=\"CR3 CR4 CR5 CR6 CR7\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e SEI is a multifunctional byproduct layer formed through parasitic reactions between the anode and electrolyte, which consists of a compact inner layer of inorganic components and a porous outer layer of organic materials. \u003csup\u003e\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e It serves as an electronic insulator and ionic conductor at the interface, preventing electrolyte decomposition while ensuring dynamic stability for reversible lithiation/de-lithiation processes.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e As a necessary route of Li\u003csup\u003e+\u003c/sup\u003e transport at the anode interface, the cognition of the Li\u003csup\u003e+\u003c/sup\u003e transport mechanism in SEI remains intricate and controversial, which is still considered the \u0026ldquo;most important but the least understood point in rechargeable LBs\u0026rdquo;.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e The primary scientific debate centers on whether the bulk SEI or only the grain boundary inside the SEI provides the pathway for Li\u003csup\u003e+\u003c/sup\u003e transport. Recent studies suggest that Li\u003csup\u003e+\u003c/sup\u003e can diffuse and migrate through the SEI; however, the precise route of Li\u003csup\u003e+\u003c/sup\u003e transport remains unclear, and the influence of liquid electrolyte utilized cannot be ignored.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn fact, the characterization of Li\u003csup\u003e+\u003c/sup\u003e diffusion in the ultrathin amorphous SEI at the surface of the anode remains an arduous task. Most of the current research on Li\u003csup\u003e+\u003c/sup\u003e transport mechanism in SEI relies on single-component analysis, such as LiF and Li\u003csub\u003e2\u003c/sub\u003eO, due to practical limitations in technology.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e For Li\u003csub\u003e2\u003c/sub\u003eO, bulk Li\u003csub\u003e2\u003c/sub\u003eO can be applied in TEM techniques as the solid electrolyte.\u003csup\u003e\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e However, the conductivity measured after pressing Li\u003csub\u003e2\u003c/sub\u003eO bulk into a dense disk is very low, which raises doubts among researchers whether it conducts Li\u003csup\u003e+\u003c/sup\u003e through the bulk phase in the natural SEI. Given this disparity, it still lacks direct experimental evidence to prove the Li\u003csup\u003e+\u003c/sup\u003e conduction behavior of Li\u003csub\u003e2\u003c/sub\u003eO within the SEI structure. However, antecedent experimental and theoretical inquiries have indicated that LiF bulk has poor ion conductivity (~\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003csup\u003e1,20,21\u003c/sup\u003e As such, from this perspective, numerous scholars harbor the belief that the presence of LiF impedes the diffusion of Li\u003csup\u003e+\u003c/sup\u003e in SEI. \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Nonetheless, recent first-principles calculations have shown that Li\u003csup\u003e+\u003c/sup\u003e can diffuse either along the surface of LiF or through the interface, implying the possibility that LiF could bolster ion transport in SEI.\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26 CR27\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Thus far, there is no empirical substantiation to scientifically verify the surface conduction properties of LiF.\u003c/p\u003e \u003cp\u003eUnlike other inorganic species in SEI that have been identified for several decades, LiH has only recently been discovered as a key component with high abundance. \u003csup\u003e\u003cspan additionalcitationids=\"CR30 CR31\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e Up to date, the contribution of LiH in the SEI to the electrochemical performance of the battery remains controversial.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e Previous study has shown that LiH may induce the deteriorated cyclability of Li metal batteries.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e Meanwhile, the positive impact of LiH on Li\u003csup\u003e+\u003c/sup\u003e transport in SEI and stabilizing lithium metal anode have also been proposed.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e To gain an objective and comprehensive understanding of the role of LiH in the SEI, a thorough understanding of the Li\u003csup\u003e+\u003c/sup\u003e conduction behavior in LiH is necessary. Nevertheless, the extent to which LiH participates in Li\u0026thinsp;+\u0026thinsp;transport within the SEI complex remains unclear and warrants further exploration. In light of these ongoing debates surrounding Li\u0026thinsp;+\u0026thinsp;transport in SEI, Therefore, all the debates related to the Li\u003csup\u003e+\u003c/sup\u003e transport in SEI emphasize the urgent need for actual experimental evidence to uncover the mysteries of the ion transport mechanism in the SEI.