In Situ, Atomic-Resolution Observation of Lithiation and Sodiation of WS2 Nanoflakes: Implications for Lithium-Ion and Sodium-Ion Batteries

Electrophoretic assisted fabrication of additive-free WS2 nanosheet anodes for high energy density lithium-ion batteries

2D WS2 nanosheets (NSs) are gaining popularity in the domain of Li-ion batteries (LIBs) due to their unique structures, which can enable reversible insertion and extraction of alkali metal ions. While synthesis methods have mostly relied on the exfoliation of bulk materials or direct growth on substrates, here we report an alternative approach involving colloidal hot-injection synthesis of 2D WS2 in 2H and 1T' crystal phases followed by their electrophoretic deposition (EPD) on the current collector. The produced 2D WS2 NSs' films do not require any additional additives during deposition, which boosts the energy density of the additive-free LIBs produced. The 1T' and 2H NSs exhibit long-term stable cyclic performance at C/5 for 600 cycles. At a high cycling rate (1C), the 2H NSs outperform the 1T' NSs, delivering a 1st cycle reversible capacity of 513 mA h g-1 with capacity retention of 73% after 100 cycles (compared to 205 mA h g-1, and 84 mA h g-1 respectively for NS-1T'). Post-cycling investigation confirms that there is no leaching or cracking of the active material on the surface of anodes after 100 cycles at C/5, which enables mechanical stability, and impressive battery performance of the WS2 NS electrodes.

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.

Mo3S13 Chalcogel: A High-Capacity Electrode for Conversion-Based Li-Ion Batteries

Despite large theoretical energy densities, metal-sulfide electrodes for energy storage systems face several limitations that impact the practical realization. Here, we present the solution-processable, room temperature (RT) synthesis, local structures, and application of a sulfur-rich Mo3S13 chalcogel as a conversion-based electrode for lithium-sulfide batteries (LiSBs). The structure of the amorphous Mo3S13 chalcogel is derived through operando Raman spectroscopy, synchrotron X-ray pair distribution function (PDF), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analysis, along with ab initio molecular dynamics (AIMD) simulations. A key feature of the three-dimensional (3D) network is the connection of Mo3S13 units through S-S bonds. Li/Mo3S13 half-cells deliver initial capacity of 1013 mAh g-1 during the first discharge. After the activation cycles, the capacity stabilizes and maintains 312 mAh g-1 at a C/3 rate after 140 cycles, demonstrating sustained performance over subsequent cycling. Such high-capacity and stability are attributed to the high density of (poly)sulfide bonds and the stable Mo-S coordination in Mo3S13 chalcogel. These findings showcase the potential of Mo3S13 chalcogels as metal-sulfide electrode materials for LiSBs.

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

Resolving the Origins of Superior Cycling Performance of Antimony Anode in Sodium-ion Batteries: A Comparison with Lithium-ion Batteries

Alloying-type antimony (Sb) with high theoretical capacity is a promising anode candidate for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Given the larger radius of Na+ (1.02 Å) than Li+ (0.76 Å), it was generally believed that the Sb anode would experience even worse capacity degradation in SIBs due to more substantial volumetric variations during cycling when compared to LIBs. However, the Sb anode in SIBs unexpectedly exhibited both better electrochemical and structural stability than in LIBs, and the mechanistic reasons that underlie this performance discrepancy remain undiscovered. Here, using substantial in situ transmission electron microscopy, X-ray diffraction, and Raman techniques complemented by theoretical simulations, we explicitly reveal that compared to the lithiation/delithiation process, sodiation/desodiation process of Sb anode displays a previously unexplored two-stage alloying/dealloying mechanism with polycrystalline and amorphous phases as the intermediates featuring improved resilience to mechanical damage, contributing to superior cycling stability in SIBs. Additionally, the better mechanical properties and weaker atomic interaction of Na-Sb alloys than Li-Sb alloys favor enabling mitigated mechanical stress, accounting for enhanced structural stability as unveiled by theoretical simulations. Our finding delineates the mechanistic origins of enhanced cycling stability of Sb anode in SIBs with potential implications for other large-volume-change electrode materials.

