Fast-Charging Degradation Mechanism of Two-Dimensional FeSe Anode in Sodium-Ion Batteries

Transition-metal chalcogenides (TMCs) are recognized as promising sodium-ion battery anodes for their high theoretical specific capacity and low sodium metal plating risk. Nevertheless, their unsatisfactory rate capability along with unstable cyclability are bottlenecks to their implementation, especially for fast-charging applications. Herein, we report two-dimensional FeSe nanosheets (FeSe-NS) and monitor the fast-charging degradation mechanism of FeSe-NS with in situ magnetometry as the central role. Specifically, first, combining in situ XRD and in situ magnetometry with theoretical calculations, we reshape the sodium storage mechanism of FeSe-NS, namely, the "insertion–conversion–space charge" sodium storage mechanism. Then, with the aid of in situ magnetometry and ex situ characterizations, we reveal that the sluggish kinetics and inferior reversibility of the conversion reaction (Fe2+ ↔ Fe0) in the medium-voltage region are barriers to the fast-charging performance of FeSe-NS. Therefore, we propose that enhancing the kinetics and reversibility of the conversion reaction is the key point for constructing a fast-charging TMC anode.