JP6332033B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  Lithium ion secondary batteries are widely used as power sources for small devices because they have a small volume, a large mass capacity density, and a high voltage can be taken out. For example, it is used as a power source for mobile devices such as mobile phones and notebook computers. Also, in recent years, in addition to small mobile device applications, large secondary devices that require a long life with a large capacity, such as electric vehicles (EV) and power storage fields, are being considered due to consideration for environmental issues and increased awareness of energy conservation. Application to batteries is expected.

In the lithium ion secondary battery currently on the market, a positive electrode active material based on LiMO ₂ having a layered structure (M is at least one of Co, Ni, and Mn) or LiMn ₂ O ₄ having a spinel structure is used. Commonly used. A lithium ion secondary battery having a positive electrode containing such a positive electrode active material mainly uses a charge / discharge region of 4.2 V or less (hereinafter referred to as a positive electrode having an operating potential of 4.2 V or less with respect to lithium metal). May be described as “4V class positive electrode”). Further, a carbon material such as graphite is used as the negative electrode active material.

On the other hand, it is known that a material obtained by substituting a part of Mn of LiMn ₂ O ₄ with Ni or the like exhibits a charge / discharge region as high as 4.5 to 4.8 V with respect to lithium metal. Specifically, spinel compounds such as LiNi _0.5 Mn _1.5 O ₄ are not redox of conventional Mn ³⁺ and Mn ⁴⁺ , but Mn exists in the state of Mn ⁴⁺ and redox of Ni ²⁺ and Ni ⁴⁺ Therefore, a high operating voltage of 4.5 V or higher is shown. Such a material is called a 5V class active material and is expected to be a promising positive electrode material because it can improve the energy density by increasing the voltage.

  However, when the potential of the positive electrode is increased, the electrolyte is oxidized and decomposed to generate gas, by-products accompanying the decomposition of the electrolyte are generated, and metal ions such as Mn and Ni in the positive electrode active material are eluted. As a result, there are problems such as precipitation on the negative electrode, which causes capacity deterioration due to charge / discharge cycles at high temperatures and generation of a large amount of gas, which has been an obstacle to practical use.

  In a battery using a conventional 4V class positive electrode, a technique of adding an additive to an electrolytic solution is widely performed in order to improve battery performance or suppress deterioration. However, in the 4V class positive electrode, the oxidative decomposition of the electrolyte on the positive electrode is not the main cause of the deterioration, so the additive used in the 4V class battery is not always effective for the 5V class battery. The effect may be different.

Various compounds have been reported as additives to the electrolytic solution. For example, Patent Documents 1 to 4 disclose bis (fluorosulfonyl) imide lithium salts represented by Li ⁺ [(FSO ₂ ) ₂ N] ^−. Patent Document 1 describes that LiFSI can be applied as an electrolyte of a non-aqueous solvent, and Patent Document 2 can improve output characteristics after high-temperature storage by using in combination with a specific compound. Is described. Patent Document 3 describes a nonaqueous electrolytic solution containing a sulfonylimide lithium containing a fluorine atom, a fluorinated diether, and a fluorinated monoether. Patent Document 4 describes a nonaqueous electrolytic solution for a secondary battery containing a fluorinated cyclic carbonate and lithium bisfluorosulfonylimide.

  However, Patent Documents 1 to 4 can sufficiently solve problems such as high temperature cycle characteristics and gas generation in a battery using a 5 V class active material that exhibits a high voltage of 4.5 V or higher with respect to lithium metal. It wasn't. For example, the electrolytic solution described in Patent Document 4 has a large practical problem because a large amount of gas is generated during a high-temperature cycle although not specifically mentioned.

JP 2012-94454 A Japanese Patent No. 3878206 JP 2011-187163 A JP 2010-129449 A

  The present invention solves the above-mentioned problems in a lithium ion secondary battery using a 5V class active material, and provides a lithium ion secondary battery with a high capacity retention rate of charge / discharge cycles at high temperatures and reduced gas generation. With the goal.

The present invention is a lithium ion secondary battery comprising a positive electrode and a non-aqueous electrolyte,
The positive electrode includes a positive electrode active material having an operating potential of 4.5 V or more with respect to lithium metal,
The non-aqueous electrolyte is
(A) N (SO ₂ F) ₂ anion (FSI anion);
(B) a cyclic carbonate;
(C) A fluorinated ether represented by the following formula (1), a fluorinated phosphate ester represented by the following formula (2), and a sulfone compound represented by the following formula (3) or the following formula (4) At least one selected from the group consisting of:
The present invention relates to a lithium ion secondary battery.

（式（１）において、Ｒ₁₀₁及びＲ₁₀₂は、それぞれ独立に、アルキル基又はフッ化アルキル基を示し、Ｒ₁₀₁及びＲ₁₀₂のうち少なくとも一つはフッ化アルキル基である）、

(In Formula (1), R ₁₀₁ and R ₁₀₂ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R ₁₀₁ and R ₁₀₂ is a fluorinated alkyl group),

（式（２）において、Ｒ^１、Ｒ^２、Ｒ^３は、それぞれ独立に、アルキル基またはフッ化アルキル基を示し、これらのうち少なくとも１つがフッ化アルキル基である。）、

(In Formula (2), R ¹ , R ² and R ³ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of them is a fluorinated alkyl group),

（式（３）中、Ｒ_１及びＲ_２は、それぞれ独立に、置換または無置換のアルキル基を示す。）、

(In Formula (3), R ₁ and R ₂ each independently represent a substituted or unsubstituted alkyl group),

（式（４）中、Ｒ_３は、置換または無置換のアルキレン基を示す。）。

(In formula (4), R ₃ represents a substituted or unsubstituted alkylene group).

  According to the present invention, the cycle characteristics of a lithium ion secondary battery containing a 5V class active material can be improved and gas generation can be suppressed. Note that in this specification, a positive electrode active material having an operating potential of 4.5 V or higher with respect to lithium metal may be referred to as a “5 V class active material”.

It is sectional drawing which shows an example of the secondary battery which concerns on this embodiment. It is a figure which shows the comparison of the capacity | capacitance maintenance factor of the secondary battery produced by the Example and the comparative example. It is a figure which shows the comparison of the volume increase rate of the secondary battery produced by the Example and the comparative example.

The lithium ion secondary battery of this embodiment is
A positive electrode and a non-aqueous electrolyte,
The positive electrode includes a positive electrode active material (hereinafter sometimes referred to as “5 V class active material”) having an operating potential of 4.5 V or more with respect to lithium metal,
The non-aqueous electrolyte is
(A) N (SO ₂ F) ₂ anion (FSI anion);
(B) a cyclic carbonate;
(C) A fluorinated ether represented by the following formula (1), a fluorinated phosphate ester represented by the following formula (2), and a sulfone compound represented by the following formula (3) or the following formula (4) At least one selected from the group consisting of:
including.

（式（１）において、Ｒ₁₀₁及びＲ₁₀₂は、それぞれ独立に、アルキル基又はフッ化アルキル基を示し、Ｒ₁₀₁及びＲ₁₀₂のうち少なくとも一つはフッ化アルキル基である）、

(In Formula (1), R ₁₀₁ and R ₁₀₂ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R ₁₀₁ and R ₁₀₂ is a fluorinated alkyl group),

（式（２）において、Ｒ^１、Ｒ^２、Ｒ^３は、それぞれ独立に、アルキル基またはフッ化アルキル基を示し、これらのうち少なくとも１つがフッ化アルキル基である。）、

(In Formula (2), R ¹ , R ² and R ³ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of them is a fluorinated alkyl group),

（式（３）中、Ｒ_１及びＲ_２は、それぞれ独立に、置換または無置換のアルキル基を示す。）、

(In Formula (3), R ₁ and R ₂ each independently represent a substituted or unsubstituted alkyl group),

（式（４）中、Ｒ_３は、置換または無置換のアルキレン基を示す。）。

(In formula (4), R ₃ represents a substituted or unsubstituted alkylene group).

