Method for manufacturing silicon anode material for lithium-ion secondary battery

Abstract

The present invention relates to a method for manufacturing a silicon anode material for a lithium-ion secondary battery from waste silicon kerf, can provide a silicon anode material for a lithium-ion secondary battery, an anode including same, and a lithium-ion secondary battery, the material comprising a plate-shaped silicon composite in which a composite layer including an oxide layer and a carbon-containing layer is formed on plate-shaped silicon acquired from waste silicon kerf, has an excellent filling rate when composited with graphite, is capable of storing a greater amount of lithium on a volumetric basis, and has outstanding economic feasibility by using waste silicon kerf.

Description

Method for manufacturing silicon anode material for lithium-ion secondary batteries

[0001] The present invention relates to a silicon-containing negative electrode material for a lithium-ion secondary battery and a method for manufacturing a lithium-ion secondary battery including the same, and more specifically, to a silicon negative electrode material for a lithium-ion secondary battery having excellent charge capacity and lifespan in which a composite layer including an oxide layer and carbon is formed on a plate-shaped waste silicon kerf, and a method for manufacturing a lithium-ion secondary battery including the same.

[0002]

[0003] Graphite is a representative anode material for lithium-ion secondary batteries, forming a layered structure and having a mechanism for lithium ions to be inserted and charged between the layers. Although graphite-based anode materials have a theoretical capacity of 372 mAh / g, they actually show a capacity of 360 mAh / g. As there is a demand for higher capacity, research and development on materials with higher capacity than graphite-based anode materials is actively underway.

[0004] Among materials with higher capacity than graphite, silicon (Si)-based anode materials have a theoretical capacity of 4,200 mAh / g. Since silicon anodes are more than 10 times larger than graphite, they are attracting attention as a major material capable of replacing graphite anodes. However, silicon anodes suffer from a short lifespan. This is because during the charge-discharge process, where four lithium ions bind to and then detach from a single silicon silicon molecule, a volume change of up to 300% occurs. Cracks develop in the parts of the expanded silicon that fail to return to their original state. Furthermore, during this process, the silicon separates into fine nanoparticles, severing the electrical connection with the electrolyte or liquid, making it impossible to recharge with lithium.

[0005] As this problem with silicon anode materials is a widely known fact, research and development are actively underway to suppress cracks occurring in silicon anode materials during charging and discharging. For example, Korean Registered Patent No. 2476118 discloses a technology for increasing cycle life by producing nano-level fine silicon particles, and Korean Published Patent No. 2019-0083613 discloses a technology for improving the lifespan of silicon anode materials by coating amorphous carbon onto porous silicon clusters.

[0006] However, despite these technological developments, the use of silicon anode materials in lithium-ion secondary batteries remains limited to small quantities, and silicon anode technology capable of replacing the majority of graphite anode materials is still in demand worldwide.

[0007]

[0008] The present invention can provide a method for manufacturing a silicon anode material with significantly improved filling rate and lifespan by using plate-shaped silicon formed from waste silicon kerf to solve the problem of silicon-based anode materials, and the silicon-based anode material manufacturing method of the present invention also has excellent commercial and economic feasibility.

[0009] The present invention provides a method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery from a waste silicon kerf, comprising: a disintegration step of a plate-shaped silicon formed from a waste silicon kerf; an oxidation step of oxidizing the surface of the plate-shaped silicon to form a silicon oxide with an oxide layer; a carbonization step of carbonizing the outer surface of the silicon oxide to form a silicon carbide layer; and a carbon coating step of coating the surface of the silicon carbide layer with conductive carbon to form a plate-shaped silicon composite with a carbon coating film.

[0010] In the carbonization step of the method for manufacturing a silicon anode material according to the present invention, the silicon carbide layer may be characterized by being formed in which some of the oxygen in the oxide layer is replaced with carbon.

[0011] The method for manufacturing a silicon anode material of the present invention may further include a secondary disintegration step for disintegrating silicon oxide after the oxidation step.

[0012] In the method for manufacturing a silicon anode material according to the present invention, the silicon carbide layer formation step and the carbon coating step can occur sequentially within a single heating process.

[0013] The method for manufacturing a silicon anode material according to the present invention may further include a mixing step after the carbon coating step, wherein a plate-shaped silicon composite and graphite are mixed to form a mixture.

[0014] The method for manufacturing a silicon anode material according to the present invention may further include a pretreatment step for removing moisture and lubricant from waste silicon cuffs prior to the disintegration step.

[0015] The method for manufacturing a silicon anode material of the present invention may further perform a process of burning residual carbon after forming a silicon carbide layer.

[0016] In the method for manufacturing a silicon anode material of the present invention, the silicon carbide layer may be characterized by having an average thickness of 1 to 5 nm.

[0017] The method for manufacturing a silicon anode material for a lithium-ion secondary battery according to the present invention forms a multilayer structure including an oxide layer and a carbon-containing layer on plate-shaped silicon formed from a waste silicon kerf, and when combined with graphite, it has an excellent filling rate and can charge more lithium based on the same volume, and also has excellent economic efficiency by using a waste silicon kerf.

[0018]

[0019] FIG. 1 is a flowchart schematically illustrating a method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery according to an embodiment of the present invention.

[0020] Figure 2 is a flowchart that includes a pretreatment step, a doping step, and a secondary disintegration step in addition to Figure 1.

[0021] Figure 3 is an SEM image of plate-shaped silicon used in a method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery according to an embodiment of the present invention.

[0022] Figure 4 is an example diagram showing the appearance of plate-shaped silicon particles formed in the oxidation step of Figure 1.

[0023] Figure 5 is an example diagram showing the appearance of plate-shaped silicon particles formed during the silicon carbide layer formation step of Figure 1.

[0024] Figure 6 is an example diagram showing the appearance of plate-shaped silicon particles formed in the carbon coating step of Figure 1.

