Secondary batteries

Abstract

Provided is a secondary battery capable of obtaining excellent battery characteristics. This secondary battery comprises: a flexible exterior member; and a positive electrode, a negative electrode, and an electrolyte solution which are housed inside the exterior member. The electrolyte solution contains a solvent and an electrolyte salt, and the solvent contains propyl acetate and propyl propionate. The ratio of the content of propyl acetate in the solvent to the sum of the content of propyl acetate in the solvent and the content of propyl propionate in the solvent is 0.1-0.5.

Description

[Technical Field]

[0001] This technology relates to secondary batteries. [Background technology]

[0002] With the widespread use of various electronic devices such as mobile phones, development of rechargeable batteries is progressing as a power source that is small, lightweight, and provides high energy density. These rechargeable batteries contain an electrolyte along with a positive electrode and a negative electrode, and various studies are being conducted on the configuration of these batteries.

[0003] Specifically, in secondary batteries using a film outer casing, the electrolyte contains cyclic carbonate esters and linear carboxylic acid esters (see, for example, Patent Document 1). In addition, a mixed solvent electrolyte of cyclic carbonate esters and propionic acid esters is used as the electrolyte for secondary batteries (see, for example, Patent Documents 2 to 6). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-215959 [Patent Document 2] Special Publication No. 2010-529634 [Patent Document 3] Special Publication No. 2010-530118 [Patent Document 4] Japanese Patent Publication No. 2014-209491 [Patent Document 5] Special Publication No. 2010-539670 [Patent Document 6] Special Publication No. 2017-530500 [Overview of the project]

[0005] Although various studies have been conducted on the configuration of secondary batteries, their battery characteristics are still not satisfactory, and there is room for improvement.

[0006] There is a need for a secondary battery that can achieve excellent battery characteristics.

[0007] A secondary battery according to one embodiment of this technology comprises a flexible outer casing member and a positive electrode, a negative electrode, and an electrolyte housed inside the outer casing member. The electrolyte contains a solvent and an electrolyte salt, the solvent containing propyl acetate and propyl propionate. The ratio of the propyl acetate content in the solvent to the sum of the propyl acetate content in the solvent and the propyl propionate content in the solvent is 0.1 or more and 0.5 or less.

[0008] According to one embodiment of this technology, a secondary battery contains an electrolyte inside a flexible outer casing, and the solvent in the electrolyte contains propyl acetate and propyl propionate. The ratio of the propyl acetate content in the solvent to the sum of the propyl acetate content in the solvent and the propyl propionate content in the solvent is 0.1 or more and 0.5 or less, so that excellent battery characteristics can be obtained.

[0009] Furthermore, the effects of this technology are not necessarily limited to those described herein, but may include any of the series of effects related to this technology described later. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a perspective view showing the configuration of a secondary battery in one embodiment of this technology. [Figure 2] Figure 2 is a cross-sectional view showing the configuration of the battery element shown in Figure 1. [Figure 3] Figure 3 is a perspective view showing the configuration of the secondary battery in Modification Example 1. [Figure 4] Figure 4 is a cross-sectional view showing the configuration of the battery element shown in Figure 3. [Figure 5] Figure 5 is a plan view showing the configuration of the positive electrode shown in Figure 4. [Figure 6]FIG. 6 is a plan view showing the configuration of the negative electrode shown in FIG. 4. [Figure 7] FIG. 7 is a block diagram showing the configuration of an application example of the secondary battery.

Mode for Carrying Out the Invention

[0011] Hereinafter, with respect to one embodiment of the present technology, a detailed description will be given while referring to the drawings. The order of description is as follows. 1. Secondary battery 1-1. Configuration 1-2. Operation 1-3. Manufacturing method 1-4. Action and effect 2. Modification example 3. Applications of secondary batteries

[0012] <1. Secondary battery> First, a secondary battery according to one embodiment of the present technology will be described.

[0013] The secondary battery described here is a secondary battery in which a battery capacity is obtained by utilizing the occlusion and release of an electrode reactant, and includes an electrolytic solution together with a positive electrode and a negative electrode.

[0014] The type of the electrode reactant is not particularly limited, but specifically, it is a light metal such as an alkali metal and an alkaline earth metal. Specific examples of the alkali metal include lithium, sodium, potassium, etc., and specific examples of the alkaline earth metal include beryllium, magnesium, calcium, etc.

[0015] It is preferable that the charging capacity of the negative electrode is larger than the discharging capacity of the positive electrode. That is, it is preferable that the electrochemical capacity per unit area of the negative electrode is larger than the electrochemical capacity per unit area of the positive electrode. This is to suppress the deposition of the electrode reactant on the surface of the negative electrode during charging.

[0016] In the following example, we will consider the case where lithium is the electrode reactant. A secondary battery that obtains battery capacity by utilizing the intercalation and deintercalation of lithium is a so-called lithium-ion secondary battery. In this secondary battery, lithium is intercalated and deintercalated in an ionic state.

[0017] <1-1. Structure> Figure 1 shows a perspective view of the secondary battery, while Figure 2 shows a cross-sectional view of the battery element 20 shown in Figure 1. However, in Figure 1, the outer film 10 and the battery element 20 are shown separated from each other, and the cross-section of the battery element 20 along the XZ plane is shown with a dashed line. In Figure 2, only a portion of the battery element 20 is shown.

[0018] As shown in Figures 1 and 2, this secondary battery comprises an outer film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42.

[0019] [Exterior film] As shown in Figure 1, the outer film 10 is a flexible (or pliable) outer component that houses the battery element 20 inside. Since the outer film 10 has a sealed bag-like structure with the battery element 20 housed inside, it houses the positive electrode 21, negative electrode 22, and electrolyte, which will be described later.

[0020] A secondary battery using an outer film 10, which is a flexible outer component, is a so-called laminate film type secondary battery.

[0021] Here, the outer film 10 is a single film-like component that is folded in the folding direction F. The outer film 10 is provided with a recess 10U (deep-drawn portion) for housing the battery element 20.

[0022] Specifically, the outer film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out. When the outer film 10 is folded, the outer edges of the opposing fusion layers are fused together. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0023] However, the composition (number of layers) of the outer film 10 is not particularly limited; it may consist of one or two layers, or four or more layers.

[0024] [Battery element] As shown in Figures 1 and 2, the battery element 20 is a power generation element that includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), and is housed inside the outer film 10.

[0025] This battery element 20 is a so-called wound electrode body. That is, the positive electrode 21 and the negative electrode 22 are wound around a winding axis P, facing each other via a separator 23. This winding axis P is a virtual axis extending in the Y-axis direction.

[0026] The three-dimensional shape of the battery element 20 is not particularly limited. Here, since the three-dimensional shape of the battery element 20 is flattened, the shape of the cross-section of the battery element 20 intersecting the winding axis P (cross-section along the XZ plane) is a flattened shape defined by the major axis J1 and the minor axis J2. The major axis J1 is a virtual axis that extends in the X-axis direction and has a length greater than the length of the minor axis J2. The minor axis J2 is a virtual axis that extends in the Z-axis direction intersecting the X-axis direction and has a length less than the length of the major axis J1. Here, since the three-dimensional shape of the battery element 20 is a flattened cylinder, the shape of the cross-section of the battery element 20 is a flattened, approximately elliptical shape.

