JP2020181744A - Electrolyte solution and lithium ion secondary battery - Google Patents

Abstract

To provide an electrolyte solution suitable for providing a secondary battery superior in battery characteristic.SOLUTION: An electrolyte solution comprises: LiPF6; 1,2-dialkoxyethane; and a fluorine-substituted ether represented by the following general formula (A) below, where the mole ratio of 1,2-dialkoxyethane to LiPF6 is equal to or smaller than 4. The general formula (A) CnHaFbOCmHcFd. In the general formula (A), n is an integer of 1 or larger, a is an integer of 0 or larger, and b is an integer of 1 or larger, satisfying 2n+1=a+b, and m is an integer of 1 or larger, c is an integer of 0 or larger, and d is an integer of 0 or larger, satisfying 2m+1=c+d.SELECTED DRAWING: None

Description

The present invention relates to an electrolytic solution and a lithium ion secondary battery including the electrolytic solution.

Generally, a lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution as main components. The positive electrode is provided with a positive electrode active material involved in charging and discharging, and the negative electrode is provided with a negative electrode active material involved in charging and discharging. Then, as the electrolytic solution, a solution in which the electrolyte is dissolved in a non-aqueous solvent is generally adopted.

Here, as the electrolytic solution, LiPF ₆ is adopted as the electrolyte, and a mixed solvent using a mixed solvent in which a chain carbonate such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate and a cyclic carbonate such as ethylene carbonate are used in combination as a non-aqueous solvent is adopted. It is common to use a liquid.

Actually, in Patent Documents 1 to 3, LiPF ₆ is adopted as an electrolyte, and a chain carbonate such as ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate and ethylene carbonate which is a cyclic carbonate are used in combination as a non-aqueous solvent. A lithium ion secondary battery including an electrolytic solution using a solvent is specifically described.

JP-A-2015-185509 JP-A-2015-179625 International Publication No. 2014/08608

As described above, as the non-aqueous solvent of the electrolytic solution used in the lithium ion secondary battery, a carbonate-based solvent is generally used as the main solvent. However, the industry demands a lithium-ion secondary battery having better battery characteristics.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a suitable electrolytic solution in order to provide a lithium ion secondary battery having excellent battery characteristics.

The present inventor has studied a technique for further improving the battery characteristics of a lithium ion secondary battery. Specifically, the relationship between the negative electrode and the electrolytic solution was examined.

Conventionally, it is known that a carbonate-based solvent generally used forms a SEI (Solid Electrolyte Interphase) film on the surface of a negative electrode by reduction decomposition on the surface of the negative electrode. The SEI coating can prevent direct contact between the negative electrode active material and the electrolytic solution, but it is considered that the SEI coating produced from a carbonate-based solvent has _three CO groups.

Here, for example, in the case of a lithium ion secondary battery including a Si-containing negative electrode active material having relatively low resistance to oxidation, it is estimated that oxygen of _three CO groups in the SEI coating oxidizes the Si-containing negative electrode active material. To. Further, since the Si-containing negative electrode active material has a large degree of expansion and contraction during charging and discharging, the SEI coating is destroyed and a new portion not covered with the SEI coating (hereinafter, such a portion is referred to as a "new surface". ) Can occur. Here, there is a concern that the Si-containing negative electrode active material may be oxidized when the Si-containing negative electrode active material on the new surface comes into contact with the carbonate-based solvent.

Therefore, the present inventor has aimed to use as a non-aqueous solvent for the electrolytic solution, an ether that is unlikely to undergo reductive decomposition on the surface of the negative electrode and has a low oxidizing ability.

When the present inventor selected 1,2-dimethoxyethane as one aspect of ether and tried to dissolve LiPF ₆ which is generally used as an electrolyte in the ether, it was difficult to dissolve LiPF ₆ at a sufficient concentration. I found out.

Therefore, the present inventor diligently examined the type of electrolyte and the type of non-aqueous solvent used in combination with 1,2-dimethoxyethane. As a result, LiPF ₆ was adopted as the electrolyte and 1,2-dimethoxyethane was used as the non-aqueous solvent. It has been found that an electrolytic solution in which a mixed solvent in which a mixture of 1 and fluorine-substituted ether is used in combination and the molar ratio of 1,2-dimethoxyethane to LiPF ₆ is within a specific range is suitable.

Based on these findings, the present invention has been completed.

The electrolytic solution of the present invention contains LiPF ₆ , 1,2-dialkoxyethane and a fluorine-substituted ether represented by the following general formula (A), and the molar ratio of the 1,2-dialkoxyethane to the LiPF _{6 is} Is 4 or less.
Formula _{(A) C n H a F} b OC m H c F d
In the general formula (A), n is an integer of 1 or more, a is an integer of 0 or more, b is an integer of 1 or more, and 2n + 1 = a + b is satisfied. m is an integer of 1 or more, c is an integer of 0 or more, and d is an integer of 0 or more. Satisfy 2m + 1 = c + d.

The lithium ion secondary battery provided with the electrolytic solution of the present invention has excellent battery characteristics.

Hereinafter, embodiments for carrying out the present invention will be described. Unless otherwise specified, the numerical range "a to b" described in the present specification includes the lower limit a and the upper limit b in the range. Then, a numerical range can be constructed by arbitrarily combining these upper and lower limit values and the numerical values listed in the examples. Further, a numerical value arbitrarily selected from these numerical values can be used as a new upper limit or lower limit numerical value.

The electrolytic solution of the present invention contains LiPF ₆ , 1,2-dialkoxyethane and a fluorine-substituted ether represented by the following general formula (A) (hereinafter, may be simply referred to as "fluorine-substituted ether"). It is characterized in that the molar ratio of the 1,2-dialkoxyethane to the LiPF ₆ is 4 or less.
Formula _{(A) C n H a F} b OC m H c F d
In the general formula (A), n is an integer of 1 or more, a is an integer of 0 or more, b is an integer of 1 or more, and 2n + 1 = a + b is satisfied. m is an integer of 1 or more, c is an integer of 0 or more, and d is an integer of 0 or more. Satisfy 2m + 1 = c + d.

LiPF ₆ cannot be dissolved in a non-aqueous solvent of 1,2-dialkoxyethane alone at a sufficient concentration. However, LiPF ₆ can be dissolved in a mixed solvent in which 1,2-dimethoxyethane and a fluorine-substituted ether are used in combination at a sufficient concentration.

As the alkoxy group of 1,2-dialkoxyethane, those having 1 to 6 carbon atoms are preferable, those having 1 to 4 carbon atoms are more preferable, and those having 1 to 2 carbon atoms are further preferable.
One type of 1,2-dialkoxyethane may be used, or a plurality of types may be used in combination.

