Nonaqueous electrolyte for lithium-ion secondary battery for hybrid vehicle driving power source, and lithium-ion secondary battery for hybrid vehicle driving power source including the same

Abstract

To provide a non-aqueous electrolyte capable of reducing initial output resistance of a lithium ion secondary battery for HEV drive power supply and suppressing an increase in output resistance when the lithium ion secondary battery is in a storage state.SOLUTION: A non-aqueous electrolyte disclosed herein is a non-aqueous electrolyte for a lithium ion secondary battery for hybrid automobile drive power supply and contains a non-aqueous solvent, LiPF6, a lithium bis-(oxalate-)borate and difluorophosphate lithium. The non-aqueous solvent contains carbonates and methyl acetate. Conductivity of the non-aqueous solvent is 24.0 or more. Conductivity of the non-aqueous electrolyte at 25°C is 9.2 S / cm or more. Viscosity of the non-aqueous electrolyte at 25°C is 3.5 mPa s or less. In the non-aqueous electrolyte, a value of a concentration (mol / L) of {the conductivity (S / cm)×the viscosity (mPa s)} / LiPF6 is 26.0 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte for a lithium ion secondary battery for use as a driving power source for a hybrid vehicle, and also to a lithium ion secondary battery for use as a driving power source for a hybrid vehicle that includes the non-aqueous electrolyte. [Background technology]

[0002] In recent years, lithium ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as vehicle drive power sources for electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0003] As components of the non-aqueous electrolyte of a lithium ion secondary battery, carbonates are often used as the non-aqueous solvent and LiPF6 is often used as the electrolyte salt (see, for example, Patent Document 1). Patent Document 1 also describes the use of methyl acetate as an additional non-aqueous solvent and lithium bis(oxalato)borate as an additive. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 149539 Summary of the Invention [Problem to be solved by the invention]

[0005] Currently, the required characteristics of lithium-ion secondary batteries used as vehicle power sources vary depending on the type of vehicle. Specifically, while high capacity is primarily required for lithium-ion secondary batteries used as BEV power sources, high output is primarily required for lithium-ion secondary batteries used as HEV power sources. Therefore, lithium-ion secondary batteries used as vehicle power sources must be developed according to the type of vehicle, and their nonaqueous electrolytes must also be developed according to the type of vehicle. For HEV power source applications, there is a need for the development of nonaqueous electrolytes that can provide lithium-ion secondary batteries with high output over a long period of time.

[0006] Therefore, an object of the present invention is to provide a nonaqueous electrolyte solution that can reduce the initial output resistance of a lithium ion secondary battery for use as a power source for driving an HEV, and that can suppress an increase in output resistance during storage of the lithium ion secondary battery. [Means for solving the problem]

[0007] The nonaqueous electrolyte disclosed herein is a nonaqueous electrolyte for a lithium-ion secondary battery used as a driving power source for a hybrid vehicle, and contains a nonaqueous solvent, LiPF6, lithium bis(oxalato)borate, and lithium difluorophosphate. The nonaqueous solvent contains carbonates and methyl acetate. The dielectric constant of the nonaqueous solvent is 24.0 or higher. The electrical conductivity of the nonaqueous electrolyte at 25°C is 9.2. m The viscosity of the non-aqueous electrolyte at 25°C is 3.5 mPa·s or less. In the non-aqueous electrolyte, the conductivity ( m The value of {(mPa·s / cm)×(same viscosity (mPa·s)} / LiPF6 concentration (mol / L) is 26.0 or more.

[0008] According to this configuration, it is possible to provide a nonaqueous electrolyte solution that can reduce the initial output resistance of a lithium ion secondary battery for use as a power source for driving an HEV, and that can suppress an increase in output resistance during storage of the lithium ion secondary battery. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically showing the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described. It should be noted that matters not mentioned in this specification but necessary for carrying out the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. It should be noted that a numerical range expressed as "A to B" in this specification includes A and B.

[0011] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between positive and negative electrodes. Furthermore, in this specification, the term "hybrid vehicle" refers to a vehicle equipped with an internal combustion engine such as a gasoline engine and a motor operated by a secondary battery as power sources.

