High stability of Mo-F dual-doped O3-type NaNi1/3Fe1/3Mn1/3O2 cathode material for sodium-ion battery

Aiming for promoting the capacity and cycling stability of O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM), the cathode material of sodium ion batteries (SIBs) with the most potential for industrialization, Na(Ni1/3Fe1/3Mn1/3)0.99Mo0.01O1.99F0.01 (NFM-MoF), Na(Ni1/3Fe1/3Mn1/3)0.99Mo0.01O2 (NFM-Mo) and NaNi1/3Fe1/3Mn1/3O1.99F0.01 (NFM-F) were successfully synthesized by solid phase method. The electrochemical performance of the doped samples was compared with that of the pristine NFM. The mechanism of the single- or dual-doping was explored by XRD Rietveld refinement and XPS. The NFM-MoF exhibits the highest initial specific discharge capacity of 137mAhg−1 at 1 C with a retention of 91.97% after 100 cycles and the smallest polarization (0.32 V), which could be contributed to the highest Ni2+ content and highest DNa+ (7.335 ×10−13 cm2·s−1) caused by the synergistic effect of Mo and F dual-doping. The highest Fe3+ content in the NFM-Mo leads to the higher initial discharge capacity of 136 mAhg−1 at 1 C with a relative lower retention of 83.38%, which is due to the structure instability resulting from Fe migration and the lowest Mn content, than that of the NFM-F (85.31%). It illustrates that the Mn4+ plays a crucial role in stabilizing the structure during Na+ migrating. The superiority of the discharge capacity of the NFM-Mo become narrow with rates rise, due to its lower DNa+ (4.229 ×10−13 cm2·s−1) than that of the NFM-F DNa+ (5.681 ×10−13 cm2·s−1). It verifies that it is a diffusion control process at high rates. Mo and F dual-doping in NFM was proved to amplify the benefit of Ni2+ increasing, meanwhile restrain undesirable Fe increasing due to the synergistic effect.