Origin of preferential magnesium electrodeposition on separators: Synergistic effects of interface-accelerating desolvation and spatial confinement

Preferential magnesium (Mg) electrodeposition on separators is a ubiquitous yet poorly understood phenomenon in rechargeable Mg-metal batteries, posing a fundamental challenge to their development. In this work, the synergy effects of interface-accelerating desolvation and spatial confinement have been demonstrated as the essential causation of this counterintuitive experimental phenomenon. At the molecular level, the imide ring (–CO–NR–CO–, in which R represents the phenyl) groups in an artificially introduced polyimide (PI) interlayer facilitate the strong electrostatic affinity towards Mg2+, which accelerates the desolvation process for Mg2+ solvation structures at the inner Helmholtz plane. At the nucleation scale, the wedge-like concave geometry formed at the PI/current collector interface provides energetically favorable sites for Mg nucleation. This unique architecture reduces the critical nucleus size, thereby significantly lowering nucleation energy barriers. As a result, the satisfactory Coulombic efficiency for Mg plating/stripping (98.22 %) and cycle lifespan (1200 cycles, above 100 days) have been achieved, outperforming most of the previous results. This work pioneers a molecular-level understanding of separator-directed Mg deposition and resolves a long-standing confusion in Mg-metal batteries.