Active Learning Guided Computational Discovery of Plant-Based Redoxmers for Organic Nonaqueous Redox Flow Batteries.

Organic nonaqueous redox flow batteries (O-NRFBs) are promising energy storage devices due to their scalability and reliance on sourceable materials. However, finding suitable redox-active organic molecules (redoxmers) for these batteries remains a challenge. Using plant-based compounds as precursors for these redoxmers can decrease their costs and environmental toxicity. In this computational study, flavonoid molecules have been examined as potential redoxmers for O-NRFBs. Flavone and isoflavone derivatives were selected as catholyte (positive charge carrier) and anolyte (negative charge carrier) molecules, respectively. To drive their redox potentials to the opposite extremes, in silico derivatization was performed using a novel algorithm to generate a library of > 40000 candidate molecules that penalizes overly complex structures. A multiobjective Bayesian optimization based active learning algorithm was then used to identify best redoxmer candidates in these search spaces. Our study provides methodologies for molecular design and optimization of natural scaffolds and highlights the need of incorporating expert chemistry awareness of the natural products and the basic rules of synthetic chemistry in machine learning.

Liquid Redoxmers for Nonaqueous Redox Flow Batteries.

Redoxmers are organic active molecules storing electrochemical energy in nonaqueous redox flow batteries (NRFBs). Increasing the solubility of redoxmers is an important approach for increasing energy density of NRFBs as effective redoxmer concentration determines how much electricity can be stored in a given volume. Molecular engineering redoxmers towards liquid forms is regarded as one promising strategy as liquid redoxmers represent an extreme scenario where fluidity is maintained at maximum concentration using a minimum amount of supporting solvents. In this Perspective, recent examples of liquid redoxmers as well as their development strategy will be discussed.

Concentration-dependent Cycling of Phenothiazine-based Electrolytes in Nonaqueous Redox Flow Cells.

Increasing redox-active species concentrations can improve viability for organic redox flow batteries by enabling higher energy densities, but the required concentrated solutions can become viscous and less conductive, leading to inefficient electrochemical cycling and low material utilization at higher current densities. To better understand these tradeoffs in a model system, we study a highly soluble and stable redox-active couple, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI). We measure the physicochemical properties of electrolytes containing 0.2-1 M active species and connect these to symmetric cell cycling behavior, achieving robust cycling performance. Specifically, for a 1 M electrolyte concentration, we demonstrate 94% materials utilization, 89% capacity retention, and 99.8% average coulombic efficiency over 435 h (100 full cycles). This demonstration helps to establish potential for high-performing, concentrated nonaqueous electrolytes and highlights possible failure modes in such systems.

Softening by charging: how collective modes of ionic association in concentrated redoxmer/electrolyte solutions define the structural and dynamic properties in different states of charge.

Understanding the physical and chemical processes occurring in concentrated electrolyte solutions is required to achieve redox flow batteries with high energy density. Highly concentrated electrolyte solutions are often studied in which collective crowded interactions between molecules and ions become predominant. Herein, experimental and computational methods were used to examine non-aqueous electrolyte solutions in two different states of charge as a function of redoxmer concentration. As the latter increases and the ionic association strengthens, the electric conductivity passes through a maximum and the solution increasingly gels, which is seen through a rapid non-linear increase in viscosity. We establish that the structural rigidity of ionic networks is closely connected with this loss of fluidity and show that charging generally yields softer ionic assemblies with weaker attractive forces and improved dynamical properties.

Fluorination Enables Simultaneous Improvements of a Dialkoxybenzene-Based Redoxmer for Nonaqueous Redox Flow Batteries.

