KR100477971B1 - Negative active material for lithium secondary battery - Google Patents

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery, wherein the negative electrode active material has a Raman spectral intensity ratio Ra (I (1360) / I (1580)) of 0.01 to 0.45, which is a Raman spectral intensity ratio between an I (1360) plane and an I (1580) plane. Coating the core and the core and having a turbostratactic or anti-onion ring structure, Ra (I (1360) / I (1580)), which is a Raman spectral intensity ratio between the I (1360) plane and the I (1580) plane A first active ingredient comprising a semicrystalline carbon shell comprising a plate-like crystalline carbon fine particle powder having a mean particle diameter (D50) of 0.46 to 1.5 and 0.1 to 15 mu m; And a second active ingredient comprising crystalline carbon consisting of 15 to 25 μm secondary particles formed by assembling primary particles of 1 to 10 μm.

The negative electrode active material for a lithium secondary battery of the present invention is excellent in cycle life characteristics and low temperature characteristics.

Description

Negative active material for lithium secondary battery {NEGATIVE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY}

[Industrial use]

The present invention relates to a negative electrode active material for lithium secondary batteries, and more particularly to a negative electrode active material for lithium secondary batteries excellent in cycle life and low temperature characteristics.

[Prior art]

Although lithium metal was initially used as a negative electrode active material of a conventional lithium-based secondary battery, its capacity was greatly reduced during charging and discharging, and as a result, lithium ions were precipitated to form a dendrite phase, which shortened the life of the battery. Resulted. In order to improve this problem, a lithium alloy has been proposed, but it does not significantly improve the problems that occur when using lithium metal.

Then, a carbon-based compound capable of intercalating / deintercalating lithium ions was used as a negative electrode active material. As such a carbon-based negative electrode active material, a crystalline carbon material such as artificial graphite such as spherical or fibrous form or bulk graphite, natural graphite, or an amorphous carbon material such as coke was used. The bulk graphite refers to graphite having a shape in which primary particles in a flat state are bonded so that an orientation plane becomes non-equilibrium.

Among these, spherical or fibrous crystalline carbon materials are expensive to manufacture and have difficulty in increasing charging and discharging capacity, and bulk graphite has low surface orientation and is capable of random insertion of random lithium ions, thereby providing excellent high rate life characteristics. In addition, the impregnation of the electrolyte in the micropores between the binding particles is easy to show high ionic conductivity, but at low temperatures (-20 ° C), these micropores hinder the activity of lithium ions, resulting in poor ionic conductivity resulting in poor lifetime characteristics. There are disadvantages. In addition, amorphous carbon, such as coke, has a drawback in that the electrochemical performance of electrolytes varies greatly, and natural graphite or plate graphite, which is relatively inexpensive, has a low cost, high capacity, and low temperature characteristics, but has a high rate of life due to the plate structure. The characteristics were poor, the efficiency was low, and the workability of the pole plate when processing in the form of the pole plate was disadvantageous.

Since there are advantages and disadvantages according to the type of the carbon-based material, in order to take advantage of each of the advantages in recent years, a method of using a mixture of carbon-based materials having different physical properties have been studied. For example, Japanese Patent Application Laid-Open Nos. 8-180873, 9-27316, 11-191408, and 2000-164215 disclose that mixing and discharging different carbonaceous materials improves charge and discharge capacity and lifespan. However, since these methods are used as a lithium-based secondary battery negative electrode active material, the life / low temperature characteristics have not yet reached a satisfactory level or are not mentioned.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a negative electrode active material for a lithium secondary battery excellent in cycle life characteristics and low temperature characteristics.

In order to achieve the above object, the present invention provides a crystalline carbon core and Ra (I (1360) / I (1580)) of the Raman spectral intensity ratio between the I (1360) plane and the I (1580) plane 0.01 to 0.45 Coated, having a turbostratonic or half-onion ring structure and having a Raman spectral intensity ratio Ra (I (1360) / I (1580)) between the I (1360) and I (1580) planes of 0.46 to 1.5, with an average A first active ingredient comprising a semicrystalline carbon shell comprising a plate-like crystalline carbon fine particle powder having a particle diameter (D50) of 0.1 to 15 µm; And it provides a negative electrode active material for a lithium secondary battery comprising a second active component comprising crystalline carbon consisting of 15 to 25 ㎛ secondary particles formed by assembling the primary particles of 1 to 10 ㎛.

