JP2012015297A - Electrode for electrochemical element, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for an electrochemical element that has low resistance and has excellent withstand voltage characteristics.SOLUTION: The electrode for an electrochemical element comprises a current collector, a conductive layer formed on a surface of the current collector and including conductive carbon particles and a first binder, and an active material layer formed on a surface of the conductive layer and including active material particles and a second binder. The conductive carbon particles include a small-particle group and a large-particle group. The volume particle size distribution of the small-particle group has a peak in 3-7 μm range, the volume particle size distribution of the large-particle group has a peak in 10-20 μm range, and the weight ratio of the small-particle group to the large-particle group (small-particle group/large-particle group) is 95/5-50/50. An interface between the conductive layer and the active material layer has a roughness with a maximum height Rmax of 10 μm or more.

Description

  The present invention relates to an electrode for an electrochemical element having low resistance and excellent large current characteristics. For example, a capacitor such as an electric double layer capacitor (EDLC) or a pseudo capacitor (P-EDLC), a lithium ion battery, a nickel hydrogen storage battery, and the like It is related with the electrode used for the battery.

  In recent years, technology development using electrochemical elements such as secondary batteries and electric double layer capacitors (EDLC) has been progressing mainly in automobiles from the viewpoints of energy saving, environmental conservation and use of alternative energy to petroleum, and hybrid cars (HEV) And the development of PEV (electric vehicle) is accelerating. Further, high-performance secondary batteries and EDLC are also being used in SSD (Solid state drive) type hard disks.

  The electrode for electrochemical devices generally has a current collector and an active material layer formed on the surface thereof. However, when the active material layer is formed directly on the surface of the current collector, the electrode resistance may increase, the current collector may be oxidized, or hydrogen embrittlement may occur. Therefore, it has been proposed to form a conductive layer containing conductive carbon particles as the first layer on the surface of the current collector and to form an active material layer as the second layer on the surface (Patent Document 1, 2).

JP 2009-38387 A JP 2005-508081 A

  As the performance of electrochemical devices has increased, there has been an increasing demand for the development of electrodes for electrochemical devices having excellent large current characteristics. However, simply forming a conductive layer on the surface of the current collector makes it difficult to obtain an electrode with a low resistance enough to satisfy the recently demanded large current characteristics. In addition, the voltage applied to the electrochemical element is gradually increasing, and high withstand voltage characteristics are required for the electrodes.

  In view of the above situation, an object of the present invention is to provide an electrode for an electrochemical element that can satisfy a sufficiently large current characteristic and is excellent in low resistance and withstand voltage characteristics.

  That is, one aspect of the present invention is an active material formed on a surface of a current collector, a conductive layer formed on the surface of the current collector and including conductive carbon particles and a first binder, and an active material. An active material layer including particles and a second binder, wherein the conductive carbon particles include a small particle group and a large particle group, and the peak of the volume particle size distribution of the small particle group is in the range of 3 to 7 μm. The volume particle size distribution peak of the large particle group is in the range of 10 to 20 μm, and the weight ratio of the small particle group to the large particle group (small particle group / large particle group) is 95/5 to 50/50. In addition, the interface between the conductive layer and the active material layer relates to an electrode for an electrochemical element having a roughness with a maximum height Rmax of 10 μm or more.

The present invention also provides an effective method for producing a low-resistance electrode for electrochemical devices.
That is, according to another aspect of the present invention, (i) a first slurry containing the conductive carbon particles, the first binder, and the first liquid component is applied to the surface of the current collector, and the conductive material is electrically conductive. A step of forming a coating film, (ii) a step of heating the conductive coating film by radiant heat and volatilizing the first liquid component from the conductive coating layer, (iii) after step (ii), on the surface of the conductive coating film, Applying a second slurry containing active material particles, a second binder, and a second liquid component to form an active material coating; (iv) a laminate of a conductive coating and an active material coating; And a step of drying with at least one of radiant heat and hot air.

  Since the electrode for an electrochemical element according to the present invention has an extremely low resistance at the interface between the conductive layer and the active material layer, it is excellent in low resistance and large current characteristics. Further, peeling at the interface between the conductive layer and the active material layer hardly occurs, and the withstand voltage characteristics are also excellent. According to the manufacturing method according to the present invention, an electrode for an electrochemical element that is excellent in low resistance, large current characteristics and withstand voltage characteristics can be efficiently manufactured.

It is a schematic longitudinal cross-sectional view which shows an example of the conventional electrode structure. It is a schematic longitudinal cross-sectional view which shows another example of the conventional electrode structure. It is a schematic longitudinal cross-sectional view which shows an example of the electrode structure which concerns on this invention. It is a cross-sectional SEM photograph example of an example of the electrode which concerns on a comparative example. It is a cross-sectional SEM photograph example of another example of the electrode which concerns on a comparative example. It is a cross-sectional SEM photograph example of an example of the electrode which concerns on the Example of this invention. FIG. 2b shows a peak line and a valley line for the roughness curve of FIG. 2c. It is a figure which shows the space | interval (Rmax-n) of the peak line of FIG. 3, and a valley bottom line.

  The electrode for an electrochemical element according to the present invention is formed on the surface of the current collector, the conductive layer formed on the surface of the current collector and including the conductive carbon particles and the first binder, and An active material layer including active material particles and a second binder.

  In order to obtain an electrode that has low resistance enough to satisfy the demanded large current characteristics and has excellent withstand voltage characteristics in recent years, an interdiffusion layer must be formed between the conductive layer and the active material layer. is important. The interdiffusion layer is formed by diffusing the conductive carbon particles and the first binder contained in the conductive layer toward the active material layer, and diffusing the active material particles and the second binder contained in the active material layer toward the conductive layer. Is done.

  The existence of such an interdiffusion layer can be confirmed by measuring the maximum height Rmax of the roughness of the interface between the conductive layer and the active material layer. When the interdiffusion layer is formed, the maximum height Rmax is 10 μm or more. When the maximum height Rmax is 10 μm or more (for example, 13 μm or more, 14 μm or more, or 15 μm or more), that is, when an interdiffusion layer is formed, the interface resistance between the conductive layer and the active material layer is extremely small. In addition, the adhesion between the conductive layer and the active material layer is increased, and an electrode having desired low resistance and withstand voltage characteristics can be obtained. However, if the maximum height Rmax exceeds 25 μm, the electrode characteristics may become unstable. Therefore, it is preferably 25 μm or less.

