Electrolyte for energy storage devices, energy storage devices, and methods for manufacturing energy storage devices

Abstract

It enables the energy storage device to achieve its full capacity even in low-temperature environments. [Solution] The electrolyte for the energy storage device comprises an electrolyte containing lithium ions and a mixed solvent of sulfolanes and carboxylic acid esters, prepared through a preparation process. The sulfolanes include sulfolanes, sulfolane derivatives, or both. The carboxylic acid esters are dicarboxylic acid esters having a linear carbon skeleton with two or more carbon atoms, dicarboxylic acid esters having a cyclic carbon skeleton, or both.

Description

[Technical Field]

[0001] The present invention relates to an electrolyte for an energy storage device and an energy storage device equipped with this electrolyte. [Background technology]

[0002] There are energy storage devices that use an electrolyte containing lithium ions, such as lithium-ion secondary batteries and lithium-ion capacitors. Lithium-ion secondary batteries have Faraday reaction electrodes at the positive and negative electrodes, in which lithium ions are reversibly inserted and removed. One electrode of a lithium-ion capacitor is a Faraday reaction electrode in which lithium ions are reversibly inserted and removed, while the counter electrode is a polarizing electrode that utilizes the energy storage effect of an electric double layer formed at the interface between activated carbon and the electrolyte, for example.

[0003] An electrolyte is interposed between the positive and negative electrodes of these energy storage devices. Hereinafter, the electrolyte used in these energy storage devices will be referred to as the electrolyte for energy storage devices. The electrolyte for energy storage devices uses carbonate-based solvents such as propylene carbonate and diethyl carbonate, or carboxylic acid esters such as γ-butyrolactone (see Patent Documents 1 and 2).

[0004] However, propylene carbonate, diethyl carbonate, and γ-butyrolactone decompose through electrochemical reactions, generating gases and deposits within the energy storage device. The generation of gases and deposits can lead to a degradation of the energy storage device's capacity, an increase in resistance, and even valve opening of the case. Therefore, energy storage devices using electrochemically stable sulfolane as the solvent for the electrolyte have been proposed (see, for example, Patent Document 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-326023 [Patent Document 2] Japanese Patent Publication No. 2001-217150 [Patent Document 3] Japanese Patent Application Publication No. 06-275468 [Overview of the project] [Problems that the invention aims to solve]

[0006] In recent years, the electrification of automobiles has progressed, and energy storage devices that use electrolytes containing lithium ions, such as lithium-ion secondary batteries and lithium-ion capacitors, are becoming promising for automotive applications. Energy storage devices for automotive applications are expected to be used in low-temperature environments, such as -20°C. However, sulfolane has a freezing point of 28°C. Therefore, energy storage devices that use sulfolane as the solvent for the electrolyte may become unusable in low-temperature environments, as their charge and discharge capacity will become zero.

[0007] The present invention was proposed to solve the above problems, and its objective is to provide an electrolyte for an energy storage device, an energy storage device, and a method for manufacturing an energy storage device that exhibits capacity even in low-temperature environments. [Means for solving the problem]

[0008] To solve the above problems, the electrolyte for the energy storage device of this embodiment comprises an electrolyte containing lithium ions and a mixed solvent of sulfolanes and carboxylic acid esters, wherein the sulfolanes include sulfolanes, sulfolane derivatives, or both, and the carboxylic acid ester is a dicarboxylic acid ester having a linear carbon skeleton with 2 or more carbon atoms, a dicarboxylic acid ester having a cyclic carbon skeleton, or both.

[0009] The mixed solvent may further contain a monocarboxylic acid ester.

[0010] The monocarboxylic acid ester may have a chain-like structure.

[0011] The mixed solvent may contain two or more of the dicarboxylic acid esters.

[0012] The dicarboxylic acid ester having a cyclic carbon skeleton may be dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 1,2-cyclohexanedicarboxylate, diethyl 1,2-cyclohexanedicarboxylate, dimethyl 1,3-cyclohexanedicarboxylate, dimethyl 1,3-cyclopentanedicarboxylate, or trimethyl 1,3,5-cyclohexanetricarboxylate.

[0013] A dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms may have a carbon skeleton with three to six carbon atoms.

[0014] Dicarboxylic acid esters having a linear carbon skeleton with two or more carbon atoms may have the carbon skeleton containing a methyl group, an ethyl group, a propyl group, or a butyl group as a branched chain.

[0015] Furthermore, in order to solve the above problems, the energy storage device of this embodiment comprises a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte contains an electrolyte containing lithium ions and a mixed solvent of sulfolanes and carboxylic acid esters, the sulfolanes are sulfolanes, sulfolane derivatives, or both, and the carboxylic acid esters are dicarboxylic acid esters having a linear carbon skeleton with 2 or more carbon atoms, dicarboxylic acid esters having a cyclic carbon skeleton, or both.

[0016] Furthermore, in order to solve the above problems, the method for manufacturing the energy storage device of this embodiment includes a preparation step of preparing an electrolyte, an element formation step of forming an element having a positive electrode and a negative electrode, and an impregnation step of impregnating the element with the electrolyte, wherein in the preparation step, an electrolyte and a solvent are mixed, the electrolyte contains lithium ions, the solvent contains sulfolanes and carboxylic acid esters, the sulfolanes are sulfolanes, sulfolane derivatives or both, and the carboxylic acid esters are dicarboxylic acid esters having a linear carbon skeleton with 2 or more carbon atoms, dicarboxylic acid esters having a cyclic carbon skeleton, or both. [Effects of the Invention]

[0017] According to the present invention, the energy storage device can achieve its capacity even in a low-temperature environment, even when using a solvent containing sulfolanes as the electrolyte. [Modes for carrying out the invention]

[0018] The following describes embodiments for carrying out the present invention. However, the present invention is not limited to the embodiments described below.

[0019] (Electrolyte for energy storage devices) The electrolyte of this embodiment is an electrolyte for energy storage devices. The electrolyte for energy storage devices is used in energy storage devices that use lithium ions as the electrolyte. The electrolyte for energy storage devices is prepared by mixing an electrolyte, a solvent, and additives. The electrolyte consists of lithium ions and anions, that is, the electrolyte contains lithium salts that undergo ion dissociation.

[0020] (solvent) The solvent is a mixed solvent containing sulfolanes and specific dicarboxylic acid esters (hereinafter referred to as "specific dicarboxylic acid esters"). The sulfolanes are sulfolanes, sulfolane derivatives, or both. The specific dicarboxylic acid esters are dicarboxylic acid esters having a linear carbon skeleton with two or more carbon atoms (hereinafter referred to as "linear-skeleton dicarboxylic acid esters"), dicarboxylic acid esters having a cyclic carbon skeleton (hereinafter referred to as "cyclic-skeleton dicarboxylic acid esters"), or both.

[0021] Sulfolane derivatives are sulfolanes in which substituents such as alkyl groups are added to a five-membered ring. They are as stable as sulfolane and have a freezing point higher than -20°C, considering their use in vehicles. For example, sulfolane derivatives include 3-methylsulfolane, which has a freezing point of 6°C, or 3-ethylsulfolane, which has a freezing point of -3°C. Sulfolane has a freezing point of 28°C. Preferably, both sulfolane and sulfolane derivatives are used as solvents. By using both, the freezing point of the solvent can be lowered compared to using sulfolane alone.

