Safeguarding Lithium-ion Battery Separators
UL applied materials science to develop a testing approach that establishes a more robust evaluation of lithium-ion battery separators. These tests help ensure that lithium-ion battery design is safe for its intended use.
WHY THE LITHIUM-ION BATTERY SEPARATOR RECOGNITION PROGRAM MATTERS
The separator is a critical component within a lithium-ion battery 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 and infrastructure 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.
Research has shown that a thinner separator is critical to achieving higher energy and power density in a lithium-ion battery.1 Over the past two decades, the ongoing reduction of separator thickness and addition of active materials have contributed to a threefold increase in the energy density of lithium-ion batteries.2 These batteries now offer unmatched performance, which has helped lithium-ion become the battery chemistry of choice for a range of applications — first for consumer electronics, but more and more for automobiles and the electric power grid.3 Evidence of the growing role of lithium-ion batteries can be seen in the size of the market, which reached $11.7 billion in 2012 and is projected by Frost & Sullivan to double by 2016.4
One of the primary contributors to the safety risks of lithium-ion batteries is their use of flammable electrolyte solutions.5 This underscores the critical role the separator plays in preventing electronic contact of the electrodes while enabling free ionic transport and isolating the electronic flow within the battery.6 The reduction of the separator thickness to 18μm or less means that some lithium-ion batteries are now more susceptible to manufacturing defects and external damage.7 Specifically, this translates to a potentially increased risk of internal short circuits, which can lead to thermal runaway — the rapid buildup of heat inside the battery — that ignites the flammable electrolyte or raises the vapor pressure until the cell bursts.8
WHAT DID UL DO?
UL applied materials science to develop a comprehensive testing approach that establishes a more robust evaluation of lithium-ion battery separators. The key characteristics of the separator as it relates to potential safety risks and performance were identified. We then developed a Separator Recognition Program comprising a suite of tests — some of which already existed while others were developed specifically for the program — including permeability, thickness, material composition, tensile strength, puncture strength, dimensional stability, shutdown temperature and melting temperature.9 Component Recognition is a service UL offers that covers the evaluation of components or materials intended for incorporation and use in other end-use products or systems.10
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 some consistency in the evaluation of all the important separator parameters.11 Our recognition program outlines eight specific parameters of a lithium-ion battery separator that our research and experience show to be critical to both battery safety and performance.12 For five of the parameters, existing test protocols are ideal. For separator thickness, dimensional stability and shutdown temperature, we needed to create new test protocols.13
KEY PARAMETERS AND TESTS IN THE UL SEPARATOR RECOGNITION PROGRAM14
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. To implement the program, we are conducting and witnessing tests, and we also publish parameters in the UL online directory. Still in its early stages, the program is not yet fully utilized by the industry. However, today, lithium-ion battery and separator manufacturers can submit separators to UL for safety testing and certification. 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.