Structure and Dynamics in Ionic Liquid and Highly Concentrated Electrolytes

Licentiatavhandling, 2020

The electrolyte is a crucial part of a battery in terms of longevity and safety. However, the state-of-the-art electrolytes for lithium-ion batteries are based on organic solvents and Li-salts (typically 1M concentration) and are known to be volatile and to degrade at higher temperature. In the search for a safer electrolyte, highly concentrated electrolytes (HCEs) and ionic liquids (ILs) have been proposed as alternatives. The high salt concentration in HCEs (typically >4M) results in an increased electrochemical stability whereas ionic liquids, consisting only of ions, are known to have a negligible vapour pressure and high thermal stability. A common feature for HCEs and ILs is an ordering on mesoscopic length scales, normally not found in simple liquids, resulting from correlations between the ions. This nanostructure can be expected to influence the ion transport and a key to develop these new electrolyte concepts is to understand the structure and dynamics on the molecular level and how this links to macroscopic transport properties.

The thesis focuses on the understanding of mesoscopic structure and dynamics in ILs and HCEs with the help of neutron and X-ray scattering with the aim to identify how local dynamical processes are influenced by the nanostrucutre. I have investigated an archetypal HCE system where the Li-salt LiTFSI is dissolved in acetonitrile and a model ionic liquid. Varying the Li-salt concentration in the HCE we can link the local processes to the development of the structure. The ion transport in the HCE takes place by the means of a jump diffusion and is highly dependent on the salt concentration and temperature of the system. For the ionic liquid we investigate the response of structure and dynamics to changes in both pressure and temperature with a particular focus on state points (P,T) where the macroscopic dynamics i.e. conductivity is constant. A conned diusion was found with a diusion coecient in agreement with macroscopic conductivity, thus providing a link between the microscopic and macroscopic dynamics.