JP7685870B2 - 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 containing an oxide containing Li, La and Zr, Li ₇ La ₃ Zr ₂ O ₁₂ , an electrolyte salt, an ionic liquid and a polymer is known (Patent Document 1).

Patent No. 6562184

In the prior art, an electrolyte composition in which a basic oxide made of Li ₇ La ₃ Zr ₂ O ₁₂ and a polymer 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 a 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 comprises an oxide containing Li, La and Zr, an electrolyte salt, an ionic liquid and a polymer, the oxide having a garnet structure, with a portion of Li being substituted with Mg and a portion of La being substituted with Sr, the ionic liquid containing a pyrrolidinium cation and the polymer containing -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 gelation because interactions between the components are less likely to occur. 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.

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 polymer. 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 a garnet-type structure 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 the pentavalent M cation in the basic composition is replaced with a tetravalent cation, with part of the Li replaced with Mg and part of the La replaced with Sr. The oxide 18 has a crystal structure of, for example, a cubic system (space group Ia-3d (- indicates an overline that means a reversal operation), JCPDS: 84-1753).

The oxide 18 is preferably one in which 2% or more of the Li element is substituted with Mg. In particular, it is more preferable that the molar ratio of each element satisfies all of the following (1) to (3), or the molar ratio of each element satisfies all of the following (4) to (6).
(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.

In addition to Li, La, Zr, Mg, and Sr, oxide 18 may 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 lanthanides (excluding La).

The median diameter of the circle-equivalent diameter of the oxide 18 appearing in the cross section of the electrolyte layer 14 is 0.5-10 μm, 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 cationic component of the ionic liquid has a pyrrolidinium cation. The pyrrolidinium cation is, for example, a five-membered ring compound represented by formula (1).

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. The number of carbon atoms of the alkyl group (including the substituent) represented by R ¹ and R ² is preferably 1 to 5, more preferably 1 to 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 component of the ionic liquid is not particularly limited. Examples of the anion component include inorganic anions such as BF ₄ ^- and N(SO ₂ F) ₂ ^- , and organic anions such as B(C ₆ H ₅ ) ₄ ^- , CH ₃ SO ₃ ^- , CF ₃ SO ₃ ^- , N(SO ₂ CF ₃ ) ₂ ^- , and N(SO ₂ C ₄ F ₉ ) ₂ ^- . Examples of the ionic liquid 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, since this makes it easier to control the coordination (interaction) between the lithium ion and the anion contained in the electrolyte.

An example of the polymer contained in the electrolyte layer 14 is a binder that binds the oxide 18. The polymer includes a vinylidene fluoride-based polymer that contains -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 types of the copolymerizable monomers are polymerized to vinylidene fluoride to form a copolymer.

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

The electrolyte layer 14 may contain a solvent that dissolves the polymer. The electrolyte layer 14 is obtained by forming an electrolyte composition containing an oxide 18, an electrolyte 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 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 can be determined by gas chromatography-mass spectrometry (GC-MS).

The solvent contained in the electrolyte composition is an aprotic polar solvent. The classification of solvents 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 receive protons, and "aprotic" that do not have hydrogen atoms capable of hydrogen bonding, and the latter are further divided into "protophilic" that are highly basic and easily solvate with cations, and "protophilic" that are weakly basic and difficult to solvate with cations. An aprotic solvent is a solvent in which an anion is not thought to exist stably after losing a proton. Oxide 18 is easily dispersed in a polar solvent.

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

Ionic liquids depend on the type of cation, but some bases make it easier for protons to be released from the cation. When protons are released from the cation, the ionic conductivity of the electrolyte decreases, and the released protons react with ^OH- to generate water. As mentioned above, the generated water reacts with oxides to further strengthen its basicity.

