Negative active material for rechargeable lithium battery and rechargeable lithium battery

Abstract

Disclosed are a negative electrode active material for a rechargeable lithium battery and a rechargeable lithium battery including the negative electrode active material. The negative electrode active material includes: secondary particles, a plurality of natural graphite primary particles are agglomerated in the secondary particles, the secondary particles include pores; and amorphous carbon positioned in the pores, wherein, in a cross section of the negative electrode active material, based on 100% of the total area of ​​the negative electrode active material, the area of ​​the amorphous carbon is about 10% to about 30%.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material for a rechargeable lithium battery and a rechargeable lithium battery including the negative electrode active material. Background Art

[0002] Due to the increasing demand for mobile devices or portable batteries, continuous technological development has been carried out to achieve a high capacity of rechargeable lithium batteries.

[0003] For the positive electrode active material of a rechargeable lithium battery, lithium-transition metal oxides such as LiCoO2, LiMn2O4, LiNi 1-x Co x O2 (0 < x < 1), etc., having a structure capable of intercalating lithium ions have been used.

[0004] For the negative electrode active material, Si-based active materials including Si and Sn, or various carbon-based materials including artificial graphite, natural graphite, and hard carbon capable of intercalating and deintercalating lithium ions have been used.

[0005] The above information disclosed in this background art section is only for enhancing the understanding of the background of the invention, and thus, it may include information that does not form the prior art known to those of ordinary skill in the art in the country. Summary of the Invention

[0006] One embodiment provides a negative electrode active material for a rechargeable lithium battery that exhibits good safety and excellent cycle life characteristics.

[0007] Another embodiment provides a method for preparing a negative electrode active material.

[0008] Yet another embodiment provides a rechargeable lithium battery including a negative electrode active material.

[0009] According to one embodiment, there is provided a negative electrode active material including: secondary particles in which a plurality of natural graphite primary particles are agglomerated, the secondary particles including pores; and amorphous carbon located in the pores, wherein, in a cross-section of the negative electrode active material, based on 100% of the total area of the negative electrode active material, the area of the amorphous carbon is about 10% to about 30%.

[0010] The primary particles may have a particle size of about 5 μm to about 15 μm.

[0011] The secondary particles may have a particle size of about 8 μm to about 24 μm.

[0012] The natural graphite may be flake graphite.

[0013] The natural graphite may be flake graphite having a major axis and a minor axis, and the length of the major axis is about 5 μm to about 15 μm.

[0014] The secondary particles also include amorphous carbon on the surfaces of the secondary particles.

[0015] The negative active material may include amorphous carbon in an amount of about 1 wt % to about 9 wt % based on 100 wt % of the total weight of the negative active material.

[0016] Another embodiment provides a method for preparing a negative electrode active material, the method comprising the following steps: mixing an amorphous carbon precursor with secondary particles of porous, agglomerated natural graphite primary particles to prepare a mixture; controlling the pressure of the mixture; maintaining the resulting product at a temperature at which the viscosity of the amorphous carbon precursor is about 100 cP to about 1000 cP; pulverizing the obtained material; and heat treating the obtained material.

[0017] The amorphous carbon precursor may have a softening point of about 200°C to about 300°C.

[0018] The temperature at which the viscosity of the amorphous carbon precursor is about 100 cP to about 1000 cP may be about 350° C. to about 400° C.

[0019] The heat treatment may be performed at about 950°C to about 1400°C.

[0020] Controlling the pressure of the mixture may be performed using a pressurization process by applying a pressure of about 10 MPa to about 100 MPa or a decompression process by reducing the pressure at a pressure of about -0.1 MPa to about -1.0 MPa. According to one embodiment, the step of controlling the pressure may be a pressurization process by applying pressure.

[0021] The method may further include screening the resulting product after controlling the pressure and before maintaining the temperature.

[0022] Yet another embodiment provides a rechargeable lithium battery including: a negative electrode including a negative active material; a positive electrode including a positive active material; and an electrolyte.

[0023] A negative active material for a rechargeable lithium battery may exhibit excellent charge rate characteristics and discharge rate characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a diagram schematically showing a structure of a rechargeable lithium battery according to one embodiment.

[0025] Figure 2 is a SEM photograph of the negative electrode active material according to Example 1. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments are described in detail.

[0027] However, these embodiments are exemplary, and the present invention is not limited thereto but is defined by the scope of the appended claims.

[0028] According to one embodiment, a negative active material for a rechargeable lithium battery includes: secondary particles, a plurality of natural graphite primary particles are aggregated in the secondary particles, the secondary particles include pores (or pores); amorphous carbon is positioned in the pores. Here, in a cross section of the negative active material for a rechargeable lithium battery, based on 100% of the total area of ​​the negative active material, the area of ​​the amorphous carbon is about 10% to about 30%.

[0029] In one embodiment, the area may be the area of ​​a cross section of a negative active material for a rechargeable lithium battery. For example, a SEM photograph of a polished cross section (CP) of the negative active material is evaluated, and the SEM photograph may be classified as follows: a dark area with a low value is amorphous carbon, and a bright area with a high value is graphite. The area for each region and the total area of ​​the negative active material may be measured using an Image J analysis tool, or may be measured using any device or process known in the relevant art.

[0030] Thus, the negative active material according to one embodiment includes amorphous carbon positioned in the pores of the secondary particles, that is, the negative active material according to one embodiment is an active material in which amorphous carbon is filled in the pores formed in the secondary particles prepared by agglomerating the primary particles. In addition, the natural graphite may be a secondary particle having a cabbage shape and having pores inside by randomly accumulating and agglomerating at least one primary particle.

[0031] The degree of filling of amorphous carbon can be obtained from the area of ​​amorphous carbon in the negative electrode active material. When the area of ​​amorphous carbon is about 10% to about 30% based on 100% of the total area of ​​the negative electrode active material, the internal density of the negative electrode active material can be improved, and the expansion of the active material during charge and discharge can be effectively suppressed. In addition, since the amorphous carbon is fully filled in the pores, the empty space in the pores in the active material can be minimized, so the side reaction between the active material and the electrolyte caused by impregnation of the electrolyte into the pores of the active material can be effectively suppressed, thereby improving the cycle life characteristics.

