CA2237679A1 - Double layer capacitor with porous carbon electrodes and method for manufacturing these electrodes - Google Patents

Abstract

A double electric layer capacitor comprising at least two electrodes (4, 5), substantially of porous carbon, the electrodes being substantially saturated with electrolyte and separated by means of a porous separator (6) with ionic conductivity. The capacitor is especially characterized in that the electrodes (4, 5) in the form of a porous structure are made of materials with a carbon content exceeding 95 % mass and a pore volume exceeding 55 % of the electrode material volume, a certain part of the pores having a size less than 10 nm.

Description

~ CA 02237679 l998-0j-l4 1 ( a ~D \I~
DO~3LE ~AYER CAPACITOR WITH POROUS CARBON ELECTRODES AND
METHOD FOR MANUFACTURING THESE ELECTRODES.

The present invention relates to a double layer capacitor comprislng at least two electrodes, substantially o~ porous carbon, the electrodes being substantially saturated with electrolyte and separated by means of a porous separator with ionic conductivity, the electrodes consisting of an interconnecting solid carbon network.
Such electric devices, more specifically accumulating constructions for electricity, can be used e.g as a short time or reserve source o~ electric current ~or a radio electronic apparatus, for memory units of personal compu-ters, video and other devices.

The invention also relates to a method of manufacturing an electrode having a solid carbon skeleton network with a plur~lity o~ pores and a capacitor electrode material.
One c~ the main directions o~ the development o~ high-e~iciency capacitors with double electric layer is to make new electrode carbon materials with such a combination o~
properties as an optimal pore size, mechanical strength and high chemical purity.

Previously known are capacitors with a double electric layer (e.g. Japanese patent application No. 3-62296.1991), comprising two polarized electrodes divided by a separator, which are placed in a hermetic ~rame. The electrodes are made of active carbon and a binding agent, which consists of carbon black and ceramic powder. The electrode material has a porous structure, resulting in a specific electric capacitance not more than 25F/cm3.
The deficiencies of such capacitors are:

- considerable leakage currents due to a great content o~
ash in the electrode material (3-8~);
AMENDEO SHEET

CA 02237679 1998-0~-14

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- increased variation in capacitance characteristics due to changeS in microporosity properties o~ the electrode mate-rial in the process of manufacture of the electrodes and the capacitor assembly;
- the electrode material has low mechanical strength (this limits the use of these capacitors in constructions, which are working under conditlons of high mechanical stress, e.g. vibrations).

Further, previously known are capacitors with double elec-tric layer, comprising a ~rame o~ stainless steel; the frame comprises a bottom and a lid joined by a washer creating a hermetic container. In the frame, two polarized electrodes, saturated with electrolyte and separated by a porous separator, are situated. The electrodes are made of active carbon (80~ mass) and a binding agent, which con-sists o~ ash (lO~mass) and polytetrafluorethylene (10~
mass). The material in the ~orm o~ paste is applied to an e,ectrically conductive underlayer and is then rolled and dried. From~ the resulting sheet product the prescribed size electrodes are cut.

Such capacitors can operate over a wide range of tempera-tures. The electrode material provides speci~ic electric capacitance within the limits of 20-25 F/cm; However, these capacitors have all the deficiencies of the preceeding ones.

An electrode for a double-layer capacitor consisting o~ an interconnecting solid network is known from EP-A1- 0 660 345.

The object of the present invention is to obtain a simulta-neous increase in capacitor specific electric capacitance, decreased variation of the actual capacitance values and decrease in leakage currents. In addition, the purpose of the invention is to obtain an increase in electrode strength and mechanical stability. This will allow an extension of the field of use for the capacitors, ~or A~UEND~o SHET

CA 02237679 1998-0~-14

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example in constructions working under conditions of mecha-nical impact or vibration To obtain this technical result a capacitor of the kind mentioned in the beginning of the description is characterized in that the electrodes have a pore volume exceeding 55~ of the electrode material volume, the part of the pores having a size less than 10 nm being 35-S0~ of the electrode material volume The carbon content of the electrode is more than 95~ mass, preferably more than 99~ mass The material has a total pore volume pre~e-rably in the range from 55 to 80~ of the electrode volume;
this makes it possible to obtain a high electric capaci-tance.

~ The invention also relates to a method of manufacturing an electrode having a solid carbon skeleton network with a plurality of pores, characterized by the steps of, -moulding an electrode blank o~ a metal carbide powder and, ~ as a binding agentl organic binders and carbon, in the form o~ ca~bon black or as a pyrolysis product, the am.ount 5f binding agent being 5-50 g per 100 g of metal carbide powder, - saturating the moulded blank by liquid metal at a tempe-rature exceeding the melting temperature but not exceeding 300~ C above this temperature in a vacuum furnace.