Since 1,2-dialkoxyethane is a chelate compound, it has a high coordination ability with respect to metal ions (complex formation constant or complex stability constant). Therefore, it is considered that 1,2-dialkoxyethane preferentially coordinates with the lithium ion of LiPF _{6 in} the electrolytic solution to form a stable complex.
Here, it is considered that 1,2-dialkoxyethane that is not involved in complex formation may be present in the electrolytic solution in which the molar ratio of 1,2-dialkoxyethane to LiPF ₆ exceeds 4.

In the electrolytic solution of the present invention, the molar ratio of 1,2-dialkoxyethane to LiPF ₆ is 0.5 to 3.5, 0.7 to 3.2, 1.0 to 3.0, and 1. .5-2.7, 1.8-2.5 can be exemplified.

In the general formula (A), 1 to 18, 1 to 12, 1 to 8 and 1 to 6 can be exemplified independently as n and m, respectively. From the viewpoint of boiling point, it is preferable to satisfy n + m ≧ 4. In the general formula (A), those satisfying a <b, those satisfying c <d, or those satisfying a + c <b + d are preferable. In the general formula (A), a = 0 and / or c = 0 may be used.
One type of fluorine-substituted ether may be used, or a plurality of types may be used in combination.

Examples of the fluorine-substituted ether represented by the general formula (A) include CF ₃ CH ₂ OCF ₂ CH ₃ , CF ₃ CH ₂ OCF ₂ CHF ₂ , CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , and CF ₃ CF ₂ CH _2. OCF ₂ CHF ₂ , CF ₃ CHFCF ₂ OCH ₂ CF ₃ , C ₃ F ₇ OCH ₃ , C ₄ F ₉ OCH ₃ , C ₆ F ₁₃ OCH ₃ , C ₂ F ₅ CF (OCH ₃ ) C ₃ F ₇ Examples thereof include ₄ F ₉ OC ₂ H ₅ , C ₃ HF ₆ CH (CH ₃ ) OC ₃ HF ₆ .

In the electrolytic solution of the present invention, examples of the molar ratio of the fluorine-substituted ether to LiPF ₆ are 0.5 or more, 1 or more, and 1.5 or more. More specifically, 0.5 to 10, 1 to 7, and 1.5 to 5 can be exemplified as the range of the molar ratio of the fluorine-substituted ether to LiPF ₆ .

From the viewpoint of its chemical structure, the fluorine-substituted ether is considered to have a lower coordination ability with respect to metal ions (complex formation constant or complex stability constant) than 1,2-dialkoxyethane. Therefore, it is considered that the fluorine-substituted ether does not affect the stability of the complex formed of 1,2-dialkoxyethane and lithium ion.

In the electrolytic solution of the present invention, the compounding ratio of 1,2-dialkoxyethane and fluorine-substituted ether is not particularly limited. The volume ratio of 1,2-dialkoxyethane to fluorine-substituted ether is in the range of 10:90 to 90:10, in the range of 20:80 to 80:20, and in the range of 30:70 to 70:30. It can be exemplified. The molar ratio of 1,2-dialkoxyethane to fluorine-substituted ether is in the range of 10:90 to 90:10, in the range of 20:80 to 80:20, and in the range of 30:70 to 70:30. It can be exemplified.

The concentration of LiPF ₆ in the electrolytic solution of the present invention was in the range of 0.3 to 4 mol / L, in the range of 0.5 to 3 mol / L, in the range of 0.7 to 2.5 mol / L, and 0. The range of 9 to 2 mol / L and the range of 1 to 1.5 mol / L can be exemplified.

The electrolytic solution of the present invention includes an electrolyte other than LiPF ₆ and a non-aqueous solvent other than 1,2-dialkoxyethane and a fluorine-substituted ether (hereinafter referred to as a third solvent) as long as the gist of the present invention is not deviated. There is.) May be added. Further, various additives may be added to the electrolytic solution of the present invention.

Examples of the ratio of LiPF ₆ to the total electrolyte contained in the electrolytic solution of the present invention include 60 to 100 mol%, 70 to 99 mol%, 80 to 98 mol%, and 90 to 95 mol%.

The ratios of 1,2-dialkoxyethane and fluorine-substituted ether to the total non-aqueous solvent contained in the electrolytic solution of the present invention are 60 to 100% by volume, 70 to 98% by volume, 80 to 95% by volume, and 85 to 90% by volume. %, 60 to 100 mol%, 70 to 98 mol%, 80 to 95 mol%, 85 to 90 mol% can be exemplified.

The third solvent may be any solvent that can be used as a non-aqueous solvent for the lithium ion secondary battery. Further, as the third solvent, a solvent having a coordination ability with respect to metal ions (complex formation constant or complex stability constant) lower than that of 1,2-dialkoxyethane is preferable. Examples of the third solvent include chain ethers, cyclic ethers and hydrocarbons.
When the molar ratio of 1,2-dialkethaneethane to LiPF ₆ is low, the fluorine-substituted ether or the third solvent containing oxygen may coordinate with the lithium ion of LiPF _{6 in} the electrolytic solution. Presumed to be.

As the chain ether, those represented by R ¹ OR ² are preferable. R ¹ and R ² are independently chain alkyl groups or cyclic alkyl groups, respectively. Examples of the number of carbon atoms of the chain alkyl group include 1 to 6, 2 to 5, and 3 to 4. Examples of the number of carbon atoms of the cyclic alkyl group include 3 to 8, 4 to 7, and 5 to 6.

Examples of the chain ether include diethyl ether, di-n-propyl ether, di-i-propyl ether, ethyl n-propyl ether, ethyl i-propyl ether, di-n-butyl ether, di-iso-butyl ether, and di-sec. -Butyl ether, di-tert-butyl ether, n-butyl methyl ether, iso-butyl methyl ether, sec-butyl methyl ether, tert-butyl methyl ether, n-butyl ethyl ether, iso-butyl ethyl ether, sec-butyl ethyl ether , Tert-butyl ethyl ether, n-butyl n-propyl ether, iso-butyl n-propyl ether, sec-butyl n-propyl ether, tert-butyl n-propyl ether, di-n-pentyl ether, diisopentyl ether. , Methyl neopentyl ether, ethyl neopentyl ether, di-n-hexyl ether, methyl cyclopentyl ether, methyl cyclohexyl ether, ethyl cyclopentyl ether, ethyl cyclohexyl ether can be exemplified.

The cyclic ether is preferably a 4- to 8-membered ring ether which may be substituted with an alkyl group, more preferably a 5- to 7-membered ring ether which may be substituted with an alkyl group, and is substituted with an alkyl group. A 6-membered ring ether may be more preferred.
The cyclic ether preferably has an oxygen number of 2 or 1. Examples of the cyclic ether having an oxygen number of 2 include 1,4-dioxane.
Examples of the cyclic ether having an oxygen number of 1 include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran, 3-methyltetrahydropyran, 4-methyltetrahydropyran, and oxepane.

