Lithium-ion battery

Abstract

To realize a high-performance lithium-ion battery using a flame-retardant electrolyte.SOLUTION: A lithium-ion battery 100 includes a battery element 10 for the lithium-ion battery, having a positive electrode 11, a negative electrode 12, and a separator 13 disposed therebetween, an outer casing 30 in which the battery element 10 is housed, and an electrolyte 20 housed within the outer casing 30. For example, the electrolyte 20 in such a lithium-ion battery 100 may include a deep eutectic solvent containing an amide compound or an imide compound and a lithium salt, and a carbonate compound. The positive electrode 11 has a mass loading of 6.5 mg / cm2 and a density within the range of 2.7 g / cm3 or more and 3.3 g / cm3 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Known types of batteries include lithium ion batteries that use an electrolyte solution, such as a lithium ion battery that uses a non-aqueous electrolyte solution in which lithium hexafluorophosphate is dissolved as an electrolyte in a solvent such as ethylene carbonate or diethyl carbonate, and a lithium ion battery that further adds a phosphate ester to the non-aqueous electrolyte solution (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-307768 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 bag or a can. Lithium-ion batteries that use an electrolyte containing a lithium salt and a flammable organic solvent may ignite or explode due to a short circuit or other cause. To address this issue, a technology using an electrolyte containing a flame retardant has been proposed. However, while such electrolytes provide flame retardancy, they also have high viscosity and low ionic conductivity, which can lead to insufficient discharge capacity and, therefore, may not be able to achieve 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 embodiment, the present invention relates to a lithium ion battery comprising 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 outer casing in which the battery element is housed, and an electrolyte solution housed in the outer casing, the electrolyte solution including a deep eutectic solvent containing an amide compound or an imide compound and a lithium salt, and a carbonate compound, and the positive electrode has a basis weight of 6.5 mg / cm. 2 and the density is 2.7 g / cm 3 More than 3.3g / cm 3 The following ranges of lithium ion batteries are provided: [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. [Figure 2] FIG. 10 is a diagram showing an example of the relationship between discharge current and discharge capacity. [Figure 3] FIG. 10 is a graph showing an example of the relationship between the number of cycles and the discharge capacity. 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 thin batteries that use a bag-shaped laminate film as an exterior body, and coin batteries, cylindrical batteries, and prismatic batteries that use a can of a predetermined shape as an 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] 1(A) and 1(B) is an example of a thin 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 nickel manganese cobalt oxide is 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 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. For example, the solvent may be N-methylpyrrolidone. Such materials may be kneaded with the positive electrode active material to form a positive electrode material (also referred to as a positive electrode mixture, positive electrode slurry, positive electrode paste, or the like). The positive electrode 11 may include a current collector such as aluminum foil, stainless steel foil, or titanium foil, on which the positive electrode material is laminated. For example, a positive electrode material is applied to a current collector, dried, and then pressed to form the positive electrode 11. A positive electrode tab 11a made of a strip of metal or the like is connected to the positive electrode 11 for external connection.

[0014] The negative electrode 12 uses a negative electrode material containing a negative electrode active material. Examples of the negative electrode active material include metallic lithium and carbon materials capable of absorbing and releasing lithium. In addition to the negative electrode active material, the negative electrode material may also include a conductive material such as a carbon material, a thickener, a binder, a solvent, and other components. Examples of conductive materials include acetylene black and carbon black. Examples of thickeners include carboxymethyl cellulose. Examples of binders include acrylic, styrene butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride. Examples of solvents include pure water. These materials may be kneaded with the negative electrode active material to form a negative electrode material (also referred to as a negative electrode mixture, negative electrode slurry, negative electrode paste, and the like). The negative electrode 12 may include a current collector such as copper foil, on which the negative electrode material is laminated. For example, the negative electrode 12 is formed by applying the negative electrode material to a current collector, drying it, and then pressing it. A negative electrode tab 12a made of a strip of metal or the like is connected to the negative electrode 12 for external connection.

[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 made of a polyolefin or cellulose porous film, a woven fabric, a nonwoven fabric, or the like. Alternatively, the separator 13 may be made of ceramic or the like.