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

In contrast, the electrolyte composition contains an ionic liquid containing a pyrrolidinium cation, and an oxide 18 containing Li, La, and Zr, with some of the Li replaced with Mg and some of the La replaced with Sr. The role of the oxide 18 is unclear, but it is presumed that the protons of the pyrrolidinium cation are difficult to remove, and the interaction between the oxide 18 and the pyrrolidinium cation makes it difficult for the vinylidene fluoride polymer to become polyenated, reducing gelation of the electrolyte composition. Furthermore, since electrochemical reactions derived from HF are difficult 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 of polymer (volume %) relative to the combined amount of oxide 18 and ionic liquid is preferably 10 volume % or less (excluding 0 volume %). That is, combined amount of oxide and ionic liquid: amount of polymer=(100-Y):Y, where 0<Y≦10. This is to ensure the formability of the electrolyte layer 14 by the polymer and to reduce a decrease in the ionic conductivity of the electrolyte layer 14. The polymer content (volume %) can be determined from the area % of the cross section of the electrolyte layer 14 determined by SEM-EDS analysis in the same manner as above.

The 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, a Li-Al alloy, Li ₄ Ti ₅ O ₁₂ , graphite, In, Si, a Si-Li alloy, and SiO. As with the electrolyte layer 14, the composite layers 13 and 17 may include a polymer.

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 polymer 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 polymer 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 polymer 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 to _{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 pulverized and mixed in a ball mill for 15 hours. The slurry taken out of the pot was dried, and then pre-fired (1100°C for 10 hours) on a MgO plate. The pre-fired powder and ethanol were put into a nylon pot, and were pulverized 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. The sintered body was pulverized in an Ar atmosphere to obtain a powder of the oxide in the example (hereinafter referred to as "LLZ-Sr").

_{Li2CO3 ,} La(OH _{) 3} , _Ta2O5 , _{and ZrO2} were weighed out to obtain _{Li6.6La3Ta0.4Zr1.6O12 ,} and then mixed, pre _- fired, fired, and pulverized in the same manner as in preparing LLZ _- Sr to obtain an oxide powder in the comparative example (hereinafter referred to as "LLZ-Ta").

(Preparation of Electrolyte)
The ionic liquid N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (P13-FSI, special grade reagent) was combined with the electrolyte salt LiN(SO ₂ F) ₂ at 1 mol/dm ³ or 3 mol/dm ³ to obtain various electrolyte solutions.

The ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI, special reagent grade) was combined with the electrolyte salt LiN(SO ₂ F) ₂ at 1 mol/dm ³ or 3 mol/dm ³ to obtain various electrolyte solutions.

Example 1
A composite powder was obtained by mixing an electrolyte containing P13-FSI with a salt concentration of ³ mol/dm3 and LLZ-Sr in a mortar in an Ar atmosphere so that the oxide:electrolyte ratio was 61:39 (volume ratio). 18 g of the composite powder, 0.864 g of polyvinylidene fluoride (PVDF), and 7.776 g of dimethyl carbonate (DMC) were mixed in an Ar atmosphere to obtain the slurry in Example 1.

Example 2
A composite powder was obtained by mixing an electrolyte containing P13-FSI with a salt concentration of 1 mol/ ^dm3 and LLZ-Sr in a mortar in an Ar atmosphere so that the oxide:electrolyte ratio was 61:39 (volume ratio). 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 a slurry in Example 2.

Example 3
A slurry in Example 3 was obtained in the same manner as in Example 1, except that dimethyl carbonate was replaced with propylene carbonate.

Example 4
The 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).

Example 5
The 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.

(Comparative Example 1)
A composite powder was obtained by mixing an electrolyte containing EMI-FSI with a salt concentration of 1 mol/ ^dm3 and LLZ-Sr in a mortar in an Ar atmosphere so that the oxide:electrolyte ratio was 61:39 (volume ratio). 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.

(Comparative Example 2)
A slurry in Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that the electrolytic solution was changed to an electrolytic solution containing EMI-FSI and having a salt concentration of 3 mol/dm ^3.

(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).

(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.

(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. The state of the slurry was visually confirmed 1 hour, 8 hours, and 24 hours after the slurry was prepared, and the presence or absence of gelation (non-fluidity) and coloration was judged. The results are shown in Table 1. Slurries in which at least a part had gelled were marked with +, and slurries in which no gelation had occurred at all were marked with -.

