KR100441611B1 - Polymer electrolyte using absorbent and preparation method thereof - Google Patents

Abstract

The present invention relates to a polymer electrolyte having conductivity to lithium ions by providing a space in which a liquid component and a lithium salt can be sucked by introducing an absorbent into a polymer membrane, a method for manufacturing the same, and a lithium secondary battery using the polymer electrolyte. . The absorbent may be a polymer or inorganic material having a particle size of 40 μm or less, and the polymer binder may be one having low solubility in a liquid electrolyte. The polymer electrolyte not only has an ionic conductivity of about 10 ⁻⁴ S / cm or more at room temperature, but also a polymer electrolyte prepared by laminating or pressing the liquid electrolyte decomposed by moisture and then added immediately before battery packaging. The process is not affected by humidity and temperature, and has excellent mechanical strength and low reactivity with lithium metal, which makes it suitable for use as an electrolyte for lithium secondary batteries.

Description

Polymer electrolyte using absorbent and preparation method thereof

The present invention relates to a polymer film that can be used in an electrolytic cell, particularly a secondary battery. More specifically, the introduction of a liquid component and a lithium salt (hereinafter referred to as "liquid electrolyte") to the polymer membrane may provide a migration path of ions moving between the positive electrode and the negative electrode during the charge and discharge of the secondary battery. have.

The battery has three basic components: a positive electrode, a negative electrode, and an electrolyte. The material of the negative electrode is often a compound in which lithium metal or lithium ions can be intercalated. Polymeric materials can be used. The material of the positive electrode is a material capable of intercalating lithium ions, preferably lithium cobalt oxide (Li _x CoO ₂ ), lithium nickel oxide (Li _x NiO ₂ ), lithium nickel cobalt oxide (Li _x (CoNi) Oxides or polymer materials such as O ₂ ), spinel-type lithium manganese oxide (Li _x Mn ₂ O ₄ ), manganese dioxide (MnO ₂ ), and the like. By introducing a liquid electrolyte into the polymer membrane, an ion conductive matrix can be formed.

The battery using the polymer electrolyte has a lower risk of leakage and excellent electrochemical stability than the battery using the liquid electrolyte, and thus, various types of assembly are possible and automation of the manufacturing process is easy. Therefore, since the discovery that a polymer such as polyoxyethylene may have a metal ion conductivity when the polymer contains a polar heterogeneous element capable of electrical interaction with metal ions, research on ion conductive polymers, that is, polymer electrolytes, has been actively conducted. It has been going on.

However, only pure polymer of polyoxyethylene has very low ion conductivity of about 10 ^-8 S / cm at room temperature, so that the temperature of about 100 ° C is necessary to show a conductivity of 10 ^-3 S / cm applicable to the battery. Because of the problem of reaching, the main flow of polymer electrolyte research was to improve conductivity.

As it has been found that the conduction of ions in the polymer electrolyte requires the movement of the polymer chain, attempts to improve the conductivity of the ions have been made in the direction of increasing the flexibility of the polymer chain, Blonsky. Et al. Have introduced an phosphazene bond into the polymer backbone to produce an electrolyte having an improved conductivity of 10 ⁻⁵ S / cm ( J. Am. Chem. Soc. , 106 , 6854 (1984)), The electrolyte was poor in terms of conductivity and mechanical strength.

In addition, various attempts have been made to modify the structure of the polymer or to add an inorganic material to reduce the crystallinity of the polymer. However, a pure polymer electrolyte (not including a liquid electrolyte) composed of only a polymer and a metal salt does not exhibit sufficient conductivity. It is true.

In contrast, the gel-type electrolyte of US Pat. No. 5,219,679 contains a liquid electrolyte in the polymer backbone, and exhibits properties of the polymer in terms of mechanical properties and has conductivity close to that of the liquid electrolyte. It suggests practical possibilities. That is, there is no separate activation process for adding a liquid electrolyte, and a part of the liquid electrolyte is already contained in the process of preparing the polymer electrolyte (casting a mixture of the polymer solution and the liquid electrolyte). However, the electrolyte of US Pat. No. 5,219,679 includes a polymer such as polyacrylonitrile that is reactive toward lithium metal, so that reaction products accumulate between the electrolyte and the lithium electrode during storage and use of the battery. As a result, the interface resistance is continuously increased.