[0016] 1(A) and 1(B), the positive electrode 11, negative electrode 12, and separator 13 of the battery element 10 are housed in an exterior body 30, for example, an exterior body 30 formed by laminating the edges of two exterior films 30a together to form a bag, so that the tips of the positive electrode tab 11a and the negative electrode tab 12a protrude. For the exterior film 30a of the exterior body 30, a laminate film is used in which insulating layers of polyamide, polypropylene, or the like are laminated on both sides of a metal layer of aluminum, stainless steel, or the like.

[0017] The exterior body 30 contains the battery element 10 and an electrolyte solution 20 that serves as a conductive medium for lithium ions. For example, the battery element 10 is housed together with the electrolyte solution 20 in a bag-shaped exterior body 30 formed by welding the edges of three sides of two rectangular exterior films 30a and leaving one side open, with the positive electrode tab 11a and negative electrode tab 12a protruding outside the exterior body 30, and the open side is welded. The battery element 10 is impregnated with the electrolyte solution 20, and the electrolyte solution 20 permeates into the battery element 10. Using this method, a lithium ion battery 100 as shown in FIGS. 1(A) and 1(B) is manufactured, i.e., a lithium ion battery 100 in which the battery element 10 and the electrolyte solution 20 are housed in the exterior body 30 and the positive electrode tab 11a and negative electrode tab 12a protrude outside the exterior body 30.

[0018] 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.

[0019] Here, a lithium-ion battery 100 in the form of a thin battery is illustrated as an example of a lithium-ion battery. However, other known lithium-ion batteries include coin-shaped batteries in which a battery element having a positive electrode, a negative electrode, and a separator between them is housed together with an electrolyte in a coin-shaped battery can (also referred to as an "external can"), and cylindrical or prismatic batteries in which the battery element is housed together with an electrolyte in a cylindrical or prismatic battery can. In any form of lithium-ion battery, 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 a separator interposed therebetween (i.e., two or more positive or negative electrodes). Furthermore, the battery element may be one in which a predetermined number of positive electrodes, negative electrodes, and separators are stacked in one direction in a predetermined order, or one in which the stacked layers are wound or folded.

[0020] In the following, one or both of the positive electrode and negative electrode in the battery elements of various lithium ion batteries including the lithium ion battery 100 (for example, the positive electrode 11 and negative electrode 12 of the lithium ion battery 100) will also be referred to as "electrodes".

[0021] Conventionally, known electrolytes for lithium ion batteries contain a lithium salt such as lithium hexafluorophosphate and an organic solvent such as ethylene carbonate. However, because the organic solvent is flammable, electrolytes with such compositions have the risk of fire, explosion, or the like due to short circuiting or the like. In response to this, a technology using an electrolyte to which a flame retardant such as a phosphate ester has been added is also known. However, while such electrolytes to which a flame retardant has been added provide flame retardancy, they also have high viscosity and low ionic conductivity, which can lead to insufficient discharge capacity and other problems that may prevent the production of high-performance lithium ion batteries.

[0022] In view of this, it is proposed here to use an electrolyte solution containing, for example, a deep eutectic solvent and a carbonate compound 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 0.01 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 known as "FEC") in a volume ratio of 1:1. As another example, a deep eutectic solvent containing MAc and LiTFSI in a molar ratio of 4:1 is mixed with FEC in a volume ratio of 1:0.05 or 1:0.1.

[0028] 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.

[0029] When a deep eutectic solvent containing an amide or imide compound and a lithium salt is combined 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 lithium-ion batteries using a deep eutectic solvent with a carbonate compound added, the carbonate compound such as FEC is decomposed to form a good SEI film on the electrode surface. For example, in a lithium-ion battery using lithium nickel manganese cobalt oxide as the positive electrode active material and graphite as the negative electrode active material, the deep eutectic solvent may be co-intercalated into the negative electrode during initial charge / discharge cycles, resulting in poor charge / discharge performance. Therefore, adding FEC to the deep eutectic solvent forms a good SEI film on the negative electrode surface during charge / discharge cycles, thereby preventing deterioration of charge / discharge performance.

