Negative active material for lithium secondary battery, secondary battery comprising the same, and method for preparing the same

Abstract

The present invention relates to a negative active material for a lithium secondary battery and a method of manufacturing the same, the negative active material for a lithium secondary battery including a silicon-based base material and a coating layer coated on a surface of the silicon-based base material, the coating layer including a carbon layer and at least one nano carbon bead, the carbon layer and the nano carbon bead including sulfur, and a content of a coating material in the coating layer can be 0.10 to 1.50 wt%.

Description

Technical Field

[0001] This invention relates to a battery, and more specifically, to a negative electrode active material for a lithium secondary battery, a secondary battery including the same, and a method for preparing the same. Background Technology

[0002] With increasing public concern about the depletion of fossil fuels and the environmental pollution caused by their use, environmentally friendly energy sources are gaining significant attention as a solution. Among these environmentally friendly energy sources, electricity is receiving increasing attention, particularly lithium-ion batteries.

[0003] Lithium-ion batteries are widely used in small applications such as mobile phones and laptops, as well as medium-to-large applications such as electric vehicles and energy storage systems. As the application scope of lithium-ion batteries expands, the development of new materials for high capacity and high output becomes increasingly important. However, improving electrochemical characteristics remains a technical problem to be solved, considering solutions for enhancing electrochemical properties (such as extending the efficient lifespan in small applications and improving energy characteristics in medium-to-large applications such as electric vehicles). Specifically, in the structure of a lithium-ion battery, the negative electrode material plays a role in storing lithium ions, and it is a factor related to the capacity and lifespan of the lithium-ion battery. Various forms of carbon-based materials have been used as negative electrode materials in the past, including artificial graphite, natural graphite, and hard carbon, which can intercalate / deintercalate lithium ions. Graphite has a relatively low discharge voltage of 0.2V compared to lithium, therefore batteries using graphite as the negative electrode active material exhibit a discharge voltage as high as 3.6V, giving lithium-ion batteries an advantage in energy density. Furthermore, graphite's excellent reversibility ensures a long lifespan for lithium-ion batteries, thus making it the most widely used material.

[0004] Natural graphite is inexpensive and exhibits similar electrochemical properties to synthetic graphite, making it highly effective as a negative electrode material. However, the flake-like shape of natural graphite results in a large surface area and exposed edges, leading to electrolyte penetration or decomposition reactions when used as a negative electrode active material. This can cause edge peeling or damage, resulting in significant irreversible reactions. When natural graphite is used to form electrode plates, the graphite active material is flattened and oriented on the current collector, making it difficult for the electrolyte to impregnate and reducing charge-discharge characteristics.

[0005] To overcome the limitations of graphite, research on novel anode materials continues, with increasing attention paid to silicon-based anode materials. Silicon has the following advantages: it is abundant worldwide, has a higher theoretical capacity of approximately 4,200 mAh / g compared to graphite, has a lower reaction potential with lithium, and is an environmentally friendly material.

[0006] However, the silicon-based anode material has an irregular shape because it is made by mechanically crushing bulk silicon. This irregular shape will cause irregular volume expansion and contraction during the charging and discharging process of silicon, resulting in a decrease in battery performance. Furthermore, the uneven electrode fabrication process may also lead to a decrease in battery performance.

[0007] To solve the aforementioned problems, efforts are needed to suppress the side reactions between the negative electrode material and the electrolyte and to further improve the conductivity. Summary of the Invention

[0008] (a) Technical problems to be solved The technical problem to be solved by the present invention is to provide a negative electrode active material that inhibits side reactions with the electrolyte and further improves conductivity.

[0009] Another technical problem to be solved by the present invention is to provide a method for preparing a negative electrode active material with the aforementioned advantages.

[0010] (II) Technical Solution An embodiment of the present invention provides a negative electrode active material comprising: a silicon-based substrate and a coating applied to the surface of the silicon-based substrate, the coating comprising a carbon layer and at least one carbon nanosphere, the carbon layer and the carbon nanosphere comprising sulfur, and the content of the coating material being 0.10 to 1.50 wt% based on 100% of the total active material.

[0011] According to one embodiment, the negative electrode active material can satisfy the following formula 1: <Formula 1> 0.50 ≤ Coating material content [weight % (wt%)] × BET specific surface area [m²] 2 / g]≤9.50.

[0012] According to one embodiment, the negative electrode active material can satisfy the following formula 2: <Formula 2> 1.40≤Average particle size (D50) [μm] of secondary particles × content of coating material [weight% (wt%)]≤15.0.

[0013] According to one embodiment, the negative electrode active material can satisfy the following formula 3: <Formula 3> 2.0≤(Sulfur content [weight% (wt%)] / Coating material content [weight% (wt%)])×100≤120.0.

[0014] In one embodiment, the negative electrode active material may include 0.03 to 0.3% by weight of the sulfur. In one embodiment, the D10 particle size of the secondary particles may be 2.0 to 6.5 μm. In one embodiment, the average particle size (D50) of the secondary particles may be 6.7 to 9.5 μm.

