Carbon current collector, energy storage device, and method for manufacturing a carbon current collector

Abstract

To provide: a carbon collector having a low side reaction and low resistance; a power storage device; and a manufacturing method of the carbon collector.SOLUTION: A carbon collector contains at least expanded graphite and carbon fiber pieces as carbon materials and may also contain fibrous carbon. With respect to the entire expanded graphite, carbon fiber pieces and fibrous carbon, a ratio of expanded graphite is more than 60 mass% and less than 90 mass%, a ratio of carbon fiber pieces is more than 0 mass% and less than 30 mass%, and an average particle diameter of expanded graphite is more than 7 μm and less than 25 μm. The carbon collector also includes a binder which binds the carbon materials.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a carbon current collector, an energy storage device, and a method for manufacturing a carbon current collector. [Background technology]

[0002] Conventionally, in energy storage devices, it has been proposed to reduce the interfacial resistance between the electrode and the current collector, thereby reducing the resistance of the energy storage device, by using carbon-coated foil, which is made by coating a metal foil with carbon nanoparticles together with a predetermined binder resin, as the current collector (see, for example, Patent Document 1). Furthermore, it has been proposed to reduce the weight of the current collector by using a resin current collector containing a conductive carbon material and a resin, and in doing so, to reduce the resistance in the thickness direction of the current collector by adjusting the type and composition of the resin current collector and the conductive carbon material (see, for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-74261 [Patent Document 2] Japanese Patent Publication No. 2019-153587 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, while the current collectors described in Patent Documents 1 and 2 above have low resistance, the activity of carbon nanoparticles and carbon fillers has not been considered, and there is a risk of side reactions caused by carbon nanoparticles and carbon fillers during charging and discharging. Similarly, carbon paper, which is used as a gas diffusion substrate for fuel cell electrodes, has low resistance, but there is a risk of side reactions when used as a current collector in an energy storage device. Such side reactions in current collectors cause a capacitive imbalance between the positive and negative electrodes, leading to a decrease in charge-discharge cycle characteristics. For this reason, there has been a demand for carbon current collectors with low side reactions and low resistance, and energy storage devices using them.

[0005] This disclosure is made to solve these problems, and its main objective is to provide a carbon current collector, an energy storage device, and a method for manufacturing a carbon current collector that exhibits low side reactions and low resistance. [Means for solving the problem]

[0006] To achieve the above-mentioned objectives, the inventors conducted diligent research. They discovered that in a carbon current collector in which carbon materials are bound together with a binder, by including at least expanded graphite and carbon fiber pieces, and possibly further including fibrous carbon, and by appropriately adjusting the mixing ratio of these materials and the average particle size of the expanded graphite, it is possible to achieve both low side reactions and low resistance, thus completing this disclosure.

[0007] In other words, the carbon current collector of this disclosure is The carbon material comprises at least expanded graphite and carbon fiber fragments, and may also contain fibrous carbon, wherein the proportion of expanded graphite is greater than 60% by mass and less than 90% by mass, the proportion of carbon fiber fragments is greater than 0% by mass and less than 30% by mass, the average particle size of the expanded graphite is greater than 7 μm and less than 25 μm, and the carbon material comprises a binder for binding the carbon material. It is.

[0008] Alternatively, the carbon current collector of this disclosure is The carbon material comprises at least expanded graphite and carbon fiber fragments, and may also contain fibrous carbon. Thermogravimetric analysis is performed in air at a heating rate of 2°C / min, and in the differential curve obtained by differentiating the weight change [μg] with respect to elapsed time, a peak in weight loss appears in the range of 700°C to 775°C, and a change in weight loss appears at higher temperatures. When the weight change at the temperature T [°C] of the weight loss peak is defined as H(T) [μg / sec], and the weight change at the temperature T+X [°C], which is midway between the start of the change in weight loss and the end of the change in the differential curve, is defined as H(T+X) [μg / sec], the value of H(T+X) / H(T) is 0.13 or more and 0.48 or less, and the material includes a binder for binding the carbon material. It is.

[0009] The energy storage device of the present disclosure includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, an ion conductive medium interposed between the positive electrode and the negative electrode, and is provided with the above-described carbon current collector is provided as at least one of the current collector of the positive electrode and the current collector of the negative electrode. It is such a thing.

[0010] The method for manufacturing the carbon current collector of the present disclosure uses a carbon mixture containing at least expanded graphite and carbon fiber sheets, and may contain fibrous carbon. With respect to the whole of the expanded graphite, the carbon fiber sheets, and the fibrous carbon, the proportion of the expanded graphite is more than 60% by mass and less than 90% by mass, and the proportion of the carbon fiber sheets is more than 0% by mass and less than 30% by mass, and the average particle diameter of the expanded graphite is more than 7 μm and less than 25 μm, and a binder to produce a carbon current collector. It is such a thing.

Effects of the Invention

[0011] The carbon current collector, energy storage device, and method for manufacturing the carbon current collector described herein can achieve both low side reactions and low resistance. The reasons for these effects are presumed to be, for example, as follows: The expanded graphite is oriented so that its basal surface is parallel to the surface of the carbon current collector, covering the surface of the carbon current collector with the less reactive basal surface and preventing the intrusion of carrier ions into the interior, thereby suppressing side reactions. Furthermore, by being oriented as described above, the expanded graphite is responsible for electron conduction in the direction parallel to the surface of the carbon current collector (in-plane direction). The carbon fiber pieces promote electron conduction inside the carbon current collector by being responsible for electron conduction over relatively long distances. The fibrous carbon promotes electron conduction inside the carbon current collector by being responsible for electron conduction between the expanded graphite and the carbon fiber pieces, between the expanded graphites themselves, and between the carbon fiber pieces themselves. When the proportion of expanded graphite is greater than 60% by mass but less than 90% by mass, the proportion of carbon fiber fragments is greater than 0% by mass but less than 30% by mass, and the average particle size of expanded graphite is greater than 7 μm but less than 25 μm, it is presumed that the balance of the above-mentioned functions is good, and therefore both low side reactions and low resistance can be achieved. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing an example of the configuration of the carbon current collector 10. [Figure 2] A schematic diagram showing an example of a manufacturing method for the carbon current collector 10. [Figure 3] A schematic diagram showing an example of the configuration of the energy storage device 20. [Figure 4] Graphs showing the side reaction capacity and volume resistivity for experimental examples 1-26. [Figure 5] SEM image showing the surface of the carbon current collector in Experimental Example 1. [Figure 6] SEM image showing the surface of the carbon current collector in Experimental Example 2. [Figure 7] SEM image showing the surface of the carbon current collector in Experimental Example 1. [Figure 8] SEM image showing the surface of the carbon current collector in Experimental Example 10. [Figure 9] Differential curves of thermogravimetric analysis for experimental examples 1, 2, and 15-17. [Figure 10] Charge / discharge curve for Experimental Example 1. [Modes for carrying out the invention]

[0013] [Carbon current collector] The carbon current collector of this disclosure comprises at least expanded graphite and carbon fiber pieces as carbon materials. The carbon current collector may further comprise fibrous carbon as carbon materials. The carbon current collector comprises a binder for binding the carbon materials together.

[0014] Expanded graphite is graphite in which the interlayers of graphene that constitute graphite are expanded. For example, it may be obtained by inserting ions or molecules into the interlayers of graphite such as flake graphite to form a graphite interlayer compound, then heating and crushing it as necessary. The average particle size of expanded graphite is greater than 7 μm and less than 25 μm. When the average particle size of expanded graphite is greater than 7 μm and less than 25 μm, both low side reactions and low resistance can be achieved. The average particle size of expanded graphite is preferably between 8 μm and 23 μm, and more preferably between 10 μm and 20 μm. In a carbon current collector, the average particle size of expanded graphite can be determined as follows. First, the surface of the carbon current collector is observed at a magnification of 1000x using a scanning electron microscope (SEM) and five observation images are obtained, and 20 expanded graphite particles are arbitrarily selected from each observation image. Next, for each expanded graphite, determine the maximum diameter LH and the maximum diameter LW perpendicular to the maximum diameter LH (see Figure 1 below), and calculate (LH + LW) / 2 as the particle size of that expanded graphite. Then, calculate the average of (LH + LW) / 2 for all 100 expanded graphites as the average particle size of the expanded graphite. Note that if it is difficult to measure the average particle size of the expanded graphite in a carbon current collector, the average particle size of the expanded graphite used in the manufacture of the carbon current collector may be used as a substitute.

