Energy storage element and energy storage device

Abstract

To provide a power storage element capable of suppressing increase in resistance after charge / discharge cycles at high current density.SOLUTION: A power storage element includes a negative electrode having a negative electrode active material layer, a separator superimposed on the surface of the negative electrode active material layer, and a non-aqueous electrolyte, and the difference in thickness of the negative electrode active material layer between the charged state and the discharged state is 0.6 μm or less, and the amount of change in thickness of the separator before and after applying a load of 2 MPa for 60 seconds at a temperature of 65°C is 2 μm or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an energy storage element and an energy storage device. [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] As the above-mentioned energy storage element, for example, a lithium ion secondary battery has been proposed which includes an electrode body formed by winding a first electrode plate as a positive electrode plate, a second electrode plate as a negative electrode plate, and a separator, and which has a power generation section where the active material coated section of the first electrode plate, the active material coated section of the second electrode plate, and the separator overlap, and a battery case that houses the electrode body (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-53055 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, energy storage devices have been applied to automobiles, such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles. Energy sources for these vehicles and the like require energy storage devices with rapid charging capabilities. However, the inventors' research has led to the following findings. Repeated charge / discharge cycles cause the negative electrode active material layer to expand and contract, causing the separator to repeatedly compress and release, resulting in the electrolyte being extruded from the separator's pores or being re-impregnated into the separator's pores. In particular, when a separator with a large change in thickness in response to compression is used, the amount of electrolyte extruded from the separator's pores increases. When repeated charge / discharge cycles are performed at a high current density, the electrolyte extruded from the separator's pores cannot re-impregnate the separator's pores at a rapid enough rate that the electrolyte becomes depleted in the separator's pores, resulting in an increase in the resistance of the energy storage device.

[0006] An object of the present invention is to provide an energy storage element capable of suppressing an increase in resistance after charge-discharge cycles at a high current density, and to provide an energy storage device capable of performing charge-discharge at a high current density and suppressing an increase in resistance after charge-discharge cycles at a high current density. [Means for solving the problem]

[0007] One aspect of the present invention is an energy storage element including a negative electrode having a negative electrode active material layer, a separator overlaid on a surface of the negative electrode active material layer, and a nonaqueous electrolyte, wherein the difference in thickness of the negative electrode active material layer between a charged state and a discharged state is 0.6 μm or less, and the change in thickness of the separator between before and after application of a load of 2 MPa at a temperature of 65°C for 60 seconds is 2 μm or more.

[0008] Another aspect of the present invention is a power storage device comprising the power storage device and a control unit that controls charging and discharging of the power storage device, wherein the maximum current density of charging and discharging the power storage device controlled by the control unit is 6 mA / cm 2 The above is the power storage device. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide an energy storage element that can suppress an increase in resistance after charge-discharge cycles at a high current density. Also, according to another aspect of the present invention, it is possible to provide an energy storage device that can be charged and discharged at a high current density and that can suppress an increase in resistance after charge-discharge cycles at a high current density. [Brief explanation of the drawings]

[0010] [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 the configuration of an electricity storage device according to one embodiment of the present invention and the state in which the electricity storage device is mounted on a vehicle. DETAILED DESCRIPTION OF THE INVENTION

[0011] First, an overview of the energy storage element and the energy storage device disclosed in this specification will be described.

[0012] One aspect of the present invention is an energy storage element including a negative electrode having a negative electrode active material layer, a separator overlaid on a surface of the negative electrode active material layer, and a nonaqueous electrolyte, wherein the difference in thickness of the negative electrode active material layer between a charged state and a discharged state is 0.6 μm or less, and the change in thickness of the separator between before and after application of a load of 2 MPa at a temperature of 65°C for 60 seconds is 2 μm or more.

