Graphite and silicon composite and lithium secondary battery including same

Abstract

The present invention relates to a graphite and silicon composite and a lithium secondary battery including same. The graphite and silicon composite includes graphite with a high surface area. By using the graphite with a high surface area, an adhesion effect with silicon particles can be increased and pressure can be absorbed inside a negative electrode material when silicon expands. In addition, employment of the graphite and silicon composite as a negative electrode active material enables the provision of a lithium secondary battery that exhibits excellent initial efficiency, suppression of volume expansion, and excellent lifespan characteristics.

Description

Graphite and silicon composite and lithium secondary battery containing the same

[0001] The present invention relates to a graphite and silicon composite and a lithium secondary battery containing the same.

[0002] With the miniaturization and high performance of information and communication devices, as well as the increasing technological development and demand for electric vehicles (EVs) and energy storage systems (ESS), the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0003] Lithium metal has been used as the negative electrode of conventional secondary batteries, but as the risk of damage to the separator and short circuits caused by the formation of lithium dendrites during the charging and discharging process, as well as the resulting ignition and explosion, has become known, it is being replaced by carbon-based compounds that allow for reversible intercalation and extraction of lithium ions while maintaining structural and electrical properties.

[0004] The above carbon-based compound has a very low discharge potential of about -3 V relative to the standard hydrogen electrode potential and exhibits excellent electrode cycle life characteristics due to highly reversible charge-discharge behavior caused by the uniaxial orientation of the graphene layer. In addition, since the electrode potential is 0 V (vs. Li / Li+) when Li ions are charged, which is nearly similar to that of pure lithium metal, there is an advantage in obtaining higher energy when configuring the battery with an oxide-based cathode.

[0005] Natural graphite, commonly used as a cathode, offers high capacity per unit weight and excellent cost competitiveness; however, it has the disadvantage of degrading rapid charging performance as the increased grain orientation during electrode rolling lengthens the lithium ion entry and exit paths. In contrast, artificial graphite exhibits a relatively lower degree of orientation during electrode rolling compared to natural graphite, resulting in superior lithium ion entry and exit characteristics and thus offering advantages in improving rapid charging performance. Furthermore, it possesses the advantage of long lifespan characteristics due to the stability of its crystal structure and low expansion properties. Although attempts are being made to secure long lifespan characteristics in lithium-ion batteries by applying artificial graphite with these advantages, it has the disadvantage of poor processability or a susceptibility to electrode detachment due to low adhesion to the cathode current collector caused by a lack of surface functional groups.

[0006] Meanwhile, in response to market demand for high-capacity batteries, silicon (Si) has a significantly higher theoretical capacity (4,200 mAh / g) compared to graphite, so various studies have been conducted to replace or composite the carbon-based compounds using it.

[0007] However, most silicon anode materials have the disadvantage that the silicon volume expands by more than 300% due to lithium insertion, which destroys the anode active material particles and prevents them from exhibiting high cycle characteristics. In addition, in the case of silicon, as the cycle continues, repeated volume expansion and contraction occur due to the lithium insertion, which can lead to pulverization of particles, contact losses caused by electrical short circuits with conducting agents and current collectors, and fading mechanisms such as the formation of an unstable solid-electrolyte-interphase (SEI) layer due to electrolyte decomposition on the newly formed surface.

[0008] Therefore, there is a need to develop a new technology capable of simultaneously realizing high capacity and long lifespan characteristics by maximizing the advantages of each material, while overcoming the limitations of adhesion and output characteristics of conventional carbon-based compounds and the disadvantages of volume expansion and reduced lifespan caused by the use of silicon.

[0009] [Prior Art Literature]

[0010] [Patent Literature]

[0011] KR 10-2024-0162627 A1

[0012] The object of the present invention is to provide a graphite and silicon composite and a lithium secondary battery containing the same.

[0013] Another objective of the present invention is to provide a graphite and silicon composite comprising high surface area graphite, wherein the use of high surface area graphite enhances the adhesion effect with silicon particles and, by using high surface area graphite, the graphite and silicon composite can absorb pressure within the cathode material when silicon expands.

[0014] Another objective of the present invention is to provide a lithium secondary battery that exhibits excellent initial efficiency, suppression of volume expansion, and excellent lifespan characteristics by using the aforementioned graphite and silicon composite as a negative electrode active material.

[0015] To achieve the above-mentioned objective, the present invention provides a specific surface area (BET) of 10 m² 2 It may be a graphite and silicon composite comprising a high surface area graphite exceeding / g; and a precursor composed of silicon particles.

[0016] In addition, the above precursor may contain 1% to 30% by weight of high surface area graphite.

[0017] In addition, the high surface area graphite may have a particle size of 0.1 μm to 50 μm.

[0018] In addition, the above high surface area graphite has a specific surface area (BET) of 30 m²2 / g may exceed

[0019] In addition, the graphite and silicon composite may further include amorphous carbon.

[0020] In addition, the amorphous carbon may be included in an amount of 1% to 50% by weight relative to the weight of the graphite and silicon composite.

