A Computational Chemical Insight into Microscopic ... - ACS Publications

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The Journal of Physical Chemistry

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

Norio Takenaka,1,2 Hirofumi Sakai,1 Yuichi Suzuki,1 Purushotham Uppula,1,3 Masataka Nagaoka1,2,3* 1

Graduate School of Information Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; 2

3

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;

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

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ABSTRACT: Sodium (Na)-ion batteries (NIB) have recently attracted special attention as a substitute for Li-ion batteries (LIB) due to the increase in the cost of Li. The performance of NIB with liquid electrolytes, e.g., propylene carbonate (PC), is strongly dependent on a stable solid electrolyte interphase (SEI) film on the anode surface. According to a recent experiment, fluoroethylene carbonate (FEC) can be an efficient electrolyte additive to improve the SEI film formation in NIB. However, the molecular mechanism of such additive effect remains unknown. To investigate this mechanism, atomistic reaction simulations in PC-based electrolyte with and without FEC additives were performed using the hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method and, successfully reproduced experimental observations such as the smaller irreversible capacity and the smoother SEI film in FEC-added electrolyte. Further, this study showed for the first time that intact FEC molecules can improve SEI film formation so as to enhance the network formation of organic species owing to the large electronegativity of their fluorine atoms. This new microscopic insight may provide an important guiding principle for use in designing the most suitable electrolytes for developing high-performance NIB.

KEYWORDS: ・Na-ion battery ・Irreversible capacity ・SEI film ・FEC ・Hybrid MC/MD reaction method

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1. INTRODUCTION Rechargeable Li-ion batteries (LIB) play an extremely important role supporting modern industry and our daily life as the power sources for various electronic devices.1-4 However, since the lithium (Li) element in LIB is a rare metal and its cost has increased with the growth in demand, there is concern that it will become more difficult to stably supply in the future. To avoid problems resulting from the short supply of Li, sodium (Na)-ion batteries (NIB) are recently attracting attention as a replacement for LIB, since the Na element is very abundant all over the world.5-9 In order to develop NIB for practical use with performance comparable with that of LIB, it is essential to optimize materials such as the electrode, electrolyte, separator and additive. The performance of secondary batteries with liquid electrolytes such as propylene carbonate (PC) 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 thin SEI film on the anode surface during the initial charging cycle in NIB, and this thin film should realistically protect the electrolyte from further reductive decomposition during the subsequent cycles, while allowing Na+ to exchange between the electrolyte and anode.5-9 This characteristic is therefore directly related to the cycle performance and safety of NIB. According to a recent experimental investigation, fluoroethylene carbonate (FEC) may be an effective additive for NIB with a PC-based electrolyte since it decreases the irreversible capacity as a result of the improvement of the SEI film formation.6 There have been many experimental studies on the additive effect of FEC molecules in both LIB and NIB.6,10-17 A common characteristic was found to be that the addition of FEC can improve cycle performance by decreasing the irreversible capacity associated with SEI film formation. Using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the SEI film was found to become smoother in FEC-added electrolyte.10,12-17 Additionally, the experimental XPS spectra showed that the F 1s peak assigned to sodium (or lithium) fluoride (NaF (or LiF)) becomes higher as the amount of added FEC increases.6,11,13-15,17 Therefore, the production of NaF (or LiF) complexes from ACS Paragon Plus Environment

