Jonggu Jeon

Complex, dynamically heterogeneous environments such as lipid membranes, electrocatalytic interfaces, soft polymer matrices, crowded biological media, and phase-separated assemblies are characterized by spatially distinct microenvironments, multiscale structural dynamics, and short-range interactions. Microscopic interactions are poorly captured by bulk descriptors. In this article, we review how ultrafast two-dimensional infrared (2D IR) spectroscopy and related vibrational methods provide direct access to these complex molecular landscapes by resolving inhomogeneous ensembles, picosecond environmental fluctuations, and exchange between local states. When combined with vibrational probes and modern molecular simulation, these spectroscopic observables reveal how local interactions, such as electrostatics, hydrogen bonding (H-bonding), and ion pairing and aggregation generate microscopic heterogeneity and govern multiscale dynamics. We organize discussions around representative classes of complex environments, including hydration and electrostatics in lipid membranes, molecular environments and dynamics in biomolecular condensates, confinement and crowding in self-assembled and interfacial systems, including electrochemical interfaces, polymers/gels and soft matter, electrolytes and ion-specific effects, and aqueous cosolvent mixtures relevant to solvation and cryopreservation. Looking ahead, ongoing advancements in vibrational probe design, multidimensional spectroscopy, and molecular simulation are expected to enable more quantitative and predictive analyses of heterogeneous molecular environments across chemistry, materials science, and biology.

ConspectusWater-in-salt electrolytes (WiSEs) are promising electrolytes for next-generation lithium-ion batteries (LIBs), offering critical advantages like nonflammability and improved safety. These electrolytes have extremely high salt concentrations and exhibit unique solvation structures and transport mechanisms dominated by the formation of ion networks and aggregates. These ion networks are central to the performance of WiSEs, govern the transport properties and stability of the electrolyte, deviating from conventional dilute aqueous or organic electrolytes.The availability of free water molecules is significantly reduced in WiSEs, leading to a shift in the solvation environment. Lithium ions (Li⁺) typically travel with their solvation shells in dilute solutions and form stronger interactions with anions, resulting in the formation of complex ion aggregates. Despite the high viscosity of WiSEs, they exhibit surprisingly high ionic conductivity attributed to the decoupling of viscosity and ionic mobility. Instead of moving through free water, Li⁺ ions are transported along the pathways formed by the ion networks, minimizing direct solvent interaction and enhancing mobility.Advanced spectroscopic techniques, such as infrared IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy, and molecular dynamics (MD) simulations have illuminated the critical role of these ion networks in facilitating transport. These studies have shown that even at extreme salt concentrations, some water molecules retain properties similar to bulk water, essential for fast ion movement. In WiSEs, bulk-like water molecules form transient hydrogen-bond networks that serve as conduits for Li⁺ ions, while anion-bound water molecules play a less active role in transport due to their slower dynamics.As the salt concentration increases, the structure of WiSEs becomes more dominated by 3D ion networks. MD simulations reveal that these networks, stabilized by chaotropic anions such as bis(trifluoromethanesulfonyl)imide (TFSI^-), disrupt the hydrogen-bonding network of water and provide a stable, interconnected structure that supports the movement of Li⁺ ions. The formation of these extensive ion networks is critical for maintaining ionic mobility and the electrochemical stability of the electrolyte.The shift from traditional vehicular transport mechanisms to structural diffusion is a hallmark of WiSEs. Li⁺ ions no longer move with their solvation shells but hop between coordination sites within the ion network. This structural diffusion mechanism enables high ionic mobility despite the reduced presence of water and the increased viscosity of the solution. In conclusion, the formation of ion networks and aggregates in WiSEs not only stabilizes the electrolyte but also drives an unconventional ion transport mechanism. By understanding and controlling these aggregates, WiSEs offer a pathway toward safer, high-performance electrolytes for LIBs and other aqueous energy storage technologies.

The development of safe and cost-effective electrolytes for rechargeable batteries is currently underway. While water-based electrolytes hold promise, their restricted electrochemical stability window poses a challenge. Combining multiple ionic species emerges as a promising strategy to broaden this stability window and optimize Li-ion battery performance. This study focuses on dual-cation electrolytes, which blend lithium and potassium acetates to enhance the electrochemical characteristics of the solution at high concentrations. We investigated the solvation structure of each ion and its interactions on a molecular level. Our analysis reveals that ion clusters and aggregates are formed through shared acetate and water molecules at high salt concentrations. Furthermore, the residence time analyses of atom pairs indicate that cations diffuse in vehicular mode at low concentrations. In contrast, they switch to a structural mode at high concentrations due to diminishing water content. This study offers a comprehensive model for exploring diverse solvation structures of cations and gaining insights into their diffusion mechanisms within water-in-bisalt electrolytes for aqueous Li-ion batteries.

