Compositionally graded high-voltage P2-type cathode with superior structural stability and redox kinetics for advanced Na-ion batteries

Layered P2-type cathodes with high voltage, large capacity, and easy synthesis show great potential for developing sodium (Na)-ion batteries (NIBs). However, the P2-O2 phase transition makes their structural degradation and the Na+/vacancy ordering lowers their redox kinetics. Here, we rationally propose a compositionally graded P2-type cathode, where nickel (Ni) and manganese (Mn) fractions decrease gradually, and cobalt (Co) content increases contiguously from the inside to the outside of a secondary particle. Inside these particles, the Ni/Mn-based compound delivers high capacity and high voltage. On the surface of particles, the Co/Mn-based solid solution offers a stable buffer matrix. Benefiting from these synergistic effects, this graded P2-type cathode shows the elimination of P2-O2 transformation even when charged to 4.4 V, which enables good structural stability, maintaining capacity retention reaching similar to 80% within 300 cycles. Moreover, the Na+/vacancy ordering superstructure is further suppressed, and the Na+ diffusion kinetics is significantly improved. The proposed graded structure with optimized chemical composition offers a new perspective for eliminating the unwanted phase transition and thus enhancing the electrochemistry of high-voltage layered cathodes for advanced NIBs.