Negative electrode active material for secondary battery, method for manufacturing same, and secondary battery using same

Abstract

The present disclosure relates to a negative electrode active material for a secondary battery, a method for manufacturing same, and a secondary battery using same, the negative electrode active material comprising a carbide carbonized by heat-treating a mixture comprising starch and isocyanate in an inert gas atmosphere. More specifically, the present invention relates to a negative electrode active material for a secondary battery, a method for manufacturing same, and a secondary battery using same, the negative electrode active material having a low specific surface area and improving the initial charging / discharging capacity of a secondary battery.

Description

Negative active material for secondary batteries and method for manufacturing the same, and secondary batteries using the same The present disclosure relates to a negative electrode active material for a secondary battery comprising a carbide obtained by carbonizing a mixture comprising starch and isocyanate by heat treatment under an inert gas atmosphere, a method for producing the same, and a secondary battery using the same. More specifically, the present disclosure relates to a negative electrode active material for a secondary battery having a low specific surface area and improved initial charge / discharge capacity of the secondary battery, a method for producing the same, and a secondary battery using the same. Amidst the growing interest in and demand for eco-friendly, green cars, the emergence of electric-powered vehicles is becoming increasingly apparent. Electric vehicles can be categorized into Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs), and Plug-in Hybrid Electric Vehicles (PHEVs), and the power that powers these motors is provided by secondary batteries. Automotive secondary batteries, unlike those used in mobile IT devices, require significantly higher output and lifespan characteristics. Among secondary batteries, secondary batteries offer a higher energy density per unit weight and superior output characteristics than nickel-metal hydride batteries, making them a popular choice as a power source for electric vehicle drive motors. Currently, graphite is mostly used as the negative active material for secondary batteries for automobiles. It exhibits a high discharge voltage of 3.6 V, which increases the energy density of secondary batteries, and its excellent reversibility ensures long life characteristics of secondary batteries, making it the most widely used. However, graphite has the problem of poor energy input / output characteristics, especially poor low-temperature output characteristics. Furthermore, during charge / discharge, the volume of graphite changes by about 10%, which adversely affects the bonding strength between the current collector and the composite layer, thereby reducing the life characteristics of the battery. To address these issues, non-graphitizable carbon with developed micropores has been proposed and is being used to some extent. Non-graphitizable carbon has a structure in which lithium ions are stored and released within numerous pores. This structure exhibits minimal volume expansion during lithium ion charging and discharging, resulting in excellent battery life characteristics. Furthermore, it is known to have excellent output characteristics, as it can store and release lithium ions through micropores present in all directions of the particle. However, non-graphitizable carbon has a large specific surface area, which requires increased solvent and binder usage during electrode slurry production. This, in turn, reduces the energy density of the battery by a corresponding amount of binder. In addition, graphitizable carbon has a high specific surface area, which increases the amount of moisture adsorbed in the air. When manufacturing a secondary battery, moisture reacts with the electrolyte to form hydrofluoric acid (HF), which increases the irreversible capacity and reduces durability. To solve the above problem, a method of carbonizing under atmospheric / pressurized pressure to develop closed pores rather than open pores and reduce the specific surface area, or a method of forming pyrolytic carbon on the surface of non-graphitizable carbon to reduce the specific surface area have been disclosed. However, there are problems such as re-contamination by tar components during carbonization of raw materials, or poor uniformity of the product between the upper and lower parts of the raw material crucible. In Korean Patent No. 10-1375688 (Patent Document 0001), a negative electrode active material including a carbonized carbide was manufactured by heat-treating a polyurethane resin in an inert gas atmosphere. Conventional negative electrode active materials for secondary batteries, such as the above, had a problem with moisture adsorption due to their relatively high specific surface area. [Prior Art Literature] (Patent Document 0001) Republic of Korea Patent No. 10-1375688 The present disclosure relates to a negative electrode active material for a secondary battery comprising a carbonized carbide obtained by heat-treating a mixture containing starch and isocyanate under an inert gas atmosphere to solve the conventional problems. The present disclosure aims to provide a carbonaceous material as an anode active material for a secondary battery, which reduces the problem of moisture adsorption due to a low specific surface area, improves the initial charge / discharge capacity of the secondary battery, thereby improving the energy density of the battery and battery characteristics such as charge / discharge output, and a method for producing the same. Starch is a polysaccharide formed by the polymerization of glucose. Because it melts and carbonizes when heat-treated in an inert gas atmosphere, it is difficult to obtain a carbonaceous material with a high degree of amorphousness. However, by mixing isocyanate with starch, a three-dimensional bond is formed, allowing the production of a carbonaceous material with a high degree of amorphousness. Accordingly, the purpose of the present disclosure is to provide a carbonaceous material for a secondary battery and a method for producing the same, which improves the energy density of a secondary battery and enhances battery characteristics of high output that is difficult to expect from graphite-based anode active materials by producing a hard carbon anode active material, which is an amorphous carbon, from a mixture containing starch and isocyanate. According to the present disclosure for achieving the above purpose, in order to manufacture a negative electrode active material for a secondary