KR101555833B1 - Electrode Assembly and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention provides an electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes a lithium-transition metal composite oxide composed of nickel, manganese and cobalt as a positive electrode active material; Wherein the negative electrode includes lithium titanium oxide (LTO) as a negative electrode active material, and the separator is a nonwoven fabric separator, and a lithium secondary battery comprising the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an electrode assembly and a lithium secondary battery including the electrode assembly,

The present invention provides an electrode assembly comprising a cathode, a cathode, and a separator disposed between the anode and the cathode, wherein the cathode comprises a lithium-transition metal composite oxide composed of nickel, manganese, and cobalt as a cathode active material; The negative electrode includes lithium titanium oxide (LTO) as a negative electrode active material, and the separator is a nonwoven fabric separator.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operational potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like. Although a nickel metal hydride (Ni-MH) secondary battery is mainly used as a power source for such an electric vehicle (EV) and a hybrid electric vehicle (HEV), a lithium secondary battery having a high energy density, a high discharge voltage, Research is being actively carried out, and some are commercialized.

The lithium secondary battery has a structure in which a non-aqueous electrolyte containing a lithium salt is impregnated in an electrode assembly having a porous separator interposed between a positive electrode and a negative electrode coated with an active material on an electrode current collector.

Conventionally, a typical lithium secondary battery uses graphite as a negative electrode active material, charging and discharging proceed while repeating a process in which lithium ions in an anode are inserted into a negative electrode and desorbed.

Particularly, in the case of the negative active material, mainly crystalline carbon and quasi-crystalline carbon are used. In this case, Li dendrite (Li dendrite) is formed at a low temperature at which a high charge / There is a problem that the battery deteriorates.

In order to prevent such problems, various studies have been made on an anode active material, and lithium-titanium oxide (LTO) is attracting attention.

This lithium titanium oxide (LTO) has a very low structural change during charging and discharging, and therefore has excellent life characteristics due to a zero-strain material, forms a relatively high voltage band, and does not generate dendrite Safety and stability, and it is also advantageous in that it has a rapid charging electrode characteristic capable of charging within a few minutes.

However, due to the property of absorbing moisture in the air, when an electrode is manufactured using the same, water contained therein is decomposed to generate a large amount of gas. Such a large amount of gas causes degradation of the performance of the battery.

Therefore, in the case of a lithium secondary battery using lithium titanium oxide as a negative electrode active material, a drying process at a high temperature is required to remove moisture, but at a high temperature, the porous polyolefin film, which is a separator commonly used in lithium secondary batteries, Causing an internal short circuit.

Therefore, there is a great need for a technique having stability at high temperature for removing moisture.

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 conducted intensive research and various experiments and have found that the lithium secondary battery according to the present invention includes a lithium-transition metal composite oxide composed of nickel, manganese, and cobalt as a cathode active material, It has been confirmed that a desired effect can be achieved when a nonwoven fabric separator is used for an electrode assembly, and the present invention has been accomplished.

Accordingly, the present invention provides an electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode includes a lithium-transition metal composite oxide composed of nickel, manganese, and cobalt as a positive electrode active material; The negative electrode includes lithium titanium oxide (LTO) as a negative electrode active material, and the separator is a nonwoven fabric separator.

As described above, when the carbonaceous material is used as the negative electrode active material, the working voltage of the carbonaceous material is similar to the lithium dendrite formation potential, resulting in a lithium dendrite problem. On the other hand, in the case of lithium titanium oxide, since the operating voltage is 1.5 V to 1.8 V, the lithium dendrite problem does not occur and the possibility of deterioration upon rapid charging is low.

Therefore, the present invention has an effect of preventing internal short-circuit due to shrinkage during high-temperature drying to remove moisture using a nonwoven fabric separator while securing structural stability by using lithium titanium oxide as a negative electrode active material.

The lithium titanium oxide may preferably be represented by the following formula 1, and specifically, Li _{0 .8} Ti _{2 .2} O ₄ , Li _{2 .67} Ti _{1 .33} O ₄ , LiTi ₂ O ₄ , Li _{1 . 33} Ti _{1 .67} O _4, etc. may be an _{Li 1 .14 Ti 1 .71 O 4} , but is not limited to these. More preferably, Li _{1 .33} Ti _{1 .67} O ₄ has a spinel structure with less change in crystal structure during charge and discharge and excellent reversibility.

Li _x Ti _y O ₄ (0.5? _X? 3; 1? Y? 2.5) (1)

The lithium-transition metal composite oxide may preferably be a compound represented by the following general formula (2).

Li _{1 + z} Ni _a Mn _b Co _{1 - (a + b)} O ₂ (2)

In the above formula, 0? Z? 0.1, 0.2? A? 0.8, 0.2? B? 0.7 and a + b <1.

Further, the present invention can further improve the stability and lifetime characteristics by using the lithium transition metal complex oxide, which is expected to have a stable life expectancy, as a cathode active material. Wherein a is preferably from 0.3 to 0.6 may be, among _{Li 1 + z Ni 1/3} Mn 1/3 Co 1/3 O 2 or _{Li 1 + z Ni 0.5 Mn 0.3} Co 0.2 O 2 ( where, 0? Z? 0.1), but is not limited thereto. Particularly, lithium nickel cobalt manganese oxide having a relatively higher content of nickel than manganese and cobalt is more preferable in terms of high capacity.

Since the lithium titanium oxide (LTO) is required to be dried at a high temperature in order to remove moisture, a nonwoven fabric separator having excellent high-temperature stability is more effective for application to such a battery.

