Controlling the structural and sodium storage properties of glucose-derived hard carbons using the pre-carbonization heating rate

Non-graphitizing (so-called "hard") carbons, derived from natural and abundant precursors such as biomass, sugars (<i>e.g.</i>, glucose and sucrose) and polysaccharides (<i>e.g.</i>, starch and cellulose), are widely investigated as negative electrode materials for alkali metal-ion storage in secondary batteries because of their suitable structural features, including the unique "closed porosity". The formation of such microstructures depends on the heating conditions, such as the temperature or holding time of either carbohydrate condensation during preliminary carbonization ("pre-carbonization") or final carbonization. Numerous studies have extensively examined the impact of condensation and carbonization temperatures on the microstructure of hard carbons and their resulting electrochemical properties. Comparatively less research attention has been devoted to the influence of the heating rate in the low-temperature region, where it can be expected that the structure of the final hard carbon is largely established and a significant impact can be expected. Hence, this work is dedicated to investigating the effect of the rate of preheating, from room temperature to 600 °C, of glucose-derived carbon on its structure and electrochemical properties after final high-temperature carbonization at 1500 °C. Hard carbons obtained using 2, 10 and 200 K min^-1 pre-carbonization temperature ramps exhibit distinct differences in plateau capacity values for sodium (226 ± 15.60 mAh g^-1, 206 ± 13.27 mAh g^-1, and 192 ± 9.44 mAh g^-1, respectively). This points to differences in closed porosity, defectiveness, and pore symmetry. The material preheated at the slowest ramp has the highest internal surface area, defectiveness, and highly asymmetrical pores, thereby promoting an increase in specific sodiation capacity (326 ± 21 mAh g^-1), particularly pronounced in the plateau region (226 ± 15.60 mAh g^-1). These findings imply the potential to regulate the microstructure of hard carbons through the initial heating rate during carbonization.