JP2024114016A - Separator for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Abstract

[Problem] The present invention provides a separator for a non-aqueous electrolyte secondary battery that has good wettability with an ionic liquid electrolyte. [Solution] The separator for a non-aqueous electrolyte secondary battery of the present invention comprises a porous membrane containing polyolefin. The porous membrane has a non-ionic surfactant on the pore wall surface or in the pore wall, the non-ionic surfactant being a polyethylene glycol derivative having 20 or less repeating units of polyethylene glycol, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative, the hydrophobic group of the non-ionic surfactant being a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination of these functional groups, and the number of carbon atoms of the hydrophobic group is 4 or more and 20 or less. [Selected Figure] Figure 1

Description

The present invention relates to a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

In recent years, non-aqueous electrolyte secondary batteries have become increasingly popular. In particular, lithium-ion batteries, which have a high energy density, are used not only in small electronic devices such as smartphones and laptops, but also in large devices such as electric vehicles and emergency power sources.
In general lithium-ion batteries, polyolefin porous membranes are used as separators. One of the reasons for this is that polyolefin porous membranes have an excellent shutdown function. The shutdown function is a function in which, when the lithium-ion battery generates abnormal heat, the separator softens and the pores in the separator are blocked, thereby blocking the movement of lithium ions between the positive and negative electrodes. Since polyolefins soften at low temperatures, polyolefin porous membranes can have an excellent shutdown function and can improve the safety of lithium-ion batteries.

The electrolyte used in typical lithium-ion batteries is an organic electrolyte solution, which is made by dissolving lithium salt in a flammable organic liquid. Lithium-ion batteries can also generate abnormal heat due to overcharging or short circuits between the positive and negative electrodes. For this reason, lithium-ion batteries are at risk of catching fire or burning.
In order to prevent ignition or combustion of lithium ion batteries, it has been proposed to use an ionic liquid electrolyte in which a lithium salt is dissolved in an ionic liquid as an electrolyte for lithium ion batteries (for example, Patent Documents 1 and 2). An ionic liquid is a liquid composed of an anion component and a cation component, and is generally non-flammable or flame-retardant. Therefore, the use of an ionic liquid electrolyte can improve the safety of lithium ion batteries.

JP 2017-195129 A JP 2018-116840 A

However, since ionic liquids have higher viscosity and surface tension than organic liquids, ionic liquid electrolytes cannot be impregnated into typical polyolefin porous membranes.
The present invention has been made in view of the above circumstances, and provides a separator for a non-aqueous electrolyte secondary battery that has good wettability with an ionic liquid electrolyte.

The present invention provides a separator for a non-aqueous electrolyte secondary battery, comprising a porous membrane containing polyolefin, the porous membrane having a non-ionic surfactant on the pore wall surface or in the pore wall, the non-ionic surfactant being a polyethylene glycol derivative having 20 or less repeating units of polyethylene glycol, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative, the hydrophobic group of the non-ionic surfactant being a functional group consisting of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination of these functional groups, and the number of carbon atoms of the hydrophobic group is 4 or more and 20 or less.

The separator for a non-aqueous electrolyte secondary battery of the present invention has the above-mentioned characteristics, and therefore has good wettability with respect to an ionic liquid electrolyte. This was made clear by experiments conducted by the present inventors.

1 is a partial cross-sectional view of a separator for a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. 1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention.

The separator for a non-aqueous electrolyte secondary battery of the present invention comprises a porous membrane containing polyolefin, the porous membrane having a non-ionic surfactant on the pore wall surface or in the pore wall, the non-ionic surfactant being a polyethylene glycol derivative having 20 or less repeating units of polyethylene glycol, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative, the hydrophobic group of the non-ionic surfactant being a functional group consisting of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination of these functional groups, and the number of carbon atoms of the hydrophobic group is 4 or more and 20 or less.

The present invention also provides a nonaqueous electrolyte secondary battery comprising the separator for a nonaqueous electrolyte secondary battery of the present invention, a positive electrode, a negative electrode, and an electrolyte solution containing an ionic liquid.
Preferably, the ionic liquid contains a cationic component and an anionic component, the cationic component is a quaternary ammonium ion, the quaternary ammonium ion has at least a partial chain structure or a cyclic structure, and the anionic component is a bis(fluorosulfonyl)amide ion.
The pores of the porous membrane preferably have an average pore diameter of 0.01 μm or more and 0.5 μm or less.

