WO 2006/068428 PCT/KR2005/004450 ORGANIC/INORGANIC COMPOSITE MICBOPOROUS MEMBRANE AND ELECTROCHEMICAL DEVICE PREPARED THEREBY Technical Field The present invention relates to a novel organic/inorganic composite porous separator that can show excellent thermal safety, electrochemical safety and lithium ion conductivity and a high degree of swelling with electrolyte, compared to conventional polyolefin-based separators, and an electrochemical device comprising the same, which ensures safety and has improved quality. 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 the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a drive voltage and an energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni-Cd batteries and H2SO4-Pb batteries) , and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have problems related to their safety, due to ignition and explosion caused by the use of organic electrolytes, and are manufactured by a complicated process. Lithium ion polymer batteries, appearing more recently, solve the above-mentioned disadvantages of 1 WO 2006/068428 PCT/KR2005/004450 secondary lithium ion batteries, and thus become one of the most potent candidates of next generation batteries. However, such secondary lithium ion polymer batteries still have low capacity compared to secondary lithium ion batteries. Particularly, they show insufficient discharge capacity at low temperature. Hence, there is an imminent need for the improvement of secondary lithium ion batteries. A lithium ion battery is manufactured by coating a cathode active material (e.g. LiCoO2) and an anode active material (e.g. graphite), which have crystal structures including interstitial volumes, onto the corresponding current collector (i.e. aluminum foil and copper foil, respectively) to provide a cathode and an anode. Then, a separator is interposed between both electrodes to form an electrode assembly, and an electrolyre is injected into the electrode assembly. During a charge cycle of the battery, lithium intercalated into the crystal structure of the cathode active material is deintercalated, and then intercalated into the crystal structure of the anode active material. On the other hand, during a discharge cycle, lithium intercalated into the anode active material is deintercalated again, and then intercalated back into the crystal structure of the cathode. As charge/discharge cycles are repeated, lithium ions reciprocate between the cathode and the anode. In this regard, a lithium ion battery is also referred to as a rocking chair battery. Such batteries have been produced by many battery producers. However, most lithium secondary batteries have different safety characteristics depending on several factors. Evaluation of and security in safety of batteries are very important matters to be considered. Particularly, users should be protected from being damaged by 2 WO 2006/068428 PCT/KR2005/004450 malfunctioning batteries. Therefore, sarety of batteries is strictly restricted in terms of ignition and combustion of batteries by safety standards. Many attempts have been made to solve the problem related to the safety of a battery. However, ignition of a battery, caused by a forced internal short circuit due to external impacts (particularly, in the case of a customer- abused battery) cannot be solved yet. Recently, US Patent No. 6,432,586 discloses a polyolefin-based separator coated with an inorganic layer such as calcium carbonate, silica, etc., so as to prevent an internal shorn circuit, caused by dendrite growth inside of a battery. However, the polyoiefin-based separator merely using conventional inorganic particles cannot provide significant improvement in the safety of a battery, when the battery experiences an internal short circuit due to external impacts. There is no mechanism for preventing such problem in the separator. Additionally, the inorganic particle layer disclosed in the above patent is not particularly defined in terms of the thickness, pore size and porosity. Moreover, the inorganic particles used in the separator have no lithium conductivity, and thus cause a significant drop in the quality of a battery. 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 schematic view showing an organic/inorganic composite porous separator according to the present invention, and the function thereof in a battery; 3 WO 2006/068428 PCT/KR2005/004450 FIG. 2a and FIG. 2b are photographs taken by a Scanning Electron Microscope (SEM) showing the organic/inorganic composite porous separator (PVdF-CTFE/BaTiCb) according to Example 1, wherein FIG. 2a and FIG. 2b show the active layer and separator substrate, respectively; FIG. 3 is a photograph taken by SEM showing the composite separator according to Comparative Example 2, wherein the composite separator comprises inorganic particles and a polymer, the polymer being present in a higher proportion than the inorganic particles; FIG. 4 is a graph showing variations in ion conductivity depending on the mixing ratio of inorganic particles and a binder polymer that form the organic/inorganic composite porous separator according to the present invention; FIG. 5a and FIG. 5b are photographs showing the results for a heat shrinking test of separators, wherein FIG. 5a and FIG. 5b show a currently used PE separator, and the organic/inorganic composite porous separator (PVdF- CTFE/BaTiO3) according to Example 1, respectively, after each of the separators is maintained at 150 °C for 1 hour; FIG. 6a and FIG. 6b are photographs showing the results for a pseudo internal short circuit