Negative electrode for electricity storage device, and electricity storage device

Abstract

A negative electrode for a power storage element according to one aspect of the present invention has a negative electrode active material layer containing natural graphite particles and a binder, the natural graphite particles having an internal porosity of 2% or less, and the natural graphite particles having an average particle diameter of 6 [mu] m to 10 [mu] m (inclusive), the content of the binder in the negative electrode active material layer is from 0.1% by mass to 2.0% by mass (inclusive), the binder is particulate, and the ratio (B / A) of the average particle diameter B of the binder to the average particle diameter A of the natural graphite particles is from 0.015 to 0.0375 (inclusive).

Description

Technical Field

[0001] This invention relates to a negative electrode for an energy storage device and an energy storage device. Background Technology

[0002] Non-aqueous electrolyte secondary batteries, represented by lithium-ion batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles, due to their high energy density. These non-aqueous electrolyte secondary batteries typically consist of a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte between the electrodes. Charging and discharging are achieved through the exchange of charge-carrying ions between the two electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double-layer capacitors, as well as energy storage components using electrolytes other than non-aqueous electrolytes, have also become widely used.

[0003] As an energy storage element, it typically comprises an electrode body consisting of a positive electrode containing a positive active material and a negative electrode containing a negative active material, superimposed on each other through a separator. This electrode body, along with the electrolyte, is housed in a container to form the energy storage element. Carbon materials, primarily graphite, are widely used as the negative active material (see Patent Document 1, etc.).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-085906 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] For energy storage components, it is required that the energy (power: W) that can be taken in per unit time during charging is large, that is, the input (electricity) during charging.

[0009] The present invention was made based on the above circumstances, and its object is to provide a negative electrode for a storage element that can increase the initial input of the storage element, and a storage element having such a negative electrode for a storage element.

[0010] Methods for solving problems

[0011] In one aspect of the present invention, the negative electrode of the energy storage element has a negative electrode active material layer containing natural graphite particles and a binder, wherein the internal porosity of the natural graphite particles is 2% or less, the average particle size of the natural graphite particles is 6 μm or more and 10 μm or less, the content of the binder in the negative electrode active material layer is 0.1% by mass or more and 2.0% by mass or less, the binder is in particulate form, and the ratio (B / A) of the average particle size B of the binder to the average particle size A of the natural graphite particles is 0.015 or more and 0.0375 or less.

[0012] Another aspect of the present invention provides an energy storage element having a negative electrode for an energy storage element according to one aspect of the present invention.

[0013] Invention Effects

[0014] In one aspect of the present invention, the negative electrode of the energy storage element can increase the initial input of the energy storage element.

[0015] In another aspect of the present invention, the initial input of the energy storage element is large. Attached Figure Description

[0016] [ Figure 1 ] Figure 1 This is a perspective view showing one embodiment of an energy storage device.

[0017] [ Figure 2 ] Figure 2 This is a schematic diagram illustrating one embodiment of an energy storage device composed of multiple energy storage elements. Detailed Implementation

[0018] First, the negative electrode for the energy storage element and the general outline of the energy storage element disclosed in this specification will be explained.

[0019] [1] In one aspect of the present invention, the negative electrode of the energy storage element has a negative electrode active material layer containing natural graphite particles and a binder, wherein the internal porosity of the natural graphite particles is 2% or less, the average particle size of the natural graphite particles is 6 μm or more and 10 μm or less, the content of the binder in the negative electrode active material layer is 0.1% by mass or more and 2.0% by mass or less, the binder is in the form of particles, and the ratio (B / A) of the average particle size B of the binder to the average particle size A of the natural graphite particles is 0.015 or more and 0.0375 or less.

[0020] The negative electrode of the energy storage element described above [1] can increase the initial input of the energy storage element. The reason is still uncertain, but it is speculated that the reason is as follows.

[0021] Generally, natural graphite is a material that can easily achieve a balance between procurement cost and output performance when used as a negative electrode active material. When natural graphite is used as a negative electrode active material, it is mostly processed into spherical shape, thereby making it easy for natural graphite particles to form internal voids. If such natural graphite particles and adhesives are used to prepare a negative electrode mixture, the adhesive will enter the internal voids of the natural graphite particles, and therefore sometimes it is not possible to properly bond the natural graphite particles together. In contrast, in the negative electrode for the energy storage device described above [1], since the internal porosity of the natural graphite particles used as the negative electrode active material is less than 2%, the adhesive enters less of the internal voids, and the adhesive can be easily disposed between the natural graphite particles. In addition, by keeping the content of the adhesive within the above range, the adhesive can properly bond the natural graphite particles together without over-coating the natural graphite particles, thus easily ensuring sufficient conductivity between the natural graphite particles. Furthermore, by ensuring that the average particle size of the natural graphite particles is within the aforementioned range, the internal voids of the natural graphite particles are small, making it difficult for the adhesive to be trapped within these internal voids. Additionally, since the adhesive is granular and the ratio (B / A) of the average particle size B of the adhesive to the average particle size A of the natural graphite particles is within the aforementioned range, the adhesive is difficult to trap within the internal voids of the natural graphite particles and can be easily disposed between them. Furthermore, the natural graphite particles can be well bonded together. That is, in the negative electrode of this energy storage element, since the adhesive is easily disposed between the natural graphite particles without excessively coating them, the natural graphite particles can be well bonded together. Therefore, the negative electrode of the energy storage element described above [1] can increase the initial input of the energy storage element.

[0022] "Graphite" refers to the average interplanar spacing (d) of the (002) plane in an X-ray diffraction pattern using CuKα rays, measured before or during charging and discharging. 002 The carbon material is 0.33 nm or larger and less than 0.34 nm. Here, the "discharge state" of the carbon material refers to the state in which the carbon material used as the negative electrode active material is discharged in a manner in which charge-carrying ions that are absorbed and released during charging and discharging are fully released. For example, in a half-cell using a negative electrode containing the above-mentioned carbon material as the working electrode and using metallic Li as the counter electrode, the state is one with an open-circuit voltage of 0.6 V or larger.

[0023] To determine the aforementioned X-ray diffraction, when preparing carbon material from the assembled energy storage element, specifically, a sample containing carbon material in the aforementioned discharge state can be prepared using the following method. First, the energy storage element is discharged at a constant current of 0.1C to the discharge termination voltage under normal use. Here, "normal use" refers to using the energy storage element under recommended or specified charge and discharge conditions. Then, the energy storage element is disassembled, and the negative electrode is removed. Using the removed negative electrode as the working electrode and metallic Li as the counter electrode, a half-cell is assembled. With the open-circuit voltage of the half-cell less than 0.6V, the half-cell is discharged at a current of 0.1C until the open-circuit voltage becomes 0.6V or higher. It should be noted that the discharge of the half-cell refers to the oxidation reaction in which charge-transporting ions are released from the carbon material (graphite) serving as the negative electrode active material. The half-cell is disassembled, the negative electrode is removed, thoroughly cleaned with dimethyl carbonate, and then dried under reduced pressure at room temperature. Next, powder from the negative electrode active material layer is collected from the negative electrode. The obtained negative electrode active material layer powder is used as a sample containing carbon material for measuring the above-mentioned X-ray diffraction. The operation from the disassembly of the energy storage element to the collection of the sample containing the carbon material to be measured is carried out in a dry air atmosphere with a dew point below -40°C as required.

