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Kuan-Yu Lin, Rui-Tong Kuo, [Tsuyoshi Miyazaki](https://orcid.org/0000-0003-3534-4404), Bing Joe Hwang, Jyh-Chiang Jiang

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[Unveiling dendrite-suppressing potential of alkali metal-based alloys in lithium metal batteries](https://mdr.nims.go.jp/datasets/861884ca-92ba-41f1-88b0-799e89ad3068)

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Template for Electronic Submission to ACS JournalsUnveiling Dendrite-Suppressing Potential of Alkali Metal-Based Alloys in Lithium Metal BatteriesKuan-Yu Lin,a,b Rui-Tong Kuo,a Tsuyoshi Miyazaki,b Bing Joe Hwang,c,d,e and Jyh-Chiang Jiang*,aa Computational Chemistry Lab, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwanb Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japanc Nano-electrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwand Sustainable Electrochemical Energy Development Center, National Taiwan University of Science and Technology, Taipei 106, Taiwane National Synchrotron Radiation Research Center (NSRRC), Hsin-chu 30076, TaiwanCorresponding Author*Email: jcjiang@mail.ntust.edu.tw (J.-C. J.)ABSTRACTWe reveal a concept of adding K and Na to dual-cation systems and investigate their effects on dendrite growth morphology and electrolyte decomposition reactivity in lithium metal batteries (LMBs). We analyze the effects of Li/K- and Li/Na-alloyed surfaces on Li ion diffusion along the deposition direction, as well as the reactivity of the electrolytes and the composition of the solid electrolyte interface (SEI) resulting from electrolyte decomposition. Compared with the Li/K system, the Li/Na-alloyed surface can significantly reduce Li diffusion along the z-direction and effectively prevent the formation of dendrite-like morphologies. Furthermore, the Li/Na-alloyed surface substantially mitigates the reactivity of the bis(fluorosulfonyl)imide (FSI–) anion and fluorinated ether (TTE) solvent, thus inhibiting the generation of excessive SEI species. Additionally, the regulated and simplified components of a hybrid LiF/NaF SEI layer in the Li/Na system are observed. This hybrid layer is expected to promote the uniform deposition of Li and exhibit excellent electrical insulating properties, thereby effectively preventing electron transfer to the electrolytes and enhancing the Coulombic efficiency of LMBs. This study presents new insights into the dendrite-suppressing potential of alkali-metal alloys for improving the stability and safety of LMBs.KeywordsLithium metal battery, Li-Na alloy, Lithium dendrites, Density function theory; Molecular dynamics simulation1. Introduction As the demand for energy storage in portable electronics and electric vehicles continues to increase, interest in Li metal batteries (LMBs) as a solution to fulfill market requirements is increasing accordingly [1-4]. Metallic Li is being extensively investigated as a promising candidate for anodes owing to its ultrahigh theoretical specific capacity (3860 mAh g−1) and low redox potential (−3.04 V vs. SHE, the standard hydrogen electrode) [5-7]. However, the utilization of Li anodes presents several challenges, including irreversible interfacial side reactions that result in an unstable solid electrolyte interface (SEI) and the uncontrolled growth of Li dendrites. The ongoing growth of dendrites during the repetitive charging and discharging process depletes active Li, thus reducing the cycle lifespan and posing significant safety risks in the use of LMBs [5-8]. Hence, researchers have attempted to devise various mitigation strategies, such as incorporating alloyed metal anodes [9-14], applying artificial coatings [15, 16], and utilizing dual-cation electrolytes [17-22]. Among these approaches, dual-cation electrolytes have actively garnered attention in the field of LMBs in recent years. Xu et al. introduced an innovative concept known as the self-healing electrostatic shield (SHES) mechanism, which has been proven to enhance Li deposition [19]. Their results showed that the addition of trace amounts of co-salts based on Cs+ or Rb+ cations into Li-based electrolytes can effectively suppress the formation of dendrites. Ma et al. revealed that the incorporation of Na+ and Li+ cations resulted