KR20190063692A - Chitosan-catechol based porous membrane, method for preparing the same and a lithium-sulfur battery including the same - Google Patents

Abstract

A chitosan-catechol-based porous separator capable of improving the durability characteristics of a battery by applying a porous film containing a chitosan-catechol-based compound as a separator of a lithium-sulfur battery, a manufacturing method thereof, and a lithium-sulfur battery having the same. The chitosan-catechol-based porous separator comprises a chitosan-catechol-based polymer compound and pores formed therein and on the surface.

Description

Chitosan-catechol-based porous membrane, a method for producing the same, and a lithium-sulfur battery including the same -

TECHNICAL FIELD The present invention relates to a porous separator and a lithium-sulfur battery including the same, and more particularly, to a porous separator which can improve the lifetime characteristics of a battery by applying a porous film containing a chitosan- The present invention relates to a porous separator for chitosan-catechol, a method for producing the same, and a lithium-sulfur battery including the same.

Lithium-sulfur batteries among the rechargeable batteries whose application fields are expanded to electric vehicles and energy storage devices are attracting attention as the next generation secondary battery technology such that a relatively high energy storage density can be realized. Such a lithium-sulfur battery means a battery using a sulfur-based material having S-S bond (Sulfur-Sulfur Bond) as a cathode active material and using lithium metal as an anode active material. In particular, the sulfur used as the main material of the cathode active material is very rich in resources, is not toxic, and has a low atomic weight.

In the lithium-sulfur battery, lithium, which is a negative electrode active material, is ionized and oxidized while discharging electrons, and the sulfur-based material, which is a positive electrode active material, receives and reduces the released electrons (lithium- And the reduction reaction of sulfur is a process in which the SS bond is converted into a sulfur anion form by accepting two electrons). At this time, the lithium cation generated by the oxidation reaction of lithium is transferred to the anode through the electrolyte, and forms a salt by binding with the sulfur anion generated by the reduction reaction of sulfur. More specifically, sulfur before the discharge is a cyclic S ₈ structure, which is converted to lithium polysulfide by a reduction reaction, and lithium sulfide (Li ₂ S) is produced when the lithium polysulfide is completely reduced .

As described above, the lithium-sulfur (Li-S) secondary battery has many advantages and has attracted much attention. However, when the intermediate product, lithium polysulfide, is eluted as an electrolyte and diffused to the cathode (shuttle reaction of lithium polysulfide), there is a problem that battery degradation is accelerated such that the lifetime of the battery is lowered. Therefore, in order to minimize the elution of lithium polysulfide, a lot of studies have been conducted such as the modification of the morphology of the positive electrode composite. That is, various studies have been made to improve the lifetime of the battery by suppressing the dissolution of lithium polysulfide by filling sulfur compounds into a variety of carbon structures such as carbon fiber, porous carbon, graphene, and CNT, or forming a complex . However, the above methods are complicated or fundamentally difficult to solve the problem of dissolving lithium polysulfide. Therefore, there is a continuing search for solutions to such problems.

Accordingly, an object of the present invention is to provide a chitosan-catechol-based porous separator which can improve the lifetime characteristics of a battery by applying a porous film containing chitosan-catechol-based compound as a separator of a lithium-sulfur battery, And a lithium-sulfur battery including the same.

In order to achieve the above object, the present invention provides a chitosan-catechol-based porous separator comprising a chitosan-catechol-based polymer compound and pores formed therein and on the surface thereof.

The present invention also provides a method for producing a chitosan-catechol-based polymer, comprising the steps of: a) reacting chitosan and a catechol compound to prepare a chitosan-catechol polymer; b) dissolving the prepared chitosan-catechol-based polymer in a solvent to prepare a chitosan-catechol-based polymer solution; c) oxidizing and hydrogeling the prepared chitosan-catechol polymer solution; And d) lyophilizing the chitosan-catechol-based hydrogel. The present invention also provides a method for preparing a chitosan-catechol porous separator.

The present invention also provides a positive electrode comprising: a positive electrode; cathode; The chitosan-catechol porous separator interposed between the anode and the cathode; And an electrolyte.

