Electrode and capacitor

Abstract

To provide an electrode and a capacitor capable of reducing a decrease in capacitance in a high-temperature environment.SOLUTION: In the capacitor 10, the electrode (positive electrode) 11 includes a mixture layer 13 containing activated carbon and a current collector layer 12 attached to the mixture layer. The mixture layer contains metal elements (excluding alkali metals) other than the elements contained in the current collecting layer, and the mass concentrations of the metal elements are 1ppm or more. As an example of the implementation procedure, it was confirmed that a small amount of Zr was contained in the positive electrode by charging and discharging using the supernatant obtained by mixing the electrolytic solution and the powder of the solid electrolytic Li7La3Zr2O12 as the electrolytic solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to electrodes and capacitors that include activated carbon. [Background technology]

[0002] Patent Document 1 discloses a prior art technique in which the mesopore volume and functional groups of activated carbon contained in a capacitor electrode are set within specific ranges in order to maintain the capacitance of the capacitor in a low-temperature environment of about -30°C to -40°C. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-141168 Summary of the Invention [Problem to be solved by the invention]

[0004] In prior art, the decomposition of the electrolyte contained in the capacitor by activated carbon is accelerated in a high-temperature environment of around 85°C, which causes the problem of the capacitance of the capacitor decreasing when the pores of the activated carbon become clogged with the decomposition products.

[0005] The present invention has been made to solve this problem, and has an object to provide an electrode and a capacitor that can reduce the decrease in capacitance in a high-temperature environment. [Means for solving the problem]

[0006] A first aspect for achieving this object is an electrode comprising a composite layer containing activated carbon and a current collecting layer adhered to the composite layer, wherein the composite layer contains metal elements (excluding alkali metals) other than the elements contained in the current collecting layer, and the mass concentration of the metal elements is 1 ppm or more.

[0007] In the second embodiment, in the first embodiment, the metal element is one or more selected from Zr, La, Ti, and Ge.

[0008] In a third embodiment, in the first embodiment, the metal element is at least one of Zr and La.

[0009] A fourth embodiment is a capacitor, which includes the electrode of any one of the first to third embodiments. [Effects of the Invention]

[0010] According to the present invention, the composite layer containing activated carbon contains a metal element (excluding alkali metals) other than the elements contained in the current collecting layer at a mass concentration of 1 ppm or more. The metal element reduces decomposition of the electrolyte by the activated carbon and reduces the generation of decomposition products, thereby reducing the decrease in capacitance in high-temperature environments. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view of a capacitor according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of a capacitor 10 according to one embodiment. Capacitor 10 includes, in order, a positive electrode 11, a separator 14, and a negative electrode 15. Charge carriers in capacitor 10 include alkali metal ions such as lithium ions, sodium ions, and potassium ions.

[0013] 1 shows a capacitor 10 having one set of positive electrode 11, separator 14, and negative electrode 15, but the present invention is not limited to this and may include multiple sets of these. Capacitor 10 is not limited to a laminated structure, but may also have a wound structure in which positive electrode 11 and negative electrode 15 are laminated and wound with separator 14 interposed therebetween.

[0014] The capacitor 10 is exemplified by an electrochemical capacitor such as a lithium ion capacitor. Examples of electrochemical capacitors include electric double layer capacitors, redox capacitors that utilize redox reactions of electrodes or redox reactions of ions in a non-aqueous electrolyte, and hybrid capacitors that combine electric double layers and redox reactions, or that combine them with secondary battery materials.

[0015] The positive electrode 11 includes a current collecting layer 12 and a composite layer 13 attached to the current collecting layer 12. There are no particular limitations on the material of the current collecting layer 12, and examples include Cu, Al, a Cu alloy, an Al alloy, and stainless steel. There are also no particular limitations on the shape of the current collecting layer 12. The current collecting layer 12 may be a porous foil having a plurality of holes penetrating the current collecting layer 12.

