WO2010027119A1

WO2010027119A1 - Cathode active material for lithium secondary batteries with high conductivity and method of preparing for the same and lithium secondary batteries comprising the same

Info

Publication number: WO2010027119A1
Application number: PCT/KR2008/005571
Authority: WO
Inventors: Seong-Bae Kim; Woo-Seong Kim; Kook-Jae Lee; Young-Su Park
Original assignee: Daejung Em Co., Ltd.
Priority date: 2008-09-03
Filing date: 2008-09-19
Publication date: 2010-03-11
Also published as: KR100938138B1

Abstract

A lithium secondary battery includes a lithium metal oxide secondary particle core formed by agglomerating lithium metal oxide primary particles; and a conductive carbonaceous material formed by applying a thermoplastic polymer to an outer surface of the core and then thermally treating the core such that the thermoplastic polymer is melt and thermally decomposed, the conductive carbonaceous material being dispersed on the outer surface of the core and in pores among a plurality of the primary particles in the core. This cathode active material has improved conductivity, thereby improving high rate discharge characteristics and low temperature characteristics of a lithium secondary battery.

Description

CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY

BATTERIES WITH HIGH CONDUCTIVITY AND METHOD OF

PREPARING FOR THE SAME AND LITHIUM SECONDARY

BATTERIES COMPRISING THE SAME Technical Field

[1] The present invention relates to a cathode active material for lithium second batteries with high conductivity, a method for preparing the same, and a lithium secondary battery having the same. More particularly, the present invention relates to a cathode active material capable of improving conductivity of the cathode active material for a lithium ion secondary battery or a lithium ion polymer battery, particularly high rate characteristics and low temperature characteristics, a method for preparing the same, and a lithium secondary battery having the same. Background Art

[2] Along with the rapid development of electronic, communication and computer industries, there are remarkable improvements in camcorders, cellular phones, notebooks and so on. Thus, the demand for lithium secondary batteries is increased day by day as a power source to drive such portable electronic communication devices. In particular, researches and studies are actively progressing not only in Korea but also in Japan, Europe and USA on lithium secondary batteries as an environment-friendly power source that may be applied to electric vehicles, uninterruptible power supplies, electromotive tools and satellites.

[3] Lithium cobalt oxide (LiCoO₂) has been frequently used as a cathode active material of a lithium secondary battery, but in these days, lithium nickel oxide (Li(Ni-Co-Al)O₂ ) and lithium metal composite oxide (Li(Ni-Co-Mn)O₂) are also used as other layered cathode active materials. In addition, spinel-type lithium manganese oxide (LiMn₂O₄) and olivine-type ferric phosphate lithium compound (LiFePO₄) with low price and high stability are increasingly consumed.

[4] The layered lithium metal composite oxide, gradually substituting for lithium cobalt oxides, has been applied to various large capacity battery models such as electric vehicles, load leveling tools and electric bicycles due to its low price and high stability, but it was not used in wireless electromotive tools or wireless aircrafts, which demand a better high rate characteristic. To overcome it, a cathode active material with a relatively smaller average diameter has been used, or lithium cobalt oxide and lithium metal composite oxide have been mixed at a certain ratio and then used. Also, when making a cathode electrode plate, the content of conductive material was increased, or the electrode plate itself was coated thin.

[5] However, if a metal composite oxide with a relatively smaller average diameter is used, characteristics may be improved to some extent, but not greatly. Also, if a metal composite oxide is mixed with lithium cobalt oxide and then used, various problems of the lithium cobalt oxide are still issued. Thus, the above schemes cannot be a fundamental solution.

[6] In addition, if the content of conductive material applied to the cathode electrode plate design is increased or the electrode plate is coated thin, energy density is deteriorated undesirably since the content of cathode active material itself is relatively decreased.

[7] As mentioned above, lithium metal composite oxides having various compositions are limitedly applied as cathode active material for lithium secondary batteries that are applied to the fields demanding high rate characteristics. Also, though being applied, they should have limitations on battery design, namely electrode design, and there is no fundamental improvement for the material itself.

[8] Meanwhile, there has been introduced a technique for coating a surface of a cathode active material with various conductive materials, for example conductive carbonaceous material, or forming a coating layer using a metal oxide with excellent conductivity. This technique improves conductivity of the surface portion of secondary particles and thus improves a high rate characteristic due to the improved conductivity of secondary particles when an electrode plate is formed by means of coating or rolling. However, this technique cannot improve conductivity of primary particles that configure the inside of secondary particles, so the fundamental conductivity of the cathode active material itself is not improved. Thus, the improvement obtained by this technique is not so great.

[9] In order to solve this problem, there have been made various attempts to improve a fundamental conductivity by enhancing the conductivity of primary particles, which correspond to the interior of secondary particle, when making olivine-type ferric phosphate lithium.

