Power storage element

Abstract

To provide a power storage element in which artificial graphite is used in a negative electrode, the power storage element having low initial resistance and also a low resistance increase rate after charge / discharge cycles.SOLUTION: A power storage element 1 according to an aspect of the present invention includes an electrode body 2 having a positive electrode and a negative electrode, and a container 3 accommodating the electrode body. The negative electrode includes: coated artificial graphite in which at least some of artificial graphite particles are coated with non-graphite carbon; and flaky graphite. A content of the non-graphite carbon in the coated artificial graphite is 0.5% by mass or more, and a content of the flaky graphite with respect to the total content of the coated artificial graphite and the flaky graphite is 10% by mass or more and 30% by mass or less. The container is constrained while sandwiched between restraint members with constant spacing in a thickness direction.SELECTED DRAWING: Figure 1

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, electricity storage elements using electrolytes other than non-aqueous electrolytes, solid electrolytes, etc., are also widely used.

[0003] A typical energy storage device includes an electrode assembly in which a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material are stacked with a separator interposed therebetween. This electrode assembly is housed in a container together with an electrolyte solution and the like to form an energy storage device. Carbon materials such as graphite are widely used as negative electrode active materials. Patent Document 1 describes a nonaqueous electrolyte secondary battery having a negative electrode containing artificial graphite and natural graphite. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-26514 Summary of the Invention [Problem to be solved by the invention]

[0005] Increasing the content of artificial graphite in a negative electrode using graphite is said to have advantages such as extending the life of the energy storage device. However, an energy storage device having a negative electrode using artificial graphite has the disadvantage of high initial resistance. Furthermore, it is desirable for the energy storage device to not only have low initial resistance, but also have a low rate of increase in resistance after charge / discharge cycles.

[0006] An object of the present invention is to provide an electricity storage element using artificial graphite in the negative electrode, which has low initial resistance and a low rate of increase in resistance after charge / discharge cycles. [Means for solving the problem]

[0007] An energy storage element according to one aspect of the present invention comprises an electrode body having a positive electrode and a negative electrode, and a container for accommodating the electrode body, wherein the negative electrode comprises coated artificial graphite in which at least a portion of artificial graphite particles are coated with non-graphitic carbon, and flake graphite, the non-graphitic carbon content in the coated artificial graphite is 0.5% by mass or more, the flake graphite content relative to the total content of the coated artificial graphite and the flake graphite is 10% by mass or more and 30% by mass or less, and the container is restrained in a state where it is sandwiched in the thickness direction by restraining members that maintain a constant gap between them. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to provide an energy storage element using artificial graphite in a negative electrode, which has low initial resistance and a low rate of increase in resistance after charge-discharge cycles. [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] (1) An energy storage element according to one aspect of the present invention comprises an electrode body having a positive electrode and a negative electrode, and a container for accommodating the electrode body, wherein the negative electrode comprises coated artificial graphite in which at least a portion of the particles of artificial graphite are coated with non-graphitic carbon, and flake graphite, the content of the non-graphitic carbon in the coated artificial graphite being 0.5% by mass or more, the content of the flake graphite relative to the total content of the coated artificial graphite and the flake graphite being 10% by mass or more and 30% by mass or less, and the container is restrained in a state where it is sandwiched in the thickness direction by restraining members that maintain a constant gap between them.

[0012] The energy storage element described in (1) above uses artificial graphite for the negative electrode, and has low initial resistance and a low rate of increase in resistance after charge-discharge cycling. The reason for this effect is unclear, but the following reason is presumed. By coating at least a portion of the surface of the artificial graphite particles with a sufficient amount of non-graphitic carbon, the initial resistance of an energy storage element using such coated artificial graphite for the negative electrode is low. Meanwhile, coating the surface with a sufficient amount of non-graphitic carbon hardens the coated artificial graphite. Therefore, a negative electrode using such coated artificial graphite is more susceptible to breakage of the electron conduction path due to repeated expansion and contraction during charge-discharge cycles than a negative electrode using artificial graphite that is uncoated or coated with a small amount of non-graphitic carbon. For this reason, it is believed that an energy storage element using coated artificial graphite for the negative electrode is more likely to have a high rate of increase in resistance after charge-discharge cycling. Therefore, by incorporating a predetermined amount of flake graphite into the negative electrode together with the coated artificial graphite, a negative electrode with relatively low porosity and sufficient electron conduction paths can be formed by pressing or the like. Such a negative electrode makes it easier to maintain the electron conduction paths even after repeated charge and discharge. Furthermore, by restraining the container of the energy storage device in a state sandwiched in the thickness direction by restraining members that maintain a constant gap, the negative electrode can be maintained in a sufficiently compressed state even after repeated charge and discharge, thereby more fully maintaining the electron conduction paths even after repeated charge and discharge. For these reasons, it is presumed that the energy storage device described in (1) above has low initial resistance and a low rate of increase in resistance after charge and discharge cycles.

