Electrolyte composition, electrolyte sheet, and secondary battery

Abstract

To provide an electrolyte composition, an electrolyte sheet, and a secondary battery that can reduce gelation.SOLUTION: An electrolyte composition includes an oxide containing Li, La, and Zr, an electrolyte salt, an ionic liquid, and a polymer, the oxide has a garnet-type structure, a part of Li is replaced with Mg, and a part of La is replaced with Sr. The ionic liquid contains pyrrolidinium cations and the polymer contains -CH2CF2-.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrolyte composition, an electrolyte sheet, and a secondary battery.

Background Art

[0002] An electrolyte composition containing an oxide Li 7 La 3 Zr 2 O 12 , an electrolyte salt, an ionic liquid, and a polymer is known (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, an electrolyte composition in which a basic oxide composed of Li 7 La 3 Zr 2 O 12 and a polymer are dispersed may gel (become non-fluid). When the electrolyte composition gels, variations occur in the dispersion state of the components of the electrolyte composition, or the sheet cannot be formed.

[0005] The present invention has been made to solve this problem, and an object thereof is to provide an electrolyte composition, an electrolyte sheet, and a secondary battery capable of reducing gelation.

Means for Solving the Problems

[0006] To achieve this object, the electrolyte composition of the present invention includes an oxide containing Li, La, and Zr, an electrolyte salt, an ionic liquid, and a polymer. The oxide has a garnet-type structure, a part of Li is substituted with Mg, and a part of La is substituted with Sr. The ionic liquid contains a pyrrolidinium cation, and the polymer contains -CH 2 CF 2 -.

[0007] The electrolyte sheet of the present invention is composed of the electrolyte composition. The secondary battery of the present invention includes a positive electrode layer, an electrolyte layer, and a negative electrode layer in this order, and the electrolyte composition is included in at least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer.

Advantages of the Invention

[0008] According to the electrolyte composition of the present invention, the interaction between the components is less likely to occur, so gelation can be reduced. According to the electrolyte sheet and the secondary battery including the electrolyte composition, the variation in the dispersion state of the components can be reduced.

Brief Description of the Drawings

[0009]

Figure 1

Embodiments for Carrying Out the Invention

[0010] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of a secondary battery 10 in one embodiment. The secondary battery 10 in this embodiment is a lithium-ion solid battery in which the power generation element is composed of a solid. That the power generation element is composed of a solid means that the skeleton of the power generation element is composed of a solid, and for example, it does not exclude a form in which a liquid is impregnated in the skeleton.

[0011] As shown in FIG. 1, the secondary battery 10 includes a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15 in this order. 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 laminating a current collector layer 12 and a composite layer 13. The current collector layer 12 is a conductive member. Examples of the material of the current collector layer 12 include metals selected from Ni, Ti, Fe, and Al, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0013] The composite layer 13 contains an active material 19 and an electrolyte composition (described later). The electrolyte composition contains an oxide 18 having ionic conductivity. In order to reduce the resistance of the composite layer 13, a conductive assistant may be included in the composite layer 13. Examples of the conductive assistant include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag.

[0014] Examples of the active material 19 include metal oxides having transition metals, sulfur-based active materials, and organic-based active materials. Examples of the metal oxide having a transition metal include metal oxides containing one or more elements selected from Mn, Co, Ni, Fe, Cr, and V and Li. The metal oxide having a transition metal is LiCoO 2 ,LiNi 0.8 Co 0.15 Al 0.05 O 2 ,LiMn 2 O 4 ,LiNiVO 4 ,LiNi 0.5 Mn 1.5 O 4 ,LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O 4 and LiFePO 4 are exemplified.

[0015] For the purpose of suppressing the reaction between the active material 19 and the oxide 18, a coating layer can be provided on the surface of the active material 19. The coating layer is Al 2 O 3 ,ZrO 2 ,LiNbO 3 ,Li 4 Ti 5 O 12 ,LiTaO 3 ,LiNbO 3 ,LiAlO 2 ,Li2 ZrO 3 , Li 2 WO 4 , Li 2 TiO 3 , Li 2 B 4 O 7 , Li 3 PO 4 and Li 2 MoO 4 are exemplified.