test of separators, wherein FIG. 6a and FIG. 6b show a currently used PE separator, and the organic/inorganic composite porous separator (PVdF-CTFE/BaTiO3) according to Example 1, respectively; FIG. 7 is a graph showing variations in voltage of each of the lithium secondary batteries including the organic/inorganic composite porous separator (PVdF- CTFE/BaTiO3) according to Example 1 and the currently used PE 4 WO 2006/068428 PCT/KR2005/004450 separator according to Comparative Example 1, after local crush that causes an artificial internal short circuit; FIG. 8a and FIG. 8b are photographs showing the results for the battery safety test, after local crush that causes an artificial internal short circuit, wherein FIG. 8a and FIG. 8b show the currently used PE separator according to Comparative Example 1 and the organic/inorganic composite porous separator (PVdF-CTFE/BaTiO3) according to Example 1, respectively; and FIG. 9a and FIG. 9b are graphs showing the results for the safety test of batteries after overcharge, wherein FIG. 9a and FIG. 9b show the currently used PE separator according to Comparative Example 1 and the organic/inorganic composite porous separator (PVdF-CTFE/BaTiO3) according to Example 1, respectively. Disclosure of the Invention The present inventors have found that an organic/inorganic composite porous separator, formed by using (1) a polyolefin-based separator substrate, (2) inorganic particles and (3) a binder polymer, improves thermal safety of a conventional polyolefin-based separator. Additionally, we have found that because the organic/inorganic composite porous separator has pore structures present both in the polyolefin-based separator substrate and in an active layer formed of the inorganic particles and the binder polymer coated on the separator substrate, it provides an increased volume of space, into which a liquid electrolyte infiltrates, resulting in improvements in lithium ion conductivity and degree of swelling with electrolyte. Therefore, the organic/inorganic composite porous separator can improve the 5 WO 2006/068428 PCT/KR2005/004450 quality and safety of an electrochemical device using the same as a separator. We have also found that when inorganic particles having piezoelectricity derived from a high dielectric constant and/or inorganic particles having lithium ion conductivity are used as the inorganic particles that form the active layer, it is possible to prevent a complete short circuit between both electrodes by the inorganic particles, even if the separator in a battery is broken due to external impacts. It is also possible to solve the problem related to safety, such as explosion of a battery, by reducing the voltage of a battery gradually due to the flow of electric current, caused by the lithium conductivity and/or piezoelectricity of the inorganic particles. Therefore, it is an object of the present invention to provide an organic/inorganic composite porous separator, a method for manufacturing the same and an electrochemical device comprising the same. According to an aspect of the present invention, there is provided an organic/inorganic composite porous separator, which comprises (a) a polyolefin-based separator substrate; and (b) an active layer formed by coating at least one region selected from the group consisting of a surface of the substrate and a part of pores present in the substrate with a mixture of inorganic particles and a binder polymer, wherein the inorganic particles in the active layer are interconnected among themselves and are fixed by the binder polymer, and interstitial volumes among the inorganic particles form a pore structure. There is also provided an electrochemical device (preferably, a lithium secondary battery) comprising the same. 6 WO 2006/068428 PCT/KR2005/004450 According to another aspect of the present invention, there is provided a method for manufacturing an organic/incrganic composite porous separator, which includes the steps of: (a) dissolving a binder polymer into a solvent to form a polymer solution; (b) adding inorganic particles having lithium ion conductivity to the polymer solution obtained from step (a) and mixing them; and (c) coating the mixture of inorganic particles with a binder polymer obtained from step (b) onto 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, followed by drying. Hereinafter, the present invention will be explained in more detail. The present invention is characterized by providing a novel organic/inorganic composite porous separator, which shows excellent thermal safety, electrochemical safety and lithium ion conductivity, and a high degree of swelling with electrolyte, compared to a polyolefin-based separator currently used as a separator for batteries. The organic/inorganic composite porous separator is obtained by coating an active layer comprising inorganic particles and a binder polymer on a polyolefin-based separator substrate. Herein, the pores present in the separator substrate itself and a uniform pore structure formed in the active layer by the interstitial volumes among the inorganic particles permit the organic/inorganic composite porous separator to be used as a separator. Additionally, if a polymer capable of being gelled when swelled with a liguid electrolyte is used as the binder polymer component, the organic/inorganic composite porous separator can serve also as an electrolyte. 