[0013] "Graphite" refers to a graphite material that has an average lattice spacing (d 002 "Non-graphitic carbon" refers to a carbon material having an average lattice spacing (d ) of the (002) plane determined by X-ray diffraction before charge / discharge or in a discharged state. 002 ) is 0.340 nm or more and 0.420 nm or less. Here, the "discharged state" of a carbon material serving as a negative electrode active material refers to a state in which the carbon material serving as a negative electrode active material is discharged so that charge-transporting ions (such as lithium ions) that can be absorbed and released during charging and discharging are sufficiently released from the carbon material serving as a negative electrode active material. For example, this refers to a state in which the open circuit voltage is 0.7 V or more in a half cell using a negative electrode containing a carbon material as a negative electrode active material as a working electrode and metallic lithium as a counter electrode.

[0014] "Artificial graphite" refers to graphite that has been artificially produced. Artificial graphite may be one in which only two peaks appear 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. In the case of natural graphite, a total of four peaks appear in a diffraction angle 2θ range of 40° to 50°: two peaks resulting from the hexagonal crystal structure and two peaks resulting from the rhombohedral crystal structure. In contrast, in the case of artificial graphite, only two peaks resulting from the hexagonal crystal structure are generally considered to appear.

[0015] X-ray diffraction measurements using CuKα radiation on carbon materials (coated artificial graphite, flake graphite, etc.) are performed on the carbon material before charging and discharging, or on the carbon material contained in the negative electrode of a storage element after processing using the following procedure. First, the storage element is discharged at a constant current of 0.1 C to the discharge end voltage during normal use. Then, it is disassembled, the negative electrode is removed, and washed with dimethyl carbonate. A test battery is assembled using the washed negative electrode as the working electrode and metallic lithium as the counter electrode. For this test battery, the closed circuit potential of the negative electrode is measured at a current of 50 mA / g per mass of negative electrode active material until the closed circuit potential is 2.0 V vs. Li / Li. +The battery is then subjected to constant-current discharge until the temperature reaches a constant value, adjusting the carbon material to a fully discharged state (a state in which the charge-transport ions involved in the charge-discharge reaction have been released). The battery is then disassembled again, and the negative electrode is removed. The removed negative electrode is then washed with dimethyl carbonate. The negative electrode active material layer containing the carbon material is then peeled from the negative electrode substrate, and the negative electrode active material layer is washed with water to remove other water-soluble components. The water-washed negative electrode active material layer is then immersed in an acid or alkaline solution to remove metals derived from the negative electrode substrate and the SEI (solid electrolyte interface) coating, etc., followed by washing with water and drying under reduced pressure to obtain the carbon material (coated artificial graphite, flake graphite, etc.). The disassembly of the energy storage device and test battery is carried out in an argon atmosphere with a dew point of -60°C or below. Here, "normal use" refers to the use of the energy storage device under the recommended or specified charge-discharge conditions for the energy storage device.

[0016] X-ray diffraction measurements using CuKα radiation are performed according to the following procedure. The carbon material to be measured is packed into an airtight X-ray diffraction sample holder under an argon atmosphere with a dew point of -60°C or lower. Powder X-ray diffraction measurements are performed using an X-ray diffractometer (Rigaku's "MiniFlex II"). The radiation source is CuKα radiation, the tube voltage is 30 kV, and the tube current is 15 mA. Diffracted X-rays are passed through a 30 μm-thick Kβ filter and detected by a high-speed one-dimensional detector (model number: D / teX Ultra 2). The sampling width is 0.01°, the scan speed is 5° / min, the divergence slit width is 0.625°, the receiving slit width is 13 mm (open), and the scattering slit width is 8 mm.

[0017] The non-graphitic carbon content of coated artificial graphite is measured by controlled atmosphere thermogravimetry-differential thermal analysis (TG-DTA). Specifically, the measurement is performed as follows. Approximately 10 mg of the coated artificial graphite sample is weighed to an accuracy of 0.01 mg and introduced into a thermogravimetric analyzer. A mixed gas of water vapor and nitrogen, adjusted to a relative humidity of 65%, is introduced into the measurement atmosphere at a flow rate of 300 ml / min. The upper limit of the measurement temperature range is 1000°C, and the temperature is increased from room temperature to 700°C at a rate of 10°C / min. After reaching 700°C, the temperature is increased so that the mass change rate is 0.001% / s. The non-graphitic carbon content is calculated by analyzing the resulting TG and DTA curves. The specific analytical method is as follows. This analytical method is based on the fact that the gasification reaction between carbon and water vapor during heating occurs at a lower temperature for non-graphitic carbon than for artificial graphite, and that this gasification reaction is endothermic. The weight of the sample (coated artificial graphite) at 850°C is defined as W0. The weight at the temperature (T1) corresponding to the behavior at which the first weight loss after 850°C begins is defined as W1, and the weight at the temperature (T2) corresponding to the behavior at which the weight loss converges is defined as W2. The amount of gasification (ΔW) is calculated as ΔW = W1 - W2. In this specification, the "temperature corresponding to the behavior at which weight loss begins" and the "temperature corresponding to the behavior at which the weight loss converges" are calculated by a standard method using diagrams. That is, in a TG curve represented by a linear graph with temperature on the horizontal axis and weight on the vertical axis, the "temperature corresponding to the behavior at which weight loss begins" is determined from the intersection of a line extrapolated from the TG curve just before the onset of weight loss with a line extrapolated from the TG curve at the portion where the rate of weight loss is maximum, and the "temperature corresponding to the behavior at which weight loss converges" is determined from the intersection of the line extrapolated from the TG curve at the portion where the rate of weight loss is maximum with a line extrapolated from the TG curve at the portion where the rate of weight loss is minimum at a temperature higher than the portion where the rate of weight loss is maximum. However, if the amount of gasification of amorphous carbon is small and it is difficult to determine the "temperature corresponding to the behavior at which weight loss converges" from the TG curve, the temperature can be determined from the DTA curve.In this case, the temperature at which the rate of change in DTA with respect to the amount of temperature change becomes steeper above the "temperature corresponding to the behavior at which weight loss begins" is defined as the first inflection point, and the temperature at which the rate of change in DTA with respect to the amount of temperature change becomes gentler above the first inflection point is defined as the second inflection point, and this second inflection point is determined as the "temperature corresponding to the behavior at which the weight loss converges." The content of non-graphitic carbon in the coated artificial graphite is calculated using the following formula. Non-graphitic carbon content in coated artificial graphite. =(ΔW / W0)×100 ={(W1-W2) / W0}×100

