JP2024059181A - Electrochemical devices, electrolytes, and additives used in electrolytes - Google Patents

Abstract

[Problem] To reduce the DC resistance during discharge of an electrochemical device. [Solution] An electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing lithium metal phosphate as an active material, and the electrolyte containing a compound represented by the following formula (1): TIFF2024059181000012.tif32149 In formula (1), R1 to R3 each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group. [Selected Figure] Figure 2

Description

The present invention relates to an electrochemical device, an electrolyte, and an additive for use in the electrolyte.

In recent years, the widespread use of portable electronic devices and electric vehicles has created a need for high-performance electrochemical devices, such as nonaqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, and capacitors. As a means for improving the performance of electrochemical devices, for example, a method of adding a specific additive to the electrolyte has been considered. For example, Patent Document 1 discloses a nonaqueous electrolyte that contains a specific acrylate compound as an additive.

International Publication No. 2012/147502

According to the study by the present inventors, it was found that even if the additive is the same, the effect of the additive varies depending on the type of positive electrode (positive electrode active material, etc.). More specifically, even among the acrylate compounds described in the above Patent Document 1, the effect of adding the acrylate compound on the direct current resistance (discharge DCR) during discharge of the electrochemical device varies greatly depending on the type of positive electrode active material.

One aspect of the present invention aims to reduce the DC resistance during discharge of an electrochemical device.

The inventors have discovered that the use of an electrolyte containing a specific acrylate compound in an electrochemical device having a positive electrode containing lithium metal phosphate as an active material reduces the direct current resistance of the electrochemical device during discharge.

式（１）中、Ｒ^１～Ｒ^３はそれぞれ独立に水素原子又はメチル基を示し、Ｘは２価の有機基を示す。
［２］ 式（１）におけるＲ^１及びＲ^２が水素原子である、［１］に記載の電気化学デバイス。
［３］ 式（１）におけるＸが炭素数１～６のアルキレン基である、［１］又は［２］に記載の電気化学デバイス。
［４］ 式（１）におけるＸがエチレン基である、［１］～［３］のいずれかに記載の電気化学デバイス。
［５］ 式（１）で表される化合物の含有量が、電解液全量を基準として０．００１質量％以上１０質量％以下である、［１］～［４］のいずれかに記載の電気化学デバイス。
［６］ 電気化学デバイスが、非水電解液二次電池又はキャパシタである、［１］～［５］のいずれかに記載の電気化学デバイス。 The present invention includes the following aspects.
[1] An electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing lithium metal phosphate as an active material, and the electrolyte containing a compound represented by the following formula (1):

In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.
[2] The electrochemical device according to [1], wherein R ¹ and R ² in formula (1) are hydrogen atoms.
[3] The electrochemical device according to [1] or [2], wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms.
[4] The electrochemical device according to any one of [1] to [3], wherein X in formula (1) is an ethylene group.
[5] The electrochemical device according to any one of [1] to [4], wherein the content of the compound represented by formula (1) is 0.001 mass % or more and 10 mass % or less based on the total amount of the electrolyte.
[6] The electrochemical device according to any one of [1] to [5], which is a non-aqueous electrolyte secondary battery or a capacitor.

式（１）中、Ｒ^１～Ｒ^３はそれぞれ独立に水素原子又はメチル基を示し、Ｘは２価の有機基を示す。
［８］ 式（１）におけるＲ^１及びＲ^２が水素原子である、［７］に記載の電解液。
［９］ 式（１）におけるＸが炭素数１～６のアルキレン基である、［７］又は［８］に記載の電解液。
［１０］ 式（１）におけるＸがエチレン基である、［７］～［９］のいずれかに記載の電解液。
［１１］ 式（１）で表される化合物の含有量が、電解液全量を基準として０．００１質量％以上１０質量％以下である、［７］～［１０］のいずれかに記載の電解液。 [7] An electrolyte solution for use in an electrochemical device having a positive electrode containing lithium metal phosphate as an active material, the electrolyte solution containing a compound represented by the following formula (1):

In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.
[8] The electrolyte solution according to [7], wherein R ¹ and R ² in formula (1) are hydrogen atoms.
[9] The electrolyte solution according to [7] or [8], wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms.
[10] The electrolyte solution according to any one of [7] to [9], wherein X in formula (1) is an ethylene group.
[11] The electrolyte solution according to any one of [7] to [10], wherein the content of the compound represented by formula (1) is 0.001 mass% or more and 10 mass% or less based on the total amount of the electrolyte solution.

