Anode active material, and anode and lithium secondary battery comprising same

Abstract

The present invention relates to an anode active material comprising: natural graphite particles; and elemental iron, elemental silicon, and elemental sulfur present on the surface, inside, or both on the surface and inside the natural graphite particles, wherein the total content of the elemental iron, the elemental silicon, and the elemental sulfur is more than 5 ppm and 100 ppm or less on the basis of the total weight of the natural graphite particles, and the content of the elemental iron is 5-50 ppm on the basis of the total weight of the natural graphite particles.

Description

Negative electrode active material, negative electrode including the same, and lithium secondary battery

[0001] The present application claims the benefit of priority based on Korean Patent Application No. 10-2024-0194792 filed December 23, 2024 and Korean Patent Application No. 10-2025-0195775 filed December 10, 2025, the entire contents of which are incorporated herein.

[0002] The present invention relates to a negative electrode active material, a negative electrode including the same, and a lithium secondary battery.

[0003] Lithium-ion batteries are used in a wide range of energy storage devices, from portable electronic devices to electric vehicles, and natural graphite is widely utilized as the anode active material, which is a core component. Natural graphite exhibits excellent electrochemical properties due to its relatively low cost, high electrical conductivity, and stable structure. However, natural graphite contains various impurities in its natural state, and these impurities can degrade the performance of lithium-ion batteries and negatively affect their stability.

[0004] Major impurities found in natural graphite include aluminum (Al), iron (Fe), sulfur (S), and silicon (Si). These substances are highly likely to trigger electrochemical reactions within the battery, shortening its lifespan and compromising its stability. Therefore, it is crucial to remove these impurities from natural graphite or control their content to below a certain level.

[0005] Currently, chemical purification processes using acids are primarily used for the purification of natural graphite. While this process can effectively remove impurities, it has a significant negative impact on the environment due to the generation of toxic byproducts. As an alternative to address this problem, a thermal purification process utilizing high temperatures to remove impurities has been proposed; however, this process has the disadvantage of significantly increasing costs due to the high energy consumption required for high-temperature treatment.

[0006] Considering these environmental issues and process costs, various studies are being conducted to minimize the purification process of natural graphite.

[0007] The present invention aims to provide a negative electrode active material having the effect of improving the initial efficiency of a lithium secondary battery, a negative electrode including the same, and a lithium secondary battery.

[0008] [1] The present invention provides a cathode active material comprising natural graphite particles; and iron, silicon, and sulfur elements present on the surface, inside, or on both the surface and inside of the natural graphite particles, wherein the total content of the iron, silicon, and sulfur elements is greater than 5 ppm and less than or equal to 100 ppm based on the total weight of the natural graphite particles, and the content of the iron element is 5 ppm to 50 ppm based on the total weight of the natural graphite particles.

[0009] [2] The present invention provides a negative electrode active material according to [1], wherein the content of the silicon element is 50 ppm or less based on the total weight of the natural graphite particles.

[0010] [3] The present invention provides a negative electrode active material in which, in [1] or [2], the content of the sulfur element is 60 ppm or less based on the total weight of the natural graphite particles.

[0011] [4] The present invention provides a cathode active material in which, in at least one of [1] to [3], the total content of the iron element, silicon element and sulfur element is 10 ppm to 90 ppm based on the total weight of the natural graphite particles.

[0012] [5] The present invention, in at least one of [1] to [4], wherein the BET specific surface area of ​​the negative electrode active material is 1.2 m 2 / g to 2.4m 2 Provides a cathode active material with a g content.

[0013] [6] The present invention is, in at least one of [1] to [5], the D of the negative electrode active material 50 This provides a negative electrode active material having a thickness of 15.0㎛ to 22.0㎛.

[0014] [7] The present invention provides a cathode active material, wherein, in at least one of [1] to [6], the cathode active material further comprises a carbon coating layer on the natural graphite particles.

[0015] [8] The present invention provides a cathode active material comprising 3% to 6% by weight of the carbon coating layer based on the total weight of the cathode active material according to [7].

[0016] [9] The present invention provides a cathode comprising a cathode active material according to at least one of [1] to [8].

[0017]

[0010] The present invention provides a lithium secondary battery comprising a cathode according to [9]; a positive electrode; a separator interposed between the cathode and the positive electrode; and an electrolyte.

