JP2019197716A - Electrolytic solution - Google Patents

Abstract

To provide a suitable electrolytic solution which adopts LiBFas an electrolyte.SOLUTION: Disclosed is an electrolytic solution which contains LiBFand a non-aqueous solvent.This electrolytic solution also contains a fluorine-containing cyclic carbonate and dipropyl carbonate within a range of a volume ratio of 5: 95-50: 50.SELECTED DRAWING: None

Description

  The present invention relates to an electrolytic solution used for a power storage device.

  In general, a power storage device such as a secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution as main components. The negative electrode is provided with a negative electrode active material involved in charge / discharge. The industry demands higher capacity of power storage devices, and various technologies are being studied as a response. As one specific high-capacity technology, it is known to employ a Si-containing negative electrode active material containing Si, which has an excellent ability to occlude charge carriers such as lithium, as a negative electrode active material of a power storage device.

  For example, Patent Document 1 describes a lithium ion secondary battery in which the negative electrode active material is silicon. Patent Literature 2 and Patent Literature 3 describe lithium ion secondary batteries in which the negative electrode active material is SiO.

In Patent Document 4, a layered silicon compound mainly composed of a layered polysilane obtained by reacting CaSi ₂ with an acid to remove Ca and synthesizing the layered silicon compound is heated at 300 ° C. or higher to release hydrogen. A material is manufactured and a lithium ion secondary battery including the silicon material as a negative electrode active material is described.

A general electrolytic solution used for a power storage device contains an electrolyte and a non-aqueous solvent. Here, LiPF ₆ is generally employed as the electrolyte, and carbonates such as ethyl methyl carbonate and dimethyl carbonate are generally employed as the non-aqueous solvent. Actually, Patent Documents 1 to 4 specifically describe lithium ion secondary batteries that employ LiPF ₆ as an electrolyte and carbonates such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate as nonaqueous solvents. Has been described.

  It is also known to employ a fluorine-containing cyclic carbonate such as fluoroethylene carbonate or difluoroethylene carbonate as part of the non-aqueous solvent. Actually, in Patent Document 5, the capacity of a secondary battery is obtained by using a fluorine-containing cyclic carbonate such as fluoroethylene carbonate or difluoroethylene carbonate as a non-aqueous solvent for an electrolyte in a secondary battery including a Si-containing negative electrode active material. It is described that the maintenance rate has been optimized (see Table 17).

JP 2014-203595 A JP 2015-185509 A JP-A-2015-179625 International Publication No. 2014/080608 JP 2009-32492 A

As described above, LiPF ₆ is generally used as the electrolyte of the electrolytic solution. However, it cannot be said that studies on an electrolytic solution using LiBF ₄ as an electrolyte have been sufficiently conducted.

The present invention has been made in view of such circumstances, and an object thereof is to provide a suitable electrolytic solution adopted LiBF ₄ as the electrolyte.

The present inventor has intensively studied an electrolytic solution that employs LiBF ₄ as an electrolyte and a fluorine-containing cyclic carbonate as a part of a non-aqueous solvent. Then, adopting the LiBF ₄ as an electrolyte, and a secondary battery having an electrolyte employing fluoroethylene carbonate and dimethyl carbonate as a nonaqueous solvent was found that not work. Further, it was found that a secondary battery including an electrolytic solution that employs LiBF ₄ as an electrolyte and employs fluoroethylene carbonate and diethyl carbonate as a nonaqueous solvent does not exhibit a sufficient capacity retention rate.

As a result of further earnest studies by the present inventors, a secondary battery including an electrolytic solution employing LiBF ₄ as an electrolyte and employing fluoroethylene carbonate and dipropyl carbonate as a non-aqueous solvent operates. It has been found that there is a suitable range for the ratio of carbonate and dipropyl carbonate.
Based on these findings, the present invention has been completed.

The electrolytic solution of the present invention is an electrolytic solution containing LiBF ₄ and a non-aqueous solvent,
Fluorine-containing cyclic carbonate and dipropyl carbonate are contained within a volume ratio of 5:95 to 50:50.