式（１）中、Ｒ^１～Ｒ^３はそれぞれ独立に水素原子又はメチル基を示し、Ｘは２価の有機基を示す。
［１３］ 式（１）におけるＲ^１及びＲ^２が水素原子である、［１２］に記載の添加剤。
［１４］ 式（１）におけるＸが炭素数１～６のアルキレン基である、［１２］又は［１３］に記載の添加剤。
［１５］ 式（１）におけるＸがエチレン基である、［１２］～［１４］のいずれかに記載の添加剤。 [12] An additive for use in an electrolyte solution of an electrochemical device having a positive electrode containing lithium metal phosphate as an active material, the additive containing a compound represented by the following formula (1):

In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.
[13] The additive according to [12], wherein R ¹ and R ² in formula (1) are hydrogen atoms.
[14] The additive according to [12] or [13], wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms.
[15] The additive according to any one of [12] to [14], wherein X in formula (1) is an ethylene group.

According to one aspect of the present invention, it is possible to reduce the DC resistance of an electrochemical device during discharge. According to another aspect of the present invention, it is possible to suppress volume expansion of an electrochemical device when it is stored at high temperatures.

FIG. 1 is a perspective view showing a nonaqueous electrolyte secondary battery as an electrochemical device according to one embodiment. FIG. 2 is an exploded perspective view showing an electrode group of the secondary battery shown in FIG. 1 .

Below, an embodiment of the present invention will be described with reference to the drawings as appropriate. However, the present invention is not limited to the following embodiment.

1 is a perspective view showing an electrochemical device according to one embodiment. In this embodiment, the electrochemical device is a nonaqueous electrolyte secondary battery. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 1 includes an electrode group 2 consisting of a positive electrode, a negative electrode, and a separator, and a bag-shaped battery exterior 3 that houses the electrode group 2. The positive electrode and the negative electrode are provided with a positive electrode current collector tab 4 and a negative electrode current collector tab 5, respectively. The positive electrode current collector tab 4 and the negative electrode current collector tab 5 protrude from the inside of the battery exterior 3 to the outside so that the positive electrode and the negative electrode can be electrically connected to the outside of the nonaqueous electrolyte secondary battery 1, respectively. The battery exterior 3 is filled with an electrolyte (not shown). The nonaqueous electrolyte secondary battery 1 may be a battery of a shape other than the so-called "laminated type" as described above (coin type, cylindrical type, laminated type, etc.).

The battery exterior body 3 may be a container formed of, for example, a laminate film. The laminate film may be a laminate film in which, for example, a resin film such as a polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper, or stainless steel, and a sealant layer such as polypropylene are laminated in this order.

Figure 2 is an exploded perspective view showing one embodiment of the electrode group 2 in the nonaqueous electrolyte secondary battery 1 shown in Figure 1. As shown in Figure 2, the electrode group 2 includes a positive electrode 6, a separator 7, and a negative electrode 8, in this order. The positive electrode 6 and the negative electrode 8 are arranged so that the surfaces facing the positive electrode mixture layer 10 and the negative electrode mixture layer 12 face the separator 7, respectively.

The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. The positive electrode current collector 9 is provided with a positive electrode current collector tab 4. The thickness of the positive electrode current collector 9 may be, for example, 1 μm or more and 50 μm or less. The thickness of the positive electrode mixture layer 10 may be, for example, 20 μm or more and 200 μm or less.

The positive electrode collector 9 is formed of, for example, aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymer, conductive glass, etc. The positive electrode collector 9 may be made of aluminum, copper, etc., whose surface has been treated with carbon, nickel, titanium, silver, etc., for the purpose of improving adhesion, conductivity, and oxidation resistance.

The positive electrode mixture layer 10 contains at least an active material (positive electrode active material). The positive electrode active material contains lithium metal phosphate. The lithium metal phosphate is a salt composed of lithium ions, metal ions other than lithium ions, and phosphate ions. The lithium metal phosphate may have an olivine structure.

The lithium metal phosphate may be a lithium transition metal phosphate and may be represented by LiM _A PO ₄ (wherein M represents a transition metal element and A represents a number greater than 0 and equal to or less than 1). M may preferably be a first transition element (3d transition metal element) and may be Mn, Fe, Co, or V. A may preferably be equal to or greater than 2/3 and equal to or less than 1. Examples of lithium metal phosphate include lithium manganese phosphate (LiMnPO ₄ ), lithium iron phosphate (LiFePO ₄ ), lithium cobalt phosphate (LiCoPO ₄ ), and lithium vanadium phosphate (Li ₃ V ₂ (PO ₄ ) ₃ ).

The content of the positive electrode active material may be 80% by mass or more, 85% by mass or more, or 90% by mass or more, and may be 99% by mass or less, based on the total amount of the positive electrode mixture layer.

The positive electrode mixture layer 10 may further contain a conductive agent. The conductive agent may be a carbon material such as carbon black, such as acetylene black or ketjen black, graphite, graphene, or carbon nanotubes. The content of the conductive agent may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and may be 30% by mass or less, 15% by mass or less, or 5% by mass or less, based on the total amount of the positive electrode mixture layer.

