Carbon-metal fluoride nanocomposites -: Structure and electrochemistry of FeF3:C

The practical electroactivity of electrically insulating iron fluoride was enabled through the use of carbon-metal fluoride nanocomposites (CMFNCs). The nanocomposites were fabricated through the use of high energy mechanical milling and resulted in nanodomains of FeF3 on the order of 1-20 nm encompassed in a matrix of carbon as characterized by transmission electron microscopy and X-ray diffraction (XRD). Electrochemical characterization of CMFNCs composed of 85/15 wt % FeF3/C resulted in a nanocomposite specific capacity as high as 200 mAh/g (235 mAh/(g of FeF3) with the electrochemical activity associated with the Fe3+ --> Fe2+ occurring in the region of 2.8-3.5 V. The CMFNCs revealed encouraging rate capability and cycle life with <10% fade after 50 cycles. Structural evolution during the first lithiation reaction was investigated with the use of ex situ and in situ XRD. Initial results suggest that x from 0 to 0.5 in LixFeF3 proceeds in a two-phase reaction resulting in a phase with significant redistribution of the Fe atoms within a structure very similar to the base FeF3. FeF3-based CMFNCs also exhibited a very high specific capacity of 600 mAh/g at 70 degrees C due to a reversible reaction at approximately 2 V. (C) 2003 The Electrochemical Society.