New thin lithium-ion batteries using a liquid electrolyte with thermal stability

New thin lithium-ion batteries (LIBs) using a laminated film bag, an organi c liquid electrolyte, and a mesophase-pitch-based carbon fiber (MCF) anode exhibited high discharge performance, a very low swelling under a high-temp erature storage, and excellent safety performance. A 1.5 M solution of LiBF 4 in an ethylene carbonate (EC)ly-butyrolactone (GBL) mixed solvent was fou nd to be a promising liquid electrolyte for the thin LIBs because the elect rolyte has a high flame point of 129 degreesC, a high boiling point of 216 degreesC, a low vapor pressure, and a high conductivity of 2.1 mS/cm at -20 degreesC. It was demonstrated that the thin LIE using B-doped MCF anode ha s a high energy density of 172 Wh/kg, high rate capability, high capacity a t -20 degreesC, and a long cycle Life of 500 cycles. The safety of thin LIB s under overcharging tests and oven tests was superior to that of conventio nal prismatic LIBs and PLBs. The very low swelling and the excellent safety performance were attributable to the stability of LiBF4-EC/GBL electrolyte against LiCoO2 cathode material. (C) 2001 Elsevier Science B.V. All rights reserved.