[0027] (positive electrode) As shown in Figure 2, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

[0028] The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. This positive electrode current collector 21A contains a conductive material such as a metal material, a specific example of which is aluminum.

[0029] The positive electrode active material layer 21B contains one or more types of positive electrode active materials that intercalate and deintercalate lithium. However, the positive electrode active material layer 21B may further contain one or more types of other materials such as positive electrode binders and positive electrode conductive agents. The method for forming the positive electrode active material layer 21B is not particularly limited, but specifically includes methods such as coating.

[0030] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A on the side where the positive electrode 21 faces the negative electrode 22.

[0031] The type of positive electrode active material is not particularly limited, but specifically, it is a lithium-containing compound, because a high voltage can be obtained. This lithium-containing compound is a compound that contains lithium along with one or more transition metal elements as constituent elements, and may further contain one or more other elements (excluding lithium and transition metal elements) as constituent elements. The type of other elements is not particularly limited, but specifically, it is an element belonging to groups 2 to 15 of the long-period periodic table. The type of lithium-containing compound is not particularly limited, but specifically, it is an oxide, a phosphoric acid compound, a silicate compound, and a borate compound, etc.

[0032] Specific examples of oxides include LiNiO2, LiCoO2, and LiCo 0.98 Al 0.01 Mg 0.01 O2, LiLiLi 0.5 Co 0.2 Mn 0.3 Examples include O2 and LiMn2O4. Specific examples of phosphorylated compounds include LiFePO4, LiMnPO4, and LiFe 0.5 Mn0.5 Examples include PO4.

[0033] The positive electrode binder contains one or more materials, such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Specific examples of polymer compounds include polyvinylidene fluoride, polyimide, and carboxymethylcellulose.

[0034] The positive electrode conductive agent contains one or more conductive materials, such as carbon materials, metallic materials, and conductive polymer compounds. Specific examples of carbon materials include graphite, carbon black, acetylene black, and Ketjen black.

[0035] (Negative electrode) As shown in Figure 2, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0036] The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. This negative electrode current collector 22A contains a conductive material such as a metallic material, a specific example of which is copper.

[0037] The negative electrode active material layer 22B contains one or more types of negative electrode active materials that intercalate and deintercalate lithium. However, the negative electrode active material layer 22B may further contain one or more types of other materials such as negative electrode binders and negative electrode conductive agents. The method for forming the negative electrode active material layer 22B is not particularly limited, but specifically includes methods such as coating.

[0038] Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may be provided on only one side of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21.

[0039] The type of the negative electrode active material is not particularly limited, but specifically, it is a carbon material, a metal-based material, etc. This is because a high energy density can be obtained.

[0040] Specific examples of the carbon material are graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite), etc.

[0041] The metal-based material is a material containing any one or two or more of metal elements and semi-metal elements that can form an alloy with lithium as constituent elements. Specific examples of the metal elements and semi-metal elements are silicon, tin, etc. This metal-based material may be a single substance, an alloy, a compound, a mixture of two or more of them, or a material containing two or more phases of them. Specific examples of the metal-based material are TiSi2 and SiO x (0 < x ≤ 2 or 0.2 < x < 1.4), etc.

[0042] 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 negative electrode binder.

[0043] (Separator) As shown in FIG. 2, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, and allows lithium ions to pass through while preventing a short circuit caused by the contact between the positive electrode 21 and the negative electrode 22. This separator 23 contains a polymer compound such as polyethylene.

[0044] (Electrolyte) The electrolyte is a liquid electrolyte and is impregnated in each of the positive electrode 21, the negative electrode 22, and the separator 23. This electrolyte contains a solvent and an electrolyte salt, and the solvent contains a non-aqueous solvent (organic solvent). The electrolyte containing a non-aqueous solvent is a so-called non-aqueous electrolyte.

[0045] Specifically, the solvent contains propyl acetate and propyl propionate, and the mixing ratio of propyl acetate to propyl propionate satisfies a predetermined relationship.

[0046] Specifically, let C1 be the content of propyl acetate in the solvent, and C2 be the content of propyl propionate in the solvent. In this case, the content ratio R, which is the ratio of content C1 to the sum of content C1 and content C2, is between 0.1 and 0.5. This content ratio R (%) is calculated based on the formula R = C1 / (C1 + C2). The value of the content ratio R is rounded to the third decimal place.

[0047] The reason why the content ratio R is 0.1 to 0.5 is that the mixing ratio of propyl acetate and propyl propionate is optimized, which suppresses heat generation and gas generation during charging and discharging.

[0048] In detail, propyl acetate has a lower viscosity than propyl propionate, making it less likely to induce heat generation during charging and discharging. However, propyl acetate is easily decomposed during charging and discharging, and therefore tends to generate gas.

[0049] On the other hand, propyl propionate is less likely to decompose during charging and discharging, and therefore does not generate much gas. However, because propyl propionate has a higher viscosity than propyl acetate, it is more likely to induce heat generation during charging and discharging.

[0050] Based on these considerations, by using propyl acetate and propyl propionate in combination, and setting the content ratio R to 0.1-0.5, the advantages of both propyl acetate and propyl propionate are achieved, thereby suppressing heat generation and gas generation during charging and discharging. As a result, the secondary battery becomes more stable even after repeated charging and discharging, and the secondary battery is less likely to swell even when using the outer film 10.

[0051] Furthermore, the solvent preferably contains ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate, and the mixing ratio of ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate satisfies a predetermined relationship.

[0052] Specifically, let C3 be the content of ethylene carbonate in the solvent, C4 be the content of propylene carbonate in the solvent, and C5 be the content of monofluoroethylene carbonate in the solvent. In this case, it is preferable that the content of C4 is greater than the content of C5, and that the content of C5 is greater than the content of C3. That is, it is preferable that the relationship C4 > C5 > C3 holds true for the content of C3 to C5.

[0053] The relationship described above holds true for the C3-C5 content because the ionic conductivity of the electrolyte improves and gas generation is further suppressed during charging and discharging.

[0054] In detail, ethylene carbonate has a higher dielectric constant than propylene carbonate, making it easier to improve the ionic conductivity of the electrolyte. However, ethylene carbonate is easily decomposed during charging and discharging, and therefore tends to generate gas.

[0055] Furthermore, propylene carbonate is less likely to decompose during charging and discharging, thus having the property of generating less gas. However, because propylene carbonate has a lower dielectric constant than ethylene carbonate, it is less likely to improve the ionic conductivity of the electrolyte.

[0056] Furthermore, monofluoroethylene protects the surfaces of the positive electrode 21 and the negative electrode 22 by forming a coating on each surface during charging and discharging. This allows monofluoroethylene to suppress gas generation by inhibiting the decomposition reaction of the electrolyte on the surfaces of the positive electrode 21 and the negative electrode 22. However, because monofluoroethylene is easily decomposed during charging and discharging, it, like ethylene carbonate, is prone to generating gas.

