Laser-irradiated coffee waste: a path to eco-friendly anode materials for lithium-ion batteries.

Hard carbon is being actively explored as a candidate anode material for next-generation batteries, offering ion storage mechanisms distinct from and potentially advantageous to those of graphite. However, conventional synthesis of hard carbon usually relies on high-temperature pyrolysis and chemical activation, which involve high energy consumption and environmental challenges. Spent coffee grounds (SCGs), generated in large amounts worldwide, represent an abundant biomass resource with high carbon content that is often discarded with limited recycling. In this study, SCGs were directly converted into hard carbon and partially graphitized structures using femtosecond laser direct writing (FsLDW) under solvent-free and non-vacuum conditions. Localized photothermal reactions induced by the ultrashort pulses promoted particle consolidation and structural rearrangement, and by varying the laser parameters, the carbonization pathway could be directed to favor either hard carbon or graphene domains. A copper substrate was deliberately employed to spontaneously generate copper compound nanoparticles, which were subsequently etched to create micro-porous carbon with tunable pore characteristics. This laser-based approach provides a controllable and one-step pathway for transforming SCGs into functional carbon anodes, highlighting the potential to selectively prepare materials suitable for both lithium-ion and sodium-ion batteries by adjusting only the laser conditions. While this study focused on lithium-ion battery applications, the tunable control of carbon structure and porosity offers the possibility of extending this strategy to sodium-ion batteries.