Scalable synthesis of N/S co-doped hard carbon microspheres as a high-performance anode material for sodium-ion batteries

Hard carbon is a promising anode material for sodium-ion batteries (SIBs) due to its abundance. However, it exhibits low reversible capacity and slow kinetics if inappropriate microstructural features are developed during synthesis. Herein, N/S co-doped phenolic resin-based hard carbon microspheres are prepared by a scalable strategy, and the electrochemical performance is assessed both in half cells and full cells. We demonstrate that the expanded interlayer spacing, the increased active sites, and the enhanced capacitive behavior result in the enhanced reversible capacity and promoted kinetics for Na+ storage. The sample with appropriate doping amount exhibits an initial charge capacity of 536.8 mAh g-1 at 50 mA g-1 and maintains 445.9 mAh g-1 after 1000 cycles at a current density of 1 A g-1 in a Na-metal half cell. Coupled with a carbon-coated Na4Fe3(PO4)2P2O7 (NFPP) cathode, the full cell exhibits a capacity of 92.5 mAh g-1 after 90 cycles, with a capacity retention of 91.6%. This work provides a facile and scalable method for synthesizing high-performance hard carbon anode materials for SIBs.