Negative electrode composition for lithium ion secondary battery, negative electrode slurry, negative electrode, and lithium ion secondary battery

Abstract

This application relates to a negative electrode composition for lithium ion secondary batteries, including a negative electrode active material, a conductive material, and an aqueous binder. The conductive material includes a dispersion containing a specific type of dispersant, and the aqueous binder includes a specific content of (meth)acrylamide (AM). This application also relates to a negative electrode slurry, a negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery, each including the negative electrode composition for lithium ion secondary batteries. The negative electrode composition for lithium ion secondary batteries can contribute to improved phase stability and suppression of volume expansion during battery charge and discharge.

Description

[Technical Field]

[0001] The present application relates to a negative electrode composition for a lithium ion secondary battery, a negative electrode slurry, a negative electrode, and a lithium ion secondary battery.

[0002] This application claims the benefit of the filing dates of Korean Patent Application No. 10-2023-0042670 filed with the Korean Intellectual Property Office on March 31, 2023, and Korean Patent Application No. 10-2024-0042404 filed with the Korean Intellectual Property Office on March 28, 2024, the entire contents of which are incorporated herein by reference. [Background technology]

[0003] The rapid increase in the use of fossil fuels has led to an increasing demand for alternative and clean energy, and one of the most actively researched fields in this area is power generation and storage using electrochemical reactions.

[0004] Currently, a typical example of an electrochemical element that uses electrochemical energy is a secondary battery, and the range of its use is expanding.

[0005] Meanwhile, with the development of mobile device technologies and the increase in demand, the demand for secondary batteries is rapidly increasing. Among such secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and widely used. In addition, research is being actively conducted to produce electrodes with higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries.

[0006] Generally, a secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator. In particular, the negative electrode contains a negative electrode active material, which may be silicon-based particles having a large charge / discharge capacity.

[0007] In particular, recently, with the demand for secondary batteries having high-energy electrodes, research has been actively conducted on methods for increasing the capacity by using silicon-based compounds such as Si / C, SiOx (x = 0), SiOx (0 < x < 2), etc., which have a capacity more than 10 times greater than graphite-based materials, as the negative electrode active material. However, in the case of silicon-based compounds, compared with conventional graphite-based materials, there is a problem that during the repeated charging and discharging process, the volume expands due to the generation of hydrogen gas, breaking the conductive path and thereby degrading the battery characteristics.

[0008] In order to solve the volume expansion during the repeated charging and discharging process described above, research on the composition of the binder has also been conducted, and as a result, research on binder polymers having strong stress has been carried out. 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 degradation of the performance of the lithium-ion secondary battery derived therefrom.

[0009] Also, in order to ensure the conductivity of the negative electrode, the secondary battery further includes a conductive material. Conventionally, carbon black etc. have been mainly used, but in order to improve the capacity of the secondary battery, single-walled carbon nanotubes (SWCNT) with an elongated shape are used.

[0010] However, when using single-walled carbon nanotubes as the conductive material, they must be used in the form of a dispersion liquid in order to uniformly arrange the single-walled carbon nanotubes within the active material layer. However, as the degree of dispersion of the dispersion liquid increases, the density difference between the conductive material containing the dispersion liquid and the active material increases, and therefore, a phenomenon (migration) occurs in which the conductive material moves to the upper layer from the current collector, resulting in problems such as a decrease in the phase stability and conductivity of the negative electrode composition.

[0011] Therefore, in order to improve the battery characteristics while controlling the dispersion of the SWCNT, various proposals have been discussed in terms of changing the components of the negative electrode composition.

Summary of the Invention

[0012] The inventors have found that by using single-walled carbon nanotubes (SWCNTs) as the conductive material, adopting a nonionic vinyl compound as a dispersant to control compatibility and dispersibility with the conductive material, and adopting an aqueous binder containing a specific amount of (meth)acrylamide to further improve dispersibility, it is possible to ultimately solve the problems of volume expansion of the negative electrode and cycle performance.

[0013] The present specification aims to provide a negative electrode composition for a lithium ion secondary battery in which the dispersibility of single-walled carbon nanotubes is controlled by using a specific type of dispersant and a specific content of a (meth)acrylamide-containing aqueous binder, thereby improving the problem of future volume expansion of the negative electrode, and to provide a negative electrode slurry, a negative electrode, and a lithium ion secondary battery containing the same. [Means for solving the problem]

[0014] One embodiment of the present specification provides a negative electrode composition for a lithium ion secondary battery, comprising: a negative electrode active material; a conductive material; and an aqueous binder, wherein the conductive material comprises a dispersion containing single-walled carbon nanotubes (SWCNTs) and a dispersant, the aqueous binder comprises 30 wt % to 80 wt % of (meth)acrylamide (AM) relative to 100 wt % of the total aqueous binder, and the dispersant comprises a nonionic vinyl compound.

