KR102621786B1 - Si Anode and Lithium Secondary Battery Comprising The Same - Google Patents

Abstract

The present invention includes a current collector and a negative electrode active material layer located on at least one surface of the current collector, wherein the negative electrode active material layer includes a Si-containing negative electrode active material, a conductive material, and a first binder in contact with the Si-containing negative electrode active material. , and a second binder in contact with the conductive material, wherein the first binder has a modulus of 10,000 to 80,000 Mpa, and the second binder has a modulus of 1,000 to 8,000 Mpa, manufacturing the same. A method and a lithium secondary battery comprising the same are provided.

Description

Si-based anode and lithium secondary battery containing the same {Si Anode and Lithium Secondary Battery Comprising The Same}

The present invention relates to a Si-based negative electrode and a lithium secondary battery containing the same, and more specifically, to a Si-based negative electrode whose volume expansion is controlled by including two types of binders with different moduli, and to a lithium secondary battery containing the same. .

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries with high energy density and voltage, long cycle life, and low discharge rate have been commercialized and are widely used.

A lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode to separate them, and an electrolyte solution that electrochemically communicates with the positive electrode and the negative electrode.

These lithium secondary batteries are typically manufactured using a compound in which lithium is inserted, such as LiCoO ₂ or LiMn ₂ O ₄ , for the positive electrode, and using a material in which lithium is not inserted, such as carbon-based or Si-based, for the negative electrode. During charging, lithium ions inserted into the positive electrode move to the negative electrode through the electrolyte, and during discharging, lithium ions move again from the negative electrode to the positive electrode. Lithium, which moves from the anode to the cathode during the charging reaction, reacts with the electrolyte to form a solid electrolyte interface (SEI), a kind of passivation film, on the surface of the cathode. This SEI can stabilize the structure of the cathode by preventing the decomposition reaction of the electrolyte by suppressing the movement of electrons required for the reaction between the cathode and the electrolyte, but because it is an irreversible reaction, it results in the consumption of lithium ions. In other words, the lithium consumed due to the formation of SEI does not return to the positive electrode during the subsequent discharge process, thereby reducing the capacity of the battery. This phenomenon is called irreversible capacity.

As a negative electrode material, carbon-based materials such as graphite have excellent stability and reversibility, but have limitations in terms of capacity, so in fields aimed at high capacity, Si-based materials with high theoretical capacity are used as negative electrode materials.

However, the crystal structure of Si particles changes as lithium ions are inserted during charging, and the volume expands about 4 times compared to before lithium is inserted, although it varies depending on the depth of charge (state of charge, SOC) and the range of use. entails

Figure 1 shows the distribution form of the active material, conductive material, and binder in a typical Si-based anode. Referring to FIG. 1, a cathode layer is located between Si particles 10, a conductive material 20, and a binder 30 that connects and secures the Si particles 10 and the conductive material 20. In a structure containing, when volume expansion of the Si particles 10 occurs during the charging and discharging process of the battery, binders of high molecular weight are generally preferred to prevent cracks occurring inside the crystals of the Si particles and destruction of the particles. . However, in such cases, contact and electrical connection between conductive material particles in relatively fine powder form are deteriorated, ultimately deteriorating lifespan characteristics.

Therefore, although Si-based materials have high capacity, they cannot fully demonstrate their performance (high capacity) when applied to batteries, and there is a continuous need to improve their lifespan characteristics.

Therefore, the present invention is to solve the above problems, and one object of the present invention is to provide a Si-based cathode with improved life characteristics by controlling volume expansion and maintaining good electrical connection by including two types of binders with different moduli. The aim is to provide a lithium secondary battery including this.

Additionally, another object of the present invention is to provide a method for manufacturing the Si-based anode.

According to one aspect of the present invention, it includes a current collector and a negative electrode active material layer located on at least one surface of the current collector, wherein the negative electrode active material layer is in contact with the Si-containing negative electrode active material, the conductive material, and the Si-containing negative electrode active material. It includes a first binder in contact with the conductive material, and a second binder in contact with the conductive material, wherein the first binder has a modulus of 10,000 to 80,000 Mpa, and the second binder has a modulus of 1,000 to 8,000 Mpa. Provided for battery cathode.

