Superior Sodium Storage Properties in the Anode Material NiCr2 S4 for Sodium-Ion Batteries: An X-ray Diffraction, Pair Distribution Function, and X-ray Absorption Study Reveals a Conversion Mechanism via Nickel Extrusion

Se-Regulated MnS Porous Nanocubes Encapsulated in Carbon Nanofibers as High-Performance Anode for Sodium-Ion Batteries

Manganese-based chalcogenides have significant potential as anodes for sodium-ion batteries (SIBs) due to their high theoretical specific capacity, abundant natural reserves, and environmental friendliness. However, their application is hindered by poor cycling stability, resulting from severe volume changes during cycling and slow reaction kinetics due to their complex crystal structure. Here, an efficient and straightforward strategy was employed to in-situ encapsulate single-phase porous nanocubic MnS0.5Se0.5 into carbon nanofibers using electrospinning and the hard template method, thus forming a necklace-like porous MnS0.5Se0.5-carbon nanofiber composite (MnS0.5Se0.5@N-CNF). The introduction of Se significantly impacts both the composition and microstructure of MnS0.5Se0.5, including lattice distortion that generates additional defects, optimization of chemical bonds, and a nano-spatially confined design. In situ/ex-situ characterization and density functional theory calculations verified that this MnS0.5Se0.5@N-CNF alleviates the volume expansion and facilitates the transfer of Na+/electron. As expected, MnS0.5Se0.5@N-CNF anode demonstrates excellent sodium storage performance, characterized by high initial Coulombic efficiency (90.8%), high-rate capability (370.5 mAh g-1 at 10 A g-1) and long durability (over 5000 cycles at 5 A g-1). The MnS0.5Se0.5@N-CNF //NVP@C full cell, assembled with MnS0.5Se0.5@N-CNF as anode and Na3V2(PO4)3@C as cathode, exhibits a high energy density of 254 Wh kg-1 can be provided. This work presents a novel strategy to optimize the design of anode materials through structural engineering and Se substitution, while also elucidating the underlying reaction mechanisms.

High-Entropy Metal Sulfide Nanocrystal Libraries for Highly Reversible Sodium Storage

Controlled synthesis of high-entropy materials offers a unique platform to explore unprecedented electrochemical properties. High-entropy metal sulfides (HEMSs) have recently emerged as promising electrodes in electrochemical energy storage applications. However, synthesizing HEMSs with a tunable number of components and composition is still challenging. Here, a HEMS library is built by using a general synthetic approach, enabling the synthesis of HEMS with arbitrary combinations of 5 to 12 out of 28 elements in the periodic table. The formation of a solid solution of HEMS is attributed to the two-step method that lowers the energy barrier and facilitates the sulfur diffusion during the synthesis. The hard soft acid base (HSAB) theory is used to precisely describe the conversion rates of the metal precursors during the synthesis. The HEMSs as cathodes in Na-ion batteries (SIBs) is investigated, where 7-component HEMS (7-HEMS) delivers a promising rate capability and an exceptional sodium storage performance with reversible a capacity of 230 mAh g-1 over 3000 cycles. This work paves the way for the multidisciplinary exploration of HEMSs and their potential in electrochemical energy storage.

Olivine-Type Fe2GeX4 (X = S, Se, and Te): A Novel Class of Anode Materials for Exceptional Sodium Storage Performance

The introduction of abundant metals to form ternary germanium-based chalcogenides can dilute the high price and effectively buffer the volume variation of germanium. Herein, olivine-structured Fe2GeX4 (X = S, Se, and Te) are synthesized by a chemical vapor transport method to compare their sodium storage properties. A series of in situ and ex situ measurements validate a combined intercalation-conversion-alloying reaction mechanism of Fe2GeX4. Fe2GeS4 exhibits a high capacity of 477.9 mA h g-1 after 2660 cycles at 8 A g-1, and excellent rate capability. Furthermore, the Na3V2(PO4)3//Fe2GeS4 full cell delivers a capacity of 375.5 mA h g-1 at 0.5 A g-1, which is more than three times that of commercial hard carbon, with a high initial Coulombic efficiency of 93.23%. Capacity-contribution and kinetic analyses reveal that the alloying reaction significantly contributes to the overall capacity and serves as the rate-determining step within the reaction for both Fe2GeS4 and Fe2GeSe4. Upon reaching a specific cycle threshold, the assessment of the kinetic properties of Fe2GeX4 primarily relies on the ion diffusion process that occurs during charging. This work demonstrates that Fe2GeX4 possesses promising practical potential to outperform hard carbon, offering valuable insights and impetus for the advancement of ternary germanium-based anodes.

