Lithium-ion secondary battery

Abstract

This lithium ion secondary battery includes: a negative electrode; a positive electrode; a non-aqueous electrolyte which is water repellant and contains a lithium salt; and an aqueous solid electrolyte which contains a lithium salt. The aqueous solid electrolyte contacts only the positive electrode, from between the negative electrode and the positive electrode. The non-aqueous electrolyte contacts at least the negative electrode, from between the negative electrode and the positive electrode.

Description

[Technical Field]

[0001] The present disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries, which have a positive electrode, a negative electrode, and an electrolyte, are widely used as high-power, high-energy-density secondary batteries. Conventional secondary batteries use organic solvent-based electrolytes to achieve high energy density.

[0003] However, organic solvents are generally flammable, making safety an important issue, and the ionic conductivity of organic solvents is lower than that of aqueous solutions, making their rapid charge-discharge characteristics insufficient.

[0004] In view of these problems, research has been conducted into secondary batteries that use aqueous electrolytes containing water. For example, Patent Document 1 proposes a lithium ion secondary battery that uses an aqueous solution containing a high concentration of alkaline salt as the aqueous liquid electrolyte. Furthermore, Patent Document 2 proposes a lithium ion secondary battery that includes a negative electrode filled with a nonaqueous solid electrolyte, a positive electrode, a separator disposed between the negative electrode and the positive electrode and filled with the nonaqueous solid electrolyte, and an aqueous liquid electrolyte. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6423453 [Patent Document 2] Japanese Patent Application Publication No. 2018-198131 Summary of the Invention

[0006] Conventional lithium-ion secondary batteries with aqueous electrolytes have the problem of low initial charge-discharge efficiency.

[0007] Therefore, an object of the present disclosure is to provide a lithium ion secondary battery that can improve the initial charge / discharge efficiency even while using an aqueous electrolyte.

[0008] One aspect of the present disclosure is a lithium ion secondary battery including a negative electrode, a positive electrode, a water-repellent nonaqueous electrolyte containing a lithium salt, and an aqueous solid electrolyte containing a lithium salt, wherein the aqueous solid electrolyte is in contact with only the positive electrode of the negative electrode and the positive electrode, and the nonaqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode.

[0009] According to the present disclosure, it is possible to provide a lithium ion secondary battery capable of improving the initial charge / discharge efficiency. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of the lithium ion secondary battery of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] A lithium ion secondary battery according to one embodiment of the present disclosure includes a negative electrode, a positive electrode, a water-repellent nonaqueous electrolyte containing a lithium salt, and an aqueous solid electrolyte containing a lithium salt, wherein the aqueous solid electrolyte is in contact with only the positive electrode of the negative electrode and the positive electrode, and the nonaqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode. By using a lithium ion secondary battery according to one embodiment of the present disclosure, it is possible to improve the initial charge / discharge efficiency. The mechanism by which this effect is achieved is not fully understood, but the following is presumed.

[0012] Generally, in lithium-ion secondary batteries using an aqueous electrolyte containing water, the reductive decomposition of water in the aqueous electrolyte (i.e., a side reaction) occurs on the negative electrode, inhibiting the progress of the charging reaction at the negative electrode and reducing the initial charge / discharge efficiency. However, in the present disclosure, an aqueous solid electrolyte is used as the aqueous electrolyte, and this aqueous solid electrolyte is coated on the positive electrode or otherwise brought into contact with only the positive electrode. This makes it possible to suppress water from leaking into the negative electrode compared to when an aqueous liquid electrolyte is used. Furthermore, because a water-repellent nonaqueous electrolyte is in contact with the negative electrode, even if water leaks into the negative electrode, the water-repellent nonaqueous electrolyte prevents water from contacting the negative electrode surface. As a result, the side reaction of water at the negative electrode is suppressed, allowing the charge / discharge reaction at the negative electrode to proceed and improving the initial charge / discharge efficiency.

[0013] An example of an embodiment of a lithium ion secondary battery according to the present disclosure will be described in detail below.

