Iron Borophosphate as a Potential Cathode for Lithium- and Sodium-Ion Batteries

Lithium iron borophosphate, Li0.8Fe­(H2O)2[BP2O8]·H2O, with a chiral 65 helical channel structure has been shown to be electrochemically active as the cathode for both Li- and Na-ion batteries. We report here, for the first time, synthesis of the illusive Li-containing iron borophosphate of a well-known structure type by employing a hydrothermal synthesis route. The compound has been characterized by single-crystal X-ray diffraction, magnetic measurement, and Mössbauer spectroscopy, which unequivocally prove the mixed valency of Fe2+/3+. The compound exhibits a sloppy voltage profile reminiscent of single-phase solid-solution-type behavior on electrochemical lithium and sodium insertion in the voltage range of 2.1–4.0 V and 1.6–4.0 V, respectively. The pure single-phase oxidized end-member Fe­(H2O)2[BP2O8]·H2O was synthesized by chemical delithiation of the as-synthesized compound, and the structure was solved by ab initio methods, followed by Rietveld refinement of the synchrotron powder X-ray diffraction data, showing a volume change of 3% with retention of the parent lattice. The oxidized phase was also characterized with magnetic and Mössbauer spectroscopy, which, besides proving the 3+ oxidation state of Fe, showed long-range antiferromagnetic ordering. The electrochemical performance of Li0.8Fe­(H2O)2[BP2O8]·H2O was studied with galvanostatic charge/discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy. The compound showed facile Li- and Na- (de)-insertion with an average voltage of 3.06 and 2.76 V for Li- and Na-ion cells, respectively, with almost 80% of the theoretical capacity achieved, and a reasonable capacity retention was observed. The results of EIS in the fabricated cells indicated higher impedances for the Na-ion cell, compared to the Li-ion cell. Variable-temperature EIS studies on pressed pellets of Li0.8Fe­(H2O)2[BP2O8]·H2O showed high Li-ion conductivity (3.0 × 10–8 S cm–1 at room temperature) with low activation energy (0.20 eV/Li+).