JP7542213B2 - Electrochemical Devices - Google Patents

Description

The present invention relates to an electrochemical device.

An example of an electrochemical device is the electric double layer capacitor. Compared to secondary batteries, electric double layer capacitors have a longer lifespan, can be charged quickly, and have superior output characteristics. For this reason, electrochemical devices such as electric double layer capacitors are widely used as backup power sources, etc.

Patent Document 1 (Patent No. 5724875) discloses an electric double layer capacitor with an operating temperature exceeding 70°C, characterized by having a coating electrode formed by applying to a current collector made of etched foil a slurry in which an electrolyte solution containing gamma-butyrolactone as a solvent, an electrode material, a conductive assistant, and a styrene-butadiene elastomer having an expansion rate of 50% or less after 100 hours in gamma-butyrolactone at 85°C as a binder are dispersed in water.

Patent No. 5724875 specification

Electrochemical devices are expected to be used at a variety of temperatures, and are required to have minimal degradation in performance even at low temperatures.

One aspect of the present disclosure relates to an electrochemical device. The electrochemical device includes a pair of electrodes and an electrolyte, at least one of the pair of electrodes includes activated carbon and a binder, and the binder has a volume change rate of 50% or more and 450% or less when immersed in the electrolyte at 85°C for 100 hours.

This disclosure provides an electrochemical device that exhibits minimal performance degradation at low temperatures.

FIG. 1 is a perspective view illustrating a schematic example of an electrochemical device according to the present disclosure.

Below, embodiments of the present disclosure are described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In the following specification, when a "range of numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

Traditionally, in electrochemical devices, emphasis has been placed on characteristics at high temperatures. It has also been generally believed that binders used in the active layer that swell little when immersed in an electrolyte are preferable. This is because it has been thought that a binder with a high degree of swelling will have a reduced binding strength. However, as a result of the investigations conducted by the present inventors, it has been newly discovered that, contrary to conventional technical common sense, under certain conditions a binder with a high degree of swelling is preferable. This invention is based on this new finding.

(Electrochemical Devices)
The electrochemical device according to the present disclosure includes a pair of electrodes and an electrolytic solution. At least one of the pair of electrodes includes activated carbon and a binder. Hereinafter, the electrolytic solution and the binder may be referred to as "electrolytic solution (S)" and "binder (B)", respectively. Hereinafter, an electrode including activated carbon and binder (B) may be referred to as "electrode (E)". The binder (B) has a volume change rate of 50% or more and 450% or less when immersed in the electrolytic solution (S) at 85° C. for 100 hours. Hereinafter, the volume change rate may be referred to as "volume change rate VC". The volume change rate VC is a value indicating the expansion rate of the binder in the electrolytic solution (S).

As described later, when the binder (B) (for example, a film made of the binder (B)) is immersed in the electrolytic solution (S) at 85° C. for 100 hours, the volume of the binder (B) before immersion in the electrolytic solution (S) is V0, and the volume of the film after immersion in the electrolytic solution (S) for 100 hours is V1, the value of the volume change rate VC can be calculated by the following formula.
Volume change rate VC (%) = 100 × (V1 - V0) / V0

The electrochemical device according to the present disclosure provides an electrochemical device with little performance degradation at low temperatures. The electrochemical device according to the present disclosure is preferably used in applications where use at temperatures below -30°C is anticipated. In other words, the lower limit of the operating temperature of the electrochemical device according to the present disclosure is below -30°C, for example, in the range of -45°C to -30°C.

The electrochemical device may be a chargeable and dischargeable electricity storage device. The electrochemical device may be an electric double layer capacitor (EDLC), a lithium ion capacitor (LIC), or the like. When the electrochemical device is an EDLC, the above-mentioned electrode (E) is used for one or both of a pair of electrodes. When the electrochemical device is an LIC, the above-mentioned electrode (E) is used for one of the pair of electrodes (positive electrode). In this case, the other of the pair of electrodes (negative electrode) may be a negative electrode used in a lithium ion secondary battery. An example of a negative electrode used in a lithium ion secondary battery includes, for example, a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

(Electrode (E))
The electrode (E) includes an active layer. The active layer includes activated carbon, which is an active material, and a binder (B) as essential components. The electrode (E) may further include a current collector that supports the active layer. The active layer may include components other than the essential components (activated carbon and binder (B)), such as a conductive agent. Examples of the conductive agent include carbon black.

