Patent Publication Number: US-2013236786-A1

Title: Electrode sheet and its preparation method and super capacitor and lithium ion battery

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrode sheet and preparing method thereof, as well as super capacitor and lithium ion battery provided with such electrode sheet. 
     BACKGROUND OF THE INVENTION 
     Super capacitors, also known as electrochemical capacitors having extraordinarily high capacity, are new energy storage devices between ordinary capacitors and secondary batteries. The amount of energy stored in super capacitors is over 10 times as great as that in conventional capacitors. Compared to batteries, super capacitors have greater power density, shorter charge and discharge time, higher charge and discharge efficiency, long cycle life, as well as wide working temperature range (−40˜75° C.), good reliability, and advantages of energy-saving and environmental conservation, thus can be widely used as backup power supply for microcomputer, solar charger, warning device, household appliances, flashbulb of camera, ignition device of aircraft, and particularly the uses in the field of motor-driven cars being investigated have attracted worldwide attention. 
     Basically, high capacity, tiny size, high energy density and high power density are required for super capacitors and lithium ion batteries. According to the energy density formula, which is described by the equation E=½CU 2 , an improvement of energy density can be achieved by increasing specific capacitance which has close relationship with its electrode materials. However, the electrode materials commonly used for producing super capacitors and lithium ion batteries have general problems of limited conductivity that cause difficulties in improving energy density of produced super capacitors and lithium ion batteries further. 
     SUMMARY OF THE INVENTION 
     In view of this, it is necessary to provide an electrode sheet of excellent conductivity. 
     An electrode sheet including a substrate and a coating layer coated on the substrate, wherein the coating layer includes graphene fluoride materials. 
     Preferably, the coating layer further includes conductive agent and binder, in addition, mass fractions of the conductive agent, binder and the graphene fluoride materials are separately represented by x, y, z, x+y+z=1, 2%&lt;x&lt;15%, 3%&lt;y&lt;15%, 70%&lt;z&lt;95%. 
     Preferably, conductive agent is at least one of acetylene black, carbon nano tube, vapor grown carbon fiber, conductive graphite and conductive carbon black; binder is at least one battery binder of polyvinylidene fluoride and polytetrafluoroethylene. 
     Preferably, thickness of the coating layer is in the range of 10 to 200 μm. 
     The above-mentioned electrode sheet made from graphene fluoride having excellent conductivity has higher energy density and higher conductivity. In addition, graphene fluoride and electrolyte have good wettability, and carbon can be formed in the discharge reaction of graphene fluoride, utilization rate of materials is approximately 100%, internal resistance will not increase during the discharge reaction, the discharge voltage remains stable until the end of discharging, so that the whole electrode sheet has great stability. 
     Moreover, it is necessary to provide a preparation method for electrode sheet of excellent conductivity. 
     A preparation method for electrode sheet comprising: preparing or providing graphene fluoride materials, mixing said graphene fluoride materials with conductive agent and binder to prepare coating agent; coating substrate with said coating agent to form a coating layer, drying then forming sheet; rolling said sheet and cutting into electrode sheets. 
     Preferably, preparation for graphene fluoride materials comprises: preparing graphene oxide with graphite raw materials; reducing said graphene oxide in liquid phase to produce graphene; obtaining said graphene fluoride materials by reacting graphene with mixed gases of N 2  and F 2  at 50˜500° C. 
     Preferably, mass fractions of said conductive agent, binder and said graphene fluoride materials are separately represented by x, y, z, x+y+z=1, 2%&lt;x&lt;15%, 3%&lt;y&lt;15%, 70%&lt;z&lt;95%. More preferably, mass ratio of conductive agent, binder and fluoride oxide graphene materials can be 1:1:8, 1:1:18, 2:1:8.5; said conductive agent is at least one of acetylene black, carbon nano tube, vapor grown carbon fiber, conductive graphite and conductive carbon black; said binder is at least one of polyvinylidene fluoride and polytetrafluoroethylene. 
     Preferably, thickness of said coating layer is in the range of 10 to 200 μm. More preferably, thickness of coating layer is in the range of 50 to 100 μm. 
     The above preparing method is easy to operate, has low demand for equipment, and can be applied widely. 
     Besides, it is necessary to provide super capacitor having high energy density, and lithium ion battery. Such super capacitor of higher energy density and higher conductivity is provided with the above electrode sheet. The above electrode sheet can also be used as negative electrode of lithium ion battery, and the lithium ion battery has higher energy density and better stability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of preparation method for electrode sheet of one embodiment of the present invention; 
         FIG. 2  is charge-discharge curve of super capacitor of Example 1 at a constant current. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     Further description of electrode sheet and preparation method thereof, as well as super capacitor and lithium ion battery will be illustrated, which combined with embodiments in the drawings. 
