TECHNICAL FIELD Cross-reference to Related Applications 5 [0001] This application claims the benefit of Korean Patent Application Nos. 2014-0131746, filed on September 30, 2014, and 2015-0124201, filed on September 2, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 10 Technical Field [0002] The present invention relates to a method of preparing a lithium secondary battery which may simultaneously improve output characteristics and lifetime characteristics of the lithium secondary battery. 15 BACKGROUND ART [0003] Recently, in line with the development of information and telecommunications industry, electronic devices are being miniaturized, light-weighted, reduced in thickness, and portable. As a result, the need for high energy density 20 batteries used as power sources of such electronic devices has increased. Currently, research into lithium secondary batteries, as batteries that may best satisfy the above need, has actively conducted. [0004] A lithium secondary battery is a battery which is 25 composed of a positive electrode, a negative electrode, and 2 an electrolyte and a separator which provide movement paths of lithium ions between the positive electrode and the negative electrode, wherein electrical energy is generated by oxidation and reduction reactions that occur when lithium 5 ions are stored in and released from the positive electrode and the negative electrode. [0005] A lithium secondary battery has an average discharge voltage of about 3.6 V to about 3.7 V, and one of the advantages of the lithium secondary battery is that it has a 10 higher discharge voltage than other alkaline batteries and a nickel-cadmium battery. In order to achieve such a high operating voltage, an electrolyte composition, which is electrochemically stable in a charge and discharge voltage range of 0 V to 4.2 V, is required. 15 [0006] Lithium ions released from a positive electrode active material, such as lithium metal oxide, during initial charging of a lithium secondary battery move to a negative electrode active material, such as a graphite-based material, to be intercalated into interlayers of the negative electrode 20 active material. In this case, since lithium is highly reactive, lithium reacts with an electrolyte and carbon constituting the negative electrode active material on a surface of the negative electrode active material, such as a graphite-based material, to form a compound such as Li2CO3, 25 Li2O, or LiOH. These compounds may form a solid electrolyte 3 interface (SEI) film on the surface of the negative electrode active material such as a graphite-based material. [0007] The SEI film may only pass lithium ions by acting as an ion tunnel. Due to the effect of the ion tunnel, the SEI 5 film may prevent the destruction of a negative electrode structure due to the intercalation of organic solvent molecules having a high molecular weight, which move with lithium ions in the electrolyte, into the interlayers of the negative electrode active material. Thus, it has been 10 reported that the decomposition of the electrolyte does not occur by preventing the contact between the electrolyte and the negative electrode active material, and stable charge and discharge may be maintained by reversibly maintaining the amount of lithium ions in the electrolyte. 15 [0008] However, the SEI film of the lithium secondary battery may be unstable due to an additive or an organic solvent included in the electrolyte, and although the SEI film may be stably formed, gas may be generated due to the decomposition of the residual additive. 20 [0009] Even in a case of using an imide-based salt which may improve high-temperature storability and low-temperature output characteristics by minimizing an increase in viscosity of the organic solvent at low temperature and improving the mobility of the lithium ions, there is a significant 25 limitation in using the lithium secondary battery due to the 4 possibility of corrosion. [0010] Thus, there is an urgent need to develop a method of improving output characteristics and lifetime characteristics of the lithium secondary battery while forming a robust and 5 uniform SEI film and selectively selecting and using an electrolyte. DISCLOSURE OF THE INVENTION TECHNICAL PROBLEM [0011] The present invention provides a method of preparing 10 a lithium secondary battery which may simultaneously improve output characteristics and lifetime characteristics of the lithium secondary battery by preparing an electrode on which an SEI film is formed through a pretreatment process, putting an electrode assembly including the electrode in a battery 15 case, and injecting an electrolyte thereinto. TECHNICAL SOLUTION [0012] According to an aspect of the present invention, there is provided a method of preparing a lithium secondary battery including: performing a pretreatment in which an SEI 20 (solid electrolyte interphase) film is formed on an electrode by impregnating the electrode in a composition for forming an SEI film, which includes a lithium salt, a non-aqueous organic solvent, and an SEI film-forming agent capable of forming the SEI film by an electrochemical oxidation or 25 reduction decomposition reaction, and applying a voltage to 5 the electrode; and preparing an electrode assembly using the electrode having the SEI film formed thereon, putting the electrode assembly in a battery case, and performing an assembly process of electrolyte injection and charging one or 5 more times. [0013] According to another aspect of the present invention, there is provided a lithium secondary battery prepared by the above method. [0014] According to another aspect of the present invention, 10 there is provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module. [0015] Particularities of other embodiments of the present invention are included in the following detailed description. 