[0018] (2) In the energy storage element described above in (1), the content of the non-graphitic carbon in the coated artificial graphite may be 1.5% by mass or more and 2.5% by mass or less.

[0019] According to the energy storage element described in (2) above, the content of non-graphitic carbon in the coated artificial graphite is within an appropriate range, which makes it possible to further reduce the rate of increase in resistance after charge-discharge cycles.

[0020] Hereinafter, an energy storage element, an energy storage device, a method for manufacturing an energy storage element according to one embodiment of the present invention, and 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.

[0021] <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 exists in a state in which it is impregnated into 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.

[0022] (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.

[0023] 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 -2 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-H-4160 (2006).

[0024] 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 energy storage element.

[0025] 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.

[0026] 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.

[0027] 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, 0<1-x-γ), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β). Lithium transition metal composite oxides with spinel crystal structures include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0028] 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).

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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 energy storage element can be increased.

[0033] 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.

[0034] The content of the binder 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 binder content within this range, the positive electrode active material can be stably maintained.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] The positive electrode can be produced, for example, by applying a positive electrode mixture paste to a positive electrode substrate directly or via an intermediate layer, and then drying. After drying, pressing or the like may be performed as necessary. The positive electrode mixture paste contains each component constituting the positive electrode active material layer, including a positive electrode active material and optional components such as a conductive agent and a binder. The positive electrode mixture paste usually further contains a dispersion medium.

[0039] (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.

[0040] 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.

[0041] 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 energy storage element.

[0042] The negative electrode active material layer contains coated artificial graphite and flake graphite. The coated artificial graphite is a component that functions as a negative electrode active material. The flake graphite is a component that functions as a conductive agent. The flake graphite may also function as a negative electrode active material. The negative electrode active material layer contains optional components such as other negative electrode active materials, other conductive agents, binders, thickeners, and fillers, as necessary. The other conductive agents, binders, thickeners, and fillers can be selected from the materials exemplified for the positive electrode above.

[0043] Coated artificial graphite has a structure in which at least a portion of the artificial graphite particles is coated with non-graphitic carbon. By using coated artificial graphite with a non-graphitic carbon content of 0.5 mass% or more, the initial resistance of the energy storage element can be reduced.

[0044] Examples of non-graphitic carbon that coats at least a portion of the artificial graphite particles serving as the core material include resin-derived materials, pitch-derived materials, petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials, with pitch-derived materials being preferred. For example, by coating artificial graphite particles with pitch as a coating material and then firing the resulting mixture, coated artificial graphite can be obtained, in which at least a portion of the artificial graphite particles are coated with non-graphitic carbon. The method for producing coated artificial graphite is not limited to the above, and it can be produced by known methods. It is also possible to use coated artificial graphite that is produced without intentionally carrying out a coating treatment and has a non-graphitic carbon content of 0.5% by mass or more due to surface deterioration or the like. However, it is preferable to use coated artificial graphite obtained through a coating treatment, from the viewpoints of being well and uniformly coated with non-graphitic carbon and having sufficiently low resistance. Artificial graphite can also be produced by known methods or purchased.

[0045] The lower limit of the non-graphitic carbon content in the coated artificial graphite is 0.5% by mass, preferably 1.0% by mass, more preferably 1.5% by mass, even more preferably 2.0% by mass, and may be 2.4% by mass. Having the non-graphitic carbon content in the coated artificial graphite at or above the lower limit makes it possible to lower the initial resistance of the energy storage device, for example. The upper limit of the non-graphitic carbon content in the coated artificial graphite is, for example, preferably 5% by mass, more preferably 3% by mass, even more preferably 2.5% by mass, and may be 2.4% by mass. Having the non-graphitic carbon content in the coated artificial graphite at or below the upper limit makes it possible to further lower the rate of increase in resistance of the energy storage device after charge-discharge cycling. The non-graphitic carbon content in the coated artificial graphite may be within the range between any of the above lower limits and any of the above upper limits.

