KR101115390B1 - Mixed Cathode Material for Lithium Secondary Battery and High Power Lithium Secondary Battery Employed with the Same - Google Patents

Abstract

The present invention provides a negative electrode material for electrode mixture consisting of a mixture containing an amorphous carbon-based compound and a flake-like crystalline carbon-based compound, the lithium secondary battery using such a negative electrode material has improved output density and energy density, excellent battery characteristics Has

Description

Mixed Cathode Material for Lithium Secondary Battery and High Power Lithium Secondary Battery Containing the Same {Mixed Cathode Material for Lithium Secondary Battery and High Power Lithium Secondary Battery Employed with the Same}

The present invention relates to a mixed negative electrode material for a lithium secondary battery, and a high output lithium secondary battery including the same, and more particularly, to an electrode mixture negative electrode material including a mixture containing an amorphous carbonaceous compound and a flake carbonaceous compound. It relates to a high output lithium secondary battery comprising.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Recently, the use of secondary batteries as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized. have. Accordingly, many studies have been conducted on secondary batteries capable of meeting various needs, and in particular, there is a high demand for lithium secondary batteries having high energy density, high discharge voltage, and output stability.

In particular, lithium secondary batteries used in electric vehicles, such as high energy density and the ability to exhibit a large output in a short time, should be able to be used for more than 10 years under the harsh conditions that charge and discharge by a large current is repeated in a short time, It is inevitably required to have superior safety and long life characteristics than small lithium secondary batteries.

In general, a negative electrode active material of a lithium ion battery used in a conventional small battery generally reversibly accepts or supplies lithium ions while maintaining structural and electrical properties, and the chemical potential of lithium ions upon insertion and desorption of lithium ions is increased. Graphite-based materials have been mainly used among carbon-based compounds having almost similar properties to those of.

However, the anode made of graphite-based materials has a theoretical maximum capacity of 372 mAh / g (844 mAh / cc), which is limited in capacity increase, making it difficult to play a sufficient role as an energy source for a rapidly changing next-generation mobile device. .

In addition, although lithium metal, which has been examined as a negative electrode material, has a very high energy density and can realize a high capacity, there are problems of safety due to dendrite growth and short cycle life during repeated charging and discharging. In addition, attempts have been made to use carbon nanotubes as negative electrode active materials, but problems such as low productivity, high price, and low initial efficiency of 50% or less have been pointed out.

As another cathode material, it has been known that silicon, tin, or alloys thereof can reversibly occlude and release a large amount of lithium through a compound formation reaction with lithium. Is going on. For example, silicon is promising as a high capacity cathode material because the theoretical maximum capacity is about 4020 mAh / g (9800 mAh / cc, specific gravity 2.23), which is much larger than graphite-based materials. However, the negative electrode material has a disadvantage that the volume change during charge and discharge is very large.

On the other hand, the amorphous carbon-based compound has a very high discharge capacity of 280 to 450 mAh / g, whereas the initial efficiency is very low, such as 70 to 80%, the energy density is not good due to the large irreversible capacity and low bulk density and electrical conductivity There is a problem. In order to compensate for each of these disadvantages, a research has been attempted to produce an electrode with a mixed negative electrode active material of a graphite-based carbon compound and an amorphous carbon-based compound. However, such a mixed negative electrode material has a low charge and discharge characteristic with respect to a large current, and in particular, a spherical crystalline carbonaceous compound is deformed by rolling during electrode production including a spherical crystalline compound, thereby increasing irreversible capacity. have.

Therefore, there is an urgent need for development of a negative electrode material having high output characteristics and high energy density.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

The inventors of the present application, after continuing in-depth research and various experiments, the lithium secondary battery using the negative electrode material for the electrode mixture consisting of a mixture containing an amorphous carbon-based compound and flaky crystalline carbon-based compound has a high output density and energy density It has been found to be improved to have excellent battery characteristics, and the present invention has been completed.

Therefore, the negative electrode material for electrode mixture according to the present invention is composed of a mixture containing an amorphous carbon compound and a flaky crystalline carbon compound.

