The present invention relates to a device for preparing a lithium composite transition metal oxide, a lithium composite transition metal oxide prepared using the same, and a method of preparing the lithium composite transition metal oxide. 10 [BACKGROUND ART] Cathode active materials, which are one material constituting lithium secondary batteries, play a critical role in determining battery capacity and performance. As cathode active materials, lithium cobalt oxides (e.g., LiCo02) that have relatively excellent overall physical properties such as excellent cycle characteristics 15 and the like are mainly used. However, cobalt used in LiCo02 is a so-called rare metal and supply of cobalt is unstable because reserves and production thereof are limited. -1- In addition, LiCoC>2 is expensive due to unstable supply of cobalt and increasing demand for lithium secondary batteries. Under these circumstances, research on cathode active materials that can replace L1C0O2 is continuously underway and use of lithium-containing manganese 5 oxides such as LiMn02, LiMii204 having a spinal crystal structure, and the like and lithium-containing nickel oxides (e.g., L1MO2) is also under consideration. However, it is difficult to apply LiNiC>2 to actual mass-production at reasonable costs in terms of characteristics according to a preparation method thereof, and lithium manganese oxides such as LiMnC>2, LiMn204, and the like have poor cycle characteristics and the like. 10 Thus, recently, research on a method of using, as a cathode active material, a lithium composite transition metal oxide including at least two transition metals selected from among nickel (Ni), manganese (Mn), and cobalt (Co) or a lithium transition metal phosphate, which are representative alternative materials, has been underway. In particular, lithium transition metal phosphates are largely divided into 15 LixM2(P04)3 having a NASICON structure and LiMP04 having an olivine structure, and have been studied as a material having higher stability at high temperature than existing LiCo02- Currently, Li3V2(P04)3 having a NASICON structure is known and, among -2- compounds having an olivine structure, LiFeP04 and Li(Mn, Fe)P04 are most widely studied. Among the compounds having an olivine structure, in particular, LiFePC>4 has a voltage of -3.5 V (vs. lithium), a high bulk density of 3.6 g/cm3, and a theoretical 5 capacity of 170 mAli/g. In addition, LiFePC>4 has higher stability at high temperature than Co and uses Fe as a raw material and thus is highly applicable as a cathode active material for lithium secondary batteries in the near future. Conventional methods of preparing such cathode active materials are largely divided into dry calcination and wet precipitation. According to dry calcination, a 10 cathode active material is prepared by mixing an oxide or hydroxide of a transition metal such as Co or the like with lithium carbonate or lithium hydroxide as a lithium source in a dried state and then calcining the resulting mixture at a high temperature of 700°C to 1000°C for 5 to 48 hours. Dry calcination is, advantageously, a widely used technology for preparing metal oxides and thus is easy to approach, but is 15 disadvantageous in that it is difficult to obtain single-phase products due to difficulties in uniform mixing of raw materials and, in the case of multi-component cathode active materials consisting of two or more transition metals, it is difficult to uniformly arrange at least two elements at the atomic level. -3- Ill wet precipitation, which is another conventional cathode active material preparation method, a cathode active material is prepared by dissolving a salt containing a transition metal such as Co or the like in water, adding alkali to the solution to precipitate the transition metal in the form of transition metal hydroxide, filtering and 5 drying the precipitate, mixing the resulting precipitate with lithium carbonate or lithium hydroxide as a lithium source in a dried state, and calcining the mixture at a high temperature of 700°C to 1000°C for 1 to 48 hours. Wet precipitation is known to easily obtain a uniform mixture by co-precipitating, in particular, two or more transition metal elements, but requires a long period of time in precipitation reaction, is 10 complicated, and incurs generation of waste acids as by-products. In addition, various methods, such as a sol-gel method, a hydrothermal method, spray pyrolysis, an ion exchange method, and the like, have been used to prepare a cathode active material for lithium secondary batteries. Meanwhile, a method of preparing cathode active material particles using 15 supercritical water has recently received much attention. JP 2001-163700 discloses a method of preparing a metal oxide for cathode active materials by allowing lithium ions to react