[0046] The lower limit of the R value in the Raman spectrum of the coated artificial graphite is preferably 0.15, more preferably 0.18, even more preferably 0.20, and even more preferably 0.25. The upper limit of the R value is preferably 0.8, more preferably 0.6, even more preferably 0.5, and even more preferably 0.4. By keeping the R value within the above range, it is possible to further reduce the initial resistance. The R value is the value at 1500 cm in the Raman spectrum. -1 More than 1700cm -1 The maximum value in the range below I G For 1200cm -1 More than 1450cm -1 The maximum value in the range below I D The ratio (I D / I G ) in the Raman spectrum of carbon materials. -1 More than 1700cm -1 The peaks appearing in the following range are called G bands and are considered to be peaks derived from the crystalline structure of graphite. -1 More than 1450cm -1 The peak appearing in the range below is called a D band and is a peak derived from a defective structure of carbon. When the R value of coated artificial graphite is within the above range, it is presumed that an appropriate amount of defective structure of carbon is present, which results in good electronic conductivity and the like.

[0047] The "Raman spectrum" is a spectrum obtained by Raman spectroscopy measurement using Horiba's "LabRAM HR Revolution" under the conditions of an excitation laser wavelength of 532 nm (YAG laser) and a grating of 600 gr / mm, and is corrected by removing the baseline. The specific measurement and correction methods are explained below. First, -1 From 4000cm -1 Raman spectroscopy is performed in the range of 1000 cm to obtain a spectrum. -1 The first minimum at the highest wavelength in the range below and 1800 cm -1The line connecting the second minimum value, which exists on the lowest wavelength side in the above range, is taken as the baseline. However, minimum values ​​caused by noise are not included in the first and second minimum values. The spectrum obtained by the above measurement is corrected by removing the above baseline. Based on the corrected spectrum excluding this baseline, the maximum value I G and maximum value I D The Raman spectrometer software Labspec 6 (Horiba, Ltd.) may also be used to correct the background. If the S / N ratio is low (noise is high), noise may be removed using Labspec 6.

[0048] The coated artificial graphite is usually in a particulate form. The average particle size of the coated artificial graphite is, for example, preferably 1 μm or more and 30 μm or less, and more preferably 2 μm or more and 20 μm or less. By setting the average particle size of the coated artificial graphite to the above lower limit or more, the production and handling of the coated artificial graphite becomes easier. By setting the average particle size of the coated artificial graphite to the above upper limit or less, a large number of electron conduction paths are more easily formed, which tends to reduce the initial resistance and the rate of increase in resistance after charge / discharge cycles. In order to obtain coated artificial graphite 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 above.

[0049] The lower limit of the content of the coated artificial graphite in the negative electrode active material layer is preferably 50% by mass, more preferably 60% by mass, even more preferably 70% by mass, and even more preferably 75% by mass. The upper limit of the content of the coated artificial graphite is preferably 90% by mass, more preferably 85% by mass, and even more preferably 82% by mass. By setting the content of the coated artificial graphite within the above range, both high energy density and manufacturability of the negative electrode active material layer can be achieved. Furthermore, by setting the content of the coated artificial graphite to be equal to or greater than the above lower limit, the advantages of the artificial graphite itself, such as a longer life, can be fully exhibited. On the other hand, by setting the content of the coated artificial graphite to be equal to or less than the above upper limit, a high-density negative electrode with low porosity can be formed by pressing or the like. The content of the coated artificial graphite in the negative electrode active material layer can be set within the range between any of the above lower limits and any of the above upper limits.

[0050] The negative electrode active material layer may contain other negative electrode active materials besides the coated artificial graphite. Examples of other negative electrode active materials include carbon materials other than the coated artificial graphite, such as natural graphite and uncoated artificial graphite; metallic lithium; 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 include titanium-containing oxides such as TiNb2O7; polyphosphate compounds; and silicon carbide. When other negative electrode active materials are used in combination, natural graphite is preferred as the other negative electrode active material. However, the content of coated artificial graphite relative to all negative electrode active materials is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 99% by mass or more, even more preferably 99.9% by mass or more, and particularly preferably 100% by mass. In this way, when the negative electrode active material is essentially composed of coated artificial graphite, the effects of low initial resistance of the energy storage element and a low rate of increase in resistance after charge-discharge cycling can be particularly pronounced. Note that the above "all negative electrode active materials" does not include flake graphite, which can also function as a negative electrode active material.

[0051] Flake graphite is graphite that has a scaly appearance. Both natural graphite and artificial graphite can be used as the scaly graphite. The scaly graphite may be obtained by graphitizing raw materials such as coke and resin carbide through firing to form a scaly shape. Furthermore, the scaly shape may be thin flakes like fish scales, and may be curved or have rounded particle ends. Thin graphite flakes in shapes similar to circles, ellipses, or polygons may also be used.