battery, starch is mixed with isocyanate and then carbonized by heat treatment under an inert gas atmosphere, thereby achieving excellent properties of the negative electrode active material. The present disclosure relates to a negative active material for a secondary battery comprising a carbide carbonized by heat-treating a mixture containing starch and isocyanate under an inert gas atmosphere. In one embodiment, the mixture may further comprise a phosphorus flame retardant. In one embodiment, the starch may be at least one selected from the group consisting of corn starch, rice starch, barley starch, wheat starch, cassava starch, potato starch and sweet potato starch, and corn starch is preferably used, but is not limited thereto. In one embodiment, the starch may have an average diameter (D50) of 1 to 20 μm, preferably 5 to 10 μm, but is not limited thereto. In one embodiment, the starch may have a spherical particle shape. In one embodiment, the negative electrode active material for a secondary battery after the heat treatment may have an amorphous particle form. In addition, in one embodiment, the isocyanate is selected from the group consisting of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenyl isocyanate, toluene diisocyanate (TDI), 2,2'-diphenylmethane diisocyanate (2,2'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 4,4'-diphenylmethane diisocyanate (4,4'-MDI, monomeric MDI), polymeric diphenylmethane diisocyanate (polymeric MDI), orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI), and triphenylmethane. It may be at least one selected from the group consisting of triisocyanate (TPTI), but is not limited thereto. In one embodiment, the isocyanate is preferably at least one selected from the group consisting of 4,4'-diphenylmethane diisocyanate (4,4'-MDI, monomeric MDI), polymeric diphenylmethane diisocyanate (polymeric MDI), and polyethylene polyphenyl isocyanate, but is not limited thereto. In one embodiment, the heat treatment may include a pre-carbonization step of a primary heat treatment and a carbonization step of a secondary heat treatment. In one embodiment, the preliminary carbonization step of the first heat treatment may be performed at a temperature of 600 to 1400°C, and specifically, may be performed at a temperature of 600 to 1000°C, but is not limited thereto. In one embodiment, the preliminary carbonization step of the first heat treatment may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the carbonization step of the secondary heat treatment may be performed at a temperature of 1000 to 1400°C, preferably at 1100 to 1400°C, and more preferably at 1100 to 1200°C, but is not limited thereto. In one embodiment, the carbonization step of the secondary heat treatment may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the first heat treatment pre-carbonization step and the second heat treatment carbonization step may be performed sequentially. In one embodiment, the isocyanate may be present in an amount of 15 to 40 parts by weight per 100 parts by weight of the mixture comprising the starch and isocyanate. In one embodiment, the mixture may further comprise a phosphorus flame retardant. In one embodiment, the phosphorus flame retardant may be selected from TCPP (Tris(2-ChloroPropyl) Phosphate), TCEP (Tris(2-Chroroethyl) Phosphate), TEP (triethyl phosphate), or TMP (Trimethyl phosphate), with TCPP (Tris(2-ChloroPropyl) Phosphate) being preferred but not limited thereto. In one embodiment, the mixture comprising starch and isocyanate may further comprise a catalyst. Additionally, the present disclosure relates to a method for manufacturing a negative electrode active material for a secondary battery. In one embodiment, the method for manufacturing the negative active material for a secondary battery includes a carbonization step of mixing starch and isocyanate to make a precursor and heat-treating the manufactured precursor under an inert gas atmosphere. In one embodiment, the starch and isocyanate may be uniformly mixed at a certain ratio to obtain a precursor. The mixing method is preferably mixing using an impeller, but is not limited thereto. In one embodiment, a carbonization step may be performed by heat-treating the starch and isocyanate under an inert gas atmosphere. In one embodiment, a carbonization step may be performed by mixing the starch and isocyanate at a temperature between 20 and 60°C for 50 to 60 minutes and then heat-treating them under an inert gas atmosphere. In one embodiment, the carbonization step may include a pre-carbonization step and a carbonization step. In one embodiment, the pre-carbonization step may be carried out at a temperature of 600 to 1400°C, preferably at 600 to 1000°C, and more preferably at 600 to 800°C, but is not limited thereto. In one embodiment, the pre-carbonization step may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the carbonization step may be performed at a temperature of 1000 to 1400°C, preferably at 1100 to 1400°C, and more preferably at 1100 to 1200°C, but is not limited thereto. In one embodiment, the carbonization step may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the pre-carbonization step and the carbonization step may be performed sequentially. In one embodiment, the pre-carbonization step is performed under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixed gas thereof. In one embodiment, the method for manufacturing the negative electrode active material for a secondary battery may include a pulverization step for adjusting the particle size to a size suitable for manufacturing a secondary battery electrode before the pre-carbonization step, after the pre-carbonization step, or after the carbonization step. In one embodiment, the above-mentioned fine grinding step can be performed using a conventional grinder that uses a mechanical grinding method, and in particular, various grinding devices such as a ball mill, pin mill, rotor mill, or jet mill can be used, but are not limited thereto. In one embodiment, when performing a pulverization step after the preliminary carbonization step, it is preferable to use a jet mill, but this is not limited thereto. In one embodiment, the average diameter (D50) of the pulverized negative active material particles may be 1 to 50 μm, preferably 3 to 50 μm, more specifically 3 to 20 μm, and most specifically 6 to 15 μm, but is not limited thereto. In one embodiment, the method for manufacturing the negative electrode active material for the secondary battery may include a carbonization step of heat treating at a temperature of 1000 to 1400°C for 30 to 120 minutes after a preliminary carbonization step and a fine grinding step. In one embodiment, the carbonization step is a step for improving the conductivity of carbon after