  The maximum height Rmax of the roughness of the interface between the conductive layer and the active material layer can be measured in a cross section perpendicular to the surface direction of the current collector of the electrode. In the cross section, the boundary between the conductive carbon particles and the active material particles is observed. The locus of the point moving along the boundary (hereinafter, roughness curve) corresponds to the interface between the conductive layer and the active material layer. The maximum height Rmax of the roughness curve is obtained by a predetermined method described later.

  The electrochemical element mainly means a capacitor or a battery, and includes, for example, an electric double layer capacitor (EDLC), a pseudo capacitor (P-EDLC), a lithium ion battery, a nickel metal hydride storage battery, and the like. The structure of the capacitor or battery is not particularly limited, and includes a coin type, a wound type, a laminated type, and the like.

Next, the structure of the electrode for an electrochemical device according to the prior art and the present invention will be described with reference to schematic longitudinal sectional views of the electrode.
1a and 1b show a conventional electrode structure, and FIG. 1c shows an electrode structure of the present invention. Here, a case where the active material layer 2 is provided on one surface of the current collector 1 will be described. However, the electrode according to the present invention is not limited to the form of FIG. 1 c, and the case where the active material layer 2 and the conductive layer 3 are provided on one side of the current collector 1 and the active material layer 2 and the both sides of the current collector 1 are provided. The case where the conductive layer 3 is provided is included.

  In FIG. 1 a, the active material layer 2 is formed directly on one main surface of the current collector 1. The structure shown in FIG. 1a is most often used in batteries and EDLCs, and many portable devices and personal computers have electrochemical elements having this structure. However, with this structure, the current collection performance is limited, and it is becoming difficult to meet the recent demand for large current characteristics.

  In FIG. 1 b, the conductive layer 3 is formed on one main surface of the current collector 1, and the active material layer 2 is formed on the surface of the conductive layer 3. However, since no interdiffusion layer is formed between the active material layer 2 and the conductive layer 3, the interface resistance between the conductive layer and the active material layer is large, and the adhesion between the conductive layer and the active material layer is large. Also lower. Therefore, desired low resistance and withstand voltage characteristics cannot be obtained, and for example, peeling between layers tends to occur after 50 to 100 cycles of charge and discharge.

  In FIG. 1 c, the conductive layer 3 is formed on one main surface of the current collector 1, the active material layer 2 is formed on the surface of the conductive layer 3, and the active material layer 2 and the conductive layer 3 An interdiffusion layer 4 is formed between them. When the interdiffusion layer 4 is enlarged, a boundary between the conductive carbon particles and the active material particles is observed, and a roughness curve corresponding to the interface between the conductive layer and the active material layer can be obtained from the boundary. When the maximum roughness Rmax of the roughness curve is 10 μm or more, the electrode resistance can be reduced to 1/3 to 1/10 of the case where the interdiffusion layer 4 is not provided. In addition, the charge / discharge cycle characteristics of the electrochemical device are greatly improved.

Next, the electrode for an electrochemical element according to the present invention will be described in more detail.
The electrode includes a current collector, a conductive layer formed on the surface of the current collector, and an active material layer formed on the surface of the conductive layer. The current collector is usually in the form of a sheet, and the conductive layer and the active material layer may be formed only on one main surface of the current collector, or may be formed on both main surfaces.

  A metal foil is preferably used for the current collector. The thickness of the metal foil is, for example, 8 to 60 μm, preferably 20 to 40 μm. Examples of constituent elements of the metal foil include Al, Ni, and Cu. Aluminum foil is preferably used for the capacitor electrode and the positive electrode of the lithium ion battery. Also, copper foil is preferably used for the negative electrode of the lithium ion battery. The metal foil may be a plain foil that has not been subjected to an etching treatment, or may be an etching foil. The plain foil can be expected to have high withstand voltage characteristics. The etching foil is excellent in adhesiveness with the conductive layer. The current collector may have a three-dimensionally processed structure. For example, a punching foil or a lath wire mesh current collector may be used.

The conductive layer includes conductive carbon particles and a first binder.
It is preferable to use a graphite material for the conductive carbon particles. Graphite material is a general term for carbon materials having a graphite region, and any of natural graphite (phosphorus-like graphite, earth-like graphite, etc.), heat-treated natural graphite, and artificial graphite can be used.

The average particle diameter of the conductive carbon particles (median diameter D ₅₀ in the volume particle size distribution) is preferably _{5 to 20} μm. If the average particle size is too large, the particle gap may increase and the resistance value may increase, and if it is too small, it may be difficult to form irregularities at the interface between the conductive layer and the active material layer.

  In the present invention, in order to set the maximum height Rmax of the roughness of the interface between the conductive layer and the active material layer to 10 μm or more, the conductive carbon particles include mixed particles including a small particle group and a large particle group. Is used. By mixing the conductive carbon particles with large particles and small particles, an interdiffusion layer having a maximum height Rmax of 10 μm or more, which is dense and excellent in adhesion, is formed between the conductive layer and the active material layer. It becomes easy. As a result, the adhesion between the conductive layer and the active material layer increases, and the resistance of the electrode decreases.

  More specifically, the volume particle size distribution of the conductive carbon particles has peaks on the small particle size side and the large particle size side, respectively. The conductive carbon particles having such a particle size distribution can be regarded as mixed particles including a small particle group and a large particle group. Here, it is also important that the volume particle size distribution peak of the small particle group is in the range of 3 to 7 μm, and the volume particle size distribution peak of the large particle group is in the range of 10 to 20 μm. It is considered that when the conductive carbon particles have such a particle size distribution, a dense interdiffusion layer is formed.

