Electrolyte Sheet and Method for Manufacturing a Secondary Battery

Abstract

An electrolyte sheet manufacturing method is provided which involves a step for forming a composition containing a polymer, Li[TFSI], oxide particles and a dispersion medium into a sheet and volatilizing the dispersion medium at at least 100°C.

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, with the popularization of portable electronic devices, electric vehicles, etc., high-performance secondary batteries have been required. Since conventional secondary batteries used electrolytes containing flammable organic solvents, there were concerns about safety. Therefore, as a secondary battery with higher safety, solid batteries using solid electrolytes have been developed. As solid electrolytes, organic polymer solid electrolytes, inorganic solid electrolytes, etc. are known, and these solid electrolytes are often formed into sheets and used in solid batteries.

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

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a secondary battery using a solid electrolyte as an electrolyte layer, it is preferable that the physical strength (tensile strength, etc.) in the electrolyte layer is excellent from the viewpoint of obtaining a secondary battery having excellent battery characteristics. However, as a result of the study by the present inventors, it has been found that the strength of the electrolyte layer may be insufficient depending on the composition, manufacturing conditions, etc. of the electrolyte layer.

[0006] One aspect of the present invention aims to provide a method for manufacturing an electrolyte sheet having excellent tensile strength, which is suitably used as an electrolyte layer of a secondary battery.

Means for Solving the Problems

[0007] The inventors of the present invention used a specific compound for the electrolyte salt contained in the electrolyte sheet and formed it into a sheet shape under specific conditions, thereby obtaining an electrolyte sheet having excellent tensile strength, and found that this is suitably used as an electrolyte layer of a secondary battery, and completed the present invention.

[0008] 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 100°C or higher.

[0009] In this manufacturing method, by using Li[TFSI] as the electrolyte salt and volatilizing the dispersion medium from the composition (slurry) containing the components of the electrolyte sheet at 100°C or higher, the electrolyte sheet has excellent tensile strength. Further, by using Li[TFSI] as the 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 is suppressed, and the appearance is excellent. Furthermore, since the ionic conductivity of this electrolyte sheet is also excellent, it can be suitably used as an electrolyte layer of a secondary battery.

[0010] The composition may further contain at least one solvent selected from the group consisting of an ionic liquid and a glyme represented by the following formula (1). R 1 O-(CH2CH2O) k -R 2 (1) [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.]

[0011] Another aspect of the present invention provides 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; and a step of disposing an electrolyte sheet obtained by the above manufacturing method between the positive electrode and the negative electrode.

Advantages of the Invention

[0012] According to one aspect of the present invention, an electrolyte sheet excellent in tensile strength, which is suitably used as an electrolyte layer of a secondary battery, can be manufactured.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with appropriate reference to the drawings. However, the present invention is not limited to the following embodiments. In the following embodiments, the constituent elements (including steps, etc.) are not essential unless otherwise specified. The sizes of the constituent elements in each figure are conceptual, and the relative size relationships between the constituent elements are not limited to those shown in each figure.

[0015] The numerical values and ranges thereof in this specification do not limit the present invention. The numerical range indicated by "~" in this specification indicates a range 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 may be replaced with the upper limit value or the lower limit value of other stepwise descriptions. Also, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

[0016] In this specification, the following abbreviations may be used. [FSI] - :N(SO2F)2 - , bis(fluorosulfonyl)imide anion [TFSI] - :N(SO2CF3)2 - , bis(trifluoromethanesulfonyl)imide anion [BOB] - :B(O2C2O2)2 - , bisoxalate borate anion [f3C] - :C(SO2F)3 - , tris(fluorosulfonyl)carboanion

[0017] FIG. 1 is a perspective view showing a secondary battery according to an embodiment. As shown in FIG. 1, the secondary battery 1 includes an electrode group 2 composed of a positive electrode, a negative electrode, and a sheet-like electrolyte layer (electrolyte sheet), and a bag-shaped battery exterior 3 that houses the electrode group 2. Positive electrode current collector tabs 4 and negative electrode current collector tabs 5 are provided on 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 to the outside of the battery exterior 3 so that the positive electrode and the negative electrode can be electrically connected to the outside of the secondary battery 1, respectively.

[0018] The battery exterior body 3 may be formed of, for example, a laminate film. The laminate film may be a laminated film in which 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 are laminated in this order.

[0019] FIG. 2 is an exploded perspective view showing an embodiment of the electrode group 2 of the secondary battery 1 shown in FIG. 1. As shown in FIG. 2, the electrode group 2A includes a positive electrode 6, an electrolyte sheet 7, and a negative electrode 8 in this order. The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode active material layer 10 provided on the positive electrode current collector 9. A positive electrode current collector tab 4 is provided on 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 active material layer 12 provided on the negative electrode current collector 11. A negative electrode current collector tab 5 is provided on the negative electrode current collector 11 of the negative electrode 8.

[0020] The positive electrode current collector 9 may be formed of a metal such as aluminum, titanium, tantalum, or an alloy thereof. Since the positive electrode current collector 9 is lightweight and has a high weight energy density, it is preferably formed of aluminum or an alloy thereof. The thickness of the positive electrode current collector 9 may be 10 μm or more and may be 100 μm or less.

[0021] In one embodiment, the positive electrode active material layer 10 contains a positive electrode active material and a binder.

