KR102695392B1 - Negative active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery including the same - Google Patents

Abstract

The present invention relates to 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 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 the 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 68% or more, a method for producing the same, and a lithium secondary battery comprising the anode active material.

Description

Negative active material for rechargeable lithium battery, method of preparing the same, and lithium secondary battery including the same {NEGATIVE ACTIVE MATERIAL FOR RECHARGEABLE LITHIUM BATTERY, METHOD OF PREPARING THE SAME, AND RECHARGEABLE LITHIUM BATTERY INCLUDING THE SAME}

The present invention relates to an anode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the anode active material for a lithium secondary battery.

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, improvements in the performance of lithium secondary batteries in terms of stability at high temperatures, high output, and long life characteristics are required.

Currently, crystalline graphite materials are used as negative active materials for 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 through a graphitization process, so the manufacturing cost is high and the lithium storage capacity is somewhat lower than that of natural graphite due to the limitation of the degree of graphitization. However, its use is increasing due to its relatively excellent life characteristics and output characteristics.

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

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

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 semi-crystalline carbon coating layer on the surface of the spherical natural graphite particles as repeated charge and discharge progresses. In particular, the edge sites, which are the active sites of the flaky natural graphite fragment particles, are known to further promote the electrolyte decomposition reaction.

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 a current collector, and further, due to the compression deformation of the spherical natural graphite particles, the gaps (or micro-spaces, pores) between 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.

In the case of the above spherical natural graphite, the problem of easy compression during electrode manufacturing can be improved by densifying, but when densifying in 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. Accordingly, a method of controlling the shape of the active material particles together with densifying the spherical natural graphite to increase the contact area between the active material particles is necessary, but a method that can solve all of the above-mentioned problems has not been presented so far.

Furthermore, although 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, considering the high-cost manufacturing process and the problems of large amounts of carbon dioxide ( _CO2 ) being emitted during the manufacturing process, the development of a natural graphite anode active material that has equivalent or higher performance than the artificial graphite in terms of high-temperature stability, high-power characteristics, and long-life characteristics is very important in the lithium secondary battery-related industry.

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 a high-cost manufacturing process and a large amount of 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.

Korean Patent Publication No. 10-2018-0035693 (Published on: 2018.04.06) Korean Patent Publication No. 10-2014-0099987 (Published on: 2014.08.14) Korean Patent Publication No. 10-1999-0085720 (Publication Date: 1999.12.15.)

The present invention is to solve the problem of spherical natural graphite as an anode active material of a conventional lithium secondary battery as described above, and has a controlled active material particle shape so as to increase particle strength along with stabilization of edge sites, which are active sites of the flaky natural graphite fragment particles constituting the surface and interior of the spherical natural graphite particles, and to increase the contact area between the anode active material particles, thereby providing a lithium secondary battery anode active material having stable charge/discharge cycle life characteristics at high temperatures and excellent high-rate charge/discharge characteristics and a method for producing the same, an anode of a lithium secondary battery including the anode active material, and a lithium secondary battery having the same.

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 C-O-P or C-P-O bonds on the edge plane of all or at least some of the flaky natural graphite fragment particles constituting the surface or the 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 68% or more, and the tap density is 1.19 g/cm3 or more.

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.

In addition, the present invention provides a negative electrode active material for a lithium secondary battery including 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 C-O-P or C-P-O 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 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 A negative active material for a lithium secondary battery is provided, characterized in that the percentage of particles having a particle size of 0.95 or less is 68% or more and the tap density is 1.19 g/cm3 or more.

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.

At this time, the amorphous or semi-crystalline carbon coated on the surface of the spherical natural graphite particles is characterized by being 0.1 to 10 wt% based on the total weight of the negative electrode active material.

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

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 particles in which phosphorus (P) atoms are selectively bonded only to the surface of the edge plane rather than the basal plane through C-O-P or C-P-O bonds.

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 C-O-P or C-P-O 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 68% or more, the tap density is 1.19 g/cm3 or more, and the orientation index of the spherical natural graphite is 0.08 or more in the electrode manufactured with an electrode density of 1.6 g/cm3. there is.

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 percentage of the number 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 68% or more, and the tap density is 1.19 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.

At this time, the amorphous or semi-crystalline carbon coating layer formed on the surface of the modified spherical natural graphite particles may contain 0.1 to 10 wt% based on the total weight of the negative electrode active material.

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 shape or randomly, 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 a portion 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.

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-assembled structure of flake-like 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 a portion of the flake-like 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.

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.

In addition, in the step (A) or step (a) of the method for producing 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.

In addition, in step (A) or step (a) of the method for producing 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.

In addition, the process of adsorbing a phosphorus compound in step (B) or step (b) of the method for producing 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.

In addition, in the step (C) or step (c) of the method for producing 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.

In addition, the heat treatment in step (C) or step (c) of the method for producing 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.

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.

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 can be performed by a cold isostatic pressing method.

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

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

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.

