In Situ Revealing the Electroactivity of PO and PC Bonds in Hard Carbon for High-Capacity and Long-Life Li/K-Ion Batteries

The low capacity and unsatisfactory rate capability of hard carbon still restricts its practical application for Li/K-ion batteries. Herein, a low-cost and large-scale method is developed to fabricate phosphorus-doped hard carbon (PHC-700) by crosslinking phosphoric acid and epoxy resin and followed by annealing at 700 °C. H₃PO₄ acts not only as a crosslinker to solidify epoxy resin for promoting the degree of graphitization and lowering the specific surface area, but also as phosphorus source for forming PC and PO bonds, thus providing more active sites for Li/K storage. As a result, the PHC-700 electrode delivers a highly reversible capacity of 1294.8 mA h g⁻¹ at 0.1 A g⁻¹ and a capacity of 214 mA h g⁻¹ after 10 000 cycles at 10 A g⁻¹. As for potassium-ion batteries, PHC-700 exhibits a reversible capacity of 381.9 mA h g⁻¹ at 0.1 A g⁻¹ and a capacity of 260 mA h g⁻¹ after 1000 cycles at 0.2 A g⁻¹. In situ Raman and in situ NMR measurements reveal that the P-containing bonds can enhance the adsorption to alkali metal ions, and the PC bond can participate in electrochemical redox reaction by forming Li_x PC_y. Additionally, P-doped hard carbon shows better structural/interfacial stability for improved long-term cycling stability.