Commercial Carbon Fibers as Host for Sodium Deposition to Achieve High Volumetric Capacity

Abstract The advancement of flexible electronic devices necessitates the utilization of electrode materials that offer robustness and high capacity. In this paper, it is revealed that commercially available carbon fibers with specific microcrystalline structures not only have high mechanical strength but also a high volumetric capacity of up to 300 mAh cm⁻³, surpassing conventional carbon materials. When multiple structural parameters of carbon fiber reach certain thresholds, a breakthrough in sodium storage capacity and rate performance can be achieved. This study further elucidates the mechanism whereby this specific carbon fiber primarily utilizes an all‐plateau sodium deposition mechanism, which occurs in pore‐like grain boundaries. Through in situ spectroscopy and synchrotron techniques, the reversible deposition process of metallic sodium has been revealed at different scales. Theoretical calculations and thermodynamic principles further confirm the desolvation and deposition mechanisms in carbon fibers. As a result, this research discovers the modulating effects and patterns of crystallinity, defect, and orientation of carbon materials on sodium storage sites and diffusion kinetics, thereby achieving controlled sodium storage. This work shows that commercial carbon fibers can serve as robust hosts for sodium deposition and enhances the theoretical understanding of how the microcrystalline structure of carbon materials relates to sodium storage properties.