  Rechargeable Zn batteries offer safe, inexpensive energy storage, but when
deeply discharged to compete with lithium-ion batteries, they are plagued by
parasitic reactions at the Zn anodes. We apply super-concentrated alkaline
electrolytes to suppress two key parasitic reactions, hydrogen evolution and
ZnO passivation. An electrolyte with 15 M KOH displays a broad electrochemical
window (>2.5 V on Au), a high ZnO solubility (>1.5 M), and an exceptionally
high ionic conductivity (>0.27 S/cm at 25 C). Spectroscopies and ab-initio
molecular dynamics simulation suggest K+-OH- pairs and a tightened water
network to underpin the stability. The simulation further reveals unique
triggered proton hopping that offsets the lack of water wires to sustain the
conductivity. Low hydrogen evolution, confirmed via online mass spectroscopy,
and slow passivation enable a NiOOH||Zn battery to deliver a cumulative
capacity of 8.4 Ah cm-2 and a Zn-air battery to last for over 110 hours.
