Separator for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries

Abstract

To provide a separator for a nonaqueous electrolyte secondary battery, having good wettability of an ionic liquid electrolyte.SOLUTION: A separator for a nonaqueous electrolyte secondary battery of the present invention includes a porous membrane including polyolefin. The porous membrane includes a zwitterionic surfactant on a pore wall surface or in a pore wall. The zwitterionic surfactant is a betaine derivative or a sulfobetaine derivative. A hydrophobic group of the zwitterionic surfactant is an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, alkylsulfonyl group or alkenylsulfonyl group, or a functional group comprising a combination of these functional groups, the hydrophobic group having a carbon number of 10 or more and 16 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a separator for non-aqueous electrolyte secondary batteries and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, the use of non-aqueous electrolyte secondary batteries has been rapidly expanding. Lithium-ion batteries, in particular, which have a high energy density, are being installed not only in small electronic devices such as smartphones and laptops, but also in large devices such as electric vehicles and emergency power supplies. In typical lithium-ion batteries, a porous polyolefin membrane is used as the separator. One reason for this is that the porous polyolefin membrane possesses excellent shutdown capabilities. The shutdown function is a mechanism in which, when a lithium-ion battery overheats abnormally, the separator softens, and the pores in the separator close, thereby blocking the movement of lithium ions between the positive and negative electrodes. Since polyolefins soften at low temperatures, a porous polyolefin membrane can provide excellent shutdown capabilities, thereby improving the safety of lithium-ion batteries.

[0003] The electrolyte in typical lithium-ion batteries is an organic electrolyte solution in which lithium salts are dissolved in a flammable organic liquid. Furthermore, lithium-ion batteries can overheat due to overcharging or short circuits between the positive and negative electrodes. Therefore, lithium-ion batteries pose a risk of fire and combustion. To prevent lithium-ion batteries from igniting or burning, it has been proposed to use an ionic liquid electrolyte, which is an ionic liquid in which a lithium salt is dissolved, as the electrolyte for lithium-ion batteries (for example, Patent Documents 1 and 2). Ionic liquids are liquids composed of anionic and cationic components and are generally non-flammable or flame-retardant. Therefore, the safety of lithium-ion batteries can be improved by using an ionic liquid electrolyte. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-195129 [Patent Document 2] Japanese Patent Publication No. 2018-116840 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, because ionic liquids have higher viscosity and surface tension than organic liquids, ionic liquid electrolytes cannot be impregnated into typical polyolefin porous membranes. This invention has been made in view of these circumstances, and provides a separator for a non-aqueous electrolyte secondary battery with good wettability of an ionic liquid electrolyte. [Means for solving the problem]

[0006] The present invention provides a separator for a non-aqueous electrolyte secondary battery, comprising a porous membrane containing a polyolefin, wherein the porous membrane has a zwitterionic surfactant on its pore wall surface or within the pore wall, the zwitterionic surfactant is a betaine derivative or a sulfobetaine derivative, the hydrophobic group of the zwitterionic surfactant is a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, or an alkenylsulfonyl group or a combination thereof, and the number of carbon atoms of the hydrophobic group is 10 to 16. [Effects of the Invention]

[0007] The separator for non-aqueous electrolyte secondary batteries of the present invention, having the above-described characteristics, exhibits good wettability to ionic liquid electrolytes. This was revealed by experiments conducted by the inventors of the present invention. [Brief explanation of the drawing]

[0008] [Figure 1] This is a partial cross-sectional view of a separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. [Figure 2] This is a schematic cross-sectional view of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. [Modes for carrying out the invention]

[0009] The separator for a non-aqueous electrolyte secondary battery of the present invention comprises a porous membrane containing a polyolefin, wherein the porous membrane has a zwitterionic surfactant on its pore wall surface or within the pore wall, the zwitterionic surfactant is a betaine derivative or a sulfobetaine derivative, the hydrophobic group of the zwitterionic surfactant is a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, or an alkenylsulfonyl group or a combination thereof, and the number of carbon atoms of the hydrophobic group is 10 or more and 16 or less. Preferably, the pores of the porous membrane have an average pore diameter of 0.01 μm or more and 0.5 μm or less. This helps to suppress the occurrence of short circuits between the positive electrode and the negative electrode.

[0010] The present invention also provides a non-aqueous electrolyte secondary battery comprising the separator for non-aqueous electrolyte secondary battery of the present invention, a positive electrode, a negative electrode, and an electrolyte containing an ionic liquid. Preferably, the ionic liquid comprises a cationic component and an anionic component, wherein the cationic component is a quaternary ammonium ion, the quaternary ammonium ion has at least a portion of a chain-like or cyclic structure, and the anionic component is a bis(fluorosulfonyl)amide ion.

