Method for preparing negative electrode active material for lithium secondary battery and negative electrode active material for lithium secondary battery prepared thereby

Abstract

The present invention relates to a method for preparing a negative electrode active material for a lithium secondary battery and a negative electrode active material prepared thereby, the method comprising: a first step of dispersing silicon-based particles in a solvent to form a silicon dispersion; a second step of adding a ferric salt and a tannic acid to the silicon dispersion and stirring to form a reaction layer of the ferric salt and the tannic acid on the surface of the silicon-based particles; a third step of performing heat treatment on the reaction layer of the ferric salt and the tannic acid to form an amorphous carbon coating; and 4, carrying out acid treatment on the amorphous carbon coating to form the porous amorphous carbon coating.

Description

Technical Field

[0001] This application claims the priority of Korean Patent Application No. 10-2022-0174210, filed on December 13, 2022, and Korean Patent Application No. 10-2023-0164782, filed on November 23, 2023, the disclosures of which are incorporated herein by reference.

[0002] The present invention relates to a method for preparing a negative electrode active material for a lithium secondary battery including a porous amorphous carbon coating on a surface, a negative electrode active material prepared therefrom, a negative electrode including the negative electrode active material, and a lithium secondary battery including the negative electrode. Background Art

[0003] In recent years, with the increase in the development of technologies and demands for mobile devices, the demand for batteries as an energy source has increased significantly, and accordingly, various studies have been conducted on batteries that can meet various demands. In particular, as a power source for such devices, research on lithium secondary batteries having excellent life and cycle characteristics and high energy density has been actively carried out.

[0004] A lithium secondary battery refers to a battery including an electrode assembly and a non-aqueous electrolyte containing lithium ions, the electrode assembly having a positive electrode containing a positive electrode active material capable of inserting / extracting lithium ions, a negative electrode containing a negative electrode active material capable of inserting / extracting lithium ions, and a microporous separator disposed between the positive electrode and the negative electrode.

[0005] Carbon-based active materials and silicon-based active materials are mainly used as the negative electrode active material, and the silicon-based active material has attracted much attention because it has a capacity about 10 times higher than that of the carbon-based active material. However, since the conductivity of silicon particles is very low, a large amount of conductive agent is required in the negative electrode, and thus there is a problem that the amount of the negative electrode active material is relatively reduced, resulting in a decrease in the energy density of the negative electrode.

[0006] To solve this problem, a technique of forming a carbon coating on the surface of silicon particles is being used. However, in the case of using a conventional asphalt coating or a chemical vapor deposition (CVD) method using a carbonization gas to introduce a carbon coating, since pores are not formed in the coating, the diffusivity of lithium is reduced, and ultimately other problems such as deterioration of rate characteristics and life characteristics occur, and this problem needs to be solved. Summary of the Invention

[0007] Technical Problem

[0008] One aspect of the present invention provides a method for preparing a negative electrode active material for a lithium secondary battery including a porous amorphous carbon coating on a surface, and a negative electrode active material prepared therefrom.

[0009] Another aspect of the present invention provides a lithium secondary battery, in which the rate performance and the life characteristics are improved by including the above-described negative electrode active material.

[0010] Technical solution

[0011] According to an embodiment of the present invention, the present invention provides:

[0012] A method for preparing a negative electrode active material for a lithium secondary battery, comprising:

[0013] In the first step, silicon-based particles are dispersed in a solvent to form a silicon dispersion;

[0014] In the second step, an iron salt and tannic acid are added to the silicon dispersion and stirred, so as to form a reaction layer of the iron salt and tannic acid on the surface of the silicon-based particles;

[0015] In the third step, the reaction layer of the iron salt and tannic acid is heat-treated to form an amorphous carbon coating; and

[0016] In the fourth step, the amorphous carbon coating is acid-treated to form a porous amorphous carbon coating.

[0017] In addition, the present invention provides a negative electrode active material for a lithium secondary battery, which includes silicon-based particles; and

[0018] a porous amorphous carbon coating formed on the surface of the silicon-based particles,

[0019] wherein the Brunauer-Emmett-Teller (BET) specific surface area of the negative electrode active material is in the range of 10 m 2 / g to 100 m 2 / g.

[0020] In addition, the present invention provides a negative electrode for a lithium secondary battery, which includes the negative electrode active material for a lithium secondary battery.

[0021] Furthermore, the present invention provides a lithium secondary battery, which includes the negative electrode for a lithium secondary battery.

[0022] Advantageous effects

[0023] The method for preparing a negative electrode active material for a lithium secondary battery according to the present invention provides a method for forming a carbon coating on silicon-based particles with relatively simple processes as compared with conventional methods. Specifically, by using tannic acid extracted from plants without using materials such as carbonization gas and pitch that require complex processing and are flammable, a method for forming a carbon coating in an environmentally friendly manner is provided.

