KR100965276B1 - Negative Electrode Slurry Containing Conductive Agent of Improved Dispensability and Lithium Secondary Battery Prepared Therewith - Google Patents

Abstract

The present invention is a negative electrode slurry for a secondary battery obtained by dispersing a negative electrode mixture including a negative electrode active material, a conductive agent and a binder in a solvent, the affinity domain for the solvent for producing the slurry and the affinity domain for the carbon-based conductive agent in one molecule It provides a negative electrode slurry comprising a block copolymer having together and each of the domains consisting of a nonionic structure in an amount of 0.01 to 200% by weight based on the total weight of the conductive agent.

The negative electrode slurry improves the dispersibility of the conductive agent in the solvent for preparing the negative electrode slurry, thereby causing problems due to non-uniform distribution of the conductive agent in the negative electrode mixture, that is, lowering the rate characteristic of the battery, voltage drop, and rapid lithium metal precipitation at the electrode. There is an effect that can solve the problems such as, and the block copolymer added for the purpose of improving dispersibility does not cause side reactions by not involved in the electrochemical reaction of the battery.

Description

Negative Electrode Slurry Containing Conductive Agent of Improved Dispensability and Lithium Secondary Battery Prepared Therewith

1 and 2 are graphs showing the results of a coin half cell test on the cell of Example 1 and the cell of Comparative Example 1;

3 and 4 are graphs showing the results of experiments to confirm the rate characteristics and cycle performance for the battery of Example 2 and the battery of Comparative Example 2.

The present invention relates to a negative electrode slurry having improved dispersibility of a conductive agent and a lithium secondary battery prepared therefrom, and more particularly, to a negative electrode slurry for a secondary battery in which a negative electrode mixture including a negative electrode active material, a conductive agent and a binder is dispersed in a solvent. In order to improve the dispersibility of a conductive agent in a solvent for preparing a slurry, the affinity domain for the solvent and the affinity domain for the conductive agent are together in one molecule, and the two domains each have a nonionic structure. Provided are a negative electrode slurry containing a predetermined amount of a block copolymer, and a lithium secondary battery prepared therefrom.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Among them, many researches have been conducted and commercialized and widely used for lithium secondary batteries with high energy density and discharge voltage. It is used.

Lithium secondary batteries use lithium transition metal oxides such as LiCoO ₂ as the cathode active material and carbon materials such as graphite as the anode active material, and a polyolefin-based porous separator is interposed between the anode and the cathode to have a lithium salt such as LiPF ₆ . It is prepared by impregnating an aqueous electrolyte. During charging, lithium ions of the positive electrode active material are released and inserted into the carbon layer of the negative electrode. On discharge, lithium ions of the negative electrode carbon layer are released and inserted into the positive electrode active material. It acts as a medium for moving ions. Such a lithium secondary battery should basically be stable in the operating voltage range of the battery and have a performance capable of transferring ions at a sufficiently high speed.

The carbon material used as a negative electrode active material of a lithium secondary battery may be used by itself since it exhibits conductivity in itself. However, graphite, carbon black, and metal powder are preferably used to further improve conductivity to increase battery operation efficiency. Additives, such as these, are added. In addition, binders such as polyvinylidene fluoride (PVdF) and polyvinyl alcohol are further added to the negative electrode in order to assist the bonding between the negative electrode active material and the conductive agent and the current collector. Such a conductive agent, a binder, and the like are mixed with a negative electrode active material in a solvent such as N-methyl-2-pyrrolidone (NMP) and prepared in the form of a slurry, which is applied to a metal foil current collector such as copper at a predetermined thickness. After drying, pressing and molding to prepare a negative electrode for a secondary battery.

By the way, in the case of using the carbon-based conductive agent in the production of the negative electrode, since the carbon-based conductive agent has a low affinity to a solvent such as NMP in the process of manufacturing the negative electrode by the initial high viscosity mixing, it is dispersed dispertically and is distributed unevenly A voltage drop is caused, and the smooth movement of lithium ions is blocked, resulting in a decrease in the rate characteristic of the negative electrode. In addition, as the battery repeats the charge / discharge cycle, the movement of lithium ions is disturbed, resulting in the promotion of the deposition of lithium metal.

