JP7666984B2 - Electrolyte composition, electrolyte sheet and secondary battery - Google Patents

Description

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

An electrolyte composition is known that contains an oxide with a garnet structure containing Li, La, and Zr, an electrolyte salt, an ionic liquid, a binder, and a solvent (Patent Document 1).

Patent No. 6682708

In the prior art, oxides with a garnet structure containing Li, La, and Zr are basic, so the electrolyte composition in which the oxide and organic compound are dispersed may gel (become non-fluid). When the electrolyte composition gels, the dispersion state of the components of the electrolyte composition becomes uneven and it becomes impossible to mold the sheet.

The present invention has been made to solve this problem, and aims to provide an electrolyte composition, an electrolyte sheet, and a secondary battery that can reduce gelation.

To achieve this object, the electrolyte composition of the present invention includes an oxide having a garnet structure containing Li, La, and Zr, an electrolyte salt, an ionic liquid, a binder, and a solvent. The oxide has a particle size of 1.4 μm or more at which the cumulative frequency in a volume-based particle size distribution is 10%, the ionic liquid includes an imidazolium cation and a sulfonylimide anion, the solvent is an aprotic and protophobic polar solvent, and the binder includes -CH ₂ CF ₂ -.

The electrolyte sheet of the present invention is made of an electrolyte composition. The secondary battery of the present invention includes, in order, a positive electrode layer, an electrolyte layer, and a negative electrode layer, and at least one of the positive electrode layer, the electrolyte layer, and the negative electrode layer includes the electrolyte composition.

The electrolyte composition of the present invention reduces the basicity of the oxide and makes it difficult for the components to interact with each other, thereby reducing gelation. An electrolyte sheet and a secondary battery containing the electrolyte composition can reduce variation in the dispersion state of the components.

1 is a cross-sectional view of a secondary battery according to an embodiment. 1 is an example of an integrated distribution of oxides.

Below, 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. In this embodiment, the secondary battery 10 is a lithium ion solid-state battery in which the power generating element is made of a solid. The power generating element being 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.

As shown in FIG. 1, the secondary battery 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 contains an active material 19 and an electrolyte composition (described later). The electrolyte composition contains an oxide 18 having ion conductivity. 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.

The active material 19 may be, for example, a metal oxide having a transition metal, a sulfur-based active material, or an organic active material. The metal oxide having a transition metal may be _, for _example , a metal oxide containing Li and one or more elements selected from Mn, Co, Ni, Fe, Cr, and V. The metal oxide _{having a transition} metal may be, _for example, _LiCoO2 , _{LiNi0.8Co0.15Al0.05O2} , _LiMn2O4 , _LiNiVO4 , _{LiNi0.5Mn1.5O4} , LiNi1 _{/ 3Mn1} / _3Co1 / _3O4 , or _LiFePO4 .

A coating layer may be provided on the surface of the active material 19 in order to suppress a reaction between _the active material _{19 and the} oxide 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, tetracyaquinodimethane, and phenazine oxide.

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

In addition to Li, La, and Zr, oxide 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 oxide 18 has, for example, a cubic crystal structure (space group Ia-3d (- indicates an overline meaning a reversal operation), JCPDS: 84-1753). The oxide 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), and the molar ratio of each element satisfies all of the following (1) to (3), or contains both Mg and element A, and the molar ratio of each element satisfies all of the following (4) to (6). The element A is preferably Sr in order to increase the ionic conductivity of the oxide 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.

The oxide 18 is a powder having a particle diameter (hereinafter referred to as "D ₁₀ ") at which the cumulative frequency in the volumetric particle size distribution is 10% is 1.4 μm or more. The volumetric particle size distribution of the oxide 18 is measured by a laser diffraction/scattering particle size distribution measuring device (e.g., Microtrac MT3300EX II). The sample used for measuring the particle size distribution is, for example, the oxide 18 as it is taken out from the electrolyte layer 14. An appropriate amount of the sample is placed in a solvent (0.2 wt % sodium hexametaphosphate aqueous solution), dispersed for 3 minutes using an ultrasonic disperser (e.g., SD-600 manufactured by Nippon Seiki Seisakusho), and set in the measuring device. When calculating the particle size distribution, the sample refractive index is set to 1.81, and the solvent refractive index is set to 1.33.