[0011] One embodiment of the present invention will be described below with reference to the drawings. The configurations shown in the drawings and the following description are illustrative, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0012] Separator for non-aqueous electrolyte secondary batteries Figure 1 is a partial cross-sectional view of the separator for a non-aqueous electrolyte secondary battery according to this embodiment. The separator 4 for a non-aqueous electrolyte secondary battery of the present embodiment includes a porous membrane 8 containing a polyolefin, the porous membrane 8 has a zwitterionic surfactant on a pore wall surface or in a pore wall, the zwitterionic surfactant is a betaine derivative or a sulfobetaine derivative, a hydrophobic group of the zwitterionic surfactant is a functional group constituted by an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination thereof, and the number of carbon atoms in the hydrophobic group is 10 or more and 16 or less.

[0013] The separator 4 for a non-aqueous electrolyte secondary battery is a separator used for producing a non-aqueous electrolyte secondary battery or a separator incorporated in a non-aqueous electrolyte secondary battery. The film thickness of the separator 4 is not particularly limited, but is preferably 5 µm or more and 30 µm or less. Setting the film thickness of the separator 4 to 5 µm or more can improve the electrical insulation of the separator 4, and can improve the safety of the non-aqueous electrolyte secondary battery. In addition, setting the film thickness of the separator 4 to 30 µm or less can shorten the movement distance (diffusion distance) of ions between the positive electrode and the negative electrode, and can reduce the internal resistance of the non-aqueous electrolyte secondary battery.

[0014] The porosity of the separator 4 is not particularly limited, but is preferably 20 volume% or more and 80 volume% or less. Setting the porosity of the separator 4 to 20 volume% or more can increase the electrolyte retention amount, and can improve the charge-discharge cycle characteristics of the non-aqueous electrolyte secondary battery. In addition, setting the porosity of the separator 4 to 80 volume% or less can improve the electrical insulation of the separator 4, and can improve the safety of the non-aqueous electrolyte secondary battery.

[0015] The air permeability of separator 4 (air permeability by the Gurley test machine method) is not particularly limited, but is preferably 50 to 500 sec / 100cc. By setting the air permeability of separator 4 to 50 sec / 100cc or higher, the electrical insulation properties of separator 4 can be improved, thereby improving the safety of the non-aqueous electrolyte secondary battery. Furthermore, by setting the air permeability of separator 4 to 500 sec / 100cc or lower, the ionic liquid electrolyte can be quickly impregnated into separator 4.

[0016] The separator 4 comprises a porous membrane 8 containing a polyolefin. This allows the separator 4 to have an excellent shutdown function, providing a separator with superior safety. Examples of polyolefins used as the material for the porous membrane 8 include polyethylene, polypropylene, and polybutene. The porous membrane 8 may contain one of these polyolefins individually or in a mixture of several types. The porous membrane 8 may also contain components other than polyolefins. Examples of components other than polyolefins include polyethers, polyesters, polyamides, polyimides, polyamide-imides, and polyphenylene sulfides. The proportion of polyolefin contained in the porous membrane 8 is not particularly limited, but it is preferably 50% by mass or more of the total porous membrane, and more preferably 80% by mass or more. This improves the shutdown function of the separator 4 and enhances the safety of the non-aqueous electrolyte secondary battery. The porous membrane 8 may be a polyolefin porous membrane made of polyolefin. Furthermore, the porous membrane 8 may have a laminated structure in which a polyethylene layer and a polypropylene layer are laminated.

[0017] The porous membrane 8 may be a microporous membrane having pores of 0.5 μm or less. The pores of the porous membrane 8 can have an average pore diameter of, for example, 0.01 μm or more and 0.5 μm or less. If the average pore diameter is too small, the internal resistance of the battery will increase, and if the average pore diameter is too large, a short circuit between the positive and negative electrodes will be more likely. The average pore diameter can be calculated based on the results of pore diameter distribution measurement by the mercury intrusion method. The porous membrane 8 is, for example, a dry one-component microporous membrane, a wet two-component porous membrane, or a wet three-component microporous membrane. Dry one-component microporous membranes are produced, for example, by cooling and stretching a molten polymer to form a film, with micropores formed by stretching during the cooling process. Wet two-component porous membranes are produced, for example, by cooling and stretching a molten polymer that has been pre-mixed with a plasticizer to form a film, and then extracting the plasticizer. Wet three-component microporous membranes are produced, for example, by cooling and stretching a molten polymer that has been pre-mixed with a plasticizer and filler fine particles to form a film, and then extracting the plasticizer and filler particles.

