Carbon-Free Crystal-like Fe1-xS as an Anode for Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) as a new electrochemical energy storage system have been considered as a desirable candidate in the post-lithium-ion battery era. Nevertheless, the study on this realm is in its infancy; it is urgent to develop electrode materials with high electrochemical performance and low cost. Iron sulfides as anode materials have aroused wide attention by virtue of their merits of high theoretical capacities, environmental benignity, and cost competitiveness. Herein, we constructed carbon-free crystal-like Fe1-xS and demonstrated its feasibility as a PIB anode. The unique structural feature endows the prepared Fe1-xS with plentiful active sites for electrochemical reactions and short transmission pathways for ions/electrons. The Fe1-xS electrode retained capacities of 420.8 mAh g-1 after 100 cycles at 0.1 A g-1 and 212.9 mAh g-1 after 250 cycles at 1.0 A g-1. Even at a high rate of 5.0 A g-1, an average capacity of 167.6 mAh g-1 was achieved. In addition, a potassium-ion full cell is assembled by employing Fe1-xS as an anode and potassium Prussian blue as a cathode; it delivered a discharge capacity of 127.6 mAh g-1 at 100 mA g-1 after 50 cycles.