  Among the chloride-based Li-ion solid electrolytes, Li$_2$ZrCl$_6$ (LZC) have
emerged as potential candidates due to their affordability, moisture stability,
and high ionic conductivity. LZC synthesized by solid-state heating exhibits
limited Li-ion conductivity while the mechanochemical ball-milled material is
more conductive. In this computational study, we integrate thermodynamic
modeling, using cluster-expansion Monte Carlo, and kinetic modeling, using
molecular dynamics, to investigate whether cation disorder can be achieved in
LZC, and how it affects Li-ion transport. Our results indicate that fast Li-ion
conductivity is induced by the activation of Li/vacancy disorder, which itself
depends on the degree of Zr disorder. We find that the very high-temperature
scale at which equilibrium Zr-disorder can form precludes any equilibrium
synthesis processes for achieving fast Li-ion conductivity, rationalizing why
only non-equilibrium synthesis methods, such as ball milling leads to good
conductivity. We identify as the critical mechanism the lack of Li/vacancy
disorder near room temperature when Zr is well-ordered. Our simulations further
show that the Li/vacancy order-disorder transition temperature is lowered by Zr
disorder, which is necessary for creating high Li diffusivity at room
temperature. The insights obtained from this study raise a challenge for the
large-scale production of these materials and the potential for the long-term
stability of their properties.
