WO2025110483A1 - Carbon nanotube and carbon black composite network-type silicon negative electrode material for lithium-ion secondary battery - Google Patents

Abstract

The present invention can provide a silicon negative electrode material that includes a plate-shaped silicon aggregate in which a silicon composite having a multi-layer structure is aggregated and bonded, wherein an oxide layer, a silicon carbide layer, and a carbon black layer are formed on the plate-shaped silicon, and carbon nanotubes capable of imparting electrical conductivity between silicon composite particles are included in the plate-shaped silicon, and thus can solve the problem of lifespan degradation caused by volume change of the silicon negative electrode material and is suitable for manufacturing a lithium-ion secondary battery having a high capacity and high output performance. In addition, the silicon negative electrode material of the present invention uses silicon kerf waste as a raw material, and is thus economically efficient.

Description

Silicon anode material for lithium-ion secondary battery using carbon nanotube and carbon black composite network

The present invention relates to a silicon negative electrode material for a lithium ion secondary battery, which comprises a plate-shaped silicon particle, an oxide layer, a carbon-based multilayer structure, and carbon nanotubes.

As electric vehicles are becoming more widespread worldwide, there is an increasing demand for improved performance of lithium-ion secondary batteries, which are the batteries of electric vehicles. Research is being conducted in various fields to improve the performance of lithium-ion secondary batteries, including changes in materials and structures, and in particular, research and development on silicon as a negative electrode material is active.

The typical anode material for lithium-ion secondary batteries is graphite-based anode material, and it is used in most electric vehicles. However, due to the nature of the material, there are clear limitations in terms of performance improvement. On the other hand, silicon-based anode materials have a capacity per unit weight that is about 10 times higher than that of graphite-based anode materials, which can significantly increase the driving range of electric vehicles and also has the advantage of being advantageous for fast charging.

Despite these advantages of silicon anode materials, silicon anode materials are currently only used in some cases along with carbon anode materials. The biggest problem with silicon anode materials is the volume expansion that occurs during repeated charging and discharging. Due to the atomic structure of silicon, four lithium ions combine and leave during charging and discharging, and as this process is repeated, the volume expands by 2 to 3 times. After the volume expansion, the silicon anode material may not return to its original state, and in this case, the electrical connection with the electrolyte may be lost due to cracks in the anode or separation of the anode material.

Since the volume expansion problem of silicon anode materials is already a well-known problem, continuous research is being conducted, such as developing a core-shell structure to suppress the volume expansion of silicon anode materials, as in Korean Patent No. 1818813 and Japanese Patent No. 7288054. However, there are still issues to be resolved before silicon anode materials can be used as the main anode material to replace graphite anode materials.

The present invention solves problems such as volume expansion, reduced charge/discharge efficiency, and short lifespan that occur in silicon anode materials, and also solves problems such as recycling of discarded silicon materials and increased costs due to the use of silicon materials by using waste silicon kerf as a silicon raw material.

The present invention provides a silicon anode material for a lithium secondary battery including a platelet silicon aggregate formed by agglomeration of a plurality of platelet silicon complexes, wherein the platelet silicon complex includes an oxide layer formed by oxidizing the surface of platelet silicon particles, a silicon carbide layer formed on the outer surface of the oxide layer, a plurality of carbon nanotubes formed on the surface of the silicon carbide layer and bonded to form a mesh network structure, and a carbon black layer coating the silicon carbide layer and the plurality of carbon nanotubes formed on the surface and bonded to form a mesh network structure.

In the silicon negative electrode material for a lithium secondary battery of the present invention, carbon nanotubes that are combined to form a mesh-like network structure can connect a plurality of adjacent plate-shaped silicon complexes.

In the silicon negative electrode material for a lithium secondary battery of the present invention, the silicon carbide layer can be formed by oxidizing the surface of plate-shaped silicon particles, and having a plurality of carbon nanotubes form a mesh network structure on the surface of the oxide layer, and some of the oxygen in the oxide layer can be replaced with carbon.

In the silicon negative electrode material for a lithium secondary battery of the present invention, the silicon carbide layer and the carbon black layer can be formed continuously in a single heating process.

In the silicon negative electrode material for a lithium secondary battery of the present invention, the carbon nanotubes may have an average diameter of 1.0 to 15 nm.

In the silicon negative electrode material for a lithium secondary battery of the present invention, the plate-like silicon aggregate may be in a spherical or granule shape.

In the silicon negative electrode material for a lithium secondary battery of the present invention, the plate-shaped silicon aggregates may have an average diameter of 2 to 50 μm.

The silicon negative electrode material for a lithium secondary battery of the present invention may further contain graphite.

The present invention can provide an anode comprising the silicon anode material for a lithium secondary battery of the present invention.

The present invention can provide a lithium secondary battery including a negative electrode according to the present invention.

The silicon anode material for a lithium secondary battery of the present invention forms an oxide layer, a silicon carbide layer, and a carbon black layer on plate-like silicon, and includes carbon nanotubes capable of imparting electrical conductivity between silicon composite particles, thereby imparting constant electrical conductivity even when the volume of the silicon anode material changes, thereby enabling uniform charging and discharging, and preventing loss of electrical conductivity of individual composite particles, thereby significantly improving the lifespan of the silicon anode material. In addition, by using waste silicon cuff as a raw material, it is possible to solve the problem of recycling of discarded silicon materials and increased costs due to the use of silicon materials.

FIG. 1 is a photograph showing plate-shaped silicon particles used as a silicon anode material raw material for a lithium secondary battery according to one embodiment of the present invention.

Figure 2 shows a cross-sectional structure of a silicon negative electrode material for a lithium battery according to one embodiment of the present invention.

FIG. 3 is a photograph showing the shape of a plate-shaped silicon aggregate formed after drying a mixed dispersion in which plate-shaped silicon particles having a silicon oxide layer formed thereon are mixed with a carbon nanotube dispersion during the manufacturing process for a silicon anode material for a lithium secondary battery according to one embodiment of the present invention.

