WO2010002084A1

WO2010002084A1 - Anode for secondary battery having negative active material with multi-component metal oxide nanofiber web structure and secondary battery using the same, and fabrication method of negative active material for secondary battery

Info

Publication number: WO2010002084A1
Application number: PCT/KR2008/007898
Authority: WO
Inventors: Il-Doo Kim; Jae-Min Hong
Original assignee: Korea Institute Of Science And Technology
Priority date: 2008-06-30
Filing date: 2008-12-31
Publication date: 2010-01-07
Also published as: KR100907486B1

Abstract

There are provided an anode for a secondary battery having a negative active material with a multi-component metal oxide nanofiber web structure in which a cycle characteristic and a life characteristic are improved and stability is improved when charging and discharging are performed by a large current, a secondary battery using the same, and a method of fabricating a negative active material for a secondary battery. The method of fabricating the negative active material for a secondary battery includes spinning a solution in which no less than two kinds of metal-salt precursors and a polymer are mixed with each other onto an anode colector to form a web of composite fiber in which the no less than two kinds of metal-salt precursors and the polymer are mixed with each other, performing a thermal compression or thermal pressure process on the web of the composite fiber, and performing thermal treatment on the thermal compressed or thermal pressed web of the composite fiber to remove the polymer from the web of the composite filder.

Description

ANODE FOR SECONDARY BATTERY HAVING NEGATIVE ACTIVE MATERIAL WITH MULTI-COMPONENT METAL OXIDE NANOFIBER WEB STRUCTURE

AND SECONDARY BATTERY USING THE SAME, AND FABRICATION METHOD OF NEGATIVE ACTIVE MATERIAL FOR SECONDARY BATTERY

This nonprovisional application claims priority under 35 U. S. C. § 1 19(a) on Patent Application No. 10-2008-0062957 filed in Korea on June 30, 2008, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

[01 ] The present invention relates to an anode for a secondary battery having a negative active material with a multi-component metal oxide nanofiber web structure in which a cycle characteristic and a life characteristic are improved and stability is improved when charging and discharging are performed by a large current, a secondary battery using the same, and a method of fabricating a negative active material for a secondary battery.

Description of the Background Art

[02] As the portability and miniaturization of electronic products such as an MP3 player, a digital camera, a mobile telephone, a digital camcorder, a personal digital assistant (PDA), and a laptop computer are accelerated in 2000's, request for the miniaturization, lightness, and high effectiveness of the secondary battery used as a power supply have been increased.

[03] In addition, recently, due to the lack of oil and environmental contamination, an effort to replace an existing internal combustion engine vehicle by an electric vehicle (EV) or a hybrid electric vehicle (HEV) is being steadily made. Therefore, an environment friendly battery having a large output (a high C-rate characteristic) within a short time, having high energy density, and having high stability although charging and discharging are repeated by a large current is required.

[04] In general, a battery includes a cathode, an anode, an electrolyte, and a separator. The active materials that constitute the cathode and the anode most significantly affect the characteristics of the battery.

[05] As the negative active materials, various materials such as a lithium metal, a lithium metal alloy, a carbon material, silicon, a tin oxide, and a transition metal oxide are reported. However, currently, the carbon material having a small change in electric potential for the intercalation and deintercalation of lithium and having high reversibility is commonly used.

[06] However, in the currently used carbon material (graphite), one item of lithium LiC₆ is theoretically intercalated into six carbon atoms so that the theoretical maximum capacity is limited to 372mAh/g and that there are limitations on increasing the capacity. In particular, in order to achieve stable and long cycle and life characteristics for high output and input characteristics, the negative active material having a small amount of deterioration in capacity, a small increase of internal resistance, and a small amount of volume expansion during the charge and discharge of pulses by the large current is required. However, the existing carbon material does not satisfy such characteristics.

[07] Silicon, tin, or an alloy and an oxide thereof can be used as the negative active material used for a second generation battery. In particular, in SnO₂ (bulk SnO₂ with theoretical capacity of 1 ,494mAh/g) having a rutile structure (a = 4.7372A and c = 3.816A), not common intercalation/deintercalation mechanism but alloying/dealloying mechanism is used as charging and discharging mechanism. [08] However, significant stress is generated by a change in a volume in an alloying/dealloying process, resulting in separation of the active material from an electrode or destruction of an internal structure by the stress so that electrical contact deteriorates and that an electrode characteristic deteriorates.

[09] In order to solve such problems, a significant effort is being made to improve the cycle characteristic and to guarantee long term stability through a method of adding tin oxide powder to carbon powder to fabricate an anode material (Korean Unexamined Patent Application Publication No. 2005- 0087609) and a method of fabricating tin based nanopowder capped with monomer (Korean Unexamined Patent Application Publication No. 2007- 0005149). However, a remarkable result is not obtained.

[10] On the other hand, an interest in an electro-spinning method as one of methods of fabricating functional nanofiber is increasing. The nanofiber obtained by the electro-spinning method has high porosity and the high ratio of a surface area to a volume so that improved property can be expected. However, in the case of the metal oxide-polymer composite fiber web obtained by the electro-spinning, a rapid shrinkage of volume occurs during decomposition of polymers under the thermal treatment at a temperature higher than 450⁰C so that a crack is generated or that the fiber web is easily separated from a lower substrate.

SUMMARY OF THE INVENTION

[1 1 ] In order to solve the above problems, it is an object of the present invention to provide a negative active material in which a specific surface area (a reaction area) is significantly increased through the web structure of the nanofiber including particles of metal oxide having a ternary or higher component component system so that high rate charging and discharging can be performed under high energy density and large current, and a method of fabricating the same.

[12] It is another object of the present invention to provide a negative active material in which the volumetric expansion and reduction of the negative active material generated during charging and discharging processes are minimized so that a cycle characteristic can be maximized, that long term life stability is improved, and high rate charging and discharging and a high output (a high C-rate) can be performed, and a method of fabricating the same.

[13] It is still another object of the present invention to provide an anode for a secondary battery in which adhesive force between a negative active material and a collector is significantly increased to improve mechanical, thermal, and electrical stability, and a secondary battery using the same.

[14] It is still another object of the present invention to provide a method of fabricating a next generation negative active material that can be applied to a thick film as well as a thin film through a simple, low-priced, and rapid process and that can be easily commercialized.