\u003c/p\u003e \u003cp\u003eIn this article, the Li\u003csup\u003e+\u003c/sup\u003e conduction pathway within the SEI has been experimentally identified for the first time. The designed isotope tracing experiment based on high-resolution solid-state nuclear magnetic resonance (ssNMR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) demonstrated the bulk Li\u003csup\u003e+\u003c/sup\u003e conduction behavior of Li\u003csub\u003e2\u003c/sub\u003eO and LiH within the SEI. Furthermore, the application of nanobattery technology based on in-situ transmission electron microscopy (TEM) facilitated the visualization of Li\u003csup\u003e+\u003c/sup\u003e conduction behaviors of Li\u003csub\u003e2\u003c/sub\u003eO, LiH, and LiF, thereby providing direct experimental evidence of the bulk Li\u003csup\u003e+\u003c/sup\u003e conduction within Li\u003csub\u003e2\u003c/sub\u003eO, surface Li\u003csup\u003e+\u003c/sup\u003e conduction of LiF, and electrochemical decomposition of LiH (as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These experimental observations are in excellent agreement with the results obtained from density functional theory (DFT) simulations. The established fundamental understanding of transport mechanism will aid in the innovative design of new electrolytes and additives specifically for the high-quality SEI construction, which could lead to a leap progress on interface engineering of rechargeable lithium batteries.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"2. Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Capturing the trails of Li\u003csup\u003e+\u003c/sup\u003e transport in SEI\u003c/h2\u003e \u003cp\u003eThe isotopic(\u003csup\u003e6\u003c/sup\u003eLi) tracing experiments based on high-resolution NMR technique was elaborately designed to distinguish the Li\u003csup\u003e+\u003c/sup\u003e conduction pathways in SEI. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, firstly, by cycling Li-Cu half cells with \u003csup\u003e6\u003c/sup\u003eLi-labeled lithium foil and \u003csup\u003e6\u003c/sup\u003eLi-labeled electrolyte (1 M \u003csup\u003e6\u003c/sup\u003eLiTFSI- Ethylene carbonate/dimethyl carbonate (EC/DMC)) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), the pristine SEI rich in \u003csup\u003e6\u003c/sup\u003eLi was obtained on the surface of Cu. Here, the \u003csup\u003e6\u003c/sup\u003eLiTFSI was synthesized via the reaction between \u003csup\u003e6\u003c/sup\u003eLi foil and bistrifluoromethanesulfonimide in ultrapure water. The battery was cycled at a deposition capacity of 1.3 mAh for 5 cycles with an average coulombic efficiency of approximately 90%, which means that the corresponding capacity converted to SEI (including dead lithium) during charge and discharge processes was approximately 0.65 mAh. The Cu foil with original SEI rich in \u003csup\u003e6\u003c/sup\u003eLi was washed 5 times with DMC and then dried for 12 h in a vacuum chamber attached to the glovebox. The SEI on the Cu foils disassembled from 40 identical coin cells was collected and labeled as sample 1. The total mass of sample 1 was 6.4 mg. The large amount of SEI component maintains the confidence of the ssNMR measurement. Then, an all-solid-state \u003csup\u003e7\u003c/sup\u003eLi-Cu battery (ASSLCB) was assembled using the Cu foil with the \u003csup\u003e6\u003c/sup\u003eLi-labeled SEI. The \u003csup\u003e7\u003c/sup\u003eLi metal and the solid-state electrolyte (\u003csup\u003e7\u003c/sup\u003eLiSiPSCl) of the ASSLCBs were commercially available. It is important to note that the all-solid-state system, rather than the normal liquid system, is implemented in this step to avoid the frequently physical Li exchange between the liquid electrolyte and the SEI, which would interfere with our analysis based on the isotopic tracing technique. Upon discharge of the all-solid-state lithium-copper batteries (ASSLCBs) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), the \u003csup\u003e6\u003c/sup\u003eLi present in the SEI was partially replaced by \u003csup\u003e7\u003c/sup\u003eLi from the solid electrolyte, leaving a trail for Li\u003csup\u003e+\u003c/sup\u003e transport in the bulk SEI. And then, the SEI on Cu were harvested by carefully scraping the deposits on the Cu foils after disassembling all-solid-state \u003csup\u003e7\u003c/sup\u003eLi-Cu batteries, which was marked as sample 2 (total mass is 12.2 mg).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe single-pulse \u003csup\u003e6\u003c/sup\u003eLi MAS NMR spectrum of sample 1 is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, and the peaks located at -0.8 ppm, 1.3 ppm, and 2.4 ppm are assigned to LiF, LiOH, and Li\u003csub\u003e2\u003c/sub\u003eS, respectively. The sharp peak located near 0.2 ppm is attributed to Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e/Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/ and/or CH\u003csub\u003e3\u003c/sub\u003eOLi.