[0052] The average particle size of the flake graphite is preferably 1 μm or more and 15 μm or less. The average aspect ratio of the flake graphite is preferably 3.0 or more. The average aspect ratio is the average value of the aspect ratios of any 10 flake graphite particles. The aspect ratio refers to the ratio (major axis / minor axis) of the length of the major axis (longest part) to the length (thickness) of the minor axis perpendicular to the major axis, as observed in a cross-sectional SEM (Scanning Electron Microscope) image. Here, "cross-sectional view" refers to a cross-sectional view when the flake graphite is present in a layer, or to a side view when the flake graphite is placed on a horizontal surface, when the flake graphite is present alone.

[0053] It is preferable that the flake graphite has high crystallinity. High crystallinity results in softness, which improves the pressability of the negative electrode active material layer (ease of densification by pressing), making it easier to obtain a negative electrode active material layer with low porosity. For example, the average lattice spacing (d 002 ) is preferably 0.337 nm or less.

[0054] The content of flake graphite relative to the total content of the coated artificial graphite and flake graphite is 10% by mass or more and 30% by mass or less, preferably 12% by mass or more and 28% by mass or less, more preferably 15% by mass or more and 25% by mass or less, and even more preferably 17% by mass or more and 23% by mass or less. By setting the content of flake graphite relative to the total content of the coated artificial graphite and flake graphite within the above range, the resistance increase rate after charge-discharge cycles of the energy storage element can be reduced. Furthermore, by relatively increasing the content of flake graphite relative to the total content of the coated artificial graphite and flake graphite (for example, more than 10% by mass), a high-density negative electrode with particularly low porosity can be formed by pressing or the like.

[0055] The content of flake graphite in the negative electrode active material layer is preferably 5% by mass or more and 30% by mass or less, more preferably 9% by mass or more and 28% by mass or less, even more preferably 12% by mass or more and 25% by mass or less, and even more preferably 15% by mass or more and 23% by mass or less. By setting the content of flake graphite in the negative electrode active material layer within the above range, the resistance increase rate after charge-discharge cycles of the energy storage device can be further reduced. Furthermore, by making the content of flake graphite in the negative electrode active material layer relatively large, a high-density negative electrode with particularly low porosity can be formed by pressing or the like.

[0056] The negative electrode active material layer may contain a conductive agent other than flake graphite. However, the content of flake graphite relative to all conductive agents is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 99% by mass or more, even more preferably 99.9% by mass or more, and particularly preferably 100% by mass. In this way, when the conductive agent is essentially composed of flake graphite, the effects of low initial resistance of the energy storage element and low rate of increase in resistance after charge-discharge cycles can be particularly pronounced.

[0057] The content of the binder in the negative electrode active material layer is preferably 0.1% by mass to 10% by mass, more preferably 0.3% by mass to 5% by mass, even more preferably 0.5% by mass to 3% by mass, and even more preferably 1.0% by mass to 2.5% by mass. By setting the binder content within the above range, it is possible to stably hold the coated artificial graphite, flake graphite, etc.

[0058] The content of the thickener in the negative electrode active material layer is, for example, preferably from 0.1% by mass to 6% by mass, more preferably from 0.5% by mass to 3% by mass, and may be 2% by mass or less or 1.5% by mass or less.

[0059] When the negative electrode active material layer contains a filler, the content of the filler in the negative electrode active material layer is, for example, 0.1% by mass or more and 5% by mass or less, and may be 1% by mass or less, 0.1% by mass or less, or even 0% by mass.

[0060] 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 coated artificial graphite, scaly graphite, other negative electrode active materials, other conductive agents, binders, thickeners, and fillers.

[0061] The upper limit of the porosity of the negative electrode active material layer is preferably 40%, more preferably 35%, and even more preferably 30%. By setting the porosity of the negative electrode active material layer to the above upper limit or less, the initial resistance of the energy storage device and the resistance increase rate after charge / discharge cycling can be further reduced, and the energy density can also be increased. The lower limit of the porosity of the negative electrode active material layer may be, for example, 20% or 25%. The porosity of the negative electrode active material layer can be set within the range between any of the above lower limits and any of the above upper limits. The porosity of the negative electrode active material layer can be adjusted by the content of coated artificial graphite, flake graphite, etc., the average particle size, whether or not pressing is used during production, the pressing pressure, etc.

[0062] The "porosity (%)" of the negative electrode active material layer is calculated by the formula (1-V2 / V1) x 100, where V1 is the apparent volume of the negative electrode active material layer (volume including voids) and V2 is the sum of the actual volumes of the materials constituting the negative electrode active material layer. V2, the sum of the actual volumes of the materials constituting the negative electrode active material layer, can be calculated from the content of each material in the negative electrode active material layer and the true density of each material.

[0063] The negative electrode can be produced by, for example, 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 coated artificial graphite, flake graphite, and an optional binder. The negative electrode mixture paste usually further contains a dispersion medium.

[0064] (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 shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of non-aqueous electrolyte retention. As the material for the substrate layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide and aramid are preferred from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used as the substrate layer of the separator.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] (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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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. 3More 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.

[0076] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the 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, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, and cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, and sulfur dioxide. Examples of the additives include dimethyl ether, 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, and lithium difluorophosphate. These additives may be used alone or in combination of two or more.

[0077] 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.

