On the Oxidation State of Manganese in Li-Ion Cells: A New Perspective

It has been long recognized that dissolution of transition metals, particularly Mn, at the positive electrode is the starting point for a major performance degradation mechanism in Li-ion batteries (LIBs).^1-4 We present recent X-ray absorption spectroscopy data on components from Li_xMn₂O₄ spinel – graphite (LMO-GR) cells with 1M LiPF₆in an ethylene carbonate : diethyl carbonate (1:2 v/v) electrolyte, after 20 days of cycling at 50 °C with 100% depth of discharge and C/5 rate.

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.

References

1. D. H. Jang, & S. M. Oh, J. Electrochem. Soc. 144, 3342 (1997).

2. Y. Xia, Y. Zhou, & M. Yoshio, J. Electrochem. Soc.144, 2593 (1997).

3. G. Amatucci, C. N. Schmuts, A. Blyr, C. Sigala, A. S. Gozdz, D. Larcher, & J.-M. Tarascon, J. Power Sources 69, 11 (1997).

4. N. Kumagai, S. Komaba, Y. Kataoka, & M. Koyanagi, Chem. Lett.1154 (2000).

Acknowledgements

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.