<p>Fast charging of high-energy batteries is critical for transportation electrification but remains challenging because the rapid rise in cell overpotential easily exceeds electrolytes’ fixed electrochemical stability window. Here we design a self-adaptive electrolyte with a dynamically expanding electrochemical stability window that increases in real time during charging, outpacing the rise in overpotential as the charging current intensifies. The self-adaptive electrolyte is a single-phase solution of salt and complementary oxidation- and reduction-resistant solvents at the cloud point composition but can undergo solvent separation to dynamically redistribute solvent components during charging. The oxidation-resistant solvents concentrate at the positive electrode and reduction-resistant solvents accumulate at the negative electrode, broadening the electrolyte stability window in real time during charging. Proof-of-concept experiments validate the versatility of this design in both aqueous zinc-metal and non-aqueous lithium-metal batteries, achieving high Coulombic efficiencies of negative electrodes and enhanced oxidative stability for positive electrodes.</p>