Non-Graphitic Carbon with Enhanced Plateau Capacity and Reduced Slope Capacity for Sodium Storage Prepared with the Assistance of Phosphoric Acid

The relatively low energy density of sodium-ion batteries can be effectively mitigated by boosting the low-voltage plateau capacity of anode materials. Herein, a non-graphitic carbon (NGC) is prepared from pre-carbonized semi-coke through H3PO4 treatment followed by carbonization at 1400 °C. The obtained NGC possesses an increased content of pseudo-graphitic structure and a decreased content of disordered structure thanks to the multiple effects of catalysis, activation, and heteroatom doping of H3PO4 on semi-coke, which reach a balance at an H3PO4/semi-coke mass ratio of 1 and a treatment temperature of 600 °C. By comparison, the resulting NGC (PHC-600) shows an increase of 11.6% in the content of pseudo-graphitic structure, as well as a decrease of 11.8% in the content of highly disordered structure. As expected, PHC-600 exhibits a remarkable reversible capacity of 302.2 mAh g–1 at 20 mA g–1, along with an outstanding ICE of 85.0%, which is notably better than that of its control sample (275.6 mAh g–1 and 81.1%). Notably, PHC-600 shows a significantly increased plateau capacity of 217.7 mAh g–1 in contrast to a diminished sloping capacity of 84.5 mAh g–1 for its counterpart, which exhibits 173.4 and 102.2 mAh g–1, respectively. These changes are attributed to a weakened “defect adsorption” effect and an enhanced “interlayer intercalation” effect.