Lithium ion secondary battery

Abstract

An object of the present invention is to provide, for example, a lithium ion secondary battery having a relatively high volume energy density, a relatively low melting point of an electrolytic solution, and relatively high flame retardancy.SOLUTION: The lithium ion secondary battery of the present invention is a lithium ion secondary battery containing an electrolytic solution and a negative electrode active material, wherein the electrolytic solution contains a chloroether-based solvent represented by the following general formula (1), and the negative electrode active material contains graphite. Wherein R1 is each independently a hydrogen atom or a chlorine atom, R2 is each independently a hydrogen atom, a methyl group or a chlorine atom, at least one of R1 is a chlorine atom, or at least one of R2 is a chlorine atom, and n is an integer of 0 to 4; ) SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery. [Background technology]

[0002] Lithium ion secondary batteries have high energy density and are widely used in a variety of applications, such as mobile phones, laptop computers, electric vehicles, etc. Lithium ion secondary batteries basically comprise a positive electrode, a negative electrode, an electrolyte, and a separator, and various studies have been conducted to improve the battery characteristics, such as improving each component (for example, Patent Document 1).

[0003] Lithium-ion secondary batteries can be discharged and charged as secondary batteries by the movement of lithium ions between the positive and negative electrodes and the efficient intercalation (insertion) of lithium ions into the negative electrode active material. From the perspective of improving the performance of lithium-ion secondary batteries, research is being conducted into technologies that enable more efficient insertion of lithium ions.

[0004] For example, in a lithium-ion secondary battery consisting of an anode made of a carbon-based material and a cathode made of a lithium-containing transition metal oxide, lithium ions in the cathode are inserted between the carbon material of the anode during charging, and the reverse reaction occurs during discharging. The reversible charge-discharge reaction of the carbon-based anode requires mixed carbonate solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolytes, but these solvents are highly flammable. To reduce the flammability of the electrolyte, electrolytes containing halogens or phosphorus have been investigated, but only a few solvents, such as cyclic phosphate esters, are capable of reversible charge-discharge reactions at the anode, such as electrolytes using EC-based solvents.

[0005] Carbonate-based solvents and ether-based solvents are used as electrolytes in lithium-ion secondary batteries. Ether-based solvents have a major advantage in that they do not freeze even in low-temperature environments due to their low melting points. However, like carbonate-based solvents, ether-based solvents also have the problem of being highly flammable. Furthermore, while ether-based solvents have excellent resistance to reduction, they have the disadvantage of poor resistance to oxidation. In research using ether-based solvents, lithium metal has been actively used as the anode active material, taking advantage of the high reduction resistance of ether-based solvents. Lithium metal anodes have a theoretical capacity of 3861 mAh / g and a very low electrode potential (-3.04 V vs. standard hydrogen electrode), making them attractive as secondary battery materials with high energy density. However, batteries using lithium metal have the problem that metallic lithium precipitates in the form of dendrites during charging, and with repeated charge / discharge, the dendrites grow to the cathode, causing short circuits. This poses a high risk, and ensuring the safety of lithium-ion batteries remains an unresolved issue. In lithium-ion secondary batteries, the problems associated with the use of lithium metal as a negative electrode, particularly the problem of dendrite precipitation, have been addressed by using carbon materials as the negative electrode active material. In this case, the nonaqueous solvent for the electrolyte is limited to cyclic carbonates such as ethylene carbonate. However, as mentioned above, these solvents are highly flammable, and therefore measures to reduce the flammability of the electrolyte are also required. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-179341 [Patent Document 2] Special Publication No. 2015-533020 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in consideration of the above circumstances, and has an object, for example, to provide a lithium ion secondary battery with a relatively high volumetric energy density. Another object of the present invention is to provide a lithium ion secondary battery with a relatively low melting point of the electrolyte. Another object of the present invention is to provide a lithium ion secondary battery with relatively high flame retardancy. Another object of the present invention is to provide a lithium ion secondary battery with a low resistance to reductions in charge capacity and discharge capacity and in coulombic efficiency. Another object of the present invention is to provide a lithium ion secondary battery with high durability against repeated use. [Means for solving the problem]

