A negative electrode composition, a negative electrode for a lithium secondary battery containing the same, and a lithium secondary battery containing the negative electrode

Abstract

The present application relates to a negative electrode composition, a negative electrode for a lithium secondary battery including the same, and a lithium secondary battery including the negative electrode.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0185213, filed with the Korean Intellectual Property Office on December 22, 2021, and all of its contents are incorporated herein by reference. This application relates to a negative electrode composition, a negative electrode for a lithium secondary battery including the same, and a lithium secondary battery including the negative electrode.

Background Art

[0002] Due to the rapid increase in the use of fossil fuels, the demand for alternative and clean energy has been increasing. As part of this, the fields of power generation and power storage using electrochemical reactions are the most actively studied.

[0003] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its usage areas are increasingly expanding.

[0004] As the technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density, voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. In addition, research on methods for manufacturing high-density electrodes with a higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries has been actively underway.

[0005] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and desorbs lithium ions emitted from the positive electrode, and silicon-based particles having a large discharge capacity can be used as the negative electrode active material.

[0006] In recent years, in response to the demand for high-density energy batteries, research has been actively conducted on methods to increase capacity by using silicon-based compounds such as Si / C and SiOx as negative electrode active materials, which have more than 10 times the capacity of graphite-based materials. Although silicon-based compounds are high-capacity materials and have a larger capacity than conventionally used graphite, they have the problem of rapidly expanding in volume during the charging process, disrupting the conductive path and degrading battery performance.

[0007] Therefore, various methods have been discussed to suppress volume expansion itself or prevent the conduction path from being interrupted, such as methods to adjust the driving potential, methods to further coat a thin film on the active material layer, and methods to adjust the particle size of the silicon-based compound, in order to resolve the problems when using silicon-based compounds as negative electrode active materials. However, these methods can actually degrade the performance of the battery, so their application is limited, and there are still limitations to the commercialization of negative electrode battery manufacturing with a high silicon-based compound content.

[0008] In particular, research is progressing on the composition of binders in relation to volume expansion, and studies are underway to use binder polymers with strong stress on the sides in order to suppress the volume expansion of the negative electrode active material, which undergoes large volume changes, during charging and discharging. However, these binder polymers alone have limitations in suppressing the increase in electrode thickness due to the contraction and expansion of the negative electrode active material and the resulting degradation of lithium secondary battery performance.

[0009] Furthermore, to solve the problems associated with the volume expansion of the negative electrode containing the silicon-based active material described above, an aqueous binder that possesses both dispersibility and adhesion properties is used. While the aqueous binder has the advantage of improving dispersibility, its poor adhesion leads to problems such as electrode detachment due to the volume expansion of the active material.

[0010] Furthermore, while rubber-based binders can be applied to improve adhesion, it is known that in the case of silicon-based active materials, the inclusion of rubber-based binders leads to dispersibility problems, and this also has its limitations.

[0011] Therefore, even when using high-capacity materials to create high-capacity batteries, research is needed on binders that do not disrupt the conductive network due to the volume expansion of the active material, and that also have excellent adhesive properties. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2009-080971 [Overview of the project] [Problems that the invention aims to solve]

[0013] This application relates to a negative electrode composition, a negative electrode for a lithium secondary battery containing the same, and a lithium secondary battery containing the negative electrode. [Means for solving the problem]

[0014] One embodiment of this specification provides a negative electrode composition comprising a silicon-based active material; a negative electrode conductive material; and a negative electrode binder, wherein the negative electrode binder comprises a main binder comprising a water-based binder and a secondary binder comprising a rubber-based binder, the negative electrode binder comprises 80 to 99 parts by weight of the main binder and 1 to 20 parts by weight of the secondary binder based on 100 parts by weight of the negative electrode binder, and the secondary binder comprises 80 parts by weight or more of butadiene (BD) based on 100 parts by weight of the secondary binder.

[0015] Another embodiment provides a negative electrode for a lithium secondary battery, comprising a negative electrode current collector layer and a negative electrode active material layer containing the negative electrode composition according to this application, formed on one or both sides of the negative electrode current collector layer.

[0016] Finally, the present invention provides a lithium secondary battery comprising a positive electrode; a negative electrode for a lithium secondary battery according to this application; a separation membrane provided between the positive electrode and the negative electrode; and an electrolyte. [Effects of the Invention]

[0017] A negative electrode composition according to one embodiment of the present invention is characterized in that, when using a silicon-based active material, which is a high-capacity material, to manufacture a high-capacity battery, the problems associated with the volume expansion of the silicon-based active material are solved by applying a specific negative electrode binder.

[0018] In particular, the negative electrode binder comprises a main binder containing a water-based binder and a sub-binder containing a rubber-based binder, and the negative electrode binder is characterized by containing 80 to 99 parts by weight of the main binder and 1 to 20 parts by weight of the sub-binder based on 100 parts by weight of the negative electrode binder, and containing 80 parts by weight or more of butadiene (BD) based on 100 parts by weight of the sub-binder.

[0019] In this application, the aqueous main binder acts as a matrix with polymer chains, and the rubber sub-binder is provided in particulate form between the main binders; that is, the rubber sub-binder and the aqueous main binder do not bond together.

[0020] Specifically, the negative electrode composition according to this application improves the dispersibility for dispersing the active material via a main binder, even when using a silicon-based active material, and further includes a sub-binder of a specific composition to improve adhesion, thereby solving the problems of adhesion strength and disconnection of the conductive network due to volume expansion in the initial and later stages of batteries using silicon-based active materials.

[0021] In other words, the negative electrode composition according to this application has a high content of silicon-based active material particles, enabling the creation of a high-capacity and high-density negative electrode. Furthermore, to solve problems such as volume expansion caused by the high content of silicon-based active material particles, the aforementioned problems were solved by using a binder of a specific composition and content. [Brief explanation of the drawing]

[0022] [Figure 1] This figure shows a stacked structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. [Figure 2] This figure shows a stacked structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. [Figure 3] This figure shows a stacked structure of a lithium secondary battery according to one embodiment of the present application. [Modes for carrying out the invention]

[0023] Before describing the present invention, we will first define some terms. In this specification, when a part "includes" a component, this means, unless otherwise stated, that it may include other components rather than excluding them.

[0024] In this specification, "p~q" means the range "p or greater and q or less". In this specification, "specific surface area" is measured by the BET method, specifically calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mino II from BEL Japan. In other words, in this application, BET specific surface area may mean the specific surface area measured by the above measurement method.

[0025] In this specification, "Dn" refers to the particle size distribution, and means the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. Alternatively, the particle size distribution may be measured using the laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the difference in diffraction patterns due to particle size as the particles pass through the laser beam is measured to calculate the particle size distribution.

[0026] In this specification, when a polymer contains a monomer as a monomer unit, it means that the monomer participates in the polymerization reaction and is included as a repeating unit within the polymer. In this specification, when a polymer contains a monomer, this is interpreted as meaning that the polymer contains monomers as monomer units.

[0027] In this specification, unless otherwise specified, the term "polymer" is understood to be used in a broad sense, including copolymers.

