Anode active material, anode and lithium secondary battery

Abstract

The present invention relates to an anode active material, an anode comprising same, and a lithium secondary battery, the anode active material comprising; a core including a silicon carbon composite; an oxide layer which is provided on at least a portion of the core and includes silicon oxide, and of which at least 50% has a thickness of greater than 5 nm; and a carbon layer provided on at least a portion of the oxide layer.

Description

Negative active material, negative electrode, and lithium secondary battery

[0001] This specification claims the benefit of Korean Patent Application No. 10-2023-0098692, filed with the Korean Intellectual Property Office on July 28, 2023, and Korean Patent Application No. 10-2024-0098727, filed with the Korean Intellectual Property Office on July 25, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] This application relates to a negative electrode active material, a negative electrode, and a lithium secondary battery.

[0003] The recent rapid proliferation of battery-powered electronic devices, including mobile phones, laptops, electric vehicles, power tools, and vacuum cleaners, has led to a rapid increase in demand for compact, lightweight, and relatively high-capacity and / or high-output secondary batteries. In particular, lithium secondary batteries, with their lightweight design and high energy density, are attracting significant attention as power sources for electronic devices. Accordingly, active research and development efforts are underway to improve the performance of lithium secondary batteries.

[0004] In general, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. In addition, an active material layer including a positive electrode active material and a negative electrode active material may be formed on a current collector on the positive electrode and the negative electrode, respectively. In general, a lithium-containing metal oxide such as LiCoO2 or LiMn2O4 is used as a positive electrode active material for the positive electrode, and a carbon-based active material or a silicon-based active material that does not contain lithium is used as an negative electrode active material for the negative electrode.

[0005] Among negative active materials, silicon-based materials are attracting attention for their higher capacity and superior fast-charge characteristics compared to carbon-based materials. However, silicon-based materials exhibit significant volumetric expansion and contraction during charge and discharge, and their high irreversible capacity leads to low initial efficiency.

[0006] Accordingly, the development of negative electrode active materials that can improve the performance of lithium secondary batteries is required.

[0007] The present invention relates to a negative electrode active material capable of improving the life characteristics of a lithium secondary battery, a negative electrode including the negative electrode active material, and a secondary battery including the same.

[0008] One embodiment of the present invention provides a negative electrode active material comprising: a core comprising a silicon carbon composite; an oxide layer provided on at least a portion of the core and comprising silicon oxide, wherein at least 50% of the thickness of the oxide layer is greater than 5 nm; and a carbon layer provided on at least a portion of the oxide layer.

[0009] According to one embodiment of the present invention, the Si:C element ratio on the surface of the negative electrode active material of the above-described embodiment is 1:1 to 1:4.

[0010] According to one embodiment of the present invention, the Si:C element ratio in the entire negative active material of the above-described embodiment is 0.9:1.1 to 1.1:0.9.

[0011] According to one embodiment of the present invention, the silicon oxide of the oxide layer of the above-described embodiment is SiOx (x is 0.1 or more and less than 2).

[0012] According to one embodiment of the present invention, the negative active material of the above-described embodiment includes silicon crystal grains having a particle size of 8 ㎛ or less.

[0013] One embodiment of the present invention provides a negative electrode including a negative electrode active material, a conductive material, and a binder according to the above-described embodiments.

[0014] One embodiment of the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, and a separator according to the above-described embodiment.

[0015] One embodiment of the present invention provides a battery module including a lithium secondary battery according to the above-described embodiment.

[0016] One embodiment of the present invention provides a battery pack including a lithium secondary battery according to the above-described embodiment.

[0017] One embodiment of the present invention provides a battery pack including a battery module according to the above-described embodiment.

[0018] According to embodiments of the present invention, by using a silicon-carbon composite as a core material of an anode active material, high capacity and efficiency can be achieved, and at the same time, the life characteristics of the battery can be improved and problems such as gas generation can be prevented due to the presence of an oxide layer and a carbon layer. Specifically, the oxide layer and the carbon layer can prevent the inferiority of life characteristics due to the high reactivity of silicon even when silicon crystal grains are small, and can prevent the problem of silicon generating gas when it comes into contact with water in an aqueous process.

[0019] Hereinafter, the present invention will be described in more detail to facilitate understanding. The present invention may be embodied in various different forms and is not limited to the embodiments described herein. In this regard, terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Rather, they should be interpreted in the sense and concept that aligns with the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention.

[0020] It should be understood that the terms “include,” “comprising,” or “having” used in this specification are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0021] Furthermore, when we say that a part of a layer or the like is "above" or "on" another part, this includes not only cases where it is "directly above" that part, but also cases where there is another part in between. Conversely, when we say that a part is "directly above" another part, it means that there is no other part in between. Furthermore, when we say that a part is "above" or "on" a reference part, it means that it is located above or below that reference part, and does not necessarily mean that it is located "above" or "on" in the direction opposite to gravity.

