Cu/Ti co-doping boosting P2-type Fe/Mn-based layered oxide cathodes for high-performance sodium storage

Environmentally friendly P2-type layered iron manganese oxides appear to be one of the most potential cathode materials for sodium-ion batteries (SIBs). However, their practical application is hindered by the unfavorable phase transitions, dissolution of transition metals, and poor air stability. One effective strategy by either single-cation doping or high-cost Li involved co-doping is used to alleviate the problems. Here, low-cost Cu/Ti co-doping is introduced to boost P2-Na_0.7Cu_0.2Fe_0.2Mn_0.5Ti_0.1O₂ as an air and electrochemical stable cathode material for SIBs. The resulting electrode delivers an initial capacity of 130 mAh g^-1 at 0.1C within 2.0-4.2 V, a reversible discharge capacity of 61.0 mAh g^-1 at a high rate of 5C and a capacity retention ratio exceeding 71.1% after 300 cycles. In particular, the co-doped crystal structure is well-maintained after 1 month of exposure to air, and even 3 days of soaking in water. Furthermore, the enhancement is elucidated by the effectively mitigated P2-Z and the completely suppressed P2-P'2 phase transitions, the decreased volume variation proved by in-situ X-ray diffraction (XRD), as well as the lowered transition-metal dissolution evidenced by inductively coupled plasma optical emission spectrometer (ICP-OES) and X-ray photoelectron spectroscopy (XPS). The low-lost Cu/Ti doping strategy could thus be effective for designing and preparing environmentally friendly and high-performance cathode materials for SIBs.