Secondary battery

Abstract

Provided is a secondary battery capable of stably charging and discharging while suppressing corrosion of a lithium bis (fluorosulfonyl) imide to a current collector or the like. A lithium ion secondary battery having a positive electrode, a negative electrode, a separator, and an electrolyte solution, the electrolyte solution containing an acetamide derivative represented by formula (1) and lithium bis (fluorosulfonyl) imide. (In formula (1), R1 and R2 each independently represent an optionally substituted alkyl group or alkoxy group having 1-5 carbon atoms, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a fused ring. > # imgabs0 #

Description

Technical Field

[0001] The present invention relates to secondary batteries. Background Art

[0002] In secondary batteries, lithium bis(fluorosulfonyl)imide (LiFSI) is sometimes used as an electrolyte salt of an electrolytic solution. In such a case, the electrolytic solution sometimes corrodes metals such as aluminum used as a positive electrode current collector. Therefore, it is required to suppress the corrosion of LiFSI on the current collector and the like.

[0003] In Patent Document 1, it is described that an asymmetric borate ester, an asymmetric phosphate ester, etc. are added to an electrolytic solution for the purpose of suppressing the corrosion of LiFSI on a current collector and the like.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-504145 Summary of the Invention

[0007] Technical Problem to be Solved by the Invention

[0008] There is a desire for a secondary battery that uses LiFSI as an electrolyte salt of an electrolytic solution, can suppress the corrosion of a current collector and the like, and can stably perform charge and discharge.

[0009] The present invention has been made in view of the above technical problems, and an object thereof is to provide a secondary battery that can suppress the corrosion of lithium bis(fluorosulfonyl)imide on a current collector and the like and can stably perform charge and discharge.

[0010] Technical Solution for Solving the Technical Problem

[0011] The secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator, and an electrolytic solution, and the electrolytic solution contains an acetamide derivative represented by formula (1) and lithium bis(fluorosulfonyl)imide.

[0012] [Chemical Formula 1]

[0013]

[0014] (In formula (1), R1 and R2 each independently represent an alkyl group or an alkoxy group having 1 to 5 carbon atoms that may have a substituent, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a condensed ring.)

[0015] Effects of the Invention

[0016] According to the present invention, a secondary battery can be provided that can suppress corrosion of lithium bis(fluorosulfonyl)imide on a current collector or the like and can stably perform charge and discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 FIG. is a cross-sectional view showing an example of the secondary battery according to the present embodiment.

[0018] Figure 2 is Figure 1 an enlarged view of region A of.

[0019] Figure 3 FIG. is a cutaway view showing a different example of the secondary battery according to the present embodiment.

[0020] Figure 4 is Figure 3 a schematic view of a cross-section taken along line VI-VI of. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to this embodiment.

[0022] (Secondary Battery)

[0023] Figure 1 FIG. is a cross-sectional view showing an example of the secondary battery according to the present embodiment. Figure 1 The secondary battery 1 shown is a cylindrical lithium ion secondary battery. As Figure 1 shown, the secondary battery 1 includes a housing 10 and an electrode body 200.

[0024] The housing 10 is a case that houses the electrode body 200 and an electrolyte (not shown) inside. The housing 10 includes a battery can 11, a lid body 12, a thermistor element 13, a safety valve mechanism 14, a gasket 15, a positive electrode lead 16, a negative electrode lead 17, a center pin 19, and an insulating plate 18.

[0025] The battery can 11 is a cylindrical member that includes an end face that serves as the negative electrode of the secondary battery 1. That is, the battery can 11 is formed as a cylinder with one end face closed and the other end face open. The battery can 11 is a conductor and is made of, for example, iron (Fe) plated with nickel (Ni) on the surface.

[0026] The lid body 12 is a disk-shaped member that includes a protrusion that serves as the positive electrode of the secondary battery 1. The lid body 12 is provided on the open end face of the battery can 11. The lid body 12 is made of a conductor and is made of, for example, the same material as the battery can 11.

[0027] Here, in the following description, the direction in which the cylindrical portion of the battery can 11 extends is sometimes described as the longitudinal direction of the secondary battery 1. Additionally, in the following description, the positive electrode of the secondary battery 1 refers to the protrusion of the lid body 12, and the negative electrode of the secondary battery 1 refers to the enclosed end face of the battery can 11.

[0028] The thermistor element 13 is an element whose resistance increases as the temperature rises. The thermistor element 13 is disposed on the negative electrode side with respect to the lid body 12. When the secondary battery 1 becomes high temperature due to a short circuit or the like, the resistance of the thermistor element 13 increases, restricting the current.

[0029] The safety valve mechanism 14 is a mechanism whose shape changes according to the air pressure inside the housing 10. The safety valve mechanism 14 is disposed on the negative electrode side with respect to the thermistor element 13. The safety valve mechanism 14 is electrically connected to the lid body 12 via the thermistor element 13. The safety valve mechanism 14 has a protrusion on the negative electrode side, and in the case where the air pressure inside the housing 10 is normal, it contacts and is electrically connected to the positive electrode lead 16 via the protrusion. On the other hand, when the air pressure inside the housing 10 rises, the protrusion reverses toward the positive electrode side and separates from the positive electrode lead 16. Thereby, the positive electrode lead 16 and the lid body 12 are electrically disconnected.