[0021] In addition, the graphite and silicon composite may be spherical particles with a core-shell structure.

[0022] In addition, the outer shell of the graphite and silicon composite may be made of amorphous carbon.

[0023] In addition, the graphite and silicon composite may have a diameter of 1 μm to 50 μm.

[0024] Another aspect of the present invention for achieving the above-mentioned purpose comprises the step of manufacturing silicon particles by grinding silicon; said silicon particles and having a specific surface area (BET) of 10 m² 2 The invention may relate to a method for manufacturing a graphite and silicon composite, comprising the steps of: mixing high surface area graphite exceeding 1g to prepare a slurry; spray-drying the slurry to prepare a precursor; mixing the precursor with amorphous carbon and heat-treating it to carbonize the amorphous carbon; and grinding and classifying to produce a cathode material.

[0025] In addition, the silicon particles may be 5 nm to 1 µm in size.

[0026] In addition, the slurry may contain 1% to 30% by weight of high surface area graphite.

[0027] In addition, the amorphous carbon may be included in an amount of 1% to 50% by weight relative to the total weight mixed with the precursor.

[0028] In addition, the step of carbonizing the amorphous carbon can be carried out under a nitrogen atmosphere at a temperature of 800°C to 1,000°C.

[0029] Another invention for achieving the above-mentioned purpose may be a negative electrode material for a lithium secondary battery comprising the silicon-carbon composite.

[0030] Another invention for achieving the above-mentioned purpose may be a lithium secondary battery comprising the negative electrode material for the lithium secondary battery.

[0031] The present invention is a graphite and silicon composite comprising high surface area graphite. By using high surface area graphite, the adhesion effect with silicon particles can be enhanced, and by using high surface area graphite, pressure can be absorbed inside the cathode material when silicon expands.

[0032] In addition, by using the above graphite and silicon composite as a negative electrode active material, excellent initial efficiency, suppression of volume expansion, and excellent lifespan characteristics can be exhibited.

[0033] Figure 1 is the result of the discharge capacity in a 50-cycle experiment of a battery comprising a graphite and silicon composite according to one embodiment of the present invention.

[0034] Figure 2 is the result of the capacity retention rate in a 50-cycle experiment of a battery including a graphite and silicon composite according to one embodiment of the present invention.

[0035] The present invention has a specific surface area (BET) of 10 m² 2 The invention relates to a graphite and silicon composite comprising a high surface area graphite with a surface area of ​​greater than 1 / g; and a precursor composed of silicon particles.

[0036] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0037] As mentioned above, with the recent rapid growth of the electric vehicle (EV) and energy storage system (ESS) markets, the demand for secondary batteries with high energy density and fast charging characteristics is continuously increasing. Existing commercially available graphite anode materials have a theoretical capacity of only about 372 mAh / g, which limits the ability to improve driving range and achieve battery miniaturization.

[0038] As an alternative, silicon (Si), which has a theoretical capacity more than 10 times higher than graphite (approx. 4,200 mAh / g), is attracting attention as a next-generation anode material. However, silicon undergoes rapid volume expansion of approximately 400% due to reactions with lithium ions during the charging and discharging process. This volume expansion causes pulverization of active material particles, disruption of the electrode conductive network, and the formation of an unstable solid electrolyte interface (SEI) layer, which is a major cause of the rapid degradation of the battery's lifespan characteristics and efficiency.

[0039] Therefore, there is an urgent need to develop a new cathode material structure and a method for manufacturing the same that can effectively control volume expansion and secure electrical conductivity while maintaining the high capacity characteristics of silicon.

[0040] Accordingly, the present invention has a specific surface area (BET) of 10 m² 2 The present invention relates to a graphite and silicon composite comprising high surface area graphite having a specific surface area exceeding / g; and a precursor composed of silicon particles. In the manufacture of conventional graphite and silicon composites, the graphite mainly used has a specific surface area of ​​0.4 m² 2 / g to 15 m 2 / g or 10 m 2Graphite having a specific surface area (BET) of less than 1 / g was used. The use of graphite having a specific surface area within the above range is intended to exhibit an appropriate level of conductivity, prevent an increase in the initial irreversible capacity during charge and discharge due to the specific surface area, and ensure that adverse reactions with the electrolyte occur to an appropriate degree so as not to degrade lifespan characteristics. However, conventional use of graphite having a specific surface area within the above range applies primarily to cases where graphite is mainly included and a small amount of silicon particles are included to control rapid volume expansion while utilizing the high capacity characteristics of silicon when manufacturing graphite-silicon composites. This limits the utilization of the high capacity characteristics resulting from the use of silicon.

[0041] Accordingly, in the present invention, the specific surface area (BET) is 30 m² 2 A cathode material can be provided that exhibits excellent initial efficiency and lifespan characteristics while suppressing the volume expansion of silicon particles by using a precursor composed of high surface area graphite exceeding / g; and silicon particles.

