Phase transformation and grain-boundary segregation in Al-Doped Li7La3Zr2O12 ceramics

Cubic phase garnet-type Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for highly safe Li-ion batteries. Aldoped LLZO (Al-LLZO) has been widely studied due to the low cost of Al2O3. The reported ionic conductivities were variable due to the complicated Al3+-Li+ substitution and Li(x)AlOy segregation in Al-LLZO ceramics. This work prepared Li7-3xAlxLa3Zr2O12 (x = 0.00-0.40) ceramics via a conventional solid-state reaction method. The AC impedance and corresponding distribution of relaxation times (DRT) were analyzed combined with phase transformation, cross-sectional microstructure evolution, and grain boundary element mapping results for these Al-LLZO ceramics to understand the various ionic transportation levels in LLZO with different Al-doping amounts. The low conductivity in low Al-doped (0.12-0.28) LLZO originates from the slow Li+ ion migration (1.4-0.25 mu s) in the cubic-tetragonal mixed phase. On the other hand, LiAlO2 and LaAlO3 segregation occur at the grain boundaries of high Al-doped (0.40) LLZO, resulting in a gradual Li+ ion jump (6.5 mu s) over grain boundaries and low ionic conductivity. The Li6.04Al0.32La3Zr2O12 ceramic delivers the optimum Li+ ion conductivity of 1.7 x 10(-4) S cm(-1) at 25 degrees C.

Automated summary

Cubic phase garnet-type Li7La3Zr2O12 (LLZO) is a safe solid electrolyte for Li-ion batteries, while Al-doped LLZO (Al-LLZO) has been widely studied for its low cost, but the ionic conductivity is affected by Al3+-Li+ substitution and Li(x)AlOy segregation.