Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor

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 Li3YCl6and demonstrate a method of controlling its Li+conductivity by tuning the defect concentration with synthesis andheat treatments at select temperatures. Leveraging complementaryinsights 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 loweringLi+migration barriers and increasing Li site connectivity in mechanochemically synthesized Li3YCl6. We harness paramagneticrelaxation enhancement to enable89Y 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 degrees 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

Automated summary

In this study, we uncovered the presence of a high concentration of stacking faults in the superionic conductor Li3YCl6 and demonstrated a method of controlling its Li+ conductivity by tuning the defect concentration with synthesis and heat treatments. We used a combination of calculations, experiments, and analysis to identify the nature of planar defects and the role of nonstoichiometry in enhancing Li+ conduction in mechanochemically synthesized Li3YCl6.