Negative electrode composition, negative electrode for lithium secondary battery comprising negative electrode composition, and lithium secondary battery comprising same

Abstract

The present invention relates to a negative electrode composition, a negative electrode for a lithium secondary battery comprising the negative electrode composition, and a lithium secondary battery comprising the same, the present invention improves the energy density of a lithium secondary battery by using a negative electrode active material containing a silicon oxide containing Si nanocrystals having an average particle diameter (D50) of 0.1-5 nm in combination with a negative electrode conductive material and a negative electrode aqueous binder in an appropriate composition, and reduces the degree of deterioration during operation of the lithium secondary battery. And the service life characteristic can be further improved.

Description

Technical Field

[0001] The present invention relates to a negative electrode composition, a negative electrode for a lithium secondary battery comprising the negative electrode composition, and a lithium secondary battery comprising the negative electrode composition.

[0002] This application claims priority to and the benefit of Korean Patent Application Nos. 10-2023-0114515 and 10-2024-0116069, filed on August 30, 2023, and August 28, 2024, respectively, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative or clean energy is increasing, and as part of this trend, the most active research area is the field of power generation and storage using electrochemical reactions.

[0004] Currently, representative examples of electrochemical devices using such electrochemical energy include secondary batteries, and their fields of use are increasingly expanding.

[0005] With the technological development and increase in demand for mobile devices, the demand for secondary batteries as energy sources has increased rapidly. Among such secondary batteries, lithium secondary batteries with high energy density and voltage, long cycle life and low self-discharge rate have been commercialized and widely used. In addition, research on methods for preparing high-density electrodes with higher energy density per unit volume as electrodes for the above-mentioned high-capacity lithium secondary batteries is being actively carried out, and there is a trend towards higher and higher loads in order to improve energy density. However, since the electrode resistance and battery resistance also increase accordingly, it is difficult to ensure performance such as high output and fast charging. In order to solve this problem, attempts have been made to physically expand the reaction surface by manufacturing patterned electrodes to cause a decrease in resistance, and the patterned electrodes induce reactions not only on the electrode surface but also in the depth direction, thereby improving performance.

[0006] Generally speaking, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material for inserting and extracting lithium ions from the positive electrode. As the negative electrode active material, silicon-based particles with high discharge capacity can be used.

[0007] Although graphite is commonly used as a negative electrode active material for lithium secondary batteries, its low mass capacity of 372 mAh / g makes it difficult to increase the capacity of lithium secondary batteries. Therefore, to increase the capacity of lithium secondary batteries, non-carbon-based negative electrode materials such as silicon, tin, and their oxides have been developed as negative electrode materials with higher energy densities than graphite. However, despite their high capacity, these non-carbon-based negative electrode materials suffer from low initial efficiency, resulting in high lithium consumption and irreversible capacity loss during initial charge and discharge.

[0008] In addition, lithium secondary batteries have a size required by their use and need to be designed within a limited space. Although consumer demand for increased energy density and improved high-output performance is increasing, when using high-capacity positive electrode materials, the content of negative electrode materials has to be increased to match the demand, which limits the improvement of battery efficiency within a limited space. Therefore, it is necessary to develop a battery with improved performance such as efficiency and service life within a limited space.

[0009] [Related technical literature]

[0010] [Patent Document]

[0011] (Patent Document 1) Korean Patent Application Publication No. 10-2023-0050258 Summary of the Invention

[0012] [Technical Issues]

[0013] The present invention relates to a negative electrode composition capable of maximally improving the energy density and service life of a lithium secondary battery, a negative electrode for a lithium secondary battery comprising the negative electrode composition, and a lithium secondary battery comprising the negative electrode composition.

[0014] [Technical solution]

[0015] An exemplary embodiment of the present invention provides a negative electrode composition, the negative electrode composition comprising: a negative electrode active material comprising silicon oxide; a negative electrode conductive material; and a negative electrode aqueous binder,

[0016] Based on 100 parts by weight of the solid content of the negative electrode composition, the content of the negative electrode active material is 70 parts by weight or more, the content of the negative electrode conductive material is 0.3 parts by weight or more, and the content of the negative electrode aqueous binder is 9 parts by weight or more.

[0017] The negative electrode conductive material includes a linear conductive material, and the content of the linear conductive material is 0.3 parts by weight or more and 3 parts by weight or less based on 100 parts by weight of the solid content of the negative electrode composition.

[0018] The silicon oxide comprises Si nanocrystals, and

[0019] The average particle size (D50) of the Si nanocrystals is greater than or equal to 0.1 nm and less than or equal to 5 nm.

[0020] An exemplary embodiment of the present invention provides a negative electrode for a lithium secondary battery, comprising a negative electrode active material layer on at least one surface of a negative electrode current collector layer, the negative electrode active material layer comprising the negative electrode composition or a cured product thereof.

[0021] An exemplary embodiment of the present invention provides a lithium secondary battery, the lithium secondary battery comprising:

[0022] positive electrode;

[0023] A negative electrode for the above-mentioned lithium secondary battery;

[0024] a separator disposed between the positive electrode and the negative electrode; and

[0025] electrolytes.

[0026] Another exemplary embodiment of the present invention provides a battery module or a battery pack including the above-mentioned lithium secondary battery.

[0027] Finally, yet another exemplary embodiment of the present invention provides a battery pack including the above-mentioned battery module.

[0028] [Beneficial Effects]

[0029] According to one embodiment of the present invention, the negative electrode composition uses a silicon oxide with a high specific capacity in combination with a linear conductive material and a negative electrode aqueous binder in an appropriate composition, which can maximize the improvement of the energy density and cycle performance of the lithium secondary battery, and by using silicon oxide containing Si nanocrystals with a small average particle size (D50), the degree of degradation during the operation of the lithium secondary battery is reduced, and the service life characteristics can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 FIG. 1 is a diagram illustrating a stacked structure of a negative electrode for a lithium secondary battery according to an exemplary embodiment of the present invention.

[0031] Figure 2 is a diagram illustrating a stack structure of a lithium secondary battery according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0032] Before describing the present invention, some terms are first defined.

[0033] In the present specification, when a part “comprises” one constituent element, unless otherwise specifically stated, this does not mean that other constituent elements are excluded but means that other constituent elements may further be included.

[0034] In this specification, description that one member is placed “on” another member includes not only a case where the one member is adjacent to the other member but also a case where other members exist between the two members.

[0035] In this specification, “p to q” means a range of “p or more and q or less”.

[0036] In this specification, "specific surface area" is measured by the BET method, specifically, using a BELSORP-mini II manufactured by BEL Japan Co., Ltd., and is calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K). That is, in this specification, the BET specific surface area may refer to the specific surface area measured by this measurement method.

[0037] In this specification, "Dn" means particle size distribution, and means the particle diameter at n% point according to the cumulative distribution of the number of particles of the particle diameter. That is, D50 is the particle diameter (average particle diameter) at 50% point according to the cumulative distribution of the number of particles of the particle diameter, D90 is the particle diameter at 90% point according to the cumulative distribution of the number of particles of the particle diameter, and D10 is the particle diameter at 10% point according to the cumulative distribution of the number of particles of the particle diameter. On the other hand, the average particle diameter can be measured using a laser diffraction method. The laser diffraction method is generally capable of measuring particle diameters from the submicron region to about several mm, and can obtain results with high reproducibility and high resolution.

[0038] The determination of the average particle size can be confirmed using a Microtrac device (manufacturer: Microtrac, model: S3500) using water and Triton-X100 dispersant. Specifically, the average particle size of the positive electrode active material can be measured in the range of a refractive index of 1.5 to 1.7, and the average particle size of the negative electrode active material can be measured under the condition of a refractive index of 1.97 or 2.42. For example, after the particles are dispersed in a dispersion medium, the resulting dispersion is introduced into a commercially available laser diffraction particle size measuring device, and the dispersion is irradiated with an ultrasonic wave of about 28 kHz and an output of 60 W, and then a volume cumulative particle size distribution diagram is obtained, and then the average particle size can be determined by obtaining a particle size corresponding to 50% of the volume cumulative amount.

