Negative electrode active material, negative electrode, and lithium secondary battery

Abstract

The present invention relates to a negative electrode active material including a core containing a silicon carbon composite; an oxide layer containing silicon oxide and provided on at least a portion of the core, wherein 50% or more of the oxide layer has a thickness exceeding 5 nm; and a carbon layer provided on at least a portion of the oxide layer, as well as to a negative electrode and a lithium secondary battery including the same.

Description

[Technical Field]

[0001] This specification asserts the benefit as of the filing date of Korean Patent Application No. 10-2023-0098692, filed with the Korean Intellectual Property Office on 28 July 2023, and Korean Patent Application No. 10-2024-0098727, filed with the Korean Intellectual Property Office on 25 July 2024, and all content disclosed in the documents of said Korean Patent Applications is incorporated herein.

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

[0003] In recent years, with the rapid proliferation of battery-powered electronic devices such as mobile phones, laptops, electric vehicles, power tools, and vacuum cleaners, the demand for small, lightweight, and relatively high-capacity and / or high-power rechargeable batteries has been rapidly increasing. In particular, lithium-ion batteries, being lightweight and possessing high energy density, have attracted considerable attention as a power source for electronic devices. As a result, research and development efforts to improve the performance of lithium-ion batteries are being actively pursued.

[0004] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, an electrolyte, an organic solvent, and the like. Furthermore, active material layers containing positive electrode active material and negative electrode active material can be formed on a current collector at both the positive and negative electrodes. Generally, lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are used as the positive electrode active material, while lithium-free carbon-based active materials and silicon-based active materials are used as the negative electrode active material.

[0005] Among negative electrode active materials, silicon-based active materials are attracting attention because they have a higher capacity and superior fast charging characteristics compared to carbon-based active materials. However, silicon-based active materials have the disadvantage of low initial efficiency due to a large degree of volume expansion / contraction during charging and discharging, resulting in a large irreversible capacity.

[0006] Therefore, there is a need to develop an anode active material that can improve the performance of lithium secondary batteries.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention relates to an anode active material that can improve the life characteristics of a lithium secondary battery, an anode containing the anode active material, and a secondary battery containing the same.

Means for Solving the Problems

[0008] One embodiment of the present invention provides an anode active material including a core containing a silicon-carbon composite; an oxide layer provided on at least a part of the core and containing silicon oxide, wherein more than 50% of the thickness of the oxide layer exceeds 5 nm; and a carbon layer provided on at least a part of the oxide layer.

[0009] According to one embodiment of the present invention, the Si:C element ratio on the surface of the anode 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 anode 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 anode active material of the above-described embodiment contains silicon crystal grains having a particle size of 8 μm or less.

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

[0014] One embodiment of the present invention provides a lithium secondary battery including the anode, a cathode, 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 embodiment described above.

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

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

[0018] According to embodiments of the present invention, by using a silicon-carbon composite as the core material of the negative electrode active material, high capacity and efficiency can be achieved, and the presence of an oxide layer and a carbon layer can improve the battery's lifespan and prevent problems such as gas generation. Specifically, the oxide layer and carbon layer can prevent a decrease in lifespan characteristics due to the high reactivity of silicon, even when the silicon crystal grains are small, and can prevent the problem of gas generation when silicon comes into contact with water in aqueous processes. [Modes for carrying out the invention]

[0019] The present invention will be described in more detail below to aid in understanding the invention. The present invention may be realized in various different forms and is not limited to the embodiments described herein. In this regard, terms and words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather should be interpreted in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0020] In this specification, terms such as “include,” “provide,” or “have” indicate the presence of implemented features, figures, stages, components, or combinations thereof, and should be understood not to preemptively exclude the possibility of the presence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0021] Furthermore, when a layer or other part is said to be "above" or "above" another part, it includes not only the case where it is "directly above" the other part, but also the case where another part exists in between. In contrast, when a part is said to be "directly above" another part, it means that there is no other part in between. Also, when a part is said to be "above" or "above" a reference part, it means that it is located above or below the reference part, and does not necessarily mean that it is located "above" or "above" in the opposite direction of gravity.

