Surface functionalization to abate the irreversible capacity of hard carbons derived from grapefruit peels for sodium-ion batteries

Carbon materials obtained from biomass are on the route to become the anode material for Na Ion Batteries; however, the large irreversible capacity limits their application, while the high synthesis temperature might have a great impact on the process sustainability. In this work, carbon materials derived from Grapefruit peels are proposed as sustainable anode materials for energy storage in Na-Ion batteries. The low temperature carbonization produces highly porous carbon presenting high irreversible capacity for Na intercalation. In order to increase the reversible capacity, a surface functionalization process with Melamine, Urea and Citric acid is proposed. The functionalization processes modify the disorder within the material and add functional groups on the surface leading to the attainment of higher reversible capacity; particularly the Urea functionalized sample possess a much higher reversible capacity and capacity retention at higher C-rates due to a less reactive surface and enhanced Na diffusivity. Thus, the functionalization has an important effect on the transport of sodium inside the pores but also on the increase of the disordered nature which permits stable sodium intercalation capacity, around 180 mA h g−1 after 60 charge/discharge cycles at several C-rates, which is a drastic improvement of the properties of unfunctionalized materials obtained at low carbonization temperature. This strategy fully exploited can lead to new avenues for the reutilization of biomass waste into energy storage applications.