[0013] Even when a separator with a large change in thickness under compression at high temperatures is used, the energy storage device can suppress an increase in resistance after charge / discharge cycling at high current densities by keeping the difference in thickness between the charged and discharged states of the negative electrode active material layer within a specific range. While the reason for this is unclear, the following is presumed. In general, in energy storage devices, when the negative electrode active material layer expands due to the movement of ions into and out of the negative electrode active material during charge and discharge, the separator is compressed, and the electrolyte impregnated in the separator's pores is forced out of the electrode body. In particular, when the thickness change of the separator after applying a load of 2 MPa for 60 seconds at a temperature of 65°C is 2 μm or more, the separator is likely to be compressed in the thickness direction, resulting in a significant occurrence of the above-mentioned phenomenon. In contrast, in this energy storage device, the difference in thickness between the charged and discharged states of the negative electrode active material layer is 0.6 μm or less, so even if a separator with a large change in thickness under compression at high temperatures is used, the separator is unlikely to be compressed due to expansion of the negative electrode active material layer during charging and discharging, and the electrolyte is unlikely to be forced out of the electrode body. Therefore, with this energy storage device, even when charging and discharging are repeated at high current densities, it is presumed that the electrolyte is unlikely to be depleted in the pores of the separator, thereby suppressing an increase in resistance.

[0014] The negative electrode active material layer preferably contains a negative electrode active material, and the negative electrode active material is preferably graphite. The use of graphite as the negative electrode active material can improve capacity and high-rate charge / discharge performance. Furthermore, since graphite is a negative electrode active material with a high expansion rate during charging among carbon materials, when graphite is used as the negative electrode active material, a large amount of electrolyte is extruded from the separator. Therefore, the effect of suppressing an increase in resistance after charge / discharge cycles at high current density is significantly achieved.

[0015] "Graphite" refers to graphite that has an average lattice spacing (d 002) is a carbon material having a particle size of 0.33 nm or more and less than 0.34 nm. The "discharged state" of a carbon material such as graphite means a state in which the carbon material, which is the negative electrode active material, is discharged so that ions such as lithium that can be absorbed and released during charging and discharging are sufficiently released from the carbon material. For example, in a half cell using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic lithium (Li) as the counter electrode, this is a state in which the open circuit voltage is 0.7 V or more.

[0016] The separator has an air permeability of 150 seconds / 100 cm 3 It is preferable that the separator has an air permeability of 150 seconds / 100 cm or less. 3 By keeping the air permeability at or below 100 cm under a constant pressure difference, sufficient ions such as lithium can be supplied to the negative electrode during charge and discharge at a high current density, thereby suppressing the increase in resistance after charge and discharge cycles at a high current density and further increasing the output. 3 This indicates the time it takes for air to pass through a sample of a certain area, and is a value measured in accordance with JIS-P8117 (2009).

[0017] The porosity of the separator is preferably 55% by volume or more and 70% by volume or less. By having the porosity of the separator in this range, the strength of the separator can be maintained while further suppressing an increase in resistance after charge-discharge cycles at high current densities. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0018] The energy storage element is preferably used as a power source for a hybrid vehicle. Hybrid vehicle power sources are generally charged and discharged at high current densities, making it particularly important to suppress an increase in resistance. Therefore, the energy storage element, in which an increase in resistance after charge-discharge cycles at high current densities is suppressed, is particularly useful as a power source for a hybrid vehicle.

[0019] A "hybrid vehicle" is a vehicle that has two or more power sources (prime movers), and typically has an internal combustion engine and an electric motor as its power sources.

[0020] Another aspect of the present invention is a power storage device comprising the power storage device and a control unit that controls charging and discharging of the power storage device, wherein the maximum current density of charging and discharging the power storage device controlled by the control unit is 6 mA / cm 2 The above is the power storage device.

[0021] The electricity storage device can be charged and discharged at a high current density, and can suppress an increase in resistance after charge-discharge cycles at a high current density.

[0022] The "maximum current density" refers to the value obtained by dividing the maximum current (upper limit of the controlled current) controlled by the control unit when charging and discharging the energy storage element by the area where the positive electrode active material layer and the negative electrode active material layer face each other.

[0023] 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.

[0024] <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 laminated 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 exists 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.

[0025] (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.

[0026] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials 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, vapor-deposited films, meshes, and porous materials, 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.

[0027] 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.

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

[0029] 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.

[0030] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material 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, and carbon materials such as graphite and 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.

[0031] The negative electrode active material is preferably graphite. By using graphite as the negative electrode active material, it is possible to achieve high capacity and improved high-rate charge-discharge performance. Furthermore, since graphite is a negative electrode active material with a high expansion rate during charging among carbon materials, when graphite is used as the negative electrode active material, a large amount of electrolyte is extruded from the separator. Therefore, the effect of suppressing an increase in resistance after charge-discharge cycles at high current density is significantly achieved.