[0014] FIG. 1 is a schematic cross-sectional view showing an example of a lithium-ion secondary battery according to the present embodiment. The lithium-ion secondary battery 1 shown in FIG. 1 includes a positive electrode 10, a negative electrode 12, a separator 14, an aqueous solid electrolyte 16, a water-repellent nonaqueous electrolyte 18, a positive electrode lead 20, a negative electrode lead 22, and a battery case 24 that houses these components. The aqueous solid electrolyte 16 is coated on the positive electrode 10 and is in contact with only the positive electrode 10 of the positive electrode 10 and the negative electrode 12. The separator 14 is wrapped around the negative electrode 12. The water-repellent nonaqueous electrolyte 18 is coated on the negative electrode 12 and the separator 14 and is in contact with the negative electrode 12 and the separator 14. In FIG. 1, the water-repellent nonaqueous electrolyte 18 is in contact with only the negative electrode 12 of the positive electrode 10 and the negative electrode 12, but it may be in contact with both the positive electrode 10 and the negative electrode 12.

[0015] The positive electrode 10 has a positive electrode current collector 26 and a positive electrode composite layer 28 disposed on the positive electrode current collector 26. A positive electrode lead 20 is connected to the positive electrode current collector 26. The positive electrode lead 20 is housed in the battery case 24 such that the tip of the positive electrode lead 20 protrudes outside the battery case 24.

[0016] The positive electrode current collector 26 may be a foil of a metal that is electrochemically and chemically stable within the potential range of the positive electrode 10, or a film having such a metal disposed on its surface. The form of the positive electrode current collector 26 is not particularly limited, and may be, for example, a porous body such as a mesh body, punched sheet, or expanded metal of the metal. Examples of materials for the positive electrode current collector 26 include stainless steel, Al, aluminum alloy, and Ti. From the viewpoints of current collection performance, mechanical strength, and the like, the thickness of the positive electrode current collector 26 is preferably, for example, 3 μm or more and 50 μm or less.

[0017] Positive electrode mixture layer 28 includes a positive electrode active material. Positive electrode mixture layer 28 may also include a binder, a conductive material, etc. Positive electrode 10 can be manufactured, for example, by applying a positive electrode mixture slurry including a positive electrode active material, a binder, a conductive material, etc., onto positive electrode current collector 26, drying and rolling the coating, and forming positive electrode mixture layer 28 on positive electrode current collector 26.

[0018] Examples of the positive electrode active material include lithium-containing transition metal oxides containing lithium (Li) and transition metal elements such as cobalt (Co), manganese (Mn), and nickel (Ni). Other examples of the positive electrode active material include transition metal sulfides, metal oxides, lithium-containing polyanion compounds containing one or more transition metals such as lithium iron phosphate (LiFePO) and lithium iron pyrophosphate (LiFePO), sulfur-based compounds (LiS), and oxygen-containing metal salts such as oxygen and lithium oxide. For example, from the viewpoint of charge / discharge efficiency, lithium-containing transition metal oxides are preferred as the positive electrode active material.

[0019] From the viewpoint of, for example, charge / discharge efficiency, the lithium-containing transition metal oxide preferably contains at least one element selected from the group consisting of Ni, Co, Mn, and aluminum (Al). Among these elements, it is preferable to contain at least Ni, at least Co, at least two elements of Ni and Mn, at least three elements of Ni, Co, and Mn, or at least three elements of Ni, Co, and Al. The lithium-containing transition metal oxide may contain additional elements other than these elements, such as zirconium (Zr), boron (B), magnesium (Mg), scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), lead (Pb), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), and silicon (Si).

[0020] Specific examples of lithium-containing transition metal oxides include Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M yO4, LiMPO4, Li2MPO4F (in each chemical formula, M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3) can be mentioned. The lithium-containing transition metal oxide may be used alone or in combination of multiple kinds. From the viewpoint of increasing the capacity, it is preferable that the lithium-containing transition metal oxide contains 80 mol% or more of Ni with respect to the total amount of transition metals other than lithium. Also, from the viewpoint of the stability of the crystal structure, the lithium-containing transition metal oxide is Li a Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.8 ≤ b < 1, 0 < c < 0.2, 0 < d ≤ 0.1, b + c + d = 1) is more preferable.

[0021] In addition, the lithium-containing transition metal oxide may be a Li-excess type transition metal oxide, a lithium-containing transition metal halide, etc. The Li-excess type transition metal oxide is represented by, for example, the general formula Li 1+x Me 1-x O2 (0 < x). Also, the lithium-containing transition metal halide is not particularly limited as long as it is a lithium-containing transition metal oxide containing a halogen atom. However, for example, from the viewpoint of the structural stability of the lithium-containing transition metal oxide, etc., it is preferable to include a lithium-containing transition metal oxide containing a fluorine atom.

