Two-Dimensional Tin Disulfide Nanosheets for Enhanced Sodium Storage

In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

Constructing Hollow Microcubes SnS2 as Negative Electrode for Sodium-ion and Potassium-ion Batteries

Sodium/potassium-ion batteries (NIBs and KIBs) are considered the most promising candidates for lithium-ion batteries in energy storage fields. Tin sulfide (SnS2) is regarded as an attractive negative candidate for NIBs and KIBs thanks to its superior power density, high-rate performance and natural richness. Nevertheless, the slow dynamics, the enormous volume change and the decomposition of polysulfide intermediates limit its practical application. Herein, microcubes SnS2 were prepared through sacrificial MnCO3 template-assisted and a facile solvothermal reaction strategy and their performance was investigated in Na and K-based cells. The unique hollow cubic structure and well-confined SnS2 nanosheets play an important role in Na+/K+ rapid kinetic and alleviating volume change. The effect of the carbon additives (Super P/C65) on the electrochemical properties were investigated thoroughly. The in operando and ex-situ characterization provide a piece of direct evidence to clarify the storage mechanism of such conversion-alloying type negative electrode materials.

Flexible ultrathin Nitrogen-Doped carbon mediates the surface charge redistribution of a hierarchical tin disulfide nanoflake electrode for efficient capacitive deionization

Constructing pseudocapacitive electrodes with high specific capacities is indispensable for increasing the large-scale application of capacitive deionization (CDI). However, the insufficient CDI rate and cycling performance of pseudocapacitive-based electrodes have led to a decline in their use due to the corresponding volumetric expansion and contraction that occurs during long-term CDI processes. Herein, hierarchical porous SnS2 nanoflakes are encapsulated inside an N-doped carbon (NC) matrix to achieve efficient CDI. Benefiting from the synergistic properties of the pseudocapacitive SnS2 nanoflakes and few-layered N-doped carbon, the heterogeneous interface simultaneously provides more available vigorous sites and demonstrates rapid charge-transfer kinetics, resulting in a superior desalination capability (49.86 mg g-1 at 1.2 V), rapid desalination rate (1.66 mg g-1 min-1) and better cyclic stability. Computational research reveals a work function-induced surface charge redistribution of the SnS2@NC heterojunction, which can lead to an auspicious surface electronic structure that reduces the adsorption energy to improve the diffusion kinetics toward sodium adsorption. This work contributes to providing a thoughtful understanding of the interface engineering between transition metal dichalcogenides and NC to construct high-performance CDI electrode materials for further industrialization.

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

Solid-state electrolytes (SEs) have attracted overwhelming attention as a promising alternative to traditional organic liquid electrolytes (OLEs) for high-energy-density sodium-metal batteries (SMBs), owing to their intrinsic incombustibility, wider electrochemical stability window (ESW), and better thermal stability. Among various kinds of SEs, inorganic solid-state electrolytes (ISEs) stand out because of their high ionic conductivity, excellent oxidative stability, and good mechanical strength, rendering potential utilization in safe and dendrite-free SMBs at room temperature. However, the development of Na-ion ISEs still remains challenging, that a perfect solution has yet to be achieved. Herein, we provide a comprehensive and in-depth inspection of the state-of-the-art ISEs, aiming at revealing the underlying Na⁺ conduction mechanisms at different length scales, and interpreting their compatibility with the Na metal anode from multiple aspects. A thorough material screening will include nearly all ISEs developed to date, i.e., oxides, chalcogenides, halides, antiperovskites, and borohydrides, followed by an overview of the modification strategies for enhancing their ionic conductivity and interfacial compatibility with Na metal, including synthesis, doping and interfacial engineering. By discussing the remaining challenges in ISE research, we propose rational and strategic perspectives that can serve as guidelines for future development of desirable ISEs and practical implementation of high-performance SMBs.

Investigation of SnS2 -rGO Sandwich Structures as Negative Electrode for Sodium-Ion and Potassium-Ion Batteries

Sodium-ion and potassium-ion batteries (NIBs and KIBs) are considered promising alternatives to replace lithium-ion batteries (LIBs) in energy storage applications due to the natural abundance and low cost of Na and K. Nevertheless, a critical challenge is that the large size of Na⁺ /K⁺ leads to a huge volume change of the hosting material during electrochemical cycling, resulting in rapid capacity decay. Among negative candidates for alkali-metal-ion batteries, SnS₂ is attractive due to the competitively high specific capacity, low redox potential and high abundance. Porous few-layer SnS₂ nanosheets are in situ grown on reduced graphene oxide, forming a SnS₂ -rGO sandwich structure via strong C-O-Sn bonds. This nano-scaled sandwich structure not only shortens Na⁺ /K⁺ and electron transport pathways but also accommodates volume expansion, thereby enabling high and stable electrochemical cycling performance of SnS₂ -rGO. This work explores the influence of different conductive carbons (Super P and C65) on the SnS₂ -rGO electrode. In addition, the effects of the electrolyte additive fluoroethylene carbonate (FEC) on the electrochemical performance in NIBs and KIBs is evaluated. This work provides guidelines for optimized electrode structure design, electrolyte additives and carbon additives for the realization of better NIBs and KIBs.