The chitosan-catechol-based porous separator according to the present invention, the method for producing the same, and the lithium-sulfur battery including the same can be manufactured by applying a porous film containing a chitosan-catechol compound as a separation membrane of a lithium- It is possible to suppress the diffusion, thereby improving the lifetime characteristics of the battery.

FIG. 1 is an SEM image showing the adsorption capacity of a chitosan-catechol film (CHI-C film) and a conventional polypropylene separation membrane, both of which are not pores.
Fig. 2 is an XPS graph showing the adsorption capacity of a chitosan-catechol film (CHI-C patch) membrane in which pores are not formed and a conventional polypropylene separation membrane.
3 is an SEM image of a surface membrane of a chitosan-catechol film to which pores are not applied as a separation membrane of a lithium-sulfur battery.
4 is a chemical structural formula of a chitosan-catechol-based polymer compound according to an embodiment of the present invention.
5 is an SEM image of the porous separator of chitosan-catechol system according to the present invention.

Hereinafter, the present invention will be described in detail.

The chitosan-catechol porous separator according to the present invention is preferably for a lithium-sulfur battery, and comprises a chitosan-catechol-based polymer compound and is characterized by having pores formed therein and on its surface.

The applicant of the present application concluded that the chitosan-catechol compound containing an amine group has excellent lithium polysulfide adsorption ability after inhibiting the elution of lithium polysulfide and thereby improving the lifetime of the battery . FIG. 1 is an SEM image showing the adsorption capability of a chitosan-catecholate film (CHI-C film) having no pores and a conventional polypropylene separation membrane, and FIG. 2 is a SEM image showing a chitosan- 1 (a) and 2 (b) show adsorptivity against carbon, FIGS. 1 and 2 (b) show adsorption capacity against nitrogen, and FIG. 1 And 2 (c) show the adsorption capacity for sulfur (sulfide).

As shown in Figs. 1 and 2, the polysulfide adsorption ability of the chitosan-catechol compound was confirmed to be excellent. However, when the chitosan-catechol-based compound is applied as a separation membrane of a lithium-sulfur battery, the present applicant has confirmed that ion permeation is not smooth due to the absence of pores in the chitosan-catechol separation membrane. FIG. 3 is an SEM image of a surface membrane of a chitosan-catechol film without pores applied as a separation membrane of a lithium-sulfur battery. FIG. 3 shows that pores are not formed in the separation membrane of the chitosan-catechol system.

On the basis of the above-mentioned results, the present inventors have found that although the chitosan-catechol compound has excellent lithium polysulfide adsorbing ability, when the chitosan-catechol compound is used as a separation membrane of a lithium-sulfur battery, And it was confirmed that the battery was not normally driven by the high resistance. The Applicant has thus completed a chitosan-catechol-based porous separator for lithium-sulfur batteries inherent to the present invention which maintains the polysulfide adsorbing ability by forming pores in the chitosan-catechol-based film membrane, but increases the ion permeability and reduces the resistance of the battery will be.

That is, FIG. 4 is a chemical structural formula of a chitosan-catechol-based polymer compound according to an embodiment of the present invention. In the chitosan-catechol-based polymer compound, a large number of amine groups adsorbing (lithium) , And a plurality of phenol groups containing at least two hydroxyl groups (-OH) present by forming an amide bond with the amine group.

Examples of the amine group-containing chitosan-catechol-based polymer compound contained in the above-mentioned chitosan-catechol porous separator include conventional chitosan-catechol compounds such as the compounds shown in FIG. In addition, the weight average molecular weight (Mw) of the chitosan-catechol-based polymer compound may be 1,000,000 (1 MDa) to 2,500,000 (2.5 MDa), preferably 1,700,000 (1.7 MDa) to 2,200,000 (2.2 MDa) (2 MDa).

5 is an SEM image of the porous separator of chitosan-catechol system according to the present invention. As shown in FIG. 5, the porosity of the pores formed in the chitosan-catechol porous separator is 40 to 90%, preferably 60 to 80%, and the porosity of the pores formed in the chitosan-catechol porous separator is less than 40% , The lithium ion transfer is not normally performed and acts as a resistance component, which may cause a problem. If it exceeds 90%, the mechanical strength may be lowered.

