WO2016067428A1 - Lithium ion battery and method for manufacturing same - Google Patents

Lithium ion battery and method for manufacturing same Download PDF

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Publication number
WO2016067428A1
WO2016067428A1 PCT/JP2014/078979 JP2014078979W WO2016067428A1 WO 2016067428 A1 WO2016067428 A1 WO 2016067428A1 JP 2014078979 W JP2014078979 W JP 2014078979W WO 2016067428 A1 WO2016067428 A1 WO 2016067428A1
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Prior art keywords
binder
electrode mixture
ion battery
lithium ion
electrode
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PCT/JP2014/078979
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French (fr)
Japanese (ja)
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和明 直江
利光 野口
祐介 加賀
新平 尼崎
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株式会社日立製作所
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Priority to PCT/JP2014/078979 priority Critical patent/WO2016067428A1/en
Priority to JP2016556136A priority patent/JPWO2016067428A1/en
Publication of WO2016067428A1 publication Critical patent/WO2016067428A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/058—Construction or manufacture
    • H01M10/0587—Construction or manufacture of accumulators having only wound construction elements, i.e. wound positive electrodes, wound negative electrodes and wound separators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a lithium ion battery and a manufacturing method thereof.
  • Patent Document 1 JP 2013-149407 A
  • the electrode film formed on the surface of the current collector foil and a binder for bonding the current collector foil and the electrode film are provided, and the binder concentration on the side of the current collector foil of the electrode film is opposite to that of the current collector foil. Describes a lithium ion secondary battery characterized by being higher than the binder concentration on the side.
  • the binder has a function of linking the active material particles and the conductive auxiliary particles in the electrode mixture, and a function of bonding the active material particles and the conductive auxiliary particles to the current collector, but does not participate in the battery reaction, It inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector, thereby increasing the internal resistance of the lithium ion battery. Therefore, it is necessary to reduce the amount of binder in the electrode mixture.
  • Patent Document 1 discloses lithium ions that do not need to increase the amount of binder even if the thickness of the electrode mixture is increased by suppressing the non-uniform distribution of the binder in the electrode mixture caused by drying of the electrode mixture. A battery is described. However, there is no mention of a binder distribution that can reduce the amount of the binder, and further increase in the output of the lithium ion battery cannot be realized.
  • the present invention provides a lithium ion battery that can reduce the amount of binder in the electrode mixture and achieve high output.
  • the lithium ion battery according to the present invention can be used in an electrode mixture by adjusting the thickness of the electrode mixture or the volume-based center particle diameter of the active material particles contained in the electrode mixture.
  • the binder contained is segregated to the collector side, and an electrode mixture having a binder distribution in which the binder amount on the collector side of the electrode mixture is larger than the binder amount on the surface side of the electrode mixture is used.
  • the present invention it is possible to provide a lithium ion battery capable of reducing the amount of binder in the electrode mixture and realizing high output.
  • FIG. 1 is a perspective view schematically showing a configuration of a lithium ion battery according to Example 1.
  • FIG. It is a perspective view which shows typically the structure of the winding body by Example 1.
  • FIG. 3 is a plan view schematically showing the configuration of an electrode according to Example 1.
  • FIG. 3 is a cross-sectional view schematically showing an example of an electrode analysis site for analyzing the amount of binder in the electrode mixture according to Example 1; It is a graph which shows binder distribution of the electrode samples 1, 2, 3, 4, and 5 from which T / D by Example 1 differs. It is a side view which shows typically the measuring apparatus of the peel strength by Example 1.
  • FIG. It is a graph which shows the peel strength of the electrode samples 1, 2, 3, 4, 5 from which T / D by Example 1 differs.
  • FIG. 6 is a perspective view schematically showing a configuration of a lithium ion battery having a rectangular outer can according to Example 2.
  • FIG. 6 is a plan view schematically showing the configuration of an electrode when a wound type wound body according to Example 2 is disassembled.
  • FIG. 6 is a plan view schematically showing the configuration of an electrode when a rectangular wound body according to Example 2 is disassembled.
  • the constituent elements are not necessarily indispensable unless otherwise specified and clearly considered essential in principle. Needless to say.
  • FIG. 1 is a perspective view schematically showing a configuration of a lithium ion battery according to the first embodiment.
  • a lithium ion battery has a wound body WRF inside an outer can CS made mainly of, for example, iron or stainless steel, and inside the outer can CS and inside the wound body WRF.
  • the electrolyte solution EL is filled.
  • FIG. 2 is a perspective view schematically showing the configuration of the wound body according to the first embodiment.
  • the wound body WRF includes a positive electrode PER, a separator SP, and a negative electrode NER that are wound around an axis CR.
  • the separator SP has a function as a spacer that prevents electrical contact between the positive electrode PER and the negative electrode NER and allows lithium ions to pass therethrough.
  • the separator SP for example, polyethylene, polypropylene, or a combination of these materials can be used.
  • Electrolytic solution is a non-aqueous electrolytic solution.
  • a lithium ion battery is a battery that performs charging and discharging by using insertion of lithium ions into an active material and desorption of lithium ions from the active material, and the lithium ions move through the electrolyte.
  • Lithium is a strong reducing agent and reacts violently with water to generate hydrogen gas. Therefore, in a lithium ion battery in which lithium ions move in the electrolytic solution, an aqueous solution cannot be used as the electrolytic solution. For this reason, in the lithium ion battery, a nonaqueous electrolytic solution is used as the electrolytic solution.
  • Examples of the electrolyte of the nonaqueous electrolytic solution include LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiB (C 6 H 5 ) 4 , CH 3 SO 3 Li, or CF 3 SO 3 Li, or a mixture thereof.
  • the organic solvent for example, ethylene carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate, or ethyl methyl carbonate can be used.
  • examples of the organic solvent include 1,2-dimethoxyethane, 1,2-diethoxyethane, ⁇ -butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl Sulfolane, acetonitrile, propionitrile or the like can be used. Further, a mixed solution of the organic solvent described above can be used.
  • FIG. 3 is a plan view schematically showing the configuration of the electrode according to the first embodiment.
  • the electrode ER is composed of a current collector EP and an electrode mixture EC formed on the current collector EP, and the electrode mixture EC contains an active material AS, a conductive additive CA, and a binder BD.
  • the active material AS is a material capable of occluding and releasing lithium ions, and when a positive electrode active material is used for the active material AS, the electrode ER can be used as a positive electrode (positive electrode PER in FIG. 2).
  • the positive electrode active material for example, a lithium-containing transition metal oxide represented by lithium cobaltate, lithium nickelate, lithium manganate, or the like, or a mixture thereof can be used.
  • the current collector EP can be a metal foil or a net-like metal made of a conductive metal such as aluminum, for example.
  • the electrode ER can be used as a negative electrode (negative electrode NER in FIG. 2).
  • the negative electrode active material include carbon materials such as hard carbon, soft carbon, and graphite, oxides such as silicon oxide, niobium oxide, and titanium oxide, and lithium and alloys such as silicon, tin, germanium, lead, and aluminum. It is possible to use materials typified by the material that forms the film, or a mixture thereof.
  • a metal foil or a net-like metal made of a conductive metal such as copper can be used for the current collector EP.
  • binder BD for example, polyvinyl fluoride, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyimide, or a mixture thereof can be used.
  • the binder BD has a function of connecting the active material particles and the conductive auxiliary particles in the electrode mixture EC, and the active material particles and the conductive auxiliary particles are bonded to the current collector EP, whereby the lithium ion battery is charged and discharged. Even if it repeats, it has a function which prevents an active material particle and a conductive support agent particle from dropping from electrode mixture EC or current collector EP.
  • the binder contained in the electrode mixture segregates on the current collector side, and the electrode mixture has a binder amount on the current collector side of the electrode mixture.
  • the binder distribution is larger than the binder amount on the opposite side of the body.
  • T thickness of the electrode mixture
  • D volume-based center particle diameter of the active material particles
  • T / D ⁇ 2.0 the amount of binder on the current collector side of the electrode mixture is It becomes 1.5 times or more of the binder amount on the surface side of the mixture.
  • the electrode mixture in which the binder is segregated on the current collector side increases the adhesion strength to the current collector. Therefore, even if the amount of the binder in the electrode mixture is reduced, it is possible to have adhesion strength equivalent to that of the electrode mixture having a uniform binder distribution as typified by Patent Document 1. Because the binder that inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector is reduced, the internal resistance of the lithium ion battery can be reduced. The output of the lithium ion battery can be increased.
  • the internal resistance of a lithium ion battery depends on the total amount of binder contained in the entire electrode mixture, so that even if only an electrode mixture with a reduced binder is applied to a part of the electrode mixture, the lithium ion battery has high resistance. The output effect can be obtained.
  • Lithium manganese cobalt nickel composite oxide, graphite powder, and PVDF were used for the active material, conductive additive, and binder constituting the positive electrode mixture, respectively.
  • Example 1 N-methylpyrrolidone (NMP) was used as a solvent.
  • the positive electrode slurry was applied to the surface of a current collector made of aluminum with a bar coater, and the solvent was dried in a hot air drying oven at 120 ° C. to prepare a positive electrode mixture.
  • a coating method various other coating methods such as a die coater, a gravure coater, a brush coating, or a dipping can be used. Thereafter, the density of the positive electrode mixture was adjusted to 2.1 g / cc by a hot roll press.
  • Example 1 five types of positive electrode mixtures were prepared by changing the coating amount of the positive electrode mixture and the center particle diameter of the reference volume of the active material particles. Specifically, the ratio T / D is 1.1, 1.6, 2.1, 4.1 when the thickness of the electrode mixture is T and the volume-based center particle diameter of the active material particles is D. , And 5.4 were produced. Below, the electrode samples corresponding to each positive electrode mixture are described as electrode samples 1, 2, 3, 4, and 5.
  • the thickness T of the electrode mixture can be measured by a cross-sectional image of a micrometer or a scanning electron microscope (SEM), for example.
  • SEM scanning electron microscope
  • various particles such as a particle gauge method or a laser diffraction / scattering method can be used.
  • the volume-based center particle diameter D of the active material particles can be measured by the diameter distribution measuring method.
  • Quantitative evaluation of the binder distribution in the electrode mixture can be performed using, for example, an energy dispersive X-ray spectroscopy (EDX) apparatus mounted on the SEM.
  • EDX energy dispersive X-ray spectroscopy
  • FIG. 4 is a cross-sectional view schematically showing an example of an electrode analysis portion for EDX analysis of the binder amount in the electrode mixture according to the first embodiment.
  • an analysis region having a width of about 2 ⁇ m and a length of about 50 ⁇ m is arranged on the current collector EP side, the center (current collector EP) of the electrode mixture EC. Between the side and the surface side) and the surface side, and perform EDX analysis in each region.
  • the fluorine content is calculated as an index of the binder amount in the region, and the relative change in the fluorine content is respectively measured on the current collector EP side, the center, and the surface side of the electrode mixture EC. evaluate.
  • FIG. 5 is a graph showing the binder distribution of the electrode samples 1, 2, 3, 4, and 5 having different T / D values according to the first embodiment.
  • the binder amount is standardized by the value on the surface side of the electrode mixture.
  • each plot on the current collector side, center, and surface side of the electrode mixture represents an average value of fluorine content when EDX analysis is performed with six fields of view. .
  • the amount of binder increases from the current collector side of the electrode mixture toward the surface side, and the binder segregates on the surface side.
  • the amount of the binder increases from the surface side of the electrode mixture toward the current collector side, and the binder is segregated on the current collector side.
  • the amount of the binder on the current collector side of the electrode mixture is 1.5 times or more the amount of the binder on the surface side.
  • the cause of the binder distribution changing depending on the relative relationship between the thickness T of the electrode mixture and the volume-based center particle diameter D of the active material particles is considered as follows.
  • the thickness T of the electrode mixture is larger than the volume-based center particle diameter D of the active material particles (for example, T / D ⁇ 4.1)
  • D of the active material particles for example, T / D ⁇ 4.1
  • the temperature rise inside the electrode mixture is small.
