Patent Publication Number: US-2005130042-A1

Title: Materials for positive electrodes of lithium ion batteries and their methods of fabrication

Description:
CROSS REFERENCES  
      This application is a continuation-in-part of the following U.S. patent applications: “Compounds of Lithium Nickel Cobalt Metal Oxide and the Methods of their Fabrication, having a U.S. patent application Ser. No. 10/823,931, filed on Apr. 14, 2004; “Improved Lithium Nickel Cobalt Oxides and Their Methods of Fabrication, having a U.S. patent application Ser. No. 10/841,760, filed on May 8, 2004; and “Positive Electrodes for Rechargeable Batteries”, having a U.S. patent application Ser. No. 10/918,580, filed on Aug. 13, 2004. These applications are incorporated herein by reference. 
    
    
      This application claims priority from the following Chinese patent applications: 
          “Materials for Positive Electrodes of Lithium Ion Batteries and Their Method of Fabrication” filed on “Dec. 11, 2003”, having a Chinese Application No. 200310112600.9. The above application is incorporated herein by reference.        

     FIELD OF INVENTION  
      This invention relates to materials for positive electrodes of lithium ion batteries and their methods of fabrication. Particularly, it relates to materials and their methods of fabrication for positive electrodes of lithium ion batteries with excellent cycling and high temperature properties.  
     BACKGROUND  
      As electronic devices, especially portable electronic devices, become more complex and smaller, demands made on the power sources for these devices have increased accordingly. These power supplies are required to have higher capacities, longer cycling life, excellent high temperature properties, and excellent high rate discharge and overcharge resistant properties. In the last decade, lithium ion batteries have developed rapidly and have been widely used because of their excellent overall properties.  
      The properties of the materials for positive electrodes directly affect the effectiveness of lithium ion batteries. At present, the most widely used active materials for positive electrodes are LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , and LiNi 1-x Co x O 2  (0&lt;x&lt;1). During the repeat charging and discharging cycles of the batteries, the lattices of these active materials for positive electrodes can easily deform and the material can decompose and produce oxygen, especially when the charging voltage reaches above 4.2V. This phenomenon increases a battery&#39;s internal pressure, causes the destructive oxidization of the non-aqueous electrolyte, and, lowers the thermo-stability and safety of batteries.  
      The trace water in the non-aqueous electrolyte of lithium ion batteries also reacts with lithium salts such as LiPF6 in the electrolyte to produce HF acid. HF acid reacts with the surface of the active materials for positive electrodes, damaging the crystal structure of the active materials. This results in the deterioration of the cycling properties of batteries, which becomes more apparent at high temperatures.  
      In addition, during the charging and discharging process of the lithium ion batteries, the diffusion rate of the lithium ions in the crystal lattices of these active materials is significantly lower than that in the electrolyte solution. This causes a large gradient in the concentration of lithium ions at the surface between the materials for positive electrodes and the electrolyte solution, and, lower ion conductivity. Therefore, the capacity of these batteries easily decreases during the cycling process.  
      In order to improve the electrochemical properties of these materials, Japanese Patent 2001-143703 disclosed the method of adding extra-fine alumina powder to lithium cobalt oxide, and heating treating the mixture such that the particles of aluminum oxide and the lithium cobalt oxide were in close and tight contact. However, this method of mixing solid materials can easily lead to the non-uniform mixing of these two substances resulting in a non-uniform material for the positive electrodes.  
      U.S. Pat. No. 6,555,269 B2 disclosed a positive active material for a rechargeable lithium battery and a method of preparing that material. The positive active material included a LiCoO.sub.2 core and a metal selected from a group consisting of Al, Mg, Sn, Ca, Ti, Mn and mixtures thereof. The metal has a concentration gradient from a surface of the core to the center of the core. The method of preparing a positive active material for a rechargeable lithium battery included the steps of dissolving a metal compound in alcohol to prepare a metal compound solution in a sol state, coating LiCoO.sub.2 with the metal compound solution in the sol state and sintering the coated LiCoO.sub.2 at 150 to 500° C.  
