Negative electrode active material for concentration gradient secondary battery and method for manufacturing the same

Abstract

To provide a negative electrode active material for a concentration gradient secondary battery and a method for manufacturing the same, which can suppress the deterioration of battery performance due to volume expansion of silicon.SOLUTION: The present invention relates to a negative electrode active material for a secondary battery, which includes silicon which forms a concentration gradient that increases from the surface portion toward the center of the active material, and carbon which forms a concentration gradient that decreases from the surface portion toward the center of the active material, and a method for manufacturing the same.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material for a concentration gradient secondary battery and a method for manufacturing the same, and more specifically to a negative electrode active material for a concentration gradient secondary battery containing silicon formed with a concentration gradient that increases from the surface toward the center of the active material, and a method for manufacturing the same. [Background technology]

[0002] Lithium secondary batteries, which have attracted considerable attention in recent years as energy storage devices, exhibit high energy density and are used in electric vehicles, drones, and electronic devices. However, graphite, a commercially available anode active material, has a low capacity per unit mass of 372 mAh / g, making it impossible to increase its capacity. Therefore, much effort has been devoted to developing high-capacity conversion-type anode active materials (e.g., silicon, tin, etc.). Among these, silicon, which has the highest capacity (3,579 mAh / g), exhibits a volume change of up to 300% during the charge / discharge process and suffers from stability issues such as detachment from the electrode, unstable interface formation, and mechanical crushing. These issues result in a decrease in the battery's Coulombic Efficiency (CE) and a rapid capacity loss. Therefore, for practical use of silicon active materials, it is essential to form a composite with carbon.

[0003] Conventional technologies involve the formation of a composite between silicon and carbon through point or surface contact, which presents a problem of easily breaking the carbon-silicon bond during volume expansion. Simply mixing silicon and carbon presents the drawback of easily losing physical contact due to the different expansion rates of carbon and silicon during charging and discharging, resulting in the disruption of the electron transport pathway within the electrode. Furthermore, methods involving coating carbon on silicon or dispersing carbon within silicon also present the problem of peeling of the coating layer or carbon detachment from the silicon due to the difference in expansion rates between silicon and carbon. This leads to a rapid decrease in capacity and increases the possibility of various side reactions, significantly adversely affecting battery performance. Both of these technologies form a silicon-carbon composite layer through simple physical bonds, which presents a problem of weak bond strength and vulnerability to breakage. Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention has been devised to solve the above-mentioned problems, and aims to provide a negative electrode active material for a concentration-gradient secondary battery that can suppress the deterioration of battery performance due to volume expansion of silicon, and a method for manufacturing the same. [Means for solving the problem]

[0005] As a means for achieving the above-mentioned object, the present invention discloses a negative electrode active material for a secondary battery, which comprises: silicon forming a concentration gradient that increases from the surface portion toward the center of the active material; and carbon forming a concentration gradient that decreases from the surface portion toward the center of the active material.

[0006] Here, the silicon content in the center of the active material particle may be 95 to 100%, and the carbon content in the surface of the active material particle may be 95 to 100%.

[0007] Here, the silicon concentration gradient may be from -3 to 0.

[0008] In addition, as a means for achieving the above-mentioned object, the present invention discloses a method for manufacturing a negative electrode active material for a secondary battery, the method including the steps of: mixing a coating precursor material with a carrier solvent to prepare a coating precursor solution; introducing silicon into a furnace and heating the inside of the furnace; and flowing the coating precursor solution into the heated furnace.

[0009] Here, the coating precursor material may be tetramethylsilane, tris(dimethylamino)silane, trimethyl(phenyl)silane, trimethyl(propargyl)silane, trimethyl(trifluoromethyl)silane, tert-butyldimethyl(2-propynyloxy)silane, trimethyl(methylthio)silane, The silane may include any one or more substances selected from the group consisting of trimethyl(phenylthio)silane, vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane, bromotrimethylsilane, 1-phenyl-2-trimethylsilylacetylene, and phenylsilane.

