Deciphering the formation process and electrochemical behavior of novel P2/O3 biphasic layered cathode with long cycle life for sodium-ion batteries

Sodium manganese layered oxides with mixed structures are regarded as the most attractive cathode materials for sodium ion batteries (SIBs) due to desired electrochemical performance . However, developing novel mixed layered oxides with long cycle life and revealing the formation process of mixed structure is quite challenging. Herein, the novel P2/O3–Na 0.8 Mn 0.55 Ni 0.35 Cu 0.1 O 2 with long cycle life was designed and prepared by solid-state reaction. The formation process of P2/O3 biphasic structure was first revealed by the high-temperature in-situ XRD . More importantly, the XRD Rietveld refinement exhibits that the Na interlayer distance is increased in the O3 phase, which is beneficial to Na + insertion/extraction. Benefitting from the unique P2/O3 biphasic structure and Ni/Cu co-doping, the P2/O3–Na 0.8 Mn 0.55 Ni 0.35 Cu 0.1 O 2 exhibits a high reversible capacity of 148.7 mAh g −1  at 0.1C and a high capacity retention of 94.2% after 200 cycles at 1C. The combination of in-situ XRD and electrochemical analyses suggest the reversible structural evolution and fast Na  +  diffusion kinetics of P2/O3–Na 0.8 Mn 0.55 Ni 0.35 Cu 0.1 O 2 during the Na  +  insertion/extraction processes. Furthermore, the Na full cell of hard carbon//P2/O3–Na 0.8 Mn 0.55 Ni 0.35 Cu 0.1 O 2 can deliver a remarkable cycle stability of 97.75% retention after 500 cycles at 1C, implying the application of this novel P2/O3–Na 0.8 Mn 0.55 Ni 0.35 Cu 0.1 O 2 cathode for SIBs.