Magnetic resonance studies of chemically intercalated LixV2O5 aerogels

Abstract

Li 7 , V51 solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements have been performed upon chemically lithiated LixV2O5 aerogels, with compositions of 1.00<x<5.84. These compounds can intercalate reversibly large amounts of Li+ and, therefore, are of interest as battery cathodes. Still, the mechanism regarding the electron transfer from an inserted lithium metal to a host aerogel V2O5 and details regarding the lithium cation environments are not fully understood. LixV2O5 crystals are known to exhibit various structural phase changes and, when multiple phases are present, the capability of the material to intercalate reversibly appears to be adversely affected. On the other hand, aerogels have no such multiphase behavior and aerogel based cathodes exhibit greater stability upon cycling. NMR shows that neither the structure nor the dynamics vary greatly with the amount of lithium content, and that the lithiated aerogel is best described as a single-phase material. Characterization of lithium and vanadium sites is performed through analysis of both NMR and EPR spectra. Li7 line shapes are affected by first-order quadrupolar, magnetic dipolar interactions and motional narrowing. At and above room temperature, relaxation is governed primarily by a quadrupolar mechanism. NMR derived activation energies and diffusion coefficients are different from those of bronzes and electrochemically intercalated V2O5. V51 NMR lines, indicative of the presence of V5+ at all compositions, undergo diamagnetic shifts of up to about 50 ppm with an increase in lithium content. These results imply the presence of oxidized impurities or electronic charge delocalization. Additionally, EPR measurements provide evidence of VO2+ impurities and indirect evidence of nonbridging oxygen at high lithium contents.