[0032] Examples of the graphite include natural graphite and artificial graphite. The negative electrode active material of the energy storage device is preferably natural graphite, and more preferably solid natural graphite. When the negative electrode active material contains natural graphite, the effect of suppressing an increase in resistance after charge-discharge cycles at high current densities of the energy storage device can be further enhanced. Natural graphite is a general term for graphite extracted from natural resources. Specific examples of natural graphite include flake graphite, lump graphite (flake graphite), and earthy graphite. Natural graphite may be spherical natural graphite particles obtained by spheroidizing flake graphite or the like. Natural graphite may exhibit four peaks in a 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 four peaks are believed to be two peaks derived from a hexagonal crystal structure and two peaks derived from a rhombohedral crystal structure. In the case of artificial graphite, it is generally believed that only two peaks resulting from the hexagonal crystal structure appear. In the X-ray diffraction pattern, the ratio of the peak intensity resulting from the (012) plane to the peak intensity resulting from the (100) plane ((012) / (100)) is preferably 0.3 or more, more preferably 0.4 or more. The peak intensity ratio ((012) / (100)) is preferably 0.6 or less. Here, the (100) plane is derived from the hexagonal crystal structure, and the (012) plane is derived from the rhombohedral crystal structure.

[0033] The term "solid" in solid natural graphite means that the interior of a particle of natural graphite is filled and substantially no voids exist. More specifically, "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 of voids within the particle to the area of ​​the entire particle (void ratio) is 2% or less. In a preferred embodiment, the area ratio of voids in solid natural graphite may be 1% or less. 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 negative electrode fixed with resin, and a measurement sample is prepared. The negative electrode to be measured is prepared using the following procedure. The storage element is discharged at a current of 0.1C to the discharge end voltage during normal use. Next, the storage element is disassembled, the negative electrode is removed, and after thorough cleaning with dimethyl carbonate, it is dried under reduced pressure at room temperature and cut to a specified area to be used as the measurement sample. The process from disassembling the storage element to preparing the measurement sample is carried out in a dry air atmosphere with a dew point of -40°C or below. (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 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 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, 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 The "area ratio of voids within a particle to the total particle area (porosity)" is determined by calculating the average value of all void area ratios R1, R2, R3, ... calculated by the binarization process.

[0034] "Non-graphitic carbon" refers to carbon that has an average lattice spacing (d 002) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0035] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0036] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0037] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm to 100 μm. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm to 1 μm. By setting the average particle size of the negative electrode active material to the above lower limit or above, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to the above upper limit or below, 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. Examples of pulverization methods include methods using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow jet mill, or a sieve. Wet pulverization in the presence of water or an organic solvent such as hexane can also be used. As a classification method, a sieve, an air classifier, or the like is used as needed in both dry and wet methods. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As a classification method, a sieve, an air classifier, or the like is used as needed in both dry and wet methods. When the negative electrode active material is a metal such as metallic Li, the negative electrode active material may be in the form of a foil.

[0038] 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.

[0039] When the negative electrode active material contains graphite, the lower limit of the graphite content relative to the total negative electrode active material contained in the negative electrode active material layer is preferably 50% by mass, more preferably 70% by mass, and even more preferably 90% by mass. The negative electrode active material contained in the negative electrode active material layer may consist essentially of graphite. In this case, the advantages of graphite can be fully exhibited. On the other hand, when the graphite content is high, the expansion coefficient of the negative electrode active material layer increases, and the effect of suppressing an increase in resistance after charge-discharge cycles at high current density is more pronounced.

[0040] The conductive agent is not particularly limited as long as it is a material that is conductive. Carbon materials such as the above-mentioned graphite, graphitizable carbon, and non-graphitizable carbon also have conductivity, but are not included in the conductive agent in the negative electrode active material layer. Examples of conductive agents other than the above-mentioned carbon materials include other carbonaceous materials, metals, conductive ceramics, etc. Examples of other carbonaceous materials include other non-graphitic carbons and graphene-based carbons. Examples of other non-graphitic carbons 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 carbons 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 and used. Furthermore, these materials may be used in combination. For example, a composite material of carbon black and CNTs may be used. Of these, carbon black is preferred from the viewpoint of electron conductivity and coatability, and acetylene black is particularly preferred.

[0041] When the negative electrode active material layer contains a conductive agent, the content of the conductive agent is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By setting the content of the conductive agent within the above range, the energy density of the secondary battery can be increased.

