JP2022171030A - Solid electrolyte, electrolyte composition, electrolyte sheet and power storage device - Google Patents

Abstract

To provide: a solid electrolyte which is able to be reduced in gelation; an electrolyte composition; an electrolyte sheet; and a power storage device.SOLUTION: This solid electrolyte has a garnet structure that contains Li, La, Zr and O. In a mixture comprising N-methyl pyrrolidone and a solid electrolyte, a content of the solid electrolyte is 24.5 wt.% relative to the mixture; and liquid which is a supernatant of the mixture diluted with ten parts of oure water has a hydrogen ion index of pH 8 or less. This electrolyte composition contains this solid electrolyte, ionic liquid that contains imidazolium cations, a lithium salt, and a polymer that contains -CH2CF2-. This electrolyte sheet is formed of this electrolyte composition. This power storage device comprises an electrolyte layer that is formed of this electrolyte composition.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a solid electrolyte, an electrolyte composition, an electrolyte sheet and an electricity storage device.

A solid electrolyte with a garnet structure containing Li, La, Zr and O is known (Patent Document 1).

Japanese Patent No. 6682709

In the prior art, when a solid electrolyte with a garnet-type structure containing Li, La, Zr and O is mixed with an organic compound, the organic compound may react with the solid electrolyte and gel (non-fluidize). When the solid electrolyte is in a gel state, dispersion occurs in the state of dispersion of the solid electrolyte.

The present invention has been made to solve this problem, and an object of the present invention is to provide a solid electrolyte, an electrolyte composition, an electrolyte sheet, and an electricity storage device that can reduce gelation.

In order to achieve this object, the solid electrolyte of the present invention is a solid electrolyte having a garnet-type structure containing Li, La, Zr and O, wherein in a mixture comprising N-methylpyrrolidone and a solid electrolyte, is contained in an amount of 24.5% by weight, and the hydrogen ion exponent of the liquid obtained by diluting the supernatant of the mixture 10 times with pure water is pH 8 or less.

The electrolyte composition of the present invention includes a solid electrolyte, an ionic liquid containing imidazolium cations, a lithium salt, and a polymer containing —CH ₂ CF ₂ —. The electrolyte sheet of the present invention comprises an electrolyte composition. The electricity storage device of the present invention includes an electrolyte layer made of an electrolyte composition.

Since the solid electrolyte of the present invention is less basic, interaction between the solid electrolyte and the organic compound is less likely to occur, and gelation can be reduced. According to the electrolyte composition containing the solid electrolyte, the electrolyte sheet containing the electrolyte composition, and the electric storage device, the dispersion state of the solid electrolyte can be reduced.

1 is a cross-sectional view of an electricity storage device in one embodiment; FIG. It is a schematic diagram of a mixture.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an electricity storage device 10 according to one embodiment. The power storage device 10 according to the present embodiment is a secondary battery composed of a solid-state battery in which a power generation element is solid. The fact that the power generation element is solid means that the skeleton of the power generation element is solid, and does not exclude, for example, a form in which the skeleton is impregnated with a liquid.

As shown in FIG. 1, the electricity storage device 10 includes a positive electrode layer 11, an electrolyte layer 14 and a negative electrode layer 15 in order. The positive electrode layer 11, the electrolyte layer 14 and the negative electrode layer 15 are housed in a case (not shown).

The positive electrode layer 11 is formed by stacking a current collecting layer 12 and a composite layer 13 . The current collection layer 12 is a member having conductivity. Examples of the material of the current collecting layer 12 include metals selected from Ni, Ti, Fe and Al, alloys containing two or more of these elements, stainless steel, and carbon materials.

The composite layer 13 consists of an electrolyte composition. The electrolyte composition includes solid electrolyte 18, active material 19, polymer and electrolytic solution. In order to lower the resistance of the composite layer 13, the composite layer 13 may contain a conductive aid. Carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt and Ag are exemplified as conductive aids.

The active material 19 is exemplified by metal oxides containing transition metals, sulfur-based active materials, and organic-based active materials. Metal oxides containing transition metals are exemplified by metal oxides containing one or more elements selected from Mn, Co, Ni, Fe, Cr and V and Li. Metal _oxides with _{transition metals include LiCoO2} , _{LiNi0.8Co0.15Al0.05O2} , _LiMn2O4 , _LiNiVO4 , _{LiNi0.5Mn1.5O4} , _{LiNi1 / 3} Mn _1/3 Co _1/3 O ₄ and LiFePO ₄ are exemplified.

A coating layer can be provided on the surface of the active material 19 for the purpose of suppressing the reaction between the active material 19 and the solid electrolyte 18 . _The coating layer is _Al2O3 , _{ZrO2 , LiNbO3 , Li4Ti5O12} , _{LiTaO3 , LiNbO3 , LiAlO2} , _Li2ZrO3 , _{Li2WO4 , Li2TiO3 , Li2B4 O7} , _Li3PO4 and _{Li2MoO4 are exemplified} .

