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

Abstract

A lithium ion secondary battery according to the present invention is provided with an electrolyte solution together with a positive electrode and a negative electrode, and the electrolyte solution contains a nitrile compound that contains one or more cyano groups in each molecule, and a fluorinated alcohol represented by formula (1). The content of the nitrile compound in the electrolyte solution is 0.5% by weight to 5% by weight (inclusive), and the content of the fluorinated alcohol in the electrolyte solution is 0.05% by weight to 1% by weight (inclusive).　(1): R1R2R3COH (In the formula, each of R1, R2 and R3 represents one of a hydrogen group, an alkyl group and a fluorinated alkyl group, provided that at least one of the R1, R2 and R3 moieties represents a fluorinated alkyl group.)

Description

Electrolyte for lithium ion secondary battery and lithium ion secondary battery

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

[0002] Due to the widespread use of various electronic devices such as mobile phones, development of lithium-ion secondary batteries is underway as a power source that is small, lightweight, and has high energy density. These lithium-ion secondary batteries contain a positive electrode, a negative electrode, and an electrolyte (electrolyte for lithium-ion secondary batteries), and various studies have been conducted on the configuration of these lithium-ion secondary batteries.

[0003] Specifically, in lithium ion secondary batteries, the electrolyte solution contains alcohols such as ethanol, and the content of alcohols in the electrolyte solution is regulated (see, for example, Patent Document 1).

[0004] JP 2015-133236 A

[0005] Although various studies have been conducted on the configuration of lithium ion secondary batteries, the battery characteristics of the lithium ion secondary batteries are still insufficient and there is room for improvement.

[0006] There is a demand for an electrolyte solution for a lithium ion secondary battery and a lithium ion secondary battery that can provide excellent battery characteristics.

[0007] An electrolyte solution 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 formula (1). 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.

[0008] R1R2R3COH (1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, provided that at least one of R1, R2, and R3 is a fluorinated alkyl group.)

[0009] A lithium ion secondary battery according to an embodiment of the present technology includes an electrolyte solution together with a positive electrode and a negative electrode, and the electrolyte solution has a configuration similar to the configuration of the electrolyte solution for a lithium ion secondary battery according to the embodiment of the present technology described above.

[0010] According to the electrolyte solution for a lithium ion secondary battery or the lithium ion secondary battery of one embodiment of the present technology, the electrolyte solution for a lithium ion secondary battery contains a nitrile compound and a fluorinated alcohol, and 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, so that excellent battery characteristics can be obtained.

[0011] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

[0012] Fig. 1 is a perspective view showing a configuration of a lithium ion secondary battery according to an embodiment of the present technology. Fig. 2 is a cross-sectional view showing a configuration of a battery element shown in Fig. 1. Fig. 3 is a block diagram showing a configuration of an application example of a lithium ion secondary battery. Fig. 4 is a cross-sectional view showing a configuration of a test lithium ion secondary battery.

[0013] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Electrolyte for lithium ion secondary battery 1-1. Configuration 1-2. Manufacturing method 1-3. Action and effect 2. Lithium ion secondary battery 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Action and effect 3. Modification 4. Use of lithium ion secondary battery

[0014] 1. Electrolyte for Lithium-Ion Secondary Battery First, an electrolyte for a lithium-ion secondary battery (hereinafter simply referred to as "electrolyte") according to an embodiment of the present technology will be described.

[0015] This electrolyte is used in a lithium ion secondary battery, which is an electrochemical device. However, the electrolyte may also be used in other electrochemical devices other than lithium ion secondary batteries. The type of other electrochemical device is not particularly limited, but specifically, it may be a capacitor or the like.

[0016] <1-1. Structure> The electrolytic solution is a liquid electrolyte and is used as a medium for lithium ions in lithium ion secondary batteries. This electrolytic solution contains a nitrile compound and a fluorinated alcohol.

[0017] [Nitrile Compound] A nitrile compound is a general term for compounds containing one or more cyano groups (—CN) in the molecule. The type of nitrile compound may be one type or two or more types.

[0018] The nitrile compound contains one or more cyano groups and a central group into which the one or more cyano groups are introduced. The type of the central group is not particularly limited, but specifically, it is a group in which one or more hydrogen groups have been removed from a hydrocarbon group, and the number of hydrogen groups removed from the hydrocarbon group is determined depending on the number of cyano groups introduced into the central group.

[0019] A hydrocarbon group is a general term for a group composed of carbon and hydrogen. This hydrocarbon group may be linear, cyclic, or a combination of linear and cyclic groups.

[0020] A specific example of a nitrile compound containing one cyano group in the molecule (mononitrile compound) is acetonitrile.

[0021] Specific examples of nitrile compounds containing two cyano groups in the molecule (dinitrile compounds) include succinonitrile, glutaronitrile, adiponitrile, and 3,3'-(ethylenedioxy)dipropionitrile.

[0022] Specific examples of nitrile compounds (trinitrile compounds) containing three cyano groups in the molecule include 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.

[0023] Of course, specific examples of nitrile compounds may also be compounds containing four or more cyano groups in the molecule.

[0024] Among these, the nitrile compound is preferably a compound containing two cyano groups in the molecule, i.e., a dinitrile compound, because in a lithium ion secondary battery using the electrolyte, a good coating is likely to be formed on the surface of the negative electrode, thereby suppressing gas generation during storage of the lithium ion secondary battery.

[0025] [Fluorinated Alcohol] A fluorinated alcohol is an alcohol into which a fluorine group (—F) has been introduced, and more specifically, is a compound represented by formula (1). The type of fluorinated alcohol may be one type or two or more types.

[0026] R1R2R3COH (1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, provided that at least one of R1, R2, and R3 is a fluorinated alkyl group.)

[0027] 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 fluorinated alkyl group.

[0028] The alkyl group may be linear or branched. The number of carbon atoms in the alkyl group is not particularly limited, but is preferably 1 to 4, because this improves the solubility and compatibility of the fluorinated alcohol.

