Phosphorous Acid Route Synthesis of Iron Tavorite Phases, LiFePO4(OH)xF1-X [0 ≤ X ≤ 1] and Comparative Study of Their Electrochemical Activities


New synthesis routes were employed for the synthesis of three derivatives of iron hydroxo-, fluoro-, and mixed hydroxo-fluoro phosphates LiFePO 4(OH)xF1-x where 0 ? x ? 1 with the tavorite structure type, and their detail electrochemical activities have been presented. The hydrothermal synthesis of the pure hydroxo-derivative, LiFePO4OH, using phosphorous acid as a source of phosphate yielded good quality crystals from which the crystal structure was solved for the first time using SC-XRD (single crystal X-ray diffraction). The fluoro derivative, LiFePO4F, was prepared as a very fine powder at low temperature in a solvent-less flux-based method employing phosphorous acid and mixed alkali metal nitrates. A mixed anionic hydroxo-fluoro iron tavorite phase, LiFePO 4(OH)0.32F0.68, was also synthesized by a hydrothermal route. The electrochemical performance of the three phases was studied with galvanostatic charge-discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). All three phases showed facile Li-insertion through the reduction of Fe3+ to Fe2+ at an average voltage in the range of 2.4-2.75 V, through the variation of the anion from pure OH to pure F. An increase of 0.35 V was observed as a result of F substitution in the OH position. Also, good cyclability and capacity retention were observed for all three phases and a reversible capacity of more than 90% was achieved for LiFePO4F. The results of EIS indicated that lithium ion mobility is highest in the mixed anion.



Keywords and Phrases

Cyclic voltammetry; Electrochemical impedance spectroscopy; Hydrothermal synthesis; Iron; Lithium alloys; Lithium compounds; Phosphorus; X ray diffraction; Capacity retention; Comparative studies; Electrochemical activities; Electrochemical performance; Galvanostatic charge discharges; Hydrothermal routes; Reversible capacity; Single crystal x-ray diffraction; LiFePO; Phosphorous acid; Route synthesis; Tavorite, Iron compounds

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