Electrolyte solution for lithium ion secondary battery and lithium ion secondary battery

Abstract

This lithium ion secondary battery is provided with a positive electrode, a negative electrode, and an electrolyte solution that contains: a nitrile compound that contains one or more cyano groups in the molecule; and a fluorinated alcohol represented by formula (1). The content of the nitrile compound in the electrolyte solution is 0.5% by weight or more and 5% by weight or less, and the content of the fluorinated alcohol in the electrolyte solution is 0.05% by weight or more and 1% by weight or less. R1R2R3COH (1) (R1, R2, and R3 are each any one of a hydrogen group, an alkyl group, and a fluoroalkyl group, provided that at least one of R1, R2, and R3 is a fluoroalkyl group)

Description

Technical Field

[0001] The present technology relates to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery. Background Art

[0002] A variety of electronic devices such as mobile phones are becoming popular. Therefore, as a small, lightweight, and high-energy-density power source, the development of lithium ion secondary batteries is underway. The lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte (electrolyte for a lithium ion secondary battery). Various studies have been conducted on the configuration of the lithium ion secondary battery.

[0003] Specifically, in a lithium ion secondary battery, the electrolyte contains alcohols such as ethanol, and the content of the alcohols in the electrolyte is specified (for example, refer to Patent Document 1).

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-133236 Summary of the Invention

[0007] Although various studies have been conducted on the configuration of the lithium ion secondary battery, the battery characteristics of the lithium ion secondary battery are still insufficient, and thus there is room for improvement.

[0008] An electrolyte for a lithium ion secondary battery and a lithium ion secondary battery that can achieve excellent battery characteristics are desired.

[0009] The electrolyte for a lithium ion secondary battery according to one embodiment of the present technology includes: a nitrile compound containing one or more cyano groups in the molecule; and a fluorinated alcohol represented by the formula (1). The content of the nitrile compound is 0.5% by weight or more and 5% by weight or less, and the content of the fluorinated alcohol is 0.05% by weight or more and 1% by weight or less.

[0010] R1R2R3COH …(1)

[0011] (R1, R2, and R3 are each any one of a hydrogen group, an alkyl group, and a fluoroalkyl group. Among them, at least one of R1, R2, and R3 is a fluoroalkyl group.)

[0012] The lithium ion secondary battery according to one embodiment of the present technology includes a positive electrode, a negative electrode, and an electrolyte, and the electrolyte has the same configuration as that of the electrolyte for a lithium ion secondary battery according to one embodiment of the present technology described above.

[0013] A lithium-ion secondary battery electrolyte or a lithium-ion secondary battery according to an embodiment of the present technology. The lithium-ion secondary battery electrolyte contains a nitrile compound and a fluorinated alcohol. The content of the nitrile compound is 0.5 wt% or more and 5 wt% or less, and the content of the fluorinated alcohol is 0.05 wt% or more and 1 wt% or less, so that excellent battery characteristics can be obtained.

[0014] In addition, the effects of the present technology are not necessarily limited to the effects described herein, and may be any of a series of effects associated with the present technology described later. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a perspective view showing the configuration of a lithium-ion secondary battery in an embodiment of the present technology.

[0016] Figure 2 It shows Figure 1 A cross-sectional view showing the configuration of the battery element shown.

[0017] Figure 3 It is a block diagram showing the configuration of an application example of a lithium-ion secondary battery.

[0018] Figure 4 It is a cross-sectional view showing the configuration of a lithium-ion secondary battery for testing. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Next, while referring to the attached Figure 1 An embodiment of the present technology will be described in detail. In addition, the order of description is as follows.

[0020] 1. Electrolyte for Lithium-Ion Secondary Battery

[0021] 1-1. Configuration

[0022] 1-2. Manufacturing Method

[0023] 1-3. Action and Effect

[0024] 2. Lithium-Ion Secondary Battery

[0025] 2-1. Structure

[0026] 2-2. Operation

[0027] 2-3. Manufacturing Method

[0028] 2-4. Action and Effect

[0029] 3. Modification Examples

[0030] 4. Uses of Lithium-Ion Secondary Batteries

[0031] <1. Electrolyte for Lithium-Ion Secondary Battery>

[0032] First, an electrolyte for a lithium ion secondary battery according to an embodiment of the present technology (hereinafter, simply referred to as "electrolyte") will be described.

[0033] This electrolyte is used for a lithium ion secondary battery as an electrochemical device. However, the electrolyte can also be used for other electrochemical devices different from lithium ion secondary batteries. The type of other electrochemical devices is not particularly limited, and specifically, it is a capacitor or the like.

[0034] <1-1. Configuration>

[0035] The electrolyte is a liquid electrolyte and is used as a medium for lithium ions in a lithium ion secondary battery. This electrolyte contains a nitrile compound and a fluorinated alcohol.

[0036] [Nitrile compound]

[0037] The nitrile compound is a general term for compounds containing one or more cyano groups (-CN) in the molecule. The type of nitrile compound can be only one kind, or two or more kinds.

[0038] This nitrile compound contains one or more cyano groups and a central group to which the one or more cyano groups are introduced. The type of this central group is not particularly limited, and specifically, it is a group in which one or more hydrogen groups are detached from a hydrocarbon group, and the number of hydrogen groups detached from the hydrocarbon group is determined according to the number of cyano groups introduced into the central group.

[0039] The hydrocarbon group is a general term for groups composed of carbon and hydrogen. This hydrocarbon group can be linear, cyclic, or a state in which the linear and cyclic are combined with each other.

[0040] Specific examples of the nitrile compound containing one cyano group in the molecule (mononitrile compound) are acetonitrile and the like.

[0041] Specific examples of the nitrile compound containing two cyano groups in the molecule (dinitrile compound) are succinonitrile, glutaronitrile, adiponitrile, and 3,3'-(ethylenedioxy)dipropionitrile and the like.

[0042] Specific examples of the nitrile compound containing three cyano groups in the molecule (trinitrile compound) are 1,2,3-propanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,3,4-hexanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-cyclohexanetricarbonitrile, and 1,3,5-benzenetricarbonitrile and the like.

[0043] Of course, specific examples of the nitrile compound can also be compounds containing four or more cyano groups in the molecule.

[0044] Among them, the nitrile compound is preferably a compound containing two cyano groups in the molecule, that is, preferably a dinitrile compound. This is because, in a lithium-ion secondary battery using an electrolyte, since a good coating film is easily formed on the surface of the negative electrode, gas generation is suppressed during the storage of the lithium-ion secondary battery.

[0045] [Fluorinated alcohol]

[0046] A fluorinated alcohol is an alcohol into which a fluorine group (-F) is introduced. More specifically, it is a compound represented by the formula (1). The type of fluorinated alcohol can be only one kind, or two or more kinds.

