JP2021022437A - Electrolyte and lithium ion secondary battery - Google Patents

Abstract

To provide a suitable electrolytic solution in order to provide a lithium ion secondary battery having excellent battery characteristics.SOLUTION: An electrolytic solution includes (FSO2)2 NLi, 1,2-dimethoxyethane, dialkyl ether, and nonionic surfactant, and the proportion of nonionic surfactant is 5 mass% or less of the total electrolytic solution.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.

Specifically, LiPF ₆ is generally used as the electrolyte of the electrolytic solution, and chain carbonates such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate and cyclic carbonates such as ethylene carbonate are used as the non-aqueous solvent of the electrolytic solution. It is common to use a mixed solvent in combination.

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.

It is also known to use 1,2-dimethoxyethane, which is a chain ether, as a non-aqueous solvent. Patent Document 4 discloses a lithium ion secondary battery including an electrolytic solution in which LiPF ₆ is used as an electrolyte and a mixed solvent in which ethylmethyl carbonate, ethylene carbonate and 1,2-dimethoxyethane are used in combination as a non-aqueous solvent is used. It is described as.

An electrolyte solution using (FSO ₂ ) ₂ NLi as the electrolyte is also known. Patent Document 5 describes a lithium ion secondary battery comprising an electrolytic solution in which (FSO ₂ ) ₂ NLi and LiPF ₆ are used in combination as an electrolyte and a mixed solvent in which ethylene carbonate, dimethyl carbonate and fluoroethylene carbonate are used in combination as a non-aqueous solvent is used. The battery is specifically described.

JP-A-2015-185509 JP-A-2015-179625 International Publication No. 2014/08608 JP-A-2019-36455 Japanese Unexamined Patent Publication No. 2011-150958

As mentioned above, research on lithium-ion secondary batteries is being vigorously conducted. Then, the industry demands a lithium ion secondary battery having excellent 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 ether as a non-aqueous solvent for the electrolytic solution, which is considered to be less likely to undergo reductive decomposition on the surface of the negative electrode and have 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, when (FSO ₂ ) ₂ NLi was used as the electrolyte instead of LiPF ₆ , it was found that (FSO ₂ ) ₂ NLi was dissolved in 1,2-dimethoxyethane at a sufficient concentration. It can be said that (FSO ₂ ) ₂ NLi and 1,2-dimethoxyethane have excellent affinity.

However, the electrolytic solution consisting only of (FSO ₂ ) ₂ NLi and 1,2-dimethoxyethane has a relatively high viscosity. Highly viscous electrolytes may lack permeability to the separators placed between the positive and negative electrodes of the lithium ion secondary battery and may interfere with the smooth charging and discharging of the lithium ion secondary battery. The present inventor thought that there was a property.
Therefore, the present inventor has aimed to reduce the viscosity of the electrolytic solution by adding dialkyl ether, which is one aspect of the ether, to the electrolytic solution.

However, it was found that when din-propyl ether was added to an electrolytic solution consisting only of (FSO ₂ ) ₂ NLi and 1,2-dimethoxyethane, the electrolytic solution was separated into two liquid phases.
Therefore, when a study was conducted to add a surfactant in order to make the two-component phase single-phase, the electrolytic solution became single-phase by adding a specific surfactant in a specific amount, and lithium ions. It was found that it can be suitably used as an electrolytic solution for a secondary battery.

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

The electrolytic solution of the present invention is an electrolytic solution containing (FSO ₂ ) ₂ NLi, 1,2-dimethoxyethane, dialkyl ether and a nonionic surfactant, and the proportion of the nonionic surfactant is the entire electrolytic solution. It is characterized in that it is 5% by mass or less.

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 is an electrolytic solution containing (FSO ₂ ) ₂ NLi, 1,2-dimethoxyethane, dialkyl ether and a nonionic surfactant, and the proportion of the nonionic surfactant in the entire electrolytic solution is It is characterized in that it is 5% by mass or less.

Since 1,2-dimethoxyethane 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-dimethoxyethane preferentially coordinates with the lithium ion of (FSO ₂ ) ₂ NLi in the electrolytic solution to form a stable complex.

Here, it is considered that the most stable complex is formed by coordinating two molecules of 1,2-dimethoxyethane with respect to one molecule of (FSO ₂ ) ₂ NLi. Therefore, in consideration of the stability of the complex, it can be said that the molar ratio of 1,2-dimethoxyethane to (FSO ₂ ) ₂ NLi is preferably about ₂ in the electrolytic solution of the present invention.
However, when the lithium ion secondary battery is charged and discharged, Li needs to be inserted and removed at the positive electrode and the negative electrode. In that case, Li at the center of the complex needs to move beyond the stabilizing energy of the complex to the inside of the positive electrode or the inside of the negative electrode. Considering these matters, it can be said that a slightly unstable complex is more appropriate as a charge carrier.