[0010] Here, the heating temperature may be 300 to 1000°C.

[0011] Here, the coating precursor solution may flow into the furnace at a flow rate of 30 to 80 mL / min.

[0012] Here, the time for which the coating precursor solution flows into the heated furnace may be 5 to 120 minutes.

[0013] Here, the coating precursor solution flows into the heated furnace, whereby the coating precursor material is thermally decomposed and continuously deposited on the silicon surface.

[0014] Here, the coating precursor material may be mixed with the carrier solvent in an amount of 50 to 500 parts by weight based on 100 parts by weight of the silicone.

[0015] Furthermore, as a means for achieving the above-mentioned object, the present invention discloses a negative electrode for a secondary battery, which contains a negative electrode active material for a secondary battery produced by the above-mentioned method. [Effects of the Invention]

[0016] The negative electrode active material for a concentration-gradient secondary battery of the present invention has excellent mechanical properties and can efficiently relieve internally generated stress due to the introduction of an intermediate layer having continuous atomic unit bonds. Furthermore, the method of producing a negative electrode active material for a concentration gradient secondary battery of the present invention can produce a negative electrode active material having a continuous concentration gradient by a simple process of switching coating precursors. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic diagram showing the form of a negative electrode active material for a concentration gradient secondary battery according to the present invention. [Figure 2] 1 is a flowchart showing a manufacturing process of a negative electrode active material for a concentration gradient secondary battery according to the present invention. [Figure 3]1 is a schematic diagram showing a manufacturing process of a negative electrode active material for a concentration gradient secondary battery according to the present invention. [Figure 4] 1 is an XRD spectrum of a negative electrode active material for a secondary battery prepared according to an embodiment of the present invention. [Figure 5] 1 is a TEM image of a negative electrode active material for a secondary battery prepared according to an embodiment of the present invention. [Figure 6] 1 is a result of elemental analysis of a negative electrode active material for a secondary battery prepared according to an embodiment of the present invention, using an electron microscope; [Figure 7] 1 is a graph of battery capacity with cycling for a half-cell fabricated according to an embodiment of the present invention. [Figure 8] 1 is a graph showing the Si / C element ratio of a negative electrode active material for a secondary battery prepared according to an embodiment of the present invention and the battery capacity of a half cell prepared including the same according to cycles. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention is susceptible to various modifications and embodiments, and specific embodiments are shown in the drawings and will be described in detail in the detailed description. However, it is not intended to limit the present invention to the specific embodiments, and it should be understood that the present invention includes all modifications, equivalents, and alternatives within the spirit and scope of the present invention.

[0019] Throughout this specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0020] As used herein, terms of degree such as "about" and "substantially" are used to mean a numerical value or a value close to that value when a material tolerance inherent in the manufacturing process is presented, and are used to facilitate understanding of the present application and to prevent unlawful infringers from unfairly exploiting the disclosure in which precise or absolute numerical values ​​are mentioned. Furthermore, throughout the present specification, "the step of..." or "the step of..." does not mean "the step for...".

[0021] Those skilled in the art will recognize that various applications based on the gist of the present invention are possible, and therefore the scope of the present invention is not limited to the following examples. The scope of the present invention extends to parts that are obvious to those skilled in the art to easily replace or modify using conventional technology based on the matters described in the specific claims. The present invention will now be described in more detail, with reference to the accompanying drawings as necessary.

[0022] <Negative electrode active material for concentration gradient secondary batteries> As a means for achieving the above-mentioned object, the present invention discloses a negative electrode active material for a secondary battery, which comprises: silicon forming a concentration gradient that increases from the surface portion toward the center of the active material; and carbon forming a concentration gradient that decreases from the surface portion toward the center of the active material.

[0023] FIG. 1 is a schematic diagram showing the form of the negative electrode active material for a concentration gradient secondary battery of the present invention.

[0024] 1, it can be seen that the present invention includes a surface region rich in C element, a central region rich in Si element, and an intermediate region in which the C and Si have continuous atomic unit bonds. In the intermediate region, the C element may be formed in a concentration gradient that decreases toward the center, and the Si element may be formed in a concentration gradient that increases toward the center.