[15] In order to achieve the above objects, there is provided an anode for a secondary battery, comprising an anode collector and a negative active material compressed onto at least one surface of the anode collector. The negative active material is a thin layer of a belt-shaped metal oxide nanofiber web in which a web of a metal-salt precursor-polymer composite fiber formed by spinning a solution where no less than two kinds of metal-salt precursors and a polymer are mixed with each other is thermal compressed or thermal pressed to be thermal processed and the nanofiber is comprised of metal oxide nanoparticles having a ternary or higher component component system. [16] There is also provided a secondary battery comprising an anode including an anode collector and a negative active material compressed onto at least one surface of the anode collector, an electrolyte, and a cathode. The negative active material is a web thin layer of a belt-shaped metal oxide nanofiber in which a web of a metal-salt precursor-polymer composite fiber formed by spinning a solution where no less than two kinds of metal-salt precursors and a polymer are mixed with each other is thermal compressed or thermal pressed to be thermal processed. The nanofiber comprises ternary or higher component component system metal oxide nanoparticles.

[17] The nanoparticles comprise at least one of (1 ) a solid solution of no less than two kinds of metal oxides, (2) mixed phase of the phases of the phase separated no less than two kinds of metal oxides, and (3) a compound formed of the no less than two kinds of metal oxides.

[18] There is provided a method of fabricating a negative active material for a secondary battery, comprising spinning a solution in which no less than two kinds of metal-salt precursors and a polymer are mixed with each other onto an anode collector to form a web of composite fiber in which the no less than two kinds of metal-salt precursors and the polymer are mixed with each other, performing a thermal compression or thermal pressure process on the web of the composite fiber, and performing thermal treatment on the thermal compressed or thermal pressed web of the composite fiber to remove the polymer from the web of the composite fiber.

[19] Since the negative active material according to the present invention has a one-dimensional nanofiber web structure containing ternary or higher component component system ultra-fine metal oxide nanoparticles, the negative active material has a significantly increased specific surface area and can stand a volumetric change. Therefore, it is possible to minimize the reduction of capacity in accordance with the increase in a cycle that is often observed in a metal oxide based negative active material, to minimize the volumetric change generated by repeated charging and discharging reactions, to significantly improve a life characteristic, and to obtain high stability during charging and discharging with a large current. Therefore, a high capacity and high output battery can be obtained.

[20] In addition, adhesive force between a collector and a negative active material layer is significantly increased through a thermal compression or thermal pressure and spinning time is controlled so that the thickness of the negative active material layer can be easily controlled.

[21 ] In particular, since the ratio of no less than two kinds of metal-salt precursors can be easily controlled, it is possible to easily fabricate a secondary battery having various characteristics in accordance with a required specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[22] FIG. 1 schematically illustrates processes of fabricating a negative active material for a secondary battery according to the present invention;

[23] FIG. 2 illustrates the scanning electron microscope (SEM) photographs of the Sn₁^Ti_xO₂ (x = 0.1 to 0.9) nanofiber webs obtained by making the ratio of a tin precursor different from the ratio of a titanium precursor in a spinning solution and by performing thermal treatment at 450⁰C and 900°C in accordance with an embodiment 1 ;

[24] FIG. 3 illustrates the transmission electron microscope (TEM) photograph of the Sn_{0 5}Ti_{0 5}O₂ nanofiber web obtained by the embodiment 1 (in which thermal treatment temperature is 450⁰C); [25] FIG. 4 illustrates the SEM photograph of the CoSnO₃ nanofiber web obtained by an embodiment 2;

[26] FIGs. 5 and 6 illustrate the enlarged SEM photographs of FIG. 4;

[27] FIG. 7 illustrates the TEM photograph of the CoSnO₃ nanofiber obtained by the embodiment 2;

[28] FIG. 8 illustrates the diffraction pattern of the CoSnO₃ obtained by the embodiment 2;

[29] FIG. 9 illustrates the SEM photograph of the Zn₂SnO₄ nanofiber web obtained by an embodiment 3 (in which thermal treatment temperature is 450°C);

[30] FIGs. 10 and 1 1 illustrate the enlarged SEM photographs of FIG. 9;

[31 ] FIG. 12 illustrates the diffraction pattern of the Zn₂SnO₄ obtained by the embodiment 3 (in which thermal treatment temperature is 450°C);

[32] FIG. 13 illustrates the SEM photograph of the Zn₂SnO₄ nanofiber web obtained by the embodiment 3 (in which thermal treatment temperature is 600°C);

[33] FIG. 14 illustrates the enlarged SEM photograph of FIG. 13;

[34] FIG. 15 illustrates the result of measuring a change in discharge capacity for the number of cycles when the Sn_{0 4}Ti_{0 6}O₂ nanofiber web thin layer obtained by the embodiment 1 (in which thermal treatment temperature is 45O⁰C) is used as a negative active material is measured at the current density of 2C;

[35] FIGs. 16 to 20 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the CoSO₃ nanofiber web thin layer obtained by the embodiment 2 is used as a negative active material at the current densities of 2C, 4C, 8C, 16C, and 32C; [36] FIG. 21 illustrates the result of measuring a change in discharge capacity for the number of cycles when the CoSnO₃ nanofiber web thin layer obtained by the embodiment 2 is used as a negative active material is measured at the current densities of 2C, 4C, 8C, 16C, and 32C;

[37] FIGs. 22 to 24 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the Zn₂SnO₄ nanofiber web thin layer obtained by the embodiment 3 (in which thermal treatment temperature is 450⁰C) is used as a negative active material at the current densities of 2C, 4C, and 8C; and

[38] FIGs. 25 to 27 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the Zn₂SnO₄ nanofiber web thin layer obtained by the embodiment 3 (in which thermal treatment temperature is 600⁰C) is used as a negative active material at the current densities of 2C, 4C, and 8C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[39] A method of fabricating a negative active material for a secondary battery according to the present invention can be divided into (1 ) a process of fabricating a spinning solution, (2) a process of fabricating a composite fiber web of no less than two kinds of metal-salt precursors and a polymer, and (3) a process of fabricating a metal oxide nanofiber web having a ternary or higher component component system. Hereinafter, the present invention will be described in detail by process.

Fabrication of a spinning solution

[40] First, for performing spinning, a mixed solution (hereinafter, referred to as a spinning solution or an electro-spinning solution) containing no less than two kinds of metal-salt precursors (referred to as metal oxide precursors), a polymer, and a solvent is provided.

[41 ] The no less than two kinds of metal-salt precursors used for the present invention can be selected from the group consisting of precursors containing Sn, Ni, Fe, Co, Ti, Mg, Mn, Ca, Cu, Zn, In, Mo, and W ions. The precursors of the present invention are not limited to the above-described precursors but any no less than two kinds of metal-salt precursors mixed with a polymer and a solvent and spun to form a metal oxide composite having a ternary or higher component system through high temperature (for example, no less than 400°C) thermal treatment can be used.

