KR100828879B1 - Electrode active material and secondary battery using the same - Google Patents

Abstract

The present invention is a metal or metalloid core layer capable of repeatedly charging and discharging lithium; Amorphous carbon layer; And an electrode active material sequentially comprising a crystalline carbon layer, wherein an electrode active material comprising a chemical bond between metal or metalloid and carbon is formed in part or all between the metal or metalloid core layer and the amorphous carbon layer. It provides a used lithium secondary battery.

In the electrode active material of the present invention, a chemical bond between metal or metalloid and carbon is formed in some or all of the metal or metalloid core layer and the amorphous carbon layer while maintaining a high charge / discharge capacity which is an advantage of the metal or metalloid electrode active material. Therefore, it is possible to suppress the volume change of the core layer that may occur during charge and discharge of lithium, thereby improving the cycle life characteristics of the battery.

Electrode active material, crystalline carbon layer, amorphous carbon layer

Description

Electrode active material and secondary battery using the same {ELECTRODE ACTIVE MATERIAL AND SECONDARY BATTERY USING THE SAME}

1 is a STEM HAADF photograph of the cross-section of the electrode active material prepared in Example 1.

2 is a TEM photograph of the electrode active material prepared in Example 1.

3 is a result of XPS of an electrode active material prepared in Example 1. FIG.

4 is a result of FT-IR of the electrode active material prepared in Example 1. FIG.

The present invention relates to an electrode active material and a secondary battery using the electrode active material.

Lithium secondary batteries are manufactured by using a material capable of inserting and desorbing lithium ions as a negative electrode and a positive electrode, and filling an organic or polymer electrolyte between a positive electrode and a negative electrode, and when lithium ions are inserted and desorbed from a positive electrode and a negative electrode. Electrical energy is generated by oxidation and reduction.

Currently, carbonaceous materials are mainly used as electrode active materials constituting the negative electrode of a lithium secondary battery. However, in order to further improve the capacity of the lithium secondary battery, the use of a high capacity electrode active material is required.

In order to satisfy such a demand, there is an example of using a Si, Al, or the like, which is a metal that exhibits a higher charge / discharge capacity than a carbonaceous material and which can be electrochemically alloyed with lithium. However, such a metal-based electrode active material has a severe change in volume associated with lithium charging and discharging, resulting in cracking and micronization. Therefore, the capacity of the secondary battery using the metal-based electrode active material decreases rapidly as the charge-discharge cycle progresses, and the cycle Life is shortened.

Japanese Patent Application Laid-Open No. 2001-297757 proposes an electrode active material having a structure mainly composed of an α phase (for example, Si) composed of lithium and a chargeable and dischargeable element, and a β phase, which is an intermetallic compound or solid solution of this element with another element b. It was.

However, there is a problem that the conventional method described above cannot obtain sufficient and good cycle life characteristics and thus cannot be used as a practical electrode active material for lithium secondary batteries.

The present invention is a metal or metalloid core layer capable of repeatedly charging and discharging lithium; Amorphous carbon layer; And an electrode active material comprising a crystalline carbon layer sequentially, by using an electrode active material in which a chemical bond between metal or metalloid and carbon is formed in part or all between the metal or metalloid core layer and the amorphous carbon layer. An electrode active material having high charge and discharge capacity and excellent cycle life characteristics by suppressing a volume change of a core layer such as a metal generated during charge and discharge, and a secondary battery using the same.

The present invention is a metal or metalloid core layer capable of repeatedly charging and discharging lithium; Amorphous carbon layer; And an electrode active material sequentially comprising a crystalline carbon layer, wherein an electrode active material comprising a chemical bond between metal or metalloid and carbon is formed in part or all between the metal or metalloid core layer and the amorphous carbon layer. Provided the secondary battery used.

Hereinafter, the present invention will be described in detail.

Electrode active material of the present invention is a structure comprising a metal or metalloid core layer, an amorphous carbon layer and a crystalline carbon layer sequentially, that is, a metal or metalloid core layer; An amorphous carbon layer coated on the surface of the metal or metalloid core layer; And a crystalline carbon layer coated on the amorphous carbon layer. In addition, a chemical bond, for example, a covalent bond is formed between the metal or the metalloid and carbon, and the chemical bond is formed in part or all between the core layer and the amorphous carbon layer. For example, when silicon is applied as the core layer, the bond between silicon and carbon is much stronger than that between silicon and other additives, and thus the bond is not broken during charge and discharge of lithium. Therefore, even if the volume change of the core layer that can occur during charging and discharging occurs, the electrical contact between the core layer and the amorphous carbon layer is maintained, the lithium secondary battery using the electrode active material of the present invention has a high charge and discharge capacity and excellent cycle life Can be.