[0022] Linear dicarboxylic acid esters are represented by the following chemical formula (1). (chemical 1) TIFF2026121160000001.tif30161 [In the formula, n and m are each 1 or greater.]

[0023] The X structure in the above chemical formula (1) is a substructure sandwiched between carboxylic acid ester groups at both ends. This X structure has a linear carbon skeleton. An example of a linear carbon skeleton is a chain hydrocarbon. This carbon skeleton has two or more carbon atoms along the straight chain. In other words, a linear dicarboxylic acid ester has a chain hydrocarbon with two or more carbon atoms along the straight chain, and both ends of the chain hydrocarbon are substituted with carboxylic acid ester groups.

[0024] The linear carbon skeleton may have methyl groups, ethyl groups, propyl groups, butyl groups, or a combination thereof attached as branched chains. That is, examples of linear skeleton dicarboxylic acid esters include dimethyl succinate, diethyl succinate, dimethyl glutarate, diethyl glutarate, dimethyl pimephosphate, diethyl pimephosphate, dimethyl 2-methylglutarate, and diethyl 2-methylglutarate.

[0025] A cyclic dicarboxylic acid ester has a cyclic carbon skeleton and two ester bonds. The cyclic carbon skeleton is a cycloalkane structure with four or more carbon atoms forming the skeleton. That is, a cyclic dicarboxylic acid ester is a cyclobutanedicarboxylic acid diester, a cyclopentanedicarboxylic acid diester, a cyclohexanedicarboxylic acid diester, or a cyclohexanetricarboxylic acid triester. Preferably, the cyclic dicarboxylic acid ester is a compound represented by any of the following chemical formulas (2) to (5).

[0026] (chemical 2) TIFF2026121160000002.tif85161

[0027] (3) TIFF2026121160000003.tif54161

[0028] (C4) TIFF2026121160000004.tif67161

[0029] (C5) TIFF2026121160000005.tif74161

[0030] The cyclic skeleton dicarboxylic acid ester of chemical formula (2) above is 1,4-cyclohexanedicarboxylic acid diester. An example of 1,4-cyclohexanedicarboxylic acid diester is 1,4-cyclohexanedicarboxylic acid dimethyl. The cyclic skeleton dicarboxylic acid ester of chemical formula (3) above is 1,2-cyclohexanedicarboxylic acid diester. An example of 1,2-cyclohexanedicarboxylic acid diester is 1,2-cyclohexanedicarboxylic acid dimethyl or 1,2-cyclohexanediethyl.

[0031] The cyclic skeleton dicarboxylic acid ester of chemical formula (4) above is 1,3-cyclohexanedicarboxylic acid diester. For example, 1,3-cyclohexanedicarboxylic acid diester is 1,3-dimethyl cyclohexanedicarboxylic acid. The cyclic skeleton dicarboxylic acid ester of chemical formula (5) above is 1,3-cyclopentanedicarboxylic acid diester. For example, 1,3-cyclopentanedicarboxylic acid diester is 1,3-dimethyl cyclopentanedicarboxylic acid. Note that cyclohexanetricarboxylic acid triester is, for example, 1,3,5-trimethyl cyclohexanetricarboxylic acid.

[0032] In the formula, n, m, and p are between 1 and 4, and the alkyl group may have a branched chain. Furthermore, these specific dicarboxylic acid esters may be in either the cis or trans form. Particularly preferred is dimethyl 1,4-cyclohexanedicarboxylate.

[0033] This particular dicarboxylic acid ester has a solubility parameter, the SP value, of 9 or higher. Carboxylic acid esters with an SP value of 9 or higher tend to have good electron acceptor properties, resulting in a large interaction between the ester group and the sulfonyl group of sulfolane. This interaction lowers the freezing point of sulfolane, improving the capacity of energy storage devices in low-temperature environments. Although this is a hypothesis and not limited to this, it is presumed that the freezing point depression of sulfolane follows the van't Hoff coefficient.

[0034] Two or more specific dicarboxylic acid esters may be used as solvents and mixed. By combining two or more specific dicarboxylic acid esters, the capacity of the energy storage device in low-temperature environments can be further improved, or the capacity can be achieved even in even lower temperatures such as -30°C.

[0035] Incidentally, for example, in the case of dimethyl malonate, where the hydrocarbon linking the two carboxylic acid ester groups has a carbon atom of C=1, there is one α-carbon sandwiched between the carboxylic acid ester groups at both ends. The α-carbon is a carbon that readily undergoes decomposition reactions because its α-hydrogen is easily removed. Therefore, if the α-carbons for both carboxylic acid ester groups are the same carbon, the hydrogen of the carboxylic acid ester is removed in the solvent to form an enolate, and when the enolate decomposes, gases that were not observed in sulfolane alone are generated.

[0036] On the other hand, in this particular dicarboxylic acid ester, the two carboxylic acid ester groups are not bonded to the same carbon, and the α-carbons exist separately. Therefore, when a mixed solvent containing this particular dicarboxylic acid ester and sulfolanes is used, the energy storage device exhibits its capacity even in low-temperature environments. In particular, in cyclic dicarboxylic acid esters, the cyclic carbon skeleton acts as a steric obstacle that inhibits the elimination of the α-hydrogen.

[0037] The number of carbon atoms along the linear chain of the hydrocarbon in the X structure of a linear dicarboxylic acid ester is preferably 3 to 6. In this range of 3 to 6 carbon atoms, the SP value becomes significantly higher than 9. For example, dimethyl succinate has an SP value of 9.52, dimethyl glutarate has an SP value of 10.36, dimethyl adipicate has an SP value of 9.90, dimethyl pimerate has an SP value of 9.87, dimethyl suberate has an SP value of 9.69, and dimethyl azelaate has an SP value of 7.59. When the number of carbon atoms along the linear chain of the hydrocarbon in the X structure is 3 to 6, the SO2 peak shift of the sulfonyl group of the sulfolane is 2.05 cm. -1 That concludes the explanation. Furthermore, the energy storage device exhibits discharge capacity even in low-temperature environments.

[0038] Furthermore, the Raman shift of the sulfonyl group measured by Raman spectroscopy using only sulfolane as the sample will be used as the reference value. In addition, the Raman shift of the sulfonyl group measured by Raman spectroscopy using a mixed solvent of sulfolane and a specific dicarboxylic acid ester will be used as the measured value. The SO2 peak shift is the difference between this measured value and the reference value. Specifically, the reference value is a Raman shift of 1140 cm⁻¹. -1 This is the peak that appears in the vicinity of [the specified value]. In a mixed solvent of sulfolane and a specific dicarboxylic acid ester, the Raman shift of the sulfonyl group changes from the reference value due to the interaction between sulfolane and the specific dicarboxylic acid ester. The peak after this change is the measured value.

[0039] To determine the SP value, if the SP value is δ, it can be calculated using the following formula. In the formula, ΔH is the enthalpy of evaporation and V is the molar volume [cm³]. 3 [ / mol], where R is the gas constant and T is the absolute temperature. Molar volume was calculated from molecular weight and specific gravity. δ = [(ΔH - RT) / V] 1 / 2

[0040] However, the amount of sulfolanes in the solvent is preferably 50 vol% to 90 vol% of the total amount of solvent in the electrolyte. If the amount of sulfolane is insufficient, the stability of the electrolyte decreases, and gas is more likely to be generated by electrochemical decomposition. On the other hand, the amount of specific dicarboxylic acid esters in the solvent is preferably 1 vol% to 50 vol% of the total amount of solvent in the electrolyte. If the amount of specific dicarboxylic acid esters is insufficient, the Raman shift of the sulfonyl group of the sulfolane decreases, and the effect of exhibiting discharge capacity in low-temperature environments decreases.

