Description

NEGATIVE ACTIVE MATERIAL FOR SECONDARY BATTERY, AND ELECTRODE AND SECONDARY BATTERY

INCLUDING THE SAME

Technical Field

[1] The present invention relates to a negative active material for a secondary battery, and in particular, to a negative active material for a secondary battery, in which at least a portion of an edge of a core carbon material is coated with a carbide layer, and to an electrode of a secondary battery and a secondary battery including the same. Background Art

[2] With rapid popularization of electronic appliances using batteries in these days, for example, mobile phones, notebook computers or electric vehicles, the demand for secondary batteries of small size, light weight and relatively high capacity is rapidly increasing. In particular, a lithium secondary battery has light weight and high energy density, and thus is widely used as a power source of a portable electronic appliance. Accordingly, research and development is lively made to improve the performance of the lithium secondary battery.

[3] The lithium secondary battery includes an anode and a cathode, each containing an active material capable of intercalating and deintercalating lithium ions, and an organic electrolytic solution or polymer electrolytic solution filled therebetween. The lithium secondary battery generates electric energy by oxidation and reduction reactions during intercalation and deintercalation of lithium ions at the anode and the cathode.

[4] The lithium secondary battery uses mainly a transition metal compound as an active material for a cathode, for example LiCoO ₂ , LiNiO ₂ or LiMnO ₂ .

[5] And, the lithium secondary battery uses, as an active material for an anode, a crystalline carbon material having high softness, for example natural graphite or artificial graphite, or a low crystalline carbon material having a pseudo-graphite structure or turbostratic structure, obtained by carbonizing hydrocarbon or polymer at a low temperature of 1000 ⁰ C to 1500 ⁰ C.

[6] The crystalline carbon material has a high level of true density that is advantageous to pack an active material, and has excellent electric potential flatness, initial capacity and charging/discharging reversibility. However, as the number of times a battery is used increases, charging/discharging efficiency and cycle capability reduces. According to analysis, this is because when battery charging/discharging cycles are repeated, decomposition of an electrolytic solution occurs at an edge of the crystalline carbon material.

[7] Japanese Patent Laid-open Publication No. 2002-348109 discloses a carbon material- based negative active material, in which a crystalline carbon material is coated with a carbide layer to prevent decomposition of an electrolytic solution from occurring at an edge of the crystalline carbon material. In the carbon material-based negative active material, the carbide layer is formed by coating pitch on the surface of the carbon material and performing thermal treatment at 1000 ⁰ C or more. Here, coating of the carbon material with the carbide layer reduces slightly an initial capacity of a secondary battery, but improves charging/discharging efficiency and cycle capability of the secondary battery. In particular, high temperature thermal treatment makes the coating layer an artificial graphite to reduce a reduction amount of initial capacity and effectively suppress decomposition of an electrolytic solution.

[8] However, while manufacturing an electrode of a secondary battery by coating the carbon material-based negative active material on a metallic current collector, coating effect of the carbide layer is reduced. In the manufacture of an electrode of a secondary battery, a compression process is performed to closely bond the negative active material and the metallic current collector. However, during the compression process, the carbide layer coated on the edge of the carbon material is destroyed to expose the edge of the carbon material again, resulting in decomposition of an electrolytic solution.

[9] Therefore, in the manufacture of an electrode of a secondary battery using the conventional carbon material-based negative active material, it needs to newly define property parameters of the negative active material and clearly understand the correlation between the defined property parameters and electrical and chemical characteristics of the secondary battery so as to prevent deterioration of the electrical and chemical characteristics of the secondary battery caused by destruction of the carbide layer.

[10] However, the above-mentioned prior art simply specifies a mass ratio between the carbon material and the carbide layer, coating and sintering conditions of the carbide layer, crystallographic properties of the carbide layer through XRD (X-Ray Diffraction) and Raman analysis, and specific surface area conditions of the carbide layer, to effectively suppress a decomposition reaction of an electrolytic solution, but not mention any problem caused by destruction of the carbide layer that may occur in the course of manufacturing an electrode of a secondary battery, and the solution to overcome deterioration of the carbide layer. Disclosure of Invention Technical Problem

[11] The present invention is designed to solve the above-mentioned problems. Therefore,

it is an object of the present invention to provide a carbon material-based negative active material for a secondary battery with such property parameter values as to prevent deterioration of electrical and chemical characteristics of the secondary battery that may occur during a compression process performed to manufacture an electrode of the secondary battery by newly defining property parameters of the negative active material and understanding the correlation between the defined property parameters and electrical and chemical characteristics of the secondary battery.