[42] The polymer increases the viscosity of the mixed solution to form a fiber phase during spinning and controls the structure of the spun fiber by compatibility with the metal-salt precursors. No less than one of polyurethane, polyetherurethane, a polyurethane copolymer, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, polymethyl methacrylate, polymethylacrylate, a polyacryl copolymer, polyvinylacetate, a polyvinylacette copolymer, polyvinylalcohol, polyfurfuryl alcohol, polystyrene, a polystyrene copolymer, polyethylene oxide, polypropyleneoxide, a polyethyleneoxide copolymer, a polypropyleneoxide copolymer, polycarbonate, polyvinylsalt, polycaprolactone, polyvinylpyrrolidone, polyvinylfluoride, a polyvinylidenefluoride copolymer, polyamide, polyacrylonitrile, pitch, and phenol resin can be used as the polymer. However, the present invention is not limited to the above-described polymers.

[43] Dimethylformamide, acetone, thetrahydrofurane, toluene, or a mixed solvent of the above can be used as the solvent used for the present invention. However, the present invention is not limited to the above.

[44] When an example of the process of fabricating the spinning solution is described, first, polyvinylacetate having high affinity with tin acetate that is a tin precursor (tin acetate) and titanium propoxide that is a titanium precursor is dissolved in a dimethylformamide solvent to fabricate a polymer solution of 5 to 20wt%. At this time, the average molecular weight of polyvinylacetate is preferably 100,000 to 1 ,500,000 g/mol. Next, after tin acetate and titanium propoxide are added to the polymer solution with the amount of 1 to 60wt% for the polymer solution and acetic acid as a catalyst is added to the polymer solution with the amount of 0.01 to 60w% for tin acetate, a reaction is performed at a room temperature for 1 to 10 hours. If necessary, cetyltrimethyl ammonium bromide (CTAB) is added to the solution and the resultant solution is agitated for several minutes. Here, CTAB serves as additive that controls the charge characteristic of the metal-salt precursors to facilitate spinning.

Fabrication of a composite fiber web of no less than two kinds of metal-salt precursors and a polymer

[45] Next, the obtained mixed solution is spun onto a negative collector to separate the phases of no less than two kinds of metal-salt precursors from the phase of the polymer or to mix the phases of the no less than two kinds of metal-salt precursors with the phase of the polymer so that the composite fiber web of the no less than two kinds of metal-salt precursors and the polymer (hereinafter, referred to as a composite fiber web) is formed (refer to FIG. 1 ).

[46] In order to form the composite fiber web, other than the electro- spinning method, a melt-blown method, a flash spinning method, or an electrostatic melt-blown method can be used.

[47] For example, during electro-spinning, a voltage of 5 to 3OkV is applied and the discharge rate of the solution is controlled as 10 to 50μ£/min to fabricate a ultra-fine composite fiber web in which the diameter of fiber is 50 to 1 ,000nm. The electro-spinning is performed until a composite fiber web layer is formed on the collector to a thickness of 0.1 to 60μm. Since the sol- gel reaction caused by the electro-spinning depends on moisture, temperature and humidity around a spinning apparatus serve as important process variables.

[48] The negative collector used for the present invention can be formed of (1 ) Pt, Au, Pd, Ir, Ag, Rh, Ru, Ni, stainless steel, Al, Mo, Cr, Cu, Ti, or W, (2) indium doped SnO₂ (ITO) or fluorine doped SnO₂ (FTO), or (3) a metal material formed on a Si wafer. However, the present invention is not limited to the above.

Fabrication of a metal oxide nanofiber web having a ternary or higher component system

[49] Next, the as-spun composite fiber web is thermal compressed or thermal pressed at a temperature no less than the glass transition temperature of the polymer and then, is heat-treated (ex. calcining) to fabricate a metal oxide nanofiber web (hereinafter, referred to as a nanofiber web) having a ternary or higher component system (refer to FIG. 1 ).

[50] The pressure, the temperature, and the thermal compression time can be properly selected in consideration of the kind of the used polymer and the glass transition temperature of the polymer. For example, when polyvinylacetate is used as the polymer, the thermal compression is preferably performed at 70 to 120⁰C, for 30 seconds to 10 minutes, and under the pressure of 0.01 to 20MPa.

[51 ] In the above process, a flow between the phase separated metal-salt precursors and polymer is suppressed during the electro-spinning. Then, a thermal treatment process is performed to form a belt-shaped nanofiber web containing particles having a size of nanometers. [52] If a part of the polymer or the entire polymer can be melt without compressing the polymer, the polymer can be heated at a temperature slightly higher than the glass transition temperature or can be pressed using hot compression air (referred to as heat pressure). At this time, in order to prevent the polymer from rapidly volatilizing, after performing the thermal treatment process by stage at a low temperature (for example, 100 to 20O⁰C), the thermal pressure can be performed at a high temperature.

[53] By performing the thermal compression or pressure process as described above, a special structure in which the polymer in the composite fiber is partially or entirely melt, adhesive property with the collector is improved, and a specific surface area and density per a unit volume are significantly improved after performing the thermal treatment can be obtained. Therefore, it is possible to obtain the negative active material having high C- rate density. The metal oxide nanofiber web having a ternary or higher component system that did not undergo the thermal compression process is easily separated from a substrate after the thermal treatment.

[54] Next, the thermal compressed or thermal pressed composite fiber web is heat-treated to decompose the polymer and to remove the polymer from the composite fiber web, it is possible to obtain the metal oxide nanofiber web having a ternary or higher component system (refer to 1 ).

[55] Thermal treatment temperature and time are determined in consideration of the crystallizing and annealing temperatures of the used metal oxide. The thermal treatment can be performed in a temperature range of 400 to 800⁰C in accordance with the kinds of the metal-salt precursors. The metal oxide nanofiber web having an amorphous structure or ultra-fine nanoparticles is obtained by the thermal treatment process performed at a low temperature of 300 to 40O⁰C to improve the cycle characteristic. [56] Thus obtained negative active material for a secondary battery as a porous metal oxide thin layer is compressed onto at least one surface of the negative collector in the form of a belt by the thermal compression or the thermal pressure. The adhesive property between the metal oxide thin layer and the collector is significantly improved and the web structure of the nanofiber containing the ultra-fine metal oxide nanoparticles having the ternary or higher component system is obtained. Therefore, the specific surface area and the reaction area are maximized.

