Copper current collector, its manufacturing method, negative electrode and lithium secondary battery including the same

Abstract

The present invention relates to a copper current collector in which, when measuring surface residual stress, the area in which compressive residual stress is formed is 30% or more of the total measured area, and a method for producing the same.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0178735, filed December 19, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a copper current collector and a method for producing the same. [Background technology]

[0003] With the development of technologies and increasing demand for electric vehicles and energy storage systems (ESS), the demand for batteries as energy sources is rapidly increasing, and research into batteries that can meet various needs is being conducted. In particular, research into lithium secondary batteries that have high energy density, excellent lifespan, and cycle characteristics as power sources for such devices is being actively conducted.

[0004] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, etc. The negative electrode may have a structure in which a negative electrode active material layer is laminated on one or both sides of a negative electrode current collector, and the negative electrode current collector is typically made of a copper thin film.

[0005] There are two types of copper thin films: rolled copper foil, which is produced by a rolling process, and electrolytic copper foil, which is produced by an electroplating process. Rolled copper foil has advantages over electrolytic copper foil in terms of physical properties, as the rolling process imparts workability to the metal. However, it has disadvantages such as high manufacturing costs and poor quality uniformity across the width due to the need for multiple rolling processes to produce the thin film. On the other hand, electrolytic copper foil has the advantage of being easily manufactured to a very thin thickness of 6 μm to 8 μm. However, because electrolytic copper foil is produced by electroplating due to its process characteristics, tensile residual stress may form on the surface of the copper thin film. Tensile residual stress acts as a factor that increases the stress intensity factor of the metal. Therefore, when a copper current collector manufactured from the electrolytic copper foil is exposed to the electrolyte environment inside a battery for a long period of time, cracks may initiate and propagate on the surface of the current collector due to corrosion, resulting in a problem of reduced mechanical properties of the copper current collector and reduced battery life.

[0006] Therefore, there is a need to develop a technology to improve the surface fatigue corrosion of copper current collectors in the electrolyte environment inside the battery. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention is intended to solve the above problems, and provides a copper current collector capable of improving fatigue corrosion in an electrolyte environment inside a battery, and a method for manufacturing the same. [Means for solving the problem]

[0008] According to one embodiment of the present invention, there is provided a copper current collector in which, when measuring surface residual stress, an area in which compressive residual stress is formed is 30% or more of the total measured area.

[0009] In the copper current collector of the present invention, the area where compressive residual stress is formed may be 30% to 100% of the total measured area.

[0010] The maximum compressive residual stress can be −20 MPa or less, specifically −200 MPa to −20 MPa.

[0011] The thickness of the copper current collector of the present invention can be 4 μm to 20 μm.

[0012] According to an embodiment of the present invention, there is provided a method for manufacturing a copper current collector, comprising the steps of: manufacturing a copper thin film by an electrolytic plating process; and rolling the copper thin film to form a copper current collector, wherein, when measuring residual stress in the copper current collector, an area where compressive residual stress is formed is 30% or more of a total measured area.

[0013] According to the method for producing a copper current collector of the present invention, at least a portion of the surface of the copper current collector can be plastically deformed by the rolling.

[0014] The magnitude of the pressure applied to the copper thin film during the rolling may be 60% to 83% of the tensile strength of the copper thin film before the rolling, specifically, 210 MPa to 290 MPa.

[0015] The copper thin film before rolling may have a tensile strength of 350 MPa to 500 MPa.

[0016] According to another embodiment of the present invention, there is provided a negative electrode comprising the copper current collector described above.

[0017] According to another embodiment of the present invention, there is provided a lithium secondary battery including a negative electrode including the above-described copper current collector, a positive electrode, and a separator disposed between the negative electrode and the positive electrode. [Effects of the Invention]

[0018] According to the present invention, when measuring the surface residual stress of a copper current collector, the area where compressive residual stress is formed is 30% or more of the total measurement area, and the stress intensity factor of the copper current collector can be significantly reduced. As a result, the strength of the stress field formed around microcracks on the surface of the copper current collector in the electrolyte environment inside the battery is reduced, thereby preventing crack propagation due to fatigue corrosion and thereby improving the life characteristics of the battery.

