Patent Publication Number: US-9416024-B2

Title: Method of preparing material for lithium secondary battery of high performance

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 13/973,269, filed Aug. 22, 2013, which is a continuation of U.S. application Ser. No. 12/914,142, filed Oct. 28, 2010, which has now issued as U.S. Pat. No. 8,540,961, which is a continuation of U.S. application Ser. No. 11/831,522, filed Jul. 31, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 11/104,734, filed on Apr. 13, 2005, which has now issued as U.S. Pat. No. 7,648,693, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a Ni-based lithium mixed transition metal oxide and a method for preparing the same. More specifically, the present invention relates to a method for preparing a powdered lithium mixed transition metal oxide having a given composition and a specific atomic-level structure which is prepared by a solid-state reaction of Li 2 CO 3  with a mixed transition metal precursor under an oxygen-deficient atmosphere with an oxygen concentration of 10 to 50% by volume. 
     BACKGROUND OF THE INVENTION 
     Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used. 
     As cathode active materials for the lithium secondary batteries, lithium-containing cobalt oxide (LiCoO 2 ) is largely used. In addition, consideration has been made of using lithium-containing manganese oxides such as LiMnO 2  having a layered crystal structure and LiMn 2 O 4  having a spinel crystal structure, and lithium-containing nickel oxides (LiNiO 2 ). 
     Of the aforementioned cathode active materials, LiCoO 2  is currently widely used due to superior general properties including excellent cycle characteristics, but suffers from low safety, expensiveness due to finite resources of cobalt as a raw material, and limitations in practical and mass application thereof as a power source for electric vehicles (EVs) and the like. 
     Lithium manganese oxides, such as LiMnO 2  and LiMn 2 O 4 , are abundant resources as raw materials and advantageously employ environmentally-friendly manganese, and therefore have attracted a great deal of attention as a cathode active material capable of substituting LiCoO 2 . However, these lithium manganese oxides suffer from shortcomings such as low capacity and poor cycle characteristics. 
     Whereas, lithium/nickel-based oxides including LiNiO 2  are inexpensive as compared to the aforementioned cobalt-based oxides and exhibit a high discharge capacity upon charging to 4.3 V. The reversible capacity of doped LiNiO 2  approximates about 200 mAh/g which exceeds the capacity of LiCoO 2  (about 165 mAh/g). Therefore, despite a slightly lower average discharge voltage and a slightly lower volumetric density, commercial batteries comprising LiNiO 2  as the cathode active material exhibit an improved energy density. To this end, a great deal of intensive research is being actively undertaken on the feasibility of applications of such nickel-based cathode active materials for the development of high-capacity batteries. However, the LiNiO 2 -based cathode active materials suffer from some limitations in practical application thereof, due to the following problems. 
     First, LiNiO 2 -based oxides undergo sharp phase transition of the crystal structure with volumetric changes accompanied by repeated charge/discharge cycling, and thereby may suffer from cracking of particles or formation of voids in grain boundaries. Consequently, intercalation/deintercalation of lithium ions may be hindered to increase the polarization resistance, thereby resulting in deterioration of the charge/discharge performance. In order to prevent such problems, conventional prior arts attempted to prepare a LiNiO 2 -based oxide by adding an excess of a Li source and reacting reaction components under an oxygen atmosphere. However, the thus-prepared cathode active material, under the charged state, undergoes structural swelling and destabilization due to the repulsive force between oxygen atoms, and suffers from problems of severe deterioration in cycle characteristics due to repeated charge/discharge cycles. 
     Second, LiNiO 2  has shortcomings associated with the evolution of an excess of gas during storage or cycling. That is, in order to smoothly form the crystal structure, an excess of a Li source is added during manufacturing of the LiNiO 2 -based oxide, followed by heat treatment. As a result, water-soluble bases including Li 2 CO 3  and LiOH as reaction residues remain between primary particles and thereby they decompose or react with electrolytes to thereby produce CO 2  gas, upon charging. Further, LiNiO 2  particles have an agglomerate secondary particle structure in which primary particles are agglomerated to form secondary particles and consequently a contact area with the electrolyte further increases to result in severe evolution of CO 2  gas, which in turn unfortunately leads to the occurrence of battery swelling and deterioration of the high-temperature safety. 
     Third, LiNiO 2  suffers from a sharp decrease in the chemical resistance of a surface thereof upon exposure to air and moisture, and the gelation of slurries by polymerization of a n N-methyl pyrrolidone/poly(vinylidene fluoride) (NMP-PVDF) slurry due to a high pH value. These properties of LiNiO 2  cause severe processing problems during battery production. 
     Fourth, high-quality LiNiO 2  cannot be produced by a simple solid-state reaction as is used in the production of LiCoO 2 , and LiNiMO 2  cathode active materials containing an essential dopant cobalt and further dopants manganese and aluminum are produced by reacting a lithium source such as LiOH.H 2 O with a mixed transition metal hydroxide under an oxygen or syngas atmosphere (i.e, a. CO 2 -deficient atmosphere), which consequently increases production costs. Further, when an additional step, such as intermediary washing or coating, is included to remove impurities in the production of LiNiO 2 , this leads to a further increase in production costs. 
     Many prior arts focus on improving properties of LiNiO 2 -based cathode active materials and processes to prepare LiNiO 2 . However, various problems, such as high production costs, swelling due to gas evolution in the fabricated batteries, poor chemical stability, high pH and the like, have not been sufficiently solved. A few examples will be illustrated hereinafter. 
     U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a wide range of compositions including nickel-based and high-Ni LiMO 2 , the materials having high crystallinity and being used in Li-ion batteries in ethylene carbonate (EC) containing an electrolyte. Samples were prepared on a small scale, using LiOH.H 2 O as a lithium source. The samples were prepared in a flow of synthetic air composed of a mixture of oxygen and nitrogen, free of CO 2 . 
     U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO 2  substantially free of lithium hydroxides and lithium carbonates. For this purpose, lithium hydroxide and LiOH.H 2 O as a lithium source are employed and heat treatment is performed under an oxygen atmosphere free of CO 2 , additionally with a low content of H 2 O. An excess of lithium “evaporates”; however, “evaporation” is a lab-scale effect and not an option for large-scale preparation. That is, when applied to a large-scale production process, it becomes difficult to evaporate an excess of lithium, thereby resulting in problems associated with the formation of lithium hydroxides and lithium carbonates. 
     U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses a process for the production of Mn-doped LiNi 1−x Mn x O 2  (x&lt;0.45), wherein the manganese source is manganese nitrate, and the lithium source is either lithium hydroxide or lithium nitrate. 
     U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses a process to prepare LiNi 1−x Mn x O 2  by a two-step heating, involving pre-drying, cooking and the final heating. The final heating is done in an oxidizing gas such as air or oxygen. This patent focuses on oxygen. The disclosed method uses a very low temperature of 550 to 650° C. for cooking, and less than 800° C. for sintering. At higher temperatures, samples deteriorate dramatically. Excess lithium is used such that the final samples contain a large amount of soluble bases (i.e., lithium compounds). According to the research performed by the inventors of the present invention, the observed deterioration is attributable to the presence of lithium salts as impurities and melting at about 700 to 800° C., thereby detaching the crystallites. 
     WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicated preparation method very different from that disclosed in the present invention. This preparation method involves the use of lithium nitrates and lithium hydroxides and recovering the evolved noxious gasses. The sintering temperature never exceeds 800° C. and typically is far lower. 
     U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepare LiNiO 2 -based cathodes from lithium hydroxides and metal oxides at temperatures below 800° C. 
     In prior arts including the above, LiNiO 2 -based cathode active materials are generally prepared by high cost processes, in a specific reaction atmosphere, especially in a flow of synthetic gas such as oxygen or synthetic air, free of CO 2 , and using LiOH.H 2 O, Li-nitrate, Li acetate, etc., but not the inexpensive, easily manageable Li 2 CO 3 . Furthermore, the final cathode active materials have a high content of soluble bases, originating from carbonate impurities present in the precursors, which remain in the final cathode because of the thermodynamic limitation. Further, the crystal structure of the final cathode active materials per se is basically unstable even when the final cathode active materials are substantially free of soluble bases. Consequently, upon exposure to air containing moisture or carbon dioxide during storage of the active materials, lithium is released to surfaces from the crystal structure and reacts with air to thereby result in continuous formation of soluble bases. 
     Meanwhile, Japanese Unexamined Patent Publication Nos. 2004-281253, 2005-150057 and 2005-310744 disclose oxides having a composition formula of Li a Mn x Ni y M 2 O 2  (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1, 0≦z≦0.65, and x+y+z=1). These inventions provide a method of preparing the oxide involving mixing each transition metal precursor with a lithium compound, grinding, drying and sintering the mixture, and re-grinding the sintered composite oxide by ball milling, followed by heat treatment. In addition, working examples disclosed in the above prior art employ substantially only LiOH as a lithium source. Further, it was found through various experiments conducted by the inventors of the present invention that the aforesaid prior art composite oxide suffers from significant problems associated with a high-temperature safety, due to production of large amounts of impurities such as Li 2 CO 3 . 
     Alternatively, encapsulation of high Ni—LiNiO 2  by SiO x  protective coating has been proposed (H. Omanda, T. Brousse, C. Marhic, and D, M. Schleich,  J. Electrochem. Soc.  2004, 151, A922.), but the resulting electrochemical properties are very poor. In this connection, the inventors of the present invention have investigated the encapsulation by LiPO 3  glass. Even where a complete coverage of the particle is accomplished, a significant improvement of air-stability could not be made and electrochemical properties were poor. 
     Therefore, there is a strong need for the development of a method of preparing LiNiO 2 -based cathode active materials that can be produced at a low cost from inexpensive precursors, have low contents of soluble bases, and show improved properties such as low swelling when applied to commercial lithium secondary batteries, improved chemical and structural stability, superior cycle characteristics, and high capacity. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention has been made in view of the above problems and other technical problems that have yet to be resolved. 
     As a result of a variety of extensive and intensive studies and experiments in view of the problems as described above, the inventors of the present invention provide herewith a lithium mixed transition metal oxide having a given composition, prepared by a solid-state reaction under an oxygen-deficient atmosphere and using a precursor material that is cheap and easy to handle, with which it is possible to realize environmental friendliness of the preparation method, decreased production costs and improved production efficiency, where the thus-prepared lithium mixed transition metal oxide is substantially free of impurities and has superior thermal stability due to a stable atomic-level structure, and a secondary battery comprising such a lithium mixed transition metal oxide having a high capacity, excellent cycle characteristics, significantly improved storage properties and high-temperature safety. The present invention has been completed based on these findings. 
    
