Sodium sulfur batteries require efficient sulfur hosts that can capture soluble polysulfides and enable fast reduction kinetics. Herein, we design hollow, polar and catalytic bipyramid prisms of cobalt sulfide as efficient sulfur host for sodium sulfur batteries. Cobalt sulfide has interwoven surfaces with wide internal spaces that can accommodate sodium polysulfides and withstand volumetric expansion. Furthermore, results from in/ex-situ characterization techniques and density functional theory calculations support the significance of the polar and catalytic properties of cobalt sulfide as hosts for soluble sodium polysulfides that reduce the shuttle effect and display excellent electrochemical performance. The polar catalytic bipyramid prisms sulfur@cobalt sulfide composite exhibits a high capacity of 755 mAh g−1 in the second discharge and 675 mAh g−1 after 800 charge/discharge cycles, with an ultralow capacity decay rate of 0.0126 % at a high current density of 0.5 C. Additionally, at a high mass loading of 9.1 mg cm−2, sulfur@cobalt sulfide shows high capacity of 545 mAh g−1 at a current density of 0.5 C. This study demonstrates a hollow, polar, and catalytic sulfur host with a unique structure that can capture sodium polysulfides and speed up the reduction reaction of long chain sodium polysulfides to solid small chain polysulfides, which results in excellent electrochemical performance for sodium-sulfur batteries.

Transition metal oxide nanocrystals with dual-mode electrochromism hold promise for smart windows enabling spectrally selective solar modulation. We have developed the colloidal synthesis of anisotropic monoclinic Nb12O29 nanoplatelets (NPLs) to investigate the dual-mode electrochromism of niobium oxide nanocrystals. The precursor for synthesizing NPLs was prepared by mixing NbCl5 and oleic acid to form a complex that was subsequently heated to form an oxide-like structure capped by oleic acid, denoted as niobium oxo cluster. By initiating the synthesis using niobium oxo clusters, preferred growth of NPLs over other polymorphs was observed. The structure of the synthesized NPLs was examined by X-ray diffraction in conjunction with simulations, revealing that the NPLs are monolayer monoclinic Nb12O29, thin in the [100] direction and extended along the b and c directions. Besides having monolayer thickness, NPLs show decreased intensity of Raman signal from Nb-O bonds with higher bond order when compared to bulk monoclinic Nb12O29, as interpreted by calculations. Progressive electrochemical reduction of NPL films led to absorbance in the near-infrared region (stage 1) followed by absorbance in both the visible and near-infrared regions (stage 2), thus exhibiting dual-mode electrochromism. The mechanisms underlying these two processes were distinguished electrochemically by cyclic voltammetry to determine the extent to which ion intercalation limits the kinetics, and by verifying the presence of localized electrons following ion intercalation using X-ray photoelectron spectroscopy. Both results support that the near-infrared absorption results from capacitive charging and the onset of visible absorption in the second stage is caused by ion intercalation. File list (2) download file view on ChemRxiv Maintext-04-09-2020.pdf (5.47 MiB) download file view on ChemRxiv SuppResults-04-09-2020.pdf (7.23 MiB)

Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb12O29 nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb12O29 nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb12O29 nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochemical reduction accompanied by Li insertion, the NR films show increasing absorbance mostly in the near infrared region. In contrast, the NPL films show increasing absorbance in the near infrared region first followed by increasing absorbance in both visible and near infrared regions. To elucidate the influence of shape anisotropy, we used density functional theory to construct the lithiated structures of monoclinic Nb12O29 and in these structures we identified the presence of square planar sites and crystallographic shear sites for Li insertion. By calculating the theoretical spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near infrared region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallographic shear sites leads to absorption in both visible and near infrared regions, which only occurs in NPLs due to their extension in the c direction.

analysis on cryo-FIB cross-sections combined with XPS demonstrate that cycled NST-Na is dense and pore-free, flat on its surface with a stable SEI. The cycled baseline bulk microstructure is triphasic; all the metal is form of curved dendritic filaments that are interspersed with SEI and with pores. A mechanistic explanation of these combined results is put fourth, being directly supported by DFT analysis. One key scientific takeaway is that templated growth is necessary for electrochemical stability of Na metal and that dendritic growth is the default on a standard foil. Keywordsalkaline metals, anode-free batteries, dead sodium, lithium-metal batteries, Na 3 V 2 (PO 4 ) 3 , sodium-metal batteries

PCM-102 is a new organophosphine metal–organic framework (MOF) featuring diphosphine pockets that consist of pairs of offset trans-oriented P­(III) donors. Postsynthetic addition of M­(I) salts (M = Cu, Ag, Au) to PCM-102 induces single-crystal to single-crystal transformations and the formation of trans-[P2M]+ solid-state complexes (where P = framework-based triarylphosphines). While the unmetalated PCM-102 has low porosity, the addition of secondary Lewis acids to install rigid P–M–P pillars is shown to dramatically increase both stability and selective gas uptake properties, with N2 Brunauer–Emmett–Teller surface areas >1500 m2 g–1. The Ag­(I) analogue can also be obtained via a simple, one-pot peri-synthetic route and is an ideal sacrificial precursor for materials with mixed bimetallic MA/MB pillars via postsynthetic, solvent-assisted metal exchange. Notably, the M-PCM-102 family of MOFs contain periodic trans-[P2M]+ sites that are free of counter anions, unlike traditional analogous molecular complexes, since the precursor PCM-102 MOF is monoanionic, enabling access to charge-neutral metal-pillared materials. Four M-PCM-102 materials were evaluated for the separation of C2 hydrocarbons. The separation performance was found to be tunable based on the metal(s) incorporated, and density functional theory was employed to elucidate the nature of the unusual observed sorption preference, C2H2 > C2H6 > C2H4.

