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

A method for analysis of a positive electrode active material for a non-aqueous electrolyte secondary cell, comprising:

• preparing a powdery metal oxide as the positive electrode active material;
• classifying the powdery metal oxide by using a difference in particle diameter and density of its component particles to obtain a classified coarse powder having a classification ratio in a range of 0.1 to 5% and a remaining powder without the classified coarse powder, wherein the classification ratio is a mass ratio of an amount of the classified coarse powder to a total amount of the powdery metal oxide;
• measuring a content A of an impurity metal element in the powdery metal oxide before classifying and a content B of an impurity metal element in the classified coarse powder obtained by classifying;
• comparing the content B of the impurity metal element with the content A of the impurity metal element in order to evaluate an amount of particulate metal impurity contained in the powdery metal oxide before classifying;
• and
• selecting a whole of the powdery metal oxide containing the classified coarse powder and the remaining powder, of which a ratio (B/A) of the content B of the impurity metal element to the content A of the impurity metal element is 0.9or more and 1.4 or less, as the positive electrode active material,
• wherein the powdery metal oxide is classified to have a classification precision index κ of 0.7 or more,
• wherein the classification precision index κ is expressed with a ratio (Dp25/Dp75), where Dp25 is a particle diameter (μm) when a partial classification efficiency in a partial classification efficiency curve is 25% and Dp75 is a particle diameter (μm) when the partial classification efficiency in the partial classification efficiency curve is 75%.