A novel anhydrous method to prepare hard carbon with high yield for sodium-ion batteries

In order to solve the problems of low yield and initial Coulombic efficiency (ICE) for hard carbon (HC), a simple and novel anhydrous method followed by a high-temperature carbonization process has been successfully employed to synthesize HC. This method is able to obviously increase the yield and ICE of HC to 59.01 % and 77.61 %, respectively, compared with the hydrothermal method (42.25 % and 69.79 %, respectively). It has been found that HC synthesized by anhydrous treatment at 190 degrees C and calcination at 1100 degrees C has the best electrochemical performance (capacity retention of 89.28 % after 150 cycles at 100 mA g(-1)). In order to further improve the electrochemical performance of HCs, phenolic resin has been adopted to coat HCs. ICE and capacity retention after 100 cycles of coated HC are improved to 83.26 and 99.18 %, respectively. Coated HCs are more inclined to form solid electrolyte interphase (SEI) films and present smaller extents of increase in impedance after cycling. This work provides a simple and effective way which is potential for large-scale synthesis of HC with high yield and ICE.