     An electrode sheet of one embodiment comprises a substrate and a coating layer coated on the substrate, wherein the coating layer includes graphene fluoride materials. 
     Substrate is preferably metal substrate having excellent conductivity, such as aluminum substrate, copper substrate, nickel substrate, etc. 
     Thickness of the coating layer is in the range of 10 to 200 μm. Preferably, the coating layer further includes conductive agent and binder, wherein mass fractions of conductive agent, binder and graphene fluoride materials are separately represented by x, y, z, x+y+z=1, 2%&lt;x&lt;15%, 3%&lt;y&lt;15%, 70%&lt;z&lt;95%; More preferably, mass ratio of conductive agent, binder and graphene fluoride materials can be 1:1:8, 1:1:18, 2:1:8.5; conductive agent can be at least one of acetylene black, carbon nano tube, vapor grown carbon fiber, conductive graphite and conductive carbon black; binder can be at least one battery binders of polyvinylidene fluoride and polytetrafluoroethylene. 
     Such electrode sheet has higher energy density and higher conductivity owing to graphene fluoride from which the electrode sheet is made. In addition, graphene fluoride and electrolyte have good wettability, and carbon can be formed in the discharge reaction of graphene fluoride, utilization rate of materials is approximately 100%, internal resistance will not increase during the discharge reaction, the discharge voltage remains stable until the end of discharging, so that the whole electrode sheet has great stability. 
     As shown in  FIG. 1 , a preparation method for the above-mentioned electrode sheet, comprising: 
     Step S 1 : preparing or providing graphene fluoride materials, mixing said graphene fluoride materials with conductive agent and binder to prepare coating agent. 
     Wherein, graphene fluoride can be made by traditional method or the method as follows: 
     Step S 11 , providing graphite raw materials, preparing graphene oxide with said graphite raw materials: adding graphite powders, potassium persulfate and phosphorus pentoxide into concentrated sulfuric acid at 70˜100° C., stirring thoroughly and cooling for over 6 h, filtrating, washing the precipitates to neutrality, drying then adding into concentrated sulfuric acid at 0° C., after that, adding potassium permanganate and maintaining the temperature of the reaction system below 20° C. for 2 to 4 h, then keeping in an oil-bath at 35° C. for 2 to 4 h, subsequently, adding slowly solution of deionized water containing hydrogen peroxide until the reaction system becomes bright yellow in color, filtrating by applying pressure, washing precipitates with hydrochloric acid, vacuum drying to obtain graphene oxide. 
     Step S 12 , reducing said graphene oxide in liquid phase to produce graphene: dissolving graphene oxide as prepared in deionized water to obtain graphene oxide suspensions, sonicating suspensions to disperse, adding hydrazine hydrate and heating to 90˜120° C. and reacting for 24˜48, filtrating, washing the obtained precipitates successively with water and methanol, vacuum drying to produce graphene. 
     Step S 13 , obtaining graphene fluoride by reacting said graphene with mixed gases of N 2  and F 2  at 50˜500° C.: placing graphene as dried into reactor and supplying dry nitrogen for 0.5 to 4 h then supplying mixed gases of fluoride and nitrogen, reacting at 50 to 500° C. for 3 to 120 h, obtaining graphene fluoride, wherein, fluoride accounts for 5˜30% of mixed gases of fluoride and nitrogen by volume ratio. 
     Mass fractions of conductive agent, binder and graphene fluoride materials are represented by x, y and z, respectively, x+y+z=1, 2%&lt;x&lt;15%, 3%&lt;y&lt;15%, 70%&lt;z&lt;95%; conductive agent can be at least one of acetylene black, carbon nano tube, vapor grown carbon fiber, conductive graphite and conductive carbon black; binder can be at least one of polyvinylidene fluoride and polytetrafluoroethylene. 
     Step S 2 : coating substrate with said coating agent to form a coating layer, drying then forming sheet. Preferably, thickness of the coating layer is in the range of 10 to 200 μm. 
     Step S 3 : rolling said sheet and cutting into electrode sheets. 
     The above preparing method is easy to operate, has low demand for equipment, and can be applied widely. 
     The above-mentioned electrode sheet can be applied in the field of manufacturing super capacitors and lithium ion batteries due to its excellent conductivity. 
     For example, super capacitor prepared by the above electrode sheet has higher energy density and higher conductivity. During the preparation of super capacitor, electrode sheet, corresponding separator and electrolyte are assembled in glove box according to technique of manufacturing super capacitor; charge-discharge test is carried out after standing for one day. Herein, the separator used for super capacitor is preferably polypropylene separator, which can also be replaced by other separator commonly used in the prior art. Electrolyte used for super capacitor is conventional electrolyte (e.g. aqueous KOH, organic NMe 4 BF 4 ) or ionic liquid electrolyte (e.g. LiTFSI/EMITFSI). 
     Lithium ion battery provided with the above electrode sheet served as negative electrode has higher energy density and better stability. Herein, electrolyte commonly used for lithium ion battery can be organic electrolyte (e.g. LiF 6  PC EC) or ionic liquid electrolyte (e.g. LiTFSI/BMITFSI). After assembling, battery is allowed to stand for 24 h then tested. 
