Impedance Spectroscopy Analysis of Sulfide Solid Electrolyte Composites for All-Solid-State Batteries

To further improve sulfide-based all-solid-state batteries, it is essential to gain a deeper understanding of the chemical and physical phenomena that occur during battery operation. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful, nondestructive technique for determining ionic and electronic conductivities of various electrochemical systems. However, despite the potentially high informative power of this method, EIS data interpretation can be challenging. One approach to addressing this issue is to utilize physics-based electrical equivalent circuits (EECs) to fit and analyze the obtained data. In this work, physics-based EECs are proposed and validated for a model solid electrolyte composite with Li6PS5Cl (LPSCl) as the sulfide solid electrolyte (SE) and hydrogenated poly(acrylonitrile-co-butadiene) (HNBR) as the binder, tested under blocking conditions. The EECs are validated by using various current collectors (tungsten carbide-cobalt, nickel, nickel-plated copper, and gold-sputtered nickel-plated copper) as well as HNBR binder contents of 2, 5, and 10 wt % in the SE composite films. Impedances due to conduction pathways and interfaces, including the LPSCl bulk, binder-free and binder-coated grain boundaries, and interfaces between the electrode and the composite, were quantified. We demonstrate that two distinct EECs are required to accurately model the impedance response across lower and mid-high binder contents, with the main differences observed in the high-frequency region. Finally, we determine the impact of binder content on the composites’ ionic conductivity: the addition of binder decreases the ionic conductivity of the SE composite film by more than 60% for the 5 wt % HNBR SE composite and by more than 90% for the 10 wt % HNBR SE composites compared to the 2 wt % HNBR binder composites.