[0025] Figure 7 is a TEM image of the plate-like silicon composite of Figure 6.

[0026] Figure 8 is a TEM image of plate-shaped silicon particles from which the carbon coating film has been removed from the plate-shaped silicon composite of Figure 7.

[0027] Figure 9 is a graph of the full cell charge / discharge test results of Example 2.

[0028] Figure 10 is a graph of the full cell charge / discharge test results of Example 4.

[0029]

[0030] The following description of the present invention with reference to the drawings is not limited to specific embodiments and may be subject to various modifications and have various embodiments. Furthermore, it should be understood that the content described below includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the present invention.

[0031] In the following description, terms such as "first," "second," etc., are used to describe various components and are not limited in their meaning; they are used solely for the purpose of distinguishing one component from another.

[0032] Identical reference numbers used throughout this specification indicate identical components.

[0033] The singular expressions used in the present invention include the plural expressions unless the context clearly indicates otherwise. Furthermore, terms such as "comprising," "having," or "having" described below should be interpreted as indicating the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not excluding in advance the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0034]

[0035] The present invention relates to a method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery from a waste silicon kerf, and can provide a silicon negative electrode material for a lithium-ion secondary battery comprising a plate-shaped silicon composite formed by forming a composite layer including an oxide layer and a carbon-containing layer on plate-shaped silicon obtained from a waste silicon kerf, a negative electrode comprising the same, and a lithium-ion secondary battery.

[0036]

[0037] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached FIGS. 1 to 10.

[0038]

[0039] FIG. 1 is a flowchart of a method for manufacturing a silicon anode material for a lithium-ion secondary battery according to an embodiment of the present invention, FIG. 2 is a flowchart of FIG. 1 that further includes a pretreatment step, a doping step, and a secondary disintegration step, FIG. 3 is an SEM image of plate-shaped silicon used in the method for manufacturing a silicon anode material for a lithium-ion secondary battery according to an embodiment of the present invention, FIG. 4 is an example diagram showing the appearance of plate-shaped silicon after undergoing the oxidation step of FIG. 1, FIG. 5 is an example diagram showing the appearance of plate-shaped silicon particles formed in the silicon carbide layer formation step of FIG. 1, FIG. 6 is an example diagram showing the appearance of a plate-shaped silicon composite formed in the carbon coating step of FIG. 1, FIG. 7 is a TEM image of the plate-shaped silicon composite of FIG. 6. FIG. 8 is a TEM image of plate-shaped silicon particles in which the silicon carbide layer is exposed by burning the carbon coating film of the plate-shaped silicon composite of FIG. 7 at 800 degrees in oxygen. Figure 9 is a graph of the discharge capacity and lifetime of a mixed cathode of a plate-shaped silicon composite and graphite prepared according to Figure 1. Figure 10 is a graph of the discharge capacity and lifetime of a mixed cathode of a sulfur-doped plate-shaped silicon composite and graphite prepared according to Figure 2.

[0040] The present invention relates to a method for manufacturing a silicon anode for a lithium-ion secondary battery, which can lower the unit cost of the anode material by using plate-shaped silicon formed from waste silicon kerf, and can also manufacture a silicon anode material with excellent filling rate by being composited with plate-shaped graphite.

[0041] The silicon kerf used in the present invention is utilized from the process of thinly slicing metallic silicon lumps to obtain silicon wafers in the solar cell or semiconductor industry. Here, the silicon kerf is high-purity silicon with a purity of 99.9999999% to 99.999999999% used in the solar cell or semiconductor industry, and is detached as a plate-like material with nano-thickness because a wire-type saw is used. Such high-purity plate-like silicon serves as a good candidate material for negative electrode active materials for lithium secondary batteries.

[0042] Referring to FIG. 1, a method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery according to an embodiment of the present invention may include a first disintegration step (S10), an oxidation step (S20), and a carbon coating step (S30).

[0043] The first disintegration step (S10) can disintegrate plate-shaped silicon (10) formed from waste silicon kerf to have an apparent density of 0.1 to 0.4 g / cm³.

[0044] This is to facilitate easy reaction with gas in the subsequent process, and allows the plate-shaped silicon (10) to be broken down to form spaces between particles. As the average distance between particles of the plate-shaped silicon (10) increases in this way, the oxide layer (11) can be uniformly formed in the subsequent oxidation step (S20).

[0045] Here, the plate-shaped silicon (10) may be a powder formed in a plate shape as shown in FIG. 3. The plate-shaped silicon may be formed from waste silicon cuff (cutting fines) generated during the process of thinly slicing a silicon ingot for a solar cell or semiconductor.

[0046] There are generally three to four types of silicon ingot cutting methods, and in all methods, waste silicon cuffs can be obtained through the steps of classification, washing, sedimentation, and drying. To obtain waste silicon cuffs with high thickness uniformity, it is preferable to use waste silicon cuffs made using a diamond wire saw, but is not limited thereto.

[0047] Specifically, the diamond wire saw is a method of cutting silicon ingots using water or diethylene glycol as a lubricant, with diamond particles randomly embedded on the surface of a carbon steel wire called a piano wire of about 50 μm. Silicon ingots include single-crystal ingots and polycrystalline ingots, and all cutting fine particles have the advantage of being suitable for the plate-shaped silicon (10) of the present invention.

[0048] A plate-shaped silicon (10) formed from a waste silicon kerf can be formed with an average thickness of 10 to 100 nm and an average length of 10 μm or less, preferably 1 to 10 μm.

[0049] This is because if the average thickness of the plate-shaped silicon (10) is less than 10 nm, there is too much loss when forming the oxide layer (11), so the initial capacity may not even be 60% of the pre-processing capacity, which can significantly reduce economic efficiency, and if the average thickness exceeds 100 mm, there are many particles in which the ratio of the oxide layer (11) is less than 20%, so the lifespan performance improvement effect may significantly decrease.