[0018] The negative electrode active material according to the present invention comprises natural graphite in which the content of iron, silicon, and sulfur elements present inside and / or on the surface of the particles is appropriately controlled, and accordingly, a lithium secondary battery comprising the negative electrode active material has the effect of exhibiting high initial efficiency.

[0019] Conventional technologies aimed to minimize adverse reactions with the electrolyte by removing as much impurity as possible from natural graphite, but the present invention is significant in that it suggests the optimal content of specific elements that are most desirable in terms of initial efficiency, taking into account the environmental and cost burdens of the purification process performed to obtain natural graphite for cathode active materials from natural minerals.

[0020] Hereinafter, the present invention will be described in more detail to aid in understanding the invention.

[0021]

[0022] In the present invention, “ICP analysis” means that 0.1g of the cathode active material to be analyzed is mixed with 2mL of distilled water and 1mL of concentrated nitric acid, then diluted with 50mL of ultrapure water, and then analyzed using an ICP-OES (PERKIN-ELMER, Optima 7300DV) instrument.

[0023] In the present invention, the “BET specific surface area” is calculated through the BET (Brunauer-Emmett-Teller) multi-point method from the nitrogen adsorption isotherm under a 77K liquid nitrogen atmosphere obtained using a nitrogen adsorption analyzer (e.g., MicrotracBEL, BELSORP-MAX).

[0024] In the present invention, “D 50 "This refers to the particle size corresponding to 50% of the volume accumulation in the volume accumulation particle size distribution of the corresponding particle powder, and can be measured using the laser diffraction method. For example, after dispersing the cathode active material powder in a dispersion medium, it can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500 from Microtrac Inc.) and irradiated with ultrasound of approximately 28 kHz at an output of 60 W to obtain a volume accumulation particle size distribution graph, and then the particle size at the point where the volume accumulation is 50% can be measured from the obtained volume accumulation particle size distribution graph.

[0025]

[0026] The present invention will be described in detail below.

[0027]

[0028] cathode active material

[0029] The present invention relates to a negative electrode active material, specifically a negative electrode active material for a lithium secondary battery.

[0030] The above-described cathode active material comprises natural graphite particles; and iron, silicon, and sulfur elements present on the surface, inside, or both the surface and inside of the natural graphite particles, wherein the total content of the iron, silicon, and sulfur elements is greater than 5 ppm and less than or equal to 100 ppm based on the total weight of the natural graphite particles, and the content of the iron element is 5 ppm to 50 ppm based on the total weight of the natural graphite particles.

[0031] Natural graphite is widely used as a negative electrode active material because it has high crystallinity, excellent conductivity, abundant reserves, and low processing costs. However, since it is obtained from natural minerals, side reactions with the electrolyte caused by impurities can affect battery performance, so it must undergo a purification process, which includes chemical and thermal treatment processes. As such processes cause environmental pollution and increase manufacturing costs, rather than simply aiming to lower the impurity content, it was confirmed that controlling the content of iron, silicon, and sulfur elements among the impurities to a specific range can improve the performance of lithium secondary batteries while minimizing the purification process, and thus the present invention was completed.

[0032]

[0033] The higher the iron element content in the negative electrode active material, the more iron-lithium oxide is generated by lithium and the electrolyte, which can interfere with electrochemical reactions within the electrode and cause a decrease in the initial efficiency of the battery. Additionally, since the Fe metal itself acts as a conductor and can increase the risk of an internal short circuit in the battery, it is preferable that the content of the iron element (Fe) be 50 ppm or less, 45 ppm or less, or 40 ppm or less based on the total weight of the natural graphite particles. However, if Fe exists as nano iron oxide, it can act as an active material rather than an impurity by assisting in lithium intercalation. That is, since the presence of a small amount of Fe has the effect of improving capacity and helping to form a stable SEI layer, it is preferable that the content of the iron element (Fe) be 5 ppm or more, 10 ppm or more, 15 ppm or more, or 30 ppm or more based on the total weight of the natural graphite particles.

[0034] The above iron element content can be achieved by controlling the concentration and amount of acid used in chemical purification. For example, since Fe2O3 is an impurity removed by HCl, the target iron element content can be reached by controlling the concentration and amount of HCl used.

[0035] In addition, when using a thermal purification process, the iron content can be controlled by utilizing the characteristic that each material has a different boiling point. For example, natural graphite with a target iron content can be produced by controlling the temperature during purification by utilizing the fact that iron oxide has a boiling point of about 2,600°C.