Sulfur-based active materials are exemplified by S, TiS ₂ , NiS, FeS ₂ , Li ₂ S, MoS ₃ and sulfur-carbon composites. Organic active materials include radical compounds such as 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radialene compounds, tetracyanoquino Examples are dimethane and phenazine oxide.

The negative electrode layer 15 is formed by stacking a current collecting layer 16 and a composite layer 17 . The current collection layer 16 is a member having conductivity. Examples of the material of the collector layer 16 include metals selected from Ni, Ti, Fe, Cu and Si, alloys containing two or more of these elements, stainless steel, and carbon materials.

Composite layer 17 consists of an electrolyte composition. The electrolyte composition includes solid electrolyte 18, active material 20, polymer and electrolyte. In order to reduce the resistance of the composite layer 17, the composite layer 17 may contain a conductive aid. Carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt and Ag are exemplified as conductive aids. The active material 20 is exemplified by Li, Li—Al alloy, Li ₄ Ti ₅ O ₁₂ , graphite, In, Si, Si—Li alloy, and SiO.

The electrolyte layer 14 is made of an electrolyte composition. The electrolyte composition includes a solid electrolyte 18, a polymer and an electrolytic solution. The solid electrolyte 18 is an oxide containing Li, La, Zr and O and having a garnet structure and lithium ion conductivity. The basic composition of the garnet-type structure oxide is Li ₅ La ₃ M ₂ O ₁₂ (M=Nb, Ta). The solid electrolyte 18 is exemplified by Li ₇ La ₃ Zr ₂ O ₁₂ in which pentavalent M cations in the basic composition are replaced with tetravalent cations.

The solid electrolyte 18 contains Mg, Al, Si, Ca, Ti, V, Ga, Sr, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb and lanthanoids in addition to Li, La and Zr. It can contain at least one element selected from the group consisting of (excluding La). _{For example , Li6La3Zr1.5W0.5O12 , Li6.15La3Zr1.75Ta0.25Al0.2O12 , Li6.15La3Zr1.75Ta0 . _ _ _ _ _ _ 25Ga0.2O12 , Li6.25La3Zr2Ga0.25O12 , Li6.4La3Zr1.4Ta0.6O12 , Li6.5La3Zr1.75 _ _ _ _ _ _ _ _ _ Te0.25O12 , Li6.75La3Zr1.75Nb0.25O12 , Li6.9La3Zr1.675Ta0.289Bi0.036O12 , Li6.46Ga _ _ _ _ _ _ _ 0.23 La3Zr1.85Y0.15O12 , Li6.8La2.95Ca0.05Zr1.75Nb0.25O12 , Li7.05La3.00Zr1 . _ _ _ _ _ _ _ 95Gd0.05O12 , Li6.20Ba0.30La2.95Rb0.05Zr2O12 . _ _ _ _ _}

The solid electrolyte 18 has a crystal structure of, for example, a cubic system (space group Ia-3d (- indicates an overline indicating a rolling operation), JCPDS: 84-1753). The solid electrolyte 18 particularly contains at least one of Mg and element A (A is at least one element selected from the group consisting of Ca, Sr and Ba), and the molar ratio of each element is from (1) to ( A material that satisfies all of 3) or a material that contains both Mg and the element A and has a molar ratio of each element that satisfies all of the following (4) to (6) is preferable. The element A is preferably Sr because it increases the ionic conductivity of the solid electrolyte 18 .
(1) 1.33≦Li/(La+A)≦3
(2) 0≦Mg/(La+A)≦0.5
(3) 0≦A/(La+A)≦0.67
(4) 2.0≦Li/(La+A)≦2.5
(5) 0.01≦Mg/(La+A)≦0.14
(6) 0.04≦A/(La+A)≦0.17.

FIG. 2 is a schematic diagram of a mixture 21 containing a solid electrolyte 18. As shown in FIG. Mixture 21 consists of N-methylpyrrolidone (N-methyl-2-pyrrolidone) and solid electrolyte 18 . The mixture 21 is prepared so that the mass ratio of the solid electrolyte 18 to the mass of the mixture 21 at 25° C. is 24.5 wt %. The solid electrolyte 18 contained in the mixture 21 is, for example, randomly extracted from the electrolyte layer 14 .

Solid electrolytes with garnet-type structures containing Li, La, Zr and O are usually strongly basic. For example, after stirring a mixture of a strongly basic solid electrolyte immersed in N-methylpyrrolidone at a ratio of 24.5 wt%, the supernatant obtained by standing for 12 hours was diluted 10 times with pure water. The hydrogen ion exponent is pH 10 or higher. The hydrogen ion index is a value measured using pH test paper (Advantech Toyo UNIV (1-11)).

