Additive for nonaqueous electrolytic solution, nonaqueous electrolytic solution, and lithium ion secondary battery

Abstract

The present invention improves cycle characteristics of a lithium ion secondary battery including a nonaqueous electrolytic solution. An additive for a nonaqueous electrolytic solution according to an embodiment of the present invention is to be added to a nonaqueous electrolytic solution in a lithium ion secondary battery including a positive electrode, a negative electrode, and the nonaqueous electrolytic solution. The additive contains a compound having a urethane bond and an oxetane ring at a terminal.

Description

Additive for non-aqueous electrolyte, non-aqueous electrolyte, and lithium ion secondary battery

[0001] FIELD OF THE INVENTION The present invention relates to an additive for a non-aqueous electrolyte, a non-aqueous electrolyte, and a lithium ion secondary battery.

[0002] Lithium ion secondary batteries are configured with a positive electrode, a negative electrode, and a non-aqueous electrolyte, and there is a demand for improving battery characteristics such as cycle characteristics. For example, Patent Document 1 discloses that, in order to improve cycle characteristics, a copolymer of methyl methacrylate and (3-ethyloxetan-3-yl)methyl methacrylate is blended into the non-aqueous electrolyte, and vinylene carbonate is blended with the copolymer.

[0003] JP 2023-88767 A

[0004] The non-aqueous electrolyte for lithium ion secondary batteries includes liquid non-aqueous electrolytes and gel-like solid electrolytes. Generally, adding a polymer to a non-aqueous electrolyte reduces the electrical conductivity. However, the present inventors have found that adding a specific polymer to a non-aqueous electrolyte improves the cycle characteristics of lithium ion secondary batteries.

[0005] An object of an embodiment of the present invention is to provide an additive for a non-aqueous electrolyte solution that can improve the cycle characteristics of a lithium ion secondary battery containing a non-aqueous electrolyte solution, and a non-aqueous electrolyte solution and a lithium ion secondary battery using the additive.

[0006] The present invention includes the following embodiments. [1] A non-aqueous electrolyte additive for a lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the non-aqueous electrolyte additive comprising a compound having an oxetane ring at a terminal and a urethane bond. [2] The non-aqueous electrolyte additive according to [1], wherein the compound has a structure derived from polycarbonate polyol. [3] The non-aqueous electrolyte additive according to [1] or [2], wherein the compound has an average number of oxetane rings per molecule of 2.0 to 100. [4] The non-aqueous electrolyte additive according to any one of [1] to [3], wherein the compound has a weight-average molecular weight of 2,500 to 200,000.

[0007] [5] A non-aqueous electrolyte for a lithium ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the non-aqueous electrolyte comprising a supporting salt, an aprotic solvent, and a compound having an oxetane ring at a terminal and a urethane bond. [6] The non-aqueous electrolyte according to [5], further comprising vinylene carbonate and / or fluoroethylene carbonate. [7] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the non-aqueous electrolyte according to [5] or [6].

[0008] According to an embodiment of the present invention, it is possible to improve the cycle characteristics of a lithium ion secondary battery containing a non-aqueous electrolyte solution.

[0009] The non-aqueous electrolyte additive according to the present embodiment is an additive added to a non-aqueous electrolyte in a lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, and includes a compound having an oxetane ring at a terminal and a urethane bond (hereinafter also referred to as an oxetane ring-containing compound). Addition of such an oxetane ring-containing compound to the non-aqueous electrolyte can improve the cycle characteristics of the lithium ion secondary battery.

[0010] The oxetane ring-containing compound is a compound having a urethane structure in the molecule and an oxetane ring at its terminal, and is preferably a polymer based on polyurethane having an oxetane ring at its terminal. Therefore, the oxetane ring-containing compound according to one embodiment comprises a polyurethane obtained by reacting a polyol with a polyisocyanate, and an oxetane ring added to the molecular terminal of the polyurethane, and has a structure derived from the polyol and a structure derived from the polyisocyanate.

[0011] The polyol may be composed solely of one having two hydroxy groups per molecule, but it is preferable to use a polyol having three or more hydroxy groups in combination with a polyol having two hydroxy groups, which gives the oxetane ring-containing compound branches and includes three or more terminals, making it possible to increase the number of oxetane rings per molecule to three or more.

[0012] Examples of polyols include polycarbonate polyols, polyether polyols, polyester polyols, polyether ester polyols, polyolefin polyols, polyhydric alcohols, polyacrylic polyols, polyacetal polyols, polysiloxane polyols, fluorine polyols, etc. These may be used alone or in combination of two or more.

[0013] Examples of polycarbonate polyols include those obtained by reacting a polyol compound such as an aliphatic polyol or an alicyclic polyol with a carbonate derivative such as a carbonate ester or phosgene, and preferably bifunctional polyols, i.e., polycarbonate diols. The polyol compound is preferably a diol having 4 to 12 carbon atoms, such as 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 1,8-octanediol, or 1,10-decanediol, or an alicyclic diol such as isosorbide. The polyol compound and carbonate derivative in the polycarbonate polyol can each be used alone or in combination of two or more.

[0014] Specific examples of polycarbonate polyols include PCDL manufactured by Asahi Kasei Corporation. T-6001, T-6002, T-5651, T-5652, T-5650J, T-4671, T-4672, Kuraray Polyol C-590, C-1050, C-1050R, C-1090, C-2050, C-2050R, C-2070, C-2070R, C-2090, C-2090R, C-3090, C-3090R, C-4090, C-4090R, C-5090, C-5090R, C-1065N, C-2065N, C-1015N, C-2015N manufactured by Kuraray Co., Ltd., and ETERNACOLL (registered trademark) manufactured by UBE Co., Ltd. Examples include UH-50, UH-100, UH-200, UH-300, UM-90 (3 / 1), UM-90 (1 / 1), UM-90 (1 / 3), and UC-100.

[0015] The weight average molecular weight (Mw) of the polycarbonate polyol is not particularly limited and may be, for example, 300 to 5000, 500 to 4000, or 1000 to 3000. The weight average molecular weight of the polycarbonate polyol is a polystyrene-equivalent weight average molecular weight measured by gel permeation chromatography (GPC), and in detail can be measured by the method described in the Examples section.