[0016] Sulfur-based active materials include S, TiS 2 , NiS, FeS 2 , Li 2 S, MoS 3 and sulfur-carbon composites are exemplified. Organic-based active materials include radical compounds typified by 2,2,6,6-tetramethylpiperidinoxyl-4-yl methacrylate and polytetramethylpiperidinoxyl vinyl ether, quinone compounds, radicalene compounds, tetracyanoquinodimethane, and phenazine oxide.

[0017] The electrolyte layer 14 is composed of an electrolyte composition. The electrolyte composition includes an oxide 18, an electrolytic solution, and a polymer. The oxide 18 is a solid electrolyte having lithium ion conductivity with a garnet-type structure containing Li, La, Zr, and O. The basic composition of the garnet-type oxide is Li 5 La 3 M 2 O 12 (M = Nb, Ta). The oxide 18 is Li 7 La 3 Zr 2 O 12 in which part of Li is substituted with Mg and part of La is substituted with Sr. The oxide 18 has a crystal structure of, for example, cubic system (space group Ia-3d (the overline indicates the reciprocal operation), JCPDS: 84-1753).

[0018] The oxide 18 preferably has 2% or more of the Li element substituted with Mg. Particularly preferred are those that satisfy all of the following (1) to (3) for the molar ratio of each element, or those that satisfy all of the following (4) to (6) for the molar ratio of each element. (1) 1.33 ≤ Li / (La + Sr) ≤ 3 (2) 0 ≤ Mg / (La + Sr) ≤ 0.5 (3) 0 ≤ Sr / (La + Sr) ≤ 0.67 (4) 2.0 ≤ Li / (La + Sr) ≤ 2.5 (5) 0.01 ≤ Mg / (La + Sr) ≤ 0.14 (6) 0.04 ≤ Sr / (La + Sr) ≤ 0.17.

[0019] In addition to Li, La, Zr, Mg, and Sr, the oxide 18 can contain at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Y, Nb, Sn, Sb, Ba, Hf, Ta, W, Bi, Rb, and lanthanoids (excluding La).

[0020] The median diameter of the equivalent circle diameter of the oxide 18 appearing in the cross-section of the electrolyte layer 14 is 0.5 - 10 μm, preferably 4 - 10 μm, more preferably 4 - 6 μm. This is to make the surface area of the oxide 18 an appropriate size and ensure the amount of lithium ion movement between the electrolyte present on the surface of the oxide 18 and the oxide 18.

[0021] To obtain the median diameter of the oxide 18, first, an image obtained by a scanning electron microscope (SEM) of the oxide 18 appearing in 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. The equivalent circle diameter is calculated from the area of each particle of the oxide 18, and a volume-based particle size distribution is obtained. The median diameter is the equivalent circle diameter at which the cumulative value of the frequency in the particle size distribution is 50%. To ensure accuracy, the image for obtaining the particle size distribution is taken from an area of 400 μm 2 or more of the electrolyte layer 14.

[0022] The electrolyte solution contained in the electrolyte layer 14 contains an ionic liquid in which an electrolyte salt is dissolved. The ionic liquid is a compound composed of a cation and an anion and is liquid at normal temperature and pressure. Since 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 types of the electrolyte salt and the ionic liquid and the salt concentration.

[0023] The electrolyte salt is a compound used for the transfer of cations between the positive electrode layer 11 and the negative electrode layer 15. The electrolyte salt is, for example, a lithium salt. The anion of the electrolyte salt is a halide ion (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 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, etc.) are exemplified.

[0024] The anion of the electrolyte salt is N(SO 2 - having a sulfonyl group -S(=O) 2 F) 2 - ,N(SO 2 CF 3 ) 2 - ,N(SO 2 C 2 F 5 ) 2 - etc. Sulfonylimide anions have little effect on the increase in the viscosity of the electrolyte and the decrease in ionic conductivity even when the salt concentration is high, and further, due to the formation of a highly stable and low-resistance film (SEI), the reductive decomposition of the electrolyte can be reduced and the reductive side potential window can be expanded. N(SO 2 F) 2 - is abbreviated as [FSI] - : called bis(fluorosulfonyl)imide anion, and N(SO 2 CF 3 ) 2 - is abbreviated as [TFSI] - : may be called bis(trifluoromethanesulfonyl)imide anion.

[0025] The cation component of the ionic liquid has a pyrrolidinium cation. The pyrrolidinium cation is a five-membered ring compound represented by, for example, formula (1).