[0015] Carbon fiber pieces are obtained by cutting or crushing carbon fibers (excluding fibrous carbon as described later) to shorten them, and may be in powder form. For cutting, a roller cutter or guillotine cutter may be used, for example. The carbon fiber pieces may be pitch-based or PAN (polyacrylonitrile)-based, for example. The average fiber diameter of the carbon fiber pieces may be smaller than the average particle size of expanded graphite, and may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less of the average particle size of expanded graphite. The average fiber diameter of the carbon fiber pieces may be 3 μm to 20 μm, 5 μm to 15 μm or 5 μm to 10 μm. The average length of the carbon fiber pieces may be larger than the average particle size of expanded graphite, and may be 5 times or more, 10 times or more, or 15 times or more of the average particle size of expanded graphite. The average length of the carbon fiber pieces may be between 30 μm and 500 μm, between 50 μm and 300 μm, or between 200 μm and 300 μm. In a carbon current collector, the average fiber diameter and average length of the carbon fiber pieces can be determined as follows. First, the surface of the carbon current collector is observed using a SEM at 200x magnification, and 20 carbon fiber pieces are arbitrarily selected from the observed image. Next, for each carbon fiber piece, the axial length MH and the length MW perpendicular to the axial direction are determined, with length MH being the length of the carbon fiber piece and length MW being the fiber diameter of the carbon fiber piece. Then, the average value of the fiber lengths of the 20 carbon fiber pieces is used as the average length of the carbon fiber pieces, and the average value of the fiber diameters of the 20 carbon fiber pieces is used as the average fiber diameter of the carbon fiber pieces. In the case of a carbon current collector, if it is difficult to measure the average fiber diameter and average length of the carbon fiber pieces, the average fiber diameter and average length of the carbon fiber pieces used in the manufacture of the carbon current collector may be used as substitutes.

[0016] Fibrous carbon is a carbon material having a fiber diameter on the order of nanometers, and examples include carbon nanotubes and carbon nanofibers. Fibrous carbon may also be fibrous graphite having a graphite crystal structure. Fibrous carbon may be obtained, for example, by the CVD method (chemical vapor deposition) or by the PVD method (physical vapor deposition). The average fiber diameter of the fibrous carbon is preferably smaller than the average fiber diameter of the carbon fiber pieces, and may be 1 / 10 or less, 1 / 20 or less, or 1 / 50 or less of the average fiber diameter of the carbon fiber pieces. The average length of the fibrous carbon may be smaller than the average particle size of the expanded graphite, and may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less of the average particle size of the expanded graphite. The average length of the fibrous carbon may be 1 μm or more and 20 μm or less, 2 μm or more and 15 μm or less, or 3 μm or more and 10 μm or less. In a carbon current collector, the average fiber diameter and average length of the fibrous carbon can be determined as follows. First, the surface of the carbon current collector is observed using a SEM at a magnification of 3000x to obtain five observation images, and 20 fibrous carbons are arbitrarily selected from each observation image. Next, for each fibrous carbon, the axial length NH and the length in the direction perpendicular to the axial direction NW are determined, and NH is calculated as the length of the fibrous carbon, and NW as the fiber diameter of the fibrous carbon. Then, the average fiber length of the 100 fibrous carbons is calculated as the average length of the fibrous carbons, and the average fiber diameter MW of the 100 fibrous carbons is calculated as the average fiber diameter of the fibrous carbons. Note that if it is difficult to measure the average fiber diameter and average length of the fibrous carbons in the carbon current collector, the average fiber diameter and average length of the fibrous carbons used in the manufacture of the carbon current collector may be used as substitutes.

[0017] The proportion of expanded graphite is more than 60% by mass but less than 90% by mass, and the proportion of carbon fiber pieces is more than 0% by mass but less than 30% by mass, relative to the total of expanded graphite, carbon fiber pieces, and fibrous carbon. Carbon current collectors that satisfy these ranges in the proportion of expanded graphite and carbon fiber pieces can achieve both low side reactions and low resistance. The proportion of expanded graphite is preferably 65% ​​by mass or more and 85% by mass or less, and more preferably 70% by mass or more and 80% by mass or less. The proportion of carbon fiber pieces is preferably 5% by mass or more and 25% by mass or less, and more preferably 10% by mass or more and 20% by mass or less, relative to the total of expanded graphite, carbon fiber pieces, and fibrous carbon. The proportion of fibrous carbon may be 0% by mass or more and 10% by mass or less, or 5% by mass or more and 15% by mass or less.

[0018] The carbon current collector preferably does not contain carbon materials other than the expanded graphite, carbon fiber pieces, and fibrous carbon mentioned above. If it does contain carbon materials, it is preferably 10% by mass or less, preferably 5% by mass or less, and preferably 1% by mass or less, relative to the total amount of expanded graphite, carbon fiber pieces, and fibrous carbon.

[0019] The carbon current collector contains a binder that binds the carbon material. Suitable binders include, for example, water-based binders such as cellulose-based carboxymethylcellulose (CMC), styrene-butadiene copolymer (SBR), and polyvinyl alcohol. Alternatively, the binder may be a fluororesin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, or natural butyl rubber (NBR). The binder may be used alone or in combination of two or more types. The proportion of binder contained in the carbon current collector may be 30% by mass or less, 10% by mass or less, or 5% by mass or less. The proportion of binder contained in the carbon current collector may be 1% by mass or more, or 2% by mass or more.

[0020] The carbon current collector may be, for example, in the form of a sheet. Also, the carbon current collector may have flexibility or may not have flexibility. The carbon current collector may have a thickness of, for example, 5 μm or more and 500 μm or less, 10 μm or more and 500 μm or less, 10 μm or more and 200 μm or less, or 20 μm or more and 200 μm or less. Also, the carbon current collector may have a density of 0.5 g / cm 3 or more and 1.8 g / cm 3 or less, 0.5 g / cm 3 or more and 1.5 g / cm 3 or less, or 0.9 g / cm 3 or more and 1.5 g / cm 3 or less. Also, the carbon current collector preferably has a side reaction capacity per unit volume of 60 mAh / cm 3 or less, more preferably 54 mAh / cm 3 or less, and even more preferably 50 mAh / cm 3 or less. Also, the carbon current collector preferably has a volume resistivity measured by the four-probe method of 15 mΩ·cm or less, more preferably 10 mΩ·cm or less, and even more preferably 8 mΩ·cm or less.

[0021] The carbon current collector may be a self-supporting film or may be formed on a substrate. The substrate may be, for example, a conductive substrate such as aluminum, titanium, stainless steel, nickel, iron, fired carbon, a conductive polymer, or conductive glass, or an insulating substrate such as polyimide, polytetrafluoroethylene, polyethylene, or polypropylene. The total thickness of the carbon current collector and the substrate may be, for example, 10 μm or more and 540 μm or less.

[0022] A carbon current collector may be defined as a carbon current collector in which, when thermogravimetric analysis is performed in air at a heating rate of 2°C / min and the weight change [μg] is differentiated with respect to elapsed time, a peak in weight loss appears in the range of 700°C to 775°C, and a change in weight loss appears at higher temperatures. Note that a change in weight loss refers to a change that occurs at a point where the slope of the differential curve tends to change. A carbon current collector may be defined as a carbon current collector in which, when the weight change at the temperature T [°C] where the weight loss peak is defined as H(T) [μg / sec], and the weight change at the temperature T+X [°C], which is midway between the start of the change in weight loss and the end of the change in the differential curve, is defined as H(T+X) [μg / sec], the ratio of weight changes H(T+X) / H(T) is between 0.13 and 0.48. In carbon current collectors where the value of H(T+X) / H(T) is between 0.13 and 0.48, low side reactions and low resistance can be achieved simultaneously due to factors such as a suitable ratio of expanded graphite, carbon fiber pieces, and fibrous carbon, and a suitable average particle size of expanded graphite. The value of H(T+X) / H(T) is preferably between 0.15 and 0.40, and more preferably between 0.16 and 0.38. The weight change H(T) at temperature T[°C] is considered to be mainly the weight loss related to expanded graphite. The temperature T[°C] is, for example, between 700°C and 775°C, but may also be between 710°C and 760°C, between 720°C and 750°C, or between 730°C and 740°C. Furthermore, the weight change H(T+X) at temperature T+X[°C] is considered to be mainly the weight loss related to carbon fiber pieces and fibrous graphite. The temperature X [°C] is, for example, between 20°C and 100°C, but may also be between 30°C and 90°C, or between 60°C and 80°C.