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FEC is thought to affect SEI film formation in FEC-added electrolyte. However, it is difficult to identify the microscopic origins of this additive effect solely through experimental investigations since there is still no direct technique to observe the transient processes of the SEI film formation on the anode surface. To reveal the microscopic mechanism of SEI film formation in secondary batteries such as NIB, computational simulation is a useful methodology. It has been common for the microscopic reaction processes associated with SEI film formation to be investigated with quantum chemical (QC) calculations1,18-22,27 and ab initio molecular dynamics (AIMD) simulations23-28. Among such computational studies are some theoretical reports on the reductive reaction mechanism of the solvent PC and FEC molecules.1,20,27 The reduced PC molecule undergoes a ring-opening reaction and yields sodium propyl carbonate (NaPC) by binding with Na+. Then, they form organic oligomers such as 2,3-dimethylsodium butylene dicarbonate, which is Na2DMBDC as a result of combination reactions between two NaPC molecules via radical polymerization.1,20,22 Inorganic salts such as sodium carbonate (Na2CO3) are also formed via two-electron (2e) reductions of PC molecules on the anode surface.1,20,24-26 Such NaPC and Na2CO3 complexes were actually detected in experimental observations of NIB.5,6 On the other hand, there are some possible one-electron (1e) and two-electron (2e) reduction reaction pathways for the FEC molecule.27 Among these pathways, F-, CO2 and C2H3O molecules are predicted to be the most probable singly reduced products from FEC since the free energies of activation are smaller than those of other reaction pathways. Then F- yields sodium fluoride (NaF) by binding Na+, which is plainly observed in the FEC-added electrolyte of NIB.6 It is true that these theoretical studies provide microscopically indispensable information on the elementary chemical reactions associated with SEI film formation. However, to understand why SEI film formation as a whole is improved by adding FECs, it is essential to investigate their complex effects on SEI film formation while simultaneously considering the multiple elementary reaction processes. It is well-known that naïve application of traditional molecular simulations gives restricted results when investigating long-term properties that depend on rare events such as chemical reactions. ACS Paragon Plus Environment

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Under the circumstances, we have recently proposed a new atomistic reaction simulation method, which is a hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method (or Red Moon method), in order to deal with the long-term characteristics of the sub-micro scale structure produced by successive complex chemical reaction processes.29-31 It must be worth noting a characteristic contrast between first principles methods and the hybrid MC/MD reaction method. The AIMD method has been mainly applied to investigate the initial stage of SEI film formation, focusing a few reduction reactions of electrolyte solvents and lithium salts on the anode surface.24-28 Further, it should be essential to know the transport properties of ions between the electrode and the electrolyte since the methods based on classical force fields cannot treat transient changes in electronic states of the electrode.32 Such first principles studies are quite useful to investigate their microscopic processes associated with SEI film formation. In comparison, the hybrid MC/MD reaction method can simulate, at the same time, a number of complex chemical reaction processes associated with SEI film formation at several hundred times larger and several tens of thousands times longer space-time scale, considering in advance a necessary and sufficient set of elementary reaction processes. Its major goal is to microscopically elucidate dynamic effects to the heterogeneous structures on SEI film formation such as aggregation and dissolution of reduction reaction products near the anode surface. In fact, the hybrid MC/MD reaction method has been successfully applied to LIB and theoretically provided the spatial structures of SEI films that cannot be easily observed in conventional experimental investigations,30 leading to a compatible atomistic picture to microscopically solve “EC-PC mystery”:3 Despite a close structural similarity between EC and PC, it was unclear why no effective SEI film is formed in PC-based electrolyte and how such a small structural difference (a methyl) could result in two different electrochemical extremes.1-3 The hybrid MC/MD reaction method has theoretically shown at the microscopic level that the small difference between EC and PC becomes really the cause to prevent reaction products from their efficiently packing.3 In this article, to investigate the microscopic origin of the FEC additive effect on SEI film formation in NIB, we performed atomistic reaction simulations on the carbon anode in PC-based NaPF6 ACS Paragon Plus Environment