[57] Here, the average diameter of the nanofiber is 50 to 900nm, the average size of the nanoparticles that constitute the nanofiber is 1 to 100nm, and pores having a size of 1 to 100nm are formed among the nanoparticles.

The size of the nanoparticle can be changed depending on the thermal treatment temperature. If necessary, it is possible to fabricate microfiber having a diameter of 1 to 3 μm by controlling the viscosity of the polymer and the solvent.

[58] In the fine structure of the nanoparticles, the nanoparticles can contain the solid solution of no less than two kinds of metal oxides, the mixed phase of the phases of the phase separated no less than two kinds of metal oxides, and a compound formed of the no less than two kinds of metal oxides in accordance with the relative ratios of the no less than two kinds of metal-salt precursors.

[59] That is, when the relative ratios of the no less than two metal-salt precursors is within a solubility limit, the nanoparticles are formed of the solid solution of no less than two metal oxides. In addition, when the relative ratios are larger than the solubility limit, phase separation is generated in the ratio larger than the solubility limit so that the nanoparticles are formed of the mixed phase of the solid solution and the phases of the phase separated no less than two kinds of metal oxides. In addition, when the used no less than two kinds of metal-salt precursors do not form the solid solution, the nanoparticles are formed of the mixed phase of the phases of the phase separated no less than two kinds of metal oxides. Furthermore, when the used no less than two kinds of metal-salt precursors have a specific composition ratio, the nanoparticles can be formed of a new compound having the specific composition ratio.

[60] For example, when a Zn-salt precursor and a Sn-salt precursor are mixed with the polymer and the solvent to be spun and then, the thermal compression process is performed to obtain the thermal processed nanofiber web, the nanofiber web is formed of a solid solution, a mixed phase, a compound, or a phase in which the solid solution, the mixed phase, and the compound are mixed with each other in accordance with the relative amounts of the Zn-salt precursor and the Sn-salt precursor. When it is assumed that the solubility limit between ZnO and SnO₂ is 10wt%, in order to constitute the solid solution, ZnO having the hexagonal Wrutzite structure and SnO₂ having the Rutile structure are formed to have the composition ratios of Sn₁. _xZn_xO (X = 0.9 to 1 ) and Zn₁^Sn_xO₂ (X = 0.9 to 1 ) so that the ZnO having the hexagonal Wrutzite structure and the SnO₂ having the Rutile structure are soluble in the respective structures. Since the solubility limit between ZnO and SnO₂ is larger than 10wt% in the region where the composition ratios of ZnO and SnO₂ are larger than Sn₁^Zn_xO (X = 0.9 to 1 ) and Zn^_xSn_xO₂ (X = 0.9 to 1 ), the mixed phase of (ZnO)_Lx(SnO₂J_x (X = 0.1 to 0.9) in which ZnO and SnO₂ exist together is formed by the phase separation. In the composition of the mixed phase, in the composition ratio where the atom ratio of [Zn]:[Sn] is 2: 1 , the Zn₂SnO₄ compound referred to as Zinc Stannate is formed. Zn₂SnO₄ has a spinel structure having a lattice parameter of 0.8657nm.

[61 ] The no less than two metal oxides can be selected from the group consisting of SnO₂, TiO₂, Fe₂O₃, Fe₃O₄, CoO, Co₃O₄, CaO, MgO, CuO, ZO, In₂O₃, NiO, MoO₃, MnO₂, and WO₃. In addition, the nanoparticles can have (1 ) one composition ratio selected from the group consisting of Sn₁^Ti_xO₂, (ZnO)₁JSnO₂J_x, (CoO^SnO,),, (CaO^SnO,),, (ZnO)₁JCoO)_x, (MgO)₁. _x(SnO₂)_x, and (Mn0)₁._x(Sn0₂)_x (x = 0.01 -0.99) that are metal oxide complexes having a ternary system and Zn₂SnO₄, CoSnO₃, Ca₂SnO₄, CaSnO₃, ZnCo₂O₄, Co₂SnO₄, Mg₂SnO₄, and Mn₂SnO₄ that are compounds, (2) one composition ratio selected from the group consisting of (ZnO)_x(SnO₂)_y(CoO)_z, (ZnO)_x(SnO₂)_y(CaO)_z, (TiO)_x(SnO₂)_y(CaO)_z, (Ti0)_x(Sn0₂)_y(Zn0)_z,

(Mg0)_x(Sn0₂)_y(Zn0)_z, and (Mn0)_x(Sn0₂)_y(Zn0)_z (x + y + z = 1 ) that are metal oxide complexes having a four component system, or (3) the composition ratio of (NiO)_a(ZnO)_b(Fe₂O₃)_c(TiO₂)_d(SnO₂)_e (a + b + c + d + e = 1 ) that are metal oxide complexes having a five component system. The present invention is not limited to the above but various complexes can be constituted in accordance with the kinds of the used metal-salts.

[62] The present invention now will be described more fully with reference to the exemplary embodiments of the present invention. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the present invention to those skilled in the art.

[Embodiment 1 ] Fabrication of a tin-titanium oxide nanofiber web through the thermal compression process and the post-heat treatment process of a tin-titanium precursor-polyvinylacetate composite fiber web

[63] The polymer solution obtained by putting polyvinylacetate (Mw: 1 ,000,000) of 1 .5g into dimethylformamide of 7.5ml so that polyvinylacetate is dissolved into dimethylformamide for about a day was mixed with the solution obtained by dissolving tin acetate (Sn(CH₃COO)₄) and titanium propoxide (Ti(OCH(CH₃)₂)₄ in dimethylformamide (DMF) of 7.5ml so that the ratios of Sn to Ti are 1 :9, 4:6, 5:5, 6:4, and 9: 1 , respectively, in accordance with a certain ratio. At this time, small amounts of acetic acid and cetyltrimethyl ammonium bromide (CTAB) can be added to smoothly perform spinning.

[64] After the precursor in which reaction was performed was put into a syringe and was mounted in an electro-spinning device, a voltage was applied between the tip and the collector at the ends of the syringe to obtain the tin- titanium precursor-polyvinylacetate composite fiber web (refer to FIG. 1 ). Here, the voltage was 12kV, a flow rate was 30μ#/min, a total discharge amount was 500 to 10,000μ£, and the distance between the tip and the collector was about 10cm. In particular, in accordance with the amount of the acetic acid used for sol-gel reaction, a fine change in the structure of (Sn₇Ti)O₂ nanofiber could be observed. A polymer and a tin-titanium precursor were mixed within the tin-titanium precursor-polyvinylacetate composite fiber web fabricated by the electro-spinning.

