Balancing Ion Diffusion-Reduction in Chlorine-Free Electrolytes Enables Long-Life Mg Metal Batteries

Homogeneous Mg plating/stripping behaviors have been widely recognized as the distinct advantage of rechargeable Mg metal batteries over other metal batteries. However, the rapid degradation mechanism of the Mg anode in practical chlorine-free electrolytes remains unclear. Herein, we reveal that the imbalance between Mg²⁺ diffusion and reduction rates during Mg plating is the main cause of uneven Mg deposition in a Mg(HFIP)₂ electrolyte, eventually resulting in cell short-circuits. We addressed this challenge by introducing a bulky tetrabutylammonium borohydride (TBABH₄) additive in the electrolyte. In detail, the uniform coverage of TBA⁺ cations on the Mg anode surface regulates the Mg²⁺ reduction rate, ensuring homoepitaxy of the deposited Mg along the thermodynamically stable (002) crystal plane. Consequently, both Mg||Mg symmetrical cells and Mg||Mo₆S₈ full cells demonstrated doubled cycling stability and reduced overpotential. This work shed new light on stabilizing chlorine-free Mg metal batteries by balancing the ion diffusion-reduction rates.