Anode active material for lithium secondary battery, preparation method therefor, and lithium secondary battery comprising same

Abstract

The present invention relates to an anode active material for a lithium secondary battery, a preparation method therefor, and a lithium secondary battery comprising the anode active material, the anode active material comprising a plurality of spherical natural graphite particles, wherein the spherical natural graphite particles are structured such that flaky natural graphite fragment particles are coagulated and assembled in a cabbage-like shape or randomly, phosphorus (P) atoms form C-O-P or C-P-O bonds on the edge planes of all or at least some of the flaky natural graphite fragment particles constituting the surface or inside of each of the spherical natural graphite particles, the spherical natural graphite particles have a protruding / recessed surface morphology including a structurally-protruding portion protruding from the particle surface and a structurally-recessed portion recessed from the particle surface, the percentage of the number of particles having a circularity of 0.95 or less is 66% or higher, and the tap density is 1.17 g / cm3 or greater.

Description

Negative active material for lithium secondary battery, method for producing same, and lithium secondary battery including same

[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 negative electrode active material for a lithium secondary battery.

[0002]

[0003] The demand for lithium secondary batteries as an energy source for not only mobile devices but also electric vehicles is rapidly increasing, and in relation to the expansion of these application areas, there is a need for improved performance of lithium secondary batteries in terms of stability at high temperatures, high output, and long life.

[0004] Currently, crystalline graphite materials are used as negative active materials in lithium secondary batteries, and crystalline graphite is divided into artificial graphite and natural graphite. The artificial graphite is usually obtained by heating and carbonizing a carbon precursor at a high temperature of about 3000℃ or higher in an inert atmosphere, removing impurities and undergoing a graphitization process. Therefore, the manufacturing cost is high, and due to the limitation of the degree of graphitization, the lithium storage capacity is somewhat lower than that of natural graphite. However, its use is increasing due to its relatively excellent life and output characteristics.

[0005] The currently commercialized natural graphite is made by assembling natural graphite fragments in a cabbage shape or random shape into a spherical shape and coating the surface with amorphous and / or semi-crystalline carbon.

[0006] However, in the case of the amorphous and / or semi-crystalline carbon-coated spherical natural graphite, there is a problem in that the performance is significantly reduced due to gas generation and swelling caused by side reactions with the electrolyte. This problem becomes more severe when charging and discharging are repeated at high temperatures above 45°C or when maintained for a long period of time.

[0007] The above side reaction is due to an electrolyte decomposition reaction that occurs in the flaky natural graphite fragment particles constituting the surface and interior of the spherical natural graphite particles as cracks form in the amorphous and quasi-crystalline carbon coating layer on the surface of the spherical natural graphite particles as repeated charging and discharging progresses. In particular, it is known that the edge sites, which are active sites of the flaky natural graphite fragment particles, further promote the electrolyte decomposition reaction.

[0008] In addition, the spherical natural graphite particles are manufactured by a mechanical method so that the flaky natural graphite fragment particles have a cabbage-like or randomly formed and assembled structure, and when used as an anode material for a lithium secondary battery, due to the loose formation and assembly, the particles are easily compressed during the manufacturing of an electrode through rolling, so that the flaky graphite fragment particles are oriented along the surface of the current collector, and in addition, due to the compression deformation of the spherical natural graphite particles, the gaps (or micro-spaces, pores) between the active material particles in the electrode are blocked, so that the tortuosity increases, making it difficult for lithium ions to move through the electrolyte, and thus, there is a problem that the charge / discharge characteristics deteriorate.

[0009] In the case of the above spherical natural graphite, the problem of easy compression during electrode manufacturing can be improved by densifying it, but when densifying it into a spherical shape, the active material particles in the electrode make point contact, which causes a problem of increasing the internal resistance of the battery. Therefore, a method is needed to increase the contact area between the active material particles by controlling the shape of the active material particles along with densifying the spherical natural graphite, but a method that can solve all of the above-mentioned problems has not been presented so far.

[0010] Furthermore, artificial graphite, which has relatively superior high-temperature stability, high-power characteristics, and long-life characteristics compared to the spherical natural graphite, is being used as an anode active material for lithium secondary batteries for electric vehicles. However, considering the high-cost manufacturing process and the problem of large amounts of carbon dioxide (CO2) being emitted during the manufacturing process of artificial graphite, the development of a natural graphite anode active material that has performance equivalent to or higher than that of the artificial graphite in terms of high-temperature stability, high-power characteristics, and long-life characteristics is very important for the lithium secondary battery-related industry.

[0011] Accordingly, it is urgently required to propose a method that can overcome all of the problems of the existing natural graphite negative electrode active material described above, and can replace the artificial graphite negative electrode active material that has the problem of high manufacturing cost and large carbon dioxide (CO2) emissions during the manufacturing process by using natural graphite, which has a relatively large lithium storage capacity, low manufacturing cost, and abundant resources.

[0012] The present invention is to solve the problem of spheroidized natural graphite as an anode active material of a conventional lithium secondary battery, and to provide an anode active material for a lithium secondary battery having stable charge / discharge cycle life characteristics at high temperatures and excellent high-rate charge / discharge characteristics, and a method for manufacturing the same, an anode of a lithium secondary battery including the anode active material, and a lithium secondary battery including the same, which has an increased strength of spheroidized natural graphite particles by stabilizing edge sites, which are active sites of the flaky natural graphite fragment particles constituting the surface and interior of the spheroidized natural graphite particles, and an increased electrical conductivity by increasing the electrical contact between the flaky natural graphite fragment particles constituting the interior of the spheroidized natural graphite particles, and a controlled active material particle shape so as to increase the contact area between the anode active material particles.

[0013] In order to achieve the above technical problem, the present invention provides an anode active material for a lithium secondary battery comprising a plurality of spherical natural graphite particles, wherein the spherical natural graphite particles have a structure in which flaky natural graphite fragment particles are connected and assembled in a cabbage shape or random shape, and phosphorus (P) atoms form COP or CPO bonds on the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles, and the spherical natural graphite particles have a concave / convex surface morphology including a convex portion having a structure protruding from the particle surface and a concave portion having a structure sunken from the particle surface, and the number percentage of particles having a circularity of 0.95 or less is 66% or more, and the tap density is 1.17 g / cm3 or more.

[0014] The electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode composition including the negative electrode active material, binder, conductive agent, etc. is characterized in that the orientation index of the spherical natural graphite is 0.08 or more.

[0015]

[0016] In addition, the present invention provides a negative electrode active material for a lithium secondary battery comprising spherical natural graphite particles, wherein the spherical natural graphite particles have a structure in which flaky natural graphite fragment particles are connected and assembled in a cabbage shape or random shape, and phosphorus (P) atoms are modified by forming COP or CPO bonds on the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or the inside of the spherical natural graphite particles, and additionally, an amorphous carbon coating layer derived from a hard carbon precursor and / or a semi-crystalline carbon coating layer derived from a soft carbon precursor is formed on the surface of the modified spherical natural graphite particles, and the spherical natural graphite particles coated with the amorphous or semi-crystalline carbon have a shape having a concave / convex surface morphology including a convex portion having a structure protruding from the particle surface and a concave portion having a structure sunken from the particle surface, and the circularity of the particles is 0.95. A negative electrode active material for a lithium secondary battery is provided, characterized in that the percentage of particles having the following characteristics is 66% or more and the tap density is 1.17 g / cm3 or more.

[0017] The electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode composition including the negative electrode active material, binder, conductive agent, etc. is characterized in that the orientation index of the spherical natural graphite is 0.25 or more.

[0018] At this time, the amorphous or semi-crystalline carbon coated on the surface of the spherical natural graphite particles is characterized in that it is 0.1 to 5 wt% based on the total weight of the negative electrode active material.

[0019]

[0020] The above orientation index is characterized by being the area ratio ((110) / (004)) obtained by measuring the (110) plane and (004) plane of the spherical natural graphite included in the electrode by the X-ray diffraction (XRD) method and integrating each measured XRD peak.

[0021]

[0022] One embodiment of the negative active material according to the present invention is that the modified spherical natural graphite particles preferably include flake-shaped natural graphite fragment particles in which phosphorus (P) atoms are selectively bonded only to the surface of the edge plane rather than the basal plane through a COP or CPO bond.

[0023]

[0024] One embodiment of the negative active material according to the present invention may be a negative active material for a lithium secondary battery, characterized in that the edge surfaces of all or at least some of the flaky natural graphite fragment particles constituting the surface and interior of the spherical natural graphite particles are selectively adsorbed by a phosphorus compound and then surface-modified through heat treatment, so that COP or CPO bonds are formed on the edge surfaces of the flaky natural graphite fragment particles, and the modified spherical natural graphite particles are isotropically pressurized so that the particles have a shape having a concave / convex surface morphology, and the number percentage of particles having a circularity of 0.95 or less is 66% or more, the tap density is 1.17 g / cm3 or more, and the orientation index of the spherical natural graphite is 0.08 or more in an electrode manufactured with an electrode density of 1.6 g / cm3.

[0025]

[0026] One embodiment of the negative electrode active material according to the present invention may be a negative electrode active material for a lithium secondary battery, characterized in that an amorphous or semi-crystalline carbon coating layer exists on the surface of the spherical natural graphite particles that have been isotropically pressurized so that the number percentage of particles having a shape having a concave / convex surface morphology and a circularity of 0.95 or less after modification treatment using the phosphorus compound is 66% or more, and the tap density is 1.17 g / cm3 or more, and the orientation index of the spherical natural graphite in the electrode manufactured with an electrode density of 1.6 g / cm3 is 0.25 or more.

[0027] At this time, the amorphous or semi-crystalline carbon coating layer formed on the surface of the modified spherical natural graphite particles may comprise 0.1 to 5 wt% based on the total weight of the negative electrode active material.

[0028]

[0029] And, in another aspect of the present invention, a method for producing the negative active material is provided, comprising the steps of (A) preparing a solution containing spherical natural graphite particles having a structure in which flake-shaped natural graphite fragment particles are formed and assembled in a cabbage-like or random shape, a phosphorus compound, and a solvent, (B) immersing and stirring the solution to selectively adsorb the phosphorus compound to the edge plane of all or at least some of the flake-shaped natural graphite fragment particles, (C) drying and heat-treating the solution to produce modified spherical natural graphite particles, and (D) isotropically pressing and molding the modified spherical natural graphite particles and crushing the obtained molded body.

[0030]

[0031] Another embodiment of the present invention provides a method for producing an anode active material for a lithium secondary battery, the method comprising the steps of: (a) preparing a solution containing spherical natural graphite particles having a cabbage-like or randomly-arranged or assembled structure of flaky natural graphite fragment particles, a phosphorus compound, and a solvent; (b) immersing and stirring the solution to selectively adsorb the phosphorus compound onto the edge plane of all or at least some of the flaky natural graphite fragment particles; (c) drying and heat-treating the solution to produce modified spherical natural graphite particles; (d) isotropically pressing and molding the modified spherical natural graphite particles and pulverizing the obtained molded body; and (e) coating an amorphous and / or semi-crystalline carbon precursor on the surface of the spherical natural graphite particles obtained in the step (d) and heat-treating the same to form an amorphous or semi-crystalline carbon coating layer.

