FORM 2 THE PATENTS ACT, 1970 (39 of 1970) 5 & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) 10 TITLE OF INVENTION: METHOD FOR PREPARING ANODE ACTIVE MATERIAL, ANODE ACTIVE MATERIAL PREPARED THEREFROM AND LITHIUM SECONDARY BATTERY HAVING THE SAME 15 APPLICANT: LG CHEM, LTD. A company incorporated in Republic of Korea Having address: 20 128, Yeoui-daero Youngdungpo-gu Seoul 150-721, Republic of Korea The following specification particularly describes the invention and the manner in which it is 25 to be performed TECHNICAL FIELD The present invention relates to a method for preparing an anode active material comprising a silicon oxide, an anode active material prepared therefrom, and a lithium secondary battery having the anode active material. This application claims priority to Korean Patent Application Nos. 10-2011-0108782 5 and 10-2012-0118431 filed in the Republic of Korea on October 24, 2011 and October 24, 2012, respectively, the disclosures thereof are incorporated herein by reference. BACKGROUND ART Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, the intensive research 10 and development of electrochemical devices has been made. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy. 15 Among currently available secondary batteries, lithium secondary batteries developed in the early 1990’s have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni-Cd, and H2SO4-Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the 20 use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries. 25 For this, the demand for an anode material having a high capacity is increasing. In order to meet the demand, Si-based materials having a large theoretical capacity have been used as an anode active material, however, Si deteriorates the life characteristics of batteries during repeated charging/discharging processes and causes severe thickness swelling, which adversely affects the performances and safety of the batteries. Accordingly, in order to 30 maintain life characteristics and reduce thickness swelling, attempts have been made to use a 3 silicon oxide (SiOx). However, the silicon oxide forms an irreversible phase due to the insertion of lithium, and thus has a low initial efficiency. To solve such a problem, the silicon oxide is first alloyed with lithium so that it contains lithium, thereby forming less of an irreversible phase material such as lithium oxides and lithium-metal oxides during initial charging and discharging processes, and eventually improving the initial efficiency of an 5 anode active material. The lithium source, which is used to first alloy a silicon oxide with lithium, may be divided into a lithium source using metallic lithium, a lithium salt having no oxygen, and an oxygen-containing lithium salt. Among these, the metallic lithium has a great reactivity with water and may be dangerously apt to ignite, and may also react with carbon dioxide to produce lithium 10 carbonate. Also, most lithium salts not containing oxygen are formed by an ionic bond, and thus the reaction of lithium salts and silicon oxides is very restricted. Therefore, it is preferable to use lithium salts containing oxygen. However, while silicon oxides react with lithium salts containing oxygen, oxygen present in the lithium salts react with the silicon oxides, thereby making it hard to control the 15 amount of oxygen in the silicon oxides. Also, the remaining unreacted lithium sources and by-products of the reaction between the lithium sources and the silicon oxide may lead to the gelation of an anode active material-containing slurry in the preparation of an electrode. DISCLOSURE 20 Technical Problem Therefore, it is an object of the present invention to provide a method for preparing an anode active material which is easy to control the amount of oxygen present in a silicon oxide and can minimize performance deterioration due to remaining lithium sources, and by-products of the reaction between lithium sources and the silicon oxide, an anode active 25 material prepared therefrom, and a lithium secondary battery having the anode active material. Technical Solution In order to achieve the object, the present invention provides a method for preparing an anode active material, comprising (S1) forming a shell being a coating layer comprising a 30 4 carbon material on the surface of a core comprising silicon oxide particles, to obtain a silicon oxide-carbon composite having a core-shell structure; (S2) mixing the silicon oxide-carbon composite with an oxygen-containing lithium salt, followed by heat treatment to produce a silicon oxide-lithium alloy, thereby obtaining a (SiOx-Liy)-C (0 - Charge of batteries was conducted up to 5 mV at constant current, and completed when a current density reached 0.005C. - Discharge of batteries was conducted up to 1.0 V at constant current. 20 Table 1 Discharge Capacity (mAh/g) Capacity before charging (mAh/g) Initial Efficienty (%) Normalized Capacity (%) @ 50th cycle Expansion (1.6g/cc) Ex. 2 490.5 550.4 89.1 90% 38.5 Com. Ex. 5 501.8 615.7 81.5 83% 52.1 Com. Ex. 6 483.9 542.8 89.1 85% 47.1 Com. Ex. 7 429.3 512.9 83.7 80% 41.7 Com. Ex. 8 425.7 512.9 83.0 76% 49.3 As can be seen from Table 1, the secondary battery of Example 2 exhibited an initial efficiency increased by about 8% and a life characteristic increased by 7% after 50 cycles, as compared with the secondary battery of Comparative Example 5. Since Example 2 used the anode active material of Example 1 which was obtained by mixing silicon oxide-carbon with 5 LiOHH2O, followed by heat treatment, the silicon oxide-carbon particles contain lithium, and since lithium was already contained before initial charge/discharge of the battery, initial efficiency was raised. Also, when the anode active material of Example 1was used, irreversible phase materials such as lithium oxide and lithium-metal oxide were less formed, and thus structural changes due to irreversible production were reduced during 10 charge/discharge, thereby improving a life characteristic and a thickness-expansion rate. The secondary battery of Example 2 using the anode active material of Example 1 exhibited improved life and thickness characteristics, as compared with secondary battery of Comparative Example 6 using the anode active material of Comparative Example 2. This results from when silicon oxide-carbon reacts with LiOHH2O, by-products of LiOHH2O, 15 such as LiOH remained on the surface of the (SiOx-Liy)-C composite, such as LiOH being a base having a high pH value which may change the properties of a binder and deteriorate a slurry in a mixing step. Owing to such by-products of LiOHH2O, the binder cannot do its function of controlling the volume expansion of active materials, and the active materials having difficulty in uniformly mixing. Accordingly, the anode active material of Example 1 20 was washed to remove remaining by-products of LiOHH2O, thereby providing increased life and thickness characteristics. In the secondary batteries of Comparative Examples 7 and 8 each using an anode active material not having a shell of carbon, the value of x in SiOx increased owing to oxygen of LiOH, from which large amounts of oxygen reacted with lithium during a charging process 25 and irreversible phase materials increased, thereby deteriorating an initial efficiency and a life characteristic. 16 WE CLAIM: 1. A method for preparing an anode active material, comprising (S1) forming a shell being a coating layer comprising a carbon material on the surface of a core comprising silicon oxide particles, to obtain a silicon oxide-carbon composite 5 having a core-shell structure; (S2) mixing the silicon oxide-carbon composite with an oxygen-containing lithium salt, followed by heat treatment to produce a silicon oxide-lithium alloy, thereby obtaining a (SiOx-Liy)-C (0