Concentration Effect of Fluoroethylene Carbonate on Formation of

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Concentration Effect of Fluoroethylene Carbonate on Formation of Solid Electrolyte Interphase Layer in Sodium-Ion Batteries Amine Bouibes, Norio Takenaka, Takuya Fujie, Kei Kubota, Shinichi Komaba, and Masataka Nagaoka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07530 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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ACS Applied Materials & Interfaces

Concentration Effect of Fluoroethylene Carbonate on Formation of Solid Electrolyte Interphase Layer in Sodium-Ion Batteries

Amine Bouibes1, Norio Takenaka1, 2, Takuya Fujie1, Kei Kubota2, 3, Shinichi Komaba2, 3, Masataka Nagaoka1, 2, 4*

1

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

2

3

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

Department of Applied Chemistry, Tokyo University of Science 1–3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan;

4

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

*Corresponding author. E-mail: [email protected]

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ABSTRACT: Fluoroethylene carbonate (FEC) is an effective additive to improve the performance of Naion batteries (NIB). Recent experimental study showed that a small amount of FEC enhances the NIB performance, while it deteriorates by increasing the FEC amount. Toward understanding the microscopic mechanism of this observation, the dependency of the solid electrolyte interphase (SEI) film formation on the FEC concentration has been investigated in propylene carbonate (PC) based electrolyte solution by using the Red Moon method. This method was able to reproduce successfully the experimental observations where a small amount of FEC makes SEI film stable. Further, the increase of FEC amounts decreased the stability of SEI film and should lead to the decrease in the NIB lifetime during chargedischarge cycles. It was revealed that this is due to the insufficient organic dimer formation between the monomer products at the higher FEC concentration. Finally, it was reconfirmed theoretically that the appropriate adjustment of FEC additive amount is essential to develop the high-performance of NIB.

KEYWORDS: ・Na-ion battery ・FEC additive concentration ・SEI film ・Hybrid MC/MD reaction method ・Red Moon method


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1. Introduction Na-ion batteries (NIB) are considered as a promising alternative to Li-ion batteries (LIB) for largescale energy storage.1-2 Research interest to develop NIB with high performance has increased rapidly due to the abundance of sodium resources compared with those of lithium. Hence, several studies have been executed to optimize its constituent materials, e.g., electrodes, electrolytes and additives.1-5 However, NIB often suffers from the short cycle lifetime,6 which is attributed partly to the formation of unstable solid electrolyte interphase (SEI) film on the anode surface.7 In fact, during the first charge-discharge cycle, the electrolyte solution decomposes near the anode surface, and its organic and inorganic decomposition products form a passivation film called the SEI film.8-9 It should realistically protect the electrolyte from the further reductive decomposition during the subsequent charge-discharge cycles, while allowing Na+ to exchange between the electrolyte and anode. This characteristic is, therefore, directly related to the safety and lifetime of NIB. Previous experimental studies showed that the use of fluoroethylene carbonate (FEC) as electrolyte additive improves the lifetime of NIB because it enhances the SEI film formation.10-11 In fact, the FEC additive showed a high efficiency to decrease the irreversible capacity associated to the SEI film formation.11 By using soft X-ray photoelectron spectroscopy (SOXPES) measurement, it was reported that the FEC additive suppresses electrolytes decomposition leading to the decrease of the SEI film thickness.11 In addition, according to the scanning electron microscopy (SEM) images, it was shown that the SEI film surface becomes clearly smoother in FEC-added electrolyte compared to that in the FECfree one, i.e., the pristine electrolyte.12 It was also clarified that the FEC-induced SEI layer is compact and stable, and thus beneficial to obtain a uniform morphology. This characteristic of SEI film should render a significant improvement in the secondary battery performance.12 Moreover, two recent experimental studies showed that the intact FEC molecule have a direction agent role to create an ordered orientation of electrolyte on anode surface.13,14 It was suggested that this rearrangement of solvent molecules on anode surface might be effective to reproduce a better suited SEI film.13,14 ACS Paragon Plus Environment