FIG. 2 is an example of an integrated distribution (under sieve) illustrating the integrated value of frequency in the volumetric particle size distribution of oxide 18. FIG. 2 illustrates a particle diameter _D10 where the ratio of particles below this value is 10%. A solid electrolyte (oxide) with a garnet structure containing Li, La, and Zr is strongly basic. In particular, particles of oxide 18 with a small particle diameter and a large specific surface area are poor in chemical stability and have strong basicity. When the ratio of particles with a small particle diameter decreases, the chemical stability of oxide 18 increases and the basicity weakens. When oxide 18 has _D10 of 1.4 μm or more, this tendency becomes remarkable.

The thickness of the electrolyte layer 14 containing the oxide 18 is set to, for example, three times or more the particle diameter at which the cumulative value of the frequency in the particle size distribution is 100% (hereinafter referred to as "D ₁₀₀ "). This is to prevent short circuits between the electrodes. Therefore, D ₁₀ of the oxide 18 is preferably less than 5 μm. If D ₁₀ of the oxide 18 is 5 μm or more, D ₁₀₀ becomes large accordingly, and it is necessary to make the electrolyte layer 14 sufficiently thick. As the electrolyte layer 14 becomes thicker, the amount of the active material 19 per unit volume of the secondary battery 10 decreases, and the discharge capacity of the secondary battery 10 decreases. In order to ensure the discharge capacity of the secondary battery 10, D ₁₀ of the oxide 18 is preferably less than 5 μm.

An example of a method for producing the oxide 18 having a _D10 of 1.4 μm or more will be described. For example, the method for producing the oxide 18 includes a blending step of blending raw materials to obtain a blended material, a firing step of firing the blended material, and a heat treatment step of heating the obtained synthetic powder.

In the blending step, materials containing the elements that make up the oxide 18 are blended to obtain a blended material. Examples of the materials include oxides, composite oxides, hydroxides, carbonates, chlorides, sulfates, nitrates, and phosphates that contain 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 material is calcined. In this step, the blended material is calcined, for example, at 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.

It is preferable to include a step of grinding and mixing the calcined material between the calcination step and the firing step. In this step, the calcined material is ground and mixed to obtain a mixed material. By passing through the step of grinding and mixing the calcined material, it becomes easier to obtain a uniform crystal phase after the firing step. The calcined material to which a binder has been added may be ground 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 crushed in an inert gas atmosphere to obtain a synthetic powder.

In the heat treatment process, the synthetic powder is placed in a furnace where gas flows in and out, and the synthetic powder is heated in an atmosphere where the gas flows 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 _CO2 is desorbed by heat treatment, and the structure of the synthetic powder is modified to obtain oxide 18. The temperature and time of the heat treatment _can be, for example, held at a temperature range of 640°C or higher for 10 hours or more, or held at 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, so long as it does not chemically react with the synthetic powder. Examples of inert gases include nitrogen, helium, neon, argon, krypton, xenon, and radon. In particular, at least one selected from nitrogen, helium, and argon is preferred.

The inflow and outflow of the gas may be continuous or intermittent. The flow rate of the gas introduced into the furnace continuously or intermittently and the period of the gas introduced intermittently are appropriately set according to the volume of the furnace and the mass of the synthetic powder. The _CO2 concentration of the gas introduced into the furnace depends on the temperature of the heat treatment, but is preferably a concentration 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 synthetic powder. The oxide 18 after the heat treatment is immediately mixed with an organic compound or the like and formed into a sheet, and the composite layers 13, 17 and the electrolyte layer 14 are obtained.

The median diameter of the circle-equivalent diameter of the oxide 18 appearing in the cross section of the electrolyte layer 14 is preferably 4-10 μm, and more preferably 4-6 μm. This is to ensure that the surface area of the oxide 18 is of an appropriate size and to ensure the amount of lithium ions that can move between the electrolyte present on the surface of the oxide 18 and the oxide 18.