[0018] The porous membrane 8 may contain inorganic fine particles mixed with the polyolefin. Examples of inorganic fine particles include alumina particles, silica particles, and titania particles. The porous membrane 8 may contain a surfactant mixed with the polyolefin.

[0019] The separator 4 may be a laminated separator in which a porous membrane 8 and a porous resin layer are laminated. The porous resin layer may be, for example, a liquid crystal polyester resin layer, a polyphenylene ether resin layer, an aromatic polyamide resin layer, a polyimide resin layer, a polyamide-imide resin layer, a heat-resistant acrylic resin layer, or a crosslinkable polymer layer.

[0020] The separator 4 may have a porous heat-resistant layer provided on the porous membrane 8. The heat-resistant layer (porous heat-resistant layer) may be provided on only one main surface (front or back) of the porous membrane 8, or on both main surfaces (front and back) of the porous membrane 8. The heat-resistant layer is, for example, a porous layer in which a plurality of inorganic particles are bonded together by a binder. By having a heat-resistant layer in the separator 4, the heat resistance of the separator 4 can be improved, and the safety of the non-aqueous electrolyte secondary battery can be improved.

[0021] The heat-resistant layer may contain inorganic particles. Examples of inorganic particles include magnesium oxide, aluminum oxide, silicon oxide, calcium oxide, titanium oxide, magnesium hydroxide, aluminum hydroxide, calcium hydroxide, magnesium carbonate, and calcium carbonate. The heat-resistant layer may contain a binder. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylonitrile rubber, and polyacrylic acid. The heat-resistant layer may also contain a thickener. Examples of thickeners include carboxymethylcellulose. The heat-resistant layer may have surfactants on its main surface and pore wall surface.

[0022] The thickness of the heat-resistant layer is not particularly limited, but it is preferably between 1 μm and 10 μm. By making the thickness of the heat-resistant layer 1 μm or more, the heat resistance of the separator 4 can be improved, thereby improving the safety of the non-aqueous electrolyte secondary battery. Furthermore, by making the thickness of the heat-resistant layer 10 μm or less, the ion migration distance (diffusion distance) between the positive electrode and the negative electrode can be shortened, thereby reducing the internal resistance of the non-aqueous electrolyte secondary battery.

[0023] The porous membrane 8 containing polyolefin has a zwitterionic surfactant 9 (amphoteric surfactant 9) on its pore wall or within its pore walls. The zwitterionic surfactant 9 is a surfactant that dissociates when dissolved in water or the like to form an atomic group that possesses both anionic and cationic properties. The zwitterionic surfactant has both a hydrophobic group and a hydrophilic group. The presence of a zwitterionic surfactant 9 on or within the pore walls of the porous membrane 8 reduces the difference in surface tension between the pore walls of the porous membrane 8 and the ionic liquid electrolyte, thereby providing a separator with good wettability of the ionic liquid electrolyte. For example, as shown in Figure 1, the zwitterionic surfactant 9 may form a surfactant layer on the wall surface of the pores 1 of the porous membrane 8.

[0024] The zwitterionic surfactant 9 is a betaine derivative or a sulfobetaine derivative, and the hydrophobic group of the zwitterionic surfactant 9 is a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, or an alkenylsulfonyl group, or a combination thereof. The number of carbon atoms in the hydrophobic group of the zwitterionic surfactant 9 is between 10 and 16. Because the porous membrane 8 has such zwitterionic surfactant 9 on or within the pore walls, the separator 4 has good wettability with the ionic liquid electrolyte. This was revealed by experiments conducted by the inventors of the present invention.

[0025] When the zwitterionic surfactant 9 is a betaine derivative, the zwitterionic surfactant 9 may also be a compound represented by the following chemical formula (1). This allows the surfactant 9 to have appropriate hydrophilicity, providing a separator with good wettability of the ionic liquid electrolyte. Examples of the zwitterionic surfactant 9 include lauryl betaine.

[0026] [ka] [In the formula, at least one of R1, R2, and R3 represents the hydrophobic group, and n is between 1 and 5.]

[0027] When the zwitterionic surfactant 9 is a sulfobetaine derivative, the zwitterionic surfactant 9 may also be a compound represented by the following chemical formula (2). This allows the surfactant 9 to have appropriate hydrophilicity, providing a separator with good wettability of the ionic liquid electrolyte. Examples of zwitterionic surfactants 9 include decyl sulfobetaine, lauryl sulfobetaine, myristyl sulfobetaine, and palmityl sulfobetaine.