      U.S. Pat. No. 6,569,569 B1 disclosed a positive active material for a lithium secondary battery. LiaNi1-x-yCoxMyO2-zFz and LiaNi1-x-yCoxMyO2-zSz powders (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and 0≦x&lt;0.99, 0.01≦y&lt;0.01, 0.01≦z≦0.01, and 1.00≦a≦1.1) were used. An oxygen in LiaNi1-x-yCoxMyO2 was substituted with F or S to increase the durability, capacity and structural safety of the positive active material.  
      Samsung company&#39;s patent CN1274956A disclosed a method of adding substances such as Si, B, Ti, Ga, Ge, Al or their respective oxides to active materials for positive electrodes such as lithium manganese oxide and lithium nickel cobalt manganese oxide. However, again, this method of mixing solid powders can easily result in the non-uniform mixing of the two substances. In addition, without heat treatment after mixing, the additives and the active material must necessarily not be in joined as closely or tightly and the resulting material is not as effective.  
      Japanese Patent 2001-28265 disclosed a method treating the surface of Li a Ni 1-x-y Co x M y O 2  with a metal alkoxide solution, then heat-treated the resulting substance to obtain a metal oxide coating. Similarly, Japanese Patent 2001-256989 formed a metal oxide layer on the front surface of the active materials for positive electrodes by treating the active materials with a metal alkoxide solution. However, both treatments in the two patents involve the use of a metal alkoxide solution, an unsafe product that can become a hazard in the manufacturing process.  
      Due to the limitations of the prior art, it is therefore desirable to have novel fabrication methods for novel materials that have good electrochemical properties such that when these materials are used as materials for positive electrodes of batteries, the batteries have excellent cycling and high temperature properties.  
     SUMMARY OF INVENTION  
      An object of this invention is to disclose materials for positive electrodes and methods of fabricating materials for positive electrodes which, when said materials are used in lithium ion batteries, produce batteries with excellent cycling, high temperature, high rate discharge, and, overcharge resistance properties.  
      Another object of this invention is to disclose methods for fabricating materials for positive electrodes of lithium ion batteries that are simple and easy to manufacture.  
      Briefly, the present invention relates to materials for positive electrodes and their methods of fabrication. Said materials comprise of granules of an active material for the positive electrodes where the surface of each granule is coated with an oxide layer (also referred to as oxide coating). The active material is one or more of the following: oxides of lithium cobalt, oxides of lithium nickel cobalt, oxides of lithium nickel cobalt manganese, oxides of lithium manganese, LiCoO 2 , LiNi 1-x Co x O 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , and LiMn 2 O 4 . The non-oxygen component in the oxide layer is one or more of the following: aluminum, magnesium, zinc, calcium, barium, strontium, lanthanum, cerium, vanadium, titanium, tin, silicon, boron, Al, Mg, Zn, Ca, Ba, Sr, La, Ce, V, Ti, Sn, Si, and B. Said non-oxygen component of the oxide coating is between 0.01 wt. % to 10 wt. % of said respective granule of active material. The methods of fabrication for said materials includes the steps of mixing an additive with granules of an active material for positive electrodes in water or solvent, evaporating said solvent or water, and heat treating the mixture at 300° C. to 900° C. for between 1 hour to 20 hours. The additive is a compound of one or more of the following elements: aluminum, magnesium, zinc, calcium, barium, strontium, lanthanum, cerium, vanadium, titanium, tin, silicon, boron, Al, Mg, Zn, Ca, Ba, Sr, La, Ce, V, Ti, Sn, Si, and B where the element is between 0.01 wt. % to 10 wt. % of said active material.  
      An advantage of this invention is that the materials disclosed and the materials fabricated by the methods disclosed, when used for the positive electrodes in lithium ion batteries, produces batteries with excellent cycling, high temperature, high rate discharge, and overcharge resistant properties.  
      Another advantage of the methods disclosed in this invention is that the fabrication methods are simple and easy to implement in a manufacturing process. 
    
