Technical.Field The present invention relates to an electrode including an aliphatic nitrile compound. More particularly, the present invention relates to an electrode whose surface is coated with an aliphatic nitrile compound or which comprises an electrode active material comprising an aliphatic nitrile compound, as well as to a lithium secondary battery having the same electrode. Background Art In general, a non-aqueous electrolyte comprising a lithium salt such as LiPFs and a carbonate solvent reacts continuously with the surface of a cathode active material (particularly, LiCoO2) . during repeated charge/discharge cycles, resulting in the continuous formation of a resistance layer that causes an increase in resistance and interrupts conduction: of. Li+ ions Such resistance layer causes the active material particles to be isolated among themselves or from a current collector' (Al foil), thereby detracting from battery performance and life characteristics. Further, such problems increasingly and predominantly . occur at a high temperature to accelerate side reactions between an electrolyte and the surface of a cathode when a battery is stored at a high temperature (45°C or 60oC) for a long time, resulting in a significant decrease in the lifetime of a battery. Meanwhile, non-aqueous electrolyte-based secondary batteries have problems related with safety upon overcharge for the following reasons. Cathode active materials such as lithium and/or lithium ion-containing metal oxides capable of lithium' ion intercalation/ deintercalation are converted into thermally unstable substances due to the release of lithium during overcharge. When the temperature of a battery reaches the critical temperature, oxygen is liberated from such unstable substances and the free oxygen may react with the solvent of an electrolyte, etc., through a highly exothermic reaction mechanism. Therefore, such a series of exothermal reactions by heating results in thermal runaway. Generally, factors affecting the safety of a battery include: (1) heat emission due to oxidation of electrolytes; and (2) heat emission resulting from the structural collapse of a cathode due to overcharge. When overcharge proceeds, heat emission occurring from the above factors independently or simultaneously causes an increase in the internal temperature of a battery, followed by ignition or explosion of the battery. Thus, batteries show a safety problem upon overcharge. Meanwhile, when external physical impacts (for example, exposure to high temperature such as a temperature of 150°C or higher by heating) are applied to a battery while the battery is charged or overcharged, the battery is overheated due to the heat emission caused by the reaction of an inflammable electrolyte with a cathode active material, and the structure of an electrode (particularly, a cathode) is collapsed to generate oxygen, which accelerates the combustion of the electrolyte. Therefore, a separator disposed between a cathode and an anode is melted and the electrical energy induces thermal runaway, resulting in ignition and explosion of the battery. Disclosure of the Invention The present inventors have found that an aliphatic nitrile compound that forms a strong bond with a transition metal or transition metal oxide in an electrode active material can improve the safety of a battery, when the battery is overcharged and/or subjected to physical impacts applied from the exterior of the battery (for example, exposure to high temperature by heating) . Meanwhile, we have also recognized a problem in that when an aliphatic dinitrile compound is used as an additive for electrolyte, there is an increase in viscosity of the electrolyte so that diffusion of Li ions cannot be made smoothly under extreme conditions (a low temperature of between -20oC and -10°C), resulting in degradation of battery performance, at a low temperature. Therefore, the present invention has been made in view of the above-mentioned problems. It is an object of the present invention to improve the safety of a battery with no degradation of battery performance by incorporating an aliphatic dinitrile compound uniformly into an electrode so that the aliphatic nitrile compound can contribute only to the formation of a complex with an electrode active material. According to an. aspect of the present invention, there is provided an electrode comprising an aliphatic nitrile compound, preferably a compound represented by the following formula 1, whose surface is coated with the aliphatic nitrile compound or which comprises an electrode active material comprising the aliphatic nitrile compound. According to another aspect of the present invention, there is provided a lithium secondary battery having the above-described electrode. wherein R is a C2-C15 alkane. Preferably, the aliphatic nitrile compound, preferably the compound represented by formula 1 is coated uniformly on the surface of an electrode active material in an electrode. Additionally, it is preferable that the electrode according to the present invention includes a complex formed between the surface of the electrode active material