In recent years, lithium-air batteries have attracted significant interest due to increased energy density compared to currently available technologies such as lithium-ion batteries. The discharge process involves oxygen reduction to form LiO₂, followed by further reduction to Li₂O₂¹. Meanwhile, oxygen evolution from the discharge deposits occurs during charge. The poor conductivity of Li₂O₂² requires the use of high overpotentials during charge, leading to side reactions that hinder battery performance¹.

A significant amount of work has been devoted at improving battery performance via cathode³ and electrolyte engineering⁴. In addition, several experimental and computational studies have attempted to understand the nature of the charge and discharge reactions through the use of electrochemical cells^2,4-5. However, the complexity of electrochemical cells makes it difficult to gain fundamental insight of these processes. An alternative involves the use of ultrahigh vacuum (UHV) studies, where the ability to examine the influence of electrode potential on surface reactions is limited. This issue can be resolved by focusing on the effect of electric field on surface reactions since electric field is directly related to electrode potential. An example of a UHV technique is field ionization microscopy (FIM). In FIM, a Pt field emitter tip of approximately 350 Å radius is used as the substrate. The electric field on the surface is easily controllable by biasing the tip at moderate voltages (around 5 kV), to produce fields of up to 5 V/Å⁶. Reactions are carried out by adsorption of the species of interest (Li, O₂, and a solvent representative of the electrolyte) followed by application of a baseline field. Adsorbed reaction products and their morphology can be monitored by FIM and field electron microscopy (FEM). Reaction intermediates and products can be detected by pulsed field desorption time of flight mass spectrometry (PFD-MS). In PFD, reactions products are formed under a base field, followed by desorption when a field pulse is applied. By varying the base field magnitude, pulse repetition frequency, and reaction temperature, it is possible to obtain information such as activation energies and possible reaction steps⁷.

In this work, the kinetics of LiO_x consumption in acetonitrile, a low DN solvent, will be studied using PFD and FIM. Namely, the focus of this work will be to characterize the effect of electric field, temperature, and reaction time on the rate of Li₂O₂ consumption during charge, as well as characterizing the lifetime of the LiO₂ intermediate during charge.

References

1. Girishkumar, G; McCloskey, B; Luntz, A. C; Swanson, S; Wilcke, W. “Lithium-air battery: Promise and challenges”. Phys. Chem. Lett. 2010, 1, 2193–2203
2. Viswanathan, V; Thygesen, K. S; Hummelshøj, S; Nørskov, J. K; Girishkumar, G; McCloskey, B. D; Luntz, A. C. “Electrical Conductivity in Li₂O₂ and its role in determining capacity limitations in non-aqueous Li-O₂ batteries”. J Chem. Phys. 2011, 135, 214704
3. Kwak, W. J; Lau, K. C; Shin, C. D; Amine, K; Curtiss, L. A; Sun, Y. K. “AMo₂C/Carbon Nanotube Composite Cathode for Lithium-Oxygen Batteries with High Energy Efficiency and Long Cycle Life”. ACS Nano. 2015, 9, 4129–4137
4. Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. “Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium-Air Battery”. Phys. Chem. C. 2010, 114, 9178–9186
5. Laoire, C. O; Mukerjee, S; Abraham, K. M; Plichta, E. J; Hendrickson, M. A. “Elucidating the Mechanism of Oxygen Reduction for Lithium-Air Battery Applications”. Commun. 2011, 47, 9438-9440
6. Rothfuss, C. J; Medvedev, V. K; Stuve, E. M. “The influence of the surface electric field on water ionization: a two-step dissociative ionization and desorption mechanism for water ion cluster emission from a platinum field emitter tip”. Surface Science. 2003, 554-555, 133-143
7. Kruse, N. “Surface reaction kinetics on the atomic scale: studies by means of pulsed field desorption mass spectrometry”. Materials Science and Engineering A. 1999, 270, 75-82