On the other hand, the solid electrolyte 18 is prepared by stirring the mixture 21 containing the solid electrolyte 18 and then allowing the mixture 21 to stand for 12 hours. pH 7 or more and pH 8 or less. The solid electrolyte 18 has weakened basicity.

An example of a method for manufacturing the solid electrolyte 18 will be described. For example, the method of manufacturing the solid electrolyte 18 includes a blending step of blending raw materials to obtain a blended material, a firing step of firing the blended material, and a heat treatment step of heating the obtained synthetic powder.

In the compounding step, the compounded material is obtained by compounding the material containing the elements constituting the solid electrolyte 18 . The material includes, for example, Li, La, Zr, Mg, and A (A is at least one element selected from the group consisting of Ca, Sr and Ba), oxides, composite oxides, hydroxides substances, carbonates, chlorides, sulfates, nitrates, phosphates, and the like. The material is pulverized and mixed to obtain a compounded material.

It is preferable to include a calcining step of calcining the compounded material between the compounding step and the firing step. In this step, the compounded material is fired, for example, at 900-1100° C. for 2-15 hours to obtain a calcined material. By going through the calcination process, it becomes easier to obtain a garnet-type crystal structure after the calcination process.

A step of pulverizing and mixing the calcined material is preferably included between the calcining step and the firing step. In this step, the calcined material is pulverized and mixed to obtain a mixed material. By going through the step of pulverizing and mixing the calcined material, it becomes easier to obtain a uniform crystal phase after the sintering step. A calcined material to which a binder is added may be pulverized and mixed. Binders are exemplified by methyl cellulose, ethyl cellulose, polyvinyl alcohol and polyvinyl butyral.

In the sintering step, the compounded material, the calcined material, or the mixed material is molded, and then the molded body is sintered at, for example, 1000 to 1250° C. for 3 to 36 hours to obtain a sintered body. A synthetic powder is obtained by pulverizing the sintered body in an inert gas atmosphere.

In the heat treatment step, the synthetic powder is placed in a furnace into which gas flows and gas flows out, and the synthetic powder is heated in an atmosphere in which the gas flows around the synthetic powder. A solid electrolyte 18 is obtained by removing _CO2 from the _Li2CO3 film formed _on the surface of the synthetic powder when the synthetic powder reacts with _CO2 in the atmosphere by heat treatment, and transforming the structure of the synthetic powder. be done. The temperature and time of the heat treatment are exemplified by holding at a temperature range of 640° C. or higher for 10 hours or longer and holding at a temperature range of 670° C. or higher for 2 hours or longer.

The gas is at least one selected from inert gas and oxygen gas. The inert gas is not particularly limited as long as it does not chemically react with the synthetic powder. Examples of inert gases include nitrogen, helium, neon, argon, krypton, xenon, and radon. At least one selected from nitrogen, helium and argon is particularly preferred.

Gas inflow and outflow may be continuous or intermittent. The flow rate of the gas introduced into the furnace continuously or intermittently and the cycle of the gas introduced intermittently are appropriately set according to the volume of the furnace and the mass of the synthetic powder. The CO ₂ concentration of the gas introduced into the furnace is preferably lower than the CO ₂ concentration (380 ppm) in the atmosphere, for example, 100 ppm (volume) or less, although it depends on the temperature of the heat treatment. The dew point of the gas introduced into the furnace is preferably −40° C. or less, particularly −50° C. or less. This is to facilitate desorption of _CO2 from the synthetic powder. After the heat treatment, the solid electrolyte 18 is immediately mixed with an organic compound or the like and formed into a sheet to obtain the composite layers 13 and 17 and the electrolyte layer 14 .

The median diameter of the equivalent circle diameter of the solid electrolyte 18 appearing in the cross section of the electrolyte layer 14 is preferably 0.5 to 10 μm. This is because the surface area of the solid electrolyte 18 is appropriately sized, and the amount of movement of lithium ions between the solid electrolyte 18 and the electrolytic solution intervening on the surface of the solid electrolyte 18 is ensured.

In order to obtain the median diameter of the solid electrolyte 18, first, the solid electrolyte appearing on the cross section of the electrolyte layer 14 (a polished surface, a surface obtained by irradiation with a focused ion beam (FIB), a surface obtained by ion milling) is measured. Analyzing the scanning electron microscope (SEM) image of 18, the equivalent circle diameter is calculated from the area of each particle of the solid electrolyte 18, and the volume-based particle size distribution is determined. The median diameter is the circle equivalent diameter at which the integrated value of the frequency in the particle size distribution is 50%. The image for obtaining the particle size distribution has an area of 400 μm ² or more in the electrolyte layer 14 in order to ensure accuracy.

