AU2015210434A1

AU2015210434A1 - Electrochemical capacitor

Info

Publication number: AU2015210434A1
Application number: AU2015210434A
Authority: AU
Inventors: Peter William Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-08-07
Filing date: 2015-08-07
Publication date: 2017-03-02

Abstract

Abstract The present invention relates generally to electrochemical capacitors and more particularly provides a novel electrical double-layer capacitor (EDLC). The invention is directed to a method of preparing an electrode matrix for a supercapacitor comprising forming a dispersion of activated carbon in a mixture of a binder and, as solvent, tri-ethyl phosphate (TEP) to form a slurry or paste suitable as an electrode matrix. The invention is also directed to an electrical double-layer capacitor comprising an assembly of two or more carbon electrodes interleaved with ion permeable separators, which assembly is impregnated with an electrolyte, wherein the carbon electrodes include activated carbon dispersed in a binder matrix that also contains a minor proportion of carbon nanotubes as an electrically conductive constituent dispersed in the binder matrix. N uj (.5 Cw W 4 A M tM 'k I l It ik A: /x b ~ F I I 1 I

Description

2015210434 07 Aug 2015 P/00/011 Regulation 3.2 Australia

Patents Act 1990

COMPLETE SPECIFICATION STANDARD PATENT

Invention Title:

Electrochemical capacitor

The following statement is a full description of this invention, including the best method of performing it known to us: 1

ELECTROCHEMICAL CAPACITOR 2015210434 07 Aug 2015

Field of the invention

This invention relates generally to electrochemical capacitors and more 5 particularly provides a novel electrical double-layer capacitor (EDLC). In respective aspects, the invention is directed to the materials of the electrodes and of the electrolyte.

Background to the invention

New types of electrochemical capacitors known as super capacitors have come 10 to perform a useful intermediary role between electrochemical batteries and electrostatic capacitors where neither sustained energy release nor immediate power demand dominate. One type of supercapacitor is the electrical double-layer capacitor (EDLC) characterised by electrodes of carbon or derivatives, commonly activated carbon, interleaved with ion permeable membranes in an assembly that is typically 15 impregnated with a liquid or viscous electrolyte. The electrolyte may be organic or aqueous, sometimes solid state, and depends on the application, the power requirement or peak current demand, the operating voltage and the allowable temperature range.

Existing electrical double-layer capacitors have energy densities much less than >o a conventional battery, but exhibit much greater power densities so that their charge and discharge cycles are much faster than batteries. Of significance, they suffer little degradation over hundreds of thousands of charge cycles and thus will last the entire lifetime of most devices, in contrast to the limited number of charge-discharge cycles tolerated by most rechargeable batteries. EDLC’s have a particular utility in combination 25 with batteries, storing energy from other sources for load balancing purposes and then using any excess energy to optimally charge the batteries.

As noted, a common predominant material for the electrodes of EDLC’s is activated carbon, usually mesoporous IMO activated charcoal. This is dispersed in a binder that ideally should minimally occupy the pores of the activated charcoal so as to 30 optimise the active surface area of the carbon in the electrode. The more pore surface is accessible to the electrolyte, the higher the specific capacitance and conductivity. Electrolytes, which are typically ionic liquids in a compatible solvent, are also selected to 2 optimise these properties. PVDF (polyvinylidene fluoride) is a commonly employed binder, while separator membranes are typically polyethylene or polypropylene of 15-25 micron thickness and typically ceramic filled or coated. The electrode matrix also includes a minor proportion of an electrically conductive constituent, e.g. carbon black. 2015210434 07 Aug 2015 5 It is an object to the present invention to provide an improved supercapacitor of the electrical double-layer type, preferably exhibiting an improvement in specific capacitance and conductivity characteristics.

Summary of the invention

In a first aspect of the invention, it has been realised that the incursion by the 10 binder matrix into the pores of an activated carbon electrode material can be reduced, and performance of the EDLC enhanced, by employing triethyl phosphate (TEP) as a solvent in the formation of a slurry or paste for electrode formation from activated carbon and a binder.

In its first aspect, the invention accordingly provides a method of preparing an 15 electrode matrix for a supercapacitor, comprising: forming a dispersion of activated carbon in a mixture of a binder and, as solvent, tri-ethyl phosphate (TEP) to form a slurry or paste suitable as an electrode matrix.

