A comparative structural and electrochemical study of monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3

Pure monoclinic Li 3 M 2 (PO 4 ) 3 (M: Fe, V) powders (<1 μm in diameter) were obtained by an original route that involved initial homogenization of precursors in aqueous solution followed by slow evaporation and annealing under controlled atmosphere at moderate temperatures. The crystal structure of Li 3 V 2 (PO 4 ) 3 was determined for the first time through Rietveld refinements of neutron diffraction data. As for Li 3 Fe 2 (PO 4)3 , Li is distributed within three crystallographic sites, fully occupied at room temperature. The values of the temperature factors on Li(2) and Li(3) sites (five-fold coordination) were found significantly higher than that of Li(1) (four-fold coordination). Li 3 V 2 (PO 4 ) 3 shows four reversible redox phenomena upon insertion of two Li + (V 3+ /V 2+ couple), at 1.98, 1.88, 1.73 and 1.70 V vs. Li. By comparison, Li 3 Fe 2 (PO 4 ) 3 shows two reversible redox phenomena upon insertion of two Li(Fe 3+ /Fe 2+ couple), at 2.88 and 2.73 V vs. Li. Experimental capacities close to the theoretical ones were obtained after optimal composite electrode preparation through ball-milling. In situ X-ray diffraction showed very minor changes from Li 3 M 2 (PO 4 ) 3 to Li 5 M 2 (PO 4 ) 3 Additionally, Li is extracted from Li 3 V 2 (PO 4 ) 3 towards V 2 (PO 4 ) 3 (V 4+ /V 3+ and V 5+ /V 4+ couples) through four redox phenomena at 3.59, 3.67, 4.06 and 4.35 V vs. Li. Despite all these phase transitions, the [M 2 (PO 4 ) 3 ] framework is remarkably stable on cycling, particularly for M: Fe, while partial vanadium dissolution into the electrolyte occurs either on deep reduction to 1.5 V or deep oxidation to 4.6 V vs. Li.