[0028] In this specification, weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers of various degrees of polymerization (standard samples) commercially available for molecular weight measurement as standard substances. In this specification, molecular weight refers to weight-average molecular weight unless otherwise specified.

[0029] The present invention will be described in detail below with reference to the drawings so that a person with ordinary skill in the art to which the present invention pertains can easily implement it. However, the present invention can be implemented in various different forms and is not limited to the following description.

[0030] One embodiment of the present specification is a negative electrode composition comprising a silicon-based active material; a negative electrode conductive material; and a negative electrode binder, wherein the negative electrode binder includes a main binder containing an aqueous binder and a sub-binder containing a rubber-based binder, and the negative electrode binder contains 80 parts by weight or more and 99 parts by weight or less of the main binder and 1 part by weight or more and 20 parts by weight or less of the sub-binder based on 100 parts by weight of the negative electrode binder, and the sub-binder provides a negative electrode composition containing 80 parts by weight or more of butadiene (BD) based on 100 parts by weight of the sub-binder.

[0031] The negative electrode composition according to one embodiment of the present invention improves the dispersibility for dispersing the active material even when using a silicon-based active material through the main binder, and further includes a sub-binder with a specific composition to improve the adhesiveness, and can solve the problems of the initial and later adhesiveness of the battery using the silicon-based active material and the disconnection of the conductive network due to volume expansion.

[0032] In one embodiment of the present application, the silicon-based active material provides a negative electrode composition containing at least one selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloys.

[0033] The active material of the present invention includes a silicon-based active material. The silicon-based active material may be SiOx, Si / C, or Si. SiOx may include a compound represented by SiOx (0 ≤ x < 2). In the case of SiO2, since it does not react with lithium ions and cannot store lithium, x is preferably within the above range. The silicon-based active material may be Si / C or Si composed of a composite of Si and C. Also, two or more of the above silicon-based active materials may be mixed and used. The 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 can bring about excellent cycle characteristics or improvement in battery life performance of the negative electrode or secondary battery of the present invention.

[0034] Generally, silicon-based active materials are known to have a capacity more than 10 times higher than carbon-based active materials. Therefore, when applying silicon-based active materials to the negative electrode, it is expected that electrodes with high-level energy density can be realized even with a thin thickness.

[0035] In one embodiment of the present application, the silicon-based active material includes at least one selected from the group consisting of SiOx (x = 0) and SiOx (0 < x < 2), and provides a negative electrode composition containing 70 parts by weight or more of SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0036] In another embodiment, the silicon-based active material may contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less of SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0037] The silicon-based active material according to the present application contains 70 parts by weight or more of SiOx (x = 0) based on 100 parts by weight of the silicon-based active material, and has a disadvantage that its theoretical capacity is much inferior compared to the silicon-based active material of the present application when compared with a silicon-based active material using a SiOx (0 < x < 2) system as the main active material. That is, when using a SiOx (0 < x < 2) system active material, even if any treatment is performed on the active material itself, it is impossible to realize conditions equivalent to the charge and discharge capacity compared to the case of having the silicon-based active material of the present invention.

[0038] In one embodiment of the present application, pure silicon (Si) may be used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means that, as described above, when based on 100 parts by weight of the total silicon-based active material, it can mean including pure Si particles (SiOx (x = 0)) not bonded to other particles or elements within the above range.

[0039] Silicon-based active materials have significantly higher capacity compared to conventionally used graphite-based active materials, and attempts to apply them are increasing. However, their high rate of volume expansion during the charge-discharge process limits their use to situations such as mixing small amounts with graphite-based active materials.

[0040] Therefore, the present invention is characterized by using a binder under specific conditions in order to improve capacity performance, while using a high content of silicon-based active material as the negative electrode active material, and to resolve the problems of maintaining the conductive path and the bonding between the conductive material, binder, and active material due to the volume expansion described above.

[0041] On the other hand, the average particle size (D50) of the silicon-based active material in the present invention is 5 μm to 10 μm, specifically 5.5 μm to 8 μm, and more specifically 6 μm to 7 μm. When particles of the above average particle size are included within the above range, the specific surface area of ​​the particles is included within an appropriate range, and the viscosity of the negative electrode slurry is formed within an appropriate range. This allows for smooth dispersion of the particles constituting the negative electrode slurry. Furthermore, when the size of the silicon-based active material is greater than or equal to the lower limit of the above range, the composite consisting of the conductive material and binder in the negative electrode slurry provides excellent contact area between the silicon particles and the conductive material, increasing the likelihood of a sustained conductive network and increasing the capacity retention rate. On the other hand, when the average particle size satisfies the above range, excessively large silicon particles are excluded, and the surface of the negative electrode is formed smoothly, thereby preventing the phenomenon of non-uniform current density during charging and discharging.

[0042] In one embodiment of this application, the silicon-based active material generally has a characteristic BET specific surface area. The BET specific surface area of ​​the silicon-based active material is preferably 0.01 m². 2 / g~150.0m 2 / g, more comfortably 0.1m 2 / g~100.0m 2 / g, particularly preferably 0.2m 2 / g~80.0m 2 / g, most preferably 0.2m 2 / g~18.0m 2It is / g. The BET specific surface area is measured in accordance with DIN 66131 (using nitrogen).

[0043] In one embodiment of the present application, the silicon-based active material may be present, for example, in crystalline or amorphous form and is preferably not porous. The silicon particles are preferably spherical or flaky particles. As an alternative, but less preferably, the silicon particles may have a fibrous structure or be present in the form of a silicon-containing film or coating.

[0044] In one embodiment of the present application, the silicon-based active material provides a negative electrode composition containing 60 parts by weight or more based on 100 parts by weight of the negative electrode composition.

[0045] In another embodiment, the silicon-based active material may be contained in an amount of 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, and may be contained in an amount of 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 85 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0046] Even when using a silicon-based active material with a significantly high capacity within the above range, the negative electrode composition according to the present application uses a specific conductive material and binder that suppress the volume expansion rate during the charge-discharge process, and even when including the above range, does not deteriorate the performance of the negative electrode and has excellent output characteristics during charging and discharging.

[0047] In one embodiment of the present application, the silicon-based active material may have a non-spherical form, and its sphericity is, for example, 0.9 or less, for example, 0.7 to 0.9, for example, 0.8 to 0.9, for example, 0.85 to 0.9.

[0048] In the present application, the sphericity is determined by the following formula 1-1, where A is the area and P is the boundary line. [Formula 1-1] $4\pi A / P$ 2

[0049] Traditionally, graphite-based compounds were commonly used as the negative electrode active material. However, in recent years, with the increasing demand for high-capacity batteries, there has been a growing trend to mix in silicon-based compounds to increase capacity. However, in the case of silicon-based compounds, even if the properties of the silicon-based active material itself are adjusted according to this application, as described above, there is a possibility that the volume may rapidly expand during the charging / discharging process, potentially damaging the conductive paths formed within the negative electrode active material layer.

[0050] Accordingly, in one embodiment of the present application, the negative electrode conductive material may include at least one selected from the group consisting of point conductive material; planar conductive material; and linear conductive material.

