Optimization of tannin-derived hard carbon spheres for high-performance sodium-ion batteries

Abstract

The morphology (i.e., particle size and shape) of hard carbon materials remains a challenging property to tune, and most syntheses lead to large and randomly shaped particles. This indicates that a supplementary grinding step is necessary to reduce the particle size for optimal use in electrode preparations for sodium-ion batteries (NIBs). Herein, a synthesis route to obtain spherical hard carbon (HC) particles from eco-friendly tannin-polyphenol precursors is proposed. The impact of several parameters (stirring, solvent, cross-linker, and polyphenol source) on particle formation was investigated. A critical amount of cross-linker, a particular solvent, and static conditions (no stirring) were required to form spherical particles. The solvent and the cross-linker had a major effect on the material yield and the particle shape/size. Among them, the use of the hexamethylenetetramine (HMTA) cross-linker in water led to spherical particles and high polymer yield. In addition, the particle size could be tuned from 4.1 μm to ∼0.4 μm by changing the polyphenol source from Catechu to Mimosa, while the shape changed from interconnected particles to individual particles. The HC materials investigation revealed that the graphitic interlayer space was higher for Mimosa-derived HCs (3.70–3.75 Å) than Catechu-derived HCs. The higher interlayer space was found to increase the reversible capacity; therefore, Mimosa HC delivered the highest value (i.e., 330 mA h g⁻¹ for HC-MC1500). Specific surface area, oxygen-based functionalities, and active sites were lower for Mimosa-derived HCs, leading to the highest initial Coulombic efficiency of up to 88% (HC-MG1600), when tested in Na half-cells.