[0026]

Chemical formula

[0027] In formula (1), R 1 and R 2 each independently represent an alkyl group having 1 to 10 carbon atoms. The alkyl group may have a substituent. R 1 and R 2 The carbon number of the alkyl group (including substituents) represented by is preferably 1 to 5, more preferably 1 to 4. This is to ensure the ionic conductivity of the electrolyte.

[0028] The substituent is not particularly limited. 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, a sulfonamide group, and the like.

[0029] The anion component of the ionic liquid is not particularly limited. The anion component is BF 4 - , N(SO 2 F) 2 - and other inorganic anions, B(C 6 H 5 ) 4 - , CH 3 SO 3 - , CF 3 SO 3 - , N(SO 2 CF 3 ) 2 - , N(SO 2 C 4 F 9 ) 2 -Examples of the organic anions include the like. Examples of the ionic liquids include N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (P13-FSI) and N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13-TFSI). When the electrolyte salt contains a sulfonylimide anion, it is preferable that the ionic liquid also contains a sulfonylimide anion because the coordination (interaction) between the lithium ions and the anions contained in the electrolyte solution is easily controlled.

[0030] Examples of the polymer contained in the electrolyte layer 14 include a binder that binds the oxide 18. The polymer contains a vinylidene fluoride-based polymer containing -CH 2 CF 2 -. This is because the vinylidene fluoride-based polymer has high mechanical strength. The vinylidene fluoride-based polymer is not particularly limited as long as it contains -CH 2 CF 2 -. Examples of the vinylidene fluoride-based polymer include a homopolymer of vinylidene fluoride and a copolymer of vinylidene fluoride and a copolymerizable monomer.

[0031] Examples of the copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen-based copolymerizable monomers. Examples of the halogen-containing monomers include chlorine-containing monomers such as vinyl chloride; fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. Examples of the non-halogen-based 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 polymerize with vinylidene fluoride to form a copolymer.

[0032] The electrolyte composition may contain polymers other than vinylidene fluoride-based polymers. The content of the vinylidene fluoride-based polymer in the polymer is, for example, 80-100% by weight. 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 the fluorinated resin include fully fluorinated resins, partially fluorinated resins, and fluorinated resin copolymers. An example of the fully fluorinated resin is polytetrafluoroethylene. Examples of the partially fluorinated resins are polychlorotrifluoroethylene and polyvinyl fluoride. Examples of the fluorinated resin copolymers are ethylene tetrafluoroethylene perfluoroalkyl vinyl ether copolymer, ethylene tetrafluoroethylene hexafluoropropylene copolymer, ethylene tetrafluoroethylene copolymer, and ethylene chlorotrifluoroethylene copolymer.

[0033] The electrolyte layer 14 may contain a solvent that dissolves the polymer. The electrolyte layer 14 is obtained by forming a sheet-like electrolyte composition containing the oxide 18, an electrolyte salt, an ionic liquid, a polymer, and a solvent. At least a part of the solvent contained in the electrolyte composition vaporizes by, for example, drying under reduced pressure after sheet forming to obtain the electrolyte layer 14, and disappears from the electrolyte layer 14. The type and amount of the solvent remaining in the electrolyte layer 14 are determined by gas chromatography-mass spectrometry (GC-MS).

[0034] The solvent contained in the electrolyte composition is an aprotic polar solvent. The classification of the solvent follows I.M. Kolthoff, Anal. Chem. 46, 1992 (1974). In Kolthoff's classification, solvents are roughly classified into "amphoteric" which has both acidity and basicity and can donate and accept protons, and "aprotic" which does not have a hydrogen atom capable of forming a hydrogen bond. The latter is further subdivided into "proticophilic" which has strong basicity and easily solvates cations, and "aprotic" which has weak basicity and hardly solvates cations. An aprotic solvent is a solvent in which an anion is not considered to stably exist after losing a proton. The oxide 18 is easily dispersed in a polar solvent.

[0035] Among aprotic polar solvents, protic solvents include, by way of example, N,N-dimethylformamide, N,N-dimethylacetamide, hexamethylphosphoric triamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, pyridine, dioxane, tetrahydrofuran, and ether. Among aprotic polar solvents, aprotic solvents include, by way of example, 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.

[0036] The water content in the solvent and ionic liquid is preferably 200 ppm or less, more preferably 100 ppm or less, and particularly preferably 10 ppm or less, respectively. This is to reduce the reaction between the water content in the solvent and ionic liquid and oxide 18.

