Electrolyte composition, electrolyte sheet and electricity storage device

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; and with respect to particles having a particle diameter that is equal to or greater than a particle diameter at which the integrated frequencies reach 10% in the volume-based particle size distribution of solid electrolyte particles, the number average of the envelopment degree that is defined as the value of (area inside an outline) / (area inside an envelope) of each particle is 0.8 or more. 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]

[0001] The present invention Electric The present invention relates to an electrolyte composition, an electrolyte sheet, and an electricity storage device. [Background technology]

[0002] A solid electrolyte with a garnet structure containing Li, La, Zr, and O is known (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6682709 Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, when a garnet-type solid electrolyte containing Li, La, Zr, and O is mixed with an organic compound, the organic compound may react with the solid electrolyte to gel (become non-fluid). When the solid electrolyte becomes gelled, the dispersion state of the solid electrolyte becomes uneven.

[0005] The present invention has been made to solve this problem, and is capable of reducing gelation. Ruden The present invention provides an electrolyte composition, an electrolyte sheet, and an electricity storage device. [Means for solving the problem]

[0006] 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, and has a number-average envelopment index, defined as the area of ​​the particle outline divided by the area within the envelope, of 0.8 or more for particles having a particle diameter or larger at which the cumulative frequency in a volume-based particle size distribution is 10%.

[0007] 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 -CHCF-. 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. [Effects of the Invention]

[0008] The solid electrolyte of the present invention has a weakened basicity, which makes it difficult for the solid electrolyte to interact with an organic compound, thereby reducing gelation. The electrolyte composition containing the solid electrolyte, and the electrolyte sheet and electricity storage device containing the electrolyte composition can reduce variations in the dispersion state of the solid electrolyte. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of an electricity storage device according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a particle of a solid electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0010] 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 electricity storage device 10 according to this embodiment is a secondary battery formed of a solid-state battery in which the power generating element is made of a solid. "The power generating element is made of a solid" means that the skeleton of the power generating element is made of a solid, and does not exclude, for example, a form in which the skeleton is impregnated with a liquid.

[0011] 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).

[0012] The positive electrode layer 11 is formed by stacking a current collecting layer 12 and a composite layer 13. The current collecting layer 12 is a conductive member. Examples of materials for the current collecting layer 12 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0013] 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 the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0014] Examples of the active material 19 include metal oxides containing transition metals, sulfur-based active materials, and organic active materials. Examples of the metal oxides containing transition metals include metal oxides containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. Examples of the metal oxides containing transition metals include LiCoO2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiMn2O4, LiNiVO4, LiNi 0.5 Mn 1.5 O4,LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 Examples include O4 and LiFePO4.

[0015] To prevent the active material 19 from reacting with the solid electrolyte 18, a coating layer can be provided on the surface of the active material 19. The coating layer can be made of Al2O3, ZrO2, LiNbO3, Li4Ti5O 12 , LiTaO3, LiNbO3, LiAlO2, Li2ZrO3, Li2WO4, Li2TiO3, Li2B4O7, Li3PO4 and Li2MoO4 are examples.

[0016] Examples of sulfur-based active materials include S, TiS2, NiS, FeS2, Li2S, MoS3, 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.

[0017] The negative electrode layer 15 is formed by stacking a current collecting layer 16 and a composite layer 17. 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.

[0018] 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. The active material 20 is Li, Li-Al alloy, Li4Ti5O 12 , graphite, In, Si, Si—Li alloy, and SiO.

[0019] 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 Li5La3M2O 12 (M=Nb, Ta). The solid electrolyte 18 is Li7La3Zr2O, in which the pentavalent M cations in the basic composition are replaced with tetravalent cations. 12 is exemplified.

[0020] 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), in addition to Li, La, and Zr. For example, Li6La3Zr 1.5 W 0.5 O 12 ,Li 6.15 La3Zr 1.75 Ta 0.25 Al 0.2 O 12 ,Li 6.15 La3Zr 1.75 Ta 0.25 Ga 0.2 O 12 ,Li 6.25 La3Zr2Ga 0.25 O 12 ,Li 6.4 La3Zr 1.4 Ta 0.6 O 12 ,Li 6.5 La3Zr 1.75 Te 0.25 O 12 ,Li 6.75 La3Zr 1.75 Nb 0.25 O 12 ,Li 6.9 La3Zr 1.675 Ta 0.289 Bi 0.036 O 12 ,Li 6.46 Ga 0.23 La3Zr 1.85 Y 0.15 O 12 ,Li 6.8 La 2.95 Ca 0.05 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 ZrO 12 Examples include:

[0021] 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.