[0015] In one embodiment, the D90 particle size of the secondary particles can be 10.6~15.7 μm. In one embodiment, the span value of the secondary particles ((D90-D10) / D50) can be 0.80~1.40. In one embodiment, the silicon-based matrix material can be elemental silicon, silicon oxide, silicon nitride, silicon sulfide, silicon carbide, silicon-metal alloy, or combinations thereof.

[0016] In one embodiment, the carbon layer may be an amorphous carbon layer. In one embodiment, the thickness of the carbon layer may be 1.0~10 nm. In one embodiment, the at least one carbon nanosphere may be composed of at least one cluster. In one embodiment, the at least one carbon nanosphere may be at least one of spherical and elliptical shapes.

[0017] Another embodiment of the present invention provides a method for preparing a negative electrode active material for lithium secondary batteries, comprising: a step of uniformly mixing a silicon-based coating material and a sulfur-containing coating material; and a step of heat-treating the mixture to carbonize it, wherein 1 to 8 parts by weight of the coating material may be included relative to 100 parts by weight of the coating material. In one embodiment, the method for preparing a negative electrode active material for lithium secondary batteries may satisfy the following formula 4: <Formula 4> 0.37≤(Sulfur content [weight% (wt%)] / Content of coating material before carbonization [weight% (wt%)])×100≤25.0.

[0018] In one embodiment, the silicon-based coating material may include elemental silicon, silicon oxide, silicon nitride, silicon sulfide, silicon carbide, silicon-metal alloy, and combinations thereof. In one embodiment, the sulfur-containing coating material may be coal tar or pyrolyzed fuel oil (PFO).

[0019] (III) Beneficial Effects One embodiment provides a negative electrode material in which the negative electrode active material comprises a carbon layer including sulfur and at least one carbon nanosphere as a coating, thereby suppressing side reactions with the electrolyte and further improving conductivity.

[0020] Another embodiment of the present invention provides a lithium secondary battery comprising a negative electrode active material having the advantages described above.

[0021] Another embodiment of the present invention provides a method for preparing a negative electrode active material having the above-mentioned advantages. Attached Figure Description

[0022] Figure 1a and Figure 1b These are SEM and TEM images of the negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.

[0023] Figure 2a and Figure 2b These are SEM and TEM images of the negative electrode active material for a lithium secondary battery, a comparative example of the present invention. Figure 2c and Figure 2d These are SEM and TEM images of a negative electrode active material for a lithium secondary battery, which is another comparative example of the present invention.

[0024] Figures 3a to 3d This is a SEM-EDAX image of the negative electrode active material for a lithium secondary battery according to an embodiment of the present invention. Detailed Implementation

[0025] The terms "first," "second," and "third," etc., are used to describe various parts, components, regions, layers, and / or segments, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or segment from another. Therefore, the first part, component, region, layer, or segment described below may be referred to as the second part, component, region, layer, or segment without departing from the scope of the invention.

[0026] The technical terms used herein are for reference only to specific embodiments and are not intended to limit the invention. The singular forms used herein include the plural forms unless the context clearly indicates otherwise. As used in this specification, "comprising" means to embody a particular feature, region, integer, step, action, element, and / or component, and does not exclude the presence or addition of other features, regions, integers, steps, actions, elements, and / or components.

[0027] When one part is mentioned as being "above" or "on top of" another part, it can be directly above or on top of the other part, or there may be other parts in between. Conversely, when one part is mentioned as being "directly above" another part, there are no other parts in between.

[0028] In this specification, D10 refers to the particle size at 10% volume percentage in the cumulative size-distribution curve, D50 refers to the particle size at 50% volume percentage, and D90 refers to the particle size at 90% volume percentage.

[0029] Unless otherwise defined, all terms used herein, including technical and scientific terms, shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms as defined in commonly used dictionaries are further interpreted as having meanings consistent with relevant technical literature and current disclosure, and are not to be construed as having ideal or overly formal meanings unless otherwise defined.

[0030] The embodiments of the present invention will now be described in detail to enable those skilled in the art to readily implement the invention. However, the present invention is not limited to the embodiments described herein, and can be implemented in many different forms.

[0031] Figure 1a and Figure 1b These are SEM and TEM images of the negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.

[0032] Reference Figure 1a and Figure 1b An embodiment of the present invention provides a negative electrode active material for a lithium secondary battery, comprising a silicon-based substrate and a coating applied to the surface of the silicon-based substrate. The coating may include at least one of a carbon layer and carbon nanospheres. Specifically, the carbon nanospheres may be arranged on the carbon layer.

[0033] The silicon-based matrix material can be elemental silicon, silicon oxide, silicon nitride, silicon sulfide, silicon carbide, silicon-metal alloy, or combinations thereof. Specifically, in the case of silicon oxide, it can be SiO₂. x In the case of silicon carbide, it can be a Si-C complex.

[0034] The coating may be formed by applying a coating material to at least a portion of the surface of the silicon substrate. The coating may be applied to a portion or the entire surface of the silicon substrate. Specifically, the carbon nanospheres may be coated on a portion or the entirety of the coating.

