Among Li-ion-conducting solid electrolytes, LLZO has received much attention due to its high ionic conductivity (∼10^-4-10^-3 S cm^-1) and stability in contact with lithium metal over a wide voltage window (∼0-5 V) [1]. While the application of Li metal as anode has been intensively investigated [2], the integration of LLZO with high-energy cathodes such as layered mixed metal oxides is a challenge in the development of SSBs [3]. Chemical reactions occur during cell production, especially at high processing temperatures required to obtain conformal contact, and accelerate chemical reactions and diffusion processes, leading to high interface resistances.

In this study, the interface between LLZO pellets with LiCoO­₂ (LCO) cathode is modified by applying different metal oxide interface coatings as artificial solid-electrolyte interphase (SEI). The coatings are ternary Li-Me-O interlayers (Me = Nb, Al, Ti) deposited on the LLZO pellets by magnetron RF sputtering and then coated with a thin-film LCO cathode.

Co-sintering at high temperatures establishes uniform physical contact between the electrolyte and the cathode. To demonstrate the electrochemical functionality of the cells and to study the charge transport dynamics at the interface, all cells were first galvanostatically cycled. In the next step, electrochemical impedance spectroscopy was performed on different cell setups (e.g., LCO|Li-Me-O|LLZO|Li) to describe the impedance characteristics of the interfacial dynamics at the cathode interface. We are able to show that interfacial resistance is improved by coatings on the interface not only during manufacture but also during operation. The EIS measurements show two half-circles for all systems - without and with Li-Me-O coating - for the LCO/LLZO half-cell stacks, indicating two Faradaic processes at the two interfaces. The artificially applied SEIs (e.g., Li-Nb-O) result in ten times lower charge transfer resistance compared to no coating, demonstrating the effectiveness of the Li-Nb-O interphase. It is also shown that the specific charge can be more than doubled (at higher C rates), which supports the EIS measurements. The most promising interlayers were studied by synchrotron-based characterization techniques such as X-ray absorption spectroscopy. In[roy1] this way, we hope to gain deeper insights into the behavior of passivation layers under stress and the effects of concentration polarization phenomena on SSB lifetime.