Overcharge studies of carbon fiber composite-based lithium-ion cells

Prototype lithium-ion pouch cells of 5.5 Ah have been fabricated with carbon fiber composite anodes, LiCoO2 cathodes, and LiPF6 electrolyte to investigate the overcharge characteristics of these cells at the 1C rate. The cells were made with anode to cathode capacity (A/C) ratios of 1.0 and 1.1. The cells were first examined for charge–discharge characteristics at different rates in order to determine the delivered capacity, specific energy and energy density and rate capability, and to ensure that the cells are suitable for overcharge studies. The current, voltage, and temperature responses during overcharge to 12 V were recorded. Maximum temperatures of 65 and 85 °C were observed with the cells with A/C equal to 1.1 and 1.0, respectively. The overcharged cells were dissected in an inert atmosphere and their components were analyzed using scanning electron microscopy and x-ray fluorescence spectroscopy. It is believed that a relatively low amount of heat is generated with carbon fiber composite-based lithium-ion cells and a separator shutdown mechanism is operative in the cell system which prevents fire or explosion during overcharge.