Carbon metal fluoride nanocomposites - High-capacity reversible metal fluoride conversion materials as rechargeable positive electrodes for Li batteries

The structure and electrochemistry of FeF3:C-based carbon metal fluoride nanocomposites (CMFNCs) was investigated in detail from 4.5 to 1.5 V, revealing a reversible metal fluoride conversion process. These are the first reported examples of a high-capacity reversible conversion process for positive electrodes. A reversible specific capacity of approximately 600 mAh/g of CMFNCs was realized at 70degreesC. Approximately one-third of the capacity evolved in a reaction between 3.5 and 2.8 V related to the cathodic reduction reaction of Fe3+ to Fe2+. The remainder of the specific capacity occurred in a two-phase conversion reaction at 2 V resulting in the formation of a finer Fe:LiF nanocomposite. Upon oxidation, selective area electron diffraction characterization revealed the reformation of a metal fluoride. Evidence presented suggested that the metal fluoride is related to FeF2 in structure. A pseudocapacitive reaction is proposed as a possible mechanism for the subsequent Fe2+ --> Fe3+ oxidation reaction. Preliminary results of FeF2, NiF2, and CoF2 CMFNCs were used in the discussion of the electrochemical properties of the reconverted metal fluoride. (C) 2003 The Electrochemical Society.