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electrolytes without and with FEC molecules using the hybrid MC/MD reaction method. First, we investigated the number of changes in the reaction products during SEI film formation processes on the anode surface, and then obtained the mass density distributions in the resulting SEI films. Finally, based on the results, we suggest a new and illuminating insight into the microscopic FEC additive effect. 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. The present atomistic reaction simulations were executed in 1.1 mol/L NaPF6 PC-based electrolyte on the carbon anode without and with 10 vol % FEC on the basis of the experimental conditions.6 Figure 1 shows the model simulation system for the hybrid MC/MD reaction simulations. A vacuum region is assumed adjacent to the electrolyte region according to previous computational treatments.30,33 As with the previous hybrid MC/MD reaction simulations in LIB, the carbon anode was simply modeled with a graphite-like structure, and was fixed during the hybrid MC/MD reaction simulations, restricting the migration of Na+ to the inside of the anode.30 This was done in order to focus only on the microscopic difference in the SEI film formation on the anode surface between the PC-based electrolyte solutions with and without FEC molecules, assuming virtually no intercalation of Na+. To describe the electric potential difference between the electrolyte and anode, the simulations were performed with a negatively charged carbon anode.30,34 These negative charges were uniformly distributed over the carbon atoms on the graphite electrode surface in contact with the electrolyte.30,33-35 The negative charge per one carbon atom on the anode surface was set to -0.0125e with reference to the previous charging simulation in LIB.34 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 (Na+) into the bulk electrolyte solution. As a result, there are 850 PC molecules, 87 Na+ cations and 85 PF6- anions in a rectangular simulation cell with linear dimensions 3.41 nm × 3.70 nm × 30.0 nm. In the FEC-added electrolyte solution, there are 118 FEC molecules, with the number adjusted so that the partial volume of FECs becomes 10 % of the total volume of the whole electrolyte solution. In the following sections, the FEC-free and FEC-added systems are called “System I” and “System II,” respectively.

2.2. Computational Details. To investigate the SEI film formation on the anode surface in NIB, we employed a hybrid MC/MD ACS Paragon Plus Environment

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reaction method3,29-31 that repeats a combined cycle consisting of a set of MC and MD treatments, which is hereafter called the “MC/MD cycle” (the detail of hybrid MC/MD reaction method is shown in Supporting Information). The former treatment manages such a rare event stochastically as a chemical reaction, while the latter simply performs a common dynamical process in the stable phase space to naturally search for candidate molecular configurations for reaction occurrences. By repeating these MC/MD cycles, we can simulate complex chemical reaction processes such as SEI film formation, including a succession of multiple elementary reaction processes.29-31 In the method, the relative weight of selection R of one chemical reaction among all the possible reactions is approximated in proportion to the product of the exponential factor Ra ~ exp(-βEa), where Ea is its energy of activation, and the number Ncp of candidate pairs (i.e., the concentration) which are configurationally possible to generate the chemical reaction, strongly depending on the number of the reactant molecules in each MC/MD cycle. It is convenient that R can be reasonably obtained according to the instantaneous concentration of the chemical species near the anode surface during SEI film formation. In the present study, we considered the reaction scheme (Figure 2) 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 experiments1-3,5,6 and theoretical studies18-27,30. We considered only a reductive decomposition of FEC that leads to the production of NaF, CO2 and C2H3O molecules,27 although there are other possible chemical reactions such as polymerization.27,28 This is because a principal purpose of this study is to clarify the effects of NaF production from FEC reduction reactions on SEI film formation in FEC-added electrolyte. In fact, such NaF complexes are considered in experimental investigations to be a principal reaction product associated with FEC reduction reactions.6,11,13-15,17 The generalized AMBER force field (GAFF)36 was used for the electrolyte and reaction products, -

with the exclusion of the PF6 anion. The GAFF force field can reasonably reproduce the thermodynamic and transport properties in ionic liquids without modifications.37 The force field of the PF6- anion was obtained with reference to the previous theoretical study.38 The atomic point charges are ACS Paragon Plus Environment

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obtained with the RESP method at the B3LYP/6-31+G(d) level of theory. In this case, the atomistic charges of the electrolyte solvent PC molecules and FEC additives were legitimately scaled (scaling parameter 0.8) so as to reproduce the experimental dipole moments, mass densities and diffusion constants2,3,39. This treatment is very similar to those in the previous ionic liquid simulations.37,40,41 In fact, the mass density of the pure PC solvent itself was obtained to be 1.21 g/cm3 by the NPT-MD simulations at 1 atm, which was in quite good agreement with the experimental value 1.21 g/cm3.2,3 In addition, the calculated diffusion constant at 298 K was 3.7×10-10 cm2/s in pure PC solvent, which is in good agreement with the experimental value of 4.9×10-10 cm2/s.39 The SANDER module of the AMBER 9 package42 was used to execute all the atomistic simulations. 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. The cutoff distance for the Lennard-Jones (LJ) interactions was set to 12 Å. The electrostatic interactions were treated by the particle-mesh Ewald (PME) method. Then, a production run of the hybrid MC/MD reaction 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 4 ps. Whenever any reactant molecules were reduced, the same number of Na+ cations as that of injected electrons was additionally placed randomly in the electrolyte solution to neutralize the whole system.30 To check the initial structure dependence, the present simulations were performed by using a set of 10 diﬀerent initial conﬁgurations, the standard errors of which were estimated with the two-sided 95 % confidence interval (Figures 3,5-7,9). We first extracted 10 configurations with an interval of 1 ns for 10 ns equilibrium MD simulation at 700 K, and then obtained a set of 10 different initial configurations by relaxing each configuration with a 1 ns MD simulation at 298 K.


