rate capability, high-voltage performance, high-temperature or low-temperature performance, and safety. Therefore, it is worthwhile to discover new mechanisms in electrolytes. The macroscopic properties of liquids include density, viscosity, volatility, flash point, boiling point, freezing point, solubility, polarity, and toxicity. For liquid electrolytes, more properties need to be considered, such as ionic conductivity, electrochemical stability window, chemical compatibility and stability, and cost. Most of the macroscopic properties depend on the microscopic structures such as the size and shape of molecules and ions, the electron density distribution in molecules, the charge state of ions, the strength of chemical bonds, and the interaction of molecules and ions (solvation, clusters/nano segments, networks, etc.). Among them, the last one is the most complicated part since it varies with the solvent-solute combinations. It is interesting to explore the guidelines of these seemingly identical solutions, but the toolbox is limited. The X-ray diffraction in liquids has long been known. Debye and Scherrer first reported the diffraction halo in liquids in 1916 using the Laue spot method. [3] Later, it was revealed that the molecular space array in liquids can be probed by X-ray scattering. [4] Small-angle X-ray scattering (SAXS) studies date from the classical works of Guinier on Al-Cu alloy, published in 1938. [5] Since then, P. Debye, G. Porod, O. Kratky, V. Luzzati, W. Beeman, P. Schmidt, and others developed the theoretical and experimental fundamentals of the method. [6] SAXS can cover different systems in terms of the structure characterization, such as polymers, colloids, particulate systems, fractal systems and amorphous systems. Initially, SAXS was widely used to characterize polymer-based systems. Turkevich and co-workers first reported SAXS data on monodisperse metal particles synthesized via the Turkevich reaction in 1951. After that, an increasing number of SAXS papers began to focus on the field of nanoparticles. [7] New progress in refining and modeling the SAXS data began in the 1970s and is continuing today. [6] Since the experimental SAXS data could be explained well using suitable models, it has become a very popular characterization method in biology and material science, especially in size, shape and distribution analysis. In the first monograph on SAXS by Guinier and Fournet, it was already demonstrated that the method yields information on the sizes and shapes of particles and the internal structure of disordered and partially ordered systems. [8] SAXS method can probe the intermolecular spacing, cluster size, or The fundamental understanding of nanostructures of liquid electrolytes is expected to enable transformative gains in electrochemical energy storage capacities. However, the solvation structures and molecular dynamics in electrolytes are hard to probe, which limits further performance improvements in macroscopic properties such as ionic conductivity, viscosity, and stability. Small-...
