Toward high-performance hard carbon as an anode for sodium-ion batteries: Demineralization of biomass as a critical step

Biomass is a promising precursor for producing high-performance hard carbon as an anode for sodium-ion batteries (SIBs) because of its high low-voltage plateau capacity. However, the effect of residual ash in biomass on the electrochemical performance of hard carbons has rarely been investigated. This work describes an effective ash-removal approach as a critical step for preparing high-performance anodes for SIBs. A strong correlation between the ash removal techniques with structural and electrochemical properties of hard carbon was revealed. By examining various ash-removal techniques prior to carbonization and after carbonization using aqueous acid, neutral, and alkaline solutions, it was demonstrated that the removal of ash from raw cocoa pod husk (CPH) using aqueous acid and subsequent carbonization at 1300 °C can produce hard carbon with high Na+ ion uptake in the low-voltage plateau region. During the acid pretreatment, ash and some hemicellulose fractions were removed, and carbonization of the acid-treated CPH resulted in hard carbon with a high degree of graphitization and reduced surface area. When tested as an anode in SIBs, the hard carbon produced from the acid-treated CPH exhibited an exceptionally high capacity of 317 mAh g−1 and high plateau capacity of 244 mAh g−1 at 0.05 A g−1, with a high initial Coulombic efficiency of 87%. At a high current density of 250 mA g−1, a high capacity of 134 mAh g−1 was maintained after 800 cycles. Post-treatment of hard carbon did not enhance the electrochemical performance. The physicochemical and electrochemical properties of hard carbons produced with the various pre- and post-treatment techniques were presented.