[0028] [ka] [In the formula, at least one of R1, R2, and R3 represents the hydrophobic group, and n is between 1 and 5.]

[0029] The zwitterionic surfactant 9 has a hydrophobic group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, or an alkenylsulfonyl group, or a combination thereof. Here, the number of carbon atoms in the hydrophobic group is between 10 and 16. This allows the zwitterionic surfactant 9 to have appropriate hydrophobicity, and provides a separator with good wettability for the ionic liquid electrolyte.

[0030] The mass ratio (Y / X) of surfactant 9(Y) to separator 4(X) is, for example, between (1 / 100) and (20 / 100). By setting the mass ratio (Y / X) to (1 / 100) or higher, the surfactant 9 can be distributed throughout the entire pore wall surface of separator 4. As a result, the ionic liquid electrolyte can be rapidly impregnated into separator 4. Furthermore, by setting the mass ratio (Y / X) to (20 / 100) or lower, it is possible to prevent the pores of separator 4 from being blocked by the surfactant 9, thereby reducing the internal resistance of the non-aqueous electrolyte secondary battery.

[0031] A porous membrane 8 containing a zwitterionic surfactant 9 in its pore walls can be fabricated, for example, by cooling and stretching a molten polymer to which a zwitterionic surfactant has been added to form a film. This allows the pore walls of the porous membrane 8 to contain the zwitterionic surfactant 9. This zwitterionic surfactant 9 is also present on the pore wall surface, improving the wettability of the ionic liquid electrolyte to the pore wall surface of the porous membrane 8.

[0032] Furthermore, a separator 4 having a zwitterionic surfactant on its pore walls can be manufactured by impregnating a separator film containing a porous membrane 8 with a solution containing a zwitterionic surfactant 9, and then drying the separator film. This allows the pore walls of the porous membrane 8 to be coated with the zwitterionic surfactant. In addition, if the separator film includes a porous heat-resistant layer, the pore walls of the porous heat-resistant layer are also coated with the zwitterionic surfactant. The concentration of the surfactant solution used to prepare separator 4 is preferably 0.5 wt% to 10 wt%. If the concentration is too low, there is a risk that parts of the separator 4's wall surface will not be covered, and if the concentration is too high, the surfactant may clog the pores of separator 4. The solvent for the solution containing surfactant 9 is, for example, water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, N-methyl-2-pyrrolidone, etc.

[0033] Nonaqueous electrolyte secondary battery Figure 2 is a schematic cross-sectional view of the non-aqueous electrolyte secondary battery of this embodiment. The non-aqueous electrolyte secondary battery 20 of this embodiment comprises the separator 4 of this embodiment described above, a positive electrode 2, a negative electrode 3, and an electrolyte 5, the electrolyte 5 containing an ionic liquid. The non-aqueous electrolyte secondary battery 20 may also be a lithium-ion battery. Here, we will describe the case where the non-aqueous electrolyte secondary battery 20 is a lithium-ion battery. The non-aqueous electrolyte secondary battery 20 may be a coin-type battery, a pouch-type battery, or a metal can-type battery. Further, the electrode laminate composed of the positive electrode 2, the negative electrode 3, and the separator 4 may have a stacked structure or a wound structure.

[0034] The positive electrode 2 includes a positive electrode current collector sheet 6 and a porous positive electrode active material layer 7 provided on one or both main surfaces of the positive electrode current collector sheet 6. The positive electrode current collector sheet 6 is a sheet serving as a base material for providing the positive electrode active material layer 7, and is a conductor that electrically connects the positive electrode terminal of the battery to the positive electrode active material layer 7. Further, a part of the positive electrode current collector sheet 6 may serve as the positive electrode terminal of the battery. The positive electrode current collector sheet 6 is, for example, aluminum foil, stainless steel foil, or the like.

[0035] The positive electrode active material layer 7 may be provided on one surface of the positive electrode current collector sheet 6, or may be provided on both surfaces of the positive electrode current collector sheet 6 respectively. The positive electrode active material layer 7 is a porous layer in which a plurality of positive electrode active material particles are bound together by a binder. Thereby, the positive electrode active material layer 7 can have pores between the positive electrode active material particles. These pores are filled with the electrolytic solution 5, and an electrode reaction proceeds on the surfaces of the positive electrode active material particles. For example, when charging a lithium ion battery, lithium atoms in the positive electrode active material particles become lithium ions (Li + ) and are released into the electrolytic solution, and when discharging a lithium ion battery, lithium ions in the electrolytic solution are inserted as lithium atoms into the positive electrode active material particles.

