KR100378348B1 - Network polymer, method for producing the same, and solid polyelectrolyte comprising the same - Google Patents

Abstract

PURPOSE: Provided are a polymer used for polymer matrix of solid polyelectrolyte in Li secondary battery, which has a flexible structure allowing Li ion to smoothly transfer, a method for producing the same, and a solid polyelectrolyte comprising the same. CONSTITUTION: The net polymer is a poly(vinylidene fluoride-hexafluoropropylene)(PVDF-HFP) crosslinked by poly(ethylene oxide)dimethacrylate(PEODM) which is a crosslinking agent. A weight ratio of the poly(vinylidene fluoride-hexafluoropropylene) copolymer and the crosslinking agent is 10:1-2:1. The PVDF-HFP copolymer has a weight average molecular weight of 60,000-80,000.

Description

Mesh-like polymer, preparation method thereof and polymer solid electrolyte containing same

The present invention relates to a mesh-like polymer, a method for manufacturing the same, and a polymer solid electrolyte containing the same, and more particularly, a mesh-like polymer forming a matrix of a polymer solid electrolyte, such as a secondary battery, which is widely used as a power source for an electronic device. It relates to a manufacturing method and a polymer solid electrolyte containing the same.

Recently, with the development of electronic devices, especially portable electronic devices, secondary batteries for use as driving power thereof have been remarkably developed. Among them, Li secondary batteries are attracting the most attention because of their excellent characteristics such as high operating voltage, long use time and high energy density. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer solid electrolyte for use in electrochemical devices such as Li secondary batteries, and more particularly to a solid electrolyte having excellent ion conductivity of Li and a network polymer used as a matrix polymer of the electrolyte.

The ionic conductivity of the solid electrolyte greatly affects the internal resistance during charging and discharging of the battery and further affects the efficiency and rate of the battery. Therefore, the solid electrolyte of the Li battery should basically be able to prevent the short circuit of the battery, and to maintain a high ionic conductivity by moistening a large amount of electrolyte solution and to enable the smooth movement of Li ions.

As a polymer matrix of a solid electrolyte of a conventional Li secondary battery, it is common to crosslink polyethlene oxide (PEO) and use it as a main chain, as in US Pat. Nos. 4,758,483 and 4,792,504, US4,908,284. PEO is easy to prepare and easy to prepare in large quantities, but the ionic conductivity of the solid electrolyte comprising the same is low at room temperature of 10 ⁻⁵ S / cm or less. Therefore, there is a disadvantage that can not be used at room temperature and can be used only at 60 ℃ or more, usually 100 ℃ or more. That is, although it can be used as a solid electrolyte for high temperature, it has not been put to practical use because of a fatal disadvantage that cannot be used at room temperature where general electronic devices are used.

Subsequently, a poly (vinylidene fluoride) (PVDF) polymer and a solid electrolyte having a copolymer of trifluoroethylene or tetrafluoroethylene as a polymer matrix as the fluorocarbon copolymer were used, but these were also room temperature to room temperature. At lower temperatures, it is difficult to maintain uniformity, and thus the ion conductivity is 10 ⁻⁵ S / cm or less, so that it is difficult to realize practical use.

Meanwhile, US 5,418,091 discloses a method of improving the mechanical strength of an electrolyte by using a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP) as a polymer matrix and adding ultrafine SiO ₂ to the copolymer. have. When SiO ₂ is added to the copolymer, the ion conductivity and mechanical properties of the solid electrolyte are greatly improved.

However, when the fine powder of SiO ₂ or Al ₂ O ₃ is used, the surface tension of the organic electrolyte solution in the form of slurry acts around the particles to form pores as the density of the copolymer is changed, so that the pore size and uniformity It is difficult to control, which makes it inappropriate to wet a large amount of electrolyte uniformly. Therefore, the battery reaction occurs nonuniformly.

Technical problem to be solved in the present invention to solve the above problems is a polymer used in the polymer matrix of the polymer solid electrolyte of the Li secondary battery to enable the smooth movement of Li ions and have a flexible structure And to provide a method for producing the same, and to provide a polymer solid electrolyte having excellent ion conductivity, containing such a polymer.

1 is a SEM (scanning electron microscope) photograph of a polymer solid electrolyte according to an embodiment of the present invention.

2 is a SEM photograph of a polymer solid electrolyte having a conventional uncrosslinked PVDF-HFP as a polymer matrix.

In order to achieve the first technical problem of the present invention as described above, in the present invention, poly (vinylidene fluoride-hexafluoropropylene) (PVDF-) crosslinked by a crosslinking agent poly (ethylene oxide) dimethacrylate (PEODM) HFP), a network polymer is provided.