[0041] It is preferable to further include a monocarboxylic acid ester with one ester bond in addition to sulfolane and a specific dicarboxylic acid ester as the solvent for the electrolyte. With a mixed solvent containing sulfolane, a specific dicarboxylic acid ester, and a monocarboxylic acid ester, the capacity of the energy storage device in a low-temperature environment is further improved.

[0042] Examples of monocarboxylic acid esters include ethyl propionate, shown in chemical formula (6) below. (6) TIFF2026121160000006.tif27161

[0043] Furthermore, even when propylene carbonate and γ-butyrolactone are used in combination with sulfolane and specific dicarboxylic acid esters, a portion of the electrolyte decomposes, resulting in the formation of gas, deposits, or both. When 0.1 vol% of propylene carbonate or γ-butyrolactone was added to the solvent, capacity was achieved at low temperatures, but a large amount of gas and deposits were generated on the positive and negative electrodes. Therefore, it is preferable not to include propylene carbonate and γ-butyrolactone in the solvent, and it is even more preferable that the monocarboxylic acid ester has a chain-like structure.

[0044] (electrolyte) Next, the electrolyte of the electrolytic solution is lithium ions and anions. Any lithium salt that dissociates into lithium ions and anions may be used. Examples of the lithium salt include LiPF6, LiBF4, LiClO4, LiAsF6, LiTaF6, LiClO4, Li(FSO2)2N (commonly known as LiFSI), Li(SO2CF3)2N (commonly known as LiTFSI), Li(FSO2)(CF3FO2)N (commonly known as LiFTA), Li(SO2C2F5)2N (commonly known as LiBETI), LiB(C2O4)2 (commonly known as LiBOB), CF3SO3Li, LiC(SO2CF3)3, and LiPF3(C2F5)3, or a mixture thereof. The electrolyte is preferably contained at 1.0 mol or more and 3.0 mol or less per liter of the solvent.

[0045] In addition to the lithium salt, a quaternary ammonium salt or a quaternary phosphonium salt can be contained in the electrolytic solution as an electrolyte. For example, as the cation component, tetraethylammonium, triethylmethylammonium, diethyldimethylammonium, ethyltrimethylammonium, methylethylpyrrolidinium, spirobipyrrolidinium, spiro-(N,N’)-bipyrrolidinium, 1-ethyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium, and the like can be mentioned. As the anion component, BF4 - 、PF6 - 、ClO4 - 、AsF6 - 、SbF6 - 、AlCl4 - 、or RfSO3 - 、(RfSO2)2N - 、RfCO2 - (Rf is a fluoroalkyl group having 1 to 8 carbon atoms), and the like can be mentioned.

[0046] (Additive) The electrolyte may contain various additives. Examples of additives include cyclic carbonates having unsaturated bonds (vinylene carbonate, vinylethylene carbonate, etc.), sultone compounds (1,3-propanesultone, 1,3-propensultone, etc.), fluorinated carbonate esters (fluoroethylene carbonate, etc.), phosphoric acids and their derivatives (phosphoric acid, phosphorous acid, phosphoric acid esters, phosphonic acids, etc.), boric acids and their derivatives (boric acid, boric acid oxide, boric acid esters, complexes of boron with a hydroxyl group, a carboxyl group, or a compound having multiple thereof, etc.), nitrates (lithium nitrate, etc.), and nitro compounds (nitrobenzoic acid, nitrophenol, nitrophenetol, nitroacetophenone, aromatic nitro compounds, etc.). The electrolyte may also contain a gas absorbent. The gas absorbent generated from the electrode is not particularly limited as long as it does not react with the components of the electrolyte (solvent, electrolyte salt, various additives, etc.) and does not remove (adsorb, etc.) the gas. Specific examples include zeolite and silica gel.

[0047] (Energy storage device) Energy storage devices equipped with such an electrolyte include lithium-ion secondary batteries and lithium-ion capacitors. These energy storage devices comprise a positive electrode, a negative electrode, a separator, and an electrolyte. The elements of the energy storage device have the positive and negative electrodes facing each other with a separator in between. The electrolyte fills the voids within the elements.

[0048] A lithium-ion secondary battery has Faraday reaction electrodes at its positive and negative electrodes, in which lithium ions are reversibly inserted and removed. The lithium-ion secondary battery charges and discharges according to the direction of lithium ion absorption and release at the positive and negative electrodes. During charging, lithium ions are released from the positive electrode and absorbed at the negative electrode, and during discharging, lithium ions are released from the negative electrode and absorbed at the positive electrode.

[0049] A lithium-ion capacitor has a positive electrode, which is a polarizing electrode that utilizes the energy storage effect of an electric double layer formed at the interface between the positive electrode active material and the electrolyte, and a negative electrode, which is a Faraday reaction electrode in which lithium ions are reversibly inserted and removed. The counter electrode is a carbon material such as activated carbon. In the energy storage effect of the electric double layer, for example, anions of the electrolyte align at the interface with the polarizing electrode and form pairs with electrons in the polarizing electrode at a very short distance apart, forming a potential barrier at the negative electrode.

[0050] (electrode) The positive electrode consists of a positive electrode active material supported by a current collector, and the negative electrode consists of a negative electrode active material supported by a current collector. The current collectors of the positive and negative electrodes are, for example, metals such as aluminum, platinum, gold, copper, iron, nickel, titanium, or steel; conductive polymer materials such as carbon, polyaniline, polypyrrole, polythiophene, polyacetylene, poly-p-phenylene, polyphenylene vinylene, polyacrylonitrile, or polyoxadiazole; or resins in which conductive fillers are filled into non-conductive polymer materials. Aluminum or copper, which have high thermal and electronic conductivity, are particularly preferred.

[0051] (Separator) As the base material separator, cellulose and mixed papers such as kraft, Manila hemp, esparto, hemp, and rayon, as well as polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives, polyamide resins such as polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins, aliphatic polyamides, semi-aromatic polyamides, and fully aromatic polyamides, polyimide resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, and acrylic resins can be used individually or in mixtures.

[0052] (Lithium-ion rechargeable battery) (positive electrode) Examples of positive electrode active materials include lithium-containing oxides or oxyacids. Lithium-containing oxides or oxyacids include Li αM β Y γ It is represented as follows: In the case of metal oxides, for example, metal M can be one or more of Co, Ni, Mn, Ti, Si, Sn, Al, Zn, Mg, Nb, and V. In the chemical formula, Y is oxygen O. In the case of metal oxyacid salts, for example, metal M can be one or more of Fe, Mn, V, Co, and Ni. In the chemical formula, Y is, for example, PO4, SiO4, BO3, or P2O7. Note that α, β, and γ are greater than 0.

[0053] M β M δ M' ε It may be an alloy of, for example, M = one of Sn, Sb, or Si, and M' = one of Fe, Co, Mn, V, Ti, or Ni. For example, M δ M' ε These are alloys such as Sn3V2 and Sb3Co. Note that δ and ε are greater than 0.