[0019] In addition, the copper current collector according to the present invention may be manufactured by rolling a copper thin film manufactured by an electrolytic plating process. In this case, due to the characteristics of the electrolytic plating process, the copper current collector can be manufactured with a thin thickness, thereby improving the energy density of the battery. [Brief explanation of the drawings]

[0020] The drawings attached to the specification illustrate preferred embodiments of the present invention and, together with the above-described content of the invention, serve to further understand the technical concept of the present invention. Therefore, the present invention should not be interpreted as being limited solely to the matters depicted in such drawings.

[0021] [Figure 1] 1 is a surface residual stress distribution image of the copper current collector of Example 1 measured using an X-ray diffraction method. [Figure 2] 1 is a surface residual stress distribution image of the copper current collector of Example 2 measured using an X-ray diffraction method. [Figure 3] 1 is a surface residual stress distribution image of the copper current collector of Comparative Example 1 measured using an X-ray diffraction method. DETAILED DESCRIPTION OF THE INVENTION

[0022] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various different forms. However, the present embodiments are provided to fully disclose the present invention and to fully convey the scope of the invention to those skilled in the art to which the present invention pertains, and the present invention is defined only by the scope of the claims. Throughout the specification, the same reference numerals refer to the same elements.

[0023] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used as meanings that can be commonly understood by those having ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless they are clearly and specifically defined.

[0024] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular includes the plural unless otherwise stated in the text. The words "comprises" and / or "comprising" used in this specification do not exclude the presence or addition of one or more other elements in addition to the elements mentioned.

[0025] In this specification, when a part is said to include a certain component, this does not mean that it may exclude other components, but that it may further include other components, unless otherwise specified to the contrary.

[0026] In this specification, the phrase "A and / or B" means A, or B, or A and B.

[0027] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0028] In this specification, D 50 means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0029] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan or a Micromertics ASAP 2020 manufactured by Microtrac.

[0030] As used herein, the term "average diameter" refers to the average value of the longest and shortest diameters of the ligaments or pores contained in the copper current collector. For example, the average diameter can be calculated by obtaining an SEM image of the cross section of the copper current collector using a Zeiss Ultra 55 and then measuring the diameters of the ligaments or pores contained in the SEM image.

[0031] In this specification, the surface residual stress of the copper current collector can be measured by X-ray diffraction using a μ-X360 device manufactured by Pulstec.

[0032] In this specification, the tensile strength of a copper thin film refers to the stress at which the copper thin film breaks before being rolled. Specifically, the tensile strength of a copper thin film refers to the maximum tensile strength measured when the copper thin film breaks after cutting the copper thin film to a width of 12.7 mm, fastening both ends of the copper thin film to upper and lower jigs of a measuring device (UTM, manufactured by ZwickRoell). The copper thin film is then pulled in the vertical direction at a speed of 50 mm / min.

[0033] <Copper current collector> In the copper current collector according to the present invention, when measuring the surface residual stress, the area where compressive residual stress is formed is 30% or more of the total measured area.

[0034] Specifically, the area where the compressive residual stress is formed may be 30% to 100%, more specifically, 50% to 100% of the total measured area. If the ratio of the area where the compressive residual stress is formed is less than 30%, the ratio of the area of ​​the copper current collector where tensile residual stress with a high stress intensity factor is formed increases relatively. As a result, when the copper current collector is exposed to the electrolyte environment inside the battery for a long period of time, cracks may be generated and propagate on the surface of the current collector due to corrosion, which may deteriorate the mechanical properties of the copper current collector and the lifespan of the battery.

[0035] In the copper current collector of the present invention, the maximum compressive residual stress may be −20 MPa or less, specifically −200 MPa to −20 MPa, more specifically −200 MPa to −50 MPa. When the maximum compressive residual stress of the copper current collector satisfies this numerical range, the strength of the stress field formed around microcracks on the surface of the current collector is reduced, thereby preventing crack propagation due to fatigue corrosion on the surface of the current collector when a battery using the copper current collector is operated.