15 ADVANTAGEOUS EFFECTS [0016] According to a method of preparing a lithium secondary battery according to an embodiment of the present invention, output characteristics and lifetime characteristics of a lithium secondary battery may be further 20 improved by forming an SEI film on an electrode in advance through a pretreatment of the electrode, putting an electrode assembly including the electrode having the SEI film formed thereon in a battery case, and stepwise injecting an electrolyte one or more times. 25 [0017] Also, an amount of an expensive additive used may be 6 reduced by using only a small amount of the additive required for the formation of the SEI film in a pretreatment step, the release of a gas generated in the battery case may be facilitated, and various electrolytes may be selectively used 5 by the stepwise injection of the electrolyte. Thus, the performance of the lithium secondary battery may be further improved by overcoming limitations due to the typical use of a lithium salt and an additive. BRIEF DESCRIPTION OF THE DRAWINGS 10 [0018] The following drawings attached to the specification illustrate preferred examples of the present invention by example, and serve to enable technical concepts of the present invention to be further understood together with detailed description of the invention given below, and 15 therefore the present invention should not be interpreted only with matters in such drawings. [0019] FIG. 1 is a flowchart illustrating a method of preparing a lithium secondary battery according to an embodiment of the present invention; 20 [0020] FIG. 2 is a flowchart illustrating an electrode injection and charging step of step II in the method of preparing a lithium secondary battery according to the embodiment of the present invention; [0021] FIG. 3 is a graph illustrating experimental results 25 of output characteristics of lithium secondary batteries 7 prepared in Examples 1 and 2 and Comparative Examples 1 and 4; and [0022] FIG. 4 is a graph illustrating experimental results of lifetime characteristics of lithium secondary batteries 5 prepared in Examples 1 to 5 and Comparative Examples 1 to 4. MODE FOR CARRYING OUT THE INVENTION [0023] Hereinafter, the present invention will be described in more detail to allow for a clearer understanding of the present invention. 10 [0024] It will be understood that words or terms used in the specification and claims shall not be interpreted as the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their 15 meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention. [0025] A method of preparing a lithium secondary battery 20 according to an embodiment of the present invention includes: performing a pretreatment in which an SEI film is formed on an electrode by impregnating the electrode in a composition for forming an SEI film, which includes a lithium salt, a non-aqueous organic solvent, and an SEI film-forming agent 25 forming the SEI film by an electrochemical oxidation- 8 reduction decomposition reaction, and applying a voltage to the electrode (step I ) ; and preparing an electrode assembly using the electrode having the SEI film formed thereon, putting the electrode assembly in a battery case, and 5 performing a process of electrolyte injection and charging one or more times (step II). [0026] According to the method of preparing a lithium secondary battery according to the embodiment of the present invention, output characteristics and lifetime 10 characteristics of a lithium secondary battery may be further improved by forming an SEI film on an electrode in advance through a pretreatment of the electrode, putting the electrode having the SEI film formed thereon in a battery case, and stepwise injecting an electrolyte one or more times. 15 [0027] Also, an amount of an expensive additive used may be reduced by using only a small amount of the additive required for the formation of the SEI film in the pretreatment step, the release of a gas generated in the battery case may be facilitated, and various electrolytes may be selectively used 20 by the stepwise injection of the electrolyte. Thus, the performance of the lithium secondary battery may be further improved by overcoming limitations due to the typical use of a lithium salt and an additive. [0028] FIG. 1 is a flowchart illustrating the method of 25 preparing a lithium secondary battery according to the 9 embodiment of the present invention. However, FIG. 1 is only an example for describing the present invention and the present invention is not limited thereto. [0029] Hereinafter, the present invention will be described 5 in more detail for each step with reference to FIG. 1. In the method of preparing a lithium secondary battery according to the embodiment of the present invention, step I, as a pretreatment step for preparing a secondary battery, may include forming an SEI film on an electrode using a 10 composition for forming an SEI film. [0030] That is, step I is a step of forming an SEI film on a positive electrode or a negative electrode by putting the electrode, i.e., the positive electrode or the negative electrode, in a composition for forming an SEI film, which 15 includes an SEI film-forming agent capable of easily forming the SEI film due to good wettability with respect to an electrolyte as well as forming the SEI film by an electrochemical oxidation or reduction decomposition reaction, and applying a voltage to perform the electrochemical 20 reaction. [0031] In general, since an electrode used in a lithium secondary battery is strongly lipophilic, the electrode has poor wetting with an electrolyte having a hydrophilic property. In a case where activation of the battery is 25 performed in a state in which the electrolyte does not 10 sufficiently wet the electrode, since an SEI film is not properly formed on the electrode, the lifetime characteristics of the battery may be degraded. [0032] In the present invention, since the electrode 5 pretreatment is performed by using the composition for forming an SEI film which may improve the wetting with the electrolyte, an SEI film may be sufficiently formed on the electrode in advance due to the good wettability. [0033] The composition for forming an SEI film is not 10 particularly