Lithium-ion secondary battery

Abstract

The present invention improves battery performance in a lithium-ion secondary battery. A lithium-ion secondary battery according to an embodiment of the present invention comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte includes a lithium salt and an ionic liquid that contains a bis(fluorosulfonyl)imide anion as an anionic component, and the amount of the lithium salt relative to 1 kg of the ionic liquid is 1.5-8.0 mol / kg.

Description

Lithium-ion rechargeable battery

[0001] Embodiments of the present invention relate to lithium-ion secondary batteries.

[0002] Lithium-ion secondary batteries are used as high-voltage, high-energy-density energy storage devices, for example, as power sources for electronic devices such as mobile phones and laptop computers, as well as for electric bicycles and electric vehicles. Various studies are being conducted to improve the performance of such lithium-ion secondary batteries (see Non-Patent Document 1).

[0003] Polar aprotic organic solvents that readily dissolve lithium salts and are difficult to electrolyze are known as non-aqueous solvents used in the non-aqueous electrolyte of lithium-ion secondary batteries. However, the use of flame-retardant ionic liquids as an alternative is being considered. For example, Patent Document 1 discloses that the negative electrode of a lithium-ion secondary battery contains non-graphitizable carbon, and that the non-aqueous electrolyte uses an ionic liquid containing bis(fluorosulfonyl)imide anion as an anionic component. Patent Document 2 discloses that a layer made of metallic lithium, a metal that can be alloyed with lithium, or an alloy of lithium and the said metal is provided on the surface of the negative electrode, and that a lithium salt, an ionic liquid, and a hydrofluoroether are used in combination as the non-aqueous electrolyte.

[0004] Japanese Patent Publication No. 2009-026542 Japanese Patent Publication No. 2021-125416

[0005] Itsuki Konuma, and 11 others, “Unified understanding and mitigation of discomfort of phase transition in cobalt-freeLiNiO2”, Energy Storage Materials, 66, (2024) 103200

[0006] Ionic liquids containing bis(fluorosulfonyl)imide anions as anionic components are superior in terms of safety and other aspects as solvents for non-aqueous electrolytes in lithium-ion secondary batteries. Further improvements in battery performance, such as cycle characteristics and Coulomb efficiency, are required in lithium-ion secondary batteries using such ionic liquids.

[0007] In view of the above, embodiments of the present invention aim to improve battery performance in a lithium-ion secondary battery using an ionic liquid containing bis(fluorosulfonyl)imide anion as an anionic component as a non-aqueous electrolyte.

[0008] The present invention includes the embodiments shown below. [1] A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises an ionic liquid containing a bis(fluorosulfonyl)imide anion as an anionic component and a lithium salt, wherein the amount of the lithium salt per 1 kg of the ionic liquid is 1.5 to 8.0 mol / kg. [2] The lithium-ion secondary battery according to [1], wherein the ionic liquid contains at least one selected from the group consisting of pyrrolidinium, imidazolium, piperidinium, and alkylammonium as a cationic component. [3] The lithium-ion secondary battery according to [1] or [2], wherein the ionic liquid contains pyrrolidinium as a cationic component. [4] The lithium-ion secondary battery according to any one of [1] to [3], wherein the lithium salt contains lithium bis(fluorosulfonyl)imide. [5] The lithium-ion secondary battery according to any one of [1] to [3], wherein the positive electrode contains a positive electrode active material, and the positive electrode active material contains a composite oxide of lithium and a transition metal. [6] A lithium-ion secondary battery according to any one of [1] to [5], wherein the positive electrode contains a positive electrode active material, and the positive electrode active material contains lithium nickelate. [7] The lithium nickelate is of the general formula: Li ｘ Ni ｙ O ２ A lithium-ion secondary battery as described in [6], wherein x and y in the general formula satisfy 0.950 ≤ x < 1.000 and 1.000 < y ≤ 1.050.

[0009] According to embodiments of the present invention, the battery performance of a lithium-ion secondary battery can be improved.

[0010] The lithium-ion secondary battery according to this embodiment comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0011] [Non-aqueous electrolyte] The non-aqueous electrolyte contains an ionic liquid and a lithium salt.

