Electrochemical Impedance Spectroscopy of PEO-LATP Model Multilayers: Ionic Charge Transport and Transfer

Solid-state batteries are seen as a possible revolutionary technology, with increased safety and energy density compared to their liquid-electrolyte-based counterparts. Composite polymer/ceramic electrolytes are candidates of interest to develop a reliable solid-state battery due to the potential synergy between the organic (softness ensuring good interfaces) and inorganic (high ionic transport) material properties. Multilayers made of a polymer/ceramic/polymer assembly are model composite electrolytes to investigate ionic charge transport and transfer. Here, multilayer systems are thoroughly studied by electrochemical impedance spectroscopy (EIS) using poly(ethylene oxide) (PEO)-based polymer electrolytes and a NaSICON-based ceramic electrolyte. The EIS methodology allows the decomposition of the total polarization resistance (Rp) of the multilayer cell as being the sum of bulk electrolyte (migration, Rel), interfacial charge transfer (Rct), and diffusion resistance (Rdif), i.e., Rp = Rel + Rct + Rdif. The phenomena associated with Rel, Rct, and Rdif are well decoupled in frequencies, and none of the contributions is blocking for ionic transport. In addition, straightforward models to deduce Rel, Rdif, and t+ (cationic transference number) of the multilayer based on the transport properties of the polymer and ceramic electrolytes are proposed. A kinetic model based on the Butler-Volmer framework is also presented to model Rct and its dependency with the polymer electrolyte salt concentration (CLi+). Interestingly, the polymer/ceramic interfacial capacitance is found to be independent of CLi+.