Patent Publication Number: US-2018040893-A1

Title: Positive electrode active material for non-aqueous electrolyte lithium secondary batteries and production method thereof

Description:
FIELD OF TECHNOLOGY 
     The present invention relates to a positive electrode active material for non-aqueous electrolyte lithium secondary batteries comprising Li—Co—Ni—Al composite oxide and the production method thereof, particularly said positive electrode active material for large-capacity lithium secondary batteries having a long-life with superior charge-discharge cycle properties. 
     BACKGROUND ART 
     The non-aqueous electrolyte lithium secondary batteries are used for a variety of applications such as mobile devices such as laptop computers and cell phones and power sources for power tools. The applications for those batteries are expected to expand from the aspect of structuring low carbon societies and energy security, and the advancement of technology for those batteries are highly desired. 
     The demand in lithium secondary batteries as power sources of hybrid automobile or electric bicycles (indicated collectively as EV hereinafter) has been rapidly expanded recent years, and in order to use the lithium secondary batteries as power sources for EV, it is especially desired to have large capacity and long life. In view of this, the improvement in lithium secondary battery materials has been an urgent task. In the current situation where LiCoO 2  having cobalt (Co) as a main component as positive electrode material among the materials constituting lithium secondary batteries has been widely used and the demand in the power sources for EV has been rapidly expanded, it is concerned that Co, which is a rare metal, might be depleted as a resource and the cost would be high. 
     As an alternative positive electrode active material for LiCoO 2 , there are positive electrode active materials constituted of LiMn 2 O 4  or Ni—Co—Mn ternary composite oxide having Mn as a main component. However, those materials have advantages and disadvantages in battery properties and are not fully meeting their requirements as the power sources for power tools and EV. 
     In the aforementioned situations, the lithium secondary batteries using Li x Ni y Co 1-y-z Al z O 2  having Ni which has large charge-discharge voltage and capacity as a main material have been studied. Conventionally, for high capacity and improvement for charge-discharge maintenance ratio, a variety of improvement technologies for Li x Ni y Co 1-y-z Al z O 2  powders have been suggested, for example, the technology of enabling long operating life by improving conductivity by adhering oxides containing Zn and Al on the surface of the LiNiO 2  positive electrode active material (Patent Literature 1) and the technology that improves charge-discharge capacity of Li—Ni—Co—Al composite oxide and filling and preservative properties by lowering the change rate of the specific surface area before and after compression and the content of the sulfate ion (Patent Literature 2). 
     The Li x Ni y Co 1-y-z Al z O 2  positive electrode active materials disclosed in the Patent Literatures 1 and 2 are trying to obtain large capacity and long life by improving their electrical conductivities by producing a coating layer by a surface modifying agent in order to stabilize their crystal structures. However, they have not attained enough effects yet and the development of the positive electrode for the lithium secondary batteries having better battery properties than the conventional products has always been expected in the market. 
     PRIOR ART LITERATURES 
     Patent Literatures 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2011-129258 
     Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2008-166269 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The objective of the present invention is to provide the above-mentioned positive electrode active material that can be a non-aqueous electrolyte lithium secondary battery having large capacity and superior charge-discharge maintenance ratio when used as a positive electrode in a lithium secondary battery, and the production method thereof. 
     Means for Solving the Problem 
     The present inventors found out that, after having had exhaustive consideration for solving the above-mentioned objective, in order to obtain large capacity and long life, it is important to have high filling density of positive electrode active material and etc. In order to achieve this, the present inventors discovered that it is important to have: 
     ( 1 ) the positive electrode active material in particles close to spherical shape;
 
( 2 ) moreover, all the particles have the uniform composition and shape; and
 
( 3 ) furthermore, the aforementioned spherical shape particles are in a predetermined size not in extremely small fine particles and have high particle strength.
 
     Next, after having had further exhaustive consideration with keeping the aforementioned findings in mind, in order to produce a positive electrode active material composed of Li—Co—Ni—Al composite oxide having the above-mentioned particle properties, it was discovered to be important that: 
     ( 11 ) in the secondary particle Ni—Co coprecipitation hydroxide production process, it needs to be produced under inert atmosphere, not under the oxidative atmosphere which results in miniaturization of the particles;
 
( 12 ) not only the positive electrode active material strength can be further improved if the mixing of the aforementioned Ni—Co coprecipitation hydroxide particles, Li compound, and Al compound can be done in dry system, but also the cost for mixing process compared to the conventional wet system as well as that for calcining process after the mixing process can be significantly reduced; and
 
( 13 ) it is important for the secondary particles to have the particle properties of the aforementioned ( 1 )-( 3 ) and to adjust the size of Li particles in order to conduct good mixing (getting Li particles in the gaps between the secondary particles evenly and uniformly) in dry system.
 