[0036] The positive electrode active material is a substance that is directly involved in the transfer of electrons accompanied by charge transfer in the positive electrode. The positive electrode active material includes, for example, layered LiCoO2, LiNiO2, LiMnO2, LiNi x Mn y O2 (x+y=1), LiNi x Co y Mn z O2 (x+y+z=1), LiNi x Co y Al z O2 (x+y+z=1), Li2MnO3, spinel-type LiMnO2, LiNix Mn y O2(x+y=1), olivine-type LiFePO4, LiMn x Fe y Examples include PO4(x+y=1). The positive electrode active material layer 7 can contain one of these positive electrode active materials individually or in a mixture of multiple materials.

[0037] The positive electrode active material particles may have a conductive film on their surface. This improves the conductivity of the particle surface where the electrode reaction proceeds, and reduces the internal resistance of the positive electrode 2. The conductive film is, for example, a carbon film such as graphite, soft carbon, hard carbon, or carbon black.

[0038] The positive electrode active material layer 7 may contain a conductive agent. This can improve the conductivity of the positive electrode active material layer 7 and reduce the internal resistance of the positive electrode. Examples of conductive agents include graphite, soft carbon, hard carbon, and carbon black.

[0039] The positive electrode active material layer 7 may contain a binder. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylonitrile rubber, and polyacrylic acid. The positive electrode active material layer 7 may also contain a thickener. Examples of thickeners include carboxymethylcellulose.

[0040] The thickness of the positive electrode active material layer 7 is not particularly limited, but is preferably 10 to 100 μm. By making the thickness of the positive electrode active material layer 7 10 μm or more, the amount of positive electrode active material contained in the positive electrode can be increased, thereby increasing the capacity of the non-aqueous electrolyte secondary battery. In addition, the positive electrode active material layer 7 can be easily formed by coating. By making the thickness of the positive electrode active material layer 7 100 μm or less, the distance lithium ions travel (diffusion distance) between the interface between the positive electrode active material layer 7 and the positive electrode current collector sheet 6 and the surface of the positive electrode active material layer 7 can be shortened, thereby suppressing a shortage of lithium ions in the electrolyte 5 in the pores near the interface during discharge and an excess of lithium ions in the electrolyte 5 in the pores near the interface during charging.

[0041] The method for preparing the positive electrode 2 involves, for example, dispersing a positive electrode mixture, which is a mixture of positive electrode active material particles, a conductive agent, and a binder, in a dispersion medium to prepare a positive electrode slurry. This positive electrode slurry is then applied to the positive electrode current collector sheet 6, the dispersion medium is evaporated, and the positive electrode 2 is produced by pressing. Examples of dispersion media used in preparing the slurry include water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, and N-methylpyrrolidone.

[0042] The negative electrode 3 comprises a negative electrode current collector sheet 11 and a porous negative electrode active material layer 12 provided on one or both main surfaces of the negative electrode current collector sheet 11.

[0043] The negative electrode current collector sheet 11 is a base sheet for providing the negative electrode active material layer 12, and is a conductor that electrically connects the negative electrode battery terminal and the negative electrode active material layer 12. A portion of the negative electrode current collector sheet 11 may also be the negative electrode battery terminal. The negative electrode current collector sheet 11 can be made of, for example, copper foil, nickel foil, or stainless steel foil.

[0044] The negative electrode active material layer 12 may be provided on one side of the negative electrode current collector sheet 11, or on both sides of the negative electrode current collector sheet 11. The negative electrode active material layer 12 is a porous layer in which a plurality of negative electrode active material particles are bonded together by a binder. As a result, the negative electrode active material layer 12 can have pores between the negative electrode active material particles. These pores are filled with the electrolyte 5, and an electrode reaction proceeds on the surface of the negative electrode active material particles. For example, when charging a lithium-ion battery, lithium ions (Li) in the electrolyte... + ) are inserted as lithium atoms into the negative electrode active material particles, and when the lithium-ion battery is discharged, the lithium atoms in the negative electrode active material particles are released into the electrolyte 5 as lithium ions.

[0045] The negative electrode active material is a substance that directly participates in the transfer of electrons accompanied by charge transfer at the negative electrode 3. Examples of negative electrode active materials include graphite, soft carbon, hard carbon, carbon black, lithium titanate, and silicon alloy. The negative electrode active material layer 12 may contain one of these negative electrode active materials individually or in a mixture of multiple materials.

[0046] The negative electrode active material layer 12 may contain a binder. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylonitrile rubber, and polyacrylic acid. The negative electrode active material layer 12 may also contain a thickener. Examples of thickeners include carboxymethylcellulose.