[0057] Based on these considerations, by using ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate together, and by ensuring the above-mentioned relationship holds true regarding the C3-C5 content, the advantages of ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate are utilized. As a result, the ionic conductivity of the electrolyte is improved during charging and discharging, and gas generation is further suppressed. Therefore, even after repeated charging and discharging, the secondary battery becomes more stable, and the secondary battery is less likely to swell even when using the outer film 10.

[0058] Furthermore, the electrolyte solution contains succinonitrile and adiponitrile, and it is preferable that the mixing ratio of succinonitrile and adiponitrile satisfies a predetermined relationship.

[0059] Specifically, let C6 be the content of succinonitrile in the electrolyte and C7 be the content of adiponitrile in the electrolyte. In this case, it is preferable that content C7 is greater than content C6. That is, it is preferable that the relationship C7 > C6 holds true for content C6 and C7.

[0060] The relationship described above holds true for C6 and C7 content because the oxidation resistance of the electrolyte is improved and gas generation is further suppressed during charging and discharging.

[0061] In detail, electrolytes containing propyl acetate and propyl propionate are prone to oxidation during charging and discharging. However, if the electrolyte also contains succinonitrile and adiponitrile, it becomes less susceptible to oxidation during charging and discharging.

[0062] In this case, since the polarity of the electrolyte containing propyl acetate and propyl propionate is low, the solubility of succinonitrile and adiponitrile becomes an issue when succinonitrile and adiponitrile are added to the electrolyte.

[0063] Succinonitrile has properties that make it easy to suppress gas generation during charging and discharging. However, because the carbon chain of succinonitrile is shorter than that of adiponitrile, succinonitrile is less easily dissolved. Moreover, succinonitrile also has properties that can cause an increase in electrical resistance.

[0064] On the other hand, because the carbon chain of adiponitrile is longer than that of succinonitrile, adiponitrile is more easily dissolved. However, although adiponitrile has the property of suppressing gas generation during charging and discharging, its ability to suppress gas generation is lower than that of succinonitrile.

[0065] Based on these considerations, by using succinonitrile and adiponitrile in combination, and by ensuring the above-mentioned relationship holds true regarding the C6 and C7 content, the advantages of both succinonitrile and adiponitrile are achieved. As a result, during charging and discharging, the increase in electrical resistance is suppressed, the oxidation resistance of the electrolyte is improved, and gas generation is further suppressed. Therefore, even after repeated charging and discharging, the secondary battery becomes more stable, and the secondary battery is less likely to swell even when using the outer film 10.

[0066] To determine the content of C1 to C7, the electrolyte is recovered by disassembling the secondary battery, and then its content is measured by analyzing the electrolyte. The analytical method for the electrolyte is not particularly limited, but specifically, it may be one or more of the following: inductively coupled plasma (ICP) emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and gas chromatography-mass spectroscopy (GC-MS).

[0067] Here, the solvent may further contain one or more of the other compounds.

[0068] Specifically, other compounds include esters and ethers, and more specifically, carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds. This is because the dissociability of the electrolyte salt is improved, as is the mobility of the ions.

[0069] However, ethylene carbonate and propylene carbonate mentioned above are excluded from the carbonate ester compounds described here. Also, propyl acetate and propyl propionate mentioned above are excluded from the carboxylic acid ester compounds described here.

[0070] Carbonate ester compounds are linear carbonate esters, with specific examples including dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate. Carboxylic acid ester compounds are linear carboxylic acid esters, with specific examples including ethyl acetate, ethyl propionate, and trimethylethyl acetate. Lactone compounds are lactones, with specific examples including γ-butyrolactone and γ-valerolactone. Ethers may also include 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane.

[0071] Other compounds include unsaturated cyclic carbonate esters, fluorinated cyclic carbonate esters, sulfonic acid esters, phosphate esters, acid anhydrides, nitrile compounds, and isocyanate compounds.

[0072] However, the monofluoroethylene carbonate mentioned above is excluded from the fluorinated cyclic carbonate esters described here. Also, succinonitrile and adiponitrile mentioned above are excluded from the nitrile compounds described here.

[0073] Specific examples of unsaturated cyclic carbonate esters include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonate esters include difluoroethylene carbonate. Specific examples of sulfonic acid esters include propanesultone and propensultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include malononitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0074] The electrolyte salt contains one or more types of light metal salts, such as lithium salts. Specific examples of lithium salts include lithium hexafluoride phosphate (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), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2). This is because it allows for high battery capacity.

[0075] In particular, the electrolyte salt preferably contains both lithium hexafluoride phosphate, which is a phosphate, and bis(fluorosulfonyl)imide lithium, which is an imide salt. This is because it improves the ionic conductivity of the electrolyte and suppresses damage to the positive electrode 21.

[0076] In detail, lithium hexafluoride phosphate forms an insulated film on the surface of the positive electrode current collector 21A, thereby suppressing corrosion of the positive electrode current collector 21A. However, because lithium hexafluoride phosphate has low lithium ion dissociation ability, it has the property of lowering the dielectric constant of the electrolyte.

[0077] On the other hand, bis(fluorosulfonyl)imide lithium has a high ability to dissociate lithium ions, and therefore possesses the property of improving the dielectric constant of the electrolyte. However, bis(fluorosulfonyl)imide lithium has the property of corroding the positive electrode current collector 21A at high voltages. In particular, if the positive electrode current collector 21A contains aluminum, the positive electrode current collector 21A is more susceptible to corrosion at high voltages.

[0078] Based on these findings, using lithium hexafluoride phosphate and lithium bis(fluorosulfonyl)imide in combination improves the ionic conductivity of the electrolyte during charging and discharging, and suppresses damage to the positive electrode 21. Therefore, the secondary battery becomes more stable even after repeated charging and discharging.

[0079] The electrolyte salt content is not particularly limited, but specifically, it is between 0.3 mol / kg and 3.0 mol / kg relative to the solvent. This is because it allows for high ionic conductivity.

[0080] Furthermore, when the electrolyte salt contains lithium hexafluoride phosphate and lithium bis(fluorosulfonyl)imide, the mixing ratio of lithium hexafluoride phosphate and lithium bis(fluorosulfonyl)imide is not particularly limited and can be set arbitrarily.

[0081] [Positive lead and negative lead] As shown in Figures 1 and 2, the positive electrode lead 31 is the positive electrode terminal connected to the positive electrode current collector 21A of the positive electrode 21, and is led out to the outside of the outer film 10. This positive electrode lead 31 contains a conductive material such as a metal material, a specific example of which is aluminum. The shape of the positive electrode lead 31 is not particularly limited, but specifically it can be either a thin plate shape or a mesh shape.

[0082] The negative electrode lead 32, as shown in Figures 1 and 2, is the negative electrode terminal connected to the negative electrode current collector 22A of the negative electrode 22, and is led out to the outside of the outer film 10. This negative electrode lead 32 contains a conductive material such as a metallic material, a specific example of which is copper. Here, the details regarding the lead direction and shape of the negative electrode lead 32 are the same as those regarding the lead direction and shape of the positive electrode lead 31.