[0012] The nonaqueous electrolyte according to this embodiment is a nonaqueous electrolyte for a lithium ion secondary battery for use as a power source for driving an HEV. The nonaqueous electrolyte according to this embodiment contains a nonaqueous solvent, LiPF6, lithium bis(oxalato)borate, and lithium difluorophosphate. The nonaqueous solvent contains carbonates and methyl acetate. The dielectric constant of the nonaqueous solvent is 24.0 or higher. The conductivity of the nonaqueous electrolyte according to this embodiment at 25°C is 9.2 m The viscosity of the non-aqueous electrolyte according to this embodiment at 25°C is 3.5 mPa·s or less. In the non-aqueous electrolyte according to this embodiment, the conductivity ( mThe value of {(mPa·s / cm)×viscosity (mPa·s)} / LiPF6 concentration (mol / L) is 26.0 or more. First, each component contained in the nonaqueous electrolyte solution according to this embodiment will be described.

[0013] [Non-aqueous solvent] The non-aqueous solvent used in the non-aqueous electrolyte according to this embodiment contains at least a carbonate and methyl acetate. Examples of carbonates include chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate; and cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These may be used alone or in combination of two or more.

[0014] As the carbonates, it is preferable to use a combination of a chain carbonate and a cyclic carbonate. As the carbonates, a combination of EC with DMC and / or EMC is more preferable, and a combination of EC, DMC, and EMC is even more preferable. The carbonates may contain only EC, DMC, and EMC.

[0015] The non-aqueous solvent may contain only carbonates and methyl acetate, or may further contain an organic solvent other than carbonates and methyl acetate, as long as the effects of the present invention are achieved.

[0016] The dielectric constant of the non-aqueous solvent is 24.0 or higher. If the dielectric constant of the non-aqueous solvent is less than 24.0, the output resistance of the lithium-ion secondary battery will increase significantly when stored for a long period of time. The dielectric constant of the non-aqueous solvent is preferably 24.9 or higher, more preferably 25.0 or higher. The dielectric constant of the non-aqueous solvent may be 33.0 or lower, 30.0 or lower, 28.0 or lower, or 27.0 or lower. The dielectric constant of the non-aqueous solvent can be measured according to a known method. For example, the dielectric constant of the non-aqueous solvent can be determined based on the results of measuring the capacitance (electrostatic capacity) of the non-aqueous solvent using a commercially available impedance analyzer.

[0017] [LiPF6] LiPF6 functions as an electrolyte salt (in other words, a supporting salt). The concentration of LiPF6 in the non-aqueous electrolyte is, for example, 0.7 mol / L to 1.3 mol / L, preferably 1.0 mol / L to 1.2 mol / L, and more preferably 1.05 mol / L to 1.15 mol / L.

[0018] [Lithium bis(oxalato)borate and lithium difluorophosphate] Lithium bis(oxalato)borate (LiBOB) and lithium difluorophosphate (LiPO2F2) function as film-forming agents. The nonaqueous electrolyte solution according to this embodiment contains both LiBOB and LiPO2F2, which allows high-quality films to be formed on the positive and negative electrodes of the lithium-ion secondary battery, thereby significantly suppressing an increase in output resistance when the lithium-ion secondary battery is stored for a long period of time.

[0019] The higher the concentration of LiBOB in the non-aqueous electrolyte, the greater the effect of suppressing an increase in output resistance during storage, but the greater the tendency for the initial output resistance to increase. Therefore, the concentration of LiBOB in the non-aqueous electrolyte is preferably 0.1% by mass to 1.0% by mass, more preferably 0.2% by mass to 0.8% by mass, and even more preferably 0.4% by mass to 0.6% by mass.

[0020] The concentration of LiPO2F2 in the non-aqueous electrolyte is preferably 0.1 mass % to 1.0 mass %, more preferably 0.4 mass % to 0.9 mass %, and even more preferably 0.5 mass % to 0.7 mass %, because this particularly enhances the function as a film-forming agent.

[0021] The ratio of the concentration (mass %) of LiBOB to the concentration (mass %) of LiPO2F2 (LiBOB / LiPO2F2) is preferably 0.5 to 1.5, and more preferably 0.75 to 1.25.

