Synthesis of MgCl⁺ and Mg²⁺ Cation Based Novel Electrolytes: Impact of Polydentate Ethers

The development of efficient electrolytes is crucial for advancing magnesium (Mg) batteries, which hold promise for next-generation energy storage systems. Previously, electrolytes such as [Mg₂(μ-Cl)₃ ⋅ 6THF]⁺ [Ph₄Al]⁻, A, and [Mg₂(μ-Cl)₃ ⋅ 6THF]⁺ [Ph₃AlCl]⁻, B, have been studied, but their performance has been limited by issues related to ion dissociation and electrochemical stability. In this study, we report the synthesis of novel electrolytes by introducing polydentate ligands to these known systems, leading to the formation of [DME ⋅ MgCl ⋅ 3THF]⁺ [Ph₄Al]⁻ 1 and [DG ⋅ MgCl ⋅ 2THF]⁺ [Ph₄Al]⁻ 2, [Mg ⋅ 3DME]²⁺ 2[Ph₃AlCl⁻] 3 and [Mg ⋅ 2DG]²⁺ 2[Ph₃AlCl⁻] 4. These firstly discovered compounds were thoroughly characterized using X-ray crystallography and NMR spectroscopy. Our findings reveal that the choice of counter anion plays a pivotal role in the products and mechanism of the dissociation of the bridged [Mg₂(μ-Cl)₃ ⋅ 6THF]⁺ cation upon the addition of polydentate ligands. Specifically, with the [Ph₄Al]⁻ counter anion (precursor A), the dissociation results in a [MgCl]⁺ mono-cation, while with the [Ph₃AlCl]⁻ counter anion (precursor B), a [Mg]²⁺ divalent cation is formed. The resultant MgCl₂ byproduct enhances solubility, expands electrochemical windows, and improves cyclic stability, leading to superior electrochemical performance of the new electrolytes (1, 2, 3, and 4) compared to the original precursors. These insights offer valuable guidelines for the design and synthesis of advanced electrolytes for rechargeable magnesium batteries, potentially paving the way for more efficient and stable energy storage solutions.