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, Li6.4La3Zr1.4M0.6O12 (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 O2-, 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+.