The size of the pores formed in the chitosan-catechol porous separator is 10 nm to 5 탆, preferably 50 nm to 5 탆, and when the size of the pores formed in the chitosan-catechol porous separator is less than 10 nm, A problem of non-permeability may occur, and if it exceeds 5 탆, a short circuit of the battery due to the contact between the electrodes and a safety problem may occur.

The size and thickness of the chitosan-catechol porous separator are not particularly limited, and may vary depending on the shape and size of the lithium-sulfur battery to which the chitosan-catechol porous separator is applied.

Next, a method for producing the porous separator of chitosan-catechol system according to the present invention will be described. The method for preparing a chitosan-catechol-based porous separation membrane comprises the steps of: a) reacting a chitosan-based compound with a chitosan-based compound to prepare a chitosan-catechol-based polymer, b) dissolving the prepared chitosan- - preparing a catechol-based polymer solution, c) oxidizing and hydrogeling the prepared chitosan-catechol-based polymer solution, and d) freeze-drying the chitosan-catechol-based hydrogel do.

In the method for producing the chitosan-catechol porous separator (or chitosan-catechol porous film), the reaction may be carried out at a temperature of 0 to 100 ° C, preferably 20 to 30 ° C. When the reaction between the chitosan and the catechol compound is performed, the chitosan-catechol polymer compound in which the catechol reacts with some amine groups of the chitosan to form a structure with an amide bond can be prepared. More specifically, the chitosan-catechol-based high-molecular compound is a complex compound in which an amide bond structure is formed by dehydration-bonding of an amine group of chitosan and a carboxyl group of a catechol-containing reactant, to be.

The catechol may be in a structure bound to 1 to 20% of the amine groups in the chitosan, but this can be varied by controlling the reactant composition ratio. The molar ratio of the chitosan compound to the catechol compound may be 1: 1 to 1:10, and the mole ratio of the chitosan compound to the catechol compound may be in the range of Lt; RTI ID = 0.0 > substitution < / RTI > of the catechol functionality. In addition, the molecular weight of the chitosan is suitably 1,000,000 (1 MDa) to 2,500,000 (2.5 MDa), but it can be adjusted to suit the physical properties of the chitosan-catechol polymer to be produced. On the other hand, examples of the catechol compound include hydroxycapphosphoric acid and the like.

The chitosan-catechol-based polymer produced through step a) is dissolved in a solvent such as distilled water. The mass ratio of the chitosan-catechol-based polymer compound is 1 to 5 wt%, preferably 2 to 3 wt% ≪ / RTI > in a solvent. The resulting chitosan-catechol polymer solution is hydrogelized by cross-linking catechol functional groups in solution by oxidation. Examples of the oxidation method include, but are not limited to, oxidation by an oxidizing agent, oxidation in a high pH region (i.e., oxidation by pH adjustment), and oxidation by a radical formation reaction by infrared rays. Examples of the oxidizing agent include NaIO ₄ and (NH ₄ ) ₂ S ₂ O _8. However, conventional oxidizing agents can be used without limitation, and the oxidizing agent is used in an amount of not more than May be 1 to 2 times the number of moles of the catechol. The oxidation by the pH can be carried out by adjusting the pH of the chitosan-catechol polymer solution to 8.0 to 10.0. The oxidation by the radical formation reaction by infrared rays is carried out by exposing the infrared ray to the chitosan-catechol solution, Lt; / RTI >

In addition, the freeze-drying step is performed to remove moisture from the chitosan-catechol hydrogel formed by the oxidation to form a dried film (or a membrane) having pores, Lt; RTI ID = 0.0 > 5 Torr < / RTI > for 72 to 120 hours.

An electrochemical device such as a lithium-sulfur battery can be manufactured by interposing the porous separator made of chitosan-catechol thus prepared between an anode and a cathode and supplying an electrolyte, and the installation of an anode and a cathode, Can be carried out by conventional methods known in the art.

Finally, the lithium-sulfur battery according to the present invention will be briefly described. The lithium-sulfur battery includes a positive electrode, a negative electrode, a chitosan-catechol porous separator according to the present invention sandwiched between the positive and negative electrodes, .

