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 Li 3 YCl 6 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 Li 3 YCl 6 . We harness paramagnetic relaxation enhancement to enable 89 Y 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 °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.

Layered-oxide LiNi x Mn y Co1–x–y O2 (NMC) positive electrodes with high nickel content deliver high voltages and energy densities. However, a high nickel content, e.g., x = 0.8 (NMC811), can lead to high surface reactivity, which can trigger thermal runaway and gas generation. While claimed safer, all-solid-state batteries still suffer from high interfacial resistance. Here, we investigate niobate and tantalate coating materials, which can mitigate the interfacial reactivities in Li-ion and all-solid-state batteries. First-principles calculations reveal the multiphasic nature of Li–Nb–O and Li–Ta–O coatings, containing mixtures of LiNbO3 and Li3NbO4 or of LiTaO3 and Li3TaO4. The concurrence of several phases in Li–Nb–O or Li–Ta–O modulates the type of stable native defects in these coatings. Li–Nb–O and Li–Ta–O coating materials can favorably form lithium vacancies VacLi ′ and antisite defects NbLi •••• (TaLi ••••) combined into charge-neutral defect complexes. Even in defective crystalline LiNbO3 (or LiTaO3), we reveal poor Li-ion conduction properties. In contrast, Li3NbO4 and Li3TaO4 that are introduced by high-temperature calcinations can provide adequate Li-ion transport in these coatings. Our in-depth investigation of the structure–property relationships in the important Li–Nb–O and Li–Ta–O coating materials helps to develop more suitable calcination protocols to maximize the functional properties of these niobates and tantalates.