[0022] The terms and words used in this specification should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0023] As used herein, the singular expression of a term includes the plural expression unless the context clearly indicates otherwise.

[0024] In this specification, the “average particle diameter (D50)” can be defined as the particle diameter corresponding to 50% of the volume accumulation amount in the particle diameter distribution curve. The average particle diameter (D50) can be measured using, for example, a laser diffraction method. The laser diffraction method can generally measure particle diameters from the submicron range to several millimeters, and can obtain results with high reproducibility and high resolution.

[0025] The above average particle diameter (D50) can be measured using a Microtrac device (manufacturer: Microtrac model number: S3500) using water and a triton-X100 dispersant. Specifically, the average particle diameter (D50) of the positive electrode active material can be measured in the range of a refractive index of 1.5 to 1.7, and the negative electrode active material can be measured under a refractive index of 1.97 or 2.42. For example, after dispersing the particles in a dispersion medium, the particles can be introduced into a commercially available laser diffraction particle size measuring device, irradiated with ultrasonic waves of about 28 kHz at an output of 60 W, and then a volume cumulative particle size distribution graph can be obtained, thereby obtaining the particle size corresponding to 50% of the volume cumulative amount, thereby measuring.

[0026] In one embodiment of the present application, the crystal grain size can be calculated as the FWHM (Full Width at Half Maximum) value through XRD analysis. Specifically, the crystal grain size can be measured through the FWHM obtained through XRD analysis and the Debey-Scherrer equation of Equation 1-1 below.

[0027] [Formula 1-1]

[0028] FWHM=(Kλ) / (LCOSθ)

[0029] In the above formula 1-1,

[0030] L represents the grain size, K is a constant, θ is the bragg angle, and λ is the wavelength of the X-ray.

[0031] In addition, the shape of the crystal grains is diverse and can be measured three-dimensionally, and the size of the crystal grains can generally be measured using the commonly used circle method and diameter measurement method, but is not limited thereto.

[0032] The above diameter measurement method can be measured by drawing 5-10 equilibrium lines, each of which is L mm long, on a microscopic photograph of the target particle, counting the number of grains z on the lines, and averaging them. At this time, only those that are completely included are counted, and those that intersect are excluded. If the number of lines is P and the magnification is V, the average grain diameter can be calculated using the following equations 1-2.

[0033] [Formula 1-2]

[0034] Dm = (L*P*10 3 ) / (zV) (μm)

[0035] In addition, the above circle method is a method of calculating the average area of ​​crystal grains by drawing a circle of a set diameter on a microscopic photograph of the target particle and calculating the number of crystal grains within the circle and the number of crystal grains that fall on the boundary line using the following equation 1-3.

[0036] [Formula 1-3]

[0037] Fm = (Fk * 10 6 ) / ((0.67n + z) V 2 ) (μm 2 )

[0038] In the above equation 1-3, Fm represents the average particle area, Fk represents the measured area on the photograph, z represents the number of particles inside the circle, n represents the number of particles in the circular arc, and V represents the magnification of the microscope.

[0039] Hereinafter, preferred embodiments of the present invention will be described in detail. However, the embodiments of the present invention may be modified in various ways, and the scope of the present invention is not limited to the embodiments described below.

[0040]

[0041] <Cathode active material>

[0042] One embodiment of the present invention provides a negative active material comprising: a core comprising a silicon-carbon composite; an oxide layer provided on at least a portion of the core and comprising silicon oxide, wherein at least 50% of the thickness of the oxide layer is greater than 5 nm; and a carbon layer provided on at least a portion of the oxide layer. Here, at least 50% of the oxide layer means at least 50% of the area covered by the oxide layer.

[0043] According to one embodiment, the silicon carbon composite included in the core may be a Si / C-based active material.

[0044] In this specification, the silicon-carbon composite is a composite of Si and C, and is distinct from silicon carbide, denoted as SiC. Since the silicon carbide does not electrochemically react with lithium, all performances, including lifespan, may be measured as 0.

[0045] The above silicon-carbon composite may be a composite of silicon and graphite, and may also form a structure in which a core composed of silicon and graphite is surrounded by graphene or amorphous carbon. In the above silicon-carbon composite, the silicon may be silicon nanoparticles.

[0046] According to one embodiment, the silicon carbon composite includes porous carbon particles and silicon particles positioned on the surface or internal pores of the porous carbon particles.

[0047] According to one embodiment, the silicon carbon composite has a surface area of ​​0.5 to 10 m by the BET method. 2 / g, and the pore volume can be 0.005 to 0.03 cm 3 / g, and the pore size by the BET method may be 10 to 20 nm. The silicon carbon composite has a pore volume of 0.005 to 0.03 cm as measured by the mercury penetration method. 3 / g could be.

[0048] According to one embodiment, the silicon carbon composite may have a D90 particle size of 15 ㎛ to 25 ㎛, a D50 particle size of 2 ㎛ to 10 ㎛, and a D10 particle size of 0.1 ㎛ to 1 ㎛.