Synthesis and Applications of Dimensional SnS2 and SnS2/Carbon Nanomaterials

Dimensional nanomaterials can offer enhanced application properties benefiting from their sizes and morphological orientations. Tin disulfide (SnS₂) and carbon are typical sources of dimensional nanomaterials. SnS₂ is a semiconductor with visible light adsorption properties and has shown high energy density and long cycle life in energy storage processes. The integration of SnS₂ and carbon materials has shown enhanced visible light absorption and electron transmission efficiency. This helps to alleviate the volume expansion of SnS₂ which is a limitation during energy storage processes and provides a favorable bandgap in photocatalytic degradation. Several innovative approaches have been geared toward controlling the size, shape, and hybridization of SnS₂/Carbon composite nanostructures. However, dimensional nanomaterials of SnS₂ and SnS₂/Carbon have rarely been discussed. This review summarizes the synthesis methods of zero-, one-, two-, and three-dimensional SnS₂ and SnS₂/Carbon composite nanomaterials through wet and solid-state synthesis strategies. Moreover, the unique properties that promote their advances in photocatalysis and energy conversion and storage are discussed. Finally, some remarks and perspectives on the challenges and opportunities for exploring advanced SnS₂/Carbon nanomaterials are presented.

Regulating the charge carrier transport rate via bridging ternary heterojunctions to enable CdS nanorods' solar-driven hydrogen evolution

Solar-driven hydrogen generation using single-semiconductor photocatalysts for hydrogen evolution seems to be challenging due to their poor solar to fuel conversion efficiency because of their fast charge carrier recombination. The ternary heterostructure was prepared by an advanced approach to suppress the recombination of photogenerated charge carriers and has contributed a new platform for designing highly efficient photocatalytic systems. Herein, we fabricated a ternary heterojunction with ultrathin WS₂-SnS₂ nanosheets and CdS nanorods, and the photocatalytic activity was studied. The optimized CdS/SnS₂-WS₂ (6 wt%) nanostructures were found to be highly stable and exhibited the highest hydrogen evolution rate of 232.45 mmol g^-1 h^-1, which was almost 93-fold higher than that of the pristine CdS nanorods. Also, Density Functional Theory (DFT) calculations confirmed that the favorable band alignment for charge transport and superior catalytic activity of the newly fabricated ternary nanostructures make them a potential candidate for solar-driven hydrogen production.

Balanced Crystallinity and Nanostructure for SnS2 Nanosheets through Optimized Calcination Temperature toward Enhanced Pseudocapacitive Na+ Storage

Sodium ion batteries (SIBs) are expected to take the place of lithium ion batteries (LIBs) as next-generation electrochemical energy storage devices due to the cost advantages they offer. However, due to the larger ion radius, the reaction kinetics of Na⁺ in anode materials is sluggish. SnS₂ is an attractive anode material for SIBs due to its large interlayer spacing and alloying reactions with high capacity. Calcination is usually employed to improve the crystallinity of SnS₂, which could affect the Na⁺ reaction kinetics, especially the pseudocapacitive storage. However, excessively high temperature could damage the well-designed nanostructure of SnS₂. In this work, we uniformly grow SnS₂ nanosheets on a Zn-, N-, and S-doped carbon skeleton (SnS₂@ZnNS). To explore the optimal calcination temperature, SnS₂@ZnNS is calcined at three typical temperatures (300, 350, and 400 °C), and the electrochemical performance and Na⁺ storage kinetics are investigated specifically. The results show that the sample calcined at 350 °C exhibited the best rate capacity and cycle performance, and the reaction kinetics analysis shows that the same sample exhibited a stronger pseudocapacitive response than the other two samples. This improved Na⁺ storage capability can be attributed to the enhanced crystallinity and the intact nanostructure.

Tin-Based Anode Materials for Stable Sodium Storage: Progress and Perspective

Because of concerns regarding shortages of lithium resources and the urgent need to develop low-cost and high-efficiency energy-storage systems, research and applications of sodium-ion batteries (SIBs) have re-emerged in recent years. Herein, recent advances in high-capacity Sn-based anode materials for stable SIBs are highlighted, including tin (Sn) alloys, Sn oxides, Sn sulfides, Sn selenides, Sn phosphides, and their composites. The reaction mechanisms between Sn-based materials and sodium are clarified. Multiphase and multiscale structural optimizations of Sn-based materials to achieve good sodium-storage performance are emphasized. Full-cell designs using Sn-based materials as anodes and further development of Sn-based materials are discussed from a commercialization perspective. Insights into the preparation of future high-performance Sn-based anode materials and the construction of sodium-ion full batteries with a high energy density and long service life are provided.

