Preparation Of Graphene-Based Materials For Energy Storage

Graphene, a flat monolayer sp2-bonded carbon atoms, has great application prospect in energy storage devices with high capacity and power density. However, graphene with different morphology and property are essential when used in different energy storage field. Based on the different requirements of Li-ion battery, supercapacitor and Li-air battery on graphene, we firstly synthesize graphene using oxidation-reduction method and chemical vapor deposition. On the basis of preparation method of graphene, we have developed our research on the following three aspects. A new "two-step solvothermal method" to synthesize graphene/metal oxide composites is designed and developed with graphite oxide as precursor, then electrochemical performances with these graphene-based composites as anodes in Li-ion battery are investigated, to reveal the exact reaction mechanism of graphene-based composites, lithiation/de-lithiation processes are conducted using in situ TEM technology. N-doped graphene is synthesized with in situ liquid method and investigated as active materials of all-solid-state flexible microsupercapacitor.3D graphene are deposited on the skeleton of porous nickel foam with a CVD method, δ-MnO2 is then deposited onto the graphene-coated Ni foam with a facile hydrothermal method, electrochemical performances of Li-O2 cells with δ-MnO2 and Au/δ-MnO2 as cathode catalysts are investigated. In order to further decrease the charge overpotential, LiI is introduced to the electrolyte, forming a synergistic catalytic effect with δ-MnO2 to efficiently catalyze the decomposition of Li2O2, thus decreasing the charge overpotential.(1) Two methods are used to prepare graphene:oxidation-reduction method and chemical vapor deposition. A two-step method is used to synthesize graphene/metal oxides (such as Fe2O3, Mn3O4 and MFe2O4 (M=Co/Mn)) and graphene/sulfide nanostructure composites (such as CoS2 and SnS2), analyzing their formation mechanism and application prospect in Li-ion batteries. N-doped graphene is synthesized with graphite oxide as precursor, and analyzing its potential application in all-solid-state flexible microsupercapacitor. Based on the synthesis of graphene with CVD method, we have studied the formation mechanism of 3D graphene on Ni and analyzed the structure characteristic and application value of graphene in Li-air battery as a collector.(2) Electrochemical performances of Li-ion batteries with graphene/metal oxides as anodes are investigated, the metal oxides are apt to form small particles and sandwich structure with graphene when the precursors go through low temperature heating steps, this sandwich structure can improve the electrochemical performances of the composites, especially the cycling stability. On this basis, we also study the lithiation/de-lithiation mechanism of MnFe2O4/GNs and action mechanism of graphene with in situ TEM technique. The results show that, after the first cycle, the electrode carries on the reversible conversion between Fe/Mn/Li2O and MnO/Fe3O4.Dynamic image analysis shows that graphene can buffer the volume expansion and fix the small particles, revealing the reasons of excellent electrochemical performances of graphene-based composites.(3) Based on the preparation of N-doped graphene, we have explored the application of N-doped graphene in all-solid-state flexible microsupercapacitor. The research indicates that the electrode possesses high specific capacity(3.4 F cm-3), which is much higher than that of undoped graphene. Based on a single device, a doubling of output voltage and capacitance are achieved through simple serial and parallel processes, proving the consistency of devices assembled by this method. Bending performance test shows that the electrochemical performances are not affected due to different bending angle of the device, which proves that the flexible solid-state capacitors have potential applications in flexible devices.(4) δ-MnO2 with high catalyst effect is grown on 3D graphene-Ni substrate with the solvothermal method. Li-O2 cell with δ-MnO2/GNs as cathode catalyst shows excellent cycle stability, which can sustain more than 130 cycles at 400 mA g-1 with a limit of capacity of 500 mA g-1. On the basis of δ-MnO2, we attempt to introduce Au on the surface of δ-MnO2, nanostructural Li2O2 with thin slice structure instead of large micron particles with δ-MnO2 as catalyst due to inducing effect of Au and synergistic catalytic effect between Au and δ-MnO2 is apt to grow on the surface of δ-MnO2. The thin slice structural Li2O2 possesses fast electron and lithium ion transport, showing lower charge and discharge overpotential, excellent cycle stability and rate performance, the Li-O2 cell can sustain 400 cycles at 400 mA g-1 with a limit capacity of 500 mAh g-1, on the basis of this, by adding Lil in the electrolyte, we can further reduce the charge-discharge overpotential of Li-O2 battery.