KR100663180B1 - Anode active material for high energy density lithium secondary battery - Google Patents

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery, and the negative electrode active material according to the present invention is manufactured in a mixed powder form in which the distribution of powder particle size is controlled, thereby maximizing the powder content per unit volume coated on the current collector, thereby greatly improving the mixture density. In addition, it minimizes the deformation of the powder during electrode rolling to improve the spring back phenomenon, and also has the effect of improving the wettability with the electrolyte by the microchannels formed between the powders. The lithium secondary battery including the negative electrode active material of the present invention exhibits high energy density, excellent battery capacity, and stable life characteristics.

Description

Anode active material for high energy density lithium secondary battery {ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY HAVING HIGH ENERGY DENSITY}

Figure 1 is a graph showing the particle size distribution of the measured powder after analyzing the negative electrode active material (Example 5) prepared by the present invention with a particle size analyzer.

The present invention relates to a negative electrode active material that can be used in the manufacture of a high energy density lithium secondary battery.

In recent years, the market for portable electronic devices such as mobile phones, laptops and camcorders is growing rapidly, and the technology for lithium secondary batteries is rapidly growing. Ni-Cd batteries and nickel-metal hydride batteries, which have been widely used in the past, may have a memory effect that decreases their capacity when fully charged without being fully discharged. However, lithium-ion batteries have a large capacity, are light, and have no memory effect. It is convenient because it can be recharged as it is remaining.

The core technology development tasks of the lithium secondary battery include energy density improvement, long life, high efficiency, low temperature characteristics, and cost reduction.The core technologies include battery structure development, battery material characteristics improvement, electrode material and electrode plate manufacturing technology. Improvement, electrolyte solution characteristics and the like.

A lithium secondary battery generally has a structure including a negative electrode, a positive electrode, an electrolyte, and a lithium ion-transmissive separator between the electrodes. The negative electrode of the lithium secondary battery may be a negative electrode active material (eg, carbon-based material), a conductive agent (eg, carbon black), a binder (eg, polyvinylidene fluoride (PVDF)), and a solvent (eg, N-methylpyrrolidone). (NMP)) is prepared by coating, drying and compressing the negative electrode composition in the form of a slurry on the surface of the current collector.

As the technology trend of the negative electrode active material, graphitized mesophase globules, natural graphite, pseudoisotropic carbon, fine mosaic carbon, amorphous carbon, etc. are used, but the difference in voltage and capacity for each material improves the improvement. In order to achieve the high capacity through the method of mixing various surface treatment or materials with different characteristics. In addition, recently, when manufacturing a negative electrode with a negative electrode composition containing a single negative electrode active material, there is a limit in improving the packing density and high capacity of the negative electrode plate, so that two or more kinds of negative electrode active materials having different particle diameters may be mixed, or the surface of the negative electrode active material may be coated or Surface treatment techniques have been proposed.

For example, U. S. Patent No. 5,273, 842 discloses a technique of classifying the particle size distribution of the negative electrode active material to be a grading distribution and applying the same to the manufacturing of the negative electrode. However, this method has the disadvantage that the consumption of the negative electrode active material is high and the manufacturing time of the negative electrode active material itself is long.

Accordingly, an object of the present invention is to maximize the powder content per unit volume coated on the current collector to improve the mixture density, to minimize the deformation of the powder during electrode rolling to improve the spring back phenomenon, between the powder It is to provide a negative electrode active material that can improve the wettability (weettability) with the electrolyte by the fine channels to be formed.

In addition, an object of the present invention is to provide a lithium secondary battery exhibiting a high energy density, excellent battery capacity and stable life characteristics by treating the negative electrode active material on a negative electrode plate.

In accordance with the above object, in the present invention, the granular powder having a particle size of 5 μm or less is included in 1% to 15% of the total volume, and the granule powder having a particle size of d90 or more is 20 to 80 μm, the maximum in the total particle size. It provides a negative electrode active material for a lithium secondary battery, characterized in that the particle size dmax _peak corresponding to the peak position is d50 _vol or more corresponding to the average volume particles.

Hereinafter, the present invention will be described in detail.

As described above, the negative electrode active material for a lithium secondary battery according to the present invention is characterized in that the powder particle size distribution of the active material is controlled in a specific range.

