Lithium-ion secondary battery electrolyte solution and lithium-ion secondary battery

Abstract

The present invention provides a lithium-ion secondary battery electrolyte solution with which the resistance of the battery can be reduced and the capacity retention rate thereof can be improved.　The lithium-ion secondary battery electrolyte solution being characterized by containing an electrolyte salt and an organic solvent, wherein: the electrolyte salt is lithium bis(fluorosulfonyl)imide; the organic solvent contains a polar ether and a non-polar ether; the polar ether is 1,2-dimethoxyethane; and the non-polar ether is 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane.

Description

Electrolyte for lithium-ion secondary batteries and lithium-ion secondary batteries

[0001] This invention relates to an electrolyte for lithium-ion secondary batteries and a lithium-ion secondary battery.

[0002] In recent years, research and development has been conducted on rechargeable batteries that contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. In particular, lithium-ion rechargeable batteries are becoming increasingly important as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other similar vehicles.

[0003] Generally, lithium-ion secondary batteries consist of a positive electrode, a negative electrode, and an electrolyte as their main components. The positive electrode contains a positive electrode active material that is involved in charging and discharging, and the negative electrode contains a negative electrode active material that is also involved in charging and discharging. In particular, lithium metal secondary batteries (LMBs), which use metallic lithium in the negative electrode and have a higher energy density, are attracting attention. In many cases, a solution of a high concentration of lithium salt dissolved in a non-aqueous solvent is used as the electrolyte.

[0004] For example, Patent Documents 1 to 3 describe an electrolyte that uses an ether containing fluorine atoms as a non-aqueous solvent.

[0005] Patent No. 7475763 Patent No. 5343665 Patent No. 7287251

[0006] In lithium-ion secondary batteries, a localized high-concentration electrolyte with low viscosity is known, achieved by diluting a high-concentration electrolyte with a nonpolar solvent while maintaining a stable solvation structure. However, if the solvent's polarity is too low, the solution separates into two layers, and if the polarity is too high, the solvation structure changes. When charging and discharging lithium-ion secondary batteries using such an unstable solution, a stable interface may not form between the electrolyte and the negative electrode during charging and discharging, and a side reaction of electrolyte decomposition may occur. Such side reactions of the electrolyte not only drastically increase battery resistance but also deplete the electrolyte in the battery, causing a deterioration in battery life. The combination of the organic solvent used to dissolve lithium ions and the organic solvent used to dilute the organic solvent containing dissolved lithium ions is particularly important. Carbonates such as EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) are used because they are easily oxidatively decomposed on the positive electrode of the electrolyte, but there is a problem that they are easily reductively decomposed, especially when using metallic lithium as the negative electrode. On the other hand, chain ethers, such as 1,2-dimethoxyethane, are less prone to reductive decomposition at the negative electrode, are low-viscosity and versatile solvents, but are more easily oxidized than the carbonates mentioned above. In particular, when the nickel ratio of the positive electrode is high, durability during charge-discharge cycles tends to decrease. Furthermore, when these organic solvents are used as electrolytes for batteries, it is necessary to have few impurities during organic solvent synthesis and to be sufficiently dehydrated. Therefore, selecting a solvent that can achieve these conditions is also important.

[0007] This invention has been made in view of the above circumstances, and aims to provide an electrolyte for lithium-ion secondary batteries that can reduce battery resistance and improve capacity retention. Ultimately, this contributes to energy efficiency.

[0008] To achieve the above object, the inventors of the present invention have conducted intensive studies, and as a result found that, by combining a specific electrolyte salt, a polar ether and a non-polar ether, while obtaining the effect of improving reduction resistance and oxidation resistance through a solvation structure, the viscosity of the electrolyte solution for lithium ion secondary batteries can be reduced without two-layer separation, thereby reducing battery resistance and increasing capacity retention rate, which has led to the completion of the present invention. That is, the present invention provides the following means. [1] An electrolyte solution for a lithium ion secondary battery, comprising an electrolyte salt and an organic solvent, wherein the electrolyte salt is lithium bis(fluorosulfonyl)imide, the organic solvent comprises a polar ether and a non-polar ether, the polar ether is 1,2-dimethoxyethane, and the non-polar ether is 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane.