Figure 4 is an enlarged photograph of the plate-shaped silicon agglomerate photograph of Figure 3, showing a plurality of carbon nanotubes that are bonded to form a mesh-like network structure on the surface of the silicon oxide layer and are connected across the surfaces of adjacent composite particles.

Figure 5 is an enlarged photograph of the photograph in Figure 4, showing a structure in which carbon nanotubes cover about 25% of the surface of the silicon oxide layer and connect adjacent silicon complexes.

Figure 6 shows a photograph of a plate-shaped silicon aggregate in which the plate-shaped silicon aggregate of Figure 3 was carbonized to form a carbon black layer.

Figure 7 is an enlarged photograph of the plate-shaped silicon agglomerate of Figure 6, showing a silicon carbide layer formed from a silicon oxide layer and carbon nanotubes coated with a carbon black layer.

Figure 8 is a flow chart schematically illustrating a method for manufacturing a silicon negative electrode material for a lithium ion secondary battery according to one embodiment of the present invention.

Figure 9 is a graph of the results of a charge/discharge test of a half-cell according to an embodiment and a comparative example of the present invention.

The advantages and features of the present invention, and the method for achieving them, will become clear with reference to the specific contents described in detail below together with the accompanying drawings. However, the present invention is not limited to the specific contents disclosed below, but can be implemented in various different forms, and the specific contents of the present invention are provided only to make the disclosure of the present invention complete and to fully inform a person having ordinary skill in the art to which the present invention belongs of the scope of the invention, and the present invention is defined only by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

In describing the specific contents of the present invention, if it is judged that the description of known functions or configurations may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, the terms described below are terms defined in consideration of the functions of the present invention, and these may vary depending on the intention or custom of the user or operator. Therefore, the definitions should be made based on the contents throughout this specification.

The present invention can provide a silicon anode material for a lithium secondary battery including a plate-shaped silicon aggregate formed by agglomeration of a silicon composite including a silicon oxide layer, a silicon carbide layer, a plurality of carbon nanotubes bonded to plate-shaped silicon particles and forming a mesh-like network structure, and a carbon black layer coating the silicon carbide and carbon nanotubes.

In the silicon anode material of the present invention, the plate-shaped silicon composite may include an oxide layer formed by oxidizing the surface of plate-shaped silicon particles, a silicon carbide layer formed on the outer surface of the oxide layer, a plurality of carbon nanotubes formed in a mesh-like network structure on the surface of the silicon carbide layer and bonded thereto, and a carbon black layer coating the plurality of carbon nanotubes formed in a mesh-like network structure with the silicon carbide layer and bonded thereto.

In the plate-like silicon composite of the present invention, the plate-like silicon particles may be manufactured from waste silicon kerf, and the plate-like silicon particles may have an average thickness of 10 to 100 nm and an average length of 10 μm or less, preferably 1 to 10 μm. When the average thickness of the plate-like silicon particles is less than 10 nm, so that too much part is lost during the formation of an oxide layer, so that the initial capacity may not even be 30% of the amount before treatment, and when the average thickness exceeds 100 nm, the ratio of the central silicon layer may be higher than 80%, so that the performance improvement effect may not be significant. In addition, when the average length of the plate-like silicon particles exceeds 10 μm, when mixed with graphite, a lot of empty space is formed between the graphite and the plate-like silicon aggregate according to the present invention, so that the porosity in the same space increases and the filling rate decreases, so that the discharge capacity in the same volume may drop significantly.

The waste silicon kerf used in the present invention can be utilized as generated in the process of thinly slicing a lump of metallic silicon to obtain a silicon wafer in the solar cell industry or the semiconductor industry. Here, the waste silicon kerf is high-purity silicon with a purity of 99.9999999% to 99.99999999% used in the solar cell industry or the semiconductor industry, and since a wire saw, which is a wire-shaped saw, is used, it can be separated into a plate-shaped material with a nano-thickness, and this can be used as an excellent raw material for an anode active material for a lithium secondary battery as a high-purity plate-shaped silicon particle.

According to one embodiment of the present invention, the plate-shaped silicon particles may be powder particles formed in a plate shape, and may be formed from waste silicon kerfs (cutting fines) generated in the process of thinly slicing a silicon ingot for a solar cell or a semiconductor. There are generally three to four types of silicon ingot cutting methods, and all methods can obtain waste silicon kerfs through the steps of classification, washing, precipitation, and drying. In order to obtain plate-shaped silicon particles having a uniform thickness from the waste silicon kerfs, it is preferable to use those made by a diamond wire saw, but the present invention is not limited thereto. Specifically, the diamond wire saw is a method of cutting silicon ingots using water or diethylene glycol as a lubricant by randomly embedding diamond particles on the surface of a carbon steel wire called a piano wire of about 50㎛. Silicon ingots include single-crystal ingots and polycrystalline ingots, and all of the cutting fines have the advantage of being suitable as the plate-shaped silicon particles of the present invention.

In the silicon anode material of the present invention, the silicon oxide layer is formed by oxidizing the surface of plate-shaped silicon particles obtained from waste silicon cuffs, and the bonding speed between lithium and plate-shaped silicon particles is slowed down through the oxide layer, thereby dramatically improving the charge/discharge life.

According to one embodiment of the present invention, the oxide layer may further oxidize a natural oxide layer naturally formed on the surface of the plate-like silicon particles to form a thicker oxide layer, and the average thickness of the oxide layer may be 2 to 10 nm. When the average thickness of the oxide layer is less than 2 nm, an uneven oxide layer in the shape of dots may be formed on the surface of the plate-like silicon particles, and a large number of non-oxidized portions on the surface of the plate-like silicon particles occur between the dot-like oxide layers, so that the effect of improving the life performance due to the uniform oxide layer may not be preferably exhibited. In addition, when the oxide layer exceeds 10 nm, the irreversible capacity, which is the main reason for the decrease in the initial discharge capacity, may significantly increase by more than 30%, which may cause severe lithium consumption.

