Positive electrode active material powder, positive electrode, lithium ion battery, and method of producing positive electrode

A positive electrode active material powder includes a first particle group and a second particle group. The first particle group consists of a plurality of first particles. Each of the first particles includes 1 to 10 single-particles. The second particle group consists of a plurality of second particles. Each of the second particles includes an aggregated particle. The aggregated particle is formed by aggregation of 50 or more primary particles. The positive electrode active material powder has a flow function coefficient of more than or equal to 2.9. The flow function coefficient is a ratio of a maximum principal stress to a uniaxial collapse stress. The uniaxial collapse stress and the maximum principal stress are measured by a direct shear test for powders.