KR102378574B1 - Binder Composition Comprising Aqueous Polyurethane Dispersion, Electrode Comprising Same, and Lithium Secondary Battery Comprising the Electrode - Google Patents

Abstract

The present invention relates to a binder composition including water-dispersed polyurethane, an electrode including the same, and a lithium secondary battery including the electrode. According to the present invention, a problem of environmental pollution and toxicity caused by organic solvents can be solved by using the water-dispersed polyurethane as a binder for an electrode, and the physical properties of the water-dispersed polyurethane are improved by using a phase inversion method so that a binder material having excellent elasticity and mechanical properties can be provided. In addition, a problem of deterioration of the lifespan and capacity of a lithium-sulfur battery due to the elution of lithium polysulfide can be solved by mixing carbon quantum dots with the water-dispersed polyurethane and applying the mixture to a positive electrode of a lithium-sulfur battery.

Description

A binder composition comprising water-dispersed polyurethane, an electrode comprising the same, and a lithium secondary battery comprising the electrode

The present invention relates to a binder composition comprising a water-dispersed polyurethane, an electrode comprising the same, and a lithium secondary battery comprising the electrode, and more particularly, to an eco-friendly, mechanical property and electrochemical property including water-dispersed polyurethane It relates to an excellent binder composition, an electrode including the same, and a lithium secondary battery including the electrode.

In general, in a lithium ion battery, a binder made of a polymer material is used to impart adhesion to an active material of an electrode and a current collector and to facilitate the movement of electrons and ions. A typical electrode binder material includes a polyvinylidene fluoride (PVDF)-based polymer. However, these PVDF-based polymers have an elongation of about 10%, lack elasticity, and are difficult to manufacture because they are mixed with an organic solvent such as N-methyl-2-pyrrolidone, and environmental pollution and toxicity caused by organic solvents. may occur.

On the other hand, polyurethane (polyurethane, PU) is a thermoplastic elastomer, a polymer material having excellent physical properties such as abrasion resistance, toughness, flexibility, and chemical resistance. Accordingly, a technique for using polyurethane as a binder for battery electrodes has been proposed. For example, Korean Patent No. 10-0790849 discloses a technique in which a polyurethane binder is used in an electrode to impart adhesion to an electrode active material and a current collector in a lithium battery and to facilitate the movement of electrons and ions.

However, due to the strong hydrophobicity of polyol, which is a synthetic raw material, an organic solvent is mainly used in the manufacturing process of polyurethane, and in this case, there are problems such as environmental pollution and toxicity caused by the organic solvent. In order to solve this problem, in the above technology, an aqueous polyurethane dispersion (APUD) prepared using water instead of an organic solvent was used. There was a limitation in the increase and the mechanical properties were somewhat inferior.

Therefore, in order to meet the physical properties required for the electrode of the battery while solving the problem of environmental pollution and toxicity by applying the water-dispersible polyurethane to the electrode, it is necessary to improve the elasticity and mechanical properties.

Meanwhile, as a next-generation secondary battery, a lithium-sulfur battery (LSB), which is a type of lithium secondary battery, is in the spotlight. The lithium-sulfur battery has an advantage in that the energy density is higher than that of the conventional lithium ion battery. However, in lithium-sulfur batteries, lithium polysulfide (LPS) is generated as an intermediate due to the reduction reaction of solid sulfur (S8) in the positive electrode during discharge. Since it is continuously dissolved in the middle to reduce the active material of the positive electrode, it may cause a problem that the lifespan of the battery is shortened.

In addition, during the charging process, lithium polysulfide travels back and forth between the lithium metal surface and the positive electrode surface, resulting in a 'shuttle effect' in which the final product, sulfur, is not produced and the charging process continues. The lithium metal and the active material are damaged, making it difficult to drive the battery.

As a technology for solving the lithium polysulfide dissolution problem, Korean Patent Application Laid-Open No. 2016-0037084 discloses that by using a carbon nanotube aggregate with a three-dimensional structure coated with graphene, it blocks the melting of lithium polysulfide and conducts conductivity. can be improved. However, since the above technique requires the formation of aggregates of graphene and carbon nanotubes, there is a disadvantage in that the process is complicated and the cost is increased.

In addition, since the expansion and contraction of sulfur occurs during the cycling process in the positive electrode of a lithium-sulfur battery, the binder used for the positive electrode requires excellent elasticity and mechanical properties capable of maintaining adhesive strength and restoring force despite such expansion and contraction.

In this situation, in order to expand the application range of water-dispersed polyurethane to the positive electrode binder of lithium secondary batteries, particularly lithium-sulfur batteries, the mechanical properties of the water-dispersed polyurethane are improved while the battery life and performance due to lithium polysulfide are improved. There is a need to develop a technology that can solve the degradation problem.

It is an object of the present invention to provide a binder composition to which a water-dispersed polyurethane having improved mechanical properties is applied, and excellent in electrochemical performance and eco-friendliness.

Another object of the present invention is to provide an electrode of a lithium secondary battery comprising the binder composition.

Another object of the present invention is to provide a lithium secondary battery with improved electrochemical performance and battery life, including the electrode.