The present invention can provide a suitable electrolytic solution adopted LiBF ₄ as the electrolyte.

  Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from these numerical ranges can be used as new upper and lower numerical values.

The electrolytic solution of the present invention is an electrolytic solution containing LiBF ₄ and a non-aqueous solvent,
Fluorine-containing cyclic carbonate and dipropyl carbonate are contained within a volume ratio of 5:95 to 50:50.

  The electrolytic solution of the present invention can be used as an electrolytic solution for a power storage device. Examples of the power storage device include capacitors such as lithium ion secondary batteries and lithium ion capacitors. Hereinafter, a power storage device including the electrolytic solution of the present invention is referred to as a power storage device of the present invention, and a lithium ion secondary battery including the electrolytic solution of the present invention is referred to as a lithium ion secondary battery of the present invention.

The concentration of LiBF ₄ in the electrolytic solution of the present invention is not particularly limited. Suitable concentration ranges include 0.5-4 mol / L, 0.6-3 mol / L, 0.7-2 mol / L, 0.8-1.7 mol / L, 0.9-1.5 mol / L. It can be illustrated.

Moreover, you may use together well-known electrolyte for the electrolyte solution of this invention in the range which does not deviate from the meaning of this invention. Examples of the ratio of LiBF _{4 to} the entire electrolyte contained in the electrolytic solution of the present invention include 50 to 100 mol%, 70 to 100 mol%, and 90 to 100 mol%.

  The electrolytic solution of the present invention contains a fluorine-containing cyclic carbonate. The fluorine-containing cyclic carbonate means a cyclic carbonate having fluorine in the molecule. Fluorine-containing cyclic carbonate is excellent in oxidation resistance, but easily decomposes under reducing conditions. Accordingly, the fluorine-containing cyclic carbonate is preferentially decomposed at the interface between the negative electrode and the electrolytic solution under the charge / discharge conditions of the power storage device of the present invention. As a result, a film derived from the decomposition product of the fluorine-containing cyclic carbonate is formed on the surface of the negative electrode. Due to the presence of such a coating, direct contact of the electrolytic solution with the negative electrode can be prevented, so that excessive decomposition of the constituent components of the electrolytic solution can be suppressed. As a result, deterioration of the power storage device of the present invention can be suppressed.

  Specific examples of the fluorine-containing cyclic carbonate include compounds represented by the following general formula (A).

R ¹ and R ² are each independently hydrogen, an alkyl group, a halogen-substituted alkyl group, or a halogen. However, at least one of each R ¹ and each R ² includes F.

  Specific examples of the fluorine-containing cyclic carbonate represented by the general formula (A) include fluoroethylene carbonate, 4- (trifluoromethyl) -1,3-dioxolan-2-one, and 4,4-difluoro. -1,3-dioxolan-2-one, 4-fluoro-4-methyl-1,3-dioxolan-2-one, 4- (fluoromethyl) -1,3-dioxolan-2-one, 4,5- Difluoro-1,3-dioxolan-2-one, 4-fluoro-5-methyl-1,3-dioxolan-2-one, 4,5-difluoro-4,5-dimethyl-1,3-dioxolane-2- ON can be mentioned, and among these, fluoroethylene carbonate is preferred.

The propyl group of dipropyl carbonate may be an n-propyl group or an i-propyl group. By selecting dipropyl carbonate as the solvent to be mixed with the fluorine-containing cyclic carbonate, it can be said that the affinity between LiBF ₄ and the non-aqueous solvent is improved, and the compatibility with the constituent components of the power storage device is improved. It can be said that the optimization of the coating film which can be formed on the surface of this can be achieved. These functions and effects, as will be described later, did not show satisfactory characteristics in a power storage device including an electrolytic solution in which dimethyl carbonate or diethyl carbonate was selected instead of dipropyl carbonate as a solvent to be mixed with a fluorine-containing cyclic carbonate, This is also supported by the fact that even when dibutyl carbonate was selected instead of dipropyl carbonate as the solvent to be mixed with the fluorine-containing cyclic carbonate, even the electrolyte solution could not be produced.

