Countersolvent Relaxes Strong Mg−O Coulomb Force for Improved Rechargeable Magnesium Batteries

Abstract Rechargeable magnesium batteries are hindered by the strong coulombic force of Mg2+ with solvents, causing sluggish desolvation, solvent decomposition, and unstable Mg metal interphases. The current strategies mainly rely on direct competition with the solvent for Mg2+ coordination. However, strong coordination may tightly attach to Mg2+, increasing desolvation energy. Herein, unlike coordination additives, we report 2-(4-methylpiperazin-1-yl)aniline (2-MPA), whose chair-conformed methylpiperazine moiety disfavors direct occupation of Mg2+ coordination sites. Meanwhile, its ortho-linked molecular backbone polarizes the N−H bond and strengthens hydrogen-bonding interactions with solvents. Thus, 2-MPA acts as a countersolvent to preferentially interact with solvents, effectively softening the Mg2+−O(ether) bonds, thereby relaxing the Mg2+ solvation structure. Consequently, the electrolyte enables Mg plating/stripping for 800 h with a low overpotential of 0.17 V, achieves an average coulombic efficiency of 96% over 1500 cycles, and improves the cycling performance of a Cu2−xSe||Mg full cell.