[0024] "Natural graphite" refers to graphite obtained from natural resources. It should be noted that particles obtained by coating the surface of natural graphite particles with materials other than natural graphite are also considered natural graphite particles. In the above X-ray diffraction pattern, natural graphite exhibits four peaks within a diffraction angle 2θ ranging from 40° to 50°. These four peaks are two from a hexagonal crystal structure and two from a rhombohedral crystal structure. In the case of synthetic graphite, typically only two peaks originating from the hexagonal crystal structure appear. In the X-ray diffraction pattern, the ratio of the peak intensity originating from the (012) plane to the peak intensity originating from the (100) plane ((012) / (100)) is preferably 0.3 or more, more preferably 0.4 or more. The aforementioned peak intensity ratio ((012) / (100)) is preferably 0.6 or less. Here, the (100) plane originates from a hexagonal crystal structure, and the (012) plane originates from a rhombohedral crystal structure.

[0025] The "internal porosity" of natural graphite particles refers to the area ratio of the voids within the particles relative to the total area of ​​the particles in a cross-section of a natural graphite particle observed using a scanning electron microscope (SEM) (cross-sectional SEM image). The "internal porosity" of natural graphite particles (the area ratio of voids within the particles relative to the total area of ​​the particles) can be determined through the following steps.

[0026] (1) Preparation of the test sample

[0027] The negative electrode, which is the object of measurement, is fixed with thermosetting resin. For the resin-fixed negative electrode, the cross-section is exposed by ion milling to prepare the test sample. It should be noted that when the assembled storage element includes the negative electrode, the negative electrode is prepared by the following steps, which are also repeated below. The storage element is discharged at a constant current of 0.1C to the discharge termination voltage used in normal operation, thus reaching a discharged state. The storage element in this discharged state is disassembled, the negative electrode is removed, thoroughly cleaned with dimethyl carbonate, and then dried under reduced pressure at room temperature. The operation from the disassembly of the storage element to the preparation of the negative electrode is carried out in a dry air atmosphere with a dew point below -40°C.

[0028] (2) Obtaining cross-sectional SEM images

[0029] For obtaining cross-sectional SEM images, a JSM-7001F (manufactured by Nippon Electron Ltd.) was used as a scanning electron microscope. Cross-sectional SEM images were obtained by observing secondary electron images. The accelerating voltage was set to 15 kV. The magnification was set to a level where 3 to 15 natural graphite particles appeared in a single field of view. The obtained cross-sectional SEM images were saved as image files. Furthermore, various conditions such as spot diameter, working distance, irradiation current, brightness, and focusing were appropriately set to ensure that the outlines of the natural graphite particles were clearly defined.

[0030] (3) Shearing of the contours of natural graphite particles

[0031] Using the image cropping function of Adobe Photoshop Elements 11, the outlines of natural graphite particles were cropped from the acquired cross-sectional SEM image. This cropping was performed by using the Quick Selection Tool to select the area outside the natural graphite particle outline, and then editing the area outside the natural graphite particles to a black background. If fewer than three natural graphite particles could be cropped at this point, the cross-sectional SEM image was acquired again, and this process was repeated until at least three natural graphite particles could be cropped.

[0032] (4) Binarization

[0033] For the image of the first natural graphite particle in the sheared natural graphite particles, the image analysis software PopImaging 6.00 was used to perform binarization, setting the concentration 20% lower than the highest concentration as the threshold. Through binarization, the area of ​​the side with higher concentration was calculated, which was then used as the "area S1 of the voids within the particle".

[0034] Next, for the same image of the first natural graphite particle as before, a concentration of 10% is used as a threshold for binarization. Through binarization, the outer edge of the natural graphite particle is determined, and the area inside this outer edge is calculated, which is then used as the "area S0 of the particle as a whole".

[0035] Using the calculated S1 and S0, calculate the ratio of S1 to S0 (S1 / S0), and then calculate the "area ratio R1 of the voids within the particle relative to the total area of ​​the particle" in the first natural graphite particle.

[0036] For the images of the second and subsequent natural graphite particles in the cut natural graphite particles, the above binarization process is also performed, and the areas S1 and S0 are calculated. Based on the calculated areas S1 and S0, the area ratios R2, R3, ... of the voids within each natural graphite particle relative to the total area of ​​the particle are calculated.

[0037] (5) Determination of internal porosity

[0038] The average value of the area ratios R1, R2, R3, ... of the internal voids of all natural graphite particles relative to the total area of ​​the particles, calculated through binarization, is obtained to determine the "internal porosity" of the natural graphite particles.

[0039] It should be noted that alternatives to the scanning electron microscope used in "Obtaining Cross-Sectional SEM Images", the image editing software used in "Shearing the Profile of Natural Graphite Particles", and the image analysis software used in "Binarization Processing", devices and software capable of performing equivalent measurements, image editing, and image analysis can be used.

[0040] The “average particle size” of natural graphite particles refers to the value (median particle size) calculated according to JIS-Z-8819-2 (2001) as the 50% cumulative distribution of the volume basis, based on the particle size distribution determined by laser diffraction scattering of the diluted solution obtained by solvent dilution of particles in accordance with JIS-Z-8825 (2013). It should be noted that, regarding the median particle size determined above, in the negative cross-sectional SEM image obtained using a scanning electron microscope (SEM), it was confirmed that it is approximately consistent with the arithmetic mean of the equivalent circle diameter determined by extracting 100 natural graphite particles, avoiding extremely large and extremely small natural graphite particles. The negative cross-sectional SEM image was obtained by the steps described in “(1) Preparation of the sample for measurement” and “(2) Acquisition of cross-sectional SEM image” in the determination of the “internal porosity” of natural graphite particles above.

[0041] [2] In the negative electrode of the energy storage element described above [1], the content of the adhesive in the negative electrode active material layer may be less than 1.0% by mass.

[0042] In the negative electrode of the energy storage element described above [2], since the content of the adhesive in the negative electrode active material layer is less than the upper limit, the input maintenance rate during charge-discharge cycles can be increased. The reason is uncertain, but it is speculated that the adhesive swells easily by being impregnated with electrolytes or the like. On the other hand, by making the content of the adhesive in the negative electrode active material layer less than the upper limit, even if the adhesive swells, the amount of adhesive coating on the natural graphite particles can be maintained within a suitable range. Therefore, the surface resistance of the natural graphite particles increases and decreases, which increases the input maintenance rate during charge-discharge cycles.

