Study On High-performance Electrode Materials For Lithium Ion Batteries

The lithium ion batteries have many advantages such as high voltage and energy density, long cycle life and no memory effect. It has become the prefeered mobile power. The cathodes and anodes have great effect on perfoemance of lithium ion batteries.Graphene possesses a two-dimensional (2D) hexagonal structure with carbon atoms connected by sp2 bonding. It has plenty of fascinating properties such as very low resistivity, high mobility of charge carries at room temperature, promising a wide range of potential applications in energy storage. However, graphene suffers from large irreversible capacity, low initial coulombic efficiency when investigated as anode material of lithium-ion batteries. Electrical measurements show that N-doped graphene can effectively modulate the electrical properties of graphene. In this paper, nitrogen-doped graphene nanosheets (N-GNS) were prepared by a simple method, XRD, SEM, XPS and electrochemical techniques are employed to characterize the prepared product .Result of XPS shows that the doping level of nitrogen in the graphene was 2%. The charge-transfer resistance of N-GNS electrode is found to be smaller than graphene after 5 cycles through AC impedance. The graphene after N doping offer more structural defects as active sites to store Li, thus it is expected to improve the capacity and rate capability. The N-GNS delivered a more reversible capacity of around 900 mAh·g^-1 at a current density of 42 mA·g^-1, while it was only 600 mAh·g^-1 for graphene. Highly stable capacities of over 250 mAh·g^-1 could still be obtained at higher current densities of 2.1 A·g^-1 (2.5 C), which is approximately 5 times higher than the value of GNS electrode (around 50 mAh·g^-1).Graphene can not only used in active material for energy storage, but also can composite with other materials because of its high electric conductivity. It can improve the electron transport performance of materials, thus improve the electrons and ions transport performance of composite materials.LiFePO₄ is emerging as a promising cathode material for lithium-ion batteries because of low cost and environmental compatibility. In addition, LiFePO₄ has a large theoretical capacity of 170 mAh·g^-1, a flat discharge potential of 3.4 V versus Li/Li+,the good cycle stability, and the excellent thermal stability. However, it is difficult to attain the full capacity because of the lower electronic conductivity (¹0^-9 S·cm^-1) which leads to poor rate capability. In this paper, LiFePO₄ was prepared via hydrothermal process followed by heat treatment. The effects of synthesis conditions such as the concentration of reactants,pyrolysis temperature,sintering atmosphere on the physic-electrochemical properties of the materials prepared were investigated. To improve the performance of LiFePO₄, mixed (electron and ion) conducting LiFePO₄/Graphene composites have been prepared through a facile hydrothermal route followed by heat treatment. The structure, morphology, and electrochemical performance of materials were also studied by means of XRD,SEM,AC impedance and electrochemical techniques.The optimum conditions obtained from this work is that the concentration of FeSO4·7H2O was 0.75 mol/L and the product was heated at 600℃under H2/Ar atmosphere. LiFePO₄/Graphene composites was prepared through a facile hydrothermal route followed by heat treatment. It was found that introduction of graphene had no effect on the structure of LiFePO₄. An effective three-dimensional conducting network was formed by bridging graphene nanosheets, which can facilitate electron transport effectively. The LiFePO₄/Graphene composites exhibited a discharge capacity of 160.3 mAh·g^-1 at 0.1 C and 81.5 mAh·g^-1 at 10 C, respectively. All the results show that addition of graphene improve the kinetics and rate performance of LiFePO₄.It was reported that VN has high theoretical capacity as a anode material for lithium-ion batteries. However, it suffers from low conductivity at room temperature which limits it for further application. The addition of graphene is one of the useful methods to improve the performance of VN. In this paper, we presented a facile sol-gel process followed by heat treatment under ammonia atmosphere for preparation of a series of VN/Graphene hybrid composites.The results show that the electrochemical properties of all composites are better than pure VN. VN / Graphene (70: 30 in wt.%) reached the highest value of 410 mAh·g^-1 at a current density of 42 mA·g^-1, while it was only 110 mAh·g^-1 for pristine VN.