     The present invention is based on the aforementioned findings and the positive electrode active material for non-aqueous electrolyte lithium secondary batteries of the present invention is ( 1 ) a non-aqueous electrolyte lithium secondary battery composed of Li—Ni—Co—Al composite oxide as a positive electrode, characterized in that the initial discharge capacity is 192 mAh/g or higher when discharged at the discharge rate of 0.1 C at the measuring temperature of 20° C., ( 2 ) the discharge maintenance ratio after discharging the aforementioned non-aqueous electrolyte lithium secondary battery at the discharge rate of 1 C at the measuring temperature of 20° C. for 100 times can be 94% or higher, ( 3 ) the composition formula of the aforementioned composite oxide can be Li x Ni y Co 1-y-z Al z O 2  (where 0.9≦x≦1.1, 0.8≦y≦95, 0.00≦z≦0.05), and ( 4 ) the aforementioned composite oxide particles can have the aspect ratio≧0.9. 
     Also, the production method of the aforementioned positive electrode active material for non-aqueous electrolyte lithium secondary battery of the present invention ( 5 ) is characterized in producing Ni and Co coprecipitation hydroxide by stirring the aqueous solution in which Ni compound and Co compound are dissolved under N 2  gas atmosphere while adding alkali, after dehydrating and drying the coprecipitation hydroxide, mixing Li compound and Al compound in dry system, then calcining the mixture under oxidation atmosphere. At this time, ( 6 ) the stirring of the aqueous solution of Ni compound and Co compound dissolved can be done at a speed at which N 2  gas is taken into the aqueous solution, ( 7 ) the Li compound particles that are mixed with Ni and Co coprecipitation hydroxide are 10-50 μm, and ( 8 ) the calcining can be done at 710-790° C. for 5-20 hours. 
     Effects of the Invention 
     The positive electrode active material composed of Li—Ni—Co—Al composite oxide in the present invention achieved to make a lithium secondary battery having the initial discharge capacity of the battery of 192 mAh/g or higher, improved charge-discharge maintenance ratio, and large capacity/long life by being used for non-aqueous electrolyte lithium secondary battery. 
     Also, in the positive electrode active material of the present invention, since the ratio between the long axis and short axis in each positive electrode active material particle (the aspect ratio) is 0.9 or higher and the particle shape is close to spherical shape, the filling density of the positive electrode active material can be high when manufactured to be a positive electrode of the lithium secondary battery. 
     Further, in the producing method of the present invention, Ni and Co coprecipitation hydroxide particles (secondary particles) can be made to a shape close to spherical shape and to have high strength. 
     Moreover, the positive electrode active material having uniform composition and spherical shape can be obtained by synergic action of the aforementioned coprecipitation hydroxide and particle properties by adjusting the particle size of Li compound which is mixed with the coprecipitation hydroxide even in the dry system mixing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Examples showing the charge-discharge curves of the lithium secondary batteries using the positive electrode active material of the present invention and the conventional (existing) positive electrode active material respectively. 
         FIG. 2  Examples showing the cycle properties when the lithium secondary batteries using the positive electrode active material of the present invention and the conventional positive electrode active material respectively are repeatedly charged and discharged. 
         FIG. 3  SEM (Scanning Electron microscope) photographs of Ni—Co coprecipitation hydroxide in the positive electrode active material obtained in an embodiment of the present invention. 
         FIG. 4  SEM photographs of Ni—Co coprecipitation hydroxide in the positive electrode active material obtained in another embodiment of the present invention. 
         FIG. 5  SEM photographs of Ni—Co coprecipitation hydroxide in the conventional positive electrode active material. 
         FIG. 6  SEM photographs of the positive electrode active material of the present invention. 
         FIG. 7  SEM photographs of the conventional positive electrode active material. 
         FIG. 8  The conditions for the temperatures at calcining in the production method of the present invention and the conventional method. 
     