[0049] The silicon particles formed on the surface and internal pores of the above carbon particles may be silicon nanoparticles, and they may be crystalline, semi-crystalline, amorphous, or a combination thereof.

[0050] According to one embodiment of the present invention, an oxide layer including silicon oxide is provided on at least a portion of the core, and a thickness of 50% or more of the oxide layer is greater than 5 nm, greater than 7 nm, greater than 8 nm, greater than 9 nm, or greater than 10 nm.

[0051] According to one embodiment of the present invention, an oxide layer including silicon oxide is provided on at least a portion of the core, and a thickness of at least 50% of the oxide layer is at least 10 nm.

[0052] According to one embodiment of the present invention, an oxide layer including silicon oxide is provided on at least a portion of the core, and a thickness of 50% or more of the oxide layer is 100 nm or less, 80 nm or less, 50 nm or less, 40 nm or less, or 30 nm or less.

[0053] When the thickness of 50% or more, preferably 70% or more, and more preferably 90% or more of the above oxide layer exceeds 5 nm, it is possible to prevent the exposure of silicon or to reduce the reactivity of silicon, thereby preventing inferior life characteristics or gas generation due to contact with water during an aqueous process. When the thickness of 50% or more of the oxide layer including silicon oxide is 5 nm or less, there is a concern that internal silicon particles may be exposed to the outside, thereby deteriorating the aqueous processability, and in this case, the electrode condition is not good, resulting in a deterioration in the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0054] According to this temporary state, the oxide layer includes silicon oxide.

[0055] By including silicon oxide in the above oxide layer, contact between water and silicon is prevented during the preparation of an aqueous slurry, thereby improving the electrode condition, thereby improving the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0056] In particular, when the above oxide layer contains silicon oxide and the thickness of more than 50% of the oxide layer exceeds 5 nm, there is a high probability that most of the silicon is passivated by the oxide layer, thereby improving the aqueous processability, and thus improving the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0057] According to one embodiment, the silicon oxide is SiO x (0 <x≤2)를 포함한다. 상기 SiO x (0 <x≤2)는 Si 및 SiO2등의 다양한 산화 상태를 포함하는 형태일 수 있다. 즉, 상기 x는 상기 SiO x (0 <x≤2) 내에 포함된 Si에 대한 O의 개수비에 해당한다. 다시 말하면, 상기 SiO x (0 <x≤2)는 한 가지 상태가 아닌 여러 산화 상태가 혼재되어 있는 상태로서, XPS에서 에칭(etching)하면서 산소(O)의 함량을 통해 x의 평균값을 얻을 수 있다.

[0058] For example, the silicon oxide is SiO x (0.1≤ x < 2). The above x may be 1 or more and less than 2, for example, 1.5 or more and less than 2.

[0059] According to one embodiment, the silicon oxide is in an amorphous state. The SiO x (0 <x≤2)는 섬형(island type) 또는 박막형의 층을 이루며 존재할 수 있고, 이에 한정되지 않고 다양한 형태로 존재할 수 있다.

[0060] According to one embodiment, the thickness of the oxide layer may be 200 nm or less, for example, 100 nm or less, 50 nm or less, 30 nm or less, or 20 nm or less. If the thickness of the oxide layer is too thin, it is difficult to expect the effect of blocking contact with water, and if it is too thick, the resistance may increase and the discharge capacity may not be developed. The content of the oxide layer may be 1 to 5 parts by weight based on 100 parts by weight of the negative electrode active material. Such a content range is advantageous in providing the effect of blocking contact with water by the oxide layer and at the same time providing an appropriate discharge capacity.

[0061] The negative active material according to one embodiment includes a carbon layer provided on at least a portion of the oxide layer.

[0062] Specifically, the carbon layer, together with the aforementioned oxide layer, can prevent exposure of silicon or reduce the reactivity of silicon, thereby preventing poor life characteristics or gas generation due to contact with water during an aqueous process. Furthermore, the carbon layer can impart conductivity to the negative electrode active material, thereby improving the initial efficiency, life characteristics, and battery capacity characteristics of the secondary battery.

[0063] In one embodiment, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0064] In one embodiment, the carbon layer may be an amorphous carbon layer. The amorphous carbon can appropriately maintain the strength of the carbon layer, thereby suppressing expansion of the silicon-carbon composite.

[0065] Additionally, the carbon layer may or may not additionally include crystalline carbon.

[0066] The above crystalline carbon can further improve the conductivity of the negative electrode active material. The above crystalline carbon can include at least one selected from the group consisting of fluorene, carbon nanotubes, and graphene.

[0067] The amorphous carbon can appropriately maintain the strength of the carbon layer, thereby suppressing expansion of the silicon-carbon composite. The amorphous carbon can be a carbon-based material formed using at least one carbide or hydrocarbon selected from the group consisting of tar, pitch, and other organic materials as a source for chemical vapor deposition.

