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Solid polymer electrolytes have been prepared by encapsulating plasticized polymer electrolytes based on poly(tetraethylene glycol diaerylate) into the pores of Celgard | mieroporous membranes. Electrolyte membranes with thicknesses of ~40 Fm have been prepared. The conductivity of the electrolyte is determined by the porosity of the membrane, and a conductivity of ~2 x I0 -4 ~I ~. cm -~ at room temperature has been demonstrated. These electrolytes showed good compatibility with Li and an electrochemical stability window spanning the range of 0.0 to ~4.5 V vs. Li+~i. The excellent chemical and electrochemical stabilities enabling the use of the membranes in rechargeable Li batteries have been confirmed by the discharge/charge cycling of Li/solid polymer eleetrolyte/LiMn~O4 cells at room temperature.

The electrochemical intercalation of Li into graphite has been studied in Li/polymer electrolyte/graphite cells using an in situ x-ray diffraction (XRD) technique. In cells containing an electrolyte of PAN(polyacrylonitrile)-EC(ethylene carbonate)-LiPF6, a minor irreversible reduction of the electrolyte is observed only during the first discharge. In these cells, Li is reversibly intercalated into graphite to form Li~ 0C6, principally at potentials between 0.2 and 0.0 V vs. Li+/Li. No evidence for the cointercalation of EC was obtained. In cells containing PAN-EC/PC(propylene carbonate)-LiPF6-based electrolyte, a massive reduction of electrolyte occurs during the first discharge at -0.8 V vs. Li+/Li, which precludes Li intercalation into graphite. In situ XRD data are consistent with the absence of the intercalation of PC or Li+(PC)~ solvates into the graphite lattice, either prior to or during the solvent reduction process. The latter appears to be a surface-catalyzed process, the extent of which is determined by a combination of thermodynamic and kinetic factors including the reduction potential of the electrolyte, and the passivating films which form on the graphite surface as a result of electrolyte reduction.