removing low molecular weight gases generated in the pre-carbonization step, thereby optimizing the properties as a negative electrode material for secondary batteries. In one embodiment, the carbonization step is performed under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixed gas thereof. In one embodiment, the heat treatment temperature of the carbonization step may be 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto. In one embodiment, the negative active material may have a specific surface area of ​​1.9 to 2.7 m2 / g. In one embodiment, the negative active material may have an average pore size of 1.0 to 20 nm, as shown in FIG. 5, and is preferably 1.0 to 5 nm, but is not limited thereto. Additionally, in one embodiment, the negative active material may have an average interlayer spacing (d002) of the (002) plane as determined by X-ray diffraction analysis (XRD) of 3.4 to 4.3 Å, preferably 3.7 to 4.0 Å, but is not limited thereto. In one embodiment, the negative active material may have a crystal diameter Lc(002) in the C-axis direction of 0.8 to 2.0 nm, preferably 0.9 to 1.0 nm, but is not limited thereto. In one embodiment, the negative active material may have an R value of 1.3 to 2, and preferably an R value of 1.7 to 2, but is not limited thereto. In one embodiment, the negative active material may have a charge capacity per unit weight of 370 to 500 (mAh / g), preferably 390 to 480 (mAh / g), and more preferably 400 to 420 (mAh / g), but is not limited thereto. In one embodiment, the negative active material may have a discharge capacity per unit weight of 270 to 380 (mAh / g), preferably 290 to 350 (mAh / g), and more preferably 320 to 350 (mAh / g), but is not limited thereto. In one embodiment, the negative active material may have an average interlayer spacing (d002) of 3.7 to 4.0 Å on the (002) plane as determined by X-ray diffraction analysis, a crystallite diameter Lc(002) in the C-axis direction of 0.9 to 1.0 nm, and an R value of 1.7 to 2. The negative active material for a secondary battery manufactured by the manufacturing method of the present disclosure has properties within the above range, thereby forming a structure with a low moisture adsorption rate and facilitating charge / discharge, thereby improving the initial charge / discharge efficiency of the secondary battery. In addition, it was confirmed that the structure of this negative active material for a secondary battery is formed by a bonding reaction between starch and isocyanate, and includes a microstructure close to an amorphous state and fine, uniform pores. Additionally, the present disclosure relates to a secondary battery including the negative electrode active material for the secondary battery. According to the carbonaceous material for a secondary battery and the method for producing the same of the present disclosure, a carbonaceous material including a carbonized carbide obtained by heat-treating a precursor mixed with starch and isocyanate under an inert gas atmosphere is produced, thereby providing the advantages of a low specific surface area of ​​the carbonaceous material and a high charge / discharge capacity compared to polyurethane resins widely used in the past. In addition, a secondary battery including the above-mentioned negative active material has the advantage of being able to improve the initial discharge capacity, efficiency, and output of the battery. Figure 1 is a scanning electron microscope photograph showing corn starch used as a negative electrode active material of a secondary battery according to the present disclosure. Figure 2 is a scanning electron microscope photograph of a negative electrode active material of a secondary battery according to the present disclosure. Figure 3 is a scanning electron microscope photograph of a negative electrode active material manufactured using a polyurethane resin made as Comparative Example 1 of a negative electrode active material of a secondary battery according to the present disclosure. FIG. 4 is a graph analyzing mesopores on the surface of a negative electrode active material manufactured according to Example 1 of the present disclosure and a negative electrode active material manufactured according to Comparative Example 1. Figure 5 is a graph analyzing micropores on the surface of the negative active material of a secondary battery according to the present disclosure. Figure 6 is an initial charge / discharge graph of the negative electrode active material of a secondary battery according to the present disclosure. Figure 7 is a graph showing the evaluation of room temperature discharge output characteristics of a negative electrode active material manufactured according to Example 1 of the present disclosure and a negative electrode active material manufactured according to Comparative Example 1. According to the present disclosure for achieving the above purpose, in order to manufacture an anode active material for a secondary battery, starch is mixed with isocyanate and then carbonized by heat treatment under an inert gas atmosphere, thereby achieving excellent properties of an anode active material that do not require additional post-treatment such as carbon coating. In one embodiment, the mixture may further comprise a phosphorus flame retardant. In one embodiment, the starch may be at least one selected from the group consisting of corn starch, rice starch, barley starch, wheat starch, cassava starch, potato starch and sweet potato starch, and corn starch is preferably used, but is not limited thereto. In one embodiment, the starch may have an average diameter (D50) of 1 to 20 μm, preferably 5 to 10 μm, but is not limited thereto. In one embodiment, the starch may have a spherical particle shape. In one embodiment, the negative electrode active material for a secondary battery after the heat treatment may have an amorphous particle form. In one embodiment, the isocyanate is selected from the group consisting of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenyl isocyanate, toluene diisocyanate (TDI), 2,2'-diphenylmethane diisocyanate (2,2'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 4,4'-diphenylmethane diisocyanate (4,4'-MDI, monomeric MDI), polymeric diphenylmethane diisocyanate (polymeric MDI), orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI) and triphenylmethane triisocyanate (TPTI). It may be at least one selected from the group consisting of, but is not limited to. In one embodiment, the isocyanate is preferably at least one selected from the group consisting of 4,4'-diphenylmethane diisocyanate (4,4'-MDI, monomeric MDI), polymeric diphenylmethane diisocyanate (polymeric MDI), and polyethylene polyphenyl isocyanate, but is not limited thereto. In one embodiment, the heat