The positive electrode mixture layer 10 may further contain a binder. Examples of the binder include resins such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubbers such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluororubber, isoprene rubber, butadiene rubber, and ethylene-propylene rubber; styrene-butadiene-styrene block copolymers or hydrogenated products thereof, EPDM (ethylene-propylene-diene terpolymer), styrene-ethylene-butadiene-ethylene copolymer, and styrene-isoprene-styrene block copolymer. Examples of suitable materials include thermoplastic elastomers such as polybutadiene or hydrogenated products thereof; soft resins such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, and propylene-α-olefin copolymer; fluorine-containing resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer, and polytetrafluoroethylene-vinylidene fluoride copolymer; resins having nitrile group-containing monomers as monomer units; and polymer compositions having ionic conductivity for alkali metal ions (e.g., lithium ions).

The binder content may be, for example, 0.1% by mass or more, 1% by mass or more, or 1.5% by mass or more, and 20% by mass or less, 10% by mass or less, or 5% by mass or less, based on the total amount of the positive electrode mixture layer.

There are no particular limitations on the separator 7, so long as it provides electronic insulation between the positive electrode 6 and the negative electrode 8 while allowing ions to pass through, and is resistant to oxidation on the positive electrode 6 side and reduction on the negative electrode 8 side. Examples of materials for such separator 7 include resins, inorganic substances, etc.

Examples of resins include olefin-based polymers, fluorine-based polymers, cellulose-based polymers, polyimides, nylons, etc. From the viewpoint of being stable against the electrolyte and having excellent liquid retention properties, the separator 7 is preferably a porous sheet or nonwoven fabric formed of a polyolefin such as polyethylene or polypropylene.

Examples of inorganic substances include oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate. Separator 7 may be a separator in which fibrous or particulate inorganic substances are attached to a thin-film substrate such as a nonwoven fabric, woven fabric, or microporous film.

The negative electrode 8 includes a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. The negative electrode current collector 11 is provided with a negative electrode current collector tab 5. The thickness of the negative electrode current collector 11 may be, for example, 1 μm or more and 50 μm or less. The thickness of the negative electrode mixture layer 12 may be, for example, 20 μm or more and 200 μm or less.

The negative electrode current collector 11 is formed of copper, stainless steel, nickel, aluminum, titanium, baked carbon, conductive polymer, conductive glass, aluminum-cadmium alloy, etc. The negative electrode current collector 11 may be made of copper, aluminum, etc., whose surface is treated with carbon, nickel, titanium, silver, etc., for the purpose of improving adhesion, conductivity, and reduction resistance. The thickness of the negative electrode current collector 11 is, for example, 1 to 50 μm in terms of electrode strength and energy density.

The negative electrode mixture layer 12 contains at least an active material (negative electrode active material). Examples of the negative electrode active material include carbon-based active materials and silicon-based active materials. Examples of the carbon material constituting the carbon-based active material include amorphous carbon materials, natural graphite, composite carbon materials in which a coating of amorphous carbon material is formed on natural graphite, and artificial graphite (obtained by firing resin raw materials such as epoxy resins and phenolic resins, or pitch-based raw materials obtained from petroleum, coal, etc.).

From the viewpoint of increasing the capacity of electrochemical devices, the carbon-based active material is preferably a graphite-based active material made of graphite. In graphite, the carbon net surface interlayer distance (d002) in wide-angle X-ray diffraction is preferably less than 0.34 nm, and more preferably 0.3354 nm or more and 0.337 nm or less. Carbon materials (graphite) that satisfy such conditions are sometimes called pseudoanisotropic carbon.

The silicon-based active material may be a simple substance of silicon, or may be a compound containing silicon and other elements. The compound may be an alloy containing silicon and at least one selected from the group consisting of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. The compound may be an oxide, nitride, or carbide of silicon. Examples of oxides of silicon include SiOx ( _SiO , _SiO2 , etc.) and LiSiO. Examples of _nitrides of silicon include _Si3N4 and _Si2N2O . Examples of carbides of silicon include SiC.

In addition to the above-mentioned negative electrode active materials, the negative electrode active material may further include a negative electrode active material composed of a metal composite oxide, an oxide or nitride of a Group 4 element such as tin or germanium, lithium alone, or a lithium alloy such as a lithium aluminum alloy.

The content of the negative electrode active material may be 80% by mass or more, 85% by mass or more, or 90% by mass or more, and may be 99% by mass or less, based on the total amount of the negative electrode mixture layer.

The negative electrode mixture layer 12 may further contain a conductive agent. The conductive agent and its content may be the same as the conductive agent and its content in the positive electrode mixture layer described above.

The negative electrode mixture layer 12 may further contain a binder. The binder and its content may be the same as the binder and its content in the positive electrode mixture layer described above.

The negative electrode mixture layer 12 may further contain a thickener to adjust the viscosity. The thickener is not particularly limited, and may be carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein, salts thereof, or the like. The thickener may be one of these alone or a mixture of two or more of them.