[0047] R1R2R3COH …(1)

[0048] (Each of R1, R2, and R3 is any one of a hydrogen group, an alkyl group, and a fluoroalkyl group. Among them, at least one of R1, R2, and R3 is a fluoroalkyl group.)

[0049] As described above, each of R1, R2, and R3 is not particularly limited as long as it is any one of a hydrogen group (-H), an alkyl group, and a fluoroalkyl group.

[0050] The alkyl group can be linear or branched. The number of carbon atoms of the alkyl group is not particularly limited, and among them, it is preferably 1 to 4. This is because the solubility and compatibility of the fluorinated alcohol are improved.

[0051] Specific examples of the alkyl group are methyl, ethyl, propyl, and butyl, etc. Among them, as described above, the alkyl group is not limited to linear, and can also be branched. Therefore, for example, propyl can be n-propyl or isopropyl. In addition, for another example, butyl can be n-butyl, sec-butyl, isobutyl, or tert-butyl.

[0052] A fluoroalkyl group is a group in which one or more hydrogen groups in the alkyl group are replaced by fluorine groups. The details (constitution and number of carbon atoms) related to the alkyl group are as described above.

[0053] Specific examples of the fluoroalkyl group are perfluoromethyl, perfluoroethyl, perfluoropropyl, and perfluorobutyl, etc. However, the specific examples of the fluoroalkyl group are not limited to perfluoro groups, and therefore can also be monofluoromethyl, monofluoroethyl, monofluoropropyl, and monofluorobutyl, etc.

[0054] Here, as described above, one or more of R1, R2, and R3 are fluoroalkyl groups. As described above, this is because, since a fluorinated alcohol is an alcohol into which one or more fluorine groups are introduced, it needs to contain one or more fluorines as constituent elements. Thus, compounds in which each of R1, R2, and R3 is any one of a hydrogen group and an alkyl group are excluded from the fluorinated alcohols described herein.

[0055] Among them, two or more of R1, R2, and R3 are preferably fluoroalkyl groups. This is because, in a lithium-ion secondary battery using an electrolyte solution, a good coating film is easily formed on the surface of the negative electrode, so the resistance is sufficiently reduced.

[0056] Specific examples of the fluorinated alcohol are CF3CH2OH, CF2HCH2OH, CFH2CH2OH, CF3CF2CH2OH, CF3CFHCH2OH, CF3CH2CH2OH, CF2HCF2CH2OH, (CF3)2CHOH, CF3C(CH3)HOH, (CF3)3COH, (CF3)2C(CH3)OH, (CF3)C(CH3)2OH, CF3CF2CF2CH2OH, CF3CF2CH2CH2OH, CF3CH2CH2CH2OH, CF3CF2CH(OH)CF3, CF3CF2CH(OH)CH3, CF3CH2CH(OH)CF3, CF3CH2CH(OH)CH3, and CH3CH2CH(OH)CF3, etc.

[0057] [Content]

[0058] In this electrolyte solution, in order to improve the battery characteristics of a lithium-ion secondary battery using this electrolyte solution, the relationship between the content of the nitrile compound and the content of the fluorinated alcohol is optimized. More specifically, regarding the relationship between the content of the nitrile compound and the content of the fluorinated alcohol, two conditions described below are satisfied.

[0059] First, the content C1 of the nitrile compound in the electrolyte solution is 0.5% by weight to 5% by weight.

[0060] Second, the content C2 of the fluorinated alcohol in the electrolyte solution is 0.05% by weight to 1% by weight.

[0061] The reason for satisfying the two conditions for the contents C1 and C2 is that since the relationship between the contents C1 and C2 is optimized, the resistance is reduced in a lithium-ion secondary battery using the electrolyte solution.

[0062] Specifically, the nitrile compound has a function of suppressing the decomposition reaction of the electrolyte solution. Thus, if the electrolyte solution contains the nitrile compound, the decomposition reaction of the electrolyte solution is suppressed, and thus the generation of gas caused by the decomposition reaction of the electrolyte solution is suppressed.

[0063] However, if the electrolyte solution contains the nitrile compound, the decomposition reaction of the electrolyte solution is suppressed. On the other hand, the resistance of a lithium-ion secondary battery using this electrolyte solution increases. Thus, a trade-off relationship is generated between suppressing gas generation and suppressing resistance increase, that is, a relationship in which if one characteristic is improved, the other characteristic deteriorates.

[0064] Regarding this point, if the electrolyte contains a nitrile compound and a fluorinated alcohol, and two conditions are satisfied for the contents C1 and C2, then during charging and discharging of a lithium-ion secondary battery using this electrolyte, due to the synergistic effect of the nitrile compound and the fluorinated alcohol, a favorable coating film is formed on the surface of the negative electrode. This coating film functions as a protective film covering the surface of the highly reactive electrode and has a low resistance.

[0065] The reason why the resistance of this coating film is considered to be low is as follows. If the electrolyte contains a nitrile compound and a fluorinated alcohol, then on the surface of the negative electrode, the fluorinated alcohol is preferentially reduced compared to the nitrile compound. In this case, a coating film containing lithium ions is formed, and more specifically, a coating film containing lithium alkoxide or the like is formed. Therefore, it can be considered that even when a coating film is formed on the surface of the negative electrode, since a migration path for lithium ions is ensured in this coating film, the resistance of this coating film becomes low.

[0066] In addition, the lithium ions described here are substances that move between the positive electrode and the negative electrode during the operation (charging and discharging) of a lithium-ion secondary battery, and are so-called electrode reaction substances.

[0067] Therefore, even if the electrolyte contains a nitrile compound, while suppressing the excessive increase in the resistance of the electrolyte, the decomposition reaction of the electrolyte on the surface of the negative electrode is also suppressed. Therefore, since the trade-off relationship related to the suppression of gas generation and the suppression of resistance increase is broken, the resistance is reduced in a lithium-ion secondary battery using this electrolyte.

[0068] The magnitude relationship between the contents C1 and C2 is not particularly limited, and thus can be arbitrarily set. Among them, the content C1 is equal to or more than the content C2, and therefore it is preferred that the ratio (=C1 / C2) of the content C1 to this content C2 is 1 or more. In particular, since the content C1 is larger than the content C2, it is more preferred that the ratio of the content C1 to this content C2 is greater than 1. This is because the resistance is sufficiently reduced in a lithium-ion secondary battery using this electrolyte.

[0069] Specifically, if the ratio is less than 1 because the content C1 is less than the content C2, then a coating film mainly from the fluorinated alcohol, that is, a coating film having fluorine characteristics, is likely to be formed on the surface of the negative electrode. As a result, the transport resistance of each of the lithium ions, the solvent described later, and the solvated lithium ions increases, and therefore the resistance of the coating film may increase.

[0070] In contrast, if the ratio is 1 or more because the content C1 is equal to or more than the content C2, then the above-mentioned coating film having fluorine characteristics is difficult to be formed on the surface of the negative electrode. As a result, the transport resistance of each of the lithium ions, the solvent, and the solvated lithium ions is reduced, and therefore an increase in the resistance of the coating film is suppressed.

