ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY, MANUFACTURING METHOD THERE-FOR, ANODE FOR LITHIUM SECONDARY BATTERY COMPRISING SAME, AND LITHIUM SECONDARY BATTERY 5 BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a negative active material for rechargeable lithium secondary batteries capable of inhibiting battery side reactions and gas 10 generation and improving battery performance since moisture formed during an oxidation-reduction (redox) reaction is effectively absorbed into a surface of the negative active material, a method of preparing the same, and a rechargeable lithium secondary battery including the same. 15 Description of the Related Art Rechargeable lithium secondary batteries (e.g., lithium ion batteries), nickel-hydrogen batteries, and other secondary batteries have been recognized to be of growing 20 importance as vehicle-mounted power sources, or power sources for portable terminals such as laptop computers. In particular, rechargeable lithium secondary batteries which are lightweight and may have a high energy density may be desirably used as high-output power sources for vehicle 25 mounting, and thus demand for rechargeable lithium secondary 2 batteries is expected to increase in the future. A rechargeable lithium secondary battery is manufactured by installing a porous separation film between a positive electrode and a negative electrode, followed by 5 injecting a liquid electrolyte between the positive electrode and the negative electrode. Here, a material in which lithium ions are intercalatable and deintercalatable is used as the negative electrode or a negative active material and negative electrodes. In this case, electricity 10 may be produced or consumed by a redox reaction caused by intercalation/deintercalation of lithium ions into/from the negative and positive electrodes. Specifically, various types of carbon-based materials, in which lithium ions are intercalatable and 15 deintercalatable and which include synthetic graphite, natural graphite, and hard carbon, have been applied as the negative active materials in the case of the rechargeable lithium secondary batteries. Among the carbon-based materials, graphite has a discharge voltage of -0.2 V lower 20 than lithium, and thus secondary batteries in which graphite is used as a negative active material may have a high discharge voltage of 3.6 V. In addition, graphite has been most widely used since it may be advantageous in terms of the energy density of rechargeable lithium secondary 25 batteries, and may ensure long lifespan of the rechargeable 3 lithium secondary batteries due to excellent reversibility thereof. However, such graphite active materials have a problem in that they have low capacity with respect to the energy density of electrode plates per unit volume since 5 graphite has a low density (a theoretical density of 2.2 g/cc), and side reactions with an organic electrolyte solution used at a high discharge voltage may easily occur upon manufacture of the electrode plates, resulting in swelling of the batteries, and thus battery capacity 10 degradation. To solve the above problems regarding such carbonbased negative active materials, Si-based negative active materials having a much higher capacity than graphite, and negative active materials using oxides such as tin oxide, 15 lithium vanadium-based oxide, and lithium titanium-based oxide have been developed and researched. However, the high-capacity, Si-based negative active materials undergo serious changes in volume during charge/discharge cycles, and thus lifespan characteristics 20 may be deteriorated due to particle splitting. In addition, oxide negative electrodes do not show satisfactory battery performance, and thus research on the oxide negative electrodes continues to be conducted. Among these, lithium titanium oxide (hereinafter referred to as 25 “LTO”) does not form a solid electrolyte interface (SEI) 4 layer due to poor reactivity with an electrolyte solution. Therefore, LTO is advantageous in terms of an irreversible reaction, and thus has very stable lifespan characteristics. Owing to excellent reversibility, LTO may also be desirably 5 used to charge and discharge the secondary batteries at a high speed during intercalation/deintercalation of lithium (Li) ions. However, LTO has a high content of moisture, and thus has a drawback in that battery performance may be degraded due to the presence of moisture, and thus gas 10 generation. Therefore, there is a demand for development of methods capable of inhibiting generation of gases caused by moisture in the LTO-based negative active material itself, thereby preventing degradation of battery performance. 