Iron Boro-Phosphate: A Mixed Polyanionic Compound As a New Cathode Material for Na-Ion Battery
Authors: Yaghoobnejad Asl Hooman , Choudhury Amitava 
Published: February 2020

Lithium ion batteries are currently the best electrochemical energy storage devices for small and medium scale applications including electronic devices and electric vehicles (EVs). However for large scale stationary applications of batteries, for example, in smart grid, safety, cost and environmental friendliness of materials supersedes the high energy density requirement of mobile application. During the last two decades, polyanionic (Phosphate, sulfate, silicate, borate etc.) compounds of transition metals have gained enormous attention as cathode materials for Li-ion batteries due to their inherently higher safety and voltage tunability by virtue of the inductive effect of the polyanion (1). On the other hand high cost of Li has forced scientists to look for other alternative alkali ion insertion batteries.
As a consequence, polyanion-based Na-ion batteries have emerged as alternatives to Li-ion counterparts due to the inherent safety of polyanion, much lower cost of sodium and its elemental abundance in the geo- and hydrosphere (2, 3). Towards this goal we have successfully synthesized a new mixed polyanionic cathode material based on iron, NaFeB(PO4)2(H2O)2, which can topotactically and reversibly (de)intercalate Na+ ions upon subsequent charge and discharge cycles. Single-crystal X-ray diffraction was used to solve the crystal structure of the compound and the electrochemical activity was studied using galvanostatic charge-discharge tests. In this compound BO4 and PO4 tetrahedra are polymerized through a common oxygen atom to form a macro-anionic boro-phosphate network. It is to be noted here that some computationally predicted boro-phosphate structures have been touted as good cathode materials (4). The crystal structure has been solved in P65
space group and is composed of FeO6 octahedra, PO4 and BO4 tetrahedral units. The connectivity between the polyhedra forms empty channels in the c-axis of the crystal. The formula unit contains one Fe(II) site which can be electrochemically oxidized to Fe(III) giving rise to a theoretical capacity of 89.5 mAh.g-1. The first discharge curve of this compound is shown in Figure 1 which demonstrates that the compound can deliver almost 80% of its theoretical capacity with a sloppy voltage-composition curve, reminiscent of solid-solution type behavior, over a voltage range of 3.25 — 1.70 V vs. Na+/Na.
Performance of the cells fabricated using the above cathode materials shows satisfactory capacity retention with cycling at different C-rates. Also cheap and scalable synthesis procedures makes this material a viable cathode candidate for Na-ion battery.
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Figure 1