JP2015125949A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery with improved ion conductivity of an electrolyte and excellent high rate characteristics.SOLUTION: The lithium ion secondary battery contains, as an additive compound to an electrolyte, 0.1-10 mass% of at least one of compounds represented by chemical formula (1) and chemical formula (2) [R1 to R3 are C1-4 alkyl group; R4 to R7 are each H or a C1-4 alkyl group; and R8 and R9 are each a C1-4 alkyl group]. Preferably, the electrolyte further contains, as solvents, an annular carbide and a low viscosity solvent.

Description

The present invention relates to a lithium ion secondary battery.

  With the development and popularization of portable information devices such as mobile phones, notebook personal computers, and video cameras in recent years, the demand for secondary batteries that are small, lightweight, and have high energy density has greatly increased. Considering higher performance. In the future, the use of large batteries for automobiles is expected to expand. As such a secondary battery, a lithium ion secondary battery is particularly attracting attention.

  The general structure of a lithium ion secondary battery is that a positive electrode active material layer made of a positive electrode active material powder such as lithium-cobalt composite oxide, a conductive powder, and a binder is formed on the surface of a positive electrode current collector made of aluminum foil. A negative electrode formed by forming a negative electrode active material layer made of a copper foil and a negative electrode active material layer made of a copper foil on a surface of a negative electrode current collector made of a porous film. The power generation element is impregnated with an electrolytic solution.

As the electrolytic solution, a solution in which a Li electrolyte such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , or Li ₂ SiF ₆ is dissolved in an aprotic organic solvent is used. In the lithium ion secondary battery, the electrolyte plays a role of transferring ions between the positive electrode and the negative electrode. In order to improve the high rate characteristics of the battery, it is necessary to increase the ion transfer rate between the positive electrode and the negative electrode as much as possible. The ion conductivity of the electrolytic solution is increased, the viscosity of the electrolytic solution is decreased, and the substance caused by diffusion It is necessary to make movement easier.

As an electrolytic solution satisfying such requirements, generally, LiPF _{6 is used in} a mixed solvent of a high dielectric constant carbonate solvent such as propylene carbonate or ethylene carbonate and a low viscosity carbonate solvent such as diethyl carbonate, methyl ethyl carbonate or dimethyl carbonate. What dissolved lithium salt, such as, is used.

  However, while using the battery, there was a problem that the electrolytic solution was decomposed on the surface of the positive electrode or the negative electrode, and the decomposition product lowered the ionic conductivity. A method of adding a fluorinated silane compound to the electrolytic solution has been proposed as a method for preventing degradation of ionic conductivity due to decomposition products, but the ionic conductivity is still insufficient to improve high rate characteristics ( Patent Document 1).

JP 2004-171981 A

  This invention is made | formed in view of the said subject, and it aims at providing the lithium ion secondary battery which improved the ionic conductivity of electrolyte solution and was excellent in the high rate characteristic.

  A lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator, and an electrolytic solution, and the electrolytic solution includes at least one of compounds represented by chemical formula (1) and chemical formula (2). To do.

〔化学式（１）において、Ｒ１、Ｒ２およびＲ３は炭素数１〜４のアルキル基を表し、Ｒ４、Ｒ５、Ｒ６およびＲ７は水素原子または炭素数１〜４のアルキル基を表す。〕

[In the chemical formula (1), R1, R2 and R3 represent an alkyl group having 1 to 4 carbon atoms, and R4, R5, R6 and R7 represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ]

〔化学式（２）において、Ｒ８およびＲ９は炭素数１〜４のアルキル基を表す。〕

[In chemical formula (2), R8 and R9 represent a C1-C4 alkyl group. ]

The SO ₃ CF ₃ group of the compound represented by the chemical formula (1) or the chemical formula (2) is arranged in the electrolytic solution, and further, lithium ions are attracted to the negative charge resulting from the electron withdrawing property of the CF ₃ group. It is estimated that the ionic conductivity is improved by the movement of lithium ions along the SO ₃ CF ₃ group. That is, it is estimated that the high rate characteristics are improved by smooth movement of lithium ions.

