Negative electrode slurry, negative electrode containing the same, and lithium secondary battery

Abstract

This specification relates to a negative electrode slurry containing a negative electrode active material, single-walled carbon nanotubes (SWCNT), an organic binder, and an organic solvent, a negative electrode containing the same, and a secondary battery. The negative electrode slurry according to this specification contains a negative electrode active material using silicon-based particles with a large charge-discharge capacity, generates a small amount of hydrogen gas during the production and storage of the slurry, and has excellent capacity retention during charge and discharge.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0183268, filed with the Korean Intellectual Property Office on December 23, 2022, and all of its content is incorporated herein by reference.

[0002] This specification relates to a negative electrode slurry, a negative electrode including the same, and a lithium secondary battery.

Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy has been increasing. As part of this, the field that is most actively studied is the field of power generation and power storage using electrochemical reactions.

[0004] Currently, as a typical example of an electrochemical device using such electrochemical energy, a secondary battery is cited, and its use area is increasingly expanding.

[0005] On the other hand, as the technology development and demand for mobile devices increase, the demand for secondary batteries has been rapidly increasing. As a result, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. Accordingly, in order to manufacture an electrode having a high energy density per unit volume as an electrode for a high-capacity lithium secondary battery, research is actively underway.

[0006] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. In particular, the negative electrode contains a negative electrode active material, and silicon-based particles having a large charge-discharge capacity can be used as the negative electrode active material.

[0007] In particular, in response to the recent demand for secondary batteries having high-energy electrodes, as a negative electrode active material, Si / C having a capacity more than 10 times larger than that of a graphite-based material, SiO x(0 < x < 2) and other silicon-based compounds are being actively studied for methods to increase capacity. However, in the case of silicon-based compounds, compared with conventional graphite-based materials, hydrogen gas is generated by the oxidation reaction with water during storage in a slurry state, and the volume expands during charge and discharge, blocking the conduction path, thereby reducing the battery characteristics.

[0008] To solve the above problems, research on the composition of the binder is also underway, and as a result, research on binder polymers with strong stress is being advanced. However, these binder polymers alone have limitations in preventing the increase in the thickness of the electrode due to the shrinkage and expansion of the negative electrode active material and the resulting performance degradation of the lithium secondary battery. In addition, to solve the problems associated with the volume expansion of the negative electrode having the silicon-based active material as described above, an aqueous binder having high dispersibility is used.

[0009] In addition, to ensure the conductivity of the negative electrode, the secondary battery further includes a conductive material. Conventionally, carbon black and the like have been mainly used, but in order to improve the capacity of the secondary battery, single-walled carbon nanotubes (SWCNTs) with an elongated shape are used. Usually, a negative electrode slurry is produced with a dispersion in which the single-walled carbon nanotubes are completely dispersed, and a negative electrode active material layer is produced with the negative electrode slurry. However, when the single-walled carbon nanotubes are completely dispersed, there is a problem that a carbon nanotube structure is not easily formed. In recent years, methods have been proposed to pre-disperse the single-walled carbon nanotubes to a certain level in a dispersion medium so that most exist as a carbon nanotube structure.

[0010] However, when the single-walled carbon nanotubes are pre-dispersed to a certain level in a dispersion medium, there is a problem that the carbon nanotube structure is not maintained in an aqueous system containing an aqueous binder.

[0011] Therefore, to solve the above problems, various solutions have been discussed, and among them, research on the negative electrode binder is one of the solutions.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] The inventors of the present invention have found that by using a silicon-based active material with a large charge-discharge capacity as the negative electrode active material, using a pre-dispersed liquid in which single-walled carbon nanotubes are pre-dispersed in a dispersion medium as the conductive material, and realizing a negative electrode slurry using an organic binder so that the carbon nanotube structure embodied in the pre-dispersed liquid is maintained, the above problems can be solved.

[0013] In addition, the inventors of the present invention have confirmed that by using N-methyl-2-pyrrolidinone (NMP) as the dispersion medium, it is easier to maintain the carbon nanotube structure, and the mutual safety with the binder is improved.

[0014] Accordingly, this specification intends to provide a negative electrode slurry including the above technical features, a negative electrode including the same, and a lithium secondary battery.

MEANS FOR SOLVING THE PROBLEMS

[0015] One embodiment of this specification is a negative electrode slurry including a negative electrode active material; single-walled carbon nanotubes (SWCNT); an organic binder; and an organic solvent, wherein the single-walled carbon nanotubes include a pre-dispersed liquid dispersed in N-methyl-2-pyrrolidinone (NMP).