  The electrolytic solution of the present invention contains a fluorine-containing cyclic carbonate and dipropyl carbonate within a volume ratio of 5:95 to 50:50. When the ratio of the fluorine-containing cyclic carbonate is too high or too low, the capacity retention rate of the power storage device may be reduced.

  The volume ratio of the fluorine-containing cyclic carbonate and dipropyl carbonate is preferably in the range of 8:92 to 40:60, more preferably in the range of 10:90 to 30:70, and in the range of 15:85 to 25:75. The inside is more preferable.

  Moreover, you may use together well-known nonaqueous solvents other than a fluorine-containing cyclic carbonate and dipropyl carbonate in the range which does not deviate from the meaning of this invention in the electrolyte solution of this invention. Examples of the ratio of the total amount of fluorine-containing cyclic carbonate and dipropyl carbonate to the entire non-aqueous solvent contained in the electrolytic solution of the present invention include 50 to 100% by volume, 70 to 100% by volume, and 90 to 100% by volume.

  In the electrolytic solution of the present invention, known additives may be blended without departing from the spirit of the present invention.

Next, the lithium ion secondary battery of the present invention will be described as a representative example of the power storage device of the present invention. The lithium ion secondary battery of this invention is equipped with the electrolyte solution of this invention, a negative electrode, and a positive electrode.
A specific aspect of the negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector.

  A current collector refers to a chemically inert electronic conductor that keeps current flowing through an electrode during discharge or charging of a secondary battery such as 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 to be 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 Examples of such a metal material can be given. The current collector may be covered with a known protective layer. What collected the surface of the electrical power collector by the well-known method may be used as an electrical power collector.

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

  The negative electrode active material layer includes a negative electrode active material and, if necessary, a binder and a conductive additive.

  The electrolytic solution of the present invention is decomposed during charging / discharging of the lithium ion secondary battery of the present invention to form a film containing lithium and fluorine on the surface of the negative electrode. Such a film suppresses excessive decomposition of the electrolytic solution and also acts as a protective film for a negative electrode active material that may be deteriorated. In particular, it acts as a suitable protective film when a Si-containing negative electrode active material is employed as the negative electrode active material. Therefore, it is preferable to employ a Si-containing negative electrode active material as the negative electrode active material. The negative electrode active material layer may contain a known negative electrode active material other than the Si-containing 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 Si ratio becomes excessive, so that the volume change at the time of charging / discharging becomes too large, and the cycle characteristics of the 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 further preferably 0.7 ≦ x ≦ 1.2.

  Specific examples of the Si-containing material include a silicon material disclosed in International Publication No. 2014/080608 (hereinafter simply referred to as “silicon material”).

The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. For example, the silicon material is subjected to a process of synthesizing a layered silicon compound containing polysilane as a main component by reacting CaSi ₂ with an acid, and further a process of heating the layered silicon compound at 300 ° C. or higher to desorb hydrogen. It is manufactured.

The production method of the silicon material is represented by the following ideal reaction formula when hydrogen chloride is used as the acid.
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 a 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 upper reaction, the layered silicon compound is hardly manufactured as containing only Si ₆ H ₆ , and the layered The silicon compound is 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 product. In the above chemical formula, inevitable impurities such as Ca that can remain are not taken into consideration. A 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-like silicon bodies are laminated in the thickness direction. In order to efficiently occlude and release lithium ions, the plate-like 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 in the longitudinal direction of the plate-like silicon body is preferably within a range of 0.1 μm to 50 μm. The plate-like silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. This laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, the plate-like silicon body preferably has a state in which 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, still more 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 Scherrer formula using the half-value width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart after performing X-ray diffraction measurement on the silicon material. .

  The abundance and size of the plate-like silicon body, amorphous silicon and silicon crystallite contained in the silicon material mainly depend on the heating temperature and the 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 a powdered material that is an aggregate of particles. The average particle diameter of the Si-containing negative electrode active material is preferably in the range of 1 to 30 μm, and more preferably in the range of 2 to 20 μm. If a Si-containing negative electrode active material having an average particle size that is too small is used, the manufacturing operation may be 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 / discharged appropriately.
The average particle diameter herein means a D ₅₀ in the case of measuring a sample in a conventional laser diffraction particle size distribution analyzer.

