JP7374664B2 - Solid electrolyte sheet and all-solid lithium secondary battery - Google Patents

Description

The present invention relates to a solid electrolyte sheet that has excellent shape retention and can be made to have a large area, and an all-solid lithium secondary battery that includes the solid electrolyte sheet and has excellent discharge characteristics.

In recent years, with the development of portable electronic devices such as mobile phones and notebook personal computers, and the practical use of electric vehicles, there has been a need for small, lightweight, high-capacity, and high-energy-density secondary batteries. It has become to.

Currently, lithium secondary batteries that can meet this demand, especially lithium ion secondary batteries, use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) as positive electrode active materials. Graphite or the like is used as the negative electrode active material, and an organic electrolyte containing an organic solvent and a lithium salt is used as the nonaqueous electrolyte.

With the further development of equipment to which lithium ion secondary batteries are applied, there is a need for lithium ion secondary batteries to have a longer lifespan, higher capacity, and higher energy density. There is also a high demand for safety and reliability of lithium-ion secondary batteries with increased capacity and high energy density.

However, the organic electrolyte used in lithium-ion secondary batteries contains an organic solvent, which is a flammable substance, so when an abnormal situation such as a short circuit occurs in the battery, the organic electrolyte can generate abnormal heat. there is a possibility. In addition, with the recent trend toward higher energy density of lithium ion secondary batteries and an increase in the amount of organic solvent in organic electrolytes, there is a demand for even greater safety and reliability of lithium ion secondary batteries.

Under the above circumstances, all-solid-state lithium secondary batteries that do not use organic solvents are attracting attention. All-solid-state lithium secondary batteries use a molded solid electrolyte that does not use organic solvents instead of conventional organic solvent-based electrolytes, and there is no risk of abnormal heat generation of the solid electrolyte, ensuring high safety. We are prepared.

On the other hand, solid electrolyte molded bodies are brittle and have poor workability, making it difficult to form solid electrolytes into thin films and to enlarge the area. As a result, the solid electrolyte is difficult to handle during battery manufacture, and the molded solid electrolyte becomes thick, resulting in a decrease in lithium ion conductivity of the solid electrolyte, resulting in a decrease in battery performance.

On the other hand, techniques to solve such problems are also being considered. For example, in Patent Documents 1 and 2, a solid electrolyte sheet having both lithium ion conductivity and strength is obtained by filling the voids of a porous base material such as a nonwoven fabric with a solid electrolyte, and this solid electrolyte It has been proposed to construct an all-solid-state secondary battery using sheets.

Among these, in Patent Document 1, when fixing a solid electrolyte in the voids of a porous base material, a method is adopted in which the solid electrolyte is attached to an adhesive attached to the surface of the skeleton of the porous base material. . Moreover, in Patent Document 2, a method is adopted in which the relationship between the particle diameter of the solid electrolyte and the opening diameter of the nonwoven fabric is adjusted.

JP2015-153460A (claims, etc.) JP 2016-139482 (claims, etc.)

Although the techniques described in Patent Documents 1 and 2 make it possible to increase the strength to a certain extent compared to sheets made only of solid electrolytes, they are required for solid electrolyte sheets for all-solid lithium secondary batteries. There is still room for improvement in satisfying the shape retention requirements.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a solid electrolyte sheet that has excellent shape retention and can be made into a large area, and a solid electrolyte sheet that has the solid electrolyte sheet and has excellent discharge characteristics. The purpose of the present invention is to provide a solid-state lithium secondary battery.

The solid electrolyte sheet of the present invention uses an insulating porous base material as a support, the insulating porous base material is composed of resin fibers, and the basis weight of the insulating porous base material is x( g/m ² ), where y (N) is the tensile strength of the insulating porous base material, y/x≧0.45, and solid electrolyte particles are present inside the insulating porous base material. It is characterized by containing a binder.

In another aspect of the solid electrolyte sheet of the present invention, an insulating porous base material is used as a support, the insulating porous base material is composed of liquid crystal polyester fibers, and the insulating porous base material is made of liquid crystal polyester fibers. The material is characterized by containing solid electrolyte particles and a binder inside the material.

Furthermore, the all-solid-state lithium secondary battery of the present invention is characterized by having a positive electrode, a negative electrode, and the solid electrolyte sheet according to any aspect of the present invention inserted between the positive electrode and the negative electrode. It is something.

According to the present invention, it is possible to provide a solid electrolyte sheet that has excellent shape retention and can be made to have a large area, and an all-solid lithium secondary battery that includes the solid electrolyte sheet and has excellent discharge characteristics.

FIG. 1 is a plan view schematically showing an example of a solid electrolyte sheet of the present invention. 1 is a cross-sectional view schematically showing an example of an all-solid-state lithium secondary battery of the present invention.

The solid electrolyte sheet of the first aspect of the present invention uses an insulating porous base material as a support, the insulating porous base material is made of resin fibers, and the insulating porous base material is made of resin fibers. When the basis weight is x (g/m ² ) and the tensile strength of the insulating porous base material is y (N), y/x≧0.45, and the inside of the insulating porous base material contains solid electrolyte particles and a binder.

