Power storage element

Abstract

To provide a power storage element comprising a negative electrode having a high discharge capacity per unit volume of the negative electrode active material layer, even when no or little pressure is applied to the negative electrode active material layer, when graphite is used as the negative electrode active material.SOLUTION: One aspect of the present invention is a power storage element which comprises a negative electrode having a negative electrode substrate and a negative electrode active material layer directly or indirectly laminated onto at least one surface of the negative electrode substrate, in which the negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an energy storage element. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium-ion non-aqueous electrolyte secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc., due to their high energy density. The non-aqueous electrolyte secondary batteries generally include an electrode assembly having 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 ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium-ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] In order to increase the energy density of such energy storage elements and improve charge / discharge efficiency, carbon materials such as graphite, which have a large charge / discharge capacity, are used as the negative electrode active material of the above-mentioned energy storage elements (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-222933 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been a demand for further improvement in the discharge capacity per volume of a negative electrode containing such a carbon material. To address this demand, the discharge capacity per volume of the negative electrode active material layer can be increased by pressing the negative electrode active material layer to increase its density. However, pressing the negative electrode active material layer at high pressure can cause problems such as significant expansion of the negative electrode during initial charging due to residual stress from the press. There is a demand for an energy storage device including a negative electrode that has a large discharge capacity per volume of the negative electrode active material layer even when no or only a small pressure is applied to the negative electrode active material layer.

[0006] The present invention has been made in light of the above circumstances, and aims to provide an energy storage device including a negative electrode that has a high discharge capacity per volume of the negative electrode active material layer even when no or only a small pressure is applied to the negative electrode active material layer when graphite is used as the negative electrode active material. [Means for solving the problem]

[0007] One aspect of the present invention, which has been made to solve the above-mentioned problems, is an energy storage element including a negative electrode having a negative electrode substrate and a negative electrode active material layer laminated directly or indirectly on at least one surface of the negative electrode substrate, wherein the negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles. [Effects of the Invention]

[0008] According to the present invention, when graphite is used as the negative electrode active material, it is possible to obtain an energy storage element having a negative electrode with a high discharge capacity per volume of the negative electrode active material layer, even when no or only a small pressure is applied to the negative electrode active material layer. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing the appearance of an energy storage device according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an energy storage device configured by assembling a plurality of energy storage elements according to one embodiment of the present invention. [Figure 3] FIG. 3 is a graph showing the relationship between the content of solid graphite particles and the discharge capacity per volume of the negative electrode active material layer in Examples and Comparative Examples. [Figure 4] FIG. 4 is a graph showing the relationship between the content of solid graphite particles and the expansion rate of the negative electrode active material during initial charging in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] First, an overview of the energy storage element disclosed in this specification will be described.

[0011] An energy storage device according to one aspect of the present invention includes a negative electrode having a negative electrode substrate and a negative electrode active material layer laminated directly or indirectly on at least one surface of the negative electrode substrate, the negative electrode active material layer containing a negative electrode active material, and the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles.

[0012] In the energy storage device, the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles, so that the discharge capacity per volume of the negative electrode active material layer before pressing is higher than when either hollow graphite particles or solid graphite particles are contained alone, and a synergistic effect of the hollow graphite particles and the solid graphite particles can be obtained. Therefore, it is possible to obtain an energy storage device including a negative electrode having an excellent discharge capacity per volume of the negative electrode active material layer without attempting to increase the density by high-pressure pressing.

[0013] Here, the content of the hollow graphite particles is preferably 80% by mass or less relative to the total content of the hollow graphite particles and the solid graphite particles, which allows the negative electrode active material layer to have a higher discharge capacity per volume.

[0014] Here, it is preferable that the negative electrode active material layer is not substantially pressed. With this configuration, it is possible to increase the discharge capacity per volume of the negative electrode active material layer while suppressing problems that may arise from pressing the negative electrode active material layer (for example, expansion of the negative electrode during initial charging).

[0015] Here, the density of the negative electrode active material layer is 1.30 g / cm 3 More than 1.55g / cm 3 It is preferable that the density of the negative electrode active material layer containing hollow graphite particles and solid graphite particles as the negative electrode active material is 1.30 g / cm or less. 3 More than 1.55g / cm 3 The effect of applying this configuration can be more suitably exhibited in the following energy storage elements.

[0016] Here, the ratio Q2 / Q1, which is the ratio of the surface roughness Q2 of the region of the negative electrode substrate where the negative electrode active material layer is not laminated to the surface roughness Q1 of the region of the negative electrode substrate where the negative electrode active material layer is laminated, is preferably 0.90 or more. In an energy storage device where Q2 / Q1 of the negative electrode substrate is 0.90 or more, the effect of applying this configuration can be more suitably exhibited.

[0017] Here, the median diameter of the solid graphite particles is preferably 4 μm or less. By making the median diameter of the solid graphite particles 4 μm or less, the above-mentioned performance improvement effect (for example, the effect of increasing the discharge capacity per volume of the negative electrode active material layer when not pressed) can be more effectively exhibited. In addition, the output of the energy storage device can be further improved.

[0018] Here, the aspect ratio of the solid graphite particles is preferably from 1 to 5. When the aspect ratio of the solid graphite particles is from 1 to 5, the above-mentioned performance-improving effect (for example, the effect of increasing the discharge capacity per volume of the negative electrode active material layer when not pressed) can be more effectively exhibited.

[0019] An energy storage device according to an embodiment of the present invention will be described in detail. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

[0020] <Energy storage element> An energy storage element according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode and the negative electrode are typically stacked or wound with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a container, and the container is filled with a non-aqueous electrolyte. The non-aqueous electrolyte is interposed between the positive electrode and the negative electrode. A non-aqueous electrolyte secondary battery will be described as an example of such an energy storage element.

[0021] [Negative electrode] The negative electrode includes a negative electrode substrate and a negative electrode active material layer laminated directly or indirectly on at least one surface of the negative electrode substrate. The negative electrode may also include an intermediate layer disposed between the negative electrode substrate and the negative electrode active material layer.

[0022] (negative electrode substrate) The negative electrode substrate is electrically conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, or alloys thereof, are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils and vapor-deposited films, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil. The term "electrically conductive" refers to a material having a volume resistivity of 1×10 as measured in accordance with JIS-H0505 (1975). 7 "Non-conductive" means that the volume resistivity is 1×10 7 This means that the resistance is greater than Ω·cm.

