JP2014182889A - Electrolytic solution, and lithium ion secondary battery including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic solution which makes possible to obtain a lithium ion secondary battery exhibiting good cycle characteristics.SOLUTION: An electrolytic solution comprises a nonaqueous solvent containing fluorine-containing cyclic carbonate, and an electrolyte. The electrolytic solution includes 1-7 vol.% of the fluorine-containing cyclic carbonate to 100 vol.% of the nonaqueous solvent. Further, the electrolytic solution includes at least one compound of lithium borates expressed by the general formula (1) at a concentration of 0-0.1 mol/L, exclusive, and it does not include a silicon fluoride compound nor 1,3-propane sultone.

Description

  The present invention relates to an electrolytic solution used for a non-aqueous secondary battery such as a lithium ion secondary battery.

  Lithium ion secondary batteries are small and have a large capacity, and are therefore used as batteries for various devices such as mobile phones and notebook computers. A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution as main components. Each electrode has an active material layer and a current collector on which the active material layer is bound. The positive electrode active material layer included in the positive electrode generally contains a composite oxide of lithium and a transition metal such as lithium manganese composite oxide, lithium cobalt composite oxide, and lithium nickel composite oxide. In recent years, the use of materials containing silicon or tin has been studied for the negative electrode active material contained in the negative electrode.

  During charging / discharging of the lithium ion secondary battery, a film may be formed on the surface of the negative electrode active material. The presence of this coating prevents direct contact between the electrolytic solution and the negative electrode active material, thereby suppressing deterioration of the electrolytic solution. However, since the volume change of the negative electrode active material accompanying the insertion or extraction of lithium ions occurs during charging / discharging of the lithium ion secondary battery, the coating may crack due to the volume change of the negative electrode active material. When a crack occurs in the coating, the electrolytic solution directly contacts the negative electrode active material and the electrolytic solution deteriorates. As a result, charge / discharge cycle characteristics (hereinafter simply referred to as “cycle characteristics”) may be deteriorated.

  In recent years, various studies have been made on electrolyte solutions for the purpose of improving the cycle characteristics of lithium ion secondary batteries. For example, Patent Document 1 discloses an electrolytic solution containing 0.1 to 3% by mass of 1,3-propane sultone and fluoroethylene carbonate (hereinafter sometimes referred to as “FEC”). Patent Document 2 discloses an electrolytic solution containing a silicon fluoride compound and fluoroethylene carbonate.

JP 2011-049114 A JP 2010-238506 A

  There is a high demand for improving the cycle characteristics of lithium ion secondary batteries, and lithium ion secondary batteries that exhibit better cycle characteristics are eagerly desired. This invention is made | formed in view of this situation, and it aims at providing the electrolyte solution from which the lithium ion secondary battery which shows the outstanding cycling characteristics is obtained.

  The inventor conducted extensive studies on the electrolyte used in the lithium ion secondary battery. As a result of many trials and errors, it was found that a lithium ion secondary battery using an electrolytic solution containing a specific component in a specific blending amount exhibits excellent cycle characteristics, and the present invention has been completed.

  The present invention is an electrolytic solution containing a non-aqueous solvent containing a fluorine-containing cyclic carbonate and an electrolyte, and contains 1 to 7% by volume of the fluorine-containing cyclic carbonate with respect to 100% by volume of the non-aqueous solvent. One or more lithium borate salts represented by (1) are contained at a concentration of more than 0 mol / L and not more than 0.1 mol / L, and no silicon fluoride compound or 1,3-propane sultone is contained.

In the general formula (1), R ¹ and R ² are each independently represented by any one of the following general formulas (1-1) to (1-6).

In General Formulas (1-1) to (1-6), X ^{01 to} X ¹⁸ are each independently hydrogen, an alkyl group, a halogen, or a halogenated alkyl group.

