Adsorption of Aromatic Decomposition Products from Phenyl-Containing Magnesium-Ion Battery Electrolyte Solutions

Abstract

A series of solutions containing PhMgCl in THF solvent are examined electrochemically to investigate the adsorption of an electron-insulating layer. Solutions containing no additional salt, as well as those containing added MgCl2 or Al(OPh)3, show an initial anodic current response on a platinum working electrode poised between 2 and 4 V (vs Mg2+/0) followed by electrode passivation (minimal current on continued cycling) due to adsorption of aromatic polymer decomposition products on the platinum. On the other hand, PhMgCl–AlCl3 solutions do not demonstrate the adsorption of an aromatic species and show continuous electrolyte degradation. Once an adsorbed layer is formed, a significant increase in electrode impedance is observed (103 to 105 Ω) with no additional growth of the insulating film. A phenyl radical (Ph•) is deemed the culprit, as adding a phenyl anion source (Ph–) provides apparent 5 V (vs Mg2+/0) stability to nonpassivating electrolyte solutions. Through gel permeation chromatography, the adsorbed polymeric species has an average molar mass of either 50–300 g/mol or 200–800 g/mol depending on electrolysis conditions and electrode composition. Overall, this work shows that electrolytes containing phenyl constituents are nonideal for testing potential rechargeable Mg-ion battery components.