lithium-ion secondary batteries

Abstract

A lithium-ion secondary battery comprises: a negative electrode, a positive electrode, a water-repellent non-aqueous electrolyte containing a lithium salt, and an aqueous solid electrolyte containing a lithium salt, wherein the aqueous solid electrolyte contacts only the positive electrode of the negative electrode and the positive electrode, and the non-aqueous electrolyte contacts at least the negative electrode of the positive electrode.

Description

Technical Field

[0001] The present invention relates to a lithium ion secondary battery. Background Art

[0002] Lithium-ion secondary batteries, which consist of a positive electrode, a negative electrode, and an electrolyte, and are charged and discharged by the movement of lithium ions between the positive and negative electrodes, are widely used as high-output, high-energy-density secondary batteries. Conventional secondary batteries typically use organic solvent-based electrolytes to achieve high energy density.

[0003] However, organic solvents are generally flammable, making safety a key issue. Furthermore, organic solvents have lower ion conductivity than aqueous solutions, leading to insufficient rapid charge and discharge characteristics.

[0004] In view of such problems, research is underway on secondary batteries using aqueous electrolytes containing water. For example, Patent Document 1 proposes a lithium-ion secondary battery using an aqueous solution containing a high concentration of an alkali salt as an aqueous liquid electrolyte. Furthermore, Patent Document 2 proposes a lithium-ion secondary battery comprising: a negative electrode filled with a non-aqueous solid electrolyte; a positive electrode; a separator sandwiched between the negative and positive electrodes and filled with a non-aqueous solid electrolyte; and an aqueous liquid electrolyte.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 6423453

[0008] Patent Document 2: Japanese Patent Application Publication No. 2018-198131 Summary of the Invention

[0009] Conventional lithium-ion secondary batteries having aqueous electrolytes have problems such as low initial charge and discharge efficiency.

[0010] Therefore, an object of the present invention is to provide a lithium ion secondary battery capable of improving initial charge and discharge efficiency while using an aqueous electrolyte.

[0011] One embodiment of the present invention is a lithium-ion secondary battery comprising: a negative electrode, a positive electrode, a water-repellent non-aqueous electrolyte containing a lithium salt, and an aqueous solid electrolyte containing a lithium salt, wherein the aqueous solid electrolyte is in contact only with the positive electrode of the negative electrode and the positive electrode, and the non-aqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode.

[0012] According to the present invention, a lithium ion secondary battery capable of improving initial charge and discharge efficiency can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a schematic cross-sectional view showing an example of the lithium ion secondary battery according to this embodiment. DETAILED DESCRIPTION

[0014] A lithium-ion secondary battery according to one embodiment of the present invention comprises: a negative electrode, a positive electrode, a water-repellent non-aqueous electrolyte containing a lithium salt, and an aqueous solid electrolyte containing a lithium salt, wherein the aqueous solid electrolyte is in contact only with the positive electrode of the negative electrode and the positive electrode, and the non-aqueous 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 invention, the initial charge and discharge efficiency can be improved. Although the mechanism of this effect is not entirely clear, it can be inferred as follows.

[0015] Typically, for lithium ion secondary batteries using aqueous electrolytes containing water, since the reduction decomposition (i.e., side reaction) of the water in the aqueous electrolyte is carried out on the negative electrode, the charging reaction of the negative electrode is hindered, and the initial charge and discharge efficiency is reduced. However, in the present invention, an aqueous solid electrolyte is used as an aqueous electrolyte, and the aqueous solid electrolyte is applied to the positive electrode, etc., and is only in contact with the positive electrode, so that compared with the case of using an aqueous liquid electrolyte, it is possible to suppress the backflow of water to the negative electrode side. In addition, a water-repellent non-aqueous electrolyte contacts the negative electrode, so even if water flows back to the negative electrode side, the contact of water with the negative electrode surface can be suppressed by a water-repellent non-aqueous electrolyte. Thus, the side reaction of the water in the negative electrode can be suppressed, and therefore the charge and discharge reaction of the negative electrode can be carried out, improving the initial charge and discharge efficiency.

[0016] Hereinafter, an example of an embodiment of the lithium ion secondary battery of the present invention will be described in detail.

