Correlating capacity loss of stoichiometric and nonstoichiometric lithium manganese oxide spinel electrodes with their structural integrity

It is known that stoichiometric spinel, LiMn2O4, when used as a lithium-ion battery cathode? exhibits significant capacity fade on cycling at room tem perature, whereas by making the spinel slightly nonstoichiometric the capac ity retention on cycling is improved to a great extent. To help understand this difference in performance, X-ray diffraction (XRD) was used to investi gate the spinel structure during lithium extraction and reinsertion. The st oichiometric spinel shows degradation during the first lithium extraction w hich becomes more severe at high rates. The XRD pattern measured at the end of 50 cycles showed significant loss of structural integrity, with several prominent peaks that were not present prior to cycling. In contrast, the n onstoichiometric spinel showed no evidence of degradation up to at least 30 0 cycles, even at high rates. It is suggested that in the case of stoichiom etric spinel, lambda- MnO2, which forms upon extraction of lithium, accumul ates during cycling and, because it becomes disconnected from the active el ectrode, capacity fade is significant. (C) 1999 The Electrochemical Society . S0013-4651(99)03-071-2. All rights reserved.