Secondary battery

Abstract

A secondary battery includes: a flexible outer packaging member including a heat fusion layer; and a battery element housed inside the outer packaging member, including a positive electrode, a negative electrode, and an electrolytic solution. The outer packaging member is sealed in a heat fusion portion formed by heat fusing the heat fusion layers to each other, the heat fusion layer containing polypropylene. The electrolytic solution contains a solvent containing a chain carboxylic acid ester and an electrolyte salt. The thickness of the heat fusion layer is 25 μm or more and 60 μm or less, the length of the heat fusion portion is 160 mm or more and 650 mm or less, the width of the heat fusion portion is 3 mm or more and 6 mm or less, and a dimension ratio defined by the thickness of the heat fusion layer, the length of the heat fusion portion, and the width of the heat fusion portion satisfies a condition expressed by formula (1). The stretching amount of the outer packaging member is 7.8 mm or less. The content of the chain carboxylic acid ester in the solvent is 30 vol% or more and 60 vol% or less. 0.16 ≤ (T × L) / W ≤ 0.32 … (1) ((T × L) / W is the dimension ratio. T is the thickness of the heat fusion layer (cm). L is the length of the heat fusion portion (cm). W is the width of the heat fusion portion (cm)).

Description

TECHNICAL FIELD

[0001] The present technology relates to a secondary battery. BACKGROUND

[0002] As various electronic devices such as mobile phones are becoming widespread, development of a secondary battery as a power source that is small and light and that can achieve a high energy density is being pursued.

[0003] The secondary battery has a battery element including a positive electrode, a negative electrode, and an electrolyte solution. As a secondary battery, a secondary battery in which a battery element is housed inside a flexible outer packaging member is known.

[0004] Various studies have been made on the structure of a secondary battery having the flexible outer packaging member. Specifically, in order to achieve excellent heat resistance and the like, in a secondary battery using an electrochemical cell packaging material having a multilayer structure including a heat-adhesive resin layer (polypropylene) (for example, see Patent Literature 1). In addition, in order to achieve excellent long-term storage properties and the like, in a secondary battery using a bag-shaped single cell case, a resin block is provided at the inner side end portion of a bag-shaped single cell case fusion portion (for example, see Patent Literature 2).

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2010-086744

[0008] Patent Literature 2: Japanese Patent Application Publication No. 2000-277066 SUMMARY

[0009] Various studies have been made on a secondary battery having a flexible outer packaging member, but the cycle characteristics, expansion characteristics, and resistance characteristics of the secondary battery are still insufficient, and thus there is room for improvement.

[0010] The present technology was completed in view of the above-described problems, and aims to provide a secondary battery that can achieve excellent cycle characteristics, excellent expansion characteristics, and excellent insulation characteristics.

[0011] The secondary battery of one embodiment of the present technology includes a flexible outer packaging member including a heat-sealed layer and a battery element including a positive electrode, a negative electrode, and an electrolyte solution housed inside the outer packaging member. The outer packaging member is sealed in a heat-sealed portion formed by heat-sealing the heat-sealed layers to each other, and the heat-sealed layer contains polypropylene. The electrolyte solution contains a solvent and an electrolyte salt, and the solvent contains a chain carboxylic acid ester. The thickness of the heat-sealed layer is greater than or equal to 25 μm and less than or equal to 60 μm, the length of the heat-sealed portion is greater than or equal to 160 mm and less than or equal to 650 mm, the width of the heat-sealed portion is greater than or equal to 3 mm and less than or equal to 6 mm, and a dimension ratio defined by the thickness of the heat-sealed layer, the length of the heat-sealed portion, and the width of the heat-sealed portion satisfies a condition represented by Formula (1). The stretching amount of the outer packaging member is less than or equal to 7.8 mm. The content of the chain carboxylic acid ester in the solvent is greater than or equal to 30 vol% and less than or equal to 60 vol%.

[0012] 0.16 ≤ (T x L) / W ≤ 0.32 … (1)

[0013] ((T x L) / W is a dimension ratio. T is the thickness of the heat-sealed layer (cm). L is the length of the heat-sealed portion (cm). W is the width of the heat-sealed portion (cm).)

[0014] Definitions of each of the "thickness of the heat-sealed layer", the "length of the heat-sealed portion", the "width of the heat-sealed portion", and the "stretching amount of the outer packaging member" will be described later.

[0015] The secondary battery according to one embodiment of the present technology includes a battery element (the solvent of the electrolyte solution contains a chain carboxylic acid ester) housed inside a flexible outer packaging member (the heat-sealed layer contains polypropylene) sealed in a heat-sealed portion. In addition, the thickness of the heat-sealed layer, the length and the width of the heat-sealed portion, the dimension ratio of the thickness, the length, and the width thereof, the stretching amount of the outer packaging member, and the content of the chain carboxylic acid ester in the solvent satisfy the above conditions. Thus, excellent cycle characteristics, excellent swelling characteristics, and excellent insulation characteristics can be obtained.

[0016] Note that the effects of the present technology are not limited to the effects described here, and can be any of the effects described below in connection with the series of effects associated with the present technology. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a perspective view illustrating the structure of a secondary battery in one embodiment of the present technology.

[0018] Figure 2 is a cross-sectional view illustrating Figure 1 the structure of the battery element illustrated in FIG. 1.

[0019] Figure 3 is a cross-sectional view illustratingFigure 1 a plan view of the structure of the secondary battery shown.

[0020] Figure 4 is a block diagram showing the structure of an example of application of the secondary battery. Figure 1 a sectional view of the structure of the secondary battery shown.

[0021] Figure 5 is a block diagram showing the structure of an example of application of the secondary battery. DETAILED DESCRIPTION

[0022] Hereinafter, one embodiment of the present technology will be explained in detail with reference to the drawings. Note that the order of explanation is as follows.

[0023] 1. Secondary battery

[0024] 1-1. Overall structure

[0025] 1-2. Structure of main part

[0026] 1-3. Operation

[0027] 1-4. Manufacturing method

[0028] 1-5. Action and effect

[0029] 2. Modified example

[0030] 3. Use of secondary battery

[0031] <1. Secondary battery>

[0032] First, a secondary battery of one embodiment of the present technology will be explained.

[0033] The secondary battery explained here is a secondary battery that obtains battery capacity by intercalation and deintercalation of an electrode reactant, and has a positive electrode, a negative electrode, and an electrolyte that is a liquid electrolyte. In the secondary battery, in order to prevent precipitation of an electrode reactant on the surface of the negative electrode during charging, the charging capacity of the negative electrode is greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of the negative electrode is set to be greater than the electrochemical capacity per unit area of the positive electrode.

[0034] The kind of the electrode reactant is not particularly limited, and specifically, it is a light metal such as an alkali metal and an alkaline earth metal. The alkali metal is lithium, sodium, and potassium, and the alkaline earth metal is beryllium, magnesium, and calcium.

[0035] Hereinafter, a case where the electrode reactant is lithium will be described. A secondary battery that obtains battery capacity by intercalation and deintercalation of lithium is a so-called lithium-ion secondary battery. In the lithium-ion secondary battery, lithium is intercalated and deintercalated in an ionic state.

[0036] <1-1. Overall structure>

[0037] Figure 1 A perspective structure of the secondary battery is shown, Figure 2 A perspective structure of the secondary battery is shown, Figure 1 A cross-sectional structure of the battery element 10 is shown. In addition, Figure 1 A state in which the battery element 10 and the outer packaging film 20 are separated from each other is shown, and Figure 2 Only a part of the battery element 10 is shown.

[0038] As Figure 1 shown, the secondary battery is provided with the battery element 10, the outer packaging film 20, the positive electrode lead 14, and the negative electrode lead 15. The secondary battery described herein is a laminate film type secondary battery that uses a flexible (or soft) outer packaging member (the outer packaging film 20) as an outer packaging member for housing the battery element 10.