The electrolytic solution contained in the electrolyte layer 14 contains an ionic liquid in which a lithium salt is dissolved. Ionic liquids are compounds composed of cations and anions, and are liquid at normal temperature and pressure. Since the ionic liquid constitutes the electrolytic solution, the flame retardancy of the electrolytic solution can be improved. Various physical properties and functions of the electrolytic solution are determined by the types and salt concentrations of the lithium salt and ionic liquid.

A lithium salt is a compound used for the transfer of cations between the positive electrode layer 11 and the negative electrode layer 15 . Anions of lithium salts include halide ions (I ⁻ , Cl ⁻ , Br ⁻ etc.), SCN ⁻ , BF ₄ ⁻ , BF ₃ (CF ₃ ) ⁻ , BF ₃ (C ₂ F ₅ ) ⁻ , PF ₆ ⁻ , ClO ₄ ⁻ , SbF ₆ ⁻ , N(SO ₂ F) ₂ ⁻ , N(SO ₂ CF ₃ ) ₂ ⁻ , N(SO ₂ C ₂ F ₅ ) ₂ ⁻ , B(C ₆ H ₅ ) ₄ ⁻ , B _{( O2C2H4} ) _2- ^, _{C ( SO2F} ⁾ _3- ^, _{C ( SO2CF3} ⁾ _3- ^, _{CF3COO- , CF3SO2O-} ^, _C6F5SO2O- , B(O ₂ C ₂ O ₂ ) ₂ ⁻ , RCOO ⁻ (R is an alkyl group having 1 to 4 carbon atoms, a phenyl group or a naphthyl group), and the like.

The anion of the lithium salt is an anion such as N(SO ₂ F) ₂ ⁻ , N(SO ₂ CF ₃ ) ₂ ⁻ , N(SO ₂ C ₂ F ₅ ) ₂ ⁻ having a sulfonyl group —S(=O) ₂ —. Sulfonylimides are preferred. The sulfonylimide anion has little effect of increasing the viscosity of the electrolyte and decreasing the ionic conductivity even at high salt concentrations, and reduces reductive decomposition of the electrolyte by forming a film (SEI) with high stability and low resistance. This is because the potential window on the reduction side can be extended. N(SO ₂ F) ₂ ^— is abbreviated as [FSI] ⁻ : bis(fluorosulfonyl)imide anion, and N(SO ₂ CF ₃ ) ₂ ^— is abbreviated as [TFSI] ⁻ : bis(trifluoromethanesulfonyl)imide. Sometimes called an anion.

The ionic liquid preferably contains imidazolium as a cation species. The imidazolium cation is, for example, a compound represented by formula (1).

In formula (1), R ¹ to R ⁵ each independently represent a hydrogen group or an alkyl group. The alkyl group may have a substituent. The number of carbon atoms in the alkyl group (including substituents) represented by R ¹ -R ⁵ is preferably 1-10, more preferably 1-5, still more preferably 1-4. This is to ensure the ionic conductivity of the electrolytic solution.

There are no particular restrictions on the substituents. Substituents include alkyl groups, cycloalkyl groups, aryl groups, hydroxyl groups, carboxyl groups, nitro groups, trifluoromethyl groups, amide groups, carbamoyl groups, ester groups, carbonyloxy groups, cyano groups, halogeno groups, alkoxy groups, Examples include an aryloxy group and a sulfonamide group.

Sulfonylimide is suitable for the anionic species of the ionic liquid. The sulfonylimide anions are exemplified by N(SO ₂ F) ₂ ⁻ , N(SO ₂ CF ₃ ) ₂ ⁻ , N(SO ₂ C ₂ F ₅ ) ₂ ⁻ , N(SO ₂ C ₄ F ₉ ) ₂ ^{− and} the like. be done. It is preferable that the anion species of the ionic liquid and the anion species of the lithium salt are the same, because the coordination (interaction) between the lithium ions and the anions contained in the electrolytic solution can be easily controlled.

Ionic liquids are exemplified by 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). . An ionic liquid containing an imidazolium cation and a sulfonylimide anion and having a lithium salt dissolved therein (electrolytic solution) is preferable because high ionic conductivity can be ensured.

The polymer contained in the electrolyte layer 14 is, for example, a binder that binds the solid electrolyte 18 . Polymers include vinylidene fluoride-based polymers containing —CH ₂ CF ₂ —. A vinylidene fluoride-based polymer is preferable because of its high mechanical strength. _The vinylidene fluoride _- based polymer is not particularly limited as long as it contains --CH.sub.2CF.sub.2--. Vinylidene fluoride-based polymers are exemplified by vinylidene fluoride homopolymers and copolymers of vinylidene fluoride and copolymerizable monomers.

Examples of copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen copolymerizable monomers. Halogen-containing monomers are exemplified by chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkylvinyl ether. Examples of non-halogen copolymerizable monomers include olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, esters or salts thereof; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene.

