Achieving Long-Term Cyclability in Sodium-Ion Batteries: Site-Selective Doping to Inhibit Irreversible Phase Transitions in P2-Na_2/3Ni_1/3Mn_2/3O₂ Cathode

The typical P2-type Na_2/3Ni_1/3Mn_2/3O₂ exhibits a high theoretical capacity for sodium-ion batteries (SIBs). However, its P2-O2 phase transition during deep charging causes severe structural degradation and capacity decay. In this work, we propose a site-selective doping strategy based on multielement synergy to suppress irreversible phase transitions. The alkali metal site doping by Sr doping as an interlayer pillar prevents cracks along the <i>a</i><i>b</i>-plane and restrains interlaminar slip during deep desodiation. Y³⁺ and Mo⁶⁺ doping in transition metal layers stabilizes the transition metal bond and effectively prevents Na-O plate collapse during sodium deintercalation, dissipating strain accumulation and thereby inhibiting intergranular cracking. Additionally, Y³⁺/Mo⁶⁺ doping activates additional Mn redox, effectively limits electron delocalization and charge order in transition metal layers, and creates a disordered sodium vacancy configuration, thus reducing the migration barrier of Na⁺. Benefiting from this, the site-selectively doped P2-Na_0.65Sr_0.02Ni_0.30Mn_0.67Y_0.01Mo_0.02O₂ cathode exhibits excellent electrochemical performance, delivering a high reversible capacity of 90 mAh g^-1 at 200 C and maintaining 85.8% capacity retention after 2500 cycles at 20 C, significantly surpassing the pristine P2-NaNM cathode material. This work demonstrates the rational design of ultrastable layered cathode materials for sodium-ion batteries, contributing to the development of high-performance and long-life energy storage systems.