High-Viscosity Phase Inversion Separators for Freestanding and Direct-on-Electrode Manufacturing in Lithium-Ion Batteries

Separators play a critical role in lithium-ion batteries (LIBs) by facilitating lithium-ion (Li-ion) transport while enabling safe battery operation. However, commercial separators made from polypropylene (PP) or polyethylene (PE) impose a discrete processing step in current LIB manufacturing as they cannot be manufactured with the same slot-die coating process used to fabricate the electrodes. Moreover, commercial separators cannot accommodate newer manufacturing processes used to produce leading-edge microbatteries and flexible batteries with customized form factors. As a path toward rethinking LIB fabrication, we have developed a high-viscosity polymer composite separator slurry that enables the fabrication of both freestanding and direct-on-electrode films. A streamlined phase inversion process is used to impart porosity in cast separator films upon drying. To understand the impacts of material composition and rheology on phase inversion processing and separator performance, we investigated four different separator formulations. We used either diethylene glycol (DEG) or triethyl phosphate (TEP) as a nonsolvent, and either silica (SiO₂) or alumina (Al₂O₃) as an inorganic additive in a polyvinylidene fluoride-<i>co</i>-hexafluoropropylene (PVDF-HFP) matrix. Through a down-selection process, we developed a TEP-SiO₂ separator formulation that matched or outperformed a commercial Celgard 2325 (PP/PE/PP) separator and a Beyond Battery ceramic-coated PE (CC/PE/CC) separator under rate and cycle life tests in LiFePO₄|Li₄Ti₅O₁₂ (LFP|LTO) and LiNi_0.5Mn_0.3Co_0.2O₂|graphite (NMC-532|graphite) coin cells at C/10-1C rates. Our TEP-SiO₂ slurry had a viscosity of 298 Pa s at a 1 s^-1 shear rate and shear-thinning behavior. When deposited directly onto an LTO anode and cycled against an LFP cathode, the direct-on-electrode TEP-SiO₂ separator increased the specific capacity by 58% and 304% at 2C rates relative to the PP/PE/PP and CC/PE/CC separators, respectively. Additionally, the freestanding TEP-SiO₂ separator maintained dimensional stability when heated to 200 °C for 1 h and demonstrated a higher elastic modulus and hardness than the PP/PE/PP and CC/PE/CC separators when measured with nanoindentation.