KR102803278B1 - Negative electrode and secondary battery comprising the same - Google Patents

Abstract

The present invention relates to an anode comprising: a negative current collector; and a negative active material layer formed on the negative current collector, wherein the negative active material layer comprises a negative active material including a silicon-based active material, a binder including a first binder polymer and a second binder polymer, and a conductive material, wherein the first binder polymer comprises at least one selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and polyacrylamide, the second binder polymer comprises polyethylene glycol, and the binder comprises the first binder polymer and the second binder polymer in a weight ratio of 79:21 to 99.8:0.2.

Description

Negative electrode and secondary battery comprising the same {NEGATIVE ELECTRODE AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a negative electrode and a secondary battery including the same.

Recently, with the rapid spread of electronic devices that use batteries, such as mobile phones, laptop computers, and electric vehicles, the demand for small, lightweight, and relatively high-capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries are attracting attention as a power source for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts are actively being conducted to improve the performance of lithium secondary batteries.

In general, a lithium secondary battery includes a cathode, an anode, a separator interposed between the cathode and the anode, an electrolyte, an organic solvent, etc. In addition, the cathode and the anode may have an active material layer formed on a current collector including a cathode active material or an anode active material. In general, a lithium-containing metal oxide such as LiCoO ₂ or LiMn ₂ O ₄ is used as a cathode active material for the cathode, and accordingly, a carbon-based active material or a silicon-based active material that does not contain lithium is used as an anode active material for the anode.

In particular, silicon-based active materials among negative active materials are noteworthy because they have a capacity that is about 10 times higher than carbon-based active materials, and have the advantage of being able to realize high energy density even with thin electrodes due to their high capacity. However, silicon-based active materials are not commonly used due to problems such as volume expansion due to charge and discharge and the resulting decrease in life characteristics.

Meanwhile, in the negative electrode containing a silicon-based active material, there have been attempts to use binder polymers such as polyacrylic acid and polyvinyl alcohol that have strong stress in order to alleviate volume expansion due to charge and discharge of the silicon-based active material. However, since the flexibility of the negative electrode is reduced due to the strong stress of these binder polymers, there are problems such as cracks occurring in the negative electrode or damage to the negative electrode occurring when applying a roll-to-roll process.

Korean Patent Publication No. 10-2017-0074030 relates to an anode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. Although the invention discloses an anode active material including a porous silicon-carbon composite, it has limitations in solving the aforementioned problems.

Korean Patent Publication No. 10-2017-0074030

One object of the present invention is to provide a cathode having excellent flexibility and adhesion to a current collector.

In addition, another object of the present invention is to provide a secondary battery including the aforementioned negative electrode.

The present invention provides an anode including: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material including a silicon-based active material, a binder including a first binder polymer and a second binder polymer, and a conductive material, wherein the first binder polymer includes at least one selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and polyacrylamide, the second binder polymer includes polyethylene glycol, and the binder includes the first binder polymer and the second binder polymer in a weight ratio of 79:21 to 99.8:0.2.

In addition, the present invention provides a secondary battery including the above-mentioned negative electrode; a positive electrode opposite to the negative electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

The negative electrode of the present invention uses a silicon-based active material, and includes a binder comprising a first binder polymer and a second binder polymer at a specific weight ratio. The first binder polymer can exert excellent stress to alleviate volume expansion/contraction of the silicon-based active material, and the second binder polymer can improve the flexibility of the negative electrode to an excellent level by including polyethylene glycol. Therefore, the negative electrode of the present invention has excellent flexibility, and thus can be preferably used in a roll-to-roll process or in the manufacture of a cylindrical cell, and can have excellent electrode adhesion and life characteristics.

The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as having meanings and concepts that conform to the technical idea of the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best manner.

The terminology used in this specification is for the purpose of describing exemplary embodiments only and is not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

In this specification, it should be understood that the terms “comprise,” “include,” or “have,” etc., are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

In this specification, the average particle diameter (D ₅₀ ) can be defined as a particle diameter corresponding to 50% of the volume accumulation amount in the particle diameter distribution curve of the particles. The average particle diameter (D ₅₀ ) can be measured using, for example, a laser diffraction method. The laser diffraction method can generally measure particle diameters from the submicron range to several mm, and can obtain results with high reproducibility and high resolution.

Hereinafter, the present invention will be described in detail.