[0036] The content of the silicon element (Si) may be 50 ppm or less, 40 ppm or less, 30 ppm or less, or 10 ppm or less based on the total weight of the natural graphite particles. Since a higher Si content in the negative electrode active material causes volume expansion by reacting with lithium, which can damage the SEI layer of the electrode and reduce the initial efficiency of the battery, it is desirable to control the Si content within the above range. However, considering the limitations of the purification process, it may be 0.1 ppm or more.

[0037] The content of the silicon element can be achieved by controlling the concentration and amount of acid reacting with the silicon element or by changing temperature conditions, similar to the method used to control the content of the iron element. Silicon oxide is removed using HF, and the boiling point of SiO2 is about 2,200°C.

[0038] The content of the sulfur element (S) may be 60 ppm or less, 50 ppm or less, 40 ppm or less, or 10 ppm or less based on the total weight of the natural graphite particles. Since a higher S content in the negative electrode active material reacts with the electrolyte to form lithium sulfide as a byproduct, which accelerates the decrease in conductivity and electrolyte decomposition and may lower the initial efficiency of the battery, it is desirable to control the S content within the above range. However, since acid leaching or a heat treatment process at a high temperature is required to remove S during the purification process, it may be 1 ppm or more when considering the efficiency of the purification process.

[0039] The above sulfur content can be achieved by controlling the concentration and amount of acid or base reacting with the sulfur or by changing temperature conditions, similar to the method used to control the iron content. Sulfur can exist in various compounds, and it is known that FeS is removed by HNO3 and H2SO4, and FeS2 is removed by NaOH. The melting point of FeS is approximately 1,200°C.

[0040] The total content of the iron, silicon, and sulfur elements may be 90 ppm or less, 80 ppm or less, or 60 ppm or less based on the total weight of the natural graphite particles, and may be 10 ppm or more, 15 ppm or more, or 30 ppm or more. The above numerical ranges may be combined with one another without limitation. Specifically, the total content of the iron, silicon, and sulfur elements may be 10 ppm to 90 ppm, more specifically 15 ppm to 80 ppm, and even more specifically 15 ppm to 60 ppm based on the total weight of the natural graphite particles.

[0041] The effect of improving the initial efficiency of a lithium secondary battery can be preferably realized by the content of the iron, silicon, and sulfur elements described above.

[0042] The content of the above iron, silicon, and sulfur elements can be measured by ICP analysis.

[0043] The BET specific surface area of ​​the above cathode active material is 1.2 m² 2 / g to 2.4m 2 / g, specifically 1.3m 2 / g to 2.2m 2 / g, more specifically 1.8m 2 / g to 2.2m 2 It can be / g. When the BET specific surface area of ​​the cathode active material is within the above range, it is desirable in terms of storage and lifespan characteristics.

[0044] D of the above negative electrode active material 50It may be 15.0㎛ to 22.0㎛, specifically 16.0㎛ to 21.0㎛, more specifically 17.0㎛ to 20.0㎛.

[0045] The above-mentioned cathode active material may further include a carbon coating layer on the above-mentioned natural graphite particles. The carbon coating layer may be an amorphous carbon coating layer. The carbon coating layer may contribute to improving the structural stability of the natural graphite particles and preventing adverse reactions between the cathode active material and the electrolyte.

[0046] The above-described cathode active material may contain the carbon coating layer in an amount of 3% to 12% by weight, specifically 3% to 8% by weight, and more specifically 3% to 6% by weight, based on the total weight of the above-described cathode active material. Although the presence of the carbon coating layer can improve the structural stability of the cathode active material, if the carbon coating layer is excessive, there is a concern that the initial efficiency will decrease and high-temperature storage performance will deteriorate due to an increase in the specific surface area during cathode rolling; therefore, it is desirable to form the carbon coating layer with an amount within the above-described range.

[0047] The above carbon coating layer can be formed by mixing a carbon coating layer precursor with natural graphite particles and then heat-treating it.

[0048] The carbon coating layer precursor may include at least one selected from polymer resin and pitch. Specifically, the polymer resin may include at least one selected from the group consisting of sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, polyacrylonitrile resin, polyamide resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin, vinyl chloride resin, and polyvinyl chloride. The pitch may include at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, and mesophase pitch. The heat treatment process for forming the carbon coating layer can be carried out at 1,000°C to 1,500°C in order to promote the uniform formation of the carbon coating layer.