When the negative electrode mixture layer 12 contains a thickener, its content is not particularly limited. From the viewpoint of the coatability of the negative electrode mixture layer, the content of the thickener may be 0.1 mass% or more, 0.2 mass% or more, or 0.5 mass% or more based on the total amount of the negative electrode mixture layer. From the viewpoint of suppressing a decrease in battery capacity or an increase in resistance between the negative electrode active materials, the content of the thickener may be 5 mass% or less, 3 mass% or less, or 2 mass% or less based on the total amount of the negative electrode mixture layer.

式（１）中、Ｒ^１～Ｒ^３はそれぞれ独立に水素原子又はメチル基を示し、Ｘは２価の有機基を示す。 The electrolytic solution contains at least a compound represented by the following formula (1), and may further contain an electrolyte salt and a non-aqueous solvent.

In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.

In other words, in one embodiment, the compound represented by formula (1) is an additive used in the electrolyte of the electrochemical device 1.

From the viewpoint of further reducing the discharge DCR, ^R1 and ^R2 are preferably hydrogen atoms. ^R3 is preferably a hydrogen atom.

X may be, for example, a divalent hydrocarbon group or an alkylene group. The alkylene group may be linear or branched. The divalent hydrocarbon group and the alkylene group may have, for example, 1 to 6 carbon atoms. The lower limit of the carbon number may be 2 or more. The upper limit of the carbon number may be 5 or less or 4 or less. The alkylene group represented by X may be a methylene group, an ethylene group, a propylene group, a butylene group, or a pentylene group, and is preferably an ethylene group.

X may be, for example, a divalent group in which a part of a divalent hydrocarbon group is substituted with a heteroatom. The heteroatom may be, for example, an oxygen atom. X may be, for example, a divalent group in which a part of a divalent hydrocarbon group is substituted with an oxygen atom to have an ether structure. X may be, for example, a divalent group represented by the following formula (2).
-X ¹ -O-X ² - (2)
In formula (2), ^X1 and ^X2 each independently represent an alkylene group. The alkylene group may be linear or branched. The number of carbon atoms of the alkylene group represented by ^X1 and ^X2 may each independently be 1 to 6, 1 to 5, 1 to 4, 1 to 3, or 1 to 2.

From the viewpoint of further reducing the discharge DCR, the content of the compound represented by formula (1) is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, even more preferably 0.01% by mass or more, particularly preferably 0.05% by mass or more, even more preferably 0.1% by mass or more, based on the total amount of the electrolyte, and may be preferably 10% by mass or less or 8% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, particularly preferably 2% by mass or less, even more preferably 1% by mass or less.

The electrolyte salt may be, for example, a lithium salt. The lithium salt may be, for example, at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiB(C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , CF ₃ SO ₂ OLi, LiN(SO ₂ F) ₂ (Li[FSI], lithium bisfluorosulfonylimide), LiN(SO ₂ CF ₃ ) ₂ (Li[TFSI], lithium bistrifluoromethanesulfonylimide), and LiN(SO ₂ CF ₂ CF ₃ ) _2. The lithium salt preferably contains LiPF ₆ from the viewpoint of further improving the solubility in the solvent, the charge/discharge characteristics, output characteristics, cycle characteristics, etc. of the secondary battery.

From the viewpoint of excellent charge/discharge characteristics, the concentration of the electrolyte salt is preferably 0.5 mol/L or more, more preferably 0.7 mol/L or more, and even more preferably 0.8 mol/L or more, based on the total amount of the nonaqueous solvent, and is preferably 1.5 mol/L or less, more preferably 1.3 mol/L or less, and even more preferably 1.2 mol/L or less.

The non-aqueous solvent may be, for example, a chain carbonate compound such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl butyl carbonate. Or a cyclic carbonate compound such as ethylene carbonate, propylene carbonate, or butylene carbonate. Or a chain carboxylic acid ester compound such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, or propyl propionate. Or a cyclic carboxylic acid ester compound such as γ-butyrolactone. Or a chain ether compound such as dimethoxymethane, dimethoxyethane, or diethoxyethane. Or a cyclic ether compound such as tetrahydrofuran, tetrahydropyran, or dioxolane. Or a nitrile compound such as acetonitrile, or a sulfur compound such as sulfolane. The non-aqueous solvent may be one of these alone or a mixture of two or more of them, and is preferably a mixture of two or more of them.

The electrolyte may further contain other materials in addition to the compound represented by formula (1), the electrolyte salt, and the non-aqueous solvent. The other materials may be, for example, an unsaturated cyclic carbonate, a fluorine-containing cyclic carbonate, a cyclic carboxylic acid ester, a compound containing a nitrogen atom, a sulfur atom, or a nitrogen atom and a sulfur atom other than the compound represented by formula (1), etc.

The unsaturated cyclic carbonate may be, for example, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate (4,5-dimethylvinylene carbonate), ethylvinylene carbonate (4,5-diethylvinylene carbonate), diethylvinylene carbonate, vinylethylene carbonate, etc., and vinylene carbonate is preferred from the viewpoint of further reducing the discharge DCR.