[0015] In yet another embodiment, there is provided a negative electrode slurry comprising the negative electrode composition for a lithium ion secondary battery and a solvent.

[0016] In yet another embodiment, an electrode is provided, 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 comprises the negative electrode slurry or a dried product thereof.

[0017] Finally, there is provided a lithium ion secondary battery comprising: a first electrode; a second electrode; a separator interposed between the first electrode and the second electrode; and an electrolyte, wherein one of the first electrode and the second electrode is the negative electrode. [Effects of the Invention]

[0018] The negative electrode composition for a lithium ion secondary battery according to one embodiment of the present invention controls the dispersibility of single-walled carbon nanotubes in the conductive material, and uses an aqueous binder containing high-strength (meth)acrylamide to improve the compatibility and / or phase stability between the conductive material and the aqueous binder, thereby preventing volume expansion of the negative electrode active material, thereby providing future lithium ion secondary batteries with high performance and long life.

[0019] Furthermore, the negative electrode slurry, negative electrode, and lithium ion secondary battery according to an embodiment of the present invention contain the negative electrode composition for a lithium ion secondary battery, and therefore exhibit the above-described effects as they are, thereby ensuring the safety and stability of the lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0020] Before describing the present invention, some terms will first be defined. In this specification, when a part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified.

[0021] In this specification, "p to q" means a range of "not less than p and not more than q." In this specification, when x parts by weight is based on 100 parts by weight, it may be used interchangeably with wt%.

[0022] In this specification, when a polymer contains a certain monomer as a repeating unit, it means that the monomer participates in a polymerization reaction and is included as a repeating unit in the polymer.

[0023] In this specification, when a polymer is said to contain a monomer, this is to be interpreted as the same as saying that the polymer contains the monomer as a monomer unit.

[0024] In this specification, the term "polymer" is understood to be used in a broad sense, including copolymers, unless otherwise specified as a "homopolymer."

[0025] In this specification, the 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 (standard samples) with various degrees of polymerization commercially available for molecular weight measurement as standard substances.

[0026] In this specification, unless otherwise specified, the molecular weight means the weight average molecular weight. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail with reference to the accompanying drawings so that those skilled in the art can easily practice the present invention. However, the present invention may be embodied in many different forms and should not be construed as limited to the following description.

[0027] <Negative electrode composition for lithium ion secondary batteries> One embodiment of the present specification provides a negative electrode composition for a lithium ion secondary battery, comprising: a negative electrode active material; a conductive material; and an aqueous binder, wherein the conductive material comprises a dispersion containing single-walled carbon nanotubes (SWCNTs) and a dispersant, the aqueous binder comprises 30 wt % to 80 wt % of (meth)acrylamide (AM) relative to 100 wt % of the total aqueous binder, and the dispersant comprises a nonionic vinyl compound.

[0028] The negative electrode composition for a lithium ion secondary battery according to one embodiment of the present invention controls the dispersibility of single-walled carbon nanotubes in the conductive material, and uses an aqueous binder containing high-strength (meth)acrylamide to improve the compatibility and / or phase stability between the conductive material and the aqueous binder, thereby preventing volume expansion of the negative electrode active material, thereby providing future lithium ion secondary batteries with high performance and long life.

[0029] In the negative electrode composition for a lithium ion secondary battery according to the above embodiment, the (meth)acrylamide first repeating unit satisfies the above content range, thereby improving phase stability, improving mechanical properties, and further improving cycle and / or life performance in future batteries.

[0030] As used herein, the term "(meth)acrylamide" may include methacrylamide or acrylamide.

[0031] In this specification, the aqueous binder may further contain (meth)acrylic acid (AA) in addition to (meth)acrylamide.

[0032] In this specification, the aqueous binder may additionally contain (meth)acrylonitrile in addition to (meth)acrylamide and (meth)acrylic acid.

[0033] In this specification, the water-based binder refers to a binder based on water in a solvent or electrolyte.

[0034] In this specification, unless otherwise specified, the aqueous binder refers to a copolymer containing two or more repeating units.

[0035] In this specification, the copolymer is a concept that encompasses all of alternating copolymers, random copolymers, block copolymers, and graft copolymers, unless otherwise specified.

[0036] According to one embodiment of the present specification, the linear dispersion liquid may contain, relative to 100 parts by weight of the total single-walled carbon nanotubes, more than 0 parts by weight and 1 part by weight or less of single-walled carbon nanotubes having a length of more than 0 μm and less than 0.2 μm, and 15 parts by weight or more of single-walled carbon nanotubes having a length of 10 μm or more and less than 100 μm.

[0037] The negative electrode composition for a lithium ion secondary battery according to the above embodiment can control dispersibility and efficiently improve conductivity. Specifically, when the amount of single-walled carbon nanotubes having a length of more than 0 μm and less than 0.2 μm is 1 part by weight or less per 100 parts by weight of the total single-walled carbon nanotubes, damage to the single-walled carbon nanotubes due to excessive linear dispersion can be suppressed, and a decrease in the connectivity of the conductive network can be prevented. When the amount of single-walled carbon nanotubes having a length of 10 μm to 100 μm is 15 parts by weight or more per 100 parts by weight of the total single-walled carbon nanotubes, the volume expansion of the electrode during charging and discharging can be achieved, resulting in excellent connectivity of the conductive network, thereby improving the lifespan.

