The importance of A-site cation chemistry in superionic halide solid electrolytes

Halide solid electrolytes do not currently display ionic conductivities suitable for high-power all-solid-state batteries. We explore the model system A₂ZrCl₆ (A = Li, Na, Cu, Ag) to understand the fundamental role that A-site chemistry plays on fast ion transport. Having synthesised the previously unknown Ag₂ZrCl₆ we reveal high room temperature ionic conductivities in Cu₂ZrCl₆ and Ag₂ZrCl₆ of 1 × 10^-2 and 4 × 10^-3 S cm^-1, respectively. We introduce the concept that there are inherent limits to ionic conductivity in solids, where the energy and number of transition states play pivotal roles. Transport that involves multiple coordination changes along the pathway suffer from an intrinsic minimum activation energy. At certain lattice sizes, the energies of different coordinations can become equivalent, leading to lower barriers when a pathway involves a single coordination change. Our models provide a deeper understanding into the optimisation and design criteria for halide superionic conductors.