Electrolyte for lithium-ion secondary batteries and lithium-ion secondary batteries

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

[Technical Field]

[0001] This technology relates to electrolytes for lithium-ion secondary batteries and lithium-ion secondary batteries. [Background technology]

[0002] With the widespread use of various electronic devices such as mobile phones, the development of lithium-ion secondary batteries is progressing as a power source that is small, lightweight, and provides high energy density. These lithium-ion secondary batteries consist of a positive electrode, a negative electrode, and an electrolyte (electrolyte for lithium-ion secondary batteries), and various studies are being conducted on the configuration of these lithium-ion secondary batteries.

[0003] Specifically, in lithium-ion secondary batteries, the electrolyte contains alcohols such as ethanol, and the amount of alcohols contained in the electrolyte is specified (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-133236 [Overview of the Initiative]

[0005] Although various studies have been conducted on the configuration of lithium-ion secondary batteries, their battery characteristics are still not satisfactory, and there is room for improvement.

[0006] There is a need for electrolytes and lithium-ion secondary batteries that can achieve excellent battery characteristics.

[0007] An electrolyte for a lithium-ion secondary battery according to one embodiment of this technology comprises a nitrile compound containing one or more cyano groups in its 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, and the content of the fluorinated alcohol is 0.05% by weight or more and 1% by weight or less.

[0008] R1R2R3COH ···(1) (Each of R1, R2, and R3 is either a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.)

[0009] A lithium-ion secondary battery according to one embodiment of this technology includes an electrolyte along with a positive electrode and a negative electrode, and the electrolyte has the same configuration as the electrolyte for the lithium-ion secondary battery according to the embodiment of this technology described above.

[0010] According to one embodiment of this technology, the electrolyte for lithium-ion secondary batteries or the lithium-ion secondary battery contains a nitrile compound and a fluorinated alcohol, with the nitrile compound content being 0.5% by weight or more and 5% by weight or less, and the fluorinated alcohol content being 0.05% by weight or more and 1% by weight or less, thereby obtaining excellent battery characteristics.

[0011] Furthermore, the effects of this technology are not necessarily limited to those described herein, but may include any of the series of effects related to this technology described later. [Brief explanation of the drawing]

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

Embodiments for Carrying Out the Invention

[0013] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The order of description is as follows. 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. Variation 4. Applications 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 be used in other electrochemical devices different from the lithium ion secondary battery. The type of other electrochemical devices is not particularly limited, but specifically, it is a capacitor or the like.

[0016] <1-1. Configuration> 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.

[0017] [Nitrile Compound] Nitrile compounds are a general term for compounds that contain one or more cyano groups (-CN) in their molecule. A nitrile compound can consist of only one type or two or more types.

[0018] This nitrile compound contains one or more cyano groups along with a central group to which these one or more cyano groups are introduced. The type of this central group is not particularly limited, but specifically it is a group from 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 according to 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 in the form of a chain, a ring, or a combination of both.

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

[0021] Specific examples of nitrile compounds (dinitrile compounds) that contain two cyano groups in their molecule include succinonitrile, glutalonitrile, adiponitrile, and 3,3'-(ethylenedioxy)dipropionitrile.

[0022] Specific examples of nitrile compounds (trinitrile compounds) that contain three cyano groups in their molecule include 1,2,3-propanetricarbonitric, 1,3,5-pentanetricarbonitric, 1,3,4-hexanetricarbonitric, 1,3,6-hexanetricarbonitric, 1,3,5-cyclohexanetricarbonitric, and 1,3,5-benzenetricarbonitric.

[0023] Of course, specific examples of nitrile compounds may also include compounds that contain four or more cyano groups in their molecule.

[0024] In particular, the nitrile compound is preferably a compound containing two cyano groups in its molecule, that is, a dinitrile compound. This is because, in lithium-ion secondary batteries using an electrolyte, a good film is more easily formed on the surface of the negative electrode, thereby suppressing gas generation during storage of the lithium-ion secondary battery.

[0025] [Fluorinated alcohol] Fluorinated alcohols are alcohols to which a fluorine group (-F) has been introduced, and more specifically, are compounds represented by formula (1). There may be only one type of fluorinated alcohol, or two or more types.

[0026] R1R2R3COH ···(1) (Each of R1, R2, and R3 is either a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.)

[0027] Each of R1, R2, and R3 is not particularly limited, as long as it is one of a hydrogen group (-H), an alkyl group, or a fluorinated alkyl group, as described above.

[0028] The alkyl group may be linear or branched. The number of carbon atoms in the alkyl group is not particularly limited, but it 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 structures but may also be branched. For example, a propyl group may be an n-propyl group or an isopropyl group. Another example is a butyl group which 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 are replaced by fluorine groups. Details regarding alkyl groups (composition and number of carbon atoms) are as described above.

