The effects of dual modification on structure and performance of P2-type layered oxide cathode for sodium-ion batteries

As the competitive cathode material of sodium-ion batteries (SIBs), P2-type Na0.67Ni0.33Mn0.67O2 (NMO) possesses high theoretical specific capacity and high operating voltage. However, it suffers from severe capacity fading caused by the unfavorable P2–O2 phase transition during charge/discharge cycling process. Moreover, the HF formed by the reaction of electrolyte salts with trace water molecules causes the voltage decay of NMO. Herein, the dual-modification strategy of Mg substitution for transition metal in bulk structure and NaTi2(PO4)3 surface coating layer is designed to solve above defects. It has been found that Mg substitution can stabilize the bulk structure and enlarge the slab-spacing, while the NASICON-type NaTi2(PO4)3 coating layer can stabilize the interface, prevent the HF impact and facilitate the Na+ migration at the cathode-electrolyte interface. Additionally, the combination of Mg substitution and NaTi2(PO4)3 coating can restrain particle crack and exfoliation. The results show that the as-prepared material by dual-modification technology exhibits excellent cycling stability (77.4% capacity retention after 200 cycles at a rate of 1 C) and good rate performance (106.8 mAh g−1 at a rate of 5 C). Accordingly, the dual-modification strategy provides a new horizon for design and development of the high-performance layered oxide cathode materials of SIBs.