Secondary battery

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

secondary battery

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

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight power sources that can provide high energy density. These secondary batteries contain a positive electrode, a negative electrode, and an electrolyte solution, and various studies have been conducted on the configuration of these secondary batteries.

[0003] Specifically, in a secondary battery using a film exterior member, the electrolyte contains a cyclic carbonate ester and a chain carboxylate ester (see, for example, Patent Document 1). Also, a mixed solvent electrolyte of a cyclic carbonate ester and a propionate ester has been used as the electrolyte for a secondary battery (see, for example, Patent Documents 2 to 6).

[0004] Japanese Patent Application Laid-Open No. 2019-215959 Japanese Patent Application Laid-Open No. 2010-529634 Japanese Patent Application Laid-Open No. 2010-530118 Japanese Patent Application Laid-Open No. 2014-209491 Japanese Patent Application Laid-Open No. 2010-539670 Japanese Patent Application Laid-Open No. 2017-530500

[0005] Although various studies have been conducted on the configuration of secondary batteries, the battery characteristics of the secondary batteries are still insufficient and there is room for improvement.

[0006] There is a demand for a secondary battery that can provide excellent battery characteristics.

[0007] According to an embodiment of the present disclosure, there is provided a secondary battery including a flexible exterior member, a positive electrode, a negative electrode, and an electrolyte solution housed within the exterior member. The electrolyte solution includes a solvent and an electrolyte salt, and the solvent includes 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 to 0.5.

[0008] According to a secondary battery of one embodiment of the present technology, an electrolytic solution is contained inside a flexible exterior member, a solvent in the electrolytic solution contains propyl acetate and propyl propionate, and 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, thereby achieving excellent battery characteristics.

[0009] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

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

[0011] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Secondary battery 1-1. Configuration 1-2. Operation 1-3. Manufacturing method 1-4. Actions and effects 2. Modifications 3. Uses of secondary battery

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

[0013] The secondary battery described here is a secondary battery that obtains battery capacity by utilizing the absorption and desorption of electrode reactants, and is equipped with a positive electrode, a negative electrode, and an electrolyte.

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

[0015] The charge capacity of the negative electrode is preferably larger than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is preferably larger than the electrochemical capacity per unit area of ​​the positive electrode. This is to prevent deposition of electrode reactants on the surface of the negative electrode during charging.

[0016] In the following, we will take the case where the electrode reactant is lithium as an example. A secondary battery that obtains battery capacity by utilizing the absorption and desorption of lithium is called a lithium ion secondary battery. In this secondary battery, lithium is absorbed and desorbed in the ionic state.

[0017] <1-1. Configuration> Fig. 1 shows a perspective configuration of a secondary battery, and Fig. 2 shows a cross-sectional configuration of the battery element 20 shown in Fig. 1. However, Fig. 1 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a dashed line. Fig. 2 shows only a part of the battery element 20.

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

[0019] 1, the exterior film 10 is a flexible (or pliable) exterior member, and houses the battery element 20. The exterior film 10 has a sealed bag-like structure with the battery element 20 housed therein, and therefore houses the positive electrode 21, the negative electrode 22, and the electrolyte solution described below.

[0020] A secondary battery using the exterior film 10, which is a flexible exterior member, is a so-called laminate film type secondary battery.

[0021] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (deeply drawn portion) for accommodating the battery element 20.

[0022] Specifically, the exterior 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, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0023] However, the configuration (number of layers) of the exterior film 10 is not particularly limited, and may be one layer, two layers, or four or more layers.

[0024] [Battery Element] As shown in FIGS. 1 and 2 , the battery element 20 is a power generating element including a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), and is housed inside the exterior 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 while facing each other with a separator 23 interposed therebetween. This winding axis P is an imaginary axis extending in the Y-axis direction.

[0026] The three-dimensional shape of battery element 20 is not particularly limited. Here, the three-dimensional shape of battery element 20 is flat, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by a major axis J1 and a minor axis J2. This major axis J1 is an imaginary axis that extends in the X-axis direction and has a length greater than the length of minor axis J2. Furthermore, minor axis J2 is an imaginary axis that extends in the Z-axis direction intersecting with the X-axis direction and has a length smaller than the length of major axis J1. Here, the three-dimensional shape of battery element 20 is a flat cylindrical shape, and therefore the shape of the cross section of battery element 20 is a flat, approximately elliptical shape.

[0027] (Positive Electrode) As shown in FIG. 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. The positive electrode current collector 21A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum.

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

[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 includes a lithium-containing compound, etc. This is because a high voltage can be obtained. The lithium-containing compound is a compound containing lithium and one or more transition metal elements as constituent elements, and may further include 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 includes elements belonging to Groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specifically includes oxides, phosphate compounds, silicate compounds, borate compounds, etc.

[0032] A specific example of the oxide is LiNiO 2 , LiCoO 2 , LiCo 0.98 Al 0.01 Mg 0.01 O 2 , LiNi 0.5 Co 0.2 Mn 0.3 O 2 and LiMn 2 O 4 Specific examples of phosphate compounds include LiFePO 4 , LiMnPO 4 and LiFe 0.5 Mn 0.5 P.O. 4 And so on.

