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

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 batteries using carbon nanotubes and carbon black composite network

[0001] The present invention relates to a silicon negative electrode material for a lithium ion secondary battery, which includes a plate-shaped silicon particle, an oxide layer, a carbon-based multilayer structure, and carbon nanotubes.

[0002]

[0003] With the global expansion of electric vehicles, demand is growing for improved performance of lithium-ion secondary batteries, the primary batteries used in electric vehicles. Research is being conducted on various aspects of improving lithium-ion secondary battery performance, including changes to materials and structures. In particular, research and development on silicon as a cathode material is actively underway.

[0004] Graphite is the most common anode material for lithium-ion secondary batteries and is used in most electric vehicles. However, due to the nature of the material, performance improvements are limited. In contrast, silicon anodes offer approximately 10 times the capacity per unit weight of graphite anodes, significantly increasing the driving range of electric vehicles and facilitating fast charging.

[0005] Despite these advantages of silicon anodes, they are currently only used in limited quantities alongside carbon-based anodes. The biggest problem with silicon anodes is their volume expansion, which occurs during repeated charging and discharging. Due to the atomic structure of silicon, four lithium ions combine and release during charging and discharging, and this process repeats, resulting in volume expansion of up to two to three times. After this volume expansion, the silicon anode may not be able to return to its original state. This can lead to cracks in the anode or separation of the anode material, leading to a loss of electrical connection with the electrolyte.

[0006] 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 challenges to be solved before silicon anode materials can be used as the main anode material to replace graphite anode materials.

[0007]

[0008] The present invention solves problems such as volume expansion, reduced charge / discharge efficiency, and short lifespan that occur in silicon anode materials, and can also solve 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.

[0009] The present invention provides a silicon anode material for a lithium secondary battery comprising a platelet silicon aggregate formed by agglomeration of a plurality of platelet silicon complexes, wherein the platelet silicon complex comprises 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 bonded to form a mesh-like network structure on the surface of the silicon carbide layer, and a carbon black layer coating the silicon carbide layer and the plurality of carbon nanotubes bonded to form a mesh-like network structure.

[0010] In the silicon anode 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.

[0011] In the silicon anode 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 forming a network structure of multiple carbon nanotubes on the surface of the oxide layer, where some of the oxygen in the oxide layer is replaced with carbon.

[0012] 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.

[0013] 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.

[0014] In the silicon negative electrode material for a lithium secondary battery of the present invention, the plate-shaped silicon aggregate may be spherical or granule-shaped.

[0015] 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.

[0016] The silicon negative electrode material for a lithium secondary battery of the present invention may further include graphite.

[0017] The present invention can provide an anode comprising the silicon anode material for a lithium secondary battery of the present invention.

[0018] The present invention can provide a lithium secondary battery including a negative electrode according to the present invention.

[0019] 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.

[0020]

[0021] 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.

[0022] 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.

[0023] 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 in a process for manufacturing a silicon anode material for a lithium secondary battery according to one embodiment of the present invention.

[0024] Figure 4 is an enlarged photograph of the plate-like silicon aggregate 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 surface of adjacent composite particles.

[0025] 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.

[0026] Figure 6 shows a photograph of a plate-shaped silicon aggregate in which the plate-shaped silicon aggregate of Figure 3 has been carbonized and a carbon black layer has been formed.

[0027] Figure 7 is an enlarged photograph of the plate-shaped silicon aggregate of Figure 6, showing a silicon carbide layer formed from a silicon oxide layer and carbon nanotubes coated with a carbon black layer.

[0028] Figure 8 is a flow chart schematically showing a method for manufacturing a silicon negative electrode material for a lithium ion secondary battery according to one embodiment of the present invention.

[0029] 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.

[0030]

[0031] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the specific details described below together with the accompanying drawings. However, the present invention is not limited to the specific details disclosed below and may be implemented in various different forms. The specific details of the present invention are provided only to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims. Like reference numerals designate like elements throughout the specification.

[0032] When describing the specifics of the present invention, if a description of a known function or configuration is deemed to unnecessarily obscure the gist of the invention, such detailed description will be omitted. Furthermore, the terms described below are defined in consideration of the functions of the present invention and may vary depending on the intent or custom of the user or operator. Therefore, their definitions should be based on the overall content of this specification.

[0033] The present invention can provide a silicon anode material for a lithium secondary battery, which includes a plate-shaped silicon aggregate formed by agglomerating 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.

[0034] 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 on the surface of the silicon carbide layer and bonded to form a mesh-like network structure, and a carbon black layer coating the plurality of carbon nanotubes formed on the silicon carbide layer and bonded to form a mesh-like network structure.

