Layered metal oxides have attracted widespread attention as cathodes for Na-ion batteries (NIBs) because of easy synthesis, high specific capacity, and high energy density. However, most reported layered oxides suffer from complex phase transitions upon a large amount of Na deintercalation. Here we report a P2-type Na_0.72[Li_0.24Mn_0.76]O₂, exhibiting exceptionally high initial charge capacity of ∼210 mAh/g (0.72 Na) based on a pure anionic redox reaction (ARR). Surprisingly, global P2 structure can be maintained with minimal volume change (1.35%) upon complete removal of Na⁺ ions. This is due to the reduced Coulombic repulsion associated with ARR and consequent suppression of the phase transition as observed in other P2 materials. Here we reveal for the first time that ARR has the functionality of stabilizing the structure, in addition to its role in increasing its already known capacity. This would pave the way for the further improvement of high-energy-density NIBs.