[65] The collector on which the obtained polymer-tin-titanium precursor composite fiber web is laminated was thermal compressed by the press(ex. lamination machine) heated to 120°C under the pressure of 1 .5kgf/cm² (21 .34 psi) for 5 minutes (refer to FIG. 1 ). At this time, it was noted that the structure of the surface changed in accordance with the degree of applied pressure. In addition, a thermal pressure temperature can vary with the used polymer. The tin-titanium precursor/polymer (PVAc) composite fiber has a connection structure through the entire melting process of the polymer (PVAc) generated in the thermal compression process.

[66] FIG. 2 illustrates the scanning electron microscope (SEM) photographs of the (Sn₁^Ti_x)O₂ (x = 0.1 to 0.9) nanofiber webs obtained by performing the thermal treatment at 450 to 900°C for 30 minutes through the thermal compression process. As a result of analyzing an energy dispersive spectroscopy (EDS) composition, it was noted that the composition ratios of (Sn_{0 1}Ti_{0 9})O₂, (Sn_{0 4}Ti_{0 6})O₂, (Sn_{0 5}Ti_{0 5})O₂, (Sn_{0 6}Ti₀.₄)O₂, and (Sn_0-9Ti_{0 1})O₂ were well formed after the thermal treatment. The composition ratios are determined by the ratios of the tin acetate (Sn(CH₃COO)₄) and titanium propoxide (Ti(OCH(CH₃)₂)₄ precursors that are put into the spinning solution at an initial stage. As illustrated in FIG. 2, it is noted that the size of the nanoparticles that constitute the nanofiber web significantly increases when the thermal treatment temperature after the thermal compression process increases from 450°C to 900⁰C. Therefore, it is noted that the size of the particles of (Sn₁^Ti_x)O₂ (x = 0.1 to 0.9) can be controlled by controlling the thermal treatment temperature.

[67] FIG. 3 illustrates the transmission electron microscope (TEM) photograph of the tin-titanium oxide (Sn_{0 5}Ti_{0 5}O₂) nanofiber web obtained by performing the thermal treatment at 450⁰C through the thermal compression process, in which it is clearly noted that the size of the tin-titanium oxide nanoparticles is increased to before and after 2 to 1 5nm.

[68] In accordance with the ratio of (Sn₁^Ti_x)O₂ (x = 0.01 to 0.99), it is possible to form the solid solution in which SnO₂ and TiO₂ are completely mixed with each other. In the ratio larger than the solubility limit, the phases of SnO₂ and TiO₂ are separated from each other so that a mixed phase can exist. Since the phase separation is determined by the states of the respective materials and the negative active material characteristics of the respective metal oxides can be expected, it is possible to design a material having a new composition ratio.

[Embodiment 2] Fabrication of a cobalt-tin oxide nanofiber web through the thermal compression process and the post-thermal treatment process of a cobalt-tin precursor-polvvinylacetate composite fiber web [69] The polymer solution obtained by putting polyvinylacetate (Mw: 1 ,000,000) of 2.4g into dimethylformamide of 15ml so that polyvinylacetate is dissolved into dimethylformamide for about a day was mixed with the solution obtained by dissolving cobalt salt (Co(II)CI₂) of 1 .6g and tin acetate (Sn(CH₃COO)₄ of 4.4g in dimethylformamide (DMF) of 1 5ml.

[70] After the blue precursor in which reaction was performed was put into a syringe and was mounted in an electro-spinning device, a voltage was applied between the tip and the collector at the ends of the syringe to obtain a cobalt-tin precursor-polyvinylacetate composite fiber web. Here, the voltage was 10.5kV, a flow rate was 10μ#/min, a total discharge amount was 500 to 10,000μ£, and the distance between the tip and the collector was about 12.5cm. According to the present embodiment, the acetic acid was not additionally used. The polymer and the cobalt-tin precursor were mixed with the cobalt-tin precursor-polyvinylacetate composite fiber web fabricated by the electro-spinning.

[71 ] The collector on which the obtained polymer-cobalt-tin precursor composite fiber web is laminated was thermal compressed by the press heated to 120⁰C under the pressure of 1 .5kgf/cm² (21 .34 psi) for 5 minutes. At this time, it was noted that the structure of the surface changed in accordance with the degree of applied pressure. In addition, a thermal pressure temperature can vary with the used polymer. The cobalt-tin precursor/polymer (PVAc) composite fiber has an inter-connected structure through the entire melting process of the polymer (PVAc) generated in the thermal compression process.

[72] FIG. 4 illustrates the SEM photograph observed after thermal compressing the cobalt-tin precursor/PVAc composite fiber web formed on the collector by the electro-spinning at 120⁰C under the pressure of 1 .5kgf/cm² (21 .34 psi) for 5 minutes and then, thermal processing the coblt.tin precursor/PVAc composite fiber web at 450°C, in which a porous CoSnO₃ nanofiber web structure is well formed. FIGs. 5 and 6 illustrate the enlarged SEM photographs of FIG. 4, in which the porous CoSnO₃ nanofiber web structure comprised of ultra-fine nanoparticles is clearly shown.

[73] FIG. 7 illustrates the TEM photograph of the cobalt-tin oxide (CoSnO₃) nanofiber web heat-treated at 450°C through the thermal compression process, in which the cobalt-tin oxide nanoparticles in the range of 5 to 15nm are clearly shown. FIG. 8 illustrates the diffraction pattern of the cobalt-tin oxide (CoSnO₃) of FIG. 7, in which a fine structure where crystallization is slightly performed is shown.

[74] In the case of the cobalt-tin oxide (CoSnO₃), like in the (Sn₁^Ti_x)O₂ (x = 0.01 to 0.09) of the above-described embodiment 1 , the ratios of cobalt salt (Co(II)CI₂) and the tin acetate (Sn(CH₃COO)₄) precursors are controlled. Therefore, a solid solution in the form of (CoO)₁._x(SnO₂)_x (x = 0.01 to 0.99) can exist, the phases of the cobalt salt (Co(II)CI₂) and the tin acetate (Sn(CH₃COO)₄) precursors are separated from each other in the ratios larger than the solubility limit so that a mixed phase can exist, or CoSnO₃ and Co₂SnO₄ that are the stannate compounds having specific composition ratios can exist.