A novel interlayer-expanded tin disulfide/reduced graphene oxide nanocomposite as anode material for high-performance sodium-ion batteries

As a kind of negative electrode material for sodium-ion batteries (SIBs), tin-based active compounds have attracted numerous research efforts in recent years due to relatively high theoretical capacity. However, sluggish reaction kinetics for large-radius sodium ions hinders the practical application of layered tin-based anodes such as tin disulfide (SnS₂) in SIBs. In this study, polyethylene glycol (PEG) is introduced as an intercalant and reduced graphene oxide (rGO) is utilized as the substrate to prepare a novel PEG-SnS₂/rGO composite with expanded layer spacing (0.921 nm) through a facile hydrothermal process. SnS₂ flakes in a size range of 50-100 nm are uniformly grown on the graphene sheet, the CS covalent bonding demonstrates a tight connection between the active SnS₂ particles and the graphene skeleton, which is conductive to convenient charge transfer during the electrochemical process. Owing to the significantly improved sodium ions transport kinetics and fast electronic conductive network, the PEG-SnS₂/rGO composite presents a high capacitance contribution of 90.69% at a scan rate of 0.6 mV s^-1. It delivers a high reversible capacity of 960.6 mAh g^-1 at 0.1 A g^-1, good cycling performance with 770 mAh g^-1 remained after 100 charge/discharge cycles, and excellent rate capability with an ultrahigh capacity of 720 mAh g^-1 at 5 A g^-1. This work provides new insights into the design of a kinetically favorable anode material for SIBs.

Atmospheric pressure chemical vapor deposition growth of vertically aligned SnS2 and SnSe2 nanosheets

Laminated metal dichalcogenides are candidates for different potential applications ranging from catalysis to nanoelectronics. However, efforts are still needed to optimize synthesis methods aiming to control the number of layers, morphology, and crystallinity, parameters that govern the properties of the synthesized materials. Another important parameter is the thickness and the length of the samples with the possibility of large-scale growth of target homogeneous materials. Here, we report a chemical vapor deposition method at atmospheric pressure to produce vertically aligned tin dichalcogenide based-materials. Tin disulfide (SnS2) and tin diselenide (SnSe2) vertically aligned nanosheets have been synthesized and characterized by different methods showing their crystallinity and purity. Homogenous crystalline 2H-phase SnS2 nanosheets with high purity were synthesized with vertical orientation on substrates; sulfur vacancies were observed at the edges of the sheets. Similarly, in the crystalline 2H phase SnSe2 nanosheets selenium vacancies were observed at the edges. Moreover, these nanosheets are larger than the SnS2 nanosheets, show lower nanosheet homogeneity on substrates and contamination with selenium atoms from the synthesis was observed. The synthesized nanomaterials are interesting in various applications where the edge accessibility and/or directionality of the nanosheets play a major role as for example in gas sensing or field emission.

Nano Sn2 S3 Embedded in Nitrogenous-Carbon Compounds for Long-Life and High-Rate Cycling Sodium-Ion Batteries

Metallic tin (Sn) compounds are viewed as promising candidates for sodium-ion batteries (SIB) anode materials yet suffer from large volume expansion and limited electrode kinetics. Manufacturing rational structure is a crucial factor to achieve high-efficiency sodium storage for SIBs. In this study, nano Sn₂ S₃ embedded in nitrogenous-carbon compounds (nano-Sn₂ S₃ /C) was designed for SIB anode materials via a facile three-step strategy: precipitation, heat treatment and vulcanization with no templating agent. Density functional theory calculations suggested that Sn₂ S₃ displayed a low Na⁺ diffusion energy barrier and the Sn-S bonds could be rebuilt during the sodiation/de-sodiation process. Notably, electrochemical measurements coupled with ex-situ X-ray diffraction and ex-situ transmission electron microscopy were proposed to reveal the underlying Na⁺ storage mechanisms. Sn₂ S₃ acted as a high-capacity composition, while the porous nitrogenous-carbon matrix served as a rigid-conductive frame to accommodate the volume expansion and prevented the aggregation of nano Sn₂ S₃ . The rationally generated architectures benefited greatly in rate capacity and structural stability. As expected, the as-prepared nano Sn₂ S₃ /C exhibited remarkable rate capabilities with a specific capacity of 603 and 160 mAh g^-1 under typical conditions at 0.2 and 4 A g^-1 , respectively. This work may trigger new enthusiasm for engineering high-performance SIB anode materials.

Sheet-to-layer structure of SnSe2/MXene composite materials for advanced sodium ion battery anodes

The synergistic SnSe₂/MXene composite was synthesized through electrostatic assembly and exhibits a good electrochemical performance for use in a sodium-ion battery.

