Positive electrode mixture layer and lithium ion secondary battery

Abstract

To provide a positive electrode mixture layer enabling the charge / discharge cycle life thereof to be improved, and a lithium ion secondary battery.SOLUTION: A positive electrode mixture layer includes an active material, a solid electrolyte, and a binder. The active material includes chemical compound including Li, Ni, Mn, Co and O, the solid electrolyte includes an oxide of a garnet type structure including Li, La and Zr. The positive electrode mixture layer further includes ion liquid including imidazolium cation and sulfonyl imide anion. A ratio of the number of Ni atoms with respect to a total number of respective atoms of Li, Ni, Mn, Co and O present in one molecule of the chemical compound is 12.5% or less. An amount of oxide with respect to the positive electrode mixture layer is 1-25 vol%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode mixture layer and a lithium ion secondary battery. [Background technology]

[0002] A positive electrode mixture layer is known that includes a composite in which an active material made of a compound containing Li, Ni, Mn, Co, and O is combined with a solid electrolyte made of an oxide with a garnet structure containing Li, La, and Zr, and a binder (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-175830 Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, the resistance of lithium ions to move across the solid-solid interface between the compound and the oxide is high, so the amount of lithium ions that move across the interface gradually decreases during charging and discharging, resulting in insufficient charge-discharge cycle life.

[0005] The present invention has been made to solve this problem, and an object of the present invention is to provide a positive electrode mixture layer and a lithium ion secondary battery that can improve the charge / discharge cycle life. [Means for solving the problem]

[0006] To achieve this objective, the positive electrode mixture layer of the present invention includes an active material, a solid electrolyte, and a binder. The active material includes a compound containing Li, Ni, Mn, Co, and O, and the solid electrolyte includes an oxide with a garnet structure containing Li, La, and Zr. The positive electrode mixture layer further includes an ionic liquid containing an imidazolium cation and a sulfonylimide anion. The ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound is 12.5% ​​or less, and the amount of oxide relative to the positive electrode mixture layer is 1-25 vol%. The lithium-ion secondary battery of the present invention includes the positive electrode mixture layer. [Effects of the Invention]

[0007] The positive electrode mixture layer of the present invention contains an ionic liquid, which forms a liquid-solid interface between the compound and the oxide. The ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound is 12.5% ​​or less, and the amount of oxide relative to the positive electrode mixture layer is 1-25 vol%, which weakens the basicity of the compound and oxide and reduces decomposition of the imidazolium cation contained in the ionic liquid. The resistance to lithium ion migration across the liquid-solid interface between the compound and the oxide is maintained low, thereby improving the charge-discharge cycle life of the positive electrode mixture layer and lithium-ion secondary battery. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of a lithium ion secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of a lithium-ion secondary battery (hereinafter referred to as "secondary battery") 10 in one embodiment. The secondary battery 10 in this embodiment is a solid-state battery in which the power generating element is made of a solid. "The power generating element is made of a solid" means that the skeleton of the power generating element is made of a solid, and does not exclude, for example, a form in which the skeleton is impregnated with a liquid.

[0010] 1, a 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).

[0011] The positive electrode layer 11 is formed by stacking a current collecting layer 12 and a positive electrode mixture layer 13. The current collecting layer 12 is a conductive member. Examples of materials for the current collecting layer 12 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, stainless steel, and a carbon material.

[0012] The positive electrode mixture layer 13 contains a solid electrolyte, an active material, an ionic liquid, and a binder. In order to reduce the resistance of the positive electrode mixture layer 13, the positive electrode mixture 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.

[0013] The solid electrolyte contains a garnet-type structure oxide 18 having lithium ion conductivity, which contains Li, La, Zr, and O. The basic composition of the garnet-type structure oxide 18 is Li5La3M2O 12 (M=Nb, Ta). Oxide 18 is Li7La3Zr2O, in which the pentavalent M cations in the basic composition are replaced with tetravalent cations. 12 The oxide 18 has a crystal structure of, for example, a cubic system (space group Ia-3d (- indicates an overline that indicates a reversal operation), JCPDS: 84-1753). The oxide 18 exhibits basicity.