  Fluorine-containing cyclic carbonates such as FEC are components that are relatively easily reduced and decomposed among the components of the electrolytic solution. For this reason, when charging and discharging are performed with a lithium ion secondary battery using an electrolytic solution containing FEC, a solid electrolyte interface film (SEI (Solid Electrolyte Interface Interface) containing FEC decomposition products on the surface of the negative electrode active material or the surface of the positive electrode active material. ) Film) is easily formed. For example, the formation of the FEC-derived coating prevents further direct contact between the electrolyte and the active material. If it does so, since the electrolyte in electrolyte solution and another solvent will become difficult to decompose | disassemble and degradation of electrolyte solution will be prevented, it is thought that the cycle characteristic of a lithium ion secondary battery is maintained as a result.

  The FEC-derived film is thinly formed on the entire surface of the negative electrode active material. Since the coating derived from FEC is thin, it can easily follow the volume change of the negative electrode active material accompanying the insertion or release of lithium ions and can exist stably. Therefore, even if charging / discharging of the lithium ion secondary battery is repeated, direct contact between the negative electrode active material and the electrolytic solution is suppressed over a long period of time, and deterioration of the electrolytic solution is suppressed. Moreover, since the coating film to be formed is thin, an increase in the electrical resistance of the negative electrode accompanying the film formation can be suppressed to a low level.

  Also on the surface of the positive electrode active material, the same phenomenon as that of the negative electrode occurs, and it is possible to suppress deterioration of the electrolytic solution and to suppress an increase in electric resistance of the positive electrode.

  The above lithium borate salt is easily reductively decomposed. For this reason, the lithium borate salt is reduced and decomposed during the operation of the lithium secondary battery including the electrolytic solution containing the lithium borate salt. Then, a thin and stable coating derived from lithium borate is formed on the entire surface of the negative electrode active material or the positive electrode active material. Then, it can be said that the coating derived from lithium borate has the same action as the coating derived from fluorine-containing cyclic carbonate such as FEC.

  Since the electrolytic solution of the present invention contains both the fluorine-containing cyclic carbonate and the above-mentioned specific lithium borate salt, the cycle characteristics are improved in a wide temperature range when used as an electrolytic solution for a lithium ion secondary battery. Therefore, it is not necessary to use the silicon fluoride compound or 1,3-propane sultone described in Patent Documents 1 and 2 for the electrolytic solution.

  The lithium ion secondary battery using the electrolytic solution of the present invention exhibits excellent cycle characteristics.

  Below, the form for implementing the electrolyte solution of 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 the numerical value range can be used as upper and lower numerical values.

  The electrolytic solution of the present invention is an electrolytic solution containing a non-aqueous solvent containing a fluorine-containing cyclic carbonate and an electrolyte, and contains 1 to 7% by volume of the fluorine-containing cyclic carbonate with respect to 100% by volume of the non-aqueous solvent, One or more lithium borate salts represented by the following general formula (1) are contained at a concentration of more than 0 mol / L and not more than 0.1 mol / L, and do not contain a silicon fluoride compound or 1,3-propane sultone. And

In the general formula (1), R ¹ and R ² are each independently represented by any one of the following general formulas (1-1) to (1-6).

In General Formulas (1-1) to (1-6), X ^{01 to} X ¹⁸ are each independently hydrogen, an alkyl group, a halogen, or a halogenated alkyl group.

  The fluorine-containing cyclic carbonate is not particularly limited as long as it is a cyclic carbonate containing at least one fluorine. The fluorine-containing cyclic carbonate may contain halogens other than fluorine, but those represented by the following general formula (2) are preferred.

(In General Formula (2), R ^{3 to} R ⁶ are each independently hydrogen, an alkyl group, fluorine or a fluorinated alkyl group, and at least one of R ^{3 to} R ⁶ is fluorine or fluorinated. Represents an alkyl group)
When R ^{3 to} R ⁶ are an alkyl group or a fluorinated alkyl group, the number of carbon atoms is preferably 1 or 2. Particularly preferred fluorine-containing cyclic carbonates are those in which at least one fluorine is bonded to one or more carbons constituting the cyclic structure, and specifically represented by the following formulas (2-I) to (2-III). Is done.

