Method for manufacturing an electrolyte sheet and a secondary battery

Abstract

One aspect of the present invention provides a method for manufacturing an electrolyte sheet, comprising: a step of shaping a composition containing a polymer, Li[TFSI], oxide particles, and a dispersion medium into a sheet shape, and volatilizing the dispersion medium at a temperature of 100°C or higher.

Description

Technical Field

[0001] The present invention relates to an electrolyte sheet and a method for manufacturing a secondary battery. Background Art

[0002] In recent years, due to the popularization of portable electronic devices, electric vehicles, etc., high-performance secondary batteries have been required. Conventional secondary batteries have concerns about safety because they use electrolytes containing flammable organic solvents. Therefore, as a secondary battery with higher safety, a solid-state battery using a solid electrolyte has been developed. As the solid electrolyte, an organic polymer solid electrolyte, an inorganic solid electrolyte, etc. are known, but most of these solid electrolytes are formed into sheets and used in solid-state batteries.

[0003] For example, Patent Document 1 discloses a method for manufacturing a solid electrolyte sheet, which includes: a step of coating a solid electrolyte slurry on a substrate; and a step of drying the solid electrolyte slurry coated on the substrate to form a solid electrolyte layer on the substrate.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-062709 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] In a secondary battery using a solid electrolyte as an electrolyte layer, from the viewpoint of obtaining a secondary battery having excellent battery characteristics, it is preferable that the physical strength (tensile strength, etc.) in the electrolyte layer is excellent. However, according to the research of the present inventors, it has been found that depending on the composition, manufacturing conditions, etc. of the electrolyte layer, the strength of the electrolyte layer sometimes becomes insufficient.

[0009] An object of one aspect of the present invention is to provide a method for manufacturing an electrolyte sheet that is suitable for use as an electrolyte layer of a secondary battery and has excellent tensile strength.

[0010] Means for Solving the Problems

[0011] The present inventors have found that by using a specific compound as the electrolyte salt contained in the electrolyte sheet and molding it into a sheet under specific conditions, an electrolyte sheet having excellent tensile strength can be obtained, and this electrolyte sheet is suitable for use as an electrolyte layer of a secondary battery, and thus the present invention has been completed.

[0012] One aspect of the present invention provides a method for manufacturing an electrolyte sheet, which includes a step of forming a composition containing a polymer, Li[TFSI], oxide particles, and a dispersion medium into a sheet shape, and volatilizing the dispersion medium at a temperature of 100 °C or higher.

[0013] In this manufacturing method, by using Li[TFSI] as an electrolyte salt and volatilizing the dispersion medium from a composition (slurry) containing the components of the electrolyte sheet at a temperature of 100 °C or higher, the electrolyte sheet has excellent tensile strength. In addition, by using Li[TFSI] as an electrolyte salt, even when the dispersion medium is volatilized at a high temperature of 100 °C or higher, discoloration of the electrolyte sheet due to the influence of heating can be suppressed, and the appearance is excellent. Further, the ionic conductivity in this electrolyte sheet is also excellent, so it can be suitably used as an electrolyte layer of a secondary battery.

[0014] The composition may further contain at least one solvent selected from the group consisting of an ionic liquid and a (poly)ethylene glycol dialkyl ether represented by the following formula (1).

[0015] R 1 O-(CH2CH2O) k -R 2 (1)

[0016] [In formula (1), R 1 and R 2 each independently represent an alkyl group having 1 to 4 carbon atoms, and k represents an integer of 3 to 6. ]

[0017] Another aspect of the present invention provides a method for manufacturing a secondary battery, which includes the following steps: a step of forming a positive electrode mixture layer on a positive electrode current collector to obtain a positive electrode; a step of forming a negative electrode mixture layer on a negative electrode current collector to obtain a negative electrode; and a step of disposing the electrolyte sheet obtained by the above manufacturing method between the positive electrode and the negative electrode.

[0018] Advantages of the Invention

[0019] According to one aspect of the present invention, an electrolyte sheet suitable for use as an electrolyte layer of a secondary battery and having excellent tensile strength can be manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a perspective view showing a secondary battery according to an embodiment.

[0021] Figure 2 is a perspective view showing Figure 1 an exploded perspective view of an embodiment of an electrode assembly of the secondary battery shown.

[0022] Figure 3It is a schematic cross-sectional view showing an embodiment of a method for manufacturing an electrolyte sheet.

[0023] Figure 4 It is a schematic cross-sectional view showing an embodiment of a laminated sheet.

[0024] Figure 5 It is an exploded perspective view showing an embodiment of an electrode assembly of a bipolar secondary battery. Detailed Embodiments

[0025] Hereinafter, embodiments of the present invention will be described while appropriately referring to the attached Figure 1 However, the present invention is not limited to the following embodiments. In the following embodiments, its constituent elements (including steps, etc.) are not essential except in the case of special description. The sizes of the constituent elements in each figure are conceptual, and the relative size relationships between the constituent elements are not limited to the relationships shown in each figure.

[0026] The numerical values and their ranges in this specification do not limit the present invention. The numerical ranges expressed by "~" in this specification indicate ranges including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range can be replaced with the upper limit value or the lower limit value described in other stepwise descriptions. In addition, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range can also be replaced with the value shown in the examples.

[0027] In this specification, the following abbreviations are sometimes used.

[0028] [FSI] - : N(SO2F)2 - , bis(fluorosulfonyl)imide anion

[0029] [TFSI] - : N(SO2CF3)2 - , bis(trifluoromethanesulfonyl)imide anion

[0030] [BOB] - : B(O2C2O2)2 - , bis(oxalato)borate anion

[0031] [f3C] - : C(SO2F)3 - , tris(fluorosulfonyl)carbon anion

[0032] Figure 1 It is a perspective view showing a secondary battery according to an embodiment. As Figure 1As shown, the secondary battery 1 includes an electrode group 2 composed of a positive electrode, a negative electrode, and a sheet-shaped electrolyte layer (electrolyte sheet), and a bag-shaped battery exterior body 3 that houses the electrode group 2. A positive electrode current collector tab 4 and a negative electrode current collector tab 5 are provided in the positive electrode and the negative electrode, respectively. The positive electrode current collector tab 4 and the negative electrode current collector tab 5 protrude from the inside of the battery exterior body 3 to the outside in such a manner that the positive electrode and the negative electrode can be electrically connected to the outside of the secondary battery 1.

[0033] The battery exterior body 3 can be formed of a laminated film, for example. The laminated film can be a laminated film formed by sequentially laminating a resin film such as a polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper, or stainless steel, and a sealant layer such as polypropylene.

[0034] Figure 2 It represents Figure 1 An exploded perspective view of an embodiment of the electrode group 2 of the secondary battery 1 shown. As Figure 2 shown, the electrode group 2A sequentially includes a positive electrode 6, an electrolyte sheet 7, and a negative electrode 8. The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. A positive electrode current collector tab 4 is provided in the positive electrode current collector 9 of the positive electrode 6. The negative electrode 8 includes a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. A negative electrode current collector tab 5 is provided in the negative electrode current collector 11 of the negative electrode 8.

