JP7773309B2 - Electrolyte composition, electrolyte sheet and electricity storage device - Google Patents

Description

The present invention relates to 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).

Patent No. 6682709

In prior art, when a garnet-type solid electrolyte containing Li, La, Zr, and O is mixed with an organic compound, the organic compound can react with the solid electrolyte, causing it to gel (become non-fluid). This gelling can cause variations in the 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 an electrolyte composition, an electrolyte sheet, and an electricity storage device that can reduce gelation.

To achieve this objective, the solid electrolyte of the present invention is a garnet-type solid electrolyte containing Li, La, Zr, and O, in which, for particles having a particle size equal to or greater than the particle size at which the cumulative frequency in the volume-based particle size distribution is 10%, the number-average envelope ratio, defined as the area of the particle outline divided by the area within the envelope, is 0.8 or greater.

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

The solid electrolyte of the present invention has weakened basicity, making it less likely for the solid electrolyte to interact with organic compounds, thereby reducing gelation. An electrolyte composition containing the solid electrolyte, an electrolyte sheet containing the electrolyte composition, and an electricity storage device can reduce variation in the dispersion state of the solid electrolyte.

FIG. 1 is a cross-sectional view of an electricity storage device according to an embodiment. FIG. 2 is a cross-sectional view of a particle of a solid electrolyte.

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 in one embodiment. The electricity storage device 10 in this embodiment is a secondary battery that is a solid-state battery in which the power generation element is made of a solid. "The power generation element is made of a solid" means that the skeleton of the power generation element is made of a 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, in order, a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15. 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 made up of a current collecting layer 12 and a composite layer 13 superimposed on each other. The current collecting layer 12 is a conductive member. Examples of materials for 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 is made of an electrolyte composition. The electrolyte composition includes a solid electrolyte 18, an active material 19, a polymer, and an electrolyte solution. To reduce the resistance of the composite layer 13, the composite layer 13 may contain a conductive additive. Examples of conductive additives include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

Examples of the active material 19 include a metal oxide containing a transition metal, a sulfur-based active material, and an organic active material. Examples of the metal oxide containing a transition metal include a metal oxide containing Li and one or more elements selected from _Mn , Co, Ni, Fe, _Cr , and _V. Examples of _{the metal} oxide containing a transition metal include _LiCoO2 , _{LiNi0.8Co0.15Al0.05O2} , _LiMn2O4 , _LiNiVO4 , _{LiNi0.5Mn1.5O4} , LiNi1 _{/ 3Mn1} / _3Co1 / _{3O4 ,} and _LiFePO4 .

A coating layer can be provided on the surface of the active material 19 in order to suppress a reaction between the active material 19 and the solid electrolyte 18. Examples of _the coating layer include _{Al2O3 , ZrO2} , _LiNbO3 , _Li4Ti5O12 , _LiTaO3 , _{LiNbO3 , LiAlO2 , Li2ZrO3 , Li2WO4 , Li2TiO3 , Li2B4O7 , Li3PO4 , and Li2MoO4} .

Examples of sulfur-based active materials include S, TiS ₂ , NiS, FeS ₂ , Li ₂ S, MoS ₃ , and sulfur-carbon composites. Examples of organic active materials include radical compounds such as 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radialene compounds, tetraciaquinodimethane, and phenazine oxide.

The negative electrode layer 15 is made up of a current collecting layer 16 and a composite layer 17 superimposed on each other. The current collecting layer 16 is a conductive member. Examples of materials for the current collecting 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.

The composite layer 17 is made of an electrolyte composition. The electrolyte composition includes a solid electrolyte 18, an active material 20, a polymer, and an electrolyte solution. To reduce the resistance of the composite layer 17, the composite layer 17 may contain a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. Examples of the active material 20 include Li, a Li-Al alloy, Li ₄ Ti ₅ O ₁₂ , graphite, In, Si, a 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 electrolyte solution. The solid electrolyte 18 is a lithium ion conductive oxide with a garnet structure containing Li _, La, _Zr , and O. The basic composition of the oxide with _a garnet structure is _Li5La3M2O12 (M = Nb, Ta). An example of the solid electrolyte ₁₈ is _Li7La3Zr2O12 , in which the _pentavalent M cation in the basic composition is replaced with a tetravalent _cation .