    
     
       BRIEF DESCRIPTION OF THE 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: 
         FIG. 1  is a schematic view showing a crystal structure of a conventional Ni-based lithium transition metal oxide; 
         FIG. 2  is a schematic view showing a crystal structure of a Ni-based lithium mixed transition metal oxide prepared by a method according to one embodiment; 
         FIGS. 3 and 4  are graphs showing a preferred composition range of a Ni-based lithium mixed transition metal oxide prepared by a method according to an embodiment; 
         FIG. 5  is an FESEM (Field Emission Scanning Electron Microscope) image (×2000) showing LiNiMO 2  according to Example 1. 5A: 850° C.; 5B: 900° C.; 5C: 950° C.; and 5D: 1,000° C.; 
         FIG. 6  is an FESEM image showing commercial LiMO 2  (M=Ni 0.8 CO 0.2 ) according to Comparative Example 1. 6A: FESEM image of a sample as received, and 6B: FESEM image of a sample after heating to 850° C. in air; 
         FIG. 7  is an FESEM image showing the standard pH titration curve of commercial high-Ni LiNiO 2  according to Comparative Example 2. A: Sample as received, B: After heating of a sample to 800° C. under an oxygen atmosphere, and C: Copy of A; 
         FIG. 8  is a graph showing a pH titration curve of a sample according to Comparative Example 3 during storage of the sample in a wet chamber. A: Sample as received, B: After storage of a sample for 17 hrs, and C: After storage of a sample for 3 days; 
         FIG. 9  is a graph showing a pH titration curve of a sample according to Example 2 during storage of the sample in a wet chamber. A: Sample as received, B: After storage of a sample for 17 hrs, and C: After storage of a sample for 3 days; 
         FIG. 10  is a graph showing lengths of a-axis and c-axis of crystallographic unit cells of samples having different ratios of Li:M in Experimental Example 3; 
         FIG. 11  is an SEM image of a sample according to Example 4; 
         FIG. 12  shows the Rietveld refinement on X-ray diffraction patterns of a sample according to Example 4; 
         FIG. 13  is an SEM micrograph (×5000) of a precursor in Example 5, which is prepared by an inexpensive ammonia-free process and has a low density; 
         FIG. 14  is a graph showing electrochemical properties of LiNiMO 2  according to the present invention in Experimental Example 1. 12A: Graph showing voltage profiles and rate characteristics at room temperature (1 to 7 cycles); 7B: Graph showing cycle stability at 25° C. and 60° C. and a rate of C/5 (3.0 to 4.3V); and 7C: Graph showing discharge profiles (at C/10 rate) for Cycle 2 and Cycle 31, obtained during cycling at 25° C. and 60° C.; 
         FIG. 15  is a graph showing DSC (differential scanning calorimetry) values for samples of Comparative Examples 3 and 4 in Experimental Example 2. A: Commercial Al/Ba-modified LiNiO 2  of Comparative Example 3, and B: Commercial AlPO 4 -coated LiNiO 2  of Comparative Example 4; 
         FIG. 16  is a graph showing DSC values for LiNiMO 2  according to Example 3 in Experimental Example 2; 
         FIG. 17  is a graph showing electrophysical properties of a polymer cell according to one embodiment in Experimental Example 3; and 
         FIG. 18  is a graph showing swelling of a polymer cell during high-temperature storage in Experimental Example 3. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with an aspect of the present invention, the above can be accomplished by the provision of a method for preparing a lithium mixed transition metal oxide, comprising subjecting Li 2 CO 3  and a mixed transition metal precursor to a solid-state reaction under an oxygen-deficient atmosphere with an oxygen concentration of 10 to 50% by volume to thereby prepare a lithium mixed transition metal oxide having a composition represented by Formula I below:
 
Li x M y O 2   (I)
 
