Reversible Ca Research Articles

A CaI2‐Based Electrolyte Enabled by Borate Ester Anion Receptors for Reversible Ca−Organic and Ca−Se Batteries

AbstractPassivating solid electrolyte interphases (SEIs) in Ca metal anodes constitute a long‐standing challenge, as they block Ca2+ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum‐based electrolytes to mitigate such interfacial issues by producing Ca2+‐conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca2+ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI2‐dimethoxyethane electrolyte supports reversible Ca/Ca2+ redox reactions via forming CaI2‐involved SEI, inspired by our density functional theory calculations where CaI2 species is predicted to possess the lowest Ca2+ diffusion barrier among a range of inorganic phases. We further materialize this finding by introducing a serial of borate ester anion receptors, resulting in the formation of CaI2/borides hybrid SEIs with an enhanced Ca2+ conductivity. Consequently, the resultant electrolytes realize a 7‐fold reduction in deposition/stripping overpotential compared to anion receptor‐free one, allowing for the construction of reversible Ca‐metal full cells with high‐capacity selenium and organic cathodes.

A CaI2-Based Electrolyte Enabled by Borate Ester Anion Receptors for Reversible Ca-Organic and Ca-Se Batteries.

Passivating solid electrolyte interphases (SEIs) in Ca metal anodes constitute a long-standing challenge, as they block Ca2+ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum-based electrolytes to mitigate such interfacial issues by producing Ca2+-conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca2+ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI2-dimethoxyethane electrolyte supports reversible Ca/Ca2+ redox reactions via forming CaI2-involved SEI, inspired by our density functional theory calculations where CaI2 species is predicted to possess the lowest Ca2+ diffusion barrier among a range of inorganic phases. We further materialize this finding by introducing a serial of borate ester anion receptors, resulting in the formation of CaI2/borides hybrid SEIs with an enhanced Ca2+ conductivity. Consequently, the resultant electrolytes realize a 7-fold reduction in deposition/stripping overpotential compared to anion receptor-free one, allowing for the construction of reversible Ca-metal full cells with high-capacity selenium and organic cathodes.

Complex Hydride-Based Gel Polymer Electrolytes for Rechargeable Ca-Metal Batteries.

Rechargeable Ca batteries offer the advantages of high energy density, low cost, and earth-abundant constituents, presenting a viable alternative to lithium-ion batteries. However, using polymer electrolytes in practical Ca batteries is not often reported, despite its potential to prevent leakage and preserve battery flexibility. Herein, a Ca(BH4)2-based gel-polymer electrolyte (GPE) is prepared from Ca(BH4)2 and poly(tetrahydrofuran) (pTHF) and tested its performance in Ca batteries. The electrolyte demonstrates excellent stability against Ca-metal anodes and high ionic conductivity. The results of infrared spectroscopy and 1H and 11B NMR indicate that the terminal ─OH groups of pTHF reacted with BH4 - anions to form B─H─(pTHF)3 moieties, achieving cross-linking and solidification. Cyclic voltammetry measurements indicate the occurrence of reversible Ca plating/stripping. To improve the performance at high current densities, the GPE is supplemented with LiBH4 to achieve a lower overpotential in the Ca plating/stripping process. An all-solid-state Ca-metal battery with a dual-cation (Ca2+ and Li+) GPE, a Ca-metal anode, and a Li4Ti5O12 cathode sustained >200 cycles, confirming their feasibility. The results pave the way for further developing lithium salt-free Ca batteries by developing electrolyte salts with high oxidation stability and optimal electrochemical properties.

High‐Content Carbon Layer Confined Small‐Molecule/Covalent Sulfur Cathode Enables Long‐Life Calcium–Sulfur Batteries in Hybrid‐Ion Electrolyte