[0039] [Outer packaging film]

[0040] As Figure 1 shown, the outer packaging film 20 includes two film-like members (an upper side film 20X and a lower side film 20Y) and has a bag-like structure formed by joining (heat sealing) the upper side film 20X and the lower side film 20Y to each other. As described above, since the outer packaging film 20 houses the battery element 10 inside, the positive electrode 11 and the negative electrode 12 and the electrolyte described later are housed inside. Here, a recessed portion 20U (so-called deep-drawing portion) for housing the battery element 10 is provided on the upper side film 20X.

[0041] A sealing film 31 is interposed between the upper side film 20X and the positive electrode lead 14 and between the lower side film 20Y and the positive electrode lead 14, respectively. A sealing film 32 is interposed between the upper side film 20X and the negative electrode lead 15 and between the lower side film 20Y and the negative electrode lead 15, respectively. The sealing films 31, 32 are members for preventing external gas from intruding into the inside of the outer packaging film 20 and contain any one or two or more of a high molecular compound such as polyolefin having adhesion to the positive electrode lead 14 and the negative electrode lead 15, respectively. The polyolefin is polyethylene, polypropylene, modified polyethylene, modified polypropylene, or the like. In addition, one or both of the sealing films 31, 32 can be omitted.

[0042] Note that the detailed structure of the outer packaging film 20 (the upper side film 20X and the lower side film 20Y) will be described later (refer to Figure 3 and Figure 4 ).

[0043] [Battery element]

[0044] As Figure 1 and Figure 2As shown, the battery element 10 is housed inside the outer packaging film 20 and includes a positive electrode 11, a negative electrode 12, a separator 13, and an electrolyte (not shown). The electrolyte is impregnated in each of the positive electrode 11, the negative electrode 12, and the separator 13.

[0045] The battery element 10 is a structure in which a positive electrode 11 and a negative electrode 12 are stacked on top of each other with a separator 13 in between, and the positive electrode 11, negative electrode 12, and separator 13 are wound around a winding axis (wound electrode body). Therefore, the positive electrode 11 and negative electrode 12 are wound opposite each other with a separator 13 in between. It should be noted that the winding axis mentioned above is an imaginary axis extending in the Y-axis direction.

[0046] Here, the three-dimensional shape of the battery element 10 is a flat shape. That is, the shape of the cross-section (along the XZ plane) of the battery element 10 intersecting the winding axis is a flat shape defined by the major axis and the minor axis, more specifically, a flat, approximately elliptical shape. The major axis is an imaginary axis extending in the X-axis direction and having a length greater than the minor axis, and the minor axis is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and having a length less than the major axis.

[0047] (positive electrode)

[0048] like Figure 2 As shown, the positive electrode 11 includes a positive current collector 11A having one and two opposite sides and two positive active material layers 11B disposed on both sides of the positive current collector 11A. Alternatively, the positive active material layer 11B may be disposed only on one side of the positive current collector 11A on the side opposite the negative electrode 12.

[0049] The positive current collector 11A comprises one or more conductive materials, such as metals, aluminum, nickel, and stainless steel. The positive active material layer 11B comprises one or more positive active materials capable of lithium insertion and extraction, and may also include a positive binder and a positive conductive agent.

[0050] There are no particular limitations on the type of positive electrode active material. Specifically, it refers to lithium-containing compounds such as lithium transition metal compounds. These lithium transition metal compounds are compounds containing lithium and one or more transition metal elements, and may also contain one or more other elements. The other elements can be any elements other than transition metal elements; there are no particular limitations. Specifically, they are elements belonging to groups 2 to 15 of the long-period periodic table. There are no particular limitations on the type of lithium transition metal compounds. Specifically, they include oxides, phosphoric acid compounds, silicate compounds, and borate compounds, etc.

[0051] Specific examples of oxides are LiNiO2, LiCoO2, and LiCo. 0.98 Al 0.01Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 O2 and LiMn2O4, etc. Specific examples of phosphoric acid compounds are LiFePO4, LiMnPO4, and LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0052] The positive electrode binder contains one or more of the following: synthetic rubber and polymeric compounds. Synthetic rubbers include styrene-butadiene rubber, fluorinated rubber, and ethylene propylene diene monomer (EPDM) rubber. Polymeric compounds include polyvinylidene fluoride (PVDF), polyimide, and carboxymethyl cellulose.

[0053] The positive electrode conductive agent contains one or more of the following conductive materials: carbon materials, such as graphite, carbon black, acetylene black, and Ketjen black. Alternatively, the conductive material can also be a metallic material or a polymer compound.

[0054] There is no particular limitation on the method for forming the positive electrode active material layer 11B. Specifically, it can be any one or more of the following methods: coating method, etc.

[0055] (negative electrode)

[0056] like Figure 2 As shown, the negative electrode 12 includes a negative electrode current collector 12A having one opposite side and two negative electrode active material layers 12B disposed on both sides of the negative electrode current collector 12A. Alternatively, the negative electrode active material layer 12B may be disposed only on one side of the negative electrode current collector 12A on the side of the negative electrode 12 opposite to the positive electrode 11.

[0057] The negative electrode current collector 12A comprises one or more conductive materials, such as copper, aluminum, nickel, and stainless steel. The negative electrode active material layer 12B comprises one or more negative electrode active materials capable of lithium insertion / extraction, and may also include a negative electrode binder and a negative electrode conductive agent. Details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and details regarding the negative electrode conductive agent are the same as those regarding the positive electrode conductive agent.

[0058] The type of negative electrode active material is not particularly limited. Specifically, it can be one or both of carbon materials and metallic materials. Carbon materials include easily graphitized carbon, difficult-to-graphitize carbon, and graphite, such as natural graphite and artificial graphite. Metallic materials are materials containing any one or more metallic elements and half-metallic elements capable of forming alloys with lithium, such as silicon and tin, such as silicon and tin. Furthermore, metallic materials can be monomers, alloys, compounds, mixtures of two or more of these, or materials containing two or more of these phases.

[0059] Specific examples of metallic materials include SiB4, SiB6, Mg2Si, Ni2Si, TiSi2, MoSi2, CoSi2, NiSi2, CaSi2, CrSi2, Cu5Si, FeSi2, MnSi2, NbSi2, TaSi2, VSi2, WSi2, ZnSi2, SiC, Si3N4, Si2N2O, and SiO. v (0<v≤2), LiSiO, SnO w (0<w≤2), SnSiO3, LiSnO, and Mg2Sn, etc. Additionally, SiO v The value of v can also satisfy 0.2 < v < 1.4.

[0060] There is no particular limitation on the method of forming the negative electrode active material layer 12B. Specifically, it can be any one or more of the following methods: coating, gas phase, liquid phase, spraying, and firing (sintering).

[0061] (Septum)

[0062] like Figure 2 As shown, the separator 13 is an insulating porous membrane located between the positive electrode 11 and the negative electrode 12, which prevents contact (short circuit) between the positive electrode 11 and the negative electrode 12 while allowing lithium ions to pass through. The separator 13 contains any one or more of the following polymer compounds: polytetrafluoroethylene, polypropylene, and polyethylene.

[0063] (electrolyte)

[0064] The electrolytic solution contains any one or two or more of chain carboxylic acid esters. The "chain carboxylic acid ester" refers to an ester of a straight-chain saturated fatty acid.

[0065] More specifically, the electrolytic solution contains a solvent containing a chain carboxylic acid ester and an electrolyte salt. The electrolytic solution containing a chain carboxylic acid ester as a nonaqueous solvent (organic solvent) is a so-called nonaqueous electrolytic solution.

[0066] The electrolytic solution (solvent) contains a chain carboxylic acid ester because the swelling of the secondary battery can be suppressed. In detail, the chain carboxylic acid ester has a property that gas is less likely to be generated by a decomposition reaction at the time of charge and discharge, as compared with a cyclic carbonate and a chain carbonate and the like described later. Thus, in the electrolytic solution containing a chain carboxylic acid ester, the decomposition reaction of the chain carboxylic acid ester is less likely to proceed at the time of charge and discharge, and thus gas is less likely to be generated by the decomposition reaction of the chain carboxylic acid ester. Thus, gas is less likely to accumulate inside the bag-like outer packaging film 20, and thus the secondary battery is less likely to swell.

