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

Abstract

The present disclosure relates to a negative electrode active material for a secondary battery, which includes a carbonized product obtained by heat-treating a mixture containing starch and isocyanate 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, which has a low specific surface area and improves the initial charge / discharge capacity of the secondary battery, a method for producing the same, and a secondary battery using the same.

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material for a secondary battery, which includes a carbonized material obtained by heat-treating a mixture containing starch and isocyanate 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, which has a low specific surface area and improves the initial charge / discharge capacity of the secondary battery, a method for producing the same, and a secondary battery using the same. [Background technology]

[0002] In recent years, interest in and demand for environmentally friendly vehicles (green cars) has been growing, making the emergence of electric motor-driven vehicles more visible. Vehicles driven by electric motors can be categorized into electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), and the power source for driving the motors is a secondary battery. Unlike batteries used in mobile IT devices, secondary batteries for automobiles are required to have extremely high output and life characteristics. In particular, secondary batteries have a higher energy density per weight and superior output characteristics than nickel-metal hydride batteries, and are therefore attracting attention as the power source for the drive motors of electric vehicles.

[0003] Currently, graphite is the most widely used negative electrode active material in automotive secondary batteries. It exhibits a high discharge voltage of 3.6 V, high energy density, and excellent reversibility, ensuring long battery life. However, graphite suffers from poor energy input / output characteristics, particularly poor low-temperature output characteristics. Furthermore, graphite undergoes a volume change of approximately 10% during charging and discharging, which can adversely affect the bonding strength between the current collector and the composite layer, resulting in reduced battery life.

[0004] To solve these problems, non-graphitizable carbon with developed micropores has been proposed and is being used in some cases. Non-graphitizable carbon has a structure in which lithium ions are stored and released within numerous pores, and there is almost no volume expansion during lithium ion charging and discharging, resulting in excellent battery life characteristics.

[0005] It is also known to have excellent output characteristics because it can store and release lithium ions through the micropores present in all directions of the particles.However, non-graphitizable carbon has a large specific surface area, which means that increased amounts of solvent and binder are required when producing electrode slurry, resulting in a decrease in the energy density of the battery.

[0006] Furthermore, non-graphitizable carbon has the disadvantage that its high specific surface area increases the amount of moisture it adsorbs from the atmosphere, and during the production of secondary batteries, the moisture reacts with the electrolyte to form hydrofluoric acid (HF), increasing the irreversible capacity and reducing durability.

[0007] As a method for solving the above problem, a method of carbonizing under normal pressure / 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 recontamination with tar components during carbonization of the raw material, or poor uniformity of the products between the top and bottom of the raw material crucible.

[0008] In Korean Patent No. 10-1375688 (Patent Document 1), a negative electrode active material containing a carbonized material was prepared by heat treating a polyurethane resin in an inert gas atmosphere. The above-mentioned conventional negative electrode active materials for secondary batteries have a problem of moisture adsorption due to their relatively high specific surface area. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent No. 10-1375688 Summary of the Invention [Problem to be solved by the invention]

[0010] The present disclosure is intended to solve the problems of the related art, and relates to a negative electrode active material for a secondary battery, which contains a carbonized product obtained by heat-treating a mixture containing starch and isocyanate under an inert gas atmosphere.

[0011] The present disclosure aims to provide a carbonaceous material for a negative electrode active material for a secondary battery, which has a low specific surface area that reduces the problem of moisture adsorption, improves the initial charge / discharge capacity of the secondary battery, improves the energy density of the battery, and improves battery characteristics such as charge / discharge output, and a method for producing the same.

[0012] Starch is a type of polysaccharide produced by the polymerization of glucose, and 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 to form three-dimensional bonds with the starch, it is possible to produce a carbonaceous material with a high degree of amorphousness.

[0013] Therefore, an object of the present disclosure is to provide a carbonaceous material for a negative electrode active material for a secondary battery, which is produced by producing a hard carbon negative electrode active material, which is amorphous carbon, from a mixture containing starch and isocyanate, thereby improving the energy density of the secondary battery and achieving high-power battery characteristics that are difficult to achieve with graphite-based negative electrode active materials, and a method for producing the same. [Means for solving the problem]

[0014] According to the present disclosure, in order to achieve the above-mentioned object, a negative electrode active material for a secondary battery is produced by mixing starch with isocyanate and then carbonizing the mixture by heat treatment under an inert gas atmosphere, thereby achieving excellent physical properties of the negative electrode active material.