AbstractCalcium‐sulfur (Ca‐S) battery is a promising candidate for large‐scale energy storage system. However, the development of long‐life room‐temperature Ca–S batteries is hindered by the lack of effective sulfur cathode and the incompatibility between electrodes and electrolytes, leading to irreversible sulfur conversion, inefficient calcium plating/stripping and improper cathode/anode‐electrolyte interfaces (CEI/SEI). Herein, a turbo carbon layer confined small‐molecule/covalent sulfur cathode (S@C NS) with a record‐high content of 65.7% and enlarged carbon layer space is prepared via a C/S co‐deposition process. By using such a cathode and optimizing interfacial electrochemistry in LiPF6/Ca(BF4)2‐based hybrid electrolyte, a record‐breaking long‐life room‐temperature Ca‐S battery is reported. The capacity reaches 824.6 mAh g−1 at 100 mA g−1 with a retention of 47.9% after 145 cycles, and 462.1 mAh g−1 at even 500 mA g−1. Mechanism studies demonstrated that a Li/Ca‐based hybrid CEI with porous microstructure is found to facilitate the transfer kinetics of Ca2+ across the CEI; Turbo carbon layers with large inter‐layer space and their strong confinement on small‐molecule sulfur enabled reversible solid‐state sulfur conversion in situ inside the cathode; An effective Li/Ca‐based hybrid SEI formed on anode, allowing reversible Ca plating/stripping with low deposition potential and small overpotential.

Optogenetic Control of Oncogenic Signaling in B-Cell Malignancies

Background: B-cell receptor (BCR) signals are essential determinants of survival and proliferation throughout normal B-cell development. In B-cell malignancies, these signals are frequently generated by oncogenic mimics of the BCR signaling pathway. For instance, oncogenes in B-ALL, derived from B-cell precursors, typically mimic survival signals from a constitutively active pre-BCR, while tonic and chronic active BCR signaling were identified in mature B-cell lymphomas. Oncogenic BCR-signaling represents an important target of therapeutic intervention: for instance, small molecule inhibitors of SYK (e.g. entospletinib) and BTK (e.g. ibrutinib) have been developed to disrupt oncogenic BCR signaling in mature B-cell lymphomas. Besides SYK and BTK tyrosine kinases, oncogenic BCR-signaling leads to activation of PLCG2, which initiates the release of Ca 2+ from the ER into the cytoplasm. Thereby, calcium flux is decoded by NFATC1 or NF-kB based on fast (NFATC1) or slow (NF-kB) frequencies of Ca 2+-signals. Significance: To interrogate Ca 2+-signals as critical integration point of oncogenic BCR-signaling, we engineered cell lines and PDX with a GCaMP6s-biosensor, which allows tracing of calcium signaling in single cells over time ( Figure A). To cover B-cell malignancies from multiple stages of B-cell development, we engineered B-ALL (pro- and pre-B cells), mantle cell lymphoma (MCL, naïve B-cells), Burkitt's lymphoma (germinal center), DLBCL (post-GC), multiple myeloma (terminally differentiated plasma cells) and Hodgkin's disease (“crippled” BCR-deficient GC-B cells). Strikingly, B-cell malignancies exhibit autonomous Ca 2+-oscillations, which decreased in their frequency from 20 mHz (pro- and pre-B), 11 mHz (naïve), 4 mHz (germinal center) to 0 mHz in post-GC and terminally differentiated plasma cells based on the differentiation stage of their cell of origin ( Figure A). Results: Given the striking differences in oncogenic BCR-signaling as measured by autonomous Ca 2+ oscillations, we developed an optogenetic system to control the frequency and amplitude of Ca 2+ oscillations by blue light pulses ( Figure B). To model frequency-modulated Ca 2+ signaling in B-cells, we engineered murine pre-B cells carrying an optogenetic tool termed OptoCRAC, which enables reversible activation and inactivation of the plasma membrane Ca 2+ channel Orai1 by intermittent blue light irradiation. Time-lapse imaging of a fluorescent Ca 2+ reporter (R-CaMP1.07) confirmed rapid and reversible Ca 2+ influx in response to intermittent blue light pulses. To assess the phenotypic consequence of high-frequency Ca 2+ oscillations, we developed an LED-array-based optogenetic platform for cell culture plates. While low-frequency Ca 2+ oscillations (0.5 mHz) did not impact cell viability, high-frequency Ca 2+ oscillations (20 mHz) rapidly induced NFAT activation (5 min), and eventually cell death within 72 h ( Figure B). Mechanism and conclusions: To elucidate the mechanistic contribution of NFATC1 in B-cell death induced by fast Ca 2+ oscillations, we tested whether genetic deletion of Nfatc1 is sufficient to rescue B-cell death upon fast Ca 2+ oscillation induced by 20 mHz intermittent blue light pulses. As expected, while B-ALL cells retaining intact Nfatc1 rapidly decreased cell viability, the cells with Cre-mediated deletion persistently remained in cell culture upon 20 mHz Ca 2+ oscillation. In addition to genetic deletion of Nfatc1, we also found that inducible expression of mutant activators of NF-kB signaling were able to rescue B-cells from cell death induced by high-frequency oscillations: Concurrent expression of Card11 L232LI, MYD88 L265P and IKK2 S177E/S181E not only reduced the frequency of autonomous Ca 2+ oscillations but also enabled B-cell survival despite delivery of light-pulses and Ca 2+ oscillations at a fast 20 mHz frequency. We conclude that autonomous Ca 2+ oscillations represent a critical integration point of oncogenic BCR-signaling in B-cell malignancies and that optogenetic control of BCR-downstream Ca 2+ oscillations will reveal previously unrecognized vulnerabilities that can be exploited to increase efficacy of BCR-signaling inhibitors (e.g. ibrutinib, entospletinib, idelalisib) that are currently being used for the treatment of B-cell lymphomas.

