JP7180861B2 - Electrolyte, lithium ion secondary battery, compound - Google Patents

Description

TECHNICAL FIELD The present invention relates to electrolyte solutions, lithium-ion secondary batteries, and compounds.

In recent years, various types of batteries have been developed in connection with power sources for small portable devices such as mobile phones, late-night power storage systems, household distributed power storage systems based on solar power generation, and power storage systems for electric vehicles. is being developed.
Lithium-ion secondary batteries are attracting attention as such batteries (Patent Document 1).

JP-A-7-006786

In lithium-ion secondary batteries, studies are underway to increase the battery voltage. One of the means for achieving higher voltage batteries is to use a positive electrode that operates at a high potential.
On the other hand, in a lithium ion secondary battery including a positive electrode that operates at a high potential, a solvent typified by a carbonate-based solvent contained in the electrolyte is oxidatively decomposed on the surface of the positive electrode, and charging of the lithium ion secondary battery is hindered. It was confirmed in the discharge evaluation that the discharge capacity was not sufficient.
In addition, in lithium ion secondary batteries, the positive electrode active material contained in the positive electrode is also required to have excellent structural stability. The term “excellent in structural stability” means that the shape of the positive electrode active material contained in the positive electrode changes little when the positive electrode and the electrolytic solution are brought into contact with each other.

In view of the above circumstances, the present invention provides an excellent discharge capacity during charging and discharging when applied to a lithium ion secondary battery (especially a lithium ion secondary battery including a positive electrode that operates at a high potential), and a positive electrode An object of the present invention is to provide an electrolytic solution in which the active material has excellent structural stability.
Another object of the present invention is to provide a lithium ion secondary battery and a compound.

As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved by using an electrolytic solution containing a predetermined compound, and have completed the present invention.
That is, the above problems can be solved by means shown below.

(1) An electrolytic solution containing a compound represented by Formula (1) described below, a solvent, and a lithium salt.
(2) The electrolytic solution according to (1), wherein the compound represented by formula (1) is a compound represented by formula (2) described below.
(3) The electrolytic solution according to (1) or (2), wherein X represents a group represented by formula (A).
(4) The electrolytic solution according to any one of (1) to (3), wherein L represents a divalent conjugated linking group.
(5) The electrolyte solution according to any one of (1) to (4), wherein the content of the compound represented by formula (1) is 0.01 to 10% by mass with respect to the total mass of the electrolyte solution. .
(6) Any one of (1) to (5), wherein the solvent is selected from the group consisting of carbonate solvents, ester solvents, ether solvents, ketone solvents, alcohol solvents, and aprotic solvents. The electrolytic solution described in .
(7) A lithium ion secondary battery containing the electrolytic solution according to any one of (1) to (6).
(8) A compound represented by formula (2) described later.

According to the present invention, when applied to a lithium ion secondary battery (especially a lithium ion secondary battery including a positive electrode that operates at a high potential), it has excellent discharge capacity during charging and discharging, and the structure of the positive electrode active material An electrolytic solution having excellent stability can be provided.
According to the present invention, a lithium ion secondary battery and a compound can be provided.

1 is the ¹ H NMR spectrum of Compound 1. FIG. ¹³ C NMR spectrum of Compound 1. FIG. 1 is an infrared absorption (IR) spectrum of compound 1; (A) is a diagram (cyclic voltammogram) showing the results of cyclic voltammetry measurement using a mixture containing compound 1, and (B) is the result of cyclic voltammetry measurement using a mixture that does not contain compound 1. It is a figure which shows. (A) is the XPS measurement result of the positive electrode before cyclic voltammetry measurement. (B) is the XPS measurement result of the positive electrode obtained by performing cyclic voltammetry measurement in a mixed solution containing no compound 1. FIG. (C) is an XPS measurement result of the positive electrode obtained by performing cyclic voltammetry measurement in a mixed solution containing Compound 1. FIG. (A) is an XPS measurement result of a positive electrode obtained by performing cyclic voltammetry measurement in a mixed solution containing no compound 1. FIG. (B) is an XPS measurement result of the positive electrode obtained by performing cyclic voltammetry measurement in a mixed solution containing Compound 1. FIG. 4 is a diagram showing changes in charge capacity due to cycling of a cell containing a mixed solution containing compound 1 and a cell containing a mixed solution not containing compound 1. FIG. (A) is a SEM (scanning electron _microscope ) observation _view of a _{LiMnxNiyCozO2} electrode before being immersed in _a mixed solution. (B) is an SEM observation view of _{a LiMnxNiyCozO2 electrode} after being immersed in a mixed solution containing Compound _1. FIG. (C) is an SEM observation view of _{the LiMnxNiyCozO2 electrode} after being immersed in a mixed solution containing no compound _1. FIG.