The Lithium Storage Mechanism of Zero-Strain Anode Materials with Ultralong Cycle Lives

At present, graphite is a widely used anode material in commercial lithium-ion batteries for its low cost, but the large volume expansion (about 10%) after fully lithiated makes the material prone to cracking and even surface stripping in the cycle. Therefore, the development of zero-strain anode materials (volume change <1%) is of great significance. LiAl5O8 is a zero-strain insertion anode material with a high theoretical specific capacity. However, the Li+ storage mechanism remains unclear, and the cycle life as well as fast-charging capability need to be greatly improved to meet the practical requirements. In this study, LiAl5O8 nanorods are prepared by utilizing aluminum ethoxide nanowires as a soft template and doped with the Zr element to further improve the Li+ diffusion coefficient and electronic conductivity, which in turn improves cycle and rate performances. The Zr-doped LiAl5O8 presents a high reversible capacity of 227.2 mAh g-1 after 20,000 cycles under 5 A g-1, which significantly outperforms the state-of-the-art anode materials. In addition, the Li+ storage mechanisms of LiAl5O8 and Zr-doped LiAl5O8 are clearly clarified with a variety of characterization techniques including nuclear magnetic resonance. This work greatly promotes the practical process of zero-strain insertion anode materials.

Loss-free pulverization by confining copper oxide inside hierarchical nitrogen-doped carbon nanocages toward superb potassium-ion batteries

Taking the advantages of hierarchical nitrogen-doped carbon nanocages (hNCNCs) with nanocavities for encapsulation and multiscale micro-meso-macropores/high conductivity for mass/electron synergistic transportation, a conversion-type CuO anode material is confined inside hNCNCs for potassium storage. The so-obtained yolk-shelled CuO@hNCNC hybrids have tunable CuO contents in the range of 11.7-63.7 wt%. The unique architecture leads to the loss-free pulverization of the active components during charge/discharge, which increases the surface-controlled charge storage, shortens the K+ solid diffusion lengths with an enlarged K+ diffusion coefficient, and meanwhile enhances the rate capability and durability. Consequently, the optimized CuO@hNCNC delivers a high specific capacity of 498 mA h g-1 at 0.1 A g-1 and 194 mA h g-1 at 10.0 A g-1 based on the total mass of CuO@hNCNC, and a long-term stability. The capacity based on the CuO active component reaches a record-high 522 mA h g-1 at 1.0 A g-1 after 2000 cycles, which is ca. 2.5 times the state-of-the-art value in the literature. The evolution of the cycling performance with CuO loading is well understood based on the loss-free pulverization. This study demonstrates a new strategy to turn the generally harmful pulverization of active components into a beneficial factor for K+ storage, which paves the way for exploring high-performance anodes for rechargeable batteries.

Is there a common reaction pathway for chromium sulfides as anodes in sodium-ion batteries? A case study about sodium storage properties of MCr2S4 (M = Cr, Ti, Fe)

We present new insights into the electrochemical properties of three metal sulfides MCr2S4 (M = Cr, Ti, Fe) probed as anode materials in sodium-ion batteries for the first time. The electrodes deliver decent reversible capacities and good long-term cycle stability, e.g., 470, 375, and 524 mAh g−1 are obtained after 200 cycles applying 0.5 A g−1 for M = Cr, Ti, and Fe, respectively. The reaction mechanisms are investigated via synchrotron-based X-ray powder diffraction and pair distribution function analyses. The highly crystalline educts are decomposed into Na2S nanoparticles and ultra-small metal particles during initial discharge without formation of intermediate NaCrS2 domains as previously reported for CuCrS2 and NiCr2S4. After a full cycle, the structural integrity of MCr2S4 (M = Cr, Ti, Fe) is not recovered. Thus, the Na storage properties are attributed to redox reactions between nanoscopic to X-ray amorphous conversion products with only local atomic correlations M···S/S···S in the charged and M···M/Na···S in the discharged state.