One or more of the copolymerizable monomers are polymerized to vinylidene fluoride to form a copolymer. A copolymer of vinylidene fluoride and hexafluoropropylene is particularly preferred because it can widen the potential window.

The polymer may contain polymers other than the vinylidene fluoride-based polymer. The content of the vinylidene fluoride polymer in the polymer is, for example, 80-100 mass %. Examples of other polymers include fluorinated resins (excluding vinylidene fluoride-based polymers), polyolefins, rubber-like polymers such as styrene-butadiene rubber, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, and cellulose ethers. Examples of fluorinated resins include fully fluorinated resins, partially fluorinated resins, and fluorinated resin copolymers. A fully fluorinated resin is exemplified by polytetrafluoroethylene. Partially fluorinated resins are exemplified by polychlorotrifluoroethylene and polyvinyl fluoride. Fluorinated resin copolymers include tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene/hexafluoropropylene copolymer, ethylene tetrafluoroethylene copolymer, and ethylene/chlorotrifluoroethylene copolymer. Polymers are exemplified.

The electrolyte layer 14 may contain a solvent that dissolves the polymer. The electrolyte layer 14 is obtained by molding an electrolyte composition containing a solid electrolyte 18, a lithium salt, an ionic liquid, a polymer and a solvent into a sheet. At least part of the solvent contained in the electrolyte composition disappears from the electrolyte layer 14 by vaporizing, for example, drying under reduced pressure after sheet molding for obtaining the electrolyte layer 14 . The type and amount of solvent remaining in the electrolyte layer 14 are determined by gas chromatography-mass spectrometry (GC-MS).

The solvent is preferably an aprotic polar solvent, more preferably an aprotic and protonic polar solvent. Classification of solvents (aprotic or protonic) is described in I. M. Kolthoff, Anal. Chem. 46, 1992 (1974). According to Kolthoff's classification, solvents are roughly divided into "amphoteric" solvents that have both acidic and basic properties and can exchange protons, and "aprotic" solvents that do not have hydrogen atoms capable of hydrogen bonding. , are subdivided into “protonic”, which is strongly basic and easily solvated with cations, and “protonic”, which is weakly basic and difficult to solvate with cations. In an aprotic and protonic polar solvent, protons and hydrogen bonds hardly contribute to the reaction, and the solid electrolyte 18 is easily dispersed.

Among aprotic polar solvents, protic solvents include N,N-dimethylformamide, N,N-dimethylacetamide, hexamethylphosphoric acid triamide, dimethylsulfoxide, N-methyl-2-pyrrolidone, pyridine, dioxane, tetrahydrofuran. , ethers are exemplified. Among aprotic polar solvents, protonic solvents are exemplified by propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, sulfolane, acetonitrile, acetone, isobutylmethylketone, nitromethane, methylethylketone, and tetramethylsilane.

The electrolyte composition contains one or more of these polar solvents. The water contained in the solvent and the ionic liquid is preferably 200 ppm or less. This is to reduce the reaction between the water contained in the solvent or the ionic liquid and the solid electrolyte 18 .

In the electrolyte composition in which the solid electrolyte 18 and the organic compound are mixed, the presumed mechanism by which the solid electrolyte 18 reacts with the organic compound to gel (non-fluidize) is as follows. First, a small amount of water contained in the electrolyte composition reacts with a basic solid electrolyte having a garnet structure containing Li, La and Zr to generate LiOH and Li ₂ O ₃ on the surface of the solid electrolyte. This increases OH ^- in the system and strengthens its basicity.

When hydrogen is bonded to the 2-position carbon of the imidazolium cation of the ionic liquid, the ionic conductivity of the electrolytic solution decreases when the 2-position proton is eliminated by a base. A proton released from the imidazolium cation reacts with OH- ^to generate water. The generated water reacts with the solid electrolyte as described above, and becomes more basic.

Under basic conditions, the vinylidene fluoride-based polymer tends to form a polyene structure due to elimination of HF. The electrolyte composition is gelled by the polyene conversion of the vinylidene fluoride polymer. Furthermore, due to the electrochemical reaction derived from the desorbed HF, an unintended SEI is formed and the resistance of the SEI increases.

On the other hand, the solid electrolyte 18 having a hydrogen ion exponent of pH 8 or less obtained by diluting the supernatant 22 of the mixture 21 10 times with pure water is weakly basic. Reaction can be reduced. Since the basicity in the system is less likely to become strong, the proton at the 2-position of the imidazolium cation is less likely to detach, and the drop in ionic conductivity of the electrolytic solution due to detachment of the proton is reduced. In addition, the vinylidene fluoride-based polymer is less prone to polyene formation due to desorption of HF, so gelling of the electrolyte composition is reduced. Furthermore, since the electrochemical reaction derived from HF hardly occurs, the SEI resistance can be kept low. Especially when the solvent contained in the electrolyte composition is an aprotic and protonic polar solvent, protons and hydrogen bonds hardly contribute to the reaction, so that gelation can be further reduced.

