The relationship between the redox activity and electrochemical stability of solid electrolytes for solid-state batteries

TL;DR

This paper investigates the redox activity and stability of solid electrolytes in solid-state batteries, revealing that indirect decomposition pathways extend their stability window and influence battery cycling performance.

Contribution

It uncovers that solid electrolyte decomposition occurs via (de)lithiated states, explaining stability and redox activity, guiding future material and interface design.

Findings

Decomposition pathways are indirect, involving (de)lithiated states.

Electrochemical stability window is larger than predicted.

Metastable phases contribute to reversible capacity.

Abstract

All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite, garnet and NASICON type solid electrolytes, that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is significantly larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.