  • the solvent evaporates on the surface of the electrode mixture, and evaporation proceeds as the solvent inside the electrode mixture moves to the surface.
  • the binder dissolved in the solvent also moves to the surface side of the electrode mixture together with the solvent, and precipitates as the solvent evaporates. As a result, binder segregation occurs on the surface side of the electrode mixture.
  • the thickness T of the electrode mixture is smaller than the volume-based center particle diameter D of the active material particles (for example, when T / D ⁇ 2.1), in the drying process, The temperature gradient in the thickness direction is small, and not only the surface of the electrode mixture but also the inside of the electrode mixture rises uniformly to the temperature of the drying furnace. This is because the thermal resistance of the electrode mixture is reduced as compared with the case where T / D is large. If the volume-based center particle diameter D of the active material particles is constant and the thickness T of the electrode mixture is reduced, the thickness T of the electrode mixture is reduced, so that the thermal resistance of the electrode mixture is reduced.
  • the gap between the active material particles of the electrode mixture is increased so that heat is transferred by thermal convection of the solvent. Therefore, the thermal resistance of the electrode mixture is reduced.
  • the solvent evaporates in the entire area of the electrode mixture including the surface of the current collector.
  • the binder is not only on the surface of the active material particles but also on the surface of the current collector. Precipitate equally.
  • the binder is uniformly deposited inside the electrode mixture. However, due to the binder deposited on the surface of the current collector, binder segregation occurs on the current collector side as a whole.
  • Patent Document 1 adds a step of solidifying a binder by bringing a solvent for precipitating the binder into contact with the electrode material paste as a method of suppressing the non-uniform distribution of the binder in the electrode film caused by drying of the electrode film. A method of drying an electrode film in a state where a binder is solidified is described.
  • the binder moves from the surface side of the electrode film to the current collector foil side where solidification has not progressed, and the amount of binder on the current collector foil side is It is described that the difference between the binder concentration on the current collector foil side of the electrode film and the binder concentration on the surface side is within 50%, although the amount of the binder on the side is larger.
  • Example 1 in order to control the binder distribution by the relative relationship between the thickness T of the electrode mixture and the volume-based center particle diameter D of the active material particles, the binder described in Patent Document 1 is solidified. The process to do is unnecessary. Therefore, the binder distribution can be controlled at a lower cost. Further, in Patent Document 1, the difference between the binder concentration on the current collector foil side of the electrode film and the binder concentration on the surface side is within 50%, whereas in Example 1, T / D ⁇ 1.6. As in electrode samples 1 and 2 that satisfy the above conditions, an electrode mixture in which the amount of the binder on the current collector side is 1.5 times or more the amount of the binder on the surface side can be produced. Furthermore, like the electrode sample 1 satisfying T / D ⁇ 1.1, an electrode mixture in which the amount of binder on the current collector side is increased to 2.6 times the amount of binder on the surface side can be produced.
  • FIG. 6 is a side view schematically showing a peel strength measuring apparatus according to the first embodiment.
  • the peel strength of the electrode mixture with respect to the current collector was measured using electrode samples 1, 2, 3, 4, and 5 having different T / D.
  • FIG. 7 is a graph showing the peel strength of electrode samples 1, 2, 3, 4, and 5 having different T / D values according to Example 1.
  • Electrode Samples 1, 2, and 3 in which the amount of binder on the current collector side of the electrode mixture is larger than the amount of binder on the surface side are electrode samples 4 in which the amount of binder on the current collector side of the electrode mixture is smaller than the amount of binder on the surface side It can be seen that the peel strength is higher than 5. After the test, when all of the electrode samples 1, 2, 3, 4, 5 were observed, peeling progressed at the interface between the electrode mixture and the current collector. It can be said that the adhesion to the electric body is the weakest. Therefore, it is considered that when the binder segregates on the current collector side, the amount of the binder for bonding the active material particles and the current collector relatively increases, and thus the peel strength is effectively increased.
  • the binder amount on the current collector side of the electrode mixture is 2.61 times the binder amount on the surface side
  • the binder amount on the current collector side of the electrode mixture is 0.89 of the binder amount on the surface side. It can be seen that the peel strength is significantly increased to 18.8 times that of the electrode sample 4 which is doubled.
  • the thickness T of the electrode mixture of the electrode sample 1 is 12.1 ⁇ m
  • the volume-based center particle diameter D of the active material particles is 11.0 ⁇ m. Since the peel strength is improved as the binder amount on the current collector side of the electrode mixture is larger than the binder amount on the surface side, the binder amount on the current collector side of the electrode mixture is 2. If it is 61 times or more, it is considered that the peel strength further increases.
  • the electrode mixture having a binder distribution in which the binder segregates on the current collector side increases the adhesion strength. This means that if an electrode having a binder distribution such as electrode samples 1, 2, and 3 is used, the same adhesion strength can be maintained even with a small amount of binder compared to an electrode having a binder distribution such as electrode samples 4 and 5. Means.
  • a positive electrode mixture in which the binder was reduced to less than 3.5% by weight was prepared, and its adhesion strength and battery characteristics were evaluated.
  • Table 1 shows the weight percentages of the active material, the conductive additive, and the binder in the electrode samples 6, 7, and 8 used for the evaluation of peel strength and battery characteristics.
  • the active material active material particles having a volume-based center particle diameter D of 11.0 ⁇ m are used, and the thickness T (T / D) of the electrode mixture with respect to the volume-based center particle diameter D of the active material particles is 1.5.
  • T / D thickness of the electrode mixture with respect to the volume-based center particle diameter D of the active material particles
  • FIG. 8 is a graph showing the peel strength of the electrode samples 6, 7, and 8 having different binder weight percentages according to the first embodiment.
  • the peel strength of the electrode sample 7 having a binder of 2.6% by weight is 139 N / m
  • the peel strength of the electrode sample 8 having a binder of 1.5% by weight is 115 N / m
  • T / D is 1.5.
  • the binder had an adhesion strength equal to or higher than the peel strength of 40 N / m of the electrode mixture (electrode sample 4) having 3.5% by weight and no binder segregation.
  • the binder can be reduced from 3.5% by weight to 1.5% by weight. Is 3.5% by weight, and it is considered that the electrode mixture (electrode sample 4) having no binder segregation has an adhesion strength equal to or higher than the peel strength of 40 N / m. Further, when T / D ⁇ 1.5, when the data of the electrode samples 1 and 2 in FIG. 5 are interpolated, the amount of the binder on the current collector side of the electrode mixture becomes 1.8 times or more of the amount of the binder on the surface side. It can be estimated that
  • the electrode mixture may fall off the current collector as the battery is charged / discharged.
  • the chargeable / dischargeable capacity is lowered, so that both the capacity and output of the battery are lowered.
  • the adhesion strength of the electrode mixture can be increased by segregating the binder to the current collector foil side of the electrode mixture, the amount of the binder can be reduced without deteriorating battery characteristics. It becomes.
  • a positive electrode mixture using lithium manganese cobalt nickel composite oxide as an active material, graphite powder as a conductive additive and PVDF as a binder was formed on the surface of a current collector made of aluminum.
  • a negative electrode mixture using amorphous carbon as an active material, carbon black as a conductive additive, and PVDF as a binder was formed on the surface of a current collector made of copper.
  • a porous polypropylene having a thickness of 20 ⁇ m was used as the separator.
  • an organic solvent a mixed solution of ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate in which the electrolytic salt LiPF 6 was dissolved was used.
  • the density of the positive electrode mixture and the negative electrode mixture was adjusted to 2.1 g / cc and 1.4 g / cc, respectively, by a hot roll press.
  • the positive electrode, the negative electrode, and the separator were punched into a diameter of 14 mm, 16 mm, and 16 mm, respectively, and then laminated, and then the electrolyte was impregnated as a whole to produce a lithium ion battery for evaluation.
  • the produced lithium ion battery was charged at a constant current of 2 mA up to 4.1 V, and then charged at a constant voltage to obtain a fully charged state. From a fully charged state, the C rate is discharged at each current value of 0.2 C to 5.0 C, the cell voltage after 10 seconds is measured, and the slope of the cell voltage with respect to the discharge current (C rate unit) is measured as a lithium ion battery.
  • C rate unit the slope of the cell voltage with respect to the discharge current
  • the C rate is an expression of a current value based on a current value for discharging the battery capacity in one hour. For example, the current value when discharging over 1 hour is 1C, the current value when discharging over 0.5 hours is 2C, and the current value when discharging over 0.2 hours is 5C.
  • FIG. 9 is a graph showing the internal resistance of a lithium ion battery manufactured using electrode samples 6, 7, and 8 having different binder weight percentages according to the first embodiment.
  • the internal resistance was reduced by reducing the amount of binder.
  • the internal resistance was greatly reduced to 0.877 times that of the electrode sample 6 with 3.5% by weight of binder.
  • the binder contained in the electrode mixture segregates on the current collector side, and the binder distribution on the current collector side of the electrode mixture has a binder distribution larger than the binder amount on the surface side. It has the electrode which consists of an electrode mixture which has.
  • the electrode mixture with the binder segregated on the current collector side increases the adhesion strength to the current collector, so even if the total amount of binder contained in the entire electrode mixture is reduced, it is equivalent to the electrode mixture with a uniform binder distribution
  • the adhesion strength can be as follows.
  • the binder that inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector is reduced, the internal resistance of the lithium ion battery can be reduced. The output of the lithium ion battery can be increased.
  • the present invention is applicable to the negative electrode mixture, and the amount of the binder on the current collector side of the negative electrode mixture is the amount of the binder on the surface side. It may have more binder distribution. Also in the negative electrode mixture, as described above, when the binder segregates on the current collector side, the adhesion strength of the negative electrode mixture to the current collector increases, so the amount of the binder in the negative electrode mixture can be reduced, and lithium The output of the ion battery can be increased.
  • Example 2 the binder contained in the electrode mixture is segregated on the current collector side, and the electrode mixture having a binder distribution in which the binder amount on the current collector side of the electrode mixture is larger than the binder amount on the surface side is changed to lithium. It was selectively formed at a location where the active material was likely to fall off during charge / discharge of the ion battery.
  • the active material occludes or releases lithium ions due to charge / discharge of the lithium ion battery, causing a volume change.
  • the stress caused by this volume change exceeds the adhesive force by the binder, the active material falls off from the electrode mixture. Since the dropped active material cannot contribute to charging / discharging, the characteristics of the lithium ion battery are deteriorated.
  • the electrode mixture inside (inside) the wound body WRF close to the axial core CR depends on the surrounding electrode mixture and the current collector. Since it is restrained, even if stress occurs in the electrode mixture due to charge / discharge, the active material is unlikely to fall off from the electrode mixture. On the other hand, since the electrode mixture outside (outside, near the outer periphery) of the wound body WRF is not constrained from the surroundings, the active material is easily dropped from the electrode mixture.
  • FIG. 10 is a perspective view schematically showing a configuration of a lithium ion battery having a rectangular outer can according to the second embodiment.
  • the configuration of the wound body is almost the same as in FIGS.
  • the radius of curvature of the current collector and the electrode mixture formed on the surface thereof is not uniform.
  • the electrode mixture is stretched, so that the binder that bonds the active material particles is easily cut, and as a result, the active material is also likely to fall off.
  • the binder contained in the electrode mixture segregates on the current collector side, and the electrode mixture having a binder distribution in which the amount of the binder on the current collector side is larger than the amount of the binder on the surface side, It is characterized in that it is selectively formed at a location where the active material is likely to fall off during charging / discharging of the ion battery.
  • the T / D is reduced and the binder is collected by reducing the coating pressure of the electrode slurry and reducing the thickness T of the electrode mixture at the above location.
  • a binder distribution segregated on the electric body side can be selectively formed.
  • FIG. 11 is a plan view schematically showing the configuration of the electrode when the wound type wound body according to the second embodiment is disassembled.