[0022] The terms and words used herein should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather should be interpreted in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best explain their invention.

[0023] In this specification, singular expressions of terms include plural expressions unless the context clearly indicates otherwise.

[0024] In this specification, "average particle size (D50)" can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size (D50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters, and can obtain highly reproducible and high-resolution results.

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

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

[0027] [Formula 1-1] FWHM = (Kλ) / (LCOSθ)

[0028] In the above formula 1-1, L represents the crystal grain size, K is a constant, θ is the Bragg angle, and λ is the wavelength of the X-ray.

[0029] Furthermore, the shape of the crystal grains varies and can be measured three-dimensionally. Generally, the crystal grain size can be measured using the commonly used circle method or diameter measurement method, but is not limited to these methods.

[0030] The aforementioned diameter measurement method involves drawing 5 to 10 equilibrium lines, each with a length of L mm, on a micrograph of the target particle, and then counting and averaging the number of crystal grains z along each line. In this process, only grains that fit entirely within the line are counted, and those that do not are excluded. If the number of lines is P and the magnification is V, the average grain size can be calculated using the following equation 1-2.

[0031] [Formula 1-2] Dm=(L*P*10 3 ) / (zV)(μm)

[0032] Furthermore, the circle method involves drawing a circle of a predetermined diameter on a micrograph of the target particles, and then determining the average area of ​​the crystal grains by the number of crystal grains that fall within the circle and the number of crystal grains that fall on the boundary line. This can be calculated using the following equations 1-3.

[0033] [Formula 1-3] Fm=(Fk*10 6 ) / ((0.67n+z)V 2 )(μm 2 )

[0034] In equations 1-3 above, Fm represents the average particle area, Fk represents the measurement area on the photograph, z represents the number of particles within the circle, n represents the number of particles along the arc, and V represents the microscope magnification.

[0035] Preferred embodiments of the present invention will be described in detail below. However, 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 later.

[0036] <Negative electrode active material> One embodiment of the present invention provides a negative electrode active material comprising: a core containing a silicon-carbon composite; an oxide layer containing silicon oxide provided on at least a portion of the core, wherein the oxide layer has a thickness of 50% or more of the oxide layer that exceeds 5 nm; and a carbon layer provided on at least a portion of the oxide layer. Here, 50% or more of the oxide layer means 50% or more of the area covered by the oxide layer.

[0037] According to one embodiment, the silicon-carbon composite contained in the core may be a Si / C-based active material.

[0038] In this specification, the silicon-carbon composite is a composite of Si and C, and is distinguished from silicon carbide represented by SiC. The silicon carbide does not react electrochemically with lithium, and all performances such as lifespan can be measured to be 0.

[0039] The silicon-carbon composite may be a composite of silicon and graphite or the like, and may form a structure surrounded by graphene or amorphous carbon or the like around a core in which silicon and graphite or the like are composite. The silicon in the silicon-carbon composite may be silicon nanoparticles.

[0040] According to one embodiment, the silicon-carbon composite includes porous carbon-based particles and silicon particles located on the surface or internal pores of the porous carbon-based particles.

[0041] According to one embodiment, the silicon-carbon composite has a surface area of 0.5 m 2 / g to 10 m 2 / g by the BET method, a pore volume of 0.005 cm 3 / g to 0.03 cm 3 / g, and the pore size by the BET method may be 10 nm to 20 nm. The silicon-carbon composite may have a pore volume of 0.005 cm 3 / g to 0.03 cm 3 / g measured by the mercury intrusion method.

[0042] According to one embodiment, the silicon-carbon composite may have a D90 particle size of 15 μm to 25 μm, a D50 particle size of 2 μm to 10 μm, and a D10 particle size of 0.1 μm to 1 μm.

[0043] The silicon particles formed on the surface and internal pores of the carbon-based particles may be silicon nanoparticles, and may be crystalline, quasi-crystalline, amorphous, or a combination thereof.