There is no particular limitation on the activated carbon (activated carbon particles) that is the active material, and known activated carbon used in electrochemical devices may be used. Activated carbon may be produced, for example, by subjecting a raw material to heat treatment to carbonization, and then subjecting the resulting carbonized material to activation treatment. Examples of raw materials include wood, coconut shells, pulp waste liquid, coal or coal-based pitch obtained by thermal decomposition thereof, heavy oil or petroleum-based pitch obtained by thermal decomposition thereof, phenolic resin, petroleum coke, and coal coke. Examples of activation treatments include gas activation using gas such as steam, and chemical activation using alkali such as potassium hydroxide. The activated carbon particles obtained by the above activation treatment may be subjected to a pulverization treatment. After the pulverization treatment, a classification treatment may be performed. For example, a ball mill, a jet mill, or the like is used for the pulverization treatment.

There is no particular limit to the proportion (content) of activated carbon in the active layer. The proportion may be 60% by mass or more and 95% by mass or less, and in a preferred example, 70% by mass or more and 90% by mass or less.

(Binder (B))
As described above, the electrode binder (B) has a volume change rate VC of 50% or more and 450% or less when immersed in the electrolyte (S) at 85 ° C. for 100 hours. The definition and measurement method of the volume change rate VC will be described in the examples. The volume change rate VC may be 60% or more, 80% or more (for example, more than 80%), 90% or more, or 100% or more. The volume change rate VC may be 300% or less, 150% or less, 100% or less, 80% or less, or less than 80%. The volume change rate VC may be in a range in which these lower and upper limits are combined in a compatible combination. For example, the volume change rate VC may be in a range of 60% or more and 150% or less, a range of 60% or more and 100% or less, a range of 60% or more and less than 80%, a range of more than 80% and 450% or less, or a range of more than 80% and 150% or less.

The volume change rate VC can be changed by the type of binder (B) and the structure of the binder (B). In addition, the volume change rate VC of the mixed binder can be changed by mixing multiple types of binders with different volume change rates VC. Binders (polymers) with different volume change rates for various solvents are commercially available from various manufacturers. Therefore, by selecting such binders, it is possible to achieve an appropriate volume change rate. When the binder (B) is a mixture of multiple types of polymers (binders), the volume change rate VC is measured in the state of the mixture.

The binder (B) may contain at least one selected from the group consisting of styrene-butadiene rubber and acrylic rubber. That is, the essential components of the binder (B) may be styrene-butadiene rubber, acrylic rubber, or styrene-butadiene rubber and acrylic rubber. The binder (B) may be composed only of these.

The electrode (E) may contain a polymer (binder) other than styrene-butadiene rubber and acrylic rubber. Examples of such a polymer (binder) include resin materials such as polytetrafluoroethylene (PTFE) and carboxymethyl cellulose (CMC).

Styrene-butadiene rubber is a copolymer (e.g., a copolymer of styrene and butadiene) whose main monomers are styrene and butadiene, and may be a modified version of such a copolymer. The volume change rate VC can be changed by changing the polymerization ratio of styrene and butadiene, the average molecular weight (e.g., weight average molecular weight) of the styrene-butadiene rubber, etc.

Acrylic rubber is a polymer in which an acrylic acid ester is the main monomer. Examples of acrylic rubber include copolymers of two or more monomers containing an acrylic acid ester and another monomer, and also include modified versions of these copolymers. Examples of other monomers include 2-chloroethyl vinyl ether and acrylonitrile. Examples of acrylic acid esters include ethyl acrylate, butyl acrylate, and methoxyethyl acrylate. Two or more types of acrylic acid esters may be mixed and used. When considering use at temperatures below -30°C, the acrylic acid ester may include butyl acrylate or methoxyethyl acrylate. The acrylic rubber may be fluorinated. The volume change rate VC can be changed by changing the type of acrylic acid ester, the type of other monomer, the copolymerization ratio of the monomers, the average molecular weight of the acrylic rubber (for example, weight average molecular weight), etc.