When the chitosan-catechol porous separator according to the present invention is applied to a lithium-sulfur battery, lithium polysulfide, which is generated from an anode and eluted as an electrolyte, diffuses into a cathode, unlike the case of using a conventional separator (Due to the adsorption of polysulfide on the porous separator), the life characteristics of the lithium-sulfur battery are improved. The positive electrode, the negative electrode and the electrolyte are conventional ones well known in the art, and a detailed description thereof will be described later, and the description of the chitosan-catechol porous separator will be replaced with the above description.

Hereinafter, the positive electrode, the negative electrode and the electrolyte applied to the lithium-sulfur battery according to the present invention will be described.

anode

The positive electrode used in the present invention is prepared by preparing a positive electrode composition comprising a positive electrode active material, a conductive material and a binder, diluting the positive electrode composition with a predetermined solvent (dispersion medium), coating the slurry directly on the positive electrode collector, The anode layer can be formed by drying. Alternatively, the slurry may be cast on a separate support, and then the film may be peeled off from the support to laminate the cathode current collector on the anode current collector to produce a cathode layer. In addition, the anode can be fabricated in a variety of ways using methods well known to those skilled in the art.

The conductive material serves as a path through which electrons move from the positive electrode collector to the positive electrode active material, thereby not only imparting electron conductivity, but also electrically connecting the electrolyte and the positive electrode active material to form lithium ions (Li +) in the electrolyte. Sulfur, and react to the reaction. Therefore, if the amount of the conductive material is insufficient or fails to perform its role properly, the portion of the sulfur in the electrode that does not react increases and eventually causes a reduction in capacity. In addition, since high-rate discharge characteristics and charge-discharge cycle life are adversely affected, it is necessary to add an appropriate conductive material. The conductive material is preferably added in an amount of 0.01 to 30% by weight based on the total weight of the cathode composition.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite; Carbon black such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Concrete examples of commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack, Armak Company products, Vulcan XC-72 Cabot Company products and Super-P (Timcal) products can be used.

The binder is used to adhere the positive electrode active material well to the current collector. The binder should be well dissolved in a solvent, and it should not only constitute a conductive network between the positive electrode active material and the conductive material, but also have adequate electrolyte impregnation properties. The binder may be any binder known in the art and specifically includes a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); Rubber-based binders including styrene-butadiene rubber, acrylonitrile-butadiene rubber, and styrene-isoprene rubber; Cellulosic binders including carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, and regenerated cellulose; Polyalcohol-based binders; Polyolefin binders including polyethylene and polypropylene; Polyamide-based binder, polyester-based binder, and silane-based binder.

The content of the binder may be 0.5 to 30% by weight based on the total weight of the cathode composition, but is not limited thereto. If the content of the binder resin is less than 0.5% by weight, the physical properties of the positive electrode may deteriorate and the positive electrode active material and the conductive material may fall off. When the amount of the binder resin is more than 30% by weight, the ratio of the active material and the conductive material is relatively decreased The battery capacity may be reduced, and the efficiency may be lowered by acting as a resistance element.

cathode

As the negative electrode, any material capable of intercalating and deintercalating lithium ions can be used. Examples of the negative electrode include a metallic material such as lithium metal and lithium alloy, and carbon materials such as low crystalline carbon and highly crystalline carbon. Examples of low crystalline carbon are soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber High-temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes. Alloys containing silicon and oxides such as Li ₄ Ti ₅ O ₁₂ are also well known cathodes.

The negative electrode may include a binder. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile Various kinds of binder polymers such as polyacrylonitrile, polymethylmethacrylate and styrene-butadiene rubber (SBR) may be used.

The negative electrode may further include a negative electrode collector for supporting the negative electrode active layer including the negative electrode active material and the binder. The negative electrode current collector may be specifically selected from the group consisting of copper, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium or silver, and an aluminum-cadmium alloy may be used as the alloy. In addition, fired carbon, a nonconductive polymer surface treated with a conductive agent, or a conductive polymer may be used.

