JP2021153010A - Lithium secondary battery - Google Patents

Abstract

To provide a lithium secondary battery which is allowed to have a longer life.SOLUTION: A lithium secondary battery according to the present invention comprises: a positive electrode; a negative electrode; and an electrolyte sheet provided between the positive and negative electrodes. The electrolyte sheet has a porous separation film, and an oxide layer provided on at least one surface of the separation film, and containing oxide particles and a binder. The oxide particles are held in part of pores of the separation film on the surface side.SELECTED DRAWING: Figure 3

Description

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery in which an oxide layer is provided on the surface of a separation membrane.

In recent years, with the spread of portable electronic devices, electric vehicles, etc., high-performance secondary batteries are required. Among them, the lithium secondary battery has a high energy density, and is therefore attracting attention as a power source for electric vehicle batteries, power storage batteries, and the like. Specifically, lithium secondary batteries as batteries for electric vehicles include zero-emission electric vehicles that do not have an engine, hybrid electric vehicles that have both an engine and a secondary battery, and plug-in hybrid electricity that charges directly from the power system. It is used in electric vehicles such as automobiles. Further, a lithium secondary battery as a power storage battery is used in a stationary power storage system or the like that supplies power stored in advance in an emergency when the power system is cut off.

Lithium secondary batteries with higher energy densities are required and are being developed for use in such a wide range of applications. In particular, lithium secondary batteries for electric vehicles are required to have a long life in addition to high input / output characteristics and high energy density. As a factor of shortening the life, there is consumption of lithium ions due to side reactions such as oxidation and reduction of the electrolytic solution. In addition, the life may be deteriorated due to hydrogen fluoride (hydrogen fluoride), water content, etc. in the electrolytic solution. As the latter method for measures against deterioration of life, a lithium secondary battery using an oxide such as glass as a separation film is known (for example, Non-Patent Documents 1 and 2). Further, a lithium secondary battery in which an oxide layer is provided on the surface of the separation membrane is also known (for example, Patent Document 1).

Special Table 2017-519619

Nature Catalysis, Volume 1, pp. 255-262, 2018 Journal of Membrane Science, Vol. 449, pp. 169-175, 2014

However, in the configuration in which an oxide such as glass is used for the separation membrane, since the oxide such as glass is distributed throughout the separation membrane, there is a concern that it will be reduced by the negative electrode of the lithium secondary battery. For example, even if an oxide such as glass absorbs hydrogen fluoride, water, etc. in the electrolytic solution, hydrogen fluoride, water, etc. are reduced by the reducing power of the negative electrode, and a film containing Li grows on the negative electrode. There is. When the coating film containing Li grows, the lithium ion occlusion and release reaction at the negative electrode is inhibited. Further, even if the oxide layer is provided on the surface of the separation membrane, the same problem will occur if the oxide layer is opposed to the negative electrode.

Further, if the oxide layer is provided only on the surface of the separation membrane, the permeation of the electrolytic solution into the separation membrane is hindered by the oxide layer, and the oxide layer having a higher affinity with the electrolytic solution than the separation membrane. The electrolyte will be absorbed. As a result, there is a concern that the distribution of the electrolytic solution in the separation membrane becomes non-uniform due to the decrease in the permeation rate of the electrolytic solution into the separation membrane. If the distribution of the electrolytic solution on the separation membrane becomes non-uniform, the flow of lithium ions becomes uneven during charging / discharging, and a non-uniform charging / discharging reaction occurs, which accelerates the deterioration of the battery.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a lithium secondary battery capable of extending its life.

In order to solve the above problems, the lithium secondary battery of the present invention is a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte sheet provided between the positive electrode and the negative electrode, and the electrolyte sheet. Has a porous separation membrane and an oxide layer containing an oxide particle and a binder provided on at least one surface of the separation membrane, and the oxide particles are the above-mentioned in the pores of the separation membrane. It is characterized in that it is held on a part of the surface side.

According to the present invention, the life of the lithium secondary battery can be extended.

Issues, configurations, and effects of the present invention other than those described above will be clarified by the following description of embodiments for carrying out the invention.

It is a perspective view which shows the lithium ion secondary battery which is an example of the lithium secondary battery which concerns on embodiment. It is an exploded perspective view which shows the electrode group in the lithium ion secondary battery shown in FIG. It is a schematic cross-sectional view which shows the electrode group in the lithium ion secondary battery shown in FIG. It is an SEM photograph which observed one surface of the electrolyte sheet in Example 3. 3 is an SEM photograph of the other surface of the electrolyte sheet in Example 3.

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

Numerical values and their ranges herein are not intended to limit the present invention. In the present specification, the numerical range indicated by using "~" indicates a range including the numerical values before and after "~" as the minimum value and the maximum value, respectively. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value described in another stepwise description. Further, in the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.

[Embodiment]
FIG. 1 is a perspective view showing a lithium ion secondary battery which is an example of the lithium secondary battery according to the embodiment. The lithium ion secondary battery 1 shown in FIG. 1 includes an electrode group 2 composed of a positive electrode, a negative electrode, and an electrolyte sheet, and a bag-shaped battery exterior body 3 accommodating the electrode group 2. The positive electrode and the negative electrode are provided with a positive electrode current collecting tab 4 and a negative electrode current collecting tab 5, respectively. The positive electrode current collecting tab 4 and the negative electrode current collecting tab 5 project from the inside of the battery exterior 3 to the outside so that the positive electrode and the negative electrode can be electrically connected to the outside of the lithium ion secondary battery 1, respectively.

The battery exterior 3 may be formed of, for example, a laminated film. The laminated film may be, for example, a laminated film in which a resin film such as polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper or stainless steel, and a sealant layer such as polypropylene are laminated in this order.

FIG. 2 is an exploded perspective view showing an electrode group in the lithium ion secondary battery shown in FIG. FIG. 3 is a schematic cross-sectional view showing an electrode group in the lithium ion secondary battery shown in FIG. As shown in FIGS. 2 and 3, the electrode group 2A (2) includes a positive electrode 6, a negative electrode 8, and an electrolyte sheet 7 provided between the positive electrode 6 and the negative electrode 8, and these are laminated in this order. There is. The positive electrode 6 has a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. The positive electrode current collector 9 is provided with a positive electrode current collector tab 4. The negative electrode 8 has a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. The negative electrode current collector 11 is provided with a negative electrode current collector tab 5.

As shown in FIG. 3, the electrolyte sheet 7 has a porous separation membrane 20 and an oxide layer 30 provided on the surface 20a of the separation membrane 20 facing the positive electrode 6, and the separation membrane 20 and oxidation. It has a laminated structure of the material layer 30. The oxide layer 30 contains oxide particles and a binder (not shown). In the electrolyte sheet 7, although not shown, the electrolytic solution is held in the pores of the separation membrane 20 and the oxide layer 30.

The average pore size D of the separation membrane 20 and the average particle size d of the oxide particles contained in the oxide layer 30 satisfy the relational expression of D / 3 ≦ d <D. As a result, the oxide particles can penetrate into the pores of the separation membrane 20, but the free movement of the oxide particles in the pores of the separation membrane 20 is hindered. Therefore, although not shown, the oxide particles contained in the oxide layer 30 are retained only in a part of the pores of the separation membrane 20 on the surface 20a side facing the positive electrode 6 of the separation membrane 20 and are contained in the pores. The oxide particles of the above are prevented from moving to the surface 20b side of the separation membrane 20 facing the negative electrode 8.

Therefore, in the lithium ion secondary battery 1, the average particle size d of the oxide particles is small enough to satisfy the relational expression of D / 3 ≦ d <D, so that hydrogen fluoride in the electrolytic solution by the oxide layer 30 is satisfied. And the absorption of water and the like can be promoted. Further, since the oxide particles contained in the oxide layer 30 have a large amount of oxygen and hydroxyl groups, they have high affinity with the electrolyte solvent (polar solvent molecules having oxygen). Since both polar functional groups (δ +, δ−) are attracted to each other, the wettability of the electrolytic solution solvent to the oxide particles is very high. Further, in the portion of the pores of the separation membrane 20 where the oxide particles are retained, the capillary force is increased due to the narrowing of the voids.
Therefore, the oxide particles are held in a part of the pores of the separation membrane 20 on the surface 20a side facing the positive electrode 6 of the separation membrane 20, so that the separation membrane 20 is separated from the surface 20a facing the positive electrode 6. The permeation rate of the electrolytic solution into the membrane 20 can be improved. As a result, the distribution of the electrolytic solution on the separation membrane 20 can be made uniform, and it is possible to suppress uneven charging / discharging of the lithium ion flow and non-uniform charging / discharging reaction.

Further, in the lithium ion secondary battery 1, the oxide layer 30 does not face the negative electrode 8, and the oxide particles in the pores may move to the surface 20b side of the separation membrane 20 facing the negative electrode 8. Be hindered. Therefore, hydrogen fluoride, water, and the like absorbed by the oxide particles are reduced by the reducing power of the negative electrode, and it is possible to suppress the growth of a film containing Li on the negative electrode.

Therefore, in the lithium secondary battery according to the embodiment, the absorption of hydrogen fluoride, water, etc. in the electrolytic solution by the oxide layer can be promoted, and the distribution of the electrolytic solution in the separation membrane can be made uniform. Therefore, the life can be extended. Further, when the oxide layer is not provided on the surface of the separation film facing the negative electrode, the life can be extended by suppressing the growth of the film containing Li on the negative electrode. In addition to this, when the average pore size D of the separation membrane 20 and the average particle size d of the oxide particles contained in the oxide layer 30 satisfy the relational expression of D / 3 ≦ d <D, Li is placed on the negative electrode. The growth of the coating film containing the above can be effectively suppressed, and the life extension can be further promoted.