[0030] 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 it may not provide sufficient charge / discharge characteristics. This is thought to be due to the following reasons.

[0031] That is, when a deep eutectic solvent is added to a lithium-ion battery, the surface tension of the deep eutectic solvent relative to the electrode of the battery element is high, resulting in a large contact angle and relatively low wettability and permeability (or impregnation or absorption) of the electrode. As a result, the contact area between the deep eutectic solvent, which functions as a lithium ion conductive medium, and the electrode of the battery element is insufficient, suppressing the movement of lithium ions between the deep eutectic solvent and the electrode, which may result in insufficient charge / discharge characteristics. Furthermore, while carbonate compounds such as FEC contribute to lowering the viscosity of the electrolyte, the carbonate compound not used to form the SEI coating decomposes during charge / discharge, and the decomposition products may deteriorate the charge / discharge characteristics.

[0032] Therefore, the following configuration is adopted to realize a high-performance lithium-ion battery using a flame-retardant electrolyte. That is, in this case, the electrolyte of the lithium ion battery is a deep eutectic solvent as described above to which a carbonate compound such as FEC is added. Furthermore, among the electrodes of the lithium ion battery, a positive electrode having a basis weight of 6.5 mg / cm is used. 2 and the density is 2.7 g / cm 3 More than 3.3g / cm 3 The following ranges are used:

[0033] As described above, by adding a carbonate compound such as FEC to a deep eutectic solvent, an electrolyte solution that is flame-retardant, has a relatively low viscosity, and is capable of forming an SEI film is realized. Furthermore, the basis weight of the positive electrode of a lithium-ion battery using such an electrolyte solution is 10.3 mg / cm 2 (for example, 10.3 mg / cm 3) when a conventional electrolyte solution (such as one containing lithium hexafluorophosphate and ethylene carbonate) is used. 2 ) is lower than 6.5 mg / cm 2 Furthermore, the density of the positive electrode having such a weight is set to 2.7 g / cm 3 More than 3.3g / cm 3The positive electrode has such a weight and density, which are set within the following ranges. By setting the weight and density of the positive electrode in this range, the wettability and permeability of the electrolyte solution containing the deep eutectic solvent and the carbonate compound to the positive electrode are improved. This ensures a sufficient contact area between the deep eutectic solvent, which functions as a conductive medium for lithium ions, and the positive electrode, allowing lithium ions to move smoothly between the deep eutectic solvent and the positive electrode. As a result, deterioration of the charge / discharge characteristics of the lithium ion battery is suppressed.

[0034] In order to improve the charge-discharge characteristics of a lithium-ion battery, it may be effective to improve the wettability of the positive electrode with the electrolyte, although this depends on the types and mass ratios of the active materials of the positive and negative electrodes, i.e., the positive electrode capacity and the negative electrode capacity. Therefore, adjusting the basis weight and density of the positive electrode as described above and improving the wettability of the positive electrode with the electrolyte may be effective in improving the charge-discharge characteristics of a lithium-ion battery.

[0035] Furthermore, in an electrolyte in which a carbonate compound such as FEC is added to a deep eutectic solvent, the amount of the carbonate compound added is adjusted. For example, the volume ratio of the deep eutectic solvent to the carbonate compound such as FEC is adjusted to 1:1, and the amount of the carbonate compound added is adjusted to a relatively small amount so that the volume ratio is in the range of 1:0.05 to 1:0.1. Adjusting the amount of the carbonate compound such as FEC added to a relatively small amount reduces the amount of excess carbonate compound that is not used to form the SEI coating and is decomposed during charge and discharge, thereby preventing the decomposition products from deteriorating the charge and discharge characteristics.