[0042] Accordingly, the above-mentioned high-surface-area graphite has a specific surface area (BET) of 10 m² 2 Exceeding / g and 20 m 2 Exceeding / g and 30 m 2 Exceeding / g and 35 m 2 Exceeding / g and 40 m 2 Exceeding / g and 45 m 2 Exceeding / g and 50 m 2 Exceeding / g and 55 m 2 Exceeding / g and 60 m 2 Exceeding / g and 65 m 2 Exceeding / g and 70 m 2 Exceeding / g and 75 m 2 Exceeding / g and 80 m 2 Exceeding / g and 85 m 2 Exceeding / g and 90 m 2 Exceeding / g and 95 m2 It may exceed / g. Additionally, preferably, the specific surface area (BET) is 11 m² 2 / g to 300 m 2 / g and 15 m 2 / g to 300 m 2 / g and 20 m 2 / g to 300 m 2 / g and 30 m 2 / g to 300 m 2 / g and 35 m 2 / g to 300 m 2 / g and 40 m 2 / g to 300 m 2 / g and 45 m 2 / g to 300 m 2 / g and 50 m 2 / g to 300 m 2 / g and 55 m 2 / g to 300 m 2 / g and 60 m 2 / g to 300 m 2 / g and 65 m 2 / g to 300 m 2 / g and 70 m 2 / g to 300 m 2 / g and 75 m 2 / g to 300 m 2 / g and 80 m 2 / g to 300 m 2 / g and 85 m 2 / g to 300 m 2 / g and 90 m 2 / g to 300 m 2 / g and 95 m 2 / g to 300 m 2 / g and 100 m 2 / g to 300 m 2 / g and 100 m 2 / g to 290 m 2 / g and 100 m 2 / g to 280 m 2 / g and 100 m2 / g to 270 m 2 / g and 100 m 2 / g to 260 m 2 / g and 100 m 2 / g to 250 m 2 / g and 100 m 2 / g to 240 m 2 / g and 100 m 2 / g to 230 m 2 / g and 100 m 2 / g to 220 m 2 / g and 100 m 2 / g to 210 m 2 / g and 100 m 2 / g to 200 m 2 It may be / g. The present invention forms a composite structure in which nanoscaled silicon is dispersed within a high-surface-area graphite and an amorphous carbon matrix. That is, the precursor is in the form where high-surface-area graphite is located internally and nanoscaled silicon particles surround the outside; however, when finally manufactured into a cathode material, it is in a form mixed with an amorphous carbon matrix. The high-surface-area graphite acts as a buffer that absorbs the volume expansion of silicon through a large number of internal pores, and the amorphous carbon coating layer firmly holds the particles from the outside, thereby suppressing particle collapse caused by expansion. This enables the present invention to achieve an excellent retention rate despite the application of a high silicon content.

[0043] The above high surface area graphite has a particle size of 0.1 μm to 50 μm, 0.1 μm to 49 μm, 0.1 μm to 48 μm, 0.1 μm to 47 μm, 0.1 μm to 46 μm, 0.1 μm to 45 μm, 0.1 μm to 44 μm, 0.1 μm to 43 μm, 0.1 μm to 42 μm, 0.1 μm to 41 μm, 0.1 μm to 40 μm, 0.1 to 39 μm, 0.1 μm to 38 μm, 0.1 μm to 37 μm, 0.1 μm to 36 μm, 0.1 μm to 35 μm, and 0.1 μm to 34 μm. The diameter may be 0.1 μm to 33 μm, 0.1 to 32 μm, 0.1 μm to 31 μm, 0.1 μm to 30 μm, 0.2 μm to 30 μm, 0.3 μm to 30 μm, 0.4 μm to 30 μm, or 0.5 μm to 30 μm. If the diameter of the graphite is less than 0.5 μm, the specific surface area of ​​the particles increases excessively, causing an increase in viscosity during slurry preparation and potentially lowering initial efficiency due to adverse reactions with the electrolyte. On the other hand, if the diameter of the graphite exceeds 30 μm, the graphite particles are too large relative to the target size of the precursor to be formed through spray drying, making it difficult to form a uniform spherical assembly. In addition, it is difficult to secure contact with silicon particles, which can weaken the conductive network, and the inability to effectively disperse the volume expansion of silicon during charging may degrade the lifespan characteristics of the electrode.

[0044] The above precursor comprises high surface area graphite in an amount of 1 wt% to 30 wt%, 2 wt% to 30 wt%, 3 wt% to 30 wt%, 4 wt% to 30 wt%, 5 wt% to 30 wt%, 6 wt% to 30 wt%, 7 wt% to 30 wt%, 8 wt% to 30 wt%, 9 wt% to 30 wt%, 10 wt% to 30 wt%, 11 wt% to 30 wt%, 12 wt% to 30 wt%, 13 wt% to 30 wt%, 14 wt% to 30 wt%, 15 wt% to 30 wt%, 16 wt% to 30 wt%, and 17 wt%. It may be included in an amount of up to 30 wt%, 18 wt% to 30 wt%, 19 wt% to 30 wt%, or 20 wt% to 30 wt%. Within the above-described range, the silicon particles and the high surface area graphite may be aggregated together to form an assembly in the form of secondary particles. In particular, as described below, even when a high content (e.g., 80 wt%) of silicon particles is included, uniform dispersion and spheroidization between particles are made possible by mixing the high surface area graphite together within the above-described range and proceeding with spray drying.

