Hexafluorophosphate-Based Solutions for Mg Batteries and the Importance of Chlorides

The selection of viable conventional magnesium salts in electrolyte solutions for Mg secondary batteries is very limited. Reversible magnesium deposition was demonstrated with only MgTFSI₂, in ethereal solutions. A recent report has suggested that Mg can be reversibly deposited from a solution of Mg(PF₆)₂ in tetrahydrofuran and acetonitrile. In this paper, we dispute that claim and show that PF₆^- anions passivate Mg anodes and completely inhibit any Mg deposition/dissolution process. We show that addition of chlorides suppresses the passivation phenomena and allows reversible Mg deposition/dissolution processes to commence. The Mg deposits have been examined via elemental analysis, scanning electron microscopy, and X-ray diffraction measurements, depicting a highly oriented, preferential Mg growth. This study evaluates the feasibility of employing PF₆-based electrolytes for Mg batteries and exemplifies the aptitude of chlorides for suppressing passivation phenomena.