[0022] [Other ingredients] The nonaqueous electrolyte according to this embodiment may contain other components than those described above, as long as the effects of the present invention are achieved. Examples of such components include additives such as gas generating agents (e.g., biphenyl (BP) and cyclohexylbenzene (CHB)) and thickeners.

[0023] Next, the physical properties of the nonaqueous electrolyte according to this embodiment will be described. These physical properties are important for reducing the initial output resistance of a lithium ion secondary battery for use as a driving power source for an HEV and for suppressing an increase in output resistance during storage.

[0024] 〔conductivity〕 The electrical conductivity (hereinafter also referred to as "electrical conductivity (C)") of the non-aqueous electrolyte according to this embodiment at 25°C is 9.2 m S / cm or more. Conductivity (C) is 9.2 m If the conductivity is less than 10.0 S / cm, the initial output resistance of the lithium ion secondary battery increases. In other words, the initial output of the lithium ion secondary battery decreases. m S / cm or more, more preferably 10.5 m S / cm or more, and more preferably 11.0 m S / cm or more. The conductivity (C) is 16.0 m S / cm or less, 14.0 m The conductivity (C) may be 0.05 S / cm or less. The conductivity (C) can be determined according to a known method. For example, the conductivity (C) can be measured at 25°C using a commercially available conductivity meter.

[0025] 〔viscosity〕 In this specification, the "viscosity" of the non-aqueous electrolyte refers to the "shear viscosity (mPa·s)." The viscosity of the non-aqueous electrolyte according to this embodiment at 25°C (hereinafter also referred to as "viscosity (V)") is 3.5 mPa·s or less. If the viscosity (V) exceeds 3.5 mPa·s, the initial output resistance of the lithium-ion secondary battery increases. The viscosity (V) is preferably 3.1 mPa·s or less, and more preferably 3.0 mPa·s or less. The viscosity (V) may be 2.4 mPa·s or more, or may be 2.6 mPa·s or more. The viscosity (V) can be determined according to a known method. For example, the viscosity (V) can be measured at 25°C using a commercially available rotational viscometer (particularly a B-type viscometer).

[0026] [(Conductivity x viscosity) / LiPF6 concentration] In the nonaqueous electrolyte according to this embodiment, the value (hereinafter also referred to as "value (D)") expressed by the formula: {conductivity (C) (S / cm) × viscosity (V) (mPa·s)} / LiPF concentration in the nonaqueous electrolyte (mol / L)} is 26.0 or higher. This value (D) is an index of the degree of dissociation of the electrolyte salt LiPF. A higher conductivity (C) and a lower viscosity (V) are advantageous for initial output resistance. However, if the product of these values ​​is too small, the value (D) becomes too small, resulting in a significant increase in output resistance during long-term storage of the lithium-ion secondary battery. Therefore, by combining the specified conductivity (C), viscosity (V), and value (D), it is possible to reduce the initial output resistance of a lithium-ion secondary battery for use as a driving power source for an HEV and suppress the increase in output resistance during storage. The value (D) is preferably 28.5 or higher, more preferably 29.0 or higher, and even more preferably 30.0 or higher. The value (D) may be 35.0 or less, 33.0 or less, or 32.0 or less.

[0027] In the non-aqueous electrolyte according to this embodiment, the dielectric constant of the non-aqueous solvent is 24.9 or more, and the electrical conductivity (C) is 10.5 mIn the non-aqueous electrolyte solution according to this embodiment, the dielectric constant of the non-aqueous solvent is preferably 25.0 or more and 27.0 or less, and the conductivity (C) is preferably 11.0 or more. m S / cm or more 14.0 m It is more preferable that the viscosity (V) is 2.6 mPa·s or more and 3.0 mPa·s or less, and the value (D) is 30.0 or more and 32.0 or less.

[0028] The physical properties of these non-aqueous electrolytes change at different rates depending on the type of organic solvent used as the non-aqueous solvent. Therefore, to obtain a non-aqueous electrolyte that satisfies the above physical properties, a method can be adopted in which several types of non-aqueous electrolytes with different compositions are actually prepared, and the change in each physical property and the degree of change caused by the organic solvent used are first determined. Based on this, the volume ratio of each organic solvent is adjusted while taking into account the effects of LiPF6, LiBOB, and LiPO2F2. This method does not require excessive trial and error, and a non-aqueous electrolyte that satisfies the above physical properties can be obtained through a limited number of experiments.

