High Thiophene-S doped soft carbons for sodium storage

S-doped carbon materials with both low cost and high capacity are one of the best choices for large-scale sodium storage. However, conventional S doping strategies often fail to obtain a high S concentration and accurately control the doping forms of sulfur, which restricts the Na-storage performance and mechanism revelation of materials. Here, low-cost pitch-derived soft carbons with ultrahigh surface-S doping (12.19 at%) and thiophene-S ratio approaching 95 % (CSC95) are accurately synthesized by a modified sulfate pyrolysis strategy. Density functional theory calculations indicate that thiophene-S is an ideal doping form that can form reversible Na-S bonds, which is beneficial for improving the Na-adsorption and storage ability for the materials. The rich thiophene-S regulates the local charge density to improve the electrical conductivity of CSC95. More importantly, the highly reversible redox reactions between thiophene-S and Na+ dominate the Na-storage process. In addition, the hierarchically porous structure of CSC95 provides moderate surface area and ion diffusion channels to ensure the fast Na-storage kinetics. As expected, CSC95 deliver a high Na-storage capacity of 574.5 and 187.7 mAh g−1 at 0.2 and 10 A g−1, respectively, and remarkable cyclability of 77.4 % capacity retention for 800 cycles at 10 A g−1. Moreover, Na3V2(PO4)2F3@C (NVPF@C)//CSC95 full cells display gravimetric energy density of 217 Wh kg−1 at 0.1C and 89.4 % capacity retention after 120 cycles at 1C.