Revealing Asymmetric Phase Transformation of the BiVO₄ Anodes for Potassium-Ion Batteries by In Situ Transmission Electron Microscopy

Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), thanks to the cost-effectiveness of potassium resources and a favorable redox potential of approximately −2.936 V. The monoclinic BiVO₄, known for its layered structure, shows noteworthy electrochemical properties when utilized as an anode material for both LIBs and sodium-ion batteries. However, the fundamental electrochemical reaction mechanisms of the BiVO₄ anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO₄ anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO₄ during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO₄ nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO₄ material is transformed into an amorphous K_xBiVO₄ phase after the first cycle. More interestingly, the anode materials near and far from the potassium source exhibit opposite volume-changing trends under the same voltage potential. Also, this phenomenon should be attributed to the mass flow of the unstable K-Bi alloy under the electric field. Our findings provide significant insights into the electrochemical mechanism of BiVO₄ nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.