Positive electrode active material for nonaqueous electrolyte secondary battery

A positive electrode active material for a nonaqueous electrolyte secondary battery, comprising particles of a lithium-transition metal composite oxide that contains nickel in the composition thereof and has a layered structure, wherein

• a ratio D50/DSEM of a 50% particle size D50 in volume-based cumulative particle size distribution to an average particle size DSEM based on electron microscopic observation is in a range of 1 to 4,
• wherein the DSEM is to be determined as follows:
• observing an image at a magnification of 1,000 to 10,000 using a scanning electron microscope (SEM), selecting 100 particles having recognizable outlines, calculating the equivalent spherical diameters of the selected particles using an image processing software, and determining the arithmetic average of the obtained equivalent spherical diameters as DSEM;
• and
• a ratio D90/D10 of a 90% particle size D90 to a 10% particle size D10 in volume-based cumulative particle size distribution is 4 or less.