Thin Li-ion batteries with a laminated film bag as a casing were developed by using a liquid electrolyte and a graphitized boron-doped mesophase-pitch-based carbon fiber (B-MCF) anode. The thin Li-ion batteries exhibited excellent discharge performance, long cycle life, and very low swelling under high-temperature storage. A 1.5 M solution of LiBF4 in an ethylene carbonate (EC)/gamma -butyrolactone (GBL) (1:3 by volume) mixed solvent is advantageous for use as the electrolyte in the laminated film bag because of its high flame point of 129 degrees C, high boiling point of 216 degrees C, low vapor pressure, and high conductivity of 2.1 mS/cm at -20 degrees C. The B-MCF anode in the LiBF4-EC/GBL electrolyte exhibited a high reversible capacity of 345 mAh/g, a high coulombic efficiency of 94% at the first cycle, and high rate capability. It was demonstrated that the thin Li-ion battery with a thickness of 3.6 mm has a high energy density of 172 Wh/kg, high rate capability between 0.2 and 3C rate discharge, a high capacity ratio of 50% at 1C rate discharge and -20 degrees C, and a long cycle life of more than 500 cycles at 1C rate charge-discharge cycling. The B-MCF anode led to the high rate discharge performance and the long cycle life of the thin Li-ion batteries using the LiBF4-EC/GBL electrolyte. The very low swelling and small evolution of gas under the high-temperature storage at 85 degreesC were attributable to the stability of LiBF4-EC/GBL electrolyte against the fully charged LiCoO2 cathode material.