The material of the nonwoven fabric separator is preferably selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides such as aramid, polyacetal, polycarbonate, polyimide, polyether ketone , Polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, polytetrafluoroethylene, polyfluorinated vinylidene, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, polyparaphenylenebenzobis Oxazole, polyarylate, and glass. In the present invention, More preferably, it can be formed of polytetrafluoroethylene or polyester having a relatively high melting point because it is dried at a high temperature of about 150 to 200 ° C to remove water. In some cases, the nonwoven fabric separating film may be formed by using fibers made of two or more materials.

The nonwoven fabric separator is disposed between the positive electrode and the negative electrode and preferably has an average diameter of 0.5 to 10 mu m, more preferably 1 to 7 mu m, Is formed to include pores of 0.1 to 70 μm. It is difficult to manufacture nonwoven fabrics having many pores having a long diameter of less than 0.1 μm, and when the long diameter of the pores exceeds 70 μm, there may arise a problem of lowering the insulating property due to the pore size. The thickness of the nonwoven fabric separator is preferably 5 to 300 μm.

The porosity of the nonwoven fabric separator may be preferably 45 to 90%, and more preferably 50 to 70%. When the porosity is lower than the above range, the wettability and the output characteristics are deteriorated. If the porosity is higher than the above range, the separation membrane can not function.

The present invention provides a secondary battery including the electrode assembly, wherein the secondary battery is preferably a lithium secondary battery.

The secondary battery according to the present invention comprises the above-described positive electrode, negative electrode, separator, and nonaqueous electrolyte containing a lithium salt.

The positive electrode is prepared by applying a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode collector, followed by drying and pressing. If necessary, a filler may be further added to the mixture.

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. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface 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.

The conductive material is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or 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.

The binder is a component that assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 50 wt% based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

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

On the other hand, the negative electrode is manufactured by applying, drying and pressing an anode active material on an anode current collector, and may optionally further include a conductive material, a binder, a filler, and the like as described above.

The negative electrode current 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 electrical conductivity without causing chemical changes in the battery, and examples of the anode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, a 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 present invention also provides a secondary battery having a structure in which the electrode assembly is impregnated with a lithium salt-containing electrolyte.

The electrolyte solution containing the lithium salt is composed of an electrolyte solution and a lithium salt. The electrolyte solution may be a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte, but is not limited thereto.

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, sulfates and the like 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 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide.

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), and the like.

In a preferred _{embodiment, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing non-aqueous electrolyte.

The present invention also provides a battery module including the secondary battery as a unit cell, and a battery pack including the battery module.

The battery pack can be used as a power source for a medium and large-sized device requiring high temperature stability, long cycle characteristics, and high rate characteristics.

Preferred examples of the above medium to large devices include a power tool that is powered by an electric motor and moves; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; And a power storage system, but the present invention is not limited thereto.

As described above, the electrode assembly and the secondary battery using the nonwoven fabric separator according to the present invention are low in cost compared to conventional separators, have excellent output characteristics due to high porosity, and are excellent in high-temperature stability, thereby eliminating moisture due to the use of lithium titanium oxide It is possible to prevent an internal short circuit due to shrinkage during high temperature drying.

1 is a graph showing output characteristics of a secondary battery according to Example 1;
FIG. 2 is a graph showing output characteristics of a secondary battery according to Comparative Example 1. FIG.

&Lt; Example 1 >

As a cathode active material _{LiNi 1/3 Mn 1/3} Co 1/3 O after applying the positive electrode material mixture slurry containing the positive electrode current collector ₂ and rolled and dried to prepare a positive electrode and a secondary battery, the negative electrode active material Li _{1 .33} to Ti after application of the _first negative electrode material mixture slurry containing _.67 O ₄ on the negative electrode collector to prepare a rolled and dried to a secondary battery negative electrode. A non-woven fabric separator comprising polyester was disposed between the positive electrode and the negative electrode, and a lithium secondary battery comprising 1 M of LiPF ₆ and 7: 3 of PC: EC was used.

&Lt; Comparative Example 1 &

A secondary battery was manufactured using the method of Example 1 except that graphite was used as the negative electrode active material.

<Experimental Example>

The output characteristics of the batteries of Example 1 and Comparative Example 1 were tested and shown in FIGS. 1 and 2, respectively.

1 and 2, the battery of Comparative Example 1 using a carbon-based anode active material rapidly changes the output characteristic according to the SOC and narrows the SOC range in which a predetermined output or more is realized. On the other hand, The SOC region having a wide output characteristic is constant, and it is confirmed that the battery of Comparative Example 1 exhibits superior output characteristics.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (16)

An electrode assembly comprising an anode, a cathode, and a separator disposed between the anode and the cathode,
The positive electrode contains Li _{1 + z} Ni _0.5 Mn _0.3 Co _0.2 O ₂ (where 0 <z? 0.1) as the positive electrode active material;
The negative electrode contains Li _1.33 Ti _1.67 O ₄ as an anode active material,
Wherein the separation membrane is a nonwoven fabric separation membrane made of polyester and having a porosity of 50 to 70%
Wherein the nonwoven fabric separator has a pore diameter of 0.1 to 70 mu m and a thickness of 5 to 300 mu m. delete delete delete delete delete delete delete delete delete A secondary battery comprising the electrode assembly according to claim 1. 12. The secondary battery according to claim 11, wherein the secondary battery is a lithium secondary battery. A battery module comprising a secondary battery according to claim 12 as a unit cell. A battery pack comprising the battery module according to claim 13. A device comprising a battery pack according to claim 14. 16. The device of claim 15, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a system for power storage.

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