[0067] Specific examples of the chain carboxylic acid ester are not particularly limited, and are methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, and ethyl trimethylacetate, and the like.

[0068] In addition, the content of the chain carboxylic acid ester in the solvent is 30% to 60% by volume. This is because the swelling of the secondary battery can be sufficiently suppressed. In addition, since the amount of penetration of the chain carboxylic acid ester into the heat-seal layer 21 described later is reduced, the sealability of the outer packaging film 20 can be ensured.

[0069] The content of the chain carboxylic acid ester described herein is a value obtained by analyzing the electrolytic solution using a high-frequency inductively coupled plasma (ICP) emission spectroscopy after the completion of the secondary battery.

[0070] Note that the solvent can contain any one or two or more of other solvents (nonaqueous solvents) in addition to the chain carboxylic acid ester.

[0071] The other solvents are esters and ethers, and the like. In addition, the chain carboxylic acid ester described above is not included in the esters described herein.

[0072] The esters are carbonic acid ester compounds and the like, and the carbonic acid ester compounds are cyclic carbonates and chain carbonates and the like. Specifically, the cyclic carbonates are ethylene carbonate and propylene carbonate and the like, and the chain carbonates are dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate and the like.

[0073] The ethers are lactone-based compounds, and the lactone-based compounds are lactones and the like. Specifically, the lactones are γ-butyrolactone, γ-valerolactone, and the like. Further, the ethers can be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and the like.

[0074] The content of the other solvent in the solvent (vol%) is not particularly limited and can be arbitrarily set in accordance with the content of the chain carboxylic acid ester in the solvent described above.

[0075] Further, the solvent can contain any one or two or more of the additives. The additives are unsaturated cyclic carbonates, halogenated carbonates, sulfonic acid esters, phosphoric acid esters, acid anhydrides, nitrile compounds, isocyanate compounds, and the like. This is because the chemical stability of the electrolyte solution is improved. Note that the content of the additive in the electrolyte solution is not particularly limited and can be arbitrarily set.

[0076] Specifically, the unsaturated cyclic carbonates are vinylene carbonate (1,3-dioxol-2-one), vinyl ethylene carbonate (4-vinyl-1,3-dioxolan-2-one), and methylene ethylene carbonate (4-methylene-1,3-dioxolan-2-one), and the like. The halogenated carbonates are fluoroethylene carbonate (4-fluoro-1,3-dioxolan-2-one), difluoroethylene carbonate (4,5-difluoro-1,3-dioxolan-2-one), and the like. The sulfonic acid esters are 1,3-propane sultone, and the like. The phosphoric acid esters are trimethyl phosphate, triethyl phosphate, and the like.

[0077] The acid anhydrides are cyclic dicarboxylic acid anhydrides, cyclic disulfonic acid anhydrides, and cyclic carboxylic acid sulfonic acid anhydrides, and the like. The cyclic dicarboxylic acid anhydrides are succinic anhydride, glutaric anhydride, maleic anhydride, and the like. The cyclic disulfonic acid anhydrides are ethane disulfonic anhydride, propane disulfonic anhydride, and the like. The cyclic carboxylic acid sulfonic acid anhydrides are sulfobenzoic anhydride, sulfopropionic anhydride, sulfobutyric anhydride, and the like.

[0078] The nitrile compounds are acetonitrile, succinonitrile, adiponitrile, and the like. The isocyanate compounds are hexamethylene diisocyanate, and the like.

[0079] The electrolyte salt contains any one or two or more of light metal salts such as lithium salts. The lithium salts are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), and the like.

[0080] There is no particular limit to the content of the electrolyte salt, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent. This is because it allows for higher ionic conductivity.

[0081] [Positive and negative leads]

[0082] The positive lead 14 is the positive terminal connected to the positive electrode 11 (positive current collector 11A) and contains one or more conductive materials such as aluminum. The negative lead 15 is the negative terminal connected to the negative electrode 12 (negative current collector 12A) and contains one or more conductive materials such as copper, nickel, and stainless steel. The positive lead 14 and the negative lead 15 are each in the shape of one or more of the following: a thin plate shape and a mesh shape.

[0083] Here, as Figure 1 As shown, the positive electrode lead 14 and the negative electrode lead 15 are led out from the inside of the outer packaging film 20 in the same direction. Alternatively, the positive electrode lead 14 and the negative electrode lead 15 can be led out in different directions.

[0084] Furthermore, the number of positive electrode leads 14 is 1. However, the number of positive electrode leads 14 is not particularly limited, and can therefore be 2 or more. In particular, when the number of positive electrode leads 14 is 2 or more, the resistance of the secondary battery decreases. The same explanation regarding the number of positive electrode leads 14 applies to the number of negative electrode leads 15; therefore, the number of negative electrode leads 15 is not limited to 1, and can also be 2 or more.

[0085] <1-2. Structure of the main parts>

[0086] Figure 3 It shows Figure 1 The planar structure of the secondary battery shown. Figure 4 It shows Figure 1 The cross-sectional structure of the secondary battery is shown. Additionally, in... Figure 3 In the diagram, the outer packaging film 20 is shaded, and the illustrations of the sealing films 31 and 32 are omitted. Figure 4 Only a portion of the outer packaging film 20 (the portion near the heat-sealed portion 20S described later) is shown in the diagram. Please refer to the already described sections below. Figure 1 as well as Figure 2 .

[0087] Here, as described above, the outer packaging film 20 includes an upper film 20X and a lower film 20Y. In this outer packaging film 20, with the upper film 20X and the lower film 20Y overlapping each other, the outer peripheral portions of the opposing parts are joined together (thermal welded), so the outer packaging film 20 has a bag-shaped sealed (sealed) structure.

[0088] More specifically, as shown in Figure 3 and Figure 4 the outer packaging film 20, i.e., the upper side film 20X and the lower side film 20Y, respectively, includes a heat fusion layer 21. Thereby, in a state where the upper side film 20X and the lower side film 20Y are overlapped with each other, the outer peripheral edge portions of the heat fusion layers 21, which are opposed to each other, are heat fused to each other. Thereby, since the outer packaging film 20 has a heat fusion portion 20S formed by heat fusion of the heat fusion layers 21 to each other, the outer packaging film 20 is sealed in the heat fusion portion 20S. The heat fusion portion 20S is formed in the outer peripheral edge portions of the outer packaging film 20 due to heat fusion of the outer peripheral edge portions of the heat fusion layers 21 to each other.

[0089] Here, since the outer packaging film 20 is a laminate film including the heat fusion layer 21, it has a multi-layer structure including the heat fusion layer 21. This is because the sealability of the outer packaging film 20 is improved. Thereby, external components such as moisture are less likely to intrude from the outside of the outer packaging film 20 into the inside, and internal components such as volatilized components of the electrolytic solution are less likely to be released from the inside of the outer packaging film 20 to the outside.

[0090] Here, the outer packaging film 20 has a multi-layer structure in which the heat fusion layer 21, the metal layer 22 and the protective layer 23 are sequentially stacked, and the heat fusion layer 21, the metal layer 22 and the protective layer 23 are preferably arranged in this order from the inside. This is because the sealability of the outer packaging film 20 can be further improved.

[0091] In this case, as shown in Figure 4 the heat fusion portion 20S has a multi-layer structure in which the outer packaging film 20 (the upper side film 20X and the lower side film 20Y) is heat fused to each other by the heat fusion layer 21. More specifically, the heat fusion portion 20S has a 6-layer structure in which the protective layer 23, the metal layer 22, the heat fusion layer 21, the heat fusion layer 21, the metal layer 22 and the protective layer 23 are sequentially stacked, and the heat fusion layers 21, which are adjacent to each other, are heat fused to each other.

[0092] The heat fusion layer 21 is a layer which can be heat fused using a heat fusion method, and contains polypropylene as an insulating high molecular compound. Note that the polypropylene can be unmodified polypropylene, modified polypropylene or both.

