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

Abstract

The present invention relates to a negative electrode active material for a secondary battery and a method for manufacturing the same, the negative electrode active material comprising: silicon forming an increasing concentration gradient from a surface portion of the active material toward a center portion; and carbon that forms a concentration gradient that decreases from the surface portion of the active material toward the center portion of the active material.

Description

Technical Field

[0001] This invention relates to a concentration gradient type negative electrode active material for secondary batteries and a method for manufacturing the same, and more specifically, to a concentration gradient type negative electrode active material for secondary batteries comprising silicon forming an increasing concentration gradient from the surface of the active material towards the center and a method for manufacturing the same. Background Technology

[0002] In recent years, lithium-ion batteries, which have attracted much attention as energy storage devices, are widely used in electric vehicles, drones, and electronic devices due to their high energy density. However, graphite, currently a commercially available anode active material, is difficult to increase in capacity due to its low capacity per unit mass (372 mAh / g). Therefore, much effort has been devoted to the development of conversion-type anode materials (silicon, tin, etc.) as high-capacity anode active materials. Among them, silicon, which has the highest capacity (3,579 mAh / g), undergoes a volume change of up to 300% during charge and discharge, resulting in stability problems such as shedding from the electrode, forming unstable interfaces, and mechanical breakage. These problems lead to a decrease in battery coulombic efficiency (CE) and a sharp drop in capacity. Therefore, in order to practically apply silicon active materials, it is necessary to form a complex with carbon.

[0003] Existing technologies form silicon-carbon composites through point or surface contact, but these suffer from the problem of easily breaking bonds between carbon and silicon during volume expansion. In the case of simple mixing of silicon and carbon, the different expansion ratios of carbon and silicon during charging and discharging lead to the easy loss of physical contact and the disruption of electron transport paths within the electrodes. Furthermore, methods of coating carbon onto silicon or dispersing carbon within silicon also suffer from coating detachment or carbon detachment from silicon due to the difference in expansion ratios between silicon and carbon. This increases the likelihood of rapid capacity loss and various side reactions, thus negatively impacting battery performance. All of the above technologies form silicon-carbon composite layers through simple physical bonds, resulting in weak bond strength and susceptibility to breakage. Summary of the Invention

[0004] (Technical issue)

[0005] In order to solve the above problems, this invention proposes a concentration gradient type negative electrode active material for secondary batteries and its manufacturing method, which can suppress the decline in battery performance caused by the volume expansion of silicon.

[0006] (Technical Solution)

[0007] In order to achieve the above-mentioned objectives, the present invention discloses a negative electrode active material for a secondary battery, comprising: silicon, forming an increasing concentration gradient from the surface portion of the active material toward the center portion; and carbon, forming a decreasing concentration gradient from the surface portion of the active material toward the center portion.

[0008] In the active material particles, the silicon content in the central part can be 95-100%, and the carbon content in the surface part can be 95-100%.

[0009] The silicon concentration tilt can be from -3 to 0.

[0010] Furthermore, as a means to achieve the above-mentioned objectives, the present invention discloses a method for manufacturing a negative electrode active material for a secondary battery, the method comprising: a step of mixing a coating precursor material and a carrier solvent to manufacture a coating precursor solution; a step of injecting silicon into a furnace and heating the furnace; and a step of flowing the coating precursor solution into the heated furnace.

[0011] The coating precursor material may include, selectively, 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. It is at least one substance from the group consisting of trimethyl(phenylthio)silane, vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane, bromotrimethylsilane, 1-Phenyl-2-trimethylsilylacetylene, and phenylsilane.

[0012] The heating temperature can be 300 to 1000°C.

[0013] The coating precursor solution can flow into the furnace at a flow rate of 50-300 mL / min.

[0014] The time for the coating precursor solution to flow into the heated furnace can be 5 to 120 minutes.

[0015] As the coating precursor solution flows into the heated furnace, the coating precursor material can be thermally decomposed and continuously deposited on the surface of the silicon.

