H₂O-Mg²⁺ Waltz-Like Shuttle Enables High-Capacity and Ultralong-Life Magnesium-Ion Batteries

Mg-ion batteries (MIBs) are promising next-generation secondary batteries, but suffer from sluggish Mg²⁺ migration kinetics and structural collapse of the cathode materials. Here, an H₂O-Mg²⁺ waltz-like shuttle mechanism in the lamellar cathode, which is realized by the coordination, adaptive rotation and flipping, and co-migration of lattice H₂O molecules with inserted Mg²⁺, leading to the fast Mg²⁺ migration kinetics, is reported; after Mg²⁺ extraction, the lattice H₂O molecules rearrange to stabilize the lamellar structure, eliminating structural collapse of the cathode. Consequently, the demo cathode of Mg_0.75V₁₀O₂₄·nH₂O (MVOH) exhibits a high capacity of 350 mAh g^-1 at a current density of 50 mA g^-1 and maintains a capacity of 70 mAh g^-1 at 4 A g^-1. The full aqueous MIB based on MVOH delivers an ultralong lifespan of 5000 cycles The reported waltz-like shuttle mechanism of lattice H₂O provides a novel strategy to develop high-performance cathodes for MIBs as well as other multivalent-ion batteries.