[0036] An example of a lithium ion battery using the above electrolyte and positive electrode and the results of its characteristic evaluation will be described below. Example 1 (positive electrode) A positive electrode slurry was obtained by mixing lithium nickel manganese cobalt oxide as the positive electrode active material, acetylene black as the conductive material, an emulsion solution of polyvinylidene fluoride as the binder, and N-methylpyrrolidone as the mixed solvent in a kneader. The obtained positive electrode slurry was mixed into a powder having a basis weight of 6.5 mg / cm. 2 The coating was applied to an aluminum current collector foil and dried so that the density was 2.7 g / cm. 3 After pressing, a positive electrode portion and a positive electrode terminal portion were cut out, and a positive electrode tab with a heat-sealing resin was resistance-welded to the positive electrode terminal portion to obtain a positive electrode with a connected positive electrode tab.

[0037] (Negative electrode) Graphite was used as the negative 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, and these were mixed in a kneader to obtain a negative electrode slurry. The obtained negative electrode slurry was mixed into a powder having a basis weight of 5.0 mg / cm. 2 The paste was applied to a copper current collector foil and dried so that the density was 1.3 g / cm. 3 The negative electrode portion and the negative electrode terminal portion were cut out, and a negative electrode tab with a heat-sealing resin was resistance-welded to the negative electrode terminal portion to obtain a negative electrode with a negative electrode tab connected thereto.

[0038] (separator) A cellulose-based separator was used as the separator. (electrolyte) MAc and LiTFSI were mixed in a molar ratio of 4:1 to obtain a deep eutectic solvent. FEC was then mixed with the obtained deep eutectic solvent in a volume ratio of 1:1. This resulted in an electrolyte solution in which FEC, a carbonate compound, was added to a deep eutectic solvent containing MAc and LiTFSI.

[0039] (Lithium-ion battery) A laminate, which was formed by stacking a positive electrode with a positive electrode tab, a separator, and a negative electrode with a negative electrode tab in this order, was housed together with an electrolyte in a separately prepared exterior body made of a laminate film so that the positive and negative electrode tabs protruded, and the exterior body was sealed. The procedure for housing the laminate and the electrolyte in the exterior body and sealing the exterior body so that the positive and negative electrode tabs protrude outside the exterior body can be, for example, as follows. That is, three sides of two rectangular laminate films (exterior films) facing each other are welded to form a bag-shaped exterior body with one side open. The laminate is housed in the bag-shaped exterior body together with the electrolyte so that the positive and negative electrode tabs protrude, and the open side of the bag-shaped exterior body on the protruding side of the positive and negative electrode tabs is sealed. For example, this procedure was adopted, and the laminate and the electrolyte were housed in the exterior body so that the positive and negative electrode tabs protrude outside the exterior body. The lithium-ion battery of Example 1 was produced by assembling using this procedure.

[0040] <Example 2> As a positive electrode, the density after pressing is 2.9 g / cm 3 A lithium ion battery of Example 2 was produced in the same manner as in Example 1, except that a positive electrode of

[0041] Example 3 As a positive electrode, the density after pressing is 3.3 g / cm 3 A lithium ion battery of Example 3 was produced in the same manner as in Example 1, except that a positive electrode of

[0042] Example 4 The electrolyte used was a deep eutectic solvent (MAc and LiTFSI in a molar ratio of 4:1) and FEC in a volume ratio of 1:0.1. The positive electrode had a density of 2.9 g / cm after pressing. 3 A lithium ion battery of Example 4 was produced in the same manner as in Example 1, except that a positive electrode of

[0043] <Example 5> The electrolyte used was a deep eutectic solvent (MAc and LiTFSI in a molar ratio of 4:1) and FEC in a volume ratio of 1:0.05. The positive electrode had a density of 2.9 g / cm after pressing. 3 A lithium ion battery of Example 5 was produced in the same manner as in Example 1, except that a positive electrode of

[0044] <Comparative Example 1> As a positive electrode, the weight after application and drying is 10.3 mg / cm 2 and the density after pressing is 3.1 g / cm 3 A lithium ion battery of Comparative Example 1 was produced in the same manner as in Example 1, except that a positive electrode of

[0045] <Comparative Example 2> As a positive electrode, the density after pressing is 3.5 g / cm 3 A lithium ion battery of Comparative Example 2 was produced in the same manner as in Example 1, except that a positive electrode of

[0046] <Characteristics evaluation> The properties of the prepared positive electrodes and lithium ion batteries were evaluated. It was difficult to prepare a lithium ion battery using the positive electrodes prepared under the conditions of Comparative Example 2 because the electrode plates were significantly warped and electrode breakage occurred. For Examples 1-5 and Comparative Example 1, excluding Comparative Example 2, the following contact angle measurements and charge / discharge measurements were performed.