[0038] In the present specification, the linear dispersion liquid may be prepared by mixing single-walled carbon nanotubes and a dispersant (and optionally a dispersion medium, a viscosity modifier, etc.); and milling the mixture.

[0039] In this specification, the linear dispersion liquid is prepared by preparing single-walled carbon nanotubes and then preparing the linear dispersion liquid using a high-pressure homogenizer or the like.

[0040] In this specification, "100 parts by weight of total single-walled carbon nanotubes" refers to the solid content, i.e., the solid content of the dispersion. The solid content and the weight parts of each component based on the solid content can be measured by common analytical means used in the art, such as liquid chromatography or gas chromatography.

[0041] In the present specification, the milling may be performed by a milling method using a ball mill, a bead mill, a disc mill, a basket mill, or a high pressure homogenizer, and preferably, a high pressure homogenizer may be used, which can effectively achieve dispersion without causing damage to the single-walled carbon nanotubes.

[0042] In this specification, milling using a high-pressure homogenizer may be performed by, for example, pressurizing the mixture with a plunger pump of the high-pressure homogenizer and forcing it through a gap in a homogenizing valve, thereby using forces such as cavitation, shear, impact, and explosion when passing through the gap.

[0043] In this specification, the particle size of single-walled carbon nanotubes (SWCNTs) may be measured using a particle size analyzer (Malbon Co., Ltd.) that utilizes the laser diffraction method. The laser diffraction method generally enables measurement of particle sizes (especially lengths) over a wide range from submicrons to several mm, and can provide analytical results with high reproducibility and high resolution.

[0044] In one embodiment of the present specification, the negative electrode active material may include one or more selected from the group consisting of silicon-based active materials and carbon-based active materials.

[0045] In another embodiment of the present specification, the negative electrode active material may be a silicon-based active material.

[0046] Since the negative electrode composition for a lithium ion secondary battery according to the above embodiment contains a silicon-based active material as a negative electrode active material, it can have a capacity more than 10 times higher than that of the carbon-based active material. Accordingly, when applying the silicon-based active material to the negative electrode, it is possible to embody an electrode having a thickness thinner than that when the carbon-based active material is included alone and having a high level of energy density.

[0047] In another embodiment of the present specification, the negative electrode active material may include a silicon-based active material and a carbon-based active material.

[0048] In another embodiment of the present specification, when the negative electrode active material includes a silicon-based active material and a carbon-based active material, the weight ratio between the silicon-based active material and the carbon-based active material may be within a range of 2:98 to 30:70.

[0049] The negative electrode active material according to the above embodiment may further provide an effect of having a main component of a carbon-based active material, having less volume expansion of the active material during charge and discharge, less swelling, and excellent conductive connectivity of the negative electrode.

[0050] In one embodiment of the present specification, the silicon-based active material may include one or more of SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloy.

[0051] In the present specification, in the case of SiO2 where x is 2 in the SiOx, it is not included, but since this SiO2 does not react with lithium ions, it cannot store lithium. Therefore, x is preferably within the range of the above embodiment.

[0052] In the present specification, the silicon-based active material may be Si / C or Si composed of a composite of Si and C. In the present specification, two or more kinds of the silicon-based active materials may be mixed and used.

[0053] In one embodiment of the present specification, the negative electrode active material may be 60 parts by weight or more based on 100 parts by weight of the negative electrode composition for a lithium ion secondary battery.

[0054] In one embodiment of the present specification, the negative electrode active material may contain 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, and may be 95 parts by weight or less, preferably 90 parts by weight or less, based on 100 parts by weight of the negative electrode composition for a lithium ion secondary battery.

[0055] In the present specification, the silicon-based active material may exist in, for example, a crystalline or amorphous form. Specifically, the silicon particles of the silicon-based active material may preferably be spherical particles, but are not limited thereto.

[0056] In one embodiment of the present specification, when the negative electrode active material consists of a silicon-based active material alone, the silicon-based active material contains one or more selected from the group consisting of SiOx (x = 0) and SiOx (0 < x < 2), and may contain 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0057] 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, or may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0058] In another embodiment of the present invention, when the silicon-based active material contains SiOx (x = 0), the average particle size (D ,

[0058] , , ) may be 3 μm to 10 μm.

[0059] When the above range is satisfied, the active material is structurally stable during charge and discharge, and the problem of increased volume expansion / contraction due to excessively large particle size is prevented, and the problem of reduced initial efficiency due to excessively small particle size is prevented.

[0060] The particle size of the negative active material can be adjusted by methods such as, but not limited to, a ball mill, a jet mill, or an airflow distribution method.