[0083] [Sealing film] The sealing film 41 is inserted between the outer film 10 and the positive lead 31, and the sealing film 42 is inserted between the outer film 10 and the negative lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0084] The sealing film 41 is a sealing member that prevents outside air and other elements from entering the interior of the outer film 10. This sealing film 41 contains a polymer compound such as polyolefin that has good adhesion to the positive electrode lead 31, and a specific example of the polyolefin is polypropylene.

[0085] The structure of the sealing film 42 is the same as that of the sealing film 41, except that it is a sealing member that adheres to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as a polyolefin that adheres to the negative electrode lead 32.

[0086] <1-2. Operation> The secondary battery operates as described below.

[0087] During charging, lithium ions are released from the positive electrode 21 of the battery element 20, and these lithium ions are absorbed into the negative electrode 22 via the electrolyte. Conversely, during discharging, lithium ions are released from the negative electrode 22 of the battery element 20, and these lithium ions are absorbed into the positive electrode 21 via the electrolyte.

[0088] <1-3. Manufacturing method> When manufacturing a secondary battery, the positive electrode 21 and negative electrode 22 are prepared and the electrolyte is prepared according to the example procedure described below. The secondary battery is then assembled using the positive electrode 21, negative electrode 22, and electrolyte, and the assembled secondary battery is subjected to a stabilization treatment.

[0089] [Fabrication of the positive electrode] First, a positive electrode mixture is prepared by mixing the positive electrode active material, positive electrode binder, and positive electrode conductive agent together. Next, a paste-like positive electrode mixture slurry is prepared by adding the positive electrode mixture to a solvent. This solvent may be an aqueous solvent or an organic solvent. Finally, a positive electrode active material layer 21B is formed by applying the positive electrode mixture slurry to both sides of the positive electrode current collector 21A. After this, the positive electrode active material layer 21B may be compression molded using a roll press or the like. In this case, the positive electrode active material layer 21B may be heated, or the compression molding may be repeated multiple times. As a result, a positive electrode 21 is produced by forming a positive electrode active material layer 21B on both sides of the positive electrode current collector 21A.

[0090] [Fabrication of the negative electrode] The negative electrode 22 is manufactured using the same procedure as that used for manufacturing the positive electrode 21 described above. Specifically, a paste-like negative electrode mixture slurry is prepared by adding a negative electrode mixture, which is a mixture of negative electrode active material, negative electrode binder, and negative electrode conductive agent, to a solvent. Then, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A to form a negative electrode active material layer 22B. After this, the negative electrode active material layer 22B may be compression molded. As a result, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, thus manufacturing the negative electrode 22.

[0091] [Preparation of electrolyte solution] An electrolyte salt is added to a solvent containing propyl acetate and propyl propionate. This disperses or dissolves the electrolyte salt in the solvent, thus preparing the electrolyte solution.

[0092] In this case, the mixing ratio of propyl acetate and propyl propionate is adjusted so that the content ratio R is 0.1 to 0.5 after the secondary battery is completed (after the stabilization treatment described later).

[0093] Furthermore, when preparing the electrolyte, ethylene carbonate, propylene carbonate, and monofluorocarbonate may be added to the solvent so that an appropriate relationship is established regarding the C3 to C5 content, as described above.

[0094] Furthermore, when preparing the electrolyte, succinonitrile and adiponitrile may be added to the solvent to which the electrolyte salt has been added, so as described above, that an appropriate relationship is established regarding the C6 and C7 content.

[0095] [Assembly of rechargeable batteries] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

[0096] Next, the positive electrode 21 and the negative electrode 22 are stacked on top of each other via the separator 23, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to produce a wound body (not shown). Subsequently, the wound body is pressed using a press or the like to form it into a flattened shape. The wound body after this formation has the same configuration as the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with electrolyte.

[0097] Next, after housing the wound body inside the recessed portion 10U, the outer film 10 (fusion layer / metal layer / surface protection layer) is folded so that the outer films 10 face each other. Subsequently, using an adhesive method such as heat fusion, the outer edges of two sides of the opposing fusion layers are joined together, thereby housing the wound body inside the bag-shaped outer film 10.

[0098] Finally, after injecting the electrolyte into the bag-shaped outer film 10, the outer edges of the remaining sides of the opposing fused layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the outer film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the outer film 10 and the negative electrode lead 32.

[0099] As a result, the electrolyte is impregnated into the wound material, thus creating the battery element 20, which is a wound electrode body. Therefore, the battery element 20 is sealed inside the bag-shaped outer film 10, and the secondary battery is assembled.

[0100] [Stabilization process for secondary batteries] The assembled secondary battery is then charged and discharged. Various conditions such as ambient temperature, number of charge / discharge cycles, and charge / discharge conditions can be set arbitrarily. As a result, a coating is formed on the surface of the positive electrode 21 and the negative electrode 22, thereby electrochemically stabilizing the state of the battery element 20. Thus, the secondary battery is completed.

[0101] <1-4. Mechanism and Effects> According to this secondary battery, the electrolyte is housed inside the outer film 10, and the solvent in the electrolyte contains propyl acetate and propyl propionate, with a content ratio R of 0.1 to 0.5.

[0102] In this case, as described above, heat generation and gas generation are suppressed during charging and discharging. As a result, the secondary battery is more likely to operate stably even after repeated charging and discharging, and the secondary battery is less likely to swell even when using the outer film 10. Therefore, excellent battery characteristics can be obtained.

[0103] In particular, if the solvent further contains ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate, and an appropriate relationship (C4>C5>C3) is established regarding the C3-C5 content, the ionic conductivity of the electrolyte is improved during charging and discharging, and gas generation is further suppressed. As a result, the secondary battery operates more stably even after repeated charging and discharging, and the secondary battery is less likely to swell even when using the outer film 10. Therefore, a higher effect can be obtained.

[0104] Furthermore, if the electrolyte also contains succinonitrile and adiponitrile, and an appropriate relationship (C7 > C6) is established regarding the C6 and C7 content, the increase in electrical resistance during charging and discharging will be suppressed, the oxidation resistance of the electrolyte will be improved, and gas generation will be further suppressed. As a result, the secondary battery will operate more stably even after repeated charging and discharging, and the secondary battery will be less likely to swell even when using the outer film 10. Therefore, a higher level of effectiveness can be obtained.

[0105] Furthermore, if the electrolyte salt contains lithium hexafluoride phosphate and bis(fluorosulfonyl)imide lithium, the ionic conductivity of the electrolyte is improved during charging and discharging, and damage to the positive electrode 21 is suppressed. Therefore, the secondary battery is more likely to operate stably even after repeated charging and discharging, resulting in a greater effect.

[0106] Furthermore, if the secondary battery is a lithium-ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the intercalation and deintercalation of lithium, thus achieving a higher level of efficiency.

[0107] <2. Variant> The configuration of the secondary battery can be modified as appropriate, as described below. However, the variations described below may be combined with each other.

[0108] [Example 1] In Figures 1 and 2, the secondary battery is equipped with a battery element 20 which is a wound electrode body. However, as shown in Figures 3 to 6, the secondary battery may be equipped with a battery element 50 which is a stacked electrode body instead of the battery element 20 which is a wound electrode body.

