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Study on Layered LiNi1/3Co1/3Mn1/3O2 and Modified Spinel LiMn2O4

Author: YuZuoYuan
Tutor: LiuYeXiang；HuGuoRong
School: Central South University
Course: Non-ferrous metallurgy
Keywords: Lithium ion batteries layered LiNi1/3Co1/3Mn1/3O2 spinel LiMn2O4 doping modification surface coating
CLC: TM912
Type: PhD thesis
Year: 2005
Downloads: 991
Quote: 1
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Abstract


Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials. LiCoO2 has been widely used as a cathode material in commercial lithium ion battery production. But, due to the high cost and toxicity of LiCoO2, many efforts have been made to replace it. On the base of reviewing the development of cathode materials for lithium ion batteries, this dissertation emphasizes on layered LiNi1/3Co1/3Mn1/3O2 cathode materials possessed superior performance. The synthesis, doping, electrochemical behavior and kinetics of lithium insertion-extraction process of layered LiNi1/3Co1/3Mn1/3O2 cathode materials were studied in detail. Then the doping and surface modification of spinel LiMn2O4 were studied, too.The effect of synthesis method and synthesis conditions, such as precipitating agent, precursors synthesis, heat treatment and Li/M（M=Ni+Co+Mn） molar ration, on morphology, lattice parameter, specific surface area, particle distribution and electrochemical performance of layered LiNi1/3Co1/3Mn1/3O2 were studied. On this basis, the optimized flowsheet was obtained, i.e. triple carbonate of nickel-cobalt-manganese precursor was prepared by co-precipitation method with NH4HCO3+Na2CO3 as precipitating agent, and mixed with Li2CO3 （ Li/M ration of 1.05）, then the mixture powders were calcined at 950℃ for 20h. The results showed that the initial discharge capacity of Li/LiNi1/3Co1/3Mn1/3O2 cell was 190.29 mAh/g in 2.5-4.6V and at 0.1C rate. And initial discharge capacity of C/LiNi1/3Co1/3Mn1/3cell was 145.5mAh/g in the voltage window 2.75-4.2V and at 1C rate, its capacity retained 98.41% after 100 cycles. Its performance was close to that of the material reported by BATT program of the U.S. Development of Energy （DOE）. So this material is determined to be a promising cathode material for lithium ion batteries.Furthermore, X-ray photoelectron spectroscopy （XPS） studies showed that the predominant oxidation states of Co, Ni, Mn in the compound were +3, +2, +4, respectively with small content of Ni3+and Mn3+, meantime the reason was demonstrated with the theory of crystalline field. Cyclic voltammetry（CV） test showed that the major oxidation peak at 3.95V and reduction peak at 3.69V corresponded to the redox process of Ni2+ /Ni4+ couple.Layered LiNi1/3Co1/3Mn1/3-xMxO2 （M=A1, Mg, Cr） cathode materials were synthesized by co-precipitation method. The results showed thatLiNii/3Coi/3Mni/3.xMxO2 （ M= Mg2\ Al3+, x=0.05） exhibited the initial capacity of 139.23 mAh/g, 151.64 mAh/g in the voltage range of 2.8-4.3V and at 1.0 C rate, its capacity retained 98.8%, 96.7% after 20 cycles, respectively. According to the element radium and chemical stability of doped metal ions, the occupation in the structure and the role of them during charge-discharge cycling were discussed.The kinetics behaviors related to Li deintercalation/intercalation process of Layered LiNii/3Coi/3Mni/3O2 were studied by means of electrochemical impedance spectroscopy （EIS） measurements. It was found that the exchange current density （io） and diffusion coefficient （Du+） were changed with delithiation voltage, and the max value （1.03mA ?cm 2 ） of exchange current density and the min value （1.23 x 10"12 cm2 ?s’1） of diffusion coefficient （Du+） were obtained near the voltage plats. Moreover, both of them were affected by doping modification.Spinel LiMn2O4 cathode materials were modified by doping multiple components with CrF3, AIF3 or MgF2. The results indicated that cation and anion co-doping can improve the cycling performance at elevated temperature （55 T!） to some extent, but improvement extent is poorer than that at room temperature. LiMn2.xMgx04.2xF2x （x=0.15） doped by MgF2 exhibited the specific capacity of 113.09mAh/g after cycling 35 times at room temperature, and its capacity retained 98.08%. But its capacity retained 92.62% after cycling 20 times at elevated temperature （55°C） .Surface modification of spinel LiMn2O4 was studied. It was found that LiMn2O4 samples coated by metal oxide enhanced the cycling performance at elevated temperature, but the kind and content of coated metal oxides affected on the electrochemical performance of LiMn2O4. The results showed that LiMn2O4 coated with SiO2/Al2C>3 behaved best and exhibited initial specific capacity of 120.6mAh/g and good cycling performance, and its capacity retained 94.62% after cycling 20 times at elevated temperature, but pure spinel LiMn2O4 only retained 78.83% at the same condition.

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