[0032] If the area of ​​amorphous carbon is less than 10% based on 100% of the total area of ​​the negative electrode active material, the amount of amorphous carbon positioned in the pores is too small, thereby reducing the internal density, making it impossible to suppress the expansion of the active material, and impregnation of the electrolyte into the pores occurs, thereby failing to obtain the effect of suppressing side reactions with the electrolyte or improving the cycle life characteristics.

[0033] Alternatively, if the area of ​​amorphous carbon is greater than 30% based on 100% of the total area of ​​the negative active material, the negative active material is excessively filled in the pores of the negative active material, making it impossible to meet the efficiency of the battery for practical use, and disadvantages associated with high temperature storage characteristics may occur.

[0034] Flake natural graphite is generally composed of giant particles of about 80 μm, and if such giant particles are used for agglomeration, many pores are formed inside, which promotes side reactions with the electrolyte and deterioration of cycle life characteristics due to electrode expansion during charging and discharging, and primary particles with a large particle size are produced, which is not desirable.

[0035] In contrast, one embodiment includes small particles having a particle size of about 5 μm to about 15 μm as primary particles and secondary particles in which the primary particles are agglomerated, so that the occurrence of internal pores can be slightly reduced. In addition, amorphous carbon is positioned in the internal pores to fill the internal pores, so that both effects of slightly reducing the formation of internal pores by using fine primary particles and by filling amorphous carbon in the internal pores can be obtained, and the density can be better improved.

[0036] The particle size of the primary particles may be about 5 μm to about 15 μm, for example, about 5 μm to about 13 μm, about 5 μm to about 12 μm, or about 5.5 μm to about 11.5 μm. The particle size of the secondary particles may be about 8 μm to about 24 μm, for example, about 10 μm to about 24 μm, about 11 μm to about 24 μm, about 12 μm to about 24 μm, about 13 μm to about 24 μm, about 13 μm to about 23 μm, or about 13 μm to about 20 μm.

[0037] When the particle sizes of the primary particles and the secondary particles are within the above range, the expansion of the negative electrode active material including them can be effectively suppressed, and the tap density of the negative electrode active material can be further increased. In addition, the advantages of excellent power characteristics at high rates can be obtained.

[0038] The particle size of the primary particles and the secondary particles can be the average particle size of the particle diameter. Here, the average particle size is a value obtained by measuring a plurality of particles and adding the plurality of particles to a particle size analyzer, and refers to the particle size (D50) at which the cumulative volume is 50% by volume in the cumulative size distribution curve. That is, unless otherwise defined, the average particle size (D50) refers to the diameter of the particles at which the cumulative volume is 50% by volume in the particle size distribution.

[0039] The average particle size D50 can be measured by techniques known to those skilled in the art, such as using a particle size analyzer, a transmission electron microscope, or a scanning electron microscope. Another method is to measure it using a measuring device with dynamic light scattering, analyze the data to count the number of particles for each particle size, and then calculate to obtain the average particle size D50.

[0040] The number of primary particles constituting the secondary particles is not particularly limited as long as they can form secondary particles, but for example, 2 to 50, 2 to 40, 2 to 30, 2 to 20, 2 to 10 or 2 to 4 primary particles may be agglomerated to form secondary particles.

[0041] The natural graphite may be flake graphite.

[0042] In one embodiment, the natural graphite may be flaky graphite having a major axis and a minor axis, and the length of the major axis may be about 5 μm to about 15 μm. When the major axis of the natural graphite is within the above range, it may be randomly oriented, and the mobility of lithium ions may be increased to improve rate characteristics. In addition, it may have a suitable edge portion, thereby appropriately maintaining efficiency.

[0043] In one embodiment, the ratio of the thickness of the amorphous carbon filled in the pores of the secondary particles to the thickness of the primary particles can be about 2.0 or less, preferably about 0.1 to about 2.0. Here, the pores of the secondary particles refer to the space between the primary particles (specifically, the flaky natural graphite primary particles), and the thickness of the amorphous carbon filled in the pores of the secondary particles refers to the distance between the flaky natural graphite primary particles. When the ratio of the thickness of the amorphous carbon filled in the pores of the secondary particles to the thickness of the primary particles is 2.0 or less, the amount of amorphous carbon filled in the pores of the secondary particles can be appropriately controlled to further improve efficiency and have a higher pellet density in terms of electrochemical physical properties.

[0044] The thickness of the primary particles and amorphous carbon can be measured from a SEM photograph of the active material, where the length of the minor axis represents the thickness of the amorphous carbon, and the thickness of the primary particles represents the thickness of the finest particle among the primary particles in the SEM photograph.

[0045] The amorphous carbon may be soft carbon.

[0046] In one embodiment, the negative electrode active material may further include amorphous carbon positioned on the surface of the secondary particles. This allows the amorphous carbon to also be positioned on the surface of the secondary particles in the negative electrode active material. That is, the amorphous carbon may be filled in the pores of the negative electrode active material and partially positioned on the surface of the secondary particles. In this way, when the amorphous carbon is positioned on the surface of the secondary particles, the resistance may be reduced and the low temperature characteristics may be improved.

[0047] In the case of positioning amorphous carbon on the surface of the secondary particles, the thickness may be about 5 nm to about 50 nm, and according to one embodiment, for example, may be about 10 nm to about 50 nm or about 20 nm to about 50 nm. When the thickness is within this range, the side reaction with the electrolyte may be further effectively suppressed, and the charge and discharge rate characteristics may be improved.

[0048] Regardless of whether the amorphous carbon is present only in the pores of the negative electrode active material or in both the pores and the surface, the amount of the amorphous carbon may be about 1 wt % to about 9 wt % based on 100 wt % of the total amount of the negative electrode active material. When the amount of the amorphous carbon is within the above range, the inner pores may be substantially or completely filled to further effectively suppress the expansion of the active material during charge and discharge, and the amorphous carbon may be included in the active material in a very sufficient amount to further improve the rate characteristics.