[0029] As an example, a case where the non-aqueous solvent contains EC, DMC, and EMC will be described. When the volume fraction of EC is large, the conductivity (C) of the non-aqueous electrolyte decreases, the viscosity (V) of the non-aqueous electrolyte increases, and the dielectric constant of the non-aqueous solvent tends to increase. On the other hand, when the volume fraction of MA is large, the conductivity (C) of the non-aqueous electrolyte increases, the viscosity (V) of the non-aqueous electrolyte decreases, and the dielectric constant of the non-aqueous solvent tends to increase.

[0030] Therefore, the volume percentage of EC in the non-aqueous solvent is preferably 22 to 32% by volume, more preferably 24 to 29% by volume, and even more preferably 24 to 27% by volume. The volume percentage of DMC in the non-aqueous solvent is preferably 5 to 45% by volume, more preferably 10 to 40% by volume, even more preferably 30 to 36% by volume, and particularly preferably 30 to 33% by volume. The volume percentage of EMC in the non-aqueous solvent is preferably 25 to 40% by volume, more preferably 30 to 38% by volume, and even more preferably 32 to 36% by volume. The volume percentage of MA in the non-aqueous solvent is preferably 1 to 30% by volume, more preferably 1 to 10% by volume, even more preferably 3 to 10% by volume, and particularly preferably 5 to 9% by volume.

[0031] The non-aqueous solvent preferably contains 22 to 32 volume % EC, 5 to 45 volume % DMC, 25 to 40 volume % EMC, and 1 to 30 volume % MA, and more preferably contains 24 to 27 volume % EC, 30 to 33 volume % DMC, 32 to 36 volume % EMC, and 5 to 9 volume % MA.

[0032] The volume ratio of EC to DMC (EC / DMC) is preferably 0.5 to 3.0, more preferably 0.75 to 1.00, and even more preferably 0.76 to 0.80. The volume ratio of MA to DMC (MA / DMC) is preferably 0.02 to 3.0, more preferably 0.07 to 0.50, and even more preferably 0.13 to 0.30.

[0033] The nonaqueous electrolyte solution according to the present embodiment configured as described above can reduce the initial output resistance of the lithium ion secondary battery and can suppress an increase in output resistance when the lithium ion secondary battery is stored for a long period of time. Therefore, the nonaqueous electrolyte solution according to the present embodiment can provide the lithium ion secondary battery with high output power for a long period of time. This is a characteristic particularly required for lithium ion secondary batteries used as driving power sources for HEVs.

[0034] The non-aqueous electrolyte solution according to this embodiment can be used in a lithium ion secondary battery for use as a driving power source for an HEV according to a known method.

[0035] From another perspective, the lithium ion secondary battery according to this embodiment is for use as a power source for driving an HEV, and includes a positive electrode, a negative electrode, and the nonaqueous electrolyte solution described above.

[0036] As a structural example of the lithium ion secondary battery according to this embodiment, a lithium ion secondary battery including the above-described nonaqueous electrolyte solution will be described below with reference to the drawings. The lithium ion secondary battery described below is merely an example and does not limit the lithium ion secondary battery according to this embodiment. In the following drawings, members and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

[0037] The lithium-ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte 80 in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 also has an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0038] As shown in Figures 1 and 2, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. The positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0039] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0040] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0041] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a known positive electrode active material used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0042] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0043] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0044] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0045] As the positive electrode active material, a lithium nickel cobalt manganese based composite oxide is particularly preferable.

[0046] The average particle diameter (median diameter: D50) of the positive electrode active material is not particularly limited, but is preferably 0.05 μm or more and 7 μm or less, and more preferably 1 μm or more and 7 μm or less. The average particle diameter (D50) of the positive electrode active material can be determined, for example, by a laser diffraction scattering method. The BET specific surface area of ​​the positive electrode active material is preferably 1.0 m 2 / g or more.

