WO2018162325A1

WO2018162325A1 - Energy storage device and method for producing the same

Info

Publication number: WO2018162325A1
Application number: PCT/EP2018/055079
Authority: WO
Inventors: Felix Eberle; Yuta OSUGI; Keisuke ANAMI; Kohei TSUJITA
Original assignee: Lithium Energy and Power GmbH and Co KG
Current assignee: Lithium Energy and Power GmbH and Co KG
Priority date: 2017-03-06
Filing date: 2018-03-01
Publication date: 2018-09-13
Anticipated expiration: 2019-09-06
Also published as: JP2018148707A

Abstract

Provided is an energy storage device including an electrode having a current collecting substrate, an active material layer, and an intermediate layer disposed between the current collecting substrate and the active material layer. The intermediate layer contains a crosslinkable compound causing a cross-linking reaction. A method for producing an energy storage device of the present invention is a method for producing an energy storage device including laminating an intermediate layer-forming material which contains a crosslinkable compound causing a cross-linking reaction on at least one side of a current collecting substrate, and laminating an active material layer so as to cover an intermediate layer formed of the intermediate layer-forming material.

Description

DESCRIPTION

TITLE OF THE INVENTION: ENERGY STORAGE DEVICE AND METHOD FOR PRODUCING THE SAME

TECHNICAL FIELD

[0001]

The present invention relates to an energy storage device and a method for producing the same.

BACKGROUND ART

[0002]

Secondary batteries typified by lithium ion secondary batteries are widely used for electronic devices such as personal computers and

communication terminals, automobiles, and the like because of their high energy density. Abnormalities such as heat generation, ignition, and rise in pressure may occur in such an energy storage device (a secondary battery or a capacitor) due to use which is usually unforeseeable or the like.

Therefore, it is required that the energy storage device has a function of stopping charge- discharge at the time of abnormality.

[0003]

As a secondary battery having the above function, a secondary battery including an electrode in which a current is cut off as temperature rises has been developed. Specifically, a lithium battery including an electrode having an active material layer (electrode material layer) containing a thermal activation material that causes a cross-linking reaction by heat has been developed (see Patent Document l). In this lithium battery, it is said that the thermal activation material in the active material layer becomes polymerized with heat generation and a movement of lithium ions is inhibited, and therefore conductivity is lowered. Also, an electrode provided with a resin layer having a structure in which emulsion particles are crosslinked by a crosslinking agent between a substrate and an active material layer has also been developed (see Patent Document 2). In the electrode, it is said that resistance increases as the emulsion particles expand due to heat generation.

PRIOR ART DOCUMENTS PATENT DOCUMENTS

[0004]

Patent Document l: JP-A-2012-134149

Patent Document 2- WO 14/077366 A

SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0005]

However, the so-called shutdown function at the time of abnormality as described above is required to have higher performance, and there is also room for improvement in a conventional energy storage device.

[0006]

The present invention has been made based on the above

circumstances, and an object thereof is to provide an energy storage device which is improved in a function of charge- discharge stoppage at the time of abnormality, and a method for producing the energy storage device.

MEANS FOR SOLVING THE PROBLEMS [0007]

An aspect of the present invention is an energy storage device including an electrode having a current collecting substrate, an active material layer, and an intermediate layers disposed between the current collecting substrate and the active material layer. The intermediate layer contains a crosslinkable compound causing a cross-linking reaction.

[0008]

Another aspect of the present invention is a method for producing an energy storage device including laminating an intermediate layer-forming material which contains a crosslinkable compound causing a cross-linking reaction on at least one side of a current collecting substrate, and

laminating an active material layer so as to cover an intermediate layer formed of the intermediate layer-forming material.

ADVANTAGES OF THE INVENTION

[0009]

According to the present invention, it is possible to provide an energy storage device which is improved in a function of charge- discharge stoppage at the time of abnormality, and a method for producing the energy storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

Fig. 1 is a schematic perspective view showing a nonaqueous electrolyte secondary battery according to one embodiment of the present invention.

Fig. 2 is a schematic cross- sectional view showing a positive electrode of the nonaqueous electrolyte secondary battery according to one embodiment of the present invention.

Fig. 3 is a schematic view showing an energy storage apparatus configured by assembling a plurality of nonaqueous electrolyte secondary batteries according to one embodiment of the present invention.

Fig. 4 is a graph showing a change in ACR in a nonaqueous electrolyte secondary battery of example and comparative example.

MODE FOR CARRYING OUT THE INVENTION

[0011]

A nonaqueous electrolyte secondary battery (hereinafter, sometimes simply referred to as "secondary battery") as an embodiment of the energy storage device of the present invention and a method for producing the same will be described in detail.

[0012]

A nonaqueous electrolyte secondary battery 1 of Fig. 1 includes an electrode assembly 2 and a battery case 3 housing the electrode assembly 2. Fig. 1 is a perspective view of an inside of the battery case 3. The electrode assembly 2 is formed by winding a pair of electrodes (a positive electrode and a negative electrode) with a separator interposed therebetween. The positive electrode is electrically connected to a positive electrode terminal 4 through a positive electrode lead 4', and the negative electrode is electrically connected to a negative electrode terminal 5 through a negative electrode lead 5'. Further, the battery case 3 is filled with a nonaqueous electrolyte.

