Magnetic behavior of Fe nanoparticles driven by phase transition of FeF3

Typical electrodes for Li-ion batteries such as LiCoO2 operate via the insertion/de-insertion of Li ions; innovative electrodes that undergo conversion reactions have been developed to meet the requirement for greater capacities. However, batteries operating via a conversion process suffer from degradation over repeated cycles and from hysteresis in charge/discharge profiles, and their mechanism remains a topic of ongoing debate. We prepare the FeF3/C composite electrode as a model electrode to address the conversion mechanism. Magnetization measurements provide direct evidence of superparamagnetic behavior―Fe nanoparticles (NPs) are magnetized similarly to a paramagnet under an external field―after the discharge process. We quantify the size of the Fe NPs, and find a correlation between an increased Fe particle size and a decrease in discharge capacity. These findings would pave the strategy based on control of the size of Fe NPs for realizing a favorable cycling performance. Indeed, improvement of an electrolyte goes toward higher capacities after 30 cycles.