[0022] The positive electrode active material may be a lithium transition metal compound such as a lithium transition metal oxide or a lithium transition metal phosphate.

[0023] The lithium transition metal oxide may be, for example, lithium manganate, lithium nickelate, lithium cobaltate, etc. The lithium transition metal oxide is a lithium transition metal oxide in which part of the transition metals such as Mn, Ni, Co contained in lithium manganate, lithium nickelate, lithium cobaltate, etc. are substituted with one or more other transition metals or metal elements (typical elements) such as Mg, Al. That is, the lithium transition metal oxide is LiM 1O2 or LiM 1 2O4 (M 1 includes at least one transition metal). The compound may be represented by. Specifically, the lithium transition metal oxide is, for example, Li(Co 1 / 3 Ni 1 / 3 Mn 1 / 3 )O2, LiNi 1 / 2 Mn 1 / 2 O2, LiNi 1 / 2 Mn 3 / 2 O4 and the like.

[0024] From the viewpoint of further improving the energy density, the lithium transition metal oxide is preferably a compound represented by the following formula (A). Li a Ni b Co c M 2 d O 2+e (A) [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, respectively.]

[0025] The lithium transition metal phosphate may be, for example, 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), and the like.

[0026] The positive electrode active material may be ungranulated primary particles or granulated secondary particles.

[0027] The particle size of the positive electrode active material is adjusted to be equal to or less than the thickness of the positive electrode binder layer 10. When there are coarse particles having a particle size equal to or greater than the thickness of the positive electrode binder layer 10 in the positive electrode active material, the coarse particles are removed in advance by sieving classification, air current classification, etc., and the positive electrode active material having a particle size equal to or less than the thickness of the positive electrode binder layer 10 is selected.

[0028] The average particle size of the positive electrode active material is preferably 0.1 μm or more, more preferably 1 μm or more. The average particle size of the positive electrode active material is preferably 30 μm or less, more preferably 25 μm or less. The average particle size of the positive electrode active material is the particle size (D 50 ) when the ratio (volume fraction) to the volume of the entire positive electrode active material is 50%. The average particle size (D 50 ) of the positive electrode active material is obtained by measuring a suspension in which the positive electrode active material is suspended in water by the laser scattering method using a laser scattering type particle size measuring device (for example, Microtrac).

[0029] The content of the positive electrode active material may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, and may be 99% by mass or less, based on the total amount of the positive electrode binder layer.

[0030] The binder may be a polymer containing at least one selected from the group consisting of ethylene tetrafluoride, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, methyl methacrylate, and acrylonitrile as monomer units, rubbers such as styrene-butadiene rubber, isoprene rubber, and acrylic rubber. The binder is preferably polyvinylidene fluoride or a copolymer containing hexafluoropropylene and vinylidene fluoride as structural units.

[0031] The content of the binder may be 0.3% by mass or more, 0.5% by mass or more, 1% by mass or more, or 1.5% by mass or more, and may be 10% by mass or less, 8% by mass or less, 6% by mass or less, or 4% by mass or less, based on the total amount of the positive electrode binder layer.

[0032] The positive electrode mixture layer 10 may further contain a conductive material. The conductive material may be a carbon material such as carbon black, acetylene black, graphite, carbon fiber, carbon nanotube, etc. These conductive materials may be used alone or in combination of two or more.

[0033] The content of the conductive material may be 0.1% by mass or more, 1% by mass or more, or 3% by mass or more based on the total amount of the positive electrode mixture layer. From the viewpoint of suppressing an increase in the volume of the positive electrode 6 and a consequent decrease in the energy density of the secondary battery 1, the content of the conductive material is preferably 15% by mass or less, more preferably 10% by mass or less, still more preferably 8% by mass or less based on the total amount of the positive electrode mixture layer.

[0034] The positive electrode mixture layer 10 may further contain a solvent described later. In that case, the content of the solvent is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less based on the total amount of the positive electrode mixture layer.

[0035] The thickness of the positive electrode mixture layer 10 is a thickness equal to or greater than the average particle diameter of the positive electrode active material from the viewpoint of improving the conductivity of the secondary battery 1. More specifically, it is preferably 5 μm or more, more preferably 10 μm or more, still more preferably 15 μm or more, and particularly preferably 20 μm or more. The thickness of the positive electrode mixture layer 10 is preferably 100 μm or less, more preferably 80 μm or less, still more preferably 70 μm or less, and particularly preferably 50 μm or less. By setting the thickness of the positive electrode mixture layer 10 to 100 μm or less, it is possible to suppress the unevenness of charge and discharge caused by the variation in 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.

[0036] The negative electrode current collector 11 may be formed of a metal such as aluminum, copper, nickel, stainless steel, or an alloy thereof. Since the negative electrode current collector 11 is lightweight and has a high weight energy density, it is preferably formed of aluminum or an alloy thereof. The negative electrode current collector 11 is preferably copper from the viewpoints of ease of processing into a thin film and cost. The thickness of the negative electrode current collector 11 may be 10 μm or more and may be 100 μm or less.

[0037] In one embodiment, the negative electrode mixture layer 12 contains a negative electrode active material and a binder.