[0029] Specific examples of alkyl groups include methyl, ethyl, propyl, and butyl groups. However, as mentioned above, alkyl groups are not limited to linear groups and may be branched. Thus, for example, the propyl group may be an n-propyl group or an isopropyl group. As another example, the butyl group may be an n-butyl group, a sec-butyl group, an isobutyl group, or a tert-butyl group.

[0030] A fluorinated alkyl group is an alkyl group in which one or more hydrogen groups have been substituted with fluorine groups. Details of the alkyl group (structure and number of carbon atoms) are as described above.

[0031] Specific examples of the fluorinated alkyl group include a perfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, and a perfluorobutyl group, etc. However, specific examples of the fluorinated alkyl group are not limited to a perfluoro group, and may include a monofluoromethyl group, a monofluoroethyl group, a monofluoropropyl group, and a monofluorobutyl group, etc.

[0032] Here, as described above, one or more of R1, R2, and R3 are fluorinated alkyl groups. As described above, a fluorinated alcohol is an alcohol into which one or more fluorine groups have been introduced, and therefore must contain one or more fluorine atoms as a constituent element. Therefore, compounds in which each of R1, R2, and R3 is either a hydrogen group or an alkyl group are excluded from the fluorinated alcohols described here.

[0033] In particular, it is preferable that two or more of R1, R2, and R3 are fluorinated alkyl groups, because in a lithium ion secondary battery using the electrolyte solution, a good coating is easily formed on the surface of the negative electrode, thereby sufficiently reducing the electrical resistance.

[0034] Specific examples of fluorinated alcohols include CF ３　 CH ２　 OH, CF ２　 HCH ２　 OH, CFH ２　 CH ２　OH、CF ３　 CF ２　 CH ２　 OH、CF ３　 CFHCH ２　 OH、CF ３　 CH ２　 CH ２　 OH、CF ２　 HCF ２　 CH ２　 OH、(CF ３　 ) ２　 CHOH、CF ３　 C(CH ３　 )HOH、(CF ３　 ) ３　 COH、(CF ３　 ) ２　 C(CH ３　 )OH、(CF ３　 )C(CH ３　 ) ２　 OH、CF ３　 CF ２　 CF ２　 CH ２　 OH、CF ３　 CF ２　 CH ２　 CH ２　 OH、CF ３　 CH ２　 CH ２　 CH ２　 OH、CF ３　 CF ２　 CH(OH)CF ３　 、CF ３　 CF ２　 CH(OH)CH ３　 、CF ３　 CH ２　 CH(OH)CF ３　 、CF ３　 CH ２　 CH(OH)CH ３　 およびCH ３　 CH ２　 CH(OH)CF ３　 などである。

[0035] [Contents] In this electrolyte solution, the relationship between the content of the nitrile compound and the content of the fluorinated alcohol is optimized to improve the battery characteristics of a lithium-ion secondary battery using the electrolyte solution. More specifically, the relationship between the content of the nitrile compound and the content of the fluorinated alcohol satisfies the following two conditions.

[0036] First, the content C1 of the nitrile compound in the electrolyte is 0.5% by weight to 5% by weight.

[0037] Second, the content C2 of the fluorinated alcohol in the electrolyte is 0.05% by weight to 1% by weight.

[0038] The reason why the two conditions for the contents C1 and C2 are satisfied is that the relationship between the contents C1 and C2 is optimized, and therefore the electrical resistance of the lithium ion secondary battery using the electrolyte solution is reduced.

[0039] Specifically, the nitrile compound has the function of suppressing the decomposition reaction of the electrolyte solution, and therefore, when the electrolyte solution contains the nitrile compound, the decomposition reaction of the electrolyte solution is suppressed, and therefore, the generation of gas due to the decomposition reaction of the electrolyte solution is suppressed.

[0040] However, when an electrolyte solution contains a nitrile compound, the decomposition reaction of the electrolyte solution is suppressed, but the electrical resistance of the lithium-ion secondary battery using the electrolyte solution increases. This creates a trade-off between suppressing gas generation and suppressing the increase in electrical resistance, i.e., improving one characteristic results in a deterioration of the other characteristic.

[0041] In this regard, when an electrolyte solution contains a fluorinated alcohol together with a nitrile compound and the contents C1 and C2 satisfy two conditions, a good coating is formed on the surface of the negative electrode during charging and discharging of a lithium-ion secondary battery using the electrolyte solution due to the synergistic action of the nitrile compound and the fluorinated alcohol. This coating functions as a protective film covering the surface of the highly reactive electrode and has low electrical resistance.

[0042] The reason why the electrical resistance of this coating is low is thought to be as follows: When the electrolyte solution contains a fluorinated alcohol together with a nitrile compound, the fluorinated alcohol is reduced preferentially over the nitrile compound on the surface of the negative electrode. In this case, a coating containing lithium ions, more specifically, a coating containing lithium alkoxide or the like, is formed. As a result, even if a coating is formed on the surface of the negative electrode, a migration path for lithium ions is secured in the coating, and therefore the electrical resistance of the coating is thought to be low.

[0043] The lithium ions described here are substances that move between the positive electrode and the negative electrode during operation (charging and discharging) of the lithium ion secondary battery, and are so-called electrode reactants.

[0044] For these reasons, even if the electrolyte solution contains a nitrile compound, the electrical resistance of the electrolyte solution is prevented from increasing too much, and the decomposition reaction of the electrolyte solution on the surface of the negative electrode is also suppressed. Therefore, the trade-off relationship between the suppression of gas generation and the suppression of an increase in electrical resistance is overcome, and the electrical resistance of a lithium-ion secondary battery using the electrolyte solution is reduced.