[0022] The conductive material may be a known conductive material that enhances the electrical conductivity of the positive electrode mixture layer 28, such as carbon black, acetylene black, ketjen black, graphite, carbon nanofibers, carbon nanotubes, graphene, or other carbon materials. The binder may be a known binder that maintains good contact between the positive electrode active material and the conductive material and enhances the binding of the positive electrode active material to the surface of the positive electrode current collector 26, such as fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, carboxymethyl cellulose (CMC) or a salt thereof, styrene-butadiene rubber (SBR), polyethylene oxide (PEO), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP).

[0023] The negative electrode 12 has a negative electrode current collector 30 and a negative electrode composite layer 32 disposed on the negative electrode current collector 30. A negative electrode lead 22 is connected to the negative electrode current collector 30. The negative electrode lead 22 is housed in the battery case 24 such that the tip of the negative electrode lead 22 protrudes outside the battery case 24.

[0024] The negative electrode current collector 30 may be a foil of a metal that is electrochemically and chemically stable within the potential range of the negative electrode 12, or a film having such a metal disposed on its surface. The form of the negative electrode current collector 30 is not particularly limited, and may be, for example, a porous body such as a mesh, punched sheet, or expanded metal. Examples of materials for the negative electrode current collector 30 include Al, Ti, Mg, Zn, Pb, Sn, Zr, and In. These may be used alone or as an alloy of two or more elements, as long as the material contains at least one of them as a main component. Furthermore, when two or more elements are contained, the elements do not necessarily need to be alloyed. The thickness of the negative electrode current collector 30 is preferably, for example, 3 μm to 50 μm inclusive, from the viewpoints of current collection performance, mechanical strength, and the like.

[0025] Negative electrode mixture layer 32 contains a negative electrode active material. Negative electrode mixture layer 32 may also contain a binder, a conductive material, and the like. The conductive material and binder may be the same as those used for positive electrode 10. Negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, a conductive material, and the like onto negative electrode current collector 30, drying and rolling the coating, and forming negative electrode mixture layer 32 on negative electrode current collector 30.

[0026] The negative electrode active material is not particularly limited as long as it is a material that can be used as a negative electrode active material in conventional lithium-ion secondary batteries. Examples include carbon materials such as artificial graphite, natural graphite, hard carbon, soft carbon, carbon nanotubes, and activated carbon; metals such as Li, Si, and Sn, and their alloys, oxides, metal sulfides, and metal nitrides. Examples of Li alloys include lithium aluminum alloys, lithium tin alloys, lithium lead alloys, and lithium silicon alloys. Examples of metal oxides containing Li include lithium titanate (Li4Ti5O 12 Examples of metal nitrides containing Li include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride. Further examples include sulfur-based compounds.

[0027] Generally, in a lithium-ion secondary battery using an aqueous electrolyte, when a carbon material is used as a negative electrode active material, the influence of a side reaction of water is large, and the initial charge-discharge efficiency is significantly reduced. However, in the lithium-ion secondary battery of the present embodiment, contact of water with the negative electrode is suppressed, so that it is possible to improve the initial charge-discharge efficiency even when a carbon material is used as a negative electrode active material.

[0028] The aqueous solid electrolyte 16 containing a lithium salt is, for example, a solid electrolyte in which a lithium salt, an aqueous solvent, and a matrix polymer are composited together. The aqueous solid electrolyte 16 can be obtained, for example, by dissolving a lithium salt in an aqueous solvent, and then drying a precursor solution in which a matrix polymer is further mixed or dissolved.

[0029] The aqueous solvent is a solvent containing water, and may be water alone or may contain water and a solvent other than water. The water content relative to the total amount of the aqueous solvent is preferably 50% or more by volume, for example, from the viewpoint of improving the safety of lithium-ion secondary batteries.

[0030] Furthermore, the amount of water relative to the lithium salt contained in the aqueous solid electrolyte 16 is preferably 1:4 or less, more preferably in the range of 1:0.5 to 1:4, and even more preferably in the range of 1:0.5 to 1:3, in terms of the molar ratio of lithium salt to water. When the amount of water relative to the lithium salt contained in the aqueous solid electrolyte 16 is within the above range, the potential window of the aqueous solid electrolyte 16 may be expanded, for example, compared to when the amount is outside the above range, and the voltage applied to the lithium ion secondary battery 1 may be increased.

