Water-ion network structure evolution in LiTFSI aqueous electrolytes: from solvated ions to water-mediated anion ordering.

The structure-property relationships of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and water-in-salt electrolytes (WISEs) have gained significant attention in recent years. Although several studies have described the microscopic arrangement and molecular interactions of the constituents of the LiTFSI aqueous electrolyte, significant disagreement remains about its molecular organization, ranging from nanoscale heterogeneity to a homogeneous ionic network. With recent SAXS and simulation studies highlighting nanoscale TFSI clustering, the molecular-level interactions facilitating these organizations remain unclear. In this study, we analyzed the stretching vibrations of various groups in TFSI- while simultaneously probing the H2O stretching/bending region to gain insights into the fundamentals of these interactions. Small concentration increments of ≤0.01 M combined with second-derivative analysis and peak deconvolution were used to probe the continuous structural evolution of H2O around the TFSI- anion and Li+ cation from the salt-in-water (SIW) to the water-in-salt (WIS) domains. Significant changes in the -CF3 vibrational modes of TFSI-, along with the appearance of a high-wave number peak in the H2O stretching region, suggest a pivotal role of weak H2O-CF3 interactions for TFSI-TFSI ordering. The distinct splitting of the H2O stretching peaks in the high-wave number region (>3 M LiTFSI) and its resemblance to H2O in non-aqueous media/ionic liquids were discussed. The conformational analysis of TFSI- suggests a significant contribution from the cis-conformation as it approaches the WIS domain. The present study suggests that H2O remains weakly interacting close to the -CF3 group of cis- and trans-conformations of the TFSI- ion and plays a pivotal role in anion ordering. The Li+ hydration structure, its interaction with the -SO2 group, and H2O-mediated dynamic cis-trans-TFSI ordering at the microscopic scale give a unique organization. The proposed solid-like microscopic structural organization of LiTFSI in this concentrated region with H2O-mediated TFSI ordering may also help in understanding the transport behavior and/or Li+ conductivity in these electrolytes.