- heat treating the saturated blank in halogen gas, such as fluorine or chlorine, at a temperature of 800 - 1200~ C for the formation of transport channels/pores and nano porous (<10 nm) carbon structure. After such a manufacture the electrode contains practically pure carbon with a ramified system of transport channels/pores, and only minor amounts of impurities (less than 5% mass, preferably less than 1~
mass). These electrodes have a carbon structure providing high electrode mechanical strength (compressive strength more than 90 kg/cm2). The material consists of a solid network of carbon interconnected throughout the structure, resulting in mechanical rigidity and strength, and a com-A!AENDED SHEET

~ CA 02237679 1998-0~-14 . . .

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,~' bination of coarser sized transport channels/pores of the electrolyte and nano sized porosity, together making up the total porosity volume. Of importance is also the stability o~ the electrode dimensions and its pores and, as a result, a stability o~ the electrode electrical properties. Thus, the decrease in height and diameter values from intermedia-te product to finished electrode is not more than 0,05~
permitting a very limited variation in electrode speci~ic electric capacitance, resulting in actual capacitor capaci-tance in the range +- 15~, whereas known capacitors have the electric capacitance tolerance + 80 to - 20~.

The new electrodes of~er an increase in speci~ic electric ~ capacitance and actual capacitor capacitance by nearly 30~ -in comparison with known technical solutions and a decrease in leakage currents o~ 5-10 times because o~ an only minor impurity content of the electrode material. In addition, the high electrode strength makes it possible to use the ~:apacitors in devices working under vibration, impact and othe~ mechallical stresses.

The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with re~erence to the accompanying drawing, in which in ~igure l an overall capacitor picture is given (side view) and in -~ENDED SHEET

CA 02237679 1998-0~-14 WO 97/20333 PCT/EP96tO0431 treatment of a metal carbide composite. After such a treatment the electrode contains practically pure carbon with a ramified system of transport channels/pores, and only minor amounts of impurities (less than 5~ mass, preferably less than 1% mass). These electrodes have a carbon structure providing high electrode ?ch~n; cal strength (compressive strength more than 90 kg/cmZ). The material consists of a solid network of carbon intercon-nected throughout the structure, resulting in me~-h~nical rigidity and strength, and a combination of coarser sized transport channels/pores of the electrolyte and nano sized porosity, together making up the total porosity volume. Of importance is also the stability of the electrode dimen-sions and its pores and, as a result, a stability of the electrode electrical properties. Thus, the decrease in height and diameter values from intermediate product to finished electrode is not more than 0,05~ permitting a very limited variation in electrode specific electric capaci-tance, resulting in actual capacitor capacitance in the range +- 15%, whereas known capacitors have the electric capacitance tolerance + 80 to - 20%.

The new electrodes offer an increase in specific electric capacitance and actual capacitor capacitance by nearly 30%
in comparison with known t~hn; cal solutions and a decrease in leakage currents of 5-10 times because of an only minor impurity content of the electrode material. In addition, the high electrode strength makes it possible to use the WO 97/20333 pcT~r96Joot~

capacitors in devices working under vibration, impact and other mechanical stresses.

The invention will now be described in more detail with reference to examplifying embodiments thereof and also with reference to the acc~ ~-nying drawing, in which in figure l an overall capacitor picture is given (side view) and in figure 2 plots of the voltage across the load versus dis-charge time are given.

The capacitor with a double electric layer comprises a hermetic frame, comprising a bottom 1 and a lid 2, joined by a dielectric washer 3. Inside of the frame electrodes

4, 5 are situated. The electrodes are saturated with an electrolyte and separated by means of a porous separator 6.
The opposite sides 4', 5' of the double electrode layer are in contact with the bottom l and lid 2 respectively. To make assembly of the capacitor more simple there are elas-tic washers 7 encircling the electrodes peripherally.

For confirmation of the obtained tçchn; cal result 12 pieces of carbon electrodes (diameter 19.5 mm, hight 1.0 mm) and 6 pieces of button like capacitors (diameter 24.5 mm, hight 2.2 mm) were manufactured. As a separator porous polypropylene with ionic conductivity was used and as electrolyte an aqueus solution of alkali, KOH, was used.
The nominal electric capacitance of the capacitor was 20F
and the voltage was 1.0 volt.

CA 02237679 1998-0~-14 W O 97/20333 PCTrEP96/00431 The physical and mechanical properties of the electrode material were investigated and the capacitors were tested for reliability and possibility to work under actual condi-tions as a power source for electronic watches and electro-nic memory units for personal computers. The tests for the reliability were carried out at the voltage 0.9+-0.1 V. at a temperature of +70 +-5~ C. The test duration was 500 hours.

The results of the investigation of the electrode physical, chemical and m~ch~n;cal properties and of the capacitor tests are given in tables 1 and 2 and by the graphs of figure 2.