[0043] [3] In the negative electrode for the energy storage element described in [1] or [2] above, the ratio (B / A) of the average particle size B of the adhesive to the average particle size A of the natural graphite particles may be 0.018 or more and 0.030 or less.

[0044] In the negative electrode of the energy storage element described above [3], since the ratio (B / A) of the average particle size B of the adhesive to the average particle size A of the natural graphite particles is within the above range, the adhesive is not easily trapped in the internal gaps of the natural graphite particles and can be more easily disposed between the natural graphite particles. In addition, the natural graphite particles can be bonded to each other better.

[0045] It should be noted that the "average particle size" of the adhesive refers to the arithmetic mean of the equivalent circle diameters measured from 100 particles extracted from the surface SEM image of the negative electrode obtained using a scanning electron microscope (SEM), avoiding extremely large and extremely small adhesive particles. A JSM-7001F (manufactured by Nippon Electron Ltd.) was used as the scanning electron microscope to obtain the surface SEM image of the negative electrode. The surface SEM image was obtained by observing secondary electron images. The accelerating voltage was set to 5kV. The magnification was set to a magnification where there were 3 to 15 natural graphite particles appearing in one field of view. The obtained surface SEM image was saved as an image file. Furthermore, various conditions such as spot diameter, working distance, irradiation current, brightness, and focus were appropriately set to ensure that the outlines of the adhesive particles were clearly defined.

[0046] [4] In any of the above [1] to [3], the average particle size of the adhesive can be 150 nm or more and 225 nm or less.

[0047] In the negative electrode of the energy storage element described above [4], since the average particle size of the adhesive is within the above range, the adhesive is not easily trapped in the internal gaps of the natural graphite particles, and can be more easily disposed between the natural graphite particles. In addition, the natural graphite particles can be bonded to each other better.

[0048] [5] In the negative electrode of the energy storage element described in any one of [1] to [4] above, the average aspect ratio of the particles of the adhesive can be 1.5 or less.

[0049] In the negative electrode for the energy storage element described above [5], since the average aspect ratio of the adhesive particles is below the upper limit, the natural graphite particles can be bonded to each other more effectively. It should be noted that "average aspect ratio of the adhesive particles" refers to the arithmetic mean of the ratio of the longest major axis T to the longest minor axis Y in the direction perpendicular to the major axis T, measured by extracting 10 particles from the surface SEM image of the negative electrode using a scanning electron microscope (SEM), avoiding extremely large and extremely small adhesive particles. The surface SEM image of the negative electrode is obtained by the same steps as when determining the average particle size of the adhesive.

[0050] [6] Another aspect of the present invention provides an energy storage element having a negative electrode for an energy storage element as described in any one of [1] to [5] above.

[0051] The energy storage element described above [6] has a large initial input because it has a negative terminal for energy storage elements as described in any one of [1] to [5].

[0052] Hereinafter, a negative electrode for an energy storage element, an energy storage element, an energy storage device, a method for manufacturing an energy storage element, and other embodiments of the present invention will be described in detail. It should be noted that the names of the constituent components (constituent elements) used in each embodiment are sometimes different from the names of the constituent components (constituent elements) used in the prior art.

[0053] [Negative terminal for energy storage components]

[0054] An embodiment of the present invention provides a negative electrode for an energy storage element (hereinafter also simply referred to as "negative electrode") having a negative electrode substrate and a negative electrode active material layer disposed directly or through an intermediate layer on the negative electrode substrate. This negative electrode is used as the negative electrode for non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, and other non-aqueous electrolyte energy storage elements, as well as energy storage elements using electrolytes other than non-aqueous electrolytes. This negative electrode is suitable as the negative electrode for non-aqueous electrolyte energy storage elements.

[0055] (Negative electrode substrate)

[0056] The negative electrode substrate is conductive. Regarding whether it possesses "conductivity," the volume resistivity measured according to JIS-H-0505 (1975) is used as a 10... -2 The threshold value (Ω·cm) is used for determination. The material used as the negative electrode substrate can be metals such as copper, nickel, stainless steel, nickel-plated steel, or their alloys, carbonaceous materials, etc. Copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials; from a cost perspective, foil is preferred. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0057] The average thickness of the negative electrode substrate is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 30 μm or less, even more preferably 4 μm or more and 25 μm or less, and particularly preferably 5 μm or more and 20 μm or less. By making the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be improved, and the energy density per unit volume of the energy storage element can be increased.

[0058] (Intermediate layer)

[0059] The intermediate layer is disposed between the negative electrode substrate and the negative electrode active material layer. The intermediate layer reduces the contact resistance between the negative electrode substrate and the negative electrode active material layer by including conductive agents such as carbon particles. The composition of the intermediate layer is not particularly limited; for example, it may include an adhesive and a conductive agent. It should be noted that the adhesive and conductive agent in the intermediate layer can be the same as those in the negative electrode active material layer, as described later.

[0060] (Negative electrode active material layer)

[0061] The negative electrode active material layer contains natural graphite particles and a binder. Depending on the requirements, the negative electrode active material layer may include other negative electrode active materials besides natural graphite particles, conductive agents, thickeners, fillers, and other arbitrary components.

[0062] Natural graphite particles are the component that functions as a negative electrode active material. The type of natural graphite particles is not particularly limited, and examples include flake graphite, block graphite (flake graphite), and amorphous graphite. From the viewpoint of ensuring crystallinity, flake graphite or block graphite is preferred, with flake graphite being more preferred. Natural graphite particles can be obtained by spheroidizing flake graphite and then compacting it. One type of natural graphite particle can be used alone, or two or more types can be used in combination. Natural graphite particles can be particles coated with carbon materials other than natural graphite particles (e.g., non-graphitic carbon). The carbon material coating can be achieved, for example, by coating the surface of the natural graphite particles with pitch and then firing them.

[0063] The upper limit for the internal porosity of natural graphite particles is 2%, preferably 1.5%, and more preferably 1%. If the internal porosity of natural graphite particles is below the above upper limit, the binder can be easily disposed between the natural graphite particles, thereby increasing the initial input. The lower limit for the internal porosity of natural graphite particles is not particularly limited, and from a manufacturing point of view, it can be 0.1%. The internal porosity of natural graphite particles can be above any of the above lower limits and below any of the above upper limits. The internal porosity of natural graphite particles can be adjusted by compacting the natural graphite particles, etc.

[0064] The lower limit for the average particle size of natural graphite particles is 6 μm, preferably 6.5 μm, and more preferably 7 μm. On the other hand, the upper limit for the average particle size of natural graphite particles is 10 μm, and sometimes preferably 9 μm or 8 μm. If the average particle size of natural graphite particles is above the aforementioned lower limit, it is possible to suppress the excessive coating of the surface of the natural graphite particles by the binder, thereby increasing the initial input. In addition, if the average particle size of natural graphite particles is below the aforementioned upper limit, the internal voids of the natural graphite particles are small, and the binder is not easily trapped in these internal voids. That is, even a small amount of binder is easily disposed between the natural graphite particles, thereby increasing the initial input. It should be noted that, as a method for obtaining natural graphite particles with a predetermined particle size, examples include pulverizing the natural graphite material using a pulverizer, classifier, etc. As pulverizing methods, examples include using a mortar and pestle, ball mill, sand mill, vibratory ball mill, planetary ball mill, jet mill, reverse jet mill, cyclone jet mill, or sieve, etc. Wet grinding, which involves the coexistence of water or organic solvents such as hexane, can also be used during pulverization. As a classification method, sieves, air classifiers, etc., can be used in both dry and wet processes as needed.