[0051] In one embodiment of this application, the point-shaped conductive material can be used to improve the conductivity of the negative electrode and means a point-shaped or spherical conductive material that has conductivity without causing a chemical change. Specifically, the point-shaped conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and may preferably include carbon black in that it embodies high conductivity and has excellent dispersibility.

[0052] In one embodiment of this application, the point conductive material has a BET specific surface area of ​​40 m². 2 / g or more 70m 2 / g or less, preferably 45m 2 / g or more 65m 2 / g or less, more preferably 50m 2 / g or more 60m 2 It may be less than / g.

[0053] In one embodiment of this application, the point-shaped conductive material may satisfy a functional group content (volatile matter) of 0.01% or more and 1% or less, preferably 0.01% or more and 0.3% or less, and more preferably 0.01% or more and 0.1% or less.

[0054] In particular, when the functional group content of the dot-shaped conductive material satisfies the aforementioned range, functional groups are present on the surface of the dot-shaped conductive material, and when water is used as the solvent, the dot-shaped conductive material can be smoothly dispersed in the solvent. In particular, in the present invention, by using silicon particles and a specific binder, the functional group content of the dot-shaped conductive material can be reduced, thereby having an excellent effect in improving dispersibility.

[0055] One embodiment of this application is characterized by including a point-type conductive material having a functional group content within the aforementioned range together with a silicon-based active material, wherein the functional group content can be adjusted according to the degree of heat treatment of the point-type conductive material.

[0056] In one embodiment of this application, the particle size of the dot-like conductive material is 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 20 nm to 60 nm.

[0057] In one embodiment of this application, the conductive material may include a planar conductive material. The planar conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, and at the same time suppress the disruption of the conductive path due to volume expansion. The planar conductive material may also be referred to as a plate-type conductive material or a bulk-type conductive material.

[0058] In one embodiment of this application, the planar conductive material may be provided in a form bonded to the surface of the silicon-based particles. Specifically, it may be provided in a form in which the -OH groups or -O groups on the surface of the silicon-based particles and the hydrophilic groups of the planar conductive material are bonded to each other.

[0059] In one embodiment of this application, the planar conductive material may include at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and preferably plate graphite.

[0060] In one embodiment of this application, the average particle size (D50) of the planar conductive material is 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 3.5 μm to 5 μm. When this range is met, the particle size is sufficient to facilitate dispersion without causing an excessive increase in the viscosity of the negative electrode slurry. Therefore, the dispersion effect is superior when dispersed using the same equipment and time.

[0061] In one embodiment of this application, a negative electrode composition is provided in which the planar conductive material has a D10 of 0.5 μm or more and 2.0 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 6.5 μm or more and 15.0 μm or less.

[0062] In one embodiment of this application, the planar conductive material may be a high specific surface area planar conductive material with a high BET specific surface area, or a low specific surface area planar conductive material.

[0063] In one embodiment of this application, a high specific surface area planar conductive material or a low specific surface area planar conductive material can be used without limitation as the planar conductive material. In particular, the planar conductive material according to this application may be affected to some extent by dispersion in terms of electrode performance, and it may be particularly preferable to use a low specific surface area planar conductive material that does not cause dispersion problems.

[0064] In one embodiment of this application, the planar conductive material has a BET specific surface area of ​​1 m². 2 It may be more than / g.

[0065] In another embodiment, the planar conductive material has a BET specific surface area of ​​1 m². 2 / g or more 500m 2 / g or less, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 It may be less than / g.

[0066] The planar conductive material relating to this application may be a high specific surface area planar conductive material or a low specific surface area planar conductive material.

[0067] In another embodiment, the planar conductive material is a high specific surface area planar conductive material with a BET specific surface area of ​​50 m². 2 / g or more 500m 2 / g or less, preferably 80mg 2 / g or more 300m 2 / g or less, more preferably 100m 2 / g or more 300m 2 The range of / g or less may also be satisfied.

[0068] In another embodiment, the planar conductive material is a low specific surface area planar conductive material, with a BET specific surface area of ​​1 m². 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m 2 The range of / g or less may also be satisfied.

[0069] Other conductive materials may include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain multiple carbon nanotube units. Specifically, "bundle type" here refers to a secondary shape in the form of a bundle or rope, in which multiple carbon nanotube units are arranged side by side or intertwined with substantially the same orientation along their longitudinal axes. The carbon nanotube units have a graphite sheet in the shape of a cylinder with a nanoscale diameter and have an sp2 bond structure. In this case, the properties of a conductor or semiconductor can be determined by the angle and structure in which the graphite sheet is wound. Compared to entangled type carbon nanotubes, the bundle-type carbon nanotubes can be uniformly dispersed during anode manufacturing, smoothly forming a conductive network within the anode and improving the conductivity of the anode.

[0070] Exemplary, linear conductive materials may be single-walled carbon nanotubes (SWCNTs) having a large BET specific surface area, a linear shape, a very small diameter, and a very long length. Linear conductive materials such as SWCNTs cannot be stretched through dispersion and have a strong ability to return to their original form upon drying. As a result, linear conductive materials such as SWCNTs have a strong tendency to return to their original form upon drying, and therefore commonly exist in a form that encloses or connects negative electrode active materials or secondary aggregates. Bonding can be achieved by adsorption via van der Waals forces.

[0071] In one embodiment of this application, a negative electrode composition is provided in which the negative electrode conductive material is contained in an amount of 10 to 40 parts by weight based on 100 parts by weight of the negative electrode composition.

[0072] In another embodiment, the negative electrode conductive material may be included in an amount of 10 to 40 parts by weight, preferably 10 to 30 parts by weight, and more preferably 10 to 25 parts by weight, based on 100 parts by weight of the negative electrode composition.

[0073] One embodiment of this application provides a negative electrode composition in which the negative electrode conductive material includes a planar conductive material and a linear conductive material.

[0074] In one embodiment of this application, a negative electrode composition is provided in which the negative electrode conductive material comprises 80 to 99.9 parts by weight of the planar conductive material and 0.1 to 20 parts by weight of the linear conductive material, based on 100 parts by weight of the negative electrode conductive material.

[0075] In another embodiment, the negative electrode conductive material may include 80 to 99.9 parts by weight of the planar conductive material, preferably 85 to 99.9 parts by weight, and more preferably 95 to 98 parts by weight, based on 100 parts by weight of the negative electrode conductive material.

[0076] In another embodiment, the negative electrode conductive material may include 0.1 to 20 parts by weight, preferably 0.1 to 15 parts by weight, and more preferably 2 to 5 parts by weight, of the linear conductive material based on 100 parts by weight of the negative electrode conductive material.

[0077] In one embodiment of this application, the negative electrode conductive material includes a planar conductive material and a linear conductive material, each satisfying the aforementioned composition and proportion, thereby not significantly affecting the life characteristics of existing lithium secondary batteries. In particular, when a planar conductive material and a linear conductive material are included, the number of charge and discharge points increases, resulting in excellent output characteristics at a high C rate and reduced high-temperature gas generation.

[0078] The negative electrode conductive material described in this application has a completely different structure from the positive electrode conductive material applied to the positive electrode. Specifically, the negative electrode conductive material described in this application plays the role of capturing the contact points between silicon-based active materials, where the volume expansion of the electrodes is very large due to charging and discharging, while the positive electrode conductive material plays the role of a buffer that provides partial conductivity while acting as a buffer during rolling, and its structure and role are completely different from the negative electrode conductive material of the present invention.