Subsequently, each configuration of the lithium secondary battery according to the embodiment will be described in detail.

1. 1. Electrolyte sheet The electrolyte sheet has a porous separation membrane and an oxide layer containing oxide particles and binders provided on at least one surface of the separation membrane. The oxide particles are retained in a part of the surface side of the pores of the separation membrane.

(1) Separation Membrane The separation membrane is not particularly limited as long as it is porous and can hold oxide particles on a part of the surface side of the pores. It has heat resistance that can withstand heating when evaporating the solvent of the slurry composition applied to the surface (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent), does not react with the slurry composition, and Those that do not swell due to the slurry composition are preferable.
Examples of the separation membrane include non-woven fabrics made of biologically-derived cellulose fibers and polymer fibers, resin films, and the like. Examples of the non-woven fabric made of biologically-derived cellulose fibers include non-woven fabrics made of pulp and the like.
Examples of the non-woven fabric made of polymer fibers include non-woven fabrics made of synthetic fibers such as polyethylene, polypropylene, polyimide, and cellulose acetate. Examples of the resin film include a film made of a resin (general-purpose engineer plastic) such as polyethylene terephthalate, polytetrafluoride ethylene, polyimide, polyethersulfone, and polyetherketone.

The separation membrane preferably has a heat resistant temperature that can withstand heating when the solvent of the slurry composition applied to the surface of the separation membrane is evaporated when the oxide layer is formed. The heat-resistant temperature of the separation membrane is the lower of the softening point (the temperature at which plastic deformation begins) or the melting point of the separation membrane when the separation membrane is made of resin. The heat resistant temperature of the separation membrane is preferably 50 ° C. or higher, more preferably 100 ° C. or higher, further preferably 150 ° C. or higher, and for example, 400 ° C. or lower, from the viewpoint of compatibility with grime used for the electrolyte sheet. It may be there. If such a heat-resistant temperature separation membrane is used, a solvent described later can be preferably used.

It is desirable that the thickness of the separation membrane be as small as possible while maintaining the strength that can withstand the tensile force in the coating apparatus. For example, the tensile strength required to convey the separation membrane in the coating device is 0.1 N / cm ² or more, so a higher tensile strength is required. At the same time, the thickness of the separation membrane must be thinner than the electrolyte sheet.

The thickness of the separation membrane is not particularly limited as long as it is thinner than the electrolyte sheet, but is, for example, 5 μm or more and 100 μm or less, preferably 10 μm or more and 50 μm or less, more preferably 15 μm or more and 40 μm or less, and further preferably 15 μm or more and 30 μm or less. , Even more preferably 15 μm or more and 25 μm or less. When the thickness of the separation membrane is at least the lower limit of these ranges, the positive electrode and the negative electrode can be reliably insulated. When the thickness of the separation membrane is less than or equal to the upper limit of these ranges, the battery resistance of the separation membrane can be reduced to a desired range, and the entire pores of the separation membrane can be filled with the electrolyte by supplying a small amount of electrolyte. It becomes advantageous in.

The average pore diameter of the separation membrane is not particularly limited as long as the oxide particles can be retained in a part of the pores of the separation membrane on the surface side of the separation membrane, but is, for example, 0.01 μm or more and 2 μm or less. It is preferably 0.05 μm or more and 1 μm or less, and more preferably 0.1 μm or more and 0.5 μm or less. When the average pore diameter of the separation membrane is equal to or greater than the lower limit of these ranges, the oxide particles contained in the oxide layer can easily enter the pores from the surface of the separation membrane. It becomes easy to hold it on a part of the surface side of the separation film in. When the average pore diameter of the separation membrane is not more than the upper limit of these ranges, free movement of the oxide particles in the pores of the separation membrane can be effectively prevented.

Here, the average pore diameter of the separation membrane refers to the median diameter obtained from the pore distribution on the surface and inside of the separation membrane, and is a value measured by the porosimeter method by press-fitting with mercury or water.

The porosity of the separation membrane is not particularly limited, and the larger the porosity, the smaller the battery resistance, which is preferable. For example, it is 40% or more and 95% or less. When the porosity of the separation membrane is at least the lower limit of this range, the battery resistance of the separation membrane can be reduced to a desired range. When the porosity of the separation membrane is not more than the upper limit of this range, the problem that the strength of the separation membrane is lowered and the tensile strength thereof is reduced can be suppressed.

(2) Oxide layer The oxide layer is a layer containing oxide particles and a binder provided on at least one surface of the separation membrane.

For the oxide layer, a slurry composition containing at least oxide particles, a binder, and a slurry solvent is applied to at least one surface of the separation membrane, and the solvent (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent) is evaporated. It is formed by letting it. The slurry composition is obtained in a slurry composition preparation step in which oxide particles and binders and a slurry solvent for dissolving them are mixed.

Hereinafter, the oxide particles, the binder, the slurry solvent for dissolving them, the slurry composition, and the oxide layer will be described in detail.

a. Oxide particles Oxide particles are not particularly limited, but are, for example, inorganic oxide particles. The inorganic oxide may be, for example, an inorganic oxide containing Li, Mg, Al, Si, Ca, Ti, Zr, La, Na, K, Ba, Sr, V, Nb, B, Ge, etc. as constituent elements. .. The oxide particles are preferably at least one selected from the group consisting of _{SiO 2} , Al ₂ O ₃ , glass, AlOOH, MgO, CaO, ZrO ₂ , TiO ₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , and BaTIO _3. Seed particles. Since the oxide particles have polarity, the dissociation of the electrolyte in the electrolyte sheet can be promoted and the battery characteristics can be improved.

Oxide particles generally include primary particles (particles that do not form secondary particles) that integrally form a single particle, and multiple primary particles, judging from their apparent geometrical morphology. May include secondary particles formed by the aggregation of.

The average particle size of the oxide particles is not particularly limited, but is, for example, 0.001 μm or more and 10 μm or less, preferably 0.005 μm or more and 5 μm or less, more preferably 0.005 μm or more and 1 μm or less, and further preferably 0.005 μm. It is 0.1 μm or less. This is because when the average particle size of the oxide particles is at least these lower limits, the oxide layer is less likely to fall off from the separation membrane. When the average particle size of the oxide particles is equal to or less than these upper limits, the absorption of hydrogen fluoride, water, etc. in the electrolytic solution by the oxide particles can be effectively promoted, and the oxide particles are separated into a separation membrane. This is because it becomes easy to penetrate into the pores of.

Here, the average particle size of the oxide particles is measured by a laser diffraction method, and corresponds to a particle size in which the volume accumulation is 50% when the volume cumulative particle size distribution curve is drawn from the small particle size side.

The average primary particle size of the oxide particles (average particle size of the primary particles) is not particularly limited, but is preferably 0.005 μm (5 nm) or more, more preferably 0.01 μm (10 nm), from the viewpoint of further improving the conductivity. ) Or more, more preferably 0.015 μm (15 nm) or more. The average primary particle size of the oxide particles is preferably 1 μm or less, more preferably 0.1 μm or less, still more preferably 0.05 μm or less from the viewpoint of thinning the electrolyte sheet. The average primary particle size of the oxide particles is preferably 0.005 μm to 1 μm, more preferably 0.01 μm, from the viewpoint of thinning the electrolyte sheet and suppressing the protrusion of the oxide particles from the surface of the electrolyte sheet. It is ~ 0.1 μm, more preferably 0.015 μm to 0.05 μm.

The average primary particle size of the oxide particles can be measured by observing the oxide particles with a transmission electron microscope or the like.

The specific surface area of the oxide particles is not particularly limited, for ^example, a ^{2m 2 / g~380m 2 / g,} 5m 2 / g~100m 2 / g, 10m 2 / g~80m 2 / g, or 15 m ² It may be / g to 60 m ^{2 / g.} If the specific surface area is ^{2m 2} / ^g~380m 2 / g, there is a tendency that excellent in discharge characteristics of the secondary battery. From the same viewpoint, the specific surface area of the oxide particles, ^{5 m} 2 / g or more, ^{10 m} 2 / g or more, or may be ^{15 m} 2 / g or more, 100 m ² / g or less, ^{80 m} 2 / g or less, or ^{60 m} 2 / It may be less than or equal to g.

The specific surface area of the oxide particles means the specific surface area of the entire oxide particles including the primary particles and the secondary particles, and is measured by the BET method.

The shape of the oxide particles is not particularly limited, but may be, for example, agglomerates or substantially spherical. The aspect ratio of the oxide particles is not particularly limited, but is preferably 10 or less, more preferably 5 or less, still more preferably 2 or less, from the viewpoint of facilitating the thinning of the electrolyte sheet.

The aspect ratio of the oxide particles is the length in the major axis direction of the particles (maximum length of the particles) and the length in the minor axis direction of the particles (particles) calculated from the scanning electron micrograph of the oxide particles. Is defined as the ratio to the minimum length of. The length of the particles is obtained by statistically calculating the above-mentioned photograph using a commercially available image processing foot (for example, image analysis software manufactured by Asahi Kasei Engineering Co., Ltd., A-kun (registered trademark)).

The oxide particles may be surface-treated with a surface treatment agent. The surface treatment agent is not particularly limited, and examples thereof include silicon-containing compounds and the like. Silicon-containing compounds include, for example, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, dimethoxydiphenylsilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, tetraethoxysilane, methyltriethoxysilane. , Dimethyldiethoxysilane, alkoxysilanes such as n-propyltriethoxysilane, 2- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxy Epyl group-containing silanes such as silane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, N-2- (aminoethyl) -3-aminopropylmethyldimethoxysilane, N-2- (Aminoethyl) Amino group-containing silanes such as -3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, silazane such as hexamethyldisilazane, and dimethylsilicone oil. Or siloxane or the like.

As the oxide particles surface-treated with the surface treatment agent, those produced by a known method may be used, or commercially available products may be used as they are.

