A study was made of LixNi1-yFeyO2 lithium-ion cell cathode materials. Through a series of experiments including cyclic voltammetry, X-ray diffractometry, Mossbauer spectroscopy, in situ X-my diffractometry, and in situ Mossbauer spectroscopy, it was determined that the phenomenon commonly referred to as "irreversible capacity" in LixNi1-yFeyO2 lithium-ion cell cathode materials is a misnomer. Cells utilizing LixNi1-yFeyO2 with 0.05 less than or equal to y less than or equal to 0.10 as the cathode active material(vs. a lithium metal anode) have been reversibly cycled between 0.64 less than or equal to x less than or equal to 1.00 (100 mAh/g). The recovery of the "irreversible capacity" involved a discharge through a similar to 2 V plateau during which it is believed that a surface layer of Li>1Ni1-yFeyO2 is present. Also, a new transition has been observed between two O3 phases (belonging to the space group R (3) over bar m) during charge beginning at the start of charge and reaching completion by x approximate to 0.82 (50 mAh/g). The cause of this transition as well as the formation of thr Li>1Ni1-yFeyO2 surface layer during discharge are explained in terms of the mobility of lithium ions in die intercalation host being related to the availability of monovacancy and divacancy hopping paths.