The average particle diameter of the small particle group (median diameter D ₅₀ in the volume particle size distribution) is preferably 3 to 7 μm, and the average particle diameter of the large particle group is preferably 10 to 20 μm. It is considered that the large particle group mainly contributes to the improvement of conductivity in the conductive layer, and the small particle group mainly contributes to the adhesion of the mutual diffusion layer.
The volume particle size distribution can be measured with a laser diffraction particle size distribution measuring device (Microtrac MT3300EX II manufactured by Nikkiso Co., Ltd.).

  It is also important that the weight ratio between the small particle group and the large particle group (small particle group / large particle group) is 95/5 to 50/50, preferably 90/10 to 70/30. It is considered that the structure of the interdiffusion layer at the interface between the conductive layer and the active material layer becomes dense when the small particle group occupies 50 to 95% by weight of the conductive carbon particles contained in the conductive layer.

  The weight ratio between the small particle group and the large particle group is obtained by waveform-separating the obtained volume particle size distribution, determining the volume ratio between the small particle group and the large particle group, and converting the volume ratio into a weight ratio. Can do. However, when both the small particle group and the large particle group are graphite materials, the volume ratio can be regarded as the weight ratio.

  The ratio (D1 / D2) between the average particle diameter D1 of the small particle group and the average particle diameter D2 of the large particle group is preferably 1.5 or more and 5 or less, and particularly preferably 2 or more and 3.5 or less. By setting such a particle size ratio, the denseness of the interdiffusion layer is improved and the strength of the conductive layer itself is greatly improved.

  The conductive carbon particles may contain carbon black in addition to graphite. As the carbon black, acetylene black, ketjen black and the like can be used. The conductive layer containing carbon black is advantageous in that volume resistance and sheet resistance are easily reduced. The amount of carbon black is preferably 20 to 110 parts by weight with respect to 100 parts by weight of conductive carbon particles contained in the conductive layer.

  When the electrode is required to have a withstand voltage characteristic of 2.5 V or more, in the conductive layer, the first binder does not have a carbon-carbon double bond and has a melting point of 160 ° C. or higher, or a thermosetting type. Resins are preferably used. Such a resin is excellent in withstand voltage characteristics and heat resistance. For example, at least one selected from the group consisting of acrylate resins, polyimide resins, polyamideimide resins, fluororesins and cellulose resins is stable and preferable. Moreover, an acrylate resin, a polyamide-imide resin, and a cellulose resin are particularly preferable in that a high binding force can be obtained with a small amount. Of these, acrylate resins are most preferred because they are excellent in redox resistance.

In recent years, in the field of electrochemical devices such as EDLC, the development of an electrolyte solvent with excellent withstand voltage characteristics such as HCF ₂ CH ₂ -O-CF ₂ CF ₂ H has been promoted. However, there is a demand for high withstand voltage characteristics (high voltage exceeding 2.5 V for capacitors and about 4.2 to 4.5 V for lithium ion batteries). The above resin is also excellent from the viewpoint of satisfying such a requirement.

  The acrylate resin is a general term for resins containing units of acrylic acid or its alkyl ester or methacrylic acid or its alkyl ester. According to the fluorinated acrylate resin obtained by substituting a part of the hydrogen atoms contained in the main chain or alkyl ester group of the acrylate resin with fluorine, the oxidation-reduction resistance of the electrode can be further improved, and excellent chargeability can be achieved. Discharge cycle characteristics can be obtained. For example, an emulsion in which a fluorinated acrylate resin is dispersed in water is commercially available.

  As the fluororesin, for example, polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) can be used. The fluororesin is excellent in acid resistance and alkali resistance.

  The cellulose resin is not particularly limited, but ethyl cellulose is preferable from the viewpoint of the stability of the slurry that is the precursor of the conductive layer, the film formability of the slurry coating film (conductive coating film), the leveling property, the adhesiveness, and the like.

  As the thermosetting resin, those having a Tg of 260 ° C. or higher are preferable, and for example, an addition type thermosetting imide (polyimide resin) having an allyl group at both ends and a polyamideimide resin are preferable. When using a thermosetting resin, it is preferable to finally perform a heat treatment at 200 to 300 ° C. for about 30 minutes as a curing treatment, if necessary.

  On the other hand, when the electrode is required to have a withstand voltage characteristic of 2.1 V or more and 2.7 V or less, the first binder is preferably at least one selected from the group consisting of an olefin resin, a synthetic rubber, and a fluororesin. Used. Examples of the synthetic rubber include styrene butadiene rubber (SBR), nitrile rubber (NBR), and ethylene / propylene rubber (EPM, EPDM). Examples of the olefin-based resin include polyolefins such as polyethylene (PE), polypropylene (PP), modified PE, and modified PP. Synthetic rubber is excellent in that it has excellent adhesive strength and can impart appropriate flexibility to the electrode. Olefin resins are excellent in that they are inexpensive. However, the carbon-carbon double bond contained in the synthetic rubber may react with oxygen in EDLC to which a high voltage exceeding 2.5 V is applied or in a lithium ion battery to which a high voltage of about 4.5 V is applied. In some cases, the durability characteristics are slightly deteriorated. In addition, an electrode containing (modified) polyolefin may deteriorate when heated to 130 ° C. or higher.

  From the viewpoint of imparting an appropriate viscosity to the slurry that is a precursor of the conductive layer, carboxymethyl cellulose (CMC) may be included in the first binder. For example, when an emulsion in which a fluorinated acrylate resin is dispersed in water and CMC are used in combination, CMC dissolves in water and imparts an appropriate viscosity to the slurry, making it easy to prepare a uniform slurry. .

  By using the first binder, for example, a highly heat-resistant EDLC electrode that has a withstand voltage characteristic of 2.1 V or more or exceeds 2.5 V and can be dried at a high temperature of about 160 ° C. can be obtained. . Moreover, the electrode for lithium ion batteries to which the voltage exceeding 4.2V is applied is obtained.

  In the conductive layer, the amount of the first binder is preferably 1 to 20 parts by weight and more preferably 3 to 10 parts by weight or 3 to 6 parts by weight per 100 parts by weight of the conductive carbon particles. Moreover, when using together CMC and another binder, 0.5-30 weight% is suitable for the weight ratio of CMC to the whole 1st binder.