[0025] That is, the negative electrode active material for a concentration-gradient secondary battery of the present invention can achieve high electrical conductivity and structural flexibility by forming an abundance of elemental carbon in the surface region, and can achieve a high capacity negative electrode active material while structurally supporting the negative electrode active material by forming an abundance of elemental silicon in the center region. Furthermore, the negative electrode active material for a concentration-gradient secondary battery of the present invention can strengthen the bond between the active material and the coating layer by introducing an intermediate portion having continuous atomic unit bonds. A negative electrode active material including such an intermediate portion having continuous atomic unit bonds has excellent mechanical properties, efficiently relieves internal stress, and can prevent structural collapse and deterioration of battery performance due to volume expansion.

[0026] Here, the silicon content in the center of the active material particle may be 95 to 100%, and the carbon content in the surface of the active material particle may be 95 to 100%.

[0027] Here, the silicon concentration gradient may be −3 to 0. The silicon concentration gradient can be derived by the following formula.

[0028]

number

[0029] Here, x is the distance from the center (0,0) of the negative electrode active material particle for a secondary battery of the present invention. Furthermore, f(x) is a silicon concentration function depending on the distance from the center of the negative electrode active material particle for a secondary battery of the present invention, and can be expressed as a linear or quadratic function. Furthermore, f'(x) is obtained by differentiating the silicon concentration function (f(x)) depending on the distance from the center of the negative electrode active material particle for a secondary battery, which is expressed as a linear or quadratic function. The silicon concentration gradient can be calculated by substituting the distance within the negative electrode active material particle for x. As an example, in the case of Example 1 described later, the silicon concentration function depending on the distance within the negative electrode active material particle for a secondary battery is f(x)=0.0358x 2-3.7372x+99.029, and f'(x)=0.0716x-3.7372. The silicon concentration gradient when the distance (from the center (0 nm)) within the negative electrode active material particle in Example 1 is 30 nm is -1.5892.

[0030] In the present invention, the silicon concentration gradient of -3 to 0 may mean that the silicon particles in the negative electrode active material particle for a secondary battery form a concentration gradient that decreases from the center to the surface of the active material, in other words, a concentration gradient that increases from the surface to the center of the active material. Furthermore, the above-mentioned silicon concentration gradient range may mean that a continuous concentration gradient is formed in the negative electrode active material particle for a secondary battery of the present invention without a sudden increase or decrease in the silicon concentration.

[0031] Here, the carbon concentration gradient may be 0 to 3, and can be similarly derived from Equation 1 above.

[0032] <Method of manufacturing a negative electrode active material for a concentration gradient secondary battery> Furthermore, the present invention discloses a method for producing a negative electrode active material for a secondary battery as a means for achieving the above-mentioned object.

[0033] 2 is a flowchart showing a manufacturing process of a negative electrode active material for a concentration gradient secondary battery according to the present invention, which includes the steps of preparing a coating precursor solution by mixing a coating precursor material with a carrier solvent, introducing silicon into a furnace and heating the furnace, and introducing the coating precursor solution into the heated furnace.

[0034] The production method of the present invention will be explained in more detail below by dividing it into each step.

[0035] First, the method for preparing a negative electrode active material for a concentration gradient secondary battery according to the present invention includes a step of preparing a coating precursor solution by mixing a coating precursor material with a carrier solvent.

[0036] Here, the coating precursor material may be tetramethylsilane, tris(dimethylamino)silane, trimethyl(phenyl)silane, trimethyl(propargyl)silane, trimethyl(trifluoromethyl)silane, tert-butyldimethyl(2-propynyloxy)silane, trimethyl(methylthio)silane, or trimethyl(phenylthio)silane. The silane may include, but is not limited to, one or more substances selected from the group consisting of 1-phenyl-2-trimethylsilane, vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane, bromotrimethylsilane, 1-phenyl-2-trimethylsilylacetylene, and phenylsilane.