[0049]

[0050] cathode

[0051] The present invention provides a cathode comprising the aforementioned cathode active material, specifically a cathode for a lithium secondary battery.

[0052] Specifically, the cathode comprises a cathode current collector; and a cathode active material layer provided on at least one surface of the cathode current collector, and the cathode active material layer may include the aforementioned cathode active material.

[0053] The above-mentioned negative current collector can be any negative current collector commonly used in the field without limitation, and is not particularly limited as long as it has high conductivity without causing chemical changes in the lithium secondary battery, for example. For example, the above-mentioned negative current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, preferably copper.

[0054] The above-mentioned negative current collector may have fine irregularities formed on its surface to strengthen the bonding force of the negative active material, and may be in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0055] The above-mentioned cathode current collector may have a thickness of 3㎛ to 500㎛.

[0056] The above-mentioned negative electrode active material layer may further include a silicon-based active material together with the aforementioned negative electrode active material.

[0057] The above silicon-based active material is, for example, silicon (Si) and silicon oxide (SiO₂). x , 0 <x<2) 및 실리콘-탄소 복합체 중 적어도 1종을 포함할 수 있다.

[0058] The above negative electrode active material or a mixture of the above negative electrode active material and the silicon-based active material may be included in the above negative electrode active material layer in an amount of 80% to 99% by weight, preferably 88% to 98% by weight.

[0059] Furthermore, the description of the cathode active material is as previously stated.

[0060] The above-mentioned negative electrode active material layer may further include a binder, a conductive material, and / or a thickener in addition to the aforementioned negative electrode active material.

[0061] The above binder is a component that assists in bonding between the active material and / or the current collector, and can typically be included in the negative active material layer in an amount of 1% to 30% by weight, preferably 1% to 10% by weight.

[0062] The binder may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, and fluororubber, preferably at least one selected from polyvinylidene fluoride and styrene-butadiene rubber.

[0063] As the above-mentioned thickener, any thickener conventionally used in lithium secondary batteries can be used, and an example is carboxymethylcellulose (CMC).

[0064] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be included in the negative electrode active material layer in an amount of 1% to 30% by weight, preferably 1% to 10% by weight.

[0065] The above conductive material is not particularly limited as long as it possesses conductivity without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0066] The thickness of the above-mentioned negative electrode active material layer may be 10㎛ to 300㎛, specifically 50㎛ to 200㎛, but is not limited thereto.

[0067] The above-mentioned negative electrode active material layer may be prepared by applying, rolling, and drying a negative electrode slurry, prepared by adding a negative electrode active material, optionally a binder, a thickener, and / or a conductive material to a solvent, onto the above-mentioned negative electrode current collector. At this time, the solvent may include water or an organic solvent such as NMP (N-methyl-2-pyrrolidone), and more specifically, may be water.

[0068]

[0069] lithium secondary battery

[0070] The present invention provides a lithium secondary battery comprising the aforementioned negative electrode.

[0071] The above lithium secondary battery may include the aforementioned negative electrode; a positive electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

[0072] The above anode can be opposite to the above cathode.

[0073] The above positive electrode may include a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector.

[0074] The above positive current collector can be any positive current collector commonly used in the field without limitation, and is not particularly limited as long as it has high conductivity without causing chemical changes in the secondary battery, for example. For example, the above positive current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, preferably aluminum.

[0075] The above positive current collector may have fine irregularities formed on its surface to strengthen the bonding force of the positive active material, and may be in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0076] The above positive current collector can generally have a thickness of 3㎛ to 500㎛.

[0077] The above positive active material layer may include a positive active material.

[0078] The above-mentioned cathode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium composite metal oxide comprising lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium composite metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), or a lithium-nickel-manganese-based oxide (e.g., LiNi 1-Y Mn Y O2(here, 0 <Y<1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2(here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2(here, 0 <Y2<1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r1 )O2(where, 0<p<1, 0<q<1, 0<r1<1, p+q+r1=1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0<p1<2, 0<q1<2, 0<r2<2, p1+q1+r2=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2Examples include )O2 (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are each atomic fractions of independent elements, such that 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1), etc.), and any one or more of these compounds may be included. Among these, the lithium composite metal oxides are LiCoO2, LiMnO2, LiNiO2, and lithium nickel manganese cobalt oxide (for example, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), or lithium nickel-cobalt-aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 It may be )O2, etc., and considering the significant improvement effect resulting from controlling the type and content ratio of constituent elements forming the lithium composite metal oxide, the lithium composite metal oxide is Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 It may be O2, etc., and any one of these or a mixture of two or more may be used.

