Laminated Thin Li-Ion Batteries Using a Liquid Electrolyte

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-MCF͒ 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 LiBF 4 in an ethylene carbonate ͑EC͒/␥-butyrolactone ͑GBL͒ ͑1:3 by volume͒ 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 Ϫ20°C. The B-MCF anode in the LiBF 4 -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 Ϫ20°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 LiBF 4 -EC/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 LiBF 4 -EC/GBL electrolyte against the fully charged LiCoO 2 cathode material.Prismatic Li-ion batteries have been developed in order to achieve progress in terms of higher capacity, lighter weight, thinness, and safety. Recently, thin-film and prismatic polymer Li-ion batteries using polymer gel electrolytes were commercialized for some portable electronic devices. Polymer Li-ion batteries have the advantages of thinness and light weight due to the use of the laminated film bag. 1,2 However, polymer Li-ion batteries are subject to some problems in that their performance is inferior in some respects to that of Li-ion batteries. We developed a laminate-type thin Li-ion battery using a graphitized mesophase-pitch-based carbon fiber ͑MCF͒ anode whose performance and safety are superior to those of polymer Li-ion batteries and prismatic Li-ion batteries. 3 We named the thin Li-ion battery the advanced lithium-ion battery. The battery has been produced by A&T Battery Corporation for cellular phones since 2000. By using a high-viscosity organic liquid electrolyte with thermal stability, the MCF anode, and the laminated film bag, the thin Li-ion batteries with high performance and safety were fabricated as thin cells with thicknesses of 1.2 and 3.6 mm. 4 Herein, we report the cell performance and the characteristics of materials for the thin Li-ion batteries using a graphitized boron-doped mesophase-pitch-based carbon fiber ͑B-MCF͒ anode. ExperimentalThe thin Li-ion batteries were constructed by using the B-MCF anode, a LiCoO 2 cathode, an organic liquid electrolyte, a separator, and an aluminum-plastic laminated film bag as the c...

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