Local structure and lithium transport in Zr- and Hf-Doped LiTa2PO8 ceramics

Among the various ceramic oxide-based Li-ion conductors, LiTa 2 PO 8 has emerged as a promising candidate for application as an electrolyte in solid state lithium batteries. Here, the effects on local structure of substitution at the Ta site in the systems Li 1+ x Ta 1− x M x PO 8 (M = Hf or Zr) are investigated using a combination of neutron total scattering, 6 Li and 31 P solid state NMR and Raman spectroscopies, supported by microstructural and electrical measurements. In both the Hf and Zr substituted systems a limited solid solution up to x = 0.05 is observed. Structural analysis of the x = 0.05 compositions reveals Li-ions to be distributed over four 5-coordinate sites, which link to form chain-like diffusion channels in the <001> direction. Reverse Monte Carlo analysis of total neutron scattering data shows subtle differences in the Li-ion distribution in the two systems with the Hf substituted system exhibiting a more 2-dimensional character. Despite these structural differences the conductivity and activation energies of the two systems are very similar with room temperature values around 0.9 × 10 −3 S cm −1 and 1.6 × 10 −3 S cm −1 for the total and bulk conductivities, respectively, which are particularly high for oxide-based systems. There is some evidence of Li-ion clustering in the two systems which may suggest cooperative motion of Li-ions in the conduction process.