Solid-state batteries are next-generation energy storage systems that potentially can overcome the limits of conventional Li-ion batteries with respect to their energy density and by providing higher safety. From a materials perspective, sulfide-based solid electrolytes (SE) and cathode active materials (CAM) such as LiNixCoyMn1-x-yO2 (NCM) are considered as promising candidates to be employed in solid-state battery cathodes. However, their poor chemical compatibility leads to severe degradation at the SE|CAM interface: Oxygen release of the NCM at high state of charge leads to rock-salt-phase formation and a cathode electrolyte interphase (CEI) by oxidation of the solid electrolyte. These degradation effects are highly detrimental for the cell performance since they significantly increase the cell resistance and require the use of protective active material coatings.
Inspired by the natural CEI, we developed a new coating concept to tackle interfacial degradation in sulfide-based composite cathodes. An artificial CEI as coating layer is created prior to cell assembly that mimics the natural CEI but is prepared in a controlled and optimized way. During a simple, well scalable and cost-effective dry coating process, Li3PS4 is used as coating precursor on high-Ni polycrystalline NCM. We show that the composition and morphology of the coating layer can be tuned by tailored annealing which significantly impacts the electrochemical performance. The coating’s effect is systematically investigated with a combination of analytical analysis methods such as X-Ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) as well as electrochemical measurements. Further STEM-EDX investigations give additional insights into the coating morphology on the nanoscale. Depending on the annealing temperature, the artificial CEI coating can either result in a worsened performance or lead to the intended superior interfacial stability. In case of an optimized annealing process, interfacial degradation is found to be reduced compared to widely used LiNbO3 coatings. We believe that the concept of an artificial CEI offers great potential for further optimization and can serve as a cheap and effective coating approach to mitigate degradation between Ni-rich CAMs and sulfide electrolytes in solid-state batteries.