[0030] The gasket 15 is an annular member that fixes the lid body 12, the thermistor element 13, and the safety valve mechanism 14 to the battery can 11. The gasket 15 is disposed on the open end face of the battery can 11. The gasket 15 makes the battery can 11 and the lid body 12 fit tightly, making the inside of the housing 10 airtight. The gasket 15 is an insulator.

[0031] The positive electrode lead 16 is a terminal connected to the positive electrode 210 of the electrode body 200 described later. The positive electrode lead 16 is electrically connected to the lid body 12 via the safety valve mechanism 14 and the thermistor element 13. The positive electrode lead 16 is a conductor, for example, made of aluminum.

[0032] The negative electrode lead 17 is a terminal connected to the negative electrode 220 of the electrode body 200 described later. The negative electrode lead 17 is electrically connected to the battery can 11. The negative electrode lead 17 is a conductor, for example, made of nickel.

[0033] The insulating plate 18 is an insulating plate-like member. One insulating plate 18 is provided on each of the positive electrode side and the negative electrode side of the secondary battery 1 so as to cover the electrode body 200 described later.

[0034] The center pin 19 is provided on the central axis of the electrode body 200. The center pin 19 is a linear member having a length in the longitudinal direction of the secondary battery 1. The material of the center pin 19 is not particularly limited, for example, it is metal.

[0035] Figure 2 is Figure 1 An enlarged view of region A of. AsFigure 2 As shown, the electrode body 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode body 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked with the separator 230 therebetween. In Figure 1 this example, the electrode body 200 is disposed inside the battery can 11 and has a structure wound around the center pin 19. In other words, in the electrode body 200, the positive electrode 210, the negative electrode 220, and the separator 230 are stacked in the radial direction of the secondary battery 1 around the center pin 19. The positive electrode 210 and the negative electrode 220 included in the electrode body 200 are layered components for the charge and discharge reactions of the secondary battery according to the present embodiment.

[0036] The positive electrode 210 includes a positive electrode current collector layer 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode current collector layer 211 is stacked between the positive electrode active material layers 212.

[0037] The positive electrode current collector layer 211 is a conductor layer and can be made of, for example, aluminum foil.

[0038] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material layer 212 contains a positive electrode active material, a binder, and a conductive additive. The positive electrode active material layer 212 is not limited to the materials listed above and may also contain, for example, a dispersant.

[0039] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or spinel type crystal structure. Specific examples of the lithium-containing composite oxide are LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13)O2, LiMn2O4, etc. The lithium-containing phosphoric acid compound is a phosphoric acid compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphoric acid compound has, for example, a crystal structure such as an olivine type. Specific examples of the lithium-containing phosphoric acid compound are LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.3 Mn 0.7 PO4, etc.

[0040] The binder contained in the positive electrode active material layer 212 can be any material. For example, it includes any one or more of synthetic rubber and polymer compounds. Synthetic rubber is, for example, styrene-butadiene rubber, fluorine rubber, ethylene propylene diene monomer rubber, etc. Polymer compounds are, for example, polyvinylidene fluoride, polyimide, etc.

[0041] The conductive additive contained in the positive electrode active material layer 212 can be any material. For example, it includes carbon. Carbon is, for example, graphite, carbon black, acetylene black, Ketjen black, etc. In addition, as long as the conductive additive is a material having conductivity, it is not limited thereto, and it can also be a metal material, a conductive polymer, etc.

[0042] The negative electrode 220 includes a negative electrode current collector layer 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector layer 221 is laminated between the negative electrode active material layers 222.

[0043] The negative electrode current collector layer 221 is a conductor, and for example, a copper foil or the like can be used.

[0044] The negative electrode active material layer 222 is a layer containing a negative electrode active material. The negative electrode active material layer 222 is not limited to being composed only of the negative electrode active material. For example, it can also contain a conductive additive and a binder.

[0045] The negative electrode active material includes, for example, carbon materials, metals, semimetals, alloys or compounds of silicon, alloys or compounds of tin (Sn), and other materials capable of intercalating and deintercalating lithium.

[0046] Carbon materials that can be used as the negative electrode active material can, for example, include graphite, non-graphitizable carbon, graphitizable carbon, etc. More specifically, carbon materials include, for example, pyrolytic carbon, coke, vitreous carbon fiber, fired bodies of organic polymer compounds, activated carbon, carbon black, etc. Coke includes pitch coke, needle coke, petroleum coke, etc. Here, the fired body of the organic polymer compound is obtained by firing and carbonizing a polymer compound such as phenolic resin or furan resin at an appropriate temperature.

[0047] As metals and metalloids that can be used as negative electrode active materials, for example, tin, lead (Pb), aluminum, indium (In), silicon, zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf) can be cited. Among them, silicon, germanium, tin, and lead are preferred. In addition, silicon and tin have a large ability to intercalate and deintercalate lithium and can obtain a high energy density, so they are more preferred.

[0048] As alloys of silicon that can be used as negative electrode active materials, for example, alloys containing at least one selected from the group consisting of tin, nickel, copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony, and chromium (Cr) as a second constituent element other than silicon can be cited. In addition, as compounds of silicon that can be used as negative electrode active materials, for example, compounds containing oxygen (O) or carbon (C) can be cited, and in addition to silicon, the above-mentioned second constituent element can also be contained.

