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

New thin lithium-ion batteries (LIBs) using a laminated film bag, an organic liquid electrolyte, and a mesophase-pitch-based carbon fiber (MCF) anode exhibited high discharge performance, a very low swelling under a high-temperature storage, and excellent safety performance. A 1.5 M solution of LiBF4 in an ethylene carbonate (EC)/γ-butyrolactone (GBL) mixed solvent was found to be a promising liquid electrolyte for the thin LIBs because the electrolyte has a high flame point of 129°C, a high boiling point of 216°C, a low vapor pressure, and a high conductivity of 2.1 mS/cm at −20°C. It was demonstrated that the thin LIB using B-doped MCF anode has a high energy density of 172 Wh/kg, high rate capability, high capacity at −20°C, and a long cycle life of 500 cycles. The safety of thin LIBs under overcharging tests and oven tests was superior to that of conventional 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.