The present invention will be described in detail below.
As used herein, the term "to" is used to include the numerical values before and after it as lower and upper limits.

A feature of the present invention is the use of a compound represented by Formula (1) described later (hereinafter, also simply referred to as "compound (1)"). X in formula (1) is a group capable of electropolymerization. Therefore, when a lithium ion secondary battery is charged and discharged using an electrolytic solution containing compound (1), polymerization of compound (1) proceeds via the X portion, and a film can be formed on the positive electrode. By forming this film, oxidative decomposition of the solvent (for example, the carbonate-based solvent) is suppressed, and as a result, the discharge capacity during charging and discharging is excellent.
Compound (1) is a Schiffs base containing a nitrogen atom, and can coordinate to the positive electrode active material via this nitrogen atom. Surprisingly, the structural stability of the positive electrode active material is improved by the coordination of the compound (1) to the positive electrode active material.
Furthermore, the nitrogen atoms in compound (1) can be expected to have a function of trapping hydrofluoric acid derived from lithium salt, and as a result, electrode corrosion is further reduced.

Below, the compound (1) will be described in detail first, and then other components contained in the electrolytic solution will be described in detail.

<Compound (1)>
The electrolytic solution contains compound (1).

R ¹ and R ² each independently represent a substituent. R ¹ and R ² may combine with each other to form a ring.
The types of substituents are not particularly limited, and examples include halogen atoms, alkyl groups (including cycloalkyl groups), alkenyl groups (including cycloalkenyl groups and bicycloalkenyl groups), alkynyl groups, aromatic groups (e.g., aromatic hydrocarbon group, aromatic heterocyclic group), cyano group, hydroxyl group, nitro group, carboxyl group, alkoxy group, aryloxy group, silyloxy group, heterocyclic oxy group, acyloxy group, carbamoyloxy group, alkoxycarbonyloxy group, Aryloxycarbonyloxy, amino groups (including anilino groups), acylamino groups, aminocarbonylamino groups, alkoxycarbonylamino groups, aryloxycarbonylamino groups, sulfamoylamino groups, alkyl and arylsulfonylamino groups, mercapto groups, alkylthio arylthio groups, heterocyclicthio groups, sulfamoyl groups, sulfo groups, alkyl and arylsulfinyl groups, alkyl and arylsulfonyl groups, acyl groups, aryloxycarbonyl groups, alkoxycarbonyl groups, carbamoyl groups, aryl and heterocyclic azo groups, Imido groups, phosphino groups, phosphinyl groups, phosphinyloxy groups, phosphinylamino groups, silyl groups, and combinations thereof.
In addition, the above substituent may be further substituted with another substituent.

R ¹ and R ² may combine with each other to form a ring.
The type of ring formed by combining R ¹ and R ² is not particularly limited, and may be an aliphatic ring or an aromatic ring.
In addition, the ring formed by combining R ¹ and R ² with each other may be a single ring or a condensed ring. A condensed ring is preferred because at least one of the effects of better structural stability can be obtained (hereinafter also simply referred to as "the effect of the present invention is excellent").
The number of rings contained in the condensed ring is not particularly limited, preferably 2 to 5, more preferably 3 to 4, and even more preferably 3.
The number of carbon atoms contained in the ring formed by combining R ¹ and R ² is not particularly limited, but is preferably 6-20, more preferably 10-15.
Examples of the ring formed by combining R ¹ and R ² include acenaphthene.