    
     DESCRIPTION OF DRAWINGS  
      The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of preferred embodiments of this invention when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a XPS graph of an embodiment of the materials for positive electrodes.  
       FIG. 2  is a XRD graph of an embodiment of the materials for positive electrodes.  
       FIG. 3  is a scanning electron micrograph of an embodiment of the materials for the positive electrode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The presently preferred embodiments of the present invention direct to a class of materials for positive electrodes of lithium ion batteries. The materials comprise of granules of active material for positive electrodes of lithium ion batteries, where the surface of each granule is covered, i.e., coated with an oxide layer (also referred to as oxide coating). The active material for the positive electrode for the embodiments is comprised of one or more of the following: oxide of lithium cobalt oxide, LiCoO 2 , oxide of lithium nickel cobalt, LiNi 1-X Co X O 2 , LiNi 1-X Co X O 2  where (0&lt;x&lt;1), oxide of lithium nickel cobalt manganese, LiNi 1/3 Co 1/3 Mn 1/3 O 2 , oxide of lithium manganese oxide, and, LiMn 2 O 4 . The non-oxygen component in the oxide layer of said granules is between 0.01 wt. % to 10 wt. % of the respective granules of active material for the positive electrodes. The non-oxygen component of said oxide coating is one or more of the following metals: aluminum, magnesium, zinc, calcium, barium, strontium, lanthanum, cerium, vanadium, titanium, tin, Al, Mg, Zn, Ca, Ba, Sr, La, Ce, V, Ti, and Sn, or the following nonmetals: silicon, boron, Si, and B. In other words, the oxide layer is the oxide of one or more of the following metals: aluminum, magnesium, zinc, calcium, barium, strontium, lanthanum, cerium, vanadium, titanium, tin, Al, Mg, Zn, Ca, Ba, Sr, La, Ce, V, Ti, Sn or the following nonmetal: silicon, boron, Si, and B. The average granule diameter of the granules of active material for positive electrodes for some embodiments of this class of materials is between 4 microns and 12 microns. A preferred selection is for the average granule diameters to be between 10 microns and 12 microns.  
      The oxide coating of the embodiments reacts well with the HF acid. It produces solid materials that do not damage batteries and reduces the corrosion of the active materials of positive electrodes by HF acid. As a result, the cycling properties of the batteries are improved, especially at high temperatures.  
      The coating of oxide on the surface of the active material of positive electrodes forms a transitional layer between the active material of positive electrodes and the electrolyte. This transitional layer reduces the diffusion rate of lithium ions at the surface of active material of positive electrodes. As a result, the diffusion rate of lithium ions in the electrodes and electrolyte are stabilized and the cycling properties of the batteries involved are improved.  
      The preferred embodiments of the present invention also discloses methods for fabricating materials for positive electrodes of lithium ion batteries. In one method, there are the following steps: 
          mixing uniformly granules of an active material for positive electrodes and an additive in either water or an organic solvent;     evaporating said organic solvent or water to obtain a composite of said granules of active material and additive; and     heating said composite at between 300° C. to 900° C. for between 1 hour to 20 hours to obtain said material for said positive electrode.        