[0009] The electrolyte solution for a lithium ion secondary battery according to [1] can improve the solubility of the electrolyte salt and suppress the occurrence of side reactions by combining a specific electrolyte salt and only linear ethers (polar ether and non-polar ether) of two types of organic solvents. Accordingly, the resistance of the battery can be reduced and the capacity retention rate can be increased. Furthermore, the electrolyte solution contributes to improvement in energy efficiency.

[0010] [2] The electrolyte solution for a lithium ion secondary battery according to [1], wherein the concentration of the lithium bis(fluorosulfonyl)imide is 1.0 to 2.0 mol / L.

[0011] The electrolyte solution for a lithium ion secondary battery according to [2] can suppress side reactions between lithium metal and the electrolyte solution by adjusting a specific electrolyte salt to a specific concentration, and can suppress corrosion of a positive electrode current collector and elution of transition metals from a positive electrode active material. By stably maintaining the solvation of 1,2-dimethoxyethane (DME), which is a polar ether, with lithium ions, free DME not solvated with lithium ions can be prevented from decomposing at the positive electrode and the negative electrode, thereby improving not only reduction resistance but also oxidation resistance. Accordingly, the resistance of the battery can be further reduced and the capacity retention rate can be further increased. Furthermore, the electrolyte solution contributes to improvement in energy efficiency.

[0012] [3] The electrolyte solution for a lithium-ion secondary battery according to [1] or [2], wherein the concentration of said lithium bis(fluorosulfonyl)imide is 1.8 to 2.0 mol / L.

[0013] In the electrolyte solution for a lithium-ion secondary battery according to [3], by setting the concentration of the specific electrolyte salt to a further limited range, side reactions between lithium metal and the electrolyte solution can be further suppressed, and corrosion of the positive electrode current collector and elution of transition metals from the positive electrode active material can be further suppressed. Accordingly, the resistance of the battery can be further reduced, and the capacity retention rate can be further increased.

[0014] [4] The electrolyte solution for a lithium-ion secondary battery according to any one of [1] to [3], wherein the molar percentage of said 1,2-dimethoxyethane is 20 to 40 mol%.

[0015] In the electrolyte solution for a lithium-ion secondary battery according to [4], since the molar percentage of the polar ether falls within a specific range, the solubility of the electrolyte salt can be further improved. Accordingly, the resistance of the battery can be further reduced, and the capacity retention rate can be further increased.

[0016] [5] The electrolyte solution for a lithium-ion secondary battery according to any one of [1] to [4], wherein the number of moles of said 1,2-dimethoxyethane is 1.8 to 2.2 times the number of moles of said lithium bis(fluorosulfonyl)imide.

[0017] In the electrolyte solution for a lithium-ion secondary battery according to [5], the molar ratio of the electrolyte salt to the polar ether is controlled within an appropriate range. Accordingly, as a locally highly concentrated electrolyte solution, the conductivity of lithium ions can be improved.

[0018] [6] The electrolyte solution for a lithium-ion secondary battery according to any one of [1] to [5], wherein the molar percentage of said lithium bis(fluorosulfonyl)imide is 10 to 20 mol%, and the molar percentage of said 1,2-dimethoxyethane is 20 to 40 mol%.

[0019] In the electrolyte solution for a lithium-ion secondary battery according to [6], the molar ratio of the electrolyte salt to the polar ether is controlled within an appropriate range. As a locally highly concentrated electrolyte solution, the conductivity of lithium ions can be improved.

[0020] [7] The electrolyte for lithium-ion secondary batteries according to any one of [1] to [6], wherein the molar percentage of 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane is 30 to 70 mol%.

[0021] The electrolyte for lithium-ion secondary batteries according to [7] has the molar percentage of nonpolar ether controlled within an appropriate range. This reduces reactivity with lithium metal and improves the durability of the battery.

[0022] [8] The electrolyte for lithium-ion secondary batteries according to any one of [1] to [7], wherein the molar percentage of 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane is 30 to 35 mol%.

[0023] The electrolyte for lithium-ion secondary batteries according to [8] has the molar percentage of nonpolar ether controlled to a more appropriate range. As a result, reactivity with lithium metal can be further reduced, and the durability of the battery can be further improved.