Figure 1 shows the method employed to measure circularity.
Figure 2 shows the XPS analysis results of a highly oriented pyrolytic graphite sample manufactured according to Experimental Example 1 of the present invention.
Figure 3 shows the XPS analysis results of a highly oriented pyrolytic graphite sample manufactured according to Experimental Example 2 of the present invention.
Figures 4a and 4b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 2.
Figures 5a and 5b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 3.
Figures 6a and 6b are scanning electron microscope (SEM) images of the negative active material according to Example 1.
Figures 7a and 7b are scanning electron microscope (SEM) images of the negative active material according to Example 2.
Figure 8 is a scanning electron microscope (SEM) photograph of the negative active material according to Example 3.
Figure 9 is a scanning electron microscope (SEM) image of the negative active material according to Example 4.
Figure 10 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 1.
Figure 11 is a scanning electron microscope (SEM) image of the negative active material according to Comparative Example 4.
Figures 12a and 12b are scanning electron microscope (SEM) images of the negative active material according to Comparative Example 5.
Figure 13 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 6.
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 2.
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.
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.
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.
Figure 18 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 4.
Figure 19 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 7.
Figure 20 is a circularity distribution curve for the negative electrode active materials according to Example 2 and Comparative Example 3.
Figure 21 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 1 to 4 and Comparative Example 1.
Figure 22 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 Example 2 and Comparative Example 2.
Figure 23 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 Example 2 and Comparative Example 4.
Figure 24 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 5.
Figure 25 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 6.
Figure 26 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 7.
Figure 27 shows the charge/discharge cycle characteristics at 45°C of the negative electrode manufactured using the negative electrode active material according to Example 2 and Comparative Example 7.
FIGS. 28a and 28b show the full cell application cycle life characteristics and the change in coulombic efficiency according to the cycle during charge and discharge at 30°C for electrodes manufactured using the respective negative active materials according to Example 2 and Comparative Examples 1 and 2.

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.

Since embodiments according to the concept of the present invention can have various changes and can take various forms, 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 it includes all modifications, equivalents, or substitutes included in the spirit and technical scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. It should be understood that, as used herein, the terms "comprises" or "has" and the like are intended to specify the presence of a described feature, number, step, operation, component, part, or combination thereof, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

Hereinafter, the present invention will be described in detail.

The negative 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 C-O-P or C-P-O 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 68% or more, the tap density is 1.19 g/cm3 or more, and the orientation index of the spherical natural graphite in an electrode manufactured with an electrode density of 1.6 g/cm3 is It is characterized by a value of 0.08 or higher.

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

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 Fig. 1, equivalent to the ratio P _D /P _r , where 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.

Accordingly, circularity is defined as follows.

Circularity = (4πA _p )/P _r ²

Here, the circularity of the particle P is obtained by determining the perimeter P _D of a disk D having an area equal to the area A _p of the particle P observed in the photograph. The perimeter P _r of the particle P is also determined.

Circularity has a value from 0 to 1. A perfect circle has a circularity of 1, and a particle with a very pointed or irregular shape has a circularity value close to 0.

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 and the flake-shaped natural graphite fragment particles constituting the surface or interior of the spherical natural graphite are selectively adsorbed with a phosphorus compound, and then surface modified through heat treatment, and then the spherical graphite particles are densified through isotropic pressurization, and the spherical natural graphite particles have a shape having a concave/convex surface morphology, and the percentage of particles having a circularity of 0.95 or less is 68% or more.

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 when manufacturing an 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 68% or more, the movement of lithium ions through the gaps (or microspaces, pores) between the anode active material particles in the electrode is smooth, and the contact area between the anode active material particles increases, thereby improving the electrical conductivity, thereby improving the charge/discharge characteristics of a lithium secondary battery anode using the anode active material.

One embodiment of the negative electrode active material according to the present invention is a lithium secondary battery negative electrode active material characterized in that an amorphous or semi-crystalline carbon coating layer is formed on the surface of spherical natural graphite particles, which have a shape having a concave/convex surface morphology through a pressurization treatment after the above-described modification treatment and in which the percentage of particles having a circularity of 0.95 or less is 68% or more, and in which the tap density is 1.19 g/cm3 or more and the orientation index of the spherical natural graphite is 0.25 or more in the electrode manufactured with an electrode density of 1.6 g/cm3.

The amount of amorphous or semi-crystalline carbon coating on the surface of the spherical natural graphite particles subjected to the above-mentioned modification treatment and pressure molding treatment may be 0.1 to 10 wt%, and preferably 0.1 to 5 wt%, based on the total weight of the negative electrode active material.

When amorphous or semi-crystalline carbon is coated on the surface of the spherical natural graphite particles that have been isotropically pressurized 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, it is preferable that the orientation index be 0.25 or more.

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

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.

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.

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 pulverization by collision between the inner surface of the rotary processing machine and the flaky natural graphite powder, friction processing, shearing processing of the powder by shear stress, etc., so that spherical natural graphite particles can be ultimately manufactured.

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

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

The average particle diameter (D50) of the above-mentioned spherical natural graphite particles may be 10 to 20 μm, and specifically, 12 to 18 μm. The above-mentioned 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, so that the assembly process is easy and the electrochemical characteristics can be improved.

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, the spherical graphite particles are easily compressed during the rolling process for manufacturing an electrode 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 a current collector, and further, the compression deformation of the spherical natural graphite particles blocks the gaps between active material particles in the electrode, so that the tortuosity increases, making it difficult for lithium ions to move through an electrolyte, thereby deteriorating the charge/discharge characteristics.

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

That is, by selectively adsorbing the phosphorus compound to 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 C-O-P or C-P-O bonds on the edge surface of the flaky natural graphite fragment particles through heat treatment, and then subjecting the spherical graphite particles to a densification process by isotropic pressing, it is possible to manufacture a negative electrode active material having the characteristics of increased particle strength, a concave/convex surface morphology, and a particle circularity of 0.95 or less of 68% or more.

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 forming treatment.

Since the above spherical natural graphite particles are physically assembled using mechanical energy through pulverization by collision between flake-shaped natural graphite powders, friction processing between powders, shearing processing of powders by shear stress, etc., there are fine gaps between the flake-shaped natural graphite fragment particles that constitute the above spherical natural graphite particles.

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 to the edge surface of each of at least some of the flaky natural graphite fragment particles existing inside the spherical natural graphite particles as well as the flaky natural graphite fragment particles constituting the surface of the spherical natural graphite particles.

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.