[0079] Furthermore, the negative electrode conductive material described in this application is applied to silicon-based active materials and has a completely different structure from conductive materials applied to graphite-based active materials. In other words, conductive materials used in electrodes with graphite-based active materials simply have smaller particles compared to the active material, thus improving output characteristics and imparting some conductivity. As with the present invention, its structure and role are completely different from negative electrode conductive materials applied together with silicon-based active materials.

[0080] In one embodiment of this application, the planar conductive material used as the negative electrode conductive material described above has a different structure and role from carbon-based active materials generally used as negative electrode active materials. Specifically, the carbon-based active material used as the negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or point-like form for use in order to facilitate the storage and release of lithium ions.

[0081] On the other hand, the planar conductive material used as the negative electrode conductive material is a substance having a planar or plate-like form, and may be described as plate-like graphite. In other words, it is a substance included to maintain conductive pathways within the negative electrode active material layer, and does not play a role in lithium storage and release, but rather refers to a substance that secures conductive pathways in a planar manner within the negative electrode active material layer.

[0082] In other words, in this application, the use of plate-shaped graphite as a conductive material means that it was processed into a planar or plate shape and used not to store or release lithium, but as a material to secure a conductive path. In this case, the negative electrode active material included together has high capacity characteristics for lithium storage and release, and plays a role in storing and releasing all lithium ions transmitted from the positive electrode.

[0083] On the other hand, in this application, the use of a carbon-based active material as an active material means that it was processed into a point-like or spherical shape and used as a substance that plays a role in storing or releasing lithium.

[0084] In one embodiment of this application, the negative electrode binder comprises a main binder containing an aqueous binder and a secondary binder containing a rubber-based binder, and the negative electrode composition contains 85 to 95 parts by weight of the main binder and 5 to 15 parts by weight of the secondary binder, based on 100 parts by weight of the negative electrode binder, and provides a negative electrode composition containing 80 parts by weight or more of butadiene (BD) based on 100 parts by weight of the secondary binder.

[0085] In one embodiment of this application, the negative electrode binder includes a main binder containing an aqueous binder. The main binder simultaneously possesses dispersibility for dispersing the negative electrode active material in a negative electrode slurry state containing the negative electrode composition, and adhesive strength for binding to the negative electrode current collector layer and the negative electrode active material layer after drying, and its adhesive strength is not particularly high. In other words, the main binder containing an aqueous binder according to this application can mean a binder having a surface adhesive configuration.

[0086] In one embodiment of this application, the aqueous binder is soluble in an aqueous solvent such as water and comprises at least one selected from the group consisting of polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacrylamide (PAM). Preferably, it may comprise at least one selected from the group consisting of polyvinyl alcohol and polyacrylic acid, more preferably polyvinyl alcohol and polyacrylic acid, in terms of having excellent resistance to volume expansion / contraction of the silicon-based active material.

[0087] The aqueous binder may also contain a binder in which hydrogen atoms are replaced with Li, Na, or Ca, etc., in order to further disperse in an aqueous solvent such as water during the production of the anode slurry for forming the anode active material layer, and to coat the active material more smoothly and improve the binding strength.

[0088] In one embodiment of this application, the main binder has a Young's modulus of 0.3 × 10⁻⁶ 2 It may be MPa or higher.

[0089] In another embodiment, the main binder has a Young's modulus of 0.3 × 10⁻⁶ 2 MPa or higher, preferably 0.5 × 10 2 MPa, more favorably 1 × 10 2 It is MPa or higher, and 2 × 10 2 MPa or less, preferably 1.5 × 10⁻⁶ 2 MPa or less, more preferably 1.3 × 10 2 It may be satisfied if it is below MPa.

[0090] The method for measuring Young's modulus involves placing the main binder solution in a coated container and drying it at room temperature for an extended period to remove moisture. The dehydrated film is then vacuum-dried at 130°C for 10 hours to obtain a dry film. Subsequently, the dried film is cut or punched into 6mm x 100mm sample shapes to collect samples, and the tensile strength (Young's modulus) can be measured using a UTM device.

[0091] The Young's modulus of the main binder varies depending on the measurement method, speed, and measurement conditions of the binder, but the Young's modulus of the main binder may represent the value measured in a dry room with a dew point of -5°C to 10°C and a temperature of approximately 20°C to 22°C.

[0092] In this application, the dew point refers to the temperature at which condensation begins when moist air is cooled, because the partial pressure of water vapor in the air becomes equal to the saturated vapor pressure of water at that temperature. In other words, it can be said to refer to the temperature at which dew begins to form when the temperature of a gas containing water vapor is lowered and the relative humidity reaches 100%.

[0093] The aforementioned dew point is -5°C to 10°C, and temperatures of around 20°C to 22°C can generally be defined as a dry room, with humidity levels corresponding to very low levels.

[0094] In this application, the main binder may be a PAM-based binder, in which case the PAM-based binder is a binder whose main component is PAM, and the ratio of PAM, PAA, and PAN may be adjusted for use, and the same Young's modulus as described above can be satisfied by appropriately changing the composition.

[0095] The aforementioned aqueous binder is hydrophilic and generally does not dissolve in the electrolyte or electrolyte solution used in secondary batteries. Such properties allow the aqueous binder to be given strong stress or tensile strength when applied to a negative electrode or lithium secondary battery, thereby effectively suppressing the volume expansion / contraction problem associated with the charging and discharging of silicon-based active materials.

[0096] In one embodiment of this application, a negative electrode composition is provided in which the weight-average molecular weight of the main binder is 100,000 g / mol or more and 1,000,000 g / mol or less.

[0097] In one embodiment of this application, the rubber-based binder is defined as a substance different from the aqueous-based binder, which does not dissolve well in aqueous solvents such as water, but can be smoothly dispersed in aqueous solvents. Specifically, the rubber-based binder may include at least one selected from the group consisting of styrene butadiene rubber (SBR), hydrogenated nitrile butadiene rubber (HNBR), acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, and fluororubber. Preferably, it may include at least one selected from the group consisting of styrene butadiene rubber and hydrogenated nitrile butadiene rubber, and more preferably styrene butadiene rubber, due to its ease of dispersion and excellent phase stability.

[0098] In one embodiment of this application, a negative electrode composition is provided that contains 80 parts by weight or more of butadiene (BD) based on 100 parts by weight of the auxiliary binder.

[0099] In another embodiment, the amount of butadiene (BD) may be 80 parts by weight or more, 81 parts by weight or more, based on 100 parts by weight of the auxiliary binder, and may be 99 parts by weight or less, preferably 90 parts by weight or less.

[0100] Generally, secondary binders are materials with very high wettability of the electrolyte compared to aqueous binders. When the secondary binder is located near the silicon-based anode surface, the FEC solvent or LiPF6 salt that can form the SEI layer can be rapidly supplied, resulting in lower anode resistance.

