Method of manufacturing secondary battery

Abstract

To provide a novel secondary battery by providing a novel separator configuration since there are a polyolefin-based material and a polyimide-based material as representative materials of a separator but the polyolefin-based material has a disadvantage that wettability with respect to an electrolyte is low and the polyimide-based material has a disadvantage that cost is high.SOLUTION: In order to improve wettability with respect to an electrolyte, polyamide acid (also called polyamic acid) is dissolved in an organic solvent to adjust a concentration and applied to a separator of a polyolefin-based material. In the dried separator, the wettability with respect to the electrolyte is improved. The wettability with respect to the electrolyte is improved, thereby sufficiently reducing the quantity of the electrolyte to be spread over gaps of the separator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] One embodiment of the present invention relates to a lithium-ion secondary battery and a manufacturing method thereof.

[0002] One embodiment of the present invention is not limited to the above fields, and relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, a vehicle, and a manufacturing method thereof. The above-described semiconductor device, display device, light-emitting device, power storage device, lighting device, electronic device, and vehicle can use the lithium-ion secondary battery of one embodiment of the present invention as a necessary power source. For example, the above-described electronic device includes an information terminal device equipped with a lithium-ion secondary battery. Furthermore, the above-described power storage device includes a stationary power storage device.

[0003] A lithium-ion secondary battery (sometimes referred to as a lithium-ion battery) is a battery that uses lithium ions as the carrier ion. Lithium-ion batteries are secondary batteries that can be used repeatedly by recharging. [Background technology]

[0004] In recent years, various types of energy storage devices have been actively developed, such as lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion secondary batteries has expanded rapidly in conjunction with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0005] In particular, there is a high demand for lithium-ion secondary batteries for mobile electronic devices, which have a large discharge capacity per weight and excellent cycle characteristics.

[0006] Patent Document 1 discloses a secondary battery that uses an electrolyte solution containing a mixed solvent of ethylene carbonate and propionate carbonate as the main component and a fluorine-substituted ether, and that uses a polyolefin separator. It also describes that separators made of polyolefins such as polyethylene or polypropylene are not wetted by the mixed solvent of ethylene carbonate and propionate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-10095 Summary of the Invention [Problem to be solved by the invention]

[0008] Lithium ion secondary batteries still have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost.

[0009] The separator material for lithium-ion secondary batteries is required to have high electrical insulation properties in order to maintain a certain distance and prevent short circuits between the positive and negative electrodes.

[0010] In addition, separator materials are often made of porous structures that allow the electrolyte to pass through numerous micropores. The passage of the electrolyte through the micropores of the separator gives the separator high ionic conductivity. Furthermore, separator materials that are less susceptible to deterioration when in contact with the electrolyte are desired. Furthermore, the separator material is selected taking into consideration its interaction with the lithium ions dissolved and dissociated in the electrolyte.

[0011] The separator also has a shutdown function that prevents the growth of lithium dendrites and melts in the event of abnormal heat generation. The shutdown function is a function in which, if abnormal heat generation occurs due to some kind of observation, part of the separator melts, blocking the micropores and stopping the separator's ion permeability. When the shutdown function is activated, the temperature rise is suppressed even if a short circuit occurs. If the temperature continues to rise, the secondary battery will enter a dangerous state known as thermal runaway. Depending on the separator material, after the shutdown function is activated, the high resistance may continue, or the high resistance may not be maintained and the resistance may gradually decrease.

[0012] Typical separator materials include polyolefin-based materials and polyimide-based materials. Polyolefin-based materials have the disadvantage of low wettability with the electrolyte, while polyimide-based materials have the disadvantage of being expensive.

[0013] Therefore, one of the objectives is to provide a new separator structure and a new secondary battery.

[0014] Another objective is to minimize the amount of electrolyte used by increasing the wettability of the separator with a new separator structure.

[0015] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0016] To increase the wettability of the separator to the electrolyte, polyamic acid (also called polyamic acid) is dissolved in an organic solvent, the concentration is adjusted, and the solution is applied to a polyolefin-based separator. The dried separator has increased wettability to the electrolyte. Furthermore, by increasing the wettability to the electrolyte, the amount of electrolyte that needs to be sufficiently distributed throughout the separator gaps can be minimized. Furthermore, to ensure that the secondary battery functions adequately even with a small amount of electrolyte, the lithium salt dissolved in the electrolyte is at a concentration greater than 1.0 mol / L, specifically greater than 1.5 mol / L.

[0017] The wettability of the separator is evaluated by dropping the electrolyte onto the separator in an atmosphere of 25° C., taking a photograph 45 seconds later, and determining the contact angle from the shape of the droplet captured on the photograph.

[0018] The amount of electrolyte used in the secondary battery can be determined by calculation in advance.

[0019] The porosity of the separator can be determined, and the amount of electrolyte can be calculated from the porosity. The porosity of the positive electrode active material layer and the porosity of the negative electrode active material layer can also be determined. When the sum of these void volumes is taken as 1, the amount of electrolyte is preferably less than 2, and more preferably 0.1 to 1.5. When fabricating a laminated secondary battery, the standard amount of electrolyte is 2 to 3 when the sum of the void volumes is taken as 1.

[0020] Porosity (also called porosity or porosity) can be measured by the Archimedes method, the mercury porosity method, and the weight porosity method.