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On the other hand, many theoretical studies focused on the FEC additive to understand its microscopic mechanism. Several studies investigated the reduction reaction processes of FEC molecules associated with SEI film formation using density functional theory (DFT) and DFT-based molecular dynamics (DFT-MD) method.15-18 Recent theoretical study reported that FEC molecules protect the solvent molecules from the reductive decomposition and promote alternate pathways for the decomposition, leading to qualitatively different and potentially stable SEI products.15,16 Also, the decomposition of FEC molecule was investigated using DFT calculation and it was reported that fluorine atoms form the strong binding with the positive residues of multiple organic SEI film compounds and play as joints connecting them.17 Further, by using Red Moon (RM) method (a hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method), the microscopic mechanism of FEC additive on SEI film formation was investigated, reproducing the atomic structures of SEI film.18 In fact, it was shown that the FEC molecules themselves enhance the network formation of organic products owing to the large electronegativity of their fluorine atoms leading to the suppression of the unstable film growth and making SEI film surface smoother.18 However, a recent experimental study reported that NIB performance depends on the amount of FEC additive. Indeed, the concentration dependency of FEC additive was investigated experimentally by using NaPF6 in PC electrolyte to improve the electrode performance of carbon anode in Na half-cell. Then, the optimal performance was critically sensitive with the small amount of FEC additive, while the performance is deteriorated by increasing the FEC amount.11 These new experimental observations open intriguing questions about the dependency of the SEI film formation on the FEC concentration. However, it is restrictive to identify experimentally the microscopic mechanism of the effect of FEC concentration since there is no direct experimental technique to observe the intermediate process of SEI film formation on the anode surface. The main goal of the present study is to shed light on these experimental observations and investigate the microscopic origin of the effect of FEC additive concentration on the SEI film formation in NIB by using the RM method.19-21 So far, this new atomistic reaction simulation has shown a great ACS Paragon Plus Environment

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applicability to treat a long-term successive complex chemical reaction processes in order to reproduce the heterogeneous atomic structures of SEI film on anode surface. In fact, a number of application works have demonstrated that this method is enough predictive and so quite useful to understand the microscopic mechanism concerning with the SEI film formation in secondary batteries.18,22,23 This paper is organized as follows. In Section 2 we describe both the used model systems and computational details. In Section 3, we present the obtained results and discussions. Section 4 is the conclusion.


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2. Model systems and computational details By reference to the previous experimental and theoretical studies,11,18 the present atomistic reaction simulations were executed in 1.1 mol/L NaPF6/PC-based electrolyte on the carbon anode with six different concentrations, i.e., 0, 1, 3, 5, 7, 10 vol% of FEC additive. Assuming the anode surface was negatively charged (-2 e),18 the system was neutralized by adding two Na+ ions into the bulk electrolyte solution. The present model of the pristine electrolyte solution, then, consists of 850 PC molecules, 87 Na+ cations and 85 PF6− anions. In the FEC-added electrolyte solutions, there included 12, 36, 59, 84, 118 FEC molecules corresponding to 1, 3, 5, 7, 10 vol% of the whole electrolyte solution respectively. In order to consider a reaction processes necessary and sufficient to simulate the SEI film formation, the reaction scheme was based on both experimental and theoretical studies.10-11,15-18,24-28 Experimentaly, it was generally accepted that the SEI film adopts a hybrid structure mainly formed from the decomposed compounds of PC solvent which result in Na2CO3 as inorganic salts, and larger amount of sodium alkyl carbonates as organic ones.6, 10-11 In addition, we based the scheme on the main reductive reaction of LiPF6 in considering NaPF6 salt decomposition.24-26 However, from the experimental observations, it was shown that the decomposition of salts is very small during SEI film formation, comparing to solvent decomposition.6,10-11 On the other hand, several theoretical studies focused on the elementary reaction processes during SEI film formation.15-18 It was concluded that PC molecules are reduced first by one electron process and associated with a cation (Na+) forming NaPC with ring-opening reaction with lower activation energy.15,27 Then, NaPC radicals were also reduced forming Na2CO3 salts or dimer organic products (Na2DMBDC) which have been proposed as one of the major components in the anode SEI film28, resulting from combination reactions of pairs of them via the radical dimerization.27 However, NaPC radicals were also observed experimentally as final SEI film products.10-11 Also, the reduction reaction processes of FEC molecules were previously investigated with Li+ and Na+ cations.15-17 Quantum chemical study suggested that the decomposition of Na+-FEC was favorable to the production of NaF, CO2, and C2H3O molecules due to the lower activation energy barriers.15 This result was also confirmed using DFT-MD method regarding to its low activation free energy in electrolyte solution, and is consistent ACS Paragon Plus Environment