[0016] Examples of polyether polyols include those obtained by addition polymerization of alkylene oxides to polyhydric alcohols, polytetramethylene ether glycol, polythioether polyols, etc. Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 2-methyl-1,3-propanediol, glycerin, trimethylolpropane, pentaerythritol, etc. Examples of alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, etc.

[0017] Examples of polyester polyols and polyether ester polyols include esters of polyhydric alcohols or polyether polyols with polycarboxylic acids or polycarboxylic acid anhydrides. Examples of polycarboxylic acids and polycarboxylic acid anhydrides include succinic acid, glutaric acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, tetrahydrofuran acid, endomethinetetrahydrofuran acid, hexahydrophthalic acid, and acid anhydrides thereof. Examples of polyester polyols include castor oil polyols and polycaprolactone polyols.

[0018] Polyolefin polyol refers to a polymer or copolymer of a diolefin having 4 to 12 carbon atoms, such as butadiene or isoprene, and is a compound having multiple hydroxy groups. The copolymer may be, for example, a copolymer of a diolefin having 4 to 12 carbon atoms and an α-olefin having 2 to 22 carbon atoms. Examples of polyolefin polyols include polybutadiene polyol and polyisoprene polyol, and these may be saturated aliphatic polyols obtained by hydrogenating the double bonds in their molecules.

[0019] Preferred examples of the polyhydric alcohol used as the polyol include low-molecular-weight polyols having a molecular weight of 400 or less, such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 2-methyl-1,3-propanediol, glycerin, trimethylolpropane, pentaerythritol, bisphenol A, and hydrogenated bisphenol A.

[0020] As the polyisocyanate, various polyisocyanate compounds having two or more isocyanate groups in one molecule can be used, such as aliphatic polyisocyanates, alicyclic polyisocyanates, aromatic polyisocyanates, araliphatic polyisocyanates, and modified products thereof, and any of these may be used alone or in combination of two or more.

[0021] Examples of aliphatic polyisocyanates include tetramethylene diisocyanate, dodecamethylene diisocyanate, pentamethylene diisocyanate (PDI), hexamethylene diisocyanate (HDI), 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate. These may be used alone or in combination of two or more.

[0022] Examples of alicyclic polyisocyanates include isophorone diisocyanate (IPDI), hydrogenated xylylene diisocyanate, dicyclohexylmethane 4,4'-diisocyanate (hydrogenated MDI), 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatomethyl)cyclohexane, and these may be used alone or in combination of two or more.

[0023] Examples of aromatic polyisocyanates include tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polymeric MDI, 4,4'-dibenzyl diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate (XDI), 1,3-phenylene diisocyanate, and 1,4-phenylene diisocyanate. These may be used alone or in combination of two or more.

[0024] Examples of the aromatic aliphatic polyisocyanate include dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, α,α,α,α-tetramethylxylylene diisocyanate, and the like, and these may be used alone or in combination of two or more.

[0025] Examples of modified polyisocyanates include isocyanurate modified products, allophanate modified products, biuret modified products, adduct modified products, and carbodiimide modified products.

[0026] The polyisocyanate is preferably an alicyclic polyisocyanate and / or an aromatic polyisocyanate, more preferably an alicyclic polyisocyanate. Therefore, the oxetane ring-containing compound preferably has a structure derived from an alicyclic polyisocyanate and / or a structure derived from an aromatic polyisocyanate, more preferably a structure derived from an alicyclic polyisocyanate.

[0027] The oxetane ring-containing compound preferably has a polycarbonate polyol-derived structure as a polyol-derived structure in addition to a polyisocyanate-derived structure. That is, it is preferable to use a polycarbonate polyol as the polyol, and more preferably to use a polycarbonate diol. Since carbonates are generally used as aprotic solvents in non-aqueous electrolytes, the oxetane ring-containing compound having a polycarbonate polyol-derived structure can enhance its affinity with non-aqueous electrolytes.

[0028] As the polyol, a polyether polyol may be used together with a polycarbonate polyol. That is, the oxetane ring-containing compound according to one embodiment preferably has, in addition to a polyisocyanate-derived structure, a polyol-derived structure including a polycarbonate polyol-derived structure and a polyether polyol-derived structure. More preferably, the polyol-derived structure includes a polycarbonate diol-derived structure and a trifunctional polyether polyol-derived structure. In this case, the polyol preferably contains a polycarbonate polyol (preferably a polycarbonate diol; the same applies hereinafter) as the main component. For example, 100% by mass of the polyol preferably contains 70 to 99% by mass of a polycarbonate polyol and 1 to 30% by mass of a trifunctional polyether polyol, more preferably 80 to 98% by mass of a polycarbonate polyol and 2 to 20% by mass of a trifunctional polyether polyol, and even more preferably 90 to 97% by mass of a polycarbonate polyol and 3 to 10% by mass of a trifunctional polyether polyol.

[0029] As described above, the oxetane ring-containing compound has an oxetane ring at its terminal. Preferably, the oxetane ring is provided at the polyisocyanate-side terminal of the polyurethane. A method for introducing an oxetane ring at the polyisocyanate-side terminal includes, for example, reacting a compound having an oxetane ring and an active hydrogen group with the isocyanate group (NCO) at the terminal of a polyurethane having a polyisocyanate-derived structure at the terminal. Examples of the active hydrogen group include a hydroxy group, a primary amino group, and a secondary amino group. Examples of compounds having an oxetane ring and an active hydrogen group include 3-hydroxymethyl-3-ethyloxetane and 3-oxetanol. When the active hydrogen group is a hydroxy group, the oxetane ring is added to the polyisocyanate-derived structure via a urethane bond. When the active hydrogen group is an amino group, the oxetane ring is added to the polyisocyanate-derived structure via a urea bond.

