Microscopic Formation Mechanism of Solid Electrolyte Interphase Film

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Microscopic Formation Mechanism of Solid Electrolyte Interphase Film in Lithium-Ion Batteries with Highly Concentrated Electrolyte Norio Takenaka, Takuya Fujie, Amine Bouibes, Yuki Yamada, Atsuo Yamada, and Masataka Nagaoka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11650 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Microscopic Formation Mechanism of Solid Electrolyte Interphase Film in Lithium-Ion Batteries with Highly Concentrated Electrolyte

Norio Takenaka1,2, Takuya Fujie1, Amine Bouibes1,3, Yuki Yamada2,4, Atsuo Yamada2,4, Masataka Nagaoka1,2,3* 1

Graduate School of Informatics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan;

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ESICB, Kyoto University, Kyodai Katsura, Nishikyo-ku, Kyoto 615-8520, Japan;

Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Honmachi, Kawaguchi 332-0012, Japan; 4

Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

*Corresponding author. E-mail: [email protected] Tel./fax: +81-52-789-5623 URL: http://www.ncube.human.nagoya-u.ac.jp/

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ABSTRACT: The performance of lithium-ion batteries (LIB) with liquid electrolytes is strongly dependent on a stable solid electrolyte interphase (SEI) film formation on the anode surface. According to recent experiment studies, the use of highly concentrated (HC) electrolyte can be quite useful to improve the battery performance, enhancing the SEI film formation. However, its molecular mechanism remains still unknown. To investigate such film formation mechanism, we performed the atomistic reaction simulations in acetonitrile (AN)-based electrolyte solutions using the Red Moon method (a hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method). The present simulations were able to successfully reproduce the experimental observations where the reaction products produced by the reduction of salts mainly form the SEI film in the HC electrolyte. Further, it was revealed that such stable SEI film can be formed in a stepwise fashion; i) the diffusive transfer of reduction products, ii) the Li salt-based passivation film formation, and iii) the formation of the solvent-based film layer. This new microscopic insight should provide an important guiding principle in designing the most effective electrolytes to develop high-performance LIB with the HC electrolyte.

KEYWORDS: ・Li-ion battery ・Highly concentrated electrolyte ・SEI film ・Hybrid MC/MD reaction method ・Red Moon method


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1. INTRODUCTION Rechargeable Li-ion battery (LIB) is extensively used as popular power sources for providing the portable energy to popular electronic devices and consumer products.1-4 At the same time, the LIB has also been attracting more interest to use in automotive and stationary power applications. To respond to such social demands, therefore, it is necessary to develop the higher performance LIB. The performance of LIB with liquid electrolytes is strongly dependent on the formation of solid electrolyte interphase (SEI) film on the electrode surface, which is formed as a result of reduction or oxidation of the electrolyte.1-4 In particular, a number of reaction products associated with electrolyte reduction form a SEI film on the anode surface during the initial charging cycle in LIB, and this film should realistically protect the electrolyte from further reductive decomposition during the subsequent charge-discharge cycles, while allowing Li+ to exchange between the electrolyte and anode. This characteristic is therefore directly related to the lifetime and safety of LIB. Such battery performance is highly influenced by the selection of electrolyte solvents, salts and additives. Recently, Yamada et al. have reported that the highly concentrated (HC) acetonitrile (AN) electrolyte with the Li salts such as lithium bis(fluorosulfonyl)amide (LiFSA) and lithium bis(trifluoromethanesulfonyl)amid (LiTFSA) shows the strong electrochemical stability against the decomposition of electrolyte on the electrode surface.5-8 According to the experimental analyses, it was clarified that the HC AN electrolyte has two important roles relating to the electrochemical stability. One is to suppress the corrosion protection on the Al current conductor at the positive electrode.8 Al corrosion was remarkably suppressed over the concentration of 5.0 mol L-1 at which all solvent molecules coordinate to Li+ and free solvent molecules are eliminated. Another is to improve the reduction stability so as to enable reversible charge-discharge cycling owing to the formation of stable SEI film on the graphite at the negative electrode. The experimental x-ray photoelectron spectroscopy (XPS) analysis showed that this SEI film is mainly formed by the reductive decomposition products of the Li salts.6,7 Namely, the reduction stability in the HC electrolyte can be attributed to the formation of a Li salt-based SEI film on the graphite anode.5-7