[0016] Furthermore, one embodiment of this specification includes a current collector layer; and a negative electrode active material layer provided on one or both surfaces of the current collector layer, wherein the negative electrode active material layer includes the negative electrode slurry or a dried product thereof, to provide a negative electrode.

[0017] Finally, one embodiment of the present specification provides a lithium secondary battery including a first electrode, a second electrode, a separation membrane interposed between the first electrode and the second electrode, and an electrolyte, wherein either the first electrode or the second electrode is the negative electrode.

Advantages of the Invention

[0018] The negative electrode slurry according to the present application uses an organic solvent to prevent the generation of hydrogen gas due to the oxidation reaction with water in a slurry state containing a silicon compound. Also, since the dispersion stability of NMP-dispersed SWCNT is superior to that of water-dispersed SWCNT, it has excellent conductive connectivity of the electrode, and due to the superiority of the connectivity of the conductive network during charge and discharge, it has excellent cycle characteristics.

[0019] Therefore, the negative electrode slurry according to the present application has a large charge-discharge capacity, a small amount of hydrogen gas generated during slurry production and storage, and during the repeated process of charge and discharge, the conductive path can be maintained by maintaining the carbon nanotube structure, and it has excellent phase stability.

[0020] In addition, the negative electrode containing the negative electrode slurry has a high energy density per unit volume and improved conductivity.

[0021] In addition, the secondary battery containing the negative electrode provides effects such as having a high energy density and voltage, improved capacity, long cycle life, and low self-discharge rate.

Embodiments for Carrying Out the Invention

[0022] Before explaining the present invention, several terms are first defined.

[0023] In this specification, when a certain part "includes" a certain component, this means that, unless otherwise stated to the contrary, it does not exclude other components, but may further include other components.

[0024] In this specification, "p~q" means a range of "p or more and q or less".

[0025] In this specification, the "specific surface area" is measured by the BET method, specifically calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BEL Japan's BELSORP-mini II. That is, in this application, the BET specific surface area can mean the specific surface area measured by the above measurement method.

[0026] In this specification, "Dn" means 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. On the other hand, the average particle size can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500), and the diffraction pattern difference due to the particle size when the particles pass through the laser beam is measured to calculate the particle size distribution.

[0027] In one embodiment of this application, the particle size or diameter can mean the average diameter or representative diameter of each particle forming the metal powder.

[0028] In this specification, the meaning that a polymer contains a certain monomer as a monomer unit means that the monomer participates in the polymerization reaction and is included as a repeating unit in the polymer. In this specification, when a polymer is said to contain a monomer, this is interpreted in the same way as the polymer containing the monomer as a monomer unit.

[0029] In this specification, the "polymer" is understood to be used in a broad sense including copolymers unless specified as "homopolymer".

[0030] In this specification, the weight average molecular weight (Mw) and the number average molecular weight (Mn) are the polystyrene equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers (standard samples) of various degrees of polymerization commercially available for molecular weight measurement as the standard substances. In this specification, the molecular weight means the weight average molecular weight unless otherwise specified.

[0031] Hereinafter, for those with ordinary knowledge in the technical field to which the present invention belongs to easily implement the present invention, a detailed description will be given with reference to the drawings. However, the present invention can be embodied in various different forms and is not limited to the following description.

[0032] <Negative electrode slurry> One embodiment of this specification is a negative electrode slurry containing a negative electrode active material; single-walled carbon nanotubes (SWCNT); an organic binder; and an organic solvent, wherein the single-walled carbon nanotubes contain a pre-dispersion liquid dispersed in N-methyl-2-pyrrolidone (NMP), and a negative electrode slurry is provided.

[0033] Since the negative electrode slurry according to the present application contains the pre-dispersion liquid containing the organic binder and single-walled carbon nanotubes, it is superior in the phase stability of the carbon nanotube negative electrode slurry compared to the aqueous system containing an aqueous binder. As a result, it is excellent in the conductivity network connectivity during the repeated process of charge and discharge, and less hydrogen gas is generated during the production and storage of the slurry.

[0034] In this specification, the organic binder plays a role in suppressing the negative electrode active material and / or the conductive material in order to prevent the twisting and deformation of the negative electrode structure in the volume expansion and relaxation of the negative electrode active material. In addition, the organic binder has better compatibility with single-walled carbon nanotubes than the aqueous binder and can maintain phase stability. In this specification, the organic binder is a polymer soluble in an organic solvent, and there is no particular limitation as long as it satisfies the organic binder that plays such a role.

[0035] In the present specification, the mixing method for forming the negative electrode slurry is not particularly limited, and examples include a ball mill, a sand mill, a pigment disperser, an ultrasonic disperser, a homogenizer, a planetary mixer, a Hobart mixer, etc. It is preferable to knead using a homogenizer and / or a planetary mixer as appropriate.