[0023] Figure 1 is a schematic diagram showing an example of a carbon current collector 10. Figure 1 shows the carbon current collector 10 as viewed from the surface. This carbon current collector 10 comprises expanded graphite 12, carbon fiber pieces 14, and fibrous carbon 16. The average particle size of the expanded graphite 12 is greater than 7 μm and less than 25 μm. The particle size of the expanded graphite 12 is the value of (LH + LW) / 2 obtained from the maximum diameter LH and the maximum diameter LW in the direction perpendicular to it. In the carbon current collector 10, the proportion of expanded graphite 12 is greater than 60 mass% and less than 90 mass%, and the proportion of carbon fiber pieces 14 is greater than 0 mass% and less than 30 mass%.

[0024] [Manufacturing method for carbon current collectors] The present disclosure is a method for manufacturing a carbon current collector, in which a carbon current collector is produced using a carbon mixture containing at least expanded graphite and carbon fiber pieces, and a binder. The carbon mixture may further contain fibrous carbon. This method for manufacturing a carbon current collector may, for example, produce the carbon current collector described above.

[0025] The expanded graphite used in the manufacture of carbon current collectors is the same as the expanded graphite described for carbon current collectors. The average particle size of the expanded graphite is greater than 7 μm and less than 25 μm. Preferably, the average particle size of the expanded graphite is between 8 μm and 23 μm, and more preferably between 10 μm and 20 μm. The average particle size of the expanded graphite can be determined in the same way as the average particle size of expanded graphite in carbon current collectors, except that the expanded graphite before being made into a carbon current collector is the object of observation.

[0026] The carbon fiber pieces used in the manufacture of carbon current collectors are the same as the carbon fiber pieces described for carbon current collectors. The average fiber diameter of the carbon fiber pieces may be smaller than the average particle size of expanded graphite, or it may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less of the average particle size of expanded graphite. The average fiber diameter of the carbon fiber pieces may be 3 μm or more and 20 μm or less, or 5 μm or more and 15 μm or less, or 5 μm or more and 10 μm or less. The average length of the carbon fiber pieces may be larger than the average particle size of expanded graphite, or it may be 5 times or more, 10 times or more, or 15 times or more of the average particle size of expanded graphite. The average length of the carbon fiber pieces may be 30 μm or more and 500 μm or less, or 50 μm or more and 300 μm or less. The average fiber diameter and average length of carbon fiber pieces can be determined in the same way as the average fiber diameter and average length of carbon fiber pieces in a carbon current collector, except that the carbon fiber pieces before they are made into a carbon current collector are the ones being observed.

[0027] The fibrous carbon used in the manufacture of carbon current collectors is the same as the fibrous carbon described for carbon current collectors. The average fiber diameter of the fibrous carbon is preferably smaller than the average fiber diameter of the carbon fiber pieces, and may be 1 / 10 or less, 1 / 20 or less, or 1 / 50 or less of the average fiber diameter of the carbon fiber pieces. The average length of the fibrous carbon may be smaller than the average particle size of expanded graphite, and may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less of the average particle size of expanded graphite. The average length of the fibrous carbon may be 1 μm or more and 20 μm or less, 2 μm or more and 15 μm or less, or 3 μm or more and 10 μm or less. The average fiber diameter and average length of the fibrous carbon can be determined in the same way as the average fiber diameter and average length of the fibrous carbon in carbon current collectors, except that the fibrous carbon before it is made into a carbon current collector is the object of observation.

[0028] The carbon mixture is prepared such that the proportion of expanded graphite is greater than 60% by mass but less than 90% by mass, and the proportion of carbon fiber pieces is greater than 0% by mass but less than 30% by mass, relative to the total amount of expanded graphite, carbon fiber pieces, and fibrous carbon. The proportion of expanded graphite is preferably 65% ​​by mass or more and 85% by mass or less, and more preferably 70% by mass or more and 80% by mass or less. The proportion of carbon fiber pieces is preferably 5% by mass or more and 25% by mass or less, and more preferably 10% by mass or more and 20% by mass or less, relative to the total amount of expanded graphite, carbon fiber pieces, and fibrous carbon. The proportion of fibrous carbon may be 0% by mass or more and 10% by mass or less, or 5% by mass or more and 15% by mass or less.

[0029] The carbon mixture preferably does not contain any carbon material other than the expanded graphite, carbon fiber fragments, and fibrous carbon mentioned above. If it does contain other carbon materials, it is preferably 10% by mass or less, preferably 5% by mass or less, and preferably 1% by mass or less, relative to the total amount of expanded graphite, carbon fiber fragments, and fibrous carbon.

[0030] The carbon compound may be mixed with a binder to form a current collector composite, or a suitable solvent may be added to form a current collector slurry or current collector paste. The binder can be the same as the binder described for carbon current collectors, and suitable examples include water-based binders such as cellulose-based carboxymethylcellulose (CMC), styrene-butadiene copolymer (SBR), and polyvinyl alcohol. The proportion of binder in the current collector composite may be 30% by mass or less, 10% by mass or less, or 5% by mass or less. The proportion of binder in the current collector composite may be 1% by mass or more, or 2% by mass or more. The solvent may be an aqueous solvent such as water. Alternatively, an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran may be used.

[0031] In the method for manufacturing a carbon current collector, a carbon mixture may be applied to a substrate together with a binder, and drying and compression may be performed as needed to produce the carbon current collector. The substrate may be a conductive substrate such as aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, or conductive glass, or an insulating substrate such as polyimide, polytetrafluoroethylene, polyethylene, or polypropylene. The substrate may be removed after the formation of the carbon current collector, or it may be used together with the carbon current collector without being removed. Examples of coating methods include roller coating using an applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any thickness and shape can be achieved using any of these. The drying temperature may be, for example, 80°C to 200°C, or 100°C to 150°C. The drying time may be, for example, 10 minutes to 24 hours, or 30 minutes to 12 hours. Compression can be performed using, for example, a roll press. Compression may be performed, for example, so that the thickness of the carbon current collector is 5 μm or more and 500 μm or less, 10 μm or more and 500 μm or less, 10 μm or more and 200 μm or less, or 20 μm or more and 200 μm or less. Furthermore, compression may be performed, for example, so that the density of the carbon current collector is 0.5 g / cm³. 3 More than 1.8g / cm 3 The following may be performed: 0.5 g / cm³ 3 More than 1.5g / cm 3 The following may be performed: 0.9 g / cm³ 3 More than 1.5g / cm 3 You may also proceed as follows:

[0032] Figure 2 is a schematic diagram showing an example of a method for manufacturing a carbon current collector 10. In this manufacturing method, first, a current collector slurry 18 is prepared using expanded graphite 12, carbon fiber pieces 14, and fibrous carbon 16 (Figure 2A). For the expanded graphite 12, an average particle size of more than 7 μm and less than 25 μm is used. The current collector slurry 18 is formulated such that the proportion of expanded graphite 12 is more than 60% by mass and less than 90% by mass, and the proportion of carbon fiber pieces 14 is more than 0% by mass and less than 30% by mass, and a binder and solvent are added to prepare it. Next, the prepared current collector slurry 18 is applied to the surface of a substrate 19 (Figure 2B). Then, the carbon current collector 10 is obtained by drying and compressing the current collector slurry 18 as needed (Figure 2C). In this manufacturing method, the carbon current collector 10 is formed on the substrate 19. The base material 19 may be removed, or it may be used together with the carbon current collector 10 without being removed.