      The active materials of the embodiments are granules and are one or more of the following: oxides of lithium cobalt, oxides of lithium nickel cobalt, oxides of lithium nickel cobalt manganese, oxides of lithium manganese, LiCoO 2 , LiNi 1-x Co x O 2 , LiNi 1-x Co x O 2  where o&lt;x&lt;1, LiNi 1/3 Co 1/3 Mn/ 1/3 O 2 , and LiMn 2 O 4 .  
      The average granule diameter of the granules of active material for positive electrodes for some embodiments is between 4 microns and 12 microns. A preferred selection is for the average granule diameters to be between 10 microns and 12 microns.  
      The additive in some embodiments is the compound of one or more of the following metal element: aluminum, magnesium, zinc, calcium, barium, strontium, lanthanum, cerium, vanadium, titanium, tin, Al, Mg, Zn, Ca, Ba, Sr, La, Ce, V, Ti, and Sn, or the following nonmetal element: silicon, boron, Si, and B. The weight of the all the metal or nonmetal elements listed above in said additive is between 0.01 wt. % to 10 wt. % of the weight of the granules of active material for positive electrodes.  
      Embodiments of this invention provide the methods for the fabrication of materials for the positive electrodes using a liquid phase method. After heat treatment, the additives produce an oxide coating on the surface of the granules of active materials for the positive electrodes. Coated by this layer, active materials for the positive electrodes have smaller structural changes during the charging and discharging process.  
      Raw material for making the active materials for the positive electrodes can be either bought from the market or fabricated in house. Lithium cobalt oxide, for example, can be prepared by heat-treating Li 2 CO 3 , LiOH, or LiNO 3  with Co 3 O 4  at 600° C. to 900° C. for 5 to 20 hours. The average granule diameter of the granules of active material is selected to be between 4 microns to 12 microns with an optimal range of between 10 microns to 12 microns. The larger the granules, the more apparent are the effects of the additives. If the granule diameter of the granules of active materials of positive electrodes is too small, it can easily form mixtures with the additives such that the additives do not form a uniform coating on the surface of the granules of active material. In addition, the oxide obtained from heat treatment of the additives can easily penetrate into the granules of active material for the positive electrodes to form a body-phase mixture. This reduces the capacity of the batteries. The average granule diameter, D 50 , is measured using a laser scattering granular analyzer.  
      The additives in the embodiments, after being heated, decompose into its corresponding oxide. Research results showed that compounds of Al, i.e., aluminum, are significantly more effective than the other additives and thus is the preferred additive. Any aluminum compound that can dissolve in water or organic solvent and can decompose into Al 2 O 3  after heating, such as aluminum nitrate, aluminum isopropanol and aluminum butanol, can be used. The preferred selection is aluminum nitrate since this compound is safer than aluminum isopropanol and aluminum butanol.  
      In the embodiments, the metal or non-metal elements in the compounds of metal or non-metal elements are 0.01 wt. % to 10 wt. % of granules of the active material for the positive electrodes. That is, the total weight of the metal or nonmetal elements in the compounds of metal or nonmetal elements for the additives is 0.01 wt. % to 10 wt. % of the granules of active materials for positive electrodes. After heat treatment, the additives decompose and form an oxide. The non-oxygen component of the oxide layer, that is, the compound of the metal or non-metal element stated above, is then 0.01 wt. % to 10 wt. % of the granules of the active materials for the positive electrodes. For example, the weight of the Al element should be 0.01 wt. % to 10 wt. % of the granules of active materials for the positive electrodes when the additive is aluminum nitrate. If weight of the metal or non-metal element in the additive is less than 0.01 wt %, it cannot form a complete coating layer on the entire granules of active materials. As a result, no clear additive effect can be achieved. Increasing the weight of the additive element to greater than 10 wt % does not produce more apparent improvement to the cycling properties. Instead, the performance of batteries using such materials in positive electrodes is negatively impacted as their capacities decrease noticeably.  
