Aluminum-Ion Electrolytes with Weakly Coordinating Anions

Abstract

The conventional chloroaluminate ionic liquid electrolytes for rechargeable aluminum (Al) batteries are challenged by intrinsic limitations including corrosivity and poor chemical stability due to the active chloride content. Here, we report the first synthesis and characterizations of an Al “simple salt” electrolyte composed of aluminum hexafluorophosphate (Al(PF6)3) in dimethyl sulfoxide (DMSO). Al(PF6)3 salt was synthesized via the reaction between triethylaluminium (Et3Al) and ammonium hexafluorophosphate, and purified via recrystallization. The single crystal X-ray diffraction reveals that the Al3+ cation is solvated with six DMSO molecules (Al(DMSO)6(PF6)3) in the salt crystal structure. The 0.25 M Al(PF6)3 solution in DMSO demonstrates high ionic conductivity at approximately 1.3×10-2 S cm-1. With characterizations including nuclear magnetic resonance spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS), we demonstrate the reversibility of Al deposition-stripping in the electrolyte, which can be improved by the addition of trace amount of Et3Al as the electrolyte additive. The side reaction involving the reductive decomposition of DMSO to form aluminum oxide during Al deposition is identified by a combination of XPS and gas chromatography/electron ionization-mass spectrometry. Another Al-ion electrolyte with weakly coordinating tetrakis(hexafluoroisopropyloxy) borate ((B[hfip]4)-) anion in DME was also synthesized and characterized. These Al-ion electrolytes with weakly coordinating anions exhibit distinct chemical and electrochemical properties comparing with chloroaluminate ionic liquid electrolytes. They create an opportunity to develop new Al electrolyte system.