The interest in secondary Li/O₂ batteries has grown rapidly over the past two decades, as they exhibit a practically achieveable specific energy of about 1,700 Wh/kg, which equals that of gasoline and is well beyond those of conventional lithium ion, Ni metal hydride and Zn/air batteries.^1,2 The large energy density is due to the very light-weight cell components: a Li metal anode, a porous C cathode (gas diffusion electrode, GDE) and gaseous O₂ as active cathode material. During discharge LiO₂ and Li₂O₂ (as well as Li₂O) are formed via the oxygen reduction reaction (ORR) in the porous carbon matrix. These products are re-oxidized upon recharge.

The major drawback of this seemingly simple system, however, is the reactivity of the organic electrolytes with almost all battery components and hence several decomposition reactions. The resulting side products are deposited in the GDE pores, leading to a minimized active electrode surface and ultimately cell failure upon continued cycling.³

Therefore, information on the electrolyte-dependent ORR and/or decomposition products deposited in the GDE during discharge is needed for an in-depth investigation of the causes for early battery failure. In order to achieve this, galvanostatic cycling and CV measurements were conducted with 1 M mixtures of LiTFSI/DMSO and LiTFSI/TEGDME to analyze the electrolyte influence on the cycling behavior. At different states of charge, the GDE surface and pores were investigated by SEM and XRD to obtain information about the product deposits on and in the carbon structure.

Galvanostatic cycling measurements show a larger discharge capacity and lower discharge overpotential with LiTFSI/DMSO as well as a better recharge behavior in the first cycle (see Fig. 1.a). The TEGDME-based electrolyte on the other hand exhibits a higher long-term stability, whereas the discharge capacity of LiTFSI/DMSO decreases drastically already upon the second discharge (see Fig. 1.b). CV measurements confirm these absolute and relative trends of the discharge capacities (see inset in Fig. 1.b).

A SEM investigation of the GDE surfaces after first discharge reveals a considerable difference in discharge product morphology depending on the electrolyte. This is in agreement to previously published reports:^4,5 with DMSO as solvent the product consists of toroidal particles with diameters of approximately 200 nm (see Fig. 1.c), whereas discharge with the TEGDME-based electrolyte yields a product layer on the porous cathode surface (see Fig. 1d). Differential capacity analysis confirmed the difference in discharge product formation: with LiTFSI/TEGDME two reduction processes were detected to contribute to the discharge capacity, whereas discharge with LiTFSI/DMSO contains only one reduction process. X-ray diffraction after the first discharge with LiTFSI/DMSO shows the presence of crystalline Li₂O₂ as well as a minor amount of LiOH in the GDE (see Fig. 1.e). The discharge with the TEGDME-based electrolyte, on the other hand, did not yield crystalline Li₂O₂ but only a small amount of LiOH. FIB-SEM measurements after the first discharge with each electrolytes furthermore showed that the GDE pores were almost completely blocked by bulk product.

Despite the absence of crystalline Li₂O₂ a comparison of SEM images after recharge with both electrolytes revealed a better cathode surface as well as pore recovery when using LiTFSI/TEGDME. This fact is most probably the reason for the significantly higher cycling stability with this electrolyte.

In order to obtain a more complete picture on the obtained results, the replacement of LiTFSI by other Li⁺ ion conducting salts, e.g., LiClO₄ and LiNO₃, as well as the use of electrolyte additives, such as the promising tetrathiafulvalene and dimethylphenazine,⁶ will be investigated and compared to LiTFSI/DMSO and LiTFSI/TEGDME.

Acknowledgments

The Research Council of Norway is acknowledged for the financial support within the framework of the research project "Optimized electrode-electrolyte interfaces for Li-air batteries" (contract number 240 866). The Research Council of Norway is also acknowledged for the support to the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50.

References

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2. Yao et al., Chem. Int. Ed. (2016) 55, 11344-11353.
3. Sharon et al., J. Chem. (2015) 55, 508-520.
4. Aetukuri et al., Chem. (2014) 7, 50-56.
5. Mitchell et al., Phys. Chem. Lett. (2013) 4, 1060-1064.
6. Lim et al., Nature Energy (2016) 1, 16066.