Anode‐Free Batteries In article number 2106005, Hui Dong, David Mitlin, and co‐workers report an intermetallic for anode‐free batteries. The winter scene shows a birthday cake with lit candles that are melting: the cake represents the complex structure of a cycled sodium‐metal anode with the protruding candles being the dendrites. A slice, obtained by cryogenic focused ion beam (FIB) microscopy, further reveals the underlying porosity that develops during repeated plating and stripping of the metal. The thick non‐uniform red frosting on the cake represents the unstable solid electrolyte interphase (SEI) that drives the dendrite growth.

abundance of sulfur. The lower cost and greater availability of sodium as compared to lithium precursors is spurring the incremental focus on Na-S batteries. The traditional high temperature Na-S batteries operating at 300-350 °C comprise the molten electrodes and the solid inorganic β-alumina electrolyte. This mature design is known to have safety issues and a relatively low theoretical energy 760 W h kg −1 (2Na + 3S → Na 2 S 3 ). [2] Instead, there is a strong incentive to develop room-temperature (RT) Na-S batteries which in principle allow the two-electron reduction of sulfur to Na 2 S, with a higher theoretical energy of 1273 Wh kg −1 and less of a safety concern. [3] In practice, RT Na-S cells are likewise held back by several primary challenges including polysulfide (Na 2 S x , 4 ≤ x ≤ 8) dissolution and crossover in liquid electrolytes. Other concerns are the insulating nature of sulfur (σ e = 5 × 10 −30 S cm −1 ) and associated sluggish sulfur redox kinetics, as well as the large volume expansion (170%) of the cathode on cell discharge. [4] Sodium ions have lower solid-state diffusivity and reactivity with solid S than lithium ions. Consequently the electrochemical redox processes in Na-S are more sluggish than in the Li-S counterpart. [5] For Na-S cells, the galvanostatic plateaus are more sloping and less well-defined, while the charge-discharge voltage hysteresis is This is the first report of molybdenum carbide-based electrocatalyst for sulfur-based sodium-metal batteries. MoC/Mo 2 C is in situ grown on nitrogen-doped carbon nanotubes in parallel with formation of extensive nanoporosity. Sulfur impregnation (50 wt% S) results in unique triphasic architecture termed molybdenum carbide-porous carbon nanotubes host (MoC/Mo 2 C@PCNT-S). Quasi-solid-state phase transformation to Na 2 S is promoted in carbonate electrolyte, with in situ time-resolved Raman, X-ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo 2 C@PCNT-S cathodes deliver among the most promising rate performance characteristics in the literature, achieving 987 mAh g −1 at 1 A g −1 , 818 mAh g −1 at 3 A g −1 , and 621 mAh g −1 at 5 A g −1 . The cells deliver superior cycling stability, retaining 650 mAh g −1 after 1000 cycles at 1.5 A g −1 , corresponding to 0.028% capacity decay per cycle. High mass loading cathodes (64 wt% S, 12.7 mg cm −2 ) also show cycling stability. Density functional theory demonstrates that formation energy of Na 2 S x (1 ≤ x ≤ 4) on surface of MoC/Mo 2 C is significantly lowered compared to analogous redox in liquid. Strong binding of Na 2 S x (1 ≤ x ≤ 4) on MoC/Mo 2 C surfaces results from charge transfer between the sulfur and Mo sites on carbides' surface.

High donor number (DN) solvents in Li-O 2 batteries that dissolve superoxidei ntermediates in lithium peroxide( Li 2 O 2 )f ormation facilitate high capacities at high rates and avoid early cell death. However,t heir beneficial characteristics also result in an instability towardsh ighly reactive superoxide intermediates.F urthermore,L i-O 2 batteries would deliver as uperior energy density,b ut the multiphase electrochemical reactions are difficult to achieve when operating with only solid catalysts.Herein we demonstrate that vanadium(III) acetylacetonate (V(acac) 3 )i sa ne fficient soluble catalyst that can address these problems.During discharge, V(acac) 3 integrates with the superoxide intermediate,a ccelerating O 2 reduction kinetics and reducing side reactions.During charge,V(acac) 3 acts as aredox mediator that permits efficient oxidation of Li 2 O 2 .T he cells with V(acac) 3 exhibit low overpotential, high rate performance,a nd considerable cycle stability.