     wherein: 
     M=M′ 1-k A k , wherein M′ is Ni 1−a-b (Ni 1/2 Mn 1/2 ) a Co b , 0.65≦a+b≦0.85 and 0.1≦b≦0.4; 
     A is a dopant; 
     0≦k&lt;0.05; and 
     x+y≈2 and 0.95≦x≦1.05. 
     Therefore, since a high-Ni lithium mixed transition metal oxide having a given composition is prepared by a simple solid-state reaction under an oxygen deficient atmosphere including for example, air, using a raw material that is cheap and easy to handle, so the present invention can enables industrial-scale production of the lithium mixed transition metal oxide with significantly decreased production costs and high production efficiency. Further, the high-Ni lithium mixed transition metal oxide produced according to the method has a very stable atomic-level structure and is substantially free of water-soluble bases such as Li 2 CO 3 . Therefore, the lithium mixed transition metal oxide of the present invention can exert excellent storage stability, decreased gas evolution and thereby excellent high-temperature stability, and a secondary battery comprising such a lithium mixed transition metal oxide can exert a high capacity and high cycle stability. 
     As used herein, the term “high-Ni” means that a content of nickel is relatively high, relative to other transition metals present which constitute the lithium mixed transition metal oxide, such as nickel, manganese, cobalt, and the like. Hereinafter, where appropriate throughout the specification, the term “high-Ni lithium mixed transition metal oxide in accordance with the present invention” is used interchangeably with the term “LiNiMO 2 ”. Therefore, NiM in LiNiMO 2  is a suggestive expression representing a complex composition of Ni, Mn and Co in Formula I. 
     Conventional Ni-based cathode active materials contain large amounts of water-soluble bases such as lithium oxides, lithium sulfates, lithium carbonates (Li 2 CO 3 ), and the like. These water-soluble bases may be bases, such as Li 2 CO 3  and LiOH, present in LiNiMO 2 , or otherwise may be bases produced by ion exchange (H +  (water)← →Li +  (surface, an outer surface of the bulk)), performed at the surface of LiNiMO 2 . The bases of the latter case are usually present at a negligible level. 
     The former water-soluble bases may be produced due to the presence of unreacted lithium raw materials upon sintering of lithium mixed transition metal oxides. This is because as production of conventional lithium mixed transition metal oxides involves an addition of relatively large amounts of lithium and a low-temperature sintering process so as to prevent the disintegration of a layered crystal structure, the resulting particles have relatively large amounts of grain boundaries as compared to the cobalt-based oxides, and a sufficient reaction of lithium ions is not realized upon sintering. 
     In addition, even when an initial amount of Li 2 CO 3  is low, Li 2 CO 3  may also be produced during fabrication of the battery or storage of electrode active materials. These water-soluble bases react with electrolytes in the battery to thereby cause gas evolution and battery swelling, which consequently result in severe deterioration of the high-temperature safety. 
     On the other hand, since the cathode active material prepared by the method in accordance with the present invention stably maintains the layered crystal structure by a specific composition of transition metal elements and a reaction atmosphere, despite the use of Li 2 CO 3  as a raw material, it is possible to carry out the sintering process at a high-temperature, thereby resulting in small amounts of grain boundaries. In addition, as retention of unreacted lithium on surfaces of particles is prevented, the particle surfaces are substantially free of water-soluble bases such as lithium carbonates, lithium sulfates, and the like. In the present invention, a content of water-soluble bases such as Li 2 CO 3  includes all of Li 2 CO 3  remaining upon production of the lithium mixed transition metal oxide, or Li 2 CO 3  produced during fabrication of the battery or storage of electrode active materials. 
     In the present invention, the content of the water-soluble bases is measured by pH titration. As used herein, the phrase “is (are) substantially free of water-soluble bases” refers to an extent that upon titration of 200 mL of a solution containing the lithium mixed transition metal oxide with 0.1M HCl, a HCl solution used to reach a pH of less than 5 is preferably consumed in an amount of less than 20 mL, more preferably less than 10 mL. Herein, 200 mL of the aforementioned solution contains substantially all kinds of the water-soluble bases in the lithium mixed transition metal oxide, and is prepared by repeatedly soaking and decanting 10 g of the lithium mixed transition metal oxide. At this time, there are no significant influences of parameters such as a total soaking time of the powder in water. In addition, the content of the water-soluble bases may be preferably below 0.07% by weight of the cathode active material. 
     One of the important features of the present invention is that a desired lithium mixed transition metal oxide is prepared by a solid-state reaction of Li 2 CO 3  and a mixed transition metal precursor under an oxygen-deficient atmosphere. 
     In this connection, it was found through various experiments conducted by the inventors of the present invention that when conventional high-nickel LiMO 2  is sintered in air containing a trace amount of CO 2 , LiMO 2  decomposes with a decrease of Ni 3+  as shown in the following reaction below, during which amounts of Li 2 CO 3  impurities increase.
 
