Nano Crystalline Manganese Spinel LiMn2O4 Prepared by Ultrasonic Spray Pyrolysis

초록

Introduction Manganese spinel LiMn2O4 and its derivatives are the most attractively studied cathode materials for lithium secondary batteries because of their three dimensional Li+ diffusion, low cost, abundance, and nontoxicity [1,2]. Recently, nanocrystalline metal oxides have been studied as promising alternative cathode materials to overcome the severe capacity loss and improve structural stability in the 3 V range, since this type of materials are much more stable compared with microcrystalline one that suffer from the structural transition during repeated electrochemical cycling; Synthesis of nanocrystalline cathode materials have been reported in the literature by use of various solution methods such as sol-gel, coprecipitation, and hydrothermal. In this work, nano-sized spinel LiMn2O4 powders have been synthesized by ultrasonic spray pyrolysis and studied nanocrystalline cathode features in the 3 V range. Experimental Nano-sized LiMn2O4 was prepared by the ultrasonic spray pyrolysis. A stoichiometric amount of lithium nitrate and manganese nitrate were dissolved in distilled water. The dissolved solution was added into a continuously agitated aqueous citric acid solution. The starting solution was atomized using an ultrasonic nebulizer with a resonant frequency of 1.7 MHz. The aerosol stream was introduced into the vertical quartz reactor heated at 500 oC. The flow rate of air used as a carrier gas was 10 L min-1. The prepared powders were further calcined at 500 oC for 5 h in air. Results and discussion Figure 1 shows X-ray diffraction patterns of LiMn2O4 powders prepared different calcinations temperature. All samples can be indexed based on a cubic spinel structure with a space group of Fd3m. No impurity peaks are observed from the XRD pattern. Figure 2 shows SEM and TEM images of the nanocrystalline LiMn2O4 powders. The powders have spherical particle shape with a mean diameter of 2 ㎛. In Figure 2b, the particle seen in a bright field mode clearly shows that the spherical particle is not hollow judging from the solid contrast observed throughout the particle. Figure 3 shows the charge/discharge curves of the Li/LiMn2O4 cell. The LiMn2O4 electrode calcined 500˚C exhibited stable cycling behaviors on the 3 V region. The initial discharge capacity of the LiMn2O4 electrode was 125 mAh/g, and the capacity retention after 50 cycles was 97 %.