【DESCRIPTION】 【Technical Field】 The present invention relates to a negative electrode active material for lithium secondary batteries for enhancing and stably expressing battery performance, a 5 method of preparing the same, and a lithium secondary battery including the same. 【Background Art】 Secondary batteries such as lithium secondary batteries (for example, lithium ion batteries) and nickel 10 metal hydride batteries are increasingly, importantly considered as power sources installed in vehicles or power sources of portable terminals such as laptops. In particular, lithium secondary batteries which are light and may provide high energy density may be preferably used as a 15 high-output power source for vehicles, and thus, demand therefor is expected to continuously increase. In regard to lithium secondary batteries, materials in which intercalation and deintercalation of lithium ions may be performed are used as a positive electrode and a 20 negative electrode active material, a liquid electrolyte is injected after disposing a porous separator between a positive electrode and a negative electrode, and electricity is generated or consumed by oxidation-reduction reaction 2 according to intercalation and deintercalation of lithium ions in the negative electrode and the positive electrode. In particular, in lithium secondary batteries, various carbon-based material types including artificial graphite, natural graphite, hard carbon, etc., in which 5 intercalation and deintercalation of lithium is possible, have been used as negative electrode active materials. Since graphite among carbon-based materials has a low discharge voltage of -0.2 V with respect to lithium, a battery using graphite as a negative electrode active material exhibits a 10 high discharge voltage of 3.6 V and there are also advantages in energy density of lithium secondary batteries. In addition, long-term lifespan of lithium secondary batteries is guaranteed due to excellent reversibility. However, graphite active materials have a low capacity with 15 respect to energy density per unit volume of an electrode plate due to low graphite density (theoretical density: 2.2 g/cc) upon manufacture into an electrode plate, and problems such as swelling in a battery and consequent capacity reduction, due to side reaction with an organic electrolyte, 20 which easily occurs in high discharge voltage. In order to address the problems of the carbon-based negative electrode active materials, Si-based negative electrode active materials and negative electrode active materials using oxides such as tin oxides, lithium vanadium-25 3 based oxides, lithium titanium-based oxides, having a high capacity, compared to graphite, are being developed and researched. However, high-capacity Si-based negative electrode materials suffer extreme volume change during 5 charge/discharge and thus particles are split, whereby lifespan characteristics are poor. In addition, in the cases of oxide negative electrodes, satisfactory battery performance is not exhibited and thus research thereinto are underway. In 10 particular, lithium titanium oxides (hereinafter referred to as “LTO”) among the oxide-based negative electrode active materials exhibit high electricity capacity maintenance ratio and stable lifespan characteristics, e.g., change in a crystal structure does not occur also in an over-charge 15 state. However, there is a problem of battery degradation due to high moisture content in an active material itself. [Related Art Document] [Patent Document] Korean Patent Application Pub. No. 1020080018737 20 (published on 28 February 2008) 【Disclosure】 【Technical Problem】 Therefore, the present invention has been made in 4 view of the above problems, and it is an object of the present invention to provide a negative electrode active material for lithium secondary batteries which may enhance battery performance and stably express the battery performance by preventing loss of a solid electrolyte 5 interface (SEI) layer through formation of a stable lithium fluoride (LiF) film, without concern for side reaction occurrence, due to decrease of a moisture amount in an active material and, at the same time, prevention of adsorption of outside moisture, and a method of preparing 10 the same. It is another object of the present invention to provide a lithium secondary battery that may stably express battery performance enhanced through inclusion of the negative electrode active material. 15 【Technical Solution】 In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a negative electrode active material 20 for lithium secondary batteries, the negative electrode active material comprising a core including lithium titanium oxide represented by Formula 1 below and a coating layer located in a surface of the core and including fluorine: 25 5 [Formula 1] LixTiyO4, (wherein 0.8≤x≤1.4 and 1.6≤y≤2.2). The lithium titanium oxide represented by Formula 1 may be Li4Ti5O12 having a spinel structure. 