treatment may include a pre-carbonization step of a primary heat treatment and a carbonization step of a secondary heat treatment. In one embodiment, the preliminary carbonization step of the first heat treatment may be performed at a temperature of 600 to 1400°C, and specifically, may be performed at a temperature of 600 to 1000°C, but is not limited thereto. In one embodiment, the preliminary carbonization step of the first heat treatment may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the carbonization step of the secondary heat treatment may be performed at a temperature of 1000 to 1400°C, preferably at 1100 to 1400°C, and more preferably at 1100 to 1200°C, but is not limited thereto. In one embodiment, the carbonization step of the secondary heat treatment may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the first heat treatment pre-carbonization step and the second heat treatment carbonization step may be performed sequentially. In one embodiment, the isocyanate may be at least 10 parts by weight, at least 15 parts by weight, at least 20 parts by weight, at least 25 parts by weight, or at least 30 parts by weight, relative to 100 parts by weight of the mixture comprising the starch and the isocyanate. In addition, the isocyanate may be at most 50 parts by weight, at most 45 parts by weight, at most 40 parts by weight, at most 35 parts by weight, or at most 30 parts by weight, relative to 100 parts by weight of the mixture comprising the starch and the isocyanate. Specifically, the isocyanate may be at least 15 to 40 parts by weight, and the isocyanate may be at least 25 to 35 parts by weight, relative to 100 parts by weight of the mixture comprising the starch and the isocyanate. In one embodiment, when the content of isocyanate is 15 to 40 parts by weight based on 100 parts by weight of the mixture containing the starch and isocyanate, sufficient bonding with the starch is formed, and when manufactured into a secondary battery, excellent electrical properties are achieved. In one embodiment, the mixture may further comprise a phosphorus flame retardant. Additionally, in one embodiment, the phosphorus flame retardant may be selected from TCPP (Tris(2-ChloroPropyl) Phosphate), TCEP (Tris(2-Chroroethyl) Phosphate), TEP (triethyl phosphate), or TMP (Trimethyl phosphate), and TCPP (Tris(2-ChloroPropyl) Phosphate) is preferred, but is not limited thereto. In one embodiment, the phosphorus-based flame retardant may be 0 parts by weight or more, 1 part by weight or more, 2 parts by weight or more, 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more, based on 100 parts by weight of the mixture. In addition, the phosphorus-based flame retardant may be 10 parts by weight or less, 9 parts by weight or less, 8 parts by weight or less, 7 parts by weight or less, 6 parts by weight or less, or 5 parts by weight or less, based on 100 parts by weight of the mixture. In one embodiment, the mixture comprising starch and isocyanate may further comprise a catalyst. Next, a method for manufacturing a negative electrode active material for a secondary battery is described in detail. In one embodiment, the method for manufacturing a negative electrode active material for a secondary battery includes a carbonization step of mixing the above-described starch and isocyanate to produce a precursor and heat-treating the produced precursor under an inert gas atmosphere. In one embodiment, the starch and isocyanate may be uniformly mixed at a certain ratio to obtain a precursor. The mixing method is preferably mixing using an impeller, but is not limited thereto. In one embodiment, a carbonization step may be performed by heat-treating the starch and isocyanate under an inert gas atmosphere. In one embodiment, a carbonization step may be performed by mixing the starch and isocyanate at a temperature between 20 and 60°C for 50 to 60 minutes and then heat-treating them under an inert gas atmosphere. In one embodiment, the carbonization step may include a pre-carbonization step and a carbonization step. In one embodiment, the pre-carbonization step may be performed at a temperature of 600 to 1400°C, preferably at 600 to 1000°C, and more preferably at 600 to 800°C, but is not limited thereto. In one embodiment, the pre-carbonization step may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the carbonization step may be performed at a temperature of 1000 to 1400°C, preferably at 1100 to 1400°C, and more preferably at 1100 to 1200°C, but is not limited thereto. In one embodiment, the carbonization step may be performed for 30 to 360 minutes, and is preferably performed for 30 to 120 minutes, but is not limited thereto. In one embodiment, the pre-carbonization step and the carbonization step may be performed sequentially, but are not limited thereto. In one embodiment, the pre-carbonization step is performed under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixed gas thereof. When the pre-carbonization step is performed at 600℃ or higher, the problem of contamination of the inside of the electric furnace and the surface of the product due to residual gas generated during the carbonization step can be resolved. In addition, if the preliminary carbonization step is performed at 1000℃ or lower, it is possible to prevent an increase in manufacturing costs due to excessive heat supply. In one embodiment, the method for manufacturing a negative electrode active material for a secondary battery may include, but is not limited to, a pulverization step for adjusting the particle size to a size suitable for manufacturing a secondary battery electrode before, after, or after the pre-carbonization step. In one embodiment, the above-mentioned fine grinding step can be performed using a conventional grinder that uses a mechanical grinding method, and in particular, various grinding devices such as a ball mill, pin mill, rotor mill, or jet mill can be used, but are not limited thereto. In general, the jet mill grinding process, which is easy to finely grind, has the advantage of solving the problem of difficulty in reducing the particle size to 60㎛ or less due to the low specific gravity of polyurethane resin and the limitation in increasing the impact between particles when performed after the pre-carbonization step. In addition, the pin mill and rotor mill processes have the advantage of solving the problem of limited rotational force and difficulty in reducing particle size due to low specific gravity when performed after the pre-carbonization step. Therefore, when performing the fine grinding step using a jet mill, pin mill, and rotor mill, it is preferable to perform it after the pre-carbonization step or after the carbonization step, but is not limited thereto. In one embodiment, when performing a