In situ X-ray absorption study of cycled ambigel v₂O ₅·nH₂O(n ≈ 0.5) Composite Cathodes

We examined the oxidation state and local structure of hydrated ambigel V₂O₅·0.5H₂O composite cathodes cycled in a nonaqueous electrolyte lithium cell. Changes in the oxidation state and local structure were monitored by X-ray absorption spectroscopy under in situ conditions. The cathodes had initial discharge capacities in the range 440-480 mAh/g, which correspond to an intercalation of 3.1-3.4 Li per V ₂O₅·0.5H₂O. Analyses of X-ray absorption near-edge structure data reveal that the average oxidation state of vanadium for discharged cathodes increased with cycling in the range of 1-17 cycles and then remained unchanged with further cycling. The oxidation state for charged cathodes remained relatively unchanged with cycling in the range of 1-40 cycles. The lack of strong contributions from higher coordination spheres in the Fourier transforms of extended X-ray absorption fine structure spectra of charged and discharged cathodes indicate that the amorphous nature of the material is retained during cycling. However, a significant increase in the amplitude of the V-O contribution is observed with prolonged cycling for both discharged and charged cathodes, which is likely due to the formation of irreversible phases with increased local symmetry for the V-O coordination geometry. The observed results for the ambigel material cycled in the range of 1-16 cycles are consistent with results previously observed for an aerogel material.