Alternate Crystal Structure Achieving Ionic Conductivity above 1 mS cm–1 in Cost-Effective Zr-Based Chloride Solid Electrolytes

The realization of practical, commercial all-solid-state Li batteries requires the solid electrolyte to possess not only high ionic conductivity (above 1 mS cm^-1 at 25 °C) but also low cost (below $50/kg). Unlike most of the present solid electrolytes, the recently reported Zr-based chloride solid electrolytes generally cost less than $50/kg, but their ionic conductivities at 25 °C are below 1 mS cm^-1. Here, a Li-ion conductivity of 1.35 mS cm^-1 at 25 °C and an estimated material cost of $11.09/kg are achieved simultaneously in a Li₃Zr_0.75OCl₄ solid electrolyte. Unlike other Zr-based chloride systems, Li₃Zr_0.75OCl₄ does not exhibit the trigonal structure, but is isostructural with Li₃ScCl₆, whose monoclinic structure allows for much faster ion transport. With such desirable characteristics, the all-solid-state cell formed by LiNi_0.8Mn_0.1Co_0.1O₂ and Li₃Zr_0.75OCl₄ shows a capacity retention above 80.9% for 700 cycles at 25 °C and 5 C (975 mA g^-1).