Researchers have created a material that consists of carefully structured molecules designed to be particularly electrochemically stable in order to prevent the battery from losing energy to unwanted reactions. In this type of battery, called nonaqueous redox flow, energy is stored in negatively and positively charged solutions inside large tanks.
The molecular makeup of the energized solutions in the tanks plays a major role in how much energy the battery is able to produce. This work focused on designing the ideal molecule to dissolve in the positively charged tank. To maximize efficiency, researchers structured the molecule to hold as much energy as possible while also being stable enough to limit superfluous reactions.
The molecule’s reversibility, or its ability to be repeatedly charged and discharged, is the very property that allows flow batteries to function. During charging, molecules stored in the positively charged tank shed electrons through a process called oxidation. A problem arises when these now unstable, positively charged molecules begin to react with their surroundings, sapping the charge that would otherwise be stored in the tank and used for power. When it loses an electron, the molecule has a natural tendency to find another electron for a complete pair, and if they form a bond, it can no longer produce electricity.
The researchers were able to shut down a common energy-depleting side reaction using a process called bicyclic substitution that protects the most reactive parts of the molecule’s atomic scaffolding — somewhat like using insulation to cover exposed wires. Bicyclic substitution itself is not new, but this research was the first to apply it to battery materials. Previously, battery scientists used bulkier protective atomic chains to increase stability; however, these shields tended to suffocate the battery. Only half of the reactive molecular regions could be covered without eliminating the molecule’s ability to give off any energy at all. The battery suffered only a minimal loss of capacity after 150 cycles of charging and draining the battery, proving the high stability of the molecule.