Structure and dynamics of highly concentrated LiTFSI/acetonitrile electrolytes

Elucidating the Quenching Mechanism of Tris(2,2'-bipyridyl)ruthenium(II) Complex in the Water-Glycerol Binary System Based on the Microscopic Structure of the Media

Kinetic studies on the photochemical quenching reaction of the tris(2,2'-bipyridyl) ruthenium(II) complex ([Ru(bpy)3]2+) in water-glycerol binary media were conducted based on the Einstein-Smoluchowski (E-S) theory. Dynamic and static quenching behaviors were analyzed by comparing results from time-resolved spectroscopy and emission spectroscopy. While the dynamic quenching reaction aligns well with the E-S theory, static quenching was observed, leading to a notable increase in the overall photoquenching reaction rate constant. Employing chromatography and infrared spectroscopy, we correlated the microscopic molecular structure of the binary solvent system and the solvation environment around the emitters with the reaction mechanism. This correlation was found to correspond to ion pair formation and the confinement effect of the emitter, respectively.

Molecular Level Origin of Ion Dynamics in Highly Concentrated Electrolytes

Single-ion conducting liquid electrolytes are key to achieving rapid charge/discharge in Li secondary batteries. The Li+ transference (or transport) numbers are the defining properties of such electrolytes and have been discussed in the framework of concentrated solution theories. However, the connection between macroscopic transference and microscopic ion dynamics remains unclear. Molecular dynamics simulations were performed to obtain direct information regarding the microscopic behaviors in highly concentrated electrolytes, and the relationships between these behaviors and the transference number were determined under anion-blocking conditions. Various solvents with different donor numbers (DNs) were used along with a Li salt of the weakly Lewis basic bis(fluorosulfonyl)amide anion for electrolyte preparation. Favorable ordered Li+ structuring and a continuous Li+ conduction pathway were observed for the fluoroethylene carbonate-based electrolyte due to its low DN. The properties were less pronounced at higher DNs, e.g., for the dimethyl sulfoxide-based electrolyte. The τLi-solventlife/τdipolerelax ratio was introduced as a factor for ion dynamics, and the two mechanisms of ion transport were considered an exchange mechanism (τLi-solventlife/τdipolerelax < 1) and a vehicle mechanism (translational motion of solvated Li+) (τLi-solventlife/τdipolerelax ≥ 1). Vehicle-type transport was dominant with high DNs, while exchangeable transport was preferable at lower DNs. These findings should aid the further selection of solvents and Li salts to prepare single-ion conducting electrolytes.

Ionic conduction mechanism in high concentration lithium ion electrolytes

The conduction mechanism of a family of high concentration lithium electrolytes (HCEs) is investigated. It is found in all HCEs that the molecular motions are regulated by the anion size and correlated to the HCE ionic resistivity. From the results, a mechanism involving highly correlated ionic networks is derived.

Structure-Dynamics Interrelation Governing Charge Transport in Cosolvated Acetonitrile/LiTFSI Solutions

Concentrated ionic solutions present a potential improvement for liquid electrolytes. However, their conductivity is limited by high viscosities, which can be attenuated via cosolvation. This study employs a series of experiments and molecular dynamics simulations to investigate how different cosolvents influence the local structure and charge transport in concentrated lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI)/acetonitrile solutions. Regardless of whether the cosolvent's dielectric constant is low (for toluene and dichloromethane), moderate (acetone), or high (methanol and water), they preserve the structural and dynamical features of the cosolvent-free precursor. However, the dissimilar effects of each case must be individually interpreted. Toluene and dichloromethane reduce the conductivity by narrowing the distribution of Li⁺-TFSI^- interactions and increasing the activation energies for ionic motions. Methanol and water broaden the distributions of Li⁺-TFSI^- interactions, replace acetonitrile in the Li⁺ solvation, and favor short-range Li⁺-Li⁺ interactions. Still, these cosolvents strongly interact with TFSI^-, leading to conductivities lower than that predicted by the Nernst-Einstein relation. Finally, acetone preserves the ion-ion interactions from the cosolvent-free solution but forms large solvation complexes by joining acetonitrile in the Li⁺ solvation. We demonstrate that cosolvation affects conductivity beyond simply changing viscosity and provide fairly unexplored molecular-scale perspectives regarding structure/transport phenomena relation in concentrated ionic solutions.