[0047] The thickness of the negative electrode active material layer 12 is not particularly limited, but is preferably 10 to 100 μm. By making the thickness of the negative electrode active material layer 12 10 μm or more, the amount of negative electrode active material contained in the negative electrode 3 can be increased, thereby increasing the capacity of the lithium-ion battery. In addition, the negative electrode active material layer 12 can be easily formed by coating. By making the thickness of the negative electrode active material layer 12 100 μm or less, the distance lithium ions travel (diffusion distance) between the interface between the negative electrode active material layer 12 and the negative electrode current collector sheet 11 and the surface of the negative electrode active material layer 12 can be shortened, thereby suppressing a shortage of lithium ions in the electrolyte in the pores near the interface during charging and an excess of lithium ions in the electrolyte in the pores near the interface during discharge.

[0048] The method for preparing the negative electrode 3 involves, for example, dispersing a negative electrode mixture, which is a mixture of negative electrode active material particles, a conductive agent, and a binder, in a dispersion medium to prepare a negative electrode slurry. This negative electrode slurry is then applied to the negative electrode current collector sheet 11, the dispersion medium is volatilized, and the negative electrode 3 is produced by pressing. Examples of dispersion media used in preparing the slurry include water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, and N-methylpyrrolidone.

[0049] The separator 4 is the aforementioned separator for a non-aqueous electrolyte secondary battery and is placed between the positive electrode 2 and the negative electrode 3. By providing the separator 4, it is possible to prevent short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. In addition, the separator 4 has a large number of pores, and these pores are filled with the electrolyte 5. As a result, lithium ions conducting through the electrolyte 5 between the positive electrode 2 and the negative electrode 3 can pass through the separator 4, thereby reducing the internal resistance of the lithium-ion battery.

[0050] The nonionic surfactant contained in the separator 4 may dissolve into the electrolyte 5. Since the nonionic surfactant does not become an ion even when dissolved in the electrolyte 5, it is possible to suppress the influence of the nonionic surfactant on the battery performance of the non-aqueous electrolyte secondary battery 20. In addition, by including the nonionic surfactant in the separator 4, it is possible to suppress an increase in the surfactant concentration in the electrolyte 5, thereby suppressing the influence of the surfactant on the battery performance of the lithium-ion battery.

[0051] The electrolyte 5 is a lithium ion conduction medium between the positive electrode and the negative electrode, and contains lithium ions. By filling the pores of the positive electrode 2, the negative electrode 3, and the separator 4 with the electrolyte 5, lithium ions can be conducted between the positive electrode 2 and the negative electrode 3.

[0052] The electrolyte 5 contains an ionic liquid. Since ionic liquids are generally non-flammable or flame-retardant, the inclusion of an ionic liquid in the electrolyte 5 can improve the safety of the lithium-ion battery.

[0053] The ionic liquid contained in electrolyte solution 5 contains both cationic and anionic components. The cationic component of the ionic liquid is preferably a chain-like or cyclic quaternary ammonium ion. By using a chain-like or cyclic quaternary ammonium ion as the cationic component of the ionic liquid, the viscosity of the ionic liquid is suppressed, and the electrolyte 5 can be quickly impregnated into the positive electrode 2, negative electrode 3, and separator 4. Examples of cyclic quaternary ammonium ions include imidazolium ions, pyrrolidinium ions, pyridinium ions, and piperidinium ions.

[0054] The anionic component of the ionic liquid is preferably a bis(fluorosulfonyl)amide ion. By using a bis(fluorosulfonyl)amide ion as the anionic component of the ionic liquid, the viscosity of the ionic liquid is suppressed, and the electrolyte 5 can be quickly impregnated into the positive electrode 2, negative electrode 3, and separator 4.

[0055] The proportion of ionic liquid in the electrolyte 5 is not particularly limited, but it is preferably 30 to 90% by mass of the total electrolyte. By setting the proportion of ionic liquid to 30% by mass or more, the heat resistance of the electrolyte 5 can be improved, thereby improving the safety of the lithium-ion battery. Furthermore, by setting the proportion of ionic liquid to 90% by mass or less, the electrolyte 5 can sufficiently contain lithium ions, which are charge carriers, thereby reducing the internal resistance of the lithium-ion battery.

[0056] The electrolyte 5 contains a lithium salt as a lithium ion source. Examples of lithium salts include lithium hexafluoride phosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)amide, and lithium bis(fluorosulfonyl)amide.