Below, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

Separator for Non-Aqueous Electrolyte Secondary Battery FIG. 1 is a partial cross-sectional view of a separator for a non-aqueous electrolyte secondary battery according to the present embodiment.
The separator 4 for a non-aqueous electrolyte secondary battery of this embodiment includes a porous film 8 containing polyolefin. The porous film 8 has a nonionic surfactant 9 on the pore wall surface or in the pore wall, the nonionic surfactant 9 being a polyethylene glycol derivative having 20 or less repeating units of polyethylene glycol, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative, the hydrophobic group of the nonionic surfactant being a functional group composed of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination of these functional groups, and the number of carbon atoms of the hydrophobic group is 4 or more and 20 or less.

The separator 4 for a nonaqueous electrolyte secondary battery is a separator used in the production of a nonaqueous electrolyte secondary battery or a separator incorporated in a nonaqueous electrolyte secondary battery.
The thickness of the separator 4 is not particularly limited, but is preferably 5 μm or more and 30 μm or less. By making the thickness of the separator 4 5 μm or more, the electrical insulation of the separator 4 can be improved, and the safety of the nonaqueous electrolyte secondary battery can be improved. Furthermore, by making the thickness of the separator 4 30 μm or less, the migration distance (diffusion distance) of ions between the positive electrode and the negative electrode can be shortened, and the internal resistance of the nonaqueous electrolyte secondary battery can be reduced.

The porosity of the separator 4 is not particularly limited, but is preferably 20% by volume or more and 80% by volume or less. By making the porosity of the separator 4 20% by volume or more, the amount of electrolyte retained can be increased, and the charge/discharge cycle characteristics of the nonaqueous electrolyte secondary battery can be improved. In addition, by making the porosity of the separator 4 80% by volume or less, the electrical insulation of the separator 4 can be improved, and the safety of the nonaqueous electrolyte secondary battery can be improved.

The air permeability of the separator 4 (air permeability measured by the Gurley tester method) is not particularly limited, but is preferably 50 to 500 sec/100 cc. By making the air permeability of the separator 4 50 sec/100 cc or more, the electrical insulation of the separator 4 can be improved, and the safety of the nonaqueous electrolyte secondary battery can be improved. In addition, by making the air permeability of the separator 4 500 sec/100 cc or less, the ionic liquid electrolyte can be quickly impregnated into the separator 4.

The separator 4 includes a porous film 8 containing polyolefin. This allows the separator 4 to have an excellent shutdown function, and a separator with excellent safety can be provided. The polyolefin, which is the material of the porous film 8, is, for example, polyethylene, polypropylene, polybutene, etc. The porous film 8 can contain these polyolefins alone or in a mixture of multiple types. The porous film 8 can also contain components other than polyolefin. The components other than polyolefin are, for example, polyether, polyester, polyamide, polyimide, polyamideimide, polyphenylene sulfide, etc. The proportion of polyolefin contained in the porous film 8 is not particularly limited, but is preferably 50% by mass or more of the entire porous film, and more preferably 80% by mass or more. This allows the shutdown function of the separator 4 to be improved, and the safety of the nonaqueous electrolyte secondary battery to be improved.
The porous membrane 8 may be a polyolefin porous membrane made of polyolefin.
The porous film 8 may also have a laminated structure in which a polyethylene layer and a polypropylene layer are laminated.

The porous membrane 8 may be a microporous membrane having pores of 0.5 μm or less. The pores of the porous membrane 8 may 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 increases, and if the average pore diameter is too large, a short circuit tends to occur between the positive and negative electrodes. The average pore diameter can be calculated based on the results of pore size distribution measurement by mercury intrusion porosimetry.
The porous film 8 is, for example, a dry one-component microporous film, a wet two-component microporous film, or a wet three-component microporous film.
A dry one-component microporous membrane is produced, for example, by cooling and stretching a molten polymer to form a film, and micropores are formed by the stretching during the cooling process. A wet two-component porous membrane is produced, for example, by cooling and stretching a molten polymer previously mixed with a plasticizer to form a film, and then extracting the plasticizer. A wet three-component microporous membrane is produced, for example, by cooling and stretching a molten polymer previously mixed with a plasticizer and filler particles to form a film, and then extracting the plasticizer and filler particles.

The porous membrane 8 may contain inorganic particles mixed with the polyolefin. The inorganic particles may be, for example, alumina particles, silica particles, titania particles, or the like.
The porous membrane 8 may include a surfactant mixed into the polyolefin.

