WO2025084071A1 - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

A non-aqueous electrolyte secondary battery according to the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode contains LiMn_(1-y)Fe_yPO₄ (0<y<1) as a positive electrode active material, and the non-aqueous electrolyte contains a compound represented by formula (1). In formula (1), Q represents an alkylene group, an alkenylene group, or the like, and X represents a group represented by formula (2a) or formula (2b). R¹ represents an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a hydroxyl group, a lithium oxy group, or the like.　

Description

Non-aqueous electrolyte secondary battery

The present invention relates to a non-aqueous electrolyte secondary battery.

As interest in solving environmental problems and realizing a sustainable, recycling-oriented society grows, extensive research is being conducted on non-aqueous electrolyte secondary batteries, such as lithium-ion batteries. Lithium-ion batteries have high voltage and energy density, and are therefore used as power sources for laptop computers, mobile phones, electric vehicles, and other devices.

In lithium-ion batteries, a method of adding various additives to the electrolyte is commonly used to improve battery performance. One such additive is one that decomposes during the first charge and discharge and forms a film called a solid electrolyte interface (SEI) on the surface of the negative electrode. The formation of the SEI is thought to play a major role in suppressing the deterioration of secondary batteries when they are repeatedly charged and discharged, and in improving various battery performances.

For example, Patent Document 1 discloses a method for improving cycle characteristics, etc., in a lithium secondary battery that uses lithium cobalt oxide or lithium manganese oxide as the positive electrode active material by adding 1,3-propane sultone and/or 1,4-butane sultone as an additive to the electrolyte.

Patent Document 2 discloses a method for improving storage stability at high temperatures in an electricity storage device that uses a lithium NCM composite metal oxide as the positive electrode active material by adding a cyclic sulfonate ester compound as an additive to the electrolyte.

Patent Document 3 discloses a method for improving storage stability at high temperatures by adding 1,3-dioxanes as additives to the electrolyte in an electricity storage device that uses lithium NCM composite metal oxide or lithium-containing olivine-type phosphate as the positive electrode active material.

Patent No. 3978881 International Publication No. 2012/147818 International Publication No. 2018/116879

Recently, among the aforementioned lithium-containing olivine-type phosphates, lithium manganese iron phosphate, which has superior stability, has been attracting attention as a positive electrode active material. However, for non-aqueous electrolyte secondary batteries that contain this lithium manganese iron phosphate as a positive electrode active material, a formulation suitable for improving storage stability at high temperatures has not yet been established. One aspect of the present disclosure relates to improving the storage stability at high temperatures of non-aqueous electrolyte secondary batteries that contain lithium manganese iron phosphate as a positive electrode active material.

で表される化合物を含有し、
　式（１）中、Ｑが、スルホニル基の硫黄原子とともに環構造を形成する基であって、１以上の置換基を有していてもよい炭素数４～６のアルキレン基、及び１以上の置換基を有していてもよい炭素数４～６のアルケニレン基から選ばれる基を示し、Ｘが、下記式（２ａ）又は下記式（２ｂ）： One aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery according to the following items:
Item 1.
A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte,
The positive electrode contains LiMn _{(1-y) FeyPO4} as a positive electrode active material, and y is _a value that satisfies 0<y<1;
The nonaqueous electrolyte solution has the following formula (1):

The compound represented by the formula:
In formula (1), Q represents a group which forms a ring structure together with the sulfur atom of the sulfonyl group and is selected from an alkylene group having 4 to 6 carbon atoms which may have one or more substituents and an alkenylene group having 4 to 6 carbon atoms which may have one or more substituents, and X represents a group represented by the following formula (2a) or the following formula (2b):

で表される基を示し、
　式（２ａ）及び式（２ｂ）中、Ｒ^１が、フッ素原子を置換基として有していてもよい炭素数１～４のアルキル基、フッ素原子を置換基として有していてもよい炭素数２～４のアルケニル基、フッ素原子を置換基として有していてもよい炭素数２～４のアルキニル基、フッ素原子を置換基として有していてもよい炭素数６～１０のアリール基、ヒドロキシ基、又はリチウムオキシ基を示す、
非水電解液二次電池。

represents a group represented by
In formula (2a) and formula (2b), R ¹ represents an alkyl group having 1 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkenyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkynyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an aryl group having 6 to 10 carbon atoms which may have a fluorine atom as a substituent, a hydroxy group, or a lithium oxy group;
Nonaqueous electrolyte secondary battery.

Item 2.
Item 2. The nonaqueous electrolyte secondary battery according to item 1, wherein in formula (1), Q is an alkylene group having 4 carbon atoms which may have one or more substituents, or an alkenylene group having 4 carbon atoms which may have one or more substituents.

Item 3.
Item 3. The nonaqueous electrolyte secondary battery according to item 1 or 2, wherein in formulas (2a) and (2b), R ¹ is an alkyl group having 1 to 2 carbon atoms which may have a fluorine atom as a substituent, an alkenyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkynyl group having 3 to 4 carbon atoms which may have a fluorine atom as a substituent, or a phenyl group which may have a fluorine atom as a substituent.

Item 4.
Item 4. The nonaqueous electrolyte secondary battery according to any one of items 1 to 3, wherein X is a group represented by formula (2a):

Item 5.
Item 4. The nonaqueous electrolyte secondary battery according to any one of items 1 to 3, wherein X is a group represented by formula (2b).

で表される化合物であり、式（１ａ）中、Ｘが、式（１）中のＸと同義である、
項１～５のいずれかに記載の非水電解液二次電池。 Item 6.
The compound represented by formula (1) is represented by the following formula (1a):

In the formula (1a), X has the same meaning as X in the formula (1),
Item 6. The nonaqueous electrolyte secondary battery according to any one of Items 1 to 5.

で表される化合物であり、式（１ｂ）中、Ｘが、式（１）中のＸと同義である、
項１～５のいずれかに記載の非水電解液二次電池。 Item 7.
The compound represented by formula (1) is represented by the following formula (1b):

In the formula (1b), X has the same meaning as X in the formula (1),
Item 6. The nonaqueous electrolyte secondary battery according to any one of Items 1 to 5.

