From Bulk to Interface: Solvent Exchange Dynamics and Their Role in Ion Transport and the Interfacial Model of Rechargeable Magnesium Batteries.

Multivalent battery chemistries have been explored in response to the increasing demand for high-energy rechargeable batteries utilizing sustainable resources. Solvation structures of working cations have been recognized as a key component in the design of electrolytes; however, most structure-property correlations of metal ions in organic electrolytes usually build upon favorable static solvation structures, often overlooking solvent exchange dynamics. We here report the ion solvation structures and solvent exchange rates of magnesium electrolytes in various solvents by using multimodal nuclear magnetic resonance (NMR) analysis and molecular dynamics/density functional theory (MD/DFT) calculations. These magnesium solvation structures and solvent exchange dynamics are correlated to the combined effects of several physicochemical properties of the solvents. Moreover, Mg2+ transport and interfacial charge transfer efficiency are found to be closely correlated to the solvent exchange rate in the binary electrolytes where the solvent exchange is tunable by the fraction of diluent solvents. Our primary findings are (1) most battery-related solvents undergo ultraslow solvent exchange coordinating to Mg2+ (with time scales ranging from 0.5 µs to 5 ms), (2) the cation transport mechanism is a mixture of vehicular and structural diffusion even at the ultraslow exchange limit (with faster solvent exchange leading to faster cation transport), and (3) an interfacial model wherein organic-rich regions facilitate desolvation and inorganic regions promote Mg2+ transport is consistent with our NMR, electrochemistry, and cryogenic X-ray photoelectron spectroscopy (cryo-XPS) results. This observed ultraslow solvent exchange and its importance for ion transport and interfacial properties necessitate the judicious selection of solvents and informed design of electrolyte blends for multivalent electrolytes.

Guiding maps of solvents for lithium-sulfur batteries via a computational data-driven approach.

Practical realization of lithium-sulfur batteries requires designing optimal electrolytes with controlled dissolution of polysulfides, high ionic conductivity, and low viscosity. Computational chemistry techniques enable tuning atomistic interactions to discover electrolytes with targeted properties. Here, we introduce ComBat (Computational Database for Lithium-Sulfur Batteries), a public database of ∼2,000 quantum-chemical and molecular dynamics properties for lithium-sulfur electrolytes composed of solvents spanning 16 chemical classes. We discuss the microscopic origins of polysulfide clustering and the diffusion mechanism of electrolyte components. Our findings reveal that polysulfide solubility cannot be determined by a single solvent property like dielectric constant. Rather, observed trends result from the synergistic effect of multiple factors, including solvent C/O ratio, fluorination degree, and steric hindrance effects. We propose binding energy as a proxy for Li+ dissociation, which is a property that impacts the ionic conductivity. The insights obtained in this work can serve as guiding maps to design optimal lithium-sulfur electrolyte compositions.

Non-polar ether-based electrolyte solutions for stable high-voltage non-aqueous lithium metal batteries.

The electrochemical instability of ether-based electrolyte solutions hinders their practical applications in high-voltage Li metal batteries. To circumvent this issue, here, we propose a dilution strategy to lose the Li+/solvent interaction and use the dilute non-aqueous electrolyte solution in high-voltage lithium metal batteries. We demonstrate that in a non-polar dipropyl ether (DPE)-based electrolyte solution with lithium bis(fluorosulfonyl) imide salt, the decomposition order of solvated species can be adjusted to promote the Li+/salt-derived anion clusters decomposition over free ether solvent molecules. This selective mechanism favors the formation of a robust cathode electrolyte interphase (CEI) and a solvent-deficient electric double-layer structure at the positive electrode interface. When the DPE-based electrolyte is tested in combination with a Li metal negative electrode (50 µm thick) and a LiNi0.8Co0.1Mn0.1O2-based positive electrode (3.3 mAh/cm2) in pouch cell configuration at 25 °C, a specific discharge capacity retention of about 74% after 150 cycles (0.33 and 1 mA/cm2 charge and discharge, respectively) is obtained.

MISPR: an open-source package for high-throughput multiscale molecular simulations.

