Nonaqueous electrolyte secondary battery, separator used therein, and method for manufacturing the nonaqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery that can quickly and more securely shut down the passage of ions between positive and negative electrodes when the battery temperature rises and sustain the shutdown state even at high temperatures.SOLUTION: A non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a composite separator 40 arranged between the positive electrode and the negative electrode. The composite separator is formed by sandwiching a thermally expandable capsule 43 between layers of a laminate composed of at least two separator sheets 41, 42. A composite separator suitable for the non-aqueous electrolyte secondary battery and a manufacturing method for the non-aqueous electrolyte secondary battery are provided.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a nonaqueous electrolyte secondary battery, a separator used therein, and a method for producing a nonaqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have a high energy density and are excellent in storage performance and low-temperature operation, and are widely used in portable electronic devices such as mobile phones and notebook computers. In addition, larger batteries are being used in automobiles and other transportation equipment, and their use as storage devices for nighttime electricity and electricity generated by natural energy sources is also being promoted.

[0003] In a non-aqueous electrolyte secondary battery, if an internal short circuit occurs due to impurity metals contained in the electrodes, the temperature of the battery will rise locally, which can trigger so-called thermal runaway. As a technique for dealing with this thermal runaway, Patent Document 1 describes the addition of thermally expandable capsules to a separator soaked in an electrolyte solution that is disposed between the positive and negative electrodes of a lithium-ion battery. According to the technique described in Patent Document 1, the thermally expandable capsules expand when the battery temperature rises, forming a space without electrolyte solution within the separator disposed between the electrodes, physically shutting out the movement of lithium ions and suppressing thermal runaway. Patent Document 2 describes a non-aqueous electrolyte secondary battery that contains a gas generating agent on the surface or inside of at least one of the positive electrode active material layer, the electrolyte layer, and the negative electrode active material layer, and that generates gas from the gas generating agent when the temperature of the secondary battery reaches 60°C or higher but lower than 300°C. Patent Document 2 also discloses a battery form in which 1H-tetrazole (gas generating agent) sandwiched between two separators is used instead of the separator. According to the technology described in Patent Document 2, when the temperature of the non-aqueous electrolyte secondary battery rises, gas is generated from the gas generating agent and diffuses. This diffused gas enters the pores of the positive electrode active material layer, the separator, the negative electrode active material layer, etc., narrowing or blocking the migration path of ions moving between the positive electrode active material layer and the negative electrode active material layer, thereby increasing the internal resistance, and it is said that it is possible to suppress further temperature rise, overcurrent, and the like. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-26674 A [Patent Document 2] JP 2008-226807 A Summary of the Invention [Problem to be solved by the invention]

[0005] The thermally expandable capsules and gas generating agents described in the above-mentioned patent documents are foreign substances that are detrimental to improving the performance of the battery under normal conditions, and therefore it is required to minimize their use. However, the method described in Patent Document 1 does not allow control of the thermal expansion direction of the thermally expandable capsule, and there are limitations to achieving both a reduction in the amount of thermally expandable capsule used and an improvement in the shutdown function (faster shutdown function, more efficient shutdown function, etc.). Furthermore, in the technology described in Patent Document 2, the gas generated from the gas generating agent diffuses without restriction within the battery, so a large amount of gas is required for shutdown, and there are limitations to speeding up the shutdown. Furthermore, in both the methods of Patent Documents 1 and 2, when the battery reaches a high temperature state and thermal melting of the separator itself progresses, it is difficult to stably maintain the shutdown state that has been formed. The present invention aims to provide a nonaqueous electrolyte secondary battery that can quickly and reliably shut down the movement of ions between positive and negative electrodes when the battery temperature rises, even when the amount of thermally expandable capsules used is reduced, and that can maintain this shutdown state even at high temperatures; a composite separator suitable for this nonaqueous electrolyte secondary battery; and a method for manufacturing this nonaqueous electrolyte secondary battery.

