# Patent

## Title
Method of manufacturing electrode for lithium secondary battery and electrode manufactured using the same

## Abstract

Disclosed are a method of manufacturing an electrode for secondary batteries that includes surface-treating a current collector so as to have a morphology wherein a surface roughness Ra of 0.001 μm to 10 μm is formed over the entire surface thereof to enhance adhesion between an electrode active material and the current collector and an electrode for secondary batteries that is manufactured using the method.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of International Application No. PCT/KR2013/003171, filed on Apr. 16, 2013, which claims benefit of Korean Patent Application No. 10-2012-0039086, filed on Apr. 16, 2012, the disclosures of which are incorporated herein by reference.


TECHNICAL FIELD
The present invention relates to a method of manufacturing an electrode for secondary batteries in which an electrode active material is coated on a current collector and an electrode manufactured using the same and, more particularly, to a method of manufacturing an electrode for secondary batteries that includes surface-treating a current collector so as to have a morphology wherein a surface roughness Ra of 0.001 μm to 10 μm is formed over the entire surface thereof, whereby adhesion between an electrode active material and the current collector is enhanced and an electrode manufactured using the same.
BACKGROUND ART
As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous separator is interposed between a cathode and an anode, each of which includes an active material coated on an electrode current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte.
In such lithium secondary batteries, charging and discharging processes are performed while lithium ions of a cathode are repeatedly intercalated into and deintercalacted from an anode. Although there are differences in theoretical capacities of batteries according to kinds of electrode active materials, in most cases, charge and discharge capacities are deteriorated as cycles proceed.
Such phenomenon is mainly attributed to non-functioning of active materials due to separation of electrode active material components or separation between an electrode active material and a current collector by change in volume of an electrode occurring as charging and discharging of a battery proceed. In addition, in the intercalation and deintercalation processes, lithium ions intercalated into an anode are unable to be properly deintercalated therefrom and thus an anode active site is reduced and, accordingly, charge and discharge capacities and lifespan characteristics of a battery are deteriorated as cycles proceed.
With regard to this, binders provide adhesion between electrode active materials and adhesion between an electrode active material and an electrode current collector and suppress volumetric expansion according to charging and discharging of a battery, which is an important factor determining battery performance.
However, when a large amount of binder is used during manufacture of a secondary battery to enhance adhesion, the amount of a conductive material or an electrode active material relatively decreases and thus conductivity of an electrode is reduced or battery capacity is reduced. In addition, if an electrode slurry is too dilute, it is not easy to perform coating of an electrode.
Therefore, there is an urgent need to develop technology of using an appropriate amount of binder and imparting high adhesion between an electrode active material and a current collector, whereby secondary battery performance may be enhanced.
DISCLOSURE
Technical Problem
The present invention aims to address the aforementioned problems of the related art and to achieve technical goals that have long been sought.
As a result of a variety of extensive and intensive studies and experiments, the inventors of the present invention confirmed that, as described below, when a current collector is surface-treated so as to have a particular morphology over the entire surface thereof and then coated with an electrode active material, desired effects may be achieved, thus completing the present invention.
Technical Solution
In accordance with one aspect of the present invention, provided is a method of manufacturing an electrode for secondary batteries that includes an electrode active material coated on a current collector, the method including surface-treating a current collector so as to have a morphology wherein a surface roughness Ra of 0.001 μm to 10 μm is formed over the entire surface thereof, whereby adhesion between the electrode active material and the current collector is enhanced.
In general, an electrode mixture is prepared in the form of a slurry obtained by mixing an electrode active material, a conductive material, a binder, and the like with an organic solvent. In this case, as described above, to prevent an electrode active material and a current collector from being separated from each other according to change in volume of an electrode occurring during charge and discharge, the amount of binder is increased and, in this case, the amount of the electrode active material or the conductive material relatively decreases and thus conductivity of an electrode is reduced or battery capacity is reduced.
Thus, the current collector surface-treated so as to have a predetermined morphology using the manufacturing method according to the present invention is provided at a surface thereof with fine irregularities and thus has an increased surface area. Accordingly, adhesion between an electrode active material and a current collector is significantly increased and thus overall performance of a secondary battery, such as charging and discharging cycle characteristics and the like, may be enhanced.
In particular, the current collector may have a morphology wherein a surface roughness Ra of 0.1 μm to 1 μm is formed over the entire surface thereof.
When the surface roughness is too small, it is difficult to form fine irregularities and thus it may be difficult to distribute stress when the volume of an electrode active material expands. On the other hand, when the surface roughness is too great, distribution and alleviation effects of stress of an electrode active material in largely formed irregularities may be reduced.
In particular, an interval between the irregularities formed at the surface of the current collector may be 0.001 μm to 10 μm, and the depth of a valley between the irregularities may be 0.001 μm to 10 μm.
A method of forming fine irregularities through surface treatment of the current collector is not particularly limited so long as the method is known in the art. However, to form a particular surface morphology as in the present invention, the current collector may be subjected to rolling using a roller provided at a surface thereof with a pattern.
The pattern formed in the roller may be embossed or have an intaglio shape and, in particular, may be embossed. Such a pattern is not particularly limited so long as the pattern is a shape that enables the surface of the current collector to be scratched. However, a vertical cross-section of the pattern may have, in particular, a polygonal, circular, oval, or slit shape.
In particular, as illustrated in FIGS. 1A to 1C, a roller 100 in which a liner shape is embossed, a roller 101 in which a tetragonal shape is embossed, and a roller 102 in which a triangular shape is embossed may be used, and patterns corresponding to the shapes may be formed in current collectors 200, 201 and 202.
According to the shape of the pattern, a surface area of the current collector formed may vary and thus a current collector in which a particular pattern that maximizes adhesion is formed may be used.
Distance, depth and the like of the pattern formed in the roller may be determined such that a current collector having a particular morphology according to the present invention may be formed.
The present invention also provides an electrode for secondary batteries manufactured using the manufacturing method described above. The electrode may be at least one of a cathode and an anode.
The cathode may include, as a cathode active material, a spinel-structure lithium metal oxide represented by Formula 1 below:

LixMyMn2−yO4−zAz  (1)

wherein 0.9≦x≦1.2, 0<y<2, and 0≦z<0.2; M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti, and Bi; and A is at least one monovalent or divalent anion.
More particularly, the lithium metal oxide may be represented by Formula 2 below:

LixNiyMn2−yO4  (2)

wherein 0.9≦x≦1.2 and 0.4≦y≦0.5. More particularly, the lithium metal oxide may be LiNi0.5Mn1.5O4 or LiNi0.4Mn1.6O4.
The anode may include, as an anode active material, a lithium metal oxide represented by Formula 3 below:

LiaM′bO4−cAc  (3)

wherein M′ is at least one element selected from the group consisting of Ti, Sn, Cu, Pb, Sb, Zn, Fe, In, Al, and Zr; 0.1≦a≦4 and 0.2≦b≦4 in which a and b are determined according to oxidation number of M′; 0≦c<0.2 in which c is determined according to oxidation number of A; and A is at least one monovalent or divalent anion.
The lithium metal oxide may be represented by Formula 4 below:

LiaTibO4  (4).

More particularly, the lithium metal oxide may be Li1.33Ti1.67O4 or LiTi2O4.
The cathode is manufactured by coating a cathode active material on a cathode current collector and drying and pressing the coated cathode current collector. As desired, a conductive material, a binder, a filler, and the like as described above may be further selectively used in addition to the cathode active material.
The cathode current collector is generally fabricated to a thickness of 3 to 500 μm. The cathode current collector is not particularly limited so long as it does not cause chemical changes in the fabricated battery and has high conductivity. For example, the cathode current collector may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The cathode current collector may have fine irregularities at a surface thereof to increase adhesion between the cathode active material and the cathode current collector. In addition, the cathode current collector may be used in any of various forms including films, sheets, foils, nets, porous structures, foams, and non-woven fabrics.
As the cathode active material, the materials as defined above may be used. In addition, examples of the cathode active material may include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Li1+xMn2−xO4 where 0≦x≦0.33, LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; Ni-site type lithium nickel oxides having the formula LiNi1−xMxO2 where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≦x≦0.3; lithium manganese composite oxides having the formula LiMn2−xMxO2 where M=Co, Ni, Fe, Cr, Zn, or Ta, and 0.01≦x≦0.1 or the formula Li2Mn3MO8 where M=Fe, Co, Ni, Cu, or Zn; spinel-structure lithium manganese composite oxides having the formula LiNixMn2−xO4 where 0.01≦x≦0.6; LiMn2O4 where some of the Li atoms are substituted with alkaline earth metal ions; disulfide compounds; and Fe2(MoO4)3.
The conductive material is typically added in an amount of 1 to 50 wt % based on the total weight of a mixture including a cathode active material. There is no particular limit as to the conductive material, so long as it does not cause chemical changes in the fabricated battery and has conductivity. Examples of conductive materials include, but are not limited to, graphite such as natural or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metallic fibers; metallic powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.
The binder is a component assisting in binding between an active material and a conductive material and in binding of the active material to a current collector. The anode binder may be typically added in an amount of 1 to 50 wt % based on a total weight of a mixture including a cathode active material. Examples of the binder include, but are not limited to, polyvinylidene fluoride, polyvinyl alcohols, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluorine rubber, and various copolymers.
The filler is optionally used as a component to inhibit cathode expansion. The filler is not particularly limited so long as it is a fibrous material that does not cause chemical changes in the fabricated secondary battery. Examples of the filler include olefin-based polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber.
On the other hand, the anode is manufactured by coating an anode active material on an anode current collector and drying and pressing the coated anode current collector. As described, a conductive material, a binder, a filler, and the like as described above may be further selectively used in addition to the anode active material.