In order to achieve the above object, the present invention provides a binder composition for an electrode comprising a water-dispersed polyurethane and carbon quantum dots.

In the present invention, the water-dispersed polyurethane is prepared by reacting a urethane raw material containing a polyol and polyisocyanate to prepare a prepolymer (prepolymer); and dropping water to the prepolymer to prepare water in which the prepolymer is dispersed; and adding a chain extender to water in which the prepolymer is dispersed to form an aqueous dispersion polyurethane.

In the present invention, the carbon quantum dots may be prepared by hydrothermal treatment (hydrothermal treatment) of water-dispersed polyurethane at 100 to 500 ℃.

In the present invention, the carbon quantum dot is a polyfunctional carbon quantum dot comprising an electrically conductive core made of carbon, and a highly polar shell including at least one functional group of a hydroxyl group, a carboxyl group, and an amine group. can

In the present invention, the carbon quantum dots may be included in an amount of 1 to 15% by weight based on the total weight of the binder composition.

The present invention also provides an electrode of a lithium secondary battery comprising the binder composition and the electrode active material.

The present invention also provides a cathode comprising at least one cathode active material selected from the group consisting of the binder composition, and a cyclic elemental sulfur material (S8), a sulfur-based compound, and a mixture thereof; an anode including an anode active material; a separator positioned between the anode and the cathode; And it provides a lithium secondary battery comprising an electrolyte.

According to the present invention, the problem of environmental pollution and toxicity caused by organic solvents can be solved by using the water-dispersed polyurethane as a binder for the electrode, and the physical properties of the water-dispersed polyurethane are improved by using the phase inversion method to improve elasticity and mechanical properties An excellent binder material can be provided. In addition, when carbon quantum dots are simply synthesized from the water-dispersed polyurethane and mixed with water-dispersed polyurethane and applied to the positive electrode of a lithium-sulfur battery, the problem of battery life and capacity degradation due to the dissolution of lithium polysulfide is solved. can

1 schematically shows a method for producing a water-dispersed polyurethane according to an embodiment of the present invention.
2 schematically shows a process (a) of a conventional prepolymer dropping method and particles (b) formed therefrom.
3 schematically shows a step (a) of a phase inversion method used in an embodiment of the present invention and particles (b) formed therefrom.
4 schematically shows a composite of water-dispersible polyurethane and polyfunctional carbon quantum dots according to the present invention.
5 schematically shows a method for producing a water-dispersed polyurethane according to an embodiment of the present invention.
6 is a graph showing the results of FT-IR analysis of water-dispersed polyurethane according to an embodiment of the present invention.
7 is a graph of differential scanning calorimetry analysis results of water-dispersed polyurethane according to an embodiment of the present invention.
8 shows the GPC results of the water-dispersed polyurethane according to an embodiment of the present invention.
9 is a graph showing a distribution graph of the particle size of the water-dispersed polyurethane according to an embodiment of the present invention.
10 shows the measurement result of the zeta potential of the water-dispersed polyurethane according to an embodiment of the present invention.
11 shows images before and after UV light (365 nm) irradiation for the WPU-CQD composite according to an embodiment of the present invention.
12 shows a UV-Vis analysis result of Li ₂ S ₆ in the presence of a WPU-CQD composite according to an embodiment of the present invention.
13 shows the results of nanoindentation of the WPU-CQD composite according to an embodiment of the present invention.
14 is a graph showing a Nyquist plot obtained from electrochemical impedance spectroscopy for a Li-S battery according to an embodiment of the present invention.
15 is a graph showing a CV and constant current charge/discharge analysis result of a Li-S battery according to an embodiment of the present invention.
16 is a graph showing a slope value calculated from the CV result of the Li-S battery according to an embodiment of the present invention.
17 is a graph showing a result of repeating a charge/discharge cycle 50 times and measuring a change in specific capacity for a Li-S battery according to an embodiment of the present invention.
18 is a graph showing a result of measuring specific capacity change under various rate-rate conditions for a Li-S battery according to an embodiment of the present invention.
19 is a graph showing a result of repeating a charge/discharge cycle 1,000 times and measuring a change in specific capacity for a Li-S battery according to an embodiment of the present invention.

Hereinafter, specific implementation forms of the present invention will be described in more detail. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In general, the nomenclature used herein is those well known and commonly used in the art.

The present invention relates to a binder composition for an electrode comprising an aqueous polyurethane dispersion (APUD), an electrode of a lithium secondary battery comprising the composition, and a lithium secondary battery comprising the electrode.

According to the present invention, the problem of environmental pollution and toxicity caused by organic solvents can be solved by using the water-dispersed polyurethane as a binder for the electrode, and the physical properties of the water-dispersed polyurethane are improved by using the phase inversion method, so that elasticity and mechanical properties This excellent binder material can be provided. In addition, when applying to the positive electrode of a lithium-sulfur battery by simply synthesizing carbon quantum dots from the water-dispersed polyurethane and mixing it with water-dispersed polyurethane, the battery life and capacity decrease due to the dissolution of lithium polysulfide can solve

The present invention provides a binder composition for an electrode comprising a water-dispersed polyurethane.

