The Chemical Stability of Polymers in Lithium-Air Batteries

To continue the technological advances in electric vehicles and other portable devices that lithium-ion batteries have enabled, it is important to investigate newer battery chemistries with much higher energy densities. Of the new battery chemistries currently being investigated, lithium-air (oxygen) batteries are particularly promising because of their high gravimetric energy densities; an order of magnitude greater than lithium-ion. However, several challenges such as electrolyte and positive electrode (e.g., carbon) instability has plagued possible commercialization. Our group has focused on developing solid-state polymer electrolytes for lithium-air batteries to address the failings of current organic liquid electrolytes such as their volatility, flammability, and instability in lithium-air cells. Before utilizing polymer electrolytes in lithium-air, it is important to study their chemical stability in a lithium-air environment. Using a subset of polymers (e.g., poly(acrylonitrile), poly(vinylidene fluoride)) that ubiquitously serve as polymer electrolytes and binders in lithium-ion batteries, we explore their stability in contact with commercial lithium peroxide to understand which polymer backbones and side group functionalities are especially susceptible to nucleophilic attack by the highly reactive superoxide and peroxide species in a lithium-air cell. The presence of an electronegative functional group on the polymer side chain and the presence of nearby hydrogen atoms that become electron deficient due to the electronegative functionality appears to make the polymer more susceptible to degradation in the presence of lithium peroxide. Using knowledge gained from this study, we can better design new polymer electrolytes that are not only ionically conducting, but are stable against chemical attack in a lithium-air cell. This bottom-up approach of understanding the stability of polymers and delineating reactive polymer functional groups will make it possible for smarter design of electrolytes and binders, and bring these novel batteries closer to commercial realization.