Spherical Mg/Cu Co-Doped NaFe(PO)PO Cathode Materials with Mitigated Diffusion-Induced Stresses and Enhanced Cyclic Stability.

NaFe(PO)PO (NFPP) has been regarded as the promising cathode material for sodium-ion batteries (SIBs). However, the practical applications of NFPP are hindered by its high-volume changes, poor intrinsic electron conductivity and sluggish Na+ ions diffusion kinetics. Herein, a spray-drying and solid-state reaction method have been utilized to fabricate the spherical trace amount Mg/Cu co-doped NaFe(PO)PO (NFMCPP). The Mg/Cu co-doping can effectively mitigate the lattice volume change and promote the electronic conductivity of NFMCPP by reducing band gap between the conduction and valence bands. While, the unique spherical structured NFMCPP with a carbon film even coated on its surface ensures rapid electron transport. Moreover, small NFMCPP particles with spherical geometry demonstrate an alleviated diffusion-induced stress and enhanced structural stability, due to the high sphericity structure enables fluent Na extraction/insertion, leads to a low high-stress concentration and uniform stress/strain distribution during extensive (de)sodiation process. Consequently, the optimized spherical NFMCPP cathode materials exhibit an excellent rate capability and cyclic stability.