  In the development of low cost, sustainable, and energy-dense batteries,
chloride-based compounds are promising catholyte materials for solid-state
batteries owing to their high Na-ion conductivities and oxidative stabilities.
The ability to further improve Na-ion conduction, however, requires an
understanding of the impact of long-range and local structural features on
transport in these systems. In this study, we leverage different synthesis
methods to control polymorphism and cation disorder in Na-Y-Zr-Cl solid
electrolytes and interrogate the impact on Na-ion conduction. We demonstrate
the existence of a more conductive P2$_1$/n polymorph of Na$_2$ZrCl$_6$ formed
upon ball milling. In Na$_3$YCl$_6$, the R$\bar{3}$ polymorph is shown to be
more conductive than its P2$_1$/n counterpart owing to the presence of
intrinsic vacancies and disorder on the Y sublattice. Transition metal ordering
in the Na$_{2.25}$Y$_{0.25}$Zr$_{0.75}$Cl$_6$ composition strongly impacts
Na-ion transport, where a greater mixing of Y$^{3+}$ and Zr$^{4+}$ on the
transition metal sublattice facilitates ion migration through partial
activation of Cl rotations at relevant temperatures. Overall, Na-ion transport
sensitively depends on the phases and transition metal distributions stabilized
during synthesis. These results are likely generalizable to other halide
compositions and indicate that achieving control over the synthetic protocol
and resultant structure is key in the pursuit of improved catholytes for high
voltage solid-state sodium-ion batteries.