In the silicon anode material of the present invention, the silicon carbide layer can be formed by substituting some of the oxygen in the silicon oxide layer with carbon through reduction and carbonization starting from the outer surface of the silicon oxide layer so as to surround the silicon oxide layer. The silicon oxide layer is amorphous and forms a passage for lithium and electrons to move while increasing the irreversible capacity when combined with lithium, but the silicon carbide layer reduces the increase in irreversible capacity to less than 1/10 of the oxide layer and forms a passage for lithium and electrons while controlling the diffusion or movement speed of lithium, thereby further improving the high-speed charge/discharge performance and lifespan.

According to one embodiment of the present invention, the thickness of the silicon carbide layer may be 1 to 5 nm. If it is less than 1 nm, the silicon carbide layer is formed in an uneven dot shape, so that the exposed area of the oxide layer increases, making it difficult to expect an improvement in life performance. If it is more than 5 nm, the discharge speed of lithium is significantly reduced compared to when there is no silicon carbide layer, so that the high-speed charge/discharge performance may be significantly reduced.

In the silicon anode material of the present invention, carbon nanotubes are bonded to the outer surface of the silicon carbide layer, and a plurality of carbon nanotubes can form a mesh-like network structure.

In addition, in the silicon anode material of the present invention, the carbon nanotubes that form a mesh-like network structure on the outer surface of the silicon carbide layer can connect a plurality of adjacent plate-like silicon composites. The carbon nanotubes can be bonded such that one or more strands span the surfaces of a plurality of adjacent silicon carbide layers, and the plurality of carbon nanotubes bonded across the surfaces of the plurality of adjacent silicon carbide layers in this way can play a role in imparting electrical conductivity between the plate-like silicon composite particles.

In the silicon anode material of the present invention, the carbon nanotubes bonded to the surface of the silicon carbide layer can improve the uniform charge and discharge speeds and the life of the anode. Specifically, in the plate-like silicon composite, the central plate-like silicon expands by more than twice in volume when lithium is charged and returns to the original volume when discharged, and the carbon nanotubes provide equal electrical conductivity both when the volume is increased and when it returns to the original volume, thereby helping to maintain a uniform charge and discharge speed and preventing individual particles of the plate-like silicon composite from losing electrical conductivity and no longer being able to perform the charge and discharge role, thereby extending the life of the silicon anode.

According to one embodiment of the present invention, the carbon nanotubes can cover 5 to 30% of the surface area of the silicon carbide layer, and the outer surface of the silicon carbide layer in the region where the carbon nanotubes and the carbon nanotubes are not combined is coated with a carbon black layer, thereby imparting high electrical conductivity to each individual particle of the plate-like silicon composite, while further improving the electrical conductivity between the plate-like silicon composite and the carbon nanotubes.

According to another embodiment of the present invention, the carbon nanotubes may have an average diameter of 1.0 to 15 nm, preferably 6 to 12 nm, and a length of 50 to 300 μm, preferably 100 to 300 μm. 1.0 nm is the minimum average diameter of the carbon nanotubes, and if the diameter of the carbon nanotubes exceeds 15 nm, the flexibility may decrease and the adhesion to the surface of the oxide layer formed on the plate-like silicon particles may decrease. If the length of the carbon nanotubes is short, the effect of improving the electrical conductivity between the plate-like silicon composite particles by the carbon nanotubes may decrease, and if the length of the carbon nanotubes is long, the uniformity of the components in the carbon nanotube dispersion used in the process of manufacturing the silicon anode material may decrease, or the size or shape of the aggregated particles of the plate-like silicon composites may become excessively non-uniform.

In the silicon anode material of the present invention, the carbon black layer is a layer formed by wrapping and coating a silicon carbide layer and carbon nanotubes, and plays a role in maintaining the formation amount and formation relationship of SEI (Solid electrolyte interface), which is an interface between graphite and an electrolyte (or electrolyte) in a conventional lithium ion secondary battery, and can eliminate the inconvenience of having to change the electrolyte (or electrolyte) due to a change in material, and can improve electrical conductivity together with carbon nanotubes.

According to one embodiment of the present invention, the carbon black layer may have an average thickness of 3 to 20 nm. When the average thickness of the carbon black layer is less than 3 nm, the carbon coating layer is formed unevenly in the form of dots on the surface, so that many uncoated areas occur, and thus the lifespan improvement effect may not be significant. When the average thickness exceeds 20 nm, excessive coating may increase the number of pores inside the carbon coating layer, so that lithium fills the pores and does not escape again, which may significantly increase the irreversible capacity.

According to one embodiment of the present invention, the plate-like silicon aggregates formed up to the carbon black layer may preferably have an average diameter of 2 to 50 μm, more preferably 2 to 30 μm, and most preferably 2 to 25 μm. If the average diameter of the plate-like silicon aggregates is small, it may be difficult to homogeneously mix with commercial graphite particles used in lithium ion secondary batteries, and if the average diameter of the plate-like silicon aggregates is large, many empty spaces may be formed when mixed with graphite, which may reduce the filling rate of the electrode. The plate-like silicon aggregates having a preferred size range are particles having a similar aspect ratio to commercial graphite, and thus have the advantage of being able to utilize a process using an existing dry milling machine or dry high-speed rotary mixer as is without any special process design change when mixed with graphite.

The silicon anode material of the present invention may further include graphite mixed with the plate-shaped silicon aggregate, and the graphite may improve the charging capacity and lifespan of the battery.

According to one embodiment of the present invention, the graphite particles may have a size of 10 to 30 μm, preferably 12 to 26 μm, and may not be particularly limited as long as it is within the size range of commercial graphite particles used in lithium ion secondary batteries.

According to another embodiment of the present invention, the platelet silicon aggregate and the graphite may be mixed in a weight ratio of 1 to 20:80 to 99, more preferably 5 to 15:85 to 90. When the content of the platelet silicon aggregate is lower than 1 wt%, the effect of increasing the charge capacity of the silicon anode material due to mixing of graphite falls within the charge capacity deviation, making it difficult to expect the effect due to the graphite, and when it exceeds 20 wt%, when charging lithium into the anode, the thickness of the entire electrode increases by more than 30% compared to when only graphite is used, so that the binder cannot withstand it, and cracks may occur at the connection between the copper current collector and the silicon composite, which may rapidly reduce the charge/discharge life.

