KR101517046B1 - Cathode for Lithium Secondary Battery - Google Patents

Abstract

The present invention relates to a cathode for a secondary battery having improved cycle characteristics, including a mixture of a lithium nickel-manganese-cobalt composite oxide and lithium iron phosphate as a cathode active material, and a surfactant as an additive.

Description

[0001] The present invention relates to a cathode for a lithium secondary battery,

The present invention relates to a positive electrode for a secondary battery having excellent output characteristics, and more particularly, to a lithium-nickel-manganese-cobalt composite oxide and a lithium iron phosphate as a positive electrode active material and a surfactant And a positive electrode for a secondary battery.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life and low self- It has been commercialized and widely used.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material of a lithium secondary battery, and a lithium-containing manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, (LiNiO ₂ ) is also being considered.

Of the above cathode active materials, LiCoO ₂ has excellent properties such as excellent cycle characteristics and is widely used at present. However, LiCoO ₂ has low safety, is expensive due to the resource limit of cobalt as a raw material, and is used as a power source for fields such as electric vehicles There is a limit.

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have attracted much attention as a cathode active material capable of replacing LiCoO ₂ because they have the advantage of using manganese rich in resources and environment friendly as a raw material. However, these lithium manganese oxides have disadvantages such as small capacity and poor cycle characteristics.

On the other hand, LiNiO _2, such as a lithium nickel based oxide is time, the reversible capacity of the bar, the doped LiNiO ₂ having a high discharge capacity was filled with a cobalt-containing oxide while the costs are cheaper than 4.3V is the capacity of LiCoO ₂ (about 165 lt; / RTI > (mAh / g). Therefore, in spite of a slightly low average discharge voltage and volumetric density, the compatibilized battery including LiNiO ₂ cathode active material has an improved energy density. Therefore, in order to develop a high capacity battery, Research is actively under way.

However, the LiNiO ₂ cathode active material exhibits a rapid phase transition of the crystal structure in accordance with the volume change accompanied by the charge / discharge cycle, causing swelling of the battery, and when exposed to air and moisture, The NMP-PVDF slurry starts to polymerize due to the high pH, and gelation of the slurry occurs.

Thus, a lithium transition metal oxide in which a part of nickel is substituted with another transition metal such as manganese or cobalt has been proposed. However, the metal-substituted nickel-based lithium-transition metal oxide is advantageous in that it has excellent cycle characteristics and capacity characteristics. In this case, however, the cycle characteristics are rapidly deteriorated during long-term use and the swelling phenomenon due to gas generation, There arises a problem such as stability.

In order to solve each of these problems, studies have been made to manufacture an electrode using a mixed cathode active material. For example, Japanese Patent Application Laid-Open Nos. 2002-110253 and 2004-134245 disclose a technique of mixing a lithium manganese composite oxide with a lithium nickel cobalt manganese composite oxide or the like in order to increase the regenerative output and the like. However, the cathode active material still has a disadvantage of poor cycle life of lithium manganese oxide and limitations in safety improvement.

Therefore, there is a high need for a secondary battery technology suitable for high capacity.

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

The inventors of the present application have found that when a lithium nickel-manganese-cobalt composite oxide is mixed with lithium iron phosphate and used as a cathode active material and a surfactant having a predetermined property as an additive is added to form a cathode, And completed the present invention.

The positive electrode according to the present invention is composed of a mixture of a lithium nickel-manganese-cobalt composite oxide and lithium iron phosphate as a positive electrode active material and a surfactant for improving cycle characteristics as an additive.

Firstly, as the cathode active material in the present invention, a lithium iron phosphate having an olivine crystal structure excellent in thermal stability and a lithium nickel-manganese-cobalt composite oxide having a large capacity and improved cycle characteristics by replacing a part of nickel with manganese and cobalt Is used.

Lithium iron phosphate and lithium nickel-manganese-cobalt composite oxides are respectively known. However, few attempts have been made to actually use such a mixture as a cathode active material.

In the cathode active material in the form of a mixture, it is important to uniformly mix the active material components constituting the mixture. To accomplish this, each of the active material components must be appropriately dispersed.

In the present invention, a surfactant is included as an additive in the positive electrode in order to enhance the dispersibility of the mixture of the cathode active material and improve the cycle characteristics. Such a surfactant promotes the dispersion of lithium iron phosphate, thereby allowing the lithium iron phosphate to be uniformly mixed with the lithium nickel-manganese-cobalt composite oxide as well as the binder.