[12] It is another object of the present invention to provide an electrode of a secondary battery manufactured using the carbon material-based negative active material with optimum values of the newly defined property parameters, and a secondary battery including the same. Technical Solution

[13] In order to achieve the above-mentioned objects, a negative active material for a secondary battery according to the present invention includes a core carbon material, and a carbide layer formed on at least a portion of an edge of the core carbon material, and the negative active material has a difference (an amount of compression density change) of 0.5g/cc or more between a first compression density measured when a pressure of 63.704 MPa is applied for 2 seconds and a second compression density measured when a pressure of 6.3704 MPa is applied for 2 seconds.

[14] In the present invention, the pressure of 63.704 MPa is a pressure applied to a negative active material when 2g of the negative active material is put into a hole cup of φ 1.4cm and a force of It is applied to the negative active material using a press machine. The pressure of 6.3704 MPa is a pressure applied to a negative active material when 2g of the negative active material is put into a hole cup of φ 1.4cm and a force of 0.11 is applied to the negative active material using a press machine.

[15] Preferably, the core carbon material is a high crystalline natural graphite having a spherical shape.

[16] Alternatively, the core carbon material may be any one selected from the group consisting of natural graphite having an oval, wavy, scale-like or whisker-like shape, artificial graphite, mesocarbonmicro beads, mesophase pitch fine powder, isotropic pitch fine powder and resin coal, and low crystalline carbon fine powder having a pseudo-graphite structure or turbostratic structure, or mixtures thereof.

[17] Preferably, the carbide layer is a low crystalline carbide layer formed by coating the core carbon material with pitch or tar derived from coal or petroleum, or mixtures thereof and performing carbonization of the coated layer.

[18] In order to achieve the above-mentioned objects, an electrode of a secondary battery according to the present invention includes a metallic current collector and a negative

active material coated onto the metallic current collector, wherein the negative active material has a difference (an amount of compression density change) of 0.5g/cc or more between a first compression density measured when a pressure of 63.704 MPa is applied for 2 seconds and a second compression density measured when a pressure of 6.3704 MPa is applied for 2 seconds.

[19] In order to achieve the above-mentioned objects, a secondary battery according to the present invention includes an anode current collector coated with a negative active material, a cathode current collector coated with a positive active material, a separator interposed between the anode current collector and the cathode current collector, and an electrolytic solution filled in the separator, wherein the negative active material has a difference (an amount of compression density change) of 0.5g/cc or more between a first compression density measured when a pressure of 63.704 MPa is applied for 2 seconds and a second compression density measured when a pressure of 6.3704 MPa is applied for 2 seconds.

[20] Preferably, efficiency of the secondary battery is 93% or more at 1st cycle, and a discharging capacity retention rate is 95% or more at 30th cycle based on a discharging capacity at 2nd cycle. Mode for the Invention

[21] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[22] A negative active material for a secondary battery according to the preferred embodiments of the present invention includes a core carbon material, and a carbide layer formed on at least a portion of an edge of the core carbon material, and the negative active material has a difference (an amount of compression density change) of 0.5g/cc or more between a first compression density measured when a pressure of 63.704 MPa is applied for 2 seconds and a second compression density measured when a pressure of 6.3704 MPa is applied for 2 seconds.

[23] A formula for calculating the amount of compression density change is represented as the following Math Figure 1.

[24] MathFigure 1

[Math.l] dP.D. = PD _h - PD ₁

[25] In the above Math Figure 1, dP.D. is an amount of compression density change, PD _h is a first compression density measured when a pressure of 63.704 MPa is applied to a negative active material for 2 seconds, and PD ₁ is a second compression density measured when a pressure of 6.3704 MPa is applied to the negative active material for 2 seconds.

[26] The pressure of 63.704 MPa is a pressure applied to a negative active material when

2g of the negative active material is put into a hole cup of φ 1.4cm and a force of It is applied to the negative active material using a press machine. And, the pressure of 6.3704 MPa is a pressure applied to a negative active material when 2g of the negative active material is put into a hole cup of φ1.4cm and a force of O.lt is applied to the negative active material using a press machine.