Time-dependent electric fields applied to ionic systems can induce both a dielectric and a conductive response, leading to the generation of macroscopic polarization and current, respectively. It has long been recognized that it is not possible to determine the two types of responses separately. However, this aspect is often not adequately accounted for in dielectric and absorption spectroscopies of ionic systems. To clarify this, we theoretically investigate the dielectric and conductive responses of ionic systems containing polyatomic ions based on linear response theory. We derive general expressions for the frequency-dependent dielectric functions, conductivity, and absorption coefficient, including those measured experimentally. Furthermore, we show that the dielectric and conductive responses cannot be uniquely distinguished even at the theoretical level and, therefore, cannot represent experimentally measured quantities. Instead, dielectric and absorption spectra of ionic systems should be expressed in terms of the generalized dielectric function that encompasses both dielectric and conductive responses. We propose a computational method to calculate this generalized dielectric function reliably. Model calculations on concentrated aqueous solutions of NaCl, a monatomic salt, and LiTFSI, a polyatomic salt, show that the dielectric and linear absorption spectra of the two systems based on the generalized dielectric function are significantly different from purely dielectric counterparts in the far-IR, terahertz, and lower-frequency regions. Moreover, the spectra are mainly determined by the autocorrelations of total dipole and total current, but dipole-current cross-correlation can also significantly contribute to the spectra of the LiTFSI solution. The present theoretical approach could be extended to nonlinear spectroscopy of ionic liquids and electrolyte solutions.

Understanding chemical exchange in carbonate-based electrolytes employed in Li-ion batteries (LIBs) is crucial for elucidating ion transport mechanisms. Ultrafast two-dimensional (2D) IR spectroscopy has been widely used to investigate the solvation structure and dynamics of Li-ions in organic carbonate-based electrolytes. However, the interpretation of cross-peaks observed in picosecond carbonyl stretch 2D-IR spectra has remained contentious. These cross-peaks could arise from various phenomena, including vibrational couplings between neighboring carbonyl groups in the first solvation shell around Li-ions, vibrational excitation transfers between carbonyl groups in distinct solvation environments, and local heating effects. Therefore, it is imperative to resolve the interpretation of 2D-IR cross-peaks to avoid misinterpretations regarding ultrafast dynamics found in LIB carbonate-based electrolytes. In this study, we have taken a comprehensive investigation of carbonate-based electrolytes utilizing 2D-IR spectroscopy and molecular dynamics (MD) simulations. Through meticulous analyses and interpretations, we have identified that the cross-peaks observed in the picosecond 2D-IR spectra of LIB electrolytes predominantly arise from intermolecular vibrational excitation transfer processes between the carbonyl groups of Li-bound and free carbonate molecules. We further discuss the limitations of employing a picosecond 2D-IR spectroscopic technique to study chemical exchange and intermolecular vibrational excitation transfer processes, particularly when the effects of the molecular photothermal process cannot be ignored. Our findings shed light on the dynamics of LIB electrolytes and resolve the controversy related to 2D-IR cross-peaks. By discerning the origin of these features, we could provide valuable insights for the design and optimization of next-generation Li-ion batteries.

Lithium-ion batteries face insufficient capacity at low temperatures. The lithium-ion desolvation process in the vicinity of a solid electrolyte interphase (SEI) layer is considered the major problem. Thus, an accurate determination of lithium-ion solvation structures is a prerequisite for understanding this process. Here, using a cryostat combined with an FTIR spectrometer, we found that as the temperature decreased, the number of coordinating carbonates in the first solvation shell of the lithium ion increased with a decreased population of the contact ion pair (CIP). More specifically, we found that two or more carbonate molecules replace a single PF₆^- anion upon CIP dissociation. This experimental finding shows that the prevailing notion that four carbonate molecules coordinate each lithium ion to form a tetrahedral structure is invalid for describing lithium-ion solvation structures. We anticipate that the present work will elucidate one of the molecular origins behind the low performance of lithium-ion batteries at low temperatures.

Time-resolved multidimensional spectroscopy is an advanced spectroscopic technique that can be used to study the molecular structure and dynamics of chromophores in condensed phases by probing multiple resonances of chemical and biophysical systems. To achieve such a multidimensional measurement, the nonlinear optical response of materials should be measured for varying time delays between multiple optical pulses. Inevitably, the data acquisition time dramatically increases with the number of dimensions and the length of each time-delay scan. Therefore, technical breakthroughs toward improved data acquisition rates and time resolutions have long been sought for more versatile and extensive use of coherent multidimensional spectroscopy. Here, we present a tutorial description of the concepts and methods of coherent multidimensional spectroscopy with multiple repetition-frequency-stabilized lasers. Pulse trains from two lasers with slightly different repetition frequencies enable an asynchronous optical sampling (ASOPS), i.e., an automatic scan of the time delay between the pulses. By combining mechanical delay lines for interferometric pulse-pair generation and signal field detection with an incoherent ASOPS scheme for probing slow population dynamics of chromophores in condensed phases, we show that the dynamic range of time-delay scan and data acquisition speed for coherent multidimensional spectroscopy could be dramatically enhanced. In this tutorial paper, we summarize the current developments in this and related research areas, and provide perspectives on all optically controlled multidimensional spectroscopy with multiple synchronized lasers.

How atoms and electrons in a molecule move during a chemical reaction and how rapidly energy is transferred to or from the surroundings can be studied with flashes of laser light. However, despite prolonged efforts to develop various coherent spectroscopic techniques, the lack of an all-encompassing method capable of both femtosecond time resolution and nanosecond relaxation measurement has hampered various applications of studying correlated electron dynamics and vibrational coherences in functional materials and biological systems. Here, we demonstrate that two broadband (>300 nm) synchronized mode-locked lasers enable two-dimensional electronic spectroscopy (2DES) study of chromophores such as bacteriochlorophyll a in condensed phases to measure both high-resolution coherent vibrational spectrum and nanosecond electronic relaxation. We thus anticipate that the dual mode-locked laser-based 2DES developed and demonstrated here would be of use for unveiling the correlation between the quantum coherence and exciton dynamics in light-harvesting protein complexes and semiconducting materials.