[0016] The coating precursor material is mixed with the carrier solvent at a ratio of 50 to 500 parts by weight relative to 100 parts by weight of the silicon.

[0017] Furthermore, as a means to achieve the above-mentioned objectives, the present invention discloses a negative electrode active material for secondary batteries manufactured by the above-described method.

[0018] (Invention Effects)

[0019] The concentration gradient type negative electrode active material for secondary batteries of the present invention, by introducing an intermediate layer with continuous atomic unit bonds, has excellent mechanical properties and can effectively alleviate internal stress.

[0020] Furthermore, the method for manufacturing a concentration gradient type negative electrode active material for secondary batteries of the present invention can manufacture a negative electrode active material with a continuous concentration gradient through a simple process of changing the coating precursor. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the concentration gradient type negative electrode active material of the present invention for secondary batteries.

[0022] Figure 2 This is a flowchart illustrating the manufacturing process of the concentration gradient type negative electrode active material for secondary batteries according to the present invention.

[0023] Figure 3 This is a schematic diagram illustrating the manufacturing process of the concentration gradient type negative electrode active material for secondary batteries according to the present invention.

[0024] Figure 4 This is an XRD spectrum of a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention.

[0025] Figure 5 This is a TEM image showing a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention.

[0026] Figure 6 This shows the results of electron microscopy elemental analysis of a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention.

[0027] Figure 7 This is a battery capacity diagram illustrating a half-cell manufactured according to an embodiment of the present invention during cycling.

[0028] Figure 8 This is a diagram showing the Si / C element ratio of the negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention, and the cycle capacity of a half-cell manufactured therefrom. Detailed Implementation

[0029] This invention can be modified in various ways and has various embodiments, and specific embodiments are shown in the accompanying drawings and described in detail in the description. However, this is not intended to limit the invention to specific embodiments, but should be understood to include all modifications, equivalents and substitutes within the spirit and technical scope of the invention.

[0030] Throughout this specification, when a part is referred to as "including" a certain successful element, unless otherwise expressly stated, this does not mean that other constituent elements are excluded, but rather that other components may be included.

[0031] The terms "approximately," "substantially," etc., used in this specification to indicate degree refer to the meaning of the inherent manufacturing value or permissible error of the material mentioned, used to approximate that value, and are used to prevent unlawful infringers from improperly using the disclosures made in precise or absolute form to aid in understanding the invention. Furthermore, throughout this specification, "~step" or "~step" does not mean "for the ~step."

[0032] Those skilled in the art can make various applications based on the spirit of this invention; therefore, the scope of this invention is not limited to the following embodiments. The scope of this invention is based on the matters set forth in the specific claims and extends to the obvious parts that can be easily replaced or modified by those skilled in the art using existing technology.

[0033] The invention will now be described in more detail with reference to the accompanying drawings.

[0034] <Concentration gradient type positive electrode active material for secondary batteries>

[0035] As a means to achieve the above-mentioned objective, the present invention discloses a negative electrode active material for a secondary battery, comprising: silicon, forming an increasing concentration gradient from the surface portion of the active material toward the center portion; and carbon, forming a decreasing concentration gradient from the surface portion of the active material toward the center portion.

[0036] Figure 1 This is a schematic diagram illustrating the concentration gradient type negative electrode active material of the present invention for secondary batteries.

[0037] refer to Figure 1 It can be confirmed that the present invention includes a C-rich surface portion, a Si-rich central portion, and an intermediate portion having continuous C and Si atomic unit bonds. In the intermediate portion, the C element can form a concentration gradient that decreases towards the central portion, while the Si element can form a concentration gradient that increases towards the central portion.

