On the Oxidation State of Manganese in Li-Ion Cells: A New Perspective
Results from the Mn K edge (6539 eV) XANES analysis indicate average oxidation states near +3 for Mn cations in the graphite electrodes and separators from cells that were subjected to a 48 hour stand (in either discharged or charged state) subsequent to the high-temperature galvanostatic cycling, see Fig. 1. This suggests that Mn metal or in oxidation state +2 can only be minor fractions of the Mn existing outside the positive electrode of a Li-ion battery. Our results run counter to the prevailing view that Mn2+ cations arising from a disproportionation reaction of two Mn3+cations at the positive electrode, followed by dissolution into and migration through the electrolyte, and ending with deposition at the negative electrode, are widely responsible for the performance degradation observed in graphite-LMO LIBs. We will discuss the possible sources of discrepancy between our results and previously published data, as well as the implications of our results for measures to mitigate the Mn dissolution related LIB performance degradation mechanism, see Fig. 2.
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Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.