Increased lithium-ionic conductivity with Nb- and Ta-doping in garnet-structured solid electrolyte oxides

Garnet-type oxide materials show high Li-ion conductivity and may be used as solid-state electrolytes in lithium-ion batteries to address safety concerns. In this study, Nb-doped Li₇Nd_2.8Ca_0.2Zr_1.8Nb_0.2O₁₂ (LNdCZNbO) and Ta-doped Li₇Nd_2.8Ca_0.2Zr_1.8Ta_0.2O₁₂ (LNdCZTaO) garnet-type compositions were prepared to examine the impact of Nb- and Ta-doping on ionic conductivity of Li₇Nd₃Zr₂O₁₂ (LNdZO). XRD patterns showed the crystallization of major phase of these garnet-structured oxides in tetragonal symmetry. Impedance measurements were recorded from room temperature to 450°C using a Novocontrol make impedance analyzer (Alpha-A high-performance frequency analyzer) in the frequency range of 1 Hz–40 MHz. The maximum total conductivity of parent composition LNdZO was 5.12 × 10⁻⁵ S cm⁻¹ at 25°C. The compositions of LNdCZNbO and LNdCZTaO showed conductivity 7.05 × 10⁻⁴ and 8.23 × 10⁻⁴ S cm⁻¹, respectively, at 25°C. On higher temperature of 350°C, these doped compositions, LNdCZNbO and LNdCZTaO, showed enhanced conductivity of 3.30 × 10⁻³ and 2.63 × 10⁻³ S cm⁻¹, respectively, as compared to the parent LNdZO composition’s conductivity of 4.42 × 10⁻⁴ S cm⁻¹. Analysis of the Cole–Cole plots fitting showed the nature of Li ionic conduction and the existence of bulk and grain boundary impedances in these compositions. The activation energy was found to be higher for the compositions of LNdCZNbO (0.18 ± 0.01 eV) and LNdCZTaO (0.17 ± 0.01 eV) in comparison with the activation energy of undoped composition Li₇Nd₃Zr₂O₁₂ (0.14 ± 0.00 eV) due to the change in garnet lattice by doping of Ca and Nb/Ta.