Interfacial Potentiodynamics of “Water-in-Salt”\nElectrolytes in Aqueous Lithium-Ion Batteries Using Nonlinear Spectroscopy\nand Molecular Simulations

The molecular arrangement\nof the electrode/electrolyte interface\nis crucial for enlarging the electrochemical stability of “water-in-salt”\nelectrolytes in aqueous lithium-ion batteries. Using <i>in situ</i> vibrational sum-frequency generation (SFG) spectroscopy and molecular\ndynamics (MD) simulations, we investigated the interfacial structure\nand molecular orientation of water molecules and anions of a LiTFSI-based\n“water-in-salt” electrolyte as a function of electrode\npotential. Shifting the electrode potential from positive to negative\nvalues induces significant interfacial changes in the structure of\nthe electrolyte species, transitioning from anion surface excess to\na water-rich interface. Furthermore, TFSI anions undergo a conformational\nchange from <i>cis</i> to <i>trans</i> at the\ninterface at positive electrode potentials. The results of comprehensive\nMD simulations support the experimental observations, demonstrating\na potentiodynamic change in the number densities of water molecules,\nTFSI anions, and Li-ions at the interface accompanied by increasing\ndihedral angles of TFSI anions at higher positive electrode potentials.