Therefore, some prior arts have attempted to solve the above problems by separately adding dispersants having high affinity for the solvent for the slurry and the carbon-based conductive agent, respectively. However, secondary batteries have a fundamental problem in that electrochemical reactions occur in the positive electrode and the negative electrode, and dispersants used in these prior arts cause side reactions due to partial involvement in such electrochemical reactions.

Accordingly, an object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

That is, it is an object of the present invention to increase dispersibility of a conductive agent in a solvent for preparing a negative electrode slurry without causing adverse reactions by not being involved in the electrochemical reaction of a battery, thereby lowering rate characteristics, voltage drop, and rapid lithium metal precipitation. It is to provide a negative electrode slurry that can solve such problems.

Still another object of the present invention is to provide a lithium secondary battery manufactured using such a negative electrode slurry.

The negative electrode slurry for secondary batteries according to the present invention for achieving the above object is a negative electrode slurry for secondary batteries obtained by dispersing a negative electrode mixture containing a negative electrode active material, a conductive agent and a binder in a solvent, the affinity domain for the solvent for producing the slurry and carbon It is composed of a block copolymer having affinity domains for a system-based conductive agent in one molecule and each of the domains having a nonionic structure, in an amount of 0.01 to 200% by weight based on the total weight of the conductive agent. It is.

As a solvent for the negative electrode slurry, NMP, which is a polar solvent, is generally used, but since the carbon-based conductive agent has an inert surface, the affinity for the solvent is very low. This low affinity leads to a decrease in dispersibility and thus causes problems as described above.

On the other hand, in the present invention, the block copolymer includes affinity domains for the solvent and affinity domains for the inert carbon-based conductive agent in one molecule, so that the dispersibility of the conductive agent in the solvent for preparing the slurry is This improves the rate characteristic of the battery, thereby suppressing voltage drop and precipitation of lithium metal.

In addition, a substance (dispersant) added for the purpose of improving dispersibility generally improves dispersibility by chemical adsorption or physical adsorption mechanism using the surface property of the substance. Although chemical adsorption causes electrons on the surface to move and cause adsorption by ionic interaction or electron sharing, electrochemically positive or negatively charged materials are likely to adversely affect the action of the battery in any way.

On the other hand, in the present invention, since both domains of the block copolymer added for the purpose of improving the dispersibility of the conductive agent to the solvent are made of a nonionic structure, the block copolymer is formed on the negative electrode prepared using the slurry. Even when remaining, it does not participate in the electrochemical reaction of the battery and does not cause side reactions.

In general, a block copolymer is a copolymer in which a chain in which one kind of monomer is connected in a certain amount and a chain in which a certain amount of another monomer having a different characteristic is connected are chemically bound. The block copolymer in the present invention is a copolymer in which a chain made of a monomer having a high affinity for a solvent and a chain made of a monomer having a high affinity for a carbon-based conductive agent form a chemical bond.

As described above, the block copolymer is added in an amount of 0.01 to 200% by weight, and preferably 0.5 to 100% by weight, based on the amount of the conductive agent added to the solvent for preparing the slurry. If the addition amount of the block copolymer is too small, it is difficult to expect the effect of the addition, on the contrary, too large is not preferable because there is a problem that not only decreases the energy density of the electrode but also increases the resistance of the electrode.

Particularly preferred examples of the block copolymers include polyester-polyethylene glycol copolymers. The block copolymer includes both an affinity domain for the solvent and an affinity domain for the carbon-based conductive agent. Such block copolymers are known as dispersants such as pigments in the field of painting, and typically, DisperBYK ^™ , Solsperse ^™ and the like are commercially available.