Graphical Abstract

Dual Regulation of Metal Doping and Adjusting Cut-Off Voltage for MoSe2 to Achieve Reversible Sodium Storage

MoSe₂ , as a typical 2D material, possesses tremendous potential in Na-ion batteries (SIBs) owing to larger interlayer distance, more favorable band gap structure, and higher theoretical specific capacity than other analogs. Nevertheless, the low intrinsic electronic conductivity and irreversible conversion of discharged products of Mo/Na₂ Se to MoSe₂  seriously hamper its electrochemical performance. Herein, through a facile hydrothermal method combined with calcination process, Sn-doped MoSe₂  nanosheets grown on graphene substrate in the vertical direction are fabricated. Benefiting from the improved electronic conductivity contributed by the abundant defects and expanded interlamellar spacing of MoSe₂  originated from Sn doping, combined with a smart strategy of raising discharge cut-off voltage to 0.2 V during the actual performance testing for SIBs, the as-fabricated anode material delivers superior Na-ions storage performance in terms of electrons/ions transfer, reversible sodium storage as well as cycle stability. An ultra-stable reversible specific capacity of 268.5 mAh g^-1 at 1 A g^-1 can be maintained after 1600 cycles. Moreover, the great sodium storage property in the SIB full-cell system of the as-obtained nanocomposite illustrates practical potential. Density functional theory calculation and in situ/ex situ measurements are employed to further reveal the storage mechanism and process of Na-ions.

MOF-Derived Long Spindle-like Carbon-Coated Ternary Transition-Metal-Oxide Composite for Lithium Storage

Fe₃O₄ is a promising alternative for next-generation lithium-ion batteries (LIBs). However, its poor cycle stability due to the large volume effect during cycling and poor conductivity hinders its application. Herein, we have successfully designed and prepared a carbon-coated ternary transition-metal-oxide composite (noted as (FeCoNi)₃O₄@C), which is derived from FeCoNi-MOF-74 (denoted as FeCoNi-211-24). (FeCoNi)₃O₄@C perfectly inherited the long spindle-shaped precursor structure, and (FeCoNi)₃O₄ particles grew in situ on the precursor surface. The ordered particles and the carbon-coated structure inhibited the agglomeration of particles, improving the material's cycle stability and conductivity. Therefore, the electrode exhibited excellent electrochemical performance. Specifically, (FeCoNi)₃O₄@C-700 presented excellent initial discharge capacity (763.1 mAh g^-1 at 0.2 A g^-1), high initial coulombic efficiency (73.8%), excellent rate capability, and cycle stability (634.6 mAh g^-1 at 0.5 A g^-1 after 505 cycles). This study provides a novel idea for developing anode materials for LIBs.

Interface Engineering of NixSy@MnOxHy Nanorods to Efficiently Enhance Overall-Water-Splitting Activity and Stability