  Among these, from the viewpoint of stability to water, 4-fluoro-1,3-dioxolan-2-one represented by the formula (2-I), that is, fluoroethylene carbonate (hereinafter sometimes referred to as FEC) is used. preferable.

  When the total amount of the non-aqueous solvent is 100% by volume, the fluorine-containing cyclic carbonate is preferably contained in the non-aqueous solvent in the range of 1 to 7% by volume, and more preferably in the range of 3 to 5% by volume. preferable. If the content rate of a fluorine-containing cyclic carbonate exists in these ranges, the cycling characteristics of a secondary battery can be maintained suitably. Furthermore, if the content ratio of the fluorine-containing cyclic carbonate is within these ranges, the viscosity of the non-aqueous solvent can be kept low, and the movement of the electrolyte ions becomes easy, so that the secondary battery does not increase in resistance. The capacity can be suitably maintained.

  The non-aqueous solvent contains other organic solvent together with the fluorine-containing cyclic carbonate. The other organic solvent is preferably an aprotic organic solvent, for example, a cyclic carbonate, a chain carbonate, or an ether is preferable, and it is particularly preferable to use several kinds of solvents in combination.

  When the total amount of the non-aqueous solvent is 100% by volume, the cyclic carbonate is preferably contained in the non-aqueous solvent in the range of 0 to 50% by volume, more preferably in the range of 10 to 40% by volume. It is particularly preferred that it is contained in the range of 35% by volume.

  When the total amount of the non-aqueous solvent is 100% by volume, the chain carbonate is preferably contained in the non-aqueous solvent in the range of 50 to 99% by volume, more preferably in the range of 60 to 80% by volume, 65 It is particularly preferred that it is contained in the range of ~ 75% by volume.

  If the cyclic carbonate is contained in the non-aqueous solvent, the conductivity of the non-aqueous solvent is increased and the conductivity of the electrolyte is improved. However, if the ratio of the cyclic carbonate is too high, the viscosity of the non-aqueous solvent is increased and the ion migration is increased. Is hindered, and the conductivity of the electrolyte is rather deteriorated.

  When the chain carbonate is contained in the non-aqueous solvent, the conductivity of the non-aqueous solvent is not so high, but the viscosity of the non-aqueous solvent is kept low. Therefore, by blending both cyclic carbonate and chain carbonate in a non-aqueous solvent in a well-balanced manner, the conductivity of the non-aqueous solvent can be increased to some extent, and the viscosity of the non-aqueous solvent can be kept low. Can be prepared.

  Specific examples of the cyclic carbonate include propylene carbonate, ethylene carbonate (hereinafter sometimes referred to as EC), butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-gammabutyrolactone, acetyl-gammabutyrolactone, and gamma valerolactone. Is mentioned. As the non-aqueous solvent, the cyclic carbonate may be used alone, or two or more kinds may be used.

  Specific examples of the chain carbonate include dimethyl carbonate (hereinafter sometimes referred to as DMC), diethyl carbonate, ethyl methyl carbonate (hereinafter sometimes referred to as EMC), dibutyl carbonate, dipropyl carbonate, and alkyl propionate. Esters, malonic acid dialkyl esters and acetic acid alkyl esters. As the non-aqueous solvent, the chain carbonate may be used alone, or two or more kinds may be used.

  The non-aqueous solvent may contain ethers. Specific examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. The blending ratio of the ethers to the non-aqueous solvent is not particularly limited, but can be appropriately blended without departing from the gist of the present invention.

  A particularly preferable non-aqueous solvent in the present invention is a mixed solvent of a fluorine-containing cyclic carbonate, EC, EMC and DMC.