[0037] The presumed mechanism by which the electrolyte composition in which the organic compound is dispersed gels (becomes non-fluid) is as follows. First, a small amount of water contained in the electrolyte composition reacts with a basic oxide containing Li, La, and Zr, and LiOH or Li 2 O 3 is generated on the surface of the oxide. As a result, the OH - in the system increases and the basicity strengthens.

[0038] Although it depends on the cation species of the ionic liquid, there are some in which protons are easily desorbed from the cation by a base. When a proton is desorbed from the cation, the ionic conductivity of the electrolytic solution decreases, and the desorbed proton reacts with OH - to generate water. The generated water reacts with the oxide as described above to further increase the basicity.

[0039] Vinylidene fluoride-based polymers tend to form polyene structures by the elimination of HF under basic conditions. The electrolyte composition gels due to the polyenation of vinylidene fluoride-based polymers. Furthermore, an unintended SEI is formed by an electrochemical reaction derived from the eliminated HF, and the resistance of the SEI increases.

[0040] In contrast, the electrolyte composition includes an ionic liquid containing a pyrrolidinium cation and an oxide 18 containing Li, La, and Zr, with a part of Li substituted by Mg and a part of La substituted by Sr. Although the role of the oxide 18 is unclear, it is presumed that the protons of the pyrrolidinium cation are difficult to eliminate, and the polyenation of vinylidene fluoride-based polymers is less likely to occur due to the interaction between the oxide 18 and the pyrrolidinium cation, reducing the gelation of the electrolyte composition. Furthermore, since an electrochemical reaction derived from HF is less likely to occur, the resistance of the SEI can be kept low.

[0041] In the electrolyte layer 14 (electrolyte composition), the content (volume %) of the ionic liquid with respect to the total amount of the oxide 18 and the ionic liquid is preferably 50 volume % or less (excluding 0 volume %). That is, oxide:ionic liquid = (100 - X):X, where 0 < X ≤ 50. This is to ensure ionic conductivity while reducing the occurrence of bleeding of the ionic liquid by the ionic liquid intervening between the oxide 18s.

[0042] 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 areas of the oxide 18 and the ionic liquid by specifying the distribution of La, Zr, S or analyzing the contrast of the backscattered electron image by image analysis, and regards the ratio of the areas in the cross-section of the electrolyte layer 14 as the ratio of the volumes in the electrolyte layer 14 to obtain the content (volume %) of the ionic liquid.

[0043] The lithium ion conductivity of the electrolyte composition is determined by the types of oxide 18, electrolyte salt and ionic liquid, salt concentration, etc. 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 secondary battery 10 containing the electrolyte composition.

[0044] Since the electrolyte composition contains anions derived from the electrolytic solution, the lithium ion conductivity of the electrolyte composition is calculated by multiplying the total ion conductivity calculated by the alternating current impedance method of a symmetric cell in which current collectors are adhered to both sides of the electrolyte composition formed in a sheet shape by the transport number of lithium ions. The transport number of lithium ions is determined by the alternating current impedance method and the steady state direct current method.

[0045] The transport number is calculated as follows. First, the resistance value R S0 of the cell is analyzed by alternating current impedance measurement. The conditions for alternating current impedance measurement are a temperature of 25 °C, a voltage of 10 mV, and a frequency of 7 MHz - 100 mHz.

[0046] Next, the initial current value I 0 immediately after applying a constant voltage V to the cell is measured, 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 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.

[0047] The resistance value R S0 and the initial resistance value R 0 are substituted into the following formula B to calculate the interfacial resistance R INT . R INT =R 0 -R S0 ···B Next, the current value I after applying a constant voltage V to the cell and reaching a steady state is measured, and the resistance value R P in the steady state of the cell is calculated according to the following formula C. R P =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.

[0048] After the cell reaches the steady state, the resistance value R of the cell is analyzed by AC impedance measurement under the above-mentioned conditions. S The resistance value R S The resistance value R P And the interfacial resistance R INT Are substituted 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 polymer with respect to the combined amount of the oxide 18 and the ionic liquid is preferably 10 volume % or less (excluding 0 volume %). That is, the combined amount of the oxide and the ionic liquid: the amount of the polymer = (100 - Y): Y, where 0 < Y ≤ 10. This is to ensure the moldability of the electrolyte layer 14 by the polymer and to reduce the decrease in the ionic conductivity of the electrolyte layer 14. The content (volume %) of the polymer can be specified from the area % of the cross-section of the electrolyte layer 14 determined by analysis using SEM-EDS as described above.

