WO2023176203A1 - Ion-conducting solid composition and solid-state secondary battery - Google Patents

Abstract

The purpose of the present invention is to provide an ion-conducting solid composition which exhibits both high ion conductivity and high molding processing properties and which can prevent oxidative degradation or reductive degradation of a solid electrolyte by forming an interfacial film having high oxidative/reductive degradation resistance at the surface of metallic lithium. Another purpose is to provide a solid state secondary battery which is obtained using this type of ion-conducting solid composition, has a high energy density, and has a long cycle life.　The present invention is: an ion-conducting solid composition that contains at least an ion-conducting amorphous inorganic substance and an ion-conducting polymeric substance having a fluorine atom in a main chain and/or a side chain; and a solid state secondary battery that includes at least a positive electrode, a negative electrode, and an ion-conducting solid composition or ion-conducting solid composition-containing solid electrolyte, in which at least a part of the surface of the ion-conducting solid composition is coated with lithium fluoride.

Description

Ion conductive solid composition and solid secondary battery

The present invention relates to an ion-conductive solid composition and a solid secondary battery.

Solid electrolytes offer various advantages to secondary batteries compared to conventional non-aqueous electrolytes. For example, solid electrolytes have high flame retardancy and can provide high safety to lithium ion secondary batteries. Solid electrolytes can also provide advantages in high energy density, good charge/discharge cycle stability, and electrochemical stability over a wide range of charge/discharge rate conditions. Therefore, efforts are being made to put into practical use all-solid-state batteries that do not use electrolytes and only use solid electrolytes. However, there are various challenges to putting all-solid-state batteries into practical use.

The first problem is the contact state between the electrolyte and the electrode. Due to the nanoscale unevenness of the inorganic solid electrolyte layer surface and the electrode surface, fine pores exist at the interface between the inorganic solid electrolyte layer surface and the electrode surface. Two problems can arise from such interfacial pores. The interfacial pores increase the resistance of the solid electrolyte layer/electrode interface. The interfacial pores also make the lithium flux at the solid electrolyte layer/electrode interface non-uniform, concentrate the lithium concentration and current density at the solid electrolyte layer/electrode contact, and increase the formation of lithium dendrites. Although inorganic solid electrolytes such as inorganic sulfides and oxides have high ionic conductivity (>10 ⁻⁴ S/cm) at room temperature, this high ionic conductivity may not be utilized efficiently.

The second problem is the processability of inorganic solid electrolytes such as inorganic sulfides and oxides into thin film electrolyte layers. A solid electrolyte layer with excessive thickness increases the electrical resistance of the solid electrolyte layer due to the electronic resistance of the solid electrolyte. The increase in pore volume and pore diameter between solid electrolyte particles within the solid electrolyte layer increases the formation of lithium dendrites. Furthermore, when a nonionically conductive polymer binder is added between solid electrolyte particles, the ionic conductivity and electronic conductivity of the solid electrolyte layer decrease. On the other hand, a polymer electrolyte using an inorganic or organic polymer compound can improve the above-mentioned problems regarding voids at the electrolyte layer/electrode interface and formation of a thin electrolyte layer. However, polymer electrolytes have inferior ionic conductivity compared to inorganic solid electrolytes.
In this way, in order to manufacture large all-solid-state batteries and realize the use of large all-solid-state batteries (for example, in electric vehicles), it is necessary to suppress the occurrence of lithium precipitation without sacrificing the ionic conductivity of the solid electrolyte layer. A solid electrolyte is required that has elastoplasticity to the extent that it is possible to form a good solid electrolyte layer/electrode interface and to form a thin film electrolyte layer.

Thirdly, inorganic solid electrolytes such as inorganic sulfides and oxides have a narrow redox stable potential range compared to the operating voltage of lithium metal negative electrodes and positive electrodes. It can be easily oxidatively decomposed or reductively decomposed on the surface of the positive electrode during charging and discharging. Oxidative decomposition products or reductive decomposition products of inorganic solid electrolytes have extremely low ionic conductivity and may cause an increase in the resistance of a lithium ion secondary battery during charge/discharge cycles.

In order to solve these problems, various all-solid electrolytes have been proposed so far. For example, Patent Document 1 proposes a composite of an inorganic solid electrolyte and an ionic liquid as a solid electrolyte that can solve the first problem. Patent Document 2 proposes an inorganic solid electrolyte and an ion-conductive polymer as a solid electrolyte with excellent moldability that can solve the second problem.

Japanese Patent Application Publication No. 2014-82091 JP 2018-515893 Publication

The all-solid electrolytes proposed in Patent Document 1 and Patent Document 2 can solve some of the above-mentioned problems, but there is a strong demand for proposals for all-solid electrolytes that can solve all three of the above-mentioned problems. ing. Therefore, the present invention aims to improve the oxidative decomposition or reduction of solid electrolytes by forming an interfacial film that has both high ionic conductivity and high moldability, and has high oxidation/reductive decomposition resistance on the lithium metal surface. An object of the present invention is to provide an ion-conducting solid composition that can prevent decomposition. A further object of the present invention is to provide a solid state secondary battery that utilizes such an ion conductive solid composition and has a high energy density and a long cycle life.