  The lithium ion secondary battery according to the present invention is characterized in that the above-described compound is contained in an amount of 0.1 to 10% by mass in the electrolytic solution.

  By setting the addition amount in the above range, it is possible to improve the high rate characteristics more effectively.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery excellent in the high rate characteristic can be provided.

It is a schematic cross section of a lithium ion secondary battery. It is the schematic diagram which showed the mechanism of the effect expression by containing the compound shown by Chemical formula (1). It is the schematic diagram which showed the mechanism of the effect expression by containing the compound shown by Chemical formula (2). It is a schematic diagram when it is easy to take a micelle structure such as an electrolyte solution solvent and a surfactant and it is difficult to take an array structure excellent in ionic conductivity.

  Hereinafter, preferred embodiments of the present embodiment will be described with reference to the drawings. However, the lithium ion secondary battery according to the present invention is not limited to the following embodiments. In addition, the dimensional ratio of drawing is not restricted to the ratio of illustration.

(Lithium ion secondary battery)
Next, the electrode and the lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a power generation element 30, a case 50 that houses the power generation element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 30.

  The power generation element 30 is configured such that a pair of electrodes 10 and 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a positive electrode current collector 12. In the negative electrode 20, a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. The positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18 contain an electrolyte solution. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

(Electrolyte)
The electrolytic solution contains at least one of a solvent, an electrolyte, and a compound represented by chemical formula (1) and chemical formula (2) as an additive compound.

〔化学式（１）において、Ｒ１、Ｒ２およびＲ３は炭素数１〜４のアルキル基を表し、Ｒ４、Ｒ５、Ｒ６およびＲ７は水素原子または炭素数１〜４のアルキル基を表す。〕

[In the chemical formula (1), R1, R2 and R3 represent an alkyl group having 1 to 4 carbon atoms, and R4, R5, R6 and R7 represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ]

〔化学式（２）において、Ｒ８およびＲ９は炭素数１〜４のアルキル基を表す。〕

[In chemical formula (2), R8 and R9 represent a C1-C4 alkyl group. ]

  It is preferable that the solvent contains a cyclic carbonate and a low viscosity solvent. The cyclic carbonate is characterized by having a dielectric constant of 20 or more so as to promote dissociation of lithium salt as a denatured material. A low viscosity solvent refers to an organic solvent having a viscosity of 1.0 cP or less so as to improve the mobility of lithium ions.

  As the cyclic carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used. Among them, ethylene carbonate is preferably included. You may mix and use ethylene carbonate with propylene carbonate and butylene carbonate.

  Further, as a low viscosity solvent, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2- Diethoxyethane or the like can be used, and two or more of these low-viscosity solvents may be mixed and used.

  The ratio of the cyclic carbonate and the low viscosity solvent in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

The electrolytes include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiPOF ₂ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN. (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) _{2 and the} like may be mentioned, and two or more kinds may be mixed and used. Good. In particular, it is preferable to include LiPF ₆ that increases conductivity.

When LiPF ₆ is dissolved in a non-aqueous solvent, the concentration of the electrolyte in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L. When the concentration of the electrolyte is 0.5 mol / L or more, the conductivity of the nonaqueous electrolytic solution can be sufficiently secured, and a sufficient capacity can be easily obtained during charging and discharging. In addition, by suppressing the electrolyte concentration to within 2.0 mol / L, it is possible to suppress an increase in the viscosity of the non-aqueous electrolyte, to sufficiently secure the mobility of lithium ions, and to obtain a sufficient capacity during charging and discharging. It becomes easy.

Even when LiPF ₆ is mixed with other electrolytes, the lithium ion concentration in the non-aqueous electrolyte is preferably adjusted to 0.5 to 2.0 mol / L, and the lithium ion concentration from LiPF ₆ is 50 mol%. More preferably, it is contained.