[0109] Figure 3 shows the perspective view of the secondary battery in modified example 1 and corresponds to Figure 1. Figure 4 shows the cross-sectional view of the battery element 50 shown in Figure 3 and corresponds to Figure 2. Figure 5 shows the planar configuration of the positive electrode 51 shown in Figure 4, and Figure 6 shows the planar configuration of the negative electrode 52 shown in Figure 4. However, only a portion of the battery element 50 is shown in Figure 4.

[0110] The configuration of the secondary battery in Modification 1 (Figures 3 to 6) is the same as that of the secondary battery described above (Figures 1 and 2), except as explained below.

[0111] As shown in Figures 3 to 6, this secondary battery comprises an outer film 10, a battery element 50, a plurality of positive electrode terminals 61, a plurality of negative electrode terminals 62, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42.

[0112] As shown in Figures 3 and 4, the battery element 50 includes a positive electrode 51, a negative electrode 52, a separator 53, and an electrolyte (not shown), and as described above, is a laminated electrode body. That is, the positive electrode 51 and the negative electrode 52 are alternately stacked with the separator 53 in between. The number of each of the positive electrode 51, negative electrode 52, and separator 53 is not particularly limited and can be set arbitrarily.

[0113] The positive electrode 51 includes a positive electrode current collector 51A and a positive electrode active material layer 51B. The configuration of the positive electrode current collector 51A is the same as that of the positive electrode current collector 21A, and the configuration of the positive electrode active material layer 51B is the same as that of the positive electrode active material layer 21B.

[0114] Here, as shown in Figure 5, a portion of the positive electrode current collector 51A protrudes, and therefore the positive electrode current collector 51A includes a portion that protrudes outward from the positive electrode active material layer 51B (hereinafter referred to as the "protruding portion of the positive electrode current collector 51A"). Since the positive electrode active material layer 51B is not provided in this protruding portion of the positive electrode current collector 51A, this protruding portion functions as a positive electrode terminal 61. Details of the positive electrode terminal 61 will be described later.

[0115] The negative electrode 52 includes a negative electrode current collector 52A and a negative electrode active material layer 52B. The configuration of the negative electrode current collector 52A is the same as that of the negative electrode current collector 22A, and the configuration of the negative electrode active material layer 52B is the same as that of the negative electrode active material layer 22B.

[0116] Here, as shown in Figure 6, a portion of the negative electrode current collector 52A protrudes, and therefore the negative electrode current collector 52A includes a portion that protrudes outward from the negative electrode active material layer 52B (hereinafter referred to as the "protruding portion of the negative electrode current collector 52A"). Since the negative electrode active material layer 52B is not provided in this protruding portion of the negative electrode current collector 52A, this protruding portion functions as a negative electrode terminal 62. Details of the negative electrode terminal 62 will be described later.

[0117] The configuration of separator 53 is the same as that of separator 23. The configuration of the electrolyte is as described above.

[0118] As shown in Figure 5, the positive electrode terminal 61 is electrically connected to the positive electrode 51, and more specifically, to the positive electrode current collector 51A. Furthermore, as described above, in the battery element 50, the positive electrode 51 and the negative electrode 52 are alternately stacked via a separator 53, so the battery element 50 contains multiple positive electrodes 51. Thus, the secondary battery has multiple positive electrode terminals 61. The material used to form the positive electrode terminal 61 is not particularly limited, but specifically, it is the same as the material used to form the positive electrode current collector 51A.

[0119] Here, as described above, the protruding portion of the positive electrode current collector 51A functions as the positive electrode terminal 61, and therefore the positive electrode terminal 61 is physically integrated with the positive electrode current collector 51A. This is because the connection resistance between the positive electrode current collector 51A and the positive electrode terminal 61 decreases, thus reducing the overall electrical resistance of the secondary battery.

[0120] Since the multiple positive terminals 61 are joined to each other, they form a single lead-shaped joint 61Z.

[0121] As shown in Figure 6, the negative electrode terminal 62 is electrically connected to the negative electrode 52, and more specifically, to the negative electrode current collector 52A. Furthermore, as described above, in the battery element 50, the positive electrode 51 and the negative electrode 52 are alternately stacked via a separator 53, so the battery element 50 contains multiple negative electrodes 52. Thus, the secondary battery has multiple negative electrode terminals 62. The material used to form the negative electrode terminal 62 is not particularly limited, but specifically, it is the same as the material used to form the negative electrode current collector 52A.

[0122] Furthermore, the negative terminal 62 is positioned so as not to overlap with the positive terminal 61 when the positive terminal 51 and the negative terminal 52 are stacked alternately via the separator 53.

[0123] Here, as described above, the protruding portion of the negative electrode current collector 52A functions as the negative electrode terminal 62, and therefore the negative electrode terminal 62 is physically integrated with the negative electrode current collector 52A. This is because the connection resistance between the negative electrode current collector 52A and the negative electrode terminal 62 decreases, which in turn reduces the overall electrical resistance of the secondary battery.

[0124] Since the multiple negative terminals 62 are joined to each other, they form a single lead-shaped joint 62Z.

[0125] The manufacturing method for the secondary battery in Modification 1 (Figures 3 to 6) is the same as the manufacturing method for the secondary battery described above (Figures 1 and 2), except as described below.

[0126] The procedure for manufacturing the positive electrode 51 is almost the same as the procedure for manufacturing the positive electrode 21. In this case, the positive electrode active material layer 51B is formed by applying the positive electrode mixture slurry to both sides (excluding the positive electrode terminal 61) of the positive electrode current collector 51A, which has the positive electrode terminal 61 integrated into it.

[0127] The procedure for manufacturing the negative electrode 52 is almost the same as the procedure for manufacturing the negative electrode 22. In this case, the negative electrode active material layer 52B is formed by applying the negative electrode mixture slurry to both sides (excluding the negative electrode terminal 62) of the negative electrode current collector 52A, which has the negative electrode terminal 62 integrated into it.

[0128] When assembling a secondary battery, a laminate (not shown) is first created by alternately stacking positive electrodes 51 and negative electrodes 52 via a separator 53. This laminate has the same configuration as the battery element 50, except that the positive electrode 51, negative electrode 52, and separator 53 are not impregnated with electrolyte, and the junctions 61Z, 61Z have not yet been formed.

[0129] Next, a joint 61Z is formed by joining multiple positive electrode terminals 61 to each other using a joining method such as welding, and then a positive electrode lead 31 is connected to the joint 61Z using the same joining method. Similarly, a joint 62Z is ​​formed by joining multiple negative electrode terminals 62 to each other using a joining method such as welding, and then a negative electrode lead 32 is connected to the joint 62Z using the same joining method.

[0130] Even when using this stacked electrode body, the battery element 50, the same effect can be obtained because the battery capacity is obtained by utilizing the intercalation and deintercalation of lithium.

[0131] [Differentiation 2] A porous membrane separator 23 was used. However, although not specifically shown in the diagram, a laminated separator containing a polymer compound layer may also be used.