The negative electrode material according to the present invention exhibits high discharge capacity and rate characteristics by including an amorphous carbon-based compound and at the same time includes flaky crystalline carbon-based compounds to fill the void space between the amorphous carbon-based compounds to increase energy density. It is possible to improve the output characteristics by high electrical conductivity, whereby the electrochemical cell comprising the same can exhibit excellent battery characteristics.

In general, crystalline carbon-based compounds have a two-dimensional path for lithium ions to be inserted and desorbed into the layered structure of carbon, whereas amorphous carbonated compounds are not limited to such routes for lithium ions to be inserted. Since it is relatively large, it exhibits an excellent rate characteristic. However, the amorphous carbon-based compound has a very high disadvantage as the irreversible capacity is about 20 to 30%. On the other hand, the negative electrode material according to the present invention comprises a flaky crystalline carbon-based compound having a relatively low irreversible capacity in the amorphous carbon-based compound, thereby compensating the above disadvantages.

The amorphous carbon-based compound is not particularly limited as long as the carbon atoms have an amorphous crystal structure and exhibit excellent rate characteristics. For example, hard carbon, coke, and pyrolyzed phenol resins or furan resins may be used. Needle coke or soft carbon carbonized with pitch may be used. The amorphous carbon-based compound has a relatively high discharge capacity of 280 to 450 mAh / g and contributes to the high output characteristics of the battery due to the excellent rate characteristics.

The amorphous carbon-based compound preferably has an average particle diameter of 8 to 25 μm. When the average particle diameter is too large, there is a problem that the bulk density of the negative electrode material is lowered. On the contrary, when the average particle diameter is too small, the irreversible capacity of the negative electrode material This is not preferable because it is high and it is difficult to expect a desired discharge capacity.

On the other hand, a representative example of the crystalline carbon-based compound may be graphite (graphite), the graphite-based crystalline carbon, for example, artificial graphite, or edge (edge) portion of the potato shape or MCMB (MesoCarbon MicroBead) shape Natural graphite having a surface treatment in order to make it smooth.

As a particularly preferable crystalline carbon compound in the present invention, graphite crystalline carbon of natural graphite and / or artificial graphite having a high degree of graphitization may be used. The graphite crystalline carbon is a carbon having a complete layered crystal structure, and has excellent electrical conductivity and energy density, good electric potential flatness, and an irreversible capacity of about 10 to 15%. Excellent reversibility

According to the present invention, scaly crystalline carbonaceous compounds in form of such crystalline carbonated compounds are used. The flaky crystalline carbon-based compound is a flat flake-shaped particle, the degree of crystal deformation is minimized by rolling during electrode production, thereby preventing an increase in irreversible capacity that may occur when using a spherical crystalline carbon-based compound can do.

In addition, the flaky crystalline carbon-based compound is located in the empty space between the amorphous carbon-based compound in the negative electrode material to provide a smooth electrical path when the insertion and desorption of lithium ions, thereby improving the output characteristics of the negative electrode material, At the same time, by itself, a large amount of lithium ions are inserted and removed, thereby improving electrical conductivity and energy density.

Therefore, the average particle diameter of the flaky crystalline carbon-based compound is a size that can be inserted into the empty space between the amorphous carbon-based compound to provide an electrical path of lithium ions, preferably having a size of 3 to 10 ㎛ It may be.

In one preferred example, the flaky crystalline carbon-based compound may have a ratio (dL / t) of the length (dL) and the thickness (t) of the major axis is 5 or more.

If the ratio of the length and thickness of the long axis of the flaky crystalline carbonaceous compound is less than 5, it is difficult to have a flaky structure, so that it is difficult to be inserted into the empty space between the amorphous carbonaceous compounds, and the deformation of the particles occurs by rolling during electrode production. It is not desirable as it can easily lead to an increase in irreversible capacity.

The mixing ratio of the amorphous carbonaceous compound and the flake-like crystalline carbonaceous compound may be appropriately adjusted according to the compositional state of the other battery components, the design of the charge / discharge power ratio, and the like, and preferably in a weight ratio of 50: 50 to 99: 1, and more preferably 80: 20 to 95: 5.

