Tonen Li-Ion Battery Separator Features Improved Permeability and Thermal Properties

21 May 2007

Tonen’s new separator films (in red) offer significant improvements in porosity (Gurley units—lower is better) and meltdown temperature (°C—higher is better). Click to enlarge.

The new separator film for lithium-ion batteries being introduced by Tonen Chemical, an affiliate of ExxonMobil Chemical, features significant improvements in porosity and thermal properties compared to its predecessors. (Earlier post.)

The improved porosity leads to lower internal resistance, desirable for smaller, high-power batteries. The improved meltdown property extends the integrity of the film longer during a thermal event, giving the cell more time to dissipate the heat, thereby increasing the possibility of avoiding a more serious event.

These separators offer the potential to make lithium-ion batteries lighter, smaller and more durable, contributing to system cost reduction and improvements in design flexibility. Tonen and ExxonMobil are targeting the new separators for use in batteries for hybrids and electric vehicles.

The new separators are the result of close collaboration between Tonen Chemical, with more than 20 years’ experience in separator research and process technology, and ExxonMobil Chemical, a global leader in polyolefin technology. Tonen supplied the microporous film for the first lithium-ion battery introduced in 1991 by Sony. Tonen’s current R&D manager, Koichi Kono—was involved with the original project.

Tonen and ExxonMobil extended the meltpoint by 40 degrees C. Click to enlarge.

Thermal properties. The potential for certain lithium-ion battery packs to burst into flame was highlighted by the recent spate of high-profile laptop battery pack recalls.  Automotive battery cell, module and pack makers are thus spending a great deal of time and effort on thermal management, including exploring different chemistries, additives and extra physical layers (e.g., Matsushita’s “Heat Resistant Layer” technology).

A shutdown separator can contribute to the overall thermal management solution. The separator is the film between cathode and anode materials, and the material through which the lithium ions flow. A shutdown separator closes off its micropores when an internal temperature threshold (130° C) is exceeded, essentially shutting down the activity of the battery.

Higher permeability (larger pore size, higher porosity) leads to lower internal resistance. Click to enlarge.

In a scenario where the internal temperature is rising rapidly, however, the separator can melt, thereby eliminating the barrier. ExxonMobil Chemical and Tonen developed a film that retained the original shutdown temperature, but that held its integrity for an additional 40 degrees C. By not melting until a higher temperature is reached, the new separator film provides more time for the malfunctioning cell to dissipate its heat, reducing the potential for transferring the heat to its neighbors and setting off a larger thermal event.

Porosity. In addition to its enhanced thermal stability, the new film offers improved porosity—51% in the current product. The new films have Gurley unit ratings of between 100-300 sec/100cc. The earlier films from Tonen had Gurley ratings of between 500 and 650 sec/100cc. A Gurley unit describes the number of seconds required for 100 cubic centimeters of air to pass through 1.0 square inch of a given material at a given pressure differential.