Composite Separators with Very High Garnet Content for Solid-State Batteries

Lithium-metal solid-state batteries are attractive as next generation of Li-ion batteries due to higher safety and potentially higher energy density. To improve processability, solid-composite separators combine advantages of inorganic and polymer separators in hybrid structure. We report a systematic approach to fabricate composite separators with high content (90–95 wt %) of ceramic Li-ion conducting Li_6.45Al_0.05La₃Zr_1.6Ta_0.4O₁₂ (LLZO) powder embedded in a polyethylene oxide (PEO)-LiTFSI (20 : 1) matrix and understand factors affecting their properties and performance. Separators with good mechanical flexibility and excellent thermal stability were obtained, by optimizing materials and processing parameters. It was found that PEO molecular weight strongly influences the microstructure and electrochemical properties of the separators. In optimized separator with 90 wt % of LLZO and PEO with Mw 300,000 g/mol, a total ionic conductivity of 1.4×10⁻⁵ S/cm at 60 °C was achieved. The ceramic-rich separator showed excellent long-term cycling stability for more than 460 cycles (1000 h) at 0.1 mA/cm² in Li/Li symmetrical cells and achieved a critical current density of 0.25 mA/cm². The separators also enabled initial discharge capacities of more than 160 mAh/g in full cells with Li metal anode and composite solid-state LiNi_0.6Co_0.2Mn_0.2O₂ cathode, although rapid capacity fade was observed after 10 cycles in fully solid-state configuration.