[0035] The positive electrode current collector 9 can be formed of a metal such as aluminum, titanium, tantalum, or an alloy thereof. In order to be lightweight and have a high weight energy density, the positive electrode current collector 9 is preferably formed of aluminum or an alloy thereof. The thickness of the positive electrode current collector 9 can be greater than or equal to 10 μm and can also be less than or equal to 100 μm.

[0036] In one embodiment, the positive electrode mixture layer 10 contains a positive electrode active material and a binder.

[0037] The positive electrode active material can be a lithium transition metal compound such as a lithium transition metal oxide or a lithium transition metal phosphate.

[0038] The lithium transition metal oxide can be, for example, lithium manganate, lithium nickelate, lithium cobaltate, etc. The lithium transition metal oxide can also be a lithium transition metal oxide in which a part of the transition metals such as Mn, Ni, Co contained in lithium manganate, lithium nickelate, lithium cobaltate, etc. is replaced by one or more other transition metals or metal elements such as Mg, Al (typical elements). That is, the lithium transition metal oxide can be a compound represented by LiM 1 O2 or LiM 1 2O4 (M 1 contains at least one transition metal). Specifically, the lithium transition metal oxide can be Li(Co 1 / 3 Ni 1 / 3 Mn1 / 3 )O2, LiNi 1 / 2 Mn 1 / 2 O2, LiNi 1 / 2 Mn 3 / 2 O4, etc.

[0039] From the viewpoint of further improving the energy density, the lithium transition metal oxide is preferably a compound represented by the following formula (A).

[0040] Li a Ni b Co c M 2 d O 2+e (A)

[0041] [In the formula, M 2 is at least one selected from the group consisting of Al, Mn, Mg, and Ca, and a, b, c, d, and e are numbers satisfying 0.2 ≦ a ≦ 1.2, 0.5 ≦ b ≦ 0.9, 0.1 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.2, -0.2 ≦ e ≦ 0.2, and b + c + d = 1.]

[0042] The lithium transition metal phosphate can be LiFePO4, LiMnPO4, LiMn x M 3 1-x PO4 (0.3 ≦ x ≦ 1, M 3 is at least one element selected from the group consisting of Fe, Ni, Co, Ti, Cu, Zn, Mg, and Zr), etc.

[0043] The positive electrode active material can be primary particles without granulation or secondary particles obtained by granulation.

[0044] The particle size of the positive electrode active material is adjusted to be less than or equal to the thickness of the positive electrode binder layer 10. When there are coarse particles with a particle size greater than or equal to the thickness of the positive electrode binder layer 10 in the positive electrode active material, the coarse particles are removed in advance by screening classification, air classification, etc., and the positive electrode active material with a particle size less than or equal to the thickness of the positive electrode binder layer 10 is selected.

[0045] The average particle size of the positive electrode active material is preferably greater than or equal to 0.1 μm, more preferably greater than or equal to 1 μm. The average particle size of the positive electrode active material is preferably less than or equal to 30 μm, more preferably less than or equal to 25 μm. The average particle size of the positive electrode active material is the particle size (D 50 ) when the ratio (volume fraction) with respect to the total volume of the positive electrode active material is 50%. The average particle size (D 50)It can be obtained by using a laser scattering particle size measuring device (e.g., Microtrac) and measuring a suspension obtained by suspending the positive electrode active material in water by the laser scattering method.

[0046] Based on the total amount of the positive electrode mixture layer, the content of the positive electrode active material can be greater than or equal to 60% by mass, greater than or equal to 70% by mass, greater than or equal to 80% by mass or greater than or equal to 90% by mass, and can be less than or equal to 99% by mass.

[0047] The binder can be: a polymer containing at least one selected from the group consisting of tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, methyl methacrylate, and acrylonitrile as monomer units, styrene-butadiene rubber, isoprene rubber, acrylate rubber, etc. The binder is preferably polyvinylidene fluoride or a copolymer containing hexafluoropropylene and vinylidene fluoride as structural units.

[0048] Based on the total amount of the positive electrode mixture layer, the content of the binder can be greater than or equal to 0.3% by mass, greater than or equal to 0.5% by mass, greater than or equal to 1% by mass or greater than or equal to 1.5% by mass, and can be less than or equal to 10% by mass, less than or equal to 8% by mass, less than or equal to 6% by mass or less than or equal to 4% by mass.

[0049] The positive electrode mixture layer 10 can further contain a conductive material. The conductive material can be carbon materials such as carbon black, acetylene black, graphite, carbon fiber, carbon nanotube, etc. These conductive materials can be used alone or in combination of two or more.

[0050] Based on the total amount of the positive electrode mixture layer, the content of the conductive material can be greater than or equal to 0.1% by mass, greater than or equal to 1% by mass or greater than or equal to 3% by mass. From the viewpoint of suppressing the volume increase of the positive electrode 6 and the accompanying decrease in the energy density of the secondary battery 1, based on the total amount of the positive electrode mixture layer, the content of the conductive material is preferably less than or equal to 15% by mass, more preferably less than or equal to 10% by mass, and further preferably less than or equal to 8% by mass.

[0051] The positive electrode mixture layer 10 can further contain the solvent described later. In this case, based on the total amount of the positive electrode mixture layer, the content of the solvent is preferably greater than or equal to 3% by mass, more preferably greater than or equal to 5% by mass, and further preferably greater than or equal to 10% by mass. In addition, it is preferably less than or equal to 30% by mass, more preferably less than or equal to 25% by mass, and further preferably less than or equal to 20% by mass.

[0052] From the viewpoint of improving the conductivity of the secondary battery 1, the thickness of the positive electrode mixture layer 10 is a thickness greater than or equal to the average particle diameter of the positive electrode active material. More specifically, it is preferably greater than or equal to 5 μm, more preferably greater than or equal to 10 μm, further preferably greater than or equal to 15 μm, and particularly preferably greater than or equal to 20 μm. The thickness of the positive electrode mixture layer 10 is preferably less than or equal to 100 μm, more preferably less than or equal to 80 μm, further preferably less than or equal to 70 μm, and particularly preferably less than or equal to 50 μm. By setting the thickness of the positive electrode mixture layer 10 to be less than or equal to 100 μm, it is possible to suppress charge-discharge unevenness caused by the deviation of the charging level of the positive electrode active material near the surface of the positive electrode mixture layer 10 and near the surface of the positive electrode current collector 9.

[0053] The negative electrode current collector 11 can be formed of a metal such as aluminum, copper, nickel, stainless steel, or an alloy thereof. In order to be lightweight and have a high weight energy density, the negative electrode current collector 11 is preferably formed of aluminum or an alloy thereof. From the viewpoints of ease of processing into a thin film and cost, the negative electrode current collector 11 is preferably copper. The thickness of the negative electrode current collector 11 can be greater than or equal to 10 μm and can be less than or equal to 100 μm.

[0054] In one embodiment, the negative electrode mixture layer 12 contains a negative electrode active material and a binder.