[0035] In one embodiment, the coating may contain 0.10 to 1.50 wt% of a coating material within the negative electrode active material. The coating material may be, for example, coal tar, pyrolyzed fuel oil (PFO), or combinations thereof. Specifically, the negative electrode active material may include 0.20 to 1.46 wt% of the coating material. The content of the coating material refers to the content of the coating material in the negative electrode active material after a carbonization process at a specified temperature range, such as above 900°C.

[0036] If the content of the coating material deviates from the upper limit, a thicker amorphous carbon layer is formed, which hinders lithium-ion migration and causes irreversible damage, leading to a decrease in the initial efficiency and initial capacity of the battery, resulting in a shorter battery life. If the content of the coating material deviates from the lower limit, the amorphous carbon layer is insufficient, resulting in a high specific surface area, which fails to ensure sufficient conductivity, leading to a decrease in initial efficiency and initial capacity, and a shorter battery life.

[0037] In one embodiment, the coating may include a carbon layer and at least one carbon nanobead. The carbon layer and the carbon nanobeads may be formed by coating with a coating material (e.g., a coating material derived from coal tar). The carbon layer may be a layered coating with a high carbon content. The carbon nanobeads may be in a particle shape with a high carbon content.

[0038] In one embodiment, the carbon layer and carbon nanospheres may include sulfur. Since the carbon layer and carbon nanospheres include sulfur, the sulfur doped in the carbon lattice provides excess electrons, thereby increasing the electron concentration and improving conductivity.

[0039] In one embodiment, the carbon layer and carbon nanospheres, based on 100% by weight of the entire negative electrode active material composition, may include 0.03 to 0.3% by weight of sulfur. Specifically, it may include 0.03 to 0.25% by weight of sulfur. If the sulfur content deviates from the upper limit, a thicker sulfur-containing amorphous carbon layer is formed, which hinders lithium-ion migration and causes irreversible damage, leading to a decrease in the initial efficiency and initial capacity of the battery, and a poorer battery life. If the sulfur content deviates from the lower limit, a smaller sulfur-containing amorphous carbon layer is formed, exposing the surface of the substrate and resulting in a high specific surface area. This may prevent the assurance of sufficient conductivity, leading to a decrease in the initial efficiency and initial capacity of the battery, and a poorer battery life.

[0040] In one embodiment, the ratio of sulfur content to coating material content can be 0.02 to 1.20. The ratio of sulfur content to coating material content refers to the value obtained by dividing the sulfur content by the amount of coating material remaining in the negative electrode active material after the carbonization process.

[0041] If the content of the coating material deviates from the upper limit, a thicker amorphous carbon layer will form, hindering lithium-ion migration and causing irreversible damage. This leads to a decrease in the initial efficiency and capacity of the battery, resulting in a shorter battery life. If the content of the coating material deviates from the lower limit, an insufficient amorphous carbon layer will form, exposing the surface of the substrate and resulting in a high specific surface area. This makes it difficult to ensure sufficient conductivity, leading to a decrease in initial efficiency and capacity, and a shorter battery life.

[0042] In one embodiment, the carbon layer may be an amorphous carbon coating. In another embodiment, the thickness of the carbon layer may be in the range of 1.0~10 nm. Specifically, the thickness of the carbon layer may be in the range of 1.0~8.0 nm. If the thickness of the carbon layer deviates from the upper limit, the amorphous layer acts as a resistor, thereby hindering the migration of lithium ions and causing irreversible damage, resulting in a decrease in initial efficiency and initial capacity, and a deterioration in battery life. If the thickness of the carbon layer deviates from the lower limit, the amorphous carbon layer is insufficient, thereby exposing the surface of the substrate, resulting in a high specific surface area, which cannot ensure sufficient conductivity. Furthermore, due to the inability to ensure sufficient conductivity, there is a decrease in initial efficiency and initial capacity, and a deterioration in battery life.

[0043] In one embodiment, the carbon nanospheres may be composed of at least one cluster. A cluster refers to an aggregate of particles; the carbon nanospheres may be single particles or have an aggregate shape formed by the aggregation of particles.

[0044] In one embodiment, at least one carbon nanosphere may have at least one shape selected from spherical and elliptical. The particle size of the at least one carbon nanosphere may be 2.0 nm to 1.0 μm. Specifically, the particle size of the carbon nanosphere may be 3.0 nm to 0.9 μm. If the particle size of the carbon nanosphere deviates from the upper limit, the coating material will be difficult to disperse, resulting in uneven particle size and uneven amorphous coating, leading to a deterioration in battery performance. If the particle size of the carbon nanosphere deviates from the lower limit, the specific surface area will increase, resulting in a decrease in initial efficiency.

[0045] In one embodiment, at least one of the carbon nanospheres may be disposed on at least one of the silicon-based substrate and the carbon layer. Specifically, the carbon nanospheres may be disposed on the silicon-based substrate, or on the carbon layer, or a portion may be disposed on the silicon-based substrate and another portion on the carbon layer.