  • the amount of binder on the current collector side is larger than the inside of the wound body (inside) near the shaft core, outside the wound body (outside, near the outer periphery).
  • Increasing the ratio of the electrode mixture having a binder distribution larger than the amount can improve the adhesion strength of the electrode mixture outside the wound body where the active material is easily removed. As a result, the characteristic reliability with respect to the charge / discharge cycle of the lithium ion battery can be improved.
  • FIG. 12 is a plan view schematically showing the configuration of the electrode when the rectangular wound body according to the second embodiment is disassembled.
  • the binder distribution in which the amount of binder on the current collector side is larger than the amount of binder on the surface side at the corner (where the radius of curvature is small) when forming the wound body By increasing the ratio of the electrode mixture having the ratio, the adhesion strength of the electrode mixture at the corner (where the radius of curvature is small) can be improved when forming a wound body from which the active material is easily removed. . As a result, the characteristic reliability with respect to the charge / discharge cycle of the lithium ion battery can be improved.
  • the wound-type lithium ion battery is taken as an example to describe the technical idea of the present invention.
  • the technical idea of the present invention is not limited to the wound-type lithium ion battery.
  • it can be widely applied to an electricity storage device (for example, a battery or a capacitor) including a positive electrode, a negative electrode, and a separator that electrically separates the positive electrode and the negative electrode.
  • the present invention can be widely used in the manufacturing industry for manufacturing batteries represented by, for example, lithium ion batteries.

Abstract

Provided is a lithium ion battery which is capable of achieving higher output power by reducing the amount of a binder in an electrode mixture. In order to solve the above-described problem, a lithium ion battery according to the present invention uses an electrode mixture which has a binder distribution wherein the amount of a binder on the collector side of the electrode mixture is larger than the amount of the binder on the surface side of the electrode mixture by adjusting the thickness of the electrode mixture and the volume-based median diameter of active material particles contained in the electrode mixture, thereby segregating the binder contained in the electrode mixture on the collector side.

Description

リチウムイオン電池およびその製造方法Lithium ion battery and manufacturing method thereof
　本発明は、リチウムイオン電池およびその製造方法に関する。 The present invention relates to a lithium ion battery and a manufacturing method thereof.
　本技術分野の背景技術として、特開２０１３－１４９４０７号公報（特許文献１）がある。この公報には、「集電箔の表面に形成された電極膜と、集電箔と電極膜とを接着させるバインダとを備え、電極膜の集電箔側のバインダ濃度が集電箔の反対側のバインダ濃度よりも高いことを特徴とするリチウムイオン二次電池」が記載されている。 As a background art in this technical field, there is JP 2013-149407 A (Patent Document 1). In this publication, “the electrode film formed on the surface of the current collector foil and a binder for bonding the current collector foil and the electrode film are provided, and the binder concentration on the side of the current collector foil of the electrode film is opposite to that of the current collector foil. Describes a lithium ion secondary battery characterized by being higher than the binder concentration on the side.
特開２０１３－１４９４０７号公報JP 2013-149407 A
　リチウムイオン電池の特性として、更なる高出力化が求められている。高出力化を実現するためには、例えば電極合剤中のバインダ量を低減することが有効である。バインダは、電極合剤中の活物質粒子と導電助剤粒子とを結ぶ機能、並びに活物質粒子および導電助剤粒子を集電体に接着する機能があるが、電池反応には関与せず、活物質粒子と電解液間のリチウムイオンの伝導、または活物質粒子と集電体間の電子の伝導を阻害し、リチウムイオン電池の内部抵抗を増加させる。そのため、電極合剤中のバインダ量を低減することが必要とされる。 As a characteristic of lithium ion batteries, higher output is required. In order to achieve high output, for example, it is effective to reduce the amount of binder in the electrode mixture. The binder has a function of linking the active material particles and the conductive auxiliary particles in the electrode mixture, and a function of bonding the active material particles and the conductive auxiliary particles to the current collector, but does not participate in the battery reaction, It inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector, thereby increasing the internal resistance of the lithium ion battery. Therefore, it is necessary to reduce the amount of binder in the electrode mixture.
　前記特許文献１には、電極合剤の乾燥によって生じる電極合剤中でのバインダの不均一分布を抑制することで、電極合剤の厚みを増加させてもバインダ量を増やす必要のないリチウムイオン電池が記載されている。しかし、バインダ量を減らすことが可能なバインダ分布については言及が無く、リチウムイオン電池の更なる高出力化は実現できない。 Patent Document 1 discloses lithium ions that do not need to increase the amount of binder even if the thickness of the electrode mixture is increased by suppressing the non-uniform distribution of the binder in the electrode mixture caused by drying of the electrode mixture. A battery is described. However, there is no mention of a binder distribution that can reduce the amount of the binder, and further increase in the output of the lithium ion battery cannot be realized.
　そこで、本発明は、電極合剤中のバインダ量を低減し、高出力化を実現できるリチウムイオン電池を提供する。 Therefore, the present invention provides a lithium ion battery that can reduce the amount of binder in the electrode mixture and achieve high output.
　上記課題を解決するために、本発明によるリチウムイオン電池は、電極合剤の厚さ、または電極合剤に含まれる活物質粒子の体積基準の中心粒径を調整することにより、電極合剤に含まれるバインダを集電体側に偏析させて、電極合剤の集電体側のバインダ量が電極合剤の表面側のバインダ量よりも多いバインダ分布を有する電極合剤を用いる。 In order to solve the above problems, the lithium ion battery according to the present invention can be used in an electrode mixture by adjusting the thickness of the electrode mixture or the volume-based center particle diameter of the active material particles contained in the electrode mixture. The binder contained is segregated to the collector side, and an electrode mixture having a binder distribution in which the binder amount on the collector side of the electrode mixture is larger than the binder amount on the surface side of the electrode mixture is used.
　本発明によれば、電極合剤中のバインダ量を低減し、高出力化を実現できるリチウムイオン電池を提供することができる。 According to the present invention, it is possible to provide a lithium ion battery capable of reducing the amount of binder in the electrode mixture and realizing high output.
　上記した以外の課題、構成および効果は、以下の実施の形態の説明により明らかにされる。 Issues, configurations, and effects other than those described above will be clarified by the following description of embodiments.
実施例１によるリチウムイオン電池の構成を模式的に示す斜視図である。1 is a perspective view schematically showing a configuration of a lithium ion battery according to Example 1. FIG. 実施例１による捲回体の構成を模式的に示す斜視図である。It is a perspective view which shows typically the structure of the winding body by Example 1. FIG. 実施例１による電極の構成を模式的に示す平面図である。3 is a plan view schematically showing the configuration of an electrode according to Example 1. FIG. 実施例１による電極合剤中のバインダ量を分析する電極の分析箇所の一例を模式的に示す断面図である。FIG. 3 is a cross-sectional view schematically showing an example of an electrode analysis site for analyzing the amount of binder in the electrode mixture according to Example 1; 実施例１によるＴ／Ｄが互いに異なる電極試料１、２、３、４、５のバインダ分布を示すグラフ図である。It is a graph which shows binder distribution of the electrode samples 1, 2, 3, 4, and 5 from which T / D by Example 1 differs. 実施例１によるピール強度の測定装置を模式的に示す側面図である。It is a side view which shows typically the measuring apparatus of the peel strength by Example 1. FIG. 実施例１によるＴ／Ｄが互いに異なる電極試料１、２、３、４、５のピール強度を示すグラフ図である。It is a graph which shows the peel strength of the electrode samples 1, 2, 3, 4, 5 from which T / D by Example 1 differs. 実施例１によるバインダの重量％が互いに異なる電極試料６、７、８のピール強度を示すグラフ図である。It is a graph which shows the peeling strength of the electrode samples 6, 7, and 8 from which the weight% of the binder by Example 1 differs mutually. 実施例１によるバインダの重量％が互いに異なる電極試料６、７、８を使用して作製したリチウムイオン電池の内部抵抗を示すグラフ図である。It is a graph which shows the internal resistance of the lithium ion battery produced using the electrode samples 6, 7, and 8 from which the weight% of the binder by Example 1 differs mutually. 実施例２による角型の外装缶を有するリチウムイオン電池の構成を模式的に示す斜視図である。6 is a perspective view schematically showing a configuration of a lithium ion battery having a rectangular outer can according to Example 2. FIG. 実施例２による捲回型の捲回体を解体したときの電極の構成を模式的に示す平面図である。6 is a plan view schematically showing the configuration of an electrode when a wound type wound body according to Example 2 is disassembled. FIG. 実施例２による角型の捲回体を解体したときの電極の構成を模式的に示す平面図である。6 is a plan view schematically showing the configuration of an electrode when a rectangular wound body according to Example 2 is disassembled. FIG.
　以下の実施の形態において、便宜上その必要があるときは、複数のセクションまたは実施の形態に分割して説明するが、特に明示した場合を除き、それらはお互いに無関係なものではなく、一方は他方の一部または全部の変形例、詳細、補足説明等の関係にある。 In the following embodiments, when necessary for the sake of convenience, the description will be divided into a plurality of sections or embodiments. However, unless otherwise specified, they are not irrelevant to each other, and one is the other. There are some or all of the modifications, details, supplementary explanations, and the like.
　また、以下の実施の形態において、要素の数等（個数、数値、量、範囲等を含む）に言及する場合、特に明示した場合および原理的に明らかに特定の数に限定される場合等を除き、その特定の数に限定されるものではなく、特定の数以上でも以下でもよい。 Further, in the following embodiments, when referring to the number of elements (including the number, numerical value, quantity, range, etc.), especially when clearly indicated and when clearly limited to a specific number in principle, etc. Except, it is not limited to the specific number, and may be more or less than the specific number.
　また、以下の実施の形態において、その構成要素（要素ステップ等も含む）は、特に明示した場合および原理的に明らかに必須であると考えられる場合等を除き、必ずしも必須のものではないことは言うまでもない。 Further, in the following embodiments, the constituent elements (including element steps) are not necessarily indispensable unless otherwise specified and clearly considered essential in principle. Needless to say.
　また、「Ａからなる」、「Ａよりなる」、「Ａを有する」、「Ａを含む」と言うときは、特にその要素のみである旨明示した場合等を除き、それ以外の要素を排除するものでないことは言うまでもない。同様に、以下の実施の形態において、構成要素等の形状、位置関係等に言及するときは、特に明示した場合および原理的に明らかにそうでないと考えられる場合等を除き、実質的にその形状等に近似または類似するもの等を含むものとする。このことは、上記数値および範囲についても同様である。 In addition, when referring to “consisting of A”, “consisting of A”, “having A”, and “including A”, other elements are excluded unless specifically indicated that only that element is included. It goes without saying that it is not what you do. Similarly, in the following embodiments, when referring to the shapes, positional relationships, etc. of the components, etc., the shapes are substantially the same unless otherwise specified, or otherwise apparent in principle. And the like are included. The same applies to the above numerical values and ranges.
　また、以下の実施の形態で用いる図面においては、平面図であっても図面を見易くするためにハッチングを付す場合もある。また、以下の実施の形態を説明するための全図において、同一機能を有するものは原則として同一の符号を付し、その繰り返しの説明は省略する。以下、本実施の形態を図面に基づいて詳細に説明する。 Also, in the drawings used in the following embodiments, hatching may be added to make the drawings easy to see even if they are plan views. In all the drawings for explaining the following embodiments, components having the same function are denoted by the same reference numerals in principle, and repeated description thereof is omitted. Hereinafter, the present embodiment will be described in detail with reference to the drawings.
　本実施例１によるリチウムイオン電池について図１から図９を用いて説明する。 The lithium ion battery according to Example 1 will be described with reference to FIGS.
　図１は、本実施例１によるリチウムイオン電池の構成を模式的に示す斜視図である。 FIG. 1 is a perspective view schematically showing a configuration of a lithium ion battery according to the first embodiment.