15 [Prior-art Document] [Patent Document] Korean Unexamined Patent Publication No. 2008-0018737 (published on February 28, 2008) 20 SUMMARY OF THE INVENTION Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a negative active material capable of inhibiting battery side reactions and gas 25 generation and improving battery performance since moisture 5 formed during a redox reaction is effectively absorbed into a surface of the negative active material, and a method of preparing the same. It is another object of the present invention to 5 provide a rechargeable lithium secondary battery capable of showing improved battery performance, which includes the negative active material. In accordance with the present invention, the above and other objects can be accomplished by the provision of 10 a negative active material for rechargeable lithium secondary batteries which includes a core including a lithium titanium oxide represented by the following Formula 1, and a coating layer positioned on a surface of the core, wherein the coating layer includes an acid anhydride 15 physically absorbed (i.e., physisorbed) into the core: [Formula 1] LixTiyO4 wherein 0.8≤x≤1.4, and 1.6≤y≤2.2. In the negative active material, the core may contain 20 at least one surface functional group selected from the group consisting of O- and CO2 -. In addition, the lithium titanium oxide of Formula 1 may be Li4Ti5O12 having a spinel structure. Additionally, the acid anhydride may include at least 25 one selected from the group consisting of a carboxylic 6 anhydride, a maleic anhydride, and an acetic anhydride. Further, the coating layer may be included in an amount of 0.5 to 3 parts by weight, based on 100 parts by weight of the core. In accordance with another 5 aspect of the present invention, there is provided a method of preparing a negative active material for rechargeable lithium secondary batteries, which includes preparing a core containing at least one surface functional group selected from the group consisting of O- and CO2 10 - by mixing a lithium source and a titanium source so as to prepare a lithium titanium oxide of Formula 1, and calcining the resulting mixture at a temperature of 750 to 800°C, which is less than a typical calcination temperature, and physisorbing an acid anhydride 15 of an organic acid onto a surface of the core by treating the core containing the surface functional group with a solution including the organic acid, and drying the core. In the method, the organic acid may be a carboxylic acid containing 1 to 3 carboxyl groups in a molecule 20 thereof. In addition, the organic acid may include at least one selected from the group consisting of acetic acid, propionic acid, stearic acid, pyruvic acid, acetoacetic acid, glyoxylic acid, oxalic acid, malonic acid, maleic acid, 25 glutaric acid, adipic acid, phthalic acid, trimellitic acid, 7 and a mixture thereof. Additionally, the organic acid may be included in an amount of 0.5 to 3% by weight, based on the total weight of the solution including the organic acid. 5 Further, the drying may be performed at 60 to 130°C under vacuum. It is yet another object of the present invention to provide a rechargeable lithium secondary battery which includes a positive electrode including a positive active 10 material, a negative electrode including a negative active material and arranged to face the positive electrode, and an electrolyte solution interposed between the positive electrode and the negative electrode, wherein the negative active material includes a core including a lithium titanium 15 oxide of Formula 1, and a coating layer positioned on a surface of the core, and the coating layer includes an acid anhydride physisorbed onto the core. Specific content of the other exemplary embodiments of the present invention are encompassed in the following 20 detailed description. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly 25 understood from the following detailed description taken 8 in conjunction with the accompanying drawings, in which: FIG. 1 is an exploded perspective view showing a rechargeable lithium secondary battery according to one exemplary embodiment of the present invention; 5 FIG. 2 is a graph illustrating results obtained by determining whether gases are generated according to the temperature of a negative active material used in Experimental Example 2 of the present invention; and FIG. 3 is a graph illustrating experimental results 10 obtained by measuring input/output characteristics of rechargeable lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 and 2 as described in Experimental Example 3 of the present invention. 15 [Brief description of main parts in the drawings] 1: rechargeable lithium secondary battery 3: negative electrode 5: positive electrode 7: separator 20 9: electrode assembly 10, 13: lead members 15: cases DETAILED DESCRIPTION OF THE INVENTION 25 Hereinafter, preferred embodiments of the present 9 invention will be described in detail with reference to the accompanying drawings so as to enable those skilled in the art to easily embody the present invention. However, it should be understood that the present invention may be 5 embodied in various different forms, but is not limited to the above-described embodiments. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments. The singular forms 10 “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, 15 steps, operations, elements, components and/or groups thereof, but do not preclude the presence or addition of one or more other features, whole numbers, steps, operations, elements, components and/or groups thereof. The present invention is characterized in that a 20 negative active material is prepared by immersing an LTObased negative active material pretreated to contain a surface functional group in a solution including an organic acid such as maleic acid, drying the LTO-based negative active material, and physisorbing the organic acid onto a 25 surface of LTO in the form of an acid anhydride, and thus 10 may be useful in inhibiting battery side reactions and gas generation and improving battery performance since moisture formed during a redox reaction is effectively absorbed into a surface of the negative active material. 