Freestanding MoO₂/Mo₂C imbedded carbon fibers for Li-ion batteries

Flexible and freestanding MoO₂/Mo₂C imbedded carbon fibers (MoO₂/Mo₂C ICFs) have been successfully synthesized via an integrated procedure including electrospinning, thermo-plastication in air and reduction/carbonization at high temperature. A series of techniques such as SEM, TEM, N₂ adsorption-desorption analysis, XRD, TGA, IR and XPS have been employed to systemically characterize the MoO₂/Mo₂C ICFs. In particular, it is observed that the MoO₂/Mo₂C ICFs derived from phosphomolybdic acid have more highly porous structures than those derived from molybdic acid. Most impressively, the obtained MoO₂/Mo₂C ICFs are directly used as binder- and current collector-free anode materials for LIBs, which exhibit desirable rate capability and satisfactory cycling performance. The electrochemical investigations illustrated that the MoO₂/Mo₂C ICFs could deliver an initial discharging capacity of 1422.0 mA h g^-1 with an original coulombic efficiency of 63.3%, and the subsequent reversible capacity could reach as high as 1103.6 mA h g^-1 even after 70 cycles at a current density of 0.1 A g^-1. Such a capacity is larger than the theoretical capacity of MoO₂ (838 mA h g^-1) and pure carbon fibers (460.5 mA h g^-1). More importantly, the MoO₂/Mo₂C ICFs exhibited an excellent rate performance with a capacity of 445.4 mA h g^-1 even at a charging current density of 1.6 A g^-1. The remarkable enhancement in rate capability and long cycling performance resulted from a synergistic effect between the MoO₂ nanoparticles and porous carbon fiber matrix. This methodology can be widely extended to fabricate other metal oxide/carbon composites for significant energy storage and conversion applications.