Improved double layer energy storage device.

This invention relates to an improved electrical double layer energy storage device. More particularly, in one aspect, this invention relates to a novel process for balancing the leakage current of individual cells when stacked in series in an electrical double layer energy storage device while in another aspect, it relates to a process for increasing the capacity of a double layer energy storage device.

The process of this invention has three advantages over the prior art. First, this invention provides a process for controlling the gas evolution when charging electrical double layer energy storage devices. Second, it provides a process for balancing the voltage and leakage current of individual energy storage cells when charged in series. Finally, it provides a process for increasing the energy storage capacity of double layer energy storage devices.

The evolution of gas when charging an electrical energy storage device is a serious problem. In sealed cells, the gas evolution can cause rupture of the individual cell cases while in vented cells, where the cells are enclosed within a closed system, gas evolution can cause rupture of the container or even produce explosive atmospheres of various gas mixtures within the container.

Variances in the leakage current of energy storage cells is a significant problem when stacking individual cells in series. It is necessary to stack individual cells in series to increase the total voltage output of an electrical energy storage device since every energy storage cell has a fixed voltage rating. Moreover, since the same amount of current will flow through each cell when stacked in series, the overall voltage output of a device composed of a plurality of individual cells should theoretically equal the sum of the separate voltage capabilities of the individual cells. Thus, in order to maximize the efficiency, capacity and voltage output of an electrical energy storage device, the voltage characteristics of each cell should match since a defective cell can limit the voltage output of the entire device.

However, individual cells tend to become unbalanced when charged in series. Some cells have low leakage currents while other cells have high leaka-q currents. If current is continued until all the cells are charged, those with low leakage currents will overcharge experiencing gas evolving reactions while other cells are still below the decomposition voltage of the electrolyte. For example, during the overcharge of an aqueous electrolytic cell, oxygen will be formed at the positive electrode while hydrogen is formed at the negative electrode.

The term leakage current as used herein, means the continuous current that passes through a fully charged cell to maintain steady state voltage conditions. In other words, leakage current can be measured by the amount of current required to maintain a constant voltage in the cell after full charge is obtained.

Previously used techniques for controlling these problems have not proven entirely satisfactory. Typically, electrical storage devices are protected from overcharge by limiting the extent of charge. For example, conventional electrical energy storage devices are charged to a voltage significantly below their total capacity. Thus, even when the voltage across any single cell is significantly increased to compensate for a less efficient cell, the possibility of overcharge is highly diminished. However, in order to accomplish this, the electrical energy storage device is conventionally charged to approximately 50-60% of the amount that is theoretically possible when adding the voltage ratings of the individual cells. For example, an electrical energy storage device having three unit cells which is theoretically capable of being charged to three volts (one volt per cell) is only charged between 1.5 and 2 volts. It is clear that this technique is both economically and electrically inefficient since additional unit cells must be used to obtain a specific total voltage output.

SUMMARY OF THE INVENTION

The objects of this invention are to provide a novel mechanism for a) controlling the gas evolution; b) balancing the leakage current; and c) increasing the capacity of electrical double layer energy storage devices.

We have found that the addition of voltage regulating agents to the electrolyte will balance the voltage between individual cells and maintain the voltage below the decomposition potential of the electrolyte. By maintaining the voltage below the decomposition potential of the electrolyte or solvent, the electrolyte will not decompose evolving gases such as hydrogen or oxygen.

Moreover, the voltage regulating agent provides a mechanism for uniformly charging individual cells stacked in series by balancing the leakage current of the cells and allowing for the voltages to approach equality.

Finally, the voltage regulating agent also increases the capacity of electrical double layer energy cells.

According to this invention, an improved electrical double layer energy storage cell is provided comprising: <ul id="ul0001" list-style="none"><li>a) a positive electrode and a negative electrode wherein at least one electrode is comprised of carbon which is substantially free of adsorbed gases, in admixture with an electrolyte and a voltage regulating agent;</li><li>b) a porous ionically conductive separator between the electrodes; and</li><li>c) a first and a second ion-insulating connector impervious to the electrolyte wherein the first connector is electronically contacted to the positive electrode and the second connector is electronically contacted to the negative electrode.</li></ul>

Also included herein is an electrical double layer device comprising a plurality of cells.

