Sacrificial Anion Reduction Mechanism for Electrochemical Stability

Jun 25, 2014 - From Figure 3a, the average distance of Li–NAN is 2.04 Å, which is in good agreement with the available data.(13) The Li–CAN (C at...
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Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte Keitaro Sodeyama,*,†,‡ Yuki Yamada,§,† Koharu Aikawa,‡,∥ Atsuo Yamada,§,† and Yoshitaka Tateyama*,‡,†,⊥ †

Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan ∥ Department of Chemistry, Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ⊥ PRESTO and CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 333-0012, Japan ‡

S Supporting Information *

ABSTRACT: Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes in lithium-ion battery (LIB). Here we theoretically investigated low (LC) and high (HC) concentration systems of LiN(SO2CF3)2 (Li-TFSA) salt in acetonitrile (AN) solution, to elucidate the mechanism of improving the low reduction stability of AN at the HC condition, by density functional theory based molecular dynamics (DFT-MD) sampling of the solvation character with extra electron(s). We demonstrated that TFSA anions sacrificially accept the reductive electron at the HC condition, which is ascribed to formation of specific network structure and the resulting shift of electron affinity of the anions. We also found that, even in the LC condition, TFSA eventually decomposes with one electron reduction. This sacrificial anion reduction hinders two electron reductive decomposition of AN, leading to improved electrochemical stability. The mechanism may give a guiding principle for the design of better LIB electrolytes.

1. INTRODUCTION

Theoretically, there have been a few studies on the concentration dependence of Li-TFSA electrolytes so far. Molecular dynamics (MD) simulations with classical force field proposed the presence of an aggregate (AGG) solvate structure in which some TFSA anions coordinate to two or more Li+ ions.13,14 However, classical MD is difficult to treat the electronic charge modulation by the change of solvation environment. Quantum chemical calculations with polarized continuum model for the solvent effect15−17 suggested the optimized structure of Li-TFSA cluster system with two AN solvent molecules, though it is less predictive upon the change of the solvation shell in the AGG solvates structure. For the electrolyte exploration, the density functional theory based molecular dynamics (DFT-MD) simulation with explicit solvent18−20 is essential including electronic states analysis for reductive decomposition of electrolyte and structural fluctuations with AGG solvation. In this article, we investigated the mechanism of the improvement of reduction stability of Li-TFSA/AN systems depending on the salt concentration by using first-principles DFT-MD calculations with explicit AN solvents, though the computational cost is significantly high. Discussions based on

Lithium-ion batteries (LIBs) have been widely used because of the higher volume and gravimetric energy density,1−4 and the improvements of durability, safety, and power density are highly demanded for future applications. The electrochemical stability of electrolytes during charge−discharge cycle is of great importance for the improvements because the reductive or oxidative decomposition of the electrolytes degrades the durability of LIB. Among the plenty of electrolytes, acetonitrile (AN) is a promising candidate because of the high oxidation stability and Li+ ion conductivity. In spite of the advantages, it has crucial deficiencies that stability against reduction is weak and that toxic CN− is easily formed by the decomposition.5,6 Recently, some of the authors have reported that high concentration (HC) LiN(SO 2 CF 3 ) 2 (lithium bis(trifluoromethanesulfonyl)amide, Li-TFSA) salt in AN electrolyte shows strong electrochemical stability against reductive decomposition, though the low concentration (LC) solution is still weak for the reduction.7 The HC Li-salt systems were also reported in the other electrolytes.8−12 The new concept of the Li-salt concentration control (solvent-in-salt) to overcome the limitation of potential window of electrolyte has a large impact on the exploration for better LIB electrolyte. However, the atomistic origin of the improved reduction stability in HC system has been still an open question. © 2014 American Chemical Society

Received: February 3, 2014 Revised: June 9, 2014 Published: June 25, 2014 14091

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We deal with one-, two-, and three-electron reduction in addition to the neutral charge system (Figure 2). To describe

the LUMO levels of electrolyte molecules and anions are frequently used for the electrochemical stability estimation. However, it cannot take into account the relaxation of the reductive electron and whether the decomposition occurs. The technique also suffers from the HOMO−LUMO gap underestimation. Therefore, we adopt DFT-MD calculation with an extra electron, which is referred as the ΔSCF type analysis. The ΔSCF treatment can include the relaxation effect of excess electrons and the decomposition after the reduction of electrolytes. We explored which molecule accepts the reduced electron in HC and LC systems with extra one electron (1e) and discussed the origin of reduction stability of the Li-TFSA/ AN system. Reductive decomposition of AN solvent molecules in LC system was also investigated with extra two (2e) and three (3e) electrons.