[0068] The above-mentioned other organic carbonates may be carbonates of organic carbonates selected from sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or kedohexose and combinations thereof.

[0069] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, etherene, ethylene, acetylene, propane, butane, butene, pentane, isobutane, or hexane, etc. The aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, etc.

[0070] In one embodiment, the carbon layer may be included in an amount of 0.1 to 50 parts by weight, 0.1 to 30 parts by weight, or 0.1 to 20 parts by weight based on 100 parts by weight of the total negative active material. More specifically, it may be included in an amount of 0.5 to 15 parts by weight, or 1 to 10 parts by weight. When the above range is satisfied, conductivity can be improved while preventing a decrease in capacity and efficiency of the negative active material.

[0071] In one embodiment, the thickness of the carbon layer is 1 nm to 500 nm, specifically 5 nm to 300 nm, and more specifically 5 nm to 100 nm. When the above range is satisfied, the conductivity of the negative electrode active material is improved, the volume change of the negative electrode active material is easily suppressed, and the side reaction between the electrolyte and the negative electrode active material is suppressed, thereby improving the initial efficiency and / or lifespan of the battery.

[0072] Specifically, the carbon layer can be formed by a chemical vapor deposition (CVD) method using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0073] Since the carbon layer is formed after the formation of the aforementioned oxide layer, it can be provided on an oxide layer including silicon oxide, or it can be provided on a surface of the aforementioned core where the oxide layer is not provided. In addition, the carbon layer can completely cover the oxide layer, but can be formed only on a portion of the oxide layer, so both the oxide layer and the carbon layer can be observed on the surface of the negative active material.

[0074] According to one embodiment of the present invention, the negative active material may include silicon crystal grains having a particle size of 8 μm or less. The silicon crystal grains may be included in a core including the aforementioned silicon-carbon composite. In the present invention, by including silicon crystal grains having a particle size of 8 μm or less, not only is it advantageous in improving life performance, but also gas generation in an aqueous process can be prevented by the aforementioned oxide layer and carbon layer. According to one example, the negative active material may include silicon crystal grains having a particle size of 8 μm or less, for example, 5 μm or less, 1,000 nm or less, or on the order of several tens of nm.

[0075] According to one embodiment of the present invention, the Si:C element ratio on the surface of the negative electrode active material of the above-described embodiment is 1:1 to 1:4, for example, 1:2 to 1:4, or 1:3 to 1:4. Since the negative electrode active material layer has a carbon layer on the surface, this means that the carbon ratio is higher at the surface. When the negative electrode active material has such an element ratio, this means that the surface of the negative electrode active material is well coated with carbon, and this can be measured through XPS. The element ratio can be adjusted by adjusting the amount of carbonized material or changing the heat treatment time during the carbon layer introduction process.

[0076] According to one embodiment of the present invention, the Si:C element ratio in the entire negative active material of the above-described embodiment is 0.9:1.1 to 1.1:0.9, and may be, for example, 1:0.95 to 1:1.1. If the Si content increases further, there is an advantage in increasing the capacity, but there may be a problem that the particles grow significantly due to deposition on the surface rather than inside the carbon pores, which may have a negative effect on the lifespan, processability, etc. The above-described Si:C element ratio range is advantageous in satisfying all of the capacity, lifespan, and processability. The negative active material according to the above embodiment is mainly composed of three elements, Si, C, and O, and excluding the influence of the remaining elements, C is measured through a CS analyzer, O is measured through an ONH analyzer, and then the value obtained by subtracting the ratio from the entire value can be calculated as the Si content.

[0077] According to one embodiment, the average particle diameter (D50) of the negative active material may be 0.1 ㎛ to 30 ㎛, specifically 1 ㎛ to 20 ㎛, and more specifically 1 ㎛ to 10 ㎛, 2 ㎛ to 10 ㎛. When the above range is satisfied, the structural stability of the active material during charge and discharge can be ensured, the problem of the volume expansion / contraction level increasing as the particle diameter becomes excessively large can be prevented, and the problem of the initial efficiency decreasing due to the particle diameter being excessively low can be prevented.

[0078] According to one embodiment, the BET specific surface area of ​​the negative active material is 20 m 2 / g or less, for example, 10m 2 / g or less is preferable. For example, the BET specific surface area of ​​the negative active material is 0.11 m 2 / g or more than 10m 2 / g or less, e.g. 4 m 2 / g to 6 m 2 / g. The specific surface area can be measured by the BET (Brunauer-Emmett-Teller; BET) method. For example, it can be measured by the BET six-point method using a porosimetry analyzer (Bell Japan Inc, Belsorp-Ⅱ mini) and a nitrogen gas adsorption flow method.

[0079]

[0080] <Method for manufacturing negative active material>

[0081] The present invention provides a method for manufacturing a negative electrode active material, and more specifically, a method for manufacturing a negative electrode active material according to the above-described embodiment.

