Improved Electromechanical Stability of the Li Metal/Garnet Ceramic Interface by a Solvent-Free Deposited OIPC Soft Layer

Ceramic electrolyte-based solid-state batteries suffer from instability at the Li metal–ceramic interface, resulting in poor and irregular lithium electrodeposition and high interfacial resistance. Here, we report the deposition, by spin coating, of an organic ionic plastic crystal (OIPC) soft layer on the surfaces of Li metal and a ceramic garnet Li7La3Zr2O12 (LLZO) electrolyte. This soft interfacial layer facilitates enhancement of Li-ion transport between Li metal and the ceramic electrolyte, increasing slightly the total conductivity of the composite OIPC–LLZO solid electrolyte (up to 1.1 × 10–3 S·cm–1) and reducing the area-specific resistance (ASR) (by up to five times, e.g., from 640 to 120 Ω·cm2). Such an achievement is crucial for the integration of solid inorganic electrolytes in all-solid-state batteries as well as the development of stable and efficient devices. The deposition of the OIPC thin layer (500 nm) was carried out by solvent-free spin coating, thus preventing any potential issues resulting from metallic lithium reacting with organic solvents. At room temperature, a solid and homogeneous soft layer was deposited between the Li metal anode and the LLZO ceramic electrolyte. The interfacial resistance was studied via SEM and EIS, and the evolution of Li transport between the two materials was followed by employing Li-ion stripping–plating experiments. Finally, this interfacial soft layer was integrated in a full cell (consisting of Li/OIPC/LLZO/OIPC/LFP) and demonstrated improved galvanostatic cycling performances due to the lower ASR.