[0012] The ionic liquid is a solvent for dissolving the lithium salt, is liquid at room temperature (25°C), and is obtained by a combination of an anion component and a cation component. In this embodiment, as the ionic liquid, one containing bis(fluorosulfonyl)imide anion (FSO ２ ), ２ N － (also referred to as "FSI anion" or "FSA") as the anion component is used.

[0013] The anion component contained in the ionic liquid may be only FSI anion, or may contain other anions together with FSI anion. Examples of other anions include, for example, BF ４ － , PF ６ － , SbF ６ － , NO ３ － , CF ３ SO ３ － , (CF ３ SO ２ ), ２ N － (hereinafter referred to as "TFSI"), (C ２ F ５ SO ２ ), ２ N － , (CF ３ SO ２ ), ３ C － , CF ３ CO ２ － , C ３ F ７ CO ２ － , CH ３ CO ２ － , (CN ２ N －Examples include the above, and one or more of these may be used. The anionic component contained in the ionic liquid is preferably mainly composed of FSI anions, more preferably containing 70% by mass or more of FSI anions, even more preferably 90% by mass or more, and may even be 100% by mass.

[0014] As the cationic component of the ionic liquid, it is preferable to use a cation that forms an ionic liquid with a melting point of 50°C or lower. Examples of cationic components include compounds that contain one or more elements from N, P, S, O, C, and Si in their structure and have a chain-like or cyclic structure such as a five-membered ring or a six-membered ring as their backbone. Examples of such cyclic structures include heteromonocyclic structures such as furan rings, thiophene rings, pyrrole rings, pyridine rings, oxazole rings, isoxazole rings, thiazole rings, isothiazole rings, furazan rings, imidazole rings, pyrazole rings, pyrazine rings, pyrimidine rings, pyridazine rings, pyrrolidine rings, and piperidine rings, as well as condensed heterocyclic structures such as benzofuran rings, isobenzofuran rings, indole rings, isoindole rings, indidine rings, and carbazole rings.

[0015] Among these cations, those containing nitrogen are preferred because they are chemically and electrochemically stable. Preferred examples of nitrogen-containing cations include pyrrolidinium such as 1-methyl-1-propylpyrrolidinium, imidazolium such as 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium, piperidinium such as 1-methyl-1-propylpiperidinium, and alkylammonium such as triethylammonium. One or more of these may be used. More preferably, pyrrolidinium such as 1-methyl-1-propylpyrrolidinium (MPPy) is used as the cationic component, which can enhance the effect of improving cycle characteristics.

[0016] The cationic component preferably consists mainly of at least one cation (more preferably pyrrolidinium) selected from the group consisting of pyrrolidinium, imidazolium, piperidinium, and alkylammonium. More preferably, the cationic component contains 70% by mass or more of the above at least one cation (preferably pyrrolidinium), even more preferably 90% by mass or more, and may even be 100% by mass.

[0017] Examples of lithium salts to be dissolved in ionic liquids include LiPF ６ LiBF ４ LiClO ４ LiAsF ６ , LiCl, LiBr, LiCF ３ SO ３ , LiI, LiAlClO ４ LiC (CF ３ SO ２ ) ３ ,LiN(C ２ F ５ SO ２ ) ２ LiBC ４ O ８ Examples include lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and one or more of these may be used. Among these, LiFSI and / or LiTFSI are preferred as lithium salts, and LiFSI is more preferred.

[0018] In this embodiment, the lithium salt content in the non-aqueous electrolyte is 1.5 to 8.0 mol / kg per 1 kg of ionic liquid. That is, 1.5 to 8.0 mol of lithium salt is added per 1 kg of ionic liquid. By increasing the lithium salt concentration in this way, battery performance can be improved. However, when the lithium salt concentration is increased, a trade-off relationship is observed between cycle characteristics and average Coulomb efficiency. If the concentration is too high, the average Coulomb efficiency improves, but the cycle characteristics tend to deteriorate. Also, if the lithium salt concentration is too high, the non-aqueous electrolyte becomes highly concentrated and difficult to handle, and in some cases, undissolved lithium salt may remain. For this reason, in this embodiment, the lithium salt concentration is set to 8.0 mol / kg or less. The amount of lithium salt per 1 kg of ionic liquid is preferably 2.0 to 7.0 mol / kg, and more preferably 2.5 to 6.0 mol / kg.

