Lithium garnets have emerged as contenders for use as an SSE, owing to a wide electrochemical window (0-6V vs Li/Li + ), chemical stability with Li metal and (in recent years) high ionic conductivity (> 0.1 mS cm -1 ) . These materials, however, form a poorly conductive tetragonal phase at high Li content (as outlined below) and suffer from atmospheric H + /Li + exchange (which ultimately forms insulating Li2CO3 passivating layers due to instability of the Li dopant in high concentrations) . This is in addition to the common SSE problems of high interfacial resistance and time-consuming synthesis.