Plasma-enhanced fluorination of multi-walled carbon nanotubes for CFx cathode materials with ultrahigh electrochemical performance

As one of the most promising lithium primary batteries, lithium/fluorinated carbon (Li/CFx) batteries are irreplaceable in military, aerospace, medical, and other markets due to their advantages such as the high energy density, long shelf life, and wide operating temperature. However, it is difficult to enable CFx cathode materials to achieve simultaneously high-capacity and excellent rate-capability. Moreover, the high-temperature direct F2 fluorination method for the commercial CFx material preparation is hazardous, eco-unfriendly, and costly. Herein, we developed an efficient, safe, and cost-effective synthesis approach, in which plasma fluorinated multi-walled carbon nanotubes (PFCNTs) were synthesized by using CF4 plasma as a fluorine source under a low fluorination temperature (300 °C) within excessively short time (110 min). The plasma not only induced ionization of CF4 gas through the electron impact reaction but also promoted the diffusion of fluorine-containing ions in multi-walled carbon nanotubes. The PFCNT possessed un-fluorinated graphitic inner layers, electrochemically active semi-ionic C–F bonds, and few inactive C–F2 and conductive sp2 C=C bonds, endowing it excellent electronic and electrochemical properties. PFCNTs exhibited a high specific capacity of 819.3 mAh/g with an energy density of 2050 Wh/kg. Furthermore, a Li/PFCNT coin cell delivered a capacity of 527.4 mAh/g and a maximum power density of 58 206 W/kg at an ultrahigh current density of 30 A/g. The underling fluorination mechanism was also analyzed by combining with the plasma diagnostic tool of optic emission spectroscopy.