LiM 3+ O 2 +CO 2   →a Li 1−x M 1+x1   3+,2+ O 2   +b Li 2 CO 3   +c O 2  
 
     This is believed to be due to that the decomposition of some Ni 3+  into Ni 2+  upon sintering results in destabilization of the crystal structure, which consequently leads to an oxide form having excessive cation mixing, i.e. Li-deficient form of Li 1−a Ni 1+a O 2  having transition metal cations misplaced on lithium sites of the crystal structure, and lithium ions, released from partial collapse of the crystal structure, react with CO 2  in air. 
     For these reasons, the conventional prior art suffered from problems in that the use of Li 2 CO 3  as a raw material brings about the evolution of CO 2  due to decomposition of Li 2 CO 3 , which then thermodynamically hinders further decomposition of Li 2 CO 3  necessary for the reaction even at a low partial pressure of CO 2 , consequently resulting in no further progression of the reaction. In addition, an excessive addition of Li 2 CO 3  is accompanied by a problem of residual Li 2 CO 3  after the reaction. 
     Therefore, in order to prevent such problems associated with the lithium-deficiency and cation mixing and in order to increase a relative amount of Ni 3+ , conventional prior arts conducted the production reaction using an excessive amount of LiOH.H 2 O as a lithium source, with a ratio of M(OH) 2  and Li of 1:1.05 to 1.15 (M(OH) 2 :Li-compound) under a high-oxygen atmosphere. 
     However, LiOH.H 2 O (technical grade) contains primarily &gt;1% Li 2 CO 3  by weight of impurities that are not decomposed or removed during the sintering process under an oxygen atmosphere and therefore remain in the final product. Further, an excess of the residual Li 2 CO 3  accelerates the electrolyte decomposition to thereby result in the evolution of gas. Therefore, the conventional method suffered from various problems such as disintegration of secondary particles into single primary crystallites, lowered storage stability, and deterioration of the high-temperature safety resulting from the gas evolution due to the reaction of the residual Li 2 CO 3  with the electrolyte in the battery. 
     Further, the lithium mixed transition metal oxide prepared by a conventional method has a layered crystal structure as shown in  FIG. 1 , and desertion of lithium ions from the reversible lithium layers in the charged state brings about swelling and destabilization of the crystal structure due to the repulsive force between oxygen atoms in the MO layers (mixed-transition metal oxide layers), thus suffering from the problems associated with sharp decreases in the capacity and cycle characteristics, resulting from changes in the crystal structure due to repeated charge/discharge cycles. 
     As a result of a variety of extensive and intensive studies and experiments, the inventors of the present invention have found that when the lithium mixed transition metal oxide is prepared by a solid-state reaction of Li 2 CO 3  with the mixed transition metal precursor under an oxygen-deficient atmosphere, it is possible to produce a cathode active material containing the lithium mixed transition metal oxide substantially free of Li 2 CO 3 . 
     Specifically, under the oxygen-deficient atmosphere, desorption of some oxygen atoms takes place from the MO layers, which leads to a decrease in an oxidation number of Ni, thereby increasing amounts of Ni 2+  ions. As a result, some of the Ni 2+  ions are inserted into the reversible lithium layers, as shown in  FIG. 2 . However, contrary to conventionally known or accepted ideas in the related art that intercalation/deintercalation of lithium ions will be hindered due to such insertion of Ni 2+  ions into the reversible lithium layers, an insertion of an effective amount of Ni 2+  ions can prevent destabilization of the crystal structure that may occur due to the repulsive force between oxygen atoms in the MO layers, upon charge. As used herein, “an effective amount of Ni 2+  ions”, can include about 3 to about 7 mole percent of the total amount of Ni ions present. Therefore, stabilization of the crystal structure is achieved to result in no occurrence of further structural collapse by oxygen desorption. Further, it is believed that the lifespan characteristics and safety are simultaneously improved, due to no further formation of Ni 2+  ions with maintenance of the oxidation number of Ni ions inserted into the reversible lithium layers, even when lithium ions are released during a charge process. Hence, it can be said that such a concept of the present invention is a remarkable one which is completely opposite to and overthrows the conventional idea. 
     Thus, the present invention can fundamentally prevent the problems that may occur due to the presence of the residual Li 2 CO 3  in the final product (active material), and provides a highly economical process by performing the production reaction using a relatively small amount of inexpensive Li 2 CO 3  as a reactant and an oxygen-deficient atmosphere such as air. Further, the sintering and storage stabilities are excellent due to the stability of the crystal structure, and thereby the battery capacity and cycle characteristics can be significantly improved simultaneously with a desired level of rate characteristics. 
     However, under an atmosphere with excessive oxygen-deficiency, an excessive amount of Ni 2+  ions transfer to the reversible lithium layers during a synthesis process, thereby resulting in hindrance of the intercalation/deintercalation of lithium ions, and therefore the performance of the battery cannot be exerted sufficiently. On the other hand, if the oxygen concentration is excessively high, a desired amount of Ni 2+  ions cannot be inserted into the reversible lithium layers. Taking into consideration such problems, the synthetic reaction may be carried out under an atmosphere with an oxygen concentration of preferably 10% to 50% by volume, and more preferably 10% to 30% by volume. In a specific embodiment, the reaction may be carried out under an air atmosphere. 
     Another feature of the present invention is that raw materials produced by an inexpensive or economical process and being easy to handle can be used, and particularly Li 2 CO 3  which is difficult to employ in the prior art can be used itself as a lithium source. 
     As an added amount of Li 2 CO 3  as the lithium source decreases, that is, a ratio (Li/M) of lithium to the mixed transition metal source (M) decreases, an amount of Ni inserted into the MO layers gradually increases. Therefore, if excessive amounts of Ni ions are inserted into the reversible lithium layers, a movement of Li ions during charge/discharge processes is hampered, which thereby leads to problems associated with a decrease in the capacity or deterioration of the rate characteristics. On the other hand, if an added amount of Li 2 CO 3  is excessively large, that is, the Li/M ratio is excessively high, the amount of Ni inserted into the reversible lithium layers is excessively low, which can undesirably lead to structural instability, thereby presenting decreased safety of the battery and poor lifespan characteristics. Further, at a high Li/M value, amounts of unreacted Li 2 CO 3  increase to thereby result in a high pH-titration value, i.e. production of large amounts of impurities, and consequently the high-temperature safety can deteriorate. 
     Therefore, in one embodiment, an added amount of Li 2 CO 3  as the lithium source may be from 0.95 to 1.04:1 where the ratio of Li 2 CO 3 :mixed transition metal raw material, is a w/w ratio, based on the weight of the mixed transition metal as the other raw material. 
     As a result, the product is substantially free of impurities due to a lack of surplus Li 2 CO 3  in the product (the cathode active material) by adding a stoichiometric amount (i.e., by not adding an excess) of the lithium source, so there are no problems associated with residual Li 2 CO 3  and a relatively small amount of inexpensive Li 2 CO 3  is used to thereby provide a highly economical process. 
     As the mixed transition metal precursor, M(OH) 2  or MOOH (M is as defined in Formula I) can specifically be used. As used herein, the term “mixed” means that several transition metal elements are well mixed at the atomic level. 
     In prior art processes, as the mixed transition metal precursors, mixtures of Ni-based transition metal hydroxides are generally employed. However, these materials commonly contain carbonate impurities. This is because Ni(OH) 2  is prepared by co-precipitation of a Ni-based salt such as NiSO 4  with a base such as NaOH in which the technical grade NaOH contains Na 2 CO 3  and the CO 3  anion is more easily inserted into the Ni(OH) 2  structure than the OH anion. 
     Further, in order to increase an energy density of the cathode active material, conventional prior art processes employed MOOH having a high tap density of 1.5 to 3.0. However, the use of such a high-tap density precursor makes it difficult to achieve the incorporation of the reactant (lithium) into the inside of the precursor particles during the synthetic process, which then lowers the reactivity to thereby result in production of large amounts of impurities. Further, for preparation of MOOH having a high tap density, co-precipitation of MSO 4  and NaOH should be carried out in the presence of excess ammonia as a complexing additive. However, ammonia in waste water causes environmental problems and thus is strictly regulated. It is, however, not possible to prepare the mixed oxyhydroxide having a high density by an ammonia-free process that is less expensive, is more environmentally friendly and is easier to proceed this process. 
     However, according to the research performed by the inventors of the present invention, it was found that even though the mixed transition metal precursor prepared by the ammonia-free process exhibits a relatively low tap density, a lithium mixed transition metal oxide prepared using the thus-prepared precursor which has an excellent sintering stability makes it is possible to prepare a mixed transition metal oxide having a superior reactivity. 
     In this way, the lithium mixed transition metal oxide, which was prepared by the method according to the present invention, as discussed hereinbefore, can maintain a well-layered crystal structure due to the insertion of some Ni ions into the reversible lithium layers, thus exhibiting excellent sintering stability. Accordingly, the present invention can employ the mixed transition metal precursor having a low tap density, as the raw material. 
     Therefore, since the raw material, i.e. the mixed transition metal precursor, is environmentally friendly, can be easily prepared at low production costs and also has a large volume of voids between primary particles, e.g. a low tap density, it is possible to easily realize the introduction of the lithium source into the inside of the precursor particles, thereby improving the reactivity, and it is also possible to prevent production of impurities and reduce an amount of the lithium source (Li 2 CO 3 ) to be used, so the method of the present invention is highly economical. 
     As used herein, the term “ammonia-free process” means that only NaOH without the use of aqueous ammonia is used as a co-precipitating agent in a co-precipitation process of a metal hydroxide. That is, the transition metal precursor is obtained by dissolving a metal salt MSO 4  (M is a metal of a composition to be used) in water, and gradually adding a small amount of a precipitating agent NaOH with stirring. At this time, the introduction of ammonia lowers the repulsive force between particles to thereby result in densification of co-precipitated particles, which then increases a density of particles. However, when it is desired to obtain a hydroxide having a low tap density as in the present invention, there is no need to employ ammonia. 
     In one specific embodiment, the tap density of the mixed transition metal precursor can be from 1.1 to 1.6 g/cm 3 . If the tap density is excessively low, a chargeable amount of the active material decreases, so the capacity per volume may be lowered. On the other hand, if the tap density is excessively high, the reactivity with the lithium source material is lowered and therefore impurities may be undesirably formed. 
     The solid-state reaction includes a sintering process preferably at 600 to 1,100° C. for 3 to 20 hours, and more preferably 800 to 1,050° C. for 5 to 15 hours. If the sintering temperature is excessively high, this may lead to non-uniform growth of particles, and reduction of the volume capacity of the battery due to a decreased amount of particles that can be contained per unit area, arising from an excessively large size of particles. On the other hand, if the sintering temperature is excessively low, an insufficient reaction leads to the retention of the raw materials in the particles, thereby presenting the risk of damaging the high-temperature safety of the battery, and it may be difficult to maintain a stable structure, due to the deterioration of the volume density and crystallinity. Further, if the sintering time is too short, it is difficult to obtain a lithium nickel-based oxide having high crystallinity. On the other hand, if the sintering time is too long, this may undesirably lead to excessively large particle diameter and reduced production efficiency. 
     The method in accordance with the present invention enables the production of a desired lithium transition metal oxide by a single heat treatment and is thus also desirable in terms of economic efficiency of the manufacturing process. 
     In addition, various parameters may occur as the process for preparation of the lithium mixed transition metal oxide is scaled-up. A few grams of samples in a furnace behave very differently from a few kg of samples, because the gas transport kinetics at a low partial pressure is completely different. Specifically, in a small-scale process, Li evaporation occurs and CO 2  transport is fast, whereas in a large-scale process, these processes are retarded. Where the Li evaporation and CO 2  transport are retarded, a gas partial pressure in the furnace increases, which in turn hinders further decomposition of Li 2 CO 3  necessary for the reaction, consequently resulting in retention of the unreacted Li 2 CO 3 , and the resulting LiNiMO 2  decomposes to result in the destabilization of the crystal structure. 
     Accordingly, when it is desired to prepare the lithium mixed transition metal oxide on a large-scale using the method of the present invention, the preparation process is specifically carried out under a high rate of air circulation. As used herein, the term “large scale” means that a sample has a size of 5 kg or more because similar behavior is expected in 100 kg of sample when the process has been correctly scaled-up, i.e., a similar gas flow (m 3 /kg of sample) reaches 100 kg of sample. 
     In order to achieve high air circulation upon the production of the lithium transition metal oxide by the large-scale mass production process, preferably at least 2 m 3  (volume at room temperature) of air, and more preferably at least 10 m 3  of air, per kg of the final lithium mixed transition metal oxide, may be pumped into or out of a reaction vessel. As such, even when the present invention is applied to a large-scale production process, it is possible to prepare the lithium mixed transition metal oxide which is substantially free of impurities. 
     In an embodiment of the present invention, a heat exchanger may be employed to minimize energy expenditure upon air circulation by pre-warming the in-flowing air before it enters the reaction vessel, while cooling the out-flowing air. 
     In a specific example, air flow of 2 m 3 /kg corresponds to about 1.5 kg of air at 25° C. The heat capacity of air is about 1 kJ/kg° K and the temperature difference is about 800K. Thus, at least about 0.33 kWh is required per kg of the final sample for air heating. Where the air flow is 10 m 3 , about 2 kWh is then necessary. Thus, the typical additional energy cost amounts to about 2 to 10% of the total cathode sales price. The additional energy cost can be significantly reduced when the air-exchange is made by using a heat exchanger. In addition, the use of the heat exchanger can also reduce the temperature gradient in the reaction vessel. To further decrease the temperature gradient, it is recommended to provide several air flows into the reaction vessel simultaneously. 
     In accordance with another aspect of the present invention, there is provided a lithium mixed transition metal oxide prepared by the aforementioned method, and a cathode active material for a secondary battery comprising the same. 
     The lithium mixed transition metal oxide in accordance with the present invention can maintain a well-layered structure due to the insertion of MO layer (mixed-transition metal oxide layers)-derived Ni 2+  ions into reversible lithium layers (lithium intercalation/deintercalation layers), even when lithium ions are released during a charge process. As a result, the lithium mixed transition metal oxide exhibits very excellent sintering stability and no occurrence of Li 2 CO 3  impurities resulting from reduction and decomposition of Ni 3+  ions, and is substantially free of water-soluble bases such as lithium carbonates and lithium sulfates. Accordingly, the lithium mixed transition metal oxide of the present invention exhibits excellent storage stability, decreased gas evolution and thereby excellent high-temperature stability simultaneously with the feasibility of industrial-scale production at low production costs. 
     The cathode active material in accordance with the present invention may be comprised only of the lithium mixed transition metal oxide having the above-specified composition and the specific atomic-level structure or, where appropriate, it may be comprised of the lithium mixed transition metal oxide in conjunction with other lithium-containing transition metal oxides. 
     Examples of the lithium-containing transition metal oxides that can be used in the present invention can include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCoO 2 ) and lithium nickel oxide (LiNiO 2 ), or compounds substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Li 1+y Mn 2−y O 4  (0≦y≦0.33), LiMnO 3 , LiMn 2 O 3 , and LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); vanadium oxides such as LiV 3 O 8 , V 2 O 5 , and Cu 2 V 2 O 7 ; Ni-site type lithium nickel oxides of Formula LiNi 1−y M y O 2  (M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≦y≦0.3); lithium manganese composite oxides of Formula LiMn 2−y M y O 2  (M=Co, Ni, Fe, Cr, Zn, or Ta, and 0.01≦y≦0.1), or Formula Li 2 Mn 3 MO 8  (M=Fe, Co, Ni, Cu, or Zn); LiMn 2 O 4  wherein a portion of Li is substituted with alkaline earth metal ions; disulfide compounds; and Fe 2 (MoO 4 ) 3 , LiFe 3 O 4 , and the like. 
     In accordance with yet another aspect of the present invention, there is provided a lithium secondary battery comprising the aforementioned lithium mixed transition metal oxide as a cathode active material. The lithium secondary battery can comprise a cathode, an anode, a separator and a lithium salt-containing non-aqueous electrolyte. General methods for preparing the lithium secondary battery are known in the art and therefore detailed description thereof will be omitted herefrom. 
     EXAMPLES 
     Now, the present invention will be described in more detail with reference to the following Examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention. 
     For reference, the content of water-soluble bases contained in the powder in the working examples was measured according to the following method. 
     Contents and Characterization of Water-Soluble Bases (pH Titration) 
     First, 5 g of a cathode active material powder was added to 25 mL of water, followed by brief stirring. About 20 mL of a clear solution was separated and pooled from the powder by soaking and decanting. Again, about 20 mL of water was added to the powder and the resulting mixture was stirred, followed by decanting and pooling. The soaking and decanting were repeated at least 5 times. In this manner, a total of 100 mL of the clear solution containing water-soluble bases was pooled. A 0.1M HCl solution was added to the thus-pooled solution, followed by pH titration with stirring. The pH was recorded as a function of time. Experiments were terminated when the pH reached a value of less than about 3, and a flow rate was appropriately selected within a range in which titration takes about 20 to about 30 min. The content of the water-soluble bases was measured as an amount of acid that was used until the pH reaches a value of less than about 5. Characterization of water-soluble bases was made from the pH profile. 
     Example 1 
     A mixed oxyhydroxide of Formula MOOH (M=Ni 4/15 (Mn 1/2 Ni 1/2 ) 8/15 Co 0.2 ) as a mixed transition metal precursor and Li 2 CO 3  were mixed in a stoichiometric ratio (Li:M=1.02:1), and the mixture was sintered in air at various temperatures of 850 (Ex. 1A), 900 (Ex. 1B), 950 (Ex. 1C), and 1,000° C. (Ex. 1D) for 10 hours, to prepare a lithium mixed transition metal oxide. Herein, secondary particles were maintained intact without being collapsed, and the crystal size increased with an increase in the sintering temperature. 
     X-ray analysis shows that all samples have a well-layered crystal structure. Further, a unit cell volume did not exhibit a significant change with an increase in the sintering temperature, thus representing that there was no significant oxygen-deficiency and no significant increase of cation mixing, in conjunction with essentially no occurrence of lithium evaporation. 
     The crystallographic data for the thus-prepared lithium mixed transition metal oxide are given in Table 1 below, and FESEM images thereof are shown in  FIG. 5 . From these results, it was found that the lithium mixed transition metal oxide is LiNiMO 2  having a well-layered crystal structure with the insertion of nickel at a level of 3.9 to 4.5% into a reversible lithium layer. Further, it was also found that even though Li 2 CO 3  was used as a raw material and sintering was carried out in air, proper amounts of Ni 2+  ions were inserted into the lithium layer, thereby achieving the structural stability. 
     Particularly, Sample B, sintered at 900° C. (Ex. 1B), exhibited a high c:a ratio and therefore excellent crystallinity, a low unit cell volume and a reasonable cation mixing ratio. As a result, Sample B showed the most excellent electrochemical properties, and a BET surface area of about 0.4 to about 0.8 m 2 /g. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Example 1(A-D) 
                 (A) 
                 (B) 
                 (C) 
                 (D) 
               