[0047] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVDF).

[0048] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass to 97% by mass, and even more preferably 85% by mass to 96% by mass. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 2% by mass to 12% by mass. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 3% by mass to 13% by mass. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 1.5% by mass to 10% by mass.

[0049] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0050] The weight of the positive electrode active material layer 54 is preferably 6 g / cm in total on both sides of the positive electrode current collector 52. 2 More than 15g / cm 2 The following is the result.

[0051] The negative electrode current collector 62 constituting the negative electrode sheet 60 may be a known negative electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0052] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0053] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0054] The average particle diameter (median diameter: D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm, and more preferably 5 μm to 20 μm. The average particle diameter (D50) of the negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0055] The negative electrode active material layer 64 may contain components other than the active material, such as a binder or a thickener. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0056] The content of the negative electrode active material in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer 64 is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0057] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0058] The capacity ratio of the negative electrode 60 to the positive electrode 50 (negative electrode / positive electrode) is preferably 1.2 to 2.0.

[0059] The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0060] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability of the separator 70 measured by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0061] The nonaqueous electrolyte solution according to the present embodiment described above is used as the nonaqueous electrolyte solution 80. Note that Fig. 1 does not strictly show the amount of nonaqueous electrolyte solution 80 injected into the battery case 30.

[0062] The lithium ion secondary battery 100 configured as described above has a high output, and maintains this high output for a long period of time.

[0063] The lithium ion secondary battery 100 is a lithium ion secondary battery for use as a power source for driving an HEV. The lithium ion secondary battery 100 can be used as a power source for driving an HEV in accordance with a known method. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0064] As an example, a rectangular lithium ion secondary battery 100 including a flat wound electrode assembly 20 has been described. However, the lithium ion secondary battery disclosed herein can also be configured as a lithium ion secondary battery including a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). The lithium ion secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, etc.

[0065] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0066] <Preparation of non-aqueous electrolyte solutions for each example and comparative example> Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and methyl acetate (MA) were prepared as organic solvents. These were mixed in the volume ratios (volume %) shown in Table 1 to prepare nonaqueous solvents. LiPF6 was dissolved in the prepared nonaqueous solvent at a concentration of 1.1 mol / L (1.1 M), and LiPO2F2 and lithium bis(oxalato)borate were also dissolved in the concentrations (mass %) shown in Table 1. In this manner, nonaqueous electrolytes for each example and comparative example were prepared.

[0067] <Dielectric constant measurement of non-aqueous solvents> The capacitance of the non-aqueous solvents used in each example and comparative example was measured at 25° C. using a commercially available impedance analyzer, and the dielectric constant was calculated based on the results. The results are shown in Table 1.

[0068] <Conductivity measurement of non-aqueous electrolyte> In a glove box under an argon atmosphere, the conductivity ( m The results are shown in Table 1.

[0069] <Viscosity measurement of non-aqueous electrolyte> The viscosity (mPa·s) of the nonaqueous electrolyte solution of each example and comparative example was measured using a commercially available Brookfield viscometer at 25° C. in a glove box under an argon atmosphere. The results are shown in Table 1.

[0070] Furthermore, using these measurement results, the formula: {conductivity ( m The value of {viscosity (mPa·s)} / LiPF6 concentration (mol / L) was calculated. The results are shown in Table 1.

[0071] <Preparation of Lithium-ion Secondary Batteries for Evaluation> LiNi as a positive electrode active material powder 0.5 Co 0.2 Mn 0.3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 90:5:5 to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of an aluminum foil and then dried to form a positive electrode active material layer. The total basis weight of both sides at this time was 10 mg / cm. 2 Next, the positive electrode active material layer was 3 A positive electrode sheet was obtained by rolling and pressing the sheet so that the thickness became

[0072] A negative electrode active material layer was prepared by mixing natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener with ion-exchanged water in a mass ratio of C:SBR:CMC = 97:2:1. This slurry was applied to both sides of copper foil and then dried to form a negative electrode active material layer. The total basis weight of both sides was 9 mg / cm. 2 Next, the negative electrode active material layer was coated with a 1.2 g / cm 3 A negative electrode sheet was obtained by rolling and pressing the sheet so that the thickness of the negative electrode sheet became 1 / 3.