In order to increase the strength of a solid electrolyte sheet having a support, it is sufficient to use a support with a high tensile strength (y). For example, in the case of a support made of a porous base material, generally if the basis weight is increased, Tensile strength can be increased. However, as the basis weight of the porous base material increases, the ionic conductivity tends to decrease, so if a solid electrolyte sheet with such a porous base material is used, the discharge characteristics of an all-solid-state lithium secondary battery will deteriorate. Resulting in.

Therefore, in the solid electrolyte sheet of the present invention, the support holding the solid electrolyte particles is made of an insulating porous base having a tensile strength per basis weight (y/x) of 0.45 N/(g/m ² ) or more. We decided to construct it from wood. In other words, since the insulating porous base material has a high tensile strength of the fibers constituting the base material, there is no need to increase the tensile strength by increasing the basis weight or thickness, and the ions of the solid electrolyte sheet It can enhance conductivity and shape retention, and improve the discharge characteristics of all-solid-state lithium secondary batteries.

An insulating porous base material whose tensile strength per basis weight satisfies the above value can be formed using, for example, liquid crystal polyester fibers.

That is, in the solid electrolyte sheet of the second aspect of the present invention, the insulating porous base material serving as the support is composed of liquid crystal polyester fibers. With such an insulating porous base material, the tensile strength per basis weight can be increased to, for example, 0.45 N/(g/m ² ) or more, which improves the ionic conductivity and shape retention of the solid electrolyte sheet. , it is possible to improve the discharge characteristics of an all-solid-state lithium secondary battery.

Furthermore, in the solid electrolyte sheet of the present invention, in both the first and second aspects, in addition to using the insulating porous base material, a binder that binds the solid electrolyte particles to each other is used. , is contained inside the insulating porous base material together with the solid electrolyte particles.

In the solid electrolyte sheet of the present invention, the overall strength can be increased by the above-mentioned action of the insulating porous base material and the action of the binder. It is possible to prevent a decrease in ion conductivity due to cracks in the sheet, improve shape retention, and make it possible to increase the area. Specifically, the area of the solid electrolyte sheet can be 1 cm ² or more, preferably 10 cm ² or more.

By constructing an all-solid lithium secondary battery using the solid electrolyte sheet of the present invention, it is possible to provide an all-solid lithium secondary battery with high capacity, high energy density, and excellent discharge characteristics.

Embodiments of the present invention will be described in detail below.

<Solid electrolyte sheet>
FIG. 1 shows a plan view schematically showing an example of a solid electrolyte sheet. The solid electrolyte sheet 10 shown in FIG. 1 includes an insulating porous base material 11, solid electrolyte particles 12 filled in gaps (pores) between fibers constituting the insulating porous base material 11, and a binder 13. have. Then, the solid electrolyte particles 12 or the solid electrolyte particles 12 and the insulating porous base material 11 are bound and fixed by the binder 13.

In the solid electrolyte sheet of the first aspect, the tensile strength per basis weight of the insulating porous base material is 0.45 N/(g/m ² ) or more, preferably 0.7 N/(g/m ² ). It is more preferably 1N/(g/m ² ) or more. This results in an insulating porous base material that is less likely to impede ion conduction and has high overall strength, so that the ion conductivity and strength of the solid electrolyte sheet can be favorably increased. Note that the upper limit of the tensile strength per basis weight of the insulating porous base material is usually about 7N/(g/m ² ).

Furthermore, also in the solid electrolyte sheet of the second embodiment, the tensile strength per basis weight of the insulating porous base material is preferably within the above range.

The tensile strength per basis weight of the insulating porous substrate referred to in this specification is determined by cutting out a test piece with a length of 50 mm and a width of 15 mm, and conducting a tensile test at a tensile speed of 120 mm/min with a distance between chucks of 30 mm. This is the required value. In addition, when the insulating porous base material has directionality, for example, when it has MD direction and TD direction, the tensile strength in the TD direction may be lower than the tensile strength in the MD direction. The value in the direction in which the tensile strength decreases (TD direction) may be taken as the tensile strength of the base material.

Further, in the solid electrolyte sheet of the first embodiment and the solid electrolyte sheet of the second embodiment, the basis weight of the insulating porous base material is preferably 10 g/m ² or less, more preferably 7 g/m 2 or less. m ² or less, particularly preferably 5 g/m ² or less. By using an insulating porous base material with a small basis weight, the ionic conductivity within the solid electrolyte sheet is improved, which can further improve the discharge characteristics of all-solid-state lithium secondary batteries. can. Further, in order to maintain the shape of the solid electrolyte sheet, the basis weight of the insulating porous base material is preferably 2 g/m ² or more.

Examples of the insulating porous base material made of resin fibers include nonwoven fabrics, woven fabrics, and meshes, with nonwoven fabrics being preferred.