[0054] For example, lithium-containing oxides or oxyacid salts are used as positive electrode active materials, such as lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and lithium titanate (Li4Ti5O4). 12 ), lithium cobalt oxide (LiCoO2), lithium vanadium phosphate (Li3V2(PO4)3), lithium iron manganese phosphate (LiFe 1 / 2 Mn 1 / 2 PO4), Lithium nickel cobalt aluminum oxide (LiNi 0.8 Co 0.15 Al 0.05 O2), Lithium nickel manganese cobalt oxide (LiNi x Mn y Co z Examples include O2 (satisfying x+y+z=1), lithium manganese phosphate (LiMnPO4), LiVOPO4, LiV3O5, LiV3O8, MoV2O8, Li2FeSiO4, Li2MnSiO4, etc. Among these, lithium manganese oxide (LiMn2O4) is preferred.

[0055] Examples of positive electrode active materials include layered rock salt type LiMO2, layered Li2MnO3-LiMO2 solid solution, and spinel type LiM2O4, or they may be used together with lithium-containing oxides or oxyacid salts as positive electrode active materials. In each chemical formula, the metal M is Mn, Fe, Co, Ni, or a combination thereof.

[0056] Specific examples of these include LiCoO2, LiNiO2, and LiNi 4 / 5 Co 1 / 5 O2, LiLiLi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiLiLi 1 / 2 Mn 1 / 2 O2, LiFeO2, LiMnO2, Li2MnO3-LiCoO2, Li2MnO3-LiNiO2, Li2MnO3-LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li2MnO3-LiNi 1 / 2 Mn 1 / 2 O2, Li2MnO3-LiNi 1 / 2 Mn 1 / 2 O2-LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiMn2O4, LiMn 3 / 2 Ni 1 / 2 O4 is one example.

[0057] Positive electrode active materials include sulfur and Li2S, TiS2, MoS2, FeS2, VS2, and Cr. 1 / 2 V 1 / 2 Sulfides such as S2, selenides such as NbSe3, VSe2, NbSe3, Cr2O5, Cr3O8, VO2, V3O8, V2O5, V6O 13 Oxides such as stencil 2.5 Co 0.4 Nitride metal compounds such as N can be used, or they may be used together with lithium-containing oxides or oxyacid salts as positive electrode active materials.

[0058] A conductive additive may be mixed with the positive electrode active material to form the positive electrode. The conductive additive improves the electrical conductivity of the positive electrode active material. Examples of conductive additives include carbon materials. Suitable carbon materials include carbon nanotubes and conductive carbon black having a hollow shell structure (e.g., Ketjenblack®), but carbon nanofibers, carbon black such as acetylene black, amorphous carbon, carbon fibers, natural graphite, artificial graphite, activated carbon, mesoporous carbon, nanoporous carbon, graphene, fullerene, or mixtures thereof are also applicable.

[0059] (Negative electrode) Examples of negative electrode active materials include carbon materials. These carbon materials include, for example, non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbons, cokes such as pitch coke, needle coke, and petroleum coke, graphites, glassy carbons, or sintered organic polymer compounds obtained by sintering phenolic resins or furan resins at an appropriate temperature.

[0060] Furthermore, suitable negative electrode active materials include oxides such as FeO, Fe2O3, Fe3O4, MnO, MnO2, Mn2O3, Mn3O4, CoO, Co3O4, NiO, Ni2O3, TiO, TiO2, TiO2(B), TiNb2O7, Nb2O5, CuO, NiO, SnO, SnO2, SiO2, RuO2, WO, WO2, WO3, MoO3, ZnO, metals such as Sn, Si, Al, and Zn, and materials such as LiVO2, Li3VO4, and Li4Ti5O. 12 , Sc2TiO5, Fe2TiO5, Li2Na2Ti6O 14 Li2BaTi6O 14 Li2SrTi6O 14 such complex oxides, Li 2.6 Co 0.4 Examples include nitrides such as N, Ge3N4, Zn3N2, and Cu3N, as well as Y2Ti2O5S2 and MoS2.

[0061] Among these, examples of negative electrode active materials include lithium-containing oxides or oxyacids. Lithium-containing oxides or oxyacids are Li α M β Yγ It is represented as follows. In the case of metal oxides, for example, as metal M, one or more of Co, Ni, Mn, Ti, Si, Sn, Al, Zn, Mg, Nb, and V can be cited. In the chemical formula, Y is oxygen O. In the case of metal oxyacid salts, for example, as metal M, one or more of Fe, Mn, V, Co, and Ni can be cited. In the chemical formula, Y is, for example, PO4, SiO4, BO3, or P2O7. β M δ M' ε It may be an alloy of any of the following: for example, M = one of Sn, Sb, or Si, and M' = one of Fe, Co, Mn, V, Ti, or Ni. α, β, γ, δ, and ε are any numbers greater than 0.

[0062] For example, negative electrode active materials containing lithium include lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and lithium titanate (Li4Ti5O4). 12 ), lithium cobalt oxide (LiCoO2), lithium vanadium phosphate (Li3V2(PO4)3), lithium iron manganese phosphate (LiFe 1 / 2 Mn 1 / 2 PO4), Lithium nickel cobalt aluminum oxide (LiNi 0.8 Co 0.15 Al 0.05 O2), Lithium nickel manganese cobalt oxide (LiNi x Mn y Co z These include O2 (which satisfies x+y+z=1), lithium manganese phosphate (LiMnPO4), LiVOPO4, LiV3O5, LiV3O8, MoV2O8, Li2FeSiO4, Li2MnSiO4, etc. Among these, lithium titanate (Li4Ti5O 12 ) is preferable.

[0063] The negative electrode active material may be pre-doped with lithium ions. For example, an electrical short-circuit method can be used in the pre-doping process. In the electrical short-circuit method, a battery element is assembled in which the electrodes of the negative electrode active material layer and the lithium metal layer are facing each other with a separator in between. This battery element is immersed in an electrolyte solution, and the negative electrode active material layer and the lithium metal layer are externally short-circuited. This external short-circuiting charges the negative electrode active material layer, thereby doping the negative electrode active material with lithium ions. Then, by disassembling the battery element, the negative electrode active material with pre-doped lithium ions can be obtained. Alternatively, a method can be used in which lithium is brought into contact with the negative electrode to short-circuit and pre-dope the negative electrode active material with lithium ions.

[0064] (Lithium-ion capacitor) (positive electrode) The positive electrode active material is mainly composed of carbon material. The carbon material that constitutes the positive electrode active material has a porous or fibrous structure and possesses electrical double-layer capacitance. The carbon material may be subjected to opening treatment or activation treatment. An appropriate amount of conductive additive may be mixed into the positive electrode active material layer as needed.

[0065] The carbon material used as the positive electrode active material is a single element or a mixture of several elements selected from the following. Examples of porous positive electrode active materials include natural plant tissues such as coconut shells, synthetic resins such as phenol, activated carbon derived from fossil fuels such as coal, coke, or pitch, carbon black such as Ketjenblack, acetylene black, or channel black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized Ketjenblack, or mesoporous carbon.

[0066] Examples of fibrous cathode active materials include carbon nanotubes and carbon nanofibers. Carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) with a single layer of graphene sheet, or multi-walled carbon nanotubes (MWCNTs) with two or more layers of graphene sheet coiled coaxially and having multilayered tube walls, or a mixture of these.