The ratio of the mass of the oxide particles to the total mass of the oxide particles and the binder is not particularly limited, but is, for example, 1% by mass or more and 90% by mass or less, preferably 5% by mass or more and 50% by mass or less, more preferably. It is 5% by mass or more and 30% by mass or less. This is because when the mass ratio is equal to or higher than these lower limits, it is possible to prevent the oxide layer from falling off from the separation membrane. This is because when the mass ratio is not more than these upper limits, it is possible to prevent the flow of lithium ions from being obstructed by the binder.

When the slurry composition does not contain an electrolytic solution, the residue excluding the oxide particles in the slurry composition and the solvent to be vaporized (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent) is the composition of the binder alone. , Or the total composition of the binder and the cellulose fibers. If the slurry composition contains an electrolyte, the rest of the slurry composition, excluding the oxide particles and the solvent to be evaporated, is the total composition of the binder and the electrolyte, or the binder, the electrolyte, and the cellulose fibers. It becomes the composition such as the total composition of.

b. Binder The binder is not particularly limited, but is, for example, a polymer having a first structural unit selected from the group consisting of ethylene tetrafluoride and vinylidene fluoride, polyacrylic acid, polyimide, styrene-butadiene rubber, carboxymethyl cellulose and the like. Those containing alkali salts, chitosan and the like are preferable.

In addition to the first structural unit, the structural unit constituting the polymer includes a second structural unit selected from the group consisting of hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate. May be. That is, the first structural unit and the second structural unit may be contained in one kind of polymer to form a copolymer, and each of them is contained in another polymer and has a first structural unit. And a second polymer having a second structural unit may constitute at least two kinds of polymers.

The polymer having the first structural unit is not particularly limited, and may be, for example, polyethylene tetrafluoride, polyvinylidene fluoride, or a copolymer of vinylidene fluoride and hexafluoropropylene.

As the binder, a water-soluble binder such as polyacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose and an alkali salt thereof, and chitosan can also be used. Water can be used as the slurry solvent for these water-soluble binders. Further, polyacrylic acid, carboxymethyl cellulose and an alkali salt thereof, and chitosan are preferable because they are oxygen-containing binders. Further, from the viewpoint of cost and availability, polyvinylidene fluoride and a copolymer of vinylidene fluoride and hexafluoropropylene are preferable.

c. Slurry solvent The slurry solvent is not particularly limited as long as it can disperse the above-mentioned binder and oxide particles, and may be an organic solvent or water. Examples of the slurry solvent include 1-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylethyleneurea, γ-butyrolactone, methyl ethyl ketone, ethyl acetate, butyl carbitol acetate, toluene, cyclohexane, and sulfolane. And water and the like.

The ratio of the mass of the slurry solvent to the total mass of 100 parts by mass of the binder and the oxide particles is not particularly limited, but is, for example, 10 parts by mass to 90 parts by mass. The ratio of the mass of the slurry solvent to the total mass of 100 parts by mass of the binder and the oxide particles is preferably 20 parts by mass or more, more preferably 30 parts by mass or more, and further preferably 40 parts by mass or more. The ratio of the mass of the slurry solvent to the total mass of 100 parts by mass of the binder and the oxide particles is preferably 70 parts by mass or less, more preferably 60 parts by mass or less, and further preferably 50 parts by mass or less.

d. Slurry composition In the slurry composition preparation step, the method of adding each component of the oxide particles, the binder, and the slurry solvent is not particularly limited, but from the viewpoint of further suppressing the generation of lumps, the slurry composition preparation step is, for example, , The first step of dry mixing the oxide particles and the binder to obtain a mixture, the second step of adding a slurry solvent to the mixture to obtain a mixture dispersion, and mixing the mixture dispersion to obtain a slurry composition. It may include a third step of obtaining a product. In the second step, the slurry solvent may be added to the mixture in a predetermined amount at a time, but from the viewpoint of further suppressing the generation of lumps, the slurry solvent is divided into a plurality of times so as to be a predetermined amount. May be added. When the slurry solvent is added in a plurality of times, the second step and the third step may be repeated.

Mixing (kneading) of each component in the slurry composition preparation step can be performed by appropriately combining a disperser such as a normal stirrer, a raker, a triple roll, a ball mill, and a bead mill. The mixing conditions (kneading conditions) are not particularly limited and can be appropriately set according to the type and content of each component.

In the slurry composition preparation step, the mixing of each component of the oxide particles, the binder, and the slurry solvent is preferably a mixing by kneading. Here, kneading means kneading in a state where the proportion of the slurry solvent is lower (that is, the proportion of the solid content is higher) than that of the finally obtained slurry composition. By kneading in a state where the proportion of the slurry solvent is low, it tends to be possible to obtain a mixture in which each component is uniformly dispersed. The content of the slurry solvent in the slurry composition at this time is not particularly limited, but is, for example, 40 parts by mass or more with respect to 100 parts by mass of the total mass of the oxide particles and the binder. By increasing the solid content in this way, it is possible to apply a strong shearing force to the material, and it is possible to eliminate the agglomeration of binders, oxide particles and the like and disperse them.

The determination of completion in the slurry composition preparation step is not particularly limited, but for example, a smaller amount of sample than the mixed slurry composition is taken, and the sample itself or a sample to which a solvent is added is used as a grain gauge (grind). One guideline is to measure the size of the coarse particles when the sample is dropped on a meter) and scraped off the sample with a scraper to confirm that each component is uniformly dispersed. The size of the coarse particles measured above needs to be less than or equal to the thickness of the electrolyte sheet, and preferably less than 1/2 the thickness of the electrolyte sheet to obtain a smooth electrolyte sheet with less unevenness. Is possible and suitable.

The slurry composition may further contain an electrolytic solution. This makes it possible to omit the step of adding the electrolytic solution after forming the oxide layer on the surface of the separation membrane.

The electrolytic solution contains a lithium salt (electrolyte salt) and an electrolytic solution solvent capable of dissolving the lithium salt, and the lithium salt is dissolved in the electrolytic solution solvent.

Lithium salts are compounds used to transfer cations between the positive and negative electrodes. Lithium salts have a low degree of dissociation at low temperatures, easily diffuse in grime, and do not thermally decompose at high temperatures, so that the environmental temperature in which a secondary battery can be used is wide, which is preferable.

Anionic component of the lithium salt is not particularly limited, for example, halide ions ^{(I -, Cl -, Br} - , _{^{etc.), SCN -, BF 4 -}} , BF 3 (CF 3) -, BF 3 (C 2 F _{^{5) -, PF 6 -,}} ClO 4 -, SbF 6 -, N (SO 2 F) 2 -, N (SO 2 CF 3) 2 -, N (SO 2 C 2 F 5) 2 -, B (C _{^{6 H 5) 4 -, B}} (O 2 C 2 H 4) 2 -, C (SO 2 F) 3 -, C (SO 2 CF 3) 3 -, CF 3 COO -, CF 3 SO 2 O -, C ₆ F ₅ SO ₂ O ⁻ , B (O ₂ C ₂ O ₂ ) ₂ ^−, etc. may be used. The anion component of the electrolyte salt is preferably an anion component represented by the general formula (III) described later, which is exemplified by an anion component such as _{N (SO 2} F) ₂ ⁻ , N (SO ₂ CF ₃ ) ₂ ^−. PF ₆ ⁻ , BF ₄ ⁻ , B (O ₂ C ₂ O ₂ ) ₂ ⁻ , or ClO ₄ ⁻ .

In the following, the following abbreviations may be used.
[FSI] ⁻ : N (SO ₂ F) ₂ ⁻ , bis (fluorosulfonyl) imide anion [TFSI] ⁻ : N (SO ₂ CF ₃ ) ₂ ⁻ , bis (trifluoromethanesulfonyl) imide anion [BOB] ⁻ : B (O ₂ C ₂ O ₂ ) ₂ ⁻ , bisoxalate borate anion [f3C] ⁻ : C (SO ₂ F) ₃ ⁻ , tris (fluorosulfonyl) carbanion

The lithium salt is not particularly limited, but is, for example, LiPF ₆ , LiBF ₄ , Li [FSI], Li [TFSI], Li [f3C], Li [BOB], LiClO ₄ , LiBF ₃ (CF ₃ ), LiBF ₃ ( C ₂ F ₅ ), LiBF ₃ (C ₃ F ₇ ), LiBF ₃ (C ₄ F ₉ ), LiC (SO ₂ CF ₃ ) ₃ , CF ₃ SO ₂ OLi, CF ₃ COOLi, and R'COOLi (R' Is at least one selected from the group consisting of an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

The lithium salt is preferably an imide-based lithium salt. The imide-based lithium salt may be, for example, Li [TFSI], Li [FSI] or the like.

The electrolyte solvent used for the lithium salt is not particularly limited, and examples thereof include a compound (glime) represented by the general formula (I), a carbonic acid ester solvent, and the like. The electrolytic solution preferably contains a compound (grime) represented by the general formula (I) as the electrolytic solution solvent.

In the general formula (I), ^{R A} and ^{R B} each independently represent an alkyl group having 1 to 4 carbon atoms, y is an integer of 1-6. The alkyl group as R ^A and ^{R B} are methyl group, ethyl group, n- propyl group, an isopropyl group, n- butyl group, an isobutyl group, or a t- butyl group. Among these, the alkyl group is preferably a methyl group or an ethyl group.

Examples of the grime include triethylene glycol dimethyl ether (sometimes referred to as "triglyme" or "G3"), tetraethylene glycol dimethyl ether (sometimes referred to as "tetraglyme" or "G4"), and pentaethylene glycol dimethyl ether ("penta"). Glyme "or" G5 "), hexaethylene glycol dimethyl ether (sometimes referred to as" hexaglyme "or" G6 ") and the like. These may be used individually by 1 type or in combination of 2 or more type. Among these, the grime is preferably triglime or tetraglime, and more preferably tetraglime.