The binder acts as a paste for the anode active material, mutual adhesion between the active materials, adhesion between the active material and the current collector, buffering effect on expansion and contraction of the active material, and the like. Specifically, the binder is the same as that described above for the positive electrode binder. The negative electrode may be a lithium metal or a lithium alloy. As a non-limiting example, the cathode may be a thin film of lithium metal, and may be a thin film of one selected from the group consisting of lithium, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Or more.

Electrolyte

The electrolyte constituting the lithium-sulfur battery of the present invention includes solvents (Solvents) and lithium salts (Lithium Salt), and may further include additives, if necessary. As the solvent, a conventional non-aqueous solvent serving as a medium through which ions involved in an electrochemical reaction of a battery can move can be used without any particular limitation. Examples of the non-aqueous solvent include a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent and an amphoteric solvent.

More specifically, examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate ), Ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, Propanolate, propionate, ethyl propionate,? -Butyrolactone, decanolide, valerolactone, mevalonolactone, and carprolactone. The ether solvents include di Ethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetra 2-methyltetrahydrofuran, and polyethylene glycol dimethyl ether. Examples of the ketone-based solvent include cyclohexanone, and the alcohol-based solvent includes ethyl alcohol and isopropyl alcohol. Examples of the non-protonic solvent include nitriles such as acetonitrile, amides such as dimethylformamide Dioxolanes such as Dole, 1,3-dioxolane (DOL), and sulfolane. The nonaqueous solvent may be used singly or in combination of two or more. When two or more of them are mixed, the mixing ratio may be appropriately adjusted according to the performance of the objective battery, and 1,3-dioxolane and dimethoxyethane In a volume ratio of 1: 1.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as set forth in the appended claims. Such changes and modifications are intended to be within the scope of the appended claims.

[Example 1] Production of chitosan-catechol-based porous separator

First, a chitosan-catechol (Chi-C) polymer compound was prepared by reacting chitosan and hydroxycapphosphoric acid (catechol compound) at room temperature (amide bond). The resulting chitosan-catechol polymer compound was dissolved in distilled water, and then oxidized in an aqueous solution to hydrogel the chitosan-catechol polymer compound. Finally, the chitosan-catechol polymer compound was lyophilized at a temperature below -40 ° C., A chitosan-catechol-based porous separator having pores of a meter unit was prepared.

Claims (11)

Chitosan-catechol-based polymer compound,
Wherein the porous membrane has pores formed therein and on the surface thereof. The chitosan-catechol porous separator according to claim 1, wherein the porosity of the pores is 40 to 90%. The chitosan-catechol porous separator according to claim 1, wherein the pore size is 10 nm to 5 탆. The chitosan-catechol-based porous separator according to claim 1, wherein the weight average molecular weight (Mw) of the chitosan-catechol-based polymer compound is 1,000,000 (1 MDa) to 2,500,000 (2.5 MDa). [Claim 2] The chitosan-catechol porous separator according to claim 1, wherein the porous separator for chitosan-catechol is a lithium-sulfur battery. a) preparing a chitosan-catechol-based polymer by reacting chitosan with a catechol compound;
b) dissolving the prepared chitosan-catechol-based polymer in a solvent to prepare a chitosan-catechol-based polymer solution;
c) oxidizing and hydrogeling the prepared chitosan-catechol polymer solution; And
d) freeze-drying the chitosan-catechol-based hydrogel. [Claim 6] The method according to claim 6, wherein the reaction mixture ratio of the chitosan and the catechol compound is 1: 1 to 1:10 by mole ratio. [Claim 6] The method according to claim 6, wherein the chitosan-catechol-based polymer compound is dissolved in a solvent in a concentration of 1 to 5 wt% as a mass ratio. 7. The method of claim 6, wherein the oxidation is carried out by an oxidation method selected from the group consisting of oxidation by an oxidizing agent, oxidation by pH control, and oxidation by radical formation reaction by infrared rays. (2). [7] The method of claim 6, wherein the lyophilization is performed by maintaining a vacuum of less than 5 Torr at -40 [deg.] C or less for 72 to 120 hours. anode; cathode; A chitosan-catechol porous separator interposed between the anode and the cathode; And an electrolyte.

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