JP5846040B2 - Electrolytic solution and lithium ion secondary battery including the same - Google Patents

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 in a wide range of fields such as mobile phones and notebook computers. In recent years, use of a battery mounted on a vehicle has been studied. A lithium ion secondary battery mainly includes a positive electrode, a negative electrode, and an electrolytic solution. Each electrode has an active material and a current collector coated with the active material.

  The positive electrode active material contained in the positive electrode includes, for example, a metal 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 a negative electrode active material containing silicon (Si) or tin (Sn) as a negative electrode active material contained in the negative electrode has been studied. A negative electrode active material containing Si or Sn expands or contracts in volume due to occlusion / release of lithium ions. A film is formed on the surface of the negative electrode active material during charge and discharge. The coating prevents the electrolytic solution from coming into direct contact with the negative electrode active material and suppresses the deterioration of the electrolytic solution. However, in this film, cracks may occur due to a 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, the electrolytic solution deteriorates, and charge / discharge cycle characteristics (hereinafter simply referred to as “cycle characteristics”) may be deteriorated.

  In order to improve the cycle characteristics, Patent Document 1 shows that the film formed on the surface of the negative electrode active material is stabilized by adding lithium bis (oxalate) borate (LiBOB) to the electrolytic solution. Yes. Patent Document 2 discloses an electrolytic solution containing 0.1 to 3% by mass of fluorine-containing ethylene carbonate.

  Patent Document 3 discloses that a stable film is formed on the surface of the negative electrode active material by adding a silicon fluoride compound such as 1,2-bis (dimethylfluorosilyl) ethane together with LiBOB to the electrolytic solution. Is disclosed. However, the blending ratio of fluoroethylene carbonate (FEC) in the electrolytic solution is 10% by weight or less, and even when converted to a volume ratio with respect to the whole solvent, it is less than 10% by volume.

JP 2010-010095 A JP 2011-049114 A JP 2010-238506 A

As a standard non-aqueous solvent, a mixed solvent prepared by mixing ethylene carbonate (EC), which is a cyclic carbonate, and ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), which are chain carbonates, and LiPF ₆ as an electrolyte. There is a standard electrolyte dissolved. One means for improving the cycle characteristics of a lithium ion secondary battery using this standard electrolyte solution is to change EC to fluoroethylene carbonate (FEC), which is also a cyclic carbonate. Note that the content of FEC assumed here is larger than that disclosed in Patent Document 3.

  FEC is a component that has a high oxidation-reduction potential and is easily reductively 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 coating (SEI) containing a reduction product of FEC is formed on the surface of the negative electrode active material or the surface of the positive electrode active material. Film) is easily formed. The FEC-derived coating prevents the direct contact between the electrolytic solution and the active material, thereby improving the cycle characteristics. The reason is considered as follows.

  Fluorine-containing cyclic carbonates such as FEC are components that have a high redox potential and are easily reduced and decomposed. When a part of the fluorine-containing cyclic carbonate is reduced and decomposed, the electrolyte and the remaining solvent in the electrolytic solution are hardly decomposed any more. Therefore, it is considered that the deterioration of the electrolytic solution is suppressed and the cycle characteristics of the battery are improved. Further, the decomposed fluorine-containing cyclic carbonate forms a thin and stable film on the entire surface of the negative electrode active material in the initial use of the lithium ion secondary battery. A thin film easily follows the volume change of the negative electrode active material associated with insertion and extraction of lithium ions, and stress is less likely to concentrate. Therefore, even if charging / discharging is repeated, direct contact between the negative electrode active material and the electrolytic solution is suppressed over a long period of time, and further deterioration of the electrolytic solution is suppressed. That is, even when a non-aqueous solvent containing a large amount of fluorine-containing cyclic carbonate is used as the electrolytic solution, deterioration of the electrolytic solution can be suppressed, and further generation of a film can be suppressed. Moreover, since the coating film to be formed is a thin film, the electrical resistance of the negative electrode can be kept low.

  Also on the surface of the positive electrode active material, a phenomenon similar to that of the negative electrode occurs, and further deterioration of the electrolytic solution can be suppressed, further generation of the coating film can be suppressed, and the electrical resistance of the positive electrode can be suppressed low.

