Energy storage element

Abstract

A power storage element according to one aspect of the present invention is provided with an electrode body having a negative electrode and a positive electrode. The negative electrode has a negative-electrode active material layer containing a negative-electrode active material. The negative-electrode active material contains solid graphite. The negative-electrode active material layer has a BET specific surface area of 2.1 m2 / g or less. The pressure applied to the electrode body is 0.1 MPa or more.

Description

[Technical Field]

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

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. In addition, as electricity storage elements other than non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors, and electricity storage elements using electrolytes other than non-aqueous electrolytes, are also widely used.

[0003] In order to improve the energy density of such an electricity storage element, carbon materials such as graphite are used as the negative electrode active material of the electricity storage element (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 such energy storage elements, when graphite is used as the negative electrode active material, expansion and contraction of the negative electrode active material during charging and discharging causes the negative electrode active materials to press against each other, increasing the distance between the negative electrode active materials and causing cracks in the negative electrode active material, which can result in an increase in DC resistance after charge-discharge cycles.

[0006] An object of the present invention is to provide an energy storage device that can suppress an increase in DC resistance after charge / discharge cycles. [Means for solving the problem]

[0007] An energy storage device according to one aspect of the present invention includes an electrode assembly having a negative electrode and a positive electrode, the negative electrode having a negative electrode active material layer containing a negative electrode active material, the negative electrode active material containing solid graphite, and a BET specific surface area of ​​the negative electrode active material layer of 2.1 m 2 / g or less, and the pressure applied to the electrode body is 0.1 MPa or more. [Effects of the Invention]

[0008] According to the energy storage device of the present invention, an increase in DC resistance after charge / discharge cycles can be suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing an embodiment of an energy storage element. [Figure 2] FIG. 2 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements. 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 an electrode assembly having a negative electrode and a positive electrode, the negative electrode having a negative electrode active material layer containing a negative electrode active material, the negative electrode active material containing solid graphite, and a BET specific surface area of ​​the negative electrode active material layer of 2.1 m 2 / g or less, and the pressure applied to the electrode body is 0.1 MPa or more.

[0012] An energy storage element according to one aspect of the present invention can suppress an increase in DC resistance after charge / discharge cycling. The reason for this effect is unclear, but the following reason is presumed. In this energy storage element, by using solid graphite, which is relatively hard to crack among graphites, as the negative electrode active material, cracking of the negative electrode active material is unlikely to occur even when the negative electrode active material expands and contracts during charge / discharge, and the decrease in electronic conductivity associated with cracking of the negative electrode active material and the increase in DC resistance due to the growth of a coating on the newly formed surface can be reduced. In addition, when the BET specific surface area of ​​the negative electrode active material layer is 2.1 m,2 / g or less, the effect of suppressing an increase in DC resistance due to contact between the surface of the negative electrode active material layer and the electrolyte can be improved. Furthermore, it is thought that by applying a pressure of 0.1 MPa or more to the electrode body, expansion and contraction of the negative electrode active material due to charge and discharge is suppressed, further reducing the increase in DC resistance after charge and discharge cycles. Therefore, it is presumed that the energy storage element can suppress an increase in DC resistance after charge and discharge cycles.

[0013] In the present invention, solid graphite refers to particulate graphite particles having an aspect ratio (b / a) of less than 5, where b is the long diameter of the particle relative to a the short diameter of the particle, and the interior of the graphite particle is dense and substantially free of voids. More specifically, "solid" refers to a particle cross-section observed in an SEM image obtained using a scanning electron microscope (SEM), in which the area ratio of voids within the particle to the entire particle area (porosity) is 2% or less. In the present invention, graphite having an aspect ratio (b / a) of less than 5 and a porosity of more than 2% is referred to as "hollow graphite."

[0014] The following method can be used to measure the aspect ratio of graphite particles. The graphite particles to be measured are prepared according to the following procedure. If graphite particles or negative electrodes before assembly of an energy storage device are available, they are used as they are. When preparing from an assembled energy storage device, the energy storage device is discharged at a current of 0.1 C to the discharge end voltage during normal use, and then disassembled in a dry air atmosphere with a dew point of −40°C or lower. After removing the negative electrode, the portion not facing the positive electrode is cut out. Adhering electrolyte and the like are washed away with dimethyl carbonate (DMC), and the sample is dried under reduced pressure at room temperature for 24 hours to prepare a measurement sample. The measurement sample may also be prepared by disassembling a discharged energy storage device in a dry air atmosphere with a dew point of −40°C or lower, removing the negative electrode, cutting out the portion not facing the positive electrode, immersing the portion in a solvent capable of dissolving the binder to dissolve it in the solvent, separating the negative electrode active material from the binder-containing solution by filtration, and then drying under reduced pressure at room temperature for 24 hours. The surface of the measurement sample or a cross-section exposed by fixing it with a thermosetting resin and using a cross-section polisher or similar tool is observed using a scanning electron microscope (SEM) using the same procedure as for the "area ratio of voids within a particle to the total particle area (porosity)" described below. The longest diameter b and the longest short diameter a perpendicular to the long diameter b are measured for 100 random graphite particles, and the aspect ratio (b / a) of each graphite particle is calculated, and the average value is calculated. Alternatively, the short diameter a and long diameter b of 100 random graphite particles can be measured separately, their average values ​​calculated, and the aspect ratio (b / a) can be calculated from these average values.