As shown in Table 1, the slurry in Example 1-3 did not gel or discolor even after 24 hours. The slurries in Examples 4 and 5 did not gel or discolor after 8 hours, but gelled and discolored (turned brown) after 24 hours. The slurry in Comparative Example 1-4 gelled and discolored after 1 hour.

The slurries of Examples 4 and 5 and Comparative Examples 1 and 2 contained NMP, which is an aprotic and protophilic polar solvent. Comparing Examples 4 and 5, which contain NMP, with Comparative Examples 1 and 2, the slurries of Examples 4 and 5, which contain pyrrolidinium cations, did not gel or discolor until 8 hours later, while the slurries of Comparative Examples 1 and 2, which contain imidazolium cations, gelled and discolored after 1 hour. It is presumed that the coloring and gelation of the slurry is due to the polyenization of vinylidene fluoride caused by the elimination of HF. It is presumed that in Examples 4 and 5, which contain pyrrolidinium cations and LLZ-Sr, the components of the slurry are less likely to interact with each other than in Comparative Examples 1 and 2, and therefore gelation and discoloration did not occur.

The slurry of Comparative Example 3, like the slurries of Comparative Examples 1 and 2, contained imidazolium cations and DMF, an aprotic and protophilic polar solvent. It is presumed that the slurry of Comparative Example 3 also experienced polyenation of vinylidene fluoride due to the elimination of HF, resulting in gelation and coloration.

The slurries of Example 5 and Comparative Example 4 contained pyrrolidinium cations and NMP. Comparing Example 5 and Comparative Example 4, the slurry of Example 5 containing LLZ-Sr did not gel or discolor until 8 hours later, while the slurry of Comparative Example 4 containing LLZ-Ta gelled and discolored after 1 hour. From the results of Examples 2, 3, 5 and Comparative Example 4, it is presumed that LLZ-Sr has the effect of suppressing the desorption of HF, and suppressed the gelation and discoloration of the slurry caused by the polyenation of vinylidene fluoride.

The slurries of Examples 1-3 and 4 and 5 contained pyrrolidinium cations. Comparing Examples 1-3 and 4 and 5, the slurry of Example 1-3, which contains DMC or PC, an aprotic and protophilic polar solvent, did not gel or discolor even after 24 hours, while the slurries of Examples 4 and 5, which contain NMP, an aprotic and protophilic polar solvent, gelled and discolored after 24 hours. Since aprotic and protophilic polar solvents are less basic than protophilic polar solvents, it is presumed that this is advantageous in reducing interactions between the components of the slurry and reducing gelation and discoloration.

According to this example, it has been revealed that an electrolyte composition containing an oxide having a garnet structure containing Li, La, and Zr, in which part of the Li is replaced with Mg and part of the La is replaced with Sr, an ionic liquid containing a pyrrolidinium cation, an electrolyte salt, and a vinylidene fluoride polymer can reduce gelation. When the electrolyte composition contains a solvent, it has been revealed that the solvent is preferably an aprotic polar solvent, and in particular, an aprotic and protophobic polar solvent is more preferable. 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 (4)

An electrolyte composition comprising an oxide containing Li, La and Zr, an electrolyte salt, an ionic liquid, and a polymer,
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 comprises a pyrrolidinium cation;
the polymer comprises -CH ₂ CF ₂ - ;
the content of the ionic liquid relative to the total amount of the oxide and the ionic liquid is 50 volume % or less . The electrolyte composition according to claim 1, wherein the pyrrolidinium cation is N-methyl-N-propylpyrrolidinium. An electrolyte sheet comprising the electrolyte composition according to claim 1 or 2. The cathode layer, the electrolyte layer, and the anode layer are included in the stack,
A secondary battery comprising the electrolyte composition according to claim 1 or 2 in at least one of the positive electrode layer, the electrolyte layer and the negative electrode layer.

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