[0006] The present inventors have conducted extensive research in view of the above problems. As a result, they have found that by adopting a separator sandwiched between two separators that are in close contact with each other by the action of an electrolyte and a thermally expandable capsule as a separator to be disposed between a positive electrode active material layer and a negative electrode active material layer in a non-aqueous electrolyte secondary battery, the thermally expandable capsule can quickly expand two-dimensionally (planarly) along the space between the two separators when the battery temperature rises, thereby shutting down the battery, and that this shutdown state can be sufficiently maintained even if the thermal melting of the separator itself progresses. Furthermore, it has become clear that the above effect can be obtained even if a small amount of the thermally expandable capsule, which is a foreign body that is disadvantageous to improving battery performance, is used because the expansion direction of the thermally expandable capsule is controlled two-dimensionally. The present invention has been completed through further investigation based on these findings. [Means for solving the problem]

[0007] The above problems were solved by the following means. [1] A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a composite separator disposed between the positive electrode and the negative electrode, The composite separator of the non-aqueous electrolyte secondary battery is formed by sandwiching a thermally expandable capsule between layers of a laminate constituted by at least two separator sheets. [2] The nonaqueous electrolyte secondary battery according to [1], wherein the thermal expansion initiation temperature of the thermally expandable capsule is 5°C or more lower than the thermal melting temperature of the separator sheets sandwiching the thermally expandable capsule. [3] The nonaqueous electrolyte secondary battery according to [1] or [2], wherein the particle size of the thermally expandable capsules is 1.5 times or more the pore size of the separator sheets sandwiching the thermally expandable capsules. [4] The nonaqueous electrolyte secondary battery according to any one of [1] to [3], wherein the thermal expansion starting temperature of the thermally expandable capsule is 10° C. or more lower in the electrolyte than in the air. [5] The nonaqueous electrolyte secondary battery according to any one of [1] to [4], wherein in a plan view, the ratio of the total area of ​​the portion in which the thermally expandable capsules are arranged to the total area of ​​the composite separator is 50% or less. [6] The nonaqueous electrolyte secondary battery according to any one of [1] to [5], wherein the composite separator is in a state where it is pressurized in a lamination direction of the separator sheets. [7] The nonaqueous electrolyte secondary battery according to any one of [1] to [6], wherein the separator sheets constituting the composite separator are microporous membranes, and the separator sheets constituting the composite separator are in close contact with each other due to the action of an electrolyte. [8] The nonaqueous electrolyte secondary battery according to any one of [1] to [7], wherein the separator sheets are bonded to each other around and / or in the vicinity of the composite separator. [9] A composite separator for a non-aqueous electrolyte secondary battery, comprising a thermally expandable capsule sandwiched between layers of a laminate constituted by at least two separator sheets.

[10] A method for producing a nonaqueous electrolyte secondary battery, comprising disposing the composite separator for a nonaqueous electrolyte secondary battery according to [9] between a positive electrode and a negative electrode.

[0008] In the description of the present invention, a numerical range expressed using "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits. In the present invention, the term "non-aqueous electrolyte" refers to an electrolyte that does not substantially contain water. That is, the "non-aqueous electrolyte" may contain a small amount of water within a range that does not impede the effects of the present invention. In the present invention, the "non-aqueous electrolyte" has a water concentration of 200 ppm (by mass) or less, preferably 100 ppm or less, and more preferably 20 ppm or less. Note that it is practically difficult to make a non-aqueous electrolyte completely anhydrous, and it usually contains 1 ppm or more of water. In the present invention, the term "nonaqueous electrolyte secondary battery" broadly includes secondary batteries that use a nonaqueous electrolyte. Effect of the Invention

[0009] The nonaqueous electrolyte secondary battery of the present invention can shut down the movement of ions between the positive and negative electrodes quickly and reliably when the battery temperature rises, even if the amount of thermally expandable capsule used is reduced, and can maintain this shutdown state even at high temperatures. The composite separator for a nonaqueous electrolyte secondary battery of the present invention is suitable as a separator for the nonaqueous electrolyte secondary battery of the present invention. According to the manufacturing method for a nonaqueous electrolyte secondary battery of the present invention, a nonaqueous electrolyte secondary battery can be obtained that can shut down the movement of ions between the positive and negative electrodes quickly and reliably when the battery temperature rises, even if the amount of thermally expandable capsule used is reduced, and can maintain this shutdown state even at high temperatures. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is an explanatory diagram (longitudinal cross-sectional view) showing the operating mechanism of a lithium ion secondary battery, which is one type of nonaqueous electrolyte secondary battery. [Diagram 2] FIG. 2 is a vertical cross-sectional view showing an example of a specific structure of a nonaqueous electrolyte secondary battery. [Diagram 3] FIG. 3 is an explanatory view (longitudinal cross-sectional view) that illustrates a schematic state of a composite separator disposed in a nonaqueous electrolyte secondary battery of the present invention. [Figure 4]FIG. 4 is an explanatory view (longitudinal cross-sectional view) that typically shows a state in which the thermally expandable capsules of the composite separator are thermally expanded when the nonaqueous electrolyte secondary battery of the present invention is exposed to high temperatures. [Diagram 5] FIG. 5 is a longitudinal sectional view that shows a schematic structure of a laminated cell battery, which is a nonaqueous electrolyte secondary battery produced in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Preferred embodiments of the nonaqueous electrolyte secondary battery of the present invention will be described below, but the present invention is not limited to these embodiments except as specified in the present invention.