  However, according to the study by the present inventors, when charging / discharging is performed with a lithium ion secondary battery using an electrolytic solution containing a lot of FEC, charging / discharging is performed at room temperature in a high temperature environment of 55 ° C. In comparison, it was found that the cycle characteristics deteriorate. This is presumably due to the fact that the high temperature stability of the FEC itself is low, the FEC-derived coating dissolves in the electrolyte at a high temperature, and the like.

In addition, it is predicted that the cycle characteristics of a lithium ion secondary battery using the same can also be improved by adding LiBOB to a standard electrolytic solution in which LiPF ₆ is dissolved in the above standard non-aqueous solvent. . However, according to the study by the present inventors, in a high temperature environment of 55 ° C., there is a large difference in the cycle characteristics of the lithium ion secondary battery between the case where LiBOB is added to the standard electrolyte and the case where it is not added. It turns out that it does not occur. That is, it was found that even if FEC or LiBOB was used, the cycle characteristics could not always be improved depending on the amount used and charge / discharge conditions.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an electrolytic solution that can be used for a lithium ion secondary battery or the like to obtain a secondary battery having excellent cycle characteristics in a wide temperature range. And

  The present inventors have further studied to improve the cycle characteristics of a lithium ion secondary battery using an electrolytic solution containing a sufficient amount of FEC. By developing the results of the study, the present invention described below has been completed.

The present invention is an electrolytic solution comprising a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent,
The non-aqueous solvent contains 10% by volume or more of a fluorine-containing cyclic carbonate containing at least one fluorine when the non-aqueous solvent is 100% by volume.
Furthermore, as an additive, when the whole electrolyte is 1 L, one or more types of lithium borate represented by the following general formula (1) are included more than 0 mol / L and 0.2 mol / L or less.

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

  Like the fluorine-containing cyclic carbonate, the above-mentioned specific lithium borate salt has a high oxidation-reduction potential and has a property of being easily reductively decomposed. For this reason, a secondary battery including an electrolytic solution containing a lithium borate salt easily forms a thin and stable film derived from the lithium borate salt on the entire surface of the negative electrode active material or the positive electrode active material in the initial stage of use.

  Further, as described above, when an electrolyte solution using a non-aqueous solvent containing a fluorinated cyclic carbonate is used in a secondary battery, a coating derived from the fluorinated cyclic carbonate is formed on the entire surface of the active material. However, this film tends to dissolve in the electrolyte at high temperatures. Furthermore, it is considered that hydrogen fluoride (HF) is generated from charge and discharge at high temperature from the fluorine-containing cyclic carbonate having low high-temperature stability. Therefore, when charging / discharging is performed at a high temperature, the coating film derived from the fluorine-containing cyclic carbonate is dissolved, and the generated HF is likely to come into contact with the active material, and the active material is corroded, and it is assumed that the cycle characteristics are deteriorated. . Derived from fluorine-containing cyclic carbonate, which occurs when charging / discharging at high temperature, by adding a small amount of the specific lithium borate to the electrolyte solution using a non-aqueous solvent containing the fluorine-containing cyclic carbonate in the predetermined amount. The performance degradation of the coating can be compensated. As a result, cycle characteristics at high temperatures are improved.

On the other hand, it is considered that a coating derived from a lithium borate salt is formed in addition to a coating derived from a fluorine-containing cyclic carbonate at room temperature where the dissolution of the coating and generation of HF are difficult to occur. Formation of an excessive film leads to an increase in resistance of the electrode. However, in the electrolytic solution of the present invention, due to the synergistic effect of the fluorine-containing cyclic carbonate and the lithium borate salt, the cycle characteristics at high temperature are improved even if the addition amount of the lithium borate salt is small (0.2 mol / L or less). The For this reason, even when a coating derived from lithium borate is formed under conditions where the coating is difficult to dissolve, such as charge / discharge at room temperature, the thickness is not so large that the cycle characteristics are significantly reduced.

  In the electrolytic solution of the present invention, due to the synergistic effect of the fluorine-containing cyclic carbonate and the specific lithium borate salt, the cycle characteristics are improved in a wide temperature range when used as an electrolytic solution for a secondary battery. Therefore, it is not necessary to add a silicon fluoride compound as described in Patent Document 3.

  That is, by using the electrolytic solution of the present invention for a secondary battery such as a lithium ion secondary battery, a secondary battery having excellent cycle characteristics in a wide temperature range can be obtained. Note that the “wide temperature range” assumed in the present specification assumes a temperature exceeding 0 ° C. and not more than 65 ° C., or about 15 to 65 ° C., if specifically defined.