[0093] As described above, by heat fusing the heat fusion layers 21, which are adjacent to each other, to each other, the heat fusion layers 21 function to seal the outer packaging film 20. In addition, the heat fusion layer 21 is interposed between the positive electrode lead 14 and the metal layer 22, thereby functioning to prevent short circuit of the positive electrode lead 14 and the metal layer 22, and is interposed between the negative electrode lead 15 and the metal layer 22, thereby functioning to prevent short circuit of the negative electrode lead 15 and the metal layer 22.

[0094] The metal layer 22 is a barrier layer that blocks liquid components and gas components and the like, and contains any one or two or more of metal materials. Specifically, the metal material is aluminum or the like. Note that the type of the liquid component is not particularly limited, and specifically, is the above-described moisture or the like, and the type of the gas component is not particularly limited, and specifically, is the above-described volatile component of the electrolyte or the like.

[0095] The metal layer 22 is interposed between the heat-sealed layer 21 and the protective layer 23, thereby functioning to substantially shield the outer packaging film 20. Thus, as described above, the liquid components and the gas components are less likely to pass through (enter and exit) the outer packaging film 20 (except for the heat-sealed portion 20S), and thus the airtightness of the outer packaging film 20 can be ensured.

[0096] The protective layer 23 is a surface protective layer that is the outermost layer on the outer side of the outer packaging film 20, and contains any one or two or more of insulating high molecular compounds. Specifically, the high molecular compound is nylon or the like.

[0097] The protective layer 23 functions to protect the metal layer 22 by covering the surface of the metal layer 22, that is, functions to prevent breakage and corrosion of the metal layer 22 or the like.

[0098] Here, in correspondence with the case where the electrolyte contains a chain carboxylate, in order to improve the airtightness of the outer packaging film 20, a series of dimensional parameters related to the structure of the outer packaging film 20 (heat-sealed portion 20S) are appropriately adapted.

[0099] Specifically, as shown in Figure 3 and Figure 4 , the dimensional ratio defined by the thickness T (cm) of the heat-sealed layer 21, the length L (cm) of the heat-sealed portion 20S, and the width W (cm) of the heat-sealed portion 20S satisfies the condition expressed by formula (1). In addition, the thickness T is 25 μm to 60 μm, the length L is 160 mm to 650 mm, and the width W is 3 mm to 6 mm. As described above, this is because the airtightness of the outer packaging film 20 is improved. In this case, since the total thickness of the outer packaging film 20 does not become excessively large, the energy density per unit volume can be ensured, and short-circuiting of the positive electrode lead 14 and the metal layer 22 can be suppressed, and short-circuiting of the negative electrode lead 15 and the metal layer 22 can be suppressed.

[0100] 0.16 ≤ (T x L) / W ≤ 0.32 … (1)

[0101] ((T x L) / W is the dimensional ratio. T is the thickness (cm) of the heat-sealed layer. L is the length (cm) of the heat-sealed portion. W is the width (cm) of the heat-sealed portion.)

[0102] As shown in Figure 4As shown, the thickness T is the thickness of the heat-fusion layer 21 at the outer end 20T of the heat-fusion portion 20S, that is, the thickness of the heat-fusion layer 21 exposed to the outside at the heat-fusion portion 20S. This thickness T is the minimum value among 10 thicknesses measured at any 10 different locations. In addition, the value of thickness T is the value after rounding to the nearest whole number.

[0103] Length L is the length of the heat-fused portion 20S, that is, the length of the portion of the heat-fused layers 21 that are heat-fused together. Therefore, as Figure 3 The figure shows the outer perimeter dimensions of the heat-fused portion 20S.

[0104] Furthermore, the value of length L is the value after rounding to the nearest decimal place. Also, the length of the portion of the thermally fused layers 21 that is thermally fused together does not include the length of the portions occupied by the positive electrode lead 14 and the negative electrode lead 15. That is, the length of the portions occupied by the positive electrode lead 14 and the negative electrode lead 15 is not included in the length of the portion of the thermally fused layers 21 that is thermally fused together.

[0105] More specifically, when the outer packaging film 20 (upper film 20X and lower film 20Y) has a rectangular planar shape, the outer packaging film 20 has four sides (a pair of opposite sides and another pair of opposite sides).

[0106] In this case, the length L is the value obtained by subtracting the length L5 of the positive lead 14 and the length L6 of the negative lead 15 from the sum of the lengths L1 and L2 of the opposite pair of sides (bottom and top) and the lengths L3 and L4 of the other opposite pair of sides (left and right). That is, the length L is represented by L = (L1 + L2 + L3 + L4) - (L5 + L6).

[0107] like Figure 4 As shown, the width W is the dimension of the heat-sealed portion 20S in the direction (Y-axis direction) from the outside of the outer packaging film 20 toward the inside. Furthermore, the width W is the value after rounding to the nearest decimal place.

[0108] More specifically, the upper film 20X extends from the outside of the outer packaging film 20 inwards, and bends midway away from the lower film 20Y to form the recess 20U. In this case, since the width W is a dimension determined based on the position where the upper film 20X begins to bend (bend 20M), it is the distance from the end 20T to the bend 20M. This width W is the minimum of 10 widths measured at any 10 different locations.

[0109] In formula (1), "T" is the thickness of the heat-sealed layer 21 exposed to the outside at the heat-sealed portion 20S (the thickness of the heat-sealed layer 21 at the end portion 20T), and thus "T x L" represents the exposed area of the heat-sealed layer 21 exposed at this end portion 20T. Thus, "(T x L) / W" represents the ratio of the exposed area of the heat-sealed layer 21 to the width W of the heat-sealed portion 20S.

[0110] That is, as described above, in consideration of the fact that the liquid component and the gas component can pass in and out (infiltrate or outflow) through the heat-sealed layer 21 in the outer packaging film 20, the "exposed area of the heat-sealed layer 21" is the area of the entrance and exit of the liquid component and the gas component, and the "width W of the heat-sealed portion 20S" is the distance of the path through which the liquid component and the gas component move inside the heat-sealed layer 21 in order to pass in and out at the heat-sealed portion 20S.

[0111] As described above, this dimension ratio ((T x L) / W) is a parameter that defines the sealability of the outer packaging film 20. Thus, the dimension ratio described herein below will be referred to as the "seal ratio R". Here, the value of (T x L) and the value of the seal ratio R are each the value rounded off to the third decimal place.

[0112] In addition, regarding the structure of the outer packaging film 20, in order to improve the sealability, not only each of the thickness T, the length L, the width W, and the seal ratio R described above, but also the stretch amount Q (mm) is appropriately set. This stretch amount Q is a dimension parameter that defines the three-dimensional shape of the outer packaging film 20, and more specifically, the three-dimensional shape of the upper side film 20X provided with the recessed portion 20U. That is, the stretch amount Q is the depth of the recessed portion 20U, and more specifically, the distance (height difference) from the upper surface M1 to the lower surface M2 of the portion of the upper side film 20X that is bent in order to form the recessed portion 20U, is 7.8 mm or less. As described above, this is because the sealability is further improved. In this case, even if the upper side film 20X is bent in order to form the recessed portion 20U, the upper side film 20X is not easily broken or fractured, and thus the airtightness of the outer packaging film 20 can be ensured. Here, the stretch amount Q is the value rounded off to the second decimal place.

[0113] In particular, in the case where the outer packaging film 20 (the upper side film 20X) is a laminate film including the metal layer 22, if the stretching amount Q is too large, the thickness of the metal layer 22 has to be reduced in order to form the recessed portion 20U (bend the upper side film 20X), and thus the metal layer 22 is easily damaged (causing so-called cracking) or broken by impact. Thus, the airtightness of the outer packaging film 20 is reduced, and thus the liquid component and the gas component easily intrude into the inside of the outer packaging film 20. However, even if the outer packaging film 20 is a laminate film including the metal layer 22, the metal layer 22 is not easily damaged or broken if the stretching amount Q is within the above range. Thus, the airtightness of the outer packaging film 20 can be ensured, and thus the liquid component and the gas component do not easily intrude into the inside of the outer packaging film 20.

[0114] The measurement of the stretching amount Q is described below. Hereinafter, a case where the stretching amount Q is measured using two measurement jigs not shown will be described. Here, the two measurement jigs are each a plate-shaped member having an opening portion.