  In one embodiment of the present invention, the conductive layer contains a compound having a triazine ring (eg, triazine, 2,4,6-trimercapto-s-triazine monosodium salt, 2,4,6-trimercapto-s-triazine trisodium). Salt). The compound having a triazine ring assists the first binder, improves the binding force between the conductive carbon particles, and improves the binding force between the current collector and the conductive layer. The triazine ring has excellent oxidation resistance and suppresses gas generation in the electrochemical element. The amount of the compound having a triazine ring is preferably 1 to 12 parts by weight and more preferably 1 to 3 parts by weight with respect to 100 parts by weight of the conductive carbon particles.

  Although the thickness of a conductive layer changes with kinds of electrode, in the case of an electric double layer capacitor, it is 5-20 micrometers, for example, and in the case of a lithium ion secondary battery, it is 10-40 micrometers. When the thickness of the conductive layer is large, for example, 10 to 20 μm, it is particularly important to use a mixture of a small particle group and a large particle group as described above from the viewpoint of ensuring high conductivity. .

An active material layer is formed on the surface of the conductive layer.
The active material layer includes active material particles and a second binder. However, the constituent material of the active material layer is not limited to these. For example, a conductive aid such as carbon black may be included as an optional component.

  In the case of an electrode for an electric double layer capacitor, activated carbon is used as the active material particles. In this case, the positive electrode (anode) and the negative electrode (cathode) both contain activated carbon as an active material. On the other hand, in the case of a pseudo capacitor, various materials are used for the active material particles. For example, activated carbon is used for the positive electrode and graphite is used for the negative electrode.

Also in the case of an electrode for a lithium ion battery, various materials are used for the active material particles. A transition metal compound or the like is used for the positive electrode. As the transition metal compound, a lithium-containing transition metal oxide containing at least one selected from the group consisting of Co, Ni and Mn and an olipic acid (LiFePO ₄ ) type compound are preferably used. For the negative electrode, graphite, silicon, silicon compound, or the like is used. As the silicon compound, silicon carbide or silicon oxide is preferably used.

In the case of a nickel-metal hydride battery electrode, the active material particles are nickel hydroxide for the positive electrode and a hydrogen storage alloy for the negative electrode.
The present invention can be applied to any of these electrodes, but is most effective when applied to electrodes for electric double layer capacitors and pseudocapacitors.

  In the active material layer, it is preferable to use the material shown as the first binder for the second binder. The second binder may be made of a material different from that of the first binder, but it is preferable to use the same material from the viewpoint of improving the adhesion between the conductive layer and the active material layer. For example, when a fluorinated acrylate resin (FA) is used as the first binder, it is preferable to use a fluorinated acrylate resin (FA) as the second binder. The same applies to the use of CMC.

  In the active material layer, the amount of the second binder varies depending on the type of electrode. For example, in the case of an electric double layer capacitor or a pseudo capacitor, 3 to 10 parts by weight with respect to 100 parts by weight of the active material particles (activated carbon or graphite), Furthermore, 4-6 weight part is suitable. When the amount of the second binder is increased, the adhesion between the conductive layer and the active material layer is improved, but the direct current resistance and the alternating current resistance (ESR) tend to increase. When there is too little 2nd binder amount, there exists a tendency for the adhesiveness of active material particles to fall. When CMC and another binder are used in combination, the weight ratio of CMC to the entire second binder is preferably 0.5 to 50% by weight.

  In one embodiment of the present invention, the active material layer includes a compound having a triazine ring similar to that described above. The compound having a triazine ring assists the second binder, improves the binding force between the active material particles, and improves the binding force between the conductive layer and the active material layer. The amount of the compound having a triazine ring is preferably 1 to 3 parts by weight per 100 parts by weight of the active material particles.

  The thickness of the active material layer varies depending on the type of electrode, but is, for example, 50 to 200 μm in the case of an electric double layer capacitor. In the case of a coin-type electrochemical device, the thickness of the active material layer is 400 to 700 μm.

Next, the manufacturing method of the electrode for electrochemical devices which concerns on this invention is demonstrated.
Process (i)
A first slurry containing conductive carbon particles, a first binder, and a first liquid component is prepared. The viscosity of the first slurry is 200 to 4000 mPa · s (200 to 4000 cP), more preferably 400 to 2200 mPa · s (400 to 2200 cP) at 20 ° C., in terms of excellent workability and mass productivity. preferable. As a viscosity measuring device, a B-type viscometer (DIGITAL VISMETRON VDH-W) and No. 5 rotor of Shibaura System Co., Ltd. are used, and measurement is performed at a rotational speed of 100 rpm. The method for preparing the first slurry is not particularly limited. For example, the conductive carbon particles, the solution, dispersion or emulsion containing the first binder, and the first liquid component may be mixed using various mixing devices. .

  What is necessary is just to select suitably for a 1st liquid component according to the kind of 1st binder. For example, when an emulsion in which a fluorinated acrylate resin that is an acrylate resin is dispersed in water and CMC are used in combination, the first liquid component is water. In the case of using an emulsion or dispersion of a synthetic rubber such as polytetrafluoroethylene or SBR instead of the acrylate resin emulsion, water can be used as the first liquid component. Even when an aqueous solution in which a cellulose resin such as ethyl cellulose is dissolved in water is used, the first liquid component is water. On the other hand, when using polyvinylidene fluoride, it is preferable to use an organic component such as N-methyl-2-pyrrolidone (NMP). More specifically, the first slurry can be obtained by mixing a solution, emulsion or dispersion containing the first binder and conductive carbon particles, and adding and mixing a predetermined liquid component as necessary. It is done. A compound having a triazine ring may be added to the first slurry.

Next, the first slurry is applied to the surface of the current collector to form a conductive coating film.
When forming a conductive coating on one main surface of the current collector, use various coaters such as a slot die coater, a roll coater, and a cam coater as the coating device. Can do. On the other hand, in the case where a conductive coating film is simultaneously formed on both main surfaces of the current collector, for example, it is preferable to use a dip coater. Dip coaters are inexpensive and have high electrode production efficiency.

