Lithium ion battery

Abstract

To achieve a lithium ion battery with high productivity using a fire-retardant electrolytic solution.SOLUTION: A lithium ion battery 100 includes: a battery element 10 for a lithium ion battery having a positive electrode 11 and a negative electrode 12 and a separator 13 provided between the electrodes; an exterior body 30 in which the battery element 10 is accommodated; and an electrolytic solution 20 accommodated inside the exterior body 30. For example, such an electrolytic solution 20 of the lithium ion battery 100 used includes a deep eutectic solvent including an amide compound or an imide compound and lithium salt, a carbonate compound, and a surfactant having a perfluoroalkyl group. The surfactant is included in an amount of 0.6 weight% or more relative to the weight of a mixture including the deep eutectic solvent and carbonate compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to lithium-ion batteries. [Background technology]

[0002] One type of battery known is a lithium ion battery that uses an electrolyte solution, such as a lithium ion secondary battery that uses an electrolyte solution that contains an ionic liquid such as trimethylbutylammonium-bis(trifluoromethanesulfonyl)imide and a lithium salt such as lithium bis(trifluoromethanesulfonyl)imide but does not contain an organic solvent (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-218160 Summary of the Invention [Problem to be solved by the invention]

[0004] A lithium-ion battery is configured such that a battery element having a positive electrode, a negative electrode, and a separator therebetween is housed in an exterior container such as a can or a bag together with an electrolyte. The battery element is impregnated with the electrolyte, and the electrolyte penetrates the battery element. In lithium-ion batteries that use an electrolyte containing a lithium salt and a flammable organic solvent, there is a risk of fire, explosion, or the like due to a short circuit or the like. To address this issue, known techniques use electrolytes that do not contain flammable organic solvents or electrolytes to which a flame retardant is added. However, while such electrolytes provide flame retardancy, they have poor wettability and permeability to the battery element, which has sometimes made it difficult to efficiently produce lithium-ion batteries in which the electrolyte has sufficiently penetrated the battery element.

[0005] In one aspect, the present invention aims to realize a highly productive lithium-ion battery using a flame-retardant electrolyte. [Means for solving the problem]

[0006] In one aspect, there is provided a lithium ion battery comprising: a battery element for a lithium ion battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; an outer casing in which the battery element is housed; and an electrolyte solution housed within the outer casing, wherein the electrolyte solution comprises a deep eutectic solvent containing an amide compound or an imide compound and a lithium salt, a carbonate compound, and a surfactant having a perfluoroalkyl group, and the surfactant is contained in an amount of 0.6 wt % or more relative to the weight of the mixture containing the deep eutectic solvent and the carbonate compound. [Effects of the Invention]

[0007] In one aspect, it will be possible to realize highly productive lithium-ion batteries using flame-retardant electrolytes. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an example of a lithium ion battery. DETAILED DESCRIPTION OF THE INVENTION

[0009] First, we will discuss lithium-ion batteries. A lithium ion battery (also referred to as a "lithium ion secondary battery") has a configuration in which a battery element for a lithium ion battery, which has, for example, a positive electrode, a negative electrode, and a separator provided between them, is housed in an exterior body together with an electrolyte. Known forms of lithium ion batteries include coin-shaped batteries, cylindrical batteries, and prismatic batteries that use a can of a predetermined shape as the exterior body, as well as thin batteries that use a bag-shaped laminate film as the exterior body.

[0010] FIG. 1 is a diagram illustrating an example of a lithium ion battery. FIG. 1(A) is a schematic plan view of a main part of an example of a lithium ion battery. FIG. 1(B) is a schematic cross-sectional view of a main part of an example of a lithium ion battery. FIG. 1(B) is a schematic cross-sectional view taken along line II of FIG. 1(A).

[0011] 1A and 1B is an example of a coin-type lithium-ion battery. The lithium-ion battery 100 includes a battery element 10, an electrolyte 20, and an exterior body 30.