[0082] One embodiment of the present invention provides a method for producing a negative electrode active material of the present invention, comprising: forming a core including a silicon carbon composite; forming an oxide layer provided on at least a portion of the core and including silicon oxide; and forming a carbon layer on at least a portion of the oxide layer.

[0083] According to one embodiment, the step of forming a core including the silicon carbon composite may be manufactured by a method including the step of etching carbon particles including internal pores to expand the internal pores of the carbon particles; and the step of forming silicon particles on the surface and in the internal pores of the carbon particles having expanded internal pores.

[0084] The step of expanding the internal pores of the above carbon particles can be performed in a nitrogen (N2) atmosphere, an oxygen (O2) atmosphere, or an air atmosphere. Specifically, the flow rate of the oxygen (O2) or the air containing the oxygen can be controlled to 0.1 L / min to 10 L / min.

[0085] The step of expanding the internal pores of the above carbon particles can be performed at a temperature range of 400°C to 1200°C for 30 minutes to 4 hours.

[0086] Depending on the conditions for expanding the internal pores of the above carbon particles, the pore characteristics of the obtained porous carbon particles may vary.

[0087] An etchant may be used to expand the internal pores of the carbon particles, and for example, a basic material such as KOH may be used. For example, the carbon particles and KOH may be mixed in a weight ratio of 1:1 to 1:5 to expand the internal pores of the carbon particles.

[0088] The step of forming the above silicon particles can be performed using chemical vapor deposition (CVD). At this time, silicon nanoparticles are deposited on the surface and / or internal pores of the carbon-based particles with expanded internal pores, thereby forming a silicon coating layer in the form of a film, an island, or a mixture thereof.

[0089] The step of forming the above silicon particles can be performed by using a chemical vapor deposition (CVD) device to flow SiH4 / H2 gas onto carbon particles at 500°C to 900°C, thereby forming a silicon carbon composite.

[0090] The step of forming an oxide layer comprising silicon oxide and provided on at least a portion of the core may be formed by partially or completely oxidizing the surface of the core comprising the above-described silicon carbon composite.

[0091] According to one embodiment of the present invention, the step of forming an oxide layer including silicon oxide and provided on at least a portion of the core may include a heat treatment step performed in an atmosphere including oxygen.

[0092] Specifically, the heat treatment step may form an oxide layer by performing heat treatment at 500°C to 700°C, for example, 600°C to 700°C.

[0093] If the heat treatment process is performed at a temperature exceeding 700℃, the silicon carbon composite (SiC) grows, which is undesirable as it reduces the capacity and efficiency of the material.

[0094] The atmosphere containing oxygen may contain oxygen in an amount of more than 0 vol% and less than or equal to 10 vol%, for example, 3 vol% to 7 vol%. The atmosphere containing oxygen may contain an inert gas such as argon in addition to oxygen. The heat treatment time is not particularly limited, but may be performed for, for example, 1 hour to 12 hours, 1 hour to 8 hours, or 2 hours to 5 hours.

[0095] The thickness of the oxide layer can be controlled through heat treatment temperature, time, etc.

[0096] The step of forming a carbon layer on at least a portion of the above oxide layer can be formed by using a chemical vapor deposition (CVD) method using a carbon-based material, for example, a hydrocarbon gas, or by carbonizing a material that serves as a carbon source.

[0097] Specifically, a silicon carbon composite having an oxide layer including silicon oxide formed thereon can be formed by introducing the composite into a reactor and then chemical vapor deposition (CVD) of a hydrocarbon gas at 600°C to 700°C. The hydrocarbon gas can be at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene, and can be heat treated at 600°C to 700°C.

[0098]

[0099] <Cathode>

[0100] One embodiment of the present invention provides a negative electrode including a negative electrode active material, a conductive material, and a binder according to the above-described embodiments.

[0101] Specifically, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer may include the negative electrode active material. Furthermore, the negative electrode active material layer may further include a binder and / or a conductive material.

[0102] The above negative electrode active material layer can be formed by applying a negative electrode slurry containing a negative electrode active material, a binder, and / or a conductive material to at least one surface of a negative electrode current collector, and drying and rolling.

[0103] The above negative electrode slurry includes the negative electrode active material, a binder and / or a conductive material.

[0104] The above negative electrode slurry may further include an additional negative electrode active material.

[0105] As the above additional negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; SiO β(0 < β < 2), SnO2, vanadium oxide, lithium titanium oxide, lithium vanadium oxide, and other metal oxides capable of doping and dedoping lithium; or composites containing the above metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used. In addition, a metallic lithium thin film may be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon may be used as the carbonaceous material. Representative examples of low-crystalline carbon include soft carbon and hard carbon, and representative examples of high-crystalline carbon include natural graphite or artificial graphite in the form of amorphous, plate-like, flaky, spherical, or fiber-like forms, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

[0106] The above additional negative electrode active material may be a carbon-based negative electrode active material.