Preferential Coordination As a Means to Investigate Ca2+ Speciation and SEI Formation

Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca2+ transport.1 At present, only a small number of nonaqueous electrolytes have been identified that form sufficiently Ca2+-conductive SEIs for Ca electrodeposition, limiting the design space for Ca battery development. Thus, learning how to tailor Ca SEI formation is of central importance to identify handles for next-generation battery design. Several methods of modifying SEI to improve Ca2+ transport have been reported, including synthesizing an ex situ artificial layer,2 or the introduction of boron-containing electrolyte additives that form beneficial SEI compounds.3 Ca electrolyte modification faces a unique challenge, however, as the large size and divalent nature of Ca2+ leads to large coordination environments (up to 7 solvating molecules) in solution.4 Recent studies have begun to identify that co-reduction of these coordinating molecules has a significant impact on Ca electrochemistry. For example, strong coordination between Ca2+ and the anion TFSI- has been demonstrated to deactivate Ca plating behavior via TFSI- decomposition.5,6 However, competition between coordinating molecules can be exploited to favor specific Ca2+ speciation (i.e. BH4 - displaces bidentate TFSI---Ca2+ in favor of BH4 ---Ca2+ interactions) and unlock Ca plating behavior.6 Further mechanistic understanding is needed to identify the specific connections between Ca2+ coordination, SEI formation, and anode performance (beyond just a binary toggling of plating behavior). To address this, we have systematically introduced chelating glyme-based solvents into a baseline electrolyte, investigating the subsequent changes in SEI composition and electrochemical behavior.This study investigates the preferential coordination of Ca2+ by glymes in the baseline electrolyte 1 M Ca(BH4)2 in tetrahydrofuran (THF). THF is systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx/Ca2+ molar ratios. Microcalorimetry measurements demonstrate that the introduction of both G1 and G2 to the electrolyte is enthalpically favorable, as each glyme displaces THF from the Ca2+ coordination sphere and modulates Ca2+--BH4 - speciation, as indicated by NMR and Raman spectroscopy. Addition of a small amount of each glyme (1:1 Gx/Ca2+) increases electrolyte ionic conductivity by nearly half; however, glyme addition systematically increases the overpotentials of both Ca plating and stripping while decreasing Coulombic efficiency. The presence of glymes increases the amount of carbon present in both chemically- and electrochemically-formed SEI as indicated through XPS. We further find increased evidence of fragmentation of strongly Ca2+-coordinating molecules (G2, BH4 -) within the SEI formed during Ca deposition as compared to formed chemically. Calcium-based olefins as well as ionic phases (CaCO3, CaC2) are detected in electrochemically-formed SEI through titration gas chromatography. This work reveals factors that modulate both Ca2+ coordination and subsequent SEI formation, highlighting preferential coordination as a novel approach for Ca-based electrolyte design.1 Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138, 3536-3545 (1991).2 Hou, Z., Zhou, R., Min, Z., Lu, Z. & Zhang, B. Realizing Wide-Temperature Reversible Ca Metal Anodes through a Ca2+-Conducting Artificial Layer. ACS Energy Letters, 274-279 (2022).3 Bodin, C. et al. Boron-Based Functional Additives Enable Solid Electrolyte Interphase Engineering in Calcium Metal Battery. Batteries & Supercaps n/a, e202200433 (2022).4 Tchitchekova, D. S. et al. On the Reliability of Half-Cell Tests for Monovalent (Li+, Na+) and Divalent (Mg2+, Ca2+) Cation Based Batteries. Journal of The Electrochemical Society 164, A1384-A1392 (2017).5 Hahn, N. T. et al. Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes. ACS Applied Energy Materials 3, 8437-8447 (2020).6 Melemed, A. M., Skiba, D. A. & Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-Based Electrolytes. The Journal of Physical Chemistry C 126, 892-902 (2022).