The separator 4 may be a laminated separator in which the 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 polyamideimide resin layer, a heat-resistant acrylic resin layer, a cross-linked polymer layer, or the like.

The separator 4 can have a porous heat-resistant layer provided on the porous membrane 8. The heat-resistant layer (porous heat-resistant layer) may be provided only on one of the main surfaces (front or back) of the porous membrane 8, or may be provided on each of the 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 by a binder. By having the separator 4 have a heat-resistant layer, the heat resistance of the separator 4 can be improved, and the safety of the nonaqueous electrolyte secondary battery can be improved.

The heat-resistant layer may contain inorganic particles, such as 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 include a binder. Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, acrylonitrile rubber, and polyacrylic acid. The heat-resistant layer may include a thickener. Examples of the thickener include carboxymethyl cellulose.
The heat resistant layer may have a surfactant on its major surfaces and on the pore wall surfaces.

The thickness of the heat-resistant layer is not particularly limited, but is preferably 1 μm or more and 10 μm or less. By making the heat-resistant layer 1 μm or more thick, the heat resistance of the separator 4 can be improved, and the safety of the non-aqueous electrolyte secondary battery can be improved. In addition, by making the heat-resistant layer 10 μm or less thick, the migration distance (diffusion distance) of ions between the positive and negative electrodes can be shortened, and the internal resistance of the non-aqueous electrolyte secondary battery can be reduced.

The polyolefin-containing porous membrane 8 has a nonionic surfactant 9 on the pore wall surface or in the pore wall. The nonionic surfactant is a surfactant that does not become ions when dissolved in water or the like. The nonionic surfactant may be a substance that does not dissolve in water or is difficult to dissolve in water as long as it has a hydrophilic group and a hydrophobic group. Since the nonionic surfactant does not become ions in the electrolyte, it is possible to suppress the surfactant from affecting the positive electrode reaction or the negative electrode reaction of the nonaqueous electrolyte secondary battery.
The presence of nonionic surfactant 9 on the pore wall surface or in the pore walls of porous membrane 8 can reduce the difference in surface tension between the pore wall surface of porous membrane 8 and the ionic liquid electrolyte, thereby providing a separator with good wettability with the ionic liquid electrolyte.
For example, as shown in FIG. 1, the nonionic surfactant 9 may form a surfactant layer on the wall surface of the pores 1 of the porous membrane 8 .

The nonionic surfactant 9 is a polyethylene glycol derivative, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative, in which the number of repeating units of polyethylene glycol is 20 or less. The hydrophobic group of the nonionic 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, an alkenylsulfonyl group, or a combination of these functional groups. The carbon number of the hydrophobic group of the nonionic surfactant 9 is 4 or more and 20 or less.
By having such a nonionic surfactant 9 on or in the pore wall surface of the porous film 8, the separator 4 has good wettability with respect to the ionic liquid electrolyte. This fact became clear from experiments carried out by the present inventors.

When the nonionic surfactant 9 is a polyethylene glycol derivative in which the number of repeating units of polyethylene glycol is 20 or less, the nonionic surfactant 9 may be a compound represented by the following chemical formula (1). This allows the surfactant 9 to have appropriate hydrophilicity, and a separator 4 with good wettability with the ionic liquid electrolyte can be provided. The nonionic surfactant 9 is, for example, a polyethylene glycol ether, a polyethylene glycol ester, etc.

[In the formula, one of R ₁ and R ₂ represents a hydrogen atom and the other represents the hydrophobic group, and n is 1 or more and 20 or less.]

When the nonionic surfactant 9 is a sorbitan derivative, the nonionic surfactant 9 may be a compound represented by the following chemical formula (2). This allows the surfactant 9 to have appropriate hydrophilicity, and a separator 4 with good wettability with the ionic liquid electrolyte can be provided. The nonionic surfactant 9 is, for example, a sorbitan ester.

[In the formula, at least one of R ₁ , R ₂ , R ₃ and R ₄ represents a hydrogen atom, and at least one represents the hydrophobic group.]

When the nonionic surfactant 9 is a polyoxyethylene sorbitan derivative, the nonionic surfactant 9 may be a compound represented by the following chemical formula (3). This allows the surfactant 9 to have appropriate hydrophilicity, and a separator 4 with good wettability with the ionic liquid electrolyte can be provided. The surfactant 9 is, for example, a polyoxyethylene sorbitan ester.

[In the formula, at least one of R ₁ , R ₂ , R ₃ and R ₄ represents a hydrogen atom, at least one represents the hydrophobic group, and each of n, m, p and q is 1 or more and 20 or less.]