[0036] In one embodiment of the present invention, the negative electrode active material may contain at least one selected from the group consisting of a silicon-based active material and a carbon-based active material.

[0037] In one embodiment of the present specification, the electrode active material may consist of a mixture of a silicon-based active material and a carbon-based active material.

[0038] In the present specification, the silicon-based active material has a capacity 10 times or more higher than that of the carbon-based active material. Thus, when applying the silicon-based active material to an electrode, particularly a negative electrode, it is possible to realize an electrode having a high level of energy density even with a thinner thickness than when containing only the carbon-based active material.

[0039] In another embodiment of the present specification, when the electrode active material consists of a silicon-based active material and a carbon-based active material, the composition ratio between the silicon-based active material and the carbon-based active material may be in the range of 2:98 to 30:70.

[0040] The electrode active material according to the above embodiment can further provide the effect of having less volume expansion and less swelling of the active material during charge and discharge with the carbon-based active material as the main component, and excellent conductive connectivity of the electrode.

[0041] In one embodiment of the present specification, the silicon-based active material is SiO x (x = 0), SiO x (0 < x < 2), SiC, and Si alloy, and may contain at least one selected from the group consisting of them.

[0042] In this specification, the electrode active material contains silicon particles, which can exist, for example, in crystalline or amorphous forms. Specifically, the silicon particles may preferably be spherical particles, but are not limited thereto.

[0043] In this specification, the SiO x In the case of SiO2 where x is 2 is not included, because this SiO2 does not react with lithium ions and thus cannot store lithium. Therefore, x is preferably within the range of the above embodiment.

[0044] In this specification, the silicon-based active material may be Si / C or Si composed of a composite of Si and C.

[0045] In this specification, two or more of the silicon-based active materials may be mixed and used.

[0046] In one embodiment of this specification, the silicon-based active material includes one or more selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may contain 70 parts by weight or more.

[0047] In the negative electrode slurry, by satisfying the content range of the silicon-based active material, the binder and the conductive material can effectively bind the silicon active material and smoothly maintain the conductive connectivity between the active materials.

[0048] In another embodiment, based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) 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.

[0049] In one embodiment of the present specification, as the silicon-based active material, pure silicon (Si) may be particularly used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means that, based on a total of 100 parts by weight of the silicon-based active material, pure Si particles (SiO x (x = 0)) are included within the above range.

[0050] In the case of silicon-based active materials, when compared with conventionally used graphite-based active materials, the capacity is significantly higher, and attempts to apply them have been increasing. However, due to the high volume expansion rate during the repeated charge and discharge process, it has been limited to cases such as mixing a small amount with graphite-based active materials for use.

[0051] Therefore, in the case of the present invention, in order to improve the capacity, while using only the silicon-based active material as the negative electrode active material, in order to solve the above problems, a negative electrode precursor dispersion liquid was prepared to improve the dispersibility of carbon nanotubes and strengthen the bond with the active material to solve existing problems.

[0052] On the other hand, the average particle size (D50) of the silicon-based active material of the present invention is 3 μm to 10 μm, specifically 3.5 μm to 8 μm, and more specifically may be 3.5 μm to 7 μm. When the average particle size is included in the above range, the specific surface area of the particles is included in an appropriate range, and the viscosity of the negative electrode slurry is formed in an appropriate range. As a result, the dispersion of the particles constituting the negative electrode slurry becomes smooth. In addition, when the size of the silicon-based active material has a value equal to or greater than the lower limit value of the above range, the contact area between the silicon particles and the conductive material in the negative electrode slurry is excellent due to the composite composed of the conductive material and the negative electrode binder, and the possibility of maintaining the conductive network is high, increasing the capacity retention rate. On the other hand, when the average particle size satisfies the above range, silicon particles that are too large are excluded, and the surface of the negative electrode is smoothly formed, thereby preventing the current density unevenness phenomenon during charge and discharge.

[0053] In one embodiment of the present application, the silicon-based active material generally has a characteristic BET surface area. The BET surface area of the silicon-based active material is preferably 0.01 m 2 / g to 150.0 m 2 / g, more preferably 0.1 m 2 / g to 100.0 m 2 / g, particularly preferably 0.2 m 2 / g to 80.0 m 2 / g, most preferably 0.2 m 2 / g to 18.0 m 2 / g. The BET surface area is measured in accordance with DIN 66131 (using nitrogen).

[0054] In one embodiment of the present application, the silicon-based active material can exist, for example, in crystalline or amorphous form and is preferably not porous. The silicon particles are preferably spherical or flaky particles. As an alternative, although less preferred, the silicon particles may also have a fibrous structure or exist in the form of a silicon-containing film or coating.