  Further, the present invention can also be understood as a lithium ion secondary battery. That is, the 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 electrolyte solution of the present invention. It is characterized by providing.

  By using the electrolytic solution of the present invention for a secondary battery such as a lithium ion secondary battery, a secondary battery having excellent cycle characteristics in a wide temperature range can be obtained.

It is a diagram which shows the cycling characteristics in room temperature (25 degreeC) of the batteries 1-4 produced using the electrolyte solution of an Example or a comparative example. It is a diagram which shows the cycling characteristics in the high temperature (55 degreeC) of the batteries 1-4 produced using the electrolyte solution of an Example or a comparative example.

  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.

<Electrolyte>
The electrolytic solution of the present invention includes a specific non-aqueous solvent described below, an electrolyte salt dissolved in the non-aqueous solvent, and a specific additive described below. Each component will be described.

<Non-aqueous solvent>
The non-aqueous solvent includes a fluorinated cyclic carbonate containing at least one fluorine. The fluorinated cyclic carbonate may contain at least one fluorine and may contain other halogens, but is preferably represented by the following formula (2).

（式（２）において、Ｒ^３〜Ｒ^６は、それぞれ独立して、水素基、アルキル基、フッ素基あるいはフッ化アルキル基であり、Ｒ^３〜Ｒ^６のうちの少なくとも１つはフッ素基またはフッ化アルキル基を表す）

(In Formula (2), R ^{3 to} R ⁶ are each independently a hydrogen group, an alkyl group, a fluorine group or a fluorinated alkyl group, and at least one of R ^{3 to} R ⁶ is a fluorine group or Represents a fluorinated alkyl group)

The non-aqueous solvent may contain at least one fluorine-containing cyclic carbonate represented by the formula (2). 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 is a fluorine-containing cyclic carbonate having a structure in which at least one fluorine is bonded to one or more carbons constituting the cyclic structure, and specifically, the following formulas (2-I) to (2-III): It is represented by

  Among these, 4-fluoro-1,3-dioxolan-2-one (fluoroethylene carbonate: FEC) represented by (2-I) is preferable from the viewpoint of stability to water.

  The fluorine-containing cyclic carbonate is contained in an amount of 10% by volume or more when the entire non-aqueous solvent is 100% by volume. If it is less than 10% by volume, the formation of the coating film derived from the fluorine-containing cyclic carbonate becomes insufficient, and improvement in cycle characteristics cannot be expected. The preferable content rate of a fluorine-containing cyclic carbonate is 15-40 volume%, More preferably, it is 25-35 volume%. If the content ratio of the fluorinated cyclic carbonate is within this range, the cycle characteristics can be effectively improved, and the electrolyte can be easily moved by suppressing the viscosity of the electrolytic solution to further improve the battery capacity. Can do.

  The non-aqueous solvent may contain another organic solvent together with the fluorine-containing cyclic carbonate. The other organic solvent is preferably an aprotic organic solvent, and for example, a cyclic carbonate (excluding a fluorine-containing cyclic carbonate), a chain carbonate, an ether, or the like may be used. In particular, it is preferable to use a cyclic carbonate containing a fluorine-containing cyclic carbonate and a chain carbonate in combination. Cyclic carbonate has a high dielectric constant, and chain carbonate has low viscosity. For this reason, when electrolyte solution contains both a cyclic carbonate and a chain carbonate, the movement of electrolyte ion is not prevented and battery capacity can be improved.

When the total amount of the non-aqueous solvent is 100% by volume, the cyclic carbonate is 10 to 50% by volume, 15 to 40% by volume, and further 25 to 35% by volume, and the chain carbonate is 50 to 90% by volume, 60 to 75% by volume. % Or 65 to 75% by volume. The cyclic carbonate increases the dielectric constant of the electrolytic solution, while having a high viscosity. As the dielectric constant increases, the conductivity of the electrolyte improves. If the viscosity is high, the movement of the electrolyte ions is hindered and the conductivity is deteriorated. A chain carbonate has a low dielectric constant but a low viscosity. By blending them in a well-balanced range within the above blending ratio, it is possible to adjust the solvent dielectric constant to a certain degree and also reduce the viscosity, adjust the solvent with good conductivity, and improve the battery capacity.