Towards a Reversible Calcium Anode through Electrolyte and Interface Engineering for Calcium Metal Batteries

Lithium-ion batteries have played an important role in decarbonizing the transportation sector, but broader utilization is currently limited by practically-attainable energy densities, safety concerns, and supply chain issues. Batteries based on calcium (Ca) metal anodes offer higher theoretical energy densities, as well as significant potential improvements in scalability and sustainability. The field of Ca metal batteries is currently in its early stages, however, due to a limited number of electrolytes that can reversibly plate and strip Ca. Two important challenges to overcome are (1) the formation of passivating solid electrolyte interphases (SEI) between Ca and the electrolyte that prevent Ca2+ transport, and (2) Ca2+-anion interactions in the electrolyte that promote cluster formation and hinder electrochemistry.1,2 This talk will explore research performed in both aspects of Ca battery development and identify strategies for future advancements. First, the electrochemical signatures of three model interfaces on Ca foil electrodes were investigated in a baseline electrolyte using cyclic voltammetry;3 the long-term cycling behavior was governed to a greater extent by electrolyte properties than the initial surface layers. Subsequent efforts have focused on electrolyte engineering, and specifically, on studying the impact that the Ca2+ coordination environment has on reversible Ca plating and stripping. For example, strong coordination between Ca2+ and the anion TFSI- has been demonstrated to deactivate Ca plating behavior via TFSI- decomposition.4,5 However, competition between coordinating molecules can be exploited to favor specific Ca2+ speciation; preferential coordination between Ca2+ and BH4 - displaces bidentate TFSI---Ca2+, unlocking Ca plating behavior in tetrahydrofuran.5 Further studies into preferential coordination are ongoing in order to better identify the connections between Ca2+ coordination, SEI formation, and anode performance. We hope to identify specific electrolyte design criteria that can advance the functionality of Ca electrolytes and ultimately examine the feasibility of Ca as a novel battery chemistry.1 Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138, 3536-3545 (1991).2 Arroyo-de Dompablo, M. E., Ponrouch, A., Johansson, P. & Palacín, M. R. Achievements, Challenges, and Prospects of Calcium Batteries. Chemical Reviews (2019).3 Melemed, A. M. & Gallant, B. M. Electrochemical Signatures of Interface-Dominated Behavior in the Testing of Calcium Foil Anodes. Journal of The Electrochemical Society 167, 140543 (2020).4 Hahn, N. T. et al. Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes. ACS Applied Energy Materials 3, 8437-8447 (2020).5 Melemed, A. M., Skiba, D. A. & Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca2+ Speciation in Borohydride-Based Electrolytes. The Journal of Physical Chemistry C 126, 892-902 (2022).

Calcium Metal Batteries with Long Cycle Life Using a Hydride-Based Electrolyte and Copper Sulfide Electrode.

As potential alternatives to Li-ion batteries, rechargeable Ca metal batteries offer advantageous features such as high energy density, cost-effectiveness, and natural elemental abundance. However, challenges, such as Ca metal passivation by electrolytes and a lack of cathode materials with efficient Ca2+ storage capabilities, impede the development of practical Ca metal batteries. To overcome these limitations, the applicability of a CuS cathode in Ca metal batteries and its electrochemical properties are verified herein. Ex situ spectroscopy and electron microscopy results show that a CuS cathode comprising nanoparticles that are well dispersed in a high-surface-area carbon matrix can serve as an effective cathode for Ca2+ storage via the conversion reaction. This optimally functioning cathode is coupled with a tailored, weakly coordinating monocarborane-anion electrolyte, namely, Ca(CB11 H12 )2 in 1,2-dimethoxyethane/tetrahydrofuran, which enables reversible Ca plating/stripping at room temperature. The combination affords a Ca metal battery with a long cycle life of over 500 cycles and capacity retention of 92% based on the capacity of the 10th cycle. This study confirms the feasibility of the long-term operation of Ca metal anodes and can expedite the development of Ca metal batteries.

