KR100936571B1 - Negative active material used for secondary battery, electrode of secondary battery and secondary battery including the same - Google Patents

Abstract

The present invention discloses a negative electrode active material for a secondary battery, a secondary battery electrode and a secondary battery comprising the same. The negative electrode active material for a secondary battery according to the present invention includes a core carbon material whose part or all of the edges are covered by a carbide layer, and applies a first compression density and a pressure of 6.3704 Mpa when a pressure of 63.704 Mpa is applied for 2 seconds. The second compression density difference (compression density change amount) when applied for 2 seconds is characterized by being 0.5 g / cc or more.

When the secondary battery is manufactured from the negative electrode active material for the secondary battery according to the present invention, deterioration of characteristics of the secondary battery due to fracture of the carbide layer can be prevented even when the electrode pressing process is applied. As a result, the efficiency of the secondary battery and the capacity retention rate in a long cycle can be improved.

Secondary battery, natural graphite, pitch, carbide layer, edge, electrolyte decomposition

Description

A negative active material for a secondary battery, a secondary battery electrode and a secondary battery comprising the same {Negative active material used for secondary battery, electrode of secondary battery and secondary battery including the same}

The present invention relates to a negative electrode active material for a secondary battery, and more particularly, a negative electrode active material for a secondary battery made of a core carbon material in which part or all of an edge is covered by a carbide layer, an electrode for a secondary battery and a secondary battery comprising the same. It is about.

Recently, with the rapid spread of electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles, the demand for small, lightweight, and relatively high capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries have attracted attention as a driving power source for portable devices due to their light weight and high energy density. Accordingly, research and development efforts for improving performance of lithium secondary batteries have been actively conducted.

Lithium secondary batteries are used when lithium ions are inserted / desorbed from the positive electrode and the negative electrode in a state in which an organic or polymer electrolyte is charged between a negative electrode and a positive electrode made of an active material capable of intercalations and deintercalation of lithium ions. Produces electrical energy by oxidation and reduction reactions.

As a cathode active material of a lithium secondary battery, transition metal compounds such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), and the like are mainly used.

The negative active material is generally a crystalline carbon material such as natural graphite or artificial graphite having a high softening degree, or a pseudo-graphite structure obtained by carbonizing a hydrocarbon or a polymer at a low temperature of 1000 to 1500 ° C or A low crystalline carbon material having a turbostatic structure is used.

The crystalline carbon material has a high true density, which is advantageous for packing the active material and has excellent potential flatness, initial capacity, and charge / discharge reversibility. However, the more the battery is used, the lower the charge / discharge efficiency and cycle capacity are. have. This problem is analyzed to be due to the electrolytic solution decomposition reaction at the edge portion of the crystalline carbon material as the charge and discharge cycle of the battery increases.

Japanese Laid-Open Patent Publication No. 2002-348109 discloses a carbon-based negative electrode active material coated with a carbide layer in order to prevent the decomposition reaction of the electrolyte from occurring at the edge portion of the crystalline carbon material. In the carbon material-based negative active material, the carbide layer is formed by coating a pitch on the surface of the carbon material and performing heat treatment at 1000 ° C. or higher. When the carbide layer is coated on the carbon material, the initial capacity of the secondary battery is reduced by a small amount, but the charging and discharging efficiency and the cycle capacity characteristics are improved. In particular, when artificial graphitization of the coating material coating layer through high temperature heat treatment, it is possible to effectively suppress the decomposition reaction of the electrolyte solution while reducing the amount of initial capacity reduction.

However, when the carbon material-based negative electrode active material is coated on a metal current collector to produce an electrode of a secondary battery, a problem of reducing a coating effect of a carbide layer occurs. When manufacturing the electrode of the secondary battery, a compression process is performed for tight bonding between the negative electrode active material and the metal current collector. At this time, the carbide layer covering the edge of the carbon material is crushed, and the edge which causes the electrolyte decomposition reaction is returned. Because it is exposed.

Therefore, in order to manufacture a secondary battery using a conventional carbonaceous material negative active material, the carbide layer is broken by newly defining the physical property parameters of the negative electrode active material and clearly identifying the correlation between the defined physical property parameters and the electrochemical properties of the secondary battery. It is necessary to prevent deterioration of the electrochemical characteristics of the secondary battery.