[0045] The magnitude relationship between the contents C1 and C2 is not particularly limited and can be set arbitrarily. In particular, since the content C1 is equal to or greater than the content C2, it is preferable that the ratio of the content C1 to the content C2 (= C1 / C2) be 1 or greater. In particular, since the content C1 is greater than the content C2, it is more preferable that the ratio of the content C1 to the content C2 be greater than 1. This is because the electrical resistance is sufficiently reduced in a lithium-ion secondary battery using the electrolyte solution.

[0046] Specifically, since the content C1 is smaller than the content C2, a coating mainly derived from the fluorinated alcohol, i.e., a fluorous coating, is likely to be formed on the surface of the negative electrode when the ratio is less than 1. This increases the transport resistance of the lithium ions, the solvent described below, and the solvated lithium ions, which may increase the electrical resistance of the coating.

[0047] In contrast, when the content C1 is equal to or greater than the content C2, and thus the ratio is equal to or greater than 1, the above-described fluorous coating is less likely to be formed on the surface of the negative electrode, thereby reducing the transport resistance of the lithium ions, the solvent, and the solvated lithium ions, and suppressing an increase in the electrical resistance of the coating.

[0048] [Measurement Procedure and Calculation Procedure] When measuring the content C1 of nitrile compounds in the electrolyte solution, the lithium ion secondary battery is disassembled to recover the electrolyte solution, and the electrolyte solution is then analyzed to calculate the content of nitrile compounds. The method for analyzing the electrolyte solution is not particularly limited, but specifically includes one or more of inductively coupled plasma (ICP) atomic emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), gas chromatography mass spectrometry (GC-MS), and the like.

[0049] The procedure for measuring the content C2 of the fluorinated alcohol in the electrolyte solution is the same as the procedure for measuring the content of the nitrile compound in the electrolyte solution described above, except that the measurement target is a fluorinated alcohol instead of a nitrile compound.

[0050] [Solvent] The electrolytic solution may further contain a solvent. This solvent contains one or more non-aqueous solvents (organic solvents), and the electrolytic solution containing the non-aqueous solvent is a so-called non-aqueous electrolytic solution. The non-aqueous solvent contains esters, ethers, etc., and more specifically, contains carbonate ester compounds, carboxylic acid ester compounds, lactone compounds, etc.

[0051] Carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates include ethylene carbonate and propylene carbonate. Specific examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0052] The carboxylic acid ester compound is a chain carboxylic acid ester, etc. Specific examples of the chain carboxylic acid ester include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl trimethylacetate, ethyl trimethylethylacetate, methyl butyrate, and ethyl butyrate.

[0053] The lactone compound is lactone, etc. Specific examples of lactone include γ-butyrolactone and γ-valerolactone.

[0054] The ethers may be partially fluorinated ether compounds. Specific examples of the ethers include 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, and 1,1,2-tetrafluoroethyl 2,2,2,3,3-tetrafluoropropyl ether.

[0055] In particular, the solvent preferably contains a cyclic carbonate ester and a chain carbonate ester. This is because, in a lithium ion secondary battery using the electrolyte, a high battery capacity can be stably obtained while the electrical resistance is reduced as described above. Furthermore, in the lithium ion secondary battery, the chemical state of the electrolyte can be easily maintained and the discharge capacity is not likely to decrease sufficiently even after repeated charge and discharge.

[0056] [Electrolyte Salt] The electrolytic solution may further contain an electrolyte salt, which is a light metal salt such as a lithium salt.

[0057] A specific example of the lithium salt is lithium hexafluorophosphate (LiPF ６　 ), lithium tetrafluoroborate (LiBF ４　 ), lithium trifluoromethanesulfonate (LiCF ３　 SO ３　 ), lithium bis(fluorosulfonyl)imide (LiN(FSO ２　 ) ２　 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ３　 SO ２　 ) ２　), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF ３　 SO ２　 ) ３　 ), lithium bis(oxalato)borate (LiB(C ２　 O ４　 ) ２　 ), lithium monofluorophosphate (Li ２　 PFO ３　 ) and lithium difluorophosphate (LiPF ２　 O ２　 ) etc. This is because a high battery capacity can be obtained.

[0058] The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol / kg to 3.0 mol / kg relative to the solvent, because high ionic conductivity can be obtained.

[0059] [Additives] The electrolyte solution may further contain one or more additives, because the electrochemical stability of the electrolyte solution is improved, and therefore the decomposition reaction of the electrolyte solution is suppressed in a lithium ion secondary battery using the electrolyte solution.

[0060] The type of additive is not particularly limited, but specific examples include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonates, phosphates, acid anhydrides, and isocyanate compounds.

[0061] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0062] <1-2. Manufacturing Method> An example of a manufacturing method for an electrolytic solution is as follows. Specifically, an electrolyte salt is added to a solvent, and then a nitrile compound and a fluorinated alcohol are added to the solvent. As a result, the electrolyte salt, the nitrile compound, and the fluorinated alcohol are each dispersed or dissolved in the solvent, thereby preparing an electrolytic solution.

[0063] When producing this electrolyte solution, the amounts of the nitrile compound and the fluorinated alcohol added are adjusted so that the two conditions for the contents C1 and C2 are satisfied, as described above.

[0064] <1-3. Actions and Effects> This electrolyte solution contains a nitrile compound and a fluorinated alcohol, and satisfies two conditions for the contents C1 and C2: the content C1 is 0.5% to 5% by weight, and the content C2 is 0.05% to 1% by weight.

[0065] 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. As a result, during charge and discharge of a lithium ion secondary battery using the electrolyte, a good coating film having low electrical resistance is formed on the surface of the negative electrode due to the synergistic effect of the nitrile compound and the fluorinated alcohol. Therefore, while preventing an excessive increase in electrical resistance, the decomposition reaction of the electrolyte on the surface of the negative electrode is also suppressed, thereby breaking the trade-off relationship between suppression of gas generation and suppression of an increase in electrical resistance.