  Note that in this specification, a positive electrode including a positive electrode active material having an operating potential of 4.5 V or higher with respect to lithium metal may be referred to as a “5 V class positive electrode”.

(Nonaqueous electrolyte)
The non-aqueous electrolyte provided in the lithium ion secondary battery of this embodiment is
(A) an N (SO ₂ F) ₂ anion (hereinafter sometimes referred to as “FSI anion”) and a nonaqueous electrolytic solvent;
The non-aqueous electrolytic solvent is
(B) a cyclic carbonate, and
(C) at least selected from a fluorinated ether represented by formula (1), a fluorinated phosphate ester represented by formula (2), and a sulfone compound represented by formula (3) or formula (4) 1 type.

(FSI anion)
In the present embodiment, the nonaqueous electrolytic solution contains an FSI anion. The FSI anion is considered to form a film on the negative electrode and the positive electrode. Such a film is also referred to as a SEI film (Solid Electrolyte Interface), and has an ionic conductivity but no electronic conductivity, and thus plays a role of preventing a reaction between the active material and the electrolytic solution. The film formed on the positive electrode suppresses the decomposition reaction between the 5V class active material and the electrolytic solution. The film formed on the negative electrode suppresses the decomposition reaction between the negative electrode and the electrolytic solution, and by-products of the electrolytic solution generated at the positive electrode or transition metal ions such as Mn and Ni eluted from the positive electrode react with the negative electrode. To prevent precipitation. As a result, the capacity maintenance rate of the charge / discharge cycle is improved, and gas generation can be sufficiently suppressed. In this embodiment, the positive electrode has a positive electrode active material having an operating potential of 4.5 V or more with respect to lithium metal, but the effect of the FSI anion is 5 V class than the conventional secondary battery using a 4 V class positive electrode. A secondary battery having an active material is more highly effective. This is considered to be because an oxidative decomposition film by FSI anion is easily formed on the positive electrode because the positive electrode has a high potential.

  The FSI anion is generated when a compound containing the FSI anion is dissolved in the non-aqueous electrolyte. As a compound containing an FSI anion, a salt of an FSI anion and an alkali metal is preferable, and examples thereof include LiFSI, NaFSI, and KFSI. Among these, LiFSI is particularly preferable because it can work as an electrolyte of a lithium ion battery and can improve the ionic conductivity of the electrolytic solution. In this specification, LiFSI is described as an example, but the coating of FSI anion is also formed other than the lithium salt, and is not limited to LiFSI.

  0.1-5 mass% is preferable and, as for the addition amount of LiFSI with respect to the total mass of electrolyte solution, 0.2-3 mass% is more preferable.

(Non-aqueous electrolytic solvent)
In the present embodiment, the electrolytic solution contains a cyclic carbonate as a nonaqueous electrolytic solvent. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). Further, some or all of hydrogen in these compounds may be substituted with fluorine. Examples of the fluorinated cyclic carbonate include 4-fluoro-1,3-dioxolan-2-one {monofluoroethylene carbonate (FEC)}, (cis or trans) 4,5-difluoro-1,3-dioxolane-2. -One, 4,4-difluoro-1,3-dioxolan-2-one, 4-fluoro-5-methyl-1,3-dioxolan-2-one and the like. A cyclic carbonate can be used individually by 1 type or in mixture of 2 or more types. EC and PC are preferable because they have a high dielectric constant and excellent electrolyte solubility, and EC is more preferable. LiFSI has poor solubility in organic solvents as compared with other lithium salts such as LiPF ₆ , but the solubility of LiFSI can be further increased by including a cyclic carbonate in the electrolytic solution.

  The content of the cyclic carbonate in the total nonaqueous electrolytic solvent is preferably 1 to 50% by volume, more preferably 5 to 40% by volume, and still more preferably 10 to 40% by volume. If the content of the cyclic carbonate is too small, the conductivity of the electrolytic solution is lowered, and the cycle characteristics may be deteriorated. If the content of the cyclic carbonate is too large, the cyclic carbonate is likely to be decomposed at a high potential. Therefore, in a lithium ion secondary battery containing a 5V-grade positive electrode active material, gas generation may increase.

  In the present embodiment, the nonaqueous electrolytic solvent is a fluorinated ether represented by the formula (1) (hereinafter sometimes simply referred to as “fluorinated ether”), the formula (2), in addition to the cyclic carbonate. And a sulfone compound represented by formula (3) or formula (4) (hereinafter simply referred to as “fluorinated phosphate ester”). It may be described as “sulfone compound”.), And at least one selected from the group consisting of 2 or more is more preferable. Hereinafter, each compound will be described.

  In the present embodiment, the non-aqueous electrolytic solvent can contain a fluorinated ether represented by the following formula (1).

（式（１）において、Ｒ₁₀₁及びＲ₁₀₂は、それぞれ独立に、アルキル基又はフッ化アルキル基を示し、Ｒ₁₀₁及びＲ₁₀₂の少なくとも一つはフッ化アルキル基である）。

(In Formula (1), R ₁₀₁ and R ₁₀₂ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R ₁₀₁ and R ₁₀₂ is a fluorinated alkyl group).

The total number of carbon atoms of R ₁₀₁ and R ₁₀₂ is preferably 10 or less.

  The fluorinated alkyl group is an alkyl group having at least one fluorine atom. In formula (1), the fluorine atom content in the fluorinated alkyl group is preferably 50% or more, more preferably 60% or more, based on the total of fluorine atoms and hydrogen atoms. When the content of fluorine atoms is large, the withstand voltage is further improved, and even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, deterioration of battery capacity after cycling is more effectively reduced. Is possible.

  Of the fluorinated ethers, fluorinated ethers represented by the following formula (1-1) are more preferable.

_{^{X 1 - (CX 2 X 3}} ) n -O- (CX 4 X 5) m -X 6 (1-1)
(In formula (1-1), n and m are each independently 1 to 8. X ^{1 to} X ⁶ are each independently a fluorine atom or a hydrogen atom, provided that X ^{1 to} X ⁶ At least one is a fluorine atom, and when n is 2 or more, a plurality of X ² and X ³ are independent from each other, and when m is 2 or more, a plurality of X ⁴ and X ⁵ are Are independent of each other.)

  The fluorinated ether is more preferably a compound represented by the following formula (1-2) from the viewpoint of withstand voltage and compatibility with other electrolytes.

_{^{X 1 - (CX 2 X 3}} ) n -CH 2 O-CX 4 X 5 -CX 6 X 7 -X 8 (1-2)
(In the formula (1-2), n is 1 to 7, and X ^{1 to} X ⁸ are each independently a fluorine atom or a hydrogen atom, provided that at least one of X ^{1 to} X ³ is a fluorine atom. And at least one of X ^{4 to} X ⁸ is a fluorine atom).

In Formula (1-2), when n is 2 or more, a plurality of X ² may be the same or different from each other, and a plurality of X ³ may be the same or different from each other. Also good.

  Furthermore, it is more preferable that the fluorinated ether compound is represented by the following formula (1-3) from the viewpoint of voltage resistance and compatibility with other electrolytes.

