Between January 7 and 14, incidents involving five Boeing 787 Dreamliners around the world spurred concern over the aircraft’s design and installed safety measures. Investigations revealed that the culprits were lithium-ion (Li) batteries on-board the aircraft. They had burnt, setting off smoke detectors and leading to emergency landings or groundings.
The primary reason for burn-ups was an issue called thermal runaway. In a Li battery, during every charge-discharge cycle, there is a small amount of charge lost, like an electron that evades the desired electrochemical reaction. “The amount of loss is a geometric progression. It starts off very low, but builds up fast over thousands of cycles,” explained Dr. Venkat Viswanathan, who will be an assistant professor of mechanical engineering at the Carnegie Mellon University this fall.
“Because that charge goes away into another reaction, it generates heat. So with each cycle, the battery becomes hotter and performs worse.” As the temperature increases, any flammable components start burning. One such is the electrolyte, the substance responsible for transporting electric charges inside the battery.
Alkyl carbonates are the conventional electrolytes in Li batteries, and they are highly flammable. Finding a non-flammable substitute is difficult because the newcomer has to be just as good as the carbonates in other aspects to not stunt the battery’s performance, especially since they are also used for renewable-energy storage and in electric vehicles.
“Non-carbonate based materials studied in literature rarely report comparable ionic conductivity as these alkyl carbonates,” said Dominica Wong, a graduate student in the University of North Carolina (UNC), in an email.
In a fortuitous turn of events, a replacement candidate may have been found lurking on the underside of ships.
Serendipitous
Joseph DeSimone, a chemistry professor at UNC, was studying a perfluoropolyether (PFPE) polymer to prevent marine organisms from sticking to the bottoms of ships, when he realized that its chemical properties were similar to electrolytes in Li batteries. With further research in his lab, his team found that lithium salts were soluble in it — a prerequisite. Most polymers are incapable of that.
During experiments, the electrolyte was sandwiched between two lithium foil electrodes, and the battery was subjected to different charge-discharge rates at different temperatures.
Under these conditions, Dr. Nitash Balsara, professor of chemical and biomolecular engineering at the University of California, Berkeley, said in an email, “Current densities are significantly lower than those of laptop and cellphone batteries but comparable to what is needed for stationary energy backup,” such as in solar power plants.
Dr. Balsara and his team studied the transport of lithium ions inside the battery.
A very high transference number — 0.91-1.0 as opposed to previous bests of about 0.5 — was also reported. According to Ms. Wong, “transference number of an ion is the fraction of the total conductivity that is carried by that ion during charging and discharging.” So, a value close to unity means better charge transport.
One reason this could have occurred, according to their paper, is that the fluorine in PFPE promotes oxygen atoms in the PFPE to become reluctant donors of electrons, allowing the positive lithium ion to flow more freely through it.
The researchers also reported a lower electrochemical polarization with PFPE against alkyl carbonates, which implies a higher derivable voltage each cycle.
The issue of flammability may have been resolved, but PFPE-batteries are some way off from hitting the market. While the amount of heat generated is proportional to the electrolyte’s flammability, researchers will now check if PFPE develops other undesirable characteristics during cycling.
There is also the cost: PFPE costs at least 10-times more than alkyl carbonates. However, Ms. Wong is hopeful that they will become cheaper because they are easier to produce, being stable in the presence of moisture and oxygen.
While these batteries are ubiquitous today in portable electronics because of improved failsafe measures, there is a larger background search for viable alternative electrolytes. In fact, PFPE isn't even the first polymer to have been considered, although it is the first nonflammable one with comparable properties to alkyl carbonates.
An American collaboration of universities and research centres, called the Joint Center for Energy Storage Research (JCESR), was set up in 2013. According to Dr. Viswanathan, one of JCESR's goals will be to compile an 'electrolyte genome': a database of all known electrolytes by their properties to make it easier for scientists and manufacturers to spot the substance best suited to their research or product needs.
