A team of U.S. Army scientists is working on developing more efficient batteries. Dr. Oleg Borodin, along with Dr. Arthur von Wald Cresce, Dr. Jarslaw Knap, Dr. Xiaoming Ren and Dr. Kang Xu, all from the U.S. Army Research Laboratory, researched modeling insights into battery electrolyte structure and stability.
"Lithium-ion batteries dominate energy storage for portable electronics and are penetrating automotive and grid-storage applications," Borodin said. "Further progress depends not only on the development of a new high capacity electrode but also on tailoring electrolytes in order to support fast and yet reversible lithium transport through the bulk electrolyte and across interfaces."
Borodin is a senior computational chemist at the ARL Electrochemistry Branch.
For batteries to work, electrolytes, a substance that is sandwiched between positive and negative electrodes, must conduct electric current in ionic form, while insulating any electron current. The properties of the electrolyte pre-determine how fast the battery can deliver power or absorb change and how long the battery can last.
One of these two factors must be met to achieve stability. Electrolytes must be either "thermodynamically stable with electrodes, or form a stable passivation layer that should be electronically insulating but ionically conducting while accommodating mechanical stresses due to electrode volume changes during battery cycling", according to Borodin.
Thermodynamic stability happens when a system is in its lowest energy state, or chemical equilibrium, with its environment. While thermodynamic stability is highly desired and most ideal, it can rarely be achieved in reality, and passivation is often the approach to stability. Passivation builds up a kinetic barrier and places the system in a meta-stable equilibrium with its environment.
"The Li-ion battery operates under the principle of this meta-stability," Xu said.
In recent years, the team, led by Xu and Borodin, has produced many electrolyte and battery innovations, including a new class of high-voltage aqueous electrolytes in collaboration with Prof. Chunsheng Wang, professor of Chemical and Biomolecular Engineering at the University of Maryland’s A. James Clark School of Engineering.
"We demonstrate that depending on their chemical structures, the anions could be designed to preferentially adsorb or desorb from the positive electrode with increasing electrode potential," Borodin concluded. "This provides additional leverage to dictate the order of anion oxidation and to effectively select a sacrificial anion for decomposition."