[0044] According to one embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a portion of the core, and the thickness of 50% or more of the oxide layer is greater than 5 nm, 7 nm or more, 8 nm or more, 9 nm or more, or 10 nm or more.

[0045] According to one embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a portion of the core, and the thickness of 50% or more of the oxide layer is 10 nm or more.

[0046] According to one embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a portion of the core, and the 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.

[0047] If the thickness of 50% or more, preferably 70% or more, and more preferably 90% or more of the oxide layer exceeds 5 nm, it is possible to prevent a decrease in lifespan characteristics and gas generation due to contact with water during aqueous processes by blocking the exposure of silicon or reducing the reactivity of silicon. If the thickness of 50% or more of the oxide layer containing silicon oxide is 5 nm or less, internal silicon particles may be exposed to the outside, which may worsen aqueous processability. In this case, the electrode condition will not be good, and the charge / discharge capacity, initial efficiency, and / or lifespan characteristics of the secondary battery will decrease.

[0048] According to this embodiment, the oxide layer contains silicon oxide.

[0049] By including silicon oxide in the oxide layer, contact between water and silicon is prevented during the production of the aqueous slurry, improving the electrode condition and enhancing the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0050] In particular, when the oxide layer contains silicon oxide and the thickness of more than 50% of the oxide layer exceeds 5 nm, the probability that most of the silicon is passivated by the oxide layer is high, and the aqueous processability is improved, so that the charge-discharge capacity, initial efficiency, and / or life characteristics of the secondary battery are improved.

[0051] According to one embodiment, the silicon oxide contains SiOx (0 < x ≤ 2). The SiOx (0 < x ≤ 2) may be in a form containing various oxidation states such as Si and SiO2. That is, the x corresponds to the number ratio of O to Si contained in the SiOx (0 < x ≤ 2). In other words, the SiOx (0 < x ≤ 2) is not in a single state but in a state where multiple oxidation states are mixed, and in XPS, while etching, the average value of x can be obtained based on the content of oxygen (O).

[0052] According to one example, the silicon oxide is SiOx (0.1 ≤ x < 2). The x may be 1 or more and less than 2, for example, 1.5 or more and less than 2.

[0053] According to one embodiment, the silicon oxide is in an amorphous state. The SiOx (0 < x ≤ 2) may exist in the form of an island type or a thin film type layer, and is not limited thereto, and may exist in various forms.

[0054] 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. When the thickness of the oxide layer is excessively thin, the effect of blocking contact with water cannot be expected, and when it is excessively thick, the resistance increases and the discharge capacity is not manifested. 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 for providing the effect of blocking contact with water by the oxide layer and providing a suitable discharge capacity.

[0055] In one embodiment, the negative electrode active material includes a carbon layer provided on at least a portion of the oxide layer.

[0056] Specifically, the carbon layer, together with the aforementioned oxide layer, can block the exposure of silicon or reduce the reactivity of silicon, thereby preventing a decrease in lifespan characteristics and gas generation due to contact with water during aqueous processes. Furthermore, the carbon layer imparts conductivity to the negative electrode active material, thereby improving the initial efficiency, lifespan characteristics, and battery capacity characteristics of the secondary battery.

[0057] In one embodiment, the carbon layer may contain at least one of amorphous carbon and crystalline carbon.

[0058] In one embodiment, the carbon layer may be an amorphous carbon layer. The amorphous carbon can appropriately maintain the strength of the carbon layer and suppress the expansion of the silicon-carbon composite.

[0059] Furthermore, the carbon layer may or may not contain crystalline carbon.

[0060] The crystalline carbon can further improve the conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotubes, and graphene.

[0061] The amorphous carbon can appropriately maintain the strength of the carbon layer and suppress the expansion of the silicon-carbon composite. The amorphous carbon may be a carbon-based material formed by using at least one carbide or hydrocarbon selected from the group consisting of tar, pitch, and other organic materials as a source in chemical vapor deposition.

[0062] The aforementioned carbonized organic substances may be carbonized organic substances selected from carbonized sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose, and combinations thereof.