[Embodiment 3] Fabrication of Zn₂SnO₄ nanofiber web through thermal compression and post heat treatment of Zinc-tin precursor-polvvinylacetate composite fiber

[75] A polymer solution in which polyvinylacetate (Mw: 1 ,000,000) of 2.4g is put into dimethylformamide (DMF) 15ml to be melted for a day is mixed with a solution in which zinc acetate dehydrate 3.318g and tin acetate (Sn(CH₃COO)₄) 2.682g are melted in dimethylformamide 1 5ml. [76] The precursor in which reaction is carried out is transferee! into a syringe and the syringe is mounted to an electro-spinning apparatus, and after that voltage is applied between a tip at an end of the syringe and a collector so that zinc-tin precursor polyvinylacetate composite fiber web is obtained. The voltage is 9.5kV, flow rate is 10 μl/min, total discharged amount is 700 to 10,000 μl, and a distance between the tip and the collector is 15cm. In this embodiment, acetic acid is not used additionally. In the zinc-tin precursor polyvinylacetate composite fiber web fabricated by the electro- spinning, polymer and zinc-tin precursor are mixed.

[77] The collector on which the obtained polymer-zinc-tin precursor composite fiber webs are laminated is thermal compressed by a press heated at 120^°C under pressure of 1 .5kgf/cm² (21 .34psi) for 5 minutes. It was observed that the surface structure can be changed by the pressure applied thereto. Moreover, temperature of the thermal pressure can be determined in accordance with kind of polymers to be used.

[78] FIG. 9 illustrates the SEM photograph of porous Zn₂SnO₄ web consisting of Zn₂SnO₄ nanoparticles that is observed after heat treatment at 450⁰C after the zinc-tin precursor/PVAc composite fiber web formed on the collector by the electro-spinning is thermal compressed under pressure of 1 .5kgf/cm² (21 .34psi) for 5 minutes, and as illustrated in the drawing, it can be aware that the nanofiber web structure consisting of fine nanoparticles is formed. During the thermal compression of the zinc-tin precursor/PVAc composite fiber web, type of the nanofiber web after heat treatment may be differed according to a melted degree of polymer. For example, when time for the thermal compression is elongated at temperature higher than glass transition temperature of the polymer or the applied pressure is increased, the polymer is completely melted after the thermal compression so that nanofiber web may be not observed after the heat treatment. In this case, the nanofiber web is formed in the form of a network of fine naoparticles. The fine structure of the nanofiber web can be easily controlled during the fabrication. FIG. 10 is an enlarged image of the porous nanofiber comprised of the zinc-tin oxide nanoparticles as illustrated in FIG. 9, and as illustrated in the drawing, the nanofiber web is comprised of fine nanoparticles of 5 to 20 nm.

[79] FIG. 1 1 illustrate a TEM photograph of the porous Zn2SnO4 nanofiber web consisting of nanoparticles heat-treated at 45O⁰C through the thermal compression, and as illustrated in the drawing the nanofiber web is formed with fine grain or particles. FIG. 12 illustrates the diffraction pattern of the Zn₂SnO₄ in FIG. 1 1 having a fine structure formed with crystalline or amorphous particles with very week crystallization.

[80] FIG. 13 illustrates the SEM photograph of porous nanofiber web consisting of Zn₂SnO₄ nanoparticles that is observed after thermal treatment at 600°C after the zinc-tin precursor/PVAc composite fiber formed on the collector by the electro-spinning is thermal compressed at 12O°C under pressure of 1 .5kgf/cm² (21 .34psi) for 5 minutes, and as illustrated in the drawing, it can be aware that the nanofiber web structure consisting of fine nanoparticles is formed. FIG. 14 is an enlarged SEM photograph of Zn₂SnO₄ nanofiber web in FIG. 13, and as illustrated in the drawing, the particles are increased through the thermal treatment of 600⁰C such that the nanofiber web is formed with nanoparticles of 10 to 40nm.

[81 ] In a case of Zn₂SnO₄, as in the case (Sn₁^Ti_x)O₂ (X = 0.01 to 0.99) of the above-mentioned embodiment 1 , a ratio of zinc acetate dehydrate and tin acetate is adjusted such that a solid solution in the form of (ZnO)₁._x(SnO₂)_x (X = 0.01 to 0.99), mixed phase in which phases are separated at a ratio higher than a solubility limit, or Zn₂SnO₄, stannate compound of a specific ratio may present. [82] In the embodiments 1 , 2, and 3, embodiments of ternary system complexes formed by mixing Ti precursor, Zn precursor, Co precursor with Sn precursor are described. However, in a case of oxide in the form of solid solution containing two or more components of Sn, Ni, Fe, Co, Ti, Mg, Mn, Ca, Cu, Zn, In, Mo, and W, oxide in the form of complex, and compound forming a new phase at a specific ratio, there is no limit to a specific material.

[Experimental result 1 ] Property estimation of lithium secondary battery using tin-titanium oxide nanofiber web as a negative active material

[83] In order to certify property of the tin-titanium oxide (composition ratio: Sn_{0 4}Ti_{0 6}O₂) nanofiber web formed on a stainless steel substrate as the negative active material according to the embodiment 1 , CR2032-type coin cell structure is fabricated as follows. EC/DEC (1 /1 volume%) solution in which LiPF6 1 M is melted is used as electrolyte of a cell. A lithium foil (Foote Mineral Co.) of purity 99.99% is used as an anode to be used as a reference electrode and a counter electrode, and the tin-titanium oxide (composition ratio: Sn₀₄Ti_{0 6}O₂) nanofiber web thin layer obtained in the embodiment 1 (thermal treatment at 450⁰C) is used as a working electrode. Polypropylene film (Celgard Inc.) is used as a separator preventing electrical short between the anode and a cathode, and the cell is fabricated after forming Ar atmosphere within a globe box of Valve Amplification Company (VAC).

[84] Charging and discharging experimental apparatus used in this experiment is a model WBCS300 of WonATech Co., Ltd., and voltage change is observed under static current with a Multi-Pontentiostat System (MPS) enabling measurement through 16 channels by adding 16 boards. Intensity of current density used to charge and discharge is set to a reference of 2-C- rate by calculating theoretical capacities of respective materials. Cut-off voltage is 0.1 V to 1 .1 V in a case of tin -titanium oxide (composition ratio: Sn₀₄Ti_{0 6}O₂) nanofiber web.

