PREPARATION AND ELECTROCHEMICAL INVESTIGATION OF LIMN2-XMEXO4 (ME-NI,FE, AND X=0.5, 1) CATHODE MATERIALS FOR SECONDARY LITHIUM BATTERIES

The spinels LiMn1.5Me0.5O4 and LiMnMeO4 (Me: Ni, Fe) were prepared usi ng either a sol-gel process in the case of nickel doping, or a solid-s tate reaction in the case of iron doping, X-ray diffraction analysis r evealed differences in the patterns when the content of the dopant in the spinel increases, The pattern of LiMn1.5Ni0.5O4 was indexed as a p ure cubic spinel together with that of LiMn1.5Fe0.5O4, whereas LiMnNiO 4 was found to be indexed mainly as a hexagonal layered phase since it was difficult to distinguish between these two structures because the diffraction peaks were broad, On the other hand, the pattern correspo nding to LiMnFeO4 was indexed as an inverse spinel. The material LiMn1 .5Ni0.5O4 could intercalate both chemically and electrochemically a se cond lithium at 3 V without structural changes and with a large discha rge capacity of 160 mAh/g, This material presents also a 4.7 V plateau with 90 mAh/g capacity due to the oxidation of divalent nickel to the tetravalent slate when first charged, A tetragonal phase due to the J ahn-Teller distortion was observed in the material Li1+xMn1.5Fe0.5O4 w ith a discharge curve showing 4.1 and 2.9 V plateaus as in the case of the spinel LixMn2O4 (0 less than or equal to x less than or equal to 2) without doping. X-ray photoelectron spectroscopy has revealed that the nickel was divalent in LiMn1.5Ni0.5O4 and trivalent in LiMnNiO4, w hereas. iron was trivalent in both LiMn1.5Fe0.5O4 and LiMnFeO4. The st ructure and the electrochemical behavior of the above materials were i nvestigated and correlated with the oxidation state of the dopants. (C ) 1997 Elsevier Science S.A.