[0014] The 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), in addition to Li, La, and Zr. For example, Li6La3Zr 1.5 W 0.5 O 12 ,Li 6.15 La3Zr 1.75 Ta 0.25 Al 0.2 O 12 ,Li 6.15 La3Zr 1.75 Ta 0.25 Ga 0.2 O 12 ,Li 6.25 La3Zr2Ga 0.25 O 12 ,Li 6.4 La3Zr 1.4 Ta 0.6 O 12 ,Li 6.5 La3Zr 1.75 Te 0.25 O 12 ,Li 6.75 La3Zr 1.75 Nb 0.25 O 12 ,Li 6.9 La3Zr 1.675 Ta 0.289 Bi 0.036 O 12 ,Li 6.46 Ga 0.23 La3Zr 1.85 Y 0.15 O 12 ,Li 6.8 La 2.95 Ca 0.05 Zr 1.75 Nb 0.25 O 12 ,Li 7.05 La 3.00 Zr 1.95 Gd 0.05 O 12 ,Li 6.20 Ba 0.30 La 2.95 Rb 0.05 ZrO 12 Examples include:

[0015] 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), with the molar ratio of each element satisfying all of the following (1) to (3), or contains both Mg and element A, with the molar ratio of each element satisfying all of the following (4) to (6). Element A is preferably Sr, as it increases the ionic conductivity of 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.

[0016] The median diameter of the circle-equivalent diameter of the oxide 18 appearing in the cross section of the positive electrode mixture layer 13 is preferably 0.5 to 10 μm or less in order to make the surface area of ​​the oxide 18 appropriately large and to ensure the amount of lithium ions moving between the oxide 18 and the ionic liquid having lithium ion conductivity present on the surface of the oxide 18.

[0017] To determine 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 positive electrode mixture layer 13 (a polished surface or a surface obtained by irradiating with a focused ion beam (FIB)) is analyzed, and the circle-equivalent diameter is calculated from the area of ​​each particle of the oxide 18, and a volume-based particle size distribution is determined. The median diameter is the circle-equivalent diameter at which the cumulative frequency in the particle size distribution is 50%. To ensure accuracy, the image for determining the particle size distribution is taken from 400 μm of the positive electrode mixture layer 13. 2 The area shall be equal to or greater than this.

[0018] The active material (reactant) contained in the positive electrode mixture layer 13 includes a compound 19 containing Li, Ni, Mn, Co, and O. In the compound 19, the ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound 19 is 12.5% ​​or less. This is to weaken the basicity of the compound 19.

[0019] Compound 19 is a compound represented by the chemical formula LiNi a Mn b Co c M X O2. In the chemical formula, M represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, x represent numbers that satisfy 0.3 ≦ a < 1, 0 < b ≦ 0.7, 0 < c ≦ 0.7, 0 ≦ x ≦ 0.3, and 3a + 3b + 3c + (valence of M) × x = 3. However, a / (3 + a + b + c) ≦ 0.125.

[0020] Compound 19 is LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, LiNi 0.2 Mn 0.4 Co 0.4 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.33 Mn 0.33 Co 0.31 Mg 0.03 O2, LiNi 0.33 Mn 0.33 Co 0.31 Zn 0.03 O2 are exemplified.

[0021] For the purpose of suppressing the reaction between Compound 19 and Oxide 18, a coating layer can be provided on the surface of Compound 19. The coating layer is exemplified by Al2O3, ZrO2, LiNbO3, Li4Ti5O 12 , LiTaO3, LiNbO3, LiAlO2, Li2ZrO3, Li2WO4, Li2TiO3, Li2B4O7, Li3PO4, and Li2MoO4.

