Low-temperature sintering characteristics and electrical properties of Ca- and Bi-doped Li₇La₃Zr₂O₁₂ electrolyte containing Li₃BO₃ additive

Li₇La₃Zr₂O₁₂ (LLZ) is one of the most promising Li-ion solid electrolytes for all-solid-state Li-ion batteries. To achieve low interfacial resistance between LLZ and the active material, low-temperature sintering is required. Toward this, we co-doped Ca and Bi into LLZ and used Li₃BO₃ as an additive to examine these species as sintering aids. We successfully demonstrated that Li_6.5La_2.9Ca_0.1Zr_1.4Bi_0.6O₁₂ containing Li₃BO₃ can be sintered even at 750 °C with a relative density of 89% and conductivity of 3 × 10⁻⁴ S cm⁻¹ at 25 °C. Thermomechanical and microstructural analyses revealed that the starting temperature of the viscous flow, which was initiated by the formation of a liquid Li₃BO₃ phase, was reduced owing to the reaction between Ca²⁺ and Li₃BO₃. Additionally, the Li-Ca-Bi-O liquid phase formed between the LLZ particles during sintering improved the sinterability, which consequently promoted the necking between particles and lowered the grain boundary resistance. Additionally, we demonstrated that the developed LLZ was applicable as an electrolyte for all-solid-state lithium-ion batteries with high cycling stability. Thus, Li_6.5La_2.9Ca_0.1Zr_1.4Bi_0.6O₁₂ containing Li₃BO₃ is a promising candidate as an electrolyte for the cathode layer of all-solid-state lithium-ion batteries.