[0037] Here, the carrier solvent may be, but is not limited to, toluene, benzene, hexane, pentane, etc.

[0038] Here, the coating precursor material can be mixed with the carrier solvent in an amount of 50 to 500 parts by weight per 100 parts by weight of the silicone. The amount of the coating precursor material can be appropriately selected depending on the type of coating precursor material and the desired thickness of the intermediate portion. However, if the coating precursor material is mixed in an amount of less than 50 parts by weight per 100 parts by weight of the silicone, it may be difficult to effectively improve the mechanical properties of the negative electrode active material. Furthermore, if the coating precursor material is mixed in an amount of more than 500 parts by weight per 100 parts by weight of the silicone, it may result in a decrease in negative electrode capacity.

[0039] Next, the method for manufacturing a negative electrode active material for a concentration gradient secondary battery of the present invention includes the steps of charging silicon into a furnace and heating the inside of the furnace.

[0040] Here, the silicon may be crystalline silicon, amorphous silicon, or a mixture thereof. According to one embodiment of the present invention, it may be preferable that the silicon is crystalline silicon.

[0041] Here, the heating temperature may be 300 to 1000°C. The heating temperature may be set according to the thermal decomposition temperature of the coating precursor material, i.e., the heating temperature may be set differently depending on the type of the coating precursor material. Table 1 below shows the thermal decomposition temperatures of each coating precursor material.

[0042] [Table 1]

[0043] Referring to Table 1, when trimethyl(phenyl)silane is used as the coating precursor material, the heating temperature is preferably set to 350 to 550° C., and more preferably set to 450 to 500° C. When tris(dimethylamino)silane is used as the coating precursor material, the heating temperature is preferably set to 850 to 1100° C., and more preferably set to 950 to 1000° C. That is, in the manufacturing method of the present invention, the heating temperature can be appropriately selected depending on the coating precursor material.

[0044] Next, the method for manufacturing a negative electrode active material for a concentration gradient secondary battery of the present invention includes a step of flowing the coating precursor solution into the heated furnace. The manufacturing method of the present invention can use a single type of coating precursor solution or a mixed solution of two or more types. Furthermore, when multiple coating precursor solutions are used, the manufacturing method of the present invention can flow each coating precursor solution in multiple stages. For example, the step can include first flowing a first coating precursor solution into the heated furnace for a certain period of time, and then flowing a second coating precursor solution prepared using a coating precursor material and a carrier solvent different from the first coating precursor solution into the same furnace for a certain period of time. This is expected to have the effect of forming an intermediate portion having a complex concentration gradient.

[0045] As the coating precursor solution flows into the heated furnace, the coating precursor material is thermally decomposed and continuously deposited on the silicon surface. In other words, the method for producing a negative electrode active material for a secondary battery according to the present invention is expected to be effective in coating the silicon surface with an intermediate portion (Si and C) having a continuous concentration gradient through a simplified process without a multi-stage deposition process. Furthermore, the intermediate portion thus formed is expected to efficiently relieve internal stress due to the volume expansion of the central Si by forming continuous atomic unit bonds between Si and C. The coating precursor solution is preferably flowed into the furnace at a flow rate of 50 to 300 mL / min, more preferably 100 to 200 mL / min. If the flow rate is outside the above range, the coating precursor material may not be uniformly coated on the silicon surface.

[0046] Here, the coating precursor solution is preferably flowed into the furnace for 5 to 120 minutes, more preferably for 5 to 60 minutes. The production method of the present invention is expected to have the effect of forming an intermediate portion of a desired thickness by maintaining the concentration of the coating precursor solution constant and controlling the time for flowing the coating precursor solution.

[0047] In addition, the method for manufacturing a negative electrode active material for a secondary battery according to the present invention may further include the step of additionally injecting the carrier solvent into the furnace after the coating precursor solution is introduced into the heated furnace. In the manufacturing method according to the present invention, by introducing only the carrier solvent into the furnace in the final step, an outermost layer having a carbon ratio of 100% can be formed on the surface of the negative electrode active material.

