Negative electrode active material for lithium secondary batteries, method for manufacturing the same, and lithium secondary battery containing the same

Abstract

The present invention relates to a negative electrode active material for lithium secondary batteries, comprising natural graphite, wherein the natural graphite satisfies formula 1.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same.

Background Art

[0002] As the development and demand for technologies for mobile devices increase, the demand for secondary batteries that can be recharged and are capable of miniaturization and large capacity has been rapidly increasing. Recently, the use of secondary batteries as power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs) has become a reality.

[0003] Thereby, many studies have been conducted on secondary batteries that can meet various requirements. In particular, the demand for lithium secondary batteries having a high energy density, a high discharge voltage, and high output has been increasing. In addition, lithium secondary batteries used in electric vehicles and the like have characteristics of having a high energy density and being able to exhibit a high output in a short time, and must be used for more than 10 years under severe conditions where charge and discharge by a large current are repeated in a short time. Therefore, output characteristics and long-term life characteristics that are much superior to those of existing small lithium secondary batteries are inevitably required.

[0004] In particular, as the demand for high-density energy batteries has recently increased, research on methods for increasing the capacity by using silicon-based materials such as Si, Si-C composites, and SiOx, which have a capacity more than 10 times larger than that of graphite-based materials as negative electrode active materials, has been actively promoted. However, in the case of silicon-based materials that are high-capacity materials, although the capacity characteristics are excellent when compared with existing graphite, the volume rapidly expands during the charging process, severing the conductive path, deteriorating the battery characteristics, and thereby deteriorating the life characteristics of the battery.

[0005] Therefore, there is a need for research on a mixed negative electrode active material of graphite and a silicon-based material that can appropriately ensure both the capacity and life characteristics of the battery.

Summary of the Invention

[0006] Therefore, one problem of the present invention is to provide a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same, which can appropriately ensure both the capacity and the life characteristics of the battery. [Means for Solving the Problems]

[0007] <> One embodiment of the present invention provides a negative electrode active material for a lithium secondary battery, which contains natural graphite, and the natural graphite satisfies the following formula 1.

[0008] [Formula 1] 0.04 g / cm ,

[0014] , , 2 , , 2 ,

[0010] , , 3 , ,

[0012] , press , , , , , , 3 , tap , , , press , , , tap ,

[0009] ,

[0013] , , 3 , ,

[0008] ,

[0015] , , ,

[0011] ≦D press -D tap ≦0.085 g / cm 3

[0009] In the above formula 1, D press is the compression density of natural graphite under an applied pressure of 5.5 kgf / cm 2 and D tap is the tap density of natural graphite.

[0010] The pH of the natural graphite may be 7.0 or more.

[0011] The surface of the natural graphite may not include a coating layer containing a low-crystalline carbon material.

[0012] kThe BET specific surface area of the natural graphite may be 4.8 m 2 / g or more.

[0013] The oil absorption amount of the natural graphite may be 50.0 mL / 100 g or more.

[0014] The average particle size D50 of the natural graphite may be 16.0 to 21.0 μm.

[0015] The tap density of the natural graphite is 0.95 g / cm 3The following is also acceptable.

[0016] The degree of spheroidization of the aforementioned natural graphite may be 0.85 or higher.

[0017] The negative electrode active material for the lithium secondary battery may further contain a silicon-based active material.

[0018] The silicon-based active material may be silicon, a silicon-carbon composite, a silicon oxide, or a combination thereof.

[0019] Another embodiment of the present invention provides a method for producing a negative electrode active material for a lithium secondary battery, comprising the steps of: preparing natural graphite powder; and heat-treating the natural graphite powder at a temperature of 500 to 650°C in an inert gas atmosphere.

[0020] The inert gas may be nitrogen.

[0021] The heat treatment may be carried out for 1 to 12 hours.

[0022] The process may further include a step of mixing a silicon-based active material after the heat treatment step.

[0023] The silicon-based active material may be silicon, a silicon-carbon composite, a silicon oxide, or a combination thereof.

[0024] Another embodiment of the present invention provides a negative electrode for a lithium secondary battery containing the negative electrode active material described above.

