The Preparation And Modification Study Of Lithium Nickel Cobalt Oxide (LiNi_0.8Co_0.2O₂) For Lithium Ion Battery Cathode Material

The performances of batteries are required to enhance quickly to satisfy the development of science and technology and the miniaturization of electronic equipment. Nowadays among the comprehensive performances of many kinds of batteries, lithium ion batteries show a most promising property in the second high-energy battery. The main aims of the paper are to synthesize the cathode material LiNi0.8Co0.2O2 under different ways and conditions in order to reduce the production cost and increase the initiate capacity and energy density. It is studied by ac-impedance technique, and the influence of material doped with Mg2+, Al3+which can improve the cyclic capability is studied.The cathode material LiNi0.8Co0.2O2 synthesized with the method of co-precipitation was researched on the electrochemical properties in order to optimize the methods of preparation through researching the effects of treating temperature, synthesis time and the molar ratios of Li/(Ni+Co). It is found that the proper treating temperature is 800℃, the proper synthesis time 12 hours and the proper molar ratio of Li/(Ni+Co) 1.05 respectively. The materials obtained by the above way possesses more than 173 mAh/g of the initiate discharge capacity and can delivery 153 mAh/g which is 88.44% initiate capacity after 10 cycles. But the resulting material is not good in the cyclic capability.Based on the early work before, the cathode material LiNi0.8Co0.2O2 doped with Mg2+ is also prepared in this paper by the method of co-precipitation, and it is found that the cathode material LiNi0.755Co0.2Mg0.045O2 has a stable layered crystal structure and shows a better charge-discharge cycling properties than that of LiNi0.8Co0.2O2. At the same time, the electrochemical properties of LiNi0.755Co0.2Mg0.045O2 cathodes are studied by ac-impedance technique; the influence of material structure on electrochemical properties which can improve the cyclic capability doped with Mg2+ is discussed.In this part, the performance of the LiNi0.8Co0.2O2 cathode is modified by multiple substitutions with Mg and Al elements by sol-gel method. Single-phase solid solution of LiNi0.755Co0.2Mg0.05-xAlxO2 has been synthesized and characterized by X-ray diffraction (XRD) and examined by charge-discharge cycling. The LiNi0.75Co0.2Mg0.02Al0.03O2 compounds show a discharge capacity of 133mAh/g after 100cycles that corresponds to 78.3% capacity retention. The beneficial effect of co-doping with Mg and Al elements is testified by slow scanning cyclic voltammograms (SSCV), electrochemical impedance spectrum (EIS) on charged cathodes. These results show that co-doping with Al and Mg stabilizes the 2D-character of the crystal lattice, the phase transitions that occur during extraction or intercalation of Li+ ions are significantly suppressed, and the cyclic capability is improved significantly.