[0067] The conductive additives include Ketjenblack, acetylene black, natural graphite, artificial graphite, or fibrous carbon such as carbon nanotubes or carbon nanofibers, or composites thereof.

[0068] A carbon coating layer containing a conductive agent such as graphite may be provided between the current collector and the active material layer. A carbon coating layer can be formed by applying a slurry containing a conductive agent such as graphite and a binder to the surface of the current collector and then drying it. Additionally, phosphorus, which inhibits hydration and oxidation of the current collector, may be attached to the surface of the current collector. For example, the current collector may be immersed in an aqueous solution of phosphoric acid or a phosphate solution.

[0069] (Negative electrode) Examples of negative electrode active materials include carbon materials. These carbon materials include, for example, non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbons, cokes such as pitch coke, needle coke, and petroleum coke, graphites, glassy carbons, or sintered organic polymer compounds obtained by sintering phenolic resins or furan resins at an appropriate temperature.

[0070] Furthermore, suitable negative electrode active materials include oxides such as FeO, Fe2O3, Fe3O4, MnO, MnO2, Mn2O3, Mn3O4, CoO, Co3O4, NiO, Ni2O3, TiO, TiO2, TiO2(B), TiNb2O7, Nb2O5, CuO, NiO, SnO, SnO2, SiO2, RuO2, WO, WO2, WO3, MoO3, ZnO, metals such as Sn, Si, Al, and Zn, and materials such as LiVO2, Li3VO4, and Li4Ti5O. 12 , Sc2TiO5, Fe2TiO5, Li2Na2Ti6O 14 Li2BaTi6O 14 Li2SrTi6O 14 such complex oxides, Li 2.6 Co 0.4 Examples include nitrides such as N, Ge3N4, Zn3N2, and Cu3N, as well as Y2Ti2O5S2 and MoS2.

[0071] Among these, examples of the negative electrode active material include oxides or oxygen acid salts containing lithium. The oxides or oxygen acid salts containing lithium are represented by Li α M β Y γ . In the case of a metal oxide, for example, as the metal M, one or more of Co, Ni, Mn, Ti, Si, Sn, Al, Zn, Mg, Nb, and V can be mentioned. In the chemical formula, Y is oxygen O. In the case of a metal oxygen acid salt, for example, as the metal M, one or more of Fe, Mn, V, Co, and Ni can be mentioned. In the chemical formula, Y is, for example, PO4, SiO4, BO3, or P2O7. M β may be an alloy of M δ M’ ε . For example, M is any one of Sn, Sb, and Si, and M’ is any one of Fe, Co, Mn, V, Ti, and Ni. Here, α, β, γ, δ, and ε are arbitrary numbers greater than 0.

[0072] For example, negative electrode active materials of oxides or oxygen acid salts containing lithium include lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), lithium titanate (Li4Ti5O 12 ), lithium cobalt oxide (LiCoO2), lithium vanadium phosphate (Li3V2(PO4)3), lithium iron manganese phosphate (LiFe 1 / 2 Mn 1 / 2 PO4), lithium nickel cobalt aluminum oxide (LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel manganese cobalt oxide (LiNi x Mn y Co z O2 (satisfying x + y + z = 1)), lithium manganese phosphate (LiMnPO4), LiVOPO4, LiV3O5, LiV3O8, MoV2O8, Li2FeSiO4, Li2MnSiO4, etc. Among them, lithium titanate (Li4Ti5O 12 ) is preferable.

[0073] The negative electrode active material may be pre-doped with lithium ions. For example, an electrical short-circuit method can be used in the pre-doping process. In the electrical short-circuit method, a battery element is assembled in which the electrodes of the negative electrode active material layer and the lithium metal layer are facing each other with a separator in between. This battery element is immersed in an electrolyte solution, and the negative electrode active material layer and the lithium metal layer are externally short-circuited. This external short-circuiting process charges the negative electrode active material layer, thereby doping the negative electrode active material with lithium ions. Then, by disassembling the battery element, a negative electrode active material pre-doped with lithium ions can be obtained. Alternatively, a method can be used in which lithium is brought into contact with the negative electrode to short-circuit and pre-dope the negative electrode active material with lithium ions.

[0074] (Manufacturing method for energy storage devices) The manufacturing method for energy storage devices is as follows: The energy storage device is manufactured through a positive electrode manufacturing process, a negative electrode manufacturing process, an element formation process, a preparation process, and an impregnation process. In the positive electrode manufacturing process, the positive electrode is manufactured. In the negative electrode manufacturing process, the negative electrode is manufactured. In the element formation process, an element is formed with the positive electrode and negative electrode facing each other. In the preparation process, an electrolyte for energy storage devices is prepared. In the impregnation process, the element is impregnated with the electrolyte for energy storage devices.

[0075] (Positive electrode manufacturing process) The positive electrode current collector is formed from metal, conductive polymer material, or resin by press molding, injection molding, etc., into a film, foil, plate, mesh, expanded metal, cylindrical, or the like. The surface of the current collector may have an uneven surface formed by etching, etc., or it may be a plain surface. The positive electrode active material and the current collector are joined via a binder using crimping or a doctor blade method, etc. A mixture of carbon material and binder may be formed into a sheet and crimped onto the current collector. Through holes may also be provided on the current collector.

[0076] For example, a positive electrode is formed by coating a current collector with a slurry of positive electrode active material and a binder, resulting in a layer of positive electrode active material laminated on the current collector. A conductive additive may be added to the slurry to incorporate the conductive additive within the positive electrode active material layer. The positive electrode active material layer may be pressure-molded into pellets by a press process, then rolled onto the current collector, or bonded to it using an adhesive.

[0077] Examples of binders include rubbers such as fluoropolymers, diene-based rubbers, and styrene-based rubbers; fluorine-containing polymers such as polytetrafluoroethylene and polyvinylidene fluoride; celluloses such as carboxymethylcellulose and nitrocellulose; and other materials such as polyolefin resins, polyimide resins, acrylic resins, nitrile resins, polyester resins, phenolic resins, polyvinyl acetate resins, polyvinyl alcohol resins, and epoxy resins. These binders may be used individually or in mixtures of two or more types.

[0078] (Negative electrode manufacturing process) In the negative electrode manufacturing process, the negative electrode current collector is formed from metal, conductive polymer material, or resin by press molding, injection molding, etc., into a film, foil, plate, mesh, expanded metal, cylindrical, etc. The surface of the current collector may have an uneven surface formed by etching, etc., or it may be a plain surface. The negative electrode active material and the current collector are joined via a binder using crimping or a doctor blade method, etc. A mixture of carbon material and binder may be formed into a sheet and crimped onto the current collector. Through holes may also be provided on the current collector.

[0079] For example, a negative electrode is formed by coating a current collector with a slurry of a negative electrode active material and a binder, resulting in a layer of negative electrode active material laminated on the current collector. A conductive additive may be added to the slurry to contain the conductive additive within the negative electrode active material layer. The negative electrode active material layer may be pressure-molded into pellets by a press process, then rolled onto the current collector, or bonded to it using an adhesive.

