Pyroprotein-Derived Hard Carbon Fibers Exhibiting Exceptionally High Plateau Capacities for Sodium Ion Batteries

In this study, low-density, hard-carbon fibers incorporating multitudinous closed pores with few-nanometer-scale widths were prepared from waste silk fabric by a simple high-temperature heating process (2000 °C). The pyroprotein carbon fibers exhibited a significantly high single-plateau capacity of 300 mA h g^-1 at ~0.1 V Na⁺/Na, a high initial Coulombic efficiency of ~91.9%, and stable cycling behaviors of more than 200 cycles when used as the anode of a sodium-ion battery. Characterization using in situ X-ray diffraction patterns and ex situ field-emission transmission electron microscopy confirmed that the outstanding charge storage behaviors of the pyroprotein carbon fibers are based on sodium-metal nanoclustering in the closed pores. Moreover, full cells assembled with the pyroprotein carbon-fiber-based anode and a reported cathode demonstrated their practical electrochemical performances, including a specific energy of 262 Wh kg^-1.