The silicon negative electrode material according to the present invention can be manufactured according to the following manufacturing method, which will be described in detail with reference to the drawings.

The present invention can provide a method for manufacturing a silicon anode material for a lithium ion secondary battery, including a step of forming an oxide layer by oxidizing the surface of platelet silicon particles to form an oxide layer, a step of preparing a carbon nanotube dispersion by mixing carbon nanotubes, a binder, and a solvent, a step of preparing a mixed dispersion by dispersing platelet silicon particles, in which an oxide layer is formed in the oxide layer forming step, in the carbon nanotube dispersion, a step of drying the mixed dispersion to form a first platelet silicon composite and a first platelet silicon aggregate which is an aggregate thereof, in which a plurality of carbon nanotubes are combined to form a mesh network structure on the surface of the oxide layer of the platelet silicon particles, a carbonization step of carbonizing the outer shell of the silicon oxide layer of the first platelet silicon composite to form a silicon carbide layer, and a step of manufacturing a second platelet silicon composite and a second platelet silicon aggregate which is an aggregate thereof, in which a carbon black layer coating the silicon carbide layer of the first platelet silicon composite and a plurality of carbon nanotubes are formed.

The plate-shaped silicon aggregate included in the silicon anode material of the present invention is formed by aggregation and bonding of plate-shaped silicon complexes, and the raw material of the plate-shaped silicon complex can use plate-shaped silicon particles (11) obtained from waste silicon kerf. The plate-shaped silicon particles as the raw material are bent and rolled when a strong force is applied during the cutting process, so that silicon is separated from a single crystal into a plate shape, and can be made into a form in which many fine single crystals are weakly attached. Accordingly, the plate-shaped silicon particles can be made into a state more suitable for the anode material through a crushing pretreatment and used.

Specifically, the pretreatment of the plate-shaped silicon particles can be performed by wet milling and drying processes, and methods for wet milling the plate-shaped silicon may include a bead mill (ball mill), ultrasonic dispersion, and high-pressure homogenizer dispersion. First, the bead mill method is a method in which plate-shaped silicon particles are mixed in water or an organic solvent in a range of 5 to 30 wt%, and then rotated together with zirconia or alumina beads in a zirconia or alumina container to crush the particles by frictional force and impact force between the balls. It may be preferable that the bead mill use beads having a diameter of 0.5 to 3 mm and rotate at 1,000 to 5,000 rpm based on a container diameter of 100 mm to crush. When using a bead mill, if the diameter of the beads is less than 0.5 mm, the impact force may be weak and crushing may hardly occur, and if it exceeds 3 mm, the number of beads may be too small, which may reduce the probability of collision with the plate-shaped silicon particles, and thus the crushing time may become unnecessarily long. In addition, when the rotation speed is less than 1,000 rpm, the energy required for crushing may be insufficient due to low energy, so crushing may hardly occur, and when it exceeds 5,000 rpm, excessive high energy may cause bead wear and impurities may be mixed with the plate-shaped silicon particles. Next, the ultrasonic dispersion method is a method in which an amplifying horn and a vibrating horn are attached to an ultrasonic vibrator to apply ultrasonic vibration to the solution to disperse or destroy the grains in the solution. It is preferable that ultrasonic dispersion process plate-shaped silicon particles under the conditions of a frequency of 20 to 35 kHz, an amplitude of 20 to 200 ㎛, and a vibrator power consumption of 200 W or more. In addition, ultrasonic dispersion is possible by mixing plate-shaped silicon particles in water or an organic solvent in a range of 30 wt% or less, and then receiving ultrasonic vibrations from a plurality of vibrators while passing through a path in which a plurality of vibrators are arranged in a row, so that crushing is possible. At this time, if the power consumption of the vibrator is less than 200 W, the energy may be too low to cause almost no crushing, and if the frequency is less than 20 KHz, it may not be easy to operate because it cannot exceed the audible frequency, and if it exceeds 35 KHz, only the durability of the vibrator and vibration generator may be reduced and the crushing and working environment improvement effects may not be achieved, and if the plate silicon is mixed in water or an organic solvent in excess of 30 wt%, the viscosity may become too high and the transmission range of the ultrasonic vibration may not be wide. Next, the high-pressure homogenizer is a device that applies pressure using a pump to disperse or destroy the powder in the solution by passing the solution through a micro-nozzle in the opposite direction. A high-pressure homogenizer can be used in a way that combines collisions, such as a method of passing through a fine nozzle and then colliding with a diamond plate, a method of passing the nozzle in both directions and colliding the solutions with each other, and a method of mixing the plate-shaped silicon particles in water or an organic solvent in a range of 30 wt% or less using a high-pressure homogenizer dispersion method to obtain plate-shaped silicon particles suitable for the silicon anode material according to the present invention, and then applying pressure of 500 bar or more to pass through a fine nozzle of 50 to 200 ㎛ and colliding with or between diamond plates may be used. However, if the plate-shaped silicon particles are mixed in water or an organic solvent in a range exceeding 30 wt%, the viscosity may become too high to be easily injected into the fine nozzle, and if the pressure is less than 500 bar, the collision energy may be weak and crushing may hardly occur, and if the fine nozzle diameter is less than 50 ㎛, frequent nozzle layering may occur, and if it exceeds 200 ㎛, the collision energy may be too weak and crushing may hardly occur.

The plate-shaped silicon particles prepared according to the above pretreatment can be used to manufacture the silicon negative electrode material desired in the present invention according to the flow chart of Fig. 8, which is an example of the present invention.

