Excellent cyclability of P2-type Na–Co–Mn–Si–O cathode material for high-rate sodium-ion batteries

P2-type Na2/3Co0.25Mn0.705Si0.045O2 (Si-NCM) high-rate cathode was designed by using silicon of the nonmetallic element as dopant and developed by the simple solid-state route for sodium-ion batteries. XRD refinements confirm the P2-type hexagonal structure with space group (SG: P63/mmc), in which Si4+ ions substitute the Mn site of P2-Na2/3Co0.25Mn0.75O2 (NCM) lattice without any impurity phases of Si-related substances. Si-NCM delivers the initial capacity of 144 mAh g−1 at 0.1 C with the capacity retention of 80.1% after 100 cycles, and the discharge capacity of 120 mAh g−1 at 1 C with 83.4% retention at 200th cycle. Particularly, excellent capacity retentions of 90.2% after 260 cycles and 85.8% after 500 cycles at 5 C have been achieved. Si-doping can expedite the superior cycle stability of Si-NCM compared to NCM, which is attributed to the more powerful Si–O, TM–O and O–O bonds, more stable occupancy rate in the Nae site of unit cell and particularly ascribed to the reversible two-phase transition of P2–P3–P2 in the process of Na+ extraction and intercalation. Hence, SiO2 as dopant is a novel strategy with regard to the development of high-rate cathode materials for SIBs.