pulverization step after a preliminary carbonization step, it is preferable to use a jet mill, but this is not limited thereto. In one embodiment, the average diameter (D50) of the pulverized negative active material particles may be 1 to 50 μm, preferably 3 to 50 μm, more specifically 3 to 20 μm, and most specifically 6 to 15 μm, but is not limited thereto. When the average diameter (D50) is 3㎛ or more, the amount of fine particles less than 1㎛ increases, which increases the specific surface area of ​​the particles and increases the property of adsorbing moisture in the air, thereby solving the problem of increasing irreversible capacity due to side reactions caused by moisture in the battery reaction. In addition, when the average diameter (D50) is 3㎛ or more, the fine particles increase, which increases the void ratio between particles, lowering the packing density of the particles, and there is an advantage in that it can solve the problem of deterioration of high-temperature storage characteristics, such as ions inserted inside carbon particles being easily eluted at high temperatures of 65℃ or higher during a battery reaction. In addition, when the average diameter (D50) is 50㎛ or less, the particle interface becomes smaller, narrowing the area for ion entry and exit, which has the advantage of solving the problem of deterioration of ion input / output characteristics during battery reaction. In one embodiment, the carbonization step includes, but is not limited to, a heat treatment step at a temperature of 1000 to 1400°C for 30 to 120 minutes after the preliminary carbonization step and the pulverization step. The carbonization step is a step to improve the conductivity of carbon after removing low molecular weight gases generated in the preliminary carbonization step, thereby optimizing its properties as a negative electrode material for secondary batteries. The carbonization step is performed under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixture thereof. In one embodiment, the heat treatment temperature of the carbonization step may be 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto. When carbonized at a temperature of 1000℃ or higher, the remaining hydrogen in the carbon reacts irreversibly, which has the advantage of solving the problem of battery capacity reduction occurring in the first 5 cycles or so. When carbonizing at a temperature exceeding 1400℃, the reversible capacity, which is the storage capacity of ions, decreases, which significantly reduces the energy density during battery manufacturing. When carbonizing at a temperature below 1000℃, the specific surface area increases, which increases the property of adsorbing moisture in the air, which may cause a problem in that moisture may react in the battery reaction and increase the irreversible capacity. In addition, when carbonizing at a temperature below 1400℃, there is an advantage in that the problem of increased manufacturing and process costs can be solved because the material and composition of the electric furnace must be changed to a heat-resistant material in order for the electric furnace to withstand a heat treatment temperature exceeding 1400℃ from a commercial perspective. In one embodiment, the negative active material may have a specific surface area of ​​1.0 m2 / g or more, 1.5 m2 / g or more, 1.9 m2 / g or more, 2.0 m2 / g or more, 2.1 m2 / g or more, 2.2 m2 / g or more, 2.3 m2 / g or more, or 2.4 m2 / g or more. In addition, the negative active material may have a specific surface area of ​​10.0 m2 / g or less, 5.0 m2 / g or less, 3.5 m2 / g or less, 3.2 m2 / g or less, 2.9 m2 / g or less, 2.8 m2 / g or less, 2.7 m2 / g or less, or 2.6 m2 / g or less. The negative active material may have a specific surface area of, but is not limited to, 1.0 to 10.0 m2 / g, specifically 1.5 to 5.0 m2 / g, and more specifically 1.9 to 2.7 m2 / g. In one embodiment, the negative active material may have an average pore size of 1.0 to 20 nm, as shown in FIG. 5, and is preferably 1.0 to 5 nm, but is not limited thereto. Additionally, in one embodiment, the negative active material may have an average interlayer spacing (d002) of the (002) plane as determined by X-ray diffraction analysis (XRD) of 3.4 to 4.3 Å, preferably 3.7 to 4.0 Å, but is not limited thereto. In one embodiment, the negative active material may have a crystal diameter Lc(002) in the C-axis direction of 0.8 to 2.0 nm, preferably 0.9 to 1.0 nm, but is not limited thereto. In one implementation example, the negative active material may have an R value of 1.3 to 2, preferably an R value of 1.7 to 2, but is not limited thereto. In one embodiment, the negative active material may have a charge capacity per unit weight of 370 to 500 (mAh / g), preferably 390 to 480 (mAh / g), and more preferably 400 to 420 (mAh / g), but is not limited thereto. In one embodiment, the negative active material may have a discharge capacity per unit weight of 270 to 380 (mAh / g), preferably 290 to 350 (mAh / g), and more preferably 320 to 350 (mAh / g), but is not limited thereto. In one embodiment, the negative active material may have an average interlayer spacing (d002) of 3.7 to 4.0 Å on the (002) plane as determined by X-ray diffraction analysis, a crystallite diameter Lc(002) in the C-axis direction of 0.9 to 1.0 nm, and an R value of 1.7 to 2. The negative active material for a secondary battery manufactured by the manufacturing method of the present disclosure has properties within the above range, and thus has a low moisture adsorption rate and is formed into a structure that facilitates charging and discharging, thereby improving the initial charging and discharging efficiency of the secondary battery. In addition, it was confirmed that the structure of the negative active material for a secondary battery is formed by a bonding reaction between starch and isocyanate, and includes a microstructure close to an amorphous state and fine, uniform pores. Additionally, the present disclosure relates to a secondary battery including the negative electrode active material for the secondary battery. Hereinafter, preferred embodiments and evaluation test items of the carbonaceous material for a secondary battery negative active material of the present disclosure and the method for manufacturing the same will be described in detail. The present disclosure can be better understood by the following examples, which are intended to illustrate the present disclosure and are not intended to limit the scope of protection defined by the appended claims. <Evaluation Test Items> 1) XRD measurement ① Analysis of the average interlayer spacing of particles (d002) Obtain a graph of 2θ values ​​measured by X-ray diffraction analysis and determine the peak positions of the graph.

