Comparative analysis on the thermal, structural, and electrochemical properties of Al-doped Li₇La₃Zr₂O₁₂ solid electrolytes through solid state and sol-gel routes

Over the last decade, Li₇La₃Zr₂O₁₂ (LLZO) has shown to be one of the most promising materials as a solid electrolyte in Li-ion batteries. However, several factors can affect the final electrochemical properties of the material, such as the synthesis method and the inclusion of dopants. In this study, we conduct a comparative analysis of undoped and Al-doped LLZO prepared through solid state and sol-gel methods. An in-situ thermal investigation on the synthesis of LLZO shows how solid state synthesis forms LLZO at lower rates, however the resulting LLZO is more stable at higher temperatures. The addition of Al as a dopant further increases the thermal stability of the material by lowering its decomposition rate. Ionic conductivity measurements reveal how sol-gel LLZO samples experience several times higher conductivity than their solid state counterparts due to tightly interconnected grains. With the addition of Al, the conductivity further increases by two orders of magnitude. With that we are able to achieve the highest ionic conductivity of 4.96 × 10⁻⁴ S/cm for Al-doped sol-gel LLZO and an activation energy of 0.28(1) eV. This work provides a better understanding of how different synthesis methods affect the final properties of LLZO solid electrolytes.