KR101116551B1 - Negative electrode for lithium secondary battery and lithium secondary battery comprising same - Google Patents

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to improve the coating property of the negative electrode active material slurry by reducing the surface tension of the aqueous solvent to improve the smoothness of the negative electrode, and has excellent life characteristics. It relates to a negative electrode for a secondary battery and a lithium secondary battery comprising the same.

Description

A negative electrode for a lithium secondary battery, and a lithium secondary battery including the same {NEGATIVE ELECTRODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING SAME}

1 is a view schematically showing the structure of a lithium secondary battery of the present invention.

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to improve the coating property of the negative electrode active material slurry by reducing the surface tension of the aqueous solvent to improve the smoothness of the negative electrode, and has excellent life characteristics. It relates to a negative electrode for a secondary battery and a lithium secondary battery comprising the same.

Recently, with the trend toward miniaturization and light weight of portable electronic devices, the need for high performance and high capacity of batteries used as power sources for these devices is increasing. In addition, research has been focused on batteries having excellent safety and economical efficiency.

In general, the battery can be divided into a primary battery used for single use and a secondary battery that can be used for recharging. The primary battery may be a manganese battery, an alkaline battery, a mercury battery, or a silver oxide battery, and the secondary battery may be a lead acid battery, a Ni-MH battery, a sealed nickel-cadmium battery, a lithium metal battery, a lithium ion battery, or a lithium polymer. Batteries, lithium-sulfur batteries and the like.

Among these batteries, non-aqueous electrolyte secondary batteries using lithium-containing metal oxides exhibiting a voltage of 4V as the positive electrode active material, and carbonaceous materials capable of intercalating or deintercalating lithium, are particularly suitable for high voltage and Expectations are high as a battery with high energy density.

A lithium secondary battery is a battery manufactured by combining a separator with an organic electrolyte that provides a movement path of lithium ions between a negative electrode, a positive electrode, and a lithium ion, when lithium ions are intercalated / deintercalated at the positive electrode and the negative electrode. Electrical energy is generated by oxidation and reduction reactions. Such lithium secondary batteries can be classified into lithium ion batteries using liquid electrolytes and lithium polymer batteries using solid electrolytes, depending on the type of separator.

In the lithium secondary battery, the positive electrode and the negative electrode are made of a material capable of inserting and deinserting lithium ions. Looking at the forming material of the electrode, a lithium-containing metal oxide such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiMnO _2, and the like are used as a cathode active material.

As an anode active material of a lithium battery, the chemical potential at the time of insertion / deintercalation of lithium metal, a lithium-containing alloy, or lithium ion capable of reversibly accepting or supplying lithium ions while maintaining structural and electrical properties is substantially different from that of metallic lithium. Similar carbonaceous materials are mainly used. The use of lithium metal or an alloy thereof as a negative electrode active material is called a lithium metal battery, and the use of a carbon material is called a lithium ion battery.

Lithium metal batteries using lithium metal or lithium alloys as negative electrodes are explosive due to battery short circuits due to the formation of dendrite, and are therefore being replaced by lithium ion batteries using carbon materials having no such risk as negative electrode active materials. . Lithium ion batteries have only a movement of lithium ions during charge and discharge, and thus the electrode active material remains intact, thereby improving battery life and stability compared to lithium metal batteries.

In accordance with the trend toward higher capacity of the battery, safety requirements are further increased, and thus, a functional material should be selected to ensure safety even at a high capacity in the case of a lithium ion battery. In particular, as battery capacity is required to increase, battery capacity maintenance and battery safety must also be accompanied, so battery manufacturers continue to make efforts to secure safety with increasing battery capacity.

In the case of lithium ion batteries, a non-aqueous system in which polyvinylidene fluoride (PVDF) is dissolved in N-methyl-2-pyrrolidone (hereinafter referred to as NMP) or an acetone organic solvent is widely known as a binder of a negative electrode plate. I use it. However, when such a PVDF / NMP non-aqueous system is used as a binder, organic solvents such as NMP or acetone may contaminate the environment, and because the price is relatively high, the manufacturing cost of the lithium battery is not only increased but also mostly volatile. When used in an enclosed space, there is a problem that requires an explosion-proof installation to prevent this because of the explosion problem.