[0115] First, the upper side film 20X provided with the recessed portion 20U is prepared, and then the upper side film 20X is sandwiched using the two measurement jigs. More specifically, the upper side film 20X is placed on one of the measurement jigs, and then the other measurement jig is placed on the upper side film 20X.

[0116] In this case, the portion of the upper side film 20X provided with the recessed portion 20U, i.e., the portion protruding toward the depth direction of the recessed portion 20U, is inserted into the opening portion provided in one of the measurement jigs. Thus, the upper side film 20X is arranged on one of the measurement jigs with the protruding portion facing downward.

[0117] In addition, the other measurement jig is placed on the upper side film 20X in such a manner as not to shield the recessed portion 20U. Thus, even if the upper side film 20X is sandwiched by the two measurement jigs, the recessed portion 20U is exposed at the opening portions provided in the respective measurement jigs.

[0118] Next, the depth (mm) of the recessed portion 20U is measured using a vernier caliper or the like.

[0119] In this case, at the bottom of the recess 20U, in the vicinity of the portion where the upper side film 20X is bent (bent portion 20N), the depth can also be likely to deviate due to the deviation of the bending method of the upper side film 20X. The "bending method" refers to the bending angle of the upper side film 20X at the bent portion 20N, etc. Of course, in the case where the upper side film 20X is curved at the bent portion 20N, the above-mentioned bending method also includes the radius of curvature of the upper side film 20X at the bent portion 20N. Therefore, in order to suppress the influence of the deviation of the depth, the depth is measured at a region that is only a distance D inward from the bent portion 20N, that is, a region that is only a distance D inward from the bent portion 20N. This distance D can be arbitrarily set according to the bending method of the upper side film 20X, and is, for example, 5 mm.

[0120] Next, by measuring the depth at a plurality of portions that are different from each other, a plurality of depths are obtained. Here, as described above, since the planar shape of the upper side film 20X is a rectangle having four corner portions, by measuring the depth in the vicinity of each corner portion, four depths are obtained.

[0121] Finally, by calculating the average of the four depths, the stretching amount Q (mm) is obtained.

[0122] <1-3. Operation>

[0123] When the secondary battery is charged, lithium is deintercalated from the positive electrode 11, and this lithium is intercalated into the negative electrode 12 via the electrolyte. In addition, when the secondary battery is discharged, lithium is deintercalated from the negative electrode 12, and this lithium is intercalated into the positive electrode 11 via the electrolyte. At these charge and discharge, lithium is intercalated and deintercalated in an ionic state.

[0124] <1-4. Manufacturing method>

[0125] In the case of manufacturing the secondary battery, the positive electrode 11 and the negative electrode 12 are produced by the steps described below, and the electrolyte is prepared, and then the secondary battery is produced using the positive electrode 11, the negative electrode 12, and the electrolyte.

[0126] [Production of the positive electrode]

[0127] First, the positive electrode active material is mixed with the positive electrode binder, the positive electrode conductive agent, and the like, thereby producing a positive electrode mixture. Next, the positive electrode mixture is put into a solvent such as an organic solvent, thereby preparing a paste-like positive electrode mixture slurry. Finally, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 11A, thereby forming the positive electrode active material layer 11B. Thereafter, the positive electrode active material layer 11B can be compression-molded using a roll press or the like. In this case, the positive electrode active material layer 11B can be heated, and compression-molding can be repeated a plurality of times. Thus, the positive electrode active material layer 11B is formed on both surfaces of the positive electrode current collector 11A, thereby producing the positive electrode 11.

[0128] [Production of the negative electrode]

[0129] The negative electrode active material layer 12B is formed on both surfaces of the negative electrode current collector 12A by the same steps as those for the production of the positive electrode 11 described above. Specifically, a negative electrode active material is mixed with a negative electrode binder and a negative electrode conductive agent, and the like as necessary, thereby producing a negative electrode mixture, and then the negative electrode mixture is put into a solvent such as an organic solvent, thereby preparing a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 12A, thereby forming the negative electrode active material layer 12B. Thereafter, the negative electrode active material layer 12B can be compression-molded as in the case of compression-molding the positive electrode active material layer 11B. Thus, the negative electrode active material layer 12B is formed on both surfaces of the negative electrode current collector 12A, thereby producing the negative electrode 12.

[0130] [Preparation of electrolyte solution]

[0131] An electrolyte salt is put into a solvent containing a chain carboxylate ester. Thus, the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an electrolyte solution.

[0132] [Assembly of secondary battery]

[0133] First, the positive electrode lead 14 is connected to the positive electrode 11 (positive electrode current collector 11A) using a welding method or the like, and the negative electrode lead 15 is connected to the negative electrode 12 (negative electrode current collector 12A) using a welding method or the like.

[0134] Next, the positive electrode 11 and the negative electrode 12 are layered with the separator 13 interposed therebetween, and then the positive electrode 11, the negative electrode 12, and the separator 13 are wound, thereby producing a wound body. The wound body has the same structure as that of the battery element 10 except that the positive electrode 11, the negative electrode 12, and the separator 13 are not impregnated with the electrolyte solution. Next, the wound body is pressed by using a press or the like, and the wound body is molded into a flat shape.

[0135] Next, the upper side film 20X and the lower side film 20Y are overlapped with each other in a state where the wound body is housed inside the recessed portion 20U, and then the outer peripheral edge portions (heat fusion layers 21) of the three edges in each of the upper side film 20X and the lower side film 20Y are heat-fused to each other using a heat fusion method, thereby housing the wound body inside the bag-shaped outer packaging film 20. In this case, the three-dimensional shape of the upper side film 20X is adjusted so that the stretching amount Q satisfies the condition described above.

[0136] Note that the amount of stretching Q can be adjusted by changing the forming (shaping) conditions of the upper-side film 20X. Specifically, in the forming process of the upper-side film 20X, a raw material film not shown is used, and a shaping process is performed on the raw material film. In the shaping process, a portion of the raw material film is shaped in a concave shape by sandwiching the raw material film between a convex block and a concave block. Thus, the recessed portion 20U is formed, and the upper-side film 20X having the recessed portion 20U is obtained. In this case, the depth of the recessed portion 20U is changed by changing the height of the convex block and the depth of the concave block, respectively, and thus the amount of stretching Q is adjusted.

[0137] Here, as each of the upper-side film 20X and the lower-side film 20Y, a laminated film in which a heat-fusible layer 21 containing polypropylene, a metal layer 22, and a protective layer 23 are sequentially stacked is used, and the heat-fusible layers 21 are heat-fused to each other.

[0138] Finally, an electrolyte solution is injected into the inside of the bag-shaped packaging film 20, and then the outer peripheral edge portions (heat-fusible layers 21) of the remaining one edge in each of the upper-side film 20X and the lower-side film 20Y are heat-fused to each other using a heat-fusion method. In this case, a sealing film 31 is interposed between the packaging film 20 and the positive electrode lead 14, and a sealing film 32 is interposed between the packaging film 20 and the negative electrode lead 15. Thus, the electrolyte solution is impregnated in the wound body, and thereby the battery element 10 as a wound electrode body is produced.

[0139] In this case, in particular, among the four edges in the outer packaging film 20 (the upper-side film 20X and the lower-side film 20Y), the outer peripheral edge portions of the heat-fusible layers 21 are heat-fused to each other, and thereby a heat-fusion portion 20S is formed. Thus, the outer packaging film 20 is sealed in the heat-fusion portion 20S. In particular, in the case where the heat-fusion portion 20S is formed, the heat-fusion range and the like are adjusted so that the thickness T of the heat-fusible layer 21, the length L and the width W of the heat-fusion portion 20S, and the sealing ratio R satisfy the above-described conditions.

[0140] Therefore, the battery element 10 is enclosed in the inside of the bag-shaped outer packaging film 20, and thereby a secondary battery is assembled.

