Synthesis and characterization of submicron size particles of LiMn2O4 by microemulsion route

Among the various positive electrode materials investigated for Li-ion batteries, spinel LiMn2O4 is one of the most important materials. Small particles of the active materials facilitate high-rate capability due to large surface to mass ratio and small diffusion path length. The present work involves the synthesis of submicron size particles of LiMn2O4 in a quaternary microemulsion medium. The precursor obtained from the reaction is heated at different temperatures in the range from 400 to 900 degrees C. The samples heated at 800 and 900 degrees C are found to possess pure spinel phase with particle size < 200 nm, as evidenced from XRD, SEM, and TEM studies. The electrochemical characterization studies provide discharge capacity values of about 100 mAh g(-1) at C/5 rate, and there is a moderate decrease in capacity by increasing the rate of charge-discharge cycling. Studies also include charge-discharge cycling and ac impedance studies in temperature range from -10 to 40 degrees C. Impedance data are analyzed with the help of an equivalent circuit and a nonlinear least squares fitting program. From temperature dependence of charge-transfer resistance, a value of 0.62 eV is obtained for the activation energy of Mn3+/Mn4+ redox process, which accompanies the intercalation/deintercalation of the Li+ ion in LiMn2O4.