[0036] According to the present invention, the surface of a copper current collector may contain both compressive and tensile residual stress regions. For example, FIG. 1 shows an image of the distribution of surface residual stress on the copper current collector of Example 1, measured using X-ray diffraction. As shown in FIG. 1, the current collector may include regions where compressive residual stress is formed (regions below 0 MPa) and regions where tensile residual stress is formed (regions above 0 MPa). In this case, the compressive and tensile residual stress regions may each be separated into one or more regions and scattered across the surface of the current collector.

[0037] The copper current collector according to the present invention can be used as a negative electrode current collector because it does not cause chemical changes in the battery and has conductivity. Furthermore, the copper current collector according to the present invention can be manufactured by an electrolytic plating method, which has the advantage of relatively reducing manufacturing costs.

[0038] The thickness of the copper current collector can be 4 μm to 20 μm, specifically 5 μm to 15 μm, more specifically 5 μm to 10 μm. When the thickness of the copper current collector satisfies the above numerical range, the energy density of a battery manufactured from the copper current collector of the present invention can be improved.

[0039] <Manufacturing method of copper current collector> Next, a method for producing a copper current collector according to the present invention will be described.

[0040] A method for manufacturing a copper current collector according to the present invention includes the steps of: preparing a copper thin film by an electrolytic plating process; and rolling the copper thin film to form a copper current collector, wherein, when measuring residual stress in the copper current collector, an area where compressive residual stress is formed is 30% or more of the total measured area.

[0041] Copper current collectors manufactured by conventional electrolytic plating methods have a tendency to form tensile residual stress on the surface of the current collector due to process characteristics, which increases the stress intensity factor of the metal. As a result, when exposed to an electrolyte environment for a long period of time, cracks are generated and propagate on the surface of the current collector due to corrosion, which reduces the mechanical properties of the copper current collector and the lifespan of the battery.

[0042] Meanwhile, the copper current collector of the present invention can be manufactured by rolling a copper thin film manufactured by an electrolytic plating process. The rolling process can form compressive residual stress on the surface of the copper current collector, thereby reducing the stress intensity factor of the copper current collector. As a result, the strength of the stress field around microcracks on the surface of the current collector in an electrolyte environment can be reduced, preventing crack propagation due to fatigue corrosion and thereby improving the lifespan of the battery.

[0043] Each step of the method for manufacturing a copper current collector according to the present invention will now be described in more detail.

[0044] (1) Producing copper thin film by electroplating process The method for manufacturing a copper current collector according to the present invention is disclosed by forming a copper foil by an electrolytic plating process.

[0045] Specifically, the copper thin film may be manufactured by electroplating. The electroplating process is advantageous in terms of process flexibility and cost because it is easy to adjust the thickness of the plating foil by adjusting the magnitude of the applied current, the current application time, the temperature, etc. Furthermore, the use of the electroplating process makes it possible to realize a thin copper thin film, thereby improving the energy density of the battery.

[0046] For example, a method for producing a copper thin film using electroplating is as follows: First, a reaction vessel is prepared, which includes a rotating anode drum and a positive electrode plate disposed opposite the rotating anode drum, and the reaction vessel is filled with an aqueous solution of copper sulfate. Next, the rotating anode drum is rotated while applying electricity to the rotating anode drum and the positive electrode plate, thereby electrodepositing copper onto the surface of the rotating anode drum. Finally, the electrodeposited copper is continuously withdrawn from the reaction vessel, thereby finally producing the copper thin film.

[0047] (2) Rolling the copper thin film to form a copper current collector The copper thin film is then rolled to form a copper current collector.

[0048] The method for rolling the copper thin film is not particularly limited, and examples of the method for rolling the copper thin film include, but are not limited to, a method in which the copper thin film is passed between a pair of rollers to apply pressure, and a cold rolling method.

[0049] The rolling may cause plastic deformation of at least a portion of the surface of the copper thin film, resulting in the formation of portions with compressive residual stress and portions with tensile residual stress on the surface of the copper thin film.