[0049] The negative active material according to one embodiment may have a thickness of about 1 m 2 / g to about 3m 2 The specific surface area may be a BET specific surface area measured by the Brunauer-Emmett-Teller (BET) method. The specific surface area of ​​the negative electrode active material within the above range is larger than the specific surface area of ​​general natural graphite (which is about 5 m 2 This low specific surface area can allow further improvement in efficiency.

[0050] The negative active material according to one embodiment may be prepared by the following process.

[0051] Flake natural graphite having a particle size of about 70 μm or more is pulverized and granulated to prepare fine (small particle) primary particles. The pulverization and small granulation can be performed by using a pulverizing device.

[0052] The particle size of the prepared fine primary particles may be about 5 μm to about 15 μm, for example, about 5 μm to about 13 μm, about 5 μm to about 12 μm, or about 5.5 μm to about 11.5 μm.

[0053] The spheroidization process can be performed by using fine primary particles and a spheroidization device to agglomerate into secondary particles. The spheroidization process can be performed by adding the primary particles to the spheroidization device and rotating the inner container.

[0054] According to the spheroidization process, secondary particles including porous, agglomerated natural graphite primary particles can be prepared.

[0055] The prepared natural graphite secondary particles may be mixed with an amorphous carbon precursor to prepare a mixture.

[0056] The amorphous carbon precursor may not be particularly limited as long as the material becomes carbide, but may be a material having a softening point of about 200° C. to about 300° C. Examples of the amorphous carbon precursor may be synthetic pitch, petroleum-based pitch, or coal-based pitch.

[0057] As such, when an amorphous carbon precursor having a softening point of about 200° C. to about 300° C. is used as the amorphous carbon precursor, the inner pores of the secondary particles may be densely filled due to a high residual carbon ratio, and low resistance allows high rate characteristics to be exhibited.

[0058] The mixing process can be performed by mixing the natural graphite secondary particles and the amorphous carbon precursor in a weight ratio of about 95.5:4.5 to about 85:15 so as to maintain the natural graphite and amorphous carbon in a weight ratio of about 99:1 to about 91:9 (e.g., about 99:1 to about 92:8, about 99:1 to about 93:7, or about 99:1 to about 94:6) in the final product.

[0059] If the natural graphite secondary particles and the amorphous carbon precursor are mixed at this weight ratio to include the natural graphite and amorphous carbon in this range in the final product, the amount of amorphous carbon added to the interior of the secondary particles can be appropriately controlled, and the pores inside the secondary particles can be reduced to reduce the side reaction between the electrolyte solution and improve the degree of internal density. When a coating layer including amorphous carbon is prepared on the surface of the secondary particles, a coating layer having the above-mentioned thickness range can be achieved, and therefore, the structural stability of the natural graphite secondary particles can be obtained, the expansion of the negative electrode active material can be prevented, and the cycle life characteristics can be improved.

[0060] Thereafter, the pressure of the mixture may be controlled.

[0061] In one embodiment, the step of controlling the pressure may be performed using a pressurization process by applying pressure to the mixture or a decompression process by reducing the pressure. Thus, when the step of controlling the pressure of the mixture is performed, the size of the pores formed inside the secondary particles may be reduced, and the amorphous carbon precursor may be easily filled in the pores.

[0062] In one embodiment, it is desirable to perform the step of controlling the pressure using a pressurizing process of the mixture to improve the internal density of the pellets and the pellet density, thereby improving the cycle life characteristics and capacity.

[0063] The pressurization process can apply a pressure of about 10MPa to about 100MPa. When the pressurization process is performed under this pressure, the amorphous carbon precursor can be further well filled in the pores of the secondary particles, and the size of the pores can be reduced more. If the pressure of the pressurization process is less than 10MPa, the pores formed inside the secondary particles will not be filled. On the other hand, if the pressure is greater than 100MPa, the resulting product obtained therefrom will be very compact and hard, which is undesirable. Since the products from the pressurization process are very compact and hard, they will be difficult to perform when performing depolymerization and removing very small particles. In addition, although depolymerization is performed, it may be inappropriate because the BET specific surface area will greatly increase.

[0064] In addition, the decompression process can be to reduce the pressure with a pressure of about -0.1MPa to about -1MPa. When the decompression process is performed under this pressure, the amorphous carbon precursor can be further well filled in the pores of the secondary particles and the size of the pores can be further reduced. If the pressure of the decompression process is less than -0.1MPa, the pores of the secondary particles may not be effectively filled in the pores. On the other hand, if the pressure is greater than -1MPa, a compact and hard product can be prepared, and a decompression process below the above pressure will actually be difficult to perform. Since the product from the decompression process is extremely compact and hard, it will be difficult to perform the decompression process when depolymerization is performed.

[0065] If pressure control is performed before mixing the amorphous carbon precursor with natural graphite, the effect of improving the internal density by filling the amorphous carbon in the pores may not be obtained. This is considered to be because the amorphous carbon precursor (e.g., an amorphous carbon precursor) is impregnated inside the secondary particles due to the capillary effect at a temperature higher than the softening point of the pitch, and therefore, if the secondary particles are not close to the amorphous carbon, the pores at the interface of the two active substances act as resistance, thereby failing to obtain a sufficient capillary effect.

[0066] After controlling the pressure, removal of very small particles may be further performed to control particle size distribution (PSD). Such control of PSD may be performed by a sieving process using a sieve, which enables uniform particle size of the final active material, thereby ensuring electrode uniformity during mixing of the negative electrode active material composition in a slurry-type form and coating the negative electrode active material composition on a current collector.

[0067] Thereafter, the obtained product may be maintained at a temperature at which the viscosity of the amorphous carbon precursor will be about 100 cP to about 1000 cP. In this process, the amorphous carbon precursor has fluidity, and specifically, when the viscosity is within this range, the viscosity is very low and the fluidity is very good, so the amorphous carbon precursor filled in the pores of the secondary particles can be completely filled in the space of the pores, and the filling in the pores can occur efficiently.

[0068] The viscosity of the amorphous carbon precursor may be about 100 cP to about 1000 cP at a temperature of about 350°C to about 400°C.