7 WO 2006/068428 PCT/KR2005/004450 Particular characteristics of the organic/ inorganic composite porous separator are as follows. (1) Conventional composite separators, formed by coating inorganic particles or a mixture of inorganic particles and a binder polymer onto a conventional polyclefin separator have no pore structure or, if any, have an irregular pore structure having a pore size of several angstroms. Therefore, they cannot serve sufficiently as spacers, through which lithium ions can pass (see FIG.3). Additionally, in order to form a microporous structure, most of such conventional separators are subjected to extraction with a plasticizer so that a microporous structure can be formed in a gel type polymer electrolyte, resulting in degradation in the quality of a battery. On the contrary, the organic/inorganic composite porous separator according to the present invention has uniform pore structures both in the active layer and the polyolefin-based separator substrate, as shown in FIGs. 2 and 3, and the pore structures permit lithium ions to move smoothly therethrough. Therefore, it is possible to introduce a large amount of electrolyte through the pore structures, so as to obtain a high degree of swelling with electrolyte, resulting in improvement in the quality of a battery. (2) Although conventional polyolefin-based separators cause heat shrinking at high temperature because they have a melting point of 120-140°C (see FIG.5a), the organic/inorganic composite porous separator, comprising the inorganic particles and the binder polymer, does not cause heat shrinking due to the heat resistance of the inorganic particles (see FIG.5b). Therefore, an electrochemical device using the above organic/inorganic composite porous separator prevents a complete internal short circuit between a cathode 8 WO 2006/068428 PCT/KR2005/004450 and an anode by the organic/inorganic composite porous active layer, even when the separator is broken under extreme conditions caused by internal or external factors, such as high temperature, overcharge, external impacts, etc. Even if a short circuit occurs, the region of short circuit can be inhibited from extending throughout: the battery. As a result, it is possible to significantly improve the safety of a battery. (3) Conventional separators or polymer electrolytes are formed in the shape of free standing films and then assembled together with electrodes. On the contrary, the organic/inorganic composite porous separator according to the present invention is formed by coating the active layer directly on the surface of a polyolefin-based separator substrate, so that the pores on the surface of the polyolefin-based separator substrate and the active layer can be anchored to each other, thereby providing a firm physical bonding between the active layer and the porous substrate. Therefore, problems related to mechanical properties such as brittleness can be improved. Additionally, such increased interfacial adhesion between the porous substrate and the active layer can decrease the interfacial resistance. In fact, the organic/inorganic composite porous film according to the present invention includes the organic/inorganic composite active layer bonded organically to the polyolefin- based separator substrate. Additionally, the active layer does not affect the pore structure present in the polyolefin- based substrate, so that the structure can be maintained. Further, the active layer itself has a uniform pore structure formed by the inorganic particles (see FIGs. 2 and 3). Because the above-mentioned pore structures are filled with a liquid electrolyte injected subsequently, interfacial 9 WO 2006/068428 PCT/KR2005/004450 resistance generated among the inorganic particles or between the inorganic particles and the binder polymer can be decreased significantly. (4) Polyolefin-based separators coated with a layer comprising a metal oxide or a mixture of a metal oxide with a polymer have been disclosed according to the prior art. However, most of such conventional separators comprise no binder polymer for supporting and interconnecting inorganic particles. Even if a polymer is used in such conventional separators, the polymer should have been used in a great amount, so that such conventional separators have no pore structures or have a non-uniform pore region in the polymer, and thus cannot serve sufficiently as separators, through which lithium ions can pass (see, FIG. 4) . Additionally, there is no correct understanding with regard to the physical properties, particle diameter and homogeneity of the inorganic particles and a pore structure formed by the inorganic particles. Therefore, such separators according to the prior art have a problem in that they cause degradation in the quality of a battery. More particularly, when the inorganic particles have a relatively large diameter, the thickness of an organic/inorganic coating layer obtained under the same solid content increases, resulting in degradation in mechanical properties. Additionally, in this case, there is a great possibility of internal short circuit during charge/discharge cycles of a battery due to an excessively large pore size. Further, due to the lack of a binder that serves to fix the inorganic particles on the substrate, a finally formed film is deteriorated in terms of mechanical properties, and is not suitable to be applied in a practical battery assemblage process. For example, 10 WO 2006/068428 PCT/KR2005/004450 conventional separators according to the prior art may not be amenable to a lamination process. On the contrary, the present inventors have recognized that controlling the porosity and pore size of the organic/inorganic composite porous separator is one of