^{H- (CY 1 Y 2 -CY 3} Y 4) n -CH 2 O-CY 5 Y 6 -CY 7 Y 8 -H (1-3)

In the formula (1-3), n is 1, 2, 3 or 4. Y ^{1 to} Y ⁸ are each independently a fluorine atom or a hydrogen atom. However, at least one of Y ^{1 to} Y ⁴ is a fluorine atom, and at least one of Y ^{5 to} Y ⁸ is a fluorine atom.

In Formula (1-3), when n is 2 or more, a plurality of Y ¹ to Y ⁴ may be the same as or different from each other.

Specific examples of the fluorinated ether include CF ₃ OCH ₃ , CF ₃ OC ₂ H ₅ , F (CF ₂ ) ₂ OCH ₃ , F (CF ₂ ) ₂ OC ₂ H ₅ , and CF ₃ (CF ₂ ). _{CH 2 O (CF 2) CF} 3, F (CF 2) 3 OCH 3, F (CF 2) 3 OC 2 H 5, F (CF 2) 4 OCH 3, F (CF 2) 4 OC 2 H 5, F (CF ₂ ) ₅ OCH ₃ , F (CF ₂ ) ₅ OC ₂ H ₅ , F (CF ₂ ) ₈ OCH ₃ , F (CF ₂ ) ₈ OC ₂ H ₅ , F (CF ₂ ) ₉ OCH ₃ , CF ₃ CH ₂ OCH ₃ , CF ₃ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ OCH ₃ , CF ₃ CF ₂ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ O (CF ₂ ) ₂ H, CF ₃ CF ₂ CH _{2 O (CF 2) 2 F} , _{CF 2 CH 2 OCH 3, (} CF 3) (CF 2) CH 2 O (CF 2) 2 H, H (CF 2) 2 OCH 2 CH 3, H (CF 2) 2 OCH 2 CF 3, H (CF ₂ ) ₂ CH ₂ OCHF ₂ , H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₂ H, H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₃ H, H (CF ₂ ) ₃ CH ₂ O ( _{CF 2) 2 H, H (} CHF) 2 CH 2 O (CF 2) 2 H, (CF 3) 2 CHOCH 3, (CF 3) 2 CHCF 2 OCH 3, CF 3 CHFCF 2 OCH 3, CF 3 CHFCF 2 OCH ₂ CH ₃ , CF ₃ CHFCF ₂ CH ₂ OCHF ₂ , CF ₃ CHFCF ₂ OCH ₂ (CF ₂ ) ₂ F, CF ₃ CHFCF ₂ OCH ₂ CF ₂ CF ₂ H, H (CF ₂ ) ₄ CH ₂ O _{(CF 2) 2 H, CH} 3 CH 2 O (CF 2) 4 F, F (CF 2) 4 CH 2 O (CF 2) such as ₂ H and the like. Among these, as the fluorinated ether compound, H (CF ₂ ) ₂ CH ₂ O (CF ₂ ) ₂ H is preferable. A fluorinated ether compound may be used alone or in combination of two or more.

  The content of the fluorinated ether compound represented by the formula (1) is preferably 0% by volume to 90% by volume and more preferably 15% by volume to 80% by volume in the nonaqueous electrolytic solvent. 30 volume% or more and 70 volume% or less is more preferable, 30 volume% or more and 60 volume% or less is more preferable, and 30 volume% or more and 50 volume% or less is further more preferable. When the content of the fluorinated ether is too large, the dielectric constant of the electrolytic solution is lowered, the supporting salt cannot be dissociated, and the capacity is similarly reduced.

  Since the fluorinated ether has high oxidation resistance, the oxidative decomposition of the solvent can be suppressed in the lithium ion secondary battery containing the 5V class active material. As a result, it is possible to improve the capacity maintenance rate of the charge / discharge cycle and reduce gas generation. LiFSI forms a film on the negative electrode and the positive electrode. However, if a large amount of decomposition products of the solvent are present on the electrode, a high-quality film is hardly formed or the effect of the film is hindered. Here, by making LiFSI coexist with fluorinated ether having high oxidation resistance, the effect of the film by LiFSI can be enhanced.

  In the present embodiment, the nonaqueous electrolytic solvent can include a fluorinated phosphate ester represented by the following formula (2).

（式（２）において、Ｒ^１、Ｒ^２、Ｒ^３は、それぞれ独立に、アルキル基またはフッ化アルキル基を示し、これらのうち少なくとも１つがフッ化アルキル基である。）。

(In Formula (2), R ¹ , R ² and R ³ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of them is a fluorinated alkyl group).

The fluorinated alkyl group is an alkyl group having at least one fluorine atom. In the formula ^(2), the number of carbon atoms in R 1, ^{R 2} and ^{R 3} each independently is preferably 1-3. At least one of R ¹ , R ² and R ³ is preferably a fluorinated alkyl group in which 50% or more of the hydrogen atoms of the corresponding unsubstituted alkyl group are substituted with fluorine atoms. Also, all of R ₁ , R ₂ and R ₃ are fluorinated alkyl groups, and 50% or more of the hydrogen atoms of the unsubstituted alkyl group to which R ₁ , R ₂ and R ₃ correspond are substituted with fluorine atoms. More preferred is a fluorinated alkyl group. When the content of fluorine atoms is large, the voltage resistance is further improved, and even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, the deterioration of battery capacity after cycling is further reduced. Because it can. Further, the ratio of fluorine atoms in the substituent containing a hydrogen atom in the fluorinated alkyl group is more preferably 55% or more.

  The fluorinated phosphate ester is not particularly limited. For example, tris (trifluoromethyl) phosphate, Tris (pentafluoroethyl) phosphate, trisphosphate (2,2,2-trifluoroethyl) [Tris (2,2,2-trifluoroethyl) phosphate (TTFP)], tris (2,2,3,3-tetrafluoropropyl) phosphate (Tris (2,2 , 3,3-tetrafluoropropyl) phosphate), tris (3,3,3-trifluoropropyl) phosphate (Tris (3,3,3-trifluorofluoro) phosphate), Tris (2,2,3,3,3-pentafluoro-propyl) (Tris (2,2,3,3,3-pentafluoropropyl) phosphate) fluorinated alkyl phosphoric acid ester compounds and the like. Among these, as the fluorinated phosphate ester compound, Tris (2,2,2-trifluoroethyl) phosphate (TTFP) is preferable. Fluorinated phosphates can be used alone or in combination of two or more.

  The content of the fluorinated phosphate ester contained in the nonaqueous electrolytic solvent is not particularly limited, but is preferably 0% by volume to 95% by volume in the nonaqueous electrolytic solvent, and is preferably 10% by volume to 95% by volume. Is more preferable, 20 vol% or more and 70 vol% or less is more preferable, 20 vol% or more and 50 vol% or less is more preferable, and 20 vol% or more and 40 vol% or less is more preferable. When the content of the fluorinated phosphate ester in the nonaqueous electrolytic solvent is 10% by volume or more, the effect of increasing the voltage resistance is further improved. Moreover, the ion conductivity of electrolyte solution improves that the content rate in the nonaqueous electrolytic solvent of fluorinated phosphate ester is 95 volume% or less, and the charge / discharge rate of a battery becomes more favorable.

  Since the fluorinated phosphate ester also has high oxidation resistance, oxidative decomposition of the solvent when a 5V class active material is used can be suppressed. As a result, it is possible to improve the capacity maintenance rate of the charge / discharge cycle and reduce gas generation. LiFSI forms a film on the negative electrode and the positive electrode. However, if a large amount of decomposable product of the solvent is present on the electrode, it is difficult to form a good film or the effect of the film is hindered. That is, the effect of the film by LiFSI can be strengthened by coexisting fluorinated phosphate ester having high oxidation resistance and LiFSI.