Hereinafter, the present invention will be described in more detail.

The negative electrode active material for a lithium secondary battery of the present invention is coated with a crystalline carbon core having a Ra (I (1360) / I (1580)), which is a Raman spectral intensity ratio between the I (1360) plane and the I (1580) plane, of 0.01 to 0.45. The Raman spectral intensity ratio Ra (I (1360) / I (1580)), which has a turbostatic or half-onion ring structure and is the I (1360) plane and the I (1580) plane, is 0.46 to 1.5, and has an average particle diameter. A first active ingredient comprising a semi-crystalline carbon shell comprising a plate-like crystalline carbon fine particle powder having a (D50) of 0.1 to 15 μm and 15 to 25 μm secondary particles formed by assembling primary particles of 1 to 10 μm. A second active ingredient comprising crystalline carbon.

In the first active component, when Ra (I (1360) / I (1580)) of the crystalline carbon core is higher than 0.45, the crystallinity is low and the discharge capacity is lowered, and Ra (I (1360) of the semicrystalline carbon shell is reduced. / I (1580)) is less than 0.46, the reactivity with the electrolyte solution is increased, the initial efficiency is lowered, and if it is higher than 1.5, it is low crystallized and the discharge capacity is lowered, which is undesirable.

The tap density of the first active ingredient is 1.20 g / cc to 1.5 g / cc. When the tap density is lower than 1.20 g / cc, the volume per weight of the active material is increased, so that the binder content in the battery plate must be increased. There is a problem that the active material content is relatively reduced to decrease the capacity per volume, it is impossible to manufacture with a material higher than 1.50g / cc.

The average particle diameter of the first active ingredient is 25 ± 5 μm, and the BET (specific surface area) value is 2.0 to 4.0 m ² / g. When the BET value of the negative electrode active material is less than 2.0 m ² / g, the discharge capacity is lowered, which is not preferable, and when it exceeds 4.0 m ² / g, the initial efficiency is lowered, which is not preferable.

In general, as the anisotropy of the graphite particle array increases, the intensity of the (002) peak decreases and the intensity of the (110) peak increases, so that I ₁₁₀ / I ₀₀₂ increases, so that I ₁₁₀ / I ₀₀₂ represents the orientation of the graphite particles. It can be seen that it is an indicator. The X-ray diffraction intensity of the in-plane direction (110) plane and the c-axis direction (002) plane of the first active ingredient, I (110) / I (002), is a ratio measured by the X-ray diffraction method. 0.01 to 0.001. If the value of I (110) / I (002) is larger than 0.01, the capacity is lowered, and if it is smaller than 0.001, the high rate characteristic is lowered, which is not preferable.

The second active ingredient of the present invention comprises crystalline carbon as secondary particles having an average particle diameter of 15 to 25 μm formed by assembling primary particles having an average particle diameter of 1 to 10 μm. In the case where the average particle diameter of the secondary particles exceeds 25 µm, the life characteristics are deteriorated, which is not preferable. When the average particle diameter is smaller than 15 µm, the irreversible capacity increases, which is not preferable. In addition, if the average particle diameter of the primary particles is less than 1㎛ increase in the content of the binder during the production of secondary particles, if larger than 10㎛ it is not preferable because the problem occurs in the control of the secondary particles.

As the crystalline carbon, natural graphite or artificial graphite may be used. The tap density of the second active ingredient is preferably 0.5 to 1.0 g / cc. If the tap bilding ratio is lower than 0.5 g / cc, the content of the binder increases, so that the amount of the negative electrode active material is relatively decreased in the negative electrode, thereby decreasing the capacity per volume. .

In addition, I (110) / I (002), which is a ratio measured by X-ray diffraction of the (110) plane and the (002) plane of the second active ingredient, is from 0.02 to 0.02. 0.2. If the value of I (110) / I (002) is larger than 0.2, the capacity is lowered, and if it is smaller than 0.02, the high rate characteristic is lowered, which is not preferable.

Hereinafter, the manufacturing method of the 1st active ingredient used by this invention is demonstrated. First, the crystalline carbon is pulverized to form a plate-shaped crystalline carbon particles having a mean particle size (D50) of 5 to 50 μm (hereinafter referred to as large particles) and a plate-like crystalline carbon fine particles having a mean particle size (D50) of 0.1 to 15 μm (hereinafter finely divided particles). To be prepared). As the crystalline carbon, natural graphite or artificial graphite may be used.