The reason why a 6-membered ring ether which may be substituted with an alkyl group is preferable as the cyclic ether will be specifically described.
Since the 6-membered ring cyclic ether is unlikely to generate radicals, it is considered to have high resistance to reductive decomposition at the negative electrode during charging and discharging. The reason is as follows.

In the 6-membered ring cyclic ether, the part where radicals are most likely to be generated, in other words, the carbon in which the hydrogen radical is most easily released is the carbon adjacent to the oxygen of the ether among the carbons constituting the 6-membered ring cyclic ether. It can be said that there is.

The 6-membered ring cyclic ether has a conformation such as a chair-form or a boat-form that is thermally stable. Then, each conformation can be converted to each other. Such conformation is by carbon constituting the ring has an sp ³ hybrid orbital is formed.
However, the carbon in which the radical is generated is converted from the sp ³ hybrid orbital to the sp ² hybrid orbital. Three bonds in sp ² hybrid orbital is ideally present on the same plane.

Then, when the radical is generated in the cyclic ether 6-membered ring, since the carbon bearing the radical will have a sp ² hybrid orbital, conformations such as thermal energy stable chair-form and boat-form It cannot take a seat and has to have a three-dimensional structure that is unstable in terms of thermal energy.
It is considered that such a thermal energy unstable three-dimensional structure serves as an energy barrier for radicalizing the 6-membered ring cyclic ether.

As the 6-membered ring cyclic ether which may be substituted with an alkyl group, it is preferable that 5 of the 6 elements constituting the ring are carbon and 1 is oxygen.
As the alkyl group that can be bonded to the carbon of the 6-membered ring, those having 1 to 6 carbon atoms are preferable, those having 1 to 4 carbon atoms are more preferable, and those having 1 to 2 carbon atoms are further preferable.

The alkyl group that can be attached to the carbon of the 6-membered ring is preferably attached to the carbon that is not adjacent to the oxygen of the ether.
When an alkyl group is bonded to a carbon adjacent to oxygen of the ether, the carbon has a structure of O-CH (alkyl group) -CH ₂ . Here, in a CH group sandwiched between oxygen and an alkyl chain, it is considered that radicals are likely to be generated because radicals are stabilized due to the interaction of adjacent groups between adjacent oxygen and the adjacent alkyl chain. .. Therefore, it is presumed that the 6-membered cyclic ether in which an alkyl group is bonded to the carbon adjacent to the oxygen of the ether is relatively easy to generate radicals.

Examples of hydrocarbons include chain hydrocarbons, cyclic hydrocarbons, and aromatic hydrocarbons. As the hydrocarbon, a fluorine-substituted hydrocarbon is preferable, and a fluorine-substituted-cyclic hydrocarbon represented by the following general formula (B) is more preferable.
Formula _{(B) C n H a F} b
In the general formula (B), n is an integer of 5 or more, a is an integer of 0 or more, b is an integer of 1 or more, and 2n = a + b is satisfied.

In the general formula (B), examples of n include 5 to 12, 5 to 10, 5 to 8, and 5 to 7. In the general formula (B), it is preferable that a <b is satisfied.
In the general formula (B), a 5-membered ring or a 6-membered ring is preferable. Further, in the general formula (B), one or more substituents such as CH ₃ , CF ₃ , CF ₃ CF ₂ may be bonded to the skeleton of the 5-membered ring or the 6-membered ring.

Examples of the fluorine-substituted-cyclic hydrocarbon represented by the general formula (B) include fluorocyclopentane, 1,1,2,2,3,3,4-heptafluorocyclopentane, decafluorocyclopentane, fluorocyclohexane, and perfluoro. Examples thereof include methylcyclohexane and perfluoro (1,3-dimethylcyclohexane).

The lithium ion secondary battery provided with the electrolytic solution of the present invention is referred to as the lithium ion secondary battery of the present invention.
Specifically, the lithium ion secondary battery of the present invention includes the electrolytic solution of the present invention, a positive electrode, a negative electrode, and a separator.

The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector.

A current collector is a chemically inactive electron conductor that keeps current flowing through the electrodes during the discharge or charging of a lithium ion secondary battery. The material of the current collector is not particularly limited as long as it is a metal that can withstand a voltage suitable for the active material used. As the material of the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel. Metallic materials such as can be exemplified. The current collector may be coated with a known protective layer. A current collector whose surface is treated by a known method may be used as the current collector.

The current collector can take the form of foil, sheet, film, linear, rod, mesh or the like. Therefore, as the current collector, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, or a stainless steel foil can be preferably used. When the current collector is in the form of a foil, a sheet, or a film, the thickness is preferably in the range of 1 μm to 100 μm.

When the potential of the positive electrode is 4 V or more based on lithium, it is preferable to use aluminum as the current collector for the positive electrode.

Specifically, it is preferable to use a current collector for the positive electrode made of aluminum or an aluminum alloy. Here, aluminum refers to pure aluminum, and aluminum having a purity of 99.0% or higher is referred to as pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Examples of the aluminum alloy include Al—Cu, Al—Mn, Al—Fe, Al—Si, Al—Mg, Al—Mg—Si, and Al—Zn—Mg.

Further, as aluminum or aluminum alloy, specifically, for example, A1000 series alloys such as JIS A1085 and A1N30 (pure aluminum series), A3000 series alloys such as JIS A3003 and A3004 (Al-Mn series), JIS A8079, A8021 and the like. A8000 series alloy (Al—Fe series) can be mentioned.

The positive electrode active material layer contains a positive electrode active material capable of occluding and releasing lithium ions, and, if necessary, a binder and a conductive auxiliary agent. The positive electrode active material layer preferably contains the positive electrode active material in an amount of 60 to 99% by mass, more preferably 70 to 95% by mass, based on the total mass of the positive electrode active material layer. As the binder and the conductive auxiliary agent, those described in the negative electrode may be appropriately used in appropriate amounts.

As the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (M is .D selected from Mn and Al W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, At least one element selected from S, Si, Na, K, P, and V. A, b, c, d, e, and f are 0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ e <1. Li ₂ MnO ₃ , a lithium composite metal oxide represented by (satisfying 1.7 ≦ f ≦ 3), can be mentioned. Further, as the positive electrode active material, a metal oxide having a spinel structure such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a metal oxide having a spinel structure and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (in the formula). M is selected from at least one of Co, Ni, Mn, and Fe) and the like. Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as the basic composition, and those in which the metal element contained in the basic composition is replaced with another metal element can also be used. Further, as the positive electrode active material, a material containing no lithium ion may be used. For example, elemental sulfur, compounds complexed with sulfur and carbon, metal sulfides such as TiS _2, oxides such as V 2 _{O 5,} MnO _2, polyaniline and anthraquinone and compounds containing these aromatic in chemical structure, conjugated double Conjugated materials such as acetic acid-based organic substances and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galbinoxyl, and phenoxyl may be adopted as the positive electrode active material. When a positive electrode active material containing no lithium ions is used, it is preferable to add lithium to the positive electrode and / or the negative electrode in advance by a known method.