[0012] The battery element 10 is a battery element for a lithium ion battery. As shown in Fig. 1(B), the battery element 10 has a positive electrode 11, a negative electrode 12, and a separator 13 provided between them.

[0013] The positive electrode 11 uses a positive electrode material containing a positive electrode active material. The positive electrode active material may be a lithium-containing metal oxide or the like. For example, lithium iron phosphate or the like may be used as the positive electrode active material. The positive electrode material may include, in addition to the positive electrode active material, a conductive material such as a carbon material, and organic components such as a thickener, a binder, and a solvent. For example, the conductive material may be acetylene black or carbon black. For example, the thickener may be carboxymethyl cellulose. For example, the binder may be acrylic, styrene butadiene rubber, polytetrafluoroethylene, polyvinylidene fluoride, or the like. Such materials may be kneaded with the positive electrode active material to form a positive electrode material mixture (referred to as a "positive electrode mixture"). The positive electrode 11 may be laminated on a positive electrode current collector 11a such as aluminum foil, stainless steel foil, or titanium foil. For example, a positive electrode mixture is applied onto a positive electrode current collector 11a, dried, and then vacuum dried at a temperature of about 100°C to 200°C for about 12 hours to form a positive electrode 11 laminated on the positive electrode current collector 11a.

[0014] The negative electrode 12 uses a negative electrode material containing a negative electrode active material. The negative electrode active material may be a carbon material capable of absorbing and releasing lithium. For example, graphite is used as the negative electrode active material. In addition to the negative electrode active material, the negative electrode material may also include a conductive material such as a carbon material, and organic components such as a thickener, binder, and solvent. For example, the conductive material may be acetylene black or carbon black. For example, the thickener may be carboxymethyl cellulose. For example, the binder may be acrylic, styrene butadiene rubber, polytetrafluoroethylene, polyvinylidene fluoride, or the like. Such materials may be kneaded with the negative electrode active material to form a negative electrode material mixture (referred to as a "negative electrode mixture"). The negative electrode 12 may be laminated on a negative electrode current collector 12a such as copper foil. For example, a negative electrode mixture is applied onto a negative electrode current collector 12a, dried, and then vacuum-dried at a temperature of about 100°C to 200°C for about 12 hours to form a negative electrode 12 laminated on the negative electrode current collector 12a.

[0015] The positive electrode 11 and the negative electrode 12 are disposed so as to face each other with a separator 13 interposed therebetween. The facing positive electrode 11 and negative electrode 12 are separated by the separator 13. The separator 13 may be a polyolefin-based or cellulose-based porous film, a woven fabric, a nonwoven fabric, or the like. Alternatively, the separator 13 may be made of ceramic or the like.

[0016] The positive electrode 11, the negative electrode 12, and the separator 13 of the battery element 10 are impregnated with an electrolytic solution 20 and housed in an exterior body 30. Alternatively, the positive electrode 11, the negative electrode 12, and the separator 13 of the battery element 10 are housed in the exterior body 30 together with the electrolytic solution 20 and in a state of being impregnated in the electrolytic solution 20. For convenience, in FIG. 1(B), the electrolytic solution 20 that coexists with the battery element 10, i.e., that is present inside or on the surface of the battery element 10, is schematically shown by a dashed line.

[0017] A coin-shaped battery can (also referred to as an "external can") is used as the exterior body 30 that houses the battery element 10 and the electrolyte solution 20. FIGS. 1(A) and 1(B) show an example of the exterior body 30, which includes a positive electrode can 31, a negative electrode can 32, and a gasket 33 that electrically insulates them. The positive electrode can 31 has a flat circular bottom 31a and a side wall 31b rising from the outer periphery of the bottom 31a, with the inside of the side wall 31b being open. The negative electrode can 32 has a flat circular bottom 32a that fits into the opening of the positive electrode can 31 and a side wall 32b rising from the outer periphery of the bottom 31a, with the inside of the side wall 32b being open. The tip of the side wall 32b of the negative electrode can 32 may be bent outward from the opening, as shown in FIG. 1(B). Gasket 33 is made of various insulating materials, such as insulating resin materials such as polypropylene resin and fluororesin. Gasket 33 is interposed between side wall 31b of positive electrode can 31 and side wall 32b of negative electrode can 32 in exterior body 30, and electrically insulates positive electrode can 31 and negative electrode can 32.