[0012] [Nonaqueous electrolyte secondary battery] The nonaqueous electrolyte secondary battery of the present invention has a positive electrode, a negative electrode, and a composite separator disposed between the positive electrode and the negative electrode. The composite separator is a laminate composed of at least two separator sheets, and a thermally expandable capsule is sandwiched between the layers of the laminate. When the composite separator is a laminate of three or more separator sheets, the laminate has two or more interlayers. In this case, the thermally expandable capsule may be present between any of the interlayers. That is, the thermally expandable capsule may be present between one of two or more interlayers, or may be present between two or more layers. The constituent materials of the multiple separator sheets constituting one composite separator may be the same or different. From the viewpoint of reducing the thickness of the resulting nonaqueous electrolyte secondary battery, the composite separator preferably includes two separator sheets.

[0013] The nonaqueous electrolyte secondary battery of the present invention can employ the same configuration as a normal nonaqueous electrolyte secondary battery, except for the configuration of the separator. FIG. 1 is an explanatory diagram (conceptual diagram) showing the operating mechanism of a lithium ion secondary battery, which is one form of non-aqueous electrolyte secondary battery. The lithium ion secondary battery 10 comprises a non-aqueous electrolyte 5, a positive electrode C (positive electrode current collector 1, positive electrode active material layer 2) capable of inserting and releasing lithium ions, and a negative electrode A (negative electrode current collector 3, negative electrode active material layer 4) capable of inserting and releasing or dissolving and depositing lithium ions. A separator 9 is disposed between the positive electrode C and the negative electrode A. With this battery configuration, electrons (e - ) is supplied to the positive electrode active material layer 2, lithium ions are released from the positive electrode active material layer 2, these lithium ions move to the negative electrode side through the non-aqueous electrolyte 5, and are accumulated in the negative electrode active material layer (a). During discharge (β), the lithium ions accumulated in the negative electrode active material layer are released, these lithium ions are accumulated in the positive electrode active material layer through the non-aqueous electrolyte 5 (b), and at the same time, electrons are supplied to the operating mechanism 6 through the circuit wiring 7. This is the operating mechanism of a lithium-ion secondary battery.

[0014] The above conceptually describes the operating mechanism of a nonaqueous electrolyte secondary battery using a lithium ion secondary battery as an example. Next, a specific shape of a nonaqueous electrolyte secondary battery will be described. Specific battery shapes of a nonaqueous electrolyte secondary battery include a bottomed cylindrical shape, a bottomed prismatic shape, a thin shape, a sheet shape, and a paper shape. The nonaqueous electrolyte secondary battery of the present invention may have any shape as long as it functions as a battery by the above operating mechanism. In addition, it may have an irregular shape such as a horseshoe shape or a comb shape taking into account the shape of a system, device, etc. to be incorporated. 2 shows an example of a bottomed cylindrical nonaqueous electrolyte secondary battery 100. This battery is a bottomed cylindrical nonaqueous electrolyte secondary battery 100 in which a positive electrode sheet 14 and a negative electrode sheet 16 are stacked with a separator 12 interposed therebetween, rolled up, and housed in an exterior can 18 (this exterior can 18 also serves as a negative electrode current collector). Other components in the figure are 20 an insulating plate, 22 a sealing plate, 24 a positive electrode current collector, 26 a gasket, 28 a pressure-sensitive valve body, and 30 a current interruption element. Note that the hatching in the enlarged circle has been changed to improve visibility, but each member corresponds to the reference numeral in the overall drawing.

