Pragmatic Approaches to Correlate between the Physicochemical Properties of a Linear Poly(ethylene oxide)-Based Solid Polymer Electrolyte and the Performance in a High-Voltage Li-Metal Battery

Abstract

Given its abundance, costs and compatibility with Li, and relatively simple processing, a poly(ethylene oxide)-based solid electrolyte (PEO-based SE) is a reasonable benchmark SE system for solid-state lithium batteries. On the basis of recent pragmatic achievements in cell design and characterization methodology, the physicochemical properties of a PEO-based SE are effectively determined as a function of lithium bis-(trifluoromethyl sulfonyl)-imide (LiTFSI) salt concentration and correlated with the performance in LiNi0.6Mn0.2Co0.2O2 (NMC622)||lithium cells. Even though higher LiTFSI concentrations decrease the crystalline phases with a plasticizing effect and soften the SE membrane, the accompanying increase in amorphous phases enhances the Li+ diffusion coefficient, as galvanostatically obtained from the Sand equation. Both the increased diffusivity of Li+ and the overall amount of charge carriers lead to improved ionic conductivities with a higher LiTFSI concentration, particularly below the melting point (Tm < 60 °C). In terms of anodic behavior, neither SE decomposition nor Al current collector dissolution is relevantly affected by the LiTFSI concentration, revealing a bulk electrolyte stability of 4.6 V vs Li|Li+ and an Al dissolution tendency as low as in LiPF6-containing liquid organic electrolytes. Finally, at an operation temperature below Tm, a higher LiTFSI concentration sensitively increases the specific capacities and Coulombic efficiencies.