It has been found that TEP is superior to conventional solvents employed in the >0 processing of activated carbon into electrode matrixes in that the combination of TEP and PVDF results in less binder residue material within the micropores (<2nm) and mesopores (2 to 50nm) of the activated carbon, thus increasing the proportion of pore surface accessible and lowering the specific capacitance. The result is a greater specific capacitance and lower electrical series resistance (ESR) for a similar electrode 25 mass.

The method may further include adding into the slurry or paste a divided electrically conductive constituent so that the constituent is dispersed through the slurry or paste. The electrically conductive constituent preferably comprises carbon nanotubes, for example multiwall carbon nanotubes. 30 A suitable binder is polyvinylidene fluoride (PVDF). The activated carbon is preferably mesoporous activated carbon, i.e. pores predominantly between 2 and 50nm. 3

The preferred proportions by weight of the constituents of an advantageous slurry or paste are as follows: 2015210434 07 Aug 2015 • activated carbon: 70 to 95% w/w of the activated carbon and PVDF together; 5 • polyvinylidene fluoride (PVDF): 30 to 5% w/w of the activated carbon and PVDF together; • multiwall carbon nanotubes: 0.1 to 0.25% w/w with respect to the activated carbon/PVDF mixture; and • tri-ethyl phosphate is sufficient as solvent to dissolve the solute. I0 The multiwall carbon nanotubes are preferably of 2-1 Onm (more preferably 4- 5nm) inner diameter, 5-20nm (more preferably 12-13nm) outer diameter, and preferably greater than 1 micron in length.

In a second aspect of the invention, it has been realised that there are substantive benefits in substituting carbon nanotubes, preferably multiwall carbon 15 nanotubes, for conventional electrically conductive constituents such as carbon black in the electrode matrix.

The invention provides, in its second aspect, an electrical double-layer capacitor comprising an assembly of two or more carbon electrodes interleaved with ion permeable separators, which assembly is impregnated with an electrolyte, wherein the >0 carbon electrodes include activated carbon dispersed in a binder matrix that also contains a minor proportion of carbon nanotubes as an electrically conductive constituent dispersed in the binder matrix.

The carbon nanotubes are preferably multiwall carbon nanotubes. The binder may comprise polyvinylidene fluoride (PVDF). 25 The multiwall carbon nanotubes are preferably of 2-1 Onm (more preferably 4- 5nm) inner diameter, 5-20nm (more preferably 12-13nm) outer diameter, and preferably greater than 1 micron in length.

The proportion of carbon nanotubes in the matrix is preferably not greater than 1%, more preferably not greater than 0.5% and most preferably in the range 0.1-0.25% 30 w/w with respect to the activated carbon/PVDF mixture.

It is found that the carbon nanotubes do not interfere with the activated carbon pore availability as certain conventional conductive carbons are known to do, hence 4 allowing greater pore access by the electrolyte and a consequent higher specific capacitance. Moreover, the preferred multiwall nanotubes form a highly conductive thin film through the electrode matrix, substantially lowering the cell internal resistance and the electrical series resistance (ESR). 2015210434 07 Aug 2015 5 In a third aspect of the invention, it has been found that performance benefits can be obtained by selection of an electrolyte that predominantly comprises one or more ionic liquids, include a pyrrolidinium-based-ionic liquid, in a compatible solvent.

The invention therefore provides, in its third aspect, an electrical double-layer capacitor comprising an assembly of two or more carbon electrodes interleaved with ion 10 permeable separators, which assembly is impregnated with an electrolyte, wherein the electrolyte predominantly comprises one or more ionic liquids, including a pyrrolidinium-based ionic liquid, in a compatible solvent. A compatible solvent may be, for example, propylene carbonate or dimethyl carbonate but is advantageously a mixture of propylene carbonate and dimethyl 5 carbonate, preferably in which the propylene carbonate is greater than 50%, more preferably greater than 65% w/w.

Advantageously, the ionic liquids further include a imidazolium-based ionic liquid, preferably up to 20% v/v, more preferably 5-15% v/v.

Preferably the selected ionic liquids are predominantly tetrafluoroborates. io A suitable pyrrolidinium-based ionic liquid is spiro-(1,T)-bipyrrolidinium tetrafluoroborate (SBP-BF4)

In the second and third aspects of the invention, the ion permeable separator may typically comprise polyethylene or polypropylene, for example of 15-25 micron thickness, according to conventional practice. 25 Preferably, in each aspect of the invention, plural said electrodes and intervening separators are provided as an assembly within a sealed enclosure (or “pouch”) containing electrolyte. Respective positive and negative conductor tabs may protrude from the enclosure. There are typically plural such enclosures within a case, with the positive and negative conductor tabs suitably electrically connected to external 30 terminals.

