UC Merced

Rechargeable batteries based on multivalent ions (Mg2+, Ca2+, Al3+) can have far-reaching applications such as portable electronics, electric vehicles, and grid storage. As the most abundant metal in the Earth’s crust, Al is an ideal candidate. Metallic Al anodes can, in theory, provide exceptionally high charge capacities. However, the lack of non-corrosive electrolytes has been a bottleneck in the advancement of a practical rechargeable battery. This dissertation discusses the author’s efforts to develop and understand organic electrolytes based on aluminum trifluoromethanesulfonate (referred to herein as Al-triflate or Al(OTF)3), an active-halide-free and commercially available Al salt. To investigate these systems, a wide range of computational and experimental techniques have been utilized, including density functional theory (DFT) calculations, Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). From these, insights into Al-ion speciation and electrochemistry are brough to light, and Al electrodeposition is successfully achieved from several electrolyte systems. Ultimately, the work presented in this dissertation successfully tackles reversible Al electrodeposition from chloride-free organic electrolytes, a major hurdle that has prevented the incorporation of these systems in rechargeable Al batteries. Chapter one includes a detailed review of historic and most recent developments in non-aqueous electrolyte chemistries for rechargeable aluminum batteries, including chloroaluminate ionic liquids and organic solutions. Chapter two reports the results of research on an Al(OTF)3 in tetrahydrofuran (THF) electrolyte. Computational modelling by means of DFT and a variety of experimental techniques have revealed the predicted and measured spectroscopic and electrochemical features of aluminum ions in this electrolyte. Aluminum was found to be electrochemically active, in addition to the presence of two concentration dependent ionic environments for triflate anions (OTF-). This work also introduces a method that provides a rationale for understanding redox potentials of Al-ions by comparing DFT calculations with cyclic voltammetry experiments. Chapter three discusses the effect of lithium chloride (LiCl) as an additive to the Al(OTF)3 in THF electrolyte to investigate the role of chloride ions on speciation and the electrochemical behavior of aluminum. Successful electrodeposition of aluminum was carried out from chloride (Cl-)-free (non-corrosive) and Cl--rich (highly-corrosive) Al electrolytes at room temperature, and a method to probe the progress of the reactions involving Al(OTF)3 and LiCl by utilizing the DFT-derived and FTIR-measured vibrational frequencies of the OTF- was introduced. Chapter four discusses the effect of hydrides (H-) on Al speciation and electrochemistry by employing a LiAlH4 additive to the Al(OTF)3/THF electrolyte. Reversible room-temperature Al electrodeposition is demonstrated and compared to the chloride-based electrolyte. Using the method developed in chapter 3, and from DFT and FTIR analyses, Al-hydride speciation in these systems is explored, and a mechanism for reversible Al electrodeposition in OTF--based electrolyte is proposed. Insight into the chemical composition of the Al deposits from the OTF-- and Cl--based electrolyte is revealed using depth-profile XPS analyses.