Impact of intentional composition tuning on the sintering properties of Ca–Bi co-doped Li7La3Zr2O12 for co-fired solid-state batteries

Abstract

Li₇La₃Zr₂O₁₂ (LLZ) is a promising candidate electrolyte for co-fired all-solid-state Li-ion batteries. However, its application is hindered by the reaction between LLZ and electrode materials during high-temperature sintering. To reduce the sintering temperature, herein Li–Bi–O oxide is added as a low-melting-point material into Ca–Bi co-doped LLZ (LLZ-CaBi). Nanocomposites of Li_6.5(La_2.92−xCa_0.08)(Zr_1.42Bi_0.58)O₁₂ and Li–Bi–O are fabricated. Here, Li–Bi–O is formed in the particles by intentional composition tuning using a lower amount of La than the stoichiometric composition. Direct observation of the nanocomposite reveals Li–Ca–Bi–O, Ca–Bi–O, and Li₂CO₃ in mother LLZ–CaBi particles. Some properties improve as x increases, and after sintering at 750 °C the highest relative density of 94% and ionic conductivity of 1.2 × 10⁻³ S cm⁻¹ are achieved at x = 0.13. Li, Ca, and Bi are dissolved in the liquid phase and re-precipitated to promote further densification. The dependence of ionic conductivity on x is well explained by the relative density and alleviation of bottlenecks in the Li-ion diffusion path. Moreover, the developed LLZ–CaBi could be co-fired with LiCoO₂, and the co-fired half-cell successfully operates as an all-solid-state battery at room temperature. Thus, the composite of LLZ–CaBi with Li–Bi–O can be applied as a catholyte in solid-state batteries.