To obtain the median diameter of the oxide 18, first, a scanning electron microscope (SEM) image of the oxide 18 appearing on the 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 a circular equivalent diameter is calculated from the area of each particle of the oxide 18 to obtain a volume-based particle size distribution. The median diameter is the circular equivalent diameter at which the integrated value of the frequency in the particle size distribution is 50%. To ensure accuracy, the image for obtaining 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 an electrolyte salt is dissolved. An ionic liquid is a compound consisting of a cation and an anion, and is liquid at room 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 type and salt concentration of the electrolyte salt and ionic liquid.

The electrolyte salt is a compound used for transferring cations between the positive electrode layer 11 and the negative electrode layer 15. The electrolyte salt is, for example, a lithium salt. The anions of the electrolyte salt are 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 electrolyte salt is preferably a sulfonylimide having a sulfonyl group -S(=O) ₂ -, such as N(SO ₂ F) ₂ ^- , N(SO ₂ CF ₃ ) ₂ ^- , or N(SO ₂ C ₂ F ₅ ) ₂ ^- . This is because the sulfonylimide anion is less affected by the increase in viscosity of the electrolyte and the decrease in ionic conductivity even when the salt concentration is high, and further, by forming a highly stable and low-resistance coating (SEI), it is possible to reduce the reductive decomposition of the electrolyte and expand the reduction-side potential window. N(SO ₂ F) ₂ ^- may be abbreviated as [FSI] ^- : bis(fluorosulfonyl)imide anion, and N(SO ₂ CF ₃ ) ₂ ^- may be abbreviated as [TFSI] ^- : bis(trifluoromethanesulfonyl)imide anion.

The ionic liquid is preferably one having imidazolium as the cationic species. 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 1-10, more preferably 1-5, and even more preferably 1-4. This is to ensure the ionic conductivity of the electrolyte.

There are no particular limitations 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 sulfonylimide. Examples of the sulfonylimide anion 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 ^- . 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 an electrolyte salt dissolved therein is preferable because it can ensure high ionic conductivity.

The binder contained in the electrolyte layer 14 binds the oxide 18. The binder contains a vinylidene fluoride-based polymer containing -CH ₂ CF ₂ -. This is because vinylidene fluoride-based polymers have high mechanical strength. There are no particular limitations on the vinylidene fluoride-based polymer 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.

Examples of the copolymerizable monomer include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen copolymerizable monomers. 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. One or more of the copolymerizable monomers are polymerized with vinylidene fluoride to form a copolymer.

The binder may contain other polymers than the vinylidene fluoride polymer. The content of the vinylidene fluoride polymer in the binder is, for example, 80-100% by mass. Examples of other polymers 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 resins 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 include tetrafluoroethylene-perfluoroalkylvinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

The electrolyte composition contains a solvent that dissolves the binder. The electrolyte salt also dissolves in the solvent. The solvent is aprotic and protophobic polar solvent. The classification of solvents (aprotic and protophobic) follows I. M. Kolthoff, Anal. Chem. 46, 1992 (1974). In Kolthoff's classification, solvents are broadly divided into "amphoteric" that have both acidity and basicity and can donate and accept protons, and "aprotic" that do not have hydrogen atoms that can form hydrogen bonds, and the latter are further divided into "protophilic" that are strongly basic and easily solvate with cations, and "protophobic" that 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 oxide 18 is easily dispersed. Examples of aprotic and protophobic 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 moisture content of the solvent and ionic liquid is preferably 200 ppm or less, more preferably 100 ppm or less, and particularly preferably 10 ppm or less. This is to reduce the reaction between the moisture in the solvent and ionic liquid and the oxide 18.

The electrolyte layer 14 is obtained by forming an electrolyte composition containing an oxide 18, an electrolyte salt, an ionic liquid, a binder, 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 the sheet is formed 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 is determined by gas chromatography-mass spectrometry (GC-MS).

The presumed mechanism by which the electrolyte composition in which an organic compound is dispersed gels (becomes non-fluid) is as follows: First, a small amount of moisture contained in the electrolyte composition reacts with a basic oxide having a garnet structure containing Li, La, and Zr, producing LiOH and _Li2O3 on the surface of the _oxide . This increases ^OH- in the system, strengthening the basicity.