2. COMPUTATIONAL SECTION We investigated systems with Li-TFSA salt in AN solvent molecules. For the LC case, we adopted one Li-TFSA salt pair and 43 AN solvent molecules (Li-TFSA/43AN), which corresponds to 0.4 mol dm−3, a typical concentration. The HC system involves 10 Li-TFSA pairs and 20 AN molecules (10(Li-TFSA/2AN)), which corresponds to the experimental value7 of 4.2 mol dm−3. As an initial structure of Li-TFSA in LC system, we can classify two conformations such as solventseparated ion pair (SSIP, Li+ is free from anions) and contact ion pair (CIP, Li+ is coordinated by one TFSA anion). Since the alignments of the energy levels of AN and TFSA are qualitatively the same tendency in both conformations, according to ref 7, we focus on the SSIP structure in the present study. The ion-pairing energies are shown in Table S1 in the Supporting Information. We performed DFT-MD simulations within the Car− Parrinello’s treatment of electric and ionic dynamics21 using CPMD code.22 Total energies were calculated at the Γ point in a super cell approach by using the PBE generalized gradient corrected exchange-correlation functional.23,24 We adopted the cubic supercell with 15.74 Å of linear dimension under periodic boundary conditions (PBC) (Figure 1). A fictitious electric

Figure 2. Schematic description of the supercell calculation model of the reductive decomposition near the negative electrode in high and low electric potential cases.

the 1e reduction, we use the injection of one electron in the calculation supercell. PBC used here introduces the homogeneous background charge. For 2e (3e) reductions, we kept the supercell charge −1 and introduced one (two) extra Li atom(s) to the system. Because Li atom immediately becomes Li+, one solvent or anion molecule (AN or TFSA) receives one electron. To deal with the unpaired electron, unrestricted DFT framework is used. We adopted doublet spin multiplicities in 1e and 3e reduction systems and singlet multiplicity in the 2e reduction system. In this work, we modified the CPMD code for calculation of the free energy profile with a single job. The profile is evaluated with the Blue-moon ensemble technique,30 where we calculate the potential of mean force (PMF) at each constraint with constraint MD method and then carry out the thermodynamic integration from the initial constraint to the final. Our modified code can carry out parallel calculations of all the necessary PMFs simultaneously. This is suitable for the use of massive parallel computers like the K computer in Japan that we used. Note that one can use this modification for parallel search or sampling with different initial conditions.

Figure 1. Supercells used in the DFT-MD simulations of HC ((a) 10(Li-TFSA/2AN) corresponding to 4.2 mol dm−3) and LC ((b) LiTFSA/43AN corresponding to 0.4 mol dm−3) Li-TFSA/AN solutions.

3. RESULTS AND DISCUSSION We first investigate the equilibrium trajectories of the LC system under the neutral conditions. Figure 3a,b shows the radial distribution functions (RDFs) from the Li+ ion and N atoms of the AN solvents (NAN). In the LC case, the Li+ ion is coordinated by four NAN atoms (Figure 4a). From Figure 3a, the average distance of Li−NAN is 2.04 Å, which is in good agreement with the available data.13 The Li−CAN (C atoms of the AN solvents) distribution has a peak at 3.20 Å. This indicates that the AN solvent molecules radially coordinate to Li+ ion, and thus, the effective radius of Li+ solvation shell is around 5 Å. Since there is no peak related to TFSA anion within 4 Å, the Li-TFSA salt is completely dissolved. We have also examined the AN solvent liquid structures in the LC case.

mass of 500 au and a time step of 4 au (0.10 fs) were chosen. The energy cutoff of the plane wave is set to 90 Ry. Stefan Goedecker’s norm-conserving pseudopotentials25−27 for C, H, O, N, S, F, and Li were used. Nuclear temperature was controlled using a Nosé thermostat28,29 with a target temperature of 298 K. After equilibration, statistical averages were computed from trajectories of at least 10 ps in length. The electronic wave function was quenched to the Born− Oppenheimer surface about every 1 ps in order to maintain adiabaticity. 14092