[0048] 3 is a schematic diagram showing the steps of producing a negative electrode active material for a concentration gradient secondary battery according to the present invention. The production method of the present invention will be described in more detail below with reference to FIG.

[0049] Specifically, the manufacturing method of the present invention can involve introducing silicon into a furnace, followed by introducing an inert gas to remove oxygen from the furnace and create an inert atmosphere. The inert gas can be, for example, argon gas, but is not limited thereto, and the flow rate can be 100 to 1000 mL / min. After heating the furnace, the coating precursor solution (coating precursor material + carrier solvent) can be introduced into the furnace by bubbling with a carrier gas. The carrier gas can be the same as the inert gas, for example, argon gas, but is not limited thereto. The flow rate and inflow time of the coating precursor solution introduced into the furnace are the same as those described above. The coating precursor solution can be thermally decomposed by flowing into the heated furnace and coated on the surface of the introduced silicon. The silicon surface can be coated with Si and C contained in the coating precursor solution to form a continuous elemental bond and a concentration gradient without a separate process. After the silicon surface is coated with the coating precursor, an inert gas is introduced to create an inert atmosphere, and then a carrier solvent (e.g., toluene) is introduced into the furnace by bubbling again with the carrier gas. This allows a carbon layer with an element ratio of 95 to 100% to be formed on the outermost layer of the silicon coated with the coating precursor. Finally, the furnace interior is stabilized in an inert atmosphere, and then cooled to obtain the final negative electrode active material for a concentration-gradient secondary battery.

[0050] <Negative electrode for secondary battery and lithium secondary battery including the same> Furthermore, as a means for achieving the above-mentioned object, the present invention discloses a negative electrode for a secondary battery, which contains a negative electrode active material for a secondary battery produced by the above-mentioned method.

[0051] The negative electrode for a secondary battery of the present invention may further contain a known current collector, binder, conductive material, etc. in addition to the above-mentioned negative electrode active material for a secondary battery.

[0052] The current collector may be, for example, a metal thin film, more specifically, an aluminum thin film, a copper thin film, a nickel thin film, a stainless steel thin film, a titanium thin film, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or a combination thereof, but is not limited thereto.

[0053] The binder serves to effectively adhere the negative electrode active material particles to each other and to effectively adhere the negative electrode active material to a current collector. When a binder is added to the negative electrode active material composition, the content of the binder may be 1 wt % to 20 wt % based on the total weight of the negative electrode active material.

[0054] The binder may be a water-insoluble binder, a water-soluble binder, or a combination thereof. Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or a combination thereof. Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, copolymers of propylene and olefins having 2 to 8 carbon atoms, copolymers of (meth)acrylic acid and (meth)acrylic acid alkyl esters, or a combination thereof.

[0055] When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. The amount of such a thickener used may be 0.1 to 3 parts by weight per 100 parts by weight of the binder.

[0056] The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. Examples include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with highly developed crystalline structures; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black (Chevron Chemical Company), Denka Black (Denka Singapore Private Limited, or Gulf Oil Company), Ketjenblack, EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super-P (Timcal).

[0057] In addition, the negative electrode active material composition further contains a solvent, and a representative example of the solvent is N-methylpyrrolidone, etc. Furthermore, when a water-soluble binder is used as the binder, water can be used, but the solvent is not limited thereto.

[0058] In addition, the present invention discloses, as a means for achieving the above-mentioned object, a lithium secondary battery including a negative electrode for a secondary battery manufactured using the above-mentioned negative electrode active material.

[0059] The lithium secondary battery of the present invention can be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of separator and electrolyte used, and can be classified into a cylindrical type, a prismatic type, a coin type, a pouch type, etc. depending on the shape, and can be classified into a bulk type and a thin film type depending on the size. The structure and manufacturing method of these batteries are widely known in the art, so detailed description will be omitted.

[0060] The lithium secondary battery according to an embodiment of the present invention may include a negative electrode including a negative electrode active material manufactured according to an embodiment of the present invention, a positive electrode including a positive electrode active material, and a non-aqueous electrolyte. The positive electrode including the positive electrode active material and the non-aqueous electrolyte may be made of known materials.