[0024] In addition, since the carbon coating formed by this method has a porous structure with nanoscale pores, the diffusion of lithium ions is easy. Therefore, it can contribute to improving the rate characteristics and life characteristics of lithium secondary batteries. Detailed Description of the Invention

[0025] Hereinafter, the present invention will be described in more detail so as to be understood more clearly.

[0026] Generally, it is known that a silicon-based negative electrode active material has a capacity about 10 times higher than that of a carbon-based negative electrode active material. Therefore, when the silicon-based negative electrode active material is used for a negative electrode, it has the advantage of a thin film electrode that can achieve a high level of energy density even with a thin thickness. However, since the silicon-based negative electrode active material has a large volume change during charge and discharge, a reaction in which a solid electrolyte interface (SEI) film is easily destroyed and regenerated continuously occurs, which may lead to a shortened life. In addition, if the destruction and regeneration of the above SEI film are repeated, the following problems may also occur: as the distance between active material particles increases, electrical contact decreases, the charge transfer path is lost, and lithium ions are isolated.

[0027] To solve these problems, a technique has been introduced that forms a carbon coating on the surface of the silicon-based negative electrode active material particles by, for example, a method of asphalt coating and chemical vapor deposition (CVD) using a carbonization gas. This method has some effect in covering the entire surface of the particles, but since pores are not formed in the coating, it has the disadvantage that lithium ions are not easily diffused. In addition, due to the use of flammable and dangerous materials, the process is difficult and may have an adverse impact on the environment.

[0028] Therefore, in the present invention, a metal-phenol network (MPN) is formed on the surface of the silicon-based negative electrode active material particles, and an amorphous carbon coating is formed using an environmentally friendly and relatively simple method. In particular, in the present invention, instead of simply carbonizing the surface on which the MPN is formed, an attempt is made to reduce the tortuosity in the coating and form a nanoscale porous structure by a process of removing iron ions after carbonization.

[0029] If the surface on which the MPN is formed is carbonized, it can be expected that the diffusibility of lithium ions will be improved with the formation of a porous structure. However, an amorphous porous structure is formed, in which the tortuosity in the coating increases due to large pores, and the movement path of lithium ions becomes longer. Therefore, the effect of improving the diffusibility of lithium ions is reduced.

[0030] The present inventors focused on this point, removed the iron ions contained in the MPN by acid treatment after carbonization, reduced the tortuosity in the coating, and shortened the movement path of lithium ions. Finally, the present inventors found that the rate characteristics and life characteristics of the battery can be significantly improved.

[0031] The "BET specific surface area" in the present invention is calculated from the nitrogen adsorption isotherm in a liquid nitrogen atmosphere at 77K obtained using BELSORP-MAX (MicrotracBEL Corporation) by the Brunauer-Emmett-Teller (BET) multipoint method.

[0032] The "ICP analysis" in the present invention means that 0.1 g of the negative electrode active material to be analyzed is mixed with 2 mL of distilled water and 1 mL of concentrated nitric acid, diluted with 50 mL of ultrapure water, and then analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer Corporation, Optima 7300DV) instrument.

[0033] The expression "particle size D 50 " in the present invention refers to the particle size corresponding to 50% of the cumulative volume in the volume-based particle size distribution, which can be measured using the laser diffraction method. For example, after dispersing the negative electrode active material in a dispersion medium, the dispersion medium is placed in a commercially available laser diffraction particle size analyzer (e.g., S-3500 of Microtrac Corporation), and then ultrasonic waves of about 28 kHz are irradiated with an output power of 60 W to obtain a volume-based particle size distribution diagram, and the particle size D 50 can be measured by the following method: in the obtained volume-based particle size distribution diagram, the particle size at 50% of the cumulative volume is obtained.

[0034] Each step will be described in more detail below.

[0035] The first step

[0036] A method for preparing a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention includes a first step: forming a silicon dispersion by dispersing silicon-based particles in a solvent.

[0037] In an embodiment of the present invention, the silicon-based particles can be represented as SiO x (0 ≤ x < 2), but preferably, it can be pure silicon that has not been intentionally oxidized, that is, x can satisfy x = 0. That is to say, the silicon-based particles may contain some oxygen atoms or oxygen-containing functional groups on the surface due to natural oxidation, but the content thereof is different from that of SiO x (0 < x < 2) formed by an intentional oxidation process. By using pure silicon particles, the capacity of the negative electrode can be improved.

[0038] In addition, the silicon-based particles may contain hydroxyl groups inevitably generated by natural oxidation. The hydroxyl groups are provided on the silicon-based particles, and based on the total weight of the silicon-based particles, the content of the hydroxyl groups may be 0.1 wt% to 2 wt%.

