Patent Publication Number: US-2007122338-A1

Title: Positive active material for lithium secondary battery and a method of preparing the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a Divisional Application of U.S. patent application Ser. No. 10/275,612, filed Nov. 7, 2002. U.S. patent application Ser. No. 10/275,612 is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/KR02/00419 which has an International filing date of Mar. 11, 2002, which designated the United States of America. This application is also based on application No. 2001-12827 filed in Korean on Mar. 13, 2001. Priority is claimed to each application under 35 U.S.C. §120 or 119(a). The entire contents of all are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      (a) Field of the Invention  
      The present invention relates to a positive active material for a lithium secondary battery and a method of preparing the same, and more particularly, to a positive active material for a lithium or lithium ion secondary battery having improved cycle-life characteristics at room temperature and at a high temperature, and improved storage characteristics at a high temperature, and a method of preparing the same.  
      (b) Description of the Related Art  
      A lithium secondary battery is required to have good cycle-life characteristics, safety characteristics and storage characteristics at a high temperature. An important factor in cycle-life characteristics of a battery is the characteristics of positive and negative active materials. Recently, colossal improvements have been made in the field of negative active material, but there are still a lot of problems in the field of positive active material. Research on positive active material is particularly required, because safety and storage characteristics of a battery at a high temperature are dependant on the characteristics of the positive active material.  
      A lithium-manganese complex oxide of a spinel structure is a material that is worth consideration, because it is safer than any other positive active material and is comparatively inexpensive. But as charge-discharge cycles advance, the capacity decreases, and its use is greatly restricted, because its capacity greatly decreases with temperature increases, especially over 40° C. Although the above-mentioned problems can be ascribable to many factors, a main reason is that manganese ions of oxidation number 3(Mn 3+ ) react heterogeneously and dissolve into electrolyte.  
      A lot of research is underway to overcome the above-mentioned problems. A representative research approach entails stabilizing the spinel structure by doping the lithium-manganese complex oxide with another metal or coating the surface of the lithium-manganese complex oxide particles with another compound. Although the above-mentioned methods can improve cycle-life characteristics, initial capacity of the spinel compound decreases because the capacity of the spinel compound is proportional to the density of Mn 3+ .  
      Method of improving the cycle-life characteristics at a high temperature by coating the surface of spinel with Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 , etc and storing at 60° C. is known (U.S. Pat. No. 5,733,685). And although a method of coating the surface of spinel particles with an ion conductive lithium non-crystalline compound is also known, the above-mentioned methods have not overcome the problem of capacity decrease, nor have they overcome problems of deteriorating cycle-life characteristics at a high temperature.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a positive active material for a lithium secondary battery and a preparation method of the same, having improved cycle-life characteristics at room temperature and at a high temperature, and improved storage characteristics at a high temperature. The present invention prohibits manganese from erupting from the lithium manganese spinel oxide and compromising the chemical safety of the overlayer oxide by coating the surface of the lithium manganese spinel oxide with a metal complex oxide to prohibit the oxide of the surface from reacting with electrolyte.  
      In order to achieve these objects and others, the present invention provides a positive active material for a lithium secondary battery comprising: 
          a) lithium manganese spinel oxide core particles described by the following Formula 1; and     b) an overlayer of a lithium nickel metal oxide described by the following Formula 2, coated on the above-mentioned core particles of a). 
 
Li 1+x Mn 2-x-y M y O 4+d    [Formula 1]
    wherein,     M is selected from the group consisting of Ti, V, Cr, Co, Ni, Mg, Zr, Fe, Gd, and Ga;     x is a real number of 0˜0.12;     y is a real number of 0˜0.5; and     d is a real number of 0˜0.04; 
 
LiNi 1-z Me z O 2    [Formula 2]
    wherein,     Me is selected from the group consisting of Ni, Mn, Co, V, Ti, Zr, Cr, Al, Mg, Fe, Cu, Zn, Ga, and Gd; and     z is a real number of 0˜0.7;        

      Also, the present invention provides a lithium secondary battery comprising the positive active material. 
    
