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

Area of Science:

• Electrochemistry
• Materials Science
• Battery Technology

Background:

• Magnesium-sulfur batteries offer high theoretical energy density but face challenges with electrolyte stability and coulombic efficiency.
• Non-nucleophilic Hauser bases like hexamethyldisilazide (HMDS) are promising for magnesium electrolytes.
• Existing HMDS-based electrolytes, particularly HMDSCl, suffer from low coulombic efficiency in full cells.

Purpose of the Study:

• To develop a novel electrolyte for magnesium-sulfur batteries with improved performance.
• To investigate the electrochemical behavior of magnesium in HMDSBr-based electrolytes.
• To understand the impact of Lewis acids on magnesium plating and stripping.

Main Methods:

• Synthesized a new HMDSBr-based electrolyte in tetrahydrofuran.
• Investigated magnesium electrochemistry via plating and stripping in Mg(HMDS)Br, Mg(HMDS)Br-BEt3, and Mg(HMDS)Br-AlEt3 electrolytes.
• Utilized Nuclear Magnetic Resonance (NMR) to identify magnesium species in the electrolytes.
• Assessed electrochemical stability window, coulombic efficiency, cycle life, and ionic conductivity.

Main Results:

• The HMDSBr electrolyte demonstrated stable reversibility over 1000 cycles with high coulombic efficiency (≈99%) for magnesium plating/stripping.
• The substitution of chloride with bromide did not significantly narrow the electrochemical stability window.
• The electrolyte exhibited high anodic stability (≈2.4 V vs. Mg/Mg2+) and ionic conductivity (1.16 mS cm−1).
• Magnesium plating occurred with low overpotential (<188 mV), forming a homogeneous metallic layer, indicating balanced nucleation and growth.

Conclusions:

• HMDSBr-based electrolytes represent a significant advancement for magnesium-sulfur batteries, overcoming the coulombic efficiency limitations of HMDSCl electrolytes.
• The developed electrolyte offers excellent long-term cycling stability and high efficiency for magnesium plating and stripping.
• These findings pave the way for more efficient and stable magnesium-based energy storage systems.