Step (ii)
Next, the conductive coating film is heated by radiant heat to volatilize the first liquid component from the conductive coating film. In that case, it is preferable not to completely dry the conductive coating, but to stop the drying in a state where the conductive coating contains a certain amount of the first liquid component. When radiant heat is used, the liquid component is preferentially reduced from the inside of the conductive coating film, so that the liquid component tends to remain near the surface of the conductive coating film. In such a state, by forming an active material layer on the surface of the conductive layer, mutual diffusion easily occurs between the conductive layer and the active material layer.

  Drying is preferably stopped when the conductive coating is in a tacky dry state. The finger touch dry state means a dry state to the extent that the surface of the conductive coating does not collapse even when touched with a finger. In such a state, it is considered that the first liquid component is distributed more on the surface side than on the current collector side of the conductive coating film.

  On the other hand, in conventional drying with hot air or warm air (convection heat), liquid components are preferentially reduced from the vicinity of the surface of the conductive coating film. Therefore, mutual diffusion hardly occurs between the conductive layer and the active material layer. When radiant heat is used in this step, the electrode resistance can be reduced to ½ to 1/10 compared to when warm air is used.

  As the radiant heat, it is preferable to use thermal energy of infrared rays (0.7 to 1000 μm), particularly far infrared rays (for example, wavelength 4 to 1000 μm). For example, as a heat source that radiates far infrared rays, an electric type, a heat medium type, a gas type, or the like can be used. The temperature of the conductive coating film during drying is preferably controlled at 130 to 180 ° C., and the drying atmosphere is an atmosphere such as nitrogen or air. The temperature or radiation temperature of the coating film can be measured with a globe thermometer.

  However, if the first liquid component is an organic component such as NMP, there is a possibility of ignition, so not only radiant heat such as far-infrared rays but also hot air of about 150 ° C. (fresh air or nitrogen) is dried. It is preferable to blow to the atmosphere. On the other hand, when the first liquid component is water, the conductive coating film can be easily dried using radiant heat.

Step (iii)
A second slurry containing active material particles, a second binder, and a second liquid component is prepared. Regarding the viscosity of the second slurry, at 20 ° C., it is 800 to 4000 mPa · s (800 to 4000 cP), more preferably 1500 to 2200 mPa · s (1500 to 2200 cP), which is excellent in workability and mass productivity. Is preferable. Viscosity can be measured in the same manner as the first slurry. The method for preparing the second slurry is not particularly limited, and for example, the active material particles, the solution, dispersion or emulsion containing the second binder, and the second liquid component may be mixed using various mixing devices. The second liquid component may be selected as appropriate according to the type of the second binder. A compound having a triazine ring may be added to the second slurry.

  Next, the second slurry is applied to the surface of the conductive coating film to form an active material coating film. In that case, it is preferable to apply | coat a 2nd slurry to the surface of the electrically conductive coating film of a touch dry state as mentioned above. When a conductive coating film is formed only on one main surface of the current collector, for example, a slot die coater, a roll coater, a cam coater or the like is used as a coating apparatus. Moreover, when the conductive coating film is formed in both the main surfaces of an electrical power collector, it is preferable to apply | coat a 2nd slurry simultaneously on the surface of both conductive coating films, for example using a dip coater.

Step (iv)
The laminate of the conductive coating film and the active material coating film obtained in the above steps (i) to (iii) is dried by at least one of radiant heat and hot air. Also at this time, it is preferable in terms of drying efficiency to dry by radiant heat as much as possible. As described above, in drying by warm air convection, drying proceeds from the surface of the coating film. On the other hand, when radiant heat is used, the temperature rises from the inside of the coating film, thereby increasing the drying efficiency. When drying proceeds from the surface of the coating film, moisture inside the coating film becomes difficult to escape. In addition, when the moisture is removed, the coating film is likely to crack. Further, when drying proceeds from the surface of the coating film, a surface higel layer is formed, and thus the resistance increases.

  In the drying in this step, it is preferable that the first liquid component and the second liquid component remaining from the laminate are volatilized almost completely to complete the electrode. When the liquid component remains in the electrode, the liquid component is brought into the electrochemical device, and the performance of the electrochemical device may be deteriorated. The temperature of the laminate during drying is preferably controlled to 130 to 180 ° C., and the drying atmosphere is not particularly limited, but an air atmosphere is preferable.

  In order to increase the stability of the first and second slurries, stabilizers may be added to these slurries. For example, when the slurry contains acetylene black or ketine black, the addition of a stabilizer is effective because the life of the slurry is likely to decrease. Moreover, since the 1st slurry for forming a conductive layer contains graphite (large particle group) with a large particle size, it is easy to gelatinize and the application | coating to the surface of a collector may become difficult. In such a case, addition of a stabilizer is effective.

As the stabilizer for the slurry, a polymer having an isoprene sulfonic acid group is preferable. The sulfonic acid group may form a sodium salt, and may have a structure such as —CH (SO ₃ Na) —CH (CH ₃ ) —CH—CH ₂ —. The bonding position of the sulfonic acid group is not particularly limited. Since the isoprene sulfonic acid group has both a hydrophobic group and a hydrophilic group, it is highly effective as a surfactant for dispersing the constituent components of the slurry and is excellent in compatibility with a binder component and the like. For example, a polymer having a structure in which isoprene sulfonic acid groups and hydrophobic groups (for example, styrene groups) are arranged in a block form in one molecular chain is preferable in terms of high dispersion stability.

  The addition amount of the stabilizer to the first and second slurries is preferably 3 to 7% by weight with respect to the total amount of solids in the slurry. If it is less than 3% by weight, the effect is small, and if it exceeds 7% by weight, the electrode production cost becomes high. It is considered that the stabilizer having an isoprene sulfonic acid group facilitates formation of an interdiffusion layer and contributes to lowering the resistance of the electrode.