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Next we investigate the equilibrium trajectories of the HC system. From Figure 3c, the position and height of the Li−NAN peak are unchanged compared to the LC case. However, the peaks of O (OTFSA) and N (NTFSA) atoms of the TFSA anions appear near the Li−NAN peak. The average distances are 1.95 and 2.10 Å, respectively. As the Li−NTFSA peak is very small compared with the peaks of Li−OTFSA and Li−NAN and the coordination numbers obtained statistically have 2 for OTFSA and 2 for NAN, the coordination of OTFSA and NAN is dominant to Li+ ions in HC system. The typical first solvation shells of Li+ ion in HC case are shown in Figure 4b−e. The solvation structure with two AN molecules and one TFSA anion shown in Figure 4c is the same coordination with the structure obtained by the quantum chemistry calculation.17 We also found the structures with two and three TFSA anions coordinated to one Li+ ion (Figure 4d,e). Since TFSA has four oxygen atoms, one TFSA anion can coordinate to some Li+ ions. Thus, Li-TFSA HC system constructs the specific chained network structure (Figure 4f), which has been proposed as the AGG structure.13,14 The AN solvent liquid structures in the HC system in Figure 3d has specific peaks of NAN−NAN and NAN−OTFSA, which appears at 3.3 and 3.0 Å, respectively. These peaks are assigned to intramolecular AN and TFSA, which coordinate to the same Li+ ion (Figure 4b− d). We then first focused on the HC systems with the 1e reduction effect at around a low potential vs Li/Li+. Figure 5a,b shows the snapshots of the structures of the HC systems with 1e reduction before and after the decomposition (see also Figure S1 in the Supporting Information). During the DFTMD simulations, we observed a spontaneous decomposition of TFSA anion into the CF3 moiety and (SO2)2CF3N, while the AN decomposition did not proceed. We examined the projected density of states (PDOS) before and after the TFSA decomposition to understand the reason. Note that the “before decomposition structure” is a metastable state during the MD sampling. As shown in Figure 5a, the added electron migrates among TFSA anions. We emphasize that the negative TFSA still attracts the excess electron. The energy level that accepts the extra electron is σ* CF3−S orbital, and the single occupied molecular orbital (SOMO) level is stabilized by the dissociation of CF3 moiety (Figure 5b) in HC system. The decomposition of TFSA indicates that the CF3 moiety and decomposed product can stack on the interface between the negative electrode and electrolyte, forming a sort of solid electrolyte interface (SEI). In fact, experimental XPS study suggests that F-related species lie on the negative electrode,7 supporting this sacrificial reduction mechanism of TFSA anion. In LC system, first reduction happens at the AN solvent molecule coordinated by Li+ ion (Figure 5c). However, the C− N triple bond in AN does not decompose by 1e reduction during the sampling. The excess electron then sometimes migrates to the σ* CF3−S bond of TFSA, whose energy level is close to the C−N triple bond, and TFSA is sacrificially decomposed due to the stabilization of the σ* bond by the dissociation (Figures 5d and S2, Supporting Information). After the decomposition, the extra electron is localized at the TFSA anion, which is suggested by SOMO in Figure 5d. Accordingly, we found that the TFSA anion is preferentially reduced and decomposed in the LC system and that the reductive decomposition of AN solvent is suppressed at the first 1e charging. We also calculated the electron affinities (EA) of TFSA and AN molecules by DFT cluster boundary condition

Figure 3. RDFs from (a) Li+ ion and (b) NAN in LC system and those from (c) Li+ ion and (d) NAN in HC system.

Figure 4. Schematic pictures of Li coordination structures of (a) Li4AN in LC system, as well as (b) Li-1TFSA/3AN, (c) Li-1TFSA/ 2AN, (d) Li-2TFSA/2AN, and (e) Li-3TFSA/1AN in HC system. (f) Snapshot of typical chain network structure with some TFSA anions and Li+ ions in HC system (Li, purple; N, blue; O, red; C, gray; S, yellow; F, light green).

NAN usually has an attractive interaction with CAN in the methyl moiety of the AN solvent. The second peak of CAN corresponds to the C atoms in the CN moiety in Figure 3b. The broad feature of the peak indicates the presence of a typical liquid structure of AN. These NAN−NAN and NAN−CAN distances are similar to the RDFs in the bulk AN liquid.31,32 14093

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Figure 5. PDOSs of HC (10(Li-TFSA/2AN)) with 1e reduced system (a) before and (b) after TFSA decomposition and PDOSs of LC (Li-TFSA/ 43AN) with 1e reduced system (c) before and (d) after the decomposition. PDOSs of (e) HC and (f) LC with neutral charge systems. SOMO-1 (HOMO) level is set to zero in the 1e reduced (neutral) system. Contribution of the decomposed products to the SOMO is shown in Figure S3 in the Supporting Information.

