| In an effort to measure the electrical grain boundary characteristics of fully dense (∼99%) cubic Li 7-3xAlxLa3Zr 2O12 (x = 0.05 - 0.3), small grain size samples of 1 - 2mum were made using the Spark Plasma Sintering (SPS) method. The large internal interfacial area of SPS samples increased the grain boundary contribution to the total resistance to separate typically overlapping bulk and grain boundary impedance responses, supporting grain boundary conductivity (sigma GB) measurements. sigmaGBwas on average two orders of magnitude lower than the bulk conductivity (sigmabulk), indicating the grain boundaries of this material are resistive to Li+ transport, despite literature reports of low grain boundary resistance. sigmaGB = 3.8x10-7 +/- 11%, 7.9x10-7 +/- 11%, and 4.8x10-7 +/- 27% S/cm at 250C with corresponding EaGB = 0.44 +/- 3%, 0.49 +/- 4%, 0.46 +/- 2% eV, for 5, 20, and 30mol%Al 3+ LLZO, respectively. The electrical grain boundary thickness (delta GB) calculated using the Brick Layer Model decreased monotonically with aluminum concentration (CAl3+) to indicate a possible space-charge effect on sigmaGB, and was deltaGB = 10.2 +/- 18% , 9.9 +/- 12% , 6.0 +/- 20% nm for 5, 20, and 30mol%Al LLZO, respectively. Non-linear behavior of sigmaGB with CAl3+ was discussed in terms of competing space-charge and impurity segregation effects that simultaneously influence lithium conduction in the electrical GB. Optimum sigmaGB and sigmabulk of 7.9x10-7 +/- 11% and 4.6x10-4 +/- 4% S/cm at 250C, respectively, were found to coincide at CAl3+ = 20mol%, the critical aluminum dopant concentration for cubic LLZO phase stability. |