[0038] As the negative electrode active material, those commonly used in the field of energy devices can be used. Specifically, examples of the negative electrode active material include metallic lithium, lithium titanate (Li4Ti5O 12 ), lithium alloy or other metal compounds, carbon materials, metal complexes, and organic polymer compounds. The negative electrode active material may be used alone or in combination of two or more thereof. Examples of the carbon material include graphite such as natural graphite (scaly graphite, etc.) and artificial graphite, amorphous carbon, carbon fiber, and carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. The negative electrode active material may be silicon, tin, or a compound containing these elements (oxide, nitride, alloy with other metals) from the viewpoint of obtaining a larger theoretical capacity (for example, 500 to 1500 Ah / kg).

[0039] The average particle diameter (D 50 ) of the negative electrode active material is preferably 1 μm or more, more preferably 5 μm or more, still more preferably 10 μm or more, preferably 50 μm or less, more preferably 40 μm or less, and still more preferably 30 μm or less from the viewpoint of obtaining a well-balanced negative electrode that suppresses the increase in irreversible capacity accompanying the decrease in particle diameter and enhances the holding ability of the electrolyte salt. The average particle diameter (D 50 ) of the negative electrode active material is measured by the same method as the average particle diameter (D 50 ) of the positive electrode active material described above.

[0040] The content of the negative electrode active material may be 50% by mass or more, 55% by mass or more, or 60% by mass or more, based on the total amount of the negative electrode binder layer, and may also be 99% by mass or less, 95% by mass or less, or 90% by mass or less.

[0041] The type of the binder and its content may be the same as those of the binder used in the positive electrode binder layer 10 described above.

[0042] The negative electrode binder layer 12 may further contain a conductive material and may further contain a solvent described later. The types and contents of the conductive material and the solvent may be the same as those of the conductive material and the solvent in the positive electrode binder layer 10 described above, respectively.

[0043] From the viewpoint of improving the conductivity of the secondary battery 1, the thickness of the negative electrode binder layer 12 is equal to or greater than the average particle diameter of the negative electrode active material. More specifically, it is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. The thickness of the negative electrode binder layer 12 is preferably 100 μm or less, 80 μm or less, 70 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less. By setting the thickness of the negative electrode binder layer 12 to 100 μm or less, it is possible to suppress the unevenness of charge and discharge caused by the variation in 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.

[0044] The electrolyte sheet 7 is a sheet manufactured by a predetermined manufacturing method and has a role 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 dispersion medium at 100° C. or higher. Hereinafter, the composition containing a polymer, Li[TFSI], oxide particles, and a dispersion medium is also referred to as an "electrolyte composition".

[0045] FIG. 3 is a schematic cross-sectional view showing an embodiment of a 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. (FIG. 3(a)).

[0046] The base material 13 has heat resistance that can withstand heating when the dispersion medium is volatilized, and is not limited as long as it does not react with the electrolyte composition and does not swell by the electrolyte composition. For example, it is formed of a resin. Specifically, the base material 13 may be a film made of a resin (general-purpose engineering plastic) such as polyethylene terephthalate, polytetrafluoroethylene, polyimide, polyethersulfone, or polyether ketone.

[0047] The thickness of the base material 13 is preferably as thin as possible while maintaining a strength that can withstand the tensile force in 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) and ensuring the strength of the laminated sheet, the thickness of the base material 13 is preferably 5 μm or more, more preferably 10 μm or more, still more preferably 25 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less, still more preferably 40 μm or less.

[0048] Next, the electrolyte composition 14 is formed into a sheet shape by applying the electrolyte composition 14 onto the base material 13 (FIG. 3(b)).

[0049] The electrolyte composition 14 contains a polymer, Li[TFSI], oxide particles, and a dispersion medium.

[0050] The polymer preferably has a first monomer unit selected from the group consisting of tetrafluoroethylene and vinylidene fluoride.

[0051] The polymer is preferably one or more polymers, and among the monomer units constituting the one or more polymers, preferably, 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 included.

[0052] The first monomer unit and the second monomer unit may be included in one kind of polymer to form a copolymer. That is, in one embodiment, the electrolyte composition 14 contains at least one copolymer containing 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, etc. When the electrolyte composition 14 contains a copolymer, it may further contain other polymers.

[0053] The first monomer unit and the second monomer unit may be included in different polymers to form 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, it may further contain other polymers.

[0054] The first polymer may be a polymer consisting only of the first monomer unit, or a polymer further having other monomer units in addition to the first monomer unit. The other monomer units may contain an oxygen-containing hydrocarbon structure such as ethylene oxide (-CH2CH2O-). The first polymer may be polytetrafluoroethylene, polyvinylidene fluoride, or a polymer having the oxygen-containing hydrocarbon structure introduced into the internal molecular structure thereof.

[0055] The second polymer may be a polymer consisting only of the second monomer unit, or a polymer further 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-).

[0056] Examples of the combination of the first polymer and the second polymer include vinylidene fluoride and polyacrylic acid, polytetrafluoroethylene and polymethyl methacrylate, vinylidene fluoride and polymethyl methacrylate, and the like.

[0057] From the viewpoint of further improving the tensile strength of the electrolyte sheet 7, the content of the first monomer unit is preferably 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more, based on the total content of the first monomer unit and the second monomer unit. From the viewpoint of improving the affinity with the solvent when the electrolyte sheet 7 contains the solvent described below, the content of the first monomer unit is preferably 99% by mass or less, 98% by mass or less, 97% by mass or less, or 96% by mass or less, based on the total content of the first monomer unit and the second structural unit.