[0042] 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.

[0043] The binder content 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 active material can be stably maintained.

[0044] 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.

[0045] 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.

[0046] The upper limit of the difference in thickness between the charged and discharged states of the negative electrode active material layer is 0.6 μm, preferably 0.58 μm, and more preferably 0.55 μm. In this energy storage device, since the difference in thickness between the charged and discharged states of the negative electrode active material layer is equal to or less than the upper limit, even if a separator with a large change in thickness under high-temperature compression is used, the separator is less likely to be compressed due to expansion of the negative electrode active material layer during charging and discharging. Therefore, with this energy storage device, even when charging and discharging are repeated at high current densities, the electrolyte is less likely to be depleted in the pores of the separator, thereby suppressing an increase in resistance. Furthermore, the lower limit of the difference in thickness between the charged and discharged states of the negative electrode active material layer is not particularly limited, but is preferably 0.1 μm, more preferably 0.2 μm, from the viewpoint of ensuring the capacity of the energy storage device. The thickness of the negative electrode active material layer is the thickness of the region facing the positive electrode active material layer. Furthermore, when the negative electrode active material layer is disposed on both sides of the negative electrode substrate, the thickness of the negative electrode active material layer is the thickness on either one of the surfaces. The difference in thickness of the negative electrode active material layer between the charged state and the discharged state can be adjusted by the type of negative electrode active material, the depth of charge of the negative electrode, the coating mass of the negative electrode active material layer, the thickness of the negative electrode active material layer, the porosity of the negative electrode active material layer, etc.

[0047] The difference in thickness of the negative electrode active material layer between the charged state and the discharged state is calculated by the following procedure. (1) Preparation of negative electrodes in charged and discharged states After constant current discharge at a current of 0.5C until the state of charge (SOC) reached 0%, constant voltage discharge was performed at a voltage of 0% SOC to achieve a discharged state. The discharge was terminated when the constant voltage discharge time reached 2 hours. The storage element was disassembled, and the positive and negative electrodes were removed. The removed negative electrode was used as the discharged negative electrode. Next, a test battery was assembled using the removed positive and negative electrodes, and a current of 0.5 mA / cm was applied. 2 The battery is charged at a constant current of 100% to a voltage that brings the SOC of the original storage element to 100%, and then at a constant voltage, the battery is charged at a voltage that brings the SOC of the original storage element to 100%. The charging is terminated when the total charging time reaches 4 hours. The test battery is disassembled in this charged state, and the removed negative electrode is used as the charged negative electrode. (2) Calculation of the difference in thickness between the charged and discharged states of the negative electrode active material layer Ten 2 cm x 2 cm pieces of the negative electrode plate in the charged state and the negative electrode in the discharged state were randomly taken from the area facing the positive electrode active material layer, and the thickness of each was measured at one point near the center to determine the negative electrode thickness. The thickness of the negative electrode minus the thickness of the negative electrode substrate was determined as the thickness of the negative electrode active material layer in the charged state and the thickness of the negative electrode active material layer in the discharged state, respectively. In this case, if the negative electrode active material layer is disposed on one side of the negative electrode substrate, the obtained value was determined as the thickness of the negative electrode active material layer. In case the negative electrode active material layer is disposed on both sides of the negative electrode substrate, the obtained value was further divided by 2 to determine the thickness of the negative electrode active material layer. After determining the average thickness of the negative electrode active material layer in the discharged and charged states, the difference in thickness between the negative electrode active material layer in the charged and discharged states was calculated using the following formula: Difference in thickness between the negative electrode active material layer in the charged and discharged states [μm] = (Average thickness of negative electrode active material layer in charged state) - (Average thickness of negative electrode active material layer in discharged state)

[0048] The upper limit of the charge depth of the negative electrode is preferably 0.8, more preferably 0.7, from the viewpoint of safety during high-rate charging. The lower limit of the charge depth of the negative electrode is preferably 0.3, more preferably 0.4, from the viewpoint of ensuring sufficient capacity of the energy storage element. The charge depth of the negative electrode refers to the ratio of the charged electrical quantity per mass of graphite in a charged state to the theoretical capacity per mass of graphite. "Theoretical capacity" refers to the maximum electrical quantity that can be stored per unit mass of active material in an expected electrochemical reaction, and in this specification, the theoretical capacity of graphite is 372 mAh / g.

