Lithium ion battery

Abstract

To achieve a lithium ion battery with high performance 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 within a range of 400 ppm or more and 1000 ppm or less 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 together with an electrolyte in an exterior container such as a can or a bag. In the case of a lithium-ion battery using 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, techniques using electrolytes that do not contain flammable organic solvents or electrolytes containing flame retardants are known. However, while such electrolytes provide flame retardancy, they also have high viscosity, resulting in low ionic conductivity and insufficient discharge capacity, making it difficult to realize high-performance lithium-ion batteries.

[0005] In one aspect, the present invention aims to realize a high-performance 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 ranging from 400 ppm to 1000 ppm by 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 high-performance 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-type 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 or the like. In response to this, techniques using electrolytes that do not contain flammable organic solvents or electrolytes to which a flame retardant has been added are also 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 high viscosity and low ionic conductivity, which can result in insufficient discharge capacity and, therefore, can prevent the production of high-performance lithium ion batteries.

[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 high-performance lithium ion battery using such a flame-retardant electrolyte solution.

[0026] That is, here, an electrolyte solution containing a deep eutectic solvent, a carbonate compound, and a surfactant having a perfluoroalkyl group is used as the electrolyte solution for the lithium ion battery. 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] However, when a deep eutectic solvent containing a carbonate compound is used as an electrolyte for a lithium-ion battery, it can ensure flame retardancy, reduce viscosity, and form an SEI film, but depending on the discharge conditions, it may not be possible to obtain sufficient discharge capacity. For example, when discharging at a relatively low C rate, the discharge capacity of the lithium-ion battery may be reduced, and when discharging at a relatively high C rate, the discharge capacity of the lithium-ion battery may be lower than when discharging at a relatively low C rate.

[0035] This is thought to be due to the following reason: When a deep eutectic solvent is added with a carbonate compound, the surface tension of the deep eutectic solvent relative to the electrode of the battery element of a lithium ion battery is high, the contact angle is large, and the wettability and permeability (or impregnation) of the electrode are relatively low. Therefore, the contact area between the deep eutectic solvent, which functions as a conductive medium for lithium ions, and the electrode of the battery element is not sufficiently secured, which suppresses the movement of lithium ions between the deep eutectic solvent and the electrode, resulting in insufficient discharge capacity.

[0036] Therefore, the electrolyte for lithium-ion batteries is prepared by adding a specific surfactant to a mixture of a deep eutectic solvent and a carbonate compound. The surfactant used is a surfactant having a perfluoroalkyl group. For 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 fewer carbon atoms, is used. The surfactant added to the electrolyte may consist of a single compound having a perfluoroalkyl group with a specific number of carbon atoms in the alkyl main chain, or may contain multiple compounds having perfluoroalkyl groups with different numbers of carbon atoms in the alkyl main chain. For 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 surfactant having a perfluoroalkyl group has been 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 using a mixture containing a deep eutectic solvent and a carbonate compound, and further adding a surfactant having a perfluoroalkyl group to form an electrolyte for a lithium-ion battery, the surface tension of the electrolyte relative to the electrodes of the battery element is reduced. In other words, the wettability and permeability of the electrolyte relative to the electrodes of the battery element are improved compared to an electrolyte without the surfactant. This ensures sufficient contact area between the deep eutectic solvent, which functions as a conductive medium for lithium ions, and the electrodes of the battery element, allowing for smooth movement of lithium ions between the deep eutectic solvent and the electrodes. As a result, the lithium-ion battery is prevented from failing to obtain sufficient discharge capacity.

[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 in a range of, for example, 400 ppm to 1000 ppm based on the weight of the mixture containing the deep eutectic solvent and the carbonate compound. If the amount added is less than 400 ppm, the amount of surfactant present around the deep eutectic solvent component will be small, which may result in insufficient reduction of the surface tension of the electrolyte on the electrodes of the battery element and in improved wettability and permeability. If the amount added is more than 1000 ppm, the amount of surfactant present around the deep eutectic solvent component will be large, which may form a resistive layer that may impede the smooth movement of lithium ions between the deep eutectic solvent and the electrodes of the battery element.