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Moreover, the broad peaks situated at 2.9 ppm and 0.9 ppm are ascribed to Li\u003csub\u003e2\u003c/sub\u003eO and LiH, respectively. \u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Importantly, the NMR signal of LiH in the natural SEI is reported for the first time, and it is well consistent with the standard spectrum of LiH (Fig \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea). The peaks of various species in the spectrum of sample 2 also correspond well with the standard peaks. It should be noted that sample 2 contains the signal of solid-state electrolyte, which is inevitable during SEI harvesting. Fortunately, the sharp peak of the solid-state electrolyte is located near 1.5 ppm (Fig \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb), and it does not interfere with our analysis of the critical components. Comparing the spectrum of sample 2 with that of the pristine sample 1, a significant change can be found \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee\u003cb\u003e)\u003c/b\u003e. It is clearly seen that the peaks of LiOH (1.4 ppm), Li\u003csub\u003e2\u003c/sub\u003eS (2.4 ppm), and Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e/Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/ and/or CH\u003csub\u003e3\u003c/sub\u003eOLi (0.2 ppm) show no significant change, while the intensity of Li\u003csub\u003e2\u003c/sub\u003eO (2.9 ppm) and LiH (0.9 ppm) significantly decreases, indicating that both Li\u003csub\u003e2\u003c/sub\u003eO and LiH participate in the Li\u003csup\u003e+\u003c/sup\u003e conduction process. These results can be mutually verified via the analysis of TOF-SIMS (Fig S2). In this study, the samples were sputtered consecutively for 1240 s. And the Li isotopic distribution in Li\u003csub\u003e2\u003c/sub\u003eO or LiH was analyzed by counting all the secondary ion fragments containing only Li-O or Li-H (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, compared to sample 1, the proportion of \u003csup\u003e6\u003c/sup\u003eLi in Li\u003csub\u003e2\u003c/sub\u003eO or LiH in sample 2 decreased significantly. Correspondingly, the proportion of \u003csup\u003e7\u003c/sup\u003eLi in Li\u003csub\u003e2\u003c/sub\u003eO and LiH increased in sample 2, indicating the participation of bulk Li\u003csub\u003e2\u003c/sub\u003eO and LiH in Li\u003csup\u003e+\u003c/sup\u003e conduction. Additionally, it is noted that the peak intensity of LiF does not display noticeable change, which precludes the possibility of Li\u003csup\u003e+\u003c/sup\u003e transport in the bulk LiF. However, the possibility of surface conduction of LiF cannot be ruled out, as the surface conduction may not leave traces that can be identified by isotope tracing experiments. As such, the Li\u003csup\u003e+\u003c/sup\u003e transport mechanism of SEI was further investigated in detail by in-situ transmission electron microscopy.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 The visualization of the Li\u003csup\u003e+\u003c/sup\u003e conduction pathway in LiH, LiF and Li\u003csub\u003e2\u003c/sub\u003eO\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 The self-decomposition behavior of LiH\u003c/h2\u003e \u003cp\u003eTo visualize the Li\u003csup\u003e+\u003c/sup\u003e conduction behavior in the individual component of SEI (LiH, LiF and Li\u003csub\u003e2\u003c/sub\u003eO), nanobatteries were assembled with the structure of carbon nanotube (CNT) cathode, various inorganic particles (LiH, LiF and Li\u003csub\u003e2\u003c/sub\u003eO) as solid electrolyte (SE) casted on Li anode or aluminum current collector (Al) in the in-situ TEM experiments (as shown in Fig S3). A potential is applied to the CNT and the current collector to drive Li\u003csup\u003e+\u003c/sup\u003e through the CNT-SE interface. The microstructural evolution of the CNT and SE was monitored in real-time. In addition, it is worth mentioning that trace amounts of oxygen are unavoidable in the TEM column although the nanobattery operated under high vacuum. Upon discharge, active lithium metal will react with O\u003csub\u003e2\u003c/sub\u003e immediately, resulting in the formation of a fine Li\u003csub\u003e2\u003c/sub\u003eO layer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows the TEM images acquired during the discharge process of the CNT-LiH-Li nanobattery, along with the corresponding EDP of CNT cathode (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, c) and LiH (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, e). During discharge process, the conduction direction of Li\u003csup\u003e+\u003c/sup\u003e is from SE-Li anode to CNT cathode (the video recording part of the process can be found in the Supporting Information (Video S1). The microstructural evolution of the CNT and SE is monitored in real time. The diameter of the CNT expands significantly from 9.5 nm to 40.5 nm during the lithiation process. Interestingly, the gradually changing morphology and the fading of diffraction contrast of LiH in TEM images indicate the structure transition behavior related to the decomposition of LiH during the discharge process (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The corresponding diffraction rings of nanocrystalline Li\u003csub\u003e2\u003c/sub\u003eO indicates that the generated Li not only lithiated CNT but also oxidized to Li\u003csub\u003e2\u003c/sub\u003eO rapidly by the trace oxygen in TEM column at the same time (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). As previously reported, LiH is an unstable hydrogen storage material which decomposes to H\u003csub\u003e2\u003c/sub\u003e and lithium metal.