[0023] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery. The "average thickness of the substrate" refers to the value obtained by dividing the punched mass when a substrate of a predetermined area is punched out by the true density and punched area of ​​the substrate.

[0024] (Negative electrode active material layer) The negative electrode active material layer is disposed along at least one surface of the negative electrode substrate directly or via an intermediate layer. The negative electrode active material layer contains a negative electrode active material.

[0025] The negative electrode active material disclosed herein contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles.

[0026] In this specification, "graphite" refers to a carbon substance having an average lattice spacing d(002) of the (002) plane of less than 0.34 nm as measured by X-ray diffraction before charge / discharge or in a discharged state.

[0027] In this specification, "solid" means that the interior of a particle is filled and substantially no voids exist. More specifically, in this specification, "solid" means that, in a cross-section of a particle observed in an SEM image obtained using a scanning electron microscope (SEM), the area ratio excluding voids within the particle to the total area of ​​the particle is 95% or more. In a preferred embodiment, the area ratio of solid graphite particles can be 97% or more (e.g., 99% or more). "Hollow" means that, in a cross-section of a particle observed in an SEM image obtained using an SEM, the area ratio excluding voids within the particle to the total area of ​​the particle is less than 95%. In a preferred embodiment, the area ratio of hollow graphite particles can be 92% or less (e.g., 90% or less).

[0028] The area ratio R of the graphite particle, excluding voids within the particle, to the total particle area can be determined by the following procedure. (1) Preparation of measurement samples The graphite particle powder to be measured is fixed in a thermosetting resin. A cross-section polisher is used to expose the cross section of the graphite particles fixed in the resin, and the measurement sample is prepared. (2) Obtaining SEM images To obtain SEM images, a JSM-7001F (manufactured by JEOL Ltd.) scanning electron microscope is used. SEM images are obtained by observing secondary electron images. The acceleration voltage is 15 kV. The observation magnification is set so that between 3 and 15 graphite particles appear in one field of view. The obtained SEM image is saved as an image file. Other conditions such as spot diameter, working distance, irradiation current, brightness, and focus are set appropriately so that the contours of the graphite particles are clearly visible. (3) Cutting out the outline of the graphite particles The image cutting function of the image editing software Adobe Photoshop Elements 11 was used to cut out the outlines of the graphite particles from the acquired SEM image. This cutting out of the outlines was done by using the quick selection tool to select the area outside the outlines of the active material particles and editing everything except the graphite particles to a black background. If the outlines of fewer than three graphite particles were successfully cut out at this point, another SEM image was acquired and this was repeated until the outlines of three or more graphite particles were successfully cut out. (4) Binarization The image of the first of the cut-out graphite particles is binarized using the image analysis software PopImaging 6.00, with a threshold set to a density 20% lower than the density at which the intensity is at its maximum. The area of ​​the lower density side is calculated through the binarization process, and this is taken as the "area S1 excluding voids within the particle." Next, the same image of the first graphite particle as before is subjected to binarization processing with a threshold density of 10. Through binarization processing, the outer periphery of the graphite particle is determined, and the area inside this periphery is calculated to obtain the "area of ​​the entire particle, S0." Using the calculated S1 and S0, S1 to S0 (S1 / S0) is calculated to calculate the "area ratio R1 excluding 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 above-described binarization process to calculate the areas S1 and S0. Based on the calculated areas S1 and S0, the area ratios R2, R3, ... of each graphite particle are calculated. (5) Determination of area ratio R The average value of all the area ratios R1, R2, R3, ... calculated by the binarization process is calculated to determine the "area ratio R of graphite particles excluding voids within the particles to the area of ​​the entire particle."

[0029] (median diameter) The median diameter D1 of the hollow graphite particles is not particularly limited as long as it is larger than the median diameter D2 of the solid graphite particles (i.e., D1>D2). D1 is suitably, for example, 4 μm or more, usually 5 μm or more, and typically 6 μm or more. D1 is preferably 7 μm or more, more preferably 7.5 μm or more. In some embodiments, D1 may be 8 μm or more, or 10 μm or more (e.g., 12 μm or more). Furthermore, hollow graphite particles having a D1 of 20 μm or less can be preferably used, from the viewpoint of increasing the discharge capacity per volume of the negative electrode active material layer in an unpressed state. For example, D1 is preferably 18 μm or less, more preferably 16 μm or less. In some embodiments, D1 may be 14 μm or less, or 12 μm or less (e.g., 10 μm or less). The technology disclosed herein can be preferably implemented in an embodiment in which the median diameter D1 of the hollow graphite particles is 4 μm or more and 20 μm or less (more preferably 6 μm or more and 16 μm or less, and particularly preferably 8 μm or more and 14 μm or less).

[0030] On the other hand, the median diameter D2 of the solid graphite particles is not particularly limited as long as it is smaller than D1. From the viewpoint of increasing the discharge capacity per volume of the negative electrode active material layer in an unpressed state, solid graphite particles having a D2 of less than 8 μm can be preferably used. For example, D2 is preferably 6 μm or less, more preferably 4 μm or less. In some embodiments, D2 may be 3.6 μm or less, or may be 3.4 μm or less (e.g., 3.2 μm or less). Furthermore, from the viewpoint of ease of handling during production, production costs, etc., D2 is usually 0.5 μm or more, preferably 1 μm or more, more preferably 1.5 μm or more, and even more preferably 2 μm or more. For example, solid graphite particles having a D2 of 2.5 μm or more (e.g., 2.8 μm or more) may be used. From the viewpoint of achieving both the discharge capacity and ease of production, solid graphite particles having a D2 of 0.5 μm or more and less than 8 μm are preferred, solid graphite particles having a D2 of 1.5 μm or more and 5 μm or less are more preferred, and those having a D2 of 2 μm or more and 4 μm or less are particularly preferred.

[0031] To maximize the effect of using the hollow graphite particles and the solid graphite particles in combination, the relationship between D1 and D2 preferably satisfies 1<(D1 / D2)<10. By combining the hollow graphite particles and the solid graphite particles so as to achieve a specific median diameter ratio, the effect of increasing the discharge capacity per volume of the unpressed negative electrode active material layer can be realized at a higher level. The technology disclosed herein can be preferably implemented, for example, in an embodiment where the relationship between D1 and D2 satisfies 1.5≦(D1 / D2)≦8, more preferably 1.8≦(D1 / D2)≦6, even more preferably 2≦(D1 / D2)≦5.2, and particularly preferably 2.5≦(D1 / D2)≦4.8. In some embodiments, for example, (D1 / D2)≦4 may be satisfied, and typically (D1 / D2)≦3.5 (e.g., (D1 / D2)≦3).