[0018] The lithium ion battery 100 shown in FIGS. 1(A) and 1(B) is manufactured, for example, as follows. A positive electrode 11 provided on a positive electrode current collector 11a and a negative electrode 12 provided on a negative electrode current collector 12a are disposed opposite each other with a separator 13 interposed therebetween, with the positive electrode current collector 11a and the negative electrode current collector 12a facing outward. An electrolyte solution 20 is poured into and impregnated (e.g., vacuum impregnation) into a battery element 10 including the positive electrode 11 on the positive electrode current collector 11a, the negative electrode 12 on the negative electrode current collector 12a, and the separator 13 thus disposed, and the battery element 10 impregnated with the electrolyte solution 20 is housed in an outer casing 30. Alternatively, the battery element 10 including the positive electrode 11 on the positive electrode current collector 11a, the negative electrode 12 on the negative electrode current collector 12a, and the separator 13 may be housed in an outer casing 30 together with the electrolyte solution 20 while impregnated in the electrolyte solution 20.

[0019] When the battery element 10 is housed in the exterior body 30, the positive electrode current collector 11a is connected to the bottom 31a of the positive electrode can 31, and the negative electrode current collector 12a is connected to the bottom 32a of the negative electrode can 32. To achieve this connection, the battery element 10 is placed between the positive electrode can 31 and the negative electrode can 32 together with the electrolyte 20, and the side wall 32b of the negative electrode can 32 is inserted inside the side wall 31b of the positive electrode can 31 via a gasket 33. Then, the tip of the side wall 31b of the positive electrode can 31 is crimped to the side wall 32b of the negative electrode can 32 via the gasket 33. As a result, the positive electrode can 31 and the negative electrode can 32 are fixed in an electrically insulated state by the gasket 33, and a sealed exterior body 30 is obtained.

[0020] By this method, a lithium ion battery 100 as shown in FIGS. 1(A) and 1(B) is manufactured, in which a battery element 10 including a positive electrode current collector 11a and a negative electrode current collector 12a, and an electrolyte solution 20 are housed in a sealed outer casing 30.

[0021] When the lithium-ion battery 100 is being charged, lithium ions are conducted from the positive electrode 11 through the separator 13 to the negative electrode 12 and are absorbed therein, and when the lithium-ion battery 100 is being discharged, lithium ions are conducted from the negative electrode 12 through the separator 13 to the positive electrode 11 and are absorbed therein. In the lithium-ion battery 100, charge and discharge operations are realized by such lithium ion conduction.

[0022] Here, a lithium-ion battery 100 in the form of a coin battery is shown as an example of a lithium-ion battery. However, other known lithium-ion batteries include cylindrical and prismatic batteries in which a battery element having a positive electrode, a negative electrode, and a separator disposed therebetween is housed together with an electrolyte in a cylindrical or rectangular battery can, and thin batteries in which the battery element and the electrolyte are housed in a bag-shaped laminate film. In any of these lithium-ion battery forms, such as coin, cylindrical, prismatic, or thin, the battery element may include two or more separator layers, and the positive electrode and negative electrode may be arranged so as to face each other with the separator interposed therebetween. Furthermore, the battery element may be one in which the positive electrode, negative electrode, and separator are stacked in a predetermined order in one direction, or one in which the stack is wound or folded.

[0023] In the following, one or both of the positive electrode and negative electrode (for example, the positive electrode 11 and negative electrode 12 of the lithium ion battery 100) in the battery elements of various lithium ion batteries including the lithium ion battery 100 will also be referred to as "electrodes".