Unraveling the Beneficial Microstructure Evolution in Pyrite for Boosted Lithium Storage Performance

Pyrite FeS₂ as a high-capacity electrode material for lithium-ion batteries (LIBs) is hindered by its unstable cycling performance owing to the large volume change and irreversible phase segregation from coarsening of Fe. Here, the beneficial microstructure evolution in MoS₂ -modified FeS₂ is unraveled during the cycling process; the microstructure evolution is responsible for its significantly boosted lithium storage performance, making it suitable for use as an anode for LIBs. Specifically, the FeS₂ /MoS₂ displays a long cycle life with a capacity retention of 116 % after 600 cycles at 0.5 A g^-1 , which is the best among the reported FeS₂ -based materials so far. A series of electrochemical tests and structural characterizations substantially revealed that the introduced MoS₂ in FeS₂ experiences an irreversible electrochemical reaction and thus the in situ formed metallic Mo could act as the conductive buffer layer to accelerate the dynamics of Li⁺ diffusion and electron transport. More importantly, it can guarantee the highly reversible conversion in lithiated FeS₂ by preventing Fe coarsening. This work provides a fundamental understanding and an effective strategy towards the microstructure evolution for boosting lithium storage performances for other metal sulfide-based materials.

Boosting the lithium-ion and sodium-ion storage performances of pyrite by regulating the energy barrier of ion transport

Pyrite (FeS2) is a functional material of great importance for lithium/sodium ion batteries (LIBs/SIBs), but its sluggish dynamics greatly hinder its high performance. Here, we demonstrate an effective strategy of regulating the energy barrier of ion transport to significantly enhance the sluggish dynamics of FeS2 by Co doping. Compared to pristine FeS2, a series of Co-doped FeS2 shows enhanced alkali metal ion storage performance and most typically, the optimized Fe0.7Co0.3S2 sample displays high reversible capacities, of 1170 mA h g-1 for LIBs and 650 mA h g-1 for SIBs at a current density of 0.1 A g-1 as well as super long-life cycling stability for SIBs (1200 cycles at 5 A g-1). The evidently enhanced performances of Fe0.7Co0.3S2 for LIBs/SIBs can be attributed to its significantly decreased activation energy of ion transport, thus leading to greatly accelerated ion transport dynamics. Furthermore, galvanostatic intermittent titration technique (GITT) experiments also support this important regulation effect of Co doping on the ion transport dynamics of FeS2. The excellent ion transport dynamics induce a strong pseudo-capacitance behavior in both SIBs and LIBs, and their pseudo-capacitance contributions are more than 90% at 1.0 mV s-1. This work provides a new perspective to improve the alkali metal ion storage performance by optimizing the ion transport dynamics.

Uniform Li Deposition Sites Provided by Atomic Layer Deposition for the Dendrite-free Lithium Metal Anode

The nonuniform nucleation of lithium (Li) leads to dendritic behavior and formation of dead Li, which seriously hinders the practical application of Li metal batteries. Here, atomic layer deposition (ALD) is used to deposit uniform and conformal ZnO coating (at a low content of 5.96%) on carbon fibers to form a free-standing framework Li host material without uncontrollable dendrites. Compared with the liquid deposition process, the ALD method can achieve homogeneous and conformal ZnO coating and excellent lithiophilicity of the carbon fiber, guiding molten Li infusion into the carbon fiber skeleton to obtain the Li/C composite electrode with a flat surface, thereby minimizing the effective current density. More importantly, the converted LiZn alloy will serve as uniform and numerous nucleation sites for Li and guide synchronous growth of the Li metal along carbon fibers, displaying a dendrite-free morphology after large-current and long-term deposition/dissolution cycling. Therefore, the ALD ZnO-modified carbon fiber/Li exhibits significantly better cycle and rate performances than the liquid deposition ZnO-modified carbon fiber/Li composite anode. The electrodes display an ultralong lifespan up to 400 cycles at 3.0 mA/cm², as well as a high rate performance (with a deposition overpotential of 338 mV at 25.0 mA/cm²) at a high Li deposition areal capacity of 5.0 mA h cm^-2.

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.

Emerging Layered Metallic Vanadium Disulfide for Rechargeable Metal-Ion Batteries: Progress and Opportunities

Rechargeable metal-ion batteries (RMIBs), as one of the most viable technologies for electric vehicles (EVs) and large-scale energy storage (EES), have received extensive research attention for a long time. Electrode materials play a decisive role on capacity, energy, and power density, which directly affect the practical applications of RMIBs in EVs and EES. As an electrode material, layered metallic vanadium disulfide (VS₂ ) has theoretically and experimentally produced inspiring results because of its synthetic characteristics of continuously adjustable V valence, large interlayer spacing, weak interlayer interactions, and high surface activity. Herein, the synthetic strategies, theoretical metal-ion storage sites, diffusion kinetics, and experimental electrochemical reaction mechanisms of VS₂ for RMIBs are systematically introduced. Emphatically, the critical issues that affect the metal-ion storage properties of the VS₂ electrode and three major enhancement strategies, namely, optimizing the electrolyte and cutoff voltage, constructing a space-confined structure, and controlling the crystal structure are summarized, with the aim of promoting the development of transition-metal dichalcogenides. Finally, the challenges and opportunities for the future development of VS₂ in the energy-storage field are presented. It is hoped that this review can attract attention from researchers for investigations into emerging layered metallic VS₂ and provide insights toward the design of an excellent VS₂ electrode material for next-generation, high-performance RMIBs.

