This study, realized within the framework of the optimization of aqueous LiFePO4 composite electrodes, relies on Prosini’s approach [ J. Electrochem. Soc. 152 , A1925 (2005) ] that characterizes the LiFePO4/Li discharge behavior through simple equations. Two key parameters extracted from the LiFePO4 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.

The different contributions to the polarization of a LiFePO 4 electrode are experimentally discriminated in this work. The electrode total resistance is dominated at high rate by the contribution of the electronic and the ionic wires, the former being more important in the case of electrodes with low compaction, while the latter being more important in the case of electrodes with high compaction. A porosity in the 35%-40% range allows to minimize the electrode polarization. At low rate, the electrode resistance is dominated by the resistance to lithium insertion into the active mass and follows the predictions of M. Gaberscek and J. Jamnik ͓Solid State Ionics, 177, 2647 ͑2006͔͒. We show here that the resistance to lithium insertion decreases with the increase of the specific current, a feature that suggests an increase of the active particle conductivity with rate. The easy-handling methodology described in this work should enable a more rational optimization of the electrode formulation and processing conditions for better electrochemical performance.After the demonstration of its electrochemical activity by Padhi et al. ten years ago, 1 LiFePO 4 is now commercialized as the active material ͑AM͒ of new generation lithium-ion batteries positive electrodes. Its low cost, nontoxicity, and thermal stability have been the main LiFePO 4 assets of its fast and massive expansion. Minimization of the size 2,3 as well as conductive carbon coating of the LiFePO 4 particles 4,5 undertaken to overcome kinetic limitation in solid state transport processes enabled significant improvements of the electrochemical performance of LiFePO 4 -based electrodes. Recent studies have been carried out to free the materials from impurities, 6 or to better understand the air/water exposure effects that are important for the processing of the LiFePO 4 electrode, 7-13 especially for the design of binder formulations for an aqueous processing of the composite electrode. 13,14 To move further in the enhancement of energy and power performance, research now focuses on the optimization of the ionic and electronic wiring of the LiFePO 4 particles within the composite electrode. Several recent works have demonstrated that rate limitations to discharge capacity of LiFePO 4 -based electrodes mainly come from the ionic and the electronic wires, i.e., restrictions of the lithium ions diffusion within the electrode porosity and of the electrons transport through the electrode architecture to the active particles. [15][16][17] The electrode polarization is another very important issue in the performance of power oriented lithium ion batteries. 18 The possible different contributions to the potential drop involve ͑i͒ the electronic wires ͓the contacts at the current collector/composite electrode interface, the conductive additive/binder ͑C/B͒ network, the contacts between the C/B network and the AM mass, and finally the carbon coating͔, ͑ii͒ the ionic wires ͑the network of pores filled by the liquid electrolyte in the composite electrode͒, ͑iii͒ the resis...

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