[0065] The content of natural graphite particles in the negative electrode active material layer is preferably 60% by mass or more and 99.5% by mass or less, more preferably 80% by mass or more and 99% by mass or less, and even more preferably 90% by mass or more and 99% by mass or less. By setting the content of natural graphite particles within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved, and the initial input can be further increased.

[0066] The negative electrode active material can include other negative electrode active materials besides natural graphite particles. Examples of other negative electrode active materials besides natural graphite particles include metallic Li; metals or half-metals such as Si and Sn; metal oxides or half-metal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li₄Ti₅O₅. 12 Titanium oxides such as LiTiO2 and TiNb2O7; polyphosphate compounds; silicon carbide; artificial graphite, non-graphitic carbon, and other carbon materials. "Non-graphitic carbon" refers to the average lattice spacing (d) of the (002) plane in the above X-ray diffraction pattern.002 The carbon material is 0.34 nm or larger and 0.42 nm or smaller. Natural graphite particles are preferably the main negative electrode active material. For example, in a cross-sectional SEM image of the negative electrode, the area ratio of natural graphite particles (area occupied by natural graphite particles / area occupied by all negative electrode active materials) relative to the total area occupied by all negative electrode active materials is preferably 90% or larger, more preferably 99% or larger, and practically can be 100%. It should be noted that the aforementioned cross-sectional SEM image of the negative electrode is obtained using the same steps as when determining the "internal porosity (area ratio of voids within the particles relative to the total area of ​​the particles)" in the natural graphite particles. The area occupied by the negative electrode active material is defined as the total area of ​​the negative electrode active material particles (including the area of ​​internal voids). It should be noted that, using a cross-sectional SEM image containing 10 or more particles of negative electrode active material in a single field of view, the area ratio of natural graphite particles relative to the total area occupied by all negative electrode active materials is calculated (area occupied by natural graphite particles / area occupied by all negative electrode active materials). Furthermore, the content of natural graphite particles in the total negative electrode active material contained in the negative electrode active material layer is preferably 90% by mass or more, more preferably 99% by mass or more, and practically can be 100% by mass. That is, it is particularly preferable to use only natural graphite particles as the negative electrode active material. In this way, by primarily using natural graphite particles as the negative electrode active material, the initial input becomes larger.

[0067] The content of all negative electrode active materials in the negative electrode active material layer is preferably 60% by mass or more and 99.5% by mass or less, more preferably 80% by mass or more and 99% by mass or less, and even more preferably 90% by mass or more and 99% by mass or less. By ensuring that the content of all negative electrode active materials is within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0068] Examples of adhesives include fluoropolymers (such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)), thermoplastic resins such as polyethylene, polypropylene, polyacrylic acid, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers. Among these, SBR is preferred due to its high flexibility and ease of bonding natural graphite particles together.

[0069] The adhesive is in particulate form. The lower limit of the average particle size of the adhesive is preferably 150 nm, more preferably 160 nm, and even more preferably 170 nm. On the other hand, the upper limit of the average particle size of the adhesive is preferably 225 nm, more preferably 210 nm, and even more preferably 200 nm. If the average particle size of the adhesive is above the aforementioned lower limit, the adhesive is less likely to be trapped within the internal voids of the natural graphite particles, and can be more easily disposed between the natural graphite particles. Furthermore, when the average particle size of the adhesive is below the aforementioned upper limit, the adhesiveness between the natural graphite particles is easily improved. The average particle size of the adhesive can be above or below any of the aforementioned lower limits and below any of the aforementioned upper limits.

[0070] The lower limit of the ratio (B / A) of the average particle size B of the binder to the average particle size A of the natural graphite particles is 0.015, preferably 0.018, more preferably 0.020, and even more preferably 0.022. The upper limit of the above ratio (B / A) is 0.0375, preferably 0.035, more preferably 0.030, and even more preferably 0.028. When the above ratio (B / A) is above the lower limit, it is easier to dissolve it between the natural graphite particles. In addition, when the above ratio (B / A) is below the upper limit, the adhesion of the binder between the natural graphite particles is easily improved. The above ratio (B / A) can be above any of the above lower limits and below any of the above upper limits.

[0071] The average aspect ratio of the particles used as the binder is preferably 1.5 or less, more preferably 1.4 or less, more preferably 1.3 or less, and even more preferably 1.2 or less. By setting the average aspect ratio of the binder particles to the above-mentioned upper limit, it is easy to improve the adhesion between natural graphite particles.

[0072] The lower limit of the binder content in the negative electrode active material layer is 0.1% by mass, preferably 0.3% by mass, and more preferably 0.5% by mass. On the other hand, the upper limit of the binder content in the negative electrode active material layer is 2.0% by mass, preferably 1.5% by mass, and more preferably 1.0% by mass. If the binder content in the negative electrode active material layer is above or above the above lower limit, the natural graphite particles can be easily bonded together. In addition, by keeping the binder content in the negative electrode active material layer below or above the above upper limit, the natural graphite particles are not over-coated, thus easily ensuring the conductivity between the natural graphite particles. That is, the initial input of the energy storage element can be increased. Furthermore, from the viewpoint of increasing the input maintenance rate during the charge-discharge cycle of the energy storage element, the upper limit of the binder content in the negative electrode active material layer is further preferably less than 1.0% by mass, more preferably 0.8% by mass, and sometimes particularly preferably 0.6% by mass. The binder content in the negative electrode active material layer can be above or above any of the above lower limits and below any of the above upper limits. It should be noted that the content of the binder in the negative electrode active material layer is determined by the amount of mass reduction when the binder contained in the negative electrode active material layer is heated and decomposed using a thermogravimetric differential thermal analysis (TG / DTA) device.

[0073] There are no particular limitations on conductive agents as long as they are conductive materials. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphite carbon, and graphene-based carbon. Examples of non-graphite carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and Ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. However, natural graphite particles contained in the negative electrode active material layer are not included in the conductive agent. Examples of conductive agents include powder and fibrous forms. One of these materials can be used alone, or two or more can be used in combination. In addition, these materials can be used in combination. For example, a material obtained by combining carbon black and CNTs can be used. From the viewpoint of electronic conductivity and coating properties, carbon black is preferred, and acetylene black is preferred.

[0074] The content of the conductive agent in the negative electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the energy storage element can be improved. In one embodiment of the present invention, the conductive agent is sometimes preferably not included in the negative electrode active material layer.