[0031] Examples of solvents other than water contained in the aqueous solvent include organic solvents such as esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Furthermore, halogen-substituted solvents in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine may also be used. Specifically, from the viewpoint of improving the battery characteristics of lithium-ion secondary batteries, organic carbonates such as cyclic organic carbonates such as ethylene carbonate, propylene carbonate, vinylidene carbonate, and butylene carbonate, chain organic carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, and fluorinated organic carbonates containing fluorine as a constituent element such as fluoroethylene carbonate, fluorodimethyl carbonate, and methyl fluoropropionate are preferred. Among the above-listed solvents, cyclic organic carbonates and fluorinated organic carbonates containing fluorine as a constituent element are particularly preferred, from the viewpoint of suppressing self-discharge of batteries, for example. Among the fluorinated organic carbonates exemplified above, fluoroethylene carbonate is preferred. These organic solvents may be used alone or in combination of two or more.

[0032] The amount of organic carbonate relative to the lithium salt contained in the aqueous solid electrolyte 16, expressed as a molar ratio of lithium salt:organic carbonate, is preferably in the range of 1:0.01 to 1:2.5, and more preferably in the range of 1:0.05 to 1:2. When the amount of organic carbonate relative to lithium salt is within this range, the battery characteristics of the lithium ion secondary battery may be improved compared to when the amount is outside this range.

[0033] Any lithium salt can be used as long as it dissolves in an aqueous solvent and dissociates, causing lithium ions to exist in the aqueous solid electrolyte 16. Examples of such lithium salts include salts with inorganic acids such as perchloric acid, sulfuric acid, and nitric acid, salts with halide ions such as chloride ions and bromide ions, and salts with organic anions containing carbon atoms in their structures.

[0034] Examples of organic anions constituting the lithium salt include anions represented by the following general formulas (i) to (vi). (R 1 SO2)(R 2 SO2)N - (i) (R 1 , R 2 are each independently selected from an alkyl group or a halogen-substituted alkyl group. 1 and R 2 may be bonded to each other to form a ring. R 3 SO3 - (ii) (R 3 is selected from alkyl groups or halogen-substituted alkyl groups. R 4 CO2 - (iii) (R 4 is selected from alkyl groups or halogen-substituted alkyl groups. (R 5 SO2)3C - (iv) (R 5 is selected from alkyl groups or halogen-substituted alkyl groups. [(R 6 SO2)N(SO2)N(R 7 SO2)] 2- (v) (R 6 , R 7 is selected from alkyl groups or halogen-substituted alkyl groups. [(R 8 SO2)N(CO)N(R 9 SO2)] 2- (vi) (R 8 , R 9 is selected from alkyl groups or halogen-substituted alkyl groups. In the above general formulas (i) to (vi), the number of carbon atoms in the alkyl group or halogen-substituted alkyl group is preferably 1 to 6, more preferably 1 to 3, and even more preferably 1 or 2. The halogen in the halogen-substituted alkyl group is preferably fluorine. The number of halogen substitutions in the halogen-substituted alkyl group is equal to or less than the number of hydrogen atoms in the original alkyl group.

[0035] R 1 ~R 9 Each of the groups is, for example, a group represented by the following general formula (vii):

[0036] C n H a F b Cl c Br d I e (vii) (n is an integer greater than or equal to 1, and a, b, c, d, and e are integers greater than or equal to 0, and satisfy 2n+1=a+b+c+d+e.) Specific examples of the organic anion represented by the general formula (i) include bis(trifluoromethanesulfonyl)imide (TFSI; [N(CF3SO2)2] - ), bis(perfluoroethanesulfonyl)imide (BETI; [N(C2F5SO2)2] - ), (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([N(C2F5SO2)(CF3SO2)] -) and the like. Specific examples of the organic anion represented by the general formula (ii) include, for example, CF3SO3 - , C2F5SO3 - Specific examples of the organic anion represented by the general formula (iii) include CF3CO2 - , C2F5CO2 - Specific examples of the organic anion represented by the general formula (iv) include tris(trifluoromethanesulfonyl)carbonate ([(CF3SO2)3C] - ), tris(perfluoroethanesulfonyl)carbonate ([(C2F5SO2)3C] - ) and the like. Specific examples of the organic anion represented by the general formula (V) include sulfonylbis(trifluoromethanesulfonyl)imide ([(CF3SO2)N(SO2)N(CF3SO2)] 2- ), sulfonylbis(perfluoroethanesulfonyl)imide ([(C2F5SO2)N(SO2)N(C2F5SO2)] 2- ), sulfonyl(perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([(C2F5SO2)N(SO2)N(CF3SO2)] 2- ) and the like. Specific examples of the organic anion represented by the general formula (vi) include carbonylbis(trifluoromethanesulfonyl)imide ([(CF3SO2)N(CO)N(CF3SO2)] 2- ), carbonylbis(perfluoroethanesulfonyl)imide ([(C2F5SO2)N(CO)N(C2F5SO2)] 2- ), carbonyl(perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide ([(C2F5SO2)N(CO)N(CF3SO2)] 2- ) etc.