in the formation of thin layers of Li/Na alloy on the anode surface throughout the charging and discharging cycles [23]. This Li/Na alloy maintained a uniform diffusion field around the electrode surface, which is required for a flat electrodeposition and a dendrite-free morphology. Therefore, the synergistic effect of incorporating alkali metal cations with Li+ might significantly contribute to the long-term stability of LMBs, particularly in inhibiting dendrite formation. Despite the utilization of various alkaline-based alloys or dual-cation electrolytes to improve electrochemical deposition and prevent Li dendrite formation [9, 16-18, 23-25], fundamental studies investigating the synergistic effects of the alloyed anode surface on the reactivity of electrolytes and the formation of the SEI layer are insufficient. Moreover, during several cycles of Li charging and discharging, alloyed surfaces with varying concentrations are formed, and the effect of the concentration on the electrolyte reactivity and dendrite formation remains unknown. Therefore, this study investigates the intricate interfacial reactions between the electrolytes and alloyed anode surface under various concentrations of incorporated alkali metals. Anode-free rechargeable lithium metal batteries (AFLMBs) are also gaining significant attention due to their potential to overcome the limitations associated with traditional lithium-metal anodes in LMBs [26-29]. AFLMBs, which consist of a current collector, electrolytes, and cathode without an active anode system, offer advantages such as reduced cell weight and space, leading to improved gravimetric and volumetric energy densities, as well as reduced anode production costs. Nonetheless, challenges such as large nucleation and diffusion overpotentials, inadequate solid electrolyte interface characteristics, non-uniform Li deposition, and severe dendrite formation, collectively lead to unsatisfactory electrochemical properties for AFLMBs [28, 30, 31]. It is known that the composition of SEI components is recognized for its significant impact on the quality of Li deposition and the formation of Li dendrites. Poor SEI properties, characterized by an abundance of SEI species and a high-density presence of grain boundaries, contribute to non-uniform Li deposition and continual electrolyte consumption, leading to the accumulation of substantial amounts of inactive Li and adversely affect the coulomb efficiency [32-35]. Among these factors, the sequential development of an effective SEI layer is regarded as a crucial key for addressing the issues of non-uniform Li deposition and the formation of Li dendrites. Many experimental studies have demonstrated the utilization of dual-cation systems to enhance and improve the electrochemical deposition of Li in AFLMBs [15, 36-38]. Notably, Yu et al. demonstrated that the integration of a dual-cation system involving Mg+2 ions can significantly enhance the quality of Li deposition [15]. This improvement is attributed to the creation of an effective SEI layer formed by a Mg+2-blocking mechanism within AFLMBs. Consequently, similar to conventional LMBs, the stepwise formation of effective SEI layers is considered a pivotal strategy for preventing the Li dendrite formation in LMBs and AFLMBs. As a novel concept for understanding the effects of dual-cation systems, we are resolving the problems above and improving the electrochemical deposition of Li in LMBs and AFLMBs. Hence, in this work, we used density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to investigate Li dendrite growth and SEI layer formation by incorporating pristine Li, Li/K- and Li/Na-alloyed surfaces onto a Cu collector with Li-based electrolytes. Moreover, we explore the impact of varying concentrations of Li/K- and Li/Na-alloyed surfaces on electrolyte decomposition and SEI layer formation. Herein, we adopt the dual cation concentrations of [1M Li+ – 0.15M K+/Na+] and [1M Li+ – 0.5M K+/Na+] to denote the concentration of Li/K- and Li/Na-alloyed surfaces, respectively, which correspond to 13.8% and 33.3%.  This theoretical study will give some guidance to the effects of Li/K- and Li/Na-alloyed surfaces on the SEI formation mechanism and the stability of the SEI layer in LMBs and AFLMBs.2. Computational methods2.1.  