[10] For example, Japanese Laid-open Patent Publication No. 2007-035295 discloses a method for preparing olivine-type ferric phosphate lithium coated with carbon at a certain ratio so as to improve conductivity of olivine-type ferric phosphate lithium oxide material itself. In the above document, in the step of making ferric phosphate mixture, fine carbon powder is dispersed, and then ferric phosphate mixture is made. After that, it is reacted with lithium compound to make a material mixture, and then it is fired to make olivine-type ferric phosphate lithium coated with carbon.

[11] Also, Japanese Laid-open Patent Publication No. 2007-035358 also discloses a method for preparing olivine-type ferric phosphate lithium coated with carbon by applying sucrose thereto as a carbon source and then thermally treating it so as to improve conductivity of the olivine-type ferric phosphate lithium oxide itself.

[12] In addition to the above documents, there have been proposed various methods for preparing an olivine-type ferric phosphate lithium oxide with improved conductivity by carbon coating since the olivine-type ferric phosphate lithium oxide has a very low conductivity. However, lithium composite oxides are gradually more demanded since they substitutes with lithium cobalt oxides, but their demand is limited because lithium composite oxides have lower conductivity than lithium cobalt oxides.

[13] Thus, it is urgent to develop a cathode active material with excellent conductivity, which may adopt a lithium metal composite oxide instead of lithium cobalt oxide by improving conductivity of the lithium metal composite oxide itself, and also to develop a method for preparing such a cathode active material with excellent productivity. Disclosure of Invention Technical Problem

[14] The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a cathode active material capable of greatly improving conductivity without deteriorating basic characteristics of the cathode active material itself, and to provide a method for preparing such a cathode active material with excellent productivity and reproduction. Technical Solution

[15] In order to accomplish the above object, the present invention provides a cathode active material for a lithium secondary battery, which includes a lithium metal oxide secondary particle core formed by agglomerating lithium metal oxide primary particles; and a conductive carbonaceous material formed by applying a thermoplastic polymer to an outer surface of the core and then thermally treating the core such that the thermoplastic polymer is melt and thermally decomposed, the conductive carbonaceous material being dispersed on the outer surface of the core and in pores among a plurality of the primary particles in the core.

[16] As mentioned above in relation to the related art, conductive carbonaceous material or conductive metal oxide were conventionally applied into a film for the purpose of enhancing conductivity, but at any case, the improvement of secondary particle itself did not contribute greatly to enhancement of conductivity (intrinsic conductivity) of the material itself. Also, in case of surface reforming using metal oxide, it was found that an adverse effect such as unit capacity reduction might occur because metal oxide is electrically inactive. In addition, there have been continuously made efforts to improve conductivity of olivine-type ferric phosphate lithium oxide itself using various carbon sources, but it was checked that there was no effort to improve conductivity of material itself as for lithium metal composite oxide.

[17] Meanwhile, in the cathode active material for a lithium secondary battery according to the present invention, since primary particles that configure a secondary particle are also coated with conductive carbonaceous material, conductivity (intrinsic conductivity) of the raw material itself is improved without deteriorating basic electrochemical characteristics of the cathode active material, so high rate discharge characteristics and low temperature characteristics are excellent. The conductive carbonaceous material employed in the present invention may be obtained by thermally treating a coating layer of thermoplastic polymer material to melt and thermally decompose it.

[18] The lithium metal oxide corresponding to the above core may use LiCoO₂, Li(Ni_aCo_b

Al₀)O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l), Li(Ni_a-Co_b-Mn_c)O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l) and LiMn₂O₄, in single or in mixture, but not limitedly.

[19] The thermoplastic polymer used in the present invention may employ various kinds of thermoplastic polymers commonly used in the art, and the conductive carbonaceous material may further include metal oxide having an average diameter of 1 to 100 nm.

[20] In another aspect of the present invention, there is also provided a active material for a lithium secondary battery, which includes (Sl) firing metal hydroxide and lithium salt to make a lithium metal oxide secondary particle core in which lithium metal oxide primary particles are agglomerated; (S2) dry-coating the core with thermoplastic polymer to form a thermoplastic polymer coating layer on an outer surface of the core; and (S3) thermally treating the resultant material such that the thermoplastic polymer coating layer is melted and then thermally decomposed.

[21] The method active material for a lithium secondary battery according to the present invention employs a dry-coating process, so it is possible to form a coating layer made of various kinds of thermoplastic polymers while keeping a conductive passivation film existing on the surface of the cathode active material corresponding to the core as it was. Also, if the thermoplastic polymer reaches a melt index by thermal treatment, the thermoplastic polymer is penetrated into pores of the secondary particle. If the polymer is thermally decomposed due to successive thermal treatment, the polymer penetrated into the secondary particle forms a conductive carbonaceous material at its location. Thus, it is possible to increase conductivity at the interior of the secondary particle.

[22] In the method active material for a lithium secondary battery according to the present invention, the thermal treatment of the step (S3) may be conducted at 500 to 800⁰C for 4 to 12 hours.