[0071] [Measurement steps and calculation steps]

[0072] When measuring the content C1 of nitrile compounds in the electrolyte, after disassembling the lithium-ion secondary battery and recovering the electrolyte, the content of nitrile compounds is calculated by analyzing the electrolyte. The analysis method of the electrolyte is not particularly limited. Specifically, it is any one or two or more of high-frequency inductively coupled plasma (ICP) emission spectrometry, nuclear magnetic resonance spectrometry (NMR), and gas chromatography-mass spectrometry (GC-MS).

[0073] The step of measuring the content C2 of fluorinated alcohol in the electrolyte is the same as the step of measuring the content of nitrile compounds in the above-mentioned electrolyte, except that fluorinated alcohol is used instead of nitrile compounds as the measurement object.

[0074] [Solvent]

[0075] In addition, the electrolyte may further contain a solvent. The solvent contains any one or two or more of non-aqueous solvents (organic solvents), and the electrolyte containing the non-aqueous solvent is a so-called non-aqueous electrolyte. The non-aqueous solvent includes esters and ethers, etc. More specifically, it includes carbonate compounds, carboxylate compounds, and lactone compounds, etc.

[0076] The carbonate compounds are cyclic carbonates and chain carbonates, etc. Specific examples of cyclic carbonates are ethylene carbonate and propylene carbonate, etc. Specific examples of chain carbonates are dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, etc.

[0077] The carboxylate compounds are chain carboxylates, etc. Specific examples of chain carboxylates are methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl pivalate, ethyl pivalate, methyl butyrate, and ethyl butyrate, etc.

[0078] The lactone compounds are lactones, etc. Specific examples of lactones are γ-butyrolactone and γ-valerolactone, etc.

[0079] In addition, the ether may also be a compound in which a part of the ether is fluorinated. Specific examples of the ether are 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and 1,1,2-tetrafluoroethyl 2,2,2,3,3-tetrafluoropropyl ether, etc.

[0080] Among them, the solvent preferably contains cyclic carbonate and chain carbonate. This is because in a lithium-ion secondary battery using the electrolyte, while stably obtaining a high battery capacity, the resistance is reduced as described above. In addition, this is because in a lithium-ion secondary battery, it is easy to sufficiently maintain the chemical state of the electrolyte, and even if charge and discharge are repeated, the discharge capacity is difficult to sufficiently decrease.

[0081] [Electrolyte salt]

[0082] In addition, the electrolyte may further contain an electrolyte salt. The electrolyte salt is a light metal salt such as a lithium salt.

[0083] Specific examples of the lithium salt are lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methylate (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2), etc. This is because a high battery capacity can be obtained.

[0084] The content of the electrolyte salt is not particularly limited. Specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent. This is because high ionic conductivity can be obtained.

[0085] [Additive]

[0086] In addition, the electrolyte may further contain any one or two or more of the additives. This is because, since the electrochemical stability of the electrolyte is improved, the decomposition reaction of the electrolyte is suppressed in the lithium ion secondary battery using the electrolyte.

[0087] The type of the additive is not particularly limited. Specifically, it is an unsaturated cyclic carbonate, a fluorinated cyclic carbonate, a sulfonate, a phosphate, an acid anhydride, an isocyanate compound, etc.

[0088] Specific examples of the unsaturated cyclic carbonate are vinylene carbonate, ethylene vinyl carbonate, and methylene ethylene carbonate, etc. Specific examples of the fluorinated cyclic carbonate are fluoroethylene carbonate and difluoroethylene carbonate, etc. Specific examples of the sulfonate are propane sultone and propene sultone, etc. Specific examples of the phosphate are trimethyl phosphate and triethyl phosphate, etc. Specific examples of the acid anhydride are succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride, etc. Specific examples of the isocyanate compound are hexamethylene diisocyanate, etc.

[0089] <1-2. Manufacturing method>

[0090] One example related to the manufacturing method of the electrolyte is as follows. Specifically, after adding the electrolyte salt to the solvent, a nitrile compound and a fluorinated alcohol are added to the solvent. Thus, the electrolyte salt, the nitrile compound, and the fluorinated alcohol are respectively dispersed or dissolved in the solvent, thereby preparing the electrolyte.

[0091] In the case of manufacturing this electrolyte, as described above, the addition amounts of the nitrile compound and the fluorinated alcohol are adjusted so that the contents C1 and C2 satisfy two conditions.

[0092] <1-3. Function and Effect>

[0093] According to this electrolyte, the electrolyte contains a nitrile compound and a fluorinated alcohol, and two conditions are satisfied regarding the contents C1 and C2. That is, the content C1 is 0.5% by weight to 5% by weight, and the content C2 is 0.05% by weight to 1% by weight.

[0094] In this case, as described above, when a nitrile compound and a fluorinated alcohol are used in combination, the relationship between the contents C1 and C2 is optimized. Thus, during charge and discharge of a lithium-ion secondary battery using this electrolyte, a good coating film with low resistance is formed on the surface of the negative electrode through the synergistic effect of the nitrile compound and the fluorinated alcohol. Therefore, while suppressing excessive increase in resistance, the decomposition reaction of the electrolyte on the surface of the negative electrode is also suppressed, thus breaking the trade-off relationship related to suppressing gas generation and suppressing resistance increase.

[0095] Therefore, in a lithium-ion secondary battery using this electrolyte, excellent battery characteristics can be obtained due to the reduced resistance.

[0096] In particular, since the nitrile compound contains two cyano groups in the molecule, if the nitrile compound is a dinitrile compound, in a lithium-ion secondary battery using this electrolyte, a good coating film is easily formed on the surface of the negative electrode. Therefore, gas generation is further suppressed, and thus higher effects can be obtained.

[0097] In addition, in formula (1), two or more of R1, R2, and R3 are fluoroalkyl groups. In a lithium-ion secondary battery using this electrolyte, a good coating film is easily formed on the surface of the negative electrode. Therefore, since the resistance is sufficiently reduced, higher effects can be obtained.

[0098] In addition, if the electrolyte further contains a cyclic carbonate and a linear carbonate, in a lithium-ion secondary battery using this electrolyte, not only is the battery capacity ensured while the resistance is reduced, but also the chemical state of this electrolyte is easily maintained sufficiently, and even if charge and discharge are repeated, the discharge capacity is hardly reduced sufficiently. Therefore, higher effects can be obtained.

[0099] <2. Lithium-Ion Secondary Battery>

[0100] Next, a lithium-ion secondary battery according to an embodiment of the present technology using the above electrolyte will be described.

[0101] The lithium-ion secondary battery described herein is a secondary battery that obtains battery capacity by the insertion and extraction of lithium, and includes a positive electrode, a negative electrode, and an electrolyte. In this lithium-ion secondary battery, sufficient battery capacity is stably obtained by the insertion and extraction of lithium.