               
                   
               
             
            
               
                 Sintering temp. 
                 850° C. 
                 900° C. 
                 950° C. 
                 1,000° C. 
               
               
                 Unit cell vol. 
                 33.902 Å 3   
                 33.921 Å 3   
                 33.934 Å 3   
                 33.957 Å 3   
               
               
                 Normalized c:a ratio 
                 1.0123 
                 1.0122 
                 1.0120 
                 1.0117 
               
               
                 c:a/24 {circumflex over ( )} 0.5 
               
               
                 Cation mixing 
                 4.5% 
                 3.9% 
                 4.3% 
                 4.5% 
               
               
                 (Rietveld refinement) 
               
               
                   
               
            
           
         
       
     
     Comparative Example 1 
     50 g of a commercial sample having a composition of LiNi 0.8 Co 0.1 Mn 0.1 O 2  represented by Formula LiNi 1−x M x O 2  (x=0.3, and M=Mn 1/3 Ni 1/3 Co 1/3 ) was heated in air to 750° C. (CEx. 1A), 850° C. (CEx. 1B), 900° C. (CEx. 1C) and 950° C. (CEx. 1D) (10 hrs), respectively. 
     X-ray analysis was carried out to obtain detailed lattice parameters with high resolution. Cation mixing was observed by Rietveld refinement, and morphology was analyzed by FESEM. The results thus obtained are given in Table 2 below. Referring to Table 2, it can be seen that all of the samples heated to a temperature of T≧750° C. (CEx. 1A-D) exhibited continuous degradation of a crystal structure (increased cation mixing, increased lattice constant and decreased c:a ratio).  FIG. 6  shows a FESEM image of a commercial sample as received and a FESEM image of the same sample heated to 850° C. (CEx. 1B) in air; and it can be seen that the sample heated to a temperature of T≧850° C. (CEx. 1B-D) exhibited structural collapse. This is believed to be due to that Li 2 CO 3 , formed during heating in air, melts to thereby segregate particles. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Comp. Ex. 1 (A-D) 
                 (A) 
                 (B) 
                 (C) 
                 (D) 
               
               
                   
               
             
            
               
                 Sintering temp. 
                 750° C. 
                 850° C. 
                 900° C. 
                 950° C. 
               