  In the present embodiment, the nonaqueous electrolytic solvent can contain a sulfone compound represented by the following formula (3).

（式（３）中、Ｒ_１及びＲ_２は、それぞれ独立に、置換または無置換のアルキル基を示す。）

(In Formula (3), R ₁ and R ₂ each independently represents a substituted or unsubstituted alkyl group.)

In sulfone compound represented by formula (3), it is preferable that the carbon number _{n 2} with carbon number _n 1, _{R 2} of _{R 1} is _{1 ≦ n 1 ≦ 12,1 ≦ n} 2 ≦ 12 , respectively, 1 ≦ n ₁ ≦ 6, 1 ≦ n ₂ ≦ 6 are more preferable, and 1 ≦ n ₁ ≦ 3 and 1 ≦ n ₂ ≦ 3 are still more preferable. The alkyl group includes linear, branched, or cyclic groups.

R ₁ and R ₂ may have a substituent. Examples of the substituent include an alkyl group having 1 to 6 carbon atoms (for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group). Group), an aryl group having 6 to 10 carbon atoms (for example, a phenyl group, a naphthyl group), a halogen atom (for example, a chlorine atom, a bromine atom, a fluorine atom) and the like.

  Examples of the sulfone compound represented by the formula (3) include ethyl methyl sulfone, ethyl isopropyl sulfone, ethyl isobutyl sulfone, dimethyl sulfone, and diethyl sulfone. Of these, dimethyl sulfone, ethyl methyl sulfone, ethyl isopropyl sulfone, and ethyl isobutyl sulfone are preferable.

  In the present embodiment, the nonaqueous electrolytic solvent can contain a sulfone compound represented by the following formula (4).

（式（４）中、Ｒ_３は、置換または無置換のアルキレン基を示す。）。

(In formula (4), R ₃ represents a substituted or unsubstituted alkylene group).

In R ₃ , the alkylene group preferably has 4 to 9 carbon atoms, and more preferably 4 to 6 carbon atoms.

In R ₃ , examples of the substituent include an alkyl group having 1 to 6 carbon atoms (for example, methyl group, ethyl group, propyl group, isopropyl group, butyl group), halogen atom (for example, chlorine atom, bromine atom, fluorine atom). Atom) and the like.

  Of the sulfone compounds represented by the formula (4), the cyclic sulfone compounds represented by the following formula (4-1) are preferred.

（式（４−１）中、ｍは１〜６の整数である。）。

(In Formula (4-1), m is an integer of 1-6).

  In Formula (4-1), m is an integer of 1 to 6, and preferably an integer of 1 to 3.

  Preferred examples of the cyclic sulfone compound represented by the formula (4) include tetramethylene sulfone (sulfolane), pentamethylene sulfone, hexamethylene sulfone and the like. Preferred examples of the cyclic sulfone compound having a substituent include 3-methylsulfolane and 2,4-dimethylsulfolane. Since these materials are compatible with the fluorinated ether compound and have a relatively high dielectric constant, they have the advantage of being excellent in the dissolution / dissociation action of the lithium salt.

  In addition, a sulfone compound can be used individually by 1 type or in mixture of 2 or more types.

  The content of the sulfone compound is preferably 0% by volume or more and 75% by volume or less in the nonaqueous electrolytic solvent, more preferably 5% by volume or more and 50% by volume or less, and more preferably 5% by volume or more and 30% by volume or less. More preferably, it is more preferably 5% by volume or more and 20% by volume or less. If the content of the sulfone compound is too large, the viscosity of the electrolytic solution increases, and the capacity of the cycle characteristics at room temperature may be reduced.

  Since the sulfone compound also has high oxidation resistance, it is possible to suppress oxidative decomposition of the solvent particularly when a 5V class active material is used. As a result, it is possible to improve the capacity maintenance rate of the charge / discharge cycle and reduce gas generation. LiFSI forms a film on the negative electrode and the positive electrode. However, if a large amount of decomposition products of the solvent are present on the electrode, a high-quality film is hardly formed or the effect of the film is hindered. Here, the effect of the film | membrane by LiFSI can be strengthened by coexisting a sulfo compound with high oxidation resistance and LiFSI. Further, since the sulfone compound is a material having a relatively high dielectric constant, it is preferable from the viewpoint of easily dissolving the lithium salt.

  In the present embodiment, the nonaqueous electrolytic solvent includes at least one selected from fluorinated ethers, fluorinated phosphates, and sulfone compounds, and more preferably includes two or more. In non-aqueous electrolytic solvents, the higher the concentration of these solvents, the better the decomposition of the electrolytic solution is suppressed, but it is preferable to include two or more in order to further improve the compatibility with the cyclic carbonate and the solubility of the lithium salt. There is.

  In the present embodiment, the nonaqueous electrolytic solvent may further contain a chain carbonate. Since the cyclic carbonate has a high viscosity, the viscosity can be reduced by mixing the chain carbonate. Examples of the chain carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), and derivatives thereof (including fluorinated products). However, since chain carbonate tends to generate gas more easily than cyclic carbonate, the concentration in the electrolytic solution is preferably 10% by volume or less, more preferably 5% by volume or less, and even more preferably 2% or less.

  In the present embodiment, the nonaqueous electrolytic solvent may contain an aliphatic carboxylic acid ester, γ-lactone, a cyclic ether, a chain ether other than the above formula (1), and the like. Examples of the aliphatic carboxylic acid ester include methyl formate, methyl acetate, ethyl propionate, and derivatives thereof (including fluorinated products). Examples of γ-lactone include γ-butyrolactone and its derivatives (including fluorinated products). Examples of the cyclic ether include tetrahydrofuran, 2-methyltetrahydrofuran, and derivatives thereof (including fluorinated products). Examples of the chain ether include 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, and derivatives thereof (including fluorinated products). These can be used individually by 1 type or in mixture of 2 or more types.

  Other non-aqueous electrolytic solvents include, for example, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane. Derivatives, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, anisole, N-methylpyrrolidone, and derivatives thereof (including fluorinated products) may also be used. it can.

  In this embodiment, it is preferable that electrolyte solution further contains the cyclic sulfonate ester represented by following formula (5) as an additive. The cyclic sulfonate ester can form a film on the negative electrode.

（式（５）中、Ａ及びＢは、それぞれ独立に、アルキレン基又はフッ化アルキレン基を示す。Ｘは、単結合又は−ＯＳＯ_２−基を示す。）。

(In formula (5), A and B each independently represent an alkylene group or a fluorinated alkylene group. X represents a single bond or an -OSO ₂ -group).

  In Formula (5), carbon number of an alkylene group is 1-8, for example, Preferably it is 1-6, More preferably, it is 1-4.

  The fluorinated alkylene group represents a substituted alkylene group having a structure in which at least one hydrogen atom in the unsubstituted alkylene group is substituted with a fluorine atom. In Formula (5), carbon number of a fluorinated alkylene group is 1-8, for example, Preferably it is 1-6, More preferably, it is 1-4.

Incidentally, -OSO ₂ - groups may be either direction.

  In the formula (5), when X is a single bond, the cyclic sulfonate ester is preferably a cyclic monosulfonate ester, and the cyclic monosulfonate ester is preferably a compound represented by the following formula (5-1).

（式（５−１）中、Ｒ_１０１及びＲ_１０２は、それぞれ独立に、水素原子、フッ素原子、又は炭素数１〜４のアルキル基を示す。ｎは０、１、２、３、又は４である。）。

(In Formula (5-1), R ₁₀₁ and R ₁₀₂ each independently represent a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms. N is 0, 1, 2, 3, or 4. .)