limited so long as it is a solution including a compound which may form the SEI film on the electrode, and may specifically include an SEI film-forming agent, a lithium salt, and a non-aqueous organic solvent. [0034] The lithium salt usable in the preparation of the 15 composition for forming an SEI film is a source of lithium ions, wherein the lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions. Specific examples of the lithium salt may be LiPF6, LiAsF6, LiCF3SO3, LiBF4, LiBF6, LiSbF6, LiAlO4, LiAlCl4, 20 LiSO3CF3, or LiClO4, and any one thereof or a mixture of two or more thereof may be used. [0035] Also, the lithium salt may be included in a concentration of 0.5 mol/l to 2 mol/l in the composition for forming an SEI film. In a case in which the concentration of 25 the lithium salt is less than 0.5 mol/l, an amount of lithium 11 ions provided is not sufficient, and in a case in which the concentration of the lithium salt is greater than 2 mol/l, since viscosity of the composition for forming an SEI film may be increased, formability of the SEI film may be reduced. 5 When considering conductivity of the lithium ions in the SEI film and the formability of the SEI film, the lithium salt, for example, may be included in a concentration of 0.5 mol/l to 1.6 mol/l in the composition for forming an SEI film. [0036] Typically, an SEI film may also be formed by an 10 electrochemical oxidation-reduction reaction of an electrolyte including a lithium salt and a non-aqueous organic electrolyte. However, the SEI film formed by the decomposition of the non-aqueous organic electrolyte is thick and has a high resistance. 15 [0037] In contrast, in the present invention, since a thickness of the formed SEI film is reduced but density is increased by using an additive for forming an SEI film, an SEI film having improved properties may be formed, for example, resistance in the SEI film is decreased. 20 [0038] In the method of preparing a lithium secondary battery according to the embodiment of the present invention, the SEI film-forming agent usable in the preparation of the composition for forming an SEI film, as a compound capable of forming the SEI film by an electrochemical oxidation or 25 reduction decomposition reaction, may specifically include a 12 cyclic carbonate-based compound containing an unsaturated bond; a cyclic or chain carbonate-based compound containing a halogen atom; a lithium salt containing an oxalato complex as an anion; an imide-based lithium salt; a fluorophosphate- 5 based lithium salt; a fluoroborate-based lithium salt; a 6- membered aromatic heterocyclic compound containing at least two nitrogen atoms in a molecule; a sultone-based compound; or an acrylate-based compound, and any one thereof or a mixture of two or more thereof may be used. Unless otherwise 10 specified in the present invention, the SEI film-forming agent is a compound different from a lithium salt and a nonaqueous organic solvent used in the preparation of the composition for forming an SEI film and described below. [0039] In the SEI film-forming agent, the cyclic carbonate- 15 based compound containing an unsaturated bond may specifically be a vinylene carbonate-based compound such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, dimethyl vinylene carbonate, or vinylene ethylene carbonate; or a vinyl 20 ethylene carbonate-based compound, such as vinyl ethylene carbonate, and any one thereof or a mixture of two or more thereof may be used. [0040] Also, in the SEI film-forming agent, the cyclic or chain carbonate-based compound containing a halogen atom may 25 specifically be a cyclic carbonate-based compound containing 13 at least one halogen atom in a molecule such as fluoroethylene carbonate or difluoroethylene carbonate; and a chain carbonate-based compound containing at least one halogen atom in a molecule such as fluoromethyl methyl 5 carbonate or bis(fluoromethyl)carbonate, and any one thereof or a mixture of two or more thereof may be used. [0041] Furthermore, in the SEI film-forming agent, as the lithium salt containing an oxalato complex as an anion, any lithium salt may be used without limitation as long as an 10 anionic compound containing an oxalate group and lithium ions form a complex through coordination bonds. Specific examples of the lithium salt containing an oxalato complex as an anion may be lithium difluoro(oxalato)borate (LiODFB), lithium tetrafluoro(oxalato) phosphate (LiTFOP), lithium 15 tris(oxalate)phosphate (LTOP), or lithium bis(oxalato)borate (LiBOB), and any one thereof or a mixture of two or more thereof may be used. [0042] In the SEI film-forming agent, an imide-based lithium salt such as lithium bisfluorosulfonyl imide (LiFSI), lithium 20 bis trifluoromethanesulfonyl imide (LiTFSI), or lithium bis(perfluoroethylsulfonyl)imide (LiBETI); and a fluorophosphate-based lithium salt, such as LiBF4, lithium difluorophosphate (LiF2O2P), or lithium monofluorophosphate (Li2PO3F), may be used as the lithium salt for forming an SEL 25 film in addition to the lithium salt containing an oxalato 14 complex as an anion, and the above compounds may be used alone or in a mixture of two or more thereof. [0043] The lithium salt containing an oxalato complex as an anion, the imide-based lithium salt, or the fluorophosphate- 5 based salt, which may be used as the SEI film-forming agent, may also be used as the lithium salt in the composition for forming an SEI film. In this case, the lithium salt containing an oxalato complex as an anion, the imide-based lithium salt, or the fluorophosphate-based salt may be 10 included in an amount in which a total concentration of the lithium salt included in the composition for forming an SEI film simultaneously satisfies a condition of lithium salt concentration in the composition for forming an SEI film and a condition of the amount of the SEI film-forming agent. 