    
    
     BEST MODES OF CARRYING OUT THE INVENTION 
     The positive electrode active material of the present invention is composed of Li—Co—Ni—Al composite oxide, preferably the Li—Co—Ni—Al composite oxide expressed by composition formula Li x Ni y Co 1-y-z Al z O 2  (where 0.9≦x≦1.1, 0.8≦y≦95, 0.00≦z≦0.05) and obtains non-aqueous electrolyte lithium secondary battery having specific battery properties (large capacity and long life) by using such Li—Co—Ni—Al composite oxide. 
       FIG. 1  shows the graph indicating examples of charge-discharge curves of the lithium secondary batteries using the positive electrode active material of the present invention and the conventional positive electrode active material respectively as a positive electrode at the discharge rate of 0.1 C (C expresses C rate wherein 1 C rate is the current value discharging the entire capacity of the battery in an hour) at the measuring temperature of 20° C. 
     On  FIG. 1 , the horizontal axis shows capacity (mAh/g) and the vertical axis shows voltage (V), curve a is in the case in which the positive electrode active material of the present invention was used and curve b is in the case in which the conventional positive electrode active material was used. 
     Each charge-discharge curve was measured in the range of voltage 4.25-2.5 V and a commercial item of the present inventors (also called as an existing item) was used for the aforementioned conventional positive electrode active material. 
     As seen from  FIG. 1 , the lithium secondary battery which used the conventional positive electrode active material as a positive electrode indicated in curve b has the initial discharge capacity of about 180 mAh/g while the lithium secondary battery which used the positive electrode active material of the present invention as a positive electrode indicated in curve a has the initial discharge capacity of about 200 mAh/g, showing that the lithium secondary battery having significantly larger discharge capacity compared to the one which used the conventional positive electrode material of curve b. 
       FIG. 2  is the graph showing the cycle properties of each lithium secondary battery having the charge-discharge properties shown on  FIG. 1  when charged and discharged repeatedly at discharge rate 1 C. 
     On  FIG. 2 , the horizontal axis shows the number of charge-discharge cycles and the vertical axis shows discharge capacity maintenance ratio, that is, the discharge capacity ratio (%) after the predetermined cycle numbers of charge-discharge to the discharge capacity on the first time (initial discharge), and curve a and curve b show the cycle properties of the lithium secondary batteries using the positive electrode active material of the present invention and the conventional positive electrode active material respectively as a positive electrode in the same manner as  FIG. 1 . 
     As shown on  FIG. 2 , the discharge capacity maintenance ratio after repeating charge-discharge for 100 times is about 80% in the lithium secondary battery using the conventional positive electrode active material in curve b while it is 94% or higher when the positive electrode active material of the present invention in curb a was used. Therefore, when the positive electrode active material of the present invention is used, the lithium secondary battery having good cycle property at charge-discharge and the significantly extended battery life can be provided. 
       FIGS. 3, 4, and 5  show SEM photographs of the Ni—Co coprecipitation hydroxides (secondary particles) of the present invention ( FIGS. 3 and 4 ) and the conventional secondary particles ( FIG. 5 ) by the present inventor, respectively. 
     As it is clear from  FIGS. 3(A) , (B), and  FIG. 4 ( 1 )-( 6 )(A), (B), all of the secondary particles of the present invention are in nearly spherical shape with the aspect ratio≧0.9 and have very smooth surfaces while, it is clear from  FIGS. 5(A) , (B), the conventional secondary particles have smaller aspect ratio compared to the secondary particles of the present invention, show deformed particle shape, and do not have the uniform shape among the particles, and the rough surfaces stand out. 
       FIGS. 6 and 7  show SEM photographs of the positive electrode active material particles obtained by using the secondary particles of the present invention shown on  FIG. 3  and the conventional secondary particles shown on  FIG. 5 , respectively. 
     As seen from  FIGS. 6(A) , (B),  FIG. 7(A) , (B), the particle properties of the positive electrode active material depend largely on the particle properties of the secondary particles of the coprecipitation hydroxide, the particles of the positive electrode active material that can be obtained by using the secondary particles of coprecipitation hydroxide of the present invention shown on  FIG. 3  are in nearly spherical shape as seen on  FIG. 6(A)  and uniform particle shape as seen on  FIG. 6(B) , and the surfaces are smooth (that is, it is considered that Li and Al particles are adhered to the gaps of the surface of the secondary particles evenly and uniformly, making the surface texture smooth) while the conventional positive electrode active material that can be obtained by using the conventional secondary particles shown on  FIG. 5  shows deformed particle shape as seen on  FIG. 7(A)  and hardly has uniform particle shape among all the particles as seen on  FIG. 7(B) , and the surface texture is not as smooth compared to the positive electrode active material of the present invention. 
