Research On Surface Modification And Performance Of Cathode Material LiMn₂O₄ For Lithium-ion Battery

Among the main cathode materials, spine LiMn₂O₄ has been regarded as one of the most prospective cathode materials because of its high operating voltage, low cost, good power performance and high security. Nevertheless, the spinel LiMn₂O₄ suffered from irreversible capacity loss during cycling which was especially severe at elevated temperatures, hindering its commercialization process. In order to suppress the capacity fading and improve the electrochemical performance of LiMn₂O₄, surface modification and the related mechanism were investigated.This paper chose the urea hydrolysis homogeneous precipitation method after camparing the coating effect of precipitation method, sol-gel method and urea hydrolysis homogeneous precipitation method. The coating process was first systematically studied,including hydrolysis temperature, reagent addition sequence, heat treatment. 5Kg of the coated material has been prepared. X-ray diffraction（XRD）, scanning electron microscopy（SEM）, transmission electron microscope（TEM）, X-ray photoelectron spectroscopy（XPS）,were applied for analyzing the product’s phase composition and micro-morphology. The electrochemical performance were evaluated by galvanostatic charge-discharge, cyclic voltammetry（CV）, electrochemical impedance spectroscopy（EIS） and inductively coupled plasma spectroscopy（ICP）.LiMn₂O₄ particles were modified with La2O3 in this paper, the dissolution of manganese was effectively been inhibited because the reduced direct contact between LiMn₂O₄ and the electrolyte. It showed that the capacity retention rate of bare LiMn₂O₄ was 88.8% after 200 cycles at 2 C rate at 55 ℃, 3.0⁴.35 V, while that of the coated sample was 94.7%. After surface modification, the polarization of the sample was reduced as well as the charge transfer impedance during the cycling, as a result, the high rate discharge performance was improved.In order to further improve the cycle performance of LiMn₂O₄, spine LiNi0.5Mn1.5O4 was chosen as the coating material. The results showed that the average valence of Mn element on the surface was improved and the Jahn-Teller distortion was restrained obviously, making the cycle stability of LiMn₂O₄ improved significantly: at 55 ℃,3.0⁴.35 V, the bare and coated LiMn₂O₄ in half cells showed a capacity retention rate of88.4% and 95.0% after 250 cycles at 2 C rate respectively; 15 C specific discharge capacity was increased from 82.7 mAh/g to 87.0 mAh/g after surface modified. LiMn₂O₄ and LiNi0.5Mn1.5O4/LiMn₂O₄ were used to assemble type AA batteries respectively, cycle life of the batteries was improved from 243 to 425 cycles, storage performance at 55 ℃was also enhanced after surface modified.