DETAILED DESCRIPTION OF THE INVENTION

The electrical energy storage devices useful in the process of the present invention include those disclosed in U.S. Patent Nos. 3,536,963, 3,634,736 and 3,648,126, hereby incorporated by reference. The energy storage devices disclosed therein comprise a pair of electrodes,at least one of which is a carbon paste electrode, a separator and a ion insulating connector. The carbon paste electrode may be manufactured by mixing finely divided carbon particles with an electrolyte to form a paste and then subsequently forming an electrode from the paste.

The carbon electrodes used according to the present invention are composed of carbon particles and an electrolyte held captive therein. According to the present invention, the electrolyte contains a voltage regulating agent. Preferably, the carbon electrodes are carbon paste electrodes.

The carbon used to prepare the carbon electrode in the present invention is preferably a high surface area carbon such as obtained by processes which produce "active" carbon. "Activation" of the carbon is a process by which greatly improved adsorption properties and increased surface areas are imparted to a naturally occurring carbonaceous material. The active carbon, which is preferably utilized in the preparation of the carbon electrodes, has a surface area in the range of 100 to 2,000 m 2 /gm, and preferably in the range of 500 to 1,500 m²/gm, as measured by the Brunauer-Emmet-Teller method. The pores of the activated carbon material must be large enough to permit electrolyte penetration.

The desired surface area of carbon may be obtained by numerous activation methods known in the art. For example, any raw carbon-containing or carbonaceous material is carbonized or charred usually in the absence of air below 600°C. Typical types of carbon-containing material include but are not limited to heavy oils, waste liquors from paper mills, and nut shells.

The method used most extensively to increase the surface area of carbonized material is controlled oxidation by suitable oxidizing gases at elevated temperatures. Most of the present commercial processes involve steam or carbon dioxide activation between 800°C an 1,000°C, or air oxidation between 300°C and 600°C. Alternatively, gases such as chlorine, sulfur dioxide and phosphorus may also be used. Typically, the time required for activation varies from 30 minutes to 24 hours, depending on the oxidizing conditions and the quality of active carbon utilized. Inhibitors or accelerators can be mixed with the carbon to develop an increased activity. Other activation methods can be employed including but not limited to metallic chloride activation, electrochemical activation and dolomite activation.

It is believed that adsorbed gases from the activated carbon may be released during cycling and cause the cell to fail. Thus depending on the carbonaceous. material and activation technique utilized, it may be preferable to remove adsorbed gases from the activated carbon. One method used to accomplish this is a vacuum heat treatment. The temperature and pressure of the treatment can vary'with temperatures above 6₀0^Q_C being preferred and above 7000C being more preferred. The vacuum pressure treatment is carried out below atmospheric pressure and typically ranges from 10^-3 to 10 atmospheres (from 9,87^-3 to 9.87^-6 Bar). Other techniques for removing adsorbed gases known to those skilled in the art can be utilized.

In one method of preparing the carbon electrode, activated carbon, in the form of powder or fine particles, is mixed with an electrolyte to form a thick slurry. The use of coarse carbon particles should be avoided since the jagged feature of the coarse particles would penetrate the separator and establish an electrode-to-electrode contact between the opposing electrodes causing a short. Water or other diluents can be used to facilitate preparation of the slurry. After the slurry is formed and the carbon and the electrolyte are well dispersed, excess water or diluent can be extracted by any conventional means, such as vacuum or pressure filtration, leaving a viscous paste. The electrode may be formed from the paste by placing a batch of the paste under a ram and applying a predetermined pressure. Upon application of pressure, some liquid would generally exude from a paste.