SnSe2 Nanoparticles Chemically Embedded in a Carbon Shell for High-Rate Sodium-Ion Storage

The development of advanced anode materials is crucial to enhance the performance of sodium-ion batteries (SIBs). In this study, SnSe₂ nanoparticles chemically embedded in a carbon shell (SnSe₂@C) were fabricated from Sn-organic frameworks and evaluated as an anode material for SIBs. The structural characterization demonstrated that there existed C-Sn chemical bonds between the SnSe₂ nanoparticles and carbon shell, which could strongly anchor SnSe₂ nanoparticles to the carbon shell. Such a structure can not only facilitate charge transfer but also ensure the structural stability of the SnSe₂@C electrode. In addition, the carbon shell also helped in the dispersion of SnSe₂ nanoparticles, thus offering more redox-active sites for Na⁺ storage. The as-prepared SnSe₂@C nanocomposite could deliver good cycling stability and a superior rate capability of 324 mA h g^-1 at 2 A g^-1 for SIBs.

Cobalt-doping SnS2 nanosheets towards high-performance anodes for sodium ion batteries

Layered SnS₂ is considered as a promising anode candidate for sodium-ion batteries yet suffers from low initial coulombic efficiency, limited specific capacity and rate capability. Herein, we report a cobalt metal cation doping strategy to enhance the electrochemical performance of a SnS₂ nanosheet array anode through a facile hydrothermal method. Benefitting from this special structure and heteroatom-doping effect, this anode material displays a high initial coulombic efficiency of 57.4%, a superior discharge specific capacity as high as 1288 mA h g^-1 at 0.2 A g^-1 after 100 cycles and outstanding long-term cycling stability with a reversible capacity of 800.4 mA h g^-1 even at 2 A g^-1. These excellent performances could be ascribed to the Co-doping effect that can increase the interlayer spacing, produce rich defects, regulate the electronic environment and improve conductivity. Besides, a carbon cloth substrate can maintain the integrity of the electrode material framework and buffer its volume variation, thus boosting intrinsic dynamic properties and enhancing sodium storage performance.

Hybridization of Defective Tin Disulfide Nanosheets and Silver Nanowires Enables Efficient Electrochemical Reduction of CO2 into Formate and Syngas

Integrating the defect engineering and conductivity promotion represents a promising way to improve the performance of CO₂ electrochemical reduction. Herein, the hybridized composite of defective SnS₂ nanosheets and Ag nanowires is developed as an efficient catalyst for the production of formate and syngas toward CO₂ electrochemical reduction. The Schottky barrier in Ag-SnS₂ hybrid nanosheets is negligible due to the similar Fermi level of SnS₂ nanosheets and Ag nanowires. Accordingly, the free electrons of Ag nanowires participate in the electronic transport of SnS₂ nanosheets, and thus give rise to a 5.5-fold larger carrier density of Ag-SnS₂ hybrid nanosheets than that of SnS₂ nanosheets. In CO₂ electrochemical reduction, the Ag-SnS₂ hybrid nanosheets display 38.8 mA cm^-2 of geometrical current density at -1.0 V vs reversible hydrogen electrode, including 23.3 mA cm^-2 for formate and 15.5 mA cm^-2 for syngas with the CO/H₂ ratio of 1:1. A mechanistic study reveals that the abundant defect sites and carrier density not only promote the conductivity of the electrocatalyst, but also increase the binding strength for CO₂ , which account for the efficient CO₂ reduction.

Surface Anionization of Self-Assembled Iron Sulfide Hierarchitectures to Enhance Capacitive Storage for Alkaline-Metal-Ion Batteries

Due to the ever-growing demand for cost-effective batteries toward greener and sustainable applications, continuous effort has been devoted to tailoring the interfacial kinetics of electrode materials. Herein, surface anionization has been introduced for the hierarchical assembly of iron sulfides on three-dimensional (3D) graphene foam (denoted FeS₂@3DGF and FeS@3DGF). The surface-anchored sulfate species provide ideal electroactive sites, which is correlated with enhanced capacitive contribution and boosted energy storage. Consequently, remarkable rate capability and stable cyclability can be achieved in alkaline-metal-ion batteries. Specifically, FeS@3DGF displays superb cycling stability when evaluated as anodes for Li-ion batteries (a steady capacity of 1109 mAh g^-1 after 200 cycles at 200 mA g^-1). Moreover, superior rate capability can be achieved for Na-ion batteries (203 mAh g^-1 at 10 000 mA g^-1). These findings provide new insights into reinforcing interface kinetics during electrochemical processes and hold great promise for versatile applications in the future.

