Micropores and defects, like oxygen-containing groups, as active sites for sodium-ion storage in hard carbon have attracted considerable attention; nevertheless, most oxygen doping or oxidizing processes inevitably introduce undesired oxygen groups into a carbon framework, leading to deteriorated initial Coulombic efficiency (ICE). Here, precise carbonyl groups and closed micropores are together introduced into biomass-derived hard carbon to enhance the Na-ion storage performance. The hard carbon delivers a large reversible capacity of 354.6 mA h g-1 at 30 mA g-1, a high ICE (88.7%), as well as ultra-long cycling stability (277 mA h g-1 at 0.3 A g-1 over 1000 cycles; 243 mA h g-1 at 1 A g-1 over 5000 cycles). The rate capability and cycling stability of hard carbon in carbonate- and diglyme-based electrolytes are contrasted to demonstrate the superiority of diglyme. Cyclic voltammetry at varied scans and galvanostatic intermittent titration techniques are carried out to clarify the disparity between the two different electrolyte systems. Furthermore, the as-prepared hard carbon is utilized as the anode for sodium-ion full cells exhibiting an energy density of 166.2 W h kg-1 at 0.2 C and a long-cycle life (47.9% retention over 200 cycles at 1 C).
Keywords: anode; biomass; carbonyl groups; hard carbon; sodium-ion battery.