The electrolytes used in the process of the present invention consist of any highly ionically conductive medium such as an aqueous solution of an acid, salt or a base. Examples of suitable aqueous electrolytes include but are not limited to ammonium chloride, sodium chloride, calcium chloride, potassium chloride, potassium carbonate, sulfuric acid, fluoroboric acid, sodium hydroxide, potassium hydroxide, trifluorosulfonic acid and the like. Aqueous solutions of acids such as sulfuric acid and fluoroboric acid are preferred.

Non-aqueous electrolytes wherein water is not utilized as a solvent can also be used. Typical non-aqueous electrolytes are salts of organic and inorganic acids, ammonium and quaternary ammonium salts and the like, dissolved in appropriate solvents. Appropriate. solvents include but are not limited to nitriles such as acetonitrile and-propionitrile; sulfoxides such as dimethyl-, diethyl-, ethyl methyl- and benzylmethyl sulfoxide; amides such as dimethyl formamide; pyrrolidones such as N-methylpyrrolidone; and carbonates such as propylene carbonate.

The solvents utilized may also function as an electrolyte and as used herein are considered part of the electrolyte. Thus, the term electrolyte as used herein includes both the above described salts and the solvents. It should be noted that useful electrolytes must be capable of solubilizing the voltage regulating agent in both its oxidized and reduced form.

The electrolyte in the electrode structure serves four functions: (1) as a promoter of ion conductivity, (2) as a source of ions, and (3) as a binder for the carbon particles, and (4) as a solubilizer of the voltage regulating agent. Sufficient electrolyte should be used to accommodate these functions although a separate binder can be used to perform the binding functions.

The pressure applied to form the electrode in a preferred embodiment is dependent on many variables such as dimension of the electrode, particle size of the carbon material, particular electrolyte used, etc. Generally, the molding pressure can vary to a maximum of 100,000 psi (6,909 Bar). Preferably, the molding pressure will range from 1,000 psi (69.0 Bar) to 80,000, (5,430 Bar) and most preferably, from 1500 (103.9 Bar) t0 50,000 psi (3,454 Bar).

While it is preferred to employ two carbon electrodes in an energy storage cell, one electrode can be composed of other materials known in the art. For example, it is known that an electrode can be composed of refractory hard boron carbide or a refractory hard metal carbide or metal bromide wherein the metal may be tungsten, titanium, tantalum, niobium or zirconium. Further, one electrode can also be composed of a powdered metal selected from copper, nickel, cadmium, zinc, iron, manganese, lead, magnesium, titanium, silver, cobalt, indium, sellenium and tellurium. These metals, metal carbides, metal bromides or boron carbides must have particle sizes which do not rupture the separator and preferably have particle sizes of less than 10 microns. These metals, metal carbides, metal bromides or boron carbides can also be used in combination with the activated carbon described above. Electrodes prepared utilizing the above materials can be prepared in a manner similar to the carbon electrodes as discussed above and described more fully in U.S. Patents 3,634,736 and 3,648,126.

The ion-insulating connector can be any conducting material which is impervious to ionic conductivity and is typically carbon, copper; lead, aluminum, gold, silver, iron, nickel, tantalum or other conducting metals which are not affected by the electrochemical reaction, conducting polymers or non-conductive polymers filled with conducting material so as to make the polymer electrically conductive. Carbon or graphite loaded butyl rubbers or carbon filled polycarbonates are preferred. The ionic insulating connector is characterized by its electrical insulating property and its chemical inertness to the particular electrolyte employed at the potential impressed upon it. Its primary functions are as an electron current collector and an inter-cell ionic insulator. Typical ion-insulating connectors are more fully described in U.S. patents 3,656,027, 4,023,079 and 4,014,730.