From the above, in the electrolytic solution of the present invention, the molar ratio of 1,2-dimethoxyethane to (FSO ₂ ) ₂ NLi is preferably in the range of 0.5 to 2.5, and is preferably 0.8 to 2. The range of 0 is more preferable, the range of 1.0 to 1.9 is further preferable, the range of 1.2 to 1.7 is particularly preferable, and the range of 1.4 to 1.6 is most preferable.

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

Dialkyl ether is added for the purpose of lowering the viscosity of the electrolytic solution, but since dialkyl ether has a relatively high LUMO level, it can be said to be advantageous from the viewpoint of resistance to reductive decomposition near the negative electrode.

For reference, for dimethyl carbonate, which is a non-aqueous solvent for din-propyl ether and general electrolytes, the molecular orbital calculation program Gaussian is used, the density functional theory is B3LYP, and the basis set is 6-311G ++ g. The calculated LUMO values are shown in Table 1.
From the results in Table 1, it can be seen that the LUMO of din-propyl ether is higher than that of dimethyl carbonate, which is a non-aqueous solvent of a general electrolytic solution.

As the dialkyl ether, one type may be adopted, or a plurality of types may be used in combination.
The total number of carbon atoms in the dialkyl ether is preferably in the range of 6 to 12, and more preferably in the range of 6 to 10. Dialkyl ethers with too few carbons have a low boiling point and can be difficult to use. If a dialkyl ether having too many carbon atoms is used, the polarity of the entire electrolytic solution becomes low, which may adversely affect the ionic conductivity.

The alkyl group in the dialkyl ether may be a chain alkyl group, a branched alkyl group, a cyclic alkyl group, or an alkyl group substituted with these alkyl groups. The two alkyl groups in the dialkyl ether may be the same or different.

Examples of the dialkyl ether include those represented by the following general formula (A) having a chain alkyl group and / or a branched alkyl group.
General formula (A) C _n H _{2n + 1} OC _m H _{2m + 1}
In the general formula (A), n and m are integers of 1 or more. As n and m, 1 to 18, 1 to 12, 1 to 8 and 1 to 6 can be exemplified independently, respectively.

Examples of the dialkyl ether include those represented by the following general formula (B) having two cyclic alkyl groups.
General formula (B) C _n H _2n-1 OC _m H _2m-1
In the general formula (B), n and m are integers of 3 or more. As n and m, 3 to 8, 4 to 7, and 5 to 6 can be exemplified independently, respectively.

Examples of the dialkyl ether include those represented by the following general formula (C), which have one cyclic alkyl group and have a chain alkyl group or a branched alkyl group.
General formula (C) C _n H _2n-1 OC _m H _{2m + 1}
In the general formula (C), n is an integer of 3 or more, and m is an integer of 1 or more. Examples of n include 3 to 8, 4 to 7, and 5 to 6. Examples of m include 1 to 12, 1 to 8, and 1 to 6.

As the blending amount of the dialkyl ether in the electrolytic solution of the present invention, the molar ratio of the dialkyl ether to (FSO ₂ ) ₂ NLi is preferably 0.1 or more. As a preferable range of the molar ratio, 0.1 to 10, 0.3 to 8, 0.5 to 6, 1 to 4 can be exemplified.
If the blending amount of the dialkyl ether is too small, the electrolytic solution of the present invention has a high viscosity, and it may be difficult to permeate the electrolytic solution of the present invention into the separator or the electrode.

The addition of a nonionic surfactant makes the electrolytic solution of the present invention monophasic.
Examples of surfactants other than nonionic surfactants include cationic surfactants, anionic surfactants, and ionic surfactants with amphoteric surfactants. However, these ionic surfactants are incompatible with a mixture of the three components (FSO ₂ ) ₂ NLi, 1,2-dimethoxyethane and dialkyl ether.

The addition of the nonionic surfactant makes the electrolytic solution of the present invention monophasic, but if the amount added is excessive, the electrolytic solution of the present invention is separated into two liquid phases again. Therefore, it is necessary that the proportion of the nonionic surfactant is 5% by mass or less of the total electrolytic solution of the present invention.

Although it depends on the amount of the dialkyl ether, the proportions of the nonionic surfactant are 0.01 to 5% by mass, 0.1 to 4.5% by mass, 0.5 to 4% by mass, and 1 to 1. An example is 3.5% by mass.

Examples of the nonionic surfactant include an ester type, an alcohol type, an ether type, an ester ether type, and an amide type. As the nonionic surfactant, one type may be adopted, or a plurality of types may be used in combination.