[0038] That is, the concentration gradient type negative electrode active material for secondary batteries of the present invention achieves high conductivity and structural flexibility by forming a C-rich layer on the surface portion; and achieves high capacity by forming a Si-rich layer in the center portion, which structurally supports the negative electrode active material. Furthermore, by introducing a central portion with continuous atomic unit bonds, the concentration gradient type negative electrode active material of the present invention strengthens the bond between the active material and the coating. The negative electrode active material containing such a central portion with continuous atomic unit bonds exhibits excellent mechanical properties, effectively alleviating internal stress and thus preventing structural damage and battery performance degradation caused by volume expansion.

[0039] In the active material particles, the silicon content in the central part can be 95% to 100%, and the carbon content in the surface part of the active material particles can be 95% to 100%.

[0040] The silicon concentration tilt can be between -3 and 0. The silicon concentration tilt can be obtained by the following formula.

[0041]

Mathematical Formula 1

[0042] Concentration tilt (y) = f′(x)

[0043] Wherein, x represents the distance from the center (0,0) of the negative electrode active material particle for the secondary battery of the present invention. Furthermore, f(x) is a silicon concentration function based on the distance from the center of the negative electrode active material particle for the secondary battery of the present invention, and can be expressed as a linear to quadratic function. f'(x) is obtained by differentiating the silicon concentration function (f(x)) based on the distance from the center of the negative electrode active material particle for the secondary battery, expressed as a linear to quadratic function. Therefore, by substituting the distance within the negative electrode active material particle for the secondary battery into x, the slope of the silicon concentration can be calculated. As an example, in Embodiment 1 below, the silicon concentration function based on the distance within the negative electrode active material particle for the secondary battery is f(x) = 0.0358x. 2 -3.7372x+99.029, f'(x)=0.0716x-3.7372. According to Example 1, the silicon concentration tilt when the distance between the particles of the negative electrode active material (from the center (0nm)) is 30nm is -1.5892.

[0044] In this invention, the silicon concentration gradient of -3 to 0 means that within the negative electrode active material particles for the secondary battery, the silicon particles 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 silicon concentration gradient range indicates that, in the negative electrode active material particles for the secondary battery of this invention, there is no abrupt increase or decrease in silicon concentration, but rather a continuous concentration gradient.

[0045] Furthermore, the carbon concentration tilt can be between 0 and 3, and the carbon concentration tilt can be derived in the same way as the above mathematical formula 1.

[0046] <Method for Manufacturing Concentration Gradient Type Anode Active Material for Secondary Batteries>

[0047] Furthermore, to achieve the above objectives, the present invention discloses a method for manufacturing a negative electrode active material for secondary batteries.

[0048] Figure 2 This is a flowchart illustrating the manufacturing process of the concentration gradient type negative electrode active material for secondary batteries according to the present invention. (Reference) Figure 2 It can be confirmed that the method includes: the steps of mixing a coating precursor material with a carrier solvent to prepare a coating precursor solution; the steps of injecting silicon into a furnace and heating the furnace; and the steps of flowing the coating precursor solution into the heated furnace.

[0049] The manufacturing method of the present invention will now be further subdivided into stages and described in more detail.

[0050] First, the method for manufacturing a concentration gradient type negative electrode active material for secondary batteries according to the present invention includes the step of mixing a coating precursor material and a carrier solvent to manufacture a coating precursor solution.

[0051] The coating precursor material may include, selectively, 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 substance is at least one of the following groups, but is not limited thereto: trimethyl(phenylthio)silane, vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane, bromotrimethylsilane, 1-Phenyl-2-trimethylsilylacetylene, and phenylsilane.

[0052] The carrier solvent may be toluene, benzene, hexane, pentane, etc., but is not limited to these.

[0053] In this process, the coating precursor material is mixed with the carrier solvent at a rate of 50 to 500 parts by weight relative to 100 parts by weight of the silicon. The weight percentage of the coating precursor material can be appropriately selected based on the type of coating precursor material and the desired thickness of the intermediate portion. However, when the coating precursor material is mixed at a rate of less than 50 parts by weight relative to 100 parts by weight of the silicon, it may be difficult to effectively improve the mechanical properties of the negative electrode active material. Furthermore, when the coating precursor material is mixed at a rate of more than 500 parts by weight relative to 100 parts by weight of the silicon, it may result in a low negative electrode capacity.