Computational tools provide a unique opportunity to study and design optimal materials by enhancing our ability to comprehend the connections between their atomistic structure and functional properties. However, designing materials with tailored functionalities is complicated due to the necessity to integrate various computational-chemistry software (not necessarily compatible with one another), the heterogeneous nature of the generated data, and the need to explore vast chemical and parameter spaces. The latter is especially important to avoid bias in scattered data points-based models and derive statistical trends only accessible by systematic datasets. Here, we introduce a robust high-throughput multi-scale computational infrastructure coined MISPR (Materials Informatics for Structure-Property Relationships) that seamlessly integrates classical molecular dynamics (MD) simulations with density functional theory (DFT). By enabling high-performance data analytics and coupling between different methods and scales, MISPR addresses critical challenges arising from the needs of automated workflow management and data provenance recording. The major features of MISPR include automated DFT and MD simulations, error handling, derivation of molecular and ensemble properties, and creation of output databases that organize results from individual calculations to enable reproducibility and transparency. In this work, we describe fully automated DFT workflows implemented in MISPR to compute various properties such as nuclear magnetic resonance chemical shift, binding energy, bond dissociation energy, and redox potential with support for multiple methods such as electron transfer and proton-coupled electron transfer reactions. The infrastructure also enables the characterization of large-scale ensemble properties by providing MD workflows that calculate a wide range of structural and dynamical properties in liquid solutions. MISPR employs the methodologies of materials informatics to facilitate understanding and prediction of phenomenological structure-property relationships, which are crucial to designing novel optimal materials for numerous scientific applications and engineering technologies.

An automated framework for high-throughput predictions of NMR chemical shifts within liquid solutions.

Identifying stable speciation in multi-component liquid solutions is fundamentally important to areas from electrochemistry to organic chemistry and biomolecular systems. Here we introduce a fully automated, high-throughput computational framework for the accurate prediction of stable species in liquid solutions by computing the nuclear magnetic resonance (NMR) chemical shifts. The framework automatically extracts and categorizes hundreds of thousands of atomic clusters from classical molecular dynamics simulations, identifies the most stable species in solution and calculates their NMR chemical shifts via density functional theory calculations. Additionally, the framework creates a database of computed chemical shifts for liquid solutions across a wide chemical and parameter space. We compare our computational results to experimental measurements for magnesium bis(trifluoromethanesulfonyl)imide Mg(TFSI)2 salt in dimethoxyethane solvent. Our analysis of the Mg2+ solvation structural evolutions reveals key factors that influence the accuracy of NMR chemical shift predictions in liquid solutions. Furthermore, we show how the framework reduces the performance of over 300 13C and 600 1H density functional theory chemical shift predictions to a single submission procedure.

Role of a Multivalent Ion-Solvent Interaction on Restricted Mg2+ Diffusion in Dimethoxyethane Electrolytes.

The diffusion behavior of Mg2+ in electrolytes is not as readily accessible as that from Li+ or Na+ utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting 25Mg NMR. In MgTFSI2/DME solutions, "bound" DME (coordinating to Mg2+) and "free" DME (bulk) are distinguishable from 1H NMR. With the exchange rates between them obtained from 2D 1H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg2+) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65-70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C-H bond length. Comparison of the diffusion behaviors of Mg2+, TFSI-, DME, and Li+ reveals a relative restriction to Mg2+ diffusion that is caused by the long-range interaction between Mg2+ and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.

Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes-A Review.

Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes.

"Rocking-Chair"-Type Metal Hybrid Supercapacitors.