[0013] <Positive Electrode>

The positive electrode of a pair of electrodes of the nonaqueous electrolyte secondary battery 1 is shown in Fig. 2. A positive electrode 10 includes a positive current collecting substrate 11, a positive active material layer 12, and an intermediate layer 13 disposed between the positive current collecting substrate 11 and the positive active material layer 12. The positive current collecting substrate 11 is an example of a current collecting substrate, and the positive active material layer 12 is an example of an active material layer. The positive electrode 10 is a layer structure formed by laminating the positive current collecting substrate 11, the intermediate layer 13 and the positive active material layer 12 in this order.

[0014]

The positive current collecting substrate 11 is a substrate having electrical conductivity. As a material of the positive current collecting substrate 11, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum and an aluminum alloy are preferred from the viewpoint of balance among an electric potential resistance, a high conductivity and a cost. Further, examples of the formation form of the positive current collecting substrate 11 include a foil, a vapor deposition film, and the like, and from the viewpoint of cost, a foil is preferred. That is, an aluminum foil is preferred as the positive current collecting substrate 11. Examples of the aluminum or the aluminum alloy include A1085P, A3003P, and the like prescribed in JIS-H-4000 (2014).

[0015]

The positive active material layer 12 is formed of a so-called positive composite containing a positive active material. The positive composite for forming the positive active material layer 12 contains optional components such as an electrical conduction aid, a binder, a thickener, a filler and the like, as required.

[0016]

Examples of the positive active material include composite oxides (Li_xCo02, Li_xNi02, Li_xMn03, Li_xNi_aCo(i-_a)02, Li_xNi_aMn6Co(i-_a-6)02, and the like having a layered crNaFe02 type crystal structure, Li_xMn204,

Li_xNi_aMn(2-a)04, and the like having a spinel type crystal structure) represented by Li_xMO_y (M represents at least one kind of transition metal), and polyanion compounds (LiFeP04, LiMnP04, LiNiP04, L1C0PO4,

Li3V2(P04)3, Li2MnSi04, L12C0PO4F, and the like) represented by

Li_wMe_x(XOy)z (Me represents at least one transition metal, and X

represents, for example, P, Si, B, V and the like). Elements or polyanions in these compounds may be partially substituted with other elements or anionic species. In the positive active material layer 12, one of these compounds may be used alone, or two or more of these compounds may be used in a mixture.

[0017]

The electrical conduction aid is not particularly limited. Examples of such an electrical conduction aid include natural or artificial graphite, carbon black such as furnace black, acetylene black, and Ketjen black, metal, conductive ceramics, and the like. Examples of a shape of the electrical conduction aid include powder, fiber, or the like. "Conductive material" means a material having electrical conductivity. "Having conductivity" means that a volume resistivity measured in accordance with

JIS-H-0505 (1975) is 10⁷ Ω-cm or less.

[0018]

Examples of the binder include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyimide and the like! elastomers such as ethylene-propylene- diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber and the like! polysaccharide polymers! and the like.

[0019]

Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. When the thickener has a functional group which reacts with lithium, it is preferred to previously deactivate the functional group by methylation or the like.

[0020]

The filler is not particularly limited. As a main component of the filler, polyolefin such as polypropylene and polyethylene, silica, alumina, zeolite, glass, and the like can be mentioned.

[0021]

It is preferred that the positive active material layer 12 (active material layer) substantially does not contain a crosslinkable compound as described later. By not substantially containing the crosslinkable compound in the active material layer, the conductivity of the positive electrode during normal use can be enhanced. An upper limit of a content of the crosslinkable compound in the active material layer is preferably 10% by mass, more preferably 1% by mass, and still more preferably 0.1% by mass.

[0022]

The intermediate layer 13 covers a surface of the positive current collecting substrate 11. The intermediate layer 13 in the positive electrode 10 in Fig. 2 is a single layer. The intermediate layer 13 contains a crosslinkable compound, and further contains an electrical conduction aid, a binder, and the like. Generally, the intermediate layer is a layer having a function of reducing a contact resistance between the positive current collecting substrate and the positive active material layer, but the

intermediate layer 13 has a function of cutting off a current in the case of abnormality, as described later in detail.

[0023]

The crosslinkable compound is a compound which causes a cross- linking reaction, and usually includes a plurality of reactive groups

(crosslinkable groups). Examples of the reactive group include a vinyl group, an epoxy group, a formyl group, an amino group, a cyanate group, an isocyanate group, a hydroxy group, and the like. In addition, the reactive group also includes groups which cause a binding reaction between different groups (for example, epoxy group and hydroxy group, etc.). One type of the crosslinkable compound may be used alone, or two or more types may be used as a mixture. When two or more types of crosslinkable compounds are used, the crosslinkable compounds may have different reactive groups, respectively, between which the binding reaction takes place.