According to one embodiment of the present invention, the plate-shaped silicon particles obtained from any one of a bead mill, a disperser, and a homogenizer may be preferably recovered in a dried powder state according to the silicon cuff drying step (S110). At this time, various devices having moisture and organic solvent vaporization functions can be used as the drying device, but it is preferable to use a spray dryer or a disk dryer. First, the spray dryer is a device that disperses a dispersion solution containing crushed plate-shaped silicon particles into the air through a spray nozzle or a rotating disk nozzle (atomizer) and injects a heated gas to rotate around the nozzle so that the solution is dried in a scattered state, and thus has the advantage of obtaining a dry powder having a low apparent density. Next, the disk dryer has the advantage of high thermal efficiency and has the advantage of high thermal efficiency in that it gradually drops a dispersion solution containing crushed plate-shaped silicon particles onto a heated rotating disk to dry them, and then scrapes the crushed and dried plate-shaped silicon particles, which are the dry residue, with a ceramic knife to recover them.

According to one embodiment of the present invention, the plate-shaped silicon particles obtained after the silicon cuff drying step (S110) are in a state in which moisture and lubricant in the waste silicon cuff are removed, but a silicon cuff disintegration step (S120) can be performed to form a space between the plate-shaped silicon particles for easy reaction with gas in a post-process. The plate-shaped silicon particles obtainable by the silicon cuff disintegration and drying step have an apparent density of 1 to 2 g/cm2, and by disintegrating at 3,000 rpm in air, plate-shaped silicon particles having an apparent density of 0.1 to 0.4 g/cm2 can be obtained, and the plate-shaped silicon particles obtained through the disintegration step can have an average distance between particles that is 3 to 10 times greater than that of the plate-shaped silicon particles obtained through the disintegration and drying steps, so that plate-shaped silicon particles (11) in a state in which an oxide layer is formed more uniformly, as shown in FIG. 1, can be obtained.

Before explaining the next step, the oxidation layer formation step (S130), the plate-like silicon composite in the manufacturing method of the present invention can be divided into a first plate-like silicon composite and a second plate-like silicon composite.

In the present invention, the first plate-shaped silicon composite is a composite in which a plurality of carbon nanotubes form a mesh network structure on the surface of an oxide layer formed on plate-shaped silicon particles having an oxide layer formed thereon, and the second plate-shaped silicon composite is a composite particle in which a silicon carbide layer and a carbon black layer are formed on the first plate-shaped silicon composite, and a cross-section thereof can be expressed as a structure as shown in FIG. 2.

According to the present invention, a first plate-shaped silicon composite is aggregated and connected to form a first plate-shaped silicon aggregate, and the first plate-shaped silicon aggregate can form a second plate-shaped silicon aggregate through the steps of forming a silicon carbide layer and coating a carbon black. At this time, the second plate-shaped silicon aggregate can be a composite in which a silicon carbide layer and a carbon black layer are formed on the first plate-shaped silicon composite, and the second plate-shaped silicon composite can be in a state in which the second plate-shaped silicon composite is aggregated and connected.

The silicon anode material targeted in the present invention is an anode material including a second plate-shaped silicon aggregate, and the plate-shaped silicon composite and the aggregate thereof will be described in more detail below, starting with the oxidation layer formation step (S130).

In the present invention, the oxide layer formation step (S130) may be a step of forming a silicon oxide layer (12) on the outer surface of the silicon particle by oxidizing the surface of the plate-shaped silicon particle (11).

According to one embodiment of the present invention, the step of forming an oxide layer (S130) may be performed by using a rotary kiln, injecting an oxidizer into the plate-shaped silicon particles, and then heating at 700 to 1,100°C for 5 to 30 minutes. If the heating temperature is less than 700°C, the formation speed of the oxide layer may be too slow, so that the reaction time may become excessively long, and thus the oxide layer may not be formed entirely or may be difficult to form with a desired thickness. In addition, if the heating temperature exceeds 1,100°C, the plate-shaped silicon particles may be damaged by the excessive temperature or the process cost may unnecessarily increase, which may be inefficient. Since the rotary kiln uses a continuous heating furnace, it can shorten the working time with excellent heat efficiency, and the rotary kiln can be composed of an inlet for injecting the material to be treated into the kiln body, a heat treatment section having a kiln body for heating, and a discharge section for discharging the heated material to be treated. Since the material to be treated is continuously mixed and moved in the rotary kiln, a uniform and thick oxide layer is formed, which can reduce the difference in the oxide layer ratio between particles.

According to another embodiment of the present invention, the oxidizing agent used in the oxidation layer formation step (S130) may be at least one of oxygen, water, and hydrogen peroxide, and the heating temperature may be controlled depending on the type of the oxidizing agent. When hydrogen peroxide is used as the oxidizing agent, heating may be performed at 700 to 1,100°C, and when oxygen is used as the oxidizing agent, heating may be performed at 900 to 1,100°C.

In the present invention, the step of mixing a carbon nanotube dispersion (S140) may include a step of preparing a carbon nanotube dispersion in which carbon nanotubes and a polymer binder are homogeneously dispersed, and a step of preparing a mixed dispersion by introducing and homogeneously mixing platelet silicon particles having a silicon oxide layer formed thereon, manufactured in the step of forming an oxide layer (S130). The step of preparing a carbon nanotube dispersion is not particularly limited, such as by using a high-pressure homogenizer. The step of preparing a mixed dispersion by mixing platelet silicon particles having a silicon oxide layer formed thereon into a carbon nanotube dispersion can be performed by a rotary homogenizer or an ultrasonic disperser at 3,000 rpm or higher, and the degree of dispersion may be preferably a degree of dispersion in which 5% or less of sediment is generated after 1 hour from the start of sedimentation at a height of 10 cm in a vibration-free state at room temperature.

According to one embodiment of the present invention, a carbon nanotube dispersion can be prepared by homogeneously mixing carbon nanotubes, a polymer binder, and a solvent. The polymer binder is a component that stabilizes the dispersion state of carbon nanotubes in the carbon nanotube dispersion and helps the attachment of the carbon nanotubes to an oxide layer during drying. At least one organic polymer selected from the group consisting of aqueous polymers such as polyvinylacetate (PVA), polynylpyrrolidone (PVP), ethylene vinyl acetate (EVA), and thermoplastic polyurethane (TPU) can be used. The polymer binder can be used in a weight ratio of carbon nanotubes: polymer binder of 1:0.5 to 3, preferably 1:0.5 to 2.5, and when the content of the binder is low, a decrease in the bonding of carbon nanotubes may occur, and when the content of the binder is high, aggregation of carbon nanotubes may occur. The solvent that can be used is preferably water, distilled water, purified water, etc.