In addition to Li, La, and Zr, the solid electrolyte 18 may contain at least one element selected from the group consisting of Mg, Al, Si, Ca, Ti, V, Ga, Sr, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb, and lanthanides (excluding La). For example, Li ₆ La ₃ Zr _1.5 W _0.5 O ₁₂ , Li _6.15 La ₃ Zr _1.75 Ta _0.25 Al _0.2 O ₁₂ , Li _6.15 La ₃ Zr _1.75 Ta _0.25 Ga _0.2 O ₁₂ , Li _6.25 La ₃ Zr ₂ Ga _0.25 O ₁₂ , Li _6.4 La ₃ Zr _1.4 Ta _0.6 O ₁₂ , Li _6.5 La ₃ Zr _1.75 Te _0.25 O ₁₂ , Li _6.75 La ₃ Zr _1.75 Nb _0.25 O ₁₂ , Li _6.9 La ₃ Zr _1.675 Ta _0.289 Bi _0.036 O ₁₂ , Li _6.46 Ga _0.23 La ₃ Zr _1.85 Y _0.15 O ₁₂ , Li _6.8 La _2.95 Ca _0.05 Examples include Zr _1.75 Nb _0.25 O ₁₂ , Li _7.05 La _3.00 Zr _1.95 Gd _0.05 O ₁₂ , Li _6.20 Ba _0.30 La _2.95 Rb _0.05 Zr ₂ O ₁₂ .

The solid electrolyte 18 has, for example, a cubic crystal structure (space group Ia-3d (- indicates an overline indicating a reversal operation), JCPDS: 84-1753). The solid electrolyte 18 preferably 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), with the molar ratio of each element satisfying all of the following (1) to (3), or contains both Mg and element A, with the molar ratio of each element satisfying all of the following (4) to (6). The element A is preferably Sr, in order to increase 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 cross-sectional view of a particle 21 of the solid electrolyte 18. The particle 21 is a particle of the solid electrolyte 18 having a particle diameter (hereinafter referred to as "D ₁₀ ") or larger, at which the cumulative frequency in the volume-based particle size distribution of the solid electrolyte 18 is 10%. To determine D ₁₀ , in this embodiment, a scanning electron microscope (SEM) image of the solid electrolyte 18 appearing on a cross section of the electrolyte layer 14 (a polished surface, a surface obtained by irradiating with a focused ion beam (FIB), or a surface obtained by ion milling) is analyzed, and the circle-equivalent diameter is calculated from the area of each particle of the solid electrolyte 18 to determine the volume-based particle size distribution. _{D 10} is the circle-equivalent diameter at which the cumulative frequency (under the sieve) in the particle size distribution is 10%. To ensure accuracy, the image used to determine the particle size distribution should have an area of 400 μm ² or larger of the cross section of the electrolyte layer 14.

Next, the number average value of the degree of envelopment of particles 21 having a particle diameter of _D10 or more among the solid electrolyte 18 appearing in the cross section where the particle size distribution was measured is determined. The degree of envelopment is defined as "area of the outline 22 of the particle 21/area inside the envelope." The area inside the envelope is the area inside the envelope 23 that is tangent to the outline 22. The number average value of the degree of envelopment is the sum of the degrees of envelopment of the particles 21 divided by the number of particles 21 for which the degree of envelopment was determined.

The reasons for excluding particles having a particle diameter of less than _D10 from the calculation of the envelopment ratio are, first, to prevent erroneous detection of objects other than the solid electrolyte 18 during image recognition, and are related to image resolution, etc. Second, when a particle is cut at its edge, the apparent diameter of the particle appearing in the cross section may be smaller than the actual particle diameter. Since particles with a small apparent diameter have an envelopment ratio close to 1, excluding particles having a particle diameter of less than _D10 from the calculation of the envelopment ratio ensures accuracy.

The degree of envelopment is an index showing the degree of irregularity of the outline 22 of the particle 21; the closer the degree of envelopment is to 1, the less irregular the outline 22 of the particle 21 is. The degree of envelopment can be calculated using, for example, the image processing software imageJ. There is a relationship between the degree of envelopment of the particles 21 and the basicity of the solid electrolyte 18: when the number-average value of the degree of envelopment of the particles 21 is 0.8 or higher, the basicity of the solid electrolyte 18 is weakened.