[0025] Another embodiment of the present invention provides a lithium secondary battery including the negative electrode for the lithium secondary battery. [Effects of the Invention]

[0026] The negative electrode active material for a lithium secondary battery according to one embodiment of the present invention can improve the battery's capacity and lifespan characteristics by appropriately adjusting the physical properties represented by Equation 1. [Modes for carrying out the invention]

[0027] The terms first, second, third, etc., are used to describe various parts, components, regions, layers, and / or sections, but are not limited to these. These terms are used solely to distinguish one part, component, region, layer, or section from other parts, components, regions, layers, or sections. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section, as long as it does not fall outside the scope of the present invention.

[0028] The technical terms used herein are for the sole purpose of referring to specific embodiments and are not intended to limit the invention. The singular forms used herein also include plural forms unless the text explicitly indicates otherwise. The meaning of “including” as used in this specification does not mean to embody a particular characteristic, area, integer, step, operation, element, and / or component, thereby excluding the presence or addition of other characteristics, areas, integers, steps, operations, elements, and / or components.

[0029] When one part is described as being "on top of" another part, it means that it is either directly on top of the other part or that the other part is in between. In contrast, when one part is described as being "directly on top of" another part, there is no other part in between.

[0030] Although not defined differently, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are additionally interpreted as having the meaning consistent with the relevant technical literature and the present disclosure, and are not interpreted in their ideal or highly formal sense unless otherwise defined.

[0031] Also, unless otherwise specified, % means weight percent, and 1 ppm is 0.0001 weight percent.

[0032] In this specification, the term “these combinations” as used in a maxi expression means one or more mixtures or combinations selected from the group of components described in the maxi expression, and includes one or more of the components selected from the group of components.

[0033] The embodiments of the present invention will be described below in detail so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be realized in a variety of different forms and is not limited to the embodiments described herein.

[0034] 1.Negative electrode active material Recently, as the development of lithium-ion batteries for electric vehicles has accelerated, the demand for higher energy density has increased. Therefore, research is actively underway on methods to increase capacity by mixing silicon-based active materials such as Si, Si-C composites, and SiOx, which have more than 10 times the capacity of naturally occurring graphite-based active materials that have been commonly used as negative electrode active materials.

[0035] However, in the case of silicon-based active materials, which are high-capacity materials, while their capacity characteristics are superior to those of graphite, which is currently used, there is a problem in that their volume rapidly expands during repeated charging and discharging processes, disrupting the conductive path and degrading their lifespan characteristics.

[0036] Furthermore, conventional natural graphite-based active materials were generally coated with low-crystallinity carbon materials such as soft carbon and hard carbon to improve output and lifespan characteristics by suppressing side reactions with the electrolyte. However, such low-crystallinity carbon-coated natural graphite has a problem in that the hardness of the powder increases due to the coating, and when used in combination with silicon-based active materials, it cannot adequately act as a buffer for the volume expansion of the silicon-based active material due to the progression of charging and discharging, resulting in deterioration of capacity, initial efficiency, and lifespan characteristics.

[0037] Therefore, the inventors conducted extensive research on the physical properties of natural graphite, which can be used in combination with silicon-based active materials to achieve good battery capacity and life characteristics. As a result, they discovered that the above effects can be preferably achieved when the softness characteristics of the powder represented by the following formula 1 satisfy a specific range, and thus completed the present invention.

[0038] One embodiment of the present invention provides a negative electrode active material for a lithium secondary battery, comprising natural graphite, wherein the natural graphite satisfies the following formula 1.

[0039] [Formula 1] 0.04 g / cm³ 3 ≤D press -D tap ≤0.085 g / cm³ 3

[0040] In the above equation 1, D press This is 5.5 kgf / cm² 2 D is the compressive density of natural graphite at the applied pressure. tap This is the tap density of natural graphite. In this specification, the compressed density of natural graphite is given by a diameter of 2 cm 2 The tap density of natural graphite can be measured after placing 2g of active material powder into a circular mold and applying a specific pressure. According to ASTM B527, the tap density of natural graphite can be measured after placing 15g of active material powder into a 50mL container and tapping it at 3000 cycles @ 284 cycles / min.

[0041] By satisfying the above equation 1, natural graphite can improve the capacity and lifespan characteristics of the battery.