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3. RESULTS AND DISCUSSION 3.1. Formation process of the SEI films—Decrease in irreversible capacity with addition of FEC. We first examined the formation processes of the SEI film on the anode surface in NIB via hybrid MC/MD reaction simulations. Figure 3 shows the changes in the number per unit area of the carbon anode surface, which is the surface number density ρ nS , of the reaction products obtained by averaging over 10 different initial configurations without (System I) and with FECs (System II), where the abscissa is taken as the number of MC/MD cycles in the present atomistic reaction simulation. In both systems, accompanying the consumption of the solvent PC molecules (blue curves), the numbers of NaPC complexes (light green curves) increased during the SEI film formation process. Then, the Na2CO3 complexes (red curves) and Na2DMBDC dimers (green curves) were successively produced by consuming the NaPC complexes previously formed. In System II (Figure 3(b)), not only PF6- anions but also the FEC molecules (light blue curves) were reduced to form NaF complexes (purple curves). Finally, the composition ratio of the molecular mixture of the reaction products was nearly unchanged in the final stage of the formation process. Compared with the changes in ρ nS of the reaction products between System I and II, the production of Na2DMBDC dimers (green curves) was found to decrease considerably in System II. On the contrary, the production of NaF complexes (purple curves) increased in System II. It is reasonable to assume that such NaF complexes were mainly formed by the reductive decomposition of FEC molecules. Although the difference between the added number of FEC molecules and that of PF6- anions was small (i.e., 118 vs. 85), the PF6- anions were scarcely reduced during the formation process. This is because it is very difficult for PF6- anions to get closer to the surface of the negatively charged carbon anode. However, it should be noted that the electron tunneling regime is not considered in the present simulations when the reactant molecules are reduced in the vicinity of the anode surface. Actually, the reaction frequency of PF6- anions should increase by including the electron tunneling regime.


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Next, we estimated the MC/MD cycles for the convergence of the number density curves in the solvent PC molecules (blue curves) by using the criteria of 0.1 [nm-2] between the previous and present MC/MD cycles. As a result, the number densities were converged at 960 and 760 MC/MD cycles in System I and II, respectively. Finally, by counting the added electrons in each chemical reaction through the MC/MD simulations until the number densities of PCs converge, we estimated the integrated values of consumed electron numbers per unit area of the carbon anode surface, and obtained the calculated values of 22.8 and 19.7 [e nm-2] in System I and II, respectively. Specifically, by adding FECs, the consumed amount of electrons was found to be reduced by 14 %. In the real cell system, the initial irreversible capacity corresponds to the total amount of electrons to generate the SEI film. Therefore, the present simulations were able to reproduce the experimental observation that the irreversible capacity loss at SEI film formation decreases in FEC-added electrolyte.6

3.2 Atomistic structure of the SEI film— —Suppression of unstable film growth by FEC addition. Next, we investigated the atomistic structures of SEI films formed on the carbon anode surface in NIB via the hybrid MC/MD reaction simulations. Figure 4 shows typical snapshots of the SEI films, gas molecules and electrolytes near the carbon anode surfaces in the stable equilibrium state (~ at 2000 MC/MD cycles) in System I and II, respectively. In both systems, NaPC, Na2DMBDC, Na2CO3 and NaF complexes would be expected to form the SEI film, which can prevent the solvent PC molecules from approaching the anode surface. Additionally, there are some gas molecules such as C3H6 (and CO2 in the FEC-added electrolyte) due to the reduction reactions of PCs (and FECs) near the anode surface. Comparing the calculation results between System I (Figure 4(a)) and System II (Figure 4(b)), it was found that the aggregation of reaction products becomes more compact in System II. Then, to estimate the mass density distributions, we executed 10 sets of 1 ns MD simulations with the stable equilibrium structure (at 2000 MC/MD cycles). Figure 5 shows the time-averaged mass density distributions ρm of the SEI films, gas molecules and electrolyte solvents obtained by the ACS Paragon Plus Environment