[0141] [Stabilization of secondary battery]

[0142] The assembled secondary battery is subjected to charge and discharge. Various conditions such as the ambient temperature, the number of times of charge and discharge (cycle number), and the conditions of charge and discharge can be arbitrarily set. Thus, a coating film is formed on the surface of the negative electrode 12 or the like, and thereby the state of the secondary battery is electrochemically stabilized. Therefore, a secondary battery using the outer packaging film 20, that is, a laminated film-type secondary battery is completed.

[0143] <1-5. Effects and advantages>

[0144] According to the secondary battery, the battery element 10 (the solvent of the electrolyte contains a chain carboxylate) is housed inside the flexible outer packaging film 20 (the heat fusion layer 21 contains polypropylene), which is sealed in the heat fusion portion 20S. In addition, the thickness T is 25 μm to 60 μm, the length L is 160 mm to 650 mm, the width W is 3 mm to 6 mm, the sealing ratio R is 0.16 to 0.32, the stretching amount Q is 7.8 mm or less, and the content of the chain carboxylate in the solvent is 30 vol% to 60 vol%.

[0145] In this case, as described above, the structure (thickness T, length L, width W, and sealing ratio R) of each of the heat fusion layer 21 and the heat fusion portion 20S and the content of the chain carboxylate are appropriately matched with each other, and the three-dimensional shape of the outer packaging film 20 (the upper film 20X provided with the recessed portion 20U) is appropriately matched. Thus, even if the electrolyte contains the chain carboxylate, the chain carboxylate does not easily penetrate into the heat fusion portion 20S (heat fusion layer 21). Thus, since the heat fusion layer 21 does not easily deteriorate, the liquid component and the gas component do not easily pass through the heat fusion portion 20S. In addition, since the outer packaging film 20 (upper film 20X) does not easily excessively deform (mold), the outer packaging film 20 does not easily break or crack. Thus, since the sealing property of the outer packaging film 20 is improved, the liquid component and the gas component do not easily intrude into the inside of the outer packaging film 20.

[0146] In detail, since the liquid component such as moisture as an external component does not easily intrude into the inside of the outer packaging film 20, the liquid component does not easily react with the electrolyte. Thus, since the gas does not easily generate from the decomposition reaction of the electrolyte, the secondary battery does not easily expand.

[0147] In addition, since the gas component such as a volatile component of the electrolyte as an internal component does not easily release to the outside of the outer packaging film 20, the electrolyte does not substantially easily leak. Thus, the housed amount of the electrolyte housed in the inside of the outer packaging film 20 is easily maintained, and thus even if the charge and discharge are repeated, the charge and discharge reaction is easily stably performed.

[0148] Further, since the positive electrode lead 14 is stably insulated from the metal layer 22 by the heat fusion layer 21, the short circuit of the positive electrode lead 14 and the metal layer 22 does not easily occur, and since the negative electrode lead 15 is stably insulated from the metal layer 22 by the heat fusion layer 21, the short circuit of the negative electrode lead 15 and the metal layer 22 does not easily occur. Thus, the insulation of the positive electrode lead 14 and the metal layer 22 can be ensured, and the insulation of the negative electrode lead 15 and the metal layer 22 can be ensured.

[0149] According to the above, even if the electrolytic solution contains a chain carboxylate ester, the sealability of the flexible outer packaging film 20 can be ensured, so the insulating property can be ensured, and even if the charge and discharge are repeated, the discharge capacity is not easily reduced, and the secondary battery is not easily expanded. Therefore, excellent cycle characteristics, excellent expansion characteristics, and excellent insulating characteristics can be obtained.

[0150] In particular, if the outer packaging film 20 is a laminate film including the heat fusion layer 21, the sealability of the outer packaging film 20 can be further improved, so a higher effect can be obtained.

[0151] In this case, if the laminate film has a multilayer structure in which the heat fusion layer 21, the metal layer 22, and the insulating protective layer 23 are sequentially stacked, the sealability of the outer packaging film 20 is further improved, so a higher effect can be further obtained.

[0152] In addition, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by lithium intercalation and deintercalation, so a higher effect can be obtained.

[0153] <2. Modified Examples>

[0154] Next, modified examples of the above-described secondary battery will be described. As described below, the structure of the secondary battery can be appropriately changed. In addition, any two or more of the series of modified examples described below can be combined with each other.

[0155] [Modified Example 1]

[0156] In the above-described Figure 4 , the outer packaging film 20 (the upper side film 20X and the lower side film 20Y) is a laminate film having a 3-layer structure (the heat fusion layer 21, the metal layer 22, and the protective layer 23). However, the layer structure of the outer packaging film 20 is not particularly limited as long as it includes the heat fusion layer 21. That is, the number of layers of the outer packaging film 20 is not limited to 3 layers including the heat fusion layer 21, and can be 1 layer consisting only of the heat fusion layer 21, can be 2 layers including the heat fusion layer 21, or can be 4 layers or more including the heat fusion layer 21. In this case, the same effects can be obtained.

[0157] [Modified Example 2]

[0158] The separator 13 is used as a porous film. However, although not specifically illustrated here, a laminated separator including a high molecular compound layer can be used instead of the separator 13 as a porous film.

[0159] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a high molecular compound layer provided on one surface or both surfaces of the porous membrane. This is because the adhesion of the separator to each of the positive electrode 11 and the negative electrode 12 is improved, so that the positional displacement of the battery element 10 is less likely to occur. Thus, even if a decomposition reaction of the electrolytic solution or the like occurs, the secondary battery is less likely to expand. The high molecular compound layer contains a high molecular compound such as polyvinylidene fluoride. This is because the high molecular compound such as polyvinylidene fluoride is excellent in physical strength and electrochemically stable.

[0160] Note that one or both of the porous membrane and the high molecular compound layer can contain any one or two or more of a plurality of insulating particles. This is because the plurality of insulating particles dissipate heat when the secondary battery generates heat, so that the safety (heat resistance) of the secondary battery is improved. The insulating particles are inorganic particles and resin particles, or the like. The type of the inorganic particles is not particularly limited, and specifically, it is a particle of alumina, aluminum nitride, boehmite, silica, titanium oxide, magnesium oxide, and zirconium oxide, or the like. The type of the resin particles is not particularly limited, and specifically, it is a particle of acrylic resin and styrene resin, or the like.

[0161] In the case of producing the laminated separator, a precursor solution containing a high molecular compound and an organic solvent, or the like is prepared, and then the precursor solution is applied to one surface or both surfaces of the porous membrane. In this case, the plurality of insulating particles can be added to the precursor solution as needed.

[0162] In the case where the laminated separator is used, lithium ions are also able to move between the positive electrode 11 and the negative electrode 12, so that the same effect can be obtained.

[0163] [Modified Example 3]

[0164] An electrolytic solution is used as a liquid electrolyte. However, although not specifically illustrated here, an electrolyte layer can be used instead of the electrolytic solution as a gel-like electrolyte.

[0165] In the battery element 10 in which the electrolyte layer is used, the positive electrode 11 and the negative electrode 12 are laminated with the separator 13 and the electrolyte layer interposed therebetween, and the positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte layer are wound. The electrolyte layer is interposed between the positive electrode 11 and the separator 13, and between the negative electrode 12 and the separator 13.

[0166] Specifically, the electrolyte layer contains an electrolytic solution and a high molecular compound, and in the electrolyte layer, the electrolytic solution is held by the high molecular compound. The structure of the electrolytic solution is as described above. The high molecular compound contains polyvinylidene fluoride, or the like. In the case of forming the electrolyte layer, after a precursor solution containing the electrolytic solution, the high molecular compound, and an organic solvent, or the like is prepared, the precursor solution is applied to one surface or both surfaces of each of the positive electrode 11 and the negative electrode 12.

[0167] When this electrolyte layer is used, lithium ions can also move between the positive electrode 11 and the negative electrode 12 via the electrolyte layer, thus achieving the same effect.

[0168] <3. Uses of Secondary Batteries>

[0169] Next, we will explain the uses (application examples) of the aforementioned secondary batteries.