The fluorine-containing cyclic carbonate may be, for example, 4-fluoro-1,3-dioxolane-2-one (fluoroethylene carbonate; FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, etc., and is preferably 4-fluoro-1,3-dioxolane-2-one (fluoroethylene carbonate; FEC).

The compound containing a nitrogen atom other than the compound represented by formula (1) may be, for example, a nitrile compound such as succinonitrile. The compound containing a sulfur atom other than the compound represented by formula (1) may be, for example, a cyclic sulfonate compound such as 1,3-propane sultone, 1-propene-1,3-sultone, etc.

From the viewpoint of further reducing the discharge DCR, the electrolyte preferably further contains an unsaturated cyclic carbonate, more preferably vinylene carbonate. The content of the unsaturated cyclic carbonate (preferably vinylene carbonate) may be 0.1 mass% or more, 0.2 mass% or more, 0.3 mass% or more, 0.4 mass% or more, or 0.5 mass% or more based on the total amount of the electrolyte, and may be 5 mass% or less, 4 mass% or less, 3 mass% or less, 2 mass% or less, or 1 mass% or less.

In the electrolyte, the mass ratio of the content of the compound represented by formula (1) to the content of the unsaturated cyclic carbonate (preferably vinylene carbonate) (content of the compound represented by formula (1)/content of the unsaturated cyclic carbonate (preferably vinylene carbonate)) may be 0.1 or more, 0.2 or more, or 0.3 or more, and from the viewpoint of further reducing the discharge DCR, is preferably 0.4 or more, more preferably 0.5 or more. The mass ratio may be 2 or less, 1.5 or less, 1.2 or less, or 1 or less.

The present inventors have found that the direct current resistance during discharge of an electrochemical device can be reduced by using an electrolyte solution containing the compound represented by the above formula (1) in an electrochemical device having a positive electrode containing lithium metal phosphate as an active material. Such an effect is obtained by combining the compound represented by formula (1) with lithium metal phosphate as a positive electrode active material, and such an effect cannot be obtained, for example, by combining the compound represented by formula (1) with lithium nickel cobalt manganese oxide (NMC622) as a positive electrode active material. The present inventors speculate that the compound represented by formula (1) forms a particularly stable and good coating on a positive electrode containing lithium metal phosphate as an active material, thereby obtaining the above-mentioned effect.

In one embodiment, the electrolyte solution containing the compound represented by formula (1) described above is used in an electrochemical device having a positive electrode containing lithium metal phosphate as an active material, thereby suppressing volume expansion when the electrochemical device is stored at high temperatures. The inventors speculate that the mechanism by which such an effect is obtained is as follows. That is, it is speculated that this is because the compound represented by formula (1) coordinates with the positive electrode active material or forms a stable coating on the positive electrode side of the electrochemical device, thereby suppressing oxidative decomposition of other components in the electrolyte (other additives, nonaqueous solvents, electrolyte salts, etc.). In addition, during the initial charge, the carbon-carbon unsaturated bond structure of the compound represented by formula (1) undergoes a reduction reaction on the negative electrode side before other components in the electrolyte (other additives or nonaqueous solvents such as carbonates) to form a coating. Thereafter, the isocyanate moiety of the compound represented by formula (1) reacts to form a stable coating, and it is speculated that this is because the reduction decomposition of other components in the electrolyte (other additives, nonaqueous solvents, electrolyte salts, etc.) on the negative electrode side can also be suppressed.

Next, a method for manufacturing the nonaqueous electrolyte secondary battery 1 will be described. The method for manufacturing the nonaqueous electrolyte secondary battery 1 includes a first step of obtaining a positive electrode 6, a second step of obtaining a negative electrode 8, a third step of housing the electrode group 2 in a battery exterior body 3, and a fourth step of injecting an electrolyte into the battery exterior body 3. The first to fourth steps may be performed in any order.

In the first step, the material used for the positive electrode mixture layer 10 is dispersed in a dispersion medium using a kneader, disperser, etc. to obtain a slurry-like positive electrode mixture, and then this positive electrode mixture is applied to the positive electrode current collector 9 by a doctor blade method, a dipping method, a spray method, etc., and the dispersion medium is then evaporated to obtain the positive electrode 6. After the dispersion medium is evaporated, a compression molding process using a roll press may be provided as necessary. The positive electrode mixture layer 10 may be formed as a multi-layered positive electrode mixture layer by performing the above-mentioned process from application of the positive electrode mixture to evaporation of the dispersion medium multiple times. The dispersion medium may be water, 1-methyl-2-pyrrolidone (hereinafter also referred to as NMP), etc.

The second step may be the same as the first step described above, and the method for forming the negative electrode mixture layer 12 on the negative electrode current collector 11 may be the same as the first step described above.