[0024] [9] The electrolyte for lithium-ion secondary batteries according to any one of [1] to [8], further comprising one or more compounds selected from lithium compounds, monoether compounds and sulfonamide compounds.

[0025] The electrolyte for lithium-ion secondary batteries according to [9] further contains specific compounds as additives. As a result, the characteristics of the battery can be further improved, the resistance of the battery can be further reduced, and the capacity retention rate can be further increased.

[0026]

[10] The electrolyte for lithium-ion secondary battery according to [9], further comprising a lithium compound, wherein the lithium compound is one or more selected from lithium difluoro(oxalato)borate and lithium difluorophosphate.

[0027] The electrolyte for lithium-ion secondary batteries according to

[10] further contains a specific lithium compound as an additive. Therefore, the characteristics of the battery can be further improved, the resistance of the battery can be further reduced, and the capacity retention rate can be further increased.

[0028]

[11] A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte for lithium-ion secondary batteries described in any of [1] to

[10] .

[0029] The lithium-ion secondary battery according to

[11] is equipped with the electrolyte for lithium-ion secondary batteries of the present invention. As a result, the characteristics of the battery can be improved, the resistance of the battery can be reduced, and the capacity retention rate can be increased.

[0030]

[12] The lithium-ion secondary battery according to

[11] , characterized in that the positive electrode contains a lithium nickel cobalt manganese composite oxide, and the content of nickel in the lithium nickel cobalt manganese composite oxide is 0.80 or more when the total molar ratio of nickel, cobalt, and manganese is 1.

[0031] The lithium-ion secondary battery according to

[12] has a positive electrode containing lithium nickel cobalt manganese composite oxide (NCM). NCM is a positive electrode active material that can increase the capacity of the battery. For this reason, a battery containing NCM in the positive electrode can improve the characteristics of the battery, reduce the resistance of the battery, and increase the capacity retention rate.

[0032]

[13] The lithium-ion secondary battery according to

[11] or

[12] , characterized in that the negative electrode is lithium metal.

[0033] In the lithium-ion secondary battery described in

[12] , the negative electrode is lithium metal. Therefore, the electrolyte is required to have durable properties that can withstand high reducing forces, and the effect of improving stability due to the effects of the present invention can be further enhanced.

[0034] The electrolyte for lithium-ion secondary batteries of the present invention can reduce the resistance of the battery and improve its capacity retention rate.

[0035] This is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention.

[0036] Preferred embodiments of the present invention will be described in detail below.

[0037] [Electrolyte for Lithium-Ion Secondary Batteries] The electrolyte for lithium-ion secondary batteries of this embodiment (hereinafter also simply referred to as "electrolyte") comprises an electrolyte salt and an organic solvent. The electrolyte salt is lithium bis(fluorosulfonyl)imide (hereinafter also referred to as "LiFSI"). The organic solvent comprises a polar ether and a nonpolar ether. The polar ether is 1,2-dimethoxyethane (hereinafter also referred to as "DME"). The nonpolar ether is 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane (hereinafter also referred to as "F8DEE").

[0038] The electrolyte of this embodiment, by containing LiFSI, DME, and F8DEE, can improve the solubility of the electrolyte salt and suppress the occurrence of side reactions. As a result, the resistance of the battery can be reduced and the capacity retention rate can be increased.

[0039] LiFSI is a compound having the following structural formula, and is included as an electrolyte salt in the electrolyte solution of this embodiment.

[0040]

[0041] LiFSI is preferable as an electrolyte salt because it exhibits weak cation-anion interactions and high ionic conductivity even at high concentrations.

[0042] The concentration of LiFSI in the electrolyte is preferably 1.0 to 2.0 mol / L, more preferably 1.5 to 2.0 mol / L, and even more preferably 1.8 to 2.0 mol / L. When the LiFSI concentration is above the lower limit, practical performance as a lithium-ion secondary battery can be ensured. When the LiFSI concentration is below the upper limit, the increase in viscosity of the electrolyte can be suppressed, and sufficient impregnation can be ensured.