[27] In the present invention, the core carbon material is preferably a high crystalline natural graphite having a spherical shape. Alternatively, the core carbon material may be any one selected from the group consisting of natural graphite having an oval, wavy, scale-like or whisker-like shape, artificial graphite, mesocarbonmicro beads, mesophase pitch fine powder, isotropic pitch fine powder and resin coal, and low crystalline carbon fine powder having a pseudo-graphite structure or turbostratic structure, or mixtures thereof.

[28] Preferably, the carbide layer is a low crystalline carbide layer formed by coating the core carbon material with pitch or tar derived from coal or petroleum, or mixtures thereof and performing carbonization of the coated layer. Here, the low crystalline means that crystallinity of the carbide layer is lower than crystallinity of the core carbon material. The carbide layer fills up micropores of the core carbon material to decrease a specific surface area and reduce a site where decomposition of an electrolytic solution may occur.

[29] The amount of compression density change of 0.5g/cc or more allows for prevention of a rapid deterioration in a cycle efficiency of a secondary battery and a discharging capacity retention rate at a long cycle, caused by an excessive exposure of an edge of the core carbon material where the reaction with an electrolytic solution occurs, that is resulted from partial destruction of the carbide layer formed on a portion or the whole of the edge of the core carbon material after compression of the negative active material performed to coat the negative active material on a metallic current collector.

[30] The negative active material for a secondary battery according to the present invention can be prepared by the steps of forming a carbon material coating layer on a

granular core carbon material by wet-mixing or dry-mixing the core carbon material with pitch or tar derived from coal or petroleum, or mixtures thereof, and sintering the core carbon material having the carbon material coating layer, so that at least a portion of an edge of the core carbon material is coated with a carbide layer.

[31] Preferably, the amount of compression density change of the negative active material is controlled to 0.5g/cc or more by controlling a mixing ratio between the core carbon material and the carbon material derived from coal or petroleum, a temperature increase speed for sintering, a sintering temperature, a sintering time and so on.

[32] Preferably, the core carbon material is a high crystalline natural graphite having a spherical shape. Alternatively, the core carbon material may be any one selected from the group consisting of natural graphite having an oval, wavy, scale-like or whisker- like shape, artificial graphite, mesocarbonmicro beads, mesophase pitch fine powder, isotropic pitch fine powder and resin coal, and low crystalline carbon fine powder having a pseudo-graphite structure or turbostratic structure, or mixtures thereof.

[33] Preferably, the carbon material derived from coal or petroleum is pitch, tar, or mixtures thereof.

[34] The negative active material for a secondary battery prepared by the above- mentioned process may be mixed with a conductive material, a binder and an organic solvent into an active material paste. The active material paste may be coated on a metallic current collector such as a copper foil current collector, and then may be dried, thermally treated and compressed to manufacture an electrode (anode) of a secondary battery.

[35] And, the electrode of a secondary battery manufactured as mentioned above may be used in manufacturing a lithium secondary battery. That is, a rechargeable lithium secondary battery may be manufactured by placing a metallic current collector coated with a predetermined thickness of the negative active material of the present invention and a metallic current collector coated with a predetermined thickness of Li-based transition metal compound on the opposite sides of a separator, and impregnating the separator with an electrolytic solution for a lithium secondary battery. The methods for manufacturing an electrode of a secondary battery and a secondary battery including the same are well known to persons having ordinary skill in the art, and their detailed description is omitted.

[36] Meanwhile, the present invention is characterized by properties of a negative active material for a secondary battery. Thus, an electrode of a secondary battery and a secondary battery including the same can be manufactured using the negative active material of the present invention by various methods well known in the art. And, it is obvious that a secondary battery manufactured using the negative active material of the present invention is not limited to a lithium secondary battery.

[38] <Examples>

[39] [Example 1]

[40] Natural spherical graphite was wet-mixed with 10 weight% of pitch dissolved in tetrahydrofuran, relative to weight of the natural graphite, at normal pressure for 2 hours or more, and dried to obtain a mixture of the graphite and the pitch. The mixture was inserted into a sintering chamber, and sintered at 1100 ⁰ C for 1 hour after increasing the temperature to 1100 ⁰ C at a temperature increase speed of 10°C/min. Fine powder removal and powder classification was performed to obtain a negative active material. The measurement results showed that the negative active material of example 1 had a first compression density of 1.73g/cc, a second compression density of 1.42g/cc, and an amount of compression density change of 0.31g/cc.

