Mixed Anionic and Cationic Redox Chemistry in a Tetrathiomolybdate Amorphous Coordination Framework

We report the electrochemistry of a hitherto unexplored Na2MoS4 phase as conversion electrode material for Na‐ and Li‐cation reversible storage. The material adopts an amorphous coordination polymer structure, with mixed Mo and S valences. Ex‐situ XPS and in‐situ XRD analysis reveal a complex interplay between Mo and S redox, while excluding the formation of free sulfur, lithium sulfide or other crystalline phases. Na2MoS4 behaves as a mixed ionic‐electronic conductor, with electronic conductivity of 6.1 × 10 ‐4 S cm ‐1 , that allowed the use of this material without any conductive carbon in an electrochemical cell. A reversible capacity of 500 mAh g ‐1 is attained corresponding to 5‐electron redox exchange, with the end‐member reaching from Na5MoS4 . This study not only emphasizes on the excellent charge storage performances of Na2MoS4 for Li or Na batteries, but also enriches the emerging library and knowledge of sulfide phases with mixed anionic and cationic redox

Confining charge-transfer complex in a metal-organic framework for photocatalytic CO2 reduction in water

Abstract In the quest for renewable fuel production, the selective conversion of CO2 to CH4 under visible light in water is a leading-edge challenge considering the involvement of kinetically sluggish multiple elementary steps. Herein, 1-pyrenebutyric acid is post-synthetically grafted in a defect-engineered Zr-based metal organic framework by replacing exchangeable formate. Then, methyl viologen is incorporated in the confined space of post-modified MOF to achieve donor-acceptor complex, which acts as an antenna to harvest visible light, and regulates electron transfer to the catalytic center (Zr-oxo cluster) to enable visible-light-driven CO2 reduction reaction. The proximal presence of the charge transfer complex enhances charge transfer kinetics as realized from transient absorption spectroscopy, and the facile electron transfer helps to produce CH4 from CO2. The reported material produces 7.3 mmol g−1 of CH4 under light irradiation in aqueous medium using sacrificial agents. Mechanistic information gleans from electron paramagnetic resonance, in situ diffuse reflectance FT-IR and density functional theory calculation

Validating the reversible redox of alkali-ion disulfonyl-methanide as organic positive electrode materials

Alkali-ion reservoir organic positive electrode materials with high redox potential emerge as promising candidates for rocking-chair batteries. Despite recent major advances on coordinated Li-enolates and conjugated Li-sulfonamides, alike chemistries remain rare with clearly enormous room to explore and investigate. Herein, we report a new high-voltage redox functionality for alkali-ion storage based on aromatic di-sulfonyl methanide. The versatile paradigm chemistry of di-alkali p-phenylene-bis-diethylsulfonyl methanide (denoted as A2-p-TESO2, where A = Li+, Na+, K+) displays high voltage (3.13 vs Li+/Li), a two-electron reversible redox in both liquid phase and solid phase electrochemistry. A2-p-TESO2 also shows great stability under ambient air conditions in their alkali-ion reservoir states. This work not only suggests a new charge storage mechanism for organic batteries but also highlights the versatility of organic chemistry in designing new redox functionalities for next-generation organic rechargeable batteries

Towards the 4 V-class n-type organic lithium-ion positive electrode materials: the case of conjugated triflimides and cyanamides

Organic electrode materials have garnered a great deal of interest owing to their sustainability, cost-efficiency, and design flexibility metrics. Despite numerous endeavors to fine-tune their redox potential, the pool of organic positive electrode materials with a redox potential above 3 V versus Li+/Li0, and maintaining air stability in the Li-reservoir configuration remains limited. This study expands the chemical landscape of organic Li-ion positive electrode chemistries towards the 4 V-class through molecular design based on electron density depletion within the redox center via the mesomeric effect of electron-withdrawing groups (EWGs). This results in the development of novel families of conjugated triflimides and cyanamides as high-voltage electrode materials for organic lithium-ion batteries. These are found to exhibit ambient air stability and demonstrate reversible electrochemistry with redox potentials spanning the range of 3.1 V to 3.8 V (versus Li+/Li0), marking the highest reported values so far within the realm of n-type organic chemistries. Through comprehensive structural analysis and extensive electrochemical studies, we elucidate the relationship between the molecular structure and the ability to fine-tune the redox potential. These findings offer promising opportunities to customize the redox properties of organic electrodes, bridging the gap with their inorganic counterparts for application in sustainable and eco-friendly electrochemical energy storage devices

