Balancing\nIon Diffusion-Reduction in Chlorine-Free\nElectrolytes Enables Long-Life Mg Metal Batteries

Homogeneous Mg plating/stripping behaviors have been\nwidely recognized\nas the distinct advantage of rechargeable Mg metal batteries over\nother metal batteries. However, the rapid degradation mechanism of\nthe Mg anode in practical chlorine-free electrolytes remains unclear.\nHerein, we reveal that the imbalance between Mg<sup>2+</sup> diffusion\nand reduction rates during Mg plating is the main cause of uneven\nMg deposition in a Mg(HFIP)<sub>2</sub> electrolyte, eventually resulting\nin cell short-circuits. We addressed this challenge by introducing\na bulky tetrabutylammonium borohydride (TBABH<sub>4</sub>) additive\nin the electrolyte. In detail, the uniform coverage of TBA<sup>+</sup> cations on the Mg anode surface regulates the Mg<sup>2+</sup> reduction\nrate, ensuring homoepitaxy of the deposited Mg along the thermodynamically\nstable (002) crystal plane. Consequently, both Mg||Mg symmetrical\ncells and Mg||Mo<sub>6</sub>S<sub>8</sub> full cells demonstrated\ndoubled cycling stability and reduced overpotential. This work shed\nnew light on stabilizing chlorine-free Mg metal batteries by balancing\nthe ion diffusion-reduction rates.