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. The secondary lithium battery is the electrochemical energy storage technology of highest performance today, as yet unmatched in energy density and robustness in charge-discharge cycling.