Ionic vs Electronic Power Limitations and Analysis of the Fraction of Wired Grains in LiFePO[sub 4] Composite Electrodes

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Abstract
This study, realized within the framework of the optimization of aqueous LiFePO4LiFePO4 composite electrodes, relies on Prosini’s approach [ J. Electrochem. Soc. 152 , A1925 (2005) ] that characterizes the LiFePO4/LiLiFePO4/Li discharge behavior through simple equations. Two key parameters extracted from the LiFePO4LiFePO4 discharge curves are analyzed to determine the optimal electrode engineering and to interpret the origins of the electrode performance limitations. In particular, the calendaring step plays a critical role. Low packing results in electronic limitation, while the ionic contribution dominates for dense electrodes. The best compromise is achieved for an optimal porosity in the 30–35% volume range. A simple equation is proposed to predict the ionic limitations of rate performance from the electrode thickness and porosity, and the liquid electrolyte diffusion constant.

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