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

Abstract

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 LiNi0.6Mn0.2Co0.2O2 (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 (Tm < 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 Tm, Li salt concentration is demonstrated to have a direct link with characteristic performance aspects of e.g. NMC622||Li cells. J. Mindemark, M. J. Lacey, T. Bowden and D. Brandell, Prog. Polym. Sci., 81, 114 (2018).L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Materials Advances, 2, 3251 (2021).G. Homann, L. Stolz, M. Winter and J. Kasnatscheew, iScience, 23, 101225 (2020).L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Materials Today, 44, 9 (2021).G. Homann, L. Stolz, J. Nair, I. C. Laskovic, M. Winter and J. Kasnatscheew, Sci Rep, 10, 4390 (2020).L. Stolz, S. Röser, G. Homann, M. Winter and J. Kasnatscheew, The Journal of Physical Chemistry C, 125, 18089 (2021). Figure 1