     As well, as seen from the examples to be explained hereinafter, the positive electrode active material that can be obtained by using the secondary particles of the coprecipitation hydroxide of the present invention seen on  FIGS. 4 ( 1 )-( 6 ) shows the similar particle properties as  FIGS. 6(A) , (B). 
     The positive electrode active material having smooth surface and nearly spherical shape shown on  FIGS. 6(A) , (B) can be filled with large amount by the increased filling ratio in the battery, of course, compared to the particles having random, rough surfaces shown on  FIGS. 7(A) , (B). 
     As a result, when the positive electrode active material of the present invention was used as a positive electrode of a secondary battery, it can obtain not only the secondary battery having high energy density per volume but also long life property by increasing the filling ratio when processed into a positive electrode. 
     The above-mentioned positive electrode active material is manufactured by obtaining Ni—Co coprecipitation hydroxide (secondary particles), mixing Al compound and Li compound therewith in dry system (inter-powder mixing), and calcining by the production method of the present invention. 
     In order to produce the secondary particles (Ni—Co coprecipitation hydroxide) of the present invention with the above-mentioned particle properties, aqueous solution having Ni compound and Co compound dissolved therein is prepared (Ni—Co aqueous solution hereinafter). For Ni and Co compounds, water soluble compounds such as sulfate, nitrate, or chloride of Ni and Co are used respectively. The ratio of Ni compound and Co compound is Ni:Co=about 95:5-75:25 by molar ratio, preferably 80:20-85:15. 
     The above-mentioned Ni—Co aqueous solution is added to a reaction tank, N 2  gas is passed through the reaction tank, and the aqueous solution is stirred while alkali is added into the Ni—Co aqueous solution. 
     At this time, in the reaction tank, the pressure is about atmospheric pressure, about 55-65° C., the ammonia concentration of about 11-13 g/L, and pH about 11-13, and the stirring is suitably done at the rate that N 2  gas in the tank is taken into the aqueous solution, for example, at the stirring vane tip speed of 1.8-5.5 m/s, preferably at 3.0-5.5 m/s when a reaction tank size is 50 liter (hereinafter liter is indicated as “L” and milliliter is indicated as “mL”). 
     In the present invention, as a rough standard for such stirring conditions, the actual stirring conditions for which N 2  gas is taken into the aqueous solution can be set. 
     When the coprecipitation hydroxide of Ni and Co is generated under the condition for which N 2  gas is taken into the aqueous solution, the coprecipitation hydroxide would show the particle condition close to approximate spherical shape having the aspect ratio of 0.9 or higher. 
     When this coprecipitation hydroxide generation is done under the conventional condition for which air is taken into the aqueous solution, not N 2  gas, the particles will have random particle shape without being spherical shape due to the particle surface oxidation while the reasons are not necessarily clear. 
     As well, the reaction tank can be made such that the N 2  gas can be taken into the aqueous solution more easily by placing a baffle or center slate. 
     As well, for the alkali, NaOH or ammonia ion supply source (such as NH 4 OH or (NH 4 ) 2 SO 4 ) is used, particularly, the combined use of NaOH and (NH 4 ) 2 SO 4  having superior handling properties is preferred as it is easy to obtain and micro-adjust. 
     By this way, the particulate Ni—Co coprecipitation hydroxide obtained by dehydrating and drying the slurry containing Ni—Co coprecipitation hydroxide generated in the reaction tank is supplied as a raw material for producing the positive electrode active material of the present invention. The slurry is dried at about 95-120° C. after dehydration. 
     The particulate Ni—Co coprecipitation hydroxide after drying shows the shape close to nearly spherical shape having the aspect ratio of 0.9 or higher and smooth surface texture. 
     Next, the nearly spherical shape Ni—Co coprecipitation hydroxide (secondary particles) obtained by the above-mentioned process and Al compound and Li compound are mixed in dry system in the ratio stoichiometrically satisfying Li x Ni y Co 1-y-z Al z O 2  (where 0.9≦x≦1.1, 0.8≦y≦95, 0.00≦z≦0.05) and the raw material mixture (a precursor of the positive electrode active material of the present invention) is prepared. 
     As well, in this mixing, the general mixing apparatus, such as horizontal cylinder, V-shape, double-cone shape, or cubic shape mixing apparatus, can be used. 
     For the Al compound, Al compound that can be oxide at high temperature such as oxide, hydroxide, sulfate, and nitrate is used, and for the Li compound, Li compound that can be oxide at high temperature such as hydroxide, carbonate, and halide is used. 
     Particularly, by dry mixing, in order to make each raw material uniform that is, to make not only the shape of all particles but also the composition uniform, lithium hydroxide having an average particle diameter of about 10-50 μm, preferably about 25-35 μm, for the Li compound is suitably used. It is sufficient to use the commercially available Al compound having an average particle diameter of 10 μm or smaller. With these particle diameters, Al can be fully solid dispersed in the following calcining process and good reactivity can be obtained. Particularly, if aluminum oxide having extremely small average particle diameter is used, Li x Ni y Co 1-y-z Al z O 2  having excellent quality can be obtained. 
