High voltage EDLC cell and method for the manufacture thereof

An EDLC device in which the double layer capacitors themselves are produced lithographically. The resultant EDLC can be of any capacitance and voltage combination for a particular application. The entire lithographically formed entity can be housed in a single container and rated according to its capacitance and voltage standoff characteristics. Through material control processes, the electrochemical composites developed onto the current collector is unilaterally equalized, providing the desired distributed voltage standoffs, thus promoting the ability to design high voltage independent cells without the normal electronic balancing circuitry associated with multiple series-connected electrochemical cells.

RELATED APPLICATIONS

The present application is related to co-pending U.S. patent application Ser. No. 12/151,811 for ELECTRODE STRUCTURE FOR THE MANUFACTURE OF AN ELECTRIC DOUBLE LAYER CAPACITOR, incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to high voltage electrochemical double layer capacitors (EDLCs) and, more particularly, to a prototype design, manufacturing methods and materials to create electrodes for a high voltage EDLC, while maintaining high capacitance and low resistance thereof.

BACKGROUND OF INVENTION

An electrochemical double layer capacitor (EDLC) is an energy storage device consisting of two electrodes arranged in such a manner that one acts as the cathode and the other the anode, creating a structure that may be modeled as two capacitors in series. The device is characterized by a high power density whose value is dictated by the effective series resistance (ESR) of the device; the lower the ESR, the higher the power density. The capacitance of these devices is very large due to the effective surface of the activated carbon used to fabricate an EDLC and the spacing between the electrode and the molecules of the electrolyte that form the double layers. Indeed, an EDLC may be modeled as a conventional parallel plate capacitor using the effective surface area of the activated carbon for the surface and the molecular distance between the electrode and the electrolyte molecules as the spacing between the parallel plates. Capacitance is calculated by the formula C=kA/d, where k is a constant of proportionality, A is the effective surface area, and d is the spacing between the plates.

EDLC devices are typically manufactured by encasing the electrodes and separator material in a package, usually an aluminum housing, which is wetted with electrolyte and sealed. The resulting device is a single cell EDLC device. These devices are used in a wide variety of applications including but not limited to industrial power supplies, UPS (uninterrupted power supplies), electric vehicles, cell phones, and in many electronic appliances.

The current generation of EDLCs operates with a nominal voltage rating of approximately 2.7 volts, a limitation imposed by virtue of the electrolyte. This requires that for many applications, many EDLC cells are required to meet the needs of a particular application. Except for devices that work at or below 2.7 volts, one or more EDLC devices are used in series to provide a composite device that operates at a higher voltage. For a specific application, a series-parallel configuration is often needed, but can include inherent problems. For example, as the number of cells increases in a series configuration, it is necessary to address cell balancing to prevent premature failure of the composite device. This approach is frequently expensive and cumbersome. The configurations are also large and heavy due to many individual cells used to form the composite structure.

SUMMARY OF INVENTION

In accordance with the present invention, there is provided a single cell composite device designed to use a large number of individual cells in a single package, thus producing a “single cell” EDLC device with a higher voltage standoff than that of conventional EDLCs and that is lighter in weight, smaller in volume, and inherently free of complications that arise from cell balancing. The EDLC has a unit cell structure with alternately interleaved electrodes formed lithographically and a preformed separator disposed between the electrodes and impregnating an electrolyte therein. The unit cell has preformed electrodes with contained individual components specific to each electrode. The components are placed in precise intervals to develop a double layer effect. Extensions of individual blocks are also provided, containing additive capacitance, interfaced by allowing extra lengths of current collector to one of the electrodes and continuing forward with component placement or by sonic welding or by heat (spot) welding the extension pieces together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention includes manufacturing steps for a single high voltage EDLC cell having characteristics associated with EDLC modular equipment associated with many subassembly components. An example capacitance of at least 58 farads and a stable voltage withstanding tolerance of 15 volts is used hereinbelow. It is not intended that this example be taken as a standard; rather, it should be understood this is an arbitrary choice to illustrate the concept. The methodology may be extended to virtually any other capacitance and voltage stack.