[23] The above cathode active material for a lithium secondary battery may be used for making a cathode of a lithium secondary battery and a lithium secondary battery having such a cathode.

Brief Description of Drawings

[24] Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawing in which:

[25] FIG. 1 is a schematic view showing a coprecipitation reactor applicable to the present invention;

[26] FIG. 2 is a SEM (Scanning Electronic Microscope) photograph showing a cathode active material of the present invention, prepared according to an example 1 of the present invention, at each stage (a: before coating - a comparative example 1, b: after coating, c: after thermal treatment);

[27] FIG. 3 is a mapping SEM photograph showing a cathode active material prepared according to an example 3 of the present invention (a: a subject for mapping, b: carbon mapping, c: aluminum mapping);

[28] FIG. 4 is a graph showing granular conductivity of the cathode active material prepared according to the comparative example 1 and the example 1 of the present invention;

[29] FIG. 5 is a graph showing granular conductivity of the cathode active material prepared according to the comparative example 3 and the example 5 of the present invention;

[30] FIG. 6 is a graph showing an initial charge/discharge curve of the cathode active material prepared according to the comparative example 1 and the example 1 of the present invention; and

[31] FIG. 7 is a graph showing an initial charge/discharge curve of the cathode active material prepared according to the comparative example 3 and the example 5 of the present invention. Best Mode for Carrying out the Invention

[32] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[33] First, metal hydroxide and lithium salt are fired to make a lithium metal oxide secondary particle core in which lithium metal oxide primary particles are agglomerated (Sl).

[34] The lithium metal oxide available in the present invention may employ any lithium metal oxide used as a cathode active material of a lithium secondary battery in the art, with no limitation. For example, the lithium metal oxide may be at least one selected from the group consisting Of LiCoO₂, Li(Ni_aCo_bAl_c)O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l), Li(Ni_a-Co_b-Mn_c)O₂(0<a<l, 0<b<l, 0<c<l, a+b+c=l) and LiMn₂O₄, or their mixtures, but not limitedly.

[35] The lithium metal oxide secondary particle used as a core in the present invention is configured such that lithium metal oxide primary particles are agglomerated therein, and it is made by firing metal hydroxide and lithium salt, as explained below in detail.

[36] There are various methods for making metal hydroxide in the art, and the present invention is explained based on the coprecipitation method as an example. The above metal hydroxide employs various raw materials correspondingly according to a targeted cathode active material. As main raw materials, metal salt may use sulphate, nitrate, acetate or the like. A solution containing such metal salt is successively put under the coprecipitation circumstance to successively take slurry containing metal hydroxide in the form of reactant, and then metal hydroxide is made by means of washing, filtration and drying. FIG. 1 schematically shows a coprecipitation reactor useable in the present invention.

[37] If such metal hydroxide is used in making a lithium metal oxide as mentioned above, it is possible to restrict introduction of impurities contained in each metal salt, to control a composition to the atom level, to maximize an adding effect of different kinds of elements introduced with a small amount, and to easily make a lithium metal oxide having a uniform crystal structure with substantially no impurity.

[38] The lithium metal oxide of the present invention may be made by thermally treating the metal hydroxide, prepared by the hydroxide coprecipitation method from raw material precursors, at a certain temperature and mixing it with various kinds of lithium metal oxides into a desired composition, and then firing it under a common firing condition. The lithium metal oxide prepared as mentioned above is obtained as a lithium metal oxide secondary particle in which lithium metal oxide primary particles are agglomerated.

[39] An average diameter of the primary particles that configure the secondary particle may be variously changed depending on coprecipitation circumstances according to a composition ratio of the metal hydroxide, and it is not limited to any range.

[40] Also, an average diameter of the secondary particles may be variously changed depending on usage or production circumstances, and it may be 7 to 15 μm for example, but not limitedly. When the average diameter of the secondary particles is in the above range, the efficiency and reproduction of the dry-coating process of the polymer material are further improved.

[41] The shape of the secondary particle is not specially limited, but the efficiency of the coating process for applying polymer to a surface of the secondary particle may be further improved when the secondary particle has a spherical shape.

[42] In addition, the secondary particle prepared using the coprecipitation method as mentioned above has fine pores of various sizes in its surface according to a composition ratio and a production process of the metal hydroxide. Such pores are naturally created while primary particles are agglomerated to form a secondary particle.

[43] After that, the core is dry-coated with the thermoplastic polymer to form a thermoplastic polymer coating layer on an outer surface of the core (S2).

[44] The thermoplastic polymer used in making the cathode active material of the present invention may adopt any thermoplastic polymer that may be melted by thermal treatment and then form conductive carbonaceous material by thermal decomposition, with no limitation. The thermoplastic polymer available in the present invention has no limitation in its shape or diameter. However, soft polymer is preferred to hard polymer, and a powder or film shape is preferred to a pellet shape. As the thermoplastic polymer is softer, a coating process onto the surface of the cathode active material secondary particle is executed faster. Also, when the thermoplastic polymer has a powder or film shape, the efficiency and reproduction are more excellent.