From the viewpoint of excellent and high capacity, and durability, as the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O f (M is .D selected from Mn and Al W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V are at least one element. A, b, c, d, e, f are 0.2 ≦ a ≦ 2. , B + c + d + e = 1, 0 ≦ e <1, 1.7 ≦ f ≦ 3).) It is preferable to use the lithium composite metal oxide represented by.

In the above general formula, the values of b, c, and d are not particularly limited as long as they satisfy the above conditions, but those having 0 <b <1, 0 <c <1, 0 <d <1 are used. It is good, and at least one of b, c, and d is in the range of 0.1 ≦ b ≦ 0.95, 0.01 ≦ c ≦ 0.5, and 0.01 ≦ d ≦ 0.5. More preferably, the range is 0.3 ≦ b ≦ 0.9, 0.03 ≦ c ≦ 0.3, 0.03 ≦ d ≦ 0.3, and 0.5 ≦ b ≦ 0.9, It is more preferable that the range is 0.05 ≦ c ≦ 0.2 and 0.05 ≦ d ≦ 0.2.

The values a, e, and f may be values within the range specified by the above general formula, and preferably 0.5 ≦ a ≦ 1.5, 0 ≦ e <0.2, 1.8 ≦ f ≦ 2. 5.5, more preferably 0.8 ≦ a ≦ 1.3, 0 ≦ e <0.1, 1.9 ≦ f ≦ 2.1 can be exemplified, respectively.

As a positive electrode active material, Li _x Mn _{2-y Ay} O ₄ (A is Ca, Mg, S, Si, Na, K, Al, P, Ga) having a spinel structure, because of its high capacity and excellent durability. , At least one element selected from Ge, and at least one metal element selected from transition metal elements such as Ni. 0 <x ≦ 2.2, 0 ≦ y ≦ 1) can be exemplified. Examples of the range of the value of x include 0.5 ≦ x ≦ 1.8, 0.7 ≦ x ≦ 1.5, and 0.9 ≦ x ≦ 1.2, and the range of the value of y is 0. Examples thereof include ≦ y ≦ 0.8 and 0 ≦ y ≦ 0.6. Examples of specific spinel-structured compounds include LiMn ₂ O ₄ and Limn _1.5 Ni _0.5 O ₄ .

Specific examples of the positive electrode active material include LiFePO ₄ , Li ₂ FeSiO ₄ , LiCoPO ₄ , Li ₂ CoPO ₄ , Li ₂ MnPO ₄ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F. As another specific positive electrode active material, Li ₂ MnO _3- LiCoO ₂ can be exemplified.

From the concrete evaluation results described later, it is preferable that the positive electrode active material is charged and discharged at a potential of 4 V or more based on lithium. Such a positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c M} d D e O lithium mixed metal oxide represented by _f, a spinel structure _Li x _{Mn 2-y A} y O _{4 and the} like can be exemplified. Examples of the lithium-based potential for charging and discharging the preferred positive electrode active material include 4 to 5.5 V, 4.1 to 5 V, 4.2 to 4.8 V, and 4.3 to 4.5 V.

The negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector.
As the current collector, the one described in the positive electrode may be appropriately adopted.

The negative electrode active material layer contains a negative electrode active material capable of occluding and releasing lithium ions, and, if necessary, a binder and a conductive auxiliary agent.

As the negative electrode active material, a material that can be used for a lithium ion secondary battery may be used. From the specific evaluation results described later, graphite or Si-containing negative electrode active material can be exemplified as a suitable negative electrode active material.

As the Si-containing negative electrode active material, a Si-containing material capable of occluding and releasing lithium ions can be used.

Specific examples of the Si-containing material include Si alone and SiO _x (0.3 ≦ x ≦ 1.6). If _x of SiO _x is less than the lower limit value, the ratio of Si becomes excessive, so that the volume change at the time of charging / discharging becomes too large, and the cycle characteristics of the lithium ion secondary battery may deteriorate. On the other hand, when x exceeds the upper limit value, the Si ratio becomes too small and the energy density decreases. The range of x is more preferably 0.5 ≦ x ≦ 1.5, and even more preferably 0.7 ≦ x ≦ 1.2.

Specific examples of the Si-containing material include a silicon material disclosed in International Publication No. 2014/08608 (hereinafter, simply referred to as “silicon material”).

The silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. The silicon material undergoes, for example, a step of reacting CaSi ₂ with an acid to synthesize a layered silicon compound containing polysilane as a main component, and a step of heating the layered silicon compound at 300 ° C. or higher to release hydrogen. It is manufactured.

The ideal reaction formula when hydrogen chloride is used as the acid shows the method for producing the silicon material as follows.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl _{2
Si 6 H 6 → 6Si + 3H} 2 ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is usually used as the reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, in the step of synthesizing the layered silicon compound including the reaction in the upper stage, the layered silicon compound is rarely produced as containing only Si ₆ H ₆ , and is layered. Silicon compounds are represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an acid anion, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a compound. In the above chemical formula, unavoidable impurities such as Ca that may remain are not considered. The silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

As described above, the silicon material has a structure in which a plurality of plate-shaped silicon bodies are laminated in the thickness direction. In order to efficiently occlude and release lithium ions, the plate-shaped silicon body preferably has a thickness in the range of 10 nm to 100 nm, and more preferably in the range of 20 nm to 50 nm. The length of the plate-shaped silicon body in the longitudinal direction is preferably in the range of 0.1 μm to 50 μm. Further, the plate-shaped silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-shaped silicon body can be confirmed by observation with a scanning electron microscope or the like. Further, this laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

The silicon material preferably contains amorphous silicon and / or silicon crystallites. In particular, in the plate-shaped silicon body, it is preferable that amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, further preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scheller's equation using the half width of the diffraction peak on the Si (111) plane of the obtained X-ray diffraction chart obtained by performing X-ray diffraction measurement on the silicon material. ..

The abundance and size of the plate-shaped silicon body, amorphous silicon, and silicon crystallites contained in the silicon material mainly depend on the heating temperature and heating time. The heating temperature is preferably in the range of 350 ° C. to 950 ° C., more preferably in the range of 400 ° C. to 900 ° C.