[0046] In one embodiment, the negative electrode active material of the present invention may be composed of at least one secondary particle, wherein the D10 of the secondary particle may be 2.0~6.5 μm. Specifically, the D10 may be 2.3~6.3 μm. More specifically, the D10 may be 2.3~4.0 μm. If the D10 range deviates from the lower limit, the content of microparticles is too high, resulting in difficulty in contact and a deterioration in the electrochemical characteristics of the battery.

[0047] In one embodiment, the D50 of the secondary particles can be 6.5~10 μm. Specifically, the D50 can be 6.7~9.5 μm. More specifically, the D50 can be 6.7~9.1 μm.

[0048] If the D50 value deviates from the upper limit, dispersion problems with the graphite may occur during the preparation of the mixed electrode, affecting the electrode's quality and resulting in a decline in the battery's electrochemical properties. If the D50 value deviates from the lower limit, a decrease in yield may occur during the micronization process.

[0049] In one embodiment, the D90 of the secondary particles can be 10.0~16.0 μm. Specifically, the D90 can be 10.6~15.7 μm.

[0050] If the range of D90 deviates from the upper limit, dispersion problems with the graphite will occur during the preparation of the mixed electrode with graphite, increasing the diffusion time of lithium ions and resulting in a deterioration in the electrochemical characteristics of the battery. If the range of D90 deviates from the lower limit, this is related to D50, and there are problems with the micronization process used to achieve the range of D90.

[0051] In one embodiment, the span value of the secondary particles ((D90-D10) / D50) can be in the range of 0.80 to 1.40. Specifically, the span value can be in the range of 0.86 to 1.39. More specifically, the span value can be in the range of 1.20 to 1.39. Even more specifically, the span value can be in the range of 1.26 to 1.39.

[0052] The span value refers to the width between the minimum and maximum values ​​in the average particle size distribution obtained using laser diffraction. Specifically, the span value is the ratio of the difference between D90 and D10 to the average particle size (D50). A span value of 0.9 to 1.3 provides the advantage of achieving excellent battery-grade negative electrode active material through uniform dispersion during electrode fabrication. Specifically, the span value can be satisfied when D50 is in the range of 6.7 to 9.1 μm.

[0053] If the span value deviates from the upper limit, the particle size distribution is too wide, making it difficult to disperse evenly. If the span value deviates from the lower limit, the particle size distribution is too narrow, making it difficult to maintain uniform contact between particles.

[0054] In one embodiment, the negative electrode active material for a lithium secondary battery can satisfy the following formula 1: <Formula 1> 0.50 ≤ Coating material content [weight % (wt%)] × BET specific surface area [m²] 2 / g]≤9.50.

[0055] The content of the carbonized coating material in Formula 1 refers to the content of the coating material in the final product, specifically the content of the coating material after carbonization. Formula 1 can confirm the correlation between the appropriate coating material content of the negative electrode active material and the BET specific surface area.

[0056] Equation 1 can satisfy 0.50~9.50. Specifically, Equation 1 can satisfy 0.80~6.40, and more specifically, it can satisfy 0.88~6.38.

[0057] If the value of Equation 1 deviates from the upper limit, it means that the amorphous carbon layer is too thick, which hinders the migration of lithium ions and causes irreversible damage, leading to a decrease in the initial efficiency and initial capacity of the battery, and a worse lifespan. If the value of Equation 2 deviates from the lower limit, the amorphous carbon layer is insufficient, thus exposing the surface of the substrate and causing a high specific surface area. This results in insufficient conductivity, leading to a decrease in the initial efficiency and initial capacity, and a worse lifespan.

[0058] The negative electrode active material for lithium secondary batteries according to an embodiment of the present invention can satisfy the following formula 2: <Formula 2> 1.40≤Average particle size (D50) [μm] of secondary particles × content of coating material [weight% (wt%)]≤15.0.

[0059] In Formula 2, D50 refers to the D50 of the secondary particles, and the coating material content refers to the content of the coating material after carbonization. Formula 2 can satisfy the range of 1.40 to 15.0. Specifically, Formula 2 can satisfy the range of 14.1 to 13.6.

[0060] If the particle size deviates from the upper limit of Equation 3, the median particle size increases or the particle distribution becomes uneven. This can lead to problems such as dispersion issues with graphite during the preparation of the mixed electrode, affecting electrode quality and making it difficult to obtain normal electrochemical properties. If the particle size deviates from the lower limit of Equation 3, the amorphous carbon layer is insufficient, exposing the substrate surface and resulting in a high specific surface area. This makes it difficult to ensure sufficient conductivity, leading to a decrease in initial efficiency and capacity, and a decline in battery life.

[0061] In one embodiment, the negative electrode active material for a lithium secondary battery can satisfy the following formula 3: <Formula 3> 2.0≤(Sulfur content [weight% (wt%)] / Coating material content [weight% (wt%)])×100≤120.0.