[0080] Examples of binders include rubbers such as fluoropolymers, diene-based rubbers, and styrene-based rubbers; fluorine-containing polymers such as polytetrafluoroethylene and polyvinylidene fluoride; celluloses such as carboxymethylcellulose and nitrocellulose; and other materials such as polyolefin resins, polyimide resins, acrylic resins, nitrile resins, polyester resins, phenolic resins, polyvinyl acetate resins, polyvinyl alcohol resins, and epoxy resins. These binders may be used individually or in mixtures of two or more types.

[0081] (Element formation process) In the element formation process, the positive and negative electrodes are placed facing each other, and a separator is used to insulate them. The positive and negative electrodes and separator may be stacked alternately, or the positive and negative electrodes may be stacked with the separator in between and then wound. In the battery assembly process, the positive and negative electrodes are placed facing each other, and an electrolyte is interposed between them to produce a lithium-ion secondary battery. The electrolyte is held in place by the separator.

[0082] (Preparation process) There are no limitations on the method for preparing the electrolyte for energy storage devices, but for example, it can be prepared as follows: First, a mixed solvent is prepared by mixing sulfolane and a specific dicarboxylic acid ester in any proportion. In a dry room, under normal temperature and atmospheric pressure conditions (1 atmosphere and 20°C), the electrolyte salt is added to a 1 L volumetric flask, and then the aforementioned mixed solvent is added until the total volume reaches 1 L. Additives other than the specific dicarboxylic acid ester are added and mixed together at the same time as the specific dicarboxylic acid ester.

[0083] (Impregnation process) In the impregnation process, part or all of the element is immersed in an electrolyte for energy storage devices, causing the electrolyte to penetrate the voids in the element. In particular, by holding the electrolyte in a separator, the electrolyte is brought into close contact with the positive electrode active material and also into close contact with the negative electrode active material. The element may be depressurized to promote impregnation, and the electrolyte may be further pressurized to promote impregnation. [Examples]

[0084] The present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

[0085] (Example 1) The electrolyte for the energy storage device and the lithium-ion secondary battery of Example 1 were prepared as follows. First, the electrolyte for the energy storage device was prepared in the preparation step. A mixed solvent was prepared by mixing sulfolane and dimethyl glutarate. Dimethyl glutarate has a linear skeleton with 3 carbon atoms and carboxylic acid ester groups at both ends of the skeleton. Sulfolane (A) and dimethyl glutarate (B) were mixed in a volume ratio of A:B = 70:30. To this mixed solvent, lithium tetrafluoroborate (LiBF4) was added as an electrolyte at a concentration of 1.5 moles per liter of solvent. This yielded an electrolyte for the energy storage device consisting of sulfolane and dimethyl glutarate containing 1.5 mol / L of LiBF4.

[0086] Next, the positive electrode was fabricated in the positive electrode fabrication process. The positive electrode active material was lithium manganese oxide (LiMn2O4). A binder was added to the lithium manganese oxide and mixed. Polyvinylidene fluoride was used as the binder. A slurry was obtained by diluting the mixture with N-methylpyrrolidone. The slurry was applied to a current collector and dried, and then rolled to obtain the positive electrode. The current collector of the positive electrode was aluminum foil.

[0087] Next, in the negative electrode fabrication process, the negative electrode active material is lithium titanate (Li4Ti5O 12 The lithium titanate was further mixed with a binder. Polyvinylidene fluoride was used as the binder. A slurry was obtained by diluting the mixture with N-methylpyrrolidone. The slurry was applied to a current collector and dried, and then rolled to obtain the negative electrode. The current collector of the negative electrode was aluminum foil.

[0088] In the element formation process, an element was formed by sandwiching the positive and negative electrodes with a separator. The separator was made of rayon. Then, the process moved to the impregnation step, in which an electrolyte for energy storage devices, with sulfolane and dimethyl glutarate as solvents, was impregnated into the element. The element impregnated with the electrolyte for energy storage devices was then sealed in a laminate film, and lead wires were extended from the positive and negative electrodes. This completed the lithium-ion secondary battery of Example 1.

[0089] (Example 2) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 2. In the preparation process, a mixed solvent of sulfolane and dimethyl succinate was prepared instead of dimethyl glutarate in Example 1. Dimethyl succinate has a linear skeleton with 2 carbon atoms, and carboxylic acid ester groups are present at both ends of this skeleton. The solvent composition ratio and electrolyte were the same as those of the electrolyte for an energy storage device in Example 1.

[0090] Furthermore, the positive electrode, negative electrode, and separator in Example 2 were the same as those in Example 1, and the lithium-ion secondary battery of Example 2 was manufactured using the same manufacturing method and under the same manufacturing conditions as in Example 1.

[0091] (Example 3) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 3. In the preparation step, a mixed solvent of sulfolane and dimethyl 1,4-cyclohexanedicarboxylate was prepared instead of dimethyl glutarate in Example 1. Dimethyl 1,4-cyclohexanedicarboxylate has a cyclic carbon skeleton, with one hydrogen atom at positions 1 and 4 substituted with a carboxylic acid ester group. 75% of dimethyl 1,4-cyclohexanedicarboxylate is in the Cis form and 25% is in the Trans form. The solvent composition ratio and electrolyte were the same as those of the electrolyte for the energy storage device in Example 1.

[0092] Furthermore, the positive electrode, negative electrode, and separator in Example 3 were the same as those in Example 1, and the lithium-ion secondary battery of Example 3 was manufactured using the same manufacturing method and under the same manufacturing conditions as in Example 1.

[0093] (Comparative Example 1) Comparative Example 1 was prepared using an electrolyte for an energy storage device and a lithium-ion secondary battery. Comparative Example 1 was identical to Example 1 in terms of the electrolyte for the energy storage device, the lithium-ion secondary battery, and the manufacturing method and conditions, except that the solvent for the electrolyte differed from that of Examples 1 to 3. The solvent for the electrolyte in Comparative Example 1 consisted solely of sulfolane.

[0094] (Comparative Example 2) Furthermore, an electrolyte for an energy storage device and a lithium-ion secondary battery were prepared for Comparative Example 2. Comparative Example 2 was the same electrolyte for an energy storage device as Example 1, the same lithium-ion secondary battery as Example 1, and was prepared using the same manufacturing method and conditions as Example 1, except that the solvent for the electrolyte for the energy storage device differed from that of Examples 1 to 3. In Comparative Example 2, in the preparation step, a mixed solvent of sulfolane and propylene carbonate was prepared instead of dimethyl glutarate as in Example 1.

[0095] (Reference example 1) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Reference Example 1. Unlike Comparative Example 1, the electrolyte for the energy storage device in Reference Example 1 does not contain sulfolane as a solvent. The solvent is a mixed solvent of ethylene carbonate and diethyl carbonate. Ethylene carbonate (A) and diethyl carbonate (B) were mixed in a volume ratio of A:B = 50:50. The electrolyte for the energy storage device in Reference Example 1 is the same electrolyte for an energy storage device as that in Example 1, the same lithium-ion secondary battery as that in Example 1, and was prepared using the same manufacturing method and conditions as in Example 1, except for the difference in solvent.

[0096] (Characteristic Test 1) The lithium-ion secondary batteries of Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example 1 were placed in temperature environments of 20°C and -20°C, and the discharge capacity (mAh) at each temperature environment was measured. Specifically, charging was performed using the CC-CV (Constant Current-Constant Voltage) method at each temperature environment. During constant current charging, the batteries were charged at a C rate of 1C to a predetermined voltage, and then constant voltage charging was performed at the predetermined voltage for 1 hour. After charging was completed, the discharge capacity (mAh) was measured when the batteries were discharged at a C rate of 1C. In this case, when lithium manganese oxide was used as the positive electrode and lithium titanate as the negative electrode, the batteries were charged to 2.7V and discharged to 1.5V. The capacity retention rate was expressed as the percentage of the discharge capacity at each temperature relative to the discharge capacity at 20°C.