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with experimental observation indicating that the amount of NaF increases in the presence of added FEC.17,10-11 Consequently, the chemical reactions considered to simulate SEI film are shown in Scheme 1. The details of RM simulations were explained previously in Takenaka et al. study.18 The present simulation cell was 3.41 nm × 3.70 nm × 30.0 nm. Then, RM simulations were executed for 2000 MC/MD cycles to obtain final SEI film structures (see Figure 1). The MD simulation time for relaxation in one MC/MD cycle for a trial chemical reaction was set to 4 ps. In order to investigate the SEI film stability, we estimated the atomic potential energy ΔVorg of organic products, which are the main compounds of SEI film and form the outer layers in contact with the bulk electrolyte solution. To this purpose, we extracted the part of organic products forming the SEI film from the whole model system, while those isolated organic products in the bulk electrolyte were excluded. In addition, to make the whole system neutralized, the excess charges are suppressed by removing, in the present simulation, the excess cations/anions that located at the longest distance from the anode surface. The potential energy includes the intramolecular interaction Vintra , i.e., bond, bending and dihedral angle energies, and the nonbonding intermolecular interaction Vnonbond and ΔVorg is obtained as follows,

∆V= org

1 (Vintra + Vnonbond ) N

(1)

where N is the total number of atoms in the extracted part of the SEI film. Both Vintra and Vnonbond can be expressed by using the function forms of generalized AMBER force field.29 On the other hand, to check the initial structure dependence, the present simulations were performed by using a set of 15 different initial configurations, and then, their standard errors were estimated with the two-sided 95 % confidence interval (Figures 2, Table 1-4).


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3. Results and discussion 3.1. Effect of FEC additive concentration on SEI film formation. By executing the RM simulations, the SEI films were formed stationarily after 2000 MC/MD cycles on the carbon anode surface in NaPF6/PC-based electrolyte solution containing 0, 1, 3, 5, 7 and 10 vol% FEC additive (see Figure S1). Figure 1a and 1b show two typical snapshots of the SEI films obtained at l and 10 vol% FEC additive concentration respectively. To analyse the dependence of SEI film structures on FEC additive concentration, the averaged surface number densities of stable reaction products are shown in Table 1. The produced dimers Na2DMBDC clearly increased at the lower concentration of FEC. The amount of dimer organic products became higher at 1 vol% FEC concentration and decreased by increasing FEC concentration. On the other hand, the NaF and Na2CO3 inorganic products increased at the higher FEC concentration, enhancing the production of the gas molecules such as C3H6 and CO2 (Figure 1b). In order to investigate the molecular configurations of the reaction products in SEI films, we estimated the mass density distributions of NaPC, Na2DMBDC, Na2CO3 and NaF products along the z axis where the origin is anode surface. To this end, we executed 1 ns MD simulations (100 ps equilibration) with the stable equilibrium structure. Figure 2 shows the averaged mass density distributions (ρ) over 15 samples of SEI film products in NaPF6/PC-based electrolyte solution containing 0, 1, 3, 5, 7 and 10 vol% FEC additive respectively. We remark immediately, for all systems, that the inorganic products such as Na2CO3 complexes are present near the anode surface while the organic products, such dimers as Na2DMBDC, are extended to the outer region of the SEI film in contact with the electrolyte solution. This is in excellent agreement with recent experimental observations in NIB.30 In fact, comparing the mass density distirubutions between those six systems, the distribution curve of Na2DMBDC showed the highest peak at 1 vol% FEC concentration (Figure 2b), while the distribution peaks of inoganic products (Na2CO3 and NaF) became the lowest. Further, the peak heights of Na2DMBDC clearly decreased by increasing the FEC concentration from 1 vol% to 10 vol%. On the other hand, in Figure 2a, it was found that some high peaks of dissoluted products appear in bulk electrolyte ACS Paragon Plus Environment