[0024] Conventionally, electrolytes for lithium ion batteries have been known to contain a lithium salt and an organic solvent. However, because the organic solvent is flammable, electrolytes with such compositions have the risk of fire, explosion, or the like due to short circuits. In response to this, techniques using electrolytes that do not contain flammable organic solvents or electrolytes to which a flame retardant has been added have also been known. However, while such electrolytes that do not contain flammable organic solvents or electrolytes to which a flame retardant has been added provide flame retardancy, they have poor wettability and permeability to battery elements, which has sometimes made it difficult to produce lithium ion batteries in which the electrolyte has sufficiently permeated the battery elements with high productivity.

[0025] In view of these points, the present invention uses an electrolyte solution for a lithium ion battery having the composition shown below, thereby realizing a flame-retardant electrolyte solution and a highly productive lithium ion battery using such a flame-retardant electrolyte solution.

[0026] That is, the electrolyte used in this lithium-ion battery is a deep eutectic solvent, a carbonate compound, and a surfactant having a perfluoroalkyl group, and the surfactant having a perfluoroalkyl group is contained in a predetermined amount relative to the amount of the mixture containing the deep eutectic solvent and the carbonate compound.

[0027] A deep eutectic solvent is a compound in which a hydrogen bond donor compound and a hydrogen bond acceptor compound (at least one of which is in a solid state at around room temperature) are mixed in a predetermined ratio, and the mixture becomes a liquid at around room temperature.

[0028] The hydrogen bond donor compound of the deep eutectic solvent is an amide compound or an imide compound. Examples of the amide compound include methylacetamide, trifluoromethylacetamide, and urea. Examples of the imide compound include succinimide.

[0029] The hydrogen bond acceptor compound of the deep eutectic solvent is a lithium salt, such as lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium nitrate, or lithium perchlorate.

[0030] A deep eutectic solvent for use in an electrolyte solution for a lithium-ion battery is prepared by mixing one or more hydrogen bond donor compounds with one or more hydrogen bond acceptor compounds. As an example, a deep eutectic solvent is prepared by mixing the amide compound n-methylacetamide (also known as "MAc") and the lithium salt lithium bis(trifluoromethanesulfonyl)imide (also known as "LiTFSI") in a molar ratio of 4:1.

[0031] Deep eutectic solvents are flame-retardant. Because of their flame-retardant properties, deep eutectic solvents are suitable as materials for electrolytes in lithium-ion batteries. However, deep eutectic solvents have relatively high viscosity. Therefore, when deep eutectic solvents are used alone in electrolytes, sufficient ionic conductivity may not be obtained, resulting in increased cell resistance in lithium-ion batteries. Furthermore, when deep eutectic solvents are used alone in electrolytes, a good coating, known as a solid electrolyte interphase (SEI) coating, may not be formed on the electrode surfaces of battery elements, resulting in insufficient charge / discharge characteristics.

[0032] Therefore, a carbonate compound is further added to the deep eutectic solvent described above. Examples of carbonate compounds include fluoroethylene carbonate, ethylene carbonate, propylene carbonate, and butylene carbonate. Such carbonate compounds have a relatively high dielectric constant and a relatively low viscosity. One or more carbonate compounds are added to the deep eutectic solvent. For example, when the ratio of the volume V1 of the deep eutectic solvent to the volume V2 of the carbonate compound is V1:V2=1:X, the amount of the carbonate compound added is set so that X is in the range of 0.3 to 2. As an example, a deep eutectic solvent containing MAc and LiTFSI in a molar ratio of 4:1 is mixed with the carbonate compound fluoroethylene carbonate (also referred to as "FEC") in a volume ratio of 1:1.