According to another embodiment of the present invention, a mixed dispersion in which plate-shaped silicon particles having a silicon oxide layer formed thereon are dispersed in a carbon nanotube dispersion may contain 2 to 15 parts by weight of carbon nanotubes relative to 100 parts by weight of the plate-shaped silicon particles having a silicon oxide layer formed thereon. If the content of the carbon nanotubes is small, it may be difficult to cover the surface of the plate-shaped silicon composite to a sufficient extent for improving electrical conductivity, and if the content of the carbon nanotubes is large, the surface area of the plate-shaped silicon composite may be covered too widely, so that the effect of improving electrical conductivity may not be significant despite an increase in the content of the carbon nanotubes. As other composition components of the mixed dispersion, a polymer binder may be contained in an amount of 5 to 15 parts by weight relative to 100 parts by weight of the plate-shaped silicon particles, and a solvent may be contained in an amount of 900 to 1,000 parts by weight relative to 100 parts by weight of the plate-shaped silicon composite.

In the present invention, the dispersion drying step (S150) is a step of drying the mixed dispersion, and may include a step of manufacturing a first plate-shaped silicon aggregate. The first plate-shaped silicon aggregate is a particle formed by agglomeration and bonding of a first plate-shaped silicon complex, and the first plate-shaped silicon complex is a complex in which a plurality of carbon nanotubes (14) form a mesh network structure on the surface of the oxide layer in plate-shaped silicon particles on which a silicon oxide layer (12) is formed and are bonded by a polymer binder.

According to one embodiment of the present invention, drying may be preferably disk drying and spray drying. Disk drying is a method in which a dispersion is poured onto a high-temperature rotating circular disk and the dried powder is scraped off with a scraper, and spray drying is a method in which a dispersion is dropped onto a sprayer or a rotating disk to spread small droplets and blown with high-temperature gas so that the particles are suspended in the air and dried. More preferably, it is preferable to use spray drying to produce first plate-shaped silicon aggregate particles having a spherical or nearly spherical shape, a distorted spherical shape, or the like granule shape with fewer angular parts, as shown in the photograph of Fig. 3. The size of the particles after spray drying or the first plate-shaped silicon aggregates crushed after disk drying may have an average diameter of 2 to 50 μm, preferably 2 to 25 μm. If the size of the aggregate is small, it may be difficult to uniformly disperse it when mixed with graphite particles, and if it is too large, many empty spaces may be formed when mixed with graphite, which may reduce the filling rate of the electrode.

According to one embodiment of the present invention, a first platelet silicon composite and a first platelet silicon aggregate including the same are manufactured, in which a plurality of carbon nanotubes are bonded to form a mesh network structure on the surface of an oxide layer of platelet silicon particles on which an oxide layer is formed by disk drying or spray drying of a mixed dispersion. The structure of the first platelet silicon composite can be clearly confirmed through FIGS. 4 and 5, which are enlarged views of the first silicon aggregate formed as in FIG. 3. The carbon nanotubes can form an irregular mesh network structure by crossing and bonding to each other on the surface of the silicon oxide layer by a polymer binder, and can be bonded to form a mesh network structure that covers about 5 to 30% of the surface area of the silicon oxide layer of the first platelet silicon composite, as shown in FIG. 5. In addition, the carbon nanotubes can form a physical electrical conduction path between the composite particles by connecting a plurality of adjacent first platelet silicon composites, as shown in FIGS. 4 and 5, thereby further improving the electrical conductivity of the cathode.

In the present invention, the silicon carbide forming (S160) step may include a carbonization step of forming a silicon carbide layer (13) on the outer surface of a silicon oxide layer (12) from a carbon source. The silicon carbide layer is a layer having electrically conductive characteristics and may be formed by substituting some of the oxygen in the silicon oxide layer with carbon from the surface of the silicon oxide layer of the first plate-like silicon composite toward the inside, and more specifically, may be formed by substituting two oxygen atoms in the silicon oxide layer with one carbon.

The formation of a silicon carbide layer according to the present invention was applied by discovering that rapid substitution is possible even at a low temperature when reacting with gasified carbon from the outer surface of an amorphous silicon oxide layer to a depth of 1 to 5 nm from the inner surface, and according to one embodiment of the present invention, a silicon carbide layer can be formed by passing first plate-shaped silicon composite particles coated with carbon nanotubes and a polymer binder dispersant through a kiln heated to 900 to 1100° C. In the kiln heated to a temperature of 900° C. or higher, the polymer binder of the first plate-shaped silicon composite decomposes to generate hydrocarbon gas, some of which forms residual carbon, and the carbon generated by the decomposition of the residual carbon and hydrocarbon gas is substituted with oxygen in the silicon oxide layer to form a silicon carbide layer. As oxygen is substituted with carbon, a silicon carbide layer can be formed very quickly from the surface of the silicon oxide layer to a depth of 5 nm toward the inner surface.

According to one embodiment of the present invention, as a silicon carbide layer is formed on the outer surface of the silicon oxide layer of the first plate-like silicon composite, carbon nanotubes (14) forming a mesh network structure as shown in the structure of FIG. 2 can be bonded to the outer surface of the silicon carbide layer (13). That is, as oxygen in the oxide layer is replaced with carbon, a silicon carbide layer is also formed at the interface between the oxide layer and the carbon nanotubes bonded to the oxide layer.