Correlation between Electrolyte Chemistry and Solid Electrolyte Interphase for Reversible Ca Metal Anodes

AbstractThe development of rechargeable Ca metal batteries (RCMBs) is hindered by the Ca2+ passivating solid electrolyte interphases (SEIs). The cation solvation structure dictated by electrolyte chemistry plays a critical role in the SEIs properties. While a relatively weak cation‐solvent binding is preferred in Li metal anodes to promote anion‐derived SEIs, we demonstrate an enhanced Ca deposition/stripping reversibility under a strong cation‐solvent interaction, which is materialized in strongly‐solvating solvent and highly‐dissociated salt combinations. Such electrolyte formulations benefit the formation of solvent‐occupied solvation structure and minimize the anion reduction, resulting in organic‐rich/CaF2‐poor SEIs for reversible Ca metal anodes. Furthermore, RCMBs paired with an organic cathode using the optimized electrolytes are demonstrated as a proof‐of‐concept. Our work reveals the paradigm shift in SEIs design for Ca metal anodes, opening up new opportunities for emerging RCMBs.

Correlation between Electrolyte Chemistry and Solid Electrolyte Interphase for Reversible Ca Metal Anodes.

The development of rechargeable Ca metal batteries (RCMBs) is hindered by the Ca2+ passivating solid electrolyte interphases (SEIs). The cation solvation structure dictated by electrolyte chemistry plays a critical role in the SEIs properties. While a relatively weak cation-solvent binding is preferred in Li metal anodes to promote anion-derived SEIs, we demonstrate an enhanced Ca deposition/stripping reversibility under a strong cation-solvent interaction, which is materialized in strongly-solvating solvent and highly-dissociated salt combinations. Such electrolyte formulations benefit the formation of solvent-occupied solvation structure and minimize the anion reduction, resulting in organic-rich/CaF2 -poor SEIs for reversible Ca metal anodes. Furthermore, RCMBs paired with an organic cathode using the optimized electrolytes are demonstrated as a proof-of-concept. Our work reveals the paradigm shift in SEIs design for Ca metal anodes, opening up new opportunities for emerging RCMBs.

Investigation of Transition Metal Oxide Post-Spinels for Calcium-Ion Batteries

The dependence on lithium for rising global energy demand coupled with the scarcity of lithium necessitates the exploration of post-lithium strategies. Calcium-ion batteries are one such post-lithium strategy that can mitigate rising costs owing to calcium’s natural abundancy. A critical gap in this field is the lack of cathodes capable of intercalating calcium at high voltages and capacities while also retaining structural stability. The handful of candidates evaluated thus far have been plagued by low capacities and poor cycling performance due to intercalation–induced phase changes and instability. Transition metal oxide post–spinel–type materials have been identified as potential candidates for reversible Ca–ion storage owing to their crystal structures and high theoretical energy densities. However, experimental validation of these theoretical predictions remains largely unaddressed. In this work, post-spinel Calcium Iron Oxide (CaFe2O4) and Calcium Manganese Oxide (CaMn2O4) are explored as cathodes for calcium-ion batteries. The redox activity of each cathode is investigated using galvanostatic (GS) cycling while their structural stabilities are evaluated with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The use of GS in tandem with XRD and SEM provides insights into the evolution of crystal structure with Ca–ion–transport within each cathode. Our results reveal that these post–spinel systems can cycle with a reversible capacity of 56 mAh/g, making them promising cathode candidates for Ca–ion batteries and warrant further investigation.