[0079] The above positive active material may be included in the above positive active material layer in an amount of 80% to 99% by weight.

[0080] The above positive active material layer may further include at least one type selected from the group consisting of a binder and a conductive material together with the above positive active material.

[0081] The above binder is a component that assists in the bonding of the active material and the conductive material, and in the bonding to the current collector, and is typically added in an amount of 1 to 30 weight percent based on the total weight of the anode composite. Examples of such binders may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene ter polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0082] The above binder may be included in the above positive active material layer in an amount of 1% to 30% by weight.

[0083] The above conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0084] The above conductive material may be added to the above positive active material layer in an amount of 1% to 30% by weight.

[0085] The above separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. It can be used without special limitations as long as it is a separator typically used in lithium secondary batteries, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte wettability. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like 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, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and it may optionally be used in a single-layer or multi-layer structure.

[0086] In addition, the electrolytes used in the present invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which are usable when manufacturing lithium secondary batteries, but are not limited to these.

[0087] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0088] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, gamma-butyrolactone, ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon elemental solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; and nitriles such as R-CN (where R is a hydrocarbon elemental group with a straight, branched, or ring structure having C2 to C20, and may include a double aromatic ring or ether bond). Amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant to improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred. In this case, using a mixture of the cyclic carbonate and the chain carbonate in a volume ratio of about 1:1 to about 1:9 can result in excellent electrolyte performance.

[0089] The above lithium salt can be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO2, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. It is preferable to use the lithium salt within the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and allow lithium ions to move effectively.

[0090] As described above, the lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, rapid charging characteristics, and capacity retention rate, making it useful in fields such as portable devices like mobile phones, laptop computers, and digital cameras, as well as electric vehicles like hybrid electric vehicles (HEVs), and particularly suitable for use as a constituent battery of a medium-to-large battery module. Accordingly, the present invention also provides a medium-to-large battery module comprising such a lithium secondary battery as a unit battery.

[0091] These medium-to-large battery modules can be advantageously applied to power sources requiring high output and large capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices.

[0092]

[0093] The present invention will be explained in more detail below through specific embodiments.

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

[0095] As the negative electrode active material for Examples 1 and 2 and Comparative Examples 1 to 6, an amorphous carbon coating layer was formed on natural graphite particles with iron (Fe), silicon (Si), and sulfur (S) content as shown in Table 1 below. The amorphous carbon coating is performed by mixing pitch with spherical natural graphite and heat treating it. The coating amount and coating uniformity may vary depending on the type of pitch used, the heat treatment temperature, and the time. At this time, the content of each element was measured using the analysis method using the ICP-OES instrument described above. The amorphous carbon coating layer was prepared by mixing each natural graphite particle with pitch and then heat treating it at approximately 1,200°C, and the content of the amorphous carbon coating layer was 3 to 6 wt% based on the total weight of the negative electrode active material.

[0096] Fe [ppm]Si [ppm]S [ppm]Fe + Si + S[ppm]BET Specific Surface Area [m 2 / g]D 50 [㎛] Example 1 381 < 5 < 442.118.3 Example 2 182335761.418.3 Comparative Example 1 < 510243 < 1502.219.0 Comparative Example 2 < 513654 < 1952.117.7 Comparative Example 3 < 51 < 5 < 112.017.6 Comparative Example 4 22337 < 5 < 2651.519.6 Comparative Example 5 12121902232.219.0 Comparative Example 6 532318941.918.0

[0097] ※ In the case of Fe and S, levels below 5 ppm cannot be determined due to the detection limit of the device.

[0098]

[0099] <Experimental Example: Initial Efficiency Evaluation>

[0100] (1) Manufacturing of lithium secondary batteries

[0101] A cathode slurry was prepared by adding the cathode active material according to Example 1, carbon black as a conductive material, styrene-butadiene rubber as a binder, and CMC as a thickener to water as a solvent in a weight ratio of 96.6:1.0:1.3:1.1. The cathode slurry was coated onto a copper current collector, dried at 130°C, and rolled to produce a cathode.

[0102] As a counter electrode for the above cathode, lithium metal was prepared.