[0039] In an exemplary embodiment of the present specification, the average particle size (D50) of the Si nanocrystals can be determined by X-ray diffraction analysis, and the X-ray diffraction analysis can be performed using an X-ray diffraction (XRD) analyzer (trade name: D4-endavor, manufacturer: Bruker). Specifically, the powder sample can be sampled into a holder and XRD measurement can be performed using Cu K α X-rays. The size of the Si nanocrystals can be calculated by fitting the XRD results using the Scherrer equation, and in this case, the size of the Si nanocrystals can be measured based on Si (220) (2θ = 47.5° to 48.5°).

[0040] In this specification, the particle size or particle diameter may mean an average diameter or a representative diameter of individual particles forming the metal powder.

[0041] In this specification, a polymer comprising a monomer as a monomer unit means that the monomer participates in a polymerization reaction and is thereby contained in the polymer as a repeating unit. In this specification, when a polymer comprises a monomer, the interpretation is the same as when the polymer comprises a monomer as a monomer unit.

[0042] In this specification, "polymer" should be understood to be used in a broad sense to include copolymers, unless otherwise specified as "homopolymer".

[0043] The terms or words used in the specification should not be construed as limited to the typical or dictionary meanings, but should be construed using meanings and concepts consistent with the technical gist of the present invention based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his / her own invention.

[0044] Singular expressions of the terms used in this specification include plural expressions unless they have clearly contrary meanings in the context.

[0045] Hereinafter, preferred exemplary embodiments of the present invention will be described in detail. However, the exemplary embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the exemplary embodiments to be described below.

[0046] <Negative Electrode Composition>

[0047] A negative electrode composition according to an exemplary embodiment of the present specification is a negative electrode composition including: a negative electrode active material; a negative electrode conductive material; and a negative electrode aqueous binder, wherein, based on 100 parts by weight of the solid content of the negative electrode composition, the content of the negative electrode active material is 70 parts by weight or more, the content of the negative electrode conductive material is 0.3 parts by weight or more, and the content of the negative electrode aqueous binder is 9 parts by weight or more, the negative electrode conductive material includes a linear conductive material, and the content of the linear conductive material is 0.3 parts by weight or more and 3 parts by weight or less, based on 100 parts by weight of the solid content of the negative electrode composition, the silicon oxide includes Si nanocrystals, and the average particle size (D50) of the Si nanocrystals is 0.1 nm or more and 5 nm or less.

[0048] Because silicon oxide itself has a high capacity, it can be difficult to achieve a capacity balance with the positive electrode active material when used in excess. Furthermore, due to the material properties of silicon oxide, the larger the average particle size (D50) of the Si nanocrystals, the more lithium (Li) remains in the material, which can cause problems such as poor cycling performance and severe swelling.

[0049] Therefore, the present invention is characterized in that the contents of the negative electrode conductive material and the negative electrode binder can be appropriately combined, thereby ensuring energy density by including an excess of silicon oxide as the negative electrode active material, while suppressing volume expansion during charge and discharge, thereby also ensuring cycle performance. In particular, by maintaining the average particle size (D50) of the Si nanocrystals dispersed in the silicon oxide of the present invention to be as small as 0.1 nm to 5 nm, the stress caused by volume expansion of the negative electrode active material particles during charge and discharge can be reduced, thereby further improving cycle performance and suppressing expansion due to the prevention of particle breakage.

[0050] According to one embodiment of the present specification, the negative electrode composition may include 70 parts by weight or more of the negative electrode active material including silicon oxide based on 100 parts by weight of the solid content of the negative electrode composition.

[0051] In this specification, “solid content” refers to the content of components other than a solvent (such as water) contained in the negative electrode composition.

[0052] In an exemplary embodiment of the present specification, the content of the negative electrode active material including silicon oxide may be 70 to 99 parts by weight, specifically 70 to 90 parts by weight, and more specifically 80 to 89.6 parts by weight, based on 100 parts by weight of the solid content of the negative electrode composition in total.

[0053] In an exemplary embodiment of the present specification, the negative electrode active material may include the silicon oxide, and specifically may be composed of the silicon oxide.

[0054] That is, in an exemplary embodiment of the present specification, based on a total of 100 parts by weight of the solid content of the negative electrode composition, the content of the silicon oxide may be 70 to 99 parts by weight, specifically 70 to 90 parts by weight, and more specifically 80 to 89.6 parts by weight.

[0055] In the present specification, "silicon oxide" includes amorphous SiO x (0 < x < 2), and may include nanosized Si crystals randomly dispersed therein. The randomly dispersed nanosized Si crystals may be represented as "Si nanocrystals" according to the present invention.

[0056] Originally, in the case of including an excessive amount of silicon oxide as the negative electrode active material, there is a problem of poor battery cycle characteristics due to volume expansion during charge and discharge and a relatively reduced content of the negative electrode conductive material and the negative electrode binder. However, the negative electrode composition according to the present specification improves the energy density of the electrode by including an excessive amount of silicon oxide, and at the same time derives the optimal content of the following appropriate combination of the negative electrode conductive material and the negative electrode aqueous binder, thereby maximizing the energy density and service life performance of the battery.

[0057] According to an exemplary embodiment of the present specification, the silicon oxide may include SiO x (0 < x < 2). In this case, the SiO x (0 < x < 2) corresponds to the amorphous matrix in the silicon oxide particles. The SiO x (0 < x < 2) may be in a form including some Si and SiO2, and the Si may also form a phase. That is, the x corresponds to the ratio of O to Si contained in the SiO x (0 < x < 2). When the silicon oxide particles include the SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0058] In an exemplary embodiment of the present specification, the silicon oxide may include Si nanocrystals, and the average particle diameter (D50) of the Si nanocrystals may be 0.1 nm or more and 5 nm or less. In an exemplary embodiment of the present specification, the average particle diameter (D50) of the Si nanocrystals of the silicon oxide may be 5 nm or less, specifically 3 nm or less.

[0059] In addition, in an exemplary embodiment of the present specification, the average particle size (D50) of the Si nanocrystals contained in the silicon oxide may be 0.1 nm or more, specifically 0.5 nm or more, and more specifically greater than 1 nm.

[0060] When the average particle size (D50) of the Si nanoparticles satisfies the above range, Li ions diffuse uniformly within the Si particles, thereby having the effect of stably maintaining the structure of the negative electrode active material particles during charge and discharge. Conversely, when the average particle size (D50) of the Si nanoparticles exceeds the above range, stress is generated due to the shrinkage / expansion of the Si nanoparticles during charge and discharge, which may cause cracks in the negative electrode active material particles. Li ions cannot diffuse into the interior of the Si nanoparticles, resulting in uneven reaction. This may accelerate the degradation of the battery due to the uneven reaction, thereby reducing the service life.

[0061] In essence, the silicon oxide negative electrode active material involves very complex crystal changes in the reaction of electrochemical absorption, storage and release of lithium atoms. As the reaction of electrochemical absorption, storage and release of lithium atoms proceeds, the composition and crystal structure of Si particles change to Si (crystal structure: Fd3m), LiSi (crystal structure: I41 / a), Li2Si (crystal structure: C2 / m), Li7Si2 (Pbam), Li 22 Si5 (F23), etc., and due to the complex changes in the crystal structure, the volume of the Si particles expands by about 4 times. Accordingly, when the charge and discharge cycles are repeated, the Si particles are destroyed, and as the bonds between the lithium atoms and the Si particles are formed, the insertion sites of the lithium atoms initially possessed by the Si particles are destroyed, and as a result, the cycle life may be significantly deteriorated. That is, when the average particle size (D50) of the Si nanocrystals according to an exemplary embodiment of the present specification exceeds 5nm, when the lithium secondary battery is charged, in the process of the volume expansion of the Si particles due to the reaction with lithium, stress is caused to the surrounding materials in the negative electrode, resulting in problems such as degradation of battery performance such as shortened battery life. Therefore, the Si nanocrystals preferably meet the above-mentioned maximum average particle size (D50) range.