5 That is, the negative active material for rechargeable lithium secondary batteries according to one exemplary embodiment of the present invention includes a core including a lithium titanium oxide represented by the following Formula 1, and a coating layer positioned on a 10 surface of the core, wherein the coating layer includes an acid anhydride physisorbed onto the core: [Formula 1] LixTiyO4 wherein 0.8≤x≤1.4, and 1.6≤y≤2.2. 15 Specifically, in the negative active material, the core may be formed of Li4Ti5O12 having a spinel structure. In this case, the number of moles of oxygen in Formula 1 is represented by a fixed value of 4, but Formula 1 is not limited thereto. For example, the number of moles of oxygen 20 may be represented as multiples of the number of moles within a range in which a ratio of the number of moles of the respective elemental atoms in Formula 1 is satisfied. That is, when the number of moles of oxygen in Formula 1 is 12, the lithium titanium oxide of Formula 1 may be 25 represented by Li3xTi3yO12. Li4Ti5O12 having a spinel 11 structure may serve to prevent an SEI film from being formed on a surface of the negative electrode to an excessively large thickness, and improve electrochemical characteristics and safety of the secondary batteries under the control of thermal 5 runaway factors. In addition, the Li4Ti5O12 may promote transportation of Li ions to impart rapid charge/discharge characteristics to the secondary batteries. The core contains at least one surface functional group selected from the group consisting of O- and CO2 - to 10 allow physisorption of the acid anhydride during preparation of the negative active material. In addition, the core including the above-described lithium titanium oxide may have an average particle diameter of 3 to 15 μm in consideration of the specific surface area 15 and negative electrode mix density of the active material. Additionally, the coating layer positioned on a surface of the core includes an acid anhydride physisorbed onto the core. The acid anhydride is derived from an organic acid, 20 particularly an organic acid containing at least one carboxyl group in a molecule thereof. In this case, the acid anhydride is present in a state in which the acid anhydride is physisorbed onto a surface of the core, but when moisture is formed during a redox reaction, the acid 25 anhydride preferentially reacts with the moisture so that 12 the acid anhydride is converted into an acid. As such, since the acid anhydride reacts with the moisture immediately after moisture is formed on a surface of the active material, side reactions and gas generation caused by 5 moisture in all the secondary batteries may be inhibited. [Scheme 1] Types of the acid anhydride may vary according to the type of the organic acid used to prepare the negative active 10 material. Specifically, the acid anhydride may include a carboxylic anhydride, a maleic anhydride, an acetic anhydride, and a mixture thereof. In the negative active material, the coating layer including the acid anhydride may be included in an amount of 15 0.5 to 3 parts by weight, based on 100 parts by weight of the core. When the content of the coating layer is less than 0.5 parts by weight, it is difficult to coat the core completely, and thus the lithium titanium oxide constituting the core may be exposed to the outside, resulting in side 13 reactions caused by generated moisture and thus gas generation. On the other hand, when the content of the coating layer is greater than 3 parts by weight, reduction in initial battery efficiency and performance degradation 5 may be caused due to an increase in thickness of the coating layer. Considering that the negative active material shows significant improvement effects due to formation of the coating layer, the coating layer may also be included in an amount of 1 to 2 parts by weight, based on 100 parts by 10 weight of the core. According to another exemplary embodiment of the present invention, there is provided a method of preparing the above-described negative active material, which includes preparing a core containing at least one surface functional group selected from the group consisting of O- and CO2 15 - by mixing a lithium source and a titanium source so as to prepare a lithium titanium oxide of Formula 1 and calcining the resulting mixture at a temperature of 750 to 800°C, which is less than a typical calcination temperature, and 20 physisorbing an acid anhydride of an organic acid onto a surface of the core by immersing the core containing the surface functional group in a solution including the organic acid and drying the core. Hereinafter, respective steps of the method will be 25 described in detail, as follows. The first step includes 14 preparing a core including the lithium titanium oxide of Formula 1 containing at least one surface functional group selected from the group consisting of O- and CO2 -. To prepare the core including the lithium titanium 5 oxide, the core