Lithium-ion batteries power our modern world, including laptops, cell phones, medical equipment, and electric vehicles. Current battery research includes improving battery safety to reduce explosion risks and product recalls. Electrolytes composed of Room-Temperature Ionic Liquids (RTILs) can address these issues by reducing vapor pressure and flammability. To understand the performance of RTILs in batteries requires first understanding their structures. X-ray scattering is the foremost technique for characterizing the structures of RTILs, but interpreting experimental data can be daunting. This work presents an approximate expression for the radial distribution function, g(r), derived by assuming a Lorentzian line shape for total structure factors, S(Q). The derived form of g(r) is used to present new equations for maxima and minima in g(r) and to define a half-length, the value of r where g(r) decays to one-half its maximum value. A detailed test of the model is presented using experimentally measured and calculated S(Q) for N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C1C3pyrrTFSI).[1] Then, the model is extended to experimentally measured S(Q) for two series of RTILs based on C1C3pyrrX and and 1-methyl-3-propylimidazolium (C1C3imidX) with anions of increasing size (X− = Cl−, Br−, BF4−, PF6−, OTf−, TFSI−).[2,3] The model predicts maxima and minima in the two series within 6.0% of those calculated directly from molecular dynamics simulations. Thus, distances within "polar scattering domains" responsible for the charge alternation peak in S(Q) are quantitatively related to inter-ionic distances within polar aggregates in these RTILs. The half-length is found to increase approximately linearly as anion size increases. We argue that the half-length is a measure of polar aggregate size in these RTILs.
[1] Wheeler, R.A.; Dalbey, E.E.; “Structure factor lineshape model gives approximate nanoscale size of polar aggregates in the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluormethylsulfonyl)imide”; Phys. Chem. Chem. Phys. 2021; 23, 9061-9064; DOI: 10.1039/D0CP04907G.
[2] Wheeler, R.A.; Dalbey, E.E.; “Structure factor line shape model gives approximate nanoscale size of polar aggregates in pyrrolidinium-based ionic liquids”; Phys. Chem. Chem. Phys. 2025, 27, 4593-4602; DOI: 10.1039/D4CP04488F (Highlighted on the outside front cover as one of the best papers in this issue).
[3] Dalbey, E.E.; Sailor, J.J.; Wheeler, R.A.; “Effect of anion size on the size of polar aggregates in imidazolium-based ionic liquids”; Phys. Chem. Chem. Phys. 2025, 27, 14296-14304; DOI: 10.1039/D5CP00419E.