[0055] In one embodiment of the present specification, the carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, and soft carbon.

[0056] In the present specification, the negative electrode composition in the negative electrode slurry means the negative electrode active material, the conductive material, and the organic binder excluding the (organic) solvent, and may sometimes be referred to by the term solid content of the negative electrode slurry.

[0057] In one embodiment of the present application, the silicon-based active material provides a negative electrode composition that is 60 parts by weight or more based on 100 parts by weight of the solid content of the negative electrode slurry.

[0058] 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, based on 100 parts by weight of the solid content of the negative electrode slurry, and may be contained in an amount of 95 parts by weight or less, preferably 90 parts by weight or less.

[0059] Even when the silicon-based active material with extremely high capacity is used within the above range, the negative electrode slurry according to the present application uses a specific conductive material and a negative electrode binder that can improve the volume expansion rate during the repeated charge and discharge process, and even when the above range is included, the performance of the negative electrode is not deteriorated, and it has the characteristics of excellent output characteristics during charging and discharging.

[0060] 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, or for example, 0.85 to 0.9.

[0061] In the present application, the sphericity is determined by the following formula 1, where A is the area and P is the boundary line.

[0062] [Formula 1] 4πA / P 2

[0063] In one embodiment of the present specification, in the pre-dispersion liquid, the single-walled carbon nanotubes may be 0.4 parts by weight or more with respect to 100 parts by weight of N-methyl-2-pyrrolidone (NMP).

[0064] In one embodiment of the present specification, in the pre-dispersion liquid, the single-walled carbon nanotubes may be 1 part by weight or more with respect to 100 parts by weight of N-methyl-2-pyrrolidone (NMP).

[0065] In this specification, the aforementioned precursor dispersion may further contain a dispersant. The dispersant enables single-walled carbon nanotubes to be smoothly mixed into a dispersion medium such as NMP during the production of a conductive mixture such as the aforementioned precursor dispersion, and can improve the dispersibility of carbon nanotubes in the conductive material dispersion produced through the conductive mixture.

[0066] In other embodiments of the present invention, the dispersant may contain at least any one selected from the group consisting of hydrogenated nitrile butadiene rubber (H-NBR), polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVdF), and polyvinyl butyral (PVB). Specifically, the dispersant may be hydrogenated nitrile butadiene rubber or PVdF.

[0067] In other embodiments of the present invention, the dispersant may be contained in an amount of 10 wt% or more and 500 wt% or less based on the weight of the single-walled carbon nanotubes. Specifically, it may be 20 wt% or more, or 50 wt% or more, or may be contained in an amount of 300 wt% or less, or 200 wt% or less. When the above range is satisfied, the single-walled carbon nanotubes can be smoothly dispersed in NMP.

[0068] The single-walled carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain a plurality of carbon nanotube units. Specifically, here, "bundle type" refers to a secondary shape in the form of a bundle or rope in which a plurality of carbon nanotube units are arranged side by side with the longitudinal axes of the carbon nanotube units substantially in the same orientation or intertwined, unless otherwise specified. The single-walled carbon nanotube unit has a graphite sheet in the shape of a cylinder with a nanosize diameter and sp 2It has a bonding structure. At this time, the properties of the conductor or semiconductor can be exhibited depending on the angle and structure around which the graphite sheet is wound. The bundled carbon nanotube can be uniformly dispersed during the production of the negative electrode as compared with the entangled type carbon nanotube, and can smoothly form a conductive network in the negative electrode, so that the conductivity of the negative electrode can be improved.

[0069] Conventionally, as the conductive material in the negative electrode slurry, a dot-shaped conductive material, a planar conductive material and / or a linear conductive material have been used. Currently, however, an elongated linear conductive material (i.e., a single-walled carbon nanotube) having excellent physical properties in terms of dispersibility is used. In particular, when the linear conductive material is used alone, the tortuosity of the electrode, which is a problem of the silicon-based negative electrode, can be simplified, the electrode structure can be improved, and thus the movement resistance of lithium ions in the electrode can be reduced.

[0070] In addition, for further improvement of dispersibility, instead of the aqueous dispersion using distilled water as the dispersion medium, an amide-based polar organic solvent (particularly, NMP) is used to facilitate the maintenance of the carbon nanotube structure.

[0071] Also, when the above range is satisfied, a conductive path of the negative electrode manufactured with the negative electrode slurry can be secured in the future, the electrode resistance can be lowered, and the life characteristics of the battery can also be improved.

[0072] In another embodiment of the present specification, the content of the single-walled carbon nanotube may be 0.01 part by weight or more and 1 part by weight or less based on 100 parts by weight of the solid content of the negative electrode slurry.

