Water-based fabrication of garnet-based solid electrolyte separators for solid-state lithium batteries

Garnet-type Li7La3Zr2O12 (LLZ) is regarded as a promising oxide-based solid electrolyte (SE) for solid-state lithium batteries (SSLBs) or other advanced Li-battery concepts like Li–air or Li–S batteries. A thin free-standing LLZ sheet can be fabricated by tape-casting and used e.g. as separators in SSLBs, since tape casting is an industrially established process and enables large-scale production of such SEs. However, organic solvents and additives employed in conventional slurry recipes for tape-casting give rise to health and safety concerns and also cause a high cost for solvent recovery. Hence, development of a green, water-based processing route can reduce both manufacturing costs and environmental footprint. In this work, we developed a tape-casting process for LLZ SEs using water as solvent, the water-soluble biopolymer methylcellulose as binder and other eco-friendly polymers as plasticizers. Although a Li+/H+ exchange takes place during our procedure, we demonstrate that the Li+/H+ exchange reaction is reversible in our procedure and results in the formation of stoichiometric cubic LLZ at the end. The obtained free-standing LLZ sheets with thickness of 150 μm and relative density of approx. 90% offer an ionic conductivity of 0.15 mS cm−1 at room temperature. We thereby prove that it is feasible to use water as dispersion medium and eco-friendly polymer additives for the fabrication of thin garnet-based SE layers.