High‐Performance Quasi‐Solid‐State Calcium‐Ion Batteries from Redox‐Active Covalent Organic Framework Electrolytes

Calcium-ion batteries (CIBs) are under investigation as next-generation energy storage devices due to their theoretically high operating potentials and lower costs tied to the high natural abundance of calcium. However, the development of CIBs has been limited by the lack of available positive electrode materials. Here, for the first time, we report two functional polyanionic phosphate materials as high-voltage cathodes for CIBs at room temperature. NaV2(PO4)3 electrodes were found to reversibly intercalate 0.6 mol of Ca2+ (81 mA h g–1) near 3.2 V (vs Ca2+/Ca) with stable cycling performance at a current density of 3.5 mA g–1. The olivine framework material FePO4 reversibly intercalates 0.2 mol of Ca2+ (72 mA h g–1) near 2.9 V (vs Ca2+/Ca) at a current density of 7.5 mA g–1 in the first cycle. Structural, electronic, and compositional changes are consistent with reversible Ca2+ intercalation into these two materials.

Calcium ion batteries (CIBs) are a promising energy storage device due to the low redox potential of the Ca metal and the abundant reserves of the Ca element. However, the large radius and divalent nature of Ca2+ lead to its slow ion diffusion kinetics and the lack of suitable electrode materials for Ca storage. Here, a layered structure of Na2Ti3O7 (NTO) is presented as an anode material for nonaqueous CIBs. This NTO anode demonstrates a high discharge capacity of 165 mA h g-1 at 100 mA g-1 and a remarkable capacity retention rate of 80%, even after 2000 cycles at 500 mA g-1, surpassing the performance of all reported intercalation-type anode materials for CIBs. The NTO transfers to layered CaVIINaIXTi3O7 (CNTO) with intercalation of Ca2+ and extraction of Na+ during the first discharge process. Then, the CNTO undergoes the reversible insertion/extraction of Ca2+ during subsequent cycling. Additionally, density functional theory calculations reveal that NTO possesses a rapid two-dimensional diffusion pathway for Ca2+. Moreover, the full CIBs based on NTO as the anode further underscore its potential for CIBs. This work presents promising anode materials for CIBs, offering opportunities to promote the development of high-performance CIBs.

This study explores the development of calcium‐ion batteries (CIBs) by focusing on the design of a novel electrolyte to overcome key challenges, such as poor calcium plating/stripping. Hydrated vanadium pentoxide (H‐V₂O₅) served as the cathode, and graphite acted as the anode. Tricalcium dicitrate tetrahydrate (Ca‐citrate) salt was used for the first time in an ethylene carbonate/propylene carbonate (EC/PC) solvent system. Ca‐citrate showed marked improvement in ionic conductivity (up to 2.6 × 10 −1 S/cm) and electrochemical stability (~6 V) relative to traditional Ca‐nitrate (2.5 V) and also displayed better capacity retention. To solve the solubility limitations of Ca‐citrate in EC/PC, highly diluted (0.001 M) solution of EC/PC and modified electrolytes with cosolvents like poly(ethylene glycol) 200 (PEG 200) and trifluoroacetic anhydride (TFA) were studied. PEG 200 increased solubility and stability through hydrogen bonding, while TFA increased ionic mobility but decreased electrochemical stability. The PEG‐modified electrolyte achieved a stability window of ~4 V with a charge/discharge capacity of ~198 mAh/g. Additionally, using ethylene glycol (EG) as an alternative solvent increased the electrochemically reversible, soluble, and capacitive Ca‐citrate (up to 708 mAh/g, 85 mA/g). EG‐based electrolyte showed high columbic efficiency (~99%–100%) and stable interfacial behavior.