[0062] In addition, the minimum average particle size (D50) of the Si nanoparticles according to an exemplary embodiment of the present specification may be 0.1 nm. When the average particle size of the Si nanoparticles is less than 0.1 nm, the energy density of the negative electrode may be reduced.

[0063] Although the negative electrode composition according to an exemplary embodiment of the present specification can sufficiently improve energy density and capacity using a negative electrode active material including silicon oxide, silicon oxide has limitations: the volume of silicon oxide rapidly expands during charge and discharge, and the conductive path formed in the negative electrode active material layer is damaged, thereby reducing the performance of the battery, so the type of negative electrode conductive material used with the negative electrode active material is important.

[0064] The negative electrode composition according to an exemplary embodiment of the present specification may include 0.3 parts by weight or more of the negative electrode conductive material based on 100 parts by weight of the solid content of the negative electrode composition.

[0065] Specifically, in an exemplary embodiment of the present specification, the content of the negative electrode conductive material may be 0.3 parts by weight or more and 11 parts by weight or less, more specifically 0.4 parts by weight or more and 10 parts by weight or less, based on 100 parts by weight of the solid content of the negative electrode composition.

[0066] When the content of the negative electrode conductive material according to an exemplary embodiment of the present specification is lower than the lower limit of the above content range, it may be difficult to control the distortion of the electrode caused by the volume expansion of silicon oxide or to suppress the disconnection of the conductive path caused by the volume expansion. When the content of the negative electrode conductive material is higher than the upper limit of the above content range, the content of the negative electrode active material and the negative electrode binder is relatively reduced, so that the energy density may be reduced or the capacity may be degraded. Therefore, the negative electrode conductive material according to an exemplary embodiment of the present invention preferably meets the above content range.

[0067] In this case, the content of the negative electrode conductive material may be parts by weight of the negative electrode conductive material including only the linear conductive material and an additional solvent, or may be the content of the negative electrode conductive material including all of the linear conductive material and an additional planar or granular conductive material.

[0068] In an exemplary embodiment of the present specification, the negative electrode conductive material may include a linear conductive material in an amount of 0.3 to 3 parts by weight based on 100 parts by weight of a solid content of the negative electrode composition.

[0069] In this specification, "wire-shaped conductive material" refers to a conductive material having a one-dimensional (1D) structure with a diameter at the nanometer unit level and a high aspect ratio, or refers to a conductive material having a fibrous structure such as a cylindrical or tubular type. Examples of the wire-shaped conductive material include carbon nanotubes, etc., and the carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain a plurality of carbon nanotube units. Specifically, unless otherwise specified, the term "bundle type" used herein refers to a secondary shape in the form of a bundle or rope in which a plurality of carbon nanotube units are arranged or wound side by side with the longitudinal axes of the carbon nanotube units being substantially the same orientation. In the carbon nanotube unit, the graphene sheet has a cylindrical shape with a nanometer-sized diameter and has sp 2 In this case, the carbon nanotube unit can exhibit conductor or semiconductor properties depending on the structure and angle of the graphite sheet winding. Compared with entangled carbon nanotubes, the bundled carbon nanotubes can be evenly dispersed during the preparation of the negative electrode and can improve the conductivity of the negative electrode by smoothly forming a conductive network in the negative electrode.

[0070] Furthermore, by requiring the negative electrode conductive material according to an exemplary embodiment of the present specification to include a linear conductive material, electrode distortion caused by volume expansion of silicon oxide contained in the negative electrode active material can be controlled, thereby maintaining the initial cycle characteristics of the battery. However, since maintaining the cycle characteristics during repeated battery charge and discharge is difficult using only the linear conductive material according to an exemplary embodiment of the present invention, the present invention attempts to address this issue by combining the linear conductive material with a negative electrode aqueous binder at an optimal content ratio, as described below.

[0071] In an exemplary embodiment of the present specification, the content of the linear conductive material in the negative electrode conductive material may be 0.3 parts by weight or more and 3 parts by weight or less, specifically 0.4 parts by weight or more and 3 parts by weight or less, and more specifically 0.4 parts 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 composition in total.

[0072] When the content of the linear conductive material according to an exemplary embodiment of the present specification is lower than the lower limit of the above content range, rapid capacity degradation may occur in the initial cycle of the battery, and when the content of the linear conductive material is higher than the upper limit of the above content range, excessive inclusion of the linear conductive material does not lead to better performance. On the contrary, there is a defect that the use of expensive linear conductive material only increases manufacturing costs.

[0073] Therefore, the content of the linear conductive material according to an exemplary embodiment of the present invention preferably satisfies the above range. However, when the content of the negative electrode aqueous binder is reduced and the content of the negative electrode conductive material is excessive, although the initial cycle characteristics can be maintained, the battery capacity may rapidly deteriorate after 200 cycles. Therefore, it is preferable to prevent battery degradation by including an appropriate content of the negative electrode aqueous binder as described below.

[0074] In an exemplary embodiment of the present specification, the linear conductive material may include single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), and more specifically, may include single-walled carbon nanotubes (SWCNTs).

[0075] In an exemplary embodiment of the present specification, the BET specific surface area of the linear conductive material may be 100 m 2 / g and above 100,000m 2 / g or less, specifically 500m 2 / g or more and 10,000m 2 / g or less, and more specifically 1,000m 2 / g or more and 5,000m 2 / g or less.

[0076] In an exemplary embodiment of the present specification, the linear conductive material may have an aspect ratio of 500 or greater, specifically 1,000 or greater, and more specifically 10,000 or greater, and an aspect ratio of 1,000,000 or less, specifically 100,000 or less.

[0077] The negative electrode composition according to an exemplary embodiment of the present specification may include 9 parts by weight or more of a negative electrode aqueous binder based on 100 parts by weight of a solid content of the negative electrode composition.

[0078] In an exemplary embodiment of the present specification, the content of the negative electrode aqueous binder may be 9 parts by weight or more, specifically 9.4 parts by weight or more, and more specifically 10 parts by weight or more, based on 100 parts by weight of the total solid content of the negative electrode composition.

[0079] In an exemplary embodiment of the present specification, the content of the negative electrode aqueous binder may be 20 parts by weight or less, specifically 18 parts by weight or less, and more specifically 15 parts by weight or less, based on 100 parts by weight of the total solid content of the negative electrode composition.

[0080] In an exemplary embodiment of the present specification, the content of the negative electrode aqueous binder may be 9 parts by weight or more and 20 parts by weight or less, specifically 9.4 parts by weight or more and 18 parts by weight or less, and more specifically 10 parts by weight or more and 15 parts by weight or less, based on 100 parts by weight of the total solid content of the negative electrode composition.

[0081] When the negative electrode aqueous binder according to one embodiment of the present specification meets the above-mentioned content range, it can capture silicon oxide, which undergoes large volume expansion during charge and discharge, thereby improving the battery's service life performance. In particular, when the content of the negative electrode aqueous binder according to the present invention is less than the lower limit of the above-mentioned range, even if the amount of the negative electrode conductive material, especially the linear conductive material, added according to the present invention is at least twice the normal amount, the negative electrode conductive material may only temporarily suppress the electrode distortion caused by the volume change of the silicon oxide active material in the initial stage of the cycle and may not play a corresponding role throughout the entire cycle like the negative electrode aqueous binder according to the present invention, so the battery performance cannot be restored. In addition, when the content of the negative electrode aqueous binder exceeds the upper limit of the above-mentioned range, the content of the negative electrode active material and the negative electrode conductive material is relatively reduced, which may cause a decrease in energy density or capacity degradation.