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Moreover, in the nanobattery with the CNT-LiH-Al configuration, CNT was also lithiated with the diameter expanded from 14.7 nm to 32.0 nm during discharge process with the absence of lithium source (Video S2, Fig S4). Thus, it is reasonable to conjecture that Li and H\u003csub\u003e2\u003c/sub\u003e are generated through the decomposition of LiH during discharge process at the interface between CNT and LiH, which is validated by the decreased NMR signal of \u003csup\u003e6\u003c/sup\u003eLiH as described above (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). Correspondingly, as indicated by the TOF-SIMS results, the intensity proportion of \u003csup\u003e6\u003c/sup\u003eLiH decreases obviously (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). It is worth emphasizing that the decomposition of LiH is the consequence of electrochemical action, rather than the irradiation decomposition, as evidenced by the fact that LiH remains stable structure even under high temperature (130 ℃) with e-beam shower, as clearly demonstrated in Fig S5. Furthermore, the lower calculated Gibbs free energy (ΔG) of the LiH decomposition reaction rationalizes the decomposition behavior of LiH in that specific electrochemical environment (Fig S6a). It is worth mentioning that, as illustrated in Fig S6, the band gap of LiH (2.97 eV) is much narrower than LiF (8.72 eV) and Li\u003csub\u003e2\u003c/sub\u003eO (4.98 eV). Narrower band gap means better electronic conductivity of LiH, which may promote the electrochemical decomposition of LiH during discharge process.\u003c/p\u003e \u003cp\u003eConduction mechanism of LiH conforms to the electrochemical decomposition behavior regularity. It is worth noting that LiH is found to be abundant in the SEI, as previously reported.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The lithium metal generated by the self-decomposition process of LiH will exacerbate interface parasitic reactions and electrolyte consumption. Moreover, the unforeseen morphology of lithium metal and subsequent localized lithiation behavior on various anode materials may accelerate anode failure and trigger thermal runaway of the battery at lower temperatures, necessitating further investigation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 The surface conduction behavior of LiF\u003c/h2\u003e \u003cp\u003eIn the CNT-LiF-Li nanobattery, the lithiation of CNT during the discharge process indicates the Li\u003csup\u003e+\u003c/sup\u003e conduction capacity of LiF (Video S3). Remarkably distinct from the self-decomposition behavior of LiH, LiF maintains its stable structure throughout the lithiation process (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei, j). Specifically, CNT thickens gradually as lithiation progresses in the early stages (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). Subsequently, lithium metal deposits as the discharge capacity surpasses that of CNT. It is noteworthy that the deposit rapidly spreads along the surface of LiF, which is significantly different from the typical lithium deposition morphology (sphere or dendrite at the interface). Under normal circumstances, the deposit would receive Li\u003csup\u003e+\u003c/sup\u003e from the shortest pathway once the bulk structure dominates ionic conduction, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. However, the spread of the deposit along the surface clearly indicates that the surface is the primary pathway for Li\u003csup\u003e+\u003c/sup\u003e conduction (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Here, the signal of Li\u003csub\u003e2\u003c/sub\u003eO is captured from the spreading layer, which is immediately formed via the reaction between the deposited lithium layer and the inevitable oxygen in the TEM column (Fig S7). All of the phenomena observed in the in-situ TEM experiment solidly corroborate the typical surface conduction behavior of LiF, which is also the reason why the signal of LiF did not change significantly in SSNMR analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurthermore, theoretical simulations employing density functional theory (DFT) and climbing image nudged elastic band (CI-NEB) methods were utilized to elucidate the Li\u003csup\u003e+\u003c/sup\u003e conduction mechanism of LiF (Fig S8-12, S18-19). It is noteworthy that the activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e) was determined as the sum of carrier formation energy and the diffusion barrier, revealing crucial insights into the diffusion properties of Li\u003csup\u003e+\u003c/sup\u003e in LiF.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e Specifically, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the lowest \u003cem\u003eE\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e values for surface and bulk diffusion of LiF were found to be 1.26 eV (LiF \u003csub\u003e(110)\u003c/sub\u003e, surface) and 4.72 eV (direct interstitial), respectively. This implies that the occurrence of bulk diffusion of Li\u003csup\u003e+\u003c/sup\u003e in LiF is arduous, while surface conduction is identified as the most favorable kinetic pathway to facilitate Li\u003csup\u003e+\u003c/sup\u003e transport.