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 (R_p) of the multilayer cell as being the sum of bulk electrolyte (migration, R_el), interfacial charge transfer (R_ct), and diffusion resistance (R_dif), i.e., R_p = R_el + R_ct + R_dif. The phenomena associated with R_el, R_ct, and R_dif are well decoupled in frequencies, and none of the contributions is blocking for ionic transport. In addition, straightforward models to deduce R_el, R_dif, 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 R_ct and its dependency with the polymer electrolyte salt concentration (C_Li). Interestingly, the polymer/ceramic interfacial capacitance is found to be independent of C_Li.