[0035] 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 the oxide layer, so that the initial capacity may not even be 30% of that before treatment, and when the average thickness exceeds 100 nm, the ratio of the central silicon layer may increase to more 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.

[0036] 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 semiconductor industry. Here, the waste silicon kerf is high-purity silicon with a purity of 99.9999999% to 99.999999999% used in the solar cell industry or semiconductor industry, and since a wire 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.

[0037] 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 during the process of thinly slicing silicon ingots for solar cells or semiconductors. 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, sedimentation, 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 through a diamond wire saw, but the present invention is not limited thereto. Specifically, a diamond wire saw is a method of cutting silicon ingots using diamond particles randomly embedded on the surface of a carbon steel wire called piano wire, which is approximately 50㎛ in diameter, using water or diethylene glycol as a lubricant. 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.

[0038] 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.

[0039] According to one embodiment of the present invention, the oxide layer may further oxidize the 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, dot-shaped oxide layer may be formed on the surface of the plate-like silicon particles, and many non-oxidized portions on the surface of the plate-like silicon particles may occur between the dot-shaped oxide layers, so that the life performance improvement effect due to the uniform oxide layer may not be desirable. 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 increase significantly by more than 30%, which may cause severe lithium consumption.

[0040] 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.

[0041] 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, increasing the exposed area of ​​the oxide layer, making it difficult to expect an improvement in life performance. If it is more than 5 nm, the lithium discharge speed is significantly lower than in the case where there is no silicon carbide layer, which may significantly deteriorate the high-speed charge / discharge performance.

[0042] 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.

[0043] 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 a plurality of adjacent silicon carbide layers in this way can serve to impart electrical conductivity between the plate-like silicon composite particles.

[0044] 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 speed 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 its original volume when discharged. The carbon nanotubes provide equal electrical conductivity both when the volume is increased and when it returns to its original size, 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 function, thereby extending the life of the silicon anode.

[0045] According to one embodiment of the present invention, 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 area where the carbon nanotubes and carbon nanotubes are not bonded 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 electrical conductivity between the plate-like silicon composite and the carbon nanotubes.

[0046] 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.

[0047] 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 amount and relationship of formation of SEI (Solid electrolyte interface), which is an interface between graphite and 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.

[0048] According to one embodiment of the present invention, the carbon black layer may have an average thickness of 3 to 20 nm. If 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, resulting in many uncoated areas, which may not significantly improve the lifespan. On the other hand, if the average thickness exceeds 20 nm, excessive coating may result in many pores within the carbon coating layer, which may cause lithium to fill the pores and not escape again, which may significantly increase the irreversible capacity.

[0049] 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 mill or dry high-speed rotary mixer as is without any special process design changes when mixed with graphite.

[0050] 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.

[0051] According to one embodiment of the present invention, the graphite particles may be 10 to 30 μm, preferably 12 to 26 μm, and may not be particularly limited as long as they are within the size range of commercial graphite particles used in lithium-ion secondary batteries.

[0052] According to another embodiment of the present invention, the plate-like silicon aggregate and 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 plate-like 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 graphite. In addition, when it exceeds 20 wt%, when charging lithium in 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 drastically reduce the charge-discharge life.

[0053]

[0054] The silicon anode 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.

[0055]

[0056] 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 having an oxide layer 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 are aggregates thereof, in which a plurality of carbon nanotubes are bonded to form a mesh network structure on the surface of the oxide layer of the platelet silicon particles having an oxide layer formed thereon, a carbonization step of forming a silicon carbide layer by carbonizing the outer shell of the silicon oxide layer of the first platelet silicon composite, and a step of manufacturing a second platelet silicon composite and a second platelet silicon aggregate which are aggregates thereof, in which a carbon black layer coating the silicon carbide layer of the first platelet silicon composite and a plurality of carbon nanotubes is formed.

[0057] 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 for the plate-shaped silicon complex may be plate-shaped silicon particles (11) obtained from waste silicon kerf. The plate-shaped silicon particles as the raw material are subjected to a strong force during the cutting process, causing silicon to fall off from a single crystal into a plate shape, thereby warping and rolling, and can be formed into a form in which many fine single crystals are weakly attached. Therefore, the plate-shaped silicon particles can be made into a state more suitable for the anode material through a crushing pretreatment and used.

