KR102273114B1 - High-capacity lithium secondary battery negative electrode material with improved reversibility and method for manufacturing the same - Google Patents

Abstract

The present disclosure comprises the steps of milling nanosized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture; preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which an aqueous binder and pitch are dispersed; preparing core particles by spray-drying the solution; preparing a block by filling the core particles in a mold and pressing at a high temperature; carbonizing the manufactured block; and pulverizing and classifying the carbonized block. It relates to a method of manufacturing a negative active material for a lithium secondary battery.

Description

Anode active material for high-capacity lithium secondary battery with improved reversibility and manufacturing method thereof

The present disclosure relates to an anode active material for a lithium secondary battery and a method for manufacturing the same. Specifically, the present disclosure relates to a high-capacity anode active material for a lithium secondary battery with improved reversibility and a method for manufacturing the same.

Lithium-ion batteries are currently the most widely used secondary battery system in portable electronic communication devices, electric vehicles, and energy storage devices. These lithium-ion batteries are attracting attention because they have advantages such as high energy density, operating voltage, and relatively small self-discharge rate compared to commercial water-based secondary batteries (Ni-Cd, Ni-MH, etc.). However, in consideration of more efficient use time in portable devices, improvement of energy characteristics in electric vehicles, and the like, improvement in electrochemical characteristics still remains as a technical problem to be solved. For this reason, a lot of research and development on the four major raw materials such as an anode, a cathode, an electrolyte, and a separator is still in progress.

Among these raw materials, graphite-based materials exhibiting excellent capacity retention characteristics and efficiency are commercially available for anodes. However, the comparatively low theoretical capacity value (LiC ₆ : 372 mAh/g) and low discharge capacity ratio of graphite-based materials are somewhat insufficient to match the characteristics of high energy and high power density of batteries required by the market. Therefore, many researchers are interested in the elements of group IV (Si, Ge, Sn) on the periodic table, especially Si has a very high theoretical capacity (Li ₁₅ Si ₄ : 3600 mAh/g) and a low operating voltage (~0.1V). vs. Li/Li+), it is attracting attention as a very attractive material. However, in the case of a general Si-based anode material, it has a disadvantage in that it is difficult to apply to an actual battery because it exhibits a low discharge capacity ratio with a volume change of 300% during a cycle.

In order to solve this problem, conventionally, a technique for manufacturing a negative electrode including Si particles dispersed in _{SiO 2 has been disclosed.} This material has a very high capacity of about 2000 mAh/g and shows improved results with respect to lifetime compared to the case of the Si-only negative electrode. However, during the electrochemical reaction, SiO _{forms an intermediate phase called SiO x in addition to Si and SiO 2} during the electrochemical reaction. At this time _{, some oxygen of SiO x reacts with Li to form Li 2} O, which is an electrochemically stable phase. This Li ₂ O is not separated back into Li during discharge, and the generation of irreversible capacity due to this acts as a disadvantage as a high-capacity anode material, and there is a need for improvement.

The continuous consumption of the Li source to generate a new SEI by exposure to electrolyte according to volume expansion along with irreversible products generated during charge-discharge acts as a technical barrier to the commercialization of Si-based anode materials.

Accordingly, the present disclosure protects Si by providing a hard contact path between the Si raw material and the conductive carbon resulting from the pitch, and proposes a negative active material capable of dramatically lowering the specific surface area and a method for manufacturing the same, thereby inducing a reversible reaction, It is intended to provide a method to supplement the lifespan and high rate characteristics.

The negative active material for a lithium secondary battery according to an exemplary embodiment of the present disclosure includes nano-sized Si particles and graphite, and may have a specific surface area of 6 m ² /g or less.

The negative active material for a lithium secondary battery may be coated with carbon.

A method of manufacturing a negative active material for a lithium secondary battery according to an embodiment of the present disclosure includes milling nano-sized Si particles, graphite, and pitch to obtain a Si-graphite-pitch mixture; preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which an aqueous binder and pitch are dispersed; preparing core particles by spray-drying the solution; preparing a block by filling the core particles in a mold and pressing at a high temperature; carbonizing the manufactured block; and pulverizing and classifying the carbonized block.

The pitch may have a volatile content of less than 15%.