Each L independently represents a divalent linking group.
The type of divalent linking group is not particularly limited, and examples include -O-, -C(=O)-, -NH-, -C(=O)NH-, -C(=O)O-, alkylene groups, alkenylene groups, alkynylene groups, arylene groups, heteroarylene groups, and groups in combination of two or more thereof.
Among them, as the divalent linking group, a divalent conjugated linking group is preferable. When L is a divalent conjugated linking group, the conductivity of the coating formed by compound (1) is superior, resulting in superior discharge capacity during charging and discharging.
A divalent conjugated linking group is intended to be a divalent linking group in which the conjugated system extends from one bonding position to the other bonding position.
The divalent conjugated linking group includes an arylene group, a heteroarylene group, -CR ³ =CR ³ -, -C≡C-, -N=N-, -arylene group -Y-, and -heteroarylene group -Y -, and groups in which two or more of these are combined.
^R3 represents a hydrogen atom or a substituent. The definition of the substituents is the same as the substituents represented by R ¹ and R ² described above.
Y represents -O-, -S- or -NH-.

Although the number of carbon atoms in the arylene group is not particularly limited, it is preferably 6 to 30, more preferably 6 to 20, even more preferably 6 to 10, from the standpoint of better effects of the present invention.
Examples of rings constituting an arylene group include benzene ring, naphthalene ring, anthracene ring, phenanthrene ring, fluorene ring, triphenylene ring, pyrene ring, naphthacene ring, and biphenyl ring (two phenyl groups are linked in any ), and a terphenyl group (three phenyl groups may be linked in any linking manner).

A heteroarylene group may have a monocyclic structure or a condensed ring structure. In the case of a condensed ring structure, it may be a structure composed of a plurality of aromatic heterocycles, or a structure composed of a combination of an aromatic hydrocarbon ring and an aromatic heterocycle. .
Heteroatoms contained in the heteroarylene group include oxygen, sulfur and nitrogen atoms.
Examples of the aromatic heterocyclic ring contained in the heteroarylene group include furan ring, thiophene ring, pyrrole ring, oxazole ring, isoxazole ring, oxadiazole ring, thiazole ring, isothiazole ring, thiadiazole ring, imidazole ring and pyrazole ring. ring, triazole ring, furazane ring, tetrazole ring, pyridine ring, pyridazine ring, pyrimidine ring, pyrazine ring, triazine ring, tetrazine ring, benzofuran ring, isobenzofuran ring, benzothiophene ring, indole ring, indoline ring, isoindole ring, benzoxazole ring, benzothiazole ring, indazole ring, benzimidazole ring, quinoline ring, isoquinoline ring, cinnoline ring, phthalazine ring, quinazoline ring, quinoxaline ring, dibenzofuran ring, dibenzothiophene ring, carbazole ring, acridine ring, phenanthridine ring , phenanthroline ring, phenazine ring, naphthyridine ring, purine ring, and pteridine ring.

Each X independently represents a group selected from the group consisting of a group represented by formula (A), a group represented by formula (B), and a group represented by formula (C). Among them, the group represented by the formula (A) is preferable from the viewpoint of easy synthesis of the compound (1). * in Formulas (A) to (C) represents a bonding position. More specifically, * represents the bonding position with L in formula (1).
These groups are electropolymerizable groups, and can be polymerized during charging and discharging of the lithium ion secondary battery, as described above.

The group represented by formula (A) is preferably a group represented by formula (A-1), and the group represented by formula (B) is preferably a group represented by formula (B-1), As the group represented by formula (C), a group represented by formula (C-1) is preferable.
In the case of the group represented by formula (B-1), polymerization proceeds at the 2- and 5-positions of the thiophene ring to form polythiophene. In the case of the group represented by formula (C-1), polymerization proceeds at positions 2 and 5 of the pyrrole ring to form polypyrrole.

In the compound represented by the formula (1), when X is a group represented by the formula (X), L preferably represents a divalent conjugated linking group, and X is the formula (B) or the formula (C ), L preferably represents a divalent linking group.

As the compound represented by Formula (1), the compound represented by Formula (2) is preferable because the effects of the present invention are more excellent.

In Formula (2), the definitions of L and X are as described above.

Among them, the energy level of the highest occupied molecular orbital (HOMO) of the compound (1) is higher than the energy level of the highest occupied molecular orbital (HOMO) of the solvent described later, in that the discharge capacity during charging and discharging is superior. High is preferred. In other words, the energy level of the highest occupied molecular orbital of compound (1) is preferably higher than the energy level of the highest occupied molecular orbital of the solvent described later.
If the energy level relationship of the highest occupied molecular orbital of the compound (1) and the solvent is the above relationship, the compound (1) is easily oxidized first, and as a result, the compound (1)-derived of the film is likely to be formed.
The energy level of the highest occupied molecular orbital of compound (1) is preferably −6.0 eV or higher, more preferably −5.0 eV or higher. Although the upper limit is not particularly limited, it is -3.0 eV or less in many cases.
Gaussian09, a quantum chemical calculation program based on density functional theory (DFT), is used to calculate the energy level of the highest occupied orbital of compound (1). B3LYP is used for mixed functional theory, and 6-31G(d, p) is used for basis functions.