However, the above-mentioned prior art shows that the mass ratio of carbon material and carbide layer, the coating and firing conditions of the carbide layer, the crystallographic properties of the carbide layer through XRD and Raman analysis, the specific surface area conditions of the carbide layer are necessary to effectively suppress the decomposition reaction of the electrolyte. The present invention is described in detail, and no mention is made about the problem caused by the fracture of the carbide layer in the electrode manufacturing process and its solution.

The present invention has been made to solve the above-described problems of the prior art, by defining a physical property parameter of the negative active material for the secondary battery newly and grasping the correlation between the defined physical property parameter and the electrochemical characteristics of the secondary battery, It is an object of the present invention to provide a carbon-based negative electrode active material having a physical property parameter value that does not deteriorate the electrochemical characteristics of a secondary battery even when a compression process is performed to manufacture a battery electrode.

It is another object of the present invention to provide a secondary battery electrode and a secondary battery including the same, which are manufactured using a carbonaceous material negative electrode active material having a newly defined property parameter value optimized.

The negative electrode active material for a secondary battery according to the present invention for achieving the above technical problem, the core portion of the edge portion of the core material is covered with a carbide layer, the first compression when applying a pressure of 63.704Mpa for 2 seconds The second compression density difference (compression density change amount) when the density and the pressure of 6.3704Mpa is applied for 2 seconds is characterized by being 0.5 g / cc or more.

In the present invention, the pressure of 63.704MPa is a pressure applied to the negative electrode active material when a 2g of the negative electrode active material is placed in a hole cup of Φ1.4cm and a load of 1t is applied using a press machine, and the pressure of 6.3704MPa is a negative electrode active material. It is the pressure applied when 2g is put in a hole cup of Φ1.4cm and a load of 0.1t is applied to the negative electrode active material using a press.

Preferably, the core carbon material is spherical natural graphite having high crystallinity.

Alternatively, the core carbon material may be a natural graphite, artificial graphite, mesocarbon micro beads, mesopeze pitch fine powder, isotropic pitch fine powder, resin coal, and pseudo-graphite having an elliptical shape, a crushed shape, a scale shape, or a whisker shape. one or a mixture thereof selected from the group consisting of low crystalline carbon fine powder having a pseudo-graphite structure or a turbostrattic structure.

Preferably, the carbide layer is a low crystalline carbide layer formed by coating the core carbon material with pitch, tar or mixtures thereof derived from coal or petroleum and then carbonizing and firing.

The secondary battery electrode according to the present invention for achieving the above technical problem includes a metal current collector and a negative electrode active material bound on the metal current collector, the negative electrode active material when the pressure of 63.704Mpa applied for 2 seconds The second compression density difference (compression density change amount) when the first compression density and the pressure of 6.3704 Mpa is applied for 2 seconds is characterized by being 0.5 g / cc or more.

A secondary battery according to the present invention for achieving the above technical problem, a negative electrode current collector coated with a negative electrode active material, a positive electrode current collector coated with a positive electrode active material, a separator interposed between the negative electrode current collector and the positive electrode current collector and the separator In the negative electrode active material, the first compression density when the pressure of 63.704Mpa is applied for 2 seconds and the second compression density difference (the amount of change in the compression density) when the pressure of 6.3704Mpa is applied for 2 seconds It is characterized by more than 0.5g / cc.

Preferably, the secondary battery has an efficiency of at least 93% of a first cycle, and a capacity retention rate of a 30th cycle of at least 95% of a discharge capacity of a second cycle.

When the secondary battery is manufactured from the negative electrode active material for the secondary battery according to the present invention, even when the electrode pressing process is applied, the deterioration of characteristics of the secondary battery due to the fracture of the carbide layer can be prevented. As a result, the efficiency of the secondary battery and the capacity retention rate in a long cycle can be improved.

Hereinafter, a preferred embodiment of the present invention will be described in detail. Prior to this, terms or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention. Therefore, the exemplary embodiments described herein are only exemplary embodiments of the present invention and do not represent all of the technical ideas of the present invention, and various equivalents and modifications that may substitute them at the time of the present application may be used. It should be understood that there may be.