[0033] [Energy storage devices] The energy storage device of this disclosure comprises a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an ion-conducting medium interposed between the positive electrode and the negative electrode, wherein the carbon current collector described above is provided as at least one of the current collectors of the positive electrode and the negative electrode. Examples of energy storage devices include electric double-layer capacitors, hybrid capacitors, pseudo-electric double-layer capacitors, lithium or sodium alkali metal secondary batteries, alkali metal ion batteries, and air batteries. Examples of carrier ions conducted by the ion-conducting medium include alkali metal ions such as Li, Na, and K, and group 2 ions (alkaline earth metal ions) such as Mg, Ca, and Sr, of which lithium ions are preferred. The energy storage device is preferably a lithium secondary battery, and particularly a lithium-ion secondary battery. In the following, for the sake of convenience of explanation, the case in which the energy storage device is a lithium-ion secondary battery equipped with the carbon current collector described above as the current collector of the negative electrode will be mainly described.

[0034] The positive electrode may be formed by applying and drying, on the surface of a current collector, a slurry or paste obtained by adding an appropriate solvent to a positive electrode mixture obtained by mixing, for example, a positive electrode active material, a conductive material, and a binder, and compressing it as necessary to increase the electrode density. Examples of the positive electrode active material include sulfides containing a transition metal element (transition metal sulfides) and compounds containing lithium and a transition metal element (lithium transition metal composite compounds). Among these, oxides containing lithium and a transition metal element (lithium transition metal composite oxides) are preferred. Examples of the transition metal sulfides include TiS2, TiS3, MoS3, FeS2, etc. Examples of the lithium transition metal composite compounds include lithium manganese composite oxides having a basic composition formula of Li (1-x) MnO2 (0 < x < 1, etc., the same applies hereinafter), lithium manganese composite oxides having a basic composition formula of Li (1-x) Mn2O4, etc., lithium cobalt composite oxides having a basic composition formula of Li (1-x) CoO2, etc., lithium nickel composite oxides having a basic composition formula of Li (1-x) NiO2, etc., lithium nickel cobalt manganese composite oxides having a basic composition formula of Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), etc., and lithium iron phosphate. Note that the "basic composition formula" means that other elements may be included. The positive electrode active material is preferably an oxide containing one or more of nickel, manganese, and cobalt. For example, LiCoO2, LiNiO2, LiMnO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc. are preferred.

[0035] As the conductive material for the positive electrode, for example, graphite such as natural graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjenblack, carbon whiskers, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used. As the binder, for example, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene rubber (EPDM), sulfonated EPDM rubber, and natural butyl rubber (NBR) can be used. In addition, aqueous binders such as cellulose-based or aqueous dispersions of styrene-butadiene rubber (SBR) can be used. As the solvent, for example, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. Alternatively, a dispersant, thickener, etc., may be added to water, and the active material may be slurryed with latex such as SBR. As a thickener, for example, polysaccharides such as carboxymethylcellulose and methylcellulose can be used alone or as a mixture of two or more. As a coating method, for example, roller coating such as applicator roll coating, screen coating, doctor blade method, spin coating, and bar coating can be used, and any thickness and shape can be achieved using any of these. As a current collector, aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, etc., as well as aluminum and copper whose surfaces have been treated with carbon, nickel, titanium, or silver for the purpose of improving adhesion, conductivity, and oxidation resistance, can be used. For these, the surface can also be oxidized. As for the shape of the current collector, examples include foil, film, sheet, net, punched or expanded, lath, porous, foam, and fiber group formation. The thickness of the current collector is, for example, 1 to 500 μm.

[0036] The negative electrode may be formed, for example, by mixing a negative electrode active material, a conductive material, and a binder, adding a suitable solvent to form a paste-like negative electrode composite, applying and drying it on the surface of a current collector, and compressing it as needed to increase the electrode density, or by forming it by closely adhering the negative electrode active material and the current collector. Examples of negative electrode active materials include inorganic compounds such as tin compounds, carbonaceous materials capable of intercalating and releasing lithium ions, composite oxides containing multiple elements, and conductive polymers. Examples of carbonaceous materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Examples of composite oxides include Li4Ti5O 12 Examples include lithium titanium composite oxides such as LiV2O3 and lithium vanadium composite oxides such as LiV2O3. Of these, carbonaceous materials such as graphites are preferred as the negative electrode active material. Furthermore, the conductive material, binder, solvent, etc. used in the negative electrode can be those exemplified for the positive electrode. The current collector of the negative electrode shall be the carbon current collector described above.

[0037] The ion-conducting medium may be, for example, a non-aqueous electrolyte containing a supporting salt and an organic solvent. Examples of supporting salts include lithium salts such as LiPF6, LiBF4, LiClO4, LiAsF6, Li(CF3SO2)2N, and LiN(C2F5SO2)2, of which LiPF6 and LiBF4 are preferred. The concentration of this supporting salt in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. A concentration of 0.1 mol / L or more in which the supporting salt is dissolved allows for a sufficient current density to be obtained, and a concentration of 5 mol / L or less allows for greater stability of the electrolyte. Furthermore, flame retardants such as phosphorus-based and halogen-based agents may be added to this non-aqueous electrolyte. As the organic solvent, for example, an aprotic organic solvent can be used. Examples of such organic solvents include cyclic carbonates, linear carbonates, cyclic esters, cyclic ethers, and linear ethers. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of linear carbonates include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of cyclic ester carbonates include gamma-butyrolactone and gamma-valerolactone. Examples of cyclic ethers include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of linear ethers include dimethoxyethane and ethylene glycol dimethyl ether. These may be used individually or in combination. In addition, non-aqueous electrolytes may include nitrile solvents such as acetonitrile and propylnitrile, as well as ionic liquids and gel electrolytes.

[0038] The energy storage device may include a separator between the positive and negative electrodes. The separator is not particularly limited as long as its composition can withstand the operating range of the energy storage device, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric or polyphenylene sulfide nonwoven fabric, and microporous films of olefin resins such as polyethylene or polypropylene. These may be used individually or in combination.

[0039] The shape of the energy storage device is not particularly limited, but examples include coin-shaped, button-shaped, sheet-shaped, laminated, cylindrical, flat, and rectangular shapes. Furthermore, multiple such energy storage devices may be connected in series to create larger devices for use in electric vehicles, etc. Figure 3 is a schematic diagram showing an example of an energy storage device 20. The energy storage device 20 comprises a positive electrode sheet 23 with a positive electrode composite material 22 formed on a current collector 21, a negative electrode sheet 26 with a negative electrode composite material 25 formed on the surface of a current collector 24, a separator 28 provided between the positive electrode sheet 23 and the negative electrode sheet 26, and a non-aqueous electrolyte 29 filling the space between the positive electrode sheet 23 and the negative electrode sheet 26. In the energy storage device 20, the separator 28 is sandwiched between the positive electrode sheet 23 and the negative electrode sheet 26, these are wound together and inserted into a cylindrical case 32, and a positive electrode terminal 34 connected to the positive electrode sheet 23 and a negative electrode terminal 36 connected to the negative electrode sheet 26 are arranged to form the device. The carbon current collector 10 described above is used as the current collector 24 of the negative electrode sheet 26.

[0040] The carbon current collector, the method for manufacturing the carbon current collector, and the energy storage device of this embodiment described above can achieve both low side reactions and low resistance. The reason for obtaining such effects is presumed to be as follows: The expanded graphite is oriented so that its basal surface is parallel to the surface of the carbon current collector, covering the surface of the carbon current collector with the less reactive basal surface and preventing the intrusion of carrier ions into the interior, thereby suppressing side reactions. Furthermore, by being oriented as described above, the expanded graphite is responsible for electron conduction in a direction parallel to the surface of the carbon current collector. The carbon fiber pieces promote electron conduction inside the carbon current collector by being responsible for electron conduction over relatively long distances. The fibrous carbon promotes electron conduction inside the carbon current collector by being responsible for electron conduction between the expanded graphite and the carbon fiber pieces, between the expanded graphites themselves, and between the carbon fiber pieces themselves. When the proportion of expanded graphite is greater than 60% by mass but less than 90% by mass, the proportion of carbon fiber fragments is greater than 0% by mass but less than 30% by mass, and the average particle size of expanded graphite is greater than 7 μm but less than 25 μm, it is presumed that the balance of the above-mentioned functions is good, and therefore both low side reactions and low resistance can be achieved. In addition, since the current collector is a carbon current collector rather than a metal current collector, there are advantages such as the ability to reduce weight and the elimination of the effort required for metal recovery during disposal and recycling.