[0008] For example, the present invention provides the following aspects. [1] A lithium ion secondary battery including an electrolyte solution and a negative electrode active material, The electrolytic solution contains a chloroether-based solvent represented by the following general formula (1): The negative electrode active material comprises graphite. [ka] (In the formula, R 1 are each independently a hydrogen atom or a chlorine atom, R 2 are each independently a hydrogen atom, a methyl group, or a chlorine atom, R 1 At least one of R is a chlorine atom, or 2 at least one of which is a chlorine atom, n is an integer from 0 to 4. [2] R 1 at least one of is a chlorine atom, R 2 and each independently represent a hydrogen atom or a methyl group. [3] R 1 is a chlorine atom, and R 2 The lithium ion secondary battery according to the above [1] or [2], wherein is a hydrogen atom. [4] The lithium ion secondary battery according to any one of the above [1] to [3], wherein n is 1 or 2. [5] The lithium ion secondary battery according to any one of the above [1] to [4], wherein the graphite is artificial graphite or natural graphite. [6] the electrolyte solution contains a lithium salt; The lithium ion secondary battery according to any one of the above [1] to [5], wherein the lithium salt is one or more selected from the group consisting of LiPF6, Li(FSO2)2N, LiBF4, and Li(CF3SO2)2N, LiClO4. [7] The lithium ion secondary battery further comprises a positive electrode active material, The lithium ion secondary battery according to any one of the above [1] to [6], wherein the positive electrode active material contains an oxide or a sulfur-containing compound. [8] the positive electrode active material contains a sulfur-containing compound, The lithium ion secondary battery according to [7] above, wherein the sulfur-containing compound is Li2S or S. [Effects of the Invention]

[0009] According to the present invention, for example, a lithium ion secondary battery having a relatively high volumetric energy density can be provided. Furthermore, according to the present invention, for example, a lithium ion secondary battery having a relatively low melting point of the electrolyte can be provided. According to the present invention, for example, a lithium ion secondary battery having relatively high flame retardancy can be provided. According to the present invention, for example, a lithium ion secondary battery having a charge capacity and a discharge capacity that are resistant to reductions in charge capacity and discharge capacity, and a coulombic efficiency that are resistant to reductions in charge capacity and discharge capacity can be provided. According to the present invention, for example, a lithium ion secondary battery having high durability against repeated use of the battery can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an example of the lithium ion secondary battery of this embodiment. [Figure 2] FIG. 2 shows charge curves (curves moving from the lower left to the upper right in the figure) and discharge curves (curves moving from the upper left to the lower right in the figure) for the first, third, and fifth cycles obtained by conducting a charge-discharge test using the lithium ion secondary battery of Example 1. [Figure 3] FIG. 3 is a diagram showing charge curves and discharge curves for the first, third, and fifth cycles obtained by carrying out a charge-discharge test using the lithium ion secondary battery of Example 2. [Figure 4] FIG. 4 is a diagram showing charge curves and discharge curves for the first, third, and fifth cycles obtained by carrying out a charge-discharge test using the lithium ion secondary battery of Example 3. [Figure 5] FIG. 5 is a diagram showing charge curves and discharge curves for the first, third, and fifth cycles obtained by carrying out a charge / discharge test using the lithium ion secondary battery of Comparative Example 1. As shown in FIG. [Figure 6] FIG. 6 is a diagram showing charge curves and discharge curves for the first, third, and fifth cycles obtained by carrying out a charge-discharge test using the lithium ion secondary battery of Comparative Example 2. [Figure 7] FIG. 7 is a graph showing the change in charge capacity (mAh / g) obtained by carrying out charge-discharge tests using the lithium ion secondary batteries of Example 1 and Comparative Example 3. As shown in FIG. [Figure 8] FIG. 8 is a graph showing the change in coulombic efficiency (%) obtained by carrying out charge-discharge tests using the lithium ion secondary batteries of Example 1 and Comparative Example 3. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] One embodiment of a specific aspect of the present invention (hereinafter referred to as "this embodiment") will be described in detail below, but the present invention is not limited to this embodiment. Regarding the numerical ranges described in this specification, the upper and lower limits can be arbitrarily combined. For example, when a numerical range is described as "preferably 30 to 100, more preferably 40 to 80," the ranges "30 to 80" and "40 to 100" are also included in the numerical ranges described in this specification. Furthermore, when a numerical range is described as "preferably 30 or more, more preferably 40 or more, and preferably 100 or less, more preferably 80 or less," the ranges "30 to 80" and "40 to 100" are also included in the numerical ranges described in this specification. In addition, as a numerical range described in this specification, for example, "60 to 100" means a range of "60 or more (60 or more) to 100 or less (100 or less)."