In addition to determining the degree of envelopment from the cross-section of the electrolyte layer 14 or the composite layers 13 and 17, the degree of envelopment can also be determined by sparsely arranging the solid electrolyte 18 (powder) so that there are spatial intervals (so that it does not overlap) and performing image analysis of the projection of the particles. When determining the degree of envelopment from the projection of the solid electrolyte 18 (powder), image analysis is performed on at least the number of particles appearing within an area of 400 ^μm2 of the cross-section of the electrolyte layer 14. Alternatively, the degree of envelopment can be determined by solidifying the solid electrolyte 18 (powder) by a method such as mixing the solid electrolyte 18 (powder) with a binder and forming it into a sheet or embedding the solid electrolyte 18 (powder) in a synthetic resin, and then analyzing an image of the solid electrolyte 18 appearing on the cross-section of the solid (a polished surface, a surface obtained by FIB irradiation, or a surface obtained by ion milling).

An example of a method for manufacturing solid electrolyte 18 will be described. For example, the method for manufacturing solid electrolyte 18 includes a blending process in which raw materials are blended to obtain a blended material, a firing process in which the blended material is fired, and a heat treatment process in which the obtained synthetic powder is heated.

In the blending process, materials containing the elements that make up the solid electrolyte 18 are blended to obtain a blended material. Examples of materials include oxides, composite oxides, hydroxides, carbonates, chlorides, sulfates, nitrates, and phosphates containing the elements Li, La, Zr, Mg, and A (A is at least one element selected from the group consisting of Ca, Sr, and Ba). The materials are pulverized and mixed to obtain the blended material.

It is preferable to include a calcination step between the blending step and the firing step, in which the blended materials are calcined. In this step, the blended materials are calcined, for example, at 900-1100°C for 2-15 hours to obtain a calcined material. By undergoing the calcination step, it becomes easier to obtain a garnet-type crystal structure after the firing step.

It is preferable to include a step of pulverizing and mixing the calcined material between the calcination 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 crystalline phase after the firing step. The calcined material to which a binder has been added may also be pulverized and mixed. Examples of binders include methyl cellulose, ethyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

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

In the heat treatment process, the synthetic powder is placed in a furnace with gas inflow and outflow, and heated in an atmosphere with gas flowing around the synthetic powder. The synthetic powder reacts with _CO2 in the atmosphere to form a _Li2CO3 film on the surface of the synthetic powder, and the heat treatment desorbs _CO2 from the _Li2CO3 film, modifying the structure of the synthetic powder to obtain a solid electrolyte 18. Examples of the heat treatment temperature and time include holding the temperature in a temperature range of 640°C or higher for 10 hours or more, and holding the temperature in a temperature range of 670°C or higher for 2 hours or more.

The gas is at least one selected from an inert gas and oxygen gas. There are no particular limitations on the inert gas, 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.

The gas inflow and outflow may be continuous or intermittent. The flow rate of the gas continuously or intermittently introduced into the furnace and the period of intermittent gas introduction are appropriately set according to the volume of the furnace and the mass of the synthesized powder. The _CO2 concentration of the gas introduced into the furnace depends on the heat treatment temperature, but is preferably lower than the _CO2 concentration in the atmosphere (380 ppm), for example, 100 ppm (volume) or less. The dew point of the gas introduced into the furnace is preferably -40°C or less, particularly -50°C or less. This is to promote the desorption of _CO2 from the synthesized 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, 17 and the electrolyte layer 14.

The median diameter (hereinafter referred to as "D ₅₀ ") of the circle-equivalent diameters of the solid electrolyte 18 appearing in the cross section of the electrolyte layer 14 is preferably 0.5 to 10 μm. This is to ensure that the surface area of the solid electrolyte 18 is of an appropriate size and to ensure the amount of lithium ion movement between the solid electrolyte 18 and the electrolytic solution present on the surface of the solid electrolyte 18.

To determine the _D50 of the solid electrolyte 18, first, an SEM image of the solid electrolyte 18 appearing on a cross section of the electrolyte layer 14 (a polished surface, a surface obtained by FIB irradiation, or a surface obtained by ion milling) is analyzed, and the equivalent circle diameter is calculated from the area of each particle of the solid electrolyte 18 to determine the volumetric particle size distribution. _D50 is the equivalent circle diameter at which the integrated value of frequency in the particle size distribution is 50%. To ensure accuracy, the image used to determine the particle size distribution should have an area of 400 ^μm2 or more of the electrolyte layer 14.