[0043] The molar percentage of LiFSI in the electrolyte is preferably 10 to 20 mol%, more preferably 15 to 20 mol%, and even more preferably 18 to 20 mol%. When the molar percentage of LiFSI is above the lower limit, practical performance as a lithium-ion secondary battery can be ensured. When the molar percentage of LiFSI is below the upper limit, the increase in viscosity of the electrolyte can be suppressed, and sufficient impregnation can be ensured.

[0044] The electrolytic solution of the present embodiment may contain an electrolyte salt other than LiFSI. Examples of electrolyte salts other than LiFSI include lithium salts commonly used in electrolytic solutions. Examples of such lithium salts include LiCl, LiBr, LiI, LiClO ４ , LiBF ４ , LiB １０ Cl １０ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiC ４ BO ８ , LiCF ３ CO ２ , LiCH ３ SO ３ , LiCF ３ SO ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , (CF ３ SO ２ ) ２ NLi, chloroborane lithium, lithium lower aliphatic carboxylate, lithium tetraphenylborate, lithium imide, and the like.

[0045] The content of the electrolyte salt other than LiFSI is preferably, for example, 0 to 3% by mass relative to the total mass of the electrolytic solution. When the content of the electrolyte salt other than LiFSI is within the above numerical range, the cycle life performance of a lithium ion secondary battery can be ensured.

[0046] DME is a compound having the following structural formula, and is contained as a polar ether in the electrolytic solution of the present embodiment.

[0047]

[0048] The electrolyte of this embodiment can improve the solubility of LiFSI by including DME. The molar percentage of DME in the electrolyte is preferably 20 to 40 mol%, more preferably 30 to 40 mol%, and even more preferably 35 to 40 mol%. If the molar percentage of DME is above the lower limit, LiFSI can be sufficiently dissolved. If the molar percentage of DME is below the upper limit, a sufficient concentration of LiFSI can be ensured.

[0049] The ratio of moles of DME to moles of LiFSI is preferably 1.8 to 2.2 times, more preferably 1.9 to 2.1 times, and most preferably 2.0 times. When the ratio of moles of DME to moles of LiFSI is within the above range, the solvation of DME, which is a polar ether, with lithium ions can be stably maintained. Therefore, it is possible to prevent the decomposition of free DME that is not solvated with lithium ions at the positive or negative electrode, thereby improving not only the reduction resistance but also the oxidation resistance. As a result, the resistance of the battery can be further reduced and the capacity retention rate can be further increased.

[0050] F8DEE is a compound having the following structural formula, and is included as a nonpolar ether in the electrolyte of this embodiment.

[0051]

[0052] F8DEE is a fluorinated diether represented as FxDEE (where x is the number of fluorine atoms), and as shown in the structural formula above, it has a nonpolar structure because the fluorine atoms are attached in symmetrical positions. F8DEE does not particularly affect the solvation structure between lithium ions and DME, and has low reactivity with lithium metal, thus extending the battery's lifespan. Furthermore, F8DEE suppresses side reactions between lithium metal and the electrolyte, improving the battery's lifespan characteristics, and reduces the viscosity of the electrolyte, improving its impregnation into electrodes and separators, thereby improving the conductivity of lithium ions.

[0053] The molar percentage of F8DEE in the electrolyte is preferably 30 to 70 mol%, more preferably 30 to 55 mol%, even more preferably 30 to 40 mol%, and particularly preferably 30 to 35 mol%. If the molar percentage of F8DEE is above the lower limit, improved battery life characteristics can be ensured. If the molar percentage of F8DEE is below the upper limit, the stability of the electrolyte can be improved.

[0054] In addition to the LiFSI, DME, and F8DEE mentioned above, the electrolyte of this embodiment may also contain a monoether compound as a chain ether. Furthermore, the electrolyte may further contain an amide compound as an organic additive.

[0055] In the aforementioned localized high-concentration electrolyte, DME stably forms solvation with lithium ions, making it difficult to further dissolve lithium salt compounds as inorganic additives to the electrolyte. However, among these, lithium compounds that are soluble and have an electrode protective effect include lithium difluoro(oxalato)borate (hereinafter also referred to as "LiFOB") and lithium difluorophosphate (hereinafter also referred to as "LiDFP"). Among these lithium compounds, LiFOB and LiDFP are preferred because they can be dissolved in the localized high-concentration electrolyte and can further enhance the effects of the present invention with a small amount of addition.