On the other hand, Scrosati et al . Prepared a polyelectrolyte in the form of a gel using polymethylmethacrylate, which is less reactive with lithium metal ( Electrochim. Acta , 140 , 991 (1995)). The electrolyte, which uses polymethyl methacrylate as a polymer component, has a low reactivity with the lithium surface and has a slight increase in resistance at the electrode surface during storage, but exhibits a weakness in mechanical strength and is sufficient to form a film. In order to obtain strength, the polymer content must be increased, and in this process, the conductivity drops to 10 ^-4 S / cm or less. In addition, since the gel-type electrolyte contains a large amount of liquid components, vaporization of the liquid components occurring on the surface of the electrolyte is inevitable, so that the change of composition and the decrease in conductivity due to the loss of the liquid components during the storage period are only concerned. In addition, the lithium salt contained in the liquid electrolyte is decomposed by reacting with moisture in the air, which has the disadvantage of requiring an extremely dehumidifying atmosphere with extremely low moisture.

U.S. Patent No. 5,296,318, U.S. Patent No. 5,418,091 and the like provide a hybrid polymer electrolyte system to supplement the above problems. Hybrid polymer electrolytes are liquid electrolytes that are sensitive to the effects of moisture while retaining the advantages of gel-type polymer electrolytes (containing a large amount of liquid electrolyte and having conductivity similar to that of liquid electrolyte because conduction of lithium ions proceeds through the liquid phase). By adding the battery just before packaging, it was possible to minimize the influence of moisture in the electrolyte manufacturing process. However, since the liquid electrolyte is added after the polymer membrane is prepared, a site for sucking the liquid component or a driving force for penetrating the liquid component into the polymer membrane is required in the polymer membrane. To this end, dibutyl phthalate was added as a plasticizer in the preparation of the polymer membrane, and after the assembly of the battery, the plasticizer was extracted with an organic solvent such as alcohol or ether to make a liquid inhalation spot. However, since the extraction process of dibutyl phthalate uses a chemical method, it has a fatal disadvantage of low reproducibility, low battery yield, and difficult automation for mass production.

Therefore, the present invention synthesizes the polymer membrane by adding an absorbent (absorbent) that can absorb the liquid electrolyte in the polymer matrix, and the liquid electrolyte is introduced in the activation process after the battery assembly is completed, thereby solving the problems of the prior art. It is to be solved.

In the present specification, the polymer membrane means a polymer membrane in a dry state and does not contain a liquid electrolyte, and the polymer electrolyte means to include ionic conductivity by including a liquid electrolyte in the polymer membrane.

The battery assembly is a process of laminating, pressing, or the like, together with a positive electrode and a negative electrode separately prepared with a polymer electrolyte interposed therebetween. When the polymer membrane is prepared by the above method, since the liquid electrolyte is added after the assembly of the battery, the limitation of the dehumidification condition in the process can be minimized. In addition, the site that can absorb the liquid electrolyte is already made at the stage of manufacturing the polymer membrane, and since the extraction process of the plasticizer is not necessary, the process can be simplified, which lowers the manufacturing cost and facilitates automation and increases the yield. There are advantages to it.

1 is a schematic cross-sectional view of a battery using the polymer electrolyte of the present invention,

2 is a view showing the experimental results according to the linear potential scanning method for measuring the electrochemical stability of the polymer electrolyte of the present invention.

Description of the main symbols in the drawings

1: polyelectrolyte 11: absorbent powder

2: anode 22: anode current collector

3: negative electrode 33: negative electrode current collector

The polymer electrolyte of the present invention includes an absorbent capable of absorbing a liquid electrolyte, a polymer binder, and an ion conductive liquid electrolyte.

More specifically, the polymer electrolyte of the present invention is prepared by introducing an absorbent into the polymer binder to prepare a polymer membrane, and then injecting a liquid electrolyte to have a lithium ion conductivity of 10 ⁻⁴ S / cm or more at room temperature.