[0030] The oxetane rings may be added to all or some of the terminals of the oxetane ring-containing compound. For example, a compound having an oxetane ring and an active hydrogen group may be reacted with some of the isocyanate groups at the terminals of the polyurethane, and a compound having an active hydrogen group but no oxetane ring may be reacted with the remaining isocyanate groups. In this way, the number of oxetane rings per molecule of the oxetane ring-containing compound can be adjusted.

[0031] The number of oxetane rings per molecule of the oxetane ring-containing compound is not particularly limited, but is preferably 2.0 to 100 on average, more preferably 2.0 to 50, even more preferably 2.1 to 30, and even more preferably 4.0 to 15. Increasing the number of oxetane rings per molecule can enhance the effect of improving cycle characteristics. On the other hand, increasing the number of oxetane rings per molecule requires increasing the branching of the molecular chain of the oxetane ring-containing compound, which increases the molecular weight and leads to an increase in the viscosity of the non-aqueous electrolyte. Therefore, the number of oxetane rings per molecule is preferably 100 or less. The number of oxetane rings per molecule can be measured by the method described in the Examples section.

[0032] The weight-average molecular weight (Mw) of the oxetane ring-containing compound is not particularly limited, but is preferably 2,500 to 200,000. A weight-average molecular weight of 200,000 or less can suppress the viscosity of the non-aqueous electrolyte. The weight-average molecular weight of the oxetane ring-containing compound is more preferably 5,000 to 150,000, and even more preferably 10,000 to 100,000. The weight-average molecular weight of the oxetane ring-containing compound is a polystyrene-equivalent weight-average molecular weight measured by gel permeation chromatography (GPC), and is measured in detail by the method described in the Examples section.

[0033] The method for producing the oxetane ring-containing compound is not particularly limited. For example, the following method can be mentioned. First, a polyol and a polyisocyanate are reacted at 30°C to 130°C for about 0.5 to 10 hours to synthesize an NCO-terminated polyurethane. Next, a compound having an oxetane ring and an active hydrogen group is added and reacted with the NCO-terminated polyurethane at 30°C to 130°C for about 0.5 to 10 hours. Thereafter, the mixture is cooled to 5°C to 45°C as necessary to obtain the oxetane ring-containing compound.

[0034] In the above reaction, any organic solvent such as diethyl carbonate, N-methyl-2-pyrrolidone (NMP), etc. can be used as the solvent. Furthermore, a catalyst can be used in the urethane reaction, and examples of such catalysts include amine compounds and metal catalysts such as tin octylate and bismuth octylate.

[0035] When synthesizing an NCO-terminated polyurethane, the molar ratio [NCO] / [OH] of the isocyanate groups (NCO) of the polyisocyanate to the hydroxy groups (OH) of the polyol is set to be greater than 1, for example, 1.1 to 2.0. The ratio of the total isocyanate group equivalents of the polyisocyanate to the total active hydrogen group equivalents of all active hydrogen group-containing compounds including the polyol and the compound having an oxetane ring and an active hydrogen group (total isocyanate group equivalents / total active hydrogen group equivalents) is preferably 0.9 to 1.0 so that no isocyanate groups remain.

[0036] The additive for a non-aqueous electrolyte solution according to this embodiment contains the above-mentioned oxetane ring-containing compound, and may consist solely of the oxetane ring-containing compound, or may contain the oxetane ring-containing compound together with other additives and an organic solvent.

[0037] Examples of other additives include vinylene carbonate and / or fluoroethylene carbonate, which will be described later. In this case, the amount of vinylene carbonate and / or fluoroethylene carbonate is not particularly limited, and may be, for example, 10 to 1,000 parts by mass or 30 to 500 parts by mass per 100 parts by mass of the oxetane ring-containing compound.

[0038] When the additive for a non-aqueous electrolyte solution contains an organic solvent, the organic solvent is preferably an aprotic solvent as described below. In this case, the amount of the organic solvent is preferably an amount that results in a concentration of the oxetane ring-containing compound of 10 to 50 mass %, more preferably 20 to 40 mass %.

[0039] The nonaqueous electrolyte according to this embodiment contains the oxetane ring-containing compound, a supporting salt, and an aprotic solvent. The nonaqueous electrolyte is a liquid electrolyte obtained by dissolving the supporting salt and the oxetane ring-containing compound in the aprotic solvent, which is a nonaqueous solvent.

[0040] The supporting salt is a substance that increases the conductivity of the aprotic solvent, and is, for example, LiPF ６ , LiBF ４ , LiAsF ６ , LiSbF ６ , LiN(SO ２ F) ２ (i.e., lithium bis(fluorosulfonyl)imide: LiFSI), LiAlCl ４ , LiClO ４ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ２ F ５ SO ２ ) ２ Examples of the supporting salt include lithium salts such as lithium carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, and LiF. These supporting salts may be used alone or in combination of two or more.

[0041] The supporting salt preferably contains a fluorine-containing lithium salt, more preferably LiPF ６ , LiBF ４ , LiAsF ６ , LiSbF ６ and LiN(SO ２ F) ２ The present invention is to contain at least one substance selected from the group consisting of:

[0042] The concentration of the supporting salt in the non-aqueous electrolyte is not particularly limited, but is preferably 0.5 to 2.0 mol / L, more preferably 0.6 to 1.8 mol / L, and may be 0.8 to 1.6 mol / L.

[0043] Examples of aprotic solvents include cyclic carbonates such as ethylene carbonate (ethylene carbonate, EC), propylene carbonate (PC), and butylene carbonate (BC); chain carbonates such as diethyl carbonate (diethyl carbonate, DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate, and ethyl propionate; γ-lactones such as γ-butyrolactone; chain ethers such as 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME); tetrahydrofuran (THF), 2-methylpropanol (DMF), and the like. fluorine derivatives thereof; dimethyl sulfoxide (DMSO), 1,3-dioxolane, formamide, acetamide, dimethylformamide (DMF), acetonitrile, propylnitrile, nitromethane, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid esters, etc. These aprotic organic solvents may be used alone or in combination of two or more.

