Structural and Li-Ion Conduction Properties of Ti- and W-Substituted LiTa₂PO₈

Li + -conductive oxide solid electrolytes (SEs) are chemically stable and could potentially lead to safer all-solid-state batteries. However, practical applications of oxide SEs are limited in the current stage, and one of the crucial problems arises from 1 or 2 orders lower ionic conductivity of oxide SEs than sulfide and chloride SEs. Among the various oxide SEs, LiTa 2 PO 8 exhibits a high Li + conductivity of 2.5 × 10 –1 mS cm –1 at room temperature. Because most high Li + conductive oxide SEs such as Li 7– x La 3 Zr 2– x Nb x O 12 and Li 1+ x Al x Ti 2– x (PO 4 ) 3 have been achieved by chemical substitutions from parent compounds (Li 7 La 3 Zr 2 O 12 and LiTi 2 (PO 4 ) 3 ), chemical substitutions for LiTa 2 PO 8 will be an effective way for further increasing Li + conductivity. Here, we synthesized single-phase Ti- and W-substituted LiTa 2 PO 8 and investigated their Li + conductivities. Both of these substitutions decreased unit cell volume. Ti-substitutions decreased activation energy for Li + conduction in the bulk ( E a ) and almost doubled both the total Li + conductivity (σ total ) and bulk Li + conductivity (σ bulk ) . W-substitutions also decreased E a, but both the σ total and the σ bulk slightly decreased. The calculated E a of σ bulk by bond valence energy assuming single-ion hopping increased in both substitutions due to their smaller unit cell volume. On the other hand, experimentally obtained E a decreased for both substitutions. This discrepancy may arise from the ionic correlation of the Li + ion in LiTa 2 PO 8, providing further insight into enhancing the ionic conductivity.