[0082] Therefore, when the negative electrode conductive material and the negative electrode aqueous binder according to an exemplary embodiment of the present invention satisfy the above content ranges, even if the content of the negative electrode active material containing silicon oxide according to the present invention is too high, the energy density of the electrode can be maximized while improving the service life performance.

[0083] The negative electrode aqueous binder according to an exemplary embodiment of the present specification plays the role of capturing the negative electrode active material and the negative electrode conductive material in order to prevent the distortion and structural deformation of the negative electrode structure during the volume expansion and relaxation of the silicon oxide active material. Although all common negative electrode aqueous binders that meet the above-mentioned role can be applied, specifically, the negative electrode aqueous binder can be a polyacrylamide (PAM)-based binder.

[0084] Compared with carbon-based active materials (especially graphite), the volume expansion of silicon oxide used as the negative electrode active material is large, so that the negative electrode conductive network may be degraded during charge and discharge, and in the case of existing SBR / CMC binders as binders, the mechanical rigidity is low, which may lead to serious problems such as expansion and battery performance degradation during charge and discharge.

[0085] In contrast, when a PAM-based binder is applied to silicon oxide as in the present invention, mechanical rigidity is excellent compared to existing binders, thereby achieving excellent conductive connectivity during charge and discharge, and expansion properties can be suppressed.

[0086] In an exemplary embodiment of the present specification, the PAM-based adhesive refers to a main component composed of a poly(meth)acrylamide-based monomer, and may further contain poly(meth)acrylic acid (PAA)-based, polyvinyl alcohol (PVA)-based, poly(meth)acrylonitrile (PAN)-based monomers as other components.

[0087] In the present specification, a polymerization initiator is used to prepare the PAM-based adhesive, and ammonium persulfate may be used as an example of the polymerization initiator, but the polymerization initiator is not limited thereto.

[0088] In this specification, the adhesive containing a plurality of compounds having a specific ratio (expressed by parts by weight or weight ratio) may mean that each compound (eg, acrylamide, acrylic acid, and acrylonitrile) is contained as a monomer for the adhesive polymer.

[0089] In this specification, the adhesive contains a plurality of compounds as monomers, and the monomer with the largest content is regarded as a representative, and thus may be named as a "monomer" type compound.

[0090] In the present specification, "(meth)acrylic..." may refer to methacrylic and / or acrylic.

[0091] In an exemplary embodiment of the present specification, when the PAM-based adhesive includes a subsidiary component as an additional monomer to form a copolymer, the ratio of each monomer is not particularly limited as long as the adhesive is a desired water-based adhesive.

[0092] In an exemplary embodiment of the present specification, based on 100 parts by weight of the solid content of the negative electrode composition, the contents of the negative electrode active material, the negative electrode conductive material, and the negative electrode aqueous binder in the negative electrode composition may be 70 parts by weight or more and 99 parts by weight or less, 0.3 parts by weight or more and 11 parts by weight or less, and 9 parts by weight or more and 20 parts by weight or less, respectively.

[0093] Specifically, based on 100 parts by weight of the solid content of the negative electrode composition, the contents of the negative electrode active material, the negative electrode conductive material and the negative electrode aqueous binder in the negative electrode composition may be greater than 70 parts by weight and less than 90 parts by weight, greater than 0.3 parts by weight and less than 11 parts by weight, and greater than 9 parts by weight and less than 20 parts by weight, respectively.

[0094] More specifically, based on 100 parts by weight of the solid content of the negative electrode composition, the contents of the negative electrode active material, the negative electrode conductive material and the negative electrode aqueous binder in the negative electrode composition may be greater than 80 parts by weight and less than 89.6 parts by weight, greater than 0.4 parts by weight and less than 10 parts by weight, and greater than 9.4 parts by weight and less than 18 parts by weight, respectively.

[0095] In an exemplary embodiment of the present specification, based on 100 parts by weight of the solid content of the negative electrode composition, the content of the silicon oxide, the negative electrode conductive material, and the negative electrode aqueous binder in the negative electrode composition may be 70 parts by weight or more and 99 parts by weight or less, 0.3 parts by weight or more and 11 parts by weight or less, and 9 parts by weight or more and 20 parts by weight or less, respectively.

[0096] Specifically, based on 100 parts by weight of the solid content of the negative electrode composition, the contents of the silicon oxide, the negative electrode conductive material and the negative electrode aqueous binder in the negative electrode composition may be greater than 70 parts by weight and less than 90 parts by weight, greater than 0.3 parts by weight and less than 11 parts by weight, and greater than 9 parts by weight and less than 20 parts by weight, respectively.

[0097] More specifically, based on 100 parts by weight of the solid content of the negative electrode composition, the contents of the silicon oxide, the negative electrode conductive material and the negative electrode aqueous binder in the negative electrode composition may be greater than 80 parts by weight and less than 89.6 parts by weight, greater than 0.4 parts by weight and less than 10 parts by weight, and greater than 9.4 parts by weight and less than 18 parts by weight, respectively.

[0098] When the composition of the negative electrode composition according to an exemplary embodiment of the present specification satisfies the above combination range, the energy density and cycle performance of the lithium secondary battery may be improved to the maximum extent.

[0099] On the basis of applying the above contents, the contents described below may be applied to the negative electrode composition according to an exemplary embodiment of the present specification.

[0100] In the negative electrode composition according to an exemplary embodiment of the present specification, the Si nanocrystals contained in the silicon oxide can be formed by heat treatment at 500° C. to 1500° C. during the preparation of the negative electrode active material, and can be formed by heat treatment at 600° C. to 1300° C., more specifically 700° C. to 1200° C. When the negative electrode active material is prepared under temperature conditions outside the above range, the Si nanocrystals dispersed in the silicon oxide may grow to exceed the average particle size (D50) range of the Si nanocrystals according to the present specification, and thus the above problem may occur. When the temperature is too low, it may be difficult to prepare the desired negative electrode active material.

[0101] In one exemplary embodiment of the present specification, a carbon layer may be provided on at least a portion of the surface of the silicon oxide. In this case, the carbon layer may cover at least a portion of the surface, i.e., partially cover the surface of the silicon oxide, or cover the entire surface of the silicon oxide. When the carbon layer is provided on at least a portion of the surface of the silicon oxide, conductivity is imparted to the negative electrode active material, and the initial efficiency, service life characteristics, and battery capacity characteristics of the secondary battery may be improved.

[0102] In an exemplary embodiment of the present specification, the carbon layer may include amorphous carbon, and the carbon layer may further include crystalline carbon.

[0103] In an exemplary embodiment of the present specification, the crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene, and the crystalline carbon may further improve conductivity of the negative electrode active material.

[0104] In an exemplary embodiment of the present specification, the amorphous carbon may be a carbide of at least one selected from the group consisting of tar, pitch, and other organic materials, or may be a carbon-based material formed using hydrocarbons as a source in a chemical vapor deposition process, and the amorphous carbon may suppress the expansion of the silicon oxide by appropriately maintaining the strength of the carbon layer.

[0105] In the present specification, the carbide of other organic materials may be carbide of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose, and carbide of an organic material selected from a combination thereof.

[0106] In this specification, the hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon in the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane, etc. Examples of the aromatic hydrocarbon in the substituted or unsubstituted aromatic hydrocarbon include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, benzofuran, pyridine, anthracene, phenanthrene, etc.

[0107] In an exemplary embodiment of this specification, the carbon layer may be an amorphous carbon layer.

