Nanostructured iron ((iii) oxyhydroxide/(vi) oxide) composite as a reversible Li, Na and K-ion insertion electrode for energy storage devices

Abstract

A nanostructured composite of iron(III) oxyhydroxide and iron(VI) oxide cathode that demonstrates the reversible insertion of diverse alkali ions (Li, Na and K) was prepared using a low temperature synthesis. The XRD patterns indicated an amorphous phase and low-intensity peaks, probably attributed to the iron(III) oxyhydroxide and potassium ferrate (K₂FeO₄) respectively. The observed Fourier transform infrared (FTIR) spectra also revealed characteristic absorption bands corresponding to both the K₂FeO₄ (peak 804 cm⁻¹ with a shoulder at 781 cm⁻¹) and FeOOH. Chromite analysis estimated the Fe(VI) content to be 14% and SEM images revealed secondary structures were formed from particles of sizes 10–20 nm. The HRTEM image revealed lattice fringe and fringe-free regions and the respective SAED patterns indicated the incorporation of K₂FeO₄ (Fe(VI)) into the nanocrystalline Fe(III) compound. The XANES pre-edge and main peaks corresponded to the characteristic Fe(III) and Fe(VI) peaks, whereas the EXAFS results indicated the Fe(III) state to be more associated with iron(III) oxyhydroxide. The composite cathode delivered discharge capacities of 320, 218 and 150 mA h g⁻¹ in the second cycle versus lithium, sodium and potassium test cells respectively. Specifically, the composite cathode demonstrated impressive rate capabilities at high current densities versus lithium. The feasibilities of realizing the reversible intercalation of Li-ions and relatively bigger Na/K-ions may be related to the presence of iron(III) oxyhydroxide and iron(VI) oxide in the composite cathode with secondary structure morphologies.