When the nonionic surfactant 9 is a glycerol derivative, the nonionic surfactant 9 may be a compound represented by the following chemical formula (4). This allows the surfactant 9 to have appropriate hydrophilicity, and a separator 4 with good wettability with the ionic liquid electrolyte can be provided. The surfactant 9 is, for example, a glycerol ester.

[In the formula, at least one of R ₁ , R ₂ and R ₃ represents a hydrogen atom, and at least one represents the hydrophobic group.]

When the nonionic surfactant 9 is a polyoxyethylene glycerol derivative, the nonionic surfactant 9 may be a compound represented by the following chemical formula (5). This allows the nonionic surfactant 9 to have appropriate hydrophilicity, and a separator 4 with good wettability with the ionic liquid electrolyte can be provided. The surfactant 9 is, for example, a polyoxyethylene glycerol ester.

[In the formula, at least one of R ₁ , R ₂ and R ₃ represents a hydrogen atom, at least one represents the hydrophobic group, and each of n, m and p is 1 or more and 20 or less.]

The nonionic 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, an alkenylsulfonyl group, or a combination thereof. Here, the hydrophobic group has 4 to 20 carbon atoms. This allows the nonionic surfactant 9 to have appropriate hydrophobicity, and a separator with good wettability with the ionic liquid electrolyte can be provided.

The mass ratio (Y/X) of the surfactant 9 (Y) to the separator 4 (X) is, for example, from (1/100) to (20/100). By making the mass ratio (Y/X) (1/100) or more, the surfactant 9 can be present over the entire pore wall surface of the separator 4. As a result, the ionic liquid electrolyte can be quickly impregnated into the separator 4. In addition, by making the mass ratio (Y/X) (20/100) or less, the pores of the separator 4 can be prevented from being blocked by the surfactant 9, and the internal resistance of the nonaqueous electrolyte secondary battery can be reduced.

The porous membrane 8 containing the nonionic surfactant 9 in the pore walls can be produced, for example, by cooling and stretching a molten polymer to which a nonionic surfactant has been added to form a film. This allows the pore walls of the porous membrane 8 to contain the nonionic surfactant 9. This nonionic surfactant 9 is also present on the pore wall surfaces, and can improve the wettability of the ionic liquid electrolyte to the pore wall surfaces of the porous membrane 8.

Furthermore, the separator 4 having a nonionic surfactant on the pore wall surface can be produced by impregnating a separator film including a porous membrane 8 with a solution containing a nonionic surfactant 9 and drying the separator film. This allows the pore wall surface of the porous membrane 8 to be coated with the nonionic surfactant. Furthermore, when the separator film includes a porous heat-resistant layer, the pore wall surface of the porous heat-resistant layer is also coated with the nonionic surfactant.
The concentration of the surfactant solution used in producing the separator 4 is preferably 0.5 wt % to 10 wt %. If the concentration is too low, there is a risk that some parts of the wall surface of the separator 4 will not be covered, whereas if the concentration is too high, there is a risk that the surfactant will clog the pores of the separator 4.
The solvent of the solution containing the surfactant 9 is, for example, water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, N-methyl-2-pyrrolidone, or the like.

Nonaqueous Electrolyte Secondary Battery FIG. 2 is a schematic cross-sectional view of the nonaqueous electrolyte secondary battery of this embodiment.
The nonaqueous electrolyte secondary battery 20 of the present embodiment includes the separator 4 of the present embodiment described above, a positive electrode 2, a negative electrode 3, and an electrolytic solution 5, and the electrolytic solution 5 contains an ionic liquid. The nonaqueous electrolyte secondary battery 20 may be a lithium ion battery. Here, a case where the nonaqueous electrolyte secondary battery 20 is a lithium ion battery will be described.
The nonaqueous electrolyte secondary battery 20 may be a coin-type battery, a pouch-type battery, or a metal can-type battery. In addition, the electrode laminate composed of the positive electrode 2, the negative electrode 3, and the separator 4 may have a stack structure or a wound structure.

The positive electrode 2 has 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 battery terminal and the positive electrode active material layer 7. A part of the positive electrode current collector sheet 6 may be the positive electrode battery terminal. The positive electrode current collector sheet 6 is, for example, an aluminum foil, a stainless steel foil, or the like.