The anode current collector is generally fabricated to a thickness of 3 to 500 μm. The cathode current collector is not particularly limited so long as it does not cause chemical changes in the fabricated battery and has conductivity. For example, the anode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or aluminum-cadmium alloys. Similar to the cathode current collector, the anode current collector may also have fine irregularities at a surface thereof to increase adhesion between the anode active material and the anode current collector. In addition, the anode current collector may be used in any of various forms including films, sheets, foils, nets, porous structures, foams, and non-woven fabrics.
Examples of the anode active material include, but are not limited to, carbon such as hard carbon and graphite-based carbon; metal composite oxides such as LixFe2O3 where 0≦x≦1, LixWO2 where 0≦x≦1, SnxMe1−xMe′yOz where Me: Mn, Fe, Pb, or Ge; Me′: Al, B, P, Si, Groups I, II and III elements, or halogens; 0<x≦1; 1≦y≦3; and 1≦z≦8; lithium metals; lithium alloys; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li—Co—Ni-based materials; titanium oxides; lithium titanium oxides.
In an embodiment, when lithium titanium oxide (LTO) is used as the anode active material, LTO has low electrical conductivity and thus may have the electrode structure described above. In addition, in this case, due to high potential of LTO, a spinel lithium manganese composition oxide having relatively high potential, such as LiNixMn2−xO4 where 0.01≦x≦0.6, may be used as the cathode active material.
In addition, the present invention provides a secondary battery in which an electrode assembly including the cathode, the anode, and a separator disposed therebetween is impregnated with a lithium salt-containing electrolyte.
The separator is disposed between the cathode and the anode and an insulating thin film having high ion permeability and mechanical strength is used as the separator. The separator typically has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator, sheets or non-woven fabrics made of an olefin polymer such as polypropylene, glass fibers or polyethylene, which have chemical resistance and hydrophobicity, are used. When a solid electrolyte such as a polymer is employed as the electrolyte, the solid electrolyte may also serve as a separator.
The lithium salt-containing electrolyte is composed of an electrolyte and a lithium salt. As the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like may be used.
For example, the non-aqueous organic solvent may be an aprotic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate, or the like.
Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, and polymers containing ionic dissociation groups.
Examples of the inorganic solid electrolyte include nitrides, halides and sulfates of lithium (Li) such as Li3N, LiI, Li5NI2, Li3N—LiI—LiOH, LiSiO4, LiSiO4—LiI—LiOH, Li2SiS3, Li4SiO4, Li4SiO4—LiI—LiOH, and Li3PO4—Li2S—SiS2.
The lithium salt is a material that is readily soluble in the non-aqueous electrolyte. Examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, and imide.
In addition, in order to improve charge/discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, or the like may be added to the non-aqueous electrolyte. In some cases, in order to impart incombustibility, the electrolyte may further include a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride. In addition, in order to improve high-temperature storage characteristics, the electrolyte may further include carbon dioxide gas, fluoro-ethylene carbonate (FEC), propene sultone (PRS), or the like.
In an embodiment, a lithium salt-containing non-aqueous electrolyte may be prepared by adding a lithium salt such as LiPF6, LiClO4, LiBF4, LiN(SO2CF3)2, or the like to a mixed solvent of a cyclic carbonate such as EC or PC, which is a high dielectric solvent, and a linear carbonate such as DEC, DMC, or EMC, which is a low-viscosity solvent.
The present invention also provides a battery module including the secondary battery as a unit battery and a battery pack including the battery module.
The battery pack may be used as a power source of medium and large devices that require high-temperature stability, long cycle characteristics, high rate characteristics, and the like.
Examples of the medium and large devices include, but are not limited to, electric motor-driven power tools; electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles such as e-bikes and e-scooters; electric golf carts; and systems for storing power.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIGS. 1A to 1C are views illustrating current collectors having predetermined morphologies according to the present invention and surfaces of rollers for formation of morphologies of these current collectors.