[0032] The value obtained by subtracting D2 from D1 (i.e., D1-D2) is preferably 2 μm or more, and more preferably 4 μm or more. Furthermore, D1-D2 is preferably 18 μm or less, more preferably 15 μm or less, and even more preferably 12 μm or less. For example, D1-D2 may be 10 μm or less, or may be 6 μm or less.

[0033] In this specification, the term "median diameter" refers to the value (D50) at which the volume-based cumulative distribution calculated in accordance with JIS-Z8819-2 (2001) is 50%. Specifically, it can be a value measured by the following method. Measurements are performed using a laser diffraction particle size distribution analyzer (Shimadzu Corporation's "SALD-2200") and Wing SALD-2200 measurement control software. A scattering measurement mode is employed, and a laser beam is irradiated onto a wet cell through which a dispersion liquid in which the measurement sample is dispersed in a dispersion solvent is circulated, to obtain a scattered light distribution from the measurement sample. The scattered light distribution is then approximated by a log-normal distribution, and the particle diameter at which the cumulative degree reaches 50% is taken as the median diameter (D50).

[0034] (aspect ratio) The aspect ratio As1 of the hollow graphite particles disclosed herein is not particularly limited. Hollow graphite particles having an aspect ratio As1 of 1 or more are typically used. The aspect ratio As1 is suitably 1.1 or more, for example, 1.2 or more. In some embodiments, the aspect ratio As1 may be 1.4 or more. The aspect ratio As1 of the hollow graphite particles is suitably approximately 5.0 or less, preferably 4.0 or less, more preferably 3.0 or less, and particularly preferably 2.0 or less (e.g., 1.8 or less). In some embodiments, the aspect ratio As1 may be 1.5 or less, or may be 1.3 or less. By setting the aspect ratio As1 of the hollow graphite particles within the above range, the discharge capacity per volume of the negative electrode active material layer in an unpressed state can be more effectively increased.

[0035] The aspect ratio As2 of the solid graphite particles disclosed herein is not particularly limited, but the lower limit is 1.0, preferably 1.2. From the viewpoint of improving the packing property of the negative electrode active material, for example, solid graphite particles having an aspect ratio As2 of 1.5 or more can be preferably used as the solid graphite particles in the negative electrode active material layer of the above embodiment. In some embodiments, the aspect ratio As2 of the solid graphite particles is preferably, for example, 2 or more, and typically 2.5 or more. On the other hand, the upper limit of the aspect ratio As2 of the solid graphite particles is 5.0, preferably 4.5. For example, graphite particles having an aspect ratio As2 of 4.0 or less are preferred, those having an aspect ratio As2 of 3.5 or less are more preferred, and those having an aspect ratio As2 of 3.2 or less (e.g., 3.0 or less) are particularly preferred. By setting the aspect ratio As2 of the solid graphite particles within the above range, the discharge capacity per volume of the negative electrode active material layer in an unpressed state can be more effectively increased. In addition, the graphite particles are closer to a spherical or spindle shape, which makes it difficult for current concentration to occur, thereby suppressing uneven expansion of the negative electrode. Furthermore, the spherical or spindle shape makes it difficult for adjacent graphite particles to get caught on each other, allowing the graphite particles to slide against each other to a moderate degree, and even if the graphite particles expand, they tend to maintain a state close to close packing. In this way, even if the graphite particles expand, they expand relatively uniformly and slide against each other to a moderate degree, maintaining a negative electrode active material layer with a high graphite particle packing rate, thereby suppressing expansion of the negative electrode that occurs during initial charging.

[0036] In a preferred embodiment, the aspect ratio As2 of the solid graphite particles is greater than the aspect ratio As1 of the hollow graphite particles. The relationship between As1 and As2 preferably satisfies 1<(As2 / As1)≦5. The technology disclosed herein can be preferably implemented, for example, in an embodiment where the relationship between As1 and As2 satisfies 1.2≦(As2 / As1)≦4, more preferably 1.5≦(As2 / As1)≦3. The value obtained by subtracting As1 from As2 (i.e., As2−As1) is preferably 0.5 or more, more preferably 1 or more. Furthermore, As2−As1 is preferably 3 or less, more preferably 2.5 or less, and even more preferably 2 or less. The above-described effects can be more effectively achieved by combining the hollow graphite particles and the solid graphite particles so as to achieve a specific ratio (As2 / As1).

[0037] In this specification, the term "aspect ratio" refers to the A / B value, which is the ratio of the longest diameter A of a particle to the longest diameter B in the direction perpendicular to diameter A, in the cross section of the particle observed in an SEM image obtained using a scanning electron microscope. The aspect ratio can be determined as follows. (1) Preparation of measurement samples A measurement sample is used in which the cross section used in determining the above-mentioned area ratio R1 is exposed. (2) Obtaining SEM images To obtain SEM images, a JSM-7001F (manufactured by JEOL Ltd.) scanning electron microscope is used. SEM images are obtained by observing secondary electron images. The acceleration voltage is 15 kV. The observation magnification is set so that 100 to 1000 graphite particles appear in one field of view. The obtained SEM image is saved as an image file. Other conditions such as spot diameter, working distance, irradiation current, brightness, and focus are set appropriately so that the contours of the graphite particles are clearly visible. (3) Determining the aspect ratio From the acquired SEM image, 100 graphite particles are randomly selected, and for each, the longest diameter A of the graphite particle and the longest diameter B in the direction perpendicular to diameter A are measured, and the A / B value is calculated. The aspect ratio of the graphite particle is determined by calculating the average of all the calculated A / B values.

[0038] (Type of graphite particles) The hollow graphite particles and solid graphite particles can be selected from various known graphite particles having an appropriate median diameter and shape. Examples of such known graphite particles include natural graphite particles and artificial graphite particles. Here, natural graphite is a general term for graphite extracted from natural minerals, and artificial graphite is a general term for artificially produced graphite.