The electrolytic solution is a solution in which an electrolyte is dissolved in a non-aqueous solvent. The electrolyte is not particularly limited as long as it is known to act as an electrolyte in a normal non-aqueous secondary battery, but an alkali metal fluoride soluble in a non-aqueous solvent is preferable, and a salt containing lithium is particularly preferable. preferable. To illustrate a specific _{electrolyte, LiPF 6, LiBF 4, LiAsF} 6, NaPF 6, NaBF 4, and NaAsF ₆ and the like. The electrolyte may be used alone or in combination of two or more. The concentration of the electrolyte in the electrolytic solution may be about 0.5 to 1.7 mol / L.

  The electrolytic solution of the present invention contains a lithium borate salt represented by the following general formula (1).

In the general formula (1), R ¹ and R ² are each independently represented by any one of the following general formulas (1-1) to (1-6).

In General Formulas (1-1) to (1-6), X ^{01 to} X ¹⁸ are each independently hydrogen, an alkyl group, a halogen, or a halogenated alkyl group.

  As represented by the general formula (1), the lithium borate salt contains boron as a central element, and elements other than oxygen are not directly bonded to boron. Further, as represented by the general formula (1), the lithium borate salt has two ring structures sharing boron, so-called spiro ring structure.

In the general formulas (1-1) to (1-6), the alkyl group is represented by C _n H _{2n + 1} , and the carbon number n is preferably 1 to 4, 1 to 3, and more preferably 1 or 2. The halogen contained in the halogen and the halogenated alkyl group is preferably F or Cl, and particularly preferably F. The two rings having common boron may have the same chemical structure or different chemical structures, but at least one of the rings is preferably represented by the general formula (1-1). Specific examples of the lithium borate salt include compounds represented by the following formulas (1-I) to (1-X). In particular, in the general formula (1), R ¹ and R ² both correspond to lithium bis (oxalate) borate (formula (1-I) represented by the general formula (1-1). Hereinafter, the case of LiBOB Is particularly preferred. A single compound may be used as the lithium borate salt, or two or more compounds may be used in combination.

  The lithium borate salt is preferably contained in the electrolyte at a concentration of more than 0 mol / L and not more than 0.1 mol / L, and more preferably 0.02 to 0.07 mol / L. If the concentration of lithium borate in the electrolyte exceeds 0.1 mol / L, the initial internal resistance of the battery may increase. Lithium borate is a compound that is easily reduced and decomposed, and the reduced decomposition product becomes a coating component formed on the entire surface of the negative electrode active material and the positive electrode active material. For this reason, when lithium borate is excessively contained in the electrolytic solution, the coating becomes thick, which may increase the electrical resistance of the active material. In addition, since some lithium borate salts generate carbon dioxide by being reduced and decomposed, a large amount of carbon dioxide is generated when the lithium borate salt is excessively contained in the electrolyte solution. May be deformed. For these reasons, the concentration of the lithium borate salt in the electrolytic solution is preferably 0.1 mol / L or less, and more preferably 0.07 mol / L or less. If the lithium borate salt is present in the electrolyte even in a small amount, the cycle characteristics in the charge and discharge of the secondary battery are improved.

  The electrolytic solution of the present invention described above can be suitably used as an electrolytic solution for various secondary batteries. In particular, it is preferably used as an electrolyte solution for a lithium ion secondary battery using a lithium-metal composite oxide as a positive electrode active material and a silicon-based material containing silicon as a negative electrode active material.

  The lithium ion secondary battery using the electrolytic solution of the present invention will be described below.

  A lithium ion secondary battery of the present invention includes a negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and the electrolytic solution of the present invention. .

As the negative electrode active material, a material capable of inserting and extracting lithium ions can be used. Accordingly, there is no particular limitation as long as it is a simple substance, alloy, or compound that can occlude and release lithium ions. For example, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Ge, Sn, Pb, Sb and Bi Examples include materials containing at least one kind. Specifically, tin-based materials such as Cu-Sn alloy and Co-Sn alloy, carbon-based materials such as various graphites, silicon simple substance, SiO _x disproportionate to silicon simple substance and silicon dioxide (0.3 ≦ x ≦ 1.6) and the like. One or more of these materials can be used as the negative electrode active material. In particular, the cycle characteristics of a lithium ion secondary battery in which a negative electrode provided with a silicon-based material having a remarkable volume change associated with charge / discharge as a negative electrode active material and the electrolytic solution of the present invention are significantly improved.