[0015] The present disclosure relates to a negative electrode active material for a secondary battery, which contains a carbonized product obtained by heat-treating a mixture containing starch and an isocyanate under an inert gas atmosphere. In one embodiment, the mixture may further include a phosphorus-based flame retardant.

[0016] 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 is preferably, but not limited to, corn starch.

[0017] 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 morphology. In one embodiment, after the heat treatment, the negative electrode active material for a secondary battery may have an amorphous particle morphology.

[0018] In one embodiment, the isocyanate is hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenylisocyanate, 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 The diisocyanate may be, but is not limited to, at least one selected from the group consisting of orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI), and triphenylmethane triisocyanate (TPTI).

[0019] 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 polyphenylisocyanate, but is not limited to this.

[0020] In one embodiment, the heat treatment may include a pre-carbonization step of primary heat treatment and a carbonization step of secondary heat treatment. In one embodiment, the primary heat treatment (pre-carbonization step) may be performed at a temperature of 600 to 1400°C, specifically, 600 to 1000°C, but is not limited thereto.

[0021] In one embodiment, the primary heat treatment pre-carbonization step may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto.

[0022] In one embodiment, the carbonization step of the secondary heat treatment may be performed at a temperature of 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto.

[0023] In one embodiment, the carbonization step of the secondary heat treatment may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto.

[0024] In one embodiment, the pre-carbonization step of the primary heat treatment and the carbonization step of the secondary heat treatment may be performed sequentially. In one embodiment, the amount of the isocyanate may be 15 to 40 parts by weight based on 100 parts by weight of the mixture containing the starch and the isocyanate.

[0025] In one embodiment, the mixture may further include a phosphorus-based flame retardant. In one embodiment, the phosphorus-based flame retardant may be selected from TCPP (Tris(2-chloropropyl)phosphate), TCEP (Tris(2-chloroethyl)phosphate), TEP (Triethyl phosphate), or TMP (Trimethyl phosphate), and is preferably, but not limited to, TCPP (Tris(2-chloropropyl)phosphate). In one embodiment, the mixture comprising starch and isocyanate may further comprise a catalyst.

[0026] The present disclosure also relates to a method for producing a negative electrode active material for a secondary battery. In one embodiment, the method for preparing a negative electrode active material for a secondary battery includes mixing starch and isocyanate to prepare a precursor, and carbonizing the precursor by heat-treating it under an inert gas atmosphere.

[0027] In one embodiment, the starch and the isocyanate can be mixed uniformly at a predetermined ratio to obtain the precursor. The mixing method is preferably, but not limited to, mixing with an impeller.

[0028] In one embodiment, a carbonization step may be performed in which the starch and isocyanate are heat treated under an inert gas atmosphere. In one embodiment, the starch and isocyanate may be mixed at 20 to 60° C. for 50 to 60 minutes, followed by a carbonization step of heat treating the mixture in an inert gas atmosphere.

[0029] 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 600 to 1000°C, and more preferably 600 to 800°C, but is not limited thereto.

[0030] In one embodiment, the pre-carbonization step may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto.

[0031] In one embodiment, the carbonization step may be performed at a temperature of 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto.

[0032] In one embodiment, the carbonization step may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto. In one embodiment, the pre-carbonization step and the carbonization step may be performed sequentially.

[0033] In one embodiment, the pre-carbonization step is carried out under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixture thereof.

[0034] In an embodiment, the method for preparing a negative electrode active material for a secondary battery may include a pulverization step before the pre-carbonization step, after the pre-carbonization step, or after the carbonization step, to adjust the particle size to a size suitable for manufacturing an electrode for a secondary battery.

[0035] In one embodiment, the fine grinding step may be performed using a conventional grinder that uses a mechanical grinding method, and various grinding devices may be used, including, but not limited to, a ball mill, a pin mill, a rotor mill, or a jet mill. In one embodiment, when the pulverization step is performed after the pre-carbonization step, it is preferable to use a jet mill, but the present invention is not limited thereto.

[0036] In one embodiment, the average diameter (D50) of the pulverized negative electrode 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.

[0037] In one embodiment, the method for manufacturing a negative electrode active material for a secondary battery may include a carbonization step of performing heat treatment at a temperature of 1000 to 1400° C. for 30 to 120 minutes after the preliminary carbonization step and the pulverization step.

[0038] In one embodiment, the carbonization step is a step for improving the conductivity of the carbon after removing low molecular weight gases generated in the pre-carbonization step, and optimizing the properties as a negative electrode material for a secondary battery.

[0039] In one embodiment, the carbonization step is carried out under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixture thereof.

