Berkeley Lab battery researcher Gao Liu and his team earned a coveted R&D 100 Award this year for their invention of an electrically conductive rubbery adhesive that can be mixed with particles of silicon to form a battery's negative (-) electrode, or anode. Lithium-ion batteries whose anodes are built with this "conducting polymer binder" can have 30 percent more energy storage capacity than those with conventional anodes made with aggregated carbon particles. Now, by literally tinkering at the edges of this new polymer material, researchers have raised its performance another notch.
That was the promise of the technology, says Liu: "In addition to developing this binder, we developed a method to engineer and test ways to improve it. We are continuing to use those tools, and now we are approaching an ideal design."
In a paper published this month in the on-line edition of the Journal of the American Chemical Society, Wanli Yang, a beamline physicist at Berkeley Lab, and Gao Liu, both lead authors, and their colleagues describe how they modified the original binder, which was already an excellent conductor of electrons, to boost its capacity to transport positively (+) charged lithium ions. Because the flow of positive and negative charges in a battery is always balanced, the performance limits of the original polymer binder were determined by its less-than-ideal transport of lithium ions.
During a charging cycle, lithium ions are transported within the binder to the embedded silicon particles through the uptake of an electrolyte, which consists primarily of organic solvents filled with lithium ions in solution. By modifying the chemical structure of their original binder — adding "side chains" of ether molecules — they tripled its uptake of electrolyte solution. As a result of the improved ion flow, the specific capacity of the silicon anode made with the new binder rose to 3,750 mAh/g from the 2,100 mAh/g achieved by the original version. That 80 percent improvement meets the theoretical limit of a silicon anode's storage capacity. "It means we are using 100 percent of the silicon particles embedded in the conducting polymer binder," says Liu. "That makes it pretty close to the 'ideal' binder."
Liu's original polymer binder, which he calls PFM or PFFOMB, was notable for its combined traits of adhesion, elasticity, and electrical conductivity. The adhesion made the polymer stick to particles of silicon, which is preferable to graphite as an anode material because it can store ten times as much charge as carbon. Conductivity was essential, because without it the binder would simply insulate the silicon. The elasticity was crucial, because silicon literally swells to four times its size when it draws in lithium ions during battery charging, and then shrinks back to its original volume upon discharge. After just a few charge/discharge cycles, this breath-like movement would break conventional binders, ruining the battery. PFM's ability to accommodate this motion solved that problem, making higher-capacity silicon anodes for lithium ion batteries a practical alternative.
The improved binder, which the team calls PEFM, not only enhances lithium ion flow, it also maintains the elasticity and electron conductivity of the original; and as a bonus, the electrical traits of the added side chains actually improve the binder's adhesion to the silicon particles. "An ideal binder system should provide inherent electronic conductivity, mechanical adhesion and flexibility, and sufficient electrolyte uptake to warrant high ionic conductivity,'' says Liu. "The polymer we developed meets these challenges of an ideal binder system."
Liu says his team will continue to fine-tune its conducting polymer binders. The next goal is to find materials that offer comparable performance at lower cost. Significant testing will be required to determine that batteries made with the new silicon composite anodes can last as long as those made with graphite. To fully meet the needs of the next generation of electric vehicles and plug-in hybrids, the improved anodes must be coupled to improved cathodes, separators, electrolytes, and other components to make the truly "ideal" lithium ion batteries of the future.
The work was funded by the Office of Vehicle Technologies of the U.S. Department of Energy, under the Batteries for Advanced Transportation Technologies (BATT) program and by a University of California Discovery Grant.