Electronic and Ionic Wirings Versus the Insertion Reaction Contributions to the Polarization in LiFePO[sub 4] Composite Electrodes

Abstract

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...

Factors determining the packing-limitation of active materials in the composite electrode of lithium-ion batteries

“…This result differs from the literature that sparse LiFePO 4 -based composite electrodes are suffer from capacity decrease due to the electronic transportation limitations [18], which can be ascribed relatively poor electronic conductivity of the active material LiFePO 4 as compared with NCM. When the electrode density is further increased to 3.36 g cm À3 , the specific capacity decreases and finally yields the specific capacity of 70 mAh g À1 at the minimum.…”

Direct Observation of Li‐Ion Transport in Electrodes under Nonequilibrium Conditions Using Neutron Depth Profiling

“…Typically, the ionic and electronic conductivity strongly depend on the porosity of the electrode matrix having an optimum around 35% porosity [ 25 ] which is typically aimed for in electrode preparation. Given the current results it is interesting to fi nd how nucleation, electronic, and ionic conduction play a role at variable cycling rates in other two phase materials.…”

Basic Electrochemical Performance of Pure LiMnPO4: a Comparison with Selected Conventional Insertion Materials

“…In the case of LFP-based electrode we briefly commented on this feature some time ago and termed it an "activation" phenomenon. 48 Later on, Lestriez et al 43 performed a more systematic analysis of the measured galvanostatic curves of LFP electrodes and confirmed the existence of the phenomenon. They have also observed a similar non-linear characteristic for the case of nano-sized Si-based negative electrode.…”

“…Namely, it is well known that the measured electrochemical performance of Li ion insertion electrodes is strongly affected by the electrode morphology (electrode thickness, porosity and packing density) which has impact on the course of overpotential curve and, consequently, on the obtained capacity. For the case of LFP based electrodes, this impact was systematically and thoroughly addressed by Lestriez et al 43,44 and later effectively demonstrated in a major practical high-rate improvement of Li 4 Ti 5 O 12 45 and of LFP 46 based electrodes. In the present specific case the effect of different wiring contribution could manifest itself through the higher density (approx.…”