Optimizing hard carbon materials for sodium-ion batteries: Insights from particle size and soft carbon-coating strategy

Hard carbon (HC) has excellent sodium storage capacity as anode materials of sodium-ion batteries (SIBs), and the properties of its precursor are also important to the electrochemical performance. In this work, bamboo- derived HC materials with four different particle sizes from 1.5 to 10 mu m are prepared by combination of acid etching and carbonization at 1300 degrees C, in order to identify the effects of particle size on the structure and electrochemical performance. Two HC materials with the medium sizes of 3.5 mu m and 6.5 mu m show higher specific capacities than the others. To enhance the performance further, two hybrids of HC/soft carbon are prepared by coating pitch-derived soft carbon films onto the above two HC materials with sizes of 3.5 mu m and 6.5 mu m. Comparing the two hybrids, the one originated from 3.5 mu m HC delivers a higher reversible specific capacity of 335.3 mAh g- 1 at 30 mA g- 1 and an initial coulombic efficiency of 86.8 %. Its capacity retention is 95.2 % after 150 cycles at 300 mA g- 1 , showing an excellent cycling stability. This work provides encouraging anode materials of SIBs for the industrial application.