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 (NaxMn1-yMyO2, M: doped elements) with mixed structures are regarded as attractive cathode materials for sodium ion batteries (SIBs) due to their controllable phase structure and 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-Na0.8Mn0.55Ni0.35Cu0.1O2 with long cycle life was designed and prepared. The formation process of P2/O3 biphasic structure was first revealed by the high-temperature in-situ XRD. The XRD Rietveld refinement exhibits 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-Na0.8Mn0.55Ni0.35Cu0.1O2 exhibits a capacity of 148.7 mAh g−1 at 0.1 C and a high capacity retention of 94.2% over 200 cycles at 1 C. The combination of in-situ XRD and electrochemical analyses suggest the reversible structural evolution and fast Na+ diffusion kinetics of P2/O3-Na0.8Mn0.55Ni0.35Cu0.1O2 during Na+ insertion/extraction. The full cell of hard carbon//P2/O3-Na0.8Mn0.55Ni0.35Cu0.1O2 can deliver a capacity of 107 mAh g−1 with remarkably cycle stability of 97.7 % retention after 500 cycles at 1 C, implying the potential application of this novel P2/O3-Na0.8Mn0.55Ni0.35Cu0.1O2 cathode for SIBs