[0040] In one embodiment, the heat treatment temperature in 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.

[0041] In one embodiment, the negative electrode active material has a specific surface area of ​​1.9 to 2.7 m 2 / g. In one embodiment, the negative electrode active material may have an average pore size of 1.0 to 20 nm, preferably 1.0 to 5 nm, as shown in FIG. 5, but is not limited thereto.

[0042] In one embodiment, the negative electrode active material may have an average layer spacing (d002) of the (002) plane 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.

[0043] In one embodiment, the negative electrode active material may have a crystallite size 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 electrode active material may have an R value of 1.3 to 2, and preferably has an R value of 1.7 to 2, but is not limited thereto.

[0044] In one embodiment, the negative electrode 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.

[0045] In one embodiment, the negative electrode 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.

[0046] In one embodiment, the negative electrode active material may have an average layer spacing (d002) of 3.7 to 4.0 Å, 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, as determined by X-ray diffraction analysis.

[0047] The negative electrode active material for a secondary battery manufactured by the manufacturing method of the present disclosure has physical properties within the above ranges, and thus has a low moisture adsorption rate and a structure that is easy to charge and discharge, thereby improving the initial charge and discharge efficiency of the secondary battery. Furthermore, it has been confirmed that the structure of such a negative electrode active material for a secondary battery is formed by a bonding reaction between starch and isocyanate, resulting in a near-amorphous microstructure containing fine and uniform pores. The present disclosure also relates to a secondary battery including the negative electrode active material for a secondary battery. [Effects of the Invention]

[0048] According to the carbonaceous material for negative electrode active material for secondary batteries and the method for producing the same disclosed herein, a negative electrode active material containing a carbide is produced by heat-treating a precursor obtained by mixing starch and isocyanate in an inert gas atmosphere, thereby providing the negative electrode active material with the advantages of a low specific surface area and a higher charge / discharge capacity than polyurethane resins, which have been widely used in the past. Furthermore, a secondary battery containing the negative electrode active material has the advantage that the initial discharge capacity, efficiency, and output of the battery can be improved. [Brief explanation of the drawings]

[0049] [Figure 1] 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] 1 is a scanning electron microscope photograph of a negative electrode active material of a secondary battery according to the present disclosure. [Figure 3] 1 is a scanning electron microscope photograph of a negative electrode active material produced using a polyurethane resin produced as Comparative Example 1 of the negative electrode active material of the secondary battery according to the present disclosure. [Figure 4] 1 is a graph showing an analysis of mesopores on the surface of a negative electrode active material prepared according to Example 1 of the present disclosure and the surface of a negative electrode active material prepared according to Comparative Example 1. [Figure 5] 1 is a graph showing an analysis of micropores on the surface of the negative electrode active material of a secondary battery according to the present disclosure. [Figure 6] 1 is an initial charge / discharge graph of a negative electrode active material of a secondary battery according to the present disclosure. [Figure 7] 1 is a graph evaluating the room-temperature discharge output characteristics of the negative electrode active material produced according to Example 1 of the present disclosure and the negative electrode active material produced according to Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0050] According to the present disclosure, to achieve the above-mentioned object, in order to prepare 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 physical properties of the negative electrode active material that do not require post-treatment such as additional carbon coating. In one embodiment, the mixture may further include a phosphorus-based flame retardant.

[0051] 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 is preferably, but not limited to, corn starch.

[0052] 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 morphology. In one embodiment, after the heat treatment, the negative electrode active material for a secondary battery may have an amorphous particle morphology.

[0053] In one embodiment, the isocyanate is selected from the group consisting of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenylisocyanate, 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 The diisocyanate may be, but is not limited to, at least one selected from the group consisting of orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI), and triphenylmethane triisocyanate (TPTI).

[0054] 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 polyphenylisocyanate, but is not limited to this.

[0055] In one embodiment, the heat treatment may include a pre-carbonization step of primary heat treatment and a carbonization step of secondary heat treatment. In one embodiment, the primary heat treatment (pre-carbonization step) may be performed at a temperature of 600 to 1400°C, specifically, 600 to 1000°C, but is not limited thereto.

[0056] In one embodiment, the primary heat treatment pre-carbonization step may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto.

[0057] In one embodiment, the carbonization step of the secondary heat treatment may be performed at a temperature of 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto.

[0058] In one embodiment, the carbonization step of the secondary heat treatment may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto. In one embodiment, the pre-carbonization step of the primary heat treatment and the carbonization step of the secondary heat treatment may be performed sequentially.