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atomistic reaction simulations in System I (Figure 5(a)) and System II (Figure 5(b)). Those SEI films consist of NaPC, Na2DMBDC, Na2CO3 and NaF complexes. It should be noted that ρm of the SEI films are shown by removing their dissolved reaction products that exist in isolation apart from the SEI film. As shown in Figure 5, the distribution curve of SEI film showed a clear peak in both electrolytes, indicating that there is a dense interface structure (1 ≤ z ≤ 2 nm) in the SEI film, which was discussed in our previous study of LIB.30 Due to the presence of such a dense interface structure, the SEI film can prevent solvent molecules from directly contacting the anode surface. Comparing ρm between System I (Figure 5(a)) and System II (Figure 5(b)), it is seen that the mass density distribution of the SEI film became wider in System I. This means that the reaction products in the SEI film were distributed considerably apart from the anode surface. To compare the thicknesses of the SEI films in Systems I and II, 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 of this comparison, the thicknesses were estimated to be 5.7 nm and 3.9 nm in System I and II, respectively, an order of the magnitude similar to that obtained in experimental measurement of SEI films in LIB.1-3,43,44 Specifically, by adding FECs, the thickness was found to be reduced by 32 % within the present approximation. Such drastic change is caused by suppressing the unstable growth of the SEI film.11 On the other hand, the surface structure of the SEI film in contact with the electrolyte became smooth in System II (see the general features of those snapshots in Figures. S1 and S2 in Supporting Information). This is consistent with the feature common in the previous experimental observations in FEC-added electrolyte.10-17. Therefore, such suppression of unstable SEI film growth must be one of the most important effects of FEC additive at the microscopic level. As a result, SEI film can be efficiently formed in FEC-added electrolyte so that the total amount of consumed electrons, which is an irreversible capacity, might decrease.


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3.3. Microscopic mechanism of FEC additive effect—Indirect role of intact FEC molecules during SEI film formation. Although the production of NaF complexes increased with the addition of even a small amount of FECs (see Figure 3(b)), their effects on the SEI film formation was unclear. For the purpose of clarifying their effect, we executed artificial atomistic reaction simulations by assuming non-reactive FEC additives that do not react during SEI film formation. In these simulations, we employed the same initial configurations and calculation model as in the simulations in System II. There was only one difference in that the reaction pathway from FEC to NaF, CO2 and C2H3O was prohibited in the reaction scheme of the hybrid MC/MD reaction simulations. The new system is called “System III.” By comparing the calculation results with the reduction reactions of FECs to those without them, we can clarify the importance of NaF complexes produced from the FEC molecules during SEI film formation. Figure 6 shows the changes in the surface number density ρ nS of reaction products in the SEI films through the hybrid MC/MD reaction simulations in System III. It is a matter of course that the number density of FEC (light blue curve) was constant during the hybrid MC/MD reaction simulations. By limiting the reduction reactions of FECs, the final ρ nS value of the NaF complexes was found to be 0.12 [nm-2], drastically reduced in comparison to that in System II (2.30 [nm-2]). In addition, the composition ratio in the molecular mixture of the reaction products was almost unchanged after the SEI film formation was completed. In this case, the number density of PC solvent molecules converged at 720 MC/MD cycles, and accordingly, the integrated value of consumed electrons per unit area of the carbon anode was obtained to be 17.8 [e nm-2], which is obviously smaller than that in System I, i.e., 22.8 [e nm-2]. This means that the irreversible capacity should decrease due to the lack of NaF complex production if the FEC reduction reactions can be restrained. To investigate the molecular configurations of the reaction products in SEI films, Figure 7 shows the mass density distributions ρ m of NaPC, Na2DMBDC, Na2CO3 and NaF complexes using the three atomistic models, i.e., (a) System I, (b) System II and (c) System III. It should be noted that ρm s of ACS Paragon Plus Environment