[0056] Examples of monoether compounds include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (hereinafter also referred to as "TTE"), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and the like.

[0057] Examples of amide compounds include sulfonamide compounds. Examples of sulfonamide compounds include methanesulfonamide, ethanesulfonamide, N,N-dimethyltrifluoromethanesulfonamide, and N,N-dimethylsulfamoylfluoride.

[0058] If the electrolyte of this embodiment contains an additive, the additive content is preferably 0.1 to 1% by mass relative to the total mass of the electrolyte.

[0059] [Method for Manufacturing the Electrolyte] The method for manufacturing the electrolyte of this embodiment is not particularly limited and can be manufactured by weighing appropriate amounts of each component and mixing them, just like a normal electrolyte. For example, the electrolyte of this embodiment can be obtained by mixing LiFSI and DME in an appropriate ratio, adding an appropriate amount of F8DEE, and adding an appropriate amount of additive as needed and mixing thoroughly.

[0060] The temperature during the production of the electrolyte is not particularly limited and can be, for example, 5 to 30°C. The atmospheric pressure during the production of the electrolyte is not particularly limited and can be, for example, normal pressure (0.1 MPa). The dew point during the production of the electrolyte is preferably, for example, -76°C or lower at atmospheric pressure. The lower limit of the dew point is not particularly limited, but for example, -196°C at atmospheric pressure. The oxygen concentration during the production of the electrolyte is preferably, for example, 1 ppm or lower by mass. The lower limit of the oxygen concentration is preferably 0.1 ppm by mass.

[0061] [Lithium-ion secondary battery] The lithium-ion secondary battery of this embodiment (hereinafter also simply referred to as "battery") comprises a positive electrode, a negative electrode, a separator, and the electrolyte for the lithium-ion secondary battery of this embodiment described above. The battery of this embodiment may include other battery elements as needed.

[0062] The battery of this embodiment can use the same battery elements as known batteries, except that the electrolyte contains the above-mentioned LiFSI, DME, and F8DEE. The battery of this embodiment may be in any configuration, such as coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the battery of this embodiment can be applied to a wide range of applications, such as mobile devices like mobile phones and laptop computers, and in-vehicle applications.

[0063] The following describes the battery of this embodiment, specifically a battery using the electrolyte of this embodiment (a coin-type lithium-ion secondary battery).

[0064] As shown in Figure 1, the battery 1 of this embodiment comprises a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0065] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 form the outer shape of the battery 1. Between the positive electrode can 10 and the negative electrode can 20, the positive electrode 2 and the negative electrode 3 are provided via the separator 4, which is impregnated with electrolyte. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0066] Positive electrode 2 includes a positive electrode active material. The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in ordinary lithium-ion secondary batteries. Examples of positive electrode active materials include cobalt-based composite oxides, nickel-based composite oxides, manganese-based composite oxides, iron-based composite oxides, vanadium-based composite oxides, and the like.

[0067] Cobalt-based composite oxides include LiCoO ２ Examples include LiNiO ２ Examples include LiMnO ２ Examples include LiCo ｘ Ni １－ｘ O ２ CoNi composite oxides represented by (0 < x < 1), and LiCo ｘ Mn １－ｘ O ２ CoMn composite oxides represented by (0 < x < 1), and LiNi ｘ Mn １－ｘ O ２ (0<x<1), LiNi ｘ Mn ２－ｘ O ４ NiMn composite oxides represented by (0 < x < 2), and LiNi １－ｘ－ｙ Co ｘ Mn ｙ O ２ The composite oxides may also be NiCoMn oxides represented by (0 < x < 1, 0 < y < 1, 0 < x + y < 1). These lithium-containing composite oxides may have some of the metal elements such as Co, Ni, and Mn replaced by one or more metal elements such as Mg, Al, Zr, Ti, and Cr.

[0068] Furthermore, examples of iron-based composite oxides include LiFeO ２ LiFePO ４ Examples include, for example, V ２ O ５ Examples are given.

