Disorder-mediated ionic conductivity in irreducible solid electrolytes

Solid state batteries currently receive extensive attention due to their potential to outperform lithium ion batteries in terms of energy density when featuring next generation anodes such as lithium metal or silicon. However, most highly conducting solid electrolytes decompose at the low operating voltages of next-generation anodes leading to irreversible lithium loss and increased cell resistance. Such performance losses may be prevented by designing electrolytes which are thermodynamically stable at low operating voltages (anolytes). Here, we report on the discovery a new family of irreducible (i.e. fully reduced) electrolytes by mechanochemically dissolving lithium nitride into the Li2S antifluorite structure, yielding highly conducting crystalline Li2+xS1-xNx phases reaching > 0.2 mS cm-1 at ambient temperatures. Combining impedance spectroscopy experiments and ab initio density functional theory calculations we clarify the mechanism by which the disordering of the sulfide and nitride ions in the anion sublattice boosts ionic conductivity in Li2+xS1-xNx phases by a factor 10^5 compared to the Li2S host structure. This advance is achieved through a novel theoretical framework, leveraging percolation analysis with local-environment-specific activation energies and is widely applicable to disordered ion conductors. The same methodology allows us to rationalize how increasing nitrogen content in Li2+xS1-xNx antifluorite-like samples leads to both increased ionic conductivity and lower conductivity-activation energy. These findings pave the way to understanding disordered solid electrolytes and eliminating decomposition-induced performance losses on the anode side in solid-state batteries.