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these complexes are shown by removing those fragmental complexes that exist in isolation apart from the SEI film. In the resulting SEI films, it was found that inorganic salts such as Na2CO3 and NaF complexes tend to be closer to the anode surface, while organic salts such as NaPC and Na2DMBDC complexes are mainly in the outer layer. Comparing those ρ m s in the three systems, the amount of NaF complexes (purple curve) clearly increased in System II due to the reduction reactions of FEC. On the other hand, the curves of the other distributions in System III (Figure 7(c)) were found to be similar to those in System II (Figure 7(b)). The SEI film thickness in System III was estimated to be 4.3 nm, which is similar to that in System II (i.e., 3.9 nm). Specifically, the FEC addition must lead to a compact SEI film regardless of whether or not FEC reduction reactions take place. The major difference in ρ m between System II (Figure 7(b)) and III (Figure 7(c)) was the increase in the amount of NaF complexes in System II. As shown in Figure 8, such F- anions (yellow) have a tendency to be located closer to CO32- anions (red) inside the SEI film in System II. To show numerically this tendency, the coordination numbers of F- anions were estimated around O atoms in Na2CO3 and organic salts (NaPC and Na2DMBDC) by using 3.4 Å, i.e., the interatomic distance between O and F atom (the sum of each vdW radius) as a criterion. As a result, their average coordination numbers over 10 different configurations were found to be 0.99 ± 0.13 and 0.66 ± 0.06 around Na2CO3 and organic salts, respectively. This clearly shows that F- anions prefer forming such inorganic clusters consisting of Na2CO3 and NaF complexes rather than coordinating to the organic salts. This is considered because that a methyl group in PC enhances the separation of inorganic salts from organic salts.30 It can be understood, therefore, that the produced NaF complexes do not contribute to the aggregation of organic salts, although such inorganic clusters themselves should become larger in size as the amount of added FECs increases. To clarify the microscopic origin leading to a compact SEI film in FEC-added electrolyte, we calculated the average migration distances in the z direction of reaction products during the hybrid


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MC/MD reaction simulation. The migration distance at MC/MD cycle i, Mz (i) , was defined using this equation:

M z (i ) =

1 Ni

Ni

∑ (z

i j

− z 0j )

2

(1)

j =1

where Ni is the number of reaction products at MC/MD cycle i, and zij is the z coordinate of the jth reaction product at MC/MD cycle i, while z0j is the initial z coordinate when the jth reaction product is produced during the simulation. Figure 9 shows Mz (i) of the inorganic salts (NaF and Na2CO3 complexes) and organic salts (NaPC and Na2DMBDC) with respect to the MC/MD cycles in the present atomistic reaction simulations using the three atomistic models, i.e., (a) System I, (b) System II and (c) System III. The final migration distance, Mz (2000) , and its statistical error bar for inorganic salts were found to decrease in System II due to the production increase of NaF complexes from FEC reduction reactions. This can be attributed to the size increase in inorganic clusters formed by combining NaF and Na2CO3 complexes. On the other hand, Mz (2000) of organic salts decreased in both FEC-added electrolytes (Systems II and III), also reducing their statistical error bars (see Figures 9(b) and (c)). In the present simulations, some reaction products were found to drift away into the bulk electrolyte region without participating in SEI film formation.30 We regarded such reaction products that exist outside the SEI film (z > 7 nm) at 2000 MC/MD cycles as the dissolved reaction products. To investigate the dissolution properties of reaction products in the three systems (i.e., Systems I, II and III), Table 1 shows the number densities of dissolved reaction products by counting their numbers per unit area of the carbon anode surface. In all the systems, the number densities of NaPC complexes were highest, meaning that they are more soluble in electrolytes than other complexes. This feature is consistent with such experimental observation in NIB that NaPC complexes are plainly detected in the separator as a main component of the dissolved reaction products.6 Then, it was found that the dissolution amounts of NaPC and Na2DMBDC complexes in both FEC-added electrolytes (i.e., Systems ACS Paragon Plus Environment

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II and III) relatively decrease in comparison to those in System I. It is, thus, clarified numerically that the FEC addition should suppress the dissolution of NaPC and Na2DMBDC complexes either with or without FEC reduction reactions. All these can be reasonably understood by the theoretical observation that, while the SEI film was growing, the intact FEC molecules coordinate with the Na+ and positive residues in the organic salts owing to the strong electronegativity of their fluorine atoms (see the typical snapshot in Figure S3 in Supporting Information) so as to suppress the dissolution of the NaPC and Na2DMBDC complexes into bulk electrolytes. Such enhancement by intact FECs in the network formation among organic salts must also lead to the suppression of unstable SEI film growth.


