Solvation shell dynamics explains charge transport characteristics of LIB electrolytes

Electrolytes made up of LiPF6 salt in various mixtures of carbonate solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) are generally used in lithium ion batteries (LIBs). The electrolyte composed of EC and DMC in the ratio of 3:7 exhibits peak ionic conductivity. Despite its wide usage in LIBs, the reason for observed peak is not well established. We probe 1 M LiPF6 electrolytes of various EC:DMC solvent compositions ranging from 1:9 to 9:1 by moles using the molecular dynamics method to find that electrolyte of EC:DMC = 3:7 offers highest ionic conductivity. Though bulk properties such as viscosity and dielectric constant, and various structural properties vary with composition of the solvents, these properties do not explain the observed peak in ionic conductivity. However, the residence times of solvent molecules in ionic solvation shells show their minima for 3:7 electrolyte. It suggests that faster dynamics of solvation shell of ions (frequent exchange of solvent molecules) results in better ionic conductivity for 3:7 electrolyte. Insights gained in the present work, viz. the relation between ionic conductivity and solvation time of ions can be of help in designing novel high-performance electrolytes for next generation LIBs. (C) 2021 Elsevier B.V. All rights reserved.