[0062] Formula 3 refers to the ratio of sulfur content after carbonization to the content of coating material. Formula 3 can satisfy 2.0~120.0. Specifically, Formula 3 can satisfy 2.05~118.6.

[0063] If the value deviates from the upper limit of Equation 3, the amorphous carbon layer is insufficient, thus exposing the surface of the substrate, resulting in a high specific surface area and insufficient conductivity. When applied to batteries, this leads to a decrease in initial efficiency and capacity, and a reduction in battery life. If the value deviates from the lower limit of Equation 3, the resulting amorphous carbon layer is too thick, hindering lithium-ion migration and causing irreversible damage, leading to a decrease in initial efficiency and capacity, and a reduction in battery life.

[0064] In one embodiment, the negative electrode active material for lithium secondary batteries may have an expansion rate of less than 60.0%. Specifically, the expansion rate may be in the range of 40.0% to 60.0%. More specifically, the expansion rate may be in the range of 44.0% to 58.0%.

[0065] The expansion rate is defined as the percentage of [(electrode thickness after 50 cycles - Cu current collector thickness) - (fresh electrode thickness - Cu current collector thickness)] divided by (fresh electrode thickness - Cu current collector thickness). If the expansion rate deviates from the upper limit, excessive expansion of the negative electrode material may occur, causing deformation of the electrode structure inside the cell, resulting in decreased stability, hindering the smooth migration of lithium ions, and reducing battery life.

[0066] An embodiment of the present invention provides a method for preparing a negative electrode active material for lithium secondary batteries, comprising: a step of uniformly mixing a silicon-based coating material and a sulfur-containing coating material; and a step of heat-treating the resulting mixture to perform carbonization. The silicon-based coating material is the same as the aforementioned silicon-based base material, provided it does not contradict this method.

[0067] The sulfur-containing coating material is a substance containing sulfur, such as coal tar. The sulfur-containing coating material can be in powder, liquid, or gaseous form; specifically, it can be a sulfur-containing liquid substance.

[0068] In one embodiment, the coating material has a surface tension of 25-45 dynes / cm at room temperature, specifically 30-40 dynes / cm. The silicon-based substrate used as the coated material can have a higher surface tension than the coating material, for example, 65-75 dynes / cm. Therefore, through the surface tension difference between the coating material and the coated material, the coating material can be uniformly coated onto the surface of the coated material. As for the residual coating material remaining after surface coating, it is formed into spherical or elliptical shapes during the carbonization process to reduce surface energy, thereby forming carbon nanospheres.

[0069] In one embodiment, the coating material may include 1 to 8 parts by weight relative to 100 parts by weight of the material to be coated. Specifically, the coating material may include 2 to 7 parts by weight relative to 100 parts by weight of the material to be coated, and more specifically, the coating material may include 3 to 5 parts by weight relative to 100 parts by weight of the material to be coated.

[0070] If the particle size deviates from the upper limit of the range, it will cause interparticle agglomeration or coarsening, resulting in an increase in particle size and a thicker carbon nanofiber amorphous layer. This thicker amorphous layer hinders lithium-ion migration and causes irreversible damage, leading to a decrease in initial efficiency and capacity, and a reduced lifespan. If the particle size deviates from the lower limit of the range, the amorphous carbon layer is insufficient, exposing the substrate surface and resulting in a high specific surface area. This makes it difficult to ensure sufficient conductivity, leading to a decrease in initial efficiency and capacity, and a reduced lifespan.

[0071] In one embodiment, the step of uniformly mixing the silicon-based coating material and the sulfur-containing coating material can be performed by a dry mixing step. The dry mixing step can be a mechanical mixing process, which can be performed by a method selected from ball milling, mechanofusion milling, shaker milling, planetary milling, attritor milling, disk milling, shape milling, nauta milling, nobilta milling, planetary mixing, 3D-mixer, V-mixer, mechano-fusion, hybridizer, paste mixer, and combinations thereof.

[0072] The step of heat-treating the mixed result to carbonize it can be performed by simultaneously heat-treating the result after mixing the coating material and the coated material, and carbonizing the coating material on the surface of the coated material.

[0073] In one embodiment, the step of heat-treating the mixed product to perform carbonization can be carried out in the range of 800~1,000°C. Specifically, the range can be 850~950°C.

[0074] If heat treatment is performed outside the upper limit of the range, it will cause Si grain growth, resulting in a decrease in battery capacity and efficiency. If heat treatment is performed outside the lower limit of the range, it will be difficult to achieve uniform amorphization of the coating material.

[0075] In one embodiment, the method for preparing the negative electrode active material for lithium secondary batteries can satisfy the following formula 4: <Formula 4> 0.37≤(Sulfur content [weight% (wt%)] / content of coating material before carbonization [weight% (wt%)])×100≤25.0.

[0076] Formula 4 refers to the ratio of sulfur content [weight% (wt%)] to the content of coating material before carbonization [weight% (wt%)]. Formula 4 can be 0.37~25.0, specifically, Formula 4 can be 0.43~24.9.

