Electrochemical Behavior of Iron(III) Fluoride Trihydrate as a Cathode in Lithium-Ion Batteries

The tetragonal compound FeF3(H2O)(3) is synthesized through a facile liquid-phase method. FeF3(H2O)(3)/C is prepared by mechanical milling with carbon black and investigated for its application as a cathode material. This material exhibits two types of thermodynamic lithiation scheme, at 3.0 and 1.5 V, which correspond to a Li+-intercalation process (1 Li+) and a conversion reaction (> 1Li(+)), respectively. A reversible capacity of about 300 mAhg(-1) can be achieved at a current density of 10 mAg(-1). In particular, the intercalation/deintercalation process above 2.0 V exhibits good cycling performance and rate properties. The relatively high diffusion coefficients (D-cv = 7.27 x 10(-13)-1.07 x 10(-12) m(2) s(-1)) of Li+ through the FeF3(H2O)3 lattice are calculated by using the Randles-Sevcik equation, which reveal fast Li+-ion migration in this process. In contrast, the conversion reaction is strongly dependent on the current density. Electrochemical impedance spectroscopy indicates that the FeF3(H2O)(3)/C cathode forms a relatively stable solid electrolyte interphase film, with a low Schottky contact resistance in comparison with that of the FeF3/C cathode, which makes it a potential candidate for cathode applications.