A part or all of the compound (grime) represented by the general formula (I) in the electrolytic solution may form a complex with the lithium salt.

The molar ratio of the compound (glime) represented by the general formula (I) to the lithium salt (the amount of substance of the compound (glime) represented by the general formula (I) / the amount of substance of the lithium salt) is 0.1 to 10. Is fine. The molar ratio may be 1 or more, 2 or more, or 3 or more, or 5 or less, 4 or less, or 3 or less from the viewpoint of further improving the charge / discharge characteristics.

It is possible to add an electrolyte solution in which a hydrolyzable electrolyte salt (for example, LiPF ₆ or LiBF ₄ ) is dissolved in a carbonate ester solvent to the electrolyte sheet. In this case, the slurry composition may be devised so as not to mix water as much as possible. For example, since cellulose fibers contain water, they may not be used, or water-free reinforcing materials such as starch, cellulose acetate, and carboxylmethyl cellulose may be substituted.

When the slurry composition contains an electrolytic solution, electrolysis with respect to the total mass (mass of the electrolyte sheet) of the components excluding the solvent to be evaporated (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent) in the slurry composition. The ratio of the mass of the liquid (content of the electrolytic solution) is not particularly limited, but is, for example, 10% by mass or more and 90% by mass or less.

The slurry composition may further contain cellulose fibers. The cellulose fiber acts as a reinforcing material and can prevent the oxide layer from falling off from the separation membrane. Further, since the cellulose fiber contains oxygen, it is also preferable in that it has an ability to hold an electrolytic solution.

The cellulose fibers are not particularly limited, but are, for example, natural cellulose fibers such as coniferous wood pulp, broadleaf wood pulp, esparto pulp, Manila hemp pulp, sisal hemp pulp, and cotton pulp; these natural cellulose fibers are spun with an organic solvent. The obtained recycled cellulose fiber such as lyocell may be used. When the electrolyte sheet contains cellulose fibers, strength can be imparted to the electrolyte sheet.

The shape of the cellulose fiber is not particularly limited, but it may be an elongated shape having a certain length. The aspect ratio of the cellulose fibers is not particularly limited, but may be 10 or more, preferably more than 10, more preferably 20 or more, still more preferably 50 or more, particularly preferably 50 or more, from the viewpoint of facilitating the suppression of heat shrinkage of the electrolyte sheet. Is 100 or more.

The aspect ratio of the cellulose fiber can be obtained by the same method as the above-mentioned method for calculating the aspect ratio of the oxide particles. That is, the length and fiber diameter of the cellulose fibers can be obtained and statistically calculated from the scanning electron micrograph of the cellulose fibers.

The average length of the cellulose fibers is not particularly limited, but is preferably equal to or larger than the average particle size of the oxide particles, and more preferably the average of the oxide particles, for example, from the viewpoint of facilitating the suppression of heat shrinkage of the electrolyte sheet. The particle size is 2 times or more, 5 times or more, 10 times or more, 20 times or more, 50 times or more, or 100 times or more. The average length of the fibers is preferably 1 μm or more, more preferably 5 μm or more, still more preferably 10 μm or more. The average length of the fibers is preferably 10,000 μm or less, 5000 μm or less, 3000 μm or less, 2000 μm or less, 1500 μm or less from the viewpoint of suppressing protrusion from the electrolyte sheet when forming a secondary battery and smoothing the electrolyte sheet. , 1000 μm or less, 500 μm or less, 100 μm or less, or 50 μm or less. As for the average length of the fibers in the present specification, in the scanning electron micrograph, three fibers in which the entire fiber is shown in the photograph are selected so as to avoid duplication of measurement, and the length of the fiber is determined for each photograph. The integrated value is the average value obtained by allocating the total number of fibers.

The average fiber diameter of the fibers is not particularly limited, but is preferably, for example, preferably less than or equal to the thickness of the electrolyte sheet, and more preferably less than or equal to 1/3 of the thickness of the electrolyte sheet. The average fiber diameter of the fibers is preferably 10 μm or less, more preferably 5 μm or less, still more preferably 1 μm or less. By setting the average fiber diameter of the fibers to be less than or equal to the thickness of the electrolyte sheet, the electrolyte sheet can be thinned to a desired thickness, and the movement of oxide particles can be suppressed even with a small amount of fiber added. can. The average fiber diameter of the fibers may be, for example, 0.01 μm or more. For the average fiber diameter of the fibers in the present specification, three fibers in which the entire fiber is shown in the photograph are selected in the scanning electron micrograph so as to avoid duplication of measurement, and the total fiber diameter is integrated for each photograph. The value is the average value obtained by allocating the value by the total number of fibers.

When a slurry composition containing cellulose fibers is prepared in the slurry composition preparation step, a mixture dispersion of oxide particles and binder and a fiber dispersion in which cellulose fibers are dispersed in a fiber dispersion solvent are usually mixed. The method is used.

Examples of the fiber dispersion solvent include a solvent similar to the slurry solvent, and a solvent similar to the slurry solvent is preferable.

The ratio of the mass of the fiber dispersion solvent to 100 parts by mass of the cellulose fibers in the fiber dispersion is not particularly limited, but is 30 parts by mass to 2000 parts by mass. The ratio of the mass of the fiber dispersion solvent to 100 parts by mass of the cellulose fibers in the fiber dispersion is preferably 50 parts by mass or more, more preferably 100 parts by mass or more, and further preferably 150 parts by mass or more. The ratio of the mass of the fiber dispersion solvent to 100 parts by mass of the cellulose fibers in the fiber dispersion is preferably 1000 parts by mass or less, more preferably 500 parts by mass or less, and further preferably 300 parts by mass or less.

When the slurry composition contains cellulose fibers, cellulose with respect to the total mass (mass of the electrolyte sheet) of the components excluding the solvent to be evaporated in the slurry composition (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent). The ratio of the mass of the fibers (cellulose fiber content) is not particularly limited, but is, for example, 0.1% by mass or more and 10% by mass or less, preferably 2% by mass or more and 8% by mass or less, and more preferably 3% by mass. It is 6% by mass or less, more preferably 4% by mass or more and 5% by mass or less. When the mass ratio of the cellulose fibers is at least the lower limit of these ranges, the fibers are uniformly blended in the electrolyte sheet, and the movement of oxide particles can be effectively suppressed. When the mass ratio of the cellulose fibers is not more than the upper limit of these ranges, more oxide particles having high heat resistance can be blended in the electrolyte sheet, and the fluidity of the finally obtained slurry composition can be obtained. It may be possible to make the electrolyte sheet thinner. In addition, the cellulose fibers and the binder have a function of strongly fixing the oxide particles on the separation membrane. The same is true in the pores.

In the slurry composition preparation step, the order in which the fiber dispersion liquid and the electrolytic solution are mixed with respect to the mixture dispersion liquid of the oxide particles and the binder is not particularly limited, and the fiber dispersion liquid and the electrolytic solution may be in that order, and the electrolytic solution may be used. And the fiber dispersion, and the fiber dispersion and the electrolytic solution may be mixed with the mixture dispersion of the oxide particles and the binder at the same time. The order of mixing the fiber dispersion and the electrolytic solution with respect to the mixture dispersion of the oxide particles and the binder is preferably the order of the fiber dispersion and the electrolytic solution from the viewpoint of further suppressing the generation of lumps.

When the fiber dispersion and the electrolytic solution are mixed with the mixture dispersion of oxide particles and binder, the mixing (kneading) of each component is carried out by a normal stirrer, a raker, a triple roll, a ball mill, etc. Dispersers such as a bead mill can be combined as appropriate. The mixing conditions (kneading conditions) are not particularly limited, and can be appropriately set according to the type and content of each component.

In the slurry composition preparation step, the viscosity is adjusted when preparing a mixture dispersion of oxide particles and a binder, when preparing a fiber dispersion, when preparing an electrolytic solution, and when preparing a slurry composition. From the viewpoint of the above, solvents such as a slurry solvent, an electrolytic solution solvent, and a fiber dispersion may be added all at once and mixed, but the solvent may be divided and added and mixed, or the solvent may be divided and added. The step of mixing may be performed a plurality of times.

The total concentration of the constituent components in the slurry composition excluding the solvent to be evaporated (for example, the slurry solvent or the slurry solvent and the fiber dispersion solvent) may be, for example, 20% by mass to 80% by mass. This is because the larger the holding amount of the electrolytic solution, the higher the ionic conductivity of the separation membrane, and the easier the charge / discharge reaction proceeds.

The slurry composition may contain other components. Examples of other components include resin fibers, glass fibers, flake-like inorganic oxides, and the like. The ratio of the mass of the other components to the total mass of the components excluding the solvent to be evaporated in the slurry composition is not particularly limited, but is, for example, 0.1% by mass to 20% by mass.

In the method for producing a slurry composition according to the embodiment, the occurrence of lumps can be suppressed. Therefore, it is possible to reduce the thickness of the oxide layer provided on the surface of the separation membrane (for example, 0.1 μm to 10 μm). Even in that case, sufficient strength can be imparted to the oxide layer by containing the cellulose fiber in the oxide layer.

