Modulating Surface Oxygen Coordination to Achieve Suppressed Phase Transitions and Enhanced Cyclic Stability in Na_0.67Mn_0.5Fe_0.5O₂ Cathodes for High-Energy and Low-Cost Na-Ion Batteries

The layered iron manganese oxide cathodes accompanied by anionic redox reaction (ARR) activity show large promise of high-energy and economical sodium-ion batteries. However, the adverse surface oxygen lattice evolution caused by irreversible ARR tends to lead to poor cyclic stability and severe voltage decay, which limits its commercial application. In this work, using Na_0.67Mn_0.5Fe_0.5O₂ (NMFO) as the model compound, an optimization strategy by modulating surface oxygen coordination through a simultaneous surface Li doping and Li₃PO₄ coating is proposed to achieve both triggered and reversible ARR processes. As revealed by neutron diffraction techniques and transmission electron microscopy tests, Li ions and Li₃PO₄ are successfully doped and coated on the surface of the NMFO cathode, respectively. The optimized cathode expectedly shows not only enhanced specific capacity but also improved cyclic stability. The excellent electrochemical properties are ascribed to the suppressed detrimental P2-O2 phase transition, enhanced ARR reversibility, and improved thermal structural stability. More broadly, this work demonstrates the feasibility of modulating surface oxygen coordination to activate and stabilize the ARR ion-storage process.