Iron-induced formation of hierarchical open-pore hard carbon rich in oxygen functional groups for high-performance Na+ storage

Abstract

The hard carbon negative electrode of a Na⁺ battery derived from biomass faces significant constraints on its practical implementation, primarily stemming from inferior initial coulombic efficiency (ICE) and sluggish electrochemical kinetics, mainly due to irreversible sodium capture in its disordered regions and closed pores. This work proves that an Fe-catalyzed carbonization strategy can be used to construct hierarchical open-pore hard carbon rich in oxygen-containing functional groups. By introducing FeCl₃ in the initial carbonization process, more oxygen atoms are anchored and firmly retained via the catalytic effect of iron, and further transformed into oxygen functional groups beneficial to sodium storage at high temperatures. Meanwhile, the iron-catalyzed hierarchical porous structure (micropore–mesopore) facilitates electrolyte infiltration and the reversible adsorption–desorption of sodium ions (Na⁺). Electrochemical tests showed that FeCl₃-modified hard carbon exhibited 89.6% ICE, and its capacity remained at 94.6% after 500 cycles, which was better than that of hard carbon derived from the original biomass. Through further research, we found that the hard carbon with a graded pore structure constructed by iron catalysis exhibits a “adsorption-intercalation-pore filling” mechanism for Na⁺ storage. Further, commercial sodium vanadate phosphate was used as the matching cathode, and the specific charge–discharge capacity of the assembled battery reached 198.8/197.1 mAh g⁻¹ after 90 cycles at a current density of 0.5C. This research presents a valuable reference for the role of catalysts in improving the Na⁺ storage behaviors of hard carbon derived from biomass.