[85] FIG. 15 illustrates the result of measuring a change in discharge capacity for the number of cycles when the Sn₀₄Ti_{0 6}O₂ nanofiber web obtained by the embodiment 1 (thermal treatment at 45O⁰C and composition ratio: Sn_{0 4}Ti_{0 6}O₂) is used as a negative active material is measured at the current density of 2C. FIG. 15 illustrates property that high capacity higher than 300mAh/g is observed upto 500 cycles. The C-rate is defined by current flowing when all capacity is discharged within one hour. Therefore, it can say that the higher a magnitude of maximum current to be spontaneously discharged is the higher the C-rate is. This becomes a very importance factor as a power supply of an electronic and mechanical apparatus requiring a spontaneous high output. Therefore, it is notified that the tin -titanium oxide nanofiber web can be as an anode material of a secondary battery with high capacity as well as a secondary battery with a high output.

[Experimental result 2] Property estimation of lithium secondary battery using cobalt -tin oxide nanofiber web as a negative active material

[86] In order to certify property of the cobalt-tin oxide (composition ratio: CoSnO₃) nanofiber web formed on a stainless steel substrate as the negative active material according to the embodiment 2, CR2032-type coin cell structure is fabricated as follows. EC/DEC (1 /1 volume%) solution in which LJPF6 1 M is melted is used as electrolyte of a cell. A lithium foil (Foote Mineral Co.) of purity 99.99% is used as an anode to be used as a reference electrode and a counter electrode, and the cobalt -tin oxide (composition ratio: CoSnO₃) nanofiber web thin layer obtained in the embodiment 2 is used as a working electrode. Polypropylene film (Celgard Inc.) is used as a separator preventing electrical short between the anode and a cathode, and the cell is fabricated after forming Ar atmosphere within a globe box of Valve Amplification Company (VAC).

[87] Charging and discharging experimental apparatus used in this experiment is a model WBCS300 of WonATech Co., Ltd., and voltage change is observed under static current. Intensity of current density used to charge and discharge is set to a reference from 2 C-rate to 32-C-rate by calculating theoretical capacities of respective materials. Cut-off voltage is 0.1 V to 2.5V in a case of cobalt-tin oxide nanofiber web.

[88] FIGs. 16 to 20 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the CoSO₃ nanofiber web thin layer obtained by the embodiment 2 is used as a negative active material at the current densities of 2C, 4C, 8C, 16C, and 32C. In a first discharging reaction, a high capacity no less than 1500 mAh/g appears. Moreover, irreversible decrease of capacity appears in the first cycle, and steady cycle characteristics appear after the second cycle. FIG. 16 shows that the discharged capacity is 500mAh/g even after 100 cycles when the measurement is made at 2-C. As illustrated in FIG. 20, in the measurement at 32-C, capacity no less than 230 mAh/g is maintained even after 100 cycle so that excellent capacity characteristic appear at a high C-rate.

[89] FIG. 21 illustrates the result of measuring a change in discharge capacity for the number of cycles when the CoSnO₃ nanofiber web thin layer obtained by the embodiment 2 is used as a negative active material is measured at the current densities of 2C, 4C, 8C, 16C, and 32C. It can be notified that an initial discharging amount is decreased as the C-rate is high and the discharged amount is gradually decreased as the number of cycle increases. However, it is sufficient to notice the characteristic of maintaining a high capacity no less than 230mAh/g after 100 cycles even at the high C- rate such as 2 to 32-C. Therefore, the CoSnO₃ nanofiber web can be used as an anode material of a high output secondary battery as well as a high capacity secondary battery.

[Experimental result 3] Property estimation of lithium secondary battery using zinc-tin oxide nanofiber web as a negative active material

[90] In order to certify property of the zinc-tin oxide (composition ratio: Zn₂SnO₄) nanofiber web formed on a stainless steel substrate as the negative active material according to the embodiment 3, CR2032-type coin cell structure is fabricated as follows. EC/DEC (1 /1 volume%) solution in which LJPF6 1 M is melted is used as electrolyte of a cell. A lithium foil (Foote Mineral Co.) of purity 99.99% is used as an anode to be used as a reference electrode and a counter electrode, and the Zn₂SnO₄ nanofiber web thin layer obtained in the embodiment 3 (thermal treatment at 450⁰C) is used as a working electrode. Polypropylene film (Celgard Inc.) is used as a separator preventing electrical short between the anode and a cathode, and the cell is fabricated after forming Ar atmosphere within a globe box of Valve Amplification Company (VAC).

[91 ] Charging and discharging experimental apparatus used in this experiment is a model WBCS300 of WonATech Co., Ltd., and voltage change is observed under static current with a Multi-Pontentiostat System (MPS). Intensity of current density used to charge and discharge is set to a reference from 2-C-rate to 8-C-rate by calculating theoretical capacities of respective materials. Cut-off voltage is 0.05V to 3.0V in a case of Zn₂SnO₄ nanofiber web.

[92] FIGs. 22 to 24 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the Zn₂SnO₄ nanofiber web thin layer obtained by the embodiment 3 (in which thermal treatment temperature is 450⁰C) is used as a negative active material at the current densities of 2C, 4C, and 8C. As illustrated in FIG. 22, in a case of measuring at 2-C, a first discharging reaction shows a high capacity of about 1 500 mAh/g. Moreover, irreversible decrease of capacity appears in the first cycle, steady cycle characteristics appear after the second cycle, and the discharged capacity maintains 250mAh/g even after 100 cycles. As illustrated in FIG. 24, in the measurement at 8-C, the capacity no less than 240mAh/g is maintained even after 100 cycle so that excellent capacity characteristic appear at a high C-rate.

[93] FIGs. 25 to 27 illustrate the results of measuring the charging and discharging cycle characteristics in the case where the Zn₂SnO₄ nanofiber web thin layer obtained by the embodiment 3 (in which thermal treatment temperature is 600°C) is used as a negative active material at the current densities of 2C, 4C, and 8C. In a case of the thermal treatment at 600 °C, size of the nanoparticles is greater than that of nanoparticles constituting the nanofiber web heat-treated at 450 ⁰C (See FIGs. 10 and 14). As illustrated in FIG. 25, in a case of measuring at 2-C, a high capacity of about 1000 mAh/g appears in a first discharging reaction. Moreover, irreversible decrease of capacity appears in the first cycle as in the case of Zn₂SnO₄ heat-treated at 450 ⁰C, and steady cycle characteristics appear from the second cycle. The discharged capacity maintains 220mAh/g even after 100 cycle. As illustrated in FIG. 27, in the measurement at 8 C, the capacity of 80 mAh/g is maintained even after 100 cycles. As such, it is notified that the characteristics of the secondary battery are significantly changed by the size of the nanoparticles in accordance with the thermal treatment temperature in the nanofiber web structure thermal processed through the thermal compression process. [94] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be comprised within the scope of the following claims.