The positive electrode active material layer 7 may be provided on one side of the positive electrode current collector sheet 6, or may be provided on both sides of the positive electrode current collector sheet 6. The positive electrode active material layer 7 is a porous layer in which a plurality of positive electrode active material particles are bonded by a binder. This allows the positive electrode active material layer 7 to have pores between the positive electrode active material particles. The pores are filled with the electrolyte 5, and an electrode reaction proceeds on the surface of the positive electrode active material particles. For example, when charging a lithium ion battery, lithium atoms in the positive electrode active material particles are released into the electrolyte as lithium ions (Li ⁺ ), and when discharging the lithium ion battery, lithium ions in the electrolyte are inserted into the positive electrode active material particles as lithium atoms.

The positive electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer in the positive electrode. Examples of the positive electrode active material include 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 ,} LiNi _x Mn _{y O2} (x + y = 1 ₎ , olivine type _LiFePO4 , LiMn _x Fe _{y PO4} (x + y = 1), etc. The positive electrode active material layer 7 may contain one type of these positive electrode active materials alone or a mixture of two or more types.

The positive electrode active material particles can have a conductive coating on their surfaces. This can improve the conductivity of the particle surface where the electrode reaction proceeds, and can reduce the internal resistance of the positive electrode 2. The conductive coating can be, for example, a carbon coating such as graphite, soft carbon, hard carbon, or carbon black.

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 the conductive agent include graphite, soft carbon, hard carbon, and carbon black.

The positive electrode active material layer 7 may contain a binder. Examples of the binder 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 the thickener include carboxymethyl cellulose.

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, and the capacity of the nonaqueous electrolyte secondary battery can be increased. 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 migration distance (diffusion distance) of lithium ions between the vicinity of the interface between the positive electrode active material layer 7 and the positive electrode current collector sheet 6 and the vicinity of the surface of the positive electrode active material layer 7 can be shortened, and it is possible to suppress 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 charge.

The method of producing the positive electrode 2 is, for example, to prepare a positive electrode slurry by 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. The positive electrode 2 can be produced by applying this positive electrode slurry to the positive electrode current collector sheet 6, volatilizing the dispersion medium, and performing a pressing process. Examples of the dispersion medium used to prepare the slurry include water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, and N-methylpyrrolidone.

The negative electrode 3 has a negative electrode current collector sheet 11 and a porous negative electrode active material layer 12 provided on one or both major surfaces of the negative electrode current collector sheet 11.

The negative electrode current collector sheet 11 is a sheet that serves as a base material 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. In addition, a part of the negative electrode current collector sheet 11 may be the negative electrode battery terminal. The negative electrode current collector sheet 11 is, for example, copper foil, nickel foil, stainless steel foil, etc.

The negative electrode active material layer 12 may be provided on one side of the negative electrode current collector sheet 11, or may be provided 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 by a binder. This allows the negative electrode active material layer 12 to have pores between the negative electrode active material particles. The 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 into the negative electrode active material particles as lithium atoms, and when discharging the lithium ion battery, lithium atoms in the negative electrode active material particles are released into the electrolyte 5 as lithium ions.

The negative electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer in the negative electrode 3. Examples of the negative electrode active material include graphite, soft carbon, hard carbon, carbon black, lithium titanate, and silicon alloys. The negative electrode active material layer 12 can contain one type of these negative electrode active materials alone or a mixture of multiple types.

The negative electrode active material layer 12 may contain a binder. Examples of the binder 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 the thickener include carboxymethyl cellulose.

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, and the capacity of the lithium ion battery can be increased. 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 migration distance (diffusion distance) of lithium ions between the vicinity of the interface between the negative electrode active material layer 12 and the negative electrode current collector sheet 11 and the vicinity of the surface of the negative electrode active material layer 12 can be shortened, and it is possible to suppress 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 discharging.

The method of producing the negative electrode 3 is, for example, to prepare a negative electrode slurry by 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. The negative electrode 3 can be produced by applying this negative electrode slurry to the negative electrode current collector sheet 11, volatilizing the dispersion medium, and performing a pressing process. Examples of the dispersion medium used to prepare the slurry include water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl acetate, ethyl acetate, toluene, hexane, dimethylformamide, and N-methylpyrrolidone.

The separator 4 is the above-mentioned separator for a non-aqueous electrolyte secondary battery, and is disposed between the positive electrode 2 and the negative electrode 3. By providing the separator 4, it is possible to prevent a short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. The separator 4 also has many pores, which are filled with the electrolyte 5. This allows lithium ions conducting the electrolyte 5 between the positive electrode 2 and the negative electrode 3 to pass through the separator 4, thereby reducing the internal resistance of the lithium ion battery.