Next, a series of steps in the method for producing an electrode according to the present invention will be exemplified.
[First form]
First, the conductive coating film A is formed on one main surface of the current collector and heated with far infrared rays to dry the conductive coating film A to a dry-to-touch state. Next, the active material coating film A is formed on the surface of the conductive coating film A, and the whole is heated and completely dried with far infrared rays. Thereafter, the current collector is inverted, a conductive coating film B is formed on the other main surface of the current collector, and the conductive coating film B is dried to the touch dry state by heating with far infrared rays. Next, the active material coating film B is formed on the surface of the conductive coating film B, and the whole is heated and completely dried with far infrared rays. In this method, the predetermined process is repeated for each side of the current collector.

[Second form]
First, conductive coatings A and B are formed on both main surfaces of the current collector at the same time using a dip coater or sequentially using a slot die coater, roll coater, cam coater, etc., and heated with far infrared rays. Then, the conductive coating films A and B are dried to the dry touch state. Next, simultaneously or sequentially, the active material coatings A and B are formed on the surfaces of the conductive coatings A and B, and the whole is heated and completely dried with far infrared rays. This method is advantageous in that the conductive coating films A and B can be dried simultaneously and the active material coating films A and B can be dried simultaneously.

  In addition, when drying by hot air convection is performed instead of drying by far infrared rays, drying takes a long time, and a desired interdiffusion layer cannot be formed between the conductive layer and the active material layer. .

Next, an electric double layer capacitor will be described as an example of an electrochemical element including an electrode according to the present invention.
The electric double layer capacitor includes an electrode group including an electrode containing activated carbon as the active material and a porous separator interposed between the electrodes. The electrode group may be a laminated type or a wound type. In the case of a laminated type, a plurality of electrodes having active material layers on both sides are stacked with separators interposed between the electrodes, and a pair of electrodes having an active material layer on one side is disposed on the outermost side of the laminated body. Place with the outside. On the other hand, in the case of a wound type, a pair of strip-shaped electrodes are prepared, and a separator is interposed between them, and the winding is performed along the longitudinal direction of the electrodes. Then, the obtained electrode group is accommodated in a predetermined case, and the electrode group is impregnated with a nonaqueous electrolytic solution. As a result, the nonaqueous electrolyte is impregnated in the pores of the electrode and the separator.

The nonaqueous electrolytic solution is composed of a nonaqueous solvent in which a supporting salt is dissolved. As the supporting salt, for example, a quaternary ammonium borofluoride salt (for example, tetraethylammonium borofluoride, triethylmethylammonium borofluoride) or the like is used. The non-aqueous solvent includes a cyclic carbonate which may have a fluorine atom, a chain carbonate which may have a fluorine atom (for example, propylene carbonate), and a chain ether which may have a fluorine atom. , A cyclic ether optionally having a fluorine atom, a lactone optionally having a fluorine atom, a sulfolane derivative optionally having a fluorine atom, a non-fluorine ester solvent not having a fluorine atom, Nitrile solvents, furans, oxolanes and the like can be mentioned. Among these, it is preferable to use a non-aqueous solvent containing a fluorinated alkyl ether in order to obtain a capacitor having excellent withstand voltage characteristics. The fluorinated alkyl ether is a general term for compounds in which at least a part of hydrogen atoms of a dialkyl ether is substituted with a fluorine atom. Specific examples include HCF ₂ CH ₂ —O—CF ₂ CF ₂ H, CF ₃ CF ₂ CH ₂ —O—CF ₂ CF ₂ H, and the like.

  Although the ratio of the fluorinated alkyl ether in the entire non-aqueous solvent depends on the device to which the capacitor is applied, it is preferably 5% by weight or more, and more preferably 10-30% by weight. If it is less than 5% by weight, the effect of improving the withstand voltage characteristics is small, and if it exceeds 30% by weight, the cost tends to increase. In addition, internal resistance may increase or gas generation may become apparent.

  By using the above electrode and non-aqueous electrolyte, for example, an electric double layer capacitor having a rated voltage of 2.1 V or more or exceeding 2.5 V, and further having a rated voltage of 3 V or more and excellent in withstand voltage characteristics can be obtained. Is possible. However, if the applied voltage exceeds 3.5 V, the first or second binder may be decomposed depending on the type and cause gas generation. Therefore, the rated voltage is preferably 3.5 V or less. On the other hand, by using a binder having high withstand voltage characteristics, it is possible to achieve withstand voltage characteristics of about 4.7V.

  Next, the present invention will be described more specifically based on examples, but the present invention is not limited to the following examples.

Example 1
EDLC electrodes having various conductive layer and active material layer compositions were prepared and evaluated.
(A) First Slurry According to the composition of the conductive layer shown in Table 1, the materials were mixed with a mixer to prepare a first slurry having a viscosity at 20 ° C. of 1000 mPa · s.

Details of the materials are shown below.
Graphite (small particle group): Volume particle size distribution has a peak at 6 μm (average particle size 6 μm), 90% by volume or more is a particle particle size of 3-7 μm Graphite (large particle group): Volume particle size distribution has a peak at 18 μm (Average particle size 18 μm), 90% by volume or more particles having a particle size of 10 to 20 μm Slurry stabilizer: water-soluble polymer having isoprene sulfonic acid groups

First binder FA: Fluorinated acrylate emulsion SBR: Styrene butadiene latex CMC: Carboxymethylcellulose PI: Solvent-type polyimide PTFE: Polytetrafluoroethylene PVDF: Polyvinylidene fluoride PAI: Polyamideimide EC: Ethylcellulose

(B) Second Slurry According to the composition of the active material layer shown in Table 2, the materials were mixed with a mixer to prepare a second slurry having a viscosity at 20 ° C. of 1500 mPa · s.

Details of the materials are shown below.
Second binder TA: triazine The details of the other second binder are the same as those of the first binder.
Active material: activated carbon, average particle size 8 μm
AB: Acetylene black, primary particle size 0.03 μm
KB: Kettin black, primary particle size 0.03 μm

  The first slurry was applied to one main surface of an aluminum foil having a thickness of 20 μm to form a conductive coating film having a thickness of 35 μm, and first drying was performed. Then, the 2nd slurry was apply | coated to the surface of a conductive coating film, the active material coating film of thickness 130 micrometers was formed, and 2nd drying was performed. Then, the 1st slurry was apply | coated to the other main surface of aluminum foil, the 35-micrometer-thick conductive coating film was formed, and 1st drying was performed. Then, the 2nd slurry was apply | coated to the surface of a conductive coating film, the active material coating film of thickness 130 micrometers was formed, and 2nd drying was performed. The first drying and the second drying were performed in the same manner, and radiation or convection drying was performed as described below.