Hybrid supercapacitors that follow a "rocking-chair"-type mechanism were developed by coupling divalent metal and activated carbon electrodes in nonaqueous electrolytes. Conventional supercapacitors require a large amount of electrolyte to provide a sufficient quantity of ions to the electrodes, due to their Daniell-type mechanism that depletes the ions from the electrolyte while charging. The alternative "rocking-chair"-type mechanism effectively enhances the energy density of supercapacitors by minimizing the necessary amount of electrolyte, because the ion is replenished from the metal anode while it is adsorbed to the cathode. Newly developed nonaqueous electrolytes for Mg and Zn electrochemistry, based on bis(trifluoromethylsulfonyl)imide (TFSI) salts, made the metal hybrid supercapacitors possible by enabling reversible deposition on the metal anodes and reversible adsorption on an activated carbon cathode. Factoring in gains through the cell design, the energy density of the metal hybrid supercapacitors is projected to be a factor of 7 higher than conventional devices thanks to both the "rocking-chair"-type mechanism that minimizes total electrolyte volume and the use of metal anodes, which have substantial merits in capacity and voltage. Self-discharge was also substantially alleviated compared to conventional supercapacitors. This concept offers a route to build supercapacitors that meet dual criteria of power and energy densities with a simple cell design.

Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes.

Through coupled experimental analysis and computational techniques, we uncover the origin of anodic stability for a range of nonaqueous zinc electrolytes. By examination of electrochemical, structural, and transport properties of nonaqueous zinc electrolytes with varying concentrations, it is demonstrated that the acetonitrile-Zn(TFSI)2, acetonitrile-Zn(CF3SO3)2, and propylene carbonate-Zn(TFSI)2 electrolytes can not only support highly reversible Zn deposition behavior on a Zn metal anode (≥99% of Coulombic efficiency) but also provide high anodic stability (up to ∼3.8 V vs Zn/Zn(2+)). The predicted anodic stability from DFT calculations is well in accordance with experimental results, and elucidates that the solvents play an important role in anodic stability of most electrolytes. Molecular dynamics (MD) simulations were used to understand the solvation structure (e.g., ion solvation and ionic association) and its effect on dynamics and transport properties (e.g., diffusion coefficient and ionic conductivity) of the electrolytes. The combination of these techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties in nonaqueous zinc electrolytes.

Accelerating Electrolyte Discovery for Energy Storage with High-Throughput Screening.

Computational screening techniques have been found to be an effective alternative to the trial and error of experimentation for discovery of new materials. With increased interest in development of advanced electrical energy storage systems, it is essential to find new electrolytes that function effectively. This Perspective reviews various methods for screening electrolytes and then describes a hierarchical computational scheme to screen multiple properties of advanced electrical energy storage electrolytes using high-throughput quantum chemical calculations. The approach effectively down-selects a large pool of candidates based on successive property evaluation. As an example, results of screening are presented for redox potentials, solvation energies, and structural changes of ∼1400 organic molecules for nonaqueous redox flow batteries. Importantly, on the basis of high-throughput screening, in silico design of suitable candidate molecules for synthesis and electrochemical testing can be achieved. We anticipate that the computational approach described in this Perspective coupled with experimentation will have a significant role to play in the discovery of materials for future energy needs.

The coupling between stability and ion pair formation in magnesium electrolytes from first-principles quantum mechanics and classical molecular dynamics.

In this work we uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potentials, e.g., at the metal anode. Through comprehensive calculations using both first-principles as well as well-benchmarked classical molecular dynamics over a matrix of electrolytes, covering solvents and salt anions with a broad range in chemistry, we elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. We find that Mg electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg(2+) → Mg(+)), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI(-) exhibits a significant bond weakening while paired with the transient, partially reduced Mg(+). In contrast, BH4(-) and BF4(-) are shown to be chemically stable in a reduced ion pair configuration. Furthermore, we observe that higher order glymes as well as DMSO improve the solubility of Mg salts, but only the longer glyme chains reduce the dynamics of the ions in solution. This information provides critical design metrics for future electrolytes as it elucidates a close connection between bulk solvation and cathodic stability as well as the dynamics of the salt.

Diffusional motion of redox centers in carbonate electrolytes.