The silicon-based material is not particularly limited as long as it contains silicon and can occlude and release lithium ions. The silicon-based material is preferably silicon and / or a silicon compound, particularly preferably a silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6) disproportionate to silicon alone and silicon dioxide.

The silicon oxide preferably has a Si phase and a SiO ₂ phase. The Si phase is a phase that can occlude and release Li ions made of silicon alone, and expands and contracts as Li ions are occluded and released. The SiO ₂ phase is a phase composed of SiO ₂ and has a role of buffering expansion and contraction of the Si phase. A silicon-based material in which the Si phase is coated with the SiO ₂ phase is preferable. Furthermore, it is preferable that the coated particles obtained by coating a plurality of refined Si phases with a SiO ₂ phase aggregate to form an aggregate. This aggregate can suitably suppress the volume change of the entire silicon-based material.

The mass ratio of the SiO ₂ phase to the Si phase in the silicon-based material is preferably in the range of 1 to 3. When the mass ratio is 1 or more, the expansion and contraction of the silicon-based material is preferably alleviated, and as a result, the occurrence of cracks in the negative electrode active material layer containing the silicon-based material can be preferably suppressed. When the mass ratio is 3 or less, the lithium ion secondary battery can be maintained at a high electric capacity because the insertion and removal of Li ions in the Si phase in the negative electrode active material is not hindered. The silicon-based material may be composed only of the Si phase and the SiO ₂ phase. Further, as the negative electrode active material, it may be used in combination known active materials in addition to silicon-based material as a main component and Si phase and SiO ₂ phase.

Disproportionation of silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6) proceeds by applying energy by a method such as heating or milling. When heating silicon oxide, it may be performed under the condition of 800 ° C. or higher in the absence of oxygen. Specifically, the non-crystalline SiO ₂ is prepared by subjecting the powder containing silicon oxide to oxygen-free conditions by filling with an inert gas such as vacuum or argon, and performing treatment at 800 to 1200 ° C. for 1 to 5 hours. A silicon oxide powder containing two phases of a phase and a crystalline Si phase is obtained. When silicon oxide is milled, part of the shear stress energy of the milling contributes to atomic diffusion at the solid phase interface of the silicon oxide, thereby forming a SiO ₂ phase and a Si phase. Specifically, oxygen-free conditions may be obtained by filling with an inert gas such as vacuum or argon, and powder containing silicon oxide may be milled with a ball mill, attritor, jet mill, vibration mill, high energy ball mill, or the like. Further heat treatment after milling may further promote disproportionation of the silicon oxide. Moreover, the powder containing silicon oxide may be milled together with a carbon-based material such as graphite powder.

  The negative electrode has a current collector and a negative electrode active material layer bound to the surface of the current collector.

  The current collector refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, magnesium, 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.

  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 10 μm to 100 μm.

  The negative electrode active material layer includes a negative electrode active material and, if necessary, a binder and / or a conductive aid.

  The binder serves to bind the active material and the conductive additive to the surface of the current collector. 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, and alkoxysilyl group-containing resins. be able to. 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: 0.05 to 1: 0.5. 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. Examples of the conductive assistant include carbon black, graphite, acetylene black, ketjen black (registered trademark), and vapor grown carbon fiber (VGCF) which are carbonaceous fine particles. 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 aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the formability of the negative electrode active material layer is deteriorated and the energy density of the electrode is lowered.

  A positive electrode used for a lithium ion secondary battery has a positive electrode active material capable of inserting and extracting lithium ions. The positive electrode has a current collector and a positive electrode active material layer bound to the surface of the current collector. The positive electrode active material layer includes a positive electrode active material and, if necessary, a binder and / or a conductive aid. The current collector, binder and conductive additive of the positive electrode are the same as those described for the negative electrode.