[0031] Specific examples of fluorinated alkyl groups include perfluoromethyl, perfluoroethyl, perfluoropropyl, and perfluorobutyl groups. However, specific examples of fluorinated alkyl groups are not limited to perfluoro groups; monofluoromethyl, monofluoroethyl, monofluoropropyl, and monofluorobutyl groups may also be used.

[0032] Here, as described above, one or more of R1, R2, and R3 are fluorinated alkyl groups. This is because, as described above, fluorinated alcohols are alcohols into which one or more fluorine groups have been introduced, and therefore must contain one or more fluorine as a constituent element. Consequently, 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. This is because, in lithium-ion secondary batteries using an electrolyte, a good film is more easily formed on the surface of the negative electrode, thereby sufficiently reducing electrical resistance.

[0034] Specific examples of fluorinated alcohols include 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.

[0035] [Content] In this electrolyte, the relationship between the nitrile compound content and the fluorinated alcohol content has been optimized to improve the battery characteristics of lithium-ion secondary batteries using this electrolyte. More specifically, the relationship between the nitrile compound content and the fluorinated alcohol content satisfies the following two conditions, which are described below.

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

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

[0038] The reason why two conditions regarding the C1 and C2 content are met is that the relationship between the C1 and C2 content is optimized, which reduces the electrical resistance in lithium-ion secondary batteries using electrolyte.

[0039] In detail, nitrile compounds have the function of suppressing the decomposition reaction of the electrolyte. As a result, when an electrolyte contains nitrile compounds, the decomposition reaction of that electrolyte is suppressed, and thus the generation of gases caused by the decomposition reaction of that electrolyte is suppressed.

[0040] However, if the electrolyte contains nitrile compounds, the decomposition reaction of the electrolyte is suppressed, but at the same time, the electrical resistance of the lithium-ion secondary battery using that electrolyte increases. This creates a trade-off relationship between suppressing gas generation and suppressing the increase in electrical resistance; in other words, improving one characteristic worsens the other.

[0041] In this regard, if the electrolyte contains fluorinated alcohol along with a nitrile compound, and two conditions regarding the C1 and C2 content are met, then during the charging and discharging of a lithium-ion secondary battery using that electrolyte, a good film is formed on the surface of the negative electrode due to the synergistic effect of the nitrile compound and the fluorinated alcohol. This film functions as a protective film covering the surface of the electrode, which has high reactivity, and also 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 contains fluorinated alcohol along with nitrile compounds, the fluorinated alcohol is preferentially reduced over the nitrile compounds on the surface of the negative electrode. In this case, a coating containing lithium ions, more specifically a coating containing lithium alkoxide, is formed. As a result, even though a coating is formed on the surface of the negative electrode, a migration path for lithium ions is secured within that coating, which is thought to result in a low electrical resistance of the coating.

[0043] The lithium ions described here are substances that move between the positive and negative electrodes during the operation (charging and discharging) of a lithium-ion secondary battery, and are what are known as electrode reaction materials.

[0044] Therefore, even if the electrolyte contains nitrile compounds, the increase in the electrical resistance of the electrolyte is suppressed, while the decomposition reaction of the electrolyte on the surface of the negative electrode is also suppressed. Thus, the lay-off relationship between the suppression of gas generation and the suppression of the increase in electrical resistance is broken, and the electrical resistance decreases in lithium-ion secondary batteries using electrolytes.

[0045] The relative sizes of the C1 and C2 content are not particularly limited and can be set arbitrarily. In particular, since the C1 content is greater than or equal to the C2 content, it is preferable that the ratio of the C1 content to the C2 content (=C1 / C2) is 1 or greater. Especially since the C1 content is greater than the C2 content, it is even more preferable that the ratio of the C1 content to the C2 content is greater than 1. This is because the electrical resistance is sufficiently reduced in lithium-ion secondary batteries using electrolyte.

[0046] More specifically, because the C1 content is smaller than the C2 content, if the ratio is less than 1, a film mainly derived from fluorinated alcohols, i.e., a fluorescein film, is more likely to form on the surface of the negative electrode. This increases the transport resistance of lithium ions, the solvent (described later), and the solvated lithium ions, which can increase the electrical resistance of the film.

[0047] In contrast, if the C1 content is greater than or equal to the C2 content, and the ratio is 1 or greater, the fluorescein coating described above is less likely to form on the surface of the negative electrode. As a result, the transport resistance of lithium ions, solvent, and solvated lithium ions decreases, and the increase in the electrical resistance of the coating is suppressed.