[0069] The holding step may be performed for about 0.5 hours to about 4 hours. When the holding is performed within this range, the occurrence of empty pores inside the secondary particles may be minimized.

[0070] Thereafter, the resulting material is pulverized to prepare a pulverized product. The pulverization may be performed for about 1 hour to about 2 hours.

[0071] The resulting product is heat treated to prepare a negative electrode active material. The heat treatment may be performed at a temperature of about 950° C. to about 1400° C. (e.g., about 1000° C. to about 1400° C. or about 1200° C. to about 1300° C.). The heat treatment may be performed for about 1 hour to about 5 hours, for example, about 1 hour to about 4 hours, about 1 hour to about 3 hours, or about 2 hours to about 3 hours.

[0072] When the heat treatment is performed in this temperature range and time range, the amorphous carbon precursor may be effectively filled in the pores inside the secondary particles.

[0073] According to the heat treatment, the amorphous carbon precursor is converted into amorphous carbon to be located inside the pores or on the inside of the pores and on the surfaces of the secondary particles.

[0074] Another embodiment provides a rechargeable lithium battery including a negative electrode including the negative active material, a positive electrode, and an electrolyte.

[0075] The negative electrode may include a current collector and a negative active material layer formed on the current collector.

[0076] In the negative active material layer, the negative active material may be included in an amount of 95 wt % to 99 wt % based on the total weight of the negative active material layer.

[0077] The negative electrode active material layer may include a binder and may optionally include a conductive material. In the negative electrode active material layer, the amount of the binder may be about 1wt% to about 5wt% based on the total weight of the negative electrode active material layer. When the negative electrode active material layer also includes a conductive material, the negative electrode active material layer includes about 90wt% to about 98wt% of the negative electrode active material, about 1wt% to about 5wt% of the binder, and about 1wt% to about 5wt% of the conductive material.

[0078] The binder adheres the negative active material particles to each other well, and also adheres the negative active material to the current collector. The binder may include a non-water-soluble binder, a water-soluble binder, or a combination thereof.

[0079] The non-water-soluble binder may be ethylene-propylene copolymer, polyacrylonitrile, polystyrene, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or a combination thereof.

[0080] The water-soluble binder can be styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyepichlorohydrin, polyphosphazene, ethylene propylene diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol or a combination thereof.

[0081] When a water-soluble binder is used as a negative electrode binder, a cellulose compound may also be used as a thickener to provide viscosity. Cellulose compounds include one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and their alkali metal salts. The alkali metal may be Na, K, or Li. The thickener may be included in an amount of 0.1 to 3 parts by weight based on 100 parts by weight of the negative electrode active material.

[0082] The conductive material is included to provide electrode conductivity, and any electrically conductive material can be used as the conductive material unless it causes a chemical change. Examples of the conductive material include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, superconductive acetylene black (denka black), carbon fiber, etc.; metal-based materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0083] The current collector may include one selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof, but is not limited thereto.

[0084] The positive electrode may include a positive current collector and a positive active material layer formed on the positive current collector. The positive active material may include a compound (lithiated intercalation compound) that reversibly intercalates and deintercalates lithium ions. Specifically, the positive active material may include one or more composite oxides of a metal selected from cobalt, manganese, nickel, and combinations thereof and lithium. A more specific example may be a compound represented by one of the following chemical formulas. Li a A 1-b X b D2(0.90≤a≤1.8, 0≤b≤0.5); Li a A 1-b X b O 2-c D c(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a HAVE BEEN 1-b X b O 2-c D c (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a HAVE BEEN 2-b X b O 4-c D c (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a Ni 1-b- c Co b X c D α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α≤2);Li a Ni 1-b-c Co b X c O 2-α T α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α<2);Li a Ni 1-b-c Co b X c O 2-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α<2);Li a Ni 1-b-c Mr b X c D α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α≤2);Li a Ni 1-b-c Mr b X c O 2-α T α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α≤2);Li a Ni 1-b-c Mr b X c O 2-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0≤α≤2);Li a Ni b HAVE BEEN c G dO2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li a Ni b Co c Mn d G e O2 (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li a NeG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a CoG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-b G b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn b O4(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-g G g PO4(0.90≤a≤1.8, 0≤g≤0.5); QO2; QS2; LiQS2; V2O5; LiV2O5; LiZO2; LiNiVO4; Li (3-f) J2(PO4)3(0≤f≤2);Li (3-f) Fe2(PO4)3(0≤f≤2); and Li a FePO4(0.90≤a≤1.8).

[0085] In the chemical formula, A is selected from Ni, Co, Mn and combinations thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from O, F, S, P and combinations thereof; E is selected from Co, Mn and combinations thereof; T is selected from F, S, P and combinations thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V and combinations thereof; Q is selected from Ti, Mo, Mn and combinations thereof; Z is selected from Cr, V, Fe, Sc, Y and combinations thereof; and J is selected from V, Cr, Mn, Co, Ni, Cu and combinations thereof.

[0086] The compound may have a coating layer located on the surface, or may be mixed with another compound having a coating layer. The coating layer may include at least one coating element compound selected from the group consisting of an oxide of a coating element, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, and a hydroxycarbonate of a coating element. The compound used for the coating layer may be amorphous or crystalline. The coating element included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer may be provided in a method that does not adversely affect the properties of the positive active material by using these elements in the compound. For example, the method may include any coating method (such as spraying, dipping, etc.), but since it is well known in the relevant art, it is not described in more detail.

[0087] In the positive electrode, the amount of the positive active material may be 90 wt % to 98 wt % based on the total weight of the positive active material layer.

[0088] In one embodiment, the positive electrode active material layer may further include a binder and a conductive material, and the amount of the binder and the conductive material may be about 1 wt % to about 5 wt % based on the total amount of the positive electrode active material layer.

[0089] The binder improves the bonding properties between the positive electrode active material particles and the bonding properties between the positive electrode active material particles and the current collector. Examples of the binder may include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc., but are not limited thereto.

[0090] The conductive material is included to provide electrode conductivity. Any electrically conductive material can be used as the conductive material unless it causes a chemical change in the battery. Examples of the conductive material include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; metal-based materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0091] The positive electrode current collector may be aluminum foil, nickel foil, or a combination thereof, but is not limited thereto.