\u003c/p\u003e \u003cp\u003eAccordingly, most of controversies surrounding LiF can be reasonably resolved. The ion conductivity of LiF relies on surface conduction predominantly than bulk conduction. As a constituent of the solid electrolyte interphase (SEI), LiF generated from the interfacial side reaction exists in nanocrystalline form within the SEI, possessing an ultra-high specific surface area that effectively promotes the surface conduction of Li\u003csup\u003e+\u003c/sup\u003e. This may explain the improvement in battery electrochemical performance observed when a LiF-rich SEI is obtained via the modulation of electrolytes.\u003csup\u003e\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e However, artificial compact films or particles of LiF cannot provide sufficient efficient pathways for Li\u003csup\u003e+\u003c/sup\u003e conduction due to their relatively lower specific surface area and longer transport distance. Therefore, it is impractical to evaluate the actual ion conductivity of LiF using artificial films or particles prepared using conventional methods.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 The bulk diffusion conduction behavior of Li\u003csub\u003e2\u003c/sub\u003eO\u003c/h2\u003e \u003cp\u003eThe ssNMR and TOF-SIMS analyses have demonstrated the active participation of Li\u003csub\u003e2\u003c/sub\u003eO bulk phase in Li\u003csup\u003e+\u003c/sup\u003e conduction. Visualization of Li\u003csup\u003e+\u003c/sup\u003e conduction behavior in Li\u003csub\u003e2\u003c/sub\u003eO was achieved through the designed CNT-Li\u003csub\u003e2\u003c/sub\u003eO-Li nanobattery based on in-situ transmission electron microscopy. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ek and Video S4, Li\u003csub\u003e2\u003c/sub\u003eO single crystal maintains stable structure during lithiation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003en, o). Whereas, due to the restricted Li\u003csup\u003e+\u003c/sup\u003e capacity of CNT, lithium metal continuously deposits at the interface between Li\u003csub\u003e2\u003c/sub\u003eO and CNT during late-stage discharge. The results obtained by ssNMR, TOF-SIMS, and in-situ TEM observations indicate that Li\u003csup\u003e+\u003c/sup\u003e conduction in Li\u003csub\u003e2\u003c/sub\u003eO follows a bulk conduction mechanism, which is clearly different from the surface conduction mechanism of LiF. Corresponding DFT calculation results (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Fig S13-17, Fig S20-21) indicate that the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e of bulk vacancy (3.71 eV) of Li\u003csub\u003e2\u003c/sub\u003eO is much lower than the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e of LiF (7.41 eV), implying that bulk conduction of Li\u003csub\u003e2\u003c/sub\u003eO is the kinetically favorable pathway with the ability to conduct ions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which is consistent with the experimental results. Although surface conduction in Li\u003csub\u003e2\u003c/sub\u003eO has not been observed experimentally, the possibility of Li\u003csub\u003e2\u003c/sub\u003eO possessing surface conduction capability cannot be ruled out from a theoretical calculation perspective, and specific factors such as the actual distance of ion transfer (particle surface morphology) may limit this capability.\u003c/p\u003e \u003cp\u003eAs one of the main components of the SEI, the Li\u003csub\u003e2\u003c/sub\u003eO in nanoscale with high abundance guarantees the rapid transport of Li\u003csup\u003e+\u003c/sup\u003e. It is believed that the reported ionic conductivity of a dense nano-crystalline Li\u003csub\u003e2\u003c/sub\u003eO film (~\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is reliable and closely approximates the realistic value for a single Li\u003csub\u003e2\u003c/sub\u003eO, which is sufficient to maintain fast conduction within the limited transport distance of the SEI (ranging From nanometers to hundreds of nanometers).\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e Conversely, the low conductivity (below 10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) observed in conventional bulk Li\u003csub\u003e2\u003c/sub\u003eO pellets is possibly attributed to the unavoidable presence of cracks and voids among the particles, which severely interfere with the continuous conduction during testing, leading to significantly lower measured conductivity values. Previous calculations suggest that the diffusion barrier for bulk conduction is much higher than that for interface conduction.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e Considering the abundance of interfaces and grain boundaries in the SEI complex,\u003csup\u003e\u003cspan additionalcitationids=\"CR38 CR39 CR40\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e it is proposed that the Li\u003csup\u003e+\u003c/sup\u003e conductivity of Li\u003csub\u003e2\u003c/sub\u003eO in the intricate SEI is significantly higher than that observed in the single crystal investigated in this experiment or previously reported for Li\u003csub\u003e2\u003c/sub\u003eO films.