Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI^–) and chloride-containing electrolyte in acetonitrile exhibit current densities that are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two-electron transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical–electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg₂(μ–Cl)₃·6THF]⁺ present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature

Structure, Stability, and Properties of the Intergrowth Compounds ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where <i>m</i> = <i>n</i> = 1–20

Intergrowth compounds of ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where 1 ≤ <i>m = n ≤ </i>20, with the same atomic composition but different <i>c</i>-axis lattice parameters and number of interfaces per volume were synthesized using the modulated elemental reactant technique. A <i>c</i>-axis lattice parameter change of 1.217(6) nm as a function of one unit of <i>m</i> = <i>n</i> was observed. In-plane X-ray diffraction shows an increase in distortion of the rock salt layer as a function of <i>m</i> and a broadening of the NbSe₂ reflections as <i>n</i> increases, indicating the presence of different coordination environments for Nb (trigonal prismatic and octahedral) and smaller crystallite size, which were confirmed via scanning transmission electron microscopy investigations. The electrical resistivities of all 12 compounds exhibit metallic temperature dependence and are similar in magnitude as would be expected for isocompositional compounds. Carrier concentration and mobility of the compounds vary within a narrow range of 2–6 × 10²¹ cm^–3 and 2–6 cm² V^–1 s^–1, respectively. Even at a thickness of 12 nm for the SnSe and NbSe₂ blocks, the properties of the intergrowth compounds cannot be explained as composite behavior, due to significant charge transfer between them. Upon being annealed at 500 °C, the higher order <i>m</i> = <i>n</i> compounds were found to convert to the thermodynamically stable phase, the (1,1) compound. This suggests that the capacitive energy of the interfaces stabilizes these intergrowth compounds

Effect of Local Structure of NbSe₂ on the Transport Properties of ([SnSe]_1.16)₁(NbSe₂)_<i>n</i> Ferecrystals

([SnSe]_1.16)₁(NbSe₂)_<i>n</i> ferecrystals were synthesized through the modulated elemental reactants technique by increasing the number of Nb|Se layers in the precursor from 1 to 4. The <i>c</i>-lattice parameter of the intergrowth was observed to change as a function of <i>n</i> by 0.635(2) nm. The <i>c</i>-lattice parameter of SnSe was observed to be 0.588(8) nm and independent of <i>n</i>. The electrical resistivity does not decrease as <i>n</i> increases as expected from simple models, but instead the trend in the resistivity is (1,3) > (1,4) ≥ (1,1) > (1,2). The carrier concentration increases with <i>n</i> as expected, so the unusual trend in resistivity is a result of the carrier mobility decreasing with increasing <i>n.</i> In-plane X-ray diffraction line widths and STEM images of the (1,4) compound show that it has small in-plane grain sizes and a large diversity of stacking sequences respectively, providing a potential explanation for the reduced carrier mobility

The Influence of Interfaces on Properties of Thin-Film Inorganic Structural Isomers Containing SnSe–NbSe₂ Subunits

Inorganic isomers ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>([SnSe]_1+δ)_<i>p</i>(NbSe₂)_<i>q</i>([SnSe]_1+δ)_<i>r</i>(NbSe₂)_<i>s</i> where <i>m</i>, <i>n</i>, <i>p</i>, <i>q</i>, <i>r</i>, and <i>s</i> are integers and <i>m</i> + <i>p</i> + <i>r</i> = <i>n</i> + <i>q</i> + <i>s</i> = 4 were prepared using the modulated elemental reactant technique. This series of all six possible isomers provides an opportunity to study the influence of interface density on properties while maintaining the same unit cell size and composition. As expected, all six compounds were observed to have the same atomic compositions and an almost constant <i>c</i>-axis lattice parameter of ≈4.90(5) nm, with a slight trend in the <i>c</i>-axis lattice parameter correlated with the different number of interfaces in the isomers: two, four and six. The structures of the constituents in the <i>ab</i>-plane were independent of one another, confirming the nonepitaxial relationship between them. The temperature dependent electrical resistivities revealed metallic behavior for all the six compounds. Surprisingly, the electrical resistivity at room temperature decreases with increasing number of interfaces. Hall measurements suggest this results from changes in carrier concentration, which increases with increasing thickness of the thickest SnSe block in the isomer. Carrier mobility scales with the thickness of the thickest NbSe₂ block due to increased interfacial scattering as the NbSe₂ blocks become thinner. The observed behavior suggests that the two constituents serve different purposes with respect to electrical transport. SnSe acts as a charge donor and NbSe₂ acts as the charge transport layer. This separation of function suggests that such heterostructures can be designed to optimize performance through choice of constituent, layer thickness, and layer sequence. A simplistic model, which predicts the properties of the complex isomers from a weighted sum of the properties of building blocks, was developed. A theoretical model is needed to predict the optimal compound for specific properties among the many potential compounds that can be prepared

Structural Changes in 2D BiSe Bilayers as <i>n</i> Increases in (BiSe)_1+δ(NbSe₂)_<i>n</i> (<i>n</i> = 1–4) Heterostructures

(BiSe)_1+δ(NbSe₂)_<i>n</i> heterostructures with <i>n</i> = 1–4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with <i>n</i> = 1 to a square basal plane for <i>n</i> = 2–4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the <i>n</i> = 1 compound, is dramatically reduced for the <i>n</i> = 2–4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d_3/2, 5d_5/2 doublet peaks narrowed toward higher binding energies as <i>n</i> increased from 1 to 2, also consistent with a reduction in the number of antiphase boundaries. Temperature-dependent electrical resistivity and Hall coefficient measurements of nominally stoichiometric samples in conjunction with structural refinements and XPS data suggest a constant amount of interlayer charge transfer independent of <i>n</i>. Constant interlayer charge transfer is surprising given the changes in the BiSe in-plane structure. The structural flexibility of the BiSe layer may be useful in designing multiple constituent heterostructures as an interlayer between structurally dissimilar constituents