[0045] The above silicon particles have a diameter of 5 nm to 1 µm, 5 nm to 900 nm, 5 nm to 850 nm, 5 nm to 800 nm, 5 nm to 750 nm, 5 nm to 700 nm, 5 nm to 650 nm, 5 nm to 600 nm, 5 nm to 550 nm, 5 nm to 500 nm, 10 nm to 500 nm, 15 nm to 500 nm, 20 nm to 500 nm, 25 nm to 500 nm, 30 nm to 500 nm, 35 nm to 500 nm, 40 nm to 500 nm, 45 nm to 500 nm, 50 nm to 500 nm, 55 nm to 500 nm, and 60 nm to 500 nm. The diameter may be 65 nm to 500 nm, 70 nm to 500 nm, 75 nm to 500 nm, 80 nm to 500 nm, 85 nm to 500 nm, 90 nm to 500 nm, 95 nm to 500 nm, or 100 nm to 500 nm. The silicon particles expand by approximately 300 to 400% during charging. If the diameter of the particles is large (above a few μm), cracks may occur because they cannot withstand the internal stress generated during expansion. However, if silicon particles within the aforementioned range are used, even if they expand, the stress applied to the particles themselves becomes lower than the fracture strength of the material, allowing them to expand / contract without breaking. In other words, when the diameter falls within the aforementioned range, physical destruction caused by expansion is minimized, thereby significantly extending battery life.

[0046] The graphite and silicon composite of the present invention described above may be spherical particles with a core-shell structure. To form the spherical particles with the core-shell structure, a precursor composed of high surface area graphite and silicon particles and amorphous carbon may form the core, and the shell may include amorphous carbon.

[0047] The amorphous carbon may be included in an amount of 1% to 50% by weight relative to the total weight of the graphite and silicon composite, which is a spherical particle with a core-shell structure as described above. The amorphous carbon can fill the voids between the silicon and graphite particles and form the outermost shell. Additionally, it can serve as a buffering matrix that provides binding force between high-surface-area graphite and silicon particles and suppresses volume expansion of silicon particles, as well as a protective layer that blocks adverse reactions with the electrolyte. At this time, if the content of the amorphous carbon is less than 1% by weight, a problem may arise in that the surface of the precursor containing high-surface-area graphite and silicon particles cannot be uniformly coated to form a core-shell particle structure. Furthermore, the outermost carbon layer may fracture due to inability to withstand the stress generated during the volume expansion of silicon particles, and the lifespan characteristics may rapidly deteriorate due to problems such as internal destruction of the active material. If the content of the amorphous carbon exceeds 50 weight%, the discharge capacity and energy density of the entire cathode material may decrease as the proportion of amorphous carbon with low reversible capacity becomes excessively high. In addition, the thickness of the coating layer may become excessively thick, which may increase the diffusion resistance of lithium ions. Therefore, in order to effectively control the volume expansion of high-capacity silicon while simultaneously securing excellent electrical conductivity and energy density, the content of the amorphous carbon may be included within the aforementioned range.

[0048] The thickness of the shell formed by the above amorphous carbon is 0.01 μm to 5 μm, 0.01 μm to 4.5 μm, 0.01 μm to 4.0 μm, 0.01 μm to 3.5 μm, 0.01 μm to 3.0 μm, 0.01 μm to 2.5 μm, 0.01 μm to 2.0 μm, 0.01 μm to 1.5 μm, 0.01 μm to 1.0 μm, 0.05 μm to 1.0 μm, 0.15 μm to 1.0 μm, 0.2 μm to 1.0 μm, 0.25 μm to 1.0 μm, 0.3 μm to 1.0 μm, and 0.35 μm to 1.0 μm. The thickness may be 0.4 µm to 1.0 µm, 0.45 µm to 1.0 µm, or 0.5 µm to 1.0 µm. If the thickness of the shell is less than the above numerical range, the surface protection of the silicon particles is insufficient, which may result in a decrease in initial efficiency. On the other hand, if the thickness of the shell exceeds the above numerical range, the weight specific gravity of the amorphous carbon shell increases excessively, significantly reducing the energy density per weight of the anode material, and the high-rate characteristics and output performance may be degraded due to the increased diffusion resistance of lithium ions passing through the thickened shell. Therefore, by ensuring the shell thickness range within the above-described range, sufficient SEI stability and structural support can be secured, and capacity loss and output degradation can be prevented.

