Phase-Steering in Single-Crystal Layered Transition-Metal Oxide Cathodes by Initial Manganese Valence

Controlling transient phase evolution during high-temperature synthesis remains a significant challenge in the development of structurally robust layered oxide cathodes. Here, we demonstrate that the initial manganese oxidation state acts as a decisive structural director for phase bifurcation. Using a precursor-free model platform to decouple intrinsic redox kinetics from structural inheritance, we reveal that lower-valence precursors (Mn2+/Mn3+) kinetically trap the system in a metastable spinel intermediate (Li2Mn2O4). This pathway induces core-shell segregation, sluggish interdiffusion, and defect accumulation, resulting in mechanical fragility and rapid electrochemical degradation. In contrast, starting with Mn4+ stabilizes a structurally coherent monoclinic intermediate (Li2MnO3), which drastically lowers activation energy barriers and enables an energetically favorable topotactic transformation into a robust, homogeneous single-crystalline lattice. Furthermore, we show that introducing excess lithium thermodynamically steers the phase equilibrium of lower-valence precursors toward the layered-compatible Li2MnO3 intermediate, bypassing kinetic bottlenecks to achieve bulk homogenization. By establishing a quantitative, mechanistic link between precursor redox states, transient intermediate chemistry, and final electrochemical performance, this work provides a kinetically guided framework for the precise engineering of advanced energy storage materials.