[0022] In addition to Compound 19, the active material is, for example, inverse spinel-type oxides such as LiNiVO4, LiCoPO4, LiCoVO4, etc., LiMn 1.5 Ni 0.5 O4, LiMn2O4, LiCo 0.3Ni 0.7 Spinel-type oxides such as O2, olivine-type oxides such as Fe2(SO4)3 and LiFePO4, LiCoO2, Li 1+x (Fe, Mn, Co) 1-X It may contain layered oxides such as O2. The amount of active material other than Compound 19 is 0 vol % or more and less than 50 vol % based on the total volume of the active material including Compound 19.

[0023] The ionic liquid contained in the positive electrode mixture layer 13 has an electrolyte salt dissolved therein. The ionic liquid is a compound consisting of cations and anions, and is liquid at room temperature and normal pressure. The ionic liquid with the electrolyte salt dissolved therein constitutes the electrolyte, which can improve the flame retardancy of the electrolyte. The various physical properties and functions of the electrolyte are determined by the type and salt concentration of the electrolyte salt and ionic liquid.

[0024] 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 anion of the electrolyte salt is a halide ion (I - ,Cl - ,Br - etc.),SCN - ,BF4 - ,BF3(CF3) - ,BF3(C2F5) - ,PF6 - ,ClO4 - ,SbF6 - ,N(SO2F)2 - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - ,B(C6H5)4 - ,B(O2C2H4)2 - ,C(SO2F)3 - ,C(SO2CF3)3 - ,CF3COO - ,CF3SO2O - ,C6F5SO2O - ,B(O2C2O2)2 - ,RCOO - (R is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a naphthyl group).

[0025] The anion of the electrolyte salt is N(SO2F)2, which has a sulfonyl group -S(=O)2-. - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - Sulfonylimides such as N(SO2F)2 are preferred because the sulfonylimide anion has little effect on the increase in viscosity of the electrolyte and the decrease in ionic conductivity even when the salt concentration is high. - is abbreviated as [FSI] - : It is called bis(fluorosulfonyl)imide anion, N(SO2CF3)2 - abbreviated as [TFSI] - : It is sometimes called bis(trifluoromethanesulfonyl)imide anion.

[0026] The ionic liquid is preferably one having imidazolium as the cation species. The imidazolium cation is, for example, a compound represented by formula (1).

[0027] [ka]

[0028] In formula (1), R 1 -R 5 R each independently represents a hydrogen atom or an alkyl group. The alkyl group may have a substituent. 1 -R 5 The number of carbon atoms in the alkyl group (including the substituent) represented by the formula (I) is preferably 1 to 10, more preferably 1 to 5, and even more preferably 1 to 4. This is to ensure the ionic conductivity of the electrolyte.

[0029] The substituent is not particularly limited, and examples of the substituent include an alkyl group, a cycloalkyl group, an aryl group, a hydroxyl group, a carboxyl group, a nitro group, a trifluoromethyl group, an amide group, a carbamoyl group, an ester group, a carbonyloxy group, a cyano group, a halogeno group, an alkoxy group, an aryloxy group, and a sulfonamide group.

[0030] The anion species of the ionic liquid is preferably sulfonylimide. The sulfonylimide anion is N(SO2F)2 - ,N(SO2CF3)2 - ,N(SO2C2F5)2 - ,N(SO2C4F9)2 - 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 preferred because it can ensure high ionic conductivity.

[0031] The lithium ion concentration of the electrolyte is 1 mol / dm 3 More than 3 mol / dm 3 The above is more preferable. This is because it ensures ion conductivity while widening the oxidation-side potential window of the electrolyte solution contained in the positive electrode mixture layer 13. This is presumably due to the interaction between the oxide 18 and the electrolyte solution. 3 In the above electrolytes, all solvent molecules are Li + Even if coordinated to Li + Since the stable solvation state of Li cannot be satisfied, coordination with counter anions, so-called ionic association, is formed. In an electrolyte where ionic association is dominant and there is no uncoordinated solvent, the highest occupied molecular orbital (HOMO) of the electrolyte decreases and the oxidation potential increases. + Since the positive electrode mixture layer 13 is coordinated to the cathode, an increase in the oxidation potential of the electrolyte appears as an expansion effect of the oxidation-side potential window of the positive electrode mixture layer 13.