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solution without FEC additive (in the range larger than ca. 4 Å). To investiguate closely the dissolution properties of SEI film products, the percentage of dissoluted reaction products was numerically estimated by counting the number of SEI film products disolved in bulk electrolyte solution. In fact, both the SEI film region and its thickness were estimated from its total mass density distribution. The isolated reaction products, which are located outside of the estimated region of SEI film, were considered as dissolute products in bulk electrolyte solution. Table 2 shows the averaged percentage over 15 samples of the number densities of dissolved reaction products obtained by counting their numbers per the total number production and per unit area of the carbon anode surface. In all the systems, the dissolution amount of NaPC products was the largest among them, meaning that they are more soluble in the electrolyte solution than the other products. This feature is consistent with experimental observations in NIB where the NaPC complexes are plainly detected in the separator as a main component of the dissolved reaction products.10 In addition, the dissolution amounts of NaPC, Na2DMBDC and Na2CO3 complexes clearly decreased in the FEC-added electrolytes regardless of the concentration of FEC additive in comparison to those in the FEC-free electolyte. It is, therefore, clarified numerically that the addition of FEC molecules suppresses the dissolution of SEI film products even when the concentration is quite small (1 vol%). 3.2. Structural stability of SEI film depending on FEC additive concentration. The structural stability of SEI film must be closely related to the lifetime and self-discharge of NIB. To further understand such structural stability, we have focused on the cavity inside the SEI film. In order to estimate numerically such cavities in the SEI film, we calculated the fractional accessible volume (FAV),31 which is the ratio of the cavity volume to the total one. The FAV was calculated within the denser region in the obtained SEI film where its mass density distribution is greater than the half of its peak heigh (Figure S2). Otherwise, we can notice that the gas region is separated from dense region of SEI film (Figure S2). Table 3 shows the FAVs at the six FEC additive concentrations. It was found that the averaged FAV decreases in the low FEC concentration range to 3 vol%, leading to the denser structures of SEI films. On the other hand, as the FEC concentration increases from 3 vol%, the averaged cavity size increased. ACS Paragon Plus Environment

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In fact, the cavity size increased inside the whole of SEI film when the production of Na2DMBDC decreased (Figure S3). Thus, we can assume that such cavities inside the dense region of SEI film were mainly related to the formation of dimer organic products. Indeed, the constitutional residues such as polar −CO3 groups of the organic salts enclosed the Na2CO3 and NaF complexes, while the remaining nonpolar −C3H6 groups mainly formed the cavities inside the SEI films (see Figure S3). Furthermore, the dercease in the number of organic products leads to the sparse SEI film sturucture with the larger cavity size (see Figure S3). Such large cavity should decrease the interaction energies among SEI film compounds leading to the smaller stability of porous SEI film. Consequently, its durability should decrease under any mechanical impact, e.g., the collision of Na+ cations and electrolyte solvent molecules.22 For a deep mechanistic understunding of the SEI films, we investiguated their potential energies by focusing on the organic products (NaPC and Na2DMBDC) forming the organic layers in the SEI film which control the stability of whole SEI film. In fact, according to the experimental studies, the porous organic layer covers most of the SEI film surface.32-34 It was reported also that the stability of the SEI film structure is related to the formation of organic layer which allows cation transport at the electrode/electrolyte interface.34 Accordingly, some theoretical studies were based on organic porous layers to investigate the stability of SEI film.35,36 Takahashi et al. validated experimentally the theoretical model of organic porous layers, and investigated the mechanical stability of SEI film to illustrate that the crack formation is related to the diffusion of cation at the electrode/electrolyte interface.35 Also, Laresgoiti et al. have assumed a homogeneous organic layer as a theoretical SEI film model and validated it experimentally. In fact, by combining this theoretical model and experimental data, Laresgoiti et al. simulated the stress of a homogeneous SEI which was formed on graphite particle and found that fatigue failure of the SEI is the main cause of battery capacity fade.36 In addition, Bedrov et al. have assumed the organic dimer products as the theoretical model of SEI film to investigate its mechanical properties.37 These previous theoretical and experimental studies indicate that the stability of the whole SEI film is mainly controlled by the outer organic layers which are in contact with the electrolyte during the sodiation/disodiation process.


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Accordingly, Table 4 shows FEC concentration dependence of potential energy per atom of organic SEI film products averaged over 15 samples, i.e., ΔVorg (see Eq. (1)). It was found that ΔVorg has the minimum at 1 vol% of FEC additive, showing the highest stability of SEI film. Then, by increasing the FEC concentration, ΔVorg clearly increased, meaning that the SEI film becomes less stable. In fact, Dahbi et al. investigated carbon electrodes in the NaPF6/PC electrolyte solution with different FEC additive concentration and reported such result that the battery capacity can be maintained by using a low content of FEC additive, while decreases by increasing its amount.11 As with the experimental observation, the present simulation results revealed that a small amount of FEC improves the SEI film stability, leading to long life time and good passivation of NIB during charge-discharge cycles. Consequently, this result, in excellent agreement with experimental result, is supportive toward microscopic understanding the FEC additive concentration effect. 3.3. Negative impact of high FEC concentration on organic dimer formation The FEC additive concentration was found to control the production of organic products, which are related to the stability of SEI film. According to the present RM simulations, the number of organic dimers (Na2DMBDC) increased as the FEC concentration decreases, while that of the NaPC organic monomers remained almost constant at the different FEC concentrations (see Table 1). To investigate the microscopic mechanism of such strong dependency of the organic dimer production on the FEC additive concentration, we examined the change in the number of candidate pairs of dimerization reactions CDimer per anode surface, which satisfy the reaction condition during the RM simulations. Those candidate pairs for the dimerization reaction were detected when the interatomic distance between two C atoms each belonging to any independent NaPC monomers is less than 3.816 Å, which is the sum of each vdW radius (see Figure S4). Figure 3 shows the averaged CDimer over 15 samples at different FEC additive concentrations. It was found that CDimer decreases by increasing the FEC concentration, meaning that the collision frequency between NaPCs is reduced at the high FEC concentration. In particular, in the initial stage of SEI film formation (< 300 MC/MD cycle), CDimer drastically increased in general and, then decreased in the following stage, keeping the relative order of magnitude of CDimer at different FEC concentrations. ACS Paragon Plus Environment