According to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery containing lithium iron manganese phosphate as a positive electrode active material can be provided, which has excellent storage stability at high temperatures.

FIG. 1 is a cross-sectional view showing an example of a nonaqueous electrolyte secondary battery.

Below, we will explain in detail some examples relating to one aspect of the present invention.

The nonaqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte. The positive electrode contains LiMn _{(1-y) FeyPO4} as a positive _electrode active material, where y is a value satisfying 0<y<1. The nonaqueous electrolyte contains a compound represented by formula (1).

<Nonaqueous electrolyte secondary battery>
FIG. 1 is a cross-sectional view showing a schematic example of a nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery 1 shown in FIG. 1 includes alternately stacked negative electrodes 4 and positive electrodes 9, a nonaqueous electrolyte 5 disposed between the negative electrodes 4 and the positive electrodes 9, and a separator 6 provided in the nonaqueous electrolyte 5. The multiple negative electrodes 4 and positive electrodes 9 are stacked such that the main surface of the negative electrode 4 and the main surface of the positive electrode 9 face each other via the separator 6. The nonaqueous electrolyte secondary battery 1 includes multiple layers of negative electrodes 4 and multiple layers of positive electrodes 9, but in FIG. 1, some of the repeated structures are omitted. The negative electrode 4 has a sheet-shaped negative electrode collector 3 and a negative electrode active material layer 2 provided on both sides of the negative electrode collector 3. The positive electrode 9 has a sheet-shaped positive electrode collector 8 and a positive electrode active material layer 7 provided on both sides of the positive electrode collector 8.

(Positive electrode)
The positive electrode active material layer contains LiMn _{(1-y) FeyPO4} as a positive _electrode active material. Here, in the formula, y is a numerical value that satisfies 0<y<1. From the viewpoint of increasing the energy density, y may be a numerical value that satisfies 0.1<y<0.9, 0.1<y<0.8, 0.1<y<0.7, 0.1<y<0.6, 0.1<y<0.5, 0.2<y<0.9, 0.2<y<0.8, 0.2<y<0.7, 0.2<y<0.6, 0.2<y<0.5, 0.3<y<0.9, 0.3<y<0.8, 0.3<y<0.7, 0.3<y<0.6, or 0.3<y<0.5.

The positive electrode active material may be doped with one or more metals selected from Mg, Ca, Sr, Al, Ti, Cr, Zn, and W. A portion of the Mn and/or Fe contained in the positive electrode active material may be substituted with one or more metals selected from Mg, Ca, Sr, Al, Ti, Cr, Zn, and W.

The positive electrode active material layer may contain other components such as a binder and a conductive assistant described below in addition to LiMn _{(1-y) FeyPO4 as} the positive electrode active material. The content of LiMn _{(1-y) FeyPO4} in the positive _electrode active material layer may be, for example, 60 mass% or more, or 80 mass% or more, and may be 99.9 mass% or less, based on the total mass of the positive electrode active material layer.

The positive electrode current collector may contain a material having electronic conductivity. Examples of materials having electronic conductivity include conductive materials such as carbon, titanium, chromium, molybdenum, ruthenium, rhodium, tantalum, tungsten, osmium, iridium, platinum, gold, and aluminum, as well as alloys containing two or more conductive materials (metals) (e.g., stainless steel). The material having electronic conductivity constituting the positive electrode current collector may contain carbon, aluminum, or stainless steel from the viewpoint of high electronic conductivity, excellent stability in the electrolyte, and excellent oxidation resistance. From the viewpoint of economy, the material having electronic conductivity constituting the positive electrode current collector may contain aluminum.

The positive electrode current collector may be a foil. In other words, the positive electrode current collector may be in the form of a foil. When the positive electrode current collector is a foil, from the viewpoint of achieving a further increase in capacity, the positive electrode current collector may have a primer layer provided on the surface of the foil. When the positive electrode current collector has a primer layer, the adhesion between the positive electrode active material layer and the positive electrode current collector may be improved. The primer layer may be formed, for example, by applying a binder containing a carbon-based conductive assistant onto the surface of the foil. The thickness of the primer layer may be 0.1 μm to 50 μm.

The positive electrode current collector may have a three-dimensional shape. Examples of positive electrode current collectors having a three-dimensional shape include foamed metal, mesh, woven fabric, nonwoven fabric, and expanded metal. When the positive electrode current collector has a three-dimensional shape, an electrode with high capacity density can be produced even if the material used to produce the electrode (e.g., binder) has low adhesion to the positive electrode current collector. This makes it possible to further improve the high-rate charge/discharge characteristics.

(Negative electrode)
The negative electrode has a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector usually contains a metal such as aluminum, copper, nickel, and stainless steel. From the viewpoint of ease of processing and economic efficiency, the negative electrode current collector may contain copper. The negative electrode current collector may be a foil. In other words, the negative electrode current collector may be in the form of a foil. The surface of the negative electrode current collector may be roughened.

The negative electrode active material layer includes a negative electrode active material. The negative electrode active material is a material capable of absorbing and releasing lithium. Examples of the negative electrode active material include carbon materials such as graphite and amorphous carbon, oxide materials such as indium oxide, silicon oxide, tin oxide, lithium titanate, zinc oxide, and lithium oxide, lithium metal, and metal materials capable of forming an alloy with lithium. Examples of metal materials capable of forming an alloy with lithium include copper, tin, silicon, cobalt, manganese, iron, antimony, and silver. The negative electrode active material may include two or more selected from these metals. The content of the negative electrode active material in the negative electrode active material layer may be, for example, 60 mass% or more, or 80 mass% or more, or 99.9 mass% or less, based on the total mass of the negative electrode active material layer.

