Defining the Reversible Limit of Anionic Redox via Interlayer Li Ordering

Unlocking the latent capacity from lattice oxygen is pivotal for high-energy sodium-ion batteries. However, the practical deployment of anionic redox chemistry (ARC) is impeded by its chaotic structural irreversibility and voltage hysteresis. While current design principles predominantly target in-plane Li topology, the decisive role of long-range interlayer Li ordering in regulating oxygen activities remains an elusive "blind spot." Here, the reversible limit of anionic redox is defined by establishing a critical structure-performance correlation with c-axis Li ordering. Using P2-Na_0.7Li_0.1Cu_0.2Mn_0.7O₂ (NLCM) as a model, the effects of interlayer Li stacking are decoupled from the in-plane structures. Advanced operando diagnostic analyses reveal that turbostratic Li disorder acts as a kinetic trigger for the uncontrolled and parasitic Li migration, leading to "pathological" excess capacity and rapid degradation. Crucially, a highly ordered Li stacking framework is identified as a rigid structural lock that strictly defines the thermodynamic reversible boundary of ARC. By achieving interlayer Li ordering, deleterious cation migration and over-activation of anionic capacity are suppressed to achieve highly reversible anionic redox, delivering lower voltage hysteresis and 86% capacity retention over 200 cycles. This work transcends the conventional 2D design descriptors, offering a feasible protocol for taming ARC through 3D crystallographic regulation.