Electrochemical Impedance Spectroscopy (EIS) to Diagnose the Effect of Particle Morphology and As a Tool to Model the Aging Process of Li Batteries

Energy storage technology based on lithium is widely used in the world since their first commercialization in 1990. There are different types of technology depending on the positive electrode (LiCoO 2 , LiMnO 2 , LiNiMnCoO 2 , and LiFePO 4 ) and electrolyte materials. Although being considered as one of the leading technology for mobile and stationary power, Li batteries are not free from aging phenomena which have to be modeled and controlled. Using LiFePO 4 as our positive electrode material, diagnostic tools are being developed to study the aging of electrode materials (anode and cathode) for Li and Li – ion coin cells in order to predict their state of life. The lithium iron phosphate (LiFePO 4 ) is a cathode material with low conductivity, but good thermal stability and reversible lithium intercalation processes. The graphitic carbon layer forming around LiFePO 4 (LFP), as part of the synthesis process, induces a better conductivity and also improves resistance to aging. By comparing nano-sized and micron-sized materials, particle size also appears to be a parameter of great importance. Through Sol Gel [1] and plasma [2] methods, it was then possible to synthesize respectively micrometer (0.7 μm) and nanometer-sized LiFePO 4 materials with or without graphitic carbon layer to assess the effect of carbon layers and particle size on the aging process. Temperature, C-rate and the state of charge were chosen as parameters of accelerated aging. Electrochemical impedance spectroscopy (EIS) was our main tool of analysis. It is a rapid and versatile method to monitor charge (charge transfer and electrolyte resistance) and mass transfer parameters (diffusion coefficient) of cells. An equivalent circuit model based on diffusion in spherical coordinates was also developed to interpret the Nyquist diagram obtained at the beginning of cell life [3]. Post-mortem physicochemical analysis of the electrodes (anode and cathode) were carried out with Scanning Electron Microscope (SEM) in order to detect changes in the particle size and shape on the carbon anode, following the formation of the solid electrolyte interface (SEI), and a passivation layer on the LiFePO 4 cathode. Energy-Dispersive x-ray spectroscopy (EDX) analysis was also conducted to determine the distribution map of atoms on the surface of pristine and aged electrodes. Data from accelerated aging study will be further used to validate an aging model as a tool to predict the state of life of the lithium an