Pristine Li and Li/K and Li/Na alloyed surfacesAll density functional theory (DFT) calculations in this study were performed using the Vienna Ab initio Simulation Package (VASP) with the projector-augmented wave (PAW) method proposed by Perdew–Burke–Ernzerhof (PBE) for the exchange–correlation energy functional [39-41]. The van der Waals (vdW) D3 correction proposed by Grimme was considered to include the vdW correction [42]. The energy cutoff for the plane-wave basis expansion was employed to be 400 eV, and the Monkhorst–Pack scheme was used for k-point sampling and Brillouin zone integration. We considered (5x5x5) and (3x3x1) k-point mesh for Cu and Li bulk and their surface optimization, respectively. The energy and force convergence criteria for structural optimization were set to 10-4 eV and 10-2 eV/Å, respectively. A Li and Cu bulk was optimized first. Then, the Li (100) and Cu (100) crystallographic planes were cleaved from the optimized bulk structure as previous XRD calculation results pointed out that the Li (100) and Cu (100) facets are the preferred orientations. Herein, we constructed five layers of Cu (100) surface in a p(44) supercell as a Cu collector model, in which the bottom layer was replaced with helium atoms to prevent interactions between the neighboring slabs. Then, we constructed a pristine Li surface by incorporating three Li (100) layers onto the Cu (100) surface, followed by adding a 25 Å vacuum layer. Furthermore, we constructed low (13.8%) and high (33.8%) concentrations of K and Na atoms on the Li/K- and Li/Na-alloyed anode surfaces, respectively, by depositing them onto the Cu (100) surface as shown in Fig. S1a. The number of constituent atoms is provided in Fig. S1b. The deposited pristine and Li/K- and Li/Na-alloyed layers on the Cu (100) surface were considered by AIMD simulations to reach equilibrium. The AIMD simulations were equilibrated in a canonical ensemble (NVT) at 400 K, spanning a total duration of 5 ps with a time step of 1 fs. The 3x3x1 Monkhorst-Pack mesh is considered in all MD simulations.2.2. Solvents-Salts stability on the anode surface The considered Li salt (LiFSI), and the solvent molecules (DME and TTE) underwent initial optimization in a cubic box with 15 Å sides and carried out a Γ-point calculation. The optimized structures of both the Li salt and solvents are depicted in Fig. S2. Afterward, the optimized geometries of molecules were further utilized to construct the electrolyte cell using PACKMOL software [43]. The electrolyte cell was composed of 1.5 M LiFSI in a DME/TTE) mixture (2:3 v/v%), and the numbers in the simulation cell were chosen, corresponding to densities of 0.86 and 1.53 g cm−3 for DME and TTE, respectively. Subsequently, these optimized molecular geometries were used to construct the electrolyte cell, establishing contact with the anode surfaces. In addition, the convergence criterion for electronic self-consistent iterations was set to 10−4 eV. Modeled structures of pristine Li, Li/K, and Li/Na systems in the presence of electrolytes can be observed in Fig. S3. Within a canonical ensemble (NVT) maintained at 400 K, the AIMD simulations underwent equilibration, extending over a duration of 20 ps and utilizing a time step of 1 fs. Bader charge analysis was used to estimate the charge transfer between the electrolyte and the anode surface. Z-direction trajectory analyses of the anode atoms from the AIMD simulations were used to explore the reactivity between the electrolyte and the surface.3. Results and discussion3.1.  Effects of Li/K and Li/Na alloys on Li dendrite growthTo investigate the impacts of Li/K and Li/Na alloys on Li dendrite growth and SEI layer formation, we have conducted AIMD simulations using a canonical ensemble (NVT) to equilibrate the system at 400 K. The simulations were performed for 20 ps at time intervals of 1 fs, which allowed us to analyze the Li trajectories of the pristine Li, Li/K, and Li/Na systems. Fig. 1a shows the trajectories of Li for the pristine system and the systems with 13.8% and 33.3% K and Na. In the pristine system, Li preferentially migrated along the z-direction, resulting in the formation of a dendrite-like morphology and a nonuniform Li surface, as shown in the AIMD simulation. To