[0015] The materials, electrolytes, members, etc. used in the nonaqueous electrolyte secondary battery of the present invention are not particularly limited except for the configuration of the separator. These materials, members, etc. can be appropriately applied to those used in normal nonaqueous electrolyte secondary batteries. In addition, for the method of producing the nonaqueous electrolyte secondary battery of the present invention, a normal method can be appropriately adopted except for the configuration of the separator. For example, JP-A-2016-201308, JP-A-2008-226807, etc. can be appropriately referred to. The composite separator, which is a characteristic feature of the nonaqueous electrolyte secondary battery of the present invention, will be described below.

[0016] <Composite separator> The composite separator used in the non-aqueous electrolyte secondary battery of the present invention is a laminate composed of at least two separator sheets as described above, and a thermally expandable capsule is sandwiched between the layers of this laminate. The composite separator used in the present invention has pores, similar to the ordinary separator used in the non-aqueous electrolyte secondary battery, and functions as a positive / negative electrode separator that insulates between the positive and negative electrodes while allowing the electrolyte and ions to pass through during normal battery use. In addition, when some battery abnormality occurs and the battery temperature rises, an ordinary separator melts by heat, which blocks the pores and blocks the ion conduction between the positive and negative electrodes, thereby stopping the battery function. However, in the composite separator of the present invention, the thermally expandable capsules expand quickly before the separator melts, and the movement of ions between the positive and negative electrodes is shut down quickly and more reliably. In addition, this shutdown state can be maintained even at high temperatures. The composite separator used in the present invention will be described below in the form of a laminate consisting of two separator sheets and a thermally expandable capsule sandwiched between the layers. However, the composite separator used in the present invention is not limited to these forms except as specified in the present invention.

[0017] FIG. 3 is a longitudinal sectional view showing a schematic diagram of one embodiment of a composite separator arranged in a non-aqueous electrolyte secondary battery of the present invention. As shown in FIG. 3, the composite separator 40 has a structure in which two separator sheets 41 and 42 are laminated, and a thermally expandable capsule 43 is sandwiched between the layers of this laminate. The two separator sheets 41 and 42 are usually microporous membranes (microporous membranes) and are in close contact with each other due to the action of the electrolyte. That is, the separator sheets soaked in the electrolyte act as an adhesive, and the separator sheets are in close contact with each other in a laminated state. This is the same phenomenon as the adhesion of tissue paper that is made to absorb water. Therefore, in a non-aqueous electrolyte secondary battery, the composite separator soaked in the electrolyte is in a state in which the two separator sheets 41 and 42 are in close contact with each other due to the action of the electrolyte in the portion where the thermally expandable capsule 43 is not arranged. The two separator sheets 41 and 42 may be made of the same or different materials.

[0018] In the nonaqueous electrolyte secondary battery of the present invention, the composite separator 40 arranged in the state shown in Fig. 3 expands two-dimensionally along the interface between the two separator sheets 41, 42 when the battery temperature rises to a predetermined temperature or higher. The state after this thermal expansion is shown typically in Fig. 4. As shown in Fig. 4, when the battery temperature rises for some reason, the composite separator in the nonaqueous electrolyte secondary battery of the present invention creates a state where there is no electrolyte between the stacked separator sheets, and the battery function stops.

[0019] In the non-aqueous electrolyte secondary battery of the present invention, the thermal expansion start temperature of the thermally expandable capsule (the foaming start temperature in the atmosphere (1 atm)) is preferably lower than the thermal melting temperature of the separator sheets sandwiching the thermally expandable capsule, and more preferably is 5°C or more lower than the thermal melting temperature of the separator sheets. By achieving such a relationship, the movement of ions between the positive and negative electrodes can be shut down quickly and reliably when the battery temperature rises. The thermal expansion start temperature of the thermally expandable capsule is usually 70 to 140°C, and preferably 70 to 120°C. The thermal melting temperature of the separator sheets is usually 120 to 160°C, and preferably 120 to 130°C. Here, the thermal expansion starting temperature of the thermally expandable capsule is the temperature at which the volume of the thermally expandable capsule expands to more than twice its original size at 1 atmosphere (1 atm). The thermal melting temperature of the separator sheet is synonymous with the melting point of the constituent material of the separator sheet. When the constituent material of the separator sheet is two or more types, the melting point of the material with the lowest melting point is regarded as the thermal melting temperature of the separator sheet in relation to the thermal expansion starting temperature of the thermally expandable capsule.