[0022] 2 is a cross-sectional view of a particle 21 of the solid electrolyte 18. The particle 21 has a particle diameter (hereinafter referred to as "D") of the solid electrolyte 18 at which the cumulative frequency in the volume-based particle size distribution of the solid electrolyte 18 is 10%. 10 These particles are larger than the size of the D 10 In this embodiment, to determine the particle size distribution D, an image of the solid electrolyte 18 that appears on the cross section of the electrolyte layer 14 (a polished surface, a surface obtained by irradiating a focused ion beam (FIB), or a surface obtained by ion milling) is analyzed using a scanning electron microscope (SEM), and the circle-equivalent diameter is calculated from the area of ​​each particle of the solid electrolyte 18, thereby determining the volume-based particle size distribution. 10 is the equivalent circle diameter at which the cumulative frequency (below the sieve) in the particle size distribution is 10%. In order to ensure accuracy, the image for determining the particle size distribution is taken from a 400 μm 2 The area shall be equal to or greater than this.

[0023] Next, among the solid electrolytes 18 appearing on the cross section where the particle size distribution was measured, D 10The number average value of the degree of envelopment of particles 21 having the above particle diameter is calculated. The degree of envelopment is defined as "area of ​​outline 22 of 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 particles 21 divided by the number of particles 21 for which the degree of envelopment is calculated.

[0024] D 10 The reason for excluding particles with a particle diameter less than this from the calculation of the envelopment index is, firstly, to prevent erroneous detection of objects other than the solid electrolyte 18 during image recognition, and is due to factors such as image resolution. Secondly, if a particle is cut at its edge, the apparent diameter of the particle that appears in the cross section may be smaller than the actual particle diameter. Since particles with a small apparent diameter have an envelopment index close to 1, D 10 This is to ensure accuracy by excluding particles with a particle size less than this from the calculation of the envelopment degree.

[0025] The degree of envelopment is an index showing the degree of irregularity of the outline 22 of the particle 21, and 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, for example, using image processing software ImageJ. There is a relationship between the degree of envelopment of the particle 21 and the basicity of the solid electrolyte 18 in that when the number-average value of the degree of envelopment of the particle 21 is 0.8 or higher, the basicity of the solid electrolyte 18 is weakened.

[0026] The degree of envelopment can be determined not only from the cross section of the electrolyte layer 14 or the composite layers 13 and 17, but also by sparsely disposing 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 the degree of envelopment is determined from the projection of the solid electrolyte 18 (powder), the degree of envelopment is determined by analyzing the image of the cross section of the electrolyte layer 14 within 400 μm 2 Alternatively, the solid electrolyte 18 (powder) may be solidified by mixing it with a binder and forming it into a sheet, or by embedding it in a synthetic resin, and then the image of the solid electrolyte 18 that appears on the cross section of the solid (a polished surface, a surface obtained by FIB irradiation, or a surface obtained by ion milling) may be analyzed to determine the degree of envelopment.

[0027] The following describes an example of a method for manufacturing the solid electrolyte 18. For example, the method for 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.

[0028] In the blending step, materials containing the elements that constitute the solid electrolyte 18 are blended to obtain a blended material. Examples of the 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.

[0029] It is preferable to include a calcination step between the blending step and the firing step, in which the blended material is calcined at, for example, 900-1100°C for 2-15 hours to obtain a calcined material. By passing through the calcination step, it becomes easier to obtain a garnet-type crystal structure after the firing step.

[0030] 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 passing through the step of pulverizing and mixing the calcined material, a uniform crystalline phase can be easily obtained after the firing step. The calcined material to which a binder has been added may be pulverized and mixed. Examples of binders include methyl cellulose, ethyl cellulose, polyvinyl alcohol, and polyvinyl butyral.

[0031] In the sintering process, the compounded, calcined or mixed material is molded, and then the molded body is sintered at, for example, 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.