[0021] The invention of the fabrication method disclosed in this specification is a method for fabricating a secondary battery, comprising the steps of: applying a first slurry containing a first binder onto a positive electrode current collector to form a positive electrode having a positive electrode active material layer; applying a solution containing a polyamic acid entirely or selectively to the surface of a first separator to form a second separator; applying a second slurry containing a second binder onto a negative electrode current collector to form a negative electrode having a negative electrode active material layer; housing the stack of the positive electrode, the second separator, and the negative electrode in an exterior housing having an opening; calculating the porosity of the positive electrode active material layer, the porosity of the second separator, and the porosity of the negative electrode active material layer; dripping an amount of electrolyte into the opening of the exterior housing such that the sum of the void volumes calculated from the porosities of the positive electrode active material layer, the second separator, and the negative electrode active material layer is 0.1 to 1.5, where 1 is the sum of the void volumes.

[0022] In the above configuration, the second separator has a contact angle with the electrolyte that is greater than 0 degrees and less than 35 degrees when measured 45 seconds after a droplet of the electrolyte is dropped on the surface in a room temperature environment.

[0023] In the above configuration, the first separator is made of a polyolefin-based material, specifically polypropylene.

[0024] The positive electrode is fabricated by applying a slurry containing a positive electrode active material and a binder onto a current collector and then drying the applied slurry. The slurry is sometimes called an electrode slurry or an active material slurry, and when forming a positive electrode active material layer, it is sometimes called a positive electrode slurry, and when forming a negative electrode active material layer, it is sometimes called a negative electrode slurry. A conductive additive may be added to the slurry, and a typical example of a carbon material used as a conductive additive is acetylene black (also called AB).

[0025] The cathode active material is not particularly limited as long as it is a lithium oxide, and lithium cobalt oxide (also called LiCoO2: LCO), lithium iron phosphate (also called LiFePO4: LFP), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium manganese phosphate (LiMnPO4), lithium iron manganese phosphate (LiFe Mn b PO4: a + b is 1 or less, 0 < a < 1, 0 < b < 1), any one or more of which can be used. When using lithium iron phosphate, it is preferable to use a carbon-coated one.

[0026] In addition, the cathode active material can also be a NiCoMn-based (also called NCM) represented by LiNi x Co y Mn z O2 (x > 0, y > 0, z > 0). As an example, x, y, and z preferably satisfy x:y:z = 5:2:3 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 8:1:1 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 9:0.5:0.5 or values in the vicinity thereof.

[0027] In addition, the cathode active material can also be a NiCoAl-based (also called NCA) represented by LiNi x Co y Al z O2 (x > 0, y > 0, z > 0). As an example, x, y, and z preferably satisfy x:y:z = 7:2:1 or values in the vicinity thereof.

[0028] In addition, the anode active material is preferably one that can insert and desorb lithium, and examples include carbon-based materials such as graphite, silicon, or alloy-based materials such as silicon oxide. Among the alloy-based materials, in particular, the theoretical capacity of silicon is about 10 times higher than that of graphite, and silicon-based active materials are regarded as promising as host materials for lithium.

[0029] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0030] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0031] The electrolyte used in secondary batteries contains an organic solvent and a solute, specifically an electrolyte salt, dissolved in the organic solvent. The electrolyte serves as a medium for transporting carrier ions, typically lithium ions. Because lithium ion secondary batteries require a voltage of 4 V or higher, organic solvents that do not decompose even when voltages of 4 V or higher are applied are used. However, a drawback of organic solvents is their high flammability. When a solvent with a low flash point is used, there is a risk of fire if a short circuit occurs for some reason.

[0032] The electrolyte contains a fluorinated chain carbonate, a fluorinated cyclic carbonate, and a lithium salt.

[0033] An organic solvent with a high flash point and excellent thermal stability is a fluorinated cyclic carbonate, specifically fluoroethylene carbonate (FEC).

[0034] The structural formula of FEC is shown below (H11). In FEC, the electron-withdrawing substituent is an F group.

[0035] [ka]

[0036] The flash point of FEC is 128°C.

[0037] In this specification, the flash point refers to the lowest temperature at which a solvent produces vapors in a concentration sufficient to ignite near the liquid surface in the air. There are three known methods for measuring flash point: the Pensky-Martens closed cup flash point (PM method), the Cleveland open cup flash point (COC method), and the Tag closed cup flash point test method.

[0038] Other fluorinated cyclic carbonates that can be used include difluoroethylene carbonate (abbreviated as DFEC or F2EC), trifluoroethylene carbonate (abbreviated as F3EC), tetrafluoroethylene carbonate (abbreviated as F4EC), and pentafluoroethylene carbonate (abbreviated as F5EC). DFEC has isomers such as cis-4,5 and trans-4,5. Since all fluorinated cyclic carbonates have electron-withdrawing substituents, the solvation energy of lithium ions in organic solvents containing fluorinated cyclic carbonates is thought to be low.

[0039] The structural formula of 4,5-difluoroethylene carbonate is shown below (H12). In 4,5-difluoroethylene carbonate, the electron-withdrawing substituent is an F group.

[0040] [ka]

[0041] The structural formula of 4,4-difluoroethylene carbonate is shown below (H13). In 4,4-difluoroethylene carbonate, the electron-withdrawing substituent is an F group.

[0042] [ka]

[0043] The structural formula of F3EC is shown below (H14). In F3EC, the electron-withdrawing substituent is an F group.

[0044] [ka]

[0045] The structural formula of F4EC is shown below (H15). In F4EC, the electron-withdrawing substituent is an F group.

[0046] [ka]

[0047] The above-mentioned fluorinated cyclic carbonates are sometimes collectively referred to as cyclic molecules.

[0048] The electrolyte solution disclosed herein is a mixture of FEC and a fluorinated chain carbonate, specifically methyl 3,3,3-trifluoropropionate (MTFP). MTFP has a flash point of -2°C. The structural formula of MTFP is shown below (H21). In MTFP, the electron-withdrawing substituent is a CF3 group.

[0049] [ka]

[0050] An example of a fluorinated chain carbonate is methyl 2,2-difluoropropionate. The structural formula of methyl 2,2-difluoropropionate is shown below (H22). In methyl 2,2-difluoropropionate, the electron-withdrawing substituent is a CF2 group.

[0051] [ka]

[0052] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The structural formula of trifluoromethyl propionate is shown below (H23). In trifluoromethyl propionate, the electron-withdrawing substituent is a CF3 group.

[0053] [ka]

[0054] An example of a fluorinated chain carbonate is methyl pentafluoropropionate. The structural formula of methyl pentafluoropropionate is shown below (H24). In methyl pentafluoropropionate, the electron-withdrawing substituent is a CF3 group.

[0055] [ka]

[0056] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The structural formula of trifluoromethyl 3,3,3-trifluoropropionate is shown below (H25). In trifluoromethyl 3,3,3-trifluoropropionate, the electron-withdrawing substituent is a CF3 group.

[0057] [ka]

[0058] The above-mentioned fluorinated chain carbonates are sometimes collectively referred to as chain molecules.

[0059] Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents. Organic compounds having electron-withdrawing substituents have lower solvation energy of lithium ions than organic compounds without electron-withdrawing substituents. For this reason, fluorine-containing organic compounds are suitable for organic solvents, and secondary batteries exhibiting good initial characteristics can be provided using such organic solvents. It is more preferable that the organic solvent contains both the fluorinated cyclic carbonate and the fluorinated chain carbonate.

[0060] The blend ratio of FEC and MTFP will now be explained. When the total volume of FEC and MTFP is 100 vol%, the volume ratio of FEC to MTFP should satisfy y:100-y (where 5≦y≦30, preferably 10≦y≦20). In other words, it is preferable that the amount of MTFP is greater than that of FEC. An organic solvent containing FEC and MTFP exhibits an appropriate viscosity as an electrolyte. This allows for the provision of a secondary battery with good initial characteristics. Furthermore, because FEC and MTFP can maintain an appropriate viscosity as an electrolyte, the low-temperature characteristics of the secondary battery are improved.

[0061] The above volume ratio is measured before mixing as an organic solvent. Even if some volatilization occurs during mixing, it is not a problem as long as good initial properties can be exhibited. The organic solvent should be mixed at room temperature.

[0062] Examples of the electrolyte salt include lithium salt, sodium salt, and potassium salt, and among these, it is preferable to use lithium salt.

[0063] The lithium salt used is lithium hexafluorophosphate (LiPF6). Other lithium salts that can be used include one or more selected from LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiClO4, and LiH2PO4.

[0064] The lower limit of the content of the electrolyte salt in the electrolytic solution is a volume molar concentration of 1 mol / L (1 M) or more, and specifically, a concentration higher than 1.5 M is preferable. The upper limit is 3 M. Although a volume molar concentration of 3 M or more increases the viscosity, it can be set to 3 M or more as long as it functions satisfactorily when used in a secondary battery. [Effects of the Invention]

[0065] According to one embodiment of the present invention, a novel separator and a novel electrolyte solution are combined, the amount of electrolyte solution used in one secondary battery is reduced, and a highly safe secondary battery with a low risk of ignition or explosion can be manufactured.

[0066] Note that the above-described effects do not preclude the existence of other effects. Furthermore, the above-described effects are considered to be independent of each other, and one embodiment of the present invention does not necessarily exhibit all of the above-described effects. Furthermore, effects other than those described above can be extracted from the description of this specification and the like. [Brief explanation of the drawings]

[0067] [Figure 1] FIG. 1 shows an example of a production flow showing one embodiment of the present invention. [Figure 2] FIG. 2A is an exploded perspective view of a secondary battery illustrating one embodiment of the present invention, and FIG. 2B is a cross-sectional view. [Figure 3] 3A and 3B are perspective views of the secondary battery manufacturing process. [Figure 4] 4(A) and 4(B) are schematic diagrams of a secondary battery. [Figure 5] 5A to 5D are diagrams illustrating an example of an electronic device. [Figure 6] 6A to 6D are diagrams illustrating examples of electronic devices. [Figure 7] 7(A) and 7(B) are diagrams illustrating the nail penetration operation. [Figure 8] 8(A) to 8(C) are photographs taken from the side after dropping. DETAILED DESCRIPTION OF THE INVENTION

[0068] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0069] In this specification, the words "first" and "second" are used for the convenience of understanding the technical content or to identify each component. Therefore, the words "first" and "second" do not limit the number of each component. Furthermore, the words "first" and "second" do not limit the order of each component. Furthermore, the words "first" and "second" or identifying symbols used in this specification may not match the words or identifying symbols in the claims.

[0070] (Embodiment 1) In this embodiment, a manufacturing method of a lithium-ion battery which is one embodiment of the present invention will be described with reference to drawings.

[0071] A lithium-ion battery according to one embodiment of the present invention has a separator with high wettability, and by using the separator, the amount of electrolyte used is reduced, specifically, to half or less of the standard amount. In addition to the separator and electrolyte, the lithium-ion battery has a negative electrode and a positive electrode, and also includes an exterior body that covers the negative electrode and the positive electrode. Depending on the shape of the exterior body, the lithium-ion battery is called a laminated lithium-ion battery, a coin-cell lithium-ion battery, or a cylindrical lithium-ion battery, but the present invention is not limited to the shape of the exterior body.

[0072] <<Separator manufacturing method>> The method for producing the separator will be explained with reference to FIG.

[0073] <Step S101> First, a commercially available separator is prepared. A polyolefin material is preferable, and polypropylene is used in this embodiment. Polypropylene can be represented by the following chemical formula:

[0074] [ka]

[0075] <Step S102> Next, in step S102 shown in FIG. 1, a mixture of polyamic acid (also called polyamic acid) dissolved in an organic solvent to adjust the concentration is prepared and applied to the separator. Examples of organic solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). In this embodiment, NMP is used to dilute the polyamic acid in an amount between two and five times the amount of the polyamic acid. Using a diluted mixture reduces unevenness in application. Alternatively, if a uniform application can be achieved, the polyamic acid alone may be applied without dilution.

[0076] To coat the entire surface of the separator, the separator may be immersed in the mixture placed in a container, or the mixture may be selectively coated. For example, the mixture may be coated only in the center of a single separator sheet, leaving the periphery uncoated.

[0077] In this embodiment, the separator before the mixed liquid is applied is called a first separator, and the separator after drying is called a second separator.

[0078] <Step S103> 1, the mixed liquid is applied to the first separator and then dried. After reducing the pressure, the mixed liquid is dried using a forced air dryer to evaporate the excess organic solvent.

[0079] <Step S104> Through the above process, a second separator can be obtained, with the polyamic acid coated on the entire surface or a portion of the surface, as shown in step S104 in Figure 1. The second separator has improved wettability with the electrolyte compared to the first separator. Drying using a forced air dryer does not cause imidization, so polyimide is not formed, but the surface condition of the second separator after coating is improved.

[0080] An example of a lithium ion battery produced using the second separator will be described below.

[0081] FIG. 2(A) shows an example of an exploded perspective view of a secondary battery.

[0082] The positive electrode 200 is disposed below the second separator 203 so as to overlap it, and the negative electrode is disposed above the second separator 203 so as to overlap it. The negative electrode has a structure in which a negative electrode active material layer 205 and a negative electrode current collector 206 are stacked.

[0083] FIG. 2(B) shows an example of a cross-sectional view of a secondary battery.

[0084] As shown in FIG. 2B, a positive electrode active material layer 202 is formed on a positive electrode current collector 201, and a second separator 203 is laminated on the positive electrode active material layer 202 so as to be in contact with the positive electrode active material layer 202.

[0085] The positive electrode active material of the positive electrode active material layer 202 is not particularly limited as long as it is a lithium oxide, and examples thereof include lithium cobalt oxide (LiCoO2: also called LCO), lithium iron phosphate (LiFePO4: also called LFP), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate (LiFe a Mn bFor PO4: a + b is 1 or less, 0 < a < 1, 0 < b < 1), any one or more of them can be used. When using lithium iron phosphate, it is preferable to use a carbon-coated one. When preparing the positive electrode slurry, the positive electrode active material, a binder, a solvent, and a conductive assistant are mixed. As the binder, polyvinylidene fluoride (PVDF) is used, and as the solvent, N-methyl-2-pyrrolidone (NMP) is used.

[0086] The positive electrode active material layer 202 is formed by coating a previously prepared positive electrode slurry on the positive electrode current collector 201 and performing drying or pressing. The protruding portion of the positive electrode current collector 201 is called a tab, which becomes the portion connected to the later lead electrode, and the positive electrode slurry is not coated on the tab portion. A conductive assistant may be added to the slurry, and a typical carbon material used as the conductive assistant is acetylene black (also called AB).

[0087] Acetylene black has an average particle diameter of primary particles of about several tens of nm. The primary particles are fused to form a structure, and the particles are composed of agglomerates in which the structures or the structures are combined, and it is possible to form an electron conduction path between the particles. However, the structure of acetylene black is about several hundreds of nm, and it is not suitable for forming a long-distance electron conduction path. If a large amount of acetylene black is used to reduce the contact resistance, the ratio of the active material to the entire electrode decreases, and the discharge capacity of the secondary battery decreases.

[0088] Also, acetylene black used as a conductive assistant is a material that easily aggregates, and it is preferably mixed so as to be uniformly dispersed.

[0089] Also, a negative electrode active material layer 205 is formed on the negative electrode current collector 206 and is disposed on the second separator 203 so as to contact the negative electrode active material layer 205. When preparing the negative electrode slurry, the negative electrode active material, a binder, a solvent, and a conductive assistant are mixed.

[0090] The negative electrode active material layer 205 is formed by applying a previously prepared negative electrode slurry onto the negative electrode current collector 206 and drying or pressing it. The protruding portion of the negative electrode current collector 206 is called a tab, which will later be connected to a lead electrode, and the tab area is prevented from being coated with the negative electrode slurry.

[0091] The negative electrode active material of the negative electrode active material layer 205 is carbon particles or a silicon-based material. Examples of carbon particles include graphite, carbon with a layered structure similar to graphite, amorphous carbon, and hard carbon. Carbon fibers may also be used instead of carbon particles. The carbon particles used in this specification specifically refer to graphite particles, which are abundant in nature and therefore inexpensive, making them a preferred negative electrode active material.

[0092] Silicon particles can be used as the silicon-based material. Silicon particles are silicon powders used as negative electrode active materials in lithium-ion secondary batteries, and refer to particles with an average particle diameter of approximately 100 nm, sometimes called nanosilicon particles. The silicon particles used are preferably prepared by pulverizing silicon raw materials and adjusting them to a uniform particle diameter. The silicon particles contain at least one of silicon, silicon oxide, and silicon alloy.

[0093] 2(A) shows an example in which the area of ​​the second separator 203 overlapping the positive electrode is the same as the area of ​​the second separator 203, but the area of ​​the second separator 203 is made larger to allow for a manufacturing margin. Also, the area of ​​the negative electrode is made larger than the area of ​​the positive electrode 200. Of course, to prevent short circuits, the area of ​​the second separator 203 is made larger than the area of ​​the negative electrode.

[0094] Although Figures 2(A) and 2(B) show an example in which one positive electrode 200, one second separator 203, and a negative electrode are stacked as one set, in many cases, multiple stacks are arranged in one exterior body to increase capacity.

[0095] When a plurality of electrodes are stacked, it is preferable to process the second separator 203 into a bag shape and arrange it so as to encase either the positive electrode or the negative electrode. For example, as shown in FIG. 3(A), the second separator 203 is folded in half so as to sandwich the positive electrode, and sealed with a sealing portion 514 outside the region overlapping with the positive electrode active material layer 202, thereby ensuring that the positive electrode active material layer 202 is supported within the second separator 203. Then, as shown in FIG. 3(B), the positive electrodes and negative electrodes wrapped in the second separator 203 are arranged alternately. Note that FIG. 3(B) illustrates a negative electrode tab 282 and a positive electrode tab 281.

[0096] 3(B), it is preferable to provide the positive electrode active material layer 202 on not only one surface but also both surfaces of the positive electrode current collector 201. It is also preferable to provide the negative electrode active material layer 205 on not only one surface but also both surfaces of the negative electrode current collector 206. The positive electrode current collector 201 or the negative electrode current collector 206, which is the outermost one, may be coated on one surface.

[0097] Furthermore, the polyamic acid need not necessarily be applied to the entire surface of the second separator, but may be applied partially, for example. As an example, FIG. 4A shows an example in which polyamic acid is selectively applied to the separator. In this example, polyamic acid is partially applied in advance to the area overlapping the positive or negative electrode and near the injection position, thereby forming a first region 203a of the separator 203 to be selectively applied. The uncoated portion is the second region 203b, and this region is shown in the figure. In FIG. 4A, the region with low wettability is the second region 203b, and the region with high wettability is the first region 203a. The exterior body 509 is bag-shaped with an opening near the tab 282, and the exterior body 509 is illustrated with a dashed line in FIG. 4A. In reality, the stack is placed through the opening of the bag-shaped exterior body 509 with the lead 511 welded to the tab 282. 4(A), in order to clearly show the boundary of the separator, the negative electrode and lead 510 are indicated by dashed lines. Then, the electrolyte is dropped from the opening of the exterior body 509.

[0098] The electrolyte solution is preferably allowed to penetrate not only the separator but also the positive electrode active material layer and the negative electrode active material layer. Therefore, the porosity of the positive electrode active material layer, the porosity of the second separator, and the porosity of the negative electrode active material layer are determined, and when the sum of the void volumes calculated from the porosities of the positive electrode active material layer, the second separator, and the negative electrode active material layer is taken as 1, an amount of electrolyte solution that is 0.1 to 1.5 is dropped into the opening of the exterior body.

[0099] It is preferable to use as little electrolyte as possible, and by using second separator 203 that has high wettability with the electrolyte, it is possible to realize a separator that allows the electrolyte to spread even with a smaller amount than before.

[0100] Furthermore, the lithium salt dissolved in the electrolyte solution is made to have a concentration higher than 1.0 mol / L, specifically higher than 1.5 mol / L.

[0101] After pouring the electrolyte solution into the opening of the exterior body 509, the pressure is reduced, thereby allowing the electrolyte solution to spread selectively over the second separator 203. After reducing the pressure, the opening of the exterior body 509 is sealed to form the sealing portion 118.

[0102] Alternatively, the electrolyte may be added dropwise after reducing the pressure.

[0103] After sealing, an inspection may be performed by irradiating ultrasonic waves. Irradiating ultrasonic waves can also spread the electrolyte solution on the second separator 203. After sealing, the second separator 203 is left standing for 24 hours to spread the electrolyte solution on the second separator 203. After that, an aging treatment or degassing treatment may be performed. After the aging treatment or degassing treatment, ultrasonic waves may be irradiated to spread the electrolyte solution on the second separator 203.

[0104] 4(B) can be fabricated in which one positive electrode 200, one second separator 203, and one negative electrode are housed as one set in an exterior body 509. In this way, a secondary battery can be fabricated that uses a small amount of electrolyte solution per secondary battery, has a low risk of ignition or explosion, and is highly safe.

[0105] This embodiment mode can be freely combined with other embodiment modes.

[0106] (Embodiment 2) In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on an electronic device will be described with reference to FIGS.

[0107] Examples of electronic devices to which the secondary batteries described in the previous embodiments are applied include television devices (also called televisions or television receivers), monitors for computers, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, sound reproduction devices, and large game machines such as pachinko machines.

[0108] Figure 5(A) shows an example of a wearable device. Wearable devices use secondary batteries as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for wearable devices that can be charged wirelessly as well as via wired charging, with an exposed connector.

[0109] For example, a secondary battery according to one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 5A. The eyeglasses-type device 4000 includes a frame 4000a and a display portion 4000b. Mounting a secondary battery on temple portions of the curved frame 4000a makes it possible to provide an eyeglasses-type device 4000 that is lightweight, has a good weight balance, and can be used for a long time. By including the secondary battery according to one embodiment of the present invention, a highly safe secondary battery can be realized.

[0110] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a headset-type device 4001. The headset-type device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b and / or the earphone unit 4001c. By including the secondary battery according to one embodiment of the present invention, a highly safe secondary battery can be realized.

[0111] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided inside a thin housing 4002a of the device 4002. By including the secondary battery according to one embodiment of the present invention, a highly safe secondary battery can be realized.

[0112] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By including the secondary battery according to one embodiment of the present invention, a highly safe secondary battery can be realized.

[0113] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted inside the belt portion 4006a. By including the secondary battery according to one embodiment of the present invention, a highly safe secondary battery can be realized.

[0114] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. By including the secondary battery of one embodiment of the present invention, a highly safe secondary battery can be realized.

[0115] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0116] Furthermore, since the wristwatch device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0117] FIG. 5(B) shows a perspective view of the wristwatch type device 4005 removed from the wrist.

[0118] 5C shows a side view of the display portion 4005. FIG. 5C shows a state in which a secondary battery 913 is built inside the display portion 4005. The secondary battery 913 is the secondary battery described in Embodiment 3. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0119] 5(D) shows an example of a wireless earphone. Here, the wireless earphone is shown having a pair of main bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0120] The main bodies 4100a and 4100b each have a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also have a display unit 4104. They also preferably have a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also have a microphone.

[0121] The case 4110 has a secondary battery 4111. It also preferably has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, etc.

[0122] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, they can send sounds picked up by the microphones to the other electronic devices, and the sound data after processing by the electronic devices can be sent back to the main units 4100a and 4100b for playback. This allows them to be used as, for example, translation devices.

[0123] Further, the secondary battery 4103 included in the main body 4100a can be charged from the secondary battery 4111 included in the case 4110. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiments can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material 100 obtained in Embodiment 1 for its positive electrode has high energy density, and by using the secondary battery 4103 and the secondary battery 4111, a highly safe secondary battery can be realized.

[0124] 6A shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also includes a lithium-ion battery 2107. Use of the secondary battery of the present invention makes it possible to realize a highly safe mobile phone 2100.

[0125] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0126] The operation button 2103 can be provided with various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system built into the mobile phone 2100.

[0127] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0128] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0129] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0130] FIG. 6(B) shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a lithium-ion battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. Use of the secondary battery of the present invention makes it possible to realize a highly safe unmanned aerial vehicle 2300.

[0131] Fig. 6(C) shows an example of a robot. The robot 6400 shown in Fig. 6(C) includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a moving mechanism 6408, a transmitting / receiving device, a computing device, etc.

[0132] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0133] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0134] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0135] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. By using the secondary battery of the present invention, the robot 6400 can be made highly safe.

[0136] 6(D) shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0137] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, if an object that may become entangled in the brush 6304, such as a wire, is detected by image analysis, the cleaning robot 6300 can stop rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal area. Use of the secondary battery of the present invention enables the cleaning robot 6300 to be highly safe.

[0138] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0139] (Embodiment 3) By using the configuration of the secondary battery described in the first embodiment and reducing the amount of electrolyte, the safety of the secondary battery can be improved.

[0140] In this embodiment, a nail penetration test used for safety testing will be described below.

[0141] The nail penetration test is a test in which a secondary battery is fully charged and a nail with a specified diameter selected from 2mm to 20mm is thrust into the secondary battery at a specified speed. A fully charged battery is one in which the charge rate, expressed as State Of Charge (hereinafter referred to as SOC), is 100%.

[0142] <Secondary battery in nail penetration test> Next, the state of the secondary battery in the nail penetration test will be described with reference to FIGS. 7(A) and 7(B), etc. The nail penetration test involves inserting a nail 1003 into a fully charged secondary battery 1004 at a predetermined speed. The diameter of the nail 1003 is 2 mm or more and 10 mm or less. FIG. 7(A) shows a cross-sectional view of the secondary battery 1004 with the nail 1003 inserted. The secondary battery 1004 has a structure in which a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530 are housed in an outer casing 531. The positive electrode 503 has a positive electrode current collector 501 and positive electrode active material layers 502 formed on both sides thereof. The negative electrode 506 has a negative electrode current collector 513 and negative electrode active material layers 512 formed on both sides thereof. The positive electrode active material layer is a layer containing at least a positive electrode active material, for example, a conductive material and / or a binder. The negative electrode active material layer is a layer containing at least a negative electrode active material, for example, a conductive material and / or a binder. Fig. 7(B) shows an enlarged view of the nail 1003, the positive electrode current collector 501, and their vicinity, and also clearly shows the positive electrode active material 100 and the conductive material 553 contained in the positive electrode active material layer 502.

[0143] As shown in Figures 7(A) and 7(B), a nail 1003 is driven into a secondary battery 1004, and the nail 1003 penetrates the positive electrode 503 and the negative electrode 506, causing an internal short circuit. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode 506, and electrons (e - ) flows to the positive electrode 503, and Joule heat is generated at the internal short circuit point and its vicinity. In addition, due to the internal short circuit, carrier ions, typically lithium ions (Li + ) are released into the electrolyte as indicated by the white arrow. However, because the temperature of the secondary battery rises rapidly due to Joule heat generated by the internal short circuit, the electrolyte begins to reductively decompose on the surface of the anode before all the lithium ions are released from the anode. This is an electrochemical reaction, and is called the reduction reaction of the electrolyte by the anode.

[0144] Furthermore, when the temperature of the secondary battery 1004 rises due to Joule heat, if lithium cobalt oxide is used as the positive electrode active material, the lithium cobalt oxide may undergo a phase change (i.e., a structural change) to an H1-3 type crystal structure or an O1 type crystal structure, and heat may be generated as a result of the phase change. If an internal short circuit occurs, heat may be generated one after another.

[0145] As shown in FIGS. 7A and 7B, the electrons (e - ), the tetravalent Co in the charged lithium cobalt oxide is reduced to trivalent or divalent, and this reduction reaction releases oxygen from the lithium cobalt oxide, which then decomposes the electrolyte 530 through an oxidation reaction caused by the oxygen. This is an electrochemical reaction, and is called an oxidation reaction of the electrolyte by the positive electrode. The rate at which current flows into the positive electrode active material 100, etc., is thought to affect the electrochemical reaction, and the insulating properties of the positive electrode active material can slow the rate at which current flows.

[0146] <Characteristics of secondary batteries in nail penetration tests> The temperature rise of the secondary battery when subjected to a nail penetration test, i.e., the difference between the temperature before the nail penetration test and the maximum temperature reached after the nail penetration test (also referred to as the temperature rise ΔT), is preferably 100°C or less, more preferably 70°C or less, and even more preferably 50°C or less. The temperature is measured within 5 cm, preferably within 2 cm, of the nail hole, and specifically, the value output by a temperature sensor placed within 5 cm, preferably within 2 cm, of the nail hole. When the temperature sensor is placed in contact with the exterior body of the secondary battery, this temperature is equal to the temperature of the exterior body.

[0147] The temperature of the nail 1003 is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. This is a value output by a temperature sensor disposed on the nail 1003.

[0148] Furthermore, the maximum temperature of the secondary battery during the nail penetration test is preferably 150°C or lower, more preferably 100°C or lower, and even more preferably 80°C or lower. More preferably, it is lower than the temperature at which oxidation of the electrolyte by the positive electrode occurs. Even more preferably, the maximum temperature is lower than the flash point of the mixed organic solvent used in the electrolyte. When the flash point of the mixed organic solvent is unknown, the flash points of the individual organic solvents can be used as a reference.

[0149] When ignition does not occur by the above-described nail penetration test, it can be called a highly safe secondary battery.

[0150] In the present embodiment, lithium cobaltate has been described as the positive electrode active material, but as long as it is a lithium oxide, it is not particularly limited, and lithium iron phosphate (also called LiFePO4: LFP), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), lithium manganese phosphate (LiMnPO4), lithium iron manganese phosphate (LiFe a Mn b PO4: a + b is 1 or less, 0 < a < 1, 0 < b < 1), any one or more of which can be used.

[0151] This embodiment can be freely combined with other embodiments.

Examples

[0152] In this example, the electrolyte was dropped onto the separator and the contact angle was measured.

[0153] In this example, a separator made of polypropylene was used as a comparative example, with a porosity of 55% and a thickness of 25 μm.

[0154] In addition, a polypropylene separator was impregnated with a solvent containing dissolved polyamic acid in a petri dish and dried to prepare Sample 1 (coated separator). The volume ratio of polyamic acid to solvent (NMP) was 1:3. In this example, after impregnation, a vacuum was applied to the bell jar for 1 minute, and the separator was then hung in a forced air dryer to remove excess solvent. After hanging at 25°C for 10 minutes, the separator was heated to 80°C in the forced air dryer and dried for 1 hour while still hanging. Sample 1 (coated separator) was then recovered.

[0155] Also, polyamic acid alone was placed in a petri dish, and a polypropylene separator was impregnated with the polyamic acid and dried to prepare Sample 2 (coated separator).

[0156] The collected samples 1 and 2 were measured using Fourier transform infrared spectroscopy (FTIR), and it was confirmed that the polyamic acid had not been imidized by the drying process.

[0157] The electrolyte solution to be dropped contained fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (MTFP), and when the total content of FEC and MTFP was 100 vol%, the volume ratio of FEC to MTFP in the electrolyte was y:100-y (where 5≦x≦30). More specifically, an organic solvent containing FEC and MTFP in a volume ratio of FEC:MTFP=20:80 was prepared.

[0158] Lithium hexafluorophosphate (LiPF6) was dissolved in each of these organic solvents to a concentration of 1 mol / L, and the resulting solution was used as an electrolyte.

[0159] The electrolyte solution was dropped onto each of the comparative example, sample 1, and sample 2 in an air atmosphere at room temperature, and the contact angle was measured.

[0160] When a drop of liquid is dropped onto a solid surface, the liquid becomes round due to surface tension, and the angle between the tangent of the drop and the solid surface is called the contact angle, which is commonly used as an index of wettability.