  The cyclic carbonate has a fluorine-containing cyclic carbonate as an essential component, and in addition, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-gammabutyrolactone, acetyl-gammabutyrolactone, and gamma. One or more selected from the group of valerolactone may be included. In order to suppress a decrease in cycle characteristics at room temperature, it is desirable that the non-aqueous solvent does not contain a cyclic carbonate that does not contain fluorine, such as EC, as the cyclic carbonate. Particularly preferred is a non-aqueous solvent that contains a fluorine-containing cyclic carbonate as a main component and does not substantially contain other cyclic carbonates.

  The chain carbonate is not particularly limited as long as it is a chain. For example, at least one selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dibutyl carbonate, dipropyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester and acetic acid alkyl ester may be used. it can.

  The non-aqueous solvent may contain ethers, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane. Etc. The ethers are preferably 10% by volume or less, and more preferably 0.1 to 7% by volume when the entire non-aqueous solvent is 100% by volume.

  A particularly preferable non-aqueous solvent in the present invention is a mixed solvent of fluorine-containing cyclic carbonate (particularly FEC), EMC and DMC. In this case, it is preferable that the non-aqueous solvent is 100% by volume, EMC is 20 to 40% by volume, further 25 to 35% by volume, DMC is 30 to 50% by volume, further 35 to 45% by volume, and the balance is FEC. . Since the dielectric constant of the electrolytic solution is increased and the viscosity is relatively low, the conductivity of the electrolytic solution is increased, and as a result, the battery capacity is increased.

<Electrolyte salt>
The electrolytic solution is formed by dissolving an electrolyte salt in a non-aqueous solvent. The electrolyte salt is preferably an alkali metal fluoride that is soluble in a non-aqueous solvent. Particularly preferred is a salt containing lithium such as lithium fluoride. Specifically, LiPF ₆ , LiBF ₄ , LiAsF ₆ , NaPF ₆ , NaBF ₄ , NaAsF _{6 and the} like are preferable, and at least one selected from these may be used. The concentration of the electrolyte salt may be about 0.5 to 1.7 mol / L with respect to 1 L of the electrolyte.

<Additives>
The electrolytic solution of the present invention contains a lithium borate salt represented by the following general formula (1) as an additive.

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

  As represented by the formula (1), the lithium borate salt contains boron (B) as a central element, and an element (such as halogen) other than oxygen (O) is not directly bonded to B. Further, the lithium borate salt has two ring structures including B in common, as represented by the formula (1). The reason why such a structure is preferable as the additive of the electrolytic solution of the present invention is as follows.

In the formula (1), assuming that a halogen group is directly bonded to B instead of —O—R ¹ —O—, B electrons are biased toward a halogen group having a high electronegativity. Slightly positively charged. In this case, B tends to attract oxygen electrons forming a ring structure, and the ring is easily opened between B and O. Such a lithium borate salt has a very high redox potential, and is considered to be easily reduced and decomposed prior to the reductive decomposition of the fluorine-containing cyclic carbonate.

  On the other hand, the lithium borate salt represented by the formula (1) has two ring structures including B in common, and other elements (except for O) are not directly bonded to B. Therefore, the bias of B electrons is alleviated. Such a lithium borate salt forms a network structure even when it is reduced and decomposed, so that a film having high thermal stability can be obtained.

In Formula (1-1) to Formula (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 group and the halogenated alkyl group is preferably F, Cl, particularly F in the group 17 of the periodic table. The two rings having B in common may have the same structure or different structures, but at least one of the rings is preferably represented by the formula (1-1). Specifically, such lithium borate salt is represented by the following formula (1-I) to formula (1-X), and it is desirable to include one or more of these compounds.

As represented by Formula (1-I) and Formulas (1-III) to (1-X), in Formula (1), at least one of R ¹ and R ² is represented by Formula (1-1). The lithium borate salt is preferably included. This is because the above-described network structure is easily formed. Among them, it is essential that R ¹ and R ² in Formula (1) both essentially contain lithium bis (oxalate) borate (corresponding to Formula (1-I)) represented by Formula (1-1). Particularly desirable.