In the formula (5), X is -OSO 2 _- For group, a cyclic sulfonic acid ester comprises a cyclic disulfonic acid ester is preferably a cyclic disulfonic acid ester is a compound represented by the following formula (5-2).

（式（５−２）中、Ｒ_２０１乃至Ｒ_２０４は、それぞれ独立に、水素原子、フッ素原子、又は炭素数１〜４のアルキル基を示す。ｎは１、２、３、又は４である。また、ｎが２以上の場合、複数個存在するＲ_２０３は互いに同一であっても異なっていてもよく、複数個存在するＲ_２０４は、互いに同一であっても異なっていてもよい。）。

(In Formula (5-2), R _{201 to} R ₂₀₄ each independently represent a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms. N is 1, 2, 3, or 4. When n is 2 or more, a plurality of R ₂₀₃ may be the same or different from each other, and a plurality of R ₂₀₄ may be the same or different from each other. .

Examples of the cyclic sulfonate ester include 1,3-propane sultone, 1,2-propane sultone, 1,4-butane sultone, 1,2-butane sultone, 1,3-butane sultone, 2,4-butane sultone, 1,3 -Monosulfonic acid ester such as pentane sultone (when X in formula (5) is a single bond), disulfonic acid ester such as methylenemethane disulfonic acid ester, ethylenemethane disulfonic acid ester (X in formula (5) is- OSO ₂ - for group). Among these, 1,3-propane sultone (PS), 1,4-butane sultone (BS), and methylene methane disulfonate (MMDS) are preferable from the viewpoint of film formation effect, availability, and cost. It is preferable because a good film is formed.

  The content of the cyclic sulfonic acid ester in the electrolytic solution is preferably 0.1 to 10% by mass, more preferably 0.2 to 5% by mass, and 0.3 to 3% by mass. Is more preferable. If the content is too low, the effect as a film cannot be obtained sufficiently, and if it is too high, the internal resistance may increase.

  By causing LiFSI and cyclic sulfonic acid ester to coexist, it is possible to obtain a greater effect than when they are used alone. The cyclic sulfonic acid ester forms an excellent film on the negative electrode, but hardly forms a film on the positive electrode. On the other hand, LiFSI can form a film on both the negative electrode and the positive electrode, but the effect of the film on the negative electrode is lower than that of the cyclic sulfonate ester. Therefore, it is considered that a good quality film can be formed in a balanced manner on both the negative electrode and the positive electrode by mixing LiFSI and cyclic sulfonic acid ester.

In the present embodiment, the nonaqueous electrolytic solution is obtained by dissolving an electrolyte made of a lithium salt in a nonaqueous electrolytic solvent. Examples of the lithium salt is not particularly limited, for example, (except for compounds containing FSI _anion) lithium imide _{salt, LiPF 6, LiAsF 6, LiAlCl} 4, LiClO 4, LiBF 4, LiSbF 6 , and the like. Among these, LiPF ₆ and LiBF ₄ are preferable. The lithium imide salt, for _{example, LiN (C k F 2k +} 1 SO 2) (C m F 2m + 1 SO 2) (k and m are each independently 1 or 2). A lithium salt can be used individually by 1 type, and can also be used in combination of 2 or more type. The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / L. By setting the concentration of the lithium salt within this range, it is easy to adjust the density, viscosity, electrical conductivity, and the like to an appropriate range.

(Positive electrode active material)
The positive electrode in the present embodiment includes a positive electrode active material (5 V class active material) having an operating potential of 4.5 V or higher with respect to lithium metal. That is, the positive electrode active material used in the present embodiment has a charge / discharge region at 4.5 V or higher with respect to lithium metal.

The 5V class active material is preferably a lithium-containing composite oxide. Examples of the 5V class active material of the lithium-containing composite oxide include spinel-type lithium manganese composite oxide, olivine-type lithium manganese-containing composite oxide, reverse spinel-type lithium manganese-containing composite oxide, Li ₂ MnO ₃ -based solid solution, and the like. Can be mentioned.

  In particular, as the positive electrode active material, it is preferable to use a lithium manganese composite oxide represented by the following formula (6).

_{Li a (M x Mn 2-} x-y A y) (O 4-w Z w) (6)
(In formula (6), 0.4 ≦ x ≦ 1.2, 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2, 0 ≦ w ≦ 1, and M is Co, Ni, Fe, At least one selected from the group consisting of Cr and Cu, A is at least one selected from the group consisting of Li, B, Na, Mg, Al, Ti, Si, K and Ca, and Z is At least one of F and Cl).

  As M, it is more preferable to include only Ni or one or more of Co and Fe containing Ni as a main component. A is more preferably one or more of B, Mg, Al, and Ti. Z is more preferably F. Such a substitution element serves to stabilize the crystal structure and suppress the deterioration of the active material.

The average particle diameter (D ₅₀ ) of the positive electrode active material is preferably 1 to ₅₀ μm, and more preferably 5 to 25 μm. The average particle diameter (D ₅₀ ) of the positive electrode active material can be measured by a laser diffraction scattering method (microtrack method).

The 5V class active material is a positive electrode active material other than the above formula (6) as long as it is a positive electrode active material having a charge / discharge region of 4.5 V (vs. Li / Li ⁺ ) or more with respect to lithium metal. It doesn't matter. It is considered that the quality and stability of the film formed on the surface of the positive electrode active material are dominated by the potential and are not directly influenced by the composition of the active material.

Another example of the 5V class active material is represented by, for example, Li _x MPO ₄ F _y (0 ≦ x ≦ 2, 0 ≦ y ≦ 1, M is at least one of Co and Ni). Si-containing composite oxide represented by Li _x MSiO ₄ (0 ≦ x ≦ 2, M: at least one of Mn, Fe and Co); Li _x [Li _a M _b Mn _1-ab ] O ₂ (0 ≦ x ≦ 1, 0.02 ≦ a ≦ 0.3, 0.1 <b <0.7, M is at least Ni, Co, Fe and Cr A layered composite oxide represented by the following formula: and the like. One type of positive electrode active material may be used alone, or two or more types may be used in combination. In addition to the 5V class active material, a 4V class active material may be included.

(Negative electrode active material)
Although it does not restrict | limit especially as a negative electrode active material, For example, carbon materials, such as graphite and amorphous carbon, can be used. From the viewpoint of energy density, it is preferable to use graphite as the negative electrode active material. Further, as the negative electrode active material, in addition to the carbon material, for example, a material that forms an alloy with Li, such as Si, Sn, and Al, a Si oxide, a Si composite oxide containing a metal element other than Si and Si, Sn An oxide, a Sn composite oxide containing a metal element other than Sn and Sn, Li ₄ Ti ₅ O ₁₂ , a composite material in which these materials are coated with carbon, or the like can also be used. A negative electrode active material can be used individually by 1 type, and can also be used in combination of 2 or more type.

(electrode)
The positive electrode has, for example, a positive electrode active material layer formed on at least one surface of a positive electrode current collector. A positive electrode active material layer is comprised by the positive electrode active material which is a main material, a binder, and a conductive support agent, for example. The negative electrode is configured, for example, by forming a negative electrode active material layer on at least one surface of a negative electrode current collector. A negative electrode active material layer is comprised by the negative electrode active material which is a main material, a binder, and a conductive support agent, for example.

