JP2005235684A - Gel polymer electrolyte and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gel polymer electrolyte in which deterioration of battery performance due to hydrogen fluoride can be prevented surely, and provide a lithium secondary battery in which this gel polymer electrolyte is equipped. <P>SOLUTION: This is the gel polymer electrolyte in which lithium ions can be conducted, and this is adopted while a matrix polymer formed by polymerizing substrate monomers having either one or both of an acrylate group and a methacrylate group and a reactive monomer having a dioxolane ring and a non-aqueous electrolytic solution are mixed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a gel polymer electrolyte and a lithium secondary battery, and more particularly to a gel polymer electrolyte that improves battery characteristics and has little change in mechanical strength with time.

In the electrolyte solution of a lithium secondary battery, a salt containing fluorine such as LiPF ₆ is generally used as a lithium salt. This LiPF ₆ reacts with moisture as shown in the following formula, and is fluorinated. It has the property of producing halogen acids such as hydrogen.

2LiPF ₆ + 12H ₂ O → 12HF + 2LiP (OH) ₆ (1)

  The above reaction is likely to proceed when, for example, a trace amount of water is mixed in the electrolyte solution of the lithium secondary battery, and the lithium secondary battery is used or left at a high temperature. Conventionally, the generated hydrogen fluoride HF has a problem that battery constituent materials such as a current collector and a positive electrode active material are deteriorated. Deterioration of battery constituent materials often adversely affects battery performance such as an increase in internal resistance of the battery. Further, in a lithium secondary battery using a polyethylene oxide (PEO) -based polymer electrolyte, there is a problem that the polymer matrix is cut by the generated HF and the polymer electrolyte is liquefied.

Conventionally, with respect to the above problems caused by hydrogen fluoride, as described in the following Patent Documents 1, 2, and 3, respectively, the generation of hydrogen fluoride is suppressed or the generated hydrogen fluoride is captured ( (Neutralization) has been taken to reduce the adverse effects of hydrogen fluoride as much as possible.
JP 11-354153 A JP 2002-343364 A JP 09-245832 A

In Patent Document 1, a lithium imide salt is used instead of LiPF ₆ as a lithium salt, or a mixture of a fluorine-containing salt such as LiPF ₆ and a lithium imide salt is used to suppress generation of hydrogen fluoride. ing. However, since lithium imide salt has a higher thermal decomposition temperature than LiPF ₆ , the effect of suppressing the generation of HF may be obtained, but the battery characteristics have a problem that it does not reach that of a battery using LiPF ₆ salt.
Further, in Patent Document 2, the reactivity of hydrogen fluoride HF is suppressed by reacting the generated HF with SiO ₂ . Further, in Patent Document 3, HF is neutralized by adding 1,4,8,11-tetraazacyclotetradecane, and the reactivity of hydrogen fluoride HF is suppressed. These Patent Documents 2 and 3 also seem to have an HF suppressing effect, but there is anxiety on the battery performance such as the reactivity of the additive and the stability of the reaction product.

  The present invention has been made in view of the above circumstances, and includes a gel polymer electrolyte that can reliably prevent deterioration of battery performance due to hydrogen fluoride, and has a small decrease in mechanical strength due to liquefaction, and the gel polymer electrolyte. An object of the present invention is to provide a lithium secondary battery.

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 has a substrate monomer having one or both of an acrylate group and a methacrylate group and a dioxolane ring having a structure shown in the following [Chemical Formula 1]. A matrix polymer obtained by polymerizing a reactive monomer and a non-aqueous electrolyte are mixed.
However, in Chemical Formula 1], ^{R 1} is a cyclohexane ring leads CH ₃ or ^{R 2, R 2} is _{CH 3, C 2 H 5,} CH (CH 3) 2 or ^{R 1} and of the cyclohexane ring leads, And R ³ is H or CH ₃ .

  The gel polymer electrolyte of the present invention is the gel polymer electrolyte described above, and the content of the reactive monomer with respect to the total of the substrate monomer and the reactive monomer is in the range of 3% by mass to 30% by mass. It is characterized by being.

  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 0.1% 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 a positive electrode, a negative electrode, and the gel polymer electrolyte according to any one of claims 1 to 3. It is characterized by that.