[0069] As a positive electrode active material, among the above-mentioned composite oxides, nickel-based composite oxides or cobalt-based composite oxides are preferred because they can achieve high capacity. In particular, lithium nickel cobalt manganese composite oxide (NCM) is preferred, and NCM in which the nickel content is 0.80 or more when the total molar ratio of nickel, cobalt, and manganese is 1 is more preferred. An example of an NCM with a nickel content of 0.80 or more is NCM811 (LiNi ０．８０ Co ０．１０ Mn ０．１０ O ２ ), LiNi ０．８４ Co ０．０９ Mn ０．０７ O ２ These are some examples.

[0070] The positive electrode 2 can be manufactured by preparing a positive electrode composite material by mixing a conductive agent, binder, etc., with the positive electrode active material as needed, and then pressing this composite material onto a current collector (not shown). Preferably, stainless steel mesh, aluminum foil, etc., can be used as the current collector. Preferably, carbon nanotubes (CNTs), acetylene black, ketchen black, etc., can be used as the conductive agent. Preferably, tetrafluoroethylene, polyvinylidene fluoride, etc., can be used as the binder.

[0071] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode mixture is not particularly limited. The content of the positive electrode active material in the positive electrode mixture is preferably 75 to 100% by mass, and more preferably 90 to 99% by mass. The content of the conductive agent in the positive electrode mixture is preferably 0.1 to 15% by mass, and more preferably 0.1 to 5% by mass. The content of the binder in the positive electrode mixture is preferably 0.1 to 10% by mass, and more preferably 0.1 to 5% by mass.

[0072] In battery 1, the negative electrode 3 relative to the positive electrode 2 can be any known material that functions as a negative electrode active material and is capable of intercalating and releasing lithium, such as metallic materials like lithium metal and lithium alloys, carbon-based materials like graphite and MCMB (mesocarbon microbeads), or silicon-based materials like silicon (Si), Si alloys, and silicon oxide. Among these, lithium metal is preferred as the negative electrode 3.

[0073] The separator 4 is impregnated with the electrolyte of this embodiment. The separator 4 separates the positive electrode 2 and the negative electrode 3 and provides a passage for lithium ions to move. Any separator 4 commonly used in lithium batteries can be used. In other words, any separator 4 with low resistance to ion movement of the electrolyte and excellent electrolyte impregnation ability can be used. Examples of separator 4 include glass fiber, polyester, Teflon®, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), or combinations thereof. The separator 4 may be in the form of a nonwoven or woven fabric.

[0074] As the separator 4, for example, a separation membrane may be used that includes a coating layer containing a metal oxide or polymer substance such as alumina to ensure heat resistance or mechanical strength, on a microporous membrane of a polyolefin polymer such as polyethylene or polypropylene. Such a separation membrane can be used in a single-layer or multi-layer structure. Examples of coating agents included in the coating layer include alumina, conductive particles (carbon, carbon nanotubes (CNTs)), and binder resins (styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF)). As the separator 4, a separation membrane in which a polyethylene microporous membrane is coated with alumina is preferred because it has excellent electrolyte impregnation properties.

[0075] The battery container (positive electrode container 10, negative electrode container 20) can employ known battery components.

[0076] The method for manufacturing the battery 1 of this embodiment is not particularly limited. For example, an electrode assembly can be manufactured by sequentially stacking a positive electrode 2, a separator 4, and a negative electrode 3, placing this assembly into a battery container (positive electrode container 10, negative electrode container 20), injecting the electrolyte of this embodiment into the separator 4, and sealing it with a cap plate and gasket.

[0077] The battery 1 of this embodiment includes a separator 4 impregnated with the electrolyte of this embodiment, thereby reducing the battery's resistance and improving its capacity retention rate.

[0078] The present invention is not limited to the embodiments described above, and any modifications or improvements that can achieve the objectives of the present invention are included within the scope of the present invention.