[0037] Examples of organic anions other than those represented by the general formulae (i) to (vi) above include anions such as bis(1,2-benzenediolate(2-)-O,O')borate, bis(2,3-naphthalenediolate(2-)-O,O')borate, bis(2,2'-biphenyldiolate(2-)-O,O')borate, and bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate.

[0038] The anion constituting the lithium salt is preferably an imide anion. Specific examples of suitable imide anions include the imide anions exemplified as the organic anions represented by the general formula (i) above, as well as bis(fluorosulfonyl)imide (FSI; [N(FSO2)2] - ), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTI; [N(FSO2)(CF3SO2)] - ) etc.

[0039] As the lithium salt having a lithium ion and an imide anion, for example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethanesulfonyl)imide (LiBETI), lithium (perfluoroethanesulfonyl)(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide (LiFSI), and lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiFTI) are preferred, with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) being more preferred, in terms of being able to effectively suppress self-discharge of the battery. These may be used alone or in combination of two or more.

[0040] Specific examples of other lithium salts include CF3SO3Li, C2F5SO3Li, CF3CO2Li, C2F5CO2Li, (CF3SO2)3CLi, (C2F5SO2)3CLi, (C2F5SO2)2(CF3SO2)CLi, (C2F5SO2)(CF3SO2)2CLi, [(CF3SO2)N(SO2)N(CF3SO2)]Li2, [(C2F5SO2)N(SO2)N(C2F5SO2)]Li2, [(C2F5SO2)N(SO2)N(CF3SO2)]Li2, [(CF3SO2)N(CO)N(CF3SO2)]Li2, [(C2F5SO2)N (CO)N(CFSO)]Li, lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate, lithium perchlorate (LiClO), lithium chloride (LiCl), lithium bromide (LiBr), lithium hydroxide (LiOH), lithium nitrate (LiNO), lithium sulfate (LiSO), lithium sulfide (LiS), lithium hydroxide (LiOH), etc. These may be used alone or in combination of two or more.

[0041] Examples of the matrix polymer include polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), etc. Also usable are those obtained by mixing monomers, acrylonitrile and acrylic acid, and thermally polymerizing them to form a polymer.

[0042] The content of the matrix polymer is, for example, preferably 1% by mass or more and 15.0% by mass or less, and more preferably 3% by mass or more and 10% by mass or less, relative to the total amount of the aqueous solid electrolyte 16. By setting the content within this range, for example, gelation or solidification of the aqueous solid electrolyte 16 becomes easier.

[0043] The aqueous solid electrolyte 16 may be coated on the entire cathode 10, or may be coated on at least the cathode composite layer 28, as shown in Fig. 1. The aqueous solid electrolyte 16 can be obtained, for example, by dissolving a lithium salt in an aqueous solvent, and then mixing or dissolving a matrix polymer in the resulting precursor solution, applying the precursor solution to the cathode 10, or by immersing the cathode 10 in the precursor solution, coating the precursor solution on the cathode 10, and then drying the solution.

[0044] The water-repellent nonaqueous electrolyte 18 containing a lithium salt may be a nonaqueous solid electrolyte or a nonaqueous liquid electrolyte. For example, a nonaqueous solid electrolyte is a solid electrolyte formed by combining a lithium salt, an organic solvent, and a matrix polymer, while a nonaqueous liquid electrolyte is a liquid electrolyte formed by dissolving an organic solvent in a lithium salt. A nonaqueous solid electrolyte is more preferable from the viewpoint of suppressing water penetration because it coats the entire negative electrode. A nonaqueous solid electrolyte is prepared, for example, by dissolving a lithium salt in an organic solvent and then heating and drying the resulting precursor solution, which is then mixed with or dissolved in a matrix polymer.