When the content of the flaky crystalline carbon-based compound is too large, the proportion of the amorphous carbon-based compound is relatively reduced, so that the output characteristics may be lowered. On the contrary, when the content of the flaky crystalline carbon-based compound is too small, the negative electrode material It is not preferable because the interparticle conductivity constituting the mixture and the bulk density of the electrode are lowered, so that it is difficult to expect an improvement in the desired energy density.

The present invention also provides a negative electrode mixture comprising the negative electrode material, a binder and a conductive material.

The binder is a component that assists in mutual bonding of the negative electrode material and the conductive material and bonding to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the mixture including the negative electrode material. Examples of such binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose Roses, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers And polymer high polymerized polyvinyl alcohol.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Specific examples of commercially available conductive materials include Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack and EC, which are acetylene black series. Family (Armak Company), Vulcan XC-72 (manufactured by Cabot Company) and Super P (manufactured by Timcal).

The negative electrode mixture may also optionally include a filler, an adhesion promoter, and the like.

The filler is not particularly limited as long as it is a fibrous material that does not cause chemical change in the battery as a component that inhibits expansion of the negative electrode. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The adhesion promoter is an auxiliary component added to improve adhesion of the active material to the current collector, and may be added in an amount of 10 wt% or less, for example, oxalic acid, adipic acid, Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

In addition, the negative electrode on which the negative electrode mixture is applied on the current collector, for example, may be prepared by coating the negative electrode mixture on the current collector. Specifically, the negative electrode mixture may be added to a predetermined solvent to prepare a slurry, and then, the slurry mixture may be coated on a current collector such as a metal foil, dried, and rolled to prepare a predetermined sheet-shaped electrode.

The invention also relates to an electrochemical cell comprising the negative electrode mixture comprising a negative electrode applied on a current collector.

Generally, the negative electrode is prepared by adding a negative electrode mixture to a solvent such as NMP to prepare a slurry, and then applying the same to a current collector to dry and press. Preferred examples of the solvent used in the preparation of the electrode slurry include dimethyl sulfoxide (DMSO), alcohol, N-methylpyrrolidone (NMP), acetone, and the like. Up to 400% by weight can be used and removed during drying.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The method of evenly applying the negative electrode mixture paste on the negative electrode current collector may be selected from known methods or performed by a new suitable method in consideration of the properties of the material. For example, it is preferable to disperse the paste onto the current collector and then to disperse the paste uniformly using a doctor blade or the like. In some cases, a method of distributing and dispersing in one process may be used. In addition, die casting, comma coating, screen printing, or the like may be used. Alternatively, a separate substrate may be formed and then joined with a current collector by pressing or lamination. You can also Drying of the paste applied on the current collector is preferably dried for 1 to 3 days in a vacuum oven at 50 to 200 ℃.

The electrochemical cell is a device for providing electricity through an electrochemical reaction, preferably a lithium secondary battery containing a lithium salt-containing nonaqueous electrolyte. The lithium secondary battery may have a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated in an electrode assembly having a structure in which a separator is interposed between the negative electrode and the positive electrode.

Other components of the lithium secondary battery according to the present invention will be described in detail below.

The positive electrode is produced by, for example, applying a positive electrode mixture containing a positive electrode active material onto a positive electrode current collector and then drying it.

The cathode current collector generally has a thickness of 3 to 500 mu m. 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 adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y} Mn _2-y O ₄ (where y 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-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and y = 0.01 to 0.3; Formula LiMn _2-y M _y O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta and y = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where 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. However, the present invention is not limited to these.

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 lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymerizers 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 material that is readily soluble 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, in the electrolyte solution, for the purpose of improving the charge and discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride and the like may be added. . 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.