[0044] The aprotic solvent preferably includes at least one solvent selected from the group consisting of cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, and chain ethers. More preferably, the aprotic solvent includes at least one solvent selected from the group consisting of cyclic carbonates and chain carbonates. In one embodiment, the aprotic solvent may include at least one solvent selected from the group consisting of EC, PC, BC, DEC, DMC, EMC, and DPC, or may include at least one solvent selected from the group consisting of EC, PC, and BC and at least one solvent selected from the group consisting of DEC, DMC, EMC, and DPC.

[0045] The content of the oxetane ring-containing compound in the non-aqueous electrolyte is not particularly limited, and is preferably 0.1 to 5.0 mass %, more preferably 0.3 to 4.0 mass %, and even more preferably 0.5 to 3.0 mass %, based on the total amount of the supporting salt, the aprotic solvent, and the oxetane ring-containing compound.

[0046] The non-aqueous electrolyte solution according to one embodiment preferably further contains vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC) as an additive. By further adding vinylene carbonate and / or fluoroethylene carbonate to the non-aqueous electrolyte solution containing the oxetane ring-containing compound, the cycle characteristics can be further improved.

[0047] The content of vinylene carbonate and / or fluoroethylene carbonate in the nonaqueous electrolyte solution (i.e., the amount of either VC or FEC when either one is contained, or the total amount of both when both are contained) is not particularly limited, and is preferably 0.1 to 5.0 mass %, more preferably 0.5 to 4.0 mass %, and even more preferably 1.0 to 3.0 mass %, relative to the total amount of the supporting salt, aprotic solvent, and oxetane ring-containing compound.

[0048] The non-aqueous electrolyte according to this embodiment is liquid at room temperature (25° C.), and preferably has a viscosity of 5 to 100 mPa·s, more preferably 6 to 50 mPa·s, and even more preferably 8 to 30 mPa·s at 25° C. The viscosity of the non-aqueous electrolyte is measured by the method described in the Examples section.

[0049] As described above, the non-aqueous electrolyte is liquid, and is not a solid electrolyte that is gelled, so it is preferable that the non-aqueous electrolyte does not contain an inorganic oxide. Furthermore, since the non-aqueous electrolyte is not gelled by heating and contains an aprotic solvent, it is not necessary to use an ionic liquid, but this does not exclude the addition of a small amount of an ionic liquid as long as the effect is not impaired.

[0050] In addition to the above components, additives such as acid anhydride, sulfonic acid ester, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, and t-butylbenzene may be added to the non-aqueous electrolyte solution as appropriate.

[0051] The lithium ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, and the nonaqueous electrolyte, and the nonaqueous electrolyte is in a liquid state. In the lithium ion secondary battery, the oxetane ring of the oxetane ring-containing compound contained in the nonaqueous electrolyte is preferably essentially unreacted and remains as the oxetane ring, at least at the time of completion of the battery (in a new state). The oxetane ring may react to form a crosslinked structure by repeated charge and discharge.

[0052] The positive electrode contains at least a positive electrode active material. The positive electrode may be, for example, a current collector made of a metal such as aluminum foil, on one or both sides of which a positive electrode mixture layer containing the positive electrode active material is formed.

[0053] The positive electrode mixture layer can be formed by applying a positive electrode mixture-containing paint to a current collector, drying the paint, and compressing and molding the applied paint. The positive electrode mixture-containing paint can be obtained by dispersing and kneading a positive electrode active material together with a conductive agent such as carbon black or graphite and a binder such as polyvinylidene fluoride (PVDF) in a dispersion medium such as N-methyl-2-pyrrolidone (NMP).

[0054] The positive electrode active material is not particularly limited as long as it is capable of inserting and extracting lithium ions. Examples include CuO, Cu ２ O, MnO ２ , V ２ O ５ , CrO ３ , MoO ３ , Fe ２ O ３ , Ni ２ O ３ , CoO ３ Metal oxides such as Li ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ Mn ２ O ４ , LiFePO ４ and composite oxides of lithium and transition metals such as TiS ２ , MoS ２ , NbSe ３ and conductive polymer compounds such as polyacene, polyparaphenylene, polypyrrole, and polyaniline. Among the above, composite oxides of lithium and one or more transition metals selected from cobalt, nickel, and manganese, which are generally called high-voltage oxides, are preferred in terms of lithium ion release properties and ease of obtaining high voltage. Specific examples of composite oxides of lithium with cobalt, nickel, and manganese include LiCoO ２ , LiMnO ２ , LiMn ２ O ４ , LiNiO ２ , LiNi ｘ Co （１－ｘ） O ２ , LiMn ａ Ni ｂ Co ｃ O ２ (a+b+c=1) and the like. In addition, these lithium composite oxides are doped with a small amount of elements such as fluorine, boron, aluminum, chromium, zirconium, molybdenum, and iron, and the particle surfaces of the lithium composite oxides are coated with carbon, MgO, Al ２ O ３ , SiO ２It is also possible to use a cathode active material that has been surface-treated with, etc. Two or more of the above cathode active materials can be used in combination.

[0055] The negative electrode contains at least a negative electrode active material. For example, the negative electrode may be one in which a negative electrode mixture layer containing the negative electrode active material is formed on one or both sides of a current collector made of a metal such as copper foil.

[0056] The negative electrode mixture layer can be formed by applying a negative electrode mixture-containing paint to a current collector, drying the paint, and compressing and molding the paint. The negative electrode mixture-containing paint can be obtained by dispersing and kneading a negative electrode active material together with a binder such as styrene butadiene rubber (SBR) in a dispersion medium such as water. The negative electrode mixture-containing paint may further contain a thickener such as carboxymethyl cellulose salt (CMC) or a conductive agent such as carbon black. The negative electrode mixture layer may also be formed by a method such as vapor deposition, CVD, or sputtering.

[0057] The negative electrode active material is not particularly limited as long as it can insert and extract metallic lithium or lithium ions. Examples include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon. Metallic materials such as metallic lithium, alloys, and tin compounds, lithium transition metal nitrides, crystalline metal oxides, amorphous metal oxides, silicon compounds, and conductive polymers can also be used. Specific examples include silicon monoxide (SiO), tin oxide, indium oxide, zinc oxide, lithium oxide, and Li ４ Ti ５ O １２ , NiSi ５ C ６ etc.