[0033] Carbonate compounds such as FEC as described above may be flammable. On the other hand, deep eutectic solvents containing the above-described amide or imide compounds and lithium salts are flame-retardant. In the electrolyte solution of a lithium ion battery, the deep eutectic solvent functions as a lithium ion conductive medium and also as a flame retardant. Therefore, even if a certain amount of a flammable carbonate compound is added to the deep eutectic solvent, the flame-retardant properties of the electrolyte solution of a lithium ion battery are ensured.

[0034] When a deep eutectic solvent containing an amide compound or an imide compound and a lithium salt is added with a relatively low-viscosity carbonate compound such as FEC to form an electrolyte for a lithium ion battery, the viscosity of the electrolyte is reduced compared to when a deep eutectic solvent is used alone. Furthermore, in a lithium ion battery using an electrolyte in which a carbonate compound is added to a deep eutectic solvent, the carbonate compound such as FEC is decomposed, forming a good SEI coating on the electrode surface.

[0035] However, when a deep eutectic solvent to which a carbonate compound has been added is used as an electrolyte for a lithium-ion battery, while it can ensure flame retardancy, reduce viscosity, and form an SEI film, it may have poor wettability and permeability (or impregnation) to the electrodes of the battery element. That is, a deep eutectic solvent to which a carbonate compound has been added may have a high surface tension with respect to the electrodes of the battery element of a lithium-ion battery, resulting in a large contact angle and poor wettability and permeability to the electrodes. Therefore, when the electrodes of the battery element are impregnated with the electrolyte, the electrolyte that does not wet and penetrate the electrodes may overflow, or it may take a long time for the electrolyte to penetrate the electrodes, making it difficult to produce lithium-ion batteries with high productivity.

[0036] Therefore, as an electrolyte for a lithium-ion battery, a mixture of a deep eutectic solvent and a carbonate compound is used, to which a predetermined amount of a predetermined surfactant is further added. A surfactant having a perfluoroalkyl group is used as the surfactant. As an example, a surfactant having a perfluoroalkyl group with a low carbon number in the alkyl main chain, preferably a surfactant having a perfluoroalkyl group with an alkyl main chain having 6 or less carbon atoms, is used. The surfactant to be added may consist of a single type of compound having a perfluoroalkyl group with a specific number of carbon atoms in the alkyl main chain, or may contain multiple types of compounds having perfluoroalkyl groups with different numbers of carbon atoms in the alkyl main chain. As an example, a nonionic surfactant having a perfluoroalkyl group is used as the surfactant.

[0037] In this way, a mixture containing a deep eutectic solvent and a carbonate compound to which a predetermined amount of a surfactant having a perfluoroalkyl group is further added is used as an electrolyte for various lithium ion batteries, such as the electrolyte 20 for the lithium ion battery 100 shown in Figure 1 above.

[0038] Perfluoroalkyl groups form strong carbon-fluorine bonds due to the strong electronegativity of fluorine, which weakens the intermolecular forces between perfluoroalkyl groups and reduces their interactions with other substances. Therefore, surfactants with perfluoroalkyl groups can achieve low surface tension even at low concentrations.

[0039] Therefore, by further adding a predetermined amount of a surfactant having a perfluoroalkyl group to a mixture containing a deep eutectic solvent and a carbonate compound to form an electrolyte solution for a lithium ion battery, the surface tension of the electrolyte solution relative to the electrodes of the battery element is reduced. That is, the wettability and permeability of the electrolyte solution relative to the electrodes of the battery element are improved compared to an electrolyte solution without the addition of the surfactant. This makes it possible to produce lithium ion batteries in which the electrolyte solution is sufficiently permeated and contacts the electrodes of the battery element, with good productivity, while suppressing overflow and prolonged permeation that occur when the electrolyte solution is impregnated into the electrodes.

[0040] It is preferable that the surfactant having a perfluoroalkyl group has 6 or less carbon atoms in the alkyl main chain of the perfluoroalkyl group. If the number of carbon atoms in the alkyl main chain of the perfluoroalkyl group exceeds 6, the molecular weight and molecular bulk of the surfactant will increase, which may make it difficult to achieve low surface tension at a low concentration. Furthermore, in addition to nonionic surfactants having a perfluoroalkyl group, cationic, anionic, or amphoteric surfactants may also be used as the surfactant.

