Exploring new hydrated delta type vanadium oxides for lithium intercalation

Three hydrated double layered vanadium oxides, namely Na_0.35V₂O₅·0.8(H₂O), K_0.36(H₃O)_0.15V₂O₅ and (NH₄)_0.37V₂O₅·0.15(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.