Rational Design of Mid‐Ni Layered Oxides via Precursor‐Encoded Spatial Distribution of Redox‐Active Li ₂ MnO ₃ Domains

ABSTRACT Li‐rich layered oxides (LLOs) promise exceptional energy density via anionic redox but are plagued by instability stemming from irreversible oxygen release. Implementing this strategy in cost‐effective, Co‐free mid‐Ni cathodes is attractive, yet requires precise control over the Li 2 MnO 3 domains. Here, we demonstrate that a previously overlooked parameter—the precursor's Mn oxidation state, precisely modulated by the drying atmosphere (oxygen‐rich vs. oxygen‐free)—dictates the final spatial distribution of Li 2 MnO 3 domains. An oxidized precursor (oxygen‐rich) triggers the preferential formation of Li 2 MnO 3 domains segregated at the particle surface (OR‐LNR). In contrast, an oxygen‐free atmosphere ensures a uniform Mn oxidation state, resulting in a homogeneous domain distribution throughout the bulk (OF‐LNR). This homogeneous architecture is proven crucial for enhancing oxygen redox reversibility. Consequently, OF‐LNR exhibits superior full‐cell cycling stability compared to the surface‐segregated OR‐LNR. Mechanistic analysis reveals that the surface‐segregated domains initiate a catastrophic degradation cascade, including internal void formation, impedance growth, and severe cathode‒anode crosstalk. This work establishes a new precursor engineering principle, identifying Mn oxidation state control as a critical strategy for the rational design of high‐energy, durable LLO cathodes.