[0102] In addition, the charging capacity of the negative electrode is preferably greater than the discharging capacity of the positive electrode. That is, the electrochemical capacity per unit area of the negative electrode is preferably greater than that of the positive electrode. This is to prevent lithium metal from depositing on the surface of the negative electrode during charging.

[0103] <2-1. Structure>

[0104] Figure 1 The three-dimensional structure of the lithium-ion secondary battery is shown, and Figure 2 shows Figure 1 the cross-sectional structure of the battery element 20 shown. However, in Figure 1 it, the state where the outer packaging film 10 and the battery element 20 are separated from each other is shown, and the cross-section of the battery element 20 along the XZ plane is shown by a dashed line.

[0105] As Figure 1 and Figure 2 shown, the lithium-ion secondary battery includes an outer packaging film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42. The lithium-ion secondary battery described herein is a laminated film type lithium-ion secondary battery using a flexible or pliable outer packaging film 10.

[0106] [Outer packaging film]

[0107] As Figure 1 shown, the outer packaging film 10 is an outer packaging member that houses the battery element 20 and has a bag-like structure that is sealed with the battery element 20 housed inside. Thus, the outer packaging film 10 houses the electrolyte together with the positive electrode 21 and the negative electrode 22 described later inside.

[0108] Here, the outer packaging film 10 is a sheet-like member that is folded in the folding direction F. A recessed portion 10U (deep drawing portion) for housing the battery element 20 is provided in the outer packaging film 10.

[0109] Specifically, the outer packaging film 10 is a three-layer laminated film having a welding layer, a metal layer, and a surface protective layer laminated in order from the inside. In the state where the outer packaging film 10 is folded, the outer peripheral edge portions of the welding layers facing each other are welded to each other. The welding layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protective layer contains a polymer compound such as nylon.

[0110] However, the structure (number of layers) of the outer packaging film 10 is not particularly limited, and thus it may be one layer or two layers, or four layers or more.

[0111] [Battery element]

[0112] As Figure 1 and Figure 2As shown, the battery element 20 is a power generation element including a positive electrode 21, a negative electrode 22, a separator 23, and an electrolytic solution (not shown), and is housed inside the outer packaging film 10.

[0113] The battery element 20 is a so-called wound electrode body. That is, the positive electrode 21 and the negative electrode 22 are laminated with each other with the separator 23 therebetween, and are opposed to each other with the separator 23 therebetween and wound around a winding axis P. The winding axis P is an imaginary axis extending in the Y-axis direction.

[0114] The three-dimensional shape of the battery element 20 is not particularly limited. Here, since the three-dimensional shape of the battery element 20 is flat, the shape of the cross-section of the battery element 20 intersecting the winding axis P (the cross-section along the XZ plane) is a flat shape defined by a major axis J1 and a minor axis J2. The major axis J1 is an imaginary axis extending in the X-axis direction and having a length larger than the length of the minor axis J2, and the minor axis J2 is an imaginary axis extending in the Z-axis direction intersecting the X-axis direction and having a length smaller than the length of the major axis J1. Here, since the three-dimensional shape of the battery element 20 is a flat cylindrical shape, the cross-sectional shape of the battery element 20 is a flat approximate ellipse.

[0115] (Positive electrode)

[0116] As Figure 2 shown, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

[0117] The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. The positive electrode current collector 21A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum or the like.

[0118] Here, the positive electrode active material layer 21B is provided on both surfaces of the positive electrode current collector 21A and contains any one or two or more of positive electrode active materials that intercalate and deintercalate lithium. However, the positive electrode active material layer 21B may be provided only on one surface of the positive electrode current collector 21A on the side where the positive electrode 21 faces the negative electrode 22. In addition, the positive electrode active material layer 21B may also contain any one or two or more of other materials such as a positive electrode binder and a positive electrode conductive agent. The method for forming the positive electrode active material layer 21B is not particularly limited, and specifically, it is a coating method or the like.

[0119] The positive electrode active material contains a lithium-containing compound. The lithium-containing compound is a compound containing one or two or more transition metal elements as constituent elements together with lithium, and further, may contain one or two or more other elements as constituent elements. The type of the other element is not particularly limited as long as it is an element other than a transition metal element (except lithium), and specifically, it is an element belonging to Groups 2 to 15 of the long-period type periodic table. The type of the lithium-containing compound is not particularly limited, and specifically, it is an oxide, a phosphate compound, a silicate compound, a borate compound, etc.

[0120] Specific examples of the oxide are LiNiO2, LiCoO2, LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2 and LiMn2O4, etc. Specific examples of the phosphate compound are LiFePO4, LiMnPO4, LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0121] The positive electrode binder contains any one or two or more of compounds such as synthetic rubber and polymer compounds. Specific examples of the synthetic rubber are styrene-butadiene rubber, fluorine rubber, ethylene-propylene-diene rubber, etc. Specific examples of the polymer compound are polyvinylidene fluoride, polyimide, carboxymethyl cellulose, etc.

[0122] The positive electrode conductive agent contains any one or two or more of conductive materials such as carbon materials, and specific examples of the conductive materials are graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube, etc. However, the conductive material may also be a metal material and a conductive polymer compound, etc.

[0123] (Negative electrode)

[0124] As Figure 2As shown, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0125] The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. The negative electrode current collector 22A contains a conductive material such as a metal material, and specific examples of the conductive material are copper and the like.

[0126] Here, the negative electrode active material layer 22B is provided on both surfaces of the negative electrode current collector 22A and contains any one or two or more of the negative electrode active materials that intercalate and deintercalate lithium. However, the negative electrode active material layer 22B may also be provided only on one surface of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21. In addition, the negative electrode active material layer 22B may also contain any one or two or more of other materials such as a negative electrode binder and a negative electrode conductive agent. In addition, the method for forming the negative electrode active material layer 22B is not particularly limited. Specifically, it is a coating method or the like.

[0127] The negative electrode active material contains any one or two or more of a carbon material and a metal-based material, etc. This is because a high energy density can be obtained.

[0128] Specific examples of the carbon material are graphitizable carbon, non-graphitizable carbon, and graphite, etc. The graphite may be natural graphite, artificial graphite, or both.

[0129] The metal-based material is a material containing any one or two or more of a metal element and a metalloid element that can form an alloy with lithium as constituent elements. Specific examples of the metal element and the metalloid element are silicon and tin, etc. The metal-based material may be a monomer, an alloy, a compound, a mixture of two or more of them, or a material containing two or more of their phases. Specific examples of the metal-based material are TiSi2 and SiO x (0 < x ≤ 2 or 0.2 < x < 1.4), etc.

[0130] The details of the negative electrode binder and the negative electrode conductive agent are the same as the details of the positive electrode binder and the positive electrode conductive agent, respectively.