[0049] The graphite and silicon composite may have a diameter of 1 μm to 50 μm. Within this range, uniform mixing of particles of different sizes is induced, which minimizes empty space within the electrode, thereby increasing the energy density per unit volume of the battery. Additionally, by using particles having the aforementioned diameter, a suitable pore structure can be formed that allows the electrolyte to smoothly penetrate the particle surface. This secures a diffusion path for lithium ions, enabling stable charging and discharging even at high rate speeds.

[0050] A method for manufacturing a graphite and silicon composite according to another embodiment of the present invention comprises the step of grinding silicon to produce silicon particles; said silicon particles and a specific surface area (BET) of 10 m² 2 The method may include the steps of: preparing a slurry by mixing high surface area graphite exceeding 1g; preparing a precursor by spray-drying the slurry; mixing the precursor with amorphous carbon and heat-treating it to carbonize the amorphous carbon; and preparing a cathode material by grinding and classifying.

[0051] The first step of the method for manufacturing the graphite and silicon composite described above is to produce silicon particles by grinding silicon. The ground silicon particles can be manufactured to have a diameter of 5 nm to 1 μm as described above. The silicon particles can be produced using any method capable of physically grinding particles, for example, by grinding the silicon particles to a desired size using a method such as a bead mill, but is not limited to the methods described above, and any method capable of grinding the silicon particles to within the diameter range described above can be used without limitation.

[0052] After grinding the above silicon to form silicon particles, the specific surface area (BET) is 10 m² 2A slurry can be prepared by mixing high surface area graphite exceeding 1g / g. The high surface area graphite may be used having a specific surface area within the range described above and may be included in an amount of 1% to 30% by weight relative to the total weight mixed with silicon particles. The range of specific surface area and weight range are as described above. A slurry was prepared by mixing the ground silicon particles and the high surface area graphite. To prepare the slurry, the ground silicon and the high surface area graphite are mixed, and at this time, a solvent and a dispersant may be used together to prepare the slurry. The solvent may be selected from the group consisting of ethanol, NMP (N-Methyl-2-pyrrolidone), IPA (Isopropanol), acetone, water, and mixtures thereof. The above dispersant may be selected from the group consisting of stearic acid, PAA (Polyacrylic Acid), polyvinylpyrrolidone, polyethylene glycol, oleic acid, SDBS (Sodium Dodecylbenzenesulfonate), Triton X-100, and mixtures thereof, but is not limited to the above examples, and any dispersant that can be used in a slurry may be used without limitation.

[0053] The above-mentioned dispersant may be included in an amount of 0.1% to 10% by weight relative to the total weight of the ground silicon, but is not limited to the above figures and may be included in an amount necessary for the preparation of the slurry. However, the above-mentioned dispersant is not mandatory, and the slurry can be prepared using only ground silicon, high surface area graphite, and a solvent without the dispersant.

[0054] Subsequently, the above slurry can be spray-dried to produce a mechanically assembled precursor. Specifically, the mixture of the pulverized silicon particles and high-surface-area graphite is dispersed in a solvent, and a slurry state in which silicon and graphite are uniformly dispersed in the solvent is prepared using a bead mill or the like. Then, the mixture is sprayed in the form of small liquid droplets using a high-pressure pump or a rotating disc, and the solvent rapidly evaporates within a high-temperature drying chamber, causing the silicon and graphite particles to clump together and assemble into spherical precursor particles.

[0055] As the graphite and silicon precursor particles of the present invention contain a greater amount of silicon particles relative to graphite to form the particles as described above, the finally formed spherical precursor particles may have a shape in which graphite is located inside and silicon particles surround it.

[0056] The above precursor can be mixed with amorphous carbon and heat-treated to carbonize the amorphous carbon. The content range for mixing the above precursor and amorphous carbon is the same as the content range of the amorphous carbon described above. The carbonization process can be carried out under a nitrogen atmosphere at a temperature of 800°C to 1,000°C. The above carbonization process can complete the core-shell structure and overcome structural instability and low initial efficiency that are problematic when silicon particles are included. At the heat treatment temperature of the above carbonization process, the mixed amorphous carbon softens or changes into a liquid state, penetrates into the micropores of the precursor, forms a shell that uniformly surrounds the entire particle, and is finally carbonized to form the inner and outer shells of the particle with hard, amorphous carbon. As the amorphous carbon forms a high-strength shell, it can prevent the particle from breaking even if stress is generated due to volume expansion when the internal silicon particles are filled. In addition, by forming a uniform carbon layer on the outside, the electrolyte can be blocked from coming into contact with silicon particles, thereby increasing the initial Coulomb efficiency and suppressing the formation of the SEI layer.

[0057] The cathode material after the carbonization process is completed can be physically crushed and classified to produce a cathode material with a thickness of 1 μm to 50 μm.

[0058] When a lithium secondary battery is manufactured using the aforementioned anode material, it is possible to manufacture a high-capacity anode material compared to existing commercially available or developed silicon anode materials, and this can be used as an anode material capable of maximizing the high-capacity characteristics of silicon particles. In addition, although lifespan performance generally deteriorates as the proportion of Si increases, the anode material of the present invention can secure a lifespan performance of 80% or more despite having a high proportion of Si, thus enabling it to be provided as a high-lifespan anode material.

