Enabling ionic transport in Li3AlP2: the roles of defects and disorder†

Lithium phosphides are an emerging class of Li⁺ ion conductors for solid state battery applications. Despite potentially favorable characteristics as a solid electrolyte, stoichiometric crystalline Li₃AlP₂ has been reported to be an ionic insulator. Using a combined computational and experimental approach, we investigate the underlying reasons for this and show that ion transport can be induced via defects and structural disorder in this material. Lithium vacancies are shown to promote diffusion, and a low barrier to Li⁺ hopping of 0.2–0.3 eV is revealed by both simulations and experiment. However, polycrystalline pellets exhibit low ionic conductivity (≈10⁻⁸ S cm⁻¹) at room temperature, attributed to crystalline anisotropy and the presence of resistive grain boundaries. These aspects can be overcome in nanocrystalline Li₃AlP₂, where ionic conductivity values approaching 10⁻⁶ S cm⁻¹ and low electronic conductivities are achieved. This approach, leveraging both defects and structural disorder, should have relevance to the discovery of new, or previously overlooked, ion conducting materials.