The electrolyte solution contained in the electrolyte layer 14 includes an ionic liquid in which a lithium salt is dissolved. Ionic liquid is a compound consisting of cations and anions, and is liquid at room temperature and pressure. Because the ionic liquid constitutes the electrolyte solution, the flame retardancy of the electrolyte solution can be improved. The various physical properties and functions of the electrolyte solution are determined by the type and salt concentration of the lithium salt and ionic liquid.

The lithium salt is a compound used for the exchange of cations between the positive electrode layer 11 and the negative electrode layer 15 . The 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(O ₂ C ₂ H ₄ ) ₂ ^- , C(SO ₂ F) ₃ ^- , C(SO ₂ CF ₃ ) ₃ ^- , CF ₃ COO ^- , CF ₃ SO ₂ Examples include O ⁻ , C ₆ F ₅ SO ₂ O ⁻ , 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).

The anion of the lithium salt ^is preferably a sulfonylimide having a sulfonyl group -S(=O) _{2- ,} such as N( _SO2F ) _2- ^, N( _SO2CF3 ) _2- ^, _or N( _SO2C2F5 ) _{2- .} This is because sulfonylimide anions are less susceptible to increases in electrolyte viscosity and decreases in ionic conductivity even when the salt concentration is high, and furthermore, they form a highly stable and low-resistance coating (SEI), which reduces reductive decomposition of the electrolyte and expands the reduction-side potential window. ^N ( _SO2F ) _2- is sometimes referred to as [FSI] ^- : bis(fluorosulfonyl)imide anion, _and N( _SO2CF3 ⁾ _2- is sometimes referred to as [TFSI] ^- : bis(trifluoromethanesulfonyl)imide anion.

Ionic liquids with imidazolium as the cation species are preferred. The imidazolium cation is, for example, a compound represented by formula (1).

In formula (1), R ¹ -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 the substituent) represented by ^{R 1} -R ⁵ is preferably 1 to 10, more preferably 1 to 5, and even more preferably 1 to 4. This is to ensure the ionic conductivity of the electrolyte solution.

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

The anion species of the ionic liquid is preferably a sulfonylimide. Examples of sulfonylimide anions include N(SO ₂ F) ₂ ⁻ , N(SO ₂ CF ₃ ) ₂ ⁻ , N(SO ₂ C ₂ F ₅ ) ₂ ⁻ , and N(SO ₂ C ₄ F ₉ ) ₂ ⁻ . It is preferable that the anion species of the ionic liquid and the anion species of the lithium salt are the same, since this makes it easier to control the coordination (interaction) between the lithium ions and the anions contained in the electrolyte solution.

Examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). Ionic liquids (electrolytes) containing imidazolium cations and sulfonylimide anions and in which lithium salts are dissolved are preferred because they can ensure high ionic conductivity.

The polymer contained in the electrolyte layer 14 is, for example, a binder that binds the solid electrolyte 18. The polymer includes a vinylidene fluoride polymer containing -CH ₂ CF ₂ -. Vinylidene fluoride polymers are preferred because of their high mechanical strength. There are no particular restrictions on the vinylidene fluoride polymer as long as it contains -CH ₂ CF ₂ -. Examples of vinylidene fluoride polymers include a homopolymer of vinylidene fluoride and a copolymer of vinylidene fluoride and a copolymerizable monomer.

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

One or more copolymerizable monomers are polymerized with 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 vinylidene fluoride polymers. The content of vinylidene fluoride polymers in the polymer is, for example, 80-100% by mass. Examples of other polymers include fluorinated resins (excluding vinylidene fluoride polymers), polyolefins, rubbery 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. An example of a fully fluorinated resin is polytetrafluoroethylene. Examples of partially fluorinated resins include polychlorotrifluoroethylene and polyvinyl fluoride. Examples of fluorinated resin copolymers include tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

The electrolyte layer 14 may contain a solvent that dissolves the polymer. The electrolyte layer 14 is obtained by forming an electrolyte composition containing a solid electrolyte 18, lithium salt, ionic liquid, polymer, and solvent into a sheet. At least a portion of the solvent contained in the electrolyte composition is vaporized by vacuum drying or the like after forming the sheet to obtain the electrolyte layer 14, and disappears from the electrolyte layer 14. The type and amount of solvent remaining in the electrolyte layer 14 can be determined by gas chromatography-mass spectrometry (GC-MS).