[0058] From the viewpoint of further improving the affinity with the solvent when the electrolyte sheet 7 contains the solvent described below, the content of the second monomer unit is preferably 1% by mass or more, 3% by mass or more, or 4% by mass or more, based on the total content of the first monomer unit and the second monomer unit. From the viewpoint of further improving the tensile strength of the electrolyte sheet 7, the content of the second monomer unit is preferably 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less, based on the total content of the first monomer unit and the second monomer unit.

[0059] The content of the polymer may be 10% by mass or more, 15% by mass or more, or 20% by mass or more, and may be 60% by mass or less, 55% by mass or less, or 50% by mass or less, based on the total non-volatile content of the electrolyte composition 14. The non-volatile content of the electrolyte composition 14 is the component obtained by removing the dispersion medium from the electrolyte composition 14. Thereby, the content of the polymer in the obtained electrolyte sheet 7 becomes the same as the above-described content (the same applies to the following components).

[0060] Li[TFSI] is an electrolyte salt and is lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2). The content of Li[TFSI] may be 1% by mass or more and may be 50% by mass or less based on the total amount of non-volatile components of the electrolyte composition 14. By using Li[TFSI] as the electrolyte salt,

[0061] The electrolyte sheet 7 may further contain other electrolyte salts in addition to Li[TFSI]. The other electrolyte salts may be at least one selected from the group consisting of lithium salts (excluding Li[TFSI]), sodium salts, calcium salts, and magnesium salts.

[0062] The anion of the other electrolyte salts is 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 - .

[0063] The lithium salt may 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 (where R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0064] The sodium salt may 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 (where R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0065] The calcium salt may 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 (where R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0066] 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).

[0067] From the viewpoint of suitably producing the electrolyte sheet 7, the content of other electrolyte salts may be 10% by mass or more and may be 60% by mass or less based on the total amount of the non-volatile components of the electrolyte composition 14.

[0068] The electrolyte composition 14 further contains oxide particles. By containing oxide particles in the electrolyte composition 14, the tensile strength of the obtained electrolyte sheet 7 can be further increased, and the ionic conductivity of the electrolyte sheet 7 can also be improved.

[0069] 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 kind of particle selected from the group consisting of SiO2, Al2O3, AlOOH, MgO, CaO, ZrO2, TiO2, Li7La3Zr2O 12 , and BaTiO3. Since the oxide particles have polarity, they can promote the dissociation of the electrolyte in the electrolyte sheet 7 and also improve the battery characteristics.

[0070] The oxide particles may 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.

[0071] The oxide particles may have a hydrophobic surface. Usually, the oxide particles have hydroxyl groups on their surfaces and tend to exhibit hydrophilicity. The oxide particles having a hydrophobic surface have fewer hydroxyl groups on the surface than the oxide particles having no hydrophobic surface. Therefore, when using oxide particles having a hydrophobic surface, when the electrolyte sheet 7 contains an ionic liquid described later (for example, N(SO2F)2 as an anion component - , N(SO2CF3)2 -In an ionic liquid having etc., since the ionic liquid is hydrophobic, it is expected that the affinity between the oxide particles and the ionic liquid will be 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. Further, in a secondary battery including an electrolyte sheet containing oxide particles having a hydrophobic surface, the discharge characteristics can be particularly improved.

[0072] Oxide particles having a hydrophobic surface can be obtained, for example, by treating oxide particles showing hydrophilicity with a surface treatment agent capable of imparting a hydrophobic surface. That is, oxide particles having a hydrophobic surface mean oxide particles surface-treated with a surface treatment agent. The surface treatment agent is preferably a silicon-containing compound.

[0073] The oxide particles may be surface-treated with a silicon-containing compound. That is, the oxide particles may be those in which the surface of the oxide particles and the silicon atom of the silicon-containing compound are bonded 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.

[0074] The halogen element in the halogen-containing alkylsilane may be chlorine, fluorine, etc. The chlorine-containing alkylsilane (alkylchlorosilane) may be methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, n-octyldimethylchlorosilane, etc. The fluorine-containing alkylsilane (fluoroalkylsilane) may be trifluoropropyltrimethoxysilane, tridecafluorooctyltrimethoxysilane, etc.

[0075] The alkoxysilane may be methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, dimethoxydiphenylsilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, tetraethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, n-propyltriethoxysilane, or the like.

[0076] The epoxy group-containing silane may be 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, or the like.

[0077] The amino group-containing silane may be N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, or the like.

[0078] The silazane may be hexamethyldisilazane or the like. The siloxane may be dimethyl silicone oil or the like. Those having a reactive functional group (e.g., carboxyl group, etc.) at one end or both ends thereof may also be used.

[0079] The oxide particles having a hydrophobic surface (surface-treated oxide particles) may be those produced by a known method or commercially available products may be used as they are.

[0080] The oxide particles generally may include primary particles (particles that do not constitute secondary particles) that integrally form a single particle as judged from the apparent geometric form, and secondary particles formed by aggregation of a plurality of primary particles.

[0081] From the viewpoint of excellent discharge characteristics of the secondary battery, the specific surface area of the oxide particles is 2 m 2 / g or more and 5 m2 above / g, 10 m 2 above / g, 15 m 2 above / g, or 50 m 2 may be above / g, 500 m 2 below / g, 400 m 2 below / g, 350 m 2 below / g, 300 m 2 below / g, 200 m 2 below / g, 100 m 2 below / g, 90 m 2 below / g, 80 m 2 below / g, or 60 m 2 may be below / g. The specific surface area of the oxide particles means the specific surface area of the entire oxide particles including primary particles and secondary particles, and is measured by the BET method.