[0055] As the negative electrode active material, substances commonly used in the field of energy devices can be used. Specifically, for example, metallic lithium, lithium titanate (Li4Ti5O 12 ), lithium alloy or other metal compounds, carbon materials, metal complexes, and organic polymer compounds can be cited. The negative electrode active material can be used alone or in combination of two or more of the above. As the carbon material, natural graphite (such as flake graphite), artificial graphite, amorphous carbon, carbon fiber, and carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black can be cited. From the viewpoint of obtaining a larger theoretical capacity (for example, 500 to 1500 Ah / kg), the negative electrode active material can be silicon, tin, or a compound (oxide, nitride, alloy with other metals) containing these elements.

[0056] From the viewpoint of obtaining a well-balanced negative electrode that suppresses the increase in irreversible capacity as the particle diameter decreases and improves the electrolyte salt retention ability, the average particle diameter (D 50 ) of the negative electrode active material is preferably greater than or equal to 1 μm, more preferably greater than or equal to 5 μm, further preferably greater than or equal to 10 μm, preferably less than or equal to 50 μm, more preferably less than or equal to 40 μm, and further preferably less than or equal to 30 μm. The average particle diameter (D 50)Measured by the same method as the average particle diameter (D 50 ) of the above positive electrode active material.

[0057] Based on the total amount of the negative electrode binder layer, the content of the negative electrode active material can be greater than or equal to 50% by mass, greater than or equal to 55% by mass or greater than or equal to 60% by mass. Additionally, it can be less than or equal to 99% by mass, less than or equal to 95% by mass or less than or equal to 90% by mass.

[0058] The type and content of the binder can be the same as those of the binder used in the above positive electrode binder layer 10.

[0059] The negative electrode binder layer 12 can further contain a conductive material or can further contain a solvent described later. The types and contents of the conductive material and the solvent can be the same as those of the conductive material and the solvent in the above positive electrode binder layer 10, respectively.

[0060] From the viewpoint of improving the conductivity of the secondary battery 1, the thickness of the negative electrode binder layer 12 is greater than or equal to the average particle diameter of the negative electrode active material. More specifically, it is preferably greater than or equal to 10 μm, more preferably greater than or equal to 15 μm, and further preferably greater than or equal to 20 μm. The thickness of the negative electrode binder layer 12 is preferably less than or equal to 100 μm, less than or equal to 80 μm, less than or equal to 70 μm, less than or equal to 50 μm, less than or equal to 40 μm or less than or equal to 30 μm. By setting the thickness of the negative electrode binder layer 12 to be less than or equal to 100 μm, charge-discharge unevenness caused by the deviation of the charging level of the negative electrode active material near the surface of the negative electrode binder layer 12 and near the surface of the negative electrode current collector 11 can be suppressed.

[0061] The electrolyte sheet 7 is a sheet manufactured by a predetermined manufacturing method and functions as an electrolyte layer in the secondary battery 1. In one embodiment, the electrolyte sheet 7 includes a step of forming a composition containing a polymer, Li[TFSI], oxide particles, and a dispersion medium into a sheet shape and volatilizing the above dispersion medium at a temperature greater than or equal to 100°C. Hereinafter, the composition containing a polymer, Li[TFSI], oxide particles, and a dispersion medium is referred to as an "electrolyte composition".

[0062] Figure 3 It is a schematic cross-sectional view showing one embodiment of the manufacturing method of the electrolyte sheet 7. In this manufacturing method, first, a base material 13 for forming the electrolyte composition into a sheet shape is prepared. ( Figure 3 (a)).

[0063] The base material 13 is not particularly limited as long as it has heat resistance capable of withstanding heating during volatilization of the dispersion medium, does not react with the electrolyte composition, and does not swell due to the electrolyte composition, and is formed of, for example, a resin. Specifically, the base material 13 may be a film formed of a resin (general engineering plastic) such as polyethylene terephthalate, polytetrafluoroethylene, polyimide, polyethersulfone, or polyetherketone.

[0064] The thickness of the base material 13 is preferably as thin as possible while maintaining the strength capable of withstanding the tensile force of the coating apparatus. From the viewpoint of reducing the volume of the laminated sheet including the electrolyte sheet 7 and the base material 13 (details will be described later) while ensuring the strength of the laminated sheet, the thickness of the base material 13 is preferably greater than or equal to 5 μm, more preferably greater than or equal to 10 μm, still more preferably greater than or equal to 25 μm. In addition, it is preferably less than or equal to 100 μm, more preferably less than or equal to 50 μm, still more preferably less than or equal to 40 μm.

[0065] Next, the electrolyte composition 14 is formed into a sheet shape by coating the electrolyte composition 14 on the base material 13 ( Figure 3 (b)).

[0066] The electrolyte composition 14 contains a polymer, Li[TFSI], oxide particles, and a dispersion medium.

[0067] The polymer preferably has a first monomer unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride.

[0068] The polymer is preferably one or more polymers. Among the monomer units constituting the one or more polymers, a first monomer unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride and a second monomer unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate are preferably included.

[0069] The first monomer unit and the second monomer unit may be included in one polymer to form a copolymer. That is, in one embodiment, the electrolyte composition 14 contains at least one copolymer including both the first monomer unit and the second monomer unit. The copolymer may be a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and maleic acid, a copolymer of vinylidene fluoride and methyl methacrylate, or the like. When the electrolyte composition 14 contains a copolymer, other polymers may further be contained.

[0070] The first monomer unit and the second monomer unit may also be contained in different polymers respectively, and constitute at least two polymers, namely, a first polymer having the first monomer unit and a second polymer having the second monomer unit. That is, in one embodiment, the electrolyte composition 14 contains at least two or more polymers, namely, a first polymer containing the first monomer unit and a second polymer containing the second monomer unit. When the electrolyte composition 14 contains the first polymer and the second polymer, other polymers may also be further contained.

[0071] The first polymer may be a polymer composed only of the first monomer unit, or a polymer having other monomer units in addition to the first monomer unit. The other monomer units may include an oxygen-containing hydrocarbon structure such as ethylene oxide (-CH2CH2O-). The first polymer may be polytetrafluoroethylene, polyvinylidene fluoride, or a polymer having the above oxygen-containing hydrocarbon structure introduced into its molecular structure.

[0072] The second polymer may be a polymer composed only of the second monomer unit, or a polymer having other monomer units in addition to the second monomer unit. The other monomer units may be an oxygen-containing hydrocarbon structure such as ethylene oxide (-CH2CH2O-).

[0073] Examples of the combination of the first polymer and the second polymer include polyvinylidene fluoride and polyacrylic acid, polytetrafluoroethylene and polymethyl methacrylate, polyvinylidene fluoride and polymethyl methacrylate, etc.