[0049] The negative electrode layer 15 is formed by laminating a current collector layer 16 and a composite layer 17. The current collector layer 16 is a conductive member. Examples of the material of the current 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.

[0050] The composite layer 17 contains an active material 20 and an electrolyte composition. The electrolyte composition contains an oxide 18. In order to lower the resistance of the composite layer 17, a conductive aid may be included in the composite layer 17. Examples of the conductive aid include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. The active material 20 is Li, Li-Al alloy, Li 4 Ti 5 O 12, graphite, In, Si, Si-Li alloy, and SiO are exemplified. Similar to the electrolyte layer 14, the composite layers 13 and 17 may contain a polymer.

[0051] The secondary battery 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 an electrolyte salt is dissolved and the oxide 18 to form a slurry. After tape casting, it is dried to obtain a green sheet (electrolyte sheet) for the electrolyte layer 14.

[0052] The active material 19 is mixed with a mixture of an ionic liquid in which an electrolyte salt is dissolved and the oxide 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, it is dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 11.

[0053] The active material 20 is mixed with a mixture of an ionic liquid in which an electrolyte salt is dissolved and the oxide 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, it is dried to obtain a green sheet (negative electrode sheet) for the negative electrode layer 15.

[0054] After cutting the electrolyte sheet, the positive electrode sheet, and the negative electrode sheet into predetermined shapes, they are stacked in the order of the positive electrode sheet, the electrolyte sheet, and the negative electrode sheet, and are pressure-bonded to each other to be integrated. Terminals (not shown) are connected to the current collector layers 12 and 16, and are enclosed in a case (not shown), whereby a secondary battery 10 including a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15 is obtained in order.

Example

[0055] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0056] (Preparation of Oxide) Li 6.95 Mg 0.15 La 2.75 Sr 0.25 Zr 2.0 O 12 So that it becomes Li2 CO 3 , MgO, La(OH) 3 , SrCO 3 , ZrO 2 were weighed. Li 2 CO 3 was made about 15 mol% in excess in terms of elements in consideration of the volatilization of Li during firing. The weighed raw materials and ethanol were put into a nylon pot together with zirconia balls and pulverized and mixed with a ball mill for 15 hours. After drying the slurry taken out from the pot, it was calcined temporarily (at 1100 °C for 10 hours) on a plate made of MgO. The powder after temporary calcination and ethanol were put into a nylon pot and pulverized and mixed with a ball mill for 15 hours.

[0057] After drying the slurry taken out from the pot, it was put into a mold with a diameter of 12 mm, and a molded body with a thickness of about 1.5 mm was obtained by press molding. A hydrostatic pressure of 1.5 t / cm 2 was further applied to the molded body using a cold isostatic pressing machine (CIP). The molded body was covered with a temporarily calcined powder having the same composition as the molded body and fired (at 1100 °C for 4 hours) in a reducing atmosphere to obtain a sintered body of the oxide. The lithium ion conductivity of the sintered body determined by the alternating current impedance method was 1.0×10 -3 S / cm. The measurement conditions for the lithium ion conductivity were 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 to obtain the oxide powder in the example (hereinafter referred to as "LLZ-Sr").

[0058] Li 6.6 La 3 Ta 0.4 Zr 1.6 O 12 So as to become 2 CO 3 , La(OH) 3 , Ta 2 O 5 , ZrO 2 were weighed, and then, in the same manner as in the case of preparing LLZ-Sr, they were mixed, calcined temporarily, fired, and pulverized to obtain the oxide powder in the comparative example (hereinafter referred to as "LLZ-Ta").

[0059] (Preparation of Electrolyte Solution) To the ionic liquid N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (P13-FSI, reagent grade), the electrolyte salt LiN(SO 2 F) 2 was compounded at 1 mol / dm 3 or 3 mol / dm 3 to obtain various electrolyte solutions.

[0060] To the ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI, reagent grade), the electrolyte salt LiN(SO 2 F) 2 was compounded at 1 mol / dm 3 or 3 mol / dm 3 to obtain various electrolyte solutions.

