Positive electrode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

A method of using a positive electrode active material comprising a composite oxide that contains at least lithium, a transition metal, and oxygen, wherein:

• when a volume-based particle size frequency distribution of primary particles of the composite oxide is separated into a plurality of peaks, these peaks include specific peaks comprising:
• a main peak exhibiting a maximum value of peak area, and at least one peak having an area in a range of from 0.1 to 1, in terms of area ratio, with respect to the area of the main peak, and
• ratios of primary particle size (large particle size/small particle size) at peak tops of adjacent specific peaks are all in a range of from 1.2 to 8;
• the method comprising using the positive electrode active material to inhibit or avoid thermal runaway in a nonaqueous electrolyte secondary battery.