  As the Si-containing negative electrode active material, a carbon layer coating-Si-containing negative electrode active material obtained by coating a Si-containing negative electrode active material with a carbon layer may be employed. 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. As a method for producing such a carbon layer coating-Si-containing negative electrode active material, a mechanical milling method in which a strong pressure is applied to the mixture of the Si-containing negative electrode active material and the carbon powder and then integrated by stirring, An example is a CVD (chemical vapor deposition) method in which carbon generated from a carbon source is deposited on a Si-containing negative electrode active material.

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

  As the organic substance, those that can be thermally decomposed and carbonized by heating in a non-oxidizing atmosphere are used, for example, saturated aliphatic hydrocarbons such as methane, ethane, propane, butane, isobutane, pentane, 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, chlorobenzene, indene and benzofuran , Aromatic hydrocarbons such as pyridine, esters such as ethyl acetate, butyl acetate and amyl acetate, fatty acids and the like.

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

  The thermal CVD process is desirably performed with the Si-containing negative electrode active material in a fluid state. By doing in this way, the whole surface of Si containing negative electrode active material can be made to contact 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 contact the Si-containing negative electrode active material with an organic substance while stirring. For example, if a rotary furnace having a baffle plate is used, the Si-containing negative electrode active material remaining on the baffle plate is agitated by dropping from a predetermined height as the rotary furnace rotates, and in this case, it contacts organic substances. Thus, a more uniform carbon layer can be formed on the entire Si-containing negative electrode active material.

  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 within the range of 1 nm to 100 nm, more preferably within the range of 5 to 50 nm, and even more preferably within 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, and more preferably 70 to 95% by mass with respect to the mass of the entire negative electrode active material layer.

  Examples of the binder include fluorine-containing resins 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 carboxymethylcellulose. What is necessary is just to employ | adopt well-known things, such as a styrene butadiene rubber.

  A cross-linked 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. 2016/063882 may be used as a binder.

  Examples of diamines used in the crosslinked polymer include alkylene diamines such as ethylene diamine, propylene diamine, and hexamethylene diamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophorone diamine, and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine, and naphthalenediamine are exemplified.

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

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be optionally added when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent. The conductive auxiliary agent may be any chemically inert electronic high conductor, and examples thereof include carbon black, graphite, Vapor Grown Carbon Fiber, and various metal particles. The Examples of carbon black include acetylene black, ketjen black (registered trademark), furnace black, and channel black. These conductive assistants can be added to the active material layer alone or in combination of two or more.

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

  In order to form the negative electrode active material layer on the surface of the current collector, a conventionally known method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method is used. A negative electrode active material may be applied to the surface of the electric body. Specifically, a negative electrode active material, a binder, a solvent, and, if necessary, a conductive additive are mixed to form a slurry, and 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 product may be compressed.

One aspect of the lithium ion secondary battery of the present invention includes a negative electrode, a positive electrode, a separator, and the electrolytic solution of the present invention.
The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector. As the current collector, those described for the negative electrode may be appropriately employed.

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

  Specifically, it is preferable to use a positive electrode current collector made of aluminum or an aluminum alloy. Here, aluminum refers to pure aluminum, and aluminum having a purity of 99.0% or more is referred to as pure aluminum. An alloy obtained by adding various elements to pure aluminum is referred to as 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.

  Specific examples of aluminum or aluminum alloy include A1000 series alloys (pure aluminum series) such as JIS A1085 and A1N30, A3000 series alloys (Al-Mn series) such as JIS A3003 and A3004, JIS A8079, A8021, etc. A8000-based alloy (Al-Fe-based).

  The positive electrode active material layer includes a positive electrode active material that can occlude and release lithium ions, and a binder and a conductive aid as necessary. The positive electrode active material layer preferably contains the positive electrode active material in an amount of 60 to 99% by mass, and more preferably 70 to 95% by mass with respect to the mass of the entire positive electrode active material layer. What is necessary is just to employ | adopt suitably what was demonstrated with the negative electrode as a binder and a conductive support agent.

