Influence of electrolyte additives on the formation of gaseous decomposition products in lithium-ion batteries

Fossil fuels are a limited energy source on earth, with much of it consumed by the automotive
sector. In recent years an alternative to conventional gasoline powered cars emerged in the
form of electric vehicles which are powered by electricity stored in batteries. Lithium-ion
batteries are known for their high energy density and long cycle life suitable for automotive
applications to fulfill consumer wishes such as high driving range at low cost. However,
especially in the beginning, lithium-ion batteries and their reputation suffered from safety issues
mainly caused by the decomposition of the electrolyte. Since the scale of batteries in
automotive vehicles adopted to much larger cells compared to mobile phones or laptops, it is
of utmost importance to improve safety in lithium-ion battery technology by identifying and
mitigating the effects of electrolyte decomposition in the battery cell.
The operando gas chromatography mass spectrometry (GC/MS) is a novel approach to
determine the composition of the gas-phase in lithium-ion batteries during operation. This
method provides the possibility to detect small decomposition products next to the electrolyte
solvent itself. By identifying and quantifying these analytes, it is possible to accurately deduce
the mechanism of decomposition reactions in state-of-the-art electrolytes. In this thesis, the
operando GC/MS will be further improved to increase the efficiency of the method.
Furthermore, the operating principle of electrolyte additives are investigated. For this purpose,
gas analysis is combined with electrochemical methods like impedance spectroscopy and
cycling data as well as post-mortem analysis like X-ray photoelectron spectroscopy (XPS).
First, these methods are used on the known electrolyte additives vinylene carbonate (VC) and
fluoroethylene carbonate (FEC) and the results are compared with reaction mechanisms from
the literature. Then, the lesser-known additive tris(trimethylsilyl)phosphite (TMSP) is
investigated with the same mode of operation which provides a confirmation and summary of
the mechanisms published in relation to this additive and in addition gives new insight into the
mechanism of the electrolyte additive.
These findings can be used to refine electrolyte compositions and tackle issues like longevity
and energy density of batteries. This can be achieved by stabilizing the electrolyte beyond the
4.5 V threshold that currently impedes the use of higher voltage electrode materials.
Furthermore, operando gas chromatography can be established as an important tool to
accurately determine the state-of-health (SOH) of batteries not only in an experimental stage
but also for real-time monitoring of industrially manufactured battery products.