[0042] The value of Equation 1 above can indicate the softness property of the active material powder. Specifically, a larger value of Equation 1 means that the active material powder is softer. In this case, if the value of Equation 1 for natural graphite is sufficiently large as within the range described above, the natural graphite will contract and absorb the pressure generated by the volume expansion of the silicon-based active material during charging and discharging of the negative electrode, thereby reducing the volume expansion rate of the electrode during charging and discharging. This can improve the capacity and lifespan characteristics of the battery. However, if the value of Equation 1 for natural graphite is excessively large, the graphite itself may be kept in a compressed state during the manufacturing of the electrode, which may actually degrade the capacity and lifespan characteristics of the battery.

[0043] The value in Equation 1 above is, more specifically, 0.05 to 0.085 g / cm³. 3 Or 0.06 to 0.085 g / cm³ 3 That's fine.

[0044] Furthermore, the pH of the natural graphite may be 7.0 or higher, and more specifically, 7.1 or 7.2 or higher. By satisfying the above range for the pH of the natural graphite, acidic groups present on the surface of the natural graphite are removed, thereby improving the capacity and life characteristics of the battery. In this specification, the pH of the natural graphite can be measured using a Metrohm, 780 pH meter after thoroughly dispersing 10 g of active material powder with 100 mL of distilled water in an ultrasonic cleaner for 30 minutes.

[0045] Furthermore, the BET specific surface area of ​​the aforementioned natural graphite is 4.8 m². 2 It may be greater than or equal to / g, more specifically, 4.82m 2It may be greater than or equal to / g. By satisfying the above range, the BET specific surface area of ​​natural graphite ensures a certain level of pores, maintains softness, mixes with silicon-based active materials, suppresses the volume expansion rate of electrodes during charging and discharging, and improves the capacity and life characteristics of the battery. In this specification, the BET specific surface area of ​​natural graphite can be measured using the BET method (Surface area and Porosity analyzer) (Micromeritics, ASAP2020).

[0046] Furthermore, the oil absorption capacity of the natural graphite may be 50.0 mL / 100 g or more, and more specifically, 51.0 mL / 100 g or 52.0 mL / 100 g or more. By satisfying the above range for the oil absorption capacity of the natural graphite, the electrolyte impregnation and lithium ion conductivity can be increased, thereby improving the capacity and life characteristics of the battery. In this specification, the oil absorption capacity of the natural graphite can be measured by adding 30 g of active material powder and linseed oil (Asahi Research Institute, S-500).

[0047] Furthermore, the average particle size D50 of the natural graphite may be 16.0 to 21.0 μm, and more specifically, 17.0 to 20.0 μm. If the average particle size of the natural graphite is excessively small, it may be necessary to increase the amount of binder during electrode manufacturing, or, under the same conditions, the electrode adhesion strength may decrease. If the average particle size of the natural graphite is excessively large, it may be necessary to increase the lithium diffusion distance, which may lead to a decrease in input / output characteristics. The average particle size D50 of the natural graphite can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve. The average particle size D50 can be measured, for example, using the laser diffraction method.

[0048] Furthermore, the tap density of the aforementioned natural graphite is 0.95 g / cm³. 3 It may also be less than or equal to the following, more specifically, 0.93 g / cm³. 3The following is also acceptable. Generally, a higher tap density is preferable for the active material in order to maximize the electrode energy density. However, the negative electrode active material according to the present invention maintains a soft state of the active material powder by satisfying the above range of tap density, and the aforementioned battery performance improvement effect can be more preferably realized by suppressing the volume expansion of the silicon-based active material.

[0049] On the other hand, the degree of spheroidization of the natural graphite may be 0.85 or higher. By satisfying the above range for the degree of spheroidization of the natural graphite, the planes forming the graphite layer are oriented in various directions, thereby further maximizing capacity and output characteristics and improving electrode energy density. In this specification, the degree of spheroidization is a numerical representation of the degree to which a particle is close to being spherical, and refers to the value obtained by dividing the circumference of a circle having the same area as the particle projection shape by the actual circumference of the particle projection shape, using a flow-type particle analyzer. Such a degree of spheroidization can be measured using an analyzer for obtaining an optical image (Fluid Imaging Technologies, Flowcam 8100) and an analysis S / W (visual spreadsheet).

[0050] On the other hand, the negative electrode active material for lithium secondary batteries according to the present invention may not include a low-crystallinity carbon material-containing coating layer on the surface of the natural graphite. The technical significance of the negative electrode active material of the present invention not including a separate coating layer has been described above and will be omitted here.