Metallic-State SnS2 Nanosheets with Expanded Lattice Spacing for High-Performance Sodium-Ion Batteries

Metallic-state 2D SnS₂ nanosheets with expanded lattice spacing and a defect-rich structure were synthesized by the intercalation of Ni into the van der Waals gap of SnS₂ . The expanded lattice spacing efficiently enhanced the electrochemical performance of the SnS₂ for sodium-ion batteries owing to the change electron state density and energy band structure. In operando synchrotron XRD and theoretical calculations were used to gain insight into the influence of foreign metal-ion doping and its location. The optimized architecture obtained by in situ uniform growth of nanosheets on carbon fibers significantly enhanced the electrochemical performance. The inherent advantages of this architecture are shorter paths for ion insertion and extraction, larger contact area for more sodium diffusion pathways, and superior electrolyte penetration. Benefiting from the Ni intercalated SnS₂ bilayer, the internal adjustment of the electronic state and the enlarged interlayer spacing significantly enhanced the electron transport kinetics, which can be explained by the metallic-state properties. The integrated electrode exhibited an initial high reversible capacity of 795 mAh g^-1 at 0.1 A g^-1 , with a stable capacity retention of 666 mAh g^-1 after 100 cycles. Good rate capability was also exhibited with specific capacities of 691, 564, 437 mAh g^-1 at current densities of 200, 500, and 1000 mA g^-1 , respectively.

Sandwich-like SnS2/Graphene/SnS2 with Expanded Interlayer Distance as High-Rate Lithium/Sodium-Ion Battery Anode Materials

SnS₂ materials have attracted broad attention in the field of electrochemical energy storage due to their layered structure with high specific capacity. However, the easy restacking property during charge/discharge cycling leads to electrode structure instability and a severe capacity decrease. In this paper, we report a simple one-step hydrothermal synthesis of SnS₂/graphene/SnS₂ (SnS₂/rGO/SnS₂) composite with ultrathin SnS₂ nanosheets covalently decorated on both sides of reduced graphene oxide sheets via C-S bonds. Owing to the graphene sandwiched between two SnS₂ sheets, the composite presents an enlarged interlayer spacing of ∼8.03 Å for SnS₂, which could facilitate the insertion/extraction of Li⁺/Na⁺ ions with rapid transport kinetics as well as inhibit the restacking of SnS₂ nanosheets during the charge/discharge cycling. The density functional theory calculation reveals the most stable state of the moderate interlayer spacing for the sandwich-like composite. The diffusion coefficients of Li/Na ions from both molecular simulation and experimental observation also demonstrate that this state is the most suitable for fast ion transport. In addition, numerous ultratiny SnS₂ nanoparticles anchored on the graphene sheets can generate dominant pseudocapacitive contribution to the composite especially at large current density, guaranteeing its excellent high-rate performance with 844 and 765 mAh g^-1 for Li/Na-ion batteries even at 10 A g^-1. No distinct morphology changes occur after 200 cycles, and the SnS₂ nanoparticles still recover to a pristine phase without distinct agglomeration, demonstrating that this composite with high-rate capabilities and excellent cycle stability are promising candidates for lithium/sodium storage.

Reversible uptake and release of sodium ions in layered SnS2-reduced graphene oxide composites for neuromorphic devices

With the advent of brain-inspired computing for complex data processing, emerging nonvolatile memories have been widely studied to develop neuromorphic devices for pattern recognition and deep learning. However, the devices still suffer from limitations such as nonlinearity and large write noise because they adopt a stochastic switching approach. Here, we suggest a biomimetic three-terminal electrochemical artificial synapse that is operated by a conductance change in response to intercalation of sodium (Na+) ions into a layered SnS2-reduced graphene oxide (RGO) composite channel. SnS2-RGO can reversibly uptake and release Na+ ions, so the conductance of the channel in artificial synapse can be controlled effectively and thereby it can emulate essential synaptic functions including short-term plasticity, spatiotemporal signal processing, and transition from short-term to long-term plasticity. The artificial synapse also shows linear and symmetric potentiation/depression with low cycle-to-cycle variation; these responses could improve the write linearity and reduce the write noise of devices. This study demonstrates the feasibility of next-generation neuromorphic memory using ion-based electrochemical devices that can mimic biological synapses with the migration of Na+ ions.

