Electrochemical Cycling and Lithium Insertion in Nanostructured FeF_3 Cathodes

The cycle lives for cathodes of nanocrystalline iron trifluoride (FeF_3) were measured in rechargeable lithium batteries at different depths of discharge. When the discharge was limited to less than one Li^+ ion per FeF_3, both the cycle life and energy efficiency were considerably greater than when converting FeF_3 into Fe and LiF in deep discharge. An in situ X-ray diffractometry (XRD) study of the FeF_3 cathode during its initial discharge to LiFeF_3 showed a continuous change of the FeF_3 diffraction pattern, indicating Li^+ insertion into the rhombohedral FeF_3 causing distortion of its lattice parameters. Electrochemical cycling is most reversible when this mechanism occurs in the absence of other changes in the crystal structure