Abstract:
Disclosed is a positive active material for a lithium secondary battery having high capacity and long durability properties, and particularly to a powder of LiaNi1−x−yCoxMyO2−zFz and LiaNi1−x−yCoxMyO2−zSz (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0≦x&lt;0.99, 0.01≦y≦0.1, 0.01≦z≦0.1, and 1.00≦a≦1.1) an oxygen in LiaNi1−x−yCoxMyO2 is substituted with F or S whereby the durability, capacity and structural safety of the positive active material is increased.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on application No. 99-22764, filed in the Korean Industrial Property Office on Jun. 17, 1999, the content of which is incorporated hereinto by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to an active material for a lithium secondary battery and a method of preparing the same, more particularly, to an active material for a lithium secondary battery in which an oxygen (O) in a Li a Ni 1−x−y Co x M y O 2  is substituted with F or S, and a method of preparing the same. 
     (b) Description of the Related Art 
     Due to technological advances in appliance miniaturization and weight reduction, and increased functionality of cordless portable appliances such as video cameras, personal phones, and personal computers, there are escalating requirements for the sources of electric power used to driving these appliances. Particularly, there have been advances in developing and studying rechargeable lithium secondary batteries around the world, anticipating the need for a battery with a high energy density. 
     A lithium secondary battery uses as an anode and a cathode materials which can intercalate and deintercalate lithium ions, and is prepared by filling organic or polymer electrolyte between the cathode and the anode to move the lithium ions. The battery generates electric energy by a redox reaction when lithium ions intercalate and deintercalate in the cathode and in the anode. 
     Lithium secondary batteries use carbon materials or lithium metals as anodes and intercalatable/deintercalatable chalcogenide compounds as cathodes. Carbon materials are substituted for lithium metals because the latter, when used as an anode, has the disadvantage of educing dendrites with the associated danger of explosion and a reduction in recharging efficiency. 
     On the other hand, complex metal oxides such as LiCoO 2 , LiMn 2 O 4 , LiNi 1−x Co x O 2  (0&lt;X&lt;1), and LiMnO 2  are now being studied for a cathode use because chrome oxide, MnO 2 , etc. that were initially used have problems with low recharge efficiency and safety. 
     Manganic positive active materials such as LiMn 2 O 4 , LiMnO 2 , etc, or cobaltic positive active materials such as LiCoO 2 , etc, had been developed, but they have the limits of capacity of 120 mAh/g and 160 mAh/g respectively when recharged at 4.3 V. Also, LiCoO 2  has been widely used due to having the high voltage capacity, excellent electrode properties, and an electro-conductivity of 10 −2  to 1 S/cm at ambient temperature, but it has low stability when recharged and discharged at a high current rate. 
     There have been developments in the study of nickelic positive active materials that show a discharge capacity more than 20% greater than cobaltic positive active materials. 
     Lithium secondary batteries using nickelic positive active materials have the potential to make high capacity batteries due to their high discharge capacity, but more development of nickelic active materials is needed in order to overcome defects associated with their low durability and the structural instability of LiNi 1−x Co x O 2  (0&lt;x&lt;1). 
     Synthesizing methods employing solid state processes, co-precipitation methods, polymer chelating agents, etc, have been developed and researched thus far on LiNi 1−x M x O 2  (0&lt;x&lt;1) powder with some Ni substituted with Co, Mn, etc, for improving structural safety features, discharge capacities, and life span properties of the basic nickel based cathode compound, LiNiO 2 . 
     LiNiO 2  has disadvantages in that it is difficult to synthesize, is not practical to use in a battery because of poor durability, and its capacity decreases suddenly during continuous discharge-recharge cycles due to instabilities caused by its repeated structural change from monoclinic to hexagonal and back, in spite of having a discharge capacity of 200 mAh/g at 1.0 C. 
     To solve these problems, Co is added to LiNiO 2  in order to stabilize the structure, but this causes the problem that the capacity of LiNiO 2  decreases relative to the amount of Co added, and this quantity must be more than 30 mole %. 
     To improve the structural stability, LiNi 1−x M x O 2  (M is a metal such as Co or Mn, etc, 0&lt;x&lt;1) and LiNi 1−x Co x M y O 2  (M is a metal such as Al, Mg, Sr, La, Ce, etc, 0&lt;x&lt;1, 0.01&lt;y&lt;0.1) were developed. However, these nikelic positive active materials also have defects of structural instabilities, and this defect causes the stability of the system of lithium secondary battery to decrease. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a positive active material for a Li secondary battery, wherein Li a Ni 1−x−y Co x M y O 2−z F z  and Li a Ni 1−x−y Co x M y O 2−z S z  (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0≦x&lt;0.99, 0.01≦y≦0.1, 0.01≦z≦0.1, and 1.00≦a≦1.1) powders in which an oxygen (O) in Li a Ni 1−x−y Co x M y O 2  is substituted with F or S are synthesized to improve the durability, capacity, and structural stability of the battery. 
     It is another object to provide a method of preparation of the positive active material for a Li secondary battery. 
     In order to achieve these other objects, the present invention provides positive active materials for Li secondary battery in which an oxygen (O) in Li a Ni 1−x−y Co x M y O 2  (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0≦x&lt;0.99, 0.01≦y≦0.1, 0.01≦z≦0.1, and 1.00≦a≦1.1) is substituted with F or S, that is, positive active materials selected from the group consisting of the following formulae 1 and 2: 
     