may be prepared by mixing a lithium source and a titanium source at an atomic ratio between lithium and titanium (4 lithium atoms and 5 titanium atoms in the case of Li4Ti5O12), stirring and drying the resulting mixture to prepare a precursor, and calcining the precursor. The 10 lithium source may be a solution obtained by dissolving lithium salts, such as lithium hydroxide, lithium carbonate and lithium oxide, in water, and the titanium source may be titanium oxide, etc. Accordingly, the core containing a surface functional group selected from O- and CO2 15 -, both of which are generated when the lithium source is added, may be prepared by mixing the lithium and titanium sources used to prepare the lithium titanium oxide, and calcining the resulting mixture at a temperature of 750 to 800°C, which is less than a typical 20 calcination temperature. The second step includes physisorbing the acid anhydride of the organic acid onto a surface of the core containing the surface functional group. Specifically, the core containing the surface 25 functional group is immersed in a solution including the 15 organic acid, and then dried at 60°C or higher, or 60 to 130°C under vacuum to physisorb the organic acid onto a surface of the core in the form of an acid anhydride. In this case, the organic acid may be an organic acid containing 5 one or more carboxyl groups, or 1 to 3 carboxyl groups in a molecule thereof. Specifically, the organic acid may include a monocarboxylic acid such as acetic acid, propionic acid, stearic acid, pyruvic acid, acetoacetic acid, or glyoxylic acid; or a polyvalent carboxylic acid 10 such as oxalic acid, malonic acid, maleic acid, glutaric acid, adipic acid, phthalic acid, or trimellitic acid, which may be used alone or in combination of two or more thereof. Among these, acetic acid or maleic acid may be more preferred in an aspect of significant improvement effects. 15 To treat the surface-treated core with the organic acid, the organic acid may be used in a solution phase in which the organic acid is dissolved in water, etc. Specifically, the acid anhydride of the organic acid may be used in a ratio of 0.1 to 3 moles, based on one mole of the 20 lithium titanium oxide, in consideration of the content of the acid anhydride in the finally prepared negative active material capable of showing the effects according to exemplary embodiments of the present invention. Therefore, the solution including the organic acid may include the 25 organic acid in a concentration of 0.1 to 2 moles. 16 Considering this fact, the organic acid may be included in an amount of 0.5 to 3% by weight, based on the total weight of the solution including the organic acid. In addition, the treatment of the surface-treated core 5 with the organic acid may be performed using a method such as immersion, spraying, coating, etc. Among these, immersion may be used to treat the core with the organic acid in consideration of surface treatment of the core and ease of processing. 10 Next, the drying after treatment with the organic acid may be performed at 60 to 130°C under vacuum. When the drying temperature is less than 60°C, the organic acid may be present on a surface of the core in the form of an organic acid rather than an acid anhydride. On the other 15 hand, when the drying temperature is greater than 130°C, the acid anhydride and the lithium titanium oxide may chemically react with each other, resulting in ineffective moisture absorption. The results of the preparation method as described 20 above show that the organic acid is physisorbed onto a surface of the core including the LTD of Formula 1 in the form of an acid anhydride. When the acid anhydride is physisorbed onto the surface of the core as described above, there is a difference in that the acid anhydride may 25 preferentially absorb moisture before the moisture reacts 17 with LTD since the acid anhydride is uniformly distributed on a surface of the core, compared to when the acid anhydride may be simply mixed with the core to prepare a negative electrode, or when a surface of the core is treated 5 with the acid anhydride itself to prepare a negative active material. Owing to such a difference, the negative active material may have superior effects in terms of battery lifespan and rate performance. In addition, when the LTObased core is surface-treated in the form of an acid 10 anhydride as described above, the acid anhydride may be much purer since a smaller amount of residual substances is present on a surface of the core, compared to when the LTObased core is surface-treated in the form of an acid or a salt thereof. Therefore, the acid anhydride may be more 15 favorable in terms of reduction in side reactions. According to yet another exemplary embodiment of the present invention, there is provided a rechargeable lithium secondary battery including the negative active material prepared by the above-described preparation method. 