One embodiment of the present invention is an ion-conducting solid composition comprising at least an ion-conducting amorphous inorganic substance and an ion-conducting polymeric substance having a fluorine atom in its main chain and/or side chain. It is. At least a portion of the surface of the ion-conductive solid composition is coated with lithium fluoride.
It is preferable that the ion conductive amorphous inorganic substance is contained in an amount of 80% or more based on the mass of the ion conductive solid composition.
Further, it is preferable that the ion conductivity of the ion conductive solid composition is 1×10 ⁻² [S·m ⁻¹ ] or more.

Furthermore, a second embodiment of the present invention is a solid secondary battery including at least a solid electrolyte containing the ion conductive solid composition, a positive electrode, and a negative electrode.
Preferably, at least a portion of the interface between the solid electrolyte and the negative electrode is coated with lithium fluoride.

The ion conductive solid composition of the present invention has both high ion conductivity and high moldability. In addition, the ion-conductive solid composition of the present invention can form an interfacial film with high oxidation/reduction decomposition resistance on the lithium metal surface. It is possible to prevent this. Further, a solid secondary battery using the ion-conductive solid composition of the present invention has high energy density, excellent cycle characteristics, and long life.

Embodiments of the present invention will be described below. The ion-conducting solid composition of one embodiment includes at least an ion-conducting amorphous inorganic substance and an ion-conducting polymeric substance having fluorine atoms in its main chain and/or side chain, At least a portion of the surface of the conductive solid composition is coated with lithium fluoride.

As used herein, ion conductivity refers to a phenomenon in which charges are transported by the movement of ions (anions or cations). Further, the solid composition refers to a composite material that is a mixture of two or more substances and is solid or semi-solid (gel-like) in the room temperature range. That is, an ion-conductive solid composition is a composite material that can create an environment in which charges are transported by anion or cation movement in a solid or semi-solid in the room temperature range.
The ion-conducting solid composition of one embodiment includes an ion-conducting amorphous inorganic substance and an ion-conducting polymeric substance having fluorine atoms in its main chain and/or side chain. In this specification, an amorphous substance means that it is not crystalline, and specifically refers to a substance in which atoms and molecules constituting a solid are not regularly arranged. In this specification, an amorphous substance may be referred to as an amorphous substance, amorphous, or the like. Furthermore, inorganic substances refer to all substances other than organic substances. In one embodiment, an ion-conducting amorphous inorganic material is an amorphous inorganic material that has the property of transporting charge through the movement of anions or cations. Examples of ionically conductive amorphous inorganic materials include perovskites (e.g., Li ₃ xLa _(2/3)-x TiO ₃ , 0≦x≦0.67), lithium superionic conductor compounds (e.g., Li _2+2x Zn _1-x GeO ₄ , 0≦x≦1; Li ₁₄ ZnGe ₄ O ₁₆ ), Thiolisicone® compounds (for example, Li _4-x A _1-y B _y S ₄ , A is Si, Ge or Sn, B is P, Al, Zn, Ga; Li ₁₀ SnP ₂ S ₁₂ ), garnet (for example, Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₅ La ₃ M ₂ O ₁₂ , M is Ta or Nb), Nasicon type lithium Ionic conductors (e.g. Li _1.3 A _10.3 Ti _1.7 (PO ₄ ) ₃ ), oxidized glasses or glass ceramics (e.g. Li ₃ BO ₃ -Li ₂ SO ₄ , Li ₂ O-P ₂ O ₅ , Li ₂ O-SiO ₂ ), sulfide glasses or glass ceramics (e.g. 75Li ₂ S-25P ₂ S ₅ , Li ₂ S-SiS ₂ , LiI-Li ₂ SB ₂ S ₃ ), phosphates (For example, Li _1-x Al _x Ge _2-x (PO ₄ ) ₃ (LAGP), Li _1+x Ti _2-x Al _x (PO ₄ )), α-silver iodide (α-AgI), lithium iodide (LiI) K-priderite (K _1.5 Mg _0.75 Ti _7.25 O ₁₆ ), Na-β-alumina (Na ₂ O.11Al ₂ O ₃ ), stabilized zirconia (e.g. Zr _0.85 Ca _{0 .15} ) O ₂ ), Nasicon (Na ₃ Zr ₂ Si ₂ PO ₁₂ ), Bismuth oxide ((Bi _0.75 Y _0.25 ) ₂ O ₃ ), Li _3.6 Si _0.6 P _0.4 O ₄ , _Li6PS5Cl , _Ag3SI , and _{Ag6I4WO4 ,} and _two or _more of these inorganic substances can be used in combination. The ionically conductive amorphous inorganic substance alone has a transference number close to 1, preferably 0.9 or more, more preferably 0.99 or more.