[0101] However, the auxiliary binder according to this application contains butadiene (BD) in the aforementioned amount based on 100 parts by weight of the auxiliary binder. Depending on the electrolyte, an SEI layer is formed on the surface of the negative electrode active material layer, and in this case, the formation of the SEI layer begins with a radical reaction. Butadiene (BD) has conjugation bonds (1.5 bonds) in which double bonds and single bonds change rapidly, and free radicals are present, resulting in high adhesion to the active material.

[0102] Therefore, as in the present application, when butadiene (BD) is included in the specified amount based on 100 parts by weight of the auxiliary binder, an SEI layer is formed by radicals from the butadiene component included in the specified amount by weight. This SEI layer is linked to the auxiliary binder, preventing the phenomenon of the SEI layer falling off or cracking, and furthermore, there is the advantage that there is no need to generate an SEI layer. As a result, the lithium secondary battery has the characteristic of being able to further prevent an increase in the rate of increase of electrode resistance.

[0103] In one embodiment of this application, since the aqueous binder has high stress, when the aqueous binder is used alone, there is a risk of warping of the negative electrode, crack formation due to warping, and deterioration of life characteristics. The rubber binder can generally dissolve well in electrolytes or electrolytes used in secondary batteries, and when used in combination with the aqueous binder, it can relieve the stress of the aqueous binder to a certain level.

[0104] In one embodiment of this application, the auxiliary binder has a tensile strain of 30% or more, preferably 100% or more, more preferably 150% or more, most preferably 200% or more, and may also have a tensile strain of 1000% or less, preferably 900% or less, and even more preferably 800% or less.

[0105] In this case, the tensile strain of the auxiliary binder can be specifically realized within the range that satisfies the above-mentioned range by adjusting the ST / BD ratio of the SBR binder to an appropriate range.

[0106] The sub-binder according to this application solves the problem of high stress when using only the main binder described above by applying a sub-binder that satisfies the aforementioned tensile strain ratio, thereby reducing the stress of the water-based binder to a certain level when used in combination with the main binder.

[0107] The aforementioned method for measuring tensile strain involves placing a secondary binder solution in a coated container and drying it at room temperature for an extended period to remove moisture. The dehydrated film is then vacuum-dried at 130°C for 10 hours, according to the electrode drying temperature, to obtain a dry film. Subsequently, the dried film is cut or punched into 6mm x 100mm sample shapes to collect samples, and the tensile strain can be measured using a UTM device.

[0108] The tensile strain of the aforementioned sub-binder varies depending on the measurement method, speed, and measurement conditions of the binder, but the tensile strain of the sub-binder is the same as that of the Young's modulus measurement conditions for the aforementioned main binder.

[0109] In one embodiment of this application, the Young's modulus of the negative electrode binder, including the main binder and the sub-binder, may be 90 MPa or more and 110 MPa or less, and the tensile strain may be in the range of 20% or more and 45% or less.

[0110] In other words, in order to solve the problem of high stress when using only the main binder, by applying a sub-binder that satisfies the tensile strain rate described above, the overall negative electrode binder has the characteristic of relaxing the stress of the water-based binder to a certain level when the main and sub-binders are used together, while simultaneously ensuring adhesive strength.

[0111] The anode composition of the present invention, by using an anode binder containing a main binder containing the aqueous binder and a sub-binder containing the rubber binder in a specific weight ratio, can effectively resolve the volume expansion / contraction problem of the silicon-based active material, improve lifespan characteristics, resolve the warping problem during thin-film anode manufacturing, and further improve adhesive strength.

[0112] Furthermore, when the negative electrode binder and negative electrode conductive material include planar conductive material and linear conductive material, the adhesive strength problem can be improved, and the internal resistance of the negative electrode can also be improved.

[0113] In one embodiment of this application, the negative electrode binder may include 80 to 99 parts by weight of the main binder and 1 to 20 parts by weight of the sub-binder, based on 100 parts by weight of the negative electrode binder.

[0114] In another embodiment, the negative electrode binder may be in the range of 80 to 99 parts by weight, preferably 87 to 97 parts by weight, and more preferably 89 to 96 parts by weight, based on 100 parts by weight of the negative electrode binder.

[0115] In another embodiment, the negative electrode binder may be 1 to 20 parts by weight of the sub-binder, preferably 3 to 13 parts by weight, and more preferably 4 to 11 parts by weight, based on 100 parts by weight of the negative electrode binder.

[0116] As described above, the negative electrode binder according to this application has the characteristic that, by satisfying the above content of the main binder and sub-binder, it can improve dispersibility and solve the problem of adhesive strength even when using a silicon-based active material.

[0117] In one embodiment of this application, the negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogens of these substances are substituted with Li, Na, Ca, etc., or it may contain various copolymers thereof.

[0118] In one embodiment of this application, the negative electrode binder is provided in an amount of 5 to 30 parts by weight based on 100 parts by weight of the negative electrode composition.

[0119] In one embodiment of this application, the negative electrode binder may contain 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode composition, or it may contain 5 parts by weight or more, or 10 parts by weight or more.

[0120] When using a Si-based negative electrode compared to existing carbon-based negative electrodes, a main binder containing an aqueous binder is applied in the aforementioned parts by weight, and a sub-binder can be used within a certain range. In particular, the butadiene content of the sub-binder satisfies the aforementioned range, and even with the inclusion of a low-content sub-binder, it exhibits excellent bonding strength with the conductive material / binder.

[0121] One embodiment of this application provides a negative electrode for a lithium secondary battery, comprising a negative electrode current collector layer; and a negative electrode active material layer containing the negative electrode composition according to this application, formed on one or both sides of the negative electrode current collector layer.

[0122] Figure 1 shows a laminated structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. Specifically, a negative electrode 100 for a lithium secondary battery can be seen, which includes a negative electrode active material layer 20 on one surface of a negative electrode current collector layer 10.

[0123] Figure 2, representing another embodiment, shows a laminated structure of a negative electrode for a lithium secondary battery, specifically a negative electrode 100 for a lithium secondary battery that includes a negative electrode active material layer 20 on both sides of a negative electrode current collector layer 10.

[0124] As mentioned above, there are two types: one in which the negative electrode active material layer is coated on one side of the negative electrode current collector layer (see Figure 1), and one in which the negative electrode active material layer is coated on both sides of the negative electrode current collector layer (see Figure 2). In this case, the composition of the negative electrode active material layer coated on both sides may be the same or different.

[0125] In one embodiment of this application, when a negative electrode active material layer is coated on both sides, the negative electrode active material layer containing the negative electrode composition according to this application can be used without limitation as long as it is coated on only one of the two sides, and the other side may contain a silicon-based negative electrode active material or a carbon-based negative electrode active material, which are commonly included.

[0126] In one embodiment of this application, the negative electrode may be formed by coating one or both sides of a current collector with a negative electrode slurry containing the negative electrode composition.

[0127] In one embodiment of this application, the negative electrode slurry may include a negative electrode composition and a slurry solvent.

[0128] In one embodiment of this application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0129] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0130] The solid content of the negative electrode slurry can mean the content of the negative electrode composition contained in the negative electrode slurry, and can mean the content of the negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0131] When the solid content of the negative electrode slurry satisfies the aforementioned range, the viscosity during negative electrode active material layer formation is appropriate, and the particle aggregation phenomenon of the negative electrode composition can be minimized, enabling efficient formation of the negative electrode active material layer.

