Key Factors Influencing Initial Coulombic Efficiency and Plateau Region in N, P-co-Doped Hard Carbon: Insights from Chemical States and Microstructure

Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs), but its moderate capacitance and sluggish Na+ storage kinetics limit its practical application. Optimizing HCs is crucial, yet the complexity and diversity of atomic arrangements, defects, pore structures, and carbon chemical states pose significant challenges. In this study, we advance the understanding of intrinsic storage mechanisms by investigating a set of nitrogen and phosphorus codoped HCs (NPHCs) with subtle variations in chemical states and microstructures. These materials are synthesized via a one-step pyrolysis process using p-phenylenediamine and phosphonitrilic chloride trimers as precursors. Comprehensive compositional and structural analyses reveal that the plateau region is associated with the presence of closed pore structures in the HC, while the initial Coulombic efficiency (ICE) is inversely proportional to the concentration of doping elements. Specifically, nitrogen and phosphorus codoping effectively enhances capacity and accelerates electrochemical kinetics, but excessive heteroatom doping results in a lower ICE. Optimized concentrations of phosphorus and nitrogen codoping significantly improve cycling stability, with the codoped HCs achieving a capacity retention of 78% after 384 days of cycling at 0.5 C (1 C = 300 mA·g–1).