Positive electrode active material for secondary cell and nonaqueous electrolyte secondary cell using the same, and method for analysis of positive electrode active material for secondary cell

Abstract

A positive electrode active material for use in a non-aqueous electrolyte secondary cell comprises a powdery metal oxide (LiCoO 2 , LiNiO 2 , LiMn 2 O 4 or the like). When the positive electrode active material is classified with a classification precision index κ of 0.7 or greater so as to obtain a coarse powder having a classification ratio in a range of 0.1% to 5%, a ratio (B/A) of the content (B) of an impurity metal element in the coarse powder obtained by the classification to the content (A) of the impurity metal element in the powder before the classification is 1.5 or less. The contents of the impurity metal elements are compared with respect to Ca, Mn, Fe, Cr, Cu, Zn and the like (exclusive of the metal element constituting the powdery metal oxide). The positive electrode active material for a secondary cell serves to improve cell performance capabilities and production yields.

What is claimed is:  
     
         1 . A positive electrode active material for a non-aqueous electrolyte secondary cell, comprising: 
 a powdery metal oxide,    wherein, when the powdery composite metal oxide is classified through the use of a difference in particle diameter and density of its component particles to obtain coarse powder having a classification ratio in a range of 0.1 to 5%, a ratio (B/A) of the content B of an impurity metal element in the coarse powder obtained by the classification to the content A of an impurity metal element in the powdery metal oxide before the classification is 1.5 or below.    
     
     
         2 . The positive electrode active material for a secondary cell according to  claim 1 , 
 wherein the powdery metal oxide is classified to have a classification precision index κ of 0.7 or more.    
     
     
         3 . The positive electrode active material for a secondary cell according to  claim 1 , 
 wherein the powdery metal oxide is classified by a classifier based on a balance between centrifugal force and fluid resistance by a forced vortex.    
     
     
         4 . The positive electrode active material for a secondary cell according to  claim 1 , 
 wherein the impurity metal element is at least one kind of element (excepting the metal elements constituting the powdery metal oxide) selected from Mg, Ca, Ba, Sr, Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re, Os, Ir, Tl, Pb and Bi.    
     
     
         5 . The positive electrode active material for a secondary cell according to  claim 4 , 
 wherein the impurity metal element is at least one kind of element (excepting the metal elements constituting the powdery metal oxide) selected from Fe, Cr, Cu, Zn, Mg and Ca.    
     
     
         6 . The positive electrode active material for a secondary cell according to  claim 1 , 
 wherein the positive electrode active material comprises a composite metal oxide containing lithium and at least one kind of element selected from cobalt, nickel and manganese.    
     
     
         7 . The positive electrode active material for a secondary cell according to  claim 6 , 
 wherein the positive electrode active material comprises at least one kind of composite metal oxide selected from the following general formulae:   LiA a O x     (where, A denotes at least one kind of element selected from Co, Ni and Mn, and a and x denote numerals falling in a range of 0.8≦a≦1.1, 1.6≦x≦2.4), and   LiB b O y     (where, B denotes an element containing at least Mn selected from Mn, Co and Ni, and b and y denote numerals falling in a range of 1.5≦b≦2.1, 3.6≦y4.4).    
     
     
         8 . A non-aqueous electrolyte secondary cell, comprising: 
 a positive electrode containing a positive electrode active material consisting essentially of a powdery metal oxide,    a negative electrode disposing with the positive electrode through a separator,    a cell casing for accommodating the positive electrode, the separator and the negative electrode, and    a non-aqueous electrolyte filling in the cell casing,    wherein, when the positive electrode active material is classified through the use of a difference in particle diameter and density of component particles of the powdery metal oxide to obtain coarse powder having a classification ratio in a range of 0.1 to 5%, a ratio (B/A) of the content B of an impurity metal element in the coarse powder obtained by the classification to the content A of an impurity metal element in the powdery metal oxide before the classification is 1.5 or below.    
     
     
         9 . The non-aqueous electrolyte secondary cell according to  claim 8 , 
 wherein the impurity metal element is at least one kind of element (excepting the metal elements constituting the powdery metal oxide) selected from Mg, Ca, Ba, Sr, Sc, Y, Ti, Zr, Hf, V, Cr, Nb, Mo, Ta, W, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Re, Os, Ir, Tl, Pb and Bi.    
     
     
         10 . The non-aqueous electrolyte secondary cell according to  claim 8 , 
 wherein the positive electrode active material comprises a composite metal oxide containing lithium and at least one kind of element selected from cobalt, nickel and manganese.    
     
     
         11 . The non-aqueous electrolyte secondary cell according to  claim 8 , 
 wherein the positive electrode active material comprises at least one kind of composite metal oxide selected from the following general formulae:   LiA a O x     (where, A denotes at least one kind of element selected from Co, Ni and Mn, and a and x denote numerals falling in a range of 0.8≦a≦1.1, 1.6≦x≦2.4), and   LiB b O y     (where, B denotes an element containing at least Mn selected from Mn, Co and Ni, and b and y denote numerals falling in a range of 1.5≦b≦2.1, 3.6≦y4.4).    
     
     
         12 . The non-aqueous electrolyte secondary cell according to  claim 8 , 
 wherein the secondary cell is a lithium-ion secondary cell.    
     
     
         13 . A method for analysis of a positive electrode active material comprising a powdery metal oxide to be used for a non-aqueous electrolyte secondary cell, comprising: 
 obtaining coarse powder having a classification ratio in a range of 0.1 to 5% by classifying the powdery metal oxide through the use of a difference in particle diameter and density of its component particles;    measuring the content A of an impurity metal element in the powdery metal oxide before the classification and the content B of an impurity metal element in the coarse powder obtained by the classification; and    evaluating the amount of the particulate metal impurity contained in the powdery metal oxide before the classification based on a ratio (B/A) of the content B of the impurity metal element to the content A of the impurity metal element.    
     
     
         14 . The method for analysis of a positive electrode active material for a secondary cell according to  claim 13 , 
 wherein the powdery metal oxide is classified to have a classification precision index κ of 0.7 or more.    
     
     
         15 . The method for analysis of a positive electrode active material for a secondary cell according to  claim 13 , 
 wherein the powdery metal oxide is classified by a classifier based on a balance between centrifugal force and fluid resistance by a forced vortex.