An Electrically Conducting Li-Ion Metal–Organic Framework

Metal–organic frameworks (MOFs) have emerged as an important, yet highly challenging class of electrochemical energy storage materials. The chemical principles for electroactive MOFs remain, however, poorly explored because precise chemical and structural control is mandatory. For instance, no anionic MOF with a lithium cation reservoir and reversible redox (like a conventional Li-ion cathode) has been synthesized to date. Herein, we report on electrically conducting Li-ion MOF cathodes with the generic formula Li2-M-DOBDC (wherein M = Mg2+ or Mn2+; DOBDC4– = 2,5-dioxido-1,4-benzenedicarboxylate), by rational control of the ligand to transition metal stoichiometry and secondary building unit (SBU) topology in the archetypal CPO-27. The accurate chemical and structural changes not only enable reversible redox but also induce a million-fold electrical conductivity increase by virtue of efficient electronic self-exchange facilitated by mix-in redox: 10–7 S/cm for Li2-Mn-DOBDC vs 10–13 S/cm for the isoreticular H2-Mn-DOBDC and Li2-Mg-DOBDC, or the Mn-CPO-27 compositional analogues. This particular SBU topology also considerably augments the redox potential of the DOBDC4– linker (from 2.4 V up to 3.2 V, vs Li+/Li0), a highly practical feature for Li-ion battery assembly and energy evaluation. As a particular cathode material, Li2-Mn-DOBDC displays an average discharge potential of 3.2 V vs Li+/Li0, demonstrates excellent capacity retention over 100 cycles, while also handling fast cycling rates, inherent to the intrinsic electronic conductivity. The Li2-M-DOBDC material validates the concept of reversible redox activity and electronic conductivity in MOFs by accommodating the ligand’s noncoordinating redox center through composition and SBU design

Through-Space Charge Modulation Overriding Substituent Effect: Rise of the Redox Potential at 3.35 V in a Lithium-Phenolate Stereoelectronic Isomer

Raising the operating potential of the organic positive electrode materials is a crucial challenge if they are to compare with lithium-ion inorganic counterparts. Although many efforts have been directed on tuning through substituent electronic effect, the chemistries than can operate above 3 V vs Li+/Li0, and thus be air stable in the Li-reservoir form (alike the conventional inorganic Li-ion positive electrode materials) remain finger-counted. Herein, we report on a new n-type organic Li-ion positive electrode material—the tetralithium 2,5-dihydroxy-1,4-benzenediacetate—with a remarkably high redox potential of 3.35 V vs Li+/Li0 attained notably in the solid phase. The origin of the high-energy content in this quinone derivative is found in a stereoelectronic chameleonic effect with an intramolecular conformation change and charge modulation leading to a redox potential increase of 650 mV in the solid state as compared to the same chemistry tested in solution (2.70 V vs Li+/Li0). The conformational dependent electroactivity rationale is supported by electrochemical and crystallography analysis, comparative infrared spectroscopy, and DFT calculation. We identify and make a linear correlation between the enolate vibrational modes and the redox potential, with general applicability for possibly other phenolate redox chemistries. Owing to these effects, this lithiated quinone is stable in ambient air and can be processed and handled alike the conventional inorganic Li-ion positive electrode materials. Whereas intrinsic to high voltage operation stability issues remain to be solved for practical implementation, our fundamental in nature and proof-of-concept study highlights the strong amplitude of through-space charge modulation effects in designing new organic Li-ion positive electrode chemistries with practical operating potential

