A facile approach using MgCl2 to formulate high performance Mg2+ electrolytes for rechargeable Mg batteries

Abstract

Rechargeable Mg batteries have been regarded as a viable battery technology for grid scale energy storage and transportation applications. However, the limited performance of Mg²⁺ electrolytes has been a primary technical hurdle to develop high energy density rechargeable Mg batteries. In this study, MgCl₂ is demonstrated as a non-nucleophilic and cheap Mg²⁺ source in combination with Al Lewis acids (AlCl₃, AlPh₃ and AlEtCl₂) to formulate a series of Mg²⁺ electrolytes, representing the simplest method to prepare Mg²⁺ conductive electrolytes (no precursor synthesis, free of recrystallization and giving quantitative yield). These electrolytes are characterized by high oxidation stability (up to 3.4 V vs. Mg), improved electrophile compatibility and electrochemical reversibility (up to 100% coulombic efficiency). Three electrolyte systems (MgCl₂–AlCl₃, MgCl₂–AlPh₃, and MgCl₂–AlEtCl₂) were fully characterized by multinuclear NMR (¹H, ²⁷Al{¹H} and ²⁵Mg{¹H}) spectroscopies and electrochemical analysis. Single crystal X-ray diffraction and NMR studies consistently established molecular structures of the three electrolytes sharing a common Mg²⁺-dimer mono-cation, [(μ-Cl)₃Mg₂(THF)₆]⁺, along with an anion (AlCl₄⁻, AlPh₃Cl⁻ and AlEtCl₃⁻ respectively). Clean and dendrite free Mg bulk plating and viable battery performance were validated through representative studies using the MgCl₂–AlEtCl₂ electrolyte. The reaction mechanism of MgCl₂ and the Al Lewis acids in THF is discussed to highlight the formation of the electrochemically active [(μ-Cl)₃Mg₂(THF)₆]⁺ dimer mono-cation in these electrolytes and their improved performance compared to reported electrolytes using nucleophilic Mg²⁺ sources.