58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

            The original objective of this project was to test the hypothesis that sol-gel encapsulated ionic liquids (ionogels) can be prepared with high aluminum ion conductivity (~10^-3 S/cm at room temperature) and high aluminum ion concentration (>1 mol/liter), and that aluminum reversibly reacts with nanocrystalline V₂O₅ or MnO₂ from these liquids, paving the way for rechargeable aluminum batteries.

            This hypothesis was supported by both our preliminary results prior to proposal submission, as well as work published by other groups. The majority of this prior work showed that an electrochemical cell using an aluminum anode, a V₂O₅ cathode, and an electrolyte based on the 1-ethyl-3-methylimidazolium chloride:aluminum chloride ionic liquid behaved as a rechargeable battery, albeit with lower than expected energy density. However, after further experimentation, we were able to show that the previous reports were incorrect, and that the electrochemical cell was not acting as a rechargeable battery. Instead, all of the electrochemical activity was due to corrosion of the stainless steel that was used as a current collector, and this corrosion was a result of the high concentration of chloride in the ionic liquid. These results were published in the Journal of the Electrochemical Society.

            Due to the deleterious corrosion induced by the chloride, we have begun work on finding an alternative, halogen-free electrolyte that allows for aluminum electrodeposition. We are currently working on two systems, both using aluminum triflate as the aluminum salt because triflate is a much weaker Lewis base than chloride, and so should be far less active in the solution. In the first system, we have been using a series organic solvents based on polyethylene glycol to better understand the fundamental physics and chemistry of aluminum ions in solution, while in the second we have been working with ionic liquids based on the bis(trifluoromethane)sulfonimide (TFSI) anion (primarily 1-ethyl-3-methylimidazolium TFSI and 1-butyl-1-methylpyrrolidinium TFSI) to understand how the addition of aluminum triflate affect the viscosity, electrochemical window, and ionic conductivity of the ionic liquids.

            In addition to our current work on electrolytes, further experimental work, coupled with some computational work, on the V₂O₅ and MnO₂ cathodes has shown that transition oxides are unlikely to be viable cathode materials for aluminum-ion rechargeable batteries. Therefore, we have spent the last five months working to identify other possible cathodes for an aluminum battery, and we have settled on two possibilities, both based on sulfur. The first is molybdenum disulfide, which, due to its layered structure, allows for the possibility of ion intercalation between the sulfur sheets. The second possibility we are investigating is a catholyte based on polysulfides, similar to recent work on lithium-sulfur batteries.

Impact

            While this work has so far only led to one publication, it has had a number of impacts on both the PI’s research and the graduate student supported by the grant. In terms of impact on the PI’s research, this grant has allowed the group’s research efforts to move in a completely new direction. Prior to this grant the majority of the work in the PI’s lab was focused on nanowire synthesis for solar cells, while our current research is now equally balanced between the solar research and battery materials. Furthermore, in the last year we have been able to gather enough results to justify research proposals that are being submitted to NSF, DOE, and DoD. Finally, both the grant and the publication have further strengthened the PI’s tenure package, which was submitted this past summer.

            In terms of impact on the supported graduate student, this grant has enabled him to focus on research, rather than splitting time between research and work as a teaching assistant, and as a result the work accomplished so far, plus the work currently in progress, will serve as the majority of his Ph.D. dissertation, which will be written and defended within the next year.