     For the mixing conditions at the dry mixing, it is desirable to mix under normal temperature, normal pressure, and closed condition (the raw material insertion part on the powder mixing apparatus is closed, etc.) for about 0.5-1.5 hours. 
     The raw material mixture prepared in the way as mentioned above is calcined under an oxidizing atmosphere, 710-790° C., preferably 730-780° C., for 5-20 hours. 
     After calcining, it can be rapid cooled outside the calcining furnace or gradually cooled inside the calcining furnace. 
     For the rising temperature conditions until it reaches the aforementioned temperature, it is not particularly limited, however, it rises in 5-15 hours from the beginning of temperature rising in the furnace, preferably in about 8-12 hours. 
     The present invention can make the speed of the temperature rising twice as much fast as it takes conventionally and keep calcining temperature low since the reactivity with each raw material mixed is improved. 
     Examples of temperature rising and calcining mode of the present invention and the conventional method are shown on  FIG. 7 . On  FIG. 7 , a indicates the embodiment of the method of the present invention and b indicates the embodiment of the conventional method. 
     EXAMPLES 
     [Synthesis of Ni—Co Coprecipitation Hydroxide (Secondary Particles)] 
     Synthesis of Ni—Co Coprecipitation Hydroxide [1]: 
     Ni—Co aqueous solution was prepared in the Ni:Co molar ratio of NiSO 4  and CoSO 4  of 84:16 under room temperature. 
     On the other hand, pure water was placed in a SUS reaction tank (internal capacity  50 L) with a lid and an overflow port and a stirring machine was operated at 60° C. While keeping these conditions, N 2  gas was introduced and the Ni—Co aqueous solution and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were dropped therein, and stirring had been continued at the stirring vane tip speed of 4.1 m/s. 
     During this stirring, it was confirmed that N 2  gas passed through the reaction tank was continuously taken into the aqueous solution. 
     Also, (NH 4 ) 2 SO 4  was controlled to drop so that the ammonia concentration in the aqueous solution in the tank was 12.0 g/L and NaOH was controlled to drop so that pH of the aqueous solution in the tank was 12.0. 
     The precipitated material obtained was removed in a slurry condition, dehydrated, dried at 110° C. for 16 hours, and Ni—Co coprecipitation hydroxide [1] was obtained. 
     The secondary particles [1] obtained were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 3(A) , (B). 
     Synthesis of Ni—Co Coprecipitation Hydroxide [11]: 
     ( 1 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 80:20 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 3.0 m/s, Ni—Co coprecipitation hydroxide [111] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [111] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 1 ) (A), (B). 
     ( 2 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 80:20 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 5.5 m/s, Ni—Co coprecipitation hydroxide [112] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [112] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 2 ) (A), (B). 
     Synthesis of Ni—Co Coprecipitation Hydroxide [12]: 
     ( 3 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 95:5 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 3.0 m/s, Ni—Co coprecipitation hydroxide [121] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [121] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 3 ) (A), (B). 
     ( 4 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 95:5 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 5.5 m/s, Ni—Co coprecipitation hydroxide [122] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [122] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 4 ) (A), (B). 
     Synthesis of Ni—Co Coprecipitation Hydroxide [13]: 
     ( 5 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 75:25 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 3.0 m/s, Ni—Co coprecipitation hydroxide [131] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [131] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 5 )(A), (B). 
     ( 6 ) Except that the Ni:Co molar ratio of NiSO 4  and CoSO 4  was 75:25 and the Ni—Co aqueous solution obtained and (NH 4 ) 2 SO 4  and NaOH aqueous solutions were stirred under N 2  gas being introduced at the stirring vane tip speed of 5.5 m/s, Ni—Co coprecipitation hydroxide [132] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. During this stirring, N 2  gas was continuously taken into the aqueous solution. 
     The secondary particles obtained [132] were particles having nearly spherical shape having the aspect ratio of 0.9 or higher and had smooth surfaces as seen on  FIGS. 4 ( 6 ) (A), (B). 
     Synthesis of Ni—Co Coprecipitation Hydroxide [2]: 
     At the production of the above-mentioned Ni—Co coprecipitation hydroxide [1], except that NiSO 4  and CoSO 4  aqueous solutions in the reaction tank were stirred at the stirring vane tip speed of 2.7 m/s, Ni—Co coprecipitation hydroxide [2] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. 
     During this stirring, the amount of N 2  gas passing through the reaction tank was very small compared to the stirring in the above [1], however, it was confirmed that the N 2  gas was slightly taken into (about 40-60% of the above [1], about 50% on average, of the bubble generation due to the N 2  gas being taken into by the baffle effect in the reaction tank was confirmed). 