The negative electrode active material for a secondary battery according to a preferred embodiment of the present invention includes a core carbon material whose part or all of the edges are covered by a carbide layer, and has a first compression density and 6.3704 Mpa when a pressure of 63.704 Mpa is applied for 2 seconds. The second compression density difference (compression density change amount) when the pressure is applied for 2 seconds is characterized by being 0.5 g / cc or more.

The calculation formula of the change amount of the compression density is shown in Equation 1 below.

<Equation 1>

dP.D. = PD _h -PD _i

In Equation 1, dP.D. is a change in compression density, PD _h is the first compression density measured after applying a pressure of 63.704MPa to the negative electrode active material for 2 seconds, PD _i is a pressure of 6.3704MPa to the negative electrode active material It is the 2nd compression density measured after applying for 2 second.

The pressure of 63.704MPa is a pressure applied to the negative electrode active material when 2g of the negative electrode active material is placed in a hole cup of Φ 1.4 cm and a load of 1 ton is applied by using a press. The pressure of 6.3704MPa is a pressure applied when 2g of the negative electrode active material is placed in a hole cup of Φ 1.4 cm and a load of 0.1 t is applied to the negative electrode active material using a press.

In the present invention, the core carbon material is preferably spherical high crystalline natural graphite. Alternatively, the core carbon material may be formed of natural graphite, artificial graphite, mesocarbon micro beads, mesopez pitch fine powder, isotropic pitch fine powder, resin charcoal, and pseudo-graphite having an oval shape, a crushed shape, a scale shape, a whisker shape, or the like. It may be any one selected from the group consisting of low crystalline carbon fine powder having a pseudo-graphite structure or a turbostrattic structure or a mixture thereof.

Preferably, the carbide layer is a low crystalline carbide layer formed by coating a core carbon material with pitch, tar or mixtures thereof derived from coal or petroleum and then carbonizing and firing the same. Here, low crystallinity means that the carbide layer has a lower crystallinity than the core carbon material. The carbide layer serves to reduce the specific surface area by filling the pores of the core carbon material and to reduce the decomposition reaction site of the electrolyte.

If the change in the compressive density is 0.5 or more in relation to the numerical range of the change in the compressive density, when the negative electrode active material is compressed, a part of the edge of the core carbon material or a part of the carbide layer covering all of the edges may be broken to react with the electrolyte. Is exposed above the limit, it is possible to prevent the problem that the efficiency of the secondary battery and the capacity retention rate in the long-term cycle rapidly deteriorate.

In the negative electrode active material for a secondary battery according to the present invention described above, the carbon material derived from coal-based or petroleum-based carbonaceous material having a particle form is wet or dry mixed to form a carbon material coating layer on the surface of the core carbon material. And, by firing the core carbon material having the carbon material coating layer is formed by proceeding the step of forming a carbide layer on part or all of the edge of the core carbon material.

Preferably, the amount of change in the compression density of the negative electrode active material is controlled to be 0.5 or more by controlling the mixing ratio of the core carbon material and the carbon material derived from coal or petroleum, heating rate for firing, firing temperature, firing time and the like.

Preferably, the core carbon material is spherical high crystalline natural graphite. Alternatively, natural graphite, artificial graphite, mesocarbon micro beads, mesopeze pitch fine powder, isotropic pitch fine powder, resin coal, and pseudo-graphite having elliptical shape, crushed shape, scale shape, whisker shape and the like. Any one or a mixture thereof selected from the group consisting of low crystalline carbon fine powder having a structure or a turbostratactic structure can be used as the core carbon material.

Preferably, pitch, tar or a mixture thereof is used as the carbon material derived from coal or petroleum.

The negative electrode active material for a secondary battery manufactured by the above-described method may be mixed with a conductive material, a binder, and an organic solvent to prepare an active material paste. Then, the active material paste may be applied to a metal current collector such as a copper foil, and then dried, heat treated, and pressed to prepare an electrode for a secondary battery (cathode).