[0092] The electrolyte may include a non-aqueous organic solvent and a lithium salt.

[0093] The nonaqueous organic solvent serves as a medium for transporting ions participating in the electrochemical reaction of the battery.

[0094] The non-aqueous organic solvent may include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent or an aprotic solvent.

[0095] Carbonate solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Ester solvents may include methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, mevalonolactone, caprolactone, etc. Ether solvents may include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Ketone solvents may include cyclohexanone, etc. Alcohol solvents may include ethanol, isopropanol, etc., and aprotic solvents may include nitriles such as R-CN (wherein R is a hydrocarbon group having a C2 to C20 linear, branched or cyclic structure, and may include a double bond, an aromatic ring or an ether bond), dioxolanes such as 1,3-dioxolane, and cyclopentane, etc.

[0096] The nonaqueous organic solvent may be used alone or in a mixture. When the organic solvent is used in a mixture, the mixing ratio may be controlled according to desired battery performance.

[0097] The carbonate-based solvent may include a mixture having a cyclic carbonate and a linear (chain) carbonate. The cyclic carbonate and the linear carbonate are mixed together in a volume ratio of 1:1 to 1:9, which may provide enhanced performance of the electrolyte.

[0098] In addition to the carbonate-based solvent, the organic solvent may further include an aromatic hydrocarbon-based organic solvent. Here, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of 1:1 to 30:1.

[0099] The aromatic hydrocarbon organic solvent may be an aromatic hydrocarbon compound of Chemical Formula 1.

[0100] [Chemical formula 1]

[0101]

[0102] In Chemical Formula 1, R1 to R6 are the same or different and are selected from hydrogen, halogen, C1 to C10 alkyl, halogenated alkyl, and combinations thereof.

[0103] Specific examples of aromatic hydrocarbon organic solvents can be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluoroform, Benzene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

[0104] The electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound represented by Chemical Formula 2 to improve battery cycle life.

[0105] [Chemical formula 2]

[0106]

[0107] In Chemical Formula 2, R7 and R8 are the same or different and are selected from hydrogen, halogen, cyano (CN), nitro (NO2) and C1 to C5 fluorinated alkyl, provided that at least one of R7 and R8 is selected from halogen, cyano (CN), nitro (NO2) and C1 to C5 fluorinated alkyl, and R7 and R8 are not hydrogen at the same time.

[0108] Examples of ethylene carbonate compounds may include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, etc. When such an additive for improving cycle life is also used, the amount may be controlled within a suitable range.

[0109] Lithium salts dissolved in organic solvents provide lithium ions to the battery, enable basic operation of rechargeable lithium batteries, and improve the transport of lithium ions between the positive electrode and the negative electrode. Examples of lithium salts include those selected from LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F2y+1 SO2) (wherein, x and y are natural numbers, for example, integers of 1 to 20), at least one supporting salt of LiCl, LiI, and LiB(C2O4)2 (lithium bis(oxalato)borate; LiBOB). The concentration of the lithium salt may be in the range of 0.1 M to 2.0 M. When the lithium salt is included in the above concentration range, the electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

[0110] In addition, depending on the type of rechargeable lithium battery, a separator may be provided between the positive electrode and the negative electrode. Such a separator may include polyethylene, polypropylene, polyvinylidene fluoride, and multilayers thereof (such as polyethylene / polypropylene double-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, or polypropylene / polyethylene / polypropylene three-layer separator).

[0111] Figure 1 is an exploded perspective view of a rechargeable lithium battery according to an embodiment. The rechargeable lithium battery according to an embodiment is illustrated as a prismatic battery, but is not limited thereto and may include batteries of various shapes such as a cylindrical battery or a pouch-type battery.

[0112] Reference Figure 1 , the rechargeable lithium battery 100 according to the embodiment includes a wound electrode assembly 40 including a positive electrode 10, a negative electrode 20, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20, and a case 50 accommodating the electrode assembly 40. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte solution (not shown).

[0113] Hereinafter, examples of the present invention and comparative examples are described. However, the present invention is not limited to the following examples.

[0114] (Comparative Example 1)

[0115] For flake natural graphite having an average particle size D50 of 70 μm, fine primary particles having an average particle size D50 of 13 μm were prepared using a pulverizing device (product name: QWJ60). Secondary particles in which flake natural graphite primary particles were agglomerated and included pores were prepared using fine primary particles and a spheroidizing device (product name: F500).

[0116] The secondary particles were mixed with a petroleum pitch (product name: LQ250) amorphous carbon precursor at a weight ratio of 90:10 to prepare a mixture. A petroleum pitch amorphous carbon precursor having a softening point of 250° C. was used as the petroleum pitch amorphous carbon precursor.

[0117] The mixture was heat-treated in a sintering furnace at 1250° C. for 1.5 hours to prepare a negative electrode active material including secondary particles having an average particle size D50 of 15 μm, in which primary particles having an average particle size D50 of 13 μm were agglomerated. The secondary particles included pores in which soft carbon amorphous carbon was filled, and the negative electrode active material had an average particle size D50 of 1.5 μm. 2 Here, the amount of amorphous carbon is 7 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0118] 97.5 wt % of the negative active material, 1.0 wt % of carboxymethyl cellulose, and 1.5 wt % of styrene-butadiene rubber were mixed in distilled water to prepare a negative active material slurry composition.

[0119] The negative active material slurry composition was coated on a Cu current collector and dried, and then compressed to prepare a negative electrode for a rechargeable lithium battery.

[0120] A half-cell was fabricated using a negative electrode, a lithium metal counter electrode and an electrolyte solution. As the electrolyte, 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio) was used.

[0121] (Example 1)

[0122] For flake natural graphite having an average particle size D50 of 70 μm, fine primary particles having an average particle size D50 of 13 μm were prepared using a pulverizing device (product name: QWJ60). Secondary particles in which flake natural graphite primary particles were agglomerated and included pores were prepared using fine primary particles and a spheroidizing device (product name: F500).