Between the electrodes is a separator generally made of a highly porous material which functions as an electronic insulator between the electrodes while affording free and unobstructed movement to the ions in the electrolyte. The pores of the separator must be small enough to prevent electrode-to-electrode contact between the opposing electrodes, since such a condition would result in a short circuit and consequent rapid depletion of the charges accumulated on the electrode. The separator can also be a nonporous ion-conducting material including but not limited to ion exchange membranes such as synthetic; resins containing sulfonic, carboxylic, phenol or substituted amino groups and the like. Generally, any conventional battery separator is suitable including but not limited to porous polyvinyl chloride, porous polyolefins, glass fiber filter paper, cellulose acetate, mixed esters of cellulose and fiberglass cloth. It is preferred to saturate the separator with electrolyte prior to incorporation into a cell. This can be accomplished by soaking the separator in the electrolyte.

We have found that the addition of a voltage regulating agent to the electrolyte of a cell will increase the cell current leakage. This becomes particularly important when the leakage current of one cell is different than the current leakage of another cell when charged in series. Since previously known cells with low leakage currents often reach higher voltages than cells with high leakage currents, the cells with the low leakage currents will overcharge and evolve gas. By increasing the leakage current of the cells, accomplished in the present invention by the addition of a voltage regulating agent, all of the cells will charge more uniformly.

According to this invention, a particular voltage regulating agent can be chosen which will activate at a determined voltage, the oxidation potential of the regulating agent, which is below the decomposition potential of the electrolyte and before the state in which the cells are fully charged. Once activated, the voltage regulating agent both increases the cell's leakage current and decreases the cell's ability to exceed the decomposition potential of the electrolyte. In other words, the voltage regulating agent acts as a moderator preventing the overcharging of a unit cell and the breakdown of the electrolyte which causes the gas evolution. Further, the voltage regulating agent has no detrimental effect at voltages below the specific agent's oxidation potential. By employing the voltage regulating agent in the electrolyte of each of the unit cells stacked in series, the opportunity for a single unit cell to overcharge, evolve gas and destroy the energy storage device is significantly diminished.

Suprisingly, the voltage regulating agent also increases the overall capacitance of the cells. It is believed that the energy storage of the voltage regulating agent's red/ox potential is recoverable when current is reversed or drawn from the cell. Thus, electrical double layer energy cells can be prepared with increased capacitances by adding a voltage regulating agent to the electrolyte prior to making the carbon electrode.

The voltage regulating agents are generally ionic, capable of becoming electrically charged ions or radicals. They are characterized by their ability to undergo reduction/oxidation reactions and remain soluble in the electrolyte in both states. It is particularly important that the voltage regulating agent remain soluble in the electrolyte in both the oxidized and the reduced state and will neither gas nor plate on an electrode. Examples of voltage regulating agents include but are not limited to halide ions such as bromide, chloride and iodide with bromide being preferred. Further examples include multivalent ions of the metals tin, iron, chromium and the like. Preferably, the voltage regulating agents will have an oxidation potential of between 0.6 volts and 1.2 volts.

The voltage regulating agents can be added to the electrolyte in various forms known to those skilled in the art. For example, the halides can be added in the form of hydrogen, sodium, potassium or ammonium compounds. Typically, the metal ions can be added in the form of sulfates, halides and the like. The voltage regulating agent can be added to the electrolyte in varying concentrations depending upon the specific electrolyte employed. Typically, the molar concentration of ions will range from 0.01 to 10 moles, preferably 0.1 to 3.0 and most preferably 0.5 to 1.5 moles of ion per liter of electrolyte.

The following Examples illustrate the invention:-

SPECIFIC EMBODIMENTS

Example 1

Activated carbon type APL from Calgon Corporation was obtained having the following specifications: <ul id="ul0002" list-style="none"><li>Total Surface Area: 1,000-1,1₀₀m²/g</li><li>Through 325 Mesh (45µm) - 65-75%</li><li>Pore Volume Within Particle 0.9-1.22 cc/g</li><li>Apparent Density 0.41 g/cc</li><li>Real Density 2.10-2.35 g/cc</li><li>Moisture Content 2% Maximum</li><li>Iodine Value 900</li><li>Total Ash (maximum) 8.5%.</li></ul>

Approximately 300 grams of APL carbon was placed into a stainless steel container which was lined with graphoil and had a graphoil gasket. The stainless steel container was placed in a muffle furnace and connected to a vacuum system. The carbon was heated for 2 hours at 800°C at a vacuum of 26-28 mm Hg°. The stainless steel container was then put into a dry box with an argon atmosphere for 30 minutes to remove all oxygen before opening the container.