with transition metal ions in a supercritical or subcritical state in a batch-type reactor and a continuous reactor. KR 2007-008290, which was filed by the present applicant prior to the filing of the present application, discloses a method of preparing a -4- lithium iron phosphate having an olivine crystal structure using a supercritical hydrothermal method. However, in existing supercritical devices, a reaction fluid, which is an intermediate product generated due to reaction between raw materials, rapidly gels and 5 thus the reactants are not uniformly mixed. In addition, fluidity of the reaction fluid is deteriorated and thus clogging of the inside of a mixer frequently occurs. As a result of previous studies, it was found that, when a reaction fluid in a gel state is strongly mixed, a sol-state reaction fluid having a uniform mixing state and very high fluidity may be obtained. However, a fixed mixer of a generally used supercritical device is 10 inserted into a tube and thus mixing effects that are strong enough to solate a reaction fluid may not be obtained, and the fixed mixer rather acts as resistance and thus disturbs flow of the reaction fluid and therefore the above-described problems cannot be addressed. Therefore, there is a high need to develop a technology that addresses the 15 clogging problem by enhancing fluidity of a reaction fluid and enables uniform mixing of raw materials, in preparation of a lithium composite transition metal oxide using supercritical or subcritical water. [DISCLOSURE] -5- [Technical Problem] Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved. As a result of a variety of intensive studies and various experiments, the 5 inventors of the present invention confirmed that, when a mixer to mix raw materials is applied using ring-shaped vortex pairs that rotate in opposite directions in a reaction space optimized for a device for preparing a lithium composite transition metal oxide using existing supercritical or subcritical water, the raw materials are uniformly mixed and a reaction fluid in a gel state is solated by a strong force and thus fluidity of the 10 reaction fluid is secured and the clogging problem is addressed, whereby manufacturing efficiency may be enhanced, thus completing the present invention. [TECHNICAL SOLUTION] In accordance with one aspect of the present invention, provided is a device for preparing a lithium composite transition metal oxide for lithium secondary batteries by 15 using supercritical or subcritical water, including first and second mixers continuously arranged in a direction in which a fluid proceeds, wherein the first mixer has a closed structure including: -6- a hollow fixed cylinder; a rotating cylinder having the same axis as that of the hollow fixed cylinder and having an outer diameter that is smaller than an inner diameter of the fixed cylinder; an electric motor to generate power for rotation of the rotating cylinder; 5 a rotation reaction space, as a separation space between the hollow fixed cylinder and the rotating cylinder, in which ring-shaped vortex pairs periodically arranged along a rotating shaft and rotating in opposite directions are formed; first inlets through which raw materials are introduced into the rotation reaction space; and 10 a first outlet to discharge a reaction fluid formed from the rotation reaction space. FIG. 1 is a side view of a conventional supercritical device. Referring to FIG. 1, the conventional supercritical device largely includes a pre-mixer 1 and a main mixer 2. The pre-mixer 1 includes a plurality of inlets 10, 11 15 and 12 through which raw materials are introduced into a case and an outlet 20 to discharge a reaction fluid, and the main mixer 2 includes an inlet 42 through which the -7- reaction fluid is introduced, inlets 40 and 41 through which supercritical water or subcritical water is introduced, and an outlet 50 to discharge the prepared lithium composite transition metal oxide. The outlet 20 of the pre-mixer 1 and the inlet 42 of the main mixer 2 mean opposite ends of a single tube, the pre-mixer 1 and the main 5 mixer 2 are connected to each other via a tube, and a fixed mixer 30 is included in the tube. The raw materials introduced into the pre-mixer 1 are mixed by the fixed mixer 30 and transferred to the main mixer 2 via the tube, followed by mixing with supercritical water or subcritical water introduced via the inlets 40 and 41 in the main 10 mixer 2, thereby obtaining a lithium composite transition metal oxide. In this regard, the fixed mixer 30 to mix raw materials has very weak mixing power and is positioned in a narrow tube and thus acts as resistance and, accordingly, a reaction fluid in a gel state cannot be solated. Consequently, clogging of the tube that connects the outlet 20 of the pre-mixer 1 and the inlet 42 of the main