[0059] In one embodiment, the amount of the isocyanate may be 10 parts by weight or more, 15 parts by weight or more, 20 parts by weight or more, 25 parts by weight or more, or 30 parts by weight or more, relative to 100 parts by weight of the mixture containing the starch and isocyanate. Furthermore, the amount of the isocyanate may be 50 parts by weight or less, 45 parts by weight or less, 40 parts by weight or less, 35 parts by weight or less, or 30 parts by weight or less, relative to 100 parts by weight of the mixture containing the starch and isocyanate. Specifically, the amount of the isocyanate may be 15 to 40 parts by weight, or 25 to 35 parts by weight, relative to 100 parts by weight of the mixture containing the starch and isocyanate.

[0060] 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 starch and isocyanate, sufficient bonding with starch is formed, resulting in excellent electrical properties when used in a secondary battery.

[0061] In one embodiment, the mixture may further include a phosphorus-based flame retardant. In one embodiment, the phosphorus-based flame retardant may be selected from TCPP (Tris(2-chloropropyl)phosphate), TCEP (Tris(2-chloroethyl)phosphate), TEP (Triethyl phosphate), or TMP (Trimethyl phosphate), and is preferably, but not limited to, TCPP (Tris(2-chloropropyl)phosphate).

[0062] 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, relative to 100 parts by weight of the mixture, and 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, relative to 100 parts by weight of the mixture. In one embodiment, the mixture comprising starch and isocyanate may further comprise a catalyst.

[0063] Next, a method for manufacturing a negative electrode active material for a secondary battery will be described in detail. In one embodiment, the method for manufacturing a negative electrode active material for a secondary battery includes mixing the above-described starch and isocyanate to prepare a precursor, and then carbonizing the precursor by heat-treating it under an inert gas atmosphere.

[0064] In one embodiment, the starch and the isocyanate can be mixed uniformly at a predetermined ratio to obtain the precursor. The mixing method is preferably, but not limited to, mixing with an impeller.

[0065] In one embodiment, a carbonization step may be performed in which the starch and isocyanate are heat treated under an inert gas atmosphere. In one embodiment, the starch and isocyanate may be mixed at 20 to 60° C. for 50 to 60 minutes, followed by a carbonization step of heat treating the mixture in an inert gas atmosphere.

[0066] 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 600 to 1000°C, and more preferably 600 to 800°C, but is not limited thereto.

[0067] In one embodiment, the pre-carbonization step may be performed for 30 to 360 minutes, preferably 30 to 120 minutes, but is not limited thereto.

[0068] In one embodiment, the carbonization step may be performed at a temperature of 1000 to 1400°C, preferably 1100 to 1400°C, and more preferably 1100 to 1200°C, but is not limited thereto.

[0069] In one embodiment, the carbonization step may be performed for 30 to 360 minutes, preferably 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 to this.

[0070] In one embodiment, the pre-carbonization step is carried out under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixture thereof.

[0071] When the preliminary carbonization step is carried out at 600°C or higher, the problem of contamination of the inside of the electric furnace and the surface of the product by residual gases generated in the carbonization step can be eliminated. Furthermore, when the preliminary carbonization step is carried out at 1000° C. or less, it is possible to prevent an increase in production costs due to the supply of more heat than necessary.

[0072] In one embodiment, the method for producing a negative electrode active material for a secondary battery may include, but is not limited to, a pulverization step before the pre-carbonization step, after the pre-carbonization step, or after the carbonization step to adjust the particle size to a size suitable for production as an electrode for a secondary battery.

[0073] In one embodiment, the fine grinding step may be performed using a conventional grinder that uses a mechanical grinding method, and various grinding devices may be used, including, but not limited to, a ball mill, a pin mill, a rotor mill, or a jet mill.

[0074] Generally, the jet mill grinding process, which is easy to perform fine grinding, has the advantage of solving the problem of making it difficult to reduce the particle size to 60 μm or less when it is performed after the preliminary carbonization step, because the proportion of polyurethane resin is low and there is a limit to increasing the impact between particles.

[0075] In addition, the pin mill and rotor mill processes have the advantage that they can solve the problem of difficulty in reducing particle size when performed after the preliminary carbonization step due to limited rotational force and a low particle ratio.

[0076] Therefore, when the pulverization step is carried out using a jet mill, a pin mill, or a rotor mill, it is preferably carried out after the pre-carbonization step or after the carbonization step, but is not limited to this. In one embodiment, when the pulverizing step is carried out after the pre-carbonization step, it is preferable to use a jet mill, but this is not limiting.