[0061] (Example 1) The electrolyte solution with a salt concentration of 3 mol / dm 3 containing P13-FSI and LLZ-Sr were mixed in a mortar in an Ar atmosphere so that the volume ratio of oxide:electrolyte solution was 61:39 to obtain a composite powder. 18 g of the composite powder, 0.864 g of vinylidene fluoride (PVDF), and 7.776 g of dimethyl carbonate (DMC) were mixed in an Ar atmosphere to obtain the slurry in Example 1.

[0062] (Example 2) The electrolyte solution with a salt concentration of 1 mol / dm 3 containing P13-FSI and LLZ-Sr were mixed in a mortar in an Ar atmosphere so that the volume ratio of oxide:electrolyte solution was 61:39 to obtain a composite powder. 18 g of the composite powder, 0.864 g of vinylidene fluoride, and 7.776 g of propylene carbonate (PC) were mixed in an Ar atmosphere to obtain the slurry in Example 2.

[0063] (Example 3) The slurry in Example 3 was obtained in the same manner as in Example 1 except that dimethyl carbonate was replaced with propylene carbonate.

[0064] (Example 4) A slurry in Example 4 was obtained in the same manner as in Example 2, except that propylene carbonate was replaced with N-methyl-2-pyrrolidone (NMP).

[0065] (Example 5) A slurry in Example 4 was obtained in the same manner as in Example 1, except that dimethyl carbonate was replaced with N-methyl-2-pyrrolidone.

[0066] (Comparative Example 1) An electrolyte solution containing EMI-FSI with a salt concentration of 1 mol / dm 3 and LLZ-Sr were mixed in a mortar in an Ar atmosphere to obtain a composite powder. 18 g of the composite powder, 0.864 g of vinylidene fluoride, and 7.776 g of N-methyl-2-pyrrolidone were mixed in an Ar atmosphere to obtain a slurry in Comparative Example 1.

[0067] (Comparative Example 2) A slurry in Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that the electrolyte solution was replaced with an electrolyte solution containing EMI-FSI with a salt concentration of 3 mol / dm 3 .

[0068] (Comparative Example 3) A slurry in Comparative Example 3 was obtained in the same manner as in Comparative Example 1, except that N-methyl-2-pyrrolidone was replaced with N,N-dimethylformamide (DMF).

[0069] (Comparative Example 4) A slurry in Comparative Example 4 was obtained in the same manner as in Example 5, except that LLZ-Sr was replaced with LLZ-Ta.

[0070] (Test Method and Results) The slurries in the examples and comparative examples were each placed in a beaker and left in a container with an Ar atmosphere at 25°C. One hour, eight hours, and 24 hours after the slurries were prepared, the states of the slurries were visually confirmed, and the presence or absence of gelation (non-fluidization) and coloring was determined. The results are shown in Table 1. Those in which at least a part of the slurry was gelled were denoted as +, and those in which the slurry was not gelled at all were denoted as -.

[0071]

Table 1

[0072] As shown in Table 1, the slurries in Examples 1-3 did not gel or color even after 24 hours. The slurries in Examples 4 and 5 did not gel or color after 8 hours, but gelled and colored (changed to brown) after 24 hours. The slurries in Comparative Examples 1-4 gelled and colored after 1 hour.

[0073] The slurries of Examples 4 and 5 and Comparative Examples 1 and 2 contained NMP, an aprotic and protic polar solvent. Comparing Examples 4 and 5 containing NMP in the slurry with Comparative Examples 1 and 2, the slurries of Examples 4 and 5 containing pyrrolidinium cations did not gel or color until 8 hours later, while the slurries of Comparative Examples 1 and 2 containing imidazolium cations gelled and colored after 1 hour. The coloring and gelation of the slurry are presumed to be due to the polyene formation of vinylidene fluoride by the elimination of HF. Examples 4 and 5 containing pyrrolidinium cations and LLZ-Sr are presumed to have less interaction between the components of the slurry and did not gel or color compared to Comparative Examples 1 and 2.

[0074] The slurry of Comparative Example 3 contained imidazolium cations and DMF, an aprotic and protic polar solvent, similar to the slurries of Comparative Examples 1 and 2. It is presumed that polyene formation of vinylidene fluoride by the elimination of HF also occurred in the slurry of Comparative Example 3, resulting in gelation and coloring.