[0048] [Measurement Procedure and Calculation Procedure] To measure the nitrile compound content C1 in the electrolyte, the lithium-ion secondary battery is disassembled, the electrolyte is recovered, and then the nitrile compound content is calculated by analyzing the electrolyte. The analytical method for the electrolyte is not particularly limited, but specifically, it may be one or more of the following: inductively coupled plasma (ICP) emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and gas chromatography-mass spectroscopy (GC-MS).

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

[0050] [solvent] Furthermore, the electrolyte may also contain a solvent. This solvent contains one or more types of non-aqueous solvents (organic solvents), and the electrolyte containing such a non-aqueous solvent is a so-called non-aqueous electrolyte. Non-aqueous solvents include esters and ethers, and more specifically, carbonate ester compounds, carboxylic acid ester compounds, and lactone compounds.

[0051] Carbonate ester compounds include cyclic carbonate esters and linear carbonate esters. Specific examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate. Specific examples of linear carbonate esters include dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.

[0052] Carboxylic acid ester compounds include linear carboxylic acid esters. Specific examples of linear carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl trimethylacetate, ethyl trimethylacetate, methyl butyrate, and ethyl butyrate.

[0053] Lactone compounds include lactones, among others. Specific examples of lactones include γ-butyrolactone and γ-valerolactone.

[0054] Furthermore, ethers may be compounds in which a portion of the ether is fluorinated. Specific examples of 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 cyclic carbonate esters and linear carbonate esters. This is because, in lithium-ion secondary batteries using the electrolyte, a high battery capacity can be stably obtained while the electrical resistance decreases as described above. Furthermore, in lithium-ion secondary batteries, the chemical state of the electrolyte is more easily maintained, and the discharge capacity does not decrease significantly even after repeated charging and discharging.

[0056] [Electrolyte salts] Furthermore, the electrolyte may also contain an electrolyte salt. This electrolyte salt is a light metal salt, such as a lithium salt.

[0057] Specific examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF3SO2)3), lithium bis(oxalato)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2). These are used because they provide high battery capacity.

[0058] The electrolyte salt content is not particularly limited, but specifically, it is between 0.3 mol / kg and 3.0 mol / kg relative to the solvent. This is because it allows for high ionic conductivity.

[0059] [Additives] Furthermore, the electrolyte may also contain one or more of the additives. This is because the electrochemical stability of the electrolyte is improved, thereby suppressing the decomposition reaction of the electrolyte in a lithium-ion secondary battery using that electrolyte.

[0060] The types of additives are not particularly limited, but specifically include unsaturated cyclic carbonate esters, fluorinated cyclic carbonate esters, sulfonic acid esters, phosphate esters, acid anhydrides, and isocyanate compounds.

[0061] Specific examples of unsaturated cyclic carbonate esters include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonate esters include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propanesultone and propensultone. 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 method for producing an electrolyte solution is described below. 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, nitrile compound, and fluorinated alcohol are dispersed or dissolved in the solvent, thus preparing the electrolyte solution.

[0063] When manufacturing this electrolyte, the amounts of nitrile compound and fluorinated alcohol added are adjusted so that two conditions regarding the C1 and C2 content are met, as described above.

[0064] <1-3. Mechanism and Effects> According to this electrolyte, the electrolyte contains a nitrile compound and a fluorinated alcohol, and two conditions are met regarding the C1 and C2 content. Specifically, the C1 content is 0.5% to 5% by weight, and the C2 content 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 C1 and C2 content is optimized. As a result, during the charging and discharging of a lithium-ion secondary battery using an electrolyte, a good film with 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, thus breaking the lay-off relationship between suppressing gas generation and suppressing the increase in electrical resistance.

[0066] These factors suggest that in lithium-ion secondary batteries using an electrolyte, the electrical resistance decreases, resulting in superior battery characteristics.

[0067] In particular, because nitrile compounds contain two cyano groups within their molecule, if the nitrile compound is a dinitrile compound, a good film is more easily formed on the surface of the negative electrode in lithium-ion secondary batteries using an electrolyte. Therefore, gas generation is further suppressed, resulting in a higher efficiency.

[0068] Furthermore, if two or more of R1, R2, and R3 in equation (1) are fluorinated alkyl groups, a good film is more easily formed on the surface of the negative electrode in a lithium-ion secondary battery using an electrolyte. As a result, the electrical resistance is sufficiently reduced, and a higher effect can be obtained.

[0069] Furthermore, if the electrolyte also contains cyclic carbonate esters and chain carbonate esters, a lithium-ion secondary battery using that electrolyte will not only maintain battery capacity while reducing electrical resistance, but will also be able to maintain the chemical state of the electrolyte more effectively, and the discharge capacity will not decrease significantly even after repeated charging and discharging. Therefore, a higher level of effectiveness can be achieved.

