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.