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To reveal the microscopic mechanism of SEI film formation in LIB, the computational chemical investigations are indispensable as complementary approaches for experimental analyses. Some computational chemical studies have provided the important insight into the elementary reaction processes and initial stage in the whole SEI film formation process.1,9-17 In fact, the ab initio molecular dynamics (AIMD) method is useful to identify the elementary reactions associated with the SEI film formation of LIB.13-17 Sodeyama et al. have applied the AIMD method to the LiTFSA/AN electrolyte, and demonstrated that TFSA- anions sacrificially accept the electron in the HC electrolyte, which is ascribed to the formation of specific network structure and the resulting shift of electron affinity of the TFSA- anions.6,17 This sacrificial anion reduction should hinder the two electron reductive decomposition of AN, leading to improved electrochemical stability on the anode surface.17 While quantum chemical analyses provide microscopically indispensable information on the elementary chemical reactions, to understand the long-term characteristic of the SEI film produced by the successive complex chemical reaction processes, more long-term atomistic reaction simulations must be done, considering a necessary and sufficient set of multiple elementary reaction processes simultaneously. To treat such complicated phenomena, we have recently proposed a new atomistic reaction simulation method, which is a Red Moon method (a hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method).18-22 The main objective of applying Red Moon method here is to investigate the microscopic insights into the SEI film formation such as the aggregation and dissolution of reaction products near the electrode surface. In fact, this method was successfully applied so far to elucidate the microscopic mechanism of several secondary battery systems and theoretically provided the spatial structures of SEI films that cannot be easily observed in conventional experimental investigations.19,20 Further, it was shown that the fluoroethylene carbonate (FEC) additive improves the sodium-ion battery performance by making SEI film surface smooth while the intact FEC molecules enhance the aggregation of organic products.20 The validity of these theoretical elucidations was confirmed recently by a systematic experimental study with soft x-ray photoelectron spectroscopy (SOXPES) ACS Paragon Plus Environment

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measurement,23 where the electrochemical decomposition of PC solvent is concluded to be suppressed before the FEC reduction, indicating that the intact FEC molecules really suppress the unstable film growth. It can be said therefore that the Red Moon method has a high predictive accuracy and is so quite useful in understanding the microscopic mechanism concerning with the SEI film formation in secondary batteries. In this article, to investigate the microscopic origin of the salt concentration effect on SEI film formation in LIB, we performed atomistic reaction simulations on the graphite anode in the HC LiFSA/AN electrolyte solutions using the Red Moon method. First, we investigated the SEI film formation processes in the HC electrolyte, and then obtained the mass density distributions of the resulting SEI films. Next, we estimated the salt concentration effect on the SEI film formation by comparing the simulation results with the different salt concentrations. Finally, we proposed a new and illuminating insight into the microscopic mechanism of the SEI film formation in this HC electrolyte solution. We explain both the computational model systems and computational details in Sec. 2, provide the results and discussion in Sec. 3 and state our conclusion in Sec. 4.


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2. METHODS 2.1. Model Systems. By reference to the previous experimental studies,5-8 the present atomistic reaction simulations were executed in the LiFSA/AN electrolyte solutions on the carbon anode by using five different salt concentrations of 1.0, 2.0, 3.0, 4.0 and 5.0 mol L-1. To investigate the bulk properties of electrolyte, we performed the MD calculations with the NPT ensemble at 1 atm. The number of molecules, molecular ratio and mass densities were summarized in Table 1. As a result, the molecular ratios and mass densities were similar to the experimental values in the same electrolyte solution8 (The values are shown in parenthesis in Table 1). By combining the electrolyte with the carbon anode, the simulation model system was prepared for the Red Moon method. A vacuum region was assumed adjacent to the electrolyte region according to previous computational treatments.19,20,24 The carbon anode was simply modeled with a graphite-like structure, and was fixed during the Red Moon simulations, restricting the migration of Li+ to the inside of the anode to focus on only the SEI film formation.19,20 To describe the electric potential difference between the electrolyte and anode, the simulations were performed with a negatively charged carbon anode.19,20,24-26 These negative charges were uniformly distributed over the carbon atoms on the graphite surface in contact with the electrolyte. With reference to the previous charging simulation in LIB, the negative charge per one carbon atom on the anode surface was set to -0.0125 e.19,20,25 Since the number of carbon atoms on the anode surface was 160 in the present model system, the sum of negative charges became -2 e. Accordingly, the system was neutralized by adding two positive ions (Li+) into the bulk electrolyte solution. The present simulation cell was 3.41 nm × 3.70 nm × 30.0 nm.