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, and 0 ≦ e <1. And 1.5 ≦ f ≦ 3 are satisfied.) Li ₂ MnO ₃ represented by the following formula. 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 (wherein M is selected from at least one of Co, Ni, Mn, and Fe). 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 a basic composition, and a metal element contained in the basic composition may be substituted with another metal element. Further, a positive electrode active material that does not contain lithium ions may be used. For example, sulfur alone, compounds in which sulfur and carbon are combined, metal sulfides such as TiS ₂ , oxides such as V ₂ O ₅ and MnO ₂ , compounds containing polyaniline and anthraquinone, and aromatics in their chemical structures, conjugated two Conjugated materials such as acetic acid organic materials and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, phenoxyl, etc. may be adopted as the positive electrode active material. When using a positive electrode active material that does not contain lithium ions, 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, It is at least one element selected from Sc, Sn, In, Y, Bi, 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, 1.7 ≦ f ≦ 3.) It is preferable to employ a lithium composite metal oxide represented by the following formula.

  In the above general formula, the values of b, c, and d are not particularly limited as long as the above conditions are satisfied, but those in which 0 <b <1, 0 <c <1, 0 <d <1 are satisfied. In addition, at least one of b, c and d is in the range of 10/100 <b <90/100, 10/100 <c <90/100, 5/100 <d <70/100. More preferably, the ranges are 20/100 <b <80/100, 12/100 <c <70/100, 10/100 <d <60/100, 30/100 <b <70/100, More preferably, the ranges are 15/100 <c <50/100 and 12/100 <d <50/100.

  About a, e, and f, what is necessary is just a numerical value within the range prescribed | regulated by the said general formula, Preferably 0.5 <= a <= 1.5, 0 <= e <0.2, 1.8 <= f <= 2 0.5, more preferably 0.8 ≦ a ≦ 1.3, 0 ≦ e <0.1, 1.9 ≦ f ≦ 2.1, respectively.

From the viewpoint of excellent and high capacity, and durability, as a positive electrode active material, the _{Li x Mn 2-y A y} O 4 (A spinel structure, Ca, Mg, S, Si , Na, K, Al, P, Ga And 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). Examples of the value range of x include 0.5 ≦ x ≦ 1.8, 0.7 ≦ x ≦ 1.5, and 0.9 ≦ x ≦ 1.2. The value range of y is 0 ≦ y ≦ 0.8, 0 ≦ y ≦ 0.6 can be exemplified. Specific examples of the spinel structure compound 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 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, such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester, polyacrylonitrile and other synthetic resins, cellulose, amylose and other polysaccharides, fibroin And porous materials, nonwoven fabrics, woven fabrics, and the like using one or more electrical insulating materials such as natural polymers such as keratin, lignin, and suberin, and ceramics. The separator may have a multilayer structure.

A specific method for producing the lithium ion secondary battery of the present invention will be described.
For example, a separator is sandwiched between a positive electrode and a negative electrode to form an electrode body. The electrode body may be any of a stacked type in which a positive electrode, a separator and a negative electrode are stacked, or a wound type in which a positive electrode, a separator and a negative electrode are stacked. After connecting the current collector of the positive electrode and the current collector of the negative electrode to the positive electrode terminal and the negative electrode terminal that communicate with the outside using a current collecting lead or the like, the electrolyte solution of the present invention is added to the electrode body and the lithium ion secondary Use batteries.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape, and a laminate shape 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 generated by a lithium ion secondary battery for all or a part of its power source. For example, the vehicle may be 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. Examples of devices equipped with lithium ion secondary batteries include various home appliances driven by batteries such as personal computers and portable communication devices, office devices, and industrial devices in addition to vehicles. Furthermore, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydroelectric power generation and other power system power storage devices and power smoothing devices, power supplies for ships and / or auxiliary power supply sources, aircraft, Power supply for spacecraft and / or auxiliary equipment, auxiliary power supply for vehicles that do not use electricity as a power source, power supply for mobile home robots, power supply for system backup, power supply for uninterruptible power supply, You may use for the electrical storage apparatus which stores temporarily the electric power required for charge in the charging station for electric vehicles.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, this invention is not limited by these Examples.

