Aqueous Eutectic Electrolytes Suppress Oxygen and Hydrogen Evolution for Long-Life Zn||MnO2 Dual-Electrode-Free Batteries

Aqueous Zn ²⁺ /Zn||MnO ₂ /Mn ²⁺ batteries - operating on electrodeposition/dissolution - offer promising high-voltage, high-capacity grid storage capabilities but require acidic conditions for MnO ₂ /Mn ²⁺ conversion that induces problematic zinc corrosion. Here we present a global approach that identifies deep eutectic aqueous-organic electrolytes that strategically disrupt water's hydrogen bonding network, simultaneously enhancing MnO ₂ reversibility at the cathode without OER while enabling stable zinc cycling at the anode without HER. Such non-flammable electrolytes regulate the cation solvation structure and phase of the deposited MnO ₂ and its morphology, promoting layered structures with enhanced ion transport pathways that significantly improve stripping efficiency. These deep eutectics increase the oxygen evolution overpotential by 0.6 V, well above the MnO ₂ deposition potential, which completely suppresses unwanted O ₂ evolution. Moreover, they alter the local environment at the cathode interface to create localized interfacial pH gradients that influence critical processes, including optimizing proton transport and MnO ₂ stripping. Our Zn ²⁺ /Zn||MnO ₂ /Mn ²⁺ dual-electrode-free battery achieves high Coulombic efficiency for extended cycling (>5000 cycles) without external acid addition, advancing high-energy-density zinc-manganese battery development through rational electrolyte design.