The first binder is polyacrylic acid (PAA), polyvinyl alcohol (PVA), or a mixture of two or more thereof with a weight average molecular weight of 250,000 to 800,000, and the second binder is polyacrylic acid (PAA) with a weight average molecular weight of 20,000 to 200,000. ), polyacrylic acid copolymer, or a mixture of two or more of them.

In the second binder, the polyacrylic acid copolymer may contain a repeating unit derived from acrylic acid and a repeating unit derived from malic acid, acrylonitrile styrene, or urethane that can be polymerized therewith.

The Si-containing negative electrode active material may include Si, SiO, SiO ₂ or a mixture of two or more thereof.

The first binder may be included in an amount of 5 to 25 parts by weight based on 100 parts by weight of the Si-containing negative electrode active material.

The second binder may be included in an amount of 10 to 70 parts by weight based on 100 parts by weight of the conductive material.

The negative electrode further includes lithium metal deposited on the Si-containing negative electrode active material or a negative electrode active material layer containing the same, and the lithium metal has a capacity of 10 to 35% based on the capacity of the Si-containing negative electrode active material. It can be deposited and pre-lithiated.

According to another aspect of the present invention, (S1) dispersing a Si-containing negative electrode active material in a first dispersion medium together with a first binder having a modulus of 10,000 to 80,000 Mpa to obtain a first preliminary slurry; (S2) dispersing the conductive material in a second dispersion medium together with a second binder having a modulus of 1,000 to 8,000 MPa to obtain a second preliminary slurry; (S3) mixing the first preliminary slurry and the second preliminary slurry to obtain a negative electrode slurry; and (S4) coating the negative electrode slurry on at least one surface of a current collector and performing drying and rolling. A method for manufacturing a negative electrode for a lithium secondary battery is provided.

The first and second dispersion media may each include water, methanol, ethanol, or a mixture of two or more thereof.

According to another aspect of the present invention, a lithium secondary battery including the negative electrode as described above is provided.

The Si-based cathode according to one aspect of the present invention includes a first binder that has a large modulus to suppress the volume expansion of Si particles, and a second binder that has a small modulus to maintain electrical contact within the electrode by complying with the volume expansion of Si particles. By doing so, volume expansion is controlled, adhesion is improved, and excellent lifespan characteristics can be exhibited.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention along with the contents of the above-described invention. Therefore, the present invention is limited to the matters described in such drawings. It should not be interpreted in a limited way.
Figure 1 shows the distribution form of Si particles, conductive material, and binder in a conventional Si-based anode.
Figure 2 shows the distribution form of Si particles, a conductive material, and two types of binders in a Si-based anode according to an embodiment of the present invention.

Hereinafter, terms or words used in this specification and claims should not be construed as limited to their usual or dictionary meanings, and the inventor appropriately defines the concept of terms in order to explain his or her invention in the best way. It must be interpreted with meaning and concept consistent with the technical idea of the present invention based on the principle that it can be done.

One embodiment of the present invention includes a current collector and a negative electrode active material layer located on at least one surface thereof, wherein the negative electrode active material layer includes a Si-containing negative electrode active material, a conductive material, and two types of binders, that is, a first binder and a second binder. It relates to a negative electrode for a lithium secondary battery containing.

Figure 2 shows the distribution form of Si particles, a conductive material, and two types of binders in a Si-based anode according to an embodiment of the present invention.

Referring to Figure 2, the negative electrode of the present invention includes Si-containing negative electrode active material 10; A conductive material (20) positioned between the Si-containing negative electrode active material (10); a first binder (30) surrounding and contacting the Si-containing negative electrode active material (10); and a second binder 40 in contact with the conductive material 20.

The Si-containing negative electrode active material 10 is a component that causes an electrochemical reaction by combining with lithium ions moving from the positive electrode during the charging reaction. For example, Si, SiO, SiO ₂ or a mixture of two or more of these is used as the negative electrode active material. can be used

In addition, in addition to the Si-containing negative electrode active material, Sn-based materials such as Sn, SnO, SnO2, etc., if necessary; Carbon-based materials such as artificial graphite, natural graphite, amorphous hard carbon, and low-crystalline soft carbon; metal complex oxides such as lithium titanium oxide; Alternatively, a mixture of two or more of these may be additionally used.