(Example 1)
Fluoroethylene carbonate and di-n-propyl carbonate were mixed at a volume ratio of 10:90 to obtain a mixed solvent. LiBF ₄ was dissolved in a mixed solvent to produce an electrolyte solution of Example 1 having a LiBF ₄ concentration of 1 mol / L.

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

CaSi ₂ was added to a concentrated hydrochloric acid solution at 0 ° C. under stirring conditions and reacted for 1 hour. Water was added to the reaction solution, filtration was performed, and a yellow powder was collected by filtration. The yellow powder was washed with water, further washed with ethanol, and then dried under reduced pressure to obtain a layered silicon compound containing layered polysilane. Next, the layered silicon compound was heated at 800 ° C. in an argon atmosphere to release hydrogen to produce a silicon material. By heating the silicon material at 880 ° C. in a propane gas atmosphere, a carbon-coated silicon material which is a carbon layer coating-Si-containing negative electrode active material was produced.

  A polyacrylic acid having a weight average molecular weight of 800,000 was dissolved in N-methyl-2-pyrrolidone to prepare a polyacrylic acid solution containing 10% by mass of polyacrylic acid. Further, 4,4'-diaminodiphenylmethane 0.2 g (1.0 mmol) was dissolved in 0.4 mL of N-methyl-2-pyrrolidone to prepare a 4,4'-diaminodiphenylmethane solution. Under stirring conditions, the total amount of the 4,4′-diaminodiphenylmethane solution was added dropwise to 7 mL of a polyacrylic acid solution (corresponding to 9.5 mmol in terms of acrylic acid monomer), and the resulting mixture was stirred at room temperature for 30 minutes. Stir. Thereafter, using a Dean Stark apparatus, the mixture was stirred at 130 ° C. for 3 hours to advance the dehydration reaction, thereby producing a binder solution.

  72.5 parts by mass of a carbon-coated silicon material as a Si-containing negative electrode active material, 13.5 parts by mass of acetylene black as a conductive additive, and the above binder solution in an amount of 14 parts by mass as a binder, and 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 negative electrode current collector. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. The copper foil coated with the slurry was dried at 80 ° C. for 15 minutes to remove N-methyl-2-pyrrolidone. Then, the negative electrode in which the negative electrode active material layer with a thickness of 25 micrometers was formed was manufactured by pressing.

94 parts by mass of LiNi _0.5 Co _0.3 Mn _0.2 O ₂ as a positive electrode active material, 3 parts by mass of acetylene black as a conductive additive, 3 parts by mass of polyvinylidene fluoride as a binder, and an appropriate amount N-methyl-2-pyrrolidone was mixed to produce a slurry. An aluminum foil was prepared as a positive electrode current collector. The slurry was applied in the form of a film on the surface of the aluminum foil using a doctor blade. N-methyl-2-pyrrolidone was removed by drying the aluminum foil coated with the slurry at 80 ° C. for 20 minutes. Then, it pressed and heat-processed for 120 hours at 120 degreeC under pressure reduction, and the positive electrode in which the positive electrode active material layer was formed was manufactured.

As a separator, a porous film made of polypropylene was prepared.
A separator was sandwiched between the positive electrode and the negative electrode to produce an electrode body. The electrode assembly and the electrolytic solution of Example 1 were accommodated in a bag made of an aluminum laminate film, and the bag was sealed to manufacture a lithium ion secondary battery of Example 1.

(Example 2)
The electrolyte solution and lithium ion secondary battery of Example 2 were produced in the same manner as in Example 1 except that a mixed solvent in which fluoroethylene carbonate and di-n-propyl carbonate were mixed at a volume ratio of 20:80 was used. did.

(Comparative Example 1)
The electrolytic solution and the lithium ion secondary battery of Comparative Example 1 were produced in the same manner as in Example 1, except that a mixed solvent in which fluoroethylene carbonate and di-n-propyl carbonate were mixed at a volume ratio of 90:10 was used. did.