A variety of absorbents that can absorb liquids are known, and they can be classified into those that can absorb water and oil (oil). The excellent water absorption is mainly used for disposable diapers, sanitary napkins, and the like, and the oil absorbent is mainly used to remove petroleum spilled out to sea or factories, or to remove organic solvents in laboratories. Absorbents for this purpose are mainly used porous polymers or inorganic materials. The porous polymer may be a mesh-like polymer having a bulky functional group introduced into a side chain, or a polypropylene or a polyethylene introduced by adjusting a variable of a manufacturing process, and a pulp, which is a natural polymer. ), Cellulose and cork can also be used.

As the absorbent of the present invention, any material in powder form may be used to improve the absorbency of the liquid electrolyte. Preferably, polymer particles, mineral particles, mesoporous molecular sieves, and commercially available absorbent particles may be used. More preferably, synthetic / natural polymer particles such as polyethylene particles, polypropylene particles, polystyrene particles, polyurethane particles, pulp, cellulose particles, cork particles, wood flour and the like; Mineral particles having a phyllosilicate structure such as clay, paragonite, montmorillonite, mica, etc .; Synthetic oxide particles such as zeolite, porous silica and porous alumina; And MCM-41 (Mobil Co., Ltd.) having a pore diameter of 2 to 30 nm with an oxide / polymer material such as silica and Mesoporous molecular sieves such as MCM-48 from Mobil; Or one or more mixtures selected from the group consisting of commercially available absorbers such as Absorbent W Product (Absorption Corp.), Oclansorb (Hi Point Industries), PP Spillow spill control (Aldrich), Leak-Sorb spill absorbent (Aldrich) Can be used.

The amount of the absorbent added is 30 to 95% by weight, more preferably 50 to 90% by weight based on the weight of the dry polymer membrane containing no liquid electrolyte. If the added amount is 95% by weight or more, the mechanical strength of the formed polymer membrane is lowered, and if it is 30% by weight or less, there is a problem that the ability to absorb the liquid electrolyte is lowered. In order not to reduce the mechanical strength and the uniformity of the polymer film to be formed, the absorbent agent is preferably 40 µm or less, more preferably 20 µm or less.

Almost all ordinary polymers can be used as the polymer binder, including polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, vinylidene fluoride and maleic anhydride. (maleic anhydride) copolymer, polyvinylchloride, polymethylmethacrylate, polymethacrylate, cellulose triacetate, polyurethane, polysulfone, poly Polyether, polyolefine such as polyethylene or polypropylene, polyethylene oxide, polyisobutylene, polybutyldiene, polyvinylalcohol, polyacrylonitrile, poly Polyimide, polyvinyl formal, acrylonitrilebutyldiene high (acrylonitrilebutyldiene rubber), ethylene-propylene-diene-monomer, tetra (ethylene glycol) diacrylate, polydimethylsiloxane, polycarbonate and polysilicone Preference is given to one or more mixtures or copolymers thereof selected from the group consisting of:

The solvent of the polymer binder, which is a solvent for dissolving the polymer binder, is N-methylpyrrolidinone, dimethylformamide, dimethylacetamide, tetrahydrofuran, acetonitrile ), Cyclohexanone, chloroform, dichloromethane, hexamethylphosphoramide, dimethylsulfoxide, acetone, and dioxane Preference is given to one or a mixture of two or more selected.

The liquid electrolyte to be absorbed in the polymer film containing the absorbent is prepared by dissolving lithium salt in an organic solvent. In this specification, the absorption of the liquid electrolyte into the polymer membrane is defined as activation.

The organic solvent is preferred to have high polarity and no reactivity with lithium metal in order to enhance dissociation of ions by increasing the polarity of the electrolyte and to facilitate conduction of ions by lowering the local viscosity around the ions. Examples of such organic solvents are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone , Dimethylsulfoxide, 1,3-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, N, N Dimethylformamide (N, N-dimethylformamide), diglyme, triglyme, tetraglyme and the like. In particular, it is preferable to use two or more mixed solutions composed of a high viscosity solvent and a low viscosity solvent as the organic solvent.