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To understand its origin, we calculated the averaged migration distances Mr(i) of NaPCs as a function of the MC/MD cycle i: = M r (i )

1 Ni

Ni

∑r j =1

i j

− r j0 ; r j = ( x j , y j , z j ) ,

(2)

where Ni is the number of NaPC products at the MC/MD cycle i, and rji is the position vector of the j-th NaPC product at the MC/MD cycle i, while rj0 is the initial position vector when the j-th reaction product is produced during the simulation. Figure 4 shows that the migration distances of NaPCs at the same MC/MD cycle become smaller little by little in the increasing order of the FEC concentration from 1 to 10 vol%. This observation clearly indicates that the slower NaPC diffusion leads to the decrease in the collision frequency for the dimerization reaction. In fact, the present simulation results are consistent with the experimental tendency, considering that the FEC solvent shows the higher viscosity than PC one.38 On the other hand, in the early stages of SEI film formation, since the intact FEC molecules increase naturally near the anode surface by increasing FEC concentration, their coordination with positive residues of NaPCs should be enhanced at the higher concentration of FEC. To demonstrate numerically this tendency, the coordination numbers of F and three O atoms in the FEC molecules were estimated around O atoms in the NaPC products by using 3.41 Å and 3.32 Å, that is, the interatomic distance between O and F atoms and that between O and O atoms, respectively (the sums of each vdW radius). Figure 5 shows the averaged coordination number per unit anode surface (Ncoord) of the intact FEC molecules around the NaPC products averaged over 15 samples at the different FEC additive concentrations. During the initial SEI film formation processes, Ncoord greadually increased, and their converged values became larger at the higher FEC concentration. At the same time, one FEC molecule could form a bridge between two different NaPCs because a FEC molecules has two negative parts (-F and –CO3 groups) (see Figure S5). Such bridge formation could be essential to supress the dissolution of NaPC products even if the concentration of FEC might be smaller (see Table 2). However, the increase in number of bridges at the higher FEC concentration should prevent the contact between NaPCs so as to decrease the reaction freqeuncy of the dimer formation (see Figure S6). ACS Paragon Plus Environment

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The present analyses revealed the two microscopic effects of FEC molecules to suppress the formation of organic dimers at higher FEC additive concentration. The increase in the FEC amount decreased the diffusion of NaPCs due to the higher viscosity of FEC solvent than PC one (see Figure 4). In addition, the increase of the FEC bridges between NaPCs prevents the contact between them (see Figure S6). It is concluded, therefore, that these two effects at the high FEC concentration revealed here lead to the decrease in the organic dimer formation and unstable SEI film by reducing the collision frequency between NaPCs. In addition, the concentration of FEC additive showed a similar impact on LIB as that on NIB performance. Recent experimental study showed that the cycling performance was enhanced by adding small amount of FEC additive. However, the higher concentrations of FEC lead to the deterioration of LIB performance. Using electrochemical impedance spectroscopy (EIS) combining with scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS), it was revealed that the deterioration of LIB performance is mainly due to the poor cohesion and flexibility of the SEI film formed on the graphite electrode which leads to larger charge transfer resistance as well as the SEI film resistance.40 The present study must be supportive very much to undestund the essential and microscopic reasons of poor cohesion and flexibility of SEI film at higher FEC concentration. On the other hand, comparing the SEI film formation of NIB with that of LIB, it was shown that sodium SEI compounds as the organic species were highly soluble than those of Li-SEI.41 Thus, it can be considered that FEC additive is more effective on the formation of sodium SEI film because of higher effeciency of FEC molecules to suppress the dissolution of SEI film products. However, the optimal adjustment of FEC additive amount is necessary and essential to improve the high-performance NIB with FEC additive. We believe that this must be accomplished through the collaborative works of theoretical and experimental studies.