[0020] In the non-aqueous electrolyte secondary battery of the present invention, the particle size of the thermally expandable capsule constituting the composite separator is preferably larger than the pore size of the separator sheet sandwiching the thermally expandable capsule, and more preferably the particle size of the thermally expandable capsule is 1.5 times or more the pore size of the separator sheet sandwiching the thermally expandable capsule. By making such a relationship, the thermally expandable capsule is less likely to enter the separator sheet, and when the thermally expandable capsule thermally expands, the expansion direction can be more reliably controlled to the direction along the interface between the stacked separator sheets. In other words, the expanding thermally expandable capsule expands two-dimensionally (in the planar direction) between the layers of the laminate in which the separator sheets are in close contact with each other, so that a state in which there is no electrolyte at the heat generating site and its surroundings can be quickly created. In addition, in the non-aqueous electrolyte secondary battery, a certain pressure (usually "atmospheric pressure" + "0.05 to 0.1 MPa") is usually applied in the stacking direction of the composite separator, so that the thermally expandable capsule is less likely to expand in the stacking direction of the separator sheets. The above "particle size of the thermally expandable capsule" is the average particle size of the thermally expandable capsule used. This average particle size is published by the manufacturer or distributor of the thermally expandable capsule. If the average particle size published by the manufacturer is unknown, the "particle size of the thermally expandable capsule" shall be the median diameter based on volume (d50). Furthermore, the "pore diameter of the separator sheet" is determined by randomly observing 20 pores (hole entrances) observed on the surface of the separator sheet using an electron microscope, measuring the maximum width of each of the 20 pores on the surface of the separator sheet in the direction perpendicular to the maximum diameter direction, and taking the arithmetic mean of the 20 measured values. The "maximum diameter" of a pore means the maximum distance from a point on the inner circumference of the pore on the surface of the separator sheet to another point on the inner circumference. The pore diameter of the separator sheet is determined when the separator sheet is in a dry state.

[0021] The particle size of the thermally expandable capsules used in the present invention is preferably 1 to 20 μm, more preferably 1 to 15 μm, and even more preferably 1 to 12 μm. The particle size may be 1 to 10 μm, 1 to 5 μm, and preferably 1 to 3 μm. The pore size of the separator sheet is preferably 1 μm or less. For example, it may be 0.01 to 1 μm, or may be 0.01 to 0.8 μm, preferably 0.01 to 0.5 μm, more preferably 0.02 to 0.2 μm, and even more preferably 0.03 to 0.1 μm.

[0022] Next, preferred embodiments of the constituent materials of the composite separator used in the present invention will be described.

[0023] <Separator sheet> As the separator sheet constituting the composite separator, a separator generally used in a non-aqueous electrolyte secondary battery can be used without particular limitation as long as the effect of the present invention is not impaired. For example, the constituent material of the separator sheet can be a porous polymer material, an inorganic material, an organic-inorganic hybrid material, or glass fiber. The volume ratio of the gaps in the separator sheet, i.e., the porosity, is preferably 20% to 90%, more preferably 35% to 80%.

[0024] Examples of the polymer material include cellulose nonwoven fabric, polyethylene, polypropylene, etc., and a separator sheet using these materials in combination can also be used. A laminate of two or more types of microporous films with different pore sizes, porosities, pore blocking temperatures, etc. is also preferred. Examples of the inorganic material include oxides such as alumina and silicon dioxide; nitrides such as aluminum nitride and silicon nitride; and sulfates such as barium sulfate and calcium sulfate.

[0025] <Thermal expansion capsule> The thermally expandable capsule is usually formed of a thermoplastic resin containing a foaming agent inside. The foaming agent is not particularly limited as long as it expands at the desired temperature. For example, chemical foaming agents such as azo compounds, nitroso compounds, hydrazine derivatives, semicarbazide compounds, tetrazole compounds, isocyanate compounds, bicarbonates, carbonates, nitrites, hydrides, sodium bicarbonate and acid, hydrogen peroxide and yeast, zinc powder and acid, etc.; and physical foaming agents such as butane, pentane, hexane, dichloroethane, dichloromethane, freon, air, carbon dioxide gas, and nitrogen gas. The thermally expandable capsule used in the present invention preferably expands in volume when the internal pressure becomes sufficient to expand the capsule, for example, by vaporizing the low boiling point liquid therein. The temperature at which the volume expands can be controlled by selecting a low boiling point liquid to be filled therein, the boiling point of which is close to the desired temperature. In order to allow the volume expansion to proceed more quickly, it is also preferable to form the outer shell of the capsule from a thermoplastic resin or the like having a softening point below a predetermined temperature. The capsule can be formed by a known method such as a coacervation method.