Moreover, polyethylene terephthalate (PET), aramid, polyarylate (liquid crystal polyester, etc.), etc. can be used as the resin constituting the fiber. However, even if the strength of the fiber itself is high, it does not necessarily mean that the strength of a base material such as a nonwoven fabric will be high; it is necessary to use a base material in which the fibers are properly intertwined with each other to increase the strength. Furthermore, even if the strength of the base material is high, if the basis weight is too low, it will be difficult to hold the solid electrolyte particles, so it is desirable to use a base material with a basis weight above a certain level.

In the solid electrolyte sheet of the second embodiment, an insulating porous base material made of liquid crystal polyester fibers is used. Also, in the solid electrolyte sheet of the first embodiment, it is preferable to use an insulating porous base material made of liquid crystal polyester fibers.

Examples of the liquid crystal polyester constituting the insulating porous base material include those constituted by a combination of any of the following structural units (1) to (6).

Among these liquid crystal polyesters, a copolymer having a structural unit derived from parahydroxybenzoic acid and a structural unit derived from 2-hydroxy-6-naphthoic acid [the combination of (5) above] is preferred.

The resin constituting the insulating porous base material preferably has a heat resistance temperature of 200°C or higher, thereby improving the heat resistance of the solid electrolyte sheet and, by extension, the heat resistance of the all-solid lithium secondary battery using the same. improves. The liquid crystal polyester satisfies the above-mentioned heat resistance temperature.

The heat resistant temperature of the resin as used herein means the melting temperature determined using a differential scanning calorimeter (DSC) according to the method specified in JIS K 7121, when the resin has a melting point (melting temperature). If it does not have a melting point, it means a softening point determined according to the method specified in JIS K 7206.

Examples of the insulating porous base material made of liquid crystal polyester fibers include nonwoven fabrics, woven fabrics, meshes, etc., and nonwoven fabrics are preferred.

Nonwoven fabrics made of fibers such as liquid crystal polyester can be manufactured, for example, by a melt blowing method. With the melt blow method, it is easy to reduce the diameter of the resin fibers that make up the nonwoven fabric, so while maintaining good ion permeability, the fibers are often entangled with each other, and the tensile strength per basis weight is It is possible to obtain a nonwoven fabric with high strength that satisfies the value of .

The resin fibers constituting the insulating porous base material may contain additives (inorganic fillers, antioxidants, ultraviolet absorbers, etc.) that are normally added to nonwoven fabrics, etc., as necessary.

The diameter of the liquid crystal polyester fibers constituting the insulating porous base material is preferably 2 to 10 μm.

Further, in the solid electrolyte sheet of the first embodiment and the solid electrolyte sheet of the second embodiment, the thickness of the insulating porous base material is preferably 50 μm or less, more preferably 30 μm or less, and 20 μm or less. The following is particularly preferred. By using such a thin insulating porous base material, the thickness of the solid electrolyte sheet can be reduced, which reduces the internal resistance of the all-solid-state lithium secondary battery and further improves the discharge characteristics. be able to. In the present invention, the tensile strength per basis weight of the insulating porous base material is 0.45 N/(g/m ² ) or more, or by being composed of liquid crystal polyester fibers, Even if the thickness is reduced, the shape of the solid electrolyte sheet can be maintained well.

The solid electrolyte constituting the solid electrolyte particles that can be used in the solid electrolyte sheet is not particularly limited as long as it has lithium ion conductivity, and examples include sulfide-based solid electrolytes, hydride-based solid electrolytes, and oxide-based solid electrolytes. Solid electrolytes etc. can be used.

Examples of sulfide-based solid electrolytes include Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS ₂ , Li ₂ S-P ₂ S ₅ -GeS ₂ , Li ₂ S-B ₂ S _3- based glass, and the like. In addition to the above, LGPS type (Li ₁₀ GeP ₂ S _12, etc.) and argyrodite type (Li ₆ PS ₅ Cl, etc.), which have recently attracted attention as having high lithium ion conductivity, can also be used. Among these, argyrodite-based materials, which have particularly high lithium ion conductivity and high chemical stability, are preferably used.

Examples of the hydride solid electrolyte include LiBH ₄ and a solid solution of LIBH ₄ and the following alkali metal compound (for example, one in which the molar ratio of LiBH ₄ and the alkali metal compound is 1:1 to 20:1). Can be mentioned. Examples of the alkali metal compounds in the solid solution include lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbiF, RbCl, etc.), and cesium halides (CsI, CsBr, CsF, CsCl, etc.). , lithium amide, rubidium amide, and cesium amide.

Examples of the oxide solid electrolyte include Li ₇ La ₃ Zr ₂ O ₁₂ , LiTi(PO ₄ ) ₃ , LiGe(PO ₄ ) ₃ , and LiLaTiO ₃ .

Among the solid electrolytes mentioned above, it is more preferable to use a sulfide-based solid electrolyte with high lithium ion conductivity.

Although one type of solid electrolyte particles can be used alone, two or more types can also be used in combination. In addition, when using two or more types of solid electrolyte particles as exemplified above, each solid electrolyte particle may be mixed, or each solid electrolyte particle may be layered in different regions in the thickness direction of the solid electrolyte sheet. It may be present so as to form.