[0051] The negative electrode active material for the lithium secondary battery may further contain a silicon-based active material.

[0052] The silicon-based active material is not particularly limited as long as it is commonly used in the industry. For example, the silicon-based active material may be, but is not necessarily limited to, silicon, silicon-carbon composites, silicon oxides, or combinations thereof.

[0053] 2. Method for producing negative electrode active material Another embodiment of the present invention provides a method for producing a negative electrode active material for a lithium secondary battery, comprising the steps of: preparing natural graphite powder; and heat-treating the natural graphite powder at a temperature of 500 to 650°C in an inert gas atmosphere.

[0054] The following describes in detail, step by step, a method for producing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention.

[0055] First, prepare the natural graphite powder.

[0056] In this case, the prepared natural graphite may be natural graphite raw material powder that has been shaped into primary spheres.

[0057] The aforementioned primary spheroidization may be carried out using a general method for spheroidizing plate-shaped natural graphite raw material powder in order to use spheroidized graphite material during the production of the negative electrode active material.

[0058] Next, the natural graphite powder is heat-treated at a temperature of 500 to 650°C under an inert gas atmosphere.

[0059] The inert gas is not particularly limited, but more specifically, it may be nitrogen. When nitrogen is used as the inert gas, there is an advantage in that the graphite surface can be kept in a stable state without oxidation.

[0060] At this time, the heat treatment may be carried out at a temperature of 500 to 650°C, or more specifically, at a temperature of 550 to 650°C. If the heat treatment temperature is excessively low, the pH may be maintained at a low level, which may lead to problems with poor lifespan characteristics. If the heat treatment temperature is excessively high, the specific surface area may increase and the amount of oil absorbed may decrease, which may lead to an increase in side reactions of the battery and deterioration of its lifespan characteristics.

[0061] The heat treatment may be carried out for 1 to 12 hours, or more specifically, 3 to 11 hours. If the heat treatment time is too short, the reaction time will be short, which may result in a low pH and poor lifespan characteristics. If the heat treatment time is too long, the performance difference may be similar, but there may be problems with increased productivity and manufacturing costs.

[0062] By appropriately controlling the gas atmosphere, heat treatment temperature, and heat treatment time as described above, the properties of the resulting negative electrode active material, as shown in Equation 1 and other properties, can be easily obtained within the scope of the present invention. This makes it possible to improve the capacity and life characteristics of the battery.

[0063] On the other hand, the step of mixing a silicon-based active material after the heat treatment step may further be included.

[0064] The silicon-based active material is not particularly limited as long as it is commonly used in the industry. For example, the silicon-based active material may be, but is not necessarily limited to, silicon, silicon-carbon composites, silicon oxides, or combinations thereof.

[0065] 3. Negative electrode and lithium secondary battery Another embodiment of the present invention provides a negative electrode for a lithium secondary battery containing the negative electrode active material described above.

[0066] A negative electrode for a lithium secondary battery according to yet another embodiment of the present invention may more specifically include a negative electrode current collector; and a negative electrode active material layer located on the negative electrode current collector and containing the aforementioned negative electrode active material for a lithium secondary battery.

[0067] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. The negative electrode current collector can also typically have a thickness of 3 to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0068] The negative electrode active material layer may selectively include a binder and / or a conductive material together with the negative electrode active material.

[0069] The binder plays a role in improving adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these can be used individually or in mixtures of two or more. The binder may be present in an amount of 1 to 30% by weight relative to the total weight of the negative electrode active material layer.

[0070] The conductive material is used to impart conductivity to the electrodes and can be used without special limitations as long as it has electronic conductivity without causing chemical changes in the battery it is configured in. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more can be used. The conductive material may usually be included in an amount of 1 to 30% by weight relative to the total weight of the negative electrode active material layer.

[0071] A negative electrode for a lithium secondary battery according to one embodiment of the present invention can be manufactured by a conventional negative electrode manufacturing method, except that the negative electrode active material described above is used.

[0072] Specifically, it can be manufactured by applying a negative electrode active material layer-forming composition, which includes the aforementioned negative electrode active material and selectively contains a binder, conductive material, and solvent, onto a negative electrode current collector, followed by rolling and drying. At this time, the types and contents of the negative electrode active material, binder, and conductive material are as described above.

