Elucidation and comparison of the effects of lithium salts on discharge chemistry of nonaqueous li-O2 Batteries

Iliksu, Merve; Sauer, Dirk Uwe (Thesis advisor); Pitsch, Heinz Günter (Thesis advisor)

Aachen : ISEA (2017, 2018)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA ; 95
Page(s)/Article-Nr.: xvi, 160 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2017

Abstract

In this work Li-O2 batteries have been investigated. Their theoretical specific energy is 3500 Wh/(kg of Li2O2). There are many challenges to overcome before commercializing the Li-O2 battery technology could take place such as poor rate capability and poor cycle life. It has been pointed out that electrolyte decomposition undermines the cell’s rechargeability and the insulating nature of the main discharge product Li2O2 limits the maximum capacity. In order to understand the electrochemical discharge behavior at different salt and solvent combinations, Galvanostatic tests were conducted with different concentrations of the salt in the solvent. Raman spectroscopy is employed to analyze the degree of order of the discharged cathode surfaces and discharge products. FT-IR measurements are used on the electrolyte and cathode to observe the formation of decomposition products. SEM images are produced to observe the changes in surface morphology pre- and post-discharge. When TEGDME (low DN solvent) is used, the discharge capacity of the cells increases until a certain concentration of LiNO3 (high DN salt), then plummets. Based on the complete ex-situ analysis using these techniques, it is proposed that an excessive amount of LiNO3 reacts with the carbon cathode surface, rendering it unsuitable for discharge product build-up. On the other hand when DMSO (high DN solvent) is used in the electrolyte, the solution mechanism is promoted by the solvent leading to a higher discharge capacity. Carbonate species are observed and is more prominent in cells with more LiTFSI, indicating that the salt promotes this manner.