Exploring new hydrated delta type vanadium oxides for lithium intercalation

Three hydrated double layered vanadium oxides, namely Na 0.35 V 2 O 5 ·0.8(H 2 O), K 0.36 (H 3 O) 0.15 V 2 O 5 and (NH 4 ) 0.37 V 2 O 5 ·0.15(H 2 O), 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 V 4+ ions up to 40%. The monitoring of the V 4+ EPR signal over time suggests a reduction of the electronic delocalization on account of the partial oxidation to V 5+ . 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. The interlayer spacing of novel double layered vanadium oxides is modulated by the difference in water contents and alkali pillars. The capacity retention of the active phases is strongly dependent on the vacuum-drying treatment of the electrodes.