2.2. Computational Details. To investigate the SEI film formation on the anode surface in LIB, we employed the Red Moon method18-22 that repeats a combined cycle consisting of a set of MC and MD treatments, which is hereafter called the “MC/MD cycle” (the details of Red Moon method is shown in Supporting ACS Paragon Plus Environment

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Information). In the present study, we considered the reaction scheme (Figure 1) consisting of a list of elementary reaction processes necessary and sufficient to simulate the SEI film formation. These processes were introduced based on both previous experimental and theoretical studies.6,17 Table 2 lists reac the potential energy changes in gas phase ∆U0 , free energies of reaction ∆Er, free energies of

activation ∆Ea and the values of exp(-β∆Ea)) of the present elementary chemical reaction processes, which are calculated at M06L/cc-pvtz level of theory. In the Red Moon method, the relative weight values were obtained as the product of exp(-β∆Ea)) and the number of candidate pairs at each MC/MD cycle (The detail is shown in Supporting Information). The present calculation level was determined by reference to the previous quantum chemical study on the structures and energies of the Li-FSA cluster model.27 By executing the systematic analyses to obtain ∆Ea, it was found that there is the activation barrier for the reductive decomposition of AN- anion with one coordinated Li+ (i.e. (Li-AN)-1 → Li-CN + CH3-). The calculated value of ∆Ea (6.17 kcal/mol) was similar to the previously reported one at B3LYP/cc-pvdz level of theory by Sodeyama et al. (around 7 kcal/mol).17 To estimate the free energies, the SMD method28 was employed with the dielectric constant of 35.688 which corresponds to that of the AN solvent, executing the normal frequency analysis. As a result, the AN solvent was stable without decomposition even if it accepts one electron from the anode as reported in the previous AIMD studies.6,17 By receiving an extra electron, the AN- anion can be decomposed into the CN- and CH3- anions with the activation barrier of 6.17 kcal/mol (see Table 2). As with the TFSA- anion,6,17 the FSA- anion can be preferentially reduced in comparison to the AN solvent because its reduction energy becomes large (-46.6 kcal/mol) than that of the AN solvent (-19.1 kcal/mol) in solution (see Table 2). To treat such preferential reduction of FSA- anion, the present simulations were executed by considering the electron transfer from AN- anion to FSA- anion as a constituent chemical process of the whole reaction scheme for the Red Moon method (Figure 1). The generalized AMBER force field (GAFF)29 was used for the electrolyte and reaction products because this force filed can reasonably reproduce the thermodynamic and transport properties in ionic ACS Paragon Plus Environment

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liquids without modifications.30 The atomic point charges were obtained with the RESP method at the M06L/cc-pvtz level of theory. The SANDER module of the AMBER 9 package was used to execute all the atomistic simulations.31 The SHAKE method was used to constrain the hydrogen-heavy atom bond distances and the integration time-step was 1 fs. The temperature was maintained at 298 K with a Berendsen thermostat. Then, a production run of the Red Moon simulation was executed for 2000 MC/MD cycles. The MD simulation time for relaxation in one MC/MD cycle for a trial chemical reaction was set to 10 ps. Whenever any reactant molecules were reduced, a Li+ cation was additionally placed randomly in the electrolyte solution to neutralize the whole system.19,20 To check the initial structure dependence, the present simulations were performed by using a set of 10 diﬀerent initial conﬁgurations, and those standard errors were estimated with the two-sided 95 % confidence interval (Figures 2,4-6, Table 3).

2.3. About approximations in the present simulations of SEI film formation. It is true that the theoretical modeling of SEI formation is extremely difficult and some approximations are almost certainly necessary. It would be, therefore, most important to spell out the modeling limitations so that the non-experts (especially experimentalists) could understand them, and future theorists might overcome these difficulties. Actually, in the present simulations, some essential approximations, other than those explained in Supporting Information, were employed in order to reduce drastically the computational cost. First, we assumed that the reduction reactions should occur when the solvent molecules (or salt anions) close to the anode surface within the sum of vdW radii (rc). However, the electron transfer (ET) rates, in fact, should further depend on the molecular orientation and the distance from the electrode surface as described in Marcus theory.13,32,33 To know numerically the effect of the distance-dependent ET rate on the SEI film formation simulations, a number of extra Red Moon simulations were executed by considering the exponentially-decaying electron tunneling probability depending on the distance from the anode surface (see section 2 in Supporting Information).


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Second, the treatment of voltage effect is considered not enough in the present simulations because the electrode charges should be redistributed so as to reproduce the electrode potential, depending on which the SEI film formation might proceed. One plausible method to estimate them is to resort to the AIMD calculations under the constant voltage, such as effective screening medium (ESM) method34. Although the first approximation was examined to show qualitatively the same characteristics to the component ratio of reaction products in the SEI film (see Supporting Information), the influence of the second one should be discussed more precisely and, therefore, is now being studied carefully by our group, especially in modeling the SEI film formation process.


