Electrochemical Reaction Mechanism for Bi 1‐ x Ba x F 3‐ x ( x =0, 0.1, 0.2, and 0.4) Electrodes in Lithium‐Ion Batteries

Abstract The electrochemical reaction mechanism for Bi 1‐ x Ba x F 3‐ x ( x =0, 0.1, 0.2, and 0.4) electrode materials, which have several crystal structures, was investigated. As the barium content x increased, the crystal structure changed from orthorhombic ( x =0) to hexagonal ( x =0.1 and 0.2) and cubic ( x =0.4). For x =0.1 and 0.2, the discharge voltage and capacity decreased during cycling. In contrast, for x =0 and 0.4, the discharge voltage increased from the 1st to the 2nd cycle. The results obtained by X‐ray diffraction measurements showed that the crystal structures of Bi 1‐ x Ba x F 3‐ x ( x =0 and 0.4) changed from orthorhombic and cubic to hexagonal after one cycle. This indicates that the transformation of the structure to hexagonal contributes to the improvement of electrochemical properties. Furthermore, when Bi 1‐ x Ba x F 3‐ x ( x =0) in which the orthorhombic and hexagonal phases coexisted was partly discharged, the hexagonal phase contributed preferentially to the discharge reaction. These results indicate that, for Bi 1‐ x Ba x F 3‐ x , regardless of the crystal structure in the pristine state, the crystal structure changes into the hexagonal phase after one discharge‐charge cycle, and this structural change improves the electrochemical performance of the electrode materials.