[0123] The secondary particles were mixed with a petroleum pitch (product name: LQ250) amorphous carbon precursor at a weight ratio of 93:7 to prepare a mixture. As the LQ250 petroleum pitch amorphous carbon precursor, LQ250 petroleum pitch amorphous carbon precursor having a softening point of 250° C. was used.

[0124] A pressurizing process was performed by applying a pressure of 30 MPa to the mixture, and the resulting product was maintained at 360° C. for 1.5 hours so that the viscosity of the LQ250 amorphous carbon precursor reached 500 cP.

[0125] Thereafter, the resulting material was pulverized for 1.5 hours, and the pulverized product was heat-treated in a sintering furnace at 1250° C. for 1.5 hours.

[0126] According to this process, a negative electrode active material including secondary particles having an average particle size D50 of 15 μm was prepared, in which primary particles having an average particle size D50 of 13 μm were aggregated. The secondary particles included pores in which soft carbon amorphous carbon was filled, and the negative electrode active material had an average particle size D50 of 2.2 μm. 2 Here, the amount of amorphous carbon is 5 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0127] 97.5 wt % of the negative active material, 1.0 wt % of carboxymethyl cellulose, and 1.5 wt % of styrene-butadiene rubber were mixed in distilled water to prepare a negative active material slurry composition.

[0128] The negative active material slurry composition was coated on a Cu current collector and dried, and then compressed to prepare a negative electrode for a rechargeable lithium battery.

[0129] A half-cell was fabricated using a negative electrode, a lithium metal counter electrode and an electrolyte solution. As the electrolyte, 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio) was used.

[0130] (Example 2)

[0131] Secondary particles in which flaky natural graphite primary particles were agglomerated and included pores were prepared using a spheroidizing apparatus (product name: F500), and fine primary particles having an average particle diameter D50 of 13 μm were prepared by the same process as in Example 1.

[0132] The secondary particles were mixed with a petroleum pitch (product name: LQ250) amorphous carbon precursor at a weight ratio of 90:10 to prepare a mixture. As the LQ250 petroleum pitch amorphous carbon precursor, an LQ250 petroleum pitch amorphous carbon precursor having a softening point of 250° C. was used.

[0133] A decompression process was performed by reducing the pressure of the mixture by -0.5 MPa, and the resultant product was maintained at 360° C. for 1.5 hours so that the viscosity of the petroleum pitch amorphous carbon precursor reached 500 cP.

[0134] Thereafter, the resulting material was pulverized for 1.5 hours, and the pulverized product was heat-treated in a sintering furnace at 1250° C. for 1.5 hours.

[0135] According to the process, a negative electrode active material including secondary particles having an average particle size D50 of 15 μm was prepared, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and soft carbon amorphous carbon was filled in the pores. The negative electrode active material had an average particle size D50 of 1.77 μm. 2Here, the amount of amorphous carbon is 7 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0136] 97.5 wt % of the negative active material, 1.0 wt % of carboxymethyl cellulose, and 1.5 wt % of styrene-butadiene rubber were mixed in distilled water to prepare a negative active material slurry composition.

[0137] The negative active material slurry composition was coated on a Cu current collector and dried, and then compressed to prepare a negative electrode for a rechargeable lithium battery.

[0138] A half-cell was fabricated using a negative electrode, a lithium metal counter electrode and an electrolyte solution. As the electrolyte, 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio) was used.

[0139] (Example 3)

[0140] A negative electrode active material was prepared by the same process as in Example 1, except that the secondary particles were mixed with the LQ250 amorphous carbon precursor at a weight ratio of 91.5:8.5 to prepare a mixture. The negative electrode active material included secondary particles having an average particle size D50 of 15 μm, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and the soft carbon was filled in the pores. The negative electrode active material had a particle size of 2.09 μm. 2 Here, the amount of amorphous carbon is 6 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0141] A negative electrode and a half cell for a rechargeable lithium battery were manufactured by the same process as in Example 1 using the negative active material.

[0142] (Example 4)

[0143] For flake natural graphite having an average particle size D50 of 70 μm, fine primary particles having an average particle size D50 of 13 μm were prepared using a pulverizing device (product name: QWJ60). Secondary particles in which flake natural graphite primary particles were agglomerated and included pores were prepared using fine primary particles and a spheroidizing device (product name: F500).

[0144] The secondary particles were mixed with a petroleum pitch (product name: LQ250) amorphous carbon precursor at a weight ratio of 90:10 to prepare a mixture. As the LQ250 petroleum pitch amorphous carbon precursor, an LQ250 petroleum pitch amorphous carbon precursor having a softening point of 250° C. was used.

[0145] A pressurization process was performed by applying a pressure of 30 MPa to the mixture, and the resulting product was maintained at 360° C. for 1.5 hours so that the viscosity of the LQ250 amorphous carbon precursor reached 500 cP.

[0146] Thereafter, the resulting material was pulverized for 1.5 hours, and the pulverized product was heat-treated in a sintering furnace at 1250° C. for 1.5 hours.

[0147] According to the process, a negative electrode active material including secondary particles having an average particle size D50 of 15 μm was prepared, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and soft carbon amorphous carbon was filled in the pores. The negative electrode active material had an average particle size D50 of 1.92 μm. 2 Here, the amount of amorphous carbon is 7 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0148] 97.5 wt % of the negative active material, 1.0 wt % of carboxymethyl cellulose, and 1.5 wt % of styrene-butadiene rubber were mixed in distilled water to prepare a negative active material slurry composition.

[0149] The negative active material slurry composition was coated on a Cu current collector and dried, and then compressed to prepare a negative electrode for a rechargeable lithium battery.

[0150] A half-cell was fabricated using a negative electrode, a lithium metal counter electrode and an electrolyte solution. As the electrolyte, 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio) was used.

[0151] (Example 5)

[0152] The negative active material was prepared by the same process as in Example 1, except that a sieving process for controlling the particle size distribution (PSD) by removing very small particles was further performed after the pressurization process. The negative active material included secondary particles having an average particle size D50 of 15 μm, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and soft carbon was filled in the pores. The negative active material had a particle size of 1.82 μm. 2 Here, the amount of amorphous carbon is 7 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0153] A negative electrode and a half cell for a rechargeable lithium battery were manufactured by the same process as in Example 2 using the negative active material.