               
                 Unit cell vol. 
                 33.902 Å 3   
                 33.920 Å 3   
                 33.934 Å 3   
                 33.957 Å 3   
               
               
                 Normalized c:a ratio 
                 1.0103 
                 1.0100 
                 1.0090 
                 1.0085 
               
               
                 c:a/24 {circumflex over ( )} 0.5 
               
               
                 Cation mixing 
                 10% 
                 12% 
                 15% 
                 18% 
               
               
                 (Rietveld refinement) 
               
               
                   
               
            
           
         
       
     
     Therefore, it can be seen that it is impossible to produce a conventional lithium mixed transition metal oxide in the air containing trace amounts of carbon dioxide, due to thermodynamic limitations. In addition, upon producing the lithium mixed transition metal oxide according to a conventional method, the use of Li 2 CO 3  as a raw material is accompanied by evolution of CO 2  due to decomposition of Li 2 CO 3 , thereby leading to thermodynamic hindrance of further decomposition of Li 2 CO 3  necessary for the reaction, consequently resulting in no further progression of the reaction. For these reasons, it was found that such a conventional method cannot be applied to the practical production process. 
     Comparative Example 2 
     The pH titration was carried out at a flow rate of &gt;2 L/min for 400 g of a commercial sample having a composition of LiNi 0.8 Co 0.2 O 2 . The results thus obtained are given in  FIG. 7 . In  FIG. 7 , Curve A (CEx. 2A) represents pH titration for the sample as received, and Curve B (CEx. 2B) represents pH titration for the sample heated to 800° C. in a flow of pure oxygen for 24 hours. From the analysis results of pH profiles, it can be seen that the contents of Li 2 CO 3  before and after heat treatment were the same therebetween, and there was no reaction of Li 2 CO 3  impurities. That is, it can be seen that the heat treatment under an oxygen atmosphere resulted in no additional production of Li 2 CO 3  impurities, but Li 2 CO 3  impurities present in the particles were not decomposed. Through slightly increased cation mixing, a slightly decreased c:a ratio and a slightly decreased unit cell volume from the X-ray analysis results, it was found that the content of Li slightly decreased in the crystal structure of LiNiO 2  in conjunction with the formation of a small amount of Li 2 O. Therefore, it can be seen that it is impossible to prepare a stoichiometric lithium mixed transition metal oxide with no impurities and no lithium-deficiency in a flow of oxygen gas or synthetic air. 
     Comparative Example 3 
     LiAl 0.02 Ni 0.78 Co 0.2 O 2  containing less than 3% by mole of an aluminum compound, as commercially available Ana-modified, high-nickel LiNiO 2 , was stored in a wet chamber (90% relative humidity; abbreviated “RH”) at 60° C. in air. The pH titration was carried out for a sample prior to exposure to moisture, and samples wet-stored for 17 hrs and 3 days, respectively. The results thus obtained are given in  FIG. 8 . Referring to  FIG. 8 , an amount of water-soluble bases was low before storage, but substantial amounts of water-soluble bases, primarily comprising Li 2 CO 3 , were continuously formed upon exposure to air. Therefore, even when an initial amount of Li 2 CO 3  impurities was low, it was revealed that the commercially available high-nickel LiNiO 2  is not stable in air and therefore rapidly decomposes at a substantial rate, and substantial amounts of Li 2 CO 3  impurities are formed during storage. 
     Example 2 
     The pH titration was carried out for a sample of the lithium mixed transition metal oxide in accordance with Example 2 prior to exposure to moisture, and samples stored in a wet chamber (90% RH) at 60° C. in air for 17 hours and 3 days, respectively. The results thus obtained are given in  FIG. 9 . 
     Upon comparing the lithium mixed transition metal oxide of Example 2 (see  FIG. 9 ) with the sample of Comparative Example 3 (see  FIG. 8 ), the sample of Comparative Example 3 (stored for 17 hours) exhibited consumption of about 20 mL of HCl, whereas the sample of Example 2 (stored for 17 hours) exhibited consumption of 10 mL of HCl, thus showing an about two-fold decrease in production of the water-soluble bases. Further, in 3-day-storage samples, the sample of Comparative Example 3 exhibited consumption of about 110 mL of HCl, whereas the sample of Example 2 exhibited consumption of 26 mL of HCl, which corresponds to an about five-fold decrease in production of the water-soluble bases. Therefore, it can be seen that the sample of Example 2 decomposed at a rate about five-fold slower than that of the sample of Comparative Example 3. Then, it can be shown that the lithium mixed transition metal oxide of Example 2 exhibits superior chemical resistance even when it is exposed to air and moisture. 
     Comparative Example 4 
     A high-nickel LiNiO 2  sample having a composition of LiNi 0.8 Mn 0.05 Co 0.15 O 2 , as a commercial sample which was surface-coated with AlPO 4  followed by gentle heat treatment, was subjected to pH titration before and after storage in a wet chamber. As a result of pH titration, 12 mL of 0.1M HCl was consumed per 10 g cathode, and the content of Li 2 CO 3  after storage was slightly lower (80 to 90%) as compared to the sample of Comparative Example 3, but the content of Li 2 CO 3  was higher than that of Example 2. Consequently, it was found that the aforementioned high-Ni LiNiO 2  shows no improvements in the stability against exposure to the air even when it was surface-coated, and also exhibits insignificant improvements in the electrochemical properties such as the cycle stability and rate characteristics. 
     Example 3 
     Samples with different Li:M ratios were prepared from MOOH (M=Ni 4/15 (Mn 1/2 Ni 1/2 ) 8/15 Co 0.2 ). Li 2 CO 3  was used as a lithium source. Specifically, 7 samples each of about 50 g with Li:M ratios ranging from 0.925 to 1.12 were prepared by a sintering process in air at a temperature of 910 to 920° C. Then, electrochemical properties were tested. 
     Table 3 below provides the obtained crystallographic data. The unit cell volume changes smoothly according to the Li:M molar ratio.  FIG. 10  shows its crystallographic map. All samples are located on a straight line. According to the results of pH titration, the content of soluble base slightly increased with an increase of the Li:M ratio, but the total amount thereof was small. Accordingly, the soluble base probably originates from the surface basicity (i.e., is present by an ion exchange mechanism) but not from the dissolution of Li 2 CO 3  impurities as observed in Comparative Example 1. 
     Therefore, this experiment clearly shows that the lithium mixed transition metal oxide prepared by the method in accordance with the present invention is in the Li stoichiometric range and additional Li is inserted into the crystal structure. In addition, it can be seen that stoichiometric samples without Li 2 CO 3  impurity can be obtained even when Li 2 CO 3  is used as a precursor and the sintering is carried out in air. 
     That is, as the Li/M ratio decreases, the amount of Ni 2+  inserted into the reversible lithium layer gradually increases. Insertion of excessively large amounts of Ni 2+  into the reversible lithium layer hinders the movement of Li +  during the charge/discharge process, thereby resulting in decreased capacity or poor rate characteristics. On the other hand, if the Li/M ratio is excessively high, the amount of Ni 2+  inserted into the reversible lithium layer is too low, which may result in structural instability leading to deterioration of the battery safety and lifespan characteristics. Further, at the high Li/M value, amounts of unreacted Li 2 CO 3  increase to thereby result in a high pH-titration value. Therefore, upon considering the performance and safety of the battery, the ratio of Li:M is particularly preferably in a range of 0.95 to 1.04 (Samples B, C and D) to ensure that the value of Ni 2+  inserted into the lithium layer is in a range of 3 to 7%. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Samples 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Li:M ratio 
                 0.925 
                 0.975 
                 1.0 
                 1.025 
                 1.05 
                 1.075 
                 1.125 
               
