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.
H. Yaghoobnejad Asl and A. Choudhury, "Phosphorous Acid Route Synthesis of Iron Tavorite Phases, LiFePO₄(OH)ₓF₁₋ₓ [0 ≤ X ≤ 1] and Comparative Study of Their Electrochemical Activities," RSC Advances, vol. 4, no. 71, pp. 37691-37700, Royal Society of Chemistry, Aug 2014.
The definitive version is available at https://doi.org/10.1039/c4ra05391e
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
International Standard Serial Number (ISSN)
Article - Journal
© 2014 Royal Society of Chemistry, All rights reserved.
01 Aug 2014