Effects of Different Doping Strategies on Cubic Li7La3Zr2O12 Solid-State Li-Ion Battery Electrolytes

Solid-state Li-ion conductors based on cubic Li7La3Zr2O12 (LLZO) garnets have received much attention in recent years as potential next-generation battery electrolytes, enabling safer and more energy-dense Li-ion batteries. Aliovalent doping of the LLZO structure is usually necessary to stabilize the cubic garnet phase and increase the ionic conductivity by increasing the concentration of Li vacancies. Here, we report on the synthesis, characterization, and testing of Li7–3xAlxLa3Zr2O12 ceramics with different amounts of Al doping (x = 0.20–0.40). Phase-pure LLZO with a cubic crystal structure was prepared by an aqueous synthesis route, and dense (>93%) ceramic samples were fabricated by conventional sintering at 1200 °C. By analyzing the composition, microstructure, and electrochemical performance, we found that the optimal Al content in LLZO is x = 0.2, the lowest content needed to stabilize the cubic structure in our series. For the composition with x = 0.2, we found a Li-ion conductivity at room temperature of 3.7 × 10–4 S cm–1 and an activation energy of Ea = 0.3 eV. At a higher doping concentration, the conductivity decreases, and the activation energy increases; for x ≥ 0.35, secondary Al-rich phases appear. These results indicate an inverse relationship between Li-ion conductivity and Al doping, where the optimal amount of doping is the minimum amount necessary to stabilize the cubic LLZO phase. Additionally, we present an analysis of the available literature on chemical modification of LLZO to compare how different doping strategies affect Li conductivity. Based on our literature review, Ga and Ta doping gives the highest conductivities (≤2 × 10–3 S cm–1). The literature analysis also supports our findings that the primary objective of the dopant is to stabilize the cubic structure rather than create Li vacancies.