Negative electrode active material particles, negative electrode for power storage element, and power storage element

Abstract

Negative electrode active material particles according to one aspect of the present invention include first graphite particles and second graphite particles that are different from the first graphite particles. The first graphite particles are constituted by natural graphite having an average particle diameter of 3-8 μm, and have a solid structure. The second graphite particles are constituted by natural graphite having an average particle diameter of 10-20 μm. The ratio of the contained amount of first graphite particles to the total contained amount of the first graphite particles and the second graphite particles is greater than 0.2 and less than or equal to 0.6 on the basis of volume.

Description

Negative electrode active material particles, negative electrode for energy storage device, and energy storage device

[0001] The present invention relates to negative electrode active material particles, a negative electrode for an electricity storage device, and an electricity storage device.

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring charge-transporting ions such as lithium ions between the electrodes. Capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as energy storage elements other than non-aqueous electrolyte secondary batteries.

[0003] A typical energy storage device includes an electrode assembly in which a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material are stacked with a separator interposed therebetween. This electrode assembly is housed in a container together with an electrolyte to form an energy storage device. Carbon materials such as graphite are used as the negative electrode active material.

[0004] Patent Document 1 describes that the use of natural graphite particles as a negative electrode active material can contribute to improving the discharge capacity of an energy storage device. It also describes a method for manufacturing a negative electrode in which a mixture paste containing graphite particles is applied to a negative electrode substrate, and then dried and pressed to form a negative electrode active material layer.

[0005] International Publication No. 2021 / 015194

[0006] According to the findings of the inventors, the internal resistance of an energy storage device can be reduced by reducing the particle size of natural graphite particles. Furthermore, by solidifying natural graphite particles, the capacity decrease during charge-discharge cycling can be suppressed. However, when small-particle-size natural graphite particles having a solid structure are used as a negative electrode active material, the durability of the negative electrode active material particles is likely to decrease due to pressing during negative electrode production. When the durability of the negative electrode active material particles decreases, cracks and chips occur in the negative electrode active material particles with charge-discharge cycling, making it difficult to fully achieve the above-mentioned effect of suppressing capacity decrease by solidifying the particles. For this reason, it has been difficult to achieve both reduced internal resistance and maintaining capacity during charge-discharge cycling in energy storage devices using negative electrodes containing natural graphite particles.

[0007] An object of the present invention is to provide negative electrode active material particles that can reduce the internal resistance of an electric storage element and suppress a decrease in the capacity retention rate during charge / discharge cycling of the electric storage element, as well as a negative electrode for an electric storage element and an electric storage element that contain such negative electrode active material particles.

[0008] A negative electrode active material particle according to one aspect of the present invention includes first graphite particles and second graphite particles different from the first graphite particles, wherein the first graphite particles are natural graphite having an average particle size of 3 μm or more and 8 μm or less and have a solid structure, and the second graphite particles are natural graphite having an average particle size of 10 μm or more and 20 μm or less, and the ratio of the content of the first graphite particles to the total content of the first graphite particles and the second graphite particles is greater than 0.2 and 0.6 or less on a volume basis.

[0009] A negative electrode for a storage device according to another aspect of the present invention includes a negative electrode active material layer containing the negative electrode active material particles according to the aspect of the present invention.

[0010] An electric storage device according to another aspect of the present invention includes the electric storage device negative electrode according to the aspect of the present invention.

[0011] According to any one aspect of the present invention, it is possible to provide negative electrode active material particles that can reduce the internal resistance of an energy storage element and suppress a decrease in the capacity retention rate during charge-discharge cycling of the energy storage element, as well as a negative electrode for an energy storage element and an energy storage element that include such negative electrode active material particles.

[0012] Fig. 1 is a perspective view showing an electric storage element using negative electrode active material particles according to one embodiment of the present invention, and Fig. 2 is a schematic view showing an electric storage device including a plurality of the electric storage elements shown in Fig. 1.

[0013] First, an outline of the negative electrode active material particles, the negative electrode for an electricity storage device, and the electricity storage device disclosed in this specification will be described.

[0014] [1] A negative electrode active material particle according to one aspect of the present invention includes first graphite particles and second graphite particles different from the first graphite particles, wherein the first graphite particles are natural graphite having an average particle size of 3 μm or more and 8 μm or less and have a solid structure, and the second graphite particles are natural graphite having an average particle size of 10 μm or more and 20 μm or less, and the ratio of the content of the first graphite particles to the total content of the first graphite particles and the second graphite particles is greater than 0.2 and equal to or less than 0.6 on a volume basis.

[0015] The negative electrode active material particles described in [1] above can reduce the internal resistance of an energy storage device and suppress a decrease in the capacity retention rate of the energy storage device during charge-discharge cycling. While the reason for this is unclear, the following reasons are presumed. First, when small-particle-sized natural graphite particles having a solid structure are used as the negative electrode active material, the internal resistance of the energy storage device is reduced, but the capacity of the energy storage device is likely to decrease during charge-discharge cycling. This is thought to be due to a decrease in the durability of the negative electrode active material particles (natural graphite particles). More specifically, the small-particle-sized natural graphite particles having a solid structure can reduce the internal resistance of an energy storage device, but due to their solid structure, they are hard and resistant to deformation, and the large number of contact points between particles increases the frictional force that occurs when the particles misalign. Therefore, in order to achieve an appropriate porosity (thickness) in the negative electrode active material layer, it is necessary to increase the pressing pressure during negative electrode production, which tends to reduce the durability of the negative electrode active material particles. When the durability of the negative electrode active material particles decreases in this way, cracks and chips occur in the negative electrode active material particles with repeated charge-discharge cycles, which can lead to a decrease in the capacity of the energy storage device. In contrast, the negative electrode active material particles described in [1] above contain first graphite particles, which are natural graphite with a small average particle size and a solid structure, and second graphite particles, which are natural graphite with a large average particle size. Therefore, during the production of the negative electrode, the first graphite particles are easily inserted into the gaps between the second graphite particles to form a negative electrode active material layer. This allows the negative electrode active material layer to be easily adjusted to an appropriate porosity while maintaining the contact area between the negative electrode active material particles in the negative electrode active material layer. In other words, the durability of the negative electrode active material particles can be maintained without increasing the pressing pressure during the production of the negative electrode. Furthermore, the negative electrode active material particles described in [1] above have a controlled ratio of the content of the first graphite particles to the content of the second graphite particles. For these reasons, it is presumed that the negative electrode active material particles described in [1] above can reduce the internal resistance of the electricity storage device and can suppress a decrease in the capacity retention rate during charge-discharge cycles of the electricity storage device.

[0016] In the present invention, "natural graphite" refers to graphite extracted from natural resources, not artificially produced graphite (artificial graphite). "Graphite" refers to graphite that has an average lattice spacing (d ００２) is 0.33 nm or more and less than 0.34 nm. Here, "discharged state" refers to a state in which the carbon material serving as the negative electrode active material has been discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode, the open circuit voltage is 0.7 V or more. Natural graphite may exhibit four peaks in the diffraction angle 2θ range of 40° to 50° in an X-ray diffraction pattern using CuKα radiation measured before charging and discharging or in a discharged state. These four peaks are believed to be two peaks derived from a hexagonal crystal structure and two peaks derived from a rhombohedral crystal structure. In the case of artificial graphite, it is generally believed that only two peaks derived from a hexagonal crystal structure appear. In an X-ray diffraction pattern using CuKα radiation, 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 peak intensity ratio ((012) / (100)) is preferably 0.6 or less. Here, the (100) plane is derived from a hexagonal crystal structure, and the (012) plane is derived from a rhombohedral crystal structure.