15 [0044] Also, in the SEI film-forming agent, the 6-membered aromatic heterocyclic compound containing at least two nitrogen atoms in a molecule may specifically be pyrimidine and 1,3,5-triazine, and any one thereof or a mixture of two or more thereof may be used. 20 [0045] Furthermore, in the SEI film-forming agent, the sultone-based compound may specifically be 1,3-propane sultone (PS), 1,4-butane sultone, or 1,3-propene sultone, and any one thereof or a mixture of two or more thereof may be used. 25 [0046] The SEI film-forming agent may be included in an 15 amount of 0.1 wt% to 10 wt% based on a total weight of the composition for forming an SEI film. When the amount of the SEI film-forming agent is within the above range, a uniform and thin film derived from the additive for forming an SEI 5 film may be formed on the electrode after the electrical reaction, and an amount of a gas generated by charging may be minimized. Also, when considering an effect of improving physical properties of the SEI film formed according to the use of the SEI film-forming agent, the SEI film-forming agent 10 may specifically be included in an amount of 0.25 wt% to 5 wt%, for example, 0.5 wt% to 3 wt%, based on the total weight of the composition for forming an SEI film. [0047] In the composition for forming an SEI film, the nonaqueous organic solvent functions as a medium through which 15 ions involved in the electrochemical reaction may move, wherein any non-aqueous organic solvent, which may be minimally decomposed by an oxidation reaction or the like during charging and discharging of the battery and may exhibit desired characteristics with the SEI film-forming 20 agent, may be appropriate. [0048] Specifically, the non-aqueous organic solvent may be a cyclic carbonate-based solvent such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); a linear carbonate-based solvent such as dimethyl carbonate 25 (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), 16 ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), and ethylpropyl carbonate (EPC); an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε- caprolactone; an ether-based solvent such as dibutyl ether or 5 tetrahydrofuran; or a ketone-based solvent, such as cyclohexanone, and any one thereof or a mixture of two or more thereof may be used. Among the above solvents, the organic solvent may be a mixture of cyclic carbonate and linear carbonate in consideration of the formability of the 10 SEI film, and, for example, may be a mixture in which the linear carbonate is mixed in a larger amount than the cyclic carbonate. Specifically, a mixing volume ratio of the linear carbonate to the cyclic carbonate may be in a range of 5:5 to 8:2. 15 [0049] The non-aqueous organic solvent may be included in such an amount that the composition for forming an SEI film has a viscosity which facilitates the formation of the SEI film. [0050] The electrode pretreated by the composition for 20 forming an SEI film may be a positive electrode or a negative electrode. [0051] Specifically, in a case in which the electrode is a positive electrode, the usable positive electrode may include a structure in which a positive electrode active material 25 layer is stacked on a current collector such as a thin film 17 formed of aluminum or an aluminum-based alloy. In this case, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material and the 5 conductive agent may be stacked on the current collector by the binder. [0052] The positive electrode active material is a compound (lithiated intercalation compound) capable of reversibly intercalating and deintercalating lithium, wherein the 10 positive electrode active material may specifically be a lithium transition metal oxide including lithium and a transition metal such as cobalt, manganese, nickel, or aluminum. Furthermore, the lithium transition metal oxide may be selected form the group consisting of lithium-nickel- 15 manganese-cobalt-based oxide, lithium-manganese-based oxide, lithium-nickel-manganese-based oxide, and lithium-manganesecobalt- based oxide, and, for example, may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiaCobMnc)O2 (where 0 [00133] A composition for forming a positive electrode was prepared by adding 96 wt% of Li(Ni0.33Co0.33Mn0.33)O2 as a positive electrode active material, 2 wt% of carbon black as 25 a conductive agent, and 2 wt% of polyvinylidene fluoride 44 (PVdF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. A 20 μm thick aluminum (Al) thin film as a positive electrode collector was coated with the composition for forming a positive electrode and dried, and the Al thin film 5 was then roll-pressed to prepare a positive electrode. [00134] Also, a composition for forming a negative electrode was prepared by adding 96 wt% of carbon powder as a negative electrode active material, 3 wt% of PVdF as a binder, and 1 wt% of carbon black as a conductive agent to NMP as a solvent. 10 A 10 μm thick copper (Cu) thin film as a negative electrode collector was coated with the composition for forming a negative electrode and dried, and the Cu thin film was then roll-pressed to prepare a negative electrode. [00135] 15 [00136] A composition for forming an SEI film was prepared by adding LiPF6, as a lithium salt, at a concentration of 1.0 M to a non-aqueous organic solvent having a composition, in which a volume ratio of ethylene carbonate (EC):ethylmethyl carbonate (EMC):dimethyl carbonate (DMC) was 3:3:4, and then 20 adding 1 wt% of vinylene carbonate (VC) based on a total weight of the composition for forming an SEI film. [00137] [00138] The composition for forming an SEI film and negative electrode thus prepared were put in a 15 mL bath and an 25 electrochemical reaction was then performed using a lithium 45 metal foil as a counter electrode. For the electrochemical reaction, a constant current of 0.05 C was applied to the negative electrode and the counter electrode within a voltage range of 2.5 V to 0.005 V, and, when the voltage was 0.005 V, 5 an SEI film was formed on the negative electrode by applying a current under a constant voltage condition until a current value became 1/20 C. [00139] Step II: Electrolyte Injection and Charging Step (Twice Injection) 