Pre‑carbonization for regulating sucrose-based hard carbon pore structure as high plateau capacity sodium-ion battery anode

Although hard carbon still suffers from low initial coulombic efficiency and a controversial sodium storage mechanism, it is widely explored and utilized as an anode material for sodium-ion batteries due to its affordability and accessibility. This work used pre‑carbonization to construct sufficient reaction time of volatile reactive molecules released from matrix in the carbon interlayers, hence optimizing the structure of the nanopore and the graphite microcrystal inside the sucrose-based hard carbon. The sucrose-based hard carbon after pre‑carbonization treatment has an expanded carbon layer spacing, an appropriate micro-mesopore ratio, and a distinct closed pore structure. The result provides evidence that the low-voltage plateau region capacity is related to two Na⁺ storage behaviors: intercalation between carbon layers and pore-filling in nanopores. Further larger interlayer distances, lower micro-mesoporous ratios, and closed pores are favorable for sodium storage in the low-voltage plateau region which is assisting to improve the initial coulombic efficiency. In comparison to previously published studies, the pre‑carbonized hard carbon at 450 °C with a heating rate of 3 °C/min exhibits an impressive plateau capacity of 277 mAh g⁻¹, increasing the contribution of the plateau capacity from 54 % to 63 %, while also enhancing cycling and rate performance. Furthermore, it has a significant initial coulombic efficiency (ICE) of 85 % and a noteworthy reversible specific capacity of 374 mAh g⁻¹ at a current density of 20 mA g⁻¹, which is noticeably better than the biomass hard carbon documented in the literature. Achieving a sustained low-voltage plateau capacity through microstructure modulation is crucial for producing hard carbon with both high specific capacity and rewarding ICE. This study presents a novel approach for the preparation sucrose based hard carbon of high plateau capacity and is expected to contribute significantly to the development of high energy density sodium-ion battery energy storage systems.