Specifically, in the negative electrode active material according to the present invention, fine powder having a size of 5 μm or less is included in a volume of 1% to 15% of the total negative electrode active material.

In the lithium secondary battery, the presence of fine particles having a particle size of 5 μm or less in the powder used as the negative electrode active material is important in terms of improving electrical conductivity by connecting the interparticle contacts while increasing the filling property in the electrode. The more contained, the greater the increase rate of the specific surface area of the powder, and the less fairness in the preparation and coating of the slurry for the electrode. That is, when the fine powder is present in more than 15% of the total negative electrode active material, the specific surface area is excessively increased to cause a lot of irreversible reaction, the battery efficiency is reduced and the processability of the battery manufacturing is reduced, if less than 1%, the filling Performance and electrical conductivity will fall.

In addition, the negative electrode active material powder according to the present invention includes granule powder having a particle size of d90 or more of 20 to 80 µm. In the lithium secondary battery, the presence of granules in the powder used as the negative electrode active material is advantageous in terms of improving the mixture density, but when a large number of relatively large particles are present within the limited electrode thickness, the nonuniformity of the electrode can be induced, and high rate characteristics Can interfere. That is, when the particle size of the granules is smaller than 20 μm, the overall average particle size is lowered to induce an increase in specific surface area and a decrease in the mixture density, and when larger than 80 μm, the electrode nonuniformity and the high rate property may be affected.

In addition, the negative electrode active material according to the present invention is characterized in that the particle size dmax _peak corresponding to the maximum peak position in the overall particle size is larger than d50 _vol corresponding to the average volume particle. The negative electrode active material of the present invention can increase the specific gravity of the granules of medium size or more in the overall particle size distribution, while minimizing the irreversible reaction, and improve the electrical conductivity and the mixture density, while improving the processability during electrode production. . However, in order to obtain the above effects, it is necessary to minimize the content of the maximum granules occupying about 10% by volume, so that the negative electrode active material according to the present invention has an average content as the volume ratio of the granules, and the distribution of the intermediate or higher particles. Has a large particle size distribution towards the granules.

In the present invention, at least one mixed powder selected from the group consisting of natural graphite, artificial graphite, and amorphous carbon may be used as the powder component constituting the negative electrode active material.

Since the particle size distribution of the powder is controlled in a specific range as described above, the negative electrode active material according to the present invention improves the filling property during electrode rolling, thereby easily increasing the mixture density, and decreases the powder deformation due to less stress given in the powder. When left under conditions, the springback of the powder trying to return to the state before the external pressure is greatly reduced. When the springback phenomenon occurs a lot, even if the mixture density increases during rolling, the mixture density decreases in the battery, making it difficult to implement a battery having a high energy density, and also increasing the pressure between electrodes in a battery designed with a constant volume. By inducing battery expansion, battery characteristics and safety may be reduced. The extent of this springback phenomenon is calculated as the thickness (T1) measured immediately after the electrode rolling and the thickness (T2) measured after vacuum drying for 12 hours after the electrode rolling, based on the electrode having a mixture density of 1.90 g / cc or less. It can be evaluated as a springback value [{(T2-T1) / T1} * 100], and advantageous battery performance can be expressed when the springback value is 8% or less. If the springback value is greater than or equal to 8%, an excessive volume expansion of the negative electrode plate causes stress on the positive electrode plate and the battery itself, and this stress causes electrical non-uniformity to cause degradation of battery performance.

In addition, the negative electrode active material according to the present invention forms a uniform microchannel between powders in the electrode by the particle size distribution of the powder, thereby improving the inflow of the electrolyte, thereby improving the reactivity between the electrolyte and the powder, that is, the wettability. Can be. The improvement of the wettability is one of very important physical properties in the high mixture density electrode, and even if the high mixture density electrode has poor wettability with the electrolyte, it is difficult to express the excellent battery characteristics due to difficulty in introducing the electrolyte solution into the electrode.

Hereinafter, the present invention will be described in more detail with reference to the following Examples and Test Examples. However, the following Examples and Test Examples are only for illustrating the present invention, but the scope of the present invention is not limited thereto.