Preparing the block by filling the core particles in a mold and pressing at a high temperature; may be performed in a vacuum atmosphere.

The vacuum degree of the vacuum atmosphere may be -50 to 90 kPa.

Preparing the block by filling the core particles in a mold and pressing at a high temperature; may be performed at a temperature 100 to 200° C. higher than the softening point of the pitch included in the core particles.

Preparing the negative active material block by filling the core particles in a mold and pressing at a high temperature; may be a step of pressing at a pressure of ^{2 ton/cm 2 or more.}

Milling the nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture; in, based on the total weight of the mixture, the nano-sized Si particles are 40-50 wt%, the graphite is 22-32 wt%, the pitch is 23 to 33% by weight may be mixed.

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the aqueous binder may be included in an amount of 7.5 to 15% by weight based on the weight of the Si-graphite-pitch mixture. .

carbonizing the manufactured block; may be carbonized at a temperature of 1000° C. or less and an inert atmosphere.

The step of pulverizing and classifying the carbonized block may be a step of pulverizing using one or more selected from the group consisting of a jet mill and a pin mill.

The step of milling the nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture; may be a step of dry milling at least one selected from the group consisting of mechano fusion and ball mill.

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the aqueous binder is one selected from the group consisting of polyvinyl alcohol (PVA) and gum arabic (Gumarabic) may be more than

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the pitch may be an undifferentiated pitch having a D50 of 1 μm or less.

According to one embodiment of the present disclosure, it is possible to provide a spherical Si-carbon composite anode material having a dense structure capable of preventing deterioration due to Si expansion even during long-term cycling.

1 shows images obtained by SEM imaging of cross-sections of negative active materials of Comparative Examples and Examples prepared in Experimental Examples disclosed herein.
2 shows a pore distribution graph of Comparative Examples and Examples prepared in Experimental Examples of the present disclosure.
3 is a graph showing the 50 cycle life characteristics.

Terms such as first, second and third are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. The meaning of "comprising," as used herein, specifies a particular characteristic, region, integer, step, operation, element and/or component, and includes the presence or absence of another characteristic, region, integer, step, operation, element and/or component. It does not exclude additions.

When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part, or the other part may be involved in between. In contrast, when a part refers to being "directly above" another part, the other part is not interposed therebetween.

In addition, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

Although not defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, they are not interpreted in an ideal or very formal meaning.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein.

The present disclosure utilizes a pitch having a fixed carbon ratio (char yield) and caking property (high β-resin value) value in order to protect Si from a side reaction with an electrolyte and control the specific surface area, and the pitch of the pitch during the manufacturing process. By introducing a high-temperature pressing process in a vacuum atmosphere that can control residual pores after effectively removing volatile matter, a spherical Si- having a dense structure that can prevent deterioration due to Si expansion even during long-term cycling It is defined as manufacturing a carbon composite negative electrode material.

Hereinafter, each step will be described in detail.

The negative active material for a lithium secondary battery according to an exemplary embodiment of the present disclosure includes nano-sized Si particles and graphite, and a specific surface area may be 6m ² /g or less. If the specific surface area (measured by the BET method) of the negative electrode active material is large, a side reaction site with the electrolyte may be provided, thereby deteriorating long-term lifespan characteristics. In particular, in the case of silicon, it is very important to control the specific surface area of the initial active material itself to a low value in order to minimize a new surface newly exposed due to volume expansion during charging and discharging. .

The volatile matter content of the coal-based pitch applied to the negative electrode active material for the lithium secondary battery is preferably less than 15% because the carbon layer effectively supports the Si surface to minimize the specific surface area and silicon exposure when the residual carbon amount after heat treatment is high.

The negative active material for a lithium secondary battery may be coated with carbon.

A method of manufacturing a negative active material for a lithium secondary battery according to an embodiment of the present disclosure includes milling nano-sized Si particles, graphite, and pitch to obtain a Si-graphite-pitch mixture; preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which an aqueous binder and pitch are dispersed; preparing core particles by spray-drying the solution; preparing a block by filling the core particles in a mold and pressing at a high temperature; carbonizing the manufactured block; and pulverizing and classifying the carbonized block.

Preparing the block by filling the core particles in a mold and pressing at a high temperature; may be performed in a vacuum atmosphere.

The vacuum degree of the vacuum atmosphere may be -50 to 90 kPa.