The method for synthesizing compound (1) is not particularly limited, and it can be synthesized by combining known methods.

The content of compound (1) in the electrolytic solution is not particularly limited, but is preferably 0.01 to 10% by weight, preferably 0.05 to 5%, based on the total weight of the electrolytic solution in terms of more excellent effects of the present invention. % by mass is more preferred, and 0.10 to 1% by mass is even more preferred.

<Solvent>
The electrolytic solution of the present invention contains a solvent.
The type of solvent is not particularly limited, and includes non-aqueous solvents such as carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, and aprotic solvents.
Examples of carbonate-based solvents include cyclic carbonate-based solvents and chain carbonate-based solvents. and butylene carbonate.
Examples of ester solvents include methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, and Caprolactone can be mentioned.
Ether solvents include, for example, dibutyl ether, tetraglyme, triglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran.
Ketone solvents include, for example, cyclohexanone.
Alcoholic solvents include, for example, ethyl alcohol and isopropyl alcohol.
Aprotic solvents include nitriles, amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes.

The content of the solvent in the electrolytic solution is not particularly limited, but is preferably 50 to 99% by weight, more preferably 85 to 98% by weight, based on the total weight of the electrolytic solution in terms of better effects of the present invention.

<Lithium salt>
The electrolyte of the present invention contains a lithium salt.
The type of lithium salt is not particularly limited, and LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiC(CF ₃ SO ₂ ) ₃ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _2k+1 [k is an integer of 1 to 8 ], LiN(SO ₂ C _k F _2k+1 ) ₂ [k is an integer of 1 to 8], LiPF _n (C _k F2 _k+1 ) _6-n [n is an integer of 1 to 5, k is an integer of 1 to 8] , LiPF ₄ (C ₂ O ₂ ), and LiPF ₂ (C ₂ O ₂ ) ₂ .

The content of the lithium salt in the electrolytic solution is not particularly limited, but is preferably 0.2 to 3.0 mol/L, more preferably 0.4 to 2.0 mol/L, in terms of more excellent effects of the present invention. preferable.

The electrolytic solution of the present invention may contain components other than the above-described compound (1), solvent, and lithium salt.

The method for preparing the electrolytic solution of the present invention is not particularly limited, and the above-described compound (1), solvent, lithium salt, and optional components to be added as necessary are mixed, and each component is uniformly dissolved or dispersed. Any method is acceptable.

The electrolytic solution of the present invention is suitably used as an electrolytic solution for lithium ion secondary batteries.
The configuration of the lithium ion secondary battery containing the electrolytic solution of the present invention is not particularly limited, and may be the same as the configuration of conventionally known lithium ion secondary batteries. Lithium ion secondary batteries containing the electrolytic solution of the present invention usually contain a negative electrode and a positive electrode, and if necessary, a separator and/or an outer package.
The shape of the lithium-ion secondary battery is not particularly limited, and examples thereof include cylindrical, prismatic, laminate, and coin shapes.
Representative members included in the lithium-ion secondary battery will be described in detail below.

<Positive electrode>
The positive electrode preferably includes a current collector and a positive electrode active material layer formed on the current collector.
Examples of current collectors include aluminum substrates.
The positive electrode active material layer preferably contains a positive electrode active material and a binder. The positive electrode active material layer may contain a conductive aid.
A compound capable of reversibly intercalating and deintercalating lithium ions is preferable as the positive electrode active material.
Among them, from the viewpoint of increasing the capacity of the battery, the lithium-based positive electrode potential at full charge of the lithium ion secondary battery is preferably 4.4 V (vs Li/Li ⁺ ) or more, and 4.5 V (vs Li/Li ⁺ ) or more is more preferable. In other words, the positive electrode active material is preferably a material that allows the positive electrode to have a lithium-based potential of 4.4 V or higher when fully charged.

