MgSO₄ as an Effective, Low-Temperature Sulfur Dopant for Carbon Materials Enabling Fast Sodium Storage

S doping is a powerful tool to boost the sodium storage performance of hard carbons. However, developing efficient, sustainable doping strategies is still a challenge. Herein we show the effectiveness of MgSO4 as S dopant compared to other traditionally used sulfates. Even at a low carbonization/doping temperature of 600 °C and using a low MgSO4:carbon precursor weight ratio of 1, the glucose-based carbon material achieved a sulfur content as high as 26 wt %. This translates into a Na storage capacity of 640 mAh g–1 at 0.1 A g–1 and still 122 mAh g–1 at 10 A g–1. This S doping tool promotes not only the redox activity at high potentials but also the low potential capacity exploitable in Na-based technologies. Indeed, DFT studies support increased disordered induced by S doping. This feature, coupled to the enhanced reversibility (ICE ∼ 77%) caused by the S doping, supports the suitability of this material as negative electrode in Na-based batteries and hybrid capacitors. Further engineering through nanostructuring improves the kinetics in the usable low-potential region, which proves the beneficial effect of the templating technique coupled to doping. The universality of the synthesis strategy is shown by successfully applying it to biomass products and wastes.