[0049] As alloys of tin that can be used as negative electrode active materials, for example, alloys containing at least one selected from the group consisting of silicon, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin can be cited. In addition, as compounds of tin that can be used as negative electrode active materials, for example, compounds containing oxygen or carbon can be cited, and in addition to tin, the above-mentioned second constituent element can also be contained.

[0050] The separator 230 is a film that insulates the positive electrode 210 and the negative electrode 220. The separator 230 is laminated between the positive electrode 210 and the negative electrode 220 in such a way that the positive electrode 210 and the negative electrode 220 do not come into direct contact. It is preferred that the material of the separator 230 is electrochemically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. The separator 230 can, for example, use a layer composed of a non-woven fabric of a polymer, a porous membrane, or fibers of glass or ceramic. The material of the separator 230 more preferably contains a porous polyolefin membrane. The separator 230 can be composed of multiple layers, or a separator obtained by laminating a porous polyolefin membrane and a heat-resistant membrane containing fibers of polyimide, glass, or ceramic can also be used.

[0051] The electrolyte is an electrolyte filled in the space surrounded by the insulating plate 18 and the battery can 11. The electrolyte is, for example, an electrolyte containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0052] The electrolyte salt contains lithium bis(fluorosulfonyl)imide (LiN(SO2F2)2). Thereby, the charge and discharge characteristics can be improved. It should be noted that the electrolyte salt may also contain other electrolyte salts used as the electrolyte salt of the lithium ion battery. The mass of the other electrolyte salt is preferably 1 / 3 or less, more preferably 1 / 4 or less, relative to the mass of lithium bis(fluorosulfonyl)imide (LiN(SO2F2)2). The other electrolyte salt is, for example, a light metal salt such as a lithium salt. Specific examples of the lithium salt are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), lithium difluorophosphate (LiPF2O2), etc.

[0053] The solvent contains an acetamide derivative represented by the formula (1). Thereby, the corrosion of lithium bis(fluorosulfonyl)imide to the positive electrode current collector layer 211 etc. can be suppressed.

[0054] [Chemical formula 2]

[0055]

[0056] (In the formula (1), R1 and R2 each independently represent an alkyl or alkoxy group having 1 to 5 carbon atoms which may have a substituent, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a condensed ring.)

[0057] "May have a substituent" means not having a substituent, or the hydrogen group is substituted by one or more substituents. As the substituent, for example, a hydrocarbon group, a halogen group such as a fluorine group, etc. can be cited.

[0058] As the compound represented by the formula (1), for example, the compounds shown in the formula (1-1) to the formula (1-5) etc. can be cited.

[0059] [Chemical formula 3]

[0060]

[0061] In the present embodiment, the molar ratio of the acetamide derivative to lithium bis(fluorosulfonyl)imide is 2 or more and 4 or less. By setting this molar ratio to 2 or more, the corrosion of lithium bis(fluorosulfonyl)imide to the positive electrode current collector layer 211 etc. can be suppressed well. In addition, by setting this molar ratio to 4 or less, the viscosity of the electrolyte solution is reduced, and thus the discharge rate characteristics can be improved.

[0062] It should be noted that the solvent may also contain other non-aqueous solvents used as electrolyte salts for lithium-ion batteries. The mass of the other non-aqueous solvents is preferably 1 / 3 or less, more preferably 1 / 4 or less, relative to the mass of the acetamide derivative represented by the formula (1). The other non-aqueous solvents include esters, ethers, etc. More specifically, the other non-aqueous solvents include carbonate-based compounds, carboxylate-based compounds, lactone-based compounds, etc. The carbonate-based compounds are cyclic carbonates, chain carbonates, etc. Specific examples of the cyclic carbonate are ethylene carbonate, propylene carbonate, etc. Specific examples of the chain carbonate are dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc. The carboxylate-based compounds are chain carboxylates, etc. Specific examples of the chain carboxylate are methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl pivalate, ethyl pivalate, methyl butyrate, ethyl butyrate, etc. The lactone-based compounds are lactones, etc. Specific examples of the lactone are γ-butyrolactone, γ-valerolactone, etc. Specific examples of the ethers are 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc. The ethers may be compounds in which a part or all of the hydrogens of 1,1,2-tetrafluoroethyl 2,2,2,3,3-pentafluoropropyl ether, etc. are substituted with fluorine.

[0063] The electrolyte solution may contain substances other than the electrolyte salt and the solvent, such as additives, etc. The mass of the substances other than the electrolyte salt and the solvent is preferably 0.1% by mass or more and 20% by mass or less relative to the mass of the additives of the electrolyte salt and the solvent.

[0064] The electrolyte solution preferably further contains at least one of unsaturated cyclic carbonates such as vinylene carbonate (ethylene carbonate), 4-methylene-1,3-dioxolan-2-one (ethylene methylene carbonate), ethylene ethyl carbonate, etc. and halogenated cyclic carbonates such as fluoroethylene carbonate (monofluoroethylene carbonate), difluoroethylene carbonate, etc. as additives. Thereby, on the interfaces of the positive electrode 210 and the negative electrode 220 with the electrolyte solution, a film with high ion conductivity is formed, and thus the discharge rate characteristics can be further improved.

