Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries

Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH4)2 in THF

Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species - the monocation, CaBH[Formula: see text] - produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca2+. The consequences for performance are discussed in light of this molecular-scale insight.

How machine learning can extend electroanalytical measurements beyond analytical interpretation

Electroanalytical measurements are routinely used to estimate material properties exhibiting current and voltage signatures. Analysis of such measurements relies on analytical expressions of material properties to describe the experiments. The need for analytical expressions limits the experiments that can be used to measure properties as well as the properties that can be estimated from a given experiment. Such analytical relations are essentially solutions of the physics-based differential equations (with properties as coefficients) describing the material behavior under certain specific conditions. In recent years, a new machine learning-based approach has been gaining popularity wherein the differential equations are numerically solved to interpret the electroanalytical experiments in terms of corresponding material properties. Since the physics-based differential equations are solved, one can additionally estimate underlying fields, e.g., concentration profile, using such an approach. To exemplify the characteristics of such a machine learning assisted interpretation of electroanalytical measurements, we use data from the Hebb-Wagner test on a magnesium spinel intercalation host. As compared to the traditional analytical expression-based interpretation, the emerging approach decreases experimental efforts to characterize relevant material properties as well as provides field information that was previously inaccessible.

Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis

Ammonia (NH3) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH3 production that can be paired with renewable energy. The first verified electrochemical method for NH3 synthesis was a process mediated by lithium (Li) in organic electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the reduction of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochemical production of NH3, a critical chemical precursor and promising energy vector.

A novel calcium fluorinated alkoxyaluminate salt as a next step towards Ca metal anode rechargeable batteries

Ca metal anode rechargeable batteries are seen as a sustainable high-energy density and high-voltage alternative to the current Li-ion battery technology due to the low redox potential of Ca metal and abundance of Ca. Electrolytes are key enablers on the path towards next-generation battery systems. Within this work, we synthesize a new calcium tetrakis(hexafluoroisopropyloxy) aluminate salt, Ca[Al(hfip)₄]₂, and benchmark it versus the state-of-the-art boron analogue Ca[B(hfip)₄]₂. The newly developed aluminate-based electrolyte exhibits improved performance in terms of conductivity, Ca plating/stripping efficiency, and oxidative stability as well as Ca battery cell performance. A marked improvement of 0.5 V higher oxidative stability can pave the path towards high-voltage Ca batteries. A critical issue of solvent quality during salt synthesis is identified as well as solvent decomposition at the Ca metal/electrolyte interface, which leads to passivation of the Ca metal anode. However, the new aluminate salt with preferable electrochemical properties over the existing boron analogue opens up a new area for future Ca battery research based on aluminium compounds.

Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-based Electrolytes

Learning how to tailor Ca2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Using an exemplar dual-salt electrolyte, Ca(BH4)2 + Ca(TFSI)2 in THF, this work examines how to modulate a critical parameter proposed to govern electroactivity, the BH4 -/Ca2+ ratio. Introduction of a more-dissociating source of Ca2+ via Ca(TFSI)2 drives re-speciation of strongly ion-paired Ca(BH4)2, confirmed by ionic conductivity, Raman spectroscopy, and reaction microcalorimetry measurements, generating larger populations of charged species and enhancing plating currents. Ca plating is possible when [Ca(TFSI)2] < [Ca(BH4)2] and thus BH4 -/Ca2+ >1, but a dramatic shut-down of plating activity occurs when [Ca(TFSI)2] > [Ca(BH4)2] (BH4 -/Ca2+ <1), directly evidencing the significance of coordination-shell chemistry on plating activity. Ca(BH4)2 + TBABH4 in THF, which enables enrichment of BH4 - concentrations compared to Ca2+, is also examined; ionic conductivity and plating currents also increase compared to Ca(BH4)2/THF, with the latter related in part to a decrease in solution resistance. These findings delineate future directions to modulate Ca2+ coordination towards achieving both high plating activity and reversibility.

Progress and Prospects of Electrolyte Chemistry of Calcium Batteries

The increasing energy storage demand of portable devices, electric vehicles, and scalable energy storage has been driving extensive research for more affordable, more energy dense battery technologies than Li ion batteries. The alkaline earth metal, calcium (Ca), has been considered an attractive anode material to develop the next generation of rechargeable batteries. Herein, the chemical designs, electrochemical performance, and solution and interfacial chemistry of Ca²⁺ electrolytes are comprehensively reviewed and discussed. In addition, a few recommendations are presented to guide the development and evaluation of Ca²⁺ electrolytes in future.