[0131] (Separator)

[0132] As Figure 2 shown, the separator 23 is an insulating porous membrane between the positive electrode 21 and the negative electrode 22, and allows lithium ions to pass through while preventing contact (short circuit) between the positive electrode 21 and the negative electrode 22. The separator 23 contains a polymer compound such as polyethylene.

[0133] (Electrolyte)

[0134] The details of the electrolytic solution are as described above. That is, the electrolytic solution contains a nitrile compound and a fluorinated alcohol, and the contents C1 and C2 satisfy two conditions.

[0135] [Positive electrode lead and negative electrode lead]

[0136] As Figure 1 and Figure 2 shown, the positive electrode lead 31 is a positive electrode terminal connected to the positive electrode current collector 21A in the positive electrode 21, and is led out to the outside of the outer packaging film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and specific examples of the conductive material are aluminum and the like. The shape of the positive electrode lead 31 is not particularly limited, and specifically, it is any one of a thin plate shape, a mesh shape, and the like.

[0137] As Figure 1 and Figure 2 shown, the negative electrode lead 32 is a negative electrode terminal connected to the negative electrode current collector 22A in the negative electrode 22, and is led out to the outside of the outer packaging film 10. The negative electrode lead 32 contains a conductive material such as a metal material, and specific examples of the conductive material are copper and the like. Here, the details of the leading-out direction and shape of the negative electrode lead 32 are the same as the details of the leading-out direction and shape of the positive electrode lead 31.

[0138] [Sealing film]

[0139] The sealing film 41 is inserted between the outer packaging film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the outer packaging film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0140] The sealing film 41 is a sealing member that prevents external gases and the like from invading the inside of the outer packaging film 10. In addition, the sealing film 41 contains a polymer compound such as polyolefin having adhesiveness to the positive electrode lead 31, and specific examples of the polyolefin are polypropylene and the like.

[0141] The structure of the sealing film 42 is the same as that of the sealing film 41 except that it is a sealing member having adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin having adhesiveness to the negative electrode lead 32.

[0142] <2-2. Operation>

[0143] The lithium ion secondary battery operates in the manner described below.

[0144] During charging, in the battery element 20, lithium is deintercalated from the positive electrode 21 in an ionic state, and this lithium is intercalated into the negative electrode 22 in an ionic state via the electrolytic solution. On the other hand, during discharging, in the battery element 20, lithium is deintercalated from the negative electrode 22 in an ionic state, and this lithium is intercalated into the positive electrode 21 in an ionic state via the electrolytic solution.

[0145] <2-3. Manufacturing Method>

[0146] In the case of manufacturing a lithium ion secondary battery, each of the positive electrode 21 and the negative electrode 22 is fabricated through the steps of an example described below. After preparing the electrolyte, the lithium ion secondary battery is assembled using the positive electrode 21, the negative electrode 22, and the electrolyte, and stabilization treatment of the lithium ion secondary battery is performed.

[0147] [Fabrication of Positive Electrode]

[0148] First, a paste-like positive electrode mixture slurry is prepared by putting a mixture (positive electrode mixture) obtained by mixing a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent into a solvent. The solvent can be an aqueous solvent or an organic solvent. Next, the positive electrode mixture slurry is coated on both sides of the positive electrode current collector 21A to form a positive electrode active material layer 21B. Finally, the positive electrode active material layer 21B can also be compression-molded using a roll press or the like. In this case, the positive electrode active material layer 21B can be heated, or compression molding can be repeated. Thus, the positive electrode active material layer 21B is formed on both sides of the positive electrode current collector 21A, thereby fabricating the positive electrode 21.

[0149] [Fabrication of Negative Electrode]

[0150] The negative electrode 22 is fabricated through the same steps as those for fabricating the positive electrode 21 described above. Specifically, a paste-like negative electrode mixture slurry is prepared by putting a mixture (negative electrode mixture) obtained by mixing a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent into a solvent, and then the negative electrode mixture slurry is coated on both sides of the negative electrode current collector 22A to form a negative electrode active material layer 22B. Thereafter, the negative electrode active material layer 22B can also be compression-molded. Thus, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, thereby fabricating the negative electrode 22.

[0151] [Preparation of Electrolyte]

[0152] Through the above steps, an electrolyte containing a nitrile compound and a fluorinated alcohol is prepared.

[0153] [Assembly of Lithium Ion Secondary Battery]

[0154] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A in the positive electrode 21 using a joining method such as a welding method, and the negative electrode lead 32 is connected to the negative electrode current collector 22A in the negative electrode 22 using a joining method such as a welding method.

[0155] Next, after forming a laminate by laminating the positive electrode 21 and the negative electrode 22 with the separator 23 interposed therebetween, a wound body (not shown) is fabricated by winding the laminate. This wound body has the same structure as that of the battery element 20 except that the electrolytic solution is not impregnated in each of the positive electrode 21, the negative electrode 22, and the separator 23. Next, the wound body is formed into a flat shape by pressing the wound body using a press or the like.

[0156] Next, after accommodating the wound body inside the recessed portion 10U, the outer packaging film 10 (welding layer / metal layer / surface protective layer) is folded so that the outer packaging films 10 face each other. Next, using a bonding method such as a hot melt bonding method, by joining the outer peripheral edge portions of both sides in the welding layers facing each other, the wound body is accommodated inside the bag-shaped outer packaging film 10.

[0157] Finally, after injecting the electrolytic solution inside the bag-shaped outer packaging film 10, using a bonding method such as a hot melt bonding method, the outer peripheral edge portions of the remaining one side in the welding layers facing each other are joined. In this case, a sealing film 41 is inserted between the outer packaging film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the outer packaging film 10 and the negative electrode lead 32.

[0158] Thereby, the electrolytic solution is impregnated in the wound body, and thus the battery element 20 as a wound electrode body is fabricated. Therefore, the battery element 20 is sealed inside the bag-shaped outer packaging film 10, and thus a lithium ion secondary battery is assembled.

[0159] [Stabilization treatment]

[0160] The assembled lithium ion secondary battery is charged and discharged. Various conditions such as the ambient temperature, the number of charge and discharge cycles (number of cycles), and the charge and discharge conditions can be arbitrarily set. Thereby, a coating film is formed on the surfaces of the positive electrode 21 and the negative electrode 22, and thus the state of the lithium ion secondary battery is electrochemically stabilized. Therefore, the lithium ion secondary battery is completed.

[0161] <2-4. Function and effect>

[0162] According to this lithium ion secondary battery, the lithium ion secondary battery includes an electrolytic solution having the above-described configuration. Therefore, for the above reasons, a good coating film with low resistance is formed on the surface of the negative electrode 22. Thus, while suppressing the excessive increase in the resistance of the electrolytic solution, the decomposition reaction of the electrolytic solution on the surface of the negative electrode 22 is also suppressed. Therefore, due to the reduction in resistance, excellent battery characteristics can be obtained.