Water-dispersed polyurethane is a non-toxic and non-combustible material, which is more environmentally friendly than organic solvent-based polyurethane. In addition, the water-dispersed polyurethane has transparency, dryness, flexibility, impact resistance, abrasion resistance and adhesion, and has excellent resistance to solvents and chemicals, and thus exhibits excellent weather resistance.

The water-dispersed polyurethane is prepared using polyol and polyisocyanate as raw materials, and molecular weight may be increased by using a chain extender.

Specifically, the water-dispersed polyurethane may be prepared using a polyol, a polyisocyanate, and a chain extender as raw materials.

Specifically, the water-dispersed polyurethane of the present invention comprises the steps of preparing a prepolymer by reacting a urethane raw material containing a polyol and a polyisocyanate; and dropping water to the prepolymer to prepare water in which the prepolymer is dispersed; and adding a chain extender to water in which the prepolymer is dispersed to form an aqueous dispersion polyurethane.

The polyisocyanate is a compound having two or more isocyanate groups in the molecule, isophorone diisocyanate (IPDI), 1,4-cyclohexylmethane diisocyanate (1,4-cyclohexylmethane diisocyanate), 4,4'-dicy Chlohexylmethane diisocyanate (4,4'-dicyclohexylmethane diisocyanate), 4,4'-diphenylmethane diisocyanate (4,4'-diphenylmethane diisocyanate), 2,4-toluene diisocyanate (2,4-toluene diisocyanate) , 2,6-toluene diisocyanate, etc. may be used.

The polyol is a compound having two or more hydroxyl groups in the molecule, and includes polyether polyol, polyester polyol, polylactone polyol, polycarbonate polyol, etc. Various types of polyol compounds used in the art may be used.

For example, as the polyether polyol, polyethylene glycol (PEG), polypropylene glycol (PPG), polytetramethylene ether glycol (PTMEG), etc. may be used, and polyester polyol Polyethylene adipate, polybutylene adipate, polyhexylene adipate, polydiethylene glycol adipate, poly(3-methyl-1,5-pentanediol adipate), etc. may be used as the examples.

The chain extender includes hydrazine, ethylene diamine (EDA), butylene diamine (BDA), 1,6-hexanediamine (1,6-hexane diamine, HDA), isophorone diamine ( Amines such as isophorone diamine (IPDA), diethylene triamine (DETA), and triethylene tetramine (TETA) may be used.

In one embodiment of the present invention, a polyol and polyisocyanate may be mixed with a diol compound including a hydrophilic ionic group as a dispersing agent. As the diol compound containing the hydrophilic ionic group, for example, a diol compound containing a carboxyl group such as dimethylol propionic acid (DMPA) or dimethylolbutanoic acid (DMBA) may be used.

The polyol and the diol compound including a hydrophilic ionic group may be used in a molar ratio of 3:7 to 7:3. When the content of the diol compound containing the hydrophilic ionic group is increased, the content of the ionic group is increased to increase the hydrophilicity of the prepolymer, so it is possible to reduce the amount of acetone as a diluting solvent, thereby securing storage stability even when the content of the solid content is increased can do. In addition, the particle size of the polyurethane may increase and the mechanical strength may increase. However, if the content of ionic groups is too high, the elongation may be deteriorated.

When a diol compound including a hydrophilic ionic group is added as described above, a hydrophilic prepolymer into which a hydrophilic ionic group is introduced as a prepolymer is formed. In this case, the step of neutralizing the hydrophilic prepolymer by adding a neutralizing agent before adding the chain extender after dispersing the prepolymer in water may be further performed.

As the neutralizing agent, triethylamine (TEA), sodium hydroxide (NaOH), potassium hydroxide (KOH), acetic acid, etc. may be used.

In one embodiment of the present invention, acetone may be used in the manufacturing process of the water-dispersible polyurethane. Specifically, after preparing the prepolymer, the step of diluting by adding acetone before dispersing it in water may be further performed, and after adding the chain extender, the step of removing acetone may be further performed.

1 schematically shows a method for producing a water-dispersed polyurethane according to an embodiment of the present invention. Referring to FIG. 1, the water-dispersed polyurethane is prepared by mixing a polyol and a diisocyanate, adding a compound containing a hydrophilic ionic group, and reacting to form a prepolymer including an isocyanate group (-NCO) at the terminal; diluting the prepolymer with acetone; dispersing the prepolymer in water; adding a chain extender to the dispersed prepolymer to extend the chain; And it may be prepared by a method comprising the step of removing the acetone.

In one embodiment of the present invention, the step of dispersing the water-dispersible polyurethane in water may be performed by a phase-inversion method.

In the present invention, the phase inversion method is not a prepolymer dropping method generally used in the process of dispersing a prepolymer in water, that is, a method of dropping the prepolymer in water, but a method of dropping water to the prepolymer. am.

2 schematically shows a process (a) of a conventional prepolymer dropping method and particles (b) formed therefrom. Referring to FIG. 2 , in the prepolymer dropping method, the prepolymer is dispersed in water to form particles. When the prepolymer is added to water, the specific surface area decreases and the formation of particles occurs relatively quickly, so the NCO present in the prepolymer is It may remain located in the center of the particle without reacting. Accordingly, the final molecular weight may be insufficient, and the particle size may also be rather small.