[0024] The crosslinkable compound is usually a compound which causes a cross-linking reaction in response to stimulation such as visible light, radiation, for example, ultraviolet rays, heat, pressure, etc. In the nonaqueous electrolyte secondary battery 1, since the intermediate layer 13 in the positive electrode 10 (at least one of a pair of electrodes) contains a crosslinkable compound, the nonaqueous electrolyte secondary battery 1 has a favorable function of charge-discharge stoppage at the time of

abnormality. An estimated mechanism by which the nonaqueous electrolyte secondary battery exhibits the above function is as follows.

When an abnormality such as heat generation, ignition or increase in pressure occurs in the secondary battery, a cross-linking reaction of the crosslinkable compound in the intermediate layer 13 takes place with the heat and the like. By crosslinking (polymerization) of the crosslinkable compound, the electrical resistance of the intermediate layer 13 increases. The reason why an electric resistance of the intermediate layer 13 is increased by the cross-linking reaction is guessed that the conductivity of the crosslinkable compound itself decreases as the crosslinkable compound becomes polymerized, or the electrical conduction aids that are in contact with each other are divided associated with the cross-linking reaction of the crosslinkable compound.

[0025]

As described above, according to the nonaqueous electrolyte secondary battery 1, a good function of electric current shutdown is exhibited at the time of abnormality. Accordingly, according to the nonaqueous electrolyte secondary battery 1, it is possible to suppress thermal runaway and internal short-circuit when an abnormality occurs. Therefore, according to the nonaqueous electrolyte secondary battery 1, a high capacity active material can be used with high safety.

[0026]

The crosslinkable compound is preferably a compound (thermally crosslinkable compound) in which a cross-linking reaction takes place by heat. Generally, when an abnormality occurs in a secondary battery, heat is generated in many cases. Therefore, when the crosslinkable compound is a compound in which the cross-linking reaction takes place by heat, a shutdown function at the time of abnormality can be more effectively exhibited.

[0027]

Examples of the thermally crosslinkable compound include epoxy compounds (poly glycerol polyglycidyl ether, sorbitol polyglycidyl ether, etc.), polyfunctional (meth)acrylates (trimethylolpropane triacrylate,

dipentaerythritol pentaacrylate, etc.), polyoxyalkylene compounds

(polyethylene glycol, polypropylene glycol, etc.), isocyanate compounds (2,4- tolylene diisocyanate, 2,6-tolylene diisocyanate, etc.), and the like. As the thermally crosslinkable compound, a thermosetting resin described later and a monomer of a thermosetting resin can also be mentioned. In addition, the thermally crosslinkable compound can be used in combination with a thermal polymerization initiator.

[0028]

The crosslinkable compound (thermally crosslinkable compound, etc.) is preferably a polymer having a plurality of branched structures. In the case where the crosslinkable compound is a monomer, the cross-linking reaction may occur at a relatively low temperature, and there is a possibility that it is difficult to control the cross-linking reaction. On the other hand, in the case of a crosslinkable compound as a polymer, the cross-linking reaction can be initiated and progressed at a temperature corresponding to the heat generation at the time of abnormality (for example, higher than 100°C or higher than 150°C). Therefore, by using the crosslinkable compound as a polymer, good charging and discharging can be carried out without progressing the cross-linking reaction during normal use, and the shutdown function can be more effectively exhibited at the time of

abnormality. Further, when the crosslinkable compound is a polymer having a plurality of branched structures, the cross-linking reaction is efficiently caused by a large number of reactive groups existing at a terminal, and a dense three-dimensional crosslinked structure is formed by crosslinking, and this enables to effectively increase the electric resistance.

[0029]

As the crosslinking compound (thermally crosslinkable compound, etc.), it is more preferred that it is an oligomer. Specifically, a lower limit of a number average molecular weight of the crosslinkable compound is preferably 200, and an upper limit thereof is preferably 3000. By setting the number average molecular weight of the crosslinkable compound to the above lower limit or more, it is possible to more appropriately cause a cross- linking reaction at a temperature corresponding to the heat generation at the time of abnormality. On the other hand, by setting the number average molecular weight of the crosslinkable compound to the above upper limit or less, it is possible to increase the resistance more effectively with the cross- linking reaction.

[0030]

As the thermally crosslinkable compound which is a polymer having a plurality of branched structures, a so-called thermosetting resin and the like can be mentioned. Examples of the thermosetting resin include melamine resins, urea resins, urethane resins, epoxy resins, alkyd resins, phthalic acid resins, allyl resins, phenol resins, benzoxazine resins, xylene resins, ketone resins, furan resins, wholly aromatic polyimides, polyamino bismaleimide resin, reaction products of bismaleimide and barbituric acids, and the like.

[0031]

Among them, the reaction product of bismaleimide and barbituric acid, that is, a compound obtained as a reaction product of bismaleimide and barbituric acid is preferred. This compound has a structure having many branches and having a vinylene group (-CH=CH-) derived from

bismaleimide and an amino group derived from barbituric acid as a terminal reactive group. These groups are crosslinked at a temperature higher than 100°C, preferably about 150°C. Therefore, by using this compound, the cross-linking reaction can proceed effectively when the abnormality such as heat generation occurs, and the electrical resistance can be increased.

[0032]

Bismaleimide is a compound having two maleimide structures.