[0048] Obtained by integration method, d002 (d002 = λ / 2sinθ) is calculated by Bragg's formula. The wavelength of the CuKa line is 0.15406 nm. At this time, the measurement range is from 5° to 80°, and the measurement speed is 5° / min. ② Comparative analysis of graphite lattice crystallinity (R value) The R value is defined as the intensity ratio of (A) and (B) at 2θ, which represents the (002) peak. (A) is the background established by drawing a straight line based on the baseline on both sides of the (002) peak, and (B) is the intensity at the point where the background moves parallel to the (002) peak and meets the (002) peak. ③ Analysis of particle crystalline size The crystallite thickness Lc(002) along the C-axis of the particle was calculated by Scherrer's equation. Lc(002) = Kλ / (B cosθ) K = 0.9 λ = wavelength (0.154056 nm) B = FWHM (Full Width at Half Maximum) 2) Surface area measurement After collecting a sample, the sample was degassed at 300℃ for 3 hours through a pretreatment device, and the specific surface area of ​​the sample was measured at a pressure range (P / P0) of 0.05 to 0.3 using a Surface Area, Pore Size Analyzer device using the nitrogen gas adsorption BET method. 3) Surface pore analysis After degassing at 300°C for 3 hours through a pretreatment device, the pores on the sample surface were analyzed by nitrogen gas adsorption using a Pore Size Analyzer (Micromeritics ASAP2020). The analysis was expressed as a total volume distribution of pores (micropores) with a diameter of 2 nm or less by the HK method, and as a total volume distribution of pores (mesopores) with a diameter of 2 to 50 nm by the BJH method. Micropore = ≤ 2 nm Mesopores = 2 ~ 50 nm Macropores = ≥ 50nm 4) Manufacturing method of measuring cell The measurement cell is a coin-type half-cell, and uses an electrode manufactured with a negative active material and binder in a ratio of 97:3 and a lithium metal foil as a counter electrode, and a separator between them, and an organic electrolyte solution of EC / EMC / DMC mixed in a ratio of 1:1:1 and an electrolyte solution containing 1M LiPF6 dissolved therein, to manufacture a 2016 type coin cell. 5) Charge / discharge characteristics evaluation Charging was performed by inserting lithium ions into the carbon electrode using a constant current method at a 0.1 C rate up to 0.005 V, and then inserting lithium ions using a constant current method from 0.005 V until the current reached 0.01 C, at which point the insertion was terminated. Discharging was performed by desorbing lithium ions from the carbon electrode using a constant current method at a 0.1 C rate, at a terminal voltage of 1.5 V, and a total of 5 charge-discharge cycles were performed. 6) Output characteristics evaluation The output characteristics evaluation measured the output characteristics during lithium-ion discharge. After 5 cycles of charge and discharge at an initial rate of 0.1 C, only the discharge (lithium-ion desorption) C rate was gradually increased thereafter, and the maintenance rate of the 10 C-rate reversible capacity compared to the 0.2 C rate reversible capacity was measured. Manufacturing of secondary batteries (a) Electrode fabrication To 97 parts by weight of the above-mentioned manufactured negative active material, 2 parts by weight of SBR (Stylene Butadiene Rubber), 1 part by weight of CMC (Carboxyl Methyl Cellulose), and distilled water were added, stirred evenly in the form of a slurry, and the slurry was uniformly coated on a copper foil. The coating was uniformly coated at 80㎛ using a doctor blade, dried in an 80℃ oven for 10 minutes, and pressed at a pressure of 0.6Mpa. The foil-shaped electrode was punched into a circle with an area of ​​1.5cm2 and dried in a 120℃ vacuum oven for 5 hours. (b) Fabrication of test cells The negative active material manufactured in the above example was used as the negative electrode of an aqueous electrolyte secondary battery. The lithium secondary battery was assembled in a glove box under an argon atmosphere as a coin-type battery of size 2016 (diameter 20 mm, thickness 16 mm). 1 mm thick metallic lithium was pressed onto the bottom of the coin-type battery can, and a polypropylene separator was formed thereon, with the negative electrode facing the lithium. At this time, the electrolyte used was prepared by adding 1 M LiPF6 salt to a solvent prepared by mixing EC (Ethylene Carbonate), DMC (Dimethyl Carbonate), and EMC (Ethyl Methyl Carbonate) in a volume ratio of 1:1:1. The electrolyte was put into the coin-type battery, the can cover was pressed, and the lithium secondary battery was assembled. (c) Battery capacity measurement The characteristics of the assembled lithium secondary batteries were analyzed by the constant current-constant voltage (CCCV) method at 25°C using an HC0105R charge-discharge tester manufactured by HNT SYSTEM. Under the constant current-constant voltage conditions, the coin cell was charged at a constant current density (0.1C standard) until the voltage of the coin cell reached 0.005 V. Thereafter, the voltage was maintained and the current was steadily decreased until 0.01 C, and the charge capacity was measured. After charging, the battery was stopped for 10 minutes and discharged. The discharge was performed at a constant current until the voltage of the coin cell reached 1.5 V, and the discharge capacity was measured. The reversible capacity was defined as the discharge capacity, and the irreversible capacity was calculated by subtracting the discharge capacity from the charge capacity. The efficiency was calculated as the percentage (%) of the discharge capacity relative to the charge capacity. The basic characteristic values ​​of coin cells were expressed as the average of the characteristic values ​​of at least six identical cells manufactured from the same sample. (d) Measurement of high-rate charge / discharge characteristics The high-rate charge-discharge characteristics of the above-mentioned assembled lithium secondary battery were analyzed at 25°C using the constant current-constant voltage (CCCV) method, as in (c). The high-rate charge-discharge characteristics were measured by changing the current density during charge and discharge, increasing the constant current density supplied or discharged for each cycle, and expressing the capacity (mAh / g) measured by charging and discharging at that current density. [Example 1] A negative electrode active material for a secondary battery was manufactured by mixing corn starch and isocyanate. 