The above lithium borate salt contains 0 mol / L and 0.2 mol / L or less, preferably 0.03 to 0.1 mol / L when the entire electrolyte is 1 L. If it exceeds 0.2 mol / L, the initial internal resistance of the battery may increase. As described above, the lithium borate salt 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 such lithium borate salt is excessively contained in the electrolytic solution, the coating becomes thicker, leading to an increase in the electrical resistance of the active material. In charging / discharging at room temperature, the thickening of the coating becomes particularly significant. Therefore, in order to maintain the cycle characteristics at room temperature, the content ratio of the lithium borate salt is 0.2 mol / L or less, further 0.1 mol / L.
It is good to keep it below L. In addition, if a small amount of the lithium borate is included, the cycle characteristics during charging and discharging at high temperatures are improved, but it is desirable that the lithium borate be 0.03 mol / L or more.

<Lithium ion secondary battery>
The electrolytic solution of the present invention described above can be suitably used as an electrolytic solution for various secondary batteries. In particular, a lithium ion secondary battery using, for example, a composite oxide of lithium and a metal as a positive electrode, for example, a silicon-based material containing silicon as a negative electrode is preferable. The lithium ion secondary battery using the electrolytic solution of the present invention will be described below.

  The 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 electrolyte solution of the present invention, Is provided.

As the negative electrode active material, a material capable of inserting and extracting lithium ions can be used. Therefore, 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 thereof include a negative electrode active material containing at least one kind. Specifically, a tin-based material such as a Cu—Sn alloy or a Co—Sn alloy, a carbon-based material such as various graphites, a simple silicon, or a silicon-based material such as SiO _x (0.5 ≦ x ≦ 1.5). One or more of these can be used. In particular, the cycle characteristics can be effectively improved by using a combination of a negative electrode having a negative electrode active material containing a silicon-based material with a remarkable volume change accompanying charge / discharge and the electrolytic solution of the present invention.

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

The silicon oxide preferably has a Si phase and a SiO ₂ phase. The Si phase is composed of simple silicon, and is a phase that can occlude and release Li ions, and expands and contracts as Li ions are occluded and released. The SiO ₂ phase is made of SiO ₂ and absorbs expansion and contraction of the Si phase. It is preferable to form a silicon-based material composed of the Si phase and the SiO ₂ phase by covering the Si phase with the SiO ₂ phase. Furthermore, it is preferable that a plurality of micronized Si phases are covered with a SiO ₂ phase to form particles integrally. In this case, the volume change of the entire silicon-based material can be effectively suppressed.

The mass ratio of the SiO ₂ phase to the Si phase in the silicon-based material is preferably 1 to 3. A mass ratio of 1 or more is preferable because expansion / contraction of the silicon-based material is alleviated and cracks generated in the negative electrode active material layer made of the silicon-based material are suppressed. A mass ratio of 3 or less is preferable because the amount of insertion and extraction of Li ions in the negative electrode active material is sufficiently performed and a high electric capacity is maintained. The silicon-based material may be composed only of the Si phase and the SiO ₂ phase. In addition, the silicon-based material mainly includes a Si phase and a SiO ₂ phase, but may contain a known active material.

A raw material powder containing the above silicon oxide may be used as a raw material for the silicon-based material. In this case, the silicon oxide in the raw material powder is preferably disproportionated into two phases of SiO ₂ phase and Si phase. In the disproportionation of silicon oxide, silicon oxide, which is a homogeneous solid having an atomic ratio of Si to O of approximately 1: 1, is separated into two phases of SiO ₂ phase and Si phase by the reaction inside the solid. . The silicon oxide powder obtained by disproportionation includes a SiO ₂ phase and a Si phase.

  Disproportionation of the silicon oxide of the raw material powder proceeds by applying energy to the raw material powder. As an example, a method of heating or milling the raw material powder can be mentioned.

When the raw material powder is heated, it is generally said that almost all of the silicon oxide is disproportionated and separated into two phases at 800 ° C. or higher if oxygen is removed. Specifically, the raw material powder containing the amorphous silicon oxide powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as vacuum or in an inert gas. A silicon oxide powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

  When milling the raw material powder, part of the mechanical energy of the milling contributes to chemical atomic diffusion at the solid phase interface of the raw material powder, and generates an oxide phase, a silicon phase, and the like. In milling, the raw material powder may be mixed using a V-type mixer, a ball mill, an attritor, a jet mill, a vibration mill, a high energy ball mill or the like in an inert gas atmosphere such as vacuum or argon gas. Further heat treatment may be performed after milling to further promote disproportionation of the silicon oxide. The raw material powder containing silicon oxide may be milled together with a carbon-based material such as graphite powder.

