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 H2O–H2O 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, Na2MnFe­(CN)6||NaTi2(PO4)3 batteries deliver a high energy density of ∼80
W h kg–1 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.