Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 15, Issue 26, Pages 11128-11138Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c3cp51927a
Keywords
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Funding
- Office of Vehicle Technologies of the U.S. Department of Energy [DE-AC02-05CH11231, 7056412]
- U.S. Department of Energy's Office of Basic Energy Science, Division of Materials Sciences and Engineering under UT-Battelle, LLC
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-98CH10886]
- Florida Energy System Consortium through University of Florida [80859]
- Office of Basic Energy Sciences, U.S. Department of Energy
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A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn4+ in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn4+. The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials.
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