[0050] The magnitude of the pressure applied to the copper thin film during the rolling may be 60% to 83%, specifically 70% to 83%, more specifically 70% to 80% of the tensile strength of the copper thin film before the rolling. When the magnitude of the pressure satisfies this numerical range, the copper thin film does not break or fracture, and the area where compressive residual stress is formed on the surface of the copper thin film is expanded, thereby significantly reducing the stress intensity factor of the current collector.

[0051] The pressure applied to the copper thin film during the rolling may be 210 MPa to 290 MPa, specifically 230 MPa to 290 MPa, more specifically 250 MPa to 280 MPa. When the rolling pressure of the copper thin film satisfies the above numerical range, the copper thin film does not break, and the area where compressive residual stress is formed on the surface of the copper thin film can be increased.

[0052] The tensile strength of the copper thin film before rolling may be 300 MPa to 500 MPa, specifically 350 MPa to 500 MPa, and more specifically 350 MPa to 450 MPa. Here, the tensile strength of the copper thin film refers to the stress at which the copper thin film breaks before rolling. For example, the tensile strength of the copper thin film refers to the maximum tensile strength measured when the copper thin film is cut to a width of 12.7 mm, the copper thin film is fastened to the upper and lower jigs of a measuring device (UTM, manufactured by ZwickRoell), and the copper thin film is pulled vertically at a speed of 50 mm / min until the copper thin film breaks. When the tensile strength of the copper thin film satisfies the above numerical range, the copper current collector has excellent mechanical properties against external forces, excellent durability, and can prevent the problem of the copper current collector being subjected to excessive stress and cracking during cell charge and discharge.

[0053] <Lithium secondary battery> The lithium secondary battery of the present invention includes a negative electrode, a positive electrode, a separator, and an electrolyte. The negative electrode includes a negative electrode current collector, and the negative electrode current collector can be the copper current collector of the present invention described above.

[0054] According to one embodiment of the present invention, the negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. According to another embodiment of the present invention, the negative electrode may not include a negative electrode active material layer formed on the negative electrode current collector. Specifically, the negative electrode may be the copper current collector itself manufactured according to the present invention, or a specific metal may be physically bonded, rolled, or deposited on the copper current collector. The deposition method may be an electrochemical deposition method or a chemical vapor deposition method for the specific metal. In this case, the specific metal bonded / rolled / deposited on the copper current collector may include one metal or an alloy of two metals selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), and indium (In).

[0055] The negative electrode active material layer may contain a negative electrode active material, and may further contain a conductive material, a binder, and the like, as needed.

[0056] The negative electrode active material may include at least one selected from the group consisting of lithium metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0057] As the carbon material capable of reversibly intercalating / deintercalating the lithium ions, any carbon-based negative electrode active material generally used in lithium ion secondary batteries can be used without particular limitation. Representative examples thereof include crystalline carbon, amorphous carbon, or both of them can be used. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0058] As the metal or an alloy of these metals and lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn or an alloy of these metals and lithium can be used.

[0059] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li X Fe2O3 (0 ≦ x ≦ 1), Li X WO2 (0 ≦ x ≦ 1) and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) can be used.

[0060] As the substance capable of doping and undoping lithium, Si, SiO x(0 < x ≤ 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. can be mentioned, and at least one of these can also be mixed with SiO2 and used. As the element Y, it can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0061] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0062] The negative electrode active material can be contained in an amount of 60% to 99% by weight, preferably 70% to 99% by weight, more preferably 80% to 98% by weight, based on the total weight of the negative electrode active material layer.

[0063] The negative electrode conductive material is a component for further improving the conductivity of the negative electrode active material. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, carbon powder such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fiber and metal fiber; carbon fluoride powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0064] The negative electrode conductive material can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0065] The negative electrode binder is a component that helps bind the negative electrode conductive material, negative electrode active material, and negative electrode current collector together. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0066] Typically, the negative electrode binder can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0067] Meanwhile, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0068] The positive electrode current collector may be any material that does not cause chemical changes in the battery and has conductivity, and is not particularly limited. For example, as the current collector, stainless steel, aluminum, nickel, titanium, fired carbon, or materials obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used.

