Dual Honeycomb-Superlattice Enables Double-High Activity and Reversibility of Anion Redox for Sodium-Ion Battery Layered Cathodes

Anion redox contributes to the anomalous capacity exceeding the theoretical limit of layered oxides. However, double-high activity and reversibility is challenging due to the structural rearrangement and potential oxygen loss. Here, we propose a strategy for constructing a dual honeycomb-superlattice structure in Na_2/3[Li_1/7Mn_5/14][Mg_1/7Mn_5/14]O₂ to simultaneously realize high activity and reversibility of lattice O redox. Theoretical simulation and electrochemical tests show that [Li_1/7Mn_5/14] superlattice units remarkably trigger the anion redox activity and enable the delivery of a record capacity of 285.9 mA g⁻¹ in layered sodium-ion battery cathodes. Nuclear magnetic resonance and in situ X-ray diffraction reveal that [Mg_1/7Mn_5/14] superlattice units are beneficial to the structure and anion redox reversibility, where Li⁺ reversibly shuttles between Na layers and transition-metal slabs in contrast to the absence of [Mg_1/7Mn_5/14] units. Our findings underline the importance of multifunctional units and provide a path to advanced battery materials.