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

[0034] The positive electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0035] (Negative Electrode) As shown in FIG. 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. The negative electrode current collector 22A contains a conductive material such as a metal material, and a specific example of the conductive material is copper.

[0037] The anode active material layer 22B contains one or more types of anode active materials that absorb and release lithium. However, the anode active material layer 22B may further contain one or more types of other materials such as anode binders and anode conductors. The method for forming the anode active material layer 22B is not particularly limited, but may be, for example, a coating method.

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

[0039] The type of negative electrode active material is not particularly limited, but specific examples include carbon materials and metal-based materials, because high energy density can be obtained.

[0040] Specific examples of carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite (natural graphite and artificial graphite).

[0041] The metallic material is a material containing, as a constituent element, one or more of metallic elements and semi-metallic elements that can form an alloy with lithium, and specific examples of the metallic element and semi-metallic element include silicon and tin. The metallic material may be a simple substance, an alloy, a compound, a mixture of two or more of these, or a material containing two or more of these phases. Specific examples of metallic materials include TiSi 2 and SiO x (0<x≦2 or 0.2<x<1.4), etc.

[0042] The details regarding the negative electrode binder are the same as those regarding the positive electrode binder, and the details regarding the negative electrode conductive agent are the same as those regarding the negative electrode binder.

[0043] 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 contact between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0044] (Electrolyte) The electrolyte is a liquid electrolyte, and is impregnated into each of the positive electrode 21, the negative electrode 22, and the separator 23. The 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 the propyl acetate and the propyl propionate satisfies a predetermined relationship.

[0046] Specifically, the content of propyl acetate in the solvent is C1, and the content of propyl propionate in the solvent is C2. In this case, the content ratio R, which is the ratio of the content C1 to the sum of the contents C1 and C2, is 0.1 to 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 two decimal places.

[0047] The content ratio R is 0.1 to 0.5 because the mixture ratio of propyl acetate and propyl propionate is optimized, thereby suppressing heat generation and gas generation during charging and discharging.

[0048] Specifically, propyl acetate has a lower viscosity than propyl propionate and is therefore less likely to generate heat during charging and discharging. However, propyl acetate is prone to decomposition during charging and discharging, which makes it more likely to generate gas.

[0049] On the other hand, propyl propionate is less likely to decompose during charging and discharging, and therefore less likely to generate gas. However, propyl propionate has a higher viscosity than propyl acetate, and therefore is more likely to induce heat generation during charging and discharging.

[0050] For these reasons, by using propyl acetate and propyl propionate in combination and setting the content ratio R to 0.1 to 0.5, the advantages of both propyl acetate and propyl propionate are achieved, suppressing heat generation and gas generation during charge and discharge. Therefore, even after repeated charge and discharge, the secondary battery can operate (discharge) stably, and the secondary battery is less likely to swell even when using the exterior film 10.

[0051] The solvent further contains ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate, and it is preferable that the mixing ratio of the ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate satisfy a predetermined relationship.

[0052] Specifically, the content of ethylene carbonate in the solvent is designated C3, the content of propylene carbonate in the solvent is designated C4, and the content of monofluoroethylene carbonate in the solvent is designated C5. In this case, it is preferable that the content C4 is greater than the content C5, and that the content C5 is greater than the content C3. In other words, it is preferable that the relationship C4>C5>C3 holds among the contents C3 to C5.

[0053] The reason why the above relationship holds for the contents C3 to C5 is that the ionic conductivity of the electrolyte is improved and gas generation is further suppressed during charge and discharge.

[0054] Specifically, ethylene carbonate has a higher dielectric constant than propylene carbonate and therefore has the property of easily improving the ionic conductivity of the electrolyte solution. However, ethylene carbonate is easily decomposed during charge and discharge, and therefore has the property of easily generating gas.

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

[0056] Furthermore, monofluoroethylene carbonate forms a coating on the surface of each of the positive electrode 21 and the negative electrode 22 during charge and discharge, thereby protecting the surfaces of the positive electrode 21 and the negative electrode 22. As a result, monofluoroethylene carbonate has the property of easily suppressing gas generation by suppressing the decomposition reaction of the electrolyte on the surface of each of the positive electrode 21 and the negative electrode 22. However, monofluoroethylene carbonate is easily decomposed during charge and discharge, and therefore has the property of easily generating gas, similar to ethylene carbonate.

[0057] For these reasons, by using ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate together and by satisfying the above-described relationship between the contents C3 to C5, the advantages of ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate are utilized. This improves the ionic conductivity of the electrolyte and further suppresses gas generation during charge and discharge. Therefore, even with repeated charge and discharge, the secondary battery operates more stably, and the secondary battery is less likely to swell even when using the exterior film 10.

[0058] The electrolyte solution preferably further contains succinonitrile and adiponitrile, and the mixture ratio of the succinonitrile and adiponitrile preferably satisfies a predetermined relationship.