[0082] From the viewpoint of improving the conductivity of the secondary battery 1, the average primary particle size (average particle size of primary particles) of the oxide particles is preferably 0.005 μm (5 nm) or more, more preferably 0.01 μm (10 nm) or more, and still more preferably 0.015 μm (15 nm) or more. From the viewpoint of thinning the electrolyte sheet 7, the average primary particle size of the oxide particles is preferably 1 μm or less, more preferably 0.1 μm or less, and still more preferably 0.05 μm or less. The average primary particle size of the oxide particles can be measured by observing the oxide particles with a transmission electron microscope or the like.

[0083] From the viewpoint of easily obtaining the electrolyte sheet 7 excellent in tensile strength and ionic conductivity, the content of the oxide particles is preferably 5% by mass or more, 10% by mass or more, or 15% by mass or more, and preferably 80% by mass or less, 70% by mass or less, or 60% by mass or less, based on the total amount of the non-volatile components of the electrolyte composition 14.

[0084] The dispersion medium may be water, an organic solvent, etc. The organic solvent may 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 so that the electrolyte composition 14 can be applied on the base material 13.

[0085] 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 glyme represented by the following formula (1). R 1 O-(CH2CH2O) k -R 2 (1) [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.]

[0086] The ionic liquid contains the following anion component and cation component. The ionic liquid in this specification is a substance that is liquid at -20°C or higher.

[0087] The anion component of the ionic liquid is not particularly limited, but anions of halogens such as Cl - , Br - , I - ; inorganic anions such as BF4 - , N(SO2F)2 - ([FSI] - ); organic anions such as B(C6H5)4 - , CH3SO2O - , CF3SO2O - , N(SO2C4F9)2 - , N(SO2CF3)2 - ([TFSI] - ), N(SO2C2F5)2 - etc. may be used. The anion component of the ionic liquid preferably contains at least one of the anion components represented by the following formula (2). N(SO2C m F 2m+1 )(SO2C n F 2n+1 ) - (2) [In the formula, m and n each independently represent an integer of 0 to 5. m and n may be the same as or different from each other, and are preferably the same as each other.]

[0088] The anion component represented by 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 anion component of the ionic liquid is more preferably N(SO2C4F9)2 - , CF3SO2O - , [FSI] - , [TFSI] - , and at least one selected from the group consisting of N(SO2C2F5)2 - , and more preferably contains [FSI] - .

[0089] The cation component of the ionic liquid is not particularly limited, but is preferably at least one selected from the group consisting of a chain quaternary onium cation, a piperidinium cation, a pyrrolidinium cation, a pyridinium cation, and an imidazolium cation.

[0090] The chain quaternary onium cation is, for example, a compound represented by the following formula (3).

Chemical formula

[0091] The piperidinium cation is, for example, a nitrogen-containing six-membered cyclic compound represented by the following formula (4). [Chemical formula] [In formula (4), R 7 and R 8 each independently represents an alkyl group having 1 to 20 carbon atoms, or an alkoxyalkyl group represented by R-O-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4). The alkyl group represented by R 7 and R 8 preferably has 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, and still more preferably 1 to 5 carbon atoms.]

[0092] The pyrrolidinium cation is, for example, a five-membered cyclic compound represented by the following formula (5). [Chemical formula] [In formula (5), R 9 and R 10 each independently represents an alkyl group having 1 to 20 carbon atoms, or an alkoxyalkyl group represented by R-O-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4). The alkyl group represented by R 9 and R 10 preferably has 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, and still more preferably 1 to 5 carbon atoms.]

[0093] The pyridinium cation is, for example, a compound represented by the following formula (6). [Chemical formula] [In formula (6), R 11 to R 15 each independently represents an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R-O-(CH2) n - (wherein R represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), or a hydrogen atom. R 11 to R 15The number of carbon atoms of the alkyl group represented by is preferably 1 to 20, more preferably 1 to 10, and still more preferably 1 to 5.

[0094] The imidazolium cation is, for example, a compound represented by the following formula (7). [Chemical formula] [In formula (7), R 16 ~R 20 are each independently an alkyl group having 1 to 20 carbon atoms, an alkoxyalkyl group represented by R-O-(CH2) n - (wherein R represents a methyl group or an ethyl group, 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.

[0095] More specifically, the ionic liquid may 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-methylimidazolium-bis(trifluoromethanesulfonyl)imide (EMI-TFSI), 1-ethyl-3-methylimidazolium-bis(fluorosulfonyl)imide (EMI-FSI), N-methyl-N-propylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py13-TFSI), N-methyl-N-propylpyrrolidinium-bis(fluorosulfonyl)imide (Py13-FSI), N-ethyl-N-methylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (Py12-TFSI), N-ethyl-N-methylpyrrolidinium-bis(fluorosulfonyl)imide (Py12-FSI), 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA), etc.

[0096] In the glyme represented by the above formula (1), in formula (1), R 1 and R 2 each independently represents an alkyl group having 4 or less carbon atoms or a fluoroalkyl group having 4 or less carbon atoms, and k represents an integer of 1 to 6. R 1 and R 2 are each independently preferably a methyl group or an ethyl group.