The solvent is preferably an aprotic polar solvent, and more preferably an aprotic and protophobic polar solvent. The classification of solvents (aprotic and protophobic) follows I. M. Kolthoff, Anal. Chem. 46, 1992 (1974). Kolthoff's classification broadly divides solvents into "amphoteric" solvents, which have both acidic and basic properties and can donate and accept protons, and "aprotic" solvents, which lack hydrogen atoms capable of hydrogen bonding. The latter are further subdivided into "protophilic" solvents, which are highly basic and readily solvate with cations, and "protophobic" solvents, which are weakly basic and difficult to solvate with cations. In aprotic and protophobic polar solvents, protons and hydrogen bonds contribute little to the reaction, and the solid electrolyte 18 is more easily dispersed.

Examples of protophilic aprotic polar solvents include N,N-dimethylformamide, N,N-dimethylacetamide, hexamethylphosphoric triamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, pyridine, dioxane, tetrahydrofuran, and ether. Examples of protophilic aprotic polar solvents include propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolane, acetonitrile, acetone, isobutyl methyl ketone, nitromethane, methyl ethyl ketone, and tetramethylsilane.

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

In an electrolyte composition in which a solid electrolyte 18 and an organic compound are mixed, the presumed mechanism by which the solid electrolyte 18 reacts with the organic compound to gel (become non-fluid) is as follows: First, a small amount of water contained in the electrolyte composition reacts with the basic solid electrolyte having a garnet structure containing Li, La, and _Zr , producing LiOH and _Li2O3 on the surface of the solid electrolyte. This increases ^OH- in the system, strengthening its basicity.

When a hydrogen atom is bonded to the carbon atom at the second position of the imidazolium cation in an ionic liquid, the ionic conductivity of the electrolyte decreases when the proton at the second position is removed by a base. The proton removed from the imidazolium cation reacts with ^OH- to generate water. As mentioned above, the generated water reacts with the solid electrolyte, further increasing its basicity.

Under basic conditions, vinylidene fluoride polymers are prone to forming polyene structures due to the elimination of HF. The polyene formation of vinylidene fluoride polymers causes the electrolyte composition to gel. Furthermore, electrochemical reactions caused by the elimination of HF can lead to the formation of an unintended SEI, which increases the resistance of the SEI.

In contrast, a solid electrolyte 18 having a number-average envelopment degree of particles 21 of 0.8 or greater has weak basicity, thereby reducing the reaction between the water contained in the electrolyte composition and the solid electrolyte 18. Because the basicity in the system is less likely to increase, the proton at the second position of the imidazolium cation is less likely to be eliminated, reducing the decrease in ionic conductivity of the electrolyte solution due to proton elimination. Furthermore, vinylidene fluoride-based polymers are less likely to undergo polyenation due to HF elimination, reducing gelation of the electrolyte composition. Furthermore, because electrochemical reactions caused by HF are less likely to occur, the resistance of the SEI can be maintained low. In particular, when the solvent contained in the electrolyte composition is an aprotic, protophobic polar solvent, protons and hydrogen bonds contribute little to the reaction, further reducing gelation.

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

The ionic liquid content (volume %) is determined by freezing the electrolyte layer 14 or embedding it in a tetrafunctional epoxy resin or similar and then analyzing a randomly selected cross-section of the electrolyte layer 14 at a magnification of 5000x using an SEM equipped with an energy dispersive X-ray spectrometer (EDS). The analysis involves identifying the distribution of La, Zr, and S and analyzing the contrast of the backscattered electron image to determine the area of the solid electrolyte 18 and the area of the ionic liquid. The ionic liquid content (volume %) is then calculated by regarding the area ratio in the cross-section of the electrolyte layer 14 as the volume ratio of the electrolyte layer 14.

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 output density of the electricity storage device 10 containing the electrolyte composition.

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

The transport number is calculated as follows: First, the resistance value R _S0 of the cell is analyzed by AC impedance measurement, which is performed under the conditions of a temperature of 25° C., a voltage of 10 mV, and a frequency of 7 MHz to 100 mHz.