Since bismaleimide includes two vinylene groups as reactive groups, a branched polymer can be formed. As the bismaleimide, a compound represented by the following formula (l) can be mentioned.

[0033]

[Chem. Formula l]

[0034]

In the formula (l), R¹ is a divalent hydrocarbon group having 1 to 20 carbon atoms, -CO", -0-, -0-0-, -S-, -S-S-, -SO", -OSO- or groups formed of a combination of these groups. A part or all of hydrogen atoms of the divalent hydrocarbon group may be substituted with a substituent.

Examples of the substituent include a halogen atom, a hydroxyl group, an amino group, a nitro group, and the like.

[0035]

Examples of bismaleimide Include N,N'-bismaleimide-4,4'- diphenylmethane, l,l'-(methylenedi-4,l-phenylene) bismaleimide, Ν,Ν'- (l , 1 '-biphenyl-4, 4'- diyObismaleimide, Ν,Ν'- (4-methyl- 1 , 3-phenylene) bismaleimide, 1 , 1 '- (3, 3'- dimethyl- 1 , 1 '-biphenyl- 4,4'- diyObismaleimide, Ν,Ν'- ethylenedimaleimide, N,N'-(l,2-phenylene)dimaleimide, N,N'-(l,3- phenylene)dimaleimide, Ν,Ν'-thiodimaleimide, N,N'-dithiodimaleimide, Ν,Ν'-ketone dimaleimide, Ν,Ν'-methylene-bis-maleinimide, bis- maleinimidomethyl- ether, l,2-bis-(maleimide)-l,2-ethanediol, N,N'-4,4'- diphenylether-bis-maleimide, 4,4'-bis(maleimide)-diphenylsulfone, and the like.

[0036]

Barbituric acids refer to barbituric acid and derivatives thereof. Barbituric acids function as monomers, polymerization initiators, chain transfer agents, chain terminators, radical scavengers, and the like. By reacting barbituric acids having such functions with bismaleimide, an oligomer or polymer having a complex hyperbranched structure is formed. As the barbituric acids, compounds represented by the following formula (2) can be mentioned.

[0037]

[Chem. Formula 2]

[0038]

In the above formula (2), R² to R⁵ are each independently a hydrogen atom or a monovalent hydrocarbon group having 1 to 6 carbon atoms. X is an oxygen atom or a sulfur atom.

[0039]

In addition, when R² to R⁵ are all hydrogen atoms and X is an oxygen atom, the compound represented by the above formula (2) is barbituric acid.

[0040] A reaction between bismaleimide and barbituric acids can be carried out by a publicly known method. For example, the crosslinkable compound can be obtained by adding bismaleimide and barbituric acid to a solvent containing Broensted base and heating the resulting mixture from 20° C to 100°C. Barbituric acids may be further added during the reaction.

Further, monomers and the like other than bismaleimide and barbituric acid may be further copolymerized.

[0041]

The crosslinkable compound preferably has a vinyl group (CH2=CH-) or a vinylene group (-CH=CH-) and an amino group. Of the vinyl group and the vinylene group, the vinylene group is preferred. The amino group may be any of a primary amino group, a secondary amino group, and a tertiary amino group, but is preferably a secondary amino group (-NH-). As described above, the vinyl group or vinylene group and the amino group normally cause a cross-linking reaction at a temperature (for example, over 100°C) corresponding to the heat generation at the time of abnormality of the secondary battery, and therefore an effective shutdown function can be exhibited. As the crosslinkable compound having a vinyl group or a vinylene group and an amino group, a polyammobismaleimide resin and the like can be mentioned besides the above-mentioned resin obtained using bismaleimide and barbituric acids as a raw material (reactant).

[0042]

A lower limit of the content of the crosslinkable compound in the intermediate layer 13 is not particularly limited, and may be, for example, 5% by mass! however, the lower limit is preferably 15% by mass, more preferably 20% by mass, and still more preferably 25% by mass. On the other hand, an upper limit of the content is also not particularly limited, and may be, for example, 80% by mass! however, the upper limit is preferably 50% by mass, more preferably 45% by mass, and still more preferably 40% by mass. When the content of the crosslinkable compound is the above-mentioned lower limit or less, the shutdown function at the time of abnormality can be more effectively exhibited. On the other hand, when the content of the crosslinkable compound is the above-mentioned upper limit or more, good conductivity can be exhibited during normal use. In the case where the intermediate layer is a plurality of layers, the content thereof is the content in each layer containing the crosslinkable compound (hereinafter the same is applied to the content in the intermediate layer).

[0043]

As the electrical conduction aid contained in the intermediate layer 13, those exemplified as the electrical conduction aid contained in the positive active material layer 12 can be mentioned. As the electrical conduction aid contained in the intermediate layer 13, natural or artificial graphite and carbon black such as furnace black, acetylene black, and Ketjen black are preferred. A shape of the electrical conduction aid may be, for example, particulate.

