Stripping and Plating a Magnesium Metal Anode in Bromide-Based Non-Nucleophilic Electrolytes

A non-nucleophilic Hauser base hexamethyldisilazide (HMDS) magnesium electrolyte possesses inherent properties required for a magnesium-sulfur battery. However, the development of full cell batteries using HMDSCl-based electrolytes is still hampered by a low coulombic efficiency. A new electrolyte formulation of non-nucleophilic HMDS magnesium containing bromide as a halide instead of chloride was obtained through a simple and straightforward synthesis route. The electrochemistry of magnesium was investigated through plating and stripping in three different HMDSBr-based electrolytes: Mg(HMDS)Br, Mg(HMDS)Br-BEt₃ , and Mg(HMDS)Br-AlEt₃ dissolved in tetrahydrofuran. The different magnesium species present in the electrolytes were determined using NMR. Weak electron-withdrawing Lewis acids, BEt₃ and AlEt₃ were used intentionally and their impact was investigated. Contrary to expectation, the substitution of chloride by bromide does not drastically narrow the electrochemical stability window. HMDSBr-based electrolytes demonstrated long-term (1000 cycles) stable reversibility and highly efficient (≈99 %) magnesium plating/tripping without a high ratio of bromide compared with the MgHMDSCl-based electrolytes. The aprotic electrolyte shows comparatively high anodic stability (≈2.4 V vs. Mg/Mg²⁺ ) and high ionic conductivity of 1.16 mS cm^-1 at room temperature. Plating of magnesium with low overpotential (<188 mV) revealed a morphology dependence on the electrolyte type with a shiny metallic homogenous layer, suggesting a rational balance between the nucleation and growth process in HMDSBr-based electrolytes.