[0073] In still another embodiment, the content of the single-walled carbon nanotube may include 0.05 part by weight or more and 0.8 part by weight or less, preferably 0.1 part by weight or more and 0.7 part by weight or less, and more preferably 0.15 part by weight or more and 0.5 part by weight or less based on 100 parts by weight of the solid content of the negative electrode slurry.

[0074] The negative electrode conductive material according to the present application has a completely different configuration from the positive electrode conductive material applied to the positive electrode. That is, in the case of the negative electrode conductive material according to the present application, it plays a role of capturing the contact points between silicon-based active materials with a very large volume expansion of the electrode during charging and discharging, while the positive electrode conductive material plays a role of imparting partial conductivity while serving as a buffer for the buffer role when rolled, and the negative electrode conductive material of the present invention has a completely different configuration and role.

[0075] In addition, the negative electrode conductive material according to the present application is applied to silicon-based active materials and has a completely different configuration from the conductive material applied to graphite-based active materials. That is, the conductive material used for an electrode having a graphite-based active material has characteristics of improving output characteristics and imparting partial conductivity because it simply has particles smaller than the active material, and the negative electrode conductive material applied together with the silicon-based active material as in the present invention has a completely different configuration and role.

[0076] In one embodiment of the present specification, the organic binder may be one or more selected from the group consisting of polyvinylidene fluoride (PVdF)-based binders, polyamideimide (PAI)-based binders, polyimide (PI)-based binders, polyacrylonitrile (PAN)-based and polyacrylamide (PAM)-based binders.

[0077] In the present specification, the organic binder may be a homopolymer or a copolymer, and in the case of a copolymer, it may be a random copolymer, a block copolymer, an alternating copolymer, or a graft copolymer.

[0078] In one embodiment of the present specification, the organic binder may further contain a repeating unit derived from at least one compound selected from the group consisting of acrylic acid and hexafluoropropylene, or may further contain a polyacrylic acid polymer. Examples of the organic binder include, but are not limited to, PVdF-AA / HFP, PAN-PAA, PAN-PAM, etc.

[0079] In one embodiment of the present specification, the organic solvent means a 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 derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyruvate, ethyl propionate may be used.

[0080] In one embodiment of the present specification, the organic solvent may be N-methyl-2-pyrrolidinone (NMP).

[0081] In the present specification, by using the same type of the organic solvent and the dispersion medium of the conductive material, an organic system can be ensured on the side surface of the entire negative electrode slurry, which is useful for maintaining the form of the carbon nanotube structure formed in the above-described dispersion liquid.

[0082] In one embodiment of the present specification, the weight ratio of the negative electrode active material, the organic binder, and the single-walled carbon nanotube may be 70 to 95 (wt%): 3 to 15 (wt%): 0.01 to 1 (wt%). In the present specification, a composition containing the negative electrode active material, the organic binder, and the single-walled carbon nanotube can be referred to as an electrode composition.

[0083] In one embodiment of the present specification, the negative electrode slurry may further contain an additional conductive material, and the content of the additional conductive material may be 20 wt% or less compared to the sum of the negative electrode active material, the organic binder, and the single-walled carbon nanotube.

[0084] In other embodiments of the present specification, the content of the additional conductive material may be more than about 0 wt%, more than about 1 wt%, more than about 3 wt%, or more than about 4 wt%, or about 20 wt% or less, about 19 wt% or less, about 18 wt% or less, or about 17 wt% or less, based on the sum of the negative electrode active material, the organic binder, and the single-walled carbon nanotubes (i.e., the electrode composition).

[0085] In the present specification, the additional conductive material, as another configuration other than the single-walled carbon nanotubes, may mean at least one of a dot-shaped conductive material, a planar conductive material, and a linear conductive material.

[0086] In the present specification, the dot-shaped conductive material can be used to improve the conductivity of an electrode, preferably a negative electrode, and means a dot-shaped or spherical conductive material having conductivity without inducing a chemical change. Specifically, the dot-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, fluorocarbons, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and preferably may contain carbon black in terms of realizing high conductivity and excellent dispersibility.

[0087] In the present specification, the planar conductive material can play a role in increasing the surface contact between silicon particles inside an electrode, preferably improving the conductivity, and at the same time suppressing the interruption of the conductive path due to volume expansion. The planar conductive material can be represented as a plate-shaped conductive material or a bulk-type conductive material. Examples of the planar conductive material may include at least any one selected from the group consisting of plate-shaped graphite, graphene, graphene oxide, and graphite flakes, and preferably may be plate-shaped graphite.

[0088] In the present specification, the linear conductive material may be a multi-walled carbon nanotube other than the single-walled carbon nanotubes.