[0069] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesive force to the positive electrode active material layer. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc.

[0070] The positive electrode active material layer can contain a positive electrode active material, and can further contain a conductive material, a binder, etc. as required.

[0071] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it can include lithium metal oxides containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the lithium metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1O4 (where 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2, etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r2, and s2 are the atomic fractions of the respective independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, p2 + q2 + r2 + s2 = 1, etc.), lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (where M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, -0.5 ≤ a ≤ 0.5, 0 ≤ x ≤ 0.5, 0 ≤ b ≤ 0.1, etc.), and any one or two or more of these compounds can be included.

[0072] Among them, in terms of being able to enhance the capacity characteristics and stability of the battery, the lithium metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15)O2 and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, or lithium nickel manganese cobalt aluminum oxide (e.g., Li(Ni 0.86 Co 0.05 Mn 0.07 Al 0.02 )O2), lithium iron phosphate (e.g., LiFePO4), etc., and any one or a mixture of two or more of these may be used.

[0073] The positive electrode active material may be contained in an amount of 60% to 99% by weight, preferably 70% to 99% by weight, and more preferably 80% to 98% by weight, based on the total weight of the positive electrode active material layer.

[0074] The positive electrode conductive material is a component for further improving the conductivity of the positive electrode active material. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. Examples of such a conductive material include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with highly developed crystal structures; conductive fibers such as carbon fiber and metal fiber; carbon fluoride powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0075] Typically, the positive electrode conductive material can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, relative to the total weight of the positive electrode active material layer.

[0076] The positive electrode binder is a component that helps bind the active material and conductive material together and to bind them to the current collector.

[0077] Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

[0078] Typically, the positive electrode binder can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, relative to the total weight of the positive electrode active material layer.

[0079] On the other hand, a separator can be disposed between the negative electrode and the positive electrode. Any separator that is typically used as a separator in a lithium secondary battery can be used without any particular limitation, and in particular, a separator that has low resistance to ion migration of the electrolyte and excellent ability to retain moisture in the electrolyte solution is preferred.

[0080] For example, the separator may be a porous polymer film containing a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used as the separator.

[0081] The lithium secondary battery according to an embodiment of the present invention may include an electrolyte. In this case, the electrolyte may be a non-aqueous electrolyte. The non-aqueous electrolyte may include an organic solvent and a lithium salt commonly used in the art, but is not particularly limited thereto.

[0082] The organic solvent can be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0083] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0084] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt is preferably contained in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0085] Meanwhile, the non-aqueous electrolyte according to the present invention may further contain an additive, although this is not essential, to further improve the physical properties of the secondary battery.

[0086] Examples of such additives include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, nitrile-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0087] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinyl ethylene carbonate (VEC).

[0088] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0089] The nitrile compound may be, for example, succinonitrile, adiponitrile, hexanetricyanide, 1,4-dicyano-2-butene, or the like.

[0090] The sultone compound may be, for example, 1,3-propane sultone, 1,3-propene sultone, or the like.

[0091] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0092] The phosphate-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0093] The borate-based compound may be, for example, tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), or the like.

[0094] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0095] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution, and may be one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bis(oxalatoborate) (LiB(C2O4)2)), and LiBF4.

[0096] Meanwhile, the additives may be used alone or in combination of two or more.

[0097] The total amount of the additives may be 1 wt % to 20 wt %, preferably 1 wt % to 15 wt %, based on the total weight of the electrolyte. When the additives are contained within this range, a stable coating is formed on the electrode, ignition during overcharge can be suppressed, and side reactions can be prevented from occurring during the initial activation process of the secondary battery, or the additives can be prevented from remaining or being precipitated.

[0098] The lithium secondary battery of the present invention may be fabricated by placing an electrode assembly formed by disposing a separator between a positive electrode and a negative electrode in a battery case, injecting an electrolyte, and sealing the battery case, or by stacking the electrode assemblies, impregnating the stack with an electrolyte, and then placing the resulting stack in a battery case and sealing the battery case.

[0099] The battery case may be one commonly used in the art, and its shape is not limited depending on the intended use of the battery. For example, it may be cylindrical, rectangular, pouch-shaped, or coin-shaped, but is not limited thereto.

