Physicochemical Interplay in Solid Polymer Electrolytes: Benchmarking, Prospects and Limits in High Voltage Batteries

Kasnatscheew, Johannes

A systematic R&D of solid electrolytes (SEs) requires reasonable benchmark systems for the intra- and interlaboratory comparison and evaluation. Given its abundance, costs, compatibility with Li and a relative simplicity in processing, the poly(ethylene oxide)-based solid electrolyte (PEO-based SE) is a reasonable benchmark SE system for solid-state lithium batteries.(1)

On the basis of recent progress in cell design and methodology,(2-5) the physicochemical properties of PEO-based SE are elaborated as a function of Li salt concentration and related with the performance in LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)||lithium cells. The overall aim is to unravel the apparently complex interplay of relevant parameter and finally to demonstrate the prospects and limits of the SPE benchmark in practical lithium-based batteries.(6)

For instance, despite the decrease of the crystalline phases with a Li salt in a plasticizing manner leading to SE membrane softening, the accompanied increase in amorphous phases enhances the Li⁺ diffusion coefficient, which can be easily obtained from the analysis of Li||Li cells with the Sand equation. Both, the increased diffusivity of Li⁺ and the overall amount of charge carriers leads to improved ionic conductivities with higher Li salt concentration, particularly below the melting point (T_m < 60 °C). In terms of anodic behavior, neither SE decomposition nor Al current collector dissolution is visibly affected by the Li salt concentration, revealing a surprisingly high bulk electrolyte stability of 4.6 V vs. Li|Li⁺ on practical, i.e. composite electrodes; and an Al dissolution tendency as low as in conventional LiPF₆/carbonate-based liquid electrolytes. Finally, at an operation temperature below T_m, Li salt concentration is demonstrated to have a direct link with characteristic performance aspects of e.g. NMC622||Li cells.

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232 Physicochemical Interplay in Solid Polymer Electrolytes: Benchmarking, Prospects and Limits in High Voltage Batteries

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Physicochemical Interplay in Solid Polymer Electrolytes: Benchmarking, Prospects and Limits in High Voltage Batteries