Redoxmers or redox-active organic materials, are one critical component for nonaqueous redox flow batteries (RFBs), which hold high promise in enabling the time domain of the grid. While tuning redox potentials of redoxmers is a very effective way to enhance energy densities of NRFBs, those improvements often accompany accelerated kinetics of the charged species, undermining stability and cycling performance. Herein, a strategy for designing redoxmers with simultaneous improvements in redox potential and stability is proposed. Specifically, the redoxmer 1,4-di-tert-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene (ANL-C46) is developed by incorporating fluorinated substitutions into the dialkoxybenzene-based platform. Compared to the non-fluorinated analogue, ANL-C46 demonstrates not only an increased (∼0.41 V) redox potential but also much enhanced stability (1.6 times) and cyclability (4 times) evidenced by electron paramagnetic resonance kinetic study, H-cell and flow cell cycling. In fact, the cycling performance of ANL-C46 is among the best of high potential (>1.0 V vs Ag/Ag+) redoxmers ever reported. Density functional theory calculations suggest that while the introduced fluorine substitutions elevate the redox potentials, they also help to depress the decomposition reactions of the charged redoxmers, affording excellent stability. The findings represent an interesting strategy for simultaneously improving energy density and stability, which could further prompt the development of high-performance redoxmers.

Amphiphile Organization in Organic Solutions: An Alternative Explanation for Small-Angle X-ray Scattering Features in Malonamide/Alkane Mixtures.

The role of different intermolecular interactions in the aggregation of amphiphiles in an organic solvent is studied for systems of relevance to liquid-liquid extraction (LLE), a chemical process used to selectively recover metals from complex mixtures. Of specific interest is the role, or lack thereof, of hydrogen bonding, which is often assumed to be a main driver of the organic phase structural organization that has been linked to separation efficacy. Toward that end, a series of malonamide extractants in n-dodecane have been studied in the absence of any extracted aqueous solutes, including water. The series of extractants includes N,N'-dimethyl-N,N'-dibutyltetradecylmalonamide (DMDBTDMA), two of its homologs, and N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA). This simplified model LLE system enables systematic investigation of the role of dipole-dipole and alkyl tail steric interactions in amphiphile aggregation. Small-angle X-ray scattering (SAXS) profiles computed from molecular dynamics trajectories are in good agreement with experimental SAXS data. Molecular dynamics simulations show that malonamide aggregation results from dipole-driven self-association and lacks characteristic aggregate sizes. Mid-q correlation peaks in the SAXS profiles emerge at high concentration for each malonamide. In those densely packed solutions, the correlation peaks are observed to result from alkyl tail-induced spacing between electron-rich polar head groups, with peak positions determined by the different alkyl tail lengths present in the malonamide molecule. This explanation of the SAXS correlation peaks contrasts with the prevailing literature, which attributes mesoscale features observed in small-angle scattering to the formation of microemulsions. Instead, this work finds that these features are present in the absence of water or any reverse micellar organization of the malonamides. As such, molecular-scale malonamide self-association and packing, rather than microemulsion-based colloidal-scale descriptions, is a more appropriate framework for these LLE systems.

Competitive Pi-Stacking and H-Bond Piling Increase Solubility of Heterocyclic Redoxmers.

Redoxmers are organic molecules that carry electric charge in flow batteries. In many instances, they consist of heteroaromatic moieties modified with appended groups to prevent stacking of the planar cores and increase solubility in liquid electrolytes. This higher solubility is desired as it potentially allows achieving greater energy density in the battery. However, the present synthetic strategies often yield bulky molecules with low molarity even when they are neat and still lower molarity in liquid solutions. Fortunately, there are exceptions to this rule. Here, we examine one well-studied redoxmer, 2,1,3-benzothiadiazole, which has solubility ∼5.7 M in acetonitrile at 25 °C. We show computationally and prove experimentally that the competition between two packing motifs, face-to-face π-stacking and random N-H bond piling, introduces frustration that confounds nucleation in crowded solutions. Our findings and examples from related systems suggest a complementary strategy for the molecular design of redoxmers for high energy density redox flow cells.

Self-Assembled Solute Networks in Crowded Electrolyte Solutions and Nanoconfinement of Charged Redoxmer Molecules.