[0016] The composite layer 13 contains activated carbon. Activated carbon obtained by carbonizing and activating a mineral-based, plant-based, or resin-based carbonaceous material is preferably used. Examples of mineral-based carbonaceous materials include coal (lignite, brown coal, bituminous coal, anthracite, etc.), cokes, infusibilized pitch, and oil carbon. Examples of plant-based carbonaceous materials include charcoal, coconut shells, sawdust, wood chips, and grass peat. Examples of resin-based carbonaceous materials include phenolic resin.

[0017] Mixture layer 13 may contain an active material other than activated carbon. Examples of the active material other than activated carbon include graphite, low-crystalline carbon, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), and carbon black.

[0018] Composite layer 13 contains a conductive additive. The conductive additive electrically connects the active materials so that the active materials are not isolated in composite layer 13. Examples of the conductive additive include graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, carbon black, carbon nanotubes, and carbon fibers.

[0019] The composite layer 13 may contain a binder and a thickener. The binder binds the active material and the conductive additive. Examples of the binder include fluororesin, acrylic resin, polyolefin, and rubber. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Examples of the acrylic resin include polyacrylic acid, polyacrylonitrile, and polyacrylamide. Examples of the polyacrylic acid include polymethacrylic acid ester, polyacrylic acid ester, and sodium polyacrylate. Examples of the polyolefin include polyethylene and polypropylene. Examples of the rubber include styrene-butadiene rubber. Examples of the thickener include carboxymethyl cellulose.

[0020] The composite layer 13 contains metal elements other than those contained in the current collecting layer 12. The metal elements are elements excluding alkali metals in Group 1 of the periodic table based on the IUPAC 1990 Recommendations, elements in Group 18, actinides, and non-metallic elements. The alkali metals are excluded in order to exclude metal elements that are charge carriers. The elements contained in the current collecting layer 12 are excluded in order to exclude elements that are formed when the current collecting layer 12 is decomposed and adheres to the composite layer 13.

[0021] The mass concentration of the metal elements contained in mixture layer 13 is preferably 1 ppm or more and 100 ppm or less. If the mass concentration of the metal elements (excluding alkali metals) contained in mixture layer 13 becomes too high, the metal elements become resistance components, which reduces the initial characteristics of capacitor 10.

[0022] The mass concentration of the metal element is determined by preparing a sample solution by decomposing the composite layer 13 with an acid, and then subjecting the sample solution to optical emission spectroscopy using a high-frequency inductively coupled plasma (ICP) as a light source. The acid used to decompose the composite layer 13 may be hydrochloric acid or dilute sulfuric acid, which decomposes metal elements with a higher ionization tendency than hydrogen, or nitric acid, hot sulfuric acid, or aqua regia, which decomposes metal elements with a lower ionization tendency than hydrogen.

[0023] The organic matter contained in the composite layer 13 can be decomposed by a wet acid decomposition method, and then nitric acid is added to remove carbon monoxide and carbon dioxide from the sample solution. If there are any undecomposed substances in the sample solution, they are removed by filtration, and then ICP atomic emission spectrometry is performed.

[0024] At least a portion of the metal elements contained in mixture layer 13 are present on the surface of the activated carbon. The metal elements are elements that constitute the inorganic coating, and it is presumed that the inorganic coating containing the metal elements is attached to the surface of the activated carbon.

[0025] Negative electrode 15 is formed by stacking current collecting layer 16 and composite layer 17. Examples of the material for current collecting layer 16 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, and stainless steel.

[0026] The composite layer 17 contains an active material and a conductive additive. Examples of the active material include carbon-based materials such as graphite, low-crystalline carbon, graphitizable carbon, and non-graphitizable carbon, and silicon-containing materials. Examples of the silicon-containing materials include Si, Si-Li alloys, and compounds containing Si and O as constituent elements (hereinafter referred to as "SiO x " where 0.5≦X≦1.5 is an example.

