Study of the Capacity Fading Mechanism for Fe-Substituted LiCoO[sub 2] Positive Electrode

Abstract

To find the origin of a large initial irreversible capacity and capacity fading with cycling for Fe-substituted the positive electrode was selected for study by ex situ X-ray diffraction, Co and Fe K-edge X-ray absorption, and Mössbauer spectroscopies. A disordering of Fe ions from 3b(0, 0, 1/2) to 6c(0, 0, 3/8) sites was detected for initial charged samples through X-ray Rietveld analysis and Co and Fe K-edge X-ray absorption near-edge structures and extended X-ray fine structures spectra. The valence state of Fe ions in the 6c site was determined to be a 3+/4+ mixed valence state from the isomer shift values obtained by Mössbauer spectra and mean M-O distance values. 50% of the iron ions become disordered after the initial charge process and more than 20% remain in 6c sites after the first and tenth discharge processes. The existence of ions on the interstitial 6c site can block fast Li conduction in the Li layer of the layered rock-salt structure This leads to a lack of reversibility in Fe-substituted positive electrode materials. © 2004 The Electrochemical Society. All rights reserved.