[0039] The particle size D of the silicon-based particles50 can be in the range of 0.3 μm to 100 μm. When the above range is satisfied, the particle breakage phenomenon caused by the volume change of the silicon-based particles during battery operation can be minimized, and the side reaction between the electrolyte and the silicon-based particles can be suppressed, so the battery life can be improved. Specifically, the particle size D of the silicon-based particles 50 can be in the range of 2 μm to 7 μm. When the above range is satisfied, the porous amorphous carbon coating can be uniformly provided while completely covering the silicon-based particles.

[0040] Generally, silicon-based particles contain a predetermined amount of hydroxyl groups on the surface due to natural oxidation, so the surface is negatively charged. Therefore, it is easy to interact with positively charged iron ions.

[0041] In one embodiment of the present invention, the solvent in the first step can be any polar solvent in which tannic acid and iron ions can be dispersed, without particular limitation, but preferably a buffer solution with a pH of 7 to 12. And, considering the ease of pH adjustment, it is desirable that the solvent contains water. It is desirable that the pH of the solvent is close to 12. Specifically, the pH of the solvent can be in the range of 8 to 12, and more specifically, it can be in the range of 10 to 12. To achieve this pH, as the solvent in the first step, a mixed solution of distilled water and at least one buffer selected from the group consisting of sodium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, arginine hydrochloride, sodium citrate, trisodium citrate dihydrate, monosodium L-glutamate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium lactate, potassium dihydrogen phosphate, sodium hydroxide, meglumine, glycine, citric acid, and acetic acid can be used. Specifically, a mixed solution of disodium hydrogen phosphate and sodium hydroxide in distilled water can be used. When using a buffer solution that remains close to pH 12 even after adding tannic acid and iron salts as the solvent, it is desirable because it is beneficial to form a tris-complex through the interaction between Fe 3+ and the phenol in tannic acid.

[0042] Since the silicon-based particles are dispersed before adding tannic acid and iron salts, in the second process described below, when a reaction layer is formed on the surface of the silicon-based particles, by preventing agglomeration between particles, the effect of ensuring uniform coating is achieved. The dispersion can be carried out, for example, by stirring at a speed of 1,000 rpm to 5,000 rpm, specifically 1,500 rpm to 4,000 rpm, and more specifically 2,000 rpm to 3,000 rpm at 10 °C to 50 °C, specifically 15 °C to 40 °C, and more specifically 20 °C to 30 °C.

[0043] The second step

[0044] The method for preparing the negative electrode active material for a lithium secondary battery according to an embodiment of the present invention includes a second step: adding an iron salt and tannic acid to the silicon dispersion and stirring to form a reaction layer of the iron salt and tannic acid on the surface of the silicon-based particles.

[0045] The reaction layer of the iron salt and tannic acid may include a metal-phenol network. Through the metal-phenol network, the entire silicon-based particles can be covered by the reaction layer having a thin thickness.

[0046] As described above, since tannic acid is not only a nature-friendly material obtained from plants, but also contains a large number of phenolic groups, is easy to interact with metal ions, and has a large molecular weight, it has the advantage of remaining a large amount of carbon even after carbonization. Moreover, since iron ions among metal ions are trivalent ions, they can form up to three ionic bonds with -OH functional groups, so the advantage is to promote stable coating, is relatively harmless to the human body compared with other metal ions such as nickel (Ni), titanium (Ti), and tin (Sn), and has a low price. Therefore, an iron salt is added in the present invention to form an MPN.

[0047] The iron salt refers to a salt containing iron ions or ferrous ions, and may be at least one selected from the group consisting of FeCl3, FeCl2, Fe(NO3)3, FeSO4, and Fe2(SO4)3, preferably FeCl3. Since chloride ions as anions are less likely to cause environmental pollution compared with anions derived from other iron salts, FeCl3 is preferred.

[0048] In the second step, the weight ratio of the iron salt to the tannic acid may be in the range of 1:1 to 1:50, preferably 1:1 to 1:20, more preferably 1:1.5 to 1:3. Based on the weight ratio, it is desirable that the tannic acid is at least 1 time the iron salt, because at this time the tannic acid and the iron salt react sufficiently to form a network. However, if the amount of the tannic acid is greater than the amount of the iron salt that can react, the coating cannot be carried out properly and the tannic acid remains as an unreacted substance in the solution. Therefore, it is desirable that the amount of the tannic acid is not greater than 50 times the amount of the iron salt.

[0049] In addition, based on the total weight of the silicon dispersion, the total amount of the iron salt and the tannic acid may be in the range of 0.1 wt% to 50 wt%, preferably 1 wt% to 10 wt%, more preferably 2 wt% to 6 wt%. From the aspect of completely coating the silicon surface, it is desirable that the total amount of the iron salt and the tannic acid is 0.1 wt% or more. However, considering that if the coating amount is too large, the charge-discharge capacity of the silicon composite decreases, it is desirable that the total amount of the iron salt and the tannic acid is not greater than 50 wt%.