[0015] The "area ratio of voids within a particle to the area of ​​the entire particle (porosity)" of a graphite particle can be determined by the following procedure. (1) Preparation of measurement samples The negative electrode to be measured is fixed with thermosetting resin. A cross-section polisher is used to expose the cross section of the resin-fixed negative electrode, and a measurement sample is prepared. The negative electrode to be measured is prepared using the following procedure. If a negative electrode before assembly into the energy storage element can be prepared, it is used as is. When preparing 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, until it is in a discharged state. The discharged energy storage element is disassembled, and the negative electrode is removed. Components (electrolyte, etc.) adhering to the negative electrode are thoroughly washed with dimethyl carbonate, and then dried under reduced pressure at room temperature for 24 hours. The entire 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. Here, normal use refers to the case where the storage element is used under the charging and discharging conditions recommended or specified for the storage element, and if a charger is provided for the storage element, the charger is used to use the storage element. (2) Obtaining SEM images To obtain SEM images, a JSM-7001F SEM (manufactured by JEOL Ltd.) 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 graphite 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 concentration 20% lower than the concentration at which the intensity is at its maximum. The area of ​​the higher concentration side is calculated through binarization, and this is taken as the "area S1 of the voids within the particle." Next, the image of the first graphite particle is binarized using a concentration threshold of 10%. The outer periphery of the graphite particle is determined by the binarization process, and the area inside the periphery is calculated as the "area of ​​the entire particle, S0." Using the calculated S1 and S0, the ratio of S1 to S0 (S1 / S0) is calculated to calculate the "area ratio R1 of voids within a 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 the voids in each graphite particle are calculated. (5) Determination of void area ratio Of all the void area ratios R1, R2, R3, ... calculated by the binarization process, the numerical values ​​for which the void area ratio Rn is 2% or less are extracted and their average value is calculated to determine the "area ratio of voids within a particle to the area of ​​the entire particle (porosity)" of solid graphite. Of all the void area ratios R1, R2, R3, ... calculated by the binarization process, the numerical values ​​for which the void area ratio Rn is more than 2% are extracted and their average value is calculated to determine the "area ratio of voids within a particle to the area of ​​the entire particle (porosity)" of hollow graphite. In addition, instead of the scanning electron microscope used for "obtaining an SEM image," the image editing software used for "cutting out the contours of graphite particles," and the image analysis software used for "binarization processing," devices and software capable of equivalent measurements, image editing, and image analysis may be used.

[0016] The "BET specific surface area of ​​the negative electrode active material layer" is determined by immersing a sample of the negative electrode active material layer to be measured in liquid nitrogen to cool it, and then supplying nitrogen gas to the sample, whereby nitrogen molecules are physically adsorbed onto the particle surfaces, and measuring the pressure and the amount of nitrogen adsorption at that time. Specifically, the BET specific surface area is measured by the following method: Using a specific surface area measuring device manufactured by Yuasa Ionics Corporation (trade name: MONOSORB), the amount of nitrogen adsorption (m 2 The amount of adsorption thus obtained is divided by the mass (g) of the sample to be measured, and the value is calculated as the BET specific surface area (m 2 / g). Cooling is performed using liquid nitrogen before the measurement. Also, preheating is performed at 120°C for 15 minutes before cooling. The amount of sample to be measured is 0.5g ± 0.01g. The sample of the negative electrode active material layer to be used for measuring the BET specific surface area is prepared in the following manner. The storage element is discharged at a constant current of 0.05C to the lower limit voltage for normal use. The storage element is disassembled, and the negative electrode is removed and used as the working electrode. A half cell is assembled using metallic Li as the counter electrode. At a current of 10mA per 1g of negative electrode active material, the potential of the working electrode is 2.0V vs. Li / Li. + Constant-current charging was performed until the value reached 0.5. Note that charging refers to applying a current that increases the potential of the working electrode. The half-cell was disassembled, the working electrode removed, and thoroughly washed with dimethyl carbonate. After drying under reduced pressure at room temperature for 24 hours, the negative electrode active material layer was collected and used as the test sample. 1.00 g of the test sample (negative electrode active material layer) powder was placed in a sample tube and dried under reduced pressure at 120°C for 12 hours to thoroughly remove moisture. The tube was then cooled using liquid nitrogen and evacuated. The adsorption isotherm was measured using the nitrogen gas adsorption method within a relative pressure range of P / P0 (P0 = approximately 770 mmHg) of 0 to 1. Five points were extracted from the resulting adsorption isotherm in the P / P0 = 0.05 to 0.3 region, and a BET plot was performed. The BET specific surface area was calculated from the y-intercept and slope of the line. From disassembly of the energy storage device to collection of the negative electrode active material layer, the process was carried out in an argon atmosphere with a dew point of -60°C or below.