[41]

[42] [Example 2]

[43] A negative active material was prepared in the same way as the example 1, except that 7 weight% of pitch was used relative to weight of the natural graphite and a temperature increase speed for mixture sintering was 5°C/min. The measurement results showed that the negative active material of example 2 had a first compression density of 1.76g/cc, a second compression density of 1.36g/cc, and an amount of compression density change of 0.40g/cc.

[44]

[45] [Example 3]

[46] A negative active material was prepared in the same way as the example 1, except that 5 weight% of pitch was used relative to weight of the natural graphite and the temperature increase speed was l°C/min. The measurement results showed that the negative active material of example 3 had a first compression density of 1.92g/cc, a second compression density of 1.43g/cc, and an amount of compression density change of 0.49g/cc.

[47]

[48] [Example 4]

[49] A negative active material was prepared in the same way as the example 1, except that 3 weight% of pitch was used relative to weight of the natural graphite and the temperature increase speed was 0.3°C/min. The measurement results showed that the negative active material of example 4 had a first compression density of 2.00g/cc, a second compression density of 1.38g/cc, and an amount of compression density change of 0.62g/cc.

[50]

[51] [Example 5]

[52] A negative active material was prepared in the same way as the example 1, except that 1 weight% of pitch was used relative to weight of the natural graphite and the temperature increase speed was 0.15°C/min. The measurement results showed that the negative active material of example 5 had a first compression density of 2.1 lg/cc, a second compression density of 1.40g/cc, and an amount of compression density change of 0.7 lg/cc.

[53]

[54] <Manufacture of an electrode of a secondary battery and a coin cell>

[55] An electrode of a secondary battery was manufactured using each negative active material prepared according to examples 1 to 5. lOOg of a negative active material was put into a 500m# reactor, and a small amount of N-methylpyrrolidone (NMP) and a binder (PVDF) were added. They were mixed by a mixer. The mixture was uniformly coated on a copper foil for an anode current collector, dried, heated and compressed with density of 1.65g/cnf to manufacture an anode of a secondary battery. And, 2016 coin cell battery was manufactured using each anode manufactured according to examples 1 to 5 and a Li electrode (an opposite electrode), and then tested to evaluate charging/discharging characteristics of the negative active material.

[56]

[57] <Evaluation of charging/discharging characteristics of a coin cell>

[58] A charging/discharging test was performed from 1st cycle to 30th cycle. The charging and discharging test was performed each cycle such that voltage was controlled to the range of 0.01 to 1.5V, and charging was made with a charging current of 0.5mA/cnf until voltage is 0.01V and continued until the charging current is 0.02mA/cnf while maintaining the voltage at 0.01V, and discharging was made with a discharging current of 0.5mA/cnf .

[59]

[60] The following Table 1 shows the measurement results about an amount of compression density change of each negative active material prepared according to examples 1 to 5 and charging/discharging characteristics of a coin cell manufactured using each negative active material. In Table 1, note that a discharging capacity retention rate is measured at 30th cycle based on a discharging capacity at 2nd cycle.

[61] Table 1

[Table 1] [Table ]

[62] Referring to the above Table 1, it is found that an amount of compression density change of a negative active material is related to performance of a secondary battery. That is, an amount of compression density change is not significantly related to a discharging capacity at 1st cycle (i.e. a initial capacity), but as the amount of compression density change is smaller, efficiency at 1st cycle and a discharging capacity retention rate at 30th cycle is rapidly deteriorated.

[63] Here, a small amount of compression density change means a possibility that a surface area of natural graphite may have been exposed, where a decomposition reaction of an electrolytic solution occurs, due to destruction of a carbide layer coated on the natural graphite during a compression process performed to meet the electrode density requirements.

[64] It is found through Table 1 that examples 4 and 5 with an amount of compression density change of 0.5g/cc or more have higher efficiency at 1st cycle and discharging capacity retention rate at 30th cycle than examples 1 to 3, and consequently better performance.

[65] That is, if a negative active material has an amount of compression density change of

0.5g/cc or more, a secondary battery manufactured using the negative active material

has an efficiency of 93.7% or more at 1st cycle and a discharging capacity retention rate of 95.2% or more at 30th cycle, and consequently excellent performance.

[66] As such, the preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Industrial Applicability

[67] A secondary battery manufactured using the negative active material according to the present invention can prevent deterioration of characteristics caused by destruction of the carbide layer during a compression process in the manufacture of an electrode of a secondary battery. As a result, the secondary battery has the improved cycle efficiency and discharging capacity retention rate at a long cycle.