               
                 Unit cell vol. 
                 34.111 Å 3   
                 34.023 Å 3   
                 33.923 Å 3   
                 33.921 Å 3   
                 33.882 Å 3   
                 33.857 Å 3   
                 33.764 Å 3   
               
               
                 c:a ratio 
                 1.0116 
                 1.0117 
                 1.0119 
                 1.0122 
                 1.0122 
                 1.0123 
                 1.0125 
               
               
                 Cation mixing 
                 8.8% 
                 6.6% 
                 4.7% 
                 4.0% 
                 2.1% 
                 2.5% 
                 1.4% 
               
               
                 pH 
                 3 
                 3.5 
                 5 
                 9 
                 15 
                 19 
                 25 
               
               
                   
               
            
           
         
       
     
     Example 4 
     Li 2 CO 3  and a mixed oxyhydroxide of Formula MOOH (M=Ni 4/15 (Mn 1/2 Ni 1/2 ) 8/15 Co 0.2 ) were introduced into a furnace with an about 20 L chamber and sintered at 920° C. for 10 hours, during which more than 10 m 3  of air was pumped into the furnace, thereby preparing about 5 kg of LiNiMO 2  in one batch. 
     After sintering was complete, a unit cell constant was determined by X-ray analysis, and a unit cell volume was compared with a target value (Sample B of Example 1: 33.921 Å 3 ). ICP analysis showed that a ratio of Li and M is very close to 1.00, and the unit cell volume was within the target range.  FIG. 11  shows a scanning electron microscope (SEM) image of the thus-prepared cathode active material and  FIG. 12  shows results of Rietveld refinement. Referring to these drawings, it can be seen that the sample exhibits high crystallinity and well-layered structure, a mole fraction of Ni 2+  inserted into a reversible lithium layer is 3.97%, and the calculated value and the measured value of the mole fraction of Ni 2+  are approximately the same. 
     Meanwhile, upon performing pH titration, less than 10 mL of 0.1M HCl was consumed to titrate 10 g of a cathode to achieve a pH of less than 5, which corresponds to a Li 2 CO 3  impurity content of less than about 0.035 wt %. Hence, these results show that it is possible to achieve mass production of substantially Li 2 CO 3 -free LiNiMO 2  having a stable crystal structure from the mixed oxyhydroxide and Li 2 CO 3  by a solid-state reaction. 
     Example 5 
     More than 1 kg of MOOH (M=Ni 4/15 (Mn 1/2 Ni 1/2 ) 8/15 Co 0.2 ) was prepared by ammonia-free coprecipitation of MSO 4  and NaOH at 80° C. under the pH-adjustment condition.  FIG. 13  shows an SEM micrograph of the thus-prepared precursor hydroxide. The aforementioned MOOH exhibited a narrow particle diameter distribution, and a tap density of about 1.2 g/cm 3 . A lithium mixed transition metal oxide was prepared using MOOH as a precursor. Sintering was carried out at 930° C. The lithium mixed transition metal oxide prepared using such a precursor did not exhibit the disintegration of particles as shown in Comparative Example 2. Therefore, from the excellent sintering stability of LiMO 2 , it can be seen that LiMO 2  can be prepared from the mixed oxyhydroxide having a low tap density. 
     Experimental Example 1 
     Test of Electrochemical Properties 
     Coin cells were fabricated using the lithium mixed transition metal oxide of Examples 3 and 5, and LiNiMO 2  of Comparative Examples 2 to 4 (M=(Ni 1/2 Mn 1/2 ) 1−x Co x  and x=0.17 (Comparative Example 5) and x=0.33 (Comparative Example 6), respectively, as a cathode, and a lithium metal as an anode. Electrochemical properties of the thus-fabricated coin cells were tested. Cycling was carried out primarily at 25° C. and 60° C., a charge rate of C/5 and a discharge rate of C/5 (1 C=150 mA/g) from 3 to 4.3 V. 
     Experimental results of the electrochemical properties for the coin cells of Comparative Examples 2 to 4 are given in Table 4 below. Referring to Table 4, the cycle stability was poor with the exception of Comparative Example 3 (Sample B). It is believed that Comparative Example 4 (Sample C) exhibits the poor cycle stability due to the lithium-deficiency of the surface. Whereas, even though Comparative Example 2 (Sample A) and Comparative Example 3 (Sample B) were not lithium-deficient, only Comparative Example 3 (Sample B) exhibited a low content of Li 2 CO 3 . The presence of such Li 2 CO 3  may lead to gas evolution and gradual degradation of the performance (at 4.3 V, Li 2 CO 3  slowly decomposes with the collapse of crystals). That is, there are no nickel-based active materials meeting both the excellent cycle stability and the low-impurity content, and therefore it can be shown that no commercial product is available in which the nickel-based active material has excellent cycle stability and high stability against exposure to air, in conjunction with a low level of Li 2 CO 3  impurities and low production costs. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Example 
                 Sample A 
                 Sample B 
                 Sample C 
               
               
                   
               
             
            
               
                   
                 LiNi 0.8 Co 0.2 O 2   
                 Al/Ba-modified 
                 AlPO 4 -coated 
               
               
                 Substrate 
                 Comp. Ex. 2 
                 Comp. Ex. 3 
                 Comp. Ex. 4 
               
               
                 Stoichiometric 
                 Stoichiometric 
                 Stoichiometric 
                 Li-deficient at 
               
               
                 Li:M 
                   
                   
                 surfaces 
               
               
                 Li 2 CO 3  impurities 
                 High 
                 High 
                 Low 
               
               
                 Capacity at 25° C. 
                 193, 175 
                 195, 175 
                 185, 155 
               
               
                 C/10, C/1 
                 mAh/g 
                 mAh/g 
                 mAh/g 
               
               
                 Capacity loss 
                 30% per 100 
                 11% per 100 
                 &gt;30% per 100 
               
               
                   
                 cycles 
                 cycles 
                 cycles 
               
               
                   
               
            
           
         
       
     
     On the other hand, the cells of Comparative Examples 5 and 6 exhibited a crystallographic density of 4.7 and 4.76 g/cm 3 , respectively, which were almost the same, and showed a discharge capacity of 157 to 159 mAh/g at a C/10 rate (3 to 4.3 V). Upon comparing with LiCoO 2  having a crystallographic density of 5.04 g/cm 3  and a discharge capacity of 157 mAh/g, a volume capacity of the cell of Comparative Example 5 is equal to a 93% level of LiCoO 2 , and the cell of Comparative Example 6 exhibits a crystallographic density corresponding to a 94% level of LiCoO 2 . Therefore, it can be seen that a low content of Ni results in a poor volume capacity. 
     Table 5 below summarizes electrochemical results of coin cells using LiNiMO 2  in accordance with Example 3 as a cathode, and  FIG. 14  depicts voltage profiles, discharge curves and cycle stability. A crystallographic density of LiNiMO 2  in accordance with Example 3 was 4.74 g/cm 3  (cf. LiCoO 2 : 5.05 g/cm 3 ). A discharge capacity was more than 170 mAh/g (cf. LiCoO 2 : 157 mAh/g) at C/20, thus representing that the volume capacity of LiNiMO 2  was much improved as compared to LiCoO 2 . Electrochemical properties of LiNiMO 2  in accordance with Example 5 were similar to those of Example 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Capacity retention after 100 
                 Primary charge 
                   
               
               
                 cycles (extrapolated) 
                 capacity 
               
               
                 C/5-C/5 cycle, 3.0-4.3 V 
                 3.0-4.3 V, C/10 
                 Discharge capacity 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 25° C. 
                 60° C. 
                 — 
                 25° C., 
                 25° C., 
                 60° C., 
               
               
                   
                   
                   
                 C/1 
                 C/20 
                 C/20 
               
               
                 &gt;96% 
                 &gt;90% 
                 &gt;190 
                 152 
                 173 
                 185 
               
               
                   
                   
                 mAh/g 
                 mA/g 
                 mAh/g 
                 mAh/g 
               
               
                   