[0050] The stirring in the second step can be carried out, for example, at a speed of 1,000 rpm to 5,000 rpm, specifically 1,500 rpm to 4,000 rpm, and more specifically 2,000 rpm to 3,000 rpm at a temperature of 10°C to 50°C, specifically 15°C to 40°C, and more specifically 20°C to 30°C.

[0051] In addition, after the stirring in the second step, a step of extracting the silicon-based particles having a reaction layer of iron salt and tannic acid formed on the surface is carried out. This step can be carried out by the following method: operating a centrifuge at 11,000 rpm for 5 minutes at room temperature to remove the unreacted solution, redispersing the silicon-based particles in distilled water, and then repeating the same operation three times to wash the unreacted products.

[0052] The third step

[0053] A method for preparing a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention includes a third step: heat-treating the reaction layer of the iron salt and tannic acid to form an amorphous carbon coating. Specifically, the amorphous carbon coating can be formed by carbonizing by applying heat to the silicon-based particles having the reaction layer of the iron salt and tannic acid formed on the surface.

[0054] The heat treatment in the third step can be carried out in a mixed gas atmosphere of argon (Ar) and hydrogen (H2), specifically in a mixed gas atmosphere where the volume ratio of Ar to H2 is in the range of 3:1 to 10:1.

[0055] In addition, the heat treatment in the third step can be carried out at 800°C to 1,100°C, preferably 850°C to 1,050°C, and more preferably 900°C to 1,000°C. It is desirable that the heat treatment temperature is 800°C or higher because the tannic acid-iron complex in the reaction layer can be reduced to smoothly form the amorphous carbon coating. However, since the carbon coating may undergo a decomposition reaction above 1,100°C, it is desirable that the heat treatment temperature does not exceed 1,100°C.

[0056] In addition, the heat treatment in the third step can be carried out for 1 hour to 10 hours, preferably 2 hours to 5 hours, and more preferably 2 hours to 4 hours. In order to form the amorphous carbon coating, it is necessary to ensure sufficient reaction time to reduce the tannic acid-iron complex. Therefore, the heat treatment must be carried out for more than 1 hour. However, since the reaction is completed after a certain time, it is desirable that the heat treatment is carried out for no more than 10 hours.

[0057] The fourth step

[0058] The method for preparing a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention includes a fourth step: subjecting the amorphous carbon coating to an acid treatment to form a porous amorphous carbon coating. The acid treatment in the fourth step can remove iron ions.

[0059] As described above, an amorphous porous structure is formed in the amorphous carbon coating formed in the third step, in which the tortuosity in the coating is high due to macropores. In addition, since it is in a state containing iron ions, when used in a battery in this state, there is a risk of short circuit due to the dissolution of iron, and since iron ions are present in the electrolyte, the SEI film becomes thicker. Therefore, there is a risk that the lithium ions consumed for forming the SEI may increase, thereby shortening the battery life.

[0060] Therefore, it is desirable to remove iron ions in the fourth step to reduce the tortuosity in the coating, thereby shortening the migration path of lithium ions and eliminating the possibility of problems due to iron dissolution during subsequent battery operation. The effect of removing other impurities can also be promoted.

[0061] The acid treatment can be carried out by immersing the silicon-based particles having the amorphous carbon coating formed thereon in an acidic aqueous solution containing at least one acid selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid at a concentration of 0.5 wt% to 30 wt%, preferably 1 wt% to 20 wt%, more preferably 5 wt% to 10 wt%. Specifically, the immersion can be carried out at room temperature (25 °C) for 10 minutes to 120 minutes.

[0062] The final negative electrode active material can be obtained by dispersing the acid-treated silicon-based particles in an excessive amount of distilled water and then filtering. The negative electrode active material prepared in this way has the following characteristics.

[0063] Negative electrode active material

[0064] The negative electrode active material for a lithium secondary battery according to an embodiment of the present invention includes silicon-based particles; and a porous amorphous carbon coating formed on the surface of the silicon-based particles, wherein the BET specific surface area of the negative electrode active material can be in the range of 10 m 2 / g to 100 m 2 / g, preferably 19 m 2 / g to 100 m 2 / g, more preferably 27 m 2 / g to 100 m 2 / g, most preferably 90 m 2 / g to 100 m 2 / g.

