Properties, crystal structure, and performance of o-LiMnO2 as cathode material for Li secondary batteries

Orthorhombic LiMnO2 (o-LiMnO2) was prepared as a cathode material for Li secondary batteries by the solid-state, sol-gel, and reflux heating methods. Reflux heating yielded a single phase whose composition we were able to control. The particle size distribution of this o-LiMnO2 was uniform. Its discharge capacity at 25 degrees C was the lowest and its cycle performance the most stable among the LiMnO2 produced by the three methods. The discharge capacity increased with cycle number and stabilized at about 30 - 40 cycles at 25 degrees C. We investigated the relationship between the Li+ diffusion coefficient ((D) over tilde (Li+)) and the crystal structure during the cycles. The (D) over tilde (Li+) for Li1-xMnO2 at the stable cycle region is larger than that at the initial cycle and was almost constant during the charge - discharge process at 25 degrees C. The (D) over tilde (Li+) at 30, 40 cycles corresponded almost exactly to that of the spinel Li1-xMn2O4. Refinement of the sample was performed in 30 cycles by assuming two phases of space group F d(3) over bar m. The electron-density images suggest that the covalent bond of Mn-O was stronger for the sample synthesized by reflux heating than those synthesized by the solid-state method. The high-crystallinity o-LiMnO2 leads to better stability and slower conversion to spinel LiMn2O4. (c) 2006 The Electrochemical Society.