As the positive electrode active material, from the viewpoint of structural stability of the positive electrode active material,
A layered oxide represented by the following formula (3);
LiNi _x Co _y Ma _1-x-y O ₂ (3)
(Wherein, Ma represents one or more selected from the group consisting of Mn and Al, and 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, and x + y ≤ 1.)
A spinel-type oxide represented by the following formula (4);
_{LiMn2-xMbxO4 ( 4} )
(In the formula, Mb represents one or more selected from the group consisting of transition metals, and 0.2≦x≦0.7.)
an oxide represented by the following formula (5a);
Li ₂ McO ₃ (5a)
(In the formula, each Mc independently represents one or more selected from the group consisting of transition metals.)
An oxide represented by the following formula (5b) and LiMdO ₂ (5b)
(In the formula, each Md independently represents one or more selected from the group consisting of transition metals.)
Li-excess layered oxide represented by the following formula (5);
zLi ₂ McO ₃ -(1-z)LiMdO ₂ (5)
(Wherein, Mc each independently represents one or more selected from the group consisting of transition metals, Md each independently represents one or more selected from the group consisting of transition metals, and 0.05 ≤ z ≤ 0.95.)
Olivine-type oxide represented by the following formula (6);
_{LiMe1- xFexPO4} ( ₆ )
(In the formula, Me represents one or more selected from the group consisting of Mn and Co, and 0 ≤ x ≤ 1.)
An oxide represented by the following formula (7);
_{Li2MfPO4F (} 7)
(In the formula, Mf represents one or more selected from the group consisting of transition metals.)
It is preferably one or more selected from the group consisting of.

The binder plays a role of facilitating adhesion of the positive electrode active material to each other or a role of facilitating adhesion of the positive electrode active material to the current collector. Carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethanes, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resins , nylon, polyvinylidene fluoride, polytetrafluoroethylene, polyimide, and polyimidamide.

<Negative Electrode>
The negative electrode preferably includes a current collector and a negative electrode active material layer formed on the current collector.
Current collectors include, for example, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymeric substrates coated with conductive metals, and combinations of two or more of these. mentioned.
Examples of negative electrode active materials include substances capable of reversibly intercalating/deintercalating lithium ions, metallic lithium, alloys of metallic lithium, substances capable of doping and dedoping lithium, and transition metal oxides.

Materials capable of reversibly intercalating/deintercalating lithium ions include, for example, crystalline carbon, amorphous carbon, and combinations thereof. Crystalline carbon includes, for example, amorphous graphite, plate-like graphite, scale-like graphite, spherical graphite, and fibrous graphite. Amorphous carbon includes, for example, soft carbon, hard carbon, mesophase pitch carbides, and calcined coke.
Examples of metal lithium alloys include lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al or Sn. Alloys with metals can be used.
Materials capable of doping and dedoping lithium include, for example, Si, SiO _x (0<x<2), Si—C composites, Si—Q alloys (Q is an alkali metal, alkaline earth metal, Group 13 to Group 16 elements, transition metals, rare earth elements or combinations thereof, but not Si), Sn, SnO ₂ , Sn—C composite, Sn—R (R is an alkali metal, alkaline earth metal, Group 13 to 16th group elements, transition metals, rare earth elements, or combinations thereof, but not Sn).

The negative electrode active material layer may contain a binder. Examples of the binder include those exemplified as the binder contained in the positive electrode active material layer described above.

<Separator>
Any separator commonly used in conventional lithium ion secondary batteries can be used. Materials constituting the separator include, for example, glass fiber, polyester, Teflon (registered trademark), polyethylene, polypropylene, polytetrafluoroethylene (PTFE), and polyimide.

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

<Synthesis Example 1>
Acetonaphthenequinone (1.49 g) and 4,4'-oxydianiline (6.30 g) were dissolved in toluene (120 mL) under a nitrogen atmosphere. Sulfuric acid (0.03 mL) was added dropwise to the resulting solution and the resulting mixture was refluxed for 5 hours under a nitrogen atmosphere. After that, the resulting mixture was returned to room temperature to obtain an orange precipitate. The precipitate was collected by filtration, dissolved in ethyl acetate (200 mL), hexane (400 mL) was added, and the resulting mixture was stirred for 30 minutes. The resulting mixture was allowed to stand overnight at -15°C. A precipitate obtained in the mixed solution was collected by filtration, washed with hexane, and dried to synthesize the following compound 1 (BIANODA).
The ¹ H NMR spectrum, ¹³ C NMR spectrum and IR spectrum of Compound 1 thus obtained are shown in FIGS. 1 to 3, respectively.

