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

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• Rechargeable magnesium-ion batteries (MIBs) offer high energy density but face challenges with cathode stability.
• Copper phthalocyanine derivatives are explored as cathode materials, but their electrochemical behavior requires detailed investigation.

Purpose of the Study:

• To investigate the initial self-conditioning mechanisms of [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) composite cathodes in MIBs.
• To elucidate the chemical transformations and interfacial phenomena occurring during the first charge-discharge cycles.

Main Methods:

• Electrochemical cycling of CuDEPP cathodes.
• Surface analysis using scanning electron microscopy (SEM).
• Spectroscopic characterization including energy-dispersive X-ray spectroscopy (EDX), total reflection X-ray fluorescence spectrometry (TXRF), Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and orbitrap SIMS (Orbi-SIMS).

Main Results:

• Copper is released from CuDEPP into the electrolyte at potentials > 3.1 V vs Mg²⁺/Mg, indicating a transmetalation process forming [5,15-bis(ethynyl)-10,20-diphenylporphinato]magnesium(II) (MgDEPP).
• The typical electro-polymerization self-conditioning observed for monovalent ions is absent; the alkynyl group remains unaltered.
• ToF-SIMS analysis reveals an accumulation of fluorine-rich copper and magnesium species on the electrode surface, suggesting passivation layer or cathodic electrolyte interphase (CEI) formation.

Conclusions:

• The initial cycling of CuDEPP cathodes in MIBs involves copper dissolution and transmetalation, leading to MgDEPP formation.
• The absence of electro-polymerization and the formation of a passivation layer are key differences compared to monovalent ion systems.
• Understanding these processes is crucial for designing stable and efficient MIB cathodes.