The ion-conducting solid composition of one embodiment includes, in addition to the ion-conducting amorphous inorganic substance, an ion-conducting polymeric substance having fluorine atoms in its main chain and/or side chain. In this specification, a polymer substance is defined as a molecule with a large molecular weight and a substance having a structure composed of many repetitions of units obtained substantially or conceptually from molecules with a small molecular weight. Ru. In this specification, a polymer substance may be simply referred to as a polymer, and may also be referred to as a polymer compound, polymer, or the like. The polymeric substance used in one embodiment has a fluorine atom in its main chain and/or side chain, and has the above-mentioned ion conductive property. The ion-conducting polymeric substance used in one embodiment may have any shape such as linear, branched, net-like, comb-like, or brush-like. It is preferable that the polymer substance having such a shape has a fluorine atom in either the main chain, the side chain, or both. The main structure constituting the main chain or side chain of the ion-conductive polymer substance can include polyolefin, polyester, polyamine, polyamide, polyaramid, polyurethane, polyether, acrylic resin, polysiloxane, or epoxy resin, etc. It may have a copolymer structure having two or more of these structures. Furthermore, two or more polymeric substances having these structures can be selected and used in combination. The ion-conducting polymer substance used in one embodiment has a fluorine atom in a part of the main chain and/or side chain having the above structure. In addition, the main chain and/or side chain terminals of the ion-conductive polymer substance may be substituted with a group selected from cyano, thiol, amide, amino, sulfonic acid, epoxy, carboxyl, or hydroxyl group. good. Ion-conducting polymeric substances having fluorine atoms in the main chain and/or side chains include perfluoropolyether (PFPE), polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), polychlorotrifluoroethylene ( PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and other fluororesins, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE) , fluorinated resin copolymers such as ethylene-chlorotrifluoroethylene copolymer (ECTFE) can be suitably used.

The ionically conductive polymeric material has a number average molecular weight in the range of 500 g/mol - 50,000 g/mol, preferably 1,000 g/mol - 40,000 g/mol, more preferably 100 g/mol - 10,000 g/mol. It is preferable.
The ion-conducting polymeric material has a glass transition temperature of -50°C or lower, preferably -70°C or lower. That is, the ion-conductive polymer substance is preferably glassy or amorphous at room temperature. Further, it is preferable that the ion conductive polymer substance has a relatively low melting point near room temperature. The melting point of the ion conductive polymer substance is 150°C or lower, preferably 100°C or lower, and more preferably 50°C or lower.

In one embodiment, the content ratio of the ion conductive amorphous inorganic substance in the ion conductive solid composition is 80% or more, preferably 90% or more based on the mass of the ion conductive solid composition. It is preferable that there be.
The ion conductive solid composition of one embodiment can be produced by mixing an ion conductive amorphous inorganic substance and an ion conductive polymer substance at the above content ratio. . These components can be manufactured by mixing preferably without using a solvent or the like. These components can be mixed using a machine such as a ball mill, a planetary mixer, an extruder, a kneader, or a kneader. In some cases, an ion-conducting polymer substance and a suitable solvent are mixed to obtain a solution or suspension, and an ion-conducting amorphous inorganic substance is added to this liquid and mixed. It is also possible to obtain ionically conductive solid-state compositions of the embodiments. At this time, the solvent used may be removed by evaporation using an appropriate method, or may remain in the ion-conductive solid composition as it is. The resulting ion-conducting solid composition may be in any solid form, such as a clay-like solid, a paste-like solid, or a gel-like solid.

An ion-conducting solid electrolyte can be formed by adding an appropriate amount of electrolyte to the ion-conducting solid composition of one embodiment. Electrolytes that can be added to the ion-conductive solid composition include lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and lithium hexafluoroantimonate. (LiSbF ₆ ), lithium tetraphenylborate tris(1,2-dimethoxyethane) (LiB(C ₆ H ₅ ) ₄ ), lithium perchlorate (LiClO ₄ ), lithium bistrifluoromethanesulfonimide (LiTFSI), lithium bis (fluorosulfonyl)imide (LiFSI), 1-ethyl-3methylimidazolium bis(fluorosulfonylimide) (EMIFSI), 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPYFSI), etc. It is preferable to use a lithium salt having an anion diameter of a certain size. As the electrolyte, potassium salts such as potassium trifluoromethanesulfonate, or sodium salts such as sodium trifluoromethanesulfonate can also be used. The concentration of the electrolyte in the ionically conductive solid composition can be about 0.5-10 mol%, preferably 0.8-5 mol%, and more preferably about 1.0-3 mol%. The ion conductive solid electrolyte obtained by adding an appropriate amount of electrolyte to the ion conductive solid composition of the first embodiment has an ionic conductivity of 1×10 ⁻² [S·m ⁻¹ ] or more. has. It is preferable that the ionic conductivity of the ion conductive solid electrolyte is high.

In addition, the ion-conducting solid composition of the first embodiment also includes a dispersant for substantially uniformly dispersing the ion-conducting amorphous inorganic material in the ion-conducting polymer material, and an ion-conducting solid composition. Various agents for adjusting the mechanical strength of the composition (e.g., plasticizers, reinforcing agents), various agents for improving the properties of the ion-conducting solid composition (e.g., heat resistant agents, ultraviolet absorbers, antistatic agents) ) can be added as appropriate. The solid electrolyte, which includes the ion-conducting solid composition of one embodiment, an electrolyte, and optionally other agents, is preferably formed into a membrane shape. A membrane is a relatively thin layer having a generally planar structure with a predetermined area. The area and thickness of the solid electrolyte can be determined as appropriate depending on the use of the solid secondary battery, desired output, etc., which will be described later. For example, the thickness of the solid electrolyte can be any desired in the range of 10 μm to 1,000 μm. It is preferable to obtain a solid electrolyte having an areal capacity in the range of 2 mAh/cm ³ to 5 mAh/cm ³ by appropriately changing the size of the solid electrolyte.