[0059] Specifically, the content of succinonitrile in the electrolyte solution is designated as C6, and the content of adiponitrile in the electrolyte solution is designated as C7. In this case, the content C7 is preferably greater than the content C6. That is, it is preferable that the relationship C7 > C6 holds between the contents C6 and C7.

[0060] The reason why the above relationship holds for the contents C6 and C7 is that the oxidation resistance of the electrolyte is improved and gas generation is further suppressed during charge and discharge.

[0061] Specifically, an electrolyte containing propyl acetate and propyl propionate is susceptible to oxidation during charging and discharging. However, if the electrolyte further contains succinonitrile and adiponitrile, the electrolyte is less susceptible to oxidation during charging and discharging.

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

[0063] Succinonitrile has the property of easily suppressing gas generation during charge and discharge. However, since the carbon chain of succinonitrile is shorter than that of adiponitrile, succinonitrile has the property of being less soluble. Moreover, succinonitrile also has the property of potentially increasing electrical resistance.

[0064] On the other hand, since the carbon chain of adiponitrile is longer than that of succinonitrile, adiponitrile has the property of being easily dissolved. However, although adiponitrile has the property of suppressing gas generation during charge and discharge, the ability of adiponitrile to suppress gas generation is lower than that of succinonitrile.

[0065] For these reasons, by using succinonitrile and adiponitrile in combination and by satisfying the above-described relationship between the contents C6 and C7, the advantages of both succinonitrile and adiponitrile are achieved. This suppresses an increase in electrical resistance during charge and discharge, improves the oxidation resistance of the electrolyte, and further suppresses gas generation. Therefore, even with repeated charge and discharge, the secondary battery operates more stably, and even when using the exterior film 10, the secondary battery is less likely to swell.

[0066] In addition, when specifying the contents C1 to C7, the secondary battery is disassembled to recover the electrolyte solution, and the electrolyte solution is then analyzed to measure the contents C1 to C7. The method for analyzing the electrolyte solution is not particularly limited, but specifically, any one or more of inductively coupled plasma (ICP) optical emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), gas chromatography mass spectrometry (GC-MS), and the like are used.

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

[0068] Specifically, the other compounds are esters and ethers, and more specifically, carbonate ester compounds, carboxylic acid ester compounds, lactone compounds, etc. This is because the dissociation property of the electrolyte salt is improved and the mobility of ions is also improved.

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

[0070] The carbonate ester compound is a chain carbonate ester, and specific examples of the chain carbonate ester include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. The carboxylic acid ester compound is a chain carboxylic acid ester, and specific examples of the chain carboxylic acid ester include ethyl acetate, ethyl propionate, and ethyl trimethylacetate. The lactone compound is a lactone, and specific examples of the lactone include γ-butyrolactone and γ-valerolactone. The ether may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, or the like.

[0071] Other compounds include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, nitrile compounds, and isocyanate compounds.

[0072] However, the above-mentioned monofluoroethylene carbonate is excluded from the fluorinated cyclic carbonates described herein, and the above-mentioned succinonitrile and adiponitrile are excluded from the nitrile compounds described herein.

[0073] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. 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 light metal salts such as lithium salts. Specific examples of lithium salts include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 ) 2 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2 ) 3 ), lithium bis(oxalato)borate (LiB(C 2 O 4 ) 2 ), lithium monofluorophosphate (Li 2 PFO 3 ) and lithium difluorophosphate (LiPF 2 O 2 ) etc. This is because a high battery capacity can be obtained.

[0075] Among these, it is preferable that the electrolyte salt contains both lithium hexafluorophosphate, which is a phosphate, and lithium bis(fluorosulfonyl)imide, which is an imide salt, because this improves the ionic conductivity of the electrolyte solution and suppresses damage to the positive electrode 21.

[0076] Specifically, lithium hexafluorophosphate forms a non-conductive film on the surface of the positive electrode current collector 21 A, and therefore has the property of suppressing corrosion of the positive electrode current collector 21 A. However, lithium hexafluorophosphate has a low ability to dissociate lithium ions, and therefore has the property of lowering the dielectric constant of the electrolyte solution.

[0077] On the other hand, because lithium bis(fluorosulfonyl)imide has a high lithium ion dissociation ability, it has the property of improving the dielectric constant of the electrolyte. However, lithium bis(fluorosulfonyl)imide has the property of corroding the positive electrode current collector 21A at high voltages. In particular, when the positive electrode current collector 21A contains aluminum, the positive electrode current collector 21A is easily corroded at high voltages.

[0078] For these reasons, the combined use of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide improves the ionic conductivity of the electrolyte during charge and discharge and suppresses damage to the positive electrode 21. Therefore, the secondary battery is more likely to operate stably even after repeated charge and discharge.

[0079] The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent, because high ionic conductivity can be obtained.

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

[0081] 1 and 2 , the positive electrode lead 31 is a positive electrode terminal connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The shape of the positive electrode lead 31 is not particularly limited, but is specifically either a thin plate shape or a mesh shape.