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Enlightenment for SEI design\u003c/h2\u003e \u003cp\u003eSEI is a crucial factor determining the safety, the power capability, the morphology of lithium deposition, the calendar and cycle life of the batteries. Rational design of SEI has emerged as a fundamental obstacle to achieving progress in high energy density lithium-based batteries. As revealed in this study, we suggest that an SEI with high ionic conductivity and good stability should contain a high proportion of LiF and Li\u003csub\u003e2\u003c/sub\u003eO to ensure efficient ion transport, stable interfaces, and minimal parasitic reactions at the anode. Conversely, LiH is an unstable component in SEI that can generate lithium metal through self-decomposition, inducing the structural instability of SEI. Furthermore, the resulting morphology of the lithium metal cannot be controlled, which will trigger various failure mechanisms that depend on the properties of the anode material. Therefore, it is recommended that the ideal SEI should have a high content of LiF and Li\u003csub\u003e2\u003c/sub\u003eO, and as low a LiH content as possible, which has been well confirmed by our previous research. The dual-salt electrolyte (0.5 M lithium difuoro(oxalate)borate and 0.5 M lithium bis (trifluoromethane sulfonyl) imide in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (EC/DEC/EMC)) was used to generate a stable SEI rich in LiF and Li\u003csub\u003e2\u003c/sub\u003eO with a low amount of LiH, which enables better cyclability and Coulombic efficiency compared to the cases using the traditional electrolyte of 1 M LiPF\u003csub\u003e6\u003c/sub\u003e EC/DEC/EMC.\u003csup\u003e29\u003c/sup\u003e Furthermore, the proportion of LiH in SEI of the traditional electrolyte increased dramatically (from 0.74\u0026ndash;16.55%) upon cycle (from 1st to 20th ), and that proportion of dual-salt electrolyte also increased obviously (from 2.74\u0026ndash;16.94%) from 3th to 120th cycle. The significantly increased LiH identified in SEI of both batteries using various electrolyte upon the process of anode failure, mutually validating our above argument.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Conclusions","content":"\u003cp\u003eThe transport mechanism of Li\u003csup\u003e+\u003c/sup\u003e within SEI is successfully deciphered in this study. The empirical evidence obtained demonstrates that SEI achieves actual Li\u003csup\u003e+\u003c/sup\u003e conduction through the synergistic actions of LiH, LiF, and Li\u003csub\u003e2\u003c/sub\u003eO. Isotopic tracing experiments using high-resolution \u003csup\u003e6\u003c/sup\u003eLi solid-state nuclear magnetic resonance (ssNMR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicate the bulk-phase Li\u003csup\u003e+\u003c/sup\u003e conduction behavior of Li\u003csub\u003e2\u003c/sub\u003eO and LiH. In-situ nanobattery transmission electron microscopy (TEM) reveals that Li\u003csub\u003e2\u003c/sub\u003eO conducts Li\u003csup\u003e+\u003c/sup\u003e via bulk diffusion, which is consistent with the ssNMR results. Interestingly, it is clearly observed that LiH participates in the Li\u003csup\u003e+\u003c/sup\u003e conduction through self-decomposition, which may induce different effects on performance in various electrode system. Most importantly, the rapid surface Li\u003csup\u003e+\u003c/sup\u003e conduction behavior of LiF is revealed, confirming the significant advantage of abundant LiF in SEI for promoting Li\u003csup\u003e+\u003c/sup\u003e transport and rationalizing the empirical conclusion that fluorinated SEI can improve the electrochemical performance of batteries. We anticipate that the fundamental understanding and in-depth insights into the Li\u003csup\u003e+\u003c/sup\u003e conduction mechanism in SEI provided by this study will make a significant contribution to the rational design of SEI and the leapfrog improvement of battery performance in future work.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the Finance science and technology project of Hainan province (ZDKJ202014), National Natural Science Foundation of China (22179135), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22010600), Taishan Scholars Program for Young Expert of Shandong Province (tsqn202103145), Shandong Energy Institute (SEI I202108), the Key Scientific and Technological Innovation Project of Shandong (No.2022CXGC020301). Special thanks to Dr. Yang Haijun and Dr. Xie Jin from Tsinghua University.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJ.S., S.D. and G.C. conceived this work and designed the experiment.\u0026nbsp;J.Y., J.L., Y.T. and J.H.\u0026nbsp;performed the in-situ TEM experiment. J.S., F.L. and G.H. performed the ssNMR experiment. J.Y., Y.T., and J.H. performed the theoretical calculation. J.S. and S.D. wrote the draft of the manuscript. J.Y., F.L., G.H., Y.T., S.Z., and J.M. contributed feedback and comments for revising the manuscript. G.C. directed and supervised the study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eTan, J., Matz, J., Dong, P., Shen, J. \u0026amp; Ye, M. 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Y.\u003cem\u003e et al.