[0061] In this specification, "D n " means particle size distribution, and refers to the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D 50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution by particle size, and D 90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D 10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. Alternatively, particle size distribution can be measured using the laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in diffraction patterns due to particle size is measured to calculate the particle size distribution.

[0062] 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.

[0063] In one embodiment of the present specification, the conductive material may be 0.03 parts by weight or more and 40 parts by weight or less based on 100 parts by weight of the negative electrode composition.

[0064] In another embodiment of the present specification, the conductive material may be included in an amount of 0.03 parts by weight or more and 40 parts by weight or less, preferably 0.05 parts by weight or more and 30 parts by weight or less, and more preferably 0.5 parts by weight or more and 25 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0065] In this specification, the conductive material and the aqueous binder contained in the negative electrode composition for a lithium ion secondary battery may refer to a negative electrode conductive material and a negative electrode aqueous binder, respectively.

[0066] In this specification, the negative electrode conductive material has a completely different structure from the positive electrode conductive material used in the positive electrode. That is, the negative electrode conductive material according to the present application controls the contact points between the silicon-based active material, which experiences a large volume expansion of the negative electrode during charge and discharge, while the positive electrode conductive material acts as a buffer during rolling and also provides some conductivity, and therefore has a completely different structure and role from the negative electrode conductive material of the present invention.

[0067] Furthermore, when the negative electrode conductive material according to the present application contains a silicon-based active material, it may have a structure that is completely different from that of a conductive material used in a graphite-based active material (i.e., a carbon-based active material containing only graphite). That is, a conductive material used in an electrode containing a graphite-based active material simply has smaller particles than the active material, and therefore has the properties of improving output characteristics and imparting some conductivity, and is completely different in structure and role from a negative electrode conductive material that is used together with a silicon-based active material as in the present invention.

[0068] In this specification, the term "carbon-based active material is used as an active material" means that the carbon-based active material is processed into a dotted or spherical shape and used as a material that stores or releases lithium.

[0069] That is, in one embodiment of the present application, the carbon-based active material, artificial graphite or natural graphite, has a dot-like shape and a BET specific surface area of ​​0.1 m 2 / g or more 4.5m 2 / g or less. The plate-shaped graphite, which is a planar conductive material, is planar and has a BET specific surface area of ​​5m 2 / g or more.

[0070] In one embodiment of the present invention, the nonionic vinyl compound may be one or more selected from the group consisting of polyvinyl pyrrolidone (PVP) and polyvinyl butyral (PVB). Preferably, the dispersant may be polyvinyl pyrrolidone (PVP).

[0071] When the dispersant is used in a negative electrode composition for a lithium ion secondary battery, the phase stability is improved during the preparation of a negative electrode slurry, thereby improving the life performance of a negative electrode including the negative electrode slurry.

[0072] In particular, among non-ionic vinyl compounds, PVP has excellent solubility in water and excellent mutual binding strength with SWCNTs. It is not sensitive to interactions with single-walled carbon nanotubes (SWCNTs) in response to pH changes due to changes in the slurry composition caused by the type and content of aqueous binders and conductive materials, and has excellent phase stability.

[0073] In another embodiment of the present invention, the dispersant may further comprise an elastomer such as hydrogenated nitrile butadiene rubber (H-NBR).

[0074] In another embodiment of the present invention, the dispersant may be present in an amount of 0.5 parts by weight or more and 5 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0075] In another embodiment of the present invention, the dispersant is used for the purpose of dispersing the SWCNT conductive material. The upper limit is not significantly limited, and it can be used to the extent that it does not affect the physical properties of the binder. An amount of 20 parts by weight or less, or 10 parts by weight or less per 100 parts by weight of the binder is suitable. The lower limit of the dispersant is suitable to be 20 parts by weight or more, 50 parts by weight or more, 100 parts by weight or more, or 150 parts by weight or more per 100 parts by weight of the SWCNT. If the dispersant is used in an amount less than 20 parts by weight per 100 parts by weight of the SWCNT, it is difficult to effectively disperse the SWCNT, and dispersion stability after dispersion may be reduced.

[0076] In one embodiment of the present invention, the water-based binder may be a copolymer comprising a first repeat unit of (meth)acrylamide (AM) and a second repeat unit of (meth)acrylic acid (AA).

[0077] When the copolymer is included in an aqueous binder, the solubility in a solvent (a solvent used in preparing a slurry) can be further improved.

[0078] In another embodiment of the present invention, when the aqueous binder is a copolymer composed of two types of repeating units of (meth)acrylamide (AM) and (meth)acrylic acid (AA), the weight ratio of AM:AA may be 30-80:20-70.

[0079] When this weight ratio is satisfied, the aqueous binder can ensure improved mechanical properties and cycle characteristics, and can control the dispersibility with single-walled carbon nanotubes (SWCNTs).Furthermore, when this weight ratio is satisfied, the aqueous binder can further improve its solubility in a solvent (e.g., water used in preparing the slurry).

