The Influence of Electric Field on Lithium-Oxygen Battery Cathode Reactions
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 increased capacity in lithium-air batteries is due to charge/discharge reactions involving the formation of lithium oxide (LiOx) species1 However, issues such as loss of capacity due to irreversible oxidation reactions2, low capacity compared to the maximum theoretical capacity, high charging overpotentials, and the presence of parasitic reactions have limited their commercial deployment. Most of these issues occur in the cathode, where the lithium-oxygen deposits form.

Recent work has focused in solving these issues via cathode engineering3-4 and improved electrolyte formulations5. In addition, there has been an increasing body of work aiming to understand the processes occurring in the cathode6-7. Most of these studies involve the use of electrochemical cells with subsequent characterization of the electrode. However, the complexity of electrochemical cells makes it difficult to gain insight regarding the nature of the cathode reactions. Instead, ultrahigh vacuum (UHV) surface science studies can be used to gain further insight regarding the surface reactions. In UHV studies, 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.

The effects of electric field on kinetics can be studied in a UHV system based on field ionization microscopy (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/Å8-9. The experiment is detailed in Fig. 1. 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 for a certain time and temperature. Adsorbed reaction products (LiOx) 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) and ramped field desorption (RFD). PFD-MS enables a survey scan of all species, while RFD probes the effects of electric field and temperature on surface reactions.

In this work, the formation of LiOx deposits as a function of field, temperature, and coverage are studied with FIM and RFD. Reactions are conducted on both Pt and carbon-coated Pt tips, the latter to simulate a Li-O2 cathode. The differing field dependencies of LiOx deposit formation for different x will be used to estimate relative rates of LiOx formation in the Li-O2 battery.

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Figure 1