[0050] In addition, when the average length of the plate silicon (10) exceeds 10㎛, when mixed with graphite particles, a large empty space is formed between the graphite and the plate silicon composite (1) particles, so the porosity in the same space increases and the filling rate decreases, so the discharge capacity in the same volume may decrease significantly.

[0051] Meanwhile, the plate-shaped silicon (10) can be bent and curled as the silicon separates from the single crystal into a plate shape due to the strong force applied during the cutting process, and can be formed in a form with many fine single crystals weakly attached. Accordingly, the plate-shaped silicon (10) can be made into a state more suitable for the cathode material through crushing pretreatment.

[0052] To this end, with reference to FIG. 2, the method for manufacturing a silicon anode material for a lithium-ion secondary battery according to the present invention may include a pretreatment step (S1) prior to a first disintegration step (S10).

[0053] The pretreatment step (S1) can produce plate-shaped silicon particles by wet milling and drying the plate-shaped silicon (10) prior to the first disintegration step (S10).

[0054] At this time, the wet milling method for the plate-shaped silicon (10) can be a bead mill (ball mill), ultrasonic dispersion, or high-pressure homogenizer dispersion method.

[0055] First, the bead mill is a method of crushing by mixing plate-shaped silicon (10) in water or an organic solvent in a range of 5 to 30 weight percent and then rotating it together with zirconia or alumina beads in a zirconia or alumina container through friction and impact force between the balls. It is preferable to use beads with a diameter of 0.5 to 3 mm and crush them by rotating at 1,000 to 5,000 rpm based on a container diameter of 100 mm.

[0056] When using a bead mill, if the diameter of the bead is less than 0.5 mm, the impact force is weak and there may be almost no crushing, and if it is more than 3 mm, the number of beads is too small, so the crushing time may be unnecessarily long due to the reduced probability of collision with plate silicone (10).

[0057] In addition, if the rotational speed is less than 1000 rpm, there may be insufficient energy for crushing due to low energy, so crushing may not occur, and if it exceeds 5000 rpm, bead wear may occur due to excessive high energy and may be mixed with plate-shaped silicon as impurities.

[0058] Ultrasonic dispersion is a method of dispersing or destroying granules in a solution by attaching an amplification horn and a vibration horn to an ultrasonic transducer and applying ultrasonic vibrations to the solution. Ultrasonic dispersion is preferably performed on plate-shaped silicon (10) under conditions of a frequency of 20 to 35 KHz, an amplitude of 20 to 200 μm, and a transducer power consumption of 200 W or more. Additionally, ultrasonic dispersion can be performed by mixing plate-shaped silicon (10) in water or an organic solvent in a range of 30% or less, and then passing it through a channel in which a plurality of transducers are arranged in a line, receiving ultrasonic vibrations from the plurality of transducers to crush it.

[0059] In this case, if the power consumption of the vibrator is less than 200W, crushing may be almost impossible due to the excessively low energy. Additionally, if the frequency is below 20 KHz, operation may be difficult as it fails to exceed the audible frequency range, and if it exceeds 35 KHz, it only reduces the durability of the vibrator and vibration generator without any effect on crushing or improvement of the working environment.

[0060] In addition, if plate-shaped silicon (10) is mixed in water or an organic solvent in a range of more than 30% by weight, the viscosity becomes too high and the transmission range of ultrasonic vibrations may not be wide.

[0061] A high-pressure homogenizer is a device that uses a pump to apply pressure and passes a solution through a micro-nozzle in the opposite direction to disperse or destroy powder within the solution. There are also methods of combining collisions in the high-pressure homogenizer, such as a method of colliding with a diamond plate after passing through a micro-nozzle, or a method of passing the solution through a nozzle in both directions to cause the solutions to collide with each other. In the present invention, it is preferable to use a method in which plate-shaped silicon (10) is mixed in water or an organic solvent in a range of 30% by weight or less, then pressurized to 500 bar or more, passed through a micro-nozzle of 50 to 200 μm, and collide with a diamond plate or collide with each other.

[0062] At this time, if plate-shaped silicone (10) is mixed in water or an organic solvent in a range of more than 30% by weight, the viscosity becomes too high and it may not be easy to inject into a fine nozzle. Also, if the pressure is less than 500 bar, the impact energy is weak and there may be almost no crushing.

[0063] In addition, if the micro-nozzle diameter is less than 50㎛, nozzle clogging may occur frequently, making process operation difficult, and if it exceeds 200㎛, the collision energy becomes too weak, so crushing may not occur at all.

[0064] In addition, it is preferable that the crushed plate-shaped silicon particles obtained from one of the above bead mill, disperser, and homogenizer be recovered in a dried powder state. At this time, various devices with functions for vaporizing moisture and organic solvents can be used for drying, but it is preferable to use a spray dryer or a disc dryer.

[0065] Here, the spray dryer is a device that disperses a dispersion solution containing crushed plate-shaped silicon particles into the atmosphere through a spray nozzle or a rotating disc nozzle (Atomizer) and dries the solution while it is atomized by injecting heated gas to rotate around the nozzle; it has the advantage of obtaining a dry powder with low apparent density, but has poor thermal efficiency.

[0066] The disc dryer has high thermal efficiency and operates by dropping a dispersion solution containing crushed plate-shaped silicon particles onto a heated rotating disc in small increments to dry them, and then recovering the resulting residue—the crushed and dried plate-shaped silicon particles—by scraping them off with a ceramic knife. While the disc dryer offers good thermal efficiency, it has the disadvantage of recovering powder with a high apparent density.

