Influence of the cr content on the Li deinsertion behavior of the LiCryMn2-yO4 (0 <= y <= 1) compounds - I. Separation of bulk and superficial processes at high voltage

After a first charge-discharge cycle within the 4.3-5.2 V range, leading to the formation of a passivation layer, LixCryMn2-yO4 materials with y less than or equal to 0.75 present a good reversiblity contrary to those with y > 0.75. The use of a LiPF6-based, rather than a LiClO4-based electrolyte, e nables an easier separation of successive Li deinsertion processes, and str ongly decreases the current of electrolyte oxidation above 5.1 V. However, regardless of the electrolyte, the separation of bulk and superficial pheno mena on the active material can be done using methods which depend on the e volution of the electrolyte oxidation rate with the voltage. Thanks to thes e separation methods, the influence of various experimental parameters on t he irreversible and cyclable capacities was examined. It is shown that the cyclable capacity does not depend on the scan rate provided that the end-of -charge voltage is high enough to reach the end of the deintercalation proc ess, Furthermore, an optimized composition of the composite electrode is su ggested from the variations of both capacities with the mass of active mate rial and the carbon black content. (C) 2001 The Electrochemical Society.