[45] The content of the thermoplastic polymer used in the present invention may be variously selected according to usage of a cathode active material or a battery. For example, the content of the thermoplastic polymer may be 0.1 to 10 parts by weight, based on 100 parts by weight of the core, but not limitedly. If the content of the thermoplastic polymer is less than 0.1 parts by weight, the content of carbonaceous material obtained after thermal composition is insufficient, so a conductivity improving efficiency is not good. If the content exceeds 10 parts by weight, the content of carbonaceous material obtained after thermal decomposition is greatly increased, so specific capacity of the cathode raw material itself tends to be deteriorated.

[46] Selectively, as an alternative of the above polymer materials, any solid material having a melt index above a certain temperature is available, and for example, sugars, cellulose or low molecular materials with a molecular weight of 10,000 or less are all available. In this case, it is preferred that hardness of such materials is similar to that of the thermoplastic polymer, in aspect of the dry-coating process.

[47] In the cathode active material of the present invention, the above coating layer may further include metal oxide with an average diameter of 1 to 100 nm, in addition to polymer material. When the average diameter is in the above range, the coating layer may ensure excellent productivity and maximized performance of conductive carbonaceous material formed later, without causing capacity reduction of a battery. In more detail, if metal oxide with a size of several nanometers is included, the secondary particle corresponding to the core may have a spherical shape more easily, and the polymer material forming the coating layer gives an effect of a binding agent to enhance efficiency of the coating process. Also, in addition to the improved conductivity caused by the conductive carbonaceous material formed after the coating layer is melted, it is expected that the metal oxide additionally improve characteristics.

[48] For example, in case aluminum oxide (Al₂O₃) is further included and mixed, powder characteristics are improved, bonding of the polymer material is reinforced, and improvement of high temperature characteristics is additionally expected, though capacity of the battery is slightly decreased. As known in the art, since the electrically inactive metal oxide coexists with the conductive carbonaceous material dispersed in the outer surface of the secondary particle and in the pores of the interior primary particles, the surface of the lithium metal oxide corresponding to the core is relatively limitedly contacted with electrolyte at the high temperature storage or high temperature cycle, so the characteristics are improved. Also, it is known in the art that metal oxides such as titanium oxide (TiO₂), yttrium oxide (Y₂O₃), magnesium oxide (MgO) and zinc oxide (ZnO) also exhibit similar characteristics. In addition, lithium metal oxide may be further included, and in this case, it is expected that rapid charge/discharge characteristics or cycle characteristics is improved without reduction of the capacity of the battery. The lithium metal oxide may employ various kinds of lithium metal oxides such as layered lithium metal composite oxide, lithium cobalt oxide and spinel-type lithium manganese oxide, for example.

[49] However, the coating material used with the conductive carbonaceous material is not limited to the above, but various kinds of nano-size metal oxide materials can be used in combination according to the desired improvement in its functionality such as stability and high temperature characteristics.

[50] Generally, the coating method can be classified into a dry-coating method and a wet- coating method, as well known in the art. In most conventional cases, the wet-coating method was applied for the purpose of uniform dispersion of coating material. Namely, in general cases, a dispersion solution or a suspension solution in which a coating material is dispersed or an organic solution or an aqueous solution in which a coating material is dissolved is sprayed or impregnated into a cathode active material, and then dried for coating. However, the wet-coating method has a limit in forming a film-type coating layer. In addition, in case a coating solution using water as a solvent is applied, a conductive passivation film of a lithium salt form, formed on the surface of the cathode active material, is removed, so the conductivity of the cathode active material itself is deteriorated. Also, drying and pulverizing processes are additionally required, which becomes a drawback in mass production.

[51] On the contrary, the dry-coating method used in the present invention is executed such that a coating material is applied to a surface of a cathode active material corresponding to the core in a mechanical way, which may gives a shearing force, a collision force or a compression force according to a used equipment, so this method allows simple blending as well as coating.

[52] Generally, a cathode active material obtained by firing lithium and metal hydroxide as a raw material precursor at a high temperature necessarily requires pulverization and distribution since sintering happens by some hydroxide precursors with bad spherical shape or an excessive amount of lithium. However, it was substantially impossible to pulverize the metal hydroxide as a raw material precursor into an average diameter while keeping the spherical shape.

[53] However, in the coating method of the present invention using a mechanical way, the lithium metal oxide corresponding to the core is made into spherical shape and disintegrated at the same time by the nanometer-size metal oxide and/or the thermoplastic polymer configuring conductive carbonaceous material, so the powder characteristics may be improved.

[54] Subsequently, after the thermoplastic polymer coating layer is melted, the resultant material is thermally treated for thermal decomposition (S3).