[0074] From the viewpoint of further improving the tensile strength of the electrolyte sheet 7, based on the total content of the first monomer unit and the second monomer unit, the content of the first monomer unit is preferably greater than or equal to 50% by mass, greater than or equal to 70% by mass, greater than or equal to 80% by mass, greater than or equal to 90% by mass, or greater than or equal to 95% by mass. From the viewpoint of improving the affinity with the solvent when the solvent described later is contained in the electrolyte sheet 7, based on the total content of the first monomer unit and the second structural unit, the content of the first monomer unit is preferably less than or equal to 99% by mass, less than or equal to 98% by mass, less than or equal to 97% by mass, or less than or equal to 96% by mass.

[0075] From the viewpoint of further improving the affinity with the solvent when the following solvent is included in the electrolyte sheet 7, based on the total content of the first monomer unit and the second monomer unit, the content of the second monomer unit is preferably greater than or equal to 1% by mass, greater than or equal to 3% by mass or greater than or equal to 4% by mass. From the viewpoint of further improving the tensile strength of the electrolyte sheet 7, based on the total content of the first monomer unit and the second monomer unit, the content of the second monomer unit is preferably less than or equal to 50% by mass, less than or equal to 40% by mass, less than or equal to 30% by mass, less than or equal to 20% by mass, less than or equal to 10% by mass or less than or equal to 5% by mass.

[0076] Based on the total amount of the non-volatile components of the electrolyte composition 14, the content of the polymer can be greater than or equal to 10% by mass, greater than or equal to 15% by mass or greater than or equal to 20% by mass, and can be less than or equal to 60% by mass, less than or equal to 55% by mass or less than or equal to 50% by mass. It should be noted that the non-volatile components of the electrolyte composition 14 are the components obtained by removing the dispersion medium from the electrolyte composition 14. Thus, the content of the polymer in the obtained electrolyte sheet 7 becomes the same as the above content (the same applies to the following components).

[0077] Li[TFSI] is an electrolyte salt and is lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2). Based on the total amount of the non-volatile components of the electrolyte composition 14, the content of Li[TFSI] can be greater than or equal to 1% by mass and can also be less than or equal to 50% by mass. By using Li[TFSI] as the electrolyte salt,

[0078] The electrolyte sheet 7 can further contain other electrolyte salts in addition to containing Li[TFSI]. As the other electrolyte salts, at least one selected from the group consisting of lithium salts (excluding Li[TFSI]), sodium salts, calcium salts and magnesium salts can be used.

[0079] The anion of the other electrolyte salt can be a halide ion (I - , Cl - , Br - , etc.), SCN - , BF4 - , BF3(CF3) - , BF3(C2F5) - , PF6 - , ClO4 - , SbF6 - , [FSI]-, [TFSI] - , N(SO2C2F5)2 - , B(C6H5)4 - , B(O2C2H4)2- , [f3C]-, C(SO2CF3)3 - , CF3COO - , CF3SO2O - , C6F5SO2O - , [BOB]-, etc. The anion is preferably PF6 - , BF4 - , [FSI] - , [TFSI] - , [BOB] - or ClO4 - .

[0080] The lithium salt can be at least one selected from the group consisting of LiPF6, LiBF4, Li[f3C], Li[BOB], LiClO4, LiCF3BF3, LiC2F5BF3, LiC3F7BF3, LiC4F9BF3, Li[C(SO2CF3)3], LiCF3SO3, LiCF3COO, and LiRCOO (R is an alkyl group, phenyl group or naphthyl group having 1 to 4 carbon atoms).

[0081] The sodium salt can be at least one selected from the group consisting of NaPF6, NaBF4, Na[FSI], Na[TFSI], Na[f3C], Na[BOB], NaClO4, NaCF3BF3, NaC2F5BF3, NaC3F7BF3, NaC4F9BF3, Na[C(SO2CF3)3], NaCF3SO3, NaCF3COO and NaRCOO (R is an alkyl group, phenyl group or naphthyl group having 1 to 4 carbon atoms).

[0082] The calcium salt can be at least one selected from the group consisting of Ca(PF6)2, Ca(BF4)2, Ca[FSI]2, Ca[TFSI]2, Ca[f3C]2, Ca[BOB]2, Ca(ClO4)2, Ca(CF3BF3)2, Ca(C2F5BF3)2, Ca(C3F7BF3)2, Ca(C4F9BF3)2, Ca[C(SO2CF3)3]2, Ca(CF3SO3)2, Ca(CF3COO)2 and Ca(RCOO)2 (R is an alkyl group, phenyl group or naphthyl group having 1 to 4 carbon atoms).

[0083] The magnesium salt may be at least one selected from the group consisting of Mg(PF6)2, Mg(BF4)2, Mg[FSI]2, Mg[TFSI]2, Mg[f3C]2, Mg[BOB]2, Mg(ClO4)2, Mg(CF3BF3)2, Mg(C2F5BF3)2, Mg(C3F7BF3)2, Mg(C4F9BF3)2, Mg[C(SO2CF3)3]2, Mg(CF3SO3)2, Mg(CF3COO)2, and Mg(RCOO)2 (where R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0084] From the viewpoint of appropriately manufacturing the electrolyte sheet 7, based on the total amount of the non-volatile components of the electrolyte composition 14, the content of other electrolyte salts may be greater than or equal to 10% by mass and may be less than or equal to 60% by mass.

[0085] The electrolyte composition 14 further contains oxide particles. By the electrolyte composition 14 containing oxide particles, the tensile strength of the obtained electrolyte sheet 7 can be further improved, and the ionic conductivity of the electrolyte sheet 7 can also be improved.

[0086] The oxide particles are, for example, particles of an inorganic oxide. The inorganic oxide may be, for example, an inorganic oxide containing Li, Mg, Al, Si, Ca, Ti, Zr, La, Na, K, Ba, Sr, V, Nb, B, Ge, etc. as constituent elements. The oxide particles are preferably at least one particle selected from the group consisting of SiO2, Al2O3, AlOOH, MgO, CaO, ZrO2, TiO2, Li7La3Zr2O 12 and BaTiO3. Since the oxide particles have polarity, the dissociation of the electrolyte in the electrolyte sheet 7 can be promoted, and the battery characteristics can be improved.

[0087] The oxide particles may also be particles of an oxide of a rare earth metal. Specifically, the oxide particles may be scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, lutetium oxide, etc.

[0088] The oxide particles may also have a hydrophobic surface. The oxide particles usually have hydroxyl groups on their surfaces and tend to exhibit hydrophilicity. Compared with oxide particles without a hydrophobic surface, the hydroxyl groups on the surface of oxide particles with a hydrophobic surface are reduced. Therefore, if oxide particles with a hydrophobic surface are used, in the case where the electrolyte sheet 7 contains an ionic liquid described later (for example, having N(SO2F)2 - 、N(SO2CF3)2 -Ionic liquids such as etc. as anionic components), since the ionic liquid is hydrophobic, it can be predicted that the affinity between the oxide particles and the ionic liquid is improved. Therefore, the liquid retention property of the ionic liquid in the electrolyte sheet 7 is further improved, and as a result, the ionic conductivity of the electrolyte sheet 7 is further improved. In addition, in a secondary battery having an electrolyte sheet containing oxide particles having a hydrophobic surface, the discharge characteristics can be particularly improved.

