Improving the performance and rechargeability of Lithium-air batteries, which offer up to ten times more energy density than Li-ion batteries, has been the focus of much research in the last decade.^{1, 2} One of the ways to improve these batteries is to gain a deeper understanding of the oxygen reduction reactions (ORR) in non-aqueous electrolytes in order to find an optimum combination of Li salt and solvent that provides chemical stability and rechargeability. In this paper, we report the results of a study of the physical and electrochemical properties of N,N-dimethylacetamide (DMAc)-based electrolytes containing either lithium or tetrabutylammonium (TBA) salts (LiX and TBAX, where X= PF₆^-, CF₃SO₃^-, ClO₄^- and NO₃^-). The chemical structures of the salt solutions were determined from NMR and FTIR spectroscopic data. The spectra revealed that DMAc (CH₃CON(CH₃)₂) solvates the lithium salts via bonding between its carbonyl (C=O) group and Li⁺ to form ion pairs. In the case of Li salts with the anions PF₆^-, ClO₄^- or NO₃, the ion pairs are solvent separated while in the case of CF₃SO₃^-, it is a contact ion pair. On the other hand, the nature of the interactions between DMAc and the TBA⁺ ions appeared to be dipolar in agreement with its soft Lewis acidity.³ The conductivities of these electrolytes were measured as a function of temperature from -20 to 40 ^oC and correlated with their concentrations and viscosities. The conductivities plotted versus temperature showed a linear fit for Vogel-Tamman-Fulcher (VTF) relationship indicating a solvent-assisted ion conduction mechanism. Diffusion coefficient and solubility of O₂ in the various electrolytes measured using a microelectrode technique are reported in Table 1. Cyclic voltammograms (CV) for ORR on microelectrode in TBA/DMAc electrolytes (Figure 1A) showed steady-state limiting current behavior indicating that the ORR process is electrochemically reversible. In all Li/DMAc electrolytes the CVs (Figure 1B) on microelectrodes showed an electrochemical irreversible behavior. The irreversibility of the ORR process in Li/DMAc electrolytes is primarily caused by the deposition of insoluble reduction products on the electrode surface. The results obtained in this study correlates with the high donor number of DMAc (27.8) and comply with the Hard Soft acid base (HSAB) theory.^{3, 4}

References:

1. Abraham, K. Journal of The Electrochemical Society 2015, 162, (2), A3021-A3031.
2. Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.; Kozinsky, B.; Liedtke, R.; Ahmed, J.; Kojic, A. Journal of the Electrochemical Society 2011, 159, (2), R1-R30.
3. Laoire, C. O.; Mukerjee, S.; Abraham, K.; Plichta, E. J.; Hendrickson, M. A. The Journal of Physical Chemistry C 2010, 114, (19), 9178-9186.4. Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. Journal of The Electrochemical Society 2014, 161, (3), A381-A392.

Acknoledgement: Partial financial support for this work was provided by the US Army Cerdec thorugh subcontract No: GTS-S-15-015. A scholarship for Amell Alsudairi by the Saudi Arabian Cultural Mission is appreciated.