\u003c/em\u003e The Crucial Role of Electrode Potential of a Working Anode in Dictating the Structural Evolution of Solid Electrolyte Interphase. \u003cem\u003eAngewandte Chemie\u003c/em\u003e \u003cstrong\u003e61\u003c/strong\u003e, e202208743, doi:10.1002/anie.202208743 (2022).\u003c/li\u003e\n\u003cli\u003eHan, B.\u003cem\u003e et al.\u003c/em\u003e Poor Stability of Li2 CO3 in the Solid Electrolyte Interphase of a Lithium-Metal Anode Revealed by Cryo-Electron Microscopy. \u003cem\u003eAdvanced materials\u003c/em\u003e \u003cstrong\u003e33\u003c/strong\u003e, e2100404, doi:10.1002/adma.202100404 (2021).\u003c/li\u003e\n\u003cli\u003eHan, B.\u003cem\u003e et al.\u003c/em\u003e Additive stabilization of SEI on graphite observed using cryo-electron microscopy. \u003cem\u003eEnergy \u0026amp; Environmental Science\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 4882-4889, doi:10.1039/d1ee01678d (2021).\u003c/li\u003e\n\u003cli\u003eCheng, D.\u003cem\u003e et al.\u003c/em\u003e Unveiling the Stable Nature of the Solid Electrolyte Interphase between Lithium Metal and LiPON via Cryogenic Electron Microscopy. \u003cem\u003eJoule\u003c/em\u003e \u003cstrong\u003e4\u003c/strong\u003e, 2484-2500, doi:10.1016/j.joule.2020.08.013 (2020).\u003c/li\u003e\n\u003cli\u003eLin, R., He, Y. \u0026amp; Wang, C. Characterization of the structure and chemistry of the solid\u0026ndash;electrolyte interface by cryo-EM leads to high-performance solid-state Li-metal batteries. \u003cem\u003eNature nanotechnology\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 23685-23693, doi:10.1021/acsami.5b07517 (2022).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eThe preparation of \u003csup\u003e6\u003c/sup\u003eLiTFSI\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe lithium metal (\u003csup\u003e6\u003c/sup\u003eLi,\u0026nbsp;purchased from Shanghai Sigma-Aldrich Co., Ltd) was reacted\u0026nbsp;and bistrifluoromethanesulfonimide (purchased from Shanghai Aladdin Bio-chem Technology Co., Ltd) in ultrapure\u0026nbsp;water\u0026nbsp;in argon atmosphere. And then the product was transferred to a vacuum oven for full drying (110\u0026nbsp;℃).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eElectrochemical tests were conducted using a coin-type cell (LIR 2032) built in a glove box filled with argon. The half-cells were assembled with a polyethylene film (Celgard) as the separator, a lithium metal electrode (\u003csup\u003e6\u003c/sup\u003eLi), and a copper current collector. And the liquid electrolyte is 1 M \u003csup\u003e6\u003c/sup\u003eLiTFSI in 1:1 EC/DMC (by weight).\u0026nbsp;The half-cells were cycled with 0.9 mA cm\u003csup\u003e−\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e. For sample 1, the SEI layers formed in each half cell were harvested by carefully scraping the SEI layer from the Cu electrodes after disassembling the cells that were in a state of full charge (i.e., all Li was stripped from the Cu electrode) in the Ar glovebox without scraping the Cu metal. The scraped off layers were washed using the solvent of electrolyte, i.e., DMC for five times, and then dried in a vacuum chamber attached to the glovebox for 12 h. For sample 2, the Cu foil as well as the attached SEI was washed using the solvent of electrolyte, i.e., DMC for five times after disassembling the coin cells, and dried in a vacuum chamber attached to the glovebox for 12 h. Then, that sample was applied in a new solid-state battery with configuration of \u003csup\u003e7\u003c/sup\u003eLi-solid state electrolyte-Cu (SEI). The solid-state battery was discharged with 0.2 mA cm\u003csup\u003e−\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e. The SEI layers formed in each cell were harvested by carefully scraping the SEI layer from the Cu electrodes after disassembling the all solid-state batteries (i.e., all Li was stripped from the Cu electrode) in the Ar glovebox without scraping the Cu metal.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003essNMR\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003essNMR experiments were performed on a Bruker AVANCE NEO 800 spectrometer with a standard-bore magnet under a magnetic field of 18.8 T in combination of a 3.2 mm HX DVT probe\u0026nbsp;at \u003csup\u003e6\u003c/sup\u003eLi and \u003csup\u003e7\u003c/sup\u003eLi observed frequencies of 117.79 MHz and 311.07 MHz, respectively. For \u003csup\u003e6\u003c/sup\u003eLi,single-pulse NMR experiments were performed with a π/12 pulse of 0.65 μs, 10 s recycle delay and 20 kHz MAS rate. For \u003csup\u003e7\u003c/sup\u003eLi,single-pulse NMR experiments were performed with a π/2 pulse of 4.6 μs, 10 s recycle delay and 20 kHz MAS rate.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTOF-SIMs\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) depth profiles and 3D-compositional analysis were obtained by using a TOF-SIMS 5-100 (ION-TOF GmbH) instrument in a positive mode, which was equipped with Ar-gas cluster sputtering ion source (5 keV) and Bi\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e ion beam as the analysis beam (30 keV).