Correlation between lithium-ion diffusion and coordination environment in solid polymer electrolytes: a molecular dynamics study

Abstract

Lithium-ion diffusion in solid polymer electrolytes (SPEs) is a pivotal characteristic that significantly influences overall lithium-ion battery performance. This characteristic can be affected by the coordination environment of lithium ions within the polymer matrix. However, the correlation between lithium-ion diffusion and its coordination environment in biopolymer-based SPEs such as carboxymethyl chitosan (CMCS) remains understudied. In this study, we used molecular dynamics (MD) simulations to investigate this correlation. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) was used as the lithium salt in the simulated systems. All MD simulations were conducted using the GROMACS package with the general AMBER force field (GAFF). The coordination structures around Li+ were successfully estimated using the radial distribution function obtained from the MD simulations. These results indicate a preference for Li+ coordination with oxygen atoms, both from the CMCS polymer chains (OCMCS) and TFSI− ions (OTFSI-). The coordination number between Li+ and OCMCS decreases as the concentration of LiTFSI increases. The diffusion coefficients of Li+ varied depending on the concentration of LiTFSI and demonstrated a sensitivity to the coordination structure of Li+. A high diffusion coefficient of Li+ ions was observed at low LiTFSI concentrations, where Li+ was primarily coordinated with oxygen atoms from the CMCS polymer chains.