In one embodiment, at least a portion of the surface of the ion-conductive solid composition is preferably coated with lithium fluoride. It is possible to add an appropriate amount of the above-mentioned electrolyte to the ion-conducting solid composition of one embodiment and use this as an ion-conducting solid electrolyte to form a solid state secondary battery to be described later. can. When this solid state secondary battery is charged and discharged, especially during charging, the ion conductive polymer substance contained in the ion conductive solid composition of the first embodiment is reductively decomposed near the negative electrode surface, and lithium fluoride is (LiF) is generated. The generated LiF covers at least a portion of the negative electrode side surface of the solid electrolyte, forming a high quality solid electrolyte interface (Solid Electrolyte Interface, SEI). The presence of SEI on at least a portion of the surface of the solid electrolyte containing the ion-conducting solid composition of one embodiment makes the ion-conducting solid electrolyte electronically insulated, resulting in apparent durability. sexuality will be acquired.

A second embodiment of the present invention is a solid secondary battery that includes at least a solid electrolyte containing the ion-conductive solid composition of the first embodiment, a positive electrode, and a negative electrode.
The solid electrolyte used in the second embodiment is obtained by adding an electrolyte to the ion conductive solid composition of the first embodiment. Here, the electrolyte is lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium hexafluoroantimonate (LiSbF ₆ ), lithium tris tetraphenylborate ( 1,2-dimethoxyethane) (LiB(C ₆ H ₅ ) ₄ ), lithium perchlorate (LiClO ₄ ), lithium bistrifluoromethanesulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), 1- Lithium salts such as ethyl-3-methylimidazolium bis(fluorosulfonylimide) (EMIFSI), 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPYFSI), or potassium trifluoromethanesulfonate, etc. One or more of potassium salts and sodium salts such as sodium trifluoromethanesulfonate can be used in combination. The solid secondary battery of the second embodiment is a "solid lithium secondary battery" when a lithium salt is used as the electrolyte, and a "solid potassium secondary battery" when a potassium salt is used as the electrolyte. When sodium salt is used, it becomes a "solid sodium secondary battery."

The positive electrode used in the second embodiment is in the form of a thin plate or sheet in which a positive electrode active material layer is formed by coating or rolling a mixture containing a positive electrode active material on a positive electrode current collector, which is a metal foil such as aluminum foil, and drying the positive electrode current collector. This is a battery component. That is, the positive electrode is composed of a positive electrode current collector and a positive electrode active material layer containing a positive electrode active material coated on both or one side of the positive electrode current collector. In the second embodiment, the positive electrode active material layer preferably includes a positive electrode active material and a binder. A positive electrode active material is a material used in a positive electrode among substances involved in a reaction that generates electrical energy. Further, a binder is generally a substance for binding positive electrode active material particles in order to bring the particle-shaped positive electrode active materials into electrical contact with each other.

The positive electrode active material used in the second embodiment preferably contains a lithium-nickel-based composite oxide as the positive electrode active material. Lithium-nickel composite oxide has the general formula Li _x Ni _y Me _(1-y) O ₂ (where Me is Al, Mn, Na, Fe, Co, Cr, Cu, Zn, Ca, K, It is a transition metal composite oxide containing lithium and nickel and is represented by at least one metal selected from the group consisting of Mg and Pb. In particular, it is preferable to include a lithium-manganese complex oxide. Examples of the lithium-manganese-based composite oxide include lithium manganate (LiMnO ₂ ) having a zigzag layered structure and spinel-type lithium manganate (LiMn ₂ O ₄ ). The positive electrode active material particularly includes a lithium nickel manganese cobalt composite oxide having a layered crystal structure represented by the general formula Li _x Ni _y Co _z Mn _(1-y-z) O ₂ . Here, x in the general formula satisfies 1≦x≦1.2, y and z are positive numbers satisfying y+z<1, and the value of y is 0.5 or more. Note that as the proportion of manganese increases, it becomes difficult to synthesize a single-phase composite oxide, so it is desirable that 1-y-z≦0.4. In order to obtain a high capacity battery, it is particularly preferable that y>1-yz and y>z. A lithium-nickel-based composite oxide having this general formula is a lithium-nickel-cobalt-manganese composite oxide (hereinafter sometimes referred to as "NCM"). NCM is a lithium-nickel composite oxide that is suitably used to increase the capacity of batteries. For example, in the general formula Li _x Ni _y Co _z Mn _(1.0-yz) O ₂ , a composite oxide with x=1, y=0.8, and z=0.1 is called "NCM811", The composite oxide with x=1, y=0.5, and z=0.2 is called "NCM523".
Note that sulfur can also be used as a positive electrode of a solid state secondary battery.