From the viewpoint of achieving high energy density, the negative electrode active material may include a carbon material such as graphite, and a Si-based active material selected from Si, a Si alloy, and a Si oxide. From the viewpoint of achieving both cycle characteristics and high energy density, the negative electrode active material may include graphite and a Si-based active material. In these cases, the mass ratio of the Si-based active material to the total mass of the carbon material and the Si-based active material may be 0.5 mass% or more, 1 mass% or more, or 2 mass% or more, and may be 95 mass% or less, 50 mass% or less, or 40 mass% or less. The mass ratio of the Si-based active material to the total mass of the carbon material and the Si-based active material may be 0.5 mass% or more, 1 mass% or more, or 2 mass% or more, and 95 mass% or less. The mass ratio of the Si-based active material to the total mass of the carbon material and the Si-based active material may be 0.5 mass% or more, 1 mass% or more, or 2 mass% or more, and 40 mass% or less.

(Other ingredients)
The positive electrode active material layer and the negative electrode active material layer may further contain a binder. Examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber (SBR), carboxymethyl cellulose (CMC), polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide, polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylic acid, and copolymers thereof. The positive electrode active material layer and the negative electrode active material layer may contain the same or different binders. When the positive electrode active material layer contains a binder, the binder contained in the positive electrode active material layer may contain polyvinylidene fluoride (PVDF). When the negative electrode active material layer contains a binder, the binder contained in the negative electrode active material layer may contain CMC and/or SBR. The content of the binder in the positive electrode active material layer may be, for example, 0.1 mass % or more, 20 mass % or less, or 10 mass % or less, based on the total mass of the positive electrode active material layer. The content of the binder in the negative electrode active material layer may be, for example, 0.1 mass % or more, 20 mass % or less, or 10 mass % or less, based on the mass of the negative electrode active material layer.

The positive electrode active material layer and the negative electrode active material layer may further contain a conductive assistant. The conductive assistant may be a material containing a conductive material such as carbon. Examples of materials containing carbon include carbonaceous fine particles such as graphite, carbon black, acetylene black, and ketjen black, as well as carbon fibers. The carbon-containing material may also function as a negative electrode active material. A carbon-containing material used in combination with another negative electrode active material may be considered as a conductive assistant. The content of the conductive assistant in the positive electrode active material layer may be, for example, 0.1% by mass or more, 20% by mass or less, or 10% by mass or less, based on the mass of the positive electrode active material layer. The content of the conductive assistant in the negative electrode active material layer may be, for example, 0.1% by mass or more, 20% by mass or less, or 10% by mass or less, based on the total mass of the negative electrode active material layer.

(Separator)
The separator 6 may be a porous film. The porous film may contain, for example, a resin selected from the group consisting of polyethylene, polypropylene, and fluororesin. The separator 6 may be a single layer or may have multiple layers.

The specific configurations, such as the shape and thickness, of each component that constitutes a non-aqueous electrolyte secondary battery, such as a lithium ion battery, can be appropriately determined by a person skilled in the art.

<Non-aqueous electrolyte>
The non-aqueous electrolyte contains a compound represented by formula (1). When the non-aqueous electrolyte contains a compound represented by formula (1), the storage stability at high temperatures of a non-aqueous electrolyte secondary battery containing a combination of the non-aqueous electrolyte and a positive electrode active material containing lithium manganese iron phosphate is improved. The reason why the storage stability at high temperatures is improved by the non-aqueous electrolyte containing the compound represented by formula (1) is not necessarily clear, but it is thought to be due to the formation of a passivation film called CEI on the surface of the positive electrode.

Q in formula (1) is a group that forms a ring structure together with the sulfur atom of the sulfonyl group (-S(=O) ₂ -) in formula (1). Q represents a group selected from an alkylene group having 4 to 6 carbon atoms, which may have one or more substituents, and an alkenylene group having 4 to 6 carbon atoms, which may have one or more substituents. In the compound represented by formula (1), the number of carbon atoms of the alkylene group and alkenylene group as Q is preferably 4. In these cases, the battery resistance can be further reduced. A group represented by -X in formula (1) is bonded to a carbon atom at any position in the alkylene group and alkenylene group as Q. The alkylene group and alkenylene group as Q may further have a substituent other than the group represented by -X.

　前記式（２ａ）及び（２ｂ）中、Ｒ^１は、フッ素原子を置換基として有していてもよい炭素数１～４のアルキル基、フッ素原子を置換基として有していてもよい炭素数２～４のアルケニル基、フッ素原子を置換基として有していてもよい炭素数２～４のアルキニル基、フッ素原子を置換基として有していてもよい炭素数６～１０のアリール基、ヒドロキシ基、又はリチウムオキシ基を示す。Ｒ^１としてのアルキル基、アルケニル基、アルキニル基、及びアリール基は、それぞれ、置換基として１つ以上のフッ素原子を有していてもよい。Ｒ^１としての各基が置換基としてフッ素原子を有すると、電池抵抗をより低減することができる。電荷移動抵抗が低下する観点から、Ｘは、前記式（２ａ）で表される基であることが好ましい。 X (a group represented by -X) in formula (1) represents a group represented by the following formula (2a) or formula (2b).

In the formulas (2a) and (2b), R ¹ represents an alkyl group having 1 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkenyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkynyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an aryl group having 6 to 10 carbon atoms which may have a fluorine atom as a substituent, a hydroxyl group, or a lithium oxy group. The alkyl group, alkenyl group, alkynyl group, and aryl group as R ¹ may each have one or more fluorine atoms as a substituent. When each group as R ¹ has a fluorine atom as a substituent, the battery resistance can be further reduced. From the viewpoint of reducing the charge transfer resistance, X is preferably a group represented by the formula (2a).

In the formulas (2a) and (2b), the number of carbon atoms in the alkyl group represented by R ¹ is 1 to 4. The number of carbon atoms in the alkyl group represented by ^{R 1} may be 1 to 3, or 1 to 2. The alkyl group may be linear or branched. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a tert-butyl group, a trifluoromethyl group, and a 1,1,1-trifluoroethyl group. From the viewpoint of facilitating a reduction in the battery resistance, the alkyl group may be an unsubstituted methyl group or a methyl group having a fluorine atom as a substituent.