When hydrogen is bonded to the carbon at the 2-position of the imidazolium cation in an ionic liquid, the ionic conductivity of the electrolyte decreases when the proton at the 2-position is eliminated by a base. The proton eliminated from the imidazolium cation reacts with ^OH- to generate water. As mentioned above, the generated water reacts with oxides to further increase its basicity.

Under basic conditions, vinylidene fluoride-based polymers used as binders are prone to forming polyene structures due to the desorption of HF. The polyene conversion of vinylidene fluoride-based polymers causes the electrolyte composition to gel. Furthermore, an unintended SEI is formed due to an electrochemical reaction caused by the desorbed HF, and the resistance of the SEI increases.

In contrast, the oxide 18 having a D ₁₀ of 1.4 μm or more has a weak basicity, so that the reaction between the moisture contained in the electrolyte composition and the oxide 18 can be reduced. Since the basicity in the system is unlikely to become strong, the proton at the second position of the imidazolium cation is unlikely to be eliminated, and the decrease in the ionic conductivity of the electrolyte solution caused by the elimination of the proton is reduced. In addition, the polar solvent contained in the electrolyte composition hardly contributes to the reaction due to protons and hydrogen bonds, and the vinylidene fluoride-based polymer is unlikely to be polyenized due to the elimination of HF, so that the gelation of the electrolyte composition is reduced. Furthermore, since the electrochemical reaction derived from HF is unlikely to occur, the resistance of the SEI can be kept low.

In the electrolyte layer 14 (electrolyte composition), the content (volume %) of the ionic liquid relative 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, 0 < X ≦ 50. This is to reduce the occurrence of seepage of the ionic liquid while ensuring ionic conductivity by the ionic liquid interposed between the oxides 18.

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

The lithium ion conductivity of the electrolyte composition is determined by the types of oxide 18, electrolyte salt, and ionic liquid, the salt concentration, etc. The lithium ion conductivity of the electrolyte composition at 25° C. is preferably 4.0×10 ⁻⁵ S/cm or more in order to ensure the output density of secondary battery 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 ion conductivity calculated by the AC impedance method of 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 by 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 _I0 is measured immediately after applying a constant voltage V to the cell, and the initial resistance value _R0 of the cell is calculated according to 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.

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 after the cell reaches a steady state is measured. 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 transport number t _Li is calculated by substituting the resistance value R _S , the resistance value R _P and the interface resistance R _INT into the following formula D. t _Li = R _S / (R _P - R _INT ) ... D
In the electrolyte layer 14 (electrolyte composition), the amount (volume %) of the binder relative to the combined amount of the oxide 18 and the ionic liquid is preferably 10 volume % or less (excluding 0 volume %). That is, combined amount of the oxide and the ionic liquid: amount of the binder=(100-Y):Y, where 0<Y≦10. This is because the binder ensures the formability of the electrolyte layer 14 and reduces the decrease in the ionic conductivity of the electrolyte layer 14. The content (volume %) of the binder 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 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 includes an active material 20 and an electrolyte composition. The electrolyte composition includes an oxide 18. In order to reduce the resistance of the composite layer 17, the composite layer 17 may include 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, Li-Al alloy, Li ₄ Ti ₅ O ₁₂ , graphite, In, Si, Si-Li alloy, and SiO. As with the electrolyte layer 14, the composite layers 13 and 17 may include a binder.

The secondary battery 10 is manufactured, for example, as follows. A mixture of an ionic liquid in which an electrolyte salt is dissolved and an oxide 18 is mixed with a solution in which a binder is 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.

The active material 19 is mixed with a mixture of an ionic liquid in which an electrolyte salt is dissolved and an oxide 18, and then a solution in which a binder is dissolved in a solvent is mixed to create a slurry. After tape casting on the current collecting layer 12, it is dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 11.

