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DOE OSTI2023

Model simulations of Plum Island Ecosystems LTER low marsh site using ELM-PFLOTRAN

Model simulations using the E3SM Land Model (ELM) coupled to the PFLOTRAN reactive transport model via the Alquimia interface. The simulations were conducted for a tidal salt marsh at the Plum Island Ecosystems LTER near Rowley, Massachusetts, USA. Model simulations were forced using site-specific tidal cycles and salinity, and the simulations used a biogeochemical reaction network including aerobic decomposition, sulfate reduction, iron reduction, and methanogenesis. Model outputs include simulated carbon stocks, carbon dioxide and methane fluxes, and porewater concentrations of key solutes related to sulfur, iron, and carbon cycling. The model simulations included a saline simulation (with tidal sulfate inputs), a fresh simulation (with low salinity and low sulfate inputs), and a saline simulation with lower vegetation productivity to represent the effect of salinity on vegetation. These simulations were conducted to demonstrate that a new model framework incorporating subsurface redox and biogeochemical interactions into a land surface model could reproduce measured surface greenhouse gas fluxes and biogeochemical dynamics in tidal marsh ecosystems, and to test whether including redox interactions in a land surface model would allow the model to resolve contrasts in biogeochemical cycling and greenhouse gas production between saline and freshwater wetlands.The data package includes gzipped tar archives (which can be expanded using standard tar and gzip utilities) of model outputs from three model configurations: saline subsurface and reduced vegetation productivity related to salinity; saline subsurface with vegetation productivity not reduced; and freshwater. Also included are code for the modified E3SM model, Alquimia interface, and PFLOTRAN reactive transport simulator in gzipped tar format; plain text parameter and configuration files; python code files for visualizing model output and defining model configurations; and model output, tide and salinity forcing, and configuration files in netCDF format. See the README.md file in the data package for a detailed description of all files contained in the package. All files are in netCDF (.nc), gzipped tar archive (.tar.gz or .tgz), or text (all other files).Updated: May 13, 2024. Model output, E3SM code, PFLOTRAN input files, and python codes for visualizing results were updated to reflect changes made for the manuscript revision. The updated archive reflects the code and model output from the final accepted manuscript. Changes included updated reaction parameters reflecting improved parameterization and additional comparisons with field measurements. E3SM code changes included better support for multiple grid cells and improved flow and transport parameterization.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Modeling prebiotic chemistries with quantum accuracy at classical costs

Molecular Dynamics (MD) simulations using classical force-fields are commonly employed in numerous scientific investigations. However, many natural processes involve bond breaking and quantum forces. This complexity is compounded by the presence of multiple competing length and timescales. For example, accurately modeling the thermodynamics and dynamics of a chemical reaction requires accounting for the concerted movements of numerous solvent molecules and ions with their own fast or slow timescales. While widely used static Density Functional Theory (DFT) calculations at 0 temperature can be beneficial for such investigations, they do not account for dynamics, and lack precision in describing the molecular environments. They particularly fail at correct, rigorous treatments of finite-temperature fluctuations, and thus generalization to experimentally relevant conditions. In PNAS Benayad et al develop a scalable, generalizable approach for designing Neural Network Potentials (NNPs) that can handle chemical reactivity in solvated systems with quantum accuracy at classical costs. Specifically, they study phosphoester bond formation and rupture, which is fundamentally relevant to the Phosphorus-Oxygen bond formation central to life, and especially for the RNA world hypothesis. The framework developed here has the potential to generalize to different chemical reactions of energy and biological relevance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Liquid-to-gas transfer of sodium in a liquid cathode glow discharge

Abstract Plasma-liquid interactions have been extensively studied with a focus on the transport of reactive species from the plasma to the liquid phase and their induced liquid phase chemistry and resulting applications. While solute transfer from the liquid to the gas phase in plasmas has been widely used in analytical chemistry, the underlying processes remain relatively unexplored. We report spatially and temporally resolved absolute density measurements of sodium in a plasma with a NaCl solution cathode using two-photon absorption laser induced fluorescence (TaLIF). The observed non-linear increase in sodium density with solution conductivity is shown to correlate with droplet generation as visualized by Mie scattering. The findings are explained by droplet generation by electrospray induced by Taylor cone formation as underpinning mechanism for the introduction of sodium in the plasma. An analytical sheath model combined with a scaling law shows an increase in electric field force with solution conductivity that is consistent with the observed non-linear increase in sodium density in the plasma with solution conductivity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Teaching a neural network to attach and detach electrons from molecules

