Coupling Se-Vacancy-Rich FeSe ₂ /Bi ₂ Se ₃ Heterojunction and Microhydration-Guided Water-in-Oil Electrolyte for Ultrahigh-Performance Hybrid-Ion Batteries

Magnesium/sodium hybrid-ion batteries (MNHBs), combining dendrite-free, high-capacity Mg anodes with fast Na⁺ cathode kinetics, are appealing for post-lithium-ion storage. However, adoption is limited by sluggish Mg²⁺ diffusion and a lack of ideal electrolytes. Here we present a synergistically engineered MNHB coupling a Se-vacancy-rich FeSe₂/Bi₂Se₃ heterojunction cathode with an optimized trace water-in-oil electrolyte. The vacancy-tailored heterointerface accelerates Mg²⁺/Na⁺ migration, preserving structural integrity, supported by first-principles calculations. Molecular dynamics reveal that controlled microhydration strengthens [Mg(H₂O)_<i>n</i>]²⁺ coordination, weakens Mg²⁺-Na⁺ pairing, and increases the diffusivity. Electrochemical measurements reveal a high capacity 487 mAh g^-1, excellent rate capability, a high Coulombic efficiency of >99.7% after 1000 cycles at 1.0 A g^-1, and ultralong cycling stability ≥ 3500 cycles at 1.5 A g^-1. <i>In-situ</i>/<i>ex-situ</i> characterizations reveal low polarization, fast diffusion, and reversible phase transitions. These findings establish a clear mechanistic understanding and a broadly applicable strategy to overcome kinetic and interfacial limitations in secondary batteries.