KR20140037074A - Carbon electrodes and electrochemical capacitors - Google Patents

Abstract

The present invention relates to a carbon electrode for a capacitor having a conditioned carbon element in combination with a high concentration of electrolyte tetrafluoroborate salt and a non-aqueous aprotic solvent providing an operating voltage of up to 4.5 V, and a capacitor used with the carbon electrode. It is about.

Description

Carbon Electrodes and Electrochemical Capacitors {CARBON ELECTRODES AND ELECTROCHEMICAL CAPACITORS}

The present invention relates to an electrochemical capacitor containing conditioned carbon for the electrode, and the electrode. More particularly, the capacitor provides a synergistic improvement when using the conditioned carbon for the electrode of the present invention in combination with a non-aqueous non-acetonitrile electrolyte with a high concentration of electrolyte salt which is a quaternary tetrafluoroborate salt.

Activated carbon is a preferred material for use in the production of electrodes for carbon electrode capacitors. These activated carbons are made from a number of different sources, such as coconut shells, wood, sugars, cellulosic materials and phenolic resins. These materials are converted to carbon under steam controlled conditions and then carbon is "activated" in a second step with steam, or catalyzed with KOH, or carbon dioxide and KOH, resulting in very high surface areas such as 1000 to 2400 m ² / g. Increase the high surface area. These activated carbons generally contain about 2% oxygen and trace amounts of inorganic salts after they have been completely dried. This oxygen is probably present with some nitrogen compounds as quinones, hydroquinones, phenols, carboxylic acids, furans and possibly ketones, etc., all of which are higher voltages above 3 V because the voltage will increase above 3.3 V. Under conditions will undergo electrochemical oxidation / reduction. At lower voltages these functional groups actually improve the energy storage capacity of carbon, which is desirable at voltages below 3.2V. However, since the voltage is pushed above 3.2 V, high leakage current, ESR (resistance) increase, and performance loss due to gas formation occur.

High energy density capable electrochemical double layer capacitors, known as "super-capacitors", are assembled from a variety of materials. In general, it is desirable to construct super-capacitors with stable, non-reactive electrolytes and lightweight materials, as described in US Pat. No. 5,260,855 to Kaschmitter et al., The entire teachings of which are incorporated herein by reference. Supercapacitors of this type include carbon-based electrodes, which can be made from organic gels.

US Pat. No. 6,902,683 to Smith et al., Incorporated herein by this reference, discloses complex salts formed by mixing a tetraalkyl ammonium salt of hydrogen fluoride with an imidazolium compound in a nitrile solvent operated at a temperature of -60 to 150 ° C. Relates to an electrolyte.

Papers incorporated herein by reference [ Ue in J. electrochem . Soc . Vol. 11, Nov. 1994 entitled " Electrochemical Properties of Organic Liquid electrolyte s based on Quaternary Onium Salts for Electrical Double - Layer Capacitors " ] disclose high dielectric constant solvents and onium salts for double layer capacitors. Particularly studied in this paper were quaternary onium tetrafluoroborate salts which showed greater solubility in solvents with good stability and conductivity.

US Pat. No. 5,418,682 to Warren et al., Incorporated herein by this reference, discloses a process for preparing a tetraalkyl ammonium tetrafluoroborate salt for use as a solvent with a dinitrile mixture as a solvent.

US Pat. Nos. 6,535,373 and 6,902,684 to Smith et al., Incorporated herein by reference, disclose similar electrolytes that predominantly utilize nitrile solvents.

According to one embodiment of the present invention, there is provided an improved carbon electrode for a higher voltage electrochemical double layer capacitor (EDLC) and a method of manufacturing the same.

Carbon electrodes were prepared from discs formed from carbon particles heat treated at 850 ° C. to 1300 ° C., preferably at about 1050 ° C. to 1190 ° C. for about 30 to 60 minutes under inert atmosphere or vacuum. This carbon was formed into a Kynar binder and a 4 mm sheet, and then cut into electrode disks. This electrode was then tested using a voltage ramp of 0.5 increments against the disc in a capacitor test cell of 2 to 4.4 volts, holding for 3 cycles in each step under inert atmosphere.

According to another feature of the invention, the electrolyte of EDLC comprises a higher concentration (2.0-4.0 M; 20-55 wt%) of one or more tetraalkyl ammonium tetrafluoroborate salts and aprotic non-acetonitrile non-aqueous Solvent.

Advantageously, an aluminum metal collector is used for the capacitor electrode.

A general object of the present invention is to provide an EDLC having an operating voltage of up to 4.0 to 4.5V.

According to a feature of the invention, the treated activated carbon particles are formed into sheets and then cut into the disc of the electrode; The electrode includes a metal collector, carbon on the top and bottom surfaces of the collector, a second conductive carbon layer located on the heat treated carbon layer, and a porous inert separator between the conductive carbon layers. This is located.

It is a general object of the present invention to produce carbon electrodes for capacitors containing discs comprising ash, silica or post-treated activated carbon particles which do not contain functional oxygen or sulfur compounds.

Another object of the present invention is to provide a capacitor having a carbon electrode operating at higher voltage (> 3.0 to 4.5V).