In addition, PVDF should be used at least 6 to 8% by weight based on the total weight of the negative electrode active material in order to give a sufficient binding force between the electrode plate and the active material due to poor adhesion. As the amount of binder used increases, the amount of the negative electrode active material decreases so that the capacity of the battery cannot be increased, and fluorine and lithium ions of PVDF react to form LiF, which is one of the causes of thermal runaway. This reduces the safety of the lithium ion battery. In particular, as the lithium ion battery becomes higher in capacity, such a phenomenon is increased, which makes it difficult to secure battery safety.

Recently, in order to overcome this problem, in manufacturing a negative electrode plate, an aqueous binder in which styrene-butadiene rubber (hereinafter referred to as "SBR") is dispersed in water together with carboxymethyl cellulose (hereinafter referred to as "CMC") as a thickener. There are many attempts to use the system. The SBR binder can be dispersed in water in the form of an emulsion, which does not require the use of an organic solvent, and because of its strong adhesive strength, it is advantageous to reduce the content of the binder and increase the content of the negative electrode active material to high capacity of the lithium battery.

However, when the aqueous solvent is used, the applicability of the negative electrode active material slurry is deteriorated due to the high surface tension of the aqueous solvent, which causes a problem of deterioration of the smoothness of the negative electrode. In addition, due to the poor smoothness of the negative electrode, the surface of the negative electrode becomes nonuniform, resulting in a decrease in reaction surface area and a decrease in lifetime characteristics.

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to improve the coating property of the negative electrode active material slurry by reducing the surface tension of the aqueous solvent to improve the smoothness of the negative electrode, and has excellent life characteristics. It is to provide a cathode for a secondary battery.

Another object of the present invention is to improve the smoothness of the negative electrode by reducing the surface tension of the aqueous solvent by using the negative electrode, and to provide a lithium secondary battery having excellent life characteristics.

In order to achieve the above object, the present invention provides a negative electrode for a lithium secondary battery comprising a negative electrode active material, an SBR binder and a thickener, a surfactant, and an aqueous solvent.

In order to achieve the above technical problem, the present invention provides a positive electrode in which a positive electrode active material slurry is described in a positive electrode current collector, a negative electrode in which a negative electrode active material slurry is described in a negative electrode current collector, and a separator and a non-aqueous organic solvent interposed therebetween. Lithium salt is dissolved in the electrolyte, and the negative electrode provides a lithium secondary battery comprising a negative electrode active material, an SBR binder, a thickener, a surfactant and an aqueous solvent.

Hereinafter, the present invention will be described in more detail.

The negative electrode according to an embodiment of the present invention comprises a thickener, a surfactant and an aqueous solvent for controlling the viscosity of the SBR binder and the negative electrode active material slurry used to apply the negative electrode active material slurry on the negative electrode active material slurry and the negative electrode current collector. Include.

The negative electrode active material of the present invention is a crystalline or amorphous carbon, or a carbon-based negative electrode active material (thermally decomposed carbon, coke, graphite) of carbon composites, burned organic polymer compound, carbon fiber, tin oxide compound, lithium metal or lithium alloy Can be made. Preferred negative electrode active material is crystalline carbon, Lc is 150 kPa or more, preferably 150 to 3000 kPa, d (002) is 3.35 to 3.38 kPa, true density is 2.2 g / cm ³ or more, preferably 2.2 to 2.3 g / It is more preferable that they are crystalline carbon and graphite which are cm <^3> , BET (specific surface area) values are 0.5-50 m <2> / g, and an average particle diameter (D50) is 1-30 micrometers.

The SBR binder can eliminate problems that may occur when using a fluorine-containing binder such as PVDF, a copolymer of vinylidene chloride and hexafluoropropylene, and the like. Moreover, the adhesive force with an electrode plate is also superior to the said fluorine-containing binder.

The content of the SBR binder is preferably 0.8 to 5% by weight, more preferably 1 to 5% by weight, and most preferably 1 to 2% by weight based on the total weight of the negative electrode active material. If the content of the SBR binder is less than 0.8 wt%, the binder content is too small to provide insufficient adhesion between the negative electrode active material and the current collector. If the content exceeds 5 wt%, the amount of the negative electrode active material decreases by an amount exceeding the battery capacity. It is difficult to achieve high capacity.