[0063] 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, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, or hexane. Examples of the aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene.

[0064] In one embodiment, the carbon layer may be included in amounts 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 electrode active material. More specifically, it may be included in amounts of 0.5 to 15 parts by weight, or 1 to 10 parts by weight. When these ranges are met, conductivity can be improved and a decrease in the capacity and efficiency of the negative electrode active material can be prevented.

[0065] In one embodiment, the thickness of the carbon layer may be 1 nm to 500 nm, more specifically 5 nm to 300 nm, and more specifically 5 nm to 100 nm. When this range is met, the conductivity of the negative electrode active material is improved, volume changes of the negative electrode active material are easily suppressed, side reactions between the electrolyte and the negative electrode active material are suppressed, and the initial efficiency and / or lifespan of the battery are improved.

[0066] Specifically, the carbon layer may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0067] Since the carbon layer is formed after the formation of the oxide layer described above, it may be provided on an oxide layer containing silicon oxide, or on a surface of the core where the oxide layer is not provided. Furthermore, the carbon layer may completely surround the oxide layer, but it can also be formed only on a part of the oxide layer, so both the oxide layer and the carbon layer may be observed on the surface of the negative electrode active material.

[0068] According to one embodiment of the present invention, the negative electrode active material may include silicon crystal grains with a particle size of 8 μm or less. The silicon crystal grains may be included in a core containing the silicon-carbon composite described above. The present invention is advantageous not only for improving lifetime performance by including silicon crystal grains with a particle size of 8 μm or less, but also for preventing gas generation in aqueous processes due to the oxide layer and carbon layer described above. For example, the negative electrode active material may include silicon crystal grains with a particle size of 8 μm or less, for example, 5 μm or less, 1,000 nm or less, or at the level of several tens of nanometers.

[0069] According to one embodiment of the present invention, the Si:C elemental ratio on the surface of the negative electrode active material in 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 its surface, this means that the proportion of carbon is even higher on the surface. When such an elemental ratio is present, it means that the surface of the negative electrode active material is well coated with carbon, which can be measured by XPS. The elemental ratio can be adjusted by adjusting the amount of carbonized material during the introduction of the carbon layer or by changing the heat treatment time.

[0070] According to one embodiment of the present invention, the Si:C elemental ratio in the entire negative electrode active material of the embodiment described above is 0.9:1.1 to 1.1:0.9, and may be, for example, 1:0.95 to 1:1.1. While a higher Si content is advantageous for increasing capacity, it may lead to problems such as the particles not being deposited in the carbon pores but being deposited on the surface, causing large particle growth, which can adversely affect lifespan and processability. The aforementioned range of Si:C elemental ratio is advantageous for satisfying capacity, lifespan, and processability. The negative electrode active material according to the embodiment described above is mainly composed of three elements: Si, C, and O. After eliminating the influence of the remaining elements, C is measured via a CS analyzer and O is measured via an ONH analyzer, and the Si content can be calculated by subtracting their proportions from the total.

[0071] According to one embodiment, the average particle size (D50) of the negative electrode active material may be 0.1 μm to 30 μm, more specifically 1 μm to 20 μm, and more specifically 1 μm to 10 μm or 2 μm to 10 μm. When the above range is satisfied, the structural stability of the active material during charging and discharging can be ensured, the problem of volume expansion / contraction levels becoming large due to excessively large particle size can be prevented, and the problem of initial efficiency decreasing due to excessively low particle size can be prevented.

[0072] According to one embodiment, the BET specific surface area of ​​the negative electrode active material is 20 m². 2 / g or less, for example, 10m 2 It is preferable that the amount is less than or equal to / g. For example, the BET specific surface area of ​​the negative electrode active material is 0.11m². 2 / g or more 10m 2 / g or less, for example, 4m 2 / g~6m 2 It may also be / g. The specific surface area can be measured by the BET (Brunauer-Emmett-Teller; BET) method. For example, it can be measured using the BET 6-point method by nitrogen gas adsorption flow using a porosimetry analyzer (Bell Japan Inc, Belsorp-II mini).

