Investigating Li-ion diffusion in LiTa2-xNbxPO8 using μ+SR

Li-ion solid-state electrolytes with high ionic conductivities are essential for advancing solid-state batteries with increased energy density. However, many of the most conductive candidates suffer from poor air stability. Recently, LiTa2PO8 (LTPO) has emerged as a promising, air-stable contender, exhibiting total ionic conductivities in the 10-4 S/cm range at room temperature. While aliovalent substitution in LTPO has been shown to increase conductivity, the effects of isovalent substitution remain unexplored. In this proposal, we aim to investigate the transport properties of a new family of compounds, LiTa2-xNbxPO8 (0 ≤ x ≤ 0.8), synthesised by conventional solid-state methods. Partial substitution of Ta5+ with Nb5+ – which shares an identical ionic radius – has been found to enhance pellet densification and increase total Li-ion conductivity by an order of magnitude. Despite these improvements, the underlying relationship between structure and Li-ion mobility is still unclear. To address this, we propose μ+SR experiments on three selected compositions in the series to directly probe intrinsic Li-ion diffusion parameters. μ+SR will provide complementary insights to EIS by disentangling grain boundary effects, contributing not only to a deeper understanding of the role of isovalent substitution in tuning ion transport and but also clarifying key composition–structure–property relationships in oxide SSEs.