[0054] Next, the method for manufacturing a concentration gradient type negative electrode active material for secondary batteries according to the present invention includes the steps of injecting silicon into a furnace and heating the furnace.

[0055] The silicon may be crystalline silicon, amorphous silicon, or a mixture thereof. According to one embodiment of the present invention, the silicon is preferably crystalline silicon.

[0056] The heating temperature can be between 300 and 1000°C. The heating temperature can be set according to the thermal decomposition temperature of the coating precursor material; that is, the heating temperature can be set differently depending on the type of coating precursor material. Table 1 below shows the thermal decomposition temperatures of various coating precursor materials.

[0057] Table 1

[0058]

[0059] Referring to Table 1, when trimethyl(phenyl)silane is used as the coating precursor, the heating temperature is preferably set to 350 to 550°C, more preferably to 450 to 500°C. Furthermore, when tris(dimethylamino)silane is used as the coating precursor, the heating temperature is preferably set to 850 to 1100°C, more preferably to 950 to 1000°C. In other words, the manufacturing method of the present invention can appropriately select the heating temperature based on the coating precursor.

[0060] Next, the method for manufacturing a concentration gradient type negative electrode active material for secondary batteries according to the present invention includes the step of flowing the coating precursor solution into the heated furnace. The manufacturing method of the present invention can use a single type or a mixture of two or more types of coating precursor solutions as the coating precursor solution. Furthermore, when the manufacturing method of the present invention uses multiple coating precursor solutions, each coating precursor solution can be flowed into the furnace in multiple stages. For example, the step can first flow a first coating precursor solution into the heated furnace for a certain period of time, and then flow a second coating precursor solution, manufactured using a different coating precursor material and carrier solvent than the first coating precursor solution, into the same furnace for a certain period of time. Thus, the effect of forming an intermediate portion with a complex concentration gradient can be expected.

[0061] As the coating precursor solution flows into the furnace, the coating precursor material can be thermally decomposed and continuously deposited onto the silicon surface. In other words, the method for manufacturing the negative electrode active material for secondary batteries of the present invention can be expected to coat the silicon surface with an intermediate portion (Si and C) having a continuous concentration gradient using a simplified process, without requiring a multi-step deposition process. Furthermore, the intermediate portion thus formed, with the formation of continuous atomic unit bonds between Si and C, is expected to effectively alleviate the internal stress caused by the volume expansion of Si in the central portion. Here, the coating precursor solution is preferably introduced into the furnace at a flow rate of 50 to 300 mL / min, more preferably 100 to 200 mL / min. If the flow rate exceeds the above range, the coating precursor material may not be uniformly coated onto the silicon surface.

[0062] The coating precursor solution is preferably introduced into the furnace over a period of 5 to 120 minutes, more preferably 5 to 60 minutes. The manufacturing method of the present invention maintains the same concentration of the coating precursor solution and controls the time of its introduction, thereby enabling the formation of an intermediate portion with the desired thickness.

[0063] Furthermore, the method for manufacturing the negative electrode active material for secondary batteries according to the present invention may further include the step of additionally flowing the carrier solvent into the furnace after the coating precursor solution is flowed into the heated furnace. In the manufacturing method of the present invention, by only flowing the carrier solvent into the furnace in the final stage, an outermost layer with a carbon ratio of 100% can be formed on the surface of the negative electrode active material.

[0064] Figure 3 This is a schematic diagram illustrating the manufacturing process of the concentration gradient type negative electrode active material for secondary batteries according to the present invention. See also Figure 3 The manufacturing method of the present invention will be described in more detail.