[0012] The lithium ion secondary battery of the present embodiment is a lithium ion secondary battery including an electrolyte solution and a negative electrode active material, wherein the electrolyte solution includes a chloroether-based solvent represented by the following general formula (1), and the negative electrode active material includes graphite. [ka] (In the formula, R 1 are each independently a hydrogen atom or a chlorine atom, R 2 are each independently a hydrogen atom, a methyl group, or a chlorine atom, R 1 At least one of R is a chlorine atom, or 2 at least one of which is a chlorine atom, n is an integer from 0 to 4.

[0013] According to this embodiment, it is possible to provide a lithium ion secondary battery having a relatively high volumetric energy density, a relatively low melting point of the electrolyte, and relatively high flame retardancy. More specifically, in addition to carbonate-based solvents, ether-based solvents have been investigated as electrolytes for lithium-ion secondary batteries. Ether-based solvents have a major advantage in that they do not freeze even in low-temperature environments due to their low melting points. However, like carbonate-based solvents, ether-based solvents have the problem of being highly flammable, and while they have excellent reduction resistance, they also have the problem of relatively low oxidation resistance. In contrast, chloroether-based solvents represented by general formula (1) have improved oxidation resistance due to chlorination, and at the same time, they also have a relatively high flash point. While the use of carbon materials as negative electrode active materials has been proposed, and there are several types of carbon materials, when crystalline graphite is used as the negative electrode active material, a carbonate-based solvent is usually used as the electrolyte. The inventors decided to use graphite, among carbon materials, as the negative electrode active material, and investigated the use of an ether-based solvent as the electrolyte, which has advantages such as a low melting point. As a result of the investigation, it was found that when an ether-based solvent is used as the electrolyte together with graphite as the negative electrode active material, the solvent may be intercalated between the graphite layers together with lithium ions during charging (hereinafter, this phenomenon is referred to as solvent co-intercalation), resulting in insufficient battery performance. Furthermore, once solvent co-intercalation occurs, the graphite layers are destroyed, and the battery capacity may be significantly reduced. The present inventors therefore investigated methods for preventing solvent co-intercalation and surprisingly discovered that the use of a chloroether-based solvent represented by the general formula (1) above can prevent solvent co-intercalation. Specifically, by including a chloroether-based solvent represented by the general formula (1) above in the electrolyte and graphite in the negative electrode active material, the intercalation of the chloroether-based solvent between the layers of the graphite negative electrode active material is suppressed, allowing lithium ions to be efficiently intercalated. As a result, a battery with a high volumetric energy density can be obtained. Furthermore, the use of an ether-based solvent as the electrolyte can contribute to the use of batteries in low-temperature environments by lowering the melting point of the electrolyte and to making the battery more flame-retardant. Furthermore, according to the lithium ion secondary battery of this embodiment, it is possible to provide a battery in which the charge capacity and discharge capacity are unlikely to decrease and the coulomb efficiency is unlikely to decrease.

[0014] In addition, in the prior art, in a lithium secondary battery using an ether-based solvent as the electrolyte and a carbon material as the negative electrode active material, when crystalline graphite is used as the negative electrode active material, the electrolyte may decompose, so there is a technology (JP Patent Publication No. 2015-533020) that limits the carbon material to amorphous hard carbon and / or soft carbon. That is, although an ether-based solvent and crystalline graphite are not usually combined in the manufacture of lithium ion secondary batteries, in the lithium ion secondary battery of this embodiment, by combining a chloroether-based solvent represented by the above general formula (1) with graphite, it is surprisingly possible to solve the conventional problems and achieve high battery performance.

[0015] The configuration of this embodiment will be described below. 1. Electrolyte In this embodiment, the electrolytic solution contains a chloroether solvent represented by the following general formula (1). [ka] (In the formula, R 1 are each independently a hydrogen atom or a chlorine atom, R 2 are each independently a hydrogen atom, a methyl group, or a chlorine atom, R 1 At least one of R is a chlorine atom, or 2 at least one of which is a chlorine atom, n is an integer from 0 to 4.

