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Preparation and Supercapacitor Properties of Ball Milled Graphite and Ball Milled Graphite/Manganese Oxide Composite

Author: CaoJianYun
Tutor: WangYaMing
School: Harbin Institute of Technology
Course: Materials Science
Keywords: graphite high energy ball milling supercapacitor high voltage manganese oxide
CLC: TB332
Type: Master's thesis
Year: 2011
Downloads: 58
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Abstract

Supercapacitors exhibit broad potential applications in fields such as electric vircles, wind and solar enrgy storage due to their high power density, long cycle life and environment friendly. The main challenges of supercapacitors remain to promote the energy density to narrow the gap with the secondary batteries and reduce the cost of electrode materials to extend the application fields. In this work, a graphite supercapacitor electrode material was prepared by high energy ball milling under air atmosphere with low cost natural flake graphite as starting material. In addition, a graphite/manganese oxide composite was fabricated directly through in-situ reaction between ball-milled graphite and KMnO₄ under microwave irradiation. Finally, the novel high voltage aqueous supercapacitors were constructed using those two types of materials as obtained. The morphology, crystal structure, chemical composition and functional groups were analysised by XRD, SEM, XPS, TEM and FT-IR techniques. The surface area and porosity of ball milled graphite were analyzed by means of N2 adsorption. Cyclic voltammetry （CV） and galvanostatic charge/discharge were used to determine the supercapacitor properties of the materials.The results show that a sharp decrease in intensity and a broadening of the （0 0 2） peak were found in graphite material with the increasing ball-milling time, indicating a transformation from the highly orderd graphite crystal sutructure to amorphous carbon. The morphology evolution process of graphite with increasing of ball-milling time can be divided into two characterstic stages: during the initial period （0³0h）, the flake natural graphite particles with a size of 15μm were crushed into irregular shaped nanoparticles （< 100nm）; in the later period （30⁷5h）, with the particle size further decreasing to 50nm, most of the them agglomerated into the large sub-particles. A special microstructure of graphite nano sheets embeded in amorphous carbon was obtained after 30h ball milling. With the futher milling, the long range order graphite crystall stucture was almost disappeared, and the graphene layers at the particles edge tend to stack and bend to arch bridge or concentric cylinder structure, while the interior of particles was composed of many disordered basic structure units and amorphous carbon.Specific surface area and external surface area firstly increased and then decreased with the increase of millling time, the graphite obtained by 30h milling showed the highest surface area of 562m²/g. The micropore surface area firstly increased and then kept almost constant after 40h milling. Pore size distribution changed from mesopore, macropore to micropore with the increse of milling time. Proportionality factor R_ext （the ratio of extnal surface area to BET surface area） decreased monotonically with milling time. After 75h ball milling in air atmosphere, the oxgen functional groups including alkoxy and carboxyl were detected in graphite, and the oxgen content increased up to 11.5 at. %Specific capacitance of ball milled graphite increased with the increasing of milling time, and the capacitance of 75h milled graphite was 72F/g and 116F/g in 1 M Na₂SO₄ and 6 M KOH at scan rate 5mV/s, respectively. Rate capability of ball milled graphite decreased with the increasing milling time, thus R_ext was regared as an important factor to affect the capability. A novel aqueous high voltage supercapacitor was construsted using 75h milled graphite, whose energy density increased from 2.7Wh/kg to 9.1Wh/kg with the cell voltage rising from 1.0V to 1.8V. The capacitance rentention was 73% after 9000 cycles when the cell voltage was set as 1.6V.Graphite/manganese oxide composite was prepared through in-situ reaction between ball-milled graphite and KMnO₄ under microwave irradiation. Reaction product transformed from well crystallied Mn₃O₄ to amorphous MnO₂ with the decreasing C/KMnO₄ reactant ratio. Composites with different composition showed irregular shaped morphology with paticle size less than 100nm.Graphite/manganese oxide composite showed good supercapacitor behaviour, with specific capacitance increasing with the increase of manganese oxide content. The specific capacitance of （C/KMnO₄）_5.0 was 57F/g, and （C/KMnO₄）_5.0 reached 137F/g. Rate capability decreased with the increasing of manganese oxide content. High voltage aquous supercapacitor was constructed with （C/KMnO₄）_3.1 composite, and the single electrode specific capacitance was 69F/g and energy density was 7.8Wh/kg, with capacitance rentention 76% after 10000 cycles.

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