[0017] In the present invention, "having a solid structure" means that the interior of a graphite particle is filled and substantially no voids exist. More specifically, "solid" means that, in a cross section of the particle observed in an SEM image obtained using a scanning electron microscope (SEM), the internal porosity (area ratio) of voids within the particle relative to the entire particle area is 3% or less. In a preferred embodiment, the internal porosity of graphite particles having a solid structure may be 1% or less. The "internal porosity" of a graphite particle refers to the area ratio of voids within the particle relative to the entire particle area in a cross section of the particle observed in an SEM image obtained using a scanning electron microscope. The "internal porosity (area ratio of voids within the particle relative to the entire particle area)" of a graphite particle is determined by the following procedure. (1) Preparation of Measurement Sample The measurement target may be graphite particles or a negative electrode containing graphite particles. When the measurement target is a negative electrode, the sample is prepared by the following procedure. If a negative electrode can be prepared before assembling the energy storage element, it is used as is. When preparing a negative electrode from an assembled energy storage element, the energy storage element is first discharged at a constant current of 0.1 C to the discharge end voltage during normal use. The element is disassembled, the negative electrode is removed, thoroughly washed with dimethyl carbonate, and then dried under reduced pressure at room temperature for 24 hours. The process from disassembling the energy storage element to preparing the negative electrode to be measured is carried out in a dry air atmosphere with a dew point of -40°C or below. Multiple graphite particles or negative electrodes to be measured are fixed with thermosetting resin. The cross section of the resin-fixed negative electrode is exposed by ion milling to prepare a measurement sample. (2) Obtaining SEM Images: A JSM-7001F SEM (manufactured by JEOL Ltd.) is used to obtain SEM images. The SEM images are secondary electron images. The accelerating voltage is 5 kV. The observation magnification is set so that the number of graphite particles appearing in one field of view is between 3 and 15. The obtained SEM image is saved as an image file. Other conditions such as spot diameter, working distance, irradiation current, brightness, and focus are appropriately set so that the contours of the graphite particles become clear.(3) Cutting out the contours of graphite particles The contours of graphite particles are cut out from the acquired SEM image using the image cutout function of image editing software Adobe Photoshop Elements (registered trademark) 11. This contour cutout is performed by using the quick selection tool to select the area outside the contours of the graphite particles and editing everything except the graphite particles to a black background. If the contours of fewer than three graphite particles are cut out at this time, an SEM image is acquired again and this is repeated until the contours of three or more graphite particles are cut out. (4) Binarization Processing The image of the first graphite particle among the cut-out graphite particles is binarized using image analysis software PopImaging (registered trademark) 6.00, with a threshold value set to a concentration 20% lower than the concentration at which the intensity is maximum. By the binarization processing, the area of ​​the high-concentration side is calculated and determined as the "area S1 of the voids within the particle." Next, the image of the first graphite particle, as before, is subjected to binarization processing using a concentration of 10% as a threshold. The outer edge of the graphite particle is determined by the binarization processing, and the area inside the outer edge is calculated to determine the "area S0 of the entire particle." The calculated S1 and S0 are used to calculate the ratio of S1 to S0 (S1 / S0), thereby calculating the "area ratio R1 of voids within the particle to the area of ​​the entire particle" for the first graphite particle. The images of the second and subsequent graphite particles among the cut-out graphite particles are also subjected to the binarization processing, to calculate the area S1 and area S0. The void area ratios R2, R3, ... of each graphite particle are calculated based on the calculated area S1 and area S0. (5) Determination of void area ratio: The "area ratio of voids within the particle to the area of ​​the entire particle (internal porosity)" is determined by calculating the average value of all the void area ratios R1, R2, R3, ... calculated by the binarization processing. It should be noted that instead of the scanning electron microscope used in the "obtaining an SEM image," the image editing software used in the "cutting out the contours of graphite particles," and the image analysis software used in the "binarization process," devices and software capable of performing measurements, image editing, and image analysis equivalent to these may be used.

[0018] In the present invention, the "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% (D50: median diameter) based on the particle size distribution measured in accordance with JIS-Z-8825 (2013) using a laser diffraction / scattering method for a diluted solution of particles diluted with a solvent. It has been confirmed that the average particle size based on the above measurement is substantially consistent with the average secondary particle size, which is the average value of the particle sizes of 50 particles extracted from an SEM image of the particles, excluding extremely large and extremely small particles. The particle size of each secondary particle based on the measurement from this SEM image is determined as follows: The shortest diameter passing through the center of the minimum circumscribing circle of each secondary particle is defined as the minor diameter, and the diameter passing through the center and perpendicular to the minor diameter is defined as the major diameter. The particle size of each secondary particle is defined as the average of the major and minor diameters. When there are two or more shortest diameters, the longest diameter perpendicular to the minor diameter is defined as the minor diameter.

[0019] The average particle size of graphite particles used as the negative electrode active material is measured on graphite particles before charge / discharge, or on graphite particles contained in the negative electrode of a storage battery element after treatment using the following procedure. First, the storage battery element is discharged at a constant current of 0.1 C to the end-of-discharge voltage during normal use. The device is then disassembled, the negative electrode is removed, and a test battery is assembled using the removed negative electrode as the working electrode and metallic lithium as the counter electrode. Pure metallic lithium is used for the metallic lithium electrode. The test battery is then discharged at a constant current of 50 mA per gram of negative electrode active material until the closed-circuit voltage reaches 1.5 V, adjusting the graphite particles to a fully discharged state. The battery is then disassembled again, and the negative electrode is removed. The removed negative electrode is then washed with dimethyl carbonate. The negative electrode layer containing the graphite particles is then peeled from the negative electrode substrate, and the negative electrode active material layer is washed with a binder-soluble solvent to remove the binder. The washed negative electrode active material layer is immersed in an acid or alkaline solution to remove metals derived from the negative electrode substrate, a solid electrolyte interface (SEI) coating, and the like. The layer is then washed with water and dried under reduced pressure at room temperature for 24 hours to obtain graphite particles. The disassembly of the energy storage element and test battery is carried out in an argon atmosphere with a dew point of -60°C or lower. "Normal use" refers to the case where the energy storage element is used under the charge / discharge conditions recommended or specified for the energy storage element, and, if equipment for using the energy storage element is available, the energy storage element is used using that equipment.

[0020] In measuring the average particle size of graphite particles, if two or more types of particles with different average particle sizes are contained, a particle size distribution with multiple peaks (particle size distribution with particle size on the horizontal axis and volumetric abundance ratio on the vertical axis) may be obtained. If there is no overlap between the particle size distributions of different particles, the average particle size is calculated for each particle size distribution to which each particle belongs. If there is an overlap between the particle size distributions of different particles, the particle size distribution to which each particle belongs is separated and the average particle size is measured. The particle size distributions can be separated by known methods. Numerical analysis software may be used to separate the particle size distributions. Examples of numerical analysis software include MATLAB (registered trademark) (The MathWorks), ORIGIN PRO (registered trademark) (Lightstone), etc.

[0021] The multiple types of graphite particles contained in the negative electrode active material particles are identified based on particle size distribution. That is, particles belonging to each particle size distribution in the negative electrode active material particles are considered to be of the same type. The above-mentioned internal porosity is measured for each type of particle after identifying the type of particle in the negative electrode active material particles based on the particle size distribution. If there is an overlap between particle size distributions, the internal porosity is measured for particles belonging to the areas other than the overlap. From this perspective, the particles to be measured for internal porosity are preferably those falling within the 20% range around the average particle size of each particle size distribution, and more preferably those falling within the 10% range around the average particle size.

[0022] In the present invention, the volume-based "content ratio" of graphite particles is calculated based on the volume-based particle size distribution (particle size distribution with particle size on the horizontal axis and volume-based abundance ratio on the vertical axis) measured by laser diffraction / scattering on a diluted solution obtained by diluting particles with a solvent, in accordance with JIS-Z-8825 (2013). When the measurement sample contains two or more types of particles with different average particle sizes, the particle size distributions to which each particle belongs are separated by the method described above as appropriate, and an integral value is calculated for each particle size distribution. The volume-based "content ratio" is calculated by comparing the above-mentioned integral values.

[0023] [2] In the negative electrode active material particles described in [1] above, the second graphite particles may have a solid structure.

[0024] The negative electrode active material particles described in the above [2] can further suppress the decrease in the capacity retention rate during charge-discharge cycles of the electricity storage device.

