Heterogeneity and Nanostructure of Superconcentrated LiTFSI-EmimTFSI Hybrid Aqueous Electrolytes: Beyond the 21 m Limit of Water-in-Salt Electrolyte

Water-in-Salt Electrolyte Stabilizes Pyrazine Radical: Suppression of Its Aggregation by Interaction between Pyrazine and Li(H2O)n. J Am Chem Soc 2025;147:16812-16825. [PMID: 40327745 DOI: 10.1021/jacs.4c09561] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Stabilizing radical intermediates of redox-active organic molecules in aqueous media is crucial for advancing applications in energy storage, catalysis, and electrosynthesis. This study investigates the stabilization of protonated radical intermediates of pyrazine derivatives in water-in-salt electrolytes (WISEs) with 7-8 m LiTFSI. Strong interactions between pyrazine derivatives and Li+-coordinated water (Li(H2O)n+) in WISEs prevent molecular aggregation and protect radical intermediates from disproportionation and oxygen-induced degradation. Voltammetric results show that higher concentrations of LiTFSI enhance both the stability and redox reversibility of dimethylpyrazine (DMP) radical intermediates, with protonation identified as a key stabilizing factor. Notably, these stabilizing effects were absent in solutions containing concentrated LiCl or LiNO3. Fourier-transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulations confirmed reduced DMP aggregation in LiTFSI-based electrolytes, driven by interactions with Li(H2O)n+, while no similar solvation structure modification occurred with LiNO3. The protonated radical intermediates in LiTFSI-based WISEs exhibited greater resistance to oxygen-induced degradation compared to conventional acidic solutions. Additionally, substitution of methyl or ethyl groups on the pyrazine ring destabilized the corresponding radical intermediates in LiTFSI-based WISEs, primarily due to the alkyl inductive effect, as evidenced by electrochemical and UV-visible absorption spectroscopy. Charge-discharge tests in an H-cell further demonstrated significantly improved Coulombic efficiency of pyrazine redox reactions in LiTFSI-based WISEs compared to acidic Salt-in-Water electrolytes, underscoring the importance of radical intermediate stabilization.

Stability and structure of the aqueous LiTFSI-LiCl interface

It has recently been demonstrated that aqueous lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium chloride (LiCl) solutions can form stable liquid-liquid biphasic systems when both electrolyte phases have sufficiently high concentrations. In this work, we combine molecular dynamics simulations and experimental analysis to investigate what drives the formation of the interface and how the interfacial molecular structure correlates with its thermodynamic stability. We observe that at the liquid-vapour interface, TFSI- anions exhibit surfactant-like properties, leading to a reduction in surface tension and an increase in interfacial thickness. In contrast, the interfacial stability of the LiTFSI-LiCl biphasic systems increases with the concentration of both salts, as evidenced by the increasing surface tension and decreasing interfacial thickness. The opposing effects that the ionic concentration has on the thermodynamic stability of the different interfaces are linked to the anions' interfacial adsorption/desorption, which in turn affects the number and strength of water-water hydrogen bonds, the interfacial molecular structure and the diffusion of cations across the interface. Finally, calculations and experiments indicate that the liquid-liquid separation is driven primarily by the concentration of LiCl, and is the result of a 'salting out' effect.

Regulation of Molecular Microheterogeneity in Electrolytes Enables Ampere-Hour-Level Aqueous LiMn2O4||Li4Ti5O12 Pouch Cells

Aqueous batteries are attractive due to their high safety and fast reaction kinetics, but the narrow electrochemical stability window of H2O limits their applications. It is a big challenge to broaden the electrochemical operation window of aqueous electrolytes while retaining fast reaction kinetics. Here, a new organic aqueous mixture electrolyte of manipulatable (3D) molecular microheterogeneity with H2O-rich and H2O-poor domains is demonstrated. H2O-poor domains molecularly surround the reformed microclusters of H2O molecules through interfacial H-bonds, which thus not only inhibit the long-range transfer of H2O but also allow fast and consecutive Li+ transport. This new design enables low-voltage anodes reversibly cycling with aqueous-based electrolytes and high ionic conductivity of 4.5 mS cm-1. LiMn2O4||Li4Ti5O12 full cells demonstrate excellent cycling stability over 1000 cycles under various C rates and a low temperature of -20 °C. 1 Ah pouch cell delivers a high energy density of 79.3 Wh kg-1 and high Coulombic efficiency of 99.4% at 1 C over 200 cycles. This work provides new insights into the design of electrolytes based on the molecular microheterogeneity for rechargeable batteries.