70 g of industrial corn starch, 25 g of MDI (50 wt% polymeric diphenylmethane diisocyanate (polymeric MDI) and 50 wt% 4,4'-MDI), and 5 g of TCPP (Tris(2-chloropropyl)phosphate) were stirred for 30 minutes to prepare a precursor mixture. The precursor was heated to 600°C in a nitrogen gas atmosphere without drying and maintained at 600°C for 1 hour to perform preliminary carbonization, thereby obtaining a secondary battery negative active material precursor with a carbonization yield of 38%. The obtained negative active material precursor was finely pulverized using a jet mill to an average particle size of about 6 to 12 μm, and the maximum particle size was set to not exceed 50 μm. The finely pulverized negative electrode active material precursor was placed in a ceramic crucible, heated to 1200°C at a heating rate of 5°C / min in a nitrogen gas atmosphere, and maintained at 1200°C for 1 hour to undergo a carbonization process, thereby producing a carbon material usable as a negative electrode active material for lithium secondary batteries. The negative electrode active material for secondary batteries produced in Example 1 was subjected to the above-described <evaluation test items>, and the results are shown in Tables 1 to 3 below. [Comparative Example 1] 100 g of polyol (AKP SSP-104) containing 7 wt% of hydroxyl groups and 175 g of 4,4'-MDI were stirred at 4000 rpm for 10 seconds to prepare a cured polyurethane resin. The polyurethane resin was pulverized using a crusher to a particle size of 0.1 to 2 mm, and then the pulverized material was heated to 700°C in a nitrogen gas atmosphere and maintained at 700°C for 1 hour to perform preliminary carbonization, thereby obtaining a lithium secondary battery negative active material precursor with a carbonization yield of 38%. The obtained negative active material precursor was finely pulverized using a jet mill to an average particle size of about 6 to 12 μm, and the maximum particle size was set to not exceed 50 μm. The finely pulverized negative electrode active material precursor was placed in a ceramic crucible, heated to 1200°C at a heating rate of 5°C / min. in a nitrogen gas atmosphere, and maintained at 1200°C for 1 hour to undergo a carbonization process, thereby producing a carbon material usable as a negative electrode active material for secondary batteries. The results are shown in Tables 1 to 3 below. [Comparative Example 2] The same procedure as in Example 1 was followed except that no isocyanate compound was used. The results are shown in Tables 1 and 3. [Example 2] The same procedure as in Example 1 was followed except that 60 g of corn starch and 35 g of 4,4'-MDI were used. The results are shown in Tables 1 and 3. [Example 3] The same procedure as in Example 1 was followed, except that 65 g of corn starch and 30 g of 4,4'-MDI were used. The results are shown in Tables 1 and 3. [Example 4] The same procedure as in Example 1 was followed, except that 75 g of corn starch and 20 g of 4,4'-MDI were used. The results are shown in Tables 1 and 3. [Example 5] The same procedure as in Example 1 was followed except that 80 g of corn starch and 15 g of 4,4'-MDI were used. The results are shown in Tables 1 and 3. [Example 6] The same procedure as in Example 1 was followed, except that the carbonization temperature was set to 1100°C. The results are shown in Tables 1 and 3. [Example 7] The same procedure as in Example 1 was repeated except that potato starch was used instead of corn starch. The results are shown in Tables 1 and 3. [Example 8] The same procedure as in Example 1 was repeated except that the phosphorus flame retardant TCPP was changed to toluene diisocyanate. The results are shown in Tables 1 and 3. [Example 9] The same experiment was conducted as in Example 1 except that the phosphorus flame retardant was not added. The results are shown in Tables 1 and 3. Sample name Classification 1st firing 2nd firing Total yield (%) Conditional yield (%) Conditional yield (%) Example 1 Corn starch 70g, MDI 25g and TCPP 5g 600℃ 38.6 1 200℃ 88.2 3 4.0 Comparative example 1 Polyol (AKP SSP-104) 100g and 4,4'-MDI 175g 700℃ 35.9 1 200℃ 87.4 3 1.4 Comparative example 2 Example 1 without MDI 600℃ 27.1 1 200℃ 87.1 2 4.8 Example 2 Corn starch 60g, MDI 35g and TCPP 5g 600℃ 41.4 1 200℃ 86.7 3 6.0 Example 3 Corn starch 65g, MDI 30g and TCPP 5g40.287.735.2Example 4Corn starch 75g, MDI 20g and TCPP 5g34.889.731.2Example 5Corn starch 80g, MDI 15g and TCPP 5g33.690.630.5Example 6Secondary firing temperature 1100℃ in Example 1 39.01100℃88.834.6Example 7Corn starch changed to potato starch in Example 1 40.11200℃86.434.6Example 8Flame retardant changed to toluene diisocyanate (TDI) in Example 1 29.788.426.2Example 9Flame retardant not added in Example 1 29.488.125.9 XRDD002(Å)R value(B / A)CrystalliteSize (Å)Example 13.771.989.45Comparative example 13.721.629.46 Sample specificationCharge capacity (mAh / g)Discharge capacity (mAh / g)Specific surface area (m 2 / g)Example 1 Corn starch 70g, MDI 25g and TCPP 5g412.6327.82.5Comparative example 1 Polyol (AKP SSP-104) 100g and MDI 175g333.5278.52.9Comparative example 2 Example 1 without MDI 305.4251.63.9Example 2 Corn starch 60g, MDI 35g and TCPP 5g416.1343.72.6Example 3 Corn starch 65g, MDI 30g and TCPP 5g413.6333.92.4Example 4 Corn starch 75g, MDI 20g and TCPP 5g398.9302.22.4Example 5 Corn starch 80g, MDI 15g and TCPP 5g404.5297.72.2Example 6 Secondary firing temperature 1100℃ in Example 1477.0337.72.2Example 7 In Example 1, corn starch was changed to potato starch427.1325.21.9Example 8 In Example 1, flame retardant was changed to toluene diisocyanate (TDI)371.9297.42.7Example 9 In Example 1, flame retardant was not added388.0300.52.7 As shown in Table 1 above, it can be confirmed that the carbonization yield decreases as the weight ratio of isocyanate decreases. In addition, in the case of the negative active material manufactured according to the embodiment of the present disclosure, it can be confirmed that the charge capacity and discharge capacity are significantly increased compared to the negative active material using a resin-based raw material, as in Comparative Example 1 of Table 3, and it was confirmed that the discharge capacity tends to decrease as the weight ratio of isocyanate decreases, as in Examples 2, 3, 4, and 5. In addition, in the case of the negative active material manufactured according to the embodiment of the present disclosure, it can be confirmed that the specific surface area is reduced compared to the negative active material using a resin-based raw material, as shown in Comparative Example 1 of Table 3. Figures 1, 2, and 3 show scanning electron microscope photographs of corn starch used in the present disclosure, the negative electrode active material manufactured according to Example 1, and the resin-based hard carbon manufactured according to the manufacturing method of Comparative Example 1. Corn starch has spherical particles as shown in Figure 1, but the negative electrode active material manufactured according to Example 1 in Figure 2 was confirmed to have an uneven particle shape without a specific shape. It was confirmed that the negative electrode active material manufactured by the manufacturing method of the present disclosure had the same particle shape as the resin-based negative electrode active material shown in Figure 3. In addition, in the case of the negative active material manufactured according to Example 1 of the present disclosure, as shown in FIG. 4, mesopores are not developed on the carbon surface, so the moisture content is low, and the amount of moisture adsorbed is also reduced, so that the irreversible capacity is reduced and the initial charge / discharge efficiency is increased, etc., and the electrochemical characteristics are also significantly improved. Figure 7 shows a graph of room temperature discharge output characteristics of a negative electrode active material manufactured according to Example 1 of the present disclosure and a negative electrode active material manufactured according to Comparative Example 1. As a result of performing high-speed charging and discharging by gradually increasing the current intensity, it was confirmed that the discharge capacity retention rate was at the same level as that of the resin-based negative electrode active material. While the preferred embodiments of the present disclosure have been described above, it is clear that the present disclosure can utilize various variations and equivalents, and that the above embodiments can be appropriately modified and applied in the same manner. Accordingly, the above description does not limit the scope of the present disclosure, which is defined by the limitations of the following claims.