  The negative electrode active material constitutes a negative electrode active material layer that covers at least the surface of the current collector. Generally, a negative electrode is formed by covering a current collector with a negative electrode active material layer. As the current collector, for example, a metal mesh such as copper or copper alloy, a porous body, or a metal foil may be used.

  In addition to the negative electrode active material, the negative electrode active material layer preferably contains a binder, a conductive additive, and the like.

  The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used. The blending ratio of the binder 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.

  As the conductive auxiliary agent, a material generally used for an electrode of a lithium ion secondary battery may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides conductive carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary agent may be used. One of these may be used alone or in combination of two or more. The blending ratio of the conductive assistant is, in mass ratio, negative electrode active material: conductive assistant = 1: 0.01 to 1: 0.5. 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 electrode material 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 occluding and releasing lithium ions. The positive electrode is preferably composed of a current collector and a positive electrode active material layer that has a positive electrode active material and covers the surface of the current collector. The positive electrode active material may constitute a positive electrode material together with a binder and / or a conductive aid. The conductive auxiliary agent and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

As the positive electrode active material, LiCoO ₂ , LiNi _p Co _q Mn _r O ₂ (0 <p <1, 0 + p <q <1-p, 0+ (p + q) <r <1- (p + q)), Li ₂ MnO ₂ , Li ₂ MnO ₃ , LiNi _s Mn _t O ₂ (0 <s <1, 0 + s <t <1-s), LiFePO ₄ , Li ₂ FeSO ₄ , or a lithium-containing metal oxide having a basic composition or 1 each. Examples thereof include solid solution materials containing two or more species. Any metal oxide may have the above basic composition, and a compound in which a metal element contained in the basic composition is substituted with another metal element can also be used. 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 obtained by introducing S into an organic compound such as simple sulfur (S), polyacrylonitrile (PAN), or TiS _2. It is also possible to use metal sulfides such as polyaniline, conjugated diacetate-based organic substances, and the like. 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.

  The positive electrode current collector can employ a metal mesh, a porous body, a metal foil, or the like, but is not particularly limited as long as it has a shape according to the purpose. The current collector is not particularly limited as long as it is generally used for a positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel.

  As the binder resin and the conductive auxiliary agent, those similar to those described in the above negative electrode can be used. In addition, what is necessary is just to set the compounding ratio of a positive electrode active material, a binder, and a conductive support agent according to the conventional lithium ion secondary battery.

  Usually, the active material and binder resin are mixed with a conductive aid and an appropriate amount of an organic solvent as necessary, and the resulting slurry is rolled, dip-coated, doctor blade, spray-coated, curtained. An electrode can be produced by coating on a current collector by a method such as a coating method and curing the binder resin. This is not limited depending on the type of active material, and as long as it is a sheet-like sulfur-modified compound, the positive electrode active material layer may be directly pressure-bonded to the current collector to form a positive electrode.

  A separator is used as needed. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

  A separator is sandwiched between the positive electrode and the negative electrode as necessary to form an electrode body. Lithium ion secondary battery in which a non-aqueous electrolyte is impregnated in the 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 leading to the outside using a current collecting lead or the like 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 stacked shape, a coin shape, and a laminated shape can be adopted.

  Such a lithium ion secondary battery may be charged and discharged within a voltage range suitable for the type of active material contained in the electrode. However, the fluorine-containing cyclic carbonate and the lithium borate salt having a specific structure contained in the electrolytic solution of the present invention produce a film at around 1.6 to 1.7 V. Therefore, when the oxidation-reduction potential of Li is used as a reference potential, the charge / discharge voltage range of the lithium ion secondary battery is preferably 2 to 4.5 V, more preferably 2.25 to 4.25 V.

  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 of the electrolytic solution of the present invention.

  In order to evaluate the characteristics of the non-aqueous electrolyte, four types of lithium ion secondary batteries with different types of electrolyte were prepared, and the cycle characteristics of the batteries were measured.

