Amorphous iron phosphate: potential host for various charge carrier ions

In response to the ever-increasing global demand for viable energy-storage systems, sodium and potassium batteries appear to be promising alternatives to lithium ion batteries because of the abundance, low cost and environmental benignity of sodium/potassium. Electrical energy storage via ion-intercalation reactions in crystalline electrodes is critically dependent on the sizes of the guest ions. Herein, we report on the use of a porous amorphous iron phosphate synthesized using ambient temperature strategies as a potential host that stores electrical energy through the feasible insertion of mono-/di-/tri-valent ions. A combination of ex situ studies reveals the existence of a reversible amorphous-to-crystalline transition in this versatile electrode during electrochemical reactions with monovalent sodium, potassium and lithium. This reconstitutive reaction contributes to realizing specific capacities of 179 and 156 mAhg−1 versus sodium and potassium at current densities of 10 and 5 mAg−1, respectively. This finding facilitates the feasible development of several amorphous electrodes with similar phase behavior for energy-storage applications. Cation redox hosts are realized that undergo crystalline–amorphous transitions with large cations on electrochemical or chemical reactions. Jaekook Kim from Chonnam National University and co-workers based in South Korea and China show that porous amorphous iron phosphate can act as a host for various charge-carrier ions with large ionic radii (specifically, monovalent sodium and potassium). This is significant because low-cost, environmentally friendly energy storage systems are needed to replace currently used lithium-ion batteries and the storage of electrical energy based on ion-intercalation reactions in crystalline electrodes depends critically on guest ion size. Divalent and trivalent cations were also shown to be able enter and exit the host. These results are promising for the development of various amorphous electrodes possessing a similar phase behavior that offer potential solutions for alternative energy storage. Unlike crystalline electrodes wherein ion insertion is crucially dependent on the presence of energetically equivalent sites, nanostructured amorphous iron(III) phosphate hosts prepared by room temperature strategies and possessing porous properties facilitate the insertion of alkali ions with different sizes and also higher charge carriers including divalent cations (Mg2+−0.72Å, Zn2+-0.74 Å) or trivalent cations (Al3+−0.53 Å). This versatile cathode stores electrical energy by a reversible amorphous to crystalline reconstitutive reaction that occurs during electrochemical reaction with monovalent sodium, potassium and lithium. The study presents opportunities to develop amorphous electrodes with similar phase behavior for energy storage applications.