JP4157055B2 - Gel polymer electrolyte and lithium secondary battery - Google Patents

Description

  The present invention relates to a gel polymer electrolyte and a lithium secondary battery, and particularly to a gel polymer electrolyte capable of improving battery characteristics such as liquid retention and charge / discharge characteristics of a non-aqueous electrolyte.

  2. Description of the Related Art Polymer electrolytes used for so-called polymer lithium secondary batteries are roughly classified into two types: physical polymer type and chemical polymer type. The physical polymer type is different in that a polymer that has been polymerized in advance is used as a starting material, whereas the chemical polymer type is that an unpolymerized monomer or oligomer is used as a starting material, and these are polymerized together with a non-aqueous liquid. That is, a physical polymer type polymer electrolyte forms a polymer / electrode composite by dissolving a raw material polymer in a solvent to form a solution, and applying the solution to the positive electrode and the negative electrode and then removing the solvent. The polymer electrode assembly is impregnated with a non-aqueous electrolyte. On the other hand, a chemical polymer type polymer electrolyte has a positive electrode and a negative electrode configured beforehand, and a mixed solution consisting of a monomer or oligomer as a raw material and a non-aqueous electrolyte is added to these positive and negative electrodes, It is formed by polymerizing monomers or oligomers by polymerization or the like.

As examples of physical polymer types, for example, as described in Patent Document 1 and Patent Document 2 below, a number of types in which a fluorine-based sheet polymer is formed and this sheet polymer is impregnated with a non-aqueous electrolyte are proposed. Has been. Moreover, as an example of the chemical polymer type, as described in Patent Document 3 below, for example, a polymer electrolyte made of polyethylene oxide (PEO) or polypropylene oxide (PPO) is known.
Japanese Patent Laid-Open No. 9-97618 JP 2000-17124 A JP 2001-155773 A

  However, in the conventional fluorinated physical polymer type polymer electrolyte, it is difficult to completely hold the nonaqueous electrolyte in the polymer due to the low interface resistance unique to the fluorinated polymer. There was a problem of bleeding.

  On the other hand, fluoropolymers are promising as materials for polymer electrolytes because they have low resistance to lithium ion transport due to low interface resistance, and are also excellent in insulation and chemical resistance. For this reason, conventional PEO or PPO Attempts have been made to introduce a fluorine-based polymer into a polymer electrolyte of a chemical polymer type. However, this case has the following four problems. That is, first, due to the low interfacial resistance unique to the fluoropolymer, it does not mix well with the non-aqueous electrolyte. Second, it cannot be polymerized with PEO or the like due to the high electronegativity of fluorine. Thirdly, the mechanical strength inherent to substrate monomers such as PEO decreases with the introduction of the fluoropolymer. Fourth, the liquid retention of the non-aqueous electrolyte after polymerization is greatly reduced.

  The present invention has been made in view of the above circumstances, and is excellent in liquid retention of a nonaqueous electrolytic solution, excellent in mechanical strength, and excellent in lithium ion conductivity and battery characteristics such as charge / discharge characteristics. An object of the present invention is to provide a gel polymer electrolyte that can be improved, and a lithium secondary battery including the gel polymer electrolyte.

In order to achieve the above object, the present invention employs the following configuration.
The gel polymer electrolyte of the present invention is a gel polymer electrolyte capable of conducting lithium ions, and comprises 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester, 2-propenoic acid α, ω-poly. A substrate monomer selected from (oxy-1 / 2-methyl-2,1-ethanediyl) ester or 1,1,1-tris [propenoyloxypoly (ethyleneoxy) methyl] propane and the following [Chemical Formula 1] A matrix polymer obtained by polymerizing a fluorine-based monomer having the structure shown in FIG. 6 and a nonaqueous electrolytic solution are mixed. However, in the above [Chemical Formula 1], R ¹ is H or CH ₃ , R ² is H or F, and n is in the range of 1 or more and 10 or less.

  Moreover, the gel polymer electrolyte of the present invention is the gel polymer electrolyte described above, and the content of the fluorinated monomer with respect to the total of the substrate monomer and the fluorinated monomer is from 0.1% by mass to 30% by mass. It is a range.

  The gel polymer electrolyte of the present invention is the gel polymer electrolyte described above, and the content of the substrate monomer with respect to the total of the substrate monomer and the nonaqueous electrolytic solution is 0.01% by mass or more and 20% by mass or less. It is a range.