[0058] Pretreatment of platelet silicon particles can be specifically performed through wet milling and drying processes, and methods for wet milling platelet silicon include bead mill (ball mill), ultrasonic dispersion, and high-pressure homogenizer dispersion. First, the bead mill method is a method in which platelet 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 and impact forces between the balls. It may be preferable to crush the beads using beads with a diameter of 0.5 to 3 mm and rotating at 1,000 to 5,000 rpm based on a container diameter of 100 mm. 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 lower the probability of collision with the platelet silicon particles, and thus the crushing time may be unnecessarily long. Also, 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 not be performed at all, and when it exceeds 5,000 rpm, excessively high energy may cause bead wear and impurities may be mixed with the plate-like silicon particles. Next, the ultrasonic dispersion method is a method of attaching an amplifying horn and a vibrating horn to an ultrasonic vibrator to apply ultrasonic vibration to the solution to disperse or destroy the particles in the solution. It is preferable that ultrasonic dispersion process plate-like silicon particles under the conditions of a frequency of 20 to 35 kHz, an amplitude of 20 to 200 μm, and a vibrator power consumption of 200 W or more. In addition, ultrasonic dispersion is possible by mixing plate-like 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 200W, 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, it may only reduce the durability of the vibrator and vibration generator and have no effect on crushing and improving the working environment, and if the plate silicon is mixed in water or an organic solvent in an amount exceeding 30% by weight, 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 colliding the solution with a diamond plate after passing through a fine nozzle, a method of colliding the solution with each other by passing the nozzle in both directions, and a method of using a high-pressure homogenizer dispersion method to obtain plate-shaped silicon particles suitable for the silicon anode material according to the present invention, in which plate-shaped silicon particles are mixed in water or an organic solvent in an amount of 30 wt% or less, and then pressurized to 500 bar or more to pass through a fine nozzle of 50 to 200 μm and collide with a diamond plate or with each other, but if the plate-shaped silicon particles are mixed in water or an organic solvent in an amount 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, resulting in almost no crushing. If the fine nozzle diameter is less than 50 μm, frequent nozzle layering may occur, and if it exceeds 200 μm, the collision energy may be too weak, resulting in almost no crushing.

[0059] 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.

[0060] According to one embodiment of the present invention, it may be preferable to recover the platelet silicon particles obtained from the silicon cuff 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 a spray dryer or a disk dryer is preferably used. First, a spray dryer is a device that disperses a dispersion solution containing crushed platelet silicon particles into the atmosphere 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, a disk dryer has the advantage of high thermal efficiency and has the advantage of high thermal efficiency by gradually dropping a dispersion solution containing crushed platelet silicon particles onto a heated rotating disk to dry it, and then scraping the crushed and dried platelet silicon particles, which are the dried residue, with a ceramic knife to recover them.

[0061] According to one embodiment of the present invention, the plate-like silicon particles obtained after the silicone cuff drying step (S110) are in a state in which moisture and lubricant in the waste silicone cuff have been removed, but in order to facilitate reaction with gas in a subsequent process, a silicon cuff disintegration step (S120) may be performed to form spaces between the plate-like silicon particles. The plate-like silicon particles obtained by disintegrating and drying the silicon cuff have an apparent density of 1 to 2 g / cm3, and by disintegrating at 3,000 rpm in air, plate-like silicon particles having an apparent density of 0.1 to 0.4 g / cm3 can be obtained, and the plate-like 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-like silicon particles obtained through the disintegration and drying steps, so that plate-like silicon particles (11) in a state in which an oxide layer can be formed more uniformly, as shown in FIG. 1, can be obtained.

[0062] 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.

[0063] In the present invention, the first plate-like silicon composite is a composite in which a plurality of carbon nanotubes form a mesh-like network structure on the surface of the oxide layer in plate-like silicon particles having an oxide layer formed thereon, and the second plate-like silicon composite is a composite particle in which a silicon carbide layer and a carbon black layer are formed on the first plate-like silicon composite, and the cross-section thereof can be expressed as a structure as shown in FIG. 2.

[0064] 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 a silicon carbide layer formation and carbon black coating step. 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.

[0065] The silicon anode material targeted in the present invention is a cathode 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 oxide layer formation step (S130).

[0066] 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).

[0067] According to one embodiment of the present invention, the oxide layer formation step (S130) may be performed by using a rotary kiln, injecting an oxidizing agent into the plate-like 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 becomes too slow, so that the reaction time becomes 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-like silicon particles may be damaged by the excessive temperature or the process cost may be unnecessarily increased, 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 feeding 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 ratio of the oxide layer between particles.

[0068] According to another embodiment of the present invention, the oxidizing agent used in the oxide layer formation step (S130) may be one or more of oxygen, water, and hydrogen peroxide, and the heating temperature may be controlled depending on the type of oxidizing agent. When hydrogen peroxide is used as the oxidizing agent, the heating temperature may be 700 to 1,100°C, and when oxygen is used as the oxidizing agent, the heating temperature may be 900 to 1,100°C.