In addition, the secondary battery electrode thus prepared can be used for the production of a lithium secondary battery. That is, a metal current collector in which the negative electrode active material is bound to a predetermined thickness and a metal current collector in which the Li-based transition metal compound is bound to a predetermined thickness are opposed to each other with a separator therebetween, and the separator is impregnated with an electrolyte for a lithium secondary battery. If possible, it is also possible to manufacture a lithium secondary battery that can be repeatedly charged and discharged. Since the secondary battery electrode and the secondary battery manufacturing method are well known to those skilled in the art to which the present invention pertains, a detailed description thereof will be omitted.

On the other hand, the present invention is characterized in the physical properties of the negative electrode active material for secondary batteries. Therefore, when manufacturing a secondary battery electrode and a secondary battery including the same using the negative electrode active material according to the present invention can be applied to a variety of methods known in the art. In addition, it is apparent that the type of secondary battery in which the negative electrode active material according to the present invention may be utilized is not limited to the lithium secondary battery.

<Actual example>

Example 1

The pitch melted with tetrahydrofuran in spherical natural graphite was mixed by 10% by weight relative to the weight of natural graphite, wet stirred at normal pressure for 2 hours or more, and dried to obtain a mixture. Then, the mixture was introduced into a firing chamber, and the temperature was raised to 1,100 ° C. at a heating rate of 10 ° C./min, and then calcined at 1,100 ° C. for 1 hour, and a classification and fine powder removal process was performed to prepare a negative electrode active material. As a result of measuring the first compression density, the second compression density and the compression density change of the negative electrode active material thus prepared, the values were 1.73 g / cc, 1.42 g / cc and 0.31 g / cc, respectively.

Example 2

Except for adjusting the mixing ratio of the natural graphite pitch to 7% by weight based on the weight of the natural graphite, and adjusting the temperature increase rate for baking the mixture to 5 ℃ / min, the rest of the process conditions are applied in the same manner as in Example 1 An active material was prepared. As a result of measuring the first compression density, the second compression density and the compression density change of the negative electrode active material thus prepared, the values were 1.76 g / cc, 1.36 g / cc and 0.40 g / cc, respectively.

Example 3

A negative electrode active material was prepared by applying the same process as in Example 1 except that the mixing ratio of the natural graphite was adjusted to 5% by weight based on the weight of the natural graphite and the temperature increase rate was adjusted to 1 ° C./min. As a result of measuring the first compression density, the second compression density and the compression density variation of the negative electrode active material thus prepared, the values were 1.92 g / cc, 1.43 g / cc and 0.49 g / cc, respectively.

Example 4

Except for adjusting the mixing ratio of the natural graphite pitch to 3% by weight based on the weight of the natural graphite, and the temperature increase rate was adjusted to 0.3 ℃ / min was applied in the same manner as in Example 1 to prepare a negative electrode active material. As a result of measuring the first compression density, the second compression density and the compression density change of the negative electrode active material thus prepared, the values were 2.00 g / cc, 1.38 g / cc and 0.62 g / cc, respectively.

Example 5

The negative electrode active material was prepared by applying the same process as in Example 1 except that the mixing ratio of natural graphite was adjusted to 1 wt% based on the weight of natural graphite and the temperature increase rate was adjusted to 0.15 ° C./minute. As a result of measuring the first compression density, the second compression density and the compression density change of the negative electrode active material thus prepared, the values were 2.11 g / cc, 1.40 g / cc and 0.71 g / cc, respectively.

<Production of Secondary Battery Electrode and Coin Cell>

Secondary battery electrodes were prepared using the respective negative electrode active materials for secondary batteries prepared in Examples 1 to 5 as raw materials. First, 100 g of the negative electrode active material was placed in a 500 ml reactor, and a small amount of N-methylpyrrolidone (NMP) and polyvinylidene fluoride (PVDF) were added as a binder and mixed. Subsequently, the mixture was kneaded with a mixer, and then coated, dried and heated on a copper thin film, which is a negative electrode current collector, and pressed at a density of 1.65 g / cm ³ to prepare an electrode for a secondary battery. Then, in order to evaluate the charge and discharge characteristics of the negative electrode active material, a coin cell of a 2016 standard using Li as a counter electrode was manufactured for each example.