The chemical bond between the metal or metalloid and carbon is preferably formed in a proportion of 0.5 to 25 parts by weight based on 100 parts by weight of the electrode active material. If the chemical bond is less than 0.5 parts by weight, even if a chemical bond is present, it is difficult to secure an electrical path due to volume expansion because the chemical bond between the metal or metalloid and carbon is not sufficiently formed between the core layer and the amorphous carbon layer. When the volume of the core layer is expanded, the volume of the amorphous carbon layer is so small that the volume change cannot be sufficiently absorbed, resulting in poor cycle characteristics. On the other hand, since the chemical bond between metal or metalloid and carbon is very strong, even if lithium is inserted, it does not react. Therefore, when the chemical bond is formed by more than 25 parts by weight, the capacity is reduced because the inactive portion is large. .

In the present invention, examples of the metal or metalloid constituting the metal or metalloid core layer include metals or metalloids or alloys thereof including at least one selected from Si, Al, Sn, Sb, Bi, As, Ge, and Pb. Although it may be mentioned, it is not particularly limited as long as it can electrochemically recharge and discharge lithium.

Examples of amorphous carbon include a coal tar pitch, a petroleum pitch, and a carbon-based material made by heat treatment using various organic materials as raw materials.

Crystalline carbon includes natural graphite and artificial graphite having a high degree of graphitization. Examples of the graphite-based material include MCMB (MesoCarbon MicroBead), carbon fiber, and natural graphite.

Metal or metalloid core layer: amorphous carbon layer: crystalline carbon layer = 90 to 10 parts by weight: 0.1 to 50 parts by weight: preferably 9 to 90 parts by weight. If the metal or metalloid core layer is less than 10 parts by weight, the reversible capacity is small, meaning as a high-capacity electrode active material, and if the crystalline carbon layer is less than 9 parts by weight, it is difficult to secure sufficient conductivity, and the amorphous carbon layer is less than 0.1 part by weight. This is because if it does not play a sufficient role in the expansion inhibition and exceeds 50 parts by weight, there is a fear of capacity decrease and conductivity decrease.

The amorphous carbon layer preferably has an interlayer distance d002 of carbon of 0.34 nm or more and a thickness of 5 nm or more. The electrode active material of the present invention forms a strong chemical bond between the metal or metalloid core and the amorphous carbon layer and the metal or metalloid and carbon, so that the amorphous carbon layer during charge and discharge of the metal or metalloid core layer It maintains electrical contact with the resulting volume expansion. At this time, when the interlayer distance d002 of carbon is less than 0.34 nm, carbon cannot be classified into an amorphous carbon layer because of its excellent crystallinity, and its crystallinity is high, making it difficult to chemically bond with the metal or metalloid of the core layer. If the thickness of the amorphous carbon layer is less than 5 nm, the chemical bond between the metal or the metalloid and the amorphous carbon is maintained by the stress due to the volume change generated during charging and discharging, but the bond in the amorphous carbon layer is broken, resulting in poor cycle characteristics. Therefore, the thickness of the amorphous carbon layer is preferably 5 nm or more.

The crystalline carbon layer preferably has an interlayer distance d002 of carbon of 0.3354 nm or more and 0.35 nm or less. The lower limit is theoretically the lowest possible interlayer distance of graphite, and the lower limit does not exist, and carbon having an interlayer distance exceeding the upper limit does not have good conductivity and thus lowers conductivity of the coating layer, thereby preventing smooth lithium charge and discharge characteristics.

The thickness of the crystalline carbon layer is not particularly limited but is preferably 1 micron or more and 10 microns or less. If the thickness of the layer is less than 1 micron, it is difficult to secure sufficient conductivity between the particles. If the thickness is more than 15 microns, the proportion of carbonaceous contained in the electrode active material becomes high, and thus high charge and discharge capacity cannot be obtained.

Method for producing an electrode active material of the present invention, a metal or metalloid core layer that can be repeatedly charged and discharged with lithium; Amorphous carbon layer; And a first step of providing an electrode active material sequentially comprising a crystalline carbon layer; And a second step of heat treating the electrode active material at 600 to 1600 ° C.