Hydrogen fluoride or fluorine ions produced by the reaction of LiPF ₆ with moisture is preferably 50 ppm or less, more preferably 30 ppm or less in the non-aqueous electrolyte. Therefore, the water content in the non-aqueous electrolyte is preferably 50 ppm or less, and more preferably 30 ppm or less.

  Although the mechanism of the effect expression by containing at least one of the compounds represented by the chemical formula (1) and the chemical formula (2) is not clear, the present inventors consider as follows.

FIG. 2 is a schematic diagram showing a mechanism of effect expression by containing the compound represented by the chemical formula (1). In the compound represented by the chemical formula (1), an aromatic ring is interposed between the Si atom and the SO ₃ CF ₃ group, and the interaction of high-density packing due to the π-electron conjugated system between the Si atoms and between the aromatic rings. Therefore, it is considered that an association state is easily obtained in the electrolytic solution. By forming such an association state, SO ₃ CF ₃ groups are also arranged, and lithium ions are attracted to the negative charge due to the electron withdrawing property of the CF ₃ groups, and the arranged SO ₃ CF ₃ groups are attracted to the arranged SO ₃ CF ₃ groups. It is presumed that the ionic conductivity is improved by the movement of lithium ions along the line.

FIG. 3 is a schematic diagram showing a mechanism of effect expression by containing the compound represented by the chemical formula (2). The compound represented by the chemical formula (2) has a structure in which SO ₃ CF ₃ groups are arranged symmetrically with Si atoms in between, and it is easy to assume an association state in the electrolyte solution due to the interaction and symmetrical structure of Si atoms. Conceivable. By forming such an association state, SO ₃ CF ₃ groups are arranged, and lithium ions are attracted to the negative charge due to the electron withdrawing properties of the CF ₃ groups, and the arranged SO ₃ CF ₃ groups are attracted to the arranged SO ₃ CF ₃ groups. It is presumed that the ionic conductivity is improved by the movement of lithium ions along the line.

On the other hand, in the structure where the SO ₃ CF ₃ group is not symmetrically arranged with the Si atom in between (for example, trimethylsilyl trifluoromethanesulfonate), it is easy to take a micellar structure such as an electrolyte solvent and a surfactant. It is thought that it is difficult to obtain an array structure with excellent conductivity. As a result, the movement of lithium ions is not smooth, and it is difficult to improve the high rate characteristics. FIG. 4 shows a schematic diagram.

  Thus, by using the compound of this embodiment as an additive, the ionic conductivity in the electrolytic solution is improved, that is, the movement of lithium ions is smoothed, thereby improving the high rate characteristics. I can do it.

  The content of the above-mentioned compound is preferably 0.1 to 10% by mass.

  By setting the addition amount in the above range, it is possible to improve the high rate characteristics more effectively.

(Positive electrode)
The positive electrode 10 includes a positive electrode active material layer 14 on both surfaces of a positive electrode current collector 12. Furthermore, the positive electrode active material layer 14 is formed by applying a paint containing a positive electrode active material, a conductive additive, and a binder to the positive electrode current collector 12.

The positive electrode active material is composed of lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a lithium ion and a counter anion (for example, PF ₆ ⁻ ) of the lithium ion. Is not particularly limited as long as it can reversibly proceed, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) or Li _a Ni _x Co _y Al _1-xy O ₂ (0.98 <a <1.2, 0 <x, y <1) A composite metal oxide, olivine-type LiMPO ₄ (where M is Co, Ni , Mn, Fe, or V)).

  The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, easily graphitizable carbon, and graphite, changing a shape.

  As carbon black, acetylene black, ketjen black and the like are particularly preferable, and ketjen black is particularly preferable. By including an electron conductive porous body, pores can be formed at the interface between the positive electrode active material material particles and the binder, and the penetration of the electrolyte into the positive electrode active material layer 14 is facilitated by the pores. preferable.

