Interfacial Potentiodynamics of "Water-in-Salt" Electrolytes in Aqueous Lithium-Ion Batteries Using Nonlinear Spectroscopy and Molecular Simulations

The molecular arrangement of the electrode/electrolyte interface is crucial for enlarging the electrochemical stability of "water-in-salt" electrolytes in aqueous lithium-ion batteries. Using in situ vibrational sum-frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulations, we investigated the interfacial structure and molecular orientation of water molecules and anions of a LiTFSI-based "water-in-salt" electrolyte as a function of electrode potential. Shifting the electrode potential from positive to negative values induces significant interfacial changes in the structure of the electrolyte species, transitioning from anion surface excess to a water-rich interface. Furthermore, TFSI anions undergo a conformational change from cis to trans at the interface at positive electrode potentials. The results of comprehensive MD simulations support the experimental observations, demonstrating a potentiodynamic change in the number densities of water molecules, TFSI anions, and Li-ions at the interface accompanied by increasing dihedral angles of TFSI anions at higher positive electrode potentials.