Chemical designs, electrochemical performance, and solution and interfacial chemistry of calcium battery electrolytes are comprehensively reviewed and discussed.

Electrodeposition of neodymium and dysprosium from organic electrolytes

A new class of organic electrolytes has been developed for the electrodeposition of rare-earth metals at room temperature. These electrolytes consist of a rare-earth bis(trifluoromethylsulfonyl)imide or chloride salt and a borohydride salt, dissolved in the ether solvents 1,2-dimethoxyethane or 2-methyltetrahydrofuran. In these electrolytes, a soluble lanthanide(iii) borohydride complex [Ln(BH4)4]- is formed, which allows for the electrodeposition of neodymium- or dysprosium-containing layers. The electrochemistry of these electrolytes was characterized by cyclic voltammetry. The deposits were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDX) and X-ray photoelectron spectroscopy (XPS), and the results suggest the presence of metallic neodymium and dysprosium.

Quantifying Species Populations in Multivalent Borohydride Electrolytes

Multivalent batteries represent an important beyond Li-ion energy storage concept. The prospect of calcium batteries, in particular, has emerged recently due to novel electrolyte demonstrations, especially that of a ground-breaking combination of the borohydride salt Ca(BH₄)₂ dissolved in tetrahydrofuran. Recent analysis of magnesium and calcium versions of this electrolyte led to the identification of divergent speciation pathways for Mg²⁺ and Ca²⁺ despite identical anions and solvents, owing to differences in cation size and attendant flexibility of coordination. To test these proposed speciation equilibria and develop a more quantitative understanding thereof, we have applied pulsed-field-gradient nuclear magnetic resonance and dielectric relaxation spectroscopy to study these electrolytes. Concentration-dependent variation in anion diffusivities and solution dipole relaxations, interpreted with the aid of molecular dynamics simulations, confirms these divergent Mg²⁺ and Ca²⁺ speciation pathways. These results provide a more quantitative description of the electroactive species populations. We find that these species are present in relatively small quantities, even in the highly active Ca(BH₄)₂/tetrahydrofuran electrolyte. This finding helps interpret previous characterizations of metal deposition efficiency and morphology control and thus provides important fundamental insight into the dynamic properties of multivalent electrolytes for next-generation batteries.

Recent Advances and Perspectives on Calcium-Ion Storage: Key Materials and Devices

The urgent demand for cost-effective energy storage devices for large-scale applications has led to the development of several beyond-lithium energy storage systems (EESs). Among them, calcium-ion batteries (CIBs) are attractive due to abundant calcium resources, excellent volumetric and gravimetric capacities of Ca metal anode, and potential high energy density coming from the multivalent feature of Ca-ion. Therefore, the exploration of CIBs electrode materials and the construction of CIBs devices are gaining increasing research interest. Relevant publications cover a wide range of materials by both theoretical and experimental investigations, whereas the performances of rocking-chair CIBs have been unsatisfactory. Meanwhile, multi-ion strategies using more than one ion as the charge carrier have been demonstrated to be feasible and promising options in realizing room temperature CIBs. The summary and reflection of previous studies would provide useful information for future exploration and optimization. In this circumstance, this paper overviews the reported CIBs electrode materials, including both anode and cathode, and presents the latest progress of multi-ion strategies in CIBs. Fundamental challenges, potential solutions, and opportunities are accordingly proposed, mimicking other more mature EESs. This review may promote the development of electrode materials and accelerate the construction of low-cost and high-performance CIBs.

Hybrid Solid Electrolyte Interphases Enabled Ultralong Life Ca-Metal Batteries Working at Room Temperature

Currently, the application of calcium metal anodes is challenged by rapidly degenerated plating/stripping electrochemistry without suitable solid electrolyte interphases (SEIs) capable of fast Ca²⁺ transport kinetics and superior ability to resist anion oxidation. Here, through in situ evolved Na/Ca hybrid SEIs, symmetrical Ca//Ca batteries readily remain stable for more than 1000 h deposition-dissolution cycles (versus less than 60 h for those with pure Ca SEIs under the same condition). Coupled with a specially designed freestanding lattice-expanded graphitic carbon fiber membrane and tailored operation voltages, the proof-of-concept Ca-metal batteries reversibly run for almost 1900 cycles with ≈83% capacity retention and a high average discharge voltage of 3.16 V. The good performance not only benefits from the stable SEIs at the Ca metal surface which affords free Ca²⁺ transports and prohibits out-of-control fluridation of Ca (forming CaF₂ ion-/electron-insulating layer) but is also attributed to reversible relay insertion/extraction electrochemistry in the cathode. This work sheds new light on durable metal battery technology.