The method for preparing a network polymer according to the present invention comprises the step of crosslinking a poly (vinylidene fluoride-hexafluoropropylene) copolymer as a crosslinking agent by mixing poly (ethylene oxide0 dimethacrylate and adding a polymerization initiator. Characterized in that.

Preferably, the weight ratio of the poly (vinylidene fluoride-hexafluoropropylene) copolymer and the crosslinking agent is 10: 1 to 2: 1.

Preferably, the weight average molecular weight of the poly (vinylidene fluoride-hexafluoropropylene) copolymer is 60,000 to 80,000, and is a repeating unit of poly (ethylene oxide) dimethacrylate (PEODM) used as the crosslinking agent. The number of —CH ₂ CH ₂ O— is 4 to 23.

As the polymerization initiator, conventional initiators may be used as the photopolymerization initiator or the thermal polymerization initiator, and it is preferable to use 0.5 to 1% by weight based on the total amount of the PVDF-HFP copolymer and the crosslinking agent.

When using a photoinitiator as said polymerization initiator, it is preferable to use the initiator which reacts in the ultraviolet wavelength of 350 nm or more.

In addition, the polymer solid electrolyte according to the present invention is dissociated with a polymer matrix made of a network polymer of poly (vinylidene fluoride-hexafluoropropylene) crosslinked by poly (ethylene oxide) dimethacrylate as a crosslinking agent. An organic electrolytic solution containing a lithium compound that emits Li ions as a supporting salt is characterized by comprising a.

Preferably, the weight ratio of the polymer matrix is 20 to 40% by weight based on the solid electrolyte.

Preferably, the poly (vinylidene fluoride-hexafluoropropylene) copolymer has a weight ratio of 10: 1 to 2: 1.

Preferably, the weight average molecular weight of the poly (vinylidene fluoride-hexafluoropropylene) copolymer is 60,000 to 80,000, and is a repeating unit of poly (ethylene oxide) dimethacrylate (PEODM) used as the crosslinking agent. The number of —CH ₂ CH ₂ O— is 4 to 23.

The lithium compound is not particularly limited as long as it is a compound that dissociates to give Li ions, and at least one of LiClO ₄ , LiBF ₄ , and LiCF ₃ SO ₃ is preferably used.

As the organic electrolyte, a non-aqueous aprotic solvent having low reactivity with Li is preferably used, and more preferably, propylene carbonate (PC), ethylene carbonate (EC) or dimethylene carbonate (DMC) is used. Do.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

First, the manufacturing method of the mesh polymer of the present invention and the characteristics of the polymer will be described in detail.

The mesh polymer of the present invention is formed by crosslinking polyethylene oxide dimethacrylate (PEODM) to PVDF-HFP copolymer, as can be seen through the following scheme.

Scheme

First, the PVDF-HFP copolymer is sufficiently dissolved to form a long linear chain using acetone, and then dibutylphthalate (DBP) and PEODM, which is a crosslinking agent, are added to form a slurry by mixing a polymerization initiator at a predetermined ratio. The slurry is coated on a glass substrate and dried to evaporate acetone. Heat or light is added thereto to polymerize and crosslink.

The polymerization initiator used to improve the crosslinking ability of PEODM is a photoinitiator or a thermal initiator. The photoinitiator mainly uses BEE (benzoin ethyl ether) and the thermal initiator is AIBN (azoisobutyronitrile). use. BEE reacts and decomposes with ultraviolet light, causing continuous reaction with the crosslinking agent, breaking down the C = C double bond in the crosslinking agent, thereby creating new active radicals. Since the crosslinking agent activated by the polymerization initiator is metastable with active radicals, it is bound to the linear chain of the PVDF-HFP copolymer to form a stable polymer having a network structure.

There are various kinds of photopolymerization initiators having different reaction characteristics depending on the wavelength region of the ultraviolet ray used. In the present invention, it is particularly preferable to use a material that reacts to an ultraviolet wavelength of 350 nm or more, since side reactions such as PEODM's own reaction occur before decomposition and activation of the initiator when using ultraviolet rays having a short wavelength of 350 nm or less. to be. Since the amount of initiator also affects the properties of the polymer to be formed, it is preferable to adjust the amount within an appropriate range. Excessive use of the polymerization initiator results in the presence of unreacted residual initiator remaining as an impurity, which does not participate in the polymerization reaction, thereby lowering the electrochemical properties of the polymer solid electrolyte. On the contrary, when a small amount is used, the crosslinking reaction of the crosslinking agent is insufficient, and thus a polymer having a low crosslinking degree is formed, thereby deteriorating the mechanical properties of the polymer. Therefore, in the present invention, it is preferable to adjust the ratio of the initiator to 0.5% by weight or more and 1% by weight or less of the total amount of the reagent added during polymerization.

