Salt and Solvent effect on physicochemical properties and species organisation of Lithium fluorosulfonyl imide (FSI and TFSI) based electrolytes for Li-ion battery: Consequence on cyclability of LiNi0.8Co0.15Al0.05 (NCA) cathode

Abstract

Herein we present a comparative study of the physicochemical and thermal properties of four electrolytes formulated with LiTFSI, LiFSI in binary (3EC/7EMC) and ternary (EC/PC/3DMC) mixture of solvents, as a function of salt concentration and temperature. By modeling the change on viscosity, ionic conductivity, with Arrhenius, Eyring and Jones-Dole-Kaminsky equations, we established the determinants parameters which influence the organization of solutions, in order to understand their consequences on electrochemical performances of LiNi0.8Co0.15Al0.05 (NCA) as cathode in Li-ion battery. Results shown that (3EC/7EMC + LiTFSI) present a double solvation sphere of Li+ and weak ionic association consequently it is poorly suited to the activation and cycling of NCA. While (EC/PC/3DMC + LiFSI) benefits from both a single sphere of solvation and strong ion-ion and ion-solvent interactions, making the polarization of the interfaces in this electrolyte suitable to the Li+ diffusion and to low interface resistances. These results demonstrate that the electrolyte properties such as viscosity, conductivity and thermal stability depend on several factors. Those related to species in solutions (solvents and salt ions), and those related to operating conditions such as temperature and concentration. All these parameters impact the ionic dissociation and solvation. This study shows the close-link between the organization of species in solutions, the physicochemical and thermal properties, and their impact on electrochemical performances in a Li-ion battery.