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MXenes and sulfurized polyacrylonitrile (S-PAN) are emerging as possible contenders to resolve the polysulfide dissolution and volumetric expansion issues in sodium-sulfur batteries. Herein, we explore the interactions between Ti3C2Tx MXenes and S-PAN with traditional binders such as polyvinylidene difluoride (PVDF), poly(acrylic acid) (PAA), and carboxymethyl cellulose (CMC) in Na-S batteries for the first time. We hypothesize that the linearity and polarity of the binder significantly influence the dispersion of S-PAN over Ti3C2Tx. The three-dimensional polar CMC binder resulted in better contact surface area with both S-PAN and Ti3C2Tx. Moreover, the improved binding of the discharge products with the CMC binder effectively traps them and prevents unwanted shuttling. Consequently, the Na-S battery using the CMC binder displayed a high initial capacity of 1282 mAh/g(s) at 0.2 C and a low capacity fading of 0.092% per cycle over 300 cycles. This work highlights the importance of understanding MXene-binder interactions in sulfur cathodes.
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Two-dimensional metal dichalcogenides have demonstrated outstanding potential as cathodes for magnesium-ion batteries. However, the limited capacity, poor cycling stability, and severe electrode pulverization, resulting from lack of void space for expansion, impede their further development. In this work, we report for the first time, nickel sulfide (NiS2) hollow nanospheres assembled with nanoparticles for use as cathode materials in magnesium-ion batteries. Notably, the nanospheres were prepared by a one-step solvothermal process in the absence of an additive. The results show that regulating the synergistic effect between the rich anions and hollow structure positively affects its electrochemical performance. Crystallographic and microstructural characterizations reveal the reversible anionic redox of S2-/(S2)2-, consistent with density functional theory results. Consequently, the optimized cathode (8-NiS2 hollow nanospheres) could deliver a large capacity of 301 mA h g-1 after 100 cycles at 50 mA g-1, supporting the promising practical application of NiS2 hollow nanospheres in magnesium-ion batteries.
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Two-dimensional MXenes produce competitive performances when incorporated into lithium-sulfur batteries (LSBs), solving key problems such as the poor electronic conductivity of sulfur and dissolution of its polysulfide intermediates. However, MXene nanosheets are known to easily aggregate and restack during electrode fabrication, filtration, or water removal, limiting their practical applicability. Furthermore, in complex electrocatalytic reactions like the multistep sulfur reduction process in LSBs, MXene alone is insufficient to ensure an optimal reaction pathway. In this work, we demonstrate for the first time a loose templating of sulfur spheres using Ti3C2Tx MXene nanosheets decorated with polymorphic CoSe2 nanoparticles. This work shows that the templating of sulfur spheres using nanoparticle-decorated MXene nanosheets can prevent nanosheet aggregation and exert a strong electrocatalytic effect, thereby enabling improved reaction kinetics and battery performance. The S@MXene-CoSe2 cathode demonstrated a long cycle life of 1000 cycles and a low capacity decay rate of 0.06% per cycle in LSBs.
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Owing to its high volumetric capacity and natural abundance, magnesium (Mg) metal has attracted tremendous attention as an ideal anode material for rechargeable Mg batteries. Despite Mg deposition playing an integral role in determining the cycling lifespan, its exact behavior is not clearly understood yet. Herein, for the first time, we introduce a facile approach to build magnesiophilic In/MgIn sites in situ on a Mg metal surface using InCl3 electrolyte additive for rechargeable Mg batteries. These magnesiophilic sites can regulate Mg deposition behaviors by homogenizing the distributions of Mg-ion flux and electric field at the electrode-electrolyte interphase, allowing flat and compact Mg deposition to inhibit short-circuiting. The as-designed Mg metal batteries achieve a stable cycling lifespan of 340 h at 1.0 mA cm-2 and 1.0 mAh cm-2 using Celgard separators, while the full cell coupled with Mo6S8 cathode maintains a high capacity retention of 95.5% over 800 cycles at 1 C.
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Metallic magnesium is a promising high-capacity anode material for energy storage technologies beyond lithium-ion batteries. However, most reported Mg metal anodes are only cyclable under shallow cycling (≤1 mAh cm-2) and thus poor Mg utilization (<3%) conditions, significantly compromising their energy-dense characteristic. Herein, composite Mg metal anodes with high capacity utilization of 75% are achieved by coating magnesiophilic gold nanoparticles on copper foils for the first time. Benefiting from homogeneous ionic flux and uniform deposition morphology, the Mg-plated Au-Cu electrode exhibits high average Coulombic efficiency of 99.16% over 170 h cycling at 75% Mg utilization. Moreover, the full cell based on Mg-plated Au-Cu anode and Mo6S8 cathode achieves superior capacity retention of 80% after 300 cycles at a low negative/positive ratio of 1.33. This work provides a simple yet effective general strategy to enhance Mg utilization and reversibility, which can be extended to other metal anodes as well.
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Room-temperature sodium-sulfur batteries have potential in stationary applications, but challenges such as loss of active sulfur and low electrical conductivity must be solved. Nitrogen-doped nanocarbon host cathodes have been employed in metal-sulfur batteries: polar interactions mitigate the loss of sulfur, while the conductive nanostructure addresses the low conductivity. Nevertheless, these two properties run contrary to each other as greater nitrogen-doping of nanocarbon hosts is associated with lower conductivity. Herein, we investigate the polarity-conductivity dilemma to determine which of these properties have the stronger influence on cycling performance. Lower carbonization temperatures produce more pyridinic nitrogen and pyrrolic nitrogen, which from density functional theory calculations preferentially bind discharge products (Na2S and short-chain polysulfides). Despite its lower conductivity, the highly doped composite showed better Coulombic efficiency and stability, retaining a high capacity of 980 mAh g(S)-1 after 800 cycles. Our findings represent a paradigm shift where nitrogen-doping should be prioritized in designing shuttle-free, long-life sodium-sulfur batteries.
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The molecular design of organic battery electrodes is a big challenge. Here, we synthesize two metal-free organosulfur acenes and shed insight into battery properties using first-principles calculations. A new zone-melting chemical-vapor-transport (ZM-CVT) apparatus was fabricated to provide a simple, solvent-free, and continuous synthetic protocol, and produce single crystals of tetrathiotetracene (TTT) and hexathiapentacene (HTP) at a large scale. Single crystals of HTP showed better Li-ion battery performance and higher cycling stability than those of TTT. A two-step, three-electron lithiation mechanism instead of the commonly depicted two-electron mechanism is proposed for the HTP Li-ion battery. The superior performance of HTP is linked to unique trisulfide bonding scenarios, which are also responsible for the formation of empty channels along the stacking direction. In-depth theoretical analysis suggests that organosulfur acenes are potential prototypes for organic battery materials with tunable properties, and that the tuning of sulfur bonds is critical in designing these new materials.
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We present the first atomistic-scale simulation of the discharge process of polymeric cathode materials for electrochemical batteries in solid state. The oxidation of polyaniline (PANI) and of cyano groups (CN) functionalized PANI induced by coordination to the electrolyte anions is computed and voltage curves are estimated. To deal with the large required numbers of atoms and structures, a combination of molecular dynamics and density functional tight binding (DFTB) is used. The DFTB is benchmarked to density functional theory (DFT) calculations using different functionals to confirm its accuracy. The voltages computed with the solid state model are in good agreement with available experimental data and ab initio models based on oligomers. The solid state model also predicts substantially increased voltage with PANI functionalized with cyano groups.
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The mechanism(s) of interactions of phenol with oxygenated functional groups (OH, COO and COOH) in nanopores of activated carbon (AC) is a contentious issue among researchers. This mechanism is of particular interest because a better understanding of the role of such groups in nanopores would essentially translate to advances in AC production and use, especially in regard to the treatment of organic-based wastewaters. We therefore attempt to shed more light on the subject by employing density functional theory (DFT) calculations in which fullerene-like models integrating convex or concave structure, which simulate the eclectic porous structures on AC surface, are adopted. TEM analysis, EDS mapping and Boehm titration are also conducted on actual phenol-adsorbed AC. Our results suggest the widely-reported phenomenon of decreased phenol uptake on AC due to increased concentration of oxygenated functional groups is possibly attributed to the increased presence of the latter on the convex side of the curved carbon sheets. Such a system effectively inhibits phenol from getting direct contact with the carbon sheet, thus constraining any available π-π interaction, while the effect of groups acting on the concave part of the curved sheet does not impart the same detriment.
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We perform a comprehensive mapping of GeSi nanowire (NW) electronic band characteristics versus the alloy composition and the diameter (up to 3 nm) using hybrid density functional theory (DFT) calculations, the cluster expansion method and Monte Carlo simulations. We reveal that stable alloy GeSi NW configurations across compositions tend to exhibit asymmetric core-shell structures, which enhance spatial separation of the band edges, making them more effective for electron-hole charge separation as compared to conventional symmetric core-shell structures. More importantly, from the composition-size map of the NW band edges, we show that GeSi NWs with diameters below 3 nm are thermodynamically capable of photocatalysing water-splitting reactions (alkaline conditions) and CO2 reduction. In particular, NWs with diameters of 2 and 3 nm possess desirable properties for efficient photo-conversion; their bandgaps (1.4 to 2.0 eV) match well with the solar spectrum.
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Despite decades of concerted experimental studies dedicated to providing fundamental insights into the adsorption of aurocyanide ion, Au(CN)2(-), on activated carbon (AC) surface, such a mechanism is still poorly understood and remains a contentious issue. This adsorption process is an essential unit operation for extracting gold from ores using carbon-in-pulp (CIP) technology. We hereby attempt to shed more light on the subject by employing a range of transmission electron microscopy (TEM) associated techniques. Gold-based clusters on the AC surface are observed by Z-contrast scanning TEM imaging and energy-filtered TEM element mapping and are supported by X-ray microanalysis. Density functional theory (DFT) calculations are applied to investigate this adsorption process for the first time. Fullerene-like models incorporating convex, concave, or planar structure which mimic the eclectic porous structures on the AC surface are adopted. Pentagonal, hexagonal, and heptagonal arrangements of carbon rings are duly considered in the DFT study. By determining the favored adsorption sites in water environment, a general adsorption trend of Au(CN)2(-) adsorbed on AC surface is revealed whereby concave > convex ≈ planar. The results suggest a tendency for Au(CN)2(-) ion to adsorb on the carbon sheet defects or edges rather than on the basal plane. In addition, we show that the adsorption energy of Au(CN)2(-) is approximately 5 times higher than that of OH(-) in the alkaline environment (in negative ion form), compared to only about 2 times in acidic environment (in protonated form), indicating the Au extraction process is much favored in basic condition. The overall simulation results resolve certain ambiguities about the adsorption process for earlier studies. Our findings afford crucial information which could assist in enhancing our fundamental understanding of the CIP adsorption process.
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By means of density functional theory, we systematically investigate the insertion and diffusion of Na and Li in layered Si materials (polysilane and H-passivated silicene), in comparison with bulk Si. It is found that Na binding and mobility can be significantly facilitated in layered Si structures. In contrast to the Si bulk, where Na insertion is energetically unfavorable, Na storage can be achieved in polysilane and silicene. The energy barrier for Na diffusion is reduced from 1.06 eV in the Si bulk to 0.41 eV in polysilane. The improvements in binding energetics and in the activation energy for Na diffusion are attributed to the large surface area and available free volume for the large Na cation. Based on these results, we suggest that polysilane may be a promising anode material for Na-ion and Li-ion batteries with high charge-discharge rates.
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By means of density functional theory calculations, the cluster expansion method, and Monte Carlo simulations, we identify the stable spatial configurations (ground states) for [100] CSi, GeSi, and SnSi alloy nanowires (NWs) across compositions. In particular, we find that stable configurations of GeSiNWs and SnSiNWs exhibit core-shell segregation tendencies, while those of CSiNWs favor ordering. Moreover, we show compositional ranges where the band gaps are expected to vary linearly with composition, allowing predictable band gap fine-tuning. We also predict composition ranges where the spatial separation of near-band gap states are imminent, making it possible for electron-hole charge separation. By addressing both the issues of stability and the compositional trend of electronic band structure, our work should prove useful for designing alloy NWs of smaller dimensions.
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A stable anode-free lithium metal battery (AFLMB) is accomplished by the adoption of a facile fabricated amorphous antimony (Sb)-coated separator (SbSC). The large specific surface area of the separator elevates lithium (Li)-Sb alloy kinetic, improving Li wetting ability on pristine copper current collector (Cu). When tested with LiNi0.8 Mn0.1 Co0.1 O2 (NMC811) as cathode, the full cell with SbSC demonstrates low nucleation overpotential with compact, dendrite-free and homogeneous Li plating, and exhibits a notable lithium inventory retention rate (LIRR) of 99.8 % with capacity retention of 93.6 % after 60 cycles at 0.5â C-rate. Conversely, full cells containing pristine separator/Cu (i. e., SC) and pristine separator/Sb-coated current collector (i. e., SSbC) display poor cycling performances with low LIRRs. Density functional theory corroborates the nucleation behaviours observed during in-situ half-cell Li deposition. Functionalizing polymeric separator by metallic coating in AFLMB is a novel approach in improving the cycle life of an AFLMB by promoting homogeneous Li plating behavior. This innovative approach exemplifies a promising applicability for uniform Li-plating behavior to achieve a longer cycle life in AFLMB.
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We report a first-principles investigation of Li adsorption and diffusion in single-walled Si nanotubes (SWSiNTs) of interest to Li-ion battery anodes. We calculate Li insertion characteristics in SWSiNTs and compare them with the respective ones in carbon nanotubes (CNTs) and other silicon nanostructures. From our calculations, SWSiNTs show higher reactivity toward the adsorption of Li adatoms than CNTs and Si nanoclusters. Considering the importance of Li kinetics, we demonstrate that the interior of SWSiNTs may serve as a fast Li diffusion channel. The important advantage of SWSiNTs over their carbon analogues is a sevenfold reduction in the energy barrier for the penetration of the Li atoms into the nanotube interior through the sidewalls. This prepossesses easier Li diffusion inside the tube and subsequent utilization of the interior sites, which enhances Li storage capacity of the system. The improvements in both Li uptake and Li mobility over their analogues support the great potential of SWSiNTs as Li-ion battery anodes.
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We report a systematic study on the surface passivation and strain effects on the electronic properties of hydrogenated germanium nanowires (H-GeNWs) with different growth orientations and diameters using density functional theory calculations. We show that increasing the coverage percentage of halogen passivations--fluorine (F) and chlorine (Cl) in particular--reduces the band gap of the GeNWs drastically but not linearly, depending on the chemical environment of the passivation sites. Moreover, we find that in general, applying strain--either compression or tensile--can only induce a decreased band gap in GeNWs but exception is found in <110> GeNWs: an increased band gap can be induced which is determined to be related to their surface structures. The current work reveals that electronic response upon structural changes caused by external factors is more sensitive in <110> GeNWs than in <100> GeNWs, suggesting that GeNWs with selected growth orientation can be applied in specialized applications that require different degrees of sensitivity or robustness.
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We evaluate the boron (B) and phosphorus (P) core-surface codoped radial p-n junction characteristics in silicon nanowires (SiNWs) using density functional theory calculations. We find that the formation of radial p-n junction is energetically favorable. The stability depends on the diameter of SiNWs and the dopant concentration. Generally, a higher concentration of B-P pair dopants results in a more stable nanowire. More importantly, we predict that the radial p-n junction can evolve into a Schottky-like junction in relatively highly doped SiNWs when the diameter increases, attributing to the change of the core p-doping characteristic, that is, the core p-junction becomes metallic, while the n-junction near the surface remains semiconducting. The interfacial contact between the junctions is found to be the key for such change. Our calculated results support an experimental observation in SiNW solar cells.
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Atomically-thin monolayer WS2 is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS2 also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS2, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS2 by applying the in situ self-limited thinning of multilayer WS2 formed by sulfurization of WOx films. Through a pulsed supply of sulfur precursor vapor under a continuous H2 flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS2 to the monolayer limit without damaging the remaining bottom WS2 monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS2 top layers as opposed to a stable bottom monolayer WS2 on sapphire above a vacuum sublimation temperature of WS2. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2nd layer WS2 domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS2 on the wafer-scale.
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Electrocatalytic nitrate (NO3-)/nitrite (NO2-) reduction reaction (eNOx-RR) to ammonia under ambient conditions presents a green and promising alternative to the Haber-Bosch process. Practically available NOx- sources, such as wastewater or plasma-enabled nitrogen oxidation reaction (p-NOR), typically have low NOx- concentrations. Hence, electrocatalyst engineering is important for practical eNOx-RR to obtain both high NH3 Faradaic efficiency (FE) and high yield rate. Herein, we designed balanced NOx- and proton adsorption by properly introducing Cu sites into the Fe/Fe2O3 electrocatalyst. During the eNOx-RR process, the H adsorption is balanced, and the good NOx- affinity is maintained. As a consequence, the designed Cu-Fe/Fe2O3 catalyst exhibits promising performance, with an average NH3 FE of â¼98% and an average NH3 yield rate of 15.66 mg h-1 cm-2 under the low NO3- concentration (32.3 mM) of typical industrial wastewater at an applied potential of -0.6 V versus reversible hydrogen electrode (RHE). With low-power direct current p-NOR generated NOx- (23.5 mM) in KOH electrolyte, the Cu-Fe/Fe2O3 catalyst achieves an FE of â¼99% and a yield rate of 15.1 mg h-1 cm-2 for NH3 production at -0.5 V (vs RHE). The performance achieved in this study exceeds industrialization targets for NH3 production by exploiting two available low-concentration NOx- sources.
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Even though metal-organic frameworks (MOFs) derived from antiferromagnetic dimeric-Cu(II) building units and nonmagnetic molecular linkers are known to exhibit unexpected ferromagnetic behavior, a comprehensive understanding of the underlying mechanism remains elusive. Using a combined theoretical and experimental approach, here we reveal the origin of the long-range ferromagnetic coupling in a series of MOFs, constructed from antiferromagnetic dimeric-Cu(II) building blocks. Our studies show that the strong localization of copper vacancy states favors spontaneous spin polarization and formation of local moment. These copper vacancy-induced moments are coupled via the itinerant electrons in the conjugated aromatic linkers to establish a long-range ferromagnetic ordering. The proposed mechanism is supported by direct experimental evidence of copper vacancies and the magnetic hysteresis (M-H) loops.