The invention further extends to any two or more of the aforementioned aspects in combination. 5

As used herein, except where the context requires otherwise the term ‘comprise’ and variations of the term, such as ‘comprising’, ‘comprises’ and ‘comprised’, are not intended to exclude other additives, components, integers or steps. 2015210434 07 Aug 2015

Detailed description of drawings 5 The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a partially cut away schematic view of an electrical double layer capacitor according to an embodiment of the invention; and

Figure 2 is a schematic exploded view depicting the internal components of a io pouch of the capacitor shown in Figure 1.

Description of preferred embodiments

The illustrated electrical double layer capacitor 10 includes an outer case 12 of 15 generally rectangular configuration, and a removable cover 14. Disposed within the case in a main chamber 16, are a series of sealed enclosures or pouches 25, typically of mylar, each containing in turn a stack of multiple alternatively arranged pairs of generally planar positive electrodes 20a and negative electrodes 20b. The stack is depicted in schematic exploded view in Figure 2. Separators 40 are interposed !0 between the positive and negative electrodes of each pair of electrodes.

Separators 40 are conveniently industry standard polyethylene or polypropylene type separators, for example of 15-25 micron thickness, of the type used in standard lithium battery production. Preferably, the separator is ceramic filled or coated.

Because the electrode pairs are alternately arranged, the positive electrodes of 25 adjacent pairs are back to back, as are the negative electrodes of adjacent pairs.

Between the electrode pairs are respective aluminium foil conductors 27. Integral foil tabs 29 project from alternate ends of the upper edges of the foil conductors 27, to define positive 29a and negative 29b foil tabs.

The electrodes and separators of each pouch are disposed in an ionic liquid 30 electrolyte. 6

The foil tabs 29a, 29b at each side of each pouch 25 are gathered together, in a conventional manner not shown, to respective positive and negative conductor tabs 37, 39 that protrude from an upper sealed rim 25a of the respective pouch. The successive pouches 25 are arranged so that the positive and negative conductor tabs 37, 39 are 5 alternately on opposite sides of chamber 16, and the whole array of tabs 37, 39 is electrically connected in series, by insulated conductor links or wires 49, with interior connector lugs 34 of positive 22 and negative 32 battery terminals to the exterior of case 12. A vent 18 is also optionally provided as required. 2015210434 07 Aug 2015

Electrodes 20a, 20b comprise a coating, typically 15-30 microns in thickness, of a I0 slurry or paste mixture, to be described, on each side of a substrate. The coating comprises a matrix of an activated carbon in a suitable binder such as polyvinylidene fluoride (PVDF). It also contains a minor proportion of an electrically conductive constituent in the form of multiwall carbon nanotubes. The preferred activated carbon is a mesoporous activated carbon derived from a natural biological precursor such as 15 hemp, bagasse or rice bran and exhibits a BET determined surface area between 1950 and 2500 m2/g. The preferred porosity is provided by mesopore sizes in the range from 2nm to 50nm with the bulk typically in the 4nm to 20nm range.

The multiwall carbon nanotubes are preferably 4-5nm inner diameter and 12-13nm outer diameter, and greater than 1 micron in length. 20 A method of fabricating the electrodes 20a, 20b will now be described. The first step is the preparation of an electrode matrix according to an embodiment of the first aspect of the invention. The preparation employs as binder solvent triethyl phosphate (TEP). The activated carbon is mixed in a dispersion of TEP with the activated carbon 95% and PVDF 5% w/w, (of the activated carbon and PVDF together), and sonicated 25 for 1 hour, then stirred for 1 hour to form an initial solution. TEP is used as it has been determined to be superior in binder/activated carbon (AC) processing in that other common solvents used in the electrode fabrication industry, especially NMP, cause a reduction in specific capacitance and an increase in the ESR (Electrical Series Resistance) due to the solvent's placing the binder residue 30 material within the micropores and mesopores of the activated carbon in a far greater amount than is desired, thus affecting the amount of available pores accessible and lowering the possible specific capacitance. TEP is less adverse in this respect, allowing 7 for a greater access to and availability of the activated carbon pores and hence resulting in a greater specific capacitance for the same amount of electrode mass. 2015210434 07 Aug 2015

Multiwall carbon nanotubes (MWNT) are first dispersed in TEP (triethyl phosphate), sonificated for one hour and then vigorously stirred for one hour using a 5 homogenizer. The MWNT solution is then slowly added into the AC/PVDF-TEP solution at a rate total of between 0.10 and 0.25% w/w ratio with mixing. The mixture is vigorously stirred for 1.0 hour using a homogenizer. The ingredients are mixed thoroughly to form a paste or slurry mixture of AC/PVDF-TEP/MWNT.