The slurry composition is not particularly limited, and for example, an oxide particle, a binder, a lithium salt, a compound represented by the above general formula (I), a cellulose fiber, a slurry solvent and a fiber dispersion solvent are used. The ratio of the mass of the cellulose fibers to the total mass of the constituent components excluding the slurry solvent and the fiber dispersion solvent in the slurry composition is preferably 0.1 to 10% by mass.

e. Oxide layer The oxide layer is not particularly limited as long as it contains oxide particles and a binder, but may hold an electrolytic solution. In order to retain the electrolytic solution in the oxide layer, the electrolytic solution may be mixed with the slurry composition in advance, or a slurry composition containing no electrolytic solution is applied to the surface of the separation membrane and a solvent (for example, a slurry solvent) is applied. , Or the oxide layer formed by evaporating the slurry solvent and the fiber dispersion solvent) may be impregnated with the electrolytic solution. Since the oxide particles of the oxide layer have a high affinity with the electrolytic solution, the electrolytic solution rapidly permeates the pores of the oxide layer and the separation membrane, and once permeates the pores of the oxide layer and the separation membrane. The liquid is held firmly without leaking.

The thickness of the oxide layer needs to be 0.1 μm or more so that the separation membrane is not exposed and comes into direct contact with the positive electrode. By setting the thickness of the oxide layer to the lower limit of this range or more, it is possible to prevent pinholes and the like from being generated in the oxide layer and the exposed portion of the separation film from being generated. The thickness of the oxide layer is, for example, 10% or less of the thickness of the separation membrane, preferably 1% or less of the thickness of the separation membrane. By setting the thickness of the oxide layer to be equal to or less than the upper limit of these ranges, it is possible to prevent the volume of the battery from increasing too much.

(3) Electrolyte sheet The electrolyte sheet has a slurry composition preparation step, a slurry composition coating step of applying the slurry composition to at least one surface of the separation membrane, and a solvent to be removed from the slurry composition (for example, a slurry solvent). Alternatively, it is produced by a production method comprising a step of forming an oxide layer in which an oxide layer is formed on at least one surface of the separation membrane of the separation membrane by removing the slurry solvent and the fiber dispersion solvent).

The method of applying the slurry composition to at least one surface of the separation membrane is not particularly limited, and examples thereof include a doctor blade method, a dipping method, and a spray method.

The method for removing the solvent to be removed from the slurry composition is not particularly limited, and examples thereof include a method for evaporating the solvent by heating the slurry composition. The heating temperature in the method of evaporating the solvent can be appropriately set according to the solvent used. Further, in the method of applying the slurry composition to evaporate the solvent, the solvent is rapidly evaporated to allow the oxide particles and the binder to penetrate into the pores of the separation membrane, and the slurry composition in the separation membrane is applied. Almost no exposure can be made to the surface opposite to the surface that has been exposed.

Oxide particles in the pores of the separation membrane move very slowly or stop in the middle of moving in the pores. Normally, among the oxide particles that have penetrated into the pores of the separation membrane, only particles having a particle size of 1/2 or less of the pore diameter of the separation membrane can freely move in the pores. Further, the movement of the oxide particles in contact with at least the wall surface is hindered by the affinity of the oxide particles and the inner wall of the pores and the physical frictional force. As a result, theoretically, only 1/3 of the pore diameter of the separation membrane remains in the pores of the separation membrane so that the oxide particles can freely move. Further, as the pores of the separation membrane are bent, the space in which the oxide particles can freely move is also bent, so that the diameter of the oxide particles that can freely move in the pores of the separation membrane is further reduced. By utilizing the property of restricting the movement of oxide particles in the pores of the separation membrane as described above, the slurry composition in the separation membrane can be obtained by applying the slurry composition to one surface of the separation membrane. It is possible to realize a structure in which the oxide particles are not substantially exposed on the surface opposite to the coated surface. The average pore size D of the separation membrane and the average particle size d of the oxide particles contained in the oxide layer preferably satisfy the relational expression d <D because the oxide particles easily penetrate into the pores of the separation membrane. Since the above structure can be easily realized, it is more preferable to satisfy the relational expression of D / 3 ≦ d <D.

The thickness of the electrolyte sheet is not particularly limited, but may be, for example, 5 μm to 100 μm. The thickness of the electrolyte sheet may be, for example, 5 μm or more, 10 μm or more, or 15 μm or more, and may be 50 μm or less, 40 μm or less, or 30 μm or less. When the thickness of the electrolyte sheet is at least the lower limit of these ranges, the electrical insulation between the positive electrode and the negative electrode can be ensured. When the thickness of the electrolyte sheet is not more than the upper limit of these ranges, the resistance of ion conduction between the positive electrode and the negative electrode can be reduced.

The electrolyte sheet is not particularly limited, but has a separation membrane and an oxide layer provided on at least one surface of the separation membrane, and the oxide layer includes a binder, oxide particles, a lithium salt, and the above. It is preferable that the compound represented by the general formula (I) and the cellulose fiber are contained, and the ratio of the mass of the cellulose fiber to the total mass of the electrolyte sheet is 0.1% by mass to 10% by mass. In the electrolyte sheet, the oxide layer may be changed to one that does not contain cellulose fibers, or the compound represented by the above general formula (I) may be replaced with another electrolyte solvent.

The type and content of each component in the electrolyte sheet can be the same as the type and content of each component in the slurry composition preparation step.

The electrolyte sheet can also be continuously produced while being wound into a roll. In that case, the surface of the electrolyte sheet on the side where the oxide layer is provided comes into contact with the surface on the opposite side of the electrolyte sheet, and a part of the surface is attached to the separation membrane, so that the electrolyte sheet is damaged. Sometimes. In order to prevent such a situation, the electrolyte sheet may be one in which a protective material is provided on the surface of the electrolyte sheet on the side where the oxide layer is provided.

The protective material is not particularly limited, and a material that can be easily peeled off from the electrolyte sheet can be used, but a non-polar resin film such as polyethylene, polypropylene, and polyethylene tetrafluoride is preferable. When a non-polar resin film is used, the electrolyte sheet and the protective material do not stick to each other, and the protective material can be easily peeled off.

The thickness of the protective material is preferably 5 μm or more, more preferably 10 μm, and preferably 100 μm or less, more preferably, from the viewpoint of ensuring the strength while reducing the total volume of the electrolyte sheet and the protective material. It is 50 μm or less, more preferably 30 μm or less.

The heat-resistant temperature of the protective material is preferably −30 ° C. or higher, more preferably 0 ° C. or higher, and preferably 100, from the viewpoint of suppressing deterioration in a low temperature environment and suppressing softening in a high temperature environment. ° C or lower, more preferably 50 ° C or lower. When the protective material is provided, it is not necessary to raise the heat resistant temperature because the step of evaporating the above solvent is not essential.

The electrolyte sheet may be integrated with the reinforcing material. In this case, the electrolyte sheet can be used as a self-supporting membrane. As a result, the electrolyte sheet can be easily inserted between the positive electrode and the negative electrode by automatic transfer, so that the battery can be automatically manufactured.

2. Positive electrode The positive electrode is not particularly limited, but usually has a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector.

The positive electrode current collector is not particularly limited, and may be, for example, a current collector made of a metal such as aluminum, stainless steel, or titanium. Specifically, the positive electrode current collector may be, for example, an aluminum perforated foil having holes with a hole diameter of 0.1 mm to 10 mm, an expanded metal, a foamed metal plate, or the like. In addition to the above, the positive electrode current collector may be made of any material as long as it does not cause changes such as dissolution and oxidation during use of the battery, and its shape and manufacturing method are not limited. ..

The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 10 μm or more and 100 μm or less. The thickness of the positive electrode current collector is preferably 10 μm or more and 50 μm or less from the viewpoint of reducing the volume of the entire positive electrode, and more preferably 10 μm or more from the viewpoint of winding the positive electrode with a small curvature when forming a battery. It is 20 μm or less.

The positive electrode mixture layer is not particularly limited, but includes, for example, a positive electrode active material, a conductive agent, and a binder.

The positive electrode active material is not particularly limited, and examples thereof include lithium transition metal compounds such as lithium transition metal oxides and lithium transition metal phosphates.

The lithium transition metal oxide is not particularly limited, and may be, for example, lithium manganate, lithium nickel oxide, lithium cobalt oxide, or the like. The lithium transition metal oxide is, for example, a part of a transition metal such as Mn, Ni, Co contained in lithium manganate, lithium nickelate, lithium cobalt, etc., and one or more other transition metals. , Or a lithium transition metal oxide substituted with a metal element (typical element) such as Mg or Al. That is, the lithium transition metal oxide may ^{be a compound represented by LiM 1} O ₂ or LiM ¹ O ₄ (M ¹ contains at least one transition metal). Specifically, the lithium transition metal oxide is, for example, Li (Co _1/3 Ni _1/3 Mn _1/3 ) O ₂ , LiNi _1/2 Mn _1/2 O ₂ , LiNi _1/2 Mn _{3 /. 2} O ₄ etc. may be used.

The lithium transition metal oxide is preferably a compound represented by the formula (II) from the viewpoint of further improving the energy density.

[In formula (II), M ² is at least one selected from the group consisting of Al, Mn, Mg, and Ca, and a, b, c, d, and e are 0.2 ≦ a ≦, respectively. A number that satisfies 1.2, 0.5 ≦ b ≦ 0.9, 0.1 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.2, −0.2 ≦ e ≦ 0.2, and b + c + d = 1. Is. ]

The lithium transition metal phosphate is not particularly limited, but is, for example, LiFePO ₄ , LiMnPO ₄ , LiMn _x M ³ _1-x PO ₄ (0.3 ≦ x ≦ 1, M ³ is Fe, Ni, Co, Ti, It may be at least one element selected from the group consisting of Cu, Zn, Mg, and Zr) and the like.

The positive electrode active material may be primary particles that have not been granulated, or secondary particles that have been granulated.

The particle size of the positive electrode active material is adjusted so as to be equal to or less than the thickness of the positive electrode mixture layer. If the positive electrode active material contains coarse particles having a particle size equal to or greater than the thickness of the positive electrode mixture layer, the coarse particles are removed in advance by sieving classification, wind flow classification, etc., and the particle size is equal to or less than the thickness of the positive electrode mixture layer. Select the positive electrode active material to have.

