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

High Resolution Image Download MS PowerPoint Slide LiTa 2 PO 8, 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 LiTa 2 PO 8 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 Li 1+ x Ta 2– x Hf x PO 8, in which part of Ta 5+ in LiTa 2 PO 8 is substituted with Hf 4+ for introducing interstitial Li + ions. Li 1.2 Ta 1.8 Hf 0.2 PO 8 exhibits a total conductivity of 9.2 × 10 –4 S cm –1 at 300 K, over twice higher than that of unsubstituted LiTa 2 PO 8 . Further, pulsed-field-gradient nuclear magnetic resonance indicates that the Li + ion diffusion coefficient of Li 1.2 Ta 1.8 Hf 0.2 PO 8 in the bulk (2.0 × 10 –8 cm 2 s –1 at 330 K) is twice as high as that of LiTa 2 PO 8 (1.2 × 10 –8 cm 2 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.