Characterization of composite cellulosic separators for rechargeable lithium-ion batteries

Thin composite cellulosic separators (39-85 mu m thickness), composed of fine fibrilliform cellulosic fibres (diameter 0.5 to 5.0 mu m) embedded in microporous (pore diameter: 10-200 nm) cellulosic film and soaked in ethylene carbonate or other aprotic solvent, are found to possess fair-to-moderate physical strength, an apparent complete freedom from pinholes, and a complex impedance equal to or lower than that of conventional polyolefin separators for rechargeable lithium-ion batteries. Laboratory-scale cells that comprise LiCoO2/petroleum coke electrodes, 1.0 M LiBF4/PC:EC:BL (25:25:50 by volume; propylene carbonate:ethylene carbonate:gamma-butyrolactone) electrolyte and the cellulosic separators exhibit good initial discharge capacity and capacity retention over 41 charge/discharge cycles. The latter are indistinguishable from those obtained from a similar cell with conventional polyolefin separator.