[0079] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0080] [Preparation of Electrolyte] <Example 1> 1.7 g of LiFSI was placed in a 50 mL polypropylene (PP) container and dissolved in 1.6 g of DME. Then, 16.7 g of F8DEE as FxDEE, 0.1 g of LiFOB as additive 1, and 0.04 g of LiDFP as additive 2 were placed in the flask and stirred at room temperature for 60 minutes to prepare the electrolyte of Example 1. The moisture content of the prepared electrolyte was measured by the Karl Fischer method and was 20 ppm (by mass) or less in all cases.

[0081] <Examples 2-7, Comparative Examples 1-7> The electrolytes for Examples 2-7 and Comparative Examples 1-7 were prepared in the same manner as in Example 1, except that LiFSI, DME, LiFOB, LiDFP, TTE, and sulfonamide were placed in flasks in the amounts listed in Tables 1-2 using the FxDEE listed in Tables 1-2. A "-" in Tables 1-2 indicates that the component is not present.

[0082]

[0083]

[0084] [Preparation of test cell] (Positive electrode) 2% by mass of acetylene black (AB) as an electronically conductive material, 1.5% by mass of polyvinylidene fluoride (PVDF) as a binder, and polyvinylpyrrolidone (PVP) as a dispersant were pre-mixed with N-methyl-2-pyrrolidone (NMP) as a dispersion solvent, and wet-mixed in a rotary-orbit mixer to obtain a pre-mixed slurry. Subsequently, LiNi was used as the positive electrode active material. ０．８ Co ０．１ Mn ０．１ O ２ (NCM811) and the obtained pre-mixed slurry were mixed and dispersed using a planetary mixer to obtain a positive electrode paste. NCM811 consists of single particles, with a median diameter of 4 μm. The capacity per unit mass was 197–205 mAh / g. Next, the obtained positive electrode paste was applied to an aluminum positive electrode current collector without a primer layer, dried, and pressed with a roll press, resulting in a positive electrode composite layer with a thickness of 64 μm and a density of 3.3–3.5 g / cm³. ３ A positive electrode was obtained. Subsequently, it was dried in a vacuum at 120°C to form a positive electrode plate with a positive electrode composite layer. The obtained positive electrode plate was punched out to a size of 30 mm x 40 mm to serve as the positive electrode.

[0085] (Negative electrode) For the negative electrode, a cladding material consisting of a 10 μm thick copper foil and a 20 μm thick lithium foil was used. The negative electrode was punched out to an electrode area of ​​34 mm x 44 mm.

[0086] (Separator) As a separator, an alumina-coated polyethylene microporous membrane was used, with the alumina-coated side facing the positive electrode.

[0087] (Fabrication of Lithium-ion Secondary Batteries) A ​​container made by heat-sealing aluminum laminate for secondary batteries (manufactured by Dai Nippon Printing Co., Ltd.) into a bag shape was prepared, and after introducing the positive electrode, separator, and negative electrode into the container, 350 μl of the electrolyte for each example was poured in and the container was sealed under reduced pressure. After that, it was left at 45°C for 5 hours and then restrained with a cushioned metal plate to achieve a holding pressure of 1 MPa to prepare a test cell. A lithium-ion secondary battery of approximately 50 mAh was prepared by performing two charge-discharge cycles at 0.1C (4.3V to 2.65V).

[0088] [Capacity Retention Rate Measurement] The capacity retention rate was measured using the lithium-ion secondary battery described above. CCCV charging was performed at 0.33C (1 / 3C) to 4.3V at 25°C, followed by CV charging for 20 minutes. After standing for 10 minutes, the battery was discharged at 0.33C to 2.65V. This was considered 100% of the battery capacity. Subsequently, 99 or 198 charge-discharge cycles were performed in a constant temperature bath at 25°C with an upper voltage limit of 4.3V, charging at 0.33C, a lower voltage limit of 2.65V, and discharging at 0.33C, ranging from 4.3V to 2.65V. After the discharge was completed and the battery was left for 6 hours, the rated capacity at 1 / 3C was measured at the 100th or 200th cycle, and the capacity retention rate (%) was calculated. The results are shown in Table 3. Note that the battery in Comparative Example 7 could not be charged, so the capacity retention rate could not be calculated.