[0026] Examples of commercially available thermally expandable capsules include Expancel 051DU, 007WU, 053WU, 053DU, 054WU, 091DU, 091-080DU, 091-140-DU, 092-120DU, 093-120DU, 820WU, 642WU, 551WU, 551DU, 551-20WU, 551-20DU, 551-80WU, 551-80DU, 461WU, 461DU, and 461-20 manufactured by Nippon Ferrite Co., Ltd.; and Microcapsules F-20, F-30, F-40, F-50, F-80S, F-82, F-85, F-100, and FN-100SSD manufactured by Matsumoto Oil & Fat Co., Ltd. These consist of a copolymer shell and a foaming agent made of a low-boiling point hydrocarbon inside. When they reach a certain temperature between about 70°C and 200°C, the shell softens and the contents vaporize, causing the volume of the capsule to expand, for example, to about 40 to 60 times its original size.

[0027] The thermal expansion capsule used in the present invention preferably has a thermal expansion starting temperature measured in the electrolyte of a nonaqueous electrolyte secondary battery (in an electrolyte placed under 1 atmospheric pressure) that is at least 10° C. lower than the temperature measured in the air (1 atmospheric pressure). This enhances the thermal expansion responsiveness of the nonaqueous electrolyte secondary battery to an increase in battery temperature, making it possible to more quickly shut down the battery when an abnormality occurs. When the thermal expansion onset temperature is decreased in the electrolyte of the non-aqueous electrolyte secondary battery, the reason is considered to be that the polar solvent contained in the electrolyte plasticizes the capsule wall when the thermal expansion microcapsule is immersed in the non-aqueous electrolyte. The lower the boiling point of the polar solvent contained in the electrolyte, the more noticeable the decrease in the thermal expansion onset temperature tends to be.

[0028] In the composite separator used in the present invention, the total area of ​​the portion in which the thermally expandable capsules are arranged (the area of ​​the thermally expandable capsules projected horizontally onto the separator sheet) is preferably 50% or less in plan view relative to the total area of ​​the composite separator. In this way, the effect of the present invention can be obtained even if the thermally expandable capsules are scattered between the separator sheets in a sparse manner, in a virtually single layer. In addition, the smaller this ratio is within the range in which the effect of the present invention can be obtained, the more preferable it is, and it is more preferably 40% or less, and even more preferably 30% or less, and it can be 20% or less. Even if the amount of the thermally expandable capsules used is reduced in this way, the nonaqueous electrolyte secondary battery of the present invention, which can two-dimensionally control the thermal expansion of the thermally expandable capsules, can achieve a quick shutdown effect.

[0029] The composite separator used in the present invention is preferably in a form in which the separator sheets are bonded to each other around and / or in the vicinity of the composite separator. By adopting such a form, even if the shell of the thermally expandable capsule breaks and gas leaks from the capsule due to thermal expansion, the dissipation of gas from the lamination interface between the separator sheets can be delayed, which contributes to more stable maintenance of the shutdown state. The separator sheets can be bonded to each other by heat sealing or the like, and it is preferable to apply heat sealing or the like in a line along the periphery of the composite separator.

[0030] The composite separator has been described above in terms of a state in which it is disposed between the positive electrode and the negative electrode of a nonaqueous electrolyte secondary battery. On the other hand, the above-mentioned composite separator can be distributed independently as a composite separator for a nonaqueous electrolyte secondary battery in a state before being incorporated into the nonaqueous electrolyte secondary battery. That is, according to the present invention, a composite separator for a non-aqueous electrolyte secondary battery is provided, which is formed by sandwiching a thermally expandable capsule between layers of a laminate composed of at least two separator sheets. This composite separator for a non-aqueous electrolyte secondary battery may be in a dry state, i.e., not soaked in an electrolyte, or in a wet state, i.e., soaked in an electrolyte. The size of the composite separator for a non-aqueous electrolyte secondary battery of the present invention is not particularly limited. A large-sized composite separator may be produced, and when it is incorporated into a non-aqueous electrolyte secondary battery, it may be cut to a desired size and used.

[0031] The composite separator of the present invention can be produced by overlapping at least two separator sheets and disposing the thermally expandable capsules approximately uniformly between the desired layers. It is also preferable to form a laminate of separator sheets in which thermally expandable capsules are arranged between desired layers, and then bond the separator sheets to each other at their periphery and / or in the vicinity thereof. For example, adjacent separator sheets can be bonded to each other along the periphery of the composite separator by applying a heat seal having a width of 1 to 10 mm.