Only the large particles are spheroidized to obtain spherical crystalline carbon particles. Core particles are prepared by granulating the spherical particles and the plate-shaped fine particles remaining without spheroidization by a mechanochemical method. The mixing ratio of the spherical particles and plate-like fine particles is preferably 70 to 99.99: 0.01 to 30 weight ratio. If the mixing ratio is out of the above-mentioned range, spherical particles and fine particles are not assembled, but rather are aggregated separately so that a desired substance is not obtained. In this way, when the pulverized finely divided particles are mixed with the spherical particles, small particles are assembled on the surface to increase the discharge capacity.

Through the granulation process of the spherical particles and the plate-like fine particles, the plate-like fine particles (average particle diameter of 0.1 to 15 μm) are assembled on the surface of the large spherical particles (average particle diameter of 5 to 50 μm), thereby exposing the graphite edge surface. This reduces the number of particles and gives a graphite structure with random orientation on one particle surface.

In view of the difference between the shape spheronization process and the assembly process, the shape spheronization process uses only the large particles, that is, particles having a general normal distribution, at high rotational speeds (300 to 1000 rpm) in a short time (several minutes to several tens of minutes) It is a process of modifying the particle surface by giving a lot of shear force.

On the other hand, the granulation process adds several percent by weight of plate-shaped fine particles (<5 μm) to the spherical large particles (about 20-30 μm) and gives a lot of shear force (eg, blades). , Circular), to rotate at low rotational speeds (500 to 1000 rpm), whereby particle assembly is achieved by mechanical particle collision. Since these fine particles are included, they are also advantageous in conductivity. At this time, the particle size difference between the large particles and the fine particles is best when the difference is several tens to several hundred times.

The prepared core particles and the amorphous carbon are mixed in a solid phase in a ratio of 50 to 99.99% by weight: 0.01 to 50% by weight. If the weight of the amorphous carbon is less than 50% by weight, there is a problem in that the initial efficiency is lowered due to high reactivity with the electrolyte solution, and when it exceeds 99.99% by weight, the content of high-crystalline primary particles is reduced, so that the discharge capacity is reduced. Not. As the amorphous carbon, coal pitch or petroleum pitch may be used.

The mixture is heat treated at a temperature of 2000 to 3200 ° C. under an inert atmosphere to prepare a first active ingredient comprising a crystalline carbon core and a coating of the core and a semicrystalline carbon shell derived from amorphous carbon. When the heat treatment step is performed at less than 2000 ° C., the discharge capacity is lowered, which is undesirable, and heat treatment at a temperature higher than 3200 ° C. is practically difficult to be performed.

 The carbon shell has a turbostratic or onion ring structure on its surface and includes plate-like crystalline carbon fine particles.

The first active ingredient and the second active ingredient are mixed. This second active ingredient comprises crystalline carbon formed from secondary particles having an average particle size of 15 to 25 μm, formed by assembling primary particles having an average particle size of 1 to 10 μm. The assembling process is performed using an aglomaster or the like, and the method is well known in the art, and thus detailed description thereof will be omitted.

The mixing ratio of the first active ingredient and the second active ingredient is preferably 90 to 50% by weight: 10 to 50% by weight. When the amount of the second active ingredient exceeds 50% by weight, the low temperature property is lowered, which is not preferable, and when the amount of the second active ingredient is lower than 10% by weight, the lifespan property is lowered, which is not preferable.

The mixing process may be performed for 5 to 60 minutes, preferably 20 to 30 minutes, to uniformly mix the first active ingredient and the second active ingredient.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)

Natural graphite was pulverized to obtain a plate-shaped large particle having an average particle diameter (D50) of 30 µm and a plate-shaped fine particle (average particle diameter (D50) of 0.1 µm). The large particles were spheroidized to produce spherical crystalline carbon particles. The spherical crystalline nanoparticles were mixed with the above-mentioned plate-like fine particles obtained by grinding, and granulated by a mechanical method to produce small particles.

