High performing and sustainable hard carbons for Na-ion batteries through acid-catalysed hydrothermal carbonisation of vine shoots

This study investigates the synthesis of hard carbons via acid-assisted hydrothermal carbonisation (HTC) of vine shoots, followed by thermal annealing, to be used as Na-ion battery anodes. Various carbons with diverse pore structures, ordering degrees, 2D morphologies, and chemical compositions are obtained using 2 mol dm−3 solutions of HNO3, HCl, H2SO4, or H3PO4 as the hydrothermal media. Compared to standard ester-based liquid electrolytes, the use of a diglyme-based electrolyte (NaPF6/DGM) substantially boosts both the initial coulombic efficiency (ICE) and the specific capacity, particularly evident with the highly ordered and porous H3PO4-1000 and HCl-1000 carbons. These materials exhibit outstanding performance, storing 200 mAh g−1 at 2 A g−1 and showing ICE values of 68% and 77%, respectively. The improved stability and capacity are attributed to the formation of a more stable and thinner solid electrolyte interface (SEI), along with sodium storage into graphitic regions through a solvent co-intercalation reaction. While ester-based electrolytes limit hard carbon mesoporosity and require the promotion of turbostratic domains, ether-based electrolytes enable the development of more ordered and porous carbons, enhancing transport kinetics, sodium storage capacity, and electrode stability. Overall, acid-catalysed HTC offers a sustainable pathway for tailoring hard carbon based electrodes, repurposing biomass residues into valuable materials for green and low-cost energy storage applications.