Electrochemical Cycling and Lithium Insertion in Nanostructured FeF₃Cathodes

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.