     The secondary particles obtained [2] showed particle shape condition close to  FIG. 5(A) , (B), intermediate between  FIG. 3(A) , (B) and  FIG. 5(A) , (B). 
     Synthesis of Ni—Co Coprecipitation Hydroxide [3]: 
     At the production of the above-mentioned Ni—Co coprecipitation hydroxide [1], except that N 2  gas was not introduced into the reaction tank and stirring was continued in the air, Ni—Co coprecipitation hydroxide [3] was obtained in the same manner as the above-mentioned Ni—Co coprecipitation hydroxide [1]. 
     The secondary particles obtained [3] showed particle shape condition having deformation and rough surfaces and all the particles had non-uniform shape as seen on  FIG. 5(A) , (B). 
     Example 1 
     Each raw powder material of 950 g (molar ratio 0.97) of the Ci-Co coprecipitation hydroxide [1], 16.0 g (molar ratio 0.03) of alumina (average particle diameter: 10 μm), and 445 g (molar ratio 1.03) of lithium hydroxide (average particle diameter: 30 μm) were dry mixed with a ribbon blender for 1 hour. 
     After mixing, it was calcined under oxidation atmosphere at 750° C. for 20 hours including the time for rising temperature, and after calcining, it was removed from the furnace when the temperature inside the furnace reached 200° C., then cooled to room temperature. The conditions for the temperatures at calcining was shown on  FIG. 8 , a. 
     In this way, the positive electrode active material [1] having the composition formula of Li 1.03 Ni 0.82 Co 0.15 Al 0.03 O 2 , the molar ratio (the ratio of gram atom number of each element) of Ni:Co:Al is 82:5:3, was obtained. 
     The shape of the positive electrode active material [1] obtained is nearly spherical shape having the aspect ratio of 0.9 or higher as seen on  FIGS. 6(A) , (B). 
     Next, except that the positive electrode active material [1] was used, lithium secondary battery [1] was manufactured in a conventional way and charge-discharge was repeated for 100 times at the measurement temperature of 20° C., the voltage range in 4.25-2.5 V, and the voltage rate 1 C. 
     The initial capacity (discharge capacity) and the discharge capacity maintenance rate (the rate of discharge capacity after discharged for 100 times to the discharge capacity at the initial discharge) are shown on Table 1. 
     Example 2 
     Except that the Ni—Co coprecipitation hydroxide [2] was used, the positive electrode active material [2] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [2] obtained showed particle shape close to those shown on  FIGS. 7(A) , (B), which is intermediate between those shown on  FIGS. 6(A) , (B) and  FIGS. 7(A) , (B). 
     Except that this positive electrode active material [2] was used as a positive electrode, lithium secondary battery [2] was manufactured in the same way as Example 1. 
     For the lithium secondary battery [2] obtained, the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 1. 
     Example 3 
     Except that the Ni—Co coprecipitation hydroxide [3] was used, the positive electrode active material [3] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [3] obtained showed particle shape close to those shown on  FIGS. 7(A) , (B). 
     Except that this positive electrode active material was used, lithium secondary battery [3] was manufactured in the same way as Example 1. 
     For the lithium secondary battery [3] obtained, the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 1. 
     Comparable Example 1 
     The above-mentioned 950 g (molar ratio 0.97) of the Ni—Co coprecipitation hydroxide [1], 27.0 g (molar ratio 0.05) of alumina having average particle diameter of 10 μm, and 445 g (molar ratio 1.03) of lithium hydroxide having average particle diameter of 30 μm were wet mixed like a conventional way (a pressure tight container was used and after dissolving in water, stirred and wet mixed after reducing the pressure to 10.7 KPa), dehydrated, and dried. 
     Next, except that calcining at 790° C. for 48 hours including the time for rising temperature, the positive electrode active material [4] was manufactured in the same way as Example 1. 
     The positive electrode active material [4] obtained showed the particle shape shown on  FIGS. 7(A) , (B). 
     Except that this positive electrode active material [4] was used, lithium secondary battery [4] was manufactured in the same way as Example 1. 
     For the lithium secondary battery [4] obtained, the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 1. 
     Comparable Example 2 
     Except that using the Ni—Co coprecipitation hydroxide [1] and calcining at 790° C., the positive electrode active material [5] was manufactured in the same way as Example 1. 
     Except that this positive electrode active material was used, lithium secondary battery [5] of Comparable Example 3 was manufactured in the same way as Example 1. 
     For the lithium secondary battery [5] obtained, the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 1. 
     Comparable Example 3 
     Except that using the Ni—Co coprecipitation hydroxide [1] and calcining at 710° C., the positive electrode active material [6] was manufactured in the same way as Example 1. 
     Except that this positive electrode active material [6] was used, lithium secondary battery [6] of Comparable Example 4 was manufactured in the same way as Example 1. 
     For the lithium secondary battery [6] obtained, the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Examples 
                 Initial Capacity 
                 Discharge Capacity 
               
