Poly(vinylidene fluoride) (PVDF) Binder Degradation in Li–O₂ 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 (Li₂O₂) 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.