Lithium-Ion Battery Separators for Ionic-Liquid Electrolytes: A Review

Ionic liquids (ILs) are widely studied as a safer alternative electrolyte for lithium-ion batteries. The properties of IL electrolytes compared to conventional electrolytes make them more thermally stable, but they also have poor wetting with commercial separators. In a lithium-ion battery, the electrolyte should completely wet out the separator and electrodes to reduce the cell internal resistance. Investigations of cell materials with IL electrolytes have shown that the wetting issues in IL-electrolyte cells are most likely due to poor separator compatibility, not electrode compatibility. A compatible separator must be developed before IL electrolytes can be used in commercial lithium-ion batteries. Herein, separators for IL electrolytes, including commercial and novel separators, are reviewed. Separators with different processing methods, polymers, additives, and different IL electrolytes are considered. Collated, the separator studies show a strong correlation between ionic conductivity and membrane porosity, even more than the electrolyte type. The challenge of a suitable separator for IL electrolytes is not solved yet. Herein, it is revealed that a separator for IL electrolytes will most likely require a combination of high thermal and mechanical stability polymer, ceramic additives, and an optimized manufacturing process.