Electrolytes for capacitors

[0001] This application claims priority from U.S. provisional application Serial No. 60/353,895, filed on Jan. 31, 2002.

[0002] This invention is directed to an electrolyte for electrolytic capacitors. More particularly, the present invention relates to an electrolyte for high voltage wet tantulum or aluminum capacitors.

[0003] The present electrolyte is suitable for an electrolytic capacitor and includes water and an organic solvent having an ammonium salt of a relatively weak organic acid dissolved therein. The organic acid is used to achieve an appropriate pH, conductivity, and breakdown voltage for a particular capacitor application.

[0004] An exemplary capacitor includes an anode of a valve metal such as aluminum or tantalum provided with an oxide film on the surface as a dielectric. The oxide film is typically formed by an anodizing process. The anode is kept from contacting a cathode by a separator disposed there between. The separator is impregnated with the present electrolyte. The electrolyte has a relatively high conductivity and breakdown voltage, which ensures that the capacitor exhibits low series resistance while withstanding high voltage. As such, the electrolyte impregnated separator provides the conductivity between the anode and the cathode while supporting the rated voltage. The electrolyte impregnated separator also helps heal the dielectric oxide film on the anode during operation.

[0005] These and other aspects and advantages of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.

[0006] An electrolyte according to the present invention preferably contains the following constituents, by weight: about 1% to about 80% de-ionized water and 0% to about 80% of an organic solvent along with about 1% to about 80% isobutyric acid and about 0.5% to about 50% of concentrated ammonium salt (28%). The organic solvent includes, but is not limited to, glycols, glycol ethers, polyethylene glycols, amides, esters, nitriles, linear carbonates, cyclic carbonates, and mixtures thereof.

[0007] Suitable glycols include, but are not limited to, ethylene glycol, diethylene glycol, propylene glycol, trimethylene glycol, and mixtures thereof.

[0008] Suitable glycol ethers include, but are not limited to, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol methyl ether, diethylene glycol methyl ether, dipropylene glycol methyl ether, glycol monobutyl ether, and mixtures thereof.

[0009] Suitable amides include formamide, dimethyl formamide, diethyl formamide, ethyl formamide, dimethyl acetamide, methyl acetamide, and mixtures thereof.

[0010] Suitable nitriles include acetonitrile, propionitrile, and mixtures thereof.

[0011] Cyclic esters such as -butyrolactone, -valerolactone and N-methyl-2-pyrrolidone are also useful solvents or co-solvents as are carbonates, both linear and cyclic. Suitable linear and cyclic carbonates include dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, ethyl propyl carbonate, methyl propyl carbonate, propylene carbonate, ethylene carbonate, butylenes carbonate, vinylene carbonate, and mixtures thereof.

[0012] Isobutyric acid can act as both a solvent and a solute. While isobutyric acid is preferred, other relatively weak organic acids of the general formula of c_nH_2nO₂ (where n2 to 7) are acceptable. Examples are butyric acid, propionic acid, valeric acid (pentanoic acid), methylbutyric acid, trimethylacetic acid, and mixtures thereof, among others coming under the purview of the above formula.

[0013] Ammonium hydroxide is added to react with the acid to form an ammonium salt in situ that provides electrical conductivity. Electrolyte pH and conductivity can be adjusted by the amount of ammonium hydroxide. Ammonium hydroxide can be substituted by an ammonium salt of the corresponding acid constituent. Examples of these salts are ammonium isobutyrate, ammonium butyrate, ammonium propionate, ammonium valerate, ammonium methylbutyrate, ammonium trimethylacetate, and mixtures thereof.

[0014] The electrolytes of the present invention are useful for not only conventional electrolytic capacitors, but also those of the electrolytic/electrochemical hybrid type. Capacitor cathodes commonly used in electrolytic capacitors include etched aluminum foil in aluminum electrolytic capacitors, and those commonly used in wet tantalum capacitors such as of silver, sintered valve metal powders, platinum black, and carbon. The cathode of hybrid capacitors include a pseudocapacitive coating of a transition metal oxide, nitride, carbide or carbon nitride, the transition metal being selected from the group consisting of ruthenium, cobalt, manganese, molybdenum, tungsten, tantalum, iron, niobium, iridium, titanium, zirconium, hafnium, rhodium, vanadium, osmium, palladium, platinum, and nickel. The pseudocapacitive coating is deposited on a conductive substrate such as of titanium or tantalum. The electrolytic/electrochemical hybrid capacitor has high energy density and is particularly useful for implantable medical devices such as a cardiac defibrillator.

[0015] The anode is of a valve metal consisting of the group vanadium, niobium, tantalum, aluminum, titanium, zirconium and hafnium. The anode can be a foil, etched foil, sintered powder, or any other form of porous substrate of these metals.

[0016] A preferred chemistry for a hybrid capacitor comprises a cathode electrode of a porous ruthenium oxide film provided on a titanium substrate coupled with an anode of a sintered tantalum powder pressed into a pellet. The cathode and anode electrodes are segregated from each other by a suitable separator material impregrated with the present working electrolyte. Such a capacitor is described in U.S. Pat. Nos. 5,894,403, 5,920,455 and 5,926,362. These patents are assigned to the assignee of the present invention and incorporated herein by reference.