In Situ Transmission Electron Microscopy for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attentions in recent years due to the abundance and wide distribution of Na resource on the earth. However, SIBs still face the critical issues of low energy density and unsatisfactory cyclic stability at present. The enhancement of electrochemical performance of SIBs depends on comprehensive and precise understanding of the underlying sodium storage mechanism. Although extensive transmission electron microscopy (TEM) investigations have been performed to reveal the sodium storage property and mechanism of SIBs, a dedicated review on the in situ TEM investigations of SIBs has not been reported. In this review, recent progress in the in situ TEM investigations on the morphological, structural, and chemical evolutions of cathode materials, anode materials, and solid-electrolyte interface during the sodium storage of SIBs is comprehensively summarized. The detailed relationship between structure/composition of electrode materials and electrochemical performance of SIBs has been clarified. This review aims to provide insights into the effective selection and rational design of advanced electrode materials for high-performance SIBs.

Delicate Co-Control of Shell Structure and Sulfur Vacancies in Interlayer-Expanded Tungsten Disulfide Hollow Sphere for Fast and Stable Sodium Storage

Hollow multishelled structure (HoMS) is a promising multi-functional platform for energy storage, owing to its unique temporal-spatial ordering property and buffering function. Accurate co-control of its multiscale structures may bring fascinating properties and new opportunities, which is highly desired yet rarely achieved due to the challenging synthesis. Herein, a sequential sulfidation and etching approach is developed to achieve the delicate co-control over both molecular- and nano-/micro-scale structure of WS_{2- x} HoMS. Typically, sextuple-shelled WS_{2- x} HoMS with abundant sulfur vacancies and expanded-interlayer spacing is obtained from triple-shelled WO₃ HoMS. By further coating with nitrogen-doped carbon, WS_{2- x} HoMS maintains a reversible capacity of 241.7 mAh g^-1 at 5 A g^-1 after 1000 cycles for sodium storage, which is superior to the previously reported results. Mechanism analyses reveal that HoMS provides good electrode-electrolyte contact and plentiful sodium storage sites as well as an effective buffer of the stress/strain during cycling; sulfur vacancy and expanded interlayer of WS_{2- x} enhance ion diffusion kinetics; carbon coating improves the electron conductivity and benefits the structural stability. This finding offers prospects for realizing practical fast-charging, high-energy, and long-cycling sodium storage.

Titanium nitride as a promising sodium-ion battery anode: interface-confined preparation and electrochemical investigation

The search for new electrode materials for sodium-ion batteries (SIBs), especially for enhancing the specific capacity and cycling stability of anodes, is of great significance for the development of new energy conversion and storage materials. Here, a new type of titanium nitride composite anode (TiN@C) coated with 2D carbon nanosheets was prepared for the first time using a rationally designed topochemical conversion approach of interface-confinement. Subsequently, the electrochemical performance and Na⁺ storage mechanism of TiN@C as an anode for SIBs was investigated. The quantum-dot-sized TiN anodes exhibited shorter ionic transport pathways, while the 2D ultrathin carbon nanosheets reinforced the structural stability of the composite and provided a high electron transformation rate. As a result, the TiN/C composite anode can deliver a high reversible capacity of 170 mA h g^-1 and 149 mA h g^-1 after 5000 cycles at a current density of 0.5 A g^-1 and 1 A g^-1, indicating excellent electrochemical properties. This work provides new opportunities to explore the convenient and controllable preparation of metal nitride anodes for other energy conversion and storage applications.

Atomically Thin TaSe2 Film as a High-Performance Substrate for Surface-Enhanced Raman Scattering

An atomically thin TaSe₂ sample, approximately containing two to three layers of TaSe₂ nanosheets with a diameter of 2.5 cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10^-10  m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe₂ are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe₂ film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.

Ultrahigh Rate and Ultralong Life Span Sodium Storage of FePS3 Enabled by the Space Confinement Effect of Layered Expanded Graphite

Metal phosphorus trichalcogenides have been regarded as promising high-capacity anode materials for sodium-ion batteries (SIBs) owing to their high reversible capacity. Nevertheless, their practical application is plagued by poor diffusion kinetics and dramatic volume fluctuations during the charge-discharge process, resulting in no satisfactory rate and life span so far. Herein, we propose a space-confinement strategy to remarkably promote the cycling stability and rate capacity by embedding FePS₃ particles in the interlayer of expanded graphite (EG), which are derived from in situ transformation of graphite intercalation compounds. The layered EG not only greatly alleviates the volume fluctuations of FePS₃ by the space confinement effect so as to maintain the stability of the electrode microstructure, but it also ensures rapid Na⁺ and electron transfer during cycling. When acting as an anode for SIBs, the hybrid electrode delivers a highly reversible capacity of 312.5 mAh g^-1 at an ultrahigh rate of 50 A g^-1 while retaining an ultralong life span of 1300 cycles with a retention of 82.4% at 10 A g^-1. Moreover, the excellent performance of the assembled full battery indicates the practical application potential of FPS/EG.