Claims

What Is Claimed Is:

1 . An anode for a secondary battery, comprising:

an anode collector; and

a negative active material compressed onto at least one surface of the anode colfector,

wherein the negative active material is a web thin layer of a belt-shaped metal oxide nanofiber in which a web of a metal-salt precursor-polymer composite fiber formed by spinning a solution where no less than two kinds of metal-salt precursors and a polymer are mixed with each other is thermal compressed or thfeφmal pressed to be thermal processed, and

wherein the nanofiber comprises ternary or higher component system metal oxide nanoparticles.

2. The anode for a secondary battery as claimed in claim 1 , wherein the nairSoparticles comprise at least one of

(1 ) a solid solution of no less than two kinds of metal oxides;

(2) mixed phase of the phases of the phase separated no less than two kinds of metal oxides; and

(3) a compound formed of the no less than two kinds of metal oxides.

20

3. The anode for a secondary battery as claimed in claim 2, wherein the no less than two kinds of metal oxides are selected from a group consisting of SnO₂, TiO₂, Fe₂O₃, Fe₃O₄, CoO, Co₃O₄, CaO, MgO, CuO, ZO, In₂O₃, NiO, MoO₃, MnO₂, and WO₃.

25

4. The anode for a secondary battery as claimed in claim 1 , wherein the nanoparticles has

(1 ) one composition ratio selected from the group consisting of Sn₁^Ti_xO₂, (ZnO)₁._x(SnO₂)_x, (CoO)₁._x(SnO₂)_x, (CaO)_Lx(SnO₂J_x, (ZnO)₁JCoO)_x, (MgO)_Lx(SnO₂J_x, and (Mr5O)₁._x(SnO₂)_x (x = 0.01 -0.99) that are metal oxide complexes having a ternary system and Zn₂SnO₄, CoSnO₃, Ca₂SnO₄, CaSnO₃, ZnCo₂O₄, Co₂SnO₄, Mg₂SnO₄, and Mn₂SnO₄ that are compounds;

(2) one composition ratio selected from the group consisting of (ZnO)_x(SnO₂)_y(CoO)_z, (Zn0)_x(Sn0₂)_y(Ca0)_z, (Ti0)_x(Sn0₂)_y(Ca0)_z, (Ti0)_x(Sn0₂)_y(Zn0)_z, (M§0)_x(Sn0₂)_y(Zn0)_z, and (Mn0)_x(Sn0₂)_y(Zn0)_z (x + y + z = 1 ) that are metal oxide complexes having a four component system; or

(3) the composition ratio of (NiO)_a(ZnO)_b(Fe₂O₃)_c(TiO₂)_d(SnO₂)_e (a + b + c + d -ι- e = 1 ) that are metal oxide complexes having a five component system .

15 5. The anode for a secondary battery as claimed in claim 1 ,

wherein the average diameter of the nanofiber is 50 to 900nm, and

wherein the average size of the nanoparticles that constitute the nanofiber is 1 to 100nm.

20 6. The anode for a secondary battery as claimed in claim 1 , wherein the collector is made of

(1 ) Pt, Au, Pd, Ir, Ag, Rh, Ru, Ni, stainless steel, Al, Mo, Cr, Cu, Ti, or W;

(2) indium doped SnO₂ (ITO) or fluorine doped SnO₂ (FTO); or

(3) a metal material formed on a Si wafer.

25

7. A secondary battery, comprising:

an anode including an anode collector and a negative active material compressed onto at least one surface of the anode collector;

an electrolyte; and

5 a cathode,

wherein the negative active material is a web thin layer of a belt-shaped metal oxide nanofiber in which a web of a metal-salt precursor-polymer composite fiber formed by spinning a solution where no less than two kinds of metal-salt precursors and a polymer are mixed with each other is thermal compressed or thermal pressed tolβe thermal processed, and

wherein the nanofiber comprises ternary or higher component componet system metal oxide nanoparticles.

8. The secondary battery as claimed in claim 7, wherein the nanoparticles cclrfiprise at least one of

(1 ) a solid solution of no less than two kinds of metal oxides;

(3) a compound formed of the no less than two kinds of metal oxides.

20

9. A method of fabricating a negative active material for a secondary battery, comprising:

spinning a solution in which no less than two kinds of metal-salt precursors and a polymer are mixed with each other onto an anode collector to form a web of composite fiber in which the no less than two kinds of metal-salt precursors and the polymer are mixed with each other;

performing a thermal compression or thermal pressure process on the web of the composite fiber; and

5 performing thermal treatment on the thermal compressed or thermal pressed web of the composite fiber to remove the polymer from the web of the composite fiber.

10. The method as claimed in claim 9, wherein the no less than two kinds of mfeΦal-salt precursors are selected from the group consisting of precursors containing Sn, Ni, Fe, Co, Ti, Mg, Mn, Ca, Cu, Zn, In, Mo, and W ions.

1 1 . The method as claimed in claim 9, wherein the polymer comprises a less than one selected from the group consisting of polyurethane, polyetherurethane, a pdl^urethane copolymer, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, polymethyl methacrylate, polymethylacrylate, a polyacryl copolymer, polyvinylacetate, a polyvinylacette copolymer, polyvinylalcohol, polyfurfuryl alcohol, polystyrene, a polystyrene copolymer, polyethylene oxide, polypropyleneoxide, a polyethyleneoxide copolymer, a polypropyleneoxide copolymer, polycarbonate, polyvinylsalt, polycaprolactone, polyvinylpyrrolidone, polyvinylfluoride, a polyvinylidenefluoride copolymer, polyamide, polyacrylonitrile, pitch, and phenol resin.

12. The method as claimed in claim 9, wherein the spinning comprises one of ar25electro-spinning method, a melt-blown method, a flash spinning method, or an electrostatic melt-blown method.

13. The method as claimed in claim 9, wherein, in the thermal compression process, the polymer is partially or totally melted by pressing the metal oxide precursors and the polymer at a temperature no less than the glass transition ten5perature of the polymer.

14. The method as claimed in claim 13, wherein the pressure is 0.01 to 20MPa during the thermal compression.

10 1 5. The method as claimed in claim 9, wherein, in the thermal pressure process, the polymer is heated at a temperature no less than the glass transition temperature of the polymer to be melted or is pressed by compression air having a temperature no less than the glass transition temperature of the polymer to be melted.

15

16. The method as claimed in claim 9, wherein the thermal treatment is performed in a temperature range of 400 to 1 ,000°C in accordance with the kinds of the precursors.