WO 2006/062349 PCT7KR2005/004174 1 SURFACE-TREATED MICROPOROUS MEMBRANE AMD ELECTROCHEMICAL DEVICE PREPARED THEREBY Technical Field The present invention relates to a porous film, surfaced-treated with a polymer capable of improving adhesion to other substrates, scratch resistance and wear resistance. The present invention also relates to an electrochemical device comprising the above porous film as a separator. Background Art Recently, there is increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Among the currently available secondary batteries, lithium secondary batteries, developed in the early 1990's, have a drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni-Cd batteries and H2SO4-Pb batteries). Lithium secondary batteries have been spotlighted due to the above-mentioned advantages. In general, a lithium secondary battery is manufactured by forming an assembly of an anode, a cathode, and a separator interposed between both electrodes. In the above assembly, the separator interposed between both electrodes of the battery is a member that serves to prevent an internal short circuit caused by direct contact between the cathode and anode. Also, the separator serves as an ion WO 2006/062349 PCT7KR2005/004174 2 flow path in the battery, and contributes to the improvement of battery safety. However, conventional batteries, manufactured in the same manner as described above by using a polyolefin-based separator, frequently cause the problems of poor adhesion and separation between a separator and electrodes, and inefficient lithium ion transfer through the pores of the separator, resulting in degradation in the quality of a battery. Additionally, conventional separators are formed from a chemically stable material, which is not decomposed and does not allow any reaction upon exposure to the oxidative or reductive atmosphere inside a battery, such as polyolefin or fluoropolymer. However, such materials provide insufficient mechanical strength, and thus cause the problems of peel-off or breakage of a separator during the assemblage of a battery, resulting in a drop in the battery safety, caused by an internal short circuit of the battery. Further, conventional separators are coated with inorganic particles in order to improve the heat resistance and to provide a high dielectric constant. However, due to the poor binding force between the separator and inorganic particles, the particles are detached from the separator, and thus it is not possible to obtain desired effects. Brief Description of the Drawings The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a photograph showing the results of evaluation for the adhesion between an electrode and the organic/inorganic composite porous film (BaTiO3/PVdF-HFP) WO 2006/062349 PCT/KR2005/004174 3 coated with styrene-butadiene rubber (SBR) according to Example 1, after laminating the electrode and the porous film; FIG. 2 is a photograph showing the results of evaluation for the adhesion between an electrode and the organic/inorganic composite porous film (BaTiO3/PVdF-HFP) according to Comparative Example 1, after laminating the electrode and the porous film; FIG. 3 is a photograph showing the results of the peeling test performed by using the organic/inorganic composite porous film (BaTiO3/PVdF-HFP) coated with styrene- butadiene rubber (SBR) according to Example 1; and FIG. 4 is a photograph showing the results of the peeling test performed by using the organic/inorganic composite porous film (BaTiO3/PVdF-HFP) according to Comparative Example 1. Disclosure of the Invention Therefore, the present invention has been made in view of the above-mentioned problems. The present inventors have found that when a separator is overcoated with styrene- butadiene rubber (SBR) that imparts excellent adhesion and mechanical strength, on a surface of the separator, or on a part of pores present in the separator, the separator shows improved adhesion to other substrates, preferably to electrodes, and is prevented from peeling-off and breaking during the assemblage of an electrochemical device, so that an electrochemical device using the same separator can provide improved safety and can be prevented from degradation in the quality. Therefore, it is an object of the present invention to provide a porous film coated with styrene-butadiene rubber WO 2006/062349 PCT/KR2005/004174 4 having excellent adhesion and mechanical strength. It is another object of the present invention to provide a method for manufacturing the above porous film. It is still another object of the present invention to provide an electrochemical device using the above porous film as a separator. According to an aspect of the present invention, there is provided a porous film comprising: (a) a porous substrate having pores; and (b) a coating layer formed on at least one region selected from the group consisting of a surface of the substrate and a part of the pores present in the substrate, wherein the coating layer comprises styrene-butadiene rubber. The present invention also provides an electrochemical device using the above porous film as a separator. Hereinafter, the present invention will be explained in more detail. The porous film according to the present invention is characterized in that the surface of the porous substrate and/or a part of the pores present in the substrate is coated with styrene-butadiene rubber. Such coated porous film can improve the safety of a battery and prevent degradation in the quality of a battery by virtue of the physical properties of styrene-butadiene rubber. (1) The porous film coated with styrene-butadiene rubber on the surface of the porous substrate having pores and/or on a part of the pores present in the porous substrate can improve the safety of a battery. As described above, conventional separators generally use polyolefin polymers. However, polyolefin polymers have insufficient mechanical strength, and thus cause the problems of peel-off and breakage of separators during the assemblage of a battery, resulting in degradation in the safety of a battery, caused by an internal short circuit, or the like. WO 2006/062349 PCT/KR2005/004174 S On the contrary, the porous film according to the present invention has improved scratch resistance and maintains the pore structure present in the film over a longer period of time, by virtue of the rubbery characteristics provided by low glass transition temperature (Tg) of styrene-butadiene rubber. Therefore, an electrochemical device comprising the porous film as a separator can provide improved safety. Additionally, when the styrene-butadiene rubber used in the porous film comprises a hydrophilic functional group, the porous film can show more improved adhesion. Hence, the porous film according to the present invention maintains to be in close contact with other substrates (e.g. electrodes) continuously, so that both electrodes can be prevented from being in direct contact with each other due to a drop in external stress and degradation in the thermal safety of a separator, caused by internal or external factors. Therefore, it is possible to prevent an internal short circuit. Further, as described above, when inorganic particles are dispersed or coated on a conventional polyolefin-based separator in order to improve the heat resistance and conductivity, the inorganic particles coated on the separator are detached from the separator, and thus it is not possible to obtain desired effects. However, in the porous film according to the present invention, a styrene-butadiene rubber coating layer is introduced onto an organic/inorganic composite porous film having a pore structure formed by interstitial volumes of the inorganic particles, while maintaining the pore structure as it is. Therefore, it is possible to realize excellent adhesive property provided by styrene-butadiene rubber, while maintaining the effects of improving heat resistance and mechanical strength, provided WO 2006/062349 PCT/KR2005/004174 6 by the inorganic particles. Particularly, when styrene- butadiene rubber is coated on the surface of the porous film and infiltrates into a part of the pores present in the film, it is possible to generate synergy of the above effects. (2) The porous film coated with styrene-butadiene rubber on the surface of the porous substrate having pores and/or on a part of the pores present in the porous substrate can prevent degradation in the quality of a battery. In a conventional process for assembling a battery, for example, by interposing a separator between a cathode and an anode of a battery, the electrodes and separator are frequently separated from each other due to poor adhesion between them. Thus, during the electrochemical reaction in the battery, lithium ion transfer cannot be performed efficiently through the pores of the separator, resulting in degradation in the quality of a battery. However, in the porous film coated with styrene- butadiene rubber according to the present invention, it is possible to provide excellent adhesion by controlling the kinds and amounts of monomers during the preparation of the styrene-butadiene rubber. Therefore, continuous lithium ion transfer can be maintained, during the electrochemical reaction in the battery as well as the process for assembling a battery, due to the close contact between the porous film and electrodes, so that degradation in the battery quality can be prevented. (3) The porous film according to the present invention is obtained by coating (i) a porous substrate having pores; (ii) an organic/inorganic composite porous film, which comprises a porous film having pores, coated with a coating layer comprising a mixture of inorganic particles with a binder polymer, on the surface of the porous substrate and/or WO 2006/062349 PCT/KR2005/004174 7 on a part of the pores present in the porous substrate; and (iii) an organic/inorganic composite porous film comprising inorganic particles and a binder polymer coating layer partially or totally formed on the surface of the inorganic particles, directly with styrene-butadiene rubber. Hence, the inorganic particles are linked and fixed among themselves by the pores present on the surface of the porous substrate and the binder polymer. Additionally, interstitial volumes of the inorganic particles permit the pore structure of the active layer type or freestanding type organic/inorganic composite porous film to be maintained as it is, and the pore structure and the styrene-butadiene rubber coating layer are bonded physically and firmly with each other. Therefore, it is possible to solve the problem of poor mechanical properties, such as brittleness. Additionally, a liquid electrolyte, injected through the pore structure subsequently, significantly reduces the interfacial resistance generated among the inorganic particles and between the inorganic particles and the binder polymer. Further, smooth lithium ion transfer can be accomplished through the pores and a larger amount of electrolyte can be injected through the pore structure, resulting in improvement of the battery quality. In addition to the above advantages, a separator using the porous film according to the present invention can be prevented from peeling-off and breaking. Hence, it is possible to increase the processability during the assemblage of a battery. The coating materials for the porous film according to the present invention include styrene-butadiene rubber known to one skilled in the art, with no particular limitation. Styrene-butadiene rubber (SBR) is preferred because it shows a low infiltration ratio to an electrolyte, and thus has WO 2006/062349 PCT7KR2005/004174 8 little possibility of dissolution or deformation inside a battery. Particularly, SBR having a glass transition temperature (Tg) of room temperature (25*0) or less is preferred. Styrene-butadiene rubber (SBR) can be controlled in terms of physical properties so as to be present in a glassy state or rubbery state by adjusting the mixing ratio of a styrene group-containing monomer and a butadiene group- containing monomer, and thus helps to improve the scratch resistance of a separator and safety of a battery. Additionally, SBR may comprise various kinds and amounts of monomers having hydrophilic functional groups that can form hydrogen bonds with other substrates (e.g. electrodes) to increase the adhesion. Therefore, SBR can provide improved adhesion to an electrode. Considering the above characteristics, SBR that may be used in the present invention preferably has at least one hydrophilic functional group selected from the group consisting of maleic acid, acrylic acid, acrylate, carboxylate, nitrile, hydroxy, mercapto, ether, ester, amide, amine and acetate groups, and halogen atoms. Styrene-butadiene rubber that may be used in the present invention includes, but is not limited to, SBR obtained by polymerizing: (a) a butadiene group-containing monomer and a styrene group-containing monomer; or (b) a butadiene group-containing monomer, a styrene group- containing monomer and a hydrophilic group-containing monomer known to one skilled in the art, in a conventional manner currently used in the art. There is no particular limitation in the hydrophilic group-containing monomer, and non-limiting examples thereof include monomers containing at least one hydrophilic functional group selected from the group WO 2006/062349 PCT7KR2005/004174 9 consisting of maleic acid, acrylic acid, acrylate, carboxylic acid, nitrile, hydroxyl and acetate groups. Herein, the mixing ratio of the styrene group- containing monomer to the butadiene group-containing monomer ranges from 1:99 to 99:1, but is not limited thereto. Preferably, the styrene-butadiene rubber has a styrene group content of 50 wt% or less. Although there is' no particular limitation in the average molecular weight (MW) of the styrene-butadiene rubber, SBR preferably has a molecular weight of 10,000 —1,000,000. Also, there is no particular limitation in the form of SBR rubber, SBR rubber is present preferably in the form of an emulsion obtained by solution copolymerization. Because SBR may be used directly in the form of an emulsion or after dispersing it into water, an additional organic solvent and an additional step for removing the same are not required. The SBR coating layer formed on the porous film preferably has a thickness of 0.001~-10 micrometers, but is not limited thereto. If the thickness is less than 0.001 \mf it is not possible to improve the adhesion and mechanical strength sufficiently. On the other hand, if the thickness is greater than 10 im, the SBR coating layer may serve as a resistance layer, resulting in degradation in the quality of a battery. The coating layer formed on the porous film according to the present invention may further comprise other additives known to one skilled in the art, in addition to SBR. Non- limiting examples of such additives include a thickening agent or a silane coupling agent that can enhance the binding force. The substrate to be coated with SBR according to the WO 2006/062349 PCT/KR2005/004174 10 present invention includes any porous substrate as long as it serves as a lithium ion flow path and as a space for receiving an electrolyte, regardless of the constitutional elements and composition of the substrate. The porous substrate may be classified broadly into the following three types, but is not limited thereto. The first type is (a) a conventional separator known to one skilled in the art. The second type is (b) an organic/inorganic composite porous film, which comprises a porous film having pores, coated with a coating layer comprising a mixture of inorganic particles with a binder polymer, on the surface 'of the porous substrate and/or on a part of the pores present in the porous substrate. The third type is (c) an organic/inorganic composite porous film comprising inorganic particles and a binder polymer coating layer partially or totally formed on the surface of the inorganic particles. Combinations of the above types of separators may be used. Herein, the inorganic/organic composite porous films {b) and (c) comprise the inorganic particles linked and fixed among themselves by the binder polymer, and have a pore structure formed by interstitial volumes of the inorganic particles. Particularly, the inorganic/organic composite porous films (b) and (c) are preferred, because such porous films have little possibility of a complete short circuit between both electrode due to the presence of the inorganic particles, even if the styrene-butadiene surface coating layer is partially or totally broken in a battery by the external or internal factors. Even if any short circuit is generated, the short circuit zone is inhibited from being extended by the inorganic particles, resulting in improvement of the safety of a battery. In cases of the separator (a) and organic/inorganic WO 2006/062349 PCT/KR2005/004174 11 composite porous film (b), non-limiting examples of the porous substrate include polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetherether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfidror polyethylene naphthalene, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene, polypropylene, or combinations thereof. However, other polyolefin-based substrates known to one skilled in the art may be used. The porous substrate used in the separator (a) and the organic/inorganic composite porous film (b) may take the form of a membrane or fiber. When the porous substrate is fibrous, it may be a nonwoven web forming a porous web (preferably, spunbond type web comprising long fibers or melt blown type web) . Although there is no particular limitation in the thickness of the porous substrate used in the separator (a) and the organic/inorganic composite porous film (b), the porous substrate preferably has a thickness of between 1 μm and 100 μM, more preferably of between 5 w and 50 /on. Although there is no particular limitation in the pore size and porosity of the porous substrate, the porous substrate preferably has a porosity of between 5% and 99%. The pore size (diameter) preferably ranges from 0.01 μm to 50 μm, more preferably from 0.1 /an to 20 μm. Among the above-described three types of porous substrates, the organic/inorganic composite porous film (b) comprises a porous substrate having pores, coated with a mixture of inorganic particles with a binder polymer, while the organic/inorganic composite porous film (c) is a free WO 2006/062349 PCTYKR2005/004174 12 standing film comprising inorganic particles and a binder polymer. These types of porous substrates permit interstitial volumes to be formed among the inorganic particles, thereby serving to form micropores and to maintain the physical shape as a spacer. Herein, the binder polymer serves to fix the inorganic particles and link the inorganic particles among themselves. There is no particular limitation in selection of the inorganic particles, as long as they are electrochemically stable. In other words, there is no particular limitation in the inorganic particles that may be used in the present invention, as long as they are not subjected to oxidation and/or reduction at the range of drive voltages (for example, 0-5 V based on Li/Li+) of a battery, to which they are applied. Particularly, it is preferable to use inorganic particles having ion conductivity as high as possible, because such inorganic particles can improve the quality of an •electrochemical device by increasing the ion conductivity in an electrochemical device. Additionally, when inorganic particles having a high density are used, they are not readily dispersed during a coating step and may increase the weight of a battery to be manufactured. Therefore, it is preferable to use inorganic particles having a density as low as possible. Further, when inorganic particles having a high dielectric constant are used, they can contribute to increase the dissociation degree of an electrolyte salt in a liquid electrolyte, such as a lithium salt, thereby improving the ion conductivity of the electrolyte. Further, because the inorganic particles are characterized by their physical properties that are not changed even at a high temperature of 2001; or higher, the organic/inorganic composite porous film using the inorganic particles can have excellent heat WO 2006/062349 PCT/KR2005/004174 13 resistance. For these reasons, the inorganic particles that may be used in the organic/inorganic composite porous films (b) and (c) are selected from conventional the inorganic particles having a high dielectric constant of 5 or more, preferably of 10 or more, inorganic particles having lithium conductivity, or mixtures thereof. This is because such inorganic particles can improve the safety of a battery and can prevent degradation in the battery quality due to their heat resistance and conductivity. Particular non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb1-xLaxZr1-yTiyO3 (PLZT), PB (Mg3Nb2/3) O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TIO2 , SiC, or mixtures thereof. As used herein, "inorganic particles having lithium ion conductivity" refer to inorganic particles containing lithium elements and having a capability of conducting- lithium ions without storing lithium. Inorganic particles having lithium ion conductivity can conduct and transfer lithium ions due to defects present in their structure, and thus can improve lithium ion conductivity and contribute to improve the quality of a battery. Non-limiting examples of such inorganic particles having lithium ion conductivity include: lithium phosphate (Li3PO4); lithium titanium phosphate {LixTiy(PO4)3, 0