The present invention concerns a rechargeable iron-ion (Fe-ion) battery cell comprising an anode, a cathode, and an electrolyte interposed between the anode and the cathode, and the use thereof. The electrolyte itself may comprise the Fe-ions, and/or can obtain the Fe-ions from the anode or cathode. A Fe-ion refers to an ion containing Fe element. The cathode is preferably made of Fe element containing materials. Aqueous electrolyte and organic electrolyte may be utilized as electrolyte for Fe-ion battery fabrication. In order to further improve the performance of Fe-ion battery, ionic liquid may be utilized as electrolyte, wherein the ionic liquid electrolyte transports Fe-ions between said anode and cathode.