[0045] Examples of the organic solvent include known organic solvents used in conventional non-aqueous secondary batteries, such as the above-mentioned esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Among these, it is preferable to use esters, ethers, nitriles, amides, and mixed solvents of two or more of these, in terms of improving battery characteristics.

[0046] Examples of esters include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; and carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

[0047] Examples of ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl and chain ethers such as ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl.

[0048] The organic solvent preferably contains a halogen-substituted product in which hydrogen atoms of the above-mentioned various solvents are substituted with halogen atoms such as fluorine. Particularly preferred is at least one of fluorinated cyclic carbonates, fluorinated chain carbonates, and fluorinated ethers. Suitable examples of fluorinated cyclic carbonates include 4-fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4-difluoroethylene carbonate, 4,4,5-trifluoroethylene carbonate, and 4,4,5,5-tetrafluoroethylene carbonate. Suitable examples of fluorinated chain carbonates include 2,2,2-ethyl trifluoroacetate, methyl 3,3,3-trifluoropropionate, and methyl pentafluoropropionate. Suitable examples of fluorinated ethers include 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0049] The organic solvent preferably contains a cyclic organic solvent such as a cyclic carbonate, for example, in order to suppress a decrease in the lithium ion conductivity of the nonaqueous electrolyte 18, and more preferably contains 80% by volume or more of the cyclic organic solvent relative to the total volume of the organic solvent.

[0050] Examples of the lithium salt include known lithium salts used in conventional non-aqueous secondary batteries, such as LiPF, LiBF, LiAsF, LiClO, LiCF, SO, LiN(FSO), and LiN(ClF 2l+1 SO2)(C m F 2m+1 SO2) (l and m are integers of 1 or greater), LiC(C p F 2p+1 SO2)(C q F 2q+1 SO2)(C r F 2r+1SO2) (p, q, and r are integers of 1 or greater), Li[B(C2O4)2] (lithium bis(oxalato)borate (LiBOB)), Li[B(C2O4)F2], Li[P(C2O4)F4], Li[P(C2O4)2F2], LiPO2F2, etc. The lithium salt may be, for example, the lithium salt used in the aqueous solid electrolyte 16, as exemplified above.

[0051] The matrix polymer may be the same as that used for the aqueous solid electrolyte 16. The content of the matrix polymer is preferably, for example, 1% by mass or more and 15.0% by mass or less, and more preferably 3% by mass or more and 10% by mass or less, relative to the total amount of the nonaqueous electrolyte 18. By setting the content within this range, for example, the nonaqueous electrolyte 18 can be easily solidified.

[0052] The water-repellent nonaqueous electrolyte 18 preferably has a solubility of 2 g or less in 100 g of water at 25° C., for example, in order to effectively prevent contact between the negative electrode 12 and water. The water repellency of the nonaqueous electrolyte 18 can be increased, for example, by increasing the proportion of an organic solvent having a water-repellent substituent or a fluorinated organic solvent.

[0053] The nonaqueous electrolyte 18 may simply be coated on the surface of the negative electrode composite layer 32. However, because a side reaction with water also occurs on the negative electrode current collector 30 and the negative electrode lead 22, it is preferable that the nonaqueous electrolyte 18 be coated on the entire negative electrode 12 as shown in FIG. 1 . It is even more preferable that the negative electrode lead 22 (excluding the portion protruding from the battery case 24) is also coated with the nonaqueous electrolyte 18. Furthermore, in order to effectively prevent water from seeping into the negative electrode 12, the nonaqueous electrolyte 18 is also preferably coated on the separator 14 as shown in FIG. 1 . The nonaqueous electrolyte 18 can be obtained, for example, by coating the negative electrode 12, separator 14, or the like with a precursor solution prepared by dissolving a lithium salt in an organic solvent and then mixing or dissolving a matrix polymer therein, or by immersing an assembly of the negative electrode 12 with the separator 14 wrapped around the negative electrode 12, with the precursor solution, and then heating the resulting mixture.

[0054] The separator 14 is not particularly limited as long as it is permeable to lithium ions and electrically separates the positive electrode 10 and the negative electrode 12. For example, a porous sheet made of a resin or an inorganic material may be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Examples of materials for the separator 14 include olefin-based resins such as polyethylene and polypropylene, polyamides, polyamide-imides, and cellulose. Examples of inorganic materials for the separator 14 include glass and ceramics such as borosilicate glass, silica, alumina, and titania. The separator 14 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 14 may be a multilayer separator 14 including a polyethylene layer and a polypropylene layer, or a separator 14 having an aramid-based resin, ceramic, or other material coated on its surface.