[0017] Figure 1 : is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to this embodiment. Figure 1 The lithium-ion secondary battery 1 shown includes a positive electrode 10, a negative electrode 12, a separator 14, an aqueous solid electrolyte 16, a water-repellent non-aqueous electrolyte 18, a positive electrode lead 20, a negative electrode lead 22, and a battery case 24 for accommodating them. The aqueous solid electrolyte 16 is applied to the positive electrode 10 and is in contact only with the positive electrode 10 of the positive electrode 10 and the negative electrode 12. The separator 14 is wound around the negative electrode 12. The water-repellent non-aqueous electrolyte 18 is applied to the negative electrode 12 and the separator 14 and is in contact with the negative electrode 12 and the separator 14. Figure 1 In the embodiment, 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 may be in contact with both the positive electrode 10 and the negative electrode 12 .

[0018] The positive electrode 10 includes a positive electrode current collector 26 and a positive electrode composite material 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 to the outside of the battery case 24.

[0019] As the positive electrode current collector 26, a foil of a metal that is electrochemically and chemically stable within the potential range of the positive electrode 10, or a thin film having the metal disposed on the surface thereof, can be used. The form of the positive electrode current collector 26 is not particularly limited; for example, a porous body such as a mesh, a punched sheet, or a porous metal mesh of the metal can also be used. Examples of materials for the positive electrode current collector 26 include stainless steel, Al, Al alloys, and Ti. From the perspectives of current collection performance and mechanical strength, the thickness of the positive electrode current collector 26 is preferably, for example, not less than 3 μm and not more than 50 μm.

[0020] The positive electrode composite material layer 28 contains a positive electrode active material. Furthermore, the positive electrode composite material layer 28 may also contain a binder, a conductive material, and the like. The positive electrode 10 can be produced by coating a positive electrode composite material slurry containing, for example, a positive electrode active material, a binder, a conductive material, and the like on the positive electrode current collector 26, drying and rolling the coating to form the positive electrode composite material layer 28 on the positive electrode current collector 26.

[0021] As the positive electrode active material, for example, lithium-containing transition metal oxides containing lithium (Li) and transition metal elements such as cobalt (Co), manganese (Mn) and nickel (Ni) can be cited. As the positive electrode active material, in addition, transition metal sulfides, metal oxides, lithium iron phosphate (LiFePO4), lithium iron pyrophosphate (Li2FeP2O7) and other lithium-containing polyanionic compounds containing one or more transition metals, sulfur compounds (Li2S), oxygen, lithium oxide and other oxygen-containing metal salts can be cited. As the positive electrode active material, for example, from the viewpoint of charge and discharge efficiency, lithium-containing transition metal oxides are preferred.

[0022] Regarding lithium-containing transition metal oxides, from the viewpoints of charge-discharge efficiency and the like, it is preferable to contain at least any one of Ni, Co, Mn, and aluminum (Al). Among these elements, it is preferable to contain at least the Ni element, at least the Co element, at least the two elements of Ni and Mn, at least the three elements of Ni, Co, and Mn, or at least the three elements of Ni, Co, and Al. The lithium-containing transition metal oxide may also contain other additive elements other than these elements. For example, it may also contain 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), etc.

[0023] As a specific example of the lithium-containing transition metal oxide, for example, 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 y O4, 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). The lithium-containing transition metal oxide may be used alone or in combination of multiple kinds. From the viewpoint of achieving a high capacity, the lithium-containing transition metal oxide preferably contains 80 mol% or more of Ni relative to the total amount of transition metals other than lithium. Further, from the viewpoint of the stability of the crystal structure, it is more preferable that 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).

[0024] In addition, the lithium-containing transition metal oxide can be a lithium-excess system transition metal oxide, a lithium-containing transition metal halogen oxide, etc. The lithium-excess system transition metal oxide is represented, for example, by the general formula Li 1+x Me 1-x O2(0 < x). In addition, the lithium-containing transition metal halogen oxide only needs to be a lithium-containing transition metal oxide containing a halogen atom, and there is no particular limitation. For example, from the viewpoints of the structural stability of the lithium-containing transition metal oxide, etc., it is preferably included: a lithium-containing transition metal oxide containing a fluorine atom.

