Synthesis And Properties Of Lithium-rich Manganese-base Cathode Materials For Li-ion Batteries

With the rapid development of electric products, higher properties are required for Li-ion batteries, such as high capacity, long device lifetime, high safety performance and less cost. The aim of this work is to synthesize high capacity, low cost lithium-rich high-manganese cathode materials for Li-ion batteries, mainly on the synthesis method of layer-structure solid solution compounds xLi2MnO3·(1-x)LiMO2and its electrochemical properties.First the crystal structure of lithium-rich layer-structure solid solution compound xLi[Li/3Mn2/3]O2-(-x)LiNi/3Co/3Mn1/3O2was designed. The synthesis conditions were investigated by designed experiments. The chemical composition, particle morphology, elements valence and crystal structure of the samples were characterized by ICP, SEM, TEM, XPS and XRD. The electrochemical performance was tested by button cell charge-discharge tests.5kinds of Ni-Co-Mn hydroxide precursor compounds, which are Ni0.14C00.14Mn0.72(OH)2, Ni1/6Co1/6Mn3.4/5(OH)2, Ni0.19C00.19Mn0.62(OH)2, Ni0.22Co0.22Mn0.58(OH)2and Nio.26C00.26Mno.5o(OH)2, were further synthesized by successive chemical co-precipitation method. Two coordination agent, EDTA and NH3H2O, are discussed about their influence on the structure, chemical valence and electrochemical performance of the precursor compounds. The results show that EDTA is more beneficial to synthesize high density spherical precursor compounds, which is a new synthesis method comparing to traditional coordination agent of NH3H2O. The optimized synthesis conditions are EDTA2.5g/L, temperature50±2℃, pH11.2, stirring rate38Hz and reaction time<50h.A two-step sintering method was adopted to transform precursor compounds into lithium-rich high-manganese cathode material by high temperature lithiation. The processing conditions were investigated by homogeneous design experiments. The results indicates that optimized conditions are lithium content3%, pre-sintering temperature480℃, pre-sintering time5h, sintering temperature900℃, sintering time10h.Under optimized conditions lithium-rich high-manganese cathode material with excellent properties was synthesized and characterized. The crystal structure of the sample is typical layer hexagonal lattice of α-NaFeO2(space group R3m). The diffraction peaks of (006)/(012) and (108)/(110) in XRD pattern show an obvious separation, which indicates the material owns a well-formed2-dimension layer micro structure. Meanwhile a series of small diffraction peaks are detected between20°and25°. This is caused by the alternating of Li and Mn in the transition metal layer of Li2MnO3in ratio of1/2, which shows a ordered arrangement over the lattice.The electrochemical properties of xLi[Li1/3Mn2/3]O2-(1-x)LiMO2samples with different x values were tested. The sample with x=0.6, which is Li[Li0.20Mn0.54Ni0.13Co0.13]02, performed the highest first-cycle discharge capacity of252.3mAh/g.The influence of precursor compounds valence to cathode material electrochemical perfor-mance was also discussed by comparing the electrochemical properties of samples processed with different drying methods.