[0082] 1 and 2 , the negative electrode lead 32 is a negative electrode terminal connected to the negative electrode current collector 22A of the negative electrode 22, and is led out of the exterior film 10. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Here, the details regarding the lead-out direction and shape of the negative electrode lead 32 are the same as the details regarding the lead-out direction and shape of the positive electrode lead 31.

[0083] [Sealing Film] The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode 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 the like from entering the inside of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polyolefin is polypropylene.

[0085] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness 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 are absorbed into the negative electrode 22 via the electrolyte. During discharging, lithium ions are released from the negative electrode 22 of the battery element 20 and 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 the negative electrode 22 are fabricated and an electrolytic solution is prepared according to the procedure described below as an example. Thereafter, the positive electrode 21, the negative electrode 22, and the electrolytic solution are used to assemble a secondary battery, and the assembled secondary battery is subjected to a stabilization treatment.

[0089] [Fabrication of Positive Electrode] First, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed together to form a positive electrode mixture. Next, the positive electrode mixture is poured into a solvent to prepare a paste-like positive electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. Finally, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 21A to form the positive electrode active material layer 21B. The positive electrode active material layer 21B may then 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. This results in the positive electrode active material layer 21B being formed on both surfaces of the positive electrode current collector 21A, thereby fabricating the positive electrode 21.

[0090] [Fabrication of Negative Electrode] The negative electrode 22 is fabricated using the same procedure as the fabrication procedure for the positive electrode 21 described above. Specifically, a negative electrode mixture, in which a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent are mixed together, is poured into a solvent to prepare a paste-like negative electrode mixture slurry, and the negative electrode mixture slurry is then applied to both surfaces of the negative electrode current collector 22A to form the negative electrode active material layer 22B. The negative electrode active material layer 22B may then be compression-molded. As a result, the negative electrode active material layer 22B is formed on both surfaces of the negative electrode current collector 22A, and the negative electrode 22 is fabricated.

[0091] [Preparation of Electrolyte Solution] An electrolyte salt is added to a solvent containing propyl acetate and propyl propionate, whereby the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing an 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 below).

[0093] When preparing the electrolytic solution, ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate may be added to the solvent so that the contents C3 to C5 have an appropriate relationship, as described above.

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

[0095] [Assembly of Secondary Battery] 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 one on top of the other with the separator 23 interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 are wound together to produce a wound body (not shown). Next, the wound body is pressed using a press or the like to form a flat wound body. The wound body after this formation has a configuration similar to that of the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with an electrolyte solution.

[0097] Next, after the roll is housed inside the recess 10U, the exterior film 10 (fusion layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the facing fusion layers are joined together using an adhesive method such as heat fusion, thereby housing the roll inside the bag-shaped exterior film 10.

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

[0099] This allows the wound body to be impregnated with the electrolyte, producing the wound electrode body, the battery element 20. The battery element 20 is then sealed inside the bag-shaped exterior film 10, and the secondary battery is assembled.

[0100] [Secondary Battery Stabilization Treatment] The assembled secondary battery is charged and discharged. Various conditions, such as the ambient temperature, the number of charge / discharge cycles (number of cycles), and the charge / discharge conditions, can be set as desired. This forms a coating on the surface of the positive electrode 21 and the surface of the negative electrode 22, electrochemically stabilizing the state of the battery element 20. This completes the secondary battery.

[0101] <1-4. Actions and Effects> According to this secondary battery, an electrolyte solution is accommodated inside the exterior film 10, and the solvent in the electrolyte solution 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. This makes it easier for the secondary battery to operate stably even after repeated charging and discharging, and the secondary battery is less likely to swell even when using the exterior 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 the contents C3 to C5 satisfy the appropriate relationship (C4>C5>C3), the ionic conductivity of the electrolyte is improved and gas generation is further suppressed during charge and discharge. This makes the secondary battery more stable even after repeated charge and discharge, and the secondary battery is less likely to swell even when using the exterior film 10. Therefore, even greater effects can be achieved.

[0104] Furthermore, if the electrolyte further contains succinonitrile and adiponitrile and the contents C6 and C7 satisfy an appropriate relationship (C7 > C6), the oxidation resistance of the electrolyte is improved and gas generation is further suppressed while suppressing an increase in electrical resistance during charging and discharging. This makes the secondary battery more stable even after repeated charging and discharging, and the secondary battery is less likely to swell even when using the exterior film 10. Therefore, even greater effects can be achieved.

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

[0106] Furthermore, if the secondary battery is a lithium ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium, and therefore a greater effect can be obtained.

[0107] 2. Modifications The configuration of the secondary battery can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0108] 1 and 2, the secondary battery includes a battery element 20 that is a wound electrode body. However, as shown in FIGS. 3 to 6, the secondary battery may include a battery element 50 that is a laminated electrode body instead of the battery element 20 that is a wound electrode body.

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

[0110] The configuration of the secondary battery in Modification 1 (FIGS. 3 to 6) is similar to the configuration of the secondary battery described above (FIGS. 1 and 2), except for the points described below.