[0097] Specifically, the glyme may be monoglyme (k = 1), diglyme (k = 2), triglyme (k = 3), tetraglyme (k = 4), pentaglyme (k = 5), or hexaglyme (k = 6).

[0098] When the electrolyte composition 14 contains glyme as a solvent, part or all of the glyme may form a complex with Li[TFSI].

[0099] From the viewpoint of suitably producing the electrolyte sheet 7, the content of the solvent may be 10% by mass or more, 80% by mass or less, or 60% by mass or less based on the total amount of the non-volatile components of the electrolyte composition 14.

[0100] When the electrolyte composition 14 contains a solvent, the total content of Li[TFSI] and the solvent is preferably 10% by mass or more, more preferably 25% by mass or more, still more preferably 40% by mass or more based on the total amount of the non-volatile components of the electrolyte composition 14 from the viewpoint of further improving the conductivity of the secondary battery 1 and suppressing capacity degradation, and is preferably 80% by mass or less, more preferably 70% by mass or less from the viewpoint of suppressing a decrease in the strength of the electrolyte sheet 7.

[0101] When the electrolyte composition 14 contains a solvent, the concentration of Li[TFSI] per unit volume of the solvent is preferably 0.5 mol / L or more, more preferably 0.7 mol / L or more, still more preferably 1.0 mol / L or more from the viewpoint of improving the charge-discharge characteristics of the secondary battery 1, and is preferably 2.0 mol / L or less, more preferably 1.8 mol / L or less, still more preferably 1.6 mol / L or less.

[0102] The electrolyte composition 14 may contain other components. Examples of the other components include fibers such as cellulose fibers, resin fibers, and glass fibers. The content of the other components may be 0.1 to 20% by mass based on the total amount of the non-volatile components of the electrolyte composition 14.

[0103] In this step, in one embodiment, as shown in FIG. 3(b), the electrolyte composition 14 is applied onto one surface 13a of the base material. Thereby, a laminate 20 including the base material 13 and the electrolyte composition 14 applied onto the base material 13 is produced. Examples of the method for applying the electrolyte composition 14 onto the base material 13 include a method of applying using an applicator, a method of applying by spraying, and the like.

[0104] The thickness when applying the electrolyte composition 14 is preferably 5 μm or more, more preferably 10 μm or more, and still more preferably 15 μm or more from the viewpoints of further increasing the tensile strength and further improving the safety. The thickness when applying the electrolyte composition 14 is preferably 200 μm or less, more preferably 150 μm or less, and still more preferably 100 μm or less from the viewpoints of reducing the internal resistance of the secondary battery 1 and further improving the high-current characteristics. The thickness when applying the electrolyte composition 14 corresponds to the thickness of the electrolyte sheet 7.

[0105] Subsequently, the dispersion medium contained in the electrolyte composition 14 applied onto the base material 13 is volatilized to obtain the electrolyte sheet 7 (FIG. 3(c)). Thereby, a laminated sheet 30A including the base material 13 and the electrolyte sheet 7 formed on the base material 13 can be obtained.

[0106] In the volatilization of the dispersion medium, for example, a laminate including the base material 13 and the electrolyte composition applied onto the base material 13 may be put into a drying furnace or the like to volatilize the dispersion medium in the electrolyte composition 14.

[0107] The temperature at which the dispersion medium is volatilized (volatilization temperature) is 100°C or higher. Thereby, the electrolyte sheet 7 excellent in tensile strength can be formed. The volatilization temperature may be higher than 100°C, preferably 105°C or higher, 110°C or higher, 115°C or higher, or 120°C or higher. When the volatilization temperature is higher than 100°C, the cycle characteristics of the secondary battery can be improved when the electrolyte sheet is used in the secondary battery. Further, when the volatilization temperature is higher than 100°C, the productivity or production efficiency can be increased, and the production cost can also be reduced. The volatilization temperature may be 150°C or lower, 145°C or lower, or 140°C or lower from the viewpoints of improving the ionic conductivity of the electrolyte sheet 7, increasing the light transmittance, and reducing the haze. In this specification, the temperature at which the dispersion medium is volatilized refers to the atmospheric temperature at the time of volatilization of the dispersion medium. For example, when a drying furnace is used, it refers to the temperature inside the drying furnace.

[0108] The time for volatilizing the dispersion medium (volatilization time) can be appropriately adjusted according to the volatilization temperature and production scale.

[0109] In the step of forming the electrolyte composition 14 into a sheet shape and volatilizing the dispersion medium at 100°C or higher, as described above, after applying the electrolyte composition 14 on the base material 13 to obtain the laminate 20, the laminate 20 may be placed in a drying furnace to volatilize the dispersion medium. However, as another example, the laminated sheet 30A may be continuously obtained by volatilizing the dispersion medium in parallel while applying the electrolyte composition 14 on the base material 13 in an environment of 100°C or higher. In this case, the volatilization time of the dispersion medium starts from the time when the electrolyte composition 14 is applied on the base material 13.

[0110] When manufacturing the secondary battery 1, the electrolyte sheet 7 can be obtained by peeling the base material 13 from the laminated sheet 30A (Fig. 3(d)).

[0111] The laminated sheet can also be continuously manufactured while being wound into a roll. In that case, since the surface of the electrolyte sheet 7 comes into contact with the back surface of the base material 13 and a part of the electrolyte sheet 7 adheres to the base material 13, the electrolyte sheet 7 may be damaged. In order to prevent such a situation, as another embodiment, the electrolyte sheet 7 may be manufactured by producing a laminated sheet having a three-layer structure in which a protective material is provided on the side opposite to the base material 13 of the electrolyte sheet 7.

