Enhanced Stability in layered P2-Na0.67Fe0.5Mn0.5O2 cathode for Sodium Ion Batteries via Cu/Ti co-doping synergies engineering

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, it suffers 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 its crystal structure and the rapid decay of its capacity, which hinders its practical application. Herein, Cu/Ti co-doping P2-Na0.67Fe0.3Mn0.5Cu0.15Ti0.05O2 (NFMCTO) cathode material were prepared by sol-gel method. The doping of Ti4+ reduces the content of Mn3+, mitigates the J-T effect as well as the dissolution of Mn2+, and furthermore also excites the lattice oxygen oxidation reduction (ORR) to improve the capacity. Meanwhile, the introduction of highly reducing Cu2+ inhibits the migration of Fe3+ and realizes the high multiplicity performance of the material by promoting electron-hole separation. 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 124.2 mAh g-1 capacity after 100 cycles at 200 mA g-1, and the capacity retention rate is increased from 51.1% to 77.1%. The In-situ XRD results confirm the reversible structural evolution of the modified material and alleviated phase transitions. The In-situ EIS 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 co-doping synergistic strategy provides a new idea for the subsequent study of Fe/Mn-based cathode materials on suppressing the J-T effect and irreversible oxygen loss under high voltage.