[0066] From these facts, in a lithium ion secondary battery using the electrolyte solution, the electrical resistance is reduced, and therefore excellent battery characteristics can be obtained.

[0067] In particular, since the nitrile compound contains two cyano groups in the molecule, if the nitrile compound is a dinitrile compound, it is easy to form a good coating on the surface of the negative electrode in a lithium ion secondary battery using the electrolyte, thereby further suppressing gas generation and achieving a greater effect.

[0068] Furthermore, if two or more of R1, R2, and R3 in formula (1) are fluorinated alkyl groups, a good coating is easily formed on the surface of the negative electrode in a lithium ion secondary battery using the electrolyte solution, and therefore, the electrical resistance is sufficiently reduced, resulting in a higher effect.

[0069] Furthermore, if the electrolyte solution further contains a cyclic carbonate ester and a chain carbonate ester, in a lithium ion secondary battery using that electrolyte solution, not only will the electrical resistance be reduced while maintaining the battery capacity, but the chemical state of the electrolyte solution will be more easily maintained and the discharge capacity will be less likely to decrease even after repeated charge and discharge, thereby achieving even greater effects.

[0070] 2. Lithium-ion secondary battery Next, a lithium-ion secondary battery using the above-described electrolytic solution according to an embodiment of the present technology will be described.

[0071] The lithium-ion secondary battery described here is a secondary battery that obtains battery capacity by utilizing the absorption and desorption of lithium, and includes a positive electrode, a negative electrode, and an electrolyte solution. This lithium-ion secondary battery utilizes the absorption and desorption of lithium to obtain sufficient battery capacity stably.

[0072] The charge capacity of the negative electrode is preferably larger than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of ​​the negative electrode is preferably larger than the electrochemical capacity per unit area of ​​the positive electrode. This is to prevent lithium metal from being deposited on the surface of the negative electrode during charging.

[0073] <2-1. Configuration> Fig. 1 shows a perspective configuration of a lithium ion secondary battery, and Fig. 2 shows a cross-sectional configuration of the battery element 20 shown in Fig. 1. However, Fig. 1 shows a state in which the exterior film 10 and the battery element 20 are separated from each other, and the cross section of the battery element 20 along the XZ plane is shown by a dashed line.

[0074] 1 and 2, this lithium ion secondary battery includes an exterior 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 here is a laminate film type lithium ion secondary battery that uses a flexible or pliable exterior film 10.

[0075] 1, the exterior film 10 is an exterior member that houses the battery element 20, and has a bag-like structure that is sealed when the battery element 20 is housed inside. As a result, the exterior film 10 houses an electrolyte solution together with a positive electrode 21 and a negative electrode 22, which will be described later.

[0076] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (deeply drawn portion) for accommodating the battery element 20.

[0077] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0078] However, the configuration (number of layers) of the exterior film 10 is not particularly limited, and may be one layer, two layers, or four or more layers.

[0079] [Battery Element] As shown in FIGS. 1 and 2 , the battery element 20 is a power generating element including a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), and is housed inside the exterior film 10.

[0080] This battery element 20 is a so-called wound electrode body. That is, the positive electrode 21 and the negative electrode 22 are stacked on top of each other with a separator 23 interposed therebetween, and are wound around a winding axis P while facing each other with the separator 23 interposed therebetween. This winding axis P is an imaginary axis extending in the Y-axis direction.

[0081] The three-dimensional shape of battery element 20 is not particularly limited. Here, the three-dimensional shape of battery element 20 is flat, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by a major axis J1 and a minor axis J2. This major axis J1 is an imaginary axis that extends in the X-axis direction and has a length greater than that of minor axis J2, and minor axis J2 is an imaginary axis that extends in the Z-axis direction intersecting with the X-axis direction and has a length smaller than that of major axis J1. Here, the three-dimensional shape of battery element 20 is a flat cylindrical shape, and therefore the shape of the cross section of battery element 20 is a flat, approximately elliptical shape.

[0082] (Positive Electrode) As shown in FIG. 2, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B.

[0083] 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.

[0084] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A and contains one or more types of positive electrode active materials that absorb and release lithium. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A, on the side where the positive electrode 21 faces the negative electrode 22. The positive electrode active material layer 21B may further contain one or more types 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, but a specific example is a coating method.

[0085] The positive electrode active material contains a lithium-containing compound. This lithium-containing compound is a compound containing lithium and one or more transition metal elements as constituent elements, and may further contain one or more other elements as constituent elements. The type of other element is not particularly limited as long as it is an element other than a transition metal element (excluding lithium), but specifically, it is an element belonging to Groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specifically, it includes oxides, phosphate compounds, silicate compounds, borate compounds, etc.

[0086] A specific example of the oxide is LiNiO ２　 , LiCoO ２　 , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２　 , LiNi ０．５　 Co ０．２　 Mn ０．３　 O ２　 , LiNi ０．８　 Co ０．１５ Al ０．０５ O ２　 , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２　 , Li １．２　 Mn ０．５２ Co ０．１７５　 Ni ０．１　 O ２　 , Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２　 and LiMn ２　 O ４　 Specific examples of phosphate compounds include LiFePO ４　 , LiMnPO ４　 , LiFe ０．５　 Mn ０．５　 P.O. ４　 and LiFe ０．３　 Mn ０．７　 P.O. ４　 And so on.

[0087] The positive electrode binder contains one or more compounds selected from the group consisting of synthetic rubbers and polymeric compounds. Specific examples of synthetic rubbers include styrene-butadiene rubbers, fluorine-containing rubbers, and ethylene-propylene-diene. Specific examples of polymeric compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0088] The positive electrode conductive agent contains one or more conductive materials such as carbon materials, and specific examples of the conductive materials include 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, a conductive polymer compound, etc.

[0089] (Negative Electrode) As shown in FIG. 2, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B.

[0090] 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 a specific example of the conductive material is copper.