[0065] The additives are not limited to the substances shown above, and other additives may also be used. The other additives are not particularly limited. Specifically, they may be sulfonic esters, phosphoric esters, acid anhydrides, isocyanates, etc. Specific examples of the sulfonic ester are propane sultone, propene sultone, etc. Specific examples of the phosphoric ester are trimethyl phosphate, triethyl phosphate, etc. Specific examples of the acid anhydride are succinic anhydride, 1,2-ethanedisulfonic anhydride, 2-sulfobenzoic anhydride, etc. Specific examples of the isocyanate are hexamethylene diisocyanate, etc.

[0066] The electrolytic solution preferably further contains a hydrofluoroether as an additive. As the hydrofluoroether, for example, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether can be cited. Thereby, the ionic conductivity of the electrolytic solution is increased by reducing the viscosity of the electrolytic solution, and thus the discharge rate characteristics can be further improved.

[0067] As described above, the battery according to the present embodiment has been described, but the secondary battery according to the present embodiment is not limited to Figure 1 the secondary battery shown. Hereinafter, other examples will be described with reference to the drawings, but the same structures as Figure 1 and Figure 2 will be denoted by the same reference numerals and the description thereof will be omitted.

[0068] Figure 3 is a cutaway view showing different examples of the secondary battery according to the present embodiment. Figure 3 The secondary battery 1A shown is a laminated lithium ion secondary battery. As Figure 3 shown, the secondary battery 1A includes a battery element 20, an outer packaging member 31, and a sealing material 32.

[0069] Figure 4 is Figure 3 a schematic view of a cross section taken along line VI-VI. The battery element 20 is disposed inside the outer packaging member 31. As Figure 4 shown, the battery element 20 includes an electrode body 200A, a positive electrode lead 21, a negative electrode lead 22, and a protective material 23. The positive electrode lead 21 is a terminal led out from the inside of the battery element 20 to the outside of the outer packaging member 31. That is, the positive electrode lead 21 is the terminal that becomes the positive electrode of the secondary battery 1A. In Figure 4 , the positive electrode lead 21 is disposed near the center of the battery element 20. The negative electrode lead 22 is a terminal led out from the inside of the battery element 20 to the outside of the outer packaging member 31. That is, the negative electrode lead 22 is the terminal that becomes the negative electrode of the secondary battery 1A. In Figure 4 , the negative electrode lead 22 is disposed near the center of the battery element 20. The protective material 23 is a member that protects the outside of the battery element 20. The protective material 23 is disposed so as to be wound around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0070] The outer packaging member 31 is a housing for accommodating the battery element 20. The outer packaging member 31 includes an insulating layer, a metal layer, and an outermost layer. The outer packaging member 31 has a structure in which the insulating layer, the metal layer, and the outermost layer are laminated in this order from the inner side, i.e., the side where the battery element 20 is provided, and are bonded by lamination processing or the like. The insulating layer of the outer packaging member 31 is made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. Thus, the outer packaging member 31 can reduce the water permeability of the secondary battery 1A and can improve the airtightness. The metal layer of the outer packaging member 31 is a metal plate or foil such as aluminum, stainless steel, nickel, or iron. The outermost layer can be any material. For example, it is preferably made of a material with high strength against breakage or piercing, such as the same resin as the insulating layer or nylon.

[0071] The sealing material 32 is a member for making the outer packaging member 31 airtight. The sealing material 32 is provided between the outer packaging member 31 and the positive electrode lead 21 and the negative electrode lead 22. Preferably, the material of the sealing material 32 has adhesiveness to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the sealing material 32 uses a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. Thus, since the sealing material 32 can seal the gap between the outer packaging member 31 and the positive electrode lead 21 and the negative electrode lead 22, the inside of the outer packaging member 31 can be made airtight.

[0072] In Figure 4 the example of, the electrode body 200A is a laminate for the charge and discharge reaction of the secondary battery according to the present embodiment. The electrode body 200A includes: a positive electrode 210A having a positive electrode current collector layer 211A and a positive electrode active material layer 212A; a negative electrode 220A having a negative electrode current collector layer 221A and a negative electrode active material layer 222A; and a separator 230A. The electrode body 200A has a structure wound around the positive electrode lead 21 and the negative electrode lead 22. From the outside, i.e., the protective material 23 side, the negative electrode current collector layer 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector layer 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A are laminated in this order. In the electrode body 200A, layers other than the negative electrode current collector layer 221A, the separator 230A, and the positive electrode current collector layer 211A are not provided near the positive electrode lead 21 and the negative electrode lead 22. By adopting this structure, the positive electrode current collector layer 211A is connected to the positive electrode lead 21, and the negative electrode current collector layer 221A is connected to the negative electrode lead 22.

[0073] As described above, the secondary battery according to the present embodiment is a secondary battery having a positive electrode, a negative electrode, a separator, and an electrolytic solution, and the electrolytic solution contains an acetamide derivative represented by the formula (1) and lithium bis(fluorosulfonyl)imide.

[0074] [Chemical formula 4]

[0075]

[0076] (In the formula (1), R1 and R2 each independently represent an alkyl group or an alkoxy group having 1 to 5 carbon atoms which may have a substituent, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a condensed ring.)

[0077] Thus, corrosion of the current collector (the positive electrode current collector layer 211) and the like by lithium bis(fluorosulfonyl)imide can be suppressed. Then, a secondary battery capable of stably performing charge and discharge can be provided.