Redoxmers are electrochemically active organic molecules storing charge and energy in electrolyte fluids circulating through redox flow batteries (RFBs). Such molecules typically have solvent-repelling cores and solvent-attracting pendant groups introduced to increase solubility in liquid electrolytes. These two features can facilitate nanoscale aggregation of the redoxmer molecules in crowded solutions. In some cases, this aggregation leads to the emergence of continuous networks of solute molecules in contact, and the solution becomes microscopically heterogeneous. Here, we use small-angle X-ray scattering (SAXS) and molecular dynamics modeling to demonstrate formation of such networks and examine structural factors controlling this self-assembly. We also show that salt ions become excluded from these solute aggregates into small pockets of electrolytes, where these ions strongly associate. This confinement by exclusion is also likely to occur to charged redoxmer molecules in a "sea" of neutral precursors coexisting in the same solution. Here, we demonstrate that the decay lifetime of the confined charged molecules in such solutions can increase several fold compared to dilute solutions. We attribute this behavior to a "microreactor effect" on reverse reactions of the confined species during their decomposition.

In situ X-ray spatial profiling reveals uneven compression of electrode assemblies and steep lateral gradients in lithium-ion coin cells.

Coin cells are used extensively as test devices in battery research for evaluation of new materials and optimization of cycling protocols. In this study, in situ X-ray diffraction profilometry is used to characterize spatial distribution of the active materials, lithiation, and phase distribution in electrodes of NCM523/graphite coin cells. The X-ray data indicate uneven areal compression of the electrode assembly in such cells, which we trace to a specific design feature that leads to elastic deformation of a metal spacer. Steep lithiation gradients observed in the electrodes imply radially-dependent resistivity, for which uneven compression of the separator is a likely cause. Electrochemical model calculations suggest that variable porosity of the polymer separator would account for the salient features of spatial profiles observed in these coin cells.

Realistic Ion Dynamics through Charge Renormalization in Nonaqueous Electrolytes.

While many practically important electrolytes contain lithium ions, interactions of these ions are particularly difficult to probe experimentally because of their small X-ray and neutron scattering cross sections and large neutron absorption cross sections. Molecular dynamics (MD) is a powerful tool for understanding the properties of nonaqueous electrolyte solutions from the atomic level, but the accuracy of this computational method crucially depends on the physics built into the classical force field. Here, we demonstrate that several force fields for lithium bistriflimide (LiTFSI) in acetonitrile yield a solution structure that is consistent with the neutron scattering experiments, yet these models produce dramatically different ion dynamics in solution. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive ionic association and ion-pair clustering. We show that reasonable agreement with the experimental observations can be achieved by renormalization of the ion charges using a "titration" method suggested herewith. This simple modification produces realistic concentration dependencies for ionic diffusion and conductivity in <2 M solutions, without loss in quality for simulation of the structure.

Observation of Microheterogeneity in Highly Concentrated Nonaqueous Electrolyte Solutions.

The development of models to describe structure and dynamics of nonaqueous electrolyte solutions is challenging, and experimental observations are needed to form a foundation. Here, neutron scattering is used to probe molecular dynamics in nonaqueous organic electrolytes. Two solutions were compared: one contained symmetrical electrolyte molecules prone to crystallize, and one contained desymmetrized electrolyte molecules preferring disordered states. For the latter, calorimetry and neutron data show that a disordered fluid persists to very low temperatures at high concentrations. Upon heating, localized cold crystallization occurs, leading to burst nucleation of microcrystalline solids within fluid phases. Our findings indicate molecular clustering and point to solvation inhomogeneities and molecular crowding in these concentrated fluids.

Solvent-dependent complex reaction pathways of bromoform revealed by time-resolved X-ray solution scattering and X-ray transient absorption spectroscopy.

The photochemical reaction pathways of CHBr3 in solution were unveiled using two complementary X-ray techniques, time-resolved X-ray solution scattering (TRXSS) and X-ray transient absorption spectroscopy, in a wide temporal range from 100 ps to tens of microseconds. By performing comparative measurements in protic (methanol) and aprotic (methylcyclohexane) solvents, we found that the reaction pathways depend significantly on the solvent properties. In methanol, the major photoproducts are CH3OCHBr2 and HBr generated by rapid solvolysis of iso-CHBr2-Br, an isomer of CHBr3. In contrast, in methylcyclohexane, iso-CHBr2-Br returns to CHBr3 without solvolysis. In both solvents, the formation of CHBr2 and Br is a competing reaction channel. From the structural analysis of TRXSS data, we determined the structures of key intermediate species, CH3OCHBr2 and iso-CHBr2-Br in methanol and methylcyclohexane, respectively, which are consistent with the structures from density functional theory calculations.