[0075] The slurries of Example 5 and Comparative Example 4 contained pyrrolidinium cations and NMP. Comparing Example 5 with Comparative Example 4, the slurry of Example 5 containing LLZ-Sr did not gel or color until 8 hours later, while the slurry of Comparative Example 4 containing LLZ-Ta gelled and colored after 1 hour. From the results of Examples 2, 3, 5 and Comparative Example 4, it is speculated that LLZ-Sr has the effect of suppressing the elimination of HF and suppressing the gelation and coloring of the slurry caused by the polyene formation of vinylidene fluoride.

[0076] The slurries of Examples 1-3 and Examples 4, 5 contained pyrrolidinium cations. Comparing Examples 1-3 with Examples 4, 5, the slurries of Examples 1-3 containing DMC or PC, which are aprotic and sparsely protic polar solvents, did not gel or color even after 24 hours, while the slurries of Examples 4, 5 containing NMP, which is an aprotic and protic polar solvent, gelled and colored after 24 hours. Since aprotic and sparsely protic polar solvents are less basic than protic polar solvents, it is speculated that the interaction of the components of the slurry is less likely to occur, which is advantageously effective in reducing gelation and discoloration.

[0077] According to this example, it was revealed that an electrolyte composition containing a garnet-type structure oxide containing Li, La and Zr, in which part of Li is substituted with Mg and part of La is substituted with Sr, an ionic liquid containing pyrrolidinium cations, an electrolyte salt, and a vinylidene fluoride-based polymer can reduce gelation. When the electrolyte composition contains a solvent, it was revealed that the solvent is preferably an aprotic polar solvent, and particularly preferably an aprotic and sparsely protic polar solvent. An electrolyte sheet and a secondary battery containing this electrolyte composition can reduce the variation in the dispersion state of the components, and thus can reduce the variation in performance.

[0078] As described above, the present invention has been described based on the embodiments, but it can be easily inferred that the present invention is not limited to the above embodiments at all, and various improvements and modifications are possible without departing from the spirit of the present invention.

[0079] In the embodiment, the secondary battery 10 has been described as including a positive electrode layer 11 provided with a composite layer 13 on one side of a current collector layer 12 and a negative electrode layer 15 provided with a composite layer 17 on one side of a current collector layer 16, but it is not necessarily limited thereto. For example, it is naturally possible to apply each element in the embodiment to a secondary battery including an electrode layer (so-called bipolar electrode) in which the composite layer 13 and the composite layer 17 are respectively provided on both sides of the current collector layer 12. If the bipolar electrode and the electrolyte layer 14 are alternately laminated and housed in a case (not shown), a secondary battery having a so-called bipolar structure can be obtained.

[0080] In the embodiment, the case where the composite layers 13 and 17 and the electrolyte layer 14 all contain an electrolyte composition has been described, but it is not necessarily limited thereto. The secondary battery only needs to have at least one of the composite layers 13 and 17 and the electrolyte layer 14 containing an electrolyte composition.

[0081] In the embodiment, a secondary battery 10 including an electrode layer (positive electrode layer 11 and negative electrode layer 15) and an electrolyte layer 14 has been described by exemplifying a lithium-ion battery (secondary battery) containing an electrolyte composition, but it is not necessarily limited thereto. Examples of other secondary batteries include lithium-sulfur batteries, lithium-oxygen batteries, and lithium-air batteries.

Explanation of Reference Numerals

[0082] 10 Secondary battery 11 Positive electrode layer 14 Electrolyte layer (electrolyte sheet) 15 Negative electrode layer 18 Oxide

Claims

1. An electrolyte composition comprising an oxide containing Li, La and Zr, an electrolyte salt, an ionic liquid, and a polymer, wherein the oxide has a garnet structure, a part of the Li is substituted with Mg, and a part of the La is substituted with Sr, the ionic liquid contains a pyrrolidinium cation, The polymer contains -CH ２ CF ２ -. and the content of the ionic liquid with respect to the total amount of the oxide and the ionic liquid is 50% by volume or less.

2. The electrolyte composition according to claim 1, wherein the pyrrolidinium cation is N-methyl-N-propylpyrrolidinium.

3. An electrolyte sheet comprising the electrolyte composition according to claim 1 or 2.

4. A secondary battery comprising a positive electrode layer, an electrolyte layer, and a negative electrode layer in this order, wherein at least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer contains the electrolyte composition according to claim 1 or 2.

Images