Understanding Thermodynamic and Kinetic Contributions in Expanding the Stability Window of Aqueous Electrolytes

Because of their non-flammable nature, low toxicity, and low production cost, aqueous Li-ion batteries (LIBs) promise very tempting alternatives to the state-of-the-art LIBs that rely on highly flammable and toxic non-aqueous electrolytes. However, the intrinsic narrow electrochemical stability window (1.23 V) of water sets an upper limit on the practical voltage and energy output. Here, we report a super-concentrated (unsaturated) LiNO₃-based aqueous electrolyte that effectively expands the aqueous stability window to 2.55 V. We further revealed that a unique local structure with (Li⁺(H₂O)₂)_n polymer-like aggregation arises at the super-concentration, which assists in stabilizing the aqueous solution at extreme potentials via both thermodynamic and kinetic contributions. This fundamental revelation of liquid structure and its effect on the electrochemical stability window provides a new pathway for designing high-voltage aqueous electrolytes.