The lightest organic radical cation for charge storage in redox flow batteries.

In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density will capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules. By generating a combinatorial set of 1,4-dimethoxybenzene derivatives with different arrangements of substituents, we discovered a minimalistic structure that combines exceptional long-term stability in its oxidized form and a record-breaking intrinsic capacity of 161 mAh/g. The nonaqueous redox flow battery has been demonstrated that uses this molecule as a catholyte material and operated stably for 100 charge/discharge cycles. The observed stability trends are rationalized by mechanistic considerations of the reaction pathways.

Selective separation of trivalent f-ions using 1,10-phenanthroline-2,9-dicarboxamide ligands in ionic liquids.

1,10-Phenanthroline-2,9-dicarboxamide complexants decorated with alkyl chains and imidazolium cations have been studied for extraction of trivalent f-ions into imidazolium ionic liquids. The dicationic complexants are shown to extract Am over Eu with separation factors >50 and high extraction efficiencies. The different size selectivities for lanthanide ions were observed for these two types of complexants, highlighting the importance of the positive charge in controlling both extraction efficiencies and extraction selectivities.

Mechanistic Insight in the Function of Phosphite Additives for Protection of LiNi0.5Co0.2Mn0.3O2 Cathode in High Voltage Li-Ion Cells.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

Spontaneous aggregation of lithium ion coordination polymers in fluorinated electrolytes for high-voltage batteries.

Fluorinated carbonates are pursued as liquid electrolyte solvents for high-voltage Li-ion batteries. Here we report aggregation of [Li(+)(FEC)3]n polymer species in fluoroethylene carbonate containing electrolytes and scrutinize the causes for this behavior.

The AHA Moment: Assessment of the Redox Stability of Ionic Liquids Based on Aromatic Heterocyclic Anions (AHAs) for Nuclear Separations and Electric Energy Storage.

Because of their extended conjugated bond network, aromatic compounds generally have higher redox stability than less saturated compounds. We conjectured that ionic liquids (ILs) consisting of aromatic heterocyclic anions (AHAs) may exhibit improved radiation and electrochemical stability. Such properties are important in applications of these ILs as diluents in radionuclide separations and electrolytes in the electric energy storage devices. In this study, we systematically examine the redox chemistry of the AHAs. Three classes of these anions have been studied: (i) simple 5-atom ring AHAs, such as the pyrazolide and triazolides, (ii) AHAs containing an adjacent benzene ring, and (iii) AHAs containing electron-withdrawing groups that were introduced to reduce their basicity and interaction with metal ions. It is shown that fragmentation in the reduced and oxidized states of these AHAs does not generally occur, and the two main products, respectively, are the H atom adduct and the imidyl radical. The latter species occurs either as an N σ-radical or as an N π-radical, depending on the length of the N-N bond, and the state that is stabilized in the solid matrix is frequently different from that having the lowest energy in the gas phase. In some instances, the formation of the sandwich π-stack dimer radical anions has been observed. For trifluoromethylated anions, H adduct formation did not occur; instead, there was facile loss of fluoride from their fluorinated groups. The latter can be problematic in nuclear separations, but beneficial in batteries. Overall, our study suggests that AHA-based ILs are viable candidates for use as radiation-exposed diluents and electrolytes.

In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids.