further analyze the Li migration, we plotted the displacement of Li atoms along the z-axis relative to their initial positions as a function of time, as shown in Fig. 1b. The plot shows that numerous Li atoms exhibited significant displacement in the z-direction in the pristine Li system during the MD simulation. Notably, one of Li atoms indicated a maximum displacement of 9 Å. Such a large displacement of Li atoms in the z-direction implies an unstable Li surface for Li plating, which may favor Li dendrite formation. However, when we considered a dual-cation system, both K+ and Na+ cations demonstrated a synergetic effect in retarding Li migration along the z-direction, thus further preventing dendrite growth. In the Li/K system, a lower concentration of K (13.8%) exhibited better performance in inhibiting the z-direction diffusion of Li atoms. The maximum displacement in the z-direction reduced from 9 Å to approximately 3 Å after the simulation was performed for 20 ps. Nevertheless, a higher concentration of K (33.3%) resulted in a less satisfactory inhibition of Li diffusion compared with the 13.8% Li/K system. It can be observed that the Li diffusion was more active, and the maximum displacement in the z-direction was approximately 4 Å. In the Li/Na system, a lower Na concentration (13.8 %) exhibited comparable results to the Li/K system with 13.8% K concentration in inhibiting Li diffusion. However, in the Li/Na system with a higher concentration (33.3%), the presence of alloyed Na atoms contributed more significantly in inhibiting Li diffusion. The maximum displacement in the z-direction was less than 2 Å. This suggests that a higher concentration of Na in the dual-cation system may effectively hinder z-direction diffusion and contribute to the better stability of the deposited Li surface, thus significantly eliminating the formation of Li dendrites.3.2.  Electrolyte decomposition and SEI layer formationAfterwards, we examined the interactions of the electrolytes in contact with the anode surface in the pristine Li, Li/K, and Li/Na systems to obtain insights into the reactivity and potential chemical reactions that may occur at the electrolyte/Li interface. Table 1 summarizes the potential chemical reactions and decomposed fragments observed for the LiFSI salt and TTE solvent in different systems under various simulation times. Tables S1 and S2 provide a comprehensive list of the decomposed reactions of LiFSI and TTE, respectively. The results of 20 ps MD simulations indicate that the pristine Li and Li/K, as well as Li/Na alloyed surfaces, do not exhibit any decomposition of DME solvent. Accordingly, the subsequent discussion will focus on the decomposition of LiFSI and TTE. LiFSI exhibited high reactivity on the pristine Li surface in the pristine system. The FSI– anion decomposed within a simulation time of 70 fs, thus resulting in the cleavage of the S-F bond of LiFSI and the formation of LiF fragments. When the simulation time was extended to 85, 400, 700, and 1300 fs, the FSI– anion underwent further decomposition, thus resulting in the subsequent cleavage of the N-S and S-O bonds of the FSI– anion and the formation of Li2O, Li2S, and Li3N fragments. On the other hand, the TTE solvent exhibited slower decomposition compared to the FSI– anion, with the initial bond breakage occurring at 850 fs. Observations revealed that the C-O bond breaks at 900 fs, followed by the subsequent cleavage of the F-C bond between 1430 and 3800 fs. This series of decomposition led to the formation of alkoxide-like lithium complexes (LiCHCO). The LiF, Li2O, Li2S, Li3N, and LiCHCO fragments are the main components of the SEI layer, as shown in Fig. 2a. However, too many species within the SEI layer can be detrimental to the stability of the Li anode and may fail to prevent dendrite growth effectively. Too many SEI species have been reported to cause the formation of high-density grain boundaries [33, 34, 44, 45]. These grain boundaries can serve as preferential paths for Li diffusion, thus providing additional sites for Li deposition and promoting dendrite growth. This observation resembled the formation of a dendrite-like morphology in the pristine system, as depicted in Fig. 1a. Therefore, the composition of the SEI layer must be