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

Cubic phase garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) is a promising solid electrolyte for highly safe Li-ion batteries. Al-doped LLZO (Al-LLZO) has been widely studied due to the low cost of Al 2 O 3 . The reported ionic conductivities were variable due to the complicated Al 3+ -Li +  substitution and Li x AlO y segregation in Al-LLZO ceramics. This work prepared Li 7−3 x Al x La 3 Zr 2 O 12 ( 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 μs) in the cubic-tetragonal mixed phase. On the other hand, LiAlO 2 and LaAlO 3 segregation occur at the grain boundaries of high Al-doped (0.40) LLZO, resulting in a gradual Li + ion jump (6.5 μs) over grain boundaries and low ionic conductivity. The Li 6.04 Al 0.32 La 3 Zr 2 O 12 ceramic delivers the optimum Li +  ion conductivity of 1.7 × 10 −4  S cm −1  at 25 °C.