[0108] In an exemplary embodiment of this specification, based on 100 parts by weight of the total silicon oxide, the content of the carbon layer may be 0.1 part by weight to 50 parts by weight, 0.1 part by weight to 30 parts by weight, or 0.1 part by weight to 20 parts by weight. More specifically, the content of the carbon layer may be 0.5 part by weight to 15 parts by weight, 1 part by weight to 10 parts by weight, or 1 part by weight to 5 parts by weight. When the content of the carbon layer satisfies the above range, reduction in the capacity and efficiency of the negative electrode active material can be prevented.

[0109] In an exemplary embodiment of this specification, the thickness of the carbon layer may be 1 nm to 500 nm, specifically 5 nm to 300 nm. When the thickness of the carbon layer satisfies the above range, the conductivity of the negative electrode active material is improved, volume change of the negative electrode active material is easily suppressed, and side reactions between the electrolyte and the negative electrode active material are suppressed, thereby having the effect of improving the initial efficiency and / or service life of the battery.

[0110] In an exemplary embodiment of this specification, at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene may be used to form the carbon layer by chemical vapor deposition (CVD) method.

[0111] In an exemplary embodiment of this specification, the silicon oxide may further contain metal impurities. The metal impurities are impurities that may be contained in silicon, and based on 100 parts by weight of all silicon-based active materials, the content thereof may satisfy the range of 0.1 part by weight or less.

[0112] On the other hand, in an exemplary embodiment of this specification, the average particle diameter (D50) of the negative electrode active material containing silicon oxide may be 1 μm to 15 μm, specifically 2 μm to 12.5 μm, and more specifically 5 μm to 10 μm. In this case, the average particle diameter (D50) of the negative electrode active material is based on the average particle diameter (D50) of the final particles containing both SiO x (0 < x < 2) and Si nanocrystals.

[0113] When the average particle size (D50) of the negative electrode active material satisfies the above range, the specific surface area of the particles falls within a suitable range, and the viscosity of the negative electrode composition can be formed within a suitable range, thereby facilitating the dispersion of the particles constituting the negative electrode composition. Furthermore, since the composite of the negative electrode conductive material and the negative electrode binder in the negative electrode composition provides excellent contact area between the silicon oxide and the negative electrode conductive material, the conductive network is more likely to be maintained, thereby improving capacity retention. Excessive silicon oxide can be eliminated, resulting in a smooth negative electrode surface, thereby preventing uneven current density during charge and discharge.

[0114] In the negative electrode composition according to an exemplary embodiment of the present specification, the negative electrode conductive material may further include one or more selected from the group consisting of planar conductive materials and granular conductive materials.

[0115] In an exemplary embodiment of the present specification, the negative electrode conductive material may further include a planar conductive material.

[0116] In this specification, “planar conductive material” refers to a conductive material having a two-dimensional (2D) structure, in which atoms have a thickness of a single atomic layer or multiple (e.g., two or more) atomic layers, while forming a crystalline structure on a plane. The planar conductive material refers to a material for ensuring a conductive path in a planar shape in the negative electrode active material layer and at the same time suppressing the disconnection of the conductive path due to volume expansion, and can be expressed as a plate-like conductive material or a bulk conductive material. Specifically, the planar conductive material may include at least one selected from the group consisting of plate-like graphite, graphene, graphene oxide, and graphite flakes, and may preferably be plate-like graphite.

[0117] In an exemplary embodiment of the present specification, the average particle size (D50) of the planar conductive material may be 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. When the average particle size (D50) of the planar conductive material satisfies the above range, sufficient particle size facilitates dispersion without excessively increasing the viscosity of the negative electrode slurry. Therefore, when the particles are dispersed using the same equipment and time, the dispersion effect is excellent.

[0118] In an exemplary embodiment of the present specification, the planar conductive material may have a D10 of 0.5 μm to 1.5 μm, a D50 of 2.5 μm to 3.5 μm, and a D90 of 7.0 μm to 15.0 μm.

[0119] In an exemplary embodiment of the present specification, a high specific surface area planar conductive material having a high BET specific surface area may be used as the planar conductive material; or a low specific surface area planar conductive material may be used as the planar conductive material.

[0120] In an exemplary embodiment of the present specification, a high specific surface area planar conductive material or a low specific surface area planar conductive material can be used without restriction as the planar conductive material, but in particular, the planar conductive material according to the present specification may be affected to a certain extent by the dispersion effect in terms of electrode performance, so it is particularly desirable to use a low specific surface area planar conductive material that does not cause dispersion problems.

[0121] In another exemplary embodiment of the present specification, the BET specific surface area of the planar conductive material may be 5 m 2 / g and above and 500m 2 / g or less, preferably 5m 2 / g and above and 300m 2 / g or less, and more preferably 5m 2 / g and above and 250m 2 / g or less.

[0122] In another exemplary embodiment, the planar conductive material is a planar conductive material with a high specific surface area, and the BET specific surface area can meet 50m 2 / g and above and 500m 2 / g or less, preferably 80m 2 / g and above and 300m 2 / g or less, and more preferably 100m 2 / g and above and 300m 2 / g range below.

[0123] In another exemplary embodiment, the planar conductive material is a planar conductive material with a low specific surface area, and the BET specific surface area can meet 5m 2 / g or above and 40m 2 / g or less, preferably 5m 2 / g or above and 30m 2 / g or less, and more preferably 5m 2 / g or above and 25m 2 / g range below.

[0124] The negative electrode conductive material according to one exemplary embodiment of the present specification has a completely different composition from the conductive material used for the positive electrode. Specifically, the negative electrode conductive material according to the present invention serves to capture contact points between the negative electrode active material, where the volume expansion of the electrode is significant due to charge and discharge. The positive electrode conductive material, on the other hand, acts as a buffer during rolling and also imparts partial conductivity. Its composition and function are completely different from those of the negative electrode conductive material of the present invention.

[0125] Furthermore, the negative electrode conductive material according to one exemplary embodiment of the present specification is applied to a silicon-based negative electrode active material and has a completely different composition from the conductive material applied to a graphite-based active material. Specifically, the conductive material used in an electrode having a graphite-based active material simply has particles that are smaller than those of the active material, thereby enhancing output characteristics and imparting partial conductivity. Its composition and function are completely different from the negative electrode conductive material applied in conjunction with the silicon-based negative electrode active material in the present invention.

[0126] In an exemplary embodiment of the present specification, the negative electrode conductive material may further include a granular conductive material.

[0127] The negative electrode conductive material according to an exemplary embodiment of the present specification may contain any granular conductive material without limitation as long as it is commonly used. However, if the negative electrode conductive material does not contain a linear conductive material but contains only a granular conductive material, the conductivity and specific surface area of the granular conductive material are lower than those of a linear conductive material or a planar conductive material. Therefore, when the granular conductive material is used in the present invention, significant capacity degradation may occur due to the inability to form a conductive path (passage) in the negative electrode active material layer. If the negative electrode conductive material according to an exemplary embodiment of the present specification uses a granular conductive material together with a linear conductive material, problems such as changes in the physical properties of the negative electrode slurry and gas generation at high temperatures may occur.

[0128] In the negative electrode composition according to an exemplary embodiment of the present specification, the negative electrode binder may also include those binders known in the art to improve the bonding between the negative electrode active material particles and the adhesion between the negative electrode active material particles and the negative electrode current collector, and its non-limiting examples may also include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0129] <Negative electrode>

[0130] A negative electrode for a lithium secondary battery according to an exemplary embodiment of the present specification may include a negative electrode active material layer on at least one surface of a negative electrode current collector layer, the negative electrode active material layer including the above-mentioned negative electrode composition or a cured product thereof.

[0131] Specifically, the negative electrode for a lithium secondary battery may include a negative electrode collector and a negative electrode active material layer provided on one or both surfaces of the negative electrode collector, and the negative electrode active material layer may include the negative electrode composition or a cured product thereof.