[0025] As the conductive material, a known conductive material that improves the conductivity of the positive electrode composite material layer 28 can be used. For example, carbon materials such as carbon black, acetylene black, Ketjen black, graphite, carbon nanofibers, carbon nanotubes, and graphene can be cited. As the binder material, a known binder material that maintains a good contact state between the positive electrode active material and the conductive material and improves the adhesion of the positive electrode active material, etc., to the surface of the positive electrode current collector 26 can be used. For example, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), etc. can be cited.

[0026] The negative electrode 12 has: a negative electrode current collector 30 and a negative electrode composite material layer 32 disposed on the negative electrode current collector 30. The 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 in such a manner that the tip of the negative electrode lead 22 protrudes to the outside of the battery case 24.

[0027] As the negative electrode current collector 30, a foil of a metal that is electrochemically and chemically stable within the potential range of the negative electrode 12, and a thin film having the metal disposed on the surface layer, etc. can be used. There is no particular limitation on the form of the negative electrode current collector 30. For example, a porous body such as a net-like body, a perforated thin plate, or a porous metal net of the metal can also be used. As the material of the negative electrode current collector 30, for example, Al, Ti, Mg, Zn, Pb, Sn, Zr, In, etc. can be cited. They can be used alone as one kind, or can be an alloy of two or more kinds, as long as they are composed of a material having at least one kind as the main component. In addition, in the case of containing two or more elements, alloying is not necessarily required. From the viewpoints of current collection property, mechanical strength, etc., the thickness of the negative electrode current collector 30 is preferably, for example, 3 μm or more and 50 μm or less.

[0028] The negative electrode composite material layer 32 contains a negative electrode active material. Furthermore, the negative electrode composite material layer 32 may also contain a binder, a conductive material, and the like. The conductive and binder materials can be the same as those used for the positive electrode 10 . The negative electrode 12 can be produced by coating a negative electrode composite material slurry containing, for example, a negative electrode active material, a binder, and a conductive material onto the negative electrode current collector 30 , drying the coating, and rolling the resulting film to form the negative electrode composite material layer 32 on the negative electrode current collector 30 .

[0029] The negative electrode active material is not particularly limited as long as it is a material that can be used for the negative electrode active material of an existing lithium ion secondary battery. For example, carbon materials such as artificial graphite, natural graphite, hard carbon, soft carbon, carbon nanotubes, activated carbon, metals such as Li, Si, Sn, alloys, oxides, metal sulfides, and metal nitrides can be cited. For example, as Li alloys, lithium aluminum alloys, lithium tin alloys, lithium lead alloys, lithium silicon alloys, etc. can be cited. In addition, as metal oxides having Li, for example, lithium titanate (Li4Ti5O 12 In addition, examples of metal nitrides containing Li include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride. In addition, sulfur compounds can also be exemplified.

[0030] Typically, in lithium-ion secondary batteries using aqueous electrolytes, if a carbon material is used as the negative electrode active material, the side reaction of water has a significant impact, significantly reducing the initial charge and discharge efficiency. However, according to the lithium-ion secondary battery of this embodiment, since contact between water and the negative electrode is suppressed, even if a carbon material is used as the negative electrode active material, the initial charge and discharge efficiency can be improved.

[0031] The aqueous solid electrolyte 16 containing a lithium salt is, for example, a solid electrolyte formed by a composite of a lithium salt, an aqueous solvent, and a matrix polymer. The aqueous solid electrolyte 16 is obtained, for example, by dissolving a lithium salt in an aqueous solvent, further mixing or dissolving a matrix polymer to obtain a precursor solution, and then drying the precursor solution.

[0032] The aqueous solvent is a solvent containing water, and may be water alone or contain water and a solvent other than water. The content of water relative to the total amount of the aqueous solvent is preferably 50% by volume or more, for example, from the perspective of improving the safety of the lithium ion secondary battery.

[0033] Furthermore, the molar ratio of lithium salt to water relative to the amount of water 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. When the amount of water relative to the amount of lithium salt contained in the aqueous solid electrolyte 16 is within this range, the potential window of the aqueous solid electrolyte 16 may be expanded compared to a case outside this range, for example, thereby further increasing the voltage applied to the lithium-ion secondary battery 1.

[0034] As solvents other than water contained in aqueous solvents, examples include organic solvents such as esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. In addition, halogen-substituted solvents in which at least a portion of the hydrogen atoms in these solvents are replaced with halogen atoms such as fluorine may also be used. Specifically, from the perspective of improving the battery characteristics of lithium-ion secondary batteries, cyclic organic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, and butylene carbonate, chain organic carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, and organic carbonates such as fluorinated organic carbonates containing fluorine as a constituent element such as fluoroethylene carbonate, fluorodimethyl carbonate, and fluoromethyl propionate are preferred. In particular, in the aforementioned examples, cyclic organic carbonates and fluorinated organic carbonates containing fluorine as a constituent element are preferred, for example, from the perspective of suppressing the self-discharge of the battery. In addition, 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.

[0035] The amount of the organic carbonate relative to the lithium salt contained in the aqueous solid electrolyte 16 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, in terms of the molar ratio of lithium salt to organic carbonate. When the amount of the organic carbonate relative to the lithium salt is within the aforementioned range, the battery characteristics of the lithium ion secondary battery can sometimes be improved compared to cases outside the aforementioned range.

[0036] Any lithium salt can be used as long as it is a compound that dissolves in an aqueous solvent and dissociates, allowing lithium ions to be present 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.

[0037] Examples of the organic anions constituting the lithium salt include anions represented by the following general formulae (i) to (vi).

[0038] (R 1 SO2)(R 2 SO2)N - (i)

[0039] (R 1 、R 2 R are independently selected from alkyl or halogenated alkyl. 1 and R 2 may be bonded to each other to form a ring.)

[0040] R 3 SO3 - (ii)

[0041] (R 3 is selected from alkyl or haloalkyl.)

[0042] R 4 CO2 - (iii)

[0043] (R 4 is selected from alkyl or haloalkyl.)

[0044] (R 5 SO2)3C - (iv)

[0045] (R 5 is selected from alkyl or haloalkyl.)

[0046] [(R 6 SO2)N(SO2)N(R 7 SO2)] 2- (v)

[0047] (R 6 、R 7 is selected from alkyl or haloalkyl.)

[0048] [(R 8 SO2)N(CO)N(R 9 SO2)] 2- (vi)

[0049] (R 8 、R 9 is selected from alkyl or haloalkyl.)

[0050] In the aforementioned general formulae (i) to (vi), the number of carbon atoms in the alkyl or haloalkyl group is preferably 1 to 6, more preferably 1 to 3, and even more preferably 1 to 2. The halogen in the haloalkyl group is preferably fluorine. The number of halogen substitutions in the haloalkyl group is equal to or less than the number of hydrogen atoms in the original alkyl group.

[0051] R 1 ~R 9 Each of them is, for example, a group represented by the following general formula (vii).

[0052] C n H a Fb Cl c Br d I e (vii)

[0053] (n is an integer greater than or equal to 1, a, b, c, d, and e are integers greater than or equal to 0, and the following is true: 2n + 1 = a + b + c + d + e.)

[0054] 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)] - ) etc. As a specific example of the organic anion represented by the general formula (ii), for example, CF3SO3 - 、C2F5SO3 - As a specific example of the organic anion represented by the general formula (iii), for example, CF3CO2 - 、C2F5CO2 - As a specific example of the organic anion represented by the general formula (iv), for example, tris(trifluoromethanesulfonyl)carbonate ([(CF3SO2)3C] - ), tris(perfluoroethanesulfonyl)carbonate ([(C2F5SO2)3C] - ) etc. 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- ) etc. 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- )wait.

[0055] Examples of organic anions other than those of the general formulae (i) to (vi) include bis(1,2-benzenediolate (2-)-O,O')boric acid, bis(2,3-naphthalenediolate (2-)-O,O')boric acid, bis(2,2'-biphenylenediolate (2-)-O,O')boric acid, and bis(5-fluoro-2-olate-1-benzenesulfonic acid-O,O')boric acid.

[0056] As the anion constituting the lithium salt, an imide anion is preferred. Preferred specific examples of the imide anion include, in addition to the imide anions exemplified as the organic anions represented by the general formula (i), bis(fluorosulfonyl)imide (FSI; [N(FSO2)2] - ), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTI; [N(FSO2)(CF3SO2)] - )wait.

[0057] For lithium salts having lithium ions and imide anions, from the aspects of being able to effectively suppress the self-discharge of the battery, for example, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(perfluoroethanesulfonyl)imide (LiBETI), lithium (perfluoroethanesulfonyl) (trifluoromethanesulfonyl) imide, lithium bis(fluorosulfonyl) imide (LiFSI), lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTI), more preferably lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). It should be noted that these can be used alone or in combination of two or more.

[0058] 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, [(C2F 5SO2)N(CO)N(C2F5SO2)]Li2, [(C2F5SO2)N(CO)N(CF3SO2)]Li2, lithium bis(1,2-benzenediolate (2-)-O,O')borate, lithium bis(2,3-naphthalenediolate (2-)-O,O')borate, lithium bis(2,2'-biphenylediolate (2-)-O,O')borate, lithium bis(5-fluoro-2-olate-1-benzenesulfonate-O,O')borate, lithium perchlorate (LiClO4), lithium chloride (LiCl), lithium bromide (LiBr), lithium hydroxide (LiOH), lithium nitrate (LiNO3), lithium sulfate (Li2SO4), lithium sulfide (Li2S), lithium hydroxide (LiOH), etc. These may be used alone or in combination of two or more.

[0059] Examples of the matrix polymer include polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), and polymethyl methacrylate (PMMA). Alternatively, a polymer obtained by mixing acrylonitrile and acrylic acid as monomers and thermally polymerizing the mixture to form a high molecular weight polymer may be used.

[0060] The content of the matrix polymer is 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. Within this range, for example, the aqueous solid electrolyte 16 can be easily gelled or solidified.

[0061] The aqueous solid electrolyte 16 may also be coated on the entire positive electrode 10, but Figure 1 As shown, the aqueous solid electrolyte 16 may be applied only to at least the positive electrode composite material layer 28. For example, the aqueous solid electrolyte 16 is obtained by dissolving a lithium salt in an aqueous solvent, further mixing or dissolving a matrix polymer to obtain a precursor solution, applying the precursor solution to the positive electrode 10 or immersing the positive electrode 10 in the precursor solution, and then drying the precursor solution.

[0062] The water-repellent non-aqueous electrolyte 18 containing a lithium salt can be a non-aqueous solid electrolyte or a non-aqueous liquid electrolyte. For example, if it is a non-aqueous solid electrolyte, it is a solid electrolyte formed by a composite of a lithium salt, an organic solvent, and a matrix polymer. If it is a non-aqueous liquid electrolyte, it is a liquid electrolyte in which an organic solvent is dissolved in a lithium salt. Since the non-aqueous solid electrolyte coats the entire negative electrode, it is more preferred from the perspective of suppressing water penetration. For example, the non-aqueous solid electrolyte is prepared by dissolving a lithium salt in an organic solvent, further mixing or dissolving a matrix polymer to obtain a precursor solution, and heating and drying the precursor solution.

[0063] Examples of the organic solvent include known organic solvents used in conventional non-aqueous secondary batteries, such as the aforementioned esters, ethers, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Among them, esters, ethers, nitriles, amides, and mixed solvents of two or more thereof are preferably used from the perspective of improving battery characteristics.

[0064] Examples of esters include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl 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.

[0065] Examples of ethers include cyclic ethers such as 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, and crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, Dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl 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, diethanol diethyl ether, diethanol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether and other chain ethers, etc.

[0066] The organic solvent preferably contains a halogen substituted body obtained by replacing the hydrogen of the aforementioned various solvents with a halogen atom such as fluorine. In particular, at least one of fluorinated cyclic carbonates, fluorinated chain carbonates, and fluorinated ethers is preferred. As an ideal example of fluorinated cyclic carbonates, 4-fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4-difluoroethylene carbonate, 4,4,5-trifluoroethylene carbonate, 4,4,5,5-tetrafluoroethylene carbonate, etc. can be enumerated. As an ideal example of fluorinated chain carbonates, 2,2,2-trifluoroethyl acetate, 3,3,3-trifluoromethyl propionate, pentafluoromethyl propionate, etc. can be enumerated. As an ideal example of fluorinated ethers, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, etc. can be enumerated.

[0067] The organic solvent preferably contains a cyclic organic solvent such as a cyclic carbonate, for example, from the perspective of suppressing a decrease in lithium ion conductivity of the non-aqueous 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.

[0068] The lithium salt may be a well-known lithium salt used in conventional non-aqueous secondary batteries, for example, LiPF6, LiBF4, LiAsF6, LiClO4, LiCF3SO3, LiN(FSO2)2, LiN(ClF 2l+1 SO2)(C m F 2m+1 SO2) (l, m are integers greater than 1), LiC (C p F 2p+1 SO2)(C q F 2q+1 SO2)(C r F 2r+1 SO2) (p, q, r are integers greater than 1), Li[B(C2O4)2] (lithium bis(oxalatoborate (LiBOB)), Li B(C2O4)F2], Li[P(C2O4)F4], Li[P(C2O4)2F2], LiPO2F2, etc. In addition, the lithium salt may be, for example, the lithium salts exemplified above for use in the aqueous solid electrolyte 16.

[0069] The matrix polymer can be the same as that used for the aqueous solid electrolyte 16. The content of the matrix polymer is 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 non-aqueous electrolyte 18. Within this range, for example, the solidification of the non-aqueous electrolyte 18 becomes easier.

[0070] The water-repellent non-aqueous electrolyte 18 preferably has a solubility of 2 g or less in 100 g of water at 25° C., for example, from the perspective of effectively suppressing contact between the negative electrode 12 and water. The water repellency of the non-aqueous electrolyte 18 can be improved, for example, by increasing the proportion of an organic solvent having a water-repellent substituent or a fluorinated organic solvent.

[0071] The non-aqueous electrolyte 18 may be applied only on the surface of the negative electrode composite material layer 32, but the side reaction of water also occurs on the negative electrode current collector 30 and the negative electrode lead 22. Figure 1 As shown in FIG. 1 , it is preferably applied to the entire negative electrode 12, and more preferably, the negative electrode lead 22 (excluding the portion protruding from the battery case 24) is also coated. In addition, from the perspective of effectively suppressing the backflow of water to the negative electrode 12, as shown in FIG. Figure 1 As shown, the non-aqueous electrolyte 18 is preferably also applied to the separator 14. The non-aqueous electrolyte 18 can be obtained by, for example, applying a precursor solution prepared by dissolving a lithium salt in an organic solvent and further mixing or dissolving a matrix polymer to the negative electrode 12, the separator 14, etc., or by immersing an assembly obtained by winding the separator 14 around the negative electrode 12 attached with the negative electrode lead 22 in the precursor solution, applying the precursor solution to the negative electrode 12, etc., and then heating the assembly.

[0072] The separator 14 is not particularly limited as long as it has the function of allowing lithium ions to pass through and electrically separating the positive electrode 10 and the negative electrode 12. For example, a porous sheet composed of a resin, an inorganic material, etc. can be used. Specific examples of porous sheets include microporous films, woven fabrics, non-woven fabrics, etc. As materials for the separator 14, olefin resins such as polyethylene and polypropylene, polyamide, polyamide-imide, cellulose, etc. can be mentioned. As inorganic materials constituting the separator 14, glasses and ceramics such as borosilicate glass, silicon dioxide, aluminum oxide, and titanium dioxide can be mentioned. The separator 14 can be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. In addition, it can also be a multilayer separator 14 comprising a polyethylene layer and a polypropylene layer, or a separator 14 coated with an aromatic polyamide resin, ceramic, or the like.

[0073] From the perspective of effectively suppressing the backflow of water to the negative electrode 12 and the side reactions of water in the negative electrode 12, the separator 14 is preferably wound around the negative electrode 12 and coated with the water-repellent non-aqueous electrolyte 18 together with the negative electrode 12, but the present invention is not limited to this. For example, the separator 14 may be simply disposed between the positive electrode 10 and the negative electrode 12, or may be coated on the aqueous solid electrolyte 16. Moreover, even in such a configuration, the side reactions of water can be suppressed, thereby suppressing the reduction in the initial charge and discharge efficiency.

[0074] The battery case 24 may be made of metal, resin, or laminated film. 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 laminated films include multilayer films formed by coating stainless steel foil with a resin film. Examples of materials for the resin film include polypropylene, polyethylene, nylon, and polyethylene terephthalate.

[0075] 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.

[0076] <Example>

[0077] Hereinafter, the present invention will be further described with reference to Examples, but the present invention is not limited to these Examples.

[0078] <Example 1>

[0079] [negative electrode]

[0080] Graphite as the negative electrode active material and PVDF as the binding material are mixed in N-methyl-2-pyrrolidone (NMP) at a solid content mass ratio of 96:4 to prepare a negative electrode composite material slurry. Next, the negative electrode composite material slurry is applied to the negative electrode collector composed of copper foil, and after the coating film is dried, it is rolled by a rolling roller. Then, it is cut into the specified electrode size to obtain a negative electrode. The coating amount of the negative electrode composite material slurry and the filling density of the negative electrode composite material layer are 32.3g / m 2 、1.0gcm -3 .

[0081] [positive electrode]

[0082] Lithium iron phosphate (LFP, composition: LiFePO4) as the positive electrode active material, carbon black as the conductive material, and PVDF as the binding material are mixed in NMP at a mass ratio of 94:3:3 to prepare a positive electrode composite material slurry. Next, the positive electrode composite material slurry is applied to the positive electrode collector composed of Ti foil, and after the coating is dried, it is rolled by a rolling roller. Then, it is cut into the specified electrode size to obtain the positive electrode. The coating amount of the positive electrode composite material slurry and the filling density of the positive electrode composite material layer are 65.0 g / cm 2 、2.8gcm -3 .

[0083] [Water-repellent non-aqueous electrolyte]

[0084] The electrolyte solution was prepared by dissolving 1M LITFSI in a mixture of fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) at 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 relative to the electrolyte solution and dissolved in a solvent consisting of 10 times the amount of THF (tetrahydrofuran) and 10 times the amount of acetone as PMMA. This solution was then mixed with the electrolyte to prepare a non-aqueous electrolyte precursor solution.

[0085] Next, an assembly was prepared by winding a separator around a negative electrode attached with a negative electrode lead. The assembly was immersed in a non-aqueous electrolyte precursor solution and then dried at 60°C for 1 hour. The precursor solution applied to the assembly was converted into a non-aqueous solid electrolyte.

[0086] [Aqueous solid electrolyte]

[0087] LITFSI, LIBETI, and water were mixed at a molar ratio of 0.7:0.3:2.0 to obtain a solution in which LITFSI and LIBETI were dissolved in water. Polyvinyl alcohol (PVA) was dissolved in this solution at a concentration of 9% by mass to prepare an aqueous solid electrolyte precursor solution.

[0088] Next, the precursor solution of the aqueous solid electrolyte was applied to the surface of the positive electrode composite layer of the positive electrode attached with the positive electrode lead, and then dried at room temperature for 10 minutes to prepare the precursor solution applied to the positive electrode composite layer as the aqueous solid electrolyte.

[0089] [Test battery cell]

[0090] like Figure 1 As shown, the assembly coated with the non-aqueous electrolyte and the positive electrode coated with the aqueous solid electrolyte were housed in a battery case to produce a test battery cell.

[0091] <Example 2>

[0092] Using transition metal oxides containing Li, Ni, Co, and Al (NCA, composition: LiNi 0.92 Co 0.05 Al 0.03 A test cell was prepared in the same manner as in Example 1 except that lithium iron phosphate was replaced with lithium ion battery (O2). The packing density of the positive electrode composite material layer was 40.0 g / m 2 、2.6gcm -3 .

[0093] <Comparative Example 1>

[0094] A test battery cell was produced in the same manner as in Example 1, except that an aqueous liquid electrolyte, obtained by mixing LITFSI, LIBETI, and water at a molar ratio of 0.7:0.3:2.0, with LITFSI and LIBETI dissolved in water, was used instead of the aqueous solid electrolyte. The positive electrode was immersed in the aqueous liquid electrolyte and housed in a battery case.

[0095] <Comparative Example 2>

[0096] The assembly of Example 1 (a separator wound around a negative electrode attached with a negative electrode lead), the positive electrode of Example 1 attached with a positive electrode lead, and the aqueous liquid electrolyte of Comparative Example 1 were housed in a battery case to prepare a test battery cell.

[0097] <Comparative Example 3>

[0098] An electrolyte solution was prepared by mixing LITFSI, dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and water at a molar ratio of 1.0:0.2:0.2:1.5. The assembly of Example 1, the positive electrode with a positive lead from Example 1, and the aqueous liquid electrolyte from Comparative Example 3 were then housed in a battery case to produce a test battery cell.

[0099] <Comparative Example 4>

[0100] A test battery cell was produced in the same manner as in Example 2, except that an aqueous liquid electrolyte, obtained by mixing LITFSI, LIBETI, and water at a molar ratio of 0.7:0.3:2.0 and dissolving LITFSI and LIBETI in water, was used instead of the aqueous solid electrolyte. The positive electrode was immersed in the aqueous liquid electrolyte and housed in a battery case.

[0101] The test battery cells of each embodiment and comparative example were charged to 3.7V at 0.2C and discharged to 2.0V at 0.2+0.05C, and the initial charge capacity and discharge capacity were measured. In the case of Example 2, the cells were charged to 4.2V. Thereafter, the initial charge and discharge efficiency (%) was calculated using the following formula. The results are shown in Table 1. Initial charge and discharge efficiency (%) = (initial discharge capacity) / (initial charge capacity) × 100

[0102] [Table 1]

[0103]

[0104] Examples 1 and 2 showed higher initial charge-discharge efficiencies than Comparative Examples 1 to 3. This is believed to be because the application of the non-aqueous electrolyte cuts off contact between water and the charged negative electrode, preventing the reduction and decomposition of water (a side reaction). This result suggests that using a lithium-ion secondary battery in which the aqueous solid electrolyte contacts only the positive electrode of the negative electrode and the non-aqueous electrolyte contacts at least the negative electrode of the positive electrode can improve initial charge-discharge efficiency.

[0105] Description of Reference Signs

[0106] 1Lithium-ion secondary battery

[0107] 10 positive electrode

[0108] 12 negative electrode

[0109] 14 dividers

[0110] 16 Aqueous solid electrolyte

[0111] 18 non-aqueous electrolytes

[0112] 20 positive lead

[0113] 22 negative lead

[0114] 24 battery housing

[0115] 26 positive electrode collector

[0116] 28 positive electrode composite material layer

[0117] 30 negative electrode collector

[0118] 32 negative electrode composite material layer

Claims

1. A lithium ion secondary battery comprising: a negative electrode, a positive electrode, a water-repellent non-aqueous electrolyte containing a lithium salt, and a solid electrolyte, The solid electrolyte comprises: a lithium salt, a solvent comprising water, and a matrix polymer. The solid electrolyte is in contact with only the positive electrode of the negative electrode and the positive electrode, The nonaqueous electrolyte is in contact with at least the negative electrode of the negative electrode and the positive electrode.

2. The lithium ion secondary battery according to claim 1, wherein The content of the matrix polymer is 1% by mass or more and 15.0% by mass or less relative to the total amount of the solid electrolyte.

3. The lithium ion secondary battery according to claim 2, wherein The solid electrolyte is gelled or solidified.

4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein The non-aqueous electrolyte is a non-aqueous solid electrolyte.

5. The lithium ion secondary battery according to claim 4, wherein The non-aqueous solid electrolyte includes: the lithium salt, an organic solvent, and a matrix polymer.

6. The lithium ion secondary battery according to claim 5, wherein The organic solvent comprises a cyclic organic solvent, The cyclic organic solvent is contained in an amount of 80% by volume or more based on the total volume of the organic solvent.

7. The lithium ion secondary battery according to any one of claims 1 to 6, wherein The positive electrode has a positive electrode active material. The positive electrode active material includes a lithium-containing transition metal oxide. The lithium-containing transition metal oxide includes at least one element of Ni, Co, Mn, and Al.

8. The lithium ion secondary battery according to claim 7, wherein The elements contained in the lithium-containing transition metal oxide include: Ni, Co, two elements of Ni and Mn, three elements of Ni, Co and Mn, or three elements of Ni, Co and Al.

9. The lithium ion secondary battery according to any one of claims 1 to 8, wherein The negative electrode includes a negative electrode active material including a carbon material.

Images