It is preferable that the lithium salt has a small lattice energy and a large dissociation degree. Examples of such lithium salts include one or two or more selected from the group consisting of LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiPF ₆ , LiSCN and LiC (CF ₃ SO ₂ ) ₃ . There is a mixture. The concentration of the lithium salt is preferably 0.5M to 2M.

The liquid electrolyte is added at 30 to 90% by weight, more preferably 40 to 85% by weight based on the total amount of the electrolyte including the liquid electrolyte.

The polymer electrolyte of the present invention is easier to manufacture than the conventional polymer electrolyte, and has high ionic conductivity because the conduction of lithium ions proceeds through the liquid phase, until the liquid electrolyte is absorbed, ie, activated. It is characterized by not being affected by moisture or temperature.

The invention also relates to a method of making such a polymer electrolyte.

The polymer electrolyte of the present invention is prepared through four steps of mixing the absorbent and the polymeric binder, dissolving, casting and drying the mixture, and activating the bar. The polymer electrolyte in the dry solid state can be obtained by going through the steps of mixing, dissolving, and casting. And an activation step of absorbing the liquid electrolyte.

That is, the mixture of the absorbent and the polymeric binder is dissolved in the solvent of the polymeric binder, cast it in the form of a membrane, and then dried to form a polymer membrane having a thickness of 10 to 200 μm, and ion conductivity to the dried polymer membrane. Prepared by absorbing the liquid electrolyte.

If such a manufacturing method in more detail as follows.

First, the powdery absorbent (particle size 20 µm or less) and the polymer binder are dry mixed in a sealed container.

Such absorbent and polymeric binder are dissolved in the solvent of the polymeric binder. In order to facilitate dissolution of the polymer binder and to prevent the absorbents from agglomerating with each other, a magnetic stirrer, a mechanical stirrer, a planetary mixer, and the like are stirred. In this process, an ultrasonic stirrer may be used to prevent the absorbents from agglomerating with each other and to prevent bubbles from forming during mixing.

After the polymeric binder is completely dissolved and uniformly mixed with the absorbent, it is poured into a flat plate such as a glass plate or a Teflon plate and adjusted to a constant thickness. At this time, the thickness of the film is preferably adjusted to 10 to 200㎛. If the thickness of the membrane is 10 μm or less, the mechanical strength is lowered. If the thickness is 200 μm or more, the ion conductivity decreases, which is not preferable.

The resulting membrane is completely dried and then a liquid electrolyte is introduced.

The invention also relates to a secondary battery, in particular a lithium secondary battery, using the polymer electrolyte as an electrolyte.

That is, in the lithium secondary battery of the present invention, after dissolving the mixture of the absorbent and the polymer binder in the solvent of the polymer binder, and cast it in the form of a membrane and dried to form a polymer film having a thickness of 10 to 200㎛, separately from After assembling in the form of a battery together with the prepared positive and negative electrodes it is prepared by absorbing the liquid electrolyte. In order to be a polymer electrolyte having sufficient ion conductivity for use, an activation step of absorbing a liquid electrolyte must be performed. After the activation step, the battery can be operated as a battery. If the activation step is not performed, the room temperature ion conductivity is very low, and thus cannot be used as an electrolyte by itself.

A schematic cross-sectional view of a secondary battery using the polymer electrolyte of the present invention is shown in FIG. 1. The battery is constructed by bonding the positive electrode 2 and the negative electrode 3 to the center with the polymer electrolyte 1 obtained from the above process. The polymer electrolyte 1 includes the absorbent powder 11, and the positive electrode 2 is electrically connected to the positive electrode current collector 22 and the negative electrode 3. The assembly thus constructed undergoes an activation step of absorbing the liquid electrolyte, thereby enabling the battery to operate.

Hereinafter, the embodiment will be described in detail a method for producing a polymer electrolyte according to the present invention. However, these examples do not limit the contents of the present invention, and modifications may be made without departing from the basic spirit of the present invention. For example, a battery may be fabricated by activating a battery after assembling the battery with a separately prepared positive electrode and a negative electrode, or may also investigate the characteristics of the polymer electrolyte and the performance of the battery in which the battery is applied. have. However, in the embodiment of the present invention, the production process of the real battery was omitted in order to examine the characteristics of the polymer electrolyte only.