[0103] An electrode assembly was manufactured by interposing a porous polyethylene separator between the above-mentioned cathode and the above-mentioned lithium metal counter electrode, the electrode assembly was placed inside a coin-type case, and a non-aqueous electrolyte was injected into the case and sealed to manufacture the lithium secondary battery of Example 1.

[0104] As the above-mentioned non-aqueous electrolyte, a solution was used in which LiPF6 was dissolved at a concentration of 1.0 M in an organic solvent mixed with ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 20:80, and vinylene carbonate (VC) was added at a content of 0.5 wt%.

[0105]

[0106] Lithium secondary batteries of Examples 2 and Comparative Examples 1 to 6 were manufactured in the same manner as Example 1, except that the negative electrode active materials of Examples 2 and Comparative Examples 1 to 6 were used instead of the negative electrode active material according to Example 1.

[0107]

[0108] (2) Initial efficiency evaluation

[0109] The lithium secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 6 prepared above were charged at 25°C in CC / CV mode at 0.1C (0.005V, 0.005C cut-off) and discharged in CC mode at 0.1C (1.5V cut-off) to check the initial efficiency.

[0110] The results are shown in Table 2 below.

[0111] Initial Efficiency [%] Example 194.2 Example 294.0 Comparative Example 193.0 Comparative Example 292.0 Comparative Example 392.8 Comparative Example 492.9 Comparative Example 593.0 Comparative Example 693.1

[0112] Through Table 2, it can be confirmed that the lithium secondary batteries of Examples 1 and 2 containing the negative electrode active material according to the present invention have superior initial efficiency compared to Comparative Examples 1 to 6.

[0113] Specifically, the lithium secondary batteries of Comparative Examples 1, 2, 4, and 5, which contain natural graphite with a total content of Fe, Si, and S exceeding 100 ppm as a negative electrode active material, were measured to have lower initial efficiencies compared to the examples.

[0114] In the case of Comparative Example 3, the content of Si and S in the cathode active material is at the same level as in Example 1, and the total content of Fe, Si, and S does not exceed 100 ppm, but since the Fe content is less than 5 ppm, it can be confirmed that the initial efficiency is significantly lower than that of Example 1.

[0115] As shown in Comparative Example 4, it was found that even when the Fe content is excessively high, the initial efficiency actually decreases.

[0116] Meanwhile, Comparative Example 5, which used natural graphite as the cathode active material with an Fe content of 5 to 50 ppm but a much higher content of Si and S compared to the examples, also showed that the initial efficiency was not as good as that of the examples.

[0117] In addition, it can be seen that Comparative Example 6, in which the content of Si and S is similar to that of the Examples but the content of Fe exceeds 50 ppm, also has a poorer initial efficiency compared to the Examples.

Claims

1. Natural graphite particles; and As a negative electrode active material comprising iron, silicon, and sulfur elements present on the surface, inside, or on both the surface and inside of the above natural graphite particles, The total content of the above iron, silicon, and sulfur elements is greater than 5 ppm and less than or equal to 100 ppm based on the total weight of the above natural graphite particles, and A negative electrode active material having a content of the iron element of 5 ppm to 50 ppm based on the total weight of the natural graphite particles.

2. In Claim 1, A negative electrode active material having a silicon element content of 50 ppm or less based on the total weight of the natural graphite particles.

3. In Claim 1, A negative electrode active material having a sulfur content of 60 ppm or less based on the total weight of the natural graphite particles.

4. A negative electrode active material having a total content of the iron element, silicon element, and sulfur element of 10 ppm to 90 ppm based on the total weight of the natural graphite particles.

5. In Claim 1, The BET specific surface area of ​​the above cathode active material is 1.2 m² 2 / g to 2.4m 2 / g, negative electrode active material.

6. In Claim 1, D of the above negative electrode active material 50 A negative electrode active material having a thickness of 15.0㎛ to 22.0㎛.

7. In Claim 1, The above-described negative electrode active material further comprises a carbon coating layer on the above-described natural graphite particles.

8. In Claim 7, A negative electrode active material comprising 3% to 6% by weight of the carbon coating layer based on the total weight of the negative electrode active material.

9. A cathode comprising a cathode active material according to claim 1.

10. A lithium secondary battery comprising a cathode according to claim 9; a positive electrode; a separator interposed between the cathode and the positive electrode; and an electrolyte.