The average particle size (D ₅₀ ) of the positive electrode active material is not particularly limited, but is preferably 0.1 μm or more, more preferably 1 μm or more, for example. Further, it is preferably 30 μm or less, more preferably 25 μm or less. The average particle size (D ₅₀ ) of the positive electrode active material is the particle size when the ratio (volume fraction) to the volume of the entire positive electrode active material is 50%. The average particle size (D ₅₀ ) of the positive electrode active material is measured by using a laser scattering type particle size measuring device (for example, Microtrac) to measure a suspension in which the positive electrode active material is suspended in water by a laser scattering method. You can get it.

The content of the positive electrode active material may be 70% by mass or more, 80% by mass or more, or 85% by mass or more based on the total amount of the positive electrode mixture layer. The content of the positive electrode active material may be 95% by mass or less, 92% by mass or less, or 90% by mass or less based on the total amount of the positive electrode mixture layer.

The conductive agent is not particularly limited, and may be, for example, a carbon material such as graphite, acetylene black, carbon black, carbon fiber, or carbon nanotube. The conductive agent may be a mixture of two or more of the above carbon materials.

The content of the conductive agent may be 0.1% by mass or more, 1% by mass or more, or 3% by mass or more based on the total amount of the positive electrode mixture layer. The content of the conductive agent is preferably 15% by mass or less, more preferably 10 based on the total amount of the positive electrode mixture layer, from the viewpoint of suppressing the increase in the volume of the positive electrode and the accompanying decrease in the energy density of the lithium secondary battery. It is mass% or less, more preferably 8 mass% or less.

The binder is not particularly limited as long as it does not decompose on the surface of the positive electrode, and is, for example, a group consisting of ethylene tetrafluoride, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, and methyl methacrylate. Examples thereof include polymers containing at least one selected as a monoma unit, and rubbers such as styrene-butadiene rubber, isoprene rubber, and acrylic rubber. The binder is preferably a copolyma containing ethylene tetrafluoride and vinylidene fluoride as structural units.

The content of the binder may be 0.5% by mass or more, 1% by mass or more, or 3% by mass or more based on the total amount of the positive electrode mixture layer. The content of the binder may be 20% by mass or less, 15% by mass or less, or 10% by mass or less based on the total amount of the positive electrode mixture layer.

The positive electrode mixture layer may further contain an ionic liquid. The ionic liquid has the following anionic and cationic components. The ionic liquid in the embodiment is a substance that is liquid at −20 ° C. or higher. It is known that ionic liquids have almost no vapor pressure, unlike molecular liquids such as water and solvents, due to the strong electrostatic interaction between the constituent cations and anions.

The anion component of the ionic liquid is not particularly limited, for ^{example, Cl -, Br -, I} - and a halogen _{^{anion, BF 4 -, N (SO}} 2 F) 2 - or the like of the inorganic anion, B _{(C 6 ^{H 5) 4 -, CH 3}} SO 2 O -, CF 3 SO 2 O -, N (SO 2 C 4 F 9) 2 -, N (SO 2 CF 3) 2 -, N (SO 2 C 2 F 5 ) ₂ ^- and organic anions and the like of.

The anionic component of the ionic liquid preferably contains at least one of the anionic components represented by the general formula (III).

In the general formula (III), m and n each independently represent an integer of 0 to 5. m and n may be the same or different from each other, and are preferably the same as each other.

The anion component represented by formula (III), is not particularly limited, for ^{_{example, N (SO 2 C 4 F}} 9) 2 -, N (SO 2 F) 2 -, N (SO 2 CF 3) 2 ^-, and _{N (SO 2 C 2 F 5} ) 2 - and the like.

_{The anionic component of the ionic liquid is more preferably N (SO 2} C ₄ F ₉ ) ₂ ⁻ , CF ₃ SO from the viewpoint of further improving the ionic conductivity and the charge / discharge characteristics with a relatively low viscosity. Includes at least one selected from the group consisting of ₂ O ⁻ , N (SO ₂ F) ₂ ⁻ , N (SO ₂ CF ₃ ) ₂ ⁻ , and N (SO ₂ C ₂ F ₅ ) ₂ ^{−, more preferably.} N _{(SO 2 F)} 2 ^- containing.

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

The content of the ionic liquid in the positive electrode mixture layer is not particularly limited, but is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, based on the total amount of the positive electrode mixture layer. .. The content of the ionic liquid in the positive electrode mixture layer is preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less, based on the total amount of the positive electrode mixture layer.

An electrolyte salt may be dissolved in the ionic liquid in the positive electrode mixture layer. The electrolyte salt used in the above slurry composition can be used.

The thickness of the positive electrode mixture layer is a thickness equal to or larger than the average particle size of the positive electrode active material from the viewpoint of further improving the conductivity, and specifically, it may be 10 μm or more, 15 μm or more, or 20 μm or more. .. The thickness of the positive electrode mixture layer may be 100 μm or less, 80 μm or less, or 70 μm or less. By setting the thickness of the positive electrode mixture layer to 100 μm or less, it is possible to suppress the bias of charge / discharge due to the variation in the charge level of the positive electrode active material in the vicinity of the surface of the positive electrode mixture layer and in the vicinity of the surface of the positive electrode current collector.

3. 3. Negative electrode The negative electrode is not particularly limited, but usually has a negative electrode current collector and a negative electrode mixture layer provided on the negative electrode current collector.

The negative electrode current collector is not particularly limited, but may be, for example, a metal such as aluminum, copper, nickel, or stainless steel, or one composed of an alloy thereof. The negative electrode current collector is preferably made of aluminum or an alloy thereof from the viewpoint of being lightweight and having a high weight energy density. The negative electrode current collector is preferably made of copper from the viewpoint of ease of processing into a thin film and cost.

The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 10 μm or more and 100 μm or less, preferably 10 μm or more and 50 μm or less from the viewpoint of reducing the volume of the entire negative electrode, and a small curvature when forming a battery. From the viewpoint of winding the negative electrode, the thickness is more preferably 10 μm or more and 20 μm or less.

The negative electrode mixture layer is not particularly limited, but includes, for example, a negative electrode active material and a binder.

The negative electrode active material is not particularly limited, but a material commonly used in the energy device field can be used. Examples of the negative electrode active material include metallic lithium, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), lithium alloys, and other metal compounds, carbon materials, metal complexes, organic polymer compounds, and the like. The negative electrode active material may be one of these alone or a mixture of two or more. Carbon materials include natural graphite (scaly graphite, etc.), graphite such as artificial graphite (graphite), amorphous carbon, carbon fiber, and acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal. Examples include carbon black such as black. The negative electrode active material is silicon, tin, or a compound containing these elements (alloy with an oxide, a nitride, or another metal) from the viewpoint of obtaining a larger theoretical capacity (for example, 500 Ah / kg to 1500 Ah / kg). But it may be.

The average particle size (D ₅₀ ) of the negative electrode active material is preferably 1 μm or more from the viewpoint of obtaining a well-balanced negative electrode having an enhanced ability to retain electrolyte salts while suppressing an increase in irreversible capacity due to a decrease in particle size. It is more preferably 5 μm or more, further preferably 10 μm or more, and preferably 50 μm or less, more preferably 40 μm or less, still more preferably 30 μm or less. The average particle size (D ₅₀ ) of the negative electrode active material is the particle size when the ratio (volume fraction) to the volume of the entire negative electrode active material is 50%. The average particle size of the negative electrode active material (D ₅₀₎ is measured in the same manner as the average particle size of the positive electrode active material (D _50).

The content of the negative electrode active material may be 60% by mass or more, 65% by mass or more, or 70% by mass or more based on the total amount of the negative electrode mixture layer. The content of the negative electrode active material may be 99% by mass or less, 95% by mass or less, or 90% by mass or less based on the total amount of the negative electrode mixture layer.

The binder and its content may be the same as the binder and its content in the positive electrode mixture layer. The negative electrode mixture layer may further contain a conductive agent from the viewpoint of further lowering the resistance of the negative electrode. The conductive agent and its content may be the same as the conductive agent and its content in the positive electrode mixture layer.

The negative electrode mixture layer may further contain an ionic liquid. As the ionic liquid, an ionic liquid that can be contained in the positive electrode mixture layer can be used. The content of the ionic liquid contained in the negative electrode mixture layer is not particularly limited, but is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, based on the total amount of the negative electrode mixture layer. Is. The content of the ionic liquid contained in the negative electrode mixture layer is not particularly limited, but is preferably 30% by mass or less, more preferably 25% by mass or less, still more preferably 20% by mass or less, based on the total amount of the negative electrode mixture layer. Is.

The ionic liquid contained in the negative electrode mixture layer may contain an electrolyte salt similar to the electrolyte salt used in the positive electrode mixture layer.

The thickness of the negative electrode mixture layer is not particularly limited, but may be, for example, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the negative electrode mixture layer may be, for example, 100 μm or less, 80 μm or less, or 70 μm or less.

4. Method for Manufacturing a Lithium Secondary Battery A method for manufacturing a lithium secondary battery according to an embodiment will be described. The method for manufacturing a lithium secondary battery includes a first step of forming a positive electrode mixture layer on a positive electrode current collector to obtain a positive electrode, and a first step of forming a negative electrode mixture layer on a negative electrode current collector to obtain a negative electrode. It includes a second step and a third step of arranging the above-mentioned electrolyte sheet between the positive electrode and the negative electrode.