10 [00140] Step i) First Electrolyte Injection Step [00141] [00142] A first electrolyte was prepared by adding 0.25 ml of 1M LiPF6, as a lithium salt, and 1 wt% of 1,3-propane sultone (PS), as an additive, based on a total weight of a non- 15 aqueous electrolyte solution to a non-aqueous organic solvent having a composition in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4. [00143] 20 [00144] An electrode assembly was prepared by disposing a separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) between the positive electrode prepared in step I and the negative electrode having the SEI film formed thereon, the electrode 25 assembly was put in a battery case, and the first electrolyte 46 was then injected. [00145] Step ii) First Charging Step [00146] As a first charging process, first charging was performed at a constant current of 0.1 C up to 4.2 V in a 5 state in which an electrolyte injection hole of the battery case obtained in step i) was not sealed. [00147] Step iii) Second Electrolyte Injection Step [00148] [00149] A second electrolyte was prepared by adding 0.25 ml 10 of 1M LiFSI, as a lithium salt, based on the total weight of the non-aqueous electrolyte solution to a non-aqueous organic solvent having a composition, in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4, and the second electrolyte was 15 then injected into the electrode assembly first charged in step i i ). [00150] Step iv) Second Charging Step [00151] An electrolyte injection hole of the battery case obtained in step iii) was sealed, and, as a second charging 20 process, second charging was then performed at a constant current of 0.2 C up to 4.2 V. [00152] Example 2 [00153] A lithium secondary battery was prepared in the same manner as in Example 1 except that the electrolyte injection 25 and charging step of step II in Example 1 was performed only 47 one time as follows: [00154] Step II: Electrolyte Injection and Charging Step (Single Injection) [00155] Step i) First Electrolyte Injection Step 5 [00156] [00157] A first electrolyte was prepared by adding 0.5 ml of 1M LiPF6, as a lithium salt, and 1 wt% of 1,3-propane sultone (PS), as an additive, based on a total weight of a nonaqueous electrolyte solution to a non-aqueous organic solvent 10 having a composition in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4. [00158] [00159] An electrode assembly was prepared by disposing a 15 separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) between the positive electrode prepared in step I and the negative electrode having the SEI film formed thereon, the electrode assembly was put in a battery case, and the first electrolyte 20 was then injected. [00160] Step ii) First Charging Step [00161] As a first charging process, first charging was performed at a constant current of 0.1 C up to 4.2 V in a state in which an electrolyte injection hole of the battery 25 case obtained in step i) was not sealed. 48 [00162] Examples 3 to 8 [00163] Lithium secondary batteries were prepared in the same manner as in Example 1 except that compositions for forming an SEI film and first and/or second electrolytes were 5 prepared according to formulations listed in the following Table 1 and used. [00164] Comparative Example 1 [00165] i) Electrolyte Injection Step [00166] 10 [00167] An electrolyte was prepared by adding a total of 0.5 ml of 0.5M LiPF6 and 0.5 M LiFSI, as a lithium salt, based on a total weight of a non-aqueous electrolyte solution to a non-aqueous organic solvent having a composition in which a volume ratio of propylene carbonate (PC):ethylene carbonate 15 (EC):ethylmethyl carbonate (EMC) was 3:3:4. [00168] [00169] A composition for forming a positive electrode was prepared by adding 96 wt% of Li(Ni0.33Co0.33Mn0.33)O2 as a positive electrode active material, 2 wt% of carbon black as 20 a conductive agent, and 2 wt% of polyvinylidene fluoride (PVdF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. A 20 μm thick aluminum (Al) thin film as a positive electrode collector was coated with the composition for forming a positive electrode and dried, and the Al thin film 25 was then roll-pressed to prepare a positive electrode. 49 [00170] Also, a composition for forming a negative electrode was prepared by adding 96 wt% of carbon powder as a negative electrode active material, 3 wt% of PVdF as a binder, and 1 wt% of carbon black as a conductive agent to NMP as a solvent. 5 A 10 μm thick copper (Cu) thin film as a negative electrode collector was coated with the composition for forming a negative electrode and dried, and the Cu thin film was then roll-pressed to prepare a negative electrode. [00171] An electrode assembly was prepared by disposing a 10 separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) between the positive electrode and negative electrode thus prepared, the electrode assembly was put in a battery case, and the electrolyte was then injected. 15 [00172] ii) Charging Step [00173] An electrolyte injection hole of the battery case obtained in step i) was sealed, and, as a first charging process, charging was then performed at a constant current of 0.1 C up to 4.2 V. 20 [00174] Comparative Example 2 [00175] Step i) First Electrolyte Injection Step [00176] [00177] A first electrolyte was prepared by adding 0.25 ml of 1M LiPF6, as a lithium salt, and 1 wt% of 1,3-propane sultone 25 (PS), as an additive, based on a total weight of a non- 50 aqueous electrolyte solution to a non-aqueous organic solvent having a composition in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4. 