As binders that form the positive electrode active material layer together with the positive electrode active material, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), polyanilines, polythiophenes, polyacetylenes, and polypyrrole are used. conductive polymers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), or carboxymethyl cellulose (CMC), Examples include polysaccharides such as xanthan gum, guar gum, and pectin.

Further, the positive electrode active material layer may optionally contain a conductive additive. Examples of conductive aids that may be used in some cases include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and Ketjen black, carbon materials such as activated carbon, graphite, mesoporous carbon, fullerenes, and carbon nanotubes. In addition, electrode additives commonly used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used in the positive electrode active material layer.

The negative electrode used in the second embodiment is a thin plate or sheet formed by coating or rolling a mixture containing a negative electrode active material on a negative electrode current collector, which is a metal foil such as copper foil, and drying it to form a negative electrode active material layer. This is a battery component. That is, the negative electrode is composed of a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material coated on both surfaces of the negative electrode current collector. In the second embodiment, the negative electrode active material layer preferably includes a negative electrode active material and a binder. The negative electrode active material is a substance used in the negative electrode among substances involved in a reaction that generates electrical energy. Moreover, a binder is generally a substance for binding negative electrode active material particles in order to bring the negative electrode active materials in particle shape into electrical contact with each other.

The negative electrode active material used in the second embodiment includes a carbon-based active material. Preferably, the carbon-based active material is natural graphite, artificial graphite, hard carbon, soft carbon, or any mixture thereof. Here, graphite is a hexagonal hexagonal plate-shaped carbon material, and is sometimes referred to as graphite, graphite, or the like. Natural graphite and artificial graphite include natural graphite with an amorphous carbon coating and artificial graphite with an amorphous carbon coating. Here, amorphous carbon is a carbon material that is entirely amorphous and has a structure in which microcrystals are randomly networked, which may partially have a structure similar to graphite. That's true. Examples of amorphous carbon include carbon black, coke, activated carbon, carbon fiber, hard carbon, soft carbon, and mesoporous carbon. When using artificial graphite, it is preferable that the interlayer distance d value (d ₀₀₂ ) is 0.33 nm or more. The crystal structure of artificial graphite is generally thinner than that of natural graphite. When artificial graphite is used as a negative electrode active material for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, it is necessary to have an interlayer distance that allows insertion of lithium ions. The interlayer distance at which lithium ions can be inserted and removed can be estimated by the d value (d ₀₀₂ ), and if the d value is 0.33 nm or more, lithium ions can be inserted and removed without any problem. In the second embodiment, the carbon-based active material is preferably in the form of particles having generally uniform or irregular sizes.
As the negative electrode active material, in addition to carbon-based active materials, silicon and silicon-containing active materials, tin and tin-containing active materials can be used. It is also possible to use lithium metal and lithium alloyed metal as the negative electrode of a solid state secondary battery.

For negative electrode active materials such as carbon-based active materials, silicon-based active materials, and tin-based active materials, good electrical contact can be achieved by binding the particles together with a binder. Preferably, the binder comprises poly(meth)acrylic acid, a metal salt of poly(meth)acrylic acid, an alkyl ester of poly(meth)acrylic acid, or any mixture thereof. Compounds suitable as binders include, for example, polyacrylic acid, polymethacrylic acid; sodium polyacrylate, potassium polyacrylate, sodium polymethacrylate, potassium polymethacrylate; polyethyl acrylate, polyethyl acrylate, polyacrylic acid. These include butyl, polymethyl methacrylate, polyethyl methacrylate, and polybutyl methacrylate, and may be any mixture thereof. The content of the binder is preferably 2% by mass or more and less than 10% by mass with respect to the total solid content of the negative electrode active material layer. If the content of the binder is too large, a large portion of the surface of the active material will be covered with the binder, which may reduce ionic conductivity and electronic conductivity. Moreover, if the content of the binder is too small, there is a possibility that electrical contact between the negative electrode active material particles may not be properly made.

As a component of the binder, in addition to the above-mentioned compounds, carboxymethylcellulose (referred to as "CMC"), which is a derivative of cellulose, or a metal salt of carboxymethylcellulose (for example, sodium carboxymethylcellulose, potassium carboxymethylcellulose) is included. It is particularly preferable. Carboxymethylcellulose or a metal salt of carboxymethylcellulose plays a role in stabilizing the above binder compound and also stabilizes the electrical contact of the negative electrode active material. When CMC or a metal salt of CMC is further added as a component of the binder, the content of CMC or CMC metal salt is 0.05% by mass or more and 1.5% by mass or less based on the total solid content of the negative electrode active material layer. In particular, it is preferably 0.08% by mass or more and 0.8% by mass or less, and more preferably 0.15% by mass or more and 0.30% by mass or less.

The negative electrode active material layer may further contain a conductive additive. The conductive aid is a material for reducing the resistance of the electrode. Examples of the conductive aid include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and Ketjen black, carbon materials such as activated carbon, graphite, mesoporous carbon, fullerenes, and carbon nanotubes. In the second embodiment, it is particularly preferable to use carbon nanotubes (referred to as "CNT") as the conductive aid. CNT is a material in which a six-membered ring network of carbon atoms (graphene) has a single-wall or multi-layer coaxial tube structure, and includes single-wall carbon nanotubes (single-wall, referred to as "SWNT") and multi-wall carbon nanotubes. (multilayer, referred to as "MWNT"). Although any CNT may be used, it is particularly preferred in embodiments to use SWNT as a conductive aid.

