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Iodide redox reactions in molten NaI/AlCl3 are shown to generate surface-blocking films, which may limit the useful cycling rates and energy densities of molten sodium batteries below 150°C. An experimental investigation of electrode interfacial stability at 110°C reveals the source of the reaction rate limitations. Electrochemical experiments in a 3-electrode configuration confirm an increase of resistance on the electrode surface after oxidation or reduction current is passed. Using chronopotentiometry, chronoamperometry, cyclic voltammetry, and electrochemical impedance spectroscopy, the film formation is shown to depend on the electrode material (W, Mo, Ta, or glassy carbon), as well as the Lewis acidity and molar ratio of I-/I3 - in the molten salt electrolytes. These factors impact the amount of charge that can be passed at a given current density prior to developing excessive overpotential due to film formation on the electrode surface. The results presented here guide the design and use of iodide-based molten salt electrolytes and electrode materials for grid scale battery applications.

Highlights Zn-MnO2 batteries promise safe, reliable energy storage, and this roadmap outlines a combination of manufacturing strategies and technical innovations that could make this goal achievable. Approaches such as improved efficiency of manufacturing and increasing active material utilization will be important to getting costs as low as $100/kWh, but key materials innovations that facilitate the full 2-electron capacity utilization of MnO2, the use of high energy density 3D electrodes, and the promise of a separator-free battery with greater than 2V potential offer a route to batteries at $50/kWh or less. Abstract Large-scale energy storage is certain to play a significant, enabling role in the evolution of the emerging electrical grid. Battery-based storage, while not a dominant form of storage today, has opportunity to expand its utility through safe, reliable, and cost-effective technologies. Here, secondary Zn–MnO2 batteries are highlighted as a promising extension of ubiquitous primary alkaline batteries, offering a safe, environmentally friendly chemistry in a scalable and practical energy dense technology. Importantly, there is a very realistic pathway to also making such batteries cost-effective at price points of $50/kWh or lower. By examining manufacturing examples at the Zn–MnO2 battery manufacturer Urban Electric Power, a roadmap has been created to realize such low-cost systems. By focusing on manufacturing optimization through reduced materials waste, scalable manufacturing, and effective materials selection, costs can be significantly reduced. Ultimately, though, coupling these approaches with emerging research and development advances to enable full capacity active materials utilization and battery voltages greater than 2V are likely needed to drive costs below a target of $50/kWh. Reaching this commercially important goal, especially with a chemistry that is safe, well-known, and reliably effective stands to inject Zn–MnO2 batteries in the storage landscape at a critical time in energy storage development and deployment. Graphical abstract

Electrocatalytic reduction of waste nitrates (NO3-) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts with varying metal centers uniquely favor mono-nitrogen products (e.g., NH3), as well as provide synergistic supports for nanoparticle catalysts. But the reaction fundamentals remain largely underexplored. Herein, we report a set of 3d-, 4d-, 5d- and f-block atomically dispersed M-N-C catalysts with a well-defined M-Nx coordination. The selectivity and activity of NO3- reduction to NH3 in neutral media were thoroughly studied, with a specific focus on deciphering the role of the NO2- intermediate in the reaction cascade, wherein strong correlations (R=0.9) were found between the NO2- reduction activity and NO3- reduction selectivity for NH3. Moreover, theoretical computations identified the associative/dissociative adsorption pathways for NO2- evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for certain oxyphilic metals. The free energies for the reductive adsorption of nitrate [∗ + NO3− → ∗NO2 ], correlated strongly with experimental NH3 selectivity. This work provides a platform for designing multielement NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.

Low-cost, long-duration energy storage is critically needed for a robust electric grid powered by renewable sources. In the pursuit of meeting this need, low-temperature (3 catholyte. Among the key challenges to widespread utilization of these emerging MNaBs is reducing the overpotential on the cathode while operating at practical current densities over numerous cycles. To improve the performance of the cathode, we examined the electrochemical behavior of a variety of disk electrode materials, including W, Mo, Ta, and glassy carbon (GC) in a 3-electrode configuration. A custom cell was designed to mimic a full battery with separators for both the reference and counter electrodes. Excess molten salt was used to keep bulk concentrations practically constant over the course of the experiments. This enabled experiments that isolated the working electrode from other battery elements, such as a changing catholyte or the NaSICON interfaces. Voltammetry, electrochemical impedance spectroscopy, chronopotentiometry, and chronoamperometry were used to evaluate each material’s charge/discharge kinetics and stability. Instability on charging is hypothesized to be due to iodine (I2) adsorption on rapid iodide (I-) oxidation. Discharge, on the other hand, is limited by the transport of triiodide (I3 -), which depends on the battery’s state of charge. Insights from these studies serve as the foundation for the rational design of high-surface area electrodes for iodide-based molten salt catholytes. After initial testing in the 3-electrode cell, the performance of several high surface area current collectors was evaluated in rate tests and continuous cycling of lab-scale battery cells. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Low-cost, long-duration energy storage is a vital resource needed for a robust electric grid powered by renewables. Low-temperature (3, AlBr3, and GaCl3. To better understand the catholyte properties during cycling, we have performed electrochemical kinetics studies and mathematical modeling of the iodide-triiodide speciation. Results reveal a dependence between catholyte speciation and usable capacity and current densities for the given catholyte compositions. Additional experiments, including cyclic voltammetry and chronoamperometry, probe the interactions between the cathode current collector and the molten salts, revealing potential impacts on electrochemical kinetics and stability during cycling. Insights from these fundamental studies serve as the foundation for the informed design of high-performance cathodes in new scalable molten sodium batteries. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The electrochemical reduction of oxidized nitrogen species, nitrate (NO3 -) and nitrite (NO2 -), both of which are environmental pollutants, to benign nitrogen gas (N2) has been extensively researched for water remediation applications. Recently, a large research focus has shifted to reducing these oxidized nitrogen species to ammonia (NH3).1 In this way, a dual benefit is achieved by eliminating the environmentally hazardous nitrates, while also creating a value-added product, ammonia. Moreover, the nitrate reduction reaction (NO3RR) can aid in closing the nitrogen cycle, by reforming nitrates introduced into the environment through nitrate-based fertilizers originating from the Haber-Bosch process, back into NH3, in a carbon neutral fashion. The NO3RR is a complex, 8e-, 9H+ transfer process with multiple possible intermediate species (NO2 -, NO, N2O, N2H4, NH2OH, NH3) and rate limiting steps. The NO3RR mechanism and its activity descriptors over various metals is not fully understood, and this is especially true over atomically dispersed sites, where less than a handful of studies exist.2,3 Our previous work found that atomically dispersed nitrogen coordinated Fe-N4 and Mo-N4 sites displayed distinct associative adsorption and dissociative adsorption of the nitrate molecule in the NO3RR pathways, respectively. Specifically, Fe-N4 sites adsorb and reduce NO3 - into NH3through an 8e- pathway on the surface, while Mo-N4 sites dissociate NO3 - and release NO2 - into the bulk electrolyte. These active sites were then integrated into a single bi-metallic catalyst (FeMo-N-C), creating a catalytic cascade yielding a significantly improved yield rate and Faradaic efficiency for NH3.4 In this talk, building on our previous work, we will present a series of transition metal atomically dispersed nitrogen doped carbon (M-N-C) electrocatalysts (M = Mn, Fe, Co, Ni, Cu...) for the systematic investigation of NO3RR activities over the different metal centers. In this work we leverage physical (ICP-MS, gas physisorption), electrochemical (e.g., NO3RR) and computational (DFT) characterization to create a set of NO3RR activity descriptors. A detailed set of activity descriptors can aid in the selection of atomically dispersed metals for creating efficient reaction cascades to synergize favorable reaction pathways. Figure 1

Molten sodium batteries show remarkable promise as large-scale energy storage systems, but their widespread deployment has been limited by high operating temperatures (~300°C), which impacts their lifetime, cost, and reliability. Lowering the battery operating temperature promises substantial improvements in these areas, but the lower temperature introduces new challenges to the battery chemistry, especially related to the solid state ion-conducting separators used in these batteries. Traditionally, the high temperature operation has been required to not only maintain high Na+-ion conductivity of solid electrolytes, such as β”-Al2O3 or NaSICON, but also to improve the liquid-solid interfacial wetting of molten sodium on the solid electrolyte. We are focused on drastically reducing the operating temperature to near the melting temperature of sodium (97.8oC), which leads to poor interfacial wetting of the molten sodium. Poor wetting and ineffective charge transfer can dominate battery resistances resulting in a cell-limiting interface. Here, we will describe the rational design of engineered coatings to improve wetting of the molten sodium on NaSICON at low temperatures near 100°C, and therefore improve charge transfer across this critical interface. Enhanced mating of the separator-sodium interface by means of engineered coatings is demonstrated to result in lower interfacial resistance and higher battery performance at increased current densities. This strategy promises substantial advances in the operation of molten sodium batteries at low temperatures and opportunities to expand the utility of these batteries to meet emerging grid-scale needs. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The global implementation of secondary lithium ion (Li-ion) batteries continues to expand rapidly, finding clear utility in portable electronics and vehicle electrification. For large format, grid-scale energy storage however, concerns over the safety, reliability, cost, and lifetime of these batteries has motivated the drive to find alternatives to Li-ion systems. Here we describe the materials-driven development of a low temperature molten sodium-halide battery for grid-scale applications. Molten sodium batteries, such as sodium sulfur (NaS) and sodium nickel chloride (Na-NiCl2or ZEBRA) batteries, continue to show promise for grid-scale applications, valued for their use of earth abundant chemistries, scalability, and the absence of volatile, potentially hazardous organic electrolytes. Widespread implementation of these batteries has been limited, however, by relatively high operating temperatures (~300°C). Potentially impacting both cost and lifetime of these batteries, lowering the battery operating temperature would reduce material degradation, enable the use of lower cost battery components, and facilitate simplified, more cost effective heat management and operating strategies. The promising low temperature system described here comprises a molten sodium anode, a solid state ion-conducting separator, and a sodium iodide (NaI)-based molten salt catholyte. This NaI-based system avoids the hazards of volatile organic electrolytes, employing a fully molten catholyte designed to enable long cycle life while still offering a higher voltage (>3V) than traditional molten sodium batteries. Here, we will describe promising demonstrations of prototype battery cycling at drastically reduced operating temperatures near 100°C. In addition, we will address new materials challenges unique to operating a molten sodium battery at these reduced temperatures. In particular, we will focus on the complex goal of increasing lower temperature separator conductance in mechanically robust solid state electrolytes while minimizing resistances at interfaces between these separators and the molten anode and catholyte. Tuning separator materials chemistry and microstructure, both in the bulk and at separator interfaces, can significantly impact electrolyte wetting, reduce cell resistance, and improve battery cycling performance. Continued development of these emerging molten Na-NaI batteries promises a new route to the type of safe, cost-effective, long-lifetime batteries needed for grid-scale energy storage. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Despite the tremendous advances in batteries for portable electronics and vehicle electrification, truly effective battery technologies for large scale energy storage remain relatively underdeveloped. These systems have distinct requirements as safe, low-cost, reliable systems, critical to robust and agile energy distribution, integration of renewable energy, effective emergency response, and even emerging national defense initiatives. Here, we describe a promising new class of molten sodium-halide batteries that show exciting potential as safe, cost-effective, long-lived batteries for large-scale applications. These Na-NaI batteries comprise a molten sodium anode and a molten NaI-based halide salt catholyte, separated by highly conductive solid-state sodium-ion conducting membrane. This NaI-based system offers a higher voltage (>3V) than traditional molten sodium batteries and enables the use of fully molten catholytes desired for long cycle life. By engineering the materials chemistry of these batteries, we have driven down operating temperature to near 100°C, utilizing chemistries that avoid potential problems with thermal runaway or flammable organic electrolytes that often plague other battery technologies. This dramatic reduction of operating temperature (compared to ~300°C typical of traditional molten sodium batteries) opens the door to valuable reduction in costs and extension of battery life. This presentation will discuss specific advances in molten catholyte composition and electrochemistry, the performance of the solid-state separators at these reduced temperatures, and the efficient cycling behavior of laboratory-scale prototypes. The encouraging performance of these innovative “low” temperature molten salt batteries promises new opportunities to address the widespread need for safe, reliable, and cost-effective large scale energy storage solutions. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Large scale electrical energy storage is a key piece of the next generation electrical grid infrastructure. Sodium batteries offer great promise as safe, low cost and high energy density storage technologies that can be utilized to meet the growing demand for these types of grid scale storage systems. We are exploring a fully molten sodium battery system that operates at low to intermediate temperatures and provides inherent safety for a large scale battery application. Here we characterize the electrochemical properties of new molten salt catholyte chemistries that enable high performance cycling behavior at lower the operating temperature. We present electrochemical analysis of candidate NaI-AlCl3 molten salt catholytes at various temperatures and compositions. We find concentration dependencies on the iodide oxidation potentials as well as the observed current densities. Insight from these studies reveal catholyte compositions that can support high current densities and good cycle life that would be critical to reliable battery performance. Understanding the phase behavior of these salt compositions is key to optimizing the catholyte performance and a preliminary phase diagram is used help explain the promising electrochemical observations. For example, solids present in the catholyte at very high or very low NaI concentrations hinder mass and charge transport to electrode surfaces. Successful understanding and manipulation of this phase space will enable increases in catholyte performance, both in terms of lower operating temperatures and also increased current densities for improved molten sodium batteries for grid scale energy storage. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

As the national electrical energy grid continues to expand and evolve, there is an increasing need for safe, cost-effective and reliable large scale batteries. Current large scale battery technologies continue to suffer from limited cyclability, high cost, and potentially hazardous runaway reaction behaviors. Here, we describe a promising alternative approach to intermediate temperature molten sodium-based batteries, enabled by advanced solid state ceramic electrolytes, such as NaSICON, that offer high Na+ conductivity at temperatures well below 200 °C. Integrating molten sodium anodes, ceramic solid state electrolytes, and fully molten AlCl3-based molten salt catholytes, we create high performance, all-inorganic batteries. In particular, we explore an AlCl3-NaI molten salt catholyte, and we show that these batteries operate efficiently below 200 °C. We describe the relationships between the molten salt composition and electrochemical properties of the catholyte, revealing how these promising, intermediate temperature technologies boast coulombic efficiencies near 100% and energy efficiencies >80%. Moreover, we validate the safety of this material set using accelerated rate calorimetry to demonstrate that the system exhibits neither the runaway exothermic reactions nor hazardous pressurized gas generation that plague other large-scale battery systems. Finally, we discuss the potential benefits to long term battery reliability afforded by low-to-intermediate temperature battery operation. The encouraging performance of this emergent intermediate temperature molten salt battery shows a path to providing the reliable, safe, and cost-effective energy storage solutions demanded by our rapidly growing national electrical energy grid. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Nanoporous membranes can be used for electrodialysis-based water purification if the ionic selectivity and conductivity of the membrane can be increased. The electrodialysis process uses an applied DC electric field actively transport dissolved ions through the nanoporous membranes, concentrating the ions on one side of the membrane and purifying the water on the other side. Due to the large amount of electrical energy needed, electrodialysis is only used in a small number of water purification plants. One way to increase the efficiency of this process is to increase the ionic selectivity of the membranes. We describe here the use of inexpensive, commercially available polymer supports modified with polyelectrolyte surface chemistries. Specifically, we use a Layer-by-Layer (LbL) assembly to carefully modify nanopore surfaces with polyelectrolyte thin films. We explore how these thin films modify the surface charge influencing the ion selectivity and conductivity of these nanoporous membranes. We can further tailor the properties of the membranes through crosslinking to impact not only the chemical selectivity but also the chemical and mechanical stability of the polyelectrolyte surface modification. using various chemical crosslinking agents to increase not only the chemical selectivity of the membranes, but also their adhesion to the support membrane. We describe the preparation and characterization of these modified nanoporous membranes and evaluated the resulting ionic conductivity. Variations in the structure and composition of the starting nanoporous support membrane, polyelectrolyte deposition parameters and crosslinking chemistries are shown to controllably influence the ionic selectivity and conductivity of the nanoporous membranes as well as adhesion of the polyelectrolyte coating. Continued advances of these engineered nanoporous supports will enable the widespread use of electrodialysis as an economical water purification technology. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Development of safe, low-cost, grid-scale electrical energy storage remains a national priority, critical to agile, reliable energy distribution, transformative renewable energy integration, effective emergency response, and even successful national defense initiatives. Viable candidate energy storage systems must be able to reliably provide the high capacity and power needed to meet evolving electrical demands while remaining cost-effective and safe. With current large scale battery technologies challenged by high cost, limited cyclability, and potentially hazardous runaway reaction behaviors, there is a clear need to develop alternative battery technologies. Here, I will describe a new generation of intermediate temperature molten sodium-based batteries, enabled by the solid state electrolyte NaSICON (Na Super Ion CONductor). NaSICON provides exceptional low-to-intermediate temperature sodium ion conductivity, exhibits excellent chemical and mechanical stability, and can be produced in a range of form factors on an industrial scale. Integrating molten sodium anodes, NaSICON solid state electrolytes, and AlCl3-based molten salt catholytes, we create high performance, all-inorganic battery constructs that operate below 200oC and avoid hazards associated with runaway exothermic reactions, polymer separators, and organic electrolytes used in other batteries. I will describe the design and scalable performance (up to 250Wh) of several emerging intermediate temperature molten salt technologies including Na-NiCl2 and Na-I2. These promising, intermediate temperature technologies boast coulombic efficiencies near 100% and energy efficiencies >80% through months of stable electrochemical cycling. Moreover, accelerated rate calorimetry verifies the inherent safety of these molten salt chemistries, revealing neither the runaway exothermic reactions nor hazardous pressurized gas generation that plague other large-scale battery systems. The encouraging performance of these intermediate temperature molten salt battery systems promises new opportunities to meet the need for reliable, safe, and cost-effective solutions to growing national challenges in grid-scale electrical energy storage. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.