In the third step, a separator 7 is sandwiched between the prepared positive electrode 6 and negative electrode 8 to form an electrode group 2. Next, this electrode group 2 is housed in a battery exterior body 3.

In the fourth step, the electrolyte is injected into the battery exterior body 3. The electrolyte can be prepared, for example, by first dissolving the electrolyte salt in a solvent and then dissolving the other materials.

In another embodiment, the electrochemical device may be a capacitor. The capacitor may include an electrode group consisting of a positive electrode, a negative electrode, and a separator, and a bag-shaped battery exterior that houses the electrode group, similar to the nonaqueous electrolyte secondary battery 1 described above. The details of each component of the capacitor may be similar to those of the nonaqueous electrolyte secondary battery 1.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
[Preparation of Positive Electrode]
Carbon black (CB) (2% by mass) and carbon nanotubes (CNT) (1% by mass) as conductive agents, and PVDF (3% by mass) as binder were sequentially added and mixed to lithium iron phosphate (LiFePO ₄ ; LFP) (94% by mass) as a positive electrode active material. NMP was added as a dispersion medium to the obtained mixture, and the mixture was kneaded to prepare a slurry-like positive electrode mixture. A predetermined amount of this positive electrode mixture was evenly and homogeneously applied to an aluminum foil having a thickness of 20 μm as a positive electrode current collector. Thereafter, the dispersion medium was volatilized, and the mixture was pressed to compact the positive electrode mixture to a density of 2.3 g/cm ³ , thereby obtaining a positive electrode containing lithium metal phosphate as an active material.

[Preparation of negative electrode]
Carbon black (CB) (1% by mass) as a conductive agent, SBR (1.5% by mass) as a binder, and carboxymethyl cellulose (1.5% by mass) as a thickener were added to a carbon-based active material (artificial graphite) (96% by mass) as a negative electrode active material, and mixed. Water was added as a dispersion medium to the obtained mixture, and the mixture was kneaded to prepare a slurry-like negative electrode mixture. A predetermined amount of this negative electrode mixture was evenly and homogeneously applied to an electrolytic copper foil having a thickness of 20 μm as a negative electrode current collector. Thereafter, the dispersion medium was volatilized, and the mixture was pressed to compact the negative electrode mixture to a density of 1.65 g/cm ³ to obtain a negative electrode.

[Preparation of secondary battery]
A positive electrode cut into a square of 20.0 cm ² was sandwiched between a polypropylene porous sheet (thickness 20 μm) as a separator, and a negative electrode cut into a square of 21.8 cm ² was stacked to prepare an electrode group. This electrode group was housed in a container (battery exterior) formed of an aluminum laminate film. Next, 0.6 mL of electrolyte was added to the container, and the container was heat-sealed to prepare a lithium ion secondary battery for evaluation (hereinafter also referred to as "LFP secondary battery"). As the electrolyte, a mixed solution of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate = 3 / 2 / 5 (volume ratio) containing 1 mol / L LiPF ₆ was used to add 0.25 mass% of compound X represented by the following formula (X) and 1 mass% of vinylene carbonate (both based on the total amount of electrolyte).

Example 2
An LFP secondary battery was produced in the same manner as in Example 1, except that an electrolyte solution containing 0.25 mass % of a compound Y represented by the following formula (Y) instead of the compound X was used, based on the total mass of the electrolyte solution.

Example 3
An LFP secondary battery was produced in the same manner as in Example 1, except that an electrolyte solution containing 0.5 mass % of compound X based on the total mass of the electrolyte solution was used.

Example 4
An LFP secondary battery was produced in the same manner as in Example 1, except that an electrolyte solution containing 0.25 mass % of compound X and 0.75 mass % of vinylene carbonate was used, based on the total mass of the electrolyte solution.

Example 5
An LFP secondary battery was produced in the same manner as in Example 1, except that an electrolyte solution containing 0.5 mass % of compound X and 0.5 mass % of vinylene carbonate was used.

(Comparative Example 1)
An LFP secondary battery was fabricated in the same manner as in Example 1, except that compound X was not added to the electrolyte solution.

(Comparative Example 2)
[Preparation of Positive Electrode]
Acetylene black (AB) (4% by mass) as a conductive agent and PVDF (4% by mass) as a binder were added to lithium nickel cobalt manganese oxide (NMC622) (92% by mass) as a positive electrode active material and mixed. NMP was added as a dispersion medium to the obtained mixture and kneaded to prepare a slurry-like positive electrode mixture. A predetermined amount of this positive electrode mixture was evenly and homogeneously applied to an aluminum foil having a thickness of 20 μm as a positive electrode current collector. Thereafter, the dispersion medium was volatilized, and the mixture was pressed to compact the positive electrode mixture to a density of 2.8 g/cm ³ , thereby obtaining a positive electrode containing lithium nickel cobalt manganese oxide as an active material.