[0089] Oxide particles having a hydrophobic surface can be obtained, for example, by treating hydrophilic oxide particles with a surface treatment agent capable of imparting a hydrophobic surface. That is, oxide particles having a hydrophobic surface refer to oxide particles that have been surface-treated with a surface treatment agent. The surface treatment agent is preferably a silicon-containing compound.

[0090] The oxide particles can also be surface-treated with a silicon-containing compound. That is, the oxide particles can also be particles in which the surface of the oxide particles is bonded to the silicon atom of the silicon-containing compound via an oxygen atom. The silicon-containing compound is preferably at least one selected from the group consisting of halogen-containing alkylsilanes, alkoxysilanes, epoxy group-containing silanes, amino group-containing silanes, silazanes, and siloxanes.

[0091] The halogen element in the halogen-containing alkylsilane can be chlorine, fluorine, etc. The chloroalkylsilane (alkylchlorosilane) can be methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, n-octyldimethylchlorosilane, etc. The fluoroalkylsilane (fluoroalkylsilane) can be 3,3,3-trifluoropropyltrimethoxysilane, tridecafluorooctyltrimethoxysilane, etc.

[0092] The alkoxysilane can be methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, dimethoxydiphenylsilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, tetraethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, n-propyltriethoxysilane, etc.

[0093] The epoxy group-containing silane can be 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, etc.

[0094] The amino group-containing silane can be N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, etc.

[0095] The silazane may be hexamethyldisilazane or the like. The siloxane may be dimethyl silicone oil or the like. It may also have reactive functional groups (such as carboxyl groups, etc.) at its single end or both ends.

[0096] The oxide particles having a hydrophobic surface (surface-treated oxide particles) may be particles manufactured by known methods or commercially available products may be directly used.

[0097] The oxide particles are generally judged according to the apparent geometric form, and may include primary particles (particles that do not constitute secondary particles) that integrally form a single particle and secondary particles formed by the aggregation of a plurality of primary particles.

[0098] From the viewpoint of excellent discharge characteristics of the secondary battery, the specific surface area of the oxide particles may be greater than or equal to 2 m 2 / g, greater than or equal to 5 m 2 / g, greater than or equal to 10 m 2 / g, greater than or equal to 15 m 2 / g or greater than or equal to 50 m 2 / g, and may be less than or equal to 500 m 2 / g, less than or equal to 400 m 2 / g, less than or equal to 350 m 2 / g, less than or equal to 300 m 2 / g, less than or equal to 200 m 2 / g, less than or equal to 100 m 2 / g, less than or equal to 90 m 2 / g, less than or equal to 80 m 2 / g or less than or equal to 60 m 2 / g. The specific surface area of the oxide particles refers to the specific surface area of the entire oxide particles including primary particles and secondary particles, and is measured by the BET method.

[0099] From the viewpoint of improving the conductivity of the secondary battery 1, the average primary particle diameter (average particle diameter of primary particles) of the oxide particles is preferably greater than or equal to 0.005 μm (5 nm), more preferably greater than or equal to 0.01 μm (10 nm), and further preferably greater than or equal to 0.015 μm (15 nm). From the viewpoint of thinning the electrolyte sheet 7, the average primary particle diameter of the oxide particles is preferably less than or equal to 1 μm, more preferably less than or equal to 0.1 μm, and further preferably less than or equal to 0.05 μm. The average primary particle diameter of the oxide particles can be measured by observing the oxide particles using a transmission electron microscope or the like.

[0100] From the viewpoint of easily obtaining the electrolyte sheet 7 having excellent tensile strength and ionic conductivity, based on the total amount of the non-volatile components of the electrolyte composition 14, the content of the oxide particles is preferably greater than or equal to 5% by mass, greater than or equal to 10% by mass or greater than or equal to 15% by mass, and further preferably less than or equal to 80% by mass, less than or equal to 70% by mass or less than or equal to 60% by mass.

[0101] The dispersion medium can be water, an organic solvent, etc. The organic solvent can be N-methyl-2-pyrrolidone (NMP), cyclohexanone, methyl ethyl ketone, 2-butanol, dimethylacetamide, etc. The dispersion medium is preferably NMP. The addition amount of the dispersion medium can be appropriately adjusted in such a manner that the electrolyte composition 14 can be coated on the substrate 13.

[0102] The electrolyte composition 14 may further contain a solvent. The solvent is preferably at least one selected from the group consisting of an ionic liquid and a (poly)ethylene glycol dialkyl ether represented by the following formula (1).

[0103] R 1 O-(CH2CH2O) k -R 2 (1)

[0104] [In formula (1), R 1 and R 2 each independently represent an alkyl group having 1 to 4 carbon atoms, and k represents an integer of 3 to ⑥.]

[0105] The ionic liquid contains the following anion component and cation component. It should be noted that the ionic liquid in this specification is a substance that is liquid at -20°C or higher.

[0106] The anion component of the ionic liquid is not particularly limited and can be an anion of a halogen such as Cl - , Br - , I - , etc., an inorganic anion such as BF4 - , N(SO2F)2 - ([FSI] - ), etc., an organic anion such as B(C6H5)4 - , CH3SO2O - , CF3SO2O - , N(SO2C4F9)2 - , N(SO2CF3)2 - ([TFSI] - ), N(SO2C2F5)2 - , etc., The anion component of the ionic liquid preferably contains at least one of the anion components represented by the following formula (2).

[0107] N(SO2C m F 2m+1 )(SO2C n F 2n+1 ) - (2)

[0108] [In the formula, m and n each independently represent an integer of 0 to 5. m and n may be the same or different from each other, and are preferably the same as each other.]

[0109] The anionic component represented by the formula (2) is, for example, N(SO2C4F9)2 - , N(SO2F)2 - ([FSI] - ), N(SO2CF3)2 - ([TFSI] - ) and N(SO2C2F5)2 - . From the viewpoint of further improving the ionic conductivity in the secondary battery 1, the anionic component of the ionic liquid preferably contains at least one selected from the group consisting of N(SO2C4F9)2 - , CF3SO2O - , [FSI] - , [TFSI] - and N(SO2C2F5)2 - , and more preferably contains [FSI] - .

[0110] The cationic component of the ionic liquid is not particularly limited, but is preferably at least one selected from the group consisting of linear quaternary cations, piperidine cations, pyrrolidine cations, pyridine cations, and imidazole cations.

[0111] The linear quaternary cation is, for example, a compound represented by the following formula (3).

[0112] [Chemical formula 1]

[0113]

[0114] [In the formula (3), R 3 to R 6 each independently represent a linear alkyl group having 1 to 20 carbon atoms or a linear alkoxyalkyl group represented by R-O-(CH2) n - (R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), and X represents a nitrogen atom or a phosphorus atom. From R 3 to R 6The number of carbon atoms of the represented alkyl group is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0115] Piperidine The cation is, for example, a nitrogen-containing six-membered ring cyclic compound represented by the following formula (4).

