Understanding irreversible capacity in LixNi1-yFeyO2 cathode materials

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. (C) 2000 The Electrochemical Society. S0013-4651(00)02-093-0. All rights reserved.