[0067] As described above, the plate-shaped silicon particles that have been crushed and dried through the pretreatment step (S1) have had moisture and lubricant removed from the waste silicon cuff, but in order to facilitate reaction with gas in the subsequent process, the plate-shaped silicon particles that have been crushed and dried in the first disintegration step (S10) can be disintegrated to form spaces between the particles.

[0068] The crushed and dried plate-shaped silicon particles have an apparent density of 1 to 2 g / cm³, and by crushing at 3,000 rpm under air in the first crushing step (S10), plate-shaped silicon particles having an apparent density of 0.1 to 0.4 g / cm³ can be obtained.

[0069] Accordingly, the average distance between particles of the plate-shaped silicon particles (10) can be increased by 3 to 10 times compared to the crushed and dried plate-shaped silicon (10). Therefore, the first disintegration step (S10) can ensure that the oxide layer (11) is uniformly formed in the subsequent oxidation step (S20).

[0070] In addition, the plate-shaped silicon (10) particles that have been first broken down have a high specific surface area of ​​10 m² / g or more, and a large contact area with the electrolyte or electrolyte. While this characteristic of plate-shaped silicon ensures a high charge / discharge rate, there is a problem that silicon fragmentation occurs rapidly due to the rapid charge / discharge. To solve this, the present invention reduces the charge / discharge rate by forming an oxide layer (11) on the plate-shaped silicon (10) particles (20) in the oxidation step (S20) described below.

[0071] Referring to FIG. 4, the oxidation step (S20) can oxidize the surface of plate-shaped silicon (10) particles to create silicon oxide (3) with an oxide layer (11) formed on the outer surface.

[0072] The oxidation step (S20) can further oxidize the naturally formed oxide layer on the surface of the plate-shaped silicon particles (10) to form a thick oxide layer.

[0073] At this time, the oxide layer (11) is formed in an amorphous state, and when lithium approaches the oxide layer (11), lithium fills the gaps in the oxide layer (11), thereby forming a battery-conducting line that penetrates the oxide layer (11). The oxide layer (11) can allow lithium from the electrolyte or electrolyte to slowly diffuse into the plate-shaped silicon particles (10). Accordingly, the silicon anode material manufactured by the manufacturing method of the present invention can dramatically increase the charge-discharge lifespan, although the charge capacity is somewhat lower than that of conventional silicon, by lowering the bonding speed between lithium and the plate-shaped silicon particles (10) through the oxide layer (11).

[0074] The oxidation step (S20) can be performed by using a rotary kiln to introduce an oxidizing agent into plate-shaped silicon particles (10) and heating them to 700 to 1,100°C.

[0075] At this time, if the heating temperature is less than 700℃, the formation rate of the oxide layer (11) becomes too slow, and as the reaction time becomes excessively long, the oxide layer (11) may not be formed entirely or may be difficult to form to the desired thickness, and if it exceeds 1,100℃, the plate-shaped silicon particles (10) may be damaged by the excessive temperature or process costs may be unnecessarily increased, making it inefficient.

[0076] Here, one or more of oxygen, water, and hydrogen peroxide can be used as the oxidizing agent, and the heating temperature can be controlled depending on the type of oxidizing agent.

[0077] Specifically, when hydrogen peroxide is used as an oxidizing agent, it can be heated to 700 to 1,100°C, and when oxygen is used as an oxidizing agent, it can be heated to 900 to 1,100°C.

[0078] Here, since the rotary kiln uses a continuous heating furnace, it has good thermal efficiency and can shorten the working time. The rotary kiln is composed of an input section for introducing the workpiece into the kiln body, a heat treatment section equipped with a kiln body for heating, and an output section for discharging the heat-treated workpiece. Since the workpiece moves while continuously mixing in the rotary kiln, a uniform and thick oxide layer (1) is formed, and the difference in the ratio of the oxide layer (11) between particles can be reduced.

[0079] The average thickness of the oxide layer (11) formed in the oxidation step (S20) can be formed to be 2 to 10 nm. If the average thickness of the oxide layer (11) is less than 2 nm, non-uniform oxide layers (11) in the form of dots may be formed on the surface of the plate-shaped silicon particles (10), and as a result, many unoxidized parts may occur between the dot-shaped oxide layers (11) on the surface of the plate-shaped silicon particles (10), so the effect of a uniform oxide layer (11) cannot be achieved. In other words, the effect of improving lifespan performance may not be desirable.

[0080] In addition, if the oxide layer (11) is greater than 10 nm, the irreversible capacity, which is the main reason for the initial discharge capacity reduction, increases significantly to more than 30%, and lithium consumption may increase.

[0081] Referring to FIG. 1, the silicon carbide layer formation step (S30) can carbonize the outer edge of the plate-shaped silicon particle oxide layer (11) to create plate-shaped silicon particles in which a silicon carbide layer is formed on the outer edge of the oxide layer.

[0082] In the step (S30) of forming the silicon carbide layer (12), an electrically conductive silicon carbide layer (12) can be formed by replacing some of the oxygen in the oxide layer with carbon.

[0083] The silicon carbide layer (12) formed on the outer edge of the oxide layer (11) is created by replacing two oxygen atoms of silicon oxide with one carbon atom starting from the outermost edge of the oxide layer (11). There are two possible methods for replacing silicon oxide with silicon carbide. One method involves mixing crystalline silicon oxide with pitch and heating it at 1200 to 1400 degrees for a long time to replace the silicon with carbon that has a stronger bonding force, and burning off the residual carbon. The other method involves heating amorphous silicon oxide with pitch or another carbon source at 1000 degrees or higher for a long time to replace the oxygen atoms attached to the silicon with carbon that has a stronger bonding force, and burning off the residual carbon.