[0073] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. One of these can be used alone or in mixtures of two or more. The amount of solvent used should be sufficient to dissolve or disperse the negative electrode active material, conductive material, and binder, taking into consideration the coating thickness and yield of the slurry, and to have a viscosity that allows for excellent thickness uniformity during subsequent coating for the manufacture of the negative electrode.

[0074] Alternatively, the negative electrode may be manufactured by casting the negative electrode active material layer forming composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0075] Another embodiment of the present invention provides a lithium secondary battery including the negative electrode.

[0076] A lithium secondary battery according to yet another embodiment of the present invention may more specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive and negative electrodes, and an electrolyte.

[0077] The negative electrode is as described above.

[0078] Furthermore, the lithium secondary battery may selectively further include a battery container for housing an electrode assembly comprising a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery container.

[0079] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and the positive electrode active material layer may include positive electrode active material.

[0080] The positive electrode current collector is not particularly limited as long as it is conductive without causing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can also typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0081] As the positive electrode active material, a compound capable of reversibly intercalating and deintercalating lithium (a lithium-intercalated compound) can be used. Specifically, one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof can be used, and a concrete example thereof is a compound represented by any one of the following chemical formulas.

[0082] Li a A 1-b B b D2(In the above equation, 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5.); Li a E 1-b B b O 2-c D c (In the above equation, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05.);LiE 2-b B b O 4-c D c (In the above equation, 0 ≤ b ≤ 0.5 and 0 ≤ c ≤ 0.05.); Li a Ni 1-b-c Co b B c D α (In the above equation, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2.); Li a Ni 1-b-c Co b B c O 2-α T α (In the above equation, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2.); Li a Ni 1-b-c Co b B c O 2-α T2(In the above equation, 0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, and 0<α<2.);Li a Ni 1-b-c Mn b B c D α(In the above equation, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α ≤ 2.); Li a Ni 1-b-c Mn b B c O 2-α T α (In the above equation, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 < α < 2.); Li a Ni 1-b-c Mn b B c O 2-α T2(In the above equation, 0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05, and 0<α<2.);Li a Ni b E c G d O2(In the above equation, 0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, and 0.001≦d≦0.1.);Li a Ni b Co c Mn d GeO2(In the above equation, 0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, and 0.001≦e≦0.1.);Li a NiG b O2(In the above equation, 0.90≦a≦1.8 and 0.001≦b≦0.1.);Li a CoG b O2(In the above equation, 0.90≦a≦1.8 and 0.001≦b≦0.1.);Li a MnG b O2(In the above equation, 0.90≦a≦1.8 and 0.001≦b≦0.1.);Li a Mn2G b O4 (In the above formula, 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1.); QO2; QS2; LiQS2; V2O5; LiV2O5; LiIO2; LiNiVO4; Li (3-f) J2(PO4)3(0≦f≦2);Li (3-f) Fe2(PO4)3 (0≦f≦2); and LiFePO4.

[0083] In the above chemical formulas, A is Ni, Co, Mn or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; T is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; I is Cr, V, Fe, Sc, Y or a combination thereof; J may be V, Cr, Mn, Co, Ni, Cu or a combination thereof.

[0084] Of course, it is also possible to use a compound having a coating layer on its surface, or to use a mixture of the compound and a compound having a coating layer.

[0085] The coating layer may contain at least one coating element compound selected from the group consisting of oxides of coating elements, hydroxides of coating elements, oxyhydroxys of coating elements, oxycarbonates of coating elements, and hydroxycarbonates of coating elements. The compounds forming these coating layers may be amorphous or crystalline. The coating elements included in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The coating layer formation step may use any coating method (e.g., spray coating, immersion method, etc.) that does not adversely affect the physical properties of the positive electrode active material using such elements, and this is a matter that will be well understood by those engaged in this field, so a detailed explanation will be omitted.

[0086] The positive electrode active material layer may further include a binder and / or a conductive material along with the positive electrode active material described above.

[0087] The binder plays a role in improving adhesion between positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and can be used individually or in mixtures of two or more of these, but is not limited thereto. The binder may be present in an amount of 1 to 30% by weight relative to the total weight of the positive electrode active material layer.