Ferrocene (Fc) and N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium bistrifluoromethyl-sulfonimide (Fc1N112-TFSI) were dissolved in carbonate solvents and self-diffusion coefficients (D) of solutes and solvents were measured by (1)H and (19)F pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy. The organic solvents were propylene carbonate (PC), ethyl methyl carbonate (EMC), and a ternary mixture that also includes ethylene carbonate (EC). Results from NMR studies over the temperature range of 0-50 °C and for various concentrations (0.25-1.7 M) of Fc1N112-TFSI are compared to values of D simulated with classical molecular dynamics (MD). The measured self-diffusion coefficients gradually decreased as the Fc1N112-TFSI concentration increased in all solvents. Since TFSI(-) has fluoromethyl groups (CF3), D(TFSI) could be measured separately and the values found are larger than those for D(Fc1N112) in all samples measured. The EC, PC, and EMC have the same D in the neat solvent mixture and when Fc is dissolved in EC/PC/EMC at a concentration of 0.2 M, probably due to the interactions between common carbonyl structures within EC, PC, and EMC. A difference in D (D(PC) < D(EC) < D(EMC)), and both a higher E(a) for translational motion and higher effective viscosity for PC in the mixture containing Fc1N112-TFSI reflect the interaction between PC and Fc1N112(+), which is a relatively stronger interaction than that between Fc1N112(+) and other solvent species. In the EC/PC/EMC solution that is saturated with Fc1N112-TFSI, we find that D(PC) = D(EC) = D(EMC) and Fc1N112(+) and all components of the EC/PC/EMC solution have the same E(a) for translational motion, while the ratio D(EC/PC/EMC)/D(Fc1N112) is approximately 3. These results reflect the lack of available free volume for independent diffusion in the saturated solution. The Fc1N112(+) transference numbers lie around 0.4 and increase slightly as the temperature is increased in the PC and EMC solvents. The trends observed for D from simulations are in good agreement with experimental results and provide molecular level understanding of the solvation structure of Fc1N112-TFSI dissolved in EC/PC/EMC.

Solvation structure and energetics of electrolytes for multivalent energy storage.

By analysing X-ray pair distribution function data using a multivariate statistical approach, we isolate the cation solvation structure for monovalent (Li(+)/Na(+)/K(+)) and multivalent (Mg(2+)/Ca(2+)/Zn(2+)) electrolytes based on TFSI salts in diglyme. Parallel molecular dynamics simulations provide enhanced structural details. The data suggest that contact ion-pairs are a common feature in multivalent electrolytes.

Molecular dynamics simulations of the ionic liquid [EMIM+][TFMSI-] confined inside rutile (110) slit nanopores.

The structure and dynamics of the ionic liquid (IL) [EMIM(+)][TFMSI(-)] inside a rutile (110) slit nanopore of width H = 5.2 nm at T = 333 K are studied using classical molecular dynamics (MD) simulations. These results are compared against those obtained in our previous study (N. N. Rajput et al., J. Phys. Chem. C, 2012, 116, 5169-5181) for the same IL inside a slit graphitic nanopore of the same width. Electrostatic and dispersion interactions are present between the IL and the rutile walls, whereas only weaker van der Waals interactions are present between the IL and the graphitic walls. Our results suggest that the strength of the interactions between the pore walls and the IL can significantly affect the structure and dynamics of the confined IL. Layering effects were more pronounced for the IL inside a rutile pore as compared to inside a graphitic pore. The ions near the rutile pore walls had a liquid structure that was significantly different from that of the bulk IL; in contrast, the same ions near graphitic pore walls had a liquid structure that was similar to that of the bulk IL. Cations and anions adopted multiple orientations near the rutile walls, which contrast with the parallel orientations that were uniformly observed for the same ions near graphitic walls. The dynamics of [EMIM(+)][TFMSI(-)] inside a slit rutile pore are significantly slower than those observed inside a slit graphitic pore. Near the rutile walls, the dynamics of the ions were about an order of magnitude slower than those of ions near graphitic walls. The ions in the center of a rutile pore exhibit enhanced mobilities, but still about 2-4 times slower than those observed for ions in the center of a graphitic pore. The effects of variations in the amount of IL on the dynamics were very marked inside a rutile pore, with reductions of up to 4 times in the mobilities of the ions in the different regions of the pore; in contrast, pore loading seems to cause smaller variations in the dynamics of ILs inside a graphitic slit nanopore.