In comparison, FIG. 3 schematically shows the process (a) of the phase inversion method and the particles (b) formed therefrom. Referring to FIG. 3 , in the case of dispersing the prepolymer by dropping water using the phase inversion method according to the present invention, the viscosity of the prepolymer gradually increases and the viscosity decreases at a specific point to be dispersed in water to form particles. . In this process, the hydrophilic portion of the prepolymer is located at the interface of water and the hydrophobic portion is located inside the particle, thereby minimizing the specific surface area for water. As described above, in the phase inversion method, the prepolymer is dispersed in water for a sufficient time, and NCO, which has a relatively high hydrophilicity, is located on the surface of the particles, so that it can easily react with the chain extender. Accordingly, a product having a large molecular weight and a large particle size can be obtained.

In the present invention, the amount of water to be dripped may be 0.5 to 2 times, preferably 1 to 1.5 times, compared to the prepolymer by weight.

In the present invention, the water-dispersed polyurethane may have a weight average molecular weight of 20,000 to 200,000, preferably 50,000 to 150,000, and an average particle diameter of 50 to 200 nm, preferably 80 to 150 nm.

In the present invention, the water-dispersible polyurethane may impart elastic properties, facilitate the formation of ion channels, and improve adhesion between the active material and the current collector. In addition, the water-dispersed polyurethane may be a source for generating carbon quantum dots to be described later.

In one embodiment of the present invention, the water-dispersible polyurethane may be used in a mixed form with carbon quantum dots.

The carbon quantum dots have an advantage of being well dispersed in an aqueous system due to their small size and excellent dispersibility, and can be prepared using water-dispersible polyurethane. In addition, when applied to the electrode, it is possible to form an electrode having excellent mechanical properties by interacting with the water-dispersed polyurethane. In addition, when applied to the positive electrode of a lithium-sulfur battery, the abundant functional groups present in the carbon quantum dots adsorb lithium polysulfide, thereby solving the problems of deterioration of battery life and capacity due to lithium polysulfide.

In the present invention, the carbon quantum dots may be formed by hydrothermal treatment of water-dispersed polyurethane. The hydrothermal treatment may be performed at a temperature of 100 to 500° C., preferably 150 to 300° C., and may be performed for 1 to 10 hours, preferably 2 to 6 hours.

In the present invention, the carbon quantum dot (carbon quantum dot, CQD) may be a multifunctional carbon quantum dot (multifunctional CQD). Specifically, the polyfunctional carbon quantum dots may be in the form of an electrically conductive core made of carbon, and a highly polar shell including at least one functional group among a hydroxyl group, a carboxyl group, and an amine group.

In the multifunctional carbon quantum dot, the electrically conductive core can easily provide an electron pathway, and the highly polar shell is rich in activity capable of chemisorption and localization of lithium polysulfide. You can provide an active site. In addition, a stable and long-life lithium-sulfur battery can be manufactured by improving mechanical properties by forming a network with water-dispersed polyurethane with abundant functional groups.

4 schematically shows a composite of water-dispersible polyurethane and polyfunctional carbon quantum dots according to the present invention. Referring to FIG. 4 , a process for producing carbon quantum dots from water-dispersed polyurethane and mixing them to form a network of water-dispersed polyurethane and carbon quantum dots to adsorb lithium polysulfide while carbon quantum dots interact with water-dispersed polyurethane You can check the principle.

In the present invention, the carbon quantum dots may be included in 1 to 15% by weight based on the total weight of the binder composition, preferably 2 to 10% by weight, more preferably 3 to 8% by weight, most preferably 4 to 6% by weight % may be included. In the above range, while the effect of carbon quantum dots adsorbing LPS appears, it is possible to manufacture a battery having excellent specific capacity and charge/discharge performance, and a long lifespan. In particular, in the embodiment of the present invention, it was confirmed that this effect was optimized when the content of carbon quantum dots was 5% by weight.

The present invention also provides an electrode for a lithium secondary battery including the binder composition and a lithium secondary battery including the electrode.

The electrode for a lithium secondary battery of the present invention may include an electrode active material and the binder composition of the present invention. Here, the electrode active material may include at least one selected from a cyclic elemental sulfur material (S8), a sulfur-based compound, and a mixture thereof, and the sulfur-based compound is specifically, Li ₂ S _n (n≥ 1), an organic sulfur compound or a carbon-sulfur polymer ((C ₂ S _x ) _n , 2.5≤x≤50, n≥2), and the like.

As such, when the active material of the electrode includes a sulfur material, the electrode may be a cathode of a lithium-sulfur battery.

The lithium secondary battery of the present invention may have a structure including a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte. Preferably, the lithium secondary battery comprises: a positive electrode including a positive active material including a sulfur material and a binder composition for an electrode of the present invention; a negative electrode including a negative active material including a lithium material; a separator positioned between the anode and the cathode; and a lithium-sulfur battery including an electrolyte.

A lithium-sulfur battery is a secondary battery that uses sulfur for the positive electrode and lithium metal for the negative electrode, and the following reaction occurs in the cell of the battery during discharge.