[0032]

[0033] The above-mentioned phosphorus compound is characterized in that it is at least one selected from the group consisting of tricresyl phosphate (TCP), tributyl phosphate (TBP), triphenyl phosphate (TPP), triethyl phosphate (TEP), trioctyl phosphate, tritolyl phosphite, and tri-isooctylphosphite.

[0034]

[0035] In addition, the solution in step (A) or step (a) of the method for manufacturing the negative active material for the lithium secondary battery is characterized in that it contains 100 parts by weight of spherical natural graphite particles and 0.000001 to 1 part by weight of a phosphorus compound.

[0036]

[0037] In addition, in step (A) or step (a) of the method for manufacturing the negative active material for the lithium secondary battery, the solution may include a solvent selected from the group consisting of water, ethanol, acetone, methanol, and isopropanol.

[0038]

[0039] In addition, the process of adsorbing a phosphorus compound in step (B) or step (b) of the method for manufacturing a negative electrode active material for a lithium secondary battery can be performed by immersing and stirring the solution at room temperature for 1 minute to 10 hours and then drying it.

[0040]

[0041] In addition, in the step (C) or step (c) of the method for manufacturing the negative active material for the lithium secondary battery, the drying process of the solution may be performed by at least one spray drying method selected from rotary spraying, nozzle spraying, and ultrasonic spraying, a drying method using a rotary evaporator, a vacuum drying method, or a natural drying method.

[0042]

[0043] In addition, the heat treatment in step (C) or step (c) of the method for manufacturing the negative electrode active material for the lithium secondary battery may be performed in an atmosphere containing air or oxygen, an atmosphere containing nitrogen, argon or a mixed gas thereof, or under vacuum.

[0044]

[0045] When the above heat treatment is performed in an atmosphere containing nitrogen, argon or a mixed gas thereof, or under vacuum, it may be performed at a temperature of 200 to 1200°C, and when it is performed in an atmosphere containing air or oxygen, it may be performed at a temperature of 200 to 600°C.

[0046]

[0047] In addition, the isotropic pressing molding treatment in step (D) or step (d) of the method for manufacturing the negative electrode active material for the lithium secondary battery may be performed by a cold isostatic pressing method.

[0048]

[0049] In another aspect of the present invention, the present invention provides a lithium secondary battery comprising a negative electrode including the negative electrode active material, a positive electrode, and an electrolyte.

[0050]

[0051] Specific details of other embodiments of the present invention are included in the detailed description below.

[0052]

[0053] The negative active material for a lithium secondary battery according to the present invention can realize a lithium secondary battery having improved stability at high temperatures and excellent charge / discharge cycle characteristics and charge / discharge output characteristics at high temperatures and room temperatures.

[0054]

[0055] Figure 1 shows the method employed to measure circularity.

[0056] Figure 2 is an XPS analysis result of a highly oriented pyrolytic graphite sample manufactured according to Experimental Example 1 of the present invention.

[0057] Figure 3 shows the XPS analysis results of a highly oriented pyrolytic graphite sample manufactured according to Experimental Example 2 of the present invention.

[0058] Figures 4a and 4b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 2.

[0059] Figures 5a and 5b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 3.

[0060] Figures 6a and 6b are scanning electron microscope (SEM) images of the negative active material according to Example 1.

[0061] Figures 7a and 7b are scanning electron microscope (SEM) images of the negative active material according to Example 2.

[0062] Figure 8 is a scanning electron microscope (SEM) photograph of the negative active material according to Example 3.

[0063] Figure 9 is a scanning electron microscope (SEM) photograph of the negative active material according to Example 4-1.

[0064] Figure 10 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 1.

[0065] Figures 11a and 11b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 4.

[0066] Figure 12 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 5.

[0067] Figure 13 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode active material according to Example 2.

[0068] Figure 14 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode active material according to Example 3.

[0069] Figure 15 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode active material according to Example 4-1.

[0070] Figure 16 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode active material according to Comparative Example 1.

[0071] Figure 17 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured with an electrode density of 1.6 g / cm3 using a negative electrode active material according to Comparative Example 2.

[0072] Figure 18 is a circularity distribution curve for the negative electrode active material according to Example 2 and Comparative Example 3.

[0073] Figure 19 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of negative electrodes manufactured using negative electrode active materials according to Examples 2 and 3 and Comparative Examples 1, 2, and 4.

[0074] Figure 20 shows the full cell application cycle life characteristics using an electrolyte solution in which 1 M LiPF6 is dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) for electrodes manufactured using the respective negative active materials according to Examples 2, 3, and 4-2 and Comparative Examples 1 and 2.

[0075] Figure 21 shows the full cell application cycle life characteristics of the negative electrodes manufactured using the negative electrode active materials according to Example 3 and Comparative Example 1, using an electrolyte in which 1 M LiPF6 is dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8), and the full cell application cycle life characteristics of the negative electrodes manufactured using the negative electrode active material according to Comparative Example 1, using an electrolyte in which 0.5 wt% VC (Vinylene Carbonate) is added as an additive to the electrolyte.

[0076] Figure 22 shows the capacity change (Figure 22a) and cycle coulombic efficiency (Figure 22b) changes according to charge and discharge cycles measured in a battery using an electrolyte in which 1 M LiPF6 is dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8), for electrodes using negative active materials according to Example 3 and Comparative Examples 1 and 5, after fully charging at 30°C, maintaining at 80°C for 2 weeks, and then discharging at 30°C and recharging.

[0077] Figure 23 shows the capacity change (Figure 23a) and cycle coulombic efficiency (Figure 23b) changes according to charge / discharge cycles measured in a battery using an electrolyte in which 0.5 wt% VC (Vinylene Carbonate) was added as an additive to an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8), for electrodes using negative active materials according to Example 3 and Comparative Examples 1 and 5, after being fully charged at 30°C, maintained at 80°C for 2 weeks, and then discharged at 30°C and recharged.

[0078] Figures 24a and 24b show capacity changes and cycle coulombic efficiency changes according to cycles during charge and discharge cycles at 30°C after the initial formation process for electrodes using negative active materials according to Example 3 and Comparative Examples 1 and 5.

[0079] FIG. 25 shows the results of comparing the charge rate characteristics measured at 30°C after the formation process at 30°C and the charge rate characteristics measured at 30°C after 1000 cycles at 45°C after the formation process in a full cell using an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) for the negative electrodes manufactured using the respective negative active materials according to Example 3 and Comparative Example 1, and the electrolyte in which 0.5 wt% VC (Vinylene Carbonate) was added as an additive.

[0080] Figure 26 is a scanning electron microscope (SEM) photograph of the surface of a negative electrode manufactured using a negative electrode active material according to Comparative Example 1 after a formation process (Figure 26a) and after 1000 cycles at 45°C (Figure 26b), and of a negative electrode manufactured using a negative electrode active material according to Example 3 after a formation process (Figure 26c) and after 1000 cycles at 45°C (Figure 26d).

[0081]

[0082] In describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

[0083]

[0084] Embodiments according to the concept of the present invention may be modified in various ways and take various forms. Therefore, specific embodiments are illustrated in the drawings and described in detail in this specification or application. However, this is not intended to limit embodiments according to the concept of the present invention to specific disclosed forms, and it should be understood that all modifications, equivalents, and alternatives included within the spirit and technical scope of the present invention are included.

[0085]

[0086] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the present invention. The singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, it should be understood that the terms "comprises" or "has" indicate the presence of a described feature, number, step, operation, component, part, or combination thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0087]

[0088] Hereinafter, the present invention will be described in detail.

[0089]

[0090] The negative electrode active material for a lithium secondary battery according to the present invention comprises spherical natural graphite particles, wherein the spherical natural graphite particles have a structure in which flaky natural graphite fragment particles are connected and assembled in a cabbage shape or random shape, and phosphorus (P) atoms form COP or CPO bonds on the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles, and the spherical natural graphite particles have a shape having a concave / convex surface morphology including a convex portion having a structure protruding from the particle surface and a concave portion having a structure sunken from the particle surface, and the number percentage of particles having a circularity of 0.95 or less is 66% or more, a tap density is 1.17 g / cm3 or more, and an orientation index of the spherical natural graphite in an electrode manufactured with an electrode density of 1.6 g / cm3 is characterized in that the spherical natural graphite is 0.08 or more. Do it.

[0091]

[0092] Circularity is used as a representative parameter indicating the shape of spherical natural graphite particles included in the negative active material for a lithium secondary battery according to the present invention. The circularity indicates how close a particle is to a circle, and Figure 1 illustrates a method employed to measure circularity.

[0093] The circularity of the particle P observed by projecting the three-dimensional natural graphite negative electrode active material particle onto a two-dimensional plane is as shown in Figure 1, with a ratio P D / P r As corresponding to, here P r represents the perimeter of the observed particle, and P D represents the perimeter of a disk having the same area as the observed particle.

[0094] Accordingly, circularity is defined as follows:

[0095] Circularity = (4πA p ) / P r 2

[0096] Here, the circularity of particle P is the area A of particle P observed in the photograph. p The perimeter P of a disk D having an area equal to D It is obtained by determining the circumference P of the particle P. r It is also confirmed.

[0097] Circularity has a value from 0 to 1. The circularity of a perfect circle is 1, and the circularity of a very pointed or irregularly shaped particle has a value close to 0.

[0098]

[0099] As an example of the negative active material according to the present invention, it is preferable that the edge planes of all or at least some of the spherical natural graphite particles and the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite are selectively adsorbed by a phosphorus compound, and then surface modified through heat treatment, and then the spherical graphite particles are densified through isotropic pressure treatment, and the spherical natural graphite particles have a shape having a concave / convex surface morphology, and the number percentage of particles having a circularity of 0.95 or less is 66% or more.

[0100] Accordingly, when the spherical natural graphite is used as an anode active material for a lithium secondary battery, the electrolyte decomposition reaction is suppressed at the edge sites, which are active sites of the flaky natural graphite fragment particles, in the spherical natural graphite particles, and at the same time, the resistance to compression is increased during the manufacture of the electrode through rolling, thereby improving the orientation index characteristics. In addition, due to the particle shape in which the particle strength is increased due to densification and the number percentage of particles having a concave / convex surface morphology and a circularity of 0.95 or less is 66% or more, the movement of lithium ions through the pores (or micro-spaces, pores) between the anode active material particles in the electrode is smooth, and the contact area between the anode active material particles is increased, thereby improving electrical conductivity, thereby improving the charge / discharge characteristics of the anode of a lithium secondary battery using the anode active material.

[0101]

[0102] One embodiment of the negative electrode active material according to the present invention is a negative electrode active material for a lithium secondary battery, characterized in that an amorphous or semi-crystalline carbon coating layer is formed on the surface of spherical natural graphite particles having a shape having a concave / convex surface morphology by pressurizing after the modification treatment and a percentage of the number of particles having a circularity of 0.95 or less is 66% or more, and a tap density is 1.17 g / cm3 or more, and an orientation index of the spherical natural graphite is 0.25 or more in an electrode manufactured with an electrode density of 1.6 g / cm3.

[0103]

[0104] The amount of amorphous or semi-crystalline carbon coating on the surface of the spherical natural graphite particles subjected to pressure molding after the above modification treatment may be 0.1 to 5 wt% based on the total weight of the negative electrode active material, and preferably 0.1 to 3 wt%.