In addition, in order to evaluate the polymerizability of Compound 1, an acetonitrile solution (100 mL) containing Compound 1 (10 mg) and LiCiO ₄ (1 M) was prepared and subjected to cyclic voltammetry measurement. In the cyclic voltammetry measurements, a silver-silver chloride electrode (Ag/AgCl) was used as a reference electrode, a platinum (Pt) electrode was used as a counter electrode, and a platinum plate was used as a working electrode.
When the measurement was performed up to 50 cycles, the formation of black precipitate was confirmed as the cycles were repeated. When the obtained precipitate was subjected to ¹ H NMR measurement and IR measurement, a peak similar to compound 1 was confirmed in each spectrum. From these results, it was presumed that compound 1 was polymerized to obtain the following polymer. In the following formulas, n represents the number of repeating units and was 2 or more.

The HOMO energy level of the obtained compound 1 was calculated by the method described above and was -4.71 eV.
The HOMO energy levels of ethylene carbonate (EC) and diethyl carbonate (DEC), which are solvents used in the latter stage, were −12.90 eV and −8.2 eV, respectively.
From this calculation result, it was confirmed that Compound 1 is easily oxidized first.
Next, cyclic voltammetry measurement using compound 1 was performed. Specifically, compound 1 (2 mg/mL) was added to a mixture of EC and DEC (EC:DEC (mass ratio) = 1:1, manufactured by Sigma Aldrich) containing 1 mol of LiPF ₆ . was used to perform cyclic voltammetry measurements. For comparison, cyclic voltammetry measurement was performed using a solution to which compound 1 was not added according to the same procedure. As the electrode, an electrode in which a positive electrode active material layer containing nickel, manganese, and cobalt is arranged on a current collector was used. Specifically _{, a LiMnxNiyCozO2} (x=y=z=1/3) electrode (manufactured by Piotrek) _was used. The electrode included an aluminum current collector and _a positive electrode active material layer _{containing LiMnxNiyCozO2} disposed on the aluminum current collector _. In addition, in the cyclic voltammetry measurement, a metallic lithium electrode (manufactured by Honjo Metal Co., Ltd.) was used as the counter electrode and the working electrode.
As shown in FIG. 4(A), when compound 1 was used, the current value for oxidation decreased significantly in the second and subsequent cycles as indicated by the arrows, whereas FIG. 4(B) shows As shown, when Compound 1 was not used, the current value for oxidation was large in the second and subsequent cycles. From these results, it is believed that when compound 1 is used, compound 1 is oxidized first during the first cycle, forming a film on the electrode and, as a result, suppressing the decomposition of the solvent. be done.

Furthermore, the surface of the electrode obtained by performing cyclic voltammetry measurement using compound 1 was subjected to XPS measurement. In addition, XPS measurement of the surface of the electrode obtained by performing cyclic voltammetry measurement without using compound 1 was also performed.
The sample for XPS measurement corresponds to the sample after 100 cycles.
FIG. 5A shows the XPS measurement results of the electrode before cyclic voltammetry measurement. On the other hand, FIG. 5B shows the XPS measurement results of the electrode obtained by cyclic voltammetry measurement without using Compound 1. FIG. Comparing the two, in FIG. 5B, almost no oxygen atoms derived from the positive electrode active material layer are visible, and it is considered that a thick film is formed on the electrode.
On the other hand, FIG. 5C shows the XPS measurement results of the electrode obtained by performing cyclic voltammetry measurement using compound 1. FIG. In FIG. 5C, although the peak of oxygen atoms derived from the positive electrode active material layer is small, it can be confirmed to some extent. This suggests that a relatively thin coating is formed on the electrode.
Furthermore, FIG. 6(A) is the XPS measurement result of the electrode obtained by performing cyclic voltammetry measurement without using compound 1. FIG. As shown in this figure, bonds such as PO/P=O are confirmed in the film. It is presumed that some kind of reaction progressed between the lithium salt and the solvent, and a film containing phosphorus atoms derived from the lithium salt and oxygen atoms derived from the solvent was formed.
On the other hand, FIG. 6(B) is the XPS measurement result of the electrode obtained by performing the cyclic voltammetry measurement using the compound 1. FIG. As shown in this figure, a peak derived from PF was confirmed in the coating, which is considered to be derived from the lithium salt. From this result, when compound 1 was used, the decomposition of the lithium salt was suppressed, which is considered to be due to the formation of a film on the electrode.