[0025] [3] In the negative electrode active material particles according to [1] or [2] above, the first graphite particles and the second graphite particles may have a spheroidization degree of more than 0.6.

[0026] The spheroidization degree is calculated by the following formula based on the image projected when a graphite particle is projected: Spheroidization degree = (4π × total particle area) / (perimeter of particle) ２The sphericity is measured using a particle shape analyzer, such as a Sysmex FPIA3000 manufactured by Malvern Instruments. The sphericity is measured on graphite particles before charge / discharge or on graphite particles that have been pre-treated according to the procedure described in the measurement of average particle size.

[0027] In the negative electrode active material particles described in [3] above, the frictional force between particles is small (high fluidity) during pressing in the production of the negative electrode, so the pressing pressure during the production of the negative electrode can be reduced. Therefore, the durability of the particles can be easily maintained. As a result, the decrease in the capacity retention rate during charge / discharge cycling of the energy storage device can be further suppressed. Here, "the spheroidization degree of the first graphite particles and the second graphite particles is greater than 0.6" means that the spheroidization degree of the first graphite particles is greater than 0.6, and the spheroidization degree of the second graphite particles is greater than 0.6.

[0028] [4] In the negative electrode active material particles according to any one of [1] to [3] above, the ratio of the average particle size of the first graphite particles to the average particle size of the second graphite particles may be 0.6 or less.

[0029] By using the negative electrode active material particles described in [4] above, the negative electrode active material layer can be easily adjusted to an appropriate porosity while maintaining the contact area between particles in the negative electrode active material layer, which can further reduce the internal resistance of the energy storage device and further suppress the decrease in the capacity retention rate during charge-discharge cycling of the energy storage device.

[0030] [5] A negative electrode for a storage device according to another aspect of the present invention includes a negative electrode active material layer containing the negative electrode active material particles according to any one of [1] to [4] above.

[0031] The negative electrode for a storage device described in [5] above includes a negative electrode active material layer containing the negative electrode active material particles described in any one of [1] to [4] above, and therefore can reduce the internal resistance of the storage device and suppress a decrease in the capacity retention rate during charge-discharge cycles of the storage device.

[0032] [6] In the negative electrode for a storage element according to [5] above, the BET specific surface area of ​​the negative electrode active material layer is 3.0 m ２ / g or more 4.3m ２ / g or less.

[0033] The "BET specific surface area" is a value obtained by the following measurement procedure. 1.00 g of a sample of the negative electrode active material layer is placed in a measurement sample tube and dried under reduced pressure at 120°C for 12 hours. Next, an adsorption isotherm is measured using a nitrogen gas adsorption method using liquid nitrogen within a relative pressure P / P0 (P0 = approximately 760 mmHg) range of 0 to 0.95. The measurement device used is an autosorb iQ manufactured by Quantachrome. Five points are extracted from the region of P / P0 = 0.05 to 0.3 of the obtained adsorption isotherm, and a BET plot is performed. The BET specific surface area is calculated from the y-intercept and slope of the resulting line. The BET specific surface area is measured on a negative electrode active material layer before charge / discharge or a negative electrode active material layer that has been pre-treated using the procedure described for measuring the average particle size.

[0034] The negative electrode for an electric storage element according to the above item [6] can further reduce the internal resistance of the electric storage element.

[0035] [7] An electric storage device according to another aspect of the present invention includes the negative electrode for an electric storage device according to [5] or [6] above.

[0036] The energy storage element according to [7] above includes the negative electrode for an energy storage element according to [5] or [6] above, and therefore has low internal resistance and is less likely to experience a decrease in capacity retention rate during charge / discharge cycles.

[0037] The negative electrode active material particles, the negative electrode for an energy storage device, the energy storage device, the method for manufacturing the energy storage device, the energy storage device, and other embodiments according to one embodiment of the present invention will be described in detail below. Note that the lower and upper limits of each numerical range described in the embodiments of the present invention can be combined in any combination.

[0038] [Negative Electrode Active Material Particles] The negative electrode active material particles according to one embodiment of the present invention include first graphite particles and second graphite particles different from the first graphite particles. The first graphite particles (hereinafter also referred to as "first particles") are natural graphite particles having an average particle size of 3 μm to 8 μm and a solid structure. The second graphite particles (hereinafter also referred to as "second particles") are natural graphite particles having an average particle size of 10 μm to 20 μm. The ratio of the content of the first particles to the total content of the first particles and the second particles is greater than 0.2 and less than 0.6 on a volume basis. The negative electrode active material particles are used in the negative electrode of a storage device such as a nonaqueous electrolyte secondary battery. The negative electrode active material particles typically form a negative electrode active material layer together with other optional components in the negative electrode of the storage device. The negative electrode active material layer is formed, for example, by applying a mixture paste containing the negative electrode active material particles to a negative electrode substrate, drying the paste, and then pressing the paste.

[0039] The negative electrode active material particles are likely to form a negative electrode active material layer such that the first particles penetrate into the gaps where the second particles contact. This allows the negative electrode active material layer to be easily adjusted to an appropriate porosity while maintaining the contact area between the negative electrode active material particles in the negative electrode active material layer. In other words, the durability of the negative electrode active material particles can be maintained without increasing the pressing pressure during negative electrode production. Furthermore, the negative electrode active material particles have a controlled appropriate ratio of the content of the first particles to the content of the second particles. Therefore, the negative electrode active material particles have the effect of reducing the internal resistance of the energy storage device and suppressing a decrease in the capacity retention rate during charge / discharge cycling of the energy storage device.

[0040] (First Particles) The first particles are natural graphite particles having a solid structure. Examples of the shape of the first particles include flakes, chunks (scales), and clay. The first particles may be spherical natural graphite particles obtained by spheroidizing flake natural graphite or the like.

[0041] The lower limit of the sphericity of the first particles is preferably greater than 0.6, more preferably 0.65. When the sphericity of the first particles is equal to or greater than the lower limit, the durability of the negative electrode active material particles can be easily maintained. The lower limit of the sphericity of the first particles may be 0.7 or 0.8. On the other hand, the upper limit of the sphericity of the first particles is not particularly limited, but may be 0.99, 0.95, or 0.9 from the viewpoint of manufacturability.

[0042] The lower limit of the average particle size of the first particles is 3 μm, preferably 4 μm, more preferably 5 μm, even more preferably 6 μm, and even more preferably 7 μm. Meanwhile, the upper limit of the average particle size of the first particles is 8 μm, preferably 7.8 μm, and more preferably 7.6 μm. When the average particle size of the first particles is within the above range, the internal resistance of the energy storage element can be reduced.

[0043] (Second particles) The second particles are natural graphite particles. Examples of the shape of the second particles include the same shapes as the first particles described above. The second particles preferably have a solid structure. When both the first particles and the second particles have a solid structure, the decrease in the capacity retention rate during charge / discharge cycling of the energy storage element can be further suppressed.

[0044] The lower limit of the sphericity of the second particles is preferably greater than 0.6, more preferably 0.65. When the sphericity of the second particles is equal to or greater than the lower limit, the durability of the negative electrode active material particles can be easily maintained. The lower limit of the sphericity of the second particles may be 0.7 or 0.8. On the other hand, the upper limit of the sphericity of the second particles is not particularly limited, but may be 0.99, 0.95, or 0.9 from the viewpoint of manufacturability.

[0045] The lower limit of the average particle size of the second particles is 10 μm, preferably 11 μm, more preferably 12 μm, and even more preferably 13 μm. Meanwhile, the upper limit of the average particle size of the second particles is 20 μm, preferably 18 μm, and more preferably 16 μm. When the average particle size of the second particles is equal to or greater than the lower limit, a decrease in the capacity retention rate during charge / discharge cycles of the energy storage element can be suppressed. Furthermore, when the average particle size of the second particles is equal to or less than the upper limit, the internal resistance of the energy storage element can be reduced.