[0041] Furthermore, the aforementioned energy storage device incorporates a carbon current collector as the negative electrode current collector. Generally, many carbon materials undergo oxidation-reduction reactions at potentials near the oxidation-reduction potential of the negative electrode. Therefore, there is a high risk of side reactions occurring when a current collector containing carbon material is used as the negative electrode current collector. For this reason, the carbon current collector of this disclosure is highly significant for application as a negative electrode current collector.

[0042] It goes without saying that this disclosure is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure.

[0043] For example, in the embodiment described above, the carbon current collector was defined as having an expanded graphite ratio of more than 60% by mass but less than 90% by mass, and a carbon fiber ratio of more than 0% by mass but less than 30% by mass, relative to the total of expanded graphite, carbon fiber pieces, and fibrous carbon. However, instead of satisfying these conditions, the value of H(T+X) / H(T) in the differential curve described above may be 0.13 or more and 0.48 or less. In a carbon current collector with an H(T+X) / H(T) value of 0.13 or more and 0.48 or less, low side reactions and low resistance can be achieved simultaneously due to factors such as a suitable ratio of expanded graphite, carbon fiber pieces, and fibrous carbon, and a suitable average particle size of expanded graphite.

[0044] In the embodiments described above, the energy storage device is provided with a carbon current collector as the negative electrode current collector. However, the carbon current collector may also be provided as the positive electrode current collector, or as both the negative and positive electrode current collectors. When a carbon current collector is provided as the positive electrode current collector, the negative electrode current collector can be made of copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc. In addition, for the purpose of improving adhesion, conductivity, and reduction resistance, materials such as copper whose surfaces have been treated with carbon, nickel, titanium, or silver can also be used. These materials can also be oxidized. The shape of the current collector can be the same as that described for the positive electrode. [Examples]

[0045] The following describes experimental examples of specific studies on carbon current collectors and energy storage devices. Experimental Examples 1-5 correspond to the Implemented Examples, and Experimental Examples 6-26 correspond to the Comparative Examples.

[0046] [Experimental Example 1] (Fabrication of carbon current collectors) First, a carbon mixture was obtained by mixing 70 parts by mass of expanded graphite (average particle size 15 μm: EC1000 manufactured by Ito Graphite Industry Co., Ltd.), 20 parts by mass of carbon fiber pieces (fiber diameter 7 μm, average length 250 μm: XN-100-25M manufactured by Nippon Graphite Fiber Co., Ltd.), and 10 parts by mass of fibrous carbon (fiber diameter 150 nm: VGCF-H manufactured by Showa Denko Co., Ltd.) in a ratio of 1 part by mass of the carbon mixture, 1 part by mass of carboxymethylcellulose (CMC), and 1 part by mass of styrene-butadiene rubber (SBR) (used in aqueous dispersion form; 1 part by mass is solid content only) in a ratio of 18 parts by mass of the carbon mixture, 1 part by mass of carboxymethylcellulose (CMC), and 1 part by mass of styrene-butadiene rubber (SBR) (used in aqueous dispersion form; 1 part by mass is solid content only). A current collector slurry was obtained by coating one side of a polyimide sheet or copper foil, which served as the base material, and drying at 120°C for 8 hours. The polyimide sheet was 40 μm thick, and the copper foil was 10 μm thick. This carbon sheet was densely pressed using a roll press to produce the carbon current collector shown in Experimental Example 1.

[0047] (SEM observation) Expanded graphite, carbon fiber fragments, and fibrous carbon before being made into carbon current collectors, as well as the carbon current collectors themselves, were observed using a scanning electron microscope (Hitachi High-Technologies SU3500). The average particle size of the expanded graphite, the average fiber diameter and length of the carbon fiber fragments, and the average fiber diameter and length of the fibrous carbon were then determined before and in the carbon current collectors.

[0048] (Measurement of volume resistivity) A carbon current collector fabricated using a polyimide sheet as the base material was used as the sample for volume resistivity measurement in Experimental Example 1, along with the polyimide sheet. In the volume resistivity measurement sample of Experimental Example 1, the thickness of the carbon current collector was 14 μm. The basis weight of the carbon current collector was estimated to be about the same as that of the carbon current collector in the electrodes described later. Four needle-shaped electrodes were placed in a straight line on the surface of the current collector of this sample, and the resistance value R [Ω] was measured by measuring the potential generated at the inner probe when a constant current was passed through both outer probes. From the measured resistance value R [Ω], the volume resistivity ρ [Ω·cm] was calculated using the following relational equation (1). In equation (1), t is the thickness of the carbon current collector [cm].

[0049]

number

[0050] (Calculation of side reaction volume) A carbon current collector, fabricated using copper foil as a base material, was cut out in a φ16 mm disc shape, along with the copper foil, to serve as the electrode for Experimental Example 1. In the electrode for Experimental Example 1, the basis weight of the carbon current collector was 1.6 mg / cm³. 2 The carbon current collector had a thickness of 14 μm. This electrode was used as the working electrode, and a 300 μm thick lithium metal foil was used as the counter electrode. A polyethylene separator with a thickness of 25 μm was placed between the two electrodes to create a two-electrode evaluation cell. For the non-aqueous electrolyte, a mixed solvent containing 30 vol% ethylene carbonate (EC), 40 vol% dimethyl carbonate (DMC), and 30 vol% ethyl methyl carbonate (EMC) was used, in which LiPF6 was dissolved at a concentration of 1 M. Using the fabricated two-electrode evaluation cell, a lower voltage limit of 5 mV and a current value of 0.25 mA / cm were obtained at a temperature of 20°C. 2 The reduction was performed, and the capacity [mAh] at that time was measured. Then, this capacity was used to measure the volume [cm³] of the carbon current collector. 3 Divide by ] to get the capacity per unit volume [mAh / cm³] 3 The value of ] was calculated and this was used as the side reaction volume.

[0051] (Thermogravimetric analysis) Thermogravimetric analysis (TG) was performed on the carbon current collector described above (with the base material removed). A Pt pan was used, and Al2O3 was used as a reference. On the sample side, Al2O3 was spread at the bottom of the Pt pan, and the weight change of the carbon current collector during heating was tracked. Heating was performed at 2°C / min under an air inflow of 200 mL / min. A differential curve was created by differentiating the weight change (μg) obtained from the TG measurement with respect to elapsed time (sec), and the weight change H(T) at temperature T and the weight change H(T+X) at temperature T+X were determined, and the ratio of weight changes H(T+X) / H(T+X) was derived.

[0052] (Charge / Discharge Test) LiNi 0.5 Co 0.2 Mn 0.3A positive electrode mixture slurry was prepared by mixing 90% by mass of O2 (manufactured by Toda Kogyo), 6% by mass of carbon black as a conductive material, and 4% by mass of polyvinylidene fluoride as a binder, and adding and dispersing an appropriate amount of N-methyl-2-pyrrolidone as a dispersant. A carbon current collector made on a polyimide sheet as a base material was used, and the positive electrode mixture slurry was applied to this carbon current collector, and a coated sheet was prepared by heating and drying. The coated sheet was then passed through a roll press to increase its density and cut into 25 mm wide strips to form the positive electrode sheet. The coated portion of the positive electrode mixture was made 40 mm long, and an Al tab was fixed to the excess portion of the carbon current collector. A negative electrode mixture slurry was prepared by mixing 95% by mass of graphite as the negative electrode active material and 5% by mass of polyvinylidene fluoride as a binder, and adding and dispersing an appropriate amount of N-methyl-2-pyrrolidone as a dispersant. A carbon current collector was fabricated using a polyimide sheet as the base material. Using the polyimide sheet as is, a negative electrode mixture slurry was applied to this carbon current collector, and the sheet was heated and dried to produce a coated sheet. The coated sheet was then passed through a roll press to increase its density and cut into 27 mm wide strips to form the negative electrode sheet. The coated portion of the negative electrode mixture was 42 mm long, and Ni tabs were fixed to the excess portion of the carbon current collector. Furthermore, a laminated electrode body was fabricated by placing the positive electrode sheet and the negative electrode sheet opposite each other with a 25 μm thick polyethylene separator in between. This electrode body was sealed in an aluminum laminate bag, impregnated with a non-aqueous electrolyte, and then sealed to produce a lithium secondary battery. The non-aqueous electrolyte used was a mixed solvent containing 30 vol% ethylene carbonate (EC), 40 vol% dimethyl carbonate (DMC), and 30 vol% ethyl methyl carbonate (EMC), in which LiPF6 was dissolved at a concentration of 1 M. Using the rechargeable battery thus obtained, under temperature conditions of 25°C, the upper voltage limit was 4.1V, the lower voltage limit was 2.5V, and the current flow rate was 0.3mA / cm². 2 Constant current and constant voltage charging and discharging were performed under constant current conditions, followed by a 2-hour period of constant potential.