[0065] The BET specific surface area of the raw material silicon particles is mostly 2 m 2Less than / g. And, in order to improve the low electrical conductivity on the surface of the silicon particles and suppress particle breakage due to the volume expansion of the silicon particles, a technique of introducing a carbon coating on the silicon surface to improve the life performance is mainly used. However, the present inventors noticed that for this carbon coating, lithium ions are not easily diffused. To solve this problem, a porous amorphous carbon coating is introduced, in which a pore structure that is beneficial to lithium diffusion while maintaining the advantages of the carbon coating is formed. The degree of formation of this pore structure (open pores) can be indirectly confirmed by the BET specific surface area. That is, through the porous amorphous carbon coating, the BET specific surface area can be increased to the above numerical range. In the case where the specific surface area is increased as described above, since the diffusivity of lithium ions is improved, the rate characteristics and life characteristics of the battery can ultimately be improved.

[0066] In addition, the content of iron element in the negative electrode active material measured by inductively coupled plasma (ICP) analysis can be 20 ppm to 3,000 ppm, specifically 20 ppm to 1,200 ppm, and more specifically 50 ppm to 400 ppm. Since an iron salt is used, it contains more than 20 ppm of iron element, but since the iron ions are removed by the above acid treatment, the amount of residual iron element is not more than 3,000 ppm.

[0067] Negative electrode

[0068] The negative electrode for a lithium secondary battery according to an embodiment of the present invention includes the above negative electrode active material. Specifically, the negative electrode includes a negative electrode active material layer, and the negative electrode active material layer includes the above negative electrode active material.

[0069] The negative electrode can be a self-supporting negative electrode. In this case, the negative electrode active material layer itself serves as the negative electrode. Alternatively, the negative electrode may include a negative electrode current collector that supports the negative electrode active material layer.

[0070] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., or an aluminum cadmium alloy can be used as the negative electrode current collector. Specifically, a transition metal that adsorbs carbon well, such as copper and nickel, can be used as the negative electrode current collector.

[0071] The negative electrode active material layer can be disposed on one or both surfaces of the negative electrode current collector. Of course, for a self-supporting negative electrode, the negative electrode active material layer itself can become the negative electrode without including a negative electrode current collector.

[0072] Based on the inclusion of the above-mentioned negative electrode active material, the negative electrode active material layer may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may be at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesophase carbon microspheres.

[0073] The negative electrode active material layer may further include a binder. The binder is used to ensure the adhesion between the silicon-based negative electrode active materials or the adhesion of the silicon-based negative electrode active materials to the current collector. Commonly used binders in the art can be used, and their types are not particularly limited. For example, the binder may include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. Any one or a mixture of two or more thereof can be used. In particular, when using a water dispersion process, an aqueous binder such as SBR and CMC is desirable.

[0074] The negative electrode active material layer may further include a conductive agent. The conductive agent may be at least one selected from the group consisting of fullerene, carbon black, carbon nanotubes, graphene, and flake graphite.

[0075] Lithium secondary battery

[0076] The lithium secondary battery according to an embodiment of the present invention includes the above-mentioned negative electrode.

[0077] Specifically, the lithium secondary battery may include the negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the same as the above-mentioned negative electrode. Since the negative electrode has been described in detail above, the remaining structures will be described below.

[0078] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

[0079] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and its surface may have fine irregularities to improve the adhesion to the positive electrode active material. The positive electrode current collector can be used in various shapes, such as a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0080] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may include layered compounds such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium iron oxides such as LiFe3O4; lithium manganese oxides such as Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; nickel (Ni)-site type lithium nickel oxides represented by the chemical formula LiNi 1-c2 M c2 O2 (where M is at least one selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), and gallium (Ga), and c2 satisfies 0.01≤c2≤0.3); lithium manganese composite oxides represented by the chemical formula LiMn 2-c3 M c3 O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, chromium (Cr), zinc (Zn), and tantalum (Ta), c3 satisfies 0.01≤c3≤0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4 in which a part of lithium (Li) is replaced by alkaline earth metal ions, but the positive electrode active material is not limited thereto. The positive electrode may be Li metal.

[0081] The positive electrode active material layer may further include a positive electrode conductive agent and a positive electrode binder on the basis of including the above positive electrode active material.

[0082] In this case, the positive electrode conductive agent is used to provide conductivity to the electrode, and any conductive agent may be used without particular limitation as long as it has conductivity and does not cause adverse chemical changes in the battery. Specific examples of the positive electrode conductive agent may be graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon-based materials, such as carbon fiber; metal powders, such as copper powder, nickel powder, aluminum powder, and silver powder, or metal fibers; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and one of them may be used alone or a mixture of two or more of them may be used.

[0083] In addition, the positive electrode binder serves to improve the binding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of them may be used alone or a mixture of two or more thereof may be used.

