Doping carbon electrodes with sulfur achieves reversible sodium ion storage

We present a combination of experiments and theory to study the effect of sulfur doping in hard carbons anodes for sodium-ion batteries. Hard carbons are synthesised via a sustainable method consisting of a two step process: first hydrothermal carbonisation and pyrolysis from a carbonaceous precursor, subsequently doping the resulting hard carbon with sulfur via chemical-vapor deposition. The resulting sulfur-doped hard carbon shows enhanced sodium storage capacity with respect to the pristine material, with significantly improved cycling reversibility. Atomistic first principles simulations give insight into this behaviour, revealing that sulfur chemisorbed onto the hard carbon increases the sodium adsorption energies and facilitates sodium desorption. This mechanism would favour a reversible Na storage, confirming our experimental observations and opening a pathway towards reversible Na-ion batteries.