[55] When reaching a corresponding melt index during the thermal treatment process, the thermoplastic polymer configuring the coating layer is melted, so the melted polymer may be penetrated from the surface of the secondary particles, which forms the porous surface, to the primary particles, which forms the interior of the secondary particles, by means of the capillary phenomenon. After that, if the temperature is increased further, thermal decomposition happens at a corresponding location, namely at the exterior of the secondary particle and the interior of the secondary particle, and as a result, the decomposed thermoplastic polymer remains as conductive carbonaceous material at the exterior of the secondary particle and in the pores among the primary particles configuring the interior of the secondary particle, thereby forming a conductive carbonaceous material dispersed at the exterior of the secondary particle and in the pores of its interior. Thus, it is possible to improve the conductivity (intrinsic conductivity) of the cathode active material itself.

[56] The thermoplastic polymer configuring the coating layer preferably has excellent fluidity such that the thermoplastic polymer may be melted and penetrated into the pores in the surface of the cathode active material configuring the core in the thermal treatment process. Namely, the polymer preferably has a great melt index. For example, the thermoplastic polymer has a melt index of 1 g/10min or above. If the melt index is lower than the above range, the fluidity of the thermoplastic polymer is limited though the corresponding thermoplastic polymer is melted in a melting temperature range. Thus, the thermoplastic polymer is relatively slowly penetrated into the fine pores in the surface of the cathode active material, so the thermal treatment time is extended to obtain sufficient penetration, thereby deteriorating productivity. As the thermoplastic polymer of the present invention has better fluidity in a melting process, the thermoplastic polymer is more easily penetrated into the pores, so its melt index has no special upper limit. For example, if the melt index exceeds lOOg/lOmin, there is expected no additional effect caused by the increased fluidity, but the present invention is not limited thereto.

[57] In more detail, the same kind of polymer may more easily reach the pores among the primary particles configuring the interior of the secondary particle as it has better melting fluidity, so a polymer with better fluidity may maximize the conductivity improvement effect.

[58] In addition, in case metal oxide is further included in the coating layer, individual cathode active material particles may get rid of stress caused by the mechanical dry- coating process, by means of the thermal treatment process after coating, so it is possible to control specific capacity reduction or surface characteristic deterioration of the secondary particle, caused by the film of the electrically inactive metal oxide. The thermal treatment conditions can be suitably selected according to production circumstances such as the kind of cathode active material of the core or the polymer material that configures a conductive carbonaceous material, but not limitedly. For example, the thermal treatment may be conducted at 500 to 800⁰C for 4 to 12 hours. At the above thermal treatment temperature, the conductive carbonaceous material exhibits very excellent density, the crystal structure defect of the core may be sufficiently compensated, and the structure of the core may be stably maintained. The thermal treatment time ensures sufficient effects in the above range. If the thermal temperature time exceeds 12 hours, there is expected no additional effects though the thermal treatment time is increased further.

[59] The cathode active material for a lithium secondary battery according to the present invention may be adhered to at least one surface of a cathode electric collector using a binder resin to form a cathode of a lithium secondary battery. The binder resin and the cathode current collector may adopt any one commonly used in the art without any limitation.

[60] In addition, the cathode for a lithium secondary battery according to the present invention may be used for making a lithium secondary battery together with an anode, a separator interposed between the cathode and the anode, and an electrolyte. The anode, the separator and the electrolyte may adopt any one commonly used in the art without any limitation.

[61] Hereinafter, various preferred examples of the present invention will be described in detail for better understandings. However, the examples of the present invention may be modified in various ways, and they should not be interpreted as limiting the scope of the invention. The examples of the present invention are just for better understandings of the invention to persons having ordinary skill in the art.

[62] Example 1

[63] < Preparation of Lithium Metal Composite Oxide Core >

[64] Nickel sulfate (NiSO₄-OH₂O), manganese sulfate (MnSO₄-H₂O) and cobalt sulfate

(CoSO₄-7H₂O) were dissolved in a refined ion exchange water such that nickel, cobalt and manganese had a mole ratio of 0.5:0.2:0.3, thereby making a metal solution. Also, a sodium hydroxide and an ammonia solution were prepared.

[65] A coprecipitation reactor was used in pH 11.5 and 400 rpm to supply the metal solution at a rate of 5 L/hr and the ammonia solution at a rate of 0.5 L/hr under an inert nitrogen circumstance by means of a quantitative pump. The sodium hydroxide solution was intermittently put such that the solution in the reactor could keep 11.5 pH constantly.

[66] The reaction was executed over 48 hours to obtain slurry containing metal composite hydroxide of regular size. A centrifugal separator-type filter was used to wash and filter the slurry until a filtered solution had pH 9.0 or below, and then the obtained metal composite hydroxide powder was dried at 12O⁰C over 24 hours to make metal composite hydroxide.