[0112] FIG. 4 is a schematic cross-sectional view showing a laminated sheet according to another embodiment. As shown in FIG. 4, this laminated sheet 30B further includes a protective material 15 on the surface 7a on the side opposite to the base material 13 of the electrolyte sheet 7, as shown in FIG. 3(b).

[0113] The laminated sheet 30B can be obtained by laminating the protective material 15 so as to cover one surface on the side opposite to the base material 13 of the electrolyte sheet 7 after obtaining a laminate (laminated sheet 30A) including the base material 13 and the electrolyte sheet 7 by the method described above.

[0114] The protective material 15 may be easily peelable from the electrolyte sheet 7, and is preferably a non-polar resin film such as polyethylene, polypropylene, or polytetrafluoroethylene. When a non-polar resin film is used, the electrolyte sheet 7 and the protective material 15 do not adhere to each other, and the protective material 15 can be easily peeled off.

[0115] From the viewpoint of ensuring strength while reducing the volume of the entire laminated sheet 30B, the thickness of the protective material 15 is preferably 5 μm or more, more preferably 10 μm, and is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 30 μm or less.

[0116] 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 laminated sheet 30B.

[0117] In addition to being excellent in tensile strength, the electrolyte sheet 7 obtained by the manufacturing method described above is suppressed in coloring and has 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.

[0118] The light transmittance of the electrolyte sheet 7 can be measured by a spectrophotometer (for example, SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7361-1 (Test method for total light transmittance of plastic - transparent materials). In this case, the size of the test piece cut out from the electrolyte sheet 7 may be adjusted according to the apparatus used. The light transmittance in this specification means the average value α of the light transmittance measured every 5 nm in the wavelength range of 380 to 780 nm for one test piece, and the average value of α obtained for three test pieces.

[0119] When measuring the light transmittance of the electrolyte sheet included in the secondary battery, the secondary battery in the state of 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.

[0120] The light transmittance of the electrolyte sheet 7 may be, for example, 90% or more, 90.5% or more, 91% or more, or 91.5% or more, and may be 95% or less, 94.5% or less, 94% or less, or 93.5% or less.

[0121] The haze of the electrolyte sheet 7 can be measured by a spectrophotometer (for example, SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136 (Method for determining haze of plastic - transparent materials). At this time, among the transmitted light passing through the test piece cut out from the electrolyte sheet 7, the percentage of the transmitted light deviated from the incident light by 2.5° or more due to forward scattering is defined as the haze. The haze in this specification means the average value β of the haze measured every 5 nm in the wavelength range of 380 to 780 nm for one test piece, and the average value of β obtained for three test pieces. The size of the test piece may be adjusted according to the apparatus used, similar to the measurement of the above light transmittance.

[0122] The haze of the electrolyte sheet 7 may be, for example, 15% or more, 20% or more, 25% or more, or 30% or more, and may be 45% or less, 40% or less, 38% or less, or 36% or less.

[0123] The yellowness of the electrolyte sheet 7 can be measured by a C light source (Illuminant C) in accordance with ASTM E313-05 using a spectral haze meter (for example, SH7000 manufactured by Nippon Denshoku Industries Co., Ltd.). The yellowness described in this specification is the average value of the yellowness measured for three test pieces. The size of the test piece may be adjusted according to the apparatus used, similar to the measurement of the light transmittance described above.

[0124] The yellowness of the electrolyte sheet 7 may be, for example, 18 or less, 14 or less, 8 or less, 4 or less, or 2 or less, and may also be 0 or more, 0.1 or more, or 0.2 or more.

[0125] Next, a method for manufacturing the secondary battery 1 including the above-described electrolyte sheet 7 will be described. A method for manufacturing 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.

[0126] In the first step, for example, the positive electrode 6 is obtained by dispersing a material used for the positive electrode mixture layer in a dispersion medium using a kneader, a disperser, etc. to obtain a slurry-like positive electrode mixture, and then applying this positive electrode mixture onto the positive electrode current collector 9 by a doctor blade method, a dipping method, a spray method, etc., and then volatilizing the dispersion medium. After volatilizing the dispersion medium, a compression molding step by roll pressing may be provided as necessary. The positive electrode mixture layer 10 may be formed as a multi-layered positive electrode mixture layer by performing the steps from the application of the positive electrode mixture described above to the volatilization of the dispersion medium a plurality of times.

[0127] The dispersion medium used in the first step may be water, N-methyl-2-pyrrolidone (NMP), or the like.

[0128] In the second step, the method of forming the negative electrode mixture layer 12 on the negative electrode current collector 11 may be the same as the method of the first step described above.

[0129] In the third step, in one embodiment, the electrolyte sheet 7 obtained by the manufacturing method described above is disposed between the positive electrode 6 and the negative electrode 8. At this time, the electrolyte sheet 7 may be disposed so as to be in contact with the positive electrode mixture layer 10 in the positive electrode 6 and the negative electrode mixture layer 12 in the negative electrode 8.

[0130] 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 by, for example, lamination. Thereby, the secondary battery 1 including the positive electrode 6, the negative electrode 8, and the electrolyte sheet 7 provided between the positive electrode 6 and the negative electrode 8 can be obtained.