Examples of positive electrode active materials include LiCoO ₂ , LiNi _p Co _q Mn _r O ₂ (0 ≦ p ≦ 1, 0 ≦ q ≦ 1, 0 ≦ r ≦ 1, p + q + r = 1), Li ₂ MnO ₂ , Li ₂ MnO _3. , LiFePO ₄ , Li ₂ FeSO ₄ as a basic composition or a lithium-containing metal oxide or a solid solution material containing one or more of them. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element included in the basic composition may be replaced with another metal element, and Mg, etc. Other metal elements may be added to the basic composition to form a metal oxide. Further, as a positive electrode active material, a positive electrode active material that does not contain lithium that becomes an electrolyte ion in charge and discharge, for example, sulfur-modified compounds in which S is introduced into organic compounds such as simple sulfur (S), polyacrylonitrile (PAN), and TiS Metal sulfides such as ₂ , polyaniline, conjugated diacetate-based organic substances, and the like can also be used. When a positive electrode active material that does not contain lithium is used, it is necessary to add electrolyte ions to the positive electrode and / or the negative electrode in advance by a known method.

  As a method of forming the active material layer on the surface of the current collector, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method is used. An active material may be applied to the surface. Specifically, an active material layer-forming composition containing an active material and, if necessary, a binder and a conductive aid is prepared, and an appropriate solvent is added to the composition to make a paste, and then the collection is performed. After applying to the surface of the electric body, it is dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, and methyl isobutyl ketone. In order to increase the electrode density, the dried product may be compressed.

  A separator is used in the lithium ion secondary battery as necessary. The separator separates the positive electrode and the negative electrode and allows lithium ions to pass while preventing a short circuit of current due to contact between the two electrodes. Examples of the separator include a porous film using one or more synthetic resins such as polytetrafluoroethylene, polypropylene, and polyethylene, or a ceramic porous film.

  A separator is sandwiched between the positive electrode and the negative electrode as necessary to form an electrode body. After connecting between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal connected to the outside using a current collecting lead or the like, an electrolytic solution is added to the electrode body to form a lithium ion secondary battery. It is good to do.

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

  The lithium ion secondary battery of this invention should just be charged / discharged in the voltage range suitable for the kind of active material contained in an electrode. The fluorine-containing cyclic carbonate and the specific lithium borate salt contained in the electrolytic solution of the present invention form a film on the active material in the vicinity of 1.6 to 1.7V. Therefore, the charging / discharging voltage range of the lithium ion secondary battery of the present invention may be set to 2 V or more when the oxidation-reduction potential of Li is set as a reference potential. In addition, the lithium ion secondary battery of the present invention can suitably maintain cycle characteristics even under high potential driving conditions. The high potential driving condition means that the operating potential of lithium ions with respect to lithium metal is 4.3 V or more, and further 4.5 to 5.5 V. Note that, under the driving conditions of a general lithium ion secondary battery, the operating potential of lithium ions with respect to lithium metal is less than 4.3V.

  The lithium ion secondary battery of the present invention can suitably maintain cycle characteristics not only at room temperature but also under severe temperature conditions around 60 ° C.

  The lithium ion secondary battery 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 a 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. Examples of the lithium ion secondary battery include various home electric appliances, office equipment, industrial equipment, and the like driven by batteries, such as personal computers and portable communication devices, in addition to vehicles.

  As mentioned above, although embodiment of the electrolyte solution and lithium ion secondary battery 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 specifically described with reference to Examples and Comparative Examples. In addition, this invention is not limited by these Examples. In the following, unless otherwise specified, “part” means part by mass, and “%” means mass%.

  In order to evaluate the characteristics of the electrolytic solution, lithium ion secondary batteries having different types of electrolytic solutions were prepared, and the cycle characteristics were measured.

  An electrolytic solution was prepared as follows.

