Investigation of Charge Transfer Kinetics in Multilayer PEO/LLZO Solid-State Batteries

Lithium-metal batteries employing solid electrolytes (ceramics or polymers) could surpass the energy and power densities of current state-of-the-art lithium-ion batteries. Unfortunately, ceramic electrolyte/electrode interfaces suffer from poor interfacial contact, and polymer electrolytes show insufficient ionic conductivities for practical uses. Composite solid electrolytes, comprised of mixtures of ceramic and polymer electrolytes, could mitigate these challenges by combining high ionic conductivity with good interfacial contact. However, it is imperative to understand the kinetics of charge transfer at interfaces in composite solid electrolytes, as these can drastically affect the overall ion transport properties of such electrolytes. Here, we design a systematic study of charge transfer kinetics using multilayer LLZO/PEO (tantalum-doped lithium lanthanum zirconium oxide and poly(ethylene oxide)) solid electrolyte architectures as model systems for composite electrolytes. Electrochemical impedance spectroscopy and DC polarization measurements highlight the nonlinear charge transfer kinetics at Li/PEO as well as PEO/LLZO interfaces and show that charge transfer kinetics at each of these interfaces is limited by ion transfer in accordance with a Butler-Volmer model that incorporates a film resistance term. In addition, slow ion transport through the solid electrolyte interphase at Li/PEO interfaces and through contamination layers at LLZO/PEO interfaces are dominant sources of impedance, the latter of which can be significantly mitigated by decreasing interfacial contaminants through a high-temperature (700 °C) heat treatment of LLZO prior to battery assembly. These results provide new insights into the charge transfer kinetics at interfaces in multilayer and composite solid-state batteries and support the future design thereof.