Since the block copolymer must be used by mixing a solid conductive agent and a liquid solvent, it is preferable to add the block copolymer to the slurry in a state dissolved in an inert organic solvent. Such organic solvents include N-methyl-2-pyrrolidone (NMP), methoxy propyl acetate, butyl acetate, glycol acid, butyl ester, butyl glycol, methyl alkyl polysiloxane, alkyl benzene, propylene glycol, xylene, monophenyl Glycol, Aralkyl Modified Methyl Alkyl Polysiloxane, Polyether Modified Dimethyl Polysiloxane Copolymer, Polyether Modified Dimethyl Polysiloxane Copolymer, Polyacrylate Solution, Alkylbenzene, Diisobutyl Ketone, Organic Modified Polysiloxane, Butanol, Isobutanol, Modified Polyacrylic One or two or more mixtures selected from the group consisting of latex, modified polyurethane, and polysiloxane modified polymers can be preferably used.

In the slurry for producing a negative electrode according to the present invention, the negative electrode active material may be, for example, carbon such as hardly graphitized carbon or graphite carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _1-x Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me' Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , and metal oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The binder is added in an amount of 1 to 50% by weight based on the total weight of the negative electrode mixture. Such binders include, but are not limited to, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, poly Vinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers, and the like. These may be used as one or a mixture of two or more.

The conductive agent is a carbon-based conductive agent is added at 0.1 to 20% by weight based on the total weight of the negative electrode mixture. Such a carbon-based conductive agent is not limited to the following, but is, for example, graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, super-p, toka black and denka black; Carbon fiber etc. are mentioned. These may be used as one or a mixture of two or more.

In some cases, a filler may be selectively added to the negative electrode mixture as a component that suppresses expansion of the negative electrode. Such fillers are not particularly limited as long as they are fibrous materials without inducing chemical changes in the battery. Examples of fillers include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, can be used.

The present invention also provides a lithium secondary battery consisting of a negative electrode produced using the slurry.

The negative electrode of the lithium secondary battery is manufactured by applying a predetermined amount of the negative electrode slurry to the current collector, drying, pressing and molding.

The negative electrode current collector is generally made to a thickness of 3 to 500 ㎛. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like, aluminum-cadmium alloy, and the like can be used. In addition, it is possible to enhance the bonding strength of the negative electrode mixture by forming fine unevenness on the surface, it can be used in various forms such as film, sheet, foil, net, porous body, foam, nonwoven fabric.

The lithium secondary battery has a structure in which a non-aqueous electrolyte is impregnated with a separator interposed between the negative electrode and the positive electrode as described above.

The positive electrode is prepared by slurrying a positive electrode mixture including a positive electrode active material, a conductive agent and a binder using a solvent, applying a certain amount to a current collector as in the method of manufacturing a negative electrode, drying, pressing and molding. In some cases, a filler or the like as described above may be further added to the positive electrode mixture.

As the cathode active material, a lithium transition metal oxide capable of occluding and releasing lithium ions is used. Such lithium transition metal oxides include, but are not limited to, the following, but are, for example, layered compounds such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), or compounds substituted with one or more transition metals. ; Lithium manganese oxides such as Li _{1 + x} Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3; Formula LiMn _2-x M _x O ₂ (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like.

The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesive strength of the positive electrode mixture, and may be in various forms such as film, sheet, foil, net, porous body, foam, and nonwoven fabric.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The non-aqueous electrolyte consists of a non-aqueous electrolyte and lithium. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide side derivatives, polypropylene oxide derivatives, phosphate ester polymers, agitation lysine, polyester sulfides, polyvinyl alcohols, and polyvinylidene fluorides. , Polymers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, etc. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited thereto.

Example 1

First, 20 g of acetylene black as a carbon-based conductive agent and 6 g of polyester-polyethylene glycol copolymer (about 30% by weight of the conductive content) as a dispersant were mixed in a small mixer, and PVdF as a binder was added thereto. After mixing by adding 150 g and 1800 g of the negative electrode active material, the mixture was stirred for 2 hours while slowly adding N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry having a solid content of 50% by weight.