[0073] A polyolefin porous film was also prepared as a separator. The prepared positive electrode sheet and negative electrode sheet were laminated with a separator interposed therebetween to prepare a laminated electrode body. A current collector terminal was attached to the prepared electrode body, and the electrode body was housed in a laminate case together with the nonaqueous electrolyte solution of each Example and Comparative Example and sealed. In this way, the lithium ion secondary batteries for evaluation of each Example and Comparative Example were prepared.

[0074] <Activation process> Each of the lithium-ion secondary batteries for evaluation prepared above was placed in a thermostatic chamber at 25°C. Each lithium-ion secondary battery for evaluation was charged at a constant current of 0.1 C up to a predetermined upper voltage limit, and then discharged at a constant current down to 3.0 V. This charge / discharge cycle was repeated twice. Next, each lithium-ion secondary battery for evaluation was adjusted to a voltage of 3.7 V, placed in a thermostatic chamber at 60°C, and subjected to aging treatment for 12 hours. In this manner, the lithium-ion secondary batteries for evaluation of each example and comparative example were activated.

[0075] <Initial characteristic evaluation> Each activated lithium ion secondary battery for evaluation was charged at a constant current of 0.1 C up to a predetermined upper limit voltage, and then discharged at a constant current to 3.0 V. The discharge capacity at this time was measured and used as the initial capacity.

[0076] This initial capacity was taken as SOC 100%, and each evaluation lithium-ion secondary battery was adjusted to SOC 50% at 25°C. Thereafter, each evaluation lithium-ion secondary battery was placed in a thermostatic chamber at -10°C and discharged at a current value of 10 C for 10 seconds. The voltage change ΔV at this time was measured, and the output resistance of each evaluation lithium-ion secondary battery was calculated as the initial resistance using this voltage change ΔV and the current value. The results are shown in Table 1. Note that the lower the initial resistance, the higher the output of the lithium-ion secondary battery; here, a resistance of 33 mΩ or less was considered acceptable.

[0077] <High-temperature storage characteristic evaluation> Each activated lithium-ion secondary battery for evaluation was adjusted to an SOC of 80% in a temperature environment of 25°C. Each lithium-ion secondary battery for evaluation was placed in a thermostatic chamber at 60°C and stored for 60 days. Thereafter, the output resistance after storage was measured using the same method as for the initial resistance. The resistance increase rate (%) was calculated using the formula: (output resistance after high-temperature storage / initial resistance) x 100. The results are shown in Table 1. Note that a lower resistance increase rate means that the output decrease during long-term storage of the lithium-ion secondary battery is more suppressed; here, a rate of less than 110% was considered acceptable.

[0078] [Table 1]

[0079] From the results in Table 1, it can be seen that the non-aqueous electrolyte contains a non-aqueous solvent, LiPF6, lithium bis(oxalato)borate, and lithium difluorophosphate, the non-aqueous solvent contains carbonates and methyl acetate, the dielectric constant of the non-aqueous solvent is 24.0 or more, and the conductivity of the non-aqueous electrolyte at 25°C is 9.2 m It can be seen that when the electrical conductivity is 3.5 mPa·s or higher, the viscosity of the non-aqueous electrolyte at 25°C is 3.5 mPa·s or lower, and the value of (electrical conductivity × viscosity) / LiPF6 concentration is 26.0 or higher, the initial output resistance is small and the increase in output resistance during high-temperature storage is small.

[0080] Therefore, it is clear that the nonaqueous electrolyte solution disclosed herein can reduce the initial output resistance of a lithium ion secondary battery for use as a power source for driving an HEV, and can also suppress an increase in output resistance after long-term storage of the lithium ion secondary battery.