[0017] The "pressure applied to the electrode assembly" refers to the pressure applied in the stacking direction of the stacked positive electrode, negative electrode, and separator (Y direction in FIG. 1). The pressure applied to the electrode assembly is a value measured by the following method. (i) When a load is applied to the energy storage element by a pressure member, etc. First, while a load is applied by a pressure member or the like, the energy storage element is discharged at a constant current to the minimum voltage limit for normal use, and then placed in an X-ray computed tomography (CT) device. Scanning is performed along a direction parallel to the stacking direction of the electrode assembly (the Y direction in Figure 1 ) to confirm whether at least a portion of the surface of the electrode assembly to which the load is applied (typically a surface perpendicular to the stacking direction of the electrode assembly, the XZ plane in Figure 1 ) is in direct or indirect contact with the inner surface of the container. If the surface to which the load is applied is not in direct or indirect contact with the inner surface of the container, the pressure applied to the electrode assembly is set to 0 MPa. If the surface to which the load is applied is in direct or indirect contact with the inner surface of the container, the load applied to the electrode assembly is measured using an autograph according to the following procedure. The energy storage element with the load applied by a pressure member or the like is placed on the autograph so that the probe is in contact with the surface to which the load is applied. Using the autograph, a load sufficiently smaller than the load applied by the pressure member or the like is applied to the energy storage element in the stacking direction of the energy storage element (the Y direction in Figure 1 ). In this state, the load applied by the pressure member or the like is released while maintaining the probe position of the autograph, i.e., while maintaining the thickness of the energy storage element. At this time, the change in the load measured by the autograph is taken as the load applied to the electrode body. The value obtained by dividing this load applied to the electrode body by the area of ​​the contact surface between the container and the electrode body is taken as the pressure applied to the electrode body. Note that, although a load is usually applied to a pair of opposing surfaces of the energy storage element by a pressure member or the like, the area of ​​only one of these surfaces is taken as the area of ​​the surface to which the load is applied. (ii) When no load is applied to the energy storage element by a pressure member, etc. When the energy storage element is restrained by a restraining member or the like but no load is applied by the restraining member or the like, the pressure on the electrode body is measured using the following procedure. First, the energy storage element is discharged at a constant current to the minimum voltage limit for normal use and then placed in an X-ray CT scanner. Scanning is performed along a direction parallel to the electrode body's stacking direction (the Y direction in Figure 1) to confirm whether at least a portion of the surface perpendicular to the electrode body's stacking direction (the XZ plane in Figure 1) is in direct or indirect contact with the inner surface of the container. If the surface perpendicular to the electrode body's stacking direction is not in direct or indirect contact with the inner surface of the container, the pressure on the electrode body is recorded as 0 MPa. If the surface perpendicular to the electrode body's stacking direction is in direct or indirect contact with the inner surface of the container, an X-ray image of the electrode body is taken and the maximum thickness of the electrode body in the stacking direction is measured. The energy storage element is disassembled, the electrode body is removed, and the electrode body is placed on an autograph so that the probe is in contact with the surface perpendicular to the electrode body's stacking direction. Using an autograph, a load is gradually applied to the surface perpendicular to the stacking direction of the electrode body, compressing the electrode body to the maximum thickness in the stacking direction of the electrode body measured from the X-ray radiography. At this time, the load measured by the autograph is taken as the load applied to the electrode body. The load applied to the electrode body divided by the area of ​​the contact surface between the container and the electrode body is taken as the pressure applied to the electrode body. Note that, although a load is usually applied to a pair of opposing surfaces of the electrode body by the container, the area of ​​only one of the pair of surfaces is taken as the area of ​​the surface to which the load is applied.

[0018] It is preferable that the negative electrode active material further contains flake graphite, which can increase the electrical conductivity by increasing the number of contact points with other particles due to its shape, and can further improve the effect of suppressing an increase in DC resistance after charge-discharge cycles of the energy storage device.

[0019] Here, "flake graphite" refers to graphite having an aspect ratio (b / a) of 5 or more, where b is the long diameter of a particle and a is the short diameter of the particle. The aspect ratio of the flake graphite can be measured in the same manner as the aspect ratio of the graphite particles described above.

[0020] The content of the solid graphite in the negative electrode active material is preferably 50% by mass or more, which can further improve the effect of suppressing an increase in DC resistance after charge-discharge cycles of the energy storage device.

[0021] The difference between D90 and D10 in the particle size distribution of the negative electrode active material layer is preferably 40 μm or less. When the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer is 40 μm or less, the charge / discharge reaction becomes uniform, localized deterioration of the negative electrode active material can be suppressed, and an increase in DC resistance after charge / discharge cycling of the energy storage device can be further suppressed. Here, the "D90" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 90% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013). The "D10" refers to the value at which the volume-based cumulative distribution is 10%.