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3. RESULTS AND DISCUSSION 3.1. Formation Process of the SEI Film in HC Electrolyte Solution We investigated the SEI film formation in the HC LiFSA/AN electrolyte at 5.0 mol L-1 by using the Red Moon method because such HC electrolyte shows the good cycle performance in experiment.8 During the Red Moon simulations, the reaction products associated with the reduction of both AN solvents and FSA- anions were found to aggregate so as to form the passivation film on the anode surface. Figure 3 shows 4 typical snapshots indicating the changes in the aggregation states of the reaction products from the initial state (0 MC/MD cycle) to final one (2000 MC/MD cycle). As shown in Figure 3(b), the FS2O4N- anions (green) can be produced apart from the anode surface as a result of the reduction of FSA- anions. On the other hand, the AN- anions (yellow) without decomposition were continually produced (see Figure 3(c)). By consuming them, the CN- and CH3- anions (pink and red) were produced in the vicinity of the anode surface. Finally, the stable film can be formed on the anode surface (see Figure 3(d)). Accordingly, AN-, CH3-, CN-, FS2O4N- and F- anions would be expected to form the SEI film, which can prevent the electrolyte molecules from approaching the anode surface. Figure 2 shows the changes in the number per unit area of the carbon anode surface, which is the surface number density ρ nS , of the electrolyte solvents, salts and reaction products obtained by averaging over 10 different initial configurations at the salt concentration of 5.0 mol L-1, where the abscissa is taken as the number of MC/MD cycles in the Red Moon simulation. Accompanying the consumption of the solvent AN molecules (blue curves), the numbers of AN- anions (yellow curves) increased during the SEI film formation process. At the same time, both FS2O4N- and F- anions (green curves) were produced through the decomposition of FSA- anions. Then, the CN- and CH3- anions were successively produced by consuming the AN- anions. Finally, the composition ratio of the molecular mixture of the reaction products was nearly unchanged in the final stage of the formation process. During the initial SEI film formation (< 500 MC/MD cycles), the production rate of the CN- and CH3anions were suppressed due to the relatively smaller weight for the reaction AN－ + e- → CH3- + CNACS Paragon Plus Environment

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than those of the others, meaning that the FSA- anions can be preferentially reduced in the HC electrolyte. To investigate the microscopic structure of SEI film, we executed 10 sets of 1 ns MD simulations with the stable equilibrium structures (at 2000 MC/MD cycles), and estimated the mass density distributions of reaction products. Figure 4 shows the time-averaged mass density distributions ρ m of the AN solvents, FSA salts and SEI films consisting of AN-, CH3-, CN-, F- and FS2O4N- anions. In fact, the electrolyte molecules were found to be blocked by the produced SEI film so that their number gradually decrease as they get closer to the anode surface.19,20 In the resulting SEI films, it was found that the CH3- and CN- anions are present near the anode surface, while the FS2O4N- anions are mainly distributed in the outer region of the SEI film. In particular, the peak height of FS2O4N- anions was found to be higher than other products. It is considered therefore that they mainly form the passivation film because they are directly contact with the electrolyte so as to prevent the electrolyte molecules in bulk electrolyte from approaching the anode surface. According to the XPS analyses, it was observed that the main SEI film components are the reaction products of the salts containing –SO2– residue.6,7 Therefore, the present simulation results were clearly consistent with the experimental observations in the HC electrolyte.

3.2 Effect of Salt Concentration on SEI Film Formation To understand the effect of salt concentration on the SEI film formation, we investigated the SEI film structures at the 5 different salt concentrations of 1.0, 2.0, 3.0, 4.0 and 5.0 mol L-1 by executing Red Moon simulations, and compared them. Figure 5 shows the mass density distributions of stable reaction products obtained by using the different salt concentrations, respectively. Among their species, the ratio of FS2O4N- anions were largest at all the salt concentrations, meaning that they were main component in the SEI film of this electrolyte solution. Further, it was found that the peak height of


