The separator, while typically regarded as an inactive component of the battery, is widely recognized as being critical to cell efficacy and safety; the importance of their role in cell design and overall battery performance has become increasingly important with the move to higher energy density storage systems to meet industry needs.

The present paper outlines the many key functions that separators serve in lithium-ion batteries and describes a separator design – specifically, AlOOH-based nanocomposite membranes – which addresses several critical safety concerns for high performance lithium-ion batteries, while maintaining cell longevity. These nanoporous membranes exhibit an extremely narrow pore size distribution, outstanding dimensional stability, excellent compression resistance, and very high thermal conductivity.  Safety and cell performance characteristics are correlated with fundamental materials and separator design parameters using a range of analytical techniques, characterization methods, and cell abuse tests. Opportunities for enhanced functionality are discussed.