[0065] The manufacturing method of this invention specifically involves first injecting silicon into a furnace, then introducing an inert gas to remove oxygen and create an inert atmosphere. The inert gas can be, for example, argon, but is not limited to this, and its flow rate can be 100 to 1000 mL / min. Subsequently, the furnace is heated, and then a coating precursor solution (coating precursor material + carrier solvent) is bubbled into the furnace along with a carrier gas. The carrier gas can be the same as the inert gas, such as argon, but is not limited to this. At this time, the flow rate and inflow time of the coating precursor solution into the furnace are the same as described above. As the coating precursor solution flows into the heated furnace, it is decomposed, allowing it to be coated onto the injected silicon surface. At this point, coating of the silicon surface can be achieved without additional processes, resulting in a continuous elemental bond and concentration gradient between Si and C contained in the coating precursor solution. After the coating precursor is coated onto the silicon surface, an additional inert gas is introduced to create an inert atmosphere. Furthermore, the carrier solvent (e.g., toluene) can be bubbled back into the furnace by the carrier gas. Thus, a carbon layer with an elemental ratio of 95-100% can be formed on the outermost layer of silicon coated with the coating precursor. Finally, after stabilization in an inert atmosphere and cooling in the furnace, a final concentration gradient type negative electrode active material for secondary batteries can be obtained.

[0066] <Negative electrode for secondary batteries and lithium secondary batteries containing it>

[0067] Furthermore, as a means to achieve the above objectives, the present invention discloses a negative electrode for a secondary battery comprising a secondary battery negative electrode active material manufactured by the above method.

[0068] In addition to the aforementioned negative electrode active material for secondary batteries, the negative electrode of the present invention for secondary batteries may also include known current collectors, binders, conductive agents, etc.

[0069] The current collector can be, for example, a metal thin film, and more specifically, an aluminum thin film, a copper thin film, a nickel thin film, a stainless steel thin film, a titanium thin film, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof, but is not limited thereto.

[0070] The adhesive serves to firmly bond the negative electrode active material particles together and to firmly adhere the negative electrode active material to the current collector. When adding the adhesive to the negative electrode active material composition, the adhesive content can be from 1% to 20% by weight of the total weight of the negative electrode active material.

[0071] The adhesive can be an insoluble adhesive, a water-soluble adhesive, or a combination thereof. Insoluble adhesives can be polyvinyl chloride (PVC), carboxylated PVC, polyvinyl fluoride (PVC), ethylene oxide-containing polymers, polyvinylpyrrolidone (PVP), polyurethane, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyamide-imide, polyimide, or a combination thereof. Water-soluble adhesives can be styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, copolymers of propylene with olefins having 2 to 8 carbon atoms, copolymers of (meth)acrylic acid and (meth)acrylate alkyl esters, or a combination thereof.

[0072] When a water-soluble binder is used as the negative electrode binder, it may also contain a cellulose-based compound that imparts viscosity. As a cellulose-based compound, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts may be used. The alkali metal may be Na, K, or Li. The amount of this thickener used may be from 0.1 to 3 parts by weight per 100 parts by weight of the binder.

[0073] Conductive materials are not particularly limited, as long as they are conductive and do not cause chemical changes in the battery. Examples include: carbon powders such as carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, or thermal cracking black; graphite powders such as natural graphite, artificial graphite, or graphite with highly developed crystal structures; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorinated carbon, aluminum, and nickel powder; conductive metal oxides 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 (manufactured by Chevron Chemicals, Denka Black (Denka Singapore Private Limited), or Gulf Oil), Ketjenblack, EC series (manufactured by Armak), Vulcan XC-72 (manufactured by Cabot), and Super-P (manufactured by Timcal).

[0074] In addition, the negative electrode active material composition also contains a solvent, a representative example of which may be N-methylpyrrolidone, etc., and when a water-soluble adhesive is used as a binder, water may be used, but is not limited thereto.

