Safety films for batteries
Electrode separators that help prevent lithium-ion batteries from exploding at high temperatures have been created by Tonen Chemical Corporation, in Japan, an affiliate of USA-based ExxonMobil Chemical.
Lithium-ion batteries are lighter and smaller than nickel metal hydride alternatives, which makes them ideal for hybrid and electric vehicles (HEVs). But the threat of explosion means they are too risky for automotive use. Conventional battery separators are made from a polyethylene monolayer that often disintegrates when heated to around 150ºC, causing an electrical short and, at higher temperatures, a fire. The Tonen/Exxon separator has been created to improve the safety margin.
Made from proprietary porous polymers, the new separators insulate the cathode and anode electrodes electrically, while allowing free transport of ions between them.
‘The separator also works as a safety device, a type of fail-safe switch with fine and highly uniform micropores. If the cell becomes too hot [above 130ºC], the polymer separators melt, closing the micropores and shutting down the cell circuit ionically,’ explains Koichi Kono, Manager of R&D at Tonen.
This gives the battery a chance to cool off and prevents overheating.
When the traditional polyethylene separators are exposed to temperatures of 130ºC they shut down. Over 130ºC they start to melt completely, which accelerates overheating and leads to electrical shorts. If the temperature in the battery cell rises to 180ºC, the electrode materials begin to decompose, releasing oxygen, allowing the electrolyte to catch fire and the battery to explode.
The main benefit of the new separator is its higher melt integrity and meltdown temperature. ‘While the shutdown temperature is kept the same at about 130ºC, the meltdown temperature is improved to about 190ºC,’ says Kono.
The separators are based on Tonen’s wet processing co-extruded fabrication technology and ExxonMobil’s specially developed heat resistant polymers (the exact process and composition cannot be revealed).
‘To develop the film, we analysed 20 years of lithium-ion battery separators’ production data and adopted specific processing conditions with new film formulation on a co-extrusion line to produce separators that meet our target properties,’ Kono explains.
The separators were designed specifically for use in HEVs, he says. ‘They help make HEV batteries lighter, smaller and more durable, contributing to system cost reduction, safety and improvement in design flexibility.’
A number of lithium-ion battery manufacturers and US national laboratories are currently testing the separators, and preliminary feedback has been encouraging. ExxonMobile is now preparing to produce the films on a commercial scale.