PNNL

Abstract

We report a phase-pure super-halide-rich argyrodite (Li5.3PS4.3Cl1.7) that exhibits the highest room temperature ionic conductivity of solid electrolytes in its class, of 11.4 ± 0.7 mS cm-1. This owes to population of two additional interstitial lithium sites in the lattice induced by vacancy re-distribution, as determined by high resolution neutron diffraction. Prominent lithium density between Li sites, and a remarkably high atomic displacement parameter reflect a liquid-like diffusive behavior within the framework. By a combination of electrochemical impedance spectroscopy, pulsed-field gradient nuclear magnetic resonance, and T1 relaxation NMR measurements, we show that the augmented ionic conductivity of Li5.3PS4.3Cl1.7 in part originates from a low energy barrier (0.08 eV) at a local length scale, compared to Li6PS5Cl. The facile local hopping process is linked to a unique three-site lithium distribution, influencing correlated lithium dynamics. Our investigation also reveals the inapplicability of the widely applied Meyer-Neldel rule to the analogous Br-rich argyrodites, where we attribute the enhanced conductivity of Li5.5PS4.5Br1.5 to a higher migration entropy. Our work provides a comprehensive understanding of the intricate interplay between structure and lithium dynamics in super-halide-rich argyrodites, shedding light on their application as solid electrolytes for solid state batteries.