[0089] [BOL Resistivity Measurement] The BOL (Beginning Of Life) resistivity was measured using the lithium-ion secondary battery described above. Specifically, the battery was charged to 50% of its discharge capacity at the time of initial capacity measurement, resulting in a SOC of 50%. A 4.5C discharge was performed for 10 seconds using this voltage, and the resistance value was calculated. This resistance value was measured over a positive electrode area of ​​12 cm². ２ Divide by and obtain the resistivity (Ω·cm) ２ The resistivity was calculated. The results are shown in Table 3. Note that the battery in Comparative Example 7 could not be charged, so its resistivity could not be measured.

[0090]

[0091] As shown in Table 3, Examples 1 to 7, which applied the electrolyte of the present invention, achieved a higher capacity retention rate compared to Comparative Examples 1 to 7. The capacity retention rate after 200 cycles was 80% or higher in Examples 1 to 7, which was excellent. Furthermore, the BOL resistivity in Examples 1 to 7 was 25 Ω·cm. ２The results were as follows, and it was confirmed that the battery resistance was reduced compared to Comparative Examples 1 to 7. In particular, in Examples 5 to 7, an improvement in capacity retention and a reduction in resistance were confirmed. This is thought to be due to improved resistance to reduction and oxidation by the solvation structure formed by the combination of electrolyte salt, polar ether, and specific nonpolar ether. Furthermore, it is thought that by finding an amount of additive that can be sufficiently dissolved in a locally high-concentration electrolyte, even a small amount of additive was used, and the synergistic effect of suppressing side reactions by forming a stable film on the electrode particularly suppressed the increase in the resistance component, thereby improving battery performance.

[0092] 1...Lithium-ion secondary battery (battery) 2...Positive electrode 3...Negative electrode 4...Separator 5...Insulating packing (gasket) 10...Positive electrode can 20...Negative electrode can (negative electrode terminal)

Claims

1. An electrolyte for lithium-ion secondary batteries, comprising an electrolyte salt and an organic solvent, wherein the electrolyte salt is lithium bis(fluorosulfonyl)imide, the organic solvent comprises a polar ether and a nonpolar ether, the polar ether is 1,2-dimethoxyethane, and the nonpolar ether is 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane.

2. The electrolyte for a lithium-ion secondary battery according to claim 1, wherein the concentration of the lithium bis(fluorosulfonyl)imide is 1.0 to 2.0 mol / L.

3. The electrolyte for a lithium-ion secondary battery according to claim 1, wherein the concentration of the lithium bis(fluorosulfonyl)imide is 1.8 to 2.0 mol / L.

4. The electrolyte for lithium-ion secondary battery according to claim 1, wherein the molar percentage of 1,2-dimethoxyethane is 20 to 40 mol%.

5. The electrolyte for a lithium-ion secondary battery according to claim 1, wherein the number of moles of 1,2-dimethoxyethane is 1.8 to 2.2 times the number of moles of lithium bis(fluorosulfonyl)imide.

6. The electrolyte for a lithium-ion secondary battery according to claim 1, wherein the molar percentage of lithium bis(fluorosulfonyl)imide is 10 to 20 mol%, and the molar percentage of 1,2-dimethoxyethane is 20 to 40 mol%.

7. The electrolyte for a lithium-ion secondary battery according to claim 1, wherein the molar percentage of 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane is 30 to 70 mol%.

8. The electrolyte for lithium-ion secondary battery according to claim 1, wherein the molar percentage of 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane is 30 to 35 mol%.

9. The electrolyte for a lithium-ion secondary battery according to claim 1, further comprising one or more compounds selected from lithium compounds, monoether compounds, and sulfonamide compounds.

10. The electrolyte for a lithium-ion secondary battery according to claim 9, further comprising a lithium compound, wherein the lithium compound is one or more selected from lithium difluoro(oxalato)borate and lithium difluorophosphate.

11. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte for a lithium-ion secondary battery according to any one of claims 1 to 10.

12. The lithium-ion secondary battery according to claim 11, characterized in that the positive electrode contains a lithium nickel cobalt manganese composite oxide, and the content of nickel in the lithium nickel cobalt manganese composite oxide is 0.80 or more when the total molar ratio of nickel, cobalt, and manganese is 1.

13. The lithium-ion secondary battery according to claim 11, characterized in that the negative electrode is lithium metal.

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