The thickener used for the purpose of viscosity control of the negative electrode active material according to the present invention may be selected from the group consisting of carboxy methyl cellulose, hydroxy methyl cellulose, hydroxy ethyl cellulose and hydroxy propyl cellulose. The content of the thickener is preferably 0.8 to 5% by weight, more preferably 1 to 5% by weight, and most preferably 1 to 2% by weight based on the entire negative active material. If the content of the thickener is less than 0.8% by weight, there is a problem that the negative electrode active material flows down when coating the negative electrode active material, and when the content of the thickener exceeds 5% by weight, the coating of the negative electrode active material is difficult, and there is a problem of acting as a resistance. In addition, the content of the negative electrode active material by the excess content is reduced, there is a problem that it is difficult to achieve high capacity of the battery capacity.

The aqueous solvent according to the present invention can be used with water or an alcohol solvent which can be mixed with water.

The surfactants according to the invention generally have a hydrophilic head group and a hydrophobic tail group in one molecule. The hydrophilic head group may have an ionic group and a nonionic group. Ionic groups have electrostatic bonds and nonionic groups have hydrogen bonds.

The surfactant having such a structure binds to and stabilizes the surface of the material dispersed in the dispersion process in which other materials are dispersed in the form of particles in a uniform material, and lowers the surface tension to increase dispersibility and wettability. Therefore, when the negative electrode active material slurry by the aqueous binder system is applied to the surface of the negative electrode current collector, the applicability of the negative electrode active material slurry is improved to improve the smoothness of the negative electrode.

The surfactant may be selected from nonionic, cationic and anionic surfactants. The nonionic surfactant may be polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, polyoxyethylene fatty acid amide, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, glycerin fatty acid ester, polyoxyethylene glycerin fatty acid ester, poly At least one selected from the group consisting of copolymers of oxyethylene and polyoxypropylene, and the cationic surfactant may be selected from the group consisting of alkyl ammonium salts, alkylpyridinium salts and alkylisoquinolinium salts. have. In addition, the anionic surfactant may be selected from the group consisting of carboxylate, acyl amino acid ester salt, alkyl sulfate ester salt, alkyl sulfonic acid ester salt, alkyl phosphate ester salt.

The nonionic surfactants include polyoxyethylene octylphenyl ether, polyoxyethylene cetyl ether, polyoxyethylene lauryl ether, polyethylene glycol monostearate, polyoxyethylene glycol monolaurate, polyoxyethylene polyoxypropylene glycol, stearic Acid monoethanol amide, sorbitan monolaurate and the like can be used.

In addition, the cationic surfactant may be alkyl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, lauryl trimethyl ammonium chloride, cetyl pyridinium chloride, etc., and as anionic surfactants potassium hydroxide, sodium hydroxide, poly Oxyethylene alkylphenyl ester phosphate, sodium polyoxyethylene alkylphenyl ester phosphate, sodium polyoxyethylene lauryl ester sulfate, sodium N-lauryl-L-glutamate, sodium cetyl sulfate, sodium lauryl sulfate, sodium tetradecene sulfonate, etc. Can be used.

The molecular weight of the surfactant is preferably from 100 to 100,000, more preferably from 500 to 10,000. If the weight average molecular weight of the surfactant is less than 100, there is no effect using the surfactant, if the weight exceeds 100,000 has a disadvantage of increasing the viscosity of the negative electrode active material slurry is difficult to apply the negative electrode active material slurry to the negative electrode current collector.

The amount of the surfactant is preferably 0.1 to 10% by weight, more preferably 1 to 5% by weight based on the weight of the negative electrode active material. If the content of the surfactant is less than 0.1% by weight based on the weight of the negative electrode active material, there is a disadvantage in low dispersibility, and when the content of the surfactant exceeds 10% by weight, the content of the negative electrode active material decreases, making it difficult to increase the capacity of the battery.

Hereinafter, a lithium secondary battery employing a negative electrode for a lithium secondary battery according to the present invention will be described.

According to an embodiment of the present invention, a lithium secondary battery includes a positive electrode in which a positive electrode active material slurry is described in a positive electrode current collector, a negative electrode in which a negative electrode active material slurry is described in a negative electrode current collector, and a separator and a nonaqueous organic material interposed therebetween. A lithium salt is dissolved in a solvent, and the negative electrode active material slurry may include a negative electrode active material, an SBR binder, a CMC thickener, a surfactant, and an aqueous solvent.

In the lithium secondary battery according to an embodiment of the present invention, the negative electrode is as described above, and the positive electrode is one or one selected from the group consisting of LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiCrO ₂ , and LiMn ₂ O ₄ . The cathode active material and the binder including the above may include one or more conductive agents selected from the group consisting of carbon black, acetylene black, ketene black. The positive electrode current collector may use aluminum or an aluminum alloy.

