EPR STUDY OF ELECTROCHEMICAL LITHIUM INTERCALATION IN V2O5 CATHODES

The evolution of the cathode material of Li/V2O5 cells upon lithium intercalation is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. Below one lithium per V2O5(x < 1), where the electrochemical process is completely reversible, V(IV) ions of the epsilon-and delta-Li(x) V2O5 phases are detected by EPR. Within this intercalation range, the shape of the EPR signal is dominated by the strong exchange interactions between magnetically concentrated V(IV) ions of the Li(x) V2O5 phases. The intercalation range x > 1 is characterized by a new EPR signal attributed to magnetically diluted vanadyl ions VO2+. It is shown that these species do not belong to the Li(x) V2O5 matrix, but most probably originate from the liberation of surface vanadyl ions during the irreversible transformation of delta-Li(x) V2O5 into gamma-Li(x) V2O5. On the basis of these results, a mechanism to explain the partial irreversibility of the electrochemical processes is proposed. A third EPR signal is also observed for x greater-than-or-equal-to 1. This signal is attributed to an electron-hole defect in Li(x) V2O5, originating from a local charge compensation of the vanadyl vacancies.