  In the pursuit of urgently-needed, energy dense solid-state batteries for
electric vehicle and portable electronics applications, halide solid
electrolytes offer a promising path forward with exceptional compatibility
against high-voltage oxide electrodes, tunable ionic conductivities, and facile
processing. For this family of compounds, synthesis protocols strongly affect
cation site disorder and modulate Li+ mobility. In this work, we reveal the
presence of a high concentration of stacking faults in the superionic conductor
Li3YCl6 and demonstrate a method of controlling its Li+ conductivity by tuning
the defect concentration with synthesis and heat treatments at select
temperatures. Leveraging complementary insights from variable temperature
synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission
electron microscopy, solid-state nuclear magnetic resonance, density functional
theory, and electrochemical impedance spectroscopy, we identify the nature of
planar defects and the role of nonstoichiometry in lowering Li+ migration
barriers and increasing Li site connectivity in mechanochemically-synthesized
Li3YCl6. We harness paramagnetic relaxation enhancement to enable 89Y
solid-state NMR, and directly contrast the Y cation site disorder resulting
from different preparation methods, demonstrating a potent tool for other
researchers studying Y-containing compositions. With heat treatments at
temperatures as low as 333 K (60{\deg}C), we decrease the concentration of
planar defects, demonstrating a simple method for tuning the Li+ conductivity.
Findings from this work are expected to be generalizable to other halide solid
electrolyte candidates and provide an improved understanding of defect-enabled
Li+ conduction in this class of Li-ion conductors.