[0042] As described above, the electrolyte for the lithium-ion battery described herein is a composition in which a deep eutectic solvent containing an amide compound or imide compound and a lithium salt is added with a relatively low-viscosity carbonate compound such as FEC, and further with a surfactant having a perfluoroalkyl group. The use of a deep eutectic solvent not only functions as a conductive medium for lithium ions but also makes the electrolyte flame-retardant. The addition of a carbonate compound such as FEC reduces the viscosity of the electrolyte and forms a good SEI coating on the electrode surfaces of the battery elements. Furthermore, the addition of a surfactant having a perfluoroalkyl group reduces the surface tension of the electrolyte, improving wettability and permeability and ensuring contact with the electrodes. As a result, in a lithium-ion battery using this electrolyte, the electrolyte is flame-retardant, ionic conductivity is increased, cell resistance is reduced, and a decrease in discharge capacity is suppressed.

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

[0044] In this study, to evaluate the effect of adding a surfactant to the electrolyte and the amount of surfactant added on the discharge characteristics of lithium-ion batteries, a 2032-type two-electrode coin-type lithium-ion battery with a positive electrode made of lithium iron phosphate and a negative electrode made of graphite was fabricated and subjected to a constant current charge / discharge test.

[0045] (positive electrode) For example, a cathode mixture was obtained by kneading lithium iron phosphate as the cathode active material, acetylene black as the conductive material, carboxymethyl cellulose as the thickener, styrene butadiene rubber as the binder, and pure water as the mixed solvent. The cathode mixture was then applied to an aluminum foil current collector for the cathode, dried, and vacuum-dried at a temperature of approximately 100 to 200°C for approximately 12 hours to obtain a cathode on the aluminum foil.

[0046] (Negative electrode) For example, graphite was used as the negative electrode active material, acetylene black as the conductive material, carboxymethyl cellulose as the thickener, styrene-butadiene rubber as the binder, and pure water as the mixed solvent, and these were kneaded to obtain a negative electrode mixture. The obtained negative electrode mixture was applied to a copper foil negative electrode current collector, dried, and then vacuum-dried at a temperature of approximately 100 to 200°C for approximately 12 hours to obtain a negative electrode on the copper foil.

[0047] (separator) A cellulose-based separator was used as the separator. (electrolyte) MAc and LiTFSI were mixed at a molar ratio of 4:1 to obtain a deep eutectic solvent. FEC was then mixed with the resulting deep eutectic solvent at a volume ratio of 1:1. This resulted in a mixture of a deep eutectic solvent containing MAc and LiTFSI to which FEC, a carbonate compound, was added. A predetermined amount of a surfactant with a perfluoroalkyl group was added to the resulting mixture to produce an electrolyte. A nonionic surfactant with a perfluoroalkyl group containing six or fewer carbon atoms in the alkyl main chain was used as the surfactant. The amount of surfactant added was set to five levels: 0 ppm, 200 ppm, 400 ppm, 800 ppm, and 1000 ppm relative to the weight of the mixture containing the deep eutectic solvent (MAc and LiTFSI molar ratio 4:1) and FEC (volume ratio to the deep eutectic solvent 1:1), and five types of electrolytes with different surfactant amounts were prepared.

[0048] To compare the electrolyte with the surfactant containing perfluoroalkyl groups, we also prepared an electrolyte containing another surfactant, hexadecyltrimethylammonium bromide (CTAB). Specifically, we added CTAB at 600 ppm by weight to a mixture containing the deep eutectic solvent (MAc and LiTFSI in a molar ratio of 4:1) and FEC (in a volume ratio of 1:1 with the deep eutectic solvent).

[0049] (Lithium-ion battery) A battery element was constructed by placing a positive electrode on aluminum foil and a negative electrode on copper foil facing each other with a separator between them, with the aluminum foil and copper foil facing outward. An electrolyte was then injected into the battery element to impregnate it. As an example, the battery element was impregnated with the electrolyte by vacuum impregnation, in which the electrolyte was injected into the battery element and the pressure was reduced. The impregnated battery element was placed in a coin-type lithium-ion battery battery can, which included a positive electrode can and a negative electrode can, each with one end open. The battery element was then placed between the positive electrode can and the negative electrode can, with the aluminum foil on the positive electrode side connected to the bottom of the positive electrode can and the copper foil on the negative electrode side connected to the bottom of the negative electrode can. The side wall of the negative electrode can was inserted inside the side wall of the positive electrode can via a gasket, and the side wall of the positive electrode can was crimped to the side wall of the negative electrode can via the gasket. This resulted in a coin-type lithium ion battery containing battery elements and an electrolyte solution in a sealed battery can in which the positive electrode can and the negative electrode can were fixed in an electrically insulated state by the gasket.