[0022] The configuration of an energy storage element, the configuration of an energy storage device, and a method for manufacturing an energy storage element according to one embodiment of the present invention, as well as other embodiments, 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.

[0023] <Configuration of energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and a container for accommodating the electrode assembly and the non-aqueous electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. The non-aqueous electrolyte is present in a state contained in the positive electrode, the negative electrode, and the separator. As an example of an energy storage element, a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0024] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0025] The positive electrode substrate is electrically conductive. Whether or not it has "electrical conductivity" is determined by whether the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 7 The threshold value is Ω·cm. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, or stainless steel, or an alloy of these. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0026] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the secondary battery.

[0027] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0028] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0029] The positive electrode active material can be appropriately selected from 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 composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. 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). Lithium transition metal composite oxides with spinel crystal structure 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 used in combination.

[0030] The positive electrode active material is usually in the form of particles (powder). 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 easier 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. 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. The "average particle size" refers to the value (D50) at which the volume-based cumulative distribution in the particle size distribution becomes 50%.

[0031] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0032] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0033] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic 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 agent 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.

[0034] The content of the conductive agent in the positive 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 content of the conductive agent within this range, the energy density of the secondary battery can be increased.

[0035] 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.

[0036] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably held.

[0037] 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.

[0038] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0039] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0040] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0041] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, and nickel-plated steel, alloys thereof, and carbonaceous materials are used as the material of the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil 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.

[0042] 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.

[0043] The negative electrode active material layer contains a negative electrode active material. The negative 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 positive electrode above.

[0044] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0045] The negative electrode active material contains solid graphite. By using solid graphite, which is relatively less likely to crack than other graphites, as the negative electrode active material, cracking of the negative electrode active material is less likely to occur even when the negative electrode active material expands and contracts during charge and discharge, and an increase in direct current resistance due to a decrease in electronic conductivity associated with cracking can be reduced.

[0046] The area ratio of voids within a particle to the area of ​​the entire particle in the cross section of solid graphite observed in the above-mentioned SEM image (porosity) is 2% or less, more preferably 1.5% or less, and even more preferably 1% or less. The lower limit of this area ratio (porosity) may be 0% or 0.1%.

[0047] The solid graphite may be natural graphite or artificial graphite, but is preferably artificial graphite. When the solid graphite is artificial graphite, cracking of the negative electrode active material is less likely to occur, and the increase in DC resistance due to the decrease in electronic conductivity associated with cracking can be further reduced. Here, artificial graphite is a general term for graphite artificially produced by heat-treating raw materials such as coke to graphitize them. The artificial graphite may have two peaks in the diffraction angle 2θ range of 40° to 50° in an X-ray diffraction pattern using CuKα radiation measured before charge / discharge or in a discharged state. These two peaks are considered to be peaks derived from a hexagonal crystal structure. In the case of natural graphite, it is generally considered that two peaks derived from a hexagonal crystal structure and two peaks derived from a rhombohedral crystal structure appear in the diffraction angle 2θ range of 40° to 50°.

[0048] The lower limit of the average particle size (D50) of the solid graphite is preferably 8 μm, and in some cases 10 μm is more preferable, and 12 μm is more preferable, 14 μm is even more preferable, and 16 μm is even more preferable. On the other hand, the upper limit of the average particle size (D50) of the solid graphite is preferably 25 μm, and in some cases 22 μm is more preferable, and 20 μm is even more preferable. When the average particle size (D50) of the solid graphite is equal to or greater than the lower limit or equal to or less than the upper limit, the packing density is more optimized, and an increase in DC resistance after charge / discharge cycles is further suppressed. Here, "D50" refers to the value at which the volume-based cumulative distribution in the particle size distribution is 50%.

[0049] The negative electrode active material may contain other negative electrode active materials in addition to solid graphite. Examples of such other negative electrode active materials include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、 Examples of the material include titanium-containing oxides such as TiNbO, polyphosphate compounds, silicon carbide, graphite other than solid graphite, and carbon materials such as non-graphitizable carbon (easily graphitizable carbon or non-graphitizable carbon). In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0050] As the other negative electrode active material, among these materials, graphite other than solid graphite is preferred, flake graphite and hollow graphite are preferred, and flake graphite is more preferred. Flake graphite can increase the electrical conductivity by increasing the number of contact points with other particles due to its shape, and can further improve the effect of suppressing the increase in DC resistance after charge-discharge cycles of the energy storage element.

[0051] "Graphite" refers to a graphite material that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm.

[0052] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode is 0.7 V or higher.

[0053] The negative electrode active material is usually in the form of particles (powder). The average particle size (D50) of the negative electrode active material other than the solid graphite can be, for example, 1 nm or more and 100 μm or less. When the other negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the other negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the other negative electrode active material to be equal to or greater than the above lower limit, the production or handling of the negative electrode active material becomes easy. By setting the average particle size of the other negative electrode active material to be equal to or less than the above upper limit, the electronic conductivity of the active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and classification method can be selected, for example, from the methods exemplified for the positive electrode.

