Structural and electrochemical characterization of the spinel LiCrxMn2-xO4system

We synthesize the spinel lithium manganese oxides of the LiCrxMn2-xO4 from x=0.00 to 0.70 at 750 degrees C in air and identify them with X-ray crystal lography. We determine the crystallinity, the local structure, and the vari ation of Mn oxidation states due to the substitution of chromium by the Rie tveld and X-ray Absorption Spectroscopy (XAS) analyses. The Rietveld analys is reveals the structure of LiCrxMn2-xO4 adopts cubic spinel phase with spa ce group Fd (3) over bar m. The crystallinity of the spinel compounds impro ves as the amount of Cr increases, which depends upon variation of the oxid ation state in Mn and the existence of Mn in tetrahedral (T-d) sites. The M n ions have a mixed oxidation state between Mn3+ and Mn4+ ions. The oxidati on state of Mn atoms has more Mn4+ ions as the x-value increases. Since Mn3 + ions have Jahn-Teller distortion, the decrease of the Mn3+ 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 decr ease 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 corresp onding to a higher oxidation state. We observe the electrochemical properti es of the spinel compounds with Li//LiCrxMn2-xO4 galvanic cells. We obtain the galvanic cell data under a constant current of 265 mu A and a voltage r ange of 3.4 to 5.2V. The capacity loss on repeated charge and discharge dec reases in overall voltage range. The first discharge capacities, however, d ecrease in the range of 3.4 to 4.3V, while the capacities increase in a ran ge of 4.3 to 5.2V, with x-value due to the oxidation/reduction of Cr in add ition to those of Mn. Also the plateau at 4.5V, which is responsible for th e oxidation/reduction of Mn in T-d site, gradually disappeared.