In the present invention, a second plate-shaped silicon aggregate, such as FIGS. 6 and 7, in which a carbon black layer is formed on a first plate-shaped silicon aggregate through a carbon black coating (S170) step, can be provided as a silicon anode material. Specifically, the carbon black coating (S170) step can include a step of manufacturing a second plate-shaped silicon composite, in which a silicon carbide (13) layer and a carbon black layer (15) coating a plurality of carbon nanotubes (14) are formed, and a second plate-shaped silicon aggregate which is an aggregate thereof. In the plate-shaped silicon composite in which the silicon carbide layer is formed, the carbon nanotubes are bonded to the surface of the silicon carbide layer, and the surface of the silicon carbide layer where the carbon nanotubes and the carbon nanotubes are not bonded is exposed to the outside, and the exposed surface of the silicon carbide layer and the carbon nanotubes are coated with carbon black. The second plate-shaped silicon composite is a composite in which a silicon carbide layer is formed on the first plate-shaped silicon composite and then a carbon black layer is formed, and a state in which these are bonded and bonded can become a second plate-shaped silicon aggregate.

In the present invention, the carbon black coating (S170) step may be performed simultaneously with the silicon carbide layer formation (S160) as a single step so that the silicon carbide layer and the carbon black layer are formed continuously, or the carbon black coating (S170) step may be performed separately from the silicon carbide layer formation (S160) step.

According to one embodiment of the present invention, a method of simultaneously forming a carbon black layer and forming a silicon carbide layer in one step may be a method of continuously forming a silicon carbide layer and a carbon black layer in one heating process, wherein when a first plate-shaped silicon composite is passed through a heated kiln for forming a silicon carbide layer, a hydrocarbon gas capable of forming a carbon black layer is supplied.

According to another embodiment of the present invention, when the carbon black layer forming step is separated from the silicon carbide layer forming step, after the silicon carbide layer formation is completed on the first plate-shaped silicon composite, the composite on which the silicon carbide layer has been formed is put into a rotary kiln or a kiln, and one of a hydrocarbon gas, a liquefied natural gas, and a liquefied petroleum gas is selectively supplied while thermally decomposing at 750 to 1000°C, thereby forming a carbon black layer.

According to embodiments of the present invention, when the silicon carbide layer is formed first and the carbon black layer is formed later, the decomposition rate of the hydrocarbon gas can be increased, so that the hydrocarbon gas used for forming the same carbon black layer can be saved. In addition, when the silicon carbide layer and the carbon black layer are formed simultaneously, the hydrocarbon gas generated by the polymer decomposition has a high ratio of hydrogen and oxygen, so that the hydrocarbon decomposition rate for forming the carbon black layer can be somewhat reduced. However, in terms of integrating a plurality of processes into one and rapidly manufacturing the silicon anode material targeted by the present invention, it may be preferable to perform the silicon carbide layer forming step and the carbon black layer forming step simultaneously in one step.

The hydrocarbon gas supplied for forming the carbon black layer according to the present invention _may be at least one selected from the group consisting of C ₂ H ₂ (acetylene), C ₂ H ₆ (ethane), C ₂ H ₄ (ethylene), CH ₄ (methane), C ₃ H ₈ (propane), C ₄ H ₁₀ (butane), C ₃ H ₆ (propylene), and C 4 H ₈ (butylene), and the hydrocarbon gas may also be used by vaporizing a hydrocarbon solution composed of C, H, and O, such as ethanol, methanol, and toluene. The hydrocarbon gas supply may be 0.05 to 1.0 M/min, preferably 0.05 to 0.5 M/min.

The method for manufacturing a silicon anode material of the present invention may further include a step of mixing the silicon anode material targeted in the present invention, which is a plate-shaped silicon aggregate, with graphite after the carbon black coating (S170) step. The graphite particles may have a size of 10 to 30 ㎛, preferably 12 to 26 ㎛, and the silicon anode material of the present invention may have an aspect ratio similar to that of the graphite particles, so that the battery performance effect due to mixing of graphite may be further improved.

Furthermore, the silicon negative electrode material for a lithium ion secondary battery of the present invention can be used in the manufacture of a negative electrode of a lithium secondary battery and a lithium secondary battery including the same.

The silicon anode material for a lithium ion secondary battery of the present invention described above includes an organic shape of a multilayer structure including a silicon oxide layer, a carbon nanotube, and a carbon-containing layer in plate-like silicon particles obtained from waste silicon kerf, and when composited with graphite, has an excellent filling rate and can charge more lithium based on the same volume, and is also economical because it uses waste silicon kerf.

According to one embodiment of the present invention, a lithium ion secondary battery including the silicon negative electrode material for a lithium ion secondary battery of the present invention may have an initial discharge capacity of 500 mAh/g or more, and a residual capacity (discharge capacity) at 100 cycles of 89% or more, or 430 to 470 mAh/g.

Hereinafter, the present invention will be described through more specific examples and comparative examples. The examples below are only intended to help understanding the present invention, and the scope of the present invention is not limited by the examples below.

Any part not specifically defined in the embodiments may be interpreted according to the meaning, standard, value, analysis or measurement method (KS, JIS, ISO, ASTM, etc.) generally understood by a person skilled in the art to which the present invention pertains, and if no separate analysis method is described in the embodiments, it goes without saying that a known analysis method may be utilized.

[Manufacturing Example 1]

Manufacturing of silicon anode material containing carbon nanotubes

A polycrystalline silicon ingot was cooled, lubricated, and cut with a 50 ㎛ diameter diamond wio saw using a mixture of water and diethylene glycol to recover 5,000 ㎖ of a 5% plate-shaped silicon mixture solution. The mixture solution was injected into an atomizer plate rotating at 15,000 rpm in a spray dryer at a rate of 20 ㎖ per minute and dried at 180°C to obtain plate-shaped silicon particles.

Low-density plate-shaped silicon particles were produced by pulverizing plate-shaped silicon particles with air in a pin mill with a radius of 120 mm and 3400 rpm.

A silicon oxide layer was formed by oxidizing low-density plate-shaped silicon particles by bubbling and injecting hydrogen peroxide with nitrogen while keeping them in a rotary kiln at 800°C for 10 minutes.

25 g of carbon nanotubes (JENOTUBE 10B) with an average diameter of 10 nm and an average length of 150 μm, 50 g of polyvinylpyrrolidone, and 4925 g of distilled water were mixed and a carbon nanotube dispersion was prepared using a high-pressure homogenizer. 500 g of a silicon anode material having an oxide layer formed was added to 5,000 g of the carbon nanotube dispersion and further dispersed using a high-pressure homogenizer to prepare a mixed dispersion.

