context: Prior to its integration with the electrodes, various properties of the LLZO SE such as the microstructure, crystalline phase and ionic conductivity were characterized. To improve the Li ion conductivity in the parent LLZO structures, Nb5+ was doped at the Zr4+ sites to create Li+ vacancies, and Ba2+ was doped at the La3+ sites to increase the Li+ concentration.Fig. 1a shows the cross-sectional SEM image of the LLZO SE pellet. A dense microstructure was formed by the uniaxial pressing and the subsequent sintering steps. The relative density of the LLZO pellet was ∼95% which was the ratio of the theoretical density calculated using the lattice parameter derived from XRD and the real density measured using a He gas pycnometer. XRD analysis of the LLZO SE (Fig. 1b) reveals the cubic phase of LLZO, where most of the diffraction peaks relate to the parent Li–garnet phase “Li5La3Nb2O12” with the space group Iad. A small peak at ∼2θ = 30 degree corresponds to the presence of BaZrO3 impurities. LLZO exists in cubic and tetragonal polymorphs. The former is typically obtained at higher sintering temperatures (>1000 °C), and exhibits about two orders of magnitude higher Li-ion conduction.Fig. 1c shows the Nyquist plots of a Au|LLZO|Au quasi-blocking cell obtained at room temperature. Au metal was sputtered on either side of the LLZO pellet to obtain uniform contacts. The plots were fitted with a resistor–capacitor (constant phase element) circuit and a series capacitor. By fitting the high frequency (5–0.5 MHz) semi-circle, we obtain a capacitance of ∼2.7 × 10−10 F which could be attributed to the bulk and grain-boundary contributions to Li+ transport in the SE. In the low frequency range (<100 Hz), we observe a linear increase which reflects the quasi-blocking effect of the Au electrodes. From the low-frequency intercept of the semi-circle with the x-axis we determine a bulk resistance (RB) of ∼640 ± 1.4 Ω cm2. When using l as the thickness of the SE pellet and a as the electrode surface area, the ionic conductivity σ was calculated using the formula: σ = l/RBa, which was found to be ∼1.5 × 10−4 S cm−1. Further, σ is plotted as a function of increasing temperature T in Fig. 1d and the relationship follows an Arrhenius behavior as expressed by the following equation where σ is the conductivity of the electrolyte (S cm−1), A is the pre-exponential factor, T is the temperature (Kelvin), Ea is the activation energy expressed (eV), and kb is Boltzmann's constant. The activation energy Ea calculated from the Arrhenius plot is 0.35 eV, which is consistent with values reported in the literature. The EIS plots used to measure Ea are given in Fig. S1.†