  Examples of the binder used in the positive electrode include polyvinylidene fluoride (PVDF) and acrylic polymers. Examples of the binder used in the negative electrode include styrene butadiene rubber (SBR) and the like other than the binder that can be used in the positive electrode. When an aqueous binder such as an SBR emulsion is used, a thickener such as carboxymethyl cellulose (CMC) can be used in combination.

  As the conductive aid, for example, carbon materials such as carbon black, granular graphite, flake graphite, and carbon fiber can be used for both the positive electrode and the negative electrode. In particular, it is preferable to use carbon black having low crystallinity for the positive electrode.

  As the positive electrode current collector, for example, aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used. As the negative electrode current collector, for example, copper, stainless steel, nickel, titanium, or an alloy thereof can be used.

  For example, the electrode is prepared by dispersing and kneading an active material, a binder, and a conductive aid in a solvent such as N-methyl-2-pyrrolidone (NMP) in a predetermined blending amount, and collecting the resulting slurry. It can be obtained by applying to an electric body to form an active material layer. The obtained electrode can be compressed to a suitable density by a method such as a roll press.

(Separator)
The separator is not particularly limited, and for example, a porous film made of a polyolefin such as polypropylene or polyethylene, a fluororesin, an inorganic separator made of cellulose, glass, or the like can be used.

(Exterior body)
As the exterior body, for example, coin-shaped, rectangular, cylindrical, etc. cans and laminate exterior bodies can be used. From the viewpoint of reducing the weight and improving the battery energy density, synthetic resin and metal A laminate outer package using a flexible film made of a laminate with a foil is preferred. Since the laminate type battery is excellent in heat dissipation, it is suitable as an in-vehicle battery such as an electric vehicle.

  In the case of a laminate type secondary battery, for example, an aluminum laminate film, a SUS laminate film, a silica-coated polypropylene film, a laminate film of polyethylene, or the like can be used as the outer package. In particular, it is preferable to use an aluminum laminate film from the viewpoint of suppressing volume expansion and cost.

(Secondary battery)
The configuration of the secondary battery according to the present embodiment is not particularly limited, and for example, an electrode element in which a positive electrode and a negative electrode are opposed to each other and an electrolytic solution may be included in an exterior body. it can. The shape of the secondary battery is not particularly limited, and examples thereof include a cylindrical shape, a flat wound rectangular shape, a laminated rectangular shape, a coin shape, a flat wound laminated shape, and a laminated laminated shape.

  FIG. 1 shows a laminated secondary battery as an example of the secondary battery according to this embodiment. The secondary battery shown in FIG. 1 includes a positive electrode composed of a positive electrode active material layer 1 including a positive electrode active material and a positive electrode binder, and a positive electrode current collector 3, and a negative electrode active material layer 2 including a negative electrode active material capable of occluding and releasing lithium. And the negative electrode current collector 4 is sandwiched between the separator 5. The positive electrode current collector 3 is connected to the positive electrode tab 8, and the negative electrode current collector 4 is connected to the negative electrode tab 7. A laminated outer package 6 is used as the outer package, and the inside of the secondary battery is filled with the nonaqueous electrolytic solution according to the present embodiment.

(Method for manufacturing secondary battery)
Although the manufacturing method of the secondary battery which concerns on this embodiment is not specifically limited, For example, the method shown below is mentioned. A positive electrode tab and a negative electrode tab are connected to the positive electrode for a secondary battery and the negative electrode according to this embodiment via a positive electrode current collector and a negative electrode current collector, respectively. The positive electrode and the negative electrode are arranged opposite to each other with the separator interposed therebetween, and an electrode laminate is produced in which the positive electrode and the negative electrode are laminated. The electrode laminate is accommodated in an exterior body and immersed in an electrolytic solution. A secondary battery is manufactured by sealing the exterior body so that a part of the positive electrode tab and the negative electrode tab protrudes to the outside.

  Hereinafter, examples of the present embodiment will be described in detail, but the present embodiment is not limited to the following examples.

The abbreviations of the compounds used in the following examples are described.
EC: ethylene carbonate DMC: dimethyl carbonate FE1: H (CF ₂ ) ₂ CH ₂ OCF ₂ CF ₂ H
FE2: CH ₃ CH ₂ O (CF ₂ ) ₄ F
FP: O = P (OCH ₂ CF ₃ ) ₃
SL: sulfolane represented by C ₄ H ₈ SO ₂ DMS: dimethyl sulfone EMS: ethyl methyl sulfone EiPS: ethyl isopropyl sulfone

Example 1
(Preparation of negative electrode)
Natural graphite powder (average particle size (D ₅₀ ): 20 μm, specific surface area: 1 m ² / g) as a negative electrode active material and PVDF as a binder are uniformly dispersed in NMP at a mass ratio of 95: 5 Thus, a negative electrode slurry was produced. By applying this negative electrode slurry on both sides of a 15 μm thick copper foil serving as a negative electrode current collector and drying at 125 ° C. for 10 minutes to evaporate NMP, a negative electrode active material layer is formed and further pressed. A negative electrode was produced. In addition, the weight of the negative electrode active material layer per unit area after drying was set to 0.015 g / cm ² .

(Preparation of positive electrode)
LiNi _0.5 Mn _1.5 O ₄ powder (average particle diameter (D ₅₀ ): 10 μm, specific surface area: 0.5 m ² / g) as a positive electrode active material was prepared. A positive electrode active material, PVDF as a binder, and carbon black as a conductive additive were uniformly dispersed in NMP at a mass ratio of 93: 4: 3 to prepare a positive electrode slurry. The positive electrode slurry was applied to both surfaces of a 20 μm thick aluminum foil serving as a positive electrode current collector, and then dried at 125 ° C. for 10 minutes to evaporate NMP, thereby producing a positive electrode. In addition, the weight of the positive electrode active material layer per unit area after drying was set to 0.040 g / cm ² .

(Nonaqueous electrolyte)
EC, DMC, and fluorinated ether (FE1) represented by H (CF ₂ ) ₂ CH ₂ OCF ₂ CF ₂ H at a ratio of EC: DMC: FE1 = 40: 20: 40 (volume ratio) A non-aqueous solvent was prepared by mixing. LiPF ₆ was dissolved in a non-aqueous solvent at a concentration of 0.8 mol / L as an electrolyte. In this electrolytic solution, 1 mass% of LiFSI as an additive was dissolved with respect to the total mass of the nonaqueous electrolytic solution to prepare a nonaqueous electrolytic solution.

(Production of laminated battery)
The positive electrode and the negative electrode were cut into 1.5 cm × 3 cm. Five layers of the obtained positive electrode and six layers of the negative electrode were alternately stacked while sandwiching a polypropylene porous film as a separator. The ends of the positive electrode current collector not covered with the positive electrode active material and the negative electrode current collector not covered with the negative electrode active material are welded respectively, and further, the positive electrode terminal made of aluminum and the negative electrode terminal made of nickel are connected to the welded portion. Each was welded to obtain an electrode element having a planar laminated structure. The electrode element was wrapped with an aluminum laminate film as an outer package, and an electrolyte solution was injected therein, and then sealed while reducing the pressure to produce a secondary battery.

(First charge / discharge)
The laminated battery produced as described above was charged to 4.75 V at a constant current of 16 mA corresponding to a 5-hour rate (0.2 C) at 20 ° C., and then charged with a constant voltage of 4.75 V for 8 hours in total. Then, constant current discharge was performed up to 3.0 V at 80 mA corresponding to 1 hour rate (1 C).

