Poly(vinylidene fluoride) (PVDF) Binder Degradation in Li-O 2 Batteries: A Consideration for the Characterization of Lithium Superoxide.

We show that a common Li-O2 battery cathode binder, poly(vinylidene fluoride) (PVDF), degrades in the presence of reduced oxygen species during Li-O2 discharge when adventitious impurities are present. This degradation process forms products that exhibit Raman shifts (∼1133 and 1525 cm-1 ) nearly identical to those reported to belong to lithium superoxide (LiO2 ), complicating the identification of LiO2 in Li-O2 batteries. We show that these peaks are not observed when characterizing extracted discharged cathodes that employ poly(tetrafluoroethylene) (PTFE) as a binder, even when used to bind iridium-decorated reduced graphene oxide (Ir-rGO)-based cathodes similar to those that reportedly stabilize bulk LiO2 formation. We confirm that for all extracted discharged cathodes on which the 1133 and 1525 cm-1 Raman shifts are observed, only a 2.0 e- /O2 process is identified during the discharge, and lithium peroxide (Li2 O2 ) is predominantly formed (along with typical parasitic side product formation). Our results strongly suggest that bulk, stable LiO2 formation via the 1 e- /O2 process is not an active discharge reaction in Li-O2 batteries.