[0059] A silicon-carbon composite according to one example of the present invention has high capacity characteristics while effectively controlling volume expansion, so it can be usefully used as a negative electrode active material for a lithium secondary battery. Accordingly, the present invention provides a negative electrode active material composition for a lithium secondary battery comprising the silicon-carbon composite and a lithium secondary battery comprising the silicon-carbon composite in the negative electrode.

[0060] The above lithium secondary battery may include a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0061] The above anode can be manufactured by conventional methods known in the field. For example, an anode can be manufactured by preparing a slurry by mixing and stirring a solvent, and optionally a binder, conductive material, and dispersant with an anode active material, then applying (coating) the slurry to a current collector of a metal material, compressing it, and drying it.

[0062] The current collector of the above metal material is a highly conductive metal to which the slurry of the positive active material can easily adhere, and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery within the voltage range of the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., may be used. In addition, fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. The current collector can be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc., and may have a thickness of 3 μm to 500 μm.

[0063] The above-mentioned positive electrode active material is, for example, a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; chemical formula Li 1+y Mn 2-yLithium manganese oxides such as O4 (where y is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7, etc.; chemical formula LiNi 1-y M y Ni-site type lithium nickel oxide represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and y = 0.01 to 0.3); lithium magnesium cobalt oxide (LiMg x Co 1-x O2); Chemical formula LiMn 2-y M y Examples include lithium manganese complex oxides represented by O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and y = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); LiMn2O4 in which a portion of the Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; Fe2(MoO4)3, etc. Additionally, LiNi x Mn y Co z Ternary cathode active materials such as O2(x+y+z=1) or olivine-based cathode active materials such as LiFePO4 may also be used, but are not limited to these.

[0064] The solvents for forming the anode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, or water, and these solvents may be used alone or in a mixture of two or more. The amount of solvent used is sufficient if it is sufficient to dissolve and disperse the anode active material, binder, and conductive material, taking into account the coating thickness of the slurry and the manufacturing yield.

[0065] Various types of binder polymers may be used as the above binder, such as polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers in which the hydrogens thereof are substituted with Li, Na, or Ca, or various copolymers.

[0066] The above conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery, and examples may be used such as graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, Farness black, lamp black, thermal black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes (CNT); metal powders, such as fluorocarbon, aluminum, or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; or conductive materials, such as polyphenylene derivatives. The above conductive material may be used in an amount of 1% to 20% by weight relative to the total weight of the anode slurry.

[0067] The above-mentioned cathode can be manufactured by conventional methods known in the art, for example, by preparing a cathode active material slurry by mixing and stirring a cathode slurry composition containing additives such as the cathode active material, binder, and conductive material, applying the slurry to a current collector, drying it, and then compressing it. The solvents for forming the above-mentioned cathode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, or water, and these solvents may be used alone or in a mixture of two or more. The amount of solvent used is sufficient if it is sufficient to dissolve and disperse the cathode active material, binder, and conductive material, taking into account the coating thickness of the slurry and the manufacturing yield.

[0068] The above binder may be used to bind the negative electrode active material particles to maintain the molded body. It is not particularly limited to any conventional binder used in the preparation of a slurry for the negative electrode active material, but, for example, non-aqueous binders such as polyvinyl alcohol, carboxymethylcellulose, hydroxypropylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene, or polypropylene may be used. In particular, to control the volume expansion of the silicon-based active material and maintain the binding strength of the electrode, it may be more preferable to use any one selected from the group consisting of acrylonitrile-butadiene rubber, styrene-butadiene rubber (SBR), and acrylic rubber, or a mixture of two or more of these, which are aqueous binders. Compared to non-aqueous binders, water-based binders are more economical, environmentally friendly, and harmless to workers' health. Furthermore, because they offer superior binding effects, the ratio of active material per unit volume can be increased, enabling the production of high-capacity binders. Carboxymethylcellulose (CMC) or similar substances may also be used in combination with the above-mentioned SBR as a thickener.

[0069] The binder may be included in an amount of 10% by weight or less of the total weight of the slurry for the cathode active material, specifically in an amount of 0.1% to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is negligible and is undesirable; if it exceeds 10% by weight, there is a concern that the capacity per volume may decrease due to a relative decrease in the content of the active material as the binder content increases and is therefore undesirable.

[0070] The above conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery. However, in order to prevent the discontinuation of the conductive network due to volume changes of silicon-based particles, it may be preferable to use a mixture of linear conductive materials such as carbon nanotubes (CNT) and carbon fibers in addition to carbon black, which is a point conductive material. The above conductive material may be used in an amount of 1% to 10% by weight relative to the total weight of the slurry for the negative electrode active material.