The step of filling a mold with the core particles and high-temperature pressurizing to prepare a block; may be performed at a temperature 50 to 300° C. higher than the softening point of the pitch included in the core particles. Specifically, the process may be performed at a temperature 50 to 200° C., or more specifically, 100 to 150° C. higher than the softening point of the pitch.

If the temperature of the block manufacturing step by high-temperature pressurization is too low, viscosity cannot be imparted to the pitch forming the carbon support layer, so VM cannot be controlled in the heat treatment process after molding, leading to a decrease in specific surface area. If the temperature is too high, the raw material component adheres to the manufacturing equipment (molding mold) that forms the high-density carbon support layer, so there may be a disadvantage that the designed compositional uniformity cannot be obtained.

Preparing the negative active material block by filling the core particles in a mold and pressing at a high temperature; may be a step of pressing at a pressure of ^{2 ton/cm 2 or more.}

Milling the nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture; in, based on the total weight of the mixture, 40 to 50 wt% of nano-sized Si particles, 22 to 32 wt% of graphite, pitch may be mixed in an amount of 23 to 33% by weight.

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the aqueous binder may be included in an amount of 7.5 to 15% by weight based on the weight of the Si-graphite-pitch mixture. .

carbonizing the manufactured block; may be carbonized at a temperature of 1000° C. or less and an inert atmosphere.

The step of pulverizing and classifying the carbonized block may be a step of pulverizing using one or more selected from the group consisting of a jet mill and a pin mill.

The step of milling nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture; may be a step of dry milling at least one selected from the group consisting of mechano fusion and ball mill.

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the aqueous binder is at least one selected from the group consisting of polyvinyl alcohol (PVA) and gum arabic (Gumarabic). can

In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed; the pitch may be an undifferentiated pitch having a D50 of 1 μm or less.

Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in various different forms, and is not limited to the embodiments described herein.

Manufacturing method of Si-carbon composite negative electrode active material

First, a general method of manufacturing a Si-carbon composite anode material will be described. However, these are some examples of the fabrication of the Si-carbon composite anode material structure, and the present disclosure is not limited thereto.

The Si-carbon composite anode active material of the embodiment of the present disclosure is composed of nano-sized Si particles, graphite, pitch powder, and an aqueous binder. As the first step of the anode active material manufacturing method, a dry milling process of attaching nano-Si to graphite particles is performed so that the nano-sized Si particles can maintain electrical contact with the internal components despite volume expansion-contraction due to cycling. At this time, the main variables include the size of Si particles, graphite and pitch particles, as well as the rotation speed and time during milling. In addition, a certain amount of pitch (carbon source) is added during the milling process so that the nano Si and carbon support layer are formed and completely bound to the final product. For the milling process, a process through contact with powder, such as mechano fusion or ball mill, may be applied.

A solution for spray drying is prepared by mixing the Si-graphite-pitch mixture obtained as described above and an aqueous binder. The purpose of the spray drying process is that there is a limit to uniformly dispersing the nano Si-graphite-pitch raw material only with the dry milling process, which causes side effects such as deterioration of local performance (lifetime) and intensification of expansion when measuring electrochemical performance. to be. When ethanol, IPA, acetone, etc. are applied as solvents in the spray drying process, the binding force of Si-graphite-pitch is lowered, resulting in performance degradation, so it is limited to the water-based spray drying process.

Design an aqueous binder (PVA, arabica rubber, etc.) in consideration of the particle size of the Si-C composite negative electrode material so that the Si-graphite-pitch particles bound in the spray-drying solution during the spray drying process can be well dispersed in distilled water. It is used by properly inputting one value. D50 1um grade micronized pitch powder micronized to a specific particle size is dispersed in distilled water in which the above-described aqueous binder is dissolved, and the micronized and dispersed pitch particles are nano-Si surface coverage characteristics in the heat treatment process after spray drying to improve the electrochemical performance by significantly reducing the specific surface area of the active material. Dry-milled nano Si-graphite-pitch powder was added to distilled water in which the fine pitch was dispersed, stirred at room temperature for a certain period of time, and then spherical porous Si-carbon composite anode material primary particles (core ) is obtained.