5 [00178] [00179] An electrode assembly was prepared by disposing a separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) between the positive electrode and negative electrode prepared in 10 Comparative Example 1, the electrode assembly was put in a battery case, and the first electrolyte was then injected. [00180] Step ii) First Charging Step [00181] As a first charging process, first charging was performed at a constant current of 0.1 C up to 4.2 V in a 15 state in which an electrolyte injection hole of the battery case obtained in step i) was not sealed. [00182] Step iii) Second Electrolyte Injection Step [00183] [00184] A second electrolyte was prepared by adding 0.25 ml 20 of 1M LiFSI, as a lithium salt, based on the total weight of the non-aqueous electrolyte solution to a non-aqueous organic solvent having a composition, in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4, and the second electrolyte was 25 then injected into the electrode assembly first charged in 51 step i i ). [00185] Step iv) Second Charging Step [00186] An electrolyte injection hole of the battery case obtained in step iii) was sealed, and, as a second charging 5 process, second charging was then performed at a constant current of 0.2 C up to 4.2 V. [00187] Comparative Example 3 [00188] A lithium secondary battery was prepared in the same manner as in Comparative Example 2 except that, in the method 10 of preparing the lithium secondary battery of Comparative Example 2, LiFSI, instead of LiPF6, was used as a lithium salt during the preparation of the first electrolyte, LiPF6, instead of LiFSI, was used as a lithium salt during the preparation of the second electrolyte, and 1,3-propane 15 sultone (PS) was further used as an additive. [00189] Comparative Example 4 [00190] A lithium secondary battery was prepared in the same manner as in Comparative Example 1 except that, in the method of preparing the lithium secondary battery of Comparative 20 Example 1, an electrolyte, which was prepared by adding 0.5 ml of 1M LiPF6, as a lithium salt, and 1 wt% of 1,3-propane sultone (PS) and 1 wt% of vinylene carbonate (VC), as an additive, based on a total weight of a non-aqueous electrolyte solution to a non-aqueous organic solvent having 25 a composition in which a volume ratio of propylene carbonate 52 (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4, was used. [00191] Comparative Example 5 [00192] A lithium secondary battery was prepared in the same 5 manner as in Comparative Example 3 except that, in the method of preparing the lithium secondary battery of Comparative Example 3, an electrolyte, which was prepared by adding 0.5 ml of 1M LiPF6, as a lithium salt, and 1 wt% of vinylene carbonate (VC), as an additive, based on a total weight of a 10 non-aqueous electrolyte solution to a non-aqueous organic solvent having a composition in which a volume ratio of propylene carbonate (PC):ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:3:4, was used. [00193] The compositions for forming an SEI film and the 15 compositions of the first and second electrolytes, which were used in Examples 1 to 8 and Comparative Examples 1 to 5, are listed in the following Table 1. [00194] [Table 1] Example 1 Example 2 Composition for forming a negative electrode SEI film (Composition A) (Composition First electrolyte (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) o (PC/EC/EMC=3/3/4, Second electrolyte (PC/EC/EMC=3/3/4, LiFSI 1M) × 53 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 A) (Composition B) (Composition C) (Composition D) (Composition E) (Composition F) (Composition G) x x x x LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiPF6 0.5M, LiFSI 0.5M) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) (PC/EC/EMC=3/3/4, LiFSI 1M) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 (PC/EC/EMC=3/3/4, LiFSI 1M) x (PC/EC/EMC=3/3/4, LiFSI 1M) x (PC/EC/EMC=3/3/4, LiFSI 1M) x x (PC/EC/EMC=3/3/4, LiFSI 1M) (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) x 54 Comparative Example 5 × wt%, VC 1 wt%) o (EC/EMC/DMC =3/3/4, LiPF6 1M, VC 1 wt%) o (PC/EC/EMC=3/3/4, LiPF6 1M, PS 1 wt%) [00195] In Table 1, the mixing ratio of the solvents was based on a volume ratio, and [00196] Composition A: EC/EMC/DMC=3/3/4, LiPF6 1 M, VC 1 wt%; 5 [00197] Composition B: EC/EMC/DMC=3/3/4, LiPF6 1 M, FEC 1 wt%; [00198] Composition C: EC/EMC/DMC=3/3/4, LiPF6 0.9 M, LiODFB 0.1 M; [00199] Composition D: EC/EMC/DMC=3/3/4, LiPF6 0.9 M, LiFSI 10 0.1 M; [00200] Composition E: EC/EMC/DMC=3/3/4, LiPF6 0.9 M, LiF2O2P 0.1 M; [00201] Composition F: EC/EMC/DMC=3/3/4, LiPF6 1 M, pyrimidine 0.5 wt%; and 15 [00202] Composition G: EC/EMC/DMC=3/3/4, LiPF6 1 M, PS 0.5 wt%. [00203] Experimental Example 1: Output Characteristics Test [00204] Outputs were calculated from voltage differences which were obtained by discharging the lithium secondary 20 batteries of Examples 1 and 2 and Comparative Examples 1 and 4 at 0.5 C for 10 seconds for the state of charge (SOC) at 55 room temperature. The results thereof are presented in FIG. 3. [00205] As illustrated in FIG. 3, output characteristics of the lithium secondary batteries of Examples 1 and 2 subjected 5 to the pretreatment step of forming an SEI film on the electrode were significantly improved in comparison to those of the lithium secondary batteries of Comparative Examples 1 and 4 which were not subjected to the pretreatment step of forming an SEI film. 10 [00206] Specifically, with respect to Example 1 in which the electrolyte injection and charging step was performed twice while performing the pretreatment step of forming an SEI film on the electrode, it may be understood that the output characteristics were improved by about 15% to about 70% at 15 90% SOC in comparison to those of Comparative Examples 1 and 4. [00207] Also, when comparing Example 2 and Comparative Example 4, it may be understood that the output characteristics of Comparative Example 4, which was not 20 subjected to the pretreatment step of forming an SEI film on the electrode, were decreased by about 15% to about 20% in comparison to those of Example 2. [00208] Thus, it may be understood that the output characteristics were significantly improved when the 25 pretreatment step of forming an SEI film on the electrode was 56 performed before the electrolyte injection and charging step. [00209] Experimental Example 2: Lifetime Characteristics Test [00210] The lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 4 were charged at 1 C to 4.2 5 V/3.25 mA at room temperature under a constant current/constant