     The present invention will be described below in detail referring to preferred embodiments. 
     Example 1 
     (1) Preparation of electrode materials graphene fluoride: graphite powders→graphene oxide→graphene→graphene fluoride. Purity of the graphite powders used herein was 99.5%. 
     Preparation of graphene oxide: Graphene oxide was prepared by improved Hummers method. Firstly, 20 g of 50-mesh sieved graphite powders, 10 g of potassium persulfate and 10 g of phosphorus pentoxide were added into concentrated sulfuric acid at 80° C. while stirring thoroughly, then cooled for over 6 h, filtrated. The precipitates were washed to neutrality, and then dried. The precipitates as dried were added into 230 mL of concentrated sulfuric acid at 0° C., and then 60 g of potassium permanganate were added. The temperature of mixture was maintained below 20° C. then kept in an oil-bath at 35° C. for 2 h; subsequently, 920 mL of deionized water were slowly added. After 15 minutes, 2.8 L of deionized water were added (contains 50 mL of hydrogen peroxide having concentration of 30%), then the mixture became bright yellow in color were filtrated by applying pressure, and washed with 5 L of hydrochloric acid having concentration of 10%, filtrated, vacuum dried at 60° C. for 48 to obtain graphene oxide. 
     Preparation of graphene: 100 mg of graphene oxide and 100 mL of deionized water were added into a 250-mL round-bottom flask, the solution was yellow-brown suspensions. Then the suspensions were dispersed by sonicating at 150 W. Finally, hydrazine hydrate (1 mL, concentration of 98%) was added and heated to 100° C. to react for 48 h. After being filtrated, the obtained graphene was washed successively with 300 mL of water and 300 mL of methanol, and then dried in vacuum dry box at 80° C. for 48 h. 
     Preparation of graphene fluoride: the graphene as dried were placed into reactor, dry nitrogen was supplied firstly for 3 h, then mixed gases of fluoride and nitrogen was supplied for reacting with graphene at 250° C. for 6 h, graphene fluoride was obtained. Herein, fluoride accounts for 30% of mixed gases of fluoride and nitrogen, nitrogen was served to dilute. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 1.25 g of graphene fluoride, 0.25 g of acetylene black, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP (N-Methyl pyrrolidone) was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal aluminium foil where the thickness of coating layer was 200 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 165 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 10 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard 2000 (product from the U.S.), electrolyte was 0.5 mol/L solution of 1-ethyl-3-methylimidazolium tetrafluoroborate. 
       FIG. 2  is charge-discharge curve of super capacitor as prepared at a constant current. (Abscissa: time, unit: second (s); ordinate: voltage, unit: volt (V)), where voltage was in the range of 0˜2.5V, electric current was 1 A/g electrode sheet. It can be seen from  FIG. 2  that the charge-discharge curve of such super capacitor exhibited great linear characteristics; charge-discharge curve at a constant current shaped like an isosceles triangle indicated that there was a linear relation between potential and time shown in discharge curve, double-layer characteristics showed up obviously, small voltage drop suggested internal resistance of materials was very low, which is suitable for charging and discharging rapidly, the capacitance was 111.32 F/g. It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 118.48 F/g, discharge specific capacity was 111.32 F/g, and charge/discharge efficiency was 93.96%, the charge/discharge efficiency was superior. 
     Example 2 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 2.5 g of graphene fluoride, 0.25 g of carbon nano tube, 0.25 g of polytetrafluoroethylene were weighed and mixed. Ethanol was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat nickel foam where the thickness of coating layer was 160 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 120 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 8 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L solution of potassium hydroxide. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 239.56 F/g, discharge specific capacity was 230.69 F/g, and charge/discharge efficiency was 96.30%, the charge/discharge efficiency was superior. 
     Example 3 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 3.75 g of graphene fluoride, 0.25 g of conductive graphite, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 100 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 80 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 98.53 F/g, discharge specific capacity was 95.96 F/g, and charge/discharge efficiency was 97.39%, the charge/discharge efficiency was superior. 
     Example 4 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 9.5 g of graphene fluoride, 0.25 g of vapor grown carbon fiber, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 10 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 8 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 106.85 F/g, discharge specific capacity was 102.29 F/g, and charge/discharge efficiency was 95.73%, the charge/discharge efficiency was superior. 
     Example 5 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 6.25 g of graphene fluoride, 0.25 g of conductive carbon black, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 50 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 45 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 87.81 F/g, discharge specific capacity was 83.24 F/g, and charge/discharge efficiency was 94.80%, the charge/discharge efficiency was superior. 