[0016] In this embodiment, the chloroether-based solvent has at least one chlorine atom in the molecule. Therefore, even when graphite is used as the negative electrode active material, the presence of chlorine atoms in the molecule can prevent the chloroether-based solvent from intercalating into the solvent. Furthermore, while ether-based solvents have excellent reduction resistance, the chlorination of the chloroether-based solvent represented by the general formula (1) further improves its oxidation resistance. Furthermore, the increase in flash point due to chlorination also improves the battery's flame resistance. Furthermore, since the chloroether-based solvent does not contain fluorine atoms, it can comply with, for example, European PFAS regulations. Furthermore, a battery can be provided that is resistant to decreases in charge capacity and discharge capacity and coulombic efficiency.

[0017] In this embodiment, in general formula (1), R 1 At least one of R is preferably a chlorine atom, and all R 1 It is more preferable that R is a chlorine atom. 2 are preferably each independently a hydrogen atom or a methyl group, and all R 2 is preferably a hydrogen atom. The position of the chlorine atom in the solvent molecule is not particularly limited, but the presence of a chlorine atom at the end of the solvent molecule makes it easier to maintain the electron density of the oxygen atom in the solvent molecule constant, thereby making it easier to more effectively dissolve various compounds as an electrolyte.

[0018] In the general formula (1), n ​​is an integer of 0 to 4 (the number of ether groups is 1 to 5), but n is preferably 1 to 4, more preferably 1 or 2, and particularly preferably 1 (the number of ether groups is 2). This makes it possible to keep the viscosity of the solvent low and improve the conductivity of lithium ions.

[0019] Specific examples of the chloroether solvent used in this embodiment include 1,2-bis(2-chloroethoxy)ethane, diethylene glycol bis(2-chloroethyl)ether, bis[2-[2-(2-chloroethoxy)ethoxy]ethyl]ether, etc. As the chloroether solvent, 1,2-bis(2-chloroethoxy)ethane or diethylene glycol bis(2-chloroethyl)ether is preferred, and 1,2-bis(2-chloroethoxy)ethane is more preferred. The chloroether solvent used in this embodiment may be one type or a combination of two or more types of compounds.

[0020] In this embodiment, the electrolyte may be a mixed solvent obtained by mixing the chloroether-based solvent represented by the general formula (1) with other solvents, such as non-halogenated ethers and carbonate-based non-aqueous solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The electrolyte solution of this embodiment is preferably a non-aqueous solvent, and may be aprotic.

[0021] The content of the chloroether solvent of the general formula (1) in the electrolytic solution is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 85% by mass or more, and particularly preferably 90% by mass or more, based on 100% by mass of the total solvent in the electrolytic solution. Alternatively, the chloroether solvent of the general formula (1) may be 100% by mass.

[0022] The electrolyte solution of this embodiment may contain a lithium salt, which is a supporting electrolyte and can be dissolved in the solvent. Examples of lithium salts include Li2SO4, LiClO4, LiNO3, LiCl, LiCF3SO3, LiPF6, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI)), LiBF4, Li(CF3CF2SO2)2N, and Li(CF3SO2)2N. The lithium salt may be one or more selected from the group consisting of LiPF6, Li(FSO2)2N, LiBF4, and Li(CF3SO2)2N, LiClO4. The lithium salt may be one or more selected from the group consisting of LiPF6, LiBF4, and Li(FSO2)2N, and preferably includes Li(FSO2)2N.

[0023] In this embodiment, the molar ratio of the chloroether solvent to the lithium salt is preferably 1.6 or more. This allows for the preparation of an effective electrolyte solution. The molar ratio is also preferably 4 or more, more preferably 5 to 10, and particularly preferably 5 to 7. The electrolytic solution of this embodiment preferably contains Li(FSO2)2N and a chloroether solvent represented by the general formula (1) above.

[0024] The electrolytic solution of this embodiment may further contain an optional additive. The additive may include, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). The content of the additive is preferably 0.01 to 5 mass %, more preferably 0.01 to 4.5 mass %, even more preferably 0.01 to 4 mass %, even more preferably 0.01 to 3.5 mass %, and particularly preferably 0.01 to 3 mass %, relative to the total mass of the electrolytic solution.