[67] After that, in order to set a stoichiometric ratio with lithium salt, the metal composite hydroxide was thermally treated over 12 hours at a temperature of 300⁰C, and then it was mixed with lithium salt such that a stoichiometric ratio with lithium salt becomes 1:1.1. This mixture was fired for 24 hours at 95O⁰C in a high temperature firing furnace capable of controlling temperature, and for 24 hours at 500⁰C. After that, pulverization and distribution were conducted to make a metal composite oxide with a controlled average diameter, and then it was thermally treated for 4 hours at 500⁰C.

[68] The made metal composite oxide had a ratio of Ni:Co:Mn as 0.50:0.20:0.30.

[69] < Preparation of Cathode Active Material >

[70] The obtained metal composite oxide was used as a core, and polypropylene having a melt index of 12g/10min (24O⁰C, 2.16kg (ASTM D 1238)) was used as a coating material to make a cathode active material. 26Og of polypropylene was mixed to 13 kg of metal composite oxide such that the polypropylene ha a weight ratio of 2% to the core, and then the mixture was dry-coated to make a cathode active material according to the present invention. 700⁰C for 4 hours to make a cathode active electrode in which conductive carbonaceous material is dispersed in the interior and exterior of the secondary particle. [71] Example 2

[72] A cathode active material was made in the same way as the example 1, except that polyethyleneglycol (with a molecular weight of 20,000) was used instead of polypropylene. [73] Example 3

[74] A cathode active material was made in the same way as the example 1, except that polypropylene and aluminum oxide with an average diameter of 13 nm were used with the content of 2.0 parts by weight and 0.3 parts by weight, respectively, based on 100 parts by weight of the core. [75] Example 4

[76] A cathode active material was made in the same way as the example 3, except that the metal composite oxide corresponding to the core was made to have a composition in which a ratio of Ni:Co:Mn is 0.40:0.30:0.30. [77] Example 5

[78] A cathode active material was made in the same way as the example 3, except that the metal composite oxide corresponding to the core was made to have a composition in which a ratio of Ni:Co:Mn is 0.50:0.00:0.50. [79] Comparative Example 1 to 3

[80] The metal composite oxides corresponding to the core, obtained in the examples 1 to

3, were selected as the comparative examples 1 to 3. [81] Comparative Example 4

[82] The metal composite oxide obtained in the example 1 was simply mixed with polypropylene and then thermally treated in the same way as the example 1 to make a cathode active material containing thermally decomposed carbon. [83] Evaluation of Characteristics

[84] 1. Powder Characteristics

[85] Average diameter and tap density of the cathode active materials prepared according to the examples 1 to 5 were measured at each production stage. The measurement results are listed in the following table 1. [86] The average diameter was measured using a particle size distribution measurer

(Maters izer 2000E, produced by Malvern). While dispersing the cathode active material using ultrasonic wave, the average diameter D₅₀ was obtained by means of laser scattering. The tap density was measured from a volume change before and after

400 strokes were conducted, using 100 ml measuring cylinder. [87] Table 1 [Table 1] [Table ]

[88] a. Comparative example 1: cathode active material of the examples 1 to 3 before coating

[89] Comparative example 2: cathode active material of the example 4 before coating

[90] Comparative example 3: cathode active material of the example 5 before coating

[91] b. PP: polypropylene, PEG: polyethyleneglycol, A: aluminum oxide

[92] As seen from the table 1, in case the cathode active material (of the comparative examples 1 to 3) without a coating layer is coated only with thermoplastic polymer as in the examples 1 and 2, the cathode active material is partially made into spherical shape and disintegrated, so it would be understood that tap density is greatly lowered since a coating layer is formed on the surface of the cathode active material though the average diameter D₅₀ is decreased. In addition, in case thermoplastic polymer and nanometer- size aluminum metal oxide are mixed and used for forming a coating layer, the cathode active material is more greatly made into spherical shape and disintegrated, but it would be understood that tap density is lowered identically to the case that the coating layer is formed only with thermoplastic polymer. However, if thermal treatment is conducted to the core-shell type cathode active material obtained after coating, the thermoplastic polymer in the coating layer is melted and then exists as thermally decomposed carbonaceous material in the surface or interior of the cathode active material corresponding to the core, so it would be understood that the tap density is recovered. [93] 2. Coating Characteristics [94] i) Surface Shape

[95] In order to check shape and surface characteristics of the cathode active material obtained in the examples and the comparative examples, SEM (8564E, produced by HP) photograph was taken. SEM photographs of the example 1 and the comparative example 1 are respectively shown in FIGs. 1 and 2. Also, FIG. 3 shows a map shape of the cathode active material prepared according to the example 3.

[96] As shown in FIG. 2, it could be found that the cathode active material on the surface of which the coating layer of thermoplastic polymer is formed and the cathode active material on which conductive carbonaceous material is formed are uniformed obtained.