[0077] In one embodiment, the average diameter (D50) of the pulverized negative electrode 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.

[0078] When the average diameter (D50) is 3 μm or more, the amount of fine powder less than 1 μm generated increases, the specific surface area of ​​the particles increases, and the ability to adsorb moisture from the atmosphere increases, which has the advantage of solving the problem of side reactions caused by moisture in the battery reaction and increasing irreversible capacity.

[0079] In addition, when the average diameter (D50) is 3 μm or more, the increase in fine powder increases the void ratio between particles, lowering the particle packing density, and there is an advantage in that it can solve the problem of reduced high-temperature storage characteristics, such as the easy elution of ions inserted inside the carbon particles at high temperatures of 65°C or higher during battery reactions.

[0080] In addition, when the average diameter (D50) is 50 μm or less, the particle interface becomes small and the area for ion ingress and egress becomes narrow, which has the advantage of solving the problem of reduced ion input and output characteristics during battery reactions.

[0081] In one embodiment, after the pre-carbonization step and the pulverization step, a carbonization step of heat treatment at a temperature of 1000 to 1400° C. for 30 to 120 minutes is included, but is not limited thereto.

[0082] The carbonization step is a step for improving the conductivity of the carbon after removing low molecular weight gases generated in the preliminary carbonization step, and optimizing the properties as a negative electrode material for secondary batteries.

[0083] The carbonization step is carried out under an inert gas atmosphere, and the inert gas is preferably, but not limited to, helium, nitrogen, argon, or a mixture thereof.

[0084] In one embodiment, the heat treatment temperature in 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.

[0085] Carbonization at temperatures above 1000°C has the advantage of solving the problem of battery capacity decreasing after the first five cycles due to irreversible reactions of hydrogen remaining in the carbon.

[0086] If carbonization is performed at temperatures above 1400°C, the reversible capacity, which is the ion storage capacity, decreases, resulting in a significant drop in energy density during battery production. On the other hand, if carbonization is performed at temperatures below 1000°C, the specific surface area increases, increasing the ability to adsorb moisture from the atmosphere, which can cause the battery to react with moisture and increase the irreversible capacity.

[0087] Furthermore, from a commercial perspective, when carbonizing at temperatures below 1400°C, the materials and configuration of the electric furnace must be replaced with heat-resistant materials in order for the electric furnace to withstand heat treatment temperatures above 1400°C, which has the advantage of solving the problem of increased manufacturing and process costs.

[0088] In one embodiment, the negative electrode active material has a specific surface area of ​​1.0 m 2 / g or more, 1.5m 2 / g or more, 1.9m 2 / g or more, 2.0m 2 / g or more, 2.1m 2 / g or more, 2.2m 2 / g or more, 2.3m 2 / g or more, or 2.4m 2 The negative electrode active material may have a specific surface area of ​​10.0 m / g or more. 2 / g or less, 5.0m 2 / g or less, 3.5m 2 / g or less, 3.2m 2 / g or less, 2.9m 2 / g or less, 2.8m 2 / g or less, 2.7m 2 / g or less, or 2.6m 2 The negative electrode active material may have a specific surface area of ​​1.0 to 10.0 m / g or less. 2 / g, specifically 1.5 to 5.0 m 2 / g, more specifically 1.9 to 2.7 m 2 / g, but is not limited thereto.

[0089] In one embodiment, the negative electrode active material may have an average pore size of 1.0 to 20 nm, preferably 1.0 to 5 nm, as shown in FIG. 5, but is not limited thereto.

[0090] In one embodiment, the negative electrode active material may have an average layer spacing (d002) of the (002) plane 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.

[0091] In one embodiment, the negative electrode active material may have a crystallite size 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 electrode active material may have an R value of 1.3 to 2, and preferably has an R value of 1.7 to 2, but is not limited thereto.

[0092] In one embodiment, the negative electrode 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.

[0093] In one embodiment, the negative electrode 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.

[0094] In one embodiment, the negative electrode active material may have an average layer spacing (d002) of 3.7 to 4.0 Å, 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, as determined by X-ray diffraction analysis.

[0095] The negative electrode active material for a secondary battery manufactured by the manufacturing method of the present disclosure has physical properties within the above ranges, and thus has a low moisture adsorption rate and a structure that is easy to charge and discharge, thereby improving the initial charge and discharge efficiency of the secondary battery. Furthermore, it has been confirmed that the structure of such a negative electrode active material for a secondary battery is formed by a bonding reaction between starch and isocyanate, resulting in a near-amorphous microstructure containing fine and uniform pores. The present disclosure also relates to a secondary battery including the negative electrode active material for a secondary battery.