In one preferred example, the lithium nickel-manganese-cobalt composite oxide may be a compound represented by the following formula (1).

Li _x M _y O ₂ (1)

2로서 0.95≤x≤1.15 이다).(Where M is Ni < ₁ > _{-a b} Mn _a Co _b (0.05? A? 0.8, 0.1? B? 0.5, 0.3? 1-ab? 0.9), x + y

2, 0.95? X? 1.15).

The content of the lithium nickel-manganese-cobalt composite oxide in the cathode active material is preferably 50 to 99% by weight, and the content of lithium iron phosphate may be 1 to 50% by weight. That is, the cathode active material can be composed of a lithium nickel-manganese-cobalt composite oxide as a main component and lithium iron phosphate as an auxiliary component.

The surfactant is preferably contained in an amount of 0.01 to 1% by weight, more preferably 0.01 to 0.1% by weight based on the total weight of the positive electrode. When the amount of the surfactant is less than 0.01% by weight, the lithium iron phosphate is not effectively dispersed and is present in a coagulated form, so that it is not uniformly mixed with the lithium nickel-manganese-cobalt oxide and the binder is concentrated around the coagulated lithium iron phosphate If it exceeds 0.1% by weight, the electric conductivity and electric capacity decrease, which is undesirable.

In general, surfactants are classified as anionic surfactants, cationic surfactants, nonionic surfactants, and amphoteric surfactants. As the surfactant used in the present invention, a nonionic surfactant is preferable, and the ionic surfactant may cause corrosion of the positive electrode collector, which is not preferable.

In one preferred example, the nonionic surfactant may be a compound having a structure of the following formula (2).

(2)

(2)

Wherein R ₁ to R ₅ are each independently hydrogen or a substituted or unsubstituted C ₁ to C ₅ alkyl, and n is an integer of 1 to 100.

When alkyl is substituted, the substituent may be amino, nitro, cyano, halogen, aryl, and the like.

In one preferred example, R ₁ to R ₅ each independently may be methyl or ethyl.

A specific method for producing the positive electrode including the positive electrode active material according to the present invention will be described below.

First, a cathode active material as a mixture according to the present invention, a binder and a conductive agent are added to the dispersion in an amount of 1 to 20% by weight with respect to the cathode active material and stirred to prepare a paste, After compression, it is dried to produce a laminate-shaped electrode.

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 change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, (EPDM), sulfonated EPDM, styrene butadiene rubber, fluorine rubber, various copolymers, and the like can be given as examples of the polypropylene resin .

The conductive agent is not particularly limited as long as it has electrical conductivity without causing any chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black 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, aluminum, and nickel powder; Conductive whiskey 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.

In some cases, a filler may be selectively added as a component for suppressing the expansion of the anode. Such a filler is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

Typical examples of the dispersion include isopropyl alcohol, N-methyl pyrrolidone (NMP), and acetone.

The method of applying the paste of the electrode material to the metal material in a uniform manner can be selected from known methods in consideration of the characteristics of the material and the like or can be carried out by a new suitable method. For example, it is preferable that the paste is uniformly dispersed on a current collector by using a doctor blade or the like. In some cases, a method of performing the distribution and dispersion processes in a single process may be used. In addition, a die casting method, a comma coating method, a screen printing method, or the like may be used. Alternatively, the resin may be formed on a separate substrate, and then may be bonded to the current collector by a pressing or lamination method. . Drying of the paste applied on the metal plate is preferably performed in a vacuum oven at 50 to 200 DEG C for 1 to 3 days.

The present invention also relates to a lithium secondary battery including such a positive electrode.

The lithium secondary battery of the present invention comprises an electrode assembly in which the positive electrode faces the negative electrode with a separation membrane therebetween, and a lithium salt-containing nonaqueous electrolyte.

The negative electrode may be formed by coating a negative electrode active material on a negative electrode collector and drying the negative electrode active material. If necessary, the negative electrode may further include components such as a conductive agent, a binder, and a filler as described above.

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 collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The negative electrode active material may include, for example, carbon such as non-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 &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{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 Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The separation membrane 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 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like; Kraft paper and the like are used.

In some cases, the separation membrane may be coated with a gel polymer electrolyte to improve the stability of the cell. Representative examples of such gel polymers include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

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 a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may 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 and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can 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.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene carbonate, PRS (propene sultone), FPC (fluoro-propylene carbonate), and the like.