[0061] The subject matter claimed in this specification will be described in more detail below with reference to the accompanying drawings and examples. However, the drawings and examples presented in this specification can be modified in various ways by those skilled in the art and can have various forms, and the description of the present invention should not be construed as limiting the present invention to the specific disclosed form, but as including all equivalents and alternatives within the spirit and technical scope of the present invention. In addition, the accompanying drawings are presented to help those skilled in the art understand the present invention more accurately, and may be exaggerated or reduced in size. [Example]

[0062] Example 1 0.1 g of silicon nanoparticles were crushed using agate mortar and placed in a square alumina boat. This was then placed in a quartz furnace. To remove oxygen from the furnace, the atmosphere was replaced with argon gas for 30 minutes to create an inert atmosphere, and the argon gas flow rate was maintained at 500 mL / min. The furnace temperature was then raised to 750 °C, and 15 mL of tetramethylsilane and 15 mL of toluene were mixed and placed in a flask. Argon gas was introduced at a flow rate of 150 mL / min and bubbled through the flask for 15 minutes. After coating, argon gas was introduced at a flow rate of 500 mL / min and the inert atmosphere was maintained for 1 hour for stabilization. 30 mL of toluene was introduced into the flask and argon gas was introduced at a flow rate of 150 mL / min and bubbled through the flask for 15 minutes for coating. After coating, argon gas was introduced at a flow rate of 500 mL / min and the inert atmosphere was maintained for 1 hour for stabilization. After the furnace was cooled, a sample was obtained, and the particles were crushed using agate mortar to obtain a negative electrode active material for a concentration gradient secondary battery (hereinafter referred to as "Example 1").

[0063] Comparative Example 1 0.1 g of silicon nanoparticles were crushed using agate mortar and placed in a square alumina boat, which was then placed in a quartz furnace. To remove oxygen from the furnace, the atmosphere was replaced with argon gas for 30 minutes to create an inert atmosphere, and the argon gas flow rate was maintained at 500 mL / min. 30 mL of toluene was placed in a flask, and argon gas was bubbled in at a flow rate of 150 mL / min to coat the material for 45 minutes. After coating, argon gas was introduced at a flow rate of 500 mL / min, and the material was maintained in an inert atmosphere for 1 hour to stabilize the material. After cooling the furnace, a sample was obtained, and the particles were crushed using agate mortar to obtain a negative electrode active material for a secondary battery (hereinafter referred to as "Comparative Example 1").

[0064] Comparative Example 2 0.1 g of silicon nanoparticles were crushed using agate mortar and placed in a square alumina boat. This was then placed in a quartz furnace. To remove oxygen from the furnace, the inside of the boat was purged with argon gas for 30 minutes to create an inert atmosphere, and the argon gas flow rate was maintained at 500 mL / min. 30 mL of toluene was added to a flask, and argon gas was introduced at a flow rate of 150 mL / min and bubbled for 45 minutes. Next, 15 mL of tetramethylsilane and 15 mL of toluene were mixed and added to the flask. Argon gas was introduced at a flow rate of 150 mL / min and bubbled for 15 minutes. After coating, argon gas was introduced at a flow rate of 500 mL / min and the inert atmosphere was maintained for 1 hour for stabilization. After cooling the furnace, a sample was obtained, and the particles were crushed using agate mortar to obtain a negative electrode active material for a secondary battery (hereinafter referred to as "Comparative Example 2").

[0065] <Evaluation> 4 shows XRD spectra of negative electrode active materials for secondary batteries manufactured according to an embodiment of the present invention. More specifically, FIG. 4(a) shows the XRD spectra (20 to 90°) of Example 1 and Comparative Example 1, and FIG. 4(b) shows the XRD spectra (28.0 to 29.0°) of Example 1 and Comparative Example 1.