               
                 (Comparable Examples) 
                 (mAh/g) 
                 Maintenance Rate (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 220 
                 197 
                 94 
               
               
                 Example 2 
                 221 
                 196 
                 91 
               
               
                 Example 3 
                 219 
                 192 
                 91 
               
               
                 Comparable Example 1 
                 213 
                 178 
                 82 
               
               
                 Comparable Example 2 
                 221 
                 194 
                 74 
               
               
                 Comparable Example 3 
                 219 
                 185 
                 94 
               
               
                   
               
            
           
         
       
     
     Example 4 
     Except that the Ni—Co coprecipitation hydroxide [1] was used and lithium hydroxide having average particle diameter of 10 μm was used, the positive electrode active material [1-1] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-1] obtained had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-1] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-1] was used as a positive electrode, lithium secondary battery [1-1] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 5 
     Except that the Ni—Co coprecipitation hydroxide [1] was used, lithium hydroxide having average particle diameter of 10 μm was used, and the calcining temperature was 710° C., the positive electrode active material [1-2] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-2] had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-2] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-2] was used as a positive electrode, lithium secondary battery [1-2] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 6 
     Except that the Ni—Co coprecipitation hydroxide [1] was used, lithium hydroxide having average particle diameter of 10 μm was used, the calcining temperature was 790° C., and the calcining time was for 5 hours including the time for rising temperature, the positive electrode active material [1-3] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-3] obtained had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-3] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-3] was used as a positive electrode, lithium secondary battery [1-3] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 7 
     Except that the Ni—Co coprecipitation hydroxide [1] was used and lithium hydroxide having average particle diameter of 25 μm was used, the positive electrode active material [1-3] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-3] obtained had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-3] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-3] was used as a positive electrode, lithium secondary battery [1-3] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 8 
     Except that the Ni—Co coprecipitation hydroxide [1] was used and lithium hydroxide having average particle diameter of 35 μm was used, the positive electrode active material [1-4] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-4] obtained had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-4] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-4] was used as a positive electrode, lithium secondary battery [1-4] was manufactured in the same as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 9 
     Except that the Ni—Co coprecipitation hydroxide [1] was used and lithium hydroxide having average particle diameter of 50 μm was used, the positive electrode active material [1-5] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-5] obtained had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-5] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-5] was used as a positive electrode, lithium secondary battery [1-5] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 10 
     Except that the Ni—Co coprecipitation hydroxide [1] was used, lithium hydroxide having average particle diameter of 50 μm was used, and the calcining temperature was 710° C., the positive electrode active material [1-6] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-6] had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-6] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-6] was used as a positive electrode, lithium secondary battery [1-6] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 11 
     Except that the Ni—Co coprecipitation hydroxide [1] was used, lithium hydroxide having average particle diameter of 50 μm was used, the calcining temperature was 790° C., and the calcining time was for 5 hours including the time for rising temperature, the positive electrode active material [1-7] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-7] had the same composition formula and molar ratio of Ni:Co:Al as Example 1. 
     The shape of the positive electrode active material [1-7] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-7] was used as a positive electrode, lithium secondary battery [1-7] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 12 
     Except that the above-mentioned 950 g (molar ratio 0.97) of the Ni—Co coprecipitation hydroxide [1], 16.0 g (molar ratio 0.03) of alumina (average particle diameter: 10 μm), and 398 g (molar ratio 0.91) of lithium hydroxide (average particle diameter: 30 μm) were used, the positive electrode active material [1-8] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-8] obtained had the composition formula Li 0.91 Ni 0.82 Co 0.15 Al 0.03 O 2  and the molar ratio of Ni:Co:Al is 82:15:3. 
     The shape of the positive electrode active material [1-8] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-8] was used as a positive electrode, lithium secondary battery [1-8] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 13 
     Except that the above-mentioned 950 g (molar ratio 0.97) of the Ni—Co coprecipitation hydroxide [1], 16.0 g (molar ratio 0.03) of alumina (average particle diameter: 10 μm), and 438 g (molar ratio 1.00) of lithium hydroxide (average particle diameter: 30 μm) were used, the positive electrode active material [1-9] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-9] obtained had the composition formula Li 1.00 Ni 0.82 Co 0.15 Al 0.03 O 2  and the molar ratio of Ni:Co:Al is 82:15:3. 
     The shape of the positive electrode active material [1-9] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-9] was used as a positive electrode, lithium secondary battery [1-9] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 14 
     Except that the Ni—Co coprecipitation hydroxide [121] was used and lithium hydroxide having average particle diameter of 30 μm was used, the positive electrode active material [121-1] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [121-1] obtained had the composition formula Li 1.03 Ni 0.92 Co 0.05 Al 0.03 O 2  and the molar ratio of Ni:Co:Al is 92:5:3. 
     The shape of the positive electrode active material [121-1] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [121-1] was used as a positive electrode, lithium secondary battery [121-1] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 15 
     Except that the Ni—Co coprecipitation hydroxide [122] was used and lithium hydroxide having average particle diameter of 30 μm was used, the positive electrode active material [122-1] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [122-1] obtained had the composition formula Li 1.03 Ni 0.92 Co 0.05 Al 0.03 O 2  and the molar ratio of Ni:Co:Al is 92:5:3. 
     The shape of the positive electrode active material [122-1] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [122-1] was used as a positive electrode, lithium secondary battery [122-1] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 16 
     Except that the above-mentioned 950 g (molar ratio 0.99) of the Ni—Co coprecipitation hydroxide [1], 5.2 g (molar ratio 0.01) of alumina (average particle diameter: 10 μm), and 390 g (molar ratio 0.91) of lithium hydroxide (average particle diameter: 30 μm) were used, the positive electrode active material [1-10] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-10] obtained had the composition formula Li 0.91 Ni 0.83 Co 0.16 Al 0.01 O 2  and the molar ratio of Ni:Co:Al is 83:16:1. 
     The shape of the positive electrode active material [1-10] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-10] was used as a positive electrode, lithium secondary battery [1-10] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     Example 17 
     Except that the above-mentioned 950 g (molar ratio 0.95) of the Ni—Co coprecipitation hydroxide [1], 27.3 g (molar ratio 0.05) of alumina (average particle diameter: 10 μm), and 460 g (molar ratio 1.03) of lithium hydroxide (average particle diameter: 30 μm) were used, the positive electrode active material [1-11] was manufactured in the same way as the positive electrode active material of Example 1. 
     The positive electrode active material [1-11] obtained had the composition formula Li 1.03 Ni 0.80 Co 0.15 Al 0.05 O 2  and the molar ratio of Ni:Co:Al is 80:15:5. 
     The shape of the positive electrode active material [1-11] obtained was nearly spherical having the aspect ratio of 0.9 or higher as shown on  FIGS. 6(A) , (B). 
     Except that this positive electrode active material [1-11] was used as a positive electrode, lithium secondary battery [1-11] was manufactured in the same way as Example 1 and the initial capacity and discharge capacity maintenance rate were calculated like Example 1 and the results are shown on Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Initial Capacity 
                   