Abstract Interatomic potentials derived with Machine Learning algorithms such as Deep-Neural Networks (DNNs), achieve the accuracy of high-fidelity quantum mechanical (QM) methods in areas traditionally dominated by empirical force fields and allow performing massive simulations. Most DNN potentials were parametrized for neutral molecules or closed-shell ions due to architectural limitations. In this work, we propose an improved machine learning framework for simulating open-shell anions and cations. We introduce the AIMNet-NSE (Neural Spin Equilibration) architecture, which can predict molecular energies for an arbitrary combination of molecular charge and spin multiplicity with errors of about 2–3 kcal/mol and spin-charges with error errors ~0.01e for small and medium-sized organic molecules, compared to the reference QM simulations. The AIMNet-NSE model allows to fully bypass QM calculations and derive the ionization potential, electron affinity, and conceptual Density Functional Theory quantities like electronegativity, hardness, and condensed Fukui functions. We show that these descriptors, along with learned atomic representations, could be used to model chemical reactivity through an example of regioselectivity in electrophilic aromatic substitution reactions.

36 MATERIALS SCIENCE↗

NASA NTRS2000

Thermophysical Property Measurement and Materials Research in the NASA/MSFC Electrostatic Levitator

Containerless processing is an important tool for thermophysical property measurements and materials research. The freedom from a crucible allows processing of liquid materials in a metastable undercooled state, as well as allowing processing of high temperature and highly reactive melts. Electrostatic levitation (ESL) is a containerless method which provides a number of unique advantages, including the decoupling of positioning force from sample heating, the ability to operate in ultra-high vacuum or at moderate gas pressure (approximately 3 atm), and the ability to process non-conducting materials. ESL also has the potential to reduce internal flow velocities below those possible with electromagnetic, acoustic, or aero-acoustic techniques. The ESL facility at NASA's Marshall Space Flight Center (MSFC) is in use for thermophysical property measurements and materials research by a number of different internal and external investigators. The methods for obtaining access to the facility, as well as a summary of current capabilities and some future directions will be discussed. In electrostatic levitation, the acceleration of gravity (or residual acceleration in reduced gravity) is opposed by the action of an applied electric field on a charged sample. This positioning method is applicable to any material which can be electrically charged, whether solid or liquid, conducting, or insulating. Because the position of the sample is unstable, a 3-dimensional active control loop rapidly adjusts the applied field to maintain levitation and minimize motion of the sample. Heating lasers melt the sample, and may be adjusted to maintain specified thermal profiles. Microgravity allows electrostatic levitation to work even more effectively. With the need to cancel less than 1 milli-g, the applied field required is reduced from approximately 10 MV/m to approximately 10 kV/m. Alternatively, a microgravity ESL can position larger samples than is possible on the ground, or it can position samples which maintain their charge poorly. Microgravity also reduces the effects of buoyant convection and sedimentation. In the MSFC ESL, many different classes of experiments and measurements have been demonstrated in collaboration with many investigators. These capabilities include heat capacity measurement, phase diagram determination, and nucleation and growth experiments including nucleation rate, TTT curves, and phase selection. We have demonstrated the capability of triggering recalescence of an undercooled liquid sample without loss of levitation. Currently under development are techniques for measuring the surface tension, viscosity, and density of levitated samples, as well as other thermophysical properties. Expanded levitation capabilities including various gas environments for sample processing, sample rotation control and management of thermal gradients are under development for the MSFC ESL. Containerless processing allows research on materials which may not be processed any other way. Electrostatic levitation extends the range of containerless processing to include many materials and classes of materials which cannot be processed by conventional electromagnetic, acoustic, or aerodynamic levitation techniques.

Rogers, Jan↗

DOE OSTI2021

Ferroelectrics everywhere: Ferroelectricity in magnesium substituted zinc oxide thin films

We demonstrate ferroelectricity in Mg-substituted ZnO thin films with the wurtzite structure. Zn 1–x Mg x O films are grown by dual-cathode reactive magnetron sputtering on (111)-Pt // (0001)-Al 2 O 3 substrates at temperatures ranging from 26 to 200 °C for compositions spanning from x = 0 to x = 0.37. X-ray diffraction indicates a decrease in the c-lattice parameter and an increase in the a-lattice parameter with increasing Mg content, resulting in a nearly constant c/a axial ratio of 1.595 over this composition range. Transmission electron microscopy studies show abrupt interfaces between Zn 1–x Mg x O films and the Pt electrode. When prepared at pO 2 = 0.025, film surfaces are populated by abnormally oriented grains as measured by atomic force microscopy for Mg concentrations >29%. Raising pO 2 to 0.25 eliminates the misoriented grains. Optical measurements show increasing bandgap values with increasing Mg content. When prepared on a 200 °C substrate, films display ferroelectric switching with remanent polarizations exceeding 100 μC cm –2 and coercive fields below 3 MVcm –1 when the Mg content is between ~30% and ~7%. Substrate temperature can be lowered to ambient conditions, and when doing so, capacitor stacks show only minor sacrifices to crystal orientation and nearly identical remanent polarization values; however, coercive fields drop below 2 MV/cm. Using ambient temperature deposition, we demonstrate ferroelectric capacitor stacks integrated directly with polymer substrate surfaces.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Continued Verification of MOOSE Structural Mechanics Tools for Modeling Core Bowing Phenomena in Fast Reactors

Under the U.S. Department of Energy Office of Nuclear Energy’s Advanced Modeling and Simulation (NEAMS) Program, an integrated multiphysics approach is being developed to model the core bowing phenomena important to liquid metal-cooled fast reactors. Core bowing is an important passive safety mechanism whereby increased power (which leads to temperature and flux gradients) influences the core to bow into less reactive configurations when the restraint system is properly designed. The phenomenon includes a complex interplay of radiation transport, duct temperature calculations involving fluid flow and heat transfer, and thermo-mechanical responses to the induced temperature and flux gradients. Structural material properties are also important to determining inelastic response to longer term flux gradients which cause irradiation creep and swelling. While core bowing provides a strong negative reactivity feedback when the restraint system is designed properly, it also results in additional forces between assemblies which increase the loads required to extricate them during refueling or control rod movement. Therefore, the restraint system must be designed with these tradeoffs in mind. The first stage of the work, which commenced in FY21 and continues through FY22, assesses thermo-mechanical modeling tools for producing core bowing predictions consistent with conventional tools. The Multiphysics Object Oriented Simulation Environment (MOOSE) Tensor Mechanics and Contact Modules are employed. This status report describes work on additional thermo-mechanical benchmark verification problems with increased complexity from the examples demonstrated in FY21. Several benchmark verification examples were selected from the IAEA verification and validation report. These examples involve clusters of ducts representative of a sector of a hexagonal reactor core which bow into each other and cause contact and load pad elevations, as well as single ducts subjected to irradiation fields undergoing swelling and subsequent bowing. The MOOSE-based results were compared to both IAEA benchmark participants’ results, analytic equations as available, and NUBOW-3D, a beam model code developed by Argonne National Laboratory. In every case, the MOOSE results agreed with other simulations results, providing additional verification basis of the tools for this particular physics application.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2024

New Nonreactive Force Field for Accurate Molecular Dynamics Simulations of TATB at Extreme Conditions

Insensitive high explosives based on TATB (1,3,5-triamino-2,4,6-trinitrobenzene) are needed in applications when safety is of paramount importance, but the basic material properties that give rise to its insensitivity are not fully understood. Molecular dynamics modeling using empirical force fields (FFs) has been the main route to characterize many complicated dynamical properties of TATB single crystal, but these FFs have not been comprehensively tested at extreme conditions typical of detonation. We collect a benchmark data set of (quasi)static TATB physical properties as determined by experiments and electronic structure calculations and apply this data set to validate four existing TATB FFs along with a new TATB FF that we develop here and denote as the CEA-LLNL-Missouri (CLM) FF. Benchmark data include vibrational spectra, the TATB crystal temperature–pressure–volume equation of state and lattice parameters, properties of TATB crystal polymorphs and transitions to the gaseous and liquid states, dimer energy landscapes, the pressure-dependent elastic tensor, and the energy landscape for inelastic deformation via sliding of TATB crystal layers. As a general assessment, we find that the two existing nonreactive FFs are more accurate in describing TATB’s physical properties compared to the two variants of the ReaxFF reactive FF considered. The new CLM FF is found to consistently yield similar or better agreement with experiments and electronic structure theory than any of the existing FF models, and it presents a distinct improvement in accurately modeling TATB elasticity and equation of state. So this work is expected to help improve the accuracy of FF-based modeling of complicated dynamic responses that ultimately govern the safety and performance characteristics of this material.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Assessing entropy for catalytic processes at complex reactive interfaces

When chemical reactions are accelerated by a catalyst, entropy differences between reactants and their transient intermediates can be the driving force behind the promotion or inhibition of desired and parasitic chemical pathways. Understanding and controlling catalytic processes therefore requires both a fundamental and practicable understanding of entropy in addition to enthalpy. In unstructured media such as the vapor phase equilibrated with sparsely covered surfaces, entropy can be adequately accounted for by well-established approaches based on translational, rotational, and harmonic vibrational partition functions. However, these approximations become inadequate in more complex condensed phase environments, e.g., solid liquid interfaces of confined reaction spaces. In this chapter, we provide an overview of the state-of-art in the computational quantification of entropy and its known ramifications on catalysis. The fundamental roles of thermodynamics and kinetics in catalysis are covered in enough detail to appreciate and contextualize the computational methods employed to compute chemically accurate estimates of entropy. These methods are discussed in appropriate detail and range from the ubiquitous harmonic oscillator approximation where entropy unrelated to high frequency oscillations is typically underestimated, to enhanced free energy sampling with molecular dynamics where the desired accuracy must be weighed against the associated computational cost of obtaining it. The rising importance of machine learning and artificial intelligence in accelerating methodological progress in this field is touched upon, as well. Finally, applications, successes, and pitfalls of using these methods are provided to showcase past and present accomplishments while clarifying where improvements in both understanding and methodology are still needed.

Kollias, Loukas↗

DOE OSTI2023

High Yield Muon Catalyzed Fusion

Fusion holds great promise as a clean and abundant energy source. However, traditional thermonuclear fusion encounters significant challenges due to the extreme temperatures required to overcome the coulomb barrier for two nuclei to fuse. In contrast, muon-catalyzed fusion presents an alternative approach that can surmount this barrier at significantly lower temperatures. Muons, with properties resembling those of electrons but 200 times heavier, can effectively reduce the atomic orbital radius, enabling central nuclei to overcome the coulomb force through the strong force. By introducing muons into a mixture of deuterium and tritium (two hydrogen isotopes), fusion is facilitated, releasing a 3.5MeV alpha particle and a 14.1MeV neutron. In the majority of cases, the muon is liberated and can initiate further fusions. However, approximately 0.8% of the time, it adheres to the alpha particle and remains bound until it either decays or undergoes reactivation through collisional ionization. To maximize the number of fusions per muon, it is crucial to enhance the cycling rate and reactivation fraction. Theoretical predictions and experimental data both suggest that the sticking rate decreases with increasing density. However, there exists a discrepancy between experimental observations and theoretical estimations regarding the extent of this decrease. To address these disparities, this experiment aims to investigate the cycling rate and sticking fraction under higher temperatures and pressures than previously explored.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2023

Muon-Catalyzed Fusion

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Beam Design for Muon Catalyzed Fusion

Fusion holds great promise as a clean and abundant energy source. However, traditional thermonuclear fusion encounters significant challenges due to the extreme temperatures required to overcome the coulomb barrier for two nuclei to fuse. In contrast, muon-catalyzed fusion presents an alternative approach that can surmount this barrier at significantly lower temperatures. Muons, with properties resembling those of electrons but 200 times heavier, can effectively reduce the atomic orbital radius, enabling central nuclei to overcome the coulomb force through the strong force. By introducing muons into a mixture of deuterium and tritium (two hydrogen isotopes), fusion is facilitated, releasing a 3.5MeV alpha particle and a 14.1MeV neutron. In the majority of cases, the muon is liberated and can initiate further fusions. However, approximately 0.8% of the time, it adheres to the alpha particle and remains bound until it either decays or undergoes reactivation through collisio nal ionization. To maximize the number of fusions per muon, it is crucial to enhance the cycling rate and reactivation fraction. Theoretical predictions and experimental data both suggest that the sticking rate decreases with increasing density. However, there exists a discrepancy between experimental observations and theoretical estimations regarding the extent of this decrease. To address these disparities, this experiment aims to investigate the cycling rate and sticking fraction under higher temperatures and pressures than previously explored.

43 PARTICLE ACCELERATORS↗

DOE OSTI2025

First Principles Molecular Dynamics Simulations of Ammonia Adsorption onto MFI Zeolite Nanosheets

MFI zeolite nanosheets membranes are promising candidates for ammonia separation from nitrogen and hydrogen, yet questions remain on the origin of their high selectivity. Silanols, Si-OH, are present in high concentration at the surface of zeolite nanosheets, and force-field-based simulations indicate that surface adsorption at the silanols contributes to selectivity. Silanols can chemically react with ammonia, which may further contribute to the ability of zeolite nanosheet membranes to separate it from other gases. In this work, we use first-principles molecular dynamics techniques to simulate ammonia’s behavior within stacked MFI zeolite nanosheets. We find that at 523 K and a loading corresponding to 35 bar, conditions desired for the ammonia separation, about 30% of ammonia reacts with surface silanols. Our work explores H-bonding and proton transfer within this system.

36 MATERIALS SCIENCE↗

NASA NTRSDate not supplied

Charge Deposition/Neutralization and Dust Removal via Plasma Interactions

Electrically charged and chemically reactive lunar dust can cause serious problems to spacecraft, surface equipment, and astronaut health, so understanding its interaction with and transport through the lunar plasma environment is important to dust mitigation efforts. We explore the charging of granular material in the natural environment of the Moon, recreate those conditions in the laboratory under high vacuum, and examine dust transport and charge neutralization for application in future lunar missions. Phenomena such as charge deposition via electron and ion beams, photoionization through ultraviolet light exposure, neutralization of charge through impingement by an ionized compressed gas, and tribocharging during dust liberation from surfaces will be presented. Surface materials of interest such as floating/grounded conductors and orthofabric for spacesuits are the preliminary focus of this effort. An electrometer is used to characterize currents interacting with the surfaces in the case of the electron/ion/UV source exposure, an electrostatic voltmeter is used to measure the potential on the surfaces without inadvertently discharging them, and a charge plate monitor is used to verify the efficiency of discharging the surfaces via ionized gas impingement. After setup of the initial charge conditions on the surfaces, a quick burst of high-pressure gas is used to overcome adhesion forces and initially dislodge the dust from the surfaces, followed by a low flow of gas that is ionized via application of a strong electric field. The gas is released in a high vacuum environment so as the pressure drops from the initial compressed state through to the vacuum state, the mean free path reaches a point where ionization can occur before all the gas disperses into the vacuum. This ionized gas can then neutralize both the dust and the surface to ensure minimal resettling. Several parameters such as gas composition, electrode geometry, high voltage waveform shape/polarity, and pulse timing have been explored and will be presented here.

electrostatics↗

DOE OSTI2025

Tritium Breeder Composites for Fusion Applications

Due to the short half-life of tritium, all fusion reactors planned for long-term operation using a deuterium-tritium fuel cycle must produce sufficient tritium from reactions outside the core to maintain reactor operation. Typically, the core is surrounded by a tritium breeder blanket filled with lithium-containing materials which react under a neutron flux to form tritium that can then be collected. Here, reactor designers are forced to choose between using chemically reactive liquid breeders (e.g., Pb-Li, FLiBe, Li) or solid breeders, which often have lower lithium concentrations and questionable mechanical integrity under irradiation (e.g., Li4SiO4, Li2TiO3). To expand this pool of options, this work has sought to produce composite tritium breeder materials using lithium-containing ceramics and metal reinforcements. The resultant composite tritium breeders simultaneously feature higher lithium densities and improved mechanical integrity, including tensile strength and toughness, compared to other tritium breeders previously reported in literature.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

NASA NTRS2012

The Discharging of Roving Objects in the Lunar Polar Regions

In 2007, the National Academy of Sciences identified the lunar polar regions as special environments: very cold locations where resources can be trapped and accumulated. These accumulated resources not only provide a natural reservoir for human explorers, but their very presence may provide a history of lunar impact events and possibly an indication of ongoing surface reactive chemistry. The recent LCROSS impacts confirm that polar crater floors are rich in material including approx 5%wt of water. An integral part of the special lunar polar environment is the solar wind plasma. Solar wind protons and electrons propagate outward from the Sun, and at the Moon's position have a nominal density of 5 el/cubic cm, flow speed of 400 km/sec, and temperature of 10 eV (approx. equal 116000K). At the sub-solar point, the flow of this plasma is effectively vertically incident at the surface. However, at the poles and along the lunar terminator region, the flow is effectively horizontal over the surface. As recently described, in these regions, local topography has a significant effect on the solar wind flow. Specifically, as the solar wind passes over topographic features like polar mountains and craters, the plasma flow is obstructed and creates a distinct plasma void in the downstream region behind the obstacle. An ion sonic wake structure forms behind the obstacle, not unlike that which forms behind a space shuttle. In the downstream region where flow is obstructed, the faster moving solar wind electrons move into the void region ahead of the more massive ions, thereby creating an ambipolar electric field pointing into the void region. This electric field then deflects ion trajectories into the void region by acting as a vertical inward force that draws ions to the surface. This solar wind 'orographic' effect is somewhat analogous to that occurring with terrestrial mountains. However, in the solar wind, the ambipolar E-field operating in the collision less plasma replaces the gradient in pressure that would act in a collisional neutral gas. Human systems (roving astronauts or robotic systems created by humans) may be required to gain access to the crater floor to collect resources such as water and other cold-trapped material. However, these human systems are also exposed to the above-described harsh thermal and electrical environments in the region. Thus, the objective of this work is to determine the nature of charging and discharging for a roving object in the cold, plasma-starved lunar polar regions. To accomplish this objective, we first define the electrical charging environment within polar craters. We then describe the subsequent charging of a moving object near and within such craters. We apply a model of an astronaut moving in periodic steps/cadence over a surface regolith. In fact the astronaut can be considered an analog for any kind of moving human system. An astronaut stepping over the surface accumulates charge via contact electrification (tribocharging) v.lith the lunar regolith. We present a model of this tribo-charge build-up. Given the environmental plasma in the region, we determine herein the dissipation time for the astronaut to bleed off its excess charge into the surrounding plasma.

Jackson, T. L.↗

DOE OSTI2025

First-Principles Molecular Dynamics Simulations of Ammonia Adsorption onto MFI Zeolite Nanosheets

MFI zeolite nanosheet membranes are promising candidates for ammonia separation from nitrogen and hydrogen gases, yet questions remain on the origin of their high selectivity. Silanols, Si–OH, are present in high concentration at the surface of zeolite nanosheets, and force-field-based simulations indicate that surface adsorption at the silanols contributes to selectivity. Acidic silanol groups can chemically react with ammonia via transfer of a proton to form ammonium ions, which may further contribute to the ability of zeolite nanosheet membranes to separate ammonia from other gases. In this work, we used first-principles molecular dynamics techniques to simulate ammonia’s behavior within stacked MFI zeolite nanosheets. We found that at 523 K and a loading corresponding to 35 bar, conditions desired for the ammonia separation, about 30% of ammonia reacts with surface silanols. Our work explored H-bonding and proton transfer within this system.

ammonia↗

DOE OSTI2025

Effect of Propagating Dopant Reactivity on Lattice Oxygen Loss in LLZO Solid Electrolyte Contacted with Lithium Metal

Lithium lanthanum zirconium oxide (LLZO) is widely known as the most stable solid electrolyte against lithium metal electrodes. This thermodynamic stability can be lost by the presence of dopants which are required to stabilize the cubic phase of LLZO and can be reduced by lithium metal. However, the role of oxygen in such reactions is taken for granted. In this work, the reduction of Nb-substituted LLZO (Nb-LLZO) is explored by Li metal and shows that interfacial reactions propagate and lead to the decomposition with substantial Nb 5+ reduction deep into the bulk electrolyte. Scanning Transmission Electron Microscopy with Energy Dispersive X-ray Spectroscopy and thermogravimetric analyses show much of the reduction is due to oxygen vacancies formed, leading to increased electronic conductivity mapped with conductive Atomic Force Microscopy. Density functional theory calculations indicate oxygen release is favored by increased excess lithiation of Nb-LLZO. Electrochemical impedance of polycrystalline Nb-LLZO shows the continuous evolution of ionically resistive interphases near the lithium metal interface with Nb-LLZO while single crystals show little reactivity at room temperature and self-limiting reduction at 60°C. This work underlines the role of grain boundaries in propagating destructive solid electrolyte reactions and highlights previously unseen mechanisms involving lattice oxygen in LLZO.

Li metal, solid-state electrolytes↗