With its high theoretical capacity (1165 mA h g ⁻¹ ), low reduction potential (−2.71) and abundant resources in the earth's crust, Na anode exhibits great potential in grid-scale energy storage systems and extensive application of electric vehicles.

The future of high‐energy density electrochemical energy storage systems relies on the advancement of rechargeable batteries that utilize reactive metals as anodes. In the alkaline metal, secondary battery systems because of abundant resource, high capacity and low redox potential, potassium (K) metal secondary battery (KMB) is expected to replace the existing lithium‐ion battery as a versatile platform for high‐energy density, cost‐effective energy storage devices. However, the difficulty in processing metal K results in nonstandard electrodes and hinders the development of KMBs. Furthermore, the mobility of the K metal anode due to its unique low‐melting point character at elevated temperatures in practical conditions leads to severe instability and risks in chemical/electrochemical processes. Herein, we fabricate a processable and moldable composite K metal anode by encapsulating K into reduced graphene oxide (rGO). The composite electrode can be engineered into various shapes discretionarily with precise sizes and stabilize the K metal anode at relatively high temperatures. Remarkably, the composite anode exhibits excellent cycling performance at high current density (8 mA cm⁻²) with dendrite‐free morphology. Paired with a Prussian blue cathode, the rGO–K composite anode shows much improved electrochemical performance and extended lifetime.