In the formulas (2a) and (2b), the number of carbon atoms in the alkenyl group represented by R ¹ is 2 to 4. The alkenyl group may be linear or branched. Examples of the alkenyl group include a vinyl group, an allyl group, a methallyl group (2-methylallyl group), a 1-butenyl group, a 2-butenyl group, a 3-butenyl group, an isobutenyl group, and a 1,1-difluoro-1-propenyl group. The alkenyl group may be a vinyl group which may have a fluorine atom as a substituent, an allyl group which may have a fluorine atom as a substituent, or a methallyl group which may have a fluorine atom as a substituent. When the alkenyl group is an allyl group which may have a fluorine atom as a substituent or a methallyl group which may have a fluorine atom as a substituent, a stronger CEI is likely to be formed.

In the formulas (2a) and (2b), the number of carbon atoms in the alkynyl group represented by R ¹ is 2 to 4, and may be 3 to 4. The alkynyl group may be linear or branched. Examples of the alkynyl group include a 1-propynyl group, a 2-propynyl group, a 1-butynyl group, a 2-butynyl group, and a 3-butynyl group. The alkynyl group may be a 2-propynyl group which may have a fluorine atom as a substituent. When the alkynyl group is a 2-propynyl group which may have a fluorine atom as a substituent, a stronger CEI is likely to be formed.

In the formulas (2a) and (2b), the number of carbon atoms in the aryl group represented by ^R1 is 6 to 10. Examples of the aryl group having 6 to 10 carbon atoms include a phenyl group, a tosyl group, a xylyl group, a naphthyl group, a fluorophenyl group, and a pentafluorophenyl group. The aryl group may be a phenyl group which may have a fluorine atom as a substituent. The aryl group may be a phenyl group, a 4-fluorophenyl group, or a pentafluorophenyl group.

　式（１ａ）中、Ｘは、前述の通りに定義される。

　式（１ｂ）中、Ｘは、前述の通りに定義される。 From the viewpoint of high-temperature storage stability of the battery, the compound of formula (1) may be a compound represented by the following formula (1a) or formula (1b).

In formula (1a), X is defined as above.

In formula (1b), X is defined as above.

Specific examples of the compound represented by formula (1) above include compounds represented by the following formulas (1-1), (1-2), (1-3), (1-4), (1-5), (1-6), (1-7), (1-8), and (1-9).

The nonaqueous electrolyte may contain one type of compound represented by formula (1), or may contain two or more types of compounds.

The content of the compound represented by formula (1) in the non-aqueous electrolyte is, for example, a total of 0.005 to 10 mass% based on the total mass of the non-aqueous electrolyte. When the content of the compound represented by formula (1) is 0.005 mass% or more, the stability of the SEI is improved and better battery characteristics are obtained. When the content of the compound represented by formula (1) is 10 mass% or less, an increase in the viscosity of the non-aqueous electrolyte can be suppressed. The content of the compound represented by formula (1) may be a total of 0.005 to 5 mass% based on the total mass of the non-aqueous electrolyte.

(Non-aqueous solvent)
The non-aqueous solvent used in the non-aqueous electrolyte may be an aprotic solvent from the viewpoint of suppressing the viscosity of the non-aqueous electrolyte. The non-aqueous solvent may be at least one selected from the group consisting of cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, lactones, lactams, cyclic ethers, chain ethers, sulfones (excluding the compound represented by formula (1)), nitriles, and halogen derivatives thereof. The non-aqueous solvent may contain at least one of cyclic carbonates and chain carbonates, or may contain a combination of cyclic carbonates and chain carbonates.

Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. Examples of linear carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of aliphatic carboxylic acid esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, and methyl trimethylacetate. Examples of lactones include γ-butyrolactone. Examples of lactams include ε-caprolactam and N-methylpyrrolidone. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, and 1,3-dioxolane. Examples of linear ethers include 1,2-diethoxyethane and ethoxymethoxyethane. An example of a sulfone is sulfolane. An example of a nitrile is acetonitrile. Examples of halogen derivatives include 4-fluoro-1,3-dioxolan-2-one, 4-chloro-1,3-dioxolan-2-one, and 4,5-difluoro-1,3-dioxolan-2-one. The nonaqueous electrolyte may contain one or more nonaqueous solvents selected from these.

(Electrolytes)
The non-aqueous electrolyte usually contains a lithium salt as an electrolyte, which is an ion source of lithium ions. The lithium salt may be at least one selected from the group consisting of LiAlCl ₄ , LiBF ₄ , LiPF ₆ , LiClO ₄ , LiTFSI (lithium bistrifluoromethanesulfonimide), LiFSI (lithium bisfluorosulfonimide), LiAsF ₆ and LiSbF _6. The non-aqueous electrolyte may contain one or more electrolytes selected from these. The electrolyte may contain LiBF ₄ , LiPF ₆ or a combination thereof. When the electrolyte contains LiBF ₄ and/or LiPF ₆ , the degree of dissociation is high, which can increase the ionic conductivity of the electrolyte, and further, the oxidation-reduction resistance characteristic can suppress the performance deterioration of the non-aqueous electrolyte secondary battery due to long-term use.

When the electrolyte is LiBF ₄ , LiPF ₆ or a combination thereof, the non-aqueous solvent may contain a cyclic carbonate and a chain carbonate. For example, LiBF ₄ and/or LiPF ₆ may be combined with ethylene carbonate and diethyl carbonate.

The electrolyte concentration may be 0.1 mol/L or more, or 0.5 mol/L or more, and may be 2.0 mol/L or less, or 1.5 mol/L or less, based on the total volume of the nonaqueous electrolyte. When the electrolyte concentration is 0.1 mol/L or more, or 0.5 mol/L or more, based on the total volume of the nonaqueous electrolyte, good conductivity of the electrolyte is easily obtained, and when it is 2.0 mol/L or less, or 1.5 mol/L or less, an increase in the viscosity of the electrolyte can be suppressed. The electrolyte concentration may be 0.1 mol/L or more and 2.0 mol/L or less, or 1.5 mol/L or less, based on the total volume of the nonaqueous electrolyte.