The phosphorus compound used to manufacture the modified spherical natural graphite particles using the above-mentioned phosphorus compound may be contained in an amount of 0.000001 to 1 part by weight, 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 the 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.

According to one embodiment of the present invention, the edge planes of all or at least some of the flake natural graphite particles constituting the surface or interior of the spherical natural graphite particles are selectively adsorbed with 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 decrease in capacity as a negative electrode active material for a lithium secondary battery due to the modification.

And, in another aspect of the present invention, as a method for manufacturing the negative active material, the present invention provides a method for manufacturing the negative active material for a lithium secondary battery, 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 shape or randomly, 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 a portion of the flake-shaped natural graphite fragment particles, (C) drying and heat-treating the solution to manufacture modified spherical natural graphite particles, and (D) isotropically pressing and molding the modified spherical natural graphite particles and pulverizing the obtained molded body.

In another embodiment of the present invention, a method for producing an anode active material for a lithium secondary battery is provided, comprising the steps of: (a) preparing a solution containing spherical natural graphite particles having a cabbage-like or randomly-assembled structure of flake-like 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 a portion of the flake-like 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 subjected to the pressurization and molding after the modification and heat-treating to form an amorphous or semi-crystalline carbon coating layer.

In the step (A) or step (a) of the method for producing 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.

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

In addition, the process of adsorbing a phosphorus compound in step (B) or step (b) of the method for producing 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.

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.

In addition, the heat treatment in step (C) or step (c) of the method for producing 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.

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.

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 it 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.

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.

In step (D) or step (d) of the method for producing 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.

The pressure applied during the above isotropic pressing molding process is preferably such that a spherical natural graphite negative electrode active material can be obtained having a tap density of 1.19 g/cm3 or higher, a particle shape having a concave/convex surface morphology, and a particle number percentage of 68% or higher having a circularity of 0.95 or lower, 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 higher in an electrode manufactured with an electrode density of 1.6 g/cm3.

In the case of a negative electrode active material having an amorphous or semi-crystalline carbon coated on the surface of spherical natural graphite particles isotropically pressurized 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.

The above spherical natural graphite may not have a uniform assembled state (e.g., particle size, particle strength, density, etc.) because the flake-shaped natural graphite particles are physically assembled using mechanical energy. Accordingly, under a pressing condition exceeding a certain standard of 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 particle circularity of 0.95 or less in which the percentage of the number of particles is 68% or more may be obtained. However, the upper limit of the pressing pressure is not particularly limited.

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

In step (e) of the method for manufacturing the negative 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 the pressure molding treatment after the modification treatment and heat-treating them 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, polyvinyl pyrrolidone, 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.

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.

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

In step (e) of the method for manufacturing the negative 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, so that the charge/discharge characteristics as the negative active material are excellent.

Meanwhile, the total amount of the amorphous and/or semi-crystalline carbon coating may be included 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 active material. When the amorphous and/or semi-crystalline carbon is included within the above range, the coating by the amorphous and/or semi-crystalline carbon is effectively formed, and thus the negative active material may exhibit excellent characteristics.

When manufacturing an electrode using the negative active material according to one embodiment of the present invention, by using particles having a circularity of 0.95 or less and a shape having a concave/convex surface morphology and having a number percentage of 68% or more, the shape of the negative active material can be maintained even at an electrode density of 1.6 g/cm3 or more, micro-spaces (pores) are effectively formed between the negative active material particles, so that impregnation of an electrolyte is easy through the micro-spaces, 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.

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

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

The improvement of the properties intended in the present invention cannot be sufficiently achieved by existing methods 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 fragment particles constituting the surface or interior of the spherical natural graphite particles.

Furthermore, in another aspect of the present invention, the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, and an electrolyte including the negative electrode active material.

Lithium secondary batteries can be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, and can be classified into cylindrical, square, coin, and pouch types depending on the shape, and can be divided into bulk type and thin film type depending on the size. The structure and manufacturing method of these batteries are widely known in this field, so a detailed description is omitted.

The above negative electrode can be manufactured by mixing the above-described negative electrode active material, binder, and optionally a conductive material to manufacture 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.

Hereinafter, in order to specifically explain this specification, examples will be given and described in detail. However, the embodiments according to this specification may be modified in various different forms, and the scope of this specification is not construed as being limited to the embodiments described below. The embodiments of this specification are provided to more completely explain this specification to a person having average knowledge in the art.

Example 1

100 parts by weight of spherical natural graphite particles having an average particle diameter (D50) of 16 μm (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, isostatically pressed at a pressure of about 400 MPa, and then the molded body was pulverized to manufacture an anode active material for a lithium secondary battery.

Example 2

A negative 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.

Example 3

In the same manner as in Example 1, spherical natural graphite particles were obtained, modified and isostatically pressed, and petroleum pitch was coated on the surface of the modified and isostatically pressed spherical natural graphite particles based on 5 wt% residual carbon 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.

Example 4

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

Comparative Example 1

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 after carbonization) and used as a negative electrode active material.

Comparative Example 2

100 parts by weight of spherical natural graphite particles having an average particle diameter (D50) of 16 ㎛ and 0.1 part 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.

Comparative Example 3

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.

Comparative Example 4

A negative 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 50 MPa.

Comparative Example 5

The spherical natural graphite particles of Comparative Example 3 were placed in a cold isostatic pressing device, isostatically pressed at a pressure of about 500 MPa, and the molded body was pulverized to manufacture a negative active material for a lithium secondary battery.

Comparative Example 6

For spherical natural graphite particles obtained in the same manner as Comparative Example 5, petroleum pitch was coated on the surface of the modified and isotropically pressurized spherical natural graphite particles based on 5 wt% residual carbon after carbonization, and then heat-treated at 1200°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.

