Design and optimization of a natural graphite/iron phosphate lithium-ion cell

Abstract

This paper uses a model for a natural graphite/lithium hexafluoro phosphate (ethylene carbonate: diethyl carbonate)/iron phosphate lithium-ion cell in order to study its performance and aid in its optimization. The model is used to generate Ragone plots for various designs, where both the average power of the cell and the peak power, defined at 80% depth-of-discharge for a 30 s pulse, are evaluated. This allows us to assess the ability of this chemistry to achieve the U.S. Department of Energy goals. The model is then used to maximize the specific energy of the cell by optimizing the design for a fixed time of discharge. The cell was optimized for the porosity and thickness of the positive electrode, while holding constant the capacity ratio of the two electrodes, the thickness and porosity of the separator, the electrolyte concentration, and the porosity of the negative electrode. The effect of the capacity ratio was qualitatively examined. The optimization was performed for discharge times ranging from 10 h to 2 min in order to map the maximum performance of this chemistry under a wide operating range. The study allows us to gauge the ability of this chemistry to be used in a particular application. The optimized designs derived in this paper are expected to be a starting point for battery manufacturers and to help decrease the time to commercialization. (C) 2004 The Electrochemical Society