This negative electrode active material may be used in an amount of 60 to 99% by weight based on the total weight of the negative electrode slurry when manufacturing the negative electrode.

The conductive material 20 is a component that provides a conductive path for electrical connection between cathode materials, and is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, as a conductive material, carbon black such as carbon black, acetylene black, Ketjen black, channel black, Paneth black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; 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; Conductive materials such as polyphenylene derivatives may be used. The conductive material may be added in an amount of 0.1 to 25% by weight based on the total weight of the anode slurry composition.

The first binder 30 is in contact with the Si-containing negative electrode active material 10, and its shape may be linear. Here, 'linear' means that the main chain of the compound constituting the binder is much longer than the side chain and the number of carbon (C) atoms introduced into the side chain is less than 10.

The first binder 30 is characterized by having a modulus of 10,000 to 80,000 Mpa, specifically 20,000 to 60,000 Mpa, and more specifically 25,000 to 50,000 Mpa. That is, the first binder has a modulus of 10,000Mpa or more, which gives it strength around the Si particles even if volume expansion of the Si particles occurs during the charging and discharging process of the battery. Changes in electrode structure can be suppressed. Accordingly, the binding force between the Si particles and the current collector or other electrode material is maintained, and as a result, the lifespan characteristics of the Si-based negative electrode can be improved. If the modulus of the first binder is less than 10,000 Mpa, the strength is insufficient, making it difficult to control the volume expansion of Si, and adhesion may be insufficient during repeated charging and discharging of the battery, which may cause detachment of the active material from the current collector or cracks on the surface of the active material. there is. On the other hand, if the modulus exceeds 80,000 Mpa and the strength is too high, cracks may occur during the drying process after coating when manufacturing the anode and the active material may be detached.

In the present invention, the modulus of the first binder and the second binder described later can be measured by various methods commonly used in the field, for example, after manufacturing the binder into a thin film, the modulus of the film is measured according to ASTM D790. It can be measured according to -71.

The first binder satisfying the modulus of 10,000 to 80,000 MPa includes polyacrylic acid (PAA) and polyvinyl alcohol (PVA) with a weight average molecular weight of 250,000 to 800,000, specifically 300,000 to 500,000, and more specifically 350,000 to 450,000. Or a mixture of two or more of these may be used. In particular, the polyacrylic acid (PAA) contains acrylic acid that imparts a high modulus and has excellent adsorption to inorganic particles, and thus is suitable for styrene-butadiene rubber (SBR) or carboxymethylcellulose (CMC), which are commonly used as a binder for negative electrodes. It has excellent binding properties compared to In addition, polyacrylic acid (PAA) has a higher modulus when it is in the form of a homopolymer that does not further contain repeating units derived from other monomers, making it advantageous for use as the first binder of the present invention.

In particular, binders such as polyacrylic acid (PAA) and polyvinyl alcohol (PVA) may have properties such as viscosity, strength, and modulus depending on their weight average molecular weight.

Considering the level of adhesion and electrode capacity, the content of the first binder is advantageously 5 to 25 parts by weight, specifically 7 to 15 parts by weight, based on 100 parts by weight of the Si-containing negative electrode active material.

Meanwhile, in the cathode of the present invention, the second binder 40 has a shorter chain length than the first binder 30 and can contact the conductive material 20 in the form of point contact. The second binder 40 is characterized by having a relatively low modulus compared to the first binder, for example, 1,000 to 8,000 Mpa, specifically 3,000 to 7,000 Mpa, and more specifically 4,000 to 6,000 Mpa.

In general, in the case of Si-based cathodes, the volume expansion of Si reduces contact between adjacent particles, making it difficult to maintain electrical connection. Therefore, even if Si volume expansion occurs, it is necessary to maintain a conductive path for electrical connection within the electrode, and for this purpose, a binder capable of providing flexibility was additionally used in the present invention. That is, since the second binder has a low modulus in the above range, it can be distributed in the cathode while complying with the volume expansion of the Si particles, and from this, the conductive material in contact with the second binder is maintained despite the volume expansion of the Si particles. and maintain a conductive path within the cathode.