An analysis of the results of electrode investigation (table 1) shows that the volume of the pores with a size less than lo nm (average 43% of electrode volume) is nearly twice that parameter of carbon electrodes manufactured by means of traditional technology. The compressive strength increased more than 3 times. The specific electric capaci-tance (average 34,5 F/cm3) ~c~ by nearly 30% the spe-cific capacitance of known carbon materials (not more than 25 F/cm3).

The results of the test of reliability (table 2) show only slight variation of the nominal capacitor capacitance (+-5,3%). The explanation for this is the high mechanical strength of the carbon electrodes, having a stable ramified structure, maint~in;ng geometrical and electrode and elec-trolyte parameters during the assembly process.

After the test the capacity loss was 5,7~ (average) and the increase in inner resistance was 18% (average), satisfying high performance demands.

The results of the test of capacitors show (Fig. 2) that the duration of the performance of the capacitors as a current source was: 198 hours at the load 100 kohm, 32 hours at the load 50 kohm, 3 hours at the load 20 kohm and 2 hours at the load 0,5 kohm. These data imitate the real discharge of capacitors in operation under load in various devices, where the capacitors may be used as a power source.

According to a preferred embodiment the electrodes are produced from silicon carbide powder and, as a binding agent, a mixture consisting of carbon black, phenolformal-dehydic resin and ethylated alcohol in the following com-ponents correlation, mass.%:

Carbon black 30-50 Phenolformaldehydic resin 5-10 Ethylated alcohol 40-60 or pyrocarbon in the amount of 5-50 g per 100 g of silicon carbide. After moulding, the blank is saturated by li~uid silicon at the temperature of 1450-1700~ C. Thermo-chemical treatment by chlorine is conducted at a tempera-ture of 900-1100~ C.

CA 02237679 1998-0~-14 The method is described below:
From silicon carbide powder and the binding agent a blank of given form is moulded. During moulding silicon carbide powder is mixed with a suspension, the composition, mass.
%, of which is: carbon black 30-50, phenolformaldehydic resin 5-10, ethylated alcohol 40-60, in the amount of 5-50 g per 100 g of silicon carbide. From this charge the blank is moulded. Then for curing the resin, heat treatment at a temperature of 150~ C is conducted. As an alternative a pyrocarbon binding agent, added to silicon carbide powder or introduced by heat treatment in a natural gas current, is used.

Moulded by this method or another moulding t~chn;que the blank is placed in a vacuum furnace, where saturation by liquid silicon at a temperature of 1450-1700~ in vacuum is made. During this process a chemical interaction of liquid silicon and carbon (carbon black or pyrocarbon) with the formation of secondary silicon carbide takes place. This secondary silicon carbide forms throughout all volume of the blank a continuous structure, bonding the grains of initial silicon carbide and forming a solid silicon carbon body with residual pores filled with silicon metal. The reaction of silicon carbide formation at a temperature lower than 1450~ C does not occur and the purpose of the method is not achieved. Silicon begins to evaporate in the vacuum furnace at temperatures above 1700~ C. Thus, a porousless blank, comprising silicon carbide particles CA 02237679 1998-0~-14 WO 97/20333 PCTrEP96/00431 bonded by a structure of secondary silicon carbide and free silicon, is obtained. Then the blan~ is heat treated by chlorine at a temperature of 900-1100~ C. During chloration the free silicon metal is removed from the blank in the form of gaseous silicon chloride and thus a necessary volume of transport microporosity channels/pores are formed. Additionally, as a result o~ silicon car~ide chloration, carbon with a developed nanoporous structure is formed.

The combination of transport channels/pores and nano porosity of the resulting solid carbon network is of great importance, because it facilitates electrolyte access to large available internal electrode surfaces, made up by the nano pore walls. The solid continous carbon network also provides low internal electrical resistance.

The function of the capacitor according to the invention should be apparent from the specification given above.

The capacitor according to the invention offers considerab-le advantages compared to previously known t~chn;ques as described in the introductory part of the specification.

The invention has been described with reference to an examplifying embodiment. It will be understood, how-ever, that other embodiments and minor modifications are conceivable without departing from the inventive concept.

CA 02237679 l998-0~-l4 For example more than two electrodes may be provided in the capacitor.

Further, it is possible to produce the electrode material by means of some other method that provides a structual network of solid carbon with transport channels/pores and nano porosity resulting in the mentioned advantages. The t~ohn;~ues are preparation of a mould comprising metal car-bide, organic binders and carbon, e.g. in the form of carbon black or as a pyrolysis product, and metal infiltra-tion and high-temperature reactions, followed by thermoche-mical removal of the metal to form the wished solid carbon structure comprising transport channels/pores and nano porosity.