Radiation: The coating film is heated at 130 ° C. for 1 minute by far infrared radiation. Convection: The coating film is heated for 5 minutes by warm air at 130 ° C. The wavelength of the far infrared radiation is 4 to 1000 μm. An electric heater was used.
When a thermosetting resin was used for the binder (sample numbers 11 and 15), a heat treatment was performed at 200 ° C. for 30 minutes as a curing process in the final stage of drying.

[Evaluation]
(I) Presence / absence of interdiffusion layer An SEM photograph of a cross section perpendicular to the surface direction of the current collector of the electrode was taken to confirm the presence / absence of the interdiffusion layer. Specifically, the maximum height Rmax of the roughness of the interface between the conductive layer and the active material layer was determined.
“○” when the maximum height Rmax is 13 μm or more,
When the maximum height Rmax is 10 μm or more and less than 13 μm, “△”,
When the maximum height Rmax is less than 10 μm, “x” is shown in Table 3.

(Ii) Adhesiveness A tape was affixed to the surface of the active material layer, a spring balance was attached to one end thereof, and the active material layer was pulled at a speed of 2 mm per second. The maximum strength reading at the time of peeling was defined as the adhesion strength. The same measurement was performed three times (n = 3), and an average was taken.
“230” for 230 gf / cm or more,
“△” for 180 gf / cm or more and less than 230 gf / cm,
In the case of less than 180 gf / cm, “x” is shown in Table 3.

(Iii) Surface crack The appearance of the electrode after the second drying was observed.
"Yes" if a crack is observed,
When no cracks were observed, “No” was shown in Table 3.

(Iv) Impregnation of electrolyte solution TEABF ₄ (tetraethylammonium borofluoride) was dissolved in propylene carbonate (PC) at a concentration of 1 mol / L to prepare a nonaqueous electrolyte solution. 1 μmL of the obtained electrolytic solution was dropped on the electrode, and the time until it was visually confirmed that the liquid was impregnated was measured.
If the electrolyte is completely impregnated within 5 seconds,
When the electrolyte was completely impregnated within 15 seconds for more than 5 seconds, “△” is shown in Table 3.

(V) ESR at 3V
18 mmφ50L type 50F wound type EDLC was prepared using each electrode, and ESR was measured under the conditions of a frequency of 1 kHz and a current of 1 mA using an Agilent LCR meter “E4284A”. The results are shown in Table 3.

(Vi) Withstand voltage characteristics A voltage of 3 V was continuously applied to the EDLC used above at 70 ° C., and after 300 hours, the presence or absence of abnormalities such as EDLC swelling and leakage was examined.
If there is no blistering or leakage due to gas generation,
If a slight swelling is observed, “△”,
When blisters or liquid leakage was clearly observed, “x” was shown in Table 3.

  2a, 2b, and 2c are examples of cross-sectional SEM photographs of the electrode of sample number 8 according to the comparative example, sample number 12 according to the comparative example, and sample number 5 according to the example, respectively. In FIG. 2b, it can be seen that the interdiffusion layer is not formed between the conductive layer and the active material layer, whereas the interdiffusion layer is formed in FIG. 2c.

Next, taking the sample shown in FIG. 2c as an example, how to obtain the maximum height Rmax will be described in detail.
[Measurement of maximum height Rmax]
First, an SEM photograph of a cross section perpendicular to the surface direction of the current collector of the electrode is taken. The magnification is 1000 times and an image is taken with a digital camera. At that time, a scale reference line (10 μm) is displayed in the same image (FIG. 2c). In the cross section, the boundary between the conductive carbon particles and the active material particles is observed. This image is taken into a personal computer, and a roughness curve (curve (a)) is drawn at the boundary between the conductive carbon particles and the active material particles by image processing or the like.

  Next, the length (for example, xcm) of the scale reference line (10 μm) on the image is measured, and a standard line (b) having a length L (L = 8 × cm) equivalent to 80 μm is drawn on the two images. These two standard lines are arranged in parallel to the surface direction of the current collector, and are moved so as to be in contact with the top and bottom of the curve (a) (see FIG. 3). The obtained lines are called the summit line and the valley bottom line, respectively. A perpendicular line is drawn from the summit line to the bottom line, and the length T of the distance between the summit line and the bottom line is obtained (see FIG. 4). The length T is converted into an actual length using a scale reference line, and is set as the maximum height Rmax-n.

  The above operation is carried out for one SEM photograph at two locations, the right reference and the left reference (N = 2), and the average value of Rmax-1 and Rmax-2 is Rmax-12. In the case of the right reference, the right end of the standard line of length L is matched with the right end of the SEM photograph, and the above operation is performed. In the case of the left reference, the left end of the standard line of length L is matched with the left end of the SEM photograph. Perform the above operation. The same operation is repeated for three SEM photographs, and an average value of three Rmax-12 is obtained (N = 6), and is set as Rmax.

  Table 4 shows the data of the maximum height Rmax obtained for sample numbers 1, 2, 5 and 6. Moreover, the data of the maximum height Rmax of the electrodes (sample numbers 1-2 and 6-2) produced in the same manner as the sample numbers 1 and 6 except that the convection method is used instead of the radiation method, and further small particles Data on the maximum height Rmax of the electrode (sample number 19) produced in the same manner as in sample numbers 1, 2, 5 and 6 is also shown except that the weight ratio of the group / large particle group is 5/95.

[Discussion]
When the coating film was dried by radiant heat, all the evaluation results were good, but when it was dried with warm air, the interdiffusion layer was not formed, and any evaluation result was insufficient.

  Compared with the case where only the small particle group or only the large particle group of the graphite particles of the conductive layer is used, when these are used in combination at a predetermined ratio, the formation of the interdiffusion layer proceeds and the 3V withstand voltage characteristics are also improved. This is considered to be due to the improved adhesion between the conductive layer and the active material layer.

