Metalloporphyrin-Based Cathode for Rechargeable Magnesium-Ion Batteries: Copper Leaching and Interphase Formation

[5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) composite cathodes for rechargeable magnesium-ion batteries have been investigated before, after, and during initial cycling (pristine, first charging, and consecutive discharging). The initial self-conditioning is studied by scanning electron microscopy and spectroscopy-based methods (energy-dispersive X-ray spectroscopy, total reflection X-ray fluorescence spectrometry, and Raman spectroscopy) as well as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and orbitrap SIMS (Orbi-SIMS). This study shows that copper is released from the active material into the electrolyte solution during the initial cycling at potentials higher than 3.1 V versus Mg²⁺/Mg. The results point toward a transmetalation process transforming CuDEPP into [5,15-bis(ethynyl)-10,20-diphenylporphinato]magnesium(II) (MgDEPP). Further, the self-conditioning process via electro-polymerization observed for monovalent ions appears to be absent in the investigated Mg-based system, as the alkynyl group of the CuDEPP seems to remain unaltered during the initial charging and discharging steps. Additionally, cross-sectional and laterally resolved ToF-SIMS image analyses show an accumulation of inorganic fluorine-rich copper and magnesium species at the electrode surface including the active material exposed to the electrolyte after cycling that might be a result of a passivation layer or the formation of a cathodic electrolyte interphase.