[0046] The upper limit of the ratio of the average particle size of the first particles to the average particle size of the second particles may be 0.7, but is preferably 0.6, and more preferably 0.55. When the ratio is equal to or less than the upper limit, the first particles can more easily enter the gaps where the second particles contact, making it easier to achieve the above-mentioned effects of the present invention. The lower limit of the ratio of the average particle size of the first particles to the average particle size of the second particles is not particularly limited, but may be 0.1 or 0.2.

[0047] The lower limit of the volumetric ratio of the content of the first particles to the total content of the first particles and the second particles is greater than 0.2, preferably 0.25. Meanwhile, the upper limit of this ratio is 0.6, preferably 0.55. When this ratio is equal to or greater than the lower limit, the internal resistance of the energy storage element can be reduced. Furthermore, when this ratio is equal to or less than the upper limit, a decrease in the capacity retention rate during charge / discharge cycling of the energy storage element can be suppressed.

[0048] The negative electrode active material particles may contain other negative electrode active materials in addition to the first particles and the second particles. Examples of other negative electrode active materials include metallic lithium; metals or semimetals such as silicon and tin; metal oxides or semimetal oxides such as silicon oxide, titanium oxide, and tin oxide; Li ４ Ti ５ O １２ , LiTiO ２、 TiNb ２ O ７ polyphosphate compounds; silicon carbide; and carbon materials such as artificial graphite and non-graphitic carbon.

[0049] "Non-graphitic carbon" refers to a carbon having an average lattice spacing (d ００２ ) is 0.34 nm or more and 0.42 nm or less. Non-graphitizable carbon includes non-graphitizable carbon and graphitizable carbon. "Non-graphitizable carbon" refers to a carbon material having the above d ００２ The term "easily graphitizable carbon" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less. ００２ The term "carbon material" refers to a carbon material having a particle size of 0.34 nm or more and less than 0.36 nm.

[0050] The total content of the first particles and the second particles in the negative electrode active material particles is preferably 90% by mass or more, more preferably 99% by mass or more, and may be substantially 100% by mass. That is, the negative electrode active material particles may be substantially composed of the first particles and the second particles.

[0051] [Negative electrode for energy storage device] A negative electrode for an energy storage device according to one embodiment of the present invention (hereinafter also simply referred to as "negative electrode") has a negative electrode substrate and a negative electrode active material layer laminated on the negative electrode substrate directly or via an intermediate layer. The negative electrode active material layer contains negative electrode active material particles according to one embodiment of the present invention. Typically, the negative electrode has a portion where the negative electrode substrate is exposed. This exposed portion of the negative electrode substrate is usually connected to a negative electrode lead. The negative electrode may have a shape such as a sheet, plate, or strip.

[0052] The negative electrode contains the negative electrode active material particles, and therefore has the effect of reducing the internal resistance of the electricity storage device and suppressing a decrease in the capacity retention rate during charge / discharge cycles of the electricity storage device.

[0053] The negative electrode substrate has electrical conductivity. In this specification, "having electrical conductivity" means that the volume resistivity is 10 －２ The volume resistivity is a value measured in accordance with JIS-H-0505 (1975). On the other hand, in this specification, "not having electrical conductivity" or "having (electrical) insulation" means that the volume resistivity is 10 ７ This means that the resistance is Ω·cm or more. Examples of materials for the negative electrode substrate include metals such as copper, nickel, iron, and alloys thereof (stainless steel, etc.), and carbon materials. Among these, copper or copper alloys are preferred.

[0054] The negative electrode substrate has a shape such as a sheet, plate, or strip. Examples of the form of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The negative electrode substrate may be, for example, copper foil or copper alloy foil.

[0055] The average thickness of the negative electrode substrate may be, for example, 2 μm or more and 35 μm or less. The lower limit of the average thickness of the negative electrode substrate may be 3 μm, 4 μm, or 5 μm. The upper limit of the average thickness of the negative electrode substrate may be 30 μm or 20 μm. In this specification, "average thickness" means the average value of thicknesses measured at any five positions.

[0056] The intermediate layer is a layer disposed between the negative electrode substrate and the negative electrode active material layer. The intermediate layer contains, for example, a conductive agent and a binder. When the intermediate layer contains a conductive agent, the contact resistance between the negative electrode substrate and the negative electrode active material layer can be reduced. Examples of the conductive agent and binder used in the intermediate layer include the same conductive agent and binder used in the negative electrode active material layer described below.

[0057] The negative electrode active material layer contains, in addition to the negative electrode active material particles, optional components such as a conductive agent, a binder, a thickener, and a filler as needed. The negative electrode active material layer may be formed from a negative electrode mixture containing the negative electrode active material and other optional components. The negative electrode active material layer may be provided on only one side or on both sides of a negative electrode substrate having a shape such as a sheet.

[0058] The content of the negative electrode active material particles in the negative electrode active material layer is, for example, preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material particles within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved. In addition, the above-mentioned effects of the present invention can be easily enjoyed.

[0059] The conductive agent is usually a component made of a material having electrical conductivity. Even when the volume resistivity of the conductive agent cannot be measured directly, it is possible to measure the volume resistivity by measuring the volume resistivity of the conductive agent when the volume resistivity is 10 －２Conductive agents are materials known to have a resistivity of Ω·cm or less. Examples of conductive agents include carbon materials, metals, and conductive ceramics. Carbon materials are materials whose primary constituent element is carbon. The primary constituent element refers to the element with the highest content by mass. For example, the carbon content in the carbon material may be 80% by mass or more, 90% by mass or more, 95% by mass, 99% by mass, or 99.9% by mass or more. The carbon material is preferably a carbon material other than an uncarbonized polymer compound. Examples of carbon materials include non-graphitic carbon and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofiber, pitch-based carbon fiber, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerene. The conductive agent may be in the form of a powder or fiber. The conductive agent may be one or more kinds. The conductive agent may be a composite of these materials. For example, a composite material of carbon black and CNT may be used.

[0060] When the negative electrode active material layer contains a conductive agent, 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, and more preferably 3% by mass or more and 9% by mass or less. The content of the conductive agent in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a conductive agent.

[0061] The lower limit of the BET specific surface area of ​​the negative electrode active material layer is 3.0 m ２ / g is preferred, and 3.5m ２ / g is more preferable, and 4.0m ２ On the other hand, the upper limit of the BET specific surface area of ​​the negative electrode active material layer is 5.0 m ２ / g, but 4.3m ２ / g is preferable. When the BET specific surface area of ​​the negative electrode active material layer is within the above range, the internal resistance of the energy storage device can be reduced. Furthermore, when the ratio is equal to or less than the above upper limit, the decrease in the capacity retention rate during charge / discharge cycling of the energy storage device can be further suppressed. The BET specific surface area of ​​the negative electrode active material layer can be adjusted, for example, by selecting the types and content ratio of the first and second particles of the negative electrode active material particles so as to have a desired BET specific surface area.

[0062] Examples of the binder include a water-based binder and an organic solvent-based binder.

[0063] The aqueous binder is a binder that dissolves or disperses in water. The aqueous binder may be a binder that dissolves or disperses 1 part by mass or more in 100 parts by mass of water at 20°C. When forming a negative electrode active material layer using a negative electrode mixture paste whose dispersion medium is water or a mixed solvent mainly composed of water, an aqueous binder (a water-soluble or water-dispersible polymer material) can be used. Examples of aqueous binders include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polytetrafluoroethylene, styrene-butadiene rubber, polyethylene, polypropylene, nitrile-butadiene rubber, and cellulose.

[0064] The organic solvent-based binder is a binder that dissolves or disperses in an organic solvent (e.g., N-methylpyrrolidone). The organic solvent-based binder may be a binder that dissolves or disperses at 1 part by mass or more per 100 parts by mass of an organic solvent (e.g., N-methylpyrrolidone) at 20°C. When forming a negative electrode active material layer using a negative electrode mixture paste whose dispersion medium is an organic solvent or a mixed solvent mainly composed of an organic solvent, an organic solvent-based binder (a polymer material that is soluble or dispersible in an organic solvent) can be used. Examples of organic solvent-based binders include polyvinylidene fluoride, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of ethylene and vinyl alcohol, polyacrylonitrile, polyphosphazene, polysiloxane, polyvinyl acetate, polymethyl methacrylate, polystyrene, polycarbonate, polyamide, polyimide, polyamideimide, crosslinked polymers of cellulose and chitosan pyrrolidone carboxylate, chitosan derivatives, and the like.

