Understanding the Chemical Stability of Polymers for Lithium-Air Batteries

Recent studies have shown that many aprotic electrolytes used in lithiumair batteries are not stable against superoxide and peroxide species formed upon discharge and charge. However, the stability of polymers often used as binders and as electrolytes is poorly understood. In this work, we select a number of polymers heavily used in the Liair/Li-ion battery literature, and examine their stability, and the changes in molecular structure in the presence of commercial Li2O2. Of the polymers studied, poly(acrylonitrile) (PAN), poly(vinyl chloride) (PVC), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and poly(vinylpyrrolidone) (PVP) are reactive and unstable in the presence of Li2O2. The presence of the electrophilic nitrile group in PAN allows for nucleophilic attack by Li2O2 at the nitrile carbon, before further degradation of the polymer backbone. For the halogenated polymers, the presence of the electron-withdrawing halogens and adjacent alpha and beta hydrogen atoms that become electron-deficient due to hyperconjugation makes PVC, PVDF, and PVDF-HFP undergo dehydrohalogenation reactions with Li2O2. PVP is also reactive, but with much slower kinetics. On the other hand, the polymers poly(tetrafluoroethylene) (PTFE), Nafion, and poly(methyl methacrylate) (PMMA) appear stable against nucleophilic Li2O2 attack. The lack of labile hydrogen atoms and the poor leaving nature of the fluoride group allow for the stability of PTFE and Nafion, while the methyl and methoxy functionalities in PMMA reduce the number of potential reaction pathways for Li2O2 attack in PMMA. Poly(ethylene oxide) (PEO) appears relatively stable, but may undergo some cross-linking in the presence of Li2O2. Knowledge gained from this work will be essential in selecting and developing new polymers as stable binders and solid or gel electrolytes for lithium-air batteries.