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, LiTa2PO8 exhibits a high Li+ conductivity of 2.5 x 10-1 mS cm-1 at room temperature. Because most high Li+ conductive oxide SEs such as Li7-x La3Zr2-x Nb x O12 and Li1+x Al x Ti2-x (PO4)3 have been achieved by chemical substitutions from parent compounds (Li7La3Zr2O12 and LiTi2(PO4)3), chemical substitutions for LiTa2PO8 will be an effective way for further increasing Li+ conductivity. Here, we synthesized single-phase Ti- and W-substituted LiTa2PO8 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 (sigma total) and bulk Li+ conductivity (sigma bulk). W-substitutions also decreased E a, but both the sigma total and the sigma bulk slightly decreased. The calculated E a of sigma 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 LiTa2PO8, providing further insight into enhancing the ionic conductivity.