the factors affecting the quality of a battery. Therefore, we have varied and optimized the particle diameter of the inorganic particles or the mixing ratio of the inorganic particles with the binder polymer. In fact, it was shown by the following Experimental Examples chat the organic/inorganic composite porous separator according to the present invention, which comprises a pore structure formed by the interstitial volumes among the inorganic particles on the polyolefin-based separator substrate, has a significantly higher ion conductivity, as compared to a conventional composite separator having an artificial pore structure formed in a polymer film on the polyolefin-based separator substrate (see FIG. 4). Additionally, according to the present invention, the binder polymer used in the active layer can serve sufficiently as a binder so as to interconnect and stably fix the inorganic particles among themselves, between the inorganic particles and the surface of the heat resistant porous substrate, and between the inorganic particles and a part of the pores in the substrate, thereby preventing degradation in mechanical properties of a finally formed organic/inorganic composite porous separator. (5) The organic/inorganic composite porous separator according to the present invention can provide excellent adhesion by controlling the mixing ratio of the components forming the active layer, i.e. the mixing ratio of the inorganic particles with the binder polymer. Therefore, it is possible to facilitate assemblage of a battery. 11 WO 2006/068428 PCT/KR2005/004450 In the organic/inorganic composite porous film according to the present invention, one component present in the active layer formed on the surface of the polyolefin- based separator substrate or on a part of the pores in the substrate is inorganic particles currently used in the art. The inorganic particles permit an interstitial volume to be formed among them, thereby serving no form micropores and to maintain the physical shape as a spacer. Additionally, because the inorganic particles are characterized in that their physical properties are not changed even at a high temperature of 200 "C or higher, the organic/inorganic composite porous separator using the inorganic particles can have excellent heat resistance. There is no particular limitation in 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 ion conductivity and quality in an electrochemical device. Additionally, when inorganic particles having a high density are used, they have a difficulty in dispersion 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 12 WO 2006/068428 PCT/KR2005/004450 salt, thereby improving the ion conductivity of the electrolyte. For these reasons, it is preferable to use inorganic particles having a high dielectric constant of 5 or more, preferably of 10 or more, inorganic particles having lithium conductivity, inorganic particles having piezoelectricity, or mixtures thereof. In general, a material having piezoelectricity means one, which is an insulator under normal pressure, but allows current flow due to the change of its internal structure, when a certain range of pressure is applied thereto. The inorganic particles having piezoelectricity show a high dielectric constant of 100 or more. They are charged positively on one surface while being charged negatively on the other surface, when they are drawn or compressed under the application of a certain range of pressure. Hence, the inorganic particles having piezoelectricity cause an electric potential difference between both surfaces thereof. When the inorganic particles having the above characteristics are used in the porous active layer, a cathode and an anode are prevented from being in direct contact with each other by the inorganic particles coated on the separator, when an internal short circuit occurs between both electrodes due to external impacts such as local crush, a nail, or the like. Additionally, as shown in FIG. 1, such piezoelectricity of the inorganic particles can permit generation of a potential difference in the particles, thereby allowing electron movements, i.e. minute flow of electric current between both electrodes. Therefore, it is possible to accomplish a slow decrease in the voltage of a battery and to improve the safety of a battery (see FIG. 7). Heretofore, separators coated with conventional inorganic 13 WO 2006/068428 PCT/KR2005/004450 particles could prevent explosion of a battery due to the inorganic particles, when an internal short circuit occurred between both electrodes by external impacts. However, in the case of a battery using such conventional separators, the battery is present practically in a state of latent danger, because it is internally damaged but maintains the potential between both electrodes due to the lack of the electroconductivity of the inorganic particles. Thus, the battery may have a possibility of ignition or explosion with time, or when a secondary impact is applied thereto. In the organic/inorganic composite porous separator according to the present invention, the above-mentioned problems can be solved satisfactorily. Particular non-limiting examples of the inorganic particles having piezoelectricity include BaTiO3, Pb(Zr,Ti)O3 (PZT) , Pbi-xLaxZri-yTiyOs (PLZT), PB (Mg3Nb2/3) O3-PbTiO3 (PMN-PT), hafnia (HfO2) , 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 move lithium ions due to defects present in their structure, and thus can improve lithium ion conductivity of a battery 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;GeyPzSw, 0