[0170] There are no particular limitations on the uses of secondary batteries. As a power source, a secondary battery can be the main power source for electronic devices and electric vehicles, or it can be an auxiliary power source. The main power source is the power source used preferentially, regardless of the availability of other power sources. An auxiliary power source can be used to replace the main power source, or it can be switched from the main power source.

[0171] Specific examples of applications for rechargeable batteries are as follows: Electronic devices (including portable electronic devices) such as camcorders, digital still cameras, mobile phones, laptops, stereo headphones, portable radios, and portable information terminals. Backup power supplies and storage devices such as memory cards. Power tools such as electric drills and chainsaws. Battery packs integrated into electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars (including hybrid vehicles). Power storage systems such as household or industrial battery systems that pre-store power in preparation for emergencies. In these applications, one or multiple rechargeable batteries can be used.

[0172] Battery packs can use single cells or battery arrays. Electric vehicles are vehicles that operate (drive) using secondary batteries as a power source, as mentioned above; they can also be hybrid vehicles that have a power source other than secondary batteries. In home power storage systems, electricity stored in secondary batteries, which serve as power storage sources, can be used to operate household electrical products, etc.

[0173] Here, we will specifically illustrate one application example of a secondary battery. The structure of the application example described below is only one example and can therefore be modified as appropriate.

[0174] Figure 5 The frame structure of the battery pack is shown. The battery pack described here is a battery pack (so-called a pouch) that uses a secondary battery and is installed in electronic devices such as smartphones.

[0175] like Figure 5 As shown, the battery pack includes a power supply 41 and a circuit board 42. The circuit board 42 is connected to the power supply 41 and includes a positive terminal 43, a negative terminal 44, and a temperature detection terminal 45.

[0176] The power supply 41 includes a secondary battery. In the secondary battery, a positive electrode lead is connected to the positive electrode terminal 43, and a negative electrode lead is connected to the negative electrode terminal 44. Since the power supply 41 is capable of being connected to the outside via the positive electrode terminal 43 and the negative electrode terminal 44, it is capable of charging and discharging. The circuit substrate 42 includes a control section 46, a switch 47, a thermistor element (PTC) 48, and a temperature detection section 49. In addition, the PTC element 48 can be omitted.

[0177] The control section 46 includes a central processing unit (CPU) and a memory, and the like, and controls the operation of the entire battery pack. The control section 46 performs detection and control of the usage state of the power supply 41 as necessary.

[0178] Note that when the voltage of the power supply 41 (secondary battery) reaches an overcharge detection voltage or an overdischarge detection voltage, the control section 46 cuts off the switch 47, thereby causing the charging current not to flow through the current path of the power supply 41. The overcharge detection voltage and the overdischarge detection voltage are not particularly limited. For example, the overcharge detection voltage is 4.2 V ± 0.05 V, and the overdischarge detection voltage is 2.4 V ± 0.1 V.

[0179] The switch 47 includes a charging control switch, a discharging control switch, a charging diode, a discharging diode, and the like, and switches whether or not the power supply 41 is connected to the external device, according to the instruction of the control section 46. The switch 47 includes a metal oxide semiconductor field effect transistor (MOSFET), and the like, and the charging and discharging current is detected based on the on-resistance of the switch 47.

[0180] The temperature detection section 49 includes a temperature detection element such as a thermistor, and measures the temperature of the power supply 41 using the temperature detection terminal 45, and outputs the measurement result of the temperature to the control section 46. The measurement result of the temperature measured by the temperature detection section 49 is used in the case where the control section 46 performs charging and discharging control when abnormal heat generation occurs, and in the case where the control section 46 performs correction processing when calculating the remaining capacity, and the like.

[0181] Embodiment

[0182] An embodiment of the present technology will be described.

[0183] (Embodiments 1 to 20 and Comparative Examples 1 to 15)

[0184] As described below, a secondary battery (lithium ion secondary battery) of a laminate film type shown in FIG. 1 was produced, and the performance of the secondary battery was evaluated. Figures 1-4

[0185] [Production of Secondary Battery]

[0186] The secondary battery was produced by the following steps. ​

[0187] (Production of the positive electrode)

[0188] First, 91 parts by mass of a positive electrode active material (lithium cobaltate (LiCoO2)), 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (graphite) were mixed, thereby producing a positive electrode mixture. Next, the positive electrode mixture was put into a solvent (N-methyl-2-pyrrolidone as an organic solvent), and then the solvent was stirred, thereby preparing a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both surfaces of the positive electrode current collector 11A (aluminum foil with a thickness of 12 μm) using a coating device, and then the positive electrode mixture slurry was dried, thereby forming the positive electrode active material layer 11B. Finally, the positive electrode active material layer 11B was compression-molded using a roll press, thereby producing the positive electrode 11.

[0189] (Production of the negative electrode)

[0190] First, 93 parts by mass of a negative electrode active material (artificial graphite as a carbon material) and 7 parts by mass of a negative electrode binder (polyvinylidene fluoride) were mixed, thereby producing a negative electrode mixture. Next, the negative electrode mixture was put into a solvent (N-methyl-2-pyrrolidone as an organic solvent), and then the solvent was stirred, thereby preparing a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both surfaces of the negative electrode current collector 12A (copper foil with a thickness of 15 μm) using a coating device, and then the negative electrode mixture slurry was dried, thereby forming the negative electrode active material layer 12B. Finally, the negative electrode active material layer 12B was compression-molded using a roll press, thereby producing the negative electrode 12.

[0191] (Preparation of the electrolytic solution)

[0192] An electrolyte salt (lithium hexafluorophosphate (LiPF6)) was added to a solvent, and then the solvent was stirred. As the solvent, ethylene carbonate as a carbonate compound (cyclic carbonate) and propyl propionate (PP) as a chain carboxylate were used. The mixing ratio (volume ratio) of the solvent was determined using ICP emission spectroscopy after the completion of the secondary battery, as described later. The content of the electrolyte salt was 1 mol / kg with respect to the solvent. Thus, the electrolyte salt was dispersed or dissolved in the solvent, thereby preparing the electrolytic solution.

[0193] (Assembly of the secondary battery)

[0194] First, the positive electrode lead 14 made of aluminum was welded to the positive electrode 11 (positive electrode current collector 11A), and the negative electrode lead 15 made of copper was welded to the negative electrode 12 (negative electrode current collector 12A).

[0195] Next, the positive electrode 11 and the negative electrode 12 were stacked with the separator 13 (a microporous polyethylene film having a thickness of 15 μm) interposed therebetween, and then the positive electrode 11, the negative electrode 12, and the separator 13 were wound, whereby a wound body was produced. Next, the wound body was punched using a press, whereby the wound body was shaped into a flat shape.

[0196] Next, as the outer packaging film 20, an upper side film 20X and a lower side film 20Y having a recessed portion 20U were prepared. As each of the upper side film 20X and the lower side film 20Y, an aluminum laminated film in which a heat fusion layer 21 (a polypropylene film having a thickness T), a metal layer 22 (an aluminum foil having a thickness = 40 μm), and a protective layer 23 (a nylon film having a thickness = 25 μm) were sequentially stacked from the inside was used. The thickness T (μm) of the heat fusion layer 21 and the stretching amount Q (mm) of the outer packaging film 20 (the upper side film 20X) are shown in Tables 1 and 2.

[0197] Next, after the wound body was housed inside the recessed portion 20U, the upper side film 20X and the lower side film 20Y were overlapped with each other in a manner of sandwiching the wound body therebetween. Next, the outer peripheral edge portions (the heat fusion layer 21) of three sides in each of the upper side film 20X and the lower side film 20Y were heat fused to each other. Thereby, since three-sided heat fusion portions 20S were formed, a bag-shaped outer packaging film 20 was formed from the upper side film 20X and the lower side film 20Y, and the wound body was housed inside the bag-shaped outer packaging film 20.