and the aliphatic nitrile compound. Hereinafter, the present invention will be explained in more detail. According to the present invention, the electrode for a lithium secondary battery is characterized by comprising an aliphatic nitrile compound, preferably the compound represented by the above formula 1. Aliphatic nitrile compounds can form a strong bond with a transition metal or transition metal oxide such as cobalt exposed to the surface of an electrode active material through their cyano functional groups having high dipole moment. Particularly, the cyano functional groups can form a stronger complex on the surface of an electrode active material at a temperature of 45°C or higher (see, FIG. 1). An electrode coated with an aliphatic nitrile compound has a -strong protection surface that protects the surface of electrode from side reactions with an electrolyte. Therefore, it is possible to accomplish efficient lithium ion intercalation/deintercalation without varying viscosity of the electrolyte and ion conductivity, and to prevent the formation of a resistance layer capable of detracting from battery performance by the reaction of the electrolyte with electrode during repeated charge/discharge cycles, on the surface of electrode. As a result, it is possible to maintain battery performance. Further, according to the present invention, a lithium secondary battery having an electrode uniformly coated with an aliphatic nitrile compound on the surface of an electrode active material, and preferably comprising an aliphatic nitrile compound forming a strong complex with a transition metal and/or transition metal oxide present on the surface of electrode active material, can stabilize the transition metal and transition metal oxide to prevent a partial release of the transition metal from the electrode active material during repeated charge/discharge cycles. In addition, when external physical impacts are applied to a battery (particularly, when a battery is exposed to high temperature such as a temperature of 150°C or higher), it is possible to efficiently inhibit an exothermic reaction caused by the reaction of an electrolyte directly with the electrode surface and to retard the structural collapse of the electrode active material, thereby preventing ignition and explosion resulting from an increase in temperature inside of the battery. More particularly, because aliphatic nitrile compounds can protect the electrode surface more strongly at a high temperature of 45°C or higher than room temperature, it is possible to provide thermally stable electrodes. Although the compound represented by the above formula 1 is exemplified as an aliphatic nitrile compound that can be incorporated into an electrode according to the. present invention, another aliphatic nitrile compound having a nitrile group only at one side, compared to the compound represented by formula 1, has a great possibility for providing safety and/or battery performance in such a degree as to be equivalent to the compound represented by formula 1, and thus it is also included in the scope of the present invention. Meanwhile, alkanes present in the compound represented by formula 1 have no reactivity. Therefore, when the compound represented by formula 1 is incorporated into an electrode, a possibility for an irreversible reaction is low. As a result, addition of the compound represented by formula 1 does not cause degradation in battery performance. Because an aromatic nitrile compound decomposes at an anode during the initial charge cycle (during formation) to increase irreversible capacity and to degrade battery performance significantly, it is not preferable to incorporate an aromatic nitrile compound into an electrode and to coat an electrode with an aromatic nitrile compound. Particular examples of the compound represented by formula 1 include succinonitrile (R=C2H4) , glutaronitrile (R=C3H6) , adiponitrile (R=C4H8) , pimelonitrile (R=C5H10), octanedinitrile (R=C6H12), azelonitrile (R=C7H14), sebaconitrile (R=CbH16), 1, 9-dicyanononane (R=C9H18) , dodecanedinitrile (R=C10H20), etc., but are not limited thereto. Particularly,. succinonitrile forms the strongest protection layer among the compounds represented by formula 1. The longer the allcane is, the weaker the protection layer to be formed becomes. Therefore, it is most preferable to use succinonitrile as a coating material among the above compounds. The aliphatic nitrile compound is present in an electrode preferably in an amount of 0.1-20 wt% based on the weight of electrolyte or 1-10 wt% based on the weight of active material, more preferably in an amount of 10 wt% or less based on the weight of electrolyte or 5 wt% or less based on the weight of active material, and most preferably in an amount of 5 wt% or less based on the weight of electrolyte or 2.5 wt% or less based.on the weight of active material. In order to incorporate an aliphatic nitrile compound into an electrode, a coating solution containing an aliphatic nitrile compound may be applied on an electrode. Otherwise, an aliphatic nitrile compound may be added to slurry for electrode active