Graphene Composite Preparation And Energy Storage Applications

Graphene consisting of single layer carbon atoms with honeycomb-like hexagon patterns,has great research value and application potential in many fields such as energy storage and composite materials,due to its excellent characters.However,the realistic applications of graphene still need to be studied further.In order to develop applications of graphene,this thesis focuses on graphene composite preparation and modifications on the current collectors or electrode materials of lithium-ion batteries(LIBs)and supercapacitors,as well as the performance-enhanced graphene/polyimide(G/PI)composite and its capacitor application exploration.In the first part of the thesis,graphene-modified Al foils for LIB current collectors were explored.Firstly,we prepared the graphene partially covered Al(PG-Al)foils for LIBs.On one hand,the graphene coverage as a buffer layer increases the electrical contact of the electrode material and current collector,which improves the polarization effect,specific capacity,rate performance and inner resistance character of the LIBs.On the other hand,graphene coverage also protects the Al substrate from corrosion by the electrolyte,which results to longer cycling life of the LIBs.The PG-Al based LIB has the specific capacity of 168.8 inAh/g at 0.2 C,close to LiFePO4 theoretical capacity of 170 mAh/g,and maintains 91.8%of the initial capacity after 500 charge-discharge cycles at 1 C.Secondly,we introduced carbon black(CB)to prepare carbon/graphene modified Al(CB/G-Al)foils for LIBs.The addition of CB combined with graphene forms 3D conductive network between the electrode material and current collector,which further enhances the electrochemical performances of the LIBs.The CB/G-Al based LIB shows the specific capacity of 169.9 mAh/g,and keeps 91.8%of the initial capacity after 500 charge-discharge cycles at 1 C.In the second part of the thesis,graphene applications as modification layer and conductive attitive in the supercapacitor were studied.We simultaneously used chemical vapor deposition(CVD)graphene-modified nickel foam(NiF/G)as the current collector and high quality few-layer graphene as electrode conductive additive to prepare the activated carbon(AC)electrode for supercapacitor(AC@G@NiF/G).With respect to the NiF/G current collector,both the reduction of nickel substrate oxidation through CVD treatment and graphene buffer layer improve the electrical contact between the electrode material and current collector.Compared with CB conductive additive,less amount of graphene can achieve higher electrode conductivity.The synergistic effect of NiF/G and graphene conductive additive makes AC@G@NiF/G electrodes possess superior electrochemical performances to those of the electrodes based on the CB conductive additive or nickel foam current collector.However,graphene sheets with 2D layer structure have block effect on the electrolyte ions.As a result,with the increasing amount of graphene conductive additive,the enhancement of electrode performances will first increase and then decrease.The AC@G@NiF/G electrode with 5%graphene additive exhibits the best performances,such as specific capacity of 123.6 F/g and energy density of 17.2 Wh/kg at 1 A/g.The symmetric supercapacitor based on these electrodes almost shows no deterioration after 10000 charge-discharge cycles at 1 A/g.In the third part of the thesis,the performance-enhanced G/PI composite films and capacitor application exploration were researched.We used the high quality few-layer graphene and polyamide solution as raw materials to prepare G/PI composite films through solution blending process and heat treatment.With smaller additive amount,the addition of functional group-free graphene effectively improves the performances of the G/PI films,such as mechanical,hydrophobic and dielectric properties.The G/PI film with just 0.1%graphene has the tensile strength of 147 MPa,30%increase than that of the pure PI film.The 0.5%G/PI film shows the water surface contact angle of 99.95 °,20.5%increase than that of the PI film.The 0.05%and 0.1%G/PI films possess the dielectric constant of 4.83 and 4.96,41.6%and 45.5%increase than those of the PI film,respectively.Furthermore,the capacitor based on 0.05%G/PI as dielectric medium exhibits stable capacitance between the frequency range from 102?105 Hz,and has the capacitance of 0.52 nF and a low loss tangent of 0.008 at 1000 Hz.