Sandwich‐Like FeCl3@C as High‐Performance Anode Materials for Potassium‐Ion Batteries

Carbon materials are used as the anode materials of potassium‐ion batteries (KIBs) thanks to the feasible intercalation of potassium ions. However, their rate capability and cycling performance are still unsatisfactory. In this work, FeCl3‐intercalated expanded graphite (FeCl3‐EG) is first reported as an excellent anode material of KIBs. Attributed to the unique structure with FeCl3 sandwiched between the adjacent graphene layers, the FeCl3‐EG electrode delivers a high reversible capacity of 269.5 mAh g−1 at 50 mA g−1 and 133.1 mAh g−1 at an ultrahigh current density of 5000 mA g−1. The FeCl3‐EG electrode also exhibits an ultrastable cycling performance. Even after 500 cycles at the current density of 50 mA g−1, the FeCl3‐EG electrode can still deliver a discharge capacity of 224.1 mAh g−1 with a high capacity retention of ≈88.82%. Moreover, the FeCl3‐EG electrode is measured at an ultrahigh current density of 2000 mA g−1 for 1300 cycles, with a high capacity retention of ≈70.38%. Ex situ X‐ray diffraction, Raman, and high‐resolution transmission electron microscopy measurements are performed to investigate the potassium storage mechanism of FeCl3‐EG electrode, which confirms the FeCl3‐EG a promising anode material for high‐performance KIBs.