[0132] In one embodiment of this application, the negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. Furthermore, fine irregularities can be formed on the surface to strengthen the bonding force of the negative electrode active material, and it can be used in various forms such as film, sheet, foil, net, porous body, foam, and nonwoven fabric.

[0133] In one embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer is 20 μm or more and 500 μm or less.

[0134] However, the thickness can vary depending on the type and application of the negative electrode used, and is not limited to this.

[0135] In one embodiment of this application, the porosity of the negative electrode active material layer may be in the range of 10% to 60%.

[0136] In another embodiment, the porosity of the negative electrode active material layer may be in the range of 10% to 60%, preferably 20% to 50%, and more preferably 30% to 45%.

[0137] The aforementioned porosity varies depending on the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer. In particular, the silicon-based active material and conductive material according to this application satisfy the aforementioned range by containing a specific composition and content portion, thereby ensuring that the electrical conductivity and resistance of the electrode are within an appropriate range.

[0138] In one embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the adhesive strength of the surface of the negative electrode active material layer in contact with the negative electrode current collector layer is 290 gf / 5 mm or more and 500 gf / 5 mm or less under normal temperature and atmospheric pressure conditions.

[0139] In another embodiment, the adhesive strength of the surface of the negative electrode active material layer in contact with the negative electrode current collector layer may satisfy 290 gf / 5 mm or more and 500 gf / 5 mm or less, preferably 290 gf / 5 mm or more and 450 gf / 5 mm or less, and more preferably 290 gf / 5 mm or more and 430 gf / 5 mm or less, under normal conditions of 25°C and atmospheric pressure.

[0140] In particular, the negative electrode according to this application contains a specific negative electrode binder as the negative electrode composition described above, thereby improving adhesion as described above. Furthermore, even when the silicon-based active material expands and contracts repeatedly due to repeated charging and discharging of the negative electrode, the conductive network can be maintained by applying a specific composition of negative electrode binder and negative electrode conductive material, preventing disconnection and suppressing an increase in resistance.

[0141] The adhesive strength was measured using a peel strength meter with 3M 9070 tape at a 90° angle and a speed of 5 mm / s. Specifically, one side of the negative electrode active material layer of the lithium secondary battery negative electrode was adhered to one side of a slide glass with an adhesive film (3M 9070 tape). Then, it was adhered by moving a 2 kg rubber roller back and forth 5 to 10 times, and the adhesive strength (peel strength) was measured at a 90° angle and a speed of 5 mm / s. At this time, the adhesive strength can be measured under normal conditions of 25°C and atmospheric pressure.

[0142] Specifically, the adhesive strength was measured on a 5mm x 15cm electrode under normal atmospheric pressure and temperature of 25°C.

[0143] In one embodiment of this application, normal pressure refers to the pressure in a state where no specific pressure is applied or reduced, and can be used synonymously with atmospheric pressure. It can generally be expressed as 1 atmosphere.

[0144] One embodiment of this application provides a lithium secondary battery comprising: a positive electrode; a negative electrode for a lithium secondary battery according to this application; a separation membrane provided between the positive electrode and the negative electrode; and an electrolyte.

[0145] Figure 3 shows a laminated structure of a lithium secondary battery according to one embodiment of the present application. Specifically, a negative electrode 100 for a lithium secondary battery, including a negative electrode active material layer 20, can be seen on one side of the negative electrode current collector layer 10, and a positive electrode 200 for a lithium secondary battery, including a positive electrode active material layer 40, can be seen on one side of the positive electrode current collector layer 50. The negative electrode 100 and the positive electrode 200 for a lithium secondary battery are formed in a laminated structure with a separation membrane 30 in between. The negative electrode active material layer 20 may also be formed on both sides of the negative electrode current collector layer 10. The positive electrode active material layer 40 may also be formed on both sides of the positive electrode current collector layer 50.

[0146] A secondary battery according to one embodiment of this specification may include, in particular, the negative electrode for a lithium secondary battery described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator membrane interposed between the positive and negative electrodes, and an electrolyte, wherein the negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, a detailed explanation will be omitted.

[0147] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, which contains the positive electrode active material.

[0148] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces that have been surface-treated with carbon, nickel, titanium, silver, etc., may be used. The positive electrode current collector can usually have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0149] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented by O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions, etc., are examples, but are not limited to these. The positive electrode may also be metallic lithium (Li-metal).

[0150] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0151] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitations as long as it has electronic conductivity in the battery without causing a chemical change. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0152] Furthermore, the positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0153] The separation membrane separates the negative and positive electrodes and provides a pathway for lithium ions to move. Any membrane commonly used as a separation membrane in secondary batteries is generally acceptable, but those with low resistance to electrolyte ion movement while exhibiting excellent electrolyte moisture absorption capacity are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, a separation membrane coated with ceramic components or polymeric substances may be used, and it may be selectively used in a single-layer or multi-layer structure.

[0154] Examples of the aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0155] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0156] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0157] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used because they are high-viscosity organic solvents with high dielectric constants and readily dissociate lithium salts. When such cyclic carbonates are mixed with low-viscosity, low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate in appropriate proportions, an electrolyte with high electrical conductivity can be produced, and this method is even more preferable.

[0158] The metal salt can be a lithium salt, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte. For example, the anion of the lithium salt is F. - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3- (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - You can use one or more selected from the group consisting of the following.

[0159] In addition to the electrolyte components, the electrolyte may further contain one or more additives for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0160] One embodiment of the present invention provides a battery module and a battery pack containing the secondary battery as a unit cell. Because the battery module and battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, they can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems. [Examples]

[0161] Preferred embodiments are presented below to aid in understanding the present invention, but these embodiments are illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of this description and the technical concept. Such variations and modifications will naturally fall within the scope of the appended claims.

[0162] <Manufacturing example> <Manufacturing of negative electrode composition> Each negative electrode composition satisfying the composition and content shown in Table 1 below was manufactured.

[0163] [Table 1]

[0164] In Table 1 above, the silicon-based active material is Si (average particle size (D50): 3.5 μm), and the plate-shaped conductive material A has a BET specific surface area of ​​17 m². 2 The values ​​are / g, D10: 1.7μm, D50: 3.5μm, D90: 6.8μm, and SWCNTs have a BET specific surface area of ​​approximately 1000-1500m². 2 Substances satisfying the / g requirement and having an aspect ratio of 10,000 or higher were used. In addition, as shown in Table 1 above, SBR-1 used styrene-butadiene rubber with a butadiene component of 81% as a secondary binder, and SBR-2 used styrene-butadiene rubber with a butadiene component of 38%. The PAM (polyacrylamide) binder used as the main binder had a weight-average molecular weight (Mw) of 500,000 g / mol to 800,000 g / mol, a number-average molecular weight (Mn) in the range of 100,000 to 400,000, and a PDI value of 20 to 50.

[0165] Furthermore, the Young's modulus of PAM is 108 MPa (5% strain), with SBR-1 having a strain value of 365% (0.1 MPa) and SBR-2 having a strain value of 773% (0.15 MPa).