[0025] 2.Negative electrode In this embodiment, the negative electrode is not particularly limited as long as the negative electrode active material contains graphite. Specifically, the negative electrode of this embodiment may include a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material capable of absorbing and desorbing lithium, and may further include, for example, a conductive material, a binder, and the like.

[0026] The negative electrode current collector is not particularly limited, but may include, for example, copper (Cu) foil, SUS foil, and the like.

[0027] The negative electrode active material of this embodiment contains graphite. More specifically, carbon materials are generally classified into crystalline graphite and amorphous carbon materials, and amorphous carbon materials are divided into hard carbon and soft carbon. The choice of whether to use crystalline graphite or amorphous carbon materials as the negative electrode active material is preferably determined based on compatibility with the electrolyte. In this embodiment, as described above, a chloroether-based solvent represented by the general formula (1) is used as the electrolyte solvent, which can suppress the occurrence of solvent co-intercalation. This allows graphite to be used as the negative electrode active material, allowing the excellent properties of graphite to be fully utilized. Examples of such properties include high volumetric energy density, low cost, and structural stability.

[0028] In this embodiment, the graphite contained in the negative electrode active material is not particularly limited. Specific types of graphite include artificial graphite and natural graphite, and either type of graphite can be suitably used in this embodiment.

[0029] The negative electrode active material may contain a material other than graphite, as long as it does not impede the objectives of the present disclosure. Examples of other materials include soft carbon, hard carbon, silicon, and silicon oxide. The content of graphite as the negative electrode active material is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass, relative to 100% by mass of the total negative electrode active material.

[0030] As described above, the negative electrode active material layer may further contain, for example, a conductive material, a binder, and the like in addition to the negative electrode active material. The conductive material may be, for example, carbon nanotubes (CNT), and the binder may be, for example, polyvinylidene fluoride (PVdF). The blending amounts of the conductive material and binder are preferably 0 to 20 parts by weight, more preferably 0 to 15 parts by weight, and particularly preferably 0 to 10 parts by weight, per 100 parts by weight of the negative electrode active material.

[0031] 3. Positive electrode In this embodiment, the positive electrode is not particularly limited as long as it functions as a positive electrode, but may include a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector may include, for example, aluminum (Al) foil. The positive electrode active material layer includes a positive electrode active material and may further include, for example, a conductive material, a binder, etc.

[0032] The positive electrode active material may include an oxide or a sulfur-containing compound. Examples of oxides include LiCoO, LiNiO, LiMnO, Li(NiCoMn)O, Li(NiMn)O, LiFePO, and Li(NiCoAl)O. Examples of sulfur-containing compounds include LiS and S. For example, "(NiCoMn)" in "Li(NiCoMn)O2" indicates that the sum of the composition ratios in parentheses is 1. As long as the sum is 1, the amount of each component is arbitrary.

[0033] An example of the conductive material is acetylene black (AB). An example of the binder is PVdF. The blending amounts of the conductive material and binder can be, for example, 0.1 to 20 parts by volume, 0.1 to 15 parts by volume, or 0 to 10 parts by volume per 100 parts by volume of the positive electrode active material.

[0034] 4. Separator In this embodiment, the separator is not particularly limited as long as it functions as a separator. The separator is an electrically insulating porous member that is permeable to the electrolyte. The separator separates the positive electrode from the negative electrode. The separator may contain, for example, a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator may have, for example, a single-layer structure or a multi-layer structure. The separator may, for example, consist essentially of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order. For example, a heat-resistant layer may be formed on the surface of the separator.

[0035] 5. Battery structure The lithium ion secondary battery of this embodiment is not particularly limited, but may have, for example, the structure shown in the schematic diagram of Fig. 1. The lithium ion secondary battery 10 shown in Fig. 1 is a coin-type battery, and is formed by stacking, in order from the negative electrode side, a negative electrode side battery case 12, a gasket 14, a negative electrode 16, a separator 18, a positive electrode 20, a spacer 22, a leaf spring 24, and a positive electrode side battery case 26, and fitting the battery case 12 and the battery case 26 together. The separator 18 is impregnated with an electrolyte (not shown). The lithium ion secondary battery of this embodiment is not limited to the coin-type battery shown in Fig. 1, but may be, for example, rectangular or cylindrical. Also, the battery case may be, for example, a pouch made of aluminum (Al) laminate film instead of a metal case.