<Charge / Discharge Characteristics Evaluation of Coin Cell>

Charge and discharge tests were performed from 1 cycle to 30 cycles. The charge / discharge test of each cycle regulates the potential in the range of 0.01 to 1.5V, proceeds with charging until it becomes 0.01V at a charging current of 0.5mA / cm ² , and maintains a voltage of 0.01V and charge current is 0.02mA. Charging continued until / cm ² . The discharge current was discharged up to 1.5 V at 0.5 mA / cm ² .

Table 1 below shows the changes in the compression density of each negative electrode active material prepared according to Examples 1 to 5 and the measurement results of charge and discharge characteristics of the coin cell produced using each negative electrode active material. In Table 1 below, the discharge capacity retention rate of the 30th cycle is based on the discharge capacity of the second cycle.

TABLE 1

Referring to Table 1, the correlation between the change in the compression density of the negative electrode active material and the secondary battery performance can be confirmed. In other words, there is no correlation between the change in compression density and the discharge capacity of the first cycle (that is, the initial capacity), but the smaller the change in the compression density, the faster the efficiency of the first cycle and the charge retention rate in the 30th cycle are confirmed. Can be.

 Here, the small change in the compression density means that the surface area of the natural graphite, which causes the decomposition of the electrolyte due to fracture of the carbide layer covering the natural graphite during the pressing process for adjusting the electrode density, is high.

According to Table 1, Examples 4 to 5 in which the compression density change amount of the negative electrode active material is 0.5 g / cc or more have high battery performance because the efficiency of the first cycle and the charge retention rate in the 30th cycle are higher than those of Examples 1 to 3. It can be confirmed that it is excellent.

That is, when the amount of change in the compression density is 0.5 g / cc or more, the efficiency of the secondary battery is excellent because the efficiency of the first cycle is 93.7% or more and the capacity retention rate of the 30th cycle is 95.2% or more.

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited thereto and is intended by those skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of equivalents of the claims to be described.

Claims (8)

In the negative electrode active material comprising a core carbon material, part or all of the edge is covered by a carbide layer, The second compression density difference (compression density change amount) when the first compression density when the pressure of 63.704Mpa is applied for 2 seconds and the pressure of 6.3704Mpa for 2 seconds is 0.5g / cc or more. Negative electrode active material. The method of claim 1, The pressure of 63.704MPa is a pressure applied to the negative electrode active material when 2g of the negative electrode active material is placed in a hole cup of Φ1.4cm and a load of 1t is applied by using a press. The pressure of 6.3704MPa is a negative electrode active material for the secondary battery, characterized in that the pressure applied when applying a load of 0.1t to the negative electrode active material by using a press after putting 2g of the negative electrode active material in a hole cup of Φ1.4cm. The method of claim 1, The core carbon material is a spherical natural graphite having a higher crystallinity than the carbide layer. The method of claim 1, The core carbon material may be natural graphite, artificial graphite, mesocarbon micro beads, mesopeze pitch fine powder, isotropic pitch fine powder, resin charcoal, and pseudo-graphite having elliptical shape, crushed shape, scale shape or whisker shape. A negative active material for a secondary battery, characterized in that any one or a mixture thereof selected from the group consisting of low crystalline carbon fine powder having a structure or a turbostratactic structure. The method of claim 1, And the carbide layer is a low crystalline carbide layer formed by coating carbon on the core carbon material with pitch, tar, or a mixture thereof derived from coal or petroleum, and carbonizing and firing the same. A secondary battery electrode comprising a metal current collector coated with the negative electrode active material according to any one of claims 1 to 5. A negative electrode current collector coated with a negative electrode active material according to any one of claims 1 to 5, a positive electrode current collector coated with a positive electrode active material, a separator interposed between the negative electrode current collector and the positive electrode current collector and filled in the separator A secondary battery comprising an electrolyte solution. The method of claim 7, wherein A secondary battery, wherein the efficiency of the first cycle is 93% or more, and the capacity retention rate of the 30th cycle is 95% or more of the discharge capacity of the second cycle.