[0132] In this case, in the present specification, inclusion of “cured material” may mean a case in which the negative electrode composition is cured by heat treatment or light treatment by a method known in the art.

[0133] Figure 1 The diagram shows a stacked structure of a negative electrode for a lithium secondary battery according to an exemplary embodiment of the present specification. Specifically, it can be confirmed that the negative electrode for a lithium secondary battery 100 includes a negative electrode active material layer 20 on one surface of a negative electrode current collecting layer 10, and Figure 1 The negative electrode active material layer is shown to be formed on one surface of the negative electrode current collecting layer, but the negative electrode active material layer may be included on both surfaces of the negative electrode current collecting layer.

[0134] The negative electrode active material layer may be formed by applying a negative electrode slurry including the negative electrode composition, a negative electrode slurry-forming solvent, and / or a thickener to at least one surface of a negative electrode collector layer, and drying and rolling the negative electrode collector layer.

[0135] The negative electrode slurry may include a solvent for forming the negative electrode slurry. Specifically, from the perspective of facilitating dispersion of components, the solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, specifically distilled water.

[0136] The negative electrode current collector layer is not particularly limited as long as the negative electrode current collector has conductivity without causing chemical changes to the battery. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc. can be used. Specifically, a transition metal that adsorbs carbon well (such as copper or nickel) can be used as a current collector. Although the thickness of the negative electrode current collector layer can be 6 μm to 20 μm, the thickness of the current collector layer is not limited thereto.

[0137] The thickener may be carboxymethyl cellulose (CMC), but is not limited thereto, and any thickener used in the art may be appropriately used.

[0138] In an exemplary embodiment of the present specification, the thickener may be included in an amount of 0.5 to 25 parts by weight, specifically 0.5 to 20 parts by weight, and more preferably 1 to 20 parts by weight, based on 100 parts by weight of the total solid content of the negative electrode slurry.

[0139] In an exemplary embodiment of the present specification, the solid content of the negative electrode slurry may be 10 parts by weight to 99 parts by weight, specifically 20 parts by weight to 80 parts by weight, based on 100 parts by weight of the negative electrode slurry in total.

[0140] <Secondary Battery>

[0141] The lithium secondary battery according to an exemplary embodiment of the present specification may include a positive electrode; the negative electrode for the lithium secondary battery described above; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

[0142] Figure 2 The figure shows a stacked structure of a lithium secondary battery according to an exemplary embodiment of the present specification. Specifically, a negative electrode 100 for a lithium secondary battery can be identified, which includes a negative electrode active material layer 20 on one surface of a negative electrode current collector layer 10, and a positive electrode 200 for a lithium secondary battery can be identified, which includes a positive electrode active material layer 40 on one surface of a positive electrode current collector layer 50. The negative electrode 100 for a lithium secondary battery and the positive electrode 200 for a lithium secondary battery are stacked with a separator 30 interposed therebetween.

[0143] A lithium secondary battery according to an exemplary embodiment of the present specification may particularly include the aforementioned negative electrode. 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, wherein the negative electrode is the same as the negative electrode described above. Since the negative electrode has already been described in detail, its detailed description will be omitted.

[0144] The positive electrode may include a positive electrode current collector layer and a positive electrode active material layer formed on the positive electrode current collector layer and including a positive electrode active material.

[0145] In the positive electrode, the positive electrode current collecting layer is not particularly limited as long as it has conductivity and does not cause chemical changes to the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the thickness of the positive electrode current collecting layer can generally be 3 μm to 500 μm, and the adhesion of the positive electrode active material can be enhanced by forming fine concave and convex surfaces on the surface of the positive electrode current collecting layer. For example, the positive electrode current collecting layer can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics.

[0146] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material includes: layered compounds, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted by one or more transition metals; lithium iron oxide, such as LiFe3O4; lithium manganese oxide, such as Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5 and Cu2V2O7; 1-c2 M c2 Ni-site lithium nickel oxide represented by O2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B and Ga, and c2 satisfies 0.01≤c2≤0.3); 2- c3 M c3 Lithium manganese composite oxide represented by Li2Mn3MO8 (herein, M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and c3 satisfies 0.01≤c3≤0.1); or LiMn2O4, wherein Li in the chemical formula is partially replaced by alkaline earth metal ions, etc., but not limited thereto. The positive electrode may be Li metal.

[0147] The positive electrode active material layer may further include a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material.

[0148] In this case, the positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitation, as long as the positive electrode conductive material has electronic conductivity without causing chemical changes in the battery. Specific examples thereof include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of these or a mixture of two or more thereof can be used.

[0149] In addition, the role of the positive electrode binder is to improve the bonding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector layer. Specific examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one thereof or a mixture of two or more thereof may be used.

[0150] The diaphragm separates the negative electrode from the positive electrode and provides a lithium ion mobile path, and can be used without particular limitation, as long as the diaphragm is generally used as a diaphragm in a secondary battery, in particular, preferably has excellent electrolyte moisture retention and a diaphragm with little resistance to ion movement in the electrolyte. Specifically, a porous polymer film can be used, for example, a porous polymer film formed by a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer, or a laminated structure thereof of more than two layers. In addition, a typical porous non-woven fabric can also be used, for example, a non-woven fabric made of glass fiber with a high melting point, polyethylene terephthalate fiber, etc. In addition, a coated diaphragm comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and can be selectively used as a single layer or multilayer structure.

[0151] Examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which can be used in the preparation of lithium secondary batteries, but are not limited thereto.

[0152] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0153] As the non-aqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate can be used.

[0154] In particular, among carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, have high dielectric constants as high-viscosity organic solvents, thereby dissociating lithium salts well, and therefore can be preferably used. When the cyclic carbonate is mixed with a low-viscosity and low-dielectric-constant linear carbonate (e.g., dimethyl carbonate and diethyl carbonate) in an appropriate ratio, an electrolyte with high electrical conductivity can be prepared, and therefore such a combined use may be more preferred.

[0155] As the metal salt, a lithium salt can be used, and the lithium salt is a material that is easily soluble in the non-aqueous electrolyte, and for example, a material selected from 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 kinds of the group consisting of are used as anions of the lithium salt.

[0156] In order to improve the service life characteristics of the battery, inhibit the decline in battery capacity and improve the discharge capacity of the battery, in addition to containing the above-mentioned electrolyte components, the electrolyte may also contain one or more additives, for example: halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum chloride.

[0157] One exemplary embodiment of the present specification provides a battery module including the lithium secondary battery as a unit battery and a battery pack including the same.

[0158] Furthermore, another exemplary embodiment of the present invention provides a battery pack including the lithium secondary battery.

[0159] Since the lithium secondary battery according to an exemplary embodiment of the present specification consistently exhibits excellent discharge capacity, output characteristics, and cycle performance, the lithium secondary battery can be used as a power source for portable devices (such as mobile phones, laptop computers, and digital cameras) and 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. For example, the battery module or battery pack can be used as a power source for one or more medium-to-large-sized devices in power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0160] [Invention Mode]

[0161] Hereinafter, preferred embodiments will be presented to facilitate understanding of the present invention. However, these embodiments are provided only to illustrate the present invention. It is clear to those skilled in the art that various changes and modifications can be made within the scope and technical spirit of the present invention, and such changes and modifications naturally fall within the appended claims.

[0162] <Preparation Example>

[0163] <Preparation of Negative Electrode>

[0164] Example 1

[0165] As the negative electrode active material, silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 3 nm, SWCNT (BET specific surface area: 1,000 m 2 / g to 1,500m 2 / g, aspect ratio: 10,000 or more) and a polyacrylamide binder were mixed in a weight ratio of 88.5:0.44:11.06 to prepare a negative electrode composition. The negative electrode composition was added to distilled water as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry (solid concentration: 28 wt %).

