Potassium-ion batteries are attracting intense attention as a promising energy storage system for grid-level applications due to their much lower cost compared with that of lithium-ion batteries. However, the lack of appropriate cathode materials becomes the most challenging issue caused by the contradiction between the higher difficulty on the controlled synthesis of K-based cathode candidates and the more rigorous requirement on the size, morphology and structure modulation in order to resist the drastic variation during the K insertion/extraction. In this work, we develop a scalable two-step self-templating strategy to synthesize P3–K0.5MnO2 hollow submicrospheres, in which largely optimized K-ion storage performance with superior cycling stability can be achieved benefitting from the synergistic effect of size, morphology and hollow interior. More importantly, the feasibility of the practical applications is further demonstrated via assembling the P3–K0.5MnO2 hollow submicrospheres cathode//graphite anode potassium-ion full cell and a high energy density of 100.7 Wh kg−1 is acquired based on the mass of cathode and anode, which highly strengthens the confidence on the future development of potassium-ion battery technology.