[0044]

A lower limit of the content of the electrical conduction aid in the intermediate layer 13 is not particularly limited, and may be, for example, 5% by mass! however, the lower limit is preferably 10% by mass, more preferably 15% by mass, still more preferably 20% by mass, and particularly preferably 25% by mass. On the other hand, an upper limit of the content is also not particularly limited, and may be, for example, 80% by mass! however, the upper limit is preferably 60% by mass, more preferably 50% by mass, and still more preferably 40% by mass. When the content of the electrical conduction aid in the intermediate layer 13 is the above- mentioned lower limit or more, good conductivity can be exhibited during normal use. On the other hand, when the content of the electrical conduction aid in the intermediate layer 13 is the above-mentioned upper limit or less, a sufficient amount of the crosslinkable compound can be contained and a more effective shutdown function can be exhibited.

[0045]

As the binder contained in the intermediate layer 13, those exemplified as the binder contained in the positive active material layer 12 can be mentioned. The content of the binder contained in the intermediate layer 13 is not particularly limited, and it may be, for example, 10% by mass or more and 50% by mass or less.

[0046]

In addition, the intermediate layer 13 may further contain a thickener, a filler, and the like. However, an upper limit of the contents of the components other than the crosslinkable compound, the electrical conduction aid, and the binder in the intermediate layer 13 is preferably, for example, 20 mass%. The upper limit may be 10% by mass, 5% by mass or 1% by mass. By setting the content of the other components to the above- mentioned upper limit or less, it is possible to satisfactorily achieve the good conductivity during normal use and the shutdown function at the time of abnormality simultaneously.

[0047]

An average thickness of the intermediate layer 13 (the intermediate layer containing the crosslinkable compound) is not particularly limited; however, a lower limit is preferably 0.5 μηι, more preferably 1 μηι, and still more preferably 2 μηι. On the other hand, an upper limit of the average thickness is preferably 10 μηι, and more preferably 7 μηι. By setting the average thickness of the intermediate layer 13 to the above-mentioned lower limit or more, the shutdown function can be more enhanced. On the other hand, by setting the average thickness of the intermediate layer 13 to the above-mentioned upper limit or less, a thickness of the positive electrode can be reduced.

[0048]

The negative electrode includes a negative current collecting substrate and a negative active material layer disposed on the negative current collecting substrate directly or with an intermediate layer

interposed between the negative active material layer and the negative current collecting substrate.

[0049]

The negative current collecting substrate may have the same structure as that of the positive current collecting substrate. However, as a material of the negative current collecting substrate, a metal such as copper, nickel, stainless steel or nickel-plated steel, or an alloy thereof is used, and copper or a copper alloy is preferred. That is, a copper foil is preferred as the negative current collecting substrate. As the copper foil, a rolled copper foil, an electrolytic copper foil, and the like are exemplified.

[0050]

The negative active material layer is formed of a so-called negative composite containing a negative active material. In addition, the negative composite for forming the negative active material layer contains optional components such as an electrical conduction aid, a binder, a thickener, and a filler, as required. As optional components such as an electrical conduction aid, a binder, a thickener, and a filler, the same materials as those of the positive active material layer can be used.

[0051]

As the negative active material, a material which can absorb and release lithium ions is usually used. Specific examples of the negative active material include metals or semi-metals such as Si and Sn! metal oxides or semi-metal oxides such as Si oxide and Sn oxide! polyphosphate compounds! carbon materials such as graphite and amorphous carbon (easily graphitizable carbon or non-graphitizable carbon), and the like.

[0052]

Further, the negative composite (negative active material layer) may contain a typical nonmetallic element such as B, N, P, F, CI, Br and L a typical metallic element such as Li, Na, Mg, Al, K, Ca, Zn, Ga and Ge! or a transition metal element such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb or W.

[0053]

The constitution of the intermediate layer in the negative electrode is not particularly limited, and it can be formed of, for example, a

composition containing a resin binder and an electrical conduction aid (conductive material). The intermediate layer in the negative electrode may be formed with the same composition as the intermediate layer in the positive electrode.

[0054]

As a material of the separator, for example, a woven fabric, a nonwoven fabric, a porous resin film or the like is used. Among these materials, a porous resin film is preferred. As a main component of the porous resin film, for example, polyolefin such as polyethylene or

polypropylene is preferred from the viewpoint of strength. Further, a porous resin film obtained by combining these resins with a resin such as aramid or polyimide may be used.

[0055]

As the nonaqueous electrolyte, a publicly known electrolyte commonly used in a nonaqueous electrolyte secondary battery can be used, and a nonaqueous solvent in which an electrolyte salt is dissolved can be used.

[0056]

Examples of the nonaqueous solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); and chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC); and the like. [0057]

Examples of the electrolyte salt include a lithium salt, a sodium salt, a potassium salt, a magnesium salt, an onium salt and the like, and a lithium salt is preferred. Examples of the lithium salt include inorganic lithium salts such as LiPF₆, LiP0₂F₂, LiBF₄, LiC10₄ and LiN(S0₂F)₂;

lithium salts having a fluorinated hydrocarbon group, such as L1SO3CF3, LiN(S0₂CF₃)₂, LiN(S0₂C₂F₅)₂, LiN(S0₂CF₃)(S0₂C₄F₉), LiC(S0₂CF₃)₃, and LiC(S0₂C₂F₅)₃; and the like.