[0073] <Method for manufacturing negative electrode active material> The present invention provides a method for producing a negative electrode active material, specifically, a method for producing a negative electrode active material according to the embodiments described above.

[0074] One embodiment of the present invention provides a method for producing a negative electrode active material, comprising the steps of: forming a core containing a silicon-carbon composite; forming an oxide layer containing silicon oxide provided on at least a portion of the core; and forming a carbon layer on at least a portion of the oxide layer.

[0075] According to one embodiment, the process of forming the silicon-carbon composite core may include the steps of: etching carbon-based particles containing internal pores to expand the internal pores of the carbon-based particles; and forming silicon particles on the surface and internal pores of the carbon-based particles with expanded internal pores.

[0076] The step of expanding the internal pores of the carbon-based particles may be carried out in a nitrogen (N2) atmosphere, an oxygen (O2) atmosphere, or an air atmosphere. Specifically, the flow rate of the oxygen (O2) or air containing the oxygen may be controlled to 0.1 L / min to 10 L / min.

[0077] The step of expanding the internal pores of the carbon-based particles may be carried out at a temperature range of 400°C to 1200°C for 30 minutes to 4 hours.

[0078] Depending on the conditions for expanding the internal pores of the carbon-based particles, the resulting porous carbon-based particles may have different pore characteristics.

[0079] An etching agent may be used to expand the internal pores of the carbon-based particles, for example, a basic material such as KOH may be used. For example, the internal pores of the carbon-based particles may be expanded by mixing the carbon-based particles and KOH in a weight ratio of 1:1 to 1:5.

[0080] The step of forming the silicon particles may be carried out using chemical vapor deposition (CVD). In this case, silicon nanoparticles may be deposited on the surface and / or on the internal pores of the carbon-based particles whose internal pores have been expanded, forming a silicon coating layer in the form of a film, an island, or a mixture thereof.

[0081] The step of forming the silicon particles may be carried out by using a chemical vapor deposition (CVD) apparatus to flow SiH4 / H2 gas through carbon-based particles at 500°C to 900°C to form a silicon-carbon composite.

[0082] The step of forming an oxide layer containing silicon oxide, which is provided on at least a portion of the core, can be formed by partially or entirely oxidizing the surface of the core containing the silicon-carbon composite described above.

[0083] According to one embodiment of the present invention, the step of forming an oxide layer containing silicon oxide, which is provided on at least a portion of the core, may include a heat treatment step carried out in an oxygen-containing atmosphere.

[0084] Specifically, the heat treatment step can form an oxide layer by performing heat treatment at 500°C to 700°C, for example, 600°C to 700°C.

[0085] If the heat treatment process is carried out at temperatures exceeding 700°C, silicon-carbon composites (SiC) will grow, which is undesirable because it can actually reduce the material's capacity and efficiency.

[0086] The oxygen-containing atmosphere may contain more than 0 vol% but less than or equal to 10 vol%, for example, 3 vol% to 7 vol%. The oxygen-containing atmosphere may also contain an inert gas other than oxygen, such as argon. The heat treatment time is not particularly limited, but may be, for example, 1 to 12 hours, 1 to 8 hours, or 2 to 5 hours.

[0087] The thickness of the oxide layer may be adjusted by the heat treatment temperature, time, etc.

[0088] The step of forming a carbon layer on at least a portion of the oxide layer may be performed using a carbon-based material, for example, by chemical vapor deposition (CVD) using hydrocarbon gas, or by carbonizing a carbon source.

[0089] Specifically, a silicon-carbon composite having an oxide layer containing silicon oxide may be introduced into a reactor, and then a hydrocarbon gas may be deposited by chemical vapor deposition (CVD) at 600°C to 700°C. The hydrocarbon gas may be at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene, and may be heat-treated at 600°C to 700°C.

[0090] <Negative electrode> One embodiment of the present invention provides a negative electrode comprising a negative electrode active material, a conductive material, and a binder according to the embodiment described above.