[0163] In addition, the other functions and effects of the lithium ion secondary battery are the same as the other functions and effects of the electrolytic solution.

[0164] <3. Modification example>

[0165] The structure of the lithium ion secondary battery is as described below and can be appropriately changed. However, a series of modification examples described below can also be combined with each other.

[0166] [Modification Example 1]

[0167] Use the separator 23 as a porous membrane. However, although not specifically illustrated here, a laminated separator including a polymer compound layer may also be used.

[0168] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the adhesion of the separator to each of the positive electrode 21 and the negative electrode 22 is improved, so the winding deviation of the battery element 20 is suppressed. Thereby, even if a decomposition reaction of the electrolyte occurs, the swelling of the lithium ion secondary battery is suppressed. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0169] In addition, one or both of the porous membrane and the polymer compound layer may also contain a plurality of insulating particles. This is because when the lithium ion secondary battery generates heat, heat dissipation of the plurality of insulating particles is promoted, and thus the safety (heat resistance) of the lithium ion secondary battery is improved. The insulating particles contain any one or two or more of insulating materials such as inorganic materials and resin materials. Specific examples of the inorganic material are alumina, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of the resin material are acrylic resin and styrene resin.

[0170] In the case of manufacturing the laminated separator, after preparing a precursor solution containing a polymer compound and a solvent, etc., the precursor solution is coated on one or both surfaces of the porous membrane. In this case, if necessary, a plurality of insulating particles may also be added to the precursor solution.

[0171] Even in the case of using this laminated separator, lithium can move between the positive electrode 21 and the negative electrode 22, so the same effect can be obtained. In this case, in particular, as described above, since the safety of the lithium ion secondary battery is improved, a higher effect can be obtained.

[0172] [Modification Example 2]

[0173] Use the electrolyte as a liquid electrolyte. However, although not specifically illustrated here, an electrolyte layer as a gel electrolyte may also be used.

[0174] In the battery element 20 using an electrolyte layer, a positive electrode 21 and a negative electrode 22 are stacked with each other via a separator 23 and the electrolyte layer, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound. The electrolyte layer is interposed between the positive electrode 21 and the separator 23 and between the negative electrode 22 and the separator 23.

[0175] Specifically, the electrolyte layer includes an electrolytic solution and a polymer compound, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The constitution of the electrolytic solution is as described above. The polymer compound includes polyvinylidene fluoride and the like. When forming the electrolyte layer, after preparing a precursor solution containing the electrolytic solution, the polymer compound, a solvent, etc., the precursor solution is coated on one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0176] Even when using this electrolyte layer, lithium can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, and thus the same effect can be obtained. In this case, in particular, as described above, since leakage of the electrolytic solution is prevented, a higher effect can be obtained.

[0177] <4. Uses of Lithium-Ion Secondary Batteries>

[0178] The uses (application examples) of lithium-ion secondary batteries are not particularly limited. A lithium-ion secondary battery used as a power source can be a main power source for electronic devices, electric vehicles, etc., or an auxiliary power source. The main power source is a power source that is preferentially used regardless of the presence or absence of other power sources. The auxiliary power source can be a power source used in place of the main power source or a power source switched from the main power source.

[0179] Specific examples of the uses of lithium-ion secondary batteries are as follows: electronic devices such as cameras, digital still cameras, mobile phones, laptop computers, stereo headphones, portable radios, and portable information terminals; storage devices such as backup power sources and memory cards; power tools such as electric drills and saws; battery packs mounted on electronic devices, etc.; medical electronic devices such as pacemakers and hearing aids; electric vehicles (including hybrid vehicles) such as electric cars; power storage systems such as household or industrial battery systems that store electricity in advance for emergencies, etc. In these uses, one lithium-ion secondary battery or multiple lithium-ion secondary batteries can be used.

[0180] The battery pack can use a single cell or a battery pack. An electric vehicle is a vehicle that operates (runs) using a lithium-ion secondary battery as a driving power source, and can also be a hybrid vehicle that also has another driving source different from the lithium-ion secondary battery. In a household power storage system, the power stored in the lithium-ion secondary battery as a power storage source can be used to operate household electrical products, etc.

[0181] Here, a specific example of an application example of a lithium-ion secondary battery will be described. The structure of the application example described below is just one example, so it can be changed as appropriate.

[0182] Figure 3 It shows the block structure of a battery pack. The battery pack described here is a battery pack (so-called soft pack) using a single lithium-ion secondary battery, and is mounted on electronic devices such as smartphones.

[0183] As Figure 3 shown, this battery pack includes a power source 51 and a circuit board 52. The circuit board 52 is connected to the power source 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0184] The power source 51 includes a single lithium-ion secondary battery. In this lithium-ion secondary battery, the positive lead is connected to the positive terminal 53, and the negative lead is connected to the negative terminal 54. The power source 51 can be connected to the outside via the positive terminal 53 and the negative terminal 54, so charging and discharging can be performed. The circuit board 52 includes a control unit 56, a switch 57, a PTC element 58, and a temperature detection unit 59. However, the PTC element 58 can also be omitted.

[0185] The control unit 56 includes a central processing unit (CPU) and a memory, etc., and controls the overall operation of the battery pack. The control unit 56 detects and controls the usage state of the power source 51 as needed.

[0186] In addition, if the voltage of the power source 51 (lithium-ion secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, the control unit 56 cuts off the switch 57 so that no charging current flows in the current path of the power source 51. The overcharge detection voltage is not particularly limited. Specifically, it is 4.20V ± 0.05V, and the overdischarge detection voltage is not particularly limited. Specifically, it is 2.40V ± 0.1V.

[0187] The switch 57 includes a charge control switch, a discharge control switch, a charging diode, a discharging diode, etc., and switches the connection between the power source 51 and an external device according to the instruction of the control unit 56. The switch 57 includes a metal-oxide-semiconductor field-effect transistor (MOSFET), etc., and detects the charging current and the discharging current based on the on-resistance of the switch 57.

[0188] The temperature detection unit 59 includes a temperature detection element such as a thermistor. The temperature detection unit 59 measures the temperature of the power supply 51 using the temperature detection terminal 55, and outputs the measurement result of the temperature to the control unit 56. The measurement result of the temperature measured by the temperature detection unit 59 is used in cases where the control unit 56 performs charge and discharge control during abnormal heating, and in cases where the control unit 56 performs correction processing during the calculation of the remaining capacity, etc.

[0189] Embodiment

[0190] Embodiments of the present technology will be described.

[0191] <Examples 1 to 11 and Comparative Examples 1 to 7>

[0192] As described below, after manufacturing a lithium-ion secondary battery, the battery characteristics of the lithium-ion secondary battery were evaluated.

[0193] [Manufacture of Lithium-Ion Secondary Battery]

[0194] Here, in order to simply evaluate the battery characteristics, a test lithium-ion secondary battery was manufactured. Figure 4 The cross-sectional structure of the test secondary battery, which is a so-called coin-type lithium-ion secondary battery, is shown.