The Si-containing negative electrode active material is preferably in the form of powder, which is an aggregate of particles. The average particle size of the Si-containing negative electrode active material is preferably in the range of 1 to 30 μm, more preferably in the range of 2 to 20 μm. If a Si-containing negative electrode active material having an average particle size too small is used, the manufacturing operation may become difficult. On the other hand, a lithium ion secondary battery including a negative electrode using a Si-containing negative electrode active material having an excessively large average particle size may not be able to be charged and discharged appropriately.
The average particle size in the present specification means D ₅₀ when the sample is measured by a general laser diffraction type particle size distribution measuring device.

As the Si-containing negative electrode active material, a carbon layer-coated-Si-containing negative electrode active material in which the Si-containing negative electrode active material is coated with a carbon layer may be adopted. The carbon layer coating-Si-containing negative electrode active material is preferably one in which the carbon layer and the Si-containing negative electrode active material are integrated. Examples of the method for producing such a carbon layer-coated-Si-containing negative electrode active material include a mechanical milling method in which a mixture of a Si-containing negative electrode active material and a carbon powder is stirred and integrated after applying strong pressure. A CVD (chemical vapor deposition) method in which carbon generated from a carbon source is deposited on a Si-containing negative electrode active material can be exemplified.

The CVD method is preferable because the surface of the Si-containing negative electrode active material can be uniformly coated with a thin carbon layer. Then, among the CVD methods, the thermal CVD method in which a gaseous organic substance as a carbon source is thermally decomposed to generate carbon is preferable.

A thermal CVD step of producing a carbon layer-coated-Si-containing negative electrode active material by using a thermal CVD method will be specifically described. More specifically, in the thermal CVD step, a carbon layer formed by bringing a Si-containing negative electrode active material into contact with an organic substance under a non-oxidizing atmosphere and heating conditions and carbonizing the organic substance on the surface of the Si-containing negative electrode active material. Is the process of forming. When performing a thermal CVD step, a known CVD apparatus such as a fluidized bed reactor, a rotary furnace, a tunnel furnace, a batch type firing furnace, a rotary kiln, etc. of a hot wall type, a cold wall type, a horizontal type, a vertical type, etc. is used. It may be used.

As the organic substance, a substance that can be thermally decomposed and carbonized by heating in a non-oxidizing atmosphere is used, and for example, saturated aliphatic hydrocarbons such as methane, ethane, propane, butan, isobutane, pentane, and hexane, ethylene, Unsaturated aliphatic hydrocarbons such as propylene and acetylene, alcohols such as methanol, ethanol, propanol and butanol, benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorbenzene, inden, benzofuran , Aromatic hydrocarbons such as pyridine, esters such as ethyl acetate, butyl acetate, amyl acetate, fatty acids and the like, and examples thereof.

The processing temperature in the thermal CVD step varies depending on the type of the organic substance, but it is desirable that the temperature is 50 ° C. or more higher than the temperature at which the organic substance is thermally decomposed. However, if the heating temperature is excessively high, free carbon (soot) may be generated in the system, so it is preferable to select a condition in which free carbon (soot) is not generated. The thickness of the carbon layer formed can be controlled by the treatment time.

It is desirable that the thermal CVD step is performed by putting the Si-containing negative electrode active material in a flowing state. By doing so, the entire surface of the Si-containing negative electrode active material can be brought into contact with the organic substance, and a more uniform carbon layer can be formed. There are various methods such as using a fluidized bed to bring the Si-containing negative electrode active material into a fluidized state, but it is preferable to bring the Si-containing negative electrode active material into contact with an organic substance while stirring. For example, if a rotary furnace having a baffle plate inside is used, the Si-containing negative electrode active material staying in the baffle plate drops from a predetermined height as the rotary furnace rotates and is agitated, and at that time, it comes into contact with organic substances. Since the carbon layer is formed, a more uniform carbon layer can be formed over the entire Si-containing negative electrode active material.

Carbon layer coating-The carbon layer of the Si-containing negative electrode active material is preferably amorphous and / or crystalline, and the carbon layer preferably covers the entire surface of the Si-containing negative electrode active material particles. The thickness of the carbon layer is preferably in the range of 1 nm to 100 nm, more preferably in the range of 5 to 50 nm, and even more preferably in the range of 10 to 30 nm.

The negative electrode active material layer preferably contains the negative electrode active material in an amount of 60 to 99% by mass, more preferably 70 to 95% by mass, based on the total mass of the negative electrode active material layer.

Examples of the binder include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins and carboxymethyl cellulose. , A known material such as styrene-butadiene rubber may be used.

Further, a crosslinked polymer obtained by cross-linking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with a polyamine such as diamine disclosed in International Publication No. 2016/06382 may be used as a binder.

Diamines used in the crosslinked polymer include alkylenediamines such as ethylenediamine, propylenediamine and hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophoronediamine and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 3,5-diaminobenzoic acid, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, Examples thereof include aromatic diamines such as o-tridine, 2,4-tolylene diamine, 2,6-tolylene diamine, xylylene diamine and naphthalenediamine.

The mixing ratio of the binder in the negative electrode active material layer is preferably, in terms of mass ratio, negative electrode active material: binder = 1: 0.005 to 1: 0.3. This is because if the amount of the binder is too small, the moldability of the electrode is lowered, and if the amount of the binder is too large, the energy density of the electrode is lowered.

The conductive auxiliary agent is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be arbitrarily added when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive auxiliary agent may be a chemically inert electron high conductor, and examples thereof include carbon black, graphite, vaporized carbon fiber (Vapor Grown Carbon Fiber), and various metal particles, which are carbonaceous fine particles. To. Examples of carbon black include acetylene black, Ketjen black (registered trademark), furnace black, and channel black. These conductive auxiliary agents can be added to the active material layer alone or in combination of two or more.

The mixing ratio of the conductive auxiliary agent in the negative electrode active material layer is preferably, in terms of mass ratio, negative electrode active material: conductive auxiliary agent = 1: 0.01 to 1: 0.5. This is because if the amount of the conductive auxiliary agent is too small, an efficient conductive path cannot be formed, and if the amount of the conductive auxiliary agent is too large, the moldability of the active material layer is deteriorated and the energy density of the electrode is lowered.

Conventionally known methods such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, and a curtain coating method are used to form a positive electrode active material layer or a negative electrode active material layer on the surface of a current collector. The positive electrode active material or the negative electrode active material may be applied to the surface of the current collector using the above. Specifically, the positive electrode active material or the negative electrode active material, the binder, the solvent, and if necessary, the conductive auxiliary agent are mixed to form a slurry, and then the slurry is applied to the surface of the current collector and then dried. .. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. In order to increase the electrode density, the dried one may be compressed.