[97] In addition, as shown in FIG. 3, it could be found that aluminum and thermally decomposed carbonaceous material formed by thermal decomposition of the thermoplastic polymer that has formed the coating layer are uniformly distributed in the cathode active material prepared according to the example 3.

[98] ii) Coating Efficiency

[99] In order to determine efficiency of the coating process, the changes of powder characteristics of cathode active material before coating, cathode active material after coating (before thermal treatment), and cathode active material obtained by simply mixing the coating material were checked. Also, in order to measure the change of powder conductivity of the cathode active material itself before and after coating and after thermal treatment, a 4-probe powder resistor (LORESTA-GP, produced by Mitsubishi Chemical) was used. The measurement results are shown in FIGs. 4 and 5. The coating efficiency was check by means of electrochemical characteristic evaluation.

[100] Table 2 [Table 2] [Table ]

[101] As seen from the table 2, it could be found that, in the case of the cathode active material obtained through the coating process, tap density is greatly decreased due to the polymer existing in the coating layer formed on the surface of the cathode active material. However, in case the same amount of thermoplastic polymer is simply mixed as in the comparative example 4, tap density is measured as being not lowered. The same result was found also in the case that the cathode active material is coated with conductive carbonaceous material instead of polymer material.

[102] In addition, as seen from FIGs. 4 and 5, it could be understood that conductivity of the cathode active material itself is greatly improved in the examples of the present invention. Namely, the LiNi₀₅Mn₀₅O₂ cathode active material containing no Co as in the comparative example 3 exhibits greatly lower powder conductivity in comparison to a general lithium metal composite oxide containing Co, but it could be found that the carbon-coated cathode active material prepared according to the example 5 of the present invention exhibits greatly improved powder conductivity.

[103] 3. Electrochemical Characteristics

[104] i) Evaluation of Half Cell

[105] In order to evaluate initial specific capacity and initial efficiency of the cathode active material obtained in the examples 1 to 5 and the comparative examples 1 to 4, the cathode active material was mixed with a NMP solution obtained by melting Teflonized acetylene black as conductive material and PVDF as a binding agent to make slurry. In the slurry, a mass ratio of the cathode active material, the conductive material and the binding agent was set to 90:3:7. This slurry was applied onto a 30mm Al electric collector and then dried, and then it was compressed to a predetermined thickness and blanked into a diameter of 13 mm, thereby making a cathode.

[106] The obtained cathode was used together with a lithium foil as an anode using a separator with a thickness of 20 mm to make a 2032 standard coin-type battery. At this time, the electrolyte adopts a mixed solvent of ethylene carbonate and dimethyl carbonate (at a volume ratio of 1:3). Charge/discharge capacity of the battery was measured with a current density of 0.2C at 25 ⁰C in 2.5-4.2V voltage range using a charge/discharge cycle device, in which charge was measured under the constant current-constant voltage condition (0.02C at a final charge stage) and discharge was measured under a constant current condition. The measurement results are shown in the following table 3. Also, initial charge/discharge curves of the examples 1 and 5 and the comparative examples 1 and 3 are respectively shown in FIGs. 6 and 7.

[107] Table 3 [Table 3] [Table ]

[108] As seen from the table 3, in the examples 1 to 5, it could be understood that the conductivity of the cathode active material itself is improved rather than the comparative examples, so the initial efficiency and the specific capacity are improved with respect to the same current density. Also, in case most thermally decomposed carbonaceous material is simply mixed to the cathode active material as in the comparative example 4, it could be found that the conductivity of the cathode active material is not improved since the thermally decomposed carbonaceous material plays the same role as conductive material introduced when cathode slurry is made.

[109] ii) Evaluation of Full Cell

[110] In order to evaluate high rate characteristics and low temperature characteristics of the cathode active material obtained in the examples and the comparative examples, the cathode active material was mixed with an NMP solution obtained by dissolving carbon as conductive material and PVDF as a binding agent to make slurry. In the slurry, a mass ratio of the cathode active material, the conductive material and the binding agent was set to 90:3:7. Graphite was used as an anode, and the cathode and the anode were placed to face each other with a separator being interposed between them. Then, an aluminum pouch with a thickness of 113 mm was applied thereto, and then they were sealed in a glove box under an argon circumstance and then thermally bonded to make a pouch-type battery. The capacity of the battery was set to 1000 mAh.

[111] The battery was initially charged/discharged with a current density of 0.2C (200 mAh) at 25 ⁰C in 3.0-4.2V voltage range using a charge/discharge cycle device, and then charge/discharge experiments were conducted at various current densities. Also, after the initial charge/discharge was conducted under the same condition, 0.5C and l.OC charge/discharge was repeated, and then, after being charged, the battery was discharged at -10 ⁰C with l.OC current density to evaluate low temperature characteristics. The high rate characteristics was evaluated from a ratio of discharge capacity at 1OC current density using discharge capacity at 0.5C current density as a criterion capacity, and the low temperature characteristics was evaluated from a ratio of discharge capacity at -10 ⁰C with l.OC current density using a discharge capacity at 25 0C with l.OC current density as a criterion capacity. The following table 4 shows high rate characteristics and low temperature characteristics of the cathode active material obtained in the examples and the comparative examples.