[0032] The amount of oxide 18 relative to the positive electrode mixture layer 13 is preferably 1 to 25 vol %, more preferably 3 to 15 vol %, in order to ensure lithium ion conductivity by oxide 18 and to suppress the basicity of positive electrode mixture layer 13 by oxide 18, thereby reducing decomposition of the ionic liquid.

[0033] The amount of compound 19 relative to the positive electrode mixture layer 13 is preferably 40 to 85 vol %, more preferably 40 to 70 vol %, in order to ensure the discharge capacity of the secondary battery 10 and the low internal resistance of the positive electrode mixture layer 13.

[0034] The amount of the ionic liquid relative to the positive electrode mixture layer 13 is preferably 3 to 35 vol %, and more preferably 10 to 30 vol %. This is to ensure low internal resistance of the positive electrode mixture layer 13 and reduce the occurrence of seepage of the ionic liquid from the positive electrode mixture layer 13.

[0035] The amounts (vol%) of oxide 18, compound 19, and ionic liquid relative to the positive electrode mixture layer 13 are determined by freezing the positive electrode mixture layer 13 or embedding the positive electrode mixture layer 13 in a tetrafunctional epoxy resin or the like and then analyzing a randomly selected cross section of the positive electrode mixture layer 13 (a polished surface or a surface obtained by irradiating with a focused ion beam (FIB)) 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, Ni, and S and performing image analysis on the contrast of the backscattered electron image to identify the areas of oxide 18, compound 19, and ionic liquid. The amounts (vol%) of oxide 18, compound 19, and ionic liquid are then obtained by regarding the area ratio in the cross section of the positive electrode mixture layer 13 as the volume ratio in the positive electrode mixture layer 13.

[0036] The binder contained in the positive electrode mixture layer 13 is not particularly limited as long as it is a polymer that binds the oxide 18 and the compound 19. Examples of the binder include fluorinated resins, polyolefins, rubbery polymers such as styrene butadiene rubber, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, and cellulose ethers.

[0037] Examples of fluorinated resins include fully fluorinated resins, partially fluorinated resins, and fluorinated resin copolymers. An example of a fully fluorinated resin is polytetrafluoroethylene. Examples of partially fluorinated resins include polychlorotrifluoroethylene, polyvinylidene fluoride, and polyvinyl fluoride. Examples of fluorinated resin copolymers include tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer.

[0038] The vinylidene fluoride polymer, which is one type of fluorinated resin, is not particularly limited as long as it contains —CHCF—. Examples of the vinylidene fluoride polymer include a homopolymer of vinylidene fluoride and a copolymer of vinylidene fluoride and a copolymerizable monomer.

[0039] Examples of copolymerizable monomers include halogen-containing monomers (excluding vinylidene fluoride) and non-halogen copolymerizable monomers. Examples of halogen-containing monomers include chlorine-containing monomers such as vinyl chloride; and fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, and perfluoroalkyl vinyl ether. Examples of non-halogen copolymerizable monomers include olefins such as ethylene and propylene; acrylic monomers such as acrylic acid, methacrylic acid, and esters or salts thereof; and vinyl monomers such as acrylonitrile, vinyl acetate, and styrene. One or more copolymerizable monomers are polymerized with vinylidene fluoride to form a copolymer.

