Stabilizing high-rate potassium storage by ferromagnetism

In this work, pre-magnetized ε-Fe₂O₃ was used as prototype electrode material to reveal the synergetic effect between ferromagnetism and electrochemical performance. Upon high-rate potassiation, ε-Fe₂O₃ was unable to fully participate in conversion reaction due to "potassiation retardation". The higher the rate was, the less complete the conversion reaction became. The as-converted Fe product was magnetized by the remanent magnetization of residual ε-Fe₂O₃ and underwent a magnetic decantation process. As a result, the Fe phase was magnetically attached to the residual ε-Fe₂O₃ surface, while the diamagnetic K₂O phase was separated out. This phase separation not only suppressed the generation of KOH but also made side reactions involving - COOK species less likely to occur, thereby avoiding a large consumption of electrolyte and stabilizing the solid-electrolyte interphase layer. Driven by this synergy, ε-Fe₂O₃ showed the best cycling stability on potassium storage at 5 A g^-1. Its discharge capacity loss per cycle was as low as 0.094 ‰ from 4 to 700 cycles with Coulombic efficiencies above 99.9 %. Moreover, the results also showed that the rate influence on potassium storage was much greater than that on lithium storage. It was thus anticipated that this work would shed new light on the understanding of interrelated physicochemical properties of electrode material.