All solid-state batteries (ASSBs) are natural successors to current LIBs, as they could enable Li metal anodes, wider electrochemical windows and improved safety . However, the development of a suitable solid-state electrolyte (SSE) has proved troublesome. To date many oxide and sulphide SSEs have been studied, but often have a high ionic conductivity or a wide electrochemical window, but rarely both . Finding a suitable SSE is, therefore, a key challenge to enable the next generation of energy storage .