[0105]

[0106] When amorphous or semi-crystalline carbon is coated on the surface of the spherical natural graphite particles that have been subjected to isotropic pressure molding after the above modification treatment, the strength of the particles can additionally increase according to the formation of the amorphous and / or semi-crystalline carbon coating layer, and thus, the orientation index is preferably 0.25 or more.

[0107]

[0108] In the present invention, with respect to the orientation index of the spherical natural graphite, considering that the electrode density of most commercially available spherical natural graphite negative electrodes is approximately 1.6 g / cm3, a value measured in an electrode manufactured with an electrode density of 1.6 g / cm3 was determined as the orientation index of the spherical natural graphite.

[0109]

[0110] In the negative electrode manufactured from the above spherical natural graphite, the orientation index of the spherical natural graphite may depend on the density of the electrode.

[0111]

[0112] In the present invention, the orientation index is measured by an X-ray diffraction (XRD) method, and is characterized by being an area ratio ((110) / (004)) obtained by integrating the measured X-ray diffraction peaks of the (110) plane and the (004) plane of the spherical natural graphite included in the electrode.

[0113]

[0114] The above spherical natural graphite particles can be formed by the methods presented in Korean Patent Publication Nos. 2003-0087986 and 2005-0009245, but are not limited thereto. For example, by performing a step of repeatedly processing flaky natural graphite having an average particle diameter of 30 ㎛ or more using a rotary processing machine, the flaky natural graphite particles are assembled through crushing by collision between the inner surface of the rotary processing machine and the flaky natural graphite powder, friction processing, shear processing of the powder by shear stress, etc., so that spherical natural graphite particles can be ultimately manufactured.

[0115]

[0116] In this way, the spherical natural graphite particles can be formed by assembling and forming flaky natural graphite fragments into a cabbage shape or random shape. More preferably, the flaky natural graphite fragments can be formed by forming and assembling them into a cabbage shape on the surface and a random shape in the center.

[0117]

[0118] Additionally, the above spherical natural graphite particles may be oval in shape as well as circular.

[0119]

[0120] The average particle diameter (D50) of the above spherical natural graphite particles may be 10 to 20 μm, and specifically, 12 to 18 μm. The D50 refers to the average diameter of particles having a cumulative volume of 50% by volume in a particle size distribution. When spherical natural graphite particles having an average particle diameter within the above range are used, the flake-like natural graphite fragments are connected in a cabbage shape or random shape, facilitating the assembly process and improving the electrochemical characteristics.

[0121]

[0122] The spherical natural graphite particles manufactured by the mechanical method described above have a problem in that, when used as an anode material for a lithium secondary battery, they are easily compressed during the manufacture of an electrode through rolling due to the loose bonding and assembly of the flaky graphite fragment particles, so that the flaky graphite fragment particles are oriented along the surface of the current collector, and further, the compression deformation of the spherical natural graphite particles blocks the pores between the active material particles in the electrode, increasing the tortuosity, making it difficult for lithium ions to move through the electrolyte, thereby deteriorating the charge / discharge characteristics.

[0123]

[0124] The negative active material according to the present invention described above can be manufactured by the following method.

[0125] That is, after manufacturing spherical natural graphite particles by selectively adsorbing the phosphorus compound on the edge surface of each of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles, and then modifying the spherical natural graphite particles by forming COP or CPO bonds on the edge surface of the flaky natural graphite fragment particles through heat treatment, and then going through a process of densifying the spherical graphite particles by isotropic pressing treatment, it is possible to manufacture a negative electrode active material having the characteristics of having a concave / convex surface morphology along with increased particle strength and a percentage of particles having a circularity of 0.95 or less of 66% or more.

[0126]

[0127] An amorphous and / or semi-crystalline carbon coating layer can be additionally formed on the surface of the spherical natural graphite particles subjected to the surface modification and isotropic pressure molding treatment.

[0128]

[0129] The above spherical natural graphite particles are physically assembled using mechanical energy through pulverization by collision between flaky natural graphite powders, friction processing between powders, shear processing of powders by shear stress, etc., so that fine gaps exist between the flaky natural graphite fragment particles that constitute the above spherical natural graphite particles.

[0130]

[0131] In the case of the above phosphorus compound, since the molecular weight is very small, a solution containing the phosphorus compound can flow into the fine gaps between the flaky natural graphite fragment particles constituting the spherical natural graphite particles during the adsorption process. Accordingly, the phosphorus compound can be selectively adsorbed not only on the surface of the flaky natural graphite fragment particles constituting the surface of the spherical natural graphite particles, but also on the edge surfaces of at least some of the flaky natural graphite fragment particles present inside the spherical natural graphite particles.

[0132]

[0133] The above-mentioned phosphorus compound may include at least one selected from the group consisting of tricresyl phosphate (TCP), tributyl phosphate (TBP), triphenyl phosphate (TPP), triethyl phosphate (TEP), trioctyl phosphate, tritolyl phosphite, and tri-isooctylphosphite.

[0134]

[0135] The phosphorus compound used to manufacture the modified spherical natural graphite particles using the above phosphorus compound may be included in an amount of 0.000001 to 1 part by weight, and more preferably 0.00001 to 0.5 part by weight, based on 100 parts by weight of the spherical natural graphite particles. When the phosphorus compound exceeds 1 part by weight, resistance to charge transfer on the surface of the spherical natural graphite particles may increase, resulting in deterioration of output characteristics and cycle characteristics. When the phosphorus compound is less than 0.000001 part by weight, the surface modification effect on the surface of the spherical natural graphite particles may be insufficient.

[0136]

[0137] According to one embodiment of the present invention, the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles is selectively adsorbed by a phosphorus compound and then surface-modified through heat treatment. The phosphorus compound may be included in an amount of 0.000001 to 1 part by weight, and more preferably, 0.00001 to 0.5 part by weight, based on 100 parts by weight of the spherical natural graphite particles, so that there is almost no capacity reduction as an anode active material for a lithium secondary battery due to the modification.

[0138]

[0139] And, in another aspect of the present invention, a method for producing the negative active material is provided, comprising the steps of (A) preparing a solution containing spherical natural graphite particles having a structure in which flaky natural graphite fragment particles are formed and assembled in a cabbage shape or random shape, a phosphorus compound, and a solvent, (B) immersing and stirring the solution to selectively adsorb the phosphorus compound to the edge plane of all or at least some of the flaky natural graphite fragment particles, (C) drying and heat-treating the solution to produce modified spherical natural graphite particles, and (D) isotropically pressing and molding the modified spherical natural graphite particles and crushing the obtained molded body.

[0140]

[0141] Another embodiment of the present invention provides a method for producing an anode active material for a lithium secondary battery, the method comprising the steps of: (a) preparing a solution containing spherical natural graphite particles having a cabbage-like or randomly-arranged or assembled structure of flaky natural graphite fragment particles, a phosphorus compound, and a solvent; (b) immersing and stirring the solution to selectively adsorb the phosphorus compound onto the edge plane of all or at least some of the flaky natural graphite fragment particles; (c) drying and heat-treating the solution to produce modified spherical natural graphite particles; (d) isotropically pressing and molding the modified spherical natural graphite particles and pulverizing the obtained molded body; and (e) coating an amorphous and / or semi-crystalline carbon precursor on the surface of the spherical natural graphite particles that have been subjected to the pressurization and molding after the modification and heat-treating to form an amorphous or semi-crystalline carbon coating layer.

[0142]

[0143] In the step (A) or step (a) of the method for manufacturing the negative active material for the lithium secondary battery, the solution is characterized in that it contains 100 parts by weight of spherical natural graphite particles and 0.000001 to 1 part by weight of a phosphorus compound.

[0144]

[0145] In addition, the solution in step (A) or step (a) of the method for producing a negative electrode active material for a lithium secondary battery may include a solvent selected from the group consisting of water, ethanol, acetone, methanol, and isopropanol.

[0146]

[0147] In addition, the process of adsorbing a phosphorus compound in step (B) or step (b) of the method for manufacturing a negative electrode active material for a lithium secondary battery can be performed by immersing and stirring the solution at room temperature for 1 minute to 10 hours and then drying it.

[0148]

[0149] In addition, the drying process of the solution in step (C) or step (c) of the method for producing the negative active material for the lithium secondary battery may be performed by at least one spray drying method selected from rotary spraying, nozzle spraying, and ultrasonic spraying, a drying method using a rotary evaporator, a vacuum drying method, or a natural drying method.

[0150]

[0151] In addition, the heat treatment in step (C) or step (c) of the method for manufacturing the negative active material for the lithium secondary battery may be performed in an atmosphere containing air or oxygen, an atmosphere containing nitrogen, argon or a mixed gas thereof, or under vacuum.

[0152]

[0153] When the above heat treatment is performed in an atmosphere containing nitrogen, argon or a mixed gas thereof, or under vacuum, it may be performed at a temperature of 200 to 1200°C, and when it is performed in an atmosphere containing air or oxygen, it may be performed at a temperature of 200 to 600°C.

[0154]

[0155] If the above heat treatment is performed at a temperature exceeding 1200°C in an atmosphere containing nitrogen, argon, or a mixed gas thereof, or under vacuum, or if the heat treatment is performed at a temperature exceeding 600°C in an atmosphere containing air or oxygen, most of the adsorbed phosphorus compounds may be decomposed and removed, resulting in an insufficient modification effect on the surface of the spherical natural graphite particles.

[0156] In addition, if the heat treatment is performed at a temperature lower than 200°C, the adsorbed phosphorus compound may not be sufficiently decomposed, and thus the surface modification effect on the spherical natural graphite particles may be insufficient.

[0157]

[0158] In step (D) or step (d) of the method for manufacturing the negative active material for the lithium secondary battery, the isotropic pressing treatment for the modified spherical natural graphite particles can be performed using a cold isostatic pressing method at room temperature, which can be performed conventionally.

[0159]

[0160] The pressure applied during the above isotropic pressing molding process is preferably such that a spherical natural graphite negative electrode active material having a tap density of 1.17 g / cm3 or more and a particle shape having a concave / convex surface morphology and a particle circularity of 0.95 or less is 66% or more, and in an electrode manufactured using the spherical natural graphite negative electrode active material, an orientation index of the spherical natural graphite is 0.08 or more in an electrode manufactured with an electrode density of 1.6 g / cm3.

[0161]

[0162] In the case of a negative electrode active material having amorphous or semi-crystalline carbon coated on the surface of spherical natural graphite particles that have been isotropically press-molded after the above modification treatment, it is preferable that the orientation index of the spherical natural graphite manufactured with an electrode density of 1.6 g / cm3 using the above negative electrode active material be 0.25 or more.

[0163]

[0164] The above spherical natural graphite may not have a uniform assembled state (e.g., particle size, particle strength, density, etc.) because the flaky natural graphite particles are physically assembled using mechanical energy. Accordingly, under a pressing condition exceeding a certain standard of the pressing pressure during the isotropic pressing molding process, the degree of mechanical or physical deformation of each of the spherical natural graphite assembled particles may be different, so that a particle shape having a concave / convex surface morphology of a spherical particle shape and a circularity of 0.95 or less in which the percentage of particles is 66% or more may be obtained. However, the upper limit of the pressing pressure is not particularly limited.

[0165]

[0166] The average particle diameter (D50) of the above spherical natural graphite particles may be 10 to 20 μm, and specifically, 12 to 18 μm.

