The Cr-substituted spinel Mn oxides LiCryMn2-yO4 (0<=y<=1): Rietveld analysis of the structure modifications induced by the electrochemical lithium deintercalation

The electrochemical study of (LiCryMn1-yMnO4)-Mn-III-Mn-III-O-IV compounds, 0 less than or equal to y less than or equal to 1, between 3.5 and 5.4 V vs Li metal, shows that the Cr substitution is beneficial to the overall performances (capacity, specific energy, and cyclability) for y less than or equal to 0.62, and that y = 0.25 gives the best results. For y > 0.62, the cyclability is worse than for LiMn2O4, and for y greater than or equal to 0.75 the capacity fading becomes very fast. The electron energy loss spectroscopy study confirms that the deintercalation in the range 3.5-4.3 V corresponds to a Mn-III-Mn-IV oxidation. Further deintercalation between 4.3 and 5.1 V mainly corresponds to a Cr-III-Cr-IV oxidation with, however, the formation of a significant amount of Cr-VI for the highest Cr content y = 0.75. In the starting compounds LiM2O4, M = Cr, Mn, Rietveld refinements show that the 8a and 16d sites of the spinel structure are occupied by Li and M, respectively. The Li deintercalation up to 5.1 V induces structural defects which do not disappear completely upon the subsequent intercalation down to 3.5 V; their concentration increases upon cycling. Such defects consist of M cations occupying an 8a site (''Td defect'') or a 16c site (''16c defect''), instead of 16d. Both defects imply a lowering of the tetrahedral Li concentration which accounts for the experimental capacity loss upon cycling. (C) 1997 Academic Press.