[0080] In one embodiment of the present invention, the aqueous binder may be a copolymer further comprising (meth)acrylonitrile (AN) as a third repeating unit. In other words, the aqueous binder may be a copolymer comprising a first repeating unit derived from (meth)acrylamide (AM), a second repeating unit derived from (meth)acrylic acid (AA), and a third repeating unit derived from (meth)acrylonitrile (AN).

[0081] According to this embodiment, the solubility and / or dispersibility in a solvent (solvent used in preparing the slurry) can be further improved, and the adhesive properties can be further improved.

[0082] In another embodiment of the present invention, when the aqueous binder is a terpolymer containing three types of repeating units of (meth)acrylamide (AM), (meth)acrylic acid (AA), and (meth)acrylonitrile (AN), the weight ratio of AM:AA:AN may be 30-80:10-40:10-30.

[0083] A water-based binder satisfying this weight ratio can ensure improved mechanical properties and cycle characteristics, and can control the dispersibility with single-walled carbon nanotubes (SWCNTs) to improve network connectivity. Furthermore, a water-based binder satisfying this weight ratio can further improve solubility in a solvent (e.g., water used in preparing a slurry). Furthermore, satisfying this weight ratio can improve binding properties.

[0084] In the present specification, a polymerization initiator is used to prepare the aqueous binder (copolymer), and an example of the polymerization initiator may be ammonium persulfate, but is not limited thereto.

[0085] In the present specification, for producing the aqueous binder, after the copolymer is produced, it may be neutralized with an acid or base of a predetermined concentration, if necessary.

[0086] In one embodiment of the present specification, the amount of the aqueous binder may be 2 parts by weight or more and 30 parts by weight or less based on 100 parts by weight of the negative electrode composition.

[0087] In other embodiments of the present specification, the aqueous binder may be included in an amount of 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, or 2 parts by weight or more, 5 parts by weight or more, or 10 parts by weight or more, based on 100 parts by weight of the negative electrode composition.

[0088] In another embodiment of the present specification, the weight average molecular weight of the binder may be 100,000 g / mol or more and 1,500,000 g / mol or less.

[0089] The negative electrode composition according to the above embodiment may have excellent mechanical strength and intermolecular interaction due to the weight-average molecular weight of the binder satisfying the above range, thereby ensuring excellent binding strength of the negative electrode. Furthermore, when the weight-average molecular weight satisfies the above range, the viscosity of the aqueous binder may be controlled within an appropriate range, resulting in excellent coating properties when used to manufacture a negative electrode.

[0090] <Method of manufacturing a negative electrode composition for lithium ion secondary batteries> One embodiment of the present invention provides a method for producing a negative electrode composition for a lithium ion secondary battery, the method comprising the steps of: preparing a dispersion by dispersing single-walled carbon nanotubes (SWCNTs) as a conductive material and a dispersant; mixing the conductive material with an aqueous binder containing 30 wt% to 80 wt% of (meth)acrylamide (AM) relative to 100 wt% of the total aqueous binder; and adding and mixing a negative electrode active material, wherein the dispersant contains a nonionic vinyl compound.

[0091] In this specification, the linear dispersion liquid is prepared by preparing single-walled carbon nanotubes and a dispersant, and then preparing the linear dispersion liquid using a high-pressure homogenizer or the like.

[0092] The details of each component in the method for preparing the negative electrode composition for a lithium ion secondary battery are as described above.

[0093] According to the method for manufacturing a negative electrode composition for a lithium ion secondary battery according to the above embodiment, the dispersibility of the conductive material is controlled, and the structure of the single-walled carbon nanotubes formed by the dispersion is easily maintained. This improves phase stability and can improve the life characteristics of future batteries.

[0094] <Negative electrode slurry> In one embodiment of the present invention, there is provided a negative electrode slurry containing the negative electrode composition for a lithium ion secondary battery and a solvent.

[0095] In one embodiment of the present invention, the solvent may be, but is not limited to, water. In this specification, the solvent may be a solvent that can be combined with the negative electrode composition.

[0096] In some cases, the average particle size (D 50 In addition to appropriately adjusting the specific surface area of ​​the particles, controlling the viscosity of the negative electrode slurry within an appropriate range can improve the dispersion of the components (e.g., conductive material, binder, silicon-based active material, etc.) in the slurry. This improves the contact area between the components, maintains the conductive network, increases the capacity retention rate, and prevents non-uniform current density during charge / discharge.

[0097] <Negative electrode> Yet another embodiment of the present invention provides 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, the negative electrode active material layer including the negative electrode slurry or a dried product thereof.

[0098] In this specification, unless otherwise specified, the negative electrode current collector layer may be referred to as a negative electrode current collector layer.

[0099] In one embodiment of the present application, the solid content of the negative electrode slurry may be 3 wt % or more and 50 wt % or less.

