Lithium thiophosphate glasses and glass-ceramics as solid electrolytes: Processing, microstructure, and properties

Lithium solid electrolytes are of major interest for solid-state batteries and electrochemical capacitors (ECs). Currently, the material selection space of liquid electrolytes is dominated by lithium salts paired with organics. Improved safety, as well as the need for higher temperature and high voltage operation, opens up opportunities for glass and ceramic alternatives in these important solid-state energy storage technologies. Lithium thiophosphates in the family x Li₂S + (1-x) P₂S₅ (mol fraction) possess room temperature ionic conductivities greater than 10^-3(Ω-cm)^-1 in crystallized x = 0.70 (almost the highest in inorganic solid-state electrolytes). Within this review article, we address recent progress made in this class of material. We consider the role of densification on the Li-ion conductivity, as well as our recent data on the effect of densification on the electrochemical properties of the system. We cover the processing techniques of mechanical milling and pressure-forming, discuss microstructure, bulk versus surface conduction, and device integration. The systematic improvement in ionic conductivity with increased density suggests that bulk conduction dominates surface conduction and demonstrates that dense, rather than porous, lithium thiophosphate solid electrolytes are important in the design of solid-state batteries and ECs.