| At present LiCoO2 still dominates the lithium-ion battery cathode material market, but due to its own shortcomings and resource constraints, there is an urgent need to develop a new cathode material in the market competition. Ternary complex LiNi1/3Co1/3Mn1/3O2is the best alternative with LiCoO2 cathode material due to its excellent electrochemical properties and similar structure to that of LiCoO2. The structure, electrochemical performance, preparation technology and developing trends of LiNi1/3Co1/3Mn1/3O2 cathode material were described by author. LiNi1/3Co1/3Mn1/3O2 based on Ni-Co-Mn precursor was prepared by solid-state reaction at high temperature, followed with ion-doping and surface coating to improve its electrochemical performance. The contents of this study were described in this dissertation.First of all, theoretical basis for co-precipitation synthesis of Ni-Co-Mn precursor was discussed. According to the principle of simultaneity balance and mass conservation, the mathematic model of thermodynamic equilibrium for the system of Ni(Ⅱ)-Co(Ⅱ)-Mn(Ⅱ)-NH4+-NH3-C2O42--H2O and Ni(Ⅱ)-Co(Ⅱ)-Mn(Ⅱ)-NH4+-NH3-H2O has been built, respectively. The log[Me]T-pH diagram of the related metal ion was drawn, and effect of total oxalate concentration, total ammonia concentration and pH value on the co-precipitation of Ni, Co, Mn oxalate and hydroxide was investigated, respectively. Using oxalic acid and stoichiometric mixed solution of NiSO4, CoSO4 and MnSO4 as starting materials, triple oxalate precursor of nickel, cobalt, and manganese with homo-morphology and uniform particle size distribution was synthesized with liquid-phase co-precipitation method. The obtained oxalate precursor was characterized by chemical element analysis, XRD and DSC-TGA, respectively, and the molecular formula of precursor was determined as Ni1/3Co1/3Mn1/3C2O4·2H2O. In order to prevent oxidation of Mn2+ ion, anti-oxidative agent was added during co-precipitation. As a result, Ni2+, Co2+, and Mn2+ were co-precipitated with stoichiometric ratio. And Secondly, the mixtures of oxalate precursor and LiOH·H2O was used as raw materials to prepare layered LiNi1/3Co1/3Mn1/3O2 cathode material by solid-state reaction at high temperature. Effect of precursor pre-treatment, reaction time, sintering temperature, the molar ratio of lithium to metal and air flow on the properties of obtained cathode material was investigated in detail. The results of electrochemical performance tests show that the first discharge specific capacity of LiNi1/3Co1/3Mn1/3O2 obtained under optimum conditions can reach 168.8mAh/g at 0.1C in voltages of 2.75V-4.3V, and the capacity maintenance rate is 95.4% after 20 charge-discharge circles. In voltages of 2.75V-4.4V,4.5V,4.6V or 4.8V, the first discharge specific capacity of obtained cathode material is 176.1mAh/g,189.1mAh/g,200.8mAh/g or 223.6mAh/g, respectively. At 1C and 2C, the first discharge specific capacity is 124.8mAh/g and 104.6mAh/g, respectively in voltages of 2.75V-4.3V. Using Ni1/3Co1/3Mn1/3(OH)2 precursor and LiOH·H2O as starting material, LiNi1/3Co1/3Mn1/3O2 was prepared with solid-state reaction method, and in voltages of 2.75V-4.3V its first discharge specific capacity is 163.4mAh/g at 0.1C.Once again, LiNi1/3Co1/3-xMn1/3TixO2 and LiNi1/3Co1/3-xMn1/3MgxO2 were synthesized with oxalate co-precipitation method. Effect of Ti4+ Mg2+ ion-doping on the electrochemical properties of LiNi1/3Co1/3Mn1/3O2 cathode material was investigated. The results show that x=0.01-1amount of Ti4+ or Mg2+ ion-doping has little influence on the crystal structure and morphology of obtained sample. The charge-discharge performance at high rate is improved when the doping amount x=0.025, especially in titanium-doped battery. At 1C, the first discharge capacity of LiNi1/3Co1/3-0.025Mn1/3Ti0.025O2 and LiNi1/3Co1/3-0.025Mn1/3Mg0.025O2 is 140.3mAh/g and 129.7mAh/g, respectively, and the capacity maintenance rate is 92.5% and 91.1% after 100 charge-discharge cycles. At 2C, the first discharge capacity is decreased to 126.9mAh/g and 114.8mAh/g, respectively, and the capacity maintenance rate is 90.8% and 88.3% after 100 charge-discharge cycles. The influence of Ti4+ or Mg2+ ion-doping on the diffusion coefficient of Li+ was studied with constant potential step method. It was found that the diffusion coefficient of lithium-ion was increased after Ti4+, Mg2+ ion-doping. When the lithium potential was 3.7V, lithium-ion diffusion coefficient of un-doped sample was 2.63x10-11cm2/s. After Ti4+ and Mg2+ ion-doping, the lithium-ion diffusion coefficient was enhanced to 3.27×10-10 cm2/s and 7.63×10-11 cm2/s, respectively.Finally, the LiNi1/3Co1/3Mn1/3O2 cathode material coated with SnO2 film was firstly prepared with low-heating solid-state reaction method. It is shown this method is a kind of simple and effective method for cathode material coated modification. The results show that a uniform 20nm thick coating layer formed on the surface of cathode material was confirmed by TEM and XPS when SnO2-coated amount of 3wt%. At 0.1C, the first discharge capacity is 188.6mAh/g in voltages of 2.75V-4.6V, and the capacity maintenance rate is 88.17%after 30 charge-discharge cycles. AC impedance method was adopted to study the interfacial reaction characteristics of LiNi1/3Co1/3Mn1/3O2 cathode material. The results show that the battery capacity decays during charge-discharge cycle due to charge-transfer resistance (Rct) increasing, and Rct significantly decreases after SnO2-coating. Therefore, the mechanism model of LiNi1/3Co1/3 Mn1/3O2 cathode material coating modification was proposed.
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