[0100] In another embodiment, the solid content of the negative electrode slurry may be in the range of 3 wt % to 50 wt %, preferably 5 wt % to 45 wt %, more preferably 7 wt % to 40 wt %.

[0101] The solid content of the negative electrode slurry may refer to the content of the negative electrode composition contained in the negative electrode slurry, and may refer to the content of the negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0102] 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 the agglomeration of particles of the negative electrode composition is minimized, thereby enabling the negative electrode active material layer to be efficiently formed.

[0103] In one embodiment of the present application, the slurry solvent can be any solvent that can dissolve the negative electrode composition, and specifically, water or NMP may be used.

[0104] In one embodiment of the present 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. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. Furthermore, the surface may be provided with fine irregularities to strengthen the binding force of the negative electrode active material, and the negative electrode current collector layer may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0105] In one embodiment of the present application, there is provided a negative electrode for a lithium ion secondary battery, 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.

[0106] However, the thickness may vary depending on the type and application of the negative electrode used, and is not limited thereto.

[0107] In one embodiment of the present application, the porosity of the negative electrode active material layer may be in the range of 10% to 60%.

[0108] In another embodiment, the porosity of the negative electrode active material layer can satisfy the range of 10% to 60%, preferably 20% to 50%, more preferably 30% to 45%.

[0109] The 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 the present application have specific compositions and content parts, so that the porosity satisfies the above range. As a result, the negative electrode is characterized by having appropriate ranges of electrical conductivity and resistance.

[0110] <Lithium-ion secondary battery> In one embodiment of the present invention, there is provided a lithium ion secondary battery including: a first electrode; a second electrode; a separator interposed between the first electrode and the second electrode; and an electrolyte; wherein one of the first electrode and the second electrode is the negative electrode.

[0111] In this specification, the first electrode may be a positive electrode and the second electrode may be a negative electrode, but the opposite is also possible.

[0112] A secondary battery according to an embodiment of the present specification may include the negative electrode for a lithium-ion 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 may be the same as the negative electrode described above. Since the negative electrode has been described above, detailed description thereof will be omitted.

[0113] 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 including the positive electrode active material.

[0114] The positive electrode current collector in the positive electrode is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may typically 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 strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0115] 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; a lithium iron oxide such as LiFe3O4; 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMn 2-c3 Mc3 Examples of the lithium manganese composite oxide include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least 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 one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4, in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be metallic lithium (Li-metal).

[0116] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material described above.

[0117] In this case, the positive electrode conductive material is used to impart conductivity to the positive electrode and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and 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 powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination.

[0118] The positive electrode binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVDF), polyvinylidene 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, and various copolymers thereof. These may be used alone or in combination.

[0119] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitations. In particular, a separator with low resistance to electrolyte ion movement and excellent electrolyte moisture absorption is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, a coated separator containing a ceramic component or a polymeric material to ensure heat resistance or mechanical strength can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0120] Examples of the electrolyte include 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 manufacturing lithium secondary batteries, but are not limited to these.

[0121] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0122] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidone (NMP), 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, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0123] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferred because they are high-viscosity organic solvents with high dielectric constants and good dissociation of lithium salts. Furthermore, when such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, and thus these cyclic carbonates are more preferred.

[0124] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be 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 - One or more selected from the group consisting of:

[0125] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid 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 a decrease in the capacity of the battery, and improving the discharge capacity of the battery.

[0126] According to one embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and high cycle characteristics, and therefore may be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0127] <Method of manufacturing lithium-ion secondary batteries> In yet another embodiment of the present invention, the method for manufacturing a lithium ion secondary battery includes the steps of: mixing the negative electrode composition for a lithium ion secondary battery with a solvent to prepare a negative electrode slurry; coating the negative electrode slurry on one or both surfaces of a current collector layer; and drying the current collector layer coated with the negative electrode slurry, wherein the negative electrode composition for a lithium ion secondary battery, the negative electrode slurry, the solvent, the current collector layer, and the coating method are as described above.

[0128] In this specification, the drying is carried out by a known method such as air drying. In the present specification, the steps such as rolling that are applied after the steps included in the method for manufacturing a lithium ion secondary battery are the same as those known in the art. [Example]

[0129] In the following, preferred embodiments will be presented to aid in understanding the present invention. However, the above embodiments are merely 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 the present description and technical ideas. Naturally, such changes and modifications fall within the scope of the claims.

[0130] <Synthesis example: Synthesis of water-based binder> Synthesis Example 1 Acrylamide (AM) and acrylic acid (AA) were mixed in a weight ratio of 40:60 in a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet pipe, and then a polymerization initiator (ammonium persulfate) was added. The mixture was reacted at 75°C for 8 hours to produce a polymer aqueous solution.

[0131] Thereafter, a 0.1 molar aqueous solution of NaOH was added dropwise to the polymer aqueous solution to neutralize it, and thus a water-based binder 1 was prepared.