Examples 1-10

The powder material prepared according to the particle size distribution and content shown in Table 1 was mixed and used as a negative electrode active material mixed powder. In the case of mixing amorphous carbon or artificial graphite with natural graphite, all were mixed in the same weight ratio, and the particle size distribution of the negative electrode active material mixed powder was measured using MASTERSIZER 2000 [MALVERN INSTRUMENT (UK)]. First, 100 parts by weight of the negative active material mixed powder, 2 parts by weight of carbon black having an average particle diameter of 100 nm as conductive fine particles, 2 parts by weight of styrene butadiene rubber (SBR) as a binder, and 2 parts by weight of carboxymethyl cellulose (CMC) as a thickener. After mixing together, the mixture was sufficiently stirred to prepare a negative electrode active material slurry composition.

After coating the negative electrode active material slurry composition on the surface of the copper current collector using a doctor blade, the coating layer was hot-air dried at 110 ° C., roll-pressed to a pressure of 30 kgf / cm 2, and then in a 120 ° C. vacuum oven. A negative electrode plate having a thickness of 60 μm was prepared by vacuum drying for 12 hours.

In Table 1, the particle size distribution graph of the negative electrode active material corresponding to Example 5 is shown in FIG. 1.

Test Example

Coin-type half cells were manufactured in a conventional manner using the negative electrode plates prepared in Examples 1 to 10 and Comparative Examples 1 to 8, respectively.

For the negative electrode plate, springback characteristics were calculated from the thickness T1 measured immediately after rolling using a micrometer and the thickness T2 measured after vacuum drying for 12 hours after rolling [{(T2-T1) / T1}. * 100], the mixture density was measured immediately after rolling, and for the manufactured battery, the reversible capacity, initial efficiency, high rate characteristics, and lifetime characteristics were measured, and the results are shown in Table 2 below. When the battery is charged to 0.2C and then discharged to 0.2C, the discharge capacity is "reversible capacity", and the ratio of the discharge capacity to the charging capacity at this time is "initial efficiency" and charging at 2C to the discharge capacity at this time. After that, the ratio of the discharge capacity when discharged at 2.0C was expressed as "high rate characteristics". "Reversible capacity" is expressed in mAh / g representing the battery capacity per 1 g of the negative electrode active material and mAh / cc representing the battery capacity per 1 cc of volume when the negative electrode active material is coated, the battery capacity per volume is the combined density of the battery capacity per weight Was obtained by multiplying In other words, as the mixture density increases, the battery capacity per volume becomes larger, and the battery capacity per volume becomes a more important capacity value in the battery. "Life characteristics" is expressed as the ratio of 50 times the discharge capacity to the first discharge capacity after charging the battery at 1.0 C, then repeatedly discharged at 1.0 C 50 times.

From Table 2, the negative electrode plate prepared in the embodiment of the present invention is superior to the negative electrode plate prepared in the comparative example, the spring density is high, the mixture density is high, the battery having the negative electrode active material of the present invention has a negative electrode active material of the comparative example It can be seen that the battery initial performance and cycle life characteristics are both superior to the battery.

As described above, the negative electrode active material according to the present invention is manufactured in the form of a mixed powder in which the distribution of powder particle size is controlled, thereby maximizing the powder content per unit volume coated on the current collector, greatly improving the mixture density, and deforming the powder during electrode rolling. In addition to improving the springback phenomenon by minimizing the effect, and having the effect of improving the wettability with the electrolyte by the microchannels formed between the powders, the lithium secondary battery including the negative electrode active material of the present invention has a high Energy density, good battery capacity and stable lifetime characteristics.

Claims (3)

1 to 15% of the total volume of particulate powder having a particle size of 5 μm or less, and a granular powder having a particle size of 20 to 80 μm or more of d90 or more, The particle size dmax _peak corresponding to the maximum peak position in the overall particle size is at least d50 _vol corresponding to the average volume particle, A negative electrode active material for lithium secondary batteries, having a springback characteristic of 8% or less at a negative electrode mixture density of 1.90 g / cc or less. The negative electrode active material of claim 1, wherein the powder component is at least one selected from the group consisting of natural graphite, artificial graphite, and amorphous carbon. A lithium secondary battery comprising the negative electrode active material according to claim 1 as a negative electrode component.

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