[0166] The method for measuring the Young's modulus and tensile strain involves placing the main and secondary binder solutions in a coated container and drying them at room temperature for an extended period to remove moisture. The film, from which moisture has been removed, is then vacuum-dried at 130°C for 10 hours to obtain a dry film. Subsequently, the dried film is cut or punched into 6mm x 100mm sample shapes to collect samples, which are then measured using a UTM instrument.

[0167] The Young's modulus and tensile strain of the main / sub-binders vary depending on the measurement method, speed, and measurement conditions of the binders. However, the Young's modulus of the main binder is a value measured in a dry room with a dew point of -5°C to 10°C and a temperature of approximately 20°C to 22°C.

[0168] The binder was in an aqueous form, and the weight-average molecular weight and number-average molecular weight were measured using aqueous GPC (Gel permeation chromatography).

[0169] In Table 1 above, the content can be interpreted as the weight ratio (parts by weight) of each composition based on 100 parts by weight of the total negative electrode composition.

[0170] Manufacturing of negative electrodes A negative electrode slurry was prepared by adding distilled water as a solvent for forming the negative electrode slurry to the negative electrode composition having the composition shown in Table 1 (solid content concentration 25% by weight).

[0171] After this, a negative electrode loading amount of 76.34 mg / 25 cm was applied to an 8 μm thick copper foil (Cu foil) with a thickness of 38 μm. 2 After coating with the negative electrode active material layer, the negative electrode was dried at 130°C for 12 hours and then rolled to a porosity of 40% to produce the negative electrode. (At this time, the negative electrode active material layer was formed on both sides of the copper foil.)

[0172] <Manufacturing of secondary batteries> LiNi 0.6 Co 0.2 Mn 0.2A cathode slurry was prepared by adding O2 (average particle size (D50): 15 μm), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 97:1.5:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent for cathode slurry formation (solid content concentration 78% by weight).

[0173] As the positive electrode current collector, an aluminum current collector (thickness: 12 μm) is coated on both sides with the positive electrode slurry at a rate of 537 mg / 25 cm². 2 The cathode was fabricated by coating with the specified loading amount, rolling (roll press), and drying in a vacuum oven at 130°C for 10 hours to form a cathode active material layer (thickness: 65 μm) (cathode thickness: 77 μm, porosity: 26%).

[0174] A lithium secondary battery was fabricated by injecting an electrolyte between the positive electrode and the negative electrodes of Examples 1-3 and Comparative Examples 1-7 via a polyethylene separation membrane.

[0175] The electrolyte in question was prepared by adding 3% by weight of vinylene carbonate to an organic solvent mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DMC) in a volume ratio of 30:70, based on the total weight of the electrolyte, and adding LiPF6 as a lithium salt at a concentration of 1M.

[0176] Experimental Example 1: Evaluation of Electrode Adhesion Strength The adhesive strength of the manufactured negative electrode was evaluated. Specifically, to evaluate the adhesive strength, the negative electrode active material layer of the electrode surface was bonded to a slide glass with an adhesive film, and the adhesive strength (peel strength) was measured at a speed of 5 mm / s in a 90° angular direction. The measured results are shown in Table 2 below.

[0177] [Table 2]

[0178] When an excessive amount of a rubber-based secondary binder is applied to a water-based primary binder, the adhesive strength increases. In other words, the water-based primary binder disperses the active material, maintains viscosity in a water-based slurry state, and, after drying, imparts adhesive strength to the negative electrode current collector layer, thus possessing both dispersibility and adhesive strength. However, the rubber-based secondary binder is a binder that possesses only the characteristic of increasing adhesive strength.

[0179] Therefore, as can be seen from the results in Table 2 above, the electrode adhesion strength of Examples 1 to 6 was confirmed to be superior to that of Comparative Example 1, which used only the main binder, a water-based binder. However, it is not possible to artificially increase only the content of the secondary binder in order to improve the adhesion strength.

[0180] In silicon-based anodes (especially pure Si anodes), the high expansion / contraction rate disrupts the network between active materials, and performance degrades as the number of isolated silicon-based active materials increases. While this is partly due to issues between the active materials themselves, more importantly, it depends on the adhesion between the active materials and the anode current collector layer. If this connection is broken, the routes through which electrons can move disappear, which has a very significant impact on performance.

[0181] Therefore, a main binder with greater rigidity than a secondary binder with lower strength must be included together, and for this reason, the specific ratio of the main binder and secondary binder of the present invention is important in terms of adhesion, short-circuit prevention, isolation, and suppression.

[0182] Table 2 above shows the results of simple adhesive strength measurements. All of Examples 1 to 6 exhibited negative electrode adhesive strengths above a certain range (290 gf / 5 mm), confirming that they possessed excellent basic negative electrode adhesive strength. Comparative Example 1 contained only a main binder, which is a water-based binder. Although the binder rigidity was high, the adhesive strength was not strong and was measured low. Comparative Example 2 corresponds to a case where a CMC, a type of water-based binder that acts more as a dispersant and thickener than as a binder, and a secondary binder are included. In this case, the rigidity of the binder itself was weak, and during adhesive strength measurement, the intermediate part of the negative electrode active material layer was separated and measured, rather than the negative electrode active material layer and the negative electrode current collector layer, resulting in a low adhesive strength. Similarly, in such cases, it can be evaluated that the binder cannot easily suppress the volume expansion of the negative electrode active material.

[0183] Comparative Examples 3 to 6 contained both a main binder and a secondary binder, and were found to be in a similar range to Examples 1 to 6 in terms of adhesive strength. However, in terms of volume retention rate and resistance increase rate, which will be discussed later, they were evaluated as inferior to Examples 1 to 6.

[0184] In particular, when comparing Examples 1 to 6, in Example 5, where the dot-shaped conductive material was applied alone, the binder was impregnated and adsorbed, resulting in the lowest adhesive strength among Examples 1 to 6.

[0185] Comparative Example 7, on the other hand, is the case where only a secondary binder is included. In this case, similar to Comparative Example 2, the binder itself had low rigidity, and during adhesive strength measurement, the intermediate portion of the negative electrode active material layer was measured separately from the negative electrode active material layer and the negative electrode current collector layer, resulting in low adhesive strength. Similarly, in this case, it was evaluated that the binder could not easily suppress the volume expansion of the negative electrode active material.

[0186] Experimental Example 2: Mono-cell Life Evaluation The lifespan of the manufactured secondary batteries was evaluated using an electrochemical charge / discharge device, and the capacity retention rate was assessed. The secondary batteries were 1) charged (0.33C CC / CV charge 4.2V 0.05C cut) and discharged (0.33C CC discharge 3.0V cut), which was considered the first cycle. From the second cycle onwards, charging and discharging were performed under the conditions of 2) charging (1.0C CC / CV charge 4.2V 0.05C cut) and discharge (0.5C CC discharge 3.0V cut).

