DFT-MD Study of Sacrificial Anion Reduction and Li-Ion Diffusion Mechanisms in Superconcentrated Electrolytes
Authors: Sodeyama Keitaro , Yamada Yuki , Yamada Atsuo , Tateyama Yoshitaka 
Published: February 2020

Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes and high ion conductivity in Lithium-ion batteries (LIBs)1. For example, highly concentrated (HC) LiN(SO2CF3)2 (Li-TFSA) or LiN(SO2F)2 (Li-FSA) salt in acetonitrile (AN) electrolyte shows strong electrochemical stability against the reductive decomposition, though in low concentration (LC) solution AN is easily reduced and decomposed1. It has a large impact on the exploration for better LIB electrolyte. However, the atomistic origin of the improved reduction stability and high ion conductivity in HC system has been still an open question.

In this study, we investigated the mechanism of the improvement of the reduction stability of Li-TFSA/AN and Li-FSA/AN systems and Li-ion diffusion mechanism depending on the salt concentration by using first principles density functional theory (DFT) molecular dynamics (MD) calculations with explicit AN solvents2. For the reduction reaction, we adopt DFT-MD calculation with extra electron, which is referred as the DSCF type analysis. It can include the relaxation effect of reductive electron and the decomposition after the reduction of electrolytes, while the usual HOMO-LUMO based discussions cannot treat the effects. We explored which molecule accepts the reduced one electron (1e) in HC and LC systems and discussed the origin of reduction stability. For the Li-ion diffusion, we calculated the diffusion coefficients of the Li ions, FSA and TFSA anions, and AN solvent molecules in the LC and HC electrolytes to elucidate how Li-ion diffusion was affected by concentration.

For the reduction stability, in the HC systems under the 1e reduction condition, we found that TFSA anion sacrificially accepts reductive electron, because specific chained network structure is formed and the electron affinity of the anion shifts lower. The reduced TFSA anion was decomposed into the CF3 moiety and (SO2)2CF3N spontaneously. This result indicates that the TFSA decomposed products can stack on the interface between the negative electrode and electrolyte, forming a sort of solid electrolyte interface (SEI). In fact, experimental XPS study confirmed the F-related species on the negative electrode, and it supports this sacrificial anion reduction mechanism.

For the Li-ion diffusion mechanism, we confirmed that the calculated values of the Li-ion diffusion coefficients for the Li-FSA/AN electrolyte were on the same order of magnitude as the experimental values. From the reliable trajectories, we confirmed that in the LC electrolytes each Li-ion is coordinated only by solvent molecules, and the Li-ions diffuse in the company of the coordinated solvent molecules (vehicle-type diffusion). In the HC electrolytes, the vehicle-type diffusion is difficult because the Li-ions are coordinated both by solvent molecules and by anions arranged in a specific network structure2, which results in high viscosity. We analyzed the motions of individual Li ions in the HC electrolytes, and found Li-ion hopping between the oxygen atoms of the anions in both FSA and TFSA anion systems (Fig.1). Additional DFT-MD calculations with different solvents also suggest the Li-ion hopping diffusion mechanism. We concluded that change of the diffusion mechanism can be an origin of the high Li-ion conductivity in the HC electrolytes.

[1] Y. Yamada, K. Furukawa, K. Sodeyama, M. Yaegashi, K. Kikuchi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc.
136, 5039-5046 (2014).

[2] K. Sodeyama, Y. Yamada, K. Aikawa, A. Yamada, Y. Tateyama, J. Phys. Chem. C
118, 14091-14097 (2014).

Figure 1