[0065] The binder may be a fluororesin (polytetrafluoroethylene, polyvinylidene fluoride, etc.), a polyolefin (polyethylene, polypropylene, etc.), an elastomer (ethylene propylene diene rubber, styrene butadiene rubber, fluororubber, etc.), a polysaccharide polymer (cellulose, chitosan derivatives, etc.), etc. One or more types of binders can be used.

[0066] When the negative electrode active material layer contains a binder, the content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 5% by mass or less, and more preferably 0.4% by mass or more and 3% by mass or less. The content of the binder in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a binder.

[0067] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose and methyl cellulose. When the thickener has a functional group that reacts with lithium or the like, the functional group may be deactivated in advance by methylation or the like. The thickener may also function as a binder. One or more types of thickeners may be used.

[0068] When the negative electrode active material layer contains a thickener, 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, and more preferably 0.4% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer may be 5% by mass or less, or may be 2% by mass or less. The technology disclosed herein may also be implemented in an embodiment in which the negative electrode active material layer does not contain a thickener.

[0069] The filler is not particularly limited. The filler may be a component other than the negative electrode active material particles, conductive agent, binder, and thickener, and may be an intentionally contained component. The filler may be a component that fills gaps in the negative electrode active material layer, or may be a component that is contained for another purpose. The filler may be an organic substance such as polyolefin, or an inorganic substance such as an inorganic oxide, hydroxide, or carbonate. One or more types of fillers can be used.

[0070] When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer can be 0.1 mass % or more and 8 mass % or less, and usually 5 mass % or less is preferable, and 2 mass % or less is more preferable. The technology disclosed herein can also be implemented in an embodiment in which the negative electrode active material layer does not contain a filler.

[0071] The negative electrode active material layer may further contain other components in addition to the negative electrode active material particles, conductive agent, binder, thickener, and filler. The other components include those unintentionally generated in the negative electrode active material layer. The negative electrode active material layer may also contain unintentionally contained impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally generated components in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally contained impurities in the negative electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0072] The average thickness of one negative electrode active material layer may be, for example, 3 μm or more and 1,000 μm or less. The lower limit of the average thickness of one negative electrode active material layer may be 5 μm, 10 μm, or 20 μm. The upper limit of the average thickness of one negative electrode active material layer may be 800 μm, 500 μm, 200 μm, 100 μm, 80 μm, or 60 μm.

[0073] The porosity of the negative electrode active material layer may be, for example, 20% or more and 70% or less. The lower limit of the porosity of the negative electrode active material layer may be 25% or 30%. The upper limit of the porosity of the negative electrode active material layer may be 60% or 50%. The "porosity (%)" of the negative electrode active material layer and the positive electrode active material layer described later is calculated by dividing the apparent volume (volume including voids) of the negative (positive) electrode active material layer by V. １ The sum of the actual volumes of the materials constituting the negative (positive) electrode active material layer is V ２ In this case, (1-V ２ / V １ The sum of the actual volumes of the materials constituting the negative (positive) electrode active material layer, V ２ can be calculated from the content of each material in the negative (positive) electrode active material layer and the true density of each material.

[0074] (Method for manufacturing a negative electrode) The negative electrode can be manufactured by a known method. The negative electrode can be manufactured, for example, by applying a paste-like negative electrode mixture (negative electrode mixture paste) to a negative electrode substrate directly or via an intermediate layer, and drying the paste to form a negative electrode active material layer. The negative electrode mixture paste usually contains the above-mentioned negative electrode active material particles, other optional components, and a dispersion medium. After drying, the negative electrode active material layer may be pressed, etc.

[0075] [Electricity storage element] An electric storage element according to one embodiment of the present invention includes a positive electrode, a negative electrode according to one embodiment of the present invention, an electrolyte, and a container that accommodates these. The electric storage element may further include a separator that is interposed between the positive electrode and the negative electrode to electrically insulate the positive electrode from the negative electrode. The positive electrode, the negative electrode, and any separator typically constitute an electrode body. At least a portion of the electrolyte typically exists in a state of being impregnated into the electrode body. The electric storage element according to one embodiment of the present invention may further include other components.

[0076] The energy storage device includes a negative electrode containing the negative electrode active material particles according to one embodiment of the present invention, and therefore has the effect of having low internal resistance and being able to suppress a decrease in capacity retention rate during charge / discharge cycling.

[0077] For example, an energy storage element 1 shown in Fig. 1 according to one embodiment of the present invention includes an electrode assembly 2, an electrolyte (not shown), and a rectangular parallelepiped container 3 that accommodates these. The energy storage element 1 in Fig. 1 further includes a positive electrode lead 4, a positive electrode external terminal 5, a negative electrode lead 6, and a negative electrode external terminal 7. The positive electrode lead 4 and the negative electrode lead 6 are accommodated in the container 3 together with the electrode assembly 2 and the like. The positive electrode external terminal 5 and the negative electrode external terminal 7 are provided outside the container 3. The positive electrode constituting the electrode assembly 2 is electrically connected to the positive electrode external terminal 5 via the positive electrode lead 4. The negative electrode constituting the electrode assembly 2 is electrically connected to the negative electrode external terminal 7 via the negative electrode lead 6.

[0078] The energy storage element of the present invention may be a non-aqueous electrolyte secondary battery. Below, main components constituting the energy storage element according to one embodiment of the present invention will be described in detail, focusing on the case where the energy storage element is a non-aqueous electrolyte secondary battery (particularly a lithium ion secondary battery), but this is not intended to limit the application of the present invention.

[0079] (Positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer laminated on the positive electrode substrate directly or via an intermediate layer. Usually, the positive electrode has a portion where the positive electrode substrate is exposed. This exposed portion of the positive electrode substrate is usually connected to the above-mentioned positive electrode lead. The positive electrode may have a shape such as a sheet, plate, or strip.

[0080] The positive electrode substrate is electrically conductive. Examples of the material for the positive electrode substrate include metals such as aluminum, titanium, iron, and alloys thereof (e.g., stainless steel). Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high electronic conductivity, and cost.

[0081] The positive electrode substrate has a shape such as a sheet, plate, or strip. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, and foil is preferred. The positive electrode substrate may be, for example, aluminum foil or aluminum alloy foil.

[0082] The average thickness of the positive electrode substrate may be, for example, 3 μm or more and 50 μm or less. The lower limit of the average thickness of the positive electrode substrate may be 5 μm, 8 μm, or 10 μm. The upper limit of the average thickness of the positive electrode substrate may be 40 μm, 30 μm, or 25 μm.

[0083] The structure of the intermediate layer of the positive electrode is not particularly limited, and can be selected from the structures exemplified for the intermediate layer of the negative electrode, for example.

[0084] The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the negative electrode above. The positive electrode active material layer may be formed from a positive electrode mixture containing a positive electrode active material and other optional components. The positive electrode active material layer may be provided on only one side or on both sides of a positive electrode substrate having a shape such as a sheet.

[0085] As the positive electrode active material, a known positive electrode active material can be used. A material capable of absorbing and releasing lithium ions is typically used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides, polyanion compounds, chalcogen compounds, sulfur-based materials, and lithium oxide. One or more positive electrode active materials can be used.

[0086] Examples of the transition metal element contained in the lithium transition metal composite oxide include nickel, cobalt, and manganese. The lithium transition metal composite oxide may also contain a typical metal element such as aluminum. Examples of the lithium transition metal composite oxide include α-NaFeO ２ Examples of the lithium transition metal composite oxide include a lithium transition metal composite oxide having a crystalline structure and a lithium transition metal composite oxide having a spinel crystalline structure.