Regarding the size of the solid electrolyte particles, it is preferable that the average particle diameter is 5 μm or less, from the viewpoint of further increasing the filling property into the pores of the insulating porous base material and ensuring good lithium ion conductivity. , more preferably 2 μm or less. However, if the size of the solid electrolyte particles is too small, there is a risk that the handleability may be reduced or a larger amount of binder may be required, resulting in an increase in resistance value. Therefore, the average particle diameter of the solid electrolyte particles is preferably 0.3 μm or more, more preferably 0.5 μm or more.

The average particle diameter of the solid electrolyte particles and other particles (positive electrode active material particles, negative electrode active material particles, etc.) referred to in this specification can be determined using, for example, a laser scattering particle size analyzer (for example, "LA-920" manufactured by HORIBA). This is the number average particle diameter measured by dispersing the particles in a medium that does not dissolve or swell the particles.

The binder of the solid electrolyte sheet is preferably one that does not react with the solid electrolyte, and at least one resin selected from the group consisting of butyl rubber, chloropyrene rubber, acrylic resin, and fluororesin is preferably used.

From the perspective of ensuring good lithium ion conductivity, the proportion of the insulating porous base material in the solid electrolyte sheet (the proportion of the actual volume excluding pores) is preferably 30% by volume or less, and 25% by volume. It is more preferable that it is below. However, if the proportion of the insulating porous base material in the solid electrolyte sheet is too small, the effect of improving the shape retention of the solid electrolyte sheet may be reduced. Therefore, from the viewpoint of further increasing the strength of the solid electrolyte sheet, the proportion of the insulating porous base material in the solid electrolyte sheet is preferably 5% by volume or more, and more preferably 10% by volume or more.

Further, from the viewpoint of further improving the shape retention of the solid electrolyte sheet, the content of the binder in the solid electrolyte sheet is preferably 0.5% by mass or more, and 1% by mass based on the total amount of the solid electrolyte particles and the binder. It is preferably at least 5% by mass, and preferably at most 3% by mass, from the viewpoint of limiting the amount of binder to some extent and suppressing a decrease in lithium ion conductivity. .

The thickness of the solid electrolyte sheet is preferably 5 μm or more, and preferably 10 μm or more, from the viewpoint of optimizing the distance between the positive electrode and the negative electrode of a battery using the solid electrolyte sheet and suppressing the occurrence of short circuits and increases in resistance. More preferably, it is 50 μm or less, and more preferably 30 μm or less.

There are no particular restrictions on the method for manufacturing the solid electrolyte sheet, but a method that includes a step of dispersing solid electrolyte particles in a solvent together with a binder to form a slurry or the like, and filling the pores of an insulating porous base material with the slurry using a wet process. Preferably, it is manufactured. This improves the strength of the solid electrolyte sheet and facilitates the manufacture of a large-area solid electrolyte sheet.

As a filling method for filling the pores of the insulating porous base material with a slurry containing solid electrolyte particles and a binder, coating methods such as a screen printing method, a doctor blade method, and a dipping method can be employed.

The slurry is prepared by adding solid electrolyte particles and a binder to a solvent and mixing them. It is preferable to select a solvent for the slurry that does not easily deteriorate the solid electrolyte. In particular, sulfide-based solid electrolytes and hydride-based solid electrolytes cause chemical reactions with minute amounts of water, so hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene are typical. Preference is given to using non-polar aprotic solvents. In particular, it is more preferable to use a super dehydrated solvent with a water content of 0.001% by mass (10 ppm) or less. In addition, fluorinated solvents such as "Vertrell (registered trademark)" manufactured by Mitsui-DuPont Fluorochemicals, "Zeorolla (registered trademark)" manufactured by Nippon Zeon, and "Novec (registered trademark)" manufactured by Sumitomo 3M, Non-aqueous organic solvents such as , dichloromethane and diethyl ether can also be used.

After filling the pores of the insulating porous base material with the slurry as described above, a solid electrolyte sheet can be obtained by removing the solvent of the slurry by drying and performing pressure molding if necessary. .

As mentioned above, the method for manufacturing the solid electrolyte sheet is not limited to the wet method described above, and for example, the pores of an insulating porous base material may be filled with a mixture of solid electrolyte particles and binder particles using a dry method. The solid electrolyte sheet may be manufactured by a method in which the sheet is then subjected to pressure molding.

Alternatively, a sheet obtained by molding a mixture of a solid electrolyte and a binder may be pasted on one or both sides of a sheet in which the pores of an insulating porous base material are filled with solid electrolyte particles and a binder to form a solid electrolyte sheet. Good too. In this case, in a sheet obtained by molding a mixture of a solid electrolyte and a binder, it is preferable to use a relatively flexible sulfide-based solid electrolyte as the solid electrolyte.

<All-solid-state lithium secondary battery>
The all-solid lithium secondary battery of the present invention (hereinafter sometimes simply referred to as "battery") has a positive electrode and a negative electrode, and further includes a solid electrolyte sheet of the present invention inserted between the positive electrode and the negative electrode. It is something that you have.