[112] Table 4 [Table 4] [Table ]

[113] As seen from the table 4, it could be found that the examples 1 to 5 exhibit more excellent discharge characteristics with respect to the same increase of discharge current density than the comparative examples 1 to 3 since the conductivity of the cathode active material itself is improved. Also, it could be found that low temperature characteristics of the examples 1 to 5 are greatly improved.

[114] From the above results, it could be understood that the cathode active material obtained in the examples allows to coat the surface of primary particles with thermally decomposed conductive carbonaceous material in a very efficient way with good reproduction, and thus the conductivity of the cathode active material itself is greatly improved, which resultantly remarkably improves high rate characteristics and low temperature characteristics. Industrial Applicability

[115] The cathode active material for a lithium secondary battery according to the present invention includes a lithium metal oxide core and a conductive carbonaceous material dispersed in the exterior and interior of the core, and the conductive carbonaceous material is applied even to the primary particles that configure the interior of the secondary particle, thereby improving conductivity of the cathode active material itself and thus improving high rate discharge characteristics and low temperature characteristics of the lithium secondary battery. Also, the method for preparing a cathode active material for a lithium secondary battery according to the present invention ensures excellent reproduction and productivity in making the cathode active material of the present invention.

Claims

[1] A cathode active material for a lithium secondary battery, comprising: a lithium metal oxide secondary particle core formed by agglomerating lithium metal oxide primary particles; and a conductive carbonaceous material formed by applying a thermoplastic polymer to an outer surface of the core and then thermally treating the core such that the thermoplastic polymer is melt and thermally decomposed, the conductive carbonaceous material being dispersed on the outer surface of the core and in pores among a plurality of the primary particles in the core.

[2] The cathode active material for a lithium secondary battery according to claim 1, wherein the lithium metal oxide is at least one selected from the group consisting Of LiCoO₂, Li(Ni_aCo_bAl_c)O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l), Li(Ni_a-Co_b-Mn_c )O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l) and LiMn₂O₄, or their mixtures.

[3] The cathode active material for a lithium secondary battery according to claim 1, wherein the secondary particle has an average diameter of 7 to 15 μm.

[4] The cathode active material for a lithium secondary battery according to claim 1, wherein the thermoplastic polymer is included in the content of 0.1 to 10 parts by weight, based on 100 parts by weight of the core.

[5] The cathode active material for a lithium secondary battery according to claim 1, wherein the thermoplastic polymer has a melt index of 1 g/10min or above.

[6] The cathode active material for a lithium secondary battery according to claim 1, wherein the conductive carbonaceous material further includes metal oxide with an average diameter of 1 to 100 nm.

[7] The cathode active material for a lithium secondary battery according to claim 6, wherein the metal oxide is at least one selected from the group consisting of aluminum oxide, titanium oxide, yttrium oxide, magnesium oxide, zinc oxide and lithium metal oxide, or their mixtures.

[8] The cathode active material for a lithium secondary battery according to claim 7, wherein the lithium metal oxide is at least one selected from the group consisting of layered lithium metal composite oxide, lithium cobalt oxide and spinel-type lithium manganese oxide, or their mixtures.

[9] A method for preparing a cathode active material for a lithium secondary battery, comprising:

(51) firing metal hydroxide and lithium salt to make a lithium metal oxide secondary particle core in which lithium metal oxide primary particles are agglomerated;

(52) dry-coating the core with thermoplastic polymer to form a thermoplastic polymer coating layer on an outer surface of the core; and

(S3) thermally treating the resultant material such that the thermoplastic polymer coating layer is melted and then thermally decomposed. [10] The method for preparing a cathode active material for a lithium secondary battery according to claim 9, wherein, in the step (Sl), the metal hydroxide is made according to a copre- cipitation method. [11] The method for preparing a cathode active material for a lithium secondary battery according to claim 9, wherein, in the step (S2), the coating layer is formed by dry-coating the core with a mixture of thermoplastic polymer and metal oxide with an average diameter of

1 to 100 nm. [12] The method for preparing a cathode active material for a lithium secondary battery according to claim 9, wherein, in the step (S3), the thermal treatment is conducted at 500 to 800 ⁰C for

4 to 12 hours. [13] A cathode of a lithium secondary battery, which includes a cathode current collector and a cathode active material layer formed on at least surface of the cathode current collector and having cathode active material and binder resin, wherein the cathode active material is a cathode active material defined in any one of the claims 1 to 8.

[14] A lithium secondary battery including a cathode, an anode and a separator interposed between the cathode and the anode, wherein the cathode is a cathode defined in the claim 13.