Microstructure and conductivity of Al-substituted Li7La3Zr2O12 ceramics with different grain sizes

Al-doped Li7La3Zr2O12 (LLZ) sintered ceramics with two grain sizes were prepared to investigate the relationship between their microstructure and conducting properties. The X-ray diffraction patterns showed no difference in the peak shift and intensity between the two prepared LLZ ceramics, suggesting that their bulk conductivities were almost the same. Scanning electron microscopy and energy dispersive X-ray spectrometry were used to observe their microstructure. Neither of the ceramics had an obvious impurity phase along grain boundaries. One of LLZ (LLZ_LG) had a larger grain size of 5-20 mu m and showed intergranular fracture. The other LLZ (LLZ_SG) had a smaller grain size of primarily <1 mu m and showed intragranular fracture. LLZ_LG showed a total conductivity of 3.6 x 10(-4) S cm(-1) and an activation energy, E-a, for conduction of 32 kJ mol(-1), while LLZ_SG showed conductivity of 4.4 x 10(-4) S cm(-1) and E-a of 26 kJ mol(-1). It is noteworthy that the grain boundary composed of small grains showed lower grain boundary resistance than that composed of large grains. As a result, it is clarified that LLZ ceramics with higher ion-conductivity and lower E-a can be prepared by suppressing grain growth.