[0071] The negative electrode current collector used in the negative electrode according to one embodiment of the present invention may have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy may be used. In addition, fine irregularities may be formed on the surface to strengthen the bonding strength of the negative electrode active material, and it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0072] In addition, as a separator, a conventional porous polymer film that has been used as a separator in the past, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-methacrylate copolymer, may be used alone or in a laminate thereof, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc., may be used. To ensure heat resistance or mechanical strength, a form (SRS) in which a ceramic component is coated on one or both sides of the separator may also be used.

[0073] The lithium salt that may be included as the electrolyte used in the present invention may be any that are conventionally used in electrolytes for lithium secondary batteries without limitation, and for example, as the anion of the lithium salt, 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 - It can be any one selected from the group consisting of.

[0074] In the electrolyte used in the present invention, the organic solvent included in the electrolyte may be any of those commonly used in electrolytes for secondary batteries without limitation, and may be typically used as any one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more of these. Specifically, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents that have a high dielectric constant and effectively dissociate lithium salts in the electrolyte, so they can be used preferably. Furthermore, if low-viscosity, low-dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate are mixed with these cyclic carbonates in appropriate proportions, an electrolyte with high electrical conductivity can be produced, making it even more preferable to use.

[0075] Optionally, the electrolyte stored according to the present invention may further include film-forming additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) to control the continuous destruction and regeneration of the SEI layer due to volume expansion of the silicon-based anode. These additives can contribute to suppressing further decomposition of the electrolyte and improving the lifespan characteristics of the battery by forming a robust SEI film on the surface of the anode.

[0076] The external shape of the lithium secondary battery of the present invention is not particularly limited, but can be a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

[0077] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but can also preferably be used as a unit cell in a medium-to-large battery module comprising a plurality of battery cells.

[0078] Preferred examples of the above-mentioned medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, but are not limited to these.

[0079]

[0080] Preparation Example

[0081] Method for manufacturing cathode material

[0082] Silicon was ground into silicon particles with a diameter of 5 nm to 1 µm using a bead mill. Subsequently, the ground silicon particles and high surface area graphite were mixed, and a slurry was prepared by mixing ethanol as a solvent and stearic acid as a dispersant at 3 wt% relative to the amount of Si input. The slurry was spray-dried to produce a precursor under conditions of a chamber heater temperature of 150°C to 200°C, a nitrogen flow rate of 0.1 MPa to 1.0 MPa, and a slurry supply of 40 to 80 Hz. The precursor was mixed with amorphous carbon.

[0083] Subsequently, a carbonization process was carried out under a nitrogen atmosphere at a temperature of 800°C to 1000°C. After heat treatment, the material was physically ground and classified to produce a cathode material with a diameter of 1 µm to 50 µm.

[0084]

[0085] manufacturing of coin cells

[0086] As a negative electrode active material, a silicon negative electrode material prepared according to the above preparation example, a conductive material, and a binder were mixed and stirred in pure water to prepare an electrode slurry. After coating and drying the slurry onto a copper foil current collector using a coater, a roll pressing process was performed to improve the contact characteristics between the electrode slurry and the current collector to manufacture an electrode.

[0087] At this time, the slurry was prepared in a ratio of 96 wt% cathode active material, 1 wt% conductive material, and 3 wt% binder, using Super P as the conductive material and CMC (carboxymethylcellulose) and SBR (styrene butadiene rubber) as the binder.

[0088] A coin-type half-cell was manufactured by using Li metal as the counter electrode, interposing a separator between the cathode and the Li metal, dissolving 1.2M LiPF6 in a solvent mixed with ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in a volume ratio of 30:70, and injecting an electrolyte containing 10 wt% fluoroethylene carbonate.

[0089]

[0090] evaluation

[0091] Batteries manufactured using the cathode material of Table 1 below were charged at 25°C with a constant current (CC) of 0.1C until the voltage reached 0.005V, and then charged with a constant voltage (CV) until the charging current reached 0.01C (Cut-off current). Subsequently, discharge was carried out with a constant current (CC) of 0.1C until the voltage reached 1.5V. Afterward, the cycle was repeated 50 times by changing the CC current to 0.5C.

[0092] Additionally, the battery efficiency and lifespan characteristics were verified using a simplified method by repeating a charge-discharge cycle of charging to 0.005V and discharging to 1.5V.

[0093] The experimental results are as shown in Table 1, Figure 1, and Figure 2 below:

[0094] Classification of graphite BET(m 2 / g) Mixing ratio of high surface area graphite in precursor (wg%) 0.1C 0.5C 50cyc Charge / Discharge Efficiency Charge / Discharge Retention HNM 1100 205 805 299 1.2% 455 454 88.6% HNM 2100 56 415 67 88.5% 308 307 58.0% HNM 3100 106 215 538 9.1% 367 365 68.8% HNM 430 205 74 517 90.1% 417 416 80.3% HNM 517 205 68 508 89.4% 400 398 79.5% HNM 6-069 66198 8.9% 929 017.0% HNM 71003056051491.8%45745688.8%HNM 810010040035789.3%35535399.1%