[0019] In a non-aqueous electrolyte, the mass ratio of the ionic liquid to the lithium salt is preferably ionic liquid / lithium salt = 0.5 / 1 to 3 / 1, more preferably 0.7 / 1 to 2.5 / 1, and even more preferably 0.8 / 1 to 2.3 / 1.

[0020] [Positive electrode] The positive electrode can be one that is commonly used in lithium-ion secondary batteries and generally includes a current collector and a positive electrode active material layer.

[0021] The current collector used in the positive electrode is not particularly limited and includes, for example, aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, etc. Alternatively, aluminum or copper with a surface treated with carbon, nickel, titanium, silver, etc. may be used.

[0022] The positive electrode active material layer is a layer containing the positive electrode active material, and may also contain a conductive agent and an organic binder together with the positive electrode active material. The positive electrode active material, conductive agent, and organic binder can be those commonly used in lithium-ion secondary batteries.

[0023] As the positive electrode active material, materials that allow for the insertion and removal of lithium ions are used, such as metal oxides, composite oxides of lithium and transition metals, metal chalcogenides, and conductive polymer compounds. Examples of metal oxides include CuO and Cu. ２ O, MnO ２ MoO ３ , V ２ O ５ ,CrO ３ MoO ３ Fe ２ O ３ Ni ２ O ３ CoO ３ Examples include the metal chalcogenides, such as TiS ２ MoS ２ NbSe ３ Examples include the following. Examples of conductive polymer compounds include polyacene, poly(p-phenylene), polypyrrole, and polyaniline.

[0024] As the positive electrode active material, it is preferable to use a composite oxide of lithium and a transition metal. Examples of composite oxides of lithium and a transition metal include lithium nickelate, lithium cobaltate, lithium manganeseate, and lithium iron phosphate. The composite oxide of lithium and a transition metal may be doped in small amounts with elements such as fluorine, boron, aluminum, chromium, zirconium, molybdenum, and iron, or the particle surface of the composite oxide may be doped with carbon, MgO, and Al. ２ O ３ SiO ２ Materials with surface treatments such as those described above may also be used.

[0025] Two or more of the above-mentioned materials may be used in combination as the positive electrode active material. The amount of positive electrode active material is not particularly limited; for example, 3 to 10 mg / cm² per unit area of ​​the current collector. ２ But that's fine.

[0026] In one embodiment, the positive electrode active material preferably contains lithium nickelate, and more preferably contains a material represented by the following general formula (1). General formula (1): Li ｘ Ni ｙ O ２In equation (1), x and y satisfy 0.950 ≤ x < 1.000 and 1.000 < y ≤ 1.050.

[0027] The lithium nickelate of formula (1) is lithium nickelate in which lithium is partially defective, and can be prepared by the method described in Non-Patent Document 1. By using lithium nickelate of formula (1) as the positive electrode active material, and by using a non-aqueous electrolyte with a high concentration of lithium salt as described above, the cycle characteristics and Coulomb efficiency can be more effectively improved.

[0028] In equation (1), x and y preferably satisfy 0.960 ≤ x ≤ 0.990 and 1.010 ≤ y ≤ 1.040, more preferably 0.965 ≤ x ≤ 0.985 and 1.015 ≤ y ≤ 1.035, and even more preferably 0.970 ≤ x ≤ 0.980 and 1.020 ≤ y ≤ 1.030.

[0029] Examples of organic binders used in the positive electrode active material layer include polyvinylidene fluoride (PVDF), PVDF copolymer resins, polytetrafluoroethylene (PTFE), fluororesins, styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), styrene-acrylonitrile copolymer, carboxymethylcellulose (CMC), and polyimide resins. Examples of PVDF copolymer resins include copolymers of hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PFMV), or tetrafluoroethylene (TFE) with PVDF. Examples of fluororesins include polytetrafluoroethylene (PTFE) and fluororubber. These organic binders may be used individually or in combination of two or more. The amount of organic binder is not particularly limited and may be 0.5 to 30% by mass or 1 to 15% by mass relative to the amount of active material.