[0058] The method for producing a lithium ion secondary battery is not particularly limited, and can be a known method. For example, a positive electrode and a negative electrode are stacked with a separator interposed therebetween to form a laminate. Alternatively, the positive electrode and the negative electrode are rolled flat with the separator interposed therebetween, and then molded into a wound body. The laminate or wound body is then inserted into an exterior body such as a can or a laminate material, and a nonaqueous electrolyte is injected into the exterior body and sealed, thereby obtaining a lithium ion secondary battery.

[0059] As the separator, for example, a nonwoven fabric, a polyolefin microporous film, or the like, which is generally used in lithium ion secondary batteries, can be used.

[0060] The shape of the lithium ion secondary battery is not particularly limited, and it can be formed into, for example, a cylindrical shape, a coin shape, a square shape, or any other shape.

[0061] The present invention will be explained in more detail below based on examples and comparative examples, but the present invention is not limited thereto.

[0062] [Measurement and Evaluation Methods] (Weight-Average Molecular Weight) A sample was dissolved in tetrahydrofuran (THF), and the polystyrene-equivalent weight-average molecular weight (Mw) was measured using gel permeation chromatography (GPC) (Prominence, manufactured by Shimadzu Corporation) connected to four columns (Shodex GPC columns KF-601, KF-602, KF-603, KF-604, manufactured by Resonac Corporation) filled with polystyrene gel. The measurement conditions were a column oven temperature of 40°C, a THF flow rate of 0.6 mL / min, a sample concentration of 0.1% by mass, and a sample injection amount of 100 μL. A differential refractive index detector (Shodex RI-504, manufactured by Resonac Corporation) was used for detection.

[0063] (Number of Oxetane Rings Per Molecule of Oxetane Ring-Containing Compound) The number of terminals per molecule of the oxetane ring-containing compound is calculated, and the number of terminals is multiplied by the introduction ratio of oxetane rings to the terminals to determine the number of oxetane rings per molecule of the oxetane ring-containing compound. The method for calculating the number of terminals per molecule is as follows. When the polyol constituting the oxetane ring-containing compound is made up of only a bifunctional hydroxyl group raw material and the skeleton of the compound is not branched, the number of terminals per molecule of the oxetane ring-containing compound is two. When a trifunctional hydroxyl group raw material is used as the polyol, the number of terminals per molecule of the oxetane ring-containing compound can be determined as follows. From the charge mass ratio during synthesis of the oxetane ring-containing compound, the amount of bifunctional hydroxyl group raw material (Mw:B) present per 1 mol of trifunctional hydroxyl group raw material (Mw:A) is calculated, and the result is designated as Z mol. In this case, a repeating unit having a chemical structure in which, on average, Z units of bifunctional hydroxyl group raw material and (2 + Z) units of bifunctional isocyanate raw material (Mw: C) are bonded per molecule of trifunctional hydroxyl group raw material is formed. Therefore, when one molecule of trifunctional hydroxyl group raw material is introduced into the skeleton of an oxetane ring-containing compound, the average increase in the Mw of the entire oxetane ring-containing compound is calculated. The molecular weight D of the repeating unit per molecule of trifunctional hydroxyl group raw material is expressed by the following formula: D = A + B × Z + C × (2 + Z). When there is no trifunctional hydroxyl group raw material, the number of terminals is two. For each additional molecule of trifunctional hydroxyl group raw material, one terminal is added, increasing the number of terminals. Therefore, if the Mw of the oxetane ring-containing compound obtained by GPC measurement is E, the number of terminals per molecule of the oxetane ring-containing compound is calculated as (2 + E / D).

[0064] (Viscosity) The electrolyte solution was placed in a 20 mL sample tube, and the viscosity at 25° C. was measured using a vibration viscometer (VM-10A-L, manufactured by Sekonic).

[0065] (Battery Discharge Capacity) The fabricated lithium ion battery was charged at a constant current of 0.2 C, and then discharged at a constant current of 0.2 C. The discharge capacity per unit mass of the positive electrode active material (unit: mAh / g) was obtained by dividing the discharge capacity by the mass of the positive electrode active material. The 0.2 C current value refers to a current value that is 0.2 times the current value of 1 C at which the cell capacity can be discharged in 1 hour.

[0066] (Cycle Characteristics) The fabricated lithium-ion batteries were charged at a constant current of 0.5 C and then discharged at a constant current of 1 C. This cycle was repeated for a certain number of cycles. In the third experimental example, 300 cycles were performed, and in the other examples and comparative examples, 600 cycles were performed. The cycle characteristics were defined as the value obtained by dividing the discharge capacity at the 300th or 600th cycle by the discharge capacity at the first cycle.

[0067] Synthesis Example 1 Synthesis of Oxetane Ring-Containing Compound 1 27.6 parts by mass of polycarbonate diol (product name: ETERNACOLL UH-200, manufactured by UBE Corporation, weight average molecular weight 2000), 0.9 parts by mass of trifunctional polyether polyol (product name: DK Polyol G-480, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), 6 parts by mass of dicyclohexylmethane 4,4′-diisocyanate, 41.5 parts by mass of diethyl carbonate, and 0.05 parts by mass of Sn catalyst (product name: T-320, manufactured by Songwon Industrial Co., Ltd.) were added to a separable flask, and the mixture was heated and stirred at 80° C. for about 2 hours to allow a urethanization reaction to proceed.

[0068] Next, 1.2 parts by mass of 3-hydroxymethyl-3-ethyloxetane and 13.8 parts by mass of diethyl carbonate were added to a separable flask, and the mixture was reacted by heating and stirring at 80° C. for about 2 hours. Thereafter, 8.9 parts by mass of diethyl carbonate was added, and the mixture was cooled with water. The resulting solution was diluted with 19 parts by mass of diethyl carbonate to obtain a solution of oxetane ring-containing compound 1 (hereinafter referred to as Additive 1) with a solids content adjusted to 30% by mass.

