Explanation for the 4.8-V plateau LiCrxMn2-xO4
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Using the results of photoelectron spectroscopy (PES) measurements on LiCrxMn2-xO4 we provide an explanation for the origin of the 4.8-V plateau in Li/LiCrxMn2-xO4 cells. PES shows that the d bands derived from Mn 3d e(g) are about 0.5 eV more weakly bound than the d bands derived from Cr 3d t(2g). Within a rigid-band formalism, as lithium atoms are removed from LiCrxMn2-xO4, the electrons are first removed from Mn e,. When these electrons are gone, the next electrons available are whose in Cr t(2g), which are bound more tightly by 0.5 eV. Thus, at this point, the voltage of a Li/LiCrxMn2-xO4 cell steps up from 4.1 to 4.8 V. The oxidation states on the atoms in the compound are Li+1Crx+3Mn2-x+3Mn+4O4-2 so the step in voltage should occur when the (1-x) Mn3+ atoms are all oxidized to Mn4+, or when (1-x) Li have been removed from the compound. Using a series of carefully prepared and characterized samples, we show that this is true for single phase samples that do not exhibit cation mixing.
Obrovac, MN, Y. Gao, and JR Dahn. 1998. "Explanation for the 4.8-V plateau LiCrxMn2-xO4." Physical Review B 57(10): 5728-5733.