A3V2(PO4)3 (A = Na or Li) probed by in situ X-ray absorption spectroscopy

Two stable modifications of A(3)V(2)(PO4)(3) (A = Na or Li) were synthesized by citric acid assisted modified sol-gel synthesis. The obtained samples were phase pure Li3V2(PO4)(3) and Na3V2(PO4)(3) materials embedded in a carbon matrix. The samples were tested as half cells against lithium or sodium metal. Both samples delivered about 90 mAh g(-1) at a C/10 cycling rate. The change of vanadium oxidation state and changes in the local environment of redox center for both materials were probed by in-situ X-ray absorption spectroscopy. Oxidation state of vanadium was determined by energy shift of the absorption edge. The reversible change of valence from trivalent to tetravalent oxidation state was determined in the potential window used in our experiments. Small reversible changes in the interatomic distances due to the relaxation of the structure in the process of alkali metal extraction and insertion were observed. Local environment (vanadium-oxygen bond distances) after 1st cycle were found to be the same as in the starting material. Both structures have been found very rigid without significant changes during alkali metal extraction. (C) 2012 Elsevier B.V. All rights reserved.