If the modulus of the second binder is less than 1,000 Mpa, it is difficult to provide sufficient adhesive strength, and if it is more than 8,000 Mpa, rigidity increases rather than flexibility to comply with the volume expansion of Si, which may cut off the conductive path.

This second binder has a weight average molecular weight different from that of the first binder, for example, polyacrylic acid (PAA), polyacrylic acid having a weight average molecular weight of 20,000 to 200,000, specifically 30,000 to 170,000, and more specifically 50,000 to 150,000. Combinations, or mixtures of two or more of these may be used. When a polyacrylic acid copolymer containing repeating units derived from acrylic acid and repeating units derived from other monomers polymerizable therewith is used as the second binder, examples of monomers polymerizable with acrylic acid include malic acid, acrylonitrile, styrene, or There is urethane. Among these, urethane is particularly suitable for using the polyacrylic acid copolymer as the second binder in the present invention to maintain good contact with the conductive material.

Considering the content and specific surface area of the conductive material, the content of the second binder is preferably 10 to 70 parts by weight, specifically 20 to 50 parts by weight, based on 100 parts by weight of the conductive material.

As such, the negative electrode for a lithium secondary battery according to an embodiment of the present invention includes two types of binders with different molecular weights and moduli, along with a Si-containing negative electrode active material that provides high capacity and a conductive material located between them to provide conductivity. By using it, it is possible to efficiently control the volume expansion of Si particles and maintain electrical connection, thereby exhibiting excellent lifespan characteristics.

Additionally, another embodiment of the present invention relates to a method for manufacturing a cathode as described above, and specifically includes the following steps.

(S1) dispersing a Si-containing negative electrode active material in a first dispersion medium together with a first binder having a modulus of 10,000 to 80,000 MPa to obtain a first preliminary slurry;

(S2) dispersing the conductive material in a second dispersion medium together with a second binder having a modulus of 1,000 to 8,000 MPa to obtain a second preliminary slurry;

(S3) mixing the first preliminary slurry and the second preliminary slurry to obtain a negative electrode slurry; and

(S4) Coating the negative electrode slurry on at least one side of the current collector and performing drying and rolling.

In step (S1), water, methanol, ethanol, or a mixture of two or more thereof is used as the first dispersion medium, and a planetary mixer is used to induce contact between the first binder and the Si-containing negative electrode active material. Dispersion by kneading can be performed in a dispersion device such as ). At this time, the first binder may be used in an amount of 5 to 25 parts by weight, specifically 7 to 15 parts by weight, based on 100 parts by weight of the Si-containing negative electrode active material.

In step (S2), water, methanol, ethanol, or a mixture of two or more of these is used as the second dispersion medium, and various dispersion devices are used in consideration of the viscosity level of the slurry to induce efficient contact between the second binder and the conductive material. can be used. For example, in the case of high viscosity slurry, dispersion may be performed in a dispersing device such as a planetary mixer, and in the case of low viscosity slurry, dispersion may be performed in a dispersing device such as a bead mill. At this time, the second binder may be used in an amount of 10 to 70 parts by weight, specifically 20 to 50 parts by weight, based on 100 parts by weight of the conductive material.

Meanwhile, the current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon on the surface of copper or stainless steel. , surface treated with nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. The thickness of the current collector is not particularly limited, but may have a commonly applied thickness of 3 to 500 ㎛.

Additionally, the coating method of the cathode slurry is not particularly limited as long as it is a method commonly used in the field. For example, a coating method using a slot die may be used, and in addition, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. may be used. Drying methods include, for example, drying using warm air, hot air, or low-humidity air, vacuum drying, and drying using irradiation with (far) infrared rays or electron beams. Drying time is usually 5 to 30 minutes, and drying temperature is usually 40 to 180°C. Additionally, if formation of a multilayer active material layer is required, such coating and drying may be repeated several times.