[0111] As shown in Figures 3 to 6, this secondary battery includes an exterior 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] 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 is a laminated electrode assembly as described above. That is, the positive electrodes 51 and the negative electrodes 52 are alternately laminated with the separator 53 interposed therebetween. The number of positive electrodes 51, negative electrodes 52, and separators 53 is not particularly limited and can be set as desired.

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

[0114] 5, a portion of the positive electrode current collector 51A protrudes, and thus the positive electrode current collector 51A includes a portion (hereinafter referred to as a "protruding portion of the positive electrode current collector 51A") that protrudes outward beyond the positive electrode active material layer 51B. Since the positive electrode active material layer 51B is not provided on the protruding portion of the positive electrode current collector 51A, the protruding portion of the positive electrode current collector 51A 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 52 A and a negative electrode active material layer 52 B. The configuration of the negative electrode current collector 52 A is similar to the configuration of the negative electrode current collector 22 A, and the configuration of the negative electrode active material layer 52 B is similar to the configuration of the negative electrode active material layer 22 B.

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

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

[0118] 5, the positive electrode terminal 61 is electrically connected to the positive electrode 51, more specifically, to the positive electrode current collector 51A. As described above, the positive electrodes 51 and the negative electrodes 52 are alternately stacked with the separators 53 interposed therebetween in the battery element 50, and therefore the battery element 50 includes a plurality of positive electrodes 51. As a result, the secondary battery includes a plurality of positive electrode terminals 61. The material for forming the positive electrode terminal 61 is not particularly limited, but is specifically the same as the material for forming the positive electrode current collector 51A.

[0119] 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, and the electrical resistance of the entire secondary battery decreases.

[0120] The positive electrode terminals 61 are joined together to form a single lead-shaped joint portion 61Z.

[0121] 6 , the negative electrode terminal 62 is electrically connected to the negative electrode 52, more specifically, to the negative electrode current collector 52A. As described above, the positive electrodes 51 and the negative electrodes 52 are alternately stacked with the separators 53 interposed therebetween in the battery element 50, and therefore the battery element 50 includes a plurality of negative electrodes 52. As a result, the secondary battery includes a plurality of negative electrode terminals 62. The material for forming the negative electrode terminal 62 is not particularly limited, but is specifically the same as the material for forming the negative electrode current collector 52A.

[0122] The negative electrode terminal 62 is disposed at a position that does not overlap with the positive electrode terminal 61 when the positive electrodes 51 and the negative electrodes 52 are alternately stacked with the separators 53 interposed therebetween.

[0123] 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, and the electrical resistance of the entire secondary battery decreases.

[0124] The negative electrode terminals 62 are joined together to form a single lead-shaped joint portion 62Z.

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

[0126] The procedure for producing the positive electrode 51 is substantially the same as the procedure for producing the positive electrode 21. In this case, a positive electrode mixture slurry is applied to both surfaces (excluding the positive electrode terminal 61) of the positive electrode current collector 51A to which the positive electrode terminal 61 is integrated, thereby forming a positive electrode active material layer 51B.

[0127] The procedure for producing the negative electrode 52 is substantially the same as the procedure for producing the negative electrode 22. In this case, the negative electrode mixture slurry is applied to both surfaces (excluding the negative electrode terminal 62) of the negative electrode current collector 52A to which the negative electrode terminal 62 is integrated, thereby forming the negative electrode active material layer 52B.

[0128] When assembling a secondary battery, first, a stack (not shown) is prepared by alternately stacking positive electrodes 51 and negative electrodes 52 with separators 53 interposed therebetween. This stack has the same configuration as that of battery element 50, except that positive electrodes 51, negative electrodes 52, and separators 53 are not impregnated with an electrolyte solution and bonding portions 61Z, 61Z are not yet formed.

[0129] Next, the plurality of positive electrode terminals 61 are joined to one another using a joining method such as welding to form joints 61Z, and then the positive electrode lead 31 is connected to the joints 61Z using a similar joining method. Also, the plurality of negative electrode terminals 62 are joined to one another using a joining method such as welding to form joints 62Z, and then the negative electrode lead 32 is connected to the joints 62Z using a similar joining method.

[0130] Even when the battery element 50, which is this laminated electrode body, is used, the battery capacity is obtained by utilizing the absorption and desorption of lithium, so the same effect can be obtained.

[0131] [Modification 2] A porous film separator 23 is used. However, although not specifically shown here, a laminated separator including 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 surfaces of the porous membrane. This is because the separator's adhesion to the positive electrode 21 and the negative electrode 22 is improved, thereby preventing the battery element 20 from slipping out of its winding. This prevents swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0133] One or both of the porous film and the polymer compound layer may contain a plurality of insulating particles. This is because the plurality of insulating particles promotes heat dissipation when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The insulating particles contain one or more 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 a laminated separator is produced, 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 film. In this case, multiple insulating particles may be added to the precursor solution as needed.

[0135] Even when this laminated separator is used, the same effect can be obtained because lithium ions are able to move between the positive electrode 21 and the negative electrode 22. In this case, as described above, the safety of the secondary battery is particularly improved, and therefore, even greater effects can be obtained.