[0081] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0082] That is, the nonaqueous electrolyte of the lithium ion secondary battery for use as a driving power source for a hybrid vehicle disclosed herein and the lithium ion secondary battery for use as a driving power source for a hybrid vehicle disclosed herein comprise the following items [1] to [7]. [1] A non-aqueous electrolyte for a lithium ion secondary battery for use as a driving power source for a hybrid vehicle, A solution containing a non-aqueous solvent, LiPF, lithium bis(oxalato)borate, and lithium difluorophosphate, the non-aqueous solvent contains a carbonate and methyl acetate, The dielectric constant of the non-aqueous solvent is 24.0 or more, Conductivity at 25°C is 9.2 m S / cm or more, The viscosity at 25°C is 3.5 mPa·s or less, {The conductivity ( m The value of {(mPa·s / cm)×(the viscosity (mPa·s)} / LiPF concentration (mol / L) is 26.0 or more, Non-aqueous electrolyte. [2] The non-aqueous electrolyte solution according to item [1], wherein the non-aqueous solvent contains ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and methyl acetate. [3] The non-aqueous electrolyte solution according to item [2], wherein the non-aqueous solvent contains 24% by volume to 27% by volume of ethylene carbonate, 30% by volume to 33% by volume of dimethyl carbonate, 32% by volume to 36% by volume of ethyl methyl carbonate, and 5% by volume to 9% by volume of methyl acetate. [4] The nonaqueous electrolyte solution according to any one of items [1] to [3], wherein the concentration of LiPF6 is 1.0 mol / L to 1.2 mol / L, the concentration of lithium bis(oxalato)borate is 0.2 mass % to 0.8 mass %, and the concentration of lithium difluorophosphate is 0.4 mass % to 0.9 mass %. [5] The dielectric constant is 24.9 or more, The conductivity is 10.5 m S / cm or more, The viscosity is 3.1 mPa s or less, {The conductivity ( m The nonaqueous electrolyte solution according to any one of items [1] to [4], wherein the value of {(mPa·s / cm)×(mPa·s)} / LiPF6 concentration (mol / L) is 29.0 or more. [6] The dielectric constant is 25.0 or more and 27.0 or less, The conductivity is 11.0 m S / cm or more 14.0 m S / cm or less, The viscosity is 2.6 mPa s or more and 3.0 mPa s or less, {The conductivity ( m [5] The nonaqueous electrolyte solution according to any one of [1] to [5], wherein the value of {(mPa·s / cm)×(mPa·s)} / LiPF6 concentration (mol / L) is 30.0 or more and 32.0 or less. [7] a positive electrode; a negative electrode; The nonaqueous electrolyte solution according to any one of items [1] to [6], A lithium-ion secondary battery for use as a driving power source for a hybrid vehicle, comprising: [Explanation of symbols]

[0083] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A non-aqueous electrolyte solution for a lithium ion secondary battery for use as a driving power source for a hybrid vehicle, comprising: A non-aqueous solvent and LiPF ６ lithium bis(oxalato)borate and lithium difluorophosphate; the non-aqueous solvent contains a carbonate and methyl acetate, the dielectric constant of the nonaqueous solvent is 25.0 or more and 27.0 or less, The electrical conductivity of the nonaqueous electrolyte at 25°C is 11.0 mS / cm or more and 14.0 mS / cm or less, The viscosity of the nonaqueous electrolyte at 25°C is 2.6 mPa s or more and 3.0 mPa s or less, {The conductivity (mS / cm) x the viscosity (mPa·s)} / LiPF ６ The concentration (mol / L) is 30.0 or more and 32.0 or less, Non-aqueous electrolyte.

2. 2. The non-aqueous electrolyte solution according to claim 1, wherein the non-aqueous solvent contains ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and methyl acetate.

3. 3. The non-aqueous electrolyte solution according to claim 2, wherein the non-aqueous solvent contains 24% by volume to 27% by volume of ethylene carbonate, 30% by volume to 33% by volume of dimethyl carbonate, 32 to 36% by volume of ethyl methyl carbonate, and 5% by volume to 9% by volume of methyl acetate.

4. LiPF ６ is 1.0 mol / L to 1.2 mol / L, the concentration of lithium bis(oxalato)borate is 0.2 mass% to 0.8 mass%, and the concentration of lithium difluorophosphate is 0.4 mass% to 0.9 mass%.

5. A positive electrode and a negative electrode; The nonaqueous electrolyte solution according to claim 1; A lithium-ion secondary battery for use as a driving power source for a hybrid vehicle, comprising:

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