[0097] The capacity retention rates at various temperatures for the lithium-ion secondary batteries of Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example 1 are summarized in Table 1 below. (Table 1) TIFF2026121160000007.tif81161

[0098] In Table 1 above, SLF is sulfolane, PC is propylene carbonate, EC is ethylene carbonate, DEC is diethyl carbonate, MeGlu is dimethyl glutarate, MeSuc is dimethyl succinate, and Cyclo is dimethyl 1,4-cyclohexanedicarboxylate.

[0099] As shown in Table 1 above, the electrolyte for the energy storage device in Comparative Example 1 uses only sulfolane as a solvent, and the lithium-ion secondary battery of Comparative Example 1 was unable to exhibit discharge capacity at a temperature of -20°C. On the other hand, the electrolyte for the energy storage device in Reference Example 1 does not contain sulfolane at all, and is a mixed solvent of ethylene carbonate and diethyl carbonate. Unlike Comparative Example 1, the lithium-ion secondary battery of Reference Example 1 using this electrolyte for the energy storage device can exhibit discharge capacity at a temperature of -20°C.

[0100] The electrolyte for the energy storage device in Comparative Example 2 is a mixed solvent of sulfolane and propylene carbonate, and the lithium-ion secondary battery of Comparative Example 2 exhibits discharge capacity under a temperature environment of -20°C. However, the discharge capacity exhibited by Comparative Example 2 remains low and does not exceed the discharge capacity of Reference Example 1, being less than half of that of Reference Example 1.

[0101] In contrast, the lithium-ion secondary batteries using the electrolytes for energy storage devices of Examples 1 to 3 exceeded the discharge capacity of Reference Example 1 and exhibited good discharge capacity at a temperature of -20°C. The electrolytes for energy storage devices of Examples 1 to 3 have a composition in which sulfolane accounts for 70% of the solvent by volume. Nevertheless, Examples 1 to 3 exhibited a discharge capacity that significantly exceeded that of Reference Example 1, which does not contain sulfolane.

[0102] When an electrolyte for an energy storage device was prepared using a mixed solvent containing sulfolane and a specific dicarboxylic acid ester, it was confirmed that even under low-temperature conditions of -20°C, a better discharge capacity was achieved compared to the case without sulfolane, assuming that sulfolane, which has a freezing point of 28°C, accounted for 50% of the volume in the solvent.

[0103] (Example 4) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 4. While the mixed solvent in Example 1 was composed of sulfolane and dimethyl glutarate, ethyl propionate, a monocarboxylic acid ester, was further added to the mixed solvent in Example 4. Sulfolane (A), dimethyl glutarate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:15:15. The electrolyte for the energy storage device in Example 4 was the same as the electrolyte for the energy storage device in Example 1, the same lithium-ion secondary battery as in Example 1, and was prepared using the same manufacturing method and conditions as in Example 1, except for the difference in solvent.

[0104] (Example 5) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 5. Example 5 had the same composition as Example 4, and the mixed solvent consisted of sulfolane, dimethyl glutarate, and ethyl propionate. However, sulfolane (A), dimethyl glutarate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:20:10. The electrolyte for an energy storage device in Example 4 was the same electrolyte for an energy storage device as in Example 1, the same lithium-ion secondary battery as in Example 1, and was prepared using the same manufacturing method and conditions as in Example 1, except that the solvent was different.

[0105] (Example 6) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 6. The mixed solvent in Example 3 was composed of sulfolane and dimethyl 1,4-cyclohexanedicarboxylate, but ethyl propionate, a monocarboxylic acid ester, was further added to the mixed solvent in Example 6. Sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:15:15. The electrolyte for the energy storage device in Example 6 was the same as the electrolyte for the energy storage device in Examples 1 and 3, except for the difference in solvent, and the lithium-ion secondary battery was the same as in Examples 1 and 3, and was prepared using the same manufacturing method and conditions as in Example 1.

[0106] (Characteristic Test 2) The lithium-ion secondary batteries of Examples 4 to 6 were placed in temperature environments of 20°C and -20°C, and their capacity was measured at each temperature. The method for measuring capacity was the same as that used in Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example 1 in Characteristic Test 1.

[0107] The capacity retention rates of the lithium-ion secondary batteries of Examples 4 to 6 at each temperature are summarized in Table 2 below, along with the results of Examples 1 and 3. (Table 2) TIFF2026121160000008.tif71166

[0108] In Table 2 above, SLF is sulfolane, MeGlu is dimethyl glutarate, Cyclo is dimethyl 1,4-cyclohexanedicarboxylate, and EP is ethyl propionate.

[0109] As shown in Table 2 above, the volume retention rates of Examples 4 and 5 are even higher than those of Example 1 at a temperature of -20°C. Furthermore, the volume retention rate of Example 6 is even higher than that of Example 3 at a temperature of -20°C. Moreover, the discharge capacity of Examples 4, 5, and 6 is significantly improved compared to the case of a mixed solvent of sulfolane and ethyl propionate. In particular, Example 6, which uses a dicarboxylic acid ester having a cyclic carbon skeleton as the specific dicarboxylic acid ester, has a volume retention rate that is more than double that of Example 3, which does not contain a monocarboxylic acid ester, and its volume retention rate is 96.4%, showing almost no degradation.

[0110] This confirmed that an electrolyte for energy storage devices containing specific dicarboxylic acid esters and monocarboxylic acid esters in the mixed solvent significantly improves the capacity retention rate of energy storage devices containing sulfolane as a solvent under low-temperature conditions.

[0111] (Example 7) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared in Example 7. In Example 1, the mixed solvent was prepared by adding dimethyl glutarate as a specific dicarboxylic acid ester to sulfolane, and in Example 2, the mixed solvent was prepared by adding dimethyl succinate as a specific dicarboxylic acid ester to sulfolane. The mixed solvent in Example 7 was prepared by adding both dimethyl glutarate from Example 1 and dimethyl succinate from Example 2 to sulfolane. That is, the mixed solvent in Example 7 is composed of two types of specific ethyl dicarboxylic acid and sulfolane. Sulfolane (A), dimethyl glutarate (B), and dimethyl succinate (C) were mixed in a volume ratio of A:B:C = 70:15:15.

[0112] The electrolyte for the energy storage device in Example 7 is the same electrolyte for the energy storage device as in Examples 1 and 2, except that the solvent is different. It is also the same lithium-ion secondary battery as in Examples 1 and 2, and was manufactured using the same manufacturing method and conditions as in Examples 1 and 2.

[0113] (Characteristic Test 3) The lithium-ion secondary battery of Example 7 was placed in temperature environments of 20°C and -20°C, and its capacity was measured in each temperature environment. The method for measuring capacity was the same as that used in Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example 1 in Characteristic Test 1.

[0114] The capacity retention rates of the lithium-ion secondary battery in Example 7 at each temperature are summarized in Table 3 below, along with the results from Examples 1 and 2. (Table 3) TIFF2026121160000009.tif51166

[0115] In Table 3 above, SLF is sulfolane, MeGlu is dimethyl glutarate, and MeSuc is dimethyl succinate.