[0047] (Contact angle measurement) To evaluate the wettability of the electrolyte solution to the positive electrode, the electrolyte solution of each lithium ion battery was dropped onto each positive electrode of Examples 1 to 5 and Comparative Example 1, and the contact angle of the electrolyte solution to the positive electrode was measured. The measurement results are shown in Table 1.

[0048] [Table 1]

[0049] As shown in Table 1, the basis weight of the positive electrode of Comparative Example 1 was 10.3 mg / cm 2Compared with the case where the weight is relatively large, such as the positive electrode of Example 1-5, the weight is 6.5 mg / cm 2 It was found that the contact angle of the electrolyte was reduced by making the weight of the surface relatively small, such as 6.5 mg / cm as in Example 1-5. 2 It was found that the wettability and permeability of the electrolyte containing FEC in a deep eutectic solvent were enhanced in a positive electrode with a relatively low temperature.

[0050] The positive electrodes of Examples 1 to 5 all had a basis weight of 6.5 mg / cm 2 However, as seen in Table 1, when an electrolyte containing less FEC than that of Examples 1-3 was used, as in Example 4-5, the contact angle tended to increase with the decrease in the amount of FEC added. This is thought to be because the increase in viscosity of the electrolyte with the decrease in the amount of FEC added reduces the wettability and permeability of the electrolyte to the positive electrode compared to when the viscosity is low.

[0051] (Charge / discharge measurement (1); rate characteristics) Charge and discharge measurements were carried out on each of the lithium-ion batteries of Examples 1-5 and Comparative Example 1 in a thermostatic chamber at 23°C. As an example, the charge and discharge measurements were carried out under the conditions of a charge current of 0.1 C, a charge voltage of 4.2 V, a discharge current ranging from 0.1 C to 2 C, and a discharge voltage of 2.5 V, and the discharge capacity was measured at each discharge current. The measurement results are shown in Figure 2.

[0052] Fig. 2 is a diagram showing an example of the relationship between discharge current and discharge capacity, in which the horizontal axis represents discharge current (discharge rate) [C] and the vertical axis represents discharge capacity [mAh / g]. As can be seen from Figure 2, the basis weight of the positive electrode is relatively small, at 6.5 mg / cm 2 In the lithium ion battery of Example 1-5, the weight of the positive electrode was set to a relatively large value of 10.3 mg / cm 2 It was found that the discharge capacity at each discharge current increased compared to the lithium-ion battery of Comparative Example 1, in which the positive electrode weight was set at 1000 kJ / cm2. This is thought to be due to the decrease in the contact angle of the electrolyte as described above, and is thought to be due to the improved wettability and permeability of the electrolyte to the positive electrode in the lithium-ion battery of Example 1-5, in which the positive electrode weight was relatively small.

[0053] In addition, looking at the density of the positive electrode, from Figure 2, the density of the positive electrode is 2.7 g / cm 3 Compared with Example 1, in which the density of the positive electrode was 2.9 g / cm 3 and Example 2, in which the density of the positive electrode was 3.3 g / cm 3 In Example 3, in which the above-mentioned condition was set, a better discharge capacity was obtained in the discharge current range of 0.1C to 2C.

[0054] On the other hand, in Example 4-5, the density of the positive electrode was 2.9 g / cm 3 as in Example 2. 3 However, an electrolyte containing a smaller amount of FEC than that of Example 2 is used. As shown in Fig. 2, the discharge capacity of Examples 4-5 is improved compared to Comparative Example 1, but tends to be lower than that of Example 2 (or Examples 1-3) in a relatively high discharge current range. This is thought to be because the viscosity of the electrolyte increases with the decrease in the amount of FEC added, which reduces the wettability and permeability of the electrolyte to the positive electrode compared to when the viscosity is low, thereby reducing the contact area between the positive electrode and the electrolyte.