  In the electrode (No. 8) of the comparative example in which the conductive layer was not formed, the resistance was increased and the withstand voltage characteristics were greatly deteriorated. From this, it is recognized that the conductive layer greatly contributes to the reduction of the electrode resistance and the improvement of the withstand voltage characteristics.

  Particularly good characteristics were obtained when an acrylate resin or a cellulose resin was used as the binder. In particular, when a cellulose resin was used, the slurry stability was excellent and the film formability was good. When SBR was used, the 3V withstand voltage characteristic was slightly lowered, but the AC resistance (ESR) was smaller than when fluorinated acrylate, PTFE or PVDF was used. When fluororesins such as PTFE and PVDF were used, the impregnation property of the electrolyte solution was slightly lowered, and the amount of binder for ensuring sufficient adhesion was relatively increased.

  Particularly good characteristics were obtained when an acrylate resin, polyimide, polyamideimide or cellulose resin was used as the binder. The cellulose resin was particularly effective in improving the adhesion between the current collector and the coating film.

  When triazine was added as an auxiliary agent for the binder, the adhesiveness was further improved, and in particular, the voltage resistance characteristics were significantly improved.

  In the above embodiment, EDLC has been described. However, since the present invention can also be manufactured for pseudocapacitors and secondary battery electrodes by the same method as that for EDLC electrodes, the same effect can be expected when applied to these. .

Since the electrode for an electrochemical element according to the present invention has a low resistance and is excellent in a large current characteristic and a withstand voltage characteristic, it can be applied to various applications. Especially, it is effective in a large-sized electrochemical device applied to a large-scale industrial machine such as a construction machine such as an automobile or a crane.
According to the present invention, an electrode having a high withstand voltage characteristic of 2.5 V or higher can be obtained. However, the present invention is naturally applied to an electrochemical element that requires a maintained voltage (CCV) of 2.1 V or higher. Applicable to. An example of such an electrochemical element is a large electric double layer capacitor that can be charged and discharged with a large current of 100 A or more.

DESCRIPTION OF SYMBOLS 1 Current collector 2 Active material layer 3 Conductive layer 4 Interdiffusion layer

Claims (12)

A current collector,
A conductive layer formed on a surface of the current collector and including conductive carbon particles and a first binder;
An active material layer formed on the surface of the conductive layer and including active material particles and a second binder,
The conductive carbon particles include a small particle group and a large particle group,
The volume particle size distribution peak of the small particle group is in the range of 3 to 7 μm,
The volume particle size distribution peak of the large particle group is in the range of 10 to 20 μm,
The weight ratio between the small particle group and the large particle group (small particle group / large particle group) is 95/5 to 50/50,
The electrode for an electrochemical element, wherein an interface between the conductive layer and the active material layer has a roughness with a maximum height Rmax of 10 μm or more.   Each of the first binder and the second binder includes a thermoplastic resin that does not have a carbon-carbon double bond and has a melting point of 160 ° C. or higher, or a thermosetting resin cured product having a Tg of 260 ° C. or higher. The electrode for an electrochemical element according to claim 1.   2. The electrode for an electrochemical element according to claim 1, wherein each of the first binder and the second binder includes at least one selected from the group consisting of an olefin resin, a synthetic rubber, and a fluororesin.   The amount of the first binder is 3 to 6 parts by weight with respect to 100 parts by weight of the conductive carbon particles, and the amount of the second binder is 3 to 6 parts by weight with respect to 100 parts by weight of the active material particles. The electrode for an electrochemical element according to any one of claims 1 to 3, wherein   5. The electrode for an electrochemical element according to claim 1, wherein at least one of the conductive layer and the active material layer contains 0.5 to 3 wt% of a compound having a triazine ring.   The electrode for an electrochemical element according to any one of claims 1 to 5, wherein the conductive carbon particles are a graphite material.   The electrode for an electrochemical element according to any one of claims 1 to 6, wherein the active material particles are activated carbon, graphite, silicon, a silicon compound, a transition metal compound, or a hydrogen storage alloy. (I) applying a first slurry containing conductive carbon particles, a first binder, and a first liquid component to the surface of the current collector to form a conductive coating film;
(Ii) heating the conductive coating film by radiant heat to volatilize the first liquid component from the conductive coating film;
(Iii) After the step (ii), a second slurry containing active material particles, a second binder, and a second liquid component is applied to the surface of the conductive coating film to form an active material coating film. The process of
(Iv) drying the laminate of the conductive coating film and the active material coating film by at least one of radiant heat and hot air,
The conductive carbon particles include a small particle group and a large particle group,
The volume particle size distribution peak of the small particle group is in the range of 3 to 7 μm,
The volume particle size distribution peak of the large particle group is in the range of 10 to 20 μm,
The manufacturing method of the electrode for electrochemical elements whose weight ratio (small particle group / large particle group) of the said small particle group and the said large particle group is 95 / 5-50 / 50. In the step (ii), the dry state of the conductive coating film is a dry touch state,
The method for producing an electrode for an electrochemical element according to claim 8, wherein in the step (iii), the second slurry is applied to the surface of the conductive coating film in the dry-to-touch state.   A solution, dispersion or liquid containing a thermoplastic resin having no carbon-carbon double bond and a melting point of 160 ° C. or higher, or a thermosetting resin having a Tg of 260 ° C. or higher as the first binder or the second binder The method for producing an electrode for an electrochemical element according to claim 8 or 9, wherein the emulsion is mixed with the conductive carbon particles or the active material particles to prepare the first slurry or the second slurry.   A solution, dispersion or emulsion containing at least one selected from the group consisting of an olefin resin, a rubbery polymer and a fluororesin is mixed with the conductive carbon particles or the active material particles, and the first The method for producing an electrode for an electrochemical element according to claim 8 or 9, wherein the slurry or the second slurry is prepared.   The method for producing an electrode for an electrochemical element according to any one of claims 8 to 11, wherein a stabilizer having an isoprene sulfonic acid group is added to the first slurry and the second slurry.