[0053] [Experimental Example 2] Experimental Example 2 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 80 parts by mass of expanded graphite, 10 parts by mass of carbon fiber fragments, and 10 parts by mass of fibrous carbon.

[0054] [Experimental Example 3] As the expanded graphite, expanded graphite with a particle size of 10 μm, obtained by controlling the particle size of the expanded graphite from Experimental Example 1 using a ball mill, was used. A carbon mixture was prepared with a blending ratio of 80 parts by mass of expanded graphite, 10 parts by mass of carbon fiber pieces, and 10 parts by mass of fibrous carbon. Experimental Example 3 was carried out in the same manner as Experimental Example 1.

[0055] [Experimental Example 4] As the expanded graphite, expanded graphite with an average particle size of 25 μm (EC500 manufactured by Ito Graphite Industry Co., Ltd.) was used, and expanded graphite with a particle size of 20 μm was obtained by controlling the particle size using a ball mill. A carbon mixture was prepared with a blending ratio of 80 parts by mass of expanded graphite, 10 parts by mass of carbon fiber pieces, and 10 parts by mass of fibrous carbon. Experimental Example 4 was carried out in the same manner as Example 1.

[0056] [Experimental Examples 5-7] Experimental Example 5 was conducted in the same manner as Experimental Example 1, except that fibrous carbon was not used, and a carbon mixture was prepared with a ratio of 80 parts by mass of expanded graphite and 20 parts by mass of carbon fiber fragments. Experimental Example 6 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared with a ratio of 70 parts by mass of expanded graphite and 30 parts by mass of carbon fiber fragments. Experimental Example 7 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared with a ratio of 90 parts by mass of expanded graphite and 10 parts by mass of carbon fiber fragments.

[0057] [Experimental Examples 8-9] Experimental Example 8 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 90 parts by mass of expanded graphite, 5 parts by mass of carbon fiber fragments, and 5 parts by mass of fibrous carbon. Experimental Example 9 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 60 parts by mass of expanded graphite, 30 parts by mass of carbon fiber fragments, and 10 parts by mass of fibrous carbon.

[0058] [Experimental Examples 10-12] Experimental Example 10 was conducted in the same manner as Experimental Example 1, except that carbon fiber pieces were not used and a carbon mixture was prepared with a ratio of 90 parts by mass of expanded graphite and 10 parts by mass of fibrous carbon. Experimental Example 11 was conducted in the same manner as Experimental Example 10, except that a carbon mixture was prepared with a ratio of 80 parts by mass of expanded graphite and 20 parts by mass of fibrous carbon. Experimental Example 12 was conducted in the same manner as Experimental Example 10, except that a carbon mixture was prepared with a ratio of 70 parts by mass of expanded graphite and 30 parts by mass of fibrous carbon.

[0059] [Experimental Examples 13-14] Experimental Example 13 was conducted in the same manner as Experimental Example 1, except that expanded graphite with an average particle size of 25 μm (EC500, manufactured by Ito Graphite Industry Co., Ltd.) was used as the expanded graphite, and a carbon mixture was prepared with a mixing ratio of 80 parts by mass of expanded graphite, 10 parts by mass of carbon fiber pieces, and 10 parts by mass of fibrous carbon. Experimental Example 14 was conducted in the same manner as Experimental Example 1, except that expanded graphite with an average particle size of 7 μm (EC1500, manufactured by Ito Graphite Industry Co., Ltd.) was used as the expanded graphite, and a carbon mixture was prepared with a mixing ratio of 80 parts by mass of expanded graphite, 10 parts by mass of carbon fiber pieces, and 10 parts by mass of fibrous carbon.

[0060] [Experimental Examples 15-17] Experimental Example 15 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 10 parts by mass of expanded graphite, 80 parts by mass of carbon fiber fragments, and 10 parts by mass of fibrous carbon. Experimental Example 16 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 10 parts by mass of expanded graphite, 10 parts by mass of carbon fiber fragments, and 80 parts by mass of fibrous carbon. Experimental Example 17 was conducted in the same manner as Experimental Example 1, except that a carbon mixture was prepared using a ratio of 1 part by mass of expanded graphite, 1 part by mass of carbon fiber fragments, and 1 part by mass of fibrous carbon.

[0061] [Experimental Example 18] Experimental Example 18 was conducted in the same manner as Experimental Example 1, except that sphericalized natural graphite particles (average particle size 10 μm) were used instead of expanded graphite, and a carbon mixture was prepared with a mixing ratio of 80 parts by mass of sphericalized natural graphite, 10 parts by mass of carbon fiber fragments, and 10 parts by mass of fibrous carbon.

[0062] [Experimental Examples 19-21] In Experiment 1, fibrous carbon was used alone instead of the carbon mixture, and polyvinylidene fluoride (PVdF) was used instead of CMC and SBR. A mixture of 50 parts by mass of fibrous carbon and 50 parts by mass of PVdF was prepared, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added as a dispersion medium and mixed to prepare a raw material slurry. Experiment 19 was carried out in the same manner as Experiment 1 otherwise. Experiment 20 was carried out in the same manner as Experiment 19 except that spherical natural graphite particles (average particle size 10 μm) were used instead of fibrous carbon. Experiment 21 was carried out in the same manner as Experiment 19 except that expanded graphite (EC1500, manufactured by Ito Graphite Industry Co., Ltd.) with an average particle size of 7 μm was used instead of fibrous carbon.

[0063] [Experimental Examples 22-24] Experimental Example 22 was conducted in the same manner as Experimental Example 19, except that fibrous carbon was not used alone, but as a carbon mixture consisting of 1 part by mass of sphericalized natural graphite particles (average particle size 10 μm), 1 part by mass of carbon fiber fragments (fiber diameter 7 μm, average length 250 μm: XN-100-25M manufactured by Nippon Graphite Fiber), and 1 part by mass of fibrous carbon. Experimental Example 23 was conducted in the same manner as Experimental Example 22, except that carbon fiber fragments with a fiber diameter of 7 μm and an average length of 150 μm (XN-100-15M manufactured by Nippon Graphite Fiber) were used as the carbon fiber fragments. Experimental Example 24 was conducted in the same manner as Experimental Example 22, except that expanded graphite with an average particle size of 7 μm (EC1500 manufactured by Ito Graphite Industry) was used instead of carbon fiber fragments.

[0064] [Experimental Example 25] Experimental Example 25 was conducted in the same manner as Experimental Example 1, except that the carbon mixture material from Experimental Example 22 was used as the carbon mixture material.

[0065] [Experimental Example 26] Carbon paper (114 μm thick: Toray TGP-H-030) was used as the carbon current collector. This carbon current collector was used as is (without polyimide sheet) as the sample for volume resistivity measurement. In addition, this carbon current collector was cut into a φ16 mm disc shape (without copper foil) and used as an electrode. Experiment 26 was conducted in the same manner as Experiment 1 otherwise.

[0066] [Results and Discussion] Table 1 summarizes the side reaction capacity and volume resistivity for Experimental Examples 1-26. Note that in Table 1, the side reaction capacity for Experimental Example 26 is 173.1 mA / cm². 3 The value was normalized with 1, and the volume resistivity was normalized with the volume resistivity of 6.72 mΩ·cm from experimental example 26 as 1. Figure 4 summarizes the volume resistivity (horizontal axis) and side reaction capacity (vertical axis) for experimental examples 1 to 26.