[0089] In one embodiment of the present specification, the additional conductive material may be one or more dot-shaped conductive materials. For example, it may be carbon black, artificial graphite, or the like.

[0090] In another embodiment, the conductive material may be a negative electrode conductive material.

[0091] In the present specification, the negative electrode conductive material is applied to the negative electrode and has a completely different configuration from the positive electrode conductive material applied to the positive electrode. That is, in the case of the negative electrode conductive material, it plays a role of capturing the contacts between silicon-based active materials with a very large volume expansion of the electrode during charging and discharging, while the positive electrode conductive material, when rolled, plays a role of imparting partial conductivity while playing a role of a buffer as a buffer role. Therefore, the negative electrode conductive material and the positive electrode conductive material have different configurations and roles from each other.

[0092] When the negative electrode slurry according to one embodiment of the present invention satisfies the above weight ratio, the phase stability is improved, the binder and the conductive material are effectively bound to the negative electrode active material during electrode manufacturing, and the conductive connectivity between the active materials can be smoothly maintained.

[0093] In some cases, the type or content of the solvent can be adjusted so that the viscosity of the negative electrode slurry belongs to the range of 5000 cps to 6000 cps. Thereby, when the negative electrode slurry is coated on one or both sides of the electrode current collector layer in the future, it is excellent in coating property and storage property.

[0094] <Negative electrode> One embodiment of the present specification includes a current collector layer; and a negative electrode active material layer provided on one or both sides of the current collector layer, and the negative electrode active material layer includes any one of the above-mentioned negative electrode slurries or a dried product thereof, providing a negative electrode.

[0095] In one embodiment of the present invention, the solid content of the negative electrode slurry may satisfy 5% or more and 60% or less, preferably 5% or more and 40% or less.

[0096] In another embodiment, the solid content of the negative electrode slurry may satisfy the range of 5% or more and 40% or less, preferably 7% or more and 35% or less, and more preferably 10% or more and 30% or less.

[0097] The solid content of the negative electrode slurry means 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.

[0098] When the solid content of the negative electrode slurry satisfies the above range, the viscosity is appropriate during the formation of the negative electrode active material layer, and it has the characteristic that the particle aggregation phenomenon of the negative electrode composition can be minimized and the negative electrode active material layer can be efficiently formed.

[0099] In one embodiment of the present application, the current collector layer generally has a thickness of 1 μm to 100 μm as the negative electrode current collector layer. 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. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. may be used. Also, fine irregularities can be formed on the surface to strengthen the binding force of the negative electrode active material, and it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0100] In one embodiment of the present 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.

[0101] However, the thickness can be variously deformed according to the type and use of the negative electrode used, and is not limited thereto.

[0102] In one embodiment of the present application, the porosity of the negative electrode active material layer may satisfy the range of 10% or more and 60% or less.

[0103] In yet another embodiment, the porosity of the negative electrode active material layer may satisfy the range of 10% or more and 60% or less, preferably 20% or more and 50% or less, and more preferably 30% or more and 45% or less.

[0104] The porosity varies according to the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer, and in particular, by including the silicon-based active material and conductive material according to the present application in specific compositions and content portions, the above range is satisfied, whereby the electrode has appropriate ranges of electric conductivity and resistance.

[0105] In one embodiment of the present application, as a means for providing the negative electrode slurry on the current collector layer, conventionally known coating devices such as a comma coater, gravure coater, microgravure coater, die coater, bar coater, etc. can be used, but it is not limited thereto.

[0106] Thereafter, the drying process can proceed, and it may proceed at a temperature of 60°C to 200°C, more preferably 100°C to 180°C, and may proceed in dry air or an inert atmosphere (for example, argon, etc.), but it is not limited thereto. The thickness of the electrode (cured coating film) may be 5 μm to 300 μm, or more preferably 10 μm to 250 μm, but it is not limited thereto.

[0107] <Lithium secondary battery> One embodiment of the present application provides a lithium secondary battery including a first electrode, a second electrode, a separator interposed between the first electrode and the second electrode, and an electrolyte, wherein either the first electrode or the second electrode is the negative electrode.

[0108] The secondary battery according to an embodiment of the present specification may particularly include the negative electrode for a lithium secondary battery described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, a specific description thereof will be omitted.

[0109] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0110] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel may be used. Further, the positive electrode current collector may usually have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive force of the positive electrode active material. For example, it may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric body, etc.

