WO 2005/013395 PCT/KR2004/001897 LITHIUM ION BATTERY HAVING AN IMPROVED CONSERVED PROPERTY AT A HIGH TEMPERATURE Technical Field The present invention relates to a cathode for a battery comprising a cathode additive for improving storage properties at a high temperature, and a lithium ion battery comprising the same. Background Art In general, a lithium ion battery is used at a high drive voltage (0 to 5V) . Therefore, when a lithium ion battery is exposed under a high temperature (40°C) for a long time after being charged completely, self-discharge may occur due to a high voltage difference between a cathode and an anode. Additionally, a cathode has reactivity to a non-aqueous electrolyte so that decomposition may be generated and thus the capacity of a battery may decrease and the impedance of a battery may rapidly increase. This has been one of the most serious problems of lithium ion batteries. To solve this, many attempts have been made to decrease the reactivity of an electrode to an electrolyte by adding a small amount of additives to an anode, an electrolyte or a cathode, or by appling a coating layer formed of inorganic materials or organic materials on the powder surface of a cathode or an anode. Additionally, Japanese Patent Laid-Open No. 98-255839 discloses that alkaline earth metal hydroxides are partially incorporated into a cathode active material to prevent reduction of battery capacity after storage at a high temperature. 1 WO 2005/013395 PCT/KR2004/001897 However, there is no description with regard to the incorporation of any other metal hydroxides except for alkaline earth metal hydroxides in a cathode active material, for the purpose of improving storage properties of a battery at a high temperature. Further, although metal hydroxides are good additives for improving storage properties of a battery at a high temperature, metal hydroxides are nonconductors by nature. Accordingly, if metal hydroxides are added to cathode active materials in an excessive amount, they may decrease battery capacity and degrade storage properties of a battery at a high temperature. Additionally, because metal hydroxides are not capable of lithium ion intercalation/ deintercalation, increase of the amount of metal hydroxides added to a cathode of a battery results in decrease of the amount of a cathode active material that may be incorporated into the cathode, thereby causing decrease of battery capacity. Therefore, in order to minimize decrease of battery capacity, it is necessary to minimise the amount of metal hydroxides added to a cathode of a battery. However, it has remained undiscovered heretofore that the specific surface area of a metal hydroxide added to a cathode of a battery is related with storage properties of a battery at a high temperature, and that a small amount of metal hydroxide may be used to improve storage properties of a battery at a high temperature, based on such relationship. Disclosure of the Invention This inventors have found that storage properties of a battery at a high temperature can be improved by adding a 2 WO 2005/011395 PCT/KR2004/001897 metal hydroxide to the cathode of a battery as a cathode additive, and such improvement is related with the specific surface area of powder of metal hydroxides. In other words, when a metal hydroxide having a large specific surface area is added to a cathode of a battery as a cathode additive, storage properties of a battery at a high temperature can be improved even if the metal hydroxide is used in a relatively small amount. As a result, decrease of battery capacity due to the addition of metal hydroxides can be minimized. Therefore, the present invention has been made based on this finding, and it is an object of the present invention to provide a cathode for a battery comprising a metal hydroxide having a large specific surface area as a cathode additive, and a lithium ion battery comprising the same. According to an aspect of the present invention, there is provided a cathode for a battery comprising a metal hydroxide having a large specific surface area as a cathode additive. According to another aspect of the present invention, there is provided a lithium ion battery comprising a cathode, an anode and a non-aqueous electrolyte, wherein the cathode comprises a metal hydroxide having a large specific surface area as a cathode additive. Hereinafter, the present invention will be explained in detail. As described above, this inventors have found that use of a metal hydroxide as a cathode additive improves storage properties of a battery at a high temperature, and the larger the specific surface area of powder of a metal hydroxide is, the better such improvement becomes. It is considered that this results from the effect of the surface of a metal 3 WO 2005/013395 PCT/KR2004/001897 hydroxide on other substances in a battery such as an electrolyte. It seems that the aforesaid effect may prevent the generation of substances increasing battery resistance such as LiF, HF, etc. According to this mechanism, the effect provided by the present invention is in proportion to the surface area of a metal hydroxide as a cathode additive. This is supported by the following Examples and Comparative Examples. Therefore, according to the invention., by using a metal hydroxide having a large specific surface area as a cathode additive, it is possible to obtain excellent storage properties of a battery at a high temperature, namely to minimize decrease of battery capacity and to prevent increase of battery impedance, even if the metal hydroxide is added in a relatively small amount. As a result, a problem occurring in the prior art can be minimized; the problem being that increase of amount of a metal hydroxide added to a cathode may decrease battery capacity. According to the present invention, the specific surface area of a metal hydroxide is 1 m2/g or more, preferably 2.5 m2/g or more, and more preferably 7 m2/g or more. As described above, the larger the specific surface area of a metal hydroxide ■ used as a cathode additive is, the better the resultant effect is. Meanwhile, although it is preferable that the specific surface area of a metal hydroxide is as large as possible, the specific surface area may be limited by conditions of battery manufactured, methods for preparing metal hydroxides, etc. Considering the conditions under which cathode slurry is coated on an electrode collector to form a cathode, the specific surface area of the metal hydroxide is 4 WO 2005/013395 PCT/KR2004/001897 preferably 100 m2/g or less. Examples of metal hydroxides that may be used in the present invention include A1(OH)3, Mg(0H)2, Ca(OH)2r LiOH, NaOH, etc. In addition, the metal hydroxides are preferably used in an amount of greater than 0 wt% and not more than 10 wt%. Although such metal hydroxides can improve the storage properties of a battery at a high temperature even if they are added in a very small amount, adding them to a cathode in an amount of greater than 10 wt% may increase resistance due to the nature of metal hydroxides as a nonconductor, thereby decreasing battery capacity and deteriorating storage properties of a battery at a high temperature. Further, when such metal hydroxides are added to a cathode in an amount of greater than 10 wt%, the amount of a cathode active material capable of lithium ion intercalation/ deintercalation is decreased accordingly, and thus battery capacity is reduced. This is clearly shown in the results of the following Examples 1 to 5 and Comparative Example 1. The cathode according to the present invention can be prepared by introducing cathode materials including a cathode active material and a metal hydroxide having a large specific surface area into a solvent, mixing them to prepare cathode slurry, coating the slurry on a cathode collector, and drying the solvent. According to the present invention, lithium-containing transition metal compounds may be used as a cathode active material. Non-limitative examples of the lithium-containing transition metal compounds include LiCo02, IiiNiO2, LiMn2O4, LiMnO2, LiCoP04, LiNi1-xCoxMY02 (wherein M = Al, Ti, Mg or Zr, 0