[0054] The lower limit of the solid graphite content in the negative electrode active material is preferably 40% by mass, more preferably 45% by mass, more preferably 50% by mass, even more preferably 55%, 58%, or 60% by mass, even more preferably 65% ​​by mass, and particularly preferably 70% by mass. When the solid graphite content is equal to or greater than the lower limit, the effect of suppressing an increase in DC resistance after charge / discharge cycling of the energy storage device can be further improved. The upper limit of the solid graphite content is not particularly limited and may be 100% by mass, 90% by mass, 88%, 85%, 82%, or 80% by mass.

[0055] When the negative electrode active material contains flake graphite, the upper limit of the content of flake graphite in the negative electrode active material is, for example, preferably 20 mass%, more preferably 18 mass%, 15 mass%, or 12 mass%, and more preferably 10 mass%.

[0056] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0057] The upper limit of the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 / g or less, and 2.0m 2 / g is preferred, 1.9m 2 / g may be more preferable, and 1.8m 2 By setting the BET specific surface area of ​​the negative electrode active material layer to the above upper limit or less, the effect of suppressing an increase in resistance on the surface of the negative electrode active material layer can be improved. The lower limit of the BET specific surface area of ​​the negative electrode active material layer is 1.0 m 2 / g is preferred, 1.2m 2 / g may be more preferable, and 1.5m 2 By setting the BET specific surface area of ​​the negative electrode active material layer to the above lower limit or more, the initial DC resistance can be reduced.

[0058] The upper limit of the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer is preferably 40 μm, more preferably 38 μm. When the difference between D90 and D10 is equal to or less than the upper limit, the charge / discharge reaction becomes uniform, and localized deterioration of the negative electrode active material can be suppressed, thereby further suppressing an increase in DC resistance after charge / discharge cycling of the energy storage device. The lower limit of the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer is preferably 25 μm, more preferably 30 μm. When the difference between D90 and D10 is equal to or greater than the lower limit, the density of the negative electrode active material layer when pressed can be increased.

[0059] (separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the form of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these forms, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of non-aqueous electrolyte retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0060] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; 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 and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0061] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0062] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0063] (Load on electrode body) The electrode assembly is in a state where a load is applied in all states from SOC 0% to SOC 100%. This load is applied to the electrode assembly in the thickness direction of the stacked positive electrode, negative electrode, and separator, i.e., in the stacking direction. Note that the electrode assembly may have portions where no load is applied, such as the ends of the electrode assembly. In this secondary battery, applying a load to the electrode assembly in this manner can suppress an increase in DC resistance after charge / discharge cycles. Although the reason why applying a load to the electrode assembly can suppress an increase in DC resistance is unclear, it is presumed that suppressing expansion of the negative electrode active material can reduce a decrease in electronic conductivity due to cracking of the negative electrode active material and an increase in DC resistance due to film growth on the newly formed surface, and that reducing gaps within the electrode body makes it difficult for gas generated during charge / discharge to accumulate within the electrode body. The load can be applied to the electrode assembly using a pressure member, etc., as described below.

[0064] The lower limit of the pressure applied to the electrode body is 0.1 MPa, preferably 0.15 MPa, and more preferably 0.2 MPa. By setting the pressure at or above the lower limit, expansion and contraction of the negative electrode active material due to charge and discharge is suppressed, and an increase in DC resistance after charge and discharge cycles is further suppressed. On the other hand, the upper limit of the pressure applied to the electrode body is preferably 2.0 MPa, more preferably 1.0 MPa. By setting the pressure at or below the upper limit, excessive pressure is prevented from being applied to the positive electrode and negative electrode, and cracking of the positive electrode active material and negative electrode active material is suppressed.

[0065] (non-aqueous electrolyte) The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. The nonaqueous electrolyte may be a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0066] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0067] Examples of cyclic carbonates 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, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0068] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0069] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve 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.

[0070] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0071] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group 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.

[0072] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0073] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of additives include oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, succinic anhydride, glutaric anhydride, and maleic anhydride. , citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2-dioxo-1,3,2-dioxathiolane) , 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, etc. These additives may be used alone or in combination of two or more.

[0074] The content of the additive contained 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. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0075] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0076] 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., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, polymer solid electrolytes, and gel polymer electrolytes.

[0077] As the sulfide solid electrolyte, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 etc.

[0078] The shape of the energy storage element of this embodiment is not particularly limited, and examples thereof include a cylindrical battery, a prismatic battery, a flat battery, a coin battery, and a button battery.

[0079] FIG. 1 shows an energy storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the container. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51.