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4. Conclusion The present study investigated the microscopic effects of FEC additive concentration on the SEI film formation in NIB. For this purpose, we performed the atomistic reaction simulations using the RM method. As a result, we were able to demonstrate microscopically the process of SEI film formation and successfully obtained the SEI film structures at the different FEC concentration. It was shown that the FEC additive shows seemingly a conflict behavior of the SEI film stability depending on the FEC concentration. Namely, its structural stability was higher at small amount of FEC, while it decreased with the increase in the FEC amount, leading to decrease in the NIB lifetime during charge-discharge cycles. However, these results were clearly consistent with the experimental tendency.11 The present computational study, therefore, revealed clearly that a small amount of FEC has a positive effect so as to reduce the amount of the dissolved reaction products into the bulk electrolyte. However, it was also found that the role of FEC changes from the positive to negative effects as the FEC concentration increases. Indeed, the higher FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. As a result, the SEI film should become unstable due to the insufficient organic dimer formation. It is concluded, finally, that the appropriate adjustment of FEC additive amount is essential to improve the high-performance NIB with FEC additive.


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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).

Supporting Information Figures show the averaged surface number densities, the averaged mass density distribution of SEI film and gas molecules, typical snapshot showing the cavity size on SEI film structures, chemical reaction condition of dimerization reaction, typical snapshot of FEC molecule forming a bridge between two different NaPCs during SEI film formation and typical snapshot of SEI film at initial stage of formation. This material is available free of charge via the Internet at http://pubs.acs.org.


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Performance and the Surface Layer of Hard Carbon for Sodium‐Ion Batteries. ChemElectroChem. 2016 3, 1856-1867. [12] Zhang, X. Q., Cheng, X. B., Chen, X., Yan, C., Zhang, Q. Fluoroethylene Carbonate Additives to Render Uniform Li Deposits in Lithium Metal Batteries. Adv. Funct. Mater. 2017, 27, 1605989. [13] Horowitz, Y., Han, H. L., Soto, F. A., Ralston, W. T., Balbuena, P. B., Somorjai, G. A. Fluoroethylene Carbonate as a Directing Agent in Amorphous Silicon Anodes: Electrolyte Interface Structure Probed by Sum Frequency Vibrational Spectroscopy and Ab Initio Molecular Dynamics. Nano Lett. 2018, 18, 11451151. [14] Horowitz, Y., Steinrück, H. G., Han, H. L., Cao, C., Abate, I. I., Tsao, Toney, M., F., Somorjai, G. A. Fluoroethylene Carbonate Induces Ordered Electrolyte Interface on Silicon and Sapphire Surfaces as Revealed by Sum Frequency Generation Vibrational Spectroscopy and X-ray Reflectivity. Nano Lett. 2018, 18, 2105-2111. [15] 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. RSC Adv. 2016, 6, 65232-65242. [16] Kumar, H., Detsi, E., Abraham, D. P., Shenoy, V. B. Fundamental Mechanisms of Solvent Decomposition Involved in Solid-Electrolyte Interphase Formation in Sodium Ion Batteries. Chem. Mater. 2016, 28, 8930-8941. [17] 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. [18] Takenaka, N., Sakai, H., Suzuki, Y., Uppula, P., 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. [19] Nagaoka, M.; Suzuki, Y.; Okamoto, T.; Takenaka, N. A Hybrid MC/MD Reaction Method with Rare ACS Paragon Plus Environment