Another embodiment of the present invention relates to a lithium secondary battery including the negative electrode. Specifically, the lithium secondary battery can be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, a negative electrode as described above, and a separator interposed therebetween.

Prior to manufacturing the lithium secondary battery of the present invention, the negative electrode may be pre-lithiated. Here, prelithiation means securing initial reversibility by pre-charging the negative electrode with lithium to compensate for the irreversible reaction of the negative electrode. In other words, the pre-lithiated Si cathode uses the lithium charged by pre-lithiation first during the discharge process, so by reducing the use of lithium inserted into Si during the charging process, the consumption of lithium ions due to the irreversible reaction of the cathode is reduced. Compensation is possible.

Generally, in a lithium secondary battery, when charging, the lithium ions from the positive electrode are inserted into the negative electrode, the potential of the negative electrode is lowered to around 0V, and during discharging, the lithium ions from the negative electrode move back to the positive electrode, lowering the potential of the negative electrode to approximately 1.5V. It goes up. Although prelithiation of the cathode is helpful in improving initial efficiency, excessive lithiation is disadvantageous in terms of productivity due to side reactions of lithium and increased costs.

Accordingly, in one embodiment of the present invention, in consideration of the cathode efficiency according to the transition metal ratio in the cathode active material, the degree of pre-lithiation of the Si-based cathode can be adjusted to a level where the discharge potential of the cathode does not rise above 0.55V.

In one embodiment of the present invention, prelithiation of a Si-based negative electrode can be accomplished by attaching lithium metal to the negative electrode active material or a negative electrode layer containing the same by vapor deposition or powder coating. For example, after manufacturing a Si-based negative electrode, a pre-lithiated negative electrode can be manufactured by performing a process of contacting lithium metal or a process of depositing lithium metal.

At this time, the amount of lithium metal used is controlled so that the discharge potential of the Si-based negative electrode does not rise higher than 0.55V. Therefore, the lithium metal used in pre-lithiation can be used in an amount capable of developing a capacity of 10 to 35%, specifically 5 to 25%, based on the capacity of the negative electrode active material. Here, the capacity of the negative electrode active material is based on the final capacity measured after sufficiently injecting electrolyte (EC/EMC=3/7; LiPF6 1M) and performing 0.2C discharge three times on a half-cell basis.

In addition, in one embodiment of the present invention, the capacity ratio (N/P ratio) of the positive electrode and the negative electrode of the lithium secondary battery, specifically, the areal capacity ratio of neagative to positive electrode It can be designed to be in the range of 1.5 to 3.5, specifically 1.8 to 3.0. As a result, the charging potential of the Si-based cathode can be maintained up to 0.45V.

In a typical cathode design, the N/P ratio is set to a range of approximately 1 to 1.4 so that lithium ions released from the anode do not precipitate as lithium metal on the opposing cathode, but in one embodiment of the present invention, the capacity of the cathode compared to the anode is By further increasing to 1.5 or more, an excessive amount of Si particles, which are the negative electrode active material, can be disposed. As a result, areas where volume expansion increases during the insertion and desorption of lithium during charging and discharging in the Si particles are excluded, and the remaining areas are limitedly utilized to form the negative electrode. The charging potential can be maintained up to 0.45V.

In this way, the charge/discharge potential of the cathode according to one embodiment of the present invention can be adjusted within the range of 0.45 to 0.55 V.

In the lithium secondary battery according to an embodiment of the present invention, the positive electrode is prepared by mixing a positive electrode active material, a conductive material, a binder, and a dispersion medium to prepare a slurry, and then coating the slurry directly on a metal current collector or casting it on a separate support. A positive electrode can be manufactured by laminating the positive electrode active material film peeled from the support to a metal current collector.

Active materials used in the positive electrode include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are independently selected from Al, Ni, Co, is any one selected from the group consisting of Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently the atomic fraction of the oxide composition elements, 0=x<0.5, 0≤ =y<0.5, 0≤=z<0.5, 0<x+y+z=1) or a mixture of two or more of them.

Meanwhile, the conductive material, binder, and dispersion medium may be used in the same manner as those used in manufacturing the anode.