Claims

1. A negative electrode active material for a secondary battery comprising a carbide obtained by carbonizing a mixture containing starch and isocyanate by heat treatment under an inert gas atmosphere.

2. In paragraph 1, A negative electrode active material for a secondary battery, wherein the mixture further comprises a phosphorus flame retardant.

3. In paragraph 1 or 2, A negative electrode active material for a secondary battery, wherein the starch is at least one selected from the group consisting of corn starch, rice starch, barley starch, wheat starch, cassava starch, potato starch, and sweet potato starch.

4. In paragraph 1 or 2, The average diameter (D50) of the above carbide particles is 3 to 50 ㎛, and the specific surface area is 1.9 to 2.7 m 2 / g and an average pore size of 1.0 to 5 nm.

5. In paragraph 1 or 2, The above negative active material is a negative active material for a secondary battery, having an average interlayer spacing (d002) of the (002) plane of 3.7 to 4.0 Å as determined by X-ray diffraction analysis, a crystallite diameter Lc(002) in the C-axis direction of 0.9 to 1.0 nm, and an R value of 1.7 to 2.

6. In paragraph 1 or 2, The above starch has an average diameter (D 50 ) is a negative electrode active material for a secondary battery, which has a spherical particle shape of 5 to 10㎛ and has an amorphous particle shape after the heat treatment.

7. In paragraph 1 or 2, A negative active material for a secondary battery, wherein the isocyanate is 15 to 40 parts by weight based on 100 parts by weight of the mixture.

8. In paragraph 1 or 2, The above isocyanate is at least one selected from the group consisting of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenyl isocyanate, toluene diisocyanate (TDI), 2,2'-diphenylmethane diisocyanate (2,2'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 4,4'-diphenylmethane diisocyanate (4,4'-MDI), polymeric diphenylmethane diisocyanate (polymeric MDI), orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI) and triphenylmethane triisocyanate (TPTI). A negative electrode active material for secondary batteries.

9. A method for producing a negative electrode active material for a secondary battery, comprising a carbonization step of heat-treating a mixture containing starch and isocyanate under an inert gas atmosphere.

10. In paragraph 9, The above carbonization step A pre-carbonization step of heat treatment at a temperature of 600 to 1000℃; and A method for producing a negative electrode active material for a secondary battery, comprising a carbonization step at a temperature of 1000 to 1400°C.

11. In paragraph 9, A method for manufacturing a negative electrode active material for a secondary battery, further comprising a fine grinding step of grinding the negative electrode active material particles for a secondary battery so that the average diameter (D50) of the particles becomes 3 to 50 ㎛.

12. In paragraph 11, A method for manufacturing a negative electrode active material for a secondary battery, wherein the above-mentioned pulverization step is performed before the above-mentioned pre-carbonization step, after the above-mentioned pre-carbonization step, or after the above-mentioned carbonization step.

13. A secondary battery comprising a negative electrode active material for a secondary battery according to any one of claims 1 to 8.

14. The average particle diameter (D50) is 3 to 50㎛, and the specific surface area is 1.9 to 2.7m 2 / g and an average pore size of 1.0 to 5 nm.

15. In paragraph 14, The above negative active material is a negative active material for a secondary battery, having an average interlayer spacing (d002) of the (002) plane of 3.7 to 4.0 Å as determined by X-ray diffraction analysis, a crystallite diameter Lc(002) in the C-axis direction of 0.9 to 1.0 nm, and an R value of 1.7 to 2.

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