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Event-Driving Mechanism: Atomistic Realization of 2-Chlorobutane Racemization Process in DMF Solution. Chem. Phys. Lett. 2013, 583, 80–86. [20] 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. [21] 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. [22] Takenaka, N.; Suzuki, Y.; Sakai, H.; Nagaoka, M. On Electrolyte-Dependent Formation of Solid Electrolyte Interphase Film in Lithium-Ion Batteries: Strong Sensitivity to Small Structural Difference of Electrolyte Molecules. J. Phys. Chem. C 2014, 118, 10874-10882. [23] Takenaka, N.; Fujie, T.; Bouibes, A.; Yamada, Y.; Yamada, A.; Nagaoka, M. Microscopic Formation Mechanism of Solid Electrolyte Interphase Film in Lithium-Ion Batteries with Highly Concentrated Electrolyte. J. Phys. Chem. C 2018, 122, 2564-2571. [24] Aurbach, D.; Markovsky, B.; Shechter, A.; Ein‐Eli, Y.; Cohen, H. A Comparative Study of Synthetic Graphite and Li Electrodes in Electrolyte Solutions Based on Ethylene Carbonate‐Dimethyl Carbonate Mixtures. J. Electrochem. Soc. 1996, 143, 3809-3820. [25] Xu, K. Nonaqueous Liquid Electrolytes for Lithium-based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303-4418. [26] Kawaguchi, T.; Shimada, K.; Ichitsubo, T.; Yagi, S.; Matsubara, E. Surface-layer Formation by Reductive Decomposition of LiPF6 at Relatively High Potentials on Negative Electrodes in Lithium Ion Batteries and its Suppression. J. Power Sources, 2014, 271, 431-436. [27] Wang, Y. and 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. ACS Paragon Plus Environment

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[28] Balbuena, P. B.; Wang, Y. Lithium-Ion Batteries: Solid-Electrolyte Interphase; Imperial College Press: London, 2004,140-157. [29] Wang, J.; Wolf, R. M.; Caldwell; J. W., Kollman, P. A.; Case, D. A. Development and Testing of a General Amber Force Field. J.Comput. chem. 2004, 25, 1157-1174. [30] Soto, F. A., Yan, P., Engelhard, M. H., Marzouk, A., Wang, C., Xu, G., Chen, Z., Khalil, A., Lui, J., Sprenkle, V., El-Mellouhi, F., Balbuena, P. B., Li, X., Tuning the Solid Electrolyte Interphase for Selective Li- and Na- Ion Storage in Hard Carbon. Adv. Mater. 2017, 29, 1606860. [31] Hofmann, D.; Entrialgo-Castano, M.; Lerbret, A., Heuchel, M.,Yampolskii, Y. Molecular Modeling Investigation of Free Volume Distributions in Stiff Chain Polymers with Conventional and Ultrahigh Free Volume: Comparison between Molecular Modeling and Positron Lifetime Studies. Macromolecules 2003, 36, 8528−8538. [32] Edström, K., Herstedt, M., Abraham, D. P. A New Look at the Solid Electrolyte Interphase on Graphite Anodes in Li-Ion Batteries. J. Power Sources 2006, 153, 380-384. [33] Peled, E., Menkin, S. SEI: Past, Present and Future. J. Electrochem. Soc. 2017, 164, A1703-A1719. [34] Agubra, V. A., Fergus, J. W. The Formation and Stability of the Solid Electrolyte Interface on the Graphite Anode. J. of Power Sources 2014, 268, 153-162. [35] Takahashi, K., Srinivasan, V. Examination of Graphite Particle Cracking as a Failure Mode in Lithium-Ion Batteries: a Model-Experimental Study. J. Electrochem. Soc. 2015, 162, A635-A645. [36] Laresgoiti, I., Käbitz, S., Ecker, M., Sauer, D. U., Modeling Mechanical Degradation in Lithium Ion Batteries during Cycling: Solid Electrolyte Interphase Fracture. J. Power Sources 2015, 300, 112-122. [37] Bedrov, D., Borodin, O., Hooper, J. B. Li+ Transport and Mechanical Properties of Model Solid Electrolyte Interphases (SEI): Insight from Atomistic Molecular Dynamics Simulations. J. Phys. Chem. C 2017, 121, 16098-16109. [38] Ohtake, M., Takimoto, K., Nanbu, N., Takehara, M., Ue, M., Sasaki, Y. Physical Properties of Fluorinated Cyclic Carbonates for Secondary Lithium Batteries. In Meet. Abstr.: Electrochem. Soc. 2008, 802, 175.


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[39] Simonoff, Jeffrey S. Smoothing Methods in Statistics, 2nd edition, Springer, New York, 1998, 40-70. [40] Zhao, X.; Zhuang, Q. C. ; Xu, S. D.; Xu, Y. X.; Shi, Y. L.; Zhang, X. X. A New Insight into the Content Effect of Fluoroethylene Carbonate as a Film Forming Additive for Lithium-ion Batteries. Int. J. Electrochem. Sci. 2015, 10, 2515-2534. [41] Mogensen, R.; Brandell, D.; Younesi, R. Solubility of the Solid Electrolyte Interphase (SEI) in Sodium Ion Batteries. ACS Energy Lett. 2016, 1, 1173-1178.