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis of Arc-Discharged CNTs\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe CNTs used in this study were prepared by an electrical arc method. The anode was an asphalt-derived carbon rod, and the cathode was a high-purity graphite electrode. To facilitate the collection of CNTs and improve the purity, a wire net was placed on the top of the two electrodes in the chamber and the distance between the wire net and the electrodes was about 5 cm. The buffer gas pressure was set in the range of 0.04–0.05 MPa during the arc discharge experiment; meanwhile, the DC voltage and current for arcing were controlled at 18–20 V and 60–80 A, respectively. By manually advancing the anode that was consumed during the experiment, the distance between the two electrodes was kept at about 1–3 mm.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIn-Situ Environmental TEM of the Nanobattery Construction and Operation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe experiments were conducted in a FEI Titan G2 ETEM. CNT-SEI-Li nanobattery was constructed with the structure of a SEI nanoparticle coated Li anode and a single carbon nanotube (CNT) as the cathode for the discharge reactions. The electrochemical reaction was controlled by external biasing using a potentiostat (PiciFemto V3ST). When a negative bias was applied to the CNT against the Li counter electrode, the discharge reaction took place. For the CNT-SEI nanobattery, SEI nanoparticle was casted on the Al rod as the anode and single (CNT) as the cathode.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eComputation Methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the DFT calculations were conducted with the Vienna Ab initio simulation package (VASP) using projector augmented waves (PAW) pseudo potentials. The exchange-correction energy was described by the Perdew–Burke–Ernzerhof (PBE) model under the generalized gradient approximation (GGA). The cutoff energy of 520 eV, and the convergence criteria for the ionic and the electronic relaxation were set to 0.02 eV Å\u003csup\u003e−1\u003c/sup\u003e and 10\u003csup\u003e–5\u003c/sup\u003e eV, respectively. Climbing image nudge elastic band calculations (CINEB) was employed to explore the Li\u003csup\u003e+\u003c/sup\u003e conduction pathway. The initial state (IS) structure and final state (FS) structure were firstly optimized, then images were created and relaxed between the IS structure and FS structure to determine the minimum energy pathway of the Li\u003csup\u003e+\u003c/sup\u003e conduction. Besides, gamma-centered k-point mesh with reciprocal space resolution of 2π×0.03\u0026nbsp;Å\u003csup\u003e-1\u003c/sup\u003e were chosen. For the density of states (DOS) calculation, gamma-centered k-point mesh with reciprocal space resolution of 2π×0.02\u0026nbsp;Å\u003csup\u003e-1\u003c/sup\u003e were chosen.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-2766405/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2766405/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe solid electrolyte interphase (SEI) plays a critical role in determining the performance of lithium batteries, making it an area of increased scrutiny in research. Although the formation mechanism and structural model of SEI have been extensively reported, understanding the Li\u003csup\u003e+\u003c/sup\u003e conduction mechanism within SEI has proven challenging due to its complex structure and the lack of advanced in-situ experimental techniques. In this study, we employed isotopic tracer experiments based on high-resolution \u003csup\u003e6\u003c/sup\u003eLi NMR and TOF-SIMS techniques to illustrate the Li\u003csup\u003e+\u003c/sup\u003e conduction path in the bulk structure of natural SEI. Importantly, in-situ TEM technology was utilized to visualize the Li\u003csup\u003e+\u003c/sup\u003e conduction pathway through specific inorganic components of SEI, both in the bulk and on the surface. The critical experimental evidence provided by our work demonstrates that grain boundaries are not the sole pathway for Li\u003csup\u003e+\u003c/sup\u003e conduction in SEI, which is imperative for optimizing the performance of lithium batteries and designing future SEI.\u003c/p\u003e","manuscriptTitle":"Deciphering the lithium ion conduction mechanism in solid-electrolyte-interphase","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-08-03 15:28:36","doi":"10.21203/rs.3.rs-2766405/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":"60a34038-5a2c-47d3-9de6-5b36734a0f06","owner":[],"postedDate":"August 3rd, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":20706504,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":20706505,"name":"Physical sciences/Materials science"},{"id":20706506,"name":"Physical sciences/Nanoscience and technology"},{"id":20706507,"name":"Physical sciences/Materials science/Techniques and instrumentation"}],"tags":[],"updatedAt":"2024-01-04T17:17:56+00:00","versionOfRecord":[],"versionCreatedAt":"2023-08-03 15:28:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2766405","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2766405","identity":"rs-2766405","version":["v1"]},"buildId":"ApUGefWb6u5IBVtyqm6d5","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}