Revealing\nAsymmetric Phase Transformation of the BiVO<sub>4</sub> Anodes for\nPotassium-Ion Batteries by In Situ Transmission\nElectron Microscopy

Potassium-ion\nbatteries (PIBs) have emerged as a promising alternative\nto lithium-ion batteries (LIBs), thanks to the cost-effectiveness\nof potassium resources and a favorable redox potential of approximately\n−2.936 V. The monoclinic BiVO<sub>4</sub>, known for its layered\nstructure, shows noteworthy electrochemical properties when utilized\nas an anode material for both LIBs and sodium-ion batteries. However,\nthe fundamental electrochemical reaction mechanisms of the BiVO<sub>4</sub> anode during the potassium insertion/extraction processes\nremain unclear. Here, we constructed a BiVO<sub>4</sub> anode PIB\ninside the transmission electron microscope (TEM) to explore the real-time\npotassiation/depotassiation behaviors of BiVO<sub>4</sub> during electrochemical\ncycling. Utilizing the state-of-art in situ TEM technique, the BiVO<sub>4</sub> nanorods are found to undergo an asymmetric phase transformation\nfor the first time, where the pristine BiVO<sub>4</sub> material is\ntransformed into an amorphous K<sub><i>x</i></sub>BiVO<sub>4</sub> phase after the first cycle. More interestingly, the anode\nmaterials near and far from the potassium source exhibit opposite\nvolume-changing trends under the same voltage potential. Also, this\nphenomenon should be attributed to the mass flow of the unstable K–Bi\nalloy under the electric field. Our findings provide significant insights\ninto the electrochemical mechanism of BiVO<sub>4</sub> nanorods during\nthe potassiation/depotassiation process, with the hope of assistance\nin designing anodes for high-performance PIBs.