Preparationand Modification Of Mono-like Morphology Of LiNi_0.5Co_0.2Mn_0.3O₂ Ternary Cathode Materials For Lithium-ion Batteries

There is a large demand for lithium batteries,because of their high specific capacity,high energy density,low cost,and environmental friendly properties,and the LiCo_xNi_yMn_1-x-yO₂ series cathode materials have been successfully used in lithium batteries,power batteries,and as a new energy source for the automobile industry.Current research on cathodes is directed towards materials with high nickel content,high density,and high voltage capacity.LiNi_0.5Co_0.2Mn_0.3O₂ prepared by the co-precipitation-solid state reaction method has a mono-like morphology that is similar to lithium cobaltate,but with improved electrode performance and density.At the same time,the shortcomings of ternary materials,such as poor cycle performance,low cation mixing and low electron conductivitywere improved by doping with Al³⁺ or co-doping with Al³⁺/Mg²⁺.These materials were characterized by XRD,SEM,EDS and their electrochemical properties were evaluated.The following main experimental results were obtained:?1?The effect of different concentrations of ammonia,which was used as a complexing agent was investigated as an experimental variable in the co-precipitation-solid state reaction method sintering preparation of LiNi_0.5Co_0.2Mn_0.3O₂.The results showed that an appropriate increase in the concentration of ammonia improved the layered structure,reduced the degree of cation mixing and the dislocation effect of transition between the metal ions and cations.The cathode material LiNi_0.5Co_0.2Mn_0.3O₂ prepared with various ammonia/water concentrations was subjected to a first charge and discharge measurement and a 50 cycles performance test in the 2.8-4.4 V range at 0.2 C.The results showed that LiNi0.5Coo.2Mn0.3O₂ prepared with a 0.5 mol/L ammonia concentration had a relatively high initial discharge specific capacity?185.3 mAh/g?,and relatively high capacity retention rate of 63.3%after 50 cycles at 0.2 C.?2?XRD of Al³⁺-doped Li[?Ni0.5Co0.2Mn0.3?1-xAlx]O₂ cathode materials?x=0.01,0.02 and 0.03?showed that the amount of Al³⁺ doping had only a small effect on the magnitudes of a and c,although the c/a ratio,which characterizes the degree of ordering of the hexagonal structure was larger than in the undoped sample,and the crystal structure of the material was more and more complete. SEM also showed that doping with a small amount of A1³⁺ increased the size of the primary single crystal particles of Li[?Ni0.5Co0.2Mn0.3?1-xAlx]O₂ and improved their morphology.Electrochemical performance tests showed that although the initial specific discharge capacity of the Li[?Ni0.5Co0.2Mn0.3?1-xAlx]O₂?x=0.01,0.02 and 0.03?was slightly lower than that of undoped material,but the cycle performance was improvedin the voltage range of 2.8-4.4V,and highest for x=0.02 with a capacity retention rate of 82.5%after 50 cycles at 0.2 C.?3?The effect of Mg²⁺ concentration on the structure and electrochemical properties of Al³⁺/Mg²⁺ co-doped Li[?Ni0.5Co0.2Mn0.3?1-x-yAlxMgy]O₂?x=0.02?was investigated for y=0,0.01 and 0.02.XRD showed that there was no impurity peak in the Al³⁺/Mg²⁺ co-doped materials,and that there was an increase in the cation ordering in the structure.SEM showed that small amounts of Al³⁺ and Mg²⁺ co-doping increased the size of the primary particles,improved their morphology,and concentrated their particle size distribution.Electrochemical performance tests in the range of 2.8?4.4 V at 0.5 C showed first charge-discharge specific capacities of 183.8/160.6,181.9/160,184.6/165.8 and 184.9/167.3mAh/g,respectively for the undoped material and Li[?Ni0.5Co0.2Mn0.3?1-x-yAlAxMgy]O₂?x=0.02;y=0,0.01,0.02?.After 50 cycles,the singly-doped sample with x=0.02 Al³⁺ had the highest capacity retention rate?75.6%?followed by the x=0.02,y=0.02,Al³⁺/Mg²⁺ co-doped material with 69.9%.