Suppressing Phase Separation in Medium-Entropy Perovskite Solid-State Electrolytes

Solid-state lithium metal batteries hold great promise for next-generation energy storage, due to their safety and energy density. Entropy-engineered solid-state electrolytes (SSEs) offer enhanced ionic conductivity and interfacial stability but face phase separation issues during a conventional synthesis. Here, we report an ultrafast synthesis strategy for fabricating pure-phase medium-entropy Li_3xLn_2/3–xTiO₃ (Ln = Nd, La, Gd, 0 < x < 0.16, ME-LLTO) SSEs. High-temperature in-situ and quasi-in-situ X-ray diffraction (XRD) analyses confirm that ultrafast synthesis employs ultrafast cooling to suppress undesired phase transitions, thereby stabilizing the cubic phase while significantly reducing the formation of intermediate phases. In contrast, conventional furnace synthesis leads to persistent intermediates and the coexistence of cubic and tetragonal LLTO phases. Microstructural and spectroscopic characterizations further reveal enhanced phase purity and elemental homogeneity in ultrafast-synthesized ME-LLTO. This work establishes ultrafast synthesis as an effective and scalable strategy for synthesizing high-performance entropy-engineered SSEs by suppressing phase separation.