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.
Designing and constructing built-in electric field (BIEF) between substances is an effective way to enhance multi-polarization relaxation. Ni 3 S 4 @Co 4 S 3 /Ni 3 N@Co 2 N 0.67 (NC-S/NC-N) composites are prepared through multiple hydrothermal and nitriding steps. The influence of the BIEF constructed by the Type-II heterojunction between the corresponding substances on the wave absorption performance is investigated. A BIEF model is constructed for this material, and the transfer mechanism of electrons and holes in the BIEF is studied. The crystal structure, lattice parameters, unit cell volume, and phase composition of composites are quantitatively analyzed by XRD refinement using the Rietveld method. The electronic structure of the heterointerface is studied by using density functional theory (DFT) calculations to verify the characteristics of Type-II heterojunctions and the formation of BIEF. The BIEF constructed by Type-II heterojunction enhances the interfacial polarization intensity by regulating the interfacial charge distribution, accelerates carrier transport, and increases conductive loss. It also affects magnetic loss by modulating the spin polarization of Ni/Co magnetic components. When the matching thickness is 2.1 mm, the minimum reflection loss (RL min ) can reach −51.64 dB. Meanwhile, the maximum effective absorption bandwidth (EAB max ) can cover 5.12 GHz at 2.3 mm. The results show that the microscopic interface mechanisms provides a brand-new path.
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