Unveiling Phase Formation Mechanisms in Over-Stoichiometric Rocksalt-Type Li Superionic Conductors

Rationalizing synthetic pathways is crucial for material design and property optimization, especially for polymorphic and metastable phases. Over-stoichiometric rocksalt (ORX) compounds, characterized by their face-sharing configurations, are a promising group of materials with unique properties for energy storage application; however, their development is significantly hindered by challenges in synthesizability. Here, taking the recently identified Li superionic conductor, over-stoichiometric rocksalt Li–In–Sn–O (o-LISO) material as a prototypical ORX compound, we systematically investigate the mechanisms of phase formation. By combining ex situ and in situ synchrotron X-ray diffraction, solid-state NMR, TEM, and ab initio calculations, we reveal that the spinel-like phase with unconventional stoichiometry forms as coherent precipitates from the high-temperature-stabilized cation-disordered rocksalt phase. This process prevents direct phase decomposition and kinetically locks the system in a metastable state with desired face-sharing Li configurations. This insight enables us to further enhance the ionic conductivity of o-LISO to exceed 1 mS cm^–1 at room temperature via synthetic optimization. Our work offers valuable perspective on the rational synthesis of ORX materials and highlights important opportunities of unraveling synthesis mechanisms in complex material optimization.