[0198] Finally, after an electrolyte solution was injected inside the bag-shaped outer packaging film 20, the outer peripheral edge portions (the heat fusion layer 21) of the remaining one side in each of the upper side film 20X and the lower side film 20Y were heat fused to each other in a reduced pressure environment. Thereby, the electrolyte solution was impregnated in the wound body, and a battery element 10 was produced. In addition, by further forming one-sided heat fusion portions 20S, four-sided heat fusion portions 20S were formed in total, and thus the bag-shaped outer packaging film 20 was sealed. The length L (mm) and the width W (mm) of the heat fusion portions 20S are shown in Tables 1 and 2. Thereby, the battery element 10 was enclosed inside the bag-shaped outer packaging film 20, and a secondary battery was assembled.

[0199] In the case where the heat fusion portions 20S were formed, by changing the range in which the upper side film 20X and the lower side film 20Y were heat fused to each other, the sealing ratio R was adjusted as shown in Tables 1 and 2.

[0200] (Stabilization of secondary battery)

[0201] The secondary battery was charged and discharged in a normal temperature environment (temperature = 23°C). At the time of charging, constant current charging was performed at a current of 0.1 C until the battery voltage reached 4.2 V, and then constant voltage charging was performed at the voltage of 4.2 V until the current reached 0.05 C. At the time of discharging, constant current discharging was performed at a current of 0.1 C until the battery voltage reached 3.0 V. 0.1 C means a current value at which the battery capacity (theoretical capacity) is completely discharged in 10 hours, and 0.05 C means a current value at which the battery capacity is completely discharged in 20 hours.

[0202] Thus, the coating film is formed on the surface of the negative electrode 12 or the like, thereby stabilizing the state of the secondary battery. Therefore, the secondary battery of the laminated film type is completed.

[0203] Note that, after the completion of the secondary battery, the content (volume %) of the chain carboxylate in the solvent was measured by analyzing the electrolyte using ICP emission spectroscopy, and the measurement results are shown in Tables 1 and 2. That is, the mixing ratio (volume ratio) of the solvent was changed in the range of ethylene carbonate: propyl propionate = 80:20 to 30:70.

[0204] [Evaluation of Performance]

[0205] As the performance (sealing property) of the secondary battery (outer packaging film 20), the cycle characteristics, the swelling characteristics, and the insulation characteristics were evaluated, and the results shown in Tables 1 and 2 were obtained.

[0206] In the case of investigating the cycle characteristics, first, the discharge capacity (discharge capacity at the 1st cycle) was measured by causing the secondary battery to perform charging and discharging for one cycle in a normal temperature environment (temperature = 23°C). Next, the secondary battery was repeatedly charged and discharged in the same environment until the number of cycles reached 100 cycles, whereby the discharge capacity (discharge capacity at the 100th cycle) was measured. Finally, the capacity maintenance rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the 1st cycle) x 100 was calculated. Note that the charging and discharging conditions were the same as those at the time of stabilizing the secondary battery.

[0207] In the case of investigating the swelling characteristics, first, the secondary battery was charged in a normal temperature environment (temperature = 23°C), and then the thickness of the secondary battery (thickness before storage) was measured. Note that the charging conditions were the same as those at the time of stabilizing the secondary battery. Next, the secondary battery was stored in a high temperature and high humidity environment (temperature = 60°C, humidity = 90%) for 90 days (storage period), and then the thickness of the secondary battery (thickness after storage) was measured. Finally, the swelling rate (%) = [(thickness after storage - thickness before storage) / thickness before storage] x 100 was calculated.

[0208] In the case of investigating the insulation characteristics, a drop test was performed using the secondary battery, and then the resistance (insulation resistance (Ω)) of the secondary battery was measured. In the drop test, the secondary battery was dropped from a position at a height = 1.9 m onto a floor made of concrete in accordance with the drop test prescribed in the Electrical Appliance and Material Safety Law.

[0209] Here, in addition to evaluating the cycle characteristics, the expansion characteristics, and the insulation characteristics, the battery capacity (Ah) of the secondary battery was also measured as shown in Table 1 and Table 2.

[0210] [Table 1]

[0211]

[0212] [Table 2]

[0213]

[0214] [Investigation]

[0215] As shown in Table 1 and Table 2, the capacity retention rate, the expansion rate, and the insulation resistance, which affect the sealability of the secondary battery (outer packaging film 20), vary depending on the structure (thickness T, length L, width W, seal ratio R, and stretching amount Q) of the outer packaging film 20 and the composition (content of chain carboxylate ester in the solvent) of the electrolyte.

[0216] Specifically, in the case where the six conditions of the thickness T being 25 μm to 60 μm, the length L being 160 mm to 650 mm, the width W being 3 mm to 6 mm, the seal ratio R being 0.16 to 0.32, the stretching amount Q being 7.8 mm or less, and the content of chain carboxylate ester being 30 vol% to 60 vol% are not all satisfied (Comparative Examples 1 to 15), a trade-off relationship in which, when any one of the capacity retention rate, the expansion rate, and the insulation resistance is improved, the others are deteriorated, occurs.

[0217] On the other hand, in the case where the six conditions are all satisfied (Examples 1 to 20), the above trade-off relationship is broken, and thus the capacity retention rate, the expansion rate, and the insulation resistance are all improved. In this case, also a sufficient battery capacity is obtained.

[0218] [Summary]

[0219] As is clear from the results shown in Table 1 and Table 2, in a case where the flexible outer packaging film 20 (the heat-sealed layer 21 contains polypropylene) has the battery element 10 (the solvent of the electrolytic solution contains a chain carboxylate) housed inside, if the above six conditions are satisfied at the same time in a case where the outer packaging film 20 is sealed in the heat-sealed portion 20S, the capacity retention rate increases while ensuring the insulation resistance, and the expansion rate decreases. Thus, in the secondary battery, excellent cycle characteristics, excellent expansion characteristics, and excellent insulation characteristics are obtained.

[0220] While one embodiment and examples are described above with respect to the present technology, the structure of the present technology is not limited to the structure described in one embodiment and examples, and various modifications can be made.

[0221] Specifically, the element structure of the battery element is described with respect to a wound type, but the element structure of the battery element is not particularly limited, and can be a stacked type in which electrodes (a positive electrode and a negative electrode) are stacked, a repeated folding type in which electrodes (a positive electrode and a negative electrode) are folded in a Z shape, or other element structures.

[0222] Further, while the electrode reaction substance is described with respect to lithium, the electrode reaction substance is not particularly limited. Specifically, as described above, the electrode reaction substance can be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. Further, the electrode reaction substance can be other light metals such as aluminum.

[0223] The effects described in this specification are merely illustrative, and the effects of the present technology are not limited to the effects described in this specification. Thus, other effects can be obtained with respect to the present technology.

Claims

1. A secondary battery comprising: a flexible outer packaging member including a heat-sealed layer; and a battery element housed inside the outer packaging member, including a positive electrode, a negative electrode, and an electrolytic solution, the outer packaging member being sealed in a heat-sealed portion formed by heat-sealing the heat-sealed layers to each other, the heat-sealed layer containing polypropylene, the electrolytic solution containing a solvent and an electrolyte salt, the solvent containing a chain carboxylic acid ester, a thickness of the heat-sealed layer being 25 μm or more and 60 μm or less, a length of the heat-sealed portion being 160 mm or more and 650 mm or less, a width of the heat-sealed portion being 3 mm or more and 6 mm or less, a dimension ratio defined by the thickness of the heat-sealed layer, the length of the heat-sealed portion, and the width of the heat-sealed portion satisfying a condition represented by formula (1), a stretching amount of the outer packaging member being 7.8 mm or less, a content of the chain carboxylic acid ester in the solvent being 30 vol% or more and 60 vol% or less, 0.16 ≤ (T x L) / W ≤ 0.32 (1) wherein (T x L) / W is a dimension ratio, T is a thickness of the heat-sealed layer, L is a length of the heat-sealed portion, and W is a width of the heat-sealed portion, and T, L, and W have units of cm.

2. The secondary battery according to claim 1, wherein the outer packaging member is a laminate film including the heat-sealed layer.

3. The secondary battery according to claim 2, wherein the outer packaging member has a multilayer structure in which the heat-sealed layer, a metal layer, and an insulating protective layer are sequentially stacked.

4. The secondary battery according to any one of claims 1 to 3, wherein the secondary battery is a lithium-ion secondary battery. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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