[0080] The application of a load to the electrode assembly 2 in the energy storage device 1 of FIG. 1 can be performed, for example, by a pressure member (not shown) that applies pressure to the container 3 from the outside. The pressure member may be a restraining member that restrains the shape of the container 3. The pressure member (restraining member) is arranged, for example, to sandwich and pressurize the electrode assembly 2 from both sides (the front side and the back side in FIG. 1) in the stacking direction (the Y direction in FIG. 1) via the container 3. Both sides of the electrode assembly 2 are in contact with the inner surface of the container 3 directly or via another member not shown. Therefore, a load is applied to the electrode assembly 2 when the container 3 is not pressurized. It is preferable that both sides of the electrode assembly 2 are in contact with the inner surface of the container 3 directly or via another member not shown when the container 3 is not pressurized. Examples of the pressure member (restraining member) include a restraining band and a metal frame. As shown in Figure 2 described below, multiple storage elements 1 may be arranged side by side in the stacking direction of the electrode body 2 (Y direction in Figure 1, left and right direction in Figure 2), and the multiple storage elements 1 may be fixed using a restraining member 21 such as a frame while being pressed from both ends in the stacking direction.

[0081] <Electricity storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements 1 in an automobile power source such as an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (PHEV), a power source for electronic devices such as a personal computer or a communication terminal, or a power storage power source, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage unit.

[0082] 2 shows an example of an energy storage device 30 in which energy storage units 20, each of which is an assembly of two or more electrically connected energy storage elements 1, are further assembled. In the energy storage unit 20, a plurality of energy storage elements 1 are arranged without gaps in the stacking direction, and are constrained by fixed-size restraining members 21 so that the thickness is kept constant and pressure is applied from the stacking direction (the left-right direction in FIG. 2). The energy storage device 30 may include a bus bar (not shown) that electrically connects two or more energy storage elements 1, a bus bar (not shown) that electrically connects two or more energy storage units 20, and the like. The energy storage unit 20 or the energy storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more energy storage elements.

[0083] <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.

[0084] The method for 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 in the container, and then the inlet may be sealed.

[0085] The method for manufacturing the energy storage element may further include attaching a pressure member such as a restraint member. Also, the method for manufacturing an energy storage device including a plurality of energy storage elements may include attaching a pressure member such as a restraint member to the plurality of energy storage elements.

[0086] <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.

[0087] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy 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.

[0088] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but 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 in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode. [Example]

[0089] 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.

[0090] [Example 1] (Preparation of negative electrode) A negative electrode mixture paste (material for forming a negative electrode active material layer) was prepared by mixing the solid graphite, hollow graphite, and flake graphite listed in Table 1 as negative electrode active materials with styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. Artificial graphite with a porosity listed in Table 1 and an aspect ratio of less than 5 was used as the solid graphite, and natural graphite with a porosity listed in Table 1 and an aspect ratio of less than 5 was used as the hollow graphite. The aspect ratio of the flake graphite was 5 or greater. The porosities of the artificial graphite, natural graphite, and flake graphite were measured by the method described above for negative electrodes separately prepared containing only artificial graphite, only natural graphite, or only flake graphite as the negative electrode active material. The mass ratio of the negative electrode active material, binder, and thickener was 97:2:1 (solids equivalent).

[0091] The viscosity of the negative electrode mixture paste was adjusted by adjusting the amount of water, and the paste was prepared through a kneading process using a multi-blender mill. The negative electrode mixture paste was applied to both sides of copper foil as the negative electrode substrate and dried. A negative electrode was then obtained by roll pressing. The D10 and D90 values ​​in the particle size distribution of the negative electrode active material measured by the above method were 9.1 μm and 53.8 μm, respectively, and the BET specific surface area of ​​the negative electrode active material layer measured by the above method was 2.0 m 2 / g.

[0092] (Preparation of positive electrode) LiNi, the positive electrode active material 0.6 Co 0.2 Mn 0.2 A positive electrode mixture paste was prepared using O2, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, conductive agent, and binder was 90:5:5 (solid content equivalent). The positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate and dried. Then, a roll press was performed to obtain a positive electrode.

[0093] (Non-aqueous electrolyte) A 1.2 mol / dm 3 LiPF6 was dissolved in the non-aqueous electrolyte at a concentration of 1000 ppm to obtain a non-aqueous electrolyte.

[0094] (separator) A 25 μm thick porous polyethylene resin film was used as the separator.

[0095] (Assembling the energy storage element) The positive electrode, negative electrode, and separator were stacked to obtain an electrode assembly. The electrode assembly was then placed in a container made of a metal-resin composite film, and a nonaqueous electrolyte solution was poured into the container, which was then sealed. Restraining members were attached to both sides of the container to restrain the electrode assembly at a fixed size in the stacking direction, and a load was applied to the electrode assembly so that a pressure of 0.1 MPa was applied, thereby obtaining the energy storage element of Example 1.

[0096] [Example 2 and Comparative Examples 1 to 8] The energy storage elements of Example 2 and Comparative Examples 1 to 8 were obtained in the same manner as Example 1, except that the materials listed in Table 1 were used as the negative electrode active materials and the content ratios of the negative electrode active materials and the pressure applied to the electrode body were changed as shown in Table 2. Table 2 shows the D10 and D90 in the particle size distribution of the negative electrode active material layer of each energy storage element, as well as the BET specific surface area.