[0041] The surfactant having a perfluoroalkyl group is preferably added to the electrolyte solution so that it is contained in an amount of 0.6 wt% or more relative to the weight of the mixture containing the deep eutectic solvent and the carbonate compound. If the amount added is less than 0.6 wt%, the surfactant component is so small that the effect of reducing the surface tension of the electrolyte solution on the electrodes of the battery element and the effect of improving wettability and permeability may not be sufficiently obtained.

[0042] As described above, the electrolyte for a lithium-ion battery used herein has a composition in which a relatively low-viscosity carbonate compound such as FEC is added to a deep eutectic solvent containing an amide compound or imide compound and a lithium salt, and a predetermined amount of a surfactant having a perfluoroalkyl group is further added. The use of a deep eutectic solvent allows it to function as a conductive medium for lithium ions and makes the electrolyte flame-retardant. The addition of a carbonate compound such as FEC reduces the viscosity of the electrolyte and forms a favorable SEI coating on the electrode surface of the battery element. This achieves electrolyte components useful for lithium-ion batteries. Furthermore, the addition of a predetermined amount of a surfactant having a perfluoroalkyl group to such electrolyte components reduces surface tension, thereby achieving an electrolyte for a lithium-ion battery that improves wettability and permeability to the electrodes of the battery element.

[0043] The above composition provides a flame-retardant electrolyte solution that is useful for lithium-ion batteries, and also enables highly productive lithium-ion batteries to be produced using the flame-retardant electrolyte solution. An example of the above-mentioned electrolyte solution and the results of its characteristic evaluation will be described below.

[0044] (electrode) As an example, lithium iron phosphate was used as the positive electrode active material, acetylene black as the conductive material, carboxymethyl cellulose as the thickener, a styrene-butadiene rubber emulsion solution as the binder, and pure water as the mixed solvent. These were kneaded together to obtain a positive electrode mixture slurry (also referred to as "positive electrode slurry"). The obtained positive electrode slurry was applied to an aluminum foil current collector for the positive electrode, dried, and rolled to obtain a positive electrode on the aluminum foil. The positive electrode thus obtained was used as an electrode sample.

[0045] (Electrolyte A) MAc and LiTFSI were mixed in a molar ratio of 4:1 to obtain a deep eutectic solvent. FEC was added to the obtained deep eutectic solvent so that the volume ratio of the deep eutectic solvent to FEC was 1:1. This was used as "electrolyte A."

[0046] (Electrolyte B) Electrolyte solution B was created by adding 0.06% by weight of a nonionic surfactant with a perfluoroalkyl group whose alkyl main chain has six or fewer carbon atoms to electrolyte solution A. When the weight of electrolyte solution A is W1 and the weight of the surfactant is W2, the ratio of W1 to W2 in electrolyte solution B is W1:W2 = 100:0.6.

[0047] (Electrolyte C) Electrolyte solution C was created by adding 0.6% by weight of a nonionic surfactant with a perfluoroalkyl group having six or fewer carbon atoms in the alkyl main chain to electrolyte solution A. When the weight of electrolyte solution A is W1 and the weight of the surfactant is W2, the ratio of W1 to W2 in electrolyte solution C is W1:W2 = 100:0.6.

[0048] (Electrolyte D) Electrolyte solution D was created by adding hexadecyltrimethylammonium bromide (also known as "CTAB"), another type of surfactant, to electrolyte solution A in an amount of 0.06 wt % relative to the weight of electrolyte solution A. When the weight of electrolyte solution A is W1 and the weight of the surfactant is W2, the ratio of W1 to W2 in electrolyte solution D is W1:W2 = 100:0.06.