Since the crosslinked copolymer has a network structure and has a lot of pores, it can moisten a large amount of organic electrolyte solution, and has a large margin in crosslinking distance, so it has good elasticity, and has a high affinity with an organic solvent. Suitable for use as a matrix.

The organic electrolyte is added to the polymer matrix as described above to have ionic conductivity and act as a polymer solid electrolyte. The organic solvent used in the organic electrolyte is not particularly limited as long as it is a solvent used in a Li battery as a water-insoluble aprotic organic solvent. Examples thereof include propylene carbonate (PC), ethylene carbonate (EC), and dimethylene carbonate. (DMC) is mentioned.

At this time, it is important to properly adjust the weight ratio of the polymer matrix and the organic electrolyte. In other words, if the ratio of the polymer matrix is high and the ratio of the organic electrolyte is low, the organic electrolyte solution is not partially filled in the empty space of the network polymer, and only partially exists. Since the polymer matrix itself is non-conductive, the ion conductivity of the solid electrolyte is reduced. When the ratio of the polymer matrix is low and the ratio of the organic electrolyte exceeds the amount that the polymer matrix can impregnate, leakage or evaporation of the organic electrolyte occurs and the flexibility of the polymer matrix itself is low. Therefore, the proportion of the polymer matrix in the solid electrolyte is preferably adjusted to 20 to 40% by weight. When the ratio of the polymer matrix is less than 20% by weight, the mechanical properties of the polymer matrix are reduced, making handling difficult, and 40% by weight. This is because the mechanical properties are improved, but the pore size is reduced, so that it is difficult to impregnate a large amount of the organic electrolyte solution, thereby decreasing the ion conductivity of the solid electrolyte.

In addition, the ratio of PVDF-HFP, a basic polymer constituting the polymer matrix, and PEODM, which is used as a crosslinking agent, also has a great effect on the properties of the solid electrolyte. When the amount of PVDF-HFP is large, the network structure cannot be formed, but a linear structure is formed. When the polymer is polymerized, the fragrance easily occurs, and when the amount of the crosslinking agent is excessive, the network structure of the polymerized polymer becomes dense to form a brittle hard polymer. Therefore, the ratio of PVDF-HFP and the crosslinking agent is preferably adjusted in the range of 3: 1 to 2: 1.

The organic electrolyte contains metal ions necessary for ion conduction. The organic electrolyte of a solid electrolyte for a lithium secondary battery using ion conductivity of Li ions is a lithium compound that is dissociated in an organic solvent to give Li ions, but is not particularly limited. ₄ , LiBF ₄ and LiCF ₃ SO ₃ and the like are used.

When the organic electrolyte solution containing the lithium compound is introduced into the polymer matrix of the solid polymer electrolyte, it is absorbed into the empty space of the crosslinked polymer forming the network structure, and acts as a path for moving Li ions in the direction of the current. .

Hereinafter, the present invention will be described in more detail with reference to the following comparative examples and non-limiting examples.

Example

After putting 25 ml of acetone in a clean polyethylene container, 3 g of PVDF-HFP (Kynar 2801, manufactured by Atochem) and 1.5 g of PEODM (23 G, manufactured by Shin-Nakamura Chemical) were added thereto to completely dissolve them. Subsequently, 4 g of DBP and 0.2 g of BEE were added, followed by sufficient mixing using a magnetic stirrer. The mixed solution was coated to a thickness of about 100㎛ on the organic substrate using a doctor blade. After coating, acetone in the solution was evaporated, and the resulting sheet was subjected to photopolymerization using ultraviolet light of 350 nm. After the polymerization was completed, a polymer sheet that could be handled was obtained, and was deposited in ether to completely extract the DBP remaining in the sheet. After the DBP extraction was completed, the sheet was transferred into a dry box of an Ar atmosphere, followed by immersing the electrolyte in an EC: DMC 2: 1 organic electrolyte in which 1M LiBF ₄ was dissolved to obtain a final polymer solid electrolyte. The ion conductivity of the solid electrolyte was measured by a conventional method, and it was found to have a constant and excellent ion conductivity of 1.4 X 10 ^-3 to 2 X 10 ^-3 S / cm. In addition, flexibility and tensile strength were also superior to the conventional.