5 The coating layer may include lithium fluoride (LiF). The coating layer may include chemisorbed fluorine (F) in a core surface. The coating layer may be included in an amount of 10 0.1 to 3 parts by weight based on 100 parts by weight of the core. The negative electrode active material may be prepared by reacting the core comprising the lithium titanium oxide represented by Formula 1 with a fluorine-15 containing polymer at 300℃ or more. In accordance with another aspect of the present invention, there is provided a method of preparing a negative electrode active material for lithium secondary batteries, the method comprising reacting a core including 20 a lithium titanium oxide represented by Formula 1 and a fluorine-containing polymer at 300℃ or more. The fluorine-containing polymer may be any one selected from the group consisting of poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropene), 25 6 polytetrafluoroethylene and a mixture thereof. The fluorine-containing polymer is used in an amount of 0.007 to 0.22 mole based on 1 mole of the lithium titanium oxide. In accordance with yet another aspect of the present 5 invention, there is provided a lithium secondary battery comprising a positive electrode comprising a positive electrode active material and a negative electrode including a negative electrode active material, which are disposed opposite each other, and an electrolyte disposed 10 between the positive electrode and the negative electrode, wherein the negative electrode active material comprises a core that comprises a lithium titanium oxide represented by Formula 1 below, and a coating layer that is located in a surface of the core and includes fluorine. 15 Particulars of embodiments of the present invention are described in the detailed description below. 【Advantageous Effects】 A negative electrode active material for lithium 20 secondary batteries according to the present invention may enhance battery performance and stably express the battery performance by preventing loss of a solid electrolyte interface (SEI) layer through formation of a stable fluoride (LiF)-containing coating film on an electrode surface, 25 7 without concern for side reaction occurrence, due to decrease of a moisture amount in an active material and, at the same time, prevention of adsorption of outside moisture, and a method of preparing the same. 5 【Description of Drawings】 The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 10 FIG. 1 illustrates an exploded oblique view of a lithium secondary battery according to an embodiment of the present invention; FIG. 2 illustrates graphs representing results for negative electrode active materials prepared according to 15 Example, Comparative Example 1 and Comparative Example 2 of the present invention measured using a potential difference titration device; and FIG. 3 illustrates graphs representing gas chromatography measurement results for negative electrode 20 active materials prepared according to Example, Comparative Example 1 and Comparative Example 2 of the present invention. 【Best Mode】 25 8 Now, the present invention will be described in more detail with reference to the following examples for easy implementation by those skilled in the art. These examples are provided only for illustration of the present invention and should not be construed as limiting the scope and spirit 5 of the present invention. Terms used in the present invention are used only to explain particular embodiments and the present invention is limited thereto. Singular expressions include plural expression so long as not definitely disclosed otherwise. It 10 should be understood that, in the present invention, terms such as “includes” and “has” are used to designate presence of characteristics, numbers, steps, operations, constituents, components or combinations thereof disclosed in the specification and do not exclude presence or presence 15 possibility of one or more characteristics, numbers, steps, operations, constituents, components or combinations. The present invention is characterized in side reaction due to moisture in a battery is inhibited by reducing a moisture amount in LTO and, at the same time, 20 preventing adsorption of outside moisture through reaction of a lithium titanium oxide (hereinafter referred to as “LTO”) and a fluorine (F)-containing polymer material at high temperature upon preparation of LTO-based negative electrode active material for lithium secondary batteries, 25 9 and loss of a solid electrolyte interface (SEI) is prevented by forming stable fluorine-containing coating layer on an LTO surface, whereby battery performance enhancement and stable performance expression are possible. That is, a negative electrode active material for 5 lithium secondary batteries according to an embodiment of the present invention includes a core including a lithium titanium oxide represented by Formula 1 below and a coating layer located in a surface of the core and including fluorine: 10 [Formula 1] LixTiyO4, (wherein 0.8≤x≤1.4 and 1.6≤y≤2.2). The negative electrode active material may be prepared by reacting a core including the lithium titanium 15 oxide represented by Formula 1 with a fluorine-containing polymer at 300℃ or more. More particularly, the negative electrode active material may be prepared according to a preparation method wherein the core including the lithium titanium oxide 20 represented by Formula 1 and the fluorine-containing polymer are mixed and then reacted at 300℃ or more or 300 to 500℃ under an inactive gas atmosphere such as a nitrogen atmosphere or an argon atmosphere. In regard to preparation of the negative electrode 25 10 active material, the lithium titanium oxide represented by Formula 1 constituting the core may be Li4Ti5O12, particularly having a spinel structure. Here, although a mole number of the oxygen of Formula 1 is 4, Formula 1 is not limited thereto and the mole number may be represented 5 by multiples thereof within a range within which a mole ratio of the each atom of Formula 1 is satisfied. That is, when a mole number of the