Anodic Reaction: 2Li → 2Li ⁺ + 2e ^-

Cathodic Reaction: S + 2e ^- → S ^2-

Overall Reaction: 2Li + S → Li ₂ S

Specifically, during discharging of a lithium-sulfur battery, an oxidation reaction of lithium occurs in the negative electrode, and lithium ions move to the positive electrode and react with sulfur. As a result, solid sulfur (S8) of a cyclic structure is converted into lithium polysulfide (Lithium Polysulfide, Li ₂ S _x , 1<x≤8, LPS) of a linear structure by a reduction reaction, and when the lithium polysulfide is completely reduced Lithium sulfide (Li ₂ S) is produced.

The lithium polysulfide has high solubility in an organic electrolyte and is continuously dissolved during the discharge reaction to reduce the active material of the positive electrode, which may cause deterioration of battery life.

In addition, during the charging process, lithium polysulfide travels back and forth between the lithium metal surface and the positive electrode surface, resulting in a 'shuttle effect' in which the final product, sulfur, is not produced and the charging process continues. The lithium metal and the active material are damaged, making it difficult to drive the battery.

In addition, since sulfur itself has very low electrical conductivity, only sulfur cannot be used as a cathode material, so a technology for forming a composite and coating with a conductive material and polymer is required. In this case, the energy density of the entire cell is reduced due to materials other than sulfur Therefore, it also has a technical task to maximize the sulfur content in the battery.

The present invention is to overcome the disadvantages and limitations of such lithium-sulfur batteries. When the binder composition of the present invention is applied to the positive electrode of a lithium-sulfur battery, the shell of carbon quantum dots included in the binder composition is mutually with lithium polysulfide. By acting, lithium polysulfide is adsorbed, and problems of shuttle effect and lifetime deterioration caused by lithium polysulfide can be prevented. In addition, an electrode network having excellent mechanical properties can be formed by hydrogen bonding between the water-dispersed polyurethane of the present invention having excellent elasticity and mechanical strength and carbon quantum dots.

In the lithium-sulfur battery, as the anode active material, a material capable of reversibly intercalating or deintercalating lithium ions, may react with lithium ions to form a lithium-containing compound reversibly material, lithium metal or lithium alloy can be used.

The material capable of reversibly intercalating or deintercalating the lithium ions may be crystalline carbon, amorphous carbon, or a mixture thereof, and the material capable of reversibly forming a lithium-containing compound by reacting with the lithium ions is It may be tin oxide, titanium nitrate or silicon. In addition, the lithium alloy may be an alloy of lithium and at least one metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

In the present invention, each electrode may further include a conductive material. The conductive material may include at least one selected from the group consisting of carbon fibers, carbon nanotubes, carbon black, metal powder, and conductive metal oxides.

In the present invention, each electrode may further include a polymer binder. Examples of the polymer binder include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, tetrafluoroethylene-perfluoroalkylvinylether copolymer, vinylidene fluoride- Hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoro Roethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene copolymer, ethylene -Acrylic acid copolymer, etc. can be used individually or in mixture.

In the lithium secondary battery, the positive electrode and the negative electrode may be prepared by coating a slurry containing the active material of each electrode on a current collector.

The current collector may be made of one or more materials selected from stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum, and, if necessary, may be used by treating the surface of the material with carbon, nickel, titanium or silver.

A separator may be positioned between the anode and the cathode. The separator separates or insulates the positive electrode and the negative electrode from each other and enables lithium ions to be transported between the positive electrode and the negative electrode, and may be made of a non-conductive or insulating material. Such a separator may be an independent member such as a film, or may be a coating layer added to the positive electrode and/or the negative electrode.

The material constituting the separator is not particularly limited as long as it is a material capable of physically separating the electrodes, and for example, a polyolefin-based polymer film such as polyethylene and polypropylene, glass fiber filter paper, or a ceramic material may be used. The separator may have a thickness of about 5 to 50 μm, preferably about 5 to 25 μm, and a porous structure having a porosity of 30 to 50%.

The electrolyte of the battery is a non-aqueous solvent serving as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, carbonate-based, ester-based, ether-based, ketone-based, alcohol-based or aprotic solvents can be used. there is.

The carbonate-based solvent is specifically dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC), etc. may be used. The ester solvent is specifically methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methylpropionate, ethylpropionate, γ-butyrolactone, decanolide (decanolide), Valerolactone, mevalonolactone (mevalonolactone), caprolactone (carprolactone) and the like may be used. Specifically, the ether solvent is diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetrahydro Furan, 2-methyltetrahydrofuran, or polyethylene glycol dimethyl ether may be used. Specifically, cyclohexanone and the like may be used as the ketone-based solvent. Specifically, as the alcohol-based solvent, ethyl alcohol, isopropyl alcohol, or the like may be used. Specifically, the aprotic solvent may include nitriles such as acetonitrile, amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane (DOL), or sulfolane. The non-aqueous organic solvent may be used alone or in a mixture of one or more, and the mixing ratio may be appropriately adjusted according to the desired battery performance. In particular, a 1:1 volume ratio of 1,3-dioxolane and dimethoxyethane is used. may be desirable.