[0107] In one embodiment of the present invention, the weight ratio of the negative electrode active material and the additional negative electrode active material included in the negative electrode slurry may be 10:90 to 90:10, and specifically, 10:90 to 50:50.

[0108] The negative current collector may be any conductive material that does not cause chemical changes in the battery, and is not particularly limited thereto. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that readily absorbs carbon, such as copper or nickel, may be used as the current collector. The current collector may have a thickness of 6 μm to 20 μm, but the thickness of the current collector is not limited thereto.

[0109] The above binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoroelastomer, polyacrylic acid, and materials in which hydrogens thereof are substituted with Li, Na, Ca, etc., and may also include various copolymers thereof.

[0110] The conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, paneth black, lamp black, and thermal black; conductive fibers such as carbon fiber or metal fiber; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0111] The above-described cathode slurry may include a solvent for forming the cathode slurry. Specifically, the solvent for forming the cathode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, specifically distilled water, in order to facilitate dispersion of the components.

[0112]

[0113] Secondary battery

[0114] One embodiment of the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, and a separator according to the above-described embodiment.

[0115] The above 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.

[0116] In the above positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and 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. In addition, the positive electrode current collector may typically have a thickness of 3 to 500 ㎛, and fine unevenness may be formed on the surface of the current collector to increase the adhesive strength of the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0117] The above 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), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound having the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4(0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by O2 (wherein, M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.3); chemical formula LiMn 2-c3 M c3Lithium manganese composite oxide represented by O2 (wherein 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 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., in which a part of Li in the chemical formula is replaced with an alkaline earth metal ion, but is not limited thereto. The positive electrode may be Li-metal.

[0118] The above-described positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the positive electrode active material described above.

[0119] At this time, the positive electrode conductive material is used to provide conductivity to the electrode, and in the battery to be constructed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without any special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used.

[0120] In addition, the positive electrode binder plays a role of improving the adhesion between positive electrode active material particles and the adhesiveness between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluoroelastomer, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof.

[0121] The separator separates the negative and positive electrodes and provides a passage for lithium ions to move. Any separator commonly used in secondary batteries can be used without any particular restrictions, and in particular, one that has low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used. In addition, a conventional porous non-woven fabric, for example, a non-woven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. can also be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can optionally be used in a single-layer or multi-layer structure.

[0122] The above lithium secondary battery may additionally include an electrolyte. Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0123] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0124] As the above non-aqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolan, formamide, dimethylformamide, dioxolan, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxy methane, dioxolan derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyrropionate, ethyl propionate, etc. can be used.

[0125] In particular, among the above carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents with high dielectric constants and thus can be preferably used because they easily dissociate lithium salts. In addition, when low-viscosity, low-dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate are mixed and used in an appropriate ratio with these cyclic carbonates, an electrolyte with high electrical conductivity can be produced, so that they can be used even more preferably.

[0126] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte, for example, the anion of the lithium salt is F - , Cl - , I- , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of may be used.

[0127] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid 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 trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery.

[0128] According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same or a battery pack including the secondary battery are provided. The battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics, and thus can 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.

[0129] Hereinafter, examples will be provided to specifically explain the present specification. However, the embodiments described herein may be modified in various ways, and the scope of the present application is not limited to the embodiments described below. The embodiments of the present application are provided to more fully explain the present specification to those of ordinary skill in the art.

[0130]

[0131] Example 1

[0132] A 0.5 M sucrose aqueous solution was placed in an autoclave and reacted at 180°C for 24 hours to synthesize spherical particles. After the reaction, the obtained carbon particles were washed 2-3 times with ethanol. The carbon particles dried at 100°C for more than 12 hours and KOH were mixed in a 1:3 weight ratio and heated at 800°C for 2 hours in a nitrogen atmosphere to expand the pores. After washing with distilled water, the particles were dried at 100°C for more than 12 hours. The carbon particles with formed oxide layers were placed in the hot zone of a CVD device, and SiH4 / H2 = 5 / 95 gas was flowed at 600°C for 2 hours to produce a silicon / carbon composite. Afterwards, heat treatment was performed at 700°C for 3 hours in an O2 / Ar = 5 / 95 atmosphere to form an oxide layer. A silicon / carbon composite was placed in the hot zone of a CVD device, and methane was blown into the hot zone at 700°C using Ar as a carrier gas to react for 1 hour to form a carbon layer on the surface, thereby manufacturing a negative active material including an oxide layer and a carbon layer on the surface of the silicon / carbon composite.

[0133]

[0134] Example 2

[0135] A negative electrode active material was manufactured in the same manner as in Example 1, except that the carbon particle:KOH ratio was changed to 1:5 and the heat treatment temperature was changed to 900°C.

