METHOD FOR PREPARING NEGATIVE ELECTRODE ACTIVE MATERIAL, FOR LITHIUM SECONDARY BATTERY, COMPRISING SILICA-METAL COMPOSITE, AND NEGATIVE ELECTRODE ACTIVE MATERIAL PREPARED THEREBY

Abstract

A method for preparing a negative electrode active material for a lithium secondary battery according to one aspect of the present invention comprises the steps of: uniformly mixing silicon and metal oxide; and heating or ball-milling the mixture.

Claims

1. A method of preparing a negative electrode active material for a lithium secondary battery, comprising the steps of: uniformly mixing silicon and a metal oxide; and heating or ball-milling the mixture.

2. The method according to claim 1, wherein the method is to form a silicon oxide-metal composite.

3. The method according to claim 2, wherein the silicon oxide-metal composite is formed by attaching metal particles on silicon oxide particles.

4. The method according to claim 2, wherein the silicon oxide is SiOx (0≤x≤2).

5. The method according to claim 1, wherein the metal oxide is an oxide of one or more selected from the group consisting of Co, Cu, Ni, Mn Fe, Ti, Al, Sn, Ag, Au, Mo, Zr, CoSi.sub.2, Cu.sub.3Si, Cu.sub.5Si, MnSi.sub.2, NiSi.sub.2, FeSi.sub.2, FeSi, TiSi.sub.2, Al.sub.4Si.sub.3, Sn.sub.2Si, AgSi.sub.2, Au.sub.5Si.sub.2, MoSi.sub.2, and ZrSi.sub.2.

6. The method according to claim 1, wherein the silicon and the metal oxide are mixed in a molar ratio of 9:1 to 19:1.

7. The method according to claim 1, wherein the heating step is performed at 400° C. to 2,000° C.

8. The method according to claim 1, wherein the ball-milling step is performed at 100 rpm to 1,500 rpm.

9. The method according to claim 1, further comprising the step of treating the silicon with an acid prior to the mixing step.

10. A negative electrode active material for a lithium secondary battery prepared by the method according to claim 1.

11. A negative electrode for a lithium secondary battery comprising the negative electrode active material of claim 10.

12. A lithium secondary battery comprising the negative electrode for a lithium secondary battery of claim 11.

13. A negative electrode active material for a lithium secondary battery formed by bringing one or more metal elements into contact with surfaces of silicon oxide particles, wherein the metal element(s) is/are selected from the group consisting of Co, Cu, Ni, Mn, Fe, Ti, Al, Sn, Ag, Au, Mo, Zr, CoSi.sub.2, Cu.sub.3Si, Cu.sub.5Si, MnSi.sub.2, NiSi.sub.2, FeSi.sub.2, FeSi, TiSi.sub.2, Al.sub.4Si.sub.3, Sn.sub.2Si, AgSi.sub.2, Au.sub.5Si.sub.2, MoSi.sub.2, and ZrSi.sub.2.

14. The negative electrode active material according to claims 13, wherein the silicon oxide and the metal element are formed in a molar ratio of 1:9 to 999:1.

Description

DESCRIPTION OF DRAWINGS

[0027] FIG. 1 illustrates a flow chart of a synthesis process of a silicon oxide-metal composite according to an embodiment of the present invention.

[0028] FIG. 2 illustrates a schematic diagram of a reaction according to an embodiment of the present invention.

[0029] FIG. 3 illustrates the XRD result pattern of heat-treated ‘CoO+Si’ according to an embodiment of the present invention and a heat-treated material of only ‘CoO’ as a comparative example.

[0030] FIG. 4 illustrates the results of XPS analysis of the composite obtained according to an embodiment of the present invention.

[0031] FIG. 5 illustrates the results of SEM-EDS analysis of the composite obtained according to an embodiment of the present invention.

[0032] FIG. 6A illustrates a SEM photograph of pure silicon, FIG. 6B illustrates a SEM photograph of a silicon oxide-cobalt composite, FIG. 6C illustrates a TEM photograph of pure silicon, FIGS. 6D and 6E illustrate a TEM photograph of a silicon oxide-cobalt composite, FIGS. 6F to 6H illustrate EDS mapping images of pure silicon, and FIGS. 6I to 6L illustrate EDS mapping images of a silicon oxide-cobalt composite.

