Use of Hydrothermal Carbonization to Improve the Performance of Biowaste‐Derived Hard Carbons in Sodium Ion‐Batteries

Abstract Over the last years, hard carbon (HC) has been the most promising anode material for sodium‐ion batteries due to its low voltage plateau, low cost and sustainability. In this study, biomass waste (spent coffee grounds, sunflower seed shells and rose stems) was investigated as potential material for hard carbon preparation combining a two‐step method consisting of on hydrothermal carbonization (HTC), to remove the inorganic impurities and increase the carbon content, and a subsequent pyrolysis process. The use of HTC as pretreatment prior to pyrolysis improves the specific capacity in all the materials compared to the ones directly pyrolyzed by more than 100 % at high C‐rates. The obtained capacity ranging between 210 and 280 mAh g −1 at C/15 is similar to the values reported in literature for biomass‐based hard carbons. Overall, HC obtained from sunflower seed shell performs better than that obtained from the other precursors with an initial Coulombic efficiency (ICE) of 76 % and capacities of 120 mAh g −1 during 1000 cycles at C with a high capacity retention of 86–93 %.