[0084] In this technology, it was discovered and applied that the outer 1 to 5 nm of amorphous silicon oxide can be rapidly replaced even at lower temperatures by reacting it with gasified carbon. When a silicon particle (10), on which an oxide layer (11) with a thickness of 2 to 20 nm has been formed by the above oxidation process, is passed through a kiln heated to 900 to 1100°C while a hydrocarbon gas is flowed into the same reactor, the hydrocarbon is decomposed, carbon atoms approach the oxide layer (11), and the oxygen at the outermost layer of the amorphous oxide layer (11) is replaced with carbon even at a relatively low temperature of 900°C. Due to the characteristics of the amorphous layer, the penetration of carbon with a small atomic size is easily achieved, allowing for rapid replacement up to a depth of 5 nm.

[0085] The hydrocarbon gases available for use at this time are C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), and C4H 10 Hydrocarbon gases such as butane, C3H6 (propylene), and C4H8 (butylene) can be used. Hydrocarbon gases can be used by vaporizing hydrocarbon solutions composed of C, H, and O, such as ethanol, methanol, and toluene.

[0086] The formation of the silicon carbide layer (12) can be confirmed by checking whether silicon carbide is detected on the outermost layer of the result obtained by heating the particles that have completed the silicon carbide layer (12) formation step to 800 degrees in an oxygen atmosphere to burn off all residual carbon. Silicon (10), silicon oxide layer (11), and silicon carbide layer (12) remain in order from the inside to the outside, and can be confirmed by transmission electron microscopy or electron microscopy analysis after ion milling. In FIG. 8, a TEM image of a plate-shaped silicon particle from which the carbon coating was removed from the plate-shaped silicon composite of FIG. 6 was analyzed.

[0087] The thickness of the silicon carbide layer (12) may be 1 to 5 nm. If it is less than 1 nm, it is formed in an uneven dot shape, so the exposed area of ​​the oxide layer (11) is large, making it difficult to expect an improvement in lifespan performance. If it is more than 5 nm, the discharge rate of lithium drops significantly compared to the case without the silicon carbide layer (12), which greatly reduces high-speed charge / discharge performance.

[0088] Also, referring to FIG. 1, the carbon coating step (S40) can create a plate-shaped silicon composite (1) in which a carbon coating film (13) is formed by coating the outer surface of the silicon carbide layer (12) with conductive carbon. The carbon coating step (S40) can impart electrical conductivity and smooth electron flow to the oxide layer (11) and the silicon carbide layer (12) by forming a carbon coating film (13) on the outer surface of the silicon carbide layer (12).

[0089] Here, the carbon coating film (13) plays a role in maintaining the amount and relationship of the SEI (Solid electrolyte interface), which is the interface between graphite and the 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.

[0090] The carbon coating step (S40) can be performed by using a rotary kiln or a kiln to select and introduce one of hydrocarbon gas, liquefied natural gas, and liquefied petroleum gas into plate-shaped silicon particles forming a silicon carbide layer (12), and then pyrolyzing them at 750 to 1000°C.

[0091] Here, hydrocarbon gases are gases composed of carbon and hydrogen bonds, such as C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), and C4H 10 (butane), C3H6 (propylene), C4H8 (butylene), etc. can be used. Hydrocarbon gas can be used that is produced by vaporizing and thermally decomposing a hydrocarbon solution composed of C, H, and O, such as ethanol, methanol, and toluene.

[0092] The carbon coating step (S30) preferably forms a carbon coating film (13) by using ethylene gas as a hydrocarbon gas to pyrolyze at 750 to 800°C or using liquefied natural gas to pyrolyze at 950 to 1,000°C, but is not limited thereto.

[0093] When using ethylene gas, if the temperature is below 750°C, the decomposition rate is less than 50%, resulting in unnecessary gas consumption, and if the temperature exceeds 800°C, the decomposition rate increases, which can produce a large amount of unnecessary byproduct called carbon black.

[0094] In the case of using liquefied natural gas, if the temperature is below 950°C, the decomposition rate is less than 50%, resulting in unnecessary gas consumption, and if it exceeds 1000°C, the decomposition rate increases, which can produce a large amount of unnecessary byproduct called carbon black.

[0095] In the carbon coating step (S40), the average thickness of the carbon coating film (13) can be formed to be 3 to 20 nm.

[0096] At this time, if the average thickness of the carbon coating film (13) is less than 3 nm, the carbon coating film (13) is formed unevenly in the form of dots on the surface of the silicon carbide layer (12), and many uncoated areas occur, so the lifespan improvement effect may not be significant, and if it is more than 20 nm, the carbon coating film (13) has many voids due to excessive coating, so lithium fills the voids and does not escape again, so the irreversible capacity may increase significantly.

[0097] The silicon carbide layer formation step (S30) and the carbon coating step (S40) can be separated into individual processes and formed stepwise by passing through a kiln for each process, and the silicon carbide layer (12) and the carbon layer (13) can be formed simultaneously. Likewise, doping with residual sulfur of the oxide layer (11) can also be done simultaneously.

[0098] First, we will explain the case where the silicon carbide layer formation step (S30) and the carbon coating step (S40) are performed separately.

[0099] The silicon carbide layer formation step (S30) involves passing plate-shaped silicon particles having an oxide layer (11) formed thereon through a kiln at 900 to 1100°C, and C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), and C4H, which decompose at 900°C. 10 When a small amount of vaporized gas is injected, such as hydrocarbon gas (butane), C3H6 (propylene), C4H8 (butylene), or a hydrocarbon solution composed of C, H, and O such as ethanol, methanol, and toluene, a silicon carbide layer (12) is formed. At this time, a residence time of 1 to 30 minutes at 900 degrees or higher is suitable. If the residence time is less than 1 minute, uniform heating is not achieved within the plate-shaped silicon particles that move by clumping together due to static electricity, making it difficult to form a silicon carbide layer (12) uniformly between the particles. If the residence time is more than 30 minutes, the rate of increase in the thickness of the silicon carbide layer (12) decreases significantly, and the economic efficiency of production decreases significantly.