[0167]

[0168] In step (e) of the method for manufacturing the negative electrode active material for the lithium secondary battery, in coating an amorphous and / or semi-crystalline carbon precursor on the surface of the spherical natural graphite particles that have been subjected to a pressure molding process after the modification process and heat-treating to form an amorphous or semi-crystalline carbon coating layer, the amorphous or semi-crystalline carbon precursor is selected from the group consisting of citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, starch, phenol resin, furan resin, furfuryl alcohol, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, polyvinyl alcohol, polyvinyl chloride, coal pitch, petroleum pitch, It may include mesophase pitch, low molecular weight heavy oil, glucose, gelatin, saccharides, or a combination thereof. However, the present invention is not limited to the type of the carbon precursor.

[0169] In step (e) of the method for manufacturing the negative electrode active material for the lithium secondary battery, the amorphous and semi-crystalline carbon precursor coating can be performed by a wet or dry method.

[0170]

[0171] In step (e) of the method for manufacturing the negative electrode active material for the lithium secondary battery, the heat treatment may be performed in an atmosphere containing nitrogen, argon, or a mixed gas thereof, or under vacuum.

[0172]

[0173] In step (e) of the method for manufacturing the negative electrode active material for the lithium secondary battery, the heat treatment temperature may be performed at a temperature of 800 to 1200°C, preferably 800 to 1000°C. When the heat treatment is performed at a temperature in the above range, foreign elements corresponding to impurities can be sufficiently removed during the carbonization process of the amorphous and / or semi-crystalline carbon precursor, thereby reducing the irreversible capacity, and the surface modification state of the flaky graphite of the spherical natural graphite can be well maintained, resulting in excellent charge / discharge characteristics as the negative electrode active material.

[0174]

[0175] Meanwhile, the total amount of the amorphous and / or semi-crystalline carbon coating may be comprised in an amount of 0.1 to 10 wt%, and more preferably 0.1 to 5 wt%, based on the total amount of the negative electrode active material. When the amorphous and / or semi-crystalline carbon is comprised within the above range, the coating by the amorphous and / or semi-crystalline carbon is effectively formed, and thus the negative electrode active material may exhibit excellent properties.

[0176]

[0177] When manufacturing an electrode using a negative electrode active material according to one embodiment of the present invention, particles having a circularity of 0.95 or less and having a shape having a concave / convex surface morphology are used in which the number percentage of particles is 66% or more, so that the shape of the negative electrode active material can be maintained even at an electrode density of 1.6 g / cm3 or more, micro-pores are effectively formed between the negative electrode active material particles, so that impregnation of an electrolyte is easy through the micro-pores, and the contact area between the active material particles increases, so that the internal resistance of the electrode decreases, so that high-rate charge / discharge characteristics can be improved.

[0178]

[0179] Furthermore, according to one embodiment of the present invention, since the edge surfaces of all or at least some of the flaky natural graphite particles constituting the surface or interior of the spherical natural graphite particles are modified through the phosphorus compound, even when repeatedly charged and discharged at high temperatures, the change in the edge surface structure of the flaky graphite is small and side reactions with the electrolyte are minimized, so that the charge and discharge efficiency during the cycle can be improved.

[0180]

[0181] Therefore, the negative active material according to the present invention can simultaneously achieve improved high-rate charge / discharge characteristics, as well as excellent cycle characteristics and stability at high temperatures.

[0182]

[0183] If the existing method, such as forming an amorphous carbon coating layer on the surface of spherical natural graphite or modifying or densifying the edge surface of the flaky natural graphite particles constituting the surface or interior of the spherical natural graphite particles, is used, the improvement of the properties intended in the present invention cannot be sufficiently achieved.

[0184]

[0185] Furthermore, in another aspect of the present invention, a lithium secondary battery is provided comprising a negative electrode including the negative active material, a positive electrode, and an electrolyte.

[0186]

[0187] Lithium secondary batteries can be categorized into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries based on the type of separator and electrolyte used. They can also be categorized by shape into cylindrical, prismatic, coin-shaped, and pouch-shaped types. They can also be categorized by size into bulk and thin-film types. The structures and manufacturing methods of these batteries are widely known in the industry, so a detailed description will be omitted.

[0188]

[0189] The above negative electrode can be manufactured by mixing the above-described negative electrode active material, binder, and optionally a conductive material to prepare a composition for forming a negative electrode active material layer, and then applying the mixture to a negative electrode current collector. Since the composition of these negative electrodes is widely known in the art, a detailed description thereof will be omitted.

[0190]

[0191] Hereinafter, examples will be provided to specifically explain the present specification. However, the embodiments described herein may be modified in various ways, and the scope of the present specification is not limited to the embodiments described below. The embodiments described herein are provided to more fully explain the present specification to those of ordinary skill in the art.

[0192]

[0193] Example 1

[0194] 100 parts by weight of spherical natural graphite particles having an average particle diameter (D50) of 16 ㎛ (provided by POSCO Chemical Co., Ltd.) and 0.1 part by weight of tricresyl phosphate (TCP) were added to ethanol, stirred for 30 minutes, dried, and then heat-treated in a nitrogen atmosphere at 800°C for 30 minutes to obtain spherical natural graphite-modified particles. The obtained spherical natural graphite-modified particles were charged into a cold isostatic pressing device, isotropically pressed at a pressure of about 400 MPa, and the molded body was pulverized to produce an anode active material for a lithium secondary battery.

[0195]

[0196] Example 2

[0197] A negative electrode active material for a lithium secondary battery was manufactured in the same manner as in Example 1, except that isotropic pressing was performed at a pressure of 500 MPa.

[0198]

[0199] Example 3

[0200] A negative electrode active material for a lithium secondary battery was manufactured in the same manner as in Example 1, except that spherical natural graphite modified particles were manufactured using 100 parts by weight of the above-mentioned spherical natural graphite particles and 0.3 parts by weight of tricresyl phosphate (TCP), and isotropic pressing was performed at a pressure of about 300 MPa.

[0201]

[0202] Example 4-1

[0203] For the modified and isotropically pressed spherical natural graphite particles obtained in the same manner as Example 2, petroleum pitch was coated on the surface of the modified and isotropically pressed spherical natural graphite particles based on 5 wt% residual carbon content after carbonization, and then heat-treated at 1000°C for 1 hour in a nitrogen atmosphere and then cooled to manufacture a negative electrode active material for a lithium secondary battery coated with amorphous and semi-crystalline carbon.

[0204]

[0205] Example 4-2

[0206] For the modified and isotropically pressed spherical natural graphite particles obtained in the same manner as Example 2, petroleum pitch was coated on the surface of the modified and isotropically pressed spherical natural graphite particles based on 3 wt% residual carbon content after carbonization, and then heat-treated at 1000°C for 1 hour in a nitrogen atmosphere and then cooled to manufacture a negative electrode active material for a lithium secondary battery coated with amorphous and semi-crystalline carbon.

[0207]

[0208] Example 5

[0209] A negative electrode active material for a lithium secondary battery coated with amorphous and semi-crystalline carbon was manufactured using modified and isotropically pressed spherical natural graphite particles obtained in the same manner as in Example 3 in the same manner as in Example 4-2.

[0210]

[0211] Comparative Example 1

[0212] Amorphous carbon was coated on the surface of spherical natural graphite particles with an average particle diameter (D50) of 16 ㎛ (based on 5 wt% residual carbon content after carbonization) and used as a negative electrode active material.

[0213]

[0214] Comparative Example 2

[0215] 100 parts by weight of spherical natural graphite particles having an average particle diameter (D50) of 16 μm and 0.3 parts by weight of tricresyl phosphate (TCP) were added to ethanol, stirred for 30 minutes, dried, and then heat-treated at 800°C for 30 minutes in a nitrogen atmosphere to obtain spherical natural graphite-modified particles.

[0216]

[0217] Comparative Example 3

[0218] The negative electrode active material is a spherical natural graphite particle with an average particle diameter (D50) of 16㎛ that is not coated with amorphous carbon.

[0219]

[0220] Comparative Example 4

[0221] The spherical natural graphite particles of Comparative Example 3 were loaded into a cold isostatic pressing device, and the molded body was isostatically pressed at a pressure of about 500 MPa, and then crushed to manufacture a negative electrode active material for a lithium secondary battery.

[0222]

[0223] Comparative Example 5

[0224] Commercial artificial graphite with an average particle diameter (D50) of 20 ㎛ and amorphous carbon coated on the particle surface (based on 5 wt% residual carbon after carbonization) was used as the negative electrode active material.

[0225]

[0226] Experimental Example 1

[0227] A highly oriented pyrolytic graphite sample and 5 wt% of tricresyl phosphate (TCP) relative to the highly oriented pyrolytic graphite were added to ethanol, stirred at room temperature for 30 minutes, dried, and then heat-treated at 300°C and 400°C for 1 hour in an air atmosphere.

[0228]

[0229] FIG. 2 shows the results of X-ray photoelectron spectroscopy (XPS) analysis of highly oriented pyrolytic graphite manufactured according to Experimental Example 1. When tricresyl phosphate (TCP) was adsorbed onto the highly oriented pyrolytic graphite sample and then heat-treated at 300°C and 400°C for 1 hour in an air atmosphere, as shown in FIG. 2a, bonds related to the P element were formed on the edge plane, but no bonds related to the P element were formed on the basal plane (FIG. 2b). Accordingly, it can be seen that the phosphorus compound of the present invention is selectively adsorbed on the edge plane of the artificial graphite, and it can be seen that as the subsequent heat treatment temperature increases after drying, the phosphorus compound adsorbed on the edge plane is decomposed, and the bonds related to the P element decrease. The P element located at the edge of the graphite surface appears to form a CPO or COP bond.

[0230]

[0231] Experimental Example 2

[0232] A highly oriented pyrolytic graphite sample and 0.5 wt% of tricresyl phosphate (TCP) relative to the highly oriented pyrolytic graphite were added to ethanol, stirred at room temperature for 30 minutes, dried, and then heat-treated at 800°C in a nitrogen atmosphere for 0.5 hours.

[0233]

[0234] Figure 3 shows the P2p peak as a result of X-ray photoelectron spectroscopy (XPS) analysis of the highly oriented pyrolytic graphite manufactured according to Experimental Example 2. When tricresyl phosphate (TCP) was adsorbed onto the highly oriented pyrolytic graphite sample and then heat-treated at 800°C for 0.5 hour in a nitrogen atmosphere, as shown in Figure 3a, a bond related to the P element was formed on the edge plane, but no bond related to the P element was formed on the basal plane (Figure 3b). This appears to be consistent with the results of the highly oriented pyrolytic graphite manufactured according to Experimental Example 1.

[0235] Accordingly, it can be seen that the phosphorus compound of the present invention is selectively adsorbed to the edge surface of the highly oriented pyrolytic graphite, and the phosphorus compound adsorbed to the edge surface after drying is decomposed during a subsequent heat treatment, and the P element located on the edge surface of the highly oriented pyrolytic graphite surface appears to form a CPO or COP bond.

[0236]

[0237] Scanning electron microscope (SEM) image analysis

[0238] FIGS. 4a and 4b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 2, and FIGS. 5a and 5b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 3.