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FS2O4N- anions in the SEI film significantly increases as the salt concentration increases. Accordingly, such structural difference must be important effect of the salt concentration on the SEI film formation. Then, we defined the film thickness by using the position from the anode surface at which the mass density distribution of the SEI film becomes less than 1 % of the maximum value in the outer region. As a result, the values of film thickness were estimated to be 4.0, 4.0, 4.0, 3.8, and 3.4 nm at 1.0, 2.0, 3.0, 4.0 and 5.0 molL-1, respectively, which are the order of the magnitudes similar to that obtained in experimental measurement of SEI films in LIB.1-3,35,36 Namely, their simulation results clearly showed that the film thickness decreases as the salt concentration increases. This is considered due to the slower diffusion of salt-derived reaction products in the HC electrolytes. This tendency is consistent with some experimental observations in LIBs with the HC electrolytes.37-39 However, it is worth mentioning the fact that the absolute values of film thickness should depend also on the selection of electrode potential (electrode charges), temperature and simulation time, etc., in addition to the salt concentration. The mass densities of the reaction products inside the SEI film were estimated and shown in Table 3. It was found that the mass densities of solvent-based reaction products such as CH3- and CN- are almost the same at all the salt concentrations. This is because that their production depends on only the contact area of the anode surface because they are formed through the two electron reduction on the anode. In contrast, the mass densities of Li salt-based reaction products such as FS2O4N- and F- clearly increased as the salt concentration increases. This indicates that the difference in the salt concentration is directly related to the mass densities of Li salt-based reaction products in the SEI film. By summing the mass densities of all the SEI film components, the total mass densities of SEI film were found to be 0.452, 0.597, 0.602, 0.650 and 0.686 gcm-3 at the salt concentration of 1.0, 2.0, 3.0, 4.0 and 5.0 mol L-1, respectively. This clearly shows that the SEI film becomes dense in the HC electrolyte. Such dense film could be more stable in comparison to those at the lower salt concentrations. It is considered therefore that the lifetime of the battery with the HC electrolyte is improved by suppressing the breakdown of the SEI film during the charge-discharge cycle. In conclusion, it was clarified that the increase in the Li salt in the electrolyte solution makes the SEI film dense and stable in the HC electrolyte. ACS Paragon Plus Environment

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3.3 Microscopic Formation Mechanism of SEI Film in HC LiFSA/AN Electrolyte To clarify the microscopic formation mechanism of SEI film in HC LiFSA/AN electrolyte, we investigated the reduction mechanism of FSA salts depending on the salt concentration. Figure 6 shows the number-of-times distribution ρ nv of the first reduction events of FSA- anions that are counted as frequency per volume and plotted along z axis. In fact, one certain FSA- anion should be reduced for the first time somewhere during each Red Moon simulation at the 5 different salt concentrations of 1.0, 2.0, 3.0, 4.0 and 5.0 mol L-1. It is natural that the reduction frequency of FSA- anions increase as the salt concentration increases. In particular, it is remarkable that the amount of the FSA- anions reduced near the anode surface (z < 1.0 nm) increases rather more frequently in the HC electrolyte (see the purple curve in Figure 6). In fact, the present simulation results were consistent with the theoretical prediction based on the AIMD calculations where the Li salts can be preferentially reduced in the HC electrolyte owing to the larger electron affinity.6,17 Thus, the increase in the reduction products of FSA- anions produced near the anode surface should lead to the denser Li salt-based SEI film in this HC electrolyte. By summarizing the present simulation results, the SEI film can be formed in the stepwise manner during the whole formation process, i.e., a) initial, b) intermediate and c) final stages (see Figure 7). In the initial stage a) of the SEI film formation process, both the AN solvents (blue) and FSA salts (orange) should be reduced on the anode surface. In the HC electrolyte, the FSA salts can be preferentially reduced in comparison to AN solvent molecules. Then, they should move toward the bulk electrolyte because of the strong electrostatic repulsive interaction between the reduction product anions and the anode surface. In fact, as shown in the previous AIMD study,40 the anionic species produced on the anode surface should be dissolved into the bulk electrolyte due to the unstable adhesion to the graphite surface. As a result, their diffusive transfer must take a supportive role to realize the “near-shore mechanism” where the SEI film components become apart from anode surface and dissolved into the bulk electrolyte and form aggregates.40 Next, in the intermediate stage b), the reductive decomposition products of FSA- anions such as FS2O4N- should form aggregates apart from anode surface. As a result, ACS Paragon Plus Environment

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the Li salt-based SEI film can be formed so as to protect the electrolyte molecules from the further reduction. However, this Li salt-based passivation film may be insufficient to block completely AN solvent molecules because the molecular size of Li salt decomposed-products such as LiF2O4N might be somewhat larger than that of AN molecule. Actually, the AN solvent molecules were found to considerably penetrate inside the Li salt-based SEI film in the present simulations (see the dark blue curve in Figure 4). In the final stage c) of SEI film formation process (Figure 7), the remaining AN- anions between the anode surface and the transient Li salt-based SEI film layer reach enough close to the anode surface so as to be able to be further reduced by accepting an extra electron from the anode. As a result, the solvent-based passivation film can be formed (bright yellow in Figure 7) closest to the anode surface via the remarkable decomposition of AN solvent molecules. The two electron reduction products of AN solvent such as the CN- anions were actually observed in the experimental XPS analysis.6,7 This interior film layer should be important to completely protect the AN solvent molecules from their reduction (see Figure 4). It is through this stepwise formation processes that the stable SEI film can be formed in the HC LiFSA/AN electrolyte solution. If this stepwise mechanism of the SEI film formation is controllable, it must provide some important key factors to design the optimum film formation in the HC electrolyte in order to improve the lifetime of the secondary battery. In the LiFSA/AN electrolyte solution, the FSA- anions can be reduced during the SEI film formation, also in the bulk electrolyte apart from the anode surface by accepting one extra electron from the dissolved AN- anion (see Figure 6). Namely, the AN- anions can behave as the carriers of electrons into the FSA- anions in the bulk electrolyte during the SEI film formation and are contributing to increase the stability of AN- anions against the decomposition. Consequently, these lead to the larger collision frequency between the AN- anions and FSA- anions in the HC electrolyte to enhance the formation of the Li salt-based SEI film. In contrast, in some common electrolyte solutions including ethylene carbonate (EC) and LiPF6 salts,1-4 EC solvent is well known to be easily decomposed by accepting an electron from the anode.1,2,9-11 As a result, the effect of salt ACS Paragon Plus Environment