[0069] In the present invention, the step of mixing the 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 plate-shaped silicon particles having a silicon oxide layer formed thereon, which are prepared in the step of forming an oxide layer (S130). The step of preparing the carbon nanotube dispersion is not particularly limited, such as by using a high-pressure homogenizer. The step of preparing the mixed dispersion by mixing plate-shaped silicon particles having a silicon oxide layer formed thereon into the carbon nanotube dispersion can be performed using a rotary homogenizer or an ultrasonic disperser at 3,000 rpm or higher, and the degree of dispersion may preferably be such that the sedimentation rate is 5% or less after 1 hour from the start of sedimentation at a height of 10 cm in a vibration-free state at room temperature.

[0070] 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 carbon nanotubes adhere to the oxide layer during drying. At least one organic polymer selected from the group consisting of water-based polymers such as polyvinylacetate (PVA), polynylpynolidone (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 to polymer binder of 1:0.5 to 3, preferably 1:0.5 to 2.5. If the binder content is low, the bonding of carbon nanotubes may be deteriorated, and if the binder content is high, the aggregation of carbon nanotubes may occur. The solvent that can be used is preferably water, distilled water, purified water, etc.

[0071] 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 carbon nanotubes is small, it may be difficult to cover the surface of the plate-shaped silicon composite to a sufficient extent to improve electrical conductivity, and if the content of 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 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 1000 parts by weight relative to 100 parts by weight of the plate-shaped silicon particles.

[0072] 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-like 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.

[0073] According to one embodiment of the present invention, drying may be preferably disk drying or 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 then dried by blowing high-temperature gas so that the droplets are suspended in the air. More preferably, it is preferable to produce first plate-shaped silicon aggregate particles having a spherical, nearly spherical, or distorted oval shape with fewer angular parts, as shown in the photograph of FIG. 3, through spray drying. The average size of the particles after spray drying or the first plate-shaped silicon aggregates crushed after disk drying may be 2 to 50 μm in diameter, preferably 2 to 25 μm. If the size of the aggregates is small, homogeneous dispersion may be difficult when mixed with graphite particles, and if the size is excessively large, a lot of empty space may be formed when mixed with graphite, which may reduce the filling rate of the electrode.

[0074] According to one embodiment of the present invention, a first planar silicon composite and a first planar silicon aggregate including the same are manufactured, in which a plurality of carbon nanotubes are bonded to form a mesh-like network structure on the surface of an oxide layer of planar silicon particles on which an oxide layer is formed by disk drying or spray drying of a mixed dispersion. The structure of the first planar 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-like 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-like network structure that covers about 5 to 30% of the surface area of ​​the silicon oxide layer of the first planar 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 planar silicon composites, as shown in FIGS. 4 and 5, thereby further improving the electrical conductivity of the cathode.

[0075] In the present invention, the silicon carbide forming step (S160) 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 properties 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 atom.

[0076] 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. 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.

[0077] 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-like 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.

[0078] In the present invention, a second plate-shaped silicon aggregate, such as those shown in 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 may 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 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 aggregated can become the second plate-shaped silicon aggregate.

[0079] In the present invention, the carbon black coating (S170) step may be performed simultaneously with the silicon carbide layer formation (S160) as one 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.

[0080] 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 to form a silicon carbide layer, a hydrocarbon gas capable of forming a carbon black layer is supplied.

[0081] 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 completing the silicon carbide layer formation on the first plate-shaped silicon composite, the composite on which the silicon carbide layer has been formed is introduced into a rotary kiln or a kiln, and one of hydrocarbon gas, liquefied natural gas, and liquefied petroleum gas is selectively supplied while thermally decomposing at 750 to 1000°C to form a carbon black layer.

[0082] According to embodiments of the present invention, when the silicon carbide layer is formed first and the carbon black layer is formed next, 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.

[0083] The hydrocarbon gas supplied for forming the carbon black layer according to the present invention is C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), C4H 10 (butane), C3H6 (propylene), and C4H8 (butylene), and the hydrocarbon gas may be one or more selected from the group consisting of C, H, and O, such as ethanol, methanol, and toluene, and may be used by vaporizing a hydrocarbon solution. The hydrocarbon gas supply may be 0.05 to 1.0 M / min, preferably 0.05 to 0.5 M / min.

[0084] The method for manufacturing a silicon anode material of the present invention may further include a step of mixing the silicon anode material of the present invention, which is a plate-shaped silicon aggregate, with graphite after the carbon black coating step (S170). The graphite particles may have a size of 10 to 30 μm, preferably 12 to 26 μm, and the silicon anode material of the present invention may have an aspect ratio similar to that of the graphite particles, thereby further improving the battery performance effect by mixing graphite.