[0091] Here, the anode active material layer 22B is provided on both sides of the anode current collector 22A and contains one or more types of anode active materials that absorb and release lithium. However, the anode active material layer 22B may be provided on only one side of the anode current collector 22A, on the side where the anode 22 faces the cathode 21. The anode active material layer 22B may further contain one or more types of other materials, such as a anode binder and anode conductor. The method for forming the anode active material layer 22B is not particularly limited, but may be, for example, a coating method.

[0092] The negative electrode active material contains one or more of a carbon material and a metal-based material, etc. This is because a high energy density can be obtained.

[0093] Specific examples of carbon materials include graphitizable carbon, non-graphitizable carbon, and graphite, etc. The graphite may be natural graphite, artificial graphite, or both.

[0094] The metallic material is a material containing, as a constituent element, one or more of metallic elements and semi-metallic elements that can form an alloy with lithium, and specific examples of the metallic element and semi-metallic element include silicon and tin. The metallic material may be a simple substance, an alloy, a compound, a mixture of two or more of these, or a material containing two or more of these phases. Specific examples of metallic materials include TiSi ２　 and SiO ｘ　 (0<x≦2, or 0.2<x<1.4), etc.

[0095] The details regarding the negative electrode binder and the negative electrode conductive agent are the same as the details regarding the positive electrode binder and the positive electrode conductive agent, respectively.

[0096] 2, the separator 23 is an insulating porous film interposed 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.

[0097] (Electrolyte) Details of the electrolyte are as described above. That is, the electrolyte contains a nitrile compound and a fluorinated alcohol, and satisfies two conditions regarding the contents C1 and C2.

[0098] 1 and 2 , the positive electrode lead 31 is a positive electrode terminal connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum. The shape of the positive electrode lead 31 is not particularly limited, but is specifically either a thin plate shape or a mesh shape.

[0099] 1 and 2 , the negative electrode lead 32 is a negative electrode terminal connected to the negative electrode current collector 22A of the negative electrode 22, and is led out of the exterior film 10. This negative electrode lead 32 contains a conductive material such as a metal material, and a specific example of the conductive material is copper. Here, the details regarding the lead-out direction and shape of the negative electrode lead 32 are the same as the details regarding the lead-out direction and shape of the positive electrode lead 31.

[0100] [Sealing Film] The sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41 and 42 may be omitted.

[0101] The sealing film 41 is a sealing member that prevents outside air and the like from entering the interior of the exterior film 10. The sealing film 41 also contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polyolefin is polypropylene.

[0102] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32.

[0103] <2-2. Operation> The lithium ion secondary battery operates as described below.

[0104] During charging, in the battery element 20, lithium is released in an ionic state from the positive electrode 21, and the lithium is absorbed in an ionic state into the negative electrode 22 via the electrolyte. On the other hand, during discharging, in the battery element 20, lithium is released in an ionic state from the negative electrode 22, and the lithium is absorbed in an ionic state into the positive electrode 21 via the electrolyte.

[0105] <2-3. Manufacturing Method> When manufacturing a lithium ion secondary battery, the positive electrode 21 and the negative electrode 22 are each produced and an electrolyte solution is prepared according to the procedure described below as an example. Thereafter, the positive electrode 21, the negative electrode 22, and the electrolyte solution are used to assemble a lithium ion secondary battery, and a stabilization process is performed on the lithium ion secondary battery.

[0106] [Fabrication of Positive Electrode] First, a paste-like positive electrode mixture slurry is prepared by adding a mixture (positive electrode mixture) of a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent to a solvent. This solvent may be an aqueous solvent or an organic solvent. Next, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 21A to form the positive electrode active material layer 21B. Finally, the positive electrode active material layer 21B may be compression-molded using a roll press or the like. In this case, the positive electrode active material layer 21B may be heated, or the compression molding may be repeated multiple times. As a result, the positive electrode active material layer 21B is formed on both surfaces of the positive electrode current collector 21A, thereby fabricating the positive electrode 21.

[0107] [Fabrication of Negative Electrode] The negative electrode 22 is fabricated using the same procedure as the fabrication procedure for the positive electrode 21 described above. Specifically, a mixture (negative electrode mixture) of a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent is mixed together and poured into a solvent to prepare a paste-like negative electrode mixture slurry. The negative electrode mixture slurry is then applied to both surfaces of the negative electrode current collector 22A to form the negative electrode active material layer 22B. The negative electrode active material layer 22B may then be compression-molded. As a result, the negative electrode active material layer 22B is formed on both surfaces of the negative electrode current collector 22A, thereby fabricating the negative electrode 22.

[0108] [Preparation of Electrolyte Solution] An electrolyte solution containing a nitrile compound and a fluorinated alcohol is prepared by the procedure described above.

[0109] [Assembly of Lithium-Ion Secondary Battery] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding, and the negative electrode lead 32 is connected to the negative electrode current collector 22A of the negative electrode 22 using a joining method such as welding.

[0110] Next, the positive electrode 21 and the negative electrode 22 are stacked together with the separator 23 interposed therebetween to form a laminate, and then the laminate is wound to produce a wound body (not shown). This wound body has a configuration similar to that of the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with an electrolyte. Next, the wound body is pressed using a press or the like to form a flattened shape.

[0111] Next, after the roll is housed inside the recess 10U, the exterior film 10 (fusion layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the facing fusion layers are joined together using an adhesive method such as heat fusion, thereby housing the roll inside the bag-shaped exterior film 10.

[0112] Finally, after injecting an electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

[0113] As a result, the wound body is impregnated with the electrolyte, and a wound electrode body, that is, a battery element 20, is produced. The battery element 20 is then sealed inside the bag-shaped exterior film 10, and a lithium ion secondary battery is assembled.

[0114] [Stabilization Treatment] The assembled lithium ion secondary battery is charged and discharged. Various conditions, such as the ambient temperature, the number of charge / discharge cycles (number of cycles), and the charge / discharge conditions, can be set as desired. This forms a coating on the surface of each of the positive electrode 21 and the negative electrode 22, electrochemically stabilizing the state of the lithium ion secondary battery. This completes the lithium ion secondary battery.

