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Battery cell separator safety

UL 2591: Component certification for lithium-ion battery cell separators

A comprehensive testing approach that establishes a more robust evaluation of battery cell separators.

The importance of lithium-ion battery cell separators 
The separator is a critical component within a lithium-ion battery cell that supports its overall performance while facilitating its safe operation. As demand for lithium-ion batteries has grown across a broad array of consumer, industrial, infrastructure, and electric vehicle applications, so has the push to improve their energy density and battery life. To help improve battery performance, separators have been made progressively thinner and lighter, which has diminished the capability of some to perform critical functions such as preventing electrode contact or shutting the battery down when overheating occurs. The resultant safety risks could hinder the continued advancement of lithium-ion battery performance as well as the expansion of this sustainable energy source.

UL 2591 for battery cell separator safety
The key characteristics of the separator as it relates to potential safety risks and performance were identified. We then developed UL 2591 - Outline of Investigation for Battery Cell Separators - comprising a suite of tests — some of which already existed while others were developed specifically for the program.  First issued in 2009, it went through a major update in 2018.  The new 3rd issue of UL 2591 updates test procedures for battery cell separator materials in the areas of thickness, dimensional stability, shutdown and melting temperatures, air permeability, tensile strength and puncture strength. In addition, the revision adds new criteria for battery separator materials to address material porosity, pore size and distribution, wettability and heat of combustion. Recently UL has established a new Standard Technical Panel to govern UL 2591 and to make sure its requirements remain relevant to future industry needs.


Test Group






Pore size and pore size distribution 



Tensile Strength 

Puncture Strength 


Dimensional Stability 

Shutdown Temperature 

Melt Temperature


Heat of combustion 

Material Characterization 

FTIR, TGA, DSC, SEM/EDX, Ash Content, MFI, Sp. Gravity 


Why UL’s Separator Recognition Program matters?
UL’s Separator Recognition Program was designed to identify the key properties of a separator to help ensure that it is appropriate and safe for the scope of its intended use. We developed the program to help drive consistency in the evaluation of important separator parameters. Our recognition program outlines twelve specific parameters of a lithium-ion battery cell separator that our research and experience show to be critical to both battery safety and performance.

With this comprehensive set of parameters and corresponding test protocols, our Separator Recognition Program is intended to help manufacturers establish tighter controls on critical battery safety components.

The Separator Recognition Program extends UL’s commitment to ensure the safety of lithium-ion batteries, focusing on one specific and vital component. We are currently offering testing and certification services and publish parameters in UL Product iQ.  Importantly, there are no pass/fail criteria; instead, our program is focused on identifying the properties of tested separators. In this way, we can help manufacturers advance the performance of lithium-ion batteries while enhancing their safety.  

UL's Separator Recognition Program creates value for battery cell manufacturers by eliminating redundant testing and surveillance for the commercially available separators employed in their cells. It also adds confidence to supply chain integrity by having the separator testing and surveillance conducted where the separators are manufactured.


Download the White Paper to learn more.