[0116] As shown in Table 3 above, the volume retention rate of Example 7 is even higher than that of Examples 1 and 2 at a temperature of -20°C. Compared to Examples 1 and 2, which contain only one type of specific dicarboxylic acid ester, Example 7 has more than double the volume retention rate, with a volume retention rate of 93.0%, indicating almost no degradation.

[0117] This confirmed that an electrolyte for energy storage devices containing two or more specific dicarboxylic acid esters in the mixed solvent significantly improves the capacity retention rate of energy storage devices containing sulfolane as a solvent under low-temperature conditions.

[0118] (Example 8) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared according to Example 8. The electrolyte for the energy storage device in Example 8 has the same composition as that of Example 4. Specifically, the mixed solvent in Example 8 is sulfolane (A), dimethyl glutarate (B), and ethyl propionate (C), mixed in a volume ratio of A:B:C = 70:15:15. The amount of electrolyte added to the electrolyte for the energy storage device in Example 8 differs from that in Example 4. In the electrolyte for the energy storage device in Example 8, lithium tetrafluoroborate (LiBF4) was added at a concentration of 2.0 moles per liter of solvent. The lithium-ion secondary battery in Example 8 is the same lithium-ion secondary battery as that in Example 1, and was prepared using the same manufacturing method and conditions as in Example 1.

[0119] (Example 9) An electrolyte for an energy storage device and a lithium-ion secondary battery were prepared in Example 9. The electrolyte for the energy storage device in Example 8 had the same composition as in Examples 4 and 9, but the electrolyte concentration was 3.0 moles per liter of solvent. The lithium-ion secondary battery in Example 9 was the same lithium-ion secondary battery as in Example 1, and was prepared using the same manufacturing method and conditions as in Example 1.

[0120] (Characteristic Test 4) The lithium-ion secondary batteries of Examples 8 and 9 were placed in temperature environments of 20°C and -20°C, and their capacities were measured at each temperature. The method for measuring capacity was the same as that used in Examples 1 to 3, Comparative Examples 1 and 2, and Reference Example 1 in Characteristic Test 1.

[0121] The capacity retention rates of the lithium-ion secondary batteries in Examples 8 and 9 at each temperature are summarized in Table 4 below, along with the results for Example 4. (Table 4) TIFF2026121160000010.tif50167

[0122] In Table 4 above, SLF is sulfolane, MeGlu is dimethyl glutarate, and EP is ethyl propionate. Furthermore, M, the unit of concentration for the electrolyte LiBF4, represents the number of moles of solute per liter of solvent (molar concentration mol / L).

[0123] As shown in Table 4 above, in Examples 4, 8, and 9, although the electrolyte concentration differed from 1.5 moles to 3 moles per liter of solvent, the capacity retention rate at a temperature of -20°C was good in all cases. Therefore, it was confirmed that the capacity of the energy storage device is well-maintained when the electrolyte concentration is in the range of at least 1.5 moles to 3 moles per liter of solvent.

[0124] (Cycle testing) The lithium-ion secondary batteries of Comparative Example 1 and Examples 1 to 4, 6, and 7 were subjected to 1000 charge-discharge cycles, and the capacity retention rate was measured around 1000 cycles under a temperature environment of 20°C. The charge-discharge voltage range was from 1.5V to 2.7V, and the charge-discharge was performed at a current unit of 5CA, i.e., a current value of 20mA that allows for complete discharge from full charge in 12 minutes. As an accelerated test, the lithium-ion secondary batteries were subjected to repeated charge-discharge cycles under a temperature environment of 85°C. The capacity retention rate was calculated as the percentage of the capacity after 1000 charge-discharge cycles relative to the capacity before 1000 charge-discharge cycles.

[0125] Table 5 below summarizes the capacity retention rates of the lithium-ion secondary batteries of Comparative Example 1 and Examples 1 to 4, 6, and 7 after 1000 charge-discharge cycles. (Table 5) TIFF2026121160000011.tif91166

[0126] In Table 5 above, SLF is sulfolane, MeGlu is dimethyl glutarate, MeSuc is dimethyl succinate, Cyclo is dimethyl 1,4-cyclohexanedicarboxylate, and EP is ethyl propionate.

[0127] As shown in Table 5 above, Examples 1 to 4, 6 and 7 maintained a capacity retention rate after 1000 charge-discharge cycles at a similar level to Comparative Example 1. In other words, the effect of adding specific dicarboxylic acid esters as solvents is minimal, and the energy storage device can exhibit capacity even in low-temperature environments such as -20°C while maintaining its basic characteristics.

Claims

1. An electrolyte containing lithium ions, A mixed solvent of sulfolanes and carboxylic acid esters, Includes, The aforementioned sulfolanes include sulfolanes, sulfolane derivatives, or both. The carboxylic acid ester is a dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms, a dicarboxylic acid ester having a cyclic carbon skeleton, or both. An electrolyte for energy storage devices characterized by the following.

2. The mixed solvent further comprises a monocarboxylic acid ester. The electrolyte for an energy storage device according to claim 1, characterized by the above.

3. The monocarboxylic acid ester has a chain-like structure. The electrolyte for an energy storage device according to claim 2, characterized by the above.

4. The mixed solvent contains two or more of the dicarboxylic acid esters. The electrolyte for an energy storage device according to claim 1, characterized by the above.

5. The dicarboxylic acid ester having a cyclic carbon skeleton is dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 1,2-cyclohexanedicarboxylate, diethyl 1,2-cyclohexanedicarboxylate, dimethyl 1,3-cyclohexanedicarboxylate, dimethyl 1,3-cyclopentanedicarboxylate, or trimethyl 1,3,5-cyclohexanetricarboxylate. An electrolyte for an energy storage device according to any one of claims 1 to 4, characterized by the above.

6. A dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms has the carbon skeleton with three to six carbon atoms. An electrolyte for an energy storage device according to any one of claims 1 to 4, characterized by the above.

7. A dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms has the carbon skeleton containing a methyl group, an ethyl group, a propyl group, or a butyl group as a branched chain. An electrolyte for an energy storage device according to any one of claims 1 to 4, characterized by the above.

8. Equipped with a positive electrode, a negative electrode, and an electrolyte, The electrolyte comprises an electrolyte containing lithium ions and a mixed solvent of sulfolanes and carboxylic acid esters. The aforementioned sulfolanes are sulfolanes, sulfolane derivatives, or both. The carboxylic acid ester is a dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms, a dicarboxylic acid ester having a cyclic carbon skeleton, or both. A power storage device characterized by the following.

9. Preparation steps for preparing the electrolyte, An element formation step for forming an element having a positive electrode and a negative electrode, An impregnation step of impregnating the element with the electrolyte, Includes, In the above preparation step, the electrolyte and solvent are mixed, The aforementioned electrolyte contains lithium ions, The solvent comprises sulfolanes and carboxylic acid esters. The aforementioned sulfolanes are sulfolanes, sulfolane derivatives, or both. The carboxylic acid ester is a dicarboxylic acid ester having a linear carbon skeleton with two or more carbon atoms, a dicarboxylic acid ester having a cyclic carbon skeleton, or both. A method for manufacturing an energy storage device characterized by the following.

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