tendency. Then, we also looked into the PDOS in the neutral HC and LC systems (Figure 5e,f), just for comparison. We then confirmed that in the LC system the conduction band minimum (CBM) always consists of the π* orbital of C atoms in an AN molecule, whereas the TFSA anion often provides CBM in the HC case. The band gap of AN regions are around 5.5 eV in the present calculations, and both LC and HC systems have similar value. However, the gap of TFSA anion decreases by 1 eV in the HC case. This is attributed to the strong interaction between the TFSA anion and Li cation, as a consequence of high concentration concept. Using the Li 1s level as the reference (see, Figure S4 in the Supporting Information), it is found that the TFSA conduction bands are pushed down. This gives rise to a rather positive region in the TFSA molecule coordinated by more than two Li+ ions, which is a typical network chain structure of HC system shown in Figure 4f. It indicates the robustness of the AN anion against

calculations (Table S2 in the Supporting Information), and TFSA has larger EA than AN. It is a qualitatively consistent result with the easier reduction of TFSA in the present study. In order to evaluate the functional dependence, we calculated the electron affinities and ion pairing energies with PBE and B3LYP functionals (Tables S1 and S2 in the Supporting Information). Both functionals reproduce the same electron affinities and ion pairing energies, which are related to the total energies. Since our present model focuses on the equilibrium states after the relaxation, we used PBE functional from the viewpoint of the computational cost and the accuracy. Note that for the quantities before the equilibrium, such as electron transfer rate, the self-interaction error of the PBE may need some cautions.33 In this analysis, we have used the ΔSCF type analysis with an excess electron. However, HOMO−LUMO orbital energies are often used for the discussion on the reduction and oxidation 14094

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Figure 6. (a) Energy diagram and equilibrium structure snapshots of 2e reduced intact (Li-TFSA/AN) and one TFSA decomposed (Li-decTFSA/ AN) trajectories obtained by DFT-MD simulations with explicit AN solvents. (b) Free energy profiles, ΔA, of 1e reductive decomposition of AN along the mechanical constraint ξ with decomposed TFSA anion reduced by another extra 1e. The ξ is set to the bond length between the carbon atoms of CN and CH3 groups in AN. (c) Energy diagram and equilibrium structure snapshots of 3e reduced decomposed TFSA and intact AN (LidecTFSA/AN), and 3e reduced decomposed TFSA and decomposed AN (Li-decTFSA/decAN) trajectories. Standard deviations of total energies at each reaction coordinate in panel b and that in four conformations in panels a and c are listed in Table S4 and S5 in Supporting Information.

decomposition. Discussion on the energy level shift of the TFSA conduction band with charge analysis is shown in Figure S5 and Table S3 in the Supporting Information. The number of oxygen atoms that are able to attach to the Li+ ion should be significant to construct the network chained structure. From this viewpoint, four oxygen atoms in TFSA anion, which coordinate to Li+, is a big advantage for the strong reduction stability in HC system. This mechanism may give fundamental information to solve the electrolyte stability problems of Li+ ion batteries. We further examine the possibility of the AN decomposition, which has been observed experimentally in the LC system.34−38 We introduce another electron leading to two excess electrons in the supercell in LC case and investigate the decomposed reaction from the aspect of thermodynamics and kinetics. In this DFT-MD simulation, we take a way of putting one Li atom, which immediately ionized to Li+ ion and one excess electron, into the supercell with −1 charge because the decomposed product of AN (CN− and CH3−) can coordinate to different Li+ ions and stabilize. This extra Li atom introduction does not contradict experimental settings because in the realistic case Li+ is provided by not only Li-salts but also the positive electrode (for example, LiCoO2). We label the initial intact system as “LiTFSA/AN”. During the DFT-MD simulation under the 2e reduction condition with two Li+ in LC system, we also observed a spontaneous decomposition of TFSA. The tendency that TFSA scavenges an extra electron is the same as the 1e reduction case. However, no AN molecule decomposed, though another extra electron stays at AN. This TFSA decomposed structure is labeled as “Li-decTFSA/AN”. The snapshots of Li-TFSA/AN and Li-decTFSA/AN are shown in Figure 6a. The LidecTFSA/AN is 4.6 eV more stable than Li-TFSA/AN, while the intact TFSA decomposed immediately. To confirm the 1e reduction stability of AN molecules, we also explored reaction barrier height of the AN decomposition reaction. Figure 6b shows the free energy profile of AN decomposition under 1e reduction with decomposed TFSA anion. The reaction barrier is around 60 kcal/mol, and the product is unstable rather than the Li-decTFSA/AN. Therefore, we confirmed that the 1e reduced AN does not decompose from thermodynamics and kinetics and focused on the stability of AN against 2e reductive decomposition.