[0050] (charge / discharge) The resulting lithium-ion batteries were subjected to multiple constant-current charge-discharge tests, with the charge rate fixed and the discharge rate varied. One charge and subsequent discharge constituted one cycle. The current value was calculated based on the weight of the lithium iron phosphate positive electrode. 1C was calculated assuming a positive electrode capacity of 150 mAh / g.

[0051] The charge and discharge conditions were as follows: Charge rate: Fixed at 0.1C Discharge rate: 4 types: 0.1C, 0.5C, 1C, 2C ·Temperature: 23℃ Voltage range: 2.5V-3.7V Table 1 shows the relationship between the number of charge / discharge cycles [times] and the charge rate [C] and discharge rate [C] during each cycle.

[0052] [Table 1]

[0053] After the fourth, fifth and sixth cycles, additional discharge was carried out at a discharge rate of 0.1 C. Table 2 shows the relationship between the amount of surfactant containing perfluoroalkyl groups added [ppm] in the electrolyte of a lithium-ion battery and the discharge capacity [mAh / g] when discharged at 0.1C in the first cycle.

[0054] [Table 2]

[0055] Table 2 shows that when discharged at a relatively low C rate of 0.1C, no significant difference in discharge capacity was observed between the case where the amount of surfactant having a perfluoroalkyl group added to the lithium-ion battery electrolyte was 200 ppm and the case where the surfactant was not added, i.e., the amount added was 0 ppm (no addition). Table 2 also shows that when the amount of surfactant having a perfluoroalkyl group added to the lithium-ion battery electrolyte was 400 ppm or more, the discharge capacity when discharged at 0.1C tended to be higher than when the amount of surfactant added was 200 ppm or less. In particular, when the amount of surfactant added to the lithium-ion battery electrolyte was 800 ppm or 1000 ppm, the tendency for the discharge capacity to increase when discharged at 0.1C was significant.

[0056] As an example of the effect, the discharge capacity when discharged at 0.1C was improved by 13% by adding 400 ppm of a surfactant with a perfluoroalkyl group compared to when no surfactant was added, and by adding 800 ppm of a surfactant with a perfluoroalkyl group compared to when no surfactant was added, it was improved by 28% compared to when no surfactant was added.

[0057] Table 3 also shows the relationship between the amount of surfactant containing perfluoroalkyl groups added [ppm] in the electrolyte of a lithium-ion battery and the discharge capacity [mAh / g] when discharged at 2C on the sixth cycle.

[0058] [Table 3]

[0059] Table 3 shows that when discharged at a relatively high C rate of 2C, there was no significant difference in discharge capacity between the case where the amount of perfluoroalkyl surfactant added to the lithium-ion battery electrolyte was 200 ppm and the case where the surfactant was not added (i.e., the amount added was 0 ppm). Table 3 also shows that when the amount of perfluoroalkyl surfactant added to the lithium-ion battery electrolyte was 400 ppm, the discharge capacity when discharged at 2C tended to be higher than when the amount of surfactant added was 200 ppm or less. Table 3 also shows that the discharge capacity when discharged at 2C tended to increase as the amount of perfluoroalkyl surfactant added to the lithium-ion battery electrolyte increased from 400 ppm to 800 ppm and 1000 ppm.

[0060] As an example of the effect, the discharge capacity when discharged at 2C was improved by 8% by adding 400 ppm of a surfactant with a perfluoroalkyl group compared to when no surfactant was added, and by adding 800 ppm of a surfactant with a perfluoroalkyl group compared to when no surfactant was added, it was improved by 26% compared to when no surfactant was added.

[0061] In addition, in a lithium-ion battery using an electrolyte solution to which CTAB, a type of surfactant other than those having perfluoroalkyl groups, was added at 600 ppm relative to the weight of the mixture containing the deep eutectic solvent and FEC, the charge voltage in the first cycle did not reach 3.7 V, and charge / discharge did not proceed under the above charge / discharge conditions.

[0062] As described above, when a surfactant having a perfluoroalkyl group is added to the electrolyte of a lithium-ion battery in an amount of 400 ppm 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), a higher discharge capacity can be obtained, compared to when the amount added is smaller, regardless of whether the C rate is relatively low or high. The amount of surfactant having a perfluoroalkyl group added relative to the weight of the mixture containing a deep eutectic solvent and a carbonate compound is preferably in the range of 400 ppm to 1000 ppm, and more preferably in the range of 800 ppm to 1000 ppm. [Explanation of symbols]

[0063] 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, The surfactant is contained in a range of 400 ppm to 1000 ppm by 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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