The present invention will be better understood by reading the preferred embodiments and drawings of the present invention.

1 is a front view of a carbon disk for an electrochemical double layer capacitor electrode of the present invention.
FIG. 2 is a cross-sectional view of the electrode having the disk of FIG. 1. FIG.
3 is a cross-sectional view of another electrode of the present invention.
4 is an exploded view of the electrode of FIG. 3.

Description of preferred embodiments

According to the present invention, there is provided an improved electrochemical carbon electrode double layer capacitor (EDLC) having an electrochemically stable carbon electrode with an electrolyte having an conductivity of about 20-30 mS / cm at 25 ° C. and increasing to 65 mS / cm at 80 ° C., At least one conductive ammonium salt of the formula and non-aqueous non-aceto at a concentration of 2.0 to 4.0 M in a solvent selected from the group consisting of ethylene carbonate, propylene carbonate, gamma butyrolactone or dimethyl carbonate, or mixtures thereof A capacitor comprising a nitrile electrolyte is provided:

Wherein R, R ₁ , R ₂ and R ₃ are ethyl or methyl groups or R, R ₁ , R ₂ and R ₃ are one or two pyrrolidinyl groups.

Carbon of the electrode is activated carbon formed into a disc after heating at 850 ° C to 1300 ° C for about 30 to 60 minutes under an inert atmosphere. Preferably, the carbon is acid washed before ash is removed. Capacitors are tested at a constant voltage of 3.5 to 4.5V.

Preferred ammonium salts have a molecular weight ranging from 175 to 250. More preferred salts are diethyldimethylammonium tetrafluoroborate (DEDMA BF ₄ ) and ethylmethyl pyrrolidinyl tetrafluoroborate. Preferred solvents are ethylene carbonate (EC) combined with gammabutyrolactone (GBL) or propylene carbonate (PC) or dimethyl carbonate (DMC) in a ratio of about 40% EC to 70% GBL, PC, DMC or mixtures thereof. Solvent.

The electrode used in the capacitor is heat treated with activated carbon used in the electrode for about 30 to 60 minutes at 850 ° C to 1300 ° C, preferably 1000 to 1300 ° C, under an inert atmosphere or vacuum after acid washing to form a carbon disk for the electrode. And then the capacitor with electrodes is subjected to constant voltage cycling in the 3.5 to 5V range.

As shown in FIGS. 1 and 2, the electrode 10 of the present invention includes an aluminum metal foil collector 11 having a thickness of about 1 to 4 mils on the top and bottom. On the collector 11 there is any conductive carbon coating 12 having a thickness of about 0.5 to 1 mil. On this coating 12 is a disk 13 of about 2 to 5 mils thick that contains a higher surface area conductive carbon conditioned with a binder. A conventional porous insulating film 14 having a thickness of about 0.5 to 3 mils is used between the conductive carbon 13. The coating 12 may also comprise a treated carbon that is identical to the carbon identified in the layer 13.

The separator 14 may comprise any conventional inert separator used to fabricate the carbon electrode. A preferred separation membrane is porous Teflon. The electrodes used may be symmetrical or asymmetric.

The disc 13 with conductive carbon layer is a pretreated conductive material prepared according to the invention in combination with an inert polymeric binder such as polytetrafluoroethane, Surlyn or Kynar present in an amount of about 6% by weight. Contains carbon.

Disc layer 13 mixes about 0.02 g of treated carbon with about 6% polytetrafluoroethane or Kynar in acetone solvent and pressurizes to roll the front and back of the mixture to a layer of about 2-5 mils thick and 5 It can be prepared by forming a circle of / 8 ", die cutting and vacuum drying. The particle size of the carbon particles is about 0.5 to 10 microns. The disk can be of various shapes, i.e. square, rectangular, round, etc. It may include.

Alternatively, the activated carbon is subjected to acid ball milling of the carbon particles used to about 0.5 to 10 microns, preferably 3 to 6 microns, and then treated to form the electrode disc 13, followed by 850 It can be prepared by heat treatment in a furnace at 1300 ° C., preferably 1100 ° C. to remove oxygen and sulfur functional groups, followed by washing and drying in vacuo. The heat treatment step is carried out in an inert atmosphere.

Thereafter, after-treated activated carbon was formed into the disk 13, the electrolyte was placed in the capacitor test cell while subjected to constant voltage cycling of 2 to 4.1V.

The capacitor is provided with means for degassing the carbon disk at the electrode to remove gas formed during initial voltage cycling (less than 10 cycles).

Aluminum collector

The aluminum collectors used for the electrodes are preferably made from plain annealed aluminum foil with an aqueous conductive carbon acrylic coating such as a commercially available Acheson carbon acrylic conductive coating which is subsequently baked at 200 ° C. Aluminum after burn-in and degassing is suitable for the anode or cathode.

The post treated activated carbon is formulated with Kynar® and can be coated directly onto the aluminum collector foil in a 3-6 mm thick electrode layer. This can be done on one side or on both sides.

