An Iron-Based Fluorophosphate Cathode Material for K-Ion Batteries

The development of a tavorite structured K- transition metal (T_M)- fluorophosphate, having earth-abundant Fe as the only T_M, crystallizing in the orthorhombic crystal system and facilitating stable-cum-reversible electrochemical K-extraction/insertion, has been reported here. Synthesized using low-cost precursors, KFePO₄F has also been found to be air-stable. Detailed information pertaining to the bonding/structure, including lattice site occupancy, have been obtained via diffraction, Raman spectroscopy and FTIR, with XPS, Mössbauer and ESR revealing the oxidation states and nature of Fe in the as-synthesized condition and upon being subjected to electrochemical potassiation/depotassiation. The electrochemical K-insertion/extraction, supported by reversible Fe-redox, leads to a reversible K-storage capacity of ~102 mAh/g (within 1.5-4.0 V), along with a 1^st cycle Coulombic efficiency (CE) of ~93 % (with CE>99.9 % from 2^nd cycle onwards). Ex-situ X-ray diffraction, as well as operando synchrotron diffraction during galvanostatic cycling, indicates reversible changes in peak positions upon electrochemical K-extraction/insertion, with no evidence for structural change. When used as cathode material in K-ion 'full' cell (with hard carbon-based anode), a discharge capacity of ~68 mAh/g, along with capacity retention of ~70 % after 50 cycles, has been obtained; which confirms that this newly-developed earth-abundant Fe-based potassium fluorophosphate can be utilized for potential application in sustainable battery chemistries, like K-ion batteries.