Comparative Example 7

After coating the surface of artificial graphite (Jiangxi Zichen Technology Co., Ltd.) having an average particle size (D50) of 20㎛ with petroleum pitch (residual carbon content after carbonization: 5 wt% relative to the negative electrode active material).

Subsequently, the negative electrode active material was manufactured by heat treatment in a nitrogen atmosphere at 1200°C for 1 hour.

Experimental Example 1

A highly oriented pyrolytic graphite sample and 5 wt% of tricresyl phosphate (TCP) based on 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.

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

Experimental example 2

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 for 0.5 hour in a nitrogen atmosphere.

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 the highly oriented pyrolytic graphite sample was heat-treated at 800°C for 0.5 hour under a nitrogen atmosphere after tricresyl phosphate (TCP) was adsorbed onto it, as shown in Figure 3a, a bond related to the P element was formed at the edge plane, but no bond related to the P element was formed at the basal plane (Figure 3b). This is consistent with the results of the highly oriented pyrolytic graphite manufactured according to Experimental Example 1.

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 appears to form a C-P-O or C-O-P bond with the P element located on the edge surface of the highly oriented pyrolytic graphite surface through decomposition of the phosphorus compound during subsequent heat treatment.

Scanning electron microscope (SEM) photo analysis

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

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 was selectively modified using the above-mentioned compound, which is almost similar to that of Comparative Example 3, which is the original sample (pristine sample) without surface modification.

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 by using mechanical energy through pulverization by collision between flaky natural graphite powders, friction processing between powders, shearing processing 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, since the molecular weight is very small, a solution including the phosphorus compound may flow 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 may be selectively adsorbed onto the surface and edge surfaces of each of the flaky natural graphite fragment particles present inside the spherical natural graphite particles.

Figures 6 and 7 are scanning electron microscope (SEM) images of the negative active materials according to Examples 1 and 2, respectively.

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

In addition, referring to the SEM images of FIGS. 6b and 7b, 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 in the negative active materials according to Examples 1 and 2, the fine gaps between the flaky natural graphite fragment particles constituting the particles on the surface of the particles are almost closed.

Figures 8 and 9 are scanning electron microscope (SEM) images of the negative active materials according to Examples 3 and 4, respectively.

Referring to the SEM photographs of FIGS. 8 and 9, the particle shape having a concave/convex surface morphology of the negative active materials according to Examples 1 and 2 appears to be maintained even after amorphous or quasi-crystalline carbon coating.

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

In the case of Comparative Example 1 (Fig. 10), where amorphous or quasi-crystalline carbon is coated on the negative active material of Comparative Example 3 (Fig. 5), a spherical shape with a smooth surface is exhibited.

Figure 11 is a scanning electron microscope (SEM) image of the negative active material according to Comparative Example 4.

Referring to Fig. 11, it can be confirmed that although the tap density increased to 1.176 (see Table 1) by isotropic pressing compared to Comparative Examples 1 and 2, it maintains an almost spherical shape, and unlike the negative active materials according to Examples 1 and 2 (Figs. 6 and 7), a concave/convex surface morphology was not sufficiently formed.

Figures 12 and 13 are scanning electron microscope (SEM) images of the negative active materials according to Comparative Examples 5 and 6. Similar to Figures 7 and 9, they show particle shapes having concave/convex surface morphologies.

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 and 2 (FIGS. 6 and 7), a press molding process capable of exhibiting a tap density higher than a certain standard during isotropic pressurization is necessary.

Scanning electron microscope (SEM) images of the surface of the cathode manufactured with an electrode density of 1.6 g/cm3 according to Examples 2 and 4 and Comparative Examples 1, 2, and Comparative Example 4 are shown in FIGS. 14 to 18.

FIGS. 14 and 15 are scanning electron microscope (SEM) images of the surfaces of cathodes manufactured according to Examples 2 and 4, respectively, and FIGS. 16 to 18 are scanning electron microscope (SEM) images of the surfaces of cathodes manufactured according to Comparative Examples 1 and 2, and Comparative Example 4, respectively.

Referring to FIGS. 16 to 18, in the case of surface modification of spheroidized natural graphite (FIG. 17), the spheroidized natural graphite particles are seriously pressed due to the pressing process during electrode manufacturing, and in both the case of coating amorphous or semi-crystalline carbon on spheroidized natural graphite (FIG. 16) and the case of performing isotropic pressing at a pressure of about 50 MPa on spheroidized natural graphite after surface modification (FIG. 18), the spheroidized natural graphite particles are pressed due to the pressing process during electrode manufacturing.

Referring to FIGS. 14 and 15, it can be seen that the particle crushing phenomenon is significantly improved in the cathodes manufactured according to Examples 2 and 4.

Figure 19 is a scanning electron microscope (SEM) photograph of the negative active material according to Comparative Example 7.

Referring to Figure 19, commercial artificial graphite shows an amorphous shape in which small artificial graphite particles are assembled.

Circularity measurement

The measurements were made using the Morphologi 4, an automatic imaging system from Malvern Panalytical. The particles were evenly dispersed and arranged on the sheet, and then the images were taken directly above the sheet and analyzed. The circularity of the particles may vary depending on the observation direction. In this experimental example, 10,000 particles were measured.

The circularity of the negative active materials according to Examples 1 and 2 and Comparative Examples 3 and 4 was measured, and Fig. 20 is a drawing comparing the circularity distribution curves for the negative active materials according to Example 2 and Comparative Example 3. Referring to Fig. 20, it can be confirmed that the circularity is lowered overall due to shape change according to the isotropic pressing molding process.

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 and 2 and Comparative Examples 3 and 4 was measured. The results are shown in Table 1.