[0095] According to the experimental results in Table 1 above, as in HNM 1, it contains 20 wt% high surface area graphite, and the BET is 100 m 2In the case of / g, it was confirmed that although the initial capacity is somewhat lower compared to other conditions, a significant difference in lifespan retention rate of 88.6% can be secured. In other words, when using only silicon, as in HNM 6, it was observed that while the initial capacity is high, the capacity retention rate after 50 cycles is very low. Similarly, as the graphite content increases, it was confirmed that although the initial capacity decreases slightly, the capacity retention rate rises further. Based on the above experimental results, it can be confirmed that HNM 1 corresponds to the minimum structural critical value for controlling the expansion stress of silicon. Additionally, when a precursor was prepared by mixing 30 wt% of high-surface-area graphite and a coin cell was fabricated using this as the anode material, and the initial capacity was checked under the same experimental conditions as before, the initial capacity was 514, which was slightly lower than HNM 1, but the initial Coulomb efficiency was 91.8%, which is higher than HNM 1's 91.2%. This confirms that when using a cathode material containing 30 wt% high surface area graphite in HNM 1 and the precursor, the stability of the silicon composite can be maximized from the first cycle, thereby minimizing irreversible capacity loss.

[0096] In addition, when only graphite with a BET of 100 is used, the initial discharge capacity is 357 mAh / g and the initial Coulomb efficiency is 89.3%, which means that the irreversible capacity loss is very large. Since this value is lower than that of commonly used pure graphite anode materials (initial Coulomb efficiency of 95% or higher), it can be said that simply using only graphite with a high surface area as an anode material is not appropriate.

[0097] In addition, when comparing battery performance while maintaining the same graphite content in the precursor, the life retention rate of HNM 1 was found to be 8–9%p higher than that of samples with 17 m² / g or 30 m² / g. This difference can be interpreted as a result of including high-surface-area graphite in the precursor compared to HNM 4 and HNM 5, which absorbs the pressure generated when silicon particles expand or disperses silicon particles more effectively to maintain a more robust structure. Furthermore, HNM 1 exhibited the highest efficiency (91.2%) compared to HNM 4 and HNM 5. HNM 8, which utilized only high-surface-area graphite, showed the best life retention rate, but it was confirmed that there was a significant difference in discharge capacity compared to HNM 1.

[0098] Although preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention as defined in the following claims also fall within the scope of the present invention.

[0099] The present invention relates to a graphite and silicon composite and a lithium secondary battery containing the same.

Claims

1. Specific surface area (BET) is 10 m² 2 High surface area graphite exceeding / g; and A precursor comprising silicon particles Graphite and silicon composite.

2. In Paragraph 1, The above precursor comprises 1% to 30% by weight of high surface area graphite. Graphite and silicon composite.

3. In Paragraph 1, The above high surface area graphite has a particle size of 0.1 μm to 50 μm. Graphite and silicon composite.

4. In Paragraph 1, The above high surface area graphite has a specific surface area (BET) of 30 m² 2 / g excess Graphite and silicon composite.

5. In Paragraph 1, The graphite and silicon composite further comprises amorphous carbon. Graphite and silicon composite.

6. In Paragraph 5, The above amorphous carbon is included in an amount of 1% to 50% by weight relative to the weight of the graphite and silicon composite. Graphite and silicon composite.

7. In Paragraph 1, The graphite and silicon composite above is a spherical particle with a core-shell structure. Graphite and silicon composite.

8. In Paragraph 7, The above graphite and silicon composite has an outer shell made of amorphous carbon. Graphite and silicon composite.

9. In Paragraph 1, The graphite and silicon composite has a diameter of 1 μm to 50 μm. Graphite and silicon composite.

10. A step of manufacturing silicon particles by grinding silicon; The above silicon particles and a specific surface area (BET) of 10 m² 2 A step of preparing a slurry by mixing high surface area graphite exceeding / g; A step of preparing a precursor by spray-drying the above slurry; A step of mixing the above precursor with amorphous carbon and heat-treating to carbonize the amorphous carbon; and A step of manufacturing a cathode material by grinding and classifying Method for manufacturing a graphite and silicon composite.

11. In Paragraph 10, The above silicon particles are 5 nm to 1 µm. Method for manufacturing a graphite and silicon composite.

12. In Paragraph 10, The above slurry comprises 1% to 30% by weight of high surface area graphite. Method for manufacturing a graphite and silicon composite.

13. In Paragraph 10, The above amorphous carbon is included in an amount of 1% to 50% by weight relative to the total weight mixed with the precursor. Method for manufacturing a graphite and silicon composite.

14. In Paragraph 10, The step of carbonizing the above amorphous carbon is carried out under a nitrogen atmosphere at a temperature of 800°C to 1,000°C. Method for manufacturing a graphite and silicon composite.

15. A negative electrode material for a lithium secondary battery comprising a silicon-carbon composite according to any one of claims 1 to 9.

16. A lithium secondary battery comprising a negative electrode material for a lithium secondary battery according to paragraph 15.

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