<negative>

The present invention relates to a negative electrode, and more particularly, to a negative electrode for a lithium secondary battery.

The negative electrode of the present invention comprises: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material including a silicon-based active material, a binder including a first binder polymer and a second binder polymer, and a conductive material, wherein the first binder polymer comprises at least one selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and polyacrylamide, the second binder polymer comprises polyethylene glycol, and the binder comprises the first binder polymer and the second binder polymer in a weight ratio of 79:21 to 99.8:0.2.

In general, silicon-based active materials are known to have a capacity about 10 times higher than carbon-based active materials, and accordingly, when silicon-based active materials are applied to the anode, it is expected that an electrode with a high level of energy density can be implemented even with a thin thickness. However, silicon-based active materials have problems with volume expansion/contraction due to insertion/desorption of lithium during charge/discharge, and thus, general-purpose use of silicon-based active materials is not easy.

To prevent such problems, there have been attempts to alleviate volume expansion/contraction of the silicon-based active material due to charge/discharge by using a binder polymer having strong stress, such as polyacrylic acid or polyvinyl alcohol, as a component of the negative electrode in the negative electrode to which the silicon-based active material is applied. However, the binder polymer described above reduces the flexibility of the negative electrode due to the strong stress, so when manufacturing the negative electrode using a roll-to-roll process or manufacturing a cylindrical cell including the negative electrode, there is a problem that the negative electrode may be damaged or cracked. Such damage or cracks in the negative electrode lead to a rapid reduction in the lifespan, which is not desirable.

Accordingly, the negative electrode of the present invention uses a binder including a first binder polymer including at least one selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and polyacrylamide, and a second binder polymer including polyethylene glycol at a specific weight ratio when using a silicon-based active material. The first binder polymer can sufficiently alleviate volume expansion/contraction of the silicon-based active material while improving electrode adhesion, and the second binder polymer can impart flexibility to the negative electrode without reducing electrode adhesion. Accordingly, the negative electrode of the present invention can have excellent flexibility and electrode adhesion.

The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Specifically, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc.

The above negative electrode collector may typically have a thickness of 3 to 100 μm.

The above negative electrode current collector may form fine irregularities on the surface to strengthen the bonding strength of the negative electrode active material. For example, the above negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

The above negative electrode active material layer is formed on the negative electrode current collector.

The above negative active material layer includes a silicon-based active material.

The above silicon-based active material may include a compound represented by SiO _x (0≤x<2). In the case of SiO ₂ , since it does not react with lithium ions and thus cannot store lithium, x is preferably within the above range, and more preferably, it is preferable to contain oxygen in order to appropriately alleviate volume expansion of the silicon-based active material, and may be 0.01≤x<2.

The average particle diameter (D ₅₀ ) of the above silicon-based active material may be 1 ㎛ to 15 ㎛, preferably 3 ㎛ to 10 ㎛, in terms of minimizing side reactions with the electrolyte, reducing the effect of volume expansion of the silicon-based active material, and facilitating accessibility to the negative electrode binder for binding the active material and the current collector.

The above negative electrode active material may further include a carbon-based active material together with the above-described silicon-based active material. The carbon-based active material may provide excellent cycle characteristics or battery life performance to the negative electrode or secondary battery of the present invention.

Specifically, the carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, super P, graphene, and fibrous carbon, and preferably may include at least one selected from the group consisting of artificial graphite and natural graphite.

The average particle diameter (D ₅₀ ) of the above carbon-based active material may be 10 ㎛ to 30 ㎛, preferably 15 ㎛ to 25 ㎛, in order to ensure structural stability during charge and discharge and reduce side reactions with the electrolyte.

Specifically, it is preferable that the negative electrode active material uses both the silicon-based active material and the carbon-based active material in terms of simultaneously improving capacity characteristics and cycle characteristics, and specifically, it is preferable that the negative electrode active material contains the silicon-based active material and the carbon-based active material in a weight ratio of 5:95 to 40:60, preferably 10:90 to 25:75. The above range is preferable in terms of simultaneously improving capacity and cycle characteristics.

The above negative active material may be included in the negative active material layer at 80 wt% to 99 wt%, preferably 85 wt% to 96 wt%.

The above negative active material layer includes a binder. The binder includes a first binder polymer and a second binder polymer.