       
         Li a Ni 1−x−y Co x M y O 2−z F z   [Formula 1], 
       
     
     
       
         Li a Ni 1−x−y Co x M y O 2−z S z   [Formula 2], 
       
     
     and 
     where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0≦x&lt;0.99, 0.01≦y≦0.1, 0.01≦z≦0.1, and 1.00≦a≦1.1. 
     Also, the present invention further provides a method of preparation the positive active material selected from the group consisting of the formulae 1 and 2. 
     The method comprises a step of synthesizing Ni 1−x−y Co x M y (OH) 2  by a coprecipitation method; a step of mixing the material with LiOH, and LiF or NaS powder; and a step of producing the positive active compound of the formulae 1 and 2 by heating and cooling the mixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a graph showing the result of recharge properties after 1 cycle of coin cells made of a) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 1.95 F 0.05  and b) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 2 : and 
     FIG. 2 is a graph showing the result of recharge properties after 50 cycles of coin cells made of a) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 1.95 F 0.05  and b) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of this invention will be explained with reference to the accompanying drawings. 
     The present invention relates to a compound selected from the group consisting of compounds of the following formulae 1 and 2: 
     
       
         Li a Ni 1−x−y Co x M y O 2−z F z   [Formula 1], 
       
     
     
       
         Li a Ni 1−x−y Co x M y O 2−z S z   [Formula 2], 
       
     
     and 
     where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0≦x&lt;0.99, 0.01≦y≦0.1, 0.01≦z≦0.1, and 1.00≦a≦1.1. 
     It is preferable that spherical or quasi-spherical Ni 1−x−y Co x M y (OH) 2  powder coprecipitating metals selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti should be used for preparing the compound of the formulae 1 or 2. 
     Ni 1−x−y Co x M y (OH) 2  is synthesized by a coprecipitation method. A solution is prepared comprising metal salts selected from the group consisting of Al-salt, Mg-salt, Sr-salt, La-salt, Ce-salt, V-salt, Ti-salt, Ni-salt and Co-salt to prepare the Ni 1−x−y Co x M y (OH) 2 . At this point, preferably, the concentration of the metal salt is approximately 2.5 M, and water is used as a solvent. 
     NH 4 OH as a binder and NaOH as a precipitant are continuously supplied to the prepared metal solution in an overflowable reactor. 
     At this time, a reactor temperature is preferably maintained at about 50° C., and the pH in the reactor is preferably maintained between 11 and 12. It is also preferable that the mole ratio of supplied metal to NH 4 OH ranges from 1:0.4 to 1:1, and that these materials in the reactor are reacted while being agitated at 900 rpm. 
     A spherical or quasi-spherical Ni 1−x−y Co x M y (OH) 2  powder is obtained by drying the powder after rinsing the overflowed-reacted reaction precipitant with water or a diluted acid solution until it is neutralized. 
     A homogeneous mixture is produced by agitating the prepared Ni 1−x−y Co x M y (OH) 2  powder with LiF or NaS powder in an equivalent ratio in a mortar agitator for 10 to 30 minutes. 
     The powder mixture is then heat-treated at between 600 and 1000° C. for 10 to 30 hours, with dry air being blown in a gas atmosphere controlled furnace to synthesize the positive active powders of the above formulae 1 to 2. 