As the styrene butadiene rubber having the desired volume change rate VC and the acrylic rubber having the desired volume change rate VC, commercially available styrene butadiene rubber and acrylic rubber may be used.

There is no particular limitation on the proportion (content) of the binder in the active layer. The proportion may be 1% by mass or more and 20% by mass or less, and in a preferred example, 5% by mass or more and 10% by mass or less.

The binder (B) may contain styrene-butadiene rubber and acrylic rubber, or may be composed of only these two types. Generally, styrene-butadiene rubber has a small volume change rate VC as described above. In contrast, acrylic rubber generally has a large volume change rate VC. Therefore, by mixing the two, the target volume change rate VC can be easily achieved. The mixing ratio of styrene-butadiene rubber to acrylic rubber may be in the range of styrene-butadiene rubber:acrylic rubber = 95:5 to 10:90 by mass.

(Current collector)
There is no particular limitation on the current collector used in the electrode (E), and an appropriate current collector may be selected according to the active layer. The current collector may be a current collector used in known electrochemical devices. Examples of the current collector include sheet metals. Examples of the sheet metal include metal foils, metal porous bodies, and etching metals. Examples of the metal material may include aluminum, aluminum alloys, nickel, titanium, and the like. An example of the current collector is a metal foil made of these metal materials. The surface of the metal foil may be roughened by etching or the like.

(Electrolyte (S))
The electrolyte (S) includes a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and includes a cation and an anion. The ionic substance may include a low-melting compound (ionic liquid) that can exist as a liquid at, for example, around room temperature. The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol/L or more and 2.0 mol/L or more.

As the solvent, a high boiling point solvent is preferable. For example, lactones such as γ-butyrolactone, carbonates such as propylene carbonate, polyhydric alcohols such as ethylene glycol and propylene glycol, cyclic sulfones such as sulfolane, amides such as N-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methyl acetate, ethers such as 1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde can be used.

The electrolyte solution (S) may contain γ-butyrolactone as a solvent. The proportion of γ-butyrolactone in the solvent may be 50% by mass or more, 70% by mass or more, 90% by mass or more, or 95% by mass or more, or may be 100% by mass. In other words, only γ-butyrolactone may be used as the solvent. γ-butyrolactone has a low melting point, so it is preferably used when considering use at temperatures below -30°C.

Ionic substances include, for example, organic salts. An organic salt is a salt in which at least one of the anion and cation contains an organic substance. An example of an organic salt in which the cation contains an organic substance is a quaternary ammonium salt. Examples of organic salts in which the anion (or both ions) contains an organic substance are trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono 1,2,3,4-tetramethylimidazolinium phthalate, and mono 1,3-dimethyl-2-ethylimidazolinium phthalate.

From the viewpoint of improving the withstand voltage characteristics, the anion preferably contains an anion of a fluorine-containing acid. Examples of the anion of a fluorine-containing acid include BF ₄ ^- and/or PF ₆ ^- . The organic salt preferably contains, for example, a cation of tetraalkylammonium and an anion of a fluorine-containing acid. Specific examples include diethyldimethylammonium tetrafluoroborate (DEDMABF ₄ ), triethylmethylammonium tetrafluoroborate (TEMABF ₄ ), and the like.