The small particles and the coal-based pitch in a solid phase mixture of 90: 10% by weight, uniformly coated coal-based pitch on the surface of the primary particles, and then sorted the powder obtained by heat treatment at 2200 ℃ under Ar atmosphere to average particle diameter (D50) About 24 micrometers of 1st active ingredients were produced. The prepared first active ingredient had a tap density of 1.2 g / cc, a spherical shape, and a diffraction peak intensity ratio (I110 / I002) of the (110) plane and the (002) plane by the X-ray diffraction method was 0.69. This value is a value measured using the powder method at a scanning speed of 0.02 ° / 1 sec. The results of the measurement are shown in FIG. 1.

50% by weight of the second active ingredient of natural graphite comprising 50% by weight of the first active ingredient and secondary particles having an average particle size of 15 μm formed by assembling primary particles having an average particle size of 1 μm into a planetary mixer. The mixture was mixed for 20 minutes to prepare a negative active material for a lithium secondary battery. The second active ingredient had a tap density of 0.6 g / cc and a diffraction peak intensity ratio (I110 / I002) of the (110) plane and the (002) plane by the X-ray diffraction method was 0.0262.

Physical properties of the first active ingredient and the second active ingredient are shown in Table 1 below.

Average particle size Specific surface area Apparent density Tap density d ₀₀₂ Lc (nm) I ₁₁₀ / I ₀₀₂ Expense fee First active ingredient 24㎛ 2.83 m ² / g 0.72 g / cc 1.20 g / cc 0.3359nm 93 0.0069 1.5 Second active ingredient 21.5 ㎛ 2.75m ² / g 0.32 g / cc 0.66 g / cc 0.3359nm 410 0.0262 1.3

In addition, SEM pictures of the first active ingredient and the second active ingredient are shown in FIGS. 2 and 3, respectively. As shown in Figures 2 and 3, it can be seen that the first active ingredient has a dense spherical shape, and the second active ingredient has an irregular shape with low density. In addition, the surface and the internal SEM photograph of the prepared negative active material are shown in FIGS. 4 and 5, respectively. As shown in FIGS. 4 and 5, particles having an irregular shape having a low density are distributed on the surface of the prepared negative electrode active material, and fine particles are distributed therein. 2 The active ingredient is considered to be mainly distributed externally.

97% by weight of the negative electrode active material, 1.5% by weight of styrene butadiene rubber, and 1.5% by weight of carboxymethylcellulose were mixed in N-methyl pyrrolidone to prepare a negative electrode active material slurry, and the slurry was applied to a current collector of copper foil and Drying made a negative electrode.

(Example 2)

The same procedure as in Example 1 was carried out except that 80 wt% of the first active ingredient and 20 wt% of the second active ingredient were mixed.

(Comparative Example 1)

Only the second active ingredient of Example 1 was used as the negative electrode active material.

(Comparative Example 2)

The same procedure as in Example 1 was carried out except that 30 wt% of the first active ingredient and 70 wt% of the second active ingredient were mixed.

LiPF ₆ was used as an electrolytic salt in a mixed carbonate ester solvent prepared by the method of Examples 1 to 2 and Comparative Examples 1 to 2, and a LiCoO ₂ anode and an ethylene carbonate / dimethyl carbonate in a volume ratio of 1/1. The 750 mAh square battery was produced using the electrolyte solution melt | dissolved in the density | concentration.

<Low Temperature Characteristics Evaluation>

The prepared rectangular battery was charged to 0.5 C (375 mA) at room temperature, and then left at -20 ° C. for 4 hours to measure discharge capacity. The value calculated by comparing this discharge capacity value with the normal temperature 0.5C discharge capacity was made into the low temperature characteristic (%).

<Cycle life characteristics evaluation>

After charging the manufactured rectangular battery to 1C (750mA) at room temperature, and discharging at 1C (750mA) as 1 cycle, charging and discharging was performed up to 200 cycles, and the life (%) was discharged at the 200th cycle compared to the first cycle discharge capacity. The dose is shown.

<Tap Density Measurement>

The tap densities of the negative electrode active materials for lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were measured using MT-1000 (equipment name, Seishin) in the following manner.

A 100 mL volumetric cylinder having a volume of 100 mL was measured beforehand using a spoon to slowly add 100 mL of a powder sample through a 250 mm mesh and stopper to measure mass (M1). The mass (M) of the powder sample (M) was obtained by subtracting the mass (M0) of the measuring cylinder from the mass (M). Next, the volume cylinder (V) of the compressed sample powder was read by dropping the measuring cylinder in that state onto the rubber plate 500 times at a height of 18 mm.