  According to the gel polymer electrolyte of the present invention, battery performance deterioration due to hydrogen fluoride can be reliably prevented.

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 nonaqueous electrolytic solution is impregnated not only in the gel polymer electrolyte but also 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 comprises a matrix polymer obtained by polymerizing a substrate monomer having one or both of an acrylate group and a methacrylate group and a reactive monomer having a dioxolane ring having the structure shown in the above [Chemical Formula 1]. It is formed by mixing with a non-aqueous 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 having either one or both of an acrylate group and a methacrylate group, but preferably 2-propemenoic acid α, ω-poly (oxy-2, It is preferred to use 1-ethanediyl) ester. In addition, 2-propemenoic acid α, ω-poly (oxy-1 / 2-methyl-2,1-ethanediyl) ester and 1,1,1-tris [propenoyloxypoly (ethyleneoxy) methyl Propane or the like may be used. The non-aqueous electrolyte is retained mainly by this matrix polymer.

Next, the reactive monomer constituting the matrix polymer is preferably a monomer having the structure shown in the above [Chemical Formula 1] having a dioxolane ring. However, in the above Chemical Formula 1], ^{R 1} is a cyclohexane ring leads CH ₃ or ^{R 2, R 2} is _{CH 3, C 2 H 5,} CH (CH 3) 2 or ^{R 1} and lead the cyclohexane ring, R ³ is H or CH ₃ .

The reactive monomer shown in [Chemical Formula 1] has a double bond bonded to the R ³ group, and can be radically polymerized with the substrate monomer by this double bond to form a matrix polymer. In this way, the reactive polymer can be uniformly dispersed in the matrix polymer.
In addition, the reactive monomer according to the present invention has a dioxolane ring structure in the molecule, and this dioxolane ring has a property of cationic polymerization, such as hydrogen fluoride produced by decomposition of LIPF ₆ or the like. The ring can be easily opened by the free acid, and the free acid can be captured (neutralized). Thereby, deterioration of the battery constituent material due to HF or the like can be prevented and battery characteristics can be improved.
Furthermore, since the above dioxolane ring structure undergoes cationic polymerization in the presence of a free acid and polymerizes with other dioxolane rings, it can capture HF and further increase the mechanical strength of the matrix polymer. Thereby, liquefaction of gel polymer electrolyte can be prevented.

The dioxolane ring is cleaved only by reacting with hydrogen fluoride, and does not contribute to the polymerization reaction at the time of polymerization of the reactive monomer and the substrate monomer.
An epoxy ring is expected to have the same effect as a dioxolane ring. The dioxolane ring has a property that the energy level required for ring opening is higher than that of an epoxy ring. Thereby, there is no possibility that the dioxolane ring is cleaved during the formation of the gel polymer electrolyte, and the cleavage reaction of the dioxolane ring can be caused when hydrogen fluoride is generated.

  The content of the reactive monomer with respect to the total of the substrate monomer and the reactive monomer is preferably in the range of 3% by mass to 30% by mass. If the content of the reactive monomer is less than 3% by mass, it is not preferable because the free acid such as HF cannot be completely captured. If the content exceeds 30% by mass, the content of the substrate monomer is relatively reduced. However, it is not preferable because the non-aqueous electrolyte cannot be sufficiently contained.

  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 0.1 mass% or more and 20 mass% or less. If the content of the substrate monomer is less than 0.1% 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. Exceeding% by mass is not preferable because the content of the nonaqueous electrolytic solution 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.
As aprotic solvents, 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 Examples include aprotic solvents such as ethers, mixed solvents in which two or more of these solvents are mixed, and those conventionally known as solvents for lithium secondary batteries, particularly propylene carbonate, ethylene carbonate And any one of butylene carbonate and one of dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate are preferred.

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 reactive 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 reactive 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 the present embodiment secondly, a separator, a positive electrode, and a negative electrode are laminated to form a laminate, and the laminate is impregnated with a mixture composed of a non-aqueous electrolyte, a substrate monomer, and a reactive monomer. And the substrate monomer and the reactive monomer are polymerized 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.