[Table 1]

Tap Density Measurement

The tap density of the negative active materials obtained according to Examples 1 to 4 and Comparative Examples 1, 2, 4, and 5 was measured. The results are shown in Table 2.

The negative active materials according to Examples 1 to 4 have a higher tap density than the negative active materials obtained according to Comparative Examples 1, 2 and 4, and are 1.19 g/cm ³ or more. The value is indicated.

Meanwhile, Comparative Example 5, which was manufactured through the same pressure molding process as Example 2, exhibited the same tap density as Example 2.

[Table 2]

Measuring the orientation index

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

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 (004) plane of the spherical natural graphite negative active material included in the above electrode.

To exclude overlapping of the X-ray diffraction peaks, the electrodes for the above orientation index measurement 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, followed by drying and pressing to produce each electrode.

In the negative electrodes 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 Comparative Examples 2 and 4, it exhibits a value of 0.08 or less.

In addition, when the surface of the above-mentioned spherical graphite is coated with amorphous or quasi-crystalline carbon, the orientation index is shown to increase, and in the case of Examples 3 and 4, it is significantly higher than that of Comparative Example 1 and shows a value of 0.28 or higher.

[Table 3]

(Manufacturing of test cells)

The negative electrode active materials according to Examples 1 to 4 and Comparative Examples 1 to 6 were mixed with CMC/SBR (carboxymethyl cellulose/styrene-butadiene rubber) at 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.

An electrode assembly was manufactured by laminating the negative electrode and lithium metal as the positive electrode, with Cellguard as a separator interposed between the negative electrode and the positive electrode. Afterwards, an electrolyte solution in which 1 M LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) was added to manufacture a test cell (2032 type coin cell).

Charge/discharge cycle characteristics analysis

Using the test cell manufactured above, the charge/discharge characteristics were evaluated at 45°C by the following method.

The evaluation of charge and discharge cycle characteristics was performed after the formation process for three cycles at room temperature. The charge was performed in CC/CV mode at 0.5C rate, and the terminal voltage was maintained at 0.005 V. The discharge was performed in CC mode at 0.5C rate, and the terminal voltage was maintained at 1.5 V.

Figure 21 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 1 to 4 and Comparative Example 1.

Referring to FIG. 21, the negative electrodes manufactured using the negative electrode active materials according to Examples 1 to 4 exhibit higher Coulombic efficiency 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.

The low Coulombic efficiency during charge and discharge cycles at 45℃ is due to the side reaction between the negative electrode active material and the electrolyte. As the SEI film gradually collapses during the high-temperature charge and discharge cycle, the exposed negative electrode active material surface, especially the edge of the graphite surface, reacts and decomposes with the electrolyte solvent, causing continuous side reactions. This increases the resistance of the electrode and causes 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.

In addition, when applied to a full cell rather than a half cell that can infinitely supply lithium ions to the cathode by 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.

Thus, during charge/discharge reactions at high temperatures, the Coulombic efficiency according to cycles becomes a key indicator of the high-temperature stability of the negative electrode active material.

From this, it is shown that the negative active material according to the embodiment of the present invention has superior stability at high temperatures compared to the negative active material according to Comparative Example 1, which is generally used as a commercial negative active material.

Figure 22 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 Example 2 and Comparative Example 2.

Referring to FIG. 22, the negative electrode manufactured using the negative electrode active material according to Example 2 exhibits higher Coulombic efficiency according to cycles than the negative electrode manufactured using the negative electrode active material according to Comparative Example 2.

In the negative active materials according to Example 2 and Comparative Example 2, the surfaces of the spherical natural graphite particles and the edges of the flaky natural graphite fragment particles constituting the interior were modified in the same manner using a phosphorus compound, but in the case of Example 2, due to the isotropic pressurization after the modification treatment, the fine 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. 6b and 7b), indicating that side reactions with the electrolyte were further suppressed.

Figure 23 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 4.

Referring to FIG. 23, the negative electrode manufactured using the negative electrode active material according to Example 2 exhibits higher Coulombic efficiency according to cycles than the negative electrode manufactured using the negative electrode active material according to Comparative Example 4.

In the negative active materials according to Example 2 and Comparative Example 4, modification treatment was performed using a phosphorus compound under the same conditions, but in the case of the negative active material according to Comparative Example 4, the effect of suppressing additional side reactions with the electrolyte by isotropic pressurization was found to be insufficient.

Figure 24 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 5.

Referring to FIG. 24, the negative electrode manufactured using the negative electrode active material according to Example 2 exhibits higher Coulombic efficiency according to cycles than the negative electrode manufactured using the negative electrode active material according to Comparative Example 5.

In the negative active materials according to Example 2 and Comparative Example 5, isotropic pressure molding 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 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.

Figure 25 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of a negative electrode manufactured using a negative electrode active material according to Example 2 and Comparative Example 6.

Referring to FIG. 25, the negative electrode manufactured using the negative electrode active material according to Example 2 exhibits higher Coulombic efficiency according to cycles than the negative electrode manufactured using the negative electrode active material according to Comparative Example 6.

In the negative active materials according to Example 2 and Comparative Example 6, isotropic pressure molding was performed under the same conditions, but in the case of the negative active material according to Example 2, isotropic pressure molding was performed after modification using the above-mentioned compound, and in the case of the negative active material according to Comparative Example 6, an amorphous carbon coating layer was formed on the particle surface after the isotropic pressure molding.

Furthermore, in the case of the negative electrode manufactured using the negative electrode active material according to Comparative Example 6, the Coulombic efficiency according to the cycle during charge and discharge is shown to be low, similar to the negative electrode manufactured according to Comparative Example 5, even though amorphous and/or semi-crystalline carbon is additionally coated on the surface of the negative electrode active material manufactured according to Comparative Example 5.