[0033] FIG. 7 illustrates the charging/discharging speed of an electrode using the composite obtained according to an embodiment of the present invention and a comparative example.

[0034] FIGS. 8A to 8F illustrate an SEM image for confirming the mechanical performance of negative electrodes made of the composite and pure silicon obtained according to an embodiment of the present invention.

BEST MODE

[0035] One aspect of the present invention provides a method of preparing a negative active material for a lithium secondary battery including the steps of uniformly mixing silicon and a metal oxide; and heating or ball-milling the mixture. According to an embodiment of the present invention, the method may form a silicon oxide-metal composite. According to an embodiment of the present invention, the silicon oxide-metal composite may be formed by attaching metal particles on silicon oxide particles.

MODES OF THE INVENTION

[0036] Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be implemented in several different forms, and therefore is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted to clearly describe the present invention, and similar reference numerals are attached to similar parts throughout the specification.

[0037] Throughout the specification, when a part is said to be “connected (linked, contacted, bonded)” with another part, it includes not only the case of being “directly connected”, but also “indirectly connected” with another member interposed therebetween. In addition, when a part “includes” a certain component, this means that other components may be further provided, not excluded, unless specifically stated to the contrary.

[0038] The terms used in the present specification are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present specification, terms such as “include” or “have” are intended to designate the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification. It is to be understood that it does not preclude the possibility of the presence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0039] As described above, when silicon is used as a negative electrode active material, as the negative electrode repeats expansion and contraction during operation of the lithium secondary battery, the lifespan and electrochemical performance of the negative electrode are reduced. To solve this problem, the present invention has been accomplished to prepare a negative electrode active material more effectively and at low cost.

[0040] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0041] FIG. 1 illustrates a flow chart of a synthesis process of a silicon oxide-metal composite according to an embodiment of the present invention.

[0042] The method of preparing a negative active material for a lithium secondary battery according to an embodiment of the present invention includes the steps of: (a) uniformly mixing silicon and a metal oxide; and (b) heating or ball-milling the mixture.

[0043] The “silicon (Si)” provides a silicon component to the composite, and it is preferable to use a single Si compound. However, in some cases, it may be used as long as it can provide silicon to the silicon oxide-metal composite through heating or ball-milling, for example, SiO, SiO.sub.2, Si(OC.sub.2H.sub.5).sub.4 may be used in the form of a single substance or a mixture of two or more.

[0044] The particle diameter of the silicon may be 10 nm to 100 μm, for example, 10 nm to 200 nm, or 30 nm to 100 nm.

[0045] When the “metal oxide” is formed in the composite, as oxygen atoms are transferred to silicon, the metal can be used without special restrictions as long as it satisfies the following conditions: (i) it does not react with lithium; (ii) it does not react with water, making it suitable for slurry processing; (iii) the binding energy of the metal oxide is low; and (iv) the metal oxide is thermodynamically stable at the temperature and pressure at which the process is performed.

[0046] The metal oxide may be an oxide of one or more metal atoms selected from the group consisting of Co, Cu, Ni, Mn, Fe, Ti, Al, Sn, Ag, Au, Mo, and Zr and/or one or more silicon alloys selected from the group consisting of CoSi.sub.2, Cu.sub.3Si, Cu.sub.5Si, MnSi.sub.2, NiSi.sub.2, FeSi.sub.2, FeSi, TiSi.sub.2, Al.sub.4Si.sub.3, Sn.sub.2Si, AgSi.sub.2, Au.sub.5Si.sub.2, MoSi.sub.2 and ZrSi.sub.2, and specifically, it may be an oxide of one or more metal atoms selected from the group consisting of Co, Cu, Ni, and Mn.

[0047] The particle diameter of the metal oxide may be 5 nm to 100 μm.