The carbon was then removed from the container and an electrolyte mixture of H₂SO₄ 25% by weight and 1 molar HBr was slowly added to the carbon until the carbon was completely covered by the electrolyte. The carbon was allowed to soak in the electrolyte for approximately 10 days. The carbon was then removed to absorbent paper to remove some of the excess electrolyte so that the carbon/electrolyte mixture became a paste. The carbon was then placed in a compression fixture with the fixture placed into an hydraulic press and compressed to 10,000 psi (690 Bar) to remove excess electrolyte. A carbon aggregate was obtained which was removed from the compression fixture and sifted through a U. S. sieve size 80 mesh (177 microns) screens. The carbon was kept in a tightly closed dry box at approximately 75% relative humidity.

2.6 grams of the carbon was carefully placed into a compression die. A plunger was placed into the die and twisted so as to evenly distribute the carbon throughout the die. The die assembly was then placed into a hydraulic press and compressed to 6,000 psi (394 Bar). The die was disassembled revealing a compressed carbon disk approximately 0.125 inch (3.18 mm) thick and 1.150 (29.21 mm) inches in diameter.

A gasket was stamped out of a sheet of non-conductive butyl rubber and cemented, using Pliobond industrial adhesive from the Goodyear Tire and Rubber Company, to a flat circular sheet of a conductive butyl rubber from the Industrial Rubber Company (Compound No. 13, Mold No. 1319/1051). The gasket was stamped out of a sheet of non-conductive butyl rubber also purchased from the Industrial Electronic Rubber Company (Compound No. L723-2, Mold 171) and had an internal diameter of 1.25 inches (31.75 mm) and an external diameter of 1.575 inches (40.01 mm). The circular sheet served the function of an electronic conductor and ionic insulator. It was stamped out of a butyl sheet having a thickness of 0.008 inches (0.20 mm) and a diameter of 1.153 inches (29.29 mm).

The carbon disk was placed into the cavity created by the gasket and the circular sheet of butyl rubber. A pair of such electrode assemblies were prepared. A porous polypropylene separator obtained in sheets from the Calgon Corporation was stamped in a circular sheet having a thickness of 0.001 inches (0.025 mm) and measuring 1.275 inches (32.39 mm) in diameter. This separator serves the function of electronically insulating the opposed electrodes. The polypropylene circular sheet was interposed between a pair of the electrode assemblies and cemented with Pliobond industrial adhesive in order to form a cell. The cell was then placed under a ram and compressed slowly until 6,000 psi (394 Bar) was achieved.

Five other cells were prepared using the procedure and apparatus of above and by placing electrical connections to the upper and lower platens, electronic variables could be determined. Each cell was placed on constant potential and charged to a desired voltage (ranging from 0.7-1.IV) and then discharged at 100 m.a. constant current to O.O volts. The capacitance in coulombs/gram of active material was determined for each cell at various voltage levels as indicated in the Table I. The values shown in Table I are the average capacitance for six cells when each cell was charged to the specific voltage.

Example A

Six cells were prepared in accordance with the procedure and apparatus employed in Example 1. However, the electrolyte employed contained only 25% by weight H₂SO₄ and are provided for comparison. The results of these examples can also be found in Table I and are expressed as average values for .six cells. <tables id="tabl0001" num="0001"><img file="EP0093539A2_D0001.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0093539A2_D0002.tif" /></tables>

Example 2

Four additional cells were prepared in accordance with the procedure and apparatus employed in Example 1. The four cells were charged at constant potential to the cell voltages indicated in Table II. Leakage currents were then measured for each cell after 18 hours of charge at the constant potential shown. The results are found in Table II and again are in averages for four cells when each was charged to the potential shown.