From this, when all of the above conditions are satisfied, namely, that the degree of densification through the isotropic pressurization treatment using the above-mentioned phosphorus compound, the tap density, and the electrode orientation index are above certain critical values, and the particle shape is maintained such that the percentage of the number of particles having a circularity of 0.95 or less is 68% or more, it can be confirmed that the high-temperature stability with respect to the side reaction with the electrolyte during the charge/discharge cycle at a high temperature of 45°C is further improved.

Meanwhile, additives for film formation/control, such as VC (Vinylene Carbonate), are generally used to improve the high-temperature stability of current graphite-based negative electrodes. The additive forms a stable film on a stable electrode surface during the charge or discharge process, and suppresses exfoliation of the layered structure of carbon, which is an anode material, and direct reaction with the electrolyte, thereby contributing to improving the charge/discharge life of the battery. It is known that the SEI layer formed through VC can maintain film stability even at high temperatures. However, VC has the disadvantages of a difficult synthesis process and being expensive.

Considering the above circumstances, the effect of the present invention, which exhibits excellent high-temperature cycle stability even without using an electrolyte containing no VC additive, appears to be very meaningful from a practical and industrial point of view.

In addition, FIG. 26 shows the change in coulombic efficiency according to cycles during charge and discharge at 45°C of the negative electrode manufactured using the negative electrode active materials according to Example 2 and Comparative Example 7.

The negative electrode active material according to Comparative Example 7 is commercial artificial graphite, and the weight ratio of the negative electrode active material/SBR/CMC/carbon black in the manufactured 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.

In general, in the case of a negative electrode using a commercial artificial graphite negative electrode active material, carbon black was added to improve electrical conductivity due to the irregular amorphous shape, and it was manufactured with an electrode density of 1.55 g/cm3. It is known that the characteristics of the artificial graphite deteriorate when manufactured with an electrode density of 1.55 g/cm3 or higher.

On the other hand, in the case of a cathode using commercial natural graphite cathode active material, it is generally manufactured with an electrode density of 1.6 g/cm3.

In addition, in this evaluation, all negative electrodes using natural graphite negative electrode active materials according to Examples 1 to 4 and Comparative Examples 1 to 6 were manufactured with an electrode density of 1.6 g/cm3 without adding carbon black.

In addition, for the evaluation of the negative electrode manufactured according to Comparative Example 7, an electrolyte in which 1 M LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) or an electrolyte in which 0.5 wt% VC (vinylene carbonate) was added as an additive to the electrolyte was used.

Referring to Fig. 26, in the case of the negative electrode manufactured using the negative electrode active material (amorphous carbon-coated artificial graphite, Jiangxi Zichen Technology Co., Ltd.) according to Comparative Example 7, a large difference in characteristics is shown depending on the application of the electrolyte additive. When the electrolyte containing 0.5 wt% VC as an additive was used, the coulombic efficiency according to the cycle during charge and discharge was shown to increase somewhat.

However, the coulombic efficiency according to the cycle during charge and discharge of the negative electrode manufactured using the negative electrode active material according to Example 2 using an electrolyte without an additive shows a higher value than the coulombic efficiency according to the cycle of the negative electrode manufactured using the negative electrode active material according to Comparative Example 7.

Figure 27 shows the charge/discharge cycle characteristics at 45°C of the negative electrodes manufactured using the negative electrode active materials according to Example 2 and Comparative Example 7. Here, the negative electrode manufactured using the negative electrode active material according to Comparative Example 7 was evaluated using an electrolyte containing 0.5 wt% VC as an additive.

Referring to FIG. 27, the negative electrode manufactured using the negative electrode active material according to Example 2 exhibits very excellent high-temperature life characteristics compared to the negative electrode manufactured using the negative electrode active material according to Comparative Example 7. The capacity retention rate after 100 cycles is 88.6% for the negative electrode manufactured according to Comparative Example 7, whereas it is 99.1% for the negative electrode manufactured according to Example 2.

Referring to Fig. 26, the SEI (solid electrolyte interface) film formed on the surface of the negative electrode active material (amorphous carbon-coated artificial graphite) according to Comparative Example 7 gradually collapses during continuous charge/discharge at high temperatures, causing side reactions in which the exposed negative electrode active material surface reacts with the electrolyte solvent and decomposes, and the crystallinity of the graphite may deteriorate. This causes an increase in electrode resistance and a decrease in battery capacity, resulting in a decrease in performance.

Meanwhile, in the case of the negative electrode manufactured using the negative electrode active material according to Example 2, the Coulomb efficiency according to the cycle shows a high value approaching 100% (see Fig. 26), indicating excellent life characteristics and capacity retention rate due to the characteristics of the SEI film and interface that are very stable even at high temperatures.

Measurement of impedance spectrum (EIS, electrochemical impedance spectroscopy)

The interfacial resistance of the negative electrodes manufactured using the negative electrode active materials according to Example 2, Comparative Examples 1, 2 and 5 was measured through EIS analysis at the state of charge (SOC100) after formation cycles at room temperature and after 100 cycles at 45°C.

The results are shown in Table 4 below. Here, R _s represents the resistance of the electrolyte, R _SEI represents the film (SEI) resistance, and R _ct represents the charge transfer resistance.

Referring to Table 4, the resistance of the negative electrode manufactured using the negative electrode active material according to Example 2 after formation cycles appears to be somewhat lower than the resistance of the negative electrodes manufactured using the negative electrode active materials according to Comparative Examples 1, 2, and 5.

In particular, referring to the results measured after 100 cycles at 45°C in Table 4, in the case of the negative electrode manufactured using the negative electrode active material according to Example 2, the R _SEI and R _ct measured after the formation cycles show almost no change.

