Research On Modification Of Nickel-rich Ternary Cathode Materials For High Eenrgy Density Lithium Ion Battery

Nickel-cobalt-manganese ternary cathode material has attracted great attention due to its high discharge capacity and abundant sources of raw materials.However,due to the low content of nickel,the traditional ternary cathode materials have a low energy density because of the low specific capacitance and output voltage.In this dissertation,nickel-rich material LiNi0.6Co0.2Mn0.2O2 was selected as the research subject.The cathode materials were prepared by co-precipitation method and modified by aluminum and fluorine doping,the performance and the improvement of their electrochemical characteristics were investigated in detail,and we can improve the energy density of the materials by this way.In this paper,the optimal process conditions were investigated and the electrochemical properties of LiNi0.6Co0.2Mn0.2O2 materials are improved by Al-F co-doping.Moreover,a variety of analytical methods such as XRD?SEM?TG?EDS?XPS?CV and EIS are used to make a systematic study on the structure,morphology,and electrochemical properties of the cathode materials.The main contents and results are as follows:(1)The preparation method of nickel-rich cathode material LiNi0.6Co0.2Mn0.2O2.The precursor of Ni0.6Co0.2Mn0.2(OH)2 was synthesized by co-precipitation.The SEM results show that the precursor possesses regular crystal structure and the filamentous primary particle accumulated to grow the spherical secondary particles.The effects of sintering temperature,time and molar ratio of Li to Ni+Co+Mn on the structure and electrochemical properties of LiNi0.6Co0.2Mn0.2O2 were studied.The experiment result expresses that the optimum experimental condition are to sinter in oxygen at 500? for 5 h and 720? for 15 h with the molar ratio of Li/(Ni+Co+Mn)as 1.10.The XRD results show that the cathode material possessed a higher crystallinity,a lower cation disorder degree.The initial discharge specific capacity at 0.5 C was 165 mAh·g-1 and 89.6% of the capacity was maintained after 30 cycles in the voltage range of 2.7~4.3 V.At the rate of 1 C,2 C and 5 C discharge specific capacity are 143.1,130.2 and 60.9mAh·g-1.(2)The effects of Al and F co-doping on the materials electrochemical properties.The XRD show that Al,F co-doping can improve the crystallinity and reduce the cation disorder degree of the materials.The characteristic peaks are offset to the left and different degree of separation with different proportion of Al,F co-doping.We find that the initial discharge capacity of the material at 0.5 C is 187.9 mAh·g-1,with 98.8% capacity retention rate by 30 cycles in the voltage range of 2.7~4.3 V,when Al,F are 1% and 2%.At the rates of 1 C,2 C and 5C discharge specific capacity are 165.0,147.9 and 96.0 mAh·g-1.All this results indicate that the Al,F co-doping can improve the electrochemical properties of LiNi0.6Co0.2Mn0.2O2.