If the heat treatment temperature is less than 600 ° C., metals or metalloids of the core layer and amorphous carbon do not chemically bond, and thus carbides are not formed. If the heat treatment temperature is more than 1600 ° C., there is a problem in that the amount of carbides is increased.

In the manufacturing method, the electrode active material of the first step may be formed by directly coating an amorphous carbon layer on a surface of a metal or metalloid that can be repeatedly charged and discharged with lithium by a thin film deposition process such as CVD or PVD, or by pitch or various organic materials. Coating an amorphous carbon layer by coating a precursor such as a material and then performing heat treatment; And coating and drying the slurry containing the crystalline carbonaceous material to form a crystalline carbon layer.

In addition, the electrode active material of the first step, the step of providing a mixture by mixing the crystalline carbon and metal or metalloid forming the core layer; And mechanically alloying the mixture using a ball mill and a Mechano Fusion device to simultaneously form an amorphous carbon layer and a crystalline carbon layer on the surface of the metal or metalloid core layer. Mechanical alloying (Mechanical Alloying) is to create an alloy of uniform composition by applying a mechanical force.

The metal or metalloid and the crystalline carbon may be mixed in a ratio of metal or metalloid: crystalline carbon = 90 to 10 parts by weight: 10 to 90 parts by weight.

Electrodes in the present invention can be prepared by conventional methods known in the art. For example, a slurry may be prepared by mixing and stirring a binder and a solvent, a conducting agent, and a dispersant in an electrode active material of the present invention, and then applying the same to a current collector of a metal material, compressing, and drying the electrode. have.

The binder can be suitably used in an amount of 1 to 10 by weight and the conductive agent in an amount of 1 to 30 by weight based on the electrode active material.

Examples of binders that can be used include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and the like.

Generally, carbon black may be used as the conductive agent, and acetylene black series (such as a Chevron Chemical Company or Gulf Oil Company) may be used as a commercially available product. Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cabot Company) and Super P (MMM) There is this.

Examples of the solvent include organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, and dimethyl acetamide or water, and these solvents may be used alone or in combination of two or more thereof. . The amount of the solvent is sufficient to dissolve and disperse the electrode active material, the core-shell binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The current collector of the metal material is a metal having high conductivity, and any metal can be used as long as the slurry of the electrode active material can be easily adhered and is not reactive in the voltage range of the battery. Representative examples include meshes, foils, and the like, which are manufactured by aluminum, copper, gold, nickel or an aluminum alloy or a combination thereof.

The method of applying the slurry to the current collector is also not particularly limited. For example, it can be applied by a method such as doctor blade, dipping, brushing, and the like, but the coating amount is not particularly limited, but the thickness of the active material layer formed after removing the solvent or the dispersion medium is usually 0.005-5 mm, preferably 0.05-2 mm. The amount of the range which becomes a range is preferable.

The method of removing the solvent or the dispersion medium is not particularly limited, but the solvent or the dispersion medium may be used as quickly as possible within the speed range in which stress concentration occurs and cracks occur in the active material layer or the active material layer does not peel off from the current collector. It is preferable to use a method of adjusting to remove to volatilize. As a non-limiting example it may be dried for 0.5 to 3 days in a vacuum oven at 50 ~ 200 ℃.

The secondary battery of the present invention is characterized by using the electrode active material of the present invention, it can be produced by a conventional method known in the art. For example, a porous separator may be inserted between the positive electrode and the negative electrode to add an electrolyte solution. Secondary batteries include lithium ion secondary batteries, lithium polymer secondary batteries or lithium ion polymer secondary batteries.

The electrolyte may include a nonaqueous solvent and an electrolyte salt.

The nonaqueous solvent is not particularly limited as long as it is normally used as a nonaqueous solvent for nonaqueous electrolyte, and cyclic carbonate, linear carbonate, lactone, ether, ester or ketone can be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. Examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC) and dipropyl carbonate. (DPC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. Examples of the lactone include gamma butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, and the like. There is this. In addition, examples of the ester include n-methyl acetate, n-ethyl acetate, methyl propionate, methyl pivalate, and the like. The ketone includes polymethylvinyl ketone. These nonaqueous solvents can be used individually or in mixture of 2 or more types.