The electrode and electrode assembly structures are described in great detail because of their critical importance to the manufactured product. Other factors of importance are preparation, component design, component application sequence, electrode manipulation, block interface coupling, thermal component dissipaters, and packaging.

FIG. 1shows the layer and component definition on the electrodes. Plan views of the manufacturing stages of the electrodes are identified by reference numerals10-15, shown on the upper portion of the Figure, while the respective cross-sectional thickness of single and double layer electrodes are identified by reference numerals16-24, shown on the lower portion of the Figure. A current collector10,16is made from capacitor grade aluminum, which is 99.99% pure aluminum, available in sheets, which for this example is 30 μm thick and 50 mm×50 mm for ease of handling, material application and electrochemical properties.

A hole11is first drilled into the center of the collector work area. It is imperative the aluminum is cleaned prior to drilling, since aluminum, when exposed to an oxygen atmosphere, forms contaminates that impede material from intimately contacting its surface. Chemical etchant baths are typically used for removing these oxides, but quick response to the intended application after its removal must be ensured since the oxide layer reforms in a matter of minutes. Plasma etching is preferred to provide a continuous ride through of aluminum strip material, allowing for the immediate component application. The plasma etcher is the first process step in building the applied components onto the current collector10,16.

Individual electrochemical cells have a finite voltage standoff due to the electrolyte's breakdown limit, which is generally in the range of 2.5-2.7 volts. Compartments are formed representing leak-proof areas for the electrolyte within the EDLC cell structure. These compartments are essential in the manufacturing of a high voltage series stacked EDLC. Required properties such as operating temperature, surface adhesion, flexibility, and chemical solubility can be found in a PVDF material such as Solef 1008, manufactured by Solvay-Solexis. Gasket12can be formed onto the current collector13, gasket12having a thickness19of less than 100 μm for a single side and less than 170 μm for a double sided component20. The application of Solef 1008 can be through a lamination process or screen printing, the latter being the preferred method. Building a detailed wall of material around the perimeter of the current collector13with a controlled thickness of single and double electrodes, respectively19,20, which is specifically immune to decay from electrolyte exposure, prevents any leakage from occurring.

FIG. 2details lithographic component plates, showing front105-115and back120-130sides of two different electrodes100,150. Each PVDF component is sequentially applied onto the current collector, one electrode front105, and one back120until the required components are completed. The second electrode150front155, and back165is fabricated in similar fashion.

Differing applications dictate the relative importance of power vs. energy density. The manufacturing process may be adjusted to accommodate either; as the requirement for energy is increased, the thickness of carbon material is increased and so do the adhesive thicknesses of the PVDF component. The aluminum area underneath the adhesive can be perforated with micro holes to transfer the adhesive through the sides of the electrode. This tends to stabilize the adhesive wall area that extends above the aluminum base.

Activated carbon, the predominant ingredient in an EDLC carbon electrode, provides the massive exposed surface areas required to create the capacitive storage in high voltage EDLC applications. The volume contained within activated carbon is proportional to the specific capacitance, which is typically determined by the carbon manufacture. Calculations of the products of length, width, thickness and specific capacitance define the capacitance for one double layer electrode component area. Depending on the structural size and organization of these component areas, orienting length, width and thickness in parallel or series compartments provides greater capacitance and/or voltage standoff capability, respectively.

Building on the technical engineering merits of the aforementioned Renewable Energy Development Inc. co-pending U.S. patent application Ser. No. 12/151,811, a carbon slurry is utilized in conjunction with a specifically manufactured casting machine. The slurry is applied to the electrode current collector14(FIG. 1). The slurry is composed primarily of activated carbon, smaller quantities of conductive carbon, binders, fillers and other additives. Mixed in proper proportions and in specific orders, half mixing with itself and with a solvent yields the correct batch combination in order to apply repeated uniform thin coating to the current collector. The slurry mixture has enhanced electrical properties that reduce the bonding resistance and increases the capacitance of the slurry promoting a unique cast component solution.