[0195] Hereinafter, before describing the structure of the coin-type lithium-ion secondary battery, the manufacturing steps of the lithium-ion secondary battery will be described.

[0196] As Figure 4 shown, the lithium-ion secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an outer packaging cup 64, an outer packaging can 65, a gasket 66, and an electrolytic solution (not shown).

[0197] The test electrode 61 is housed in the outer packaging cup 64, and the counter electrode 62 is housed in the outer packaging can 65. The test electrode 61 and the counter electrode 62 are stacked on top of each other with the separator 63 in between, and the electrolytic solution is impregnated in each of the test electrode 61, the counter electrode 62, and the separator 63. The outer packaging cup 64 and the outer packaging can 65 are fastened to each other with the gasket 66, so the test electrode 61, the counter electrode 62, and the separator 63 are sealed in the outer packaging cup 64 and the outer packaging can 65.

[0198] (Manufacture of Test Electrode)

[0199] When manufacturing a lithium-ion secondary battery, first, by mixing 91 parts by mass of a positive electrode active material (a lithium-containing compound (oxide) of LiNi 0.80 Co 0.15 Al 0.05O2), 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (Ketjenblack as amorphous carbon powder) were mixed with each other to prepare a positive electrode mixture. Next, after the positive electrode mixture was put into a solvent (N-methyl-2-pyrrolidone as an organic solvent), the solvent was stirred to prepare a paste-like positive electrode mixture slurry.

[0200] Next, after the positive electrode mixture slurry was coated on one side of a positive electrode current collector 21A (aluminum foil with a thickness of 10 μm) using a coating device, the positive electrode mixture slurry was dried to form a positive electrode active material layer 21B.

[0201] Finally, after the positive electrode active material layer 21B was compression-molded using a roll press, the positive electrode current collector 21A having the positive electrode active material layer 21B formed thereon was cut into a disk shape. Thus, the positive electrode 21 was fabricated.

[0202] (Fabrication of the counter electrode)

[0203] First, 94 parts by mass of a negative electrode active material (4 parts by mass of silicon oxide as a metal-based material and 90 parts by mass of artificial graphite as a carbon material), 1.5 parts by mass of a negative electrode binder (polyvinylidene fluoride), 2.5 parts by mass of a negative electrode conductive agent (2 parts by mass of carbon nanotubes and 0.5 parts by mass of graphite), and 2 parts by mass of a thickener (carboxymethyl cellulose) were mixed with each other to prepare a negative electrode mixture.

[0204] Next, after the negative electrode mixture was put into a solvent (water as an aqueous solvent), the solvent was stirred to prepare a paste-like negative electrode mixture slurry.

[0205] Next, after the negative electrode mixture slurry was coated on one side of a negative electrode current collector 22A (copper foil with a thickness of 8 μm) using a coating device, the negative electrode mixture slurry was dried to form a negative electrode active material layer 22B.

[0206] Finally, after the negative electrode active material layer 22B was compression-molded using a roll press, the negative electrode current collector 22A having the negative electrode active material layer 22B formed thereon was cut into a disk shape. Thus, the negative electrode 22 was fabricated.

[0207] (Preparation of the electrolyte)

[0208] First, a solvent was prepared. As the solvent, a mixture of ethylene carbonate (EC) as a cyclic carbonate and ethyl methyl carbonate (EMC) as a chain carbonate was used. In this case, the mixing ratio (weight %) of the solvent was EC:EMC = 30:70.

[0209] Next, after adding an electrolyte salt (lithium hexafluorophosphate (LiPF6) as a lithium salt) to the solvent, the solvent was stirred. In this case, the content of the electrolyte salt was 1 mol / kg relative to the solvent.

[0210] Finally, after adding a nitrile compound and a fluorinated alcohol to the solvent containing the electrolyte salt, the solvent was stirred. In this case, as the nitrile compound, succinonitrile (SN; NCCH2CH2CN) as a dinitrile compound was used, and as the fluorinated alcohol, hexafluoroisopropanol ((CF3)2CHOH (HFIP)) was used. Thus, an electrolytic solution was prepared.

[0211] In the case of preparing this electrolytic solution, the addition amount of the nitrile compound was adjusted so that the content C1 (wt%) of the nitrile compound in the electrolytic solution became the value shown in Table 1, and the addition amount of the fluorinated alcohol was adjusted so that the content C2 (wt%) of the fluorinated alcohol in the electrolytic solution became the value shown in Table 1.

[0212] In addition, for comparison, an electrolytic solution was prepared by the same procedure except that the fluorinated alcohol was not used.

[0213] (Assembly of Lithium-Ion Secondary Battery)

[0214] First, the test electrode 61 was housed in the outer packaging cup 64, and the counter electrode 62 was housed in the outer packaging can 65. Next, the test electrode 61 housed in the outer packaging cup 64 and the counter electrode 62 housed in the outer packaging can 65 were laminated with each other via a separator 63 (a microporous polyethylene film with a thickness = 20 μm) impregnated with the electrolytic solution. In this case, the positive electrode active material layer 21B and the negative electrode active material layer 22B were opposed to each other with the separator 63 interposed therebetween. Next, with the test electrode 61 and the counter electrode 62 laminated with each other via the separator 63, the outer packaging cup 64 and the outer packaging can 65 were fastened to each other via a gasket 66. Thus, the test electrode 61 and the counter electrode 62 were sealed inside the outer packaging cup 64 and the outer packaging can 65, thereby assembling a lithium-ion secondary battery.

[0215] (Stabilization Treatment)

[0216] In a normal temperature environment (temperature = 23 °C), the lithium-ion secondary battery was charged and discharged for 1 cycle. During charging, after constant current charging at a current of 0.1C until the voltage reached 4.2V, constant voltage charging was performed at the voltage of 4.2V until the current reached 0.025C. During discharging, constant current discharging was performed at a current of 0.1C until the voltage reached 3.0V. 0.1C is the current value for completely discharging the battery capacity (theoretical capacity) in 10 hours, and 0.025C is the current value for completely discharging the battery capacity in 40 hours.

[0217] Thus, each of the test electrode 61 and the counter electrode 62 is electrochemically stable, thereby completing the lithium ion secondary battery.

[0218] [Evaluation of Characteristics of Lithium Ion Secondary Battery]

[0219] Through the steps described below, the battery characteristics (resistance characteristics) of the lithium ion secondary battery were evaluated, and the results shown in Table 1 were obtained.

[0220] In the case of evaluating the resistance characteristics, by using the alternating current impedance method, the electrochemical impedance (EIS (Ω)), which is an index for evaluating the resistance characteristics, was measured. This EIS is the so-called charge transfer resistance. As the measuring device, a multi-channel potentiostat VMP-3 manufactured by Bio-Logic Science Instruments was used. The measurement conditions were frequency range = 1 MHz to 10 mHz and alternating current amplitude = 10 mV.