[0136] [Modification 3] An electrolytic solution that is a liquid electrolyte is used. However, although not specifically shown here, an electrolyte layer that is a gel electrolyte may also be used.

[0137] In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound together. The 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 together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The composition of the electrolytic solution is as described above. The polymer compound contains polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolytic solution, the polymer compound, a solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of the positive electrode 21 and one or both surfaces of the negative electrode 22.

[0139] Even when this electrolyte layer is used, the same effect can be obtained because lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer. In this case, particularly, as described above, leakage of the electrolyte solution is prevented, so that a greater effect can be obtained.

[0140] <3. Uses of Secondary Batteries> There are no particular limitations on the uses (application examples) of secondary batteries. A secondary battery used as a power source may be a main power source for electronic devices, electric vehicles, etc., or an auxiliary power source. A main 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 main power source, or a power source that can be switched from the main power source.

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

[0142] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may be a hybrid vehicle that also has a driving source other than the secondary battery. The home power storage system can use the power stored in the secondary battery, which is a power storage source, to power home electrical appliances, etc.

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

[0144] Figure 3 shows the block diagram of a battery pack. The battery pack described here is a battery pack (a so-called soft pack) that uses one secondary battery, and is installed in electronic devices such as smartphones.

[0145] 3, the battery pack includes 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 source 71 includes one secondary battery. The positive electrode lead of this secondary battery is connected to a positive electrode terminal 73, and the negative electrode lead is connected to a negative electrode terminal 74. The power source 71 can be connected to the outside via the positive electrode terminal 73 and the negative electrode terminal 74, and is therefore capable of charging and discharging. The circuit board 72 includes a control unit 76, a switch 77, a thermosensitive resistor 78, and a temperature detection unit 79. A specific example of the thermosensitive resistor 78 is a PTC element, and the thermosensitive resistor 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 detects and controls the usage state of the power source 71 as necessary.

[0148] When the voltage of the power supply 71 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, the control unit 76 turns off the switch 77 to prevent the 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 4.20 V±0.05 V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.40 V±0.1 V.

[0149] Switch 77 includes a charge control switch, a discharge control switch, a charge diode, a discharge diode, etc., and switches between the presence and absence of a connection between power supply 71 and an external device in response to an instruction from control unit 76. Switch 77 includes a metal oxide semiconductor field effect transistor (MOSFET), etc., and the charge current and the discharge current are each detected based on the ON resistance of switch 77.

[0150] The temperature detection unit 79 includes a temperature detection element such as a thermistor. The 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 controls charging and discharging in the event of abnormal heat generation, and when the control unit 76 performs correction processing when calculating the remaining capacity.

[0151] An embodiment of the present technology will be described.

[0152] Examples 1 to 3 and Comparative Examples 1 and 2 As will be described below, secondary batteries were fabricated, and then the battery characteristics of the secondary batteries were evaluated.

[0153] [Fabrication of Secondary Battery] The secondary battery shown in Figures 1 and 2 was fabricated according to the procedure described below. As described above, this secondary battery is a laminate film type lithium ion secondary battery.

[0154] (Preparation of Positive Electrode) First, a positive electrode active material (a lithium-containing compound (oxide) LiCoO 2 A positive electrode mixture was prepared by mixing 91 parts by mass of a positive electrode binder (polyvinylidene fluoride), 3 parts by mass of a positive electrode conductive agent (Ketjen black, an amorphous carbon powder), and 6 parts by mass of a positive electrode mixture. The positive electrode mixture was then 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, the positive electrode mixture 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 mixture slurry was dried to form the positive electrode active material layer 21B.

[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 a strip shape, thereby producing the positive electrode 21.

[0157] (Preparation of Negative Electrode) First, 93 parts by mass of a negative electrode active material (artificial graphite, a carbon material) and 7 parts by mass of a negative electrode binder (polyvinylidene fluoride) were mixed together to prepare a negative electrode mixture. Next, 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, the negative electrode mixture slurry was applied to both surfaces of the negative electrode current collector 22A (copper foil with a thickness of 8 μm) using a coating device, and then the negative electrode mixture slurry was dried to form the negative electrode active material layer 22B.

[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 a strip shape, thereby producing the negative electrode 22.

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

[0161] Subsequently, an electrolyte salt was added to the solvent, and the solvent was stirred. The electrolyte salt was lithium hexafluorophosphate (LiPF 6 ) and lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 In this case, the content of lithium hexafluorophosphate relative to the solvent was 0.5 mol / kg, and the content of lithium bis(fluorosulfonyl)imide relative to the solvent was 0.5 mol / kg. Thus, an electrolyte solution was prepared.

[0162] Finally, succinonitrile (SN) and adiponitrile (ADN) were added to the electrolyte solution, and the resulting electrolyte solution was stirred. In this case, the amounts of succinonitrile and adiponitrile added were adjusted so that when the electrolyte solution was analyzed after completion of the secondary battery, the contents C6 and C7 (wt %) would be the values ​​shown in Table 1, as described below.

