| Abstract:Lithium nickel-based oxide is a kind of promising cathode material to power next generation of lithium-ion batteries. Compared to commerical LiCoO2, it has advantages on specific capacity, price and abundant resources. However, it is faced with problems like structural unstability, thermal unstability and meanwhile it cannot undergo overcharging. Two of the most common and useful ways to resolve these problems are doping with other foreign ions and surface modification repectively. This thesis is focused on muti-elements ions doping nickel-rich system LiNio.9Coo.1-x[Mn1/2Mg1/2]x02cathode materials. The synthesis method and properties of the materials have been invetigated. Then the effect of ions doping on the cathode materials and the basic improvement mechanism have also been discussed.The synthesis method and process of the LiNio.9Coo.1-x[Mn1/2Mg1/2]x02cathode materials were examplified by the synthesis of LiNio.9Coo.o4[Mn1/2Mg1/2]o.o602. Firstly, nickel hydroxide precusor was prepared by controlled crystallization process; then lithium nickel-based oxides active material was obtained by traditional calcination process. In the conditional experiments of the pH values, the crystallization system is fixed:ammonia concentration at0.6mol·L-1, reaction temperature at50℃, stirring rate by500rpm, feed rate at15mL·min-1. The effect of pH on the chemical compositions, morphologies and tape density was studied. It is found that precursors with proper chemical composition, near spherical morphology and a tap density of2.2g·cm-3can be obtained when the pH value is adjusted around11.5±0.02. For the calcination process, the calcination temperature, reaction time and excess amount of lithium were studies. The optimized calcination parameters are as following:reaction temperature at750℃for16h with an excess lithium amount of5%.The effect of variation of x in the LiNi0.9Coo.1-x[Mn1/2Mg1/2]x02compounds on the structure and electrochemical properties were discussed. The LiNio.9Coo.1-x[Mn1/2Mg1/2]x02particles are of almost the same spherical morphologies confirmed by SEM. The chemical compositions of this series of LiNi0.9Coo.1-x[Mn1/2Mg1/2]xO2are quite close to the designed compositions. The XPS results revealed that Ni, Co, Mn and Mg elements are at+3,+3,+4and+2chemical valence states respectively. This means that equal amount of Mn and Mg has the nominative valence state of+3, which justify the correctness of our prior assumption that equal amount of Mn and Mg should be added. In the XRD refinements, Mg ions are detected to be located at both3b and3a sites. Meanwhile, the introduction of Mg ions into the nickel-based layered structure can lower the cation mixing of Li/Ni and increase the c parameter, which has a positive effect on fast lithium intercalation/deintercalation. The substitution of Mn-Mg to Co in the compounds can also suppress the charge-transfer growth during the electrochemical cycling, which confirmed by the EIS analysis. Mn-Mg substitution to Co improves the capacity retentation capability of the nickel-based compounds, although at a cost of little capacity decrease. When x is0.06, the cathode material has the best capacity retentation capability,93.2%of capacity is maintained after50cycles at0.5C-rate with an intial discharge capacity of194.4mAhg-1. F doping into the LiNi0.9Coo.o4[Mn1/2Mg1/2]o.o602compound can further improve the cycling capability efficiently and meanwhile keep the relative high capacity at low rate (0.2C) when the amount is chosen at2%. The initial discharge capacity at0.2C rate is185mAhg-1,97.4%of the initial discharge capacity is maintained after50cycles. However, at higher rate (0.5C), F doping seems to be detrimental to the initial discharge capacity (only165mAhg-1) although with an improved cycling performance (capacity retention rate98.3%for50cycles). |
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