[0032] A nonaqueous electrolyte secondary battery can be manufactured using the composite sheet of the present invention. That is, according to the present invention, a method for manufacturing a nonaqueous electrolyte secondary battery is provided, which includes disposing the composite separator for a nonaqueous electrolyte secondary battery of the present invention between a positive electrode and a negative electrode. The composite separator for a nonaqueous electrolyte secondary battery can be disposed between the positive electrode and the negative electrode in the same manner as the disposition of a separator in a conventional nonaqueous electrolyte secondary battery.

[0033] The nonaqueous electrolyte secondary battery of the present invention can be mounted in electronic devices such as notebook computers, pen-input computers, mobile computers, electronic book players, mobile phones, cordless phone sub-units, pagers, handheld terminals, mobile fax machines, mobile copiers, mobile printers, headphone stereos, video movie machines, liquid crystal televisions, handheld cleaners, portable CDs, mini-discs, electric shavers, transceivers, electronic organizers, calculators, memory cards, portable tape recorders, radios, backup power sources, and memory cards. It can also be mounted in automobiles, electric vehicles, motors, lighting equipment, toys, game equipment, road conditioners, watches, strobes, cameras, medical equipment (pacemakers, hearing aids, shoulder massagers, etc.), etc., for consumer use. It can also be used for various military and space applications. It can also be combined with solar cells.

[0034] The present invention will be described in more detail below with reference to examples, but the present invention should not be construed as being limited thereto. EXAMPLES

[0035] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a lithium salt to a concentration of 1 M in a non-aqueous solvent containing 40 mass % ethylene carbonate and 60 mass % ethyl methyl carbonate.

[0036] [Preparation of positive electrode active material layer forming material] Lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 A composition containing 85 mass % of O4), 7 mass % of carbon black as a conductive assistant, and 8 mass % of PVDF (polyvinylidene fluoride) as a binder was prepared as a positive electrode active material layer forming material.

[0037] [Preparation of negative electrode active material layer forming material] A composition containing 92 mass % of Gr (natural graphite) as the negative electrode active material and 8 mass % of PVDF as the binder was prepared as a material for forming a negative electrode active material layer.

[0038] [Composite separator] Two polypropylene separator sheets with dimensions of 5 cm long x 6 cm wide x 20 μm thick, pore size of 1 μm or less, and thermal melting temperature of 160°C were prepared. Matsumoto Microsphere-FN-100SSD (manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd., thermal expansion start temperature in air (under 1 atmosphere) is 120-130°C, average particle size is 6-11 μm (manufacturer's published value)) was uniformly sprinkled between the two separator sheets as thermal expansion capsules and arranged in a single layer to prepare a composite separator consisting of thermal expansion capsules and two separator sheets sandwiching them. In this composite separator, the ratio of the total area of ​​the parts where the thermal expansion capsules were arranged to the total area of ​​the composite separator was 40-48% in a plan view. The obtained composite separator was used in the nonaqueous electrolyte secondary battery of Example 2. The obtained composite separator was heat-sealed at its four peripheral sides by a width of 5 mm using a heat sealer, and used in the nonaqueous electrolyte secondary battery of Example 1. The laminate was also made by stacking two separator sheets without sandwiching a thermally expandable capsule, and heat-sealing at its four peripheral sides by a width of 5 mm, and used in the nonaqueous electrolyte secondary battery of the comparative example. The fabrication of the nonaqueous electrolyte secondary battery will be described below.

[0039] [Preparation of non-aqueous electrolyte secondary battery] A laminated cell battery (laminated cell battery) shown in FIG. 5 was fabricated as follows (FIG. 5 shows a schematic diagram of a composite separator having thermally expandable capsules). The positive electrode active material layer forming material was applied onto an aluminum current collector foil (3.8 cm long x 5.3 cm wide x 80 μm thick), dried and pressed to prepare a positive electrode. The negative electrode active material layer forming material was applied onto a copper current collector foil (4 cm long x 5.5 cm wide x 80 μm thick), dried and pressed to prepare a negative electrode. The composite separator was sandwiched between the positive electrode and the negative electrode and placed in a bag made of an Al laminate film, an electrolyte was injected, and the bag was vacuum sealed to prepare a laminated cell battery. The obtained laminated cell battery was sandwiched between SUS plates (5 cm long x 7 cm wide x 1 mm thick), restrained at atmospheric pressure +0.05 MPa, and used in the following tests.