[0032] 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 it. The synthetic powder reacts with CO2 in the atmosphere, forming a Li2CO3 film on the surface of the synthetic powder. The heat treatment desorbs CO2 from the Li2CO3 film, and the structure of the synthetic powder is modified to obtain solid electrolyte 18. Examples of heat treatment temperatures and times include holding the temperature at 640°C or higher for 10 hours or more, and holding the temperature at 670°C or higher for 2 hours or more.

[0033] The gas is at least one selected from an 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 the inert gas include nitrogen, helium, neon, argon, krypton, xenon, and radon. At least one selected from nitrogen, helium, and argon is particularly preferred.

[0034] The gas inflow and outflow may be continuous or intermittent. The flow rate of the gas introduced into the furnace continuously or intermittently, and the period of the 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, 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.

[0035] The median diameter of the circle-equivalent diameter of the solid electrolyte 18 appearing in the cross section of the electrolyte layer 14 (hereinafter referred to as "D 50 The thickness (hereinafter referred to as "thickness") of the solid electrolyte 18 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 ions that can move between the solid electrolyte 18 and the electrolytic solution present on the surface of the solid electrolyte 18.

[0036] Solid Electrolyte 18D 50To determine this, first, an SEM image of the solid electrolyte 18 appearing on the 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 circle-equivalent diameter is calculated from the area of ​​each particle of the solid electrolyte 18, and the volume-based particle size distribution is determined. 50 is the circle equivalent diameter at which the cumulative value of frequency in the particle size distribution is 50%. 2 The area shall be equal to or greater than this.

[0037] The electrolyte solution contained in the electrolyte layer 14 includes an ionic liquid in which a lithium salt is dissolved. An ionic liquid is a compound consisting of a cation and an anion, and is liquid at room temperature and normal 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.

[0038] The lithium salt is a compound used for transferring cations between the positive electrode layer 11 and the negative electrode layer 15. The anion of the lithium salt is a halide ion (I - ,Cl - ,Br - etc.), SCN - ,BF4 - ,BF3(CF3) - ,BF3(C2F5) - ,PF6 - ,ClO4 - ,SbF6 - ,N(SO2F)2 - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - ,B(C6H5)4 - ,B(O2C2H4)2 - ,C(SO2F)3 - ,C(SO2CF3)3 - ,CF3COO - ,CF3SO2O - ,C6F5SO2O - ,B(O2C2O2)2 - ,RCOO -(R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0039] The anion of the lithium salt is N(SO2F)2, which has a sulfonyl group -S(=O)2-. - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - Sulfonylimides such as N(SO2F)2 are preferred. The sulfonylimide anion is less susceptible to increases in electrolyte viscosity and decreases in ionic conductivity even when the salt concentration is high. Furthermore, it forms a highly stable and low-resistance coating (SEI), which reduces the reductive decomposition of the electrolyte and expands the reduction-side potential window. - is abbreviated as [FSI] - : It is called bis(fluorosulfonyl)imide anion, N(SO2CF3)2 - abbreviated as [TFSI] - : It is sometimes called bis(trifluoromethanesulfonyl)imide anion.

[0040] The ionic liquid is preferably one having imidazolium as the cation species. The imidazolium cation is, for example, a compound represented by formula (1).

[0041] [ka]

[0042] In formula (1), R 1 -R 5 R each independently represents a hydrogen atom or an alkyl group. The alkyl group may have a substituent. 1 -R 5 The number of carbon atoms in the alkyl group (including the substituent) represented by the formula (I) 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.

[0043] The substituent is not particularly limited, and 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.

[0044] The anion species of the ionic liquid is preferably sulfonylimide. The sulfonylimide anion is N(SO2F)2 - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - ,N(SO2C4F9)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 anions contained in the electrolyte solution.

[0045] Examples of the ionic liquid include 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). An ionic liquid (electrolyte) containing an imidazolium cation and a sulfonylimide anion and having a lithium salt dissolved therein is preferred because it can ensure high ionic conductivity.

[0046] 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-based polymer containing -CHCF-. Vinylidene fluoride-based polymers are preferred because of their high mechanical strength. There are no particular restrictions on the vinylidene fluoride-based polymer as long as it contains -CHCF-. Examples of vinylidene fluoride-based polymers include homopolymers of vinylidene fluoride and copolymers of vinylidene fluoride and a copolymerizable monomer.