20 Specifically, the rechargeable lithium secondary battery includes a positive electrode including a positive active material, a negative electrode including a negative active material and arranged to face the positive electrode, and an electrolyte solution interposed between the positive 25 electrode and the negative electrode. Here, the negative 18 active material is as described above. The rechargeable lithium secondary batteries may be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries according to types 5 of separators and electrolytes used herein, and may also be classified into cylindrical secondary batteries, square secondary batteries, coin-type secondary batteries, pouch-type secondary batteries, etc. according to the shapes thereof. In addition, the rechargeable lithium 10 secondary batteries may be classified into bulk-type secondary batteries and film-type secondary batteries according to the size thereof. FIG. 1 is an exploded perspective view showing a rechargeable lithium secondary battery 1 according to one 15 exemplary embodiment of the present invention. FIG. 1 is merely an example for the purpose of illustration only, and is not intended to limit the scope of the present invention. Referring to FIG. 1, the rechargeable lithium secondary battery 1 may be prepared by arranging a negative 20 electrode 3 and a positive electrode 5, disposing a separator 7 between the negative electrode 3 and the positive electrode 5 to manufacture an electrode assembly 9, positioning the electrode assembly 9 in a case 15, and injecting an electrolyte (not shown) so that the negative 25 electrode 3, the positive electrode 5, and the separator 7 19 are impregnated with the electrolyte. Conductive lead members 10 and 13 for collecting current occurring when a battery is operating may be attached to the negative electrode 3 and the positive electrode 5, 5 respectively. The lead members 10 and 13 may conduct current generated from the positive electrode 5 and the negative electrode 3 to positive and negative electrode terminals, respectively. The negative electrode 3 may be manufactured by mixing 10 a negative active material, a binder, and optionally a conductive material to prepare a composition for forming a negative active material layer, followed by applying the composition to a negative current collector such as copper foil. 15 The negative active material is as described above. The binder serves to attach electrode active material particles to each other, and also attach an electrode active material to a current collector. Specific examples of the binder that may be used herein may include polyvinylidene 20 fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, a styrene-butadiene rubber, 25 a fluorine rubber, and various copolymers thereof. 20 In addition, preferred examples of the solvent may include dimethyl sulfoxide (DMSO), alcohol, Nmethylpyrrolidone (NMP), acetone, water, etc. The current collector may include at least one metal 5 selected from the group consisting of copper, aluminum, stainless steel, titanium, silver, palladium, nickel, and alloys and combinations thereof. In this case, the stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy may be 10 preferably used as the alloy. In addition, baked carbon, a non-conductive polymer surface-treated with a conductive material, a conductive polymer, or the like may be used. The conductive material is used to provide conductivity to an electrode and may include any materials 15 that are electrically conductive without inducing chemical changes in the battery thus configured. Examples of the conductive material that may be used herein may include metal powders and fibers such as natural graphite, synthetic graphite, carbon black, acetylene black, Ketjen black, 20 carbon fiber, copper, nickel, aluminum, silver, etc. In addition, the conductive materials such as polyphenylene derivatives may be used alone or in combination of one or more thereof. As a method of applying the prepared composition for 25 forming a negative active material layer to the current 21 collector, one of known methods may be chosen, or a new proper method may be used in consideration of characteristics of materials, etc. For example, the composition for forming a negative active material layer may 5 be distributed onto the current collector, and then uniformly dispersed using a doctor blade. In some cases, distribution and dispersion processes may be carried out as one process. In addition, methods such as die casting, comma coating, screen printing, etc. may also be used. Like 10 the negative electrode 3, the positive electrode 5 may be manufactured by mixing a positive active material, a conductive material, and a binder to prepare a composition for forming a positive active material layer, followed by applying the composition for forming a positive active 15 material layer onto a positive current collector such as aluminum foil and rolling the positive current collector. A positive electrode plate may also be manufactured by casting the composition for forming a positive active material layer onto a separate support and then laminating a film obtained 20 through peeling from the support on a metal current collector. A compound in which lithium ions are reversibly intercalatable and deintercalatable (i.e., a lithiated intercalation compound) may be used as the positive active 25 material. Specifically, a lithium-containing transition 22 metal oxide is preferably used. For example, the positive active material that may be used herein may include at least one selected from group consisting of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiaCobMnc)O2 (0