The spherical particles thus obtained are obtained as porous particles due to the nature of the spray drying process. Basically, the porous particles provide a site for side reactions with the electrolyte during battery application, resulting in deterioration of electrochemical performance. In addition, volatile matter (VM, Volatile Matter), which is removed during high-temperature heat treatment among components of pitch, leaves pores inside the material, thereby increasing the specific surface area when applied as a final battery material. In this case, since the deterioration of the Si material can be accelerated due to a side reaction with the electrolyte, etc., a technique for effectively controlling the pores is required.

In this embodiment, in order to improve this, a process of applying a specific pressure in a prescribed time range in a certain temperature range above the pitch softening point was applied so as to minimize the pores existing inside the spray-dried particles. The pitch exposed above the softening point during high-temperature press molding has a viscosity and can improve the electrochemical properties of the synthetic powder by filling the micropores of the spray drying process. In addition, by performing the pressure molding process in a vacuum atmosphere, it is possible to effectively control the VM generated above the softening point. In this case, the process design is so that the pitch with high temperature viscosity can easily fill the pores left by the VM removed in the vacuum atmosphere. proceeded.

The high-temperature pressure molding was performed by filling a self-made mold with primary particle powder and utilizing a press facility, and the semi-finished product obtained after the molding process is obtained in the form of a block.

After the high-temperature press molding process, block carbonization was performed at about 1000 ° C. in an inert atmosphere, and then the final anode active material product was manufactured by performing a sieving process through a grinding process such as a jet mill and a pin mill. .

After obtaining the active material, the internal shape was observed using FIB-SEM (Focus Ion Beam Scanning Electron Microsopes), and the specific surface area was measured using a Surface area and Porosity analyzer (Micromeritics, ASAP2020).

In addition, the Si-carbon composite anode active material synthesized for electrochemical characterization ^{was coated on the Cu current collector to have a loading of 5 mg/cm 2} , and an electrode density of 1.2 to 1.3 g/cc, and then rolled A CR2032-type half coin cell was manufactured and a charge-discharge test was performed in the operating voltage range of 0.005V to 1.0V.

The binder used for manufacturing the electrode was a polyacrylic acid (PAA) system, and the electrolyte solution was EC (ethylene carbonate) without additives: DEC (diethyl carbonate) = 1:1 (1.0M LiPF ₆ ). The current during charge-discharge was measured at 0.1C in the initial cycle. In addition, based on the first 1C-rate capacity, a 0.5C-rate current was applied during charging and discharging to measure the lifespan of 50 cycles.

To measure the expansion rate of the electrode after measuring the life span of 50 cycles, use a half coin cell to complete 50 cycles and then fully charge the coin cell to 0.005V (0.005C cutoff), then disassemble the coin cell and measure the thickness of the charging electrode to change the expansion rate were compared.

Example

Nano Si was used to prepare a powder in which nano Si-graphite-pitch particles were dry-milled. A solution was prepared by mixing a water-based binder and fine pitch in solvent water and mixing the dry-milled powder thereto. Using the solution, primary particles were granulated by a water-based spray-drying method. The Si content in the material was 45% by weight, and the binder was added in an amount of 5% by weight compared to the Si-graphite-pitch mixture.

To form the carbon support of the powder, the primary particle powder is charged into a mold of a certain size, and a pressure of about 10 tons is applied for 20 minutes at a temperature 50 to 100 ° C higher than the softening point of the pitch in a vacuum atmosphere using a uniaxial pressure molding machine. It was molded into blocks and air-cooled. The obtained block was carbonized by heat treatment at less than 1000° C. in an inert atmosphere to prevent oxidation of nano-Si, and then naturally cooled to room temperature. Then, it was pulverized to a D50 of 10 to 15 μm with a jet mill, and sieved through a #635 mesh (20 μm) to obtain a final negative electrode active material.

comparative example

A negative electrode active material was obtained in the same manner as in the above example, except that the block formation was performed in an atmospheric pressure atmosphere rather than a vacuum atmosphere by high temperature pressure molding.

Electrochemical properties of the negative active materials obtained in Examples and Comparative Examples were evaluated.

The present invention improves the pore characteristics of pitch-derived carbon by using a pressure forming process in a vacuum atmosphere when composing a Si-C composite anode material, thereby minimizing side reactions between Si and electrolyte and improving structural stability. It provides improved cycle life characteristics and improved high rate charging characteristics.