[0039] A specific example of graphite particles that can be preferably used as hollow graphite particles is natural graphite particles. Such natural graphite particles have high crystallinity and can effectively contribute to improving the discharge capacity per volume of the negative electrode active material layer of the energy storage device. Specific examples of natural graphite particles include flake graphite, block graphite (flake graphite), and amorphous graphite. In a preferred embodiment, the hollow graphite particles can be flat, flake-shaped natural graphite particles, or spherical natural graphite particles obtained by spheroidizing such flake graphite. As the solid graphite particles, natural graphite particles having a smaller median diameter than the hollow graphite particles may be used, or artificial graphite particles may be used. Such solid graphite particles have no internal cavities and can fill the gaps between the hollow graphite particles, thereby contributing to an increase in the bulk density (the density of the coated negative electrode active material layer before pressing). The hollow graphite particles and the solid graphite particles may be composite particles in which the graphite particles are bonded to particles made of other materials (e.g., other carbon materials or Si compounds), or may be non-composite particles in which the graphite particles are not bonded to particles made of other materials. The hollow graphite particles and the solid graphite particles disclosed herein are preferably used in the form of non-composite particles in which the graphite particles are not bonded to particles made of other materials. The hollow graphite particles and the solid graphite particles may also be graphite particles having a coating (e.g., an amorphous carbon coating) on ​​the surface.

[0040] In a preferred embodiment, the hollow graphite particles and the solid graphite particles can be selected such that the R value (R1) of the hollow graphite particles is smaller than the R value (R2) of the solid graphite particles (R1 < R2). Here, the "R value" refers to the ratio (I G1 ) of the peak intensity (I D1 ) of the D band to the peak intensity (I D1 ) of the G band in the Raman spectrum (I G1 / I

[0041] For example, the hollow graphite particles and the solid graphite particles can be selected such that the relationship between the R value (R2) of the solid graphite particles and the R value (R1) of the hollow graphite particles satisfies 1 < (R2 / R1) ≦ 4, more preferably 1.2 ≦ (R2 / R1) ≦ 3, even more preferably 1.3 ≦ (R2 / R1) ≦ 2.5, for example 1.4 ≦ (R2 / R1) ≦ 2.2. In a storage element provided with the hollow graphite particles and the solid graphite particles having such R values (R1, R2) in the negative electrode active material layer, the application effect of this embodiment can be more preferably exhibited. For example, when natural graphite particles are used as the hollow graphite particles, various artificial graphite particles can be preferably employed as the solid graphite particles.

[0042] Here, the "Raman spectrum" is obtained by performing Raman spectroscopy using Horiba's "HRRevolution" under the conditions of a wavelength of 532 nm (YAG laser), a grating of 600 g / mm, and a measurement magnification of 100 times. -1 ~4000cm -1 Raman spectroscopy was performed in the range of 4000cm -1 The minimum value in the range is taken as the base intensity, and normalized by the maximum intensity in the measurement range (for example, the intensity of the G band). Next, the obtained curve is fitted using a Lorentzian function, and the 1580 cm -1 G band and 1350 cm -1 The intensity of each of the D bands in the vicinity was calculated, and the peak intensity of the G band (I G1 ) and D band peak intensity (I D1 )"

[0043] The hollow graphite particles and solid graphite particles may be, for example, spherical or non-spherical. Specific examples of non-spherical shapes include spindle shapes (e.g., elliptical or oval), scale shapes, and plate shapes. As the solid graphite particles, those having a spindle shape can be particularly preferably used. The hollow graphite particles and solid graphite particles may have irregularities on the surface. The hollow graphite particles and solid graphite particles may include particles formed by agglomeration of multiple graphite particles.

[0044] The content ratio of the hollow graphite particles relative to the total content of the hollow graphite particles and the solid graphite particles is not particularly limited. The upper limit of the content ratio is preferably 90% by mass, more preferably 80% by mass, and even more preferably 75% by mass. On the other hand, the lower limit of the content ratio of the hollow graphite particles is preferably 10% by mass, more preferably 20% by mass, and even more preferably 30% by mass (e.g., 40% by mass). The technology disclosed herein can be preferably implemented in an embodiment in which the content ratio of the hollow graphite particles relative to the total content of the hollow graphite particles and the solid graphite particles is 10% by mass or more and 80% by mass or less (even more preferably 25% by mass or more and 65% by mass or less, particularly 50% by mass or more and 70% by mass or less). When the content ratio of the hollow graphite particles relative to the total content of the hollow graphite particles and the solid graphite particles is within the above range, the discharge capacity per volume of the negative electrode active material layer can be further increased.

[0045] (Other negative electrode active materials) The negative electrode active material layer disclosed herein may contain graphite particles other than the hollow graphite particles and the solid graphite particles (hereinafter referred to as third graphite particles) to the extent that the effects of the present invention are not impaired. The third graphite particles can be appropriately selected from various known graphite particles. The shape of the third graphite particles is not particularly limited, but a spherical or spindle-like shape with an aspect ratio of 2 to 5 is preferred. When the third graphite particles are contained, the total mass of the hollow graphite particles and the solid graphite particles is suitably 70 mass% or more, preferably 80 mass% or more, and more preferably 90 mass% or more of the total mass of the graphite particles contained in the negative electrode active material layer. Among these, an energy storage device in which 100 mass% of the graphite particles contained in the negative electrode active material layer are the hollow graphite particles and the solid graphite particles is preferred. By using graphite particles essentially composed of only the hollow graphite particles and the solid graphite particles, the aforementioned effects can be more effectively achieved while taking advantage of the advantages of using the hollow graphite particles and the solid graphite particles. The negative electrode active material layer disclosed herein may contain a carbonaceous active material other than the hollow graphite particles, the solid graphite particles, and the third graphite particles (hereinafter referred to as a non-graphitizable carbonaceous active material) within a range that does not impair the effects of the present invention. Examples of non-graphitizable carbonaceous active materials include non-graphitizable carbon and graphitizable carbon. Here, "non-graphitizable carbon" refers to a carbon material having an average lattice spacing d(002) of the (002) plane of 0.36 nm or more and 0.42 nm or less, as measured by X-ray diffraction before charge / discharge or in a discharged state. "Graphitizable carbon" refers to a carbon material having a d(002) of 0.34 nm or more and less than 0.36 nm. When a non-graphitizable carbonaceous active material is contained, the total mass of the hollow graphite particles and the solid graphite particles is suitably 70 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more of the total mass of the carbonaceous active material contained in the negative electrode active material layer. Among these, an energy storage element in which 100 mass % of the carbonaceous active material contained in the negative electrode active material layer is the hollow graphite particles and the solid graphite particles is preferred.