[0066] Referring to FIG. 4, it can be seen that in the examples and comparative examples, peaks appear in the crystal directions of 111 (28-29°), 220 (approximately 47.5°), 311 (approximately 57°), 400 (approximately 69°), 331 (approximately 77°), and 422 (approximately 88°). These peaks almost coincide with the peaks that appear in silicon crystal. However, in the case of Example 1, it can be seen that the peak that appears in the 111 crystal direction is shifted to the right. This is presumably because Example 1 is formed with a continuous concentration gradient of silicon and carbon in the center, which may change the inter-element interactions within the middle portion, resulting in a microstructure in the middle portion that is different from that of crystalline silicon.

[0067] 5A and 5B are TEM images of negative electrode active materials for secondary batteries prepared according to an embodiment of the present invention. More specifically, FIG. 5A is a TEM image of Example 1, and FIG. 5B is a TEM image of Comparative Example 1.

[0068] 5, in Example 1, silicon forms a concentration gradient that increases from the surface toward the center of the active material, and carbon forms a concentration gradient that decreases from the surface toward the center of the active material, resulting in an indistinct boundary between the center and the surface. In contrast, in Comparative Example 1, a coating precursor such as tetramethylsilane was not used on the surface of the silicone particles, resulting in a clearly defined boundary between the center and the surface.

[0069] 6 shows the results of elemental analysis by electron microscope of the negative electrode active material for a secondary battery produced according to one example of the present invention. More specifically, Figures 6(a) to 6(d) show the analysis of elemental distribution from the surface to the center of Example 1.

[0070] Referring to FIG. 6, it can be seen that the negative electrode active material for a secondary battery according to the present invention has a C:Si element ratio of 99.97:0.03 at the surface, and the Si element ratio gradually increases toward the center.

[0071] Furthermore, in the present invention, the coating efficiency may vary depending on the type of coating precursor. Table 2 below shows the coating ratio of each coating precursor. Specific types of the coating precursor include tetramethylsilane, 1-phenyl-2-trimethylsilylacetylene, ethynyltrimethylsilane, and trimethyl(trifluoromethyl)silane.

[0072] [Table 2]

[0073] Referring to Table 2, it can be seen that the highest coating efficiency is shown in the case of trimethyl(trifluoromethyl)silane, and that there are some differences in coating efficiency depending on the type of each coating precursor.

[0074] 7 is a graph showing the battery capacity as a function of cycles for a half-cell fabricated according to an embodiment of the present invention. The secondary battery evaluated in FIG. 7 was fabricated in the following manner.

[0075] First, the negative electrode active material obtained in Example 1 and Comparative Example 1 was mixed with acetylene black powder as a conductive material and polyacrylic acid as a binder to form a slurry, which was then cast onto a current collector to fabricate an electrode. The weight ratio of the fabricated electrode was active material:conductive material:binder = 60:20:20. Drying was performed in a vacuum at 120°C for 6 hours. A coin-type half cell was fabricated using the electrode fabricated in this way as the working electrode and a lithium metal disk as the counter electrode and reference electrode. Polypropylene was used as the separator, and 1.3M LiPF6 in EC / DEC (3 / 7, v / v) + 10% FEC was used as the liquid electrolyte.

[0076] 7, the half cell fabricated using Example 1 had an initial discharge capacity of approximately 1.05 Ah / g, and although the discharge capacity decreased slightly with increasing cycles, it maintained a discharge capacity of approximately 0.9 Ah / g up to 80 cycles. In contrast, the half cell fabricated using Comparative Example 1 had an initial discharge capacity of approximately 1.13 Ah / g, but the discharge capacity gradually decreased with increasing cycles, reaching approximately 0.6 Ah / g after approximately 80 cycles. This result is attributable to the introduction of an intermediate portion having continuous atomic unit bonds into the negative electrode active material for a secondary battery according to the present invention, which effectively relieves stress generated within the active material.