Example 1

The absorbent and the binder powder were put in a 20 mL vial, followed by dry mixing for about 5 minutes in a magnetic stirrer. 4 mL of NMP was added thereto and stirring continued until the binder was completely dissolved. Ultrasonic agitation was performed for 30 minutes during stirring to prevent the absorbent particles from agglomerating with each other. The mixed solution prepared by the above method was coated on a glass plate so as to have a thickness of about 100 μm, and then dried at room temperature for about 2 hours and further dried by a vacuum dryer for 6 hours. At this time, the temperature of the vacuum dryer was adjusted to about 50 ℃. The polymer membrane prepared by the above method was immersed in a liquid electrolyte solution for about 10 minutes, and then the weight change before and after the liquid electrolyte was completely absorbed was measured, and the conductivity was measured using an alternating current impedance method.

The characteristics and conductivity of the polymer electrolyte according to the type and content ratio of the absorbent and the binder are summarized in Table 1 below. In order to compare the ability of the polymer membrane to absorb the liquid electrolyte, the absorption capacity (Δ _ab ) was defined as follows.

Δ _ab = [amount of absorbed liquid electrolyte (mg)] / [weight of polymer membrane (mg)]

Example Absorbent Binder Liquid electrolyte Δ _ab Ion Conductivity (mS / cm) Mechanical strength Kinds g Kinds g a Paragonite 0.13 PVdF 0.28 EC / PC 1M LiPF ₆ 0.5 0.01 Very good b 〃 0.17 〃 0.26 〃 0.7 0.02 Very good c 〃 0.72 〃 0.24 〃 0.9 0.17 Very good d 〃 1.06 〃 0.26 〃 1.5 0.19 Great e 〃 1.51 〃 0.25 〃 2.2 0.39 Great f 〃 2.00 〃 0.26 〃 5.0 0.63 Great g 〃 1.98 〃 0.25 EC / DMC 1M LiPF ₆ 5.5 0.75 Good h Zeolite 1.37 〃 0.60 〃 4.7 0.68 Great i 〃 1.50 〃 0.38 〃 5.5 0.85 Great j 〃 1.65 〃 0.29 〃 5.8 0.91 Good k Montmorillonite 1.34 〃 0.58 〃 4.4 0.70 Great l 〃 1.50 〃 0.38 〃 5.8 0.82 Great m Silica 1.35 〃 0.59 〃 5.6 0.75 Great n Polypropylene 1.35 〃 0.60 〃 5.7 0.80 Great o Wood flour 1.35 〃 0.60 〃 5.5 0.79 Great p Paragonite 2.00 P (VdF-HFP) 0.26 〃 5.1 0.76 Great q 〃 1.95 PAN 0.25 〃 5.0 0.74 Great r 〃 2.00 PU 0.26 〃 5.3 0.72 Great s 〃 1.98 PVC 0.25 〃 5.5 0.78 Great t 〃 2.00 P (VdF-HFP) 0.26 〃 5.4 0.75 Great u Zeolite 2.00 P (VdF-HFP) 0.26 〃 5.3 0.78 Great v 〃 1.95 PAN 0.25 〃 5.1 0.74 Good w 〃 2.00 PU 0.26 〃 5.2 0.72 Great x 〃 1.98 PVC 0.25 〃 5.4 0.78 Great y 〃 2.00 P (VdF-HFP) 0.26 〃 5.3 0.75 Great

Example 2 (comparative example)

A polymer membrane was prepared in the same manner as in Example 1- (f), except that no absorbent was added to investigate the effect of the absorbent added when preparing the polymer membrane. The prepared polymer membrane was transparent and had good mechanical strength. However, the polymer membrane was immersed in the liquid electrolyte solution at room temperature for 10 hours, but the weight of the polymer membrane before and after impregnation was the same within the measurement error. In addition, the ion conductivity of the polymer electrolyte thus prepared could not be measured by the alternating current impedance method.

