Tailoring the Route toward Metastability for Mn-Based Disordered Rock Salts

Abstract Metastable Mn-based disordered rock-salt (DRX) cathodes have emerged as an important class of high-performance, Ni/Co-free lithium-ion battery materials based on earth-abundant, less toxic Mn. Despite growing interest, two questions remain unresolved: how to select the optimal synthetic route toward metastability and whether a composition can access multiple distinct metastable states. Here, we address these questions using Li1.2Mn0.8O2 as a model system. We show that the pathway to metastability depends on precursor sublattice compatibility: mismatched anion sublattices require more aggressive mechanochemical conditions, whereas matched sublattices enable access to the DRX phase through a milder route. Although Li1.2Mn0.8O2 prepared via different routes shares matching composition and crystal structure, the resulting metastable products exhibit distinct particle size, surface area, electrochemical performance, degradation behavior, and mechanisms. This work reports an unusually mild mechanochemical route to metastable DRXs, improving scalability while highlighting synthetic pathway selection as a critical design parameter for metastable battery compounds.