Biomass-derived hard carbon microtubes with tunable apertures for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are considered the most up-and-coming complements for large-scale energy storage devices due to the abundance and cheap sodium. However, due to the bigger radius, it is still a great challenge to develop anode materials with suitable space for the intercalation of sodium ions. Herein, we present hard carbon microtubes (HCTs) with tunable apertures derived from low-cost natural kapok fibers via a carbonization process for SIBs. The resulted HCTs feature with smaller surface area and shorter Na⁺ diffusion path benefitting from their unique micro-nano structure. Most importantly, the wall thickness of HCTs could be regulated and controlled by the carbonization temperature. At a high temperature of 1,600 °C, the carbonized HCTs possess the smallest wall thickness, which reduces the diffusion barrier of Na⁺ and enhances the reversibility Na⁺ storage. As a result, the 1600HCTs deliver a high initial Coulombic efficiency of 90%, good cycling stability (89.4% of capacity retention over 100 cycles at 100 mA·g⁻¹), and excellent rate capacity. This work not only charts a new path for preparing hard carbon materials with adequate ion channels and novel tubular micro-nano structures but also unravels the mechanism of hard carbon materials for sodium storage.