The nonionic surfactant contained in the separator 4 may be dissolved in the electrolyte solution 5. Since the nonionic surfactant does not become ions even if it is dissolved in the electrolyte solution 5, it is possible to suppress the nonionic surfactant from affecting the battery performance of the nonaqueous electrolyte secondary battery 20. Furthermore, by containing a nonionic surfactant in the separator 4, it is possible to suppress the surfactant concentration in the electrolyte solution 5 from becoming high, and it is possible to suppress the surfactant from affecting the battery performance of the lithium ion battery.

The electrolyte 5 is a lithium ion conductive medium between the positive electrode and the negative electrode, and contains lithium ions. The electrolyte 5 fills the pores of the positive electrode 2, the pores of the negative electrode 3, and the pores of the separator 4, allowing lithium ions to be conducted between the positive electrode 2 and the negative electrode 3.

The electrolyte solution 5 contains an ionic liquid. Since ionic liquids are generally non-flammable or flame-retardant, the safety of the lithium-ion battery can be improved by including an ionic liquid in the electrolyte solution 5.

The ionic liquid contained in the electrolytic solution 5 contains a cationic component and an anionic component.
The cationic component of the ionic liquid is preferably a chain or cyclic quaternary ammonium ion. By using a chain or cyclic quaternary ammonium ion as the cationic component of the ionic liquid, the viscosity of the ionic liquid is prevented from increasing, and the electrolyte 5 can be quickly impregnated into the positive electrode 2, the negative electrode 3, and the separator 4. Examples of the cyclic quaternary ammonium ion include an imidazolium ion, a pyrrolidinium ion, a pyridinium ion, and a piperidinium ion.

The anion component of the ionic liquid is preferably a bis(fluorosulfonyl)amide ion. By using a bis(fluorosulfonyl)amide ion as the anion component of the ionic liquid, the viscosity of the ionic liquid is prevented from increasing, and the electrolyte solution 5 can be quickly impregnated into the positive electrode 2, the negative electrode 3, and the separator 4.

The proportion of ionic liquid contained in the electrolyte 5 is not particularly limited, but is preferably 30 to 90% by mass of the entire electrolyte. By making the proportion of ionic liquid 30% by mass or more, the heat resistance of the electrolyte 5 can be improved, and the safety of the lithium ion battery can be improved. Furthermore, by making the proportion of ionic liquid 90% by mass or less, the electrolyte 5 can contain a sufficient amount of lithium ions, which are charge carriers, and the internal resistance of the lithium ion battery can be reduced.

The electrolyte 5 contains a lithium salt as a lithium ion source. Examples of the lithium salt include lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)amide, and lithium bis(fluorosulfonyl)amide.

The electrolyte 5 may contain components other than the ionic liquid and lithium salt, such as a film-forming agent and a diluent. Examples of components other than the ionic liquid and lithium salt include propylene carbonate, ethylene carbonate, vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl 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.

Preparation of separators for non-aqueous electrolyte secondary batteries and lithium ion batteries Separators for non-aqueous electrolyte secondary batteries were prepared in Examples 1 to 9 and Comparative Examples 1 to 3. In addition, lithium ion batteries in Examples 1 to 9 were prepared.

[Example 1]
A surfactant solution was prepared by dissolving 2% by mass of polyethylene glycol monooleyl ether (the number of repeating units n of polyethylene glycol is about 2, a polyethylene glycol derivative, a surfactant) in 98% by mass of ethanol (solvent). A polyethylene porous membrane ("Setira" (registered trademark) manufactured by Toray Industries, Inc.) having a thickness of 15 μm, an air permeability of 100 sec/cc, and a porosity of 47% was immersed in the surfactant solution, and then removed to volatilize the solvent, thereby producing a separator for a non-aqueous electrolyte secondary battery.

A positive electrode mixture of 94% _by mass of _{LiNi0.8Co0.15Al0.05O2} (positive electrode active material) _, 3% by mass of acetylene _black (conductive agent), and 3% by mass of polyvinylidene fluoride (binder) was dispersed in N-methyl-2-pyrrolidone (dispersion medium) to prepare a positive electrode slurry. The positive electrode slurry was then applied onto one side of an aluminum foil (positive electrode current collector sheet), the dispersion medium was volatilized, and the sheet was pressed to prepare a positive electrode.