An example of the electrochemical device of the present disclosure satisfies the following configuration (1) and may further satisfy at least one of the following configurations (2) to (4).
(1) The battery includes at least one electrode (E) including activated carbon and a binder (B), and an electrolyte (S). The binder (B) has a volume change rate VC of 50% or more and 450% or less when immersed in the electrolyte (S) at 85° C. for 100 hours. The volume change rate VC may be, for example, in the range of 60% or more and 450% or less, in the range of 60% or more and 150% or less, or in the range of more than 80% and 150% or less.
(2) The binder (B) contains at least one selected from the group consisting of styrene-butadiene rubber and acrylic rubber, and for example contains both of them.
(3) The solvent of the electrolytic solution (S) contains γ-butyrolactone. In this case, the ionic substance dissolved in the electrolytic solution (S) may contain the above-mentioned organic salt containing a tetraalkylammonium cation and a fluorine-containing acid anion. The ratio of γ-butyrolactone in the solvent of the electrolytic solution (S) may be the ratio described above.
(4) The lower limit of the operating temperature is −30° C. or lower.

(Separator)
A separator may be disposed between the pair of electrodes. The separator has ion permeability and serves to physically separate the pair of electrodes to prevent short circuit. Examples of the separator include nonwoven fabrics mainly composed of cellulose, glass fiber mats, and microporous films of polyolefins such as polyethylene.

The shape of the electrochemical device is not particularly limited, and may be the same as that of a known electrochemical device. For example, the shape of the electrochemical device may be cylindrical, rectangular (square), or coin-shaped (including button-shaped). In one example of a cylindrical electrochemical device, a pair of electrodes and a separator are wound to form a wound electrode group. At this time, they are wound so that the separator is disposed between the pair of electrodes. In one example of a rectangular or coin-shaped electrochemical device, a pair of electrodes are stacked with a separator sandwiched between them to form a stacked electrode group. Each of the two electrodes constituting the pair of electrodes may be composed of a plurality of electrodes. For example, when the two electrodes constituting the pair of electrodes are considered as a positive electrode and a negative electrode, the positive electrode may be composed of a plurality of positive electrode plates, and the negative electrode may be composed of a plurality of negative electrode plates. The stacked electrode group may be formed by stacking a plurality of positive electrode plates and a plurality of negative electrode plates so that a separator is disposed between them.

(Method for producing electrode (E) and electrochemical device)
The manufacturing method of the electrode (E) and the electrochemical device is not particularly limited, and they may be manufactured by a known method. An example of the manufacturing method of the electrode (E) is described below. However, as long as the electrode (E) can be manufactured, other manufacturing methods may be used.

First, activated carbon particles, a binder, a conductive agent, and a solvent (e.g., water) are mixed to obtain a slurry. The binder may be added in the form of a suspension. Next, the obtained slurry is applied to the surface of a current collector to form a coating film. Next, the coating film is dried and, if necessary, rolled to form an active layer. In this manner, the electrode (E) can be formed.

(Embodiment 1)
An example of the electrochemical device of the present disclosure will be specifically described below with reference to the drawings. The components described above can be applied to the components of the electrochemical device described below. The electrochemical device described below can be modified based on the above description. The matters described below may be applied to the above embodiment. In the embodiment described below, components that are not essential to the electrochemical device of the present disclosure may be omitted.

The configuration of the electrochemical device 10 according to the first embodiment is shown in FIG. 1. FIG. 1 is a perspective view showing the electrochemical device 10. To facilitate understanding, FIG. 1 shows the electrochemical device 10 with a portion cut away.

The electrochemical device 10 in FIG. 1 is an electric double layer capacitor. The electrochemical device 10 includes a wound type capacitor element 1. The capacitor element 1 is constructed by winding a sheet-shaped first electrode 2 and a sheet-shaped second electrode 3 with a separator 4 interposed therebetween. The first electrode 2 and the second electrode 3 are each the electrode (E) described above.

The first electrode 2 and the second electrode 3 each have a first and second metal current collector and a first and second active layer supported on the surface thereof. The first electrode 2 and the second electrode 3 each exhibit capacitance by adsorbing and desorbing ions.

The first electrode 2 and the second electrode 3 may be the same electrode (E), or may be electrodes (E) with different configurations. As described above, either one of the electrodes may be an electrode other than the electrode (E).