[0067] As shown in Table 1 and Figures 4A and 4B, in Experimental Examples 1-5, which contain at least expanded graphite and carbon fiber pieces, and satisfy all of the following conditions: the proportion of expanded graphite is greater than 60% by mass but less than 90% by mass, the proportion of carbon fiber pieces is greater than 0% by mass but less than 30% by mass, and the average particle size of the expanded graphite is greater than 7 μm but less than 25 μm, the volume resistivity is 60 mAh / cm² when compared to Experimental Example 26, which uses carbon paper as the carbon current collector. 3 The following results showed that the side reaction capacity was 10 mΩ·cm or less, demonstrating that low resistance and low side reaction can be achieved simultaneously. Of these, experimental examples 1-4, which included expanded graphite, carbon fiber pieces, and fibrous graphite, showed that even lower volume resistivity and side reaction capacity could be achieved.

[0068] In contrast, in Experimental Example 6, which contained expanded graphite and carbon fiber fragments, but with a carbon fiber fragment ratio of 30 mass%, the volume resistivity was slightly higher and the side reaction capacity was also higher than in Experimental Examples 1-5. It was inferred that in Experimental Example 6, which had a high proportion of carbon fiber fragments, the arrangement of carbon materials tended to be sparser, the electron conduction paths were more easily interrupted, and carrier ions could easily penetrate into the carbon current collector, resulting in higher volume resistivity and side reaction capacity. Furthermore, in Experimental Example 7, which also contained expanded graphite and carbon fiber fragments, but with expanded graphite at 90 mass%, the side reaction capacity was similar to that of Experimental Examples 1-5, but the volume resistivity was higher. It was inferred that in Experimental Example 7, which had a high proportion of expanded graphite, there was a lack of carbon fiber fragments and fibrous carbon that promote electron conduction inside the carbon current collector, resulting in higher volume resistivity.

[0069] Furthermore, in Experimental Example 8, which included expanded graphite, carbon fiber fragments, and fibrous graphite, but contained 90% by mass of expanded graphite, the side reaction capacity was similar to that of Experimental Examples 1-5, but the volume resistivity was high. The side reaction capacity and volume resistivity of Experimental Example 8 were similar to those of Experimental Example 7, which also contained 90% by mass of expanded graphite. It was inferred that the volume resistivity was high in Experimental Example 8 for the same reasons as in Experimental Example 7. In addition, in Experimental Example 9, which also included expanded graphite, carbon fiber fragments, and fibrous graphite, but contained 60% by mass of expanded graphite and 30% by mass of carbon fiber fragments, the side reaction capacity was high and the volume resistivity was also high. The side reaction capacity and volume resistivity of Experimental Example 9 were higher than those of Experimental Example 6, which also contained 60% by mass of expanded graphite. In Experimental Example 9, similar to Experimental Example 6, the presence of more carbon fiber fragments, coupled with less expanded graphite, resulted in a more sparse arrangement of carbon materials, more easily interrupted electron conduction paths, and easier penetration of carrier ions into the carbon current collector. This was presumed to have led to higher volume resistivity and side reaction capacity.

[0070] Furthermore, in experimental examples 10-12, which included expanded graphite and fibrous graphite but did not contain carbon fiber fragments, the volume resistivity was higher than in experimental examples 1-9. It was inferred that in experimental examples 10-12, which did not contain carbon fiber fragments, electron conduction over relatively long distances was not smooth, resulting in higher volume resistivity.

[0071] Furthermore, in Experimental Example 13, which contained expanded graphite, carbon fiber fragments, and fibrous graphite in the same ratios as in Experimental Examples 2-4, but with an average particle size of 25 μm for the expanded graphite, the side reaction capacity was low, but the volume resistivity was slightly higher. It was inferred that in Experimental Example 13, where the particle size of the expanded graphite was large, the area of ​​each expanded graphite particle covering the surface of the carbon current collector was larger, which further suppressed the penetration of carrier ions into the carbon current collector and thus suppressed side reactions. On the other hand, the arrangement of carbon materials tended to be sparser, and the electron conduction paths were more easily interrupted, resulting in a higher volume resistivity. In addition, in Experimental Example 14, which contained expanded graphite, carbon fiber fragments, and fibrous graphite in the same ratios as in Experimental Examples 2-4, but with an average particle size of 7 μm for the expanded graphite, the side reaction capacity was high, and the volume resistivity was also high. In Experimental Example 14, where the expanded graphite particles were small, it was inferred that the small area of ​​each expanded graphite particle covering the surface of the carbon current collector allowed carrier ions to easily penetrate into the carbon current collector through the gaps, resulting in a high side reaction capacity. Furthermore, the short distance of electron conduction in the in-plane direction of the expanded graphite caused the electron conduction paths to be easily interrupted, leading to a high volume resistivity.

[0072] Experimental examples 15-17, which contained expanded graphite, carbon fiber fragments, and fibrous graphite but had less expanded graphite, showed high side reaction capacity and high volume resistivity.

[0073] In Experiment 18, where spheroidized natural graphite was used instead of the expanded graphite used in Experiments 2-4, the side reaction capacity and volume resistivity were high.

[0074] In Experiment 19 with 100% fibrous graphite, Experiment 20 with 100% spheroidized natural graphite, and Experiment 21 with 100% spheroidized natural graphite, the side reaction capacity and volume resistivity were high. In particular, Experiment 21 with 100% spheroidized graphite showed a larger side reaction capacity than Experiment 19 with 100% fibrous graphite and Experiment 20 with 100% spheroidized natural graphite.

[0075] In Experiment 22, which used carbon fiber pieces with an average length of 250 μm, the side reaction capacity and volume resistivity were lower than in Experiment 23, which used carbon fiber pieces with an average length of 150 μm. From this, it was inferred that an average length of 150 μm or more for the carbon fiber pieces is preferable.

[0076] In Experiment 24, which used expanded graphite with an average particle size of 7 μm instead of the carbon fiber pieces used in Experiments 22 and 23, the side reaction capacity and volume resistivity were higher than in Experiments 22 and 23.

[0077] In Experimental Example 25, where CMC and SBR were used instead of PVdF as the binder in Experimental Example 22, the side reaction capacity and volume resistivity were higher than in Experimental Example 22. In Experimental Examples 1 to 18, CMC and SBR were used as binders, but it was expected that using PVdF instead would reduce the side reaction capacity and volume resistivity. However, in the carbon current collector of Experimental Example 25, the proportion of CMC and SBR as binders was 2% by mass, while in the carbon current collector of Experimental Example 22, the proportion of PVdF as the binder was 50% by mass. It was inferred that the lower side reaction capacity in Experimental Example 22 was due to the large amount of binder and small amount of carbon material contained in the carbon current collector. From these findings, it was found that the material and amount of binder should be appropriately adjusted to obtain the desired characteristics.

[0078] Figures 5, 6, and 8 show SEM images of the surface of carbon current collectors from Experimental Examples 1, 2, and 10. As shown in Figures 5 and 6, in the carbon current collectors of Experimental Examples 1 and 2, expanded graphite with good particle morphology covered the current collector surface, suggesting that lithium ion intercalation / release and other surface reactions were suppressed. It was also suggested that carbon fiber fragments and fibrous carbon promoted electron conduction. On the other hand, as shown in Figure 8, in the carbon current collector of Experimental Example 10, expanded graphite did not cover the current collector surface, and the expanded graphite and fibrous carbon were dispersed without any particular orientation. A similar trend was observed in experimental examples where side reactions during charging and discharging were large. The average particle size of expanded graphite can be derived from the SEM images, but the boundaries of the expanded graphite are difficult to discern in the SEM image in Figure 5, for example. In such cases, for example, a field of view in which the particle size of the expanded graphite can be easily observed can be selected, and the average particle size of the expanded graphite can be derived from the SEM image observed in that field of view (see, for example, Figure 7).