In the electrolyte layer 14 (electrolyte composition), the content (volume %) of the ionic liquid with respect to the total amount of the solid electrolyte 18 and the ionic liquid is preferably 50 volume % or less (excluding 0 volume %). That is, solid electrolyte:ionic liquid=(100−X):X, 0<X≦50. This is because the ionic liquid intervening between the solid electrolytes 18 ensures ionic conductivity while reducing the occurrence of seepage of the ionic liquid.

The content (% by volume) of the ionic liquid is 5000, which is randomly selected from the cross section of the electrolyte layer 14 after freezing the electrolyte layer 14 or embedding and solidifying the electrolyte layer 14 in a tetrafunctional epoxy resin or the like. A double field of view is analyzed using an SEM equipped with an energy dispersive X-ray spectrometer (EDS). In the analysis, the distribution of La, Zr, and S is specified, the contrast of the backscattered electron image is analyzed, the area of the solid electrolyte 18 and the area of the ionic liquid are specified, and the ratio of the area in the cross section of the electrolyte layer 14 is determined. is regarded as the volume ratio in the electrolyte layer 14 to obtain the content (% by volume) of the ionic liquid.

The lithium ion conductivity of the electrolyte composition is determined by the types and salt concentrations of the solid electrolyte 18, lithium salt and ionic liquid. The lithium ion conductivity of the electrolyte composition at 25° C. is preferably 4.0×10 ⁻⁵ S/cm or more. This is to ensure the power density of the electricity storage device 10 containing the electrolyte composition.

Since the electrolyte composition contains anions derived from the electrolyte, the lithium ion conductivity of the electrolyte composition is calculated by the AC impedance method of a symmetrical cell in which current collectors are adhered to both sides of the electrolyte composition molded into a sheet. It is calculated by multiplying the total ionic conductivity obtained by multiplying the transport number of lithium ions. The transference number of lithium ions is determined by the AC impedance method and the steady-state DC method.

The export number is calculated as follows. First, the resistance value _RS0 of the cell is analyzed by AC impedance measurement. The conditions for AC impedance measurement are a temperature of 25° C., a voltage of 10 mV, and a frequency of 7 MHz to 100 mHz.

Next, the initial current value _I0 immediately after applying the constant voltage V to the cell is measured, and the initial resistance value _R0 of the cell is calculated according to Equation A below. _R0 =V/ _I0 ...A
The measurement conditions for the initial current value are a voltage of 10 mV, a total time of 6 seconds, and a measurement interval of 0.0002 seconds.

The interface resistance _{R_INT} is calculated by substituting the resistance value _{R_S0} and the initial resistance value _{R_0} into the following equation B. R _INT =R ₀ -R _S0 B
Next, a constant _voltage V is applied to the cell, and the current value I after the steady state is reached is measured. _RP =V/I...C
The measurement conditions for the current value in the steady state are a voltage of 10 mV, a total time of 10 hours, and a measurement interval of 60 seconds.

After the cell reaches a steady state, the resistance value R _S of the cell is analyzed by AC impedance measurement under the conditions described above. The transference number t _Li is calculated by substituting the resistance value R _S , the resistance value R _P , and the interfacial resistance R _INT into the following equation D. t _Li = R _S /(R _P - R _INT ) D
In the electrolyte layer 14 (electrolyte composition), the amount (% by volume) of the binder with respect to the total amount of the solid electrolyte 18 and the ionic liquid is preferably 10% by volume or less (excluding 0% by volume). That is, the total amount of solid electrolyte and ionic liquid: amount of binder=(100−Y): Y, 0<Y≦10. This is because the binder ensures the moldability of the electrolyte layer 14 and reduces the deterioration of the ion conductivity of the electrolyte layer 14 . The binder content (% by volume) can be specified from the area % of the cross section of the electrolyte layer 14 obtained by the SEM-EDS analysis in the same manner as described above.

The power storage device 10 is manufactured, for example, as follows. A mixture of an ionic liquid in which a lithium salt is dissolved and a solid electrolyte 18 is mixed with a solution in which a polymer is dissolved in a solvent to prepare a slurry. After forming the tape, it is dried to obtain a green sheet (electrolyte sheet) for the electrolyte layer 14 .

A mixture of an ionic liquid in which a lithium salt is dissolved and a solid electrolyte 18 is mixed with an active material 19, and further mixed with a solution of a polymer dissolved in a solvent to prepare a slurry. After forming a tape on the current collecting layer 12, it is dried to obtain a green sheet for the positive electrode layer 11 (one kind of positive electrode sheet of the electrolyte sheet).

