High-Resolution ⁷Li Solid-State NMR Study of Li _xV₂O₅ Cathode Electrodes for Li-Rechargeable Batteries

The Li_xV₂O₅ cathode of Li-rechargeable battery cells under three different charge states have been studied by high-resolution solid-state ⁷Li magic angle spinning (MAS) NMR spectroscopy. In the charged and discharged states, three different ⁷Li NMR resonances, corresponding to the Li⁺ ions in the electrolyte, in the V₂O₅ cathode, and on the surface of the V₂O₅ cathode, were identified by their spin-lattice relaxation times in inversion recovery experiments. Only signals of the Li ⁺ ions in the electrolyte were observed in the over charged state. It is shown experimentally that the Li⁺ ions in the electrolyte experience a dynamics or exchange process in a time scale of milliseconds with those in the V₂O₅ cathode, in particular for the discharged state, where a severe cross relaxation effect was observed in the inversion recovery for the Li⁺ ions in the electrolyte. It is concluded that such an exchange is mediated by the Li⁺ ions on the surface of the V₂O₅ cathode. Therefore, the surface structure of the V₂O₅ cathode electrode plays an important role in the reversibility of the Li⁺ ions in the rechargeable battery.