[0040] The amount (vol%) of the binder relative to the positive electrode mixture layer 13 is preferably 10 vol% or less (excluding 0 vol%). This is because the binder ensures the formability and handleability of the positive electrode mixture layer 13 and reduces a decrease in ionic conductivity of the positive electrode mixture layer 13 due to the inclusion of the binder. As described above, the amount (vol%) of the binder relative to the positive electrode mixture layer 13 is considered to be the same as the proportion of the area of ​​the binder in the cross section of the positive electrode mixture layer 13 determined by SEM-EDS analysis.

[0041] The positive electrode mixture layer 13 includes an oxide 18 (solid electrolyte), a compound 19 (active material), an electrolyte salt, an ionic liquid, and a binder. The ionic liquid includes an imidazolium cation and a sulfonylimide anion. The ionic liquid (electrolyte) in which the electrolyte salt is dissolved is interposed between the oxide 18 and the compound 19, forming a liquid-solid interface between the oxide 18 and the compound 19.

[0042] When protons from the imidazolium cations are released by the base of the positive electrode mixture layer 13, the ionic liquid decomposes, reducing the ionic conductivity of the electrolyte. However, the ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of compound 19 is 12.5% ​​or less, and the amount of oxide 18 relative to the positive electrode mixture layer 13 is 1-25 vol%. This weakens the base of the positive electrode mixture layer 13. Since the decomposition of the imidazolium cations contained in the electrolyte can be reduced, the resistance to lithium ion migration across the liquid-solid interface between oxide 18 and compound 19 can be maintained low. This improves the charge-discharge cycle life of the positive electrode mixture layer 13.

[0043] The electrolyte layer 14 includes a solid electrolyte 20. Examples of the solid electrolyte 20 include oxide-based, sulfide-based, and hydride-based solid electrolytes. Examples of oxide-based solid electrolytes include perovskite-type, NASICON-type, LISICON-type, and garnet-type solid electrolytes. Examples of sulfide-based solid electrolytes include thiolicon-type, Li 10 GeP2S 12 type, argyrodite type, Li7P3S 11 Examples of hydride-based materials include glass and glass ceramics such as Li2S-P2S5. Examples of hydride-based materials include hydrides of alkali metals or alkaline earth metals containing at least one Group 13 element (e.g., B, Al, Ga, In, Ta) of the Group 18 Periodic Table. Examples include LiBH4 and LiAlH4. The solid electrolyte 20 contains one or more of these elements.

[0044] The negative electrode layer 15 is formed by stacking a current collecting layer 16 and a negative electrode mixture layer 17. The current collecting layer 16 is a conductive member. Examples of materials for the current collecting layer 16 include metals selected from Ni, Ti, Fe, Cu, and Si, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0045] The negative electrode mixture layer 17 includes a solid electrolyte 20 and an active material 21. To reduce the resistance of the negative electrode mixture layer 17, the negative electrode mixture layer 17 may contain a conductive additive. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon fiber, Ni, Pt, and Ag. The active material 21 may be Li, a Li-Al alloy, Li4Ti5O 12 , graphite, In, Si, a Si—Li alloy, and SiO. As with the positive electrode mixture layer 13, the electrolyte layer 14 and the negative electrode mixture layer 17 may contain an electrolytic solution or a binder.

[0046] The secondary battery 10 is manufactured, for example, as follows: A mixture of an ionic liquid in which an electrolyte salt is dissolved and a solid electrolyte 20 is mixed with a solution in which a binder is dissolved to form a slurry. After tape casting, the mixture is dried to obtain a green sheet (electrolyte sheet) for the electrolyte layer 14.

[0047] A compound 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 is mixed to form a slurry. After tape casting on the current collecting layer 12, the mixture is dried to obtain a green sheet (positive electrode sheet) for the positive electrode layer 11.

[0048] An active material 21 is mixed with a mixture of an ionic liquid in which an electrolyte salt is dissolved and a solid electrolyte 20, and then a solution in which a binder is dissolved is mixed to form a slurry. After tape casting on the current collecting layer 16, the mixture is dried to obtain a green sheet (negative electrode sheet) for the negative electrode layer 15.