However, in the negative electrodes manufactured using the negative active materials according to Comparative Examples 1, 2, and 5, a significant increase is observed after 100 cycles at 45°C.

[Table 4]

Charge rate characteristics and discharge rate characteristics analysis

The charge rate characteristics were evaluated at 30°C using test cells manufactured using the respective negative active materials according to Examples 1 to 4 and Comparative Examples 1, 2, and 4 to 6, as follows.

The cathode for evaluating the charge-rate characteristics had an electrode density of 1.6 g/cm3 and was manufactured with a loading level of 5 mg/cm2. The results are shown in Table 4.

To evaluate the charge rate characteristics, charging (lithium insertion reaction) was performed in CC/CV mode at a current density of 0.2 C to 3 C-rate, and the terminal voltage was maintained at 0.005 V. Discharging (lithium removal reaction) was performed in CC mode at a current density of 0.2 C-rate, and the terminal voltage was maintained at 1.5 V.

[Table 5]

Referring to Table 5, the charge rate characteristics measured after formation cycles show that the electrodes manufactured using the negative active materials according to Examples 1 and 2 have superior charge rate characteristics compared to the electrodes manufactured using the negative active materials according to Comparative Examples 1, 2, and 4.

In addition, referring to the results of the electrodes manufactured using the negative active materials according to Examples 3 and 4, it appears that there is a somewhat significant additional improvement in rate characteristics in the case of the negative active material whose surface is coated with amorphous carbon after the densification treatment.

In particular, referring to Table 5, the charge rate characteristics measured after 100 cycles at 45°C show that the charge rate characteristics of the electrode manufactured using the negative active material according to Example 2 are almost maintained even after 100 cycles at 45°C.

However, in the case of electrodes manufactured using the negative active materials according to Comparative Examples 1, 2, and 5, the charge rate characteristics were found to deteriorate after 100 cycles at 45°C.

This is because the solid electrolyte interface (SEI) film formed on the surface during the formation cycles gradually collapses during continuous charge/discharge at high temperatures, causing a side reaction in which the exposed negative active material surface, especially the edge of the surface, reacts and decomposes with the electrolyte solvent, and the crystallinity of the edge of the graphite surface may decrease, which is caused by an increase in the resistance of the electrode.

In addition, referring to the EIS analysis results in Table 4, it is consistent with the results that the interfacial resistance (R _SEI and R _ct ) of the electrodes manufactured using the negative active materials according to Comparative Examples 1, 2, and 5 significantly increases after 100 cycles at 45°C.

Due to the isotropic pressurization for densification of spherical natural graphite particles, the particle strength increases, thereby improving the orientation of the spherical natural graphite particles even at a high electrode density, thereby facilitating lithium insertion and de-insertion reactions. In addition, with the increase in particle strength due to densification, micro-pores are effectively formed between negative electrode active material particles having a shape of concave/convex surface morphology, thereby facilitating impregnation of the electrolyte and movement of lithium ions through the electrolyte. In addition, with the densification, the contact area between negative electrode active material particles having a shape of concave/convex surface morphology increases, thereby reducing internal resistance of the electrode.

Thereby, it can be confirmed that the high-rate charge/discharge characteristics can be improved, and for this purpose, along with the high density of the modified spherical natural graphite particles, the particle shape control must be sufficiently performed so that the percentage of particles having the concave/convex surface morphology and a circularity of 0.95 or less is 68% or more.

From this, it is preferable that the negative active material that has been modified and densified has a particle tap density of 1.19 g/cm ³ or more.

Furthermore, the edge surfaces of the flaky natural graphite fragment particles constituting the surface or interior of the spherical natural graphite particles are modified by the phosphorus compound, and at the same time, due to the isotropic pressurization, the fine gaps between the flaky natural graphite fragment particles constituting the particles on the surface of the spherical natural graphite particles are almost closed, so that structural changes in the edge surfaces of the flaky graphite and side reactions with the electrolyte can be further suppressed even when repeated charge and discharge cycles are performed at high temperatures.

Accordingly, in the negative electrode active material, negative electrode, and lithium secondary battery using the same according to the above embodiment, the charge/discharge efficiency during the cycle is improved, and the increase in internal resistance of the battery according to the progress of the cycle is minimized, so that the charge rate characteristics and discharge rate characteristics can be stably maintained.

The charge rate characteristics at 30°C of the negative electrodes manufactured using each of the negative active materials according to Example 2 and Comparative Example 7 were compared in Table 6.

The negative electrode active material according to the above Comparative Example 7 is commercial artificial graphite, and the electrode density of the manufactured negative electrode is 1.55 g/cm3, and the electrode density of the negative electrode manufactured using the negative electrode active material according to Example 2 is 1.6 g/cm3.

In order to evaluate the charge rate characteristics of the negative electrode manufactured using the negative electrode active material according to Comparative Example 7, an electrolyte was used in which 1 M LiPF ₆ 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.

[Table 6]

Referring to Table 6, the negative electrode manufactured using the negative electrode active material (amorphous carbon-coated artificial graphite) according to Comparative Example 7 exhibits lower charge rate characteristics compared to the negative electrode manufactured using the negative electrode active material according to Example 2. The negative electrode evaluation using the negative electrode active material according to Example 2 used an electrolyte that did not contain an additive.

Considering the situation in which VC is generally used as an additive to improve the stability and performance of graphite-based negative electrodes in commercial batteries, it is very important from a practical standpoint to exhibit excellent characteristics by using an electrolyte that does not contain an additive such as VC in evaluating the characteristics of the negative electrode using the negative electrode active material according to the above-described example in the present invention.

