Jon Mark Weller

Sodium-based battery storage offers a promising solution due to the abundance and low cost of sodium (Na). Among various sodium battery technologies-including Na-ion, high-temperature Na-S, and high-temperature Na-NiCl₂ (ZEBRA) batteries, Na-NiCl₂ batteries provide inherently safer and more reliable performance compared to the current lithium-ion battery (LIB) technologies for grid-storage applications. A key component of Na-NiCl₂ batteries is the sodium beta-alumina solid-electrolyte (β''-Al₂O₃, BASE), which plays vital roles in sodium ion transport and interfacial charge transfer. These functions become increasingly critical at lower operating temperatures. Poor physical contact at the liquid-solid interface between molten sodium and BASE below 200°C leads to high interfacial resistance, causing significant degradation and failure of electrochemical performance.

Solid-state batteries (SSBs) based on a solid-state electrolyte (SSE) are desirable to enable inherently safer energy storage and potentially higher energy density compared to state-of-the-art Li-ion batteries (LIBs). Until recently, Li-based SSBs have received the most attention due to their high theoretical energy densities. However, for large-scale applications focusing on stationary energy storage, Na-SSBs are extremely attractive and have the potential to exceed the energy density of current state of the art batteries, a categorically better safety profile and lower materials costs due to the high natural abundance of Na and related raw materials.

Of the many emerging technologies for low-cost, grid-scale energy storage, redox flow batteries have been an area of active investigation for decades. More recently, a focus on lower cost alternatives to the foundational Vanadium-based redox flow batteries has been a major focus of the flow battery research community. Specifically, there is great interest in redox active species based on Earth-abundant materials suitable for aqueous systems such that the overall cost per kWh of energy storage can be minimized to enable economical and reliable grid storage to utilize energy generated from intermittent, renewable sources. The use of aqueous electrolytes, ideally at mild pH, in conjunction with inexpensive and highly soluble active material presents an exciting challenge for grid-scale flow batteries. Many new aqueous-soluble, redox active compounds have garnered interest, including water-soluble organic redox species and low-cost transition-metal complexes based on Fe or other earth-abundant metals. Recently, our group has devoted significant attention to new anolytes based on Fe(II)/Fe(III) complexes with various types of low-cost and easily synthesizable phosphonate ligands. Specifically, the nature and fine-tuning of phosphonate ligands allows a high degree of control over the redox potential, rate capability and kinetics, and stability of Fe-based complexes for flow battery anolytes. The wide variety of potential ligands with different functional moieties sometimes produces complicated electrochemical behavior, which has motivated us to apply in operando analysis to inform interpretation of electrochemical and battery cycling results. This presentation focuses on one such analytical tool, namely Raman spectroscopy, applied in operando to several flow batteries based on various Fe-phosphonate anolyte complexes to understand the effects of various substituents and functional groups on the Fe(II)/Fe(III) redox behavior, potential, kinetics, and even structural transformations of these complexes during battery cycling. Examples based on recently reported nitrilotri(methylphosphonic acid) (NTMPA) and related phosphonic acid ligands and their resultant Fe-complex-based anolytes will be presented showing that online vibrational spectroscopy can provide clarity on the effects of varied functional groups on both the molecular structure of redox-active Fe-complexes and the effects of ligand binding on kinetics. This experimental tool beneficially compliments and confirms results from density functional theory to elucidate the structure of these Fe-complexes. Further, the multivariate spectral response provided by in operando Raman spectroscopy allows the state of charge of the cell to be monitored readily. Overall, we aim to highlight the utility of Raman spectroscopy in probing the chemical transformations that occur during electrochemical measurements and battery testing in real time, with potential benefits to a much wider variety of promising redox flow battery systems and the flow battery community at large. Figure 1

The lifespan of lithium (Li) metal batteries (LMBs) can be greatly improved by the formation of inorganic‐rich electrode‐electrolyte interphases (EEIs) (including solid‐electrolyte interphase on anode and cathode‐electrolyte interphase on cathode). In this work, a localized high‐concentration electrolyte containing lithium bis(fluorosulfonyl)imide (LiFSI) salt, 1,2‐dimethoxyethane (DME) solvent and 1,2‐bis(1,1,2,2‐tetrafluoroethoxy)ethane (BTFEE) diluent is optimized. BTFEE is a fluorinated ether with weakly‐solvating ability for LiFSI so it also acts as a co‐solvent in this electrolyte. It can facilitate anion decomposition at electrode surfaces and promote the formation of more inorganic‐rich EEI layers. With an optimized molar ratio of LiFSI:DME:BTFEE = 1:1.15:3, LMBs with a high loading (4 mAh cm ⁻² ) lithium nickel manganese cobalt oxide (LiNi _0.8 Mn _0.1 Co _0.1 ) cathode can retain 80% capacity in 470 cycles when cycled in a voltage range of 2.8–4.4 V. The fundamental understanding on the functionality of BTFEE revealed in this work provides new perspectives on the design of practical high‐energy density battery systems.

Cost-competitive renewable energy sources, such as wind and solar energy, are more rapidly replacing fossil fuels for power generation. As their integration into the power grid increases, the inherent intermittency exposes a great challenge in maintaining the grid stability. Therefore, the development of low-cost energy storage technology suitable for large-scale manufacturing is essential for the further deployment of renewable energy. We noticed that renewable electricity production usually follows predictable seasonal trends. For example, the longer daylight in summers corresponds to more solar power generated. If any seasonal excess can be efficiently stored and redistributed in times of needs, a power grid based on 100% renewable energy is more feasible. Unfortunately, current rechargeable battery technologies have not been optimized for seasonal storage, as the self-discharge rate ranges from approximately 3–5% for lithium-ion batteries to almost 30% for nickel metal hydride batteries in the first month. In this presentation, we will demonstrate the concept of a rechargeable molten-salt battery with earth-abundant elements and simple construction that preserve the stored energy for long periods of time. Due to its unique operating mechanism, this battery can discharge most of its stored capacity (> 90%) after a period of 1 to 8 weeks. This approach brings new possibilities to seasonal energy storage and helps the grid to overcome short supply during seasonal fluctuations.

Lithium conducting garnets such as lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) are of great interest as ceramic solid electrolytes for solid-state lithium-metal batteries. Various synthetic approaches have been investigated for LLZO such as sol-gel, co-precipitation, thin film deposition, spray pyrolysis, combustion, and molten salt synthesis (MSS), not to mention the standard solid-state reaction (SSR) method. Most of the aforementioned methods have been used to form LLZO electrolytes with high ionic conductivity and density, each with benefits as well as some drawbacks such as cost, high processing temperatures, complexity, or difficulty of producing fine powders for thin films of LLZO. Further, there are many discrepancies between important performance parameters such as ionic conductivity reported in the literature, implying that achieving the highest performance is not completely straightforward and that the details of the synthetic route have substantial effects on the resultant materials properties. Herein, Ta-doped LLZO of nominal composition Li_6.4La₃Zr_1.4Ta_0.6O₁₂ is investigated as a representative example, and is synthesized by various MSS routes as well as SSR for comparison, covering a wide temperature range (as low as 400 °C for MSS and up to 1000 °C for SSR). By using different synthesis methods for the same target material, the differences in ionic conductivity, sinterability, and other properties as a result of the peculiarities of individual synthetic approaches are investigated. Interestingly, elemental inhomogeneity on the order of individual LLZTO grains is observed to varying degrees in each of the synthesis methods investigated, with more inhomogeneity correlated with reduced performance in sintered LLZTO, implying that strict control over garnet phase formation is required to achieve optimal performance. Some advantages of MSS for LLZTO are presented and the thermodynamic and kinetic origins of compositional inhomogeneity are discussed, as are solutions and strategies to mitigate this inhomogeneity.

Tantalum-doped garnet (Li_6.5La₃Zr_1.5Ta_0.5O₁₂, LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li⁺ conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000 °C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900 °C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li⁺ ion conductivities of up to 0.6 and 0.5 mS cm^–1, respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al₂O₃ as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.

Lithium conducting garnets are attractive solid electrolytes for solid-state lithium batteries but are difficult to process, requiring high reaction and sintering temperatures with long duration. In this work, we demonstrate a synthetic route to obtain Ta-doped garnet (Li_6.4La₃Zr_1.4Ta_0.6O₁₂) utilizing La- and Ta-doped lanthanum zirconate (La_2.4Zr_1.12Ta_0.48O_7.04) pyrochlore nanocrystals as quasi-single-source precursors. Via molten salt synthesis (MSS) in a highly basic flux, the pyrochlore nanocrystals transform to Li-garnet at reaction temperatures as low as 400 °C. We also show that the pyrochlore-to-garnet conversion can take place in one step using reactive sintering, resulting in densified garnet ceramics with high room temperature ionic conductivity (0.58 mS cm^-1) and relative density (up to 94.7 %). This approach opens new avenues for lower temperature synthesis of lithium garnets using a quasi-single-source precursor and provides an alternative route to highly dense garnet solid electrolytes without requiring advanced sintering processes.