[0078] As a preferred mode, the molar ratio of the acetamide derivative to lithium bis(fluorosulfonyl)imide is 2 or more and 4 or less. By setting this molar ratio to 2 or more, corrosion of the current collector and the like by lithium bis(fluorosulfonyl)imide can be satisfactorily suppressed. In addition, by setting this molar ratio to 4 or less, the viscosity of the electrolytic solution is reduced, and thus the discharge rate characteristics can be improved.

[0079] As a preferred mode, the electrolytic solution further contains at least one of an unsaturated cyclic carbonate and a halogenated cyclic carbonate. Thus, since a film having lithium ion conductivity is generated at the interfaces of the positive electrode and the negative electrode with the electrolytic solution, the discharge rate characteristics can be further improved.

[0080] As a preferred mode, the electrolytic solution further contains a hydrofluoroether. Thus, since the viscosity of the electrolytic solution is reduced, the discharge rate characteristics can be further improved.

[0081] Hereinafter, examples of the present invention will be described. It should be noted that the present invention is not limited to the following examples.

[0082] In the following description, lithium bis(fluorosulfonyl)imide is designated as LiFSI, and a mixture of ethylene carbonate (EC) and propylene carbonate (PC) mixed at a volume ratio of 1:1 is designated as ECPC for description. In addition, Compounds A to E in the following description are as follows.

[0083] Compound A: A compound represented by the formula (1-1)

[0084] [Chemical formula 5]

[0085]

[0086] Compound B: A compound represented by formula (1-2)

[0087] [Chemical formula 6]

[0088]

[0089] Chemical formula C: A compound represented by formula (1-3)

[0090] [Chemical formula 7]

[0091]

[0092] Compound D: A compound represented by formula (1-4)

[0093] [Chemical formula 8]

[0094]

[0095] Chemical formula E: A compound represented by formula (1-5)

[0096] [Chemical formula 9]

[0097]

[0098] <Example 1-1>

[0099] In Example 1-1, an aluminum foil, a polyethylene porous membrane, and metallic lithium were overlapped, and an electrolytic solution was injected to fabricate a battery for evaluating metal corrosion. Additionally, in Example 1-1, a positive electrode, a polyethylene porous membrane as a separator, and a negative electrode were overlapped, and an electrolytic solution was injected to fabricate a battery for evaluating battery performance. Here, the battery for evaluating battery performance was designed to have a designed capacity of 5 mAh.

[0100] In Example 1-1, Compound A was used as a solvent of the electrolytic solution, and LiFSI was used as an electrolyte salt. The electrolytic solution related to Example 1-1 was prepared by mixing Compound A and LiFSI at a molar ratio of 3:1.

[0101] The positive electrode of the battery for battery evaluation test was fabricated by the following method. First, 91% by mass of lithium nickel oxide (LiNiO₂) as the positive electrode active material, 3% by mass of polyvinylidene fluoride as the binder, and 6% by mass of acetylene black as the conductive additive were mixed together to form a positive electrode mixture. Next, this positive electrode mixture was put into an organic solvent N-methyl-2-pyrrolidone as the solvent and stirred, thereby preparing a paste-like positive electrode mixture slurry. Next, using a coating device, the positive electrode mixture slurry was coated on both sides of a strip-shaped aluminum foil with a thickness of 12 μm as the positive electrode current collector. Finally, the positive electrode mixture slurry was dried to form a positive electrode active material layer. Then, the positive electrode active material layer was compression-molded using a roll press to fabricate the positive electrode.

[0102] The negative electrode of the battery for battery evaluation test was fabricated by the following method. First, 93% by mass of graphite as the negative electrode active material and 7% by mass of polyvinylidene fluoride as the binder were mixed together to form a negative electrode mixture. Next, this negative electrode mixture was put into an organic solvent N-methyl-2-pyrrolidone as the solvent and stirred, thereby preparing a paste-like negative electrode mixture slurry. Next, using a coating device, the negative electrode mixture slurry was coated on both sides of a strip-shaped copper foil with a thickness of 15 μm as the negative electrode current collector. Then, the negative electrode mixture slurry was dried to form a negative electrode active material layer. Finally, the negative electrode active material layer was compression-molded using a roll press to fabricate the negative electrode.

[0103] "Metal Corrosion Evaluation Test"

[0104] For the fabricated battery for metal corrosion evaluation test, using the aluminum foil as the working electrode, the voltage was increased from the open circuit potential to 4.2 V at a rate of 1 mV per second. After reaching 4.2 V, potentiostatic electrolysis was carried out at 4.2 V for 5 hours to evaluate whether the aluminum foil was corroded. Specifically, in the case of discoloration of the aluminum foil or leakage of the electrolyte, it was judged that the aluminum foil was corroded.

[0105] "Initial Charge and Discharge Test"

[0106] For the fabricated battery for battery evaluation test, as the initial charge and discharge test, under the following conditions, CCCV charging was carried out at a constant charging rate. After reaching the charging control voltage, charging was carried out at the charging control voltage, and charging was terminated when the current value decreased to the charging cut-off. Then, CC discharge was carried out at a constant discharge rate, and discharge was terminated when the voltage reached the discharge cut-off voltage.

[0107] Charging rate: 0.05C

[0108] Charging control voltage: 4.20V

[0109] Charging cut-off: 0.01C

[0110] Discharge rate: 0.05C

[0111] Discharge cut-off voltage: 2.5V

[0112] The first charge-discharge test was carried out on 3 battery evaluation test cells. Here, for each battery evaluation test cell, when the capacity measured in the first charge-discharge is 50% or more of the theoretical capacity calculated from the mass of the active material, it is judged that the battery evaluation test cell can be charged and discharged, and when the capacity measured in the first charge-discharge is less than 50% of the theoretical capacity calculated from the mass of the active material, it is judged that the battery evaluation test cell cannot be charged and discharged.

[0113] <Example 1-2>

[0114] In Example 1-2, except that compound B was used instead of compound A as the solvent of the electrolyte, a metal corrosion evaluation test cell was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1. Then, a battery evaluation test cell was fabricated and the first charge-discharge test was carried out.

[0115] <Example 1-3>

[0116] In Example 1-3, except that compound C was used instead of compound A as the solvent of the electrolyte, a metal corrosion evaluation test cell was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1. Then, a battery evaluation test cell was fabricated and the first charge-discharge test was carried out.

[0117] <Example 1-4>

[0118] In Example 1-4, except that compound D was used instead of compound A as the solvent of the electrolyte, a metal corrosion evaluation test cell was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1. Then, a battery evaluation test cell was fabricated and the first charge-discharge test was carried out.

[0119] <Example 1-5>

[0120] In Example 1-5, except that compound E was used instead of compound A as the solvent of the electrolyte, a metal corrosion evaluation test cell was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1. Then, a battery evaluation test cell was fabricated and the first charge-discharge test was carried out.

[0121] <Comparative Example 1-1>

[0122] In Comparative Example 1-1, a battery for metal corrosion evaluation test was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1, except that ECPC was used instead of Compound A as the solvent of the electrolyte. Then, a battery for battery evaluation test was fabricated and a first charge-discharge test was conducted.

[0123] The results of the metal corrosion evaluation tests and the first charge-discharge tests for Examples 1-1 to 1-5 and Comparative Example 1-1 are shown in Table 1.

[0124] [Table 1]

[0125] (Table 1)

[0126]

[0127] As shown in Table 1, in Examples 1-1 to 1-5, since an electrolyte containing any one of Compounds A to E was used, corrosion of the aluminum foil by LiFSI was suppressed, and all three batteries for which a battery evaluation test was conducted were able to be charged and discharged. Accordingly, the secondary batteries according to Examples 1-1 to 1-5 were able to be charged and discharged stably.

[0128] On the other hand, in Comparative Example 1-1, although corrosion of the aluminum foil by LiFSI was suppressed since an electrolyte containing ECPC was used, all three batteries for which a battery evaluation test was conducted were unable to be charged and discharged. It is considered that this is because, in Comparative Example 1-1, since the electrolyte had a high viscosity, the electrolyte was not sufficiently impregnated into the separator or the like, or the ionic conductivity of the electrolyte decreased.

[0129] <Example 2-1>

[0130] In Example 2-1, a battery for metal corrosion evaluation test was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 1-1, except that Compound A and LiFSI were mixed at a molar ratio of 2:1. Then, a battery for battery evaluation test was fabricated and a first charge-discharge test was conducted.

[0131] 《Discharge Rate Characteristics Evaluation Test》

[0132] In Example 2-1, in addition to the first charge-discharge test conducted in Example 1-1, a discharge rate characteristics evaluation test was also conducted. In the discharge rate characteristics evaluation test, the discharge rate characteristics were evaluated by performing the second to sixth charge-discharges described below on the battery for battery evaluation test that was determined to be able to be charged and discharged in the first charge-discharge test.

[0133] In the second charge-discharge cycle, under the following conditions, CCCV charging is performed at a constant charging rate. After reaching the charging control voltage, charging is carried out at the charging control voltage, and charging ends when the current value drops to the charge cut-off value. CC discharging is performed at a constant discharging rate, and discharging ends when the voltage reaches the discharge cut-off voltage. Here, the discharge capacity is measured during the second charge-discharge cycle and regarded as the 0.2C discharge capacity.

[0134] Charging rate: 0.2C

[0135] Charging control voltage: 4.20V

[0136] Charge cut-off: 0.05C

[0137] Discharging rate: 0.2C

[0138] Discharge cut-off voltage: 2.5V

[0139] From the third to the sixth charge-discharge cycles, charge-discharge tests were carried out under the same conditions as the second charge-discharge test, except that the charging rates were set to 0.5C, 1.0C, 2.0C, and 5.0C respectively. Here, the discharge capacity was measured during the sixth charge-discharge cycle and regarded as the 5C discharge capacity.

[0140] Based on the measured 0.2C discharge capacity and 5C discharge capacity, the 5C discharge capacity retention rate was calculated. Here, the 5C discharge capacity retention rate refers to the ratio of the 5C discharge capacity to the 0.2C discharge capacity. That is, if the 5C discharge capacity retention rate is high, the discharge capacity can be increased even at a high discharge rate, so it can be said that the discharge rate characteristics are improved.

[0141] <Example 2-2>

[0142] In Example 2-2, except that compound A and LiFSI were mixed at a molar ratio of 3:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was carried out. Then, a battery for battery evaluation test was fabricated and the initial charge-discharge test and the discharge rate characteristic evaluation test were carried out. That is, the electrolyte involved in Example 2-2 was the same as that in Example 1-1.

[0143] <Example 2-3>

[0144] In Example 2-3, except that compound A and LiFSI were mixed at a molar ratio of 4:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was carried out. Then, a battery for battery evaluation test was fabricated and the initial charge-discharge test and the discharge rate characteristic evaluation test were carried out.

[0145] <Example 2-4>

[0146] In Examples 2-4, except that Compound A and LiFSI were mixed at a molar ratio of 5:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated, and an initial charge-discharge test and a discharge rate characteristic evaluation test were performed.

[0147] <Comparative Example 2-1>

[0148] In Comparative Example 2-1, except that ECPC was used instead of Compound A as the solvent of the electrolyte, and the molar ratio of ECPC and LiFSI was set to 2:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated, and an initial charge-discharge test and a discharge rate characteristic evaluation test were performed.

[0149] <Comparative Example 2-2>

[0150] In Comparative Example 2-2, except that ECPC was used instead of Compound A as the solvent of the electrolyte, and the molar ratio of ECPC and LiFSI was set to 3:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated, and an initial charge-discharge test and a discharge rate characteristic evaluation test were performed. That is, the electrolyte involved in Comparative Example 2-2 was the same as that in Comparative Example 1-1.

[0151] <Comparative Example 2-3>

[0152] In Comparative Example 2-3, except that ECPC was used instead of Compound A as the solvent of the electrolyte, and the molar ratio of ECPC and LiFSI was set to 4:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated, and an initial charge-discharge test and a discharge rate characteristic evaluation test were performed.

[0153] <Comparative Example 2-4>

[0154] In Comparative Example 2-4, except that ECPC was used instead of Compound A as the solvent of the electrolyte, and the molar ratio of ECPC and LiFSI was set to 5:1, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-1, and a metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated, and an initial charge-discharge test and a discharge rate characteristic evaluation test were performed.

[0155] The results of the metal corrosion evaluation tests and battery evaluation tests related to Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-4 are shown in Table 2.

[0156] [Table 2]

[0157]

[0158] As shown in Table 2, in Examples 2-1 to 2-4, since the electrolyte containing Compound A was used, the corrosion of aluminum foil by LiFSI could be suppressed, and all of the three batteries subjected to the battery evaluation test could be charged and discharged. Thus, the secondary batteries according to Examples 2-1 to 2-4 could be charged and discharged stably.

[0159] On the other hand, in Comparative Examples 2-1 and 2-2, since ECPC was included, the corrosion of aluminum foil by LiFSI was suppressed, but all of the three batteries subjected to the battery evaluation test could not be charged and discharged. It is considered that this is because, in Comparative Examples 2-1 and 2-2, since the electrolyte had a high viscosity, the electrolyte was not sufficiently impregnated into the separator or the like, or the ionic conductivity of the electrolyte decreased.

[0160] In Comparative Example 2-3, since ECPC was included, the corrosion of aluminum foil by LiFSI was suppressed, but among the three batteries subjected to the battery evaluation test, two could not be charged and discharged. Thus, the secondary battery according to Comparative Example 2-3 could not be charged and discharged stably. It is considered that this is because, in Comparative Example 2-3, the viscosity of the electrolyte was not sufficiently reduced, the electrolyte was difficult to impregnate into the separator or the like, or the ionic conductivity of the electrolyte decreased.

[0161] In Comparative Example 2-4, since the concentration of LiFSI with respect to ECPC was low, the corrosion of aluminum foil by LiFSI could not be suppressed, and all of the three batteries subjected to the battery evaluation test could not be charged and discharged.

[0162] As shown in Table 2, in Examples 2-1 to 2-3, the molar ratio of Compound A to LiFSI was 2 or more and 4 or less, and thus the 5C discharge capacity retention rate was improved as compared with Example 2-4 in which the molar ratio was greater than 4. In Example 2-3, since the electrolyte containing Compound A was used, the 5C discharge capacity retention rate was improved as compared with Comparative Example 2-3 having the same molar concentration of LiFSI in the electrolyte.

[0163] <Example 3-1>

[0164] Example 3-1 is the same example as Example 2-2. That is, in Example 3-1, for the electrolyte according to Example 2-2, no additive was mixed, and a battery for metal corrosion evaluation test was produced and the metal corrosion evaluation test was conducted in the same manner as in Example 2-2, and then a battery for battery evaluation test was produced and the initial charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0165] <Example 3-2>

[0166] In Example 3-2, except that 1% by mass of vinylene carbonate (VC) was mixed as an additive with respect to the electrolyte involved in Example 2-2, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-2 and the metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated and the initial charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0167] <Example 3-3>

[0168] In Example 3-3, except that 1% by mass of fluoroethylene carbonate (FEC) was mixed as an additive with respect to the electrolyte involved in Example 2-2, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-2 and the metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated and the initial charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0169] <Example 3-4>

[0170] In Example 3-4, except that 1% by mass of 4-methylene-1,3-dioxolan-2-one (MDO) was mixed as an additive with respect to the electrolyte involved in Example 2-2, a battery for metal corrosion evaluation test was fabricated in the same manner as in Example 2-2 and the metal corrosion evaluation test was conducted. Then, a battery for battery evaluation test was fabricated and the initial charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0171] The results of the metal corrosion evaluation tests and the battery evaluation tests related to Examples 3-1 to 3-4 are shown in Table 3.