　図１において、リチウムイオン電池は、例えば鉄またはステンレスを主材料とする外装缶ＣＳの内部に捲回体ＷＲＦを有しており、この外装缶ＣＳの内部、および捲回体ＷＲＦの内部には、電解液ＥＬが充填されている。 In FIG. 1, a lithium ion battery has a wound body WRF inside an outer can CS made mainly of, for example, iron or stainless steel, and inside the outer can CS and inside the wound body WRF. The electrolyte solution EL is filled.
　図２は、本実施例１による捲回体の構成を模式的に示す斜視図である。 FIG. 2 is a perspective view schematically showing the configuration of the wound body according to the first embodiment.
　捲回体ＷＲＦは、軸芯ＣＲの回りに捲回された正極ＰＥＲ、セパレータＳＰ、および負極ＮＥＲから構成される。 The wound body WRF includes a positive electrode PER, a separator SP, and a negative electrode NER that are wound around an axis CR.
　セパレータＳＰは、正極ＰＥＲと負極ＮＥＲとの電気的な接触を防止し、かつ、リチウムイオンを通過させるスペーサとしての機能を有している。セパレータＳＰは、例えばポリエチレン、ポリプロピレン、またはこれら材料を組み合わせた構成物を使用することができる。 The separator SP has a function as a spacer that prevents electrical contact between the positive electrode PER and the negative electrode NER and allows lithium ions to pass therethrough. As the separator SP, for example, polyethylene, polypropylene, or a combination of these materials can be used.
　電解液は、非水電解液が使用される。リチウムイオン電池は、活物質へのリチウムイオンの挿入および活物質からのリチウムイオンの脱離を利用して充放電を行う電池であり、リチウムイオンが電解液中を移動する。リチウムは、強い還元剤であり、水と激しく反応して水素ガスを発生する。従って、リチウムイオンが電解液中を移動するリチウムイオン電池では、水溶液を電解液に使用することができない。このことから、リチウムイオン電池では、電解液として非水電解液が使用される。 Electrolytic solution is a non-aqueous electrolytic solution. A lithium ion battery is a battery that performs charging and discharging by using insertion of lithium ions into an active material and desorption of lithium ions from the active material, and the lithium ions move through the electrolyte. Lithium is a strong reducing agent and reacts violently with water to generate hydrogen gas. Therefore, in a lithium ion battery in which lithium ions move in the electrolytic solution, an aqueous solution cannot be used as the electrolytic solution. For this reason, in the lithium ion battery, a nonaqueous electrolytic solution is used as the electrolytic solution.
　非水電解液の電解質としては、例えばＬｉＰＦ６、ＬｉＣｌＯ４、ＬｉＡｓＦ６、ＬｉＢＦ４、ＬｉＢ（Ｃ６Ｈ５）４、ＣＨ３ＳＯ３Ｌｉ、若しくはＣＦ３ＳＯ３Ｌｉなど、またはこれらの混合物を使用することができる。また、有機溶媒としては、例えばエチレンカーボネート、ジメチルカーボネート、プロピレンカーボネート、ジエチルカーボネート、またはエチルメチルカーボネートなどを使用することができる。さらに、有機溶媒としては、例えば１，２－ジメトキシエタン、１，２－ジエトキシエタン、γ－ブチロラクトン、テトラヒドロフラン、１，３－ジオキソラン、４－メチル－１，３ジオキソラン、ジエチルエーテル、スルホラン、メチルスルホラン、アセトニトリル、またはプロピオニトリルなどを使用することができる。さらに、上記した有機溶媒の混合液を使用することができる。 Examples of the electrolyte of the nonaqueous electrolytic solution include LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiB (C 6 H 5 ) 4 , CH 3 SO 3 Li, or CF 3 SO 3 Li, or a mixture thereof. Can be used. As the organic solvent, for example, ethylene carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate, or ethyl methyl carbonate can be used. Further, examples of the organic solvent include 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methyl Sulfolane, acetonitrile, propionitrile or the like can be used. Further, a mixed solution of the organic solvent described above can be used.
　図３は、本実施例１による電極の構成を模式的に示す平面図である。 FIG. 3 is a plan view schematically showing the configuration of the electrode according to the first embodiment.
　電極ＥＲは、集電体ＥＰと、集電体ＥＰ上に形成された電極合剤ＥＣからなり、電極合剤ＥＣは、活物質ＡＳ、導電助剤ＣＡ、およびバインダＢＤを含有する。 The electrode ER is composed of a current collector EP and an electrode mixture EC formed on the current collector EP, and the electrode mixture EC contains an active material AS, a conductive additive CA, and a binder BD.
　活物質ＡＳは、リチウムイオンの吸蔵および放出が可能な物質であり、活物質ＡＳに正極活物質を用いた場合、電極ＥＲを正極（図２の正極ＰＥＲ）として、利用できる。正極活物質には、例えばコバルト酸リチウム、ニッケル酸リチウム、若しくはマンガン酸リチウムなどに代表されるリチウム含有遷移金属酸化物など、またはこれらの混合物を使用することができる。活物質ＡＳに正極活物質を用いた場合、集電体ＥＰには、例えばアルミニウムなどの導電性金属からなる金属箔または網状金属などを使用できる。 The active material AS is a material capable of occluding and releasing lithium ions, and when a positive electrode active material is used for the active material AS, the electrode ER can be used as a positive electrode (positive electrode PER in FIG. 2). As the positive electrode active material, for example, a lithium-containing transition metal oxide represented by lithium cobaltate, lithium nickelate, lithium manganate, or the like, or a mixture thereof can be used. When a positive electrode active material is used for the active material AS, the current collector EP can be a metal foil or a net-like metal made of a conductive metal such as aluminum, for example.
　また、活物質ＡＳに負極活物質を用いた場合、電極ＥＲを負極（図２の負極ＮＥＲ）として、利用できる。負極活物質には、例えばハードカーボン、ソフトカーボン、若しくはグラファイトなどの炭素材料、酸化シリコン、酸化二オブ、若しくは酸化チタンなどの酸化物、シリコン、スズ、ゲルマニウム、鉛、若しくはアルミニウムなどのリチウムと合金を形成する材料などに代表される材料、またはこれらの混合物を使用することができる。活物質ＡＳに負極活物質を用いた場合、集電体ＥＰには、例えば銅などの導電性金属からなる金属箔または網状金属などを使用できる。 Moreover, when a negative electrode active material is used for the active material AS, the electrode ER can be used as a negative electrode (negative electrode NER in FIG. 2). Examples of the negative electrode active material include carbon materials such as hard carbon, soft carbon, and graphite, oxides such as silicon oxide, niobium oxide, and titanium oxide, and lithium and alloys such as silicon, tin, germanium, lead, and aluminum. It is possible to use materials typified by the material that forms the film, or a mixture thereof. When a negative electrode active material is used for the active material AS, a metal foil or a net-like metal made of a conductive metal such as copper can be used for the current collector EP.
　バインダＢＤは、例えばポリフッ化ビニル、ポリフッ化ビニリデン（PolyVinylidene DiFluoride、ＰＶＤＦ）、ポリテトラフルオロエチレン、若しくはポリイミド、またはこれらの混合物などを使用することができる。バインダＢＤは、電極合剤ＥＣ中の活物質粒子と導電助剤粒子とを結ぶ機能、並びに活物質粒子および導電助剤粒子を集電体ＥＰに接着させることで、リチウムイオン電池が充放電を繰り返しても活物質粒子および導電助剤粒子が電極合剤ＥＣまたは集電体ＥＰから脱落しないようにする機能がある。 As the binder BD, for example, polyvinyl fluoride, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyimide, or a mixture thereof can be used. The binder BD has a function of connecting the active material particles and the conductive auxiliary particles in the electrode mixture EC, and the active material particles and the conductive auxiliary particles are bonded to the current collector EP, whereby the lithium ion battery is charged and discharged. Even if it repeats, it has a function which prevents an active material particle and a conductive support agent particle from dropping from electrode mixture EC or current collector EP.
　本実施例１におけるリチウムイオン電池では、電極合剤に含まれるバインダが集電体側に偏析し、電極合剤が、電極合剤の集電体側のバインダ量が電極合剤の表面側（集電体と反対側）のバインダ量よりも多いバインダ分布を有することを特徴とする。特に、電極合剤の厚みをＴ、活物質粒子の体積基準の中心粒径をＤとしたとき、Ｔ／Ｄ≦２．０を満たす場合、電極合剤の集電体側のバインダ量は、電極合剤の表面側のバインダ量の１．５倍以上になる。 In the lithium ion battery of Example 1, the binder contained in the electrode mixture segregates on the current collector side, and the electrode mixture has a binder amount on the current collector side of the electrode mixture. The binder distribution is larger than the binder amount on the opposite side of the body. In particular, when the thickness of the electrode mixture is T and the volume-based center particle diameter of the active material particles is D, when T / D ≦ 2.0, the amount of binder on the current collector side of the electrode mixture is It becomes 1.5 times or more of the binder amount on the surface side of the mixture.
　このように集電体側にバインダが偏析した電極合剤は、以下に説明する様に、集電体に対する密着強度が増加する。そのため、電極合剤中のバインダ量を低減させても、前記特許文献１に代表されるようなバインダ分布が均一な電極合剤と同等の密着強度を有することができる。活物質粒子と電解液間のリチウムイオンの伝導、または活物質粒子と集電体間の電子の伝導を阻害するバインダが低減することで、リチウムイオン電池の内部抵抗を減少させることができるため、リチウムイオン電池の高出力化を図ることができる。なお、リチウムイオン電池の内部抵抗は、電極合剤全体に含まれるバインダの総量に依存するので、電極合剤の一部にバインダが低減した電極合剤を適用するだけでも、リチウムイオン電池の高出力化の効果が得られる。 As described below, the electrode mixture in which the binder is segregated on the current collector side increases the adhesion strength to the current collector. Therefore, even if the amount of the binder in the electrode mixture is reduced, it is possible to have adhesion strength equivalent to that of the electrode mixture having a uniform binder distribution as typified by Patent Document 1. Because the binder that inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector is reduced, the internal resistance of the lithium ion battery can be reduced. The output of the lithium ion battery can be increased. Note that the internal resistance of a lithium ion battery depends on the total amount of binder contained in the entire electrode mixture, so that even if only an electrode mixture with a reduced binder is applied to a part of the electrode mixture, the lithium ion battery has high resistance. The output effect can be obtained.
　以下、本実施例１によるリチウムイオン電池の特徴の詳細をその製造方法および分析方法の一例とともに説明する。 Hereinafter, the details of the characteristics of the lithium ion battery according to the first embodiment will be described together with an example of its manufacturing method and analysis method.
　まず、正極合剤からなる電極試料の作製手順について説明する。 First, a procedure for producing an electrode sample made of a positive electrode mixture will be described.
　正極合剤を構成する活物質、導電助剤、およびバインダにはそれぞれ、リチウムマンガンコバルトニッケル複合酸化物、黒鉛粉末、およびＰＶＤＦを使用した。 Lithium manganese cobalt nickel composite oxide, graphite powder, and PVDF were used for the active material, conductive additive, and binder constituting the positive electrode mixture, respectively.
　まず、活物質、導電助剤、およびバインダがそれぞれ、９３．０重量％、３．５重量％、および３．５重量％となるようにこれらを混合し、バインダの溶解が可能な溶剤中に分散させることで、正極用スラリーを作製した。本実施例１では、溶剤にＮ－メチル－２－ピロリドン（N-methylpyrrolidone、ＮＭＰ）を使用した。 First, these are mixed so that the active material, the conductive additive, and the binder are 93.0% by weight, 3.5% by weight, and 3.5% by weight, respectively, in a solvent capable of dissolving the binder. By dispersing, a positive electrode slurry was produced. In Example 1, N-methylpyrrolidone (NMP) was used as a solvent.