[0088] The conductive material is used to impart conductivity to the electrodes and can be used without special limitations as long as it has electronic conductivity without causing chemical changes in the battery it is configured in. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these can be used alone or a mixture of two or more, but it is not limited to these. The conductive material may usually be included in an amount of 1 to 30% by weight relative to the total weight of the positive electrode active material layer.

[0089] The positive electrode can be manufactured by a conventional method for manufacturing a positive electrode.

[0090] Specifically, the positive electrode can be manufactured by applying a composition for forming a positive electrode active material layer, which optionally contains a positive electrode active material and a binder, conductive material, or solvent, onto a positive electrode current collector, followed by drying and rolling. At this time, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0091] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. One of these can be used alone or in mixtures of two or more. The amount of solvent used should be sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, taking into consideration the coating thickness and yield of the slurry, and to have a viscosity that allows for excellent thickness uniformity during subsequent coating for the manufacture of the positive electrode.

[0092] Alternatively, the positive electrode may be manufactured by casting the positive electrode active material layer forming composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0093] The separator separates the positive and negative electrodes and provides a passage for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without special limitations, and those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity are particularly preferred. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separators containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used as single-layer or multi-layer structures.

[0094] The aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0095] Specifically, the organic liquid electrolyte may contain an organic solvent and a lithium salt.

[0096] The organic solvent can be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move, without any special limitations. Specifically, the organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; tolyls such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group of C2 to C20, and may include a double-bonded aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9 can produce an electrolyte with excellent performance.

[0097] The lithium salt can be any compound capable of providing lithium ions for use in lithium secondary batteries, without any special limitations. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0098] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethyl phosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1 to 5% by weight relative to the total weight of the electrolyte. [Examples]

[0099] Embodiments of the present invention will be described in more detail below through the examples. However, the following examples are merely preferred embodiments of the present invention, and the present invention is not limited to the following examples.

[0100] Example 1 (1) Manufacturing of negative electrode active material (Preparation of natural graphite powder) Primary spheroidized natural graphite powder with an average particle size D50 of 17 μm was prepared.

[0101] (Heat treatment) Subsequently, the natural graphite powder was heat-treated at 600°C for 10 hours under a nitrogen atmosphere.

[0102] (Mixing) After this, Si nanoparticles were further mixed as a silicon-based active material to produce a negative electrode active material.

[0103] (2) Manufacturing of the negative electrode The negative electrode active material produced above was mixed with a conductive material, styrene-butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener in a mass ratio of 96:1:1.5:1.5. The mixture was then dispersed in ion-removed distilled water to produce a negative electrode active material layer composition. The composition was applied to a copper foil current collector, then dried and rolled to obtain an electrode density of 1.10 ± 0.05 g / cm³. 3 We manufactured the negative electrode.

[0104] (3) Manufacturing of lithium secondary batteries A coin-type 2032 half-cell was fabricated using the aforementioned negative electrode as the working electrode and metallic lithium as the counter electrode. In this case, a separator made of porous polypropylene film was inserted between the working electrode and the counter electrode, and the electrolyte used was a mixed solution of diethyl carbonate (DEC) and ethylene carbonate (EC) in a volume ratio of 7:3, in which 1M LiPF6 was dissolved.

[0105] Example 2 The negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1, except that the natural graphite powder was heat-treated at 600°C for 5 hours under a nitrogen atmosphere during the production of the negative electrode active material.

[0106] Example 3 The negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1, except that the natural graphite powder was heat-treated at 650°C for 10 hours under a nitrogen atmosphere during the production of the negative electrode active material.

[0107] Comparative Example 1 In the production of the negative electrode active material, the negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1, except that, after preparing natural graphite powder with an average particle size D50 of 17 μm and then performing heat treatment, the natural graphite powder and petroleum-based pitch were mixed in a weight ratio of 100:5, then mechanically mixed in a high-speed stirrer at 2000 rpm for 10 minutes, and then heat-treated in a nitrogen atmosphere at 1200°C for 5 hours.

[0108] Comparative Example 2 The negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Comparative Example 1, except that natural graphite powder and petroleum-based pitch were mixed in a weight ratio of 100:8.