[0046] The negative electrode active material layer disclosed herein may contain a negative electrode active material (hereinafter referred to as a non-carbonaceous active material) made of a material other than a carbonaceous active material, provided that the effects of the present invention are not impaired. Examples of such non-carbonaceous active materials include semimetals such as Si, metals such as Sn, oxides of these metals, and composites of these metals and carbon materials. The content of the non-carbonaceous active material is suitably, for example, 30 mass% or less, preferably 20 mass% or less, and more preferably 10 mass% or less, of the total mass of the negative electrode active material contained in the negative electrode active material layer. The technology disclosed herein can be preferably implemented in an embodiment in which the total proportion of the carbonaceous active material in the negative electrode active material layer is greater than 90 mass%. The proportion of the carbonaceous active material is more preferably 95 mass% or more, even more preferably 98 mass% or more, and particularly preferably 99 mass% or more. Among these, an energy storage device in which 100 mass% of the negative electrode active material contained in the negative electrode active material layer is a carbonaceous active material is preferred.

[0047] (optional ingredient) The negative electrode active material layer disclosed herein contains optional components such as a conductive agent, a binder (binding agent), a thickener, and a filler, as needed.

[0048] The hollow graphite particles and solid graphite particles are also conductive, and examples of conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphitized carbon, non-graphitized carbon, and graphene-based carbon. Examples of non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive material may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred. When a conductive agent is used in the negative electrode active material layer, the proportion of the conductive agent in the entire negative electrode active material layer can be approximately 8.0 mass% or less, and usually preferably approximately 5.0 mass% or less (e.g., 1.0 mass% or less). The technology disclosed herein can be preferably implemented in an embodiment in which the negative electrode active material layer does not contain the conductive agent.

[0049] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0050] The content of the binder in the negative electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the negative electrode active material particles can be stably held.

[0051] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0052] The filler is not particularly limited. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene; inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates; hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide; carbonates such as calcium carbonate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate; nitrides such as aluminum nitride and silicon nitride; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and artificial products thereof. When a filler is used in the negative electrode active material layer, the proportion of the filler in the entire negative electrode active material layer can be approximately 8.0% by mass or less, and typically approximately 5.0% by mass or less (e.g., 1.0% by mass or less). The technology disclosed herein can be preferably implemented in an embodiment in which the negative electrode active material layer does not contain the filler. In this specification, the term "main component" refers to the component that is contained in the largest amount, for example, a component that is contained in an amount of 50 mass % or more relative to the total mass.

[0053] The negative electrode active material layer disclosed herein is preferably not substantially pressed. If pressed, residual stress from pressing may cause significant expansion of the negative electrode during initial charging. Since the negative electrode active material layer containing hollow graphite particles and solid graphite particles as the negative electrode active material is not substantially pressed, the negative electrode active material is configured such that almost no stress is applied to the negative electrode active material before the electrode body is formed. Therefore, the graphite particles themselves have little residual stress, and non-uniform expansion of the negative electrode due to the release of residual stress can be suppressed. Therefore, expansion of the negative electrode during initial charging can be suppressed.

[0054] Here, "not substantially pressed" means that during production, a step of applying a pressure (linear pressure) of 10 kgf / mm or more to the negative electrode active material layer using a device designed to apply pressure to a workpiece, such as a roll press, is not performed. In other words, "not substantially pressed" also includes a step in which a slight pressure is applied to the negative electrode active material layer in other steps, such as winding the negative electrode. "Not substantially pressed" also includes a step of applying a pressure (linear pressure) of less than 10 kgf / mm.

[0055] The density of the negative electrode active material layer is not particularly limited, but the lower limit is 1.30 g / cm 3 is preferred, and 1.35 g / cm 3 more preferably 1.40 g / cm 3 On the other hand, the upper limit of the density of the negative electrode active material layer is 1.55 g / cm. 3 is preferred, and 1.50 g / cm 3 In some embodiments, the density of the negative electrode active material layer is 1.48 g / cm 3 It may be less than 1.45 g / cm 3 or less. When the density of the negative electrode active material layer is within the above range, the graphite particles themselves have little residual stress, and non-uniform expansion of the negative electrode due to the release of residual stress can be suppressed. It is presumed that, even if the graphite particles expand, the energy storage element maintains a negative electrode active material layer with a high packing rate of graphite particles due to the relatively uniform expansion, and as a result, expansion of the negative electrode that occurs during initial charging can be suppressed.

[0056] The lower limit of Q2 / Q1, which is the ratio of the surface roughness Q2 of the region of the negative electrode substrate where the negative electrode active material layer is not stacked to the surface roughness Q1 of the region of the negative electrode substrate where the negative electrode active material layer is stacked, is preferably 0.90, more preferably 0.92, and even more preferably 0.94. The more pressure is applied to the negative electrode substrate, the greater the surface roughness of the region where the negative electrode active material layer is formed, and thus the smaller the Q2 / Q1 ratio. In other words, when no pressure is applied to the negative electrode substrate, the region where the negative electrode active material layer is stacked and the region where the negative electrode active material layer is not stacked (the so-called exposed region of the negative electrode substrate) have almost the same surface roughness. In other words, the Q2 / Q1 ratio approaches 1. In the energy storage device, when the Q2 / Q1 ratio is 0.90 or greater, no or only a small amount of pressure is applied to the negative electrode active material layer. Therefore, the graphite particles themselves have little residual stress, and non-uniform expansion of the negative electrode due to the release of residual stress can be suppressed. Thus, even if the graphite particles expand, it is presumed that the expansion is relatively uniform, maintaining a high graphite particle packing ratio in the negative electrode active material layer, thereby suppressing the expansion of the negative electrode that occurs during initial charging. Meanwhile, the upper limit of the surface roughness ratio (Q2 / Q1) is usually 1. In some embodiments, the surface roughness ratio (Q2 / Q1) may be 0.99 or less, or may be 0.98 or less.

[0057] The "surface roughness" means the center line roughness Ra of the surface of the substrate (in the region where the active material layer is formed, the surface after the active material layer is removed) measured using a laser microscope in accordance with JIS-B0601 (2013).

[0058] (middle class) The intermediate layer is a coating layer on the surface of the negative electrode substrate, and contains conductive particles such as carbon particles to reduce contact resistance between the negative electrode substrate and the negative electrode active material layer. The configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles. The technology disclosed herein can be preferably implemented in an embodiment that does not include the intermediate layer.

[0059] [Positive electrode] The positive electrode includes a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer is laminated along at least one surface of the positive electrode substrate directly or via an intermediate layer.

