Tuning anionic/cationic redox chemistry in a P2-type Na0.67Mn0.5Fe0.5O2 cathode material via a synergic strategy

The anionic redox chemistry (O2−→O−) in P2-type sodium-ion battery cathodes has attracted much attention. However, determining how to tune the anionic redox reaction is still a major challenge. Herein, we tune the activity and reversibility of both the anionic and cationic redox reactions of Na0.67Mn0.5Fe0.5O2 though an integrated strategy that combines the advantages of Li2SiO3 coating, Li doping and Si doping, and the initial capacity, rate performance and cycling stability are significantly improved. The in-depth modulation mechanism is revealed by means of neutron diffraction, X-ray absorption spectroscopy, in situ X-ray diffraction, electron paramagnetic resonance spectroscopy, first-principles calculations and so on. The Li2SiO3 coating alleviates the side reactions and enhances the cycling stability. Si4+ doping lowers the Na+ diffusion barrier due to the expanded interlayer spacing. Additionally, Si4+ doping improves the structural stability, oxygen redox activity and reversibility. Li+ doping in Na sites further increases the structure stability. The electron density maps confirm the greater activity of Na and O in the modified sample. Nuclear density maps and bond-valence energy landscapes identify the Na+ migration pathway from Nae site to Naf site (the positions of the Na ions in the crystal structure). The proposed insights into the modulation mechanism of the anionic and cationic redox chemistry are also instructive for designing other oxide-based cathode materials.