Controlling Charge Transport in 2D Conductive MOFs─The Role of Nitrogen-Rich Ligands and Chemical Functionality

Two-dimensional electrically conducting metal–organic frameworks (2D-e-MOFs) have emerged as a class of highly promising functional materials for a wide range of applications. However, despite the significant recent advances in 2D-e-MOFs, developing systems that can be postsynthetically chemically functionalized, while also allowing fine-tuning of the transport properties, remains challenging. Herein, we report two isostructural 2D-e-MOFs: Ni3(HITAT)2 and Ni3(HITBim)2 based on two new 3-fold symmetric ligands: 2,3,7,8,12,13-hexaaminotriazatruxene (HATAT) and 2,3,8,9,14,15-hexaaminotribenzimidazole (HATBim), respectively, with reactive sites for postfunctionalization. Ni3(HITAT)2 and Ni3(HITBim)2 exhibit temperature-activated charge transport, with bulk conductivity values of 44 and 0.5 mS cm–1, respectively. Density functional theory analysis attributes the difference to disparities in the electron density distribution within the parent ligands: nitrogen-rich HATBim exhibits localized electron density and a notably lower lowest unoccupied molecular orbital (LUMO) energy relative to HATAT. Precise amounts of methanesulfonyl groups are covalently bonded to the N–H indole moiety within the Ni3(HITAT)2 framework, modulating the electrical conductivity by a factor of ∼20. These results provide a blueprint for the design of porous functional materials with tunable chemical functionality and electrical response

Bimetallic Anionic Organic Frameworks with Solid‐State Cation Conduction for Charge Storage Applications

A new phosphonate-based anionic bimetallic organic framework, with the general formula of A4−Zn−DOBDP (wherein A is Li+ or Na+, and DOBDP6− is the 2,5-dioxido-1,4-benzenediphosphate ligand) is prepared and characterized for energy storage applications. With four alkali cations per formula unit, the A4−Zn−DOBDP MOF is found to be the first example of non-solvated cation conducting MOF with measured conductivities of 5.4×10−8 S cm−1 and 3.4×10−8 S cm−1 for Li4- and Na4- phases, indicating phase and composition effects of Li+ and Na+ shuttling through the channels. Three orders of magnitude increase in ionic conductivity is further attained upon solvation with propylene carbonate, placing this system among the best MOF ionic conductors at room temperature. As positive electrode material, Li4−Zn−DOBDP delivers a specific capacity of 140 mAh g−1 at a high average discharge potential of 3.2 V (vs. Li+/Li) with 90 % of capacity retention over 100 cycles. The significance of this research extends from the development of a new family of electroactive phosphonate-based MOFs with inherent ionic conductivity and reversible cation storage, to providing elementary insights into the development of highly sought yet still evasive MOFs with mixed-ion and electron conduction for energy storage applications

Revealing the reversible solid-state electrochemistry of lithium-containing conjugated oximates for organic batteries

In the rising advent of organic Li-ion positive electrode materials with increased energy content, chemistries with high redox potential and intrinsic oxidation stability remain a challenge. Here, we report the solid-phase reversible electrochemistry of the oximate organic redox functionality. The disclosed oximate chemistries, including cyclic, acyclic, aliphatic, and tetra-functional stereotypes, uncover the complex interplay between the molecular structure and the electroactivity. Among the exotic features, the most appealing one is the reversible electrochemical polymerization accompanying the charge storage process in solid phase, through intermolecular azodioxy bond coupling. The best-performing oximate delivers a high reversible capacity of 350 mAh g−1 at an average potential of 3.0 versus Li+/Li0, attaining 1 kWh kg−1 specific energy content at the material level metric. This work ascertains a strong link between electrochemistry, organic chemistry, and battery science by emphasizing on how different phases, mechanisms, and performances can be accessed using a single chemical functionality