(714d) The Impact of Electrolyte Solvent on the Stability of NaO2 Discharge Product at the Aprotic Electrolyte/Solid Interfaces of Na-O2 Batteries

Aprotic alkali metal-O₂ batteries (Li, Na-O₂) have gained significant attention as next generation energy storage devices due to their high theoretical energy densities and reversible redox chemistries¹. Na-O₂ batteries are among the most promising candidates due to the kinetic stabilization of the sodium superoxide (NaO­₂) phase on the cathode surface, resulting in low overpotentials during charge.^2,3 However, under cell resting conditions, disproportionation of the NaO­₂ phase is observed due to the underlying processes at the NaO₂-electrolyte solvent interface⁴. Strategies to mitigate this disproportionation include optimizing the electrolyte solvent-solute co-ordination to limit the interaction of the electrolyte solvent with the NaO₂ phase. However, such strategies provide limited fundamental insights into the elementary steps that dictate the disproportionation of the NaO₂ phase.

Herein, we investigate the impact of the solvation strength of the ether electrolyte solvent, governed by its molecular structure, on the disproportionation of the NaO₂ phase formed at the cathode surface. Experimental electrochemical studies and detailed characterization such as X-ray diffraction (XRD), scanning electron microscopy (SEM), RAMAN spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to probe such effects by determining the variations in the NaO₂ phase as a function of cell resting in different ether electrolyte solvents with varying solvation strengths. We find that the discharged NaO₂ phase is the most stable in solvents with strong solute-solvent interactions. This results in relatively lower charge overpotentials and higher cell efficiencies compared to solvents with weak solute-solvent interactions, which lead to significant disproportionation of the NaO₂ phase. To develop an understanding of the factors that govern the interactions between different ether electrolyte solvents and the NaO­₂ phase, theoretical calculations such as density functional theory (DFT) and Ab initio molecular dynamics (AIMD) are used. These insights are critical for the rational design of sustainable and efficient Na-O₂ batteries.

References:

1) Samira, S., et.al., ACS Energy Lett, 2021, 6 (2), 665-674.

2) Velinkar, K. K., et.al., ACS Energy Lett, 2023, 8 (11), 4555-4562.

3) Von Gunten, A., et.al., Chem. Mater., 2023, 35, 15, 5945-5952.

4) Kim, J., et.al., Nat Commun, 2016, 7 (1), 10670.