Li local environments and the dynamics in the Li1+xV3O8 insertion electrode

The structural and electronic properties of the Li1+xV3O8 insertion electrode system with nominal range of have been explored through measurements of x-ray diffraction, magnetization and nuclear magnetic resonance (NMR) for 7Li. The system prepared electrochemically exhibits two kinds of phase depending on the composition: the gamma1-phase with has the octahedral (Li1) and tetrahedral (Li2) coordinations for the Li sites, and the gamma2-phase with has the octahedral coordination alone. These phases coexist for 1.5 < x < 3.5. Application of the one-dimensional chain model to the susceptibility data for the gamma2-phase provides reasonable results for the Li concentration and the exchange constant. For x = 0, the quadrupole parameters are determined precisely. Both the gamma1-phase and the gamma2-phase indicate the activation energies for the long-range motion of Li ions to be of 3 X 103 K at temperatures above 300 K, and the gamma2-phase has another narrowing process below 100 K. Magic-angle spinning (MAS) NMR spectra for the gamma1-phase reflect the site differentiation for Li; the Li1 shift originates from a negative spin transfer, while the effect for Li2 is less significant. The spin-lattice relaxation for x = 0 is explained with the quadrupole interaction mechanism. For Li1.25V3O8, which exhibits variable-range hopping-type transport anomalies at K, the phase transition for the Li local environments takes place at around Tc. The spectra for the gamma2-phase are different from those for the gamma1-phase, and the complicated relaxations likely originating from Li ions located in the cathode and the acetylene black, and the boundary between these substances, are observed.