[0239] Referring to the SEM photographs of FIGS. 4 and 5, the surface morphology of at least some of the flaky natural graphite fragment particles forming the surface of the spherical natural graphite particles as well as the edge surfaces of the flaky natural graphite fragment particles existing inside the spherical natural graphite particles is selectively modified using the phosphorus compound, and the result shows a surface morphology almost similar to that of Comparative Example 3, which is the original sample (pristine sample) without surface modification.

[0240] In particular, as shown in the enlarged SEM image for Comparative Example 3 of FIG. 5b, the spherical natural graphite particles as a pristine sample are physically assembled using mechanical energy through pulverization by collision between flaky natural graphite powders, friction processing between powders, shearing of powders by shear stress, etc., so that fine gaps exist between the flaky natural graphite fragment particles constituting the spherical natural graphite particles. Accordingly, in the case of a phosphorus compound with a small molecular weight, the molecular weight is very small, so that a solution containing the phosphorus compound is introduced into the fine gaps between the flaky natural graphite fragment particles constituting the spherical natural graphite particles during the adsorption process of the phosphorus compound as a modification process, so that the phosphorus compound can be selectively adsorbed onto the surface and the edge surface of each of the flaky natural graphite fragment particles present inside the spherical natural graphite particles.

[0241]

[0242] Figures 6 to 8 are scanning electron microscope (SEM) photographs of the negative active materials according to Examples 1 to 3, respectively.

[0243] Referring to the SEM photographs of FIGS. 6a, 7a, and 8a, unlike FIGS. 4a (Comparative Example 2) and 5a (Comparative Example 3) which show a spherical shape with a smooth surface, the negative active materials according to Examples 1 to 3 show a shape with a concave / convex surface morphology after isotropic pressing of the spherical natural graphite particles.

[0244]

[0245] In addition, referring to the SEM photographs of FIGS. 6b, 7b, and 8b, unlike FIGS. 4b (Comparative Example 2) to 5b (Comparative Example 3) in which fine gaps exist between the flaky natural graphite fragment particles constituting the spherical natural graphite particles on the surface of the spherical natural graphite particles, it can be confirmed that the negative active materials according to Examples 1 to 3 have fine gaps almost closed between the flaky natural graphite fragment particles constituting the particles on the surface of the particles.

[0246]

[0247] Figure 9 is a scanning electron microscope (SEM) photograph of the negative active material according to Example 4-1.

[0248] Referring to the SEM photograph of Fig. 9, the particle shape with concave / convex surface morphology of the negative active material according to Example 4-1 appears to be maintained even after amorphous or quasi-crystalline carbon coating.

[0249]

[0250] Figure 10 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 1.

[0251] In the case of Comparative Example 1 (Fig. 10), which is a negative electrode active material coated with amorphous carbon on the negative electrode active material of Comparative Example 3 (Fig. 5), a spherical shape with a smooth surface is exhibited.

[0252]

[0253] Fig. 11 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 4. It shows a particle shape with a concave / convex surface morphology similar to the negative active materials according to Examples 1 to 3.

[0254]

[0255] From this, it is shown that in order to have a particle shape with a concave / convex surface morphology of the negative active material according to Examples 1 to 3 (Figs. 6b, 7b and 8b), a press forming process capable of exhibiting a tap density higher than a certain standard during isotropic pressing treatment is required.

[0256]

[0257] Figure 12 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 5.

[0258] Referring to Figure 12, commercial artificial graphite shows an amorphous shape in which small artificial graphite particles are assembled.

[0259]

[0260] Scanning electron microscope (SEM) photographs of the cathode surface manufactured with an electrode density of 1.6 g / cm3 according to Examples 2 to 4-1 and Comparative Examples 1 and 2 are shown in FIGS. 13 to 15, 16, and 17.

[0261]

[0262] Figures 13 to 15 are scanning electron microscope (SEM) photographs of the surface of the cathode manufactured according to Examples 2, 3, and 4-1, respectively, and Figures 16 and 17 are scanning electron microscope (SEM) photographs of the surface of the cathode manufactured according to Comparative Examples 1 and 2, respectively.

[0263] Referring to Figures 16 and 17, when surface modification is performed on spherical natural graphite (Figure 17), the spherical natural graphite particles are severely pressed due to the pressing process during electrode manufacturing, and when amorphous or semi-crystalline carbon is coated on spherical natural graphite (Figure 16), the spherical natural graphite particles are also pressed due to the pressing process during electrode manufacturing.

[0264]

[0265] Referring to FIGS. 13 to 15, it appears that the particle crushing phenomenon is significantly improved in the cathodes manufactured according to Examples 2, 3, and 4-1.

[0266]

[0267] Circularity measurement

[0268] Measurements were performed using the Morphologi 4, an automated imaging system from Malvern Panalytical. Particles were uniformly dispersed and arranged on a sheet, photographed directly above the sheet, and analyzed. The circularity of particles may vary depending on the viewing direction. In this experimental example, measurements were performed on 10,000 particles.

[0269] The circularity of the negative active materials according to Examples 1 to 3 and Comparative Example 3 was measured, and Fig. 18 is a drawing comparing the circularity distribution curves for the negative active materials according to Example 2 and Comparative Example 3. Referring to Fig. 18, it can be confirmed that the circularity is lowered overall due to shape change according to the isotropic pressing molding process.

[0270] From the circularity distribution results investigated in the same manner, the percentage of particles having a circularity of 0.95 or less in the negative active materials according to Examples 1 to 3 and Comparative Example 3 was measured. The results are shown in Table 1.

[0271]

[0272] [Table 1]

[0273]

[0274]

[0275] Tap density measurement

[0276] The tap density of the negative active materials obtained according to Examples 1 to 3 and 4-1 and Comparative Examples 1, 2, and 4 was measured. The results are shown in Table 2.

[0277] The negative active materials according to Examples 1 to 3 and 4-1 have a higher tap density than the negative active materials obtained according to Comparative Examples 1 and 2, which is 1.17 g / cm. 3 ideal The value was indicated.

[0278] Meanwhile, Comparative Example 4, which was manufactured through the same pressure molding process as Example 2, exhibited the same tap density as Example 2.

[0279]

[0280] [Table 2]

[0281]

[0282]

[0283] Orientation index measurement

[0284] The orientation index was measured using an XRD analysis method for electrodes manufactured to have an electrode density of 1.6 g / cm3 using the negative active materials obtained according to Examples 1, 2, and 4-1 and Comparative Examples 1 and 2. The results are shown in Table 3.

[0285] The orientation index was calculated as the area ratio ((110) / (004)) obtained by integrating the X-ray diffraction peaks (Cu Kα1-line) for the (110) plane and the (004) plane of the spherical natural graphite negative electrode active material included in the above electrode.

[0286] To measure the orientation index, the electrodes were prepared by coating a slurry containing each negative active material mixed with CMC / SBR (carboxymethyl cellulose / styrene-butadiene rubber) at a weight ratio of 96:4 on aluminum foil to exclude overlapping of the X-ray diffraction peaks, followed by drying and pressing.

[0287] In the negative electrode manufactured with an electrode density of 1.6 g / cm3 according to Examples 1 and 2, the orientation index of the negative electrode active material exhibits a value of 0.08 or more, whereas in the case of Comparative Example 2, the orientation index exhibits a value of 0.08 or less.

[0288] Also, referring to Example 4-1 and Comparative Example 1, when amorphous or quasi-crystalline carbon is coated on the spherical graphite surface, the orientation index increases, but Example 4-1 shows a larger orientation index.

[0289]

[0290] [Table 3]

[0291]

[0292]

[0293] High-temperature (45℃) charge / discharge characteristics evaluation: Half Cell

[0294] The negative electrode active materials according to Examples 1 to 3, 4-2 and 5 and Comparative Examples 1 to 4 were mixed with CMC / SBR (carboxymethyl cellulose / styrene-butadiene rubber) in a weight ratio of 96:4 in distilled water to prepare a negative electrode slurry. After coating the negative electrode slurry on a copper foil, it was dried and pressed to prepare each negative electrode having a loading level of 5 mg / cm2 and an electrode density of 1.6 g / cm3.

[0295]

[0296] According to Comparative Example 5, the negative electrode active material is commercial artificial graphite, and the weight ratio of the negative electrode active material / SBR / CMC / carbon black in the negative electrode is 95.6 / 2.3 / 1.1 / 1, and the loading level and electrode density are 5 mg / cm2 and 1.55 g / cm3, respectively.

[0297]

[0298] For the evaluation of the manufactured cathode, an electrolyte solution containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) or an electrolyte solution containing 0.5 wt% VC (Vinylene Carbonate) as an additive was used.

[0299]

[0300] An electrode assembly was manufactured by stacking the above-mentioned negative electrode and lithium metal as the positive electrode, with a separator, Cellgard, interposed between the negative electrode and the positive electrode, and an electrolyte was added to manufacture a test cell (2032 type coin cell).

[0301]

[0302] The evaluation of charge and discharge cycle characteristics was performed after the formation process was performed for 3 cycles at room temperature. Charging was performed in CC / CV mode at 0.5C rate, and the final voltage was maintained at 0.005 V. Discharging was performed in CC mode at 0.5C rate, and the final voltage was maintained at 1.5 V.

[0303]

[0304] Table 4 shows the initial discharge capacity, initial efficiency, capacity retention rate and electrode expansion rate after 100 charge-discharge cycles at 45°C.

[0305]

[0306] [Table 4]

[0307]

[0308]

[0309] Referring to Table 4, the electrodes using the negative electrode active materials according to Examples 1 to 3, 4-2 and 5 exhibit superior capacity retention and expansion rate characteristics compared to the electrodes using the negative electrode active materials according to Comparative Example 1, which is commercial natural graphite, and Comparative Example 5, which is commercial artificial graphite.

[0310]

[0311] In the case where an electrolyte solution with VC added was used in the electrode using the negative electrode active material according to Comparative Example 1, the capacity retention rate and expansion rate characteristics were somewhat improved, but they were still found to be inadequate compared to the electrode using the negative electrode active material according to Examples 1 to 3, 4-2 and 5 without VC added.

[0312]

[0313] Meanwhile, in terms of electrode expansion rate characteristics, the electrodes using the negative electrode active materials according to Examples 1 to 3, 4-2 and 5 show similar characteristics to the electrodes using the negative electrode active material according to Comparative Example 5, which is commercial artificial graphite.

[0314]

[0315] Figure 19 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of negative electrodes manufactured using negative active materials according to Examples 2 and 3 and Comparative Examples 1, 2, and 4.

[0316] Referring to FIG. 19, the negative electrodes manufactured using the negative electrode active materials according to Examples 2 and 3 show almost similar Coulombic efficiencies according to cycles, and the negative electrodes manufactured using the negative electrode active materials according to Examples 2 and 3 show much higher Coulombic efficiencies according to cycles than the negative electrodes manufactured using the negative electrode active material according to Comparative Example 1, which is currently used as a commercial natural graphite negative electrode active material.

[0317]

[0318] In addition, the negative electrodes manufactured using the negative electrode active materials according to Comparative Examples 2 and 4 exhibit somewhat lower coulombic efficiency over the cycle than the negative electrodes manufactured using the negative electrode active materials according to Examples 2 and 3.