[0027] SiO x Silicon oxide (SiO) is an oxide of silicon, and is exemplified by those having a structure in which microcrystalline or amorphous silicon is dispersed in an amorphous SiO matrix. x Since SiO has poor conductivity, x A conductive layer is provided to cover the surface of SiO x It is preferable to combine SiO with a conductive material. x Granulation is an example of a means for combining the active material with the conductive material. Examples of materials for the conductive layer include metals such as Pt and Os, and carbon. Examples of conductive materials include graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, carbon black, carbon nanotubes, and carbon fibers. The proportion of silicon in the active material is preferably 10% by mass or more and 50% by mass or less.

[0028] The separator 14 is made of a porous material that is durable against the active materials and electrolyte solution contained in the positive electrode 11 and the negative electrode 15, and that is non-electron-conductive but allows alkali metal ions to pass through. Examples of the separator 14 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0029] The electrolyte solution is a solution of an electrolyte dissolved in a solvent. The electrolyte is a compound used to transfer charge carriers between the positive electrode 11 and the negative electrode 15, and is exemplified by an alkali metal salt. The anion of the alkali metal salt is a halide ion (I - , Cl - , Br - etc.), SCN - , BF4 - , BF3(CF3) - , BF3(C2F5) - , PF6 - , ClO4 - , SbF6 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , B(C6H5)4 - , B(O2C2H4)2 - , C(SO2F)3 - , C(SO2CF3)3 - , CF3COO - , CF3SO2O - , C6F5SO2O - , B(O2C2O2)2 - is exemplified.

[0030] The solvent for the electrolyte solution is not particularly limited as long as it is liquid in the temperature range in which the capacitor 10 is used. Examples of the solvent include carbonate esters, aliphatic carboxylic acid esters, phosphate esters, γ-lactones, ethers, nitriles, sulfolane, dimethyl sulfoxide, fluorous solvents, and ionic liquids. Mixtures of these solvents are also acceptable.

[0031] Capacitor 10 is manufactured, for example, as follows. A slurry is prepared by dispersing activated carbon and a conductive additive in a solution in which a binder is dissolved. The prepared slurry is applied to current collecting layer 12, and then the slurry is dried to obtain positive electrode 11 in which composite layer 13 is formed on current collecting layer 12. After drying, composite layer 13 may be rolled using a roller or the like.

[0032] Next, a slurry is prepared by dispersing an active material and a conductive additive in a solution containing a binder. The slurry is applied onto current collecting layer 16 and then dried to obtain negative electrode 15 in which composite layer 17 is formed on current collecting layer 16.

[0033] Next, an alkali metal is supplied to the negative electrode 15, and the negative electrode 15 is pre-doped with alkali metal ions in an amount corresponding to at least a portion of the irreversible capacity. Pre-doping can be exemplified by a method in which the current collecting layer 16 and the alkali metal are short-circuited with a separator disposed between the mixture layer 17 and the alkali metal, and the negative electrode 15, the separator, and the alkali metal are immersed in an electrolyte. The potential difference between the current collecting layer 16 and the alkali metal causes electrons to flow from the alkali metal to the current collecting layer 16, and at the same time, the alkali metal is ionized and released into the electrolyte, and the alkali metal ions in the electrolyte are supported by the mixture layer 17.

[0034] Pre-doping with alkali metal ions results in negative electrode 15 in which some of the atoms constituting mixture layer 17 are replaced with alkali metal atoms. During pre-doping, a current may be passed between current collecting layer 16 and the alkali metal.

[0035] After cutting the positive electrode 11, separator 14, and negative electrode 15 into predetermined shapes, the positive electrode 11, separator 14, and negative electrode 15 are stacked in this order, terminals (not shown) are connected to the current collecting layers 12 and 16, and the resultant is sealed in a case (not shown) together with an electrolyte to obtain a capacitor 10.