[0049] The lower limit of the coating mass (mass per unit area) of the negative electrode active material layer is 0.15 g / 100 cm from the viewpoint of ensuring sufficient capacity of the storage element. 2 is preferred, and 0.2 g / 100 cm 2 On the other hand, from the viewpoint of safety, the upper limit of the coating mass of the negative electrode active material layer is 1 g / 100 cm. 2 is preferred, and 0.9 g / 100 cm 2is more preferred.

[0050] The lower limit of the porosity of the negative electrode active material layer is preferably 30%, more preferably 35%. The upper limit of the porosity of the negative electrode active material layer is preferably 60%, more preferably 55%. When the porosity of the negative electrode active material layer is in the above range, the effect of suppressing an increase in resistance after charge-discharge cycles at high current density can be further improved, and the capacity retention rate of the energy storage element after charge-discharge cycles can be increased.

[0051] The intermediate layer is a layer disposed between the negative electrode substrate and the negative 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 negative 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.

[0052] (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. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the negative electrode above, for example.

[0053] 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).

[0054] 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.

[0055] The positive electrode active material layer includes a positive electrode active material. The positive electrode active material layer includes optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the negative electrode. Note that in the positive electrode active material layer, carbon materials such as graphite, graphitizable carbon, and non-graphitizable carbon are also included as conductive agents.

[0056] 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.

[0057] As the positive electrode active material, from the viewpoints of charge / discharge cycle performance, energy density, etc., lithium transition metal composite oxides having an α-NaFeO2 type crystal structure and polyanion compounds are preferred, and lithium transition metal composite oxides containing nickel, manganese, and cobalt and LiFePO4 are more preferred.

[0058] 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 and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% 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).

[0059] To obtain powder with a predetermined particle size, a pulverizer, a classifier, etc. The pulverizing method and the classifying method can be selected from the methods exemplified for the negative electrode above, for example.

[0060] 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.

[0061] 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.

[0062] (separator) The separator is laminated on the surface of the negative electrode active material layer. The separator can be appropriately selected from known separators. For example, a separator consisting of only a substrate layer, or a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of a substrate layer, can be used.

[0063] The lower limit of the thickness change of the separator after applying a load of 2 MPa for 60 seconds at 65°C is 2 μm, preferably 2.5 μm, and more preferably 3.0 μm. Separators whose thickness change after applying a load of 2 MPa for 60 seconds at 65°C or greater than the lower limit have the advantage of excellent output characteristics. However, when used as a power source for repeated charge / discharge at high current densities, such as in hybrid vehicles, the separator is likely to undergo compression in the thickness direction during charging. Therefore, in energy storage devices including separators whose thickness change after applying a load of 2 MPa for 60 seconds at 65°C or greater than the lower limit is more pronounced, suppressing separator compression and suppressing an increase in resistance after charge / discharge cycles at high current densities. On the other hand, the upper limit of the thickness change of the separator may be 5 μm or 6 μm. The change in thickness of the separator before and after applying a load of 2 MPa at a temperature of 65° C. for 60 seconds can be adjusted by the manufacturing method, material, air permeability, porosity, thickness, etc. of the separator.

[0064] The thickness change of the separator before and after applying a load of 2 MPa for 60 seconds at 65°C is a value measured for a dry separator using the following method: 300 separators cut to a size of 30 mm x 30 mm are stacked together, and a load of 2 MPa is applied in this state for 60 seconds under the following conditions, and the displacement before and after is measured. The obtained value is divided by 300 to calculate the thickness change of the separator before and after applying a load of 2 MPa for 60 seconds at 65°C. Equipment: Mize Testing Machine Co., Ltd. load cell creep testing machine 525-L Temperature: 65℃ Compressive stress: 2 MPa Compression area: φ50mm Compression time: 60 seconds

[0065] The substrate layer of the separator may be a porous resin film. In the case of the separator, when a change in thickness of 2 μm or more occurs before and after application of a load of 2 MPa for 60 seconds at a temperature of 65° C., the substrate layer of the separator may be a woven fabric, a nonwoven fabric, or the like. From the viewpoint of safety during use of the battery, the material of the substrate layer of the separator is preferably, for example, a polyolefin such as polyethylene or polypropylene, polyphenylene ether, or polyamide. A composite material of these resins may also be used as the substrate layer of the separator. The substrate layer may be composed of two or more layers.