[0060] The positive electrode substrate is electrically conductive. Examples of the material for the substrate include metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof. Among these, aluminum and aluminum alloys are preferred because of their balance of high potential resistance, high conductivity, and cost. Examples of the form of the positive electrode substrate include foil and vapor-deposited film, with foil being preferred from the standpoint of cost. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085 and A3003 as specified in JIS-H4000 (2014).

[0061] The positive electrode active material layer is formed from a so-called positive electrode mixture containing a positive electrode active material. The positive electrode mixture forming the positive electrode active material layer further contains optional components such as a conductive agent, a binder, a thickener, and a filler as necessary.

[0062] The positive electrode active material can be appropriately selected from, for example, known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually 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 having an α-NaFeO2 crystal structure, lithium transition metal oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, sulfur, and the like. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Li x Ni 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ)]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Examples of lithium transition metal oxides with spinel crystal structures include Li x Mn2O4, Li x Ni γ Mn (2-γ) Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be mixed. In the positive electrode active material layer, one of these compounds may be used alone, or two or more may be mixed. The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but the lower limit is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, more preferably 98% by mass.

[0063] The conductive agent is not particularly limited as long as it is a conductive material. Such a conductive agent can be selected from the materials exemplified for the negative electrode. When a conductive agent is used, the proportion of the conductive agent in the entire positive electrode active material layer can be approximately 1.0 mass% or more and 20 mass% or less, and usually approximately 2.0 mass% or more and 15 mass% or less (e.g., 3.0 mass% or more and 6.0 mass% or less) is preferable.

[0064] Examples of the binder include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers. When a binder is used, the proportion of the binder in the entire positive electrode active material layer can be approximately 0.50% by mass to 15% by mass, and is usually preferably approximately 1.0% by mass to 10% by mass (for example, 1.5% by mass to 3.0% by mass).

[0065] Examples of the thickener include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. Furthermore, if the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like. When a thickener is used, the proportion of the thickener in the entire positive electrode active material layer can be approximately 8% by mass or less, and typically, it is preferably approximately 5.0% by mass or less (e.g., 1.0% by mass or less). The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain the thickener.

[0066] The filler can be selected from the materials exemplified for the negative electrode. When a filler is used, the proportion of the filler in the entire positive electrode active material layer can be approximately 8.0 mass% or less, and usually approximately 5.0 mass% or less (e.g., 1.0 mass% or less) is preferable. The technology disclosed herein can be preferably implemented in an embodiment in which the positive electrode active material layer does not contain the filler.

[0067] The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. As with the negative electrode, the configuration of the intermediate layer is not particularly limited and can be formed, for example, from a composition containing a resin binder and conductive particles. The technology disclosed herein can be preferably implemented in an embodiment that does not include the intermediate layer.

[0068] [Separator] The separator may be, for example, a woven fabric, a nonwoven fabric, or a porous resin film. Among these, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. As the main component of the separator, a polyolefin such as polyethylene or polypropylene is preferred from the viewpoint of strength, and a polyimide or aramid is preferred from the viewpoint of resistance to oxidative decomposition. These resins may also be combined.

[0069] An inorganic layer may be laminated between the separator and the electrode (usually a positive electrode). This inorganic layer is a porous layer also called a heat-resistant layer. Alternatively, a separator having an inorganic layer formed on one or both sides of a porous resin film may be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components.

[0070] [Non-aqueous electrolyte] The nonaqueous electrolyte may be a known nonaqueous electrolyte commonly used in general nonaqueous electrolyte secondary batteries (storage elements). The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous electrolyte may be a solid electrolyte or the like.

[0071] The nonaqueous solvent may be a known nonaqueous solvent commonly used as a nonaqueous solvent for general nonaqueous electrolytes for energy storage devices. Examples of the nonaqueous solvent include cyclic carbonates, chain carbonates, esters, ethers, amides, sulfones, lactones, and nitriles. Among these, it is preferable to use at least a cyclic carbonate or a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. 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 not particularly limited, but is preferably, for example, 5:95 to 50:50.

[0072] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate, with EC being preferred among these.

[0073] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diphenyl carbonate, with EMC being preferred among these.

[0074] The electrolyte salt may be any known electrolyte salt commonly used in non-aqueous electrolytes for general energy storage elements, including lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts, with lithium salts being preferred.

[0075] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, and lithium salts having a hydrocarbon group in which hydrogen is substituted with fluorine, such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0076] The lower limit of the concentration of the electrolyte salt in the nonaqueous electrolyte is 0.1 mol / dm 3 is preferred, and 0.3 mol / dm 3 More preferably, 0.5 mol / dm 3 is more preferably 0.7 mol / dm 3On the other hand, the upper limit is not particularly limited, but is preferably 2.5 mol / dm 3 is preferred, and 2.0 mol / dm 3 More preferably, 1.5 mol / dm 3 is more preferable.

[0077] The non-aqueous electrolyte may contain other additives. Furthermore, the non-aqueous electrolyte may be a room temperature molten salt, an ionic liquid, or the like.

[0078] [Specific configuration of the energy storage element] The shape of the energy storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, laminated film batteries, square batteries, flat batteries, coin batteries, and button batteries.

[0079] FIG. 1 shows an energy storage element 1 (nonaqueous electrolyte energy storage element) as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the case. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic case 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 4'. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 5'.

[0080] When graphite is used as the negative electrode active material, the energy storage element can have a negative electrode with a high discharge capacity per volume of the negative electrode active material layer even when no or only a small pressure is applied to the negative electrode active material layer.

[0081] [Method of manufacturing an energy storage element] The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing a non-aqueous electrolyte, and housing the electrode assembly and the non-aqueous electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween. In the manufacturing method for the energy storage element of this embodiment, in the step of preparing the negative electrode, a negative electrode active material containing hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles is used.

[0082] In the process of preparing the negative electrode, for example, a negative electrode active material layer containing a negative electrode active material including hollow graphite particles and solid graphite particles is laminated along at least one surface of the negative electrode substrate by coating the negative electrode mixture on the negative electrode substrate and drying it. Specifically, the negative electrode active material layer is laminated, for example, by coating the negative electrode mixture on the negative electrode substrate and drying it. After drying, the negative electrode active material layer may be pressed in the average thickness direction. The pressure (linear pressure) during pressing is not particularly limited, but from the viewpoint of suppressing expansion of the negative electrode during initial charging, it is appropriate to apply a pressure of approximately 25 kgf / mm or less, preferably 20 kgf / mm or less, more preferably 16 kgf / mm or less, and even more preferably 12 kgf / mm or less. In a preferred embodiment, the negative electrode active material layer is not pressed before laminating the negative electrode and positive electrode. That is, the negative electrode active material layer is disposed on the negative electrode substrate in a "substantially unpressed" state.

