Understanding Irreversible Capacity in Li[sub x]Ni[sub 1−y]Fe[sub y]O[sub 2] Cathode Materials

A study was made of lithium‐ion cell cathode materials. Through a series of experiments including cyclic voltammetry, X‐ray diffractometry, Mössbauer spectroscopy, in situ X‐ray diffractometry, and in situ Mössbauer spectroscopy, it was determined that the phenomenon commonly referred to as "irreversible capacity" in lithium‐ion cell cathode materials is a misnomer. Cells utilizing with 0.05 ≤ y ≤ 0.10 as the cathode active material (vs. a lithium metal anode) have been reversibly cycled between 0.64 ≤ x ≤ 1.00 (100 mAh/g). The recovery of the "irreversible capacity" involved a discharge through a ∼2 V plateau during which it is believed that a surface layer of is present. Also, a new transition has been observed between two O3 phases (belonging to the space group ) during charge beginning at the start of charge and reaching completion by x ≈ 0.82 (50 mAh/g). The cause of this transition as well as the formation of the surface layer during discharge are explained in terms of the mobility of lithium ions in the intercalation host being related to the availability of monovacancy and divacancy hopping paths. © 2000 The Electrochemical Society. All rights reserved.