[0047] The copolymerizable monomer may be a halogen-containing monomer (excluding vinylidene fluoride) or a non-halogen copolymerizable monomer. Examples of the halogen-containing monomer include chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. Examples of the non-halogen copolymerizable monomer 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.

[0048] One or more copolymerizable monomers are polymerized with vinylidene fluoride to form a copolymer. In particular, a copolymer of vinylidene fluoride and hexafluoropropylene is preferred because it can widen the potential window.

[0049] The polymer may contain a polymer other than a vinylidene fluoride polymer. The content of the vinylidene fluoride polymer in the polymer is, for example, 80-100% by mass. Examples of the other polymer include fluorinated resins (excluding vinylidene fluoride polymers), polyolefins, rubber-like polymers such as styrene-butadiene rubber, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, and cellulose ethers. Examples of the fluorinated resin 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 the fluorinated resin copolymer include tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer.

[0050] 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, a lithium salt, an ionic liquid, a polymer, and a 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 is removed 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).

[0051] 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 IM Kolthoff, Anal. Chem. 46, 1992 (1974). According to Kolthoff's classification, solvents are broadly divided 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 divided into "protophilic" solvents, which are highly basic and easily 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 easily dispersed.

[0052] Examples of protophilic solvents among 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 solvents among 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.

[0053] The electrolyte composition contains one or more of these polar solvents. The water content of the solvent and the ionic liquid is preferably 200 ppm or less, respectively, to reduce the reaction between the water contained in the solvent and the ionic liquid and the solid electrolyte 18.

[0054] In an electrolyte composition in which a solid electrolyte 18 and an organic compound are mixed, the estimated 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 a basic solid electrolyte with a garnet structure containing Li, La, and Zr, producing LiOH and Li2O3 on the surface of the solid electrolyte. This causes the OH - increases, and the basicity increases.

[0055] 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 is converted to OH. - This reacts with the solid electrolyte to generate water, which then reacts with the solid electrolyte as described above, making it even more basic.

[0056] 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, the electrochemical reaction resulting from the elimination of HF results in the unintended formation of SEI, which increases the resistance of the SEI.

[0057] In contrast, since the solid electrolyte 18 with an average number value of the degree of enclosure of particles 21 of 0.8 or more has a weak basicity, the reaction between the moisture contained in the electrolyte composition and the solid electrolyte 18 can be reduced. Since the basicity in the system is difficult to become strong, the proton at the 2-position of the imidazolium cation is difficult to desorb, and the decrease in the ionic conductivity of the electrolytic solution due to the desorption of the proton is reduced. Further, since polyene formation due to the desorption of HF hardly occurs in the vinylidene fluoride-based polymer, the gelation of the electrolyte composition is reduced. Furthermore, since an electrochemical reaction derived from HF hardly occurs, the resistance of the SEI can be kept low. Particularly when the solvent contained in the electrolyte composition is an aprotic and hydrophobic polar solvent, since protons and hydrogen bonds hardly contribute to the reaction, gelation can be further reduced.

[0058] 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, where 0 < X ≤ 50. This is to reduce the occurrence of bleeding of the ionic liquid while ensuring ionic conductivity by the ionic liquid interposed between the solid electrolytes 18.

[0059] The content (volume %) of the ionic liquid is determined by freezing the electrolyte layer 14 or embedding and solidifying the electrolyte layer 14 in a tetrafunctional epoxy resin or the like, and then analyzing a randomly selected 5000-fold field of view from the cross section of the electrolyte layer 14 using an SEM equipped with an energy dispersive X-ray spectrometer (EDS). The analysis identifies the distribution of La, Zr, S, analyzes the contrast of the backscattered electron image by image analysis, identifies the area of the solid electrolyte 18 and the area of the ionic liquid, and regards the ratio of the area in the cross section of the electrolyte layer 14 as the ratio of the volume in the electrolyte layer 14 to obtain the content (volume %) of the ionic liquid.

[0060] The lithium ion conductivity of the electrolyte composition is determined by the types and salt concentrations of the solid electrolyte 18, the lithium salt, and the ionic liquid, etc. The lithium ion conductivity of the electrolyte composition at 25 °C is 4.0×10 -5The electrical conductivity is preferably S / cm or more in order to ensure the output density of the electricity storage device 10 containing the electrolyte composition.