    
     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 SEM picture of lithium manganese complex oxide coated with lithium nickel metal complex oxide of Example 1;  
       FIG. 2  is a graph of results of an EDS (Energy Dispersive X-ray Spectrometer) analysis of the surface of the lithium manganese complex oxide particles shown in  FIG. 1 ;  
       FIG. 3  is a graph of results of an EDS (Energy Dispersive X-ray Spectrometer) analysis of the inside of the lithium manganese complex oxide particles shown in  FIG. 1 ;  
       FIG. 4  is a graph illustrating the charge-discharge curve of a cell using the lithium manganese complex oxide coated with lithium nickel metal complex oxide of Example 1 as a positive active material.  
       FIG. 5  is a graph illustrating cycle-life characteristics at room temperature (25° C.) of Examples 1 to 7 and Comparative Examples 1 and 2.  
       FIG. 6  is a graph illustrating cycle-life characteristics at a high temperature (55° C.) of Examples 1 to 7 and Comparative Examples 1 and 2.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventors of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.  
      The positive active material of the present invention comprises lithium manganese spinel oxide core particles, and an overlayer of lithium nickel metal complex oxide.  
      Formula 1 shows the lithium manganese spinel oxide. It can intercalate and deintercalate lithium ions, it has a spinel structure, and it has the advantages of good safety characteristics and low price compared with other positive active materials for a lithium secondary battery.  
      In the lithium nickel metal complex oxide overlayer shown by the Formula 2, it makes no difference whether the quantity of lithium is over the stoichiometric coverage shown by Formula 2. When the lithium nickel metal complex oxide overlayer is used to produce the positive active material, it prohibits electrolyte from reacting with the lithium manganese spinel oxide of the core particles, and therefore it prohibits Mn from erupting from the spinel. Therefore, the overlayer of the lithium nickel metal complex oxide not only improves the cycle-life characteristics at room temperature and at a high temperature, but it also improves the storage characteristics at a high temperature.  
      The positive active material of the present invention can be manufactured as follows: 
          a) supplying lithium manganese spinel oxide of the following Formula 1;     b) coating the lithium manganese spinel oxide with an overlayer material compound of the following Formula 2, comprising 
            i) a lithium compound;     ii) a nickel compound; and     ii) a metal compound comprising metal Me selected from the group consisting of Ni, Mn, Co, V, Ti, Zr, Cr, Al, Mg, Fe, Cu, Zn, Ga, and Gd; and    
            c) sintering the lithium manganese spinel oxide of a) coated with the overlayer material of b): 
 
Li 1+x Mn 2-x-y M y O 4+d    [Formula 1]
    wherein,     M is selected from the group consisting of Ti, V, Cr, Co, Ni, Mg, Zr, Fe, Gd, and Ga;     x is a real number of 0˜0.12;     y is a real number of 0˜0.5; and     d is a real number of 0˜0.04; 
 
LiNi 1-z Me z O 2    [Formula 2]
    wherein,     Me is selected from the group consisting of Ni, Mn, Co, V, Ti, Zr, Cr, Al, Mg, Fe, Cu, Zn, Ga, and Gd; and     z is a real number of 0˜0.7.        

