Nitrogen/phosphorus co-doped ultramicropores hard carbon spheres for rapid sodium storage

Hard carbon (HC) is a prospective energy storage anode material in sodium-ion batteries (SIBs). However, their unimpressive rate capability and poor initial Coulombic efficiency (ICE) have driven the requirements for superior capability HC anode materials. In our work, nitrogen (N)/phosphorus (P) co-doped ultramicropores (≈0.5 nm) hard carbon spheres (NPUCS) with the boosted pyridinic-N content are successfully prepared. The ultramicropores structure (<0.7 nm) effectively prevents the electrolyte from contacting the carbon surface but allows the rapid diffusion of Na⁺ in the carbon layer, leading to high-efficiency sodium storage. Pyridinic-N has the highest reactivity, and could significantly promote the Na⁺ adsorption in HC. The NPUCS exhibits an excellent rate capability, providing capacities of 257.7 and 157.0 mA h g⁻¹ at 0.1 and 5.0 A g⁻¹ along with a high ICE to 75 %. Furthermore, when integrated into a full battery configuration, the prepared full battery displays a high energy density to 135.9 Wh kg⁻¹ at 0.1 A g⁻¹ with long-time stability (350 cycles at 0.2 A g⁻¹). These excellent electrochemical behaviors highlight the potential of our approach for the synthesis of advanced HC anode for SIBs.