Three-dimensional (3D) core-shell heterostructured Ni_xS_y@MnO_xH_y nanorods grown on nickel foam (Ni_xS_y@MnO_xH_y/NF) were successfully fabricated via a simple hydrothermal reaction and a subsequent electrodeposition process. The fabricated Ni_xS_y@MnO_xH_y/NF shows outstanding bifunctional activity and stability for hydrogen evolution reaction and oxygen evolution reaction, as well as overall-water-splitting performance. The main origins are the interface engineering of Ni_xS_y@MnO_xH_y, the shell-protection characteristic of MnO_xH_y, and the 3D open nanorod structure, which remarkably endow the electrocatalyst with high activity and stability. Exploring highly active and stable transition metal-based bifunctional electrocatalysts has recently attracted extensive research interests for achieving high inherent activity, abundant exposed active sites, rapid mass transfer, and strong structure stability for overall water splitting. Herein, an interface engineering coupled with shell-protection strategy was applied to construct three-dimensional (3D) core-shell Ni_xS_y@MnO_xH_y heterostructure nanorods grown on nickel foam (Ni_xS_y@MnO_xH_y/NF) as a bifunctional electrocatalyst. Ni_xS_y@MnO_xH_y/NF was synthesized via a facile hydrothermal reaction followed by an electrodeposition process. The X-ray absorption fine structure spectra reveal that abundant Mn-S bonds connect the heterostructure interfaces of Ni_xS_y@MnO_xH_y, leading to a strong electronic interaction, which improves the intrinsic activities of hydrogen evolution reaction and oxygen evolution reaction (OER). Besides, as an efficient protective shell, the MnO_xH_y dramatically inhibits the electrochemical corrosion of the electrocatalyst at high current densities, which remarkably enhances the stability at high potentials. Furthermore, the 3D nanorod structure not only exposes enriched active sites, but also accelerates the electrolyte diffusion and bubble desorption. Therefore, Ni_xS_y@MnO_xH_y/NF exhibits exceptional bifunctional activity and stability for overall water splitting, with low overpotentials of 326 and 356 mV for OER at 100 and 500 mA cm^-2, respectively, along with high stability of 150 h at 100 mA cm^-2. Furthermore, for overall water splitting, it presents a low cell voltage of 1.529 V at 10 mA cm^-2, accompanied by excellent stability at 100 mA cm^-2 for 100 h. This work sheds a light on exploring highly active and stable bifunctional electrocatalysts by the interface engineering coupled with shell-protection strategy.

Understanding sodium storage properties of ultra-small Fe3S4 nanoparticles - a combined XRD, PDF, XAS and electrokinetic study

Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small Fe₃S₄ nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe₃S₄ into nanocrystalline intermediates occurs accompanied by reduction of Fe³⁺ to Fe²⁺ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe⁰ finely dispersed in a nanosized Na₂S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na₂S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g^-1 after 300 cycles applying 0.5 A g^-1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe₃S₄ with smaller domains are reversibly generated in the 1^st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe₃S₄ in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.

Long-Term Stable, High-Capacity Anode Material for Sodium-Ion Batteries: Taking a Closer Look at CrPS4 from an Electrochemical and Mechanistic Point of View

Electrochemical performance of the layered compound CrPS₄ for the usage as anode material in sodium-ion batteries (SIBs) was examined and exceptional reversible long-term capacity and capacity retention were found. After 300 cycles, an extraordinary reversible capacity of 687 mAh g^-1 at a current rate of 1 A g^-1 was achieved, while rate capability tests showed an excellent capacity retention of 100%. Detailed evaluation of the data evidence a change of the electrochemical reaction upon cycling leading to the striking long-term performance. Further investigations targeted the reaction mechanism of the first cycle by applying complementary techniques, i.e., powder X-ray diffraction (XRD), pair distribution function (PDF) analysis, X-ray absorption spectroscopy (XAS), and ²³Na/³¹P magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The results indicated an unexpectedly complex reaction pathway including formation of several intercalation compounds, depending on the amount of Na inserted at the early discharge states and subsequent conversion to Na₂S and strongly disordered metallic Cr at the completely discharged state. While XAS measurements suggest no further presence of intermediates after formation of Na intercalation compounds, several different phases are detected via MAS NMR upon continued discharging. Especially the data obtained from the MAS NMR investigations therefore point toward a very complex reaction pathway. Furthermore, solid electrolyte interphase (SEI) formation, resulting in the presence of NaF, was observed. After recharging the anode material, no structural long-range order occurred, but short-range order indeed resembled the local environment of the starting material, to a certain extent.