Meanwhile, in the case of electrolytes with VC added, it is known that SEI generated by the decomposition of VC exhibits relatively high resistance (J. Korean Electrochem. Soc., Vol. 15, No. 1, 2012). Therefore, when using electrolytes with VC added, the charge/discharge rate characteristics may deteriorate.

Accordingly, the spherical natural graphite negative electrode active material according to the present invention is shown to have performances equivalent to or higher than those of commercial artificial graphite negative electrode active materials in terms of stability at high temperatures, high output characteristics, and long life characteristics.

In addition, the spherical natural graphite negative electrode active material according to the present invention has a larger reversible capacity (~365 mAh/g) than that of the artificial graphite negative electrode active material (~350 mAh/g), and when applied as the negative electrode active material of an actual lithium secondary battery, the electrode density of the negative electrode manufactured is 1.55 g/cm2 for the artificial graphite negative electrode active material and 1.6 g/cm2 for the spherical natural graphite negative electrode active material, so it has a comparatively superior characteristic compared to the artificial graphite in terms of energy density.

Furthermore, referring to Examples 1 and 2 of the present invention, the carbon coating process can be excluded in the production of the negative electrode active material, and the heat treatment temperature is low and the production process is simple, so there is an advantage of low production cost. Accordingly, there is an advantage of low production cost compared to not only artificial graphite as an existing graphite-based negative electrode active material, but also commercial natural graphite negative electrode active material.

Full-Cell Application Evaluation Comparison

A slurry of positive electrode active material was prepared by mixing 96 wt% of a cathode 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.

A negative electrode slurry was prepared by mixing a negative active material with CMC/SBR (carboxymethyl cellulose/styrene-butadiene rubber) at 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.

A coin-type full cell was manufactured using the above positive and negative electrodes.

An electrode assembly was manufactured by laminating the cathode and anode with a separator, Cellgard, interposed between them. Then, an electrolyte containing 1 M LiPF ₆ and 0.5 wt% VC (Vinylene Carbonate) as an additive in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 2:8) was added to manufacture a test cell (2032 type coin cell).

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

Full cell application cycle life characteristics were evaluated for electrodes manufactured using each negative active material according to Example 2 and Comparative Examples 1, 2, and 5, and the results are shown in Fig. 28.

Referring to FIG. 28a, a full cell using a negative electrode manufactured using a negative electrode active material according to Example 2 exhibits superior cycle life characteristics compared to a full cell using a negative electrode manufactured using each of the negative electrode active materials according to Comparative Examples 1, 2, and 5.

This is thought to be due to the excellent interfacial stability and high output characteristics of the negative electrode manufactured using the negative electrode active material according to Example 2 for charge/discharge cycles.

As shown in Fig. 28b, the coulombic efficiency according to the charge/discharge cycle in the full cell using the negative electrode manufactured using the negative electrode active material according to Example 2 is superior to the full cell using the negative electrodes manufactured using each of the negative electrode active materials according to Comparative Examples 1, 2, and 5. The interfacial stability of the negative electrode manufactured using the negative electrode active material according to Example 2 with respect to the charge/discharge cycle can also be confirmed in the comparison of the cycle coulombic efficiency at high temperatures shown in Figs. 21, 22, and 24.

After reviewing the above examples and comparative examples, the following conclusions were reached.

When the existing spherical natural graphite satisfies all of the following conditions through surface modification and mechanical pressurization, it is possible to realize a negative 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.

The above spherical natural graphite particles have a structure in which flake-shaped natural graphite fragment particles are connected and assembled in a cabbage shape or random shape, and phosphorus (P) atoms form C-O-P or C-P-O bonds on the edge plane of all or at least some of the flake-shaped 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 68% or more, and the tap density is 1.19 g/cm3 or more.

As a result, we have developed 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.

The present invention is not limited to the above embodiments, but can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains will understand that the present invention can be implemented in other specific forms without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

Claims (15)

As a negative electrode active material for a lithium secondary battery containing 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-like or random shape.
COP or CPO bonds are formed on the edge plane surface of all or at least some of the flake-shaped natural graphite 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 68.5% 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 1.201 g/cm3 or more.
In the first paragraph,
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 natural graphite fragment particles through COP or CPO bonds.
In the first paragraph,
The above spherical natural graphite particles further include a carbon coating layer on the surface,
A negative electrode active material for a lithium secondary battery, characterized in that the carbon coating layer includes 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.
In the first paragraph,
A negative electrode active material for a lithium secondary battery, characterized in that the average particle diameter of the above-mentioned spherical natural graphite particles is 10 to 20 ㎛.
(A) A step of preparing a solution containing spherical natural graphite particles having a cabbage-like or randomly-shaped or 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;
A method for manufacturing a negative electrode active material for a lithium secondary battery as described in claim 1.
(a) a step of preparing a solution containing spherical natural graphite particles having a cabbage-like or randomly-shaped or assembled structure of natural graphite fragment particles, a phosphorus compound, and a solvent;
(b) a step of selectively adsorbing a compound to 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 step (d) and performing heat treatment to form an amorphous or semi-crystalline carbon coating layer;
A method for manufacturing a negative electrode active material for a lithium secondary battery as described in Article 1.
In clause 5 or 6,
A method for producing a negative 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.
In clause 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.
In Article 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.
In Article 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.
In Article 6,
A method for producing a negative 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, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, 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, saccharides or a combination thereof.
A negative electrode comprising the negative active material of clause 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.
As a negative electrode comprising the negative active material of clause 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.
In clause 12 or 13,
The above orientation index is an area ratio ((110)/(004)) obtained by measuring the (110) plane and the (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.
A negative electrode comprising a negative active material according to any one of claims 1 to 4;
Bipolar; and
electrolyte;
A lithium secondary battery comprising: