Site-Specific Sodiation Mechanisms of Selenium in Microporous Carbon Host

Make Selenium Reactive Again: Activating Elemental Selenium for Synthesis of Metal Selenides Ranging from Nanocrystals to Large Single Crystals

The inertness of elemental selenium is a significant obstacle in the synthesis of selenium-containing materials at low reaction temperatures. Over the years, several recipes have been developed to overcome this hurdle; however, most of the methods are associated with the use of highly toxic, expensive, and environmentally harmful reagents. As such, there is an increasing demand for the design of cheap, stable, and nontoxic reactive selenium precursors usable in the low-temperature synthesis of transition metal selenides with vast applications in nanotechnology, thermoelectrics, and superconductors. Herein, a novel synthetic route has been developed for activating elemental selenium by using a solvothermal approach. By comprehensive 77Se NMR, Raman, and infrared spectroscopies and gas chromatography-mass spectrometry, we show that the activated Se solution contained HSe-, [Se-Se]2-, and Se2- ions, as well as dialkyl selenide (R2Se) and dialkyl diselenide (R-Se-Se-R) species in dynamic equilibrium. This also corresponded to the first observation of naked Se22- in solution. The versatility of the developed Se precursor was demonstrated by the successful synthesis of (i) the polycrystalline room-temperature modification of the β-Ag2Se thermoelectric material; (ii) large single crystals of superconducting β-FeSe; (iii) CdSe nanocrystals with different particle sizes (3-10 nm); (iv) nanosheets of PtSe2; and (v) mono- and dibenzyl selenides and diselenides at room temperature. The simplicity and diversity of the developed Se activation method holds promise for applied and fundamental research.

Tetrahedral Bonding Structure (Ni3 -Se) Induced by Lattice-Distortion of Ni to Achieve High Catalytic Activity in Na-Se Battery

Fabrication of transition-metal catalytic materials is regarded as a promising strategy for developing high-performance sodium-selenium (Na-Se) batteries. However, more systematic explorations are further demanded to find out how their bonding interactions and electronic structures can affect the Na storage process. This study finds that lattice-distorted nickel (Ni) structure can form different bonding structures with Na2 Se4 , providing high activity to catalyze the electrochemical reactions in Na-Se batteries. Using this Ni structure to prepare electrode (Se@NiSe2 /Ni/CTs) can realize rapid charge transfer and high cycle stability of the battery. The electrode exhibits high storage performance of Na+ ; i.e., 345 mAh g⁻1 at 1 C after 400 cycles, and 286.4 mAh g⁻1 at 10 C in rate performance test. Further results reveal the existence of a regulated electronic structure with upshifts of the d-band center in the distorted Ni structure. This regulation changes the interaction between Ni and Na2 Se4 to form a Ni3 -Se tetrahedral bonding structure. This bonding structure can provide higher adsorption energy of Ni to Na2 Se4 to facilitate the redox reaction of Na2 Se4 during the electrochemical process. This study can inspire the design of bonding structure with high performance in conversion-reaction-based batteries.

Unraveling the Conversion Evolution on Solid-State Na-SeS2 Battery via In Situ TEM

All-solid-state (ASS) Na-S batteries are promising for a large-scale energy-storage system owing to numerous merits. However, the high conversion reaction barrier impedes their practical application. In this work, the basic mechanism on how Se catalyzes the conversion reaction in the Na-S batteries is unraveled. The sodiation/desodiation of Na-SeS2 nanobatteries are systematically evaluated via in situ transmission electron microscopy (in situ TEM) with a microheating device. The real-time analyses reveal an amorphous Na-Sex Sy intermediate phase appears during the direct conversion from SeS2 to Na2 S, and a reverse reaction succeeds at 100 °C with a prior formation of Se. The absence of polysulfides and a much lower desodiation temperature in contrast to Na-S nanobatteries demonstrate that the Se incorporation significantly lowers the conversion reaction barrier. According to these findings, the ASS SeS2 batteries using a Na3 SbS4 solid electrolyte (SE) are assembled using various SE:C ratios in the composite cathodes to investigate the effect of the ion and electron transport on the electrochemical properties, including the effective transport properties, MacMullin number, and the tortuosity factor. The obtained results in turn confirm the findings from the in situ TEM. These findings are applicable to optimize other S-based active materials and improve their utilization.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Trimodal hierarchical porous carbon nanorods enable high-performance Na–Se batteries

Technical bottlenecks of polyselenide shuttling and material volume variation significantly hamper the development of emerging sodium–selenium (Na–Se) batteries. The nanopore structure of substrate materials is demonstrated to play a vital role in stabilizing Se cathodes and approaching superior Na-ion storage properties. Herein, an ideal nanorod-like trimodal hierarchical porous carbon (THPC) host is fabricated through a facile one-step carbonization method for advanced Na–Se batteries. The THPC possesses a trimodal nanopore structure encompassing micropores, mesopores, and macropores, and functions as a good accommodator of Se molecules, a reservoir of polyselenide intermediates, a buffer for volume expansion of Se species during sodiation, and a promoter for electron/ion transfer in the electrochemical process. As a result, Na–Se batteries assembled with the Se–THPC composite cathode realize high utilization of Se, fast redox kinetics, and excellent cyclability. Furthermore, the Na-ion storage mechanism of the well-designed Se–THPC composite is profoundly revealed by in situ visual characterization techniques.

Technical bottlenecks of polyselenide shuttling and material volume variation significantly hamper the development of emerging sodium–selenium (Na–Se) batteries.

S-Decorated Porous Ti3 C2 MXene Combined with In Situ Forming Cu2 Se as Effective Shuttling Interrupter in Na-Se Batteries

Given natural abundance of Na and superior kinetics of Se, Na-Se batteries have attracted much attention but still face the problem of shuttling effect of soluble intermediates. The first-principle calculations reveal the S-decorated Ti₃ C₂ exhibits increased binding energy to sodium polyselenides, suggesting a better capture and restriction on intermediates. The obtained Se@S-decorated porous Ti₃ C₂ (Se@S-P-Ti₃ C₂ ) exhibits a high reversible capacity of 765 mAh g^-1 at 0.1 A g^-1 (calculated based on Se), ≈1.2, 1.3, and 1.7 times of Se@porous Ti₃ C₂ (Se@P-Ti₃ C₂ ), Se@Ti₃ C₂ , and Se, respectively. It gives considerable capacity of 664 mAh g^-1 at 20 A g^-1 and impressive cycling stability over 2300 cycles with an ultralow capacity decay of 0.003% per cycle. The excellent electrochemical performance can be ascribed to the S-modified porous Ti₃ C₂ , which provides effective immobilization toward polyselenides, makes full use of nanosized Se, and alleviates volume expansion during sodiation/desodiation. Additionally, in situ forming Cu₂ Se can generate Cu nanoparticles through discharge process and then transform polyselenides into solid-phase Cu₂ Se, further suppressing the shuttling effect. This work provides a practical strategy to immobilize and transform sodium polyselenides for high-capacity and long-life Na-Se batteries.

Unraveling the Nature of Excellent Potassium Storage in Small-Molecule Se@Peapod-Like N-Doped Carbon Nanofibers

The potassium-selenium (K-Se) battery is considered as an alternative solution for stationary energy storage because of abundant resource of K. However, the detailed mechanism of the energy storage process is yet to be unraveled. Herein, the findings in probing the working mechanism of the K-ion storage in Se cathode are reported using both experimental and computational approaches. A flexible K-Se battery is prepared by employing the small-molecule Se embedded in freestanding N -doped porous carbon nanofibers thin film (Se@NPCFs) as cathode. The reaction mechanisms are elucidated by identifying the existence of short-chain molecular Se encapsulated inside the microporous host, which transforms to K₂ Se by a two-step conversion reaction via an "all-solid-state" electrochemical process in the carbonate electrolyte system. Through the whole reaction, the generation of polyselenides (K₂ Se_n , 3 ≤ n ≤ 8) is effectively suppressed by electrochemical reaction dominated by Se₂ molecules, thus significantly enhancing the utilization of Se and effecting the voltage platform of the K-Se battery. This work offers a practical pathway to optimize the K-Se battery performance through structure engineering and manipulation of selenium chemistry for the formation of selective species and reveal its internal reaction mechanism in the carbonate electrolyte.

For Whom a Puddle Is the Sea? Adsorption of Organic Guests on Hydrated MCM-41 Silica

Thermal and hydration effects on the mobility of compact and branched organic molecules and a bulky pharmaceutical substance loaded in submonolayer amounts onto mesoporous silica have been elucidated using ¹H and ³¹P solid-state NMR. In all cases, the ambient hydration has a stronger effect than an increase in temperature to 370 K for water-free silica. The effect of hydration depends on the guest and ranges from complete solvation to a silica-water-guest sandwich structure to a silica-guest/silica-water pattern. The mobility of the guests under different conditions has been described. The specific structure of the MCM-41 surface allows one to study very slow surface diffusion, a diffusivity of about 10^-15-10^-16 m²/s. The data reported are relevant to any nonfunctionalized silica, while the method used is applicable to any phosphor-containing guest on any host.