Study On Preparation And Electrochemical Performance Of LiMnO₂ And LiMn_1/3Ni_1/3Co_1/3O₂ Cathode Materials As Lithium Ion Batteries

With the popularization of electric utensils and electric vehicles, the industry of lithium ion batteries focus on lithium secondary batteries which show cheap, high property, and safety. The positive materials of layered manganese system is qualified. They were studied that synthesis technique, modification of materials, structure characterization, electrochemical properties, and electrode kinetics behaviors of layered LiMnO₂ and layered LiNi_1/3Co_1/3Mn_1/3O₂ materials in this work.Preparation of monoclinic LiMnO₂ by ion exchange method, and of lithium-rich spinel Li₄Mn₅O₁₂ by a low temperature (LT) solid-state reaction method were studied. The results of X-ray diffraction(XRD) showed the sample prepared by LT solid-state method was a phase of lithium-rich spinel Li₄Mn₅O₁₂. The results of electrochemical exams exhibited the capacity of both materials significantly fading with increasing cycling tests due to their unstable phase structures. The one-step solid-state reaction method of medium-high temperature (M/HT) was confirmed to prepare layered LiMnO₂ using LiOH·H₂O and Mn₂O₃ as raw materials in nitrogen flow. The reaction mechanism of preparation of orthorhombic LiMnO₂ was investigated by means of TG-DTA thermal analysis technique. The effects of physical property of layered LiMnO₂ on reaction condition such as calcination temperature, time, ball mill time, and ratio of lithium to manganese, was studied by XRD, SEM, BET. The full width of half maximum (FWHM) of crystal index(110) peak in o-LiMnO₂ phase was investigated. The results revealed the degree of crystal stacking fault or defects decreasing with the elevated reaction temperatures, and the samples prepared at 600℃have the maximum degree of crystal stacking fault or defects. The samples with distribution of uniform particle size, large BET surface area and apparent figure of quasi-sphere were prapared using pre-treating raw materials by a ball mill with acetone as dispersant.The effect of reaction factors on electrochemical property of o-LiMnO₂ materials was investigated. The relation of electrochemical properties to physical property of o-LiMnO₂ was solved by employing principle of crystal stacking fault or defect. The electrochemical property of o-LiMnO₂ is improved with increasing degree of crystal stacking fault or defect because the material activity was activated by some stacking fault or defect inside of crystal structure. The XRD investigation showed that the layered structure of active material transformed irreversibly into spinel structure after cycled many times by charge/discharge test. The result revealed that the o-LiMnO₂ maintains good cyclic stability at 55℃and over-charged resistance in increasing voltage. The electrochemical test delivered the initial charge/discharge specific capacity of 279 mAh·g^-1, 171mAh·g^-1 ,respectively, and ratio of capacity retention of 95% on the 20th cycle for o-LiMnO₂ prepared in the optimized condition of Li:Mn ratio of 1.05, ball milling 8h using acetone as dispersant, and calcining for 30h at 600℃. The Li⁺ ion transmission mechanism in interface region of o-LiMnO₂ electrode was investigated by electrochemical impedance spectroscopy (EIS) technique.The effects of phase structure and property to Sb/Al doped LiMnO₂ were studied. The cyclic stability of o-LiMnO₂ was improved by low concentration Sb doped Doping A1 value 0.1 materials showed the best electrochemical property of discharge specific capacity of 197 mAh-g(-1) and maintained above discharge capacity of 190 mAh·g^-1 after 20 cycles in2.5～4.6V. Cyclic voltammetry(CV) and electrochemical impedance spector exam demonstrated the conductive of electron and electrochemical performance was improve due to Al-doped materials. The chemical diffusion coefficients (D_Li⁺) of A1 doped value 0.1 material is10^-10 cm²Â·s^-1 , increases much as 10 time as that of layered LiMnO₂ by chronopotential test.LiNi_1/3Co_1/3Mn_1/3O₂ cathode material was prepared by high temperature calcining the mixture of lithium resource and precursor which was synthesized via a liquid coprecipitation method. The effect of reaction temperature, concentration of precipitator, sort of precipitator, and pH on property of precursor was studied. The precursors was prepared by taking 2 mol·L^-1 LiOH as precipitator and adjusting to pH value using 10.5 NH₃·H₂O at 50℃. The effect of calcining teperature and time on the property of LiNi_1/3Co_1/3Mn_1/3O² was studied, too. The technique parameters of calcination condition were optimized. The ordered LiNi_1/3Co_1/3Mn_1/3O₂ prepared by pre-calcination for 6h at 500℃, and then secondary calcination for 12h at 900℃, delivered initial discharge capacity of 141.8 mAh·g(-1) in 4.3V～2.5V, and showed good cyclic property.The effects of Li(Ni_1/3CO_1/3Mn_1/3)_1-xM_xO₂(M=A1,Ti,Mg,x=0.02 ) doped various metal element were investigated. This study optimized focus on Ti doped materials. The Li(Ni_1/3CO_1/3Mn_1/3)_0.9Ti_0.1O₂ showed the best electrochemical property among several Li(Ni_1/3CO_1/3Mn_1/3)_1-yTi_yO₂. The Li(Ni_1/3CO_1/3Mn_1/3)_0.9Ti_0.1O₂ material exhibited the first specific capacity of 215.4 mAh·g^-1 in 4.6 V～2.5V, 194.9 mAh·g^-1 in 4.5～2.5V, 184 mAh·g^-1 in 4.4～2.5V, respectively. CV test revealed cyclic property of Li(Ni_1/3CO_1/3Mn_1/3)_0.9Ti_0.1O₂ was reversible. Compared with the research results recently all over the world, the Li(Ni_1/3CO_1/3Mn_1/3)_0.9Ti_0.1O₂ cathode material as Li-ion batteries showed excellent electrochemical performance among the corresponding materials in this paper. The market expects to popularize application of this material in the future.