FORM 2 THE PATENTS ACT, 19?0 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) TITLE OF INVENTION: SURFACE-TREATMENT METHOD OF CATHODE ACTIVE MATERIAL AND CATHODE ACTIVE MATERIAL FORMED THEREFROM APPLICANT: LG CHEM, LTD. A company incorporated in Republic; of Korea 128, Yeoui-daero, Youngdungpo-gu, Seoul 150-721, Republic of Korea The following specification particularly describes the invention and the manner in which it is to be performed. TECHNICAL FIELD The present invention relates to a surface-treatment method of cathode active material and a cathode active material formed therefrom, and more particularly to a method for treating the particle surface of cathode active material for a secondary battery with a plasma generated by gases and a cathode active material formed therefrom. This application claims priority to Korean Patent Application Nos. 10-2011-0042034 and 10-2012-0043934 filed in the Republic of Korea on May 3, 2011 and April 26, 2012, respectively, the disclosure of which are incorporated herein by reference. BACKGROUND ART A cathode active material for a secondary battery has lithium impurities such as LiOH and LiCO3 which are present on the particle surface thereof. If these impurities are present in an excessive amount, not only do they cause to make an electrode slurry to be gelated during the production of an electrode slurry in the preparation of an electrode of a lithium secondary battery, but also causes a swelling phenomenon, particularly a lithium ion prismatic or polymer battery expanding, when reacted with the components of an electrolyte introduced in the lithium secondary battery. Accordingly, there is a need for minimizing the content of lithium impurities present on the particle surface of the cathode active material for a lithium secondary battery. DISCLOSURE Technical Problem The present invention is designed to solve the problems described above, therefore, it is an object of the present invention to improve the properties of a cathode active material for a lithium secondary battery by removing lithium impurities present on the particle surface thereof. Technical Solution In order to achieve the object, the present invention provides a method for treating the particle surface of a cathode active material for a lithium secondary battery, the method comprising (a) preparing a cathode active material having a lithium compound; (b) generating a plasma from gases comprising at least one of a fluorine-containing gas and a phosphorus-containing gas as a part of a reactive gas; and (c) removing lithium impurities present on the particle surface of the cathode active material with the plasma. In the present invention, the lithium compound may be selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, LiFePO4, LiFe1-XMnxPO4, LiNi1-x-yMnxCOyO2 (0