voltage (CC/CV) condition and then discharged at a constant current (CC) of 3 C to a voltage of 3.03 V to measure discharge capacities. The charge and discharge were repeated 1 to 80 cycles and the measured 10 discharge capacities are presented in FIG. 4. [00211] As illustrated in FIG. 4, it may be understood that lifetime characteristics of Examples 1 to 5 subjected to the pretreatment step of forming an SEI film on the electrode according to the present invention were significantly 15 improved in comparison to those of the lithium secondary batteries of Comparative Examples 1 to 4 which were not subjected to the pretreatment step. [00212] Specifically, with respect to Example 1 in which the electrolyte injection and charging step was performed one or 20 more times while performing the pretreatment step of forming an SEI film on the electrode, it may be understood that initial discharge capacity was similar to that of Comparative Examples 1 to 4, but discharge capacity after about 10 cycles was significantly improved in comparison to that of 25 Comparative Examples 1 to 4. 57 [00213] That is, with respect to the secondary batteries of Examples 1 to 5, slopes of the resulting graphs of lifetime characteristics from the 1st cycle to the 80th cycle were slow so that discharge capacities at the 80th cycle were hardly 5 decreased from the initial discharge capacities. In contrast, with respect to Comparative Examples 1 to 4, it may be confirmed that discharge capacities were significantly decreased after a 30th cycle and lifetime characteristics at the 80th cycle were decreased by about 5% to about 15% in 10 comparison to those of Example 1. [00214] Also, when comparing Comparative Examples 1 and 2 in which the electrolyte injection and charging step was performed twice under the same condition, it may be observed that the lifetime characteristics were decreased by about 11% 15 depending on whether the pretreatment step of forming an SEI film on the electrode was performed before the electrolyte injection and charging step or not. [00215] Thus, it may be understood that the lifetime characteristics were significantly improved when the 20 pretreatment step of forming an SEI film on the electrode was performed before the electrolyte injection and charging step. [00216] Experimental Example 3: Swelling Characteristics Test [00217] The lithium secondary batteries of Examples 2 to 8 and Comparative Examples 4 and 5 were charged at 1 C to 4.2 25 V/3.25 mA at room temperature under a constant 58 current/constant voltage (CC/CV) condition and then put in an oven while the temperature was increased to 85°C for 1 hour. Then, a thickness of each battery was measured after holding the temperature for 4 hours. Degrees of swelling from 5 initial thicknesses are presented in Table 2. [00218] [Table 2] Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Comparative Example 4 Comparative Example 5 Degree of swelling (mm) 0.31 0.25 0.44 0.50 0.36 0.40 0.38 1.40 1.55 [00219] As illustrated in Table 2, it may be understood that swelling characteristics of Examples 2 to 8 were 10 significantly reduced in comparison to those of Comparative Examples 4 and 5. [00220] As an example, the battery of Example 2 had a degree of swelling of 0.31 nm and the battery of Comparative Example 4 had a degree of swelling of 1.40 nm, wherein it was 15 confirmed that the degree of swelling of the electrode of 59 Example 2 from its initial thickness was reduced by about 351% in comparison to that of Comparative Example 4. 60 I/We Claim: 1. A method of preparing a lithium secondary battery, the method comprising: 5 performing a pretreatment in which an SEI (solid electrolyte interphase) film is formed on an electrode by impregnating the electrode in a composition for forming an SEI film and applying a voltage to the electrode; and preparing an electrode assembly using the electrode 10 having the SEI film formed thereon, putting the electrode assembly in a battery case, and performing a combination process of electrolyte injection and charging one or more times, wherein the composition for forming an SEI film 15 comprises a lithium salt, a non-aqueous organic solvent, and an SEI film-forming agent forming the SEI film by an electrochemical oxidation or reduction decomposition reaction. 2. The method of claim 1, wherein the SEI film-forming 20 agent comprises one selected from the group consisting of a cyclic carbonate-based compound containing an unsaturated bond; a cyclic or chain carbonate-based compound containing a halogen atom; a lithium salt containing an oxalato complex as an anion; an imide-based lithium salt; a fluorophosphate- 25 based lithium salt; a 6-membered aromatic heterocyclic 61 compound containing at least two nitrogen atoms in a molecule; and a sultone-based compound, or a mixture of two or more thereof. 5 3. The method of claim 1, wherein the SEI film-forming agent comprises one selected from the group consisting of vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, dimethyl vinylene carbonate, and vinyl ethylene carbonate, or a mixture of two 10 or more thereof. 4. The method of claim 1, wherein the SEI film-forming agent is included in an amount of 0.1 wt% to 10 wt% based on a total weight of the composition for forming an SEI film. 5. The method of claim 1, wherein the lithium salt comprises one selected from the group consisting of LiPF6, LiAsF6, LiCF3SO3, LiBF4, LiBF6, LiSbF6, LiAlO4, LiAlCl4, LiSO3CF3, and LiClO4, or a mixture of two or more thereof. 6. The method of claim 1, wherein the forming of the SEI film is performed by applying a voltage of 0.005 V to 4.5 V at a constant current of 0.01 C to 5 C. 25 7. The method of claim 1, wherein, in a case in which the 62 15 20 electrode is a positive electrode, the forming of the SEI film is performed by applying a voltage of 1 V to 4.5 V for 1 hour to 24 hours at a constant current of 0.01 C to 5 C. 5 8. The method of claim 1, wherein, in a case in which the electrode is a negative electrode, the forming of the SEI film is performed by applying a voltage of 0.005 V to 4.5 V for 1 hour to 24 hours at a constant current of 0.01 C to 5 C. 10 9. The method of claim 1, wherein the combination process of electrolyte injection and charging is performed one to three times. 