mixer 2 frequently 15 occurs. The inventors of the present application were aware that, when such a conventional supercritical device is used, a fixed mixer inserted into a tube acts as resistance in the tube and thus disturbs flow of a reaction fluid, clogging of the tube -8- occurs over time due to gelation of the reaction fluid, and the fixed mixer has weak mixing power and thus has to have a long length in order for sufficient mixing to secure fluidity of the reaction fluid. As a result of a variety of experiments, the inventors of the present application confirmed that, when the first mixer is applied to the 5 conventional supercritical device, the reaction fluid is uniformly mixed and fluidity thereof is sufficiently secured and thus the above-described problems are addressed and, accordingly, manufacturing efficiency may be enhanced. In a specific embodiment, a ratio of a distance between the fixed cylinder and the rotating cylinder to an outer radius of the rotating cylinder of the first mixer may be 10 greater than 0.05 to less than 0.4. When the ratio of the distance between the fixed cylinder and the rotating cylinder to the outer radius of the rotating cylinder is 0.05 or less, the distance between the fixed cylinder and the rotating cylinder is too small and thus it is difficult to form the distance. Even when it is possible to form the distance therebetween, an effective 15 volume of the rotation reaction space in which the vortex pairs are generated decreases and thus output is dramatically reduced. Meanwhile, a vortex pair substantially acts as a single fixed mixer and thus the vortex pairs periodically arranged along a rotating shaft act as fixed mixers connected to _9_ each other. Thus, as the number of the vortex pairs increases, mixing power increases and thus flow characteristics are enhanced. However, the size of the vortex pair is nearly similar to the distance between the fixed cylinder and the rotating cylinder and thus, as the ratio of the distance between 5 the fixed cylinder and the rotating cylinder to the outer radius of the rotating cylinder increases or as the distance between the fixed cylinder and the rotating cylinder increases, the number of the vortex pairs in a reactor gradually decreases. Thus, when the ratio of the distance between the fixed cylinder and the rotating cylinder to the outer radius of the rotating cylinder is 0.4 or more, the number of the 10 vortex pairs decreases and thus flow characteristics are relatively deteriorated, when compared to a case in which the ratio of the distance between the fixed cylinder and the rotating cylinder to the outer radius of the rotating cylinder is greater than 0.05 to less than 0.4. hi addition, when the ratio of the distance between the fixed cylinder and the rotating cylinder to the outer radius of the rotating cylinder is 0.4 or more, the ring- 15 shaped vortex pairs (laminar vortex) periodically arranged along the rotating axis and rotating in opposite directions according to increase in rotation rate of the rotating cylinder, wavy vortex, modulated wavy vortex, and continuous vortex of turbulent vortex do not appear and transition from a laminar vortex region to a turbulent vortex -10- region occurs right away, and thus, flow characteristics of the vortex pairs may be relatively reduced. In a specific embodiment, the reaction fluid may have a kinematic viscosity of 0.4 to 400 cP and the device may have a power consumption per unit mass of 0.05 W/kg 5 to 100 W/kg. The power consumption per unit mass may be defined as a stirring rate of the rotating cylinder. In a specific embodiment, the vortex pairs generated in the first mixer may have a critical Reynolds number of 300 or more. When the critical Reynolds number of the vortex pairs is 300 or more, a fluid flowing between the fixed cylinder and the 10 rotating cylinder that have the same center becomes unstable due to a tendency to proceed towards the fixed cylinder by centrifugal force and thus the vortex pairs may be formed over the entire rotation reaction space. In a specific embodiment, the first inlets may include at least two inlets and positions thereof are not limited, but the first inlets may be formed at a starting part of 15 the first mixer for uniform mixing of the raw materials. When two inlets are formed, a lithium source material may be introduced via one of the two inlets and a transition metal source material may be introduced via the other thereof. -11- In addition, to further uniformly mix the reaction fluid, the rotating cylinder may be provided at an outer surface thereof with protrusions to smoothly mix reactants. In a specific embodiment, the second mixer may include: a hollow case; second inlets through which reaction fluids produced in the first mixer and supercritical 5 or subcritical water are introduced into the hollow case; and a second outlet to discharge a lithium composite transition metal oxide prepared in the reactor. The second inlets may include at least two inlets as in the first inlets. When at least two inlets are formed, reaction fluids produced in the first mixer may be introduced via any one of the inlets, and supercritical or subcritical water may be 10 introduced via the other thereof. In a specific embodiment, when at least three inlets are formed as the second inlets, the at least three inlets may include an inlet through which a reaction fluid is introduced and inlets formed at opposite sides of the inlet, through which supercritical or subcritical water is introduced. 