[0221] In addition, the EIS values shown in Table 1 are standardized values. Specifically, the EIS values in Examples 1 to 4 and Comparative Examples 1 and 2 are the values standardized with the EIS value in Comparative Example 1 as 100. The EIS values in Examples 5 to 8 and Comparative Examples 3 and 4 are the values standardized with the EIS value in Comparative Example 3 as 100. The EIS values in Examples 9 to 11 and Comparative Examples 5 to 7 are the values standardized with the EIS value in Comparative Example 5 as 100.

[0222]

Table 1

[0223]

[0224] [Discussion]

[0225] As shown in Table 1, the EIS varies significantly depending on the composition of the electrolyte.

[0226] Specifically, when the electrolyte contains a nitrile compound and a fluorinated alcohol and does not satisfy the conditions that the content C1 is 0.5 wt% to 5 wt% and the content C2 is 0.05 wt% to 1 wt% (Comparative Examples 1 to 7), the EIS increases.

[0227] In contrast, when the electrolyte contains a nitrile compound and a fluorinated alcohol and satisfies the conditions that the content C1 is 0.5 wt% to 5 wt% and the content C2 is 0.05 wt% to 1 wt% (Examples 1 to 11), the EIS decreases.

[0228] In particular, when the above conditions are satisfied (Examples 1 to 11), the following tendency is obtained.

[0229] First, by using a dinitrile compound (SN) as the nitrile compound, that is, by using a nitrile compound containing two cyano groups in the molecule, the EIS is sufficiently reduced.

[0230] Second, by using HFIP as the fluorinated alcohol, that is, by using a fluorinated alcohol in which two or more of R1 to R3 shown in the formula (1) are fluoroalkyl groups, the EIS is sufficiently reduced.

[0231] Third, by including a nitrile compound, a fluorinated alcohol, and a solvent (cyclic carbonate and linear carbonate) in the solvent, while ensuring a smooth charge-discharge reaction (battery capacity), the EIS is sufficiently reduced.

[0232] [Summary]

[0233] As can be seen from the results shown in Table 1, if the electrolyte of the lithium-ion secondary battery contains a nitrile compound and a fluorinated alcohol and satisfies two conditions (C1 = 0.5 wt% to 5 wt% and content C2 = 0.05 wt% to 1 wt%) regarding the contents C1 and C2, the EIS is reduced. Therefore, since the resistance characteristics are improved, excellent battery characteristics are obtained in the lithium-ion secondary battery.

[0234] As described above, this technology has been described by taking one embodiment and examples as an example, but the structure of this technology is not limited to the structure described in one embodiment and examples, and thus various modifications can be made.

[0235] Specifically, the case where the battery structure of the lithium-ion secondary battery is a laminated film type has been described, but the battery structure of the secondary battery applied to the battery pack of this technology is not particularly limited. Specifically, the battery structure of the lithium-ion secondary battery can also be a cylindrical type, a square type, a coin type, etc.

[0236] In addition, the case where the element structure of the battery element is a wound type has been described. However, the element structure of the battery element is not particularly limited, and it can also be a stacked type, a repeatedly folded type, etc. In the stacked type, the positive electrode and the negative electrode are stacked on each other with a separator interposed therebetween, and in the repeatedly folded type, the positive electrode and the negative electrode are opposed to each other with a separator interposed therebetween and folded into a zigzag shape.

[0237] The effects described in this specification are only examples, and thus the effects of this technology are not limited to the effects described in this specification. Therefore, other effects can also be obtained regarding this technology.

[0238] In addition, this technology can adopt the following configuration.

[0239] <1> A lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte,

[0240] The electrolyte contains:

[0241] A nitrile compound containing one or more cyano groups in the molecule; and

[0242] a fluorinated alcohol represented by the formula (1),

[0243] wherein the content of the nitrile compound in the electrolyte is 0.5% by weight or more and 5% by weight or less,

[0244] and the content of the fluorinated alcohol in the electrolyte is 0.05% by weight or more and 1% by weight or less.

[0245] R1R2R3COH …(1)

[0246] (Each of R1, R2, and R3 is any one of a hydrogen group, an alkyl group, and a fluoroalkyl group. Among them, at least one of R1, R2, and R3 is a fluoroalkyl group.)

[0247] <2> The lithium ion secondary battery according to <1>, wherein,

[0248] the nitrile compound contains two of the cyano groups in the molecule.

[0249] <3> The lithium ion secondary battery according to <1> or <2>, wherein,

[0250] in the formula (1), two or more of the R1, the R2, and the R3 are the fluoroalkyl groups.

[0251] <4> The lithium ion secondary battery according to any one of <1> to <3>, wherein,

[0252] the electrolyte further contains a cyclic carbonate and a chain carbonate.

[0253] <5> An electrolyte for a lithium ion secondary battery, comprising:

[0254] a nitrile compound containing one or more cyano groups in the molecule; and

[0255] a fluorinated alcohol represented by the formula (1),

[0256] wherein the content of the nitrile compound is 0.5% by weight or more and 5% by weight or less,

[0257] and the content of the fluorinated alcohol is 0.05% by weight or more and 1% by weight or less.

[0258] R1R2R3COH …(1)

[0259] (Each of R1, R2, and R3 is any one of a hydrogen group, an alkyl group, and a fluoroalkyl group. Among them, at least one of R1, R2, and R3 is a fluoroalkyl group).

Claims

1. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte solution, The electrolyte solution contains: A nitrile compound containing one or more cyano groups in the molecule; and A fluorinated alcohol represented by formula (1), The content of the nitrile compound in the electrolyte solution is 0.5% by weight or more and 5% by weight or less, The content of the fluorinated alcohol in the electrolyte solution is 0.05% by weight or more and 1% by weight or less, R1R2R3COH …(1) Among them, R1, R2, and R3 are each any one of a hydrogen group, an alkyl group, and a fluoroalkyl group, where at least one of R1, R2, and R3 is a fluoroalkyl group.

2. The lithium-ion secondary battery according to claim 1, wherein The nitrile compound contains two of the cyano groups in the molecule.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein In formula (1), two or more of R1, R2, and R3 are the fluoroalkyl group.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein The electrolyte solution further contains a cyclic carbonate and a chain carbonate.

5. An electrolyte solution for a lithium-ion secondary battery, containing: A nitrile compound containing one or more cyano groups in the molecule; and A fluorinated alcohol represented by formula (1), The content of the nitrile compound is 0.5% by weight or more and 5% by weight or less, The content of the fluorinated alcohol is 0.05% by weight or more and 1% by weight or less, R1R2R3COH …(1) Among them, R1, R2, and R3 are each any one of a hydrogen group, an alkyl group, and a fluoroalkyl group, where at least one of R1, R2, and R3 is a fluoroalkyl group.

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