oxygen of Formula 1 is 12, Formula 1 may be represented by Li3xTi3yO12. The Li4Ti5O12 having a spinel structure may prevent an SEI film from being too 10 thickly formed on a negative electrode surface and may enhance electrochemical characteristics and stability of batteries by controlling thermal runaway factors. In addition, the core including the lithium titanium oxide preferably has an average particle diameter of 3 to 15 15 μm when a specific surface area of an active material and the density of negative electrode mixture are considered. In addition, in regard to the preparation of the negative electrode active material, the fluorine-containing polymer may be particularly poly(vinylidene fluoride) 20 (PVdF), poly(vinylidene fluoride-co-hexafluoropropene (PVdF-co-HFP), polytetrafluoroethylene (PTE) or the like, or a mixture of two or more thereof. When enhancement effects according to the present invention are considered, the content of fluorine in the 25 11 fluorine-containing polymer may be more particularly 0.1 to 3% by weight. In addition, in regard to preparation of the negative electrode active material, the core and the fluorine-containing polymer may be used in a proper content 5 considering the content of a coating layer in a finally prepared negative electrode active material. In particular, in regard to the negative electrode active material, the coating layer including fluorine is preferably included in an amount of 0.1 to 3 parts by weight 10 based on 100 parts by weight of the core. When the content of the coating layer is less than 0.1 parts by weight, complete coating of a core is difficult and thus a lithium titanium oxide constituting the core is exposed to outside, thereby continuously reacting with moisture. In addition, 15 contact force between the core and the coating layer is decreased, and expansion and thus contraction are continuously repeated according to cycle progression, whereby cracks may occur. Meanwhile, when the content of the coating layer is greater than 3 parts by weight, the 20 thickness of the coating layer is increased, whereby electrical conductivity is decreased and initial battery efficiency and performance may be decreased. When enhancement effects according to formation of the coating layer are considered, the coating layer may be particularly 25 12 included in a content of 0.1 to 1 parts by weight based on 100 parts by weight of the core. Accordingly, when the content of the coating layer is considered, a mixture of the core and the fluorine-containing polymer may include 0.007 to 0.22 mole of fluorine-containing polymer based on 1 mole 5 of the lithium titanium oxide. In addition, upon preparation of the negative electrode active material, the core and the fluorine-containing polymer are preferably reacted at 300℃ or more, or 300 to 500℃. When the reaction temperature is less than 10 300℃, reaction between the lithium titanium oxide and the fluorine-containing polymer might not sufficiently carried out, and unreacted fluorine-containing polymers may remain to decrease battery properties. In addition, when the reaction temperature exceeds 500℃, reaction products may be 15 carbonized. A negative electrode active material prepared through reaction at high temperature as described above is present in the core including the lithium titanium oxide represented by Formula 1 and a surface of the core, and includes a 20 coating layer including fluorine. Here, the fluorine-containing polymer used in the preparation process of the negative electrode active material is not physically combined or coated as it is on the core surface and is included as a fluorine-containing 25 13 compound such as lithium fluoride through reaction of fluorine atoms in the polymer and lithium in LTO constituting the core. In addition, in portions in which LiF is not formed due to non-reaction between LTO and F, the fluorine-containing polymer is fired and thus fluorine (F) 5 is chemisorbed to an LTO surface. As such, the lithium fluoride included in the coating layer has superior stability, compared to the fluorine-containing polymer, whereby loss of an SEI layer occurring during charge/discharge may be prevented, battery 10 performance may be enhanced, and stable expression thereof is possible. In addition, the lithium fluoride in the coating layer blocks influence of hydrogen fluoride (HF) formed due to moisture present in a battery except for a negative electrode on a negative electrode active material 15 and thus more stable battery performance may be expressed. In addition, fluorine included in a coating layer makes the coating layer hydrophobic and thus effectively inhibits adsorption and influx of outside moisture. In addition, side reaction due to moisture within a battery may 20 be prevented. Furthermore, since moisture contained in the LTO is used in a formation process of the coating layer, a moisture amount of LTO itself may be decreased. As a result, side reaction due to moisture is decreased upon assembly of a 25 14 battery, and thus, battery performance may be enhanced. In particular, the content of the moisture in the core of the negative electrode active material may be 500 to 2000 ppm. In the negative electrode active material prepared according to the preparation method described above, LiF 5 formed through chemical reaction of the LTO and LiF formed through chemical reaction of the LTO and the fluorine-containing polymer, and fluorine atoms chemisorbed by firing the fluorine-containing polymer are included in a surface of