WO 2006/025707 PCT/KR2005/002909 ELECTRODE ACTIVE MATERIAL WITH MULTI-ELEMENT BASED OXIDE LAYERS AND PREPARATION METHOD THEREOF Technical Field The present invention relates to an electrode active material comprising a multinary oxide coating layer, a method for preparing the same and an electrode comprising the above electrode active material. Also, the present invention relates to an electrochemical device, preferably a lithium secondary battery, including the above electrode and thus showing high capacity resulting from the application of high . voltage, long service life, excellent structural stability and thermal safety. Background Art Since lithium secondary batteries have been commercialized, the most important object in research and development into batteries is to provide a cathode active material showing excellent electrochemical characteristics including high capacity and long service life. In addition to the above electrochemical characteristics, it is urgently required for a cathode active material to have excellent thermal safety so that a battery system can ensure the safety and reliability even under abnormal conditions such as exposure to heat, combustion or overcharge. Cathode active materials currently used in lithium secondary batteries include composite metal oxides such as LiCo02, LiMn2O4, LiNiO2, LiNii-xCox02 (0 l/2LiCo02 + l/6Co304 + l/6O2 The free oxygen shows high heat-emission property, thereby causing a thermal runaway phenomenon. Further, the free oxygen may cause a highly exothermic reaction with an electrolyte in the battery, resulting in explosion of the battery. Therefore, initiation temperature and heat flow of the reaction, in which oxygen is liberated, should be controlled in order to ensure the battery safety. In one method suggested for controlling the above-heat 2 WO 2006/025707 PCT/KR2005/002909 flow and initiation temperature, a cathode active material is prepared through a pulverization process and classification process so as to control the surface area of the resultant active material. The average voltage range of an active material having a small particle size is not affected by current density (C rate), because the active material has a large surface area. On the other hand, an active material having a large particle size shows a small surface area, and thus shows an increased surface polarity when it is subjected to high rate charge/discharge, resulting in a drop in average voltage range and capacity. In order to improve the safety of a cathode active material during charge/discharge cycles, a method for doping a Ni-based or Co-based lithium oxide with a different element was suggested. For example, Japanese Laid-Open Patent No. 12-149945 discloses an active material for improving the quality of LiNiO2, the active material being represented by the formula of LiNixMyCoz02 (wherein M is at least one selected from Mn and Al, and x+y+z=l). Another method for improving the safety of a cathode active material is based on surface modification of a cathode active material. For example, Japanese Laid-Open Patent No. 9-55210 discloses a cathode active material obtained by coating a lithium nickel-based oxide with an alkoxide of Co, Al or Mn, followed by heat treatment. Additionally, Japanese Laid-Open Patent No. 11-16566 discloses a lithium-based oxide coated with a metal selected from the group consisting of Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B and Mo, or an oxide thereof. However, the above methods according to the prior art cannot increase the initiation temperature where the surface of a cathode active material reacts with an electrolyte (i.e., the exothermic reaction temperature where the oxygen bonded to the metal in the cathode active material is 3 WO 2006/025707 PCT/KR2005/002909 liberated). Moreover, the above methods cannot decrease the amount (heat flow) of oxygen decomposed by such reactions. Ultimately, cathode active materials according to the prior art cannot improve the safety 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 photograph taken by TEM (Transmission Electron Microscope), which shows the electrode active material comprising a multinary oxide coating layer according to Example 1; and FIG. 2 is a graph showing the results of DSC (Differential Scanning Calorimetry) for each of the lithium secondary batteries according to Example 1, Example 2 and Comparative Example 1. Disclosure of the Invention Therefore, the present invention has been made in view of the above-mentioned problems. We have found that when a multinary oxide coating layer comprising a combination of Al, P and a halogen element is formed on the surface of electrode active material particles capable of lithium intercalation/deintercalation, it is possible to solve the problem related with the structural instability of an electrode resulting from progress of lithium intercalation during a charge cycle, as well as to inhibit decomposition of oxygen and to prevent the heat emission caused by a reaction of between free oxygen and an electrolyte, thereby improving the thermal safety at the same time. It is an object of the present invention to provide an WO 2006/025707 PCT/KR2005/002909 electrode active material comprising a multinary oxide coating layer, an electrode using the same electrode active material, and an electrochemical device, preferably a lithium secondary battery, comprising the same electrode. It is another object of the present invention to provide a surface modification method for improving the structural stability and thermal safety of a cathode. According to an aspect of the present invention, there is provided an electrode active material comprising: (a) electrode active material particles capable of lithium intercalation/deintercalation; and (b) a multinary oxide coating layer partially or totally formed on the surface of the electrode active material particles, the multinary oxide coating layer comprising Al, P and a halogen element. There are also provided an electrode using the same electrode active material and an electrochemical device, preferably a lithium secondary battery, including the same