[Preparation of negative electrode]
Carbon black (CB) (1% by mass) as a conductive agent, SBR (1.5% by mass) as a binder, and carboxymethyl cellulose (1.5% by mass) as a thickener were added to a carbon-based active material (artificial graphite) (96% by mass) as a negative electrode active material, and mixed. Water was added as a dispersion medium to the obtained mixture, and the mixture was kneaded to prepare a slurry-like negative electrode mixture. A predetermined amount of this negative electrode mixture was evenly and homogeneously applied to an electrolytic copper foil having a thickness of 20 μm as a negative electrode current collector. Thereafter, the dispersion medium was volatilized, and the mixture was pressed to compact the negative electrode mixture to a density of 1.65 g/cm ³ to obtain a negative electrode.

[Preparation of secondary battery]
The positive electrode cut into a square of 20.0 cm ² was sandwiched between a polypropylene porous sheet (thickness 20 μm) as a separator, and a negative electrode cut into a square of 21.8 cm ² was stacked to prepare an electrode group. This electrode group was housed in a container (battery exterior) formed of an aluminum laminate film. Next, 0.5 mL of electrolyte was added to the container, and the container was heat-sealed to prepare a lithium-ion secondary battery for evaluation (hereinafter also referred to as "NMC622 secondary battery"). As the electrolyte, a mixed solution of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate = 3 / 2 / 5 (volume ratio) containing 1 mol / L LiPF ₆ was used to add 1 mass % vinylene carbonate based on the total amount of electrolyte.

(Comparative Example 3)
An NMC622 secondary battery was fabricated in the same manner as in Comparative Example 2, except that 0.25% by mass of compound X and 1% by mass of vinylene carbonate (both based on the total mass of the electrolyte) were added.

(Comparative Example 4)
An NMC622 secondary battery was fabricated in the same manner as in Comparative Example 2, except that 0.5% by mass of compound X and 1% by mass of vinylene carbonate (both based on the total mass of the electrolyte) were added.

[Initial charge/discharge]
The lithium ion secondary batteries thus produced were subjected to initial charge and discharge in the following manner. First, constant current charging was performed at a current value of 0.1C in an environment of 25°C up to an upper limit voltage of 4.0V (LFP secondary battery) or 4.2V (NMC622 secondary battery), followed by constant voltage charging at the above upper limit voltage. The charge termination condition was a current value of 0.05C. Then, constant current discharging was performed at a current value of 0.1C up to an end voltage of 2.0V (LFP secondary battery) or 2.7V (NMC622 secondary battery). Next, constant current charging was performed at a current value of 0.2C up to the above upper limit voltage, followed by constant voltage charging up to the above upper limit voltage. The charge termination condition was a current value of 0.05C. Then, constant current discharging was performed at a current value of 0.2C up to the above end voltage. This charge and discharge cycle at a current value of 0.2C was repeated three times (the unit of current value "C" means "current value (A)/battery capacity (Ah)"). The cut-off voltage for charging or discharging is different between the LFP secondary battery and the NMC622 battery, and even if the cut-off voltage for charging in the LFP secondary battery is set to 4.0 V or higher, it is not possible to extract any more Li ions from the positive electrode of the LFP battery. Therefore, the conditions (voltage) for constant current charging and constant current discharging are different between the LFP secondary battery and the NMC622 battery.

[Measurement of Discharge DCR]
For each lithium ion secondary battery after the above-mentioned initial charge/discharge, the discharge DCR (direct current resistance during discharge) was measured by the following procedure.
First, constant current charging at 0.2C was performed up to an upper limit voltage; 4.0V (LFP secondary battery), 4.2V (secondary battery with lithium nickel-cobalt manganese oxide positive electrode), followed by constant voltage charging at the above upper limit voltage. The charge termination condition was a current value of 0.02C. Then, constant current discharging was performed at a current value of 0.2C to an end voltage; 2.0V (LFP secondary battery), 2.7V (NMC622 secondary battery), with the current value at this time being I _0.2C , and the voltage change 10 seconds after the start of discharging being ΔV _0.2C . Next, constant current charging at 0.2C was performed up to the above upper limit voltage, followed by constant voltage charging at the above upper limit voltage (charge termination condition was a current value of 0.02C). Then, constant current discharging was performed up to the above end voltage at a current value of 0.5C, with the current value at this time being I _0.5C , and the voltage change 10 seconds after the start of discharging being ΔV _0.5C . The 1C current value I _1C and the voltage change ΔV _1C 10 seconds after the start of discharge were evaluated from similar charging and discharging. A linear approximation line was drawn using the least squares method on the three plots of the current value-voltage change (I _0.2C , ΔV _0.2C ), (I _0.5C , ΔV _0.5C ), and (I _1C , ΔV _1C ), and the slope was taken as the discharge DCR (Ω). The results are shown in Table 1.