The separator separates the positive electrode and the negative electrode, and allows lithium ions to pass through while preventing a short circuit due to contact between the two electrodes. As the separator, a known one may be adopted, and synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester and polyacrylonitrile, polysaccharides such as cellulose and amylose, and fibroin , Natural polymers such as keratin, lignin, and suberin, and porous materials using one or more electrically insulating materials such as ceramics, non-woven fabrics, and woven fabrics. Further, the separator may have a multi-layer structure.

A specific method for manufacturing the lithium ion secondary battery of the present invention will be described.
For example, the separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. The electrode body may be of either a laminated type in which a positive electrode, a separator and a negative electrode are stacked, or a wound type in which a laminated body of a positive electrode, a separator and a negative electrode is wound. After connecting the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside by using a current collecting lead or the like, the electrolytic solution of the present invention is added to the electrode body to add a lithium ion secondary. Use a battery.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical type, a square type, a coin type, and a laminated type can be adopted.

The lithium ion secondary battery of the present invention has excellent durability against charging and discharging under high potential conditions. Therefore, the lithium ion secondary battery of the present invention is preferably charged to a high potential. The high potential here means a potential of 4 V or more based on lithium. Examples of the high potential range include 4 to 5.5 V, 4.1 to 5 V, 4.2 to 4.8 V, and 4.3 to 4.5 V based on lithium.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy from a lithium ion secondary battery for all or part of its power source, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium-ion secondary battery is mounted on a vehicle, a plurality of lithium-ion secondary batteries may be connected in series to form an assembled battery. In addition to vehicles, devices equipped with a lithium-ion secondary battery include various battery-powered home appliances such as personal computers and mobile communication devices, office devices, and industrial devices. Further, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydraulic power generation and other power storage devices and power smoothing devices for electric power systems, power supply sources for power and / or auxiliary machinery such as ships, aircraft, and aircraft. Power supply sources for spacecraft and / or auxiliary equipment, auxiliary power sources for vehicles that do not use electricity as power sources, power sources for mobile household robots, power sources for system backup, power sources for non-disruptive power supply devices, It may be used as a power storage device that temporarily stores the electric power required for charging in a charging station for an electric vehicle or the like.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. It can be carried out in various forms with modifications, improvements, etc. that can be made by those skilled in the art, without departing from the gist of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. The present invention is not limited to these examples.

(Example 1)
Example by mixing LiPF ₆ and 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which is a fluorine-substituted ether at a molar ratio of 1: 2: 3.38 to dissolve LiPF _6. The electrolytic solution of 1 was produced.

Using the electrolytic solution of Example 1, the lithium ion secondary battery of Example 1-N was produced as follows.

Polyacrylic acid having a weight average molecular weight of 1.5 million, 3,5-diaminobenzoic acid and water were mixed to prepare a mixed aqueous solution. Here, the molar ratio of the acrylic acid monomer constituting polyacrylic acid to 3,5-diaminobenzoic acid was 16: 1.
A binder solution was produced by stirring the mixed aqueous solution at 80 ° C. for 2 hours in a nitrogen gas atmosphere. In the binder solution, the water content was 85% by mass.

80 parts by mass of carbon-coated silicon material as a Si-containing negative electrode active material, 10 parts by mass of acetylene black as a conductive auxiliary agent, the above-mentioned binder solution having a solid content of 10 parts by mass as a binder, and an appropriate amount of water. Mixing produced a slurry. A copper foil was prepared as a current collector for the negative electrode. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. Water was removed by drying the copper foil coated with the slurry. Then, a negative electrode having a negative electrode active material layer formed on the surface of the copper foil was manufactured by pressing and heating at 230 ° C.

The negative electrode was cut to a diameter of 11 mm and used as an evaluation electrode. A metal lithium foil having a thickness of 500 μm was cut into a diameter of 13 mm to form a counter electrode. As a separator, a glass filter (Hoechst Celanese) and a single-layer polypropylene celgard 2400 (Polypore Co., Ltd.) were prepared. Two types of separators were sandwiched between the counter electrode and the evaluation electrode in the order of the counter electrode, the glass filter, the celgard 2400, and the evaluation electrode to form an electrode body. This electrode body was housed in a coin-type battery case CR2032 (Hosen Co., Ltd.), and the electrolytic solution of Example 1 was further injected to obtain a coin-type battery. This was used as the lithium ion secondary battery of Example 1-N.

Using the electrolytic solution of Example 1, the lithium ion secondary battery of Example 1-P was produced as follows.

LiNi _0.5 Co _0.3 Mn _0.2 O ₂ showing a layered rock salt structure as a positive electrode active material, polyvinylidene fluoride as a binder, acetylene black as a conductive auxiliary agent, LiNi _0.5 Co _0.3 Mn _{0 .2} O ₂ was mixed so that the mass ratio of polyvinylidene fluoride and acetylene black was 94: 3: 3, and N-methyl-2-pyrrolidone as a solvent was added to prepare a slurry. An aluminum foil was prepared as a current collector for the positive electrode. The slurry was applied in a film form on the surface of the aluminum foil using a doctor blade. The solvent was removed by drying the aluminum foil coated with the slurry. By the above procedure, a positive electrode having a positive electrode active material layer formed on the surface of the aluminum foil was manufactured.

The positive electrode was used as the evaluation electrode. A metal lithium foil having a thickness of 500 μm was used as a counter electrode. As a separator, a glass filter (Hoechst Celanese) and a single-layer polypropylene celgard 2400 (Polypore Co., Ltd.) were prepared. Two types of separators were sandwiched between the counter electrode and the evaluation electrode in the order of the counter electrode, the glass filter, the celgard 2400, and the evaluation electrode to form an electrode body. This electrode body was housed in a coin-type battery case CR2032 (Hosen Co., Ltd.), and the electrolytic solution of Example 1 was further injected to obtain a coin-type battery. This was used as the lithium ion secondary battery of Example 1-P.

(Example 2)
Example by mixing LiPF ₆ and 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which is a fluorine-substituted ether at a molar ratio of 1: 3: 3.38 to dissolve LiPF _6. The electrolytic solution of No. 2 was produced.

A lithium ion secondary battery of Example 2-N and a lithium ion secondary battery of Example 2-P were produced in the same manner as in Example 1 except that the electrolytic solution of Example 2 was used.

(Example 3)
LiPF ₆ , 1,2-dimethoxyethane, CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which is a fluorine-substituted ether, and di-n-propyl ether which is a chain ether are mixed in a molar ratio of 1: 3: 1.69: 1. The electrolytic solution of Example 3 was produced by mixing with .64 and dissolving LiPF ₆ .

A lithium ion secondary battery of Example 3-N and a lithium ion secondary battery of Example 3-P were produced in the same manner as in Example 1 except that the electrolytic solution of Example 3 was used.