We investigate the thermodynamic stability of AN after all the TFSA are decomposed under 3e reduction condition, which corresponds to 1e reduction of TFSA and 2e reduction of AN. For the 3e reduction, we used −1 charged super cell with two extra Li atoms. Two types of initial structures in which only TFSA decomposed (Li-decTFSA/AN) and both TFSA and AN decomposed (Li-decTFSA/decAN) are used for DFT-MD simulations. The snapshots and energy diagrams are shown in Figure 6c. We found that Li-decTFSA/decAN is 1.63 eV more stable than Li-decTFSA/AN. The energy barrier for 2e reductive decomposition of (Li-AN)−1 → Li-CN + CH3− obtained by using the cluster model calculation is around 7 kcal/mol, which is not significantly large. We conclude that the 3e reduction mechanism, including 2e reduction for AN and 1e reduction for TFSA systems, is required for the reduction decomposition of AN solvent, which is observed experimentally. From these microscopic analyses, we can propose the reductive stability mechanisms as follows. In the HC case, the electron affinity of TFSA anion is larger than that of AN and an extra electron is accepted by the TFSA. The 1e reduced TFSA decomposes, and the decomposed product stacks on the negative electrode. When the other extra electrons are introduced from the negative electrode, the other intact TFSA is sacrificially decomposed. In the HC electrolytes, large numbers of intact TFSA exists and the decomposed products accumulate on negative electrode. Finally the accumulated products, which can form a sort of SEI film, prevents further reduction, and the AN decomposition does not start. In the LC case, an extra electron at first charging is accepted by AN. However, before AN decomposition, TFSA collects the electron, and it is decomposed. At the next charging step, when no intact TFSA remains and still the SEI film growth is not sufficient, AN accepts the extra electrons and decomposes. As is the case with the SEI, the present study extends the limitation for the decomposition of solvent at initial charge−discharge cycle. It is a practically important technology for using the solvents whose decomposition give a crucial disadvantage for application such as AN with toxic decomposed products.

4. CONCLUSIONS We theoretically investigated the mechanism of unusual electrochemical stability of AN solvents in LC and HC Li14095

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TFSA salt solutions by using DFT-MD simulations with explicit AN solvents and ΔSCF treatment. We found that TFSA anion sacrificially accepts a reductive electron and dissociates the CF3 moiety because of the formation of a specific chained network structure and the resulting electron affinity shift of the anions. We also found that even in the LC condition TFSA eventually decomposes with 1e reduction. This sacrificial decomposition may prevent 2e reductive decomposition of AN, leading to improved electrochemical stability of the electrolyte. The decomposition of AN, which is observed experimentally,7 should be initiated after all the TFSA anions are depleted. This microscopic analysis proposes that the HC system has enough TFSA anions to form the SEI film to prevent further reduction of electrolytes, but because of an insufficient number of anions in LC system, SEI film does not grow thicker and AN solvents are decomposed. Since the concepts of “sacrificial anion reduction”, “competition between 1e and 2e reductions”, and “constructing chain network structure” can be general features for HC Li-salt systems, the present mechanism could give a guiding principle for the design of LIB electrolyte to revise the conventional electrolyte design strategy based on the solvent potential window.

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ASSOCIATED CONTENT

S Supporting Information *

Computational details, ion-pairing energy of Li-TFSA, PDOS of HC and LC systems of different snapshots in DFT-MD trajectories, Kohn−Sham orbital of SOMO in LC system, reduction potential of TFSA and AN, PDOS of the decomposed products after the TFSA reduction, Electronic states of LC and HC with Li 1s level reference, charge distribution, estimation of free energy errors, and total energies of 1e reduced AN and TFSA system after relaxation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(K.S.) Phone: +81-29-851-3354 (ext. 8003). E-mail: [email protected]. *(Y.T.) Phone: +81-29-859-2626. E-mail: TATEYAMA. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI 23340089 as well as the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The calculations in this work were carried out on the supercomputer centers of NIMS, ISSP, and ITC (Oalkleaf-FX) in the University of Tokyo, Kyushu University, as well as the K computer at the RIKEN AICS through the HPCI Systems Research Projects (Proposal Numbers hp130021).

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REFERENCES

(1) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (2) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. 14096

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