Improvement of Lithium-Ion Conductivity by Hf Substitution in Lithium Tantalum Phosphate (LiTa₂PO₈) Solid Electrolyte

LiTa2PO8, with a recently discovered framework structure and a bulk conductivity of 1.6 × 10–3 S cm–1, exhibits high potential as an electrolyte material for solid-state batteries. We have improved the ionic conductivity of LiTa2PO8 via elemental substitution. In this study, we use ab initio molecular dynamics simulations to select the substituted elements and find that interstitial Li+ ions improve Li+ ion mobility in the bulk. Consequently, we synthesize Li1+xTa2–xHfxPO8, in which part of Ta5+ in LiTa2PO8 is substituted with Hf4+ for introducing interstitial Li+ ions. Li1.2Ta1.8Hf0.2PO8 exhibits a total conductivity of 9.2 × 10–4 S cm–1 at 300 K, over twice higher than that of unsubstituted LiTa2PO8. Further, pulsed-field-gradient nuclear magnetic resonance indicates that the Li+ ion diffusion coefficient of Li1.2Ta1.8Hf0.2PO8 in the bulk (2.0 × 10–8 cm2 s–1 at 330 K) is twice as high as that of LiTa2PO8 (1.2 × 10–8 cm2 s–1), with the activation energy reduced from 0.30 to 0.27 eV. This improvement in the bulk diffusion can be attributed to the higher dimensionality of the conduction paths with introduced interstitial Li+ ions.