LAUSR

Abstract The Li + concentration and Li + transport channel size of garnet-type solid electrolyte Li 7 La 3 Zr 2 O 12 are the two key factors associated with the improvement of Li ionic conductivity. Through precise control of the Li + concentration and the Li + transport channel size by Ta-doping (Li 7-X La 3 Zr 2-X Ta X O 12 , X = 0.2–1.0), synergistic regulation of the two factors is achieved. Results show that the optimum Li + concentration seems to be at about 6.31 where the Li + occupancy in LiO 6 is high and the Li + arrangement in LiO 4 is disordering. With the Li + concentration higher than 6.31, the occupancy in LiO 4 sites remains constant but the Li + arrangement in LiO 4 becomes more and more ordering. At higher Li + concentration of 6.52, Li + arrangement becomes certain of ordering which should have reduced the grain conductivity, however, the grain conductivity is still as high as 9.84 × 10 −4  S cm −1 . This indicates that LLZO with this Li + concentration have favorable Li + transport channel size. The optimum Li + transport channel size is about 12.95 ± 0.01 A. At Li + concentration of 6.31, the proper Li + concentration is the main factor for the high grain ionic conductivity of 10.1 × 10 −4  S cm −1 . While at Li + concentration of 6.52, the suitable Li + transport channel size is the main factor for the high grain ionic conductivity. In conclusion, the high grain ionic conductivity at Li + concentration of 6.31–6.52 is due to the synergistic regulation of Li + concentration and Li + transport channel size.