The lithium-ion battery fires in Boeing’s 787 Dreamliner and Tesla’s Model S electric car highlight the safety hazard inherent in the vehicular use of this energy-storage technology. The fact is, the liquid electrolytes—the often-forgotten but crucial stuff through which ions travel between anode and cathode—are quite flammable. But for battery makers, only organic solvents are cost-effective in the roles of dissolving the lithium salts, and then conducting the resulting lithium ions between electrodes.
And any resulting fire risk from using Li-ion batteries only grows with rising power output because the larger the unit—the more cells, and the bigger stack of cells—the greater the chance a fault from a short or other manufacturing defect occurring. And it is at such a hot spot that the electrolyte might spark into flames.
Now professors and students at the University of North Carolina - Chapel Hill have come up with a potential replacement electrolyte material that is nonflammable. It is still early, but the research work might in time prove to be a way to build better Li-ion batteries that do not combust at temperatures as high as 200°C (392°F), or even make feasible super-high-performance Li-air batteries.
The team has identified a class of nonflammable electrolytes that are based on functionalized perfluoropolyethers (PFPEs), a super slippery, "liquid Teflon" machine lubricant. The new formulation performed well as an electrolyte in laboratory test batteries. The polymer electrolyte also displays very high values for certain key ionic charge-mobility metrics, namely high transference numbers and low electrochemical polarization, both of which are thought to be indicative of extended battery life, the team reported recently in the Proceedings of the National Academy of Sciences.
Electrolytes are easy to forget. The technical press is chock full of reports on new Li-ion battery electrodes, and new separators, and novel electrochemistries or manufacturing techniques, but one on a new, potentially nonflammable electrolyte is rarer. For the past two decades Li-on battery makers have used as electrolytes organic solvents such as ethylene carbonate, dimethyl carbonate, or diethyl carbonate (all combustible), which are molecular “cousins of gasoline,” according to Joseph DeSimone, Chancellor’s Eminent Professor of Chemistry at UNC.
“There is a big demand for these batteries and a huge demand to make them safer,” he said. “Researchers have been looking to replace this electrolyte for years, but nobody had ever thought to use this material called perfluoropolyether, or PFPE, as the main electrolyte material in lithium-ion batteries before.”
The new electrolyte formulation came to light when a team in the materials division of DeSimone’s interdisciplinary research lab was working on developing anti-fouling coating for ship hulls, a substance that prevents marine life from sticking to the bottom of ocean-going surface vessels and submarines, said Dominica H.C. Wong, a third-year graduate student in polymer chemistry from Canada. The Office of Naval Research (ONR) supported the research.
“Joe, who’s a very creative thinker, recognized that PFPE, which we had been researching for the anti-fouling work, had a similar chemical structure to the electrolytes used in lithium-ion batteries,” Wong recalled. “Most polymers don’t mix with salt, but this one did. When we discovered that we could dissolve lithium salt in this polymer, and it was nonflammable, we decided to roll with it.”
Researchers had previously identified alternative nonflammable electrolytes for use in Li-ion batteries, but they were never fully compatible with the Li ions. “In addition to being nonflammable, PFPE exhibits very interesting properties such as its ion transport,” she said.
When the UNC team attached the PFPE to dimethyl carbonate (DMC), an electrolyte traditionally used in batteries, the resulting PFPE-DMC was a polymer that could move a battery’s ions with very high (90%) levels of lab-scale efficiency while remaining stable. That makes this alternative electrolyte stand out from the others.
The ONR, which is working on safe batteries as well, recognized the potential benefit and adapted its funding mission, allowing DeSimone to bring some battery people into the group. Nitash Balsara, Faculty Senior Scientist at Lawrence Berkeley National Laboratory and professor of chemical and biomolecular engineering at the University of California - Berkeley, and his team were asked to study the ion-transport properties of the electrolyte and determine the most compatible electrodes to use in an assembled battery. The resulting prototype cells were tested in the lab and showed good cycling performance.
Wong suspects that the fact that the electrically polar nature of the linked molecules imparts much of the duo’s special behavior in this application. On one side, the DMC electrolyte is hydrophilic and likes water whereas the polymer electrolyte component at the other end is hydrophobic, which enables the ensemble to act like a surfactant or detergent to, for example, encapsulate adjacent chemical species.
The other attraction is that unlike the existing electrolytes, “it’s stable in oxygen, which brings lithium-air batteries within possibility, which is very exciting,” Wong said.
“The Holy Grail in batteries is a lithium-air battery, which has the power density equivalent to a fuel tank,” DeSimone said. “Everyone’s been working on it, but one of the linchpins is that [regular] electrolytes aren’t compatible with oxygen.” The new PFPE electrolyte might make this long-sought class of power-dense batteries possible.
The UNC group is now focusing on optimizing the electrolyte’s conductivity and improving it battery cycling characteristics, Wong noted. The big uncertainties revolve around whether the new electrolyte can deliver battery capacity and longevity that are at least competitive with current products.
If successful, a commercial battery could also be used in extremely cold environments, including aerospace and deep-sea applications, because the electrolyte resists freezing as well as heat.
But any commercial applications are still years away.
“We’ll see what works,” said Wong. “We’re still exploring to find out what works best, but this is a really good starting point for us to go in a lot of different directions. The best part was the interdisciplinary collaboration—having the opportunity to work on scientific problems with researchers with different backgrounds and expertise.”