As the binder added to the cathode active material, a fluorine-containing binder such as PVDF, a copolymer of vinylidene chloride and hexafluoropropylene may be used.

The electrolyte solution of the present invention contains a non-aqueous organic solvent and a lithium salt. This non-aqueous organic solvent acts as a medium through which ions involved in the electrochemical reaction of the cell can move. The non-aqueous organic solvent may be used by mixing one or two or more selected from the group consisting of cyclic carbonates, acyclic carbonates, aliphatic carboxylic acid esters, acyclic ethers, cyclic ethers, alkyl phosphate esters or fluorides thereof.

In addition, the non-aqueous organic solvent may be used by mixing one or two or more selected from the group consisting of the cyclic carbonate, acyclic carbonate, aliphatic carbolic acid ester.

Ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, etc. may be used as the cyclic carbonate, and the acyclic carbonate may be dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, di Propyl carbonate, methyl ethyl carbonate, and the like can be used, and as the fatty carboxylic acid ester, methyl porate, methyl acetate, methyl propionate, ethyl propionate, and the like can be used.

In addition, as the acyclic ether, gamma-lactones, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, etc. may be used, and as the cyclic ether, tetrahydrofuran, 2 -Methyltetrahydrofuran and the like can be used. As the alkyl diacid ester, dimethyl sulfoxide, 1,2-dioxolane, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or the like can be used.

The lithium salt acts as a source of lithium ions in the battery to enable operation of the basic lithium battery. As the lithium salt, the lithium salt is LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO _3, LiSbF _6, CF ₃ SO ₃ Li, LiN (SO ₂ CF ₃ ) ₂ , LiC ₄ F ₉ SO _{3 ,} LiAlF ₄ , LiAlCl ₄ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (C _X F _{2X + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural numbers, LiCl, LiI, and the like One or two or more of them may be used in combination.

The separator according to the present invention may be a polymer membrane such as polyolefin, polypropylene, polyethylene or the like, or a multi-layered film, a microporous film, a woven fabric and a nonwoven fabric as the separator for preventing a short circuit between the anode and the cathode. .

The lithium secondary battery may include a negative electrode having a negative electrode active material slurry interposed therebetween, and a positive electrode having a positive electrode active material slurry interposed therebetween with an electrode assembly formed by winding a separator interposed between the negative electrode and the positive electrode and a case containing the electrode assembly. have.

A representative example of the lithium secondary battery of the present invention having such a configuration is shown in FIG. 1. 1 includes a positive electrode 13, a negative electrode 15, and a separator 14 positioned between the positive electrode 13 and the negative electrode 15, and an electrolyte solution between the positive electrode 13 and the negative electrode 15. A rectangular type lithium ion battery 10 including a case 11 in which a not shown is shown is shown. Of course, the lithium secondary battery of the present invention is not limited to this shape, it is natural that any formation including a cylindrical, pouch, etc., including the negative electrode of the present invention and can operate as a battery.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Example 1)

CMC thickener powder was added to the pure water of pH 7 with stirring, followed by stirring until there was no precipitate at room temperature, thereby preparing an aqueous solution having a concentration of 1.5 wt% CMC thickener. After leaving this aqueous solution at room temperature for about 3 hours, this 1.5 wt% CMC aqueous solution was added to crystalline graphite with stirring in a homogenizer and mixed uniformly. Thereafter, an SBR binder aqueous emulsion was added to the mixture, and a polyethylene glycol (nonionic surfactant) surfactant was added to prepare a negative electrode active material slurry so that the content of the surfactant was 3% by weight. The negative electrode active material slurry was coated on a copper foil having a thickness of 10 μm, dried at 70 ° C., and rolled to prepare a negative electrode.

A positive electrode active material slurry was prepared by dispersing a LiCoO ₂ positive electrode active material, a polyvinylidene fluoride binder, and a carbon conductive agent in an N-methyl-2-pyrrolidone solvent at a weight ratio of 92: 4: 4. The positive electrode active material slurry was coated on an aluminum foil having a thickness of 15 μm, dried, and rolled to prepare a positive electrode.

The anode and cathode prepared as described above and a polyethylene separator having a thickness of 16 μm were wound and compressed to insert 46 mm × 34 mm × 50 mm square cans. Electrolyte was added to this can and the lithium secondary battery was manufactured.