[0048] The mixing ratio between the silicon and the metal compound has a great influence on the physical properties of the prepared composite. For example, the silicon and the metal oxide may be mixed in a molar ratio of 9:1 to 19:1, such as 13:1. When the mixing ratio of the silicon and metal oxide is less than 8:1, the capacity of the battery may decrease due to the high ratio of the metal oxide remaining in the composite, and when the mixing ratio is more than 30:1, it is difficult to accurately measure the weight of the components during preparation, and since the metal content is too small compared to silicon, the volume expansion effect of the negative electrode cannot be sufficiently obtained.

[0049] The method of preparing a negative electrode active material for a lithium secondary battery may further include a step of pre-treating with an acid prior to step (a). In this step, impurities such as oxides present on the surface of the silicon particles may be removed by treating the prepared silicon particles with an acid such as hydrofluoric acid.

[0050] Silicon treated with the acid as described above may be washed several times with water, for example, distilled water, filtered, dried, and then used in a mixing process with the metal oxide. The drying may be performed in equipment such as, for example, a vacuum oven or a hot plate, but is not limited thereto.

[0051] In step (a), the mixing process is performed so that silicon and metal oxide particles are uniformly mixed.

[0052] In step (b), the uniform mixture of silicon/metal oxide obtained in step (a) is heated or ball-milled to perform a process of forming a silicon oxide-metal composite through a solid phase reaction. The silicon oxide-metal composite may be formed by dispersing silicon oxide particles and metal particles, so that the metal particles are attached on silicon oxide particles.

[0053] The heating step in step (b) may be performed at 400° C. to 2,000° C., for example, 700° C., under an inert atmosphere such as argon (Ar) and nitrogen (N.sub.2). When the heating step is performed at less than 400° C., it is difficult for the complex formation reaction to occur, and when the heating step is more than 2,000° C., rapid growth of silicon crystals may occur. In addition, the heating step may be performed for 15 hours to 45 hours, for example, 30 hours.

[0054] The ball-milling step in step (b) may be performed at 100 rpm to 1,500 rpm for 1 hour to 24 hours.

[0055] A method of preparing a silicon oxide-metal composite by the preparation method of the present invention enables synthesis at a relatively low temperature within a short time using a metal oxide, thus mass production is possible at low cost. In addition, in the silicon oxide-metal composite prepared by the above method, as the metal particles are uniformly attached to the surface of the silicon oxide particles, when looking at an entire negative electrode, metal atoms are uniformly distributed between silicon oxide particles. This uniform distribution can make it possible to exert a buffering effect more effectively by the metal particles. Accordingly, the negative electrode made of the silicon oxide-metal composite according to the preparation method of the present invention may have an excellent lifespan and excellent electrochemical performance.

[0056] Further, in the negative electrode made of the silicon oxide-metal composite according to the preparation method of the present invention, referring to FIGS. 8A to 8F, which is an SEM image after 100 cycles of charging and discharging, micro-cracks were hardly generated, and particles did not aggregate compared to a silicon electrode. This means that the silicon oxide-metal composite according to the preparation method of the present invention can prevent deterioration of the electrode due to volume expansion and contraction of the silicon particles.

EXAMPLES

Example 1

Preparation of Silicon Oxide-Cobalt Composite

[0057] In order to prepare a silicon oxide-cobalt composite, silicon (Si, diameter 100 nm) and cobalt oxide (CoO, diameter 50 nm) were prepared in a molar ratio of 19:1.

[0058] The prepared silicon was immersed in 500 ml of hydrofluoric acid and allowed to stand for 1 hour, and then washed three times with distilled water. Then, it was dried in a vacuum oven at 80° C. for 3 hours.

[0059] The dried silicon and cobalt oxide were put in one place, and the two materials were mixed for about 1 hour using a mortar so that they were homogeneously mixed. The prepared mixture was placed in an alumina crucible and heated at 700° C. for 30 hours under a nitrogen gas atmosphere. After heating, it was allowed to cool naturally at room temperature, thereby obtaining a silicon oxide-cobalt composite.

[0060] The obtained composite powder was analyzed using XRD (FIG. 3). As can be seen in FIG. 3, in the case of the powder obtained in Example 1, a composite including silicon (black diamond) and cobalt (red diamond) was formed, and it was found that cobalt oxide was reduced to cobalt metal.

