Understanding irreversible capacity in LixNi1-yFeyO2 cathode materials

A study was made of LixNi1-yFeyO2 lithium-ion cell cathode materials. Throu gh a series of experiments including cyclic voltammetry, X-ray diffractomet ry, Mossbauer spectroscopy, in situ X-my diffractometry, and in situ Mossba uer spectroscopy, it was determined that the phenomenon commonly referred t o as "irreversible capacity" in LixNi1-yFeyO2 lithium-ion cell cathode mate rials is a misnomer. Cells utilizing LixNi1-yFeyO2 with 0.05 less than or e qual to y less than or equal to 0.10 as the cathode active material(vs. a l ithium metal anode) have been reversibly cycled between 0.64 less than or e qual to x less than or equal to 1.00 (100 mAh/g). The recovery of the "irre versible capacity" involved a discharge through a similar to 2 V plateau du ring 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 sur face layer during discharge are explained in terms of the mobility of lithi um ions in die intercalation host being related to the availability of mono vacancy and divacancy hopping paths. (C) 2000 The Electrochemical Society. S0013-4651(00)02-093-0. All rights reserved.