Phase transition and conductivity improvement of tetragonal fast lithium ionic electrolyte Li7La3Zr2O12

• Multiple phase transition processes are observed in the temperature regime 25 °C–11,000 °C, which correspond to the tetragonal–cubic phase transition around 100 °C–150 °C and cubic–tetragonal phase transition around 800 °C–900 °C.
• A weak IF peak (labeled as PT) and a strong IF peak (labeled as PC) are observed in the tetragonal and cubic Li7La3Zr2O12, and the mechanisms are related with different distribution of lithium ions.
• (c) A novel synthesized method of undoped cubic Li7La3Zr2O12 was proposed in terms of the irreversible characteristic of tetragonal–cubic phase transition.

The phase transition processes, lithium ionic diffusion and conductivity of solid lithium ionic electrolyte Li7La3Zr2O12 with tetragonal symmetry have been carefully investigated by variable-temperature X-ray diffraction (XRD), internal friction (IF) and alternate current (AC) impedance spectroscopy, respectively. Multiple phase transition processes were observed in temperature range from 25 °C to 1000 °C, which correspond to the tetragonal–cubic phase transition around 100 °C–150 °C and the cubic–tetragonal phase transition around 800 °C–900 °C, respectively. During the IF measurement, a weak IF peak (labeled as PT) and a strong IF peak (labeled as PC) were observed in the tetragonal and cubic Li7La3Zr2O12 compounds, and the mechanism related with the dramatic difference of the relaxational strength was ascribed to the different distribution of lithium ions: order in tetragonal phase and disorder in cubic phase. The measurement of conduction property revealed an abrupt increase of conductivity in temperature range 150 °C–200 °C, which also originated from the tetragonal–cubic phase transformation. Based on the irreversible characteristic of tetragonal–cubic phase transition around 100 °C–150 °C, a novel synthesized method of cubic Li7La3Zr2O12 was proposed.