  Next, the lithium secondary battery of the present invention is the gel polymer electrolyte described above, and includes the positive electrode, the negative electrode, and the gel polymer electrolyte described in any one of the above.

  According to the gel polymer electrolyte of the present invention, the liquid retainability of the nonaqueous electrolytic solution can be maintained as in the conventional case, the mechanical strength is not lowered, and the lithium ion conductivity can be increased. In addition, according to the lithium secondary battery of the present invention, it has a gel polymer electrolyte excellent in mechanical strength and liquid retention of a non-aqueous electrolyte, and exhibits high lithium ion conductivity. It can be demonstrated.

Hereinafter, embodiments of the present invention will be described in detail.
The lithium secondary battery of the present embodiment is roughly composed of the gel polymer electrolyte according to the present invention, and a positive electrode and a negative electrode capable of inserting and extracting lithium. The gel polymer electrolyte includes at least a matrix polymer and a nonaqueous electrolytic solution. Further, the gel polymer electrolyte may be partially contained in the positive electrode and the negative electrode.

  The gel polymer electrolyte according to the present invention is disposed between the positive electrode and the negative electrode and has a function of conducting lithium ions. The gel polymer electrolyte also has a function as a separator. That is, the gel polymer electrolyte according to the present invention has a function of isolating the positive electrode and the negative electrode in place of the conventional polyolefin separator. Of course, the gel polymer electrolyte according to the present invention and the conventional separator may be used in combination. As a separator in this case, a porous polypropylene film, a porous polyethylene film, etc. can be used suitably.

  The gel polymer electrolyte according to the present invention includes a matrix polymer obtained by polymerizing a substrate monomer having one or both of an acrylate group and a methacrylate group and a fluorine-based monomer having a structure represented by the above [Chemical Formula 1]; It is formed by mixing with a water electrolyte. In this gel polymer electrolyte, the matrix polymer is gelled by impregnating the matrix polymer with the nonaqueous electrolytic solution, and the nonaqueous electrolytic solution is held in the matrix polymer. In addition, the polymer electrolyte according to the present invention forms a gelled matrix polymer by polymerizing a mixture of the matrix polymer raw material monomer and the non-aqueous electrolyte, and the non-aqueous electrolyte is held in the matrix polymer. ing.

  The matrix monomer constituting the matrix polymer may be any monomer as long as it has a methacrylate group, but preferably 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester) (2 -Propenoic acid α, ω-poly (oxy-2,1-ethandiyl) ester) is preferably used. In addition, 2-propenoic acid α, ω-poly (oxy-1 / 2-methyl-2,1-ethanediyl) ester or 1,1,1-tris [propenoyloxypoly (ethyleneoxy) methyl] as a substrate polymer Propane or the like may be used. The nonaqueous electrolyte is retained mainly by the substrate polymer.

Next, the fluorine monomer constituting the matrix polymer is preferably a monomer having the structure shown in the above [Chemical Formula 1]. However, in the above [Chemical Formula 1], R ¹ is H or CH ₃ , R ² is H or F, and n is in the range of 1 or more and 10 or less.

The fluorine-based monomer represented by [Chemical Formula 1] has a double bond bonded to the R ¹ group, and can be radically polymerized with the substrate monomer by this double bond to form a matrix polymer. Thereby, a fluorine-type polymer can be uniformly disperse | distributed and contained in a matrix polymer.
The fluorine-based monomer according to the present invention has a methylene fluoride group (CF ₂ ) having a fluorine atom in the molecule, and the methylene fluoride group itself has a characteristic of low interface resistance. . For this reason, the resistance of the fluorine-based monomer itself to the transport of lithium ions is reduced, and the lithium ion conductivity of the gel polymer electrolyte can be increased. Thereby, the battery characteristic of a lithium secondary battery can be improved. However, if the methylene fluoride group is too long, microscopic uniform arrangement becomes difficult, and the effect of addition is diminished. Therefore, n in [Chemical Formula 1] is preferably in the range of 1 to 10.
Furthermore, the fluorine monomer according to the present invention does not impair the mechanical strength of the gel polymer electrolyte. Moreover, the fluorine-type monomer which concerns on this invention can unite with a substrate monomer, can form a gel-like electrolyte, and can hold | maintain a non-aqueous electrolyte. In addition, when an electrolyte is formed with a fluorinated monomer alone, the fluorinated monomer forms a spherical polymer, making it difficult to retain the nonaqueous electrolytic solution.