[0084] The separator separates the negative electrode and the positive electrode and provides a movement channel for lithium ions. Herein, any separator may be used as the separator as long as it is commonly used in secondary batteries, without particular limitation. In particular, a separator having a high moisture retention ability for the electrolyte and a low resistance to the movement of electrolyte ions may be used. Specifically, a porous polymer membrane may be used, such as a porous polymer membrane prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, typical porous non-woven fabrics may be used, for example, non-woven fabrics made of high melting point glass fibers or polyethylene terephthalate fibers. In addition, a coated separator containing a ceramic component or a polymer component may be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure may be optionally used.

[0085] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte that can be used to prepare the lithium secondary battery, but the present invention is not limited thereto.

[0086] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0087] Examples of the non-aqueous organic solvent may be aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0088] In particular, in carbonate-based organic solvents, ethylene carbonate and propylene carbonate as cyclic carbonates have a high dielectric constant as high-viscosity organic solvents, so that lithium salts can be well dissociated in the electrolyte. Therefore, the cyclic carbonates can be preferably used. When the cyclic carbonates are mixed with linear carbonates (such as dimethyl carbonate and diethyl carbonate) having a low viscosity and a low dielectric constant in an appropriate ratio, an electrolyte having a high conductivity can be prepared. Therefore, such a combined use can be more preferable.

[0089] As the metal salt, a lithium salt can be used, and the lithium salt is a material that is easily soluble in a non-aqueous electrolyte. As the anion of the lithium salt, for example, those selected from the group consisting of F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3-, (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - and at least one selected from the group consisting thereof.

[0090] In addition to the above electrolyte components, for the purpose of improving the life characteristics of the battery, preventing the reduction of the battery capacity, and improving the discharge capacity of the battery, the electrolyte may further contain at least one additive, such as carbonate-based compounds (such as vinylene carbonate and difluoroethyl carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride.

[0091] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell and a battery pack including the battery module. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and energy storage systems.

[0092] Hereinafter, the present invention will be described in more detail based on specific examples.

[0093] Mode of the Invention

[0094] <Examples and Comparative Examples: Preparation of Anode Active Material and Anode>

[0095] Example 1.

[0096] (1) Preparation of Anode Active Material

[0097] 2 g of silicon particles having a particle size D of 4 μm to 5 μm 50 and a specific surface area of 1.5 m 2 / g were added to 100 mL of a buffer solution having a pH of 12, and stirred at a speed of 2,500 rpm at 25 °C to prepare a silicon dispersion. As the buffer solution, a mixed solution of 10.6 g of disodium hydrogen phosphate and 3.10 g of sodium hydroxide in 1 L of distilled water was used. 2 g of tannic acid and 1 g of FeCl3 were added to the prepared silicon dispersion, and stirred at a speed of 2,500 rpm at 25 °C. Then, the dispersion was transferred to a centrifuge, and the silicon-based particles were extracted by the following method: the centrifuge was run at 11,000 rpm for 5 minutes at room temperature to remove the unreacted solution, the silicon particles were redispersed in distilled water, and then the same operation was repeated three times.

[0098] The extracted particles were heat-treated in an Ar / H2 (volume ratio = 3:1) atmosphere at 950 °C for 3 hours, and then iron ions were removed by acid treatment through a process of putting the heat-treated particles into an acidic aqueous solution containing hydrochloric acid with a concentration of 10% by weight and stirring at a speed of 3,000 rpm at 40 °C for 1 hour.

[0099] Then, the acid-treated particles were washed three times with distilled water, and then the washed particles were dried to prepare the anode active material.

[0100] (2) Preparation of Anode

[0101] The prepared negative electrode active material, carbon black as a conductive agent, carboxymethyl cellulose (CMC) as a binder, and styrene-butadiene rubber (SBR) were mixed at a weight ratio of 95.8:1:1.7:1.5 to prepare a mixture. Then, 7.8 g of distilled water was added to 5 g of the mixture, and then stirred to prepare a negative electrode slurry. The negative electrode slurry was applied to a 20-μm-thick copper (Cu) metal film as a negative electrode current collector and dried. In this case, the temperature of the circulating air was 60 °C. Subsequently, the negative electrode current collector was roll-pressed and dried in a vacuum oven at 130 °C for 12 hours to prepare a negative electrode.

[0102] Example 2.

[0103] A negative electrode was prepared according to the same procedure as in Example 1, except that in the process of preparing the negative electrode active material, the amount of FeCl3 was changed to 0.1 g.

[0104] Example 3.

[0105] A negative electrode was prepared according to the same procedure as in Example 1, except that in the process of preparing the negative electrode active material, the heat treatment temperature in the process of heat-treating the extracted silicon-based particles was changed to 500 °C.

[0106] Example 4.

[0107] A negative electrode was prepared according to the same procedure as in Example 1, except that in the process of preparing the negative electrode active material, the concentration of hydrochloric acid in the acid aqueous solution in the process of acid-treating the heat-treated particles was changed to 1 wt%.

[0108] Comparative Example 1.