controlled closely to prevent the excessive formation of grain boundaries and mitigate the risk of dendrite formation. In the Li/K system, we found that adding a low concentration of K (13.8%) effectively suppressed the reactivity of LiFSI and limited the formation of decomposed fragments. At this lower K concentration, the composition of the decomposed fragments was simplified and comprised mainly LiF and KF. However, the effort to reduce the reactivity of LiFSI by introducing a high concentration of K (33.3%) does not produce substantial effects. Moreover, both low and high concentrations of Li/K alloy fail to suppress the decomposition of the TTE solvent. Therefore, under a high concentration of K, the system continues to generate a substantial number of SEI species similar to the pristine system, causing high-density grain boundaries in the SEI layer. Interestingly, in the Li/Na system, a higher concentration of Na (33.3%) resulted in a controlled and simplified SEI layer composition with NaF and LiF fragments. The incorporation of a higher concentration of Na can significantly mitigate the reactivity of FSI– anion and TTE solvent, consequently curtailing the ensuing decomposition process. The regulated and simplified SEI components could potentially prevent the excessive development of grain boundaries and mitigate the possibility of dendrite formation.Additionally, experimental studies demonstrated that the formation of a LiF/NaF hybrid SEI layer resulted in a dendrite-free structure and improved the mechanical strength of the SEI layer [24]. Moreover, we comprehensively analyzed the interfacial structures following electrolyte decomposition, as shown in Fig. 2 in which the SEI layer can be classified into two distinct regions: an inner and outer SEI layer. In the pristine Li system, a denser and thinner inner SEI layer was formed on the anode surface, whereas the outer SEI layer was less dense and broader. This implies that the inner SEI layer hinders uniform Li deposition along the x–y plane, and the less dense outer SEI layer could potentially facilitate dendrite growth. In terms of the Li/K and Li/Na systems, Figs. 2b–d shows that the inner SEI layer underwent minimal SEI component formation, whereas the outer SEI layer became much more concentrated and denser than the pristine system. The inner SEI layer offered sufficient space for Li deposition along the x–y plane on the anode surface, and the denser and more concentrated outer SEI layer allowed excellent mechanical strength to be maintained. We speculate that this SEI morphology is crucial in facilitating Li-ion deposition and inhibiting dendrite formation. Among these dual-cation systems, the addition of Na, regardless of its concentration, demonstrated superior performance in creating a larger space for Li deposition and forming a highly concentrated outer SEI layer to effectively safeguard against Li dendrite growth compared with the Li/K system. Hence, we can infer that the incorporation of Na into a dual-cation system is a promising option for LMBs.3.3.  Charge analysis during SEI layer formationTo obtain a more comprehensive understanding of the effect of dual cations on the reactivity of electrolytes and their decomposition on pristine Li, Li/K, and Li/Na surfaces, we analyzed the Bader charge for each component in the MD simulations, as depicted in Fig. 3. In the pristine Li system (Fig. 3a), the Li anode surface underwent an electron loss of approximately 25 electrons, demonstrating the high reactivity of Li anode. Additionally, the investigation revealed that the FSI– anion exhibited a more rapid electron gain and a higher electron count than the TTE solvent. In the Li/K systems (Fig. 3b–c), including K effectively migrated electron loss from the Li anode. The magnitude of the charge loss decreased to approximately 15 electrons in both 13.8% and 33.3 % Li/K systems. Compared to the pristine system, the FSI– anion in both Li/K systems exhibited lower rapid electron gain and a reduced electron amount. Notably, the number of electron transfers to the FSI- anion in the 13.8% Li/K system was lower than in the 33.3% Li/K system. This suggested that a lower concentration of K (13.8%) could effectively suppress the electron transfer and further decomposition