The first binder polymer includes at least one selected from the group consisting of polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), and polyacryl amide (PAM). The first binder polymer may be a copolymer of the aforementioned components.

The above first binder polymer has a hydrophilic property and has a property of not dissolving in electrolytes or electrolytic solutions generally used in secondary batteries. Such a property can impart strong stress or tensile strength to the aqueous binder when applied to an anode or secondary battery, and accordingly can effectively suppress the problem of volume expansion/contraction due to charge/discharge of a silicon-based active material and exhibit excellent electrode adhesiveness.

Preferably, the first binder polymer may include polyacrylic acid and polyvinyl alcohol, and more preferably, it may include a copolymer of polyacrylic acid and polyvinyl alcohol, in which case, a stronger stress may be exerted by a hydrogen bond or a covalent bond between a carboxyl group (-COOH) of polyacrylic acid and a hydroxyl group (-OH) of polyvinyl alcohol. When the first binder polymer includes polyacrylic acid and polyvinyl alcohol, or a copolymer of polyacrylic acid and polyvinyl alcohol, the first binder polymer may include polyvinyl alcohol and polyacrylic acid in a weight ratio of 50:50 to 90:10, and preferably 55:45 to 80:20.

The weight average molecular weight (M _w ) of the first binder polymer may be 100,000 to 500,000, preferably 200,000 to 400,000, in order to further improve the flexibility of the electrode while preventing dissolution into the electrolyte.

The second binder polymer comprises polyethylene glycol (PEG).

The above polyethylene glycol (PEG) can provide flexibility to the negative electrode. When only the first binder polymer is used as a binder, the stress of the binder becomes excessively high and the tensile strength of the negative electrode becomes strong, which may decrease the flexibility of the negative electrode and cause damage or cracks in the negative electrode, and there is a risk of deterioration of the life characteristics. However, the negative electrode of the present invention uses the second binder polymer together with the first binder polymer to sufficiently control the volume expansion/contraction of the silicon-based active material and implement excellent electrode adhesion while improving the flexibility of the negative electrode. Therefore, when the negative electrode is applied to a roll-to-roll process or cylindrical cell manufacturing, the processability can be improved and the occurrence of damage to the negative electrode can be reduced.

The second binder polymer may contain 40 wt% or more of the polyethylene glycol, preferably 50 wt% or more, more preferably 80 wt% or more. If the second binder polymer contains less than 40 wt% of the polyethylene glycol, there is a concern that the effect of improving the flexibility of the negative electrode may be reduced.

The weight average molecular weight (M _w ) of the second binder polymer may be 1,000 to 500,000, preferably 50,000 to 400,000, and more preferably 75,000 to 300,000. When within the above range, the adhesive strength and flexibility of the negative electrode can be further improved.

The binder contains the first binder polymer and the second binder polymer in a weight ratio of 79:21 to 99.8:0.2. If the binder contains less than 79 wt% of the first binder polymer and more than 21 wt% of the second binder polymer, the electrode adhesiveness of the negative electrode may not be improved and there is a concern that the lifespan of the negative electrode may be reduced. If the binder contains more than 99.8 wt% of the first binder polymer and less than 0.2 wt% of the second binder polymer, the effect of improving the flexibility of the negative electrode may not be exerted.

Preferably, the binder may contain the first binder polymer and the second binder polymer in a weight ratio of 83:17 to 98:2, more preferably 92:8 to 97:3, and when within the above range, the aforementioned effects of improving the flexibility of the negative electrode and the electrode adhesion can be simultaneously exhibited at an excellent level.

The above binder may be included in the negative electrode active material layer at 1 wt% to 10 wt%, preferably 2 wt% to 5 wt%, and more preferably 3 wt% to 4.5 wt%. When present in the above range, the capacity characteristics of the negative electrode can be improved while sufficiently exhibiting the effects of improving the flexibility and electrode adhesiveness of the negative electrode.

The conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

The above-mentioned conductive material may be included in the negative electrode active material layer in an amount of 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%, relative to the total weight of the negative electrode active material layer.

The thickness of the above negative active material layer may be 10 ㎛ to 200 ㎛, preferably 50 ㎛ to 150 ㎛.

The above negative electrode can be manufactured by coating a negative electrode slurry including a negative electrode active material, a binder, and a conductive agent and/or a solvent for forming a negative electrode slurry on the negative electrode current collector, and then drying and rolling.