[0132] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both sides of the porous membrane. This improves the adhesion of the separator to the positive electrode 21 and the negative electrode 22, thereby suppressing winding misalignment of the battery element 20. As a result, swelling of the secondary battery is suppressed even if a decomposition reaction of the electrolyte occurs. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0133] Furthermore, one or both of the porous membrane and the polymer compound layer may contain multiple insulating particles. This is because the multiple insulating particles promote heat dissipation when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The insulating particles consist of one or more types of insulating materials, such as inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

[0134] When fabricating a laminated separator, a precursor solution containing a polymer compound and a solvent is prepared, and then the precursor solution is applied to one or both sides of a porous membrane. In this case, if necessary, multiple insulating particles may be added to the precursor solution.

[0135] Even when using this stacked separator, lithium ions can move between the positive electrode 21 and the negative electrode 22, thus achieving a similar effect. In this case, as mentioned above, the safety of the secondary battery is improved, resulting in an even greater effect.

[0136] [Difference 3] A liquid electrolyte solution was used. However, although not specifically illustrated here, a gel-like electrolyte layer may also be used.

[0137] In the battery element 20 using an electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked on top of each other via a separator 23 and the electrolyte layer, and the positive electrode 21, negative electrode 22, separator 23, and electrolyte layer are wound together. This electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0138] Specifically, the electrolyte layer contains a polymer compound along with the electrolyte, and the electrolyte is held in place by the polymer compound. This prevents leakage of the electrolyte. The composition of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolyte, polymer compound, and solvent is prepared, and then the precursor solution is applied to one or both sides of the positive electrode 21 and to one or both sides of the negative electrode 22.

[0139] Even when this electrolyte layer is used, lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, thus achieving a similar effect. In this case, in particular, as mentioned above, leakage of the electrolyte is prevented, resulting in an even greater effect.

[0140] <3. Applications of rechargeable batteries> The uses (examples of applications) of secondary batteries are not particularly limited. Secondary batteries used as power sources may be the primary power source or auxiliary power source for electronic devices and electric vehicles. A primary power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the primary power source, or a power source that can be switched to from the primary power source.

[0141] Specific examples of secondary battery applications are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, notebook computers, headphone stereos, portable radios, and portable information terminals; backup power supplies and storage devices such as memory cards; power tools such as electric drills and electric saws; battery packs installed in electronic devices; medical electronic devices such as pacemakers and hearing aids; electric vehicles (including hybrid vehicles); and power storage systems such as household or industrial battery systems that store power in preparation for emergencies. In these applications, one secondary battery may be used, or multiple secondary batteries may be used.

[0142] In a battery pack, individual cells may be used, or a battery pack may be used. An electric vehicle is a vehicle that runs using a secondary battery as a power source, and may also be a hybrid vehicle equipped with other power sources other than the secondary battery. In a household power storage system, household electrical appliances can be used by utilizing the electricity stored in the secondary battery, which is the power storage source.

[0143] Here, we will specifically explain one example of a secondary battery application. The configuration of the application example described below is merely an example and can be modified as needed.

[0144] Figure 3 shows the block configuration of the battery pack. The battery pack described here is a single rechargeable battery pack (a so-called soft pack) and is installed in electronic devices such as smartphones.

[0145] As shown in Figure 3, this battery pack comprises a power supply 71 and a circuit board 72. The circuit board 72 is connected to the power supply 71 and includes a positive terminal 73, a negative terminal 74, and a temperature detection terminal 75.

[0146] The power supply 71 includes one secondary battery. In this secondary battery, the positive lead is connected to the positive terminal 73, and the negative lead is connected to the negative terminal 74. Since the power supply 71 can be connected to the outside via the positive terminal 73 and the negative terminal 74, it can be charged and discharged. The circuit board 72 includes a control unit 76, a switch 77, a thermal resistance element 78, and a temperature detection unit 79. A specific example of the thermal resistance element 78 is a PTC element, and the thermal resistance element 78 may be omitted.

[0147] The control unit 76 includes a central processing unit (CPU) and memory, and controls the overall operation of the battery pack. The control unit 76 also detects and controls the usage status of the power supply 71 as needed.

[0148] Furthermore, when the voltage of the power supply 71 (secondary battery) reaches the overcharge detection voltage or over-discharge detection voltage, the control unit 76 disconnects the switch 77 to prevent charging current from flowing through the current path of the power supply 71. The overcharge detection voltage is not particularly limited, but specifically it is 4.20V ± 0.05V, and the over-discharge detection voltage is not particularly limited, but specifically it is 2 It is 0.40V ± 0.1V.

[0149] The switch 77 includes a charge control switch, a discharge control switch, a charging diode, and a discharging diode, and switches the connection between the power supply 71 and external equipment according to the instructions of the control unit 76. This switch 77 includes a field-effect transistor (MOSFET) using a metal oxide semiconductor, and the charging current and discharging current are detected based on the ON resistance of the switch 77.

[0150] The temperature detection unit 79 includes a temperature detection element such as a thermistor. This temperature detection unit 79 measures the temperature of the power supply 71 using the temperature detection terminal 75 and outputs the temperature measurement result to the control unit 76. The temperature measurement result measured by the temperature detection unit 79 is used when the control unit 76 performs charge / discharge control in the event of abnormal heat generation and when the control unit 76 performs correction processing when calculating the remaining capacity. [Examples]

[0151] An example of this technology will be described below.

[0152] <Examples 1-3 and Comparative Examples 1, 2> As explained below, after fabricating a secondary battery, its battery characteristics were evaluated.

[0153] [Manufacturing of secondary batteries] The secondary batteries shown in Figures 1 and 2 were fabricated using the procedure described below. As mentioned above, these secondary batteries are laminate film type lithium-ion secondary batteries.

[0154] (Fabrication of the positive electrode) First, a positive electrode mixture was prepared by mixing 91 parts by mass of positive electrode active material (LiCoO2, a lithium-containing compound (oxide)), 3 parts by mass of positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of positive electrode conductive agent (Ketjenbrak, an amorphous carbon powder). Subsequently, the positive electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the solvent was stirred to prepare a paste-like positive electrode mixture slurry.

[0155] Next, a positive electrode slurry was applied to both sides of the positive electrode current collector 21A (aluminum foil with a thickness of 10 μm) using a coating device, and then the positive electrode active material layer 21B was formed by drying the positive electrode slurry.

[0156] Finally, the positive electrode active material layer 21B was compression-molded using a roll press, and then the positive electrode current collector 21A on which the positive electrode active material layer 21B was formed was cut into strips. This completed the production of the positive electrode 21.

[0157] (Fabrication of the negative electrode) First, 93 parts by mass of negative electrode active material (artificial graphite, a carbon material) and 7 parts by mass of negative electrode binder (polyvinylidene fluoride) were mixed together to prepare a negative electrode mixture. Subsequently, the negative electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the solvent was stirred to prepare a paste-like negative electrode mixture slurry.

[0158] Next, a negative electrode slurry was applied to both sides of the negative electrode current collector 22A (a copper foil with a thickness of 8 μm) using a coating device, and then the negative electrode active material layer 22B was formed by drying the negative electrode slurry.