First, the specific surface areas of the negative active materials prepared in Examples and Comparative Examples were compared (Table 1). In the case of the comparative example, the specific surface area was 21.9 m ² /g, and in the case of the invention example in which VM was removed in a vacuum atmosphere, the specific surface area was effectively reduced to ^{5.9 m 2 /g.} It was confirmed that the pore distribution was significantly reduced ( FIGS. 1 and 2 ).

comparative example Invention Example 1 specific surface area 21.9 5.9

As a result of checking the electrochemical properties, the life retention rate based on 50 cycles was 7.9% for the Comparative Example and 58.0% for the Inventive Example, showing improvement in performance due to a decrease in specific surface area ( FIG. 3 ). In addition, in the case of the expansion rate of Table 2, the comparative example was 154% and the invention example was 72%, so that the side reaction between Si and the electrolyte was effectively controlled through a low specific surface area (improving the pore structure), and it can be confirmed that a reversible reaction was induced accordingly. .

comparative example Invention Example 1 Electrode expansion rate (%) 154 72

The present invention is not limited to the embodiments, but may be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can use other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that this may be practiced. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (15)

delete delete milling nanosized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture;
preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which an aqueous binder and pitch are dispersed;
preparing core particles by spray-drying the solution;
preparing a block by filling the core particles in a mold and pressing at a high temperature;
carbonizing the manufactured block; and
A method of manufacturing a negative active material for a lithium secondary battery, comprising the step of pulverizing and classifying the carbonized block.
4. The method of claim 3,
A method for producing a negative active material for a lithium secondary battery, wherein the pitch of the volatile matter content is less than 15%.
4. The method of claim 3,
Preparing a block by filling the core particles in a mold and pressing at a high temperature;
A method for producing a negative active material for a lithium secondary battery, which is performed in a vacuum atmosphere.
6. The method of claim 5,
The vacuum degree of the vacuum atmosphere is -50 to 90 kPa, a method of manufacturing a negative active material for a lithium secondary battery.
4. The method of claim 3,
Preparing a block by filling the core particles in a mold and pressing at a high temperature;
A method of manufacturing a negative active material for a lithium secondary battery, which proceeds at a temperature 100 to 200° C. higher than the softening point of the pitch contained in the core particles.
4. The method of claim 3,
Preparing the negative active material block by filling the core particles in a mold and pressing at a high temperature;
2 ton/cm ^{2 A} method of producing a negative active material for a lithium secondary battery, which is pressurized at a pressure of 2 or more.
4. The method of claim 3,
Milling nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture;
Based on the total weight of the mixture, 40 to 50% by weight of the nano-sized Si particles, 22 to 32% by weight of graphite, and 23 to 33% by weight of the pitch, a method of manufacturing a negative active material for a lithium secondary battery.
4. The method of claim 3,
Preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed;
The water-based binder is contained in an amount of 7.5 to 15% by weight based on the weight of the Si-graphite-pitch mixture, a method of manufacturing a negative active material for a lithium secondary battery.
4. The method of claim 3,
carbonizing the manufactured block; is
A method of producing a negative active material for a lithium secondary battery, which is carbonized at a temperature of 1000° C. or less and an inert atmosphere.
4. The method of claim 3,
pulverizing and classifying the carbonized block;
A method for producing a negative active material for a lithium secondary battery, which is a step of pulverizing using at least one selected from the group consisting of a jet mill and a pin mill.
4. The method of claim 3,
Milling nano-sized Si particles, graphite and pitch to obtain a Si-graphite-pitch mixture;
A method of manufacturing a negative active material for a lithium secondary battery, which is a step of dry milling at least one selected from the group consisting of mechano fusion and a ball mill.
4. The method of claim 3,
In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed;
The water-based binder is at least one selected from the group consisting of polyvinyl alcohol (PVA) and gum arabic (Gumarabic), a method of manufacturing a negative active material for a lithium secondary battery.
4. The method of claim 3,
In the step of preparing a solution by mixing the Si-graphite-pitch mixture in distilled water in which the aqueous binder and the pitch are dispersed;
The pitch is an undifferentiated pitch of D50 of 1 μm or less, a method of manufacturing a negative electrode active material for a lithium secondary battery.

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