Organic materials are promising for cation storage in calcium ion batteries (CIBs). However, the high solubility of organic materials in an electrolyte and low electronic conductivity remain the key challenges for high-performance CIBs. Herein, a nitrogen-rich covalent organic framework with multiple carbonyls (TB-COF) is designed as an aqueous anode to address those obstacles. TB-COF demonstrates a high reversible capacity of 253 mAh g-1 at 1.0 A g-1 and long cycle life (0.01% capacity decay per cycle at 5 A g-1 after 3000 cycles). The redox mechanism of Ca2+/H+ co-intercalated in COF and chelating with C═O and C═N active sites is validated. In addition, a novel C═C active site was identified for Ca2+ ion storage. Both computational and empirical results reveal that per TB-COF repetitive unit, up to nine Ca2+ ions are stored after three staggered intercalation steps, involving three distinct Ca2+ ion storage sites. Finally, the evolution process of radical intermediates further elucidates the C═C reaction mechanism.

Ultra-stable Ti2O(PO4)2(H2O) as a viable new Ca2+ storage electrode material for calcium-ion batteries

The calcium-ion battery is an emerging energy storage system that has attracted considerable attention recently. However, the absence of high-performance cathode materials is one of the main challenges for the development of calcium-ion batteries. Herein, a bilayered Mg0.25V2O5·H2O as a stable cathode for rechargeable calcium-ion batteries is identified. Remarkably, an unexpected stable structure of the material for Ca2+ storage is demonstrated. It is found that the interlayer spacing shows only a tiny variation of ∼0.09 Å during Ca2+ insertion/extraction, which results in its outstanding cycling stability (capacity retention of 86.9% after 500 cycles) for Ca2+ storage. On the basis of in situ/ex situ experimental characterizations and ab initio simulation, the origin of such superior structural stability is revealed. This ultrastable cathode together with the understanding lays a strong foundation for developing high-performance calcium-ion batteries.

The rational design of porous covalent organic frameworks (COFs) with high conductivity and reversible redox activity is the key to improving their performance in sodium‐ion batteries (SIBs). Herein, we report a series of COFs (FPDC‐TPA‐COF, FPDC‐TPB‐COF, and FPDC‐TPT‐COF) based on an organosulfur linker, (trioxocyclohexane‐triylidene)tris(dithiole‐diylylidene))hexabenzaldehyde (FPDC). These COFs feature two‐dimensional crystalline structures, high porosity, good conductivity, and densely packed redox‐active sites, making them suitable for energy storage devices. Among them, FPDC‐TPT‐COF demonstrates a remarkably high specific capacity of 420 mAh g−1 (0.2 A g−1), excellent cycling stability (~87 % capacity retention after 3000 cycles, 1.0 A g−1) and high rate performance (339 mAh g−1 at 2.0 A g−1) as an anode for SIBs, surpassing most reported COF‐based electrodes. The superior performance is attributed to the dithiole moieties enhancing the conductivity and the presence of redox‐active carbonyl, imine, and triazine sites facilitating Na storage. Furthermore, the sodiation mechanism was elucidated through in situ experiments and density functional theory (DFT) calculations. This work highlights the advantages of integrating multiple functional groups into redox‐active COFs for the rational design of efficient and stable SIBs.

Covalent organic frameworks (COFs) are crystalline, nanoporous materials of interest for various applications, but current COF synthetic routes lead to insoluble aggregates which precludes processing for practical implementation. Here, we report a COF synthesis method that produces a stable, homogeneous suspension of crystalline COF nanoparticles that enables the preparation of COF monoliths, membranes, and films using conventional solution-processing techniques. Our approach involves the use of a polar solvent, diacid catalyst, and slow reagent mixing procedure at elevated temperatures which altogether enable access to crystalline COF nanoparticle suspension that does not aggregate or precipitate when kept at elevated temperatures. On cooling, the suspension undergoes a thermoreversible gelation transition to produce crystalline and highly porous COF materials. We further show that the modified synthesis approach is compatible with various COF chemistries, including both large- and small-pore imine COFs, hydrazone-linked COFs, and COFs with rhombic and hexagonal topologies, and in each case, we demonstrate that the final product has excellent crystallinity and porosity. The final materials contain both micro- and macropores, and the total porosity can be tuned through variation of sample annealing. Dynamic light scattering measurements reveal the presence of COF nanoparticles that grow with time at room temperature, transitioning from a homogeneous suspension to a gel. Finally, we prepare imine COF membranes and measure their rejection of polyethylene glycol (PEG) polymers and oligomers, and these measurements exhibit size-dependent rejection and adsorption of PEG solutes. This work demonstrates a versatile processing strategy to create crystalline and porous COF materials using solution-processing techniques and will greatly advance the development of COFs for various applications.