Extending MIEZE spectroscopy towards thermal wavelengths

A modulation of intensity with zero effort (MIEZE) setup is proposed for high-resolution neutron spectroscopy at momentum transfers up to 3 Å^-1, energy transfers up to 20 meV and an energy resolution in the microelectronvolt range using both thermal and cold neutrons. MIEZE has two prominent advantages compared with classical neutron spin echo. The first is the possibility to investigate spin-depolarizing samples or samples in strong magnetic fields without loss of signal amplitude and intensity. This allows for the study of spin fluctuations in ferromagnets, and facilitates the study of samples with strong spin-incoherent scattering. The second advantage is that multi-analyzer setups can be implemented with comparatively little effort. The use of thermal neutrons increases the range of validity of the spin-echo approximation towards shorter spin-echo times. In turn, the thermal MIEZE option for greater ranges (TIGER) closes the gap between classical neutron spin-echo spectroscopy and conventional high-resolution neutron spectroscopy techniques such as triple-axis, time-of-flight and back-scattering. To illustrate the feasibility of TIGER, this paper presents the details of its implementation at the RESEDA beamline at FRM II by means of an additional velocity selector, polarizer and analyzer.

Direct Correlation of the Salt-Reduced Diffusivities of Organic Solvents with the Solvent's Mole Fraction

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in organic solvents (especially propylene carbonate) has demonstrated extraordinary pseudocapacitive performance as an electrolyte in the supercapacitor configuration ( Nat. Energy 2019, 4, 241-248). However, the influence of the solvated ions on the diffusivity of the solvent molecules is yet to be understood. We examine the impact of LiTFSI on the diffusivity in five organic solvents: acetonitrile (ACN), tetrahydrofuran (THF), methanol (MeOH), dimethyl sulfoxide (DMSO), and propylene carbonate (PC) using a combination of neutron scattering, conductivity measurements, and molecular dynamics simulations. The extent of the diffusivity reduction in the concentration regime of ≤1 M directly correlates with the solvent mole fraction at which the solvation shells around Li⁺ ions are of similar size in all the solvents, resulting in a universal ∼50% reduction in the solvent diffusivity. These results provide guidance for formulation of the new electrolytes to enhance the performance of energy storage devices.

Synthesis and Characterization of Lithium-Conducting Composite Polymer-Ceramic Membranes for Use in Nonaqueous Redox Flow Batteries

Redox flow batteries (RFBs) are a burgeoning electrochemical platform for long-duration energy storage, but present embodiments are too expensive for broad adoption. Nonaqueous redox flow batteries (NAqRFBs) seek to reduce system costs by leveraging the large electrochemical stability window of organic solvents (>3 V) to operate at high cell voltages and to facilitate the use of redox couples that are incompatible with aqueous electrolytes. However, a key challenge for emerging nonaqueous chemistries is the lack of membranes/separators with suitable combinations of selectivity, conductivity, and stability. Single-ion conducting ceramics, integrated into a flexible polymer matrix, may offer a pathway to attain performance attributes needed for enabling competitive nonaqueous systems. Here, we explore composite polymer-inorganic binder-filler membranes for lithium-based NAqRFBs, investigating two different ceramic compounds with NASICON-type (NASICON: sodium (Na) superionic conductor) crystal structure, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃ (LAGTP), each blended with a polyvinylidene fluoride (PVDF) polymeric matrix. We characterize the physicochemical and electrochemical properties of the synthesized membranes as a function of processing conditions and formulation using a range of microscopic and electrochemical techniques. Importantly, the electrochemical stability window of the as-prepared membranes lies between 2.2-4.5 V vs Li/Li⁺. We then integrate select composite membranes into a single electrolyte flow cell configuration and perform polarization measurements with different redox electrolyte compositions. We find that mechanically robust, chemically stable LATP/PVDF composites can support >40 mA cm^-2 at 400 mV cell overpotential, but further improvements are needed in selectivity. Overall, the insights gained through this work begin to establish the foundational knowledge needed to advance composite polymer-inorganic membranes/separators for NAqRFBs.