Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 2564−2571
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*,†,‡,§ †
Graduate School of Informatics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan 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 ∥ Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡
S Supporting Information *
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.
1. INTRODUCTION The 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, © 2018 American Chemical Society
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 Received: November 27, 2017 Revised: January 22, 2018 Published: January 23, 2018 2564
DOI: 10.1021/acs.jpcc.7b11650 J. Phys. Chem. C 2018, 122, 2564−2571
Article
The Journal of Physical Chemistry C Table 1. Numbers of Molecules, Molecular Ratio and Mass Densities in LiFSA/AN Electrolyte Solutions salt concentration [mol L−1] number of molecules AN FSA− Li+ molecular ratio (AN/LiFSA) mass density [g cm−3] a
1.0
2.0
3.0
4.0
5.0
800 50 50 16 (17)a 0.878 (0.89)a
800 105 105 7.3 1.02
800 167 167 4.8 (4.7)a 1.17 (1.14)a
800 240 240 3.3 (3.1)a 1.29 (1.26)a
800 340 340 2.3 (2.1)a 1.42 (1.37)a
The values in parentheses represent the experimental ones in ref 8.
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 section 2, provide the results and discussion in section 3, and state our conclusion in section 4.
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 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) 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
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 are 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 parentheses 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 −0.0125e.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 −2e. Accordingly, the system was neutralized by adding two positive ions (Li+) into the bulk 2565
DOI: 10.1021/acs.jpcc.7b11650 J. Phys. Chem. C 2018, 122, 2564−2571
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The Journal of Physical Chemistry C 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 are shown in the Supporting Information). In the present study, we considered the reaction scheme (Figure 1) consisting of a list 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 field can reasonably reproduce the thermodynamic and transport properties in ionic 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 and 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 nonexperts (especially experimentalists) could
Figure 1. Reaction scheme for the Red Moon simulations (cyan: carbon; red: oxygen; white: hydrogen; yellow: sulfur; green: fluorine).
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 the potential energy changes in gas Table 2. Potential Energy Changes ΔUreac 0 , Free Energies of Reaction ΔEr, Free Energies of Activation ΔEa (In kcal/ mol), and the Values of exp(−βΔEa) of the Present Elementary Chemical Reaction Processes ΔUreac 0
ΔEr
ΔEa
AN + e → AN
44.6
−19.1
AN− + e− → CH3− + CN− AN− + FSA− → AN + FS2O4N− + F− FSA− + e− → FS2O4N− + F−
16.4 8.49
−74.8 −27.5
53.0
−46.6
no barrier 6.17 no barrier no barrier
chemical reaction process −
−
exp(−βΔEa) 1.00 3.22 × 10−5 1.00 1.00
phase ΔUreac 0 , free energies of reaction ΔEr, free energies of activation ΔEa, and the values of exp(−βΔEa)) of the present elementary chemical reaction processes, which are calculated at the M06L/cc-pvtz level of theory. In the Red Moon method, the relative weight values were obtained as the product of exp(−βΔEa)) and the number of candidate pairs at each MC/ MD cycle (the details are shown in the 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 ΔEa, 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 ΔEa (6.17 kcal/mol) was similar to the previously reported one at the 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
Figure 2. Changes in the surface number densities ρSn 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. 2566
DOI: 10.1021/acs.jpcc.7b11650 J. Phys. Chem. C 2018, 122, 2564−2571
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The Journal of Physical Chemistry C Table 3. Mass Densities of Chemical Reaction Products in the SEI Film (in g cm−3) salt concentration [mol L−1] species −
AN CH3− CN− FS2O4N− F−
1.0 0.17 0.037 0.065 0.16 0.019
± ± ± ± ±
2.0 0.02 0.005 0.001 0.01 0.001
0.19 0.036 0.063 0.27 0.032
± ± ± ± ±
3.0 0.02 0.001 0.002 0.01 0.001
0.16 0.036 0.062 0.32 0.037
± ± ± ± ±
4.0 0.03 0.001 0.002 0.02 0.002
0.12 0.037 0.065 0.38 0.045
± ± ± ± ±
5.0 0.02 0.003 0.005 0.02 0.002
0.075 0.037 0.065 0.46 0.054
± ± ± ± ±
0.013 0.003 0.004 0.02 0.003
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).
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 four 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 3b, 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 3c). 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 3d). 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 ρSn, 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
understand them and future theorists might overcome these difficulties. Actually, in the present simulations, some essential approximations, other than those explained in the 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 van der Waals (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 the Supporting Information). 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 the effective screening medium (ESM) method.34 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. 2567
DOI: 10.1021/acs.jpcc.7b11650 J. Phys. Chem. C 2018, 122, 2564−2571
Article
The Journal of Physical Chemistry C 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 (