of FSI- anion. Consequently, the composition of the decomposed fragments was simplified and primarily comprised LiF and KF, which is consistent with our findings in Table 1.In the Li/Na systems (Figs. 3d–e), adding Na effectively reduced electron transfer to the FSI- anion and minimized electron loss to the TTE solvent. Compared to the pristine and Li/K systems, the number and rate of electron transfer to FSI- anion and TTE solvent can be markedly reduced in the Li/Na systems. Specifically, we noted that including a higher concentration of Na (33.3%) led to a substantial decrease in electron loss to less than 10 electrons. This suggests that the incorporation of Li/Na-alloyed surfaces could serve as an effective strategy to suppress further decomposition of LiFSI and TTE electrolytes in LMBs. This suppression is likely due to the formation of NaF components within the SEI layer, which serve as barriers that impede further electron transfer to the electrolyte. Additionally, we examined the effect of an exceptional concentration of Na (50%) on inhibiting Li diffusion. The results are summarized in Figs. S4 and S5. This concentration of Na (50%) resulted in less effective inhibition of Li diffusion compared to the 13.8% and 33.3% Li/Na systems, as shown in Fig. S4. Notably, Li diffusion was more active, with the maximum displacement in the z-direction reaching up to 6 Å. Fig. S5 demonstrated excessive Na leads to minimal electron transfer and very limited decomposition of the FSI- anion. The decreased formation of LiF and NaF makes constructing an effective SEI layer challenging, limiting its ability to further inhibit Li diffusion. Therefore, the concentration of Li/Na alloyed surfaces becomes a significant factor in suppressing Li diffusion and preventing Li dendrite formation. Based on these findings, the Li/Na-alloyed surface exerted a more pronounced effect in hindering electron transfer and enhancing the electrical insulation of the SEI layer at an appropriate concentration.3.4.  Evaluation of Li ion diffusivity through outer and inner SEI layersAfter analyzing the effects of dual-cation systems on Li dendrite growth and the evolution of specific components within the SEI layer, we investigated the Li ion diffusivity across the outer to inner SEI layers. The diffusivity of Li ions between the outer and inner SEI layers significantly affects Li deposition on the anode surface. Therefore, we investigated Li ion diffusion in the pristine Li system as well as in the Li/K and Li/Na systems. The diffusion pathways and energy profiles are shown in Fig. 4. It is noticed that the energy barrier to Li ion diffusion from the outer to the inner SEI layer in the pristine system was approximately 1.4 eV. However, in the 13.8% Li/K system, the energy battier to Li ion diffusion was drastically reduced to 0.2 eV, thus indicating substantial improvement in Li ion diffusivity between the interfaces. In contrast, 33.3% Li/K system leads to an increase in the diffusion barrier, reaching approximately 1.8 eV. On the other hand, the incorporation of the Li/Na alloy, regardless of its concentration, effectively reduced the Li ion diffusion barrier energy. Meanwhile, both forward and backward Li diffusion in the presence of the Li/Na alloy were kinetically favorable compared with those in the pristine Li and Li/K systems. In particular, as shown in Fig. 4e, the inclusion of a concentration of Na (33.3%) demonstrated a diffusion barrier that was equally thermally and kinetically balanced in both forward and backward directions. This indicates that the presence of 33.3% Li/Na alloy facilitates the plating/stripping of Li ions and increases the number of recycling cycles during the charging and discharging processes, thus highlighting the superiority of the Li/Na alloy over the pristine Li and Li/K surfaces.4. ConclusionsIn summary, we obtained compelling evidence regarding the effect of Li/K and Li/Na dual-cation systems on various aspects, such as dendrite growth and the beneficial components and properties of the SEI layer in LMBs. Based on our AIMD simulations, we conclude that the formation of the Li/Na alloy in the dual-cation system is more competitive than that of the Li/K alloy because of its better inhibition of the formation of