The use of multiwall carbon nanotubes instead of conventional conductive carbons lo is beneficial because it has been found that the MWNT does not interfere with the AC porosity availability, as certain conductive carbons are known to do in binders, hence allowing for greater pores access and subsequent greater capacitance. The MWNT in this method forms a highly conductive thin film resulting in the lowering of the ESR and internal resistance. 15 An electrode current collector substrate of aluminum or other metal foil of 15 to 30 micron thickness is etched to thoroughly clean the metal and form a fresh, highly conductive and high bond metal surface. Alternatively, a substrate of woven or unwoven wet strengthened paper or polymer may be used as substitute for the foil.

The AC/PVDF-TEP/MWNT paste or slurry mixture is then coated onto the etched >0 aluminum foil using an industry standard coating applicator such as screen, knife over roller, Meyer rod, etc., to effect a coating thickness when dried of 15 to 30 microns on each side of the substrate. The coated electrode is dried for between 1 and 4 hours at 100 °C to 120 °C.

After drying the electrode may be calender pressed to increase density as is 25 conventional. The dried electrodes are then cut to the requisite size, shaped to fit into a pouch and stored in inert atmosphere storage for later use and assembly into cells.

The electrolyte predominantly comprises one or more ionic liquids, including in particular a pyrrolidinium-based ionic liquid, in a compatible solvent. An advantageous such ionic liquid is spiro-(1,1’)-bipyrrolidinium tetrafluoroborate (SBP-BF4) and the 30 preferred solvent is either dimethyl carbonate or a mix of propylene carbonate and dimethyl carbonate in a 70:30 ratio. A suitable mixture is 0.2M to 2.0M SPB-BF4, preferably 0.2M to 1.0M, in the selected solvent. 8

The tetrafluoroborate (BF4) anion pairs with the activated carbon as the ideal size and match for proper and facile conductive functioning with the aforementioned micropore and mesopore size range. 2015210434 07 Aug 2015

The use of both propylene carbonate and dimethyl carbonate in a solvent mix is 5 very effective in the wetting of the electrode activated carbon phase. The inclusion of a pyrrolidinium-based ionic liquid, especially SPB-BF4, in the solvent allows for a higher voltage operation of approximately 3.1-4.0+ volts, depending on concentration, and a wide operating temperature range -40°C to +90°C while providing excellent specific capacitance. I0 It is preferred to include in the ionic liquids of the electrolyte an imidazolium-based ionic liquid in order to provide a further enhancement of the capacitor conductivity and specific capacitance. Suitable such ionic liquids are 1-ethyl 4 methylimidazolium tetrafluoroborate (EMim-BF4) and 1-Butyl-3-methylimidazolium tetrafluoroborate (BMim-BF4). 15 Typically, the imidazolium-based ionic liquid will be in the proportion 5-20% v/v.

Suitable formulations are 0.2M to 2.0M SBP-BF4 w/ EMImBF4 10% v/v in dimethyl carbonate or propylene carbonate/dimethyl carbonate 70/30 ratio, or 0.2M to 2.0M SBP-BF4 w/BmiMBF4 10% v/v in dimethyl carbonate or propylene carbonate/dimethyl carbonate 70/30 ratio. !0 It will be appreciated that, after formation of the electrodes, the supercapacitor may be produced in any ‘off the shelf widely employed lithium battery production machinery. In brief, the steps include cutting the electrodes into individual electrodes in stackable form, stacking the cut individual electrodes in a stacker machine, and interleaving separators and conductor foils between electrodes, as earlier described and 25 depicted in Figure 2, until the number is correct for the cell size required for the desired ampere current output. These stacks are then placed in a suitable pouch 25 and appropriate electrolyte placed inside. These sit for a period to fully soak through and are then vacuum sealed, with protruding conductor tabs 37, 39 as in Figure 1. The pouches are ganged to form the specific desired voltage and current output, then the conductor 30 tabs are interconnected and the assembly placed in a suitable case 12. 9

Claims

We claim:

1. A method of preparing an electrode matrix for a supercapacitor comprising: forming a dispersion of activated carbon in a mixture of a binder and, as solvent, tri-ethyl phosphate (TEP) to form a slurry or paste suitable as an electrode matrix.
2. A method according to claim 1 further including adding into the slurry or paste a divided electrically conductive constituent so that the constituent is dispersed through the slurry or paste.
3. A method according to claim 2 wherein the electrically conductive constituent comprises carbon nanotubes.
4. A method according to claim 3 wherein the carbon nanotubes are multiwall carbon nanotubes of 2-1 Onm inner diameter, 5-20nm outer diameter, and greater than 1 micron in length.
5. A method according to any one of claims 1 to 4 wherein the binder is polyvinylidene fluoride (PVDF).
6. A method according to claim 4 and 5 wherein the proportions by weight of the constituents of the slurry or paste are as follows: • activated carbon: 70 to 95% w/w of the activated carbon and PVDF together; • polyvinylidene fluoride (PVDF): 30 to 5% w/w of the activated carbon and PVDF together; • multiwall carbon nanotubes: 0.1 to 0.25% w/w with respect to the activated carbon/PVDF mixture; and • tri-ethyl phosphate is sufficient as solvent to dissolve the solute.
7. An electrical double-layer capacitor comprising an assembly of two or more carbon electrodes interleaved with ion permeable separators, which assembly is impregnated with an electrolyte, wherein the carbon electrodes include activated carbon dispersed in a binder matrix that also contains a minor proportion of carbon nanotubes as an electrically conductive constituent dispersed in the binder matrix.
8. An electrical double-layer capacitor according to claim 7 wherein the carbon nanotubes are multiwall carbon nanotubes.
9. An electrical double-layer capacitor according to claim 8 wherein the multiwall carbon nanotubes are of 2-1 Onm inner diameter, 5-20nm outer diameter, and greater than 1 micron in length.
10. An electrical double-layer capacitor according to claim 8 wherein the multiwall carbon nanotubes are of 4-5nm inner diameter, 12-13nm outer diameter, and greater than 1 micron in length.
11. An electrical double-layer capacitor according to any one of claims 7 to 10 wherein the proportion of carbon nanotubes in the matrix is not greater than 1% w/w with respect to the mixture of activated carbon and binder.
12. An electrical double-layer capacitor according to any one of claims 7 to 10 wherein the proportion of carbon nanotubes in the matrix is in the range 0.1-0.25% w/w with respect to the mixture of activated carbon and binder.
13. An electrical double-layer capacitor according to any one of claims 7 to 12 wherein the binder of the matrix comprises polyvinylidene fluoride (PVDF).
14. An electrical double-layer capacitor according to any one of claims 7 to 13 wherein plural said electrodes and intervening separators are provided in an assembly within a sealed enclosure (or “pouch”) containing the electrolyte, and wherein respective positive and negative conductor tabs protrude from the enclosure.
15. An electrical double-layer capacitor comprising an assembly of two or more carbon electrodes interleaved with ion permeable separators, which assembly is impregnated with an electrolyte, wherein the electrolyte predominantly comprises one or more ionic liquids, including a pyrrolidinium-based ionic liquid, in a compatible solvent.
16. An electrical double-layer capacitor according to claim 15 wherein the compatible solvent is propylene carbonate or dimethyl carbonate.
17. An electrical double-layer capacitor according to claim 15 wherein the solvent is a mixture of propylene carbonate and dimethyl carbonate, in which the proportion of propylene carbonate is greater than 50% w/w.
18. An electrical double-layer capacitor according to claim 15 wherein the solvent is a mixture of propylene carbonate and dimethyl carbonate, in which the proportion of propylene carbonate is greater than 65% w/w.
19. An electrical double-layer according to any one of claims 15 to 18 wherein the ionic liquids further include a imidazolium-based ionic liquid.
20. An electrical double-layer according to any one of claims 15 to 19 wherein the one or more ionic liquids are predominantly tetrafluoroborates.
21. An electrical double-layer according to claim 20 wherein the pyrrolidinium-based ionic liquid is spiro-(1,1’)-bipyrrolidinium tetrafluoroborate (SBP-BF4).
22. An electrical double-layer capacitor according to any one of claims 15 to 21 wherein plural said electrodes and intervening separators are provided in an assembly within a sealed enclosure (or “pouch”) containing the electrolyte, and wherein respective positive and negative conductor tabs protrude from the enclosure.
23. An electrical double-layer according to any one of claims 7 to 22 wherein the ion permeable separator comprises polyethylene or propylene.