a core including the LTO, and thus, battery characteristic 10 enhancement effects are superior and stable performance expression is possible. According to another embodiment of the present invention, a lithium secondary battery including a negative electrode active material according to the preparation 15 method is provided. In particular, the lithium secondary battery includes a positive electrode comprising a positive electrode active material and a negative electrode including a negative electrode active material, which are disposed opposite each 20 other, and an electrolyte disposed between the positive electrode and the negative electrode, and the negative electrode active material is the same as that described above. The lithium secondary battery may be classified into 25 15 lithium ions batteries, lithium ion polymer battery and lithium polymer battery according to used separator and electrolyte types, a cylindrical shape, a square shape, a coin shape, a pouch shape, etc. according to shape thereof, and a bulk type and a film type according to the size 5 thereof. FIG. 1 illustrates 무 exploded oblique view of a lithium secondary battery 1 according to another embodiment of the present invention. FIG. 1 is provided to explain the present invention, but the present invention is not limited 10 thereto. Referring to FIG. 1, in regard to the lithium secondary battery 1, a negative electrode 3, a positive electrode 5, and a separator 7 between the negative electrode 3 and the positive electrode 5 are installed, 15 thereby manufacturing an electrode assembly 9. The electrode assembly 9 is located in a case 15 and an electrolyte (not shown) is injected thereinto. Accordingly, the negative electrode 3, the positive electrode 5 and the separator 7 are impregnated with an electrolyte. 20 Each of conductive lead members 10 and 13 for collecting current occurring when a battery operates may be adhered to each of the negative electrode 3 and the positive electrode 5. Each of the lead members 10 and 13 may induce current generated from the positive electrode 5 and the 25 16 negative electrode 3 to a positive electrode terminal and a negative electrode terminal. The negative electrode 3 may be manufactured by preparing a composition for forming a negative electrode active material layer through mixing of a negative electrode 5 active material, a binder and, selectively, a conductive material, and then by spreading the composition on negative electrode current collector such as copper foil. The negative electrode active material is the same as that described above. 10 The binder adheres electrode active material particles to one another, and an electrode active material to a current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl 15 cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber and various copolymers thereof. 20 In addition, preferable embodiments of the solvent include dimethyl sulfoxide (DMSO), alcohol, N-methylpyrrolidone (NMP), acetone, water, etc. The current collector may be any one metal selected from the group consisting of copper, aluminum, stainless 25 17 steel, titanium, silver, palladium, nickel, alloys thereof and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium or silver, and the alloy is preferably an aluminum-cadmium alloy. In addition, a non-conductive polymer, a conductive polymer, or 5 the like surface-treated with fired carbon and a conductive material may be used. The conductive material is used to provide conductivity to an electrode and may be any materials that do not induce chemical change and have electrical 10 conductivity. Examples of the conductive material include metal powders, metal fibers, etc. such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, copper, nickel, aluminum, silver, etc. In addition, a mixture of one or more of conductive 15 materials such as polyphenylene derivatives may be used. As a method of spreading the composition for forming a negative electrode active material layer on the current collector, any one of publicly known methods may be selected or a new proper method may be used, considering 20 characteristics of materials. Preferably, the composition for forming a negative electrode active material layer is distributed on a current collector and then uniformly dispersed using a doctor blade, etc. In some cases, distribution and dispersion processes may be carried as one 25 18 process. In addition, a method such as die casting, comma coating, screen printing, etc. may be used. The positive electrode 5 may be manufactured by mixing a positive electrode active material, a conductive material and a binder to prepare composition for forming a 5 positive electrode active material layer, and then by coating the composition for forming a positive electrode active material layer on a positive electrode current collector such as aluminum foil and then rolling the same, as in the negative electrode 3. A positive electrode plate 10 may be manufactured by casting the positive electrode active material composition on a separate support and then laminating a film obtained through peeling of the support on a metal current collector. As the positive electrode active material, a compound 15 in which reversible intercalation and deintercalation of lithium are possible (lithiated intercalation compound) may be used. In particular, a lithium-containing transition metal oxide is preferably used, and, for example, any one selected from group consisting of LiCoO2, LiNiO2, LiMnO2, 20 LiMn2O4, Li(NiaCobMnc)O2 (0