[0077] If the range of Equation 4 deviates from the upper limit, the amorphous carbon layer is insufficient, thus exposing the surface of the substrate and resulting in a high specific surface area. This leads to insufficient conductivity, causing a decrease in initial efficiency and capacity, and a reduction in battery life. If the range of Equation 4 deviates from the lower limit, the amorphous layer acts as a resistor, causing poor lithium-ion migration and irreversible damage, resulting in a decrease in initial efficiency and capacity, and a reduction in battery life.

[0078] Another embodiment of the lithium secondary battery of the present invention may include: a negative electrode comprising the aforementioned negative electrode active material, a positive electrode comprising a positive electrode active material, a separator located between the negative electrode and the positive electrode, and an electrolyte. The description of the negative electrode active material is the same as that described previously, without contradicting the previously described negative electrode active material for lithium secondary batteries.

[0079] The positive electrode can be LiCoO2, LiNiO2, or LiNi x Mn y O2, Li 1+z Ni x Mn y Co 1-x-y O2, LiNi x Co y Al zThe active material can be one of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, LiFeO2, LiFePO4, and combinations thereof, wherein x can be 0.3~0.8, y can be 0.1~0.45, and z can be independently 0~0.2. More specifically, the positive electrode can be LiFePO4, LiCoO2, NCM811, and NCM622. The negative electrode can use the aforementioned active materials for lithium secondary batteries or a negative electrode active material prepared by a method for preparing a precursor of a negative electrode active material.

[0080] The diaphragm can be any common porous polymer membrane conventionally used as a diaphragm, such as a porous polymer membrane prepared from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / propylene copolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, used alone or in layers. Alternatively, common porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc., but these are non-limiting examples.

[0081] In the electrolyte, the lithium salt that can be included as an electrolyte can be any lithium salt commonly used in electrolytes for lithium secondary batteries. For example, the anion of the lithium salt can be selected from F. - Cl - ,Br - 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 of them.

[0082] In the electrolyte, the organic solvent can be any solvent commonly used in electrolytes for lithium secondary batteries, typically selected from propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more of these solvents.

[0083] The lithium secondary battery can be disposed within a battery casing. The battery casing is a non-limiting example and can be one of the following shapes: cylindrical, square, pouch-shaped, or coin-shaped.

[0084] The embodiments of the present invention will now be described in detail to enable those skilled in the art to readily implement the invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0085] <Ingredient Characteristics> The coating material of this invention is liquid coal tar generated during the dry distillation of raw coal in a steel plant. It is fluid at room temperature and therefore cannot be measured by ASTM D3104 (softening point determination method 1, softening point above 50°C) or ASTM D36 (softening point determination method 2, softening point above 30°C). It can be a sulfur-containing liquid substance with a specific gravity of 1.1~1.3 g / cm³ (cc) and a fixed carbon content of 19~23% when carbonized at 900°C.

[0086] When the liquid substance is applied to a base material, a sulfur-containing carbon layer and carbon nanospheres (NCB) are formed on the surface of the base material due to the properties of the liquid.

[0087] <Preparation Process> In the process for preparing the anode material of the present invention, in order to form the amorphous carbon layer and the carbon nanospheres of the anode material, the coating process includes a step of uniformly dry mixing a silicon-based material to be coated and a coating material; and a carbonization process of heat treatment at a temperature of 800~1,000°C after a dry dispersion process. The mixing and dispersion are performed by methods such as planetary mixing, 3D-mixer, V-mixer, mechano-fusion, hybridizer, paste mixer, etc.

[0088] <Example 1> The silicon-based coating material uses silicon oxide (SiO2). x The coating material uses liquid coal tar. Specifically, it uses silicon oxide (SiO2). x Based on 100 parts by weight, 1 part by weight of the liquid coal tar is coated to prepare a negative electrode active material containing a substance carbonized at 900°C.

[0089] <Example 2> In addition to the silicon oxide (SiO) x Based on 100 parts by weight, 4 parts by weight of the liquid coal tar were coated to prepare the negative electrode active material in the same manner as in Example 1.

[0090] <Example 3> In addition to the silicon oxide (SiO) x Based on 100 parts by weight, 7 parts by weight of the liquid coal tar were coated to prepare the negative electrode active material in the same manner as in Example 1.

[0091] <Example 4> The silicon-based coating material uses a silicon-carbon composite, and the coating material uses liquid coal tar. Specifically, the liquid coal tar is coated onto the silicon-carbon composite to prepare a negative electrode active material containing substances carbonized at 900°C.

[0092] <Example 5> Except for coating 4 parts by weight of the liquid coal tar with 100 parts by weight of the silicon-carbon composite, the negative electrode active material was prepared in the same manner as in Example 4.

[0093] <Example 6> Except for coating 7 parts by weight of the liquid coal tar with 100 parts by weight of the silicon-carbon composite, the negative electrode active material was prepared in the same manner as in Example 4.

[0094] <Comparative Example 1> The negative electrode active material was prepared in the same manner as in Example 1, except that the liquid coal tar was not added as a coating material.

