Revealing the Impact of Anions in Lithium Oxygen Batteries

Abstract

Despite their high theoretical capacity, lithium oxygen cells cannot be fully discharged to the limit of their theoretical energy density. The main reason for that is the insoluble and insulating character of the discharge product Li2O2, when it is formed as a thin film form on the cathode, creating the passivation layers, and causing a stop of the charge transport. Furthermore, the discharge/side products can block the cathode’s pores, therefore limiting capacity too. Capacity is also limited by oxygen transport especially at higher current rates. The solubility of oxygen in the electrolyte and the diffusion is important in that case, and can be increased with the right choice of salt. A big challenge in building lithium oxygen batteries is choosing proper electrolyte for the system, which should be stable against species such as LiO2 -, LiO2, O2-, O2 2-. New combinations are necessary to improve issues such as minimizing electrolyte breakdown and/or enhancing the stability of the battery. In this work the electrochemical performances are investigated using constant current measurements and cyclic voltammetry of the lithium nitrate and bis(trifluoromethane) sulfonamide lithium salt mixtures in tetragylme. Furthermore, post mortem analysis in the form of Raman Spectroscopy, Scanning Electron Microcopy, and Energy – dispersive X ray Spectroscopy is performed. The contribution of this work is an analysis of the effect of various lithium salts, by maintaining the activity of Li+ but changing the anion. This yields insights into the extent of ion pairing and its effects on the Li-O2 battery system. Higher capacity results are coming from the increased stability and solubility of ion pair species that are formed in the electrolyte between the initial oxygen reduction reaction product (superoxide), O2 - and solvated the Li+. Figure 1. Constant current first discharge performances of LiNO3 and LiTFSI salt mixture concentrations and Scanning Electron Micrographs (scale 10µm) of their air cathode. Figure 1