[0036] Examples of means for incorporating a metal element into composite layer 13 include a procedure for sealing an electrolytic solution (non-aqueous electrolytic solution) containing the metal element into a case, and a procedure for producing composite layer 13 and then contacting a non-aqueous solvent containing the metal element with composite layer 13 and drying the resulting mixture. The non-aqueous electrolytic solution is an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent.

[0037] The method for producing a metal element-containing electrolytic solution (nonaqueous electrolytic solution) or a metal element-containing nonaqueous solvent includes a preparation step of preparing a nonaqueous solvent or a nonaqueous electrolytic solution (hereinafter referred to as "nonaqueous solvent, etc.") and a contact step of contacting a solid electrolyte containing an alkali metal with the nonaqueous solvent, etc. The solid electrolyte is Li7La3Zr2O having a garnet structure. 12 , La with perovskite structure 2 / 3-x Li x TiO3, Li with NASICON type structure 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x When the non-aqueous solvent or the like comes into contact with the solid electrolyte in the contacting step, the non-aqueous solvent or the like (e.g., residual moisture) reacts with the solid electrolyte, and the metal elements of the solid electrolyte become cations and dissolve in the non-aqueous solvent or the like.

[0038] In the contact step, a non-aqueous solvent or the like and a solid electrolyte (particles) are placed in a container and stirred, and then the solid electrolyte is separated from the non-aqueous solvent or the like. Examples of means for separating the solid electrolyte from the non-aqueous electrolytic solution include filtration, sedimentation, and centrifugation. The non-aqueous solvent or the like may be passed through a column in which a solid electrolyte is placed or a filter holding a solid electrolyte, thereby simultaneously contacting and separating the non-aqueous solvent or the like with the solid electrolyte.

[0039] The metal element contained in the composite layer 13 is preferably one or more selected from Zr, La, Ti, and Ge. This is because the metal element and oxygen bond easily to form an inorganic coating (oxide coating) on ​​the surface of the activated carbon. The metal element is particularly preferably at least one of Zr and La. This is because the electronegativity of Zr and La is smaller than that of Ti and Ge, and therefore the difference between the electronegativity of Zr and La and the electronegativity of oxygen becomes larger, thereby strengthening the ionic bonding properties of the inorganic coating. [Example]

[0040] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0041] (Preparation of electrolyte) An electrolyte solution (non-aqueous electrolyte solution) was prepared by dissolving the electrolyte lithium bis(fluorosulfonyl)imide (LiFSI) to a concentration of 1 mol / L in a non-aqueous solvent made by mixing lithium battery grade ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate in a volume ratio of 3:3:4, and further mixing in 1 wt% vinylene carbonate.

[0042] (Electrolyte solution in Example 1) Electrolyte and solid electrolyte Li7La3Zr2O 12 The powders were placed in a sealed container in an argon atmosphere at a ratio of 4:1 (mass ratio), and after intermittent stirring for 1 hour, the powders were allowed to settle and separated by standing for 24 hours. The supernatant was used as the electrolyte solution in Example 1.

[0043] (Electrolyte solution in Example 2) Li7La3Zr2O 12 Solid electrolyte Li 1.4 Al 0.4 Ti 1.6 The electrolyte solution of Example 2 was obtained in the same manner as in Example 1, except that powder of (PO4)3 was used instead.

[0044] (Electrolyte solution in Example 3) Li7La3Zr2O 12 Solid electrolyte Li1.5 Al 0.5 Ge 1.5 The electrolyte solution of Example 3 was obtained in the same manner as in Example 1, except that powder of (PO4)3 was used instead.