[0036] Although the embodiments of the present invention have been described above, the present invention is not limited to the above examples and can be modified as appropriate. [Example]

[0037] The present invention will be described in more detail below with reference to examples, but the materials, amounts used, proportions, treatment contents, treatment procedures, etc. shown in the examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. In the examples, "%" is by weight unless otherwise specified.

[0038] <Experiment 1> Lithium ion secondary batteries of each example and comparative example were fabricated as follows and evaluated. [Example 1] The electrolyte solution was prepared as follows: Dried Li(FSO2)2N (manufactured by Tokyo Chemical Industry Co., Ltd.) as a lithium salt and 1,2-bis(2-chloroethoxy)ethane (manufactured by Tokyo Chemical Industry Co., Ltd. (hereinafter also referred to as "Cl-DEE") as a solvent were mixed in a molar ratio of 1:6, and the lithium salt was dissolved to prepare the electrolyte solution. Next, a negative electrode was fabricated as follows. Cu foil (thickness: 18 μm, manufactured by Hosen Co., Ltd.) was used as the negative electrode current collector. Natural graphite (manufactured by SEC Carbon Co., Ltd.) was used as the negative electrode active material. KFL#1120 (88% N-methyl-2-pyrrolidone / 12% polyvinylidene fluoride) (manufactured by Kureha Corporation) was used as the polymer dispersion. N-methyl-2-pyrrolidone (manufactured by Nacalai Tech, Inc.) was used as the dispersion medium. The negative electrode active material, binder, and dispersion medium were then mixed to prepare a negative electrode slurry. The mixing ratio (mass ratio) of the negative electrode active material to the binder was 90:10. The negative electrode slurry was applied to the surface of the negative electrode current collector and dried to form a negative electrode active material layer. The negative electrode active material layer was then cut out to obtain a disk-shaped negative electrode with a diameter of 16 mm.

[0039] Next, a 2032-type coin cell exterior was prepared, and the resulting negative electrode, a Φ16 lithium metal counter electrode, and a Φ17 glass separator were placed inside, with the coated surface of the negative electrode facing the lithium metal through the separator. The prepared electrolyte was then added. After placing the lid, the periphery was crimped using a crimping machine to create a test cell.

[0040] The lithium ion secondary battery of Example 1 produced as described above was evaluated by the method described below, and the results are shown in Table 1.

[0041] [Example 2] A lithium ion secondary battery was produced in the same manner as in Example 1, except that the solvent of the electrolyte solution was changed from 1,2-bis(2-chloroethoxy)ethane to diethylene glycol bis(2-chloroethyl) ether (manufactured by Tokyo Chemical Industry Co., Ltd. (hereinafter also referred to as "Cl-DGDE"). The produced lithium ion secondary battery of Example 2 was evaluated by the method described below, and the results are shown in Table 1.

[0042] [Example 3] A lithium ion secondary battery was produced in the same manner as in Example 1, except that the molar ratio of the lithium salt to the solvent (Cl-DEE) in the electrolyte was changed from 1:6 to 1:9. The produced lithium ion secondary battery of Example 3 was evaluated by the method described below, and the results are shown in Table 1.

[0043] [Comparative Example 1] A lithium ion secondary battery was produced in the same manner as in Example 1, except that the solvent of the electrolyte solution was changed from 1,2-bis(2-chloroethoxy)ethane to (1,2-ethoxy)ethane (manufactured by Kishida Chemical Co., Ltd. (hereinafter also referred to as "DEE")). The produced lithium ion secondary battery of Comparative Example 1 was evaluated by the method described below, and the results are shown in Table 1.

[0044] Comparative Example 2 A lithium ion secondary battery was produced in the same manner as in Example 1, except that the solvent of the electrolyte solution was changed from 1,2-bis(2-chloroethoxy)ethane to diethylene glycol diethyl ether (manufactured by Tokyo Chemical Industry Co., Ltd. (hereinafter also referred to as "DGDE"). The produced lithium ion secondary battery of Comparative Example 2 was evaluated by the method described below, and the results are shown in Table 1.