[0075] The thickener is not particularly limited. Examples of thickeners include polymers capable of dissolving or dispersing in aqueous solvents. Examples of such polymers include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When the thickener has functional groups that react with lithium, these functional groups can be deactivated beforehand by methylation or the like. The content of the thickener in the negative electrode active material layer is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less. It should be noted that, in this specification, a thickener refers to a substance whose viscosity is 400 mPa·s or more when an aqueous solution (an aqueous solution containing 1% by mass of the thickener) is prepared by dissolving or dispersing 1 part by mass of the thickener in 99 parts by mass of water. The above viscosity can be measured using a type B viscometer under the conditions of a rotation speed of 60 rpm and a temperature of 25°C. This thickener is not included in the binder.

[0076] The filler is not particularly limited. Examples of fillers include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silica, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; carbonates such as calcium carbonate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; and mineral-derived materials or their synthetic forms, such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica. In one embodiment of the invention, it is sometimes preferable that the negative electrode active material layer does not contain fillers.

[0077] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0078] [Energy Storage Components]

[0079] An energy storage element according to one embodiment of the present invention includes an electrode body, an electrolyte, and a container; the electrode body has a positive electrode, a negative electrode, and a separator; the container houses the electrode body and the electrolyte. The electrode body is typically a stacked type consisting of multiple positive electrodes and multiple negative electrodes separated by a separator, or a wound type consisting of positive and negative electrodes stacked with a separator between them. The electrolyte exists within the positive electrode, negative electrode, and separator. A lithium-ion secondary battery will be described as an example of an energy storage element. It should be noted that the negative electrode included in the energy storage element according to one embodiment of the present invention can be the same as the negative electrode described above. Therefore, the positive electrode, separator, and electrolyte will be described below.

[0080] Positive electrode

[0081] The positive electrode has a positive electrode substrate and a layer of positive electrode active material disposed on the positive electrode substrate directly or separated from it by an intermediate layer.

[0082] The positive electrode substrate is conductive. Materials used for the positive electrode substrate include metals such as aluminum, titanium, tantalum, and stainless steel, or their alloys. Among these, aluminum or aluminum alloys are preferred from the viewpoints of high potential resistance, high conductivity, and cost. Examples of positive electrode substrates include foil, vapor-deposited film, mesh, and porous materials; from a cost perspective, foil is preferred. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and AlN30 as specified in JIS-H-4000 (2014) or JIS-H-4160 (2006).

[0083] The average thickness of the positive electrode substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 40 μm or less, even more preferably 8 μm or more and 30 μm or less, and particularly preferably 10 μm or more and 25 μm or less. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be improved, and the energy density per unit volume of the energy storage element can be increased.

[0084] The interlayer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The interlayer reduces the contact resistance between the positive electrode substrate and the positive electrode active material layer by including conductive agents such as carbon particles. The composition of the interlayer is not particularly limited; for example, it may include a binder and a conductive agent.

[0085] The positive electrode active material layer contains positive electrode active material. The positive electrode active material layer may contain any components such as conductive agents, binders, thickeners, and fillers, as needed. These components can be selected from the materials exemplified for the negative electrode described above.

[0086] As the positive electrode active material, it can be appropriately selected from known positive electrode active materials. For positive electrode active materials used in lithium-ion secondary batteries, materials capable of absorbing and releasing lithium ions are typically used. Examples of positive electrode active materials include lithium transition metal composite oxides with an α-NaFeO2-type crystal structure, lithium transition metal composite oxides with a spinel-type crystal structure, polyanionic compounds, chalcogenides, and sulfur. For example, Li[Li] can be used as a lithium transition metal composite oxide with an α-NaFeO2-type crystal structure. x Ni （1-x） O2 (0≤x<0.5), Li[Li x Ni γ Co （1-x-γ） ]O2 (0≤x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Co （1-x） O2 (0≤x<0.5), Li[Li x Ni γ Mn （1-x-γ） ]O2 (0≤x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Ni γ Mn β Co （1-x-γ-β） ]O2 (0≤x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), Li[Li x Ni γ Co β Al （1-x-γ-β） O2 (0≤x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), etc. As a lithium transition metal composite oxide with a spinel-type crystal structure, Li can be cited as an example. x Mn2O4, Li x Ni γ Mn （2-γ） Examples of polyanionic compounds include O4, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogenides include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials can be partially replaced by atoms or anions composed of other elements. The surface of these materials can also be coated with other materials. One of these materials can be used alone, or two or more can be used in combination in the positive electrode active material layer.

[0087] The positive electrode active material is typically in the form of particles (powder). The average particle size of the positive electrode active material is preferably 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the lower limit mentioned above, the manufacture or processing of the positive electrode active material becomes easier. By setting the average particle size of the positive electrode active material to the upper limit mentioned above, the electronic conductivity of the positive electrode active material layer is improved. It should be noted that when using a composite of the positive electrode active material and other materials, the average particle size of the composite is used as the average particle size of the positive electrode active material. The "average particle size" of the positive electrode active material refers to the value where the cumulative distribution based on the volumetric standard calculated according to JIS-Z-8819-2 (2001) is 50%, determined by laser diffraction scattering method on a diluted solution obtained by diluting particles with a solvent according to JIS-Z-8825 (2013). The pulverization method and grading method can be selected, for example, from the methods exemplified for the negative electrode described above.

[0088] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By setting the content of the positive electrode active material within the above range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0089] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the energy storage element can be improved.

[0090] The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 9% by mass or less. By setting the binder content within the above range, the positive electrode active material can be stably maintained.

[0091] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. When a thickener has functional groups that react with lithium, these functional groups can be deactivated beforehand through methylation or similar methods.

[0092] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, Ba, and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0093] <Septum>

[0094] The diaphragm can be appropriately selected from known diaphragms. For example, a diaphragm consisting solely of a substrate layer, or a diaphragm with a heat-resistant layer containing heat-resistant particles and an adhesive formed on one or both sides of the substrate layer, can be used. Examples of the shape of the substrate layer for the diaphragm include woven fabric, nonwoven fabric, and porous resin membranes. Among these shapes, porous resin membranes are preferred from the viewpoint of strength, while nonwoven fabrics are preferred from the viewpoint of electrolyte retention. Regarding the material of the substrate layer for the diaphragm, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shut-off function, while polyimide and aromatic polyamides are preferred from the viewpoint of resistance to oxidative decomposition. Materials composed of these resins can also be used as the substrate layer for the diaphragm.

[0095] The heat-resistant particles contained in the heat-resistant layer preferably decrease in mass by less than 5% when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere, and more preferably by less than 5% when heated from room temperature to 800°C. Inorganic compounds can be cited as materials for which the mass decreases to the predetermined level. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; insoluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; and substances derived from mineral resources or their synthetic forms, such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica. These substances can be used alone as monomers or in combination, or in mixtures of two or more. From the perspective of the safety of energy storage components, silicon oxide, aluminum oxide, or aluminosilicates are preferred among these inorganic compounds.