[0077] 8A and 8B are graphs showing the Si / C element ratios of negative electrode active materials for secondary batteries prepared according to an embodiment of the present invention and the battery capacity of half cells prepared including the negative electrode active materials according to the cycles of the cells. More specifically, FIG. 8A is a graph showing the Si / C element ratios of negative electrode active materials for secondary batteries prepared according to Example 1 and Comparative Example 2, and FIG. 8B is a graph showing the battery capacity of half cells prepared including the negative electrode active materials for secondary batteries prepared according to Example 1 and Comparative Example 2.

[0078] Referring to Figure 8(a), it can be seen that in both Example 1 and Comparative Example 2, the Si concentration is high on the outside and decreases toward the inside. However, in the case of Comparative Example 2, it can be seen that the concentration gradient changes more rapidly from the outer layer toward the inside than in Example 1. With this in mind, referring to Figure 8(b), it can be seen that Comparative Example 2 not only has a lower initial discharge capacity than Example 1, but also gradually decreases in discharge capacity as the cycle progresses.

[0079] The negative electrode active material for a concentration gradient secondary battery of the present invention has excellent mechanical properties and can efficiently relieve internally generated stress due to the introduction of an intermediate portion having continuous atomic unit bonds.

[0080] Furthermore, the method of producing a negative electrode active material for a concentration gradient secondary battery of the present invention can produce a negative electrode active material having a continuous concentration gradient by the simple process of switching the coating precursor.

[0081] The above description is merely an illustrative example of the technical concept of the present invention, and a person having ordinary knowledge in the technical field to which the present invention pertains may make various modifications and variations without departing from the essential characteristics of the present invention.

[0082] Therefore, the embodiments disclosed in the present invention are for illustrative purposes only and do not limit the technical idea of ​​the present invention. The scope of the technical idea of ​​the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto should be interpreted as being included in the scope of the present invention.

Claims

1. A negative electrode active material for a secondary battery, comprising: silicon forming a concentration gradient that increases from the surface portion toward the center of the active material; and carbon forming a concentration gradient that decreases from the surface portion toward the center of the active material.

2. the silicon content in the center of the active material particle is 95 to 100%; 2. The negative electrode active material for a secondary battery according to claim 1, wherein the carbon content in the surface portion of the active material particles is 95 to 100%.

3. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the silicon concentration gradient is −3 to 0.

4. mixing a coating precursor material with a carrier solvent to form a coating precursor solution; charging silicon into a furnace and heating the furnace interior; and and injecting the coating precursor solution into the heated furnace.

5. The coating precursor material may be tetramethylsilane, tris(dimethylamino)silane, trimethyl(phenyl)silane, trimethyl(propargyl)silane, trimethyl(trifluoromethyl)silane, or the like. Trimethyl(trifluoromethyl)silane, tert-butyldimethyl(2-propynyloxy)silane, trimethyl(methylthio)silane, trimethyl(phenylthio)silane phenylthio)silane), vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane 5. The method of claim 4, wherein the negative electrode active material comprises at least one material selected from the group consisting of 1-phenyl-2-trimethylsilylacetylene, 2-phenyl-2-trimethylsilylacetylene, 2-phenyl-2-trimethylsilylacetylene, and 2-phenylsilane.

6. The method for producing a negative electrode active material for a secondary battery according to claim 4, wherein the heating temperature is 300 to 1000°C.

7. 5. The method for producing a negative electrode active material for a secondary battery according to claim 4, wherein the coating precursor solution flows into the furnace at a flow rate of 50 to 300 mL / min.

8. 5. The method for producing a negative electrode active material for a secondary battery according to claim 4, wherein the time for which the coating precursor solution flows into the heated furnace is 5 to 120 minutes.

9. 5. The method for producing a negative electrode active material for a secondary battery according to claim 4, wherein the coating precursor solution is flowed into the heated furnace, whereby the coating precursor material is thermally decomposed and continuously deposited on the surface of the silicon.

10. 5. The method of claim 4, wherein the coating precursor material is mixed with the carrier solvent in an amount of 50 to 500 parts by weight based on 100 parts by weight of the silicon.

11. A negative electrode for a secondary battery, comprising the negative electrode active material for a secondary battery according to any one of claims 1 to 3.

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