In the lithium secondary battery according to one embodiment of the present invention, the separator is a typical porous polymer film used as a conventional separator, such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and Porous polymer films made of polyolefin-based polymers such as ethylene/methacrylate copolymers can be used alone or by laminating them. Additionally, a thin insulating film with high ion permeability and mechanical strength can be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on the surface of the separator. In addition, conventional porous nonwoven fabrics, for example, nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., can be used, but are not limited thereto.

A lithium secondary battery according to an embodiment of the present invention forms an electrode assembly with a separator between the positive electrode and the negative electrode, the electrode assembly is placed in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then an electrolyte is added. By injection, a lithium secondary battery can be completed. In another method, a lithium secondary battery can be completed by stacking the electrode assembly, impregnating it with an electrolyte solution, and sealing the resulting product in a battery case.

The electrolyte solution contains lithium salt as an electrolyte and an organic solvent for dissolving it.

The lithium salt may be used without limitation as long as it is commonly used in electrolytes for secondary batteries. For example, the anions of the lithium salt include F ^- , Cl ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , ClO ₄ ^- , PF ₆ ^- , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , CF ₃ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , ( CF ₃ SO ₂ ) ₂ CH ^- , (SF ₅ ) ₃ C ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , One or more types selected from the group consisting of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- can be used.

The organic solvent contained in the electrolyte solution may be any commonly used solvent without limitation, and representative examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate, and dimethyl sulfoxide. One or more types selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

The lithium secondary battery according to an embodiment of the present invention may be a stack type, a wound type, a stack and fold type, or a cable type.

The lithium secondary battery of the present invention can not only be used in battery cells used as power sources for small devices, but can also be preferably used as a unit cell in medium to large-sized battery modules containing multiple battery cells. Preferred examples of the medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc., and are particularly useful for hybrid electric vehicles and batteries for renewable energy storage, which are areas requiring high output. It can be used effectively.

Hereinafter, the present invention will be described in detail through examples to aid understanding. However, the embodiments according to the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the following embodiments. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1:

<Manufacture of cathode>

Si particles with an average particle diameter of 5㎛ (Wacker) as a negative electrode active material and polyacrylic acid (PAA) (Toyo, weight average molecular weight: 450,000, modulus: 43,000Mpa) as a first binder are added to water as a first dispersion medium, Dispersion by kneading was performed at 60°C for 2 hours using a planetary mixer (DSPM-100, Daesung Chemical Machinery) to obtain a first preliminary slurry. At this time, the polyacrylic acid (PAA) was used in an amount of 10 parts by weight based on 100 parts by weight of Si particles.

Meanwhile, as a conductive material, carbon black (Super C, Imerys Graphite & Carbon) and a graphite-based conductive material (KS6L, Timcal) were mixed in a ratio of 2:8, and then polyacrylic acid (PAA) (Toyo, Inc.) was used as a second binder. Weight average molecular weight: 150,000, modulus: 5,500Mpa) was added to water as a second dispersion medium, and dispersion was performed by kneading with the same planetary mixer at 60°C for 2 hours to obtain a second preliminary slurry. At this time, the polyacrylic acid (PAA) was used in an amount of 25 parts by weight based on 100 parts by weight of the total conductive material.

The modulus of the first binder and the second binder is a value measured according to ASTM D790-71 after each binder used was manufactured into a thin film with a size of 3.2mm × 12.7mm × 125mm.

The first preliminary slurry and the second preliminary slurry were mixed to obtain a negative electrode slurry (weight ratio of Si particles: first binder: conductive material: second binder = 67.8:6.8:20.3:5.1), which was coated with copper foil having a thickness of 10 μm. A negative electrode was manufactured by coating one side of the electrode, drying and rolling to form a negative electrode active material layer.

In the negative electrode active material layer formed in this way, as shown in FIG. 2, the first binder 30 is in contact with Si particles 10, which are active materials, and the second binder 40 is in contact with the used conductive material 20. is in contact with

<Manufacture of anode>

Li(Ni _0.5 Co _0.3 Mn _0.2 )O ₂ as the positive electrode active material, carbon black (Super C, Imerys Graphite & Carbon) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder at a weight ratio of 97:1.5:1.5. A positive electrode slurry was prepared by dispersing in N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was coated on one side of aluminum foil with a thickness of 15㎛, dried, and rolled to prepare a positive electrode.