[0096] From a strength perspective, the porosity of the diaphragm is preferably below 80% by volume, and from a discharge performance perspective, it is preferably above 20% by volume. Here, "porosity" refers to a volume-based value, meaning a value measured using a mercury porosimeter.

[0097] As a diaphragm, a polymer gel composed of a polymer and an electrolyte can be used. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. Using a polymer gel has the effect of suppressing leakage. As a diaphragm, porous resin membranes or nonwoven fabrics as described above can be used in conjunction with the polymer gel.

[0098] Electrolytes

[0099] As the electrolyte, a non-aqueous electrolyte or a non-aqueous electrolyte solution can be used. A non-aqueous electrolyte solution comprises a non-aqueous solvent and an electrolyte salt dissolved in that non-aqueous solvent.

[0100] As a non-aqueous solvent, a suitable selection can be made from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonates, ethers, amides, and nitriles. Solvents in which some of the hydrogen atoms in these compounds are replaced by halogens can also be used as non-aqueous solvents.

[0101] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinylene carbonate (VC), ethylene ethylene carbonate (VEC), ethylene chloride carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylethylene carbonate, and 1,2-diphenylethylene carbonate. EC is preferred.

[0102] Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), diphenyl carbonate, trifluoromethyl ethyl carbonate, and bis(trifluoroethyl) carbonate. Among these, EMC is preferred.

[0103] As a non-aqueous solvent, cyclic carbonates or chain carbonates are preferred, and a combination of cyclic carbonates and chain carbonates is more preferred. Using cyclic carbonates promotes the dissociation of electrolyte salts, thereby increasing the ionic conductivity of the non-aqueous electrolyte. Using chain carbonates reduces the viscosity of the non-aqueous electrolyte. When using a combination of cyclic carbonates and chain carbonates, the volume ratio of cyclic carbonate to chain carbonate (cyclic carbonate: chain carbonate) is preferably in the range of 5:95 to 50:50.

[0104] As an electrolyte salt, it can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Among these, lithium salts are preferred.

[0105] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2; lithium oxalate salts such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); and lithium salts with halogenated hydrocarbon groups such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0106] The electrolyte salt content in the non-aqueous electrolyte is preferably 0.1 mol / dm³ at 20°C and 1 atmosphere. 3 Above and 2.5 mol / dm 3 The following is more preferably 0.3 mol / dm 3 Above and 2.0 mol / dm 3 The following is a further preferred value: 0.5 mol / dm 3 Above and 1.7 mol / dm 3 The following is particularly preferred: 0.7 mol / dm³ 3 Above and 1.5 mol / dm 3 The following applies. By setting the electrolyte salt content within the above range, the ionic conductivity of the non-aqueous electrolyte can be improved.

[0107] In addition to non-aqueous solvents and electrolyte salts, non-aqueous electrolytes may also contain additives. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyls, alkylbiphenyls, terphenyls, partially hydrides of terphenyls, cyclohexylbenzene, and tert-butylbenzene. Aromatic compounds such as methylbenzene, tert-amylbenzene, diphenyl ether, and dibenzofuran; partially halogenated derivatives of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, glutaric anhydride, and maleic acid. Anhydride, citraconic anhydride, gluconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; vinyl sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, vinyl sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethyl sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxazothiophene), 4-methyl Sulfonyloxymethyl-2,2-dioxo-1,3,2-dioxazolthiophene, anisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sulfonyl lactone, 1,3-propane sulfonyl lactone, 1,4-butane sulfonyl lactone, 1,4-butene sulfonyl lactone, perfluorooctane, tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate, tetra(trimethylsilyl) titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives can be used individually or in combination of two or more.

[0108] The content of additives in the non-aqueous electrolyte is preferably 0.01% by mass or more and 10% by mass or less relative to the total mass of the non-aqueous electrolyte, more preferably 0.1% by mass or more and 7% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, and particularly preferably 0.3% by mass or more and 3% by mass or less. By setting the content of additives within the above range, it is possible to improve the capacity maintenance performance or cycling performance after high-temperature storage, or further improve safety.

[0109] Non-aqueous electrolytes can use solid electrolytes, or a combination of non-aqueous electrolytes and solid electrolytes.

[0110] As a solid electrolyte, any material that has ionic conductivity and is solid at room temperature (e.g., 15°C to 25°C), such as lithium, sodium, and calcium, can be selected. Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes.

[0111] Examples of sulfide solid electrolytes used in lithium-ion secondary batteries include Li₂S-P₂S₅, LiI-Li₂S-P₂S₅, and Li₂S-P₂S₅. 10 Ge-P2S 12 wait.

[0112] The shape of the energy storage element in this embodiment is not particularly limited. Examples include cylindrical batteries, square batteries, flat batteries, coin-shaped batteries, and button batteries.

[0113] Figure 1 This diagram shows an energy storage element 1, an example of a square battery. It should be noted that this is a perspective view of the interior of the container. Electrode bodies 2, having a positive and a negative electrode wound together with a separator, are housed within a square container 3. The positive electrode is electrically connected to the positive terminal 4 via a positive lead 41. The negative electrode is electrically connected to the negative terminal 5 via a negative lead 51.

[0114] [Electronic Storage Device]

[0115] The energy storage element of this embodiment can be installed as an energy storage unit (battery module) composed of multiple energy storage elements in vehicles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), as well as in electronic devices such as personal computers and communication terminals, or as a power source for energy storage. In this case, the technology of the present invention only needs to be applied to at least one energy storage element included in the energy storage unit.

[0116] Figure 2This illustrates an example of an energy storage device 30 that further incorporates energy storage units 20, where each energy storage unit 20 is formed by aggregating two or more electrically connected energy storage elements 1. The energy storage device 30 may also include a busbar (not shown) electrically connecting two or more energy storage elements 1, or a busbar (not shown) electrically connecting two or more energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may also include a status monitoring device (not shown) for monitoring the status of one or more energy storage elements.

[0117] [Manufacturing Method of Energy Storage Components]

[0118] The method for manufacturing the energy storage element according to this embodiment can be appropriately selected from known methods. This manufacturing method includes, for example, preparing an electrode body, preparing an electrolyte, and housing the electrode body and electrolyte in a container. Preparing the electrode body includes preparing a positive electrode and a negative electrode, and forming the electrode body by laminating or winding the positive and negative electrodes with a diaphragm in between.

[0119] The electrolyte can be stored in a container using any known method. For example, if a non-aqueous electrolyte is used, the non-aqueous electrolyte can be injected into the container through an inlet, and then the inlet can be sealed.

[0120] [Other Implementation Methods]

[0121] It should be noted that the energy storage element of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the configuration of other embodiments can be added to the configuration of a certain embodiment; in addition, a part of the configuration of a certain embodiment can be replaced with the configuration of other embodiments or known technology. Furthermore, a part of the configuration of a certain embodiment can be deleted. In addition, known technology can be added to the configuration of a certain embodiment.

