Positive Electrode Active Material, Method for Manufacturing Positive Electrode Active Material, and Secondary Battery

Abstract

A positive electrode active material which can improve cycle characteristics of a secondary battery is provided. Two kinds of regions are provided in a superficial portion of a positive electrode active material such as lithium cobaltate which has a layered rock-salt crystal structure. The inner region is a non-stoichiometric compound containing a transition metal such as titanium, and the outer region is a compound of representative elements such as magnesium oxide. The two kinds of regions each have a rock-salt crystal structure. The inner layered rock-salt crystal structure and the two kinds of regions in the superficial portion are topotaxy; thus, a change of the crystal structure of the positive electrode active material generated by charging and discharging can be effectively suppressed. In addition, since the outer coating layer in contact with an electrolyte solution is the compound of representative elements which is chemically stable, the secondary battery having excellent cycle characteristics can be obtained.

1 . A method for forming a positive electrode active material, the method comprising the steps of:
 preparing a particle comprising a composite oxide, the composite oxide containing lithium, cobalt, magnesium and fluorine;   coating the particle with a material containing titanium; and   heating the particle coated with the material containing titanium so that magnesium and fluorine contained in an inside of the particle are segregated on a surface of the particle and that titanium contained in the material containing titanium is diffused into the inside of the particle.   
     
     
         2 . The method according to  claim 1 , wherein the particle is coated with the material containing titanium by a sol-gel method. 
     
     
         3 . The method according to  claim 1 , wherein the particle comprising the composite oxide is prepared by a process comprising:
 mixing a source of lithium, a source of cobalt, a source of magnesium and a source of fluorine to form a mixture; and   heating the mixture at 800° C. or higher and 1100° C. or lower.   
     
     
         4 . A method for forming a positive electrode active material, the method comprising the steps of:
 preparing a particle comprising a composite oxide, the composite oxide containing lithium, cobalt, magnesium and fluorine;   coating the particle with a material containing titanium; and   heating the particle coated with the material containing titanium to cause segregation of magnesium and fluorine and diffusion of titanium, resulting in that, in a line analysis of energy dispersive X-ray spectrometry, a peak of a concentration of magnesium and a peak of a concentration of fluorine are positioned closer to a surface of the positive electrode active material than a peak of a concentration of titanium.   
     
     
         5 . The method according to  claim 4 , wherein the particle is coated with the material containing titanium by a sol-gel method. 
     
     
         6 . The method according to  claim 4 , wherein the particle comprising the composite oxide is prepared by a process comprising:
 mixing a source of lithium, a source of cobalt, a source of magnesium and a source of fluorine to form a mixture; and   heating the mixture at 800° C. or higher and 1100° C. or lower.   
     
     
         7 . The method according to  claim 4 ,
 wherein a region in which a concentration of titanium is equal to or higher than ½ of a peak of titanium is formed by the step of heating in a range from the surface of the positive electrode active material to a depth of 20 nm.   
     
     
         8 . The method according to  claim 4 ,
 wherein the surface of the positive electrode active material is positioned at a measurement point at which a measurement value of oxygen in the line analysis of energy dispersive X-ray spectrometry is 0.5×Oave, where Oave is an average value of the measurement value of oxygen in the line analysis of the energy dispersive X-ray spectrometry in a region where the measurement value of oxygen is stable.   
     
     
         9 . The method according to  claim 4 , wherein the peak of titanium is present in a region from a depth of 0.2 nm or more to a depth of 10 nm or less. 
     
     
         10 . A method for forming a positive electrode active material, the method comprising the steps of:
 preparing a particle comprising a composite oxide, the composite oxide containing lithium, cobalt, magnesium and fluorine, the particle comprising a crack portion;   coating the particle with a material containing titanium; and   heating the particle coated with the material containing titanium so that magnesium and fluorine contained in an inside of the particle are segregated on a surface of the particle and in the crack portion and that titanium contained in the material containing titanium is diffused into the inside of the particle.   
     
     
         11 . The method according to  claim 10 , wherein the particle is coated with the material containing titanium by a sol-gel method. 
     
     
         12 . The method according to  claim 10 , wherein the particle comprising the composite oxide is prepared by a process comprising:
 mixing a source of lithium, a source of cobalt, a source of magnesium and a source of fluorine to form a mixture; and   heating the mixture at 800° C. or higher and 1100° C. or lower.   
     
     
         13 . The method according to  claim 10 , wherein titanium is segregated in the crack portion. 
     
     
         14 . A method for forming a positive electrode active material, the method comprising the steps of:
 preparing a particle comprising a composite oxide, the composite oxide containing lithium, cobalt, magnesium and fluorine, the particle comprising a crack portion;   coating the particle with a material containing titanium; and   heating the particle coated with the material containing titanium to cause segregation of magnesium and fluorine and diffusion of titanium, resulting in that, in a line analysis of energy dispersive X-ray spectrometry, a peak of a concentration of magnesium and a peak of a concentration of fluorine are positioned closer to a surface of the positive electrode active material than a peak of a concentration of titanium,   wherein magnesium is segregated at least in the crack portion.   
     
     
         15 . The method according to  claim 14 , wherein the particle is coated with the material containing titanium by a sol-gel method. 
     
     
         16 . The method according to  claim 14 , wherein the particle comprising the composite oxide is prepared by a process comprising:
 mixing a source of lithium, a source of cobalt, a source of magnesium and a source of fluorine to form a mixture; and   heating the mixture at 800° C. or higher and 1100° C. or lower.   
     
     
         17 . The method according to  claim 14 ,
 wherein a region in which a concentration of titanium is equal to or higher than ½ of a peak of titanium is formed by the step of heating in a range from the surface of the positive electrode active material to a depth of 20 nm.   
     
     
         18 . The method according to  claim 14 ,
 wherein the surface of the positive electrode active material is positioned at a measurement point at which a measurement value of oxygen in the line analysis of energy dispersive X-ray spectrometry is 0.5×Oave, where Oave is an average value of the measurement value of oxygen in the line analysis of the energy dispersive X-ray spectrometry in a region where the measurement value of oxygen is stable.   
     
     
         19 . The method according to  claim 14 , wherein the peak of titanium is present in a region from a depth of 0.2 nm or more to a depth of 10 nm or less. 
     
     
         20 . The method according to  claim 14 , wherein titanium is segregated in the crack portion. 
     
     
         21 . A method for forming a positive electrode active material, the method comprising the steps of:
 preparing a particle comprising a composite oxide, the composite oxide containing lithium, cobalt, magnesium and fluorine;   coating the particle with a material containing titanium; and   heating the particle coated with the material containing titanium to cause segregation of magnesium and fluorine and diffusion of titanium,   wherein a relative value of a concentration of titanium is greater than or equal to 0.05 and less than or equal to 0.4 when a surface of the positive electrode active material is subjected to an XPS analysis and a concentration of cobalt is defined as 1.