[0057] The electrolyte 5 may contain components other than ionic liquids and lithium salts, such as film-forming agents and diluents. Examples of components other than ionic liquids and lithium salts include propylene carbonate, ethylene carbonate, vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, γ-butyrolactone, ethylene sulfite, dimethyl sulfone, ethyl methyl sulfone, sulfolane, and dimethyl sulfoxide.

[0058] Fabrication of separators for non-aqueous electrolyte secondary batteries and lithium-ion batteries Separators for non-aqueous electrolyte secondary batteries were fabricated in Examples 1-5 and Comparative Examples 1-3. Furthermore, lithium-ion batteries of Examples 1-5 were fabricated using the non-aqueous electrolyte secondary battery separators of Examples 1-5.

[0059] [Example 1] A surfactant solution was prepared by dissolving 2% by mass of decyl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent). Then, a polyethylene porous membrane (Toray Industries, Inc.'s "Cetira" registered trademark) with a film thickness of 15 μm, an air permeability of 100 sec / cc, and a porosity of 47% was immersed in this surfactant solution, removed, and the solvent was evaporated to produce a separator for a non-aqueous electrolyte secondary battery.

[0060] 94% by mass of LiNi 0.8 Co 0.15 Al 0.05 A positive electrode slurry was prepared by dispersing a positive electrode mixture, which consisted of O2 (positive electrode active material), 3% by mass of acetylene black (conductive agent), and 3% by mass of polyvinylidene fluoride (binder), in N-methyl-2-pyrrolidone (dispersion medium). This positive electrode slurry was then applied to one side of an aluminum foil (positive electrode current collector sheet), the dispersion medium was evaporated, and the positive electrode was manufactured by pressing.

[0061] A negative electrode slurry was prepared by dispersing a negative electrode mixture, consisting of 94% by mass of graphite (negative electrode active material), 5% by mass of styrene-butadiene rubber (binder), and 1% by mass of carboxymethylcellulose (thickener), in water (dispersion medium). This negative electrode slurry was then applied to one side of a copper foil (negative electrode current collector sheet), the dispersion medium was evaporated, and the negative electrode was manufactured by pressing.

[0062] An electrolyte was prepared by dissolving 25.4% by mass of lithium bis(fluorosulfonyl)amide (lithium salt) in 73.3% by mass of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (ionic liquid) and adding 1.3% by mass of fluoroethylene carbonate (film-forming agent).

[0063] Using the above-mentioned positive electrode, negative electrode, separator, and electrolyte, a CR2032 type lithium-ion battery (coin cell) was fabricated.

[0064] [Example 2] A separator for a non-aqueous electrolyte secondary battery and a CR2032 type lithium-ion battery (coin cell) were fabricated in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of lauryl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent).

[0065] [Example 3] A separator for a non-aqueous electrolyte secondary battery and a CR2032 type lithium-ion battery (coin cell) were fabricated in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of myristyl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent).

[0066] [Example 4] A separator for a non-aqueous electrolyte secondary battery and a CR2032 type lithium-ion battery (coin cell) were fabricated in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of palmityl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent).

[0067] [Example 5] A separator for a non-aqueous electrolyte secondary battery and a CR2032 type lithium-ion battery (coin cell) were fabricated in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of lauryl betaine (betaine derivative, surfactant) in 98% by mass of water (solvent).

[0068] [Comparative Example 1] A polyethylene porous membrane (Toray Industries, Inc.'s "Cetira") with a film thickness of 15 μm, an air permeability of 100 sec / cc, and a porosity of 47% was used as a separator for a non-aqueous electrolyte secondary battery.

[0069] [Comparative Example 2] A separator for a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of octyl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent).

[0070] [Comparative Example 3] A separator for a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2% by mass of stearyl sulfobetaine (sulfobetaine derivative, surfactant) in 98% by mass of water (solvent).

[0071] Investigation into the feasibility of impregnation with ionic liquid electrolyte. The above-mentioned ionic liquid electrolyte was dropped onto the separators for non-aqueous electrolyte secondary batteries of Examples 1-5 and Comparative Examples 1-3, and it was observed whether or not the ionic liquid electrolyte impregnated each separator. The survey results are shown in Table 1. Table 1 also shows the surfactants used to prepare the surfactant solutions, as well as the number of carbon atoms in the hydrophobic groups of the surfactants.

[0072] Initial charge / discharge test Initial charge-discharge tests were performed using the lithium-ion batteries of Examples 1 to 5. Specifically, the lithium-ion batteries were charged with a constant current of 0.2C until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.05C. After a 5-minute rest, constant current discharge was performed with a current of 1C until the voltage reached 2.5V. Here, the unit of current C is the current value required to fully charge or discharge the battery capacity in one hour. The initial charge capacity was defined as the initial charge capacity, and the initial discharge capacity was defined as the initial discharge capacity. The initial charge-discharge efficiency was calculated using the following equation (3). Initial charge / discharge efficiency (%) = Initial discharge capacity ÷ Initial charge capacity × 100 ... (3) Table 1 shows the initial charge-discharge efficiencies of the lithium-ion batteries in Examples 1 to 5.