[0045] The performance of the lithium ion secondary batteries of each of the Examples and Comparative Examples was measured and evaluated by the following methods.

[0046] (1) Measurement of average discharge capacity The average discharge capacity was measured using a charge / discharge tester (TOSCAT-3000) manufactured by Toyo Systems Co., Ltd. Specifically, each fabricated lithium-ion secondary battery (test cell) was first charged at a constant current / constant voltage of 0.1 C to 0.01 V. Then, it was discharged at a constant current of 0.1 C to 2.5 V (first cycle). Then, the charge / discharge cycle was repeated for a total of five cycles. The temperature during this measurement was 25°C. In this charge-discharge cycle, the average discharge capacity (mAh / g) was calculated by averaging the discharge capacities (mAh / g) from the second to fifth cycles, excluding the initial discharge capacity (first cycle).

[0047] (2) Calculation of average Coulomb efficiency The average coulombic efficiency was calculated using the charge capacity (mAh / g) and discharge capacity (mAh / g) for cycles 2 to 5 obtained in the above (1) Measurement of average discharge capacity. Specifically, the coulombic efficiency (charge capacity / discharge capacity × 100(%)) for each cycle from cycles 2 to 5 was calculated from the obtained measurements, and then averaged to obtain the average coulombic efficiency.

[0048] (3) Solvent co-insertion The occurrence of solvent co-intercalation in the lithium ion secondary batteries of each Example and Comparative Example was determined by examining the shape of the charge curve obtained from the charge-discharge cycles performed in the above (1) Measurement of average discharge capacity. Specifically, when a carbonate-based electrolyte is used, which does not allow for solvent co-intercalation, the charge curve is known to hover around 0 volts until a certain capacity is reached, after which the voltage rises sharply.In addition, when a solvent that allows for solvent co-intercalation is used, the voltage rises sharply from a low capacity stage during charge. As shown in Figures 2 to 4, the charging curves of Examples 1 to 3 show that during charging, the voltage remained near 0 volts until a capacity of about 300 mAh / g was reached, and then the voltage rose sharply, indicating that no solvent co-intercalation occurred. On the other hand, the charging curves of Comparative Examples 1 and 2, as shown in FIGS. 5 and 6, show that the voltage rises sharply from a low capacity stage during charging, indicating that solvent co-intercalation occurred.

[0049] [Table 1]

[0050] As can be seen from Table 1, the lithium-ion secondary batteries of each Example, in which the electrolyte contained a chloroether-based solvent represented by formula (1) and the negative electrode active material contained graphite, did not experience solvent co-intercalation and had high average discharge capacity and average Coulombic efficiency. In contrast, the lithium-ion secondary batteries of each Comparative Example, in which the electrolyte did not contain a chloroether-based solvent represented by formula (1), experienced solvent co-intercalation and had low average discharge capacity and average Coulombic efficiency. The lithium ion secondary batteries of each example had high volumetric energy density due to the use of graphite as the negative electrode active material, and could be used in low-temperature environments due to the use of a chloroether-based solvent as the electrolyte, and also had improved flame retardancy.

[0051] <Experiment 2> Using the lithium ion secondary batteries of Example 1 of Experiment 1 and Comparative Example 3 described below, the batteries were evaluated by carrying out many charge / discharge cycles under high temperature conditions.

[0052] Comparative Example 3 A lithium ion secondary battery of Comparative Example 3 was produced in the same manner as in Example 1, except that the solvent for the electrolyte was changed from 1,2-bis(2-chloroethoxy)ethane to a solvent containing ethylene carbonate and dimethyl carbonate in a 1:1 ratio, and further, the lithium salt was changed from Li(FSO)N to LiPF.

[0053] The performance of the lithium ion secondary batteries of Example 1 and Comparative Example 3 was measured and evaluated by the following method. (4) Measurement of charge and discharge capacity The charge capacity was measured using a charge / discharge tester (TOSCAT-3000) manufactured by Toyo Systems Co., Ltd. Specifically, each fabricated lithium-ion secondary battery (test cell) was first charged at a constant current / constant voltage of 0.1 C to 0.01 V. Then, it was discharged at a constant current of 0.1 C to 2.5 V (first cycle). Thereafter, the charge / discharge cycle was repeated for a total of 100 cycles. The charge capacity (mAh / g) and discharge capacity (mAh / g) were measured for each cycle, and the results are shown in Table 2 below. The temperature during this measurement was 70°C.