     The heat-treatment step is performed by elevating the temperature at a rate of 1 to 5° C./min, and then allowing the powder to naturally cool down after maintaining it at the heat-treatment temperature for a fixed period of time. 
     It is preferable that lithium salt is homogeneously distributed by remixing the powder of the compound of the formulae 1 and 2 at a room temperature. 
     The present invention is explained in more detail with reference to the following examples. 
     EXAMPLE 1 
     First, Ni 0.89 Co 0.1 La 0.01 (OH) 2  was synthesized by a coprecipitation method to prepare Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 1.95 F 0.05 . 
     To prepare Ni 0.89 Co 0.1 La 0.01 (OH) 2 , the solution comprising metal salt consisting of La-salt, Ni-salt, and Co-salt was prepared. At this time, the concentration of total metal was about 2.5 M and water was used as a solvent. 
     NH 4 OH as a binder and NaOH as a precipitant were continuously supplied to the prepared metal solution in an overflowable reactor. 
     The temperature of the reactor was maintained at about 50° C. and the pH of the solution in the reactor was maintained between 11 and 12. The mole ratio of NH 4 OH and supplied metal ranged between 1:0.4 and 1:1, and the materials in the reactor were reacted while being agitated at 900 rpm. 
     A spherical or quasi-spherical Ni 0.89 Co 0.1 La 0.01 (OH) 2  powder to be later dried was obtained after rinsing the overflowed-reacted reaction precipitant with water or a diluted acid solution until it was neutralized. 
     A homogeneous mixture was produced by agitating it in a mortar agitator for about 10 to 30 minutes after measuring out LiOH and LiF powder in an equivalent ratio. 
     Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 1.95 F 0.05  was prepared by heat-treating mixed powder at 700 degree. C. for 20 hours, with dry air circulating in a gas atmosphere controlled furnace. 
     EXAMPLE 2 
     Using the method for the preparation of the positive active material according to the example 1, Li 1.02 Ni 0.88 Co 0.1 La 0.02 O 1.95 F 0.05  was produced by the same method and conditions of the example 1, except for the change of mole ratio of La of positive active material in the example 1 from 0.01 to 0.02. 
     EXAMPLE 3 
     Using the method for the preparation of the positive active material according to the example 1, Li 1.02 Ni 0.89 Co 0.1 Mg 0.01 O 1.95 F 0.05  was produced by the same method and conditions of the example 1, except for the use of Ni 0.89 Co 0.1 Mg 0.01 (OH) 2 . 
     EXAMPLE4 
     Using the method for the preparation of the positive active material according to the example 3, Li 1.02 Ni 0.88 Co 0.1 Mg 0.02 O 1.95 F 0.05  was produced by the same method and conditions of the example 3, except for the change of mole ratio of Mg of positive active material in the example 3 from 0.01 to 0.02. 
     Comparative Example 1 
     To compare material prepared in the example 1, the positive active material of Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 2  was produced by the same method and conditions of the example 1, except for mixing Ni 0.89 Co 0.1 La 0.01 (OH) 2  with powder of only LiOH, not using the powder of LiF. 
     Comparative Example2 
     Li 1.02 Ni 0.88 Co 0.1 La 0.02 O 2 was produced by the same method and conditions of the preparation of the comparative example 1, except for the change of the mole ratio of La from 0.01 to 0.02. 
