Elucidating the influence of alumina mineral on sodium storage performance of biomass-derived hard carbon anode

Biomass has gained significant attention as a promising precursor for preparing hard carbon anodes in sodium-ion batteries (SIBs), yet the influence of minerals in biomass on both material pyrolysis and sodium storage processes is not clear enough. Herein, we introduce alumina into the cellulose and lignin, which are the primary components of biomass, to elucidate the influence of alumina on the structure and sodium storage performance of biomass-based hard carbons. When cellulose serves as the precursor, alumina enlarges the interlayer spacing by impeding the growth and arrangement of carbon microcrystals. With the increase in alumina content, it initially creates open pores by facilitating the evaporation of volatile and hindering micropores closure, followed by subsequently blocking them to form closed pores, providing more active sites for Na⁺ storage in the low-voltage plateau region. As a result, the carbon derived from cellulose mixed with 1 wt% alumina demonstrates both higher initial Coulombic efficiency (82.4 %) and plateau capacity (127.1 mAh g⁻¹) at 0.02 A g⁻¹, compared to the hard carbon in the absence of alumina (77.2 % and 113.5 mAh g⁻¹). Additionally, alumina with more weak acid sites can be more conducive to catalyzing hydroxyl dehydration, thereby enhancing the pore-creating effect of volatile. The similar performance enhancements are not observed in lignin-based carbons due to the lower volatile content and larger steric hindrance of lignin. This study provides new insights into the influence of alumina on structures of hard carbon, which will promote the application of alumina on cellulose-rich precursors to improve the electrochemical performance.