[0091] 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 contain the negative electrode active material. Furthermore, the negative electrode active material layer may further include a binder and / or a conductive material.

[0092] The negative electrode active material layer may 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, drying, and rolling.

[0093] The negative electrode slurry comprises the negative electrode active material, a binder, and / or a conductive material.

[0094] The negative electrode slurry may further contain additional negative electrode active material.

[0095] As the 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 alloys, Sn alloys, or Al alloys; and SiO2. β Examples include lithium-doped and dedoped metal oxides such as (0<β<2), SnO2, vanadium oxide, lithium titanium oxide, and lithium vanadium oxide; or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites. One or more of these mixtures may be used. A metallic lithium thin film may also be used as the negative electrode active material. As for the carbon material, either low-crystallinity carbon or high-crystallinity carbon may be used. Examples of low-crystalline carbon include soft carbon and hard carbon, while examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, 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.

[0096] The additional negative electrode active material may be a carbon-based negative electrode active material.

[0097] In one embodiment of the present invention, the weight ratio of the negative electrode active material contained in the negative electrode slurry to the additional negative electrode active material may be 10:90 to 90:10, and more specifically, it may be 10:90 to 50:50.

[0098] The negative electrode current collector is not particularly limited, as long as it does not cause a chemical change in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Specifically, transition metals that readily adsorb carbon, such as copper and nickel, may be used as the current collector. The thickness of the current collector may be 6 μm to 20 μm, but is not limited thereto.

[0099] The binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, and may also contain various copolymers thereof.

[0100] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0101] The negative electrode slurry may contain a solvent for forming the negative electrode slurry. Specifically, the solvent for forming the negative electrode slurry may contain at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, specifically distilled water, in order to facilitate the dispersion of the components.

[0102] <Secondary battery> One embodiment of the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, and a separator according to the embodiment described above.

[0103] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0104] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0105] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); examples include, but are not limited to, LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may also be Li metal.

[0106] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0107] 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 it has electronic conductivity without causing a chemical change in the battery that is constructed. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0108] Furthermore, the positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0109] The separator separates the negative and positive electrodes and provides a pathway for lithium ions to move. Generally, any separator commonly used in secondary batteries can be used without particular limitations, but it is especially preferable that it has low resistance to ion movement in the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymeric substances may be used, and they may be selectively used as single-layer or multi-layer structures.

[0110] The lithium secondary battery may further contain 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.

[0111] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0112] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, 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, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0113] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents because they have high dielectric constants and dissociate lithium salts well. Furthermore, when such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be produced, making them even more preferable.

[0114] As the metal salt, a lithium salt may be used, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte, for example, as the anion of the lithium salt, F - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3- , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - , and (CF3CF2SO2)2N - You may use one or more selected from the group consisting of the following:

[0115] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for purposes such as improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0116] According to another embodiment of the present invention, a battery module and a battery pack including the secondary battery as a unit cell are provided. The battery module and battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, and can therefore be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems. [Examples]

[0117] The following describes the Specification in detail with reference to examples. However, the examples provided herein may be modified in various other forms, and the scope of this application should not be construed as being limited to the examples described below. The examples of this application are provided to give a more complete explanation of this Specification to a person of average knowledge in the industry.

[0118] [Example 1] 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 resulting carbon-based particles were washed 2-3 times with ethanol. The carbon-based particles, dried at 100°C for more than 12 hours, were mixed with KOH in a 1:3 parts by weight ratio and heated in a nitrogen atmosphere at 800°C for 2 hours to expand the pores. After washing with distilled water, the particles were dried at 100°C for more than 12 hours. The carbon-based particles with the formed oxide layer were placed in the hot zone of a CVD apparatus, and SiH4 / H2=5 / 95 gas was flowed at a flow rate of 50 ml / min at 600°C for 2 hours to produce a silicon / carbon composite. Subsequently, heat treatment was performed in an O2 / Ar=5 / 95 atmosphere at 700°C for 3 hours to form an oxide layer. A silicon / carbon composite was placed in the hot zone of a CVD apparatus, and methane was blown into the 700°C hot zone using Ar as a carrier gas and reacted for 1 hour to form a carbon layer on the surface. This produced a negative electrode active material containing an oxide layer and a carbon layer on the surface of the silicon / carbon composite.

