Waste beverage coffee-induced hard carbon granules for sodium-ion batteries

Sodium-ion batteries (SIBs) have received considerable attention as sustainable and stable energy-storage devices, but their electrochemical performance is inferior to that of current lithium-ion batteries. In this study, we designed high-plateau-capacity hard carbons from waste beverage coffee (WBC) grounds as an anodes for SIBs, using a simple pyrolysis process followed by heating. The inherent structure of the basalt-like WBCs was preserved during the thermal treatment process, resulting in highly porous internal microstructures composed of severely tousled carbon building blocks. Furthermore, the hard carbon prepared by heating WBCs at 2000 °C had the lowest bulk density and consisted of poly hexagonal carbon fragments that had undergone only slight growth. These WBCs exhibited a low-voltage single-plateau capacity of ∼235 mA h g^-1 at 25 mA g^-1, with ∼84% and ∼96% of this capacity maintained at a rate 10× higher than the current rate (25 mA g^-1) for over 200 cycles, respectively. Several analytical results indicated that the carbon microstructure and the density of active closed pores are both closely correlated with the plateau capacity and sodium diffusion rate.