MODE FOR INVENTION
Example 1
To form a surface roughness Ra of 0.5 μm on a surface of an Al current collector, the Al current collector was subjected to rolling using a roll provided at a surface thereof with an embossed tetragonal pattern. Subsequently, 95 wt % of Li1.33Ti1.67O4 as an anode active material, 2.5 wt % of Super-P as a conductive material, and 2.5 wt % of PVdF as a binder were added to NMP to prepare an anode mixture and the prepared anode mixture was coated onto the Al current collector, thereby completing fabrication of an anode for secondary batteries.
Comparative Example 1
An anode for secondary batteries was manufactured in the same manner as in Example 1, except that an Al current collector was not surface-treated using a roller.
Experimental Example 1
Adhesive strength of each of the anodes manufactured according to Example 1 and Comparative Example 1 was measured. Results are shown in Table 1 below.









TABLE 1







Adhesive strength (gf/cm)



















Example 1
52



Comparative
36



Example 1










As shown in Table 1 above, it can be confirmed that the anode of Example 1 manufactured using the current collector surface-treated using a roller having an embossed pattern exhibits enhanced adhesive strength as compared to the anode of Comparative Example 1 and, consequently, overall performance of a secondary battery including the anode of Example 1 is enhanced.
INDUSTRIAL APPLICABILITY
As described above, a method of manufacturing an electrode for secondary batteries according to the present invention includes surface-treating a current collector so as to have a particular morphology over the entire surface thereof and thus may increase a surface area of the current collector and, accordingly, adhesion between a current collector and an electrode active material of the manufactured electrode is enhanced, whereby overall performance of a secondary battery, such as charging and discharging cycle characteristics and the like, may be enhanced.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, 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

1. A method of manufacturing an electrode for secondary batteries that comprises an electrode active material coated on a current collector comprising:
(i) surface-treating the current collector to provide a surface with irregularities having a morphology, wherein the surface-treating is performed so that the current collector is subjected to rolling using a roller having an embossed pattern at a surface thereof, wherein a vertical cross-section of the pattern is a polygonal, circular, oval, or slit shape, wherein a surface roughness Ra of 0.001 μm to 10 μm is formed over an entire surface of the current collector to enhance adhesion between the electrode active material and the current collector, wherein an interval between the irregularities is from 0.001 μm to 10 μm, and a depth of a valley between the irregularities is 0.001 μm to 10 μm, and;
(ii) coating a slurry obtained by mixing an electrode mixture comprising an electrode active material, a conductive material, and a binder with an organic solvent onto the current collector.

2. The method according to claim 1, wherein the surface roughness Ra of the current collector is 0.1 μm to 1 μm is formed over the entire surface thereof.

3. An electrode for a secondary battery, manufactured using the method according to claim 1.

4. The electrode according to claim 3, wherein the electrode is at least one of a cathode and an anode.

5. The electrode according to claim 4, wherein the cathode comprises, as a cathode active material, a lithium metal oxide represented by Formula 1 below:


LixMyMn2−yO4−zAz  (1)

wherein 0.9≦x≦1.2, 0<y<2, and 0≦z<0.2;
M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti, and Bi; and
A is at least one monovalent or divalent anion.

6. The electrode according to claim 5, wherein the oxide of Formula 1 is represented by Formula 2 below:


LixNiyMn2−yO4  (2)

wherein 0.9≦x≦1.2 and 0.4≦y≦0.5.

7. The electrode according to claim 6, wherein the oxide is LiNi0.5Mn1.5O4 or LiNi0.4Mn1.6O4.

8. The electrode according to claim 4, wherein the anode comprises, as an anode active material, a lithium metal oxide represented by Formula 3 below:


LiaM′bO4−cAc  (3)

wherein M′ is at least one element selected from the group consisting of Ti, Sn, Cu, Pb, Sb, Zn, Fe, In, Al, and Zr;
0.1≦a≦4 and 0.2≦b≦4 in which a and b are determined according to oxidation number of M′;
0≦c<0.2 in which c is determined according to oxidation number of A; and
A is at least one monovalent or divalent anion.

9. The electrode according to claim 8, wherein the lithium metal oxide is represented by Formula 4 below:


LiaTibO4  (4)

wherein 0.5≦a≦3 and 1≦b≦2.5.

10. The electrode according to claim 9, wherein the lithium metal oxide is Li1.33Ti1.67O4 or LiTi2O4.

11. A secondary battery comprising the electrode according to claim 3.

12. The secondary battery according to claim 11, wherein the secondary battery is a lithium secondary battery.

13. A battery module comprising the secondary battery according to claim 12 as a unit battery.

14. A battery pack comprising the battery module according to claim 13.

15. A device comprising the battery pack according to claim 14.

16. The device according to claim 15, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a system for storing power.