[0111] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; chemical formula Li 1+c1 Mn 2-c1 O4 (0 ≦ c1 ≦ 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 O2 (where M is at least any one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01 ≦ c2 ≦ 0.3) represented by Ni-site type lithium nickel oxide; chemical formula LiMn2-c3 M c3 O2 (where M is at least any one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01 ≦ c3 ≦ 0.1) or Li2Mn3MO8 (where M is at least any one selected from the group consisting of Fe, Co, Ni, Cu, and Zn).); examples include LiMn2O4 in which part of Li in the chemical formula is substituted with alkaline earth metal ions, but are not limited thereto. The positive electrode may be Li metal (Li-metal).

[0112] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the positive electrode active material described above.

[0113] At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and in the configured battery, it can be used without particular limitation as long as it has electronic conductivity without undergoing a chemical change. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances 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. Among these, one kind alone or a mixture of two or more kinds may be used.

[0114] In addition, the positive electrode binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive force 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc. Among these, one kind alone or a mixture of two or more kinds may be used.

[0115] As the separator, it separates the negative electrode and the positive electrode to provide a migration path for lithium ions, and can be used without particular limitation as long as it is usually used as a separator in a secondary battery. In particular, those with low resistance to the ion migration of the electrolyte and excellent electrolyte moisture retention ability are preferred. Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance may be used, and it may be selectively used in a single-layer or multi-layer structure.

[0116] Examples of the electrolyte 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 during the manufacture of lithium secondary batteries.

[0117] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0118] Examples of the non-aqueous organic solvent include 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, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyropionate, ethyl propionate, etc. may be used.

[0119] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are preferably used because they have a high dielectric constant as high-viscosity organic solvents and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte having high electrical conductivity can be produced and can be more preferably used.

[0120] 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, as the anion of the lithium salt, 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 - At least one selected from the group consisting of can be used.

[0121] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the reduction of the battery capacity, improving the discharge capacity of the battery, etc.

[0122] The secondary battery according to the present invention is useful in portable devices such as mobile phones, notebook computers, digital cameras, and the field of electric vehicles such as hybrid electric vehicles (HEVs), and can be particularly preferably used as a constituent battery of a medium- to large-sized battery module. Therefore, the present invention also provides a medium- to large-sized battery module including the lithium secondary battery as described above as a unit battery.

[0123] One embodiment of the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same. Since the battery module and the 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 and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

Example

[0124] Hereinafter, preferred examples are presented to assist in understanding the present invention. However, it should be apparent to those skilled in the art that these examples are illustrative of the description and that various changes and modifications can be made within the scope of the description and the scope of the technical idea. It is natural that such variations and modifications belong to the scope of the appended claims.

[0125] Example. Manufacture of negative electrode slurry <Example 1> Si with d50 = 4.5 μm was used as the negative electrode active material, NMP-dispersed SWCNT (dispersant: PvdF, weight ratio of SWCNT: PvdF = 1:1) was used as the conductive material, carbon black (Super C65, Timcal) and artificial graphite (SFG6L, Timcal) were used as additional conductive materials, and a PAI-based binder (polyamideimide-based binder) was used as the binder. An electrode composition was prepared by mixing so that Si:C65:SFG6L:SWCNT:binder = 80:5:5:0.5:10 (by weight).

[0126] The negative electrode slurry was manufactured using NMP (N-methyl-2-pyrrolidone) as the solvent, and the content of NMP added at this time was adjusted in consideration of coatability, viscosity, and solid content. The viscosity of the obtained negative electrode slurry was adjusted to be 5000 cps to 6000 cps using a rheometer HR20 (TA).

[0127] <Example 2> The negative electrode slurry was produced in the same manner as in Example 1, except that PAN-PAM (PAN:PAM = 95:5 based on the total binder weight) was used as the binder.

[0128] <Example 3> The negative electrode slurry was produced in the same manner as in Example 1, except that PAN-PAA (PAN:PAA = 95:5 based on the total binder weight) was used as the binder.

[0129] <Example 4> The negative electrode slurry was produced in the same manner as in Example 1, except that artificial graphite:SiC (weight ratio 85:15) was used as the negative electrode active material.

[0130] <Comparative Example 1> Na as the binder + The negative electrode slurry was produced in the same manner as in Example 1, except that substituted PAA and water-dispersed single-walled carbon nanotubes were used as the conductive material.

[0131] <Comparative Example 2> PAM-Na as the binder + The negative electrode slurry was produced in the same manner as in Example 1, except that substituted PAA and water-dispersed single-walled carbon nanotubes were used as the conductive material.

[0132] <Comparative Example 3> The negative electrode slurry was produced in the same manner as in Example 1, except that SBR-CMC (SBR:CMC = 8:2 based on the total binder weight) and water-dispersed single-walled carbon nanotubes were used as the binder and the conductive material, respectively.

[0133] <Comparative Example 4> Na as the binder + The negative electrode slurry was produced in the same manner as in Example 4, except that substituted PAA and water-dispersed single-walled carbon nanotubes were used as the conductive material.