(Preparation of electrolyte)
As the non-aqueous solvent, cyclic carbonates such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC), and chain carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) were prepared. LiBOB was prepared as an additive. Moreover, LiPF ₆ was prepared as an electrolyte. LiPF ₆ and, if necessary, LiBOB were dissolved in a non-aqueous solvent in which EC or FEC, EMC, and DMC were mixed at a predetermined volume ratio to obtain an electrolytic solution. LiPF ₆ was prepared to be 1 mol / L with respect to 1 L of the electrolytic solution. LiBOB was prepared to be 0.05 mol / L with respect to 1 L of the electrolyte solution. Table 1 shows the composition of each electrolytic solution. The electrolytic solution 4 corresponds to the nonaqueous electrolytic solution of the present invention.

(Battery 1)
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 silicon oxide powder (SiO) and the graphite powder (MAG) as a negative electrode active material, ketjen black (KB) as a conductive additive, and polyamideimide (PAI) as a binder. After mixing, a solvent was added to obtain a slurry mixture. N-methyl-2-pyrrolidone (NMP) was used as the solvent. The mass ratio of SiO, MAG, KB, and PAI was SiO / MAG / KB / PAI = 32/50/8/10.

  Next, the slurry mixture was formed into a film on one side of a copper foil as a current collector using a doctor blade, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and allowed to cool. Thereby, the negative electrode formed by fixing the negative electrode active material layer containing a negative electrode active material to the collector surface was formed.

Next, a composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) as a positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder Were mixed to form a slurry, and this slurry was applied to one side of an aluminum foil as a current collector, pressed and fired. The mass ratio of the composite oxide, AB, and PVDF was composite oxide / AB / PVDF = 88/6/6. As a result, a positive electrode was obtained in which the positive electrode active material layer containing the positive electrode active material was fixed to the current collector surface.

A polypropylene porous membrane as a separator was sandwiched between the positive electrode and the negative electrode. A plurality of electrode bodies composed of the positive electrode, the separator, and the negative electrode were stacked. The periphery of the two aluminum films was sealed by heat-welding except for a part to make a bag shape. The laminated electrode body was put in a bag-like aluminum film, and “electrolyte solution 1” was further put.

  Then, the opening part of the aluminum film was completely airtightly sealed while evacuating. At this time, the tips of the current collector on the positive electrode side and the negative electrode side were protruded from the edge portion of the film to be connectable to an external terminal to obtain a lithium ion secondary battery (battery 1). The battery 1 was subjected to a conditioning treatment for initial charge / discharge at 25 ° C.

(Battery 2)
The battery 2 has the same configuration as the battery 1 except that the electrolytic solution 2 was used instead of the electrolytic solution 1. After the battery was prepared, a conditioning treatment was performed under the same conditions as battery 1.

(Battery 3)
The battery 3 has the same configuration as the battery 1 except that the electrolyte solution 3 was used instead of the electrolyte solution 1. After the battery was prepared, a conditioning treatment was performed under the same conditions as battery 1.

(Battery 4)
The battery 4 has the same configuration as the battery 1 except that the electrolyte solution 4 was used instead of the electrolyte solution 1. After the battery was prepared, a conditioning treatment was performed under the same conditions as battery 1.

  In addition, in order to evaluate electrolyte solution on different temperature conditions, the batteries 1-4 were prepared 2 each, respectively.

(Cycle test)
The charge / discharge cycle test of the batteries 1 to 4 was performed. The cycle test was performed at room temperature (25 ° C.) and high temperature (55 ° C.). The charge condition of the cycle test was CC (constant current) charge up to 4.2V at 1C, and the discharge condition was CC discharge up to 2.5V at 1C. The first charge / discharge after the conditioning treatment was regarded as the first cycle, and the same charge / discharge was repeated until the 500th cycle. The discharge capacity at each cycle relative to the discharge capacity at the first cycle was calculated as “discharge capacity maintenance rate (%)”. The results are shown in FIG. 1 and FIG.

  Regarding the cycle characteristics of the battery 1 using the electrolytic solution 1, the capacity retention rate greatly decreased after exceeding 100 cycles at room temperature. However, the cycle characteristics at a high temperature of the battery 1 showed a capacity retention rate of 70% or more up to the 500th cycle. The cycle characteristics of the battery 3 using the electrolytic solution 3 were higher than those of the battery 1 at room temperature, but were similar to those of the battery 1 at high temperatures. Moreover, as for the cycle characteristics at room temperature of the battery 3, the capacity retention rate greatly decreased from around 200 cycles. The electrolytic solution 3 used for the battery 3 is obtained by adding LiBOB to the electrolytic solution 1. It has been found that the addition of LiBOB cannot suppress the rapid decrease in discharge capacity retention rate observed at room temperature, and cannot be expected to improve the cycle characteristics at high temperatures.