10. The method of claim 1, wherein the combination process 15 of electrolyte injection and charging comprises: a first injection step of preparing a battery cell by injecting a first electrolyte into the battery case; and a first charging step of charging the battery cell. 20 11. The method of claim 1, wherein the combination process of electrolyte injection and charging comprises: a first injection step of preparing a battery cell by injecting a first electrolyte into a battery case; a first charging step of charging the battery cell; 25 a second injection step of injecting a second 63 electrolyte into the charged battery cell; and a second charging step of charging the battery cell into which the second electrolyte is injected. 5 12. The method of claim 10 or 11, wherein the first electrolyte comprises at least one lithium salt selected from the group consisting of LiPF6, LiAsF6, LiCF3SO3, LiBF4, LiBF6, LiSbF6, and LiSO3CF3, the electrode comprises an aluminum current collector, 10 and an AlF3 film is formed on the current collector after the first charging step. 13. The method of claim 10 or 11, wherein the first 15 electrolyte further comprises a sulfonic acid ester-based additive. 14. The method of claim 13, wherein the sulfonic acid ester-based additive comprises one selected from the group 20 consisting of 1,3-propane sultone, 1,4-butane sultone, 2,4- butane sultone, and a cyclic disulfonic acid ester, or a mixture of two or more thereof. 15. The method of claim 11, wherein the second electrolyte 25 comprises an imide-based lithium salt. 64 16. The method of claim 15, wherein the imide-based lithium salt comprises one selected from the group consisting of lithium bisfluorosulfonyl imide, lithium 5 bistrifluoromethanesulfonyl imide, and lithium bis(perfluoroethylsulfonyl)imide, or a mixture of two or more thereof. 17. The method of claim 11, wherein the second electrolyte 10 further comprises one selected from the group consisting of a nitrile-based compound and a phosphate-based compound, or a mixture thereof. 18. The method of claim 10 or 11, wherein the first 15 charging step is performed by applying a voltage of 1.0 V to 4.5 V at a constant current of 0.01 C to 5 C. 19. The method of claim 11, wherein the first charging step is performed in a state in which the battery case is not 20 sealed after the injection of the first electrolyte. 20. The method of claim 11, wherein the second charging step is performed by applying a voltage of 2.0 V to 4.5 V at a constant current of 0.01 C to 5 C. 25 65 21. The method of claim 11, wherein the second charging step is performed in a state in which the battery case is sealed after the injection of the second electrolyte. 5 22. The method of claim 10 or 11, wherein the SEI filmforming agent comprises a cyclic carbonate-based compound containing an unsaturated bond, and the first electrolyte comprises at least one lithium salt selected from the group consisting of LiPF6, LiAsF6, 10 LiCF3SO3, LiBF4, LiBF6, LiSbF6, and LiSO3CF3. 23. The method of claim 22, wherein the first electrolyte further comprises a sulfonic acid ester-based additive. 15 24. The method of claim 11, wherein the SEI film-forming agent comprises a cyclic carbonate-based compound containing an unsaturated bond, the first electrolyte comprises at least one lithium salt selected from the group consisting of LiPF6, LiAsF6, 20 LiCF3SO3, LiBF4, LiBF6, LiSbF6, and LiSO3CF3; and a sulfonic acid ester-based additive, and the second electrolyte comprises an imide-based lithium salt. 25 25. The method of claim 1, wherein the electrode assembly 66 is any one selected from the group consisting of a jelly-roll type, a stacked type, and a stack and folding type. 26. The method of claim 1, wherein the lithium secondary 5 battery is a cylindrical type, a prismatic type, or a pouch type. 27. A lithium secondary battery prepared by the method of claim 1. 10 28. The lithium secondary battery of claim 27, comprising an electrode in which an SEI film-forming agent derived SEI film on a surface of the electrode and at least one layer of film formed by an electrochemical oxidation or reduction 15 reaction of an electrolyte on the SEI film are formed, wherein the SEI film-forming agent comprises one selected from the group consisting of a cyclic carbonatebased compound containing an unsaturated bond; a cyclic or chain carbonate-based compound containing a halogen atom; a 20 lithium salt containing an oxalato complex as an anion; an imide-based lithium salt; a fluorophosphate-based lithium salt; a 6-membered aromatic heterocyclic compound containing at least two nitrogen atoms in a molecule; and a sultonebased compound, or a mixture of two or more thereof. 25 67 29. The lithium secondary battery of claim 27, comprising an electrode in which an SEI film derived from a cyclic carbonate-based compound containing an unsaturated bond on a surface of the electrode and a sulfonic acid ester-based 5 additive derived film on the SEI film are formed. 30. The lithium secondary battery of claim 27, comprising an electrode in which an SEI film derived from a cyclic carbonate-based compound containing an unsaturated bond on a 10 surface of the electrode, a sulfonic acid ester-based additive derived film on the SEI film, and an imide-based lithium salt derived film on the sulfonic acid ester-based additive derived film are formed. 15 31. A battery module comprising the lithium secondary battery of claim 27 as a unit cell. 32. A battery pack comprising the battery module of claim 31. 20 33. The battery pack of claim 32, wherein the battery pack is used as a power source of a medium and large sized device. 34. The battery pack of claim 33, wherein the medium and 25 large sized device is selected from the group consisting of 68 an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.