     Example 6 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 7.5 g of graphene fluoride, 0.25 g of conductive graphite, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 40 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 35 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 120.03 F/g, discharge specific capacity was 116.26 F/g, and charge/discharge efficiency was 96.86%, the charge/discharge efficiency was superior. 
     Example 7 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 9 g of graphene fluoride, 0.25 g of conductive graphite, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 120 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 100 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 103.84 F/g, discharge specific capacity was 100.33 F/g, and charge/discharge efficiency was 97.10%, the charge/discharge efficiency was superior. 
     Example 8 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 2.125 g of graphene fluoride, 0.5 g of conductive graphite, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 180 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 160 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 95.66 F/g, discharge specific capacity was 92.92 F/g, and charge/discharge efficiency was 97.13%, the charge/discharge efficiency was superior. 
     Example 9 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 8.5 g of graphene fluoride, 0.25 g of conductive graphite, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 30 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 25 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of super capacitor: electrode sheet, separator and electrolyte were assembled into super capacitor in glove box according to technique of manufacturing super capacitor. Herein, separator was celgard2000 (product from the U.S.), electrolyte was 1 mol/L NMe 4 BF 4 /PC solution. 
     It can be seen from Tab. 1 that charge specific capacity of the super capacitor was 110.18 F/g, discharge specific capacity was 101.32 F/g, and charge/discharge efficiency was 97.96%, the charge/discharge efficiency was superior. 
     Example 10 
     (1) Preparation of electrode materials graphene fluoride: the same as Example 1. 
     (2) Preparation of electrode sheets: sheet→rolling sheet→electrode sheets. 
     Preparation of sheet: 5.0 g of graphene fluoride, 0.25 g of carbon nano tube, 0.25 g of polyvinylidene fluoride were weighed and mixed. NMP was dripped into the mixture to make it become pulpy. After being thoroughly stirred and mixed, the mixture was used to coat metal copper foil where the thickness of coating layer was 80 μm, then vacuum dried at 100° C. for 12 h and taken out to form said sheet. 
     Rolling sheet: The obtained sheet was rolled with rolling mill, the thickness after rolling became 50 μm. 
     Cutting sheet: the rolled sheet was cut into circular electrode sheets in the size of 12 mm with puncher, and weighed accurately. 
     (3) Assembling of lithium ion battery: lithium ion battery was assembled from electrode sheet which was served as negative electrode, corresponding positive electrode of battery, shell and electrolyte in glove box according to technique of manufacturing lithium ion battery. The electrolyte was ionic liquid electrolyte LiTFSI/BMITFSI. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 charge/discharge specific capacity and charge/discharge 
               
               
                 efficiency of super capacitor 
               
            
           
           
               
               
               
               
            
               
                   
                 Charge specific 
                 Discharge specific 
                 Charge/discharge 
               
               
                 Example 
                 capacity (F/g) 
                 capacity (F/g) 
                 efficiency 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 118.48 
                 111.32 
                 93.96% 
               
               
                 Example 2 
                 239.56 
                 230.69 
                 96.30% 
               
               
                 Example 3 
                 98.53 
                 95.96 
                 97.39% 
               
               
                 Example 4 
                 106.85 
                 102.29 
                 95.73% 
               
               
                 Example 5 
                 87.81 
                 83.24 
                 94.80% 
               
               
                 Example 6 
                 120.03 
                 116.26 
                 96.86% 
               
               
                 Example 7 
                 103.84 
                 100.33 
                 97.10% 
               
               
                 Example 8 
                 95.66 
                 92.92 
                 97.13% 
               
               
                 Example 9 
                 110.18 
                 101.32 
                 97.96% 
               
               
                   
               
            
           
         
       
     
     While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. Alternative embodiments of the present invention will become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the spirit and scope of the present invention. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.