Characterization of Charge Reactions of Li-Air Battery Cathodes Studied with Field Ionization Methods
Authors: Valdes-Espinosa Honorio , Adler Stuart B. , Stuve Eric M. 
Published: February 2020

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 Li2O2
1. Meanwhile, oxygen evolution from the discharge deposits occurs during charge. The poor conductivity of Li2O2
2 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 cells2,4-5. 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 LiOx 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.

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