Quasi-liquid grain boundary conductivity in solid electrolyte candidate lithium imide

All-solid-state batteries utilising a Li-metal anode have long promised to be the next-generation of high-performance energy storage device, with a step-change in energy density, cycling stability and cell safety touted as potential advantages compared to conventional Li-ion battery cells. A key to enabling this technology is the development of solid-state electrolytes with the elusive combination of high ionic conductivity, wide electrochemical stability and the ability to form a conductive and stable interface with Li metal. Presently, oxide and sulfide-based materials, particularly garnet and argyrodite-type structures, have proved most promising for this application. However, these still suffer from a number of challenges, including resistive lithium metal interfaces, poor lithium dendrite suppression (at high current density) and low voltage stability. Here we report the first application of lithium imide, an antifluorite-structured material, as a solid electrolyte in a Li-metal battery. Low-temperature synthesis of lithium imide produces promising Li-ion conductivity, reaching > 1 mS cm-1 at 30 ˚C using a modest post-synthetic mechanochemical treatment, as well as displaying at least 5 V stability vs Li+/Li. In situ electrochemical operation of lithium imide with Li-metal electrodes reveals a 1000-fold increase in its measured ionic conductivity, whilst appearing to remain an electronic insulator. It is postulated that stoichiometry variation at the grain boundary leads to a highly disordered quasi-liquid state, facilitating this conductivity improvement. Furthermore, the material is shown to possess impressive stability under high current density conditions (70 mA cm-2) as well as the ability to operate in Li-metal battery cells. These results not only highlight the promising performance of lithium imide, but also its potential to be the basis for a new family of antifluorite based solid electrolytes.