2016 Volume 84 Issue 9 Pages 654-661
“Abundance” is an important keyword in materials development. This is particularly the case for the energy storage sector, where materials themselves function as a storage host. The amount of materials is directly linked to the amount of energy stored in the device. In rechargeable batteries, transition metal elements are necessary to accommodate a large number of electrons/holes in a reversible redox reaction. Iron, as the fourth most abundant element in the earth’s crust, is an ideal redox center, but practical storage electrodes with Fe redox have long been the “holy grail” of the lithium-ion battery since its commercialization in 1991. In this review article, the history of replacing Co with Mn and/or Fe in lithium battery electrodes is briefly reviewed followed by recent technical achievements toward more sustainable batteries using Na+ as a guest ion, where the goal would be to discover a high voltage electrode material composed of Na and Fe without compromising the energy density. Further, our ultimate destination is set to high energy density aqueous lithium/sodium ion batteries, where the hydrate-melt electrolyte enables surprisingly high-voltage operation over 3 V. During materials identification and optimization, reaction mechanisms should be understood in a systematic way to provide a firm direction for strategic design. With this regards, important physicochemical properties of key materials will be introduced.