Researchers at Stanford University have developed a polymer that can prevent lithium ion batteries from overheating without permanently disabling them.
Lithium ion batteries have extremely high energy density, which has made them prone to combustion under adverse conditions or manufacturing defects. Lithium ion battery problems have resulted in injury and property damage, prompting to consumer product recalls, such as HP's 2011 battery recall and Sony's 2006 battery recall.
When such events occur on planes, like UPS Airlines Flight 6 in 2010, the results can be catastrophic. Consequently, the US Department of Transportation issued more stringent rules governing the transportation of lithium ion batteries in 2014, and airlines have sought further limitations.
However, rules governing the bulk shipment of lithium ion batteries by air fall to the International Civil Aviation Organization. In 2012, Congress passed a law that prevents the US from adopting lithium battery transportation rules more strict than the ICAO unless other lithium batteries have caused another transportation accident and casualties.
Technology may prove more effective than legislation in this instance.
In a paper published online Jan. 11 in Nature, Stanford researchers Zheng Chen, Po-Chun Hsu, Jeffrey Lopez, Yuzhang Li, John W. F. To, Nan Liu, Chao Wang, Sean C. Andrews, Jia Liu, Yi Cui, and Zhenan Bao describe the creation of a reversible thermoresponsive polymer than can shut down a battery that's beginning to overheat.
The polymer is highly conductive at room temperature, but its conductivity decreases by seven to eight orders of magnitude at a threshold temperature within one second. It is made of spiky nanoparticles of graphene-coated nickel. Under heat, the particles separate, thereby losing the ability to conduct electricity efficiently. The material thus turns itself off when it gets too hot, preventing a chain reaction known as thermal runaway that can lead to a fire or explosion.
The paper explains that, while other battery safety technologies have been developed, including separators, electrolyte additives, and polyfuse circuits, these defenses against overheating are either not reversible or have temperature limitations that constrain their utility. The new polymer regains its conductivity when the battery's temperature returns to an acceptable range. Safety thus doesn't demand battery suicide.
The polymer may not cover every possible hazard scenario for lithium ion batteries. But it should reduce incidents arising from overheating or shorting. Such technology may appeal to electric carmakers like Tesla Motors, which is planning to begin producing lithium ion batteries for cars and homes on a massive scale in 2017, once its Gigafactory opens. Though electric car fires are rare, they're an unwelcome spectacle when they occur.
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