Molecular structure, interactions, and dynamics of novel Li-battery electrolytes

Doctoral thesis, 2016

Lithium-ion batteries are one of the most promising candidates for energy storage in sustainable technologies such as electro-mobility or renewable energy systems. However, at present they are incapable to compete with the combustion engine to power vehicles in terms of capacity, price, and safety. The same shortcomings are also limiting the applicability in large-scale grid energy storage. A key component for improved performance is the electrolyte where a better understanding of the limiting mechanism at a molecular level is needed in order to achieve beyond state-of-the- art technology. This thesis focuses on understanding new types of electrolytes from structural and dynamical points of view. While the commonly used electrolytes consist of a mixture of organic solvents and moderate concentrations of lithium salts, recent studies suggest the use of super-concentrated electrolytes or the implementation of ionic liquids, as additives or as full replacement of the organic solvent, as promising development routes. A common feature of these new electrolytes is a complex structure with characteristic length scales exceeding those normally found in simple liquids. Due to their ionic nature and surfactant-like structure ionic liquids tend to display a mesoscopic ordering due to the competition between ionic and van der Waals interactions. Super-concentrated electrolytes on the other hand develop an ordering as a result of local coordination responsible for the solvation of the ions. This can also be expected to influence the dynamic behaviour and hence in the end the ion transport. In this thesis the structure and dynamics of these systems have been investigated using X-ray and neutron scattering techniques identifying common structural features between ionic liquids and super-concentrated electrolytes as well a multiple relaxation processes at different length scales. Moreover, the first solvation shells and molecular interactions were determined by Raman spectroscopy and the results linked to macroscopic properties, such as the phase behaviour and the ionic conductivity.