Coordination polymers (CPs) of metal ions are central to a large variety of applications, such as catalysis and separations. These polymers frequently occur as amorphous solids that segregate from solution. The structural aspects of this segregation remain elusive due to the dearth of the spectroscopic techniques and computational approaches suitable for probing such systems. Therefore, there is a lacking of understanding of how the molecular building blocks give rise to the mesoscale architectures that characterize CP materials. In this study we revisit a CP phase formed in the extraction of trivalent lanthanide ions by diesters of the phosphoric acid, such as the bis(2-ethylhexyl)phosphoric acid (HDEHP). This is a well-known system with practical importance in strategic metals refining and nuclear fuel reprocessing. A CP phase, referred to as a "third phase", has been known to form in these systems for half a century, yet the structure of the amorphous solid is still a point of contention, illustrating the difficulties faced in characterizing such materials. In this study, we follow a deductive approach to solving the molecular structure of amorphous CP phases, using semiempirical calculations to set up an array of physically plausible models and then deploying a suite of experimental techniques, including optical, magnetic resonance, and X-ray spectroscopies, to consecutively eliminate all but one model. We demonstrate that the "third phase" consists of hexagonally packed linear chains in which the lanthanide ions are connected by three O-P-O bridges, with the modifying groups protruding outward, as in a bottlebrush. The tendency to yield linear polynuclear oligomers that is apparent in this system may also be present in other systems yielding the "third phase", demonstrating how molecular geometry directs polymeric assembly in hybrid materials. We show that the packing of bridging molecules is central to directing the structure of CP phases and that by manipulating the steric requirements of ancillary groups one can control the structure of the assembly.

Radiation stability of cations in ionic liquids. 5. Task-specific ionic liquids consisting of biocompatible cations and the puzzle of radiation hypersensitivity.

In 1953, an accidental discovery by Melvin Calvin and co-workers provided the first example of a solid (the α-polymorph of choline chloride) showing hypersensitivity to ionizing radiation: under certain conditions, the radiolytic yield of decomposition approached 5 × 10(4) per 100 eV (which is 4 orders of magnitude greater than usual values), suggesting an uncommonly efficient radiation-induced chain reaction. Twenty years later, the still-accepted mechanism for this rare condition was suggested by Martyn Symons, but no validation for this mechanism has been supplied. Meanwhile, ionic liquids and deep eutectic mixtures that are based on choline, betainium, and other derivitized natural amino compounds are presently finding an increasing number of applications as diluents in nuclear separations, where the constituent ions are exposed to ionizing radiation that is emitted by decaying radionuclides. Thus, the systems that are compositionally similar to radiation hypersensitive solids are being considered for use in high radiation fields, where this property is particularly undesirable! In Part 5 of this series on organic cations, we revisit the phenomenon of radiation hypersensitivity and explore mechanistic aspects of radiation-induced reactions involving this class of task-specific, biocompatible, functionalized cations, both in ionic liquids and in reference crystalline compounds. We demonstrate that Symons' mechanism needs certain revisions and rethinking, and suggest its modification. Our reconsideration suggests that there cannot be conditions leading to hypersensitivity in ionic liquids.

Aqueous complexes for efficient size-based separation of americium from curium.

Complexation of the adjacent actinide ions americium(III) and curium(III) by the ligand N,N'-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H2bp18c6) in aqueous solution was studied to quantify and characterize its americium/curium selectivity. Liquid-liquid extraction and spectrophotometric titration indicated the presence of both fully deprotonated and monoprotonated complexes, An(bp18c6)(+) and An(Hbp18c6)(2+) (An = Am or Cm), at the acidities that would be encountered when treating nuclear wastes. The stability constants of the complexes in 1 M NaNO3 determined using competitive complexation were log ß101 = 15.49 ± 0.06 for Am and 14.88 ± 0.03 for Cm, indicating a reversal of the usual order of complex stability, where ligands bind the smaller Cm(III) ion more tightly than Am(III). The Am/Cm selectivity of bp18c6(2-) that is defined by the ratio of the Am and Cm stability constants (ß101 Am/ß101 Cm = 4.1) is the largest reported so far for binary An(III)-ligand complexes. Theoretical density functional theory calculations using the B3LYP functional suggest that the ligand's size-selectivity for larger 4f- and 5f-element cations arises from steric constraints in the crown ether ring. Enhanced 5f character in molecular orbitals involving actinide-nitrogen interactions is predicted to favor actinide(III) complexation by bp18c6(2-) over the complexation of similarly sized lanthanide(III) cations.