[0116] [Chemical formula 2]

[0117]

[0118] [In formula (4), R 7 and R 8 each independently represent an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by R-O-(CH2) n -(wherein R represents methyl or ethyl, and n represents an integer of 1 to 4). The number of carbon atoms of the alkyl group represented by R 7 and R 8 is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0119] Pyrrolidine The cation is, for example, a five-membered ring cyclic compound represented by the following formula (5).

[0120] [Chemical formula 3]

[0121]

[0122] [In formula (5), R 9 and R 10 each independently represent an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by R-O-(CH2) n -(wherein R represents methyl or ethyl, and n represents an integer of 1 to 4). The number of carbon atoms of the alkyl group represented by R 9 and R 10 is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0123] Pyridine The cation is, for example, a compound represented by the following formula (6).

[0124] [Chemical formula 4]

[0125]

[0126] [In formula (6), R 11 to R 15 each independently represent an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R-O-(CH2) n -(wherein R represents methyl or ethyl, and n represents an integer of 1 to 4) or a hydrogen atom. The alkyl group represented by R 11~R 15 The number of carbon atoms of the alkyl group represented is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0127] imidazole The cation is, for example, a compound represented by the following formula (7).

[0128] [Chemical Formula 5]

[0129]

[0130] [In formula (7), R 16 ~R 20 each independently represents an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R-O-(CH2) n - (wherein R represents methyl or ethyl, and n represents an integer of 1 to 4) or a hydrogen atom. The number of carbon atoms of the alkyl group represented by R 16 ~R 20 is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0131] More specifically, the ionic liquid can be N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide (DEME-FSI), 1-ethyl-3-methylimidazole - bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazole - bis(fluorosulfonyl)imide (EMI-FSI), N-methyl-N-propylpyrrolidine - bis(trifluoromethanesulfonyl)imide (Py13-TFSI), N-methyl-N-propylpyrrolidine - bis(fluorosulfonyl)imide (Py13-FSI), N-ethyl-N-methylpyrrolidine - bis(trifluoromethanesulfonyl)imide (Py12-TFSI), N-ethyl-N-methylpyrrolidine - bis(fluorosulfonyl)imide (Py12-FSI), 1-ethyl-3-methylimidazole dicyanamide (EMI-DCA), etc.

[0132] In the (poly)ethylene glycol dialkyl ether represented by the above formula (1), in formula (1), R 1 and R 2 each independently represents an alkyl group having 4 or fewer carbon atoms or a fluoroalkyl group having 4 or fewer carbon atoms, and k represents an integer of 1 to 6. R 1 and R 2Each is independently preferably methyl or ethyl.

[0133] Specifically, the (poly)ethylene glycol dialkyl ether may be monoglyme (k = 1), diglyme (k = 2), triglyme (k = 3), tetraglyme (k = 4), pentaglyme (k = 5), or hexaglyme (k = 6).

[0134] When the electrolyte composition 14 contains the (poly)ethylene glycol dialkyl ether as a solvent, a part or all of the (poly)ethylene glycol dialkyl ether may form a complex with Li[TFSI].

[0135] From the viewpoint of suitably manufacturing the electrolyte sheet 7, based on the total amount of the non-volatile components of the electrolyte composition 14, the content of the solvent may be greater than or equal to 10% by mass, and may be less than or equal to 80% by mass or less than or equal to 60% by mass.

[0136] When the electrolyte composition 14 contains a solvent, from the viewpoints of further increasing the conductivity of the secondary battery 1 and suppressing capacity degradation, based on the total amount of the non-volatile components of the electrolyte composition 14, the total content of Li[TFSI] and the solvent is preferably greater than or equal to 10% by mass, more preferably greater than or equal to 25% by mass, and further preferably greater than or equal to 40% by mass. In addition, from the viewpoint of suppressing a decrease in the strength of the electrolyte sheet 7, it is preferably less than or equal to 80% by mass, more preferably less than or equal to 70% by mass.

[0137] When the electrolyte composition 14 contains a solvent, from the viewpoint of improving the charge-discharge characteristics of the secondary battery 1, the concentration of Li[TFSI] per unit volume of the solvent is preferably greater than or equal to 0.5 mol / L, more preferably greater than or equal to 0.7 mol / L, and further preferably greater than or equal to 1.0 mol / L. In addition, it is preferably less than or equal to 2.0 mol / L, more preferably less than or equal to 1.8 mol / L, and further preferably less than or equal to 1.6 mol / L.

[0138] The electrolyte composition 14 may further contain other components. Examples of the other components include fibers such as cellulose fiber, resin fiber, and glass fiber. Based on the total amount of the non-volatile components of the electrolyte composition 14, the content of the other components may be 0.1 to 20% by mass.

[0139] In this step, in one embodiment, as Figure 3As shown in (b), an electrolyte composition 14 is coated on one surface 13a of a substrate. Thus, a laminate 20 including the substrate 13 and the electrolyte composition 14 coated on the substrate 13 is produced. Examples of the method of coating the electrolyte composition 14 on the substrate 13 include a method of coating using a coater and a method of coating by a sprayer.

[0140] From the viewpoints of further improving the tensile strength and further enhancing the safety, the thickness when coating the electrolyte composition 14 is preferably greater than or equal to 5 μm, more preferably greater than or equal to 10 μm, and still more preferably greater than or equal to 15 μm. From the viewpoints of reducing the internal resistance of the secondary battery 1 and further improving the high-current characteristics, the thickness when coating the electrolyte composition 14 is preferably less than or equal to 200 μm, more preferably less than or equal to 150 μm, and still more preferably less than or equal to 100 μm. The thickness when coating the electrolyte composition 14 corresponds to the thickness of the electrolyte sheet 7.

[0141] Next, the dispersion medium contained in the electrolyte composition 14 coated on the substrate 13 is volatilized to obtain an electrolyte sheet 7 ( Figure 3 (c)). Thus, a laminated sheet 30A including the substrate 13 and the electrolyte sheet 7 formed on the substrate 13 can be obtained.

[0142] In the volatilization of the dispersion medium, for example, the laminate including the substrate 13 and the electrolyte composition coated on the substrate 13 may be placed in a drying furnace or the like to volatilize the dispersion medium in the electrolyte composition 14.

[0143] The temperature (volatilization temperature) when volatilizing the dispersion medium is greater than or equal to 100 °C. Thus, an electrolyte sheet 7 having excellent tensile strength can be formed. The volatilization temperature may be higher than 100 °C, preferably greater than or equal to 105 °C, greater than or equal to 110 °C, greater than or equal to 115 °C, or greater than or equal to 120 °C. By the volatilization temperature being higher than 100 °C, when the electrolyte sheet is used in a secondary battery, the cycle characteristics of the secondary battery can be improved. In addition, by the volatilization temperature being higher than 100 °C, the productivity or production efficiency can be improved, and the production cost can also be reduced. From the viewpoints of improving the ion conductivity of the electrolyte sheet 7, improving the light transmittance, and reducing the haze, the volatilization temperature may be less than or equal to 150 °C, less than or equal to 145 °C, or less than or equal to 140 °C. The temperature when volatilizing the dispersion medium in this specification refers to the atmosphere temperature when the dispersion medium is volatilized. For example, when using a drying furnace, it refers to the temperature inside the drying furnace.