Promises and Challenges of Next-Generation "Beyond Li-ion" Batteries for Electric Vehicles and Grid Decarbonization

The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

Potential Application of Ionic Liquids for Electrodeposition of the Material Targets for Production of Diagnostic Radioisotopes

An overview of the reported electrochemistry studies on the chemistry of the element for targets for isotope production in ionic liquids (ILs) is provided. The majority of investigations have been dedicated to two aspects of the reactive element chemistry. The first part of this review presents description of the cyclotron targets properties, especially physicochemical characterization of irradiated elements. The second part is devoted to description of the electrodeposition procedures leading to obtain elements or their alloys coatings (e.g., nickel, uranium) as the targets for cyclotron and reactor generation of the radioisotopes. This review provides an evaluation of the role ILs can have in the production of isotopes.

Electrochemical Signatures of Interface-Dominated Behavior in the Testing of Calcium Foil Anodes

Fundamental research and practical assembly of rechargeable calcium (Ca) batteries will benefit from an ability to use Ca foil anodes. Given that Ca electrochemistry is considered a surface-film-controlled process, understanding the interface's role is paramount. This study examines electrochemical signatures of several Ca interfaces in a benchmark electrolyte, Ca(BH₄)₂/tetrahydrofuran (THF). Preparation methodologies of Ca foils are presented, along with Ca plating/stripping through either pre-existing, native calcium hydride (CaH₂), or pre-formed calcium fluoride (CaF₂) interfaces. In contrast to earlier work examining Ca foil in other electrolytes, Ca foils are accessible for reversible electrochemistry in Ca(BH₄)₂/THF. However, the first cyclic voltammetry (CV) cycle reflects persistent, history-dependent behavior from prior handling, which manifests as characteristic interface-derived features. This behavior diminishes as Ca is cycled, though formation of a native interface can return the CV to interface-dominated behavior. CaF₂ modification enhances such interface-dominance; however, continued cycling suppresses such features, collectively indicating the dynamic nature of certain Ca interfaces. Cell configuration is also found to significantly influence electrochemistry. With appropriate preparation of Ca foils, the signature of interface-dominated behavior is still present during the first cycle in coin cells, but higher current density compared to three-electrode cells along with moderate cycle life are readily achievable.

Pseudocapacitive Ti-Doped Niobium Pentoxide Nanoflake Structure Design for a Fast Kinetics Anode toward a High-Performance Mg-Ion-Based Dual-Ion Battery

Magnesium-ion batteries (MIBs) have received increasing attention for next-generation energy storage recently because of the natural abundance, high capacity, and dendrite-free deposition of Mg. However, their applications are hindered by irreversible Mg anode plating in conventional electrolytes and the lack of cathode materials, demonstrating high working voltage, satisfactory Mg²⁺ diffusivity, and long cycling life. In this work, we first developed a novel magnesium-ion based dual-ion battery (Mg-DIB) by utilizing expanded graphite as the cathode and Ti-doped niobium pentoxide nanoflakes (Ti-Nb₂O₅ NFs) as the anode. The Ti-Nb₂O₅ NFs showed hierarchical structures of microspheres with diameters of 4-5 μm assembled by nanoflakes. For the first time, the Mg-ion storage mechanism in Ti-Nb₂O₅ NFs was investigated. Benefiting from the hierarchical structure design and pseudocapacitive intercalation behavior of Mg ions, the Ti-Nb₂O₅ NF anode exhibited fast Mg-ion diffusion. Consequently, the Mg-DIB exhibited a high discharge capacity of 93 mA h g^-1 at 1 C (1 C corresponding to 100 mA g^-1), along with good long-term cycling performance with a capacity retention of 79% at 3 C after 500 cycles. The Mg-DIB also demonstrated a capacity retention of 77% at 5C, indicating its good rate performance. Moreover, the Mg-DIB exhibited a high discharge medium voltage of ∼1.83 V, thus enabling a high energy density of 174 W h kg^-1 at 183 W kg^-1 and 122 W h kg^-1 at a high power density of 845 W kg^-1, among the best of the reported magnesium-ion full batteries. Our work provides a new strategy to improve the performance of MIBs and other rechargeable batteries.

Current Understanding of Nonaqueous Electrolytes for Calcium-Based Batteries

Calcium metal batteries are receiving growing research attention due to significant breakthroughs in recent years that have indicated reversible Ca plating/stripping with attractive Coulombic efficiencies (90-95%), once thought to be out of reach. While the Ca anode is often described as being surface film-controlled, the ability to access reversible Ca electrochemistry is highly electrolyte-dependent in general, which affects both interfacial chemistry on plated Ca along with more fundamental Ca2+/Ca redox properties. This mini-review describes recent progress towards a reversible Ca anode from the point of view of the most successful electrolyte chemistries identified to date. This includes, centrally, what is currently known about the Ca2+ solvation environment in these systems. Experimental (physico-chemical and spectroscopy) and computational results are summarized for the two major solvent classes - carbonates and ethers - that have yielded promising results so far. Current knowledge gaps and opportunities to improve fundamental understanding of Ca2+/Ca redox are also identified.

Conversion of Co Nanoparticles to CoS in Metal-Organic Framework-Derived Porous Carbon during Cycling Facilitates Na2S Reactivity in a Na-S Battery

Room-temperature sodium-sulfur batteries have attracted wide interest due to their high energy density and high natural abundance. Polysulfide dissolution and irreversible Na₂S conversion are challenges to achieving high battery performance. Herein, we utilize a metal-organic framework-derived Co-containing nitrogen-doped porous carbon (CoNC) as a catalytic sulfur cathode host. A concentrated sodium electrolyte based on sodium bis(fluorosulfonyl)imide, dimethoxyethane, and bis(2,2,2-trifluoroethyl) ether is used to mitigate polysulfide dissolution. We tune the amount of Co present in the CoNC carbon host by acid washing. Significant improvement in reversible sulfur conversion and capacity retention is observed with a higher Co content in CoNC, with 600 mAh g^-1 and 77% capacity retention for CoNC and 261 mAh g^-1 and 56% capacity retention for acid-washed CoNC at cycle 50 at 80 mAh g^-1. Post-mortem X-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction suggest that CoS is formed during cycling in place of Co nanoparticles and CoN₄ sites. Raman spectroscopy suggests that CoS exhibits a catalytic effect on the oxidation of Na₂S. Our findings provide insights into understanding the role Co-based catalysts play in sulfur batteries.

Energy storage emerging: A perspective from the Joint Center for Energy Storage Research

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

Dawn of Calcium Batteries

Calcium batteries are a potentially sustainable, high-energy-density battery technology beyond Li ion batteries. Now the development of Ca batteries has become possible with a newly invented Ca electrolyte capable of reversible Ca deposition/stripping at room temperature.

Methods and Protocols for Reliable Electrochemical Testing in Post-Li Batteries (Na, K, Mg, and Ca)

While less mature than the Li-ion battery, technologies based on Na, K, Mg, and Ca are attracting more and more attention from the battery community. New material (cathode, anode, or electrolyte) testing for these post-Li systems commonly involves the use of an electrochemical setup called a half-cell in which metal counter and reference electrodes are used. Here we first describe the different issues that become critical when moving away from Li with respect to the cell hardware (cell design, current collector, separator, insulator) and the nature of the counter and reference electrodes. Workarounds are given, and a versatile setup is proposed to run reliable electrochemical tests for post-Li battery materials in general, in a broad range of electrolyte compositions.

Achievements, Challenges, and Prospects of Calcium Batteries

This Review flows from past attempts to develop a (rechargeable) battery technology based on Ca via crucial breakthroughs to arrive at a comprehensive discussion of the current challenges at hand. The realization of a rechargeable Ca battery technology primarily requires identification and development of suitable electrodes and electrolytes, which is why we here cover the progress starting from the fundamental electrode/electrolyte requirements, concepts, materials, and compositions employed and finally a critical analysis of the state-of-the-art, allowing us to conclude with the particular roadblocks still existing. As for crucial breakthroughs, reversible plating and stripping of calcium at the metal-anode interface was achieved only recently and for very specific electrolyte formulations. Therefore, while much of the current research aims at finding suitable cathodes to achieve proof-of-concept for a full Ca battery, the spectrum of electrolytes researched is also expanded. Compatibility of cell components is essential, and to ensure this, proper characterization is needed, which requires design of a multitude of reliable experimental setups and sometimes methodology development beyond that of other next generation battery technologies. Finally, we conclude with recommendations for future strategies to make best use of the current advances in materials science combined with computational design, electrochemistry, and battery engineering, all to propel the Ca battery technology to reality and ultimately reach its full potential for energy storage.