AC impedance plots and the enlargement of the LLZNO ceramic pellets under different sintering conditions are showed in Figure 4a, b. The fitting curve of the sample sintered at 1200 °C for 30 min (SL2) is showed in Figure 4c, and it consists of a quasi-semicircle at high frequency and a long diffusion tail at low frequency. The equivalent circuit model Rb(RgbQgb)(RelQel), in which Rb, Rgb, and Rel are resistances originating from the bulk, grain boundaries, and Ag electrodes, is used to fit the plots and is shown in Figure 4d. The total ionic conductivity of the ceramics is mainly decided by Rb plus Rgb. The total ionic conductivity and relative density of the LLZNO ceramic pellets are showed in Figure 4e and Table 2. The highest total ionic conductivity (5.09 × 10−4 S·cm−1) of the LLZNO ceramic pellets was obtained when sintered at a high temperature and for a short time (SL2, 1200 °C × 30 min), and its relative density is 87.3%. This indicates that high-performance LLZNO ceramics are obtained when sintered at high temperatures only for short sintering times. However, the total ionic conductivity and relative density of ceramic pellets decreased and impure phases occurred when the sintering time was prolonged at 1100 and 1200 °C. The lowest total ionic conductivity (0.35 × 10−4 S·cm−1) and relative density (83.4%) were obtained when the ceramic pellets were sintered at 1100 °C for 360 min (SL5). Meanwhile, a higher total ionic conductivity (3.49 × 10−4 S·cm−1) and a higher relative density (90.3%) of ceramic pellets (SL3, 1150 °C × 60 min) were obtained when sintered for 60 min from 1100 to 1200 °C, and this result indicates that, in this study, increasing the sintering temperature too much was disadvantageous for obtaining LLZNO ceramics with good performance.