[0061] 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 symmetric cell in which current collectors are attached to both sides of a sheet of the electrolyte composition, by the transport number of the lithium ion. The transport number of the lithium ion is determined by the AC impedance method and the steady-state DC method.

[0062] The transference number is calculated as follows: First, the resistance value R of the cell is measured by AC impedance measurement. S0 The conditions for measuring AC impedance are a temperature of 25°C, a voltage of 10mV, and a frequency of 7MHz-100mHz.

[0063] Next, measure the initial current I0 immediately after applying a constant voltage V to the cell, and calculate the initial resistance R0 of the cell using the following formula A: R0 = V / I0 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.

[0064] Resistance value R S0 and the initial resistance value R0 are substituted into the following equation B to obtain the interface resistance R INT Calculate R INT =R0-R S0 ···B Next, a constant voltage V is applied to the cell, and the current I is measured after the cell reaches a steady state. The resistance R of the cell in the steady state is calculated according to the following formula C: P Calculate 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.

[0065] After the cell has reached a steady state, the cell resistance R is measured by AC impedance measurement under the above conditions. S Resistance value R S , resistance value R Pand the interfacial resistance R INT Substitute into the following formula D to calculate the transference number t Li t Li =R S / (R P -R INT ) ··· D In the electrolyte layer 14 (electrolyte composition), the amount (volume %) of the binder with respect to the combined amount of the solid electrolyte 18 and the ionic liquid is preferably 10 volume % or less (excluding 0 volume %). That is, the combined amount of the solid electrolyte and the ionic liquid: the amount of the binder = (100 - Y): Y, where 0 < Y ≤ 10. This is to ensure the formability of the electrolyte layer 14 by the binder and reduce the decrease in the ionic conductivity of the electrolyte layer 14. The content (volume %) of the binder can be specified from the area % of the cross-section of the electrolyte layer 14 determined by analysis using SEM-EDS as described above.

[0066] The power storage device 10 is manufactured, for example, as follows. A solution in which a polymer is dissolved in a solvent is mixed with a mixture of an ionic liquid in which a lithium salt is dissolved and the solid electrolyte 18 to form a slurry. After tape casting and drying, a green sheet (electrolyte sheet) for the electrolyte layer 14 is obtained.

[0067] An active material 19 is mixed with a mixture of an ionic liquid in which a lithium salt is dissolved and the solid electrolyte 18, and further a solution in which a polymer is dissolved in a solvent is mixed to form a slurry. After tape casting on the current collector layer 12 and drying, a green sheet (a type of positive electrode sheet of the electrolyte sheet) for the positive electrode layer 11 is obtained.

[0068] An active material 20 is mixed with a mixture of an ionic liquid in which a lithium salt is dissolved and the solid electrolyte 18, and further a solution in which a polymer is dissolved in a solvent is mixed to form a slurry. After tape casting on the current collector layer 16 and drying, a green sheet (a type of negative electrode sheet of the electrolyte sheet) for the negative electrode layer 15 is obtained.

[0069] The electrolyte sheet, positive electrode sheet, and negative electrode sheet are each cut to a predetermined shape, and then stacked in this order, positive electrode sheet, electrolyte sheet, and negative electrode sheet, and then pressed together to form an integrated unit. 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 including, in this order, a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15. [Example]

[0070] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0071] (Preparation of solid electrolyte) Li 6.95 Mg 0.15 La 2.75 Sr 0.25 Zr 2.0 O 12 Li2CO3, MgO, La(OH)3, SrCO3, and ZrO2 were weighed out so that the composition satisfies the following formula. Li2CO3 was used in excess of approximately 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 along with zirconia balls, and then 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 then ground and mixed in a ball mill for 15 hours.

[0072] The slurry removed from the pot was dried, then poured into a mold with a diameter of 12 mm and pressed to obtain a green body with a thickness of approximately 1.5 mm. 2 A hydrostatic pressure of 1.0 × 10 was further applied to the compact. The compact was covered with calcined powder of the same composition as the compact and sintered 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 -3The lithium ion conductivity was measured under the conditions of a temperature of 25°C, a voltage of 10 mV, and a frequency of 7 MHz-100 mHz. The sintered body was pulverized in an Ar atmosphere and passed through a sieve with a mesh size of 250 μm, and the solid electrolyte powder that passed through the sieve was collected.

