Entropy-induced high conductivity in fully-reduced electrolytes for solid-state batteries with lithium metal anodes

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 potentials of next-generation anodes leading to irreversible lithium loss and increases in cell resistance. Such performance losses due to electrochemical decomposition may be prevented by designing electrolytes which are thermodynamically stable at low operating potentials (anolytes). Here, we report on the discovery a new family of fully-reduced electrolytes by dissolving lithium nitride into the Li2S antifluorite structure, yielding highly conducting crystalline Li2+xS1-xNx phases, synthesized by mechanochemistry, identified by x-ray and neutron diffraction, and reaching high ionic conductivities (> 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 increased configurational entropy boosts ionic conductivity in Li2+xS1-xNx phases by a factor 105 compared to the Li2S host structure. This advance is achieved through a novel theoretical framework, leveraging percolation analysis with local-environment-specific calculated activation barriers and is widely applicable to disordered solid electrolytes. Finally, we introduce the concept of “trapped” Li ions and how they may play an essential role when rationalizing changes in the Arrhenius prefactors from variable temperature conductivity measurements of disordered solid electrolytes. These findings pave the way to understanding disordered solid electrolytes and eliminating decomposition-induced Li losses on the anode side in solid state batteries.