A suitable casting machine, not shown, is adjusted so the precise coating thickness18, indicative of the required capacitance, can be attained at the required component location. As the aluminum current collector is fed through the casting machine, already having the walled PVDF material deposited19,20, depending on the number of components required, carbon components are deposited in precision increments, as single sided21and double sided22(FIG. 1).FIG. 2shows the components positioned on the first electrode front110and back125, along with the second electrode front160and back170.

In order to maximize the working voltage of the EDLC device, a proton conductor or separator must be positioned between individual polarized electrodes in order to electrically isolate but not chemically separate the plates and to allow only specific charged ions to pass through. The separator is also able to store a limited amount of free electrolyte not absorbed into the activated carbon. It is important to consider the required properties needed to determine the proper separator for the high voltage EDLC cell.

The separator undergoes high mechanical stress, both in vertical and horizontal directions, as the EDLC is charged and discharged in operation. The separator adds its own resistive quantities measured as a constant in one direction or the opposite direction, and as some reactive component as a frequency dependent quantity. In all cases, the separator requires careful attention to its construction. Some materials perform their best if the device is constructed prior to assembly. These types of material are readily available from manufactures such as Celgard and NKK. The parameters are predefined and operate as tested under those conditions. These types of material separators can be physically applied as they are being manufactured or they can be laminated prior to assembly. For this invention a screening process at the point of application is preferred, since precession location, material concentration, spacing, sizing and adaptability are important parameters.

An example PVDF material that can be used to manufacture such a separator is Solef 6020 made by Solvay-Solexis. This can be screen printed15onto the prepared, dried carbon component as a separate process. Desirable properties include: a maximum applied thickness of 25 μm, a pore size of 25 to 50 μm with an evenly distributed pore size distribution (PSD) and a homogenous separator matrix composition. Depending on the power or energy requirements of the final high voltage EDLC, the screening application process can compensate for other material or manufactured defects. A single sided electrode23with screened separator and a double-sided electrode24with screened separator component are shown inFIG. 1.

The screening of the separator onto the electrode is the last fundamental step in the electrode manufacturing process. The separator must be as thin as possible but yet able to provide sufficient isolation to keep the electrodes from shorting together. Only one separator is required between each electrode assembly so alternate placement from the screening applicator may be possible.FIG. 2shows a screen method described as one electrode front115and back130. In this arrangement the second electrode need not have a screened-on separator. If the second electrode die requires a screened-on separator the separator screening can be applied onto both electrodes on both front and back sides. With this arrangement the screened separators alternate components so that every other one has a separator screened onto the cast carbon material. The components are thus arranged as follows: cast carbon, cast carbon+separator, cast carbon, cast carbon+separator, etc.

As the electrode components are processed, when one side is completed, the second side begins, making it necessary to pay close attention to the positioning of the first side components. This can be monitored with sufficient precision using optical sensors, not shown, positioned on the machine. As each cast and screened layer is applied, drying time can be modified, longer or shorter as the electrode is wound onto a large round drum that may be oil heated to maintain a satisfactory drying temperature. The large drum may also be considered a storage vehicle allowing more material in a smaller length of area and not condensing it into a compact reel thus possibly damaging the integrity of the PVDF gaskets and cast carbon material.

The order of component construction is important because misplaced operations can create premature failure of the high voltage EDLC. Another reason for the proper order is the importance of component placement in relation to whether this particular block orients itself to capacitive addition or voltage interface stacking. The processes occurs as follows: aluminum strips cut to length, hole drilling, plasma cleaning, electrode first side PVDF gasket application, second side PVDF gasket application, carbon cast first side, carbon cast second side, apply separator to first side, and apply separator to second side.

Detailed information as to how the electrode components are assembled into the high voltage EDLC are referenced initially by the sample90degree lithographic component folding scheme shown inFIG. 3. The first electrode280comprises elements115and130(FIG. 2) and the second electrode290comprises elements160and170(FIG. 2) arranged90degrees offset from each other200, both electrodes280,290having the respective front sides facing up.