FIG. 2 is a cross-sectional view schematically showing an example of the all-solid-state lithium secondary battery of the present invention. The battery 1 shown in FIG. 2 includes a positive electrode 20, a negative electrode 30, and a positive electrode 20 and a negative electrode 30 in an external case formed of an external can 40, a sealed can 50, and a resin gasket 60 interposed between these. A solid electrolyte sheet 10 inserted between them is enclosed.

The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to come into contact with the sealing can 50. The opening of the outer can 40 is sealed, so that the inside of the element has a hermetically sealed structure.

Stainless steel can be used for the outer can and sealed can. In addition, polypropylene, nylon, etc. can be used as the material for the gasket, and if heat resistance is required due to battery usage, materials such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA) can be used. Heat-resistant materials with melting points exceeding 240°C such as fluororesin, polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyether sulfone (PES), polyphenylene sulfide (PPS), and polyether ether ketone (PEEK) Resins can also be used. Furthermore, when the battery is used in applications requiring heat resistance, a glass hermetic seal can also be used to seal the battery.

Next, components other than the solid electrolyte sheet of the all-solid lithium secondary battery will be explained.

(positive electrode)
The positive electrode of an all-solid-state lithium secondary battery contains a positive electrode active material and usually also contains a solid electrolyte.

The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in conventionally known lithium ion secondary batteries, that is, an active material capable of intercalating and releasing Li ions. Specific examples of positive electrode active materials include LiM _x Mn _2-x O ₄ (where M is Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al , Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and is at least one element selected from the group consisting of 0.01≦x≦0.5). Manganese composite oxide, Li _x Mn _(1-y-x) Ni _y M _z O _(2-k) F _l (M is Co, Mg, Al, B, Ti, V, Cr, Fe, Cu , Zn, Zr, Mo, Sn, Ca, Sr and W, and 0.8≦x≦1.2, 0<y<0.5, 0≦z ≦0.5, k+l<1, -0.1≦k≦0.2, 0≦l≦0.1), LiCo _1-x M _x O ₂ (where M is Al , Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba, and 0≦x≦0.5 ) Lithium cobalt composite oxide, LiNi _1-x M _x O ₂ (where M is Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn LiM _1-x N _x PO ₄ (where M is at least one element selected from the group consisting of , Sb and Ba, and 0≦x≦0.5) , Fe, Mn and Co, where N is Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba. At least one element selected from the group consisting of olivine type composite oxide represented by 0≦x≦0.5), lithium titanium composite oxide represented by Li ₄ Ti ₅ O ₁₂ , etc. Only one type of these may be used, or two or more types may be used in combination.

As for the size of the positive electrode active material, from the viewpoint of increasing the surface area and facilitating charging and discharging, the average particle diameter is preferably 50 μm or less, more preferably 30 μm or less. However, if the size of the positive electrode active material is too small, there is a risk that the handleability may deteriorate or a larger amount of binder may be required, resulting in an increase in resistance value. Therefore, the average particle diameter of the positive electrode active material is preferably 0.1 μm or more, more preferably 3.0 μm or more.

As the solid electrolyte of the positive electrode, one or more of the solid electrolytes exemplified above as the solid electrolyte constituting the solid electrolyte particles of the solid electrolyte sheet can be used. In order to improve battery characteristics, it is desirable to include a sulfide solid electrolyte.

For the positive electrode, for example, a layer (positive electrode mixture layer) consisting of a positive electrode mixture containing a positive electrode active material, a solid electrolyte, and further a conductive agent and a binder added as necessary is placed on one side of the current collector or It is possible to use a structure in which the positive electrode mixture is formed on both sides, or a molded body obtained by pressure molding the positive electrode mixture into a pellet shape (positive electrode mixture molded body).

As the binder for the positive electrode, for example, a fluororesin such as polyvinylidene fluoride (PVDF) can be used. Further, as a conductive additive for the positive electrode, for example, a carbon material such as carbon black can be used.

When a current collector is used for the positive electrode, metal foil such as aluminum or stainless steel, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

When manufacturing a positive electrode, for example, in the case of a positive electrode with a current collector, the positive electrode active material, solid electrolyte, and conductive additives and binders added as necessary are dispersed in a solvent such as xylene. The positive electrode mixture-containing composition (paste, slurry, etc.) is applied to the current collector, dried, and then subjected to pressure molding such as calendering as necessary to apply the positive electrode mixture to the surface of the current collector. A method of forming a layer of agent (positive electrode mixture layer) can be adopted.

As with the solvent used in the slurry for forming the solid electrolyte sheet, it is desirable to select a solvent that does not easily deteriorate the solid electrolyte, and the solvent used in the positive electrode mixture-containing composition is the same as the solvent used in the slurry for forming the solid electrolyte sheet. It is preferable to use the various solvents exemplified above, and it is particularly preferable to use a super dehydrated solvent with a moisture content of 0.001% by mass (10 ppm) or less.