[0163] (Assembly of Secondary Battery) First, the positive electrode lead 31 (aluminum foil) was welded to the positive electrode current collector 21A of the positive electrode 21, and the 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 stacked together with the separator 23 (a microporous polyethylene film having a thickness of 25 μm) interposed therebetween, and then the positive electrode 21, the negative electrode 22, and the separator 23 were wound to prepare a wound body. Next, the wound body was pressed using a press machine to form the wound body into a flat shape.

[0165] Next, the exterior film 10 was folded so as to sandwich the wound body housed inside the recess 10U. This exterior film 10 was an aluminum laminate film having a fusion layer (a polypropylene film having a thickness of 30 μm), a metal layer (aluminum foil having a thickness of 40 μm), and a surface protection layer (a nylon film having a thickness of 25 μm) laminated in this order from the inside. Next, the outer peripheral edges of two opposing fusion layers were heat-sealed to each other, thereby housing the wound body inside the bag-shaped exterior film 10.

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

[0167] As a result, the wound body was impregnated with the electrolyte, thereby producing the battery element 20. The battery element 20 was then sealed inside the exterior film 10, thereby assembling a secondary battery.

[0168] (Secondary Battery Stabilization Treatment) The assembled secondary battery was subjected to one cycle of charge and discharge in a room temperature environment (temperature = 23°C). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then charged at a constant voltage of 0.025 C at the same voltage of 4.2 V. During discharging, the battery was discharged at a constant current of 0.1 C until the voltage reached 3.0 V. Note that 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.025 C is the current value at which the battery capacity is fully discharged in 40 hours.

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

[0170] After the secondary battery was completed, the electrolyte solution was analyzed using ICP atomic emission spectroscopy, and the results are shown in Table 1. Table 1 shows the content C1 (wt%) of propyl acetate (PrAc) in the solvent, the content C2 (wt%) of propyl propionate (PrPr) in the solvent, the content C3 (wt%) of ethylene carbonate (EC) in the solvent, the content C4 (wt%) of propylene carbonate (PC) in the solvent, the content C5 (wt%) of monofluoroethylene carbonate (FEC) in the solvent, the content C6 (wt%) of succinonitrile (SN) in the electrolyte, the content C7 (wt%) of adiponitrile (ADN) in the electrolyte, and the content ratio R (%).

[0171] [Evaluation of Battery Characteristics] The battery characteristics, ie, discharge characteristics and swelling characteristics, were evaluated according to the procedures described below, and the results shown in Table 1 were obtained.

[0172] (Discharge Characteristics) First, a temperature sensor connected to a temperature logger was attached to the secondary battery using Kapton (registered trademark) tape. In this case, the temperature sensor was placed approximately in the center of the upper surface (approximately flat surface) of the exterior film 10.

[0173] Next, the secondary battery was charged and discharged in a thermostatic chamber (temperature = 25°C ± 1°C) while measuring the temperature of the secondary battery every second using a temperature measurement logger. During charging, the battery was charged at a constant current of 0.5 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.03 C. During discharging, the battery was discharged at a constant current of 10 C until the voltage reached 2.5 V. Note that 0.5 C is the current value at which the battery capacity is fully discharged in 2 hours, 10 C is the current value at which the battery capacity is fully discharged in 0.1 hours, and 0.03 C is the current value at which the battery capacity is fully discharged in 100 / 3 hours.

[0174] Finally, after the charge / discharge cycle was completed, the maximum temperature (°C), which is an index for evaluating the discharge characteristics, was determined by examining the maximum temperature measured using a temperature measurement logger.

[0175] During discharge at a large current, 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 reflects the time (discharge time) from the start of charging and discharging to the end of charging and discharging, and is therefore an index for evaluating the discharge characteristics.

[0176] The lower the maximum temperature, the longer the time until charging and discharging stops (the time during which discharging 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 until charging and discharging stops, 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 the thickness of the secondary battery (thickness before storage) was measured. In this case, the secondary battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then at the same voltage of 4.2 V, the secondary battery was charged at a constant voltage until the current reached 0.025 C. The thickness of the secondary battery is the dimension from the upper surface (approximately flat surface) of the exterior film 10 to the lower surface (approximately flat surface on the opposite side) of the exterior film 10.

[0178] Subsequently, the secondary battery in a charged state was stored (storage period: 2 months) in a high-temperature environment (temperature: 60° C.), and then the thickness of the secondary battery (thickness after storage) was measured.

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

[0180]

[0181] [Discussion] As shown in Table 1, the maximum temperature and swelling rate varied greatly depending on the composition of the electrolyte solution.

[0182] Specifically, when the content ratio R was smaller than 0.1 (Comparative Example 1), the swelling ratio decreased but the maximum temperature increased, whereas when the content ratio R was larger than 0.5 (Comparative Example 2), the maximum temperature decreased but the swelling ratio 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 also decreased. In this case, particularly when the electrolyte salt contained lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, the maximum temperature decreased sufficiently and the swelling rate also decreased sufficiently.

[0184] Examples 4 to 6 Secondary batteries were fabricated in the same manner as in Example 1, except that the contents C3 to C5 (wt %) were changed as shown in Table 2 and the cycle characteristics were evaluated instead of the discharge characteristics, and the battery characteristics were then evaluated.

[0185] The "Appropriate Relationship" column in Table 2 indicates whether or not the appropriate relationship (C4>C5>C3) is established for the contents C3 to C5. That is, "Established" indicates that the appropriate relationship is established, and "Not Established" indicates that the appropriate relationship is not established.

[0186] When evaluating cycle characteristics, first, the secondary battery was charged and discharged in a room temperature environment (temperature = 23°C) to measure the discharge capacity (discharge capacity at the first cycle). Next, the secondary battery was repeatedly charged and discharged in the same environment until the total number of cycles reached 100, to measure the discharge capacity (discharge capacity at the 100th cycle). Finally, the capacity retention rate, which is an index for evaluating cycle characteristics, was calculated based on the formula: capacity retention rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the first cycle) × 100. The charge and discharge conditions were the same as those during the stabilization treatment.

[0187]

[0188] As shown in Table 2, when an appropriate relationship was established for the contents C3 to C5 (Examples 1 and 4), the swollenness ratio was suppressed and the capacity retention ratio was increased compared to when an appropriate relationship was not established for the contents C3 to C5 (Examples 5 and 6).

[0189] Examples 7 to 9 Secondary batteries were fabricated in the same manner as in Example 1, except that the contents of C6 and C7 (wt %) were changed as shown in Table 3, and then the battery characteristics were evaluated.

[0190] The "Appropriate Relationship" column in Table 3 indicates whether or not the appropriate relationship (C7>C6) is established between the contents C6 and C7. That is, "Established" indicates that the appropriate relationship is established, and "Not Established" indicates that the appropriate relationship is not established.

[0191]

[0192] As shown in Table 3, when the electrolyte solution contained succinonitrile and adiponitrile and the contents C6 and C7 had an appropriate relationship (Example 1), the maximum temperature and the swelling rate were sufficiently suppressed compared to when the electrolyte solution did not contain succinonitrile and adiponitrile (Example 7) and when the electrolyte solution contained succinonitrile and adiponitrile but the contents C6 and C7 did not have an appropriate relationship (Examples 8 and 9).

[0193] [Summary] From the results shown in Tables 1 to 3, when an electrolyte solution was contained inside the exterior film 10, the solvent in the electrolyte solution contained propyl acetate and propyl propionate, and the content ratio R was 0.1 to 0.5, excellent battery characteristics were obtained in the secondary battery.

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

[0195] Specifically, the secondary battery has been described as having a laminate film structure, but the structure of the secondary battery is not particularly limited, and may be cylindrical, prismatic, coin-shaped, button-shaped, or the like.

[0196] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, positive and negative electrodes are alternately stacked with a separator interposed therebetween, while in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern while facing each other with the separator interposed therebetween.

[0197] Furthermore, although the electrode reactant is lithium in the above description, the type of the electrode reactant is not particularly limited. Specifically, as described 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 in this specification are merely examples, and the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0199] The present technology may also be configured as follows. <1> A secondary battery comprising: a flexible exterior member; and a positive electrode, a negative electrode, and an electrolyte solution housed inside the exterior member, wherein the electrolyte solution contains a solvent and an electrolyte salt, and the solvent contains propyl acetate and propyl propionate, and a ratio of the content of the propyl acetate in the solvent to the sum of the content of the propyl acetate in the solvent and the content of the propyl propionate in the solvent is 0.1 or more and 0.5 or less. <2> The secondary battery according to <1>, wherein the solvent further contains ethylene carbonate, propylene carbonate, and monofluoroethylene carbonate, and the content of the propylene carbonate in the solvent is greater than the content of the monofluoroethylene carbonate in the solvent, and the content of the monofluoroethylene carbonate in the solvent is greater than the content of the ethylene carbonate in the solvent. <3> The secondary battery according to <1> or <2>, wherein the electrolytic solution further contains succinonitrile and adiponitrile, and the content of the adiponitrile in the electrolytic solution is greater than the content of the succinonitrile in the electrolytic solution. <4> The secondary battery according to any one of <1> to <3>, wherein the electrolyte salt contains lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide. <5> The secondary battery according to any one of <1> to <4>, which is a lithium ion secondary battery.

Claims

1. a flexible exterior member; a positive electrode, a negative electrode, and an electrolyte solution housed inside the exterior member; Equipped with the electrolyte solution includes a solvent and an electrolyte salt; the solvent comprises propyl acetate and propyl propionate; a ratio of the content of the propyl acetate in the solvent to the sum of the content of the propyl acetate in the solvent and the content of the 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 the propylene carbonate in the solvent is greater than the content of the monofluoroethylene carbonate in the solvent, and the content of the monofluoroethylene carbonate in the solvent is greater than the content of the ethylene carbonate in the solvent; The secondary battery according to claim 1 .

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

4. The electrolyte salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide. The secondary battery according to claim 1 or 2.

5. It is a lithium-ion secondary battery. The secondary battery according to claim 1 or 2.