　次に、アルミニウムからなる集電体の表面に、バーコータにより上記正極用スラリーを塗布し、１２０℃の温風乾燥炉にて溶剤を乾燥させることで、正極合剤を作製した。塗布方法としては、他にもダイコータ、グラビアコータ、刷毛塗り、またはディッピングなどの種々の塗布方法を使用することができる。その後、正極合剤は、熱ロールプレスにより、密度を２．１ｇ／ｃｃに調整した。 Next, the positive electrode slurry was applied to the surface of a current collector made of aluminum with a bar coater, and the solvent was dried in a hot air drying oven at 120 ° C. to prepare a positive electrode mixture. As a coating method, various other coating methods such as a die coater, a gravure coater, a brush coating, or a dipping can be used. Thereafter, the density of the positive electrode mixture was adjusted to 2.1 g / cc by a hot roll press.
　本実施例１では、正極合剤の塗布量および活物質粒子の基準体積の中心粒径を変えることで、５種類の正極合剤を作製した。具体的には、電極合剤の厚みをＴ、活物質粒子の体積基準の中心粒径をＤとしたとき、その比Ｔ／Ｄが１．１、１．６、２．１、４．１、および５．４となる正極合剤を作製した。以下では、それぞれの正極合剤に対応する電極試料を電極試料１、２、３、４、および５と記す。 In Example 1, five types of positive electrode mixtures were prepared by changing the coating amount of the positive electrode mixture and the center particle diameter of the reference volume of the active material particles. Specifically, the ratio T / D is 1.1, 1.6, 2.1, 4.1 when the thickness of the electrode mixture is T and the volume-based center particle diameter of the active material particles is D. , And 5.4 were produced. Below, the electrode samples corresponding to each positive electrode mixture are described as electrode samples 1, 2, 3, 4, and 5.
　電極合剤の厚みＴは、例えばマイクロメータまたは走査型電子顕微鏡（Scanning Electron Microscope、ＳＥＭ）の断面画像により測定することができる。また、電極合剤中の活物質粒子は、例えばＮＭＰを用いて電極合剤に含まれるバインダを再溶解させることにより取り出すことができるので、粒ゲージ法またはレーザー回折・散乱法などの種々の粒径分布測定方法により、活物質粒子の体積基準の中心粒径Ｄを測定することができる。 The thickness T of the electrode mixture can be measured by a cross-sectional image of a micrometer or a scanning electron microscope (SEM), for example. In addition, since the active material particles in the electrode mixture can be taken out by re-dissolving the binder contained in the electrode mixture using, for example, NMP, various particles such as a particle gauge method or a laser diffraction / scattering method can be used. The volume-based center particle diameter D of the active material particles can be measured by the diameter distribution measuring method.
　電極合剤中のバインダ分布の定量評価は、例えばＳＥＭに搭載されたエネルギー分散型蛍光Ｘ線分光（Energy Dispersive X-ray spectroscopy、ＥＤＸ）装置を利用して実施することができる。 Quantitative evaluation of the binder distribution in the electrode mixture can be performed using, for example, an energy dispersive X-ray spectroscopy (EDX) apparatus mounted on the SEM.
　図４は、本実施例１による電極合剤中のバインダ量をＥＤＸ分析する電極の分析箇所の一例を模式的に示す断面図である。 FIG. 4 is a cross-sectional view schematically showing an example of an electrode analysis portion for EDX analysis of the binder amount in the electrode mixture according to the first embodiment.
　図４に示すように、例えば倍率５００倍～２,０００倍のＳＥＭ像において、幅２μｍ、長さ５０μｍ程度の分析領域を、電極合剤ＥＣの集電体ＥＰ側、中央（集電体ＥＰ側と表面側との間）、および表面側に設定し、それぞれの領域でＥＤＸ分析を実施する。電極合剤ＥＣの構成元素のうち、フッ素含有量をその領域におけるバインダ量の指標として算出し、電極合剤ＥＣの集電体ＥＰ側、中央、および表面側においてそれぞれフッ素含有量の相対変化を評価する。バインダ分布の分析精度を高めるためには、ＥＤＸ分析を複数視野で行い、その平均を算出するとよい。 As shown in FIG. 4, for example, in an SEM image having a magnification of 500 to 2,000 times, an analysis region having a width of about 2 μm and a length of about 50 μm is arranged on the current collector EP side, the center (current collector EP) of the electrode mixture EC. Between the side and the surface side) and the surface side, and perform EDX analysis in each region. Among the constituent elements of the electrode mixture EC, the fluorine content is calculated as an index of the binder amount in the region, and the relative change in the fluorine content is respectively measured on the current collector EP side, the center, and the surface side of the electrode mixture EC. evaluate. In order to increase the analysis accuracy of the binder distribution, it is preferable to perform an EDX analysis in a plurality of fields and calculate the average.
　図５は、本実施例１によるＴ／Ｄが互いに異なる電極試料１、２、３、４、５のバインダ分布を示すグラフ図である。 FIG. 5 is a graph showing the binder distribution of the electrode samples 1, 2, 3, 4, and 5 having different T / D values according to the first embodiment.
　バインダ量は電極合剤の表面側における値で規格化してある。電極試料１、２、３、４、５において、電極合剤の集電体側、中央、および表面側のそれぞれのプロットは、６視野でＥＤＸ分析を行ったときのフッ素含有量の平均値を表す。 The binder amount is standardized by the value on the surface side of the electrode mixture. In electrode samples 1, 2, 3, 4, and 5, each plot on the current collector side, center, and surface side of the electrode mixture represents an average value of fluorine content when EDX analysis is performed with six fields of view. .
　図５に示すように、Ｔ／Ｄ≧４．１となる電極試料４、５では、バインダ量は電極合剤の集電体側から表面側に向かうに従って増加し、バインダが表面側に偏析している。一方、Ｔ／Ｄ≦２．１となる電極試料１、２、３では、バインダ量は電極合剤の表面側から集電体側に向かうに従って増加し、バインダが集電体側に偏析している。特に、電極試料１、２の場合、電極合剤の集電体側のバインダ量は、表面側のバインダ量の１．５倍以上になる。電極試料２、３のデータを内挿すると、Ｔ／Ｄ≦２．０であれば、電極合剤の集電体側のバインダ量が表面側のバインダ量の１．５倍以上になると推定できる。 As shown in FIG. 5, in the electrode samples 4 and 5 where T / D ≧ 4.1, the amount of binder increases from the current collector side of the electrode mixture toward the surface side, and the binder segregates on the surface side. Yes. On the other hand, in the electrode samples 1, 2, and 3 where T / D ≦ 2.1, the amount of the binder increases from the surface side of the electrode mixture toward the current collector side, and the binder is segregated on the current collector side. In particular, in the case of the electrode samples 1 and 2, the amount of the binder on the current collector side of the electrode mixture is 1.5 times or more the amount of the binder on the surface side. When the data of the electrode samples 2 and 3 are interpolated, if T / D ≦ 2.0, it can be estimated that the binder amount on the current collector side of the electrode mixture is 1.5 times or more the binder amount on the surface side.
　このように、電極合剤の厚みＴと活物質粒子の体積基準の中心粒径Ｄとの相対関係でバインダ分布が変化する原因は次のように考えられる。 Thus, the cause of the binder distribution changing depending on the relative relationship between the thickness T of the electrode mixture and the volume-based center particle diameter D of the active material particles is considered as follows.
　電極合剤の厚みＴが活物質粒子の体積基準の中心粒径Ｄに対して大きい場合（例えばＴ／Ｄ≧４．１の場合）は、乾燥工程において、電極合剤の厚み方向に温度勾配が生じ、電極合剤の表面が乾燥炉の温度まで上昇する。しかし、一方で、電極合剤の内部の温度上昇は小さい。このとき溶剤は電極合剤の表面で蒸発し、電極合剤の内部の溶剤が表面に移動することで、蒸発が進行する。溶剤に溶解したバインダも溶剤とともに電極合剤の表面側に移動し、溶剤が蒸発することで析出する。その結果、電極合剤の表面側にバインダ偏析が生じる。 When the thickness T of the electrode mixture is larger than the volume-based center particle diameter D of the active material particles (for example, T / D ≧ 4.1), a temperature gradient in the thickness direction of the electrode mixture in the drying step And the surface of the electrode mixture rises to the temperature of the drying furnace. However, on the other hand, the temperature rise inside the electrode mixture is small. At this time, the solvent evaporates on the surface of the electrode mixture, and evaporation proceeds as the solvent inside the electrode mixture moves to the surface. The binder dissolved in the solvent also moves to the surface side of the electrode mixture together with the solvent, and precipitates as the solvent evaporates. As a result, binder segregation occurs on the surface side of the electrode mixture.
　これに対して、電極合剤の厚みＴが活物質粒子の体積基準の中心粒径Ｄに対して小さい場合（例えばＴ／Ｄ≦２．１の場合）は、乾燥工程において、電極合剤の厚み方向の温度勾配は小さく、電極合剤の表面だけでなく、電極合剤の内部も一様に乾燥炉の温度まで上昇する。これは、Ｔ／Ｄが大きい場合に比べ、電極合剤の熱抵抗が減少するためである。活物質粒子の体積基準の中心粒径Ｄを一定として、電極合剤の厚みＴを小さくすると、電極合剤の厚みＴが小さくなるため、電極合剤の熱抵抗は小さくなる。また、電極合剤の厚みＴを一定として、活物質粒子の体積基準の中心粒径Ｄを大きくすると、電極合剤の活物質粒子間の隙間は大きくなり、溶剤の熱対流により熱が伝わるようになるため、電極合剤の熱抵抗は小さくなる。 On the other hand, when the thickness T of the electrode mixture is smaller than the volume-based center particle diameter D of the active material particles (for example, when T / D ≦ 2.1), in the drying process, The temperature gradient in the thickness direction is small, and not only the surface of the electrode mixture but also the inside of the electrode mixture rises uniformly to the temperature of the drying furnace. This is because the thermal resistance of the electrode mixture is reduced as compared with the case where T / D is large. If the volume-based center particle diameter D of the active material particles is constant and the thickness T of the electrode mixture is reduced, the thickness T of the electrode mixture is reduced, so that the thermal resistance of the electrode mixture is reduced. Further, when the electrode mixture thickness T is constant and the volume-based center particle diameter D of the active material particles is increased, the gap between the active material particles of the electrode mixture is increased so that heat is transferred by thermal convection of the solvent. Therefore, the thermal resistance of the electrode mixture is reduced.
　このように、電極合剤中の温度勾配が小さい場合、溶剤は集電体の表面を含む電極合剤の内部の全域で蒸発する。このとき、溶剤の表面張力のため、溶剤は活物質粒子および集電体の表面に残留するようにして蒸発が進行し、バインダは活物質粒子の表面だけでなく、集電体の表面にも等しく析出する。電極合剤の内部でバインダは均一に析出するが、集電体の表面に析出したバインダの影響で、電極合剤全体でみると集電体側にバインダ偏析が生じる。 Thus, when the temperature gradient in the electrode mixture is small, the solvent evaporates in the entire area of the electrode mixture including the surface of the current collector. At this time, due to the surface tension of the solvent, evaporation proceeds so that the solvent remains on the surface of the active material particles and the current collector, and the binder is not only on the surface of the active material particles but also on the surface of the current collector. Precipitate equally. The binder is uniformly deposited inside the electrode mixture. However, due to the binder deposited on the surface of the current collector, binder segregation occurs on the current collector side as a whole.
　前記特許文献１には、電極膜の乾燥によって生じる電極膜内のバインダの不均一分布を抑制する方法として、バインダを析出させる溶剤を電極材ペーストに接触させることでバインダを固化する工程を追加し、バインダが固化した状態で電極膜を乾燥させる方法が記載されている。また、バインダの固化は電極膜の表面から集電箔側へと進むため、電極膜の表面側から固化の進んでいない集電箔側へバインダが移動し、集電箔側のバインダ量が表面側のバインダ量よりも多くなるが、電極膜の集電箔側のバインダ濃度と表面側のバインダ濃度との差は５０％以内であることが記載されている。 Patent Document 1 adds a step of solidifying a binder by bringing a solvent for precipitating the binder into contact with the electrode material paste as a method of suppressing the non-uniform distribution of the binder in the electrode film caused by drying of the electrode film. A method of drying an electrode film in a state where a binder is solidified is described. Also, since the binder solidification proceeds from the surface of the electrode film to the current collector foil side, the binder moves from the surface side of the electrode film to the current collector foil side where solidification has not progressed, and the amount of binder on the current collector foil side is It is described that the difference between the binder concentration on the current collector foil side of the electrode film and the binder concentration on the surface side is within 50%, although the amount of the binder on the side is larger.