[0058]

As a nonaqueous electrolyte, an ambient temperature molten salt, an ionic liquid, a polymer solid electrolyte, or the like can also be used.

[0059]

A method for producing the nonaqueous electrolyte secondary battery 1 is not particularly limited, and a combination of publicly known methods can be used. However, it is preferred to use the following production method. That is, the method for producing a nonaqueous electrolyte secondary battery according to an embodiment of the present invention includes laminating an intermediate layer-forming material which contains a crosslinkable compound causing a cross-linking reaction on at least one side of a current collecting substrate (step l), and laminating an active material layer so as to cover an intermediate layer formed of the intermediate layer-forming material (step 2).

[0060]

By undergoing the step 1 and the step 2, it is possible to obtain an electrode whose electric resistance increases at the time of abnormality. That is, according to the producing method, it is possible to obtain a secondary battery or the like improved in the function of charge- discharge stoppage at the time of abnormality.

[0061]

The intermediate layer-forming material in the step 1, contains, besides the crosslinkable compound, components for forming an

intermediate layer such as an electrical conduction aid and a binder. In addition, the intermediate layer-forming material usually contains an organic solvent in order to uniformly mix and coat the respective

components. Lamination of the intermediate layer-forming material in the step 1 can be carried out by a publicly known method such as coating. In addition, a film formed of the intermediate layer-forming material may be laminated on the current collecting substrate.

[0062]

After a lamination of the intermediate layer-forming material (step l) and before the step 2, it is preferred to dry the intermediate layer-forming material (undried intermediate layer) laminated on the current collecting substrate at 100°C or less. By drying at 100°C or lower like this, the cross- linking reaction of the crosslinkable compound in the production process can be suppressed. When the drying is carried out at 100°C or less as described above, a solvent used for the intermediate layer-forming material is preferably a solvent having a boiling point of 100°C or less. Examples of such solvents include water, methanol, ethanol, isopropanol, tert-butyl alcohol, ethyl acetate, tetrahydrofuran, acetone, ethyl methyl ketone, hexane, cyclohexane, and the like. A lower limit of the temperature at the time of drying is not particularly limited, and drying may be performed at room temperature. It is also preferred that after laminating the

intermediate layer-forming material by coating or the like, the laminated film-like intermediate layer-forming material is press-formed. By undergoing such a process, an intermediate layer which contains a crosslinkable compound causing a cross-linking reaction is formed. A solvent having a boiling point higher than 100° C may be used. Drying conditions such as pressure conditions and blowing conditions may be appropriately adjusted so that the cross-linking reaction of the crosslinkable compound does not proceed excessively.

[0063]

The step 2 is a step of laminating the active material layer so as to cover the intermediate layer formed by undergoing the above steps.

Lamination of the active material layer can be carried out by a publicly known method. By undergoing the step, an electrode having a layer structure in which the current collecting substrate, the intermediate layer, and the active material layer are laminated in this order is obtained. At least one of the pair of electrodes may be obtained by the above method.

[0064]

A process after obtaining the electrode is the same as the publicly known process. That is, a nonaqueous electrolyte secondary battery can be obtained by winding a pair of electrodes (a positive electrode and a negative electrode) with a separator interposed therebetween to obtain an electrode assembly, housing the pair of electrodes in a battery case, filling a nonaqueous electrolyte in the battery case, and then sealing the filling hole.

[0065]

The present invention is not limited to the above-mentioned embodiment, but may be implemented in aspects with various modifications and improvements besides the above embodiments. For example, in the above embodiment, the intermediate layer of the positive electrode contains a crosslmkable compound; however, an embodiment may be employed in which the intermediate layer of the positive electrode does not contain a crosslinking compound and the intermediate layer of the negative electrode contains a crosslinking compound. Both the intermediate layer of the positive electrode and the intermediate layer of the negative electrode may contain a crosslmkable compound. When the intermediate layer of the positive electrode contains the crosslmkable compound, the negative electrode does not have to have an intermediate layer. Conversely, when the intermediate layer of the negative electrode contains the crosslmkable compound, the positive electrode does not have to have the intermediate layer. Further, in the positive electrode or the negative electrode, a coating layer or the like for covering the active material layer or the like may be provided.

[0066]

Further, the electrode may have a plurality of intermediate layers, and in this case, an intermediate layer containing a crosslmkable compound and an intermediate layer not containing a crosslmkable compound may be present, or the crosslmkable compound may be contained in all intermediate layers. In the case where the electrode has a plurality of intermediate layers, it is preferred that the intermediate layer adjacent to the active material layer contains a crosslinkable compound. By doing so, the shutdown function at the time of abnormality is more effectively exhibited. Further, the intermediate layer and the active material layer may be laminated on both sides of the current collecting substrate. As described above, in the present invention, at least one of the pair of electrodes has only to include an electrode having an current collecting substrate, an active material layer, and one or a plurality of intermediate layers disposed between the current collecting substrate and the active material layer, wherein at least one intermediate layer among the one or more intermediate layers contains a crosslinkable compound causing a cross-linking reaction.

