Preparation And Electrochemical Performances Of Freestanding Composite Membrane Based On Graphene

Lithium ion battery(LIB) is a kind of new energy storage device, which has been widely used in the field of portable electronics. With the development of electric vehicles, hybrid vehicles and flexible electronics, LIBs are studying to further improve the performance, including higher energy density and power density, higher output voltage and working temperature, better mechanical performance and flexibility. However, future modifications and technology innovations of the electrode materials are the key to improve the integrated performance of LIBs. For commercialized graphite(372 m Ah g-1) anode materials, it can not meet the above-mentioned requirements with higher performance. Therefore, it is a better option to develop the new anode materials with higher energy density and power density etc. Transition metal oxides and silicon is the promising anode materials to replace graphite anode. However, it is still a significant challenge to commercialize the anode of transition metal oxides due to the huge volume change and rapid capacity fading during the electrochemical process. Graphene as a novel 2D materials, has attracted enormous attention in the areas of energy conversion and storage devices due to its high electronic conductivity and good mechanical strength, which can be used as carrier of the transition metal oxide and silicon anode materials, to buffer volume expansion and improve the electrochemical performance. In this article, 3D transition metal oxides/graphene composite and Si nanoparticles/graphene composite materials was prepared, the relationships between microstructure and properties were investigated, some meaningful results are obtained.1. Firstly, graphene oxides(GO) were prepared by modified Hummers method, which was dispersed in deionized water to form GO aqueous solution by ultrasonic dispersion method, and then the freestanding graphene membrane was obtained by vacuum filtration method followed with reduction process. Finally, Co3O4 nanosheets array could be uniformly anchored on the surface of graphene membrane by simple hydrothermal-thermal decomposition method. The SEM result indicates that Co3O4 nanosheets interconnected with each other, forming a 3D ordered porous structure, Co3O4 nanosheets was constructed with nanoparticles. When the flexible and freestanding Co3O4 nanosheets array/graphene composite membrane was used as anode materials in LIBs, it exhibits a high reversible capacity of 453 m Ah g-1 after 200 cycles, at the current density of 200 m A g-1. The composite electrode also exhibits excellent rate capability, after the current density changed from 100 m A g-1 to 2000 m A g-1 and 50 cycles, it can recover its original capacity(650 m Ah g-1) when the current density returned to 100 m A g-1. Thus, the composite membrane exhibits better electrochemical performance when compared with the pure graphene membrane and the pure Co3O4 powder anode.2. Mn O2 nanowires were firstly prepared by hydrothermal method, and then ultrasonically dispersed in GO aqueous solution to form homogeneous mixed solution. The freestanding Mn O2 nanowires/graphene composite membrane was obtained by vacuum filtration method and reduction process. The SEM result indicates that Mn O2 nanowires evenly sandwiched between the layers of graphene, formed porous structure. The electrochemical results show that the reversible capacity as high as 480 m Ah g-1 after 200 cycles at the current density of 200 m A g-1. Besides, the composite membrane also exhibits excellent rate capability, when the current density was 100, 200, 500, 1000, 2000 m A g-1, the reversible capacity was 900, 600, 400, 300, 200 m A g-1, respectively. Thus, it displays better electrochemical performance than pure Mn O2 nanowire anode, which improves cycling stability and rate capability.3. When the functionalized Si nanoparticles was ultrasonically dispersed in GO aqueous solution, the Si nanoparticles can uniformly anchored on the surface of graphene after the amidation reaction between APTES-functionalized Si nanoparticles and GO nanosheets. The prepared mixed solution was subsequently filtered and reduced to freestanding Si nanoparticles/graphene composite membrane. The SEM result indicates that the composite membrane has porous and layered structure, Si nanoparticles homogeneously sandwiched between layered structure of graphene. When it was used as anode materials in LIBs, the first discharge specific capacity was 2500 m Ah g-1, and the first charge specific capacity was 1350 m Ah g-1, the corresponding coulomb efficiency was 54 %. After 7 cycles, the coulomb efficiency increased to 98%. The reversible capacity was 660 m Ah g-1 after 1300 cycles at the current density of 400 m A g-1. The rate capability result indicates that when the current density was 100, 200, 500, 1000, 2000 m A g-1, the reversible capacity was 1718, 1206, 864, 598, 307 m Ah g-1. The electrochemical performance of Si anode can be significantly improved, such as rapid capacity fading, poor cycle stability, short cycle life and huge volume change.