  The content of the fluorinated monomer with respect to the total of the substrate monomer and the fluorinated monomer is preferably in the range of 0.3% by mass to 30% by mass. If the fluorine monomer content is less than 0.3% by mass, the effect of adding the fluorine polymer will not be sufficiently exhibited, which is not preferable. If the content exceeds 30% by mass, the liquid retainability of the non-aqueous electrolyte decreases. Therefore, it is not preferable.

  Moreover, it is preferable that the content rate of the substrate monomer with respect to the sum total of a substrate monomer and a non-aqueous electrolyte is the range of 0.01 mass% or more and 20 mass% or less. If the content of the substrate monomer is less than 0.01% by mass, the non-aqueous electrolyte cannot be sufficiently contained, and the influence of the addition of the polymer on the battery characteristics is not seen. If it exceeds 50%, the content of the non-aqueous electrolyte is relatively lowered, and the ionic conductivity of lithium ions is lowered.

Next, examples of the nonaqueous electrolytic solution constituting the gel polymer electrolyte include an organic electrolytic solution in which a lithium salt is dissolved in an aprotic solvent.
Examples of aprotic solvents include propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl Sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate , Diethylene glycol, dimethyl ether Aprotic solvents such as, or a mixed solvent obtained by mixing two or more of these solvents, and further known as a solvent for a lithium secondary battery, and in particular, propylene carbonate, ethylene carbonate, What contains any one of butylene carbonate and any one of dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate is preferable.

The lithium _{salt, LiPF 6, LiBF 4, LiSbF} 6, LiAsF 6, LiClO 4, LiCF 3 SO 3, Li (CF 3 SO 2) 2 N, LiC 4 F 9 SO 3, LiSbF 6, LiAlO 4, LiAlCl 4 _{, LiN (C x F 2x +} 1 SO 2) (C y F 2y _tens 1 _SO 2) (provided that x, y are natural numbers), LiCl, made by mixing one or more lithium salts of such LiI And those conventionally known as lithium salts for lithium secondary batteries, and those containing any one of LiPF ₆ and LiBF ₄ are particularly preferred.

Next, as the positive electrode, a positive electrode active material, a binder, and further a conductive additive as necessary are mixed and applied to a current collector made of a metal foil or a metal net to form a sheet. Can be illustrated. Moreover, what mixed the positive electrode active material, the binder, and also the conductive support material as needed, and shape | molded these to the pellet form can also be illustrated.
Examples of the positive electrode active material include LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , V ₂ O ₅ , TiS, MoS, and the like, and materials that can occlude and release lithium, such as organic disulfide compounds and organic polysulfide compounds. It can be illustrated.

In addition, as the negative electrode, a negative electrode active material, a binder, and, if necessary, a conductive additive are mixed and applied to a current collector made of a metal foil or a metal net and formed into a sheet shape. Can be illustrated. Moreover, what mixed the negative electrode active material, the binder, and the conductive support material as needed, and shape | molded these in the pellet form can also be illustrated.
As the negative electrode active material, those capable of reversibly occluding and releasing lithium ions are preferable, and examples thereof include artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon and the like. Metallic lithium can also be used as the negative electrode.

First, the lithium secondary battery of this embodiment can be manufactured by laminating a gel polymer electrolyte previously formed into a sheet shape, a positive electrode and a negative electrode, and optionally adding a non-aqueous electrolyte.
In this first production method, in order to obtain a gel polymer electrolyte, a predetermined amount of a substrate monomer, a fluorine-based monomer, and a nonaqueous electrolytic solution are mixed, and a known peroxide as a radical polymerization initiator is added to this mixture. The substrate monomer and the fluorine monomer are radically polymerized by light irradiation or heating to form a matrix polymer, and at the same time, the matrix polymer is gelled with a non-aqueous electrolyte.