[0069] The obtained oxetane ring-containing compound 1 has a polycarbonate diol-derived structure and a trifunctional polyether polyol-derived structure as polyol-derived structures, and a hydrogenated MDI-derived structure as polyisocyanate-derived structure, and is a polyurethane formed by bonding these structures via urethane bonds, and an oxetane ring is introduced to each molecular terminal of the polyurethane via a urethane bond, i.e., a polyurethane having oxetane rings at all terminals. The weight-average molecular weight of oxetane ring-containing compound 1 was 90,000. The number of oxetane rings per molecule of oxetane ring-containing compound 1 was 8.9 on average.

[0070] Synthesis Example 2 Synthesis of Oxetane Ring-Containing Compound 2 26.2 parts by mass of polycarbonate diol (product name: ETERNACOLL UH-200, manufactured by UBE Corporation, weight average molecular weight 2000), 0.9 parts by mass of trifunctional polyether polyol (product name: DK Polyol G-480, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), 7.2 parts by mass of dicyclohexylmethane 4,4′-diisocyanate, 41.1 parts by mass of diethyl carbonate, and 0.05 parts by mass of Sn catalyst (product name: T-320, manufactured by Songwon Industrial Co., Ltd.) were added to a separable flask, and the mixture was heated and stirred at 80° C. for about 2 hours to allow a urethanization reaction to proceed.

[0071] Next, 1.2 parts by mass of 3-hydroxymethyl-3-ethyloxetane and 13.7 parts by mass of diethyl carbonate were added to a separable flask, and the mixture was reacted by heating and stirring at 80°C for approximately 2 hours. 8.8 parts by mass of diethyl carbonate was then added, and the mixture was cooled with water. To the resulting solution, 1 part by mass of 3-methoxypropylamine was added, and the mixture was stirred at room temperature for 1 hour. The resulting solution was diluted with 21.2 parts by mass of diethyl carbonate to obtain a solution of oxetane ring-containing compound 2 (hereinafter referred to as Additive 2) with a solids content adjusted to 30% by mass.

[0072] The obtained oxetane ring-containing compound 2 has a polycarbonate diol-derived structure and a trifunctional polyether polyol-derived structure as polyol-derived structures, and a hydrogenated MDI-derived structure as polyisocyanate-derived structure, and is a polyurethane formed by bonding these structures via urethane bonds, with 50% of the polyurethane molecular terminals having oxetane rings and 50% having methoxy groups introduced via urea bonds. The number of oxetane rings per molecule of oxetane ring-containing compound 2 was 2.2 on average. The weight-average molecular weight of oxetane ring-containing compound 2 was 30,000.

[0073] [First Experimental Example] Additive 1 and lithium hexafluorophosphate (LiPF ６ ), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed and dissolved, and the LiPF ６ The nonaqueous electrolyte solution of Example 1 was obtained by adjusting the concentration of EC to DEC to 1.0 mol / L, the volume ratio of EC to DEC to EC / DEC=30 / 70, and the concentration of oxetane ring-containing compound 1 to 1.0 mass %, respectively. Furthermore, the nonaqueous electrolyte solution of Comparative Example 1 was obtained in the same manner as in Example 1, except that Additive 1 was not added.

[0074] The viscosity of the obtained non-aqueous electrolyte solutions of Example 1 and Comparative Example 1 was measured, and lithium ion secondary batteries were fabricated using the non-aqueous electrolyte solutions, and the discharge capacity and cycle characteristics of the batteries were evaluated. The battery fabrication method was as follows.

[0075] <Preparation of Battery> (Preparation of Positive Electrode) Positive electrode active material NCM523 (LiNi ０．５ Co ０．２ Mn ０．３ O ２A positive electrode mixture-containing paint was prepared by mixing 94 parts by mass of acetylene black (Li-400, manufactured by Denka Co., Ltd.) as a conductive agent, 3 parts by mass of PVDF (KF Polymer, manufactured by Kureha Corporation) as a binder, and N-methyl-2-pyrrolidone as a dispersion medium to a solids content of 60% by mass. The resulting positive electrode mixture-containing paint was applied to aluminum foil (thickness 15 μm) as a current collector using a coating machine, and the resulting mixture was vacuum dried at 130°C for 8 hours and then subjected to roll press treatment to obtain a positive electrode.

[0076] (Preparation of negative electrode) 95.5 parts by mass of graphite (graphite) as a negative electrode active material, 0.5 parts by mass of acetylene black (manufactured by Denka Co., Ltd., Li-400) as a conductive agent, 2 parts by mass of CMC aqueous solution (BSH-6, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) as a thickener (solid content equivalent), 2 parts by mass of SBR aqueous solution (manufactured by JSR Co., Ltd., TRD-2001) as a binder (solid content equivalent), and pure water as a dispersion medium were mixed to a solid content of 40% by mass to prepare a negative electrode mixture-containing paint. The obtained negative electrode mixture-containing paint was applied to copper foil (thickness 10 μm) as a current collector using a coating machine, and then vacuum dried at 130 ° C. for 8 hours, followed by roll press treatment to obtain a negative electrode.

[0077] (Assembly of Battery) The positive electrode and negative electrode obtained above were laminated with a polyolefin-based single-layer separator sandwiched between them as a separator, and the positive electrode terminal and the negative electrode terminal were ultrasonically welded to each positive and negative electrode. This laminate was placed in an aluminum laminate packaging material and heat-sealed, leaving an opening for injection, to form a battery with a positive electrode area of ​​18 cm. ２ , negative electrode area 19.8cm ２ The nonaqueous electrolyte solutions of Example 1 and Comparative Example 1 were injected into the obtained pre-injection batteries through the injection openings, and the openings were sealed to obtain lithium ion secondary batteries.

[0078] The results are shown in Table 1 below. Example 1, in which the oxetane ring-containing compound was added, showed an increased discharge capacity and improved cycle characteristics compared to Comparative Example 1, in which no compound was added.