[0100] The lithium secondary battery according to an embodiment of the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in a medium- to large-sized battery module including a number of battery cells. Preferred examples of the medium- to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems (ESS).

[0101] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely illustrative and are not intended to limit the scope of the present invention. It is obvious to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present description, and it goes without saying that such changes and modifications fall within the scope of the appended claims.

[0102] Examples and Comparative Examples Example 1 (1) Manufacturing of copper current collectors A reaction vessel containing a rotating negative electrode drum and a positive electrode plate facing the rotating negative electrode drum was prepared, and the reaction vessel was filled with an electrolyte mixture of copper sulfate and water. Next, while applying electricity to the rotating negative electrode drum and the positive electrode plate, the rotating negative electrode drum was rotated to electrodeposit copper onto the surface of the rotating negative electrode drum. The electrodeposited copper was then continuously extracted from the reaction vessel, resulting in a 10 μm-thick copper thin film.

[0103] The obtained copper thin film was rolled at a pressure of 250 MPa using a roll press process. The rolled copper thin film was slit and sheeted to prepare a copper current collector with a thickness of 8.5 μm.

[0104] (2) Manufacture of lithium secondary batteries Spheroidized artificial graphite, carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a weight ratio of 97.35:0.5:1.15:1 in distilled water and stirred to prepare a negative electrode slurry. The negative electrode slurry was applied to one side of the prepared copper current collector at a concentration of 10.48 mg / cm. 2 The dried negative electrode slurry was rolled and vacuum dried in a vacuum oven at 100°C for 8 hours to prepare a negative electrode.

[0105] LiCoO2, polyvinylidene fluoride (PVdF), carbon nanotubes (CNTs), and carbon black were mixed in a weight ratio of 97.59:1.18:0.24:0.09 in N-methylpyrrolidone (NMP) solvent and stirred to prepare a cathode slurry. The cathode slurry was applied to one side of a 10 μm-thick aluminum thin film at a concentration of 18.60 mg / cm. 2 The dried cathode slurry was rolled and dried in a vacuum oven at 130°C for 6 hours, and then punched out to prepare a cathode.

[0106] The negative and positive electrodes prepared as described above and a porous polyethylene separator (thickness: 12 μm) were stacked to prepare an electrode assembly.

[0107] An electrolyte was produced by dissolving LiPF6 in a solvent (mass ratio of EC:PC:EP:PP = 20:10:25:45) to a concentration of 1.2 M.

[0108] The electrode assembly was housed in a battery case, the electrolyte was poured into the battery case, and the battery case was sealed to manufacture a lithium secondary battery.

[0109] Example 2 A copper current collector (thickness: 8 μm) was produced in the same manner as in Example 1, except that the obtained copper thin film was rolled at a pressure of 280 MPa by a roll press process.

[0110] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the copper current collector manufactured by the above method was used as a negative electrode current collector.

[0111] Comparative Example 1 A copper current collector (thickness: 10 μm) was produced in the same manner as in Example 1, except that the obtained copper thin film was rolled at a pressure of 200 MPa by a roll press process.

[0112] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the copper current collector manufactured by the above method was used as a negative electrode current collector.

[0113] Comparative Example 2 A copper current collector was produced in the same manner as in Example 1, except that the obtained copper thin film was rolled at a pressure of 300 MPa by a roll press process.

[0114] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the copper current collector manufactured by the above method was used as a negative electrode current collector.

[0115] [Experimental Example 1 - Measurement of tensile strength of copper thin film before rolling] The tensile strength of the copper thin film was measured before rolling in Examples 1 and 2 and Comparative Examples 1 and 2. Specifically, both ends of the copper thin film were fastened to the upper and lower jigs of a UTM (Universal Testing Machine, manufactured by ZwickRoell), respectively, and then the copper thin film was pulled in the vertical direction to measure the tensile strength of the copper thin film before rolling. The measurement results are shown in Table 1 below.

[0116] Then, the ratio of the pressure applied to the copper thin film to the tensile strength was calculated and shown in Table 1 below.

