Role of surface modification on high-voltage phase transition and degradation of layered P2-Na2/3Ni1/3Mn2/3O2 cathode material for Na-ion batteries

Layered transition metal oxides hold promise as cathode materials for sodium-ion batteries (SIBs). The P2-type Na2/3Ni1/3Mn2/3O2 cathode presents a high theoretical capacity and energy density, yet rapid capacity degradation occurs when the high-voltage plateau corresponding to the nominal Ni4+/Ni3+ redox is activated. Mitigating this degradation is necessary to realize the theoretical capacity of this material. While surface modification is a common strategy to improve the stability of cathode materials at high voltages, its effectiveness on the layered transition metal oxides for SIBs remains elusive. In this work, we investigated the role of surface coating on the reversibility of the high-voltage plateau through a suite of surface, bulk, and electrochemical characterizations. The surface modification of the P2-type Na2/3Ni1/3Mn2/3O2 cathode suppresses the growth of high-voltage polarization with minimal impact on the detrimental structure disorder induced by the P2-O2 phase transition. Our study highlights the limited effect of surface modification on capacity retention, which is predominantly driven by bulk structure degradation.