[0070] <2. Lithium-ion rechargeable batteries> Next, we will describe a lithium-ion secondary battery, which is one embodiment of this technology, using the electrolyte described above.

[0071] The lithium-ion secondary battery described here is a secondary battery that obtains its capacity by utilizing the intercalation and deintercalation of lithium, and is equipped with an electrolyte along with a positive electrode and a negative electrode. In this lithium-ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the intercalation and deintercalation of lithium.

[0072] Furthermore, it is preferable that the charging capacity of the negative electrode is greater than the discharge capacity of the positive electrode. In other words, it is preferable that the electrochemical capacity per unit area of ​​the negative electrode is greater than the electrochemical capacity per unit area of ​​the positive electrode. This is to prevent the deposition of lithium metal on the surface of the negative electrode during charging.

[0073] <2-1. Structure> Figure 1 shows a perspective view of a lithium-ion secondary battery, while Figure 2 shows a cross-sectional view of the battery element 20 shown in Figure 1. However, in Figure 1, the outer film 10 and the battery element 20 are shown separated from each other, and the cross-section of the battery element 20 along the XZ plane is shown with a dashed line.

[0074] As shown in Figures 1 and 2, this lithium-ion secondary battery comprises an outer 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 laminate film type lithium-ion secondary battery using a flexible or pliable outer film 10.

[0075] [Exterior film] As shown in Figure 1, the outer film 10 is an outer component that houses the battery element 20, and has a sealed bag-like structure in which the battery element 20 is housed. As a result, the outer film 10 houses the electrolyte together with the positive electrode 21 and negative electrode 22, which will be described later.

[0076] Here, the outer film 10 is a single film-like component that is folded in the folding direction F. The outer film 10 is provided with a recess 10U (deep-drawn portion) for housing the battery element 20.

[0077] Specifically, the outer 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. When the outer film 10 is folded, the outer edges of the opposing fusion layers are fused together. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metallic material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0078] However, the composition (number of layers) of the outer film 10 is not particularly limited; it may consist of one or two layers, or four or more layers.

[0079] [Battery element] As shown in Figures 1 and 2, the battery element 20 is a power generation element that includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown), and is housed inside the outer 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 via a separator 23 and are wound around a winding axis P while facing each other via the separator 23. This winding axis P is a virtual axis that extends in the Y-axis direction.

[0081] 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 flattened, the shape of the cross-section of the battery element 20 intersecting the winding axis P (cross-section along the XZ plane) is a flattened shape defined by the major axis J1 and the minor axis J2. The major axis J1 is a virtual axis that extends in the X-axis direction and has a length greater than the length of the minor axis J2, while the minor axis J2 is a virtual axis that extends in the Z-axis direction intersecting the X-axis direction and has a length less than the length of the major axis J1. Here, since the three-dimensional shape of the battery element 20 is a flattened cylinder, the shape of the cross-section of the battery element 20 is a flattened, approximately elliptical shape.

[0082] (positive electrode) As shown in Figure 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. This positive electrode current collector 21A contains a conductive material such as a metal material, a specific example of which 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 intercalate and deintercalate 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. Furthermore, the positive electrode active material layer 21B may also 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 specifically, it may be a coating method.

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

[0086] 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.

[0087] The positive electrode binder contains any one or more of compounds such as synthetic rubber and polymer compounds. Specific examples of the synthetic rubber are styrene-butadiene rubber, fluorine-based rubber, ethylene-propylene diene, etc. Specific examples of the polymer compound are polyvinylidene fluoride, polyimide, carboxymethyl cellulose, etc.

[0088] The positive electrode conductive agent contains one or more types of conductive materials, such as carbon materials. Specific examples of these conductive materials include graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, and carbon nanotubes. However, the conductive material may also be a metallic material or a conductive polymer compound.

[0089] (Negative electrode) As shown in Figure 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. This negative electrode current collector 22A contains a conductive material such as a metallic material, a specific example of which is copper.

[0091] Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode current collector 22A and contains one or more types of negative electrode active materials that intercalate and deintercalate lithium. However, the negative electrode active material layer 22B may be provided on only one side of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21. Furthermore, the negative electrode active material layer 22B may also contain one or more types of other materials such as a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited, but specifically, it may be a coating method.

[0092] The negative electrode active material contains one or more types of materials, such as carbon materials and metallic materials, because it allows for a high energy density to be obtained.

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

[0094] Metallic materials are materials that contain one or more metallic elements and metalloid elements capable of forming alloys with lithium as constituent elements. Specific examples of these metallic and metalloid elements include silicon and tin. These metallic materials may be elements, alloys, compounds, mixtures of two or more of these, or materials containing two or more of these phases. Specific examples of metallic materials include TiSi2 and SiO2. x (0 <x≦2、または0.2<x<1.4)などである。

[0095] Details regarding the negative electrode binder and negative electrode conductive agent are the same as those regarding the positive electrode binder and positive electrode conductive agent.