[0122] In the above embodiments, the case where the energy storage element is used as a chargeable and dischargeable lithium-ion secondary battery has been described, but the type, shape, size, capacity, etc., of the energy storage element are arbitrary. This invention can also be applied to various secondary batteries, double-layer capacitors, lithium-ion capacitors, and other capacitors that use non-aqueous electrolyte energy storage elements, or energy storage elements that use electrolytes other than non-aqueous electrolytes.

[0123] In the above embodiments, an electrode body formed by stacking positive and negative electrodes with a separator in between has been described. However, the electrode body may also lack a separator. For example, if a non-conductive layer is formed on the active material layer of the positive or negative electrode, the positive and negative electrodes can be in direct contact.

[0124] Example

[0125] The present invention will now be described in more detail through embodiments. The present invention is not limited to the following embodiments.

[0126] [Example 1]

[0127] <Making the Negative Electrode>

[0128] Natural graphite particles are used as the negative electrode active material. These natural graphite particles are spherical graphite particles obtained by spheroidizing flake graphite and then compacting it. The average particle size A of the aforementioned natural graphite particles is 10.0 μm, and the internal porosity is 1%.

[0129] A negative electrode paste was prepared containing the aforementioned negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, with a mass ratio (solid content conversion) of 97.0:2.0:1.0, and water as a dispersion medium. It should be noted that when the binder content was varied in the following examples or comparative examples to prepare the negative electrode paste, the total content (solid content conversion) of the negative electrode active material and the binder in the negative electrode paste was 99.0% by mass. The aforementioned negative electrode paste was coated on both sides of a copper foil serving as the negative electrode substrate, dried, and then pressed to form a negative electrode with a layer of negative electrode active material disposed on the negative electrode substrate. Furthermore, the average particle size B of the aforementioned binder was 200 nm, and the aspect ratio was 1.0.

[0130] <Making of Energy Storage Components>

[0131] (The production of the positive electrode)

[0132] As the positive electrode active material, LiNi is used, which has an α-NaFeO2 type crystal structure. 0.6 Co 0.2 Mn 0.2 O2 represents lithium transition metal complex oxide.

[0133] A positive electrode paste was prepared, containing the aforementioned positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder, with a mass ratio of 93.0:4.0:3.0 (converted based on solid content), and N-methylpyrrolidone (NMP) as a dispersion medium. The positive electrode paste was coated on both sides of an aluminum foil serving as the positive electrode substrate, dried, and then pressed to fabricate a positive electrode with a layer of positive electrode active material disposed on the positive electrode substrate.

[0134] (Preparation of non-aqueous electrolyte)

[0135] In a mixed solvent obtained by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35, 1.0 mol / dm³ 3 LiPF6 was dissolved at a concentration (20°C, 1 atmosphere) to prepare a non-aqueous electrolyte.

[0136] (The manufacture of energy storage components)

[0137] An electrode body is fabricated by overlapping the positive and negative electrodes with a polyolefin microporous membrane as a separator. The electrode body is then placed in a container made of a metal-resin composite membrane, and the non-aqueous electrolyte is injected inside. Finally, the container is sealed by heat fusion to obtain the energy storage element of Example 1.

[0138] [Comparative Example 1, Examples 2-5]

[0139] Except that the content of the binder (SBR) in the negative electrode active material layer is as shown in Table 1, the energy storage devices of Comparative Example 1 and Examples 2-5 were obtained by following the same steps as in Example 1. It should be noted that the CMC content in each example is fixed at 1% by mass, and the total content of the negative electrode active material, SBR, and CMC is adjusted to 100% by mass. The same applies below.

[0140] [Comparative Examples 2-6]

[0141] As the negative electrode active material, natural graphite particles different from those in Example 1 were used. These natural graphite particles were spherical graphite particles formed by spheroidizing flake graphite without undergoing compaction treatment. The average particle size A of the aforementioned natural graphite particles was 9.7 μm, and the internal porosity was 10%. Using the aforementioned negative electrode active material, the content of the binder (SBR) in the negative electrode active material layer was as shown in Table 1. Otherwise, the energy storage devices of Comparative Examples 2-6 were obtained following the same steps as in Example 1.

[0142] [Example 6]

[0143] As the negative electrode active material, natural graphite particles different from those in Example 1 were used. These natural graphite particles were spherical graphite particles formed by spheroidizing flake graphite and undergoing compaction treatment. The average particle size A of the aforementioned natural graphite particles was 8.1 μm, and the internal porosity was 1%. Except for using the aforementioned negative electrode active material and ensuring the binder (SBR) content in the negative electrode active material layer was as shown in Table 2, the energy storage element of Example 6 was obtained following the same steps as in Example 1.

[0144] [Comparative Example 7]

[0145] As the negative electrode active material, artificial graphite particles with an average particle size A of 2.8 μm and an internal porosity of 1% were used. Using the above-mentioned negative electrode active material, the content of binder (SBR) in the negative electrode active material layer is as shown in Table 2. Otherwise, the energy storage element of Comparative Example 7 was obtained by following the same steps as in Example 1.

[0146] [Comparative Example 8]

[0147] As the negative electrode active material, natural graphite particles different from those in Example 1 were used. These natural graphite particles were spherical graphite particles formed by spheroidizing flake graphite and undergoing compaction treatment. The average particle size A of the aforementioned natural graphite particles was 15.0 μm, and the internal porosity was 1%. Using the aforementioned negative electrode active material, the content of the binder (SBR) in the negative electrode active material layer was as shown in Table 2. Otherwise, the energy storage device of Comparative Example 8 was obtained following the same steps as in Example 1.

[0148] [Comparative Example 9]

[0149] Except that the average particle size B of the binder (SBR) in the negative electrode active material layer is as shown in Table 2, the energy storage element of Comparative Example 9 was obtained by following the same steps as in Example 6.

[0150] [Examples 7-9]

[0151] Except for the content of binder (SBR) in the negative electrode active material layer as shown in Table 3, the energy storage elements of Examples 7 to 9 were obtained by following the same steps as in Example 1.

[0152] [evaluate]

[0153] (Implementation of initial charge and discharge)

[0154] For each energy storage component, perform initial charging and discharging at 25°C according to the following guidelines.

[0155] Constant current and constant voltage charging was performed with a charging current of 0.2C and a charging termination voltage of 4.1V. The charging ended when the charging current reached 0.02C. Then, a 10-minute rest period was set. Then, constant current discharging was performed with a discharging current of 0.2C and a discharging termination voltage of 3.0V. The discharge capacity at this point was taken as the "initial discharge capacity".

[0156] (Determination of initial input)

[0157] After the initial charge and discharge described above, measure the initial input according to the following guidelines.