[0078] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0079] 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, nitride solid electrolytes, and polymer solid electrolytes.

[0080] Examples of sulfide solid electrolytes include Li2S-P2S5, LiI-Li2S-P2S5, and Li 10 Ge-P2S 12 etc.

[0081] 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.

[0082] 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.

[0083] The material of the container 3 is not particularly limited, and may be, for example, a container made of metal (aluminum, stainless steel, etc.), resin, etc. As will be described in detail later, the container 3 in the energy storage device 1 according to one embodiment of the present invention is restrained in a state where it is sandwiched in the thickness direction (Y direction) by restraining members 85 (not shown in FIG. 1; see FIG. 2) that are spaced apart at a constant interval.

[0084] <Electricity storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) comprising a plurality of energy storage elements 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 source for power storage, 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.

[0085] In an energy storage element according to one embodiment of the present invention, the container is sandwiched and restrained in the thickness direction by restraining members that maintain a constant gap between them. The restraining members are members that suppress expansion of the container in the thickness direction. For example, an energy storage device 80 shown in FIG. 2 includes a plurality of energy storage elements 1 whose containers 3 are restrained in this manner. The energy storage elements 1 in the energy storage device 80 are electrically connected to each other by bus bars (not shown). The plurality of energy storage elements 1 are sandwiched between a pair of metal plates 82 with spacers 81 interposed therebetween and arranged with no gaps in the thickness direction (Y direction). The distance D between the pair of metal plates 82 is fixed constant by bolts 83 and nuts 84. In the energy storage device 80, the metal plates 82, the bolts 83, and the nuts 84 form a restraining member 85. The restraining member 85 (the metal plates 82, the bolts 83, and the nuts 84) has a strength sufficient to prevent substantial deformation or breakage due to the force of expansion of the energy storage elements 1 in the thickness direction associated with charge / discharge cycles. This strength can be adjusted by the material, size, etc. of each component of the restraining member 85. For example, the metal plate 82 is usually a member having higher rigidity than the container 3 .

[0086] In this manner, each container 3 of the multiple energy storage elements 1 is sandwiched and restrained in the thickness direction by restraining members 85 that maintain a constant distance D. That is, the container 3 of each energy storage element 1 is prevented from expanding in the thickness direction by restraining members 85. In energy storage device 80, because each container 3 of the multiple energy storage elements 1 is restrained by restraining members 85 in this manner, each energy storage element 1 has a low rate of increase in resistance after charge / discharge cycles.

[0087] In the case of a container 3 that houses a flat, wound electrode assembly 2 as shown in Fig. 1, the thickness direction of the container is the direction perpendicular to the flat surface 6 of the electrode assembly 2 (Y direction). In the case of a container that houses a laminated electrode assembly, the thickness direction is also the direction perpendicular to the flat surface of the electrode assembly.

[0088] The spacer 81 may be a member, such as a rubber plate, that can be compressed in accordance with the expansion of the energy storage element 1 in the thickness direction, or a member, such as a metal plate, that does not substantially undergo compression. Also, unlike the embodiment shown in FIG. 2, a plurality of energy storage elements 1 may be arranged without gaps in the thickness direction without using spacers 81, or only one energy storage element may be restrained by a restraining member. That is, an embodiment in which the container is restrained in a state where it is sandwiched in the thickness direction by restraining members that maintain a constant gap therebetween includes, for example, [1] A configuration in which multiple energy storage elements are arranged without gaps in the thickness direction, either directly or via spacers, and are restrained by a restraining member. [2] A configuration in which one storage element is restrained by a restraining member, either directly or via a spacer. etc.

[0089] Furthermore, the container of the energy storage element may be restrained by a restraining member other than that shown in Fig. 2. As long as the container is restrained so as to suppress expansion in its thickness direction, the container may not be pressed by the restraining member, for example, in the initial stage. Note that even if the container is not pressed by the restraining member in the initial stage, when the energy storage element expands due to expansion of the electrode body or gas generation in the energy storage element as a result of repeated charging and discharging, and the container comes into contact with the restraining member, the container will be pressed by the restraining member because the spacing between the restraining members is maintained constant.

[0090] In one embodiment of the present invention, the electrode assembly in the energy storage device preferably contacts the inner surface of the container directly or via another member, at least in the charged state. In such a case, expansion of the electrode assembly caused by repeated charge and discharge is effectively suppressed by the restraining member, thereby further reducing the rate of increase in resistance after charge and discharge cycles of the energy storage device. Examples of such other members include an insulating film that wraps the electrode assembly. The electrode assembly in the energy storage device may contact the inner surface of the container directly or via another member, throughout the entire process from the charged state to the discharged state. Whether the electrode assembly and the container are in contact directly or via another member can be confirmed, for example, by X-ray computed tomography (CT).

[0091] <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, housing the electrode assembly and the non-aqueous electrolyte in a container, and constraining the container with a constraining member. 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.

[0092] 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 can be poured into an inlet formed in the container, and the inlet can then be sealed. The container can be restrained by a restraining member, for example, by using a metal plate 82, a bolt 83, and a nut 84 as a restraining member 85 as shown in FIG. 2 to restrain the container 3.