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concentration on the SEI film formation should relatively decrease in the common EC-based electrolyte. It is concluded, therefore, that the optimum combination of solvents and Li salts must be important to improve the SEI film formation in the HC electrolyte.

4. CONCLUSIONS In this study, to investigate the microscopic mechanism of the salt concentration effect on the SEI film formation, we applied the Red Moon methodology to the LiFSA/AN electrolyte solution on the graphite anode, and found that the Li salt-based reaction products such as LiFS2O4N mainly form the passivation film on the anode surface in the HC electrolyte. This is clearly consistent with the experimental observations. In addition, it was found that the SEI film becomes denser as the salt concentration increases. Further, the present simulation results suggest that the SEI film is formed in the stepwise manner in the HC LiFSA/AN electrolyte, i.e., the Li salt-based passivation film formation is followed by that of the solvent-based film. It should be crucial to control this stepwise mechanism of SEI film formation to reasonably optimize the film properties in the HC electrolyte. In fact, in this electrolyte solution, it is significant that the AN- anions give their electrons into the FSA- anions in the bulk electrolyte so as to enhance the formation of Li salt-based passivation film. Thus, not only the increase in the salt concentration but also the optimum combination of solvents and Li salts must be important to improve the SEI film formation in the HC electrolyte. In conclusion, the present new microscopic insight on the salt concentration effect should provide some important guiding principles in designing the most suitable electrolytes and controlling the aggregate structure of SEI films to develop high-performance LIB with the HC electrolyte.


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AUTHOR INFORMATION Notes

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science Technology Agency (JST); by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) in Japan; and also by the MEXT programs “Elements Strategy Initiative for Catalysts and Batteries (ESICB)” and “Priority Issue on Post-K computer” (Development of new fundamental technologies for high-efficiency energy creation, conversion/storage and use). The calculations were partially performed using several computing systems at the Information Technology Center in Nagoya University.


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Supporting Information The details of Red Moon method, the discussion on the effect of ET rate depending on the distance from electrode, Figure S1 showing the curve of ET probability, and Figure S2 showing the mass density distributions in the extra simulations. This material is available free of charge via the Internet at http://pubs.acs.org.


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(11) Wang, Y.; Balbuena, P. B. Theoretical Insights into the Reductive Decompositions of Propylene Carbonate and Vinylene Carbonate: Density Functional Theory Studies. J. Phys. Chem. B 2002, 106, 4486–4495. (12) Purushotham, U.; Takenaka, N.; Nagaoka, M. Additive Effect of Fluoroethylene and Difluoroethylene Carbonates for the Solid Electrolyte Interphase Film Formation in Sodium-Ion Batteries: A Quantum Chemical Study. RCS Advances 2016, 6, 65232-65242. (13) Leung, K.; Qi, Y.; Zavadil, K. R.; Jung, Y. S., Dillon, A. C.; Cavanagh, A. S.; Lee, S.-H.; George, S. M. Using Atomic Layer Decomposition to Hinder Solvent Decomposition in Lithium Ion Batteries: First-Principle Modeling and Experimental Studies. J. Am. Chem. Soc. 2011, 133, 14741– 14754. (14) Ganesh, P.; Kent, P. R. C.; Jiang, D. Solid-Electrolyte Interphase Formation and Electrolyte Reduction at Li-Ion Battery Graphite Anodes: Insights from First-Principles Molecular Dynamics. J. Phys. Chem. C 2012, 116, 24476–24481. (15) Ushirogata, K.; Sodeyama, K.; Okuno, Y.; Tateyama, Y. Additive Effect on Reductive Decomposition and Binding of Carbonate Based Solvent toward Solid Electrolyte Interphase Formation in Lithium-Ion Battery. J. Am. Chem. Soc. 2013, 135, 11967–11974. (16) Okuno, Y.; Ushirogata, K.; Sodeyama, K.; Tateyama, Y. Decomposition of the Fluoroethylene Carbonate Additive and the Glue Effect of Lithium Fluoride Products for the Solid Electrolyte Interphase: An Ab Initio Study. Phys. Chem. Chem. Phys. 2016, 18, 8643-8653. (17) Sodeyama, K.; Yamada, Y.; Aikawa, K.; Yamada, A.; Tateyama, Y. Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte. J. Phys. Chem. C 2014, 118, 14091-14097. (18) Nagaoka, M.; Suzuki, Y.; Okamoto, T.; Takenaka, N. A Hybrid MC/MD Reaction Method with Rare Event-Driving Mechanism: Atomistic Realization of 2-Chlorobutane Racemization Process in DMF Solution. Chem. Phys. Lett. 2013, 583, 80–86. (19) Takenaka, N.; Suzuki, Y.; Sakai, H.; Nagaoka, M. On Electrolyte-Dependent Formation of Solid ACS Paragon Plus Environment