When using CNTs as a conductive aid, the content of CNTs is 0.01% by mass or more and 1% by mass or less, particularly 0.03% by mass or more and 0.8% by mass, based on the total solid content of the negative electrode active material layer. Hereinafter, it is further preferably 0.1% by mass or more and 0.5% by mass or less.

In addition, electrode additives commonly used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used in the negative electrode active material layer.

The solid state secondary battery of the second embodiment usually does not include a separator as a component because the solid electrolyte containing the ion conductive solid composition of the first embodiment plays the role of a separator. However, as described above, when it is better to use a separator, such as when a solvent is used when obtaining the ion-conductive solid composition of one embodiment, the separator can be included as a component. For example, a polyolefin film can be used as the separator. Polyolefin is a compound obtained by polymerizing or copolymerizing α-olefins such as ethylene, propylene, butene, pentene, and hexene. For example, in addition to polyethylene, polypropylene, polybutene, polypentene, and polyhexene, Copolymers can be mentioned. When using a polyolefin film as a separator, it is particularly advantageous if the polyolefin film has a structure that has pores that are closed when the battery temperature rises, that is, a porous or microporous polyolefin film. Because the polyolefin film has such a structure, even if the battery temperature rises, the separator can close (shut down) and interrupt the ion flow. That is, the uniaxially stretched polyolefin film contracts when the battery is heated and the pores are closed, making it possible to prevent short circuits between the positive and negative electrodes. In order to exhibit a shutdown effect, it is highly preferable to use a porous polyethylene membrane.

Additionally, a crosslinked film can be used as a separator. Porous or microporous polyolefin films have the property of shrinking when heated, so when the battery overheats, the film shrinks and shuts down. However, if the heat shrinkage rate of the film is too large, the area of the film will change significantly, which may even result in a large current flow. Since the crosslinked polyolefin film has an appropriate heat shrinkage rate, it can shrink by the amount to close the pores even when heated, without significantly changing the area.

The separator optionally used in the second embodiment may have a heat-resistant fine particle layer on one or both sides of the separator. At this time, the heat-resistant fine particle layer provided to prevent overheating of the battery is composed of inorganic fine particles that have a heat resistance of 150° C. or higher and are stable in electrochemical reactions. Such inorganic fine particles include inorganic oxides such as silica, alumina (α-alumina, β-alumina, θ-alumina), iron oxide, titanium oxide, barium titanate, and zirconium oxide; boehmite, zeolite, apatite, kaolin, Minerals such as spinel, mica, and mullite can be mentioned.

The solid state secondary battery of the second embodiment preferably does not contain a liquid electrolyte. However, a small amount of non-aqueous electrolyte may be included in the solid electrolyte for the purpose of improving the ionic conductivity and electrical conductivity of the solid electrolyte. A non-aqueous electrolyte is an electrically conductive substance made by dissolving an ionic substance in an organic solvent. The above-mentioned positive electrode and negative electrode are stacked, a solid electrolyte is arranged between them, and the solid electrolyte secondary battery element containing this and optionally a nonaqueous electrolyte is the solid state secondary battery of the second embodiment. It is one unit of the main constituent members of. Usually, a solid state secondary battery is formed by combining a laminate in which a plurality of positive electrodes and a plurality of negative electrodes are stacked on top of each other via a plurality of solid electrolytes. In the second embodiment, the liquid electrolyte that can optionally be used is mainly a non-aqueous electrolyte, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), di-n - Chain carbonates such as propyl carbonate, di-t-propyl carbonate, di-n-butyl carbonate, di-isobutyl carbonate, or di-t-butyl carbonate, and propylene carbonate (PC), ethylene carbonate (EC), etc. A mixture containing a cyclic carbonate is preferable. Liquid electrolytes include such carbonate mixtures as described above, such as lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and hexafluoroantimony. lithium acid (LiSbF ₆ ), lithium tetraphenylborate tris(1,2-dimethoxyethane) (LiB(C ₆ H ₅ ) ₄ ), lithium perchlorate (LiClO ₄ ), lithium bistrifluoromethanesulfonimide (LiTFSI), Lithium bis(fluorosulfonyl)imide (LiFSI), 1-ethyl-3methylimidazolium bis(fluorosulfonylimide) (EMIFSI), 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPYFSI), etc. A lithium salt such as the above, a potassium salt such as potassium trifluoromethanesulfonate, or a sodium salt such as sodium trifluoromethanesulfonate is dissolved therein.

In addition to this, the liquid electrolyte may contain a cyclic carbonate compound different from the above-mentioned cyclic carbonate as an additive. An example of the cyclic carbonate used as an additive is vinylene carbonate (VC). Moreover, a cyclic carbonate compound having a halogen can also be used as an additive. These cyclic carbonates are also compounds that can form protective coatings for the positive electrode and negative electrode during the charging and discharging process of solid secondary batteries. In particular, it is a compound that can prevent a positive electrode active material containing a lithium-nickel composite oxide from being attacked by a sulfur-containing compound such as a disulfonic acid compound or a disulfonic acid ester compound. Examples of the halogen-containing cyclic carbonate compound include fluoroethylene carbonate (FEC), difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, trichloroethylene carbonate, and the like. Fluoroethylene carbonate, which is a cyclic carbonate compound containing a halogen and an unsaturated bond, is particularly preferably used.