The lithium secondary battery prepared according to the present invention, particularly the lithium-sulfur battery, has excellent electrochemical properties, excellent capacity and lifespan, excellent elasticity and mechanical strength, and has advantages of being manufactured from an eco-friendly material. The lithium secondary battery may constitute a battery module as a unit battery, and the battery module may be effectively used in various fields such as an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a high-capacity storage device.

Example

The present invention will be described in more detail with reference to the following examples. However, these Examples show some experimental methods and compositions for illustrative purposes of the present invention, and the scope of the present invention is not limited to these Examples.

Preparation Example 1: Water-dispersed polyurethane synthesis

Isophorone diisocyanate (98%, IPDI), 2,2-bis(hydroxymethyl)butyric acid (98%, DMBA), trimethylamine (99%, TEA) and hydrazine (HYD) were obtained from Sigma-Aldrich ) was purchased and used. Acetone was purchased from KP Hanseok Chemical, and poly(3-methyl-1,5-pentanediol adipate) was purchased from Fine Chemical Co., Ltd. and used. All materials were used without further purification. The reaction was carried out in a 2L, four neck jacketed reactor equipped with a mechanical stirrer, thermometer, reflux condenser, temperature controller and nitrogen inlet.

5 schematically shows a method for producing a water-dispersed polyurethane according to Preparation Example 1.

First, 0.5 moles of poly(3-methyl-1,5-pentanediol adipate) and 1.4 moles of IPDI were charged to a reactor, and the mixture was heated at 85° C. for 30 minutes.

Then, 0.5 mol of DMBA was mixed well and reacted until the amount of residual NCO groups reached the theoretical value by di-n-butylamine back-titration method under nitrogen atmosphere. The theoretical amount of residual NCO groups is calculated as when all hydroxyl groups are consumed by the NCO groups. For optimal reaction, the reaction was carried out at 90° C. for 3 hours.

After obtaining a hydrophilic prepolymer as a product of the reaction, it was cooled to 50° C. and acetone was added to reduce the viscosity. Then, 0.5 mol of TEA was added and the prepolymer was neutralized at 50° C. for 45 minutes.

Next, 350 mL of distilled water was added dropwise to 300 g of the neutralized prepolymer and stirred at 100 rpm for 1 hour. After dispersing the prepolymer in water, the reaction mixture was cooled to 30° C. and 0.5 mol of hydrazine was added to extend the chain. Next, acetone was distilled at a temperature of 45° C. and vacuum conditions to obtain an aqueous dispersion polyurethane (WPU).

Experimental Example 1: Analysis of composition and thermal properties of water-dispersed polyurethane

For the water-dispersed polyurethane of Preparation Example 1, composition and thermal properties were analyzed through FT-IR and differential scanning analysis. For comparison, water-dispersed polyurethane was prepared using the method of Preparation Example 1, but by dropping the prepolymer into water when dispersing the prepolymer, and the same measurement was performed.

FT-IR analysis (Tensor 27, Bruker, Germany) was performed by scanning each sample 64 times with a resolution of 4 cm ^-1 in the range of 400 to 4000 cm ^-1 , and the results are shown in FIG. 6 .

6, -NCO band (2,270cm ^-1 ) and -OH band (3590cm ^-1 ) did not appear in the product, and it was confirmed that the synthesis of APUD was completed therefrom. In addition, the NH peak appeared almost constant, and it was confirmed that the NCO/OH ratio was almost constant in the water-dispersed polyurethane product using the two methods.

The glass transition temperature of the water-dispersed polyurethane was confirmed with a differential scanning calorimeter (PerkinElmer, DSC4000). The sample was heated from 25° C. to 450° C. at a rate of 10° C./min, cooled to 25° C., and reheated to 450° C. in a dry nitrogen atmosphere to obtain a DSC analysis result, as shown in FIG. 7 .

Referring to FIG. 7 , the glass transition temperatures of the two products were similar at -55°C (prepolymer dropping method) and -53°C (phase inversion method), respectively. From this, it can be confirmed that their thermal properties are similar.

Experimental Example 2: Analysis of molecular weight and particle size of water-dispersed polyurethane

For the water-dispersed polyurethane of Preparation Example 1, molecular weight and particle size were analyzed. For comparison, water-dispersed polyurethane was prepared using the method of Preparation Example 1, but by dropping the prepolymer into water when dispersing the prepolymer, and the same measurement was performed. The water-dispersed polyurethane of Preparation Example 1 and the solid content of the water-dispersed polyurethane prepared using the prepolymer dropping method were 44.3% and 44.6%, respectively.

The molecular weight of the aqueous dispersion polyurethane was measured by gel permeation chromatography (GPC, YL9100 liquid chromatography, Youngin chromass) using a differential refractometer detector. The molecular weight was calculated using a calibration curve obtained as a polystyrene standard in tetrahydrofuran at 30°C. The particle size and distribution and zeta potential of the water-dispersed polyurethane were measured with a particle size analyzer (ZetaSizer, Malvern Instruments). The measurement results of GPC, particle size and zeta potential are shown in FIGS. 8 to 10 and Table 1 below, respectively.

division Prepolymer drip method reversal method Mw (g/mol) 40,906 107,794 Particle size (nm) 66.36 109.42 Zeta potential (mV) -42.0 -49.3

The molecular weight of the water-dispersed polyurethane prepared by the prepolymer dropping method and the phase inversion method was 40,906 g/mol and 107,794 g/mol, respectively, and the average particle size was 66.36 nm and 109.42 nm, respectively. From this, it was confirmed that, when the phase inversion method is used, water-dispersed polyurethane having a larger particle size and molecular weight than the prepolymer dropping method can be synthesized.