(Other ingredients)
The non-aqueous electrolyte may contain other components different from the compound represented by formula (1), the non-aqueous solvent, and the electrolyte, as necessary. Examples of other components include anode protectors, cathode protectors, flame retardants, overcharge inhibitors, cyclic carbonate compounds, nitrile compounds, isocyanate compounds, compounds having an acetylene-1,2-diyl group (-C≡C-), compounds having a sulfonyl group (>S(=O) ₂ ) (excluding the compound represented by formula (1)), phosphate compounds, acid anhydrides, cyclic phosphazene compounds, cyclic dioxazole compounds, boroxine derivatives, compounds containing silicon atoms, and alkali metal salt compounds (e.g., lithium salt compounds).

Examples of the cyclic carbonate compound include 4-fluoro-1,3-dioxolan-2-one (FEC), trans- or cis-4,5-difluoro-1,3-dioxolan-2-one (DFEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and 4-ethynyl-1,3-dioxolan-2-one (EEC). The cyclic carbonate compound may be VC, FEC, VEC, or a combination thereof.

Examples of the nitrile compound include acetonitrile, propionitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, and sebaconitrile. The nitrile compound may be succinonitrile, adiponitrile, or a combination thereof.

Examples of the isocyanate compound include methyl isocyanate, ethyl isocyanate, butyl isocyanate, phenyl isocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 1,4-phenylene diisocyanate, 2-isocyanatoethyl acrylate, and 2-isocyanatoethyl methacrylate.

Examples of compounds having the acetylene-1,2-diyl group (-C≡C-) include 2-propynyl methyl carbonate, 2-propynyl acetate, 2-propynyl formate, 2-propynyl methacrylate, 2-propynyl methanesulfonate, 2-propynyl vinylsulfonate, 2-propynyl 2-(methanesulfonyloxy)propionate, di(2-propynyl)oxalate, methyl-2-propynyl oxalate, ethyl-2-propynyl oxalate, di(2-propynyl) glutarate, 2-butyne-1,4-diyl dimethanesulfonate, 2-butyne-1,4-diyl diformate, and 2,4-hexadiyn-1,6-diyl dimethanesulfonate.

Examples of the compound having a sulfonyl group (>S(=O) ₂ ) include sultones such as 1,3-propane sultone (PS), 1,3-butane sultone, 2,4-butane sultone, 1,4-butane sultone, 1,3-propene sultone, 2,2-dioxide-1,2-oxathiolan-4-yl acetate, and 5,5-dimethyl-1,2-oxathiolan-4-one 2,2-dioxide, ethylene sulfite, ethylene sulfate, hexahydrobenzo[1,3,2]dioxathiolan-2-oxide (1,2-cyclohexa Examples of suitable cyclic sulfides include 5-vinyl-hexahydro-1,3,2-benzodioxathiol-2-oxide and other cyclic sulfites; butane-2,3-diyldimethanesulfonate, butane-1,4-diyldimethanesulfonate, methylenemethane disulfonate, 1,3-propanedisulfonic anhydride and other sulfonic acid esters; divinyl sulfone, 1,2-bis(vinylsulfonyl)ethane, and bis(2-vinylsulfonylethyl)ether.

Examples of the phosphate ester compounds include trimethyl phosphate, tributyl phosphate, trioctyl phosphate, tris(2,2,2-trifluoroethyl)phosphate, bis(2,2,2-trifluoroethyl)methyl phosphate, bis(2,2,2-trifluoroethyl)ethyl phosphate, bis(2,2,2-trifluoroethyl)2,2-difluoroethyl phosphate, bis(2,2,2-trifluoroethyl)2,2,3,3-tetrafluoropropyl phosphate, bis(2,2-difluoroethyl)2,2,2-trifluoroethyl phosphate, bis(2,2,3,3-tetrafluoropropyl)2,2,2-trifluoroethyl phosphate, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoropropyl)methyl phosphate, tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphate, methyl methylenebisphosphonate, methylenebis Ethyl phosphonate, methyl ethylene bisphosphonate, ethyl ethylene bisphosphonate, methyl butylene bisphosphonate, ethyl butylene bisphosphonate, methyl 2-(dimethyl phosphoryl) acetate, ethyl 2-(dimethyl phosphoryl) acetate, methyl 2-(diethyl phosphoryl) acetate, ethyl 2-(diethyl phosphoryl) acetate, 2-propynyl 2-(dimethyl phosphoryl) acetate, 2-propynyl 2-(diethyl phosphoryl) acetate, methyl 2-(dimethoxy phosphoryl) acetate, ethyl 2-(dimethoxy phosphoryl) acetate, methyl 2-(diethoxy phosphoryl) acetate, ethyl 2-(diethoxy phosphoryl) acetate, 2-propynyl 2-(dimethoxy phosphoryl) acetate, 2-propynyl 2-(diethoxy phosphoryl) acetate, methyl pyrophosphate, and ethyl pyrophosphate.

Examples of the acid anhydrides include acetic anhydride, propionic anhydride, succinic anhydride, maleic anhydride, 3-allyl succinic anhydride, glutaric anhydride, itaconic anhydride, and 3-sulfo-propionic anhydride.

Examples of the cyclic phosphazene compounds include methoxypentafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, phenoxypentafluorocyclotriphosphazene, and ethoxyheptafluorocyclotetraphosphazene.