A negative electrode mixture consisting of 94% graphite (negative electrode active material), 5% styrene-butadiene rubber (binder), and 1% carboxymethyl cellulose (thickener) was dispersed in water (dispersion medium) to prepare a negative electrode slurry. This negative electrode slurry was then applied to one side of copper foil (negative electrode current collector sheet), the dispersion medium was volatilized, and the negative electrode was fabricated by pressing.

An ionic liquid 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).

A CR2032 type lithium ion battery (coin type battery) was fabricated using the above positive electrode, negative electrode, separator, and electrolyte.

[Example 2]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol monooleyl ether (the number of repeating units n of polyethylene glycol = approximately 7, polyethylene glycol derivative, surfactant) in 98 mass % of ethanol (solvent).

[Example 3]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol monooleyl ether (the number of repeating units n of polyethylene glycol = approximately 10, polyethylene glycol derivative, surfactant) in 98 mass % of ethanol (solvent).

[Example 4]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol monooleyl ether (the number of repeating units n of polyethylene glycol = approximately 20, polyethylene glycol derivative, surfactant) in 98 mass % of ethanol (solvent).

[Example 5]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol mono-4-octylphenyl ether (number of repeating units of polyethylene glycol = approximately 10, surfactant, polyethylene glycol derivative) in 98 mass % of water (solvent).

[Example 6]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol monolaurate (number of repeating units of polyethylene glycol = approximately 10, polyethylene glycol derivative, surfactant) in 98 mass % of water (solvent).

[Example 7]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of sorbitan monolaurate (sorbitan derivative, surfactant) in 98 mass % of ethanol (solvent).

[Example 8]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyoxyethylene sorbitan monolaurate (a polyoxyethylene sorbitan derivative, a surfactant) in 98 mass % of water (solvent).

[Example 9]
A separator for a nonaqueous electrolyte secondary battery and a CR2032 type lithium ion battery (coin battery) were produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of glycerol monolaurate (glycerol derivative, surfactant) in 98 mass % of ethanol (solvent).

[Comparative Example 1]
A polyethylene porous film ("Setira" (registered trademark) manufactured by Toray Industries, Inc.) having a thickness of 15 μm, an air permeability of 100 sec/cc, and a porosity of 47% was used as it was as a separator for a non-aqueous electrolyte secondary battery.

[Comparative Example 2]
A separator for a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of oleyl alcohol (surfactant) in 98 mass % of ethanol (solvent).

[Comparative Example 3]
A separator for a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that a surfactant solution was prepared by dissolving 2 mass % of polyethylene glycol monooleyl ether (number of repeating units of polyethylene glycol = approximately 50, polyethylene glycol derivative, surfactant) in 98 mass % of ethanol (solvent).

Investigation of Ionic Liquid Electrolyte Impregnation The above ionic liquid electrolyte was dropped onto the separators for nonaqueous electrolyte secondary batteries of Examples 1 to 9 and Comparative Examples 1 to 3, and it was observed whether the ionic liquid electrolyte was impregnated into each separator.
The results of the investigation are shown in Table 1. Table 1 also shows the surfactants used in the preparation of the surfactant solutions and the carbon numbers of the hydrophobic groups of the surfactants.

Initial charge/discharge test: An initial charge/discharge test was performed using the lithium ion batteries of Examples 1 to 9. Specifically, the lithium ion battery was charged at a constant current of 0.2 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C. After a 5-minute pause, the battery was discharged at a constant current of 1 C until the voltage reached 2.5 V. Here, the unit of current value C is the current value when the battery capacity is fully charged or discharged in 1 hour. The charge capacity at this time was taken as the initial charge capacity, and the discharge capacity at this time was taken as the initial discharge capacity, and the initial charge/discharge efficiency was calculated from the following formula (6).
Initial charge/discharge efficiency (%) = initial discharge capacity ÷ initial charge capacity × 100 (6)
The initial charge/discharge efficiencies of the lithium ion batteries of Examples 1 to 9 are shown in Table 1.

Charge/Discharge Cycle Test A charge/discharge cycle test was conducted using the lithium ion batteries of Examples 1 to 9. Specifically, constant current charging was performed at a current value of 1 C until the voltage reached 4.2 V, followed by constant voltage charging at a voltage of 4.2 V until the current value reached 0.05 C. After a 5-minute pause, constant current discharging was performed at a current value of 1 C until the voltage reached 2.5 V. This charge/discharge cycle was repeated 50 times, and the capacity retention rate over 50 cycles (hereinafter also referred to as capacity retention rate) was calculated from the following formula (7).
Capacity retention rate (%) = 1st cycle discharge capacity ÷ 50th cycle discharge capacity × 100 (7)
The capacity retention rates of the lithium ion batteries of Examples 1 to 9 are shown in Table 1.