[0066] 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.

[0067] The upper limit of the separator's air permeability is 150 seconds / 100 cm 3 is preferred, 130 seconds / 100 cm 3By setting the separator air permeability to the above upper limit or less, it is possible to enhance the effect of suppressing the decrease in capacity retention rate after charge-discharge cycles at a high current density. On the other hand, the lower limit of the separator air permeability is set to 10 sec / 100 cm from the viewpoint of maintaining the strength of the separator. 3 is preferred, 50 seconds / 100cm 3 is more preferred.

[0068] The upper limit of the porosity of the separator is preferably 70% by volume, more preferably 65% ​​by volume, from the viewpoint of maintaining the strength of the separator, while the lower limit of the porosity of the separator is preferably 55% by volume, more preferably 56% by volume, from the viewpoint of further enhancing the effect of suppressing the increase in resistance after charge-discharge cycles at high current density.

[0069] The lower limit of the average thickness of the separator is preferably 5 μm, more preferably 10 μm. The upper limit of the average thickness is preferably 40 μm, more preferably 30 μm. By making the average thickness of the separator equal to or greater than the lower limit, sufficient mechanical strength can be obtained. Furthermore, by making the average thickness of the separator equal to or less than the upper limit, the resistance as an energy storage element is reduced, thereby making it possible to obtain sufficient output performance. The average thickness of the separator is the average value obtained by measuring the thickness at 10 random points.

[0070] (Non-aqueous electrolyte) The non-aqueous electrolyte can be appropriately selected from known non-aqueous electrolytes, and includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere. 3 More 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.

[0078] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiFOB), and lithium bis(oxalato)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and cyclohexyl. Aromatic compounds such as benzene, 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, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, Glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, 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, and the like.These additives may be used alone or in combination of two or more.

[0079] 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.

[0080] An energy storage element according to one embodiment of the present invention is typically a prismatic battery. 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 a 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.

[0081] In the energy storage element of this embodiment, an increase in resistance after charge / discharge cycles at high current density is suppressed. Therefore, the energy storage element of this embodiment is suitable for use in applications where charge / discharge is performed at high current density, such as automotive power sources for electric vehicles (EVs), hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), etc., and is particularly suitable for use as a power source for HEVs. Furthermore, the energy storage element of this embodiment has a current density of 6 mA / cm 2 or more, and even 7mA / cm 2 The upper limit of the current density when the energy storage element of this embodiment is charged and discharged is, for example, 20 mA / cm. 2 may be 10 mA / cm 2 may be.

[0082] <Electricity storage device> The energy storage element of this embodiment can be mounted as an energy storage device including an energy storage unit (battery module) configured by assembling a plurality of energy storage elements in a power source for an automobile such as an EV, HEV, or 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 device.

[0083] 2, an energy storage device 100 according to one embodiment of the present invention includes an energy storage element 1 according to one embodiment of the present invention, and a control unit 102 that controls charging and discharging of the energy storage element 1. Specifically, the energy storage device 100 includes an energy storage unit 101 having a plurality of energy storage elements 1, and a control unit 102 that controls charging and discharging of the energy storage elements 1.

[0084] The control unit 102 controls the energy storage element 1 so that it is charged and discharged at a high current density. Specifically, the maximum current density for charging and discharging the energy storage element 1 controlled by the control unit 102 is 6 mA / cm. 2 The maximum current density is 7mA / cm 2 The upper limit of the maximum current density is not particularly limited, but it is preferably 20 mA / cm 2 may be 10 mA / cm 2 Since the energy storage device 100 includes the energy storage element 1 according to one embodiment of the present invention, the current density may be 6 mA / cm 2 Even when charging and discharging are repeatedly performed at a high current density of 6 mA / cm or more, an increase in resistance is suppressed. 2 It is sufficient that the energy storage element 1 can be charged and discharged at the maximum current density above 6 mA / cm. 2 There may be times when energy storage element 1 is charged or discharged at a current density less than 100 kJ / s.

[0085] The energy storage device 100 may include a bus bar (not shown) or the like that electrically connects two or more energy storage elements 1. The energy storage device 100 may also include a state monitoring device (not shown) that monitors the state of one or more energy storage elements 1.