<Charge-discharge cycle evaluation>
Solution A was prepared by adding Compound 1 (2 mg/mL) to a mixture of EC and DEC (EC:DEC (mass ratio)=1:1, manufactured by Sigma Aldrich) containing 1 mol of LiPF ₆ .
Next, using the above solution A, a coin-shaped cell (half cell) was produced. As the positive electrode in _the cell, the above-mentioned _{LiMnxNiyCozO2} (x=y=z=1/3) electrode (manufactured _{by Piotrek} ) was used. A metal lithium electrode (manufactured by Honjo Metal Co., Ltd.) was used as the negative electrode. As the separator, a polypropylene separator (25 mm, manufactured by Celgard) was used.
A charge/discharge test of the resulting cell was performed at room temperature (25° C.) using a Battery Cycler (HJ-SD8, manufactured by Hokuto Denshi Co., Ltd.). The applied voltage was in the range of 3.0 to 4.5V.
Instead of the above solution A, a mixed solution of EC and DEC (EC:DEC (mass ratio) = 1:1, manufactured by Sigma Aldrich) containing 1 mol of LiPF 6 without compound ₁ was used. Then, according to the procedure similar to the above, a cell was produced, and the charge/discharge test similar to the above was performed. Both results are shown in FIG.
As shown in FIG. 7, when the electrolyte containing Compound 1 is used (2 mg/ml additive (%)), compared to the electrolyte without Compound 1 (Control (%)), the discharge capacity is higher. was confirmed.

<Structural stability evaluation>
Solution _A and _{LiMnxNiyCozO2} (x=y=z=1/3) electrodes (manufactured by _Piotrek ) used in the _above <Evaluation of charge-discharge cycles> were prepared.
Next, _{the LiMnxNiyCozO2} electrode was immersed in the solution A for one week, and the structure of the positive electrode active material layer _before and after _the immersion was observed by SEM. An SEM _observation of the _{LiMnxNiyCozO2} electrode before immersion is shown in FIG. 8A, _and an SEM _observation of the _{LiMnxNiyCozO2 electrode} after _immersion is shown in FIG _. 8B.
_As a comparative example, LiMn _x Ni _{The yCozO2} electrodes were soaked _for 1 week. A SEM image of _{the LiMnxNiyCozO2} electrode after _immersion is shown _in FIG. 8(C).
As shown in FIG. 8(A), in _the SEM _observation of the _{LiMnxNiyCozO2 electrode} before immersion, particles of a predetermined size were observed. When _{the LiMnxNiyCozO2} electrode was immersed in the solution A _containing the compound 1, particles of a predetermined size were observed, which was substantially the same as _in FIG. 8A. On the other hand, it was confirmed that when _{the LiMnxNiyCozO2} electrode was immersed in a solution _that did not contain compound 1, _the particles were smaller than in FIG.
From these results, when the solution containing _Compound 1 and _{the LiMnxNiyCozO2} electrode were brought into contact, _the structure _{of LiMnxNiyCozO2} ( _{the structure} of the positive electrode active material) substantially changed. It was confirmed that the

Claims (6)

An electrolytic solution containing a compound represented by formula (2), a solvent, and a lithium salt.

Each L independently represents an arylene group, a heteroarylene group, a -arylene group -Y-, or a -heteroarylene group -Y-. Y represents O , S , or NH . Each X independently represents a group selected from the group consisting of a group represented by formula (A), a group represented by formula (B), and a group represented by formula (C). In formulas (A) to (C), * represents a bonding position.

The electrolytic solution according to claim 1, wherein X represents a group represented by formula (A). 3. The electrolytic solution according to claim 1, wherein the content of the compound represented by formula (2) is 0.01 to 10% by mass with respect to the total weight of the electrolytic solution. 4. The solvent according to any one of claims 1 to 3, wherein said solvent is selected from the group consisting of carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, and aprotic solvents. electrolyte. A lithium ion secondary battery containing the electrolyte solution according to any one of claims 1 to 4. A compound represented by formula (2).

Each L independently represents an arylene group, a heteroarylene group, a -arylene group -Y-, or a -heteroarylene group -Y-. Y represents O , S , or NH . Each X independently represents a group represented by Formula (A). In formula (A), * represents a bonding position.

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