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4. CONCLUSIONS In this study, to investigate the microscopic mechanism of the FEC additive effects on the SEI film formation on the carbon anode in NIB, we performed atomistic reaction simulations using the hybrid MC/MD reaction method. We were able to demonstrate the SEI film formation process microscopically with the present atomistic reaction simulation and successfully reproduced the experimental observations that the irreversible capacity should decrease with the addition of FECs into the electrolyte. Additionally, the SEI film thickness in FEC-added electrolyte was found to be drastically reduced in comparison to that in FEC-free electrolyte. On the other hand, the surface structure in the SEI film in contact with the electrolyte became smoother in FEC-added electrolyte. Such feature is common in the experimental observations. As a result, the SEI film can be efficiently formed so as to decrease the irreversible capacity by adding FECs. By executing the hybrid MC/MD reaction simulations in an artificial NaPF6-PC electrolyte system with non-reactive FEC additives, we showed that not only the irreversible capacity but also SEI film thickness decreases even when FEC reduction reactions might not occur. This is because the intact FEC molecules can enhance the network formation of organic salts so as to suppress their diffusion into the bulk electrolyte owing to the strong electronegativity of their fluorine atoms. Such enhancement of network formation by intact FECs must lead to the suppression of the undesirable growth of the SEI film in NIB. On the other hand, the production increase of NaF complexes due to FEC reduction reactions must not be essential to improve SEI film formation, since they became fairly well incorporated into the formation of inorganic clusters consisting of Na2CO3 and NaF complexes in the SEI film. Therefore, it is concluded that the improvement in SEI film formation with intact FECs must be essential in the microscopic mechanism of the FEC additive effect. Our computational chemically obtained results are consistent with the common knowledge, successfully reproducing the experimental observations with a reasonable microscopic explanation of the significance of FEC addition. Although we considered a number of principal elementary chemical reaction processes associated with SEI film formation in the present simulations, the chemical reaction network must be in fact ACS Paragon Plus Environment

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considerably smaller than those in real electric cells. If we include more chemical reaction processes, the composition and variety of chemical species in the SEI films should change so as to become closer to the actual ones. As one example, the FEC molecules may produce some polymer species such as those formed in vinylene carbonate (VC)-added electrolyte.28 The production of FEC-derived polymers should increase the ratio of the organic salts in the whole SEI film, reducing the inorganic salts such as Na2CO3. Nevertheless, the indirect stabilization role of intact FEC molecules clarified presently must be compatible with such polymerization, since both effects could enhance the aggregation of organic salts. As a result, the relative differences in the SEI film formation processes between FEC-free and FEC-added electrolytes must be robust because it was obtained by a perturbation treatment of FEC addition. It is true that there is one additive effect that will directly change the chemical reaction mechanism itself in SEI film formation. For example, in LIB, the addition of VC was found to modify the elementary chemical reaction processes in SEI film formation by decreasing their free energies of activation.1,19,23 However, other indirect additive effects also exist. In fact, it was not until an SEI film was formed that we knew such additives played an important covert role during the process of SEI film formation. The FEC additive effect in the NaPF6-PC electrolyte solution in this study was such an example and was revealed clearly by the hybrid MC/MD reaction simulation for the first time. Specifically, it can be said that there are two additive effects associated with SEI film formation: (1) modification of elementary chemical reaction processes and (2) suppression of unstable film growth. Actually, such suppression should be realized by the increases in the concentration of lithium (or sodium) salts in the electrolyte.45 In particular, the latter effect must be more essential in NIB since the SEI film formation should become unstable due to the larger van der Waals radius of Na+. In conclusion, the present new insight on additive effects should suggest some important guiding principles in designing the most suitable electrolytes and controlling the aggregate structure of SEI films in developing high-performance NIBs. ACS Paragon Plus Environment