  The binder is not particularly limited as long as it can bind the particles of the positive electrode active material and the particles of the conductive additive. For example, a fluororesin such as polyvinylidene fluoride (PVDF) can be used. Further, the binder contributes not only to the binding of the particles of the positive electrode active material and the particles of the conductive auxiliary agent, but also to the binding to the positive electrode current collector 12.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use an aluminum foil.

(Negative electrode)
The negative electrode 20 includes a negative electrode active material layer 24 on both sides of a negative electrode current collector 22. Furthermore, the negative electrode active material layer 24 is formed by applying a paint containing a negative electrode active material, a conductive additive, and a binder to the negative electrode current collector 22.

The negative electrode active material includes at least one selected from carbon materials such as natural graphite, artificial graphite (non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, etc.), MCF (mesocarbon fiber), and the like. Among these, artificial graphite is preferable because it exhibits a favorable negative electrode capacity and cycle characteristics, and from the viewpoint of improving electrode density, it is more preferable to use artificial graphite mixed with natural graphite. In addition, for example, a metal capable of forming a compound with lithium such as Al, Si and Sn, an amorphous compound mainly composed of an oxide such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O _A known negative electrode active material such as ₁₂ ) may be mixed with a carbon material.

  Furthermore, the same material as that used for the positive electrode 10 can be used for each component (conductive aid, binder) other than the negative electrode active material. Therefore, the binder contained in the negative electrode 20 contributes not only to the binding of the particles of the positive electrode active material and the particles of the conductive additive, but also to the binding to the negative electrode current collector 22. Yes.

  As the negative electrode current collector 22, various known metal foils used for current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use a copper foil.

(Separator)
As long as the separator 18 is formed of an insulating porous material, the material, the manufacturing method, and the like are not particularly limited, and a known separator used in the lithium ion secondary battery 100 can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

  The case 50 seals the power generation element 30 and the electrolyte solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolyte solution to the outside and entry of moisture or the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the synthetic resin film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene or polypropylene.

  The leads 60 and 62 are made of a conductive material such as aluminum or nickel.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

  The lithium ion secondary batteries of Examples 1 to 21, Comparative Examples 1 to 3, and Reference Examples 1 to 2 were prepared and evaluated according to the following procedure.

Example 1
First, a positive electrode was produced. Also in the production of the positive electrode, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (90% by mass) as the positive electrode active material, carbon black (6% by mass) as the conductive additive, and PVDF (4 as the binder) (Mass%) was mixed and dispersed in NMP to obtain a slurry. The obtained slurry was applied to an aluminum foil as a current collector, dried, rolled, and a positive electrode was obtained.

  Next, a negative electrode was produced. In the production of the negative electrode, first, artificial graphite (90% by mass) as a negative electrode active material, carbon black (2% by mass) as a conductive auxiliary, and polyvinylidene fluoride (hereinafter referred to as PVDF) (8% by mass) as a binder. %) Was mixed and dispersed in N-methyl-2-pyrrolidone (hereinafter referred to as NMP) as a solvent to obtain a slurry. The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. After drying, rolling was performed to obtain a negative electrode.

Next, an electrolytic solution was prepared. LiPF ₆ was added at a rate of 1.0 mol / L in a solution in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7. In the entire electrolyte solution, 0.05 mass of the compound represented by the chemical formula (1) is added as a compound in which R1, R2, R3, and R4 are CH ₃ groups, and R5, R6, and R7 are hydrogen atoms. % Was added to obtain an electrolytic solution.

〔化学式（１）において、Ｒ１、Ｒ２、Ｒ３およびＲ４はＣＨ_３基を表し、Ｒ５、Ｒ６およびＲ７は水素原子を表す。〕

[In the chemical formula (1), R 1, R 2, R ₃ and R 4 represent a CH ₃ group, and R 5, R 6 and R 7 represent a hydrogen atom. ]

  A laminated body (element body) was obtained by laminating a separator made of polyethylene between the obtained negative electrode and positive electrode. The obtained laminate is placed in an aluminum laminate pack, a non-aqueous electrolyte is injected into the aluminum laminate pack, and then vacuum-sealed to produce a lithium ion secondary battery (length: 60 mm, width: 85 mm, thickness: 3 mm). did.