[0049] The electrolyte sheet, positive electrode sheet, and negative electrode sheet are cut into predetermined shapes, and then stacked in this order, positive electrode sheet, electrolyte sheet, and negative electrode sheet, and then pressed together to form a single sheet. Terminals (not shown) are connected to the current collecting layers 12 and 16, respectively, and the sheet is sealed in a case (not shown), resulting in a secondary battery 10 including, in this order, a positive electrode layer 11, an electrolyte layer 14, and a negative electrode layer 15. [Example]

[0050] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0051] (Preparation of Electrolyte) The ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI) was added with 3 mol / dm electrolyte salt LiN(SO2F)2. 3 The electrolyte was obtained by compounding.

[0052] (Preparation of binder solution) Polyvinylidene fluoride was used as a binder and dissolved in dimethyl carbonate to obtain a binder solution.

[0053] Example 1 Active material (compound) LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 (hereinafter referred to as "NMC111"), solid electrolyte (oxide) Li7La3Zr2O 12 The electrolyte and conductive additive (carbon fiber) were weighed and mixed in a mortar, followed by the addition of a binder solution to obtain a slurry. The slurry was applied to aluminum foil and then dried under reduced pressure at 90°C for 1 hour to obtain a positive electrode layer in which a 30 μm-thick positive electrode mixture layer was laminated on a current collecting layer. The ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one NMC111 molecule was 8.3%. The proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 67 vol%, 3 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0054] Active material (natural graphite), solid electrolyte Li7La3Zr2O 12 The electrolyte and conductive additive (carbon fiber) were weighed and mixed in a mortar, and then a binder solution was added to obtain a slurry. The slurry was applied to copper foil and then dried under reduced pressure at 90°C for 1 hour to obtain a negative electrode layer in which a 40 μm-thick negative electrode mixture layer was laminated on a current collecting layer. The proportions of the active material, solid electrolyte, electrolyte, conductive additive, and binder in the negative electrode mixture layer were 65 vol%, 6 vol%, 20 vol%, 1 vol%, and 8 vol%, respectively.

[0055] Solid electrolyte Li7La3Zr2O 12 The solid electrolyte and electrolyte solution were weighed and mixed in a mortar, and then a binder solution was added to obtain a slurry. The slurry was applied to a synthetic resin film and then dried under reduced pressure at 90°C for 1 hour to obtain a 50 μm-thick electrolyte layer. The proportions of the solid electrolyte, electrolyte solution, and binder in the electrolyte layer were 62 vol%, 29 vol%, and 9 vol%, respectively.

[0056] The positive electrode layer, negative electrode layer, and electrolyte layer were cut to a predetermined size, and then the electrolyte layer, from which the film had been removed, was attached between the positive electrode mixture layer and the negative electrode mixture layer, and then sealed in an aluminum laminate film to obtain the cell of Example 1. The above operations were carried out in an Ar atmosphere.

[0057] Example 2 A cell in Example 2 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 65 vol%, 5 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0058] Example 3 A cell in Example 3 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 60 vol%, 10 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0059] Example 4 The cell of Example 4 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 55 vol%, 15 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0060] Example 5 The active material (compound) of the positive electrode mixture layer is LiNi 0.5 Mn 0.3 Co 0.2 A cell in Example 5 was obtained in the same manner as in Example 1, except that O2 (hereinafter referred to as "NMC532") was used. The ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of NMC532 was 12.5%.