In the case of a positive electrode made of a molded positive electrode mixture, a positive electrode mixture prepared by mixing a positive electrode active material, a solid electrolyte, and a conductive additive, a binder, etc. added as necessary, is used. It can be formed by compression using pressure molding or the like.

As for the composition of the positive electrode mixture in the positive electrode, for example, the content of the positive electrode active material is preferably 50 to 90% by mass, the content of the solid electrolyte is preferably 10 to 50% by mass, and the content of the binder is preferably 50 to 90% by mass. Preferably, the amount is between 0.1 and 10% by weight. Further, when a conductive additive is contained in the positive electrode mixture, the content is preferably 0.1 to 10% by mass. Further, the thickness of the positive electrode mixture layer and the thickness of the positive electrode mixture molded body in the positive electrode having a current collector are preferably 50 to 1000 μm.

As the negative electrode of the all-solid-state lithium secondary battery, a negative electrode active material used in conventionally known lithium ion secondary batteries, that is, a negative electrode containing an active material capable of intercalating and releasing Li ions, is used. Ru.

Examples of negative electrode active materials include graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads (MCMB), carbon fibers, and other materials that can occlude and release lithium. One or a mixture of two or more carbon-based materials may be used. Also, simple substances, compounds, and alloys thereof containing elements such as Si, Sn, Ge, Bi, Sb, and In; compounds that can be charged and discharged at low voltages close to those of lithium metal, such as lithium-containing nitrides or lithium-containing oxides; lithium metal ; Lithium/aluminum alloy; can also be used as the negative electrode active material.

As for the size of the negative electrode active material, from the viewpoint of increasing the surface area and facilitating charging and discharging, the average particle diameter is preferably 50 μm or less, more preferably 30 μm or less. However, if the size of the negative electrode active material is too small, there is a risk that the handleability may be reduced or a larger amount of binder may be required, resulting in an increase in resistance value. Therefore, the average particle diameter of the negative electrode active material is preferably 0.1 μm or more, more preferably 0.5 μm or more.

The negative electrode contains a negative electrode active material, a solid electrolyte, a binder such as butyl rubber, chloropyrene rubber, acrylic resin, and fluororesin, and, if necessary, a conductive additive (carbon material such as carbon black, etc.) added appropriately. The mixture is compressed by pressure molding etc. into a molded object such as a pellet (negative electrode mixture molded object), or a molded object (negative electrode mixture layer) using a current collector as a core material, Alternatively, foils of the aforementioned various alloys or lithium metal may be used alone, or may be laminated as an active material layer on a current collector.

When the negative electrode contains a solid electrolyte, one or more of the same solid electrolytes as those exemplified above as the solid electrolyte constituting the solid electrolyte particles of the solid electrolyte sheet can be used for the solid electrolyte. . In order to improve battery characteristics, it is desirable to include a sulfide solid electrolyte.

When a current collector is used for the negative electrode, copper or nickel foil, punched metal, net, expanded metal, foamed metal, carbon sheet, etc. can be used as the current collector.

A negative electrode mixture-containing composition (paste, slurry, etc.) used when manufacturing a negative electrode having a negative electrode mixture layer containing a negative electrode active material, a solid electrolyte, etc., includes, for example, a negative electrode active material, a solid electrolyte, and a binder. It is prepared by dispersing in a solvent a conductive additive, etc. to be used as necessary. In this case, the binder may be dissolved in a solvent.

As with the solvent used in the slurry for forming a solid electrolyte sheet, it is desirable to select a solvent that does not easily deteriorate the solid electrolyte as the solvent used in the negative electrode mixture-containing composition. It is preferable to use the various solvents exemplified above, and it is particularly preferable to use a super dehydrated solvent with a moisture content of 0.001% by mass (10 ppm) or less.

In the case of a negative electrode mixture molded body containing a negative electrode active material and a solid electrolyte, or a negative electrode having a negative electrode mixture layer (negative electrode mixture layer) on the surface of a current collector, the composition of the negative electrode mixture is, for example, The content of the negative electrode active material is preferably 50 to 80% by mass, the content of the solid electrolyte is preferably 20 to 50% by mass, and the content of the binder is 0.1 to 10% by mass. is preferred. Further, when a conductive additive is contained in the negative electrode mixture, the content thereof is preferably 0.1 to 10% by mass. Further, the thickness of the negative electrode mixture layer and the thickness of the negative electrode mixture molded body in the negative electrode having a current collector are preferably 50 to 1000 μm.

(electrode body)
The positive electrode and the negative electrode can be used in a battery in the form of a laminated electrode body that is laminated with the solid electrolyte sheet of the present invention interposed therebetween, or a wound electrode body that is further wound with this laminated electrode body.

Note that when forming the electrode body, it is preferable to pressure-mold the positive electrode, the negative electrode, and the solid electrolyte sheet in a laminated state from the viewpoint of increasing the mechanical strength of the electrode body.