material to form an electrode. For the purpose that the nitrile compound participates only in complex formation with a transition metal oxide of an electrode active material, a coating solution containing an aliphatic nitrile compound is applied to an electrode or an aliphatic nitrile compound is added to electrode active material-containing slurry in an adequate amount. Preferably, the electrode or slurry comprising the nitrile compound is treated at a high temperature. Then, the surface of electrode, namely the surface of electrode active material can be protected uniformly with the aliphatic nitrile compound. In addition to the above-mentioned high-temperature treatment applied to an electrode or slurry, a battery may be preferably treated at a high temperature after the assemblage thereof. The aliphatic nitrile compound is dispersed or dissolved into a solvent to provide a solution, the solution is coated on the surface of an electrode and then the solvent is dried in order to coat the electrode surface, preferably the surface of electrode active material with the aliphatic nitrile compound. The coating method may include dip coating, spray coating, or the like. - There is no particular limitation in selection of the solvent for use in the coating solution containing an aliphatic nitrile compound, as long as the solvent has good compatibility. It is preferable to use, as a solvent for coating solution, non-polar solvents such as THF (tetrahydrofuran) and polar solvents such as NMP (N- methyl-2-pyrollidone) and carbonate solvents used as a solvent for electrolyte. Although the amount of aliphatic nitrile compound varies with the amount to be coated on an electrode, the aliphatic nitrile compound may be used in the range of between 1:9 and 9:1, expressed in the weight ratio to the solvent. The method for forming an electrode by adding an aliphatic nitrile compound to slurry for electrode active material includes the steps of: mixing an aliphatic nitrile compound with an electrode active material and other additives such as a binder and conductive agent, as necessary, to form slurry for electrode active material; applying the slurry for electrode active material on a collector; and removing the solvent used in the slurry by drying, etc. In order to apply the slurry for electrode active material, die coating, roll coating, comma coating and combinations thereof may be used. Meanwhile, because the compound represented by formula 1 starts to be slightly volatilized at a high temperature of 100°C or higher and then be substantially evaporated without leaving residues at a temperature of about 150°C, it is necessary to maintain an adequate drying temperature, drying rate and vent flow for the purpose of coating an electrode smoothly with the compound represented by formula 1 from slurry containing NMP as a solvent. To prevent the compound represented by formula 1 from being volatilized and to remove residual NMP, the drying temperature preferably ranges from 90oC to 110°C. The drying rate is preferably 3 m/min or less, more preferably 2 m/min or less, but may be varied with the length of a drying furnace and the drying temperature of slurry. The vent flow is preferably 2000-3000 rpm. More particularly, when the electrode comprising the compound represented by formula 1 is dried at an excessively low temperature in order to retain the compound in the electrode, NMP content and water content in the electrode increase, thereby causing a problem in that battery performance is degraded. On the other hand, when the electrode is dried at an excessively high temperature, NMP content in the electrode decreases but the compound represented by formula 1 is substantially volatilized, and thus it is not possible to obtain a uniformly coated electrode. Therefore, it is important that the drying temperature, drying rate and vent flow are maintained within the above ranges. Meanwhile, it is preferable that aliphatic nitrile compounds form a complex with the surface of an electrode active material. Preferably, for the purpose ' of forming a complex, an electrode comprising an electrode active material whose surface is coated with an aliphatic nitrile compound is further treated at a high temperature. Particularly, the high-temperature treatment may be performed at such a temperature range as not to affect the electrode active material and binder, generally at a temperature of 180°C or lower. Otherwise, although the high-temperature treatment varies with the kind of the aliphatic nitrile compound, it may be performed at such a temperature range as to prevent evaporation of the aliphatic nitrile compound, generally at a temperature of 120°C or lower. In general, the high-temperature treatment is suitably performed at a temperature of between 60°C and 90oC. Long-time storage at a temperature of between 30°C and 40°C may result in the same effect. As a cathode active material for use in electrodes, lithium-containing transition metal oxides may be used. The cathode active material can be at least one selected from the group consisting of LiCo02, LiNiO2, LiMn2O4, LiMn02 and LiNi1-xCox02 (wherein 0