[0061] Example 6 The cell of Example 6 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 69 vol%, 1 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0062] Example 7 The cell of Example 7 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 50 vol%, 20 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0063] Example 8 The cell of Example 8 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 45 vol%, 25 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0064] Example 9 A cell in Example 9 was obtained in the same manner as in Example 1, except that the compound of the positive electrode mixture layer was NMC532 and the proportions of the compound, oxide, electrolyte, conductive additive, and binder were 45 vol%, 25 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0065] (Comparative Example 1) A cell in Comparative Example 1 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 70 vol%, 0 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0066] (Comparative Example 2) A cell in Comparative Example 2 was obtained in the same manner as in Example 1, except that the proportions of the compound, oxide, electrolyte, conductive additive, and binder in the positive electrode mixture layer were 40 vol%, 30 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0067] (Comparative Example 3) A cell in Comparative Example 3 was obtained in the same manner as in Example 1, except that the compound of the positive electrode mixture layer was NMC532 and the proportions of the compound, oxide, electrolyte, conductive additive, and binder were 40 vol%, 30 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively.

[0068] Comparative Example 4 The active material (compound) of the positive electrode mixture layer is LiNi 0.6 Mn 0.2 Co 0.2 A cell in Comparative Example 4 was obtained in the same manner as in Example 1, except that NMC622 (hereinafter referred to as "NMC622") was used, and the proportions of the compound, oxide, electrolyte, conductive additive, and binder were 69 vol%, 1 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively. The proportion of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of NMC622 was 15.0%.

[0069] (Comparative Example 5) The active material (compound) of the positive electrode mixture layer is LiNi 0.8 Mn 0.1 Co 0.1A cell in Comparative Example 5 was obtained in the same manner as in Example 1, except that NMC811 was used as the base material, and the proportions of the compound, oxide, electrolyte, conductive additive, and binder were 69 vol%, 1 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively. The proportion of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of NMC811 was 20.0%.

[0070] (Comparative Example 6) The active material (compound) of the positive electrode mixture layer is LiNi 0.8 Co 0.15 Al 0.05 A cell in Comparative Example 6 was obtained in the same manner as in Example 1, except that the composition was changed to Li, Ni, Co, Al, and O (hereinafter referred to as "NCA"), and the proportions of the compound, oxide, electrolyte, conductive additive, and binder were set to 69 vol%, 1 vol%, 20 vol%, 1 vol%, and 9 vol%, respectively. The proportion of the number of Ni atoms to the total number of Li, Ni, Co, Al, and O atoms present in one molecule of NCA was 20.0%.

[0071] (Charge / discharge test) Charge-discharge tests were conducted on the cells in the examples and comparative examples at room temperature. The test consisted of charging the cell at a constant current of 0.1 C rate until the terminal voltage reached the upper charge voltage limit (3.6 V), and then discharging at a constant current of 0.1 C rate. This cycle was repeated 10 times. A rating of A was given when the ratio of the discharge capacity at the 10th cycle to the discharge capacity at the 1st cycle (capacity retention rate) was 99% or more, B when the capacity retention rate was 95% or more but less than 99%, and C when the capacity retention rate was less than 95%.

[0072] [Table 1]

[0073] The abbreviations and proportions (vol%) of compounds contained in the positive electrode mixture layer, the proportion (%) of the number of Ni atoms relative to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound, the proportion (vol%) of oxides in the positive electrode mixture layer, and the capacity retention rate are shown in Table 1. The capacity retention rate was determined as A for the cells in Examples 1-5, B for the cells in Examples 6-9, and C for the cells in Comparative Examples 1-6.

[0074] Comparing Example 6 (Judgment B) and Comparative Examples 4-6 (Judgment C), both contain 1 vol% of oxide. On the other hand, Example 6 has a 12.5% ​​Ni atom ratio per molecule of the compound, while Comparative Examples 4-6 have a difference in that the Ni atom ratio per molecule of the compound is 15% or more. It is presumed that the basicity of the compound in Comparative Examples 4-6 increased, causing the decomposition of cations in the electrolyte, gradually increasing the resistance of lithium ions moving across the interface between the compound and the oxide during charge and discharge, resulting in a lower capacity retention rate. On the other hand, it is presumed that the basicity of the compound in Example 6 decreased, reducing the decomposition of cations in the electrolyte, resulting in a higher capacity retention rate compared to Comparative Examples 4-6.

