Analysis of the Distribution of Relaxation Times (DRT) Responses of Li-Ion Cells as a Function of Their Preparation Conditions

Abstract

The distribution of relaxation times (DRT) approach was used to analyze the electrochemical impedance spectra obtained for lithium-ion cells as a function of their state of charge as well as the cell preparation conditions. The cells were made using LiFePO4 cathodes with PEDOT:PSS binder and Li anodes. The following parameters were varied: the load of the active mass with LiFePO4, the presence or absence of carbon nanotubes, the presence or absence of the carbon coating of the current collector, die-pressing of the active mass, as well as artificial degradation of the binder using thermal treatment. The results support the view adopted in the literature that the DRT response of Li-ion cells involves 4 main regions attributed to (i) contact resistance between the particles of the active mass or the particles and the current collector; (ii) ionic resistance of the active layer including that of SEI on the surface of the electrodes; (iii) faradaic resistance of the electrochemical charge transfer at the surface of the active material, and (iv) solid-state diffusion/transport of Li+ ions in the particles of active material. Moreover, we identified yet another contribution to the DRT spectra related to the ionic conductivity of the binder and electrolyte in the active mass of the cathode.