[Evaluation of volume expansion at high temperatures]
Each lithium ion secondary battery after the initial charge and discharge was charged at a constant current of 0.1C in an environment of 60°C to an upper limit voltage of 4.0V (LFP secondary battery), 4.2V (NMC622 secondary battery), and then charged at a constant voltage of the upper limit voltage. The charge end condition was a current value of 0.05C. Then, constant current discharge was performed at a current value of 0.1C to an end voltage of 2.0V (LFP secondary battery), 2.7V (NMC622 secondary battery). Next, constant current charging was performed at a current value of 0.2C to the upper limit voltage, and then constant voltage charging was performed at the upper limit voltage. The charge end condition was a current value of 0.05C. Then, constant current discharge was performed at a current value of 0.2C to the end voltage. This charge and discharge cycle at a current value of 0.2C was repeated twice.
Thereafter, constant current charging was performed at a current value of 0.1 C in an environment of 25° C. up to the upper limit voltage, and then constant voltage charging was performed up to the upper limit voltage. The charge termination condition was a current value of 0.01 C. Then, the secondary batteries were stored in a thermostatic chamber at 60° C. for 4 weeks.
The volume (V1) of each lithium ion secondary battery before high-temperature storage and the volume (V2) of each lithium ion secondary battery left to stand for 30 minutes in a 25°C environment after high-temperature storage were measured using a specific gravity meter based on Archimedes' method (electronic specific gravity meter MDS-300, manufactured by Alpha Mirage). The measured V1 and V2 were used to calculate the volume expansion rate according to the following formula.
Volume expansion rate (%) = V2/V1 x 100
The results are shown in Table 1.

When lithium metal phosphate was used as the positive electrode active material, in Examples 1 to 5 using an electrolyte containing the compound represented by formula (1), the discharge DCR was significantly reduced compared to Comparative Example 1 using an electrolyte not containing the compound represented by formula (1). In contrast, when lithium nickel cobalt manganese oxide (NMC622) was used as the positive electrode active material, in Comparative Examples 3 and 4 using an electrolyte containing the compound represented by formula (1), the discharge DCR was increased compared to Comparative Example 2 using an electrolyte not containing the compound represented by formula (1).

In addition, in Examples 1 to 5, which use lithium metal phosphate as the positive electrode active material and an electrolyte solution containing the compound represented by formula (1), the volume expansion during storage of the electrochemical device at high temperatures can be suppressed compared to Comparative Example 1, which uses lithium metal phosphate as the positive electrode active material and an electrolyte solution not containing the compound represented by formula (1), and Comparative Examples 2 to 4, which use lithium nickel cobalt manganese oxide (NMC622) as the positive electrode active material.

1... nonaqueous electrolyte secondary battery (electrochemical device), 2... electrode group, 3... battery exterior, 4... positive electrode current collector tab, 5... negative electrode current collector tab, 6... positive electrode, 7... separator, 8... negative electrode, 9... positive electrode current collector, 10... positive electrode mixture layer, 11... negative electrode current collector, 12... negative electrode mixture layer.

Claims (15)

An electrochemical device comprising a positive electrode, a negative electrode, and an electrolyte,
The positive electrode contains lithium metal phosphate as an active material,
The electrochemical device, wherein the electrolyte solution contains a compound represented by the following formula (1):

[In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.] The electrochemical device according to claim 1 , wherein R ¹ and R ² in the formula (1) are hydrogen atoms. The electrochemical device according to claim 1, wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms. The electrochemical device according to claim 1, wherein X in formula (1) is an ethylene group. The electrochemical device according to claim 1, wherein the content of the compound represented by formula (1) is 0.001% by mass or more and 10% by mass or less based on the total amount of the electrolyte. The electrochemical device according to any one of claims 1 to 5, wherein the electrochemical device is a non-aqueous electrolyte secondary battery or a capacitor. 1. An electrolyte for use in an electrochemical device having a positive electrode containing lithium metal phosphate as an active material,
An electrolyte solution comprising a compound represented by the following formula (1):

[In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.] The electrolyte solution according to claim 7, wherein R ¹ and R ² in the formula (1) are hydrogen atoms. The electrolyte solution according to claim 7, wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms. The electrolyte solution according to claim 7, wherein X in formula (1) is an ethylene group. The electrolyte solution according to any one of claims 7 to 10, wherein the content of the compound represented by formula (1) is 0.001% by mass or more and 10% by mass or less based on the total amount of the electrolyte solution. 1. An additive for use in an electrolyte of an electrochemical device having a positive electrode containing lithium metal phosphate as an active material, comprising:
An additive comprising a compound represented by the following formula (1):

[In formula (1), R ¹ to R ³ each independently represent a hydrogen atom or a methyl group, and X represents a divalent organic group.] The additive according to claim 12, wherein R ¹ and R ² in formula (1) are hydrogen atoms. The additive according to claim 12 or 13, wherein X in formula (1) is an alkylene group having 1 to 6 carbon atoms. The additive according to claim 12 or 13, wherein X in formula (1) is an ethylene group.

Images