15 In a specific embodiment, the hollow case may be provided at an inner portion thereof with at least one stirring wheel. -12- The stirring wheel serves to stir reaction fluids and supercritical or subcriticai water while rotating inside the hollow case. Rotation of the stirring wheel is driven by introducing force of the reaction fluids and supercritical or subcriticai water and thus, when a flow rate or amount thereof increases, the stirring wheel is more rapidly rotated 5 to implement a stirring process. The present invention also provides a method of preparing a lithium composite transition metal oxide using the above-described device. In particular, the method may include: (i) forming a transition metal hydroxide by introducing raw materials and an 10 alkalifying agent into the first mixer and primarily mixing the reactants; (ii) synthesizing a lithium composite transition metal oxide by secondarily mixing the mixture of step (i) with supercritical or subcriticai water in the second mixer and drying the lithium composite transition metal oxide; and (iii) calcining the synthesized lithium composite transition metal oxide. 15 In a specific embodiment, the raw materials may be a transition metalcontaining metal precursor compound and a lithium precursor compound. -13- The transition metal-containing metal precursor compound is not particularly limited so long as it is a transition metal-containing salt and an ionizable compound, in particular a water-soluble compound. In this regard, the transition metal may be a combination of a metal with paramagnetism and a metal with diamagnetism. 5 Examples of the metal precursor compound include, without being limited to, an alkoxide, a nitrate, an acetate, a halide, a hydroxide, an oxide, a carbonate, an oxalate, a sulfate, and combinations thereof that include a transition metal. More specifically, the metal precursor compound may be a nitrate, sulfate or acetate that includes a transition metal. 10 The lithium precursor compound is not particularly limited so long as it contains lithium and is an ionizable water-soluble salt. For example, the lithium precursor compound may be lithium nitrate, lithium acetate, lithium hydroxide, lithium sulfate, or the like, more particularly a compound selected from the group consisting of lithium hydroxide and lithium nitrate. 15 In a specific embodiment, the alkalifying agent serves to provide conditions in which one or more transition metal compounds are easily hydrolyzed and precipitated as hydroxides and is not particularly limited so long as it makes a reaction solution alkaline. Non-limiting examples of the alkalifying agent include alkali metal hydroxides (NaOH, KOH, and the like), alkaline earth metal hydroxides (Ca(OH)2, -14- Mg(OH)2, and the like), and ammonia compounds (aqueous ammonia, ammonium nitrate, and the like). The alkalifying agent and the lithium precursor compound may be simultaneously mixed with water, the alkalifying agent may be mixed with water, 5 followed by introduction of the lithium precursor compound thereinto, or the alkalifying agent and the lithium precursor compound may be first mixed, followed by addition thereof to water and mixing therein. In the process of step (ii), reaction pressure and temperature should be suitable either for allowing the transition metal hydroxide precipitate produced in step (i) to 10 react with lithium ions in an aqueous solution or for allowing lithium ions in the aqueous solution to be precipitated as hydroxides. For reference, hydroxides of alkali metals, such as lithium, sodium, potassium, and the like, have high solubility in water at room temperature and atmospheric pressure, but when the density of water is decreased due to high-temperature and high-pressure conditions, the hydroxides have significantly 15 decreased solubility. For example, the solubility of KOH in water is 2.6 mol (145.8 g/100 g water) at room temperature, atmospheric pressure, and water density of 1.0 g/cm , but is decreased to 300 ppm at a temperature of 424°C and water density of 0.139 g/cm3 (262 bar) (W. T. Wofford, P.C. Dell'Orco and E. F. Gloyna, J. Chem. Eng. Data, 