               
               
                   
                 (mAh/g) 
                 Discharge Capacity 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Examples 
                 Charge 
                 Discharge 
                 Maintenance Rate (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 4 
                 226 
                 200 
                 95 
               
               
                   
                 5 
                 220 
                 193 
                 93 
               
               
                   
                 6 
                 219 
                 193 
                 92 
               
               
                   
                 7 
                 221 
                 195 
                 97 
               
               
                   
                 8 
                 224 
                 197 
                 98 
               
               
                   
                 9 
                 220 
                 192 
                 85 
               
               
                   
                 10 
                 218 
                 192 
                 84 
               
               
                   
                 11 
                 218 
                 192 
                 81 
               
               
                   
                 12 
                 220 
                 195 
                 98 
               
               
                   
                 13 
                 219 
                 195 
                 96 
               
               
                   
                 14 
                 225 
                 202 
                 94 
               
               
                   
                 15 
                 229 
                 210 
                 94 
               
               
                   
                 16 
                 219 
                 194 
                 94 
               
               
                   
                 17 
                 219 
                 194 
                 94 
               
               
                   
                   
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The positive electrode active material of the present invention is able to obtain the lithium secondary battery having high charge-discharge capacity and long life compared to the conventional lithium secondary battery by using it as a positive electrode of the lithium secondary battery. 
     Therefore, according to the positive electrode active material of the present invention, it can be used for already-known purposes such as battery source of EV requiring high capacity all the time, battery sources for personal computers and cell phones, and backup battery sources.