[0055] (Charge / discharge measurement (2); cycle characteristics) Charge and discharge measurements were carried out on each of the lithium ion batteries of Examples 1-5 and Comparative Example 1 in a thermostatic chamber at 23° C. As an example, a charge and discharge cycle test was carried out under the conditions of a charge current of 0.1 C, a charge voltage of 4.2 V, a discharge current of 0.75 C, and a discharge voltage of 2.5 V, and the discharge capacity after the cycle was measured. The measurement results are shown in FIG.

[0056] Fig. 3 is a diagram showing an example of the relationship between the number of cycles and the discharge capacity, in which the horizontal axis represents the number of cycles (times), and the vertical axis represents the discharge capacity (mAh / g). 3 , it was found that, compared with the lithium-ion batteries of Example 1 and Comparative Example 1, which used electrolytes containing relatively low amounts of FEC, the lithium-ion batteries of Examples 4-5, which used electrolytes containing relatively low amounts of FEC, showed a suppressed decrease in discharge capacity with increasing cycle count in the charge-discharge cycle range from 2.5 V to 4.2 V. FEC not used to form the SEI film decomposes when the lithium-ion battery is charged to 4.2 V, and the decomposition products can deteriorate the charge-discharge characteristics. In Examples 4-5, the reduced amount of FEC in the electrolyte reduces the excess FEC that is not used to form the SEI film and decomposes during charge and discharge, which is thought to suppress the decrease in discharge capacity with increasing cycle count.

[0057] As described above, an electrolyte solution containing a deep eutectic solvent (e.g., MAc and LiTFSI) and a carbonate compound (e.g., FEC) is used as the electrolyte solution for a lithium ion battery (e.g., the electrolyte solution 20 of the lithium ion battery 100). This makes it possible to realize an electrolyte solution that is flame-retardant, has a relatively low viscosity, and is capable of forming an SEI film.

[0058] The positive electrode of a lithium ion battery using such an electrolyte (such as the positive electrode 11 of the lithium ion battery 100) has an adjusted basis weight and density. For example, a positive electrode containing lithium nickel manganese cobalt oxide is used. For example, from the results of the above-mentioned characteristic evaluation, the basis weight of the positive electrode is 6.5 mg / cm. 2 The density of the positive electrode is preferably 2.7 g / cm 3 More than 3.3g / cm 3 The following range is preferred: A lithium-ion battery uses an electrolyte containing a deep eutectic solvent (e.g., MAc and LiTFSI) and a carbonate compound (e.g., FEC) and further uses a positive electrode with adjusted basis weight and density, which improves the wettability and permeability of the electrolyte to the positive electrode (reducing the contact angle), thereby improving the discharge capacity. This makes it possible to improve the charge / discharge characteristics of the lithium-ion battery.

[0059] Furthermore, in the electrolyte solution, the amount of the carbonate compound added relative to the deep eutectic solvent may be kept relatively low, for example, the volume ratio of the deep eutectic solvent to the carbonate compound may be in the range of 1:0.05 to 1:0.1, which makes it possible to obtain a certain effect of improving the permeability of the electrolyte solution to the positive electrode and an effect of improving the discharge capacity, while also obtaining an effect of improving the charge-discharge cycle characteristics of the lithium-ion battery. [Explanation of symbols]

[0060] 10 Battery element 11 Positive electrode 11a Positive electrode tab 12 Negative electrode 12a Negative electrode tab 13 Separator 20 Electrolyte 30 Exterior body 30a exterior film 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; Including, The positive electrode has a basis weight of 6.5 mg / cm ２ and the density is 2.7 g / cm ３ 3.3g / cm or more ３ Lithium-ion batteries, which are in the following range:

2. 10. The lithium-ion battery of claim 1, wherein the positive electrode comprises lithium nickel manganese cobalt oxide.

3. 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.

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

5. 2. The lithium ion battery according to claim 1, wherein the ratio of the volume V1 of the deep eutectic solvent to the volume V2 of the carbonate compound is V1:V2 = 1:X, where X is in the range of 0.05 or more and 0.1 or less.

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