[0144] The time (volatilization time) for volatilizing the dispersion medium can be appropriately adjusted according to the volatilization temperature and the production scale.

[0145] In the step of forming the electrolyte composition 14 into a sheet and volatilizing the dispersion medium at a temperature of 100°C or higher, as described above, after coating the electrolyte composition 14 on the substrate 13 to obtain the laminate 20, the dispersion medium can be volatilized by placing the laminate 20 in a drying furnace. However, as another example, the dispersion medium can be volatilized simultaneously while coating the electrolyte composition 14 on the substrate 13 in an environment at a temperature of 100°C or higher, thereby continuously obtaining the laminated sheet 30A. In this case, the volatilization time of the dispersion medium starts from the moment when the electrolyte composition 14 is coated on the substrate 13.

[0146] When manufacturing the secondary battery 1, the electrolyte sheet 7 can be obtained by peeling the substrate 13 from the laminated sheet 30A. Figure 3 (d).

[0147] The laminated sheet can also be continuously manufactured while being wound into a roll. In this case, sometimes the surface of the electrolyte sheet 7 comes into contact with the back surface of the substrate 13 and a part of the electrolyte sheet 7 adheres to the substrate 13, resulting in damage to the electrolyte sheet 7. To prevent such a situation, as another embodiment, the electrolyte sheet 7 can also be manufactured by producing a laminated sheet having a three-layer structure in which a protective material is provided on the side of the electrolyte sheet 7 opposite to the substrate 13.

[0148] Figure 4 Schematic cross-sectional view showing a laminated sheet according to another embodiment. As Figure 4 shown, this laminated sheet 30B further includes a protective material 15 on the surface 7a on the side of the electrolyte sheet 7 opposite to the substrate 13, as shown in Figure 3 (b).

[0149] The laminated sheet 30B can be obtained by obtaining a laminate (laminated sheet 30A) including the substrate 13 and the electrolyte sheet 7 by the above method and then laminating the protective material 15 so as to cover one surface of the electrolyte sheet 7 opposite to the substrate 13.

[0150] The protective material 15 only needs to be easily peelable from the electrolyte sheet 7, and is preferably a non-polar resin film such as polyethylene, polypropylene, or polytetrafluoroethylene. If a non-polar resin film is used, the electrolyte sheet 7 and the protective material 15 will not adhere to each other, and thus the protective material 15 can be easily peeled off.

[0151] From the viewpoint of reducing the overall volume of the laminated sheet 30B while ensuring strength, the thickness of the protective material 15 is preferably 5 μm or more, more preferably 10 μm, and preferably 100 μm or less, more preferably 50 μm or less, and further preferably 30 μm or less.

[0152] When manufacturing the secondary battery 1, the electrolyte sheet 7 can be obtained by peeling the base material 13 and the protective material 15 from the laminate 30B.

[0153] The electrolyte sheet 7 obtained by the manufacturing method described above has excellent tensile strength and suppressed coloring, and excellent appearance. The suppression of coloring and excellent appearance of the electrolyte sheet 7 can be confirmed, for example, by measuring the transmittance, yellowness, and haze of the electrolyte sheet 7.

[0154] The light transmittance of the electrolyte sheet 7 can be measured by a spectrophotometer (e.g., SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.) according to JIS K7361-1 (Test method for total light transmittance of transparent plastics). In this case, the size of the test piece cut out from the electrolyte sheet 7 can be adjusted according to the device used. The light transmittance in this specification refers to the average value α of the light transmittance measured every 5 nm in the wavelength range of 380 to 780 nm for each test piece, and the average value of α obtained for 3 test pieces.

[0155] When measuring the light transmittance of the electrolyte sheet contained in the secondary battery, the secondary battery in the state of having undergone initial charge and discharge (within 10 cycles) is disassembled in an argon atmosphere, the electrolyte sheet is taken out, dried for 24 hours or more, and then measured by the above measurement method.

[0156] The light transmittance of the electrolyte sheet 7 can be, for example, greater than or equal to 90%, greater than or equal to 90.5%, greater than or equal to 91% or greater than or equal to 91.5%, and can be less than or equal to 95%, less than or equal to 94.5%, less than or equal to 94% or less than or equal to 93.5%.

[0157] The haze of the electrolyte sheet 7 can be measured by a spectrophotometer (e.g., SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.) according to JIS K7136 (Method for determining haze of transparent plastics). At this time, the percentage of the transmitted light that is scattered forward and deviated from the incident light by 2.5° or more in the transmitted light through the test piece cut out from the electrolyte sheet 7 is defined as the haze. The haze in this specification refers to the average value β of the haze measured every 5 nm in the wavelength range of 380 to 780 nm for each test piece, and the average value of β obtained for 3 test pieces. The size of the test piece can also be adjusted according to the device used in the same manner as the measurement of the light transmittance above.

[0158] The haze of the electrolyte sheet 7 can be, for example, greater than or equal to 15%, greater than or equal to 20%, greater than or equal to 25% or greater than or equal to 30%, and can be less than or equal to 45%, less than or equal to 40%, less than or equal to 38% or less than or equal to 36%.

[0159] The yellowness of the electrolyte sheet 7 can be measured using a spectrophotometric haze meter (e.g., SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with ASTM E313-05 using an Illuminant C. The yellowness in this specification refers to the average value of the yellowness measured for three test pieces. The size of the test pieces can be adjusted according to the apparatus used in the same manner as the measurement of the light transmittance described above.

[0160] The yellowness of the electrolyte sheet 7 can be, for example, less than or equal to 18, less than or equal to 14, less than or equal to 8, less than or equal to 4, or less than or equal to 2, and can also be greater than or equal to 0, greater than or equal to 0.1, or greater than or equal to 0.2.

[0161] Next, a method for manufacturing the secondary battery 1 including the above electrolyte sheet 7 will be described. The manufacturing method of the secondary battery 1 according to one embodiment includes: a first step of forming a positive electrode mixture layer 10 on a positive electrode current collector 9 to obtain a positive electrode 6; a second step of forming a negative electrode mixture layer 12 on a negative electrode current collector 11 to obtain a negative electrode 8; and a third step of disposing the electrolyte sheet 7 between the positive electrode 6 and the negative electrode 8. The order of the first step and the second step is arbitrary.

[0162] In the first step, the positive electrode 6 is obtained, for example, as follows: After dispersing the materials for the positive electrode mixture layer in a dispersion medium using a kneader, a disperser, etc. to obtain a paste-like positive electrode mixture, the positive electrode mixture is coated on the positive electrode current collector 9 by a doctor blade method, an impregnation method, a spraying method, etc., and then the dispersion medium is volatilized. After volatilizing the dispersion medium, a compression molding step using a roll press can also be provided as needed. The positive electrode mixture layer 10 can also be formed into a multilayer structure by repeating the steps from the coating of the positive electrode mixture to the volatilization of the dispersion medium multiple times.

