Achieving Long-Term Cyclability in Sodium-Ion Batteries: Site-Selective Doping to Inhibit Irreversible Phase Transitions in P2-Na2/3Ni1/3Mn2/3O₂ Cathode

The typical P2-type Na2/3Ni1/3Mn2/3O2 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 ab-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-Na0.65Sr0.02Ni0.30Mn0.67Y0.01Mo0.02O2 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.