[0083] The step of placing the non-aqueous electrolyte in the container can be appropriately selected from known methods. For example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the non-aqueous electrolyte solution may be poured into the container through an inlet formed therein, and then the inlet may be sealed. Details of the other components constituting the energy storage device obtained by this manufacturing method are as described above.

[0084] [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.

[0085] In the above embodiment, the description has focused on the case where the energy storage element is a non-aqueous electrolyte secondary battery, but other energy storage elements may also be used. Examples of other energy storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), etc. Examples of non-aqueous electrolyte secondary batteries include lithium ion non-aqueous electrolyte secondary batteries.

[0086] The present invention can also be realized as an energy storage device including a plurality of the above-described energy storage elements. Furthermore, a battery pack can be formed by using one or a plurality of the energy storage elements (cells) of the present invention, and this battery pack can be used to form an energy storage device. The above-described energy storage device can be used as a power source for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Furthermore, the above-described energy storage device can be used in various power supply devices such as engine starting power supplies, auxiliary power supplies, and uninterruptible power supplies (UPS).

[0087] 2 shows an example of a power storage device 30 in which power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1, are further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, and a bus bar (not shown) that electrically connects two or more power storage units 20. The power storage units 20 or the power storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements. [Example]

[0088] 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.

[0089] [Examples 1 to 8 and Comparative Examples 1 to 3] (Negative electrode) A negative electrode mixture paste was prepared containing a negative electrode active material having the composition shown in Table 1 (content ratio relative to the total content of solid graphite particles and hollow graphite particles), styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener, with water as a dispersion medium. The mass ratio of the negative electrode active material, binder, and thickener was 97.8:1.0:1.2. The negative electrode mixture paste was applied to both sides of a negative electrode substrate (surface roughness: 0.86 μm) made of copper foil with a thickness of 8 μm and dried to form a negative electrode active material layer, thereby obtaining negative electrodes of Examples 1 to 8 and Comparative Examples 1 to 3. The physical properties of the negative electrode active material are shown in Table 1. The amount of negative electrode mixture (a negative electrode mixture paste from which the dispersion medium has been evaporated) applied per unit area on one side after drying was 1.0 g / 100 cm. 2 In addition, in Examples 1 to 8 and Comparative Examples 1 to 3, pressing was performed using a roll press machine so that the pressure (linear pressure) was less than 5 kgf / mm. (Energy storage element) The negative electrode shown in Table 1, the positive electrode described below, and a polyethylene separator having a thickness of 21 μm were stacked and wound together to produce the energy storage elements of Examples 1 to 8 and Comparative Examples 1 to 3. The positive electrode contained LiNi as the positive electrode active material. 0.6 Mn 0.2 Co 0.2 A positive electrode mixture paste containing O2, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive agent, with N-methyl-2-pyrrolidone (NMP) as a dispersion medium, was prepared. The mass ratio of the positive electrode active material, binder, and conductive agent was 94.5:4:1.5. The positive electrode mixture paste was applied to both sides of a positive electrode substrate made of aluminum foil with a thickness of 15 μm, pressed, and dried to form a positive electrode active material layer. After drying, the applied amount of positive electrode mixture (the dispersion medium evaporated from the positive electrode mixture paste) per unit area on one side was 1.7 g / 100 cm. 2It was made to be like this. Next, LiPF (1.2 mol / dm 3 ) was dissolved in the electrolyte to obtain a non-aqueous electrolyte. The positive electrode and the negative electrode were then stacked with a separator made of a polyethylene microporous membrane interposed therebetween to produce an electrode assembly. This electrode assembly was housed in an aluminum prismatic battery can, and a positive electrode terminal and a negative electrode terminal were attached. The non-aqueous electrolyte was poured into this container (prismatic battery can), which was then sealed to obtain energy storage elements of Examples and Comparative Examples.

[0090] (Calculation of the area ratio of graphite particles excluding voids within the particles) The area ratio R of the graphite particles excluding the voids within the particles was calculated by the method described above.

[0091] (Raman spectrum measurement) The Raman spectrum of the obtained negative electrode active material was measured by the above-mentioned method. Based on the obtained Raman spectrum, the peak intensity of the G band and the peak intensity of the D band were determined by the above-mentioned method, and the intensity ratio (I D1 / I G1 The measurement results are shown in Table 1.

[0092] (Determining the aspect ratio) The aspect ratio of the graphite particles was determined by the method described above.

[0093] (Density of negative electrode active material layer) The density of the negative electrode active material layer is determined by the amount of negative electrode mixture applied, W (g / 100 cm 2 ), when the thickness of the negative electrode active material layer before charge and discharge, which will be described later, is T (cm), it can be calculated by the following formula. Density of the negative electrode active material layer (g / cm 3 )=W / (T×100)

[0094] [evaluation] (Discharge capacity per volume of negative electrode active material layer before pressing) Discharge capacity per volume of the negative electrode active material layer before pressing (mAh / cm 3 ) was measured using the following procedure. A storage element was fabricated using the negative electrode plate prepared by the above method as the working electrode, metallic lithium as the counter electrode, and a non-aqueous electrolyte prepared by dissolving LiPF6 (1.2 mol / L) in a non-aqueous solvent mixed at a volume ratio of EC:DMC:EMC = 30:35:35 as the electrolyte solution. A charge / discharge test was carried out under the following conditions, and the quantity of electricity at the third discharge (Li desorption reaction) was measured by dividing it by the volume of the negative electrode active material layer before pressing. Charging (Li absorption reaction): Lower limit voltage 10 mV, current density 2 mA / cm 2 , and the charge termination current density was 0.04 mA / cm 2 Constant current constant voltage (CCCV) charging Discharge (Li desorption reaction): Upper limit voltage 2.0 V, current density 2 mA / cm 2 Constant current (CC) discharge