[0109] Comparative Example 3 Except for using natural graphite with an average particle size D50 of 6 μm, the negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Comparative Example 1.

[0110] Comparative Example 4 The negative electrode active material, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1, except that no heat treatment was performed after the preparation of the natural graphite powder during the manufacturing of the negative electrode active material.

[0111] Experimental Example 1: Evaluation of the physical properties of the negative electrode active material The physical properties of the negative electrode active materials produced according to the examples and comparative examples were evaluated as follows, and the results are shown in Tables 1 and 2.

[0112] (1) Evaluation of particle sizes D10, D50, and D90 Using the laser diffraction method, particle sizes D10, D50, and D90 were determined for the active material powder. Particle sizes D90, D10, and D50 can be defined as the particle sizes corresponding to 90%, 10%, and 50% of the cumulative volume in the particle size distribution curve, respectively.

[0113] (2) Evaluation of tap density Based on ASTM B527, 15 g of active material powder was placed in a 50 mL container, and the container was tapped at 3000 cycles @ 284 cycles / min to measure the packing density.

[0114] (3) Evaluation of compressive density and evaluation of Equation 1 Diameter 2cm 2 2g of active material powder is placed into the circular mold, and 5.5kgf / cm² is applied. 2 After applying the specified pressure, the density was measured to determine the compressed density (Micromeritics, GeoPyc 1360 Density analyzer). Subsequently, the difference between the measured compressed density and the tap density was calculated to derive the value in Equation 1.

[0115] (4) Evaluation of specific surface area The specific surface area of ​​the active material powder was measured using the BET method (Surface area and Porosity analyzer) (Micromeritics, ASAP2020).

[0116] (5) Evaluation of oil absorption The amount of oil absorbed was measured after adding 30g of active material powder and linseed oil (Asahi Research Institute, S-500).

[0117] (6) pH evaluation After thoroughly dispersing 10 g of active material powder with 100 mL of distilled water in an ultrasonic cleaner for 30 minutes, the pH of the separated supernatant was measured (Metrohm, 780 pH meter).

[0118] [Table 1]

[0119] [Table 2]

[0120] Referring to Tables 1 and 2, it was confirmed that in the examples, the values ​​of Formula 1, BET specific surface area, oil absorption, and pH values ​​were appropriately obtained within the range of the present invention. In contrast, in Comparative Examples 1 to 3, in which the coating process was performed without heat treatment under a nitrogen atmosphere, it was confirmed that the values ​​of Formula 1, BET specific surface area, and oil absorption were smaller than those obtained in the examples.

[0121] Furthermore, in Comparative Example 4, which did not undergo heat treatment under a nitrogen atmosphere and did not include a coating process, it was confirmed that the value of Equation 1 was larger than that of the Examples, and that the BET specific surface area, oil absorption amount, and pH value were smaller than those of the Examples.

[0122] Experimental Example 2: Evaluation of the electrochemical properties of lithium secondary batteries The electrochemical properties of lithium secondary batteries manufactured according to the examples and comparative examples were evaluated and are shown in Table 3 and Figure 1 below. The specific experimental methods are as follows.

[0123] (1) Evaluation of initial discharge capacity and initial efficiency After fabricating half a CR2032 lithium secondary battery cell, it was aged at 25°C for 30 hours, followed by charge-discharge testing. To evaluate the initial capacity, it was charged to 5mV with a constant current of 0.1C, using 500mAh / g as the reference capacity, then switched to a constant voltage and charged until the termination current reached 0.005C. After charging, there was a 10-minute rest period, and then it was discharged to 1.0V with a constant current of 0.1C, using 500mAh / g as the reference capacity.

[0124] (2) Evaluation of negative electrode volume expansion rate (%) The thickness of the negative electrode manufactured according to the examples and comparative examples was measured, and after fabricating half-cells of lithium secondary battery CR2032, a charge-discharge test was performed after aging at 25°C for 30 hours. To evaluate the expansion characteristics, the battery was charged to 5mV with a constant current of 0.5C, using a reference capacity of 500mAh / g, then switched to a constant voltage and charged until the termination current reached 0.005C. After charging, there was a 10-minute rest time, and then 30 discharge cycles were performed with a constant current of 0.5C, using a reference capacity of 500mAh / g, until the battery reached 1.2V. Subsequently, the battery was again charged to 5mV with a constant current of 0.5C, then switched to a constant voltage and charged until the termination current reached 0.005C. The battery was disassembled, the electrodes were washed with DMC solvent for 5 minutes, the electrode thickness was measured, and the negative electrode thickness expansion rate was calculated using the following formula.