An active material 20 is mixed with a mixture of an ionic liquid in which a lithium salt is dissolved and a solid electrolyte 18, and then a solution of a polymer dissolved in a solvent is mixed to prepare a slurry. After forming a tape on the current collecting layer 16, it is dried to obtain a green sheet for the negative electrode layer 15 (negative electrode sheet, which is a type of electrolyte sheet).

After the electrolyte sheet, the positive electrode sheet and the negative electrode sheet are each cut into a predetermined shape, the positive electrode sheet, the electrolyte sheet and the negative electrode sheet are stacked in this order and pressure-bonded to each other for integration. A terminal (not shown) is connected to each of the current collecting layers 12 and 16 and sealed in a case (not shown) to obtain the electricity storage device 10 including the positive electrode layer 11, the electrolyte layer 14 and the negative electrode layer 15 in this order.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Preparation of solid electrolyte)
Li ₂ CO ₃ , MgO, La(OH) ₃ , SrCO ₃ and ZrO ₂ were weighed to obtain Li _6.95 Mg _0.15 La _2.75 Sr _0.25 Zr _2.0 O ₁₂ . Considering the volatilization of Li during firing, Li ₂ CO ₃ was made excessive by about 15 mol % in terms of element. The weighed raw materials and ethanol were put into a nylon pot together with zirconia balls, and pulverized and mixed in a ball mill for 15 hours. After drying the slurry taken out from the pot, it was calcined on a plate made of MgO (at 900° C. for 1 hour). The calcined powder and ethanol were put into a nylon pot and pulverized and mixed with a ball mill for 15 hours.

After the slurry was taken out from the pot and dried, it was put into a mold with a diameter of 12 mm and press-molded to obtain a compact with a thickness of about 1.5 mm. An additional hydrostatic pressure of 1.5 t/cm ² was applied to the compact using a cold isostatic press (CIP). The molded body was covered with calcined powder having the same composition as the molded body and fired in a reducing atmosphere (at 1100° C. for 4 hours) to obtain a sintered body of solid electrolyte. The lithium ion conductivity of the sintered body determined by the AC impedance method was 1.0×10 ⁻³ S/cm. The conditions for measuring the lithium ion conductivity were a temperature of 25° C., a voltage of 10 mV, and a frequency of 7 MHz to 100 mHz. In an Ar atmosphere, the sintered body was pulverized and passed through a sieve with an opening of 250 μm, and the solid electrolyte powder that passed through the sieve was collected.

100 g of the powder passed through the sieve, 536 g of balls with a diameter of 4 mm, and 250 mL of fluorine-based inert liquid were placed in a pot, and the powder was pulverized for 6 hours with a planetary ball mill (rotational speed: 200 rpm). The slurry taken out from the pot was dried to obtain a solid electrolyte coarse powder.

Similarly, 50 g of powder passed through a sieve, 536 g of balls having a diameter of 4 mm, and 100 mL of fluorine-based inert liquid were placed in a pot, and the powder was pulverized for 4 hours with a planetary ball mill (rotational speed: 300 rpm). The slurry taken out from the pot was dried to obtain fine powder of the solid electrolyte.

(Heat treatment of coarse powder and fine powder)
A solid electrolyte coarse powder (40 g or less) contained in a tube furnace (volume: 1875 cm ³ ), nitrogen gas was introduced from one end of the tube furnace (flow rate: 10 L/min), and exhausted from the other end of the furnace. and heated at 670° C. for 2 hours. After the temperature in the furnace reached 50° C., the heat-treated coarse powder was immediately taken out from the furnace and placed in a sealed container to prevent exposure to the atmosphere. A solid electrolyte fine powder was also heat-treated under the same conditions and placed in a sealed container.

(Measurement of hydrogen ion exponent)
The heat-treated coarse powder, heat-treated fine powder, and non-heat-treated coarse powder were each immersed in N-methyl-2-pyrrolidone to form a stirred mixture, and the mixture was allowed to stand in a room at 25° C. for 12 hours. The proportion of each powder to the mixture was 24.5 wt%. After standing, the supernatant of the mixture was diluted 10-fold with pure water, and the hydrogen ion index was measured with pH test paper (Advantech Toyo UNIV (1-11)).

Hydrogen measured from a mixture containing heat-treated coarse powder (hereinafter referred to as “powder A”), heat-treated fine powder (hereinafter referred to as “powder B”), and unheat-treated coarse powder (hereinafter referred to as “powder C”) The ionic indices were pH 7-8, pH 7-8, pH 10 in order. The reason why the hydrogen ion exponents of the powders A and B have a pH range of 7-8 is that the change in color of the pH test paper was determined visually.

(Preparation of electrolytic solution)
An ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) was compounded with 3 mol/dm ³ of lithium salt LiN(SO ₂ F) ₂ to obtain an electrolytic solution.