(Cycle test)
After the first charge / discharge is completed, the laminated battery is charged to 4.75V at 1C, and then charged for 4.75V constant voltage for 2.5 hours in total, and then discharged at a constant current to 3.0V at 1C. The charge / discharge cycle was repeated 300 times at 45 ° C. The ratio of the discharge capacity after 300 cycles to the initial discharge capacity was calculated as the capacity retention rate (%). Moreover, the cell volume after the first charge / discharge and after 300 cycles was obtained, and the volume increase rate (%) of the cell after 300 cycles with respect to after the first discharge was calculated. The volume was measured using the Archimedes method from the difference in weight between water and air.

(Example 2)
In place of the non-aqueous solvent of Example 1, EC, PC, FE1, and fluorinated phosphate ester (FP) represented by O = P (OCH ₂ CF ₃ ) ₃ were replaced with EC: PC: FE1. A secondary battery was prepared and evaluated in the same manner as in Example 1 except that a nonaqueous solvent mixed at a ratio of 20: 10: 40: 30 was used.

(Example 3)
Instead of the non-aqueous solvent of Example 1, EC, PC, FE1, FP, and a cyclic sulfone compound (sulfolane, SL) represented by C ₄ H ₈ SO ₂ were replaced with EC: PC: SL: A secondary battery was prepared and evaluated in the same manner as in Example 1 except that a non-aqueous solvent mixed at a ratio of FE1: FP = 10: 10: 10: 40: 30 was used.

(Comparative Example 1)
Instead of the non-aqueous solvent in Example 1, EC and DMC were used in the same manner as in Example 1 except that a non-aqueous solvent in which EC and DMC were mixed at a ratio of EC: DMC = 4: 6 (volume ratio) was used. A secondary battery was fabricated and evaluated.

(Comparative Example 2)
A secondary battery was prepared and evaluated in the same manner as in Comparative Example 1 except that LiFSI was not added.

(Comparative Example 3)
A secondary battery was prepared and evaluated in the same manner as in Example 1 except that LiFSI was not added.

(Comparative Example 4)
A secondary battery was prepared and evaluated in the same manner as in Example 2 except that LiFSI was not added.

(Comparative Example 5)
A secondary battery was prepared and evaluated in the same manner as in Example 3 except that LiFSI was not added.

(Comparative Example 6)
A secondary battery was fabricated in the same manner as in Example 3 except that 1% by mass of methylenemethane disulfonate (MMDS) was added as an additive in the total mass of the non-aqueous electrolyte instead of LiFSI in Example 3. ,evaluated.

Example 4
In place of the additive of Example 3, as additive, LiFSI was added in an amount of 0.5% by mass in the total mass of the nonaqueous electrolytic solution and MMDS was added in an amount of 0.5% by mass in the total mass of the nonaqueous electrolytic solution. Produced and evaluated a secondary battery in the same manner as in Example 3.

(Example 5)
Instead of the non-aqueous solvent of Example 4, a non-aqueous solvent in which EC, FE1, FP, and SL were mixed at a volume ratio of EC: SL: FE1: FP = 10: 20: 40: 30 was used. A secondary battery was fabricated and evaluated in the same manner as in Example 4 except that.

  Table 1 shows the measurement results of capacity retention ratio and volume increase of Comparative Examples 1 to 6 and Examples 1 to 5. Furthermore, the graph which compared the capacity | capacitance maintenance rate and the volume increase rate of Comparative Examples 3-5 and Examples 1-3 is shown in FIG. 2 and FIG. 3, respectively. In FIG. 2 and FIG. 3, the Example which added LiFSI and the comparative example which did not add LiFSI were shown side by side for every composition of the nonaqueous solvent.

  As shown in Table 1, FIG. 2 and FIG. 3, the example in which LiFSI was added improved the capacity retention rate compared to the comparative example in which no LiFSI was added, and the volume increased due to gas generation. The rate decreased.

  Moreover, while adding a fluorinated ether (FE1), a fluorinated phosphate ester (FP), and a sulfone compound (SL) to Comparative Example 1 containing only a carbonate-based solvent, the capacity retention rate is improved and the volume increase is increased. Also decreased significantly. Although the same tendency is seen also in the comparative example to which LiFSI is not added, the effect is larger in the example in which LiFSI is added, and the volume increase amount is from 195% to 40% in Comparative Example 2 to Comparative Example 5. While it decreased to about 1/5, in Comparative Example 1 to Example 3, it was about 1/10 from 155% to 16%. From this, it is preferable that the electrolyte contains LiFSI as an additive, and further contains, as the non-aqueous solvent, one or more selected from the group consisting of fluorinated ethers, fluorinated phosphates, and sulfone compounds. It was shown that it is more preferable to include the above. More preferably, the non-aqueous solvent contains a fluorinated ether.

  Further, EC: PC: SL: FE1: FP = 10: 10: 10: 40: 30 as a non-aqueous solvent and Comparative Example 6 in which only MMDS was added as an additive and Example 3 in which only LiFSI was added And Example 4 to which both MMDS and LiFSI were added, Example 4 had a higher capacity retention rate and a smaller volume increase than Comparative Examples 6 and 3. From this, it was shown that the combined use of cyclic sulfonate ester (MMDS) and LIFSI has a synergistic effect and is more preferable.

  In addition, as cyclic carbonate, Example 4 used both PC and EC, and Example 5 used only EC. However, Example 5 was better with a smaller volume increase rate. This is probably because EC has a higher dielectric constant than PC and a high ability to dissolve LiFSI.

(Example 6)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that CH ₃ CH ₂ O (CF ₂ ) ₄ F (FE2) was used as the fluorinated ether instead of FE1.

(Example 7)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that H (CF ₂ ) ₄ CH ₂ O (CF ₂ ) ₂ H was used as the fluorinated ether instead of FE1.

(Example 8)
A secondary battery was produced and evaluated in the same manner as in Example 4 except that CF ₃ CHFCF ₂ OCH ₂ (CF ₂ ) ₂ F was used as the fluorinated ether instead of FE1.

Example 9
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that dimethyl sulfone (DMS) was used instead of SL as the sulfone compound.

(Example 10)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that ethyl methyl sulfone (EMS) was used as the sulfone compound instead of SL.

(Example 11)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that ethyl isopropyl sulfone (EiPS) was used instead of SL as the sulfone compound.

  Table 2 shows the results of Examples 6 to 11. As described above, Examples 6 to 8 are obtained by using other fluorinated ethers instead of FE1 of Example 4, and Examples 9 to 11 are different sulfone compounds instead of SL of Example 4. Is used. In either case, it was confirmed that good results were obtained in both capacity retention ratio and volume increase.

実施例６〜１１の非水溶媒の組成（体積比）は、
（EC/PC/スルホン化合物/フッ素化エーテル/FP=1/1/1/4/3）である。

The composition (volume ratio) of the non-aqueous solvent of Examples 6 to 11 is
(EC / PC / sulfone compound / fluorinated ether / FP = 1/1/1/4/3).

(Example 12)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that 1,3-propane sultone (PS) was used instead of MMDS as the cyclic sulfonate ester.

(Example 13)
A secondary battery was prepared and evaluated in the same manner as in Example 4 except that 1,4-butane sultone (BS) was used instead of MMDS as the cyclic sulfonate ester.

  Table 3 shows the measurement results of Example 12 and Example 13. As described above, Examples 12 and 13 are obtained by changing the type of the cyclic sulfonate ester of the additive in Example 4. In Examples 12 and 13, the capacity retention rate and the volume increase amount are slightly inferior to those in the case where MMDS, which is a cyclic disulfonic acid ester, is used as an additive (Example 4), but both have good characteristics. It was confirmed.