The solvent for forming the above cathode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, preferably distilled water.

<Secondary battery>

The present invention provides a secondary battery, specifically a lithium secondary battery, including the above-described negative electrode.

Specifically, a secondary battery according to the present invention includes the above-mentioned negative electrode; a positive electrode opposite to the negative electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

The above positive electrode may include a positive electrode current collector; a positive electrode active material layer formed on the positive electrode current collector.

The above positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Specifically, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc.

The above positive electrode current collector may typically have a thickness of 3 to 500 μm.

The above-mentioned positive electrode current collector may form fine irregularities on the surface to strengthen the bonding strength of the negative electrode active material. For example, the above-mentioned negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

The above positive electrode active material layer may include a positive electrode active material.

The above positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium-transition metal composite oxide including lithium and at least one transition metal selected from nickel, cobalt, manganese, and aluminum, preferably a lithium-transition metal composite oxide including lithium and a transition metal selected from nickel, cobalt, and manganese.

More specifically, the lithium transition metal composite oxide includes lithium-manganese oxides (e.g., LiMnO ₂ , LiMn ₂ O ₄ , etc.), lithium-cobalt oxides (e.g., LiCoO ₂ , etc.), lithium-nickel oxides (e.g., LiNiO ₂ , etc.), lithium-nickel-manganese oxides (e.g., LiNi _1-Y Mn _Y O ₂ (wherein, 0<Y<1), LiMn _2-z Ni _z O ₄ (wherein, 0<Z<2)), lithium-nickel-cobalt oxides (e.g., LiNi _1-Y1 Co _Y1 O ₂ (wherein, 0<Y1<1)), lithium-manganese-cobalt oxides (e.g., LiCo _1-Y2 Mn _Y2 O ₂ (wherein, 0<Y2<1), LiMn _2-z1 Co _z1 O ₄ (wherein, (0＜Z1＜2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni _p Co _q Mn _r1 )O ₂ (wherein, 0＜p＜1, 0＜q＜1, 0＜r1＜1, p+q+r1=1) or Li(Ni _p1 Co _q1 Mn _r2 )O ₄ (wherein, 0＜p1＜2, 0＜q1＜2, 0＜r2＜2, p1+q1+r2=2) etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni _p2 Co _q2 Mn _r3 M _S2 )O ₂ (wherein, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 are atomic fractions of independent elements, 0＜p2＜1, 0＜q2＜1, 0＜r3＜1, 0＜s2＜1, p2+q2+r3+s2=1), etc.), and one or more compounds among these may be included. Among these, the lithium transition metal composite oxide may be LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , lithium nickel-manganese-cobalt oxide (for example, Li(Ni _0.6 Mn _0.2 Co _0.2 )O 2 , Li(Ni 0.5 Mn 0.3 Co _0.2 )O ₂ , Li(Ni _0.7 Mn _0.15 Co _0.15 )O ₂ or Li(Ni _0.8 Mn _0.1 Co _0.1 )O ₂ , or lithium nickel cobalt aluminum oxide (for example, Li(Ni _0.8 Co _{0.15 Al 0.05} ) _{O 2 ,} etc.), and considering the prominence of the improvement effect according to the control of the type and content ratio of the constituent elements forming the lithium transition metal composite oxide _, the lithium transition metal composite oxide may be The oxide may be _Li ( _{Ni0.6Mn0.2Co0.2} ) _{O2 , Li} ( _{Ni0.5Mn0.3Co0.2)O2, Li(Ni0.7Mn0.15Co0.15 ) O2 or Li ( Ni0.8Mn0.1Co0.1} ) _O2 , and any one of these or a mixture _of two or more _{thereof may} be used.

The above-mentioned positive electrode active material may be included in the positive electrode active material layer at 80 to 99 wt%, preferably 92 to 98.5 wt%, taking into account sufficient capacity of the positive electrode active material.

The above-described positive electrode active material layer may further include a binder and/or a conductive material together with the above-described positive electrode active material.

The above binder is a component that assists in the binding of the active material and the conductive material and the binding to the current collector, and specifically, may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

The above binder may be included in the positive electrode active material layer at 1 to 20 wt%, preferably 1.2 to 10 wt%, in order to sufficiently secure binding force between components such as the positive electrode active material.