[0095] <Comparative Example 2> In addition to the liquid coal tar being treated with silicon oxide (SiO2), x Based on 100 parts by weight, 9 parts by weight were coated on top of the negative electrode active material, which was prepared in the same manner as in Example 1.

[0096] <Comparative Example 3> In addition to the silicon oxide (SiO) x Based on 100 parts by weight, the negative electrode active material was prepared in the same manner as in Example 1, except for 18 parts by weight of the liquid coal tar.

[0097] <Comparative Example 4> The negative electrode active material was prepared in the same manner as in Example 4, except that the liquid coal tar was not added as a coating material.

[0098] <Comparative Example 5> Except for coating 9 parts by weight of the liquid coal tar with 100 parts by weight of the silicon-carbon composite, the negative electrode active material was prepared in the same manner as in Example 4.

[0099] <Comparative Example 6> In addition to the silicon oxide (SiO) x Based on 100 parts by weight, 2 parts by weight of solid asphalt were coated, and the negative electrode active material was prepared in the same manner as in Example 1.

[0100] <Comparative Example 7> Except for coating 2 parts by weight of the solid asphalt with 100 parts by weight of the silicon-carbon composite, the negative electrode active material was prepared in the same manner as in Example 4.

[0101] <Fabrication of Silicon-based Individual Electrodes> As a method for preparing a silicon-based single electrode, a slurry is prepared by preparing and mixing a negative electrode active material, a binder (PAA), and a conductive material (Super P). The silicon-based single electrode uses silicon oxide (SiO2) as the coating material in Examples 1-3, Comparative Examples 1-3, and Comparative Example 6. xThe negative electrode active material comprises a binder and a conductive material in a weight ratio of 75:24:1. The slurry is uniformly coated onto a copper (Cu, approximately 10 μm thick) current collector, rolled using a roller press, and then dried to prepare the negative electrode. Specifically, the electrode is prepared with a loading of 2-3 mg / cm³. 2 The electrode density is 0.8~1.2 g / cm³ (cc).

[0102] The counter electrode uses lithium metal (Li-Metal), and the electrolyte is a solution containing 1 mole of LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7.

[0103] Using the aforementioned negative electrode, lithium metal, and electrolyte, a CR 2032 half-cell (HalfCoin Cell) was prepared according to conventional preparation methods.

[0104] <Preparation of Silicon-based Hybrid Electrodes> As a method for preparing a silicon-based hybrid electrode, a negative electrode active material, a binder (SBR-CMC), and a conductive material (Super P) are prepared. These materials are then added to distilled water and uniformly mixed to prepare a slurry. The silicon-based hybrid electrode includes the silicon-carbon composite (Si-C Composite) used as the coating material in Examples 4-6, Comparative Examples 4-5, and Comparative Example 7 as the negative electrode active material, with a weight ratio of negative electrode active material: binder: conductive material of 96.1:2.9:1.

[0105] At this point, the negative electrode active material is a mixture comprising 8% silicon-based material and 92% spheroidized natural graphite, using a negative electrode active material with a capacity of approximately 440-450 mAh / g. The slurry is uniformly coated onto a copper (Cu, approximately 10 μm thick) current collector, rolled using a roller press, and then dried to prepare the negative electrode. Specifically, the electrode is prepared with a loading of 7-8 mg / cm³. 2 The electrode density is 1.55~1.60 g / cm³ (cc).

[0106] The counter electrode uses lithium metal (Li-Metal), and the electrolyte is an electrolyte solution containing 1 mole of LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7.

[0107] Using the aforementioned negative electrode, lithium metal, and electrolyte, a CR 2032 half-cell (HalfCoin Cell) was prepared according to conventional preparation methods.

[0108] Table 1 below shows whether carbon nanospheres are formed and the size of the carbon nanospheres under different coating materials, coating materials, and coating material contents.

[0109] [Table 1] Figure 2a and Figure 2b These are SEM and TEM images of the negative electrode active material for a lithium secondary battery according to Comparative Example 2 of the present invention. Figure 2c and Figure 2d These are SEM and TEM images of the negative electrode active material for a lithium secondary battery according to Comparative Example 6 of the present invention.

[0110] Refer again Figure 1a and Figure 1b As shown in the SEM and TEM images, the negative electrode active material for lithium secondary batteries in Example 2 of the present invention has formed a carbon layer and carbon nanospheres.

[0111] Reference Figure 2a and Figure 2c ,and Figure 1a and Figure 1b In contrast, it can be confirmed that although a carbon layer was formed, carbon nanospheres were not formed, which confirms that the performance in terms of initial capacity, initial efficiency, and hybrid lifetime is poor.

[0112] Reference Figures 3a to 3d , Figure 3a This is a graph showing the elemental surface distribution (mapping) of Si, S, O, and C. Figure 3b This is a graph showing the elemental surface distribution (mapping) analysis of Si. Figure 3c This is a graph showing the elemental surface distribution analysis (mapping) of S. Figure 3d This represents the energy spectrum of each element.