An example might be the use of aluminum carbide and alu-minum metal which lowers the needed reaction temperatures in the first process step significally. So called cubic metal carbides based on Ti and other metals of group IV, V
or VI of the periodic system might also be used where gaseous metal halogenes are formed, like fluorides and chlorides.

Test results of electrode material Table 1 Elec- Total pores Volume of Specific Compressive Carbon trodes volume in pores with capaci- st~ength content electrodessizes less tance volume than 10 nm No % % F/cm3kg/cmZ % mass 1 55 45 35 95 99,1 2 70 40 30 99 99,2 3 65 50 39 94 99,3 4 60 45 36 92 99,5 38 93 99,4 6 80 35 31 97 99,2 7 55 50 33 96 99,6 8 75 50 39 100 99,1 9 65 35 30 102 99,3 38 98 99,5 11 60 40 34 97 99,2 12 58 46 35 99 99,4 Test rçsults of manufactured ca~acitors Table 2 Before test 1 After test Capaci- Actual Resi- Actual Resi- cO-C1X t00 R~-R~ x 100 tors capaci- stance capacl- stance Co R1 tance tance ~o Co,F Ro, Ohm Cl.F Rl. Ohm % %
1 19 0,3 17,8 0,35 6,3 16,6 2 20 0,25 18,6 0,3 7,0 20,0 3 19,5 0,35 18,0 0,4 7,6 14,3 4 18,5 0,25 18,0 0,3 2,8 20,0 18,0 0,3 17,0 0,35 5,6 16,6 6 19.5 0.25 18.5 0.3 5.1 20.0 , SUBSTITLITE SH~ET (RllLE 26)

Claims (13)

Claims 9

1. A double electric layer capacitor comprising at least two electrodes (4,5), substantially of porous carbon, the electrodes being substantially saturated with electrolyte and separated by means of a porous separator (6) with ionic conductivity, the electrodes (4,5) consisting of an inter-connecting solid carbon network, characterized in, that the electrodes (4,5) have a pore volume exceeding 55% of the electrode material volume, the part of the pores having a size less than 10 nm being 35-50% of the electrode material volume.

2. A capacitor according to claim 1, characterized in, that the carbon content in the porous electrodes exceeds 95%
mass, preferably 99% mass.

3. A capacitor according to claim 1 or 2, characterized in, that the volume of pores falls in the range 55-80%, preferably in the range 60-80%

4. A capacitor according to claim 1, 2 or 3, characterized in, that the compressive strength of the electrode material exceeds 90 kg/cm2.

5. A capacitor according to any one of the preceeding claims, characterized in, that the electrodes are arranged in a hermetic frame comprising a bottom (1) and a lid (2) joined by means of a dielectric washer (3).

6. A capacitor according to any one of the preceeding claims, characterized in, that elastic washers (7) are provided, encircling the electrodes peripherally.

7. A method of manufacturing an electrode having a solid carbon skeleton network with a plurality of pores, characterized by the steps of, -mouldinq an electrode blank of a metal carbide powder and, as a binding agent, organic binders and carbon, in the form of carbon black or as a pyrolysis product, the amount of binding agent being 5-50 g per 100 g of metal carbide powder, - saturating the moulded blank by liquid metal at a temperature exceeding the melting temperature but not exceeding 300° C above this temperature in a vacuum furnace.

- heat treating the saturated blank in halogen gas, such as fluorine or chlorine, at a temperature of 800 - 1200° C for the formation of transport channels/pores and nano porous (<10 nm) carbon structure.

8. A method according to claim 7, characterized by, choosing the metal from group IV ,V or VI of the periodic system or aluminum or silicon.

9. A method according to claim 8, characterized by moulding an electrode blank of silicon carbide powder and a binding agent.

10. A method according to claim 9, characterized by moulding the electrode blank from silicon carbide powder and, as a binding agent, either a mixture substantially comprising 30-50% mass of carbon black, 5-10% mass of phenolformaldehydic resin and 40-60% mass of ethylated alcohol, or pyrocarbon, the amount of binding agent preferably being 5-50 g per 100 g of silicon carbide powder.

11. A method according to claim 9 or 10, characterized by the steps of;

- saturating the electrode blank by liquid silicon at a temperature of 1450-1700° C in a vacuum furnace, - heat treating the saturated blank by chlorine at a temperature of 900-1100° C for the formation of transport channels/pores and nano porous (<10 nm) carbon structure.

12. A capacitor electrode material, substantially of porous carbon consisting of an interconnecting solid carbon network, characterized by a pore volume exceeding 55% of the electrode material volume, the part of the pores having a size less than 10 nm being 35-50% of the electrode material volume.

13. A material according to claim 14, characterized in, that the carbon content exceeds 95% mass, preferably 99%
mass.