[0158] In a constant-temperature bath at 25°C, constant-current charging was performed at a charging current of 1.0C to adjust the SOC to 50%. After being stored in a constant-temperature bath at -30°C for 4 hours, constant-current charging was performed at 5C, 10C, 15C, 20C, or 25C for 10 seconds each. "SOC" (State of Charge) refers to the state of charge of an energy storage element measured within the voltage range of a normally used energy storage element, based on its rated capacity. In this embodiment, the rated capacity is the aforementioned initial discharge capacity. After each charging cycle, constant-current discharge was performed at 1.0C to bring the SOC to 50%. The resistance value was calculated based on the slope of the current during each charging cycle and the voltage one second after the start of charging. The formula "{Difference between the voltage before charging and the upper limit voltage (4.3V)} / resistance value × upper limit voltage (4.3V)" was calculated as the initial input. It should be noted that the input refers to the energy (power) that can be taken in per unit time. The measurement results of the initial input are shown in Tables 1 and 2 as "initial input".

[0159] (Charge-discharge cycle test)

[0160] For the energy storage elements of Examples 7 to 9, after the initial input is measured as described above, charge-discharge cycle tests are performed according to the following guidelines.

[0161] The device was charged at a constant current of 8C and a charging termination voltage of 3.9V in a constant temperature bath at 60°C. Then, it was discharged at a constant current of 8C and a discharging termination voltage of 3.5V. This charge-discharge cycle was repeated for 1000 hours.

[0162] (Measurement of input after charge-discharge cycle test)

[0163] After the above charge-discharge cycle test, the input after the charge-discharge cycle test was measured using the same method as the initial input measurement. The percentage of the input after the charge-discharge cycle test relative to the initial input for the energy storage elements of Examples 7-9 is shown in Table 3 as the "input maintenance rate after cycle".

[0164] [Table 1]

[0165]

[0166] [Table 2]

[0167]

[0168] [Table 3]

[0169]

[0170] As shown in Table 1, the average particle size of the natural graphite particles in Examples 1-5 and Comparative Examples 1-6 was 6.0 μm or more and 10.0 μm or less. However, the initial input in Comparative Examples 1-6 was less than 500 W, while the initial input in Examples 1-5 was 500 W or more. In the comparison between Comparative Example 1 and Examples 1-5, it was considered that since the SBR content in Examples 1-5 was 0.1% by mass or more and 2.0% by mass or less, the natural graphite particles were not over-coated, resulting in a larger initial input. Furthermore, in the comparison between Examples 1-5 and Comparative Examples 2-6, it was considered that in Comparative Examples 2-6, the SBR was trapped within the internal voids of the natural graphite particles, weakening the adhesion between the natural graphite particles. In contrast, in Examples 1-5, since the internal porosity was 2% or less, the SBR was not trapped within the internal voids of the natural graphite particles, and the particles were configured in a manner that sufficiently ensured the conductivity between the natural graphite particles.

[0171] Furthermore, in Comparative Examples 2-6, even when the SBR content was 2.0% by mass or less, the initial input did not increase. In contrast, in Examples 1-5, the initial input increased when the SBR content was 0.1% by mass or more and 2.0% by mass or less. It is speculated that in the cases of Comparative Examples 2-6, if the SBR content was 0.1% by mass or more and 2.0% by mass or less, the optimized effect of SBR coating on the surface of natural graphite particles would be offset by the weakened adhesion between the natural graphite particles. In contrast, in the cases of Examples 1-5, even when the SBR content was 0.1% by mass or more and 2.0% by mass or less, the adhesion between the natural graphite particles could be maintained, and the optimized effect of SBR coating on the surface of natural graphite particles could be effectively achieved.

[0172] As shown in Table 2, Example 6, with an average particle size of natural graphite particles of 6 μm or more and 10 μm or less, had a higher initial input compared to Comparative Example 7, with an average particle size of natural graphite particles less than 6 μm, and Comparative Example 8, with an average particle size of natural graphite particles exceeding 10 μm. This suggests that by maintaining the average particle size of the natural graphite particles within an appropriate range, it is easy to configure the mixture in a way that ensures good adhesion between the natural graphite particles using SBR. Furthermore, Example 6, with a ratio (B / A) of the average particle size B of the adhesive to the average particle size A of the natural graphite particles of 0.015 or more and 0.0375 or less, had a higher initial input compared to Comparative Example 7, with a ratio (B / A) exceeding 0.0375, and Comparative Examples 8 and 9, with a ratio (B / A) less than 0.015. This suggests that by maintaining the ratio (B / A) of the average particle size B of the adhesive to the average particle size A of the natural graphite particles within an appropriate range, it is easy to configure the mixture in a way that ensures good adhesion between the natural graphite particles using SBR.

[0173] As can be seen from the above, the natural graphite particles have an internal porosity of less than 2%, the binder (SBR) content is more than 0.1% by mass and less than 2.0% by mass, the average particle size of the natural graphite particles is more than 6μm and less than 10μm, and the ratio of the average particle size B of the binder to the average particle size A of the natural graphite particles (B / A) is more than 0.015 and less than 0.0375. As a result, the binder does not over-coat the natural graphite particles and is easily configured between the natural graphite particles, thus enabling the natural graphite particles to be well bonded together.

[0174] As shown in Table 3, by keeping the SBR content less than 1.0% by mass, the input retention rate after charge-discharge cycles can be increased. It is believed that by keeping the binder content less than 1.0% by mass, the amount of binder coating the natural graphite particles can be maintained within a suitable range.

[0175] Industrial availability

[0176] This invention can be applied to energy storage components used as power sources in personal computers, communication terminals and other electronic devices, automobiles and the like.

[0177] Explanation of reference numerals in the attached figures

[0178] 1: Energy storage element, 2: Electrode body, 3: Container, 4: Positive terminal, 41: Positive lead, 5: Negative terminal, 51: Negative lead, 20: Energy storage unit, 30: Energy storage device.

Claims

1. A negative electrode for a power storage element, having a negative electrode active material layer containing natural graphite particles and a binder, an internal porosity of the natural graphite particles is 2% or less, an average particle diameter of the natural graphite particles is 6 μm or more and 10 μm or less, a content of the binder in the negative electrode active material layer is 0.1 mass% or more and 2.0 mass% or less, the binder is in a particle form, and a ratio of an average particle diameter B of the binder to an average particle diameter A of the natural graphite particles, B / A, is 0.015 or more and 0.0375 or less.

2. The negative electrode for a power storage element according to claim 1, wherein a content of the binder in the negative electrode active material layer is less than 1.0 mass%.

3. The negative electrode for a power storage element according to claim 1 or 2, wherein a ratio of an average particle diameter B of the binder to an average particle diameter A of the natural graphite particles, B / A, is 0.018 or more and 0.030 or less.

4. The negative electrode for a power storage element according to claim 1 or 2, wherein an average particle diameter of the binder is 150 nm or more and 225 nm or less.

5. The negative electrode for a power storage element according to claim 1 or 2, wherein an average aspect ratio of the particles of the binder is 1.5 or less.

6. A power storage element provided with the negative electrode for a power storage element according to claim 1 or 2.

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