[0154] (Comparative Example 2)

[0155] A negative active material was prepared by the same process as in Comparative Example 1, except that the secondary particles were mixed with the LQ250 amorphous carbon precursor at a weight ratio of 93:7 to prepare a mixture. The negative active material included secondary particles having an average particle size D50 of 15 μm, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and the soft carbon was filled in the pores. The negative active material had a particle size of 1.63 μm. 2 Here, the amount of amorphous carbon is 5 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0156] A negative electrode and a half cell for a rechargeable lithium battery were manufactured by the same process as in Example 1 using the negative active material.

[0157] (Comparative Example 3)

[0158] Secondary particles in which flaky natural graphite primary particles were agglomerated and included pores were prepared using a spheroidizing apparatus (product name: F500), and fine primary particles having an average particle diameter D50 of 13 μm were prepared by the same process as in Example 1.

[0159] The pressurizing process was performed by applying a pressure of 30 MPa to the secondary particles.

[0160] Thereafter, the obtained secondary particles were mixed with a petroleum pitch (product name: LQ250) amorphous carbon precursor at a weight ratio of 90:10 to prepare a mixture. As the LQ250 petroleum pitch amorphous carbon precursor, LQ250 petroleum pitch amorphous carbon precursor having a softening point of 250° C. was used.

[0161] The resulting product was maintained at 360° C. for 1.5 hours so that the viscosity of the LQ250 amorphous carbon precursor reached 500 cP.

[0162] Thereafter, the resulting material was pulverized for 1.5 hours, and the pulverized product was heat-treated in a sintering furnace at 1250° C. for 1.5 hours.

[0163] According to this process, a negative electrode active material including secondary particles having an average particle size D50 of 15 μm was prepared, in which primary particles having an average particle size D50 of 13 μm were agglomerated. The secondary particles included pores in which soft carbon amorphous carbon was filled, and the negative electrode active material had an average particle size D50 of 1.64 μm. 2 Here, the amount of amorphous carbon is 7 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0164] 97.5 wt % of the negative active material, 1.0 wt % of carboxymethyl cellulose, and 1.5 wt % of styrene-butadiene rubber were mixed in distilled water to prepare a negative active material slurry composition.

[0165] The negative active material slurry composition was coated on a Cu current collector and dried, and then compressed to prepare a negative electrode for a rechargeable lithium battery.

[0166] A half-cell was fabricated using a negative electrode, a lithium metal counter electrode and an electrolyte solution. As the electrolyte, 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio) was used.

[0167] (Comparative Example 4)

[0168] A negative electrode active material was prepared by the same process as in Example 1, except that the secondary particles were mixed with the LQ250 amorphous carbon precursor at a weight ratio of 86:14 to prepare a mixture. The negative electrode active material included secondary particles having an average particle size D50 of 15 μm, primary particles having an average particle size D50 of 13 μm agglomerated in the secondary particles. The secondary particles included pores, soft carbon was filled in the pores, and the negative electrode active material had an average particle size D50 of 1.80 μm. 2 Here, the amount of amorphous carbon is 6 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0169] A negative electrode and a half cell for a rechargeable lithium battery were manufactured by the same process as in Example 1 using the negative active material.

[0170] (Comparative Example 5)

[0171] A negative active material was prepared by the same process as in Comparative Example 2, except that the secondary particles were mixed with the LQ250 amorphous carbon precursor at a weight ratio of 83:17 to prepare a mixture. The negative active material included secondary particles having an average particle size D50 of 15 μm, and primary particles having an average particle size D50 of 13 μm were agglomerated in the secondary particles. The secondary particles included pores, and the soft carbon was filled in the pores. The negative active material had a particle size of 0.51 μm. 2 Here, the amount of amorphous carbon is 12 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0172] A negative electrode and a half cell for a rechargeable lithium battery were manufactured by the same process as in Example 1 using the negative active material.

[0173] *Measurement of pellet density

[0174] 1.0 g of each of the negative active materials according to Examples 1 to 4 and Comparative Examples 1 to 6 was added to a mold and maintained at a pressure (pressing force) of 2 tons for 30 seconds to prepare pellets, and the pellet density of the pellets was measured. The results are shown in Table 1.

[0175] Table 1

[0176] <![CDATA[BET specific surface area (m 2 / g)]]> Granule density (g / cc) Comparative Example 1 1.50 1.61 Example 1 2.20 1.80 Example 2 1.77 1.69 Example 3 2.09 1.74 Example 4 1.92 1.85 Example 5 1.82 1.79 Comparative Example 2 1.63 1.65 Comparison Example 3 1.64 1.61 Comparison Example 4 1.80 1.88 Comparative Example 5 0.51 1.56

[0177] As shown in Table 1, the BET specific surface areas of the negative active materials according to Examples 1 and Examples 3 to 5 are increased compared to the BET specific surface areas of the negative active materials of Comparative Examples 1 to 5. Specifically, Comparative Example 5, which does not control the pressure and uses an excessive amount of amorphous carbon precursor, exhibits a significantly reduced BET specific surface area of ​​the active material, which is 0.51 m 2 / g.

[0178] *Measurement of area ratio, major axis length and thickness ratio

[0179] In the cross-sections of the negative electrode active materials according to Examples 1 to 5 and Comparative Examples 1 to 5, the area ratio was measured by taking an SEM photograph as a cross-section polished SEM photograph (i.e., an SEM photograph of a cross-section for the negative electrode active material), classifying a dark region with a low value as amorphous carbon and a bright region with a high value as natural graphite, and measuring the respective areas and the total area of ​​the active material using an Image J analysis tool. Figure 2 When describing, for example, Figure 2 A dark region (eg, a circular region) with a low value in is an amorphous carbon region, and the area of ​​the dark region and the total area of ​​the negative electrode active material are measured.

[0180] The results are shown in Table 2.

[0181] In the SEM photograph, the length of the major axis of the primary particles was measured. The results are shown in Table 2. Figure 2 As shown in , the length of the major axis indicates the long direction as shown by the arrow.