[0109] A negative electrode was prepared according to the same procedure as in Example 1, except that in the process of preparing the negative electrode active material, FeCl3 was not added to the silicon dispersion.

[0110] Comparative Example 2.

[0111] Using a tube furnace in an Ar / CH4 (volume ratio = 3:1) mixed gas atmosphere, 50 g of silicon particles with a particle size D of 4 μm to 5 μm 50 and a specific surface area of 1.5 m 2 / g were subjected to chemical vapor deposition (CVD) treatment at 950 °C for 3 hours to form a carbon coating on the silicon-based particles by thermal decomposition of methane. After that, the obtained silicon-based particles were washed three times with distilled water and dried to prepare a negative electrode active material.

[0112] A negative electrode was prepared according to the same procedure as in Example 1, except that the above-prepared negative electrode active material was used as the negative electrode active material.

[0113] Comparative Example 3.

[0114] The negative electrode was prepared in the same process as in Example 1, except that the step of removing iron ions by acid-treating the heat-treated particles was omitted during the preparation of the negative electrode active material.

[0115] Comparative Example 4.

[0116] The negative electrode was prepared in the same process as in Example 1, except that NiCl2 was added to the silicon dispersion instead of FeCl3 during the preparation of the negative electrode active material.

[0117] <Experimental Example>

[0118] Experimental Example 1: Measurement of specific surface area of negative electrode active material particles

[0119] 3 g of each of the negative electrode active material powders prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were placed in BELSORP-MAX (manufactured by MicrotracBEL Corporation), and a nitrogen adsorption isotherm was obtained in a liquid nitrogen atmosphere at 77 K. Then, the BET specific surface area was calculated using the nitrogen adsorption isotherm and is listed in Table 1 below.

[0120]

[0121] From the results in Table 1, it can be confirmed that the negative electrode active materials prepared in Examples 1 to 4 have a larger specific surface area than the negative electrode active materials prepared in Comparative Examples 1 to 4. The reason is that for Examples 1 to 4, by carbonizing the surface formed with a metal-phenol network (MPN) and then acid-treating, a coating containing a nano-porous structure was formed. It can be confirmed that among them, the negative electrode active material of Example 1 contains a larger amount of iron salt compared to Example 2, so the formation of the coating is easier, the heat treatment temperature is higher compared to Example 3, so the oxygen in tannic acid is more easily reduced, making the binding force between iron ions and OH - relatively reduced, which is beneficial to the removal of iron ions, and compared to Example 4, the acid concentration is higher, and iron ions are easily removed, so it has the largest specific surface area.

[0122] On the contrary, for Comparative Example 1, a small amount of tannic acid is bound to the surface of silicon particles by hydrogen bonds, slightly increasing the specific surface area, but it can be confirmed that the specific surface area is much lower than that of the examples. For Comparative Example 2, since the CVD method was introduced, pores were not formed in the coating. For Comparative Example 3, the iron ion removal step was omitted, and for Comparative Example 4, since nickel was used instead of iron, it can be confirmed that it shows a lower specific surface area than the examples. The reason is that Fe 3+ ions can bind up to three -OH functional groups, while Ni 2+ ions can bind up to two -OH functional groups, so Ni2+ It is relatively difficult for ions to form a complex with tannic acid. Therefore, only a small amount of coating is formed on the silicon surface.

[0123] Experimental Example 2: Determination of iron content in negative electrode active material particles

[0124] 0.1 g of each of the negative electrode active material powders prepared in Examples 1 to 4 and Comparative Example 3 was mixed with 2 mL of distilled water and 1 mL of concentrated nitric acid, and then diluted with 50 mL of ultrapure water. The iron content was confirmed using an ICP-OES (PerkinElmer, Optima 7300DV) instrument and is listed in Table 2 below.

[0125]

[0126] According to the results in Table 2, comparing the negative electrode active materials of Examples 1 to 4 and Comparative Example 3 that used iron salts during the preparation process, it can be confirmed that due to the acid treatment, the iron content of Examples 1 to 4 was significantly lower than that of Comparative Example 3.

[0127] Experimental Example 3: C-rate evaluation

[0128] The negative electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were used respectively to prepare secondary batteries as follows.

[0129] Use Li[Ni 0.6 Mn 0.2 Co 0.2 O2 as the positive electrode active material. The positive electrode active material, carbon black as the conductive agent, and polyvinylidene fluoride (PVdF) as the binder were mixed in a weight ratio of 94:4:2 in an N-methyl-2-pyrrolidone solvent to prepare a positive electrode slurry.

[0130] The prepared positive electrode slurry was applied to an aluminum metal film with a thickness of 15 μm as the positive electrode current collector and dried. In this case, the temperature of the circulating air was 110 °C. Subsequently, the coated and dried positive electrode current collector was roll-pressed and dried in a vacuum oven at 130 °C for 2 hours to form a positive electrode active material layer.