Next, the initial current value I ₀ is measured immediately after applying a constant voltage V to the cell, and the initial resistance value R ₀ of the cell is calculated according to the following formula A: _{R 0} =V/I ₀ ... A
The conditions for measuring 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 resistance value R _S0 and the initial resistance value R ₀ are substituted into the following formula B to calculate the interface resistance R _INT : R _INT =R ₀ -R _S0 ... B
Next, a constant voltage V is applied to the cell, and the current value I is measured after the cell reaches a steady state. The resistance value R _P of the cell in the steady state is calculated according to the following formula C: _{R P} = V/I C
The conditions for measuring 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 above-mentioned conditions. The resistance value R _S , the resistance value R _P and the interface resistance R _INT are substituted into the following equation D to calculate the transport number t _Li : _{t Li} = R _S / (R _P - R _INT ) ... D
In the electrolyte layer 14 (electrolyte composition), the amount of binder (volume %) relative to the combined amount of the solid electrolyte 18 and the ionic liquid is preferably 10% by volume or less (excluding 0% by volume). That is, combined amount of solid electrolyte and ionic liquid: amount of binder = (100 - Y): Y, where 0 < Y ≦ 10. This is because the binder ensures the formability of the electrolyte layer 14 and reduces a decrease in the ionic conductivity of the electrolyte layer 14. The binder content (volume %) can be determined from the area % of the cross section of the electrolyte layer 14 determined by SEM-EDS analysis in the same manner as above.

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

A mixture of an ionic liquid containing dissolved lithium salt and a solid electrolyte 18 is mixed with an active material 19, and then mixed with a solution of a polymer dissolved in a solvent to create a slurry. After tape casting onto the current collecting layer 12, it is dried to obtain a green sheet (a type of cathode sheet that is an electrolyte sheet) for the positive electrode layer 11.

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

After cutting the electrolyte sheet, positive electrode sheet, and negative electrode sheet to the desired shape, they are stacked in this order, positive electrode sheet, electrolyte sheet, and negative electrode sheet, and then pressed together to form a single sheet. Terminals (not shown) are connected to the current collecting layers 12 and 16, respectively, and the device is sealed in a case (not shown), resulting in an electricity storage device 10 comprising, in order, a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15.

The present invention will be explained in more detail using examples, but the present invention is not limited to these examples.

(Preparation of solid electrolyte)
_Li2CO3 , MgO, La(OH) ₃ , _SrCO3 , and _ZrO2 were weighed to _{obtain Li6.95Mg0.15La2.75Sr0.25Zr2.0O12 . Li2CO3} was used in _excess of about ₁₅ mol _% in elemental terms, taking into account the volatilization of Li during firing. The weighed _raw materials and ethanol were placed in a nylon pot together with zirconia balls and _ground and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried and then pre-fired (at 900°C for 1 hour) on an MgO plate. The pre-fired powder and ethanol were placed in a nylon pot and ground and mixed in a ball mill for 15 hours.

The slurry removed from the pot was dried and then placed in a 12 mm diameter mold. A compact approximately 1.5 mm thick was obtained by press molding. A hydrostatic pressure of 1.5 t/ ^cm² was applied to the compact using a cold isostatic press (CIP). The compact was covered with a calcined powder of the same composition as the compact and fired in a reducing atmosphere (at 1100°C for 4 hours) to obtain a sintered solid electrolyte. The lithium ion conductivity of the sintered body determined by AC impedance spectroscopy was 1.0 × 10 ⁻³ S/cm. The lithium ion conductivity was measured under the following conditions: temperature 25°C, voltage 10 mV, and frequency 7 MHz-100 mHz. The sintered body was pulverized in an Ar atmosphere and passed through a sieve with 250 μm mesh, and the solid electrolyte powder that passed through the sieve was collected.

100 g of the sieved powder, 536 g of 4 mm diameter balls, and 250 mL of fluorine-based inert liquid were placed in a pot, and the powder was milled for 6 hours using a planetary ball mill (rotation speed: 200 rpm). The slurry removed from the pot was dried to obtain a coarse powder of solid electrolyte.

Similarly, 50 g of the sieved powder, 536 g of 4 mm diameter balls, and 100 mL of fluorine-based inert liquid were placed in a pot, and the powder was ground for 4 hours using a planetary ball mill (rotation speed: 300 rpm). The slurry removed from the pot was dried to obtain a fine powder of solid electrolyte.

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

(Calculation of the degree of envelopment)
The heat-treated coarse powder, the heat-treated fine powder, and the unheat-treated coarse powder were each sparsely adhered and fixed onto a carbon tape with an adhesive layer, and then SEM images of the powders were taken. Image analysis was used to calculate the circle-equivalent diameter from the area (projection) of each solid electrolyte particle, and the volume-based particle size distribution was determined, from which _D10 was calculated.

