Enhanced stability in a layered P2-Na0.67Fe0.5Mn0.5O2 cathode for sodium ion batteries via a synergistic Cu/Ti co-doping strategy

Abstract

Co/Ni-free P2 type Fe/Mn-based layered oxide cathodes for sodium ion batteries have attracted much attention due to their low cost, high capacity, and environmental friendliness. However, they suffer from the Jahn–Teller (J–T) effect, transition metal migration, and irreversible oxygen loss, and the irreversible phase transition caused by these problems can lead to the deterioration of their crystal structure and the rapid decay of their capacity, which hinders their practical application. Herein, a Cu/Ti co-doped P2-Na_0.67Fe_0.3Mn_0.5Cu_0.15Ti_0.05O₂ (NFMCTO) cathode material was prepared by the sol–gel method. The doping of Ti⁴⁺ reduces the ordering of Na⁺/vacancy, inhibits the dissolution of TM, and improves capacity. Meanwhile, the high bond energy of Ti–O suppresses excessive lattice oxygen redox reactions (ORRs), enhancing the structural reversibility of the material. The high reducibility of Cu²⁺ inhibits phase transitions caused by the irreversible migration of Fe³⁺, thereby enhancing the cycling stability of the material. Furthermore, after CuTi co-doping, the content of Mn³⁺ decreases, alleviating the J–T effect and the dissolution of Mn²⁺. Under the combined effect of Cu and Ti, NFMCTO shows a high initial discharge capacity of 207.2 mAh g⁻¹ at a current density of 20 mA g⁻¹, and superior rate performance (119.1 mAh g⁻¹ at 400 mA g⁻¹). Meanwhile, it still has a capacity of 124.2 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹, and the capacity retention rate is increased from 51.1% to 77.1%. In situ XRD results confirm the reversible structural evolution of the modified material and alleviated phase transitions. The in situ EIS combined with the distribution of relaxation time (DRT) analysis and GITT results also indicate that NFMCTO has improved Na⁺ ion transfer kinetics through Cu/Ti synergism, which corresponds to the excellent rate performance of the material. Finally, the testing of the NFMCTO//HC full cell further confirms the promising application of NFMCTO. This work confirms the necessity of co-doping in doping modification, and the synergistic co-doping strategy provides a new idea for the subsequent study of Fe/Mn-based cathode materials in suppressing the J–T effect and irreversible oxygen loss at high voltage.