[0159] Finally, the negative electrode active material layer 22B was compression-molded using a roll press, and then the negative electrode current collector 22A on which the negative electrode active material layer 22B was formed was cut into strips. This produced the negative electrode 22.

[0160] (Preparation of electrolyte solution) First, the solvent was prepared. The solvent used was a mixture of propyl acetate (PrAc), propyl propionate (PrPr), ethylene carbonate (EC), propylene carbonate (PC), and monofluoroethylene carbonate (FEC). In this case, as will be described later, the mixing ratio (weight ratio) of the solvent was adjusted so that when the electrolyte was analyzed after the completion of the secondary battery, the content C1-C5 (weight %) and content ratio R (%) were the values ​​shown in Table 1.

[0161] Next, an electrolyte salt was added to the solvent, and the solvent was stirred. A mixture of lithium hexafluoride phosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) was used as the electrolyte salt. In this case, the lithium hexafluoride phosphate content was 0.5 mol / kg relative to the solvent, and the lithium bis(fluorosulfonyl)imide content was also 0.5 mol / kg relative to the solvent. This prepared the electrolyte solution.

[0162] Finally, succinonitrile (SN) and adiponitrile (ADN) were added to the electrolyte, and the electrolyte was then stirred. In this case, as will be described later, the amount of succinonitrile and adiponitrile added was adjusted so that when the electrolyte was analyzed after the completion of the secondary battery, the C6 and C7 (weight %) content would be the values ​​shown in Table 1.

[0163] (Assembly of secondary batteries) First, a positive electrode lead 31 (aluminum foil) was welded to the positive electrode current collector 21A of the positive electrode 21, and a negative electrode lead 32 (copper foil) was welded to the negative electrode current collector 22A of the negative electrode 22.

[0164] Next, the positive electrode 21 and the negative electrode 22 were laminated together via a separator 23 (a microporous polyethylene film with a thickness of 25 μm), and then the positive electrode 21, the negative electrode 22, and the separator 23 were wound together to create a wound body. Subsequently, the wound body was pressed using a press machine to form a flattened shape.

[0165] Next, the outer film 10 was folded so as to sandwich the wound body housed inside the recessed portion 10U. The outer film 10 used was an aluminum laminate film in which a fusion layer (polypropylene film with a thickness of 30 μm), a metal layer (aluminum foil with a thickness of 40 μm), and a surface protection layer (nylon film with a thickness of 25 μm) were laminated in this order from the inside. Subsequently, the outer edges of two sides of the opposing fusion layers were heat-fused together to house the wound body inside the bag-shaped outer film 10.

[0166] Finally, after injecting the electrolyte into the bag-shaped outer film 10, the outer edges of the remaining side of the opposing fusion layers were heat-fused together in a reduced-pressure environment. In this case, a sealing film 41 (polypropylene film with a thickness of 5 μm) was inserted between the outer film 10 and the positive electrode lead 31, and a sealing film 42 (polypropylene film with a thickness of 5 μm) was inserted between the outer film 10 and the negative electrode lead 32.

[0167] As a result, the electrolyte was impregnated into the wound material, thus creating the battery element 20. Therefore, the battery element 20 was sealed inside the outer film 10, and the secondary battery was assembled.

[0168] (Stabilization treatment for secondary batteries) The assembled secondary battery was subjected to one charge-discharge cycle in a room temperature environment (temperature = 23°C). During charging, constant current charging was performed at a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed at that voltage of 4.2V until the current reached 0.025C. During discharging, constant current discharge was performed at a current of 0.1C until the voltage reached 3.0V. Note that 0.1C is the current value required to completely discharge the battery capacity (theoretical capacity) in 10 hours, and 0.025C is the current value required to completely discharge the battery capacity in 40 hours.

[0169] As a result, the state of the battery element 20 became electrochemically stable, and the secondary battery was completed.

[0170] Furthermore, the results of analyzing the electrolyte using ICP emission spectrometry after the completion of the secondary battery are shown in Table 1. Table 1 shows the content of propyl acetate (PrAc) in the solvent C1 (weight%), propyl propionate (PrPr) in the solvent C2 (weight%), ethylene carbonate (EC) in the solvent C3 (weight%), propylene carbonate (PC) in the solvent C4 (weight%), monofluoroethylene carbonate (FEC) in the solvent C5 (weight%), succinonitrile (SN) in the electrolyte C6 (weight%), adiponitrile (ADN) in the electrolyte C7 (weight%), and the content ratio R (%).

[0171] [Evaluation of battery characteristics] The discharge characteristics and swelling characteristics of the battery were evaluated using the procedure described below, and the results shown in Table 1 were obtained.

[0172] (Discharge characteristics) First, the temperature sensor connected to the temperature logger was attached to the secondary battery using Kapton® tape. In this case, the temperature sensor was positioned approximately in the center of the upper surface (approximately flat surface) of the outer film 10.

[0173] Next, the secondary battery was charged and discharged in a constant temperature bath (temperature = 25℃ ± 1℃) while its temperature was measured every second using a temperature logger. During charging, constant current charging was performed at a current of 0.5C until the voltage reached 4.2V, and then constant voltage charging was performed at that voltage of 4.2V until the current reached 0.03C. During discharging, constant current discharge was performed at a current of 10C until the voltage reached 2.5V. Note that 0.5C is the current value that completely discharges the battery capacity in 2 hours, 10C is the current value that completely discharges the battery capacity in 0.1 hours, and 0.03C is the current value that completely discharges the battery capacity in 100 / 3 hours.

[0174] Finally, after the charging and discharging cycle was complete, the highest temperature (°C), an index for evaluating discharge characteristics, was identified by examining the highest temperature measured using a temperature logger.

[0175] During high-current discharge, the battery element 20 generates heat, causing the temperature of the secondary battery to rise. In this case, to ensure safety, it is necessary to stop charging and discharging using a protection circuit when the temperature rises. Therefore, the maximum temperature is the temperature that reflects the time from the start of charging and discharging until charging and discharging stops (discharge time), and thus serves as an indicator for evaluating the discharge characteristics.

[0176] The lower the maximum temperature, the longer the time it takes for charging and discharging to stop (the time during which discharge is possible), making it possible to use the secondary battery stably for a long period of time. On the other hand, the higher the maximum temperature, the shorter the time it takes for charging and discharging to stop, making it difficult to use the secondary battery stably for a long period of time.

[0177] (Swelling characteristics) First, the secondary battery was charged in a room temperature environment (temperature = 23°C), and then its thickness (thickness before storage) was measured. In this case, constant current charging was performed with a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed with that voltage of 4.2V until the current reached 0.025C. The thickness of the secondary battery is the dimension from the top surface (approximately flat surface) of the outer film 10 to the bottom surface (approximately flat surface on the opposite side) of the outer film 10.

[0178] Next, a charged secondary battery was stored in a high-temperature environment (temperature = 60°C) for 2 months, and then its thickness (thickness after storage) was measured.

[0179] Finally, the swelling rate, an index for evaluating swelling characteristics, was calculated based on the formula: swelling rate (%) = [(thickness after storage - thickness before storage) / thickness before storage] × 100.