[0095] (Expansion rate of negative electrode active material during initial charging) The amount of expansion of the negative electrode active material at the time of initial charge was calculated by subtracting the "average thickness of the negative electrode active material layer before charge and discharge" from the "average thickness of the negative electrode active material layer at the time of initial charge" calculated by the following method, and then the expansion rate of the negative electrode active material at the time of initial charge was determined. (1) Measurement of the average thickness of the negative electrode active material layer before charging and discharging Ten samples of 2 cm × 1 cm area of ​​the negative electrode before fabrication of the energy storage element were prepared as measurement samples, and the thickness of each negative electrode was measured using a high-precision digimatic micrometer manufactured by Mitutoyo Corporation. The thickness of each negative electrode was measured at five locations, and the thickness of the negative electrode active material layer before charge / discharge for one negative electrode was measured by subtracting the thickness of the negative electrode substrate (8 μm) from the average value. The average thickness of the negative electrode active material layer before charge / discharge measured for the ten negative electrodes was calculated to obtain the average thickness (μm) of the negative electrode active material layer before charge / discharge. (2) Measurement of the average thickness of the negative electrode active material layer during initial charging The average thickness (μm) of the negative electrode active material layer at the time of initial charge was measured in the same manner as in the measurement of the average thickness of the negative electrode active material layer before charge and discharge, except that the energy storage elements of Examples and Comparative Examples at the time of initial charge were disassembled in a glove box filled with argon having a dew point of −60° C. or less, and the negative electrodes after cleaning with DMC were used as measurement samples. Note that the initial charge refers to the measurement of the average thickness (μm) of the negative electrode active material layer at the time of initial charge, where the current density was 2 mA / cm for the energy storage elements of Examples and Comparative Examples before charge and discharge. 2 , and the charge termination current density was 0.04 mA / cm 2 , the state of constant current and constant voltage charging with an upper limit voltage of 4.25V. (3) Expansion rate of negative electrode active material during initial charging For each of Examples 1 to 8 and Comparative Examples 1 to 3, the amount of expansion of the negative electrode active material at the time of initial charge was calculated by subtracting the average thickness of the negative electrode active material layer before charge / discharge from the average thickness of the negative electrode active material layer at the time of initial charge, and then the expansion rate (%) of the negative electrode active material at the time of initial charge was determined by dividing the amount by the average thickness of the negative electrode active material layer before charge / discharge.

[0096] Table 1 shows the evaluation results of the negative electrodes of Examples 1 to 8 and Comparative Examples 1 to 3. For Examples 1 to 5 and Comparative Examples 1 to 2, which used solid graphite particles with a median diameter of 3 μm and hollow graphite particles with a median diameter of 8 μm, Fig. 3 shows the content ratio (mass %) of the hollow graphite particles relative to the total content of the solid graphite particles and hollow graphite particles, and the discharge capacity (mAh / cm) per volume of the negative electrode active material layer before pressing. 3 ) and FIG. 4 shows the relationship between the content (mass %) of the hollow graphite particles relative to the total content of the solid graphite particles and hollow graphite particles and the expansion rate (%) of the negative electrode active material during initial charging.

[0097] [Table 1]

[0098] As shown in Table 1 and the graph in FIG. 3 , the energy storage devices of Examples 1 to 5, which used a combination of hollow graphite particles and solid graphite particles having a smaller median diameter, had a higher discharge capacity per volume of the negative electrode active material layer when not pressed than Comparative Example 2, which used the hollow graphite particles alone. Furthermore, the energy storage devices of Examples 1 to 5 also had a higher discharge capacity per volume of the negative electrode active material layer when not pressed than the estimated value predicted by the approximate formula y = 0.76x + 360 shown by the dashed line in FIG. 3 from Comparative Example 2, which used the hollow graphite particles alone, and Comparative Example 1, which used the solid graphite particles alone. For example, in Comparative Examples 2 and 1, which used either the hollow graphite particles or the solid graphite particles alone, the discharge capacity per volume of the negative electrode active material layer when not pressed was 360 mAh / cm. 3 , 436mAh / cm 3 Therefore, in Example 3, in which hollow graphite particles and solid graphite particles were mixed at a ratio of 50:50, the discharge capacity per volume of the negative electrode active material layer before pressing was calculated to be 0.76 × 50 + 360 = 398 mAh / cm from the approximation shown by the dashed line in FIG. 3 . 3 On the other hand, the discharge capacity per volume of the negative electrode active material layer in Example 3 before pressing is expected to be about 459 mAh / cm 3 This is beyond expectations. Furthermore, in Comparative Example 3, in which the hollow graphite particles were used in combination with solid graphite particles having a larger median diameter, the discharge capacity per volume of the negative electrode active material layer in the unpressed state was inferior to that in Examples 1 to 8. These results confirm that the use of a combination of hollow graphite particles and solid graphite particles having a smaller median diameter can realize an energy storage device in which the discharge capacity per volume of the negative electrode active material layer in the unpressed state is large.

[0099] Furthermore, as shown in Table 1 and FIG. 4, Examples 1 to 8, which used a combination of hollow graphite particles and solid graphite particles having a smaller median diameter, had a better effect of suppressing expansion of the negative electrode active material layer during initial charging than Comparative Example 2, which used the hollow graphite particles alone.

[0100] As described above, it was shown that the energy storage device having a negative electrode in which the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles has a high discharge capacity per volume of the negative electrode active material layer when not pressed. [Industrial Applicability]

[0101] The present invention is suitably used as an electricity storage element, including a non-aqueous electrolyte secondary battery used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0102] 1. Energy storage element 2 Electrode body 3 Cases 4 Positive terminal 4' Positive lead 5 Negative terminal 5' negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. a negative electrode having a negative electrode substrate and a negative electrode active material layer laminated directly or indirectly on at least one surface of the negative electrode substrate; the negative electrode active material layer contains a negative electrode active material, the negative electrode active material contains hollow graphite particles having a median diameter D1 and solid graphite particles having a median diameter D2 smaller than that of the hollow graphite particles, The density of the negative electrode active material layer is 1.30 g / cm ３ 1.55g / cm or more ３ The following is a storage element:

2. 2. The energy storage element according to claim 1, wherein the content of said hollow graphite particles is 80 mass % or less of the total content of said hollow graphite particles and said solid graphite particles.

3. 3. The electric storage element according to claim 1, wherein the negative electrode active material layer is not substantially pressed.

4. 4. The energy storage element according to claim 1, wherein a ratio Q2 / Q1 of a surface roughness Q2 of a region of the negative electrode substrate where the negative electrode active material layer is not laminated to a surface roughness Q1 of a region of the negative electrode substrate where the negative electrode active material layer is laminated is 0.90 or more.

5. 5. The energy storage element according to claim 1, wherein the solid graphite particles have a median diameter of 4 μm or less.

6. 6. The energy storage element according to claim 1, wherein the solid graphite particles have an aspect ratio of 1 or more and 5 or less.

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