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

Aqueous electrolytes come with an intrinsic narrow electrochemical stability window (1.23 V). Expanding this window represents significant benefits in both fundamental science and practical battery applications. Recent breakthroughs made via super-concentration have resulted in >3.0 V windows, but fundamental understanding of the related mechanism is still absent. In the present work, we examined the widened window (2.55 V) of a super-concentrated (unsaturated) aqueous solution of LiNO3 through both theoretical and spectral analyses and discovered that a local structure of intimate Li+-water interaction arises at super-concentration, generating (Li+(H2O)2)n polymer-like chains to replace the ubiquitous hydrogen bonding between water molecules. Such structure is mainly responsible for the expanded electrochemical stability window. Further theoretical and experimental analyses quantitatively differentiate the contributions to this window, identifying the kinetic factor (desolvation) as the main contributor. Such molecular-level and quantitative understanding will further assist in tailor designing more effective approaches to stabilizing water electrochemically. [Display omitted] •The widened window of a super-concentrated aqueous LiNO3 solution was demonstrated•A local structure of (Li+(H2O)2)n polymer-like chains was discovered•The local structure is mainly responsible for the expanded electrochemical window 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. The unique local structure with (Li+(H2O)2)n polymer-like aggregation in super-concentrated LiNO3 aqueous solu