[0180] [Table 1]

[0181] [Consideration] As shown in Table 1, the maximum temperature and swelling rate varied considerably depending on the electrolyte composition.

[0182] Specifically, when the content ratio R was less than 0.1 (Comparative Example 1), the swelling rate decreased, but the maximum temperature increased. Conversely, when the content ratio R was greater than 0.5 (Comparative Example 2), the maximum temperature decreased, but the swelling rate increased.

[0183] In contrast, when the content ratio R was 0.1 to 0.5 (Examples 1 to 3), the maximum temperature decreased and the swelling rate decreased. In this case, in particular, when the electrolyte salt contained lithium hexafluoride phosphate and bis(fluorosulfonyl)imide lithium, the maximum temperature decreased sufficiently and the swelling rate decreased sufficiently.

[0184] <Examples 4-6> As shown in Table 2, secondary batteries were fabricated using the same procedure as in Example 1, except that the content of C3 to C5 (weight %) was changed and the cycle characteristics were evaluated instead of the discharge characteristics. After the batteries were fabricated, their characteristics were evaluated.

[0185] In Table 2, the "Appropriate Relationship" column indicates whether an appropriate relationship (C4>C5>C3) exists for the C3-C5 content. That is, "Established" means that an appropriate relationship exists, while "Not Established" means that an appropriate relationship does not exist.

[0186] To evaluate the cycle characteristics, the discharge capacity (discharge capacity in the first cycle) was first measured by charging and discharging the secondary battery in a normal temperature environment (temperature = 23°C). Subsequently, the discharge capacity (discharge capacity in the 100th cycle) was measured by repeatedly charging and discharging the secondary battery in the same environment until the total number of cycles reached 100. Finally, the capacity retention rate (%), which is an index for evaluating cycle characteristics, was calculated based on the formula: Capacity retention rate (%) = (Discharge capacity in the 100th cycle / Discharge capacity in the first cycle) × 100. The charge and discharge conditions were the same as those used during the stabilization process.

[0187] [Table 2]

[0188] As shown in Table 2, when an appropriate relationship was established regarding the content of C3 to C5 (Examples 1 and 4), the swelling rate was suppressed while the volume retention rate increased compared to when an appropriate relationship was not established regarding the content of C3 to C5 (Examples 5 and 6).

[0189] <Examples 7-9> As shown in Table 3, secondary batteries were fabricated using the same procedure as in Example 1, except that the content of C6 and C7 (by weight) was changed, and then the battery characteristics were evaluated.

[0190] In Table 3, the "Appropriate Relationship" column indicates whether an appropriate relationship (C7 > C6) exists between the C6 and C7 content levels. "Established" indicates that an appropriate relationship exists, while "Not Established" indicates that an appropriate relationship does not exist.

[0191] [Table 3]

[0192] As shown in Table 3, when the electrolyte contains succinonitrile and adiponitrile and an appropriate relationship is established regarding the C6 and C7 content (Example 1), the maximum temperature was sufficiently suppressed and the swelling rate was also sufficiently suppressed compared to when the electrolyte does not contain succinonitrile and adiponitrile (Example 7) and when the electrolyte contains succinonitrile and adiponitrile but an appropriate relationship is not established regarding the C6 and C7 content (Examples 8 and 9).

[0193] [summary] The results shown in Tables 1 to 3 indicate that when the electrolyte is contained inside the outer film 10, and the solvent in the electrolyte contains propyl acetate and propyl propionate, with a content ratio R of 0.1 to 0.5, excellent battery characteristics were obtained in the secondary battery.

[0194] Although the present technology has been described above with reference to one embodiment and one example, the configuration of the present technology is not limited to the configuration described in the one embodiment and one example, and can be modified in various ways.

[0195] Specifically, the explanation described the case where the battery structure of the rechargeable battery is of the laminated film type. However, the battery structure of the rechargeable battery is not particularly limited, and may also be cylindrical, prismatic, coin-shaped, or button-shaped.

[0196] Furthermore, the case where the element structure of the battery element is of the wound type has been described. However, the element structure of the battery element is not particularly limited, and may also be of the stacked type or the zigzag type. In the stacked type, the positive electrode and negative electrode are stacked alternately with a separator in between, while in the zigzag type, the positive electrode and negative electrode are folded in a zigzag pattern with a separator in between, facing each other.

[0197] Furthermore, while we have described the case where the electrode reactant is lithium, the type of electrode reactant is not particularly limited. Specifically, as mentioned above, the electrode reactant may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium, and calcium. In addition, the electrode reactant may be other light metals such as aluminum.

[0198] The effects described herein are illustrative only, and therefore the effects of this technology are not limited to those described herein. Accordingly, other effects may be obtained with respect to this technology.

[0199] Furthermore, this technology can also be configured as follows: <1> A flexible exterior component, The positive electrode, negative electrode, and electrolyte housed inside the exterior member Equipped with, The electrolyte comprises a solvent and an electrolyte salt. The solvent comprises propyl acetate and propyl propionate. The ratio of the content of propyl acetate in the solvent to the sum of the content of propyl propionate in the solvent is 0.1 or more and 0.5 or less. Secondary battery. <2> The solvent further comprises ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate. The content of propylene carbonate in the solvent is greater than the content of ethylene monofluorocarbonate in the solvent, The content of ethylene monofluorocarbonate in the solvent is greater than the content of ethylene carbonate in the solvent. <1> The secondary battery described above. <3> The electrolyte further comprises succinonitrile and adiponitrile. The content of adiponitrile in the electrolyte is greater than the content of succinonitrile in the electrolyte. <1> or <2> The secondary battery described above. <4> The aforementioned electrolyte salt comprises lithium hexafluoride phosphate and lithium bis(fluorosulfonyl)imide. <1> or <3> A rechargeable battery as described in one of the following. <5> Lithium-ion rechargeable batteries, <1> or <4> A rechargeable battery as described in one of the following.

Claims

1. A flexible exterior component, The positive electrode, negative electrode, and electrolyte housed inside the exterior member Equipped with, The electrolyte comprises a solvent and an electrolyte salt. The solvent includes propyl acetate, propyl propionate, ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate. The ratio of the content of propyl acetate in the solvent to the sum of the content of propyl propionate in the solvent is 0.1 or more and 0.5 or less. The content of propylene carbonate in the solvent is greater than the content of ethylene monofluorocarbonate in the solvent, The content of ethylene monofluorocarbonate in the solvent is greater than the content of ethylene carbonate in the solvent. The capacity retention rate, which is the ratio of the discharge capacity at 100 cycles to the discharge capacity at 1 cycle, is greater than 78%. Secondary battery.

2. The electrolyte further comprises succinonitrile and adiponitrile. The content of adiponitrile in the electrolyte is greater than the content of succinonitrile in the electrolyte. The secondary battery according to claim 1.

3. The electrolyte salt comprises lithium hexafluoride phosphate and lithium bis(fluorosulfonyl)imide. A secondary battery according to claim 1 or claim 2.

4. Lithium-ion rechargeable batteries, A secondary battery according to claim 1 or claim 2.

Images