[0030] Examples of conductive agents used in the positive electrode active material layer include carbon nanotubes, conductive carbon black (acetylene black, Ketjenblack, etc.), natural graphite (scaly graphite, flake graphite, earthy graphite, etc.), artificial graphite, carbon whiskers, carbon fibers, metal powders (copper, nickel, aluminum, silver, gold, etc.), metal fibers, and conductive ceramic materials. These conductive agents may be used individually or in combination of two or more. The amount of conductive agent is not particularly limited and may be 1 to 30% by mass or 2 to 20% by mass relative to the amount of active material.

[0031] [Negative electrode] The negative electrode can be one that is commonly used in lithium-ion secondary batteries and generally includes a current collector and a negative electrode active material layer.

[0032] The current collector used in the negative electrode is not particularly limited and includes, for example, copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc. Alternatively, metals whose surfaces have been treated with carbon, nickel, titanium, silver, etc., may also be used.

[0033] The negative electrode active material layer is a layer containing the negative electrode active material, and may also contain a conductive agent and an organic binder together with the negative electrode active material. The negative electrode active material, conductive agent, and organic binder can be those commonly used in lithium-ion secondary batteries.

[0034] The negative electrode active material is not particularly limited and includes, for example, carbon materials, metallic materials, lithium transition metal nitrides, crystalline metal oxides, amorphous metal oxides, silicon compounds, conductive polymers, etc. Examples of carbon materials include natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon. Examples of metallic materials include metallic lithium and alloys, tin compounds, etc. A specific example of a negative electrode active material is Li ４ Ti ５ O １２ NiSi ５ C ６ Examples include the above. Two or more of the above-mentioned materials may be used in combination as the negative electrode active material. The amount of negative electrode active material is not particularly limited, for example, 1 to 5 mg / cm² per unit area of ​​the current collector. ２But that's fine.

[0035] The organic binder and conductive agent used in the negative electrode active material layer are not particularly limited, and examples include the same organic binder and conductive agent as those used in the positive electrode active material layer. The amounts of these are also the same as those used in the positive electrode active material layer.

[0036] A lithium-ion secondary battery may include a separator positioned between the positive and negative electrodes. The separator is a component that isolates the positive and negative electrodes, and can be one of those commonly used in lithium-ion secondary batteries. Examples of separators include porous resins made of polyethylene, polypropylene, polyolefin, polyimide, polytetrafluoroethylene, ceramics, and nonwoven fabrics.

[0037] The lithium-ion secondary battery according to this embodiment can be formed into, for example, cylindrical, coin-shaped, prismatic, or any other arbitrary shape. The basic structure of the battery is the same regardless of the shape, and it can be designed and implemented according to the purpose. For example, in the case of a cylindrical battery, a wound body in which a negative electrode and a positive electrode are wound with a separator in between is housed in a battery case, a non-aqueous electrolyte is injected, and insulating plates are placed on the top and bottom and sealed. In the case of a coin-shaped battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and housed in a coin-shaped battery case, a non-aqueous electrolyte is injected, and it is sealed.

[0038] The present invention will be described in more detail below based on examples and comparative examples, but it is not limited thereto.

[0039] The evaluation methods for the examples and comparative examples are as follows: <Charge and Discharge Test of Lithium-ion Secondary Batteries> Constant current charge and discharge curves were measured for the lithium-ion secondary batteries of the examples and comparative examples, and their electrochemical characteristics were evaluated. Specifically, a charge and discharge device manufactured by Toyo Systems (TOSCAT-3100) was used. For the charge and discharge test, the lithium-ion secondary battery was charged with a CC (Constant Current) value of 0.25C current, and then CC discharge was performed with a CC current value of 0.25C, and the charging and discharging were repeated for a predetermined number of cycles (cycle test). Both charging and discharging were performed for approximately 4 hours. The voltage range for charging and discharging was set to 2.5V to 4.5V. Note that the 0.25C current value represents a current value that is 0.25 times the current value of 1C, which is the current value that can discharge the cell capacity in 1 hour.

