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.