[0113] Reference Figures 3a to 3d This further clarifies the sulfur content in the embodiments of the present invention. By including the sulfur content within the scope of the present invention, suitable carbon nanospheres can be formed.

[0114] The characteristics of the negative electrode active materials for lithium secondary batteries in the examples and comparative examples in Table 1 can be confirmed from Table 2 below. Specifically, Table 2 shows the initial capacity, initial efficiency, and hybrid lifetime. In the case of a single electrode, the initial efficiency was calculated as (discharge capacity / charge capacity) × 100 after measuring once under the conditions of charging CC-CV 0.1C & 5mV (0.005C cut-off) and discharging 0.1C 1.5V cut-off. Furthermore, in the case of a single electrode, the specific capacity at discharge (mAh / g) was calculated after measuring three times under the conditions of charging CC-CV 0.1C & 5mV (0.005C cut-off) and discharging 0.1C 1.5V cut-off. Furthermore, in the case of hybrid electrodes, for hybrid lifetime, after activation once under the conditions of charging CC-CV 0.1C & 5mV (0.005C cutoff) and discharging 0.1C 1.5V cutoff, 50 lifetimes are measured under the condition of charging CC-CV 0.5C 1.0V cutoff, and then the (50th lifetime discharge capacity / first lifetime discharge capacity) is calculated.

[0115] For secondary particles, the particle size distribution (D10, D50, and D90) was measured using laser diffraction.

[0116] The expansion rate is evaluated as a percentage of the value obtained by dividing [(electrode thickness after 50 cycles - Cu current collector thickness) - (fresh electrode thickness - Cu current collector thickness)] by (fresh electrode thickness - Cu current collector thickness).

[0117] The BET surface area was determined using the Micromeritics ASAP2020 Surface Area and Porosity Analyzer (BET method).

[0118] [Table 2] Figures 3a to 3d These are SEM-EDAX images of the negative electrode active material for lithium secondary batteries according to embodiments of the present invention. The present invention is not limited to the above-described embodiments and / or examples, and can be prepared in various different forms. Those skilled in the art will understand that the present invention can be implemented in other specific ways without altering its technical concept and essential technical features. Therefore, it should be understood that the embodiments and / or examples described above are illustrative in all respects and not limiting.

Claims

1. A negative electrode active material for lithium secondary batteries, comprising: Silicon-based matrix material; and A coating applied to the surface of the silicon-based substrate. The coating comprises a carbon layer and at least one carbon nanosphere. The carbon layer and the carbon nanospheres include sulfur. The content of the coating material is 0.10~1.50% by weight, based on 100% by weight of the total active substance.

2. The negative electrode active material for lithium secondary batteries according to claim 1, satisfying the following formula 1: <Formula 1> 0.50 ≤ coating material content × BET specific surface area ≤ 9.50, where, The unit of the content is weight%, and the unit of the BET specific surface area is m². 2 / g.

3. The negative electrode active material for lithium secondary batteries according to claim 1, satisfying the following formula 2: <Formula 2> 1.40 ≤ average particle size D50 of secondary particles × content of coating material ≤ 15.0, where, The average particle size is in μm, and the content is in weight.

4. The negative electrode active material for lithium secondary batteries according to claim 1, satisfying the following formula 3: <Formula 3> 2.0 ≤ (sulfur content / coating material content) × 100 ≤ 120.0, where, The unit of the content is by weight.

5. The negative electrode active material for lithium secondary batteries according to claim 1, comprising 0.03 to 0.3% by weight of the sulfur.

6. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The D10 particle size of the secondary particles is 2.0~6.5μm.

7. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The average particle size D50 of the secondary particles is 6.7~9.5μm.

8. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The D90 particle size of the secondary particles is 10.6~15.7μm.

9. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The span value (D90-D10) / D50 of the secondary particles is 0.80~1.

40.

10. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The silicon-based matrix material is elemental silicon, silicon oxide, silicon nitride, silicon sulfide, silicon carbide, silicon-metal alloy, and combinations thereof.

11. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The carbon layer is an amorphous carbon layer.

12. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The thickness of the carbon layer is 1.0~10 nm.

13. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The at least one carbon nanosphere is composed of at least one cluster.

14. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The at least one carbon nanosphere is either spherical or elliptical.

15. A method for preparing a negative electrode active material for lithium secondary batteries, comprising: The step of uniformly mixing the silicon-based coating material and the sulfur-containing coating material; and The step of heat-treating the mixed product to perform carbonization. The coating material comprises 1 to 8 parts by weight relative to 100 parts by weight of the coated material.

16. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 15, wherein the following formula 4 is satisfied: <Formula 4> 0.37 ≤ (sulfur content / content of coating material before carbonization) × 100 ≤ 25.0, where, The unit of the content is by weight.

17. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 15, wherein, The silicon-based coating materials include elemental silicon, silicon oxides, silicon nitrides, silicon sulfides, silicon carbides, silicon-metal alloys, and combinations thereof.

18. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 15, in, The sulfur-containing coating material is coal tar or pyrolysis fuel oil (PFO).

Images