[0087] α-NaFeO ２ As the lithium transition metal composite oxide having a crystalline structure, Li １＋α Ma １－α O ２ (Ma is a metal element other than lithium containing one or more transition metal elements, where 0≦α<1.) Ma preferably contains one or more of Ni, Co, and Mn. The total content of Ni, Co, and Mn relative to Ma ((Ni+Co+Mn) / Ma) is preferably 90 mol% or more, and more preferably 98 mol% or more.

[0088] Examples of lithium transition metal composite oxides having a spinel crystal structure include Li βMb ２ O ４ (Mb is a metal element other than lithium containing one or more transition metal elements, and β is 0<β≦1.2). Mb preferably contains Mn. The content of Mn relative to Mb (Mn / Mb) is preferably 50 mol % or more, and more preferably 80 mol % or more.

[0089] The polyanion compound is a compound composed of a polyanion (i.e., a polyvalent oxoacid anion) and a cation. The polyanion compound preferably contains a lithium cation and a transition metal cation as the cation. Examples of the polyanion compound include LiFePO ４ , LiMnPO ４ , LiMn ｘ Fe １－ｘ P.O. ４ (0<x<1), LiNiPO ４ , LiCoPO ４ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４ The surface of the particles of the polyanionic compound may be coated with another material (for example, a carbon material).

[0090] Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide, and molybdenum dioxide.

[0091] Examples of sulfur-based materials include elemental sulfur, metal sulfides such as lithium sulfide, organic disulfide compounds, and organic sulfur compounds such as carbon sulfide compounds.

[0092] The atoms or polyanions in these materials serving as the positive electrode active material may be partially substituted with atoms or anion species of other elements, and the surfaces of these materials may be coated with other materials.

[0093] The positive electrode active material is usually in a particulate form. The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easy to manufacture or handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material.

[0094] 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 may be 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.

[0095] When the positive electrode active material layer contains a conductive agent, the content of the conductive agent in the positive electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 1% by mass to 9% by mass, and even more preferably 3% by mass to 8% by mass. The upper limit of the content of the conductive agent may be 5%, 4%, or 3% by mass. By setting the content of the conductive agent within the above range, it is possible to increase the energy density of the energy storage element, etc.

[0096] When the positive electrode active material layer contains a binder, the content of the binder in the positive electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 1% by mass to 9% by mass, and even more preferably 3% by mass to 8% by mass. The upper limit of the binder content may be 5%, 4%, or 3% by mass. By setting the binder content within the above range, it is possible to stably hold the positive electrode active material. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a binder.

[0097] When the positive electrode active material layer contains a thickener, the content of the thickener in the positive electrode active material layer is preferably 0.1 mass% or more and 8 mass% or less, more preferably 5 mass% or less, and even more preferably 2 mass% or less. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a thickener.

[0098] When the positive electrode active material layer contains a filler, the content of the filler in the positive electrode active material layer can be 0.1 mass % or more and 8 mass % or less, and usually 5 mass % or less is preferable, and 2 mass % or less is more preferable. The technology disclosed herein can also be implemented in an embodiment in which the positive electrode active material layer does not contain a filler.

[0099] The positive electrode active material layer may further contain other components in addition to the positive electrode active material, conductive agent, binder, thickener, and filler. These other components include those unintentionally generated in the positive electrode active material layer. The positive electrode active material layer may also contain unintentionally contained impurities as the other components, as long as the effects of the present invention are achieved. The upper limit of the content of the other components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally generated components in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass. The upper limit of the content of the unintentionally contained impurities in the positive electrode active material layer may be 10% by mass, 5%, 2%, 1%, 0.1%, or 0.01% by mass.

[0100] The thickness of the positive electrode active material layer is appropriately set depending on the type of positive electrode active material, the application of the energy storage device, etc. The average thickness of one positive electrode active material layer may be, for example, 5 μm or more and 1,000 μm or less. The lower limit of the average thickness of one positive electrode active material layer may be 10 μm or 20 μm. The upper limit of the average thickness of one positive electrode active material layer may be 800 μm, 500 μm, 200 μm, or 100 μm.

[0101] The porosity of the positive electrode active material layer may be, for example, 20% or more and 50% or less. The lower limit of the porosity of the positive electrode active material layer may be 25%. The upper limit of the porosity of the positive electrode active material layer may be 45%.

[0102] (Method for manufacturing positive electrode) The positive electrode can be manufactured by a known method. The positive electrode can be manufactured, for example, by applying a paste-like positive electrode mixture (positive electrode mixture paste) to a positive electrode substrate directly or via an intermediate layer, and drying the paste to form a positive electrode active material layer. The positive electrode mixture paste usually contains a positive electrode active material, other optional components, and a dispersion medium. After drying, the positive electrode active material layer may be pressed, etc.

[0103] (Separator) A known separator can be used as the separator, for example, a separator consisting of only a base layer, or a separator in which an inorganic layer containing inorganic particles and a binder is formed on one or both surfaces of a base layer, or the like.

[0104] Examples of the form of the separator substrate layer include woven fabric, nonwoven fabric, and porous resin film. Among these forms, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. The material of the separator substrate layer is not particularly limited as long as it has insulating properties, but resins such as polyolefin (polyethylene, polypropylene, etc.), polyimide, and aramid are preferred.

[0105] Examples of inorganic compounds constituting the inorganic particles include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, and magnesium oxide; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon; mineral resource-derived substances such as talc, zeolite, kaolin, bentonite, and mica, or artificial products thereof. One or more types of inorganic particles can be used. The average particle size of the inorganic particles is preferably, for example, 0.5 μm to 10 μm. The content of the inorganic particles in the inorganic layer is preferably 50% by mass to 99% by mass, more preferably 80% by mass to 98% by mass.

[0106] Examples of binders used in the inorganic layer include the same binders as those exemplified for the positive electrode active material layer.

[0107] The porosity of the separator may be, for example, 20% or more and 80% or less. The lower limit of the porosity of the separator may be 30%, 40%, or 50% from the viewpoint of discharge performance, etc. The upper limit of the porosity of the separator may be 70%, 60%, or 50% from the viewpoint of strength, etc. In this specification, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0108] The average thickness of the separator may be, for example, 10 μm or more and 40 μm or less, or 15 μm or more and 30 μm or less.

[0109] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte, or may be a combination of the porous resin film, nonwoven fabric, or the like described above and a polymer gel.

[0110] (Electrode Body) As the electrode body, for example, a wound type electrode body, a laminated type electrode body, or the like having a known structure can be used.

[0111] A wound electrode body has a structure in which a positive electrode and a negative electrode are wound in an insulated state. The wound electrode body may be cylindrical (columnar) or flat. The electrode body 2 provided in the energy storage device 1 of FIG. 1 is a flat wound electrode body. The wound electrode body can be produced, for example, by the following procedure. First, a positive electrode, a separator, and a negative electrode, each formed in a strip shape, are stacked in this order to obtain a laminate. The wound electrode body is obtained by winding this laminate.

[0112] A laminated electrode assembly has a structure in which one or more positive electrodes and one or more negative electrodes are stacked in an insulated state. For example, a laminated electrode assembly can be obtained by stacking a positive electrode, a separator, and a negative electrode, each of which is formed into a rectangular shape, in this order.

[0113] As the electrode body, for example, one having a structure in which at least one of the positive electrode and the negative electrode is folded in an accordion-like manner and stacked can also be used.

[0114] (Non-aqueous electrolyte) A known non-aqueous electrolyte can be used as the non-aqueous electrolyte. The non-aqueous electrolyte is a medium that transports charge-transporting ions (e.g., lithium ions) between the positive electrode and the negative electrode and does not substantially contain water. Examples of the non-aqueous electrolyte include a non-aqueous electrolyte solution and a solid electrolyte. A non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

[0115] (Non-aqueous Electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0116] As the non-aqueous solvent, known non-aqueous solvents can be used. Examples of non-aqueous solvents include carbonates, esters, ethers, amides, and nitriles. Examples of carbonates include cyclic carbonates and chain carbonates. Examples of esters include carboxylic acid esters, phosphate esters, and sulfonic acid esters. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogen atoms may also be used. One or more non-aqueous solvents can be used.