[0061] In contrast, when only cobalt oxide was heated at 900° C. for 30 hours and analyzed using XRD, it was confirmed that only cobalt oxide (green diamond) was included (FIG. 3).

[0062] Meanwhile, as a result of analyzing the composite powder obtained in Example 1 by XPS and SEM-EDS, it was confirmed that amorphous silicon dioxide (SiO.sub.2) was present (FIGS. 4 and 5).

Example 2

Preparation of Silicon Oxide-Cobalt Composite

[0063] A silicon oxide-cobalt composite was prepared in the same manner as in Example 1 above except that silicon (Si, diameter 100 nm) and cobalt oxide (CoO, diameter 50 nm) were prepared in a molar ratio of 13:1.

Example 3

Preparation of Silicon Oxide-Copper Composite

[0064] A silicon oxide-copper composite was prepared in the same manner as in Example 1 above except that copper oxide was prepared instead of cobalt oxide, and silicon (Si, diameter 100 nm) and copper oxide (CuO) were prepared in a molar ratio of 11:1.

Example 4

Preparation of Silicon Oxide-Copper Composite

[0065] A silicon oxide-copper composite was prepared in the same manner as in Example 1 above except that copper oxide was prepared instead of cobalt oxide, and silicon (Si, diameter 100 nm) and copper oxide (CuO) were prepared in a molar ratio of 13:1.

Experimental Example 1

Evaluation of Charge/Discharge Characteristics

[0066] Along with the four composites prepared in Examples 1 to 4, commercially available silicon (Sigma-Aldrich, USA) was prepared as Comparative Example, and their charge/discharge characteristics were evaluated. In order to evaluate electrochemical behavior, an electrode was prepared using the composites obtained in Examples 1 to 4 and the Si single compound prepared as Comparative Example, and an electrochemical test thereof was performed.

[0067] Specifically, 75% by weight of the materials of each Example and Comparative Example and 10% by weight of the brand name Super C as carbon powder were put in a mortar and mixed for 20 minutes. The mixture and 15% by weight of PAA were added to 5 ml of distilled water and mixed for 5 hours. The mixed liquid mixture was applied on a copper foil, and slurry casting was performed using a doctor blade. After drying in an oven at 80° C. for 2 hours or more and then drying in a vacuum oven at 120° C. for 12 hours, an electrode was prepared by punching with a diameter of 8 mm.

[0068] Together with the above electrodes, a polypropylene film (25 μm) was punched with a diameter of 13 mm and used as a separator, and the electrolyte was used by adding FEC at a concentration of 5% by weight to EC/DEC (volume ratio 1:1) containing 1M LiPF.sub.6. A battery was prepared by punching and using a lithium metal with a diameter of 10 mm as a counter electrode.

[0069] The charge/discharge capacity of the batteries prepared by the above method were measured using Maccor Series 4000 at room temperature, and specifically measured at a C/20 rate in the range of 0.01 to 1.5 V. At this time, the C rate was calculated based on 200 mAh/g.

[0070] As shown in FIG. 7, in the case of the materials obtained in Examples 1 to 4 of the present invention, the discharge capacity was maintained even up to 50 cycles or more, whereas in the case of Comparative Example, the discharge capacity gradually decreased. Therefore, it was confirmed that the electrochemical properties of the material according to the embodiment of the present invention are more excellent.

[0071] The above description of the present invention is for illustrative purposes only, and those of ordinary skill in the art to which the present invention pertains should be able to understand that modification into other specific forms can be easily performed without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the embodiments described above are illustrative and non-limiting in all respects. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as being distributed may also be implemented in a combined form.

[0072] The scope of the present invention is indicated by the claims to be described below, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

INDUSTRIAL APPLICABILITY

[0073] The present invention provides a method for preparing a negative electrode active material including a silicon oxide-metal composite that can be used as a negative electrode material for a lithium secondary battery, and provides a negative electrode capable of improving the characteristics of a lowered lifespan by solving the issue of irreversible capacity due to volume change, which is a problem of a conventional silicon-based negative electrode, and a lithium secondary battery including the same.