[0134] Experimental Example <Experimental Example 1: Measurement of the Amount of H2 Gas Generated in the Electrode Slurry> 5 g of the negative electrode slurries of Examples 1 to 4 and Comparative Examples 1 to 4 were respectively placed in a pouch with a size of 9 cm × 9 cm, sealed, and then left in a constant temperature chamber at 60°C for 24 hours. Next, the gas generated in the pouch was collected, and the amount of generated H2 gas was quantitatively analyzed by GC / MS. The results are as shown in Table 1 below.

[0135] <Experimental Example 2: Manufacture of Battery and Evaluation of Battery Characteristics> The negative electrode slurries of Examples 1 to 4 and Comparative Examples 1 to 4 were coated on a copper foil with a thickness of 18 μm and dried, and an active material layer with a thickness of 50 μm was formed on one side of the copper foil to produce a circular punching test electrode (negative electrode) with a diameter of 14Φ (mm).

[0136] A metal lithium foil with a thickness of 0.3 mm was used as the positive electrode, a porous polyethylene sheet with a thickness of 0.1 mm was used as the separator, and a mixed solvent with a volume ratio of 1:1 of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte, in which LiPF6 was dissolved as a lithium salt at a concentration of about 1 mol / L.

[0137] The negative electrode, positive electrode, separator, and electrolyte were sealed in a stainless steel container to produce an evaluation coin cell with a thickness of 2 mm and a diameter of 32 mm.

[0138] The coin cell was charged at a constant current of 0.05C until the voltage reached 0.01V, and discharged at a constant current of 0.05C until the voltage reached 1.5V to obtain the discharge capacity and initial efficiency. Then, the cycle characteristics were carried out at a constant current of 0.2C in the same voltage range as above, and the capacity retention rate test was advanced. The experimental results are shown in Table 1 below.

[0139]

Table 1

[0140] As shown in Table 1 above, the batteries manufactured using the negative electrode slurries containing the organic binders and NMP-dispersed single-walled carbon nanotubes (SWCNT) of Examples 1 to 4 had less hydrogen gas generation and were excellent in cycle capacity retention compared to the batteries using the negative electrode slurries of Comparative Examples 1 to 4.

Claims

1. A negative electrode slurry comprising a negative electrode active material, single-walled carbon nanotubes (SWCNTs), an organic binder, and an organic solvent, wherein the negative electrode slurry contains a pre-dispersion in which the single-walled carbon nanotubes are dispersed in N-methyl-2-pyrrolidone (NMP).

2. The negative electrode slurry according to claim 1, wherein the negative electrode active material contains at least one selected from the group consisting of a silicon-based active material and a carbon-based active material.

3. The silicon-based active material is SiO ｘ (x=0), SiO ｘ The negative electrode slurry of claim 2 , comprising at least one selected from the group consisting of (0<x<2), SiC, and a Si alloy.

4. The negative electrode slurry according to claim 2, wherein the carbon-based active material contains at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, and soft carbon.

5. The negative electrode slurry according to claim 1, wherein the organic binder is one or more selected from the group consisting of a polyvinylidene fluoride (PVdF)-based binder, a polyamideimide (PAI)-based binder, a polyimide (PI)-based binder, a polyacrylonitrile (PAN)-based binder, and a polyacrylamide (PAM)-based binder.

6. The negative electrode slurry according to claim 5, wherein the organic binder further contains a repeating unit derived from at least one compound selected from the group consisting of acrylic acid and hexafluoropropylene, or further contains a polyacrylic acid polymer.

7. The negative electrode slurry according to claim 1, wherein the organic solvent is N-methyl-2-pyrrolidone (NMP).

8. The negative electrode slurry according to claim 1, wherein the weight ratio of the negative electrode active material, the organic binder, and the single-walled carbon nanotubes is 70 to 95 (wt%): 3 to 15 (wt%): 0.01 to 1 (wt%).

9. Further comprising an additional conductive material, The negative electrode slurry according to claim 1, wherein the weight ratio of the additional conductive material is 20 wt% or less relative to the sum of the negative electrode active material, the organic binder, and the single-walled carbon nanotubes.

10. A negative electrode comprising a current collector layer; and a negative electrode active material layer provided on one or both surfaces of the current collector layer, wherein the negative electrode active material layer contains the negative electrode slurry according to any one of claims 1 to 9 or a dried product thereof.

11. A first electrode; A second electrode; A separator interposed between the first electrode and the second electrode; and An electrolyte A lithium secondary battery comprising The lithium secondary battery, wherein either the first electrode or the second electrode is the negative electrode according to claim 10.

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