The lithium secondary battery according to the present invention may be a device including not only a unit battery but also a battery module as a power source.

Preferred examples of the device include electric power tools, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric two-wheeled vehicles, electric golf carts, or electric power storage devices, But is not limited thereto.

As described above, it is possible to provide a secondary battery which satisfies all of the high capacity and low cost characteristics through the anode for the secondary battery, which is obtained by mixing the lithium nickel-manganese-cobalt composite oxide according to the present invention with lithium iron phosphate and containing the surfactant have.

1 is a graph showing cycle characteristics according to C-rate according to Experimental Example 1 of Example 1 and Comparative Example 1;
2 is a graph showing the cycle characteristics at 1C according to Experimental Example 2 of Example 1 and Comparative Example 1. Fig.

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

&Lt; Example 1 >

Active material _{(LiNi 0 .25 Mn 0 .6 Co} 0 .15 O 2: LiFePO 4 = 7: 3): Conductive material: Measured so that the amount of the binder is 80: 10: 10 (weight ratio), put into NMP, 0.5% by weight of NMP surfactant (R ₁ to R ₅ in the formula And n is an integer of 1 to 10) were added and mixed to prepare a positive electrode mixture. The positive electrode mixture was coated on the aluminum foil having a thickness of 20 μm to a thickness of 200 μm, followed by rolling and drying.

Punching the electrode in a coin shape, and a Li metal, the electrolyte to the cathode using the carbonate electrolyte LiPF ₆ dissolved in a 1 mol to prepare a battery of coin shape.

&Lt; Comparative Example 1 &

A coin-shaped cell was prepared in the same manner as in Example 1, except that the surfactant of Example 1 was not added as an additive.

<Experimental Example 1>

Charging and discharging were performed in the voltage range of 3.0 V to 4.2 V using the batteries of Examples 1 and 2 and Comparative Example 1, and the capacities according to the respective C-rates were compared.

<Experimental Example 2>

Charging and discharging were repeated under the condition of 1 C in the voltage range of 3.0 V to 4.2 V using the batteries of Examples 1 and 2 and Comparative Example 1, and the change of the capacity according to the cycle was measured. The results are shown in FIG.

The results of Experimental Examples 1 and 2 are shown in Fig. 1 and Fig.

1, which compares the capacities according to the C-rate of Example 1 and Comparative Example 1, the capacity of Example 1 to which a surfactant was added at each C-rate was compared with that of Comparative Example 1 &lt; / RTI &gt; capacity.

2, which shows the comparison of the capacities according to the cycle of Example 1 and Comparative Example 1, it can be seen that the capacity of Example 1 is larger than that of Comparative Example 1.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (11)

A mixture of a lithium nickel-manganese-cobalt composite oxide and lithium iron phosphate as a positive electrode active material, and a surfactant for improving cycle characteristics as an additive,
The lithium nickel-manganese-cobalt composite oxide is represented by the following formula 1, wherein the content of the lithium nickel-manganese-cobalt composite oxide in the cathode active material is 50 to 99 wt%, and the content of lithium iron phosphate is 1 to 50 wt% %,
Wherein the surfactant is contained in an amount of 0.01 to 1% by weight based on the total weight of the positive electrode, the surfactant is a nonionic surfactant, the nonionic surfactant is a compound having a structure of the following formula 2,
Wherein the surfactant promotes dispersion of the lithium iron phosphate, and the lithium iron phosphate is uniformly mixed with the lithium nickel-manganese-cobalt composite oxide.
Li _x M _y O ₂ (1)
(Wherein, M is _{Ni 1-ab Mn a Co b} (0.05≤a≤0.8, 0.1≤b≤0.5, 0.3≤1-ab≤0.9) and, x + y

2, 0.95? X? 1.15)

(2)
Wherein R ₁ to R ₅ are each independently C ₁ to C ₅ alkyl, and n is an integer of 1 to 100. delete delete delete delete delete The positive electrode for a lithium secondary battery according to claim 1, wherein R ₁ to R ₅ are each independently methyl or ethyl. A lithium secondary battery comprising a positive electrode according to any one of claims 1 to 7. A battery module comprising the lithium secondary battery according to claim 8 as a unit cell. A device comprising the battery module according to claim 9 as a power source. 11. The device of claim 10, wherein the device is an electric power tool, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric motorcycle, an electric golf cart, Lt; / RTI &gt;

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