[0132] Synthesis Example 2 Aqueous binder 2 was produced in the same manner as in Synthesis Example 1, except that acrylamide (AM) and acrylic acid (AA) were mixed in a weight ratio of 70:30.

[0133] Synthesis Example 3 Aqueous binder 3 was produced in the same manner as in Synthesis Example 1, except that acrylamide (AM), acrylic acid (AA), and acrylonitrile (AN) were mixed in a weight ratio of 55:30:15.

[0134] Synthesis Example 4 Aqueous binder 4 was produced in the same manner as in Synthesis Example 1, except that acrylamide (AM) and acrylic acid (AA) were mixed in a weight ratio of 80:20.

[0135] Comparative synthesis example 1 Comparative binder 1 was prepared in the same manner as in Synthesis Example 1, except that acrylamide (AM) and acrylic acid (AA) were mixed in a weight ratio of 5:95.

[0136] Comparative synthesis example 2 Comparative binder 2 was prepared in the same manner as in Synthesis Example 1, except that acrylamide (AM) and acrylic acid (AA) were mixed in a weight ratio of 85:10.

[0137] Comparative synthesis example 3 Comparative binder 3 was prepared in the same manner as in Synthesis Example 1, except that acrylamide (AM), acrylic acid (AA), and acrylonitrile (AN) were mixed in a weight ratio of 25:60:15.

[0138] Comparative Synthesis Example 4 Comparative binder 4 was prepared in the same manner as in Synthesis Example 1, except that styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed in a ratio of 2:1.

[0139] <Production Examples 1 to 3: Production of Linear Dispersion Liquid> Single-walled carbon nanotubes (SWCNT) (OCSiAl, Tuball) were used as the conductive material, and polyvinylpyrrolidone (PVP) (Zhangzhou Huafu Chemical, K15), a non-ionic vinyl compound, was added as a dispersant. Tannic acid (Sigma-Aldrich) was added as a viscosity modifier. The SWCNT:PVP:tannic acid mixture was then mixed in a weight ratio of 1:1:0.4, and the mixture was then primarily dispersed using a high-shear in-line mixer (primary mixing).

[0140] Then, using a high-pressure homogenizer (Micronox, Model: PICOMAX), the mixture was repeatedly circulated while adjusting the pressure five times to produce a linear dispersion with a specific dispersion particle size and a solid content of 1 wt% (secondary mixing). The dispersion particle size of the single-walled carbon nanotubes was measured using a particle size distribution analyzer (Malvern). The results are shown in Table 1 below.

[0141] The single-walled carbon nanotubes (SWCNTs) and the dispersant were mixed in the ratio (based on the weight ratio (wt%) relative to 100 wt% of the negative electrode composition) shown in Table 1 below.

[0142] <Examples and Comparative Examples: Production of Negative Electrode> As shown in Table 1 below, a solvent (water) was added to each negative electrode composition in consideration of coating properties and solid content, and the mixture was stirred to prepare a negative electrode slurry.

[0143] [Table 1-1] [Table 1-2]

[0144] <Experimental example: Battery characteristics experiment> Each of the negative electrode slurries listed in Table 1 was coated on a copper foil having a thickness of 15 μm, and dried. A negative electrode active material layer having a thickness of 48 μm was formed on one side of the copper foil, and then the copper foil was punched into a circle having a diameter of 14 mm to prepare a test negative electrode.

[0145] A 0.3 mm thick metallic lithium foil was used as the positive electrode, a 0.1 mm thick porous polyethylene sheet was used as the separator, and the electrolyte was a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 7:3, with the lithium salts LiPF6 and LiTFSI (Lithium bis(trisfluoromethanesulfonyl)imide) dissolved in a volume ratio of 8:2 at a concentration of approximately 1 mol / L.

[0146] The negative electrode, positive electrode, separator and electrolyte were sealed in a stainless steel container to prepare a coin cell for evaluation having a thickness of 2 mm and a diameter of 32 mm.

[0147] [Table 2]

[0148] - Initial efficiency (%): The coin cell was charged at a constant current of 0.05 C until the voltage reached 0.01 V, and then discharged at a constant current of 0.05 C until the voltage reached 1.5 V. The discharge capacity and initial efficiency were calculated and expressed as (discharge capacity / charge capacity) x 100 (%).

[0149] -Capacity retention rate (%): The coin cell was charged at a constant current of 0.05C until the voltage reached 0.01V, and then discharged at a constant current of 0.05C until the voltage reached 1.5V. Then, the cycle characteristics were measured at a constant current of 0.2C within the same voltage range as above, and the capacity retention rate was tested and calculated based on 30 cycles.

[0150] As shown in Table 2, Examples 1 to 6 used an aqueous binder containing 30 wt% to 80 wt% (meth)acrylamide based on the total aqueous binder, and a nonionic vinyl compound (PVP) as a dispersant. As a result, all of the initial efficiencies were 90% or higher, and all of the capacity retention rates were 80% or higher.