[0115] <2-4. Actions and Effects> This lithium ion secondary battery includes an electrolyte solution, and the electrolyte solution has the above-described configuration. Therefore, for the reasons described above, a good coating having low electrical resistance is formed on the surface of the negative electrode 22, which prevents the electrical resistance of the electrolyte solution from increasing too much and also suppresses the decomposition reaction of the electrolyte solution on the surface of the negative electrode 22. Therefore, the electrical resistance is reduced, and excellent battery characteristics can be obtained.

[0116] Other functions and effects of the lithium ion secondary battery are the same as those of the electrolyte.

[0117] 3. Modifications The configuration of the lithium ion secondary battery can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0118] [Modification 1] A porous film separator 23 is used. However, although not specifically shown here, a laminated separator including a polymer compound layer may also be used.

[0119] 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 separator improves adhesion to each of the positive electrode 21 and the negative electrode 22, thereby suppressing miswinding of the battery element 20. This prevents swelling of the lithium-ion secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0120] One or both of the porous film and the polymer compound layer may contain a plurality of insulating particles. This is because the plurality of insulating particles promotes heat dissipation when the lithium ion secondary battery generates heat, thereby improving the safety (heat resistance) of the lithium ion secondary battery. The insulating particles contain one or more insulating materials, such as inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

[0121] When a laminated separator is produced, a precursor solution containing a polymer compound and a solvent is prepared, and then the precursor solution is applied to one or both sides of a porous film. In this case, multiple insulating particles may be added to the precursor solution as needed.

[0122] Even when this laminated separator is used, the same effect can be obtained because lithium becomes movable between the positive electrode 21 and the negative electrode 22. In this case, as described above, the safety of the lithium ion secondary battery is particularly improved, and therefore, a greater effect can be obtained.

[0123] [Modification 2] An electrolytic solution that is a liquid electrolyte is used. However, although not specifically shown here, an electrolyte layer that is a gel electrolyte may also be used.

[0124] In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte layer are wound together. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0125] Specifically, the electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The composition of the electrolytic solution is as described above. The polymer compound contains polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolytic solution, the polymer compound, a solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0126] Even when this electrolyte layer is used, the same effect can be obtained because lithium can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer. In this case, particularly, as described above, leakage of the electrolyte solution is prevented, so that a greater effect can be obtained.

[0127] 4. Uses of Lithium-ion Secondary Batteries There are no particular limitations on the uses (application examples) of lithium-ion secondary batteries. Lithium-ion secondary batteries used as power sources may be the main power source for electronic devices, electric vehicles, and the like, or may be an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the main power source, or may be a power source that is switched from the main power source.

[0128] Specific examples of uses for lithium ion secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals; Storage devices such as backup power supplies and memory cards; Power tools such as power drills and power saws; Battery packs installed in electronic devices; Medical electronic devices such as pacemakers and hearing aids; Electric vehicles such as electric cars (including hybrid cars); Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these uses, one lithium ion secondary battery may be used, or multiple lithium ion secondary batteries may be used.

[0129] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that operates (travels) using a lithium-ion secondary battery as a driving power source, and may be a hybrid vehicle that also has a driving source other than the lithium-ion secondary battery. The home power storage system can use the power stored in the lithium-ion secondary battery, which is the power storage source, to power home electrical appliances, etc.

[0130] Here, an example of an application of a lithium ion secondary battery will be specifically described. The configuration of the application described below is merely an example and can be modified as appropriate.

[0131] Figure 3 shows the block diagram of a battery pack. The battery pack described here is a battery pack (a so-called soft pack) that uses a single lithium-ion secondary battery, and is installed in electronic devices such as smartphones.

[0132] 3, the battery pack includes a power supply 51 and a circuit board 52. The circuit board 52 is connected to the power supply 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0133] The power source 51 includes one lithium ion secondary battery. The positive electrode lead of this lithium ion secondary battery is connected to a positive electrode terminal 53, and the negative electrode lead is connected to a negative electrode terminal 54. The power source 51 can be connected to the outside via the positive electrode terminal 53 and the negative electrode terminal 54, and is therefore capable of charging and discharging. 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 may be omitted.

[0134] The control unit 56 includes a central processing unit (CPU) and memory, 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 necessary.

[0135] When the voltage of power supply 51 (lithium ion secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, control unit 56 turns off switch 57 to prevent charging current from flowing through the current path of power supply 51. The overcharge detection voltage is not particularly limited, but specifically, it is 4.20 V±0.05 V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.40 V±0.1 V.

[0136] Switch 57 includes a charge control switch, a discharge control switch, a charging diode, a discharging diode, etc., and switches between the connection and disconnection of power supply 51 and an external device in response to instructions from control unit 56. Switch 57 includes a metal oxide semiconductor field effect transistor (MOSFET), etc., and the charge current and the discharge current are detected based on the ON resistance of switch 57.

[0137] Temperature detection unit 59 includes a temperature detection element such as a thermistor. Temperature detection unit 59 measures the temperature of power supply 51 using temperature detection terminal 55 and outputs the temperature measurement result to control unit 56. The temperature measurement result measured by temperature detection unit 59 is used when control unit 56 controls charging and discharging in the event of abnormal heat generation, and when control unit 56 performs correction processing when calculating the remaining capacity.

[0138] An embodiment of the present technology will be described.

[0139] Examples 1 to 11 and Comparative Examples 1 to 7 As will be described below, lithium ion secondary batteries were fabricated, and then the battery characteristics of the lithium ion secondary batteries were evaluated.

[0140] [Fabrication of Lithium-Ion Secondary Battery] Here, a test lithium-ion secondary battery was fabricated to simply evaluate the battery characteristics. Fig. 4 shows the cross-sectional structure of the test secondary battery, which is a so-called coin-type lithium-ion secondary battery.

