Hydride-Enhanced Plating and Stripping of Aluminum from Triflate -Based Organic Electrolytes

In the quest for developing rechargeable aluminum (Al) batteries, reversible Al electrodeposition in the absence of active-halide components (haloaluminate complexes such as, Al2Cl7–, AlCl4–, solvated AlCl3, and halide-anions such as, F–, Cl– and Br–) is considered an immense challenge. For this reason, the choice of electrolyte has been primarily limited to the highly corrosive chloroaluminate systems based on aluminum trichloride (AlCl3). In this work, we demonstrate reversible room-temperature Al plating from an active-halide-free (AHF) organic electrolyte based on aluminum trifluoromethanesulfonate (Al(OTF)3) and lithium aluminum hydride (LiAlH4) in tetrahydrofuran (THF), as well as its AlCl3 counterpart. Cyclic voltammetry measurements on a gold working electrode reveal oxidative stability of ca. 1.8 V (vs Al/Al3+). From insights obtained by DFT and FTIR, ionic speciation in the electrolyte is explored, and mechanisms for the underlying electrochemical processes in the OTF–-based electrolytes are proposed. Al electrodeposition and stripping were confirmed by optical microscopy, scanning electron microscopy (SEM) and X-ray Diffraction (XRD) spectroscopy. Characterizing the Al deposits from either the OTF–- or the Cl–-based electrolytes via depth-profile X-ray Photoelectron Spectroscopy (XPS) analyses, we find that these deposits consist of metallic Al, aluminum oxide (Al2O3), and either aluminum trifluoride (AlF3) or aluminum trichloride (AlCl3) contaminants arising from a reaction with the electrolyte components which occurs during the electrodeposition process.