[0151] In contrast, in Comparative Example 5, even though a nonionic vinyl compound was used as a dispersant, a binder was used in which the (meth)acrylamide content was outside the range of the present invention (i.e., 30 wt% to 80 wt% relative to 100 wt% of the total aqueous binder), and a high initial efficiency of 91.5% was achieved, but the capacity retention rate was 68%.

[0152] In addition, in Comparative Example 7, even though a nonionic vinyl compound was used as a dispersant, a binder that was a mixture of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) that did not contain (meth)acrylamide was used, so a high initial efficiency of 91.4% was achieved, but the capacity retention rate was 67%.

[0153] In addition, in Comparative Example 8, although an aqueous binder with a (meth)acrylamide content within the range of the present invention was used, a nonionic vinyl compound was not used as a dispersant, so a high initial efficiency of 93.2% was achieved, but the capacity retention rate was 74%.

[0154] In addition, in comparison, samples 1 to 4 and 6 not only used binders in which the (meth)acrylamide content was outside the range of 30 wt% to 80 wt% relative to 100 wt% of the total aqueous binder, but also used cellulose dispersants without nonionic vinyl dispersants. All achieved initial efficiencies higher than 90%, but showed capacity retention rates of 65%, 66%, 51%, 56%, and 71%, respectively.

[0155] Therefore, considering Comparative Examples 5 and 7, it can be seen that when the (meth)acrylamide content of the aqueous binder deviates from the range of 30 wt% or more and 80 wt% or less, the less (meth)acrylamide is contained, the greater the impact on the capacity retention rate.

[0156] Furthermore, when comparing Comparative Example 5 and Comparative Example 8, it is clear that the (meth)acrylamide content factor has a greater effect on improving the capacity retention rate than the dispersant factor, and Comparative Examples 1 to 4 and 6 show that if the above two factors are not met, the deterioration of the capacity retention rate becomes more serious.

[0157] Comparing Reference Examples 1 and 2 with Examples 1 and 4, it can be seen that differences in capacity retention appear depending on whether the particle size distribution range of the single-walled carbon nanotubes according to the present invention is satisfied or not.

Claims

1. A negative electrode composition for a lithium ion secondary battery, comprising: a negative electrode active material; a conductive material; and a water-based binder, the conductive material includes a linear dispersion containing single-walled carbon nanotubes (SWCNTs) and a dispersant; The aqueous binder contains 30 wt % to 80 wt % of (meth)acrylamide (AM) based on 100 wt % of the total aqueous binder, The negative electrode composition for a lithium ion secondary battery, wherein the dispersant contains a nonionic vinyl compound.

2. 2. The negative electrode composition for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material comprises at least one selected from the group consisting of a silicon-based active material and a carbon-based active material.

3. 3. The negative electrode composition for a lithium ion secondary battery according to claim 2, wherein the silicon-based active material comprises one or more of SiOx (x=0), SiOx (0<x<2), SiC, and a Si alloy.

4. 3. The negative electrode composition for a lithium ion secondary battery according to claim 2, wherein, when the negative electrode active material is composed solely of a silicon-based active material, the silicon-based active material comprises one or more selected from the group consisting of SiOx (x = 0) and SiOx (0 < x < 2), and the amount of SiOx (x = 0) is 70 parts by weight or more based on 100 parts by weight of the silicon-based active material.

5. 3. The negative electrode composition for a lithium ion secondary battery according to claim 2, wherein the carbon-based active material comprises at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, and soft carbon.

6. 3. The negative electrode composition for lithium ion secondary batteries according to claim 2, wherein when the negative electrode active material comprises a silicon-based active material and a carbon-based active material, a weight ratio between the silicon-based active material and the carbon-based active material is in a range of 2:98 to 30:

70.

7. 2. The negative electrode composition for a lithium ion secondary battery according to claim 1, wherein the nonionic vinyl-based compound comprises at least one selected from the group consisting of polyvinyl pyrrolidone (PVP) and polyvinyl butyral (PVB).

8. 2. The negative electrode composition for a lithium ion secondary battery according to claim 1, wherein the aqueous binder is a copolymer containing (meth)acrylamide (AM) as a first repeating unit and (meth)acrylic acid (AA) as a second repeating unit.

9. The negative electrode composition for a lithium ion secondary battery according to claim 8 , wherein the aqueous binder is a copolymer further containing (meth)acrylonitrile (AN) as a third repeating unit.

10. A negative electrode slurry comprising the negative electrode composition for a lithium ion secondary battery according to any one of claims 1 to 9 and a solvent.

11. 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, the negative electrode active material layer comprising the negative electrode slurry according to claim 10 or a dried product thereof.

12. first electrode; second electrode; a separator interposed between the first electrode and the second electrode; and Electrolytes; A lithium ion secondary battery comprising: A lithium ion secondary battery, wherein one of the first electrode and the second electrode is the negative electrode according to claim 11 .

Images