Laminated Thin Li-Ion Batteries Using a Liquid Electrolyte

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Abstract
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 LiBF4LiBF4 in an ethylene carbonate (EC)/γ-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°C, high boiling point of 216°C, low vapor pressure, and high conductivity of 2.1 mS/cm at −20°C. The B-MCF anode in the LiBF4­EC/GBLLiBF4­EC/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°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 LiBF4­EC/GBLLiBF4­EC/GBL electrolyte. The very low swelling and small evolution of gas under the high-temperature storage at 85°C were attributable to the stability of LiBF4­EC/GBLLiBF4­EC/GBL electrolyte against the fully charged LiCoO2LiCoO2 cathode material. © 2001 The Electrochemical Society. All rights reserved.