[0096] Hereinafter, preferred embodiments and evaluation test items of the carbonaceous material for a negative electrode active material for a secondary battery and a 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.

[0097] <Evaluation test items> 1) XRD measurement (i) Analysis of the average layer spacing (d002) of particles A graph of the 2θ values ​​measured by X-ray diffraction analysis was obtained, and the peak positions on the graph were determined using the

[0048] integration method. d002 (d002 = λ / 2 sin θ) was calculated using the Bragg equation. The wavelength of the Cu Ka ray was set to 0.15406 nm. The measurement range was 5° to 80°, and the measurement speed was 5° / min.

[0098] (ii) Comparative analysis of graphite lattice crystallinity (R value) The R value is defined as the ratio of the intensities of (A) and (B) at 2θ representing the (002) peak.

[0099] (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 tangent point where the background is moved parallel to the (002) peak and touches the (002) peak.

[0100] (iii) Analysis of the crystalline size of the particles The crystallite thickness Lc(002) of the grain in the C-axis direction was calculated using Scherrer's equation. Lc(002)=Kλ / (B cosθ) K=0.9 λ=wavelength (0.154056nm) B=FWHM(Full Width at Half Maximum)

[0101] 2) Measurement of specific surface area The sample was collected and degassed at 300°C for 3 hours using a pretreatment device. The specific surface area of ​​the sample was then measured using a surface area and pore size analyzer using the nitrogen gas adsorption BET method in the pressure range (P / P0) of 0.05 to 0.3.

[0102] 3) Analysis of surface pores After degassing at 300°C for 3 hours using a pretreatment device, the pores on the sample surface were analyzed by nitrogen gas adsorption using a Pore Size Analyzer (Micromeritics ASAP2020).

[0103] The analysis was performed using the HK method to show the total volume distribution of pores with diameters of 2 nm or less (micropores), and using the BJH method to show the total volume distribution of pores with diameters of 2 to 50 nm (mesopores). Micropores = ≦2 nm Mesopores = 2 to 50 nm Macropores = ≥ 50 nm

[0104] 4) Manufacturing method of the measurement cell The measurement cell was a coin-shaped half cell, using an electrode made from a negative electrode active material and binder in a ratio of 97:3 and lithium metal foil as the counter electrode, with a separator in between. The cell was impregnated with an organic electrolyte solution of EC / EMC / DMC mixed in a ratio of 1:1:1 with 1M LiPF6 dissolved in it, to create a 2016-type coin cell.

[0105] 5) Evaluation of charge / discharge characteristics Charging was performed by inserting lithium ions into the carbon electrode at a constant current of 0.1 C up to 0.005 V, then inserting lithium ions at a constant current from 0.005 V, and terminating the insertion of lithium ions when the current reached 0.01 C. Discharging was performed at a constant current of 0.1 C up to a final voltage of 1.5 V to desorb lithium ions from the carbon electrode, and a total of five charge / discharge cycles were performed.

[0106] 6) Evaluation of output characteristics The output characteristics were evaluated by measuring the output characteristics during lithium ion discharge. After five cycles of charge / discharge at an initial 0.1C, the discharge (lithium ion desorption) C rate alone was gradually increased, and the retention rate of the 10C-rate reversible capacity relative to the 0.2C-rate reversible capacity was measured.

[0107] Secondary battery manufacturing (a) Preparation of electrodes To 97 parts by weight of the negative electrode active material prepared above, 2 parts by weight of SBR (Stylene Butadiene Rubber), 1 part by weight of CMC (Carboxyl Methyl Cellulose), and distilled water were added and stirred uniformly to form a slurry, which was then uniformly coated onto copper foil. The coating was performed using a doctor blade to form a uniform coating of 80 μm, dried in an oven at 80°C for 10 minutes, and pressed at a pressure of 0.6 MPa. The electrode on the foil was placed in a 1.5 cm 2 The mixture was punched into a circle and dried in a vacuum oven at 120°C for 5 hours.

[0108] (b) Preparation of test battery The negative electrode active material prepared in the above example was used in the negative electrode of an aqueous electrolyte secondary battery. The lithium secondary battery was a 2016 size (20 mm diameter, 16 mm thick) coin battery, assembled in a glove box under an argon atmosphere. A 1 mm thick lithium metal was pressed into the bottom of the coin battery can, and a polypropylene separator was formed on top of it, with the negative electrode facing the lithium. 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 1:1:1 volume ratio. This was then placed into the coin battery, and the can cover was attached and pressed to assemble the lithium secondary battery.