(Examples 2 to 21 and Comparative Examples 1 to 3)
As an additive compound contained in the electrolytic solution, R1 to R7 of the compound represented by the chemical formula (1) or R8 to R9 of the compound represented by the chemical formula (2) are changed as shown in Table 1, and the contents are shown in Table 1. Except for this change, lithium ion secondary batteries of Examples 2 to 21 were produced in the same manner as Example 1. Further, the lithium ion secondary batteries of Comparative Examples 1 to 3 were used in the same manner as in Example 1 except that the compounds represented by the chemical formulas (3) to (5) were used as additive compounds and the addition amount was 4.0% by mass. Produced.

〔化学式（２）において、Ｒ８およびＲ９は炭素数１〜４のアルキル基を表す。〕

[In chemical formula (2), R8 and R9 represent a C1-C4 alkyl group. ]

(Reference Example 1)
A lithium ion secondary battery of Reference Example 1 was produced in the same manner as in Example 4 except that the positive electrode active material was changed to LiCoO ₂ .

(Reference Example 2)
Except for changing the negative electrode active material material SiO ₂ was to prepare a lithium ion secondary battery of Example 2 in the same manner as in Example 14.

(Initial charge / discharge)
About the lithium ion secondary battery obtained in Examples 1-21, Comparative Examples 1-3, and Reference Examples 1-2, the first charge was performed in the environment set to 25 degreeC in the thermostat, and immediately after that First discharge was performed. The charging was performed at a constant current and constant voltage up to 4.2 V at 30 mA, and the discharging was performed at a constant current of 2.5 mA at 30 mA.

(Measurement of rate characteristics)
For lithium ion secondary batteries that have undergone initial charge / discharge, from 1C (current value at which discharge is completed in 1 hour when constant current discharge is performed at 25 ° C.) to 10C (when constant current discharge is performed at 25 ° C.) The discharge capacity was measured up to the current value at which discharge was completed in 0.1 hour. The ratio (%) of the discharge capacity at 5C when the discharge capacity at 1C is 100% was determined as the high rate characteristic. The results of Examples 1-21 and Reference Examples 1-2 are shown in Table 1, and the results of Comparative Examples 1-3 are shown in Table 2.

  From comparison between Examples 1 to 21 and Comparative Examples 1 to 3, the electrolyte solution can obtain high rate characteristics by containing at least one of the compounds represented by chemical formula (1) and chemical formula (2). Was confirmed.

  Moreover, by Examples 1-6 and Examples 11-16, electrolyte solution contains 0.1 mass%-10 mass% of at least one among the compounds shown by Chemical formula (1) and Chemical formula (2). It was confirmed that better high rate characteristics could be obtained.

  From Reference Examples 1 and 2, when the electrolytic solution contains at least one of the compounds represented by the chemical formula (1) and the chemical formula (2), high rate characteristics can be obtained regardless of the positive electrode active material or the negative electrode active material. It was confirmed that it was obtained.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... power generation element, 50 ... case, 52 ... metal foil, 54 ... polymer film, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (2)

In a lithium ion secondary battery having a positive electrode, a negative electrode, a separator and an electrolyte,
The lithium ion secondary battery, wherein the electrolytic solution contains at least one of compounds represented by chemical formula (1) and chemical formula (2).

[In the chemical formula (1), R1, R2 and R3 represent an alkyl group having 1 to 4 carbon atoms, and R4, R5, R6 and R7 represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ]

[In chemical formula (2), R8 and R9 represent a C1-C4 alkyl group. ]   The lithium ion secondary battery according to claim 1, wherein the compound is contained in the electrolytic solution in an amount of 0.1 to 10% by mass.

Images