(Example 4)
Example by mixing LiPF ₆ and 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which is a fluorine-substituted ether at a molar ratio of 1: 4: 3.38 to dissolve LiPF _6. The electrolytic solution of No. 4 was produced.

A lithium ion secondary battery of Example 4-N and a lithium ion secondary battery of Example 4-P were produced in the same manner as in Example 1 except that the electrolytic solution of Example 4 was used.

(Example 5)
LiPF ₆ , 1,2-dimethoxyethane, CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which is a fluorine-substituted ether, and di-n-propyl ether which is a chain ether are mixed in a molar ratio of 1: 4: 1.69: 1. The electrolytic solution of Example 5 was produced by mixing with .64 and dissolving LiPF ₆ .

A lithium ion secondary battery of Example 5-N and a lithium ion secondary battery of Example 5-P were produced in the same manner as in Example 1 except that the electrolytic solution of Example 5 was used.

(Example 6)
LiPF ₆ and 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ and fluorine-substituted ether C ₂ F ₅ CF (OCH ₃ ) C ₃ F ₇ : 1, 1, 1 , 2,2,3,4,5,5,5-decafluoro-3-methoxy-4- (trifluoromethyl) pentane was mixed at a molar ratio of 1: 4: 2.7: 0.68 and LiPF. The electrolytic solution of Example 6 was produced by dissolving ₆ .

A lithium ion secondary battery of Example 6-N and a lithium ion secondary battery of Example 6-P were produced in the same manner as in Example 1 except that the electrolytic solution of Example 6 was used.

(Comparative Example 1)
LiPF ₆ , 1,2-dimethoxyethane and di-n-propyl ether, which is a chain ether, were mixed at a molar ratio of 1: 2: 3.38 to prepare an electrolytic solution of Comparative Example 1. However, since LiPF ₆ was not dissolved in the mixed solvent, the electrolytic solution of Comparative Example 1 could not be produced.

(Comparative Example 2)
LiPF ₆ , 1,2-dimethoxyethane and di-n-propyl ether, which is a chain ether, were mixed at a molar ratio of 1: 3: 3.38 to prepare an electrolytic solution of Comparative Example 2. However, since LiPF ₆ was not dissolved in the mixed solvent, the electrolytic solution of Comparative Example 2 could not be produced.

(Comparative Example 3)
LiPF ₆ , 1,2-dimethoxyethane and toluene, which is an aromatic hydrocarbon, were mixed at a molar ratio of 1: 3: 3.38 to prepare an electrolytic solution of Comparative Example 3. However, since LiPF ₆ was not dissolved in the mixed solvent, the electrolytic solution of Comparative Example 3 could not be produced.

(Comparative Example 4)
LiPF ₆ and 1,2-dimethoxyethane were mixed at a molar ratio of 1:11 to prepare an electrolytic solution of Comparative Example 4. However, since LiPF ₆ was not dissolved in 1,2-dimethoxyethane, the electrolytic solution of Comparative Example 4 could not be produced.

(Comparative Example 5)
LiPF ₆ and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ which are fluorine-substituted ethers were mixed at a molar ratio of 1:11 to prepare an electrolytic solution of Comparative Example 5. However, since LiPF ₆ did not dissolve in CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , the electrolytic solution of Comparative Example 5 could not be produced.

(Comparative Example 6)
LiPF ₆ and tetrahydrofuran were mixed at a molar ratio of 1:11 to dissolve LiPF ₆ to produce an electrolytic solution of Comparative Example 6.

A lithium ion secondary battery of Comparative Example 6-N and a lithium ion secondary battery of Comparative Example 6-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 6 was used.

(Comparative Example 7)
Comparative example by mixing LiPF ₆ , 1,2-dimethoxyethane, and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , which is a fluorine-substituted ether, at a molar ratio of 1: 6: 3.38 to dissolve LiPF _6. The electrolytic solution of No. 7 was produced.

A lithium ion secondary battery of Comparative Example 7-N and a lithium ion secondary battery of Comparative Example 7-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 7 was used.

(Comparative Example 8)
(FSO ₂ ) ₂ NLi, 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , which is a fluorine-substituted ether, were mixed at a molar ratio of 1: 3: 3.38, and (FSO ₂ ) ₂ NLi. Was dissolved to produce the electrolytic solution of Comparative Example 8.

A lithium ion secondary battery of Comparative Example 8-N and a lithium ion secondary battery of Comparative Example 8-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 8 was used.

(Comparative Example 9)
Comparative example by mixing LiBF ₄ , 1,2-dimethoxyethane, and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , which is a fluorine-substituted ether, at a molar ratio of 1: 3: 3.38 to dissolve LiBF _4. 9 electrolytic solutions were produced.

A lithium ion secondary battery of Comparative Example 9-N and a lithium ion secondary battery of Comparative Example 9-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 9 was used.

(Comparative Example 10)
(CF ₃ SO ₂ ) ₂ NLi, 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , which is a fluorine-substituted ether, were mixed at a molar ratio of 1: 2: 3.38, and (CF ₃ SO 2) was mixed. ₂ ) The electrolytic solution of Comparative Example 10 was produced by dissolving ₂ NLi.

A lithium ion secondary battery of Comparative Example 10-N and a lithium ion secondary battery of Comparative Example 10-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 10 was used.

(Comparative Example 11)
(CF ₃ CF ₂ SO ₂ ) ₂ NLi, 1,2-dimethoxyethane and CHF ₂ CF ₂ CH ₂ OCF ₂ CHF ₂ , which is a fluorine-substituted ether, are mixed at a molar ratio of 1: 2: 3.38 and (CF). By dissolving ₃ CF ₂ SO ₂ ) ₂ NLi, the electrolytic solution of Comparative Example 11 was produced.

A lithium ion secondary battery of Comparative Example 11-N and a lithium ion secondary battery of Comparative Example 11-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 11 was used.

(Comparative Example 12)
LiPF ₆ , ethylene carbonate, and diethyl carbonate were mixed at a molar ratio of 1: 6.75: 3.71 to dissolve LiPF ₆ , thereby producing the electrolytic solution of Comparative Example 12.
The electrolytic solution of Comparative Example 12 is one aspect of a conventional general electrolytic solution.

A lithium ion secondary battery of Comparative Example 12-N and a lithium ion secondary battery of Comparative Example 12-P were produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 12 was used.

Table 1 shows a list of electrolytic solutions of Examples 1 to 6 and Comparative Examples 1 to 12.
In the table below, DME is an abbreviation for 1,2-dimethoxyethane, DPE is an abbreviation for di-n-propyl ether, THF is an abbreviation for tetrahydrofuran, and LiFSA is (FSO ₂ ) ₂ An abbreviation for NLi, LiTFSA is an abbreviation for (CF ₃ SO ₂ ) ₂ NLi, LiBETI is an abbreviation for (CF ₃ CF ₂ SO ₂ ) ₂ NLi, and EC is an abbreviation for ethylene carbonate. DEC is an abbreviation for diethyl carbonate.
In the table below, the molar ratio is the molar ratio of 1,2-dimethoxyethane to the electrolyte.