[0109] (c) Battery capacity measurement The characteristics of the lithium secondary batteries assembled above were analyzed using a HNT SYSTEM HC0105R charge / discharge tester, using the constant current-constant voltage (CCCV) method at 25°C. Under the constant current-constant voltage conditions, the coin cell was charged at a constant current density (0.1 C reference) until the voltage reached 0.005 V. The charge capacity was then measured by decreasing the current to 0.01 C while maintaining the voltage. After charging, the battery was stopped for 10 minutes and discharged. Discharge was performed at a constant current until the coin cell voltage reached 1.5 V, and the discharge capacity was measured. The reversible capacity was defined as the discharge capacity, the irreversible capacity was calculated by subtracting the discharge capacity from the charge capacity, and the efficiency was calculated as the percentage (%) of the discharge capacity relative to the charge capacity. The basic coin cell battery characteristics were averaged over six or more identical batteries fabricated from the same sample.

[0110] (d) Measurement of high-rate charge / discharge characteristics The high-rate charge / discharge characteristics of the lithium secondary battery assembled above 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 / discharge, increasing the constant current density supplied or discharged for each cycle, and expressing the capacity (mAh / g) measured by charging / discharging at that current density.

[0111] [Example 1] Corn starch and isocyanate were mixed to prepare a negative electrode active material for a secondary battery.

[0112] A precursor mixture was prepared by stirring 70 g of industrial cornstarch, 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) for 30 minutes. The precursor was heated to 600°C in a nitrogen gas atmosphere without drying and maintained at 600°C for 1 hour for pre-carbonization, yielding a negative electrode active material precursor for secondary batteries with a carbonization yield of 38%. The resulting negative electrode active material precursor was then pulverized using a jet mill to an average particle size of approximately 6 to 12 μm, with the maximum particle size not exceeding 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 produce 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.

[0113] [Comparative Example 1] 100 g of a polyol (AKP SSP-104) containing 7 wt% hydroxyl groups and 175 g of 4,4'-MDI were stirred at 4000 rpm for 10 seconds to produce a cured polyurethane resin. The polyurethane resin was crushed using a crusher to a particle size of 0.1 to 2 mm. The crushed material was then heated to 700°C in a nitrogen gas atmosphere and maintained at 700°C for 1 hour for pre-carbonization, producing a precursor for a negative electrode active material for lithium secondary batteries with a carbonization yield of 38%. The resulting negative electrode active material precursor was then finely pulverized using a jet mill to an average particle size of approximately 6 to 12 μm, with the maximum particle size not exceeding 50 μm. The finely pulverized negative electrode active material precursor was placed in a ceramic crucible and heated to 1200°C at a heating rate of 5°C / min in a nitrogen gas atmosphere. It was then maintained at 1200°C for 1 hour for carbonization, producing a carbon material suitable for use as a negative electrode active material for secondary batteries. The results are shown in Tables 1 to 3 below.

[0114] Comparative Example 2 The same procedure was carried out as in Example 1, except that no isocyanate compound was used. The results are shown in Tables 1 and 3.

[0115] [Example 2] The same procedure was carried out as in Example 1, except that the amounts of corn starch and 4,4'-MDI were changed to 60 g and 35 g, respectively. The results are shown in Tables 1 and 3.

[0116] [Example 3] The same procedure was carried out as in Example 1, except that the amounts of corn starch and 4,4'-MDI were changed to 65 g and 30 g, respectively. The results are shown in Tables 1 and 3.

[0117] [Example 4] The same procedure was carried out as in Example 1, except that the amounts of corn starch and 4,4'-MDI were changed to 75 g and 20 g, respectively. The results are shown in Tables 1 and 3.

[0118] [Example 5] The same procedure was carried out as in Example 1, except that the amounts of corn starch and 4,4'-MDI were changed to 80 g and 15 g, respectively. The results are shown in Tables 1 and 3.

[0119] [Example 6] The same procedure as in Example 1 was carried out except that the carbonization temperature was changed to 1100° C. The results are shown in Tables 1 and 3.

[0120] [Example 7] The same procedure was carried out as in Example 1, except that potato starch was used instead of corn starch. The results are shown in Tables 1 and 3.

[0121] [Example 8] The same procedure was carried out as in Example 1, except that the phosphorus-based flame retardant TCPP was replaced with toluene diisocyanate. The results are shown in Tables 1 and 3.