[0040] [Method for Evaluating Cycle Characteristics] In this specification, the discharge capacity retention rate is defined as the ratio of the discharge capacity at a specific number of cycles to the maximum discharge capacity obtained during the cycle test. This makes it possible to evaluate the retention rate of discharge performance at a specific cycle during the cycle test. The discharge capacity was determined as the discharge capacity of the battery per unit mass of positive electrode active material (unit: mAh / g) by dividing the discharge capacity during discharge in each cycle by the mass of positive electrode active material. The number of cycles was 300 cycles for Comparative Examples 1 and 3 and Examples 2 and 3, 50 cycles for Comparative Example 2, and 100 cycles for the others. The discharge capacity retention rates were then determined at 50 cycles, 100 cycles, and 300 cycles.

[0041] [Method for Evaluating Coulomb Efficiency] In this specification, Coulomb efficiency is defined as the ratio of discharge capacity to charge capacity, i.e., the value obtained by dividing discharge capacity by charge capacity (discharge capacity / charge capacity). It is preferable to use the average value of the Coulomb efficiency over a specific number of cycles when calculating the Coulomb efficiency. In this embodiment, the average value of the Coulomb efficiency from the 2nd cycle to the 100th cycle was calculated and adopted as the average Coulomb efficiency. For Comparative Example 2, the average value of the Coulomb efficiency from the 2nd cycle to the 50th cycle was adopted as the average Coulomb efficiency.

[0042] <Preparation of Cathode Active Material Li ０．９７５ Ni １．０２５ O ２ Li ０．９７５ Ni １．０２５ O ２ was prepared from a mixture of LiOH·H ２ O and Ni(OH) ２ according to the method described in Non-Patent Document 1. Specifically, LiOH·H ２ O (Fuji Film Wako Pure Chemical Industries, Ltd.) was pulverized into fine powder using a mortar and pestle, and mixed with Ni(OH) ２ (Fuji Film Wako Pure Chemical Industries, Ltd.). At that time, the mixture was mixed with Ni:Li = 1.00:0.96 (molar ratio). The mixture was pressed to form pellets. The pellets were heated while flowing oxygen gas in a tubular furnace. The heating was carried out by raising the temperature of the tubular furnace to 650 °C at a rate of 1 °C / min and holding at 650 °C for 12 hours. Then, it was cooled and the pellets were stored in a glove box filled with Ar gas.

[0043] The obtained pellets were confirmed to have a chemical composition of Li ０．９７５ Ni １．０２５ O ２ by Rietveld analysis. Specifically, synchrotron X-ray diffraction (XRD) data was obtained at beamline BL19B2 of the Spring-8 synchrotron facility. The measurement was carried out using an automatic powder diffraction system using the Debye–Scherrer method in a glass capillary, with a wavelength of 0.500 Å, and calibrated using CeO ２ as a reference sample. For the structure analysis, RIETAN-FP was used as the software for Rietveld analysis.

[0044] <Example 1> [Preparation of Cathode] As the cathode active material, Li ０．９７５ Ni １．０２５ O ２0.40 g of [material name], 0.05 g of acetylene black (Denka Co., Ltd. "HS-100") as a conductive agent, 0.05 g of polyvinylidene fluoride (PVDF, Kureha Corporation "KF-1100") as a binder, and 0.33 g of N-methyl-2-pyrrolidone (NMP) as a dispersion medium were mixed in a planetary mixer to prepare a positive electrode coating solution with a solid content (excluding NMP) of 60%. This coating solution was coated onto 20 μm thick aluminum foil using a coating machine, dried at 130°C, and then roll-pressed to obtain a coating mass of 8 mg / cm² of positive electrode active material. ２ The positive electrode was obtained.

[0045] [Fabrication of the negative electrode] A 500 μm thick lithium metal foil (manufactured by Honjo Metal Co., Ltd.) was punched out and used as the negative electrode.