[0075] Comparing Examples 8 and 9 (Judgment B) with Comparative Examples 2 and 3 (Judgment C), both have the same Ni atom ratio per compound molecule of 12.5% ​​or less. On the other hand, Examples 8 and 9 have an oxide ratio of 25 vol%, while Comparative Examples 2 and 3 have an oxide ratio of 30 vol%. Because Comparative Examples 2 and 3 have a high oxide ratio, their basicity is strengthened, causing the decomposition of cations in the electrolyte solution contained in the positive electrode mixture layer. This gradually increases the resistance to lithium ion migration across the interface between the compound and the oxide during charge and discharge, resulting in a lower capacity retention rate. On the other hand, Examples 8 and 9 have a weakened basicity and reduced decomposition of cations in the electrolyte solution, resulting in a higher capacity retention rate compared to Comparative Examples 2 and 3.

[0076] Comparing Example 1 (criterion A) and Comparative Example 1 (criterion C), both have the same ratio of Ni atoms in one compound molecule: 8.3%. On the other hand, Example 1 contains 3 vol% of oxide, while Comparative Example 1 does not contain any oxide. It is presumed that Example 1 has a higher capacity retention rate than Comparative Example 1 because the decomposition of the electrolyte is suppressed by the 3 vol% oxide.

[0077] Comparing Example 1-5 (criteria A) and Example 6-9 (criteria B), both have the same ratio of Ni atoms per molecule of 8.3-12.5%. On the other hand, Example 1-5 has an oxide ratio of 3-15 vol%, while Example 6-9 differs in that the oxide ratio is less than 3 vol% or more than 15 vol%. Since Example 1-5 has an oxide ratio of 3-15 vol%, it is presumed that the oxide suppresses decomposition of the electrolyte, resulting in a higher capacity retention rate than Example 6-9.

[0078] This example reveals that the charge-discharge cycle life can be improved when the positive electrode mixture layer contains a compound containing Li, Ni, Mn, Co, and O, an oxide having a garnet structure containing Li, La, and Zr, an ionic liquid containing imidazolium cations and sulfonylimide anions, and a binder, and the ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound is 12.5% ​​or less and the ratio of the oxide is 1-25 vol%.

[0079] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0080] In the embodiment, the secondary battery 10 has been described as including a positive electrode layer 11 in which a positive electrode mixture layer 13 is provided on one side of a current collecting layer 12, and an anode layer 15 in which an anode mixture layer 17 is provided on one side of a current collecting layer 16, but the present invention is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to a secondary battery including electrode layers (so-called bipolar electrodes) in which a positive electrode mixture layer 13 and an anode mixture layer 17 are provided on both sides of a current collecting layer 12. A so-called bipolar structure secondary battery can be obtained by alternately stacking bipolar electrodes and electrolyte layers 14 and housing the resultant battery in a case (not shown). [Explanation of symbols]

[0081] 10 Lithium-ion secondary battery 13 Positive electrode mixture layer 18 Oxides 19 compounds

Claims

1. The battery includes an active material, a solid electrolyte, and a binder. the active material includes a compound containing Li, Ni, Mn, Co, and O; the solid electrolyte is a positive electrode mixture layer containing an oxide having a garnet structure containing Li, La, and Zr, further comprising an ionic liquid comprising an imidazolium cation and a sulfonylimide anion; the ratio of the number of Ni atoms to the total number of Li, Ni, Mn, Co, and O atoms present in one molecule of the compound is 12.5% ​​or less; The amount of the oxide relative to the positive electrode mixture layer is 1 to 25 vol %.

2. 2. The positive electrode mixture layer according to claim 1, wherein the amount of said oxide relative to said positive electrode mixture layer is 3 to 15 vol %.

3. A lithium ion secondary battery comprising the positive electrode mixture layer according to claim 1 or 2.

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