[0086] When this power storage device 100 is mounted on a vehicle 110, as shown in Fig. 2, a control unit 102 and a vehicle control device 111 that controls the engine, motor, drive system, electrical system, etc. are communicatively connected via an in-vehicle communication network such as an in-vehicle LAN or CAN. The control unit 102 and the vehicle control device 111 communicate with each other, and the power storage device 100 is controlled based on information obtained from this communication. As a result, for example, when the vehicle decelerates, drive energy is converted into regenerative energy and charged into the power storage element 1. The power storage device 100 can be mounted as a power source for automobiles such as EVs, HEVs, and PHEVs.

[0087] <Method of manufacturing an energy storage element> The manufacturing method of 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. Specific aspects of the positive electrode, negative electrode, separator, and container are as described above as components of the energy storage element according to one embodiment of the present invention.

[0088] The negative electrode can be prepared, for example, by applying a negative electrode mixture paste to a negative electrode substrate directly or via an intermediate layer, and then drying. After drying, pressing or the like may be performed as necessary. The negative electrode mixture paste contains each component constituting the negative electrode active material layer, such as a negative electrode active material, a conductive agent, a binder, and a thickener. The negative electrode mixture paste usually further contains a dispersion medium.

[0089] The non-aqueous electrolyte solution can be placed in the container by any known method, for example, by injecting the non-aqueous electrolyte solution through an inlet formed in the container and then sealing the inlet.

[0090] 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.

[0091] 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. [Example]

[0092] 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.

[0093] [Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3] (Preparation of positive electrode) LiNi, the positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 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, AB, and PVDF 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.

[0094] (Preparation of negative electrode) A negative electrode mixture paste was prepared by mixing graphite (Gr) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of Gr, SBR, and CMC was 98:1:1 (solid content equivalent). The negative electrode mixture paste was applied to both sides of copper foil as a negative electrode substrate in the application mass shown in Table 1 and dried. Then, a roll press was performed to obtain a negative electrode. Table 1 shows the thicknesses of the negative electrodes and negative electrode active material layers obtained for Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3.

[0095] (electrolyte) A 1.2 mol / dm 3 LiPF6 was dissolved in the electrolyte at a concentration of 1000 ppm to obtain an electrolyte solution.

[0096] (separator) Separators were used with the material, thickness, air permeability, porosity, and thickness change before and after applying a load of 2 MPa at 65°C for 60 seconds, as shown in Table 1. In Table 1, PE stands for polyethylene, and PP stands for polypropylene. PP / PE / PP indicates a three-layer base layer in which PP, PE, and PP are laminated in that order.

[0097] (Battery assembly) A flat, wound electrode assembly was obtained using the positive electrode, negative electrode, and separator. The electrode assembly was placed in a square container, and an electrolyte solution was poured into the container and sealed. This resulted in the energy storage elements of Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3 before charge and discharge.

[0098] (Initial charge / discharge) Each of the obtained energy storage elements before charge / discharge was subjected to constant current charging at a current of 1 C at 25°C, followed by constant voltage charging. The charge was terminated until the current reached 0.05 C. The upper limit charge voltage was the voltage at which the charge depth of the negative electrode reached the value shown in Table 1. This state was defined as SOC 100%. After a 10-minute rest, the element was discharged at a constant current of 1 C to 3 V. This state was defined as SOC 0%. The above charge and discharge process constituted one cycle, and two cycles of initial charge / discharge were performed to obtain energy storage elements of Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3.

[0099] [evaluation] (Thickness Difference Between Charged and Discharged States of Negative Electrode Active Material Layer) Using the above procedure, negative electrodes in the charged state and the discharged state were sampled from Examples 1 to 6, Comparative Examples 1 to 4, and Reference Examples 1 to 3, and the difference in thickness of the negative electrode active material layer between the charged state and the discharged state was measured. The measurement results are shown in Table 1.

