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Molecules 2022, 27, 6516 15 of 32 and conversely, improve the influence of SEI film, leading to the inevitable low ICE, which is the main problem to be overcome. Ji [79] first explored the influence of P-doping on the sodium storage performance of SIB anode material, and prepared P-doped carbon nanosheets with an excellent rate performance. Compared with N, P-doping can not only provide electrochemical active sites, but also increase the layer space to facilitate the reversible intercalation of Sodium-ions and change the electronic states of the carbon sheets to promote ion adsorption in the electrolyte. Using three-dimensional well-ordered porous phosphorus doped carbon as an anode for sodium storage was also studied [87], and a theoretical calculation further confirmed that P- doping can increase the interlayer distance and adsorption capacity of Sodium-ion, reaching the capacity of 270 mAh g−1 at 0.2 A g−1 after 800 cycles and a remarkable rate capability of 140 mAh g−1 at 10 A g−1. Wu [88] prepared HC nanofibers via electrostatic spinning and simple heat treatment. By comparison (as shown in Figure 11a,b), an excellent performance of the P-doping was demonstrated, and it was confirmed that a P–C bond can increase the electron density near the Fermi-level, thus enhancing the electronic conductivity of the material. Similarly, Jiang’s Group [80] prepared S-doped nanoscale carbon fibers using an industrial waste bacterial cellulose as a carbon source. With the synergistic effect of the carbon structure, S-doping and defect sites, a larger layer spacing, and the electrochemical activity of the S–C bond play significant roles, still providing a capacity of 310 mAh g−1 after 1100 cycles at 1 A g−1. It is considered that S atoms are doped in the interlayer of carbon [89]. Hong and Zhang [90] studied the influence of S-doping on the layer spacing in two kinds of HC with a defect free structure and a defect structure by the first-principles method, and prepared an S-doped HC with a low porosity and larger layer spacing. The reversible capacity of the material remained nearly 200 mAh g−1 after 4000 cycles at 1 A g−1, and its ICE and cycling performance were significantly higher than those of the materials mentioned above. Phosphate produces electron holes near the Fermi level, which is often taken as evidence that the p-type semiconductors can improve electronic conductivity. Recently, an in-depth study [91] of the charge distribution has shown that electron holes are mainly generated by the phosphate doped in HC with a higher reducibility than the surrounding carbon atoms. This finding further explains the basic principle of enhancing the electrochemical performance of P-doped HC. In addition to the typical single-atom doping examples mentioned above, there are also many reports on double-doping and tri-atom co-doping in recent years. Wang [92] synthesized the layered carbon material co-doped with N and S by small molecule dithioox- amide using a facile self-templating strategy. Combined with the synergic characteristics of N and S doping, defect sites are formed in the layer to increase the storage capacity of sodium while expanding the layer spacing. Simultaneously, the two heteroatoms improve the electron and ion transfer rate of the material, providing an excellent performance of 213.6 mAh g−1 at 1 A g−1 for up to 2000 cycles. In order to improve the low carbon yield from starch direct pyrolysis, Huang’s Group [93] prepared an N and P co-doped porous carbon using ammonium polyphosphate (APP) as both the dopant and crosslinking agent. DFT calculations based on first-principles confirmed that the co-doping of the N and P atoms can improve the conductivity of carbon materials and the adsorption capacity of the Sodium-ions, as well as reducing the diffusion barrier of Sodium-ions between graphite layers. The research team also used a high-yield and low-cost biomass insect feces as a pre- cursor system to prepare an O, N, P, and S co-doped porous carbon material, which achieved an excellent cycling performance of capacity retention above 95% at 1000 cycles [94]. In contrast, Song’s Group [95] prepared three-dimensional porous carbon materials with N, O and P co-doping by simple pyrolysis of the poly(P-phenylenediamine) (PpPD) hydrogel with phytic acid as a dopant and crosslinking agent. The mesoporous structures formed by the three-dimensional PpPD hydrogel network gives the anode material abundant defect sites and ion transport paths, and it serves a better electrochemical performance of 332 mAh g−1 at 0.05 A g−1, 139 mAh g−1 at 10 A g−1 and a wonderful cycling stability of 98.9% retention after 1000 cycles at 5 A g−1; this design is shown in Figure 11c.PDF Image | Hard Carbons as Anodes in Sodium-Ion Batteries
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