Analysis of a thermochemical energy storage system based on the reversible Ca(OH)2/CaO reaction

The development of novel energy storage technologies is crucial for the massive deployment of large-scale renewable energy systems. This paper presents the conceptual study of an integrated system for the large-scale storage of solar thermal energy in the form of thermochemical energy based on calcium hydroxide. Calcium oxide-based storage media are very promising because they are an abundant and inexpensive resource with high energy density and the possibility of storage under ambient conditions. The equilibrium temperature of the reaction is around 500 °C, so the integrated system operates in a temperature range similar to that of concentrating solar power plants and molten salt storage, being able to reach higher temperatures in power production depending on the partial pressure of the steam generated in the reaction. The study discusses the technological challenges of the system, highlighting the importance of recovering the latent heat from condensation of the steam generated in the dehydration reaction, which represents 38% of the solar thermal energy entering the reactor during the charging phase, and proposes recovery mechanisms. The analysis shows the potential competitiveness of the technology as a large-scale energy storage system. By considering the full recovery of the latent heat of the steam, the system achieves a round-trip thermal efficiency of more than 80%, an overall thermal-to-electrical efficiency of 40% and LCOE values of 100 USD/MWhe.

Unlocking the Reversible Selenium Electrode for Non‐Aqueous and Aqueous Calcium‐Ion Batteries

AbstractCalcium‐ion batteries (CIBs) have been considered promising multivalent ion battery systems due to the natural abundance and low redox potential of calcium. The practical realization is largely hampered by the lack of reliable electrode materials. Selenium is first explored as a new conversion‐type electrode for both non‐aqueous and aqueous CIBs. The selenium provides a specific capacity of 476 mAh g–1 with an average voltage of 2.2 V versus Ca/Ca2+ at a current density of 50 mA g–1, offering a higher energy density than other reported cathode materials. Long‐term cyclic stability at a large current density of 500 mA g–1 has been achieved in non‐aqueous electrolytes through encapsulating the selenium in mesoporous carbon. The spectroscopy analysis and density functional theory calculations suggest multi‐step conversion processes involving CaSe4 and Ca2Se5 polyselenides intermediates before reaching the final CaSe phase, exhibiting a distinct reaction pathway from those in other metal‐Se batteries. Furthermore, the application of selenium is extended to the aqueous electrolyte after expanding the electrochemical window. A 1.1 V class aqueous CIB is demonstrated by coupling with a Cu‐based Prussian blue electrode. The discovery of reversible Ca–Se chemistry opens new opportunities for emerging CIBs.

Stability of Calcium Ion Battery Electrolytes: Predictions from Ab Initio Molecular Dynamics Simulations.

Multivalent batteries, such as magnesium-ion, calcium-ion, and zinc-ion batteries, have attracted significant attention as next-generation electrochemical energy storage devices to complement conventional lithium-ion batteries (LIBs). Among them, calcium-ion batteries (CIBs) are the least explored due to difficult reversible Ca deposition-dissolution. In this work, we examined the stability of four different Ca salts with weakly coordinating anions and three different solvents commonly employed in existing battery technologies to identify suitable candidates for CIBs. By employing Born-Oppenheimer molecular dynamics (BOMD) simulations on salt-Ca and solvent-Ca interfaces, we find that the tetraglyme solvent and carborane salt are promising candidates for CIBs. Due to the strong reducing nature of the calcium surface, the other salts and solvents readily decompose. We explain the microscopic mechanisms of salt/solvent decomposition on the Ca surface using time-dependent projected density of states, time-dependent charge-transfer plots, and climbing-image nudged elastic band calculations. Collectively, this work presents the first mechanistic assessment of the dynamical stability of candidate salts and solvents on a Ca surface using BOMD simulations, and provides a predictive path toward designing stable electrolytes for CIBs.

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.

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.

On the Electrochemical Intercalation of Ca2+ and Mg2+ Ions in Layered TiS2: A Fundamental Study in Alkyl Carbonate Based Electrolytes

Batteries based on naturally abundant, light metal anodes (such as Ca and Mg) and multivalent ion host cathodes can potentially achieve very high energy densities at relatively low cost and environmental impact, thus representing a compelling alternative to currently available Li-ion systems. While reversible Ca metal plating and stripping in conventional alkyl carbonate based electrolytes has been accomplished [1,2], unraveling cathode materials with fast and reversible ion mobility at high operating voltages remains a major open challenge, mainly hampered by the slow diffusion kinetics in the solid state of multivalent ions. Thus, besides the exploration of new materials, revisiting traditional layered intercalation hosts appears as a very useful tool to gain further insight into the fundamentals of divalent ion intercalation. In this context, a thorough study of the electrochemical intercalation of Ca2+ in layered TiS2, in alkyl carbonate based electrolytes, is herein presented [3]. Fundamental insights on the insertion process are acquired through X-ray diffraction. Ca2+ insertion is unambiguously proved by using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge and the reversibility of the process is demonstrated at moderate temperature. Different phases can be formed upon reduction of pristine TiS2, whose amount and composition dependon the experimental conditions employed. A comparative study with Mg2+ containing electrolytes and other conventional intercalation hosts, such as V2O5, was also carried out. Careful examination of results highlights the potential relevance of side reactions in these system and the need to use several complementary characterization techniques to unambiguously assess divalent ion intercalation. [1]A. Ponrouch, C. Frontera, F. Bardé and M. R. Palacín, Nat. Mater. (2016), 15, 169. [2] A. Ponrouch, M. R. Palacín, Curr. Opin. Electrochem. (2018), 1. [3] Tchitchekova D.S., Ponrouch A., Verrelli R., Broux T., Frontera C., Sorrentino A., Biskup N., Arroyo-de Dompablo M.E., Bardé F., Palacín M.R., Chem. Mat. (2018), 30, 847.

A Joint Computational and Experimental Evaluation of CaMn2O4 Polymorphs as Cathode Materials for Ca Ion Batteries

The identification of potential cathode materials is a must for the development of a new calcium-ion based battery technology. In this work, we have first explored the electrochemical behavior of marokite–CaMn2O4 but the experimental attempts to deinsert Ca ion from this compound failed. First-principles calculations indicate that in terms of voltage and capacity, marokite–CaMn2O4 could sustain reversible Ca deinsertion reactions; half decalciation is predicted at an average voltage of 3.7 V with a volume variation of 6%. However, the calculated barriers for Ca diffusion are too high (1 eV), in agreement with the observed difficulty to deinsert Ca ion from the marokite structure. We have extended the computational investigation to two other CaMn2O4 polymorphs, the spinel and the CaFe2O4 structural types. Full Ca extraction from these CaMn2O4 polymorphs is predicted at an average voltage of 3.1 V, but with a large volume variation of around 20%. Structural factors limiting Ca diffusion in the three polymor...

Selfsimilarity, Simulation and Spacetime Symmetries

We study intrinsic simulations between cellular automata and introduce a new necessary condition for a CA to simulate another one. Although expressed for general CA, this condition is targeted towards surjective CA and especially linear ones. Following the approach introduced by the first author in an earlier paper, we develop proof techniques to tell whether some linear CA can simulate another linear CA. Besides rigorous proofs, the necessary condition for the simulation to occur can be heuristically checked via simple observations of typical space-time diagrams generated from finite configurations. As an illustration, we give an example of linear reversible CA which cannot simulate the identity and which is 'time-asymmetric', i.e. which can neither simulate its own inverse, nor the mirror of its own inverse.

Sequence reversed peptide from CaMKK binds to calmodulin in reversible Ca 2+-dependent manner

Calmodulin (CaM) is a highly versatile Ca 2+ signaling transducer known to regulate over a hundred proteins. In this paper, we further demonstrate the versatility of CaM binding by showing that it binds to a synthetic peptide (revCKKp) made by reversing the amino acid sequence of the CaM-binding peptide (CKKp) from CaM-dependent protein kinase kinase (CaMKK) (residues 438–463). Sequence comparison between revCKKp and other CaM-binding peptides (CBPs) from the CaM target databank showed that revCKKp does not resemble any existing classes of CBPs, except CKKp [M. Zhang, T. Yuan, Molecular mechanisms of calmodulin’s functional versatility, Biochem. Cell Biol. 76 (1998) 313–323; S.W. Vetter, E. Leclerc, Novel aspects of calmodulin target recognition and activation, Eur. J. Biochem. 270 (2003) 404–414]. Furthermore, computational modeling showed that revCKKp could bind CaM in a similar manner to CKKp. Lastly, we experimentally showed that our synthetic revCKKp binds to CaM in a reversible Ca 2+-dependent manner.