Synthesis, Modification And Electrochemical Performance Of Several Manganese-based Cathode Materials

In this paper, KMn8O16, LiMnBO3and LiMn2O4, as cathode materials for lithium-ion batteries, were synthesized and modified. Especially, the KMn8O16were systematically studied by different preparation methods and further polyaniline-coating. The crystal structure, morphology and the electrochemical properties of the expected compounds were investigated. It was found that the modifications could improve effectively electrochemical performances of the original materials, which could be applied in lithium ion battery industry. The main results are listed as follows:(1) KMn8O16nanorods were prepared for the first time by a rheological phase reaction method, using KMnO4and Mn(CH3COO)2·4H2O as reactants. The KMn8O16samples annealed at different temperatures were characterized by X-ray diffraction, transmission electron microscopy, and galvanostatic charge/discharge profile measurement. The KMn8O16nanorods annealed at400℃show the highest reversible discharge capacity (153.8mAh/g even after50cycles) at current density of50mA/g and the best cycling stability. The annealed nanorods exhibit the superior electrochemical performances compared to the unannealed sample, which is confirmed by the AC impedance measurements. These results indicate that the KMn8O16nanorods could be a promising cathode material for lithium ion batteries.(2) KMn8O16nanorods were prepared through a facile hydrothermal method by using KMnO4and MnSO4as reactants. The KMn8O16samples synthesized at different temperatures (100-160℃) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and its electrochemical properties were tested by galvanostatic charge/discharge system. The effect of reaction temperature on the morphology and electrochemical properties was investigated. As electrode materials for the lithium ion battery cycled between1.5and4.2V, the KMn8O16nanorods synthesized at160℃show the highest reversible discharge capacity (160.1mAh/g even after50cycles at current density of50mA/g) and the best cycling stability. These results indicate that the KMn8O16nanorods could be a promising cathode material for lithium ion batteries.(3) KMn8O16(KMO) nanorods were synthesized via a reflux method with KMnO4and MnSO4as reactants and polyaniline-coated KMn8O16(PANI-coated KMn8O16) nanorods were prepared via oxidative polymerization of aniline in acidic medium. The microstructures and morphologies of the KMn8O16nanorods and PANI-coated KMn8O16nanorods were characterized using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The electrochemical measurements demonstrated that PANI-coated KMn8O16presented a much higher reversible discharge capacity (more than180mAh/g) and excellent cyclabilty. These results demonstrate that the polyaniline coating on the surface of a cathode material can inhance structure stabilization of material during the charging and discharging process.(4) LiMnBO3and its carbon-coated composites (LiMnBO3/C) were prepared by a rheological phase reaction method and successive annealing procedure. The citric acid was used as carbon source during synthesis process. The structure and morphology of samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. The effect of carbon coating and the annealing temperature on the structure and electrochemical properties have also investigated. The electrochemical tests showed that the carbon-coated LiMnBO3could greatly improve the discharge capacity, rate capability and cycling stability due to the improved electric conductivity. The carbon-coated sample (LiMnBO3/C) annealed at750℃behaved the best performances (the initial discharge capacity of171.6mAh/g at10mA/g) and the capacity retention was82.9%after50cycles (142.2mAh/g). These results indicate that the carbon-coated LiMnBO3could be a promising cathode material for lithium ion batteries.(5) Spinel phase LiMn1.95MxO4-yFy(M=Co and Y) were prepared by a rheological phase reaction method. The samples were characterized by X-ray diffraction, scanning electron microscopy, AC impedance, and galvanostatic charge/discharge profile measurement. Those results showed that the LiMn1.95MxO4-yFy had better cycling performance than pure LiMn2O4. The LiMn1.95Co0.03Y0.02O3.96F0.04sample showed the best cycling performance, the initial discharge capacitiy is129mAh/g, and the discharge capacity of124mAh/g at a rate of0.5C after50cycles. The loss of its capacity was only2.6%. The possible reasons for the outstanding electrochemical properties of LiMn1.95Co0.03Y0.02O3.96F0.04are also discussed.