[0182] *Evaluation of battery characteristics

[0183] The half cells according to Examples 1 to 5 and Comparative Examples 1 to 5 were charged and discharged once at 0.2 C, and the charge capacity and discharge capacity were measured. The discharge capacity is shown in Table 2.

[0184] The charging and discharging efficiency (discharging capacity / charging capacity percentage value) was obtained from the measured discharging capacity and charging capacity. The results are shown in Table 2.

[0185] In addition, the half-cells according to Examples 1 to 6 and Comparative Examples 1 to 4 were charged and discharged once at 0.2 C, and charged and discharged once at 2 C. The capacity ratio of the charge capacity at 2 C to the charge capacity at 0.2 C was obtained. The results are shown in Table 2.

[0186] Table 2

[0187]

[0188] As shown in Table 2, the batteries using the negative active materials according to Examples 1 to 5 exhibited an initial efficiency of 91% or higher and a rate characteristic of 35% or higher, which satisfied the appropriate standards for substantial practical application of an initial efficiency of 91% or higher and a rate characteristic of 35% or higher.

[0189] Meanwhile, Comparative Examples 1 to 3 exhibited an initial efficiency of 91% or more, but a rate characteristic of less than 35%, and Comparative Examples 4 and 5 exhibited a rate characteristic of 35% or more, but an initial efficiency of less than 91%. Therefore, they do not meet the appropriate standard for substantial practical application as an initial efficiency of 91% or more and a rate characteristic of 35% or more.

[0190] These results are considered to be because improvement in the internal density in the negative electrode active materials according to Examples 1 to 5 occurs when the area of ​​amorphous carbon is within 10% to 30% based on 100% of the total area of ​​the negative electrode active material.

[0191] *Measurement of battery expansion rate

[0192] The half-cells according to Examples 4 and 5 and Comparative Examples 1 and 2 were subjected to a formation charge and discharge at 0.1C. The thickness before the formation charge and discharge and the thickness after the formation charge and discharge were measured respectively. The thickness ratio (%) of the thickness after the formation charge and discharge to the thickness before the charge and discharge was calculated as the expansion rate. The results are shown in Table 3.

[0193] The half cells according to Example 4 and Comparative Examples 1 and 2 were fully charged at 0.2C, and the thickness ratio (%) of the thickness after 0.2C full charge to the thickness before charge and discharge was measured as the expansion rate.

[0194] In addition, the half-cells according to Examples 4 and 5 and Comparative Examples 1 and 2 were charged and discharged 50 times at 1C. The thickness of the battery before and after 25 cycles of charge and discharge was measured, and the thickness of the battery before and after 50 cycles of charge and discharge was measured. The thickness ratio (%) was measured by the thickness after charge and discharge after 25 cycles and the thickness before charge and discharge, respectively, and the thickness ratio (%) was measured by the thickness after charge and discharge after 50 cycles and the thickness before charge and discharge, as the expansion rate. The results are shown in Table 3.

[0195] Table 3

[0196]

[0197] As shown in Table 3, the thickness expansion ratios of the batteries according to Examples 4, 5, and 6 are much smaller than those of Comparative Examples 1 and 2.

[0198] While the present invention has been described in conjunction with what are presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A negative electrode active material for a rechargeable lithium battery, the negative electrode active material comprising: a secondary particle in which a plurality of natural graphite primary particles are aggregated, and the secondary particle includes pores; and Amorphous carbon, located in the pores, Wherein, in the cross section of the negative electrode active material, based on 100% of the total area of ​​the negative electrode active material, the area of ​​the amorphous carbon is 10% to 30%, and The negative electrode active material includes amorphous carbon in an amount of 1 wt % to 9 wt % based on 100 wt % of the total weight of the negative electrode active material.

2. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: The primary particles have a particle size of 5 μm to 15 μm.

3. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: The secondary particles have a particle size of 8 μm to 24 μm.

4. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: Natural graphite is flake graphite.

5. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: Natural graphite is flake graphite having a major axis and a minor axis, and the length of the major axis is 5 μm to 15 μm.

6. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: The secondary particles also include amorphous carbon on the surfaces of the secondary particles.

7. The negative electrode active material for a rechargeable lithium battery according to claim 6, wherein: The thickness of the amorphous carbon located on the surface of the secondary particle is 5 nm to 50 nm.

8. The negative electrode active material for a rechargeable lithium battery according to claim 1, wherein: The ratio of the thickness of the amorphous carbon filled in the pores of the secondary particles to the thickness of the primary particles is 2.0 or less.

9. A method for preparing a negative electrode active material, the method comprising the following steps: mixing an amorphous carbon precursor with secondary particles including porous, agglomerated natural graphite primary particles to prepare a mixture; Control the pressure of the mixture; maintaining the resulting product at a temperature at which the viscosity of the amorphous carbon precursor is 100 cP to 1000 cP; Crushing of the obtained material; and The obtained material is heat treated.

10. The method for preparing a negative electrode active material according to claim 9, wherein: The amorphous carbon precursor has a softening point of 200°C to 300°C.

11. The method for preparing a negative electrode active material according to claim 9, wherein: The temperature at which the viscosity of the amorphous carbon precursor is 100 cP to 1000 cP is 350° C. to 400° C.

12. The method for preparing a negative electrode active material according to claim 9, wherein: The heat treatment is performed at 950°C to 1400°C.

13. The method for preparing a negative electrode active material according to claim 9, wherein: The step of controlling the pressure of the mixture is performed using a pressurizing process by applying a pressure of 10 MPa to 100 MPa or using a depressurizing process by reducing the pressure at a pressure of -0.1 MPa to -1.0 MPa.

14. The method for preparing a negative electrode active material according to claim 9, wherein: The step of controlling the pressure is a pressurization process by applying pressure. 15 . The method for preparing a negative electrode active material according to claim 9 , further comprising screening the obtained product after controlling the pressure and before maintaining the temperature.

16. A rechargeable lithium battery, comprising: A negative electrode comprising a negative electrode active material according to any one of claims 1 to 8; A positive electrode, including a positive active material; as well as Electrolytes.

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