[0049] (Electrolyte E) Electrolyte solution E was prepared by adding 0.6 wt % of CTAB to electrolyte solution A. When the weight of electrolyte solution A is W1 and the weight of the surfactant is W2, the ratio of W1 to W2 in electrolyte solution E is W1:W2 = 100:0.6.

[0050] The comparative examples and examples using the above-described electrolytic solution AE are as follows (Comparative Examples 1-4 and Example 1). <Comparative Example 1> The contact angle measurement was carried out as follows: A 1.0 μL droplet of electrolyte A was placed on the electrode surface and allowed to settle. The angle between the electrode surface and the deposited electrolyte A was taken as the contact angle, and the contact angle was measured every second from 1 second after the deposition of electrolyte A until 59 seconds later using a contact angle meter.

[0051] The permeability was evaluated as follows: Vacuum impregnation was performed by dropping a 5.0 μL droplet of electrolyte A onto the electrode surface, leaving it to stand for 1 minute under a pressure reduced by -90 kPa from atmospheric pressure, and then releasing it to atmospheric pressure, a process repeated three times, and the presence or absence of residual droplets of electrolyte A on the electrode surface after vacuum impregnation was evaluated.

[0052] <Comparative Example 2> The contact angle measurement and permeability evaluation were carried out in the same manner as in Comparative Example 1 above, except that the electrolyte solution B was used instead of the electrolyte solution A.

[0053] Example 1 The contact angle measurement and permeability evaluation were carried out in the same manner as in Comparative Example 1 above, except that the electrolyte solution C was used instead of the electrolyte solution A.

[0054] <Comparative Example 3> The contact angle measurement and permeability evaluation were carried out in the same manner as in Comparative Example 1 above, except that the electrolyte solution D was used instead of the electrolyte solution A.

[0055] <Comparative Example 4> The contact angle measurement and permeability evaluation were carried out in the same manner as in Comparative Example 1 above, except that the electrolyte solution E was used instead of the electrolyte solution A.

[0056] The results of the contact angle measurements for Comparative Examples 1-4 and Example 1 carried out as described above are shown in Table 1.

[0057] [Table 1]

[0058] Table 1 shows the measured values ​​of the contact angle at 1 second, 2 seconds, 5 seconds, 10 seconds, 30 seconds, and 59 seconds after the droplets landed on the surface, out of the time periods from 1 second to 59 seconds after the droplets landed on the surface. In both Comparative Examples 1-4 and Example 1, the contact angle tended to decrease with increasing time elapsed from droplet landing. At 1 second, 2 seconds, 5 seconds, 10 seconds, 30 seconds, and 59 seconds after droplet landing, Example 1 had a smaller contact angle than Comparative Example 1-4. Contact angle measurements confirmed that Example 1 had a smaller contact angle at all elapsed times from 1 second to 59 seconds after droplet landing compared to Comparative Example 1-4.

[0059] Table 2 also shows the results of the permeability evaluation (presence or absence of residual droplets) of Comparative Examples 1-4 and Example 1 carried out as described above.

[0060] [Table 2]

[0061] In Table 2, the electrode surface on which droplets remained after vacuum impregnation is marked as "present," and the electrode surface on which droplets did not remain after vacuum impregnation is marked as "absent." In Comparative Example 1-4, the liquid droplets on the electrode surface did not sufficiently penetrate into the electrode even after vacuum impregnation, and liquid droplets remained on the electrode surface after vacuum impregnation. In contrast, in Example 1, the liquid droplets on the electrode surface sufficiently penetrated into the electrode after vacuum impregnation, and liquid droplets did not remain on the electrode surface after vacuum impregnation.

[0062] As can be seen from Comparative Examples 1-2 and Example 1 in Tables 1 and 2, when a surfactant having a perfluoroalkyl group was added to the electrolyte of a lithium ion battery in an amount of 0.6 wt % (Example 1) relative to the weight of a mixture containing a deep eutectic solvent (a molar ratio of MAc to LiTFSI of 4:1) and FEC (a volume ratio with the deep eutectic solvent of 1:1), it was confirmed that the contact angle with the electrode was reduced, and wettability and permeability were improved, compared to when a smaller amount was added (Comparative Example 1-2).