Examples of the cyclic dioxazole compound include 3-phenyl-1,4,2-dioxazol-5-one, 3-(2-fluorophenyl)-1,4,2-dioxazol-5-one, 3-(3-fluorophenyl)-1,4,2-dioxazol-5-one, 3-(4-fluorophenyl)-1,4,2-dioxazol-5-one, 3-(4-methoxyphenyl)-1,4,2-dioxazol-5-one, 3-(2-thienyl)-1,4,2-dioxazol-5-one, 3-(2,3,4,5,6-pentafluorophenyl)-1,4,2-dioxazol-5-one, 3-[4-(trifluoromethyl)phenyl]-1,4,2-dioxazol-5-one, and 3-(4-nitrophenyl)-1,4,2-dioxazol-5-one.

Examples of the boroxine derivatives include boroxine, trimethylboroxine, trimethoxyboroxine, triethylboroxine, triethoxyboroxine, triisopropylboroxine, triisopropoxyboroxine, tri-n-propylboroxine, tri-n-propoxyboroxine, tri-n-butylboroxine, tri-n-butyloxyboroxine, triphenylboroxine, triphenoxyboroxine, tricyclohexylboroxine, and tricyclohexoxyboroxine.

Examples of the compounds containing silicon atoms include hexamethylcyclotrisiloxane, hexaethylcyclotrisiloxane, hexaphenylcyclotrisiloxane, 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, trimethylfluorosilane, triethylfluorosilane, tripropylfluorosilane, phenyldimethylfluorosilane, triphenylfluorosilane, vinyldimethylfluorosilane, vinyldiethylfluorosilane, vinyldiphenylfluorosilane, divinyldifluorosilane, divinyldimethylsilane, trimethoxyfluorosilane, triethoxyfluorosilane, dimethyldifluorosilane, diethyldifluorosilane, trivinylfluorosilane, trivinylmethylsilane, ethylvinyldifluorosilane, methyltrifluorosilane, and ethyltrifluorosilane. , hexamethyldisiloxane, 1,3-diethyltetramethyldisiloxane, hexaethyldisiloxane, octamethyltrisiloxane, methoxytrimethylsilane, ethoxytrimethylsilane, dimethoxydimethylsilane, trimethoxymethylsilane, tetramethoxysilane, tetravinylsilane, tetraallylsilane, tetrabutenylsilane, bis(trimethylsilyl)peroxide, trimethylsilyl acetate, triethylsilyl acetate, trimethylsilyl propionate, trimethylsilyl methacrylate, trimethylsilyl trifluoroacetate, trimethylsilyl methanesulfonate, trimethylsilyl ethanesulfonate, triethylsilyl methanesulfonate, trimethylsilyl fluoromethanesulfonate, bis(trimethylsilyl)sulfate, tris(trimethylsiloxy)boron, tris(trimethylsilyl)phosphate, and tris(trimethylsilyl)phosphite.

Examples of the lithium salt compound include lithium salts having a phosphate skeleton, such as lithium difluorophosphate, lithium bisoxalatoborate (LiBOB), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium difluorooxalatoborate (LiDFOB), lithium difluorobisoxalatophosphate (LiDFOP), lithium tetrafluoroborate, lithium bisfluorosulfonylimide, lithium tetrafluoro(oxalato)phosphate, and Li ₂ PO ₃ F, and lithium salts having an S(═O) group, such as lithium trifluoro((methanesulfonyl)oxy)borate, lithium pentafluoro((methanesulfonyl)oxy)phosphate, lithium methylsulfate, lithium ethylsulfate, lithium 2,2,2-trifluoroethylsulfate, and lithium fluorosulfonate. The lithium salt compound may be one or more lithium salt compounds selected from the group consisting of lithium difluorophosphate, lithium bisoxalatoborate, lithium tetrafluoro(oxalato)phosphate, lithium difluorooxalatoborate, lithium difluorobisoxalatophosphate, lithium methylsulfate, lithium ethylsulfate, and lithium fluorosulfonate.

Other examples of the alkali metal salt compounds include sodium difluorophosphate, potassium difluorophosphate, sodium bisoxalatoborate, potassium bisoxalatoborate, sodium tetrafluoro(oxalato)phosphate, potassium tetrafluoro(oxalato)phosphate, sodium difluorobis(oxalato)phosphate, potassium difluorobis(oxalato)phosphate, sodium difluorooxalatoborate, and potassium difluorooxalatoborate.

The content of the other components may be 0.005 to 10 mass %, or 0.01 to 10 mass %, based on the total mass of the non-aqueous electrolyte. If the content of the other components is 0.005 mass % or more, better battery characteristics are likely to be obtained. If the content of the other components is 10 mass % or less, the increase in viscosity of the non-aqueous electrolyte can be further suppressed.

The following examples further illustrate the embodiments of the present invention.
1. Synthesis Example 1 of the Compound Represented by Formula (1)
3-Sulfolene (236.3 g, 2.0 mol) and 500 mL of water were placed in a 2 L four-neck flask equipped with a stirrer, a cooling tube, and a thermometer, and the temperature was raised to 40° C. to prepare a homogeneous solution. Sodium hydroxide (104.0 g, 2.6 mol) was added to the solution, and the solution was stirred for 10 hours while maintaining the temperature at 40° C. Then, the flask was cooled in an ice bath. Concentrated sulfuric acid (130.1 g, 1.3 mol) was added dropwise to the obtained reaction solution over 30 minutes to make the reaction solution acidic. The reaction solution was concentrated to precipitate a solid, and the precipitated solid was removed by filtration, and the obtained filtrate was concentrated to obtain 3-hydroxysulfolane (250.59 g, yield 92% based on 3-sulfolene).