As described above, the lithium secondary battery using the negative electrode material for electrode mixture composed of a mixture containing an amorphous carbon-based compound and a flaky crystalline carbon-based compound has the effect of having excellent battery characteristics with improved output density and energy density.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view and a side view of a shape of a flaky crystalline carbonaceous compound according to the present invention;
2 is a graph measuring electrode resistance in the batteries of Examples 1 and 2 and Comparative Examples 1 and 2 according to Experimental Example 1 of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

1-1. Manufacture of anode

LiCoO ₂ was used as the positive electrode active material, 94% by weight of LiCoO ₂ , 3.5% by weight of Super-P (conductive material) and 2.5% by weight of PVdF (binder) were added to N-methyl-2-pyrrolidone (NMP) as a solvent. After preparing the positive electrode mixture slurry, the positive electrode was prepared by coating, drying and pressing on aluminum foil.

1-2. Preparation of Cathode

A mixture of an amorphous carbonaceous compound hard carbon having an average particle diameter of 10 μm and a flake shaped crystalline carbonaceous compound artificial graphite having an average particle diameter of 4 μm in a weight ratio of 90:10, and 1% by weight of Super-P (conductive material), 5% by weight of PVdF (binder) was added to NMP as a solvent to prepare a negative electrode mixture slurry, which was then coated, dried and pressed onto copper foil to prepare a negative electrode.

Here, the cross-sectional view and side view of the scaly crystalline carbon-based compound hard carbon are shown in FIG. 1, and the value of dL / t was 8.5.

1-3. Manufacture of batteries

A lithium secondary battery was manufactured by injecting a 1M LiPF ₆ EC / EMC electrolyte solution through a separator (Celguard ^TM ) between the positive and negative electrodes of 1-1 and 1-2.

[Example 2]

An electrode and a battery were manufactured in the same manner as in Example 1, except that the amorphous carbon compound and the flaky crystalline carbon compound were 95: 5 by weight.

Comparative Example 1

An electrode and a spherical graphite compound were used instead of the flaky crystalline carbon compound, and the amorphous carbon compound and the spherical graphite compound were used in an 80: 20 weight ratio. The battery was prepared.

Comparative Example 2

An electrode and a spherical graphite compound were used instead of the flaky crystalline carbon compound, and the amorphous carbon compound and the spherical graphite compound were 90: 10 by weight. The battery was prepared.

Experimental Example 1

In order to compare the electrical conductivity between the current collector and the negative electrode mixture in the negative electrodes prepared in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, the resistance of the prepared negative electrode was measured, and the results are shown in FIG. 2.

As shown in Figure 2, it can be seen that the cathodes of Examples 1 and 2 according to the present invention exhibit much lower internal resistance than Comparative Examples 1 and 2 containing spherical crystalline carbonaceous compounds. Particularly, in the case of the negative electrode material including the negative electrode material in which the amorphous carbonaceous compound and the flaky crystalline carbonaceous compound were mixed in a weight ratio of 90:10, the electrode resistance showed a very low value of 3 mΩ or less. Able to know. This includes a crystalline carbonaceous compound in the form of amorphous carbonaceous compound to provide a smooth electrical path to improve the output characteristics of the negative electrode agent, and also by the insertion and desorption reaction of a large amount of lithium ions by itself, It is assumed that this is because the energy density is improved.

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 (6)

The mixing ratio of the graphitized carbon of the amorphous carbonaceous compound having an average particle diameter of 8 to 25 µm and the discharge capacity of 280 to 450 mAh / g and the flaky crystalline carbonaceous compound having an average particle diameter of 3 to 10 µm is 80:20. An anode material for electrode mixture comprising a mixture of from 95 to 5. The negative electrode material for electrode mixture according to claim 1, wherein the crystalline carbon compound is graphite crystalline carbon of natural graphite and / or artificial graphite. 2. The negative electrode for electrode mixture according to claim 1, wherein the flaky crystalline carbon-based compound has a flat flaky structure, and the ratio (dL / t) of the length (dL) and the thickness (t) of the major axis is 5 or more. ashes. A negative electrode mixture comprising the negative electrode material according to claim 1, a binder and a conductive material. An electrochemical cell consisting of a negative electrode mixture according to claim 4 comprising a negative electrode applied on a current collector. The electrochemical cell of claim 5, wherein the electrochemical cell is a lithium secondary battery containing a lithium salt-containing nonaqueous electrolyte.

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