The folding pattern demonstrates how individual components are aligned but is not indicative of the capacitance to be utilized in the 58F 15 volt invention example. Beginning with folding pattern200of electrode280, the first bottom component201has bare aluminum ready for block assembly, described hereinbelow. Initially electrode290slides down on top of electrode280forming the first double layer component. The next fold205moves electrode206over electrode290forming the second double layer component. The third fold210moves section211over electrode280. This becomes a repeating operation until the last folding pattern240, folds241over and the operation is complete with the bottom component245of electrode290on top of the additive capacitive block.

As each of these electrode assemblies is built, it is targeted for specific sections, being defined as block and block interface coupling areas. The most straightforward is the block area. In this area, the two electrodes are intertwined, coupling the carbon-to-carbon interface together with only one thin separator sandwiched between them. Alternately as each electrode is advanced, it is folded 90 degrees to the second electrode with each electrode being placed down parallel to the other. Every second time an electrode is placed down, an additive capacitive component is created. As long as each electrode is interfacing in this parallel fashion with continuous lengths of current collector matching, the capacitance is additive.

At the beginning of the electrode, a carbon mass-free aluminum current collector component is provided. This also occurs at the very end of the completed EDLC block. These bare aluminum segments, at least on one side, when the EDLC is first connected to a positive and negative power source, act as the terminal interface.

FIG. 4shows a single electrode capacitive segment folding view300, indicating layer detail. One side of the segment is free from material including components305or both sides, depending on the application and desired termination method. Considering only one electrode side exposed, the other side may have components consisting of carbon mixture only or an additional component such as the separator.

InFIG. 5, as the first block of components400is coming to a close405, and the requirement for a second block exists because the voltage stacking has not been accomplished, a block to block interface415is needed. Here it is necessary to ensure the initial starting electrode is not extended further, but the second electrode is extended. A serial block-to-block interface includes the second of two electrodes extended415and420. As the second electrode is extended to meet the next block, several options can be considered. The current collector segment can be left short only to connect to the next upper block420by ultrasonic welding the two together. Second, the current collector is continued further into the second block415, and components added to satisfy the makeup of the next start of components of the block, as shown inFIG. 2. The resulting concatenated linkage(s) enable the voltage dividing necessary to be able to stack up to the complete EDLC voltage requirement. The maximum length of these interfacing electrodes can be two block lengths long.

For each component area, whether the component contains carbon material or carbon material and separator insulator, a small hole410is associated with it. These holes allow the electrolyte to be filled throughout the enclosed cavity, to permeate the porous activated carbon, and saturate the separator. Since these holes are thru-connected per block only and each block remains at its own potential, the end holes must be plugged after the electrolyte fill. In order to ensure a complete fill and to keep the oxygen level as low as possible, the block is first evacuated and back filled, replacing the vacuum with nitrogen allowing no air contaminates to linger, forming a premature breakdown in the electrolyte when refilled. Once the nitrogen has saturated the carbon and separator material, the process is reversed, allowing the electrolyte to replace the nitrogen. Sealing can be provided with Solef 1008, formed into a slug and injected into the block through the holes. As the plug is injected it is heated first to expand the material entering into the cavity and then cooled to provide a smooth thin outline on the exterior of the cavity.

There can be as many individual components in a block as needed to obtain the required capacitance. The interface between blocks should be keep as short as possible, no more than one component in length. There can be as many of the block interface sections as needed to supply the voltage standoff required for a particular application.

The resultant cascading of blocks attached into one series stack can be seen inFIG. 6. The complete invention example assembly500has58farads of capacitance rated at 15 volts. Identified by the reference numbers505-530, each of the six blocks is shown having equal capacitance and structure. The total voltage540is limited to the number of blocks within the structure and the electrolyte. The electrolyte may be, but is not limited to, TEABF4/acetonitrile or TEABF4/propylene carbonate and TEMABF4/acetonitrile or TEABF4/propylene carbonate. The block voltage limits550can be either 2.7 or 2.5 volts in respect to the above electrolytes.

The electrode component590is 50 mm in width592and 50 mm in height594. The PVDF homopolymer on the perimeter596,598is no wider than 5 mm in either direction. A nominal thickness for one electrode590is 162 μm which includes double sided active carbon mix and the aluminum current collector. Measuring the surface area using the technique disclosed in co-pending patent application Ser. No. 12/151,811, activated carbon mix and the amount of material used, the specific capacitance can be resolved. From the above information the calculated capacitance=working volume of electrodes×specific capacitance, for one electrode assembly in this example is 10.14 farads.

One electrode assembly600shows the individual layers and the actual sequence of placement. From the top, one electrode605,610,615having both sides activated carbon coated, is disposed on top of the aluminum current collector, adjacent to a separator620then the second electrode625,630,635having both sides activated coated, is disposed on top of the aluminum current collector, and constitutes the complete assembly. FromFIG. 1, the nominal thicknesses605-635can be obtained and summarized, having a total electrode assembly thickness result640of 357 μm or 0.357 mm.

One block or voltage segment550shows the 35 components of each electrode or the 70 total electrode components needed for the 350 farad block assembly. The total resulting thickness of the block585is a nominal 12.6 mm. Each end of the voltage segment has an aluminum current collector component extension560,575. This is the same asFIG. 5, showing the interface415,420between blocks and the ends of the whole stack. Each stack end has a different electrode polarity extending out, thereby forming the series connectivity.

The centers of each of the electrodes have specific hole locations so that they are all equally aligned to each other555,580in a direction corresponding to the lateral outline. These are the electrolyte fill areas, one side being the fill in and the other side being the fill out, or vice versa as described previously. Once an individual block has been filled with electrolyte, the holes are plugged with a PVDF homopolymer and sealed, leaving a uniform finish on the surface. Each block section is similarly filled with electrolyte until the total stack is completed. Since each block is mechanically and electrically temporarily fitted, preliminary tests for acceptance, such as capacitance and voltage withstanding, can be measured.

A semifinal view of the complete example high voltage stack700(FIG. 7), including the extended electrical end current collector components, negative710and positive715, voltage segment interfaces705, capacitive block sections720-745are shown in exploded detail. As the high voltage EDLC is operating, continued power cycling generates unwanted heat within the structure. The inventive process provides an efficient thermal heat transfer mechanism from which internally heated mass is removed to an exterior, cooler environment.

Devices800(FIG. 8) are installed at the intersections of the voltage segment interface820. A very thin porous and thermally efficient conductive transfer material, such as a conductive epoxy, is prefabricated to help direct and dissipate unwanted heat. The heat dissipaters are added when an application requires either continual power cycling or a high level of deep drawn voltage surges requiring large masses of charged ionic particles to migrate between the EDLC's double layers.

Expanding upon the need to thermally dissipate unwanted heat within the electrode assembly, existing exposed areas of each electrode can be made to dissipate heat. Since in the formation of each laminated cell structure, the 90 degree offset folding scheme described inFIG. 3, as well as with the two exposed edges shown inFIG. 7, create a unique natural thermal dissipater. These exposed aluminum side ended areas conduct internally generated heat into the containment shell shown inFIG. 9.

The complete stack can now be compressed into its final form, allowing the electrical end components to remain free. The aluminum current collector components that remain on the left and right sides for terminal connection are connected by sonic or electrical spot welding to preformed terminal plates or pole pieces. As can be seen inFIG. 9, the complete assembled stack900includes the welded pole pieces905. These terminal plates are part of the package design915and are constructed to allow a high density polyethylene (HDPE) to be formed around the peripheral surface to ensure an airtight seal of the complete package. Aluminum or stainless steel pole pieces are used to solidify the ends into a final package, allowing the EDLC components to electrically gain access through the packaging. The solid pole pieces help commute the internal heat buildup from the inside to the outside of the package.