[0096] (Separator) As shown in Figure 2, the separator 23 is an insulating porous membrane interposed between the positive electrode 21 and the negative electrode 22, allowing lithium ions to pass through while preventing contact (short circuit) between the positive electrode 21 and the negative electrode 22. This separator 23 contains a polymer compound such as polyethylene.

[0097] (electrolyte) Details regarding the electrolyte are as described above. Specifically, the electrolyte contains a nitrile compound and a fluorinated alcohol, and two conditions are met regarding the content of C1 and C2.

[0098] [Positive lead and negative lead] As shown in Figures 1 and 2, the positive electrode lead 31 is the positive electrode terminal connected to the positive electrode current collector 21A of the positive electrode 21, and is led out to the outside of the outer film 10. This positive electrode lead 31 contains a conductive material such as a metal material, a specific example of which is aluminum. The shape of the positive electrode lead 31 is not particularly limited, but specifically it can be either a thin plate shape or a mesh shape.

[0099] The negative electrode lead 32, as shown in Figures 1 and 2, is the negative electrode terminal connected to the negative electrode current collector 22A of the negative electrode 22, and is led out to the outside of the outer film 10. This negative electrode lead 32 contains a conductive material such as a metallic material, a specific example of which is copper. Here, the details regarding the lead direction and shape of the negative electrode lead 32 are the same as those regarding the lead direction and shape of the positive electrode lead 31.

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

[0101] This sealing film 41 is a sealing member that prevents outside air and other elements from entering the interior of the outer film 10. The sealing film 41 also contains a polymer compound such as polyolefin that has good adhesion to the positive electrode lead 31, and a specific example of this polyolefin is polypropylene.

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

[0103] <2-2. Operation> Lithium-ion rechargeable batteries operate as described below.

[0104] During charging, lithium is released in an ionic state from the positive electrode 21 of the battery element 20, and this lithium is absorbed in an ionic state into the negative electrode 22 via the electrolyte. On the other hand, during discharge, lithium is released in an ionic state from the negative electrode 22 of the battery element 20, and this 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 prepared according to the example procedure described below, and the electrolyte is prepared. Then, the lithium-ion secondary battery is assembled using the positive electrode 21, the negative electrode 22, and the electrolyte, and the lithium-ion secondary battery is subjected to a stabilization treatment.

[0106] [Fabrication of the positive electrode] First, a paste-like positive electrode mixture slurry is prepared by adding a mixture (positive electrode mixture) containing positive electrode active material, positive electrode binder, and 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 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 may be compressed and 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, a positive electrode 21 is fabricated as positive electrode active material layers 21B are formed on both sides of the positive electrode current collector 21A.

[0107] [Fabrication of the negative electrode] The negative electrode 22 is manufactured using the same procedure as that used for manufacturing the positive electrode 21 described above. Specifically, a paste-like negative electrode mixture slurry is prepared by adding a mixture (negative electrode mixture) in which the negative electrode active material, negative electrode binder, and negative electrode conductive agent are mixed together to a solvent. Then, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 22A to form the negative electrode active material layer 22B. After this, the negative electrode active material layer 22B may be compression molded. As a result, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, and the negative electrode 22 is manufactured.

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

[0109] [Assembly of lithium-ion rechargeable batteries] 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, a laminate is formed by stacking the positive electrode 21 and the negative electrode 22 on top of each other via a separator 23, and then a wound body (not shown) is produced by winding the laminate. This wound body has the same configuration as the battery element 20, except that the positive electrode 21, the negative electrode 22, and the separator 23 are not impregnated with electrolyte. Subsequently, the wound body is molded into a flattened shape by pressing it with a press or the like.

[0111] Next, after housing the wound body inside the recessed portion 10U, the outer film 10 (fusion layer / metal layer / surface protection layer) is folded so that the outer films 10 face each other. Subsequently, using an adhesive method such as heat fusion, the outer edges of two sides of the opposing fusion layers are joined together, thereby housing the wound body inside the bag-shaped outer film 10.

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

[0113] As a result, the electrolyte is impregnated into the wound material, thus creating the battery element 20, which is a wound electrode body. Therefore, the battery element 20 is sealed inside the bag-shaped outer film 10, and a lithium-ion secondary battery is assembled.

[0114] [Stabilization process] The assembled lithium-ion secondary battery is then charged and discharged. Various conditions, such as ambient temperature, number of charge / discharge cycles, and charge / discharge conditions, can be set arbitrarily. As a result, a coating is formed on the surfaces of the positive electrode 21 and the negative electrode 22, thereby electrochemically stabilizing the state of the lithium-ion secondary battery. Thus, the lithium-ion secondary battery is completed.