Production Example 2: Synthesis of a compound represented by formula (1-1) In a 200 mL four-neck flask equipped with a stirrer, a cooling tube, a thermometer, and a dropping funnel, 3-hydroxysulfolane (5.4 g, 40 mmol) obtained in Production Example 1, methanesulfonyl chloride (5.4 g, 44 mmol), and 20 mL of acetonitrile were placed. The flask was cooled in an ice bath, and triethylamine (4.0 g, 40 mmol) was added dropwise while stirring the reaction solution in the flask. After the dropwise addition was completed, the reaction solution was stirred for 1 hour while maintaining the temperature at 0 to 5°C. Thereafter, water was added, and the precipitated white solid was collected by filtration. The collected solid was washed with methanol and then dried under reduced pressure to obtain a white solid compound (compound represented by formula (1-1)) (7.0 g, yield 82% relative to 3-hydroxysulfolane).
^1H -NMR (400MHz, _CD3CN ) δ (ppm): 2.53 (m, 2H), 3.11 (s, 3H), 3.16 (m, 2H), 3.36 (m, 2H), 5.39 (s, 1H)

Production Example 3: Synthesis of a compound represented by formula (1-4) In a 200 mL four-neck flask equipped with a stirrer, a cooling tube, a thermometer, and a dropping funnel, 3-hydroxysulfolane (5.4 g, 40 mmol) obtained in Production Example 1, benzenesulfonyl chloride (7.8 g, 44 mmol), and 20 mL of acetonitrile were placed. The flask was cooled in an ice bath, and triethylamine (4.0 g, 40 mmol) was added dropwise while stirring the reaction solution in the flask. After the dropwise addition was completed, the reaction solution was stirred for 1 hour while maintaining the temperature at 0 to 5°C. Thereafter, water was added, and the precipitated white solid was collected by filtration. The collected solid was washed with methanol and then dried under reduced pressure to obtain a white solid compound (compound represented by formula (1-4)) (8.2 g, yield 68% relative to 3-hydroxysulfolane).
^1H -NMR (400MHz, _CD3CN ) δ (ppm): 2.39 (m, 2H), 3.18 (m, 4H), 5.25 (m, 1H), 7.66 (m, 2H), 7.79 (m, 1H), 7.94 (m, 2H)

Production Example 4: Synthesis of Compound Represented by Formula (1-5) Sodium 2-methyl-2-propene-1-sulfonate (8.04 g, 50 mmol), N,N-dimethylformamide (0.37 g, 5 mmol) and 30 mL of dichloromethane were placed in a 200 mL four-neck flask equipped with a stirrer, a cooling tube, a thermometer and a dropping funnel. While stirring the reaction solution in the flask, thionyl chloride (7.14 g, 60 mmol) was added dropwise to the reaction solution at room temperature (25° C.). After the dropwise addition was completed, the reaction solution was stirred for 24 hours while maintaining the temperature at 20 to 25° C. Then, the mixture was separated with water and the oil layer was concentrated to obtain 2-methyl-2-propene-1-sulfonyl chloride (7.73 g, 100% yield relative to sodium 2-methyl-2-propene-1-sulfonate).
Next, 3-hydroxysulfolane (5.45 g, 40 mmol) obtained in Production Example 1, 2-methyl-2-propene-1-sulfonyl chloride (6.80 g, 44 mmol) and 20 mL of acetonitrile were placed in a 200 mL four-neck flask equipped with a stirrer, a cooling tube, a thermometer and a dropping funnel. The flask was cooled in an ice bath, and triethylamine (4.05 g, 40 mmol) was added dropwise while stirring the reaction solution in the flask. After the dropwise addition was completed, the reaction solution was stirred for 1 hour while maintaining the temperature at 0 to 5°C. Thereafter, water was added, and the precipitated white solid was collected by filtration. The collected solid was washed with methanol and then dried under reduced pressure to obtain a white solid compound (compound represented by formula (1-5)) (5.84 g, 50% yield relative to 3-hydroxysulfolane).
^1H -NMR (400MHz, _CD3CN ) δ (ppm): 1.93 (m, 3H), 2.53 (m, 2H), 3.18 (m, 2H), 3.36 (dd, 1H), 3.41 (dd, 1H), 3.97 (s, 2H), 5.15 (s, 1H), 5.22 (s, 1H), 5.42 (m, 1H)

2. Preparation of a non-aqueous electrolyte secondary battery (Example 1)
Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume composition ratio of EC:EMC = 30:70 to obtain a mixed nonaqueous solvent. LiPF ₆ was dissolved as an electrolyte in the obtained mixed nonaqueous solvent to a concentration of 1.0 mol/L. A compound represented by formula (1-1) was added as an additive to the obtained solution to prepare a nonaqueous electrolyte. The content ratio of the compound represented by formula (1-1) was 1.0 mass% based on the total mass of the nonaqueous electrolyte.
Next, a positive electrode sheet (manufactured by Yayama Co., Ltd.) containing _{LiMn0.6Fe0.4PO4} as a positive _electrode active material and _a negative electrode sheet (manufactured by Yayama Co., Ltd.) containing graphite as a negative electrode active material were prepared. The positive electrode sheet had an aluminum foil (thickness 20 μm) as a positive electrode current collector and a positive _electrode active material layer formed on both sides thereof. _The positive electrode active material layer contained _{LiMn0.6Fe0.4PO4} as a positive electrode active material, acetylene black (AB) and single-wall CNT (SWCNT ₎ as conductive assistants, _and polyvinylidene fluoride (PVDF) as a binder. The mass ratio of these was _{LiMn0.6Fe0.4PO4} :AB:SWCNT:PVDF=94:3:0.1:2.9. The negative electrode sheet had a copper foil (thickness 10 μm) as a negative electrode current collector and a negative electrode active material layer formed on both sides thereof. The negative electrode active material layer contained graphite (Gr) as a negative electrode active material, and sodium carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) as binders. The mass ratio of these was Gr:CMC:SBR=98:1:1. The positive electrode sheet and the negative electrode sheet were alternately laminated via a polypropylene separator to produce a battery element having a total of nine layers of five negative electrode sheets and four positive electrode sheets as electrodes.

The battery element produced by the above method was inserted into a bag formed from a laminate film having aluminum (thickness 40 μm) and a resin layer covering both sides of the bag, so that the ends of the positive electrode sheet and the negative electrode sheet protruded from the bag. Next, nonaqueous electrolyte was injected into the bag, and the bag was vacuum sealed to obtain a sheet-shaped nonaqueous electrolyte secondary battery. In order to increase the adhesion between the electrodes, the sheet-shaped nonaqueous electrolyte secondary battery was sandwiched between glass plates and pressurized to produce a nonaqueous electrolyte secondary battery.

