Improved Electrochemical Performance Of Transition Metal Oxide Nanocomposites As Anode Materials For Lithium Ion Batteries

The main content of the present research is to improve the initial coulombic efficiency and cyclic performance of 3d transition metal oxides, which electrochemically react with lithium through a conversion reaction, by means of synthesis of nanocomposite. Such modification is extended from some 3d transition metal oxides, such as NiO, CuO and MnO into the electrode materials basing on the interaction reaction such as TiO2.(1) Self-supported Ni-coated NiO arrays are prepared by chemical bath deposition of NiO flake arrays followed by magnetron sputtering of nickel nanoparticles. The effect of sputtering time, corresponding to the coverage of nickel nanoparticles, on the lithium storage performance of Ni-coated NiO array electrode is investigated. It is found that the optimum sputtering time is 60 s, and such electrode delivers a reversible capacity of 455,316 and 187 mA h g-1 at a current density of 2,4 and 7.18 A g-1, respectively, which are obviously higher than the bare NiO array electrode. And its initial coulombic efficiency is improved to 80% due to the catalytic activity of the introduced of nickel nanoparticles, which facilitate the decomposition of Li2O and SEI layer during the charge process.(2) Co-doped NiO nanoflake arrays with a cellular-like morphology are fabricated by one pot of low temperature chemical bath deposition in which partly of NiSO4 are replaced by COSO4. The addition of CoSO4 has little effect on the surface morphology, but significantly affects the growth of NiO film. As anode materials for lithium ion batteries, the array film has a reversible capacity of 600 mAh g-1 after 50 discharge/charge cycles at low current density of 100 mA g-1, and it retains 471 mAh g-1 when the current density is increased to 2 A g-1, highlighting the important of the synchronous consideration of adequate electrode configuration and enhancement in the lattice electronic conductivity of NiO.(3) Nickel nanoparticles modified graphene hybrid is obtained by a two-step synthetic method consisting of the liquid phase deposition of Ni(OH)2 nanoparticles, the precursor of nickel nanoparticles, onto the graphene oxide surface followed by the in-situ thermal reduction from NiO to nickel nanoparticles by graphene. As an anode material for lithium ion batteries, the nickel modified graphene electrode delivers a reversible capacity of 675 mAh g-1 after 35 cycles at a current density of 100 mA g-1, corresponding to 85% retention of the initial charge capacity. In addition, the nickel modified graphene electrode exhibits much better rate capability than its pure counterpart operated at various rates between 200 and 800 mA g-1.(4) NiO/graphene hybrid is synthesized by a liquid phase deposition method followed by annealing. The NiO nanoparticles obtained are up to 10 nm in size and are strongly anchored on the surface of graphene sheets. As active materials for Li ion batteries, the cyclic stability and rate capability of NiO are significantly improved by the incorporation of graphene because of establishment of good electronic contact between the active particles and fast electron transport during discharge-charge process. The effect on the voltage polarization after the incorporation of graphene is discussed. And the possible reasons for the large voltage hysteresis for the conversion electrodes are also discussed.(5) CuO/graphene composite is synthesized by a hydrothermal method in which graphene oxide with high density of oxygen functional groups, which are useful to hinder the growth of CuO grains are used as the precursor of graphene. The synergetic effect is beneficial for the electrochemical performances of CuO/graphene composite, such as improved initial coulombic efficiency (68.7%) and reversible capacity of 583.5 mAh g-1 with 75.5% retention of the reversible capacity after 50 cycles.(6) Keeping in mind that the Mn(OH)2 is easily oxidized to MnO(OH)2, MnO/graphene hybrid is obtained firstly by a liquid phase deposition of MnCO3/graphene oxide hybrid then followed by pyrogenation in following nitrogen. The MnO/graphene hybrid electrode shows a charge capacity of 665.5 mAh g-1 after 50 cycles at a current density of 100 mA g-1. When the current density is increased to 400 and 800 mA g-1, its reversible capacity still maintains at 454.2 and 325.6 mAh g-1, which is obviously better than that of bare MnO electrode. Such good electrochemical performance could benefit from good electronic contact of active particles and enhanced electronic conductivity.(7) TiO2/graphene hybrid is obtained by a hydrothmeal method with the presence of N2H4 after the mixing between TiO2 colloid and graphene oxide colloid. The TiO2 nanoparticles obtained are up to 20 nm in size and are strongly anchored on the surface of graphene sheets. Such unique microstructure is useful to prevent the aggregation of TiO2 nanoparticles and keeps good electrical contact between the active materials. The TiO2/graphene hybrid exhibits an enhanced lithium storage performance even with the absence of acetylene black. The hybrid electrode delivers a capacity of 188 mAh g-1 after 40 cycles at 0.2 C, corresponding to 88% capacity retention. When the current density is increased to 2.5,5 and 20 C, it still can deliver a capacity of 158.8,142.8 and 95.2 mAh g-1, respectively.