[0046] [Fabrication of Lithium-ion Secondary Batteries] A two-electrode electrochemical cell was fabricated as a lithium-ion secondary battery using a glove box with a dew point of -60°C (manufactured by UNICO Corporation). Its configuration is as follows: • Cell type: Two-electrode electrochemical cell (TJ-AC: manufactured by Nippon Tom Cell Co., Ltd.) • Cathode: Li ０．９７５ Ni １．０２５ O ２ Acetylene black / PVDF = 80:10:10 (mass%) ・Negative electrode: Lithium metal foil ・Separator: Aramid-coated polyolefin porous membrane (manufactured by Teijin Limited) ・Electrolyte: 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (MPPy-FSI) is used as the ionic liquid, and the amount of lithium salt (LiFSI) per 1 kg of the ionic liquid is 2.7 mol / kg to form the electrolyte (LiFSI-MPPyFSI). The mass ratio of ionic liquid to lithium salt is 2 / 1.

[0047] For the lithium-ion secondary battery of Example 1, charge-discharge tests were performed at room temperature, and the cycle characteristics and average Coulomb efficiency were evaluated.

[0048] <Example 2> A lithium-ion secondary battery was prepared using the same method as in Example 1, with a lithium salt (LiFSI) amount of 5.4 mol / kg per 1 kg of ionic liquid, and a charge-discharge test was performed. The mass ratio of ionic liquid to lithium salt in Example 2 was 1 / 1.

[0049] <Example 3> Instead of 1-methyl-1-propylpyrrolidinium (MPPy) as the cationic component of the ionic liquid, 1-ethyl-3-methylimidazolium (EMIm) was used, and a lithium-ion secondary battery was prepared in the same manner as in Example 2, and a charge-discharge test was performed. The mass ratio of ionic liquid to lithium salt in Example 3 was 1 / 1.

[0050] <Example 4> Using the lithium-ion secondary battery prepared in Example 2, the charge-discharge test was performed at a temperature of 50°C for evaluation.

[0051] <Comparative Example 1> A lithium-ion secondary battery was prepared using a non-aqueous electrolyte solution prepared by dissolving lithium salt LiFSI in dimethyl carbonate (DMC) at a concentration of 5.5 mol / L, and the rest of the procedure was the same as in Example 1. A charge-discharge test was then performed.

[0052] <Comparative Example 2> As a non-aqueous electrolyte, a polar organic solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 3:7 is mixed with lithium salt LiPF ６ A lithium-ion secondary battery was prepared using a solution in which the substance was dissolved at a concentration of 1.0 mol / L, and the rest of the procedure was the same as in Example 1. A charge-discharge test was then performed.

[0053] <Comparative Example 3> The lithium-ion secondary battery prepared in Comparative Example 1 was used, and the charge-discharge test was performed at a temperature of 50°C for evaluation.

[0054] <Comparative Example 4> The lithium-ion secondary battery prepared in Comparative Example 2 was used, and the charge-discharge test was performed at a temperature of 50°C for evaluation.

[0055] <Comparative Example 5> A lithium-ion secondary battery was prepared using the same method as in Example 1, with the amount of lithium salt (LiFSI) per 1 kg of ionic liquid being 0.8 mol / kg, and a charge-discharge test was performed. The mass ratio of ionic liquid to lithium salt in Comparative Example 5 was 6.7 / 1.

[0056] <Comparative Example 6> An attempt was made to produce a lithium-ion secondary battery by using 10.0 mol / kg of lithium salt (LiFSI) per 1 kg of ionic liquid, and otherwise following the same procedure as in Example 3. However, a large amount of undissolved lithium salt remained, making it impossible to perform a charge-discharge test.

[0057] Table 1 shows the results of charge-discharge tests for lithium-ion secondary batteries of Examples 1-4 and Comparative Examples 1-5. In Table 1, "LNO" is Li ０．９７５ Ni １．０２５ O ２ This indicates that an evaluation has not been conducted. Additionally, in the evaluation column, "-" indicates that an evaluation has not been performed.

[0058] As shown in Table 1, while Comparative Example 5, which used an ionic liquid, showed a tendency toward improvement in cycle characteristics compared to Comparative Examples 1 and 2, which used a polar organic solvent as the non-aqueous electrolyte, the effect was slight, and no improvement in average Coulomb efficiency was observed.