The conductive material may be used to assist and improve conductivity in a secondary battery, and is not particularly limited as long as it has conductivity without causing a chemical change. Specifically, the conductive material may include at least one selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and preferably, the conductive material may include carbon black in terms of improving conductivity.

In order to facilitate dispersion of the conductive material during the preparation of a slurry for forming a positive electrode active material layer and to further improve electrical conductivity, the specific surface area of the conductive material may be 80 m ² /g to 200 m ² /g, preferably 100 m ² /g to 150 m ² /g.

The above-mentioned conductive agent may be included in the positive electrode active material layer at 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%, in order to sufficiently secure electrical conductivity.

The thickness of the above positive electrode active material layer may be 30 ㎛ to 400 ㎛, preferably 50 ㎛ to 110 ㎛.

The above positive electrode can be manufactured by coating a positive electrode slurry including a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming a positive electrode slurry on the positive electrode current collector, and then drying and rolling.

The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a desirable viscosity when including the positive electrode active material, and optionally a binder and a conductive material. For example, the solvent for forming the positive electrode slurry may be included in the positive electrode slurry such that the concentration of the solid content including the positive electrode active material, and optionally a binder and a conductive material is 50 wt% to 95 wt%, preferably 70 wt% to 90 wt%.

The above separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. If it is a separator that is usually used as a separator in a lithium secondary battery, it can be used without any special restrictions, and in particular, it is preferable that it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used. In addition, a typical porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. can be used. In addition, a coated separator containing a ceramic component or a polymer material can be used to secure heat resistance or mechanical strength, and can be selectively used in a single-layer or multi-layer structure.

In addition, the electrolyte used in the present invention may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a secondary battery.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, the organic solvent may include: ester solvents such as methyl acetate, ethyl acetate, gamma-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; Nitriles such as R-CN (R is a linear, branched or cyclic hydrocarbon group having C2 to C20, which may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Of these, carbonate solvents are preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge/discharge performance of the battery, and a linear carbonate compound having low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate), is more preferable. In this case, when the cyclic carbonate and the linear carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

The above lithium salt can be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF ₆ , LiClO _㎛ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ . It is preferable to use the concentration of the lithium salt within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance, and lithium ions can move effectively.

The above secondary battery can be manufactured by inserting a separator between the above-described negative electrode and positive electrode and then injecting an electrolyte according to a conventional secondary battery manufacturing method.

The secondary battery according to the present invention is useful in the fields of portable devices such as mobile phones, notebook computers, digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs), and can be particularly preferably used as a component battery of a medium- or large-sized battery module. Accordingly, the present invention also provides a medium- or large-sized battery module including the secondary battery as a unit battery.

These medium and large-sized battery modules can be preferably applied to power sources that require high output and large capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

Example

Example 1: Preparation of cathode

A negative electrode active material was prepared by mixing SiO _x (0.01≤x<2) (average particle size ( _D50 ): 7㎛) as a silicon-based active material and artificial graphite (average particle size ( _D50 ): 20㎛) as a carbon-based active material in a weight ratio of 15:85.

A copolymer (weight average molecular weight (M _w ): 360,000) containing polyvinyl alcohol and polyacrylic acid in a weight ratio of 65:35 was prepared as a first binder polymer, and polyethylene glycol (weight average molecular weight (M w ): 35,000) was prepared as a second binder polymer. The first binder polymer and the second binder polymer were mixed in a weight ratio of 80:20 to form a binder.

The negative electrode active material, carbon black (SuperC65, manufacturer: Timcal) as a conductive material, and the binder were added to distilled water as a solvent for forming a negative electrode slurry at a weight ratio of 94.5:1.5:4 to prepare a negative electrode slurry (solid content concentration: 45 wt%).

The negative electrode slurry was coated on one side of a copper current collector (thickness: 20 μm) as a negative electrode collector, rolled (roll pressed), and dried in a vacuum oven at 100°C for 1 hour to form a negative electrode active material layer (thickness: 130 μm), which was used as a negative electrode according to Example 1.

Example 2

A negative electrode of Example 2 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 85:15.

Example 3

A negative electrode of Example 3 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 90:10.

Example 4

A negative electrode of Example 4 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 95:5.

Example 5

A negative electrode of Example 5 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 97.5:2.5.

Example 6

A negative electrode of Example 6 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 98.7:1.3.

Example 7

A negative electrode of Example 6 was manufactured in the same manner as Example 1, except that the weight ratio of the first binder polymer and the second binder polymer as a binder was adjusted to 99.75:0.25.