(Comparative Example 7)
A non-aqueous solvent in which FE1, FP, and SL were mixed at a ratio of SL: FE1: FP = 20: 40: 40 was prepared. However, since a large amount of undissolved lithium salt remained visually, the battery could not be evaluated. From this result, it was found that the electrolytic solution preferably contains at least a cyclic carbonate.

(Comparative Example 8)
Instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, LiMn ₂ O ₄ powder (average particle diameter (D ₅₀ ): 13 μm, specific surface area: 0.5 m ² / g) was used, and the unit area A secondary battery was fabricated and evaluated in the same manner as in Comparative Example 6 except that the weight of the positive electrode active material layer per hit was 0.050 g / cm ² and the upper limit voltage was 4.2 V.

(Comparative Example 9)
Instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, LiMn ₂ O ₄ powder (average particle diameter (D ₅₀ ): 13 μm, specific surface area: 0.5 m ² / g) was used, and the unit area A secondary battery was produced and evaluated in the same manner as in Example 3 except that the weight of the positive electrode active material layer was 0.050 g / cm ² and the upper limit voltage was 4.2 V.

As described above, Comparative Example 8 and Comparative Example 9 are secondary batteries using a 4V class active material, Comparative Example 8 uses only MMDS as an additive, and Comparative Example 9 uses only LiFSI as an additive. ing. The capacity retention rate of Comparative Example 8 was 73%, the volume increase amount was 5%, the capacity maintenance rate of Comparative Example 9 was 70%, and the volume increase amount was 6%. In the battery using LiMn ₂ O ₄ which is a 4V class active material, even if LiFSI was used, the improvement effect of the capacity maintenance rate and the volume increase amount was not recognized as compared with the case where the cyclic sulfonic acid ester was used. . This is presumably because the degradation due to the oxidative decomposition of the electrolytic solution is small in the 4V class positive electrode, and the film formation effect on the positive electrode by LiFSI is limited. From this, it was confirmed that LiFSI has a particularly remarkable effect on the 5V class active material.

(Comparative Example 10)
Secondary in the same manner as in Comparative Example 3 except that LiCoPO ₄ was used instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, the upper limit voltage was 5.1 V, and the cycle number was 100 cycles. A battery was made and evaluated.

(Example 14)
Secondary using LiCoPO ₄ instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, the upper limit voltage is 5.1 V, and the number of cycles is 100 cycles. A battery was made and evaluated.

(Comparative Example 11)
Li (Li _0.15 Ni _0.2 Mn _0.65 ) O ₂ was used instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, and the weight of the positive electrode active material layer per unit area was 0 A secondary battery was fabricated and evaluated in the same manner as in Comparative Example 3 except that 0.025 g / cm ² , the upper limit voltage was 4.7 V, the lower limit voltage was 2.5 V, and the cycle number was 100 cycles.

(Example 15)
Li (Li _0.15 Ni _0.2 Mn _0.65 ) O ₂ was used instead of LiNi _0.5 Mn _1.5 O ₄ as the positive electrode active material, and the weight of the positive electrode active material layer per unit area was 0 and .025g / cm ^2, the upper limit voltage 4.7V, the lower limit voltage was 2.5V, except that the number of cycles is 100 cycles to produce a secondary battery in the same manner as in example 1 and evaluated.

Table 4 shows the capacity retention rates after 100 cycles at 45 ° C. of Comparative Examples 10 and 11 and Examples 14 and 15. This embodiment in which LiFSI was added to olivine type LiCoPO ₄ and layered structure Li (Li _0.15 Ni _0.2 Mn _0.65 ) O ₂ was effective in improving cycle characteristics. From this, it was shown that the effect of adding LiFSI can be obtained with a 5V class positive electrode active material regardless of the structure of the positive electrode active material.

(Comparative Example 12)
As the solvent, 4-fluoro ethylene carbonate (FEC) and ethyl methyl carbonate (EMC), were mixed in 20:80 volume ratio, 1 mol / L of LiPF ₆ in the solvent mixture, 0.1 mol of LiFSI / A secondary battery was prepared and evaluated in the same manner as in Comparative Example 1 except that a non-aqueous electrolyte prepared by dissolving so as to be L was used. As a result, the increase in volume after 50 cycles at 45 ° C. was as large as 280%, and the capacity retention rate was as low as 39%. From this result, it was confirmed that in the electrolytic solution described in Patent Document 4, gas generation during a high-temperature cycle was not sufficiently suppressed, and the capacity retention rate was greatly reduced.

DESCRIPTION OF SYMBOLS 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode collector 4 Negative electrode collector 5 Separator 6 Laminate exterior 7 Negative electrode tab 8 Positive electrode tab

Claims (7)

A lithium ion secondary battery comprising a positive electrode and a non-aqueous electrolyte,
The positive electrode includes a positive electrode active material having an operating potential of 4.5 V or more with respect to lithium metal,
The non-aqueous electrolyte is
(A) N (SO ₂ F) ₂ anion (FSI anion);
(B) a cyclic carbonate;
(C) A fluorinated ether represented by the following formula (1), a fluorinated phosphate ester represented by the following formula (2), and a sulfone compound represented by the following formula (3) or the following formula (4) At least one selected from the group consisting of:
A lithium ion secondary battery comprising:

(In Formula (1), R ₁₀₁ and R ₁₀₂ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of R ₁₀₁ and R ₁₀₂ is a fluorinated alkyl group),

(In Formula (2), R ¹ , R ² and R ³ each independently represents an alkyl group or a fluorinated alkyl group, and at least one of them is a fluorinated alkyl group),

(In Formula (3), R ₁ and R ₂ each independently represent a substituted or unsubstituted alkyl group),

(In formula (4), R ₃ represents a substituted or unsubstituted alkylene group). The lithium ion secondary battery according to claim 1, wherein the FSI anion is generated from LiN (SO ₂ F) ₂ (LiFSI) in the non-aqueous electrolyte. The non-aqueous electrolyte is
The group which consists of the fluorinated ether represented by said Formula (1), the fluorinated phosphate ester represented by said Formula (2), and the sulfone compound represented by said Formula (3) or said Formula (4). The lithium ion secondary battery according to claim 1, comprising two or more compounds selected from the group consisting of:   The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte further contains a cyclic sulfonate ester.   The lithium ion secondary battery according to claim 4, wherein the cyclic sulfonic acid ester is a cyclic disulfonic acid ester. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material is represented by the following formula (6):
_{Li a (M x Mn 2-} x-y A y) (O 4-w Z w) (6)
(In formula (6), 0.4 ≦ x ≦ 1.2, 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2, 0 ≦ w ≦ 1, and M is Co, Ni, Fe, At least one selected from the group consisting of Cr and Cu, A is at least one selected from the group consisting of Li, B, Na, Mg, Al, Ti, Si, K and Ca, and Z is At least one of F and Cl). A method for producing a lithium ion secondary battery having an electrode element, an electrolytic solution, and an outer package,
A step of producing an electrode element by arranging a positive electrode having a positive electrode active material capable of inserting and extracting lithium and operating at 4.5 V or higher with respect to lithium, and a negative electrode;
The electrode element;
(A) N (SO ₂ F) ₂ anion (FSI anion);
(B) a cyclic carbonate;
(C) a fluorinated ether represented by the formula (1) defined in claim 1, a fluorinated phosphate ester represented by the formula (2) defined in claim 1 , and and a non-aqueous electrolyte solution defined formula (3) also comprising at least one selected from the group consisting of sulfone compound represented by formula (4) as defined in claim 1 in a medium,
Sealing the inside of the exterior body,
A method for producing a lithium ion secondary battery, comprising:

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