[0017] Electrolytes of present invention may also contain phosphoric acid, an inorganic phosphate or an organic phosphate as an additive to improve anode stability. The examples of organic phosphates are trimethylphosphate, triethylphosphate, triisopropylphosphate, and mxtures thereof.

[0018] Finally, electrolytes of present invention may contain a nitroaromatic depolarizer to prevent cathodic gassing during operation. Suitable nitroaromatic compounds include, but are not limited to 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid, 3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroace tophenone, 3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole, 3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol, 3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid, 3-nitrophthalic acid, 4-nitrophthalic acid, and mixtures thereof.

[0019] The present electrolyte is useful for capacitors having an operating range of about 175 volts to about 400 volts while maintaining high conductivity. The preferred ruthenium oxide/tantalum hybrid capacitor provides high energy density at voltages of at least about 175 volts, such as is required in an implantable medical device, for example, a cardiac defibrillator. For this reason, it is important that the electrolyte have a high breakdown voltage, high conductivity, suitable pH and good chemical stability over the operating life of the device.

[0020] The present electrolyte is chemically compatible over time with the other capacitor components and capacitor materials, even at temperatures of about 105 C. This means that the electrolyte does not generate gas or promote corrosion of the other capacitor components at that temperature.

[0021] The following examples describe the manner and process of a capacitor according to the present invention, and they set forth the best mode contemplated by the inventors of carrying out the invention, but they are not to be construed as limiting.

[0022] One exemplary electrolyte according to the present invention consists of the constituents listed in Table 1. The anode breakdown voltage measurements set forth in the below tables were conducted using a tantalum anode at room temperature.

	TABLE 1
	Components	Wt. %	Amount
	ethylene glycol	39.17	89.8	(ml)
	deionized water	52.88	135.0	(ml)
	isobutyric acid	6.0	16.1	(ml)
	ammonium hydroxide (28%)	1.5	4.3	(ml)
	phosphoric acid (85%)	0.06	0.09	(ml)
	p-nitrophenol	0.4	1.0	(g)

[0023] It was determined that the above electrolyte had the physical characteristics listed in Table 2.

	TABLE 2
	pH	4.9
	Conductivity	7.1	mS/cm
	Anode Breakdown	395	volts

[0024] Another exemplary electrolyte according to the present invention consists of the constituents listed in Table 3.

	TABLE 3
	Components	Wt. %	Amount
	ethylene glycol	39.71	359.4	(ml)
	deionized water	55.6	560.0	(ml)
	isobutyric acid	3.0	31.6	(ml)
	ammonium hydroxide (28%)	1.3	14.0	(ml)
	phosphoric acid (85%)	0.06	0.36	(ml)
	p-nitrophenol	0.4	4.0	(g)

[0025] It was determined that the above electrolyte had the physical characteristics listed in Table 4.

	TABLE 4
	pH	5.4
	Conductivity	6.9	mS/cm
	Anode Breakdown	400	volts

[0026] Another exemplary electrolyte according to the present invention consists of the constituents listed in Table 5.

	TABLE 5
	Components	Wt. %	Amount
	ethylene glycol	18.2	18.0 (ml)
	deionized water	9.1	10.0 (ml)
	isobutyric acid	64.5	74.7 (ml)
	ammonium hydroxide (28%)	8.2	10.0 (ml)
	phosphoric acid (85%)	0.09	0.06 (ml)

[0027] It was determined that the above electrolyte had the physical characteristics listed in Table 6.

	TABLE 6
	pH	5.2
	Conductivity	7.9	mS/cm
	Anode Breakdown	388	volts

[0028] Another exemplary electrolyte according to the present invention consists of the constituents listed in Table 7.

	TABLE 7
	Components	Wt. %	Amount
	deionized water	44.88	100.0	(ml)
	isobutyric acid	13.5	31.6	(ml)
	ammonium hydroxide (28%)	3.2	8.0	(ml)
	phosphoric acid (85%)	0.04	0.06	(ml)
	ethylene glycol	38.2	91.4	(ml)
	monomethyl ether

[0029] It was determined that the above electrolyte had the physical characteristics listed in Table 8.

	TABLE 8
	pH	5.7
	Conductivity	11.5	mS/cm
	Anode Breakdown	380	volts

[0030] Another exemplary electrolyte according to the present invention consists of the constituents listed in Table 9.

	TABLE 9
	Components	Wt. %	Amount
	deionized water	4.69	10.0	(ml)
	isobutyric acid	37.5	84.2	(ml)
	ammonium hydroxide (28%)	10.6	25.0	(ml)
	phosphoric acid (85%)	0.05	0.06	(ml)
	ethylene glycol	46.9	107.5	(ml)
	monomethyl ether

[0031] It was determined that the above electrolyte had the physical characteristics listed in Table 10.

	TABLE 10
	pH	6.7
	Conductivity	10.8	mS/cm
	Anode Breakdown	370	volts

[0032] It is appreciated that various modifications to the present inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the herein appended claims.