A first lead wire 5a and a second lead wire 5b are connected to the first electrode 2 and the second electrode 3, respectively, as lead-out members. The capacitor element 1 is housed in a cylindrical exterior case 6 together with an electrolyte (not shown). The material of the exterior case 6 may be, for example, a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the exterior case 6 is sealed by a sealing member 7. The lead wires 5a and 5b are led out to the outside so as to pass through the sealing member 7. For the sealing member 7, for example, a rubber material such as butyl rubber is used.

The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to the following examples.

Example 1
In Example 1, seven types of electrochemical devices (devices A1 to A5, CA1, and CA2) were fabricated and evaluated by changing the volume change rate VC of the binder contained in the active layer. A wound type electric double layer capacitor with a rated voltage of 2.7 V was fabricated as the electrochemical device. Two electrodes having the same configuration were used as a pair of electrodes. A specific method for fabricating the electrochemical device will be described below.

(Measurement of Volume Change Rate VC of Binder)
The volume change rate VC of the binder used in the examples was calculated as follows. First, an electrolyte (described later) and a binder used in the manufacture of an electrochemical device were prepared. Next, a binder suspension was cast in a petri dish and dried to form a film made of the binder. This film (binder) was immersed in an electrolyte at 85° C. for 100 hours, and the volume of the film before and after immersion was measured. If the volume of the film before immersion in the electrolyte is V0 and the volume of the film after immersion in the electrolyte for 100 hours is V1, the value of the volume change rate VC can be calculated by the following formula.
Volume change rate VC (%) = 100 × (V1 - V0) / V0

The volume of the film was measured using Archimedes' method. Specifically, the weight of the film in air, W0, and the weight of the film in the liquid for volume measurement, W1, were measured. If the density of the liquid for volume measurement is ρ, the volume of the film can be calculated by the following formula.
Film volume V=(W0-W1)/ρ

The volume of the membrane was measured using the above method before and after immersion in the electrolyte to determine volume V0 and volume V1.

(Preparation of electrodes)
80 parts by mass of activated carbon particles as an active material, 5 parts by mass of styrene butadiene rubber as a binder, 5 parts by mass of CMC as a thickener, and 10 parts by mass of acetylene black as a conductive agent were dispersed in water to prepare a slurry. The styrene butadiene rubber was added in the form of a suspension (water as a dispersion medium). The styrene butadiene rubber used had a volume change rate VC of 50%. Next, the obtained slurry was applied to both sides of an Al foil (thickness 30 μm) to form a coating film. The coating film was vacuum dried at 110 ° C. and then rolled to form an active layer (thickness 40 μm). In this way, an electrode (electrode (E)) was produced.

(Preparation of Electrolyte)
An electrolyte solution was prepared by dissolving diethyldimethylammonium tetrafluoroborate (DEDMABF ₄ ) in γ-butyrolactone (GBL). The concentration of DEDMABF ₄ in the electrolyte solution was 1.0 mol/L.

(Fabrication of Electrochemical Device A1)
Two of the above electrodes were prepared, and a lead wire was connected to each of them. Next, the two electrodes were wound with a cellulose nonwoven separator interposed therebetween to prepare a capacitor element. This capacitor element was housed in a specified exterior case together with an electrolyte. Next, the exterior case was sealed with a sealing member to assemble an electrochemical device (electric double layer capacitor). After that, an aging treatment was performed at 60° C. for 16 hours while applying a rated voltage. In this way, an electrochemical device A1 was obtained.

(Preparation of electrochemical devices A2, A3, and CA1)
Electrochemical devices A2, A3, and CA1 were fabricated using the same materials and methods as those used for fabricating device A1, except that the volume change rate VC of the styrene-butadiene rubber used as the binder was changed.

(Preparation of electrochemical devices A4, A5, and CA2)
Electrochemical devices A4, A5, and CA2 were fabricated using the same materials and methods as those used for fabricating device A1, except that acrylic rubber with a different volume change rate VC was used as the binder instead of styrene-butadiene rubber.

The styrene butadiene rubber and acrylic rubber used in the examples were prepared by purchasing rubbers with a specified volume change rate VC.