[0075] Furthermore, as a means to achieve the above-mentioned objectives, the present invention discloses a lithium secondary battery comprising a negative electrode for a secondary battery made of a negative electrode active material containing the above-mentioned material.

[0076] The lithium secondary batteries of this invention can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium polymer batteries according to the type of separator and electrolyte used; they can be classified into cylindrical, square, button-shaped, pouch-shaped, etc. according to their shape; and they can be classified into block batteries and thin-film batteries according to their size. Since the structure and manufacturing methods of these batteries are well known in the art, they will not be described in detail here.

[0077] A lithium secondary battery according to an embodiment of the present invention may include: a negative electrode comprising a negative electrode active material manufactured according to an embodiment of the present invention; a positive electrode comprising a positive electrode active material; and a non-aqueous electrolyte. The positive electrode comprising the positive electrode active material and the non-aqueous electrolyte may be made of known materials.

[0078] The claims made herein will now be described in more detail with reference to the accompanying drawings and embodiments. However, since those skilled in the art can make various modifications to the drawings and embodiments provided herein, and thus give them various forms, the content described herein is not limited to the specific form of disclosure of the invention, but should be considered to include all equivalents or alternatives within the spirit and scope of the invention. Furthermore, the drawings are intended to help those skilled in the art to more accurately understand the invention and may be depicted in an exaggerated or reduced form compared to actual circumstances.

[0079] {Examples and Evaluation}

[0080] <Example>

[0081] Example 1

[0082] 0.1g of silicon nanoparticles were ground in an agate mortar and placed in a square alumina boat within a quartz furnace. To remove oxygen from the furnace, argon gas was purged for 30 minutes to create an inert atmosphere, which was then maintained at a flow rate of 500 mL / min. After the furnace temperature reached 750°C, 15 mL of tetramethylsilane and 15 mL of toluene were mixed and placed in a flask. Argon gas was then introduced at a flow rate of 150 mL / min and bubbled 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. Then, 30 mL of toluene was added to the flask, and argon gas was introduced at a flow rate of 150 mL / min and bubbled 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 cooled down, samples were taken and the particles were ground using an agate mortar to obtain a concentration gradient type negative electrode active material for secondary batteries (hereinafter referred to as "Example 1").

[0083] Comparative Example 1

[0084] 0.1 g of silicon nanoparticles were ground in an agate mortar and placed in a square alumina boat within a quartz furnace. To remove oxygen from the furnace, argon gas was purged for 30 minutes to create an inert atmosphere, which was then maintained at a flow rate of 500 mL / min. 15 mL of toluene was placed in a flask, and argon gas was introduced at a flow rate of 150 mL / min while bubbling for 45 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 cooled, a sample was taken, and the particles were ground in an agate mortar to obtain a concentration gradient type negative electrode active material for secondary batteries (hereinafter referred to as "Comparative Example 1").

[0085] Comparative Example 2

[0086] 0.1 g of silicon nanoparticles were ground in an agate mortar and placed in a square alumina boat within a quartz furnace. To remove oxygen from the furnace, argon was used to purge for 30 minutes to create an inert atmosphere, which was then maintained at a flow rate of 500 mL / min. 15 mL of toluene was placed in a flask, and argon was introduced at a flow rate of 150 mL / min while bubbling for 45 minutes for coating. Then, 15 mL of tetramethylsilane and 15 mL of toluene were mixed and placed in a flask, and argon was introduced at a flow rate of 150 mL / min while bubbling for 15 minutes for coating. After coating, argon was introduced at a flow rate of 500 mL / min, and the inert atmosphere was maintained for 1 hour for stabilization. After the furnace cooled, a sample was taken, and the particles were ground in an agate mortar to obtain a concentration gradient type negative electrode active material for secondary batteries (hereinafter referred to as "Comparative Example 2").

[0087] <Assessment>

[0088] Figure 4 This shows the XRD spectrum of a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention. More specifically, Figure 4 a represents the XRD spectra (20 to 90°) of Example 1 and Comparative Example 1. Figure 4 b represents the XRD spectra (28.0 to 29.0°) of Example 1 and Comparative Example 1.