[0100] The carbon coating step (S40) involves passing plate-shaped silicon particles, on which a silicon carbide layer (12) is formed, through a kiln at 750 to 1,000°C, and C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), C4H 10 When a large amount of gas is injected to vaporize a hydrocarbon gas such as (butane), C3H6 (propylene), C4H8 (butylene), or a hydrocarbon solution composed of C, H, and O such as ethanol, methanol, and toluene, a carbon layer (13) is formed.

[0101] This explains the case where the silicon carbide layer formation step (S30) and the carbon coating step (S40) are performed simultaneously.

[0102] Plate-shaped silicon particles having an oxide layer (11) formed thereon are passed through a kiln at 900 to 1100°C, and C2H2 (acetylene), C2H6 (ethane), C2H4 (ethylene), CH4 (methane), C3H8 (propane), and C4H, which undergo decomposition at 900°C or higher. 10 When a large amount of vaporized gas is injected, such as hydrocarbon gas (butane), C3H6 (propylene), C4H8 (butylene), or a hydrocarbon solution composed of C, H, and O such as ethanol, methanol, and toluene, a silicon carbide layer (12) is formed first, and then an excess amount of carbon accumulates to form a carbon layer (13). Since the formation rate of the silicon carbide layer (12) is slower than the formation rate of the carbon layer (13), the residence time at 900 degrees or higher follows the residence time standard of the silicon carbide layer formation step (S30). More preferably, at 950 to 1100°C, it is better to use a hydrocarbon with a carbon atomic ratio of 20% or less among hydrocarbons such as CH4 (methane), ethanol, and methanol to slow down the recombination time of carbon and make it easier to penetrate the oxide layer. Recombination time refers to the time it takes for carbon atoms, atomized by the thermal decomposition of hydrocarbon molecules, to clump together into five or more carbon black or graphene structures.

[0103] As confirmed in FIG. 7, a plate-shaped silicon composite can be formed in which an oxide layer, a silicon carbide layer, and a carbon coating film are sequentially formed on the surface of plate-shaped silicon particles through a silicon carbide layer formation step (S30) and a carbon coating step (S40).

[0104] Referring to FIG. 2, the method for manufacturing a silicon anode material for a lithium-ion secondary battery according to the present invention may further include a doping step (S22) after an oxidation step (S20).

[0105] In the oxidation step (S20) described above, the oxide layer (12) has good lithium penetration, so it is necessary to improve the strength of the oxide layer (12). To this end, in the doping step (S22), sulfur can be doped into the silicon oxide (3) after the oxidation step (S20).

[0106] The doping step (S22) can be performed by mixing 0.05 to 5 wt% of sulfur powder with respect to the total weight of silicon oxide (3) using a dry mixer.

[0107] At this time, if the sulfur powder is less than 0.05 wt%, the sulfur doping effect is barely observed, and if it exceeds 5 wt%, residual sulfur that does not participate in the reaction is generated, and the residual sulfur may combine with lithium to unnecessarily increase the irreversible capacity.

[0108] The doping step (S22) may pass the mixed silicon oxide (3) and sulfur powder through a rotary kiln at 600 to 1,000°C. In the doping step (S22), sulfur is vaporized at high temperature, and some of the vaporized sulfur penetrates into the gaps of the oxide layer (12) to form covalent bonds with silicon and oxygen, while other parts adhere to the surface of the oxide layer (12) upon cooling, and other parts may vaporize.

[0109] If the doping temperature is below 600℃, doping does not occur and the doping effect is barely observed, and if it exceeds 1000℃, a reaction occurs in the transparent ceramic tube, which can cause the ceramic tube to rapidly become opaque and significantly reduce its service life.

[0110] The doping step (S22) can improve the strength and lifespan performance of the oxide layer (12) by doping sulfur onto the outer surface of the oxide layer (12) to partially change the amorphous state of the oxide layer (12) into a crystalline state.

[0111] Through the doping step (S22) as described above, 0.05 to 5 wt% of sulfur can be doped into the oxide layer (12).

[0112] Meanwhile, if sulfur is added in the range of 0.05 to 5 wt% in the doping step (S22), the sulfur attached to the surface of the oxide layer (12) in the subsequent carbon coating step (S30) is vaporized in excess of the amount required for doping, thereby forming the silicon carbide layer (12) and the carbon coating layer (13), and allowing sulfur to be doped into the silicon carbide layer (12) and the carbon coating layer (13).

[0113] Specifically, in the doping step (S22), the sulfur introduced in excess remains as sulfur powder on the surface of the oxide layer (12), and in the carbon coating step (S30), the sulfur powder on the surface of the oxide layer (12) is vaporized so that sulfur can be doped into the spaces between the carbon atoms when the carbon coating film (13) is formed.

[0114] At this time, it is preferable to dope the silicon carbide layer (12) and the carbon layer (12) with 0.01 to 1 wt% of sulfur, respectively, because if it is less than 0.01 wt%, the doping effect of sulfur is almost non-existent, and if it is more than 1 wt%, residual sulfur is formed and combines with lithium, which can unnecessarily increase the irreversible capacity.

[0115] In addition, the method for manufacturing a silicon anode material for a lithium-ion secondary battery according to the present invention may further include a secondary disintegration step (S24) after the oxidation step (S20).

[0116] The second disintegration step (S24) can disintegrate the silicon oxide (3) that has aggregated after the doping step (S22) to reduce density. The second disintegration step (S24) can crush the silicon oxide (3) with air in a fin mill with a radius of 110 to 130 mm and 3300 to 3500 rpm.

[0117] This secondary disintegration step (S24) separates aggregated silicon oxide (3) so that the particles have an apparent density of 0.1 to 0.4 g / cm³. Accordingly, the secondary disintegration step (S24) can ensure that the silicon carbide layer (12) and the carbon coating film (13) are uniformly formed in the subsequent silicon carbide layer formation step (S30) and carbon coating step (S40).