  The electrolytic solution 2 is an electrolytic solution using a non-aqueous solvent in which the EC contained in the electrolytic solution 1 is changed to FEC. The cycle characteristics at room temperature of the battery 2 using the electrolytic solution 2 were the best among the four types of batteries. This is because, at room temperature, a flexible FEC-derived film that can follow the volume change of the negative electrode active material associated with insertion and extraction of lithium ions is formed on the surface of the negative electrode active material at the initial stage of charge and discharge. This is because deterioration of the electrolytic solution caused by contact with the electrolytic solution can be suppressed over a long period of time over many cycles. However, the cycle characteristics of the battery 2 at a high temperature were the lowest among the four types of batteries. At high temperatures, it is presumed that the FEC-derived coating melts into the electrolytic solution, causing the electrolytic solution to deteriorate, and further, HF is generated as FEC deteriorates. Therefore, it is surmised that the battery 2 has lower cycle characteristics than the battery 3 using the electrolytic solution 3 containing LiBOB, as well as the battery 1 in which the electrolytic solution does not contain LiBOB and is difficult to form a film.

  The battery 4 using the electrolytic solution 4 in which LiBOB is added to the electrolytic solution 2 is inferior to the battery 2 in cycle characteristics at room temperature, but the rapid decrease in the discharge capacity maintenance ratio generated in the battery 1 and the battery 3 is suppressed. It was an excellent secondary battery. Further, the battery 4 has greatly improved cycle characteristics at a high temperature as compared with the battery 2. This is presumably because the addition of a small amount of LiBOB formed a LiBOB-derived film on the surface of the negative electrode active material and compensated for the performance deterioration of the FEC-derived film. The decrease in the cycle characteristics at room temperature is thought to be due to the LiBOB-derived film formed on the surface of the negative electrode active material. However, since the addition amount is as small as 0.05 mol / L, the cycle characteristics at room temperature are greatly reduced. It was never high. In addition, in the electrolyte solution 4, if the added amount of LiBOB is 0.03 to 0.07 mol / L, it is presumed that results equivalent to those in FIGS. 1 and 2 are obtained.

  That is, the electrolyte solution using a small amount of lithium borate such as LiBOBO is superior in cycle characteristics at both room temperature and high temperature to the electrolyte solution using a non-aqueous solvent containing a large amount of fluorine-containing cyclic carbonate such as FEC. It was found that this was an electrolytic solution from which a lithium ion secondary battery was obtained.

Claims (7)

An electrolyte solution comprising a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent,
The non-aqueous solvent is composed of 4-fluoro-1,3-dioxolan-2-one (fluoroethylene carbonate), ethyl methyl carbonate, and dimethyl carbonate,
The non-aqueous solvent contains 10% by volume or more of the fluoroethylene carbonate when the non-aqueous solvent is 100% by volume,
Further, as an additive, an electrolyte solution comprising lithium bis (oxalate) borate exceeding 0 mol / L and not more than 0.2 mol / L when the entire electrolyte is 1 L. The electrolytic solution according to claim 1, comprising 0.03 to 0.1 mol / L of the lithium bis (oxalate) borate . The non-aqueous solvent, electrolyte solution according to claim 1 or 2, wherein including fluoroethylene carbonate 15-40% by volume when the entire non-aqueous solvent is 100% by volume. The negative electrode which has a negative electrode active material which can occlude / release lithium ion, the positive electrode which has a positive electrode active material which can occlude / release lithium ion, and the electrolyte solution in any one of Claims 1-3 are provided. A lithium ion secondary battery characterized by the above. The lithium ion secondary battery according to claim 4 , wherein the negative electrode active material includes a silicon-based material including silicon.   The lithium ion secondary battery according to claim 4 or 5, wherein the negative electrode contains polyamideimide as a binder.   The negative electrode active material is SiO _x Silicon oxide represented by (0.3 ≦ x ≦ 1.6) and graphite,
  The positive electrode active material is LiNi _1/3 Co _1/3 Mn _1/3 O ₂ The lithium ion secondary battery in any one of Claims 4-6 which is complex oxide represented by these.

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