[0161] The contact angle can be determined by the θ / 2 method, the tangent method, the curve fitting method, etc. In this example, the contact angle was measured based on a photograph taken 45 seconds after the drop. The contact angle of the comparative example was 41°, that of Sample 1 was 30°, and that of Sample 2 was 34°.

[0162] Fig. 8(A) is a photograph of Sample 1 taken from the side of the separator 45 seconds after the dropping. Fig. 8(B) is a photograph of Sample 2 taken from the side of the separator 45 seconds after the dropping. Fig. 8(C) is a photograph of the comparative example taken from the side of the separator 45 seconds after the dropping.

[0163] Furthermore, when observing the surface condition of the separator from the outside, diluted Sample 1 was better than Sample 2, with less coating unevenness.

[0164] From the photograph, it can be seen that Sample 1 and Sample 2 have smaller contact angles than the comparative example, and have higher wettability than the comparative example.

[0165] As shown in this example, the separator obtained by the manufacturing method described in Embodiment 1 and the like can have higher wettability than the comparative example. Therefore, the amount of electrolyte used can be reduced, specifically, the ratio can be set to 0.1 to 1.5 when the total void volume is 1, thereby reducing manufacturing costs and achieving a highly safe secondary battery. [Explanation of symbols]

[0166] 100 Cathode active material 118 Sealing part 200 positive electrode 201 Positive electrode current collector 202 Cathode active material layer 203 Second Separator 203a First Area 203b Second Area 205 Negative electrode active material layer 206 Negative electrode current collector 281 tabs 282 tabs 501 Positive electrode current collector 502 Positive electrode active material layer 503 Positive electrode 506 negative electrode 508 Separator 509 Exterior body 510 leads 511 leads 512 Negative electrode active material layer 513 Negative electrode current collector 514 Sealing part 530 Electrolyte 531 Exterior body 553 Conductive materials 913 Secondary battery 1003 Nail 1004 Secondary battery 2100 mobile phone 2101 Housing 2102 Display section 2103 Operation button 2104 External connection port 2105 Speaker 2106 Mike 2107 Lithium-ion battery 2300 Unmanned Aircraft 2301 Lithium-ion battery 2302 rotor 2303 Camera 4000 Eyeglasses-type Device 4000a frame 4000b Display section 4001 Headset type device 4001a microphone section 4001b Flexible Pipe 4001c Earphones 4002 devices 4002a housing 4002b secondary battery 4003 Devices 4003a housing 4003b secondary battery 4005 Wristwatch-type device 4005a Display section 4005b Belt section 4006 Belt-type device 4006a Belt section 4006b Wireless power receiving unit 4100a main unit 4100b main unit 4101 Driver Unit 4102 Antenna 4103 Secondary battery 4104 Display section 4110 cases 4111 Secondary battery 6300 Cleaning Robot 6301 Housing 6302 Display section 6303 Camera 6304 Brush 6305 Operation button 6306 Secondary battery 6310 Garbage 6400 Robot 6401 Illuminance Sensor 6402 Microphone 6403 Upper Camera 6404 Speaker 6405 Display section 6406 Lower Camera 6407 Obstacle Sensor 6408 Moving mechanism 6409 Secondary battery

Claims

1. a first slurry containing a first binder is applied onto a positive electrode current collector to form a positive electrode having a positive electrode active material layer; a solution containing polyamic acid is applied to the entire surface or selectively to the surface of the first separator to form a second separator; a second slurry containing a second binder is applied onto a negative electrode current collector to form a negative electrode having a negative electrode active material layer; a laminate of the positive electrode, the second separator, and the negative electrode is housed in an exterior body having an opening; a volume of an electrolyte solution having a ratio of 0.1 to 1.5, where the sum of void volumes calculated from the porosities of the positive electrode active material layer, the second separator, and the negative electrode active material layer is 1, is dropped into the opening of the exterior body; The method for manufacturing a secondary battery further comprises sealing the opening of the exterior body under reduced pressure.

2. 2. The method for producing a secondary battery according to claim 1, wherein the second separator has a contact angle with the electrolyte of greater than 0 degrees and less than 35 degrees when measured 45 seconds after a droplet of the electrolyte is dropped on the surface of the second separator in a room temperature environment.

3. 2. The method for producing a secondary battery according to claim 1, wherein the first separator is made of polypropylene.

4. 2. The method for producing a secondary battery according to claim 1, wherein the electrolyte solution contains a fluorinated chain carbonate, a fluorinated cyclic carbonate, and a lithium salt.

5. 2. The method for producing a secondary battery according to claim 1, wherein the fluorinated cyclic carbonate contained in the electrolytic solution is fluoroethylene carbonate, the fluorinated chain carbonate contained in the electrolytic solution is methyl 3,3,3-trifluoropropionate, and when the total volume of the fluoroethylene carbonate and the methyl 3,3,3-trifluoropropionate is taken as 100 vol %, the volume ratio of the fluoroethylene carbonate to the methyl 3,3,3-trifluoropropionate is y:100-y (where 5≦y≦30).

6. 2. The method according to claim 1, wherein the lithium salt contained in the electrolyte solution is LiPF ６ , LiBF ４ , LiCF ３ SO ３ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ CF ２ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ) and LiC(CF ３ SO ２ ) ３ , LiClO ４ , LiH ２ P.O. ４ A method for producing a secondary battery comprising one or more selected from the following:

Images