Suppressing the irreversible phase transition from P2 to O2 in sodium-layered cathode via integrating P2- and O3-type structures

Layered transition metal oxides have been broadly studied due to their great potential in application as cathodes for sodium-ion batteries. However, many single-phase layered transition metal oxides, especially those crystallized in P2- or O3-type structure, possess their individual characteristics incurring unsatisfactory overall performances with respect to the reversible capacity, rate capability, and cycling stability. Here, an effective strategy of constructing the P2/O3 biphasic structure is realized in layered cathode Na0.7Ni0.4Mn0.4Ti0.2O2 through Ti substitution. Through high-resolution scanning transmission electron microscopy and X-ray diffraction, the formation of P2/O3 intergrowth structure was clarified and the proportion of the two phases was determined. Benefitting from the presence of intergrowth structure, the layered cathode provides a competitive rate capability of 100 mAh/g at a high rate of 5 C as well as a prominent cycling stability of 80.04% capacity retention after 300 cycles at 5 C. The improved performance is closely related to the highly reversible phase transition process from P2/O3 to OP4/P3 with less strain and enhanced Na+ kinetics. These findings evidence that exploring novel multiphase cathodes is an effective approach to improve the electrochemical performances of cathode for sodium-ion batteries.