Synthesis And Electrochemical Performance Of Layered Oxide Cathode Materials For Li-ion Batteries

The research work of this thesis is mainly based on the layered oxide and its derivation called Li-rich manganese based layered oxide cathode materials for Li-ion batteries. Due to the poor rate capability and cycle stability, new synthesis, surface modification and morphology adjustment are performed to improve the electrochemical performances. The main research contents and results are as follows:Ball-like LiMn0.4Ni0.4Co0.2O2particles composed of flakes are synthesized by a simplified co-precipitation method. The relationship between the sintered temperature and electrochemical performance of the layered oxides is investigated. The layered oxide with a flake thickness of80-100nm synthesized at800℃has the best electrochemical performance. An initial discharge capacity of160mAh g-1is obtained at5C (1400mA g-1) in the voltage range of2.5-4.5V, and the capacity retention is80%after50cycles. The excellent rate capability is attributed to the good crystallinity and the formation of flake primary particles. In addition, a detailed study of diffusion coefficient of Li+(DLi+) is carried out to further understand this cathode material. The value of DLi+calculated is in the range of10-11-10-12cm2s-1CaF2and LiF modified LiMn1/3Ni1/3Co1/3O2cathode materials are prepared via a wet chemical process. The well coated CaF2layer on the LiMn1/3Ni1/3Co1/3O2particle has a thickness of4-8nm determined by High-resolution transmission electron microscopy (TEM) analysis. However, surface F-doping is observed after LiF modification. Cycling stability of the CaF2-coated LiMn1/3Ni1/3Co1/3O2is improved distinctly. Discharge capacity retention of98.1%is obtained at0.1C after50cycles. Furthermore, even at a high charge-discharge rate of5C, the capacity retention maintains above85%. The LiF-modified oxide delivers a high discharge capacity of137mAh g-1at10C at room temperature and exhibits capacity retentions93.5%at1C at60℃after50cycles. Furthermore, it has reversible capacities of85.6mAh g-1at0.1C at-20℃. Electrochemical impedance spectroscopy (EIS) shows that the modified layer stabilizes the surface structure and reduces the charge transfer resistance, resulting in the improved electrochemical performances.Li-rich manganese based layered oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2are synthesized by combustion reaction using alcohol as both solvent and fuel. After comparing the morphology and electrochemical performances of the layered oxides synthesized at different temperatures, the oxide synthesized at800℃with particle size of50-150nm exhibits the best electrochemical performances. High discharge capacities of238.6and165.0mAh g-1are obtained at current densities of200and2000mA g-1in the voltage range of2.0-4.8V, respectively. In addition, Li[Li0.2Mn0.56Ni0.16Co0.08]O2cathode materials are synthesized by sol-gel process with sucrose as complexing agent. The effect of morphology on the electrochemical performances is studied under the using of different metal sources. Porosity with high specific surface area of10.09m2g-1is only observed for the oxide powder synthesized with nitrate. Simultaneously, high discharge capacity of247.8mAh g-1and135.5mAh g-1are obtained at current densities of200mA g-1and2000mA g-1, respectively.MgO-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2are synthesized via melting impregnation method followed by a solid state reaction. The2wt.%MgO coated cathode exhibits the excellent cycling stability with capacity retention of96.4%at a current density of200mA g-1after100cycles at room temperature, and94.3%after50cycles at60℃. Simultaneously, Sm2O3-modified Li[Li0.2Mn0.56Ni0.16Co0.08]O2is also synthesized via a simple wet chemical process. Similar effect is obtained as to improve the cycle stability of the cathode. After Sm2O3surface modification, high discharge capacity of214.6mAh g-1with retention of91.5%is obtained at a current density of200mA g-1between2.0V and4.8V after80cycles.Macroporous Li1.2Mn0.54Ni0.13Co0.13O2cathode materials with high crystallinity and hexagonal ordering are synthesized by aerogel template. The Li-rich layered oxide synthesized at800℃delivers high discharge capacities of244.0mAh g-1and153.9mAh g-1at current densities of200mA g-1and2000mA g-1between2.0V and4.8V, respectively. However, the cycle stability is unsatisfactory. Increasing the synthesis temperature to900℃, the particles grows with a decrease of surface area. However, the macroporous Li1.2Mn0.54Ni0.13Co0.13O2delivers a high discharge capacity of220.2mAh g-1at a current density of200mA g-1,129.8mAh g-1at a current density of2000mAg-1and almost no capacity fading after120cycles.Li[Li0.2Mn0.54Ni0.13Co0.13]O2cathode materials are treated in molten salts. Cube-like and plate-like particles are obtained after treated in LiCl and KCl molten salts at800℃, respectively. High discharge capacities of254.1mAh g-1and173.4mAh g-1are obtained at current densities of200mA g-1and2000mA g-1, respectively, for the oxide treated in KCl molten salt with large specific area of17.05 m2g1. However, the cycle stability is poor. In addition, enhanced cycle stability with capacity retention of98.7%after50cycles at1C is obtained for the oxide treated in LiCl molten salt with sacrifice of a little capacity. Such electrochemical performance change is proved to be independent of Li+diffusion coefficient through GITT.Hollow Li1.2Mn0.5Co0.25Ni0.05O2microcube is synethesized through a simple binary template method. Such special morphology efficiently increases the surface area, decreases the path of Li+diffusion, and then improves the electrochemical performances at high current density. High reversible discharge capacities of208mAh g-1and110mAh g-1are obtained at a current density of200mA g-1and2000mA g-1, respectively. It is remarkable that such binary template method is an effective way to obtain Li-rich manganese based layered cathode material with specific morphology.