Water-Locked Eutectic Electrolyte Enables Long-Cycling Aqueous Sodium-Ion Batteries.

Aqueous sodium batteries are one of the awaited technologies for large-scale energy storage, but remain poorly rechargeable because of the reactivity issues of water. Here, we present a hydrated eutectic electrolyte featuring a water-locked effect, which is exceptional in that the O-H bond of water is essentially strengthened via weak hydrogen bonding (relative to the original HO-HO hydrogen bonds) to low-donor-number anions and ligands. Even without interphase protection, both the anodic and cathodic water electrodecomposition reactions are delayed, extending the aqueous potential window to 3.4 V. Combined with the alleviated electrode dissolution, NaMnFe(CN)||NaTi(PO) batteries deliver a high energy density of ∼80 W h kg at 0.5 C and undergo over 1000 cycles with a 74.5% capacity retention and a 99.4% Coulombic efficiency at 4.2 C. This work may offer a general guide to ultimately exploit the water's innate stability for realizing the promise of aqueous battery technologies.