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

P2-type Na_2/3Co_0.25Mn_0.705Si_0.045O₂ (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-Na_2/3Co_0.25Mn_0.75O₂ (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, T_M–O and O–O bonds, more stable occupancy rate in the Na_e 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.