From the results of Comparative Example 4 and Comparative Example 5, it can be seen that it is difficult to dissolve LiPF ₆ when the non-aqueous solvent is only 1,2-dimethoxyethane or when only the fluorine-substituted ether is used. Further, from the results of Comparative Examples 1 to 3, the non-aqueous solvent is a mixed solvent of 1,2-dimethoxyethane and a chain ether, or a mixed solvent of 1,2-dimethoxyethane and an aromatic hydrocarbon. In this case as well, it is found that it is difficult to dissolve LiPF ₆ .
From the results of Examples 1 to 6, it can be said that LiPF ₆ can be dissolved in a mixed solvent in which dimethoxyethane and fluorine-substituted ether are used in combination at a sufficient concentration.

(Evaluation example 1)
The ionic conductivity of the electrolytic solutions of Examples 1 to 6 and Comparative Examples 6 to 12 was measured under the following conditions. The results are shown in Table 2.
<Ion conductivity>
The electrolytic solution was sealed in a cell equipped with a platinum electrode, and the resistance at 25 ° C. and 10 kHz was measured. The ionic conductivity was calculated from the resistance measurement results. The measuring instrument used was Solartron 147055BEC (Solartron).

Further, the electrolytic solutions of Examples 2 to 3 and Comparative Examples 6 to 7 were stored at 60 ° C. for 24 hours under an inert gas atmosphere, and then cooled to 25 ° C. under the above conditions. The ionic conductivity was measured. The above results are shown in Table 2 as the ionic conductivity after heating.

From Table 2, it can be seen that the electrolytic solution of the present invention exhibits good ionic conductivity.
The results of Example 1, Comparative Example 10 and Comparative Example 11 in which the molar ratio of 1,2-dimethoxyethane to the electrolyte is 2, and Example 2 in which the molar ratio of 1,2-dimethoxyethane to the electrolyte is 3. From the results of Comparative Example 8 and Comparative Example 9, it can be said that the electrolytic solution using LiPF ₆ as the electrolyte is excellent in ionic conductivity.
Further, from the results of the electrolytic solutions of Examples 2 to 3 and Comparative Examples 6 to 7 heated and stored at 60 ° C., the electrolytic solution of the present invention has good ions even after experiencing high temperature conditions. It can be said that it shows conductivity.

(Evaluation example 2)
The lithium ion secondary batteries of Examples 1-N to 6-N and Comparative Examples 6-N to 12-N were charged to 0.01 V at 0.2 mA under the condition of 25 ° C. Charging and discharging was performed by discharging to 1.0 V at 0.2 mA. The initial efficiency was calculated by the following formula.
Initial efficiency (%) = 100 x (discharge capacity) / (charge capacity)

Further, each lithium ion secondary battery was charged to 0.01 V at 0.5 mA under the condition of 25 ° C. and discharged to 1.0 V at 0.5 mA, and charging / discharging was repeated 30 times. The capacity retention rate was calculated by the following formula.
Capacity retention rate (%) = 100 x (discharge capacity in the 30th cycle) / (discharge capacity in the 1st cycle)
The above results are shown in Table 3.

From Table 3, it can be seen that the lithium ion secondary batteries of Examples 1-N to 6-N are excellent in both the initial efficiency and the capacity retention rate. It can be said that the electrolytic solution of the present invention is stable under charge / discharge conditions on the negative electrode side, and it can be said that deterioration of the negative electrode is suppressed.
From the results of Examples 1-N, 2-N, 4-N and Comparative Example 7-N, an electrolytic solution having a molar ratio of 1,2-dialkoxyethane to LiPF ₆ of 4 or less is preferable. Can be understood.

(Evaluation example 3)
The lithium ion secondary batteries of Examples 1-P to 6-P and Comparative Examples 6-P to 12-P were charged to 4.35 V at 0.2 mA under the condition of 25 ° C. Charging and discharging was performed by discharging to 3.0 V at 0.2 mA. The initial efficiency was calculated by the following formula.
Initial efficiency (%) = 100 x (discharge capacity) / (charge capacity)

Further, each lithium ion secondary battery was charged to 4.35 V at 0.5 mA under the condition of 25 ° C., and discharged to 3.0 V at 0.5 mA, and charging / discharging was repeated 30 times. The capacity retention rate was calculated by the following formula.
Capacity retention rate (%) = 100 x (discharge capacity in the 30th cycle) / (discharge capacity in the 1st cycle)
The above results are shown in Table 4.

From Table 4, it can be seen that the lithium ion secondary batteries of Examples 1-P to 6-P are excellent in both initial efficiency and capacity retention rate. It can be said that the electrolytic solution of the present invention is stable under charge / discharge conditions on the positive electrode side.
From the results of Examples 1-P, Example 2-P, Example 4-P and Comparative Example 7-P, an electrolytic solution having a molar ratio of 1,2-dialkoxyethane to LiPF ₆ of 4 or less is preferable. Can be understood.

From the results of Evaluation Example 2 for evaluating the relationship between the negative electrode and the electrolytic solution and the result of Evaluation Example 3 for evaluating the relationship between the positive electrode and the electrolytic solution, the lithium ion secondary battery provided with the electrolytic solution of the present invention has a negative electrode. It can be said that the capacity retention rate is excellent under any charge / discharge condition of the positive electrode and the positive electrode.
Further, from the results of Evaluation Example 2 and Evaluation Example 3, the lithium ion secondary battery provided with the electrolytic solution that does not satisfy the provisions of the present invention is satisfied under the charge / discharge conditions of the negative electrode or the charge / discharge conditions of the positive electrode. It can be understood that the battery characteristics cannot be shown.

Claims (4)

It contains LiPF ₆ , 1,2-dialkoxyethane and a fluorine-substituted ether represented by the following general formula (A), and the molar ratio of the 1,2-dialkoxyethane to the LiPF ₆ is 4 or less. Characteristic electrolyte.
Formula _{(A) C n H a F} b OC m H c F d
In the general formula (A), n is an integer of 1 or more, a is an integer of 0 or more, b is an integer of 1 or more, and 2n + 1 = a + b is satisfied. m is an integer of 1 or more, c is an integer of 0 or more, and d is an integer of 0 or more. Satisfy 2m + 1 = c + d. The electrolytic solution according to claim 1, wherein the molar ratio of the fluorine-substituted ether to the LiPF ₆ is 1 or more. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution according to claim 1 or 2. The lithium ion secondary battery according to claim 3, wherein the negative electrode includes a Si-containing negative electrode active material.