[0059] In Examples 1 and 2, where the lithium salt concentration was increased compared to Comparative Example 5, the cycle characteristics were significantly improved, and the average Coulomb efficiency also improved. In Examples 1 and 2, as well as in Comparative Examples 1 and 2 using polar organic solvents, both the cycle characteristics and average Coulomb efficiency were improved.

[0060] In Example 3, which used EMIm-FSI as the ionic liquid, the cycle characteristics and average Coulomb efficiency were superior to those of Comparative Example 2. While Example 3 showed only a slight improvement in cycle characteristics compared to Comparative Example 1, a clear difference was observed in average Coulomb efficiency.

[0061] Even when the battery evaluation temperature was set to 50°C, Example 4 showed improved cycle characteristics and average Coulomb efficiency compared to Comparative Examples 3 and 4.

[0062] <Example 5> Li as the positive electrode active material ０．９７５ Ni １．０２５ O ２ LiMnO ２ A lithium-ion secondary battery was fabricated using the same method as in Example 1, with the rest being the same.

[0063] <Comparative Example 7> A lithium-ion secondary battery was prepared using a non-aqueous electrolyte solution prepared by dissolving lithium salt LiFSI in dimethyl carbonate (DMC) at a concentration of 5.5 mol / L, and otherwise in the same manner as in Example 5.

[0064] Charge-discharge tests were performed on the lithium-ion secondary batteries of Example 5 and Comparative Example 7 at room temperature, and the cycle characteristics and average Coulomb efficiency were evaluated. The results are shown in Table 2.

[0065]

[0066] As shown in Table 2, LiMnO is used as the positive electrode active material. ２ Even when using this method, Example 5, in which an ionic liquid was used as the non-aqueous electrolyte and a high concentration of lithium salt was used, showed an improvement in cycle characteristics compared to Comparative Example 7, in which an organic solvent was used as the non-aqueous electrolyte.

[0067] <Example 6> Li as the positive electrode active material ０．９７５ Ni １．０２５ O ２ Instead, LiCoO ２ A lithium-ion secondary battery was fabricated using the same method as in Example 1, with the rest being the same.

[0068] <Comparative Example 8> A lithium-ion secondary battery was prepared using a non-aqueous electrolyte solution prepared by dissolving lithium salt LiFSI in dimethyl carbonate (DMC) at a concentration of 5.5 mol / L, and otherwise in the same manner as in Example 6.

[0069] Charge-discharge tests were performed on the lithium-ion secondary batteries of Example 6 and Comparative Example 8 at room temperature, and the cycle characteristics and average Coulomb efficiency were evaluated. The results are shown in Table 3.

[0070] As shown in Table 3, LiCoO ２ Even when using this method, Example 6, in which an ionic liquid was used as the non-aqueous electrolyte and a high concentration of lithium salt was used, showed a significant improvement in cycle characteristics and an improvement trend in average Coulomb efficiency compared to Comparative Example 8, in which an organic solvent was used as the non-aqueous electrolyte.

[0071] Furthermore, the various numerical ranges described in this specification can be any combination of their upper and lower limits, and all such combinations are described herein as preferred numerical ranges. Also, the description of a numerical range as "X to Y" means X or greater and Y or less.

[0072] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, and modifications are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

Claims

1. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte comprises an ionic liquid containing bis(fluorosulfonyl)imide anion as an anionic component and a lithium salt, and the amount of the lithium salt per 1 kg of the ionic liquid is 1.5 to 8.0 mol / kg.

2. The lithium-ion secondary battery according to claim 1, wherein the ionic liquid contains at least one selected from the group consisting of pyrrolidinium, imidazolium, piperidinium, and alkylammonium as a cationic component.

3. The lithium-ion secondary battery according to claim 1, wherein the ionic liquid contains pyrrolidinium as a cation component.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide.

5. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode includes a positive electrode active material, and the positive electrode active material includes a composite oxide of lithium and a transition metal.

6. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode comprises a positive electrode active material, and the positive electrode active material comprises lithium nickelate.

7. The lithium nickelate mentioned above has the general formula: Li ｘ Ni ｙ O ２ The lithium-ion secondary battery according to claim 6, wherein x and y in the general formula satisfy 0.950 ≤ x < 1.000 and 1.000 < y ≤ 1.050.

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