[0122] [Example 9] The same procedure was carried out as in Example 1, except that no phosphorus-based flame retardant was added. The results are shown in Tables 1 and 3.

[0123] [Table 1]

[0124] [Table 2]

[0125] [Table 3]

[0126] As shown in Table 1, it can be seen that the carbonization yield decreases as the weight ratio of isocyanate decreases. Furthermore, in the case of the negative electrode active material prepared according to the examples of the present disclosure, it was confirmed that the charge capacity and discharge capacity were significantly increased compared to the negative electrode active material using a resin-based raw material, as in Comparative Example 1 in Table 3, and it was confirmed that the discharge capacity tended to decrease as the weight ratio of isocyanate decreased, as in Examples 2, 3, 4, and 5.

[0127] Furthermore, in the case of the negative electrode active material manufactured according to the examples of the present disclosure, it can be confirmed that the specific surface area is reduced compared to the negative electrode active material using a resin-based raw material, as shown in Comparative Example 1 in Table 3.

[0128] Figures 1, 2, and 3 show scanning electron microscope images of the cornstarch used in the present disclosure, the negative electrode active material prepared in Example 1, and the resin-based hard carbon prepared by the manufacturing method of Comparative Example 1. Cornstarch has spherical particles as shown in Figure 1, but the negative electrode active material prepared in Example 1 in Figure 2 was confirmed to have a non-uniform particle morphology without a specific shape. The negative electrode active material prepared by the manufacturing method of the present disclosure was confirmed to have a particle morphology similar to that of the resin-based negative electrode active material shown in Figure 3.

[0129] Furthermore, in the case of the negative electrode active material prepared according to Example 1 of the present disclosure, as shown in FIG. 4, mesopores are not developed on the carbon surface, resulting in a low water content and a reduced amount of water adsorption, which in turn reduces the irreversible capacity and increases the initial charge / discharge efficiency, resulting in significantly improved electrochemical properties.

[0130] 7 shows a graph evaluating the room-temperature discharge output characteristics of the negative electrode active material prepared according to Example 1 of the present disclosure and the negative electrode active material prepared 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 a similar level to that of the resin-based negative electrode active material.

[0131] While the preferred embodiments of the present disclosure have been described above, it is apparent that the present disclosure is susceptible to various modifications and equivalents, and that the above embodiments can be similarly modified and applied. Accordingly, the above description should not be construed as limiting 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 carbonized product obtained by heat-treating a mixture containing starch and isocyanate under an inert gas atmosphere.

2. The negative electrode active material for a secondary battery according to claim 1 , wherein the mixture further comprises a phosphorus-based flame retardant.

3. 3. The negative electrode active material for a secondary battery according to claim 1, 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. The average diameter (D50) of the carbide particles is 3 to 50 μm, and the specific surface area is 1.9 to 2.7 m ２ 3. The negative electrode active material for a secondary battery according to claim 1, wherein the average pore size is 1.0 to 5 nm.

5. The negative electrode active material has an average layer spacing (d002) 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. The negative electrode active material for a secondary battery according to claim 1 or 2.

6. The starch has an average diameter (D ５０ 3. The negative electrode active material for a secondary battery according to claim 1, wherein the active material has a spherical particle shape of 5 to 10 μm and has an amorphous particle shape after the heat treatment.

7. 3. The negative electrode active material for a secondary battery according to claim 1, wherein the isocyanate is contained in an amount of 15 to 40 parts by weight based on 100 parts by weight of the mixture.

8. The isocyanate may be 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 3. The negative electrode active material for a secondary battery according to claim 1, wherein the negative electrode active material is at least one selected from the group consisting of methyl methyl ether (MDI), orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI), and triphenylmethane triisocyanate (TPTI).

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. The carbonization step comprises: a pre-carbonization step of heat treatment at a temperature of 600 to 1000°C; and a carbonization step at a temperature of 1000 to 1400°C.

11. The method for producing a negative electrode active material for a secondary battery according to claim 9, further comprising a pulverizing step of pulverizing the negative electrode active material for a secondary battery so that the particles have an average diameter (D50) of 3 to 50 μm.

12. The method for producing a negative electrode active material for a secondary battery according to claim 11 , wherein the pulverizing step is performed before the preliminary carbonization step, after the preliminary carbonization step, or after the carbonization step.

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

14. The average particle diameter (D50) is 3 to 50 μm, 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.

15. The negative electrode active material has an average layer spacing (d002) 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. The negative electrode active material for a secondary battery according to claim 14.

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