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 degrees 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.

Automated summary

Sodium-ion batteries (SIBs) are promising for large-scale energy storage due to abundant and cheap sodium resources, but developing anode materials with sufficient space for sodium ion intercalation remains challenging. This study presents hard carbon microtubes (HCTs) with tunable apertures derived from low-cost natural kapok fibers, which have a unique micro-nano structure, smaller surface area, and shorter Na+ diffusion path. The wall thickness of HCTs can be regulated and controlled by carbonization temperature, and HCTs carbonized at 1600 degrees C show the smallest wall thickness, leading to enhanced reversibility of Na+ storage. The 1600HCTs exhibit a high initial Coulombic efficiency, good cycling stability, and excellent rate capacity. This work not only provides a new approach for preparing hard carbon materials with suitable ion channels and novel tubular micro-nano structures, but also reveals the mechanism of hard carbon materials for sodium storage.