      The uniformly mixed composite of additives and granules of active materials of positive electrodes must be heated at 300° C. to 900° C. for 1 hour to 20 hours to produce the corresponding oxide coating. If the temperature is too high or the mixture is heated too long, the oxide produced can penetrate into the inner part of the granules of active material for positive electrodes, forming a body-phase mixture. This will lower the capacity of batteries using the materials fabricated in positive electrodes. If the temperature is too low, or mixture is heated for too short a period, the additives do not completely decompose to produce oxides. This will affect the conductivity of the resulting materials for positive electrodes. The properties of batteries using such materials in their positive electrodes will be negatively affected, as the internal resistance of these batteries will be increased.  
      The following embodiments further describe this invention in more detail.  
     Embodiment 1  
      A. Fabrication of Materials for the Positive Electrodes:  
      Li 2 CO 3  and CO 3 O 4  are uniformly mixed where Li/Co is in the molar ratio of between 1:1 and 1.1. Said mixture is then heat treated at 900° C. for 10 hours to obtain lithium cobalt oxide with an average granule diameter of 10 microns to 12 microns.  
      Aluminum nitrate is used as the additive where the aluminum element in aluminum nitrate is 0.1 wt % of the lithium cobalt oxide. This additive is dissolved in anhydrous ethanol. Lithium cobalt oxide is then added to the lithium cobalt oxide in anhydrous ethanol and mixed uniformly. The solvent is evaporated and the resulting composite is heat treated at 800° C. for 4 hours to obtain the material for the positive electrodes of this embodiment.  
      B. Fabrication of Positive Electrode Slice:  
      85 wt. % of the materials for positive electrodes that has been fabricated by the method described above, 10 wt % of graphite, the conducting agent, and 5 wt. % of PVDF (polyvinylidene fluoride), the binder, are added into a NMP (methyl-2-pyrrolidinone) solvent. The mixture is stirred well to form an emulsion in order to obtain a slurry mixture of material for the positive electrodes. This slurry mixture is uniformly coated onto both sides a belt-shaped aluminum foil. It is then dried with heat and pressed to form a belt-shaped positive electrode.  
      C. Fabrication of Negative Electrode Slice:  
      90 wt. % of graphite and 10 wt. % of PVDF are added into a NMP solvent. The mixture is stirred well to obtain a slurry mixture of negative electrode material. This slurry mixture is then uniformly coated onto both sides of a belt-shaped copper foil. It is then dried with heat and pressed to form a belt-shaped negative electrode.  
      Preparation for Electrolyte  
      The electrolyte is a solution obtained by dissolving lithium hexafluorophosphate (LiPF 6 ) in a solvent that is formed by mixing equal volumes of EC (ethylene carbonate) and DMC (dimethyl carbonate). The concentration of LiPF 6  is 1 mol/L.  
      D. Assembly of Battery:  
      In one method for the assembly of the battery, the positive electrode and negative electrode that have been fabricated as described above are wrapped many times with a multi-porous polyalkene membrane to obtain a wrapped electrode. The wrapped electrodes are placed in the outer shell of the battery. Insulating plates are also placed on the top and bottom of the electrodes. Electrolyte is then injected into the battery&#39;s outer shell containing electrodes. The battery is sealed with an insulating gasket. The safety valve, PTC component and battery cover are secured to obtain a cylindrical battery whose outer diameter is 18 mm and whose height is 65 mm.  
     Embodiment 2  
      In this embodiment, the weight of the aluminum element in aluminum nitrate is increased to 5 wt. % of the weight of lithium cobalt oxide. Except for this, all other fabrication methods and conditions remain the same as Embodiment 1.  
     Embodiment 3  
      In this embodiment, the weight of the aluminum element in aluminum nitrate is increased to 10 wt. % of the weight of lithium cobalt oxide. Except for this, all other fabrication methods and conditions remain the same as Embodiment 1.  
     Embodiment 4  
      In this embodiment, the additive is aluminum isopropanol instead of aluminum nitrate. Except for this, all other fabrication methods and conditions remain the same as Embodiment. 1.  
     Embodiment 5  
      In this embodiment, the additive is magnesium nitrate instead of aluminum nitrate. Except for this, all other fabrication methods and conditions remain the same as Embodiment. 1.  