[0093] <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.

[0094] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (e.g., a lithium ion secondary battery) has been described, but the type, shape, size, capacity, etc. of the energy storage element are arbitrary. The present invention can also be applied to various secondary batteries and capacitors such as electric double layer capacitors and lithium ion capacitors. Furthermore, the energy storage element of the present invention can also be applied to energy storage elements other than non-aqueous electrolyte energy storage elements, such as energy storage elements using an electrolytic solution containing water.

[0095] 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]

[0096] 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.

[0097] [Example 1] (Preparation of negative electrode) A negative electrode mixture paste was prepared by mixing coated artificial graphite, flake graphite, SBR as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of graphite (coated artificial graphite and flake graphite), SBR, and CMC was 97:2:1 in terms of solid content. The mass ratio of coated artificial graphite to flake graphite was 80:20 in terms of solid content. The negative electrode mixture paste was applied to both sides of copper foil as a negative electrode substrate and dried. Then, a roll press was performed to obtain a negative electrode. The coated artificial graphite was obtained by coating artificial graphite with pitch as a coating material and then firing it. The non-graphitic carbon content of the coated artificial graphite determined by the above-mentioned method was 0.56 mass%. The R value based on the Raman spectrum measured by the above-mentioned method was 0.192.

[0098] (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, AB, and PVDF was 93:4:3 in terms of solid content. 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.

[0099] (Preparation of non-aqueous electrolyte) A 1.2 mol / dm 3 A non-aqueous electrolyte was prepared by dissolving LiPF6 in the solution at a concentration of 1000 ppm.

[0100] (Fabrication of energy storage element) The positive electrode and the negative electrode were stacked with a separator interposed therebetween and wound to produce a flat, wound electrode assembly. A polyolefin microporous membrane was used as the separator. The electrode assembly was placed in a rectangular container, the nonaqueous electrolyte was poured into the container, and the container was sealed. The container was then clamped in the thickness direction by restraining members with a constant gap between them to obtain the energy storage element of Example 1. Specifically, the restraining members were composed of a metal plate, bolts, and nuts, as shown in FIG. 2, and one energy storage element was restrained by these restraining members. The metal plate had a sufficient thickness to prevent deformation even with expansion of the energy storage element in the thickness direction. In this restraining method, even if the container of the energy storage element expands in the thickness direction during charge and discharge, the thickness of the energy storage element (container) remains constant, and the pressure applied to the energy storage element (container) by the restraining members increases. This restraining method is referred to as "fixed size / non-constant pressure" in Table 1.

[0101] [Examples 2 to 3, Comparative Examples 1 to 7] The energy storage elements of Examples 2 to 3 and Comparative Examples 1 to 7 were obtained in the same manner as in Example 1, except that the content and R value of non-graphitic carbon in the coated artificial graphite, the mass ratio of the coated artificial graphite to the flake graphite, and the restraint method were as shown in Table 1. The content of non-graphitic carbon in the coated artificial graphite was adjusted by the amount of pitch used for coating. For the case with a non-graphitic carbon content of 0.35 mass%, the obtained artificial graphite was used as is. In Comparative Examples 3 to 7, the energy storage element was inserted into a metal case that served as a restraining member to restrain the container. This metal case was made of a thin metal plate that deformed in response to the expansion of the energy storage element. In this restraining method, when the energy storage element container expands in the thickness direction during charging and discharging, the metal case also deforms. As a result, the shape of the metal case (i.e., the spacing between the restraining members that restrain the container) cannot be maintained constant, and the thickness of the energy storage element (container) increases. Furthermore, when the energy storage element container expands in the thickness direction during charging and discharging, the pressure applied to the energy storage element (container) by the restraining member also increases. This type of restraining method is referred to as "non-constant size / non-constant pressure" in Table 1.

[0102] (Initial charge / discharge) Each of the obtained energy storage elements was charged at a constant current of 1.0 C to 4.25 V in a temperature environment of 25°C, and then charged at a constant voltage of 4.25 V. The charging was terminated when the total charging time reached 3 hours. After a 10-minute pause, the element was discharged at a constant current of 1.0 C to 2.75 V, followed by another 10-minute pause. These charge and discharge steps constitute one cycle, and two cycles were performed.

[0103] (Initial DC resistance) Next, each storage element was charged at a constant current of 1.0 C in a temperature environment of 25°C, and the state of charge (SOC) was adjusted to 50%. The storage element was discharged for 60 seconds at a constant current of 0.2 C, 0.5 C, or 1.0 C. After each discharge, the element was charged at a constant current of 0.2 C to adjust the SOC to 50%. The relationship between the current for each charge and the voltage 10 seconds after the start of discharge was plotted, and the initial direct current resistance (initial DCR) was calculated from the slope of the straight line obtained from the plot of the three points. The initial DC resistances thus determined are shown in Table 1 as relative values, with the value of an energy storage element using the same restraint method and the obtained artificial graphite as is as the reference (100.0%). That is, the initial DCRs of Examples 1 to 3 and Comparative Examples 1 and 2 in Table 1 are relative values ​​with the initial DCR of Comparative Example 1 as the reference, and the initial DCRs of Comparative Examples 3 to 7 are relative values ​​with the initial DCR of Comparative Example 3 as the reference.

