N-doped hard carbon with closed-pore structure via a high-pressure nitrogen doping approach as anodes for Sodium-ion batteries

The slow diffusion kinetics of Na⁺ at low potential constitute a significant hurdle to high-power applications, even though the intercalation mechanism of hard carbon (HC) anodes for sodium-ion batteries (SIBs) achieves respectable electrochemical performances. This study investigates nitrogen (N) doping effects on the electrochemical performance of HC for SIBs’ applications. X-ray photoelectron spectroscopy (XPS) analysis indicates that the N content of the high-temperature sintering process (HT-NHC) and high-pressure hydrothermal process (HP-NHC) impacts the N incorporation into the HC structure. The content of pyridinic N in HP-NHC is 9.9 mol %, which is higher than that in HT-NHC of 6.0 mol %. After 200 cycles, HP-NHC delivered a high reversible capacity of 331 mAh/g at 0.5 A/g. The improved electrochemical performance of HP-NHC is attributed to the multiplier effects of N-doping, which improves the Na⁺transfer rate. The results of theoretical calculations indicate a weak electrostatic interaction between Na⁺ and graphite and graphitic nitrogen functional groups on the surfaces in HC materials (HCMs). Moreover, the defects, pyrrolic N, N-oxides, and pyridinic N benefit Na⁺ adsorption. Pyrrolic N and other N-containing functional groups offer adequate adsorption sites for Na⁺ and hence boost the capacity of HT-NHC and HP-NHC, supporting the results of our investigation.