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 NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM), the cathode material of sodium ion batteries (SIBs) with the most potential for industrialization, Na(Ni 1/3 Fe 1/3 Mn 1/3 ) 0.99 Mo 0.01 O 1.99 F 0.01 (NFM-MoF), Na(Ni 1/3 Fe 1/3 Mn 1/3 ) 0.99 Mo 0.01 O 2 (NFM-Mo) and NaNi 1/3 Fe 1/3 Mn 1/3 O 1.99 F 0.01 (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 Ni 2+ content and highest D Na + (7.335 ×10 −13 cm 2 ·s −1 ) caused by the synergistic effect of Mo and F dual-doping. The highest Fe 3+ content in the NFM-Mo leads to the higher initial discharge capacity of 1 36 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 Mn 4+ 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 D Na + (4.229 ×10 −13 cm 2 ·s −1 ) than that of the NFM-F D Na + (5.681 ×10 −13 cm 2 ·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 Ni 2+ increasing, meanwhile restrain undesirable Fe increasing due to the synergistic effect.