[0067]

Further, in the above-mentioned embodiment, an aspect in which the energy storage device is a nonaqueous electrolyte secondary battery has been described, but other energy storage devices may be used. Examples of other energy storage devices include capacitors (electric double-layer capacitors, lithium ion capacitors), secondary batteries in which the electrolyte includes water, and the like.

[0068]

Further, the configuration of the energy storage device according to the present invention is not particularly limited, and examples thereof include a cylindrical battery, a prismatic (rectangular battery) battery, a flat battery, and the like. The present invention can also be realized as an energy storage apparatus having a plurality of the energy storage devices. An embodiment of the energy storage apparatus is shown in Fig. 3. In Fig. 3, the energy storage apparatus 30 includes a plurality of energy storage units 20. Each of the energy storage units 20 includes a plurality of energy storage devices (nonaqueous electrolyte secondary batteries l). The energy storage apparatus 30 can be mounted as a power source for automobiles such as electric vehicles (EV), hybrid automobiles (HEV), plug-in hybrid automobiles (PHEV) and the like.

EXAMPLES

[0069]

Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples.

[0070]

[Example l]

(Preparation of Positive Electrode)

An intermediate layer was formed on a surface of an aluminum foil as the current collecting substrate in the following manner. Carbon black (CB), polyvinylidene fluoride (PVDF), and a crosslinkable compound were weighed in a mass ratio of 30 : 35 : 35. As the crosslinkable compound, a reaction product of bismaleimide and barbituric acid was used. CB, PVDF, and a part of a N-methyl-2-pyrrolidone (NMP) solution of the crosslinkable compound were mixed and dispersed with a beads mill. Thereafter, the remaining NMP solution of the crosslinkable compound was added to prepare a material for forming an intermediate layer. The material for forming an intermediate layer was applied onto both sides of an aluminum foil. Thereafter, the material was dried in two stages of 90° C and 130°C to obtain an intermediate layer having an average thickness of 5 μηι per one side.

[0071]

A positive electrode paste was prepared which contains

Li(Nio.82Coo.i5Alo.o3)02, CB, and PVDF as a positive active material in a mass ratio of 94 : 3 ^: 3 (solid content basis) and uses N-methyl-2-pyrrolidone as a solvent. The positive electrode paste was applied to a surface of the intermediate layer and pressure-molded by a roller press machine to form a positive active material layer. Thereafter, the solvent was removed by drying under reduced pressure at 100°C for 12 hours to obtain a positive electrode.

[0072]

(Preparation of Nonaqueous Electrolyte Secondary Battery)

A negative electrode in which the negative active material was graphite was prepared. The positive electrode and the negative electrode were wound with a polyolefin porous resin film separator interposed therebetween to prepare an electrode assembly. After the electrode assembly was inserted into the aluminum lid of the case was welded by laser welding. After filling a nonaqueous electrolyte (a solvent composed of EC, DMC and EMC in proportions of 25 : 20 : 55 (volume ratio) containing 1 M LiPFe) through an liquid filling hole provided in the lid, the liquid filling hole was sealed to obtain a nonaqueous electrolyte secondary battery (prismatic lithium ion battery). For each of the following evaluations, a nonaqueous electrolyte secondary battery was prepared. [0073]

[Comparative Example l]

A nonaqueous electrolyte secondary battery of Comparative Example

1 was prepared in the same manner as in Example 1 except that the intermediate layer was not formed (the positive active material layer was formed directly on the surface of the aluminum foil as the current collecting substrate).

[0074]

[Comparative Example 2]

A nonaqueous electrolyte secondary battery of Comparative Example

2 was prepared in the same manner as in Comparative Example 1 except that the crosslinkable compound was contained in the positive active material layer. Specifically, a positive active material layer containing a positive active material, CB, a crosslinkable compound and PVDF in a mass ratio of 94 : 3 ^: 1 ^: 2 was formed on an aluminum foil.

[0075]

[Comparative Example 3]

A nonaqueous electrolyte secondary battery of Comparative Example

3 was prepared in the same manner as in Example 1 except that the crosslinkable compound was not contained in the intermediate layer.

Specifically, an intermediate layer containing CB and PVDF in a mass ratio of 50 : 50 was formed, and a positive active material layer was formed on the intermediate layer.

[0076]

[Evaluation] (Change in Resistance Associated with Heating)

With respect to the positive electrodes obtained in Example 1 and Comparative Examples 1, 2 and 3, an AC impedance (ACR) was measured while increasing the temperature. Specifically, two positive electrode plates were laminated with a separator made of aramid nonwoven fabric interposed between two positive electrode plates, impregnated with a solvent composed of EC and EMC in proportions of 20 : 80 (volume ratio) containing 1 M LiPF6, to prepare a positive electrode. Using AC Ohm Meter (model number: 3562) manufactured by TSURUGA ELECTRIC CORPORATION, the ACR of the resulting positive electrode was measured while increasing the temperature stepwise by 5°C or 10°C. The

measurement results are shown in Fig. 4. In addition, Fig. 4 shows relative values of each sample with reference to the ACR before the temperature increase.

