Caramelization as a Key Stage for the Preparation of Monolithic Hard Carbon with Advanced Performance in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have found broad applications due to the lower cost of stored energy per kilowatt-hour and the availability of raw materials for the production of these batteries. Nongraphitizable carbons, or hard carbons (HCs), appear to be the material of choice for negative electrodes (anodes) in SIBs. Saccharides, particularly glucose, are among the most used precursors to produce HCs. This work comprehensively explores a pretreatment step known as caramelization which precedes high-temperature pyrolysis. We synthesized monolithic HC with an ultralow specific surface area, demonstrating the initial Coulombic efficiency of 89% with a capacity of 300 mA h g-1 in a sodium half-cell. A strong correlation is revealed between the chemical composition of the intermediates and the microstructure of the HC anode materials as the final product. The pretreatment step conditions are shown to influence the properties of the HC (defectiveness, surface functionalities, and type of microporosity) and its electrochemical performance. The obtained data are important for upscaling the synthesis of HC anode materials with reproducible electrochemical characteristics.