In Situ Variable-Temperature Liquid ToF-SIMS Enables Molecular Investigation of Supercooled Water-in-Salt Electrolytes

The microstructures, including ion solvation structures and water structures within supercooled water-in-salt electrolytes, determine macroscopic properties of electrolytes, such as ion conductivity, viscosity, and freezing point, thereby affecting the low-temperature operation performance of aqueous batteries. Herein, we developed an in situ variable-temperature liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS) technique to molecularly explore the evolution of microscopic interactions within the water-in-salt lithium chloride electrolyte upon supercooling as a case study. We revealed that at -20 °C, the ion solvation structures of Li(H2O)2+ and Li(H2O)3+, which are the dominant species contributing to the Li+ ion conductivity, undergo dehydration to form Li(H2O)+. In addition, the low temperature induces the formation of more LiCl(H2O)3 complex, which was proposed to correlate with viscosity change. Moreover, the abundance of tetrahedral water clusters as a nucleation precursor for ice formation evidently increases in the supercooled water. This study demonstrates that the advanced in situ variable-temperature liquid ToF-SIMS possesses the uniqueness of critical molecular discrimination of complex microstructures as well as their temperature-induced evolution within liquids. This advances the molecular-level understanding of the correlations between microscopic interactions and macroscopic properties of supercooled electrolytes. We anticipate broad applications of the in situ variable-temperature liquid ToF-SIMS on investigations of the evolution of various microscopic interactions and associated chemical processes at low temperatures across disciplines in the near future.