Nanoscale Defluorination Mechanism and Solid Electrolyte Interphase of a MgF₂ Anode in Fluoride-Shuttle Batteries

Fluoride-shuttle batteries using fluoride ion transfer have been extensively investigated for developing post-lithium-ion batteries. One of the key issues hampering the development is the low capacity utilization rate in anodes, causing a significant reduction in the battery performance. To improve the utilization rate, it is necessary to clarify the unknown aspects regarding the (de)fluorination behavior in order to optimize the electrode design. Here, we demonstrated the characterization of Mg metal formations and the solid electrolyte interphase (SEI) of a defluorinated MgF2 anode using scanning transmission electron microscopy and electron energy loss spectroscopy techniques. Mg was mainly formed in the region where electron and F ion conductivities were supposed to be maintained enough. Nanosized Mg metals were formed even in the region with poor electron conductivity. These results imply that these conductivity paths must be efficiently increased to further improve the utilization rate and that an efficient (de)fluorination process could be achieved by designing the electrode configuration and/or elemental composition. The influence of SEI on the battery performance is currently neglected because the problems of low utilization rate are more serious. However, as research progresses, the control of the SEI composition and its properties should be an important investigation to further improve the fluoride-shuttle battery performance.