Secondly, the lithium secondary battery of this embodiment is formed by laminating a separator, a positive electrode, and a negative electrode to form a laminate, and the laminate is impregnated with a mixture of a non-aqueous electrolyte, a substrate monomer, and a fluorine monomer. It can be produced by polymerizing a substrate monomer and a fluorine monomer to form a gel polymer electrolyte. In the first manufacturing method described above, a separator may or may not be used, but in the second method, a separator is essential to separate the positive electrode and the negative electrode.
Specifically, in the second method, for example, a laminate composed of a positive electrode, a negative electrode, and a separator is housed in a battery container, and the mixture containing the constituent material of the gel polymer electrolyte is further injected into the battery container. Thus, the gel polymer electrolyte may be formed by radical polymerization inside the battery container. In this way, by pre-injecting the electrolyte constituent material including the non-aqueous electrolyte into the battery container, the electrolyte constituent material penetrates to the inside of the negative electrode and the positive electrode, and radical polymerization is performed in this state. In addition, the non-aqueous electrolyte can always be held inside the negative electrode, and the charge / discharge reaction can proceed smoothly.

  Furthermore, the lithium secondary battery of the present embodiment thirdly, a non-aqueous electrolyte, a mixture of a substrate monomer and a fluorine monomer is applied to the surfaces of the positive electrode and the negative electrode, respectively, and then the substrate monomer and the fluorine monomer are applied. It can also be produced by forming a gel polymer electrolyte on the surface of the positive and negative electrodes by polymerization and then bonding the gel polymer electrolytes together. In the third manufacturing method, the separator may or may not be used.

Hereinafter, the present invention will be described in more detail with reference to specific examples and comparative examples in which the present invention is applied to an aluminum pouch enclosed polymer battery. Needless to say, the present invention is not limited to the examples shown below.
(Preparation of positive and negative electrodes)
A positive electrode mixture was prepared by mixing 91% by mass of LiCoO ₂ , 6% by mass of graphite as a conductive agent, and 3% by mass of PVdF as a binder, and this was prepared as N-methyl-2-pyrrolidone (NMP). To make a slurry. And this slurry was apply | coated to the single side | surface of the aluminum foil which is a positive electrode electrical power collector, and after drying, it compression-formed with the roller press machine, and manufactured the positive electrode sheet (positive electrode).
Further, 90% by mass of graphite and PVdF as a binder at a ratio of 10% by mass were mixed to prepare a negative electrode mixture, which was dispersed in NMP to obtain a slurry. And this slurry was apply | coated to the single side | surface of copper foil which is a negative electrode collector, and after application | coating, it compression-formed with the roller press machine, and manufactured the negative electrode sheet (negative electrode).

(Preparation of constituent material of gel polymer electrolyte)
Non-aqueous solution prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) as a solvent in a volume ratio of EC: DEC = 2: 8, and further mixing LiPF ₆ as a lithium salt at a ratio of 1 mol / L. An electrolyte solution was prepared. This was designated as EL-0.
Next, 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester is used as a substrate monomer such that EL-0: substrate monomer = 95: 5 in terms of mass ratio with EL-0. Into EL-0, the mixture was sufficiently stirred to obtain a uniform solution, which was designated as EL-1 (Comparative Example).

Further, 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester as a substrate monomer and 2,2,2-trifluoroethyl methacrylate (the above [Chemical Formula 1] above as R ¹ ) as a fluorine monomer. H, R ² is F and n is 2), and these mixed monomers are EL so that EL-0: mixed monomer = 95: 5 in terms of mass ratio with EL-0. It was blended with -0 and sufficiently stirred to obtain a homogeneous solution, which was designated as EL-2 (Example). In addition, the mixing ratio of the substrate monomer and the fluorine monomer was mass ratio, and the substrate monomer: fluorine monomer = 9: 1.

In addition, 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester as a substrate monomer and CH ₂ ═CHCOOCH ₂ (CF ₂ ) ₇ CF ₃ (in the above [Chemical Formula 1] R as a fluorine-based monomer) ¹ is H, R ² is F, and n is 8. The mixture is mixed with Osaka Organic Stock Co., Ltd. Biscoat 17F), and these mixed monomers are mixed with EL-0 in a mass ratio of EL-0. : Mixed monomer = 95: 5 was blended in EL-0 and stirred well to obtain a homogeneous solution, which was designated as EL-3 (Example). In addition, the mixing ratio of the substrate monomer and the fluorine monomer was mass ratio, and the substrate monomer: fluorine monomer = 9: 1.

