Phosphorus-doped hard carbon with controlled active groups and microstructure for high-performance sodium-ion batteries

A series of phosphorus-doped hard carbon (PHC-Ts) was fabricated by directly annealing a phosphoric acid solidified epoxy resin over a wide temperature range to study the evolution of P-containing groups and the microstructure with increasing temperature, and to evaluate the Na-storage performance. A correlation between them has been well established. As the carbonization temperature increases, the P-containing groups undergo a transition from P-O and P-C bonds to higher active P-P bonds with a non-monotonic decrease in the interlayer distance. The high number of active P-containing groups and large interlayer distances provide more active sites and fast ion transmission channels, which jointly contribute to the excellent electrochemical performance. The obtained PHC-700 electrodes have the largest interlayer distance and highest total activity with many P-C bonds, showing a high capacity of 379.3 mA h g(-1)at 0.1 A g(-1), a superb rate capability of 158.1 mA h g(-1)at 5.0 A g(-1), and a long-term cycling stability without the loss of capacity after 6500 cycles at 5.0 A g(-1).