[0073] 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 pulverized 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 the solid electrolyte.

[0074] Similarly, 50 g of the sieved powder, 536 g of 4 mm diameter balls, and 100 mL of a fluorine-based inert liquid were placed in a pot, and the powder was pulverized 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 the solid electrolyte.

[0075] (Heat treatment of coarse and fine powders) Tube furnace (volume 1875 cm 3 Coarse powder of solid electrolyte (40 g or less) placed in a tube furnace was heated at 670 °C for 2 hours with nitrogen gas introduced into one end (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.

[0076] (Calculation 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 powder 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. 10 was calculated.

[0077] Next, the particle size distribution of the solid electrolyte was measured using ImageJ 1.52v. 10The number-average envelopment degree of particles having the above particle diameters was calculated. The number-average envelopment degrees 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.

[0078] (Preparation of Electrolyte) The ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) was dissolved in 3 mol / dm lithium salt LiN(SO2F)2. 3 The electrolyte was obtained by compounding.

[0079] 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.

[0080] 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.

[0081] (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.

[0082] (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).

[0083] (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, the slurries were visually inspected to see if they 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 -.

[0084] [Table 1]

[0085] 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.

[0086] The slurries of Examples 1 and 2 and Comparative Examples 1 and 2 contained aprotic and protonophobic polar solvents, DMC and PC. Comparing Examples 1 and 2 with Comparative Examples 1 and 2, the slurries of Examples 1 and 2 containing a solid electrolyte with an envelopment degree of 0.80 or more did not gel, while the slurries of Comparative Examples 1 and 2 containing a solid electrolyte with an envelopment degree of 0.75 gelled. The discoloration and gelation of the slurries are presumed to be due to polyenation of vinylidene fluoride caused by elimination of HF. It is presumed that gelation did not occur in slurries containing a solid electrolyte with an envelopment degree of 0.80 or more because the components of the slurry are less likely to interact with each other.

[0087] According to this embodiment, a solid electrolyte having a garnet structure containing Li, La, and Zr is used. 10 It has been revealed that solid electrolytes with a number-average envelopment index of 0.8 or more for particles with particle sizes above this range can reduce gelation. Furthermore, it has been revealed 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. Electrolyte sheets and power storage devices containing this electrolyte composition can reduce variation in the dispersion state of the constituent components, thereby reducing variation in performance.

[0088] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0089] In the embodiment, the electricity storage device 10 has been described as including the positive electrode layer 11 in which the composite layer 13 is provided on one side of the current collecting layer 12, and the negative electrode layer 15 in which the composite layer 17 is provided on one side of the current collecting layer 16, but the present invention 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 the composite layer 13 and the composite layer 17 are provided on both sides of the current collecting layer 12. If the bipolar electrodes and the electrolyte layers 14 are alternately stacked and housed in a case (not shown), a so-called bipolar structure secondary battery can be obtained.

[0090] In the embodiment, the case where the composite layers 13, 17 and the electrolyte layer 14 all contain the electrolyte composition has been described, but this is not necessarily limited to this. The secondary battery may be any battery in which at least one of the composite layers 13, 17 and the electrolyte layer 14 contains the electrolyte composition.

[0091] In the embodiment, the electricity storage device 10 including the electrode layers (positive electrode layer 11 and negative electrode layer 15) and the electrolyte layer 14 has been described using a lithium ion battery (secondary battery) containing an electrolyte composition as an example, but is not necessarily limited to this. Other examples of secondary batteries include a lithium-sulfur battery, a lithium-oxygen battery, and a lithium-air battery. [Explanation of symbols]

[0092] 10. Energy storage devices 13 Composite layer 14 Electrolyte layer (electrolyte sheet) 17 Composite layer 18 Solid electrolyte 21 particles 22 Contour 23 Envelope

Claims

1. 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 —CH ２ CF ２ and a polymer comprising 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.

2. An electrolyte sheet comprising the electrolyte composition of claim 1.

3. An electricity storage device comprising an electrolyte layer made of the electrolyte composition according to claim 1.

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