　一方、本実施例１では、電極合剤の厚みＴと活物質粒子の体積基準の中心粒径Ｄとの相対関係によってバインダ分布を制御するため、前記特許文献１に記載されているバインダを固化する工程が不要である。従って、より低コストでバインダ分布を制御することができる。さらに、前記特許文献１では、電極膜の集電箔側のバインダ濃度と表面側のバインダ濃度との差は５０％以内であるのに対し、本実施例１では、Ｔ／Ｄ≦１．６を満たす電極試料１、２のように、集電体側のバインダ量が表面側のバインダ量の１．５倍以上であるような電極合剤を作製することができる。さらに、Ｔ／Ｄ≦１．１を満たす電極試料１のように、集電体側のバインダ量が表面側のバインダ量の２．６倍まで増加した電極合剤を作製することができる。 On the other hand, in Example 1, in order to control the binder distribution by the relative relationship between the thickness T of the electrode mixture and the volume-based center particle diameter D of the active material particles, the binder described in Patent Document 1 is solidified. The process to do is unnecessary. Therefore, the binder distribution can be controlled at a lower cost. Further, in Patent Document 1, the difference between the binder concentration on the current collector foil side of the electrode film and the binder concentration on the surface side is within 50%, whereas in Example 1, T / D ≦ 1.6. As in electrode samples 1 and 2 that satisfy the above conditions, an electrode mixture in which the amount of the binder on the current collector side is 1.5 times or more the amount of the binder on the surface side can be produced. Furthermore, like the electrode sample 1 satisfying T / D ≦ 1.1, an electrode mixture in which the amount of binder on the current collector side is increased to 2.6 times the amount of binder on the surface side can be produced.
　次に、バインダ分布が変化したときの電極合剤と集電体との密着強度に関して説明する。 Next, the adhesion strength between the electrode mixture and the current collector when the binder distribution changes will be described.
　図６は、本実施例１によるピール強度の測定装置を模式的に示す側面図である。Ｔ／Ｄが互いに異なる電極試料１、２、３、４、５を用いて、集電体に対する電極合剤のピール強度を測定した。 FIG. 6 is a side view schematically showing a peel strength measuring apparatus according to the first embodiment. The peel strength of the electrode mixture with respect to the current collector was measured using electrode samples 1, 2, 3, 4, and 5 having different T / D.
　両面テープＴＰの一面を電極合剤ＥＣの表面側に、もう一面を円形板ＣＰに貼る。円形板ＣＰは、その中心点ＣＣＰのみが金属部品ＭＰに固定されており、図６の矢印に示す様に回転することができる。集電体ＥＰのうち、電極合剤ＥＣが表面に形成されていない箇所をロードセルＬＣと接続させ、金属部品ＭＰを図６の鉛直下方向に、５０ｍｍ／分の速度で移動させる。このときのロードセルＬＣにかかる平均荷重を電極合剤ＥＣの幅（２ｃｍ）で除することで、ピール強度を算出する。 ¡Attach one side of the double-sided tape TP to the surface side of the electrode mixture EC and the other side to the circular plate CP. Only the center point CCP of the circular plate CP is fixed to the metal part MP, and can be rotated as shown by the arrow in FIG. A portion of the current collector EP where the electrode mixture EC is not formed on the surface is connected to the load cell LC, and the metal part MP is moved in the vertical downward direction in FIG. 6 at a speed of 50 mm / min. The peel strength is calculated by dividing the average load applied to the load cell LC at this time by the width (2 cm) of the electrode mixture EC.
　図７は、本実施例１によるＴ／Ｄが互いに異なる電極試料１、２、３、４、５のピール強度を示すグラフ図である。 FIG. 7 is a graph showing the peel strength of electrode samples 1, 2, 3, 4, and 5 having different T / D values according to Example 1.
　電極合剤の集電体側のバインダ量が表面側のバインダ量よりも多い電極試料１、２、３は、電極合剤の集電体側のバインダ量が表面側のバインダ量よりも少ない電極試料４、５よりもピール強度が増加していることが分かる。試験後、全ての電極試料１、２、３、４、５を観察すると、電極合剤と集電体との界面で剥離が進展していたことから、電極合剤において、活物質粒子と集電体との密着が最も弱いといえる。従って、バインダが集電体側に偏析すると、活物質粒子と集電体とを接着させるバインダ量が相対的に増加するため、効果的にピール強度が増加したと考えられる。 Electrode Samples 1, 2, and 3 in which the amount of binder on the current collector side of the electrode mixture is larger than the amount of binder on the surface side are electrode samples 4 in which the amount of binder on the current collector side of the electrode mixture is smaller than the amount of binder on the surface side It can be seen that the peel strength is higher than 5. After the test, when all of the electrode samples 1, 2, 3, 4, 5 were observed, peeling progressed at the interface between the electrode mixture and the current collector. It can be said that the adhesion to the electric body is the weakest. Therefore, it is considered that when the binder segregates on the current collector side, the amount of the binder for bonding the active material particles and the current collector relatively increases, and thus the peel strength is effectively increased.
　また、電極合剤の集電体側のバインダ量が表面側のバインダ量の２．６１倍である電極試料１では、電極合剤の集電体側のバインダ量が表面側のバインダ量の０．８９倍である電極試料４よりも、ピール強度が１８．８倍まで大幅に増加していることが分かる。なお、電極試料１の電極合剤の厚みＴは１２．１μｍ、活物質粒子の体積基準の中心粒径Ｄは１１．０μｍである。電極合剤の集電体側のバインダ量が表面側のバインダ量よりも多いほど、ピール強度が向上していることから、電極合剤の集電体側のバインダ量が表面側のバインダ量の２．６１倍以上であれば、さらにピール強度が増加すると考えられる。 In the electrode sample 1 in which the binder amount on the current collector side of the electrode mixture is 2.61 times the binder amount on the surface side, the binder amount on the current collector side of the electrode mixture is 0.89 of the binder amount on the surface side. It can be seen that the peel strength is significantly increased to 18.8 times that of the electrode sample 4 which is doubled. In addition, the thickness T of the electrode mixture of the electrode sample 1 is 12.1 μm, and the volume-based center particle diameter D of the active material particles is 11.0 μm. Since the peel strength is improved as the binder amount on the current collector side of the electrode mixture is larger than the binder amount on the surface side, the binder amount on the current collector side of the electrode mixture is 2. If it is 61 times or more, it is considered that the peel strength further increases.
　このように、集電体側にバインダが偏析するバインダ分布をもつ電極合剤は、密着強度が増加する。これは、電極試料１、２、３のようなバインダ分布をもつ電極にすれば、電極試料４、５のようなバインダ分布をもつ電極に比べて、少ないバインダ量でも同等の密着強度を維持できることを意味している。 Thus, the electrode mixture having a binder distribution in which the binder segregates on the current collector side increases the adhesion strength. This means that if an electrode having a binder distribution such as electrode samples 1, 2, and 3 is used, the same adhesion strength can be maintained even with a small amount of binder compared to an electrode having a binder distribution such as electrode samples 4 and 5. Means.
　そこで、バインダが３．５重量％よりも低減した正極合剤を作製し、その密着強度および電池特性を評価した。 Therefore, a positive electrode mixture in which the binder was reduced to less than 3.5% by weight was prepared, and its adhesion strength and battery characteristics were evaluated.
Figure JPOXMLDOC01-appb-T000001
 　表１は、ピール強度および電池特性の評価に使用した電極試料６、７、８における、活物質、導電助剤、およびバインダの重量％を示している。電極試料６、７、８では、活物質および導電助剤の重量％をほぼ一定にしたまま、バインダの重量％のみを減少させた。活物質には、体積基準の中心粒径Ｄが１１．０μｍの活物質粒子を用い、活物質粒子の体積基準の中心粒径Ｄに対する電極合剤の厚みＴ（Ｔ／Ｄ）を１．５とすることで、バインダが集電箔側に偏析した電極合剤を作製した。
Figure JPOXMLDOC01-appb-T000001
 Table 1 shows the weight percentages of the active material, the conductive additive, and the binder in the electrode samples 6, 7, and 8 used for the evaluation of peel strength and battery characteristics. In the electrode samples 6, 7, and 8, only the weight percent of the binder was decreased while the weight percent of the active material and the conductive auxiliary agent was kept substantially constant. As the active material, active material particles having a volume-based center particle diameter D of 11.0 μm are used, and the thickness T (T / D) of the electrode mixture with respect to the volume-based center particle diameter D of the active material particles is 1.5. As a result, an electrode mixture in which the binder was segregated on the side of the current collector foil was produced.
　図８は、本実施例１によるバインダの重量％が互いに異なる電極試料６、７、８のピール強度を示すグラフ図である。 FIG. 8 is a graph showing the peel strength of the electrode samples 6, 7, and 8 having different binder weight percentages according to the first embodiment.
　バインダが２．６重量％である電極試料７のピール強度は１３９Ｎ／ｍ、バインダが１．５重量％である電極試料８のピール強度は１１５Ｎ／ｍであり、Ｔ／Ｄを１．５とすることで、バインダが３．５重量％でバインダ偏析がない電極合剤（電極試料４）のピール強度４０Ｎ／ｍと同等以上の密着強度を有することを確認した。 The peel strength of the electrode sample 7 having a binder of 2.6% by weight is 139 N / m, the peel strength of the electrode sample 8 having a binder of 1.5% by weight is 115 N / m, and T / D is 1.5. Thus, it was confirmed that the binder had an adhesion strength equal to or higher than the peel strength of 40 N / m of the electrode mixture (electrode sample 4) having 3.5% by weight and no binder segregation.
　図７および図８にそれぞれ示したピール強度の結果と合わせて考えると、Ｔ／Ｄ≦１．５であれば、バインダを３．５重量％から１．５重量％へ低減しても、バインダが３．５重量％でバインダ偏析がない電極合剤（電極試料４）のピール強度４０Ｎ／ｍと同等以上の密着強度を有すると考えられる。また、Ｔ／Ｄ≦１．５のとき、図５における電極試料１、２のデータを内挿すると、電極合剤の集電体側のバインダ量が表面側のバインダ量の１．８倍以上になると推定できる。 Considering together with the peel strength results shown in FIG. 7 and FIG. 8 respectively, if T / D ≦ 1.5, the binder can be reduced from 3.5% by weight to 1.5% by weight. Is 3.5% by weight, and it is considered that the electrode mixture (electrode sample 4) having no binder segregation has an adhesion strength equal to or higher than the peel strength of 40 N / m. Further, when T / D ≦ 1.5, when the data of the electrode samples 1 and 2 in FIG. 5 are interpolated, the amount of the binder on the current collector side of the electrode mixture becomes 1.8 times or more of the amount of the binder on the surface side. It can be estimated that
　電極合剤の密着強度が低いと、電池の充放電に伴い電極合剤が集電体から脱落する場合がある。電極合剤の脱落が起きると、充放電可能な容量が低下するため、電池の容量、出力がともに低下する。本実施例１では、バインダを電極合剤の集電箔側に偏析させることで、電極合剤の密着強度を増加することができるため、電池特性が低下することなく、バインダ量の低減が可能となる。 If the adhesion strength of the electrode mixture is low, the electrode mixture may fall off the current collector as the battery is charged / discharged. When the electrode mixture is dropped, the chargeable / dischargeable capacity is lowered, so that both the capacity and output of the battery are lowered. In Example 1, since the adhesion strength of the electrode mixture can be increased by segregating the binder to the current collector foil side of the electrode mixture, the amount of the binder can be reduced without deteriorating battery characteristics. It becomes.