[0131] In the secondary battery 1 thus obtained, since the electrolyte sheet 7 having excellent tensile strength and further excellent ionic conductivity is used, the battery performance such as charge and discharge performance is excellent.

[0132] As another embodiment, the electrolyte sheet obtained by the manufacturing method described above can also be used for a so-called bipolar type secondary battery. FIG. 5 is an exploded perspective view showing an embodiment of an electrode group of a bipolar type secondary battery. The electrode group 2B includes a positive electrode 6, a first electrolyte sheet 7, a bipolar electrode 16, a second electrolyte sheet 7, and a negative electrode 8 in this order. 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.

[0133] The bipolar electrode current collector 17 is formed of, for example, a single metal such as aluminum, stainless steel, or titanium, or a clad material formed by roll-bonding aluminum and copper or stainless steel and copper.

[0134] 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 of the same type or different types in terms of composition, and preferably, they are of the same type as each other.

[0135] This bipolar secondary battery also uses the above-described electrolyte sheet 7 that is excellent in tensile strength and further excellent in ionic conductivity, and thus has excellent battery performance such as charge and discharge performance.

Examples

[0136] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples.

[0137] <Example 1> Li[TFSI] was dissolved in EMI[FSI] at a concentration of 1.5 mol / L. 43 parts by mass of this solution, 23 parts by mass of SiO2 particles (AEROSIL OX50, manufactured by Nippon Aerosil Co., Ltd.), and 34 parts by mass of a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP) were mixed, and then NMP, which is a dispersion medium, was added to prepare an electrolyte composition. The electrolyte composition was applied onto a polyethylene terephthalate substrate using an applicator in a drying oven with the temperature inside the oven set to 100°C. At this time, the thickness of the dried electrolyte composition was adjusted to be 20 μm. After application, the laminate sheet on which the electrolyte sheet was formed on the substrate was obtained by allowing the dispersion medium to volatilize by standing in the drying oven at the same temperature for 30 minutes. That is, in the production of the laminate sheet of this example, the volatilization temperature of the dispersion medium was 100°C, which is the temperature inside the drying oven, and the volatilization time of the dispersion medium was 30 minutes.

[0138] <Examples 2 to 6> In Example 1, the volatilization temperature of the dispersion medium (the temperature in the drying oven) was changed to the temperature shown in Table 1, and a laminated sheet was produced without changing other conditions.

[0139] <Comparative Example 1> In Example 1, the volatilization temperature of the dispersion medium (the temperature in the drying oven) was changed to 80°C, and a laminated sheet was produced without changing other conditions.

[0140] <Comparative Example 2> In Example 1, Li[TFSI] was changed to Li[FSI], and a laminated sheet was produced without changing other conditions.

[0141] <Physical Property Evaluation of Electrolyte Sheet> For the material obtained by peeling the base material from the laminated sheets of the examples and comparative examples (electrolyte sheet), each physical property was evaluated by the following method. The respective results are shown in Table 1.

[0142] Regarding the tensile strength, the electrolyte sheet was cut into a width of 5 mm, clamped with a chuck, and then fixed to a pedestal with tape so as to have a length of 20 mm. Then, the electrolyte sheet was pulled using a force gauge (FGP-5, manufactured by Nidec-Shimpo Corporation), and the strength at the time when the electrolyte sheet broke was measured.

[0143] Regarding the light transmittance, yellowness (illuminant C), and haze, they were measured by the method described above using a spectro haze meter (SH7000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0144] Regarding the ionic conductivity, the electrolyte sheet was placed in a two-electrode sealed cell (HS cell, manufactured by Housui Co., Ltd.), and measured using an alternating current impedance measuring device (model 1260, manufactured by Solartron). At room temperature (25°C), the alternating current impedance was measured in the range of 1 Hz to 10 MHz at 10 mV. The ionic conductivity was calculated using the following formula from the resistance value obtained from the width of the arc of the Nyquist plot. The placement of the electrolyte sheet in the sealed cell was carried out in a dry room. Ionic conductivity [S / cm] = (1 / Resistance [Ω]) × (Thickness of electrolyte sheet [cm] / Area of electrolyte sheet [cm 2 )

[0145]

Table 1

Explanation of symbols

[0146] 1…Secondary battery, 6…Positive electrode, 7…Electrolyte sheet, 8…Negative electrode, 9…Positive electrode current collector, 10…Positive electrode active material layer, 11…Negative electrode current collector, 12…Negative electrode active material layer.

Claims

1. 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 105°C or higher and 150°C or lower is provided. Li[TFSI] is lithium bis(trifluoromethanesulfonyl)imide. The composition further contains an ionic liquid. The ionic liquid is 1-ethyl-3-methylimidazolium-bis(fluorosulfonyl)imide (EMI-FSI). A method for manufacturing an electrolyte sheet, wherein the concentration of Li[TFSI] per unit volume of the ionic liquid is 1.0 mol / L or higher and 1.6 mol / L or lower.

2. A step of forming a positive electrode active material layer on a positive electrode current collector to obtain a positive electrode. A step of forming a negative electrode active material layer on a negative electrode current collector to obtain a negative electrode. A step of disposing an electrolyte sheet obtained by the manufacturing method according to Claim 1 between the positive electrode and the negative electrode. A method for manufacturing a secondary battery comprising the above steps.

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