[0319]

[0320] In the negative active materials according to Examples 2 and 3 and Comparative Example 2, the surface of the spherical natural graphite particles and the edge surface of the flaky natural graphite fragment particles constituting the interior were all modified in the same manner using a phosphorus compound, but in the case of Examples 2 and 3, the electrical conductivity was improved due to internal densification caused by isotropic pressing after the modification treatment, and the microscopic gaps between the flaky natural graphite fragment particles constituting the spherical natural graphite particles on the surface of the spherical natural graphite particles were almost closed (see FIGS. 7b and 8b), indicating that side reactions with the electrolyte were further suppressed.

[0321]

[0322] In the negative active materials according to Example 2 and Comparative Example 4, isotropic pressure treatment was performed under the same conditions, but in the case of the negative active material according to Example 2, the surface of the edge surface of the flaky natural graphite fragment particles constituting the surface and interior of the negative active material particles was modified using a phosphorus compound, so that it was shown that side reactions with the electrolyte were additionally suppressed in the negative active material according to Example 2.

[0323]

[0324] The low coulombic efficiency during charge / discharge cycles at 45℃ is due to side reactions between the negative electrode active material and the electrolyte. During high-temperature charge / discharge cycles, the SEI film gradually collapses, causing side reactions to continuously occur in which the exposed negative electrode active material surface, especially the edge of the graphite surface, reacts and decomposes with the electrolyte solvent. This causes an increase in electrode resistance and gas generation inside the battery. This continuous gas generation increases the internal pressure of the lithium secondary battery at high temperatures, causing the battery thickness to expand.

[0325]

[0326] In addition, when applied to a full cell rather than a half cell that can infinitely supply lithium ions to the negative electrode using lithium metal as a counter electrode, the life characteristics deteriorate as the lithium ions and electrolyte participating in the battery reaction gradually decrease, and the output characteristics deteriorate due to the increase in resistance caused by the above-mentioned side reaction.

[0327]

[0328] Thus, during charge / discharge reactions at high temperatures, the coulombic efficiency according to cycles can be a key indicator of the stability of the negative electrode active material at high temperatures.

[0329]

[0330] High-temperature (45℃) charge / discharge characteristics evaluation: Full Cell

[0331] A slurry of positive electrode active material was prepared by mixing 96 wt% of a positive electrode active material including a compound containing lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn), 2 wt% of polyvinylidene fluoride as a binder, and 2 wt% of carbon black as a conductive material, and then dispersing the mixture in N-methylpyrrolidone. The positive electrode active material slurry was applied to aluminum foil, dried, and then rolled to prepare a positive electrode. The loading level of the positive electrode was 17.8 mg / cm2, and the electrode density was 3.4 g / cm3.

[0332]

[0333] A negative electrode slurry was prepared by mixing a negative electrode active material with CMC / SBR (carboxymethyl cellulose / styrene-butadiene rubber) in a weight ratio of 96:4 in distilled water, and the negative electrode slurry was coated on a copper foil, followed by drying and pressing to prepare a negative electrode having a loading level of 10 mg / cm2 and an electrode density of 1.6 g / cm3.

[0334]

[0335] For the evaluation of the manufactured cathode, an electrolyte solution containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) or an electrolyte solution containing 0.5 wt% VC (Vinylene Carbonate) as an additive was used.

[0336]

[0337] An electrode assembly was manufactured by laminating the separator, Cellgard, between the cathode and anode, and an electrolyte was added to manufacture a test cell (2032 type coin cell).

[0338]

[0339] After the formation stage of the coin-type lithium secondary battery at 30°C, the cycle was charged in CC-CV mode at a 1C rate and discharged in CC mode at a 1C rate, and operated in the range of 4.2 V to 2.845 V.

[0340]

[0341] The full cell application cycle life characteristics were evaluated using an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) for electrodes manufactured using the respective negative active materials according to Examples 2, 3, and 4-2 and Comparative Examples 1 and 2, and the results are shown in Fig. 20.

[0342]

[0343] Referring to Fig. 20, full cells using negative electrodes manufactured using the negative electrode active materials according to Examples 2, 3, and 4-2 exhibit similar life characteristics, and exhibit superior cycle life characteristics compared to full cells using negative electrodes manufactured using the respective negative electrode active materials according to Comparative Examples 1 and 2. This appears to be consistent with the evaluation results using the half cell described above.

[0344]

[0345] The full cell application cycle life characteristics of the negative electrodes manufactured using the negative electrode active materials according to Example 3 and Comparative Example 1 were compared with the full cell application cycle life characteristics of the negative electrodes manufactured using the negative electrode active material according to Comparative Example 1 and the electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) and the electrolyte in which 0.5 wt% VC (Vinylene Carbonate) was added as an additive to the electrolyte. The results are shown in Fig. 21.

[0346] Referring to FIG. 21, a full cell using a negative electrode manufactured using a negative electrode active material according to Example 3 in an electrolyte without a VC additive shows superior cycle life characteristics compared to a full cell using a negative electrode manufactured using a negative electrode active material according to Comparative Example 1 in an electrolyte with 0.5 wt% VC (Vinylene Carbonate) added as an additive to the electrolyte.

[0347]

[0348] High-temperature (80℃) self-discharge characteristics evaluation: Half Cell

[0349] The negative electrode active materials according to Examples 2 and 3 and Comparative Examples 1 and 2 were mixed with CMC / SBR (carboxymethyl cellulose / styrene-butadiene rubber) in a weight ratio of 96:4 in distilled water to prepare a negative electrode slurry. The negative electrode slurry was coated on copper foil, and then dried and pressed to prepare each negative electrode having a loading level of 5 mg / cm2 and an electrode density of 1.6 g / cm3.

[0350]

[0351] According to Comparative Example 5, the negative electrode active material is commercial artificial graphite, and the weight ratio of the negative electrode active material / SBR / CMC / carbon black in the negative electrode is 95.6 / 2.3 / 1.1 / 1, and the loading level and electrode density are 5 mg / cm2 and 1.55 g / cm3, respectively.

[0352]

[0353] For the evaluation of the manufactured cathode, an electrolyte solution containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) or an electrolyte solution containing 0.5 wt% VC (Vinylene Carbonate) as an additive was used.

[0354]

[0355] An electrode assembly was manufactured by stacking the above-mentioned negative electrode and lithium metal as the positive electrode, with a separator, Cellgard, interposed between the negative electrode and the positive electrode, and an electrolyte was added to manufacture a test cell (2032 type coin cell).

[0356]

[0357] The evaluation of charge and discharge cycle characteristics was performed after the formation process was performed for 3 cycles at 30°C. Charging was performed in CC / CV mode at 0.5C rate, and the final voltage was maintained at 0.005 V. Discharging was performed in CC mode at 0.5C rate, and the final voltage was maintained at 1.5 V.

[0358]

[0359] After fully charging (SOC 100%) at 30℃, the battery was maintained at 80℃ for 2 weeks and then discharged at 30℃ to evaluate the capacity loss during high-temperature storage. Then, the battery was recharged to evaluate the reversible capacity loss due to high-temperature storage and investigate the lifespan characteristics after storage.

[0360]

[0361] Table 5 shows the capacity loss ratio due to self-discharge during high-temperature storage, measured by fully charging at 30°C, maintaining at 80°C for two weeks, and then discharging at 30°C for electrodes using the negative active materials according to Examples 2 and 3 and Comparative Examples 1, 2, and 5.

[0362]

[0363] [Table 5]

[0364]

[0365]

[0366] Referring to Table 5, the capacity loss ratio due to self-discharge in the electrodes using the negative active materials according to Examples 2 and 3 is somewhat smaller than that in the electrodes using the negative active materials according to Comparative Examples 1 and 2, and shows a similar capacity loss ratio to that in the electrodes using the negative active materials according to Comparative Example 5, which is commercial artificial graphite.

[0367] In addition, it is shown that the capacity loss ratio in the electrodes using the negative active materials according to Examples 2 and 3 and Comparative Examples 1, 2 and 5 is reduced at a similar ratio when using an electrolyte with VC added.

[0368]

[0369] Table 6 shows the reversible capacity measured after fully charging at 30°C, maintaining at 80°C for two weeks, discharging at 30°C, and recharging the electrodes using the negative active materials according to Examples 2 and 3 and Comparative Examples 1, 2, and 5, and the capacity retention rate measured after performing 100 charge / discharge cycles at 30°C.

[0370]

[0371] [Table 6]

[0372]

[0373]

[0374] Referring to Table 6, in the reversible capacity measured by discharging and recharging at 30°C after maintaining at 80°C for two weeks using an electrolyte without a VC additive, the change in reversible capacity of the electrodes using the negative active materials according to Examples 2 and 3 is smaller than that of the electrodes using the negative active materials according to Comparative Examples 1, 2, and 5.

[0375] In particular, in the case of an electrode using a negative electrode active material according to Comparison 1, which is a commercial natural graphite negative electrode active material, a large decrease in reversible capacity is shown.

[0376]

[0377] When using an electrolyte with VC added, the change in reversible capacity is shown to be smaller for electrodes using negative active materials according to Examples 1 and 5 in comparison, but the effect of using an electrolyte with VC added is shown to be relatively small for electrodes using negative active materials according to Examples 2 and 3.

[0378]

[0379] After maintaining the electrode at 80°C for two weeks and performing 100 charge / discharge cycles at 30°C, the electrodes using the negative active materials according to Examples 2 and 3 showed superior capacity retention compared to the electrodes using the negative active materials according to Comparative Examples 1 and 2, and showed somewhat superior characteristics compared to the electrode using the negative active material according to Comparative Example 5, which is a commercial artificial graphite negative active material.

[0380]

[0381] It is known that during high-temperature storage in the fully charged state, the self-discharge-related characteristics occur when the SEI film formed during the formation process rapidly collapses at high temperatures, causing a side reaction in which the exposed negative electrode active material surface and the electrolyte solvent react and decompose, and as a result, the exposed graphite negative electrode active material surface (especially, the edge surface of the graphite) reacts and decomposes with the electrolyte solvent, causing a rapid side reaction in which the reversible lithium amount decreases and an irreversible reaction increases as the charge / discharge cycle progresses, resulting in a deterioration in the capacity and life characteristics of the battery.

[0382]

[0383] Accordingly, the negative electrode active material manufactured according to the present invention exhibits excellent thermal stability even when stored at a high temperature of 80°C in a fully charged state. This is because the surface of the edge, which is the active site of the flaky natural graphite fragment particles constituting the surface and the interior of the spherical natural graphite particles, is selectively modified using a phosphorus compound to form the excellent thermal stability of the SEI film, which is continuously maintained. In addition, the internal densification due to the isotropic pressurization after the modification treatment improves the electrical conductivity, and the microscopic gaps between the flaky natural graphite fragment particles constituting the spherical natural graphite particles on the surface of the spherical natural graphite particles are almost closed (see FIGS. 6b, 7b, and 8b), so that side reactions with the electrolyte are further suppressed.

[0384]

[0385] In addition, in the case of an electrode using a negative electrode active material manufactured according to the present invention, even without using an electrolyte that does not include VC (Vinylene Carbonate) as an additive for film formation / control used to improve the high-temperature stability of a graphite-based negative electrode, the electrode shows superior high-temperature storage-related characteristics compared to an electrode using a negative electrode active material according to Comparative Example 1, which is a commercial natural graphite negative electrode active material using a core solution containing VC, and shows high-temperature storage-related characteristics that are equivalent to or better than those of an electrode using a negative electrode active material according to Comparative Example 5, which is a commercial artificial graphite negative electrode active material.