　次に、電極試料６、７、８の電池特性の評価結果について説明する。 Next, the evaluation results of the battery characteristics of the electrode samples 6, 7, and 8 will be described.
　評価用のリチウムイオン電池を作製するため、活物質にリチウムマンガンコバルトニッケル複合酸化物、導電助剤に黒鉛粉末、バインダにＰＶＤＦを使用した正極合剤をアルミニウムからなる集電体の表面に作成した。また、活物質に非晶質炭素、導電助剤にカーボンブラック、バインダにＰＶＤＦを使用した負極合剤を銅からなる集電体の表面に作製した。セパレータには、厚さ２０μｍの多孔質ポリプロピレンを使用した。電解液には、電解塩ＬｉＰＦ６を溶解させた有機溶媒（エチルカーボネート、ジメチルカーボネート、エチルメチルカーボネートの混合液）を使用した。正極合剤および負極合剤は熱ロールプレスにより、密度をそれぞれ２．１ｇ／ｃｃおよび１．４ｇ／ｃｃに調整した。 In order to produce a lithium ion battery for evaluation, a positive electrode mixture using lithium manganese cobalt nickel composite oxide as an active material, graphite powder as a conductive additive and PVDF as a binder was formed on the surface of a current collector made of aluminum. . Further, a negative electrode mixture using amorphous carbon as an active material, carbon black as a conductive additive, and PVDF as a binder was formed on the surface of a current collector made of copper. As the separator, a porous polypropylene having a thickness of 20 μm was used. As the electrolytic solution, an organic solvent (a mixed solution of ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate) in which the electrolytic salt LiPF 6 was dissolved was used. The density of the positive electrode mixture and the negative electrode mixture was adjusted to 2.1 g / cc and 1.4 g / cc, respectively, by a hot roll press.
　正極、負極、およびセパレータをそれぞれ直径１４ｍｍ、１６ｍｍ、および１６ｍｍに打ち抜き、積層した後、電解液を全体に含浸させ、評価用のリチウムイオン電池を作製した。作製したリチウムイオン電池を４．１Ｖまで定電流２ｍＡで充電した後、定電圧充電を行い、満充電の状態にした。満充電の状態から、Ｃレートが０．２Ｃ～５．０Ｃの各電流値で放電させ、１０秒後のセル電圧を測定し、放電電流（Ｃレート単位）に対するセル電圧の傾きをリチウムイオン電池の内部抵抗として評価した。Ｃレートとは、電池容量を１時間で放電する電流値を基準にした電流値の表現である。例えば１時間かけて放電するときの電流値は１Ｃ、０．５時間かけて放電するときの電流値は２Ｃ、０．２時間かけて放電するときの電流値は５Ｃとなる。内部抵抗が低いほど、放電時のセル電圧の低下が小さいため、高出力で放電することができることを意味する。 The positive electrode, the negative electrode, and the separator were punched into a diameter of 14 mm, 16 mm, and 16 mm, respectively, and then laminated, and then the electrolyte was impregnated as a whole to produce a lithium ion battery for evaluation. The produced lithium ion battery was charged at a constant current of 2 mA up to 4.1 V, and then charged at a constant voltage to obtain a fully charged state. From a fully charged state, the C rate is discharged at each current value of 0.2 C to 5.0 C, the cell voltage after 10 seconds is measured, and the slope of the cell voltage with respect to the discharge current (C rate unit) is measured as a lithium ion battery. Was evaluated as internal resistance. The C rate is an expression of a current value based on a current value for discharging the battery capacity in one hour. For example, the current value when discharging over 1 hour is 1C, the current value when discharging over 0.5 hours is 2C, and the current value when discharging over 0.2 hours is 5C. The lower the internal resistance, the smaller the cell voltage drop during discharge, which means that discharge can be performed with high output.
　図９は、本実施例１によるバインダの重量％が互いに異なる電極試料６、７、８を使用して作製したリチウムイオン電池の内部抵抗を示すグラフ図である。 FIG. 9 is a graph showing the internal resistance of a lithium ion battery manufactured using electrode samples 6, 7, and 8 having different binder weight percentages according to the first embodiment.
　バインダ量を低減することで、内部抵抗が低減することを確認した。特に、バインダを１．５重量％とした電極試料８を用いたときは、バインダが３．５重量％の電極試料６に対し、内部抵抗が０．８７７倍まで大幅に低減した。 It was confirmed that the internal resistance was reduced by reducing the amount of binder. In particular, when the electrode sample 8 with 1.5% by weight of binder was used, the internal resistance was greatly reduced to 0.877 times that of the electrode sample 6 with 3.5% by weight of binder.
　このように、本実施例１によるリチウムイオン電池では、電極合剤に含まれるバインダが集電体側に偏析し、電極合剤の集電体側のバインダ量が表面側のバインダ量より多いバインダ分布を有する電極合剤からなる電極を備えることを特徴とする。集電体側にバインダが偏析した電極合剤は、集電体に対する密着強度が増加するため、電極合剤全体に含まれるバインダの総量を低減しても、バインダ分布が均一な電極合剤と同等の密着強度をもつことができる。活物質粒子と電解液間のリチウムイオンの伝導、または活物質粒子と集電体間の電子の伝導を阻害するバインダが低減することで、リチウムイオン電池の内部抵抗を減少させることができるため、リチウムイオン電池の高出力化を図ることができる。 As described above, in the lithium ion battery according to Example 1, the binder contained in the electrode mixture segregates on the current collector side, and the binder distribution on the current collector side of the electrode mixture has a binder distribution larger than the binder amount on the surface side. It has the electrode which consists of an electrode mixture which has. The electrode mixture with the binder segregated on the current collector side increases the adhesion strength to the current collector, so even if the total amount of binder contained in the entire electrode mixture is reduced, it is equivalent to the electrode mixture with a uniform binder distribution The adhesion strength can be as follows. Because the binder that inhibits the conduction of lithium ions between the active material particles and the electrolyte or the conduction of electrons between the active material particles and the current collector is reduced, the internal resistance of the lithium ion battery can be reduced. The output of the lithium ion battery can be increased.
　なお、本実施例１では、主として、正極合剤について実施の詳細を説明したが、負極合剤に対しても適用可能であり、負極合剤の集電体側のバインダ量が表面側のバインダ量よりも多いバインダ分布を有しても良い。負極合剤においても、前述したように集電体側にバインダが偏析すると、集電体に対する負極合剤の密着強度が増加するため、負極合剤中のバインダ量を低減させることが可能となり、リチウムイオン電池の高出力化を図ることができる。 In addition, in this Example 1, although the implementation details were mainly described for the positive electrode mixture, the present invention is applicable to the negative electrode mixture, and the amount of the binder on the current collector side of the negative electrode mixture is the amount of the binder on the surface side. It may have more binder distribution. Also in the negative electrode mixture, as described above, when the binder segregates on the current collector side, the adhesion strength of the negative electrode mixture to the current collector increases, so the amount of the binder in the negative electrode mixture can be reduced, and lithium The output of the ion battery can be increased.
　本実施例２によるリチウムイオン電池について図１０から図１２を用いて説明する。 The lithium ion battery according to Example 2 will be described with reference to FIGS.
　本実施例２では、電極合剤に含まれるバインダが集電体側に偏析し、電極合剤の集電体側のバインダ量が表面側のバインダ量よりも多いバインダ分布を有する電極合剤を、リチウムイオン電池の充放電中において活物質が脱落しやすい箇所に選択的に形成した。 In Example 2, the binder contained in the electrode mixture is segregated on the current collector side, and the electrode mixture having a binder distribution in which the binder amount on the current collector side of the electrode mixture is larger than the binder amount on the surface side is changed to lithium. It was selectively formed at a location where the active material was likely to fall off during charge / discharge of the ion battery.
　リチウムイオン電池の充放電により、活物質はリチウムイオンを吸蔵または放出し、体積変化を起こす。この体積変化により生じる応力が、バインダによる接着力を上回ると活物質が電極合剤から脱落する。脱落した活物質は充放電に寄与できないため、リチウムイオン電池の特性低下につながる。 The active material occludes or releases lithium ions due to charge / discharge of the lithium ion battery, causing a volume change. When the stress caused by this volume change exceeds the adhesive force by the binder, the active material falls off from the electrode mixture. Since the dropped active material cannot contribute to charging / discharging, the characteristics of the lithium ion battery are deteriorated.
　例えば図１、図２に示すような捲回型のリチウムイオン電池の場合、軸芯ＣＲに近い捲回体ＷＲＦの内部（内側）の電極合剤は、周囲の電極合剤および集電体により拘束されているため、充放電に伴い電極合剤中に応力が発生しても、活物質は電極合剤から脱落しにくい。一方、捲回体ＷＲＦの外部（外側、外周付近）の電極合剤は、周囲からの拘束がないため、活物質は電極合剤から脱落しやすい。 For example, in the case of a wound-type lithium ion battery as shown in FIGS. 1 and 2, the electrode mixture inside (inside) the wound body WRF close to the axial core CR depends on the surrounding electrode mixture and the current collector. Since it is restrained, even if stress occurs in the electrode mixture due to charge / discharge, the active material is unlikely to fall off from the electrode mixture. On the other hand, since the electrode mixture outside (outside, near the outer periphery) of the wound body WRF is not constrained from the surroundings, the active material is easily dropped from the electrode mixture.
　図１０は、本実施例２による角型の外装缶を有するリチウムイオン電池の構成を模式的に示す斜視図である。捲回体の構成は、図２および図３とほぼ同様である。 FIG. 10 is a perspective view schematically showing a configuration of a lithium ion battery having a rectangular outer can according to the second embodiment. The configuration of the wound body is almost the same as in FIGS.
　図１０に示すように、角型の捲回体ＷＲＦを有するリチウムイオン電池の場合、集電体とその表面に形成された電極合剤の曲率半径は不均一となる。特に、曲率半径の小さい捲回体ＷＲＦの角ＣＷＲＦ付近では、電極合剤が引き伸ばされるため、活物質粒子同士を接着するバインダが切断されやすく、その結果、活物質も脱落しやすい。 As shown in FIG. 10, in the case of a lithium ion battery having a rectangular wound body WRF, the radius of curvature of the current collector and the electrode mixture formed on the surface thereof is not uniform. In particular, in the vicinity of the corner CWRF of the wound body WRF having a small radius of curvature, the electrode mixture is stretched, so that the binder that bonds the active material particles is easily cut, and as a result, the active material is also likely to fall off.
　本実施例２によるリチウムイオン電池では、電極合剤に含まれるバインダが集電体側に偏析し、集電体側のバインダ量が表面側のバインダ量よりも多いバインダ分布を有する電極合剤を、リチウムイオン電池の充放電中において活物質が脱落しやすい箇所に選択的に形成することを特徴とする。例えばダイコータを電極用スラリーの塗布に用いた場合、上記箇所では電極用スラリーの塗出圧力を低減して、電極合剤の厚みＴを減少させることにより、Ｔ／Ｄを低減し、バインダが集電体側に偏析したバインダ分布を選択的に形成することができる。 In the lithium ion battery according to Example 2, the binder contained in the electrode mixture segregates on the current collector side, and the electrode mixture having a binder distribution in which the amount of the binder on the current collector side is larger than the amount of the binder on the surface side, It is characterized in that it is selectively formed at a location where the active material is likely to fall off during charging / discharging of the ion battery. For example, when a die coater is used for applying the electrode slurry, the T / D is reduced and the binder is collected by reducing the coating pressure of the electrode slurry and reducing the thickness T of the electrode mixture at the above location. A binder distribution segregated on the electric body side can be selectively formed.
　図１１は、本実施例２による捲回型の捲回体を解体したときの電極の構成を模式的に示す平面図である。 FIG. 11 is a plan view schematically showing the configuration of the electrode when the wound type wound body according to the second embodiment is disassembled.
