Structural and Electrochemical Characterization of the Spinel $LiCr_xMn_2_-_xO_4$ System

We synthesize the spinel lithium manganese oxides of the LiCr xMn2-xO4 from x=0.00 to 0.70 at 750 o C in air and identifiy them with X-ray crystallography. We determine the crystallinity, the local structure, and the variation of Mn oxidation states due to the substitution of chromium by the Rietveld and X-ray Absorption Spectroscopy (XAS) analyses. The Rietveld analysis reveals the structure of LiCr xMn2-xO4 adopts cubic spinel phase with space group Fd3m. The crystallinity of the spinel compounds improves as the amount of Cr increases, which depends upon variation of the oxidation state in Mn and the existence of Mn in tetrahedral (T d) sites. The Mn ions have a mixed oxidation state between Mn 3+ and Mn 4+ ions. The oxidation state of Mn atoms has more Mn 4+ ions as the x-value increases. Since Mn 3+ ions have Jahn-Teller distortion, the decrease of the Mn 3+ ions increases the crystallinity of compounds. A few Mn ions located in T d site instead of octahedral (O h) site cause local disorder. The Mn ions in T d site decrease with the increasing x-value, which is in agreement with results of the XAS analysis. The Mn-O bond length decreases gradually due to the raising Mn-O covalency and the absorption Mn K-edge shifts to higher energy corresponding to a higher oxidation state. We observe the electrochemical properties of the spinel compounds with Li//LiCr xMn2-xO4 galvanic cells. We obtain the galvanic cell data under a constant current of 265 μA and a voltage range of 3.4 to 5.2V. The capacity loss on repeated charge and discharge decreases in overall voltage range. The first discharge capacities, however, decrease in the range of 3.4 to 4.3V, while the capacities increase in a range of 4.3 to 5.2V, with x-value due to the oxidation/reduction of Cr in addition to those of Mn. Also the plateau at 4.5V, which is responsible for the oxidation/reduction of Mn in Td site, gradually disappeared.