[0166] The average particle size (D50) of the Si nanocrystals was calculated by fitting the XRD results using the Scherrer formula, and in this case, the reference of the nanocrystals was measured based on Si (220) (2θ=47.4° to 48.5°).

[0167] As a mixing method, after the conductive material, the binder, and water were dispersed at 2500 rpm for 30 minutes using a homomixer, a negative electrode active material was added thereto, and then the resulting mixture was dispersed at 2500 rpm for 30 minutes, thereby preparing a negative electrode slurry.

[0168] On both surfaces of a copper current collector layer (thickness: 15 μm) as a negative electrode current collector layer, a current collector of 3.675 mAh / cm 2 The negative electrode slurry was coated with a loading amount of 500 Å, and the copper current collector was roll-pressed and dried in a vacuum oven at 130° C. for 10 hours, thereby forming a negative electrode (thickness: 57 μm).

[0169] Example 2

[0170] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 2.5 nm, SWCNT (BET specific surface area: 1,000 m 2 / g to 1,500m 2 / g, aspect ratio: 10,000 or more), graphite as an additional planar conductive material (product name: SFG-6L, BET specific surface area: 17m 2 / g) and a polyacrylamide binder were mixed in a weight ratio of 80.0:0.4:9.6:10.0 to prepare a negative electrode composition.

[0171] Example 3

[0172] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 3 nm, SWCNT (BET specific surface area: 1,000 m 2 / g to 1,500m 2 / g, aspect ratio: 10,000 or more) and a polyacrylamide binder were mixed at a weight ratio of 88.5:1.2:10.3 to prepare a negative electrode composition.

[0173] Example 4

[0174] The negative electrode was formed in the same manner as in the preparation example of Example 2, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 2.5 nm, SWCNT (BET specific surface area: 1,000 m 2 / g to 1,500m 2 / g, aspect ratio: 10,000 or more), an additional particulate conductive material (carbon black), and a polyacrylamide binder were mixed in a weight ratio of 80.0:0.4:9.6:10.0 to prepare a negative electrode composition.

[0175] Comparative Example 1

[0176] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that graphite (average particle size (D50): 5.5 μm) as a carbon-based negative electrode active material, SWCNT, and polyacrylamide binder were mixed in a weight ratio of 95.6:1.0:3.4 to prepare a negative electrode composition.

[0177] Comparative Example 2

[0178] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals having an average particle size (D50) of 2.6 nm as a negative electrode active material, SWCNTs, and a polyacrylamide binder were mixed in a weight ratio of 93.8:0.47:5.73 to prepare a negative electrode composition.

[0179] Comparative Example 3

[0180] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 2.8 nm as a negative electrode active material, SWCNTs, and a polyacrylamide binder were mixed in a weight ratio of 92.7:0.927:6.373 to prepare a negative electrode composition.

[0181] Comparative Example 4

[0182] A negative electrode was formed in the same manner as in Preparation Example 1, except that silicon oxide containing Si nanocrystals having an average particle size (D50) of 0.08 nm was contained as the negative electrode active material.

[0183] Comparative Example 5

[0184] A negative electrode was formed in the same manner as in the preparation example in Example 1, except that silicon oxide containing Si nanocrystals having an average particle size (D50) of 7 nm was contained as the negative electrode active material.

[0185] Comparative Example 6

[0186] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 3 nm, SWCNT (BET specific surface area: 1,000 m 2 / g to 1,500m 2 / g, aspect ratio: 10,000 or more) and a polyacrylamide binder were mixed at a weight ratio of 88.5:0.2:11.3 to prepare a negative electrode composition.

[0187] Comparative Example 7

[0188] A negative electrode was formed in the same manner as in the preparation example of Example 1, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 3 nm as a negative electrode active material, a granular conductive material (carbon black) and a polyacrylamide binder were mixed in a weight ratio of 88.5:0.44:11.06 to prepare a negative electrode composition.

[0189] Comparative Example 8

[0190] The negative electrode was formed in the same manner as in the preparation example of Example 2, except that silicon oxide (average particle size (D50): 5.5 μm) containing Si nanocrystals with an average particle size (D50) of 2.5 nm as the negative electrode active material, a granular conductive material (carbon black), and an additional planar conductive material (product name: SFG-6L, BET specific surface area: 17 m 2 / g) and a polyacrylamide binder were mixed in a weight ratio of 80.0:0.4:9.6:10.0 to prepare a negative electrode composition.

[0191] <Manufacturing of Lithium Secondary Batteries>

[0192] A positive electrode slurry was prepared by adding NCMA active material (average particle size (D50): 9.6 μm) as a positive electrode active material, carbon nanotubes as a positive electrode conductive material, and polyvinylidene fluoride (PVdF) as a positive electrode binder in a weight ratio of 97.6:0.8:1.6 to N-methyl-2-pyrrolidone (NMP) as a solvent for forming a positive electrode slurry.

[0193] On both surfaces of an aluminum current collector (thickness: 12 μm) as a positive electrode current collector, a charge of 3.5 mAh / cm 2 The positive electrode slurry was coated with a loading amount of , and the aluminum current collector was roll-pressed and dried in a vacuum oven at 130° C. for 10 hours to form a positive electrode active material layer (thickness: 75 μm), thereby preparing a positive electrode (positive electrode thickness: 162 μm, porosity: 26%).

[0194] A full-cell lithium secondary battery of Example 1 was manufactured by interposing a polyethylene separator between the positive electrode and the negative electrode described in Example 1 and injecting an electrolyte therein.

[0195] The electrolyte was obtained by adding 0.5 wt % of vinylene carbonate based on the total weight of the electrolyte to an organic solvent in which difluoroethylene carbonate (FEC) and diethyl carbonate (DEC) were mixed at a volume ratio of 10:90, and adding LiPF6 and LiFSI as lithium salts at concentrations of 0.5 M and 1 M, respectively.

[0196] Lithium secondary battery full cells were respectively manufactured in the same manner as above, except that the negative electrodes in Examples 2 to 4 and Comparative Examples 1 to 8 were used.

[0197] <Experimental Example 1> Evaluation of Energy Density and Lifespan Characteristics

[0198] The battery energy density of the lithium secondary batteries including the negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 8 was measured, and the service life was evaluated using an electrochemical charger and discharger to evaluate the capacity retention rate at 300 cycles.

[0199] Specifically, during the evaluation of the life characteristics, the initial charge capacity was measured by charging under CCCV conditions with the C rate set to 1.0 C, the upper limit voltage and the cutoff current set to 4.2 V and 0.05 C, respectively, and the initial discharge capacity was measured under CC conditions with the C rate and the lower limit voltage set to 0.5 C and 2.5 V, respectively.

[0200] Capacity retention rate (%) = {(charge and discharge capacity of the Nth cycle) / (charge and discharge capacity of the first cycle)} x 100

[0201] The evaluation results of energy density and service life characteristics are shown in Table 1 below.

[0202]

[0203] In the prior art, the silicon content is simply increased to produce a high energy density in the negative electrode, but this leads to a decrease in the cycle capacity retention rate (less than about 79%), or when the active material and other characteristics are adjusted to ensure the capacity retention rate (above about 79%), there is a problem that the energy density cannot be ensured (less than 750Wh / L).

[0204] Since the lithium secondary battery using the negative electrode composition described in this specification is intended to simultaneously meet high energy density and excellent service life characteristics, the lithium secondary battery can only be used as the lithium secondary battery of the present invention when the energy density is above about 750Wh / L and the capacity retention rate after 300 cycles is above about 79%. This is the basis for judgment.