      The lithium manganese spinel oxide is the core particle, and it is preferable that the lithium manganese spinel oxide is manufactured by heat-treatment of the lithium compound and the compound comprising the manganese compound under air or a complex gas comprising over 2 wt % of oxygen for 1 to 50 hours at 400 to 900° C. It is preferable that the lithium compound is selected from the group consisting of LiOH, LiOH.H 2 O, LiCH 3 COO, LiCHO 2 , LiCHO 2 .H 2 O, Li 2 SO 4  and LiNO 3 , and that the manganese compound is selected from the group consisting of MnO 2 , MnCO 2 , Mn(CH 3 COO) 2 , MnSO 4 , Mn(NO 3 ) 2 , and Mn(OH) 2 .  
      The core particles selectively comprise metal M as shown in the Formula 1, In a metal compound. The metal M is selected from the group consisting of Ti, V, Cr, Co, Ni, Mg, Zr, Fe, Gd, and the metal compound comprising metal M is selected from the group consisting of carbonate salt, nitrate salt, hydrate salt, acetate salt, citric acid salt and chloride of the metal Me.  
      The i) lithium compound, ii) nickel compound, and iii) metal compound comprising metal Me are dissolved In water or organic solvent and the resulting material compound of the overlayer is coated on the core particle.  
      It is preferable that the i) lithium compound is selected from the group consisting of LiOH, LiOH.H 2 O, LiCH 3 COO, LiCHO 2 , LiCHO 2 .H 2 O, Li 2 SO 4  and LiNO 3 , and the ii) nickel compound is selected from the group consisting of carbonate salt, nitrate salt, hydrate salt, acetate salt, citric acid salt and chloride of nickel, and the iii) compound comprising metal compound comprising metal Me is selected from the group consisting of carbonate salt, nitrate salt, hydrate salt, acetate salt, citric acid salt and chloride of the metal Me.  
      It is also preferable that the quantity of the lithium nickel metal complex oxide overlayer produced after the sintering is 0.05 to 50 mol % of the lithium manganese spinel oxide. If the quantity of the lithium nickel metal complex oxide overlayer produced after the sintering is less than 0.05 mol %, there is no Improved effect of safety characteristics of the electrolyte, and if it is over 50 mol %, there is no proportionate effect of the coated quantity. The quantity of coating is different according to the composition of the coated lithium nickel metal compound or lithium manganese spinel oxide and respective material, and the purpose of use.  
      The coating can progress by two methods. One method involves making a slurry by adding the lithium manganese complex oxide powder of the a) step in aqueous solution or organic sol material compound solution, and agitating and heating the slurry for the solvent to evaporate. The overlayer forms on the surface of the core particles as the solvent evaporates.  
      The other method is by spraying an aqueous solution of the overlayer material compound or sol material of the organic solution on the core particle after making the core particles of the a) step fluid in the air, and drying the solvent. The spraying and drying are preferably done at the same time.  
      The sintering is preferably done by the following procedure. The coated lithium manganese spinel complex oxide powder is preferably heat-treated at 400 to 900° C. under air or a complex gas of which the quantity of oxygen is over 10 wt %, after the coated lithium manganese spinel complex oxide powder is dried in the oven at 50 to 150° C. The volume of the gas per weight and hour is preferably 0.05 to 3.0 l/gH, and heat-treatment time is preferably 1 to 30 hour. The heat treatment time and temperature are preferably selected within the above-mentioned coverage according to purpose, and the overlayer can be partially doped on the surface of the core particles according to the heat-treatment temperature during the heat-treatment procedure.  
      The lithium manganese complex oxide shown by the Formula 1 is coated with the lithium nickel metal complex compound shown by the Formula 2 through the above-mentioned procedure. The lithium secondary battery having the complex oxide as the positive active material shows good capacity, good cycle-life characteristics at a high temperature and good storage characteristics at a high temperature.  
      The following Examples are presented to better illustrate the invention, but are not to be construed as limiting the invention to the specific embodiments disclosed.  
     EXAMPLE 1  
     Fabrication of Core Particles  
      Li 2 CO 3  and MnO 2  were mixed at a mol ratio of Li to Mn of 0.538 for the fabrication of the spinel lithium manganese complex powder. After the materials were homogeneously mixed, the materials were heat-treated at 480° C. for 10 hours in air, and the materials were cooled to fasten the reaction, and spinel powders were fabricated by heat-treatment at 750° C. for 20 hours in air. During the heat-treatment procedure, the flow rate of the air was 0.1 l/Gh. The composition of the lithium manganese compound fabricated by the above procedure was Li 1.05 Mn 1.95 O 4 .  
     Fabrication of Material of the Overlayer  
      LiCH 3 COO was used to provide lithium as a material of the overlayer, Ni(CH 3 COO) 2  was used to provide Ni, and Co(CH 3 COO) 2  was used to provide Co. A homogeneous complex solution of the lithium nickel metal compound was fabricated by agitating the solution in which the materials were dissolved in a non-aqueous alcohol for 30 minutes. The mol ratio of Li:Ni+Co was 1:1, and the mol ratio of Ni:Co was 7:3. Also, the quantity of the overlayer was 1.5 mol % of the lithium manganese spinel oxide of the core particles.  
     Coating  
      After a slurry was prepared by adding lithium manganese spinel oxide to the complex solution of the lithium nickel metal compound, the slurry was homogeneously agitated and heated to evaporate the solvent from the slurry and deposit the overlayer.  
     Sintering  
      The lithium manganese spinel oxide coated in the coating step was heat-treated at 700° C. for 10 hours in a tube-type electric furnace. The heat-treatment was done in air and the flow rate of the air was 0.1 l/Gh.  
       FIG. 1  is a SEM picture of the lithium manganese complex oxide prepared by the above-mentioned procedure, and  FIG. 2  shows results of an EDS (Energy Dispersive X-ray Spectrometer) analysis of the surface of the lithium manganese complex oxide particles prepared by the above-mentioned procedure.  FIG. 3  shows results of an EDS (Energy Dispersive X-ray Spectrometer) analysis of the inside of the lithium manganese complex oxide particles prepared by the above-mentioned procedure.  
