Sucrose-derived hard carbon wrapped with reduced graphene oxide as a high-performance anode for sodium-ion batteries

Further improving the performance of hard carbon anodes for sodium-ion batteries (SIBs) needs a rational design of pores, which, however, remains a challenge. Herein, hard carbon is prepared by dehydration of sucrose with 95-98% sulfuric acid, followed by annealing in NH₃/Ar at 800 °C, leading to a sample named N-HC. N-HC is dominated by mesopores, evidenced by pore volumes of 0.799 cm³ g⁻¹ measured by N₂ adsorption and 0.307 cm³ g⁻¹ by CO₂ adsorption. When N-HC is further composited with ∼5 wt% graphene oxide (GO) followed by another annealing in NH₃/Ar at 800 °C, the sample (named N-HC/rGO) contains mostly ultra-micropores (<0.75 nm), showing pore volumes of 0.306 cm³ g⁻¹ measured by N₂ adsorption and 0.262 cm³ g⁻¹ by CO₂ adsorption. When being used as an anode for SIBs, N-HC/rGO shows a specific capacity of ∼500 mA h g⁻¹ at 0.01 A g⁻¹, which is much higher than that of N-HC (∼300 mA h g⁻¹). The specific capacity of N-HC/rGO remains at 190.5 mA h g⁻¹ at 1.0 A g⁻¹ after 1500 cycles. The in situ X-ray diffraction and analysis of galvanostatic charge-discharge results indicate that the insertion of Na⁺ into the carbon interlayer and filling of Na⁺ in the ultra-micropores simultaneously contribute to the high plateau capacity of N-HC/rGO. When N-HC/rGO is assembled with sodium vanadium phosphate in a full coin cell, energy and power densities of 287.4 W h kg⁻¹ and 4860 W kg⁻¹ are demonstrated.