In Situ Studies of 30% Li-Doped Bi₂₅FeO₄₀ Conversion Type Lithium Battery Electrodes

One of the important discharge mechanisms for lithium batteries is the conversion reaction mechanism, where a metal oxide (fluoride) can decompose into metallic nanoparticles embedded in a Li2O (LiF) matrix. Here, 30% Li-doped Bi25FeO40 is successfully synthesized and displays an electrochemical discharge capacity of ∼300 mAh/g above 1.5 V (vs Li/Li+). During the electrochemical cycling process, 30% Li-doped Bi25FeO40 is decomposed into metallic Bi. During the subsequent charging process, the metallic bismuth can be first converted into an amorphous bismuth oxide phase, which contributed to the electrochemical discharge activities observed between 2 and 2.5 V. At a higher charging voltage between 3.5 and 5 V, metallic Bi can be oxidized to BiOx2–O3–2x–, which contributes to the discharge activities observed above 2.5 V. Using graphite as current collectors can prevent the corrosion from O– species and the discharge capacity is greatly enhanced at the voltage region between 1.5 and 2.5 V. This work provides a deeper understanding over the role of oxygen ions during the conversion reaction process and is beneficial for the future design of battery systems based on the conversion reaction