Engineering Na⁺-layer spacings to stabilize Mn-based layered cathodes for sodium-ion batteries

Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like Na_xMnO₂ (S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na⁺ layer spacings of P2-type Na_0.67MnO₂ and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na⁺ mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g^-1 at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides.