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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Polymer Electrolytes with High Ionic Concentration for Fuel Cells and Electrolyzers

As fuel cells and electrolyzers require higher performance and durability, the design aspects of polymer electrolytes continue to evolve. One distinctive trend in achieving a remarkably improved performance of electrochemical devices is to equip polymer electrolytes with high ionic concentration. This Review presents recent implementations of polymer electrolytes with a high ionic concentration in electrochemical devices. The synthetic approaches, benefits in device performance, challenges, and prospects of polymer electrolytes are summarized. The goal of this article is to explain the working principles of polymer electrolytes with a high acid concentration in high-performing fuel cells and electrolyzers, to survey recent achievements, and to address remaining technical issues. As well as informing polymer scientists, this Review may generate interest in the broader community of emerging fields to make use of these promising materials in tackling tough scientific challenges.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Effect of Solvent Motion on Ion Transport in Electrolytes

We use concentrated solution theory to derive an equation governing solvent velocity in a binary electrolyte when a current passes through it. This equation, in combination with the material balance equation, enables the prediction of electrolyte concentration profiles and species velocities as a function of space and time. This framework is used to predict ion velocities in Li-Li symmetric cells containing a mixture of lithium bis(trifluoromethanesulfonyl)imide and poly(ethylene oxide) (LiTFSI/PEO), for which the cation transference number relative to the solvent velocity, ${t}_{+}^{0},$ can be either positive or negative, depending on salt concentration. Accounting for the solvent motion is increasingly important at higher concentrations. Especially for negative ${t}_{+}^{0},$ if solvent velocity is set to zero, the cation velocity, based on the electrode-electrolyte interface reference frame, is pointed opposite to the current flow. However, when solvent motion is taken into account, the cation velocity, based on the same reference frame, is in the same direction as the current. This analysis demonstrates the importance of accounting for solvent velocity rigorously in seemingly simple systems such as symmetric cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Using Membranes with Internal Microchannels to Prevent Drying-out during CO 2 Electrolysis

Scaling up CO 2 electrolysis is a vital aspect in the transition to manufacturing sustainable fuels and chemical compounds, satisfying the demand for chemicals and demand for storing renewable electricity. Depending on the employed catalyst, different products can be produced, such as carbon monoxide and ethylene, by applying a voltage on a CO 2 and H 2 O fed electrolyzer. CO is a desirable product according to techno-economic analysis, because it can be produced selectively using a silver catalyst and is a precursor for hydrocarbons in the Fischer–Tropsch process. In a state-of-the-art CO 2 electrolyzer, two electrodes are directly pressed against an ion-exchange membrane – this is called a zero-gap configuration. Therefore, the membrane is a crucial component for the system, since it has the role of providing a conductive medium between the electrodes. One of the challenges in CO 2 electrolysis is that water is consumed in the reaction. At high current density, this may cause the membrane's surface near the cathode to dry out, lowering efficiency or perhaps stopping the process entirely, since there is no longer a conductive medium. As a result, water management is critical for this process. In this work, we approach the drying-out challenge by studying the novel concept of a membrane with internal microchannels. These channels allow the circulation of water or an electrolyte inside the membrane, which reduces the water diffusion path and affects the membrane’s conductivity. The effects of channel geometry, location, and concentrations of electrolyte inside on water content, conductivity and overall performance are studied in a 2D COMSOL model. In addition, the effect of internal concentration of electrolyte on the membrane’s resistance, on the process performance and the K + cross-over to cathode side were investigated experimentally. Our modeling results prove that the presence of the channels can keep the membrane hydrated. The highest current densities are observed when the channel is closest to the cathode, and with smaller pores. Smaller pores are advantageous due to the trade-off between enhanced membrane conductivity and the lower conductivity of the liquid itself. If water is circulated in a large channel, it increases the membrane’s ionomer conductivity due to hydration but the overall conductivity is decreased since water is not highly conductive. Nonetheless, the results also show that a higher concentration of electrolyte inside the microchannels can significantly increase the total conductivity of the membrane, and therefore the energy efficiency of the process. These effects are most significant at higher current densities. In the experimental results, we’ve observed similar effects in terms of membrane conductivity and current density of the process – the higher the electrolyte concentration the higher the current density. Furthermore, a low concentration decreases the amount of potassium which crosses over to the cathode side, inhibiting salt deposition. We’ve concluded that a small channel, up to 90 µm wide, close to the pore with an electrolyte with a concentration of up to 10 mM could be very beneficial for the water management and energy efficiency of the process. This helps to keep the membrane hydrated at higher current densities, improves the conductivity of the membrane, and it doesn’t have significant impacts on the salt deposition.

Petrov, Kostadin Veselinov↗
DOE OSTI2023

Real time lithium metal calendar aging in common battery electrolytes

Li metal anodes are highly sought after for high energy density applications in both primary commercial batteries and next-generation rechargeable batteries. In this research, Li metal electrodes are aged in coin cells for a year with electrolytes relevant to both types of batteries. The aging response is monitored via electrochemical impedance spectroscopy, and Li electrodes are characterized post-mortem. It was found that the carbonate-based electrolytes exhibit the most severe aging effects, despite the use of LiBF 4 -based carbonate electrolytes in Li/CF x Li primary batteries. Highly concentrated LiFSI electrolytes exhibit the most minimal aging effects, with only a small impedance increase with time. This is likely due to the concentrated nature of the electrolyte causing fewer solvent molecules available to react with the electrode surface. LiI-based electrolytes also show improved aging behavior both on their own and as an additive, with a similar impedance response with time as the concentrated LiFSI electrolytes. Since I − is in its most reduced state, it likely prevents further reaction and may help protect the Li electrode surface with a primarily organic solid electrolyte interphase.

25 ENERGY STORAGE↗
DOE OSTI2022

Comparing measurement of limiting current in block copolymer electrolytes as a function of salt concentration with theoretical predictions

Optimizing electrolyte performance is crucial for the widespread adoption of electrochemical energy storage. We demonstrate that limiting current provides a robust criterion for determining the optimum electrolyte. Experiments were conducted on rigid block copolymer electrolytes comprising mixtures of polystyrene-block-poly(ethylene oxide) copolymer (SEO) and lithium bis(trifluoromethanesulfonyl) imide salt (LiTFSI) over a salt concentration range from r av = 0.04 to r av = 0.20 (r av is the molar ratio of lithium ions to ethylene oxide). Here we show that the maximum limiting current density is 4.3 mA cm –2 at r av = 0.12. The dependence of limiting current on salt concentration is in good agreement with predictions from Newman's concentrated solution theory with no adjustable parameters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Operando Measurements of Electrolyte Li-ion Concentration during fast charging with FTIR/ATR

The ability to charge a Li-ion battery at high charging rates is critical for electric vehicle adoption; however, further study of ion transport is required to develop electrolytes suitable for fast charge. Fourier transform infrared spectroscopy (FTIR) used with attenuated total reflection (ATR) enables operando measurements of liquid electrolytes. This research focused on solvation shifting of solvent infrared absorption bands in the presence of lithium ions. Lithium-shifted infrared absorption bands and non-shifted bands of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were compared to infer ion concentration changes during cycling. Lithium concentrations were calibrated using EC/EMC/LiPF 6 electrolytes with known lithium concentrations. A Li-ion half-cell with a graphite anode and EC/EMC/LiPF 6 electrolyte was observed with FTIR/ATR. The results showed that the magnitude of lithium concentration changes increased with increasing C-rate. During a galvanostatic intermittent titration technique (GITT) test, changes in lithium concentration could be observed. During intercalation, a lithium depletion occurred when a negative current was applied, and a lithium surplus occurred during deintercalation when a positive current was applied. The capability of observing lithium concentration has significant implications for the utility of operando studies and the potential to link lithium movement to battery performance.

25 ENERGY STORAGE↗
DOE OSTI2021

Effects of Applied Interfacial Pressure on Li-Metal Cycling Performance and Morphology in 4 M LiFSI in DME

Lithium-metal anodes can theoretically enable 10x higher gravimetric capacity than conventional graphite anodes. However, Li-metal anode cycling has proven difficult due to porous and dendritic morphologies, extensive parasitic solid electrolyte interphase reactions, and formation of dead Li. We systematically investigate the effects of applied interfacial pressure on Li-metal anode cycling performance and morphology in the recently developed and highly efficient 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane electrolyte. We present cycling, morphology, and impedance data at a current density of 0.5 mA/cm 2 and a capacity of 2 mAh/cm 2 at applied interfacial pressures of 0, 0.01, 0.1, 1, and 10 MPa. Cryo-focused ion beam milling and cryo-scanning electron microscopy imaging in cross section reveal that increasing the applied pressure during Li deposition from 0 to 10 MPa leads to greater than a fivefold reduction in thickness (and therefore volume) of the deposited Li. This suggests that pressure during cycling can have a profound impact on the practical volumetric energy density for Li-metal anodes. A “goldilocks zone” of cell performance is observed at intermediate pressures of 0.1–1 MPa. Increasing pressure from 0 to 1 MPa generally improves cell-to-cell reproducibility, cycling stability, and Coulombic efficiency. However, the highest pressure (10 MPa) results in high cell overpotential and evidence of soft short circuits, which likely result from transport limitations associated with increased pressure causing local pore closure in the separator. All cells exhibit at least some signs of cycling instability after 50 cycles when cycled to 2 mAh/cm 2 with thin 50 μm Li counter electrodes, though instability decreases with increasing pressure. In contrast, cells cycled to only 1 mAh/cm 2 perform well for 50 cycles, indicating that capacity plays an important role in cycling stability.

25 ENERGY STORAGE↗
DOE OSTI2023

Comparing Theoretical Salt Concentration Profiles in a Polymer Electrolyte with Experimental Measurements using Operando Raman Spectroscopy

Concentrated solution theory has furthered our understanding of ion transport in electrolytes. This theory can be used to predict salt concentration profiles under an applied current if the transport properties of the electrolyte (conductivity ( κ ), restricted diffusion coefficient ( D ), and the cation transference number with respect to the solvent velocity ( t + 0 )), and the thermodynamic factor ( T f ) are known. In this work, we provide the first study comparing the predicted salt concentration profiles with measurements based on operando Raman spectroscopy. Concentration polarization is asymmetrical; the increase in salt concentration near the positive electrode is a factor of two greater than the decrease in salt concentration near the negative electrode. We find qualitative agreement between theory and experiment. Further work is needed to resolve the quantitative differences.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Unraveling Ion Transport in Trifluoromethanesulfonimide Pentablock Copolymer Membranes in Nonaqueous Electrolytes

Cation exchange membranes provide a key function in various nonaqueous energy storage technologies. As such, understanding the role of a nonaqueous liquid electrolyte on ion transport through a membrane is imperative. This study unravels relationships between membrane ion transport and electrolyte salt concentration utilizing a trifluoromethanesulfonimide-based pentablock copolymer membrane with polar and nonpolar nonaqueous electrolyte solutions. Here, the membrane plasticized with propylene carbonate (PC) exhibits a single-ion conductivity of 9.3 × 10 –6 S/cm at 25 °C. Tailoring the nonaqueous electrolyte and electrolyte salt concentration enables the single-ion conducting membrane to maintain a high cation transport number, >0.75, and boost ion conductivity more than 10 times to 1.3 × 10 –4 S/cm at 25 °C for the PC-based electrolyte solution. The findings provide critically important design parameters for energy storage systems utilizing nonaqueous electrolytes, such as Li- and Na-ion batteries and nonaqueous flow battery technologies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Ion Transport in Concentrated Crosslinked Solid Polymer Electrolytes

Crosslinking polymers is a common approach to create mechanically stable solid materials such as polymer electrolytes for lithium batteries. In conventional liquid electrolytes, the solvent molecules move freely to accommodate the field-induced motion of ions. However, in crosslinked polymer electrolytes, the rearrangement of polymer segments is constrained by the deformation limits of the network. Herein, we develop a new transport model that accounts for both the formation of concentration gradients and the elasticity of the electrolyte. The elasticity is incorporated by adding an additional term related to the entropy of crosslinked strands to the electrochemical potential of the salt. The resulting Crosslink Model contains two adjustable parameters: $\mathcal{N}$, the average number of monomers in a strand, and λ crit , the maximum strain the network can sustain. These solid-like constraints produce singularities in the governing transport equations, fundamentally altering the concentration profiles. Plateaus in salt concentrations emerge near the electrodes, and network elasticity introduces a strain overpotential. When compared to a Baseline Model ($\mathcal{N}$ → ∞, equivalent to concentrated solution theory), which predicts steepest gradients near the electrodes, both models yield similar current–voltage relationships. Model predictions are compared to electrochemical data for a poly(ethylene oxide)-based crosslinked polymer electrolyte.

Patel, Vivaan [University of California, Berkeley,↗
DOE OSTI2021

Evolution of Protrusions on Lithium Metal Anodes Stabilized by a Solid Block Copolymer Electrolyte Studied Using Time-Resolved X-ray Tomography

The growing demand for rechargeable batteries with higher energy densities has motivated research focused on enabling the lithium metal anode. A prominent failure mechanism in such batteries is short circuiting due to the uncontrolled propagation of lithium protrusions that often have a dendritic morphology. In this paper, the electrodeposition of metallic lithium through a rigid polystyrene- b -poly(ethylene oxide) (PS- b -PEO or SEO) block copolymer electrolyte was studied using hard X-ray microtomography. In this system, protrusions were approximately ellipsoidal globules: we take advantage of this simple geometry to quantify their growth as a function of polarization time and electrolyte salt concentration. The growth of 47 different globules was tracked with time to obtain average velocities of globule growth into the electrolyte. The globule diameter was a linear function of globule height in the electrolyte with a slope of about 6, independent of time and electrolyte salt concentration.

25 ENERGY STORAGE↗
DOE OSTI2020

A High-Rate Aqueous Proton Battery Delivering Power Below -78 °C via an Unfrozen Phosphoric Acid

Lithium-sulfur batteries are attractive for automobile and grid applications due to their high theoretical energy density and the abundance of sulfur. Despite the significant progress in cathode development, lithium metal degradation and the polysulfide shuttle remain two critical challenges in the practical application of Li-S batteries. Development of advanced electrolytes has become a promising strategy to simultaneously suppress lithium dendrite formation and prevent polysulfide dissolution. Here, a new class of concentrated siloxane-based electrolytes, demonstrating significantly improved performance over the widely investigated ether-based electrolytes are reported in terms of stabilizing the sulfur cathode and Li metal anode as well as minimizing flammability. Through a combination of experimental and computational investigation, it is found that siloxane solvents can effectively regulate a hidden solvation-ion-exchange process in the concentrated electrolytes that results from the interactions between cations/anions and solvents. As a result, it could invoke a quasi-solid-solid lithiation and enable reversible Li plating/stripping and robust solid-electrolyte interphase chemistries. The solvation-ion-exchange process in the concentrated electrolytes is a key factor in understanding and designing electrolytes for other high-energy lithium metal batteries.

25 ENERGY STORAGE↗
DOE OSTI2021

Ion Pairing, Clustering and Transport in a LiFSI-TMP Electrolyte as Functions of Salt Concentration using Molecular Dynamics Simulations

Battery capacity is highly related to ion-pairing mechanisms in electrolytes, since a cluster formation can lead to dead Li formation, reducing the number of charge carriers and leading to capacity fading. We use molecular dynamics simulations to model an electrolyte comprising trimethyl phosphate (TMP) solvent and a lithium bis(fluorosulfonyl)imide (LiFSI) salt, exploring effects of salt concentration on solvation and ion-transport. We simulate the LiFSI-TMP electrolyte for salt concentrations of 0.7, 1.43 and 3.82 molar. A statistical analysis was performed to study ion-pairing, clustering, diffusivity, conductivity, and coordination of Li-ions, providing insights into relations between molecular structures and transport properties. Molecular structure of ionic components changes as concentration increases, from a predominant solvent separated ion pair (SSIP) and contact ion pair (CIP) to aggregate salt (AGG) and ionic cluster formation. Given the formation of the ionic cluster, the diffusion mechanism followed by Li-ions changes from a hopping/exchange to a vehicular mechanism as concentration increases; this is reflected in a decrease of ionic conductivities. Ionicity was also calculated to reveal how the ionic motion changes from an uncorrelated to a correlated one as the salt concentration increases. Furthermore, we also compared our results with experimental calculations performed for similar electrolyte systems

25 ENERGY STORAGE↗
DOE OSTI2022

Understanding fluorine-free electrolytes via small-angle X-ray scattering

Fluorine-free electrolytes have attracted great attention because of its low-cost and environmental friendliness. However, so far, little is known about the solution structures of these electrolytes. Here, we compare the solvation phenomenon of sodium tetraphenylborate (NaBPh 4 ) salt dissolved in organic solvents of propylene carbonate (PC), 1,2-dimethoxyethane (DME), acetonitrile (ACN) and tetrahydrofuran (THF). Small-angle X-ray scattering (SAXS) reveals a unique two-peak structural feature in this salt concentrated PC electrolyte, while solutions using other solvents only have one scattering peak. Molecular dynamics (MD) simulations further reveal that there are anion-based clusters in addition to the short-range charge ordering in the concentrated NaBPh4/PC electrolyte. Raman spectroscopy confirms the existence of considerable contact ion pairs (CIPs). Here, this work emphasizes the importance of global and local structural analysis, which will provide valuable clues for understanding the structure performance relationship of electrolytes.

25 ENERGY STORAGE↗
DOE OSTI2024

Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI) 2 ], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI) 2 ], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Science & Technology - Other Topics↗
DOE OSTI2023

Simulations with machine learning potentials identify the ion conduction mechanism mediating non-Arrhenius behavior in LGPS

Abstract Li 10 Ge(PS 6 ) 2 (LGPS) is a highly concentrated solid electrolyte, in which Coulombic repulsion between neighboring cations is hypothesized as the underlying reason for concerted ion hopping, a mechanism common among superionic conductors such as Li 7 La 3 Zr 2 O 12 (LLZO) and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP). While first principles simulations using molecular dynamics (MD) provide insight into the Li + transport mechanism, historically, there has been a gap in the temperature ranges studied in simulations and experiments. Here, we used a neural network potential trained on density functional theory (DFT) simulations, to run up to 40-nanosecond long MD simulations at DFT-like accuracy to characterize the ion conduction mechanisms across a range of temperatures that includes previous simulations and experimental studies. We have confirmed a Li + sublattice phase transition in LGPS around 400 K, below which the ab -plane diffusivity D a b ∗ is drastically reduced. Concomitant with the sublattice phase transition near 400 K, there is less cation-cation (cross) correlation, as characterized by Haven ratios closer to 1, and the vibrations in the system are more harmonic at lower temperature. Intuitively, at high temperature, the collection of vibrational modes may be sufficient to drive concerted ion hops. However, near room temperature, the vibrational modes available may be insufficient to overcome electrostatic repulsion, thus resulting in less correlated ion motion and comparatively slower ion conduction. Such phenomena of a sublattice phase transition, below which concerted hopping plays a less significant role, may be extended to other highly concentrated solid electrolytes such as LLZO and LATP.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Synthesis of UO 2 nanoparticles via coulometric titration: influence of electrolytes and ligands on synthesized particle properties

Here, this study investigates the controlled synthesis of uranium oxide nanoparticles (UO 2 NPs) via coulometric titration under ambient conditions, focusing on the impact of electrolyte anions, salt concentrations, and strongly complexing ligands on particle formation and properties. Using dilute solutions of mineral acids (HNO 3 , HCl, and HClO 4 ), we demonstrate that the choice of electrolyte anion affects particle size, polydispersity, and surface charge. Particle characterization using dynamic light scattering and transmission electron microscopy shows that particles synthesized in perchlorate are largest and show a high degree of polydispersity. In comparison, particles synthesized in chloride are smaller and more uniform in size. Synthesis in nitrate yields a wide variety of particle sizes, with the major fraction (>65 %) having a smaller size than that obtained in the other two electrolytes. The presence of more strongly coordinating ligands such as sulfate and acetate modulate hydrolysis and condensation reactions, with sulfate suppressing nanoparticle formation across a wide concentration range ([SO 4 2- ] > 5 mM) and acetate enabling stable colloidal suspensions at [HOAc] ≤ 0.1 M. Synthesis in concentrated electrolytes (2 M NaNO 3 , NaCl, or NaClO 4 ) accelerates reaction kinetics but introduces challenges, as particles showed increased polydispersity and aggregation, and were more prone to oxidation. Electrolyte effects on actinide oxide nanoparticle formation are discussed and a short comparison to established nanoparticle syntheses is drawn. This work underscores the importance of tailoring synthesis parameters to achieve desired nanoparticle properties, providing valuable insights for optimizing UO 2 NP production for various applications.

Actinide hydrolysis↗
DOE OSTI2025

Pressure effects on lithium anode/nickel-manganese-cobalt oxide cathode pouch cells through fixture design

Applying external pressure to a pouch cell results in improved performance, implicating systems-level design of batteries. Here, different formats and amounts of external pressure to Li-Li x Ni 0.8 Mn 0.1 Co 0.1 O 2 (Li-NMC811) pouch cells were studied under lean electrolyte conditions. Due to the more uniform lithium plating/stripping, a constant gap fixture that retains the distance of the frame during cycling performed greater than a constant pressure fixture that retains applied pressure to the cell. In addition, the use of flexible foam in a constant gap fixture revealed enhanced cycle life at 10 psi; however, at 30 psi, the use of a rigid plate extended cycle life to over 250 cycles, while the foam severely shortened cycle life. This discrepancy with pressure was proven to be driven by stress distribution on cell components. The failure mechanisms and the effects of pressure fixture design on cell components were unveiled, shedding light on improving high-energy battery performance through at-scale fixture design.

25 - ENERGY STORAGE↗