The ester type is preferably one in which a long chain fatty acid and a polyol are condensed to form an ester bond. Examples of long-chain fatty acids include those having 8 to 30 carbon atoms, 10 to 24 carbon atoms, and 12 to 18 carbon atoms. Examples of the polyol include dehydrated cyclized products of polyoxyethylene glycol, sugar, sugar alcohol and sugar alcohol. Examples of sugars include glucose, mannose, ribose, xylose, sucrose, lactose, and trehalose. Examples of sugar alcohols include glycerin, erythritol, threitol, arabitol, xylitol, ribitol, sorbitol, mannitol, and maltitol.

As the alcohol type, an aliphatic alcohol having a large number of carbon atoms is preferable. The carbon number of the aliphatic alcohol can be exemplified in the range of 7 to 18.

The ether type is one in which an aliphatic alcohol and a polyol are condensed to form an ether bond or a glycosidic bond, or a phenol in which a long-chain alkyl group is substituted and a polyol are condensed to form an ether bond or a glycosidic bond. Is preferable. Examples of the aliphatic alcohol in the ether type include those having 7 to 30 carbon atoms, 10 to 24 carbon atoms, and 12 to 18 carbon atoms. Examples of the long-chain alkyl group in the phenol substituted with the long-chain alkyl group include those having 8 to 30 carbon atoms, 10 to 24 carbon atoms, and 12 to 18 carbon atoms. For all explanations of polyols, the description in ester form is incorporated.

The ester ether type is preferably one in which a long chain fatty acid and an alcohol having an ether bond are condensed to form an ester bond. For the description of long chain fatty acids, the description in ester form is incorporated. Examples of the alcohol having an ether bond include polyoxyethylene glycol and those obtained by condensing a polyol to form an ether bond. The description of the polyol incorporates the description in the ester form.

The amide type is preferably one in which a long chain fatty acid and diethanolamine are condensed to form an amide bond. For the description of long chain fatty acids, the description in ester form is incorporated.

An electrolyte other than (FSO ₂ ) ₂ NLi and a non-aqueous solvent other than 1,2-dimethoxyethane and dialkyl ether may be added to the electrolytic solution of the present invention without departing from the spirit of the present invention. Further, various additives may be added to the electrolytic solution of the present invention.

Examples of the ratio of (FSO ₂ ) ₂ NLi 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 ratio of 1,2-dimethoxyethane and dialkyl ether to the total non-aqueous solvent contained in the electrolytic solution of the present invention was 60 to 100% by mass, 70 to 98% by mass, 80 to 95% by mass, 85 to 90% by mass. Alternatively, 60 to 100% by mass, 70 to 98% by mass, 80 to 95% by mass, and 85 to 90% by mass can be exemplified.

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 inert 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 covered 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 thereof is preferably in the range of 1 μm to 100 μm.

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, f are 0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ e <1. A lithium composite metal oxide represented by (satisfaction of 1.7 ≦ f ≦ 3), Li ₂ MnO 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, nitronylnitroxide, 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.

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.

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. Examples of suitable negative electrode active materials include graphite and Si-containing negative electrode active materials.
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.

As a specific example of the Si-containing material, a silicon material disclosed in International Publication No. 2014/08608 or the like (hereinafter, simply referred to as “silicon material”) can be mentioned.

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 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 decomposed by heat 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, butane, 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 , Arocarbon 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.

Examples of the diamine 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, which are carbonaceous fine particles, graphite, Vapor Grown Carbon Fiber, and various metal 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 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 lithium-ion secondary batteries 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)
374 mg of (FSO ₂ ) ₂ NLi, 270 mg of 1,2-dimethoxyethane, 409 mg of din-propyl ether and 30 mg of the nonionic surfactant sorbitan trioleate were mixed to form a single phase. The electrolytic solution of Example 1 which is a solution was produced.
In the electrolytic solution of Example 1, the molar ratio of 1,2-dimethoxyethane to (FSO ₂ ) ₂ NLi was 1.5, and the molar ratio of din-propyl ether to (FSO ₂ ) ₂ NLi was 2. It was. The proportion of the nonionic surfactant was 2.8% by mass based on the total electrolytic solution of Example 1.

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

A binder solution was prepared by mixing polyacrylic acid, 4,4'-diaminodiphenylmethane and N-methyl-2-pyrrolidone and heating and stirring.

72.5 parts by mass of carbon layer coated silicon material as Si-containing negative electrode active material, 13.5 parts by mass of acetylene black as a conductive auxiliary agent, 14 parts by mass of solid content as a binder, and the above-mentioned binder solution. , An appropriate amount of N-methyl-2-pyrrolidone was mixed to prepare a slurry. A copper foil having a thickness of 10 μm 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. N-Methyl-2-pyrrolidone was removed by drying the copper foil coated with the slurry at 80 ° C. for 15 minutes. Then, by pressing, a negative electrode having a negative electrode active material layer having a thickness of 25 μm formed was manufactured.