electrode. According to another aspect of the present invention, there is provided a method for preparing an electrode active material comprising a multinary oxide coating layer, the method comprising the steps of: (a) dissolving an aluminum precursor compound, phosphorus precursor compound and a halogen precursor compound into a solvent to provide a coating solution; (b) adding electrode active material particles to the coating solution obtained from step (a) and stirring the resultant mixture to cause the electrode active materials to be coated with the coating solution; and (c) heat treating the electrode active material coated in step (b). According to still another aspect of the present invention, there is provided a method for manufacturing an electrode comprising a multinary oxide coating layer, the method comprising the steps of: (a) dissolving an aluminum WO 2006/025707 PCT/KR2005/002909 precursor compound, phosphorus precursor compound and a halogen precursor compound into a solvent to provide a coating solution; (b) applying the coating solution to the surface of a pre-formed electrode or mixing the coating solution with electrode materials to provide an electrode; and (c) drying the electrode. Hereinafter, the present invention will be explained in more detail. The present invention is characterized in that a multinary oxide coating layer is formed on the surface of electrode active material particles capable of lithium intercalation/deintercalation, wherein the multinary oxide coating layer improves structural stability of the electrode so as to permit high-voltage charging/discharging, as well as improves thermal safety of the electrode active material under heat exposure conditions. (1) Conventional electrode active materials, particularly cathode active materials experience a rapid drop in structural stability when the lithium deintercalation amount increases during repeated charge/discharge cycles under high voltage conditions. As a result of this, bonding force between a metal and oxygen in a lithium-containing metal composite oxide is weakened. Therefore, when a battery using the conventional electrode active material is exposed to heat generated due to external and/or internal factors, oxygen is liberated and thus the battery may be ignited. However, the electrode active material according to the present invention can improve the structural stability of an electrode, because the multinary oxide coating layer formed on the surface of electrode active material particles shows excellent doping capability, maintenance and bonding force with oxygen. Therefore, the electrode active material according to the present invention can provide a battery with WO 2006/025707 PCT/KR2005/002909 excellent overall qualities, including high capacity and long service life. Additionally, the multinary oxide coating layer can inhibit liberation of oxygen by virtue of its strong bonding force with oxygen even under a significantly low content of lithium ions during a charge cycle. Therefore, it is possible to prevent a rapid increase in temperature caused by a reaction between oxygen and an electrolyte, thereby contributing to improvement in the thermal safety of a battery. (2) Additionally, the multinary oxide coating layer may be present in an amorphous form, crystalline form or a mixed form thereof. Particularly, when the outermost layer of the coating layer is amorphous, it is possible to inhibit a rapid side reaction between an electrode active material (particularly, a cathode active material) and electrolyte, and to prevent rapid transfer of lithium even under internal short circuit conditions. Therefore, the multinary oxide coating layer according to the present invention can contribute to improvement of battery safety. One component of the multinary oxide coating layer partially or totally formed on the surface of electrode active material particles according to the present invention is a substance that has such a small atom size as to facilitate doping to the surface of electrode active material particles and thus improves the structural stability of an electrode during lithium intercalation progress. Preferably, the first component is aluminum (Al) . Another component of the multinary oxide coating layer may be a substance having strong bonding force to oxygen. Preferably, the second component is phosphorus (P) , because phosphorus can inhibit liberation of oxygen caused by the structural instability of a lithium intercalation compound and can prevent heat emission caused by a reaction of free WO 2006/025707 PCT/KR2005/002909 oxygen with an electrolyte, thereby improving the safety of an electrode (particularly, a cathode). Still another component of the multinary oxide coating layer may be a substance having high electron affinity. Particularly, halogen elements (X) such as fluorine, chlorine, bromine and iodine are preferred as the third component. Etecause halogen atoms can be bonded strongly with oxygen present on the surface of an electrode and with incompletely bonded transition metals (for example, Co, Mn, Ni, etc.) so that the layered structure of the electrode surface can be maintained continuously, they can improve the structural stability and thermal safety of an electrode at the same time. As described above, a preferred composition of the multinary oxide coating layer comprises aluminum, phosphorus and a halogen element, the multinary oxide coating layer being formed on the surface of electrode active material particles to improve the structural stability and thermal safety of the electrode. Any compositions having the same characteristics and providing the same effects as described above may also be used. Additionally, a multinary (higher than ternary) coating layer comprising another element in addition to the above composition of three elements is also included in the scope of the present invention. Preferably, the multinary oxide coating layer partially or totally formed on the surface of electrode active material particles is a compound represented by the following formula 1: [Formula 1] Al1-aPaXbO4-b wherein X is a halogen element, 0