<Manufacture of lithium secondary batteries>

After interposing a polyolefin-based separator (SRS, LG Chem) with a thickness of 12㎛ between the pre-lithiated Si-based cathode and anode prepared above, ethylene carbonate (EC) and ethylmethyl carbonate (DEC) were added at 30%: An electrolyte solution containing 1.2M LiPF ₆ dissolved in a solvent mixed at a volume ratio of 70 was injected, and a lithium secondary battery with a capacity ratio (N/P ratio) of the positive and negative electrodes of 1.8 was manufactured.

Example 2:

When manufacturing the negative electrode, the same process as Example 1 was followed, except that lithium metal was deposited on the negative electrode prepared in Example 1 in an amount to develop a capacity of 30% based on the capacity of the negative electrode active material layer to pre-lithiate. A lithium secondary battery was manufactured. At this time, the capacity of the negative electrode active material was based on the final capacity measured after sufficiently injecting electrolyte (EC/EMC = 3/7; LiPF6 1M) and performing 0.2C discharge three times on a half-cell basis.

The capacity ratio (N/P ratio) of the positive and negative electrodes of the manufactured lithium secondary battery increased to 2.3 by pre-lithiation.

Example 3:

The same process as Example 1 except that a polyacrylic acid copolymer (LG Chem, weight average molecular weight: 90,000, modulus: 3,200Mpa) containing repeating units derived from acrylic acid and repeating units derived from urethane was used as the second binder. A lithium secondary battery was manufactured.

Comparative Example 1:

As the anode active material, Si particles with an average particle diameter of 5㎛ (Wacker), as a conductive material, a 2:8 mixture of carbon black (Super C, Imerys Graphite & Carbon) and a graphite-based conductive material (KS6L, Timcal), and as a binder Styrene-butadiene rubber (SBR) (BM-L302, Zeon, modulus: 1 MPa or less) and carboxymethyl cellulose (CMC) (2200, Daicel, modulus: 1 MPa or less) as a dispersant at a weight ratio of 80:15:3.5:1.5. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the negative electrode slurry obtained by dispersing in water was used to form the negative electrode active material layer.

Comparative Example 2:

As the negative electrode active material, Si particles with an average particle diameter of 5㎛ (Wacker), as a conductive material, a 2:8 mixture of carbon black (Super C, Imerys Graphite & Carbon) and a graphite-based conductive material (KS6L, Timcal), and a binder Except that the negative electrode slurry obtained by dispersing polyacrylic acid (PAA) (Toyo, weight average molecular weight: 450,000, modulus: 43,000Mpa) in water at a weight ratio of 67.8:20.3:11.9 is used to form the negative electrode active material layer. , a lithium secondary battery was manufactured by performing the same process as Example 1.

Comparative example 3:

As the negative electrode active material, Si particles with an average particle diameter of 5㎛ (Wacker), as a conductive material, a 2:8 mixture of carbon black (Super C, Imerys Graphite & Carbon) and a graphite-based conductive material (KS6L, Timcal), and a binder Except that the negative electrode slurry obtained by dispersing polyacrylic acid (PAA) (Toyo, weight average molecular weight: 150,000, modulus: 5,500Mpa) in water at a weight ratio of 67.8:20.3:11.9 is used to form the negative electrode active material layer. , a lithium secondary battery was manufactured by performing the same process as Example 1.

Experimental Example 1: Performance evaluation of lithium secondary battery

The negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 3 were manufactured as half cells, and initial efficiency was measured by performing initial (one time) charge and discharge using an electrochemical charge and discharge device. At this time, all charging and discharging conditions were performed at 0.1C. Afterwards, charging and discharging were continued to check the capacity maintenance rate. At this time, charging was done using a constant current/constant voltage (CC/CV) method until it reached 4.3V and 1/20C current with a current of 0.7C, and discharging was done at 0.5C. This was carried out using a constant current (CC) method up to a voltage of 3.2V. Charging and discharging were performed a total of 300 times.

