Investigation of « Cross-Talking » Phenomenon within Li-Ion Batteries

Abstract

Li-ion batteries are good candidates for energy storage devices in the field of transportation as they grant not only high energy and power densities to the systems but also long cycle-life. The graphite is commonly used as the anode because of its high lithium storage capacity whereas the LiFePO4 (LFP) is a cathode material of choice for transportation applications which require high level of safety. Besides, the LFP is thermally and chemically very stable, enables a good specific capacity (170 mAh/g) and is more environmentally friendly than most of the other materials of positive electrode. Nevertheless, LFP/C(gr) system suffers from a particular undesirable phenomenon, namely the "cross-talking". It consists in the iron dissolution from the LFP, the migration onto the graphite electrode and the subsequent contamination of the Solid Electrolyte Interphase (SEI), which protects the graphite from further reduction of electrolyte and a loss of active lithium. As a matter of fact, changes in physical and chemical properties of the SEI occur. Among possible alterations, permeability to either reducible species or electrons increases, enabling new SEI to grow and thus consuming more lithium. The available charge in the system LFP/C(gr) is then constantly decreased as long as the cross-talking occurs, and so is the cycle-life of the battery. Therefore, this LFP/C(gr)-specific phenomenon is a big concern especially considering the difference of capacity fading with other systems of batteries for which it is much lower. The aim of this work is to highlight, characterize and understand the cross-talking phenomenon through cycling of LFP/C(gr) coin cells. The Electrochemical Impedance Spectroscopy (EIS) technique will be used in order to assess the changes in physical and chemical properties of the LFP/C(gr) electrodes. EIS spectra from different steps of the electrodes lifetime under different conditions will be compared and discussed. By using symmetrical coin cells (LFP/LFP and C(gr)/C(gr)), it is possible to obtain EIS signal for each material, and a specific attention will be paid to the study of influence of the temperature on the impedance response. Additionally, physical parameters can be determined through EIS spectra such as activation energy of ionic conductivity and SEI thickness (from the complex capacitance representation). These electrochemical analyses will be completed by physical and chemical characterizations such as X-ray Fluorescence (XRF), Glow Discharge Optical Emission Spectroscopy (GD-OES) and RAMAN spectroscopy. Figure 1