The interfacial structure of super-concentration LiNO₃ aqueous electrolyte studied by second harmonic generation

The interfacial structure of a super-concentration LiNO₃ aqueous electrolyte was studied using non-resonant second harmonic generation (SHG) and heterodyne-detected SHG spectra. First, we investigated the electric double layer structure at the air/LiNO₃ interface. As the concentration of LiNO₃ increased, the SHG intensity first increased and then remained unchanged, while the SHG phase changed by about 5°. These results reveal that there was only a small amount of NO₃ ^- at the interface. The increase of the SHG intensity resulted from the thickening of the interfacial water molecular layer. In addition, we studied the broadening mechanism of the electrochemical stability window (ESW) for the super-concentrated LiNO₃ aqueous electrolyte. During cyclic voltammetry scanning, the potential-dependent SHG curves of the Pt/LiNO₃ interface verify that at the cathodic end of the ESW, as the concentration of LiNO₃ increased, the orientation angle θ of Pt-H changed less and the number density N_s of Pt-H gradually decreased, which indicates the decrease of the number of adsorbed H atoms on the Pt electrode surface. Therefore, the decrease of the number of free water molecules on the Pt electrode surface resulted in an expanded ESW.