Direct observation of MgO formation at cathode electrolyte interface of a spinel MgCo2O4 cathode upon electrochemical Mg removal and insertion

Abstract

Identifying a Mg2+ insertion mechanism into Mg cathode is one crucial challenge of designing Mg batteries that own comparable specific energy to the lithium ion systems. Spinel Mg cathodes are attractive candidates mainly attributed to its high intercalation voltage and theoretical capacity, however, electrochemically removing Mg from its solid structure with identified capacity dedicated to actual reversible Mg intercalation introduce complications. Work presented here applies Scanning/Transmission Electron Microscopy with Energy Loss Spectroscopy and diffraction method to investigate the cathode electrolyte interface for a spinel MgCo2O4 material electrochemically cycled in the presence of a non-aqueous, low water content Mg electrolyte. Findings at an atomic scale suggest a complete de-magnesiation is occurring for thin MgCo2O4 particles upon electrochemical charging, but a vast majority of MgCo2O4 shows formation of an amorphous MgO layer with a thickness of ∼10 nm at the cathode electrolyte interface. The amorphous MgO layer is directly jointed with the crystalline MgCo2O4 at the outer edge of the cathode. Formation of MgO at the cathode electrolyte interface is believed to be the detrimental factor preventing further electrochemical cell cycling of MgCo2O4. A possible reaction pathway of intercalation-initiated surface conversion is proposed to explain reversible Mg insertion with MgCo2O4.