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TOC GRAPHIC


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Scheme:

Scheme 1: Primary chemical reaction using in Red Moon simulations. PC+ e- + Na+ NaPC + e- + Na+ NaPC + NaPC PF6- + 2e- + 3Na+ FEC + e- + Na+

    

NaPC Na2CO3 + C3H6 Na2DMBDC PF3 + 3NaF NaF + CO2 + C2H3O

(R1) (R2) (R3) (R4) (R5)


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Figures:

Figure 1: Typical snapshot of SEI films and NaPF6/PC electrolyte solution containing 1 vol% (a) and 10 vol% (b) of FEC additive concentration from whole volume electrolyte. The solvent PC (purple) and gas molecules, i.e., C3H6 and CO2, (gray), are depicted by the stick models.


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Figure 2: Averaged mass density distribution (ρ) over 15 samples of SEI film products (red, Na2CO3; green, Na2DMBDC; dark yellow, NaPC; pink, NaF) in NaPF6/PC electrolyte solution containing (a) 0, (b) 1, (c) 3, (d) 5, (e) 7, (f) 10 vol% of FEC additive concentration.


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0,27

FEC concentration: 1 vol% 3 vol% 5 vol% 7 vol% 10 vol%

0,24 0,21

CDimer (nm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,18 0,15 0,12 0,09 0,06 0,03 0,00 0

100

200

300

400

500

600

700

800

900

1000

MC/MD cycles

Figure 3: Averaged number of NaPC candidate for dimerization reaction per anode surface (CDimer) over 15 samples at different FEC additive concentrations. The curve smoothing was carried out by adjacent averaging method considering 30 points.39


25


18


16 14 12

Μr (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

10 8 6 4 2 0 0

50

100

150

200

250

300

MC/MD cycles Figure 4: Averaged migration distance (Mr) of NaPC product up to 300 MC/MD cycles in NaPF6/PC electrolyte solution with different FEC additive concentration.


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1,2


1,1 1,0 0,9 0,8

Ncoord(nm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 0

50

100

150

200

250

300

MC/MD Cycles

Figure 5: Averaged coordination number per anode surface (Ncoord) of intact FEC molecules around NaPC products over 15 samples at different FEC additive concentrations.


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Tables:

Table 1: FEC concentration dependence of surface number densities (nm-2) averaged over 15 samples of SEI film products at 2000 MC/MD cycle.

Species

0 Na2DMBDC 6.18±0.24 NaPC 0.95±0.05 Na2CO3 5.19±0.14 NaF 0.00

1 8.82±0.48 1.03±0.08 4.53±0.16 0.24±0.08

FEC concentration (vol%) 3 5 7.15±0.56 6.71±0.20 0.95±0.16 0.80±0.07 4.29±0.24 4.63±0.11 0.40±0.08 0.66±0.07

7 5.71±0.20 0.81±0.06 5.01±0.16 1.00±0.06

10 4.79±0.20 1.13±0.25 5.05±0.10 1.34±0.10


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Table 2: FEC concentration dependence of the numbers of dissoluted species of SEI film products (%) averaged over 15 samples at 2000 MC/MD cycle.

FEC concentration (vol%) 0 1 3 5 7 Na2DMBDC 4.65±0.36 0.25±0.02 0.12±0.01 0.12±0.01 0.37±0.03 NaPC 16.10±1.28 6.41±0.51 3.39±0.26 4.02±0.31 8.05±0.78 Na2CO3 0.00 0.00 0.00 0.00 0.25±0.02 NaF 0.00 0.00 0.00 0.00 0.00 Species

10 0.50±0.03 7.92±0.63 0.00 0.00


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Table 3: FEC concentration dependence of FAV averaged over 15 samples.

Cavity (FAV)

0

1

0.14±0.02

0.09±0.01

FEC concentration (vol%) 3 5 0.07±0.01

0.12±0.01

7

10

0.13±0.01

0.18±0.02


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Table 4: FEC concentration dependence of potential energy (kcal/mol) per atom of organic (NaPC and Na2DMBDC) SEI film products averaged over 15 samples.

ΔVorg

FEC concentration (vol%) 0 1 3 5 7 10 -7.16±0.96 -9.20±0.38 -8.60±0.75 -8.03±0.71 -7.44±0.98 -6.23±0.94


31

Concentration Effect of Fluoroethylene Carbonate on Formation of

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