1995,40,968-973). -15- Accordingly, to significantly reduce the solubility of the lithium hydroxide and thus accelerate a reaction for synthesizing a lithium composite transition metal oxide, supercritical or subcritical water needs to be added and mixed. In this regard, in a specific embodiment, the supercritical or subcritical water means high-temperature and 5 high-pressure water having a pressure of 180 to 550 bar and a temperature of 200°C to 700°C. When the precipitated transition metal hydroxides and the lithium aqueous solution are instantaneously mixed with high-temperature water, the temperature of the mixture is rapidly increased to subcritical or supercritical temperature from room 10 temperature. Even after adding supercritical or subcritical water, it is necessary to continuously maintain supercritical or subcritical conditions. The temperature in the calcining process of step (iii) is not particularly limited and may be in the range of 600°C to 1200°C. When the calcination temperature is less than 600°C, growth of particles is 15 insufficient, sintering between particles hardly occurs and thus the particles have large specific surface area and low tap density. In addition, growth of crystals is insufficient and the lithium composite transition metal oxide is not sufficiently stabilized, leading to deteriorated cycle characteristics. On the other hand, when the calcination temperature -16- exceeds 1200°C, sintering between particles is excessive and thus performance of the particles as a cathode active material is deteriorated. Before, after or during any one of the steps (i) to (iii), at least one additive selected from the group consisting of a binder, a sintering aid, a doping agent, a coating 5 agent, a reducing agent, an oxidizing agent, acid, carbon or a carbon precursor, a metal oxide, and a lithium compound may be further added. In particular, a lithium composite transition metal oxide having an olivine-type crystal structure, for example, LiFeP04, may be prepared by appropriately using phosphoric acid, carbon or a carbon precursor, sucrose, or the like during the preparation process thereof. 10 The binder may be used to spherize granules and to improve particle size and may, for example, be aqueous ammonia, polyvinyl alcohol (PVA), a mixture thereof, or the like. The sintering aid may be used during high-temperature calcination of granules to reduce calcination temperature or to increase sintering density and examples thereof include, without being limited to, metal oxides such as alumina, B2O3, and 15 MgO, prccursois thereof, and Li compounds such as LiF, LiOH, and LiC03. The doping agent and the coating agent are used to coat outer surfaces of electrode active material crystals with metal oxide ultrafine particles in order to enhance durability of a calcined material when used in batteries and examples thereof include, without being -17- limited to, metal oxides such as alumina, zirconia, titania, and magnesia, and precursors thereof. The reducing agent or the oxidizing agent may be used to control atmosphere of each step to a reducing or oxidative atmosphere. The reducing agent may, for 5 example, be hydrazine, oxalic acid, sucrose, fructose, ascorbic acid (Vitamin C), hydrogen, carbon, hydrocarbon, a mixture thereof, or the like. The oxidizing agent may, for example, be oxygen, hydrogen peroxide, ozone, a mixture thereof, or the like. The acid is used in the form of a reactant such as a phosphoric acid compound, a sulfuric acid compound, or the like and may, for example, be phosphoric acid, sulfuric 10 acid, a mixture thereof, or the like. The carbon or the carbon precursor may be coated on a surface of a material prepared to increase electrical conductivity of the prepared material or to provide a reducing atmosphere and, in particular, is useful for a lithium composite transition metal oxide having an olivine-type crystal structure. The lithium compound may participate in the reaction during the calcination process to increase the 15 amount of lithium in the lithium composite transition metal oxide and may, for example, be an Li compound such as LiF, LiOH, L1NO3, LJCO3, or the like. The present invention also provides a lithium composite transition metal oxide prepared using the above-described method by using the above-described device. -18- In a specific embodiment, a lithium composite transition metal oxide that may be provided according to the present invention may be any one of compounds represented by Formulas 1 to 4 below, in particular LiFeP04, but embodiments of the present invention are not limited thereto. 5 Lii+aA,_xCx02-bXb (-0.5 Lii+aAxB2.x-A°4-bXb (-0.5 10 Lii+aA,_xCx(Y04-bXb) (-0.5 Lii+aA2-xCx(Y04-bXb)3 (-0.5 Lii+aAi.xCx02-bXb (-0.5 Lii+aAxB2-x-yCy04-bXb (-0.5 Li|+aA|_xCx(Y04-bXb) (-0.5 15 Lii+aA2.xCx(Y04.bXb)3 (-0.5