(Manufacture of lithium secondary batteries)
A positive electrode, a negative electrode, and a polypropylene separator are stacked in an aluminum laminate, and the solutions of EL-1 (Comparative Example), EL-2 (Example), and EL-3 (Example) are respectively added. The solution was poured, and a peroxide bis- (4-t-butylcyclohexyl) peroxydicarbonate was added as a polymerization initiator and heated at 70 ° C. for 4 hours to form a gel polymer electrolyte. Lithium secondary batteries PL-1 (comparative examples), PL-2 (examples), and PL-3 (examples) were produced. PL-1, PL-2, and PL-3 are manufactured from EL-1, EL-2, and EL-3, respectively.

  Further, the solutions of EL-1, EL-2, and EL-3 are respectively poured into different aluminum laminate materials, and bis- (4-t-butylcyclohexyl) peroxydi as a peroxide as a polymerization initiator. A gel polymer electrolyte for measuring mechanical strength was prepared by adding carbonate and heating at 70 ° C. for 4 hours.

With respect to the above polymer lithium secondary batteries PL-1, PL-2, and PL-3, discharge rate characteristics and cycle characteristics were measured. The discharge rate characteristics were as follows: 0.2C discharge capacity was measured at charge current 0.5C and discharge current 0.2C in the first cycle, and 2C discharge was performed at charge current 0.5C and discharge current 2C in the second cycle. The capacity was measured, and the ratio of the 2C discharge capacity to the 0.2C discharge capacity was defined as the discharge rate value. The cycle characteristics were determined by repeatedly charging and discharging under the conditions of charge current 1C and charge current 1C, and determining the ratio of the discharge capacity at the 400th cycle to the discharge capacity at the first cycle as the cycle characteristics. The results are shown in Table 1.
Furthermore, the mechanical strength of the gel polymer electrolyte for measuring the mechanical strength was measured using an F-type durometer manufactured by Kobunshi Keiki Co., Ltd. The results are also shown in Table 1.

  As shown in Table 1, the batteries of PL-2 and PL-3 (both examples) having a fluorinated monomer are compared to the PL-1 battery (comparative example) to which no fluorinated monomer is added, It can be seen that both the discharge rate and the cycle characteristics show good values. This is presumably because the fluorine-based polymer has a methylene fluoride group in the molecule, so the interface resistance is low, the resistance to lithium ion transport is reduced, and the lithium ion conductivity is improved.

  Moreover, about gel strength, it turns out that PL-2 and PL-3 show the intensity | strength excellent compared with PL-1. This difference in gel strength is also thought to be due to the addition effect of the fluoropolymer.

As described above, according to the gel polymer electrolyte according to the present invention, the battery performance of the lithium secondary battery can be improved because of excellent mechanical strength and good lithium ion conductivity. In particular, by setting the fluorine-based monomer in the range of 0.1% by mass to 30% by mass with respect to the total of the substrate monomer and the fluorine-based monomer, excellent gel strength can be obtained, and lithium ion conductivity can be improved. It can be seen that it can be improved.

Claims (4)

A gel polymer electrolyte capable of conducting lithium ions, comprising 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester, 2-propenoic acid α, ω-poly (oxy-1 / 2-methyl) A substrate monomer selected from -2,1-ethanediyl) ester or 1,1,1-tris [propenoyloxypoly (ethyleneoxy) methyl] propane, and a fluorine monomer having the structure shown in the following [Chemical Formula 1] A gel polymer electrolyte, wherein a matrix polymer obtained by polymerizing and a non-aqueous electrolyte is mixed.

However, in the above [Chemical Formula 1], R ¹ is H or CH ₃ , R ² is H or F, and n is in the range of 1 or more and 10 or less.   2. The gel polymer electrolyte according to claim 1, wherein a content of the fluorine monomer with respect to a total of the substrate monomer and the fluorine monomer is in a range of 0.1% by mass to 30% by mass.   The gel according to claim 1 or 2, wherein the content of the substrate monomer with respect to the total of the substrate monomer and the non-aqueous electrolyte is in the range of 0.01% by mass to 20% by mass. Polymer electrolyte.   A lithium secondary battery comprising a positive electrode, a negative electrode, and the gel polymer electrolyte according to any one of claims 1 to 3.