[0187] The volume retention rate after the Nth cycle was evaluated using the following formula. The results are shown in Table 3 below. Capacity retention rate (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the 1st cycle)} × 100 [Table 3]

[0188] Capacity retention rate is an experimental example used to determine how well the capacity was maintained based on the number of cycles. Specifically, it evaluates the degree to which the silicon (pure Si), the active material in a silicon-based anode, was isolated or pulverized. Using 200 cycles as a baseline, this data allows us to confirm the rate of performance degradation due to short circuits in the conductive network of the silicon active material.

[0189] As shown in Examples 1 to 6 of this application, when the sub-binder is included in an amount of 1 part by weight or more and 15 parts by weight or less relative to the negative electrode binder, it was confirmed that the capacity retention rate is evaluated as higher compared to Comparative Examples 1 to 7, as the main binder suppresses short circuits between the active materials, while the sub-binder increases the adhesion force with the negative electrode current collector layer, thereby reducing the amount of silicon active material that short-circuits and becomes isolated.

[0190] Experimental Example 3: Evaluation of the Mono-Cell Resistance Increase Rate The lifespan of the manufactured secondary batteries was evaluated using an electrochemical charge / discharge device. Resistance measurements were taken over 200 cycles, and the resistance values ​​were compared with the initial 0-cycle resistance values ​​to confirm the resistance increase rate.

[0191] Specifically, resistance evaluation was performed after the completion of the 0-cycle and 200-cycle life evaluations. First, the secondary battery was fully charged by charging (0.33C CC / CV charge, 4.2V, 0.05C cut). After discharging (0.33C CC discharge, cut at 50% of the charge capacity) to bring the charge state to 50%, discharge (2.5C CC discharge, 30s cut) was performed. Subsequently, the same resistance evaluation was performed after the 200-cycle life evaluation was carried out in the same manner.

[0192] The resistance was calculated according to the following formula, and the results measured before the life evaluation were used as the 0-cycle reference. The resistance value and resistance increase rate were calculated for each component and the results are shown in Table 4 below.

[0193] Resistance (R) = (Voltage in idle state before 2.5C discharge (V) - Voltage after 30s discharge (V)) / Discharge current (A) Resistance increase rate (%) = {(Resistance at 200th cycle) / (Resistance at 1st cycle) - 1} × 100

[0194] [Table 4]

[0195] The binder according to this application comprises a certain proportion of a main binder and a secondary binder, and in particular, the secondary binder contains 80 parts by weight or more of butadiene.

[0196] As can be seen from Table 4 above, it can be confirmed that the resistance increase rate is low in Examples 1 to 6 of this application. On the other hand, in the case of Comparative Examples 1 to 7, it was confirmed that a higher resistance increase rate was formed compared to Examples 1 to 3 of the present invention.

[0197] In conclusion, upon reviewing the results of the aforementioned experimental examples 1 to 6, it was confirmed that Examples 1 to 6, which include the binder according to the present invention, exhibit superior adhesion between the negative electrode active material layer and the negative electrode current collector layer, and furthermore, superior capacity retention rate and resistance increase rate compared to Comparative Examples 1 to 7.

[0198] In particular, when comparing Examples 1 to 6, it was found that the case including planar conductive material and linear conductive material (Examples 1 to 3) increased the number of connectable devices that could be charged and discharged, exhibited superior output characteristics at a high C-rate, and reduced the amount of high-temperature gas generated.

[0199] This can be considered an effect of using butadiene (BD) as the binder according to the present invention, which includes a specific proportion of main binder and sub-binder, and at the same time uses 80 parts by weight or more of the sub-binder. [Explanation of Symbols]

[0200] 10 ···Negative electrode current collector layer 20...Negative electrode active material layer 30...Separation membrane 40...Cathode active material layer 50 ···Positive electrode current collector layer 100 ···Negative electrode for lithium secondary batteries 200 ···Positive electrode for lithium secondary batteries

Claims

1. A negative electrode composition comprising a silicon-based active material, a negative electrode conductive material, and a negative electrode binder, The aforementioned negative electrode binder includes a main binder which is a water-based binder and a secondary binder which is a rubber-based binder. The negative electrode binder comprises 89 to 99 parts by weight of the main binder and 1 to 11 parts by weight of the sub-binder, based on 100 parts by weight of the negative electrode binder. The butadiene (BD) content in 100 parts by weight of the polymer that constitutes the rubber-based binder of the aforementioned secondary binder is 80 parts by weight or more and 90 parts by weight or less. The aforementioned aqueous binder is one that can be dissolved in an aqueous solvent. The negative electrode composition contains 60 parts by weight or more of the silicon-based active material based on 100 parts by weight of the negative electrode composition.

2. The negative electrode composition according to claim 1, wherein the silicon-based active material comprises at least one selected from the group consisting of SiOx (x=0), SiOx (0<x<2), SiC, and Si alloys.

3. The negative electrode composition according to claim 1, wherein the silicon-based active material comprises at least one selected from the group consisting of SiOx (x=0) and SiOx (0 < x < 2), and comprises 70 parts by weight or more of SiOx (x=0) based on 100 parts by weight of the silicon-based active material.

4. The negative electrode composition according to claim 1, wherein the negative electrode conductive material is contained in an amount of 10 to 30 parts by weight based on 100 parts by weight of the negative electrode composition.

5. The negative electrode composition according to claim 1, wherein the negative electrode conductive material comprises at least one selected from the group consisting of a point conductive material, a planar conductive material, and a linear conductive material.

6. The negative electrode composition according to claim 1, wherein the negative electrode conductive material includes a planar conductive material and a linear conductive material.

7. The negative electrode composition according to claim 6, wherein the negative electrode conductive material comprises 80 to 99.9 parts by weight of the planar conductive material and 0.1 to 20 parts by weight of the linear conductive material, based on 100 parts by weight of the negative electrode conductive material.

8. The negative electrode composition according to claim 1, wherein the negative electrode binder is contained in an amount of 5 to 30 parts by weight based on 100 parts by weight of the negative electrode composition.

9. The negative electrode composition according to claim 1, wherein the weight-average molecular weight of the main binder is 100,000 g / mol or more and 1,000,000 g / mol or less.

10. The negative electrode composition according to claim 1, wherein the aqueous binder comprises at least one selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polyethylene glycol, and polyacrylamide.

11. The negative electrode composition according to claim 1, wherein the rubber-based binder comprises at least one selected from the group consisting of styrene-butadiene rubber, hydrogenated nitrile-butadiene rubber, and acrylonitrile-butadiene rubber.

12. Negative electrode current collector layer, and A negative electrode active material layer comprising the negative electrode composition according to any one of claims 1 to 11, formed on one or both sides of the negative electrode current collector layer, A negative electrode for lithium secondary batteries, including...

13. The negative electrode for a lithium secondary battery according to claim 12, wherein the adhesive strength of the surface of the negative electrode active material layer in contact with the negative electrode current collector layer satisfies the condition of 290 gf / 5 mm or more and 500 gf / 5 mm or less at 25°C and normal pressure conditions.

14. The thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less. The negative electrode for a lithium secondary battery according to claim 12, wherein the thickness of the negative electrode active material layer is 20 μm or more and 500 μm or less.

15. positive electrode, The negative electrode for a lithium secondary battery according to claim 12, A separation membrane provided between the positive electrode and the negative electrode, and electrolyte, Lithium-ion batteries, including lithium-ion batteries.

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