Polyanion compounds (Phosphates, sulfates, borates, and silicates) of transition metals are receiving an ever increasing attention as potential positive electrode in the secondary alkali ion batteries. They possess certain advantages over the traditional transition metal oxides due to their higher inherent safety and structural stability granted by the covalent bonding in the polyanion. In this context iron compounds are especially attractive, thanks to the natural abundance of iron resources and its environmental friendliness. Most importantly polyanion-based compounds of iron get big boost in Li-ion insertion voltage compared to pure oxide because of the inductive effect of the polyanions. Thus far the search for polyanion compound of iron yielded promising results, among which the Olivine LiFePO₄ (3.45 V) and Triplite LiFeSO₄F (3.9 V) are currently being considered for replacing the existing transition metal oxides. The vast possibilities of creating diverse 2D layered and 3D framework crystal structures through the incorporation of different secondary anion/polyanion into the primary polyanion compounds and subsequent tuning of the cell voltage has not been explored much.

In this paper we are presenting scalable fabrication of new mixed polyanionic compounds of iron and will demonstrate their ability to function as the cathode materials for low-cost alkali-ion batteries with good current capabilities and cycle-life. This work will introduce three recently discovered iron polyanion compounds containing mixed anions: Li₃Fe(HPO₃)₃Cl, Li_xFe(H₂O)₂B(PO₄)₂∙H₂O, and LiFePO₄NO₃; a report on their electrochemical activities with respect to Li- and Na-ion cells. The last compound, LiFePO₄NO₃, is a newly discovered family of mixed polyanionic compounds akin to Li_xMPO₄CO₃ (where M = Fe and Mn) where it allows the study of the effect of variation of ligand groups on the cell voltage and performance. Moreover for the same compound reversible Na⁺ intercalation proved to be effective which makes it a possible candidate for Na-ion battery cathode material. The presentation will cover the crystal structure of the respective compounds as solved through single-crystal or synchrotron powder X-ray diffraction as well as chemical, compositional and electrochemical studies including Mӧssbauer spectroscopy and galvanostatic charge/discharge tests.