(Example 2)

The preparation was carried out in the same manner as in Example 1, except that the content of the surfactant was 0.5% by weight.

(Example 3)

In the preparation of the negative electrode was carried out in the same manner as in Example 1 except that the content of the surfactant is 11% by weight.

(Comparative Example)

The same process as in Example 1 was carried out except that no surfactant was added in the preparation of the negative electrode.

The batteries prepared by the methods of Examples 1 to 3 and Comparative Examples were subjected to a life test of 1C / 4.2V constant current-constant voltage (CC-CV), 0.1C cut-off charging, 1C / 3.0V cut- at high temperature (60 ° C.). Table 1 shows the capacity retention rate (%) [(discharge capacity at cycle) / (discharge capacity at cycle 1)] * 100 at each cycle.

division High temperature (60 ℃) 100cyc
Capacity retention rate (%) Example 1 87 Example 2 81 Example 3 83 Comparative example 82

As shown in Table 1, Example 1 using a surfactant in the range of 1 to 10% by weight shows a life characteristics superior to the capacity retention rate of the comparative example during 100 charge and discharge. Example 2 has no effect of adding a surfactant because the content of the surfactant is small, and Example 3 has a large difference from the comparative example that does not add a surfactant because the content of the surfactant is so high that the coating property of the negative electrode active material slurry is poor none.

Lithium secondary battery using the negative electrode further added to the surfactant according to the present invention reduces the surface tension of the aqueous solvent to improve the coating properties when applying the negative electrode active material slurry to improve the smoothness of the negative electrode, and the life characteristics are excellent have.

Claims (14)

Negative electrode active material; SBR binder; Thickeners; Surfactants; And Made of a negative electrode active material slurry comprising an aqueous solvent; The amount of the surfactant is a lithium secondary battery negative electrode of 0.1 to 5% by weight based on the weight of the negative electrode active material slurry. The method of claim 1, The surfactant is a lithium secondary battery negative electrode selected from nonionic, cationic and anionic surfactants. 3. The method of claim 2, The nonionic surfactant may be polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, polyoxyethylene fatty acid amide, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, glycerin fatty acid ester, polyoxyethylene glycerin fatty acid ester, poly A negative electrode for a lithium secondary battery that is selected from the group consisting of a copolymer of oxyethylene and polyoxypropylene. 3. The method of claim 2, The cationic surfactant is a lithium secondary battery negative electrode selected from the group consisting of alkyl ammonium salts, alkylpyridinium salts, alkyl isoquinolinium salts. 3. The method of claim 2, The anionic surfactant is at least one selected from the group consisting of carboxylate, acyl amino acid ester salt, alkyl sulfate ester salt, alkyl sulfonic acid ester salt, alkyl phosphate ester salt. The method of claim 1, A negative electrode for a rechargeable lithium battery having a molecular weight of 100 to 100,000 of the surfactant. The method of claim 6, A negative electrode for a lithium secondary battery having a molecular weight of the surfactant is 500 to 10,000. delete delete The method of claim 1, The negative electrode active material is composed of crystalline or amorphous carbon, or a carbon-based negative electrode active material (thermally decomposed carbon, coke, graphite) of the carbon composite, burned organic polymer compound, carbon fiber, tin oxide compound, lithium metal or lithium alloy A negative electrode for a lithium secondary battery, characterized in that. The method of claim 1, The SBR binder is a lithium secondary battery negative electrode, characterized in that 0.8 to 5% by weight based on the weight of the negative electrode active material. The method of claim 1, The thickener is a lithium secondary battery negative electrode, characterized in that 0.8 to 5% by weight based on the total weight of the negative electrode active material. The method of claim 1, The thickener is at least one selected from the group consisting of carboxy methyl cellulose, hydroxymethyl cellulose, hydroxy ethyl cellulose and hydroxy propyl cellulose, the negative electrode for a lithium secondary battery. A cathode in which a cathode active material slurry is described in a cathode current collector; A negative electrode in which a negative electrode active material slurry is described in a negative electrode current collector; A separator interposed therebetween; And An electrolytic solution in which lithium salt is dissolved in a non-aqueous organic solvent, The negative electrode active material slurry includes a negative electrode active material, an SBR binder, a thickener, a surfactant, and an aqueous solvent, The amount of the surfactant is a lithium secondary battery, characterized in that 0.1 to 5% by weight based on the weight of the negative electrode active material slurry.

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