The negative electrode slurry prepared in this manner was coated on a copper foil with a thickness of 80 μm, dried at 130 ° C. for 20 minutes, pressed, and then cut to a constant length to prepare a negative electrode.

Coin cells were prepared by interposing Celgard 2320 as a separator between the negative electrode and lithium metal prepared above and impregnating an EC / EMC solution in 1M LiPF ₆ as a non-aqueous electrolyte.

[Example 2]

A 4.5 Ah-class cell was prepared based on the method of Example 1 using manganese-based lithium metal oxide as a cathode and an anode obtained by double-coating a cathode slurry prepared in Example 1 to a thickness of 160 μm on a copper foil.

Comparative Example 1

A coin cell was prepared in the same manner as in Example 1 except that no dispersant was added.

Comparative Example 2

A 4.5 Ah cell was prepared in the same manner as in Example 2, except that no dispersant was added.

 Experimental Example 1

A coin half cell test was performed on the batteries prepared in Example 1 and Comparative Example 1, and the results are shown in FIGS. 1 and 2. 1 and 2 show a potential profile of a coin half cell of a battery according to the present invention (Example 1) and a potential profile of a battery (Comparative Example 1) without using a dispersant. Indicated.

As shown in Figures 1 and 2, the battery of Example 1 according to the present invention can be seen that the IR resistance is not large when compared with the battery of Comparative Example 1 because the cell resistance is reduced during charging.

In addition, the experiment to confirm the rate (rate) and cycle characteristics for the 4.5 Ah class cells of Example 2 and Comparative Example 2, the results are shown in Table 1 and Figures 3 and 4, respectively.

When manufacturing a cell at 4.5 Ah class as in Example 2 and Comparative Example 2, as shown in Table 1 and Figures 3 and 4, the battery of Example 2 according to the present invention compared with the battery of Comparative Example 2 It can be seen that the rate characteristics and cycle characteristics at room temperature 1C are excellent.

As described above, the negative electrode slurry according to the present invention improves the dispersibility of the conductive agent in the solvent for preparing the negative electrode slurry, thereby causing problems due to non-uniform distribution of the conductive agent in the negative electrode mixture, that is, lowering the rate characteristic of the battery, voltage There is an effect that can solve problems such as falling, rapid lithium metal precipitation. Moreover, the block copolymer added for the purpose of improving dispersibility does not participate in the electrochemical reaction of the battery and thus does not cause side reactions.

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

Claims (8)

A negative electrode slurry for secondary batteries obtained by dispersing a negative electrode mixture including a negative electrode active material, a conductive agent and a binder in a solvent, and has an affinity domain for a solvent for preparing a slurry and an affinity domain for a carbon-based conductive agent in one molecule. Each of the domains is composed of a block copolymer composed of a nonionic structure is contained in 0.01 to 200% by weight based on the total weight of the conductive agent, The block copolymer is a polyester-polyethylene glycol copolymer and is added in a dissolved state in an inert organic solvent. As the organic solvent, N-methyl-2-pyrrolidone (NMP), methoxy propyl acetate, butyl Acetate, glycol acid, butyl ester, butyl glycol, methyl alkyl polysiloxane, alkyl benzene, propylene glycol, xylene, monophenyl glycol, aralkyl modified methyl alkyl polysiloxane, polyether modified dimethylpolysiloxane copolymer, polyether modified dimethylpolysiloxane copolymer, Polyacrylate solution, alkylbenzene, diisobutyl ketone, organo modified polysiloxane, butanol, isobutanol, modified polyacrylate, modified polyurethane, and polysiloxane modified polymer, characterized in that one or two or more mixtures selected from the group consisting of Cathode slurry. The negative electrode slurry of claim 1, wherein the solvent is NMP. The negative electrode slurry of claim 1, wherein the block copolymer is added in an amount of 0.5 to 100 wt% based on the total weight of the conductive agent. delete delete delete delete A lithium secondary battery comprising a negative electrode prepared by using the negative electrode slurry according to any one of claims 1 to 3.

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