[0119] [Example 2] The negative electrode active material was manufactured in the same manner as in Example 1, except that the carbon-based particle:KOH ratio was changed to 1:5 and the heat treatment temperature was changed to 900°C.

[0120] [Example 3] The negative electrode active material was produced in the same manner as in Example 1, except that the conditions for forming the oxide layer were changed to an O2 / Ar=10 / 90 atmosphere, 800°C, and 3 hours.

[0121] [Example 4] The negative electrode 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.

[0122] [Example 5] The negative electrode active material was produced 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.

[0123] [Comparative Example 1] The negative electrode active material was produced in the same manner as in Example 1, except that the oxidation layer formation step was omitted.

[0124] [Comparative Example 2] The negative electrode active material was produced in the same manner as in Example 1, except that the carbon layer formation step was omitted.

[0125] [Comparative Example 3] The negative electrode active material was produced in the same manner as in Example 1, except that the oxidation layer formation step was changed to occur after the carbon layer formation step.

[0126] [Comparative Example 4] The negative electrode 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.

[0127] The particle size (D50) of the negative electrode active material and the particle size of the silicon crystal grains were analyzed using laser diffraction grain size analysis with a Microtrac S3500 instrument.

[0128] The specific surface area of ​​the negative electrode active material was measured using a porosimetry analyzer (Bell Japan Inc., Belsorp-II mini) by nitrogen gas adsorption flow using the BET 6-point method.

[0129] The thickness of the oxide layer and the surface Si:C ratio were measured by XPS. A Nexsa 2 XPS instrument manufactured by Thermo Fisher Scientific was used. Since this XPS instrument analyzes only the sample within an x-ray spot size (400 μm), measurements were taken at approximately 2-3 points, etching the sample surface during the measurement process. In cases where the elemental ratio differed at different locations, the maximum and minimum values ​​measured according to the positional deviations were used as the upper and lower limits, respectively.

[0130] The total Si:C ratio of the negative electrode active material and the composition of the oxide layer were derived by first measuring C using a CS analyzer and O using an ONH analyzer, and then subtracting these proportions from the total to calculate the Si content.

[0131] [Table 1]

[0132] <Experimental Example: Evaluation of Discharge Capacity, Initial Efficiency, and Lifetime (Capacity Retention Rate) Characteristics> A negative electrode and a battery were manufactured using the negative electrode active materials of the examples and comparative examples, respectively.

[0133] A mixture was prepared by mixing the aforementioned negative electrode active material, carbon black as a conductive material, and PAA (polyacrylic acid) as a binder in a weight ratio of 80:10:10. Then, 7.8g of distilled water was added to 5g of the mixture and stirred to produce a negative electrode slurry. The negative electrode slurry was applied to a copper (Cu) metal thin film, which was a negative electrode current collector with a thickness of 20μm, and dried. During this process, the temperature of the circulating air was 60°C. Next, the film was rolled (rolled in a roll press) and dried in a vacuum oven at 130°C for 12 hours to produce a negative electrode.

[0134] The manufactured negative electrode was 1.7671 cm 2A circularly cut lithium (Li) metal thin film was used as the positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and an electrolyte solution containing 0.5 parts by weight of vinylene carbonate dissolved in a mixed solution of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3, and 1M LiPF6 was injected to produce a lithium coin half-cell.

[0135] The manufactured batteries were subjected to charging and discharging tests to evaluate their discharge capacity, initial efficiency, and capacity retention rate, and these results are shown in Table 2 below.