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

After cutting the electrolyte sheet, positive electrode sheet, and negative electrode sheet into a predetermined shape, the positive electrode sheet, electrolyte sheet, and negative electrode sheet are stacked in this order and pressed together to form an integrated unit. Terminals (not shown) are connected to the current collecting layers 12 and 16, respectively, and the assembly is sealed in a case (not shown), resulting in a secondary battery 10 that includes, 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 with reference to examples, but the present invention is not limited to these examples.

(Preparation of oxide)
_{Li2CO3 ,} MgO, La(OH) ₃ , _SrCO3 , and _ZrO2 were _weighed to obtain _{Li6.95Mg0.15La2.75Sr0.25Zr2.0O12} . _Li2CO3 was weighed out at _{about 15} mol _{% excess} in terms of element, taking into account the volatilization of Li during firing. _The weighed raw materials and ethanol were put into a nylon pot together with zirconia balls, and were ground and mixed in a ball mill for 15 hours. The slurry taken out of the pot was dried, and then pre-fired (at 900°C for 1 hour) on a MgO plate. The pre-fired powder and ethanol were put into a nylon pot, and were ground and mixed in a ball mill for 15 hours.

The slurry taken out of the pot was dried, then put into a die with a diameter of 12 mm, and pressed to obtain a molded body with a thickness of about 1.5 mm. A hydrostatic pressure of 1.5 t/cm ² was further applied to the molded body using a cold isostatic press (CIP). The molded body was covered with a calcined powder of the same composition as the molded body, and fired in a reducing atmosphere (at 1100°C for 4 hours) to obtain a sintered body of oxide. The lithium ion conductivity of the sintered body obtained by the AC impedance method was 1.0 x 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 to 100 mHz. In an Ar atmosphere, the sintered body was pulverized and passed through a sieve with a mesh size of 250 μm, and the oxide powder that passed through the sieve was collected.

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

Similarly, 50 g of the powder that had passed through the sieve, 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 in a planetary ball mill (rotation speed: 300 rpm). The slurry removed from the pot was dried to obtain a fine oxide powder.

(Heat Treatment of Coarse and Fine Powders)
The coarse oxide powder (40 g or less) placed in a tube furnace (volume 1875 ^cm3 ) was heated at 670°C for 2 hours with nitrogen gas (flow rate 10 L/min) introduced into one end of the tube furnace and evacuated 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. The fine oxide powder was also heat-treated under the same conditions and placed in a sealed container.

(Measurement of particle size distribution)
The particle size distribution of the heat-treated coarse powder, the heat-treated fine powder, and the non-heat-treated coarse powder was measured using a laser diffraction/scattering particle size distribution measuring device (Microtrac MT3300EX II). The sample to measure the particle size distribution was placed in a solvent (0.2 wt% sodium hexametaphosphate aqueous solution), dispersed for 3 minutes using an ultrasonic disperser (SD-600 manufactured by Nippon Seiki Seisakusho), and set in the measuring device. The refractive index of the sample was set to 1.81, the refractive index of the solvent was set to 1.33, and D ₁₀ , D ₅₀ , D ₉₀ , and D ₁₀₀ were calculated. _{D 50} and D ₉₀ are the particle sizes at which the integrated value of the frequency in the particle size distribution (under the sieve) is 50% and 90%, respectively.

The _D10 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 3.6 μm, 1.4 μm, and 0.5 μm, respectively.

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

Example 1
Powder A and the electrolyte were mixed in a mortar in an Ar atmosphere so that the oxide: electrolyte ratio was 61:39 (volume ratio) to obtain a composite powder. 18 g of the composite powder, 0.864 g of polyvinylidene fluoride (PVDF), and 7.776 g of propylene carbonate (PC) 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 propylene carbonate was replaced with dimethyl carbonate (DMC).

Example 3
A slurry in Example 3 was obtained in the same manner as in Example 1, except that Powder A was replaced with Powder B.

Example 4
The slurry in Example 4 was obtained in the same manner as in Example 3, except that propylene carbonate was replaced with dimethyl carbonate.

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

(Comparative Example 2)
A slurry in Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that propylene carbonate was replaced with dimethyl carbonate.