[0089] refer to Figure 4It can be confirmed that peaks appeared in the crystal orientations of 111 (28 to 29°), 220 (approximately 47.5°), 311 (approximately 57°), 400 (approximately 69°), 331 (approximately 77°), and 422 (approximately 88°) in both the examples and comparative examples. This is generally consistent with the peaks appearing in silicon crystals. However, in Example 1, it can be confirmed that the peak appearing in the 111 crystal orientation is shifted to the right. It can be inferred that this is because as a continuous concentration gradient of silicon and carbon is formed in the center of Example 1, the interaction between elements inside the central part changes, resulting in a microstructure in the central part that differs from that of crystalline silicon.

[0090] Figure 5 This is a TEM image showing a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention. More specifically, Figure 5 a is a TEM image of Example 1. Figure 5 b is the TEM image of Comparative Example 1.

[0091] refer to Figure 5 In Example 1, it was confirmed that the interface between the center and the surface was not obvious, as silicon, which forms a concentration gradient increasing from the surface of the active material towards the center, and carbon, which forms a concentration gradient decreasing from the surface of the active material towards the center. In contrast, in Comparative Example 1, it was confirmed that the interface between the center and the surface was clearly visible because no coating precursor such as tetramethylsilane was used on the surface of the silicon particles.

[0092] Figure 6 This shows the results of electron microscopy elemental analysis of a negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention. More specifically, Figure 6 a to Figure 6 d. The elemental distribution from the surface to the center of Example 1 was analyzed.

[0093] refer to Figure 6 It can be confirmed that the C:Si element ratio of the negative electrode active material for secondary batteries of the present invention is 99.97:0.03 on the surface, and the Si element ratio gradually increases as it approaches the center.

[0094] Furthermore, in this invention, the coating efficiency may vary depending on the type of coating precursor. Table 2 below shows the coating ratios according to each coating precursor. The specific types of coating precursors are tetramethylsilane, 1-phenyl-2-trimethylsilylacetylene, ethynyltrimethylsilane, and trimethyl(trifluoromethyl)silane.

[0095] Table 2

[0096]

[0097] As can be seen from Table 2, the highest coating efficiency is observed when using trimethyl (trifluoromethyl)silane, and it can also be confirmed that the coating efficiency varies slightly depending on the type of coating precursor.

[0098] Figure 7 This is a battery capacity diagram illustrating a half-cell manufactured according to an embodiment of the present invention during cycling. Figure 7 The secondary battery being evaluated is manufactured in the following manner.

[0099] First, the negative electrode active material obtained in Example 1 and Comparative Example 1, acetylene black powder as a conductive agent, and polyacrylic acid as a binder were mixed to form a slurry, and the slurry was cast onto a current collector to manufacture an electrode. The weight ratio of the manufactured electrode was active material: conductive agent: binder = 60:20:20. It was dried under vacuum at 120°C for 6 hours. The electrode thus manufactured was used as the working electrode, and the lithium metal disk was used as the relative electrode and reference electrode, thereby manufacturing a button cell-type half-cell. Polypropylene was used as the separation membrane, and the liquid electrolyte was 1.3M LiPF6 in EC / DEC (3 / 7, v / v) + 10% FEC.

[0100] See Figure 7 For the half-cell manufactured using Example 1, the initial discharge capacity was approximately 1.05 Ah / g. While the discharge capacity decreased with cycling, it was confirmed that a discharge capacity of approximately 0.9 Ah / g could be maintained for up to 80 cycles. In contrast, for the half-cell manufactured using Comparative Example 1, the initial discharge capacity was approximately 1.13 Ah / g. However, the discharge capacity gradually decreased with cycling, and it was confirmed that the discharge capacity dropped to approximately 0.6 Ah / g after approximately 80 cycles. This result effectively alleviates the stress generated within the active material by introducing the negative electrode active material of the present invention into the middle portion having continuous atomic unit bonds.

