Exploring new hydrated delta type vanadium oxides for lithium intercalation

Abstract

Three hydrated double layered vanadium oxides, namely Na(0.35)V(2)O(5)0.8(H2O), K-0.36(H3O)(0.15)V2O5 and (NH4)(0.37)V(2)O(5)0.15(H2O), were obtained by using mild hydrothermal conditions. Their delta type structural frameworks were solved by high-resolution synchrotron X-ray powder diffraction and the interlayer spacings were interpreted from difference Fourier maps. The inter-slab distances are modulated by the water content and the special arrangements of the alkali and ammonium cations. The XPS measurements denote mixed valence systems with high contents of V4+ ions up to 40%. The monitoring of the V4+ EPR signal over time suggests a reduction of the electronic delocalization on account of the partial oxidation to V5+. The electrochemical performance of the active phases is strongly conditioned by the vacuum-drying process of the electrodes, showing better capacity retention when vacuum is not applied. In situ X-ray diffraction shows a structural mechanism of contraction/expansion of the bilayers upon lithium insertion/extraction where the alkali ions behave as structural stabilizers. Galvanostatic cycling at very low current density implies migration of the alkali "pillars" triggering the collapse of the structure.