[0045] (Preparation of positive electrode) Activated carbon, binder, and acetylene black (conductive additive) were mixed in a mass ratio of 80:10:10 to prepare a slurry. The slurry was applied to a current collecting layer made of aluminum foil with a thickness of 30 μm, dried, and then punched into a square with a side length of 20 mm to obtain a positive electrode. The amount of the slurry applied was 5.0 mg / cm, which is the total mass of the activated carbon, conductive additive, and binder. 2 It was adjusted so that

[0046] (Preparation of negative electrode) Graphite, binder, thickener, and acetylene black (conductive additive) were mixed in a mass ratio of 93:5:1:1 to prepare a slurry. The slurry was applied to a current collecting layer made of 8 μm thick copper foil, dried, and then punched into a square with a side length of 25 mm to obtain a negative electrode. The amount of slurry applied was 6.5 mg / cm, with the combined mass of graphite, binder, thickener, and conductive additive. 2 It was adjusted so that

[0047] (Pre-doping of negative electrode) A separator and a metallic lithium foil (counter electrode) pressed onto copper foil were attached in that order on top of the negative electrode composite layer to prepare a half-cell. The half-cell was immersed in the electrolyte, then vacuum degassed and sealed. A current was passed between the copper foil of the current collecting layer of the half-cell and the copper foil of the counter electrode. Pre-doping was performed by charging at a constant current of 0.1 C to 0 V at 25 °C, followed by charging at a constant voltage of 0.01 C.

[0048] (Fabrication of Capacitor in Example 1) The half-cell was disassembled, and a separator and a positive electrode were attached in that order on the composite layer of the removed negative electrode to prepare a cell. The electrolyte solution of Example 1 was poured into the cell, and then the cell was degassed under vacuum. A potential of 3.8 V was applied between the positive and negative electrodes to age the cell, and then the cell was sealed under vacuum to obtain the capacitor of Example 1 (lithium ion capacitor).

[0049] (Fabrication of Capacitor in Example 2) A capacitor in Example 2 was obtained in the same manner as in Example 1, except that the electrolyte in Example 2 was injected into the cell instead of the electrolyte in Example 1.

[0050] (Fabrication of Capacitor in Example 3) A capacitor in Example 3 was obtained in the same manner as in Example 1, except that the electrolyte in Example 3 was injected into the cell instead of the electrolyte in Example 1.

[0051] (Fabrication of Capacitor in Comparative Example) A capacitor in the comparative example was obtained in the same manner as in Example 1, except that an electrolyte solution (not in contact with the solid electrolyte) was injected into the cell instead of the electrolyte solution in Example 1.

[0052] (Initial capacity measurement) The capacitor was charged at room temperature at a constant current of 1C up to 3.8V. After 30 minutes of constant voltage charging at 3.8V, the charge was switched to a constant voltage discharge at 1C, and the slope of the discharge curve was calculated. The initial capacity was calculated by dividing the discharge current by the slope of the discharge curve.

[0053] (Capacity measurement and determination after high temperature test) A high-temperature test was conducted in which a capacitor was placed in an 85°C thermostatic chamber and left to stand for 125 hours with a voltage of 3.8V applied between the positive and negative electrodes. After the high-temperature test, the capacitor was cooled to room temperature and charged / discharged at a 1C rate to calculate the capacity. Capacitors whose capacity after the high-temperature test was 90% or more of the initial capacity were rated as A, and those whose capacity after the high-temperature test was less than 90% of the initial capacity were rated as B.

[0054] (ICP optical emission spectroscopy) After the high-temperature test, the capacitor was disassembled, and the removed positive electrode was decomposed in acid to prepare a sample solution. The mass concentration of metal elements (other than Li and Al) contained in the sample solution was measured using ICP atomic emission spectroscopy. The mass concentration (ppm) of the metal elements and the capacity after the high-temperature test are shown in Table 1. The detection limit for Zr, La, Ti, and Ge was 0.5 ppm. In Table 1, ND indicates that the concentration was below the detection limit.