      Part A in the  FIG. 1  represents an overlayer, and part B represents core particles of Ni. One of the composites of the overlayer was detected only on the surface of the particles (part A), and it is known that Ni was not spread to the inside of the particle but rather a new overlayer was formed.  
     Fabrication of Test Cell  
      An electrode was fabricated by using the coated lithium manganese spinel oxide as an active material. Graphite was used as a conductive agent, PVDF (polyvinylidene fluoride) was used as a binder, and the weight ratio of active material:conductive agent:binder was 85:10:5.  
      A slurry was prepared by adding the active material and conductive agent together after the binder was dissolved in NMP (n-methyl pyrrolidone). The prepared slurry was coated on the aluminum foil by a tape casting method, and the positive electrode was prepared by drying the aluminum foil coated with the prepared slurry at 130° C. in a vacuum drier for 2 hours.  
      Lithium metal was used as a negative electrode. A coin cell was fabricated by cutting the positive and negative electrodes to adequate size.  
      An electrode was used in which a 1 mol LiPF 6  solution was dissolved in a complex solution in which EC (ethylene carbonate) and EMC (ethylmethyl carbonate) were mixed to a 1:2 mol ratio.  
     Test of Properties of Test Cell  
      The cell prepared in the previous step was described as LiMn 2 O 4 /LiPF 6  (1 M) in EC+2EMC/Li, and the charge and discharge characteristics and a cycle-life characteristics were tested. The coverage of the voltage of the charge and discharge was 3.0 to 4.5V in the test of capacity case, and the test results are shown in  FIG. 4 . The test of cycle-life characteristics was done between 3.4 and 4.3V, and the cycle-life characteristics at room temperature are shown in  FIG. 5 , and the cycle-life characteristics at a high temperature are shown in  FIG. 6 . The storage characteristics at a high temperature were tested by discharge capacity from 4.3V charge at 60° C., and the test results are shown in TABLE 1.  
     EXAMPLE 2  
      The positive electrode and test cell were prepared and tested by the same method as in Example 1, except that the mole ratio of Ni and Co of the composite of the overlayer was 4:6.  
     EXAMPLE 3  
      The positive electrode and test cell were prepared and tested by the same method as in Example 1, except that the composite of the overlayer was LiNi 0.7 V 0.3 O 2 .  
     EXAMPLE 4  
      The positive electrode and test cell were prepared and tested by the same method as in Example 1, except that the quantity of the overlayer was 0.5 mol % of the lithium manganese spinel oxide of the core particle.  
     EXAMPLE 5  
      The positive electrode and test cell were prepared and tested by the same method as in Example 1, except that the coated lithium manganese spinel oxide was heat-treated at 400° C. for 10 hours in air.  
     EXAMPLE 6  
      LiCH 3 COO and Mn(CH 3 COO) 2  were mixed at the mol ratio of Li to Mn of 0.538 for the fabrication of spinel lithium manganese complex oxide powder. After the LiCH 3 COO and Mn(CH 3 COO) 2  were homogeneously mixed and heat-treated at 480° C. for 10 hours in air, the materials were cooled and remixed to fasten the reaction, and spinel powders were fabricated by heat-treatment at 700° C. for 20 hours in air. During the heat-treatment procedure, the flow rate of the air was 0.1 l/gh. The composition of the lithium manganese compound fabricated by the above procedure was Li 1.05 Mn1.95O 4 .  
      The prepared lithium manganese spinel compound was coated, sintered and used as the positive active material by the same method as in Example 1, and the positive electrode and test cell were prepared by the same method as in Example 1.  
     EXAMPLE 7  
      The positive electrode and test cell were prepared by the same method as in Example 6, except that the mole ratio of Ni and Co of the composite of the overlayer was 4:6.  
     COMPARATIVE EXAMPLE 1  
      The positive electrode and test cell were prepared by the same method as in Example 1, except that the lithium manganese spinel compound Li 1.05 Mn 1.95 O 4  of Example 1 was used as the positive active material without coating with the material of the overlayer.  
     COMPARATIVE EXAMPLE 2  
      The positive electrode and test cell were prepared by the same method as in Example 1, except that the non-coated lithium manganese spinel compound Li 1.05 Mn 1.95 O 4  of Example 6 was used as the positive active material  
                           TABLE 1                                   Mean specific capacity   Decrease ratio of capacity           after 2 cycles (mAh/g)   at 60° C. after 1 week                                                Example 1   126.7   8       Example 2   127   10.6       Example 3   127.3   15.2       Example 4   127.5   16.4       Example 5   127.4   20.5       Example 6   126.5   7.5       Example 7   126.1   5.5       Comparative   128.5   24.7       Example 1       Comparative   129.2   32.8       Example 2                  
 
       FIG. 5  is a graph illustrating capacity variation according to cycles, that is, cycle-life characteristics at room temperature (25° C.), and  FIG. 6  is a graph illustrating cycle-life characteristics at a high temperature (55° C.). The Examples 1 to 7 of the present invention showed better cycle-life characteristics than Comparative Examples 1 and 2.  
      The storage characteristics at a high temperature was tested by measuring the decrease ratios of the test cells after the test cells were charged and discharged after 1 week at 60° C. to the charged condition of 4.3V. As shown In TABLE 1, the storage characteristics of Examples 1 to 7 were better than Comparative Examples 1 and 2. Therefore, the storage characteristics of a secondary battery comprising the active material of the present Invention are shown to be great.  
      The positive active material of the present invention shows improved safety characteristics by coating the lithium manganese spinel oxide with a lithium nickel metal complex oxide, and a secondary battery comprising the positive active material shows high capacity and, cycle-life characteristics at room temperature and at a high temperature, and Improved storage characteristics at a high temperature.  
      While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.