[0172] [Table 3]

[0173]

[0174] As shown in Table 3, in Examples 3-1 to 3-4, the corrosion of aluminum foil by LiFSI was suppressed, and all of the three batteries for which the battery evaluation tests were conducted were able to be charged and discharged. Thus, the secondary batteries related to Examples 3-1 to 3-4 were able to be charged and discharged stably.

[0175] As shown in Table 3, in Examples 3-2 to 3-4, since an electrolyte in which VC, FEC, or MDO is added to the electrolyte involved in Example 2-2 is used, the 5C discharge capacity retention rate is improved compared to Example 3-1 using the same electrolyte as the electrolyte involved in Example 2-2. It is considered that this is because by adding an unsaturated cyclic carbonate or a halogenated cyclic carbonate to the electrolyte, a film with high ion conductivity is formed at the interfaces of the positive electrode and the negative electrode with the electrolyte, and the interface resistance becomes smaller.

[0176] <Example 4-1>

[0177] Example 4-1 is the same example as Example 2-2. That is, in Example 4-1, no additive was mixed in the electrolyte involved in Example 2-2. Similarly to Example 2-2, a battery for metal corrosion evaluation test was fabricated and a metal corrosion evaluation test was conducted, and then a battery for battery evaluation test was fabricated and the first charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0178] <Example 4-2>

[0179] In Example 4-2, except that 10% by mass of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was mixed as an additive with respect to the electrolyte involved in Example 2-2, a battery for metal corrosion evaluation test was fabricated and a metal corrosion evaluation test was conducted in the same manner as in Example 2-2. Then, a battery for battery evaluation test was fabricated and the first charge-discharge test and the discharge rate characteristic evaluation test were conducted.

[0180] The results of the metal corrosion evaluation tests and the battery evaluation tests related to Example 4-1 and Example 4-2 are shown in Table 4.

[0181] [Table 4]

[0182]

[0183] As shown in Table 4, in Example 4-1 and Example 4-2, the corrosion of aluminum foil by LiFSI was suppressed, and all three batteries for which the battery evaluation test was conducted were able to be charged and discharged. Thus, the secondary batteries related to Example 4-1 and 4-2 can be charged and discharged stably.

[0184] As shown in Table 4, in Example 4-1, since an electrolyte in which TTE was added to the electrolyte involved in Example 2-2 was used, the 5C discharge capacity retention rate was improved compared to Example 4-1 using the same electrolyte as the electrolyte involved in Example 2-2. It is considered that this is because by adding a hydrofluoroether to the electrolyte, the viscosity of the electrolyte is reduced and the ion conductivity is improved.

[0185] The above-described embodiments are for easily understanding the present invention and are not for limiting or interpreting the present invention. The present invention can be changed / improved without departing from its gist, and its equivalents are also included in the present invention.

[0186] The present invention can be implemented in the following manner.

[0187] <1>

[0188] A secondary battery

[0189] which has a positive electrode, a negative electrode, a separator, and an electrolyte

[0190] The electrolyte contains an acetamide derivative represented by the formula (1) and lithium bis(fluorosulfonyl)imide.

[0191] [Chemical formula 10]

[0192]

[0193] (In the formula (1), R1 and R2 each independently represent an alkyl or alkoxy group having 1 to 5 carbon atoms which may have a substituent, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a condensed ring.)

[0194] <2>

[0195] The secondary battery according to <1>

[0196] The molar ratio of the acetamide derivative to the lithium bis(fluorosulfonyl)imide is 2 or more and 4 or less.

[0197] <3>

[0198] The secondary battery according to <1> or <2>

[0199] The electrolyte further contains at least one of an unsaturated cyclic carbonate and a halogenated cyclic carbonate.

[0200] <4>

[0201] The secondary battery according to any one of <1> to <3>

[0202] The electrolyte further contains a hydrofluoroether.

[0203] Explanation of reference numerals

[0204] 1, 1A: Secondary battery; 10: Outer casing; 11: Battery can; 12: Cover body; 13: Thermistor element; 14: Safety valve mechanism; 15: Gasket; 16: Positive lead; 17: Negative lead; 18: Insulating plate; 19: Center pin; 20: Battery element; 21: Positive lead; 22: Negative lead; 23: Protective material; 31: Outer packaging component; 32: Sealing material; 200, 200A: Electrode body; 210, 210A: Positive electrode; 211, 211A: Positive electrode current collector layer; 212, 212A: Positive electrode active material layer; 220, 220A: Negative electrode; 221, 221A: Negative electrode current collector layer; 222, 222A: Negative electrode active material layer; 230, 230A: Separator.

Claims

1. A secondary battery, having a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte contains an acetamide derivative represented by formula (1) and lithium bis(fluorosulfonyl)imide, In formula (1), R1 and R2 each independently represent an alkyl group or an alkoxy group having 1 to 5 carbon atoms which may have a substituent, or a trimethylsilyl group, and R1 and R2 may be bonded to each other to form a condensed ring.

2. The secondary battery according to claim 1, wherein the molar ratio of the acetamide derivative to the lithium bis(fluorosulfonyl)imide is 2 or more and 4 or less.

3. The secondary battery according to claim 1 or 2, wherein the electrolyte further contains at least one of an unsaturated cyclic carbonate and a halogenated cyclic carbonate.

4. The secondary battery according to any one of claims 1 to 3, wherein the electrolyte further contains a hydrofluoroether.

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