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Abstract Ultrafine hollow nanoparticles‐based (7 nm) carbon nanoclusters (HNPCNCs) are developed by a facile and novel method. Benefiting from the ultrafine nanostructure, we have achieved close‐packed storage of K metallic clusters and a close‐packed K metallic clusters with a diameter of 7 nm has been observed for the first time through ex TEM. In K‐ion batteries, HNPCNCs anodes deliver an ultrahigh capacity of 269.8 mA h g−1 at 0.5 A g−1 after 200 cycles, outstanding rate performance of 150.5 mA h g−1 after 5000 cycles at 5 A g−1. Moreover, an excellent electrochemical performance has been obtained by HNPCNCs//potassium Prussian blue potassium‐ion full cells, highlighting the practicability of HNPCNCs. The unprecedented electrochemical performance is attributed to the close‐packed K metallic clusters achieved by unique ultrafine hollow nanoparticles in the carbon nanoclusters.