Engineering Unique Ball-In-Ball Structured (Ni0.33Co0.67)9S8@C Nanospheres for Advanced Sodium Storage

Constructing hollow architectures based on metal sulfides is of great interest for high-performance electrode materials for sodium-ion batteries because of their intriguing properties and various applications. However, the relatively low volumetric density and high fragile structure are the obstacles blocking the development of hollow-structured electrode materials. In this work, ball-in-ball structured (Ni_0.33Co_0.67)₉S₈@C nanospheres have been synthesized by using NiCo-glycerate as the precursor via solvothermal reaction, which was followed by a carbon coating treatment. In this structural design, hollow cavities are generated between the inner and outer balls to effectively accommodate the volume changes of the metal sulfides in the processes of charging/discharging, whereas the uniform carbon coating can increase the electrical conductivity and maintain the structural stability during repeated cycling. The Rietveld refinement, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy analyses provide evidence for an enlarged lattice parameter, weaker Co-S and Ni-S bondings, and a synergistic effect in (Ni_0.33Co_0.67)₉S₈@C toward boosting the conversion reaction and reversible formation of sulfur in the fully charged state, with sulfur trapped within the composite to additionally account for the superior cycling stability of this material. Capacitive behavior has been verified to dominate the electrochemical reaction, enabling fast charge-transport kinetics. Impressively, the double structural protection combined with the free hollow space and complete carbon layer endows the (Ni_0.33Co_0.67)₉S₈@C nanospheres with good electrochemical performance, featuring high cyclability and good rate capability.

Rational Design of Layered SnS2 on Ultralight Graphene Fiber Fabrics as Binder-Free Anodes for Enhanced Practical Capacity of Sodium-Ion Batteries

Generally, the practical capacity of an electrode should include the weight of non-active components such as current collector, polymer binder, and conductive additives, which were as high as 70 wt% in current reported works, seriously limiting the practical capacity. This work pioneered the usage of ultralight reduced graphene fiber (rGF) fabrics as conductive scaffolds, aiming to reduce the weight of non-active components and enhance the practical capacity. Ultrathin SnS2 nanosheets/rGF hybrids were prepared and used as binder-free electrodes of sodium-ion batteries (SIBs). The interfused graphene fibers endow the electrode a porous, continuous, and conductive network. The in situ phase transformation from SnO2 to SnS2 could preserve the strong interfacial interactions between SnS2 and graphene. Benefitting from these, the designed binder-free electrode delivers a high specific capacity of 500 mAh g-1 after 500 cycles at a current rate of 0.5 A g-1 with almost 100% Coulombic efficiency. Furthermore, the weight percentage of SnS2 in the whole electrode could reach up to 67.2 wt%, much higher than that of common electrode configurations using Cu foil, Al foil, or carbon cloth, significantly highlighting the ultralight characters and advantages of the rGF fabrics for using as binder-free electrodes of SIBs.

Nonaqueous Sodium-Ion Full Cells: Status, Strategies, and Prospects

With ever-increasing efforts focused on basic research of sodium-ion batteries (SIBs) and growing energy demand, sodium-ion full cells (SIFCs), as unique bridging technology between sodium-ion half-cells (SIHCs) and commercial batteries, have attracted more and more interest and attention. To promote the development of SIFCs in a better way, it is essential to gain a systematic and profound insight into their key issues and research status. This Review mainly focuses on the interface issues, major challenges, and recent progresses in SIFCs based on diversified electrolytes (i.e., nonaqueous liquid electrolytes, quasi-solid-state electrolytes, and all-solid-state electrolytes) and summarizes the modification strategies to improve their electrochemical performance, including interface modification, cathode/anode matching, capacity ratio, electrolyte optimization, and sodium compensation. Outlooks and perspectives on the future research directions to build better SIFCs are also provided.

Enhanced activity of highly conformal and layered tin sulfide (SnSx) prepared by atomic layer deposition (ALD) on 3D metal scaffold towards high performance supercapacitor electrode

Layered Sn-based chalcogenides and heterostructures are widely used in batteries and photocatalysis, but its utilizations in a supercapacitor is limited by its structural instability and low conductivity. Here, SnSx thin films are directly and conformally deposited on a three-dimensional (3D) Ni-foam (NF) substrate by atomic layer deposition (ALD), using tetrakis(dimethylamino)tin [TDMASn, ((CH3)2N)4Sn] and H2S that serves as an electrode for supercapacitor without any additional treatment. Two kinds of ALD-SnSx films grown at 160 °C and 180 °C are investigated systematically by X-ray diffractometry, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM). All of the characterization results indicate that the films deposited at 160 °C and 180 °C predominantly consist of hexagonal structured-SnS2 and orthorhombic-SnS phases, respectively. Moreover, the high-resolution TEM analyses (HRTEM) reveals the (001) oriented polycrystalline hexagonal-SnS2 layered structure for the films grown at 160 °C. The double layer capacitance with the composite electrode of SnSx@NF grown at 160 °C is higher than that of SnSx@NF at 180 °C, while pseudocapacitive Faradaic reactions are evident for both SnSx@NF electrodes. The superior performance as an electrode is directly linked to the layered structure of SnS2. Further, the optimal thickness of ALD-SnSx thin film is found to be 60 nm for the composite electrode of SnSx@NF grown at 160 °C by controlling the number of ALD cycles. The optimized SnSx@NF electrode delivers an areal capacitance of 805.5 mF/cm2 at a current density of 0.5 mA/cm2 and excellent cyclic stability over 5000 charge/discharge cycles.