The electrolyte salt is not particularly limited as long as it is usually used as an electrolyte salt for nonaqueous electrolyte. Electrolytic salt, non-limiting example, A ⁺ B ^- A salt of the structure, such as, A ⁺ comprises a Li ^+, Na ^+, an alkali metal cation or an ion composed of a combination thereof, such as K ⁺ B ^- is PF ₆ ^{- _{, BF 4 -, Cl -,}} Br -, I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO 2 ) ₃ ^- and a salt containing the same anion ion or a combination thereof. In particular, lithium salts are preferred. These electrolyte salts can be used individually or in mixture of 2 or more types.

The secondary battery of the present invention may include a separator. The separator can be used is not particularly limited, it is preferable to use a porous separator, non-limiting examples include a polypropylene-based, polyethylene, or polyolefin-based porous separator.

The lithium secondary battery of the present invention is not limited in appearance, but may be cylindrical, square, pouch type, or coin type using a can.

Hereinafter, the present invention will be described in detail with reference to the following Examples. However, the following examples are merely to illustrate the present invention and the present invention is not limited by the following examples.

(Examples 1 to 5)

50 parts by weight of Si and natural graphite are mixed by 50 parts by weight, and the ratio of 5 mm stainless steel balls to powder is 3: 3 parts by weight. Alloying was performed to prepare an electrode active material. Then, 30 minutes (Example 1), 1 hour (Example 2), 1 hour 30 minutes (Example 3), 2 hours (at 1100 ° C. in an argon (Ar) atmosphere to form a chemical bond between Si and carbon) Example 4) and heat treatment for 3 hours (Example 5), respectively.

To evaluate the electrode active material, a uniform slurry was prepared by mixing 7 parts by weight of PVDF as a binder, 15 parts by weight of acetylene black as a conductive agent, and NMP as a solvent to 100 parts by weight of the prepared electrode active material. The slurry was coated on a 20 micron copper foil with a uniform thickness of 30 μm, and then dried, rolled and punched to prepare a negative electrode.

The electrolyte was prepared by dissolving LiPF ₆ in a non-aqueous solvent having a composition of ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 1: 2 (v: v) to a concentration of 1 M.

The prepared electrode was used as a negative electrode and metal lithium was used as a counter electrode. A coin-type battery was manufactured by interposing a polyolefin-based separator between both electrodes and injecting the electrolyte solution.

(Examples 6 to 10)

In Example 1, the heat treatment temperature was set to 1000 ° C. (Example 6), 900 ° C. (Example 7), 800 ° C. (Example 8), 700 ° C. (Example 9), and 600 ° C. (Example 10). Except that the heat treatment during, the electrode active material and the battery was prepared in the same manner as in Example 1.

(Example 11)

The pitch was mixed in a weight ratio of 10: 5 to silicon having an average radius of 20 μm, and then heat-treated at 800 ° C. in an argon atmosphere to prepare an electrode active material. Thereafter, a coin-type battery was manufactured using the electrode active material prepared in the same manner as in Example 1.

(Comparative Example 1)

Except not performing heat treatment in Example 1, an electrode active material and a battery were prepared in the same manner as in Example 1.

(Experiment result)

Figure 1 is a photograph of the cross section of the electrode active material prepared in Example 1 by STEM HAADF. The white particles are silicon, and the black waves around the silicon are crystalline carbon layers. The structure in which the carbon layer surrounds the silicon can be observed, and the crystalline carbon layer surrounds the metal or metalloid core layer.

2 is a TEM photograph of the electrode active material prepared in Example 1. The left side was silicon, and the right side was a carbon layer. Silicon shows a regular arrangement of silicon atoms on the TEM, and it appears that it is a crystalline state as a regular point in the TEM SADP (Selected Area Diffraction Pattern) at the bottom right. In addition, the d001 plane is an irregular carbon layer on the right side, and it can be seen that it is an amorphous carbon layer as a plurality of rings in the TEM SADP on the lower right side. The amorphous carbon layer was about 10 nm in FIG.

3 is an XPS observation result of the electrode active material prepared in Example 1. 3, it can be seen that there is a chemical bond between silicon and silicon, a chemical bond between silicon and oxygen, and a chemical bond between silicon and carbon.

4 is an FT-IR observation result of the electrode active material prepared in Example 1. The chemical bond between silicon and carbon was found around 900 cm ^-1 . Therefore, as a result of FIGS. 3 and 4, a portion of the silicon layer and the amorphous carbon layer observed in FIG. It was confirmed that a chemical bond was formed between the crystalline silicon and the amorphous carbon.