Example 8

A negative electrode of Example 8 was manufactured in the same manner as in Example 5, except that the weight average molecular weight (Mw) of the second binder polymer was adjusted to 100,000.

Example 9

A negative electrode of Example 9 was manufactured in the same manner as in Example 5, except that the weight average molecular weight (Mw) of the second binder polymer was adjusted to 350,000.

Comparative Example 1

A negative electrode of Comparative Example 1 was manufactured in the same manner as in Example 1, except that only the first binder polymer was used as a binder.

Comparative Example 2

A negative electrode of Comparative Example 2 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer was adjusted to 77.5:22.5.

Comparative Example 3

A negative electrode of Comparative Example 3 was manufactured in the same manner as in Example 1, except that the weight ratio of the first binder polymer and the second binder polymer was adjusted to 99.88:0.12.

Experimental example

Experimental Example 1: Flexibility Evaluation

A cylindrical mandrel bending test was performed on the cathodes manufactured in Examples 1 to 9 and Comparative Examples 1 to 3.

Specifically, the cathodes of the examples and comparative examples were cut to 20 mm × 100 mm, the cathodes were wound around cylindrical rods made of SUS material having various diameters, and the minimum diameter at which cracks did not occur in the cathodes was observed with the naked eye. The results are shown in Table 1 below.

Experimental Example 2: Evaluation of Electrode Adhesion

The electrode adhesion was evaluated using the cathodes manufactured in Examples 1 to 9 and Comparative Examples 1 to 3.

Specifically, the negative electrodes of the examples and comparative examples were cut to 20 mm × 100 mm. Double-sided tape was attached to a slide glass, and the negative electrodes and the slide glass were attached with the double-sided tape so that the negative active material layer of the negative electrodes of the examples and comparative examples was in contact with the slide glass. The negative electrode current collector was separated from the negative electrode active material layer by 5 cm in a 180° direction, and the force acting between the negative electrode active material layer and the negative electrode current collector was measured using a texture analyzer device. The results are shown in Table 1 below.

Flexibility Assessment Electrode adhesion evaluation Minimum diameter (mm) in which no crack occurs in the cathode Electrode adhesion (gf/20cm) Example 1 3 25 Example 2 3 26 Example 3 3 26 Example 4 3 28 Example 5 6 31 Example 6 10 32 Example 7 22 34 Example 8 4 34 Example 9 4 31 Comparative Example 1 40 37 Comparative Example 2 3 20 Comparative Example 3 34 35

Referring to Table 1, it can be confirmed that the negative electrodes of Examples 1 to 9 using the first binder polymer and the second binder polymer within the scope of the present invention simultaneously improve the flexibility and adhesiveness of the electrode to an excellent level compared to the comparative examples.

Claims (10)

Negative current collector; and
Including a negative electrode active material layer formed on the negative electrode current collector,
The above negative active material layer includes a negative active material including a silicon-based active material, a binder composed of a first binder polymer and a second binder polymer, and a conductive material.
The first binder polymer comprises at least one selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile and polyacrylamide,
The above second binder polymer comprises polyethylene glycol,
The above binder is a negative electrode comprising the first binder polymer and the second binder polymer in a weight ratio of 79:21 to 99.8:0.2.
In claim 1,
A negative electrode in which the binder is included in the negative electrode active material layer at 1 to 10 wt%.
In claim 1,
The second binder polymer is a negative electrode containing 80 wt% or more of the polyethylene glycol.
A negative electrode according to claim 1, wherein the weight average molecular weight (Mw) of the second binder polymer is 1,000 to 500,000.
In claim 1, the first binder polymer is a negative electrode comprising polyacrylic acid and polyvinyl alcohol.
In claim 1, a negative electrode wherein the weight average molecular weight (Mw) of the first binder polymer is 100,000 to 500,000.
In claim 1, the silicon-based active material is a negative electrode including a compound represented by SiO _x (0≤x<2).
In claim 1, the negative electrode further comprises a carbon-based active material.
In claim 8, the negative electrode active material comprises the silicon-based active material and the carbon-based active material in a weight ratio of 5:95 to 40:60.
A cathode according to claim 1;
An anode opposite to the cathode;
A separator interposed between the cathode and the anode; and
A secondary battery containing an electrolyte.