[0163] The dispersion medium used in the first step can be water, N-methyl-2-pyrrolidone (NMP), or the like.

[0164] In the second step, the method for forming the negative electrode mixture layer 12 on the negative electrode current collector 11 can be the same as the method in the first step described above.

[0165] In the third step, in one embodiment, the electrolyte sheet 7 obtained by the above manufacturing method is disposed between the positive electrode 6 and the negative electrode 8. At this time, the electrolyte sheet 7 may be disposed in such a manner that the positive electrode mixture layer 10 in the positive electrode 6 is in contact with the negative electrode mixture layer 12 in the negative electrode 8.

[0166] The method of disposing the electrolyte sheet 7 between the positive electrode 6 and the negative electrode 8 is a method of laminating the positive electrode 6, the electrolyte sheet 7, and the negative electrode 8, for example, by lamination. Thus, a secondary battery 1 including the positive electrode 6, the negative electrode 8, and the electrolyte sheet 7 disposed between the positive electrode 6 and the negative electrode 8 can be obtained.

[0167] In the secondary battery 1 thus obtained, since the electrolyte sheet 7 having excellent tensile strength and excellent ionic conductivity is used, battery performance such as charge and discharge performance is excellent.

[0168] The electrolyte sheet obtained by the manufacturing method described above can also be used for a so-called bipolar secondary battery as another embodiment. Figure 5 FIG. is an exploded perspective view showing an embodiment of an electrode group of a bipolar secondary battery. The electrode group 2B sequentially includes a positive electrode 6, a first electrolyte sheet 7, a bipolar electrode 16, a second electrolyte sheet 7, and a negative electrode 8. The bipolar electrode 16 includes a bipolar electrode current collector 17, a positive electrode mixture layer 10 provided on the surface (positive electrode surface) on the negative electrode 8 side of the bipolar electrode current collector 17, and a negative electrode mixture layer 12 provided on the surface (negative electrode surface) on the positive electrode 6 side of the bipolar electrode current collector.

[0169] The bipolar electrode current collector 17 is formed of, for example, a metal monomer such as aluminum, stainless steel, or titanium, a clad material formed by rolling and joining aluminum and copper or stainless steel and copper, or the like.

[0170] The first electrolyte sheet 7 and the second electrolyte sheet 7 may be the above-described electrolyte sheets. The first electrolyte sheet 7 and the second electrolyte sheet 7 may be the same or different from each other in composition, and are preferably the same as each other.

[0171] Since the above-described electrolyte sheet 7 having excellent tensile strength and excellent ionic conductivity is also used in this bipolar secondary battery, battery performance such as charge and discharge performance is excellent.

[0172] Examples

[0173] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples.

[0174] <Example 1>

[0175] Dissolve Li[TFSI] in EMI[FSI] at a concentration of 1.5 mol / L. Mix 43 parts by mass of this solution, 23 parts by mass of SiO2 particles (AEROSIL OX50, manufactured by AEROSIL Co., Ltd., Japan) and 34 parts by mass of a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP), and then add NMP as a dispersion medium to prepare an electrolyte composition. In a drying oven with the temperature inside the oven set to 100 °C, use a coater to coat the electrolyte composition on a substrate made of polyethylene terephthalate. At this time, adjust it so that the thickness of the dried electrolyte composition is 20 μm. After coating, let it stand in the drying oven at the same temperature for 30 minutes to volatilize the dispersion medium, and obtain a laminated sheet with an electrolyte film formed on the substrate. That is, in the manufacture of the laminated sheet of this example, the volatilization temperature of the dispersion medium is the temperature inside the drying oven, which is 100 °C, and the volatilization time of the dispersion medium is 30 minutes.

[0176] <Examples 2 - 6>

[0177] In Example 1, change the volatilization temperature of the dispersion medium (the temperature inside the drying oven) to the temperature shown in Table 1, and make a laminated sheet without changing other conditions.

[0178] <Comparative Example 1>

[0179] In Example 1, change the volatilization temperature of the dispersion medium (the temperature inside the drying oven) to 80 °C, and make a laminated sheet without changing other conditions.

[0180] <Comparative Example 2>

[0181] In Example 1, change Li[TFSI] to Li[FSI], and make a laminated sheet without changing other conditions.

[0182] <Physical Property Evaluation of Electrolyte Film>

[0183] For the sheets (electrolyte films) obtained by peeling the substrates from the laminated sheets of the examples and comparative examples, evaluate each physical property by the following method. The respective results are shown in Table 1.

[0184] Regarding the tensile strength, cut the electrolyte film into a width of 5 mm, clamp it with a fixture, and then fix it to a pedestal with tape so that the length becomes 20 mm. Then, stretch the electrolyte film by using a force measuring device (FGP-5, manufactured by Nidec-Shimpo Corporation, Japan), and measure the strength at which the electrolyte film breaks.

[0185] Regarding the light transmittance, yellowness (Illuminant C) and haze, use a spectrophotometric haze meter (SH7000, manufactured by Nippon Denshoku Industries Co., Ltd.) to measure by the above method.

[0186] Regarding the ionic conductivity, the electrolyte sheet was placed in a two-electrode sealed cell (HS cell, manufactured by Takizawa Co., Ltd.), and measured using an AC impedance measurement device (Model 1260, manufactured by Solartron). The AC impedance was measured at 10 mV in the range of 1 Hz to 10 MHz at room temperature (25°C). Based on the resistance value obtained from the width of the arc of the Nyquist curve, the ionic conductivity was calculated using the following formula. It should be noted that the placement of the electrolyte sheet in the sealed cell was carried out in a drying chamber.

[0187] Ionic conductivity [S / cm] = (1 / Resistance [Ω]) × (Thickness of the electrolyte sheet [cm] / Area of the electrolyte sheet [cm 2 )

[0188] [Table 1]

[0189]

[0190] Symbol Explanation

[0191] 1: Secondary battery, 6: Positive electrode, 7: Electrolyte sheet, 8: Negative electrode, 9: Positive electrode current collector, 10: Positive electrode mixture layer, 11: Negative electrode current collector, 12: Negative electrode mixture layer.

Claims

1. A method for manufacturing an electrolyte sheet, comprising: a step of forming a composition composed of a polymer, Li[TFSI], oxide particles, an ionic liquid, and a dispersion medium into a sheet shape, and volatilizing the dispersion medium at a temperature greater than 100°C and less than 150°C. Among them, The dispersion medium is selected from water, N-methyl-2-pyrrolidone, cyclohexanone, methyl ethyl ketone, 2-butanol, and dimethylacetamide.

2. A method for manufacturing a secondary battery, comprising: a step of forming a positive electrode mixture layer on a positive electrode current collector to obtain a positive electrode; a step of forming a negative electrode mixture layer on a negative electrode current collector to obtain a negative electrode; a step of disposing the electrolyte sheet obtained by the manufacturing method according to claim 1 between the positive electrode and the negative electrode.

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