When the [group 1] metals are burned in a free supply of air the predominant product depends on the metal: Li forms the oxide Li2O (plus some Li2O2), Na forms the peroxide Na2O2 (plus someNa2O) whilst K, Rb and Cs form the superoxide MO2.
N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. (1997) p 84.