[0115] <2-4. Action and Effects> According to this lithium-ion secondary battery, the lithium-ion secondary battery is equipped with an electrolyte, and the electrolyte has the configuration described above. Therefore, for the reasons described above, a good film with low electrical resistance is formed on the surface of the negative electrode 22, so that the electrical resistance of the electrolyte does not increase too much, and the decomposition reaction of the electrolyte on the surface of the negative electrode 22 is also suppressed. As a result, the electrical resistance is reduced, and excellent battery characteristics can be obtained.

[0116] Furthermore, other effects and behaviors related to lithium-ion secondary batteries are the same as those related to the electrolyte.

[0117] <3. Variant> The configuration of the lithium-ion secondary battery can be modified as appropriate, as described below. However, the variations described below may be combined with each other.

[0118] [Example 1] A porous membrane separator 23 was used. However, although not specifically shown in the diagram, a laminated separator containing 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 sides of the porous membrane. This improves the adhesion of the separator to the positive electrode 21 and the negative electrode 22, thereby suppressing winding misalignment of the battery element 20. As a result, swelling of the lithium-ion secondary battery is suppressed even if a decomposition reaction of the electrolyte occurs. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0120] Furthermore, one or both of the porous membrane and the polymer compound layer may contain multiple insulating particles. This is because the multiple insulating particles promote 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 consist of one or more types of 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 fabricating a laminated separator, 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 membrane. In this case, if necessary, multiple insulating particles may be added to the precursor solution.

[0122] Even when using this stacked separator, lithium can move between the positive electrode 21 and the negative electrode 22, thus achieving a similar effect. In this case, as mentioned above, the safety of the lithium-ion secondary battery is improved, resulting in an even greater effect.

[0123] [Differentiation 2] A liquid electrolyte solution was used. However, although not specifically illustrated here, a gel-like electrolyte layer may also be used.

[0124] In the battery element 20 using an electrolyte layer, the positive electrode 21 and the negative electrode 22 are stacked on top of each other via a separator 23 and the electrolyte layer, and the positive electrode 21, negative electrode 22, separator 23, and electrolyte layer are wound together. This 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 along with the electrolyte, and the electrolyte is held in place by the polymer compound. This prevents leakage of the electrolyte. The composition of the electrolyte is as described above. The polymer compound includes polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolyte, polymer compound, and solvent is prepared, and then the precursor solution is applied to one or both sides of the positive electrode 21 and the negative electrode 22, respectively.

[0126] Even when this electrolyte layer is used, lithium can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer, thus achieving a similar effect. In this case, in particular, as described above, leakage of the electrolyte is prevented, thus achieving an even greater effect.

[0127] <4. Applications of Lithium-ion Rechargeable Batteries> The applications (examples of use) of lithium-ion secondary batteries are not particularly limited. Lithium-ion secondary batteries used as power sources may be the primary power source or auxiliary power source for electronic devices and electric vehicles. A primary 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 primary power source, or a power source that can be switched to from the primary power source.

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

[0129] In a battery pack, individual cells may be used, or a battery pack may be used. An electric vehicle is a vehicle that operates (drives) using a lithium-ion secondary battery as a power source, and may also be a hybrid vehicle equipped with other power sources other than the lithium-ion secondary battery. In a household power storage system, household electrical appliances can be used by utilizing the electricity stored in the lithium-ion secondary battery, which is the power storage source.

[0130] Here, we will specifically explain one example of an application of lithium-ion secondary batteries. The configuration of the application example described below is merely an example and can be modified as needed.

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

[0132] As shown in Figure 3, this battery pack comprises 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 supply 51 includes one 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. Since the power supply 51 can be connected to the outside via the positive terminal 53 and the negative terminal 54, it can be charged and discharged. 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. This control unit 56 also detects and controls the usage status of the power supply 51 as needed.

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

[0136] Switch 57 includes a charge control switch, a discharge control switch, a charging diode, and a discharge diode, and switches the connection between the power supply 51 and external equipment according to the instructions of the control unit 56. This switch 57 includes a field-effect transistor (MOSFET) using a metal oxide semiconductor, and the charging current and discharge current are detected based on the ON resistance of the switch 57.

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

[0138] An example of this technology will be described below.

[0139] <Examples 1-11 and Comparative Examples 1-7> As described below, after fabricating a lithium-ion secondary battery, its battery characteristics were evaluated.

[0140] [Manufacturing of lithium-ion secondary batteries] Here, a test lithium-ion secondary battery was fabricated to perform a simple evaluation of its battery characteristics. Figure 4 shows the cross-sectional configuration of the test secondary battery, which is a so-called coin-type lithium-ion secondary battery.