[0063] When a surfactant having a perfluoroalkyl group is used, as can be seen from the results of Comparative Example 1-2 and Example 1 in Table 1, the contact angle of the droplets tends to decrease as the amount of surfactant added increases. From this, it is thought that an electrolyte in which the amount of surfactant having a perfluoroalkyl group added exceeds 0.6 wt % in Example 1 will obtain a smaller contact angle than when the amount is less than 0.6 wt % as in Comparative Example 1-2. It is thought that an electrolyte in which the amount of surfactant having a perfluoroalkyl group added exceeds 0.6 wt % in Example 1 will obtain a small contact angle and will achieve good permeability such that no droplets remain on the electrode surface after vacuum impregnation.

[0064] In addition, electrolyte D, in which CTAB, a type of surfactant other than those containing perfluoroalkyl groups, was added at 0.06 wt % to a mixture containing a deep eutectic solvent (a 4:1 molar ratio of MAc to LiTFSI) and FEC (a 1:1 volume ratio relative to the deep eutectic solvent), did not achieve a sufficiently small contact angle and did not achieve sufficient permeability to prevent droplets from remaining on the electrode surface after vacuum impregnation, as seen in the results of Comparative Example 3 in Table 1-2. Electrolyte E, in which CTAB was added at 0.6 wt % to a mixture containing a deep eutectic solvent (a 4:1 molar ratio of MAc to LiTFSI) and FEC (a 1:1 volume ratio relative to the deep eutectic solvent), did achieve a certain degree of contact angle reduction, but not to the same extent as in Example 1, and did not achieve sufficient permeability to prevent droplets from remaining on the electrode surface after vacuum impregnation, as seen in the results of Comparative Example 4 in Table 1-2. It is preferable to use a surfactant containing a perfluoroalkyl group as the surfactant added to the mixture containing the deep eutectic solvent and FEC.

[0065] As described above, when a surfactant having a perfluoroalkyl group is added in an amount of 0.6 wt % or more relative to the weight of a mixture containing a deep eutectic solvent (e.g., MAc and LiTFSI) and a carbonate compound (e.g., FEC) as an electrolyte for a lithium-ion battery, the contact angle (or surface tension) with the electrode can be reduced, and wettability and permeability can be improved, compared to when a surfactant with a smaller amount is added. [Explanation of symbols]

[0066] 10 Battery element 11 Positive electrode 11a Positive electrode current collector 12 Negative electrode 12a Negative electrode current collector 13 Separator 20 Electrolyte 30 Exterior body 31 Positive electrode can 32 Anode can 31a, 32a bottom 31b, 32b side wall part 33 Gasket 100 Lithium-ion batteries

Claims

1. a battery element for a lithium ion battery having a positive electrode, a negative electrode, and a separator provided between the positive electrode and the negative electrode; an exterior body that houses the battery element; an electrolyte solution contained in the exterior body; Including, The electrolyte solution is a deep eutectic solvent containing an amide compound or an imide compound and a lithium salt; a carbonate compound; a surfactant having a perfluoroalkyl group; Including, A lithium ion battery, wherein the surfactant is contained in an amount of 0.6 wt % or more relative to the weight of the mixture containing the deep eutectic solvent and the carbonate compound.

2. 2. The lithium ion battery according to claim 1, wherein the electrolyte solution contains methylacetamide as the amide compound of the deep eutectic solvent and lithium bis(trifluoromethanesulfonyl)imide as the lithium salt.

3. 2. The lithium ion battery according to claim 1, wherein the electrolyte solution contains fluoroethylene carbonate as the carbonate compound.

4. 2. The lithium ion battery according to claim 1, wherein the perfluoroalkyl group of the surfactant has an alkyl main chain having 6 or less carbon atoms.

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