[0117] The term "cyclic carbonate" refers to a carbonate having a ring structure containing a carbonate group (-O-C(=O)-O-). Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, styrene carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate. The cyclic carbonate may be a saturated cyclic carbonate such as ethylene carbonate, or an unsaturated cyclic carbonate such as vinylene carbonate. "Saturated" refers to the absence of a carbon-carbon unsaturated bond (a carbon-carbon double bond or a carbon-carbon triple bond). "Unsaturated" refers to the presence of a carbon-carbon unsaturated bond. As the cyclic carbonate, saturated cyclic carbonates are preferred, and ethylene carbonate is more preferred.

[0118] The chain carbonate means a carbonate that does not have a ring structure containing a carbonate group. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. The chain carbonate may be a saturated chain carbonate such as dimethyl carbonate, or an unsaturated chain carbonate such as diphenyl carbonate. The chain carbonate is preferably a saturated chain carbonate, and more preferably ethyl methyl carbonate.

[0119] The non-aqueous solvent preferably contains a carbonate, and more preferably contains a cyclic carbonate and a chain carbonate. The content of the carbonate in the non-aqueous solvent is preferably 80% by volume or more and 100% by volume or less, and may be 99% by volume or more and 100% by volume or less, or even 100% by volume. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and increase the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0120] The electrolyte salt may be a known electrolyte salt. Examples of the electrolyte salt include lithium salt, sodium salt, potassium salt, magnesium salt, and onium salt. Among these, lithium salt is preferred. One or more types of electrolyte salt may be used.

[0121] The lithium salt is LiPF ６ , LiPO ２ F ２ , LiBF ４ , LiClO ４ Inorganic lithium salts such as LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , LiN(SO２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) (SO ２ C ４ F ９ ), imide salts such as LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ), LiPF ２ (C ２ O ４ ) ２ Lithium oxalate salts such as LiN(SO ２ F) ２ Among these, inorganic lithium salts are preferred, and LiPF ６ In some cases, an imide salt is also preferred.

[0122] The content of the electrolyte salt in the non-aqueous electrolyte solution is 0.1 mol / dm at 20°C and 1 atmosphere. ３ 2.5mol / dm or more ３ Preferably, 0.3 mol / dm or less ３ 2.0mol / dm or more ３ More preferably, 0.5 mol / dm or less ３ More than 1.7mol / dm ３ More preferably, 0.7 mol / dm or less ３ 1.5mol / dm or more ３ The following is particularly preferred: By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0123] The non-aqueous electrolyte may contain an additive in addition to the non-aqueous solvent and the electrolyte salt. One or more additives may be used. When an additive is used in the non-aqueous electrolyte, the content of the additive in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass.

[0124] (Solid Electrolyte) The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc., and is solid at room temperature (e.g., 20°C). Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, halide solid electrolytes, and polymer solid electrolytes. One or more types of solid electrolytes can be used.

[0125] (Container) The container accommodates the electrode assembly and the non-aqueous electrolyte in its internal space. Materials for the container include metal materials such as aluminum and stainless steel, and resin materials, with metal materials being preferred from the standpoint of strength, etc. Composite materials of metal and resin materials can also be used.

[0126] The shape of the container is not particularly limited, and may be cylindrical, rectangular (square), disk-like, etc. The container may also be in the shape of a sheet formed from a metal-resin composite film.

[0127] (Shape, Use, etc. of Energy Storage Element) The shape of the energy storage element according to one embodiment of the present invention is not particularly limited. The energy storage element may be, for example, a cylindrical battery, a prismatic battery, a flat battery, a coin battery, a button battery, or the like.

[0128] The use of the energy storage element according to one embodiment of the present invention is not particularly limited, and the energy storage element can be used, for example, as a power source for automobiles such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, a power source for electronic devices such as personal computers and communication terminals, a power source for power storage, and the like.

[0129] The energy storage element of the present invention may be used singly or in multiples. When the required output and required voltage are small, the energy storage element may be used singly. On the other hand, when at least one of the required output and required voltage is large, the energy storage element may be used as an energy storage device combined with other energy storage elements. In an energy storage device in which multiple energy storage elements are combined, at least one energy storage element included in the energy storage device may be an energy storage element according to one embodiment of the present invention. The energy storage device will be described in detail later.

[0130] In an energy storage element according to one embodiment of the present invention, for example, the container may be constrained so as to maintain a constant thickness, or may not be constrained in this manner. Furthermore, the container may be constrained so as to apply a constant load to the container. When the container is constrained, expansion of the container due to charge / discharge cycles, etc., may be suppressed, and deterioration of charge / discharge performance may be suppressed. When the container is constrained, a load may be applied to the electrode body in the container, or may not be applied. For example, a constraining member that performs such a constraint may be provided in the energy storage element or the energy storage device.

[0131] [Method for manufacturing energy storage element] An energy storage element according to one embodiment of the present invention can be manufactured by a known method. The method for manufacturing the energy storage element includes, for example, preparing a positive electrode, preparing a negative electrode, preparing an electrolyte, and housing the positive electrode, negative electrode, and electrolyte in a container. The manufacturing method may also include preparing a separator, and manufacturing an electrode assembly using the positive electrode, negative electrode, and separator. Housing the positive electrode, negative electrode, and electrolyte in a container may include housing the electrode assembly and electrolyte in the container.

[0132] Preparing a positive electrode may mean manufacturing a positive electrode. Manufacturing a positive electrode can be performed by the method described above. Preparing a negative electrode may mean manufacturing a negative electrode. Manufacturing a negative electrode can be performed by the method described above. Preparing an electrolyte may mean preparing an electrolyte. The electrolyte can be prepared, for example, by dissolving an electrolyte salt in a non-aqueous solvent. The positive electrode, negative electrode, separator, electrolyte, etc. may be prepared by purchasing, etc.

[0133] The electrode assembly (or the positive electrode and negative electrode) and the electrolyte can be housed in a container by a known method. When the electrolyte is a nonaqueous electrolyte solution, for example, the electrode assembly (or the positive electrode and negative electrode) is first housed in a container, and then the nonaqueous electrolyte solution is poured into the container through an inlet provided in the container. The inlet is sealed after the nonaqueous electrolyte solution is poured into the container. The method for producing the nonaqueous electrolyte storage element may further include initially charging and discharging the assembled, uncharged storage element.

[0134] The energy storage element according to one embodiment of the present invention may be manufactured by other methods. For example, when the energy storage element according to one embodiment of the present invention is an all-solid-state battery, it may be manufactured by pressing the materials for forming the positive electrode, separator, and negative electrode individually or collectively.

[0135] 2 includes a plurality of energy storage units 20. Each energy storage unit 20 includes a plurality of electrically connected energy storage elements 1. The energy storage device 30 may include a bus bar (not shown) that electrically connects the plurality of energy storage elements 1, a bus bar (not shown) that electrically connects the plurality of energy storage units 20, etc. The energy storage unit 20 or the energy storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more energy storage elements 1.

[0136] [Other Embodiments] The energy storage device of the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0137] In the above embodiment, the electric storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery, but the electric storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0138] In the above embodiment, the electrode assembly is described in which a separator is interposed between the positive electrode and the negative electrode. However, the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other, with a non-conductive layer formed on the active material layer of the positive electrode or the negative electrode. In this way, the positive electrode and the negative electrode may further include layers other than the substrate, the intermediate layer, and the active material layer. Furthermore, the positive electrode and the negative electrode may not have a layer structure.