Furthermore, the liquid electrolyte may further contain a disulfonic acid compound as an additive. A disulfonic acid compound is a compound having two sulfo groups in one molecule, and includes disulfonate compounds in which the sulfo group forms a salt with a metal ion, or disulfonic acid ester compounds in which the sulfo group forms an ester. . One or two of the sulfo groups of the disulfonic acid compound may form a salt with a metal ion, or may be in the form of an anion. Examples of disulfonic acid compounds include methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, biphenyldisulfonic acid, and Examples include salts (lithium methanedisulfonate, lithium 1,2-ethanedisulfonate, etc.), and anions thereof (methanedisulfonate anion, 1,2-ethanedisulfonate anion, etc.). Examples of disulfonic acid compounds include disulfonic acid ester compounds, such as methanedisulfonic acid, 1,2-ethanedisulfonic acid, 1,3-propanedisulfonic acid, 1,4-butanedisulfonic acid, benzenedisulfonic acid, naphthalenedisulfonic acid, Alternatively, chain disulfonic acid esters such as alkyl diesters or aryl diesters of biphenyl disulfonic acid; and cyclic disulfonic acid esters such as methylenemethane disulfonic acid ester, ethylenemethane disulfonic acid ester, and propylenemethane disulfonic acid ester are preferably used. Methylenemethane disulfonic acid ester (MMDS) is particularly preferably used.

The solid secondary battery of the second embodiment, which includes a solid electrolyte, a positive electrode, and a negative electrode, is usually sealed with an exterior body. Sealing means that at least a portion of the solid state secondary battery element is wrapped with an exterior material so as not to be exposed to outside air. The exterior body of a solid state secondary battery is either a casing having gas barrier properties and capable of sealing the solid state secondary battery element, or a bag-shaped body made of a flexible material. As the exterior body, an aluminum can, an aluminum laminate sheet made by laminating aluminum foil, polypropylene, or the like can be suitably used. That is, any material may be used for the exterior body as long as it does not expose the solid state secondary battery to the outside. The outermost layer of the exterior body has a heat-resistant protective layer made of polyester, polyamide, liquid crystal polymer, etc., and the innermost layer is polyethylene, polypropylene, ionomer, acid-modified polyethylene such as maleic acid-modified polyethylene, acid-modified polyethylene such as maleic acid-modified polypropylene, etc. Modified polypropylene, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethylene isophthalate (PEI), blends of PET and PEN, blends of PET and PEI, polyamide resins, blends of polyamide resin and PET, xylylene group-containing polyamides and A laminate film having a sealant layer made of a thermoplastic resin such as a blend of PET can be used. The exterior body may be formed by bonding or welding one or more of these laminate films together and forming multiple layers. Aluminum, tin, copper, nickel, and stainless steel can be used as the gas barrier metal layer. The thickness of the metal layer is preferably 30 to 50 μm. Particularly preferably, an aluminum laminate, which is a laminate of aluminum foil and a polymer such as polyethylene or polypropylene, can be used.
The solid state secondary battery of the second embodiment may be in various forms such as a coin type battery, a laminate type battery, and a wound type battery.

In the second embodiment, at least a portion of the interface with the solid electrolyte negative electrode is preferably coated with lithium fluoride. A suitable amount of the above electrolyte is added to the ion conductive solid composition of the first embodiment, and this is used as the ion conductive solid electrolyte to charge and discharge the solid state secondary battery of the second embodiment. Then, especially during charging, the ion-conductive polymeric substance contained in the ion-conductive solid composition of one embodiment undergoes reductive decomposition near the surface of the negative electrode, producing lithium fluoride (LiF). The generated LiF covers at least a portion of the negative electrode side surface of the solid electrolyte, forming a high quality solid electrolyte interface (Solid Electrolyte Interface, SEI). The presence of SEI on at least a portion of the surface of the solid electrolyte containing the ion-conducting solid composition of one embodiment makes the ion-conducting solid electrolyte electronically insulated, resulting in apparent durability. sexuality will be acquired.

The embodiments of the present invention have been described above. Examples of the present invention will be described below. The embodiments described above and the examples described below are merely illustrative explanations of the present invention, and are not intended to limit the technical scope of the present invention to the specific embodiments or the configurations of specific examples.
[Example 1]
(Preparation of ionically conductive amorphous inorganic material)
Li ₆ PS ₅ Cl particles were prepared as an ion conductive amorphous inorganic material.
In an argon-filled glove box, 75 g of 10 mm zirconia balls were placed into an 80 mL zirconia container. Subsequently, 10.0 g of Li ₆ PS ₅ Cl (NEI Co.) was added and the container was sealed. The container was taken out from the glove box, fixed in a ball mill, and the mixture was ground at 500 rpm for 24 hours to obtain sulfide glass particles (Li ₆ PS ₅ Cl particles).