[0141] Below, the configuration of a coin-type lithium ion secondary battery will be explained, and then the procedure for manufacturing the lithium ion secondary battery will be explained.

[0142] As shown in FIG. 4, this lithium ion secondary battery includes a test electrode 61, a counter electrode 62, a separator 63, an exterior cup 64, an exterior can 65, a gasket 66, and an electrolyte (not shown).

[0143] The test electrode 61 is housed in an exterior cup 64, and the counter electrode 62 is housed in an exterior can 65. The test electrode 61 and the counter electrode 62 are stacked together with a separator 63 interposed therebetween, and the test electrode 61, the counter electrode 62, and the separator 63 are each impregnated with an electrolyte. The exterior cup 64 and the exterior can 65 are crimped together with a gasket 66, so that the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the exterior cup 64 and the exterior can 65.

[0144] (Preparation of Test Electrode) When preparing a lithium ion secondary battery, first, a positive electrode active material (a lithium-containing compound (oxide) LiNi ０．８０ Co ０．１５ Al ０．０５ O ２　 A positive electrode mixture was prepared by mixing 91 parts by mass of a positive electrode binder (polyvinylidene fluoride), 3 parts by mass of a positive electrode conductive agent (Ketjen black, an amorphous carbon powder), and 6 parts by mass of a positive electrode mixture. The positive electrode mixture was then added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the solvent was stirred to prepare a paste-like positive electrode mixture slurry.

[0145] Next, the positive electrode mixture slurry was applied to one surface of the positive electrode current collector 21A (aluminum foil having a thickness of 10 μm) using a coating device, and then the positive electrode mixture slurry was dried to form the positive electrode active material layer 21B.

[0146] Finally, the positive electrode active material layer 21B was compression-molded using a roll press, and then the positive electrode current collector 21A on which the positive electrode active material layer 21B was formed was cut into a disk shape, thereby producing the positive electrode 21.

[0147] (Preparation of Counter Electrode) First, 94 parts by mass of a negative electrode active material (4 parts by mass of silicon oxide, which is a metallic material, and 90 parts by mass of artificial graphite, which is 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 together to prepare a negative electrode mixture.

[0148] Next, the negative electrode mixture was poured into a solvent (water, which is an aqueous solvent), and the solvent was stirred to prepare a paste-like negative electrode mixture slurry.

[0149] Next, the negative electrode mixture slurry was applied to one side of the negative electrode current collector 22A (copper foil with a thickness of 8 μm) using a coating device, and then the negative electrode mixture slurry was dried to form the negative electrode active material layer 22B.

[0150] Finally, the negative electrode active material layer 22B was compression-molded using a roll press, and then the negative electrode current collector 22A on which the negative electrode active material layer 22B was formed was cut into a disk shape, thereby producing the negative electrode 22.

[0151] (Preparation of Electrolyte Solution) First, a solvent was prepared. As the solvent, a mixture of ethylene carbonate (EC), which is a cyclic carbonate ester, and ethyl methyl carbonate (EMC), which is a chain carbonate ester, was used. In this case, the mixing ratio (wt%) of the solvent was EC:EMC = 30:70.

[0152] Next, an electrolyte salt (lithium salt, lithium hexafluorophosphate (LiPF)) was added to the solvent. ６　 )) was added, and the solvent was stirred. In this case, the content of the electrolyte salt was 1 mol / kg relative to the solvent.

[0153] Finally, the nitrile compound and the fluorinated alcohol were added to the solvent containing the electrolyte salt, and the solvent was then stirred. In this case, succinonitrile (NCCH ２　 CH ２　 CN)SN) was used as the fluorinated alcohol, and hexafluoroisopropanol ((CF ３　 )２　 CHOH(HFIP)) was used to prepare an electrolyte solution.

[0154] When preparing this electrolytic solution, the amount of the nitrile compound added was adjusted so that the content C1 (wt %) of the nitrile compound in the electrolytic solution would be the value shown in Table 1, and the amount of the fluorinated alcohol added was adjusted so that the content C2 (wt %) of the fluorinated alcohol in the electrolytic solution would be the value shown in Table 1.

[0155] For comparison, an electrolyte solution was prepared in the same manner except that no fluorinated alcohol was used.

[0156] (Assembly of Lithium-Ion Secondary Battery) First, the test electrode 61 was placed in the exterior cup 64, and the counter electrode 62 was placed in the exterior can 65. Next, the test electrode 61 placed in the exterior cup 64 and the counter electrode 62 placed in the exterior can 65 were stacked together with a separator 63 (a microporous polyethylene film with a thickness of 20 μm) impregnated with an electrolyte interposed therebetween. In this case, the positive electrode active material layer 21B and the negative electrode active material layer 22B faced each other with the separator 63 interposed therebetween. Next, with the test electrode 61 and the counter electrode 62 stacked together with the separator 63 interposed therebetween, the exterior cup 64 and the exterior can 65 were crimped together with the gasket 66. As a result, the test electrode 61 and the counter electrode 62 were sealed inside the exterior cup 64 and the exterior can 65, and a lithium-ion secondary battery was assembled.

[0157] (Stabilization Treatment) The lithium ion secondary battery was subjected to one cycle of charge and discharge in a room temperature environment (temperature = 23°C). During charging, the battery was charged at a constant current of 0.1 C until the voltage reached 4.2 V, and then charged at a constant voltage of 0.025 C at the same voltage of 4.2 V. During discharging, the battery was discharged at a constant current of 0.1 C until the voltage reached 3.0 V. 0.1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 10 hours, and 0.025 C is the current value at which the battery capacity is fully discharged in 40 hours.

[0158] As a result, the test electrode 61 and the counter electrode 62 were electrochemically stabilized, and a lithium ion secondary battery was completed.