[0104] (Resistance increase rate after charge / discharge cycles) Next, each energy storage device was stored in a thermostatic chamber at 45°C for 4 hours, and then subjected to constant-current charging at a charging current of 1.0C up to 4.25V, followed by constant-voltage charging at 4.25V. The charging was terminated when the total charging time reached 3 hours. After a 10-minute pause, the device was discharged at a constant current of 1.0C down to 2.75V, followed by another 10-minute pause. These charge and discharge cycles were counted as one cycle, and 1,200 cycles were performed. Then, for each energy storage element, the DC resistance after charge-discharge cycling was determined under the same conditions as for the initial DC resistance described above. The percentage of the increase in DC resistance after charge-discharge cycling from the initial DC resistance was calculated as the resistance increase rate after charge-discharge cycling (post-cycle DCR increase rate). The calculated post-cycle DCR increase rates are shown in Table 1.

[0105] [Table 1]

[0106] As can be seen from a comparison between Examples 1 to 3 and Comparative Examples 1 and 2, as well as a comparison between Comparative Examples 3 to 7, by coating at least a portion of the surface of the artificial graphite with non-graphitic carbon and setting the content of non-graphitic carbon in the coated artificial graphite to 0.5 mass% or more, the initial DC resistance was reduced while the resistance increase rate after charge-discharge cycling was increased. In contrast, as in the energy storage elements of Examples 1 to 3, by incorporating a predetermined amount of flake graphite together with the coated artificial graphite in the negative electrode and constraining the container at a constant size but not at a constant pressure, the resistance increase rate after charge-discharge cycling could be suppressed to 15% or less. Thus, the energy storage elements of Examples 1 to 3, which use artificial graphite in the negative electrode, were able to exhibit the effects of low initial resistance and low resistance increase rate after charge-discharge cycling.

[0107] [Reference example 1] (Preparation of negative electrode) A negative electrode mixture paste was prepared by mixing coated artificial graphite (non-graphitic carbon content 0.35% by mass), natural graphite, flake graphite, SBR as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The mass ratio of graphite (artificial graphite, natural graphite, and flake graphite), SBR, and CMC was 97:2:1 in terms of solid content. The mass ratio of artificial graphite to natural graphite and flake graphite was 60:20:20 in terms of solid content. The negative electrode mixture paste was applied to both sides of copper foil as a negative electrode substrate and dried. Thereafter, a 1.0 t / cm 2 or 2.0t / cm 2 The negative electrode of Reference Example 1 was obtained by roll pressing under a pressure of 1000 kJ / cm.

[0108] [Reference Examples 2 to 9] The negative electrodes of Reference Examples 2 to 9 were obtained in the same manner as Reference Example 1, except that the content of non-graphitic carbon in the coated artificial graphite and the mass ratio of the coated artificial graphite to natural graphite and flake graphite were as shown in Table 2.

[0109] (Porosity measurement) For each negative electrode obtained, the unpressed one, 1.0 t / cm 2 and those pressed at a pressure of 2.0t / cm 2 The porosity of each negative electrode active material layer was measured by the method described above. The measurement results are shown in Table 2.

[0110] [Table 2]

[0111] As shown in Table 2, regardless of the content of non-graphitic carbon in the coated artificial graphite, increasing the mass ratio of the coated artificial graphite tended to decrease the pressability and make it difficult to obtain a negative electrode active material layer with low porosity. In particular, in the case of coated artificial graphite with a high content of non-graphitic carbon, as in Reference Examples 6 to 9, the pressability decreased significantly when the mass ratio of the coated artificial graphite was increased, and the porosity of the negative electrode active material layer was unlikely to decrease even when pressed at a high pressure (particularly when Reference Example 9 was pressed at a pressure of 2.0 t / cm 2 (See the graphite pressed under a pressure of 0.05). When using coated artificial graphite with a high content of non-graphitic carbon, increasing the mass ratio of flake graphite significantly improves the pressing property, making it easier to obtain a negative electrode active material layer with low porosity. [Industrial Applicability]

[0112] 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]

[0113] 1. Energy storage element 2 Electrode body 3 containers 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 6 flat surface 80 Electricity storage device 81 Spacer 82 Metal plate 83 volts 84 Nut 85 Restraining member D interval

Claims

1. An electrode assembly having a positive electrode and a negative electrode, and a container for accommodating the electrode assembly, the negative electrode includes coated artificial graphite in which at least a portion of particles of artificial graphite are coated with non-graphitic carbon, and flake graphite; the content of non-graphitic carbon in the coated artificial graphite is 0.5% by mass or more; the content of the flake graphite relative to the total content of the coated artificial graphite and the flake graphite is 10% by mass or more and 30% by mass or less, The energy storage element, wherein the container is held in a thickness direction by restraining members that maintain a constant gap between the container and the restraining members.

2. 2. The energy storage element according to claim 1, wherein the content of the non-graphitic carbon in the coated artificial graphite is 1.5% by mass or more and 2.5% by mass or less.

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