[0386]

[0387] Figure 22 shows capacity changes (Figure 22a) and cycle coulombic efficiency (Figure 22b) according to charge and discharge cycles measured for electrodes using negative active materials according to Example 3 and Comparative Examples 1 and 5 in a battery using an electrolyte solution in which 1 M LiPF6 is dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8), after fully charging at 30°C, maintaining at 80°C for 2 weeks, and then discharging at 30°C and recharging.

[0388]

[0389] Referring to FIGS. 22a and 22b, the electrodes using the negative active materials according to Example 3 and Comparative Example 5 exhibit stable life characteristics and high Coulombic efficiency.

[0390] On the other hand, the electrode using the negative active material according to Comparative Example 1 exhibits a rapid decrease in capacity from the beginning of the cycle. This is because, in the case of the negative active material according to Comparative Example 1, which is commercial natural graphite, the SEI film rapidly collapses while maintained at 80°C for two weeks, causing a side reaction to occur between the electrolyte solvent and the edge of the flaky graphite existing on the surface and inside of the commercial natural graphite, and the crystallinity of the flaky graphite deteriorates.

[0391]

[0392] Figure 23 shows the capacity change (Figure 23a) and cycle coulombic efficiency (Figure 23b) according to the charge / discharge cycle measured in a battery using an electrolyte in which 0.5 wt% VC (Vinylene Carbonate) was added as an additive to an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8), for electrodes using the negative active materials according to Example 3 and Comparative Examples 1 and 5, after being fully charged at 30°C, maintained at 80°C for 2 weeks, and then discharged at 30°C and recharged.

[0393]

[0394] Referring to FIGS. 23a and 23b, in the case of an electrode using a negative active material according to Comparative Example 1, the life characteristics and coulombic efficiency were improved by using an electrolyte with VC (Vinylene Carbonate) added, but it was shown that the collapse of the SEI film and side reactions with the electrolyte solvent were not sufficiently suppressed during two weeks of maintenance at 80°C despite the use of the VC additive.

[0395]

[0396] From this, the negative electrode active material manufactured according to the present invention shows thermal stability equivalent to or greater than that of the negative electrode active material according to Comparative Example 5, which is commercial artificial graphite.

[0397]

[0398] Evaluation of charge and discharge characteristics in electrolyte containing propylene carbonate (PC): Half Cell

[0399] The negative electrode active materials according to Examples 2, 3, and 4-2 and Comparative Examples 1 and 2 were mixed with CMC / SBR (carboxymethyl cellulose / styrene-butadiene rubber) in a weight ratio of 96:4 in distilled water to prepare a negative electrode slurry. After coating the negative electrode slurry on a copper foil, it was dried and pressed to prepare each negative electrode having a loading level of 5 mg / cm2 and an electrode density of 1.6 g / cm3.

[0400]

[0401] According to Comparative Example 5, the negative electrode active material is commercial artificial graphite, and the weight ratio of the negative electrode active material / SBR / CMC / carbon black in the negative electrode is 95.6 / 2.3 / 1.1 / 1, and the loading level and electrode density are 5 mg / cm2 and 1.55 g / cm3, respectively.

[0402]

[0403] For the evaluation of the manufactured cathode, an electrolyte containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC) (EC:EMC:PC = 20:70:10 by volume) or / and an electrolyte containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC) (EC:EMC:PC = 20:60:20 by volume) were used.

[0404]

[0405] An electrode assembly was manufactured by stacking the above-mentioned negative electrode and lithium metal as the positive electrode, with a separator, Cellgard, interposed between the negative electrode and the positive electrode, and an electrolyte was added to manufacture a test cell (2032 type coin cell).

[0406]

[0407] The evaluation of charge and discharge cycle characteristics was performed after the formation process was performed for 3 cycles at 30°C. Charging was performed in CC / CV mode at 0.5C rate, and the final voltage was maintained at 0.005 V. Discharging was performed in CC mode at 0.5C rate, and the final voltage was maintained at 1.5 V.

[0408]

[0409] Table 7 shows the initial discharge capacity, initial efficiency, and capacity retention rate after 100 charge-discharge cycles at 45°C after the formation process for electrodes using the negative active materials according to Examples 3 and 4-2 and Comparative Examples 1 and 2 and an electrolyte containing 1 M LiPF6 added to a solvent having a composition of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC) = 20:70:10 by volume).

[0410]

[0411] [Table 7]

[0412]

[0413]

[0414] Referring to Table 7, the initial efficiencies were similar for the electrodes using the negative active materials according to Examples 3 and 4-2 and Comparative Examples 1 and 2, but the capacity retention rate after 100 charge-discharge cycles at 45°C following the formation process was superior for the electrodes using the negative active materials according to Examples 3 and 4-2 compared to the electrodes using the negative active materials according to Comparative Examples 1 and 2.

[0415]

[0416] In particular, the negative electrode active material according to Comparative Example 2 was surface-modified with the phosphorus compound in the same manner as the negative electrode active material according to Example 3, but in the case of the negative electrode active material according to Example 3, it is judged that the charge / discharge cycle stability for the electrolyte containing propylene carbonate (PC) was improved because an isotropic pressurization treatment was additionally performed after the surface modification treatment with the phosphorus compound.

[0417]

[0418] Table 8 shows the initial discharge capacity, initial efficiency, and capacity retention rate after 100 charge / discharge cycles at 30°C after the formation process of an electrolyte containing 1 M LiPF6 added to a solvent having a composition of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC) = 20:60:20 by volume) and an electrode using the negative active materials according to Examples 3 and 4-2 and Comparative Examples 1 and 5.

[0419]

[0420] [Table 8]

[0421]

[0422]

[0423] Referring to Table 8, the initial efficiencies of the electrodes using the negative electrode active materials according to Examples 3 and 4-2 and Comparative Example 1 were similar, but the electrode using the negative electrode active material according to Comparative Example 5, which is a commercial artificial graphite negative electrode active material, showed a very low initial efficiency.

[0424]

[0425] In addition, the capacity retention rate after 100 charge / discharge cycles at 30°C is similar for the electrodes using the negative active materials according to Examples 3 and 4-2 and Comparative Example 5, but on the other hand, the electrode using the negative active material according to Comparative Example 1, which is a commercial natural graphite negative active material, shows a very low capacity retention rate.

[0426]

[0427] Figures 24a and 24b show capacity changes and cycle coulombic efficiency according to cycles during charge and discharge cycles at 30°C after the initial chemistry process for electrodes using negative active materials according to Example 3 and Comparative Examples 1 and 5.

[0428]

[0429] Referring to FIGS. 24a and 24b, the capacity retention rates of the electrodes using the negative active materials according to Example 3 and Comparative Example 5 after 100 charge / discharge cycles at 30°C following the initial chemical reaction were similar (see Table 8), but the electrode using the negative active material according to Comparative Example 5 exhibited a low discharge capacity due to a large irreversible reaction during the initial charge / discharge, and the cycle coulombic efficiency during 100 charge / discharge cycles at 30°C was lower than that of the electrode using the negative active material according to Example 3.

[0430]

[0431] Accordingly, in a battery using an electrolyte containing propylene carbonate (PC), the negative electrode active material manufactured according to the present invention shows superior charge / discharge characteristics compared to the negative electrode active material according to Comparative Example 1, which is a commercial natural graphite negative electrode active material, and Comparative Example 5, which is a commercial artificial graphite negative electrode active material.

[0432]

[0433] From this, it is shown that in the case of the negative electrode active material manufactured according to the present invention, the initial irreversible decomposition reaction is suppressed even in an electrolyte containing propylene carbonate (PC), and a stable and strong SEI film is formed.

[0434]

[0435] Charge Rate Characteristics Evaluation: Half Cell

[0436] The charge rate characteristics were evaluated at 30°C using test cells manufactured using the respective negative active materials according to Examples 4-1, 4-2 and 5 and Comparative Examples 1 and 5, as follows.

[0437]

[0438] For electrodes manufactured using the negative active materials according to Examples 4-1 and 4-2, the amorphous carbon coating amount was adjusted to 3 wt. % and 5 wt. %, and the influence of the amorphous carbon coating amount was investigated.

[0439] In addition, the electrode density was adjusted to 1.55 g / cm3 or 1.6 g / cm3 in the electrodes manufactured using each negative active material according to the above Examples 4-2 and 5, and the influence of the electrode density and electrode composition was investigated through the presence or absence of a conductive agent added to the electrode composition.

[0440]

[0441] The electrode density of the electrode manufactured using each negative active material according to Comparative Example 1, which is commercial natural graphite, is 1.6 g / cm3, and the electrode density of the electrode manufactured using each negative active material according to Comparative Example 5, which is commercial artificial graphite, is 1.55 g / cm3.

[0442]

[0443] All of the above electrodes were manufactured with a loading level of 5 mg / cm2.

[0444] The results are shown in Table 9.

[0445]

[0446] [Table 9]

[0447]

[0448]

[0449] Referring to Table 9, the charge rate characteristics of the test cells manufactured using the negative active materials according to Examples 4-1, 4-2 and 5 appear to depend on the amorphous carbon coating amount, electrode density and electrode composition.

[0450] Referring to the charge rate characteristics of the test cells manufactured using the negative electrode active materials according to Examples 4-1 and 4-2, the amorphous carbon coating amount is superior at 3 wt. % compared to 5 wt. %, and the charge rate characteristics of the test cells manufactured using the negative electrode active materials according to Examples 4-2 and 5 are almost the same, and the characteristics are the best when the negative electrode active material according to Example 5 is used and the electrode density is maintained at 1.55 g / cm3 together with the addition of carbon black as a conductive material during electrode manufacture, and it can be seen that the characteristics are close to those of the electrode using the negative electrode active material according to Comparative Example 5, which is commercial artificial graphite.

[0451]

[0452] Charge rate characteristics evaluation: Full Cell

[0453] A slurry of positive electrode active material was prepared by mixing 96 wt% of a positive electrode active material including a compound containing lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn), 2 wt% of polyvinylidene fluoride as a binder, and 2 wt% of carbon black as a conductive material, and then dispersing the mixture in N-methylpyrrolidone. The positive electrode active material slurry was applied to aluminum foil, dried, and then rolled to prepare a positive electrode. The loading level of the positive electrode was 17.8 mg / cm2, and the electrode density was 3.4 g / cm3.

[0454]

[0455] The negative electrode was manufactured by mixing the negative electrode active material according to Example 5 and the negative electrode active material according to Comparative Example 1, which is commercial natural graphite, in a weight ratio of 7:3, and the negative electrode active material according to Comparative Example 5, which is commercial artificial graphite, and the negative electrode active material according to Comparative Example 1, which is commercial natural graphite, in a weight ratio of 7:3.

[0456]

[0457] The electrode composition of the above mixed electrode is active material (AM):binder (CMC / SBR):conductive material (CB) = 95:4:1, and the negative electrode was manufactured to have a loading level of 10 mg / cm2 and an electrode density of 1.55 g / cm3.

[0458]

[0459] For the evaluation of the manufactured cathode, an electrolyte containing 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) was used.