[0054] (5) Calculation of Coulomb efficiency The coulombic efficiency (%) was calculated by dividing the charge capacity (mAh / g) and discharge capacity (mAh / g) for each cycle obtained in (1) Measurement of charge capacity by the discharge capacity, and then multiplying the result by 100. The results are shown in Table 2 below.

[0055] [Table 2]

[0056] Graphs of the discharge capacity (mAh / g) for each cycle and the coulombic efficiency (%) for each cycle obtained by the above evaluation are shown in Figures 7 and 8, respectively. In Figure 7, the results of Example 1 are plotted with black circles, and the results of Comparative Example 3 are plotted with white circles. In Figure 8, the coulombic efficiency from the second cycle onwards is shown, with the results of Example 1 plotted with black triangles and the results of Comparative Example 3 plotted with white triangles.

[0057] As can be seen from Table 2 and FIGS. 7 and 8, the results of the cycle test at 70°C showed that the lithium ion secondary battery of Example 1 showed almost no decrease in charge capacity or discharge capacity, even under high temperature (70°C) conditions, and maintained a high and almost constant Coulombic efficiency. In contrast, the lithium ion secondary battery of Comparative Example 3 gradually decreased in charge capacity and discharge capacity with the number of cycles, and also gradually decreased in Coulombic efficiency. Therefore, it was found that the lithium ion secondary battery of Example 1 was resistant to decreases in charge capacity and discharge capacity, and in Coulombic efficiency. It was also found that the lithium ion secondary battery of Example 1 had high durability against repeated use of the battery, particularly at high temperatures. [Industrial Applicability]

[0058] According to the present invention, for example, a lithium ion secondary battery having a relatively high volumetric energy density can be provided. Furthermore, according to the present invention, for example, a lithium ion secondary battery having a relatively low melting point of the electrolyte can be provided. According to the present invention, for example, a lithium ion secondary battery having relatively high flame retardancy can be provided. According to the present invention, for example, a lithium ion secondary battery having a charge capacity and a discharge capacity that are resistant to reductions in charge capacity and discharge capacity, and a coulombic efficiency that are resistant to reductions in charge capacity and discharge capacity can be provided. According to the present invention, for example, a lithium ion secondary battery having high durability against repeated use of the battery can be provided. [Explanation of symbols]

[0059] 1: Lithium-ion secondary battery 2: Battery case (negative side) 3: Gasket 4: Negative electrode 5: Separator 6: Positive electrode 7: Spacer 8: Leaf spring 9: Battery case (positive side)

Claims

1. A lithium ion secondary battery including an electrolyte solution and a negative electrode active material, The electrolytic solution contains a chloroether-based solvent represented by the following general formula (1): The negative electrode active material comprises graphite. 【Chemistry 1】 (In the formula, R １ are each independently a hydrogen atom or a chlorine atom, R ２ are each independently a hydrogen atom, a methyl group, or a chlorine atom, R １ At least one of R is a chlorine atom, or ２ at least one of which is a chlorine atom; n is an integer from 0 to 4.

2. R １ at least one of is a chlorine atom, R ２ and each independently represent a hydrogen atom or a methyl group.

3. R １ is a chlorine atom, and R ２ The lithium ion secondary battery according to claim 2 , wherein is a hydrogen atom.

4. 3. The lithium ion secondary battery according to claim 1, wherein n is 1 or 2.

5. 3. The lithium ion secondary battery according to claim 1, wherein the graphite is artificial graphite or natural graphite.

6. the electrolyte solution contains a lithium salt; The lithium salt is LiPF ６ , Li(FSO ２ ) ２ N, LiBF ４ , and Li(CF ３ SO ２ ) ２ N, LiClO ４ 3. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is one or more selected from the group consisting of:

7. The lithium ion secondary battery further comprises a positive electrode active material, The lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material comprises an oxide or a sulfur-containing compound.

8. the positive electrode active material contains a sulfur-containing compound, The sulfur-containing compound is Li ２ The lithium ion secondary battery according to claim 7, wherein the lithium ion secondary battery is S or S.

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