Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni 3 (benzenehexathiolate) (Ni 3 BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni 3 BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm 3 in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li + ions between the 2D layers of Ni 3 BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO 2 , Nb 2 O 5 , and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors.
The conformational stability of synthesized diphenyloxalohydrazide and dibenzoyloxalohydrazide fluoro derivatives has been investigated by extensive NMR studies that are ascertained by various levels of theoretical calculations. Two-dimensional H-F HOESY NMR experiments revealed the close spatial proximity between two NMR-active nuclei, confirming the hydrogen bond (HB)-mediated interaction between them, further aiding in establishing the probable stable conformations of these molecules. The relaxed potential energy scan disclosed the energy-minimized most stable structure among the several possible multiple conformations, which is in concurrence with NMR interpretations. Atomistic molecular dynamics simulations have been employed to unequivocally establish the conformational stability and the nature of HB formation at varied temperatures. With the possibility of occurrence of a number of probable conformations, the percentage of occurrences of different types of HBs in them was determined by MD simulations. Their population analysis was carried out using a Boltzmann distribution, in addition to deriving their Gibbs free energies. The molecular interactions governing the stable conformations have not only been ascertained by experimental NMR interpretations but also corroborated by other theoretical computations, viz., quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI).
Aliovalent substitution of the B component in ABX3 metal halides has often been proposed to modify the band gap and thus the photovoltaic properties, but details about the resulting structure have remained largely unknown. Here, we examine these effects in Bi-substituted CsSnBr3. Powder X-ray diffraction (XRD) and solid-state 119Sn, 133Cs and 209Bi nuclear magnetic resonance (NMR) spectroscopy were carried out to infer how Bi substitution changes the structure of these compounds. The cubic perovskite structure is preserved upon Bi-substitution, but with disorder in the B site occurring at the atomic level. Bi atoms are randomly distributed as they substitute for Sn atoms with no evidence of Bi segregation. The absorption edge in the optical spectra shifts from 1.8 to 1.2 eV upon Bi-substitution, maintaining a direct band gap according to electronic structure calculations. It is shown that Bi-substitution improves resistance to degradation by inhibiting the oxidation of Sn.
Both antimony and bismuth can alloy with up to three molar equivalents of lithium and are therefore attractive candidates for replacing graphite in Li-ion battery anodes. Li3Sb and Li3Bi have the same cubic structure (Fm3̅m), but the ternary Li–Sb–Bi system has not been studied. We synthesized Li3(Sb x Bi1–x ) with different Sb mole fractions at room temperature by ball milling. These ternary alloys all have cubic crystal structures, as determined by X-ray diffraction (XRD), but show a tendency toward phase segregation for x = 0.25 and 0.50. For x = 0.25, the lattice parameter presents a clear positive deviation from Vegard’s law in XRD, while for x = 0.50, XRD reveals two phases after milling, with the Bi-rich minority phase diminishing after thermal annealing. Solid-state nuclear magnetic resonance spectroscopy provides evidence for a Sb-enriched environment around the Li atoms for Li3Sb0.25Bi0.75, and nuclear spin–lattice relaxation measurements of the binary and ternary alloy phases point to low activation energies and rapid Li-ion diffusion in Li3Bi.
Sodium thioantimonate (Na 3 SbS 4 ) and its W-substituted analogue Na 2.88 Sb 0.88 W 0.12 S 4 have been identified as potential electrolyte materials for allsolid-state sodium batteries due to their high Na + conductivity. Ball milling mechanochemistry is a frequently employed synthetic approach to produce such Na + -conductive solid solutions; however, changes in the structure and morphology introduced in these systems via the mechanochemistry process are poorly understood. Herein, we combined X-ray absorption fine structure spectroscopy, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy characterization techniques to provide an in-depth analysis of these solid electrolytes. We report unique changes seen in the structure and morphology of Na 3 SbS 4 and Na 2.88 Sb 0.88 W 0.12 S 4 resulting from ball milling, inducing changes in the electrochemical performance of the solid-state batteries. Specifically, we observed a tetragonal-to-cubic crystal phase transition within Na 3 SbS 4 following the ball mill, resulting in an increase in Na + conductivity. In contrast, the Na + conductivity was reduced in mechanochemically treated Na 2.88 Sb 0.88 W 0.12 S 4 due to the formation and accumulation of a WS 2 phase. In addition, mechanochemical treatment alters the surface morphology of densified Na 2.88 Sb 0.88 W 0.12 S 4 pellets, providing intimate contact at the solid electrolyte/Na interface. This phenomenon was not observed in Na 3 SbS 4 . This work reveals the structural and morphological origin of the changes seen in these materials' electrochemical performance and how mechanochemical synthesis can introduce them.
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