(Battery form)
The form of all-solid-state lithium secondary battery is one that has an outer case composed of an outer can, a sealed can, and a gasket as shown in Figure 2, that is, a form that is generally referred to as a coin-type battery or a button-type battery. Examples include those with an exterior made of resin film or metal-resin laminate film, metal exterior cans with a bottom (cylindrical or rectangular), and their openings. It may have an exterior body having a sealing structure for sealing the portion.

The all-solid-state lithium secondary battery of the present invention can be applied to the same applications as conventionally known secondary batteries, but it has excellent heat resistance because it has a solid electrolyte instead of an organic electrolyte. Therefore, it can be preferably used in applications that are exposed to high temperatures.

Hereinafter, the present invention will be described in detail based on Examples. However, the following examples do not limit the present invention.

<Positive electrode>
Using xylene as a solvent, LiNi _0.6 Co _0.2 Mn _0.2 O ₂ with an amorphous composite oxide of Li and Nb formed on the surface with an average particle size of 3 μm, and a sulfide-based solid electrolyte (Li ₆ PS ₅ Cl), carbon nanotubes as a conductive agent (“VGCF” (trade name) manufactured by Showa Denko) and an acrylic resin binder in a mass ratio of 50:44:3:3, and the solid content They were mixed so that the ratio was 50% and stirred for 10 minutes with a thin mixer to prepare a uniform slurry.This slurry was applied onto an Al foil with a thickness of 20 μm using an applicator with a gap of 200 μm. Vacuum drying was performed at 120°C to obtain a positive electrode.

<Negative electrode>
Using xylene as a solvent, graphite with an average particle size of 20 μm, sulfide solid electrolyte (Li ₆ PS ₅ Cl), and acrylic resin binder were mixed in a mass ratio of 50:47:3, and the solid content ratio was The mixture was mixed to a concentration of 50% and stirred for 10 minutes using a sinky mixer to prepare a uniform slurry. This slurry was applied onto a 20 μm thick SUS foil using an applicator with a gap of 200 μm, and vacuum dried at 120° C. to obtain a negative electrode.

Example 1
As an insulating porous base material, a nonwoven fabric made by a melt-blowing method and composed of liquid crystal polyester fibers made of fully aromatic polyester [manufactured by Kurareflex Co., Ltd., Veccles (trade name), thickness: 16 μm, basis weight: 4 g] /m ² , tensile strength: 5N]. The tensile strength per basis weight of this base material was 1.25 N/(g/m ² ).

Xylene (“super dehydrated” grade) was used as a super dehydrated solvent with a moisture content of 0.001% by mass (10 ppm) or less, and a sulfide solid electrolyte (Li ₆ PS ₅ Cl) with an average particle size of 1.0 μm was used. ), an acrylic resin binder, and a dispersant at a mass ratio of 100:3:1 and a solid content ratio of 40%, and stirred for 10 minutes with a thin mixer to form a uniform slurry. was prepared, passed through the nonwoven fabric into the slurry and pulled up, and then vacuum dried at 120° C. for 1 hour to produce a solid electrolyte sheet with a thickness of 18 μm.

Example 2
The same procedure as in Example 1 was carried out except that the insulating porous base material was changed to a nonwoven fabric made of liquid crystal polyester fibers having a thickness of 18 μm, basis weight: 6 g/m ² , and tensile strength of 10 N, and a thickness of 20 μm. A solid electrolyte sheet was prepared. The tensile strength per basis weight of the base material was 1.67 N/(g/m ² ).

Example 3
The same procedure as in Example 1 was carried out, except that the insulating porous base material was changed to a nonwoven fabric made of liquid crystal polyester fibers having a thickness of 46 μm, a basis weight of 9 g/m ² , and a tensile strength of 4.2 N. : A 50 μm solid electrolyte sheet was produced. . The tensile strength per basis weight of the base material was 0.47 N/(g/m ² ).

Comparative example 1
In the same manner as in Example 1, except that a nonwoven fabric composed of polyethylene terephthalate fibers having a thickness of 16 μm, a basis weight of 4 g/m ² , and a tensile strength of 1.5 N was used as the insulating porous base material. A solid electrolyte sheet with a thickness of 18 μm was produced. The tensile strength per basis weight of the base material was 0.38 N/(g/m ² ).

Table 1 shows the physical properties of the insulating porous substrate used in the solid electrolyte sheets of Examples 1 to 3 and Comparative Example 1.

Using the solid electrolyte sheets of Examples 1 to 3 and Comparative Example 1, ten all-solid-state lithium secondary batteries were manufactured in the following manner in an atmosphere with a controlled dew point.

Example 4
The positive electrode, the negative electrode, and the solid electrolyte sheet of Example 1 were all punched out to a size of 10 mmφ, stacked in the order of positive electrode - solid electrolyte sheet - negative electrode between upper and lower SUS pins, and placed in a PET tube at 10 tons/cm. ² was maintained under pressure to form a sealed cell (all-solid lithium secondary battery) that was not exposed to the atmosphere.