               
            
           
         
       
     
     Experimental Example 2 
     Determination of Thermal Stability 
     In order to examine the thermal stability for the lithium mixed transition metal oxide of Example 3 and LiNiMO 2  in accordance with Comparative Examples 3 and 4, DSC analysis was carried out. The thus-obtained results are given in  FIGS. 15 and 16 . For this purpose, coin cells (anode: lithium metal) were charged to 4.3 V, disassembled, and inserted into hermetically sealed DSC cans, followed by injection of an electrolyte. A total weight of the cathode was from about 50 to about 60 mg, A total weight of the electrolyte was approximately the same. Therefore, an exothermic reaction is strongly cathode-limited. The DSC measurement was carried out at a heating rate of 0.5 K/min. 
     As a result, Comparative Example 3 (Al/Ba-modified LiNiO 2 ) and Comparative Example 4 (AlPO 4 -coated LiNiO 2 ) showed the initiation of a strong exothermic reaction at a relatively low temperature. Particularly, Comparative Example 3 exhibited a heat evolution that exceeds the limit of the device. The total accumulation of heat generation was large, i.e. well above 2,000 kJ/g, thus indicating a low thermal stability (see  FIG. 15 ). 
     Meanwhile, LiNiMO 2  of Example 3 in accordance with the present invention exhibited a low total heat evolution, and the initiation of an exothermic reaction at a relatively high temperature as compared to Comparative Examples 3 and 4 (see  FIG. 16 ). Therefore, it can be seen that the thermal stability of LiNiMO 2  in accordance with the present invention is excellent. 
     Experimental Example 3 
     Test of Electrochemical Properties of Polymer Cells with Application of Lithium Mixed Transition Metal Oxide 
     Using the lithium mixed transition metal oxide of Example 3 as a cathode active material, a pilot plant polymer cell of 383562 type was fabricated. For this purpose, the cathode was mixed with 17% by weight LiCoO 2 , and the cathode slurry was an NMP/PVDF-based slurry. No additives for the purpose of preventing gelation were added. The anode was mesocarbon microbead (MCMB) anode. The electrolyte was a standard commercial electrolyte free of additives known to reduce excessive swelling. Experiments were carried out at 60° C. and charge and discharge rates of C/5. A charge voltage was from 3.0 to 4.3 V. 
       FIG. 17  shows the cycle stability of the battery of the present invention (0.8C charge, 1C discharge, 3 to 4 V, 2 V) at 25° C. An exceptional cycle stability (91% at C/1 rate after 300 cycles) was achieved at room temperature. The impedance build up was low. Also, the gas evolution during storage was measured. The results thus obtained are given in  FIG. 18 . During a 4 h-90° C. fully charged (4.2 V) storage, a very small amount of gas was evolved and only a small increase of thickness was observed. The increase of thickness was within or less than the value expected for good LiCoO 2  cathodes tested in similar cells under similar conditions. Therefore, it can be seen that LiNiMO 2  prepared by the method in accordance with the present invention exhibits very high stability and chemical resistance. 
     Example 6 
     A mixed hydroxide of Formula MOOH (M=Ni 4/15 (Mn 1/2 Ni 1/2 ) 8/15 Co 0.2 ) as a mixed transition metal precursor and Li 2 CO 3  were mixed in a molar ratio of Li:M=1.01:1, and the mixture was sintered in air at 900° C. for 10 hours, thereby preparing 50 g of a lithium mixed transition metal oxide having a composition of LiNi 0.53 Co 0.2 Mn 0.27 O 2 . 
     X-ray analysis was carried out to obtain detailed lattice parameters with high resolution. Cation mixing was observed by Rietveld refinement. The results thus obtained are given in Table 6 below. 
     Comparative Example 7 
     A lithium mixed transition metal oxide was prepared in the same manner as in Example 6, except that a ratio of Li:M was set to 1:1 and sintering was carried out under an O 2  atmosphere. Then, X-ray analysis was carried out and the cation mixing was observed. The results thus obtained are given in Table 6 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Ex. 4 
                 Comp. Ex. 7 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Li:M 
                 1.01:1 
                 1:1 
               
               
                   
                 Unit cell vol. 
                 33.921 Å 3   
                 33.798 Å 3   
               
               
                   
                 Normalized c:a ratio 
                 1.0122 
                 1.0124 
               
               
                   
                 c:a/24 {circumflex over ( )} 0.5 
               
               
                   
                 Cation mixing 
                 4.6% 
                 1.5% 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen from Table 6, the lithium mixed transition metal oxide of Example 6 in accordance with the present invention exhibited a larger unit cell volume and a smaller c:a ratio, as compared to that of Comparative Example 7. Therefore, it can be seen that the lithium mixed transition metal oxide of Comparative Example 7 exhibited an excessively low cation mixing ratio due to the heat treatment under the oxygen atmosphere. This case suffers from deterioration of the structural stability. That is, it can be seen that the heat treatment under the oxygen atmosphere resulted in the development of a layered structure due to excessively low cation mixing, but migration of Ni 2+  ions was hindered to an extent that the cycle stability of the battery is arrested. 
     Example 7 
     A lithium mixed transition metal oxide having a composition of LiNi 0.4 Co 0.3 Mn 0.30 O 2  was prepared in the same manner as in Example 6, except that a mixed hydroxide of Formula MOOH (M=Ni 1/10 (Mn 1/2 Ni 1/2 ) 6/10 Co 0.3 ) was used as a mixed transition metal precursor, and the mixed hydroxide and Li 2 CO 3  were mixed in a ratio of Li:M=1:1. The cation mixing was observed by X-ray analysis and Rietveld refinement. The results thus obtained are given in Table 7 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
             
            
               
                   
                 Li:M 
                 1:1 
               
               
                   
                 Unit cell vol. 
                 33.895 Å 3   
               
               
                   
                 Normalized c:a ratio 
                 1.0123 
               
               
                   
                 c:a/24 {circumflex over ( )} 0.5 
               
               
                   
                 Cation mixing 
                 3% 
               
               
                   
                 Capacity (mAh/g) 
                 155 
               
               
                   
                   
               
            
           
         
       
     
     Example 8 
     A lithium mixed transition metal oxide having a composition of LiNi 0.65 Co 0.2 Mn 0.15 O 2  was prepared in the same manner as in Example 6, except that a mixed hydroxide of Formula MOOH (M=Ni 5/10 (Mn 1/2 Ni 1/2 ) 3/10 Co 0.2 ) was used as a mixed transition metal precursor, and the mixed hydroxide and Li 2 CO 3  were mixed in a molar ratio of Li:M=1:1. The cation mixing was observed by X-ray analysis and Rietveld refinement. The results thus obtained are given in Table 8 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
             
            
               
                   
                 Li:M 
                 1:1 
               
               
                   
                 Unit cell vol. 
                 34.025 Å 3   
               
               
                   
                 Normalized c:a ratio 
                 1.0107 
               
               
                   
                 c:a/24 {circumflex over ( )} 0.5 
               
               
                   
                 Cation mixing 
                 7% 
               
               
                   
                 Capacity (mAh/g) 
                 172 
               
               
                   
                   
               
            
           
         
       
     
     From the results shown in Tables 7 and 8, it can be seen that the lithium mixed transition metal oxide in accordance with the present invention provides desired effects, as discussed hereinbefore, in a given range. 
     INDUSTRIAL APPLICABILITY 
     As apparent from the above description, a method for preparing a lithium mixed transition metal oxide in accordance with the present invention enables the production of a lithium mixed transition metal oxide having a given composition and a specific atomic-level structure, by a solid-state reaction of Li 2 CO 3  with a mixed transition metal precursor under an oxygen-deficient atmosphere. Therefore, it is possible to achieve an environmentally friendly preparation method, decreased production costs and improved production efficiency. Since the thus-prepared lithium mixed transition metal oxide exhibits a stable crystal structure and is substantially free of water-soluble bases such as lithium carbonates, a secondary battery comprising such a lithium mixed transition metal oxide has a high capacity, excellent cycle characteristics, and significantly improved storage properties and high-temperature safety. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.