Bipolar-type all-solid-state lithium batteries are expected to maintain higher energy density than that of the conventional stacked lithium-ion battery using organic liquid electrolytes. The main problem of the conventional stacked lithium-ion battery is the leakage of organic liquid electrolytes. Bipolar-type all-solid-state battery is composed of solid-state electrolytes as shown in Figure 1, it is possible to supply a high voltage, avoiding the aforementioned weak point of the conventional stacked lithium-ion battery. Presently, the cases of solid-state electrolytes having sufficient lithium ion conductivity and high stability in the voltage range for battery operation are limited. We reported the fabrication and characterization of quasi-solidified hybrid electrolytes composed of room temperature ionic liquids (RTILs) and 7 nm fumed-silica in our previous study. On the basis of our previous works, the quasi-solidified hybrid electrolytes can be treated as solid-state electrolytes, and it possess liquid-like Li-ion transport properties with high ionic conductivity and high stability. Therefore, the quasi-solid-state hybrid electrolyte is considered as a potential candidate for bipolar-type all-solid-state lithium-ion batteries. In this study, we assembled the bipolar-type all-solid-state Li battery using a quasi-solid-state hybrid electrolyte and the battery performance was investigated.Lithium bis(trifluoromethaesulfonyl)amide (LiTFSA) was dissolved into tetraethylene glycol dimethyl ether (G4) in equimolar ratio to prepare RTIL-like complex solution. The quasi-solid-state electrolyte (QSE) powder was fabricated according to our previously reported method [1]. In this study, we prepared the QSE powder containing 80 vol% of G4/LiTFSA liquid, and a transparent quasi-solid-state hybrid electrolyte sheet was obtained by mixing polytetrafluoroethylene (PTFE) powder with the QSE powder in 5 wt%. The cathode was made by mixing LiFePO4 (theoretical capacity of 170 mAh g-1), AB, QSE powder and PTFE, with a weight ratio of 35:10:45:10. The composite cathode, the QSE sheet and Li metal anode was assembled as one-unit cell. Two and three unit-cells, connected in series, were prepared. The charge-discharge measurements of the one-, two- and three-unit bipolar all-solid-state cells were performed at 35 oC and a rate of 0.1C.The quasi-solid-state hybrid electrolyte was a white powder and the quasi-solid-state electrolyte sheet of 200 mm thickness was prepared [1] and used to assemble all-solid-state cells. The bipolar-type all-solid-state lithium batteries with different stack structures of one-, two- and three-unit cells connected in series in one package were assembled and the battery performances were investigated. The discharge capacity of the one-unit cell reached 161 mAh g-1 at the initial cycle with coulombic efficiency of 99 %. For such one-unit cell, discharge capacity was still maintained at 160 mAh g-1 even after 50 cycles, and the capacity retention was kept 99 %. The discharge capacities of the batteries with two- and three-unit cells were 160 mAh g-1 and 165 mAh g-1, respectively, which were as high as that of one-unit cell. These results implied the cathode utilization ratios of all the bipolar batteries up to 94 %. The coulombic efficiencies for two- and three-unit cells reached 98 % and 99 %, respectively. The discharge plateaus of two- and three-unit cells were observed in the voltage range of 6.7-6.8 V and 10.0-10.2 V, which were twice and three times to the one-unit cell voltage. In summary, these results suggest that our quasi-solid-state electrolytes were successfully applied to the bipolar-type all-solid-state lithium battery.Reference [1] A. Unemoto, T. Matsuo, H. Ogawa, Y. Gambe and I. Honma, J. Power Sources, 244, 354 (2013).

Calcium-ion batteries (CIBs) represent a promising alternative to the current lithium-ion batteries (LIBs). Calcium ions can transfer two electrons per ion. Thus, in principle, the capacity of a host material can double due to the divalency of calcium, provided the host material can release and accept the transferred electrons. The final battery product is expected to be cost-effective, owing to the earth-abundance of calcium reserves. The redox potential of calcium is close to that of lithium, enabling a high cell voltage. The larger ionic radius of calcium (1.0 Å) compared to those of other divalent ions (0.60–0.74 Å) could be a disadvantage because a narrow diffusion pathway sufficient for smaller ions will not allow the passage of larger Ca ions. However, the larger ionic radius ensures a lower effective intercalant-ion charge density, which can be rather advantageous for the diffusion in the host materials as well as in the electrolytes.Recent discoveries of reversible plating or alloying of calcium provoked considerable interest in calcium-based rechargeable batteries. Theoretical calculations also predict that the energy barriers for Ca diffusion are lower than other multivalent ions in some structures, and the development of positive materials are anticipated. However, only a few cathode materials have been reported so far, only to exhibit low energy-storage capability and poor cyclability. In this talk, the synthesis, structural and electrochemical properties of new cathode materials for nonaqueous CIBs will be presented, which were recently developed in our group. Some examples include NASICON-type NaV2(PO4)3, FeV3O9∙1.2H2O, Ca0.13MoO3·(H2O)0.41. These stimulating discoveries will lead to the development of new strategies for obtaining high energy density CIBs.

Free-standing COF nanofiber in ion conductive membrane to improve efficiency of vanadium redox flow battery

The research to discover a suitable cathode material for calcium-ion batteries (CIBs) remains challenging, despite their potential advantages of high energy density and cost-effectiveness. Herein, we report K0.5V2O5 as a cathode material for CIBs. During the initial charging of the electrode, potassium ions were replaced with 0.12 Ca. The calcium-exchanged electrode exhibited a reversible discharge capacity of ∼100 mAh g–1 at 0.1 C rate at room temperature with an average voltage of ∼3.0 V (vs Ca/Ca2+). The reversible capacity is higher than any reported cathode materials without crystal water, demonstrating the feasibility of further development of high-capacity crystal-water-free CIB cathode materials.

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N2 porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature 1H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

Exploring alternative battery systems to lithium-ion batteries becomes essential and urgent with the explosive growth of the electric vehicle and energy storage markets. Benefiting from resource abundance, low redox potential of Ca metal, and prospective high volumetric capacity, rechargeable calcium-ion batteries (CIBs) have recently attracted considerable attention in the field of energy storage. There is a breakthrough in the study of reversible precipitation/stripping of Ca metal at room temperature for anodes, but the cathodes for CIBs still suffer from low reversible capacity and poor cycle stability. This paper provides a comprehensive overview and recent progress of the cathode materials for CIBs, including metal oxides, Prussian blue analogs, polyanion materials, and organic cathodes. The features and modification strategies of each type of cathode material are discussed thoroughly. Finally, current issues of CIB cathodes and prospective solutions are presented.

Sodium ion batteries meet the demand for large-scale energy storage, such as in electric vehicles, due to the material abundance of sodium. In this report, nanotube-type Na2V3O7 is proposed as a cathode material because of its fast sodium diffusivity, an important requirement for sodium ion batteries, through the investigation of ~4300 candidates via a high-throughput computation. High-rate performance was confirmed, showing ~65% capacity retention at a current density of 10C at room temperature, despite the large particle size of >5 μm. A good cycle performance of ca. 94% in capacity retention after 50 cycles was obtained owing to a small volumetric change of <0.4%.