[0097] [Table 1]

[0098] [Table 2]

[0099] [evaluation] (1-1)Initial charging / discharging Each of the obtained energy storage elements was initially charged and discharged under the following conditions. After constant current charging at a charging current of 1.0 C and a cut-off voltage of 4.20 V in a thermostatic chamber at 25°C, the elements were then subjected to constant voltage charging at 4.20 V. The charging was terminated after a total charging time of 3 hours. After this, a 10-minute rest period was provided. The elements were then discharged at a constant current of 1.0 C and a cut-off voltage of 2.75 V. (1-2) Initial DC resistance After the initial charge and discharge, each energy storage element was charged at a constant current of 1.0 C in a thermostatic chamber at 25°C until the SOC reached 50%. After storing the element in the thermostatic chamber at 25°C for at least 3 hours, it was discharged for 30 seconds at currents of 0.2 C, 0.5 C, and 1.0 C, respectively. After each discharge, the element was charged at a constant current of 0.2 C until the SOC reached 50%. The relationship between the current and the voltage 10 seconds after the start of discharge for each discharge was plotted, and the DC resistance was calculated from the slope of the straight line obtained from the plot of the three points, which was used as the initial DC resistance.

[0100] (1-3) Charge / discharge cycle test Next, the following charge-discharge cycle test was conducted. At 45°C, the battery was charged at a constant current of 1.0 C with a cut-off voltage of 4.20 V, followed by a constant voltage charge of 4.20 V. The charge was terminated after a total charge time of 3 hours. A 10-minute rest period was then provided. A constant current discharge was performed at a discharge current of 1.0 C with a cut-off voltage of 2.75 V, followed by a 10-minute rest period. This charge and discharge process constitutes one cycle, and 1,000 cycles were conducted.

[0101] (1-4) DC resistance after charge / discharge cycle test For each energy storage element after the charge-discharge cycle test, the DC resistance was measured in the same manner as for the initial DC resistance, and this was defined as the DC resistance after the charge-discharge cycle test. The increase rate [%] of DC resistance at 25°C after the charge-discharge cycle test was calculated using the following formula. After charge / discharge cycle, 25°C DC resistance increase rate [%] = (DC resistance after charge / discharge cycle test / initial DC resistance) x 100-100 Table 2 shows the increase rate of DC resistance at 25° C. after charge-discharge cycles for each of the energy storage elements of Example 1, Example 2, and Comparative Examples 1 to 8.

[0102] As shown in Table 2, the negative electrode active material contains solid graphite, and the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 It can be seen that in Examples 1 and 2, in which the content of solid graphite was 50 mass % or more and the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer was 40 μm or less, the effect of suppressing the increase in DC resistance after charge-discharge cycling of the energy storage element was particularly excellent.

[0103] On the other hand, the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 It can be seen that Comparative Examples 1 to 8, in which the BET specific surface area of ​​the negative electrode active material layer is 2.1 m / g or less and the pressure applied to the electrode body is not 0.1 MPa or more, have a low effect of suppressing the increase in DC resistance after charge-discharge cycling. 2 In Comparative Example 1, in which the negative electrode active material layer had a particle size distribution exceeding D90 / g, the pressure applied to the electrode body was less than 0.1 MPa, the solid graphite content was less than 50 mass%, and the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer exceeded 40 μm, the rate of increase in DC resistance after charge-discharge cycles was high.

[0104] [Example 3] (Preparation of negative electrode) A negative electrode mixture paste (material for forming a negative electrode active material layer) was prepared by mixing the solid graphite and flake graphite listed in Table 3 as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, carboxymethyl cellulose (CMC) as the thickener, and water as the dispersion medium. Artificial graphite with a porosity listed in Table 3 and an aspect ratio of less than 5 was used as the solid graphite. The aspect ratio of the flake graphite was 5 or more. The porosity of the artificial graphite and flake graphite was measured by the above-mentioned method for separately fabricating negative electrodes containing only artificial graphite or only flake graphite as the negative electrode active material. The mass ratio of the negative electrode active material to the binder and the thickener was 97:2:1 (solid content equivalent).

[0105] The viscosity of the negative electrode mixture paste was adjusted by adjusting the amount of water, and the paste was prepared through a kneading process using a multi-blender mill. The negative electrode mixture paste was applied to both sides of copper foil as the negative electrode substrate and dried. A negative electrode was then obtained by roll pressing. The D10 and D90 values ​​in the particle size distribution of the negative electrode active material measured by the above method were 6.7 μm and 31.6 μm, respectively, and the BET specific surface area of ​​the negative electrode active material layer measured by the above method was 2.0 m 2 / g.

[0106] (Preparation of positive electrode) LiNi, the positive electrode active material 0.6 Co 0.2 Mn 0.2 A positive electrode mixture paste was prepared using O2, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium. The mass ratio of the positive electrode active material, conductive agent, and binder was 90:5:5 (solid content equivalent). The positive electrode mixture paste was applied to both sides of aluminum foil as a positive electrode substrate and dried. Then, a roll press was performed to obtain a positive electrode.