(Example 2)
A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by formula (1-1) was changed to the compound represented by formula (1-4).

Example 3
A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by formula (1-1) was changed to the compound represented by formula (1-5).
(Comparative Example 1)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1, except that the compound represented by formula (1-1) was changed to 1,3-propane sultone (PS, manufactured by Tokyo Chemical Industry Co., Ltd.).

3. Evaluation of battery characteristics <Battery preparation before evaluation>
(Aging process)
Each nonaqueous electrolyte secondary battery was charged at 25 ° C. for 1 hour at a current corresponding to 0.1 C and held at 25 ° C. for 10 hours. Then, it was charged at a current corresponding to 0.1 C for 5 hours and held at 45 ° C. for 24 hours. Then, it was discharged at a current corresponding to 0.1 C to 3.0 V at 25 ° C., and then degassed. Then, it was charged at a current corresponding to 0.2 C to 4.2 V, discharged at a current corresponding to 0.2 C to 3 V for 3 cycles, charged at a current corresponding to 0.5 C to 4.2 V, discharged at a current corresponding to 0.5 C to 3.0 V for 3 cycles, charged at a current corresponding to 1.0 C to 4.2 V, and discharged at a current corresponding to 1.0 C to 3.0 V for 3 cycles. The battery was stabilized by aging.

(High temperature storage process)
The nonaqueous electrolyte secondary battery after the aging process was charged to 4.2 V at 1.0 C at 25° C., and then held for 30 days at 60° C. Thereafter, the nonaqueous electrolyte secondary battery was cooled to 25° C., and discharged to 3.0 V at a current equivalent to 1.0 C.

<Charge transfer resistance measurement>
The battery that had been subjected to the aging process and the high-temperature storage process was charged and discharged (4.2 V to 3.0 V) at a current equivalent to 1.0 C for two cycles, and then charged at a current equivalent to 1.0 C and discharged at a current equivalent to 1.0 C so that the state of charge (SOC) was 50%. The nonaqueous electrolyte secondary battery adjusted to an SOC of 50% was left to stand at 25° C. for two hours, and then the charge transfer resistance (Rct) of the positive and negative electrodes was measured using a Solartoron SI1260 (measurement conditions: 0.01 Hz to 1×10 ⁶ Hz, amplitude 5 mV). The results are shown in Table 1.

<Measurement of Metal Leaching Amount>
The nonaqueous electrolyte secondary battery after the charge transfer resistance measurement was discharged to 3.0 V at a current equivalent to 1.0 C, and then the nonaqueous electrolyte secondary battery was disassembled and the negative electrode was taken out. The negative electrode was washed with dimethyl carbonate (DMC) and then vacuum dried for 10 minutes. Then, the active material layer on the negative electrode was scraped off to make a powder. 60% nitric acid was added to the collected powder, and the powder was dissolved by heating to prepare a measurement solution. The masses of Fe and Mn contained in the measurement solution were measured by analysis using an inductively coupled plasma optical emission spectrometer (manufactured by Thermo Fisher). The amount of elution of Fe or Mn per unit mass of the active material layer was calculated by the following formula. The results are shown in Table 1.
Amount of eluted Fe or Mn (ppm)=(mass of Fe or Mn contained in measurement solution/mass of scraped-off negative electrode active material layer)

As described above, one aspect of the present invention provides a nonaqueous electrolyte secondary battery that contains lithium manganese iron phosphate as a positive electrode active material and has excellent storage stability at high temperatures, such as suppressing an increase in charge transfer resistance even when stored at a high temperature of 60°C for 30 days. Such a nonaqueous electrolyte secondary battery can contribute to solving environmental problems from the perspective of reducing waste by extending the battery life, and has extremely high industrial applicability.

1... Non-aqueous electrolyte secondary battery, 2... Negative electrode active material layer, 3... Negative electrode current collector, 4... Negative electrode, 5... Non-aqueous electrolyte, 6... Separator, 7... Positive electrode active material layer, 8... Positive electrode current collector, 9... Positive electrode.

Claims (7)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte,
The positive electrode contains LiMn _{(1-y) FeyPO4} as a positive electrode active material, and y is _a value that satisfies 0<y<1;
The nonaqueous electrolyte solution has the following formula (1):

The compound represented by the formula:
In formula (1), Q represents a group which forms a ring structure together with the sulfur atom of the sulfonyl group and is selected from an alkylene group having 4 to 6 carbon atoms which may have one or more substituents and an alkenylene group having 4 to 6 carbon atoms which may have one or more substituents, and X represents a group represented by the following formula (2a) or the following formula (2b):

represents a group represented by
In formula (2a) and formula (2b), R ¹ represents an alkyl group having 1 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkenyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkynyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an aryl group having 6 to 10 carbon atoms which may have a fluorine atom as a substituent, a hydroxy group, or a lithium oxy group.
Nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1, wherein in formula (1), Q is an alkylene group having 4 carbon atoms which may have one or more substituents, or an alkenylene group having 4 carbon atoms which may have one or more substituents. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein in formulas (2a) and (2b), R ¹ is an alkyl group having 1 to 2 carbon atoms which may have a fluorine atom as a substituent, an alkenyl group having 2 to 4 carbon atoms which may have a fluorine atom as a substituent, an alkynyl group having 3 to 4 carbon atoms which may have a fluorine atom as a substituent, or a phenyl group which may have a fluorine atom as a substituent. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein X is a group represented by formula (2a). The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein X is a group represented by formula (2b). The compound represented by formula (1) is represented by the following formula (1a):

In the formula (1a), X has the same meaning as X in the formula (1),
3. The nonaqueous electrolyte secondary battery according to claim 1 or 2. The compound represented by formula (1) is represented by the following formula (1b):

In the formula (1b), X has the same meaning as X in the formula (1),
3. The nonaqueous electrolyte secondary battery according to claim 1 or 2.

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