The separators of Examples 1 to 9 were capable of being impregnated with an ionic liquid electrolyte. Furthermore, the lithium ion batteries of Examples 1 to 9 exhibited excellent initial charge/discharge efficiency and capacity retention rate over 50 cycles.
In contrast, the separators of Comparative Examples 1 to 3 were not impregnated with the ionic liquid electrolyte.
The separator of Comparative Example 1 does not include a surfactant, which suggests that the difference in surface tension between the pore walls of the porous membrane and the ionic liquid electrolyte was extremely large, and the ionic liquid electrolyte could not be impregnated.
Alcohol was used as a surfactant in the preparation of the separator in Comparative Example 2. This suggests that the hydrophilicity of the surfactant was extremely low, which reduced the affinity between the surfactant and the ionic liquid electrolyte, and therefore the ionic liquid electrolyte could not be impregnated.
The separator of Comparative Example 3 was prepared using a polyethylene glycol derivative as a surfactant, the number of repeating polyethylene glycol units of which was approximately 50. This suggests that the surfactant was extremely hydrophilic, reducing the affinity between the surfactant and the polyolefin porous material, and therefore preventing the ionic liquid electrolyte from being impregnated.

1: Pore 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: Non-ionic 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 (9)

A porous membrane including a polyolefin;
The porous membrane has a nonionic surfactant on or in the pore walls,
the nonionic surfactant is a polyethylene glycol derivative having 20 or less repeating units of polyethylene glycol, a sorbitan derivative, a polyoxyethylene sorbitan derivative, a glycerol derivative, or a polyoxyethylene glycerol derivative;
The hydrophobic group of the nonionic surfactant is a functional group consisting of an alkyl group, an alkenyl group, an aryl group, a saturated or unsaturated acyl group, an alkylsulfonyl group, an alkenylsulfonyl group, or a combination of these functional groups;
The separator for a non-aqueous electrolyte secondary battery, wherein the hydrophobic group has 4 or more and 20 or less carbon atoms. 2. The separator according to claim 1, wherein the nonionic surfactant is a polyethylene glycol derivative, which is a compound represented by the following chemical formula (1):
[In the formula, one of R ₁ and R ₂ represents a hydrogen atom and the other represents the hydrophobic group, and n is 1 or more and 20 or less.] 2. The separator according to claim 1, wherein the nonionic surfactant is a sorbitan derivative, which is a compound represented by the following chemical formula (2):
[In the formula, at least one of R ₁ , R ₂ , R ₃ and R ₄ represents a hydrogen atom, and at least one represents the hydrophobic group.] 2. The separator according to claim 1, wherein the nonionic surfactant is a polyoxyethylene sorbitan derivative, which is a compound represented by the following chemical formula (3):
[In the formula, at least one of R ₁ , R ₂ , R ₃ and R ₄ represents a hydrogen atom, at least one represents the hydrophobic group, and each of n, m, p and q is 1 or more and 20 or less.] 2. The separator according to claim 1, wherein the nonionic surfactant is a glycerol derivative and is a compound represented by the following chemical formula (4):
[In the formula, at least one of R ₁ , R ₂ and R ₃ represents a hydrogen atom, and at least one represents the hydrophobic group.] 2. The separator according to claim 1, wherein the nonionic surfactant is a polyoxyethylene glycerol derivative, which is a compound represented by the following chemical formula (5).
[In the formula, at least one of R ₁ , R ₂ and R ₃ represents a hydrogen atom, at least one represents the hydrophobic group, and each of n, m and p is 1 or more and 20 or less.] The separator according to any one of claims 1 to 6, wherein the pores of the porous membrane have an average pore diameter of 0.01 μm or more and 0.5 μm or less. A battery comprising the separator according to any one of claims 1 to 6, a positive electrode, a negative electrode, and an electrolyte solution,
The electrolytic solution of the nonaqueous electrolyte secondary battery contains an ionic liquid. The ionic liquid contains a cationic component and an anionic component,
The cationic component is a quaternary ammonium ion,
The quaternary ammonium ion has at least a chain structure or a cyclic structure in part,
9. The nonaqueous electrolyte secondary battery according to claim 8, wherein the anion component is a bis(fluorosulfonyl)amide ion.