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)n2+ 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. In-situ/ex-situ 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.
Zhou et al. (2026) studied this question.