[0079] Table 2 summarizes the values ​​of temperature T, temperature T+X, weight change H(T), weight change H(T+X), and H(T+X) / H(T) obtained by thermogravimetric analysis for experimental examples 1-5 and 15-17. Figure 9 shows the differential curves of the thermogravimetric analysis for experimental examples 1, 2, and 15-17. In experimental examples 1-5, where the side reaction capacity was low and the volume resistivity was low, the value of H(T+X) / H(T) was between 0.13 and 0.48. In contrast, in experimental examples 15-17, where the side reaction capacity was high and the volume resistivity was high, the value of H(T+X) / H(T) was either less than 0.13 or greater than 0.48. From the above, it was found that if the material contains at least expanded graphite and carbon fiber pieces and H(T+X) / H(T) is between 0.13 and 0.48, both low side reactions and low resistance can be achieved. Furthermore, the absolute value of the weight change H(T+X) was particularly large in Experimental Examples 15 and 17, which had a high proportion of carbon fiber fragments, and particularly small in Experimental Example 16, which had a high proportion of fibrous graphite. On the other hand, no such trend was observed in the absolute value of the weight change H(T). From this, it was inferred that the weight change H(T+X) is related to the weight change of carbon fiber fragments and fibrous graphite, and that the weight change H(T) is related to the weight change of expanded graphite, and that the value of H(T+X) / H(T) corresponds to the ratio of expanded graphite, carbon fiber fragments, and fibrous graphite. In addition, the value of H(T+X) / H(T) was smallest in Experimental Example 2, where the average particle size of expanded graphite was 15 μm, and was large in Experimental Example 3, where the average particle size of expanded graphite was 10 μm, and in Experimental Example 4, where it was 20 μm. Therefore, it was inferred that the value of H(T+X) / H(T) also corresponds to the average particle size of expanded graphite.

[0080] Figure 10 shows the charge-discharge curves obtained from the charge-discharge test of Experimental Example 1. It was confirmed that the lithium secondary battery using the carbon current collectors from Experimental Example 1 as the current collectors for both the positive and negative electrodes exhibited stable charge-discharge behavior.

[0081] From the above, it was found that a carbon current collector in which the proportion of expanded graphite is greater than 60% but less than 90% by mass, the proportion of carbon fiber pieces is greater than 0% but less than 30% by mass, and the average particle size of the expanded graphite is greater than 7 μm but less than 25 μm is usable as a current collector and can achieve both low side reactions and low resistance. Furthermore, it was found that a carbon current collector containing at least expanded graphite and carbon fiber pieces, with an H(T+X) / H(T) value of 0.13 or more and 0.48 or less, is also usable as a current collector and can achieve both low side reactions and low resistance.

[0082] [Table 1]

[0083] [Table 2] [Industrial applicability]

[0084] This disclosure is applicable to the field of energy storage devices. [Explanation of symbols]

[0085] 10 Carbon current collector, 12 Expanded graphite, 14 Carbon fiber pieces, 16 Fibrous carbon, 18 Current collector slurry, 19 Substrate, 20 Energy storage device, 21 Current collector, 22 Positive electrode mixture, 23 Positive electrode sheet, 24 Current collector, 25 Negative electrode mixture, 26 Negative electrode sheet, 28 Separator, 29 Non-aqueous electrolyte, 32 Cylindrical case, 34 Positive electrode terminal, 36 Negative electrode terminal.

Claims

1. The carbon material comprises at least expanded graphite and carbon fiber fragments, and may also contain fibrous carbon, wherein the proportion of expanded graphite is 70% by mass or more and 80% by mass or less, the proportion of carbon fiber fragments is 10% by mass or more and 20% by mass or less, the average particle size of the expanded graphite is 10 μm or more and 20 μm or less, the average length of the carbon fiber fragments is 500 μm or less, and the material includes a binder for binding the carbon material. Carbon current collector.

2. The carbon material comprises at least expanded graphite, carbon fiber fragments, and fibrous carbon, wherein the proportion of expanded graphite is 70% by mass or more and 80% by mass or less, the proportion of carbon fiber fragments is 10% by mass or more and 20% by mass or less, the average particle size of the expanded graphite is 10 μm or more and 20 μm or less, and the material also comprises a binder for binding the carbon material. Carbon current collector.

3. The proportion of fibrous carbon to the total of the expanded graphite, carbon fiber pieces, and fibrous carbon is 0% by mass or more and 15% by mass or less. A carbon current collector according to claim 1 or 2.

4. The proportion of fibrous carbon to the total of the expanded graphite, carbon fiber pieces, and fibrous carbon is 0% by mass or more and 10% by mass or less. A carbon current collector according to claim 1 or 2.

5. The carbon fiber pieces have an average fiber diameter of 5 μm or more and 15 μm or less, an average length of 50 μm or more and 300 μm or less, and the fibrous carbon has an average fiber diameter of 100 nm or more and 200 nm or less. A carbon current collector according to any one of claims 1 to 4.

6. Thermogravimetric measurements were performed in air at a heating rate of 2°C / min. In the differential curve obtained by differentiating the weight change [μg] with respect to elapsed time, a peak in weight loss appeared in the range of 700°C to 775°C, and a change in weight loss also appeared at higher temperatures. When the weight change at the temperature T [°C] of the peak weight loss is denoted as H(T) [μg / sec], and the weight change at the temperature T+X [°C], which is midway between the start of the change in weight loss and the end of the change in the differential curve, is denoted as H(T+X) [μg / sec], the value of H(T+X) / H(T) is between 0.13 and 0.

48. A carbon current collector according to any one of claims 1 to 5.

7. The value of H(T+X) / H(T) is 0.15 or more and 0.40 or less. The carbon current collector according to claim 6.

8. It is in the form of a sheet with a thickness of 5 μm or more and 500 μm or less. A carbon current collector according to any one of claims 1 to 7.

9. The carbon material comprises at least expanded graphite and carbon fiber fragments, and may also contain fibrous carbon, wherein the proportion of expanded graphite to the total of the expanded graphite, carbon fiber fragments, and fibrous carbon is 70% by mass or more and 80% by mass or less, the average length of the carbon fiber fragments is 500 μm or less, and in the differential curve obtained by differentiating the weight change [μg] with respect to elapsed time when thermogravimetric measurement is performed at a heating rate of 2°C / min in air, a peak in weight loss appears in the range of 700°C to 775°C, and a change in weight loss appears at a higher temperature, and when the weight change at the temperature T [°C] of the weight loss peak is taken as H(T) [μg / sec], and the weight change at the temperature T+X [°C] located midway between the start of the change in weight loss and the end of the change in the differential curve is taken as H(T+X) [μg / sec], the value of H(T+X) / H(T) is 0.16 or more and 0.38 or less, and the material comprises a binder for binding the carbon material. Carbon current collector.

10. The carbon material comprises at least expanded graphite, carbon fiber fragments, and fibrous carbon, wherein the proportion of expanded graphite to the total of the expanded graphite, carbon fiber fragments, and fibrous carbon is 70% by mass or more and 80% by mass or less, and in the differential curve obtained by differentiating the weight change [μg] with respect to elapsed time when thermogravimetric measurement is performed at a heating rate of 2°C / min in air, a peak in weight loss appears in the range of 700°C to 775°C, and a change in weight loss appears at a higher temperature, and when the weight change at the temperature T [°C] of the weight loss peak is taken as H(T) [μg / sec], and the weight change at the temperature T+X [°C] located midway between the start of the change in weight loss and the end of the change in the differential curve is taken as H(T+X) [μg / sec], the value of H(T+X) / H(T) is 0.16 or more and 0.38 or less, and the material comprises a binder that binds the carbon material. Carbon current collector.

11. A positive electrode having a positive electrode active material, A negative electrode having a negative electrode active material, An ion-conducting medium interposed between the positive electrode and the negative electrode, Equipped with, The carbon current collector described in any one of claims 1 to 10 is provided as at least one of the current collectors for the positive electrode and the current collector for the negative electrode. Energy storage device.

12. A carbon current collector is manufactured using a carbon mixture comprising at least expanded graphite and carbon fiber fragments, and possibly fibrous carbon, wherein the proportion of expanded graphite is 70% by mass or more and 80% by mass or less, the proportion of carbon fiber fragments is 10% by mass or more and 20% by mass or less, the average particle size of the expanded graphite is 10 μm or more and 20 μm or less, and the average length of the carbon fiber fragments is 500 μm or less, and a binder. A method for manufacturing carbon current collectors.

13. A carbon current collector is manufactured using a carbon mixture containing at least expanded graphite, carbon fiber fragments, and fibrous carbon, wherein the proportion of expanded graphite is 70% by mass or more and 80% by mass or less, the proportion of carbon fiber fragments is 10% by mass or more and 20% by mass or less, and the average particle size of the expanded graphite is 10 μm or more and 20 μm or less, and a binder. A method for manufacturing carbon current collectors.

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