[0159] [Evaluation of Lithium-Ion Secondary Battery Characteristics] The battery characteristics (electrical resistance characteristics) of the lithium-ion secondary battery were evaluated according to the procedure described below, and the results shown in Table 1 were obtained.

[0160] When evaluating the electrical resistance characteristics, electrochemical impedance (EIS (Ω)), which is an index for evaluating the electrical resistance characteristics, was measured using an AC impedance method. This EIS is the so-called charge transfer resistance. The measurement device used was a multi-channel potentiostat VMP-3 manufactured by Bio-Logic Science Instruments. The measurement conditions were a frequency range of 1 MHz to 10 mHz and an AC amplitude of 10 mV.

[0161] The EIS values ​​shown in Table 1 are normalized values. Specifically, the EIS values ​​in Examples 1 to 4 and Comparative Examples 1 and 2 are normalized with the EIS value in Comparative Example 1 set to 100. The EIS values ​​in Examples 5 to 8 and Comparative Examples 3 and 4 are normalized with the EIS value in Comparative Example 3 set to 100. The EIS values ​​in Examples 9 to 11 and Comparative Examples 5 to 7 are normalized with the EIS value in Comparative Example 5 set to 100.

[0162]

[0163] [Discussion] As shown in Table 1, the EIS varied greatly depending on the composition of the electrolyte.

[0164] Specifically, when the electrolyte solution contained a nitrile compound and a fluorinated alcohol, and the conditions that the content C1 was 0.5 wt % to 5 wt % and the content C2 was 0.05 wt % to 1 wt % were not met (Comparative Examples 1 to 7), the EIS increased.

[0165] In contrast, when the electrolyte solution contained a nitrile compound and a fluorinated alcohol, and the content C1 was 0.5 wt % to 5 wt % and the content C2 was 0.05 wt % to 1 wt % (Examples 1 to 11), the EIS decreased.

[0166] In particular, when the above conditions were met (Examples 1 to 11), the following tendency was observed.

[0167] First, by using a dinitrile compound (SN) as the nitrile compound, that is, a nitrile compound containing two cyano groups in the molecule, the EIS was sufficiently reduced.

[0168] 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 formula (1) are fluorinated alkyl groups, the EIS was sufficiently reduced.

[0169] Third, since the solvent contains a nitrile compound and a fluorinated alcohol as well as a solvent (cyclic carbonate ester and chain carbonate ester), the EIS is sufficiently reduced while ensuring smooth charge / discharge reactions (battery capacity).

[0170] [Summary] From the results shown in Table 1, when the electrolyte solution of a lithium ion secondary battery contained a nitrile compound and a fluorinated alcohol, and two conditions for the contents C1 and C2 (C1 = 0.5 wt % to 5 wt % and the content C2 = 0.05 wt % to 1 wt %) were satisfied, the EIS decreased. Therefore, the electrical resistance characteristics were improved, and excellent battery characteristics were obtained in the lithium ion secondary battery.

[0171] The present technology has been described above with reference to an embodiment and examples. However, the configuration of the present technology is not limited to the configuration described in the embodiment and examples, and can be modified in various ways.

[0172] Specifically, although the lithium ion secondary battery has been described as having a laminate film structure, the battery structure of the secondary battery applied to the battery pack of the present technology is not particularly limited. Specifically, the lithium ion secondary battery may have a cylindrical, prismatic, or coin-shaped structure.

[0173] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, positive and negative electrodes are alternately stacked with a separator interposed therebetween, while in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern while facing each other with the separator interposed therebetween.

[0174] The effects described in this specification are merely examples, and the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0175] The present technology may also be configured as follows. <1> A lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte solution, the electrolyte solution including: a nitrile compound including 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 being 0.5% by weight or more and 5% by weight or less, and the content of the fluorinated alcohol in the electrolyte solution being 0.05% by weight or more and 1% by weight or less. R1R2R3COH ... (1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, with the proviso that at least one of R1, R2, and R3 is a fluorinated alkyl group.) <2> The lithium ion secondary battery according to <1>, the nitrile compound includes two of the cyano groups in the molecule. <3> The lithium ion secondary battery according to <1> or <2>, wherein in the formula (1), two or more of R1, R2, and R3 are the fluorinated alkyl group. <4> The lithium ion secondary battery according to any one of <1> to <3>, wherein the electrolyte further contains a cyclic carbonate ester and a chain carbonate ester. <5> An electrolyte for a lithium ion secondary battery, comprising: a nitrile compound containing one or more cyano groups in the molecule; and a fluorinated alcohol represented by formula (1), wherein 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. R1R2R3COH (1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, provided that at least one of R1, R2, and R3 is a fluorinated alkyl group.)

Claims

1. a positive electrode, a negative electrode, and an electrolyte; The electrolyte solution is a nitrile compound containing one or more cyano groups in the molecule; a fluorinated alcohol represented by formula (1); Including, the content of the nitrile compound in the electrolytic solution is 0.5% by weight or more and 5% by weight or less, The content of the fluorinated alcohol in the electrolytic solution is 0.05% by weight or more and 1% by weight or less. Lithium-ion secondary battery. R1R2R3COH...(1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, provided that at least one of R1, R2, and R3 is a fluorinated alkyl group.)

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

3. In the formula (1), two or more of R1, R2, and R3 are the fluorinated alkyl groups. The lithium ion secondary battery according to claim 1 or 2.

4. The electrolyte solution further contains a cyclic carbonate ester and a chain carbonate ester. The lithium ion secondary battery according to claim 1 or 2.

5. a nitrile compound containing one or more cyano groups in the molecule; a fluorinated alcohol represented by formula (1); Including, 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. Electrolyte for lithium-ion secondary batteries. R1R2R3COH...(1) (R1, R2, and R3 each represent a hydrogen group, an alkyl group, or a fluorinated alkyl group, provided that at least one of R1, R2, and R3 is a fluorinated alkyl group.)