[0107] (Non-aqueous electrolyte) A 1.2 mol / dm 3 LiPF6 was dissolved in the non-aqueous electrolyte at a concentration of 1000 ppm to obtain a non-aqueous electrolyte.

[0108] (separator) A 25 μm thick porous polyethylene resin film was used as the separator.

[0109] (Assembling the energy storage element) The positive electrode, negative electrode, and separator were stacked to obtain an electrode assembly. The electrode assembly was then placed in a container made of a metal-resin composite film, and a nonaqueous electrolyte solution was poured into the container, which was then sealed. Restraining members were attached to both sides of the container to restrain the electrode assembly at a fixed size in the stacking direction, and a load was applied to the electrode assembly so that the pressure was 0.1 MPa, thereby obtaining the energy storage element of Example 3.

[0110] [Comparative Examples 9 to 11] The energy storage elements of Comparative Examples 9 to 11 were obtained in the same manner as in Example 3, except that the materials listed in Table 3 were used as the negative electrode active materials, and the content ratios of the negative electrode active materials and the pressure applied to the electrode body were changed as shown in Table 4. The D10 and D90 in the particle size distribution of the negative electrode active material layer of each energy storage element, as well as the BET specific surface area, are shown in Table 4. In Comparative Examples 9 and 10, in addition to solid graphite and flake graphite, natural graphite having a porosity listed in Table 3 and an aspect ratio of less than 5 was used as hollow graphite as the negative electrode active material.

[0111] [Table 3]

[0112] [Table 4]

[0113] [evaluation] The increase rate of DC resistance after charge-discharge cycles was determined for each of the storage elements of Example 3 and Comparative Examples 9 to 11 using the same procedures as in (1-1) to (1-4) above, except that the conditions for the charge-discharge cycle test in (1-3) above were changed as follows: The conditions for the charge-discharge cycle test were as follows: At 45°C, the battery was charged at a constant current of 1.0 C with a cut-off voltage of 4.20 V, followed by a constant voltage charge of 4.20 V. The charge was terminated after a total charge time of 3 hours. This was followed by a 10-minute rest period. The battery was discharged at a constant current of 1.0 C, with 85% of the discharge capacity from the initial charge and discharge, followed by a 10-minute rest period. This charge and discharge process constitutes one cycle, and 700 cycles were performed. Table 4 shows the increase rate of DC resistance at 25° C. after charge-discharge cycles for each of the energy storage elements of Example 3 and Comparative Examples 9 to 11.

[0114] As shown in Table 4, when the negative electrode active material contains solid graphite and the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 It can be seen that even in Example 3 where the pressure applied to the electrode body is 0.1 MPa or more and the dielectric constant is 0.1 / g or less, the increase in DC resistance after charge-discharge cycles of the energy storage element is suppressed.

[0115] On the other hand, the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 It can be seen that Comparative Examples 9 to 11, in which the BET specific surface area of ​​the negative electrode active material layer is 2.1 m / g or less and the pressure applied to the electrode body is not 0.1 MPa or more, have a low effect of suppressing the increase in DC resistance after charge-discharge cycling. Surprisingly, when Comparative Example 9 and Comparative Example 11, in which no pressure is applied to the electrode body, are compared, the BET specific surface area of ​​the negative electrode active material layer is 2.1 m / g or less. 2 / g or less, the DC resistance after charge-discharge cycling increases. On the other hand, when comparing Comparative Example 10 and Example 3, in which the pressure applied to the electrode body is 0.1 MPa or more, the BET specific surface area of ​​the negative electrode active material layer is 2.1 m 2 It can be seen that in Example 3, where the Cr content is 0.15 / g or less, the increase in DC resistance after charge-discharge cycling of the energy storage element is significantly suppressed.

[0116] The above results demonstrate that the energy storage element can suppress an increase in DC resistance after charge-discharge cycles. [Industrial Applicability]

[0117] The present invention can be applied to electric storage elements used as power sources for electronic devices such as personal computers and communication terminals, and automobiles. [Explanation of symbols]

[0118] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 21 Restraining member 30 Electricity storage device

Claims

1. An electrode assembly having a negative electrode and a positive electrode, the negative electrode has a negative electrode active material layer containing a negative electrode active material, The negative electrode active material contains solid graphite, The BET specific surface area of ​​the negative electrode active material layer is 2.1 m ２ / g or less, The pressure applied to the electrode body is 0.1 MPa or more.

2. 2. The electric storage element according to claim 1, wherein the negative electrode active material further contains flake graphite.

3. 3. The energy storage element according to claim 1, wherein the content of the solid graphite in the negative electrode active material is 50% by mass or more.

4. 4. The energy storage element according to claim 1, wherein the difference between D90 and D10 in the particle size distribution of the negative electrode active material layer is 40 μm or less.

5. 5. The energy storage element according to claim 1, wherein the solid graphite is artificial graphite.

6. 6. The energy storage element according to claim 1, wherein the solid graphite is particulate, has an aspect ratio (b / a) of less than 5, where b is the long diameter of the particle and a is the short diameter of the particle, and has a porosity of 2% or less.

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