Characterization of Novel Electrolytes for Li-Batteries

Licentiatavhandling, 2014

In a world that strives toward greener and more sustainable energy solutions, the need for suitable energy storage devices has brought Lithium-ion batteries (LIB) into focus of research. Although the Lithium-ion battery is a consolidated technology for portable applications, capacity, cost, and not the least safety aspects are major concerns when it comes to its implementation into large-scale energy storage applications, such as electric vehicles. Most of the hazards present in the current technology are strongly connected to the electrolyte used, generally a combination of organic solvents and the Li-salt lithium hexafluorophosphate (LiPF6). With the target to address the safety issues and improve performance of Lithium-ion batteries new electrolyte formulations have been investigated in this thesis, both from a molecular and a macroscopic point of view. The use of ionic liquids in battery electrolytes is one way of improving the safety. At first, we studied the structural changes in ionic liquids (ILs) upon Li-salt doping by means of small-angle X-ray scattering. Due to their ionic character and surfactant-like structure ILs tend to have a mesoscopic ordering unexpected for liquids. This order has recently been associated with the alkyl chain length of the cation. Hence, a systematic study varying the cation alkyl tail and the Li-salt concentration was performed. Furthermore, we investigated the effects of mixing organic solvents and ILs. This has been shown to be beneficial since the viscosity of the IL electrolyte is considerably reduced when adding organic solvents, but the addition of an IL also reduces the flammability of organic solvents. However, no clear picture is yet available on the role played by the different components at a molecular level. Thus, we have performed a thorough characterization of the full range, from a pure IL to a pure organic solvent based electrolyte, to identify the possible local configurations around lithium, and their influence on macroscopic properties of the electrolyte. Even though ionic liquids, or mixed electrolytes for that matter, are very promising they are still liquids and lack the mechanical rigidity needed in a battery. To avoid the use of passive separator membranes the implementation of gel electrolyte configurations has been proposed. In this thesis, two different routes to create gel electrolytes have been studied, a fumed silica-IL gel electrolyte and an organic solvent casted polymer-gel electrolyte. In the former, the influence of the addition of Li-salt on the colloidal stability of the system is studied. In the latter, a novel approach of in-situ gel-casting is proposed and monitored by FT-Raman and impedance spectroscopy.