[0460]

[0461] An electrode assembly was manufactured by laminating the separator, Cellgard, between the cathode and anode, and an electrolyte was added to manufacture a test cell (Coin type full cell).

[0462]

[0463] The coin-type lithium secondary battery was charged in CC-CV mode at 0.2C, 1C, 2C, 3C and 5C rates after 3 cycles of formation stage at 30°C and discharged in CC mode at 0.2C rate, and operated in the range of 4.2 V to 2.845 V.

[0464]

[0465] The full cell application charge rate characteristics manufactured using each of the above mixed electrodes were evaluated, and the results are shown in Table 10.

[0466]

[0467] [Table 10]

[0468]

[0469]

[0470] Referring to Table 10, the charge rate characteristics of a full cell using an anode manufactured by mixing the anode active material according to Example 5 and the anode active material according to Comparative Example 1, which is commercial natural graphite, are almost the same as the charge rate characteristics of a full cell using an anode manufactured by mixing the anode active material according to Comparative Example 5, which is commercial artificial graphite, and the anode active material according to Comparative Example 1, which is commercial natural graphite.

[0471]

[0472] In general, when the thickness of the negative electrode increases in a current commercial full cell, considering the situation in which the negative electrode is manufactured and used by mixing artificial graphite and commercial natural graphite in an appropriate ratio, it is practically very important that the mixed negative electrode using the negative electrode active material according to the present invention exhibits similar charge rate characteristics to the mixed negative electrode using commercial artificial graphite.

[0473]

[0474] For the negative electrodes manufactured using the respective negative active materials according to Example 3 and Comparative Example 1, a full cell was formed using an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) and 0.5 wt% VC (Vinylene Carbonate) was added as an additive. The charge rate characteristics measured at 30°C after the formation process at 30°C were compared with the charge rate characteristics measured at 30°C after 1000 cycles at 45°C after the formation process. The results are shown in Fig. 25.

[0475]

[0476] Referring to FIG. 25, a full cell using a negative electrode manufactured using a negative electrode active material according to Example 3 and a full cell using a negative electrode manufactured using a negative electrode active material according to Comparative Example 1 showed similar charge rate characteristics measured at 30°C after the formation process at 30°C.

[0477]

[0478] On the other hand, in the case of the charge rate characteristics measured at 30°C after 1000 cycles at 45°C following the formation process, the charge rate characteristics of the full cell using the negative electrode manufactured using the negative electrode active material according to Example 3 showed little change, but the charge rate characteristics of the full cell using the negative electrode manufactured using the negative electrode active material according to Comparative Example 1 were found to deteriorate significantly.

[0479] This is because, in a full cell using an anode manufactured using an anode active material according to Comparative Example 1, the SEI (Solid electrolyte interface) film formed on the surface during the formation cycles of the anode during 1000 cycles at 45°C gradually collapses during continuous charge / discharge at high temperatures, causing a side reaction in which the exposed anode active material surface, particularly the edge of the graphite surface, reacts with the electrolyte solvent and decomposes, and the crystallinity of the edge of the graphite surface may be reduced, which leads to an increase in the resistance of the electrode.

[0480]

[0481] Figure 26 shows SEM images comparing the surface of each cathode after the formation process at 30°C and after 1000 cycles at 45°C for each full cell.

[0482]

[0483] Referring to FIG. 26, when comparing the surface of the negative electrode manufactured using the negative active material according to Comparative Example 1 after the formation process (FIG. 26a) and after 1000 cycles at 45°C (FIG. 26b), a large amount of SEI film, which is a by-product due to a side reaction with the electrolyte, is observed after 1000 cycles at 45°C, whereas when comparing the surface of the negative electrode manufactured using the negative active material according to Example 3 after the formation process (FIG. 26c) and after 1000 cycles at 45°C (FIG. 26d), it is shown that additional formation of the SEI film, which is a by-product due to a side reaction with the electrolyte, is greatly suppressed after 1000 cycles at 45°C.

[0484]

[0485] The above results show the behavior in an electrolyte containing an electrolyte additive (VC) to improve the stability of the SEI film at high temperatures, and the side reaction between the electrolyte and graphite at high temperatures is not sufficiently suppressed by the electrolyte additive (VC) alone, which results in a problem in that the layer conductivity characteristics also deteriorate as the cycle progresses.

[0486]

[0487] From this, it can be seen that the battery using the negative active material according to the present invention has excellent stability in terms of capacity retention rate and charge rate characteristics during cycling at high temperatures.

[0488]

[0489] As a result of reviewing the above examples and comparative examples, the following conclusions were reached.

[0490]

[0491] By surface modification and mechanical pressure treatment of existing spherical natural graphite, it is possible to realize a negative electrode active material and lithium secondary battery having excellent stability against side reactions with electrolytes at high temperatures, excellent charge / discharge cycle life characteristics at high temperatures, and excellent high-rate charge / discharge characteristics.

[0492]

[0493] The above spherical natural graphite particles have a structure in which flaky natural graphite fragment particles are formed and assembled in a cabbage shape or random shape, and phosphorus (P) atoms form COP or CPO bonds on the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles, and the spherical natural graphite particles have a shape having a concave / convex surface morphology, and the number percentage of particles having a circularity of 0.95 or less is 66% or more, and the tap density is 1.17 g / cm3 or more.

[0494]

[0495] This has led to the development of a cathode active material with a unique structure and shape that can solve all the problems of existing spherical natural graphite cathode active materials at once.

[0496]

[0497] The present invention is not limited to the above-described embodiments, but can be manufactured in a variety of different forms. Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

[0498]

[0499] The negative active material for a lithium secondary battery according to the present invention can realize a lithium secondary battery having improved stability at high temperatures and excellent charge / discharge cycle characteristics and charge / discharge output characteristics at high temperatures and room temperatures.

Claims

1. A negative electrode active material for a lithium secondary battery comprising spherical natural graphite particles, Each of the above spherical natural graphite particles has a structure in which natural graphite fragment particles are formed and assembled in a cabbage shape or random shape. COP or CPO bonds are formed on the edge plane surface of all or at least some of the flaky natural graphite fragment particles constituting the surface or interior of each of the spherical natural graphite particles, Each of the above spherical natural graphite particles has a concave / convex surface morphology with concave and convex portions formed on the surface, Among the above spherical natural graphite particles, the percentage of particles having a circularity of 0.95 or less is 66% or more, A negative electrode active material for a lithium secondary battery, characterized in that the tap density of the above spherical natural graphite particles is 1.17 g / cm3 or more.

2. In paragraph 1, A negative electrode active material for a lithium secondary battery, characterized in that phosphorus (P) atoms are bonded only to the surface of the edge plane, not the basal plane, of a natural graphite fragment particle through a COP or CPO bond.

3. In paragraph 1, The above spherical natural graphite particles further include a carbon coating layer of 0.1 to 5 wt% based on the total weight of the negative electrode active material on the surface, A negative electrode active material for a lithium secondary battery, characterized in that the carbon coating layer comprises at least one type of carbon selected from amorphous carbon derived from a hard carbon precursor and semi-crystalline carbon derived from a soft carbon precursor.

4. In paragraph 1, A negative electrode active material for a lithium secondary battery, characterized in that the average particle diameter of the above spherical natural graphite particles is 10 to 20 ㎛. 5.(A) A step of preparing a solution containing spherical natural graphite particles having a cabbage-shaped or randomly assembled structure of natural graphite fragment particles, a phosphorus compound, and a solvent; (B) a step of selectively adsorbing a phosphorus compound onto the edge plane of all or at least some of the natural graphite fragment particles by immersing and stirring the solution; (C) a step of drying and heat-treating the above solution to produce modified spherical natural graphite particles; and (D) a step of isotropically pressing and molding the modified spherical natural graphite particles and crushing the resulting molded body; Method for manufacturing a negative electrode active material for a lithium secondary battery. 6.(a) A step of preparing a solution containing spherical natural graphite particles having a cabbage-shaped or randomly assembled structure of natural graphite fragment particles, a phosphorus compound, and a solvent; (b) a step of selectively adsorbing a phosphorus compound on the edge plane of all or at least some of the natural graphite fragment particles by immersing and stirring the solution; (c) a step of drying and heat-treating the above solution to produce modified spherical natural graphite particles; (d) a step of isotropically pressing and molding the modified spherical natural graphite particles and crushing the resulting molded body; and (e) a step of coating an amorphous or semi-crystalline carbon precursor on the surface of the spherical natural graphite particles obtained in the above step (d) and performing heat treatment to form an amorphous or semi-crystalline carbon coating layer; Method for manufacturing a negative electrode active material for a lithium secondary battery.

7. In paragraph 5 or 6, A method for producing a negative electrode active material for a lithium secondary battery, characterized in that the above-mentioned phosphorus compound is at least one selected from the group consisting of tricresyl phosphate (TCP), tributyl phosphate (TBP), triphenyl phosphate (TPP), triethyl phosphate (TEP), trioctyl phosphate, tritolyl phosphite, and tri-isooctylphosphite.

8. In paragraph 5 or 6, The heat treatment performed in the above step (C) or step (c) is A method for producing a negative electrode active material for a lithium secondary battery, characterized in that the method is performed in an atmosphere containing air or oxygen, an atmosphere containing nitrogen, argon or a mixed gas thereof, or under vacuum.

9. In paragraph 8, The heat treatment performed in the above step (C) or step (c) is Performed at a temperature of 200 to 1200°C in an atmosphere or vacuum containing nitrogen, argon or a mixed gas thereof, or A method for producing a negative electrode active material for a lithium secondary battery, characterized in that the method is performed at a temperature of 200 to 600°C in an atmosphere containing air or oxygen.

10. In paragraph 6, The heat treatment performed in the above step (e) is A method for producing a negative electrode active material for a lithium secondary battery, characterized in that the method is performed at a temperature of 800 to 1200°C in an atmosphere containing nitrogen, argon or a mixed gas thereof or under vacuum.

11. In paragraph 6, A method for producing a negative electrode active material for a lithium secondary battery, characterized in that the amorphous or semi-crystalline carbon precursor comprises citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethylcellulose (CMC), hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, starch, phenol resin, furan resin, furfuryl alcohol, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, polyvinyl alcohol, polyvinyl chloride, coal pitch, petroleum pitch, mesophase pitch, low molecular weight heavy oil, glucose, gelatin, sugar, or a combination thereof.

12. A negative electrode comprising the negative active material of paragraph 1, A negative electrode for a lithium secondary battery, characterized in that the electrode density is 1.6 g / cm3 and the orientation index of spherical natural graphite is 0.08 or more.

13. A negative electrode comprising the negative active material of paragraph 3, A negative electrode for a lithium secondary battery, characterized in that the electrode density is 1.6 g / cm3 and the orientation index of spherical natural graphite is 0.25 or more.

14. In paragraph 12 or 13, The above orientation index is an area ratio ((110) / (004)) obtained by measuring the (110) plane and (004) plane of the spherical natural graphite included in the negative electrode by an X-ray diffraction (XRD) method and integrating each measured X-ray diffraction peak. A negative electrode for a lithium secondary battery, characterized in that the above orientation index is an area ratio ((110) / (004)) obtained by integrating each measured X-ray diffraction peak.

15. A negative electrode comprising the negative active material of any one of claims 1 to 4; Bipolar; and electrolyte; A lithium secondary battery comprising:

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