(Comparative Example 3)
A slurry in Comparative Example 3 was obtained in the same manner as in Example 1, except that propylene carbonate was replaced with N-methyl-2-pyrrolidone (NMP).

(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 slurry had gelled (become non-fluidized). The results are shown in Table 1. Slurries in which at least a part had gelled were marked with +, and slurries in which no gelling had occurred at all were marked with -.

As shown in Table 1, the slurry in Example 1-4 did not gel at all, but the slurry in Comparative Example 1-3 did gel. The slurry in Comparative Example 1-3 turned brown.

The slurries of Examples 1-4 and Comparative Examples 1 and 2 contained PC and DMC, which are aprotic and protophilic polar solvents. Comparing Example 1-4, in which PC or DMC is contained in the slurry, with Comparative Example 1-2, the slurry of Example 1-4, in which an oxide having a D ₁₀ of 1.4 μm or more is contained, did not gel, while the slurry of Comparative Examples 1 and 2, in which an oxide having a D ₁₀ of 0.5 μm is contained, gelled. It is presumed that the discoloration and gelling of the slurry are due to polyenation of vinylidene fluoride caused by the elimination of HF. It is presumed that gelling did not occur because oxides having a _{D 10} of 1.4 μm or more have a weak basicity, making it difficult for the constituent components of the slurry to interact with each other.

The slurry of Comparative Example 3 contained NMP, which is a protophilic polar solvent. Comparing Examples 1 and 2, in which an oxide having a _D10 of 3.6 μm is contained in the slurry, with Comparative Example 3, the slurries of Examples 1 and 2, in which aprotic and protophilic polar solvents (PC, DMC) are contained, did not gel, while the slurry of Comparative Example 3, in which a protophilic polar solvent (NMP) was contained, gelled. It is presumed that the aprotic and protophilic polar solvent is weakly basic compared to the protophilic polar solvent, so that interaction between the components of the slurry is less likely to occur, and gelling did not occur.

According to this example, it was revealed that an electrolyte composition containing an oxide having a garnet structure containing Li, La, and Zr and having a _D10 of 1.4 μm or more, an ionic liquid containing an imidazolium cation and a sulfonylimide anion, an electrolyte salt, a binder containing a vinylidene fluoride polymer, and an aprotic and protonic polar solvent can reduce gelation. An electrolyte sheet and a secondary battery containing this electrolyte composition can reduce the variation in the dispersion state of the components, and therefore can reduce 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 can be easily imagined that various improvements and modifications are possible within the scope of the invention without departing from its spirit.

In the embodiment, the secondary battery 10 is described as having a positive electrode layer 11 with a composite layer 13 provided on one side of a current collecting layer 12, and a negative electrode layer 15 with a composite layer 17 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 having an electrode layer (so-called bipolar electrode) in which a composite layer 13 and a composite layer 17 are provided on both sides of a current collecting layer 12. If the bipolar electrodes and electrolyte layers 14 are alternately stacked and housed in a case (not shown), a so-called bipolar structure secondary battery is 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. The secondary battery may be any battery in which at least one of the composite layers 13, 17 and the electrolyte layer 14 contains an electrolyte composition.

In the embodiment, a lithium ion battery (secondary battery) containing an electrolyte composition is used as an example to describe a secondary battery 10 having electrode layers (positive electrode layer 11 and negative electrode layer 15) and an electrolyte layer 14, 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 Secondary battery 11 Positive electrode layer 14 Electrolyte layer (electrolyte sheet)
15 negative electrode layer 18 oxide

Claims (3)

An electrolyte composition comprising an oxide having a garnet structure containing Li, La, and Zr, an electrolyte salt, an ionic liquid, a binder, and a solvent,
The oxide has a particle size of 1.4 μm or more at which the integrated value of the frequency in a volume-based particle size distribution is 10%,
the ionic liquid comprises an imidazolium cation and a sulfonylimide anion;
the solvent is an aprotic, protophobic polar solvent,
The binder is an electrolyte composition containing —CH ₂ CF ₂ —. An electrolyte sheet comprising the electrolyte composition according to claim 1. The cathode layer, the electrolyte layer, and the anode layer are included in the stack,
A secondary battery, 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.

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