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 give 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-methyl-2ethyl-1,3-dioxolan-4-yl) methyl acrylate as a reactive monomer (Medol 10 manufactured by Osaka Organic Chemical Co., Ltd.) was mixed, and these mixed monomers were blended into EL-0 such that EL-0: mixed monomer = 95: 5 in a mass ratio with EL-0, and stirred sufficiently. This was used as a homogeneous solution, which was designated as EL-2 (Example). The mixing ratio of the substrate monomer and the reactive monomer was mass ratio, and the substrate monomer: reactive monomer = 9: 1.

  Further, 2-propenoic acid α, ω-poly (oxy-2,1-ethanediyl) ester as a substrate monomer and (2-methyl-2ethyl-1,3-dioxolan-4-yl) methyl methacrylate as a reactive monomer (Medol 30 manufactured by Osaka Organic Co., Ltd.), and these mixed monomers are blended into EL-0 so that EL-0: mixed monomer = 95: 5 in terms of mass ratio with EL-0 described above. The mixture was stirred to obtain a homogeneous solution, which was designated as EL-3 (Example). The mixing ratio of the substrate monomer and the reactive monomer was mass ratio, and the substrate monomer: reactive 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 mixture was poured, and 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, the discharge rate characteristics and the low temperature discharge characteristics were measured. As for the discharge rate characteristics, 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 low-temperature discharge characteristics are as follows: charge current 0.5C at 20 ° C at the first cycle, 1C discharge capacity at 20 ° C at the discharge current 1C, charge at 0.5C charge current at 20 ° C in the first cycle 1C discharge capacity at −20 ° C. was measured at −20 ° C. with discharge current 1C, and the ratio of 1C discharge capacity at −20 ° C. to 1C discharge capacity at 20 ° C. was defined as a low temperature discharge characteristic value. .
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. This gel strength is shown by the ratio (immediately after polymerization / after 20 days) between the strength immediately after polymerization (gelation) and the strength after 20 days have passed since polymerization (gelation). The results are also shown in Table 1.

As shown in Table 1, the batteries of PL-2 and PL-3 (both examples) having reactive monomers are compared to the battery of PL-1 (comparative example) to which no reactive monomers are added, It can be seen that both the discharge rate and the low temperature discharge characteristics show good values. Furthermore, regarding gel strength, it can be seen that PL-2 and PL-3 are superior to PL-1 in strength durability.
In the gel polymer electrolyte of PL-2 and PL-3 batteries, a dioxolane ring for capturing hydrogen fluoride that adversely affects battery characteristics is uniformly introduced. For this reason, in the batteries of PL-2 and PL-3, HF generated by the decomposition of LiPF ₆ is captured by the dioxolane ring, thereby preventing the deterioration of the battery constituent material and the deterioration of the battery characteristics. it is conceivable that. Furthermore, since the dioxolane ring itself was cleaved during the capture of HF and the matrix polymer was further polymerized, it is considered that the mechanical strength was improved and the durability of the gel was improved.

As described above, according to the gel polymer electrolyte according to the present invention, it is possible to eliminate the influence of a free acid such as hydrogen fluoride without deteriorating battery performance. In addition, by setting the reactive monomer in the range of 3% by mass or more and 30% by mass or less with respect to the total of the substrate monomer and the reactive monomer, an initial gel strength equivalent to that of the substrate monomer alone can be obtained, and sufficient It can be seen that the effect of suppressing gel strength reduction can be obtained.

Claims (4)

A gel polymer electrolyte capable of conducting lithium ions, wherein a substrate monomer having one or both of an acrylate group and a methacrylate group and a reactive monomer having a dioxolane ring having a structure shown in the following [Chemical Formula 1] are polymerized A gel polymer electrolyte obtained by mixing a matrix polymer and a nonaqueous electrolytic solution.

However, in the above Chemical Formula 1], ^{R 1} is a cyclohexane ring leads CH ₃ or ^{R 2, R 2} is _{CH 3, C 2 H 5,} CH (CH 3) 2 or ^{R 1} and lead the cyclohexane ring, R ³ is H or CH ₃ .   2. The gel polymer electrolyte according to claim 1, wherein a content of the reactive monomer with respect to a total of the substrate monomer and the reactive monomer is in a range of 3% by mass to 30% by mass.   The gel according to claim 1 or 2, wherein a content of the substrate monomer with respect to a total of the substrate monomer and the non-aqueous electrolyte is in a range of 0.1% by mass or more and 20% by mass or less. 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.