     Embodiment 6  
      In this embodiment, the condition for heat treatment to fabricate the material for the positive electrodes is changed to heating at 500° C. for 10 hours. Except for this, all other fabrication methods and conditions remain the same as Embodiment. 1.  
     Embodiment 7  
      In this embodiment, the additive is silica acid ethyl ester instead of aluminum nitrate. Except for this, all other fabrication methods and conditions remain the same as Embodiment. 1.  
     COMPARISON EXAMPLE 1  
      In this embodiment, the weight of the aluminum element in aluminum nitrate is increased to 15 wt. % of the weight of lithium cobalt oxide. Except for this, all other fabrication methods and conditions remain the same as Embodiment 1.  
     COMPARISON EXAMPLE 2  
      In this embodiment, the lithium cobalt oxide is prepared in the same manner as Embodiment 1. Li 2 CO 3  and Co 3 O 4  are uniformly mixed where Li/Co is in the molar ratio of between 1:1 and 1.1. Said mixture is then heat treated at 900° C. for 10 hours to obtain lithium cobalt oxide with an average granule diameter of 10 microns to 12 microns. However, the material for the positive electrode is prepared without the addition of the additive aluminum nitrate. Except for this, all other fabrication methods and conditions remain the same as Embodiment 1, including the fabrication of the positive and negative electrodes and electrolyte and the assembly of the battery.  
     COMPARISON EXAMPLE 3  
      In this embodiment, the lithium cobalt oxide is prepared in the same manner as Embodiment 1. Li 2 CO 3  and Co 3 O 4  are uniformly mixed where Li/Co is in the molar ratio of between 1:1 and 1.1. Said mixture is then heat treated at 900° C. for 10 hours to obtain lithium cobalt oxide with an average granule diameter of 10 microns to 12 microns. However, the material for the positive electrode is prepared without heat treatment, i.e., using the non-liquid phase method, after the addition of aluminum oxide powder to lithium cobalt oxide. Except for this, all other fabrication methods and conditions remain the same as Embodiment 1, including the fabrication of the positive and negative electrodes and electrolyte and the assembly of the battery.  
      Conditions for the Testing of the Properties of Batteries  
      The testing of the properties of the charge-discharge cycles of the batteries are conducted under the following conditions:  
      Batteries were charged and discharged between 3.0V and 4.2V at a current rate of 1 C.  
      The specific discharge capacity for the positive electrode is obtained by dividing the discharge capacity by the weight of materials for positive electrode.  
      The discharge capacity measured during the second cycle is the initial discharge capacity.  
      The initial specific discharge capacity of the materials for the positive electrode is the initial discharge capacity divided by the weight of materials for positive electrode.  
      The capacity residual rate for the nth cycle is the discharge capacity of the nth cycle divided by the initial discharge capacity.  
      In these calculations, the weight of materials for positive electrode is the total weight of active materials for positive electrodes and the oxide layer.  
      Table 1 shows the test results for the charging and discharging at a current rate of 1C and at 25° C. while Table 2 shows the test results for the charging and discharging at a current rate of 1C and at 60° C.  
                           TABLE 1                                      Specific Capacity of Materials               for Positive Electrodes (mAh/g)                                     Initial           Capacity Residual Rate                                         Discharge   100th Cycle   400th Cycle   100th Cycle   400th Cycle                                                 Embodiment 1   140   136   132   97%   94%       Embodiment 2   137   134   130   98%   95%       Embodiment 3   131   128   126   98%   96%       Embodiment 4   140   134   130   96%   93%       Embodiment 5   139   133   122   96%   88%       Embodiment 6   139   135   131   97%   94%       Embodiment 7   139   132   120   95%   86%       Comparison Example 1   120   118   115   98%   96%       Comparison Example 2   140   126   112   90%   80%       Comparison Example 3   139   131   114   94%   82%                  
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Specific Capacity of Materials 
                   