[0055] In order to effectively prevent water from entering the negative electrode 12 and from causing a side reaction with water at the negative electrode 12, the separator 14 is preferably wrapped around the negative electrode 12 and coated with a water-repellent nonaqueous electrolyte 18 together with the negative electrode 12, but this is not limitative. For example, the separator 14 may simply be disposed between the positive electrode 10 and the negative electrode 12, or may be coated with an aqueous solid electrolyte 16. Even in this configuration, it is possible to prevent a side reaction with water and a decrease in the initial charge / discharge efficiency.

[0056] Examples of the battery case 24 include a metal case, a resin case, and a laminate film case. Examples of materials for the metal case include nickel, iron, and stainless steel. Examples of materials for the resin case include polyethylene and polypropylene. Examples of the laminate film include a multilayer film in which stainless steel foil is coated with a resin film. Examples of materials for the resin film include polypropylene, polyethylene, nylon, and polyethylene terephthalate.

[0057] The lithium ion secondary battery of this embodiment can be used in various forms such as a square type, a cylindrical type, a flat type, a thin type, a coin type, and a laminate type.

[0058] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0059] Example 1 [Negative electrode] Graphite as the negative electrode active material and PVDF as the binder were mixed in N-methyl-2-pyrrolidone (NMP) at a solids mass ratio of 96:4 to prepare a negative electrode composite slurry. Next, the negative electrode composite slurry was applied to a negative electrode current collector made of copper foil, the coating was dried, and then rolled with a rolling roller. The negative electrode was then cut to a predetermined electrode size to obtain a negative electrode. The application amount of the negative electrode composite slurry and the packing density of the negative electrode composite layer were 32.3 g / m, respectively. 2 , 1.0gcm -3 It was.

[0060] [Positive electrode] Lithium iron phosphate (LFP, composition: LiFePO4) as the positive electrode active material, carbon black as the conductive material, and PVDF as the binder were mixed in NMP at a mass ratio of 94:3:3 to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to a positive electrode current collector made of Ti foil, the coating was dried, and then rolled with a rolling roller. The positive electrode was then cut to a predetermined electrode size to obtain a positive electrode. The application amount of the positive electrode composite slurry and the packing density of the positive electrode composite layer were each 65.0 g / cm. 2 , 2.8gcm -3 It was.

[0061] [Water-repellent non-aqueous electrolyte] An electrolyte solution was prepared by dissolving 1M LITFSI in a mixed solution of fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) in a volume ratio of 9:1. Next, 4% by mass of polymethyl methacrylate (PMMA) and 8% by mass of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) were prepared and dissolved in a solvent consisting of 10 times the amount of THF (tetrahydrofuran) and 10 times the amount of acetone relative to the amount of PMMA. The electrolyte solution was then mixed with this solution to prepare a precursor solution for the nonaqueous electrolyte.

[0062] Next, an assembly was prepared by wrapping a separator around a negative electrode with a negative electrode lead attached, and the assembly was immersed in a precursor solution of a non-aqueous electrolyte and then dried at 60°C for 1 hour, so that the precursor solution coated on the assembly became a non-aqueous solid electrolyte.

[0063] [Aqueous solid electrolyte] LITFSI, LIBETI, and water were mixed in a molar ratio of 0.7:0.3:2.0, and polyvinyl alcohol (PVA) was dissolved in the resulting solution of LITFSI and LIBETI in water to a concentration of 9% by mass to prepare a precursor solution for the aqueous solid electrolyte.

[0064] Next, the surface of the positive electrode composite layer of the positive electrode to which the positive electrode lead was attached was coated with a precursor solution of the aqueous solid electrolyte, and then dried at room temperature for 10 minutes, thereby converting the precursor solution coated on the positive electrode composite layer into an aqueous solid electrolyte.

[0065] [Test cell] The assembly coated with the non-aqueous electrolyte and the positive electrode coated with the aqueous solid electrolyte were housed in a battery case as shown in FIG. 1 to prepare a test cell.

[0066] <Example 2> Instead of lithium iron phosphate, Li, Ni, Co, Al-containing transition metal oxide (NCA, composition: LiNi 0.92 Co 0.05 Al 0.03A test cell was fabricated in the same manner as in Example 1, except that O2 was used. The packing density of the positive electrode mixture layer was 40.0 g / m 2 , 2.6gcm -3 It was.