In the first step, as a method of forming a positive electrode mixture layer on the positive electrode current collector to obtain a positive electrode, for example, the material used for the positive electrode mixture layer is used as a dispersion medium by using a mixer, a disperser, or the like. Examples thereof include a method in which the positive electrode mixture is dispersed to obtain a slurry-shaped positive electrode mixture, the positive electrode mixture is applied onto the positive electrode current collector by a doctor blade method, a dipping method, a spray method, or the like, and then the dispersion medium is evaporated. After evaporating the dispersion medium, a compression molding step by a roll press may be performed, if necessary. The positive electrode mixture layer may be formed as a multilayer structure positive electrode mixture layer by performing the steps from the application of the positive electrode mixture to the evaporation of the dispersion medium a plurality of times.

The dispersion medium used in the first step may be water, 1-methyl-2-pyrrolidone (hereinafter, also referred to as NMP) or the like.

In the second step, the method of forming the negative electrode mixture layer on the negative electrode current collector may be the same method as in the first step described above.

In the third step, the method of arranging the electrolyte sheet between the positive electrode and the negative electrode is, for example, peeling the protective material from the electrolyte sheet provided with the protective material, and laminating the positive electrode, the electrolyte sheet, and the negative electrode by laminating. A method for obtaining a lithium secondary battery can be mentioned. In the method of arranging the electrolyte sheet between the positive electrode and the negative electrode, the electrolyte sheet is located on the positive electrode mixture layer side of the positive electrode and the negative electrode mixture layer side of the negative electrode, that is, the positive electrode current collector and the positive electrode mixture layer. , The electrolyte sheet, the negative electrode mixture layer, and the negative electrode current collector are laminated in this order. At this time, the electrolyte sheet is placed on the positive electrode and the negative electrode so that the oxide layer of the electrolyte sheet faces the positive electrode mixture layer of the positive electrode and the side opposite to the oxide layer of the electrolyte sheet faces the negative electrode mixture layer of the negative electrode. It is preferably placed between the negative electrodes.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to these examples.

[Example 1]
An evaluation coin-type battery, which is a test body of the lithium secondary battery of the present invention, was produced by the following procedure.

(Preparation of slurry composition)
First, a slurry composition used for forming an oxide layer was prepared. Specifically, first, molten silica (SFP-20M manufactured by Denka Co., Ltd.) having an average particle size of 0.4 μm is prepared as oxide particles, and as a binder, a copolyma (PVDF-) of vinylidene fluoride and hexafluoropropylene is prepared. HFP, vinylidene fluoride: hexafluoropyrene = 94% by mass: 6% by mass) were prepared.

Next, 0.5 parts by mass of molten silica and 19.5 parts by mass of copolymer were mixed at 10 rpm for 10 minutes using a mixer. Next, 30 parts by mass of a slurry solvent (1-methyl-2-pyrrolidone) was added to 20 parts by mass of the mixture of molten silica and copolyma, and the mixture was mixed at 15 rpm for 20 minutes using a mixer. Next, an equimolar electrolytic solution (LiTFSI / G4) of tetraglime (G4) (electrolyte solution solvent) and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) was added to the mixture dispersion obtained thereby. .. At this time, the amount of the equimolar electrolytic solution added was set to 80 parts by mass.

Next, the mixture dispersion obtained by adding the electrolytic solution was mixed at 40 rpm for 20 minutes using a mixer. As a result, a slurry composition was prepared. Table 1 below shows the components of the slurry composition excluding the slurry solvent and the content of each component (the ratio of the mass of each component to the total mass of the components excluding the slurry solvent in the slurry composition). The fact that each component was dispersed in the slurry composition was confirmed by measuring the size of coarse grains using a grain gauge (grind meter).

(Preparation of electrolyte sheet)
An electrolyte sheet was prepared using the above slurry composition. Specifically, first, as a separation membrane, a non-woven fabric of cellulose fibers derived from pulp having a thickness of 25 μm and an average pore diameter of 1 μm was prepared. Next, the slurry composition was applied to one surface of the non-woven fabric using an applicator. At this time, a polyimide sheet was used as a base in order to prevent leakage of slurry from the non-woven fabric.

Next, the slurry composition applied to one surface of the non-woven fabric was heated and evaporated at 80 ° C. for 1 hour to evaporate the slurry solvent, and then the underlying polyimide sheet was peeled off from the non-woven fabric. As a result, an electrolyte sheet in which the non-woven fabric and the oxide layer provided on one surface of the non-woven fabric were integrated was produced. The thickness of the electrolyte sheet was 26 μm. Therefore, the thickness of the oxide layer is determined to be 1 μm by subtracting the thickness of the original non-woven fabric of 25 μm from the thickness of the electrolyte sheet of 26 μm. The thickness of the oxide layer is shown in Table 1 below.

(Preparation of positive electrode)
First, layered lithium-nickel-manganese-cobalt composite oxide (positive electrode active material) 78.5 parts by mass, acetylene black (conductive agent, average particle size 48 nm, HS-100 manufactured by Denka Co., Ltd.) 5 parts by mass, fluoride 2.5 parts by mass of copolyma solution (binding agent, solid content 12% by mass) of vinylidene and hexafluoropropylene, and 14 parts by mass of equimolar solution of Li [TFSI] (lithium bis (trifluorosulfonyl) imide) and tetraglime. By mixing the parts, a positive electrode mixture slurry was prepared. Next, the positive electrode mixture slurry is applied onto a current collector (aluminum foil with a thickness of 20 μm) at a coating amount of 147 g / m ² and evaporated at 80 ° C. to obtain a mixture density of 2.9 g / cm ³ A positive electrode mixture layer was formed. This was punched to φ16 mm to form a positive electrode.

(Preparation of negative electrode)
First, 78 parts by mass of graphite A (negative electrode active material, graphite manufactured by Hitachi Kasei Co., Ltd.), 2.4 parts by mass of graphite B (graphite manufactured by Nippon Graphite Industry Co., Ltd.), carbon fiber (conductive agent, VGCF-manufactured by Showa Denko KK) H,) 0.6 parts by mass, 5 parts by mass of copolyma solution of vinylidene fluoride and hexafluoropropylene (binding agent, solid content 12% by mass), and Li [TFSI] (lithium bis (trifluorosulfonyl) imide) To prepare a negative mixture slurry by mixing 14 parts by mass of an equimolar solution of graphite with. Next, the negative electrode mixture slurry is applied onto a current collector (copper foil with a thickness of 10 μm) at a coating amount of 68 g / m ² and evaporated at 80 ° C. to obtain a mixture density of 1.9 g / cm ³ A negative electrode mixture layer was formed. This was punched to φ16 mm and used as a negative electrode.

(Manufacturing of coin-type batteries for evaluation)
An evaluation coin-type battery was produced using the above-mentioned electrolyte sheet, positive electrode, and negative electrode. Specifically, first, the electrolyte sheet was punched to φ18 mm so that a short circuit could be prevented by making it slightly larger than the positive electrode and the negative electrode.

Next, the positive electrode, the electrolyte sheet, and the negative electrode were arranged in this order in the CR2016 type coin cell container, and the upper part of the battery container was crimped and sealed via an insulating gasket. At this time, the electrolyte sheet is used for the positive electrode and the negative electrode so that the oxide layer of the electrolyte sheet faces the positive electrode mixture layer of the positive electrode and the side opposite to the oxide layer of the electrolyte sheet faces the negative electrode mixture layer of the negative electrode. Placed in between. Further, an electrolytic solution having the same composition as the electrolytic solution used for the electrolyte sheet was dropped onto the electrode to hold the electrolytic solution in the pores of the electrode. As a result, a coin-type battery for evaluation was produced. The arrangement positions of the oxide layers are shown in Table 1 below.

[Example 2]
In the preparation of the slurry composition, the evaluation coin-type battery under the same conditions as in Example 1 except that the mixed amount of molten silica was increased to 1 part by mass and the mixed amount of copolyma was reduced to 19 parts by mass. Was produced. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 3]
In the preparation of the slurry composition, the evaluation coin cell battery under the same conditions as in Example 1 except that the mixed amount of molten silica was increased to 5 parts by mass and the mixed amount of copolyma was reduced to 15 parts by mass. Was produced. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 4]
Except for the fact that in the preparation of the slurry composition, the mixed amount of molten silica was increased to 1 part by mass, the mixed amount of copolyma was increased to 29 parts by mass, and the amount of LiTFSI / G4 added was reduced to 70 parts by mass. , An evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 5]
Except for the fact that in the preparation of the slurry composition, the mixed amount of molten silica was increased to 1 part by mass, the mixed amount of copolyma was reduced to 9 parts by mass, and the amount of LiTFSI / G4 added was increased to 90 parts by mass. , An evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 6]
Except for the fact that in the preparation of the slurry composition, the average particle size of the molten silica was increased to 1.0 μm, the mixed amount of molten silica was increased to 1 part by mass, and the mixed amount of copolima was reduced to 19 parts by mass. , An evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 7]
In the preparation of the slurry composition, the oxide particles were changed to fumed silica having an average particle size of 7 nm (fumed silica manufactured by Sigma Aldrich), the mixed amount of fumed silica was 1 part by mass, and the mixed amount of copolyma was 19. An evaluation coin-type battery was produced under the same conditions as in Example 1 except that it was reduced to a mass portion. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 8]
In the preparation of the slurry composition, the mixing amount of molten silica was increased to 1 part by mass, the mixing amount of copolyma was reduced to 19 parts by mass, and the electrolyte was changed from LiTFSI / G4 to ethylene carbonate and ethylmethyl carbonate by volume ratio 1: Except for the change to the electrolyte (LiPF ₆ / EC-EMC) in which lithium hexafluorophosphate (LiPF ₆ ^{) having a concentration of 1 mol / dm 3} was dissolved in the mixed electrolyte solvent (EC-EMC) of 2. , An evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Example 9]
In the preparation of the slurry composition, the oxide particles were changed to fumed silica having an average particle size of 7 nm (fumed silica manufactured by Sigma Aldrich), the mixed amount of fumed silica was 1 part by mass, and the mixed amount of copolyma was 19. Lithium hexafluorophosphate (lithium hexafluorophosphate ^{) having a concentration of 1 mol / dm 3} was added to a mixed electrolyte solvent (EC-EMC) having a volume ratio of ethylene carbonate and ethyl methyl carbonate of 1: 2 from LiTFSI / G4 by reducing the amount to parts by mass. An evaluation coin-type battery was produced under the same conditions as in Example 1 except that the electrolyte solution (LiPF ₆ / EC-EMC) in which LiPF _{6) was dissolved was changed.} Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Comparative Example 1]
In the preparation of the slurry composition, a coin-type battery for evaluation was produced under the same conditions as in Example 1 except that the oxide particles were not mixed and the mixing amount of copolyma was increased to 20 parts by mass. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Comparative Example 2]
In the preparation of the slurry composition, the mixed amount of molten silica was increased to 1 part by mass, and the mixed amount of copolyma was reduced to 19 parts by mass. Further, in the production of the coin-type battery for evaluation, the oxide layer of the electrolyte sheet faces the negative electrode mixture layer of the negative electrode, and the side opposite to the oxide layer of the electrolyte sheet faces the positive electrode mixture layer of the positive electrode. , The electrolyte sheet was placed between the positive electrode and the negative electrode. Except for these points, an evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer.