               
               
                   
                 for Positive Electrodes 
                 Capacity 
               
               
                   
                 (mAh/g) 
                 Residual Rate 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Initial 
                 100th 
                   
                   
                 400th 
               
               
                   
                 Discharge 
                 Cycle 
                 400th Cycle 
                 100th Cycle 
                 Cycle 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Embodiment 1 
                 140 
                 130 
                 122 
                 93% 
                 87% 
               
               
                 Embodiment 2 
                 137 
                 127 
                 121 
                 93% 
                 88% 
               
               
                 Embodiment 3 
                 131 
                 123 
                 117 
                 94% 
                 89% 
               
               
                 Embodiment 4 
                 140 
                 130 
                 120 
                 93% 
                 86% 
               
               
                 Embodiment 5 
                 139 
                 125 
                 114 
                 90% 
                 82% 
               
               
                 Embodiment 6 
                 139 
                 129 
                 120 
                 93% 
                 86% 
               
               
                 Embodiment 7 
                 139 
                 125 
                 114 
                 90% 
                 82% 
               
               
                 Comparison 
                 120 
                 113 
                 108 
                 94% 
                 90% 
               
               
                 Example 1 
               
               
                 Comparison 
                 140 
                 119 
                 99 
                 85% 
                 71% 
               
               
                 Example 2 
               
               
                 Comparison 
                 139 
                 121 
                 104 
                 87% 
                 75% 
               
               
                 Example 3 
               
               
                   
               
            
           
         
       
     
      Table 3 shows the test results of the charge-discharge characteristics at 1C/0.5C, 2C/0.5C, 3C/0.5C and at 25° C.  
                       TABLE 3                                      Charge/Discharge Characteristics                                 1 C./0.5 C.   2 C./0.5 C.   3 C./0.5 C.                                                 Embodiment 1   98%   95%   90%           Embodiment 2   99%   96%   92%           Embodiment 3   99%   96%   92%           Embodiment 4   98%   95%   90%           Embodiment 5   97%   94%   88%           Embodiment 6   98%   95%   90%           Embodiment 7   98%   94%   89%           Comparison   99%   96%   93%           example 1           Comparison   97%   93%   84%           example 2           Comparison   96%   93%   85%           example 3                      
 
      The charged battery is also disassembled and the material for positive electrodes dried with heat. Differential scanning calorimeter (DSC) measurements of the materials are conducted in air. (The instrument used is NETSCH STA 449C). The temperature is then increased at a rate of 3° C./min for temperatures between 20° C. and 500° C. to obtain the decomposition temperature of the charged material for positive electrodes. The test results are showed in Table 4.  
                       TABLE 4                                   Decomposition temperature (° C.)                                                    Embodiment 1   248           Embodiment 2   256           Embodiment 3   258           Embodiment 5   245           Comparison Example 1   259           Comparison Example 2   238                      
 
      The data from Tables 1 to 4 shows that cycling properties, high temperature and safety characteristics, high current charge-discharge properties and overcharge resistance properties of lithium cobalt oxide are much improved after being treated with additives. The higher the content of the additive, the more apparent is the improvement of the properties. However, the reduction of the capacity is also more apparent.  
       FIG. 1  is the XPS graph of the material for positive electrodes from Embodiment 1, obtained by X-ray photoelectron spectroscopy. (The instrument used is the 5800 X-ray photoelectron spectroscope from PHI Corporation, USA). The measurement conditions are the following: Al—K α  (1486.6 eV) monochrome ray is used. The environment pressure is 2*10 −10  TORR.  FIG. 1  shows that the Al concentration is high at the surface of LiCoO 2 . This indicates that the coating at the surface of LiCoO 2  is successful.  
       FIG. 2  is the XRD graph of the material for positive electrodes from Embodiment 1, obtained with a X-Ray diffractometer. (The instrument used is D/MAX-2200/PC X-Ray diffractometer from Rigaku Corporation.) Measurement conditions are as follows: the rate of Cu Kα scanning between 2θ=10° and 80° is 4°/min; the tube pressure is 40 KV; and the tube current is 20 mA.  FIG. 2  shows that the LiCoO 2  of this embodiment has a typical diamond layer structure of α-NaFeO 2 , which belongs to R3m spatial cluster.  
       FIG. 3  is the scanning electron micrograph of the material for positive electrodes from Embodiment 1. (The instrument used is JSM-5610LV from JEOL company, Japan). The accelerate voltage of measurement is 20 KV. It can be seen from  FIG. 3  that the surface of LiCoO 2  is very smooth, the size of particle is uniform, and the average granule diameter is around 11 microns.  
      The test results using the above embodiments show the materials for positive electrodes of this invention, when used in lithium ion batteries, produces batteries with better cycling properties, excellent high temperature, high rate discharge and overcharge resistance properties. The method of fabrication is simple and easy to implement for manufacturing.  
      While the present invention has been described with reference to certain preferred embodiments, it is to be understood that the present invention is not limited to such specific embodiments. Rather, it is the inventor&#39;s contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating not only the preferred embodiments described herein but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.