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 prepare 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.

(Example 2)
The electrolytic solution and lithium ion secondary battery of Example 2 were produced in the same manner as in Example 1 except that 521 mg of din-butyl ether was used instead of din-propyl ether.
In the electrolytic solution of Example 2, the molar ratio of 1,2-dimethoxyethane to (FSO ₂ ) ₂ NLi was 1.5, and the molar ratio of din-butyl ether to (FSO ₂ ) ₂ NLi was 2. .. The proportion of the nonionic surfactant was 2.5% by mass of the entire electrolytic solution.

(Comparative Example 1)
The electrolytic solution of Comparative Example 1 was produced in the same manner as in Example 1 except that a nonionic surfactant was not used. However, since the electrolytic solution of Comparative Example 1 was separated into two liquid phases, the subsequent examination was stopped.

(Comparative Example 2)
The electrolytic solution of Comparative Example 2 was produced in the same manner as in Example 1 except that 100 mg of the nonionic surfactant was used. However, since the electrolytic solution of Comparative Example 2 was separated into two liquid phases, the subsequent examination was stopped.
The proportion of the nonionic surfactant was 8.7% by mass of the entire electrolytic solution of Comparative Example 2.

(Comparative Example 3)
The electrolytic solution of Comparative Example 3 was produced in the same manner as in Example 1 except that 30 mg of stearic acid, which is an ionic surfactant, was used instead of the nonionic surfactant. However, since the electrolytic solution of Comparative Example 3 became a suspension, the subsequent examination was stopped.

(Comparative Example 4)
The electrolytic solution of Comparative Example 4 was produced in the same manner as in Example 1 except that 30 mg of lithium stearate, which is an ionic surfactant, was used instead of the nonionic surfactant. However, since the electrolytic solution of Comparative Example 4 became a suspension, the subsequent examination was stopped.

(Comparative Example 5)
The electrolytic solution of Comparative Example 5 was produced in the same manner as in Example 1 except that 30 mg of sodium stearate, which is an ionic surfactant, was used instead of the nonionic surfactant. However, since the electrolytic solution of Comparative Example 5 became a suspension, the subsequent examination was stopped.

(Comparative Example 6)
Ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 to prepare a mixed solvent. LiPF ₆ was dissolved in a mixed solvent to prepare an electrolytic solution of Comparative Example 6 in which the concentration of LiPF ₆ was 1 mol / L. The electrolytic solution of Comparative Example 6 is a conventional general electrolytic solution.
A lithium ion secondary battery of Comparative Example 6 was produced in the same manner as in Example 1 except that the electrolytic solution of Comparative Example 6 was used.

(Evaluation example 1)
The lithium ion secondary batteries of Example 1, Example 2 and Comparative Example 6 are charged to 0.01 V at 0.2 mA and discharged to 1.0 V at 0.2 mA for the first charge and discharge. It was. Further, each lithium ion secondary battery was charged to 0.01 V at 0.5 mA and discharged to 1.0 V at 0.5 mA, and the charge / discharge cycle was repeated 30 times.
In this evaluation example, the application in which the Si-containing negative electrode active material occludes lithium is referred to as charging, and the application in which the Si-containing negative electrode active material releases lithium is referred to as discharging.

The initial efficiency and capacity retention rate were calculated by the following formulas. The results are shown in Table 2.
Initial efficiency (%) = 100 x (initial discharge capacity) / (initial charge capacity)
Capacity retention rate (%) = 100 x (discharge capacity in the 30th cycle) / (discharge capacity in the 1st cycle)

From Table 2, the lithium ion secondary battery provided with the electrolytic solution of the present invention is excellent in initial efficiency and remarkably excellent in capacity retention rate as compared with the conventional lithium ion secondary battery provided with a general electrolytic solution. You can see that.

Claims (5)

(FSO ₂ ) An electrolytic solution containing ₂ NLi, 1,2-dimethoxyethane, dialkyl ether and a nonionic surfactant, wherein the proportion of the nonionic surfactant is 5% by mass or less of the total electrolytic solution. An electrolytic solution characterized by being present. (FSO ₂ ) The electrolytic solution according to claim 1, wherein the molar ratio of 1,2-dimethoxyethane to ₂ NLi is in the range of 0.5 to 2.5. (FSO ₂ ) The electrolytic solution according to claim 1 or 2, wherein the molar ratio of the dialkyl ether to ₂ NLi is 0.1 or more. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution according to any one of claims 1 to 3. The lithium ion secondary battery according to claim 4, wherein the negative electrode includes a Si-containing negative electrode active material.