The change in internal resistance (DCR) at -30°C was measured for each of the electrochemical devices produced. Specifically, the electrochemical device was first charged at a constant current of 100 mA in an environment at -30°C until the voltage reached 2.7 V, and then the state in which a voltage of 2.7 V was applied was maintained for 7 minutes. Then, in an environment at -30°C, a constant current discharge was performed at a current of 20 mA until the voltage reached 0 V. The internal resistance of the electrical device was calculated using the initial discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) during this discharge.

Example 2
In Example 2, seven types of electrochemical devices (electrochemical devices B1 to B5, CB1 and CB2) were fabricated in the same manner as electrochemical device A1 in Example 1, except that the type of binder contained in the active layer was changed.

In Example 2, a mixture of styrene butadiene rubber (SBR) and acrylic rubber was used as the binder. Seven types of electrochemical devices (devices B1 to B5, CA1, and CA2) were fabricated and evaluated by varying only the volume change rate VC of the binder. The volume change rate VC was changed by changing the volume change rate VC of the acrylic rubber and the mixing ratio of the styrene butadiene rubber and the acrylic rubber. A suspension of styrene butadiene rubber and acrylic rubber was used to fabricate the electrodes.

The internal resistance of the fabricated electrochemical device was measured at -30°C in the same manner as in Example 1.

(Evaluation Results of Examples 1 and 2)
Table 1 shows the binders used in the production of the electrochemical device of Example 1 and the evaluation results of the internal resistance.

The binders used in the fabrication of the electrochemical device of Example 2 and the evaluation results of the internal resistance are shown in Table 2.

The internal resistances in Tables 1 and 2 are relative values when the internal resistance of electrochemical device A1 is taken as 100%. For both values, the lower the value, the better the characteristics at low temperatures. As shown in Tables 1 and 2, the internal resistance was low when the volume change rate VC was in the range of 50-450%, the internal resistance was lower when the volume change rate VC was in the range of 60-150%, and the internal resistance was particularly low when the volume change rate VC was in the range of 100-150%. Furthermore, compared to using only one of styrene-butadiene rubber and acrylic rubber as the binder, the internal resistance was reduced by using a mixture of styrene-butadiene rubber and acrylic rubber as the binder.

The reason why the internal resistance at low temperatures decreases when the volume change rate is in the range of 50 to 450% is not clear at present. However, one possible explanation is as follows.

At room temperature, electronic conduction is dominant in the internal resistance of the electrode, but at low temperatures, the influence of ionic conduction in the electrolyte is thought to be greater. When the volume change rate VC of the binder is large, the amount of electrolyte in the electrode increases, increasing the number of paths for ion movement, and it is presumed that the internal resistance at low temperatures decreases. On the other hand, if the volume change rate VC of the binder becomes too large, the contact resistance between the conductive materials in the electrode increases and electronic conduction becomes insufficient. As a result, it is presumed that the internal resistance increases even if the volume change rate VC of the binder becomes too large. Therefore, it is thought that it is possible to reduce the internal resistance at low temperatures by setting the volume change rate VC of the binder in an appropriate range.

This invention can be used in electrochemical devices.

1: Capacitor element, 2: First electrode, 3: Second electrode, 4: Separator, 5a: First lead wire, 5b: Second lead wire, 6: Outer case, 7: Sealing material, 10: Electrochemical device

Claims (5)

An electrochemical device including a pair of electrodes and an electrolyte,
At least one of the pair of electrodes includes activated carbon and a binder,
the binder has a volume change rate of 50% or more and 450% or less when immersed in the electrolyte at 85° C. for 100 hours;
The electrochemical device , wherein the binder comprises at least one selected from the group consisting of styrene-butadiene rubber and acrylic rubber . The electrochemical device of claim 1, wherein the binder includes styrene butadiene rubber and acrylic rubber. 3. The electrochemical device according to claim 1 , wherein the volume change rate of the binder is 60% or more and 150% or less. 4. The electrochemical device according to claim 1, wherein the electrolyte contains γ-butyrolactone as a solvent. 5. The electrochemical device according to claim 1, which is an electric double layer capacitor or a lithium ion capacitor.

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