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Electrolyte Interphase Film in Lithium-Ion Batteries: Strong Sensitivity to Small Structural Difference of Electrolyte Molecules. J. Phys. Chem. C 2014, 118, 10874-10882. (20) Takenaka, N.; Sakai, H.; Suzuki, Y.; Uppula, P.; U. Nagaoka, M. A Computational Chemical Insight into Microscopic Additive Effect on Solid Electrolyte Interphase Film Formation in Sodium-Ion Batteries: Suppression of Unstable Film Growth by Intact Fluoroethylene Carbonate. J. Phys. Chem. C 2015, 119, 18046-18055. (21) Suzuki, Y.; Koyano, Y.; Nagaoka, M. Influence of Monomer Mixing Ratio on Membrane Nanostructure in Interfacial Polycondensation: Application of Hybrid MC/MD Reaction Method with Minimum Bond Convention. J. Phys. Chem. B 2015, 119, 6776-6785. (22) Suzuki, Y.; Nagaoka, M. A Transformation Theory of Stochastic Evolution in Red Moon Methodology to Time Evolution of Chemical Reaction Process in the Full Atomistic System. J. Chem. Phys. 2017, 146, 204102. (23) Dahbi, M.; Nakano, T.; Yabuuchi, N.; Fujimura, S.; Chihara, K.; Kubota, K.; Son, J.-Y.; Cui, Y.-T. ; Oji, H.; Komaba, S. Effect of Hexafluorophosphate and Fluoroethylene Carbonate on Electrochemical Performance and the Surface Layer of Hard Carbon for Sodium-Ion Batteries, Chem. Electro. Chem. 2016, 3, 1856-1867. (24) Kislenko, S. A.; Samoylov, I. S.; Amorov, R. H. Molecular Dynamics Simulation of the Electrochemical Interphase between a Graphite Surface and the Ionic Liquid [BMIM][PF6]. Phys. Chem. Chem. Phys. 2009, 11, 5584–5590. (25) Hamad, I. A.; Novotny, M. A.; Wipf, D. O.; Rikvold, P. A. A New Battery-Charging Method Suggested by Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2010, 12, 2740-2743. (26) Jorn, R.; Kumar, R.; Abraham, D. P.; Voth, G. A. Atomistic Modeling of the Electrode−Electrolyte Interface in Li-Ion Energy Storage Systems: Electrolyte Structuring. J. Phys. Chem. C 2013, 117, 3747-3761.


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(27) Charles, W.; Bauschlicher, Jr.; J. B. Haskins; Eric, W. B.; John, W. L.; Oleg, B. Structure and Energetics of Li+–(BF4–)n, Li+–(FSI–)n, and Li+–(TFSI–)n: Ab Initio and Polarizable Force Field Approaches. J. Phys. Chem. B 2014, 118, 10785-10794. (28) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378-6396. (29) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of General Amber Force Field. J. Comp. Chem. 2004, 25, 1157–1174. (30) Sprenger, K. G.; Jaeger, V. W.; Pfaendtner, J. The General AMBER Force Field (GAFF) Can Accurately Predict Thermodynamic and Transport Properties of Many Ionic Liquids. J. Phys. Chem. B 2015, 119, 5882-5895. (31) Case, D. A. et al. AMBER 9; University of California: San Francisco, 2006. (32) Victoria, A. N.; Sergey, A. K.; Renat, R. N.; Michael, D. B.; Galina, A. T. Ferrocene/Ferrocenium Redox Couple at Au(111)/Ionic Liquid and Au(111)/Acetonitrile Interfaces: A Molecular-Level View at the Elementary Act. J. Phys. Chem. C 2014, 118, 6151-6164. (33) Samuel, A. D.; Oleg, B.; Marco, O.; Claire, G. E.; Joshua, L.; Allen, T.; Richard, J. Importance of Reduction and Oxidation Stability of High Voltage Electrolytes and Additives. Electrochimica Acta, 2016, 209, 498-510. (34) Otani, M.; Sugino, O. First-Principles Calculations of Charged Surfaces and Interfaces: A Plane-Wave Nonrepeated Slab Approach. Phys. Rev. B 2006, 73, 115407. (35) Peled, E.; Golodnisky, D.; Ardel, G. Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid and Polymer Electrolytes. J. Electrochem. Soc. 1997, 144, L208-L210. (36) Lu, P.; Li, C.; Schneider, E. W.; Harris, S. J. Chemistry, Impedance, and Morphology Evolution in Solid Electrolyte Interphase Films during Formation in Lithium Ion Batteries. J. Phys. Chem. C 2014, 118, 896-903. ACS Paragon Plus Environment