(Example 1)
The powder A and the electrolytic solution were mixed in a mortar in an Ar atmosphere to obtain a composite powder such that the solid electrolyte:electrolyte solution was 61:39 (volume ratio). 18 g of the composite powder, 0.864 g of vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) and 7.776 g of dimethyl carbonate (DMC) were mixed in an Ar atmosphere to obtain a slurry in Example 1.

(Example 2)
A slurry in Example 2 was obtained in the same manner as in Example 1, except that powder A was replaced with powder B.

(Comparative example 1)
A slurry in Comparative Example 1 was obtained in the same manner as in Example 1, except that powder A was replaced with powder C.

(Comparative example 2)
A slurry in Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that propylene carbonate (PC) was used instead of DMC.

(Test method and results)
The slurries in Examples and Comparative Examples were put into beakers, respectively, and left in a vessel in an Ar atmosphere at 25° C. for 12 hours. After 12 hours, it was visually confirmed whether or not the slurry had gelled (non-fluidized). The results are shown in Table 1. A case where at least a part of the slurry was gelled was indicated as +, and a case where the slurry was not gelled at all was indicated as -.

As shown in Table 1, the slurries in Examples 1 and 2 did not gel at all, but the slurries in Comparative Examples 1 and 2 gelled. The slurries in Comparative Examples 1 and 2 were discolored to brown.

The slurries of Examples 1 and 2 and Comparative Examples 1 and 2 contained DMC and PC, which are aprotic and protonic polar solvents. When comparing Examples 1 and 2 with Comparative Examples 1 and 2, the slurries of Examples 1 and 2 containing the solid electrolyte of pH 7 did not gel, while the slurries of Comparative Examples 1 and 2 containing the solid electrolyte of pH 10 did not gel. The slurry gelled. Discoloration and gelation of the slurry are presumed to be due to polyene formation of vinylidene fluoride by desorption of HF. It is presumed that the slurry containing the solid electrolyte of pH 7 did not gel because the interaction of the constituent components of the slurry was less likely to occur.

According to this example, it was clarified that a solid electrolyte having a garnet-type structure containing Li, La and Zr and having a mixture supernatant with a hydrogen ion index of pH 8 or less can reduce gelation. Furthermore, it was found that an electrolyte composition containing a solid electrolyte, an ionic liquid containing imidazolium cations, a lithium salt, and a vinylidene fluoride polymer can also reduce gelation. An electrolyte sheet and an electric storage device containing this electrolyte composition can reduce variations in the state of dispersion of constituent components, and thus can reduce variations in performance.

The present invention has been described above based on the embodiments, but the present invention is not limited to the above-described embodiments, and various modifications and improvements are possible without departing from the scope of the present invention. can be inferred.

In the embodiment, the power storage device 10 includes the positive electrode layer 11 having the composite layer 13 provided on one side of the current collecting layer 12 and the negative electrode layer 15 having the composite layer 17 provided on one side of the current collecting layer 16. Although explained, it is not necessarily limited to this. For example, it is naturally possible to apply each element in the embodiment to a secondary battery having an electrode layer (a so-called bipolar electrode) in which the composite layer 13 and the composite layer 17 are respectively provided on both sides of the current collecting layer 12 . If the bipolar electrodes and the electrolyte layers 14 are alternately laminated and housed in a case (not shown), a secondary battery having a so-called bipolar structure can be obtained.

In the embodiment, the case where all of the composite layers 13, 17 and the electrolyte layer 14 contain the electrolyte composition has been described, but the present invention is not necessarily limited to this. At least one of the composite layers 13, 17 and the electrolyte layer 14 of the secondary battery should contain the electrolyte composition.

In the embodiments, a lithium ion battery (secondary battery) containing an electrolyte composition was exemplified to describe the electricity storage device 10 including the electrode layers (the positive electrode layer 11 and the negative electrode layer 15) and the electrolyte layer 14, but the present invention is not necessarily limited to this. It is not something that can be done. Other secondary batteries include lithium-sulfur batteries, lithium-oxygen batteries, lithium-air batteries, and the like.

REFERENCE SIGNS LIST 10 power storage device 13 composite layer 14 electrolyte layer (electrolyte sheet)
17 composite layer 18 solid electrolyte 21 mixture 22 supernatant

Claims (4)

A solid electrolyte with a garnet-type structure containing Li, La, Zr and O,
In the mixture consisting of N-methylpyrrolidone and the solid electrolyte, 24.5 wt% of the solid electrolyte is contained in the mixture,
A solid electrolyte in which the hydrogen ion exponent of a solution obtained by diluting the supernatant of the above mixture 10-fold with pure water is pH 8 or less. An electrolyte composition comprising the solid electrolyte according to claim 1, an ionic liquid containing imidazolium cations, a lithium salt, and a polymer containing —CH ₂ CF ₂ —. An electrolyte sheet comprising the electrolyte composition according to claim 2 . An electricity storage device comprising an electrolyte layer comprising the electrolyte composition according to claim 2 .

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