Preferred Electrolyte

A preferred solvent comprises a combination of ethylene carbonate (EC) with at least one of the group of gamma butyrolactone (GBL), propylene carbonate or dimethyl carbonate, wherein the ethylene carbonate comprises at least 20 wt%, preferably 40 to 60 wt% %.

The salt concentration is at least 1.0 M, preferably from 1.5 to 5.2 M, and most preferably from 1.5 to 3.5 M. Mixtures of spiro or bis-pyrrolidinylammonium tetrafluoroborates also provide excellent low temperature efficiency.

This improvement in the electrolyte is even greater when the ionic salt content is greater than 2.5 to 4.0M and twice as high as the current electrolyte, which leads to much better efficiency in charge / discharge capacitance than the efficiency implied only by the conductivity values. Caused.

Ethylene carbonate (EC) has been found to be essential for achieving significant improvements in conductivity compared to current non-acetonitrile aprotic electrolytes (18-19 mS / cm) based solely on propylene carbonate (PC).

Example  One

Manufacture of electrodes

A. Carbon electrodes can be prepared as shown in FIGS. 1 and 2 by forming an acetone slurry of ash and silica removal treated 10% Kynar® resin and 90% carbon (0.5 to 10 microns). After the mixture was cast on a flat Teflon sheet, the acetone was allowed to evaporate. The dried sheet was separated from Teflon, die cut 5/8 "circular electrode and vacuum dried. The electrode was weighed to about 0.01 g and thickness was 4 mil.

B. Alternatively, the carbon electrode 20 could be prepared as shown in FIGS. 3 and 4 by making an end plate 21 from a baked aluminum sheet after coating with a thin liquid conductive carbon dispersion. . After drying, the disc 21 was drilled from the sheet to the appropriate size and placed on a roller to flatten the edges. The Surlyn® ring 23 was then heat sealed to the end plate 21. The heat treated carbon was then paste with 10% Surlyn binder and acetone. The paste was rolled into a sheet of Teflon sheeting of about 4 mm. Acetone was evaporated and then perforated to a suitable size in the Surlyn ring. Thereafter, the carbon electrode 24 was baked under vacuum to remove any moisture. The separator 22 is a 0.5 mil thick porous Teflon disc that is slightly smaller than the end plate 21 but larger than the electrode 24. After soaking the electrode 24 in a suitable electrolyte, the assembly was placed in the Surlyn ring. The separator was centered in one Surlyn ring assembly. Thereafter, another Surlyn ring assembly was placed on top of the other assembly and the entire assembly was heat sealed together.

Example  2

Post-processed OSAKA PC  Using a carbon electrode (0.008 g) EC / GBL  Electrolyte DEDMABF4 Of capacitor test cells with

Example  3

Post-processed Norit  50/50 using 30 carbons (0.008 g) EC / PC  Electrolyte DEDMABF4 Of capacitor test cells with

Claims (13)

A non-aqueous electrochemical capacitor with a sealed carbon electrode,
One or more carbon electrodes
a) metal collectors on the top and bottom surfaces;
b) optionally, a first conductive carbon layer on each upper and lower inner surface of the collector;
c) a porous second conductive carbon layer on each of said first conductive carbon layers which is degassed substantially free of oxygen and heteroatom containing functional groups;
d) a porous inert separator between the second conductive carbon layers; And
e) An improved capacitor comprising an electrolyte having a quaternary ammonium tetrafluoroborate salt at a concentration of at least 1.5 M in an aprotic solvent. The capacitor of claim 1 comprising a non-acetonitrile electrolyte. 3. The capacitor of claim 2 wherein said electrolyte comprises an aprotic solvent and at least one tetraalkyl ammonium tetrafluoroborate salt. 4. The capacitor of claim 3 wherein the salt concentration is between 2.0 and 4.8 M. The capacitor of claim 1 comprising an aluminum metal collector. 3. The capacitor of claim 2 wherein the electrolyte solvent comprises two components selected from the group consisting of ethylene carbonate, propylene carbonate, gamma butyrolactone and dimethyl carbonate. A capacitor having a carbon electrode, wherein the carbon in the electrode is thermally treated to remove oxygen and sulfur functional groups and silica by heating at a temperature of 850-1300 ° C., and at least one disc used for the electrode located in the capacitor. Capacitors with burn-in and degassing with voltage ramps up to 4.4. 8. The capacitor of claim 7, wherein the electrolyte of the electrode comprises a conductive tetraammonium salt of the formula: in an aprotic solvent:

Wherein R, R ₁ , R ₂ and R ₃ are ethyl or methyl groups or together R, R ₁ , R ₂ and R ₃ are one or two pyrrolidinyl groups. 8. The capacitor of claim 7, wherein the electrolyte solvent is at least two components of the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate and gamma butyrolactone. 8. The capacitor of claim 7, wherein the salt concentration is about 1.5 to 4.8 M. 8. The capacitor of claim 7, wherein the electrolyte solvent comprises about 40 to 60% ethylene carbonate. 8. The capacitor of claim 7, comprising an annealed aluminum collector. 8. The capacitor of claim 7, wherein the carbon in the electrode is acid washed to remove ash and salts prior to heat treatment.

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