[0205] As can be seen from Table 1 above, in the case of Examples 1 to 4 using the negative electrode composition according to the present specification, the energy density is 750 Wh / L or higher, and the capacity retention rate at 300 cycles is 79% or higher, making the lithium secondary batteries of Examples 1 to 4 useful as lithium secondary batteries having high energy density and excellent service life characteristics according to the present invention. This corresponds to the results of using the following negative electrode active material: based on 100 parts by weight of the solid content of the negative electrode composition, the content of the negative electrode active material, the negative electrode conductive material, and particularly the linear conductive material is 70 parts by weight or higher, 0.3 parts by weight or higher, and 0.3 parts by weight or higher and 3 parts by weight or lower, respectively, and the negative electrode aqueous binder content is 9 parts by weight or higher, so that each content is optimized, and at the same time, the Si nanocrystals as silicon oxide have an average particle size (D50) of 0.1 nm or higher and 5 nm or lower.

[0206] However, in the case of Example 3, as the SWCNT content increases to about 3 times that of Example 1, the energy density and cycle performance are similar to those of Example 1, but due to the high SWCNT content and no obvious improvement in performance, Example 3 is disadvantageous in terms of cost.

[0207] In addition, in the case of Example 4, carbon black (granular conductive material) was used as the second conductive material instead of the planar conductive material (SFG-6L) in Example 2. No significant difference in energy density and cycle performance was observed compared to Example 2. However, in the preliminary evaluation (manufacturing of negative electrode slurry and gas generation analysis), it was confirmed that the application of granular conductive material caused the physical properties of the negative electrode slurry to change, and gas problems occurred at high temperatures.

[0208] Since the lithium secondary battery of Comparative Example 1 using graphite as the negative electrode active material does not use a negative electrode active material containing silicon oxide, the cycle performance of the electrode does not deteriorate due to volume expansion, but the energy density is significantly reduced, so that the lithium secondary battery of Comparative Example 1 cannot be used as a lithium secondary battery with high energy density and excellent service life characteristics according to the present invention.

[0209] Compared with the negative electrode composition of Example 1, the lithium secondary battery of Comparative Example 2 has a negative electrode aqueous binder content reduced by half and has a relatively high negative electrode active material content, thereby meeting an energy density of more than 750Wh / L. However, due to the reduction in the negative electrode aqueous binder content, it becomes difficult to suppress the electrode distortion caused by the volume change of the silicon oxide active material throughout the entire cycle, and the capacity retention rate is only in the first half of the 70% range. Therefore, the lithium secondary battery of Comparative Example 2 cannot be used as a lithium secondary battery with excellent performance according to the present invention.

[0210] Compared to the negative electrode composition of Example 1, the lithium secondary battery of Comparative Example 3 reduced the negative electrode aqueous binder content by half. Similar to Comparative Example 2, it achieved an energy density of 750 Wh / L or higher. Despite containing more than twice the amount of negative electrode linear conductive material as in Example 1, the capacity retention rate was lower than 79%, rendering the lithium secondary battery of Comparative Example 3 unsuitable for use as a lithium secondary battery with excellent performance according to the present invention. This confirms that without the appropriate combination of the negative electrode active material and the negative electrode aqueous binder according to the present invention, even with the inclusion of a large amount of negative electrode linear conductive material alone, it is difficult to maintain cycling characteristics during repeated charge and discharge cycles.

[0211] The lithium secondary battery of Comparative Example 4, in which silicon oxide having significantly small Si nanocrystals is used as the negative electrode active material, satisfies a capacity retention rate of more than 79% by appropriately combining the negative electrode composition according to the present invention. However, due to the small size of the Si nanocrystals, the energy density is significantly lower than that of Example 1. Therefore, the lithium secondary battery of Comparative Example 4 cannot be used as a lithium secondary battery with excellent performance according to the present invention.

[0212] The lithium secondary battery of Comparative Example 5, in which silicon oxide having Si nanocrystals larger than the Si nanocrystal size range described in this specification is used as the negative electrode active material, is excellent in energy density, but it can be confirmed that after repeated charge and discharge, cracks are formed in the negative electrode active material, which hinders the diffusion of Li ions into the nanocrystals, and the uneven reaction accelerates the degradation of the battery, resulting in a significant decrease in the capacity retention rate.

[0213] In the case of Comparative Example 6, it was confirmed that by reducing the SWCNT content to half the level of Example 1, the capacity deteriorated rapidly from the initial stage of the cycle. Specifically, the initial cycle capacity was measured, and the initial cycle capacity of Example 1 was 88.2 mAh, and the initial cycle capacity of Example 2 was 89.0 mAh, but the initial cycle capacity of Comparative Example 6 was a significantly lower value of 86.8 mAh. Therefore, it can be confirmed that the capacity retention rate in Table 1 is also significantly low. In this case, the initial cycle capacity was measured by charging at a rate of 0.33 C under CC-CV conditions (4.2 V, current 5% cutoff) and discharging at a rate of 0.33 C under CC conditions (2.5 V).

[0214] In the case of Comparative Example 7, carbon black was used as a conductive material instead of the linear conductive material (SWCNT) in Example 1. Due to the low conductivity and specific surface area of carbon black, a conductive path (passage) could not be formed in the negative electrode active material layer. Therefore, it can be confirmed that with the occurrence of significant capacity degradation, the capacity retention rate was significantly lower than that of Example 1.

[0215] In the case of Comparative Example 8, carbon black was used as the conductive material instead of the linear conductive material (SWCNT) in Example 2, and a conductive path (passage) could not be formed in the negative electrode active material layer. From this, it can be confirmed that the capacity retention rate was significantly low with the occurrence of significant capacity degradation.

Claims

1. A negative electrode composition comprising: a negative electrode active material comprising silicon oxide; a negative electrode conductive material; and a negative electrode aqueous binder, Based on 100 parts by weight of the solid content of the negative electrode composition, the content of the negative electrode active material is 70 parts by weight or more, the content of the negative electrode conductive material is 0.3 parts by weight or more, and the content of the negative electrode aqueous binder is 9 parts by weight or more. The negative electrode conductive material includes a linear conductive material, and the content of the linear conductive material is 0.3 parts by weight or more and 3 parts by weight or less based on 100 parts by weight of the solid content of the negative electrode composition. The silicon oxide comprises Si nanocrystals, and The average particle size (D50) of the Si nanocrystals is greater than or equal to 0.1 nm and less than or equal to 5 nm. 2 . The negative electrode composition according to claim 1 , wherein the negative electrode aqueous binder is a polyacrylamide-based binder.

3. The negative electrode composition according to claim 1, wherein the contents of the negative electrode active material, the negative electrode conductive material, and the negative electrode aqueous binder in the negative electrode composition are respectively 70 parts by weight or more and 90 parts by weight or less, 0.3 parts by weight or more and 11 parts by weight or less, and 9 parts by weight or more and 20 parts by weight or less, based on 100 parts by weight of the solid content of the negative electrode composition.

4. The negative electrode composition according to claim 1, wherein the silicon oxide comprises SiO x (0 <x<2)。 5. The negative electrode composition according to claim 1, wherein the silicon oxide contains amorphous SiO x (0 < x < 2), and The Si nanocrystals are dispersed in amorphous silicon oxide. The negative electrode composition according to claim 1 , wherein D50 of the negative electrode active material is 5 μm or more and 10 μm or less. 7 . A negative electrode for a lithium secondary battery, comprising a negative electrode active material layer on at least one surface of a negative electrode current collector layer, the negative electrode active material layer comprising the negative electrode composition according to claim 1 or a cured product thereof.

8. A lithium secondary battery, comprising: positive electrode; The negative electrode for a lithium secondary battery according to claim 7; a separator disposed between the positive electrode and the negative electrode; and electrolytes. 9 . A battery module comprising the lithium secondary battery according to claim 8 . 10 . A battery pack comprising the lithium secondary battery according to claim 8 . 11 . A battery pack comprising the battery module according to claim 9 .

Images