[0136]

[0137] Example 3

[0138] A negative active material was manufactured in the same manner as in Example 1, except that the oxide layer formation conditions were changed to O2 / Ar = 10 / 90 atmosphere, 800°C, and 3 hours.

[0139]

[0140] Example 4

[0141] A negative active material was manufactured in the same manner as in Example 1, except that the heat treatment time for forming the carbon layer was increased to 3 hours.

[0142]

[0143] Example 5

[0144] A negative active material was manufactured in the same manner as in Example 1, except that SiH4 / H2 = 5 / 95 gas was flowed at a flow rate of 50 ml / min at 600°C for 1 hour.

[0145]

[0146] Comparative Example 1

[0147] A negative active material was manufactured in the same manner as in Example 1, except that the oxide layer formation step was omitted.

[0148]

[0149] Comparative Example 2

[0150] A negative electrode active material was manufactured in the same manner as in Example 1, except that the carbon layer formation step was omitted.

[0151]

[0152] Comparative Example 3

[0153] A negative active material was manufactured in the same manner as in Example 1, except that the oxide layer formation step was changed to after the carbon layer formation step.

[0154]

[0155] Comparative Example 4

[0156] A negative active material was manufactured in the same manner as in Example 1, except that an Al2O3 coating layer with a thickness of approximately 1 nm was formed as an oxide layer.

[0157]

[0158] The particle size (D50) of the above negative active material and the particle size of silicon crystals were analyzed using a laser diffraction particle size analysis method using a Microtrac S3500 device.

[0159] The specific surface area of ​​the above negative active material was measured using a BET six-point method by nitrogen gas adsorption flow using a porosimetry analyzer (Bell Japan Inc, Belsorp-Ⅱ mini).

[0160] The thickness and surface Si:C ratio of the above oxide layer were measured using XPS. The XPS instrument used was Nexsa 2 from Thermo Fisher Scientific. Since the XPS instrument can only analyze samples within the area of ​​the x-ray spot size (400 μm), approximately 2 to 3 points were measured, and the measurement was performed while etching the sample surface. At this time, when the element ratio differed by location, the maximum and minimum values ​​of the measured values ​​according to the deviation by location were expressed as the upper and lower limits, respectively.

[0161] The total Si:C ratio and the composition of the oxide layer of the above negative active material were derived by measuring C through a CS analyzer and O through an ONH analyzer, and then calculating the Si content by subtracting the ratio from the total.

[0162] Oxide layer thickness (nm) Particle size (D50) (㎛) BET specific surface area (m 2 / g) Silicon grain size (nm) Surface Si:C ratio Total Si:C ratio Oxide layer composition (x value of SiOx) Example 1 10~207.8 5.8 1.5 1:3~1:3.5 1:1.0 2 1.8 Example 2 10~207.6 4.7 1.3 1:3~1:3.5 1:1.0 7 1.75 Example 3 20~308.05 05 2 1:3~1:3.5 1:0.9 8 1.85 Example 4 10~207.9 4.8 1.7 1:3.5~1:4 1:1.0 9 1.8 Example 5 10~207.8 5.8 1.5 1:3.5~1:41: 1.2 8 1.8 Comparative example 10~57.65.51.51:3~1:3.51:1.010.1Comparative Example 210~207.7231.61:0.8~1:1.21:0.931.8Comparative Example 35~107.85.91.41:2.5~1:31:0.981.2Comparative Example 41(Al2O3)7.87.51.51:3~1:3.51:1.021.85

[0163] <Experimental Example: Evaluation of discharge capacity, initial efficiency, and lifespan (capacity retention rate) characteristics>

[0164] A negative electrode and a battery were manufactured using the negative electrode active materials of the examples and comparative examples, respectively.

[0165] The above-mentioned negative electrode active material, carbon black as a conductive agent, and PAA (poly acrylic acid) as a binder were mixed in a weight ratio of 80:10:10 to prepare a mixture. Then, 7.8 g of distilled water was added to 5 g of the above mixture and stirred to prepare a negative electrode slurry. The negative electrode slurry was applied to a copper (Cu) metal thin film as a negative electrode current collector with a thickness of 20 μm and dried. At this time, the temperature of the circulating air was 60°C. Subsequently, the negative electrode was manufactured by roll pressing and drying in a vacuum oven at 130°C for 12 hours.

[0166] The manufactured negative electrode was cut into a 1.7671㎠ circular lithium (Li) metal film as the positive electrode. A porous polyethylene separator was interposed between the positive and negative electrodes, and 0.5 parts by weight of vinylene carbonate was dissolved in a mixed solution of methyl ethyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3, and an electrolyte solution containing 1M LiPF6 was injected to manufacture a lithium coin half-cell.

[0167]

[0168] Charge and discharge were performed on the manufactured battery to evaluate the discharge capacity, initial efficiency, and capacity retention rate, which are listed in Table 2 below.

[0169] The 1st and 2nd cycles were charged and discharged at 0.1C, and from the 3rd to the 49th cycles, the charging and discharging was performed at 0.5C. The 50th cycle was terminated in a charged state (with lithium in the negative electrode).