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

Water-in-salt electrolytes (WiSEs) are a promising class of electrolytes due to their wide electrochemical stability window and nonflammability. In this study, we explore the structural organization of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) salt-based aqueous electrolytes, covering dilute to highly concentrated regions, by employing an all-atom molecular dynamics simulation. For the NaTFSI-based electrolyte, we observe that Na+ ions are mostly surrounded by water molecules at all the salt concentrations due to the very strong interaction between them. While TFSI anions weakly coordinate with Na+ ions and other TFSI anions, they also mostly prefer to be surrounded by water molecules. These interactions were found to have moderate dependence on the concentration of the NaTFSI salt. For the NaFSI-based electrolyte, while the Na+-water interaction is stronger at lower salt concentrations, the number of nearest neighbor FSI anions is found to be more than that of water at higher concentrations (≥20 m). This is because the increase in the salt concentration leads to expulsion of water molecules from the solvation shell of Na+ ions and enhances the interaction between Na+ ions and oxygen atoms of FSI. At the highest salt concentration (solubility limit), the bulk-like water structure is completely disrupted and dominated by an anionic network in the FSI-based electrolyte. In contrast, water-water hydrogen bonding network is still present even in the highly concentrated TFSI-based electrolyte. The simulated X-ray scattering pattern displays a low-q peak, revealing the presence of an intermediate range ordering due to alternating anion-rich and water/Na+-rich regions in both the electrolytes. However, the characteristic length scale corresponding to the low-q peak decreases with increasing the salt content in both the electrolytes.

Solvation Structure of Methanol-in-Salt Electrolyte Revealed by Small-Angle X-ray Scattering and Simulations

The solvation structure of water-in-salt electrolytes was thoroughly studied, and two competing structures─anion solvated structure and anion network─were well-defined in recent publications. To further reveal the solvation structure in those highly concentrated electrolytes, particularly the influence of solvent, methanol was chosen as the solvent for this proposed study. In this work, small-angle X-ray scattering, small-angle neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy were utilized to obtain the global and local structural information. With the concentration increment, the anion network formed by TFSI- became the dominant structure. Meanwhile, the hydrogen bonds among methanol were interrupted by the TFSI- anion and formed a new connection with them. Molecular dynamic simulations with two different force fields (GAFF and OPLS-AA) are tested, and GAFF agreed with synchrotron small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) results well and provided insightful information about molecular/ion scale solvation structure. This article not only deepens the understanding of the solvation structure in highly concentrated solutions, but more importantly, it provides additional strong evidence for utilizing SAXS/WAXS to validate molecular dynamics simulations.

Microstructures of Choline Amino Acid based Biocompatible Ionic Liquids

Bio-compatible ionic liquids (Bio-ILs) represent a class of solvents with peculiar properties and exhibit huge potential for their applications in different fields of chemistry. Ever since they were discovered, researchers have used bio-ILs in diverse fields such as biomass dissolution, CO2 sequestration, and biodegradation of pesticides. This review highlights the ongoing research studies focused on elucidating the microscopic structure of bio-ILs based on cholinium cation ([Ch]+ ) and amino acid ([AA]- ) anions using the state-of-the-arta b i n i t i o ${ab\hskip0.25eminitio}$ and classical molecular dynamics (MD) simulations. The microscopic structure associated with these green ILs guides their suitability for specific applications. ILs of this class differ in the side chain of the amino acid anions, and varying the side chain significantly affects the structure of these ILs and thus helps in tuning the efficiency of biomass dissolution. This review demonstrates the central role of the side chain on the morphology of choline amino acid ([Ch][AA]) bio-ILs. The seemingly matured field of bio-ILs and their employment in various applications still holds significant potential, and the insights on their microscopic structure would steer the field of target specific application of these green ILs.

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

Redox flow batteries (RFBs) represent a promising approach to enabling the widespread integration of intermittent renewable energy. Rapid developments in RFB materials and electrolyte chemistries are needed to meet the cost and performance targets. In this review, special emphasis is given to the recent advances how electrolyte design could circumvent the main thermodynamic restrictions of aqueous electrolytes. The recent success of aqueous electrolyte chemistries has been demonstrated by extending the electrochemical stability window of water beyond the thermodynamic limit, the operating temperature window beyond the thermodynamic freezing temperature of water and crystallization of redox-active materials, and the aqueous solubility beyond the thermodynamic solubility limit. They would open new avenues towards enhanced energy storage and all-climate adaptability. Depending on the constituent, concentration and condition of electrolytes, the performance gain has been correlated to the specific solvation environment, interactions among species and ion association at a molecular level.