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Acknowledgments We gratefully acknowledge Prof. Shinichi Komaba and Prof. Naoaki Yabuuchi for fruitful discussion on experimental electrochemistry. This work was partially supported by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) in Japan; by the Core Research for Evolutional Science and Technology (CREST) “Establishment of Molecular Technology towards the Creation of New Functions” of the Japan Science and Technology Agency (JST); and by the MEXT program “Elements Strategy Initiative to Form Core Research Center” (since 2012), Japan.


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Supporting Information Figures S1 and S2, showing side view snapshots of the SEI films are obtained from the 10 different initial configurations in the PC electrolyte solution without and with FECs, respectively, and Figure S3 displaying how the intact FEC molecules coordinate with the Na+ and positive residues in the organic salts during the SEI film formation process. This material is available free of charge via the Internet at http://pubs.acs.org.


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TOC GRAPHICS


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FIGURES CAPTIONS

Figure 1. Model simulation system for hybrid MC/MD reaction simulations in the FEC-added NaPF6-PC electrolyte solution (cyan: carbon, red: oxygen, white: hydrogen, orange: phosphorus, green: fluorine, blue: Na+). Here, the solvent PC molecules are depicted by a stick model, while the remaining ones are depicted by a ball model (full vdW radii).

Figure 2. Reaction scheme for hybrid MC/MD reaction simulations (cyan: carbon, red: oxygen, white: hydrogen, orange: phosphorus, green: fluorine, blue: Na+).

Figure 3. Changes in the surface number densities ρ nS of the solvent molecules (right axis) and reaction products (left axis) during the SEI film formation processes with respect to the MC/MD cycles in the atomistic reaction simulations in the PC-based NaPF6 electrolyte solution (a) without and (b) with FEC molecules. Figure 4. Typical snapshots of the SEI films and NaPF6-PC electrolytes visualized at a depth of 0 ≤ z

≤ 8 nm from the carbon anode surface ((a) without and (b) with FEC molecules) (yellow, NaPC; green, Na2DMBDC; red, Na2CO3; pink, NaF; gray, C3H6; ocher, CO2; blue, Na+; orange, PF6-; light blue, FEC; purple, PC). In these panels, the solvent PC and gas molecules (C3H6, CO2) are depicted by a stick model, while the remaining molecules are depicted by a ball model (full vdW radii).

Figure 5. Mass density distributions ρm of the product components (SEI film and gas molecules) and the electrolyte solvent itself in the present atomistic reaction simulations in the PC-based NaPF6 electrolyte solution (a) without and (b) with FEC molecules.

Figure 6. Changes in the surface number densities ρ nS of the solvent molecules (right axis) and reaction products (left axis) during the SEI film formation processes with respect to the MC/MD cycles in the atomistic reaction simulations for the PC-based NaPF6 electrolyte solution with non-reactive FEC molecules.

Figure 7. Mass density distributions ρm of the reaction products in the present atomistic reaction simulations for the PC-based NaPF6 electrolyte solution with and without FEC molecules. ACS Paragon Plus Environment

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Figure 8. Front view snapshots of the SEI film from the side of bulk electrolyte (green, SEI film; red, CO32-; yellow, F-) in FEC-added electrolyte solution. These structures are obtained from 10 product simulations done with 10 different initial conditions.

Figure 9. Changes in the average migration lengths in the z direction M z of the inorganic (NaF, Na2CO3) and organic salts (NaPC, Na2DMBDC) with respect to the MC/MD cycles in the present atomistic reaction simulations for the PC-based NaPF6 electrolyte solution with and without FEC molecules.


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FIGURES

Fig. 1


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Fig. 2


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Fig. 3


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Fig. 4


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Fig. 5


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Fig. 6


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Fig. 7


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Fig. 8


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Fig. 9


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TABLES

Table 1. Number densities of reduction reaction products dissolved from the SEI film (per unit area of anode surface) (in nm-2)


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