Enhancing waste hemp hurd-derived anodes for sodium-ion batteries through hydrochloric acid-mediated hydrothermal pretreatment

Waste hemp hurd (WHH) was used as a sustainable feedstock for producing hard carbon-based anodes for sodium-ion batteries (SIBs). Two easily scalable production pathways were tested and compared: (1) pyrolysis (at 500 degrees C) and subsequent annealing at 800, 1000 or 1200 degrees C, and (2) hydrothermal pretreatment (at 180 degrees C) and subsequent annealing at the above-mentioned highest temperatures. Results indicated that when a HCl (2 mol m-3) aqueous solution was used as hydrothermal medium, the textural, structural and surface chemistry features linked to the electrochemical performance of the resulting hard carbons improved. The WHH-derived electrode produced via HCl-mediated hydrothermal pretreatment and subsequent annealing at 1000 degrees C showed an exceptional electrochemical performance in terms of specific capacity (535 mA h g-1 at 30 mA g-1) and rate capability (372, 156, 115, and 83 mA h g-1 at 0.1, 0.5, 1, and 2 A g-1, respectively) when an ester-based electrolyte was used (NaTFSI in EC/DMC). Using an ether-based electrolyte (NaPF6 in diglyme) improved both the ICE (from 69% to 78%) and cycling stability (85% of capacity retention after 300 cycles at 1 A g-1; 91% when current rate returned to 0.1 A g-1). In summary, relatively low-cost WHH-derived carbons are able to deliver an exceptional performance, much better than that reported so far for other biomass-derived carbons, and even close to that exhibited by more expensive and complex composite and hybrid materials.