[0136] The first and second cycles were charged and discharged at 0.1C, and from the third to the 49th cycle, they were charged and discharged at 0.5C. The 50th cycle ended in a charged state (lithium was in the negative electrode). Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off) Discharge condition: CC (constant current) condition 1.5V

[0137] The discharge capacity (mAh / g) and initial efficiency (%) were derived from the results of a single charge-discharge cycle. Specifically, the initial efficiency (%) was derived by the following calculation. Initial efficiency (%) = (Discharge capacity per cycle / Charge capacity per cycle) × 100

[0138] The capacity retention rates were derived using the following calculations. Capacity retention rate (%) = (49 discharge capacity / 1 discharge capacity) × 100

[0139] <Experimental Example: Evaluation of Processability (Shear Viscosity) Characteristics> As part of the process evaluation, the change in shear viscosity at a shear rate of 1 Hz was measured for a slurry produced by mixing graphite:the aforementioned negative electrode active material:carbon black:CMC:PAA in a weight ratio of 77:20:1:1:1, and the results are shown in Table 2 below. Specifically, the change in shear viscosity (%) was derived using the following formula.

[0140] Change in shear viscosity (%) = ((Shear viscosity of slurry after 48 hours - Shear viscosity of slurry immediately after mixing) / Shear viscosity of slurry immediately after mixing) × 100

[0141] Gas generation point: 20g of slurry was placed in a 10*15cm aluminum pouch, vacuum-sealed, and the volume change was measured using Archimedes' principle. The point at which a volume change of 4mL or more occurred was defined as the gas generation point.

[0142] The characteristics evaluated as described above are shown in Table 2 below.

[0143] [Table 2]

[0144] Examples 1-5 demonstrated that by using a silicon-carbon composite having an oxide layer and a carbon layer of appropriate thickness as the negative electrode active material, the battery performance in terms of discharge capacity, initial efficiency, and capacity retention rate was better than that of Comparative Example 1, in which only a fine oxide layer was formed by spontaneous oxidation without an oxide layer formation process; Comparative Example 2, in which no carbon layer was formed; Comparative Example 3, in which the order of the oxide and carbon layers was changed; and Comparative Example 4, in which a 1 nm Al2O3 coating layer was formed. Furthermore, in Examples 1-5, by controlling gas generation during the process through the presence of the oxide and carbon layers, the change in shear viscosity was extremely low, and the gas generation time was significantly delayed. In other words, these examples confirmed that not only battery performance but also processability could be significantly improved by preventing gas generation.

Claims

1. A core containing a silicon-carbon composite; An oxide layer comprising silicon oxide, provided on at least a portion of the core, wherein the thickness of the oxide layer exceeds 5 nm by 50% or more; and A carbon layer provided on at least a portion of the oxide layer A negative electrode active material containing the above.

2. The 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. The 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. The negative electrode active material according to claim 1, wherein the silicon oxide of the oxide layer is SiOx (where x is 0.1 or more and less than 2).

5. The negative electrode active material according to claim 1, wherein the negative electrode active material includes silicon crystal grains with a particle size of 8 μm or less.

6. The negative electrode active material according to claim 1, wherein the negative electrode active material comprises silicon crystal grains with a particle size of 1,000 nm or less.

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

8. The negative electrode active material according to claim 1, wherein the thickness of 50% or more of the oxide layer is 10 nm or more.

9. The negative electrode active material according to claim 1, wherein the silicon-carbon composite comprises porous carbon-based particles and silicon particles located on the surface or in the internal pores of the porous carbon-based particles.

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

11. A lithium secondary battery comprising the negative electrode, positive electrode, and separator described in claim 10.

12. A battery module comprising the lithium secondary battery described in claim 11.

13. A battery pack comprising the lithium secondary battery described in claim 11.

14. A battery pack comprising the battery module described in claim 12.

15. The step of forming a core containing a silicon-carbon composite; A step of providing at least a portion of the core and forming an oxide layer containing silicon oxide; and Steps to form a carbon layer on at least a portion of the oxide layer. A method for producing a negative electrode active material according to any one of claims 1 to 9, including

16. The step of forming an oxide layer containing silicon oxide, provided on at least a portion of the core, includes the step of performing heat treatment in an oxygen-containing atmosphere, The method for producing a negative electrode active material according to claim 15, wherein the heat treatment is performed at 600°C to 700°C.

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