     Comparative Example 3 
     To compare material prepared in the example 3, the positive active material of Li 1.02 Ni 0.89 Co 0.1 Mg 0.01 O 2  was produced by the same method and conditions of the example 3, except for mixing Ni 0.89 Co 0.1 Mg 0.01 (OH) 2  with powder of only LiOH, not using the powder of LiF. 
     Comparative Example 4 
     Li 1.02 Ni 0.88 Co 0.1 Mg 0.02 O 2  was produced by the same method and conditions of the preparation of the comparative example 3, except for the change of the mole ratio of Mg from 0.01 to 0.02. 
     The prepared powders according to the methods of the example 1, 2, 3, and 4 and the comparative example 1, 2, 3, and 4 were structurally analyzed by X-ray diffraction (XRD), the powder particles were observed with a Scanning Electron Microscope (SEM) and the properties of the powders were confirmed by SIMS and TEM. 
     DISCHARGE PROPERTY EVALUATION 
     The discharge properties of coin type half-cell of the positive active material prepared by the method of the example 1, 2, 3, 4 and the comparative example 1, 2, 3, and 4 were evaluated. 
     For the preparation of the half-cells, 3-weight percent carbon (product name: Super P) was used as a conductor and 3-weight percent polyvinyllidenefluoride (product name: KF-1300) was used as a binder. 
     The coin type half-cell was consisted of a pole plate prepared by tape-casting positive active material, conductor, binder on an Al-foil with N-methylpyrrolidine (NMP) solvent, and lithium metal as an opposite pole plate. 
     The capacity and durability properties of the active material prepared according to the present invention were tested. 
     For a test of the discharge properties, the half-cells were tested under the condition of discharging a constant 0.1 C., 0.2 C., 0.5 C., and 1 C. between 2.75 V and 4.3 V for one to 100 times. 
     The FIGS. 1 and 2 show the results of the discharge property tests. 
     FIG. 1 shows the result of a property test of discharge for 1 cycle of a) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 1.95 F 0.05  and b) Li 1.02 Ni 0.89 Co 0.1 La 0.01 O 2 , and FIG. 2 shows the result of a property test of discharge for 50 cycles of (a) and (b) of FIG.  1 . 
     As shown in the FIG. 1, capacity of Li a Ni 1−x−y Co x M y O 2−z F z  in which an oxygen (O) in Li a Ni 1−x−y Co x M y O 2  is substituted with F decreases as much as 1 to 3% when the first discharge capacity is evaluated by using coin half-cells. However, when discharge-recharged for 50 cycles as shown in the FIG. 2, the durability property of Li a Ni 1−x−y Co x M y O 2  is about 60%, on the other hand, that of Li a Ni 1−x−y Co x M y O 2−z F z  is 74% under the condition of high current rate of 1 C. Accordingly, the durability property of Li a Ni 1−x−y Co x  M y O 2−z F z  increases as much as 14%. 
     Accordingly, the first discharge property of Li a Ni 1−x−y Co x M y O 2−z F z  in which an oxygen (O) in Li a Ni 1−x−y Co x M y O 2  is substituted with F is slightly lower than that of Li a Ni 1−x−y Co x M y O 2 , but the durability property of Li a Ni 1−x−y Co x M y O 2−z F z  increases by 14% in the evaluation of a long-period such as 50 cycles. Therefore, because Li a Ni 1−x−y Co x M y O 2−z F z  is more durable and has higher capacity than Li a Ni 1−x−y Co x M y O 2 , Li a Ni 1−x−y Co x M y O 2−z F z  is able to be used for Li-secondary batteries.