Li-Ions Transport Promoting and Highly Stable Solid-Electrolyte Interface on Si in Multilayer Si/C through Thickness Control

Lithium-ion batteries (LIBs) have been considered as promising electrochemical energy storage devices due to the high volumetric, gravimetric capacity and high power density. The charge/discharge rate and power output of LIBs largely depend on the transport property of lithium-ions (Li-ions). The Li-ions diffusion coefficient and diffusion length are the critical factors influencing the charge/discharge rate of LIBs. In this work, we present that silicon-carbon (Si-C) interfaces in an amorphous Si/C multilayer electrode promote the transport of Li-ions along the direction not only perpendicular to but also parallel to the Si-C interfaces after electrode cracking. The electrode, stacked with 5 nm amorphous carbon and 10 nm amorphous Si, has the most stable solid-electrolyte interface (SEI) formed at the cracks, even when the Si is in direct contact with the electrolyte. It exhibits highly stable cycle performance and a high retained specific capacity. Electron microscopy characterization shows that the structure contains uniform Si/C multilayer blocks of about 1 μm. A micro-size hierarchical multilayer-block design strategy with proper stacking thickness of amorphous Si and carbon is thus proposed for high-performance film LIB anodes. Furthermore, the results may be used as a reference for the design of high-performance core-shell LIB anodes.

Progressively Exposing Active Facets of 2D Nanosheets toward Enhanced Pseudocapacitive Response and High-Rate Sodium Storage

Sodium-ion batteries are gradually regarded as a prospective alternative to lithium-ion batteries due to the cost consideration. Here, three kinds of tin (IV) sulfide nanosheets are controllably designed with progressively exposed active facets, leading to beneficial influences on the Na⁺ storage kinetics, resulting in gradient improvements of pseudocapacitive response and rate performance. Interestingly, different forms of kinetics results are generated accompanying with the morphology and structure evolution of the three nanosheets. Finally, detailed density functional theory simulations are also applied to analyze the above experimental achievements, proving that different exposed facets of crystalline anodes possess dissimilar Na⁺ storage kinetics. The investigation experiences and conclusions shown in this work are meaningful to explore many other proper structure design routes toward the high-rate and stable metal-ions storage.

Fabrication of SnS2/Mn2SnS4/Carbon Heterostructures for Sodium-Ion Batteries with High Initial Coulombic Efficiency and Cycling Stability

SnS₂ has been extensive studied as an anode material for sodium storage owing to its high theoretical specific capacity, whereas the unsatisfied initial Coulombic efficiency (ICE) caused by the partial irreversible conversion reaction during the charge/discharge process is one of the critical issues that hamper its practical applications. Hence, heterostructured SnS₂/Mn₂SnS₄/carbon nanoboxes (SMS/C NBs) have been developed by a facial wet-chemical method and utilized as the anode material of sodium ion batteries. SMS/C NBs can deliver an initial capacity of 841.2 mAh g^-1 with high ICE of 90.8%, excellent rate capability (752.3, 604.7, 570.1, 546.9, 519.7, and 488.7 mAh g^-1 at the current rate of 0.1, 0.5, 1.0, 2.0, 5.0, and 10.0 A g^-1, respectively), and long cycling stability (522.5 mAh g^-1 at 5.0 A g^-1 after 500 cycles). The existence of SnS₂/Mn₂SnS₄ heterojunctions can effectively stabilize the reaction products Sn and Na₂S, greatly prevent the coarsening of nanosized Sn⁰, and enhance reversible conversion--alloying reaction, which play a key role in improving the ICE and extending the cycling performance. Moreover, the heterostructured SMS coupled with the interacting carbon network provides efficient channels for electrons and Na⁺ diffusion, resulting in an excellent rate performance.

Few-Layered Tin Sulfide Nanosheets Supported on Reduced Graphene Oxide as a High-Performance Anode for Potassium-Ion Batteries

Anodes involving conversion and alloying reaction mechanisms are attractive for potassium-ion batteries (PIBs) due to their high theoretical capacities. However, serious volume change and metal aggregation upon potassiation/depotassiation usually cause poor electrochemical performance. Herein, few-layered SnS₂ nanosheets supported on reduced graphene oxide (SnS₂ @rGO) are fabricated and investigated as anode material for PIBs, showing high specific capacity (448 mAh g^-1 at 0.05 A g^-1 ), high rate capability (247 mAh g^-1 at 1 A g^-1 ), and improved cycle performance (73% capacity retention after 300 cycles). In this composite electrode, SnS₂ nanosheets undergo sequential conversion (SnS₂ to Sn) and alloying (Sn to K₄ Sn₂₃ , KSn) reactions during potassiation/depotassiation, giving rise to a high specific capacity. Meanwhile, the hybrid ultrathin nanosheets enable fast K storage kinetics and excellent structure integrity because of fast electron/ionic transportation, surface capacitive-dominated charge storage mechanism, and effective accommodation for volume variation. This work demonstrates that K storage performance of alloy and conversion-based anodes can be remarkably promoted by subtle structure engineering.