[0100] (Increase in DC resistance (DCR) after charge / discharge cycles at high current density) For each of the energy storage elements of the Examples, Comparative Examples, and Reference Examples, the direct current resistance (DCR) increase rate after charge / discharge cycles at a high current density was measured by the following method. (1) High current density charge / discharge cycle test The battery was charged at 60°C with a constant current of 10 C to an SOC of 80%, and then discharged at a constant current of 10 C to an SOC of 20% without a break. This cycle was repeated for 250 hours. The charge and discharge current densities for the cycle were as shown in Table 1. (2) DC resistance (DCR) increase rate at 25°C and -10°C The direct current resistance (DCR) at 25°C was determined by the following method. After the initial charge / discharge and the charge / discharge cycle test at a high current density, the energy storage elements were adjusted to an SOC of 50% at a charging current of 1 C at 25°C, stored in a thermostatic chamber at 25°C for 4 hours, and then discharged for 30 seconds at a constant current of 5 C, 10 C, 15 C, 20 C, or 25 C. After each discharge, the elements were charged at a constant current of 1 C to adjust the SOC to 50%. The voltage 10 seconds after the start of discharge was plotted on the vertical axis and the discharge current on the horizontal axis, and the direct current resistance (DCR) at 25°C was determined from the slope of the resulting straight line. The direct current resistance (DCR) at -10°C was determined by the following method. After the initial charge / discharge and the charge / discharge cycle test at a high current density, the energy storage elements were adjusted to an SOC of 50% at a charging current of 1 C at 25°C, stored in a thermostatic chamber at -10°C for 4 hours, and then discharged for 30 seconds at a constant current of 5 C, 10 C, 15 C, 20 C, or 25 C. After each discharge, constant current charging was performed at a current of 1 C to adjust the SOC to 50%. The voltage 10 seconds after the start of discharge was plotted on the vertical axis and the discharge current on the horizontal axis, and the direct current resistance (DCR) at -10°C was determined from the slope of the resulting line. Then, the direct current resistance (DCR) increase rate [%] after charge-discharge cycles at high current density at 25°C and -10°C was calculated using the following formula. DC resistance (DCR) increase rate after charge-discharge cycling at high current density [%] = (DCR after charge-discharge cycling test at high current density - DCR before charge-discharge cycling test) × 100 The evaluation results are shown in Table 1.

[0101] [Table 1]

[0102] As shown in Examples 1 to 6 and Comparative Examples 1 to 4 in Table 1, in an energy storage device having a separator in which the change in thickness before and after application of a load of 2 MPa for 60 seconds at a temperature of 65°C is 2 μm or more, it is found that an increase in resistance after charge-discharge cycles at a high current density can be suppressed by keeping the difference in thickness between the charged and discharged states in the negative electrode active material layer to 0.6 μm or less. In the energy storage devices of Reference Examples 1 to 3, which include separators whose thickness changed by less than 2 μm before and after application of a load of 2 MPa for 60 seconds at 65°C, the increase in resistance after charge-discharge cycles at high current density was small, regardless of the difference in thickness between the charged and discharged states of the negative electrode active material layer. It is clear that the increase in resistance after charge-discharge cycles at high current density is a problem specific to energy storage devices whose separators whose thickness changed by 2 μm or more before and after application of a load of 2 MPa for 60 seconds at 65°C.

[0103] The above results show that in this energy storage element, when the difference in thickness between the charged and discharged states in the negative electrode active material layer is 0.6 μm or less, an increase in resistance after charge-discharge cycles at a high current density can be suppressed even when a separator is used whose thickness changes by 2 μm or more before and after application of a load of 2 MPa at a temperature of 65°C for 60 seconds. [Explanation of symbols]

[0104] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 100 Electricity storage device 102 Control section 101 Energy storage unit 110 vehicles 111 Vehicle control device

Claims

1. a negative electrode having a negative electrode active material layer; a separator overlaid on the surface of the negative electrode active material layer; Non-aqueous electrolyte It is equipped with the difference in thickness of the negative electrode active material layer between a charged state of SOC 100% and a discharged state of SOC 0% is 0.6 μm or less, the separator has a thickness change of 2 μm or more before and after application of a load of 2 MPa at a temperature of 65° C. for 60 seconds; The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains graphite.

2. The separator has an air permeability of 150 seconds / 100 cm ３ The energy storage element according to claim 1, wherein:

3. 3. The energy storage element according to claim 1, wherein the separator has a porosity of 55% by volume or more and 70% by volume or less.

4. The energy storage element according to any one of claims 1 to 3, which is used as a power source for a hybrid vehicle.

5. The energy storage element according to any one of claims 1 to 4, a control unit that controls charging and discharging of the storage element; Equipped with The maximum current density of charge and discharge in the storage element controlled by the control unit is 6 mA / cm ２ The above is the power storage device.

Images