[0117] [Table 1]

[0118] [Experimental Example 2 - Measurement of residual stress on the surface of a copper current collector] The residual stress on the surface of each of the copper current collectors manufactured in Examples 1 and 2 and Comparative Example 1 was measured. Specifically, the residual stress on the surface of each copper current collector was measured by X-ray diffraction using a μ-X360 instrument manufactured by Pulstec. The measurement results are shown in Table 2 below and Figures 1 and 2.

[0119] FIG. 1 is a distribution image of the surface residual stress of the copper current collector of Example 1 measured by X-ray diffraction.

[0120] FIG. 2 is a distribution image of the surface residual stress of the copper current collector of Example 2 measured by X-ray diffraction.

[0121] FIG. 3 is a distribution image of the surface residual stress of the copper current collector of Comparative Example 1 measured by X-ray diffraction.

[0122] As shown in FIGS. 1 to 3, the copper current collector may include a portion where compressive residual stress is formed (shown in blue) and a portion where tensile residual stress is formed (shown in red).

[0123] [Table 2]

[0124] [Experimental Example 3 - Evaluation of Life Characteristics of Lithium Secondary Battery] The lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Example 1 were charged / discharged at a temperature of 45° C. under the following conditions, and the capacity retention rate depending on the number of cycles was measured.

[0125] -Charging conditions: CC (constant current) / CV (constant voltage) mode, charging at a rate of 0.33C, cut-off at 4.2V and 0.05C -Discharge conditions: CC mode, discharge at 0.33C, cut-off at 3.0V

[0126] In this case, the capacity retention rate of the battery was measured using an IL-2C-525S manufactured by JEIO TECH Co., Ltd. The measurement results are shown in Table 3 below.

[0127] [Table 3]

[0128] As shown in Tables 1 to 3, in Examples 1 and 2, where the area where compressive residual stress was formed was 30% or more of the total measured area, it was confirmed that the battery capacity retention rate was higher than that of Comparative Example 1, where the ratio of the area where compressive residual stress was formed was outside the above range. This suggests that when secondary batteries manufactured using the copper current collectors of Examples 1 and 2 are operated, cracks do not occur on the surface of the copper current collector.

[0129] On the other hand, in the case of Comparative Example 2, the copper current collector was broken and fractured, making it impossible to measure the residual stress on the surface of the copper current collector and the capacity retention rate of the lithium secondary battery.

Claims

1. A copper current collector, wherein, when measuring surface residual stress, the area in which compressive residual stress is formed is 30% or more of the total measured area.

2. 2. The copper current collector according to claim 1, wherein the area where the compressive residual stress is formed is 30% to 100% of the total measured area.

3. 2. The copper current collector according to claim 1, wherein the maximum compressive residual stress is −20 MPa or less.

4. 2. The copper current collector according to claim 1, wherein the maximum compressive residual stress is −200 MPa to −20 MPa.

5. The copper current collector of claim 1, wherein the thickness is 4 μm to 20 μm.

6. preparing a copper thin film by electroplating; and rolling the copper thin film to form a copper current collector. The method for manufacturing a copper current collector, wherein, when measuring residual stress, an area where compressive residual stress is formed is 30% or more of a total measured area of ​​the copper current collector.

7. The method for producing a copper current collector according to claim 6 , wherein the rolling causes plastic deformation of at least a portion of the surface of the copper current collector.

8. 7. The method for producing a copper current collector according to claim 6, wherein the magnitude of the pressure applied to the copper thin film during the rolling is 60% to 83% of the tensile strength of the copper thin film before the rolling.

9. 7. The method for producing a copper current collector according to claim 6, wherein the pressure applied to the copper thin film during the rolling is 210 MPa to 290 MPa.

10. 7. The method for producing a copper current collector according to claim 6, wherein the copper thin film before rolling has a tensile strength of 350 MPa to 500 MPa.

11. A negative electrode comprising the copper current collector according to claim 1 .

12. A negative electrode comprising the copper current collector according to any one of claims 1 to 5; A positive electrode and a separator disposed between the negative electrode and the positive electrode.

Images