In addition, the zeta potentials of the water-dispersed polyurethane synthesized by the prepolymer dropping method and the phase inversion method were -42.0 mV and -49.3 mV, respectively, which were more negative than -30 mV. Through this, it was confirmed that the dispersion stability of the water-dispersed polyurethane synthesized by the phase inversion method was more excellent.

Preparation Example 2: Preparation of WPU-CQD composite using water-dispersible polyurethane (WPU)

Using the method of Preparation Example 1, but using polypropylene glycol (PPG) instead of poly (3-methyl-1,5-pentanediol adipate) water-dispersed polyurethane (waterborne polyurethane dispersion, hereinafter referred to as WPU) prepared.

The prepared WPU was hydrolyzed to obtain carbon quantum dots (CQD). Specifically, WPU was mixed with sulfuric acid, heated at 200° C. for 2 hours, and then cooled to room temperature. After mixing with distilled water, the carbon quantum dots were transferred to water by ultrasonic treatment, and filtered through a cellulose filter to obtain carbon quantum dots.

Using a magnetic stirrer, 2.5, 5.0, and 10.0 wt % of CQD, respectively, were mixed in WPU at 25° C. for 3 hours. Thus, WPU-CQD2.5, WPU-CQD5 and WPU-CQD10 were prepared as WPU-carbon quantum dot (CQD) composites (WPU-CQD).

UV light (365 nm) was irradiated to the WPU-CQD composite, and the emission color was observed. 11 shows an image of the WPU-CQD5 composite film before and after UV irradiation.

From FIG. 11 , it was confirmed that the WPU-CQD5 film emitted radiation upon UV light irradiation, and the color of the film changed from dark brown to light blue. From this, it was possible to confirm the presence of CQDs in the film.

Experimental Example 3: Affinity measurement of WPU-CQD composite and LPS

For the WPU-CQD composite of Preparation Example 2, affinity (affinity) with lithium polysulfide (LPS) was measured to evaluate the shuttle effect inhibition performance.

To evaluate the LPS adsorption performance of WPU-CQD2.5, WPU-CQD5, and WPU-CQD10, UV-Vis analysis of 0.1 mM Li ₂ S ₆ in an electrolyte (DOL/DME) containing 10 mg of WPU-CQD prepared was performed, and the results are shown in FIG. 12 .

Referring to FIG. 12 , the LPS solution exhibits absorbance in a region of 200 to 350 nm. In the absorbance graph, the peaks at 260 nm and 280 nm mean the peaks of the S ₆ ^2- moiety. After adding WPU, WPU-CQD or CQD to the Li ₂ S ₆ solution, the intensity of the S ₆ ^2- peak significantly decreased. could confirm that

In addition, as the content of CQD in the WPU matrix increased, the absorbance at 260 nm and 280 nm in the LPS solution decreased. From this, it was confirmed that WPU-CQD interacts with LPS, which is expected to inhibit the shuttle effect of LPS.

Experimental Example 4: Measurement of mechanical properties of WPU-CQD composites

The mechanical properties of WPU-CQD were evaluated using a nanoindentation test, and the results are shown in FIG. 13 , and the elastic modulus and hardness were compared therefrom.

Referring to FIG. 13 , the WPU-CQD showed an increase in elastic modulus and hardness by 150 to 200% compared to WPU without CQD, and as the content of CQD increased, the effect of improving mechanical properties was excellent. From these results, it was confirmed that WPU-CQD can effectively control the volume change of sulfur in battery cycling.

Preparation Example 3: Lithium-sulfur battery manufacturing using WPU-CQD

A slurry for an electrode was prepared using WPU-CQD of Preparation Example 2, and a cathode was prepared by coating the slurry on aluminum foil.

Specifically, sulfur/Ketjen black (S/KJB) composite (7:3, w/w), carbon black, and WPU-CQD5 binder of Preparation Example 2 were mixed in a weight ratio of 8:1:1 in a vibrating ball mill machine (PULVERISETTE 23 , Fritsch, Germany) to prepare a slurry. Then, the slurry mixed on an aluminum foil (Hosen, Japan) having a thickness of 20 μm was uniformly applied with a doctor blade, dried in a convection oven at 50° C. for 12 hours, and in a vacuum oven at 50° C. for 6 hours to prepare a sulfur anode did. The amount of sulfur loading of the prepared positive electrode was about 1.0±0.1 mg/cm ² .

A lithium-sulfur battery was prepared in which the sulfur positive electrode and the lithium negative electrode were separated by a polypropylene film (Celgard membrane 2400), and a mixed solution of DOL and DME (1:1, v/v) was used as the electrolyte.

Experimental Example 5: Measurement of electrochemical properties of Li-S cells prepared using WPU-CQD

Electrochemical impedance spectroscopy (EIS) was performed on the Li-S battery of Preparation Example 3. EIS measurement was performed by applying a 50 mV amplitude sine wave in the frequency range of 0.1 Hz to 100 kHz, and a potentiostat (VSP, BioLogic, France) was used for measurement.

