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Vanadium-based compounds are explored as promising electrode materials in lithium/sodium-ion batteries exhibiting superior energy storage properties. However, similar attempts are rarely reported for potassium-ion batteries (PIBs), in which the fundamental reaction mechanisms remain inexplicit. Herein, porous CaV4O9 nanobelts (NBs) are selected as a PIB anode to systematically investigate potassium storage mechanisms through in situ transmission electron microscopy. In situ measurements track overall electrochemical potassiation reactions of CaV4O9 and identify a polyphase state of V4O7, CaO, and K2O phases after potassiation. Unexpectedly, the potassiated products can be partially converted back to the original CaV4O9 phase with residual VO2 and CaO phases, which is different from the irreversible phase transformations in lithium/sodium storages of CaV4O9. Impressively, the cavities in NBs alternately disappear and appear with (de)potassiation, avoiding the drastic volume change and structural degradation of anodes. The reversible potassium storage and stable cycling are evaluated by electrochemical measurements and in situ X-ray diffraction analysis. This work provides a paradigm by revisiting the existing anode materials in lithium/sodium-ion batteries to seek out viable anodes for next-generation PIBs. [ABSTRACT FROM AUTHOR]