(Comparative Example 2)
An electrolytic solution and a lithium ion secondary battery of Comparative Example 2 were produced in the same manner as in Example 2 except that dimethyl carbonate was used instead of di-n-propyl carbonate.

(Comparative Example 3)
An electrolytic solution and a lithium ion secondary battery of Comparative Example 3 were produced in the same manner as in Example 2 except that diethyl carbonate was used instead of di-n-propyl carbonate.

(Comparative Example 4)
Except for employing di-n-butyl carbonate instead of di-n-propyl carbonate, an attempt was made to produce an electrolytic solution in the same manner as in Example 2, but the liquid was separated into two layers, so that a uniform mixed state was obtained. The electrolyte solution could not be produced.

(Example 3)
Using a mixed solvent in which fluoroethylene carbonate and di-n-propyl carbonate were mixed at a volume ratio of 20:80, except that the concentration of LiBF ₄ was changed to 2 mol / L, the method of Example 3 was repeated. An electrolytic solution and a lithium ion secondary battery were manufactured.

(Evaluation example 1)
The lithium ion secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3 were conditioned at a charge of 0.3 C to 3.9 V and stored at 60 ° C. for 20 hours. Then, charging / discharging which charged from 3V to 4.24V at 1C and discharged from 4.24V to 3V at 1C was repeated 100 times. The capacity maintenance rate of each lithium ion secondary battery was calculated by the following formula.
Capacity retention rate (%) = 100 × (100th discharge capacity) / (First discharge capacity)

  The results are shown in Table 1. In Table 1, FEC is an abbreviation for fluoroethylene carbonate, DPC is an abbreviation for di-n-propyl carbonate, DMC is an abbreviation for dimethyl carbonate, and DEC is an abbreviation for diethyl carbonate.

From the results of Example 2, Comparative Example 2 and Comparative Example 3 in which only the carbon number of carbonate is different, it can be seen that the combination of dipropyl carbonate and fluorine-containing cyclic carbonate is remarkably preferable. In addition, as described in Comparative Example 4, an electrolytic solution using dibutyl carbonate could not be produced.
Moreover, even if it is a combination of dipropyl carbonate and fluorine-containing cyclic carbonate from the results of Example 1, Example 2 and Comparative Example 1 in which the proportions of dipropyl carbonate and fluorine-containing cyclic carbonate are different, the battery depends on the ratio. It can be seen that the characteristics change.

(Evaluation example 2)
The lithium ion secondary batteries of Example 2 and Comparative Example 3 after completion of Evaluation Example 1 were disassembled, and the negative electrode was taken out. Elemental analysis of the surface of each negative electrode was performed using an X-ray photoelectron spectrometer. The results are shown in Table 2. The unit of numerical values in Table 2 is at%.

From the results in Table 2, it can be said that the ratio of Li and F is high on the negative electrode surface of the lithium ion secondary battery of Example 2. It is considered that a suitable film containing LiF or the like is formed on the negative electrode surface of the lithium ion secondary battery of Example 2. As a result of the protective effect such as the action of suppressing the decomposition of the electrolytic solution and the action of suppressing the deterioration of the Si-containing negative electrode active material, the lithium ion secondary battery of Example 2 has a suitable capacity. It is thought that it was able to be maintained.
Considering the results of Example 2 and Comparative Example 3 comprehensively, it can be said that the combination of fluorine-containing cyclic carbonate and dipropyl carbonate is excellent in forming a suitable film on the negative electrode surface.

Claims (4)

An electrolytic solution containing LiBF ₄ and a non-aqueous solvent,
An electrolytic solution comprising a fluorine-containing cyclic carbonate and dipropyl carbonate in a volume ratio of 5:95 to 50:50. The electrolytic solution according to claim 1, wherein the concentration of LiBF ₄ is 0.5 to 1.5 mol / L.   A power storage device comprising the electrolytic solution according to claim 1.   The power storage device according to claim 3, wherein the power storage device includes a negative electrode including a Si-containing negative electrode active material.