In addition, the batteries prepared in Examples 1 to 11 and Comparative Examples were used to measure initial capacity, 50 cycle capacity / 1 cycle capacity ratio, initial cycle efficiency, and thickness ratio after 50 cycles before thickness / charging and discharging. Shown in

Initial capacity mAh / g 50 cycle capacity / 1 cycle capacity (%) 1 cycle efficiency (%) Thickness after 50 cycles / Thickness before charge / discharge (%) Example 1 805 92.4 84.1 320 Example 2 803 92.8 83.9 343 Example 3 802 93.4 83.8 336 Example 4 804 92.9 83.9 371 Example 5 802 92.6 83.5 368 Example 6 801 92.1 84.5 315 Example 7 804 91.9 84.3 327 Example 8 802 91.8 83.8 351 Example 9 803 91.6 83.6 347 Example 10 802 91.1 83.4 363 Example 11 806 92.2 82 303 Comparative example 800 88.9 76 650

Looking at Table 1, Examples 1 to 11 showed similar initial capacities compared to Comparative Examples. However, when looking at the ratio of 50 cycle capacity / 1 cycle capacity, it can be seen that Examples 1 to 11 are 91.1 to 93.4% and the cycle characteristics are significantly improved compared to 88.9% of the Comparative Example. In addition, the first cycle efficiency was 76% in the comparative example, but Examples 1 to 11 were improved to 82 to 84.5%.

In addition, in Examples 1 to 11, the volume expansion ratio was 303 to 371% after 50 cycles, which is very small compared to 650% of the comparative example.

In the electrode active material of the present invention, a chemical bond between metal or metalloid and carbon is formed in part or all between the metal or metalloid core layer and the amorphous carbon layer while maintaining a high charge and discharge capacity which is an advantage of the metal or metalloid electrode active material. Therefore, it is possible to suppress the volume change of the core layer that may occur during charge and discharge of lithium, thereby improving the cycle life characteristics of the battery.

Claims (11)

A metal or metalloid core layer capable of repeatedly charging and discharging lithium; Amorphous carbon layer; And an electrode active material sequentially comprising a crystalline carbon layer, wherein a part of or all of the metal or metalloid core layer and the amorphous carbon layer is formed with a chemical bond between metal or metalloid and carbon. According to claim 1, wherein the metal or metalloid core layer is composed of a metal or metalloid or an alloy thereof including at least one selected from the group consisting of Si, Al, Sn, Sb, Bi, As, Ge, Pb. Electrode active material characterized in that. The electrode active material according to claim 1, wherein the chemical bond between the metal or metalloid and carbon is formed in an amount of 0.5 to 25 parts by weight based on 100 parts by weight of the electrode active material. The electrode active material according to claim 1, wherein the core layer: amorphous carbon layer: crystalline carbon layer = 90 to 10 parts by weight: 0.1 to 50 parts by weight: 9 to 90 parts by weight. The electrode active material according to claim 1, wherein the crystalline carbon layer has an interlayer distance d002 of carbon of 0.3354 nm or more and 0.35 nm or less and a thickness of 1 micron or more and 10 microns or less. The electrode active material according to claim 1, wherein the amorphous carbon layer has an interlayer distance d002 of carbon of 0.34 nm or more and a thickness of 5 nm or more. A secondary battery using the electrode active material according to any one of claims 1 to 6. A metal or metalloid core layer capable of repeatedly charging and discharging lithium; Amorphous carbon layer; And a first step of providing an electrode active material sequentially comprising a crystalline carbon layer; And a second step of heat-treating the electrode active material at 600 to 1600 ° C. 6. The method of claim 8, wherein the electrode active material of the first step, Mixing the crystalline carbon with the metal or metalloid forming the core layer; And And mechanically alloying the mixture to form an amorphous carbon layer and a crystalline carbon layer on the surface of the metal or metalloid core layer at the same time. The method of claim 8, wherein the electrode active material of the first step comprises: mixing a metal or metalloid forming a core layer with crystalline carbon to provide a mixture; And mechanically alloying the mixture using a ball mill and a Mechano Fusion device to simultaneously form an amorphous carbon layer and a crystalline carbon layer on the surface of the metal or metalloid core layer. Manufacturing method. The method of claim 10, wherein the mixing ratio of the metal or metalloid and crystalline carbon is 10 to 90: 90 to 10 by weight.

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