[0118] In addition, the method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery according to the present invention may further include a mixing step after the carbon coating step (S40).

[0119] In the mixing step, a mixture can be formed by mixing the plate-shaped silicon composite (1) and graphite after the carbon coating step (S40), and a spherical mixture or a simple mixture can be formed.

[0120] Here, graphite is a plate-like material, and even when spherical, empty spaces are formed.

[0121] The mixing step allows the plate-shaped silicon composite (1) to fill the empty space of the graphite, thereby preventing the entire electrode from expanding due to expansion when lithium is bonded and maintaining the electrical connection with the electrode due to contraction. The mixture formed by the mixing step is formed such that the plate-shaped silicon composite is bonded to the empty space of the graphite, preventing separation from the electrode when expanded by lithium. At this time, the empty space of the graphite acts as a buffer space.

[0122] The mixing step may involve introducing a plate-shaped silicon composite (1) during the process of spheroidizing graphite, or mixing the graphite and the plate-shaped silicon composite after the spheroidization or spheroidization process is complete.

[0123] Specifically, when a plate-shaped silicon composite (1) is introduced during the process of spheroidizing graphite in the mixing step, the plate-shaped silicon composite (1) is inserted between the graphite plates, and a spheroidized mixture in the form of spheroidized or spherical graphite can be formed.

[0124] On the other hand, when mixing graphite and plate-shaped silicon composite (1) that have completed the spherical or spherical process in the mixing step, a simple mixture in which plate-shaped silicon composite (1) is placed between the graphites can be formed.

[0125] At this time, the graphite undergoes a spheroidization process, as spherical graphite has low anisotropy, which is advantageous for maintaining uniformity in voltage and current distribution. On the other hand, due to the inherent anisotropy of the material itself, flake-shaped graphite suffers from poor processability caused by reduced fluidity during subsequent mixing with solvents or binders and slurrying. Furthermore, it is difficult to form a coating layer of a specified thickness, which can lead to problems such as delamination. Generally, the spheroidization process can be performed by removing rough parts of the flake-shaped carbon material and smoothing the particle surface through mechanical rotational motion.

[0126] At this time, the plate-shaped silicon composite (1) and graphite can be mixed in a weight ratio of 1 to 20: 80 to 99, and more preferably in a weight ratio of 5: 95.

[0127] At this time, if the content of the plate-shaped silicon composite (1) is lower than 1 weight%, the effect of increasing the charging capacity of the silicon anode material due to graphite mixing falls within the range of charging capacity variation, making it difficult to determine the effect, and if it exceeds 20 weight%, the number of silicon particles becomes much larger than the number of graphite particles, so the uniform dispersion effect between silicon particles and graphite may decrease.

[0128]

[0129] The method for manufacturing a silicon anode material for a lithium-ion secondary battery according to the present invention described above forms a multilayer structure including an oxide layer and a carbon-containing layer on plate-shaped silicon formed from a waste silicon kerf, and when combined with graphite, it has an excellent filling rate and can charge more lithium based on the same volume, and also has excellent economic efficiency by using a waste silicon kerf.

[0130]

[0131] According to one embodiment of the present invention, in a lithium-ion secondary battery comprising a negative electrode, a positive electrode, a separator, and an electrolyte including a silicon negative electrode material for a lithium-ion secondary battery according to the present invention, the initial discharge capacity may be 400 mAh / g or more, the residual capacity (discharge capacity) at 300 cycles may be 340 to 360 mAh / g with 85% or more, and the residual capacity (discharge capacity) at 500 cycles may be 300 to 330 mAh / g with 75% or more.

[0132] According to another embodiment of the present invention, in a lithium-ion secondary battery comprising a negative electrode, a positive electrode, a separator, and an electrolyte, wherein one or more layers of an oxide layer, a silicon carbide layer, and a carbon coating film for a lithium-ion secondary battery are sulfur-doped, the initial discharge capacity may be 400 mAh / g or more, the residual capacity (discharge capacity) at 300 cycles may be 360 ​​to 380 mAh / g with 88% or more, preferably 89% or more, and the residual capacity (discharge capacity) at 500 cycles may be 320 to 350 mAh / g with 80% or more, preferably 81% or more.

[0133] As such, the silicon anode material for a lithium-ion secondary battery according to the present invention has a low rate of decrease relative to the initial capacity during charging and discharging, and when sulfur is doped into the oxide layer of the silicon anode material, a significantly improved discharge capacity can be expected even in the same cycle.

[0134]

[0135] The present invention will be described in more detail below with reference to examples, but these are merely illustrative of preferred embodiments of the present invention and do not limit the scope of the present invention.

[0136]

[0137] [Example]

[0138] [Example 1]

[0139] 5,000 ml of a 5% plate-shaped silicon mixed solution was recovered by cooling, lubricating, and cutting a polycrystalline silicon ingot with a 50 µm diameter diamond yoser using a mixture of water and diethylene glycol. The dispersion solution was injected at a rate of 20 ml per minute into an atomizer disc rotating at 15,000 rpm in a spray dryer and dried at 180°C to obtain plate-shaped silicon particles. Low-density plate-shaped silicon particles were produced by grinding with air in a fin mill with a radius of 120 mm and 3,400 rpm. An oxide layer was formed by oxidizing the plate-shaped silicon particles by bubbling and injecting hydrogen peroxide with nitrogen while holding them in a rotary kiln at 800°C for 10 minutes. The plate-shaped silicon particles with the formed oxide layer were held in a rotary kiln at 950°C for 20 minutes while methane gas was introduced at 0.1 M / min. A plate-shaped silicon composite was produced by injecting carbon at a high speed to replace the surface of the oxide layer and form a silicon carbide layer, and allowing residual carbon to cover the outer edge of the silicon carbide layer to form a carbon layer.