The presence or absence of cracks on the electrodes, initial efficiency (%), and capacity maintenance rate (%) for each battery were calculated as follows, and the values are shown in Table 1 below.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Electrode crack occurrence evaluation ¹⁾ OK OK OK OK NG OK Initial efficiency (%) ²⁾ 91.1 102.5 91.0 91.0 91.1 91.1 Capacity maintenance rate (%) ³⁾ 95.3 96.5 95.5 86.4 66.1 71.2 1) The occurrence of cracks on the electrode was observed with the naked eye. If no cracks occurred, it was marked as “OK”, and if cracks occurred, it was marked as “NG”.
2) Initial efficiency (%) = (discharge capacity of one cycle / charge capacity of one cycle) × 100
3) Capacity maintenance rate (%) = (Discharge capacity after 300 cycles / Discharge capacity after 1 cycle) Х100

As can be seen in Table 1, the lithium secondary batteries of Examples 1 to 3 using two types of binders with different moduli when manufacturing the Si-based negative electrode are superior to Comparative Examples 1 to 3 using only one binder. Lifespan characteristics were shown. In addition, in the case of Example 2, pre-lithiation was performed, showing the best initial efficiency.

Claims (10)

It includes a current collector, and a negative electrode active material layer located on at least one side of the current collector,
The negative electrode active material layer includes a Si-containing negative electrode active material in a single layer, a conductive material, a first binder in contact with the Si-containing negative electrode active material, and a second binder in contact with the conductive material,
The first binder has a modulus of 10,000 to 80,000 Mpa,
The second binder is a negative electrode for a lithium secondary battery having a modulus of 1,000 to 8,000 Mpa. According to paragraph 1,
The first binder is polyacrylic acid (PAA), polyvinyl alcohol (PVA), or a mixture of two or more thereof having a weight average molecular weight of 250,000 to 800,000,
The second binder is a negative electrode for a lithium secondary battery, which is polyacrylic acid (PAA), polyacrylic acid copolymer, or a mixture of two or more thereof having a weight average molecular weight of 20,000 to 200,000. According to paragraph 2,
The polyacrylic acid copolymer is a negative electrode for a lithium secondary battery containing a repeating unit derived from acrylic acid and a repeating unit derived from malic acid, acrylonitrile, styrene, or urethane polymerizable therewith. According to paragraph 1,
The Si-containing negative electrode active material is a negative electrode for a lithium secondary battery comprising Si, SiO, SiO ₂ or a mixture of two or more thereof. According to paragraph 1,
The first binder is a negative electrode for a lithium secondary battery comprising 5 to 25 parts by weight based on 100 parts by weight of the Si-containing negative electrode active material. According to paragraph 1,
The second binder is a negative electrode for a lithium secondary battery containing 10 to 70 parts by weight based on 100 parts by weight of the conductive material. According to paragraph 1,
The negative electrode further includes lithium metal deposited on the Si-containing negative electrode active material or a negative electrode active material layer containing the same, and the lithium metal has a capacity of 10 to 35% based on the capacity of the Si-containing negative electrode active material. The cathode is deposited and pre-lithiated. (S1) dispersing a Si-containing negative electrode active material in a first dispersion medium together with a first binder having a modulus of 10,000 to 80,000 MPa to obtain a first preliminary slurry;
(S2) dispersing the conductive material in a second dispersion medium together with a second binder having a modulus of 1,000 to 8,000 MPa to obtain a second preliminary slurry;
(S3) mixing the first preliminary slurry and the second preliminary slurry to obtain a negative electrode slurry; and
(S4) A method of manufacturing a negative electrode for a lithium secondary battery comprising the steps of coating the negative electrode slurry on at least one side of a current collector and performing drying and rolling. According to clause 8,
A method of producing a negative electrode for a lithium secondary battery, wherein the first dispersion medium and the second dispersion medium each include water, methanol, ethanol, or a mixture of two or more thereof. A lithium secondary battery comprising the negative electrode according to any one of claims 1 to 7.

Images