[0131] The negative electrodes of Examples 1 to 4 and Comparative Examples 1 to 4, the positive electrode prepared as above, and a porous polyethylene separator were assembled using the stacking method, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio), lithium hexafluorophosphate (LiPF6 1 mol / L)) was injected into the assembled battery to prepare a secondary battery.

[0132] The prepared secondary battery is charged at a rate of 0.5C (constant current charging at 0.5C, constant voltage charging at 0.05V, cut-off at 0.05C), and discharged at specific C-rates (0.5C, 1C, 2C, 5C) (constant current discharging at the corresponding C-rate, cut-off at 1.5V) to measure the discharge capacity under each rate condition. The discharge capacity is calculated based on the weight of the negative electrode active material, and the capacity manifestation rate is calculated according to the following formula.

[0133] Capacity manifestation rate (%) = {Discharge capacity at the corresponding C-rate / Discharge capacity at 0.5C} × 100

[0134] The results are shown in Table 3 below.

[0135]

[0136] From the results in Table 3, it can be confirmed that under all rate conditions, the capacity manifestation rates of the batteries using the negative electrodes of Examples 1 to 4 are superior to those of the batteries using the negative electrodes of Comparative Examples 1 to 4. Among them, it can be confirmed that the battery using the negative electrode of Example 1 prepared with the negative electrode active material having the largest specific surface area as described above exhibits the best performance.

[0137] Experimental Example 4: Evaluation of capacity retention rate

[0138] The secondary battery prepared in Experimental Example 1 is charged and discharged under the following conditions.

[0139] • Charging conditions: Constant current charging at 0.5C to 4.25V, then charging at 4.2V until the current rate is 0.1C

[0140] • Discharging conditions: Discharging at a current rate of 0.5C to 2.8V

[0141] When the charging capacity of one cycle is 100%, the ratio of the discharging capacity of one cycle is used as the initial efficiency (%), which is listed in Table 3 below.

[0142] In addition, when the above charging and discharging are set as one cycle, 100 cycles are carried out at 25°C. Thereafter, based on 100% of the discharging capacity after 1 cycle, the discharging capacity (capacity retention rate) after 100 cycles is evaluated and shown in Table 4 below.

[0143]

[0144] From the results in Table 4, it can be confirmed that the batteries using the negative electrodes of Examples 1 to 4 not only have better initial efficiency but also have improved life characteristics compared with the batteries using the negative electrodes of Comparative Examples 1 to 4.

Claims

1. A method for preparing a negative electrode active material for a lithium secondary battery, comprising: First step, dispersing silicon-based particles in a solvent to form a silicon dispersion; Second step, adding an iron salt and tannic acid to the silicon dispersion and stirring, thereby forming a reaction layer of the iron salt and tannic acid on the surface of the silicon-based particles; Third step, performing heat treatment on the reaction layer of the iron salt and tannic acid to form an amorphous carbon coating; and Fourth step, performing acid treatment on the amorphous carbon coating to form a porous amorphous carbon coating.

2. The method according to claim 1, wherein the reaction layer of the iron salt and tannic acid contains a metal-phenol network.

3. The method according to claim 1, wherein the solvent in the first step is a buffer solution with a pH of 7 to 12.

4. The method according to claim 1, wherein the weight ratio of the iron salt to tannic acid in the second step is in the range of 1:1 to 1:

50.

5. The method according to claim 1, wherein the heat treatment in the third step is performed at 800 °C to 1,100 °C.

6. The method according to claim 1, wherein the acid treatment in the fourth step is performed by immersing the silicon-based particles formed with the amorphous carbon coating in an acidic aqueous solution containing at least one acid selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid at a concentration of 0.5 wt% to 30 wt%.

7. The method according to claim 1, wherein iron ions are removed by the acid treatment in the fourth step.

8. The method according to claim 1, wherein the silicon-based particles are represented by SiO x (0 ≤ x < 2).

9. The method according to claim 1, wherein the iron salt is at least one selected from the group consisting of FeCl3, FeCl2, Fe(NO3)3, FeSO4, and Fe2(SO4)3.

10. A negative electrode active material for a lithium secondary battery, comprising: Silicon-based particles; and A porous amorphous carbon coating formed on the surface of the silicon-based particles, wherein the BET specific surface area of the negative electrode active material is in the range of 10 m 2 / g to 100 m 2 / g.

11. The negative electrode active material for a lithium secondary battery according to claim 10, wherein the content of iron element measured by ICP analysis is in the range of 20 ppm to 3,000 ppm.

12. A negative electrode for a lithium secondary battery, comprising the negative electrode active material according to claim 10.

13. A lithium secondary battery, comprising the negative electrode according to claim 12.

Images