Roll-to-roll fabrication and characterization of ultra-thin ceramic-coated separator for high-energy-density lithium-ion batteries

Ceramic-coated separators (CCSs) used in lithium-ion batteries (LIBs) play an important role in ensuring the safety of LIBs. However, conventional CCSs fabricated by coating the slurry consisting of ceramic particles and binder have limitations due to the micrometer-thick ceramic coating layer (CCL), compromising the energy density and power capabilities of LIBs. To address this relationship, ultra-thin CCSs fabricated by the sputtering process have attracted attention due to their high thermal stability even with nanometer-thick CCL. Nevertheless, batch-type sputtering machines cannot provide roll-based CCSs for commercial LIBs. Herein, we demonstrate the continuous fabrication of ultra-thin CCS rolls with a pilot-scale direct current (DC) roll-to-roll sputtering process. The ionic conductance and thermal stability are compared with the slurry-based CCS by controlling the coating thickness through the line speed. In addition, the electrochemical performance of the pouch cell (LiNi_0.6Co_0.2Mn_0.2O₂/graphite, 550 mAh) is evaluated to confirm the applicability of the roll-to-roll sputtered ultra-thin, binder-free CCS (R2R-UB-CCS) to LIBs. Owing to the nanometer-thick CCL, the pouch cell with the R2R-UB-CCS shows higher power capabilities than the bare polyethylene (PE) separator and slurry-based CCS. Thus, the roll-to-roll sputtering process for large-scale, high-speed mass production of ultra-thin CCS has the potential for high safety and high-energy-density LIBs.