[0170] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off)

[0171] Discharge conditions: CC (constant current) condition 1.5 V

[0172] Based on the results of a single charge / discharge cycle, the discharge capacity (mAh / g) and initial efficiency (%) were derived. Specifically, the initial efficiency (%) was derived using the following calculation.

[0173] Initial efficiency (%) = (1-time discharge capacity / 1-time charge capacity) × 100

[0174] The capacity retention rate was derived by the following calculations.

[0175] Capacity retention rate (%) = (49 discharge capacity / 1 discharge capacity) × 100

[0176]

[0177] <Experimental Example: Evaluation of Shear Viscosity Characteristics>

[0178] As part of the fairness evaluation, the change in shear viscosity at a shear rate of 1 Hz of a slurry prepared by mixing graphite: the above negative electrode active material: carbon black: CMC: PAA = 77:20:1:1:1 in a weight ratio was measured and is shown in Table 2 below. Specifically, the change (%) in shear viscosity was derived by the following calculation formula.

[0179] Shear viscosity change (%) = ((Shear viscosity of slurry after 48 h - Shear viscosity of slurry immediately after mixing) / Shear viscosity of slurry immediately after mixing) × 100

[0180] Gas generation time: 20 g of slurry was placed in a 10 * 15 cm aluminum pouch, vacuum sealed, and the volume change was measured using Archimedes' principle. The time when the volume change was 4 mL or more was defined as the gas generation time.

[0181]

[0182] The characteristics evaluated as above are shown in Table 2 below.

[0183] Battery discharge capacity (mAh / g) Initial efficiency (%) Capacity retention rate (%) Shear viscosity change (%) Gas generation time (hr) Example 1 195592911766 Example 2 181090872048 Example 3 188389841870 Example 4 191290891766 Example 5 144588901952 Comparative example 1 192490883218 Comparative example 2 191090814412 Comparative example 3 187889832624 Comparative example 4 189089875010

[0184] Examples 1 to 5 showed that the battery performances of discharge capacity, initial efficiency, and capacity retention were superior to those of Comparative Example 1, in which only a fine oxide layer was formed by natural oxidation without going through an oxide layer formation process, Comparative Example 2, in which a carbon layer was not formed, Comparative Example 3, in which the order of the oxide layer and the carbon layer was reversed, and Comparative Example 4, in which a 1 nm thick Al2O3 coating layer was formed. Furthermore, Examples 1 to 5 showed an extremely low change in shear viscosity by controlling gas generation during the process due to the presence of the oxide layer and the carbon layer, and the gas generation timing could be significantly delayed. That is, it was confirmed that the Examples could significantly improve not only battery performance but also processability by preventing gas generation.

Claims

1. A core comprising a silicon carbon composite; An oxide layer provided on at least a portion of the core, and including silicon oxide, wherein a thickness of at least 50% of the oxide layer is greater than 5 nm; and A negative active material comprising a carbon layer provided on at least a portion of the above oxide layer.

2. A negative electrode active material according to claim 1, wherein the Si:C element ratio on the surface of the negative electrode active material is 1:1 to 1:

4.

3. A negative electrode active material according to claim 1, wherein the Si:C element ratio in the entire negative electrode active material is 0.9:1.1 to 1.1:0.

9.

4. In claim 1, the silicon oxide of the oxide layer is SiO x A negative active material having (x is 0.1 or more and less than 2).

5. In claim 1, the negative active material comprises silicon crystal grains having a particle size of 8 ㎛ or less.

6. A negative electrode active material according to claim 1, wherein the thickness of the oxide layer is 200 nm or less.

7. A negative electrode active material according to claim 1, wherein at least 50% of the thickness of the oxide layer is at least 10 nm.

8. A negative electrode active material according to claim 1, wherein the silicon carbon composite comprises porous carbon particles and silicon particles positioned on the surface or internal pores of the porous carbon particles.

9. A negative electrode comprising a negative electrode active material, a conductive material, and a binder according to any one of claims 1 to 8.

10. A lithium secondary battery comprising a negative electrode, a positive electrode, and a separator according to claim 9.

11. A battery module including a lithium secondary battery according to claim 10.

12. A battery pack comprising a lithium secondary battery according to claim 10.

13. A battery pack comprising the battery module of claim 11.

14. A step of forming a core including a silicon carbon composite; A step of forming an oxide layer comprising silicon oxide and provided on at least a portion of the core; and A method for producing a negative active material according to any one of claims 1 to 8, comprising a step of forming a carbon layer on at least a portion of the oxide layer.

15. In claim 14, The step of forming an oxide layer including silicon oxide and provided on at least a portion of the core includes a step of performing heat treatment in an atmosphere including oxygen, A method for manufacturing a negative electrode active material, wherein the above heat treatment is performed at 600°C to 700°C.