[0040] [Test example] Evaluation of shutdown performance Using the nonaqueous electrolyte secondary battery (lamicelle battery) prepared above, the shutdown performance when the battery temperature increased was evaluated as follows. The battery was placed on a hot plate set at 50°C, and the internal resistance was measured using an AC method at 1 kHz while the temperature of the hot plate was increased at a rate of 5°C / min. The resistance value (Ω) at 50°C was set to "1", and the rate of increase in resistance value with increasing temperature was measured. The results are shown in the table below.

[0041] [Table 1]

[0042] As shown in the above table, by using the composite separator of the present invention, the shutdown function is exerted even at a lower temperature rise. It was also found that by sealing the periphery of the composite separator, the shutdown function begins to be exerted at an even lower temperature range. It was also found that even if the thermal melting state of the separator sheets progresses due to high temperatures (180°C or higher) to the point where the separator sheets themselves are unable to maintain a high level of shutdown, the use of the composite separator of the present invention can actually further enhance the shutdown state at the above-mentioned high temperatures. This is presumably because, even if the thermal melting state of the two separator sheets progresses considerably, these sheets are held in place by the thermally expanded capsules (resin) or gas sandwiched between them, and the two thermally melted separator sheets and the thermally expanded capsules or gas act together to contribute to a stable shutdown.

[0043] Although the present invention has been described in conjunction with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless specifically stated otherwise, and believe that the appended claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0044] This application claims priority based on Japanese Patent Application No. 2019-197747, filed on October 30, 2019, the contents of which are incorporated herein by reference as part of the present specification. [Explanation of symbols]

[0045] C positive electrode 1 Positive electrode conductive material (current collector) 2 Cathode active material layer A negative electrode 3. Negative electrode conductive material (current collector) 4 Negative electrode active material layer 5 Nonaqueous electrolyte 6 Operating mechanism 7 Circuit wiring 9 Separator 10 Lithium-ion secondary battery 12 Separator 14 Positive electrode sheet 16 Negative electrode sheet 18 External can also serves as a negative electrode current collector 20 Insulating plate 22 Sealing plate 24 Positive electrode current collector 26 Gasket 28 Pressure-sensitive valve 30 Current interruption element 100 Cylindrical lithium ion secondary battery with bottom 40 Composite separator 41, 42 Separator sheet 43 Thermal expansion capsule (before and after expansion) 44 Cathode active material layer 45 Aluminum current collector foil 46 Negative electrode active material layer 47 Copper current collector foil 48 Al laminate film 49, 50 circuit wiring

Claims

1. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a composite separator disposed between the positive electrode and the negative electrode, The composite separator is formed by sandwiching a thermally expandable capsule between layers of a laminate constituted by at least two separator sheets, the nonaqueous electrolyte secondary battery is configured such that the composite separator is sandwiched between the positive electrode and the negative electrode and sealed with a laminate film, and the composite separator is in a state where it is pressurized in a stacking direction of the separator sheet.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thermal expansion initiation temperature of the thermally expandable capsule is 5° C. or more lower than the thermal melting temperature of the separator sheets sandwiching the thermally expandable capsule in the nonaqueous electrolyte secondary battery.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein a particle diameter of said thermally expandable capsules is 1.5 times or more a pore diameter of said separator sheets sandwiching said thermally expandable capsules.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thermal expansion starting temperature of the thermally expandable capsule is 10° C. or more lower in the electrolyte than in the air.

5. 5. The nonaqueous electrolyte secondary battery according to claim 1, wherein in a plan view, a ratio of a total area of ​​a portion in which the thermally expandable capsules are arranged to an entire area of ​​the composite separator is 50% or less.

6. 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the nonaqueous electrolyte secondary battery, a separator sheet constituting the composite separator is a microporous membrane, and the separator sheets constituting the composite separator are in close contact with each other due to the action of an electrolyte.

7. 7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the composite separator comprises separator sheets bonded to each other at and / or near the periphery of the separator sheet.

8. 8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte secondary battery is sandwiched and restrained between plates disposed on the outside of the laminate film, and the composite separator is in a state where it is pressurized in a stacking direction of the separator sheets by this restraining force.

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