[Comparative Example 3]
In the preparation of the slurry composition, the mixed amount of molten silica was increased to 1 part by mass, and the mixed amount of copolyma was reduced to 19 parts by mass. Further, in the preparation of the electrolyte sheet, the slurry composition is applied to both surfaces of the nonwoven fabric, and the slurry composition applied to both surfaces of the nonwoven fabric is heated and evaporated to provide oxides provided on both surfaces of the nonwoven fabric. An electrolyte sheet in which the layers were integrated was prepared. Further, in the production of the coin-type battery for evaluation, one oxide layer of the electrolyte sheet faces the positive electrode mixture layer of the positive electrode, and the other oxide layer of the electrolyte sheet faces the negative electrode mixture layer of the negative electrode. An electrolyte sheet was placed between the positive electrode and the negative electrode. Except for these points, an evaluation coin-type battery was produced under the same conditions as in Example 1. Table 1 below shows the components of the slurry composition excluding the slurry solvent, the contents of each component, and the thickness and arrangement position of the oxide layer. In Table 1 below, the thickness of the oxide layer of Comparative Example 3 is shown as the total of both oxide layers.

<SEM observation>
FIG. 4 is an SEM photograph of observing one surface of the electrolyte sheet in Example 3. In the non-woven fabric, since the slurry composition was applied from one surface shown in FIG. 4, an oxide layer was observed on one surface shown in FIG. 4, and the pores of the non-woven fabric were unclear. Non-woven cellulose fibers are exposed in the lower left of the photo. Some of the oxide particles have penetrated into the pores on the right side. On the other hand, FIG. 5 is an SEM photograph of the other surface of the electrolyte sheet in Example 3. In the non-woven fabric, the slurry composition was applied from the surface opposite to the surface shown in FIG. 5 (the surface shown in FIG. 4). Therefore, on the surface side shown in FIG. 5, reticulated cellulose fibers were observed and oxides were observed. The particles did not penetrate, and the pores of the non-woven fabric were clearly observed.

<Measurement of discharge capacity>
The evaluation coin-type batteries produced in each Example and each Comparative Example were subjected to the first charge / discharge cycle test at room temperature (25 ° C.), and the discharge capacity was measured. Specifically, in the first charge / discharge cycle test, the battery is charged with a constant current of 0.3 mA until the battery voltage reaches 4.2 V, and after reaching 4.2 V, the current is 0.03 mA at that voltage. It was charged until it became, and the amount of electricity at this time was measured as the charge capacity. Subsequently, the battery was discharged at a constant current of 0.3 mA until the battery voltage became 2.7 V, and the amount of electricity at this time was measured as the discharge capacity. The discharge capacity of all the batteries was 3 mAh, which was as designed. The results are shown in Table 2 below.

<Measurement of capacity retention rate by charge / discharge cycle test>
The charge / discharge cycle test was carried out 100 times under the same conditions as the initial charge / discharge cycle test in the measurement of the discharge capacity. Then, the ratio of the discharge capacity measured in the 100th charge / discharge cycle test to the discharge capacity measured in the first charge / discharge cycle test was calculated as the capacity retention rate (%). The capacity retention rate is shown in Table 2 below.

As shown in Tables 1 and 2 above, comparing Examples 1 to 3 in which the content of fused silica (oxide particles) and the content of copolyma (binder) in the slurry composition are different, the content of fused silica is compared. The larger the value, the larger the capacity retention rate.

It is presumed that this is because the amount of hydrogen fluoride and the like absorbed by the oxide particles increased as the content of the oxide particles increased, and the amount of reductive decomposition on the negative electrode decreased.

Further, comparing Examples 2, 4, and 5 in which the content of copolyma (binder) and the content of LiTFSI / G4 (electrolyte solution) are different, there is no significant difference in the capacity retention rate.

From this, it was found that the content of LiTFSI / G4 may be changed from 70% by mass to 90% by mass.

Comparing Examples 2 and 6 in which the average particle size of the molten silica (oxide particles) is different, the smaller the average particle size of the molten silica, the larger the capacity retention rate. This suggests that the larger the specific surface area of the oxide particles, the larger the amount of hydrogen fluoride or the like absorbed by the oxide particles.

Further, when Examples 6 and 7 in which the types of oxide particles are different between fused silica and fumed silica are compared, there is no significant difference in the capacity retention rate. This suggests that the battery life is the same even if the type of oxide particles is changed.

Comparing Examples 2 and 8 in which the types of electrolytic solutions are LiTFSI / G4 and LiPF ₆ / EC-EMC, respectively, the capacity retention rate of Example 8 is high. Similarly, when Examples 7 and 9 in which the types of electrolytic solutions are LiTFSI / G4 and LiPF ₆ / EC-EMC, respectively, are compared, the capacity retention rate of Example 9 is high.

These facts are that when the type of electrolytic solution is one containing a carbonic acid ester solvent as the electrolytic solution solvent, the capacity retention rate becomes high, and in particular, absorption by hydrogen fluoride oxide particles presumed to be produced from _{LiPF 6} Explains that the effect of

Comparing Example 1 provided with the oxide layer and Comparative Example 1 not provided with the oxide layer, it is recognized that the capacity retention rate of Comparative Example 1 is lower than that of Example 1.

Further, when comparing Example 2, Comparative Example 2 and Comparative Example 3 in which the arrangement positions of the oxide layers are different, it is recognized that the capacity retention rates of Comparative Example 2 and Comparative Example 3 are lower than those of Example 2. Be done.

In Example 2, by setting the position of the oxide layer provided on at least one surface of the non-woven fabric (separation membrane) to be the position facing the positive electrode, hydrogen fluoride, water, etc. generated from the positive electrode by the oxide particles, etc. Was able to be preferentially absorbed, and it is considered that the reductive decomposition of hydrogen fluoride, water, etc. on the negative electrode could be suppressed.

On the other hand, in Comparative Example 2 and Comparative Example 3, by setting the position of the oxide layer provided on at least one surface of the non-woven fabric to be the position facing the negative electrode, the oxide particles absorb hydrogen fluoride and the like. It is considered that the amount has decreased.

In the electrolyte sheets of Examples 1 to 9, the oxide particles were provided with an oxide layer in the pores of the separation membrane because of the relationship between the average pore size D of the separation membrane and the average particle size d of the oxide particles. It is considered that it is held on a part of the surface side of the separation membrane. Further, it is considered that the oxide particles are not exposed on the surface of the separation membrane where the oxide layer is not provided.

The present invention is not limited to the above embodiment. The above embodiment is an example, and any one having substantially the same configuration as the technical idea described in the claims of the present invention and exhibiting the same effect and effect is the present invention. It is included in the technical scope of the invention.

1 Lithium ion secondary battery 2 Electrode group 2A Electrode group 3 Battery exterior 4 Positive electrode current collector tab 5 Negative electrode current collector tab 6 Positive electrode 7 Electrode sheet 8 Negative electrode 9 Positive electrode current collector 10 Positive electrode mixture layer 11 Negative electrode current collector 12 Negative electrode Mixture layer 20 Separation film 30 Oxide layer

Claims (6)

A lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte sheet provided between the positive electrode and the negative electrode.
The electrolyte sheet has a porous separation membrane and an oxide layer containing oxide particles and binders provided on at least one surface of the separation membrane.
A lithium secondary battery characterized in that the oxide particles are retained in a part of the surface side of the pores of the separation membrane. The lithium secondary battery according to claim 1, wherein the average pore size D of the separation membrane and the average particle size d of the oxide particles satisfy the relational expression of D / 3 ≦ d <D. The binder consists of a group consisting of polyethylene tetrafluoride, polyvinylidene fluoride, and copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylic acid, polyimide, styrene-butadiene rubber, carboxymethyl cellulose and its alkali salt, and chitosan. The lithium secondary battery according to claim 1 or 2, wherein the lithium secondary battery is at least one selected. The oxide _particles are _{SiO 2, Al 2 O 3,} AlOOH, at least one MgO, _CaO, selected from the group consisting of _{ZrO 2, TiO 2, Li 7} La 3 Zr 2 O 12, and BaTiO ₃ The lithium secondary battery according to any one of claims 1 to 3, wherein the lithium secondary battery is characterized by this. The lithium secondary according to any one of claims 1 to 4, wherein the thickness of the oxide layer is 0.1 μm or more and 10% or less of the thickness of the separation film. battery. The lithium secondary battery according to any one of claims 1 to 5, wherein the thickness of the electrolyte sheet is 5 μm to 100 μm.

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