[0139] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0140] [Example 1] (Preparation of Negative Electrode) A mixture of first graphite particles, which are natural graphite particles having a solid structure, an average particle size of 7.4 μm, and a spheroidization degree of 0.67, and second graphite particles, which are natural graphite particles having a solid structure, an average particle size of 14.2 μm, and a spheroidization degree of 0.68, was used as the negative electrode active material particles. Both the first graphite particles and the second graphite particles were spherical natural graphite particles obtained by spheroidizing flake-like natural graphite. The ratio of the content of the first graphite particles to the total content of the first graphite particles and the second graphite particles was 0.5 on a volume basis. The negative electrode active material particles, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium were mixed to prepare a negative electrode mixture paste. The mass ratio of the negative electrode active material particles to SBR and CMC was 96:3.3:0.7 in terms of solid content. This negative electrode mixture paste was applied to both sides of a copper foil serving as a negative electrode substrate, dried, and roll-pressed to form a negative electrode active material layer, thereby obtaining a negative electrode. The BET specific surface area of ​​the negative electrode active material layer was 4.2 m. ２ / g.

[0141] (Preparation of Positive Electrode) As a positive electrode active material, LiFePO having an olivine crystal structure was used. ４ A lithium iron phosphate represented by the formula (I) was used. The positive electrode active material was mixed with acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium to prepare a positive electrode mixture paste. The mass ratio of the positive electrode active material, AB, and PVDF was 93.0:4.0:3.0 in terms of solid content. This positive electrode mixture paste was applied to both sides of aluminum foil, which was a positive electrode substrate, dried, and roll-pressed to form a positive electrode active material layer, thereby obtaining a positive electrode.

[0142] (Non-aqueous electrolyte) A solvent containing ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) mixed in a volume ratio of 30:35:35 was added to a 1.1 mol / dm ３ At a concentration of LiPF６ was dissolved to obtain a non-aqueous electrolyte.

[0143] (Separator) A polyethylene microporous film was used as the separator.

[0144] (Assembly of Energy Storage Element) The positive electrode, negative electrode, and separator were stacked to prepare an electrode assembly. The obtained electrode assembly was placed in a container, and the non-aqueous electrolyte was then poured into the container, followed by sealing, to obtain the energy storage element of Example 1.

[0145] [Example 2, Comparative Examples 1 to 16] Energy storage devices of Example 2 and Comparative Examples 1 to 16 were obtained in the same manner as Example 1, except that the types (natural or artificial), structures (solid or hollow), average particle sizes, volume-based content ratios (content ratios), and BET specific surface areas of the negative electrode active material layers of the first graphite particles and the second graphite particles were as shown in Table 1. The press used to form the negative electrode active material layer was adjusted so that the mass per unit area and porosity of the negative electrode active material layers of Example 2 and Comparative Examples 1 to 16 were consistent with those of Example 1. In Table 1, "hollow" means that the negative electrode active material layer does not have a solid structure, "small" in the particle size classification means that the average particle size is 3 μm or more and 8 μm or less, "large" in the particle size classification means that the average particle size is 10 μm or more and 20 μm or less, and "-" means that no second graphite particles are present.

[0146] [Evaluation] (Measurement of initial discharge capacity) For each of the energy storage elements of the Examples and Comparative Examples, the initial discharge capacity was measured by the following method. The following test was carried out in a thermostatic chamber at 25°C. The upper limit voltage was 3.5 V and the charge current was 1 mA / cm. ２ After constant current charging at 3.5 V, constant voltage charging was performed at 3.5 V. The charging was terminated 5 hours after the start of charging. After a 10-minute rest period, the battery was charged at a lower limit voltage of 2.0 V with a discharge current of 1 mA / cm. ２ The amount of electricity during this discharge was taken as the initial discharge capacity.

[0147] (Measurement of Initial DC Resistance) The initial DC resistance of each storage element was measured by the following method. The current required to charge the initial discharge capacity measured in the "Measurement of Initial Discharge Capacity" section above in one hour was defined as 1 C. From the discharged state, the cells were charged at a constant current of 0.5 C for one hour at a temperature of 25°C, and the SOC (State of Charge) was adjusted to 50%. Then, at a temperature of 25°C, constant current discharges were performed for 10 seconds at discharge currents of 0.2 C, 0.5 C, and 1.0 C. After each discharge, constant current charging was performed at a current of 0.5 C, and supplementary charging was performed to compensate for the discharged amount of electricity, so that the SOC did not deviate from 50%. The voltage at one second at each current obtained was plotted to obtain a straight line. The slope of the line was defined as the initial DC resistance (initial DCR). The results are shown in Table 1. The values ​​shown in Table 1 are relative values, with the initial DCR of Comparative Example 6 being 100.0.

[0148] (Charge-Discharge Cycle Test) A charge-discharge cycle test was conducted for each energy storage element for which the "initial discharge capacity measurement" was performed as follows. Constant-current, constant-voltage charging was performed with a charging current of 0.1 C and a charge cut-off voltage of 3.5 V. The charge was terminated until the total charge time reached 15 hours. Next, constant-current discharging was performed with a discharge current of 0.1 C and a discharge cut-off voltage of 2.0 V. A 10-minute pause was provided after discharge and after charge. Discharge, charge, and pause were all performed in a thermostatic chamber at 25°C. The above charge and discharge cycles were repeated for 50 cycles. The capacity retention rate after the charge-discharge cycles was calculated as the percentage of the discharge capacity at the 50th cycle relative to the discharge capacity at the first cycle in the charge-discharge cycle test. The results are shown in Table 1. The values ​​shown in Table 1 are relative values, with the capacity retention rate of Comparative Example 6 set to 100.0.

[0149]

[0150] As shown in Table 1, Comparative Example 1, in which small-sized solid particles were used as the first graphite particles and their content ratio was less than 0.75, had an initial DCR of 100.0. Comparative Example 2, in which small-sized hollow particles were used as the first graphite particles and their content ratio was more than 0.9, had a reduced initial DCR but a capacity retention ratio of less than 100.4. Comparative Examples 3 to 5, in which either the first graphite particles or the second graphite particles were artificial graphite, had an initial DCR of 100.0 or more. Comparative Examples 6 to 12, in which all graphite particles had a large particle size, had a large initial DCR or a capacity retention ratio of less than 100.4. Comparative Examples 13 to 16, in which all graphite particles had a small particle size, also had a large initial DCR or a capacity retention ratio of less than 100.4. In contrast, in Examples 1 and 2, in which small-diameter solid particles were used as the first graphite particles and large-diameter natural graphite particles were used as the second graphite particles and mixed in an appropriate range, the initial DCR was low and the capacity retention ratio was 100.4 or more.

[0151] The present invention can be applied to power storage elements used as power sources for electronic devices such as personal computers and communication terminals, automobiles, industrial equipment, and the like.

[0152] REFERENCE SIGNS LIST 1 Energy storage element 2 Electrode body 3 Container 4 Positive electrode lead 5 Positive electrode external terminal 6 Negative electrode lead 7 Negative electrode external terminal 20 Energy storage unit 30 Energy storage device

Claims

1. Negative electrode active material particles comprising first graphite particles and second graphite particles different from the first graphite particles, wherein the first graphite particles are natural graphite having an average particle size of 3 μm or more and 8 μm or less and have a solid structure, and the second graphite particles are natural graphite having an average particle size of 10 μm or more and 20 μm or less, and the ratio of the content of the first graphite particles to the total content of the first graphite particles and the second graphite particles is greater than 0.2 and less than 0.6 on a volume basis.

2. The negative electrode active material particles according to claim 1, wherein the second graphite particles have a solid structure.

3. The negative electrode active material particles according to claim 1 or 2, wherein the spheroidization degree of the first graphite particles and the second graphite particles is greater than 0.

6.

4. The negative electrode active material particles according to claim 1 or 2, wherein the ratio of the average particle size of said first graphite particles to the average particle size of said second graphite particles is 0.6 or less.

5. A negative electrode for a storage element, comprising a negative electrode active material layer containing the negative electrode active material particles according to claim 1 or 2.

6. The BET specific surface area of ​​the negative electrode active material layer is 3.0 m ２ / g or more 4.3m ２ The negative electrode for an electric storage element according to claim 5, wherein the molecular weight is 1 / g or less.

7. An electric storage element comprising the negative electrode for an electric storage element according to claim 5.

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