Journal
JOURNAL OF POWER SOURCES
Volume 174, Issue 2, Pages 519-523Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.jpowsour.2007.06.127
Keywords
electrode materials; EPR spectroscopy; local structural characterization; lithium-ion batteries
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Electron paramagnetic resonance spectroscopy was used to analyse the changes in local Ni, Mn environment in layered LiNi1/2Mn1/2O2 and spinel LiNi1/2Mn3/2O4 after electrochemical extraction and reinsertion of lithium. For layered LiNi1/2Mn1/2O2, the EPR signal from Mn4+ is only detected, while residual antiferromagnetic correlations between Ni2+ and Mn4+ ions gives rise to strong resonance absorption for LiNi1/2Mn3/2O4 spinel. The first charge process of layered LiNi1/2Mn1/2O2 leads to oxidation of Ni2+ ions located both in the transition metal sites and in the Li sites. The reverse process of reduction of these nickel ions was suggested to proceed between 4.4-3.0 V and 2.5-1.4 V, respectively. Lithium extraction from LiNi1/2Mn3/2O4 spinel leads to oxidation of paramagnetic Ni2+ to diamagnetic Ni4+ without significant changes in the local environment of Mn4+. For both fully delithiated compositions, Li1-xNi1/2Mn1/2O2 and Li1-xNi1/2Mn3/2O4, an EPR spectrum from localized Mn4+ ions is observed, indicating an exhaustion of paramagnetic Ni2+ ions in the vicinity of Mn4+ ions. Furthermore, it has been found that the Mn4+ environment including paramagnetic Mn4+ and Ni2+ neighbours is restored after the first cycle of charge/discharge. (C) 2007 Elsevier B.V. All rights reserved.
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