Examples 5-6, Comparative Example 2
The same procedure as in Example 4 was carried out except that the solid electrolyte sheet was changed to that of Example 2 (Example 5), that of Example 3 (Example 6), or that of Comparative Example 1 (Comparative Example 2). An all-solid-state lithium secondary battery was fabricated.

The all-solid-state lithium secondary batteries of Examples 4 to 6 and Comparative Example 2 were charged at a constant current with a current value of 0.05C until the voltage reached 4.2V while being pressurized. A charge/discharge test was conducted in which constant current discharge was performed at 0.05 C until the voltage reached 2.7 V, and the discharge capacity per 1 g of positive electrode active material was measured.

At this time, short circuits were observed in four of the batteries of Comparative Example 2 due to falling off of the solid electrolyte particles in the solid electrolyte sheet and cracking of the sheet.

Excluding the short-circuited battery, the remaining batteries were subjected to a charge/discharge test under the same conditions as above, except that the charging current value and discharging current value were each changed to 1.0C, and the discharge per 1g of positive electrode active material was performed. Capacity was measured.

Further, in charging and discharging at a current value of 0.05C, the charging and discharging efficiency was determined from the ratio of the amount of discharged electricity to the amount of charged electricity.

Table 2 shows the average values of the above measurement results.

As shown in Table 2, the all-solid lithium secondary battery of Example 4 had better charge/discharge efficiency and discharge capacity at each current value than the battery of Comparative Example 2 using a solid electrolyte sheet of the same thickness. The discharge characteristics were excellent.

The solid electrolyte sheet of Example 1 used in the battery of Example 4 has a higher tensile strength per basis weight of the insulating porous base material used for its production, and has a higher tensile strength than that of Comparative Example 1 used in the battery of Comparative Example 2. It is thought that the discharge characteristics were improved because the shape retention was higher than that of the solid electrolyte sheet.

Further, from a comparison of the discharge capacities of the batteries of Examples 4 to 6, it can be seen that the smaller the basis weight of the insulating porous base material, the better the discharge characteristics.

1 All solid lithium secondary battery 10 Solid electrolyte sheet 11 Insulating porous base material 12 Solid electrolyte particles 13 Binder 20 Positive electrode 30 Negative electrode 40 Exterior can 50 Sealed can 60 Gasket

Claims (16)

A solid electrolyte sheet having an insulating porous base material as a support,
The insulating porous base material is composed of resin fibers,
When the basis weight of the insulating porous base material is x (g/m ² ) and the tensile strength of the insulating porous base material is y (N), x is 7 or less and y/x≧0 .45,
A solid electrolyte sheet comprising solid electrolyte particles and a binder inside the insulating porous base material. The solid electrolyte sheet according to claim 1, wherein x of the insulating porous base material is 2 (g/ ^m2 ) or more . A solid electrolyte sheet having an insulating porous base material as a support,
The insulating porous base material is composed of liquid crystal polyester fibers,
A solid electrolyte sheet comprising solid electrolyte particles and a binder inside the insulating porous base material. 4. The solid electrolyte sheet according to claim 3, wherein the liquid crystal polyester is a copolymer having a structural unit derived from parahydroxybenzoic acid and a structural unit derived from 2-hydroxy-6-naphthoic acid. The solid electrolyte sheet according to any one of claims 1 to 4, wherein the resin constituting the fiber has a heat resistance temperature of 200°C or higher. The solid electrolyte sheet according to any one of claims 1 to 5, wherein the proportion of the binder in the total amount of solid electrolyte particles and binder is 0.5 to 5% by mass. The solid electrolyte sheet according to any one of claims 1 to 6, wherein the insulating porous base material is a woven fabric or a nonwoven fabric. The solid electrolyte sheet according to any one of claims 1 to 7, containing sulfide-based solid electrolyte particles as the solid electrolyte particles. The solid electrolyte sheet according to any one of claims 1 to 8, wherein the proportion of the insulating porous base material is 30% by volume or less. The solid electrolyte sheet according to claim 9, wherein the proportion of the insulating porous base material is 5% by volume or more. The solid electrolyte sheet according to any one of claims 1 to 10, wherein the solid electrolyte particles have an average particle diameter of 0.3 to 5 μm. A solid electrolyte sheet having an insulating porous base material as a support,
The insulating porous base material is composed of resin fibers,
The basis weight of the insulating porous base material is x (g/m ² ), when the tensile strength of the insulating porous base material is y (N), y/x≧0.45,
The insulating porous base material contains solid electrolyte particles and a binder,
A solid electrolyte sheet characterized in that the proportion of the insulating porous base material is 30% by volume or less. The solid electrolyte sheet according to claim 12, wherein the proportion of the insulating porous base material is 5% by volume or more. The solid electrolyte sheet according to any one of claims 1 to 13 , wherein the insulating porous base material has a thickness of 50 μm or less. The solid electrolyte sheet according to any one of claims 1 to 14 , having a thickness of 50 μm or less. An all-solid lithium secondary battery comprising a positive electrode, a negative electrode, and the solid electrolyte sheet according to any one of claims 1 to 15 inserted between the positive electrode and the negative electrode.

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