[0101] Figure 8 This is a diagram illustrating the Si / C elemental ratio of the negative electrode active material for a secondary battery manufactured according to an embodiment of the present invention, and the cycle capacity of a half-cell comprising the material. More specifically, Figure 8 a is a graph showing the Si / C element ratio of the negative electrode active material for secondary batteries manufactured according to Example 1 and Comparative Example 2. Figure 8 b is a battery capacity diagram showing the cycle life of a half-cell comprising the negative electrode active material for a secondary battery manufactured according to Example 1 and Comparative Example 2.

[0102] See Figure 8a. It can be confirmed that both Example 1 and Comparative Example 2 have a higher Si concentration at the outer edge, and the Si concentration decreases towards the interior. However, in the case of Comparative Example 2, it can be confirmed that the concentration gradient changes sharply from the outer layer to the interior compared to Example 1. Considering this, see [link to relevant documentation]. Figure 8 b. It can be confirmed that Example 1 not only exhibits a lower initial discharge capacity compared to Comparative Example 2, but also a gradually decreasing discharge capacity as cycling progresses.

[0103] The concentration gradient type negative electrode active material for secondary batteries of the present invention, by introducing it into the middle part with continuous atomic unit bonds, not only has excellent mechanical properties, but can also effectively relieve the stress that occurs inside.

[0104] Furthermore, the method for manufacturing a concentration gradient type negative electrode active material for secondary batteries of the present invention can produce a negative electrode active material with a continuous concentration gradient by simply changing the coating precursor.

[0105] The above description is merely an example of the technical concept of the present invention. Those skilled in the art can make various modifications and variations without departing from the basic characteristics of the present invention.

[0106] Therefore, the embodiments disclosed in this invention are not intended to limit the technical concept of the invention, but rather to explain it, and the scope of the technical concept of the invention is not limited by these embodiments. The scope of protection of this invention should be interpreted by the appended claims, and all technical concepts within the equivalent scope should be interpreted as being included within the scope of the claims of this invention.

Claims

1. A negative electrode active material for a secondary battery, comprising: silicon forming an increasing concentration gradient from an active material surface portion toward a center portion; and carbon forming a decreasing concentration gradient from the active material surface portion toward the center portion.

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

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

4. A manufacturing method of a negative electrode active material for a secondary battery, the manufacturing method comprising: a step of mixing a coating precursor substance with a carrier solvent to manufacture a coating precursor solution; a step of injecting silicon into a furnace and heating the furnace; and a step of flowing the coating precursor solution into the heated furnace.

5. The manufacturing method of a negative electrode active material for a secondary battery according to claim 4, wherein the coating precursor substance includes at least one substance selected from the group consisting of tetramethylsilane, tris(dimethylamino)silane, trimethyl(phenyl)silane, trimethyl(propargyl)silane, trimethyl(trifluoromethyl)silane, t-butyldimethyl(2-propynoxy)silane, trimethyl(thiomethyl)silane, trimethyl(thiophenyl)silane, vinyltrimethylsilane, ethynyltrimethylsilane, triethyl(trifluoromethyl)silane, trimethylsilane, hexamethyldisilane, bromotrimethylsilane, 1-phenyl-2-trimethylsilylethynyl, and phenylsilane.

6. The manufacturing method of a negative electrode active material for a secondary battery according to claim 5, wherein the heating temperature is 300 to 1000°C.

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

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

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

10. The manufacturing method of a negative electrode active material for a secondary battery according to claim 5, wherein the coating precursor substance is mixed with the carrier solvent at 50 to 500 parts by weight with respect to 100 parts by weight of the silicon.

11. A negative electrode for a secondary battery comprising a negative electrode active material for a secondary battery manufactured according to the method of any one of claims 5 to 10. ​

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