Enhancement of the rate performance of plasma-treated platelet carbon nanofiber anodes in lithium-ion batteries

The rate performances of lithium-ion battery (LIB) anodes using platelet carbon nanofiber (PCNF) and its graphitized version (GPCNF) are enormously enhanced by introducing carbon-fluorine (C_mF_n) functional groups on the nanofiber surfaces. The C_mF_n functional groups are selectively introduced through controlled plasma treatment in vacuo with C₄F₈ gas. Combined X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyses demonstrate that the C_mF_n functional groups had mainly semi-ionic C-F bonds, which are introduced only on the surface of PCNF and GPCNF. PCNF treated with the plasma for 60 s (PCNF-F60 s) exhibits the largest discharge capacity of 387 mA h g^-1 with increased first-cycle coulombic efficiency and a discharge capacity of 293 mA h g^-1 at the 10C rate. These are 1.13- and 1.63-fold higher, respectively, than those of pristine PCNF. The presence of C_mF_n functional groups on the PCNF and GPCNF surfaces can reduce the resistance of the anode, which is related to lithium-ion migration and charge transfer resistance. The improved migration and reduced resistance result in a marked increase in rate performance at discharge without deterioration of the first-cycle coulombic efficiency.