(644e) Computational and Experimental Investigation of Stable Polymer-Based Electrolytes for Li-O2 Batteries

To develop solid-state polymer Li-O₂ batteries with long cycle life and high roundtrip efficiency, polymer-based electrolytes that are stable against oxygen and oxygen reduction products in a basic and oxidizing environment are needed. We developed computational and experimental screening tests to probe the stability of various organic molecules. The screening results and the structures of these organic molecules can be used to assist the structural design of stable polymer-based electrolytes. Four descriptors were computed by Density functional theory and used to screen the stability of the electrolyte a priori. We investigated four possible chemical reaction mechanisms, namely, hydrogen abstraction, deprotonation, nucleophilic attack, and base-induced elimination, by computing the bond dissociation energy (BDE), deprotonation energy (~pKa), and the energetics of nucleophilic/elimination reaction pathway(s). Chemically stable molecules should have high BDE and deprotonation free energy for every hydrogen/proton and high stability against nucleophilic attack or elimination. In addition, the oxidation and reduction potentials were computed to assess the electrochemical stability of the electrolyte components. Promising candidates should be stable against oxidation in the voltage range where the battery operates. In particular, we seek electrolyte components with oxidation potentials larger than 4.5 V vs Li.

The aforementioned computational screening test was performed on a library of small molecules with representative functional groups. The calculations were validated against experiments. In particular, the oxidation potentials of a range of lithium sulfonamide salts (side-chain candidates in the polymer design) were computed and compared to the electrochemical stability windows determined experimentally. Our comprehensive computational screening methods allow us to identify stable functional groups and select promising candidates for the polymer electrolyte backbone as well as side-chain groups. A number of chemically and electrochemically stable small molecules were identified, and their molecular structures provide valuable insight into the design of stable polymer electrolytes.