Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation Studies of Nonaqueous Lithium Ion Battery Electrolytes.

Lithium ion battery (LIB) technology is undoubtedly indispensable to modern life. However, despite enormous and extended effort to improve LIB performance, our understanding of the underlying principles and mechanisms of lithium ion transport in nonaqueous LIB electrolytes remained limited until recently. There is a particular lack of knowledge of the microscopic solvation structures and fluctuation dynamics around charge carriers in real electrolytes. Typical electrolytes found in commercially available LIBs consist of lithium salts and mixed carbonate solvents, with the latter playing an essential role in promoting lithium ion transport and forming an electrically stable solid electrolyte interphase. Although a number of linear spectroscopic studies of LIB electrolytes aiming at understanding the complex nature of lithium ion solvation processes have been reported, the notion that each lithium ion is strongly solvated by carbonate molecules to form a long-lasting solvation sheath structure has remained the subject of intense debate. Here, we present the results of FTIR, fs IR pump-probe, two-dimensional IR spectroscopy, and molecular dynamics simulations reported by us and others and discuss the possible interplay of picosecond solvation dynamics and macroscopic ion transport processes within the framework of the fluctuation-dissipation relationship. Further, by measuring the time-dependent fluctuations and spectral diffusions of carbonate carbonyl stretch modes that act as excellent infrared probes for the local electrostatic environment, we show that lithium cations are not only solvated by carbonate molecules but also interact with counteranions at equilibrium depending on solvent composition. Molecular dynamics simulations support the notion that rapid chemical exchanges between carbonate solvent molecules in the first and outer solvation shells are critical for describing mobile lithium ion transport phenomena. We thus anticipate that time-resolved coherent multidimensional vibrational spectroscopy is capable of providing decisive evidence on the ultrafast solvent dynamics of various electrolytes, which is potentially helpful for designing improved and more efficient LIB electrolytes in the future.

Nanometric Water Channels in Water-in-Salt Lithium Ion Battery Electrolyte.

Lithium-ion batteries (LIBs) have been deployed in a wide range of energy-storage applications and helped to revolutionize technological development. Recently, a lithium ion battery that uses superconcentrated salt water as its electrolyte has been developed. However, the role of water in facilitating fast ion transport in such highly concentrated electrolyte solutions is not fully understood yet. Here, femtosecond IR spectroscopy and molecular dynamics simulations are used to show that bulk-like water coexists with interfacial water on ion aggregates. We found that dissolved ions form intricate three-dimensional ion-ion networks that are spontaneously intertwined with nanometric water hydrogen-bonding networks. Then, hydrated lithium ions move through bulk-like water channels acting like conducting wires for lithium ion transport. Our experimental and simulation results indicate that water structure-breaking chaotropic anion salts with a high propensity to form ion networks in aqueous solutions would be excellent candidates for water-based LIB electrolytes. We anticipate that the present work will provide guiding principles for developing aqueous LIB electrolytes.

Cyanamide as an Infrared Reporter: Comparison of Vibrational Properties between Nitriles Bonded to N and C Atoms.

Infrared (IR) probes based on terminally blocked ß-cyanamidoalanine (AlaNHCN) 1 and p-cyanamidophenylalanine (PheNHCN) 2 were synthesized, and the vibrational properties of their CN stretch modes were studied using Fourier transform infrared (FTIR) and femtosecond IR pump-probe spectroscopies in combination with quantum chemical calculations. From FTIR studies, it is found that the transition dipole strengths of the cyanamide (NHCN) group in 1 and 2 are much larger than those of the nitrile (CN) group but comparable to those of the isonitrile (NC) and azido (N3) groups in their previously studied analogs. The CN stretch frequencies in 1 and 2 are red-shifted from those in their nitrile analogs but more blue-shifted from the NC and N3 stretch frequencies in their isonitrile and azido analogs. The much larger transition dipole strength and the red-shifted frequency of the cyanamide relative to nitrile group originates from the n → π* interaction between the N atom's nonbonding (n) and CN group's antibonding (π*) orbitals of the NHCN group. Unlike aliphatic cyanamide 1, aromatic cyanamide 2 shows a complicated line shape of the CN stretch spectra. Such a complicated line shape arises from the Fermi resonance between the CN stretch mode of the NHCN group and one of the overtones of the phenyl ring vibrations and can be substantially simplified by deuteration of the NHCN into NDCN group. From IR pump-probe experiments, the vibrational lifetimes of the CN stretch mode in 1 were determined to be 0.58 ± 0.04 ps in D2O and 0.89 ± 0.09 ps in H2O and those in 2 were determined to be 1.64 ± 0.13 ps in CH3OD/dimethyl sulfoxide and 0.30 ± 0.05 and 2.62 ± 0.26 ps in CH3OH. The short time component (0.30 ± 0.05 ps) observed for 2 in CH3OH is attributed to the vibrational relaxation through Fermi resonance. These vibrational lifetimes are close to those of the nitrile and azido groups but shorter than those of the isonitrile group. Consequently, cyanamide behaves like an apparent vibrational hybrid of nitrile and isonitrile in that cyanamide is similar to nitrile in vibrational frequency and lifetime but to isonitrile in transition dipole strength. It is believed that cyanamide has the potential to be a strongly absorbing IR reporter of the conformational and environmental structure and dynamics of biomolecules in comparison to nitrile, a weak absorber.

Water Hydrogen-Bonding Network Structure and Dynamics at Phospholipid Multibilayer Surface: Femtosecond Mid-IR Pump-Probe Spectroscopy.

The water hydrogen-bonding network at a lipid bilayer surface is crucial to understanding membrane structures and its functional activities. With a phospholipid multibilayer mimicking a biological membrane, we study the temperature dependence of water hydrogen-bonding structure, distribution, and dynamics at a lipid multibilayer surface using femtosecond mid-IR pump-probe spectroscopy. We observe two distinguished vibrational lifetime components. The fast component (0.6 ps) is associated with water interacting with a phosphate part, whereas the slow component (1.9 ps) is with bulk-like choline-associated water. With increasing temperature, the vibrational lifetime of phosphate-associated water remains constant though its relative fraction dramatically increases. The OD stretch vibrational lifetime of choline-bound water slows down in a sigmoidal fashion with respect to temperature, indicating a noticeable change of the water environment upon the phase transition. The water structure and dynamics are thus shown to be in quantitative correlation with the structural change of liquid multibilayer upon the gel-to-liquid crystal phase transition.

Modulation of the Hydrogen Bonding Structure of Water by Renal Osmolytes.

Osmolytes are an integral part of living organism, e.g., the kidney uses sorbitol, trimethylglycine, taurine and myo-inositol to counter the deleterious effects of urea and salt. Therefore, knowing that the osmolytes' act either directly to the protein or mediated through water is of great importance. Our experimental and computational results show that protecting osmolytes, e.g., trimethylglycine and sorbitol, significantly modulate the water H-bonding network structure, although the magnitude and spatial extent of osmolyte-induced perturbation greatly vary. In contrast, urea behaves neutrally toward local water H-bonding network. Protecting osmolytes studied here show strong concentration-dependent behaviors (vibrational frequencies and lifetimes of two different infrared (IR) probes), while denaturant does not. The H-bond donor and/or acceptor (OH/NH) in a given osmolyte molecule play a critical role in defining their action. Our findings highlight the significance of the alteration of H-bonding network of water under biologically relevant environment, often encountered in real biological systems.