[0085] 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.

[0086] The silicon anode material for a lithium ion secondary battery of the present invention described above includes a multilayered organic shape 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 due to the use of waste silicon kerf.

[0087] 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 may be 89% or more, or 430 to 470 mAh / g.

[0088]

[0089] Hereinafter, the present invention will be described through more specific examples and comparative examples. The examples below are provided solely to aid understanding of the present invention, and the scope of the present invention is not limited by the examples below.

[0090] Parts not specifically defined in the examples can 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 a separate analysis method is not described in the examples, a known analysis method can be utilized.

[0091]

[0092] [Manufacturing Example 1]

[0093] Manufacturing of silicon anode materials containing carbon nanotubes

[0094] 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 mL 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 mL per minute and dried at 180°C to obtain plate-shaped silicon particles.

[0095] 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 a speed of 3400 rpm.

[0096] 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.

[0097] 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 silicon anode material with 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.

[0098] 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 a first plate-shaped silicon aggregate in a spherical or granule shape (Figs. 3, 4 and 5).

[0099] Afterwards, the first plate-shaped silicon aggregate was kept in a nitrogen atmosphere rotary kiln at 950℃ for 5 minutes for the carbonization step, so that the polymer binder, polyvinylpyrrolidone, was decomposed and a silicon carbide layer was formed 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℃ 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 a second plate-shaped silicon aggregate, which is the final silicon anode material. As the silicon carbide layer was formed, it was confirmed that carbon nanotubes were bonded to the surface of the silicon carbide layer (Figs. 6 and 7).

[0100] A silicon anode paste was prepared 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, which was then applied to a copper foil, dried, and punched into a circular shape to manufacture a cathode.

[0101]

[0102] [Manufacturing Example 2]

[0103] Manufacturing of silicon anode materials that do not contain carbon nanotubes

[0104] A silicon oxide layer was formed using the same method as in Manufacturing Example 1.

[0105] 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 prepare a dispersion.

[0106] 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.

[0107] 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.

[0108] A silicon anode paste was made by mixing 9.5 mass% of the silicon anode material manufactured according to 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, which was then applied to copper foil, dried, and punched into a circular shape to manufacture a cathode.

[0109]

[0110] [Example 1]

[0111] A lithium-ion battery half-cell was manufactured by assembling the negative electrode prepared according to Manufacturing Example 1, the positive electrode made by punching out lithium foil into a coin shape, and the separator, and adding electrolyte to measure charge-discharge performance. In the charge-discharge performance measurement, the capacity was measured under the condition of 1C of charge-discharge current at 25°C, and the life was measured up to 100 cycles under the condition of 1C of charge-discharge current at 25°C.

[0112] The measurement results showed a discharge capacity of 504.1 mAh / g in the first cycle test, and a low capacity decrease was observed during repeated cycles. After 100 cycles, a residual capacity of 448.6 mAh / g, or 89.0%, was observed.

[0113]

[0114] [Comparative Example 1]

[0115] A half-cell was manufactured in the same manner as in Example 1, except that the cathode prepared according to Manufacturing Example 2 was used, and charge / discharge performance was measured.

[0116] As a result of the measurement, the discharge capacity of 510.2 mAh / g was observed in the first cycle test, and the initial irreversible capacity was confirmed to be 92.2%. As the cycle progressed, the capacity continuously decreased, and at 100 cycles, a residual capacity of 426.5 mAh / g, or 83.6%, was confirmed.

[0117]

[0118] [Explanation of symbols]

[0119] 10: Second plate-like silicon composite

[0120] 11: Plate-shaped silicon particles

[0121] 12: Silicon oxide layer

[0122] 13: Silicon carbide layer

[0123] 14: Carbon nanotubes

[0124] 15: Carbon black layer

Claims

1. A silicon negative electrode 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.

2. In paragraph 1, 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.

3. In paragraph 1, 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.

4. In paragraph 1, 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.

5. In paragraph 1, The above carbon nanotube is a silicon negative electrode material for a lithium secondary battery having a diameter of 1.0 to 15 nm.

6. In paragraph 1, The above plate-shaped silicon aggregate is a silicon anode material for a lithium secondary battery having a spherical or granule shape.

7. In paragraph 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.

8. In paragraph 1, A silicon negative electrode material for lithium ion secondary batteries containing more graphite.

9. A negative electrode comprising a silicon negative electrode material for a lithium ion secondary battery of paragraph 1.

10. A lithium ion secondary battery comprising the negative electrode of clause 9.

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