The mixed dispersion was injected at a rate of 30 ml per minute into an atomizer plate rotating at 18,000 rpm in a spray dryer and dried at 200°C to obtain first plate-shaped silicon aggregates agglomerated in a spherical or granule shape (Figs. 3, 4, and 5).

Thereafter, in the carbonization step, the first plate-shaped silicon aggregate was kept in a nitrogen atmosphere rotary kiln at 950°C for 5 minutes to decompose the polymer binder, polyvinylpyrrolidone, and form a silicon carbide layer on the silicon oxide layer. After the silicon carbide layer was formed, hydrocarbon gas was then supplied to continuously form a carbon black layer. Specifically, while keeping it in the rotary kiln at 950°C for 20 minutes, methane gas was supplied at a rate of 0.1 M/min to additionally form a carbon black layer to coat the silicon carbide layer and carbon nanotubes, thereby obtaining the second plate-shaped silicon aggregate, which is the final silicon anode material. It was confirmed that carbon nanotubes were bonded to the surface of the silicon carbide layer as the silicon carbide layer was formed (Figs. 6 and 7).

By mixing 9.5 mass% of the silicon anode material (second plate-shaped silicon aggregate) manufactured by the above method, 85.5 mass% of artificial graphite having an average diameter of 15 ㎛, 4.2 mass% of a binder, and 0.8 mass% of a conductive material, a silicon anode paste was manufactured, which was applied to copper foil, dried, and then punched into a circular shape to manufacture a cathode.

[Manufacturing Example 2]

Manufacturing of silicon anode materials without carbon nanotubes

A silicon oxide layer was formed using the same method as in Manufacturing Example 1.

Afterwards, 500 g of silicon anode material forming an oxide layer was added to 5,000 g of distilled water with a viscosity of 100 cps mixed with 25 g of carboxymethyl cellulose instead of carbon nanotubes, and dispersed using a high-pressure homogenizer to produce a dispersion.

The dispersion was injected at a rate of 30 ml per minute onto an atomizer plate rotating at 18,000 rpm in a spray dryer and dried at 200°C to obtain silicon aggregates in the shape of spheres or granules.

Afterwards, the silicon agglomerate was kept in a 950℃ rotary kiln for 20 minutes, and methane gas was injected at a rate of 0.1 M/min. to form a carbon black layer, thereby obtaining a silicon anode material.

By mixing 9.5 mass% of the silicon anode material manufactured by the above method, 85.5 mass% of artificial graphite having an average diameter of 15 ㎛, 4.2 mass% of binder, and 0.8 mass% of conductive material, a silicon anode paste was made, which was then applied to copper foil, dried, and punched into a circular shape to manufacture a cathode.

[Example 1]

The negative electrode prepared according to Manufacturing Example 1, the positive electrode made by punching lithium foil into a coin shape, and the separator were assembled and an electrolyte was added to produce a lithium ion battery half-cell for measuring charge and discharge performance. In measuring charge and discharge performance, the capacity was measured under the condition of 1C of charge and discharge current at 25°C, and the life was measured up to 100 cycles under the condition of 1C of charge and discharge current at 25°C.

The measurement results showed a discharge capacity of 504.1 mAh/g in the first cycle test, a low capacity decrease was confirmed during repeated cycles, and a residual capacity of 448.6 mAh/g, or 89.0%, was confirmed at 100 cycles.

[Comparative Example 1]

A half-cell was manufactured in the same manner as in Example 1, except that the cathode prepared in Manufacturing Example 2 was used, and charge/discharge performance was measured.

The measurement results showed a discharge capacity of 510.2 mAh/g in the first cycle test and an initial irreversible capacity of 92.2%. As the cycle progressed, a continuous decrease in capacity was observed, and at 100 cycles, a residual capacity of 426.5 mAh/g, or 83.6%, was confirmed.

[Explanation of symbols]

10: Second plate silicon composite

11: Plate-shaped silicon particles

12: Silicon oxide layer

13: Silicon carbide layer

14: Carbon nanotubes

15: Carbon black layer

Claims (10)

A silicon anode material for a lithium secondary battery comprising a plate-shaped silicon aggregate formed by agglomeration of a plurality of plate-shaped silicon complexes, The above plate-shaped silicon composite is, An oxide layer formed by oxidizing the surface of plate-shaped silicon particles; A silicon carbide layer formed on the outer surface of the oxide layer; A plurality of carbon nanotubes combined to form a mesh-like network structure on the surface of the silicon carbide layer; and A silicon anode material for a lithium secondary battery comprising a silicon carbide layer and a carbon black layer coating a plurality of carbon nanotubes that are combined to form a mesh-like network structure. In the first paragraph, A silicon anode material for a lithium secondary battery, wherein the carbon nanotubes formed in the above mesh-like network structure connect the adjacent plurality of plate-like silicon complexes. In the first paragraph, The above silicon carbide layer is, In a state where a plurality of carbon nanotubes form a mesh-like network structure on the surface of an oxide layer formed by oxidizing the surface of the above plate-shaped silicon particles, A silicon negative electrode material for a lithium secondary battery, wherein the above oxide layer is formed by replacing some of the oxygen with carbon. In the first paragraph, A silicon negative electrode material for a lithium secondary battery, wherein the silicon carbide layer and the carbon black layer are formed continuously in a single heating process. In the first paragraph, The above carbon nanotube is a silicon negative electrode material for a lithium secondary battery having a diameter of 1.0 to 15 nm. In the first paragraph, The above plate-shaped silicon aggregate is a silicon anode material for a lithium secondary battery having a spherical or granule shape. In Article 6, The above plate-shaped silicon aggregate is a silicon anode material for a lithium secondary battery having an average diameter of 2 to 50 μm. In the first paragraph, A silicon negative electrode material for lithium ion secondary batteries containing more graphite. A negative electrode comprising a silicon negative electrode material for a lithium ion secondary battery according to claim 1. A lithium ion secondary battery comprising the negative electrode of clause 9.

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