Example 3

In order to measure the electrochemical stability of the polymer electrolyte, linear sweep voltammetry was performed using stainless steel (# 304) as a working electrode and lithium metal as a counter electrode and a reference electrode. The potential scan range was from the open circuit voltage to 5.4 V and the potential scan speed was 10 mV / sec. The results of the linear potential scanning method for the polymer electrolyte prepared by the method of Example 1- (g), 1- (i) and 1- (l) are shown as A, B and C in FIG.

The polymer electrolyte of the present invention has excellent mechanical strength and can be formed into a thin film, and has a high ion conductivity corresponding to a liquid electrolyte, and unlike a general gel-type polymer electrolyte, lithium salt is decomposed by a small amount of water. Since it is not introduced in the process of manufacturing the polymer membrane, a special dehumidification environment is not required, and the absorbent is electrochemically stable, and thus has a wide potential window, and the electrolyte manufacturing process is simple, so that automation for mass production is easy. In addition, since the adhesion between the positive electrode and the negative electrode is small and the volume change due to the introduction of the liquid electrolyte is small, the interface resistance between the electrolyte and the electrode can be minimized, and thus it is suitable for use as an electrolyte for a lithium secondary battery.

Claims (4)

A powdery absorbent having a particle size of 40 μm or less, a polymer binder, and an ion conductive liquid electrolyte are included, and the absorbent is added in an amount of 30 to 95 wt% based on the total weight of the dry polymer membrane containing no liquid electrolyte. And the ion conductive liquid electrolyte is added in an amount of 30 to 90 wt% based on the total weight of the total electrolyte including the liquid electrolyte. The method of claim 1, wherein the absorbent is porous polymer particles such as polyethylene, polypropylene, polystyrene, polyurethane, pulp, cellulose, cork, wood flour; Mineral particles having a phyllosilicate structure such as clay, paragonite, montmorillonite and mica; Synthetic oxide particles such as zeolite, porous silica and porous alumina; Mesoporous molecular sieve having a pore diameter of 2 to 30 nm as an oxide or a polymer material; And one or two or more mixtures selected from the group consisting of other commercially available absorbents; The polymer binder is polyvinylidene fluoride, copolymer of vinylidene fluoride and hexafluoropropylene, copolymer of vinylidene fluoride and maleic anhydride, polyvinyl chloride, polymethyl methacrylate, polymethacrylate , Cellulose triacetate, polyurethane, polysulfone, polyether, polyethylene, polypropylene, polyethylene oxide, polyisobutylene, polybutyldiene, polyvinyl alcohol, polyacrylonitrile, polyimide, polyvinyl formal, acrylo One or more mixtures or copolymers thereof selected from the group consisting of nitrile butyl diene rubber, ethylene propylene diene monomer, tetraethylene glycol diacrylate, polydimethylsiloxane, polycarbonate and silicone polymer; The ion conductive liquid electrolyte is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone, 1,3-dioxene, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide In an organic solvent consisting of one or two or more mixtures selected from the group consisting of seed, sulfolane, N, N-dimethylformamide, diglyme, triglyme and tetraglyme, LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiBF ₄ Dissolving one or more lithium salts selected from the group consisting of LiN (CF ₃ SO ₂ ) ₂ , LiPF ₆ , LiSCN, and LiC (CF ₃ SO ₂ ) ₃ at a concentration of 0.5 to 2 M; Polymer electrolyte for secondary batteries characterized in that. After dissolving the mixture of the absorbent and the polymer binder in the solvent of the polymer binder, casting it in the form of a membrane and drying it to form a polymer membrane having a thickness of 10 to 200㎛, an ion conductive liquid electrolyte on the dried polymer membrane The method for producing a polymer electrolyte for secondary batteries according to claim 1 or 2, characterized in that the absorption. After dissolving the mixture of the absorbent and the polymeric binder in the solvent of the polymeric binder, casting it in the form of a membrane and drying it to form a polymer membrane having a thickness of 10 to 200㎛, form of a battery together with the positive and negative electrodes prepared separately After assembling the lithium secondary battery characterized in that the manufacturing by absorbing the liquid electrolyte.