[0140] The manufactured plate-shaped silicon composite was mixed with a binder and a conductive material, coated onto copper foil, and perforated into a square shape. To form the battery cell, NCM811-coated aluminum foil perforated into a square shape was used as the positive electrode. A separator and electrolyte were placed between the negative and positive electrodes, and the mixture was vacuum-packed in an aluminum pouch to assemble a full lithium-ion battery.

[0141]

[0142] [Example 2]

[0143] A silicon anode material for a lithium-ion secondary battery was obtained by feeding the plate-shaped silicon composite prepared in Example 1 and spherical graphite into a dry ball mill in a weight ratio of 5:95 and mixing for 10 seconds.

[0144] A lithium-ion battery full cell was manufactured in the same manner as in Example 1 using the above silicon negative electrode material for a lithium-ion secondary battery.

[0145]

[0146] [Example 3]

[0147] Sulfur doping was performed on the surface of the oxide layer by mixing 1 wt% of sulfur powder with the plate-shaped silicon particles with an oxide layer formed in Example 1 and holding them in a rotary kiln at 800°C for 10 minutes. The plate-shaped silicon particles with the doped oxide layer were held in a rotary kiln at 950°C for 20 minutes while methane gas was introduced at a rate of 0.1 M / min. to displace the surface of the oxide layer, thereby forming a sulfur-doped silicon carbide layer. As residual carbon covered the outer surface of the silicon carbide layer to form a carbon layer, vaporized sulfur was doped to create a plate-shaped silicon composite having a sulfur-doped carbon layer.

[0148] A lithium-ion battery full-cell was manufactured using the above plate-shaped silicon composite in the same manner as in Example 1.

[0149]

[0150] [Example 4]

[0151] A silicon anode material for a lithium-ion secondary battery was obtained by feeding the plate-shaped silicon composite prepared in Example 3 and spherical graphite into a dry ball mill in a weight ratio of 5:95 and mixing for 10 seconds.

[0152] A lithium-ion battery full cell was manufactured in the same manner as in Example 1 using the above silicon negative electrode material for a lithium-ion secondary battery.

[0153]

[0154] [Experimental Example 1] Evaluation of Charge / Discharge Performance

[0155] To evaluate the charge and discharge performance of the full-cells prepared in Examples 1 to 4, capacity and lifespan were measured.

[0156] At this time, the capacity was measured under a charge / discharge current of 1C at 25℃, and the life was measured up to 500 cycles under a charge / discharge current of 1C at 25℃.

[0157] The results are shown in Figures 9 and 10.

[0158] Example 2 showed a high discharge capacity of 413.4 mAh / g in the first cycle test, and showed a near-linear decrease in capacity as the cycle progressed, then showed a residual capacity of 85.2% (352.4 mAh / g) at 300 cycles and a residual capacity of 76.8% (317.4 mAh / g) at 500 cycles (Fig. 9).

[0159] Example 4 was mixed with graphite in the same ratio as Example 2, but showed a high discharge capacity of 412.2 mAh / g in the first cycle test, and showed a near-linear decrease in capacity as the cycle progressed, showing a residual capacity of 89.5% (368.7 mAh / g) at 300 cycles and 81.6% (336.1 mAh / g) at 500 cycles (Fig. 10).

[0160] In other words, by doping sulfur into the oxide layer, the discharge capacity and lifespan performance in the same cycle (maintaining a residual capacity of 81–82% at 500 cycles) could be further improved, and the initial capacity reduction rate within 30 cycles could also be low.

[0161]

[0162] Although embodiments of the present invention have been described above with reference to the attached drawings, those skilled in the art will understand that the present invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Accordingly, the embodiments described above are illustrative in all respects and are not restrictive.

[0163]

[0164] [Explanation of the symbol]

[0165] 1: Plate-shaped silicon composite

[0166] 10: Plate-shaped silicone

[0167] 11: Oxide layer

[0168] 12: Silicon carbide layer

[0169] 13: Carbon coating layer

Claims

1. A disintegration step of plate-shaped silicon formed from waste silicon kerf; An oxidation step of oxidizing the surface of the plate-shaped silicon to form a silicon oxide layer; A carbonization step of carbonizing the outer surface of the silicon oxide to form a silicon carbide layer; and A carbon coating step for forming a plate-shaped silicon composite having a carbon coating film by coating the surface of the above silicon carbide layer with conductive carbon; A method for manufacturing a silicon anode material for a lithium-ion secondary battery, comprising 2. In Paragraph 1, A method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery, characterized in that, in the carbonization step above, the silicon carbide layer is formed by replacing some of the oxygen in the oxide layer with carbon.

3. In Paragraph 1, A method for manufacturing a silicon anode material for a lithium-ion secondary battery, further comprising a secondary dissolution step for dissolving the silicon oxide after the oxidation step.

4. In Paragraph 1, A method for manufacturing a silicon anode material for a lithium-ion secondary battery in which the silicon carbide layer formation step and the carbon coating step occur sequentially within a single heating process.

5. In Paragraph 1, A method for manufacturing a silicon anode material for a lithium-ion secondary battery, further comprising, after the carbon coating step, a mixing step of mixing the plate-shaped silicon composite and graphite to form a mixture.

6. In Paragraph 1, A method for manufacturing a silicon anode material for a lithium-ion secondary battery, further comprising a pretreatment step of removing moisture and lubricant from waste silicon cuffs before a disintegration step.

7. In Paragraph 1, A method for manufacturing a silicon anode material for a lithium-ion secondary battery, further performing a process of burning residual carbon after forming the silicon carbide layer.

8. In Paragraph 1, A method for manufacturing a silicon negative electrode material for a lithium-ion secondary battery, characterized in that the silicon carbide layer has an average thickness of 1 to 5 nm.

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