Tap density (g / cc) was calculated according to Equation 1 using the measured mass (M) and the volume (V) of the sample powder.

[Equation 1]

D = (M-M0) / V

D: tap density (g / cc)

MO: mass of the measuring cylinder

M: mass (g) of the sample powder in the measuring cylinder

V: volume of the sample powder in the measuring cylinder after 500 drops

The measured tap density values are shown in Table 2 below.

<X-ray diffraction peak intensity ratio (I ₁₁₀ / I ₀₀₂ ) measurement>

In order to obtain a reproducible I ₁₁₀ / I ₀₀₂ value, the scan rate was determined so that the intensity of the (002) plane was 10,000 counts. In other words, to measure the X-ray diffraction intensity ratio I _11O / I ₀₀₂ of the first embodiment of the negative electrode active material (110) plane and the (002) surface of the powder using a method in 0.02 ° / 1 sec scan rate.

Measurement results of tap density, I110 / I002, life characteristics and low temperature characteristics are shown in Table 2 below.

Mixing ratio of the first and second active ingredients (first active ingredient: second active ingredient) Tap Density (g / cc) I ₁₁₀ / I ₀₀₂ Lifespan (%) (200 times / 1 time) Low temperature characteristic (%) (-20 ℃ / room temperature) Example 1 50:50 0.88 0.027 88.2 55 Example 2 80:20 1.02 0.012 83.5 58 Comparative Example 1 100: 0 1.20 0.0035 77.2 68 Comparative Example 2 0: 100 0.66 0.035 83.3 30 Comparative Example 3 30:70 0.77 0.0087 90.7 42

As shown in Table 2, Comparative Examples 1 to 2 or the negative active material of Examples 1 to 2, in which the first active ingredient and the second active ingredient were mixed at an appropriate ratio, were prepared using only the first active ingredient or the second active ingredient. It can be seen that the service life and low temperature characteristics are superior to Comparative Example 3 in which the ratio is not appropriate.

The negative electrode active material for a lithium secondary battery of the present invention is excellent in cycle life characteristics and low temperature characteristics.

1 is a graph showing the X-ray diffraction intensity of the (110) plane and (002) plane of the negative electrode active material for a lithium secondary battery of the present invention.

2 is a SEM photograph of the first active ingredient used in the negative electrode active material of the present invention.

3 is a SEM photograph of a second active ingredient used in the negative electrode active material of the present invention.

Figure 4 is a SEM photograph showing the surface particles in the negative electrode active material of the present invention.

Figure 5 is a SEM photograph showing the internal particles in the negative electrode active material of the present invention.

Claims (6)

A crystalline carbon core having a Raman spectral intensity ratio Ra (I (1360) / I (1580)) of the I (1360) plane and the I (1580) plane coated between 0.01 and 0.45, and coated with the turbostratonic or half-onion It has a ring structure, and Ra (I (1360) / I (1580)), which is the Raman spectral intensity ratio between the I (1360) plane and the I (1580) plane, is 0.46 to 1.5, and the average particle diameter (D50) is 0.1 to 15 mu m. A first active ingredient comprising a semicrystalline carbon shell comprising a plate-like crystalline carbon particulate powder; And Second active ingredient comprising crystalline carbon consisting of 15 to 25 μm secondary particles formed by assembling primary particles of 1 to 10 μm A negative electrode active material for a lithium secondary battery comprising a. The negative active material of claim 1, wherein a mixing ratio of the first active ingredient and the second active ingredient is 90 to 50 wt%: 10 to 50 wt%. The negative active material of claim 1, wherein the tap density of the first active ingredient is 1.2 to 1.5 g / cc, and the tap density of the second active ingredient is 0.5 to 1.0 g / cc. According to claim 1, I (110) / I (002) is a ratio measured by the X-ray diffraction method of the X-ray diffraction intensity of the (110) plane and (002) plane of the first active ingredient is 0.001 to 0.01 (I) (110) / I (002), which is a ratio measured by X-ray diffraction method of (110) plane and (002) plane of the second active ingredient, is 0.02 to 0.2 for lithium secondary battery Negative electrode active material. The negative active material of claim 1, wherein the average particle diameter of the first active ingredient is 25 ± 5 μm. The negative active material of claim 1, wherein the BET (specific surface area) value of the first active ingredient is 2.0 to 4.0 m ² / g.