[0079]

[0080] [Second Experimental Example] (Examples 2 to 4) Additive 1 and lithium hexafluorophosphate (LiPF ６ ), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed and dissolved, and the LiPF ６ A solution was prepared so that the concentration of EC was 1.0 mol / L, the volume ratio of EC to DEC was EC / DEC = 30 / 70, and the concentration of oxetane ring-containing compound 1 was 1.0 mass %. 2.0 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of the obtained solution, to obtain nonaqueous electrolyte solutions of Examples 2 to 4.

[0081] The viscosity of the obtained non-aqueous electrolyte solutions of Examples 2 to 4 was measured, and lithium ion secondary batteries were fabricated using the non-aqueous electrolyte solutions to evaluate the discharge capacity and cycle characteristics of the batteries. In Example 2, the battery was fabricated in the same manner as in Experimental Example 1. In Example 3, lithium iron phosphate (LFP: LiFePO ) was used as the positive electrode active material. ４ In Example 4, a battery was fabricated in the same manner as in Experimental Example 1, except that lithium cobalt oxide (LCO: LiCoO ２ ) was used, and other conditions were the same as in the first experimental example to prepare a battery.

[0082] Example 5 A nonaqueous electrolyte solution of Example 5 was prepared in the same manner as in Example 2, except that 2.0 parts by mass of fluoroethylene carbonate (FEC) was used instead of 2.0 parts by mass of vinylene carbonate (VC), and the viscosity was measured. A lithium ion secondary battery was also prepared and the discharge capacity and cycle characteristics of the battery were evaluated.

[0083] Example 6 A solution was prepared using methyl ethyl carbonate (EMC) instead of diethyl carbonate (DEC) so that the volume ratio of EC to EMC in the solution was EC / EMC = 30 / 70, and the rest of the procedure was the same as in Example 2. A nonaqueous electrolyte solution of Example 6 was prepared and its viscosity was measured, and a lithium ion secondary battery was fabricated and the discharge capacity and cycle characteristics of the battery were evaluated.

[0084] Examples 7 and 8 Solutions were prepared so that the concentration of the oxetane ring-containing compound 1 was 0.5% by mass in Example 7 and 3.0% by mass in Example 8, and the rest of the procedures were the same as in Example 2. Non-aqueous electrolyte solutions of Examples 7 and 8 were prepared and their viscosities were measured, and lithium ion secondary batteries were fabricated and the discharge capacities and cycle characteristics of the batteries were evaluated.

[0085] Example 9 A solution was prepared using Additive 2 instead of Additive 1 so that the concentration of the oxetane ring-containing compound 2 in the solution was 1.0 mass %, and the rest of the procedure was the same as in Example 2. A nonaqueous electrolyte solution of Example 9 was prepared and its viscosity was measured, and a lithium ion secondary battery was fabricated and the discharge capacity and cycle characteristics of the battery were evaluated.

[0086] Comparative Example 2 A nonaqueous electrolyte solution of Comparative Example 2 was prepared in the same manner as in Example 2, except that Additive 1 was not added, and the viscosity was measured. A lithium ion secondary battery was also prepared, and the discharge capacity and cycle characteristics of the battery were evaluated.

[0087] Comparative Example 3 A nonaqueous electrolyte solution of Comparative Example 3 was prepared in the same manner as in Example 2, except that a polyether (Elexcel TA-210, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) was used instead of Additive 1, and a solution was prepared so that the concentration of the polyether in the solution was 1.0 mass %. The viscosity of the nonaqueous electrolyte solution was measured, and a lithium ion secondary battery was prepared and the discharge capacity and cycle characteristics of the battery were evaluated.

[0088] The results are shown in Table 2 below. When VC or FEC was blended as an additive, Examples 2 to 9, in which oxetane ring-containing compound 1 or 2 was added, showed improved cycle characteristics compared to Comparative Example 2, in which no compound was added. On the other hand, Comparative Example 3, in which a polyether having no oxetane ring was added, showed deteriorated cycle characteristics.

[0089]

[0090] [Third Experimental Example] (Example 10) Additive 1 and lithium hexafluorophosphate (LiPF ６ ), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed and dissolved, and the LiPF ６The nonaqueous electrolyte solution of Example 10 was prepared by adjusting the concentration of EC to 1.2 mol / L, the volume ratio of EC to DEC to EC / DEC=30 / 70, and the concentration of oxetane ring-containing compound 1 to 1.0 mass %, and the viscosity of the obtained nonaqueous electrolyte solution of Example 10 was measured, and a lithium ion secondary battery was fabricated using the nonaqueous electrolyte solution in the same manner as in Experimental Example 1, and the discharge capacity and cycle characteristics of the battery were evaluated.

[0091] Example 11 A solution was prepared using Additive 2 instead of Additive 1 so that the concentration of the oxetane ring-containing compound 2 in the solution was 1.0 mass %, and the rest of the procedure was the same as in Example 10. A nonaqueous electrolyte solution of Example 11 was prepared and its viscosity was measured, and a lithium ion secondary battery was fabricated and the discharge capacity and cycle characteristics of the battery were evaluated.

[0092] Example 12 A solution was prepared so that the concentration of the oxetane ring-containing compound 2 was 2.0 mass %, and the rest of the procedure was the same as in Example 11, whereby a nonaqueous electrolyte solution of Example 12 was prepared and its viscosity was measured. A lithium ion secondary battery was also prepared and its discharge capacity and cycle characteristics were evaluated.

[0093] Comparative Example 4 A nonaqueous electrolyte solution of Comparative Example 4 was prepared in the same manner as in Example 10, except that Additive 1 was not added, and the viscosity was measured. A lithium ion secondary battery was also prepared, and the discharge capacity and cycle characteristics of the battery were evaluated.

[0094] The results are shown in Table 3 below. In Examples 10 to 12, in which oxetane ring-containing compound 1 or 2 was added, the discharge capacity of the battery increased and the cycle characteristics improved compared to Comparative Example 4, in which no compound was added.