A Nyquist plot graph was obtained through EIS measurement and shown in FIG. 14, and the electrochemical properties of WPU-CQD were evaluated based on this.

Referring to FIG. 14 , the resistance decreased as the content of CQD in WPU-CQD increased, and R _ct in WPU-CQD5 and WPU-CQD10 was 43.7Ω and 40.8Ω, respectively, indicating similar values.

Since the lower cell resistance can facilitate the movement of ions and electrons at the anode, it can be confirmed that WPU-CQD5 and WPU-CQD10 are preferable in this respect.

Experimental Example 6: Battery performance analysis of Li-S cells prepared using WPU-CQD

In order to measure the electrochemical properties of the Li-S cell of Preparation Example 3, the cyclic voltage scanning method (scan rate) was 0.1 mV/s in the voltage (V vs. Li/Li+) range of 1.7-2.8V ( cyclic voltammetry (CV) was performed. In addition, galvanostatic charge-discharge analysis was performed under 0.2C conditions.

A graph of the CV and constant current charge/discharge analysis results is shown in FIG. 15, and the voltage difference obtained from the CV and the initial specific capacity at 0.2 C are shown in Table 2 below.

division Voltage difference (mV) Initial specific capacity (mAh/g) WPU 295 843 WPU-CQD2.5 271 975 WPU-CQD5 210 1,123 WPU-CQD10 251 1,080

As a result, the redox current density peak was the highest in the Li-S cell containing WPU-CQD5, and showed the lowest voltage difference in the charge/discharge profile. In addition, it was confirmed that the Li-S cell containing WPU-CQD5 had the highest initial specific capacity at 0.2C.

According to these results, it was confirmed that the most optimal electrochemical performance was shown in WPU-CQD5, and the kinetic reaction proceeded the most easily and the capacity was high.

Experimental Example 7: Ion transport performance measurement of Li-S cells prepared using WPU-CQD

In order to confirm the ion transport performance of the Li-S cell containing WPU-CQD5, a graph obtained by calculating the slope value from the CV result is shown in FIG. 16 .

Referring to FIG. 16 , in the case of Li-S cells including WPU-CQD5, oxidation peak current vs. WPU. The slope of the v ^1/2 plot increased by 128% in O1, while the reduction peak current vs. The slope of the v ^1/2 plot increased by 134% in R1 and 165% in R2.

According to the above results, it can be seen that the diffusion kinetics is improved, the electrochemically active spot is formed, and the stability is improved because the Li ⁺ diffusion path is fast and short in WPU-CQD5.

Experimental Example 8: Confirmation of charge/discharge characteristics of Li-S cells manufactured using WPU-CQD

For Li-S cells manufactured using the WPU-CQD composite, the change in specific capacity was measured while repeating the charging and discharging cycles 50 times.

The change in specific capacity is shown in FIG. 17 under the condition that the C-rate is 0.2C (that is, the discharge condition for 5 hours). As a result of comparing the specific capacity based on FIG. 17, the stability was better when CQD was used as compared to the case where WPU alone was used. The specific capacity was shown, and the coulombic efficiency was the highest.

In addition, the battery capacity was measured at 0.05, 0.2, 0.5, 1, 2, 5, and 10C conditions for a Li-S cell including WPU-CQD5 and shown in FIG. 18 . As a result of comparing the average specific capacity under each rate-rate condition, values of 1,432, 1,123, 857, 728, 576, 412 and 245 mAh/g were shown. In addition, it was confirmed that the capacity recovery rate was excellent at 98.6% in 2C.

Through this, when WPU-CQD5 is used for the positive electrode of a Li-S battery, it can be confirmed that the elastic property is excellent and ion/electron transport is easy, and thus the speed property is excellent.

Experimental Example 9: Confirmation of long-term stability of Li-S cells prepared using WPU-CQD

The long-term stability of Li-S cells containing WPU-CQD5 was evaluated. Charging and discharging was repeated 1,000 times under 2C conditions similar to most portable energy storage device conditions, and the specific capacity change is shown in FIG. 19 .

Referring to FIG. 19 , the initial specific capacity of the Li-S cell including WPU-CQD5 was 725 mAh/g, and decreased to 522 mAh/g after 1,000 charge/discharge cycles. From this, as a result of calculating the decrease in the capacity maintenance rate per episode, it was confirmed that it was only 0.02%.

Accordingly, it was found that excellent long-term stability can be secured when WPU-CQD5 is applied to the anode of the Li-S cell.

From the above description, those skilled in the art to which the present invention pertains will understand that the present invention may be embodied in other specific forms without changing the technical spirit or essential characteristics thereof. In this regard, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of the present invention should be construed as being included in the scope of the present invention, rather than the above detailed description, all changes or modifications derived from the meaning and scope of the claims to be described later and their equivalents.

Claims (1)

water-dispersible polyurethane; and
Multifunctional carbon quantum dots comprising an electrically conductive core made of carbon and a highly polar shell including at least one functional group selected from a hydroxyl group, a carboxyl group, and an amine group
A binder composition for an electrode comprising a.

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