High Sulfur Incorporation in Carbon Anodes via Safe Molten-Salt-Assisted Sulfate Doping for Ultrastable Sodium-Ion Batteries

Sulfur (S)-doped carbon materials have attracted growing interest as promising anode candidates for sodium-ion batteries (SIBs) due to their abundant active sites, large capacity, and fast electrochemical kinetics. However, the current S doping strategies frequently employ elemental S, thiourea, (NH4)2SO4, H2S, etc. as dopants, which have low boiling or decomposition temperatures, bringing along environmental concerns, safety hazards, and fire risks. This study provides an innovative molten-salt-assisted sulfate doping strategy to design S-rich carbon materials. Different from conventional molten-salt systems, it is found that the KCl molten salt and dopant ZnSO4 in this work undergo complicated ion exchange, eutecticum and carbothermal reduction processes, and then release SOx or S to dope the carbon framework. It is demonstrated that the participation of KCl molten salt significantly facilitates the incorporation of S atoms, and the S-doped carbon material retains a S content of 7.3% even at 900 °C, much higher than the blank sample (<3.0%). When employed as an anode for SIBs, this S-doped carbon delivers a reversible capacity of >400 mAh g–1 and demonstrates negligible capacity degradation over 1000 cycles. Notably, highly doped carbon at 900 °C has been rarely achieved, and the employment of highly stable inorganic sulfates as dopants provides many advances such as enhanced safety, low toxicity, and minimal environmental contamination.