[0095]

[0096] [Fourth Experimental Example] (Example 13) Additive 1 and lithium hexafluorophosphate (LiPF ６ ), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed and dissolved, and LiPF in the solution was ６A solution was prepared so that the concentration of EC was 1.0 mol / L, the volume ratio of EC to EMC was EC / EMC = 30 / 70, and the concentration of oxetane ring-containing compound 1 was 1.0 mass %. 1.0 part by mass of vinylene carbonate (VC) and 2.0 parts by mass of fluoroethylene carbonate (FEC) were added to 100 parts by mass of the obtained solution, to obtain a nonaqueous electrolyte solution of Example 13.

[0097] The viscosity of the obtained non-aqueous electrolyte solution of Example 13 was measured, and a lithium ion secondary battery was fabricated using the non-aqueous electrolyte solution to evaluate the discharge capacity and cycle characteristics of the battery. ０．８ Co ０．１５ Al ０．０５ O ２ ) was used as the negative electrode active material, and silicon monoxide (SiO) was used. The positive electrode was fabricated in the same manner as in the first experimental example, and the negative electrode was fabricated in the following manner.

[0098] (Synthesis of SiO negative electrode binder) A four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet tube was charged with 69.17 parts by mass of hydrogenated polybutadiene polyol (manufactured by Nippon Soda Co., Ltd. "NISSO-PB GI-1000"), 4.17 parts by mass of dimethylolpropionic acid, 25.97 parts by mass of dicyclohexylmethane 4,4'-diisocyanate (hydrogenated MDI), and 150 parts by mass of methyl ethyl ketone. The reaction was carried out at 75 ° C. for 4 hours to obtain a methyl ethyl ketone solution of a urethane prepolymer having a free isocyanate group content of 2.1% by mass relative to the non-volatile content. This solution was cooled to 45 ° C., and an aqueous sodium hydroxide solution consisting of 1.24 parts by mass of sodium hydroxide and 233 parts by mass of water was gradually added and emulsified and dispersed using a homogenizer. Subsequently, an aqueous solution prepared by diluting 1.48 parts by mass of diethylenetriamine with 37 parts by mass of water was added, and a chain extension reaction was carried out for 1 hour. The solvent was removed by heating at 50°C under reduced pressure, and an aqueous dispersion of sodium salt of polyurethane (binder 1) with a nonvolatile content of approximately 32% by mass was obtained.

[0099] (Aqueous Acetylene Black Dispersion) 100 g of acetylene black ("Li400" manufactured by Denka Co., Ltd.) was added to 300 g of a 1% by mass aqueous solution of carboxymethyl cellulose salt ("Cellogen 7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) with stirring using a high-speed dispenser, and the mixture was stirred until homogenous, thereby obtaining an aqueous acetylene black dispersion with an acetylene black concentration of 25% by mass.

[0100] (Fibrous nanocarbon aqueous dispersion) Single-walled carbon nanotubes (SWCNTs) were used, such as "TUBALL BATT" manufactured by OCSiAl (CNT purity >93%, average diameter 1.6±0.5 nm). 0.5 g of SWCNTs were mixed with 50 g of a 1% by mass aqueous solution of carboxymethyl cellulose salt (Cellogen 7A manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) in a beaker and stirred. The slurry was then circulated using a beaker, an ultrasonic homogenizer (US-600T manufactured by Nippon Seiki Seisakusho Co., Ltd.), a circulation unit, and a tube pump, while the mixture was dispersed at an output of 100 μA for 90 minutes, thereby obtaining a fibrous nanocarbon aqueous dispersion with a fibrous nanocarbon concentration of 1% by mass. (Fabrication of SiO negative electrode) SiO (average particle size 4.5 μm, specific surface area 5.5 m) was used as the negative electrode active material. ２ / g) 88.85 parts by weight, 40 parts by weight of a fibrous nanocarbon aqueous dispersion as a conductive agent and 4.0 parts by weight of an acetylene black aqueous dispersion (solid content concentration: 25% by weight), 43.3 parts by weight of a 2.0% by weight aqueous solution of carboxymethylcellulose sodium salt as a thickener, and 30 parts by weight of an aqueous dispersion of polyurethane sodium salt of binder 1, and 14 parts by weight of ion-exchanged water were used, and these were mixed with a planetary mixer to prepare a negative electrode mixture-containing paint so that the solid content was 49% by weight. The obtained negative electrode mixture-containing paint was applied to copper foil (thickness 10 μm) as a current collector using a coating machine, vacuum dried at 130 ° C. for 8 hours, and then subjected to roll press treatment to obtain a negative electrode.

[0101] Comparative Example 5 A nonaqueous electrolyte solution of Comparative Example 5 was prepared in the same manner as in Example 13, except that Additive 1 was not added, and the viscosity was measured. A lithium ion secondary battery was also prepared, and the discharge capacity and cycle characteristics of the battery were evaluated.

[0102] The results are shown in Table 4 below. Example 13, in which oxetane ring-containing compound 1 was added, showed improved cycle characteristics compared to Comparative Example 5, in which no compound was added.

[0103]

[0104] The various numerical ranges described in this specification can be arbitrarily combined with their respective upper and lower limit values, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y.

[0105] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. A non-aqueous electrolyte additive for a lithium ion secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, the non-aqueous electrolyte additive comprising a compound having an oxetane ring at a terminal and a urethane bond.

2. The additive for a non-aqueous electrolyte according to claim 1, wherein the compound has a structure derived from a polycarbonate polyol.

3. The additive for a non-aqueous electrolyte according to claim 1, wherein the compound has an average number of oxetane rings per molecule of 2.0 to 100.

4. The additive for a non-aqueous electrolyte according to claim 1, wherein the weight average molecular weight of said compound is 2,500 to 200,000.

5. A non-aqueous electrolyte for a lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the non-aqueous electrolyte including a supporting salt, an aprotic solvent, and a compound having an oxetane ring at its terminal and a urethane bond.

6. The nonaqueous electrolyte according to claim 5, further comprising vinylene carbonate and / or fluoroethylene carbonate.

7. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the nonaqueous electrolyte according to claim 5 or 6.

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