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TOC GRAPHICS


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FIGURES CAPTIONS

Figure 1. Reaction scheme for the Red Moon simulations (cyan: carbon, red: oxygen, white: hydrogen, yellow: sulfur, green: fluorine).

Figure 2. Changes in the surface number densities ρ nS of the solvent molecules (right axis) and reaction products and salts (left axis) during the SEI film formation processes with respect to the MC/MD cycles in the atomistic reaction simulations in the highly concentrated LiFSA/AN electrolyte solutions at 5.0 mol L-1.

Figure 3. Typical snapshots of SEI film formation processes in the highly concentrated LiFSA/AN electrolyte solutions at 5.0 mol L-1 (cyan: carbon, red: oxygen, white: hydrogen, yellow: sulfur, green: fluorine, blue: Li+). Here, the solvent AN molecules and FSA salts are depicted by a stick model, while Li+ cations and reaction products are depicted by a ball model (full vdW radii).

Figure 4. Mass density distributions ρm of the solvents, salts and reaction products in the atomistic reaction simulations in the highly concentrated LiFSA/AN electrolyte solutions at 5.0 mol L-1.

Figure 5. Mass density distributions ρm of the reaction products in the LiFSA/AN electrolyte solution at (a) 1.0 mol L-1, (b) 2.0 mol L-1, (c) 3.0 mol L-1, (d) 4.0 mol L-1 and (e) 5.0 mol L-1. Figure 6. Number-of-times distribution ρ nv of the first reduction events of FSA- anions that are counted as frequency per volume and plotted along z axis.

Figure 7. Schematic illustration of stepwise SEI film formation mechanism in the highly concentrated LiFSA/AN electrolyte solution.


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FIGURES

Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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TABLES

Table 1. Numbers of molecules, molecular ratio and mass densities in LiFSA/AN electrolyte solutions -1

Number of molecules

AN FSA+

Li Molecular ratio (AN/LiFSA)

-3

Mass density [gcm ] a

1.0 800 50 50

Salt concentration [mol L ] 2.0 3.0 4.0 800 800 800 105 167 240 105 167 240

5.0 800 340 340

4.8 (4.7)a

2.3 (2.1)a

16 (17)a

7.3

0.878 (0.89)a

1.02

3.3 (3.1)a

1.17 (1.14)a 1.29 (1.26)a 1.42 (1.37)a

The values in parentheses represent the experimental ones in Ref. (8).


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Table 2. Potential energy changes

, free energies of reaction ∆Er, free energies of activation

∆Ea (in kcal/mol) and the values of exp(-β∆Ea) of the present elementary chemical reaction processes Chemical reaction process AN + e- → AN-

44.6

-19.1

No barrier

1.00

AN－ + e- → CH3- + CN-

16.4

-74.8

6.17

3.22×10-5

AN- + FSA- → AN + FS2O4N- + F-

8.49

-27.5

No barrier

1.00

FSA- + e- → FS2O4N- + F-

53.0

-46.6

No barrier

1.00


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Table 3. Mass densities of chemical reaction products in the SEI film (in gcm-3) -1

Species -

AN CH3CNFS2O4NF-

1.0 0.17± 0.02 0.037 ± 0.005 0.065 ± 0.001 0.16 ± 0.01 0.019 ± 0.001

Salt concentration [mol L ] 2.0 3.0 4.0 0.19 ± 0.02 0.16 ± 0.03 0.12 ± 0.02 0.036 ± 0.001 0.036 ± 0.001 0.037 ± 0.003 0.063 ± 0.002 0.062 ± 0.002 0.065 ± 0.005 0.27 ± 0.01 0.32 ± 0.02 0.38 ± 0.02 0.032 ± 0.001 0.037 ± 0.002 0.045 ± 0.002

5.0 0.075 ± 0.013 0.037 ± 0.003 0.065 ± 0.004 0.46 ± 0.02 0.054 ± 0.003


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Microscopic Formation Mechanism of Solid Electrolyte Interphase Film

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