Emerging dual carbon fiber batteries

Dual graphite battery emerges as a promising renewable energy storage system with merits of a high working voltage, low cost and environment-friendliness. However, energy density is limited because of a low packing density of graphite. We propose for the first time dual carbon fiber batteries (DCFBs) in which carbon fiber functions as both cathode and anode. With a graphite mass loading of ∼30 mg cm−2 at pitch-based carbon fiber (PCF) cloth, the reversible discharge capacity of PCF-PCF type DCFBs (with PCF cloth cathodes/anodes) can reach 72 mAh g−1 at 0.1 C (1 C corresponding to 80 mAh g−1), resulting in an unprecedentedly high areal capacity of ∼2.2 mAh cm−2. Unlike the general applicability of commercialized carbon fiber as DCFBs anode, only PCF out of five kinds of commercialized carbon fibers can function as DCFBs cathode. Meanwhile, PCF-PCF type DCFB enables investigation into distinct electrochemical behaviors of Li+/PF6−intercalation/deintercalation in PCF. Morphological difference of PCF brought about by Li+/PF6− ions, as found out by ex-situ X-ray diffraction (XRD) and ex-situ scanning electronic microscopy/energy dispersive spectroscopy (SEM/EDS) characterizations, facilitates comprehensive understanding of electrochemical behavior of graphite. This work lays good foundations for future exploration into a new arena of dual ion batteries.