[0077]

As shown in Fig. 4, in Example 1, it can be seen that the electric resistance sharply increased with increasing temperature. In particular, in Example 1, it can be seen that the electrical resistance roughly linearly increases over the entire temperature range from 80° C to 180°C, and at 180°C, the electrical resistance increases up to two times or more larger than that at the room temperature. On the other hand, in Comparative Examples 1 and 2, substantial increase in electrical resistance was not observed. In Comparative Example 3, the electrical resistance was increased as compared with Comparative Examples 1 and 2, but no sharp increase in electrical resistance was observed as in Example 1. In Comparative Example 3, the electrical resistance was little increased after exceeding 110°C, and the resistance increase remained at about 60% at the maximum. It is understood that the secondary battery of Example 1 has an improved shutdown function against heat generation.

[0078]

(Nail Penetration Test)

The nonaqueous electrolyte secondary batteries obtained in Example 1 and Comparative Example 1 were charged and discharged at 25°C with an end-of-charge voltage of 4.35 V and an end-of- discharge voltage of 2.50 V, and an initial capacity was confirmed. Next, after charging the battery under the same conditions as above, a nail penetration test was conducted under an environment of 45°C. Specifically, a nail having a diameter of 1 mm was penetrated together with an aluminum case in the lamination direction of the electrode plates.

[0079]

In the nonaqueous electrolyte secondary battery of Comparative Example 1, the temperature increased due to nail penetration, and a container of the battery was broken. On the other hand, in the nonaqueous electrolyte secondary battery of Example 1, the temperature rose to 100° C after nail penetration, but no change occurred.

[0080]

(Discharge Capacity Test)

With respect to the positive electrode plates obtained in Example 1 and Comparative Examples 1, 2 and 3, metallic lithium was used as a counter electrode and a polyolefin-based separator was used as a separator to prepare a pouch type battery. For each of the prepared pouch type batteries, charging and discharging was carried out at 25°C with an end of charge voltage of 4.45 V and an end-of-discharge voltage of 2.50 V, and the initial capacity was confirmed. Specifically, charge- discharge of two cycles of a first cycle and a second cycle was carried out at 25°C in which for the first cycle, constant current constant voltage charge was performed at a charge current of 0.2 CmA and a charge voltage of 4.45 V and constant current discharge was performed at a discharge current of 0.2 CmA and an end-of-discharge voltage of 2.50 V, and for the second cycle, constant current constant voltage charge was performed at a charge current of 0.2 CmA and a charge voltage of 4.45 V and constant current discharge was performed at a discharge current of 1.0 CmA and an end-of-discharge voltage of 2.50 V. A rest period of 10 minutes was set between charging and discharging and between discharging and charging.

[0081]

With regard to the discharge capacity, the discharge capacity of each of the nonaqueous electrolyte secondary battery of Example 1 and

Comparative Examples 1, 2 and 3 was about 190 mAh/g after one cycle and about 180 mAh/g after two cycles. It can be seen that a remarkable decrease in capacity did not occur due to provision of the intermediate layer containing the crosslinkable compound.

INDUSTRIAL APPLICABILITY

[0082]

The present invention can be applied to nonaqueous electrolyte secondary batteries used as power sources for electronic devices such as personal computers and communication terminals, automobiles, and the like, and nonaqueous electrolytes for secondary batteries provided therein. DESCRIPTION OF REFERENCE SIGNS

[0083]

l: Nonaqueous electrolyte secondary battery

2^'· Electrode assembly

3: Battery case

4^'· Positive electrode terminal

4"· Positive electrode lead

5: Negative electrode terminal

5': Negative electrode lead

10^ Positive electrode

11: Positive current collecting substrate

12- Positive active material layer

13: Intermediate layer

20^ Energy storage unit

30: Energy storage apparatus

Claims

1. An energy storage device comprising:

an electrode including a current collecting substrate, an active material layer, and an intermediate layer disposed between the current collecting substrate and the active material layer,

wherein the intermediate layer contains a crosslinkable compound causing a cross-linking reaction.

2. The energy storage device according to claim 1, wherein the cross-linking reaction occurs by heat.

3. The energy storage device according to claim 1 or 2, wherein a content of the crosslinkable compound in the intermediate layer is 15% by mass or more and 50% by mass or less.

4. The energy storage device according to claim 1, 2 or 3, wherein the intermediate layer further contains an electrical conduction aid, and a content of the electrical conduction aid in the intermediate layer is 10% by mass or more and 50% by mass or less.

5. The energy storage device according to any one of claims 1 to 4, wherein the crosslinkable compound includes a vinyl group or a vinylene group and an amino group.

6. The energy storage device according to any one of claims 1 to 5, wherein the crosslmkable compound is a polymer having a plurality of branched structures.

7. The energy storage device according to claim 6, wherein a number average molecular weight of the crosslmkable compound is 200 or more and 3000 or less.

8. A method for producing an energy storage device comprising: laminating an intermediate layer-forming material which contains a crosslmkable compound causing a cross-linking reaction on at least one side of a current collecting substrate, and

9. The method for producing an energy storage device according to claim 8, further comprising drying the intermediate layer-forming material laminated on the current collecting substrate at 100°C or lower.