Capacity degradation of lithium rechargeable batteries

Li rechargeable cells made with structural arrangement Li/membrane/Li _xV₂O₅ were examined under different charge states using ac impedance, environmental scanning electron microscope (ESEM), transmission electron microscope (TEM) and the high-resolution nuclear magnetic resonance (NMR). These states include charged, discharged, and over cycled. The lowest internal resistance was obtained from the cell at charged state; the resistance increased when the cell was discharged; and the highest resistance was obtained from the cell at over-cycled state. From the ESEM and TEM studies, it was found that the surface of cathode electrode was porous initially; however, it was coated with an amorphous film and porous features had also disappeared from the cell at over cycled state. In addition, higher concentration of aluminum was found at the surface of the cathode electrode in over-cycled cells. From NMR studies, Li ion signals, which correspond to Li ions in the liquid electrolyte, on the surface of Li_xV₂O ₅ cathode electrode, and inside the Li_xV₂O ₅ cathode, were obtained. The mechanisms for capacity degradation and cycle lifetime of the cell are discussed.