[0141] The following section will explain the structure of a coin-type lithium-ion secondary battery, followed by a description of the manufacturing procedure for that lithium-ion secondary battery.

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

[0143] The test electrode 61 is housed in an outer cup 64, and the counter electrode 62 is housed in an outer can 65. The test electrode 61 and the counter electrode 62 are stacked on top of each other via a separator 63, and the electrolyte is impregnated into the test electrode 61, the counter electrode 62, and the separator 63, respectively. The outer cup 64 and the outer can 65 are crimped together via a gasket 66, so the test electrode 61, the counter electrode 62, and the separator 63 are sealed by the outer cup 64 and the outer can 65.

[0144] (Preparation of test electrode) When manufacturing a lithium-ion secondary battery, the first step is to create a positive electrode active material (LiNi, a lithium-containing compound (oxide)). 0.80 Co 0.15 Al 0.05A positive electrode mixture was prepared by mixing 91 parts by mass of O2, 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (Ketjenbrak, an amorphous carbon powder). Subsequently, the positive electrode mixture was 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, a positive electrode slurry was applied to one side of the positive electrode current collector 21A (aluminum foil with a thickness of 10 μm) using a coating device, and then the positive electrode active material layer 21B was formed by drying the positive electrode slurry.

[0146] Finally, the positive electrode active material layer 21B was compressed and 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 disc shape. This completed the production of the positive electrode 21.

[0147] (Preparation of the opposing pole) First, a negative electrode mixture was prepared by mixing 94 parts by mass of negative electrode active material (4 parts by mass of silicon oxide, a metallic material, and 90 parts by mass of artificial graphite, a carbon material), 1.5 parts by mass of negative electrode binder (polyvinylidene fluoride), 2.5 parts by mass of 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 thickener (carboxymethylcellulose).

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

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

[0150] Finally, the negative electrode active material layer 22B was compressed and 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 disc shape. This produced the negative electrode 22.

[0151] (Preparation of electrolyte solution) First, the solvent was prepared. The solvent used was a mixture of ethylene carbonate (EC), a cyclic carbonate ester, and ethylmethyl carbonate (EMC), a linear carbonate ester. In this case, the solvent mixing ratio (by weight) was EC:EMC = 30:70.

[0152] Next, an electrolyte salt (lithium hexafluoride phosphate (LiPF6), a lithium salt) was added to the solvent, and the solvent was then stirred. In this case, the electrolyte salt content was 1 mol / kg relative to the solvent.

[0153] Finally, a nitrile compound and a fluorinated alcohol were added to the solvent containing the electrolyte salt, and the solvent was then stirred. In this case, succinonitrile (NCCH2CH2CN)SN), a dinitrile compound, was used as the nitrile compound, and hexafluoroisopropanol ((CF3)2CHOH(HFIP)) was used as the fluorinated alcohol. This prepared the electrolyte.

[0154] When preparing this electrolyte, the amount of nitrile compound added was adjusted so that the nitrile compound content C1 (weight %) in the electrolyte was as shown in Table 1, and the amount of fluorinated alcohol added was adjusted so that the fluorinated alcohol content C2 (weight %) in the electrolyte was as shown in Table 1.

[0155] For comparison, the electrolyte was prepared using the same procedure, except that fluorinated alcohol was not used.

[0156] (Assembly of lithium-ion rechargeable batteries) First, the test electrode 61 was placed in the outer cup 64, and the counter electrode 62 was placed in the outer can 65. Next, the test electrode 61 in the outer cup 64 and the counter electrode 62 in the outer can 65 were stacked on top of each other via a separator 63 (a microporous polyethylene film with a thickness of 20 μm) impregnated with electrolyte. In this case, the positive electrode active material layer 21B and the negative electrode active material layer 22B were facing each other via the separator 63. Subsequently, with the test electrode 61 and the counter electrode 62 stacked on top of each other via the separator 63, the outer cup 64 and the outer can 65 were crimped together via a gasket 66. As a result, the test electrode 61 and the counter electrode 62 were sealed inside the outer cup 64 and the outer can 65, and the lithium-ion secondary battery was assembled.

[0157] (Stabilization process) A lithium-ion secondary battery was subjected to one charge-discharge cycle in a normal temperature environment (temperature = 23°C). During charging, constant current charging was performed at a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed at that voltage of 4.2V until the current reached 0.025C. During discharging, constant current discharge was performed at a current of 0.1C until the voltage reached 3.0V. 0.1C is the current value required to completely discharge the battery capacity (theoretical capacity) in 10 hours, and 0.025C is the current value required to completely discharge the battery capacity in 40 hours.