[0125] <Expression 1> Volume expansion rate (%) = (Electrode thickness after 30 cycles - Electrode thickness before assembly) / (Electrode thickness before assembly) × 100

[0126] (3) Evaluation of life characteristics (30 cycle retention, %) The life characteristics were tested at 25°C. The battery was charged to 5mV with a constant current of 0.5C, then switched to a constant voltage and charged until the termination current reached 0.005C. After charging, a 10-minute rest period was observed, followed by discharge at a constant current of 0.5C until the voltage reached 1.2V. Under these charge / discharge conditions, 30 charge / discharge cycles were performed, and the capacity retention rate from the 30th cycle to the 1st cycle was calculated.

[0127] [Table 3]

[0128] Referring to Table 3, it was confirmed that in the examples where the physical properties of the negative electrode active material, such as the value of Equation 1, were appropriately adjusted within the range of the present invention, the capacity, initial efficiency, volume expansion coefficient, and lifetime characteristics were all excellent. In contrast, in Comparative Examples 1 to 4, where the physical properties, such as the value of Equation 1, were outside the range of the present invention, it was confirmed that the capacity, initial efficiency, volume expansion coefficient, or lifetime characteristics were generally worse compared to the examples.

[0129] Although preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and can be implemented in various ways within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and these also naturally fall within the scope of the present invention.

[0130] Therefore, the substantial scope of the present invention should be defined by the appended claims and their equivalents.

Claims

1. Contains natural graphite, The aforementioned natural graphite is a negative electrode active material for lithium secondary batteries that satisfies the following formula 1. [Formula 1] 0.04g / cm ３ ≦D ｐｒｅｓｓ -D ｔａｐ ≦0.085g / cm ３ In the above equation 1, D ｐｒｅｓｓ This is 5.5 kgf / cm². ２ D is the compressive density of natural graphite at the applied pressure. ｔａｐ This is the tap density of natural graphite.

2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the pH of the natural graphite is 7.0 or higher.

3. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the natural graphite surface does not contain a low-crystallinity carbon material-containing coating layer.

4. The BET specific surface area of ​​the aforementioned natural graphite is 4.8 m². ２ The negative electrode active material for a lithium secondary battery according to claim 1, wherein the amount is 1 / g or more.

5. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the oil absorption capacity of the natural graphite is 50.0 mL / 100 g or more.

6. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the average particle size D50 of the natural graphite is 16.0 to 21.0 μm.

7. The tap density of the natural graphite is 0.95 g / cm ３ The negative electrode active material for a lithium secondary battery according to claim 1, which is as follows.

8. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the degree of spheroidization of the natural graphite is 0.85 or higher.

9. The negative electrode active material for a lithium secondary battery according to claim 1, further comprising a silicon-based active material.

10. The silicon-based active material is silicon, a silicon-carbon composite, a silicon oxide, or a combination thereof, as described in claim 9, for a negative electrode active material for a lithium secondary battery.

11. Steps to prepare natural graphite powder; and The process includes the step of heat-treating the aforementioned natural graphite powder at a temperature of 500 to 650°C under an inert gas atmosphere. A method for producing a negative electrode active material for lithium secondary batteries.

12. The method for producing a negative electrode active material for a lithium secondary battery according to claim 11, wherein the inert gas is nitrogen.

13. The method for producing a negative electrode active material for a lithium secondary battery according to claim 11, wherein the heat treatment is performed for 1 to 12 hours.

14. After the heat treatment step, A method for producing a negative electrode active material for a lithium secondary battery according to claim 11, further comprising the step of mixing a silicon-based active material.

15. The method for producing a negative electrode active material for a lithium secondary battery according to claim 11, wherein the silicon-based active material is silicon, a silicon-carbon composite, a silicon oxide, or a combination thereof.

16. A negative electrode for a lithium secondary battery, comprising the negative electrode active material described in any one of claims 1 to 10.

17. A lithium secondary battery comprising the negative electrode for a lithium secondary battery as described in claim 16.

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