Fluoroethylene carbonate (FEC), cyclic carbonate ethylene carbonate (EC), chain carbonate ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) were prepared as non-aqueous solvents. LiBOB was prepared as a lithium borate salt. Moreover, LiPF ₆ was prepared as an electrolyte. LiBOB and LiPF ₆ were dissolved in a non-aqueous solvent in which FEC, EC, EMC, and DMC were mixed at a predetermined volume ratio to obtain an electrolytic solution. Table 1 shows the composition of each electrolytic solution. Electrolytic solutions 1 to 3 correspond to the electrolytic solution of the present invention.

  A lithium ion secondary battery was manufactured as follows.

As a negative electrode active material, silicon oxide powder disproportionated by heat treatment (SiO _x (x is 0.3 to 1.6) manufactured by Sigma-Aldrich Japan, average particle size 5 μm) and massive artificial graphite powder (MAG: A mixed powder having a particle size of 20 μm or less was prepared.

  The above-mentioned silicon oxide powder and graphite powder as the negative electrode active material, acetylene black as the conductive auxiliary agent, and polyamideimide as the binder are mixed, and N-methyl-2-pyrrolidone as a solvent is added to form a slurry. A mixture of was obtained. The mass ratio of silicon oxide powder, graphite powder, acetylene black and polyamideimide was silicon oxide powder / graphite powder / acetylene black / polyamideimide = 32/50/8/10.

  The slurry-like mixture was formed on one side of a copper foil as a current collector using a doctor blade method, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and then allowed to cool. A negative electrode in which a negative electrode active material layer containing a negative electrode active material was fixed on the current collector surface was manufactured.

Next, a composite oxide obtained by adding 2 parts by mass of Mg to 100 parts by mass of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black as a conductive additive, and polyfluoride as a binder. Vinylidene chloride was mixed to form a slurry. The mass ratio of the composite oxide, acetylene black and polyvinylidene fluoride was composite oxide / acetylene black / polyvinylidene fluoride = 94/3/3.

  The slurry mixture was formed into a film on one surface of an aluminum foil as a current collector using a doctor blade method, pressed at a predetermined pressure, and then fired. A positive electrode was manufactured in which a positive electrode active material layer containing a positive electrode active material was fixed to the current collector surface.

  An electrode body was produced by sandwiching a polypropylene porous membrane as a separator between the positive electrode and the negative electrode, and a plurality of the electrode bodies were laminated. The periphery (excluding some) of the two aluminum films was sealed by heat welding to obtain a bag-like aluminum film. The laminated electrode body was put in a bag-shaped aluminum film, and each electrolyte solution was further injected. Then, the opening part of the bag-shaped aluminum film was sealed, evacuating. At this time, the tips of the positive and negative electrode current collectors were protruded from the edge of the bag-shaped aluminum film so that the current collector could be connected to an external terminal.

  In this way, a lithium ion secondary battery was manufactured.

  A cell test was performed using the manufactured lithium ion secondary battery.

  The cell test was conducted as follows. Charging / discharging which charged the lithium ion secondary battery with 4.5V, 1C, a constant current at 25 degreeC or 60 degreeC, and then discharged with 2.5V, 1C, constant current with the same temperature was made into 1 cycle. After a predetermined number of cycles are performed on the lithium ion secondary battery, it is charged at room temperature 4.5V, 1C, constant current and constant voltage for 2.5 hours (full charge), then 2.5V, 1C or 1 / 3C The discharge amount observed was used as the capacity of the lithium ion secondary battery.

  The result of the cell test after one cycle was taken as the initial capacity. The percentage (%) of the value obtained by dividing the capacity of the cell test after a predetermined number of cycles by the initial capacity was defined as the capacity maintenance ratio.

  The results are shown in Tables 2 to 7. However, for batteries 4 to 6 in Table 3, the lithium ion secondary battery was charged at 4.32 V, 1 C, constant current at 60 ° C., and then charged at 3.275 V, 1 C, constant current at the same temperature. Discharging was 1 cycle.