dendrite-like morphologies. Moreover, the Li/Na system exhibits the formation of a LiF/NaF hybrid SEI layer at an appropriate concentration. This hybrid SEI layer limited the reactivity of the LiFSI, TTE, and DME electrolytes, thereby inhibiting further electrolyte decomposition during cycling. This characteristic not only minimizes electrolyte consumption, but also enhances the Coulombic efficiency of LMBs. In addition, our investigation reveals the presence of a substantial space between the inner and outer SEI layers in the Li/Na system, which facilitates uniform Li deposition. More importantly, Li ion diffusion through these two layers was kinetically more favorable. This implies that Li plating/stripping in this Li/Na system is less challenging than that in the pristine Li and Li/K systems. In general, our present study provides a fundamental understanding of the effects of dual cations on dendrite growth and provides valuable guidelines for the design of future dual-cation electrolytes to pursuit safe and high-performance LMBs and AFLMBs.Appendix A. Supplementary dataSupplementary data to this article can be found.NotesThe authors declare no competing interests.Acknowledgments The authors gratefully acknowledge the financial support from the National Science and Technology Council (NSTC) (112-2923-M-011-003-MY2, 111-2926-I-011-501-G, 111-2923-E-011-001, and 111-2639-E-011-001-ASP) and the Ministry of Education of Taiwan (the Sustainable Electrochemical Energy Development Center (SEED) from the Featured Areas Research Center Program). The authors are grateful to the National Center for High-Performance Computing (NCHC) for the use of computers and facilities.References[1] M. Fichtner, et al., Rechargeable Batteries of the Future-The State of the Art from a BATTERY 2030+ Perspective, Adv. Energy Mater. 12 (2022) 2102904. https://doi.org/10.1002/aenm.202102904[2] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294. https://doi.org/10.1038/nature11475[3] D.-H. 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Xiao, Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries, Nano Lett. 16 (2016) 2011. https://doi.org/10.1021/acs.nanolett.5b05283Fig. 1. (a) Trajectories of Li, K, and Na atoms in pristine Li and low (13.8%)/high (33.3%) concentrations of Li/K and Li/Na systems after 20 ps of AIMD simulation. (b) Displacement of all Li atoms (gray line) in each system along z-axis relative to their initial positions as a function of time. Red line indicates maximum displacement of Li atom at the end of AIMD simulation.Fig. 2. Final structures of electrolyte decomposition and SEI formation on (a) pristine Li surface, (b) 13.8% Li/K alloyed surface, (c) 33.3% Li/K alloyed surface, (d) 13.8% Li/Na alloyed surface, and (e) 33.3% Li/K alloyed surface after 20 ps of AIMD simulation.Fig. 3. Total charge evolution of anode surface and electrolytes for (a) pristine Li, (b) 13.8% Li/K, (c) 33.3% Li/K, (d) 13.8% Li/Na, and (e) 33.3% Li/Na systems. Positive and negative values represent electron loss and gain, respectively.Fig. 4. Illustrations of Li diffusion pathway from outer SEI to inner SEI layer and the calculated energy profile along coordinates 1 to 5 in (a) pristine Li, (b) 13.8% Li/K, (c) 33.3% Li/K, (d) 13.8% Li/Na, and (e) 33.3% Li/Na systems.2Table 1. Comparison of decomposition time, types of bond cleavage, and formed components of SEI during LiFSI and TTE decomposition on pristine Li, Li/K, and Li/Na alloyed surface after the 20 ps of AIMD simulation. Systems LiFSI TTE Main components of SEI  Time (fs) Bond Cleavage Time (fs) Bond Cleavage  Pristine 70 S-F 850 C-F LiF, Li2O, Li2S, Li3N, LiCHCO  85 N-S 900 C-O   400 S-O 1430~1680 C-F   700 S-O 3050~3800 C-F   1300 S-O    13.8% K 100 S-F 1200 C-F LiF, KF    4300 C-O   600 S-F 6800~7100 C-F     11500 C-F  33.3% K 70 S-F 3800 C-F、C-O LiF, KF, Li2O, Li2S, LiCHFCFO  85 N-S 11200 C-F   150 S-F 13000 C-F   3600 S-O 18600 C-F  13.8% Na 60 S-F 11500 C-F LiF, NaF, Li2O, Li2S  100 S-F, N-S     3200 S-O     3500 S-O    33.3% Na 70 S-F 19850 C-F LiF, NaF11image1.tiffimage2.pngimage3.emf  LiFSI     TTE     DME     Anode  0 5 10 15 20-20-100102030 (c) 33.3% K (b) 13.8% K (a) PristineQ (|e-|)5 10 15 20(e) 33.3% Na(d) 13.8% Na   5 10 15 20  Time step (ps)5 10 15 20  5 10 15 20   image4.png