　捲回型の捲回体のリチウムイオン電池の場合、軸芯に近い捲回体内部（内側）より、捲回体外部（外側、外周付近）において、集電体側のバインダ量が表面側のバインダ量よりも多いバインダ分布を有する電極合剤の割合を多くすることで、活物質が脱落しやすい捲回体外部の電極合剤の密着強度を向上することができる。その結果、リチウムイオン電池の充放電サイクルに対する特性信頼性を向上させることができる。 In the case of a wound-type wound lithium-ion battery, the amount of binder on the current collector side is larger than the inside of the wound body (inside) near the shaft core, outside the wound body (outside, near the outer periphery). Increasing the ratio of the electrode mixture having a binder distribution larger than the amount can improve the adhesion strength of the electrode mixture outside the wound body where the active material is easily removed. As a result, the characteristic reliability with respect to the charge / discharge cycle of the lithium ion battery can be improved.
　図１２は、本実施例２による角型の捲回体を解体したときの電極の構成を模式的に示す平面図である。 FIG. 12 is a plan view schematically showing the configuration of the electrode when the rectangular wound body according to the second embodiment is disassembled.
　角型の捲回体のリチウムイオン電池の場合、捲回体を形成する時に角（曲率半径が小さい箇所）になる場所において、集電体側のバインダ量が表面側のバインダ量よりも多いバインダ分布を有する電極合剤の割合を多くすることで、活物質が脱落しやすい捲回体を形成する時に角（曲率半径が小さい箇所）になる場所の電極合剤の密着強度を向上することができる。その結果、リチウムイオン電池の充放電サイクルに対する特性信頼性を向上させることができる。 In the case of a rectangular wound lithium-ion battery, the binder distribution in which the amount of binder on the current collector side is larger than the amount of binder on the surface side at the corner (where the radius of curvature is small) when forming the wound body By increasing the ratio of the electrode mixture having the ratio, the adhesion strength of the electrode mixture at the corner (where the radius of curvature is small) can be improved when forming a wound body from which the active material is easily removed. . As a result, the characteristic reliability with respect to the charge / discharge cycle of the lithium ion battery can be improved.
　以上、本発明者によってなされた発明を実施の形態に基づき具体的に説明したが、本発明は前記実施の形態に限定されるものではなく、その要旨を逸脱しない範囲で種々変更可能であることはいうまでもない。 As mentioned above, the invention made by the present inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and various modifications can be made without departing from the scope of the invention. Needless to say.
　本実施の形態では、捲回型リチウムイオン電池を例に挙げて、本発明の技術的思想について説明したが、本発明の技術的思想は、捲回型リチウムイオン電池に限定されるものではない。例えば正極、負極、および正極と負極とを電気的に分離するセパレータを備える蓄電デバイス（例えば電池またはキャパシタなど）に幅広く適用することができる。 In the present embodiment, the wound-type lithium ion battery is taken as an example to describe the technical idea of the present invention. However, the technical idea of the present invention is not limited to the wound-type lithium ion battery. . For example, it can be widely applied to an electricity storage device (for example, a battery or a capacitor) including a positive electrode, a negative electrode, and a separator that electrically separates the positive electrode and the negative electrode.
　本発明は、例えばリチウムイオン電池に代表される電池を製造する製造業に幅広く利用することができる。 The present invention can be widely used in the manufacturing industry for manufacturing batteries represented by, for example, lithium ion batteries.
　ＡＳ　活物質
　ＢＤ　バインダ
　ＣＡ　導電助剤
　ＣＣＰ　中心点
　ＣＰ　円形板
　ＣＲ　軸芯
　ＣＳ　外装缶
　ＣＷＲＦ　角
　ＥＣ　電極合剤
　ＥＬ　電解液
　ＥＰ　集電体
　ＥＲ　電極
　ＬＣ　ロードセル
　ＭＰ　金属部品
　ＮＥＲ　負極
　ＰＥＲ　正極
　ＳＰ　セパレータ
　ＴＰ　両面テープ
　ＷＲＦ　捲回体 AS active material BD binder CA conductive auxiliary agent CCP center point CP circular plate CR shaft core CS outer can CWRF angle EC electrode mixture EL electrolytic solution EP current collector ER electrode LC load cell MP metal parts NER negative electrode PER positive electrode SP separator TP double-sided tape WRF

Claims (14)

  1. 　集電体と、前記集電体の表面に形成され、活物質、導電助剤およびバインダを含む電極合剤と、からなる電極を有し、
    　前記電極合剤は、前記電極合剤の厚み方向において、前記集電体側の第１バインダ量が、前記集電体と反対側の第２バインダ量よりも多いバインダ分布を有する部分を含む、リチウムイオン電池。     An electrode formed of a current collector and an electrode mixture formed on the surface of the current collector and including an active material, a conductive additive, and a binder;
    The electrode mixture includes a portion having a binder distribution in which the first binder amount on the current collector side is greater than the second binder amount on the opposite side to the current collector in the thickness direction of the electrode mixture. Ion battery.
  2. 　請求項１記載のリチウムイオン電池において、
    　前記第１バインダ量が、前記第２バインダ量の１．５倍以上である、リチウムイオン電池。     The lithium ion battery according to claim 1,
    The lithium ion battery, wherein the first binder amount is 1.5 times or more of the second binder amount.
  3. 　請求項１記載のリチウムイオン電池において、
    　前記第１バインダ量が、前記第２バインダ量の１．８倍以上である、リチウムイオン電池。     The lithium ion battery according to claim 1,
    The lithium ion battery, wherein the first binder amount is 1.8 times or more of the second binder amount.
  4. 　請求項１記載のリチウムイオン電池において、
    　前記バインダ分布を有する部分では、前記電極合剤の厚みをＴ、前記活物質を構成する活物質粒子の体積基準の中心粒径をＤとすると、Ｔ／Ｄが２．０以下である、リチウムイオン電池。     The lithium ion battery according to claim 1,
    In the portion having the binder distribution, T / D is 2.0 or less, where T is the thickness of the electrode mixture, and D is the volume-based center particle diameter of the active material particles constituting the active material. Ion battery.
  5. 　請求項１記載のリチウムイオン電池において、
    　前記バインダ分布を有する部分では、前記電極合剤の厚みをＴ、前記活物質を構成する活物質粒子の体積基準の中心粒径をＤとすると、Ｔ／Ｄが１．５以下である、リチウムイオン電池。     The lithium ion battery according to claim 1,
    In the portion having the binder distribution, T / D is 1.5 or less, where T is the thickness of the electrode mixture, and D is the volume-based center particle diameter of the active material particles constituting the active material. Ion battery.
  6. 　請求項１記載のリチウムイオン電池において、
    　前記電極合剤の全体に含まれるバインダが２．６重量％以下である、リチウムイオン電池。     The lithium ion battery according to claim 1,
    The lithium ion battery whose binder contained in the whole said electrode mixture is 2.6 weight% or less.
  7. 　請求項１～６のいずれか１項に記載のリチウムイオン電池において、
    　前記電極合剤と前記セパレータとが捲回体を形成し、
    　前記捲回体の外側の前記電極合剤における前記バインダ分布を有する部分の割合が、前記捲回体の内側の前記電極合剤における前記バインダ分布を有する部分の割合よりも高い、リチウムイオン電池。     The lithium ion battery according to any one of claims 1 to 6,
    The electrode mixture and the separator form a wound body,
    The lithium ion battery in which the proportion of the portion having the binder distribution in the electrode mixture outside the wound body is higher than the proportion of the portion having the binder distribution in the electrode mixture inside the wound body.
  8. 　請求項１～６のいずれか１項に記載のリチウムイオン電池において、
    　前記電極合剤と前記セパレータとが捲回体を形成し、
    　前記捲回体には、第１曲率半径を有する第１箇所と、前記第１曲率半径よりも大きい第２曲率半径を有する第２箇所とが形成され、
    　前記第１箇所における前記バインダ分布を有する部分の割合が、前記第２箇所における前記バインダ分布を有する部分の割合よりも高い、リチウムイオン電池。     The lithium ion battery according to any one of claims 1 to 6,
    The electrode mixture and the separator form a wound body,
    The winding body is formed with a first location having a first radius of curvature and a second location having a second radius of curvature greater than the first radius of curvature,
    The lithium ion battery in which the ratio of the portion having the binder distribution in the first location is higher than the ratio of the portion having the binder distribution in the second location.
  9. 　（ａ）集電体の表面に、活物質、導電助剤およびバインダを含むスラリー状の電極材料を塗布する工程、
    　（ｂ）前記電極材料を乾燥させて、前記集電体の表面に、前記活物質、前記導電助剤および前記バインダを含む電極合剤を形成する工程、
    を有し、
    　前記（ａ）工程において、前記電極材料の厚さ、または前記活物質を構成する活物質粒子の体積基準の中心粒径を制御することにより、
    　前記電極合剤の厚み方向において、前記集電体側の第１バインダ量が、前記集電体と反対側の第２バインダ量よりも多いバインダ分布を有する部分を含む前記電極合剤を形成する、リチウムイオン電池の製造方法。     (A) applying a slurry-like electrode material containing an active material, a conductive additive and a binder to the surface of the current collector
    (B) drying the electrode material to form an electrode mixture containing the active material, the conductive additive and the binder on the surface of the current collector;
    Have
    In the step (a), by controlling the thickness of the electrode material or the volume-based center particle diameter of the active material particles constituting the active material,
    In the thickness direction of the electrode mixture, the electrode mixture including a portion having a binder distribution in which the first binder amount on the current collector side is larger than the second binder amount on the side opposite to the current collector is formed. A method for producing a lithium ion battery.
  10. 　請求項９記載のリチウムイオン電池の製造方法において、
    　前記第１バインダ量が、前記第２バインダ量の１．５倍以上である、リチウムイオン電池の製造方法。     In the manufacturing method of the lithium ion battery according to claim 9,
    The method of manufacturing a lithium ion battery, wherein the first binder amount is 1.5 times or more the second binder amount.
  11. 　請求項９記載のリチウムイオン電池の製造方法において、
    　前記第１バインダ量が、前記第２バインダ量の１．８倍以上である、リチウムイオン電池の製造方法。     In the manufacturing method of the lithium ion battery according to claim 9,
    The method of manufacturing a lithium ion battery, wherein the first binder amount is 1.8 times or more of the second binder amount.
  12. 　請求項９記載のリチウムイオン電池の製造方法において、
    　前記バインダ分布を有する部分では、前記電極合剤の厚みをＴ、前記活物質を構成する活物質粒子の体積基準の中心粒径をＤとすると、Ｔ／Ｄが２．０以下である、リチウムイオン電池の製造方法。     In the manufacturing method of the lithium ion battery according to claim 9,
    In the portion having the binder distribution, T / D is 2.0 or less, where T is the thickness of the electrode mixture, and D is the volume-based center particle diameter of the active material particles constituting the active material. Ion battery manufacturing method.
  13. 　請求項９記載のリチウムイオン電池の製造方法において、
    　前記バインダ分布を有する部分では、前記電極合剤の厚みをＴ、前記活物質を構成する活物質粒子の体積基準の中心粒径をＤとすると、Ｔ／Ｄが１．５以下である、リチウムイオン電池の製造方法。     In the manufacturing method of the lithium ion battery according to claim 9,
    In the portion having the binder distribution, T / D is 1.5 or less, where T is the thickness of the electrode mixture, and D is the volume-based center particle diameter of the active material particles constituting the active material. Ion battery manufacturing method.
  14. 　請求項９記載のリチウムイオン電池の製造方法において、
    　前記電極電極材料に含まれるバインダが２．６重量％以下である、リチウムイオン電池の製造方法。     In the manufacturing method of the lithium ion battery according to claim 9,
    The manufacturing method of a lithium ion battery whose binder contained in the said electrode electrode material is 2.6 weight% or less.
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