[0073] Charge / discharge cycle test Charge-discharge cycle tests were performed using the lithium-ion batteries of Examples 1 to 5. Specifically, constant current charging was performed at a current of 1C until the voltage reached 4.2V, followed by constant voltage charging at 4.2V until the current value reached 0.05C. After a 5-minute pause, constant current discharge was performed at a current of 1C until the voltage reached 2.5V. This charge-discharge cycle was repeated 50 times, and the capacity retention rate over 50 cycles (hereinafter also referred to as the capacity retention rate) was calculated from the following equation (4). Capacity retention rate (%) = Discharge capacity at cycle 1 ÷ Discharge capacity at cycle 50 × 100 (4) Table 1 shows the capacity retention rates of the lithium-ion batteries in Examples 1 to 5.

[0074] [Table 1]

[0075] The separators in Examples 1-5 were impregnable with ionic liquid electrolyte. Furthermore, the lithium-ion batteries in Examples 1-5 exhibited excellent initial charge-discharge efficiency and capacity retention rates over 50 cycles. In contrast, the separators of Comparative Examples 1-3 were not impregnated with the ionic liquid electrolyte. The separator in Comparative Example 1 does not contain a surfactant. Therefore, it is suggested that the difference in surface tension between the pore walls of the porous membrane and the ionic liquid electrolyte was extremely large, preventing the ionic liquid electrolyte from impregnating the separator. The separator in Comparative Example 2 has eight carbon atoms in the hydrophobic group of the surfactant. Therefore, the hydrophobicity of the surfactant is extremely low, and the affinity between the surfactant and the polyolefin porous material is reduced, suggesting that the ionic liquid electrolyte could not be impregnated. The separator in Comparative Example 3 has 18 carbon atoms in the hydrophobic group of the surfactant. Therefore, the extremely high hydrophobicity of the surfactant reduced the affinity between the surfactant and the ionic liquid electrolyte, suggesting that the ionic liquid electrolyte could not be impregnated into the separator. [Explanation of Symbols]

[0076] 1: Pores 2: Positive electrode 3: Negative electrode 4: Separator 5: Electrolyte 6: Positive electrode current collector sheet 7: Positive electrode active material layer 8: Porous membrane 9: Zwitterionic surfactant 11: Negative electrode current collector sheet 12: Negative electrode active material layer 16: Positive electrode can 17: Negative electrode can 18: Gasket 20: Non-aqueous electrolyte secondary battery

Claims

1. A porous membrane containing polyolefin is provided. The porous membrane has a zwitterionic surfactant on the pore wall surface or within the pore wall, The aforementioned zwitterionic surfactant is a betaine derivative or a sulfobetaine derivative. The hydrophobic group of the aforementioned zwitterionic surfactant is a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, or an alkenylsulfonyl group, or a combination thereof. A separator for non-aqueous electrolyte secondary batteries, characterized in that the number of carbon atoms in the hydrophobic group is 10 or more and 16 or less.

2. The separator according to claim 1, wherein the zwitterionic surfactant is a betaine derivative and is a compound represented by the following chemical formula (1). 【Chemistry 1】 [In the formula, R 1 , R 2 and R 3 At least one of these represents the hydrophobic group, and n is between 1 and 5.

3. The separator according to claim 1, wherein the zwitterionic surfactant is a sulfobetaine derivative and is a compound represented by the following chemical formula (2). 【Chemistry 2】 [In the formula, R 1 , R 2 and R 3 At least one of these represents the hydrophobic group, and n is between 1 and 5.

4. The separator according to claim 1, wherein the pores of the porous membrane have an average pore diameter of 0.01 μm or more and 0.5 μm or less.

5. A separator according to any one of claims 1 to 4, comprising a positive electrode, a negative electrode, and an electrolyte, The aforementioned electrolyte is a non-aqueous electrolyte secondary battery containing an ionic liquid.

6. The aforementioned ionic liquid comprises a cationic component and an anionic component. The aforementioned cationic component is a quaternary ammonium ion. The quaternary ammonium ion has at least a chain-like or cyclic structure in part, The non-aqueous electrolyte secondary battery according to claim 5, wherein the anionic component is a bis(fluorosulfonyl)amide ion.

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