[0067] <Comparative Example 1> A test cell was produced in the same manner as in Example 1, except that an aqueous liquid electrolyte was used in place of the aqueous solid electrolyte, in which LITFSI, LIBETI, and water were mixed at a molar ratio of 0.7:0.3:2.0 to dissolve LITFSI and LIBETI in water, and the positive electrode was immersed in this aqueous liquid electrolyte and housed in a battery case.

[0068] <Comparative Example 2> The assembly of Example 1 (a separator wrapped around a negative electrode with a negative electrode lead attached), the positive electrode with a positive electrode lead attached of Example 1, and the aqueous liquid electrolyte of Comparative Example 1 were housed in a battery case to prepare a test cell.

[0069] <Comparative Example 3> An electrolyte solution was prepared by mixing LITFSI, dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and water in a molar ratio of 1.0:0.2:0.2:1.5. The assembly of Example 1, the positive electrode with the positive electrode lead of Example 1 attached, and the aqueous liquid electrolyte of Comparative Example 3 were placed in a battery case to prepare a test cell.

[0070] <Comparative Example 4> A test cell was produced in the same manner as in Example 2, except that an aqueous liquid electrolyte was used in place of the aqueous solid electrolyte, in which LITFSI, LIBETI, and water were mixed at a molar ratio of 0.7:0.3:2.0 to dissolve LITFSI and LIBETI in water, and the positive electrode was immersed in this aqueous liquid electrolyte and housed in a battery case.

[0071] The test cells of each example and comparative example were charged to 3.7 V at 0.2 C and discharged to 2.0 V at 0.2 + 0.05 C, and the initial charge and discharge capacities were measured. In the case of Example 2, the cells were charged to 4.2 V. The initial charge / discharge efficiency (%) was calculated using the following formula. The results are shown in Table 1. Initial charge / discharge efficiency (%) = (initial discharge capacity) / (initial charge capacity) x 100

[0072] [Table 1]

[0073] Examples 1 and 2 had higher initial charge-discharge efficiencies than Comparative Examples 1 to 3. This is thought to be because coating the nonaqueous electrolyte prevented contact between water and the charging negative electrode, preventing the reductive decomposition of water (a side reaction). From these results, it can be said that the initial charge-discharge efficiency can be improved by using a lithium ion secondary battery in which the aqueous solid electrolyte is in contact with only the positive electrode out of the negative and positive electrodes, and the nonaqueous electrolyte is in contact with at least the negative electrode out of the negative and positive electrodes. [Explanation of symbols]

[0074] 1. Lithium-ion secondary battery 10 positive electrode 12 Negative electrode 14 Separator 16 Water-based solid electrolyte 18 Nonaqueous electrolytes 20 Positive lead 22 Negative lead 24 Battery case 26 Positive electrode current collector 28 Positive electrode composite layer 30 Negative electrode current collector 32 Negative electrode composite layer

Claims

1. a negative electrode, a positive electrode, a water-repellent non-aqueous electrolyte containing a lithium salt, and a solid electrolyte containing a lithium salt; the solid electrolyte is in contact with only the positive electrode of the negative electrode and the positive electrode; the non-aqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode, and the solid electrolyte and the non-aqueous electrolyte are also in contact with each other; The lithium ion secondary battery includes the solid electrolyte, the lithium salt, a solvent containing water, and a matrix polymer.

2. 2. The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte is a non-aqueous solid electrolyte.

3. The lithium ion secondary battery according to claim 2 , wherein the non-aqueous solid electrolyte contains the lithium salt, an organic solvent, and a matrix polymer.

4. the organic solvent includes a cyclic organic solvent, 4. The lithium ion secondary battery according to claim 3, wherein the cyclic organic solvent accounts for 80% by volume or more of the total volume of the organic solvent.

5. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode has a positive electrode active material containing a lithium-containing transition metal oxide, and the lithium-containing transition metal oxide contains at least one element of Ni, Co, Mn, and Al.

6. 6. The lithium-ion secondary battery according to claim 5, wherein elements contained in the lithium-containing transition metal oxide include a Ni element, a Co element, two elements of Ni and Mn, three elements of Ni, Co and Mn, or three elements of Ni, Co and Al.

7. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the negative electrode has a negative electrode active material containing a carbon material.

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