[0055] [Table 1]

[0056] According to Table 1, Zr and La were detected in Example 1, Ti was detected in Example 2, and Ge was detected in Example 3. On the other hand, Zr, La, Ti, and Ge were below the detection limit in the comparative example. The metal elements detected in the positive electrode in Examples 1-3 were derived from the composition of the solid electrolyte that had come into contact with the electrolytic solution. Therefore, it is presumed that the solid electrolyte came into contact with the electrolytic solution, and the metal elements that constitute the solid electrolyte dissolved into the electrolytic solution.

[0057] As shown in Table 1, the comparative example in which Zr, La, Ti, and Ge were below the detection limit was judged to be B. It is presumed that in the comparative example, the decomposition of the electrolyte by the activated carbon in the positive electrode was accelerated during the high-temperature test at 85°C, causing the pores of the activated carbon to become clogged with the decomposition products, resulting in the ratio of the capacity after the high-temperature test to the initial capacity being less than 90%.

[0058] On the other hand, Example 1, in which Zr and La were detected, Example 2, in which Ti was detected, and Example 3, in which Ge was detected, were judged to be A. In Examples 1 to 3, the inorganic coating formed by the metal elements on the activated carbon reduced the decomposition of the electrolyte by the activated carbon, and the generation of decomposition products was reduced, so it is estimated that the ratio of the capacity after the high-temperature test to the initial capacity was 90% or more.

[0059] The tester also conducted tests different from those in the above examples, which are explained below. The tester placed a nonaqueous solvent and solid electrolyte powder in a sealed container under an argon atmosphere, stirred intermittently for 1 hour, and then left to stand for 24 hours to allow the powder to settle and separate. The tester then immersed a positive electrode composite layer in the resulting supernatant and dried it to prepare a positive electrode. A separator and a positive electrode were then attached, in that order, to the pre-doped negative electrode composite layer to prepare a cell, and the electrolyte was injected into the cell to obtain a capacitor. This capacitor also had a capacity ratio of 90% or more of its initial capacity after the high-temperature test.

[0060] Even if the capacitor was not filled with an electrolytic solution in contact with a solid electrolyte as in the examples, a capacitor using a positive electrode immersed in a non-aqueous solvent in contact with a solid electrolyte could reduce the decrease in capacitance in a high-temperature environment, as in the examples. Therefore, it was revealed that the effect of reducing the decrease in capacitance in a high-temperature environment was due to the presence of a metal element in the composite layer, not the presence of a metal element in the electrolytic solution.

[0061] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0062] In the embodiment, the capacitor 10 has been described as including a negative electrode 15 in which a composite material layer 17 is provided on one side of a current collecting layer 16, and a positive electrode 11 in which a composite material layer 13 is provided on one side of a current collecting layer 12, but the present invention is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to a capacitor including electrode layers (so-called bipolar electrodes) in which a composite material layer 13 and a composite material layer 17 are provided on both sides of a current collecting layer 12. A capacitor with a so-called bipolar structure can be obtained by alternately stacking bipolar electrodes and separators 14 and housing the resulting structure in a case (not shown).

[0063] In the embodiment, the positive electrode 11 is described in which the composite layer 13 is provided on one side of the current collecting layer 12, but this is not necessarily limited to this. It is of course possible to provide the composite layer 13 on both sides of the current collecting layer 12. [Explanation of symbols]

[0064] 10 Capacitors 11 Positive electrode 12 Current collecting layer 13 Composite material layer

Claims

1. An electrode comprising: a composite layer containing activated carbon; and a current collecting layer attached to the composite layer, the composite layer contains a metal element (excluding alkali metals) other than the elements contained in the current collecting layer, The mass concentration of the metal element is 1 ppm or more.

2. 2. The electrode according to claim 1, wherein the metal element is at least one selected from the group consisting of Zr, La, Ti and Ge.

3. 2. The electrode according to claim 1, wherein said metal element is at least one of Zr and La.

4. A capacitor comprising the electrode according to any one of claims 1 to 3.

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