Stability of Different Binders and Their Effect on the Cycling Behavior of Li-O2 Cells

Li-O₂cells are very promising energy conversion devices because of the theoretical specific capacity that they could provide (1). Unfortunately they are also very challenging regarding the instability of their components, like electrolyte solvent (2-4) and carbon electrode (5, 6) mostly due to the reactivity of superoxide ion radical during discharge or other reactive oxygen species in charge.

Also the binder used in the O₂-electrode could provide reactivity, which could have a strong effect on the electrode reactions (7, 8), even though its effect on the Li-O₂ cell behavior has been not yet thoroughly studied in the literature. For example, many recent publications still use the poly(vinylidenedifluoride) (PVdF) as binder for the oxygen electrode, despite it is well known that it is subjected to dehydrofluorination in presence of superoxide ion radical (7, 8), producing the non-rechargeable product LiOH in presence of a catalyst (7). Nevertheless, it is not yet proven if this reactivity has an effect on the discharge capacity and cyclability of Li-O₂cells using only carbon as electrode material.

In this contribution, we examine the stability of different binders (like PVdF, Lithiated Nafion and PTFE) in presence of nascent superoxide ion radical, condition similar to the real working Li-O₂ cell. Such strongly reactive species is produced in-situ in presence of the binder, using a new method developed in our group. The binder degradation products soluble in organic solvent are detected by ¹H-NMR and ¹⁹F-NMR, while the insoluble products are checked with FTIR.

O₂-electrodes based on non-catalyzed carbon (e.g. Vulcan XC72) are produced using the different binders; the electrode actual porosities are studied using the BET technique. The discharge specific capacity and the cycling behavior of Li-O₂ are obtained in test cells, with 0.5 M LiTFSI in distilled diglyme as electrolyte. Furthermore, we measure the selectivity of the discharge and charge reactions using an in-situ Online Electrochemical Mass Spectrometry (OEMS) technique. This allows monitoring the evolution rates of discharge and charge products (theoretically Li₂O₂ and oxygen in the potential window where the electrolyte is stable, obtaining e^-/O₂ ratio) and their correlation with the potential in the cell. A comparison between the discharge specific capacity, the cycling behavior and the e^-/O₂ ratio in the case of the different binders is reported. These results are finally correlated with the output of the superoxide stability test, unveiling possible artifacts present in the literature.

Acknowledgments

Support of BASF SE in the framework of its scientific network on electrochemistry and batteries is acknowledged by TUM.

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