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 BiVO 4, 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 BiVO 4 anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO 4 anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO 4 during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO 4 nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO 4 material is transformed into an amorphous K x BiVO 4 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 BiVO 4 nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.