(Preparation of ion conductive solid electrolyte)
Commercially available perfluoropolyether (PFPE, molecular weight approximately 1,000 g/mol) was used as the ion-conducting polymer material. In a glove box under an inert atmosphere, 45.45 g of PFPE and 4.55 g of commercially available lithium bistrifluoromethanesulfonimide (LiTFSI) were mixed. Next, an 80 mL zirconia cup was filled with 0.2 g of this mixture of PFPE and LiTFSI, 75 g of 10 mm zirconia balls, and 1.8 g of the sulfide glass fine particles obtained as described above. The zirconia cup was sealed so that it remained under an inert atmosphere throughout the milling time. A zirconia cup was fixed to a ball mill and mixed at 370 rpm for 2 hours to obtain an ion conductive solid electrolyte.

(Production of solid state secondary battery)
A solid state secondary battery cell was produced as follows. The ion conductive solid electrolyte (diameter 1 cm, thickness approximately 0.75 mm) obtained as described above was placed between two lithium metal negative electrodes (diameter approximately 1 cm). The assembled laminate was sandwiched between two stainless steel disks and the stainless steel disks were bolted together. Stainless steel tabs were attached to both sides and the container was sealed in a quartz glass container to obtain a lithium metal/ion conductive solid electrolyte/lithium metal solid secondary battery cell.

(Cycle charge and discharge test of solid state secondary battery cell)
The solid secondary battery cell was left standing in an atmosphere at 25° C., and the polarity was reversed every hour at a current density of 50 μA/cm ² , and this cycle was performed for 200 cycles.

(Confirmation of the presence of lithium fluoride on the surface of the solid electrolyte)
After the cycle charge/discharge test was completed, the solid secondary battery cell was disassembled, and the presence or absence of lithium fluoride on the negative electrode side surface of the ion-conductive solid electrolyte was confirmed by F1s spectrum using X-ray photoelectron spectroscopy. .

(Confirmation of the presence of lithium sulfide on the surface of the solid electrolyte)
After the cycle charge/discharge test is completed, the solid secondary battery cell is disassembled, and the presence or absence of lithium sulfide (Li ₂ S) on the negative electrode side surface of the ion-conductive solid electrolyte is determined by S2p using X-ray photoelectron spectroscopy. Confirmed by spectrum. Note that Li ₂ S is a substance that can be generated by reductive decomposition of sulfide glass particles (Li ₆ PS ₅ Cl particles) during a cycle charge/discharge test of a solid state secondary battery. Li ₂ S is a substance that has an extremely low ionic conductivity of 10 ⁻¹³ S·cm ⁻¹ . If the presence of Li ₂ S is observed on the surface of the ion-conductive solid electrolyte after the cycle charge/discharge test, it is considered that the resistance at the interface between the solid electrolyte and the negative electrode has increased.

[Comparative example 1]
In Example 1, a solid secondary battery cell was created in the same manner as in Example 1, except that PEPF was not added to the ion-conductive solid electrolyte. A cycle charge/discharge test was conducted under the same conditions as in Example 1, and then the solid secondary battery cell was disassembled to determine whether lithium fluoride was present on the negative electrode surface of the ion conductive solid electrolyte. Confirmed by F1s spectrum by line photoelectron spectroscopy. Furthermore, whether or not lithium sulfide was present on the negative electrode surface of the ion-conductive solid electrolyte was confirmed by an S2p spectrum using X-ray photoelectron spectroscopy.

 

The ion-conducting solid electrolyte using the combination of the ion-conducting amorphous inorganic material and the ion-conducting polymer material of the present invention has LiF on the surface after cycling charging and discharging of a solid secondary battery using the ion-conducting solid electrolyte. While the formation of Li 2 S was observed, almost no formation of Li ₂ S was observed. The ion-conductive solid composition of the present invention forms a high-quality SEI at the interface between the solid electrolyte and the negative electrode, and can prevent reductive decomposition of an amorphous inorganic substance. It can be seen that the solid state secondary battery of the present invention can prevent high resistance and capacity deterioration due to charge/discharge cycles, and can be a battery with a long life.

Claims (5)

1. An ion-conducting solid composition comprising at least an ion-conducting amorphous inorganic substance and an ion-conducting polymeric substance having a fluorine atom in its main chain and/or side chain,
   An ion conductive solid composition, wherein at least a portion of the surface of the ion conductive solid composition is coated with lithium fluoride.
2. The ion conductive solid composition according to claim 1, wherein the ion conductive amorphous inorganic substance is contained in an amount of 80% or more based on the mass of the ion conductive solid composition.
3. The ion conductive solid composition according to claim 1 or 2, wherein the ion conductivity of the ion conductive solid composition is 1×10 ⁻² [S·m ⁻¹ ] or more.
4. A solid secondary battery comprising at least a solid electrolyte containing the ion conductive solid composition according to any one of claims 1 to 3, a positive electrode, and a negative electrode.
5. The solid state secondary battery according to claim 4, wherein at least a portion of the interface between the solid electrolyte and the negative electrode is coated with lithium fluoride.