Electronic structure engineering through strengthened d–p orbital hybridization of sodium ferric sulfate cathode with enhanced electrochemical performance for sodium-ion batteries

The Na_2+2xFe_2-x(SO₄)₃ (NFS) cathode has garnered significant attention due to its high voltage and affordability. Nevertheless, the low intrinsic conductivity of the NFS results in poor electrochemical properties. In this work, magnesium ions with low electronegativity are strategically incorporated into the iron sites. There is an increased tendency for electrons to migrate toward the bridging oxygen atom (Fe-O-Mg). This asymmetric distribution of electrons is conducive to the reduction of band gap and the improvement of diffusion kinetics. Besides, the s-p orbital hybridization in Mg-O bonds has a lower energy, leading to a more stable lattice structure and strengthened Fe3d-O2p orbital interactions. Hence, the optimised Mg substituted NFS cathode obtains high capacity (96.2 mAh g^-1 at 0.1 C) and long cycle stability with low average capacity attenuation rates of 0.52 % and 0.57 % per cycle even at high current densities of 10 C and 20 C. Overall, regulating electronic structure through strengthened d–p orbital hybridization is an effective strategy to design cathode materials with superior electrochemical performance.