  All of the batteries 1 to 3 in Table 2 and the batteries 4 to 6 in Table 3 showed suitable capacity maintenance rates. Batteries 2 and 5 using an electrolyte solution 2 having a LiBOB concentration of 0.05 mol / L, and batteries 3 and 6 using an electrolyte solution 3 having a LiBOB concentration of 0.07 mol / L show particularly preferable capacity maintenance rates. It was.

  From Table 4, it can be seen that the battery 7 having both FEC and LiBOB shows an excellent capacity maintenance ratio as compared with the battery 8 not having LiBOB and the battery 9 not having FEC.

  Table 5 shows that under the condition that the discharge rate of the cell test is 1C, the battery 10 having both FEC and LiBOB is equivalent to the battery 11 not having LiBOB and the battery 12 not having FEC. Although it was a capacity maintenance rate, it turns out that it had the outstanding initial capacity and the capacity | capacitance after a cycle.

  Looking at Tables 6 and 7, batteries 13 and 17 having both FEC and LiBOB are batteries 14 and 18 that do not have LiBOB, batteries 15 and 19 that do not have FEC, and batteries that do not have LiBOB and FEC. Compared with 16 and 20, it can be seen that the initial capacity, the capacity after the cycle and the capacity retention ratio are equal or higher.

When ¹¹ B-NMR measurement was performed on the electrolyte solution of battery 2 at 160 MHz, a LiBOB-derived peak was confirmed in the electrolyte solution before the cell test, but the LiBOB-derived peak disappeared in the electrolyte solution after 200 cycles. . Therefore, it was confirmed that LiBOB in the electrolytic solution was decomposed by repeating charging and discharging of the lithium ion secondary battery. And it is thought that the boron derivative derived from LiBOB has adhered to the surface of the negative electrode active material or the positive electrode active material.

  The advantageous effect of the present invention is that by repeating charge and discharge of a lithium ion secondary battery, the fluorine-containing cyclic carbonate such as FEC and the lithium borate salt represented by the above general formula (1) such as LiBOB are decomposed and the negative electrode It can be understood that this is achieved by forming SEI on the surface of the active material or the positive electrode active material.

Claims (8)

An electrolytic solution containing a non-aqueous solvent containing a fluorine-containing cyclic carbonate and an electrolyte,
1 to 7% by volume of the fluorine-containing cyclic carbonate with respect to 100% by volume of the non-aqueous solvent,
Including one or more lithium borate salts represented by the following general formula (1) at a concentration of more than 0 mol / L and not more than 0.1 mol / L;
An electrolytic solution characterized by not containing a silicon fluoride compound or 1,3-propane sultone.

In the general formula (1), R ¹ and R ² are each independently represented by any one of the following general formulas (1-1) to (1-6).

In General Formulas (1-1) to (1-6), X ^{01 to} X ¹⁸ are each independently hydrogen, an alkyl group, a halogen, or a halogenated alkyl group.   The electrolytic solution according to claim 1, comprising the lithium borate salt at a concentration of 0.02 to 0.07 mol / L.   The electrolyte solution according to claim 1 or 2, wherein the non-aqueous solvent contains 3 to 5% by volume of the fluorine-containing cyclic carbonate when the entire non-aqueous solvent is 100% by volume. The lithium borate salt according to any one of claims 1 to 3, wherein R ¹ and R ² in the general formula (1) include lithium bis (oxalate) borate represented by the general formula (1-1). Electrolytic solution.   The electrolyte solution according to claim 1, wherein the fluorine-containing cyclic carbonate contains fluoroethylene carbonate.   The electrolyte solution according to claim 1, wherein the non-aqueous solvent includes fluoroethylene carbonate, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.   A negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and the electrolyte solution according to any one of claims 1 to 6. A lithium ion secondary battery characterized by the above.   The lithium ion secondary battery according to claim 7, wherein the negative electrode active material contains silicon.