[0158] As a result, both the test electrode 61 and the counter electrode 62 were electrochemically stabilized, thus completing the lithium-ion secondary battery.

[0159] [Characteristic evaluation of lithium-ion secondary batteries] The battery characteristics (electrical resistance characteristics) of the lithium-ion secondary battery were evaluated using the procedure described below, and the results shown in Table 1 were obtained.

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

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

[0162] [Table 1]

[0163] [Consideration] As shown in Table 1, the EIS varied considerably depending on the electrolyte composition.

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

[0165] In contrast, when the electrolyte contained a nitrile compound and a fluorinated alcohol, and the conditions were met such that the C1 content was 0.5% to 5% by weight and the C2 content was 0.05% to 1% by weight (Examples 1 to 11), the EIS decreased.

[0166] In particular, when the above conditions were met (Examples 1 to 11), the trends described below were observed.

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

[0168] Secondly, by using HFIP as the fluorinated alcohol, that is, by using a fluorinated alcohol in which two or more of the R1 to R3 shown in formula (1) are fluorinated alkyl groups, the EIS was sufficiently reduced.

[0169] Thirdly, the presence of solvents (cyclic carbonate esters and linear carbonate esters) along with nitrile compounds and fluorinated alcohols ensured a smooth charge-discharge reaction (battery capacity) while significantly reducing EIS.

[0170] [summary] As shown in Table 1, the EIS decreased when the electrolyte of the lithium-ion secondary battery contained a nitrile compound and a fluorinated alcohol, and two conditions were met regarding the content of C1 and C2 (C1 = 0.5% to 5% by weight and C2 = 0.05% to 1% by weight). Therefore, the electrical resistance characteristics were improved, resulting in excellent battery characteristics in the lithium-ion secondary battery.

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

[0172] Specifically, the explanation described the case where the battery structure of the lithium-ion secondary battery is of the laminated film type, 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 may be cylindrical, prismatic, coin-type, etc.

[0173] Furthermore, the case where the element structure of the battery element is of the wound type has been described. However, the element structure of the battery element is not particularly limited, and may also be of the stacked type or the zigzag type. In the stacked type, the positive electrode and negative electrode are stacked alternately with a separator in between, while in the zigzag type, the positive electrode and negative electrode are folded in a zigzag pattern with a separator in between, facing each other.

[0174] The effects described herein are illustrative only, and therefore the effects of this technology are not limited to those described herein. Accordingly, other effects may be obtained with respect to this technology.

[0175] Furthermore, this technology can also be configured as follows: <1> The system includes an electrolyte along with a positive electrode and a negative electrode. The aforementioned electrolyte is A nitrile compound containing one or more cyano groups in its molecule, A fluorinated alcohol represented by formula (1) and Includes, The content of the nitrile compound in the electrolyte is 0.5% by weight or more and 5% by weight or less. The content of the fluorinated alcohol in the electrolyte is 0.05% by weight or more and 1% by weight or less. Lithium-ion rechargeable battery. R1R2R3COH ···(1) (Each of R1, R2, and R3 is either a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.) <2> The nitrile compound contains two of the cyano groups within its molecule. <1> Lithium-ion secondary batteries as described above. <3> In formula (1) above, two or more of R1, R2, and R3 are the fluorinated alkyl groups. <1> or <2> Lithium-ion secondary batteries as described above. <4> The electrolyte further comprises a cyclic carbonate ester and a linear carbonate ester. <1> or <3> A lithium-ion secondary battery as described in any one of the following. <5> A nitrile compound containing one or more cyano groups in its molecule, A fluorinated alcohol represented by formula (1) and Includes, 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) (Each of R1, R2, and R3 is either a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.)

Claims

1. The system includes an electrolyte along with a positive electrode and a negative electrode. The aforementioned electrolyte is A nitrile compound containing one or more cyano groups in its molecule, A fluorinated alcohol represented by formula (1) and Includes, The content of the nitrile compound in the electrolyte is 0.5% by weight or more and 5% by weight or less. The content of the fluorinated alcohol in the electrolyte is 0.05% by weight or more and 1% by weight or less. Lithium-ion rechargeable battery. R1R2R3COH...(1) (Each of R1, R2, and R3 is a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.)

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

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

4. The electrolyte further comprises a cyclic carbonate ester and a linear carbonate ester. A lithium-ion secondary battery according to claim 1 or claim 2.

5. A nitrile compound containing one or more cyano groups in its molecule, A fluorinated alcohol represented by formula (1) and Includes, 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) (Each of R1, R2, and R3 is a hydrogen group, an alkyl group, or a fluorinated alkyl group. However, at least one of R1, R2, and R3 is a fluorinated alkyl group.)

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