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 SO_<i>x</i> 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.