Solid-State Battery Interface Engineering through Sintering Additives for Li-Garnet Electrolyte and Composite Cathode

Precise control of densification behavior and co-sintering compatibility is essential for the scalable fabrication and integration of oxide-based solid-state batteries. Here, the influence of sintering additives on the densification kinetics, phase evolution, microstructure, and ionic conductivity of Li₇La₃Zr₂O₁₂ (LLZO) electrolytes is systematically investigated, while their effect on densification and shrinkage behavior with LiCoO₂-LLZO composite cathodes is evaluated to assess co-sintering compatibility. Among the investigated additives in LLZO, MgO and Li₂O enable relative densities exceeding 96% while preserving high ionic conductivity and suppressing electronic conductivity. Microstructural and dilatometric analyses reveal distinct additive-dependent sintering behaviors, demonstrating the critical role of additive chemistry in governing densification and microstructural evolution. Importantly, LiOH- and Li₂O-MgO-modified systems reduce the shrinkage mismatch between composite cathode and electrolyte layers from ∼14% to ∼2.5–3.5% at 1050 °C, significantly improving shrinkage compatibility. These findings demonstrate that tailored sintering additive strategies enable shrinkage compatibility while preserving electrolyte performance, representing a key step toward realizing mechanically robust oxide-based solid-state batteries.