Lithium-air batteries have attracted significant interest due to increased energy density relative to lithium-ion batteries. Currently, the lithium-air technology suffers from low capacity, high capacity fade, and high overpotentials due to parasitic reactions. The complexity of electrochemical cells makes it difficult to gain insight regarding the nature of the cathode reactions. Likewise, traditional ultrahigh vacuum (UHV) surface science cannot simulate the electrochemical environment inherent to battery chemistry. Instead, high electric fields, on the order of 1-3 V/Å, can be used to simulate an electrochemical environment, which in turn enables field ionization (FI) methods to examine surface chemistry and deposit morphology. The electrode/electrolyte interface is prepared on a Pt field emitter tip or a carbon-covered Pt tip of radius approximately 350 Å. Reactions are carried out by adsorption of the species of interest (Li, O2, and acetonitrile, representative of the electrolyte), shown in Fig. 1. The morphology of adsorbed reaction products (LiOx) can be monitored by field ionization microscopy (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). The PFD-MS technique is capable of measuring small coverages, a wide array of products, and both kinetic (activation energies) and thermodynamic (adsorption isotherms) quantities. Ramped field and stepped field desorption probe larger coverages and can follow processes that occur throughout several multilayers on the surface, such as solvation. In this work, the formation of LiOx deposits as a function of field, temperature, and coverage are studied with FIM and RFD. The differing field dependencies of LiOx deposit formation will be used to estimate relative rates of LiOx formation in the Li-O2 battery.

Surface reactions of lithium-oxygen batteries studied under electrochemical control with field ionization methods

253rd National Meeting (2017)

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