Dual regulation of Li+ migration of Li6.4La3Zr1.4M0.6O12 (M = Sb,Ta, Nb) by bottleneck size and bond length of M−O

Bottleneck size is the minimum Li⁺ migration channel of Li₇La₃Zr₂O₁₂ (LLZO) and it greatly influences the Li⁺ conductivity. Doping different elements on the Zr site of LLZO can adjust the bottleneck size and improve the Li⁺ conductivity. However, the regulation mechanism is not clear. In this work, Li_6.4La₃Zr_1.4M_0.6O₁₂ (M = Sb, Ta, Nb) has been prepared and the bottleneck size has been adjusted by doping different pentavalent ions. The results manifest that the cell parameter and bottleneck size decrease with the rise of the radius of doped pentavalent ions. This is because larger pentavalent ion leads to larger bond length of M–O, and weaker covalent component between M⁵⁺ and O^2-, corresponding, the formal charge on the M⁵⁺ become larger, and the bond length of La–O slightly decreases due to the coulomb repulsion between La³⁺ and M⁵⁺ increase. While, the activation energy drop firstly and then rise with the rise of bottleneck size because of the migration of Li⁺ not only relate to the size of the migration channel but also to the strength of M–O covalent bonding. The bottleneck size and bond length of M–O synergistically affect the migration of Li⁺.