No abstract

Electrochemical intercalation of lithium into carbons has been studied using mesophase‐pitch‐based carbon fibers with different heat‐treatment temperatures, coke, and graphites as anodes for secondary lithium batteries. The variations in the average layer spacing and the voltage profile for the carbons with intercalating depend on the degree of graphitization. The intercalation into a more disordered carbon fiber heated at 900°C has been characterized as intercalation into the layer structure for 0

No abstract

The electrochemistry of lithium intercalation into a graphitized mesophase-pitch-based carbon fiber with a radial-like texture used as the anode material in rechargeable lithium-ion cells was characterized. The radial-like texture in the cross section of the carbon fiber contributed to the rapid diffusion of lithium ions, resulting in the high rate capability. The anode performance of the graphitized carbon fiber was superior to that of the graphite. Experimental flat-plate C/LiCoO2 lithium-ion cells using the graphitized carbon fiber anode exhibited a high mid-discharge voltage of 3.7 V, a high rate capability, and a long cycle life of more than 400 cycles at 2 mA/cm 2 during charge-discharge cycling between 4.2 and 2.7 V. The long cycle life obtained for the cell was due to no significant change in resistance associated with the passivating films on the graphitized carbon fiber with extended cycles. It was also demonstrated that A size C/LiCoO2 cells using the graphitized carbon fiber anode have excellent rate performance at discharge currents between 0.25 and 3 A, a large discharge capacity of 0.95 Ah, and a high energy density of 310 Wh/dm 3 and 120 Wh/kg.

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...