Next, the number average value of the degree of envelopment of particles having a particle diameter of D ₁₀ or more among the solid electrolytes whose particle size distributions were measured was determined using ImageJ 1.52v. The number average values of the degree of envelopment of the heat-treated coarse powder (hereinafter referred to as "powder A"), the heat-treated fine powder (hereinafter referred to as "powder B"), and the non-heat-treated coarse powder (hereinafter referred to as "powder C") were 0.93, 0.80, and 0.75, respectively.

(Preparation of Electrolyte)
The ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) was combined with the lithium salt LiN(SO ₂ F) ₂ at 3 mol/dm ³ to obtain an electrolyte solution.

Example 1
Powder A and the electrolyte solution were mixed in a mortar in an Ar atmosphere so that the solid electrolyte:electrolytic solution ratio was 61:39 (volume ratio) to obtain a composite powder. 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 the 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 DMC was replaced with propylene carbonate (PC).

(Test method and results)
The slurries in the Examples and Comparative Examples were each placed in a beaker and left in a container in an Ar atmosphere at 25°C for 12 hours. After 12 hours had passed, it was visually confirmed whether the slurries had gelled (become non-fluidized). The results are shown in Table 1. Slurries in which at least a portion had gelled were marked with +, and slurries in which no gelling had occurred at all were marked with -.

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 turned brown.

The slurries of Examples 1 and 2 and Comparative Examples 1 and 2 contained aprotic and protophobic polar solvents, DMC and PC. Comparing Examples 1 and 2 with Comparative Examples 1 and 2, the slurries of Examples 1 and 2, which contained a solid electrolyte with an envelopment degree of 0.80 or more, did not gel, while the slurries of Comparative Examples 1 and 2, which contained a solid electrolyte with an envelopment degree of 0.75, gelled. The discoloration and gelation of the slurry are presumably due to the polyenation of vinylidene fluoride caused by the elimination of HF. It is presumed that the slurries containing solid electrolytes with an envelopment degree of 0.80 or more did not gel because the components of the slurry were less likely to interact with each other.

This example demonstrates that a solid electrolyte having a garnet structure containing Li, La, and Zr, in which the number-average envelopment index of particles having a particle diameter of _D10 or greater is 0.8 or greater, can reduce gelation. Furthermore, it has been demonstrated that an electrolyte composition containing a solid electrolyte, an ionic liquid containing an imidazolium cation, a lithium salt, and a vinylidene fluoride-based polymer can also reduce gelation. An electrolyte sheet and an electricity storage device containing this electrolyte composition can reduce the variation in the dispersion state of the constituent components, thereby minimizing the variation in performance.

The present invention has been described above based on the embodiments, but the present invention is in no way limited to the above embodiments, and it is readily apparent that various improvements and modifications are possible within the scope of the present invention.

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

In the embodiment, the composite layers 13, 17 and the electrolyte layer 14 all contain an electrolyte composition, but this is not necessarily limited to this. It is sufficient for the secondary battery to contain an electrolyte composition in at least one of the composite layers 13, 17 and the electrolyte layer 14.

In the embodiment, the electricity storage device 10 including electrode layers (positive electrode layer 11 and negative electrode layer 15) and an electrolyte layer 14 has been described using a lithium ion battery (secondary battery) containing an electrolyte composition as an example, but this is not necessarily limited to this. Other examples of secondary batteries include lithium-sulfur batteries, lithium-oxygen batteries, and lithium-air batteries.

10 Electricity storage device 13 Composite layer 14 Electrolyte layer (electrolyte sheet)
17 Composite layer 18 Solid electrolyte 21 Particle 22 Contour 23 Envelope

Claims (3)

An electrolyte composition comprising a solid electrolyte having a garnet structure containing Li, La, Zr, and O,
an ionic liquid containing an imidazolium cation, a lithium salt, and a polymer containing —CH ₂ CF ₂ —;
The lithium ion conductivity of the electrolyte composition at 25°C is 4.0 × 10 ^-5 S/cm or more,
the solid electrolyte contains Mg and Sr;
the number average value of the envelope degree, defined as the area of the outline of the particle/the area within the envelope of the particle, of particles of the solid electrolyte having a particle size equal to or larger than a particle size at which the integrated value of the frequency in a volume-based particle size distribution is 10% is 0.8 or more;
The electrolyte composition , wherein the ionic liquid contains 200 ppm or less of water . An electrolyte sheet comprising the electrolyte composition of claim 1. An electricity storage device comprising an electrolyte layer made of the electrolyte composition of claim 1.