Free-Standing Garnet-Type Solid Electrolyte Separators Fabricated By Water-Based Tape-Casting

The garnet-type Li-ion-conductor Li 7 La 3 Zr 2 O 12 (LLZO) is regarded as a promising oxide-ceramic based solid electrolyte (SE) material for all-solid-state lithium ion battery (Li-ASB) owing to its wide electrochemical stability window, chemical stability versus Li metal and excellent conductivity. However, one of the challenges regarding the fabrication of such ASBs is the industrial scalability of manufacturing methods. Tape-casting of ceramic components is known e.g. from SOFC manufacturing to enable the large-scale production of free-standing ceramic components and cells with low thickness. Additionally, in conventional tape-casting, organic solvents are usually employed, which give rise to health and safety issues and lead to high cost for solvent recovery. Hence, development of a green, water-based processing route is favorable. Herein, we report an aqueous tape-casting process of LLZO separators using a water-soluble biopolymer as binder. A lattice expansion of LLZO is observed by X-ray diffraction in aqueous green tape due to the partial replacement of Li + by H + , whereas the lattice parameter of LLZO in sintered sheet decreases to the same value of pristine LLZO powder, indicating the Li-recovery in the LLZO structure after sintering. This observation confirms that the Li + /H + -exchange reaction is reversible during the aqueous processing. The obtained free-standing LLZO sheet after sintering shows high ionic conductivity observed by impedance spectrometry in Li/LLZO/Li symmetric cell at room temperature, which is comparable with reported thin garnet separators fabricated by conventional non-aqueous tape-casting. In summary, we successfully fabricate self-supporting LLZO sheet with high ionic conductivity by aqueous tape-casting process, which is enabled by reversible Li + /H + -exchange. This free-standing LLZO sheet can be later used as separator in the Li-ASBs having an electrolyte-supported configuration.