In order to determine the characteristics of the prepared solid electrolyte, a sample was taken from the SEM photograph. The result is shown in FIG.

Comparative example

A solid electrolyte was prepared according to a conventional method using SiO ₂ instead of a crosslinking agent, a sample was taken from the sample, and a SEM photograph was taken. The result is illustrated in FIG. 2.

As can be seen from FIGS. 1 and 2, in the case of the polymer solid electrolyte prepared by the conventional method, the pores are not uniform, but the polymer electrolyte of the present invention has the pores uniform and sufficient.

As described above, according to the present invention, polyethylene oxide dimethacrylate (PEODM) is added to the PVDF-HFP copolymer to crosslink the PVDF-HFP copolymer to form a network structure, thereby creating many pores. A large amount of organic electrolyte can be impregnated to increase the ionic conductivity of the solid electrolyte. In addition, in particular, the pores are sufficient and the distribution thereof is uniform, so that the movement of Li ions is uniform, thereby enabling uniform electrode reaction and improving the lifetime and capacity of the electrode. In addition, since the network structure of the polymer can prevent leakage of the organic electrolyte, complete solidification can be realized, and the polymer electrolyte membrane prepared therefrom can be easily produced in a desired shape due to the elasticity of the polymer.

Claims (17)

A mesh polymer comprising poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) crosslinked by poly (ethylene oxide) dimethacrylate (PEODM) which is a crosslinking agent. The mesh polymer according to claim 1, wherein the weight ratio of the poly (vinylidene fluoride-hexafluoropropylene) copolymer and the crosslinking agent is 10: 1 to 2: 1. The network polymer according to claim 2, wherein the weight average molecular weight of the PVDF-HFP copolymer is 60,000 to 80,000. The network polymer according to claim 1, wherein the number of -CH ₂ CH ₂ O- which is a repeating unit of the poly (ethylene oxide) dimethacrylate (PEODM) which is the crosslinking agent is 4 to 23. Mixing poly (ethyleneoxide) dimethacrylate (PEODM) as a crosslinking agent to a poly (vinylidene fluoride-hexafluoropropylene) copolymer (PVDF-HFP) and crosslinking by adding a polymerization initiator. Method of producing a network polymer. The method of claim 5, wherein the weight ratio of the poly (vinylidene fluoride-hexafluoropropylene) copolymer and the crosslinking agent is 10: 1 to 2: 1. The method of claim 6, wherein the weight average molecular weight of the PVDF-HFP is 60,000 to 80,000. The method for producing a network polymer according to claim 5, wherein the number of -CH ₂ CH ₂ O- which is a repeating unit of the poly (ethylene oxide) dimethacrylate (PEODM) which is the crosslinking agent is 4 to 23. The method of claim 5, wherein the polymerization initiator is a photopolymerization initiator or a thermal polymerization initiator, the preparation of the network polymer, characterized in that used in the range of 0.5 to 1% by weight based on the total amount of the PVDF-HFP copolymer and the crosslinking agent. Way. The method of claim 9, wherein the polymerization initiator is a photopolymerization initiator reacting at an ultraviolet wavelength of 350 nm or more. A polymer matrix made of a mesh-like polymer which is a poly (vinylidene fluoride-hexafluoropropylene) copolymer crosslinked with a poly (ethylene oxide) dimethacrylate as a crosslinking agent, A polymer solid electrolyte comprising an organic electrolyte solution containing as a supporting salt a lithium compound that dissociates to give Li ions. The polymer solid electrolyte of claim 11, wherein the weight ratio of the polymer matrix is 20 to 40 wt% based on the solid electrolyte. 12. The polymer solid electrolyte of claim 11, wherein a weight ratio of the poly (vinylidene fluoride-hexafluoropropylene) copolymer and the crosslinking agent is 10: 1 to 2: 1. The polymer solid electrolyte of claim 13, wherein the poly (vinylidene fluoride-hexafluoropropylene) copolymer has a weight average molecular weight of 60,000 to 80,000. 12. The polymer solid electrolyte of claim 11, wherein the number of -CH ₂ CH ₂ O- which is a repeating unit of the poly (ethylene oxide) dimethacrylate (PEODM) is 4 to 23. The polymer solid electrolyte of claim 11, wherein the lithium compound of the organic electrolyte is at least one of LiClO ₄ , LiBF ₄ , and LiCF ₃ SO ₃ . 12. The polymer solid electrolyte of claim 11, wherein the organic electrolyte solution comprises a water-insoluble aprotic solvent.