Example B

Four additional cells were prepared using the procedure and apparatus of Example 1. However, these cells were prepared in accordance with Example A in that no HBr was employed and are also provided for comparison. These cells were then charged at a constant potential to the cell voltage indicated in Table II. Leakage currents were measured for each cell after 18 hours of charge at the constant potential shown. The results can be seen in Table II. Again, the results in Table II are the average for four cells when being charged to the potential shown. <tables id="tabl0003" num="0003"><img file="EP0093539A2_D0003.tif" /></tables>

As can be seen from Table I, the average capacitance of the six cells of Example 1, with a voltage regulating agent incorporated into the ' electrolyte, are pronouncedly increased as compared to the average capacitance of the six cells of Example A when charged from 0.7 to 1.1 volts. Thus, the process of the present invention increases the capacitance of an electrical double-layer energy storage cell.

The four cells of Example 2 exhibit increased leakage currents when the average values are compared to the average values of the four cells of Example B which do not have a voltage regulating agent incorporated into the electrolyte. It is important to note, as seen in Table II, that the increase in leakage current exhibited by the cells of Example 2 does not occur until approximately 1.0 volts is obtained. It is believed that as the potential difference across the cell during charge approaches 1 volt, bromine ion is being oxidized to bromine gas at the positive electrode. The bromine gas is then solubilized by the electrolyte and diffuses to the negative electrode where it is reduced to bromine ion. Once this bromine to bromide cycle has been established, the potential different across the cells will not rise much above I volt providing the rate of bromine evolution does not exceed the rate of diffusion to and reduction at the negative electrode. Once this condition of equilibrium is obtained, the possibility for overcharge of the cell is significantly decreased. Thus, in a plurality of cells stacked in series where the individual cells have mismatched capacities, the cells can be charged in series without overcharge of a single cell. This prevents the excess current above that needed to charge the cell from causing dissociation of the electrolyte and prevents the accompanying gassing.

It is known that those cells in series having lower leakage currents than other cells in series will obtain a maximum charge of one volt sooner than those with higher leakage currents as long as the same amount of charge has passed through each cell. Hence, it is believed that by utilizing the process of the present invention, those cells charged in series will maintain a maximum charge of one volt via the bromine-bromide cycle until the other cells having the higher leakage currents obtain one volt at which time all of the cells will maintain a one volt maximum via the bromine-bromide cycle.

Example 3

Four cells were prepared using the procedure and apparatus of Example 1. Each cell was individually charged at constant potential to 1.0 volt. Then each cell was separately discharged to a different depth of discharge stimulating a mismatch in cell capacitance. Utilizing a constant potential power source of 4.0 volts, the four cells were charged in series. After 22.4 hours, the cell voltage were as reported in Table III.

<tables id="tabl0004" num="0004"><img file="EP0093539A2_D0004.tif" /></tables>

Example C

Four more cells were prepared in accordance with the procedure and apparatus employed in Example 3. However, in these cells, the electrolyte contain no HBr and are provided for comparison. Using the same process as above, the results are found in Table IV. <tables id="tabl0005" num="0005"><img file="EP0093539A2